Chemistry of magnetic covalent organic frameworks (MagCOFs): From synthesis to separation applications

Abstract

Magnetic covalent organic frameworks (MagCOFs) represent a new class of emerging material, primarily employed in separation applications. Due to their attractive properties such as easy and cost-effective synthesis, high porosity, large surface area, strong π-π interactions between COF shells and facile magnetic recovery, MagCOFs are being widely deployed in adsorption applications and exhibited high adsorption performance. This review emphasizes on the key advancements in the synthesis of MagCOFs and documents their potential in diverse separation applications including nitro explosive treatment, heavy metal ion adsorption and extraction of persistent organic pollutants, poly aromatic hydrocarbons and endocrine-disrupting chemicals, among others. In addition to offering up-to-date research outcomes in the field of separation science by utilizing MagCOFs, this article also highlights the challenges and possible solutions in this exciting developing area of research. This journal is

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1432 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
Cite this: Mater. Adv., 2022,
3, 1432
Chemistry of magnetic covalent organic
frameworks (MagCOFs): from synthesis
to separation applications
Priya Yadav,
ab
Manavi Yadav,
ab
Rashmi Gaur,
a
Radhika Gupta,
a
Gunjan Arora,
ac
Anju Srivastava,*
b
Anandarup Goswami, *
d
Manoj B. Gawande
e
and Rakesh K. Sharma *
a
Magnetic covalent organic frameworks (MagCOFs) represent a new class of emerging material, primarily
employed in separation applications. Due to their attractive properties such as easy and cost-effective
synthesis, high porosity, large surface area, strong p–pinteractions between COF shells and facile mag-
netic recovery, MagCOFs are being widely deployed in adsorption applications and exhibited high
adsorption performance. This review emphasizes on the key advancements in the synthesis of MagCOFs
and documents their potential in diverse separation applications including nitro explosive treatment,
heavy metal ion adsorption and extraction of persistent organic pollutants, poly aromatic hydrocarbons
and endocrine-disrupting chemicals, among others. In addition to offering up-to-date research
outcomes in the field of separation science by utilizing MagCOFs, this article also highlights the
challenges and possible solutions in this exciting developing area of research.
a
Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007, India. E-mail: rksharmagreenchem@hotmail.com
b
Department of Chemistry, Hindu College, University of Delhi, Delhi 110007, India. E-mail: dr.anjusrivastava@gmail.com
c
Department of Chemistry, Hansraj College, University of Delhi, Delhi 110007, India
d
Chemistry Division, Department of Sciences and Humanities, Vignan’s Foundation for Science, Technology and Research (VFSTR), Vadlamudi, Guntur 522 213, Andhra
Pradesh, India
e
Department of Industrial and Engineering Chemistry, Institute of Chemical Technology, Mumbai-Marathwada Campus, Jalna, Maharashtra, India
Priya Yadav
Priya Yadav is an active member
of Green Chemistry Network
Centre, University of Delhi,
India, and ACS. She received her
BSc degree from University of
Rajasthan, Jaipur in 2013 and
MSc degree from Banasthali
Vidyapith, Rajasthan in 2016.
She is currently working at the
Green Chemistry Network Centre.
She has received the award for
junior research fellowship from
University Grants Commission,
Delhi, India. She has numerous
publications in renowned journals of RSC, ACS and Wiley. Her
research interest includes fabrication of core–shell structured
magnetic nanocatalysts and their applications in diverse organic
transformations.
Radhika Gupta
Radhika Gupta is currently pur-
suing her doctoral studies under
the supervision of Professor R. K.
Sharma at Green Chemistry
Network Centre, University of
Delhi, India. She received her
Master’s degree in Chemistry from
Indian Institute of Technology,
Delhi in 2015 and Bachelor’s
degree in Chemistry from Hansraj
College, University of Delhi in 2013
where she also secured first
position. She is presently serving
as the Vice-Chair of ACS
International Student Chapter University of Delhi. Her area of
research includes green chemistry, catalysis through hydrogen
bonding interactions, immobilization of ionic liquids and
fabrication of magnetic nanocatalysts for various organic
transformations.
Received 12th November 2021,
Accepted 5th January 2022
DOI: 10.1039/d1ma01060c
rsc.li/materials-advances
Materials
Advances
REVIEW

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1433
1. Introduction
Over the decades, porous materials have gained tremendous
attention due to their magnificent potential and broad range of
applications in the field of sensing, energy storage, catalysis,
energy conversion, gas storage and separation, optoelectronics,
etc.
1–3
To date, a variety of highly porous materials having large
surface area such as hyper-cross-linked polymers, conjugated
microporous polymers and polymers with intrinsic micro-
porosity have been designed.
1
However, their linking chemistry
and the subsequent product formation primarily relies on the
kinetically controlled irreversible coupling reactions and unfor-
tunately this irreversibility results in poor self-healing ability,
structural disorder and formation of non-separable oligomers,
limiting their wider applicability. To overcome these chal-
lenges, organic porous polymers with long range ordered
structure and high crystallinity are considered to be potential
replacements.
4,5
In this context, the pioneering work by Prof.
Omar Yaghi and co-workers first introduced covalent organic
frameworks (COFs) in 2005 as a new generation of crystalline
framework materials.
6–8
COFs represent a promising class of well-defined porous and
crystalline polymeric materials synthesized via condensation
reactions of 2D and 3D organic building precursors.
9,10
In
general, they are composed of light elements such as hydrogen,
boron, oxygen, carbon, nitrogen and silicon which are linked
via strong covalent bonds in a reversible manner to generate
reticular architecture. This thermodynamically controlled and
Gunjan Arora
Gunjan Arora was born in India
in the year 1992. She received her
graduate degree in Chemistry in
2013; and post graduate degree
specialized in Inorganic Chemi-
stry in 2015 from Hansraj
College, University of Delhi, India.
She then joined Prof. Sharma’s
group to pursue her doctoral
studies. She is currently a senior
research fellow and an active
member of Green Chemistry
Network Centre, Department of
Chemistry, University of Delhi,
India. She is working in the area of Green Chemistry and
Nanocatalysis. Her research interest includes the design and
development of sustainable magnetically retrievable heterogeneous
catalysts for various organic transformations.
Anju Srivastava
Prof. Anju Srivastava is a Prof-
essor of Chemistry and Principal
of a premier institution ranked
amongst top three in the
country, Hindu College, Univer-
sity of Delhi. She received her
MSc and PhD in Synthetic
Polymer Chemistry from IIT
Delhi and has a teaching
experience of 24 years in subject
areas of organic chemistry, bio-
chemistry, environment chemistry
and analytical chemistry.
Following that she is co-editor of
several books and has been nominated as Fellow at ILLL (Institute
of Lifelong Learning) University of Delhi and conferred with the
‘Distinguished Teacher’ Award by Former President of India, Dr A.
P. J. Abdul Kalam.
Anandarup Goswami
Dr Anandarup Goswami completed
his BSc with Chemistry Hons. in
2002 from Presidency College,
Kolkata, India and obtained his
MSc in 2004 from Indian Institute
of Technology, Kanpur, India. He
then went on to receive his PhD
from Cornell University, New York,
USA and then undertook a few
postdoctoral stints in USA,
Europe, Czech Republic and then
joined Vignan’s Foundation for
Science, Technology and Research
(VFSTR) as Associate Professor in
the Division of Chemistry, Department of Sciences and Humanities in
November 2016. His main research area focuses on synthesis,
characterization, and applications of nanomaterials in catalysis,
environmental remediation, etc.
Manoj B. Gawande
Prof. Manoj B. Gawande received
his PhD in 2008 from Institute of
Chemical Technology, Mumbai,
India, and then undertook
several research stints in Germany,
South Korea, Portugal, Czech
Republic, USA, and UK. Presently,
he is an Associate Professor at
Institute of Chemical Technology,
Mumbai-Marathwada Campus,
India. Recently, he was invited
as visiting professor in chemistry
at RCPTM-CATRIN, Palacky
University, Czech Republic. His
research interests focus on single-atom catalysts, advanced
nanomaterials, as well as cutting-edge catalysis and energy
applications. He is also included in the global list of the top 2%
of scientists in the chemistry field for the years 2019 and 2020 by
Stanford University, US.
Review Materials Advances

1434 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
reversible dynamic covalent chemistry of COFs allows ‘‘proof
reading’’ and ‘‘error checking’’ during the synthesis and results
into the self-healing ability of COFs leading to the formation of
long range ordered crystalline structures.
11
Additionally, the
presence of p–pinteractions and hydrogen bonding among
COF shells strengthens their porous skeleton and provides
long-term stability.
3
Recently, COFs have attracted researchers’
interest because of their intriguing properties such as tuneable
composition and high thermal and chemical stability
(Fig. 1)
12,13
and have been widely used in a variety of applica-
tions including energy conversion
14,15
and storage,
16,17
gas
storage,
18,19
catalysis,
20–25
optoelectronics,
26,27
sensing,
28,29
drug delivery
30–32
and adsorption
33–35
to name a few.
Despite being recognized as a very promising class of
materials at an early stage of discovery of COFs, unfortunately,
the progress in the field is restricted due to some challenges.
For example, their extraction from the matrix is not very
efficient because of their low density which causes difficulty
in their separation from reaction medium.
36
In addition, their
large-scale applications are hampered due to tedious centrifu-
gation and filtration methods. Over the past few decades,
several techniques have been introduced to circumvent those
challenges such as liquid–liquid phase extraction, Solid-phase
extraction, solid-phase micro extraction, Soxhlet extraction and
many others.
37
However, most of these techniques are often
associated with numerous shortcomings, including large
amount of toxic solvent consumption, low selectivity, cumber-
some filtration and time-consuming procedures.
38
Therefore,
there is always a need to develop simple, environmentally
benign and highly sensitive synthetic techniques which not
only provide COFs with greater selectivity but also speed up the
recovery process. Recently, magnetic solid-phase extraction
(MSPE), a promising modification of SPE, has been developed
as the most prominent technique in the field of separation and
sample pre-treatment as it offers an easy recovery of adsorbent
using a simple external magnetic field, with high extraction
efficiency and large adsorption capacity.
39,40
Owing to the
appealing properties of magnetic nanoparticles (e.g. Fe
3
O
4
;
MNPs) including cost-effectiveness, reusability, easy operation,
facile synthesis, low toxicity and good magnetic performance,
they have been further explored in a variety of disciplines and
are commonly used as magnetic extraction materials.
41–55
Moreover, a wide variety of functional moieties can be grafted
over the surface of magnetic nanocomposites, which make
further modifications and reactions more convenient. The
introduction of magnetism in COFs has proven to be an
effective strategy to solve the recovery problem of COFs. Con-
sidering the broad diversity, unique capability and wide appli-
cations of both magnetic nanoparticles and COFs, we believe
that this combination will open up newer avenues in the
rational design of core–shell structures with magnetic plat-
forms and tuneable porous COF shells for wide applications.
Henceforth, after witnessing the effectiveness of MSPE, one
of the notable approaches to circumvent the issue of separation
would be to simultaneously integrate COFs with a magnetic
component which would provide the ease of separation. This
would give rise to MagCOFs that would combine the merits of
rapid separation with adequate binding sites and high adsorp-
tion performance with enhanced functionality.
38,56–66
More
recently, MagCOFs have marked a breakthrough in the current
area of COF-related research. In particular, solid spherical
magnetic core can offer a center for uniform COF growth and
crystallization in all directions, which provides a possibility for
controllable synthesis of magnetic core–shell COFs.
67
There-
fore, synthesis of MagCOFs and the investigation of their
in-depth utility in adsorption science has become an area of
prime significance in material science.
1.1 Magnetic properties of MagCOF
MagCOF composites possess various significant features that
are beneficial from green chemistry and sustainability view-
points such as effortless and economic recovery. The evaluation
of magnetic properties of the material is highly substantial. The
magnetic properties of MagCOF are investigated using vibrating
sample magnetometry (VSM) at room temperature.
68
In the
hysteresis loop, to examine the magnetism, saturation magnetiza-
tion (M
s
) is used. Also, if no hysteresis loop and coercivity or
remanence is observed in the curve, it suggests that MagCOF
composites are superparamagnetic in nature.
69
It is shown that
upon modification of MNPs with COF layer, saturation magneti-
zation value of MagCOF composite decreases than initial MNPs.
70
Fig. 1 Properties and applications of COFs.
Rakesh K. Sharma
Dr R. K. Sharma is a professor
and coordinator of Green Chemi-
stry Network Centre at University
of Delhi, India. He is also an
Honorary Professor at Deakin
University, Australia. He went to
the University of Tokyo and
Kumamoto University on JSPS
Post-Doctoral Fellowship. He has
published numerous book
chapters, reviews and research
articles in renowned inter-
national journals. He has also
written/edited various books on
Green Chemistry published by RSC, World Scientific and Wiley.
Dr Sharma is also the Honorary Secretary of the RSC (North India
Section) and faculty advisor of ACS International Student Chapter
University of Delhi. http://greenchem.du.ac.in/.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1435
Inspite of the decrease in M
s
values of MagCOF, the synthetic
material could be easily recovered from the reaction medium with
the aid of an external magnet within few seconds.
Reusability and recyclability are the prime features of mag-
netic composites that controls the economy and efficiency of
process at industrial scale. Although the benefits of using COFs
as adsorbents are enormous, but their large-scale utility is still
hampered due to their expensiveness, cumbersome filtration
and centrifugation process used for the recovery of the compo-
site. In this regard, MNPs supported COFs have been utilized as
significant adsorbents for environmental remediation. This
introduction of magnetism in COFs circumvent the issue of
separation and provide the ease of separation.
To check the recyclability of MagCOF composite, numerous
adsorption–desorption cycles are performed. In view of this,
Cai et al. synthesized magnetic adsorbent (Fe
3
O
4
@COFs) and
shows excellent stability and reusability even after ten adsorp-
tion–desorption cycles.
71
Similarly, Li and co-workers utilized
Fe
3
O
4
@COF@Zr
4+
for the extraction and determination of
organophosphorus pesticides in vegetables.
72
Subsequently,
various adsorption–desorption cycles were performed using
the same magnetic composite. It is noteworthy that, prior to
next cycle the as-synthesized MagCOF was washed with acetone
and distilled water. Thus, Fe
3
O
4
@COF@Zr
4+
can be reused for
six cycles without any significant loss in its activity. Hence, the
above examples state that MagCOF composite showed admir-
able reusability, mechanical stability and cost-effectiveness and
also advantageous from environmental viewpoints. Table 1 lists
the recyclability runs of numerous MagCOFs composites.
2. Focus of the review
In the present review, we focus on the synthesis of MagCOFs
and their utility as magnetic extractants in various fields in
combination with MSPE. The main insights of the review article
are based on the development of MagCOFs in the domain of
adsorption and separation, reflecting the need for their immo-
bilization and deliberating the ways by which the immobilized
MagCOFs can be fabricated. We have broadly discussed the
adsorption of substances of concern for human health and
environment such as endocrine-disrupting chemicals (EDCs),
enrichment of phosphopeptides, nitro explosives and persis-
tent organic pollutants (POPs), among others. The adsorption
mechanism mainly lies on the strong p–pinteractions, hydro-
phobic effect, electrostatic interaction and hydrogen bonding
between aromatic rings of MagCOFs and phenyl rings of
analytes. In recent years, numerous review articles have been
documented in the literature on COFs and many research
groups worldwide have explored the synthesis and applications
of COFs.
2,3,12,73
The literatures accumulated on COFs are
primarily focussed on (i) the current state-of-the-art on design,
synthetic strategies, structural studies (structure of 2D and
3D COFs) and applications of COFs,
1
(ii) the development of
heterogeneous single-site catalysts,
74
(iii) special emphasis has
been given on the application of COFs in various fields such
as catalytic, electrocatalytic, photocatalytic and separation
applications (water treatment or the separation of gas mixtures
and organic molecules, including chiral and isomeric
compounds)
3,11,12
and (iv) a detailed description on shape/size
selectivity as well as Schiff-base chemistry of COFs.
7
Numerous
significant research articles have been published on syn-
thesis of MagCOFs and their utility in separation applica-
tions.
60,67,69,71,75,76
The present review article consolidates the
synthetic and applicative aspects of MagCOFs where MNPs
immobilized COFs have been used for the extraction and
separation of various analytes for analytical determinations of
substances of concern areas.
3. Applications of magnetic COFs
As mentioned earlier, there is a need for sensitive and selective
adsorption of inorganic and organic substances for a range of
applications such as environmental monitoring, food quality
control and chemical threat detection. Amongst a variety of
adsorbents that have been designed and utilized in the past,
MagCOFs based magnetic adsorbents have paved their own
way on the account of their superior characteristics over others.
Recently, several researchers have begun exploring the potential
of MagCOFs as extraction materials due to their exceptional
structural tunability and properties.
67
Described below, is a series
of MagCOFs based chemical adsorbents that have been fabricated
for the selective and efficient extraction of toxic chemical
pollutants.
3.1 Extraction of endocrine-disrupting chemicals
EDCs are a new generation of environmental pollutants which
causes an antagonistic effect to the endocrinal, reproductive,
cardiological, and nervous systems.
77,78
A wide range of sub-
stances (both natural and man-made), such as polychlorinated
Table 1 Saturation magnetization values and reusability runs of various
MagCOFs
S.
No. MagCOF Saturation
magnetization (M
s
)Recyclability
runs Ref.
1Fe
3
O
4
@COF 62.5 3 148
2Fe
3
O
4
@TpBd 22 5 67
3Fe
3
O
4
@COF 42.7 12 70
4 Magnetic TpPa-1 40.1 n.d. 71
5Fe
3
O
4
@COF 48.4 10 100
6 M-COF-2 15.8 5 149
7Fe
3
O
4
@COF 42 20 114
8Fe
3
O
4
@COF@Au-b-CD 21.86 n.d. 150
9 MagCOF@MIP@CD 10.97 8 151
10 COF-(TpBd)/Fe
3
O
4
51.8 4 142
11 SPIO@COF-guanidyl 42 5 121
12 Fe
3
O
4
@iCOFs 21.8 7 122
13 Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
35.67 n.d. 123
14 mTpBd-Me
2
60.5 5 152
15 M-DAPS-COF-SH 19.6 5 131
16 Fe
3
O
4
@COF(TpPa-1) 19.5 5 153
17 Fe
3
O
4
@COF(TpBD)@
Au-MPS 40.7 n.d. 154
18 Fe
3
O
4
@TbBd 41.4 10 155
n.d. = no data.
Review Materials Advances

1436 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT),
phthalates and phenols (mainly bisphenol A (BPA)), bisphenol
AF (BPAF), 4-n-nonylphenol, 4-n-octylphenol are reported to
cause endocrine disruption. Most of these substances are
broadly used as surfactants and plasticizers in the production
of food packaging materials.
79
In view of the attempts to extract EDCs, Zhao et al. recently
designed core–shell structured MagCOFs microspheres
(Fe
3
O
4
@COF) by the reaction of 1,3,5-tris(4-aminophenyl) ben-
zene (TAPB) and terephthaldicarboxaldehyde (TPA) with Fe
3
O
4
as shown in Fig. 2.
80
The fabricated Fe
3
O
4
@COF microspheres
were deployed in the selective extraction of four endocrine-
disrupting phenols (4-n-nonylphenol, 4-n-octylphenol, BPA and
BPAF) from plastic-packaged food, drinks and environmental
water systems using a combination of MSPE with HPLC. The
presence of surface functional groups (–COOH, –CQN, –NH
2
)
in Fe
3
O
4
@COF forms p–pinteractions and hydrogen bonds
with the specific sites of target molecules, which make them
conducive for MSPE to extract target molecule from various
samples. The proposed extraction mechanism involves the
formation of hydrogen bonds between amino groups as well
as CQN groups present on the surface of Fe
3
O
4
@COF micro-
spheres and the hydroxyl groups of endocrine-disrupting
phenols. Moreover, due to the presence of amino groups on
Fe
3
O
4
@COF, the polarity and hydrophilicity of the particles
gets improved, resulting in additional enhancement in extrac-
tion proficiency. Additionally, because of the large log K
ow
(it is
a ratio value without a unit and defines solubility of substance
in water) and pK
a
values of endocrine-disrupting phenols, they
have a great tendency to escape from the water phase to the
surface of Fe
3
O
4
@COF. Finally, the efficacy further increases
due to the presence of pinteractions between aromatic rings
of COF shells and aromatic rings of endocrine-disrupting
phenols. Besides, the indicative mesoporous structure of
Fe
3
O
4
@COF also plays a vital role in the adsorption process
by providing large surface area with appropriate surface func-
tionalities. The excellent extraction performance is also attri-
buted to the hydrophobicity of the material and the ionization
of endocrine-disrupting phenols. Also, the other factors that
ascribed to high adsorption performance of MagCOFs are
hydrogen bonding, superparamagnetism, electrostatic inter-
actions and ion-exchange. Besides, the coordination reaction
between composite materials and pollutants is also a key
parameter. The method earned excellent linearity (RZ0.995)
within the concentration range of 0.05–1000 ng mL
1
. The limit
of detection (LOD) and limit of quantification (LOQ) were in
the range of 0.08 ng mL
1
to 0.21 ng mL
1
(S/N = 3) and
0.39 ng mL
1
to 0.85 ng mL
1
(S/N = 10), respectively. Besides,
the magnetic adsorbent showed high saturation magnetization
(62.5 emu g
1
) and can be used for 20 runs without any loss
in its adsorption capacity. The experimental data revealed
that MagCOFs possess great potential as MSPE adsorbent for
extracting various other pollutants from food. Hence, after the
adsorption of pollutants using MagCOFs, the external magnet
was used to assist their separation from aqueous media i.e.,
if external magnet was removed, the MagCOF composites were
dispersed again within a few seconds.
Likewise, Li and co-workers designed sensitive and efficient
core–shell MagCOFs (NiFe
2
O
4
@COFs) via facile room tempera-
ture ultrasonic approach and the resulting magnetic nano-
composites were utilized for effervescent reaction-enhanced
microextraction of endocrine disruptors in liquid matrices
(Fig. 3).
81
The equilibrium adsorption capacity of six EDCs
on magnetic adsorbent were found to be between 38.3 and
88.1 mg g
1
which were primarily attributed to p–pinteraction
between analytes and MagCOFs. Moreover, NiFe
2
O
4
@COFs
showed M
s
value of 17.1 emu g
1
. The magnetic nanocompo-
sites effervescent reaction-enhanced microextraction (MNER-
EM-HPLC-FLD) method had good linearity in the range of
0.1–0.5–200 mgL
1
, high precision (relative standard deviations
(RSDs) of 3.11–5.73%), good recoveries (83.4–106.2%) with
lower LOD (0.019–0.096 mgL
1
) and R
2
(0.8952). Additionally,
NiFe
2
O
4
@COFs could be reused for at least six cycles.
BPA is an another EDC which has been widely used to
produce epoxy resins and polycarbonate plastics.
82,83
It is a
pervasive contaminant which can interfere with endocrine
systems of humans, wildlife and environmental health.
In addition, it has been reported to induce a decrease in daily
sperm count and fertility.
84
Thus, the removal of BPA and
BPAF through selective and efficient adsorption has gained
Fig. 2 Synthesis of Fe
3
O
4
@COF and extraction of EDCs. Reproduced with
permission from ref. 80. Copyright 2019 Springer Nature Publishing AG.
Fig. 3 Schematic diagram of the MNER-EM/HPLC-FLD method. (a)
Synthesis of NiFe
2
O
4
@COF MNCs, (b) preparation of the magnetic effer-
vescent tablets and (c) pre-concentration measurements and analytical
procedures of EDCs by the MNER-EM/HPLC-FLD method. Reproduced
with permission from ref. 81. Copyright 2020 Elsevier B.V.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1437
tremendous attention in recent years. Though numerous
adsorption methods have been developed so far, nearly all of
them are associated with some drawbacks such as complicated
synthetic procedure of the materials, prolonged time period,
lower adsorption capacity of the material, slower adsorption
kinetics, and many more. In this respect, considering the rising
interest in the development of MagCOFs, Yan et al. designed
monomer mediated in situ growth strategy for the controllable
synthesis of core–shell structured MagCOF nanospheres,
Fe
3
O
4
@TpBd, for the efficient adsorption and removal of EDCs,
BPAF and BPA from aqueous solution (Fig. 4a).
67
The Fe
3
O
4
@TpBd was designed in a very systematic manner:
Fe
3
O
4
NPs were initially synthesized via a solvothermal method,
afterwards –NH
2
groups were introduced on the surface of
magnetic core to produce amino functionalized nanospheres
(Fe
3
O
4
–NH
2
) to avoid aggregation of MNPs as well as to
incorporate polar surface functionalities. Furthermore, a
monomer of TpBd, 1,3,5-triformylphloroglucinol (Tp), was
grafted over Fe
3
O
4
–NH
2
via Schiff base condensation reaction
for in situ growth of COF shell. To acquire uniform crystalline
growth of COF shell, pre-grafting of monomer played a crucial
role. The pre-grafting of monomer Tp acts as a bridge between
magnetic core and COF shell to achieve better interaction and
helps to nucleate the centers for uniform COF growth. Further,
another monomer benzidine (Bd) was grafted over uniform
core–shell through imine bond formation. Additionally,
TEM, HAADF-STEM and EDX elemental mapping data con-
firmed a typical core–shell structure of Fe
3
O
4
@TpBd nano-
spheres (with TpBd thickness being 15–65 nm) (Fig. 4b–d).
Moreover, the choice of solvent and concentration of mono-
mers acts as key factors to control the thickness and crystal-
linity of COF shells.
The as-synthesized Fe
3
O
4
@TpBd was then explored for
adsorption of BPA and BPAF and displayed higher adsorption
capacity with faster adsorption kinetics followed by facile
magnetic separation. The adsorption equilibrium was achieved
within 5 min for BPA and BPAF. The adsorption isotherms of
BPA and BPAF on Fe
3
O
4
@TpBD illustrate a typical Langmuir
adsorption with the maximum adsorption capacities of 160.6
and 236.7 mg g
1
, respectively. Fe
3
O
4
@TpBd showed excellent
reusability up to five runs, which rendered them as novel
adsorbent for aqueous solution adsorption.
In a similar fashion, extraction of BPs from human serum
sample using MNPs immobilized COF, Fe
3
O
4
@COF, was
studied by Lin and co-workers (Fig. 5).
70
The core–shell struc-
tured magnetic adsorbent was fabricated by immobilizing
two building units, TAPB and TPA on the surface of MNPs (of
size 250 nm) via Schiff base condensation reaction at room
temperature. Further, the adsorption efficiency of Fe
3
O
4
@COF
(with COF thickness being 25 nm) was investigated by eval-
uating their adsorption isotherms and adsorption kinetics.
Further, to determine the magnetic properties of bare Fe
3
O
4
and Fe
3
O
4
@COF, VSM was used. From Fig. 5c, it can be noted
that M
s
value decreases from Fe
3
O
4
(54.5 emu g
1
)toFe
3
O
4
@
COF (42.7 emu g
1
) which can be attributed to grafting of COF
shell over Fe
3
O
4
. The inset of Fig. 5c display that Fe
3
O
4
@COF
nanocomposite was dispersed in aqueous solution and separated
with the aid of external magnet within 2 minutes. Besides, thermo-
gravimetric analysis (TGA) of Fe
3
O
4
and Fe
3
O
4
@COF showed weight
loss of 6% and 48% respectively, which implies high yield of COF
shells over MNPs in the temperature range of 100–350 1C. Fe
3
O
4
@
COF showed excellent thermal stability under 300 1C(Fig.5d).
Moreover, due to high porosity, high thermal and chemical stability
and large specific surface area (181.36 m
2
g
1
)ofmagneticadsor-
bent, equilibrium was attained very quickly, indicating fast adsorp-
tion kinetics and size selectivity. The high adsorption of BPs was
achieved due to van der Waals forces and strong p–pinteractions
between COF shells and benzene rings of BPs.
The developed protocol possessed good linearity in the
range of 0.1–50 mgL
1
with R
2
Z0.9982. Besides, LODs were
Fig. 4 (a)Monomer-mediated in situ strategy for synthesis of core–shell
structured Fe
3
O
4
@TpBd nanospheres, TEM images of (b) Fe
3
O
4
,(c)Fe
3
O
4
@
TpBD and (d) HAADF-STEM image and EDX elemental mapping of
Fe
3
O
4
@TpBD. Reproduced with permission from ref. 67. Copyright 2017
Royal Society of Chemistry.
Fig. 5 (a) Preparation of the Fe
3
O
4
@COF nanocomposites and (b) MSPE
process for extraction of BPs in human serum sample, (c) VSM curves of
Fe
3
O
4
and Fe
3
O
4
@COFs and (d) TGA curve of Fe
3
O
4
and Fe
3
O
4
@COFs.
Reproduced with permission from ref. 70. Copyright 2018 Elsevier B.V.
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1438 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
found to be between 1.0 to 78.1 ng L
1
with high enrichment
factor (56–95-fold). The RSDs were less than 3.4% for inter-day
and 6.9% for intra-day. Moreover, the authors assessed the
performance of Fe
3
O
4
@COF in real samples and satisfactory
results were obtained with recoveries ranging from 93.0–
107.8%. The additional advantage of the developed adsorbent
was its ability to be recycled and reused for twelve adsorption–
desorption cycles.
3.2 Extraction of persistent organic pollutants
Over several decades, the exposure of humans and wildlife to
environmental contaminants has been a global concern. PCBs
are a class of POPs that exert various detrimental effects on
humans and wildlife, such as neurotoxicity, immunotoxicity,
reproductive toxicity and carcinogenicity.
85,86
PCBs can be
metabolized as hydroxylated PCBs (–OH PCBs) in higher organ-
isms including humans and animals via cytochrome P450
enzyme-mediated oxidation mechanism. The –OH PCBs are
produced in the environment by the oxidation of PCBs through
both metabolic transformations (in living organisms) and
abiotic reactions (with hydroxyl radicals). At present, –OH PCBs
are being detected in animal tissue sample, water, sediment
and even in human blood (which often leads to endocrine
toxicity).
87
Hence, –OH PCBs have raised serious environmental
concerns due to their toxic effects and therefore, it is quintes-
sential to develop an efficient method for the extraction of
–OH PCBs.
On a similar note, an aptamer functionalized magnetic core–
shell structured conjugated organic framework (Fe
3
O
4
@COFs-
Apt) has been developed for the selective extraction of trace
–OH PCBs from human serum sample (Fig. 6a and b).
36
An
aptamer is a single strand DNA/RNA molecule, having high
binding affinity towards target molecule through van der
Waals, electron–acceptor–donor and hydrogen-bonding inter-
actions. For the synthesis of Fe
3
O
4
@COFs-Apt, firstly, MNPs
were synthesized by two conventional methods, i.e., co-
precipitation and solvothermal method. To improve chemical
stability and to avoid agglomeration, bare MNPs were coated
with silica using tetraethyl orthosilicate (TEOS). Relatively
inexpensive and simple silica coating not only minimizes the
aggregation but also provides many potential benefits such as
chemical inertness, high binding strength with iron-oxide core,
robustness, possibility for further functionalization, etc.
88–95
Further, amino groups were introduced on the surface of
Fe
3
O
4
@SiO
2
to obtain amino-modified Fe
3
O
4
@SiO
2
@NH
2
.
Afterwards, the monomers, p-phenylenediamine (PPD) and
trimesoyl chloride (TMC) were added to obtain carboxylic acid
rich MagCOFs (Fe
3
O
4
@COFs-COOH). Thus, to enhance the
extraction efficiency, aptamers were immobilized on the sur-
face of MagCOFs (Fe
3
O
4
@COFs-Apt) via covalent linkage
between surface carboxylic groups and the amino groups of
the aptamers. Fe
3
O
4
with average size of 150 nm were obtained
by solvothermal method, denoted as Fe
3
O
4
@COFs-Apt-150 and
Fe
3
O
4
with an average size of 15 nm were obtained by co-
precipitation method and denoted as Fe
3
O
4
@COFs-Apt-15.
The N
2
adsorption–desorption isotherms revealed that
Fe
3
O
4
@COFs-Apt-15 had higher surface area and hence con-
sidered as a better adsorbent material. Subsequently, Fe
3
O
4
@
COFs-Apt-15 was deployed for the extraction of hydroxy-
20,30,40,5,50-pentachlorobiphenyl (2-OH-CB 124). To check the
selectivity of Fe
3
O
4
@COFs-Apt-15 for 2-OH-CB 124, three other
compounds such as 40-hydroxy-2,3,30,5,6-pentachlorobiphenyl
(40-OH-CB 112), 4-hydroxy-2,3,4,5-tetrachlorobiphenyl (40-OH-
CB 61) and 40-hydroxy-2,4,6-trichlorobiphenyl (40-OH-CB 30)
were selected to conduct unspecific tests. The results unveiled
that Fe
3
O
4
@COFs-Apt-15 displayed highest recovery of 490%
for 2-OH-CB 124, indicating the superior extraction efficiency of
2-OH-CB 124 (Fig. 6c). The adsorption ability for above four
compounds was also investigated with Fe
3
O
4
-15 and Fe
3
O
4
@
COFs-COOH-15 only to obtain satisfactory recoveries with
Fe
3
O
4
@COFs-Apt-15. The presence of p–pstacking interaction,
high surface area and hydrophilic property (raised from car-
boxylic group) increased the adsorption capacity of Fe
3
O
4
@
COFs-Apt-COOH-15. Meanwhile, Fig. 6c depicts that in compar-
ison with Fe
3
O
4
@COFs-Apt-15, Fe
3
O
4
@COFs-COOH-15 could
not provide satisfactory selectivity for 2-OH-CB 124 (recoveries
o25%), which indicates that aptamer played a substantial role
towards selectivity of 2-OH-CB 124 due to their highly specific
binding affinity. Under optimized conditions, Fe
3
O
4
@COFs-
Apt-15 showed great potential in determination of 2-OH-CB
124 from human serum samples with recoveries in the range of
87.7–101.5%. The MSPE-LC/MS coupled method exhibited
good linearity in the range of 0.01–40 ng mL
1
with excellent
R
2
value of 0.9973. The LOD was low (2.1 pg mL
1
) with a
satisfactory LOQ (6.7 pg mL
1
). Besides providing easy mag-
netic recoverability, the adsorbent could be recycled for ten
extraction cycles.
Poly aromatic hydrocarbons (PAHs) are a type of POPs that
have long been considered as hazardous for both human and
environment. They mainly originate from anthropogenic activ-
ities such as pyrolysis of organic materials such as oil, coal,
wood and petroleum gas. Due to highly mutagenic, carcino-
genic and teratogenic potential of PAHs, they are one of the
greatest environmental and economic threats to our society.
96
In this regard, Cai et al. fabricated a gypsophila bouquet-
shaped MagCOF, TpPa-1, by grafting COF over the surface-
modified MNPs, for enhanced extraction of PAHs from envir-
onmental samples (Fig. 7).
71
The novel TpPa-1 was synthesized
Fig. 6 (a) Synthetic illustration for the synthesis of Fe
3
O
4
@COFs-Apt, (b)
typical process for selective extraction of OH-PCBs with Fe
3
O
4
@COFs-Apt
and (c) comparison of extraction efficiency of 2-OH-CB 124, 40-OH-CB
112, 40-OH-CB 61 and 40-OH-CB 30 with Fe
3
O
4
-15, Fe
3
O
4
@COFs-
COOH-15 and Fe
3
O
4
@COFs-Apt-15. Reproduced with permission from
ref. 36. Copyright 2018 John Wiley & Sons.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1439
by facile room temperature solution-phase approach and uti-
lized the advantageous properties of both MNPs and COF.
Initially, surface-modified Fe
3
O
4
nanoparticles (of size 30 nm)
were synthesized by pre-grafting of monomer Tp onto amino
functionalized Fe
3
O
4
nanoparticles. This process helped in
acquiring uniform COF shell by generating COF centers via
Schiff base reaction. Further, another monomer, phenylenedia-
mine (Pa-1) was added under mechanical agitation to obtain
magnetic TpPa-1. The TEM image in Fig. 7 clearly depicts the
gypsophila bouquet-shaped structures of TpPa-1. Magnetic
TpPa-1 possessed superparamagnetism (40.1 emu g
1
) as well
as large surface area and high porosity.
The as-synthesized magnetic TpPa-1 was further explored for
the extraction of PAHs. Initially, magnetic TpPa-1 was directly
dispersed into filtered water sample spiked with PAHs and
further subjected to ultrasonication followed by additional
shaking for 20 min. After extraction of analytes, the sorbent
was gathered using external magnet and the supernatant was
subjected to HPLC-FLD system for further detection. Moreover,
it was anticipated that TpPa-1 might extract organic targets
containing benzene ring, amino or hydroxyl groups due to the
presence of high percentage of oxygen and nitrogen atoms as
well as large p–pframework. The fabricated nanocomposite
exhibited an excellent linear range of 2.0–200.0 ng L
1
with a
good R
2
of 0.9993, with LOD ranging from 0.24–1.01 ng L
1
.
Furthermore, the material possessed good reusability and
could be applied in real sample analysis including lake water,
tap water and river water with percentage recoveries of PAHs
over 73–110% with a lower RSD value (2–8%). Inspired by
encouraging extraction efficiencies of magnetic TpPa-1 towards
PAHs, it is expected that the bouquet shaped MagCOF could
also have strong affinity towards other chemicals. Besides,
magnetic TpPa-1 can also be utilized in wider applications by
post-synthetic modification approach.
Similarly, a new and promising class of COF has been
synthesized by Chen and group for PAHs extraction.
97
They
synthesized COF-LZU1@PEI@Fe
3
O
4
via covalent immobiliza-
tion of COF-LZU1 (imine-based COF, prepared by Schiff base
reaction of 1,3,5 triformyl benzene (Tb) and 1,4 diaminoben-
zene) on PEI-functionalized MNPs (Fig. 8a). Firstly, MNPs were
coated with polyethyleneimine (PEI) by electrostatic interac-
tions (PEI@Fe
3
O
4
). Thereafter, several active amino groups
present on PEI enhanced the covalent bonding between the
PEI layer and COF-LZU1, due to which COF shells were grown
on the surface of MNPs through Schiff base reaction. The
designed method was further applied for the extraction of six
PAHs from environmental and real samples by MSPE coupled
with HPLC. The concentration level of PAHs in an environ-
mental sample is quite low to determine directly by analytical
instrument. So, their pretreatment using MSPE was necessary
prior to HPLC. COF-LZU1@PEI@Fe
3
O
4
exhibited high surface
area, excellent thermal stability and high porosity, which
increased the loading capacity of the adsorbent. As discussed
earlier, the presence of aromatic rings and imine groups in COF
led to strong p–pstacking and hydrophobic interaction with
analytes. These characteristics made them an attractive mate-
rial for solid phase extraction of PAHs. The TEM images
showed that PEI@Fe
3
O
4
and COF-LZU1@PEI@Fe
3
O
4
display
beautiful cubic morphology with mean diameter of 40 nm
(Fig. 8b and c). Also, it can be noted that surface of PEI@Fe
3
O
4
was smooth and after modification with COF-LZU1, a rough
and thin polymer layer was observed (due to porous structure of
COF-LZU1) which confirms immobilization of COF-LZU1 over
PEI@Fe
3
O
4
.
To optimize the extraction efficiency, various parameters
were investigated, such as the effect of temperature, type of
solvent, pH of the sample solution, extraction time and
sampling volume. Due to the presence of highly hydrophobic
aromatic rings in PAHs, the addition of small amount of
acetonitrile facilitates their solubility in aqueous sample
solution. Further, it was observed that when acetonitrile con-
tent increased from 0.5% to 1%, PAH extraction efficiency
increases while it decreases when acetonitrile content increases
from 2% to 10%. Hence, results stated that a certain amount of
acetonitrile do facilitates the solubility in sample solution. Best
extraction was achieved when 1% acetonitrile was used as a
solvent. Further, sample pH was investigated from 4.0 to 9.0.
It can be noted that as pH varies from 4.0 to 9.0, extraction
efficiency of PAH increases. This is because H
+
can affect the
Fig. 8 (a) Synthesis of COF-LZU1@PEI@Fe
3
O
4
and TEM images of (b)
PEI@Fe
3
O
4
and (c) COF-LZU1@PEI@Fe
3
O
4
. Reproduced with permission
from ref. 97. Copyright 2021 Springer Nature Publishing AG.
Fig. 7 Synthesis and application of the bouquet-like magnetic TpPa-1
sorbent. Reproduced with permission from ref. 71. Copyright 2017 Amer-
ican Chemical Society.
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1440 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
interaction between PAH and adsorbent. Then, with decrease in
pH value, the –NH
2
groups present on COF-LZU1 would change
to NH
3+
and this would decrease the p-electron cloud density
and weak p–pstacking interaction by its electron-withdrawing
inductive effect. Thus, for better extraction efficiency 9.0 was
selected as sample pH value. The extraction time was 30 min
with a sample volume of 20 mL. Under optimized conditions,
the extraction performance of COF-LZU1@PEI@Fe
3
O
4
was
evaluated. The results showed that COF-LZU1@PEI@Fe
3
O
4
displayed significant extraction efficiency. The combination
of MSPE and HPLC methods showed good linear range with
R
2
higher than 0.9989. The LOQ was 0.5–50 pg mL
1
. Besides,
the adsorbent could be reused for up to six cycles. The above
research expands the scope of MagCOFs as adsorbents for
extraction of various other pollutants.
3.3 Extraction of triazole fungicides
Triazole fungicides (TFs) are the systematic broad-spectrum
fungicides which usually contain hydroxyl/ketone groups,
unique 1,2,4-triazole ring and substituted phenyl ring in the
main chain.
98
TFs are widely used in agricultural production
mainly to protect crops from various fungal diseases. Also, TFs
have been used in leather, textile industries, wood preserva-
tives, paints and antifouling agents and considered as the
largest fungicide category by the global market value.
99
However,
they can enter into multiple environmental media through sur-
face run offs and contaminate agricultural products, surface
water, human urine, soils and hair samples. Exposure to TFs
causes adverse health effects such as liver carcinogenicity, repro-
ductive and development toxicities, hepatic toxicities and endo-
crine disruption.
99
Hence, their efficient detection and effective
removal are highly imperative.
In this context, Hu et al. fabricated magnetic porous organic
polymers (MOPs) via simple azo reaction under mild reaction
conditions between monomer, TAPB and 1,3,5-trihydroxy-
benzene using Fe
3
O
4
@SiO
2
as the magnetic core (Fig. 9a).
37
The as-synthesized MOPs (having 5 nm thick polymer layer)
have been utilized as an adsorbent for the extraction of five
target TFs (triazolone, penconazole, hexaconazole, diniconazole
and tebuconazole) from vegetables by MSPE. For the desorption
ofTFs,organicsolventswithdifferentpolaritysuchasethyl
acetate, acetonitrile, mixed solvents (acetone : ethyl acetate (1 : 1/
v/v)) and acetone were used but best results were obtained when
ethyl acetate was used as a desorption solvent. The MSPE-GC-FID
showed good linearity in the range of (0.5–200 mgL
1
with R
2
4
0.9991–0.9998). The LOD for five target TFs was found to be in the
range of 0.12–0.19 mgL
1
according to the three-fold signal-to-
noise ratio. Moreover, lower RSDs value (2.6–6.8%), efficient
extraction efficiency (84.5–90.3%) and reusability up to five runs
demonstrated its excellent performance. The developed MSPE-
GC-FID was facile, rapid, selective and sensitive and proved its
incredible potential towards extraction of TFs.
3.4 Extraction of triclosan and triclocarban
Over the past few years, pharmaceutical and personal care
products (PPCPs) have emerged as one of the most hazardous
groups of environmental contaminants.
101
PPCPs include a
diverse range of chemical substances which cause potential
adverse effects to humans, wildlife and aquatic organisms.
Hence, it is essential to control the pollution level originating
from PPCPs. Among the variety of PPCPs, triclosan (TCS) and
triclocarban (TCC) are two chlorinated antimicrobial additives
which have been broadly used in detergents for sanitation.
102
However, their incomplete removal often leads to several health
and environmental problems such as endocrine-disrupting
effect, bioaccumulation in snails and algae, development of
microbial resistance and many more.
103,104
Due to its rising
concentration in environment and the abovementioned health
effects, there is a need for the development of reliable treatment
methods to remove these contaminants.
On this note, Cai et al. fabricated Fe
3
O
4
(MNPs) supported
core–shell structured MagCOF, Fe
3
O
4
@COF, for the efficient
extraction and rapid removal of TCC and TCS from water and
serum samples (Fig. 9b).
100
To fabricate Fe
3
O
4
@COF, MNPs (of
size 165 nm) were first synthesized via solvothermal method.
Further, COF shell was introduced on the surface of bare Fe
3
O
4
by Schiff base condensation reaction of monomer TAPB and
TPA in DMSO at room temperature. Additionally, the adsorp-
tion performance of Fe
3
O
4
@COF was investigated by Langmuir
and Freundlich adsorption isotherms. The Langmuir model
was more appropriate in the high concentration region while
Freundlich isotherm was more appropriate in the low concen-
tration region. Also, Langmuir and Freundlich isotherms dis-
played monolayer adsorption at high concentration range and
multilayer adsorption at low concentration, respectively. The
monolayer adsorption took place due to strong p–pstacking
interactions between the interface of TCS, TCC and Fe
3
O
4
@COF
Fig. 9 (a) Preparation of MOPs, and (b) Synthesis of Fe
3
O
4
@COFs. Repro-
duced with permission from ref. 100. Copyright 2019 American Chemical
Society.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1441
while multilayer adsorption was due to van der Waals forces, p–p
interactions among phenyl rings and space embedding inter-
action. Moreover, the adsorption kinetics followed a pseudo-
second-order kinetic model.
The quantitative analysis of TCC and TCS was determined
using UHPLC-MS/MS. The results stated good linearity in the
range of 0.05–25 ng mL
1
for TCC and 0.1–50 ng mL
1
for TCS
with R
2
greater than 0.9976. The LODs for TCC and TCS were
0.005 and 0.02 ng mL
1
, respectively. The method was further
applied to remove TCC and TCS from healthy fetal bovine serum
samples and showed excellent recoveries in the range of 92.9–
109.5% and 82.3–95.4% for TCC and TCS, respectively. Moreover,
the adsorbent was reused for ten adsorption–desorption cycles
with similar extraction efficiency. With the combination of mar-
vellous characteristics of COF and magnificent properties of
Fe
3
O
4
,Fe
3
O
4
@COF served as a potential candidate for the treat-
ment of environmental and biological samples.
3.5 Diclofenac extraction
Diclofenac sodium is a widely used non-steroidal anti-
inflammatory drug for treating several types of inflammatory
disorders.
105
Since past few years, it has gained increasing
global concern due to its frequent detection in surface, ground
and aquatic environment and even in drinking water. It has also
found to be responsible for causing aquatic ecotoxicity and severe
renal failure because of its extremely low biodegradability.
106
Thus,
there is need for the development of cost-effective, efficient and
reliable technique for the removal of diclofenac sodium from
adulterated water bodies.
Continuing the utility of MagCOFs, Shuai et al. developed a
MagCOF via salt-mediated crystallization strategy under solvent
free environment for the removal of diclofenac sodium from
water. The fabrication procedure involved the grafting of Tp
and p-phenylenediamine (PDA) on amino-functionalized MNPs
using p-toluenesulfonic acid as catalyst (Fig. 10).
107
Fig. 10b
and c shows narrow size distribution and uniform spherical
morphology of Fe
3
O
4
nanospheres with mean diameter of
10–15 nm. Also, the magnetite nanoparticles are tightly shelled
by SiO
2
. Besides, MagCOF-2 shows that MNP-NH
2
is well
encapsulated in COF framework. Adsorption isotherms dis-
played a Freundlich isotherm which was well fitted in case of
adsorption of diclofenac sodium on MagCOF-2 with a high R
2
value (B0.99) of the curve and a maximum adsorption capacity
of 565 mg g
1
. The adsorption equilibria were achieved within
20 min indicating the supremacy of MagCOF-2 towards diclofenac
sodium removal. The value of Freundlich constant (n41)
revealed that the adsorption of diclofenac sodium was favourable.
Moreover, the experimental data reflected that the effective
adsorption is caused by p–pstacking, hydrogen bonding and
electrostatic interactions. Additionally, with excellent magnetiza-
tion and high specific surface area, the as-synthesized magnetic
adsorbent could be reused for five cycles without any significant
loss of its adsorption capacity. The applicability of MagCOF-2 was
also assessed in actual water samples. Interestingly, the results
ascribed superior potential of MagCOF-2 for diclofenac sodium
removal.
Similar to the previously discussed MNPs grafted COF,
Wang et al. designed few other MagCOFs by facile impregna-
tion method for the removal of diclofenac and sulfamethazine
(SMT). For the synthesis, firstly, TPB-DMTP-COFs were synthe-
sized using the reaction between dimethoxyterephthaldehyde
(DMTP) and TAPB monomers. Secondly, the as-synthesized
TPB-DMTP-COF was reacted with a solution containing Fe
2+
and Fe
3+
followed by membrane separation. Finally, separated
solids were immersed in ammonia solution to yield Fe
3
O
4
in COF structure, (Fig. 11).
108
The adsorption activity of the
MagCOF was evaluated by adsorption kinetics and isotherms.
Adsorption kinetics revealed that equilibria were achieved
within 50 min for the removal of diclofenac and 80 min for
Fig. 10 (a) Synthesis of MagCOF-2 via a solvent free, salt-mediated
crystallization strategy, TEM images of (b) MNP-NH
2
and (c) MagCOF-2.
Reproduced with permission from ref. 107. Copyright 2019 American
Chemical Society.
Fig. 11 Synthetic steps of MagCOFs.
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1442 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
SMT removal with a maximum adsorption capacity of
40.4 mg g
1
and 55.24 mg g
1
, respectively. From the experi-
mental data it was noted that pseudo-first-order model was
better in diclofenac adsorption while SMT adsorption was well
described by pseudo-second-order kinetic model with R
2
40.99
in both cases. In addition, the adsorption process of diclofenac
and SMT was well defined by Sips isotherm model. By Sips model,
the maximum adsorption capacity of diclofenac and SMT
was found to be 109 mg g
1
and 113.2 mg g
1
, respectively.
Furthermore, the adsorption mechanism was studied by den-
sity functional theory (DFT) which revealed that the adsorption
took place by C–Hpinteraction. The adsorption energies
were calculated to be 47.8 kJ mol
1
and 63.2 kJ mol
1
for
SMT and diclofenac, respectively. When the efficacy of the
magnetic adsorbent was compared with previously reported
adsorbents, MagCOF showed higher adsorption capacity.
3.6 Paclitaxel extraction
Paclitaxel is known as a key component in the treatment of
numerous solid tumors including breast, lung and refractory
ovarian.
109
Their therapeutic effect is mediated on the poly-
merization of tubulin, thereby causing suppression/cell death
of microtubule dynamics during cell division without changing
microtubule mass.
110
Paclitaxel was approved as an anti-cancer
drug by U.S. FDA in 1992.
111
The examination of paclitaxel in
urine, plasma and other bio-fluids is essential for pharmaco-
toxicology, pharmacodynamics and pharmacokinetic evalua-
tion. However, due to the low levels of paclitaxel in bio-fluids,
their pretreatment is highly demanded prior to instrumental
analysis. In this regard, Chen et al. developed COF-1 modified
MNPs (Mag-COF-1) for the selective extraction of paclitaxel
from rat plasma samples by combining the merits of MSPE
with HPLC (Fig. 12a).
41
Mag-COF-1 was fabricated by the bio-
inspired polydopamine method through immobilization of
layered hexagonal COF-1 framework (constructed via self-
molecular dehydration reaction of BDBA), on the surface of
MNPs through catechol groups. Polydopamine played a signifi-
cant role in the immobilization process due to the presence of
catechol groups. Further, Mag-COF-1 was utilized in adsorption
of paclitaxel. Moreover, selective extraction of paclitaxel was
attributed to the hydrophobic interaction, p–pinteraction
and host–guest interaction among phenyl rings present in
Mag-COF-1 and the functional moieties present in paclitaxel.
Additionally, TEM images of M-PEI showed uniform cube like
morphology and a smooth layer was introduced over M-PEI
(M-PEI; 40 nm) in case of Mag-COF-1 (Fig. 12b and c). The
developed MSPE-HPLC method offered outstanding linearity in
the range of 0.1–200 ng mL
1
and a lower LOD (0.02 ng mL
1
).
The chromatograms of plasma samples show that after extraction
paclitaxel was efficiently enriched and showed good clean-up
capacity (Fig. 12d). Paclitaxel content in medicated plasma
samples was calculated to be 3.28 mgmL
1
.TheRSDvaluewas
less than 2.3% confirming excellent recoveries in the range of
99.4–103.7%. M-COF-1 was easy to recycle and which could be
reused up to six runs under the same conditions. Good linearity,
high accuracy and good selectivity showed that the proposed
method was efficient and sensitive for the extraction of paclitaxel.
3.7 Sulfonamides extraction
Sulfonamides (SAs) represent a most important class of syn-
thetic anti-microbials, which have been widely practiced for the
treatment of bacterial infections.
112
Also, world widely they
have been utilized to treat both human and veterinary diseases.
Owing to their high efficacy, low-cost, and broad-spectrum anti-
microbial activities, they have been used in aquaculture and
livestock. However, the uncontrolled use of SAs and their
continual discharge to aquatic environment lead to emiction,
resistant bacteria in human body, hemopoiesis and carcino-
genicity.
113
Additionally, water-borne SAs can enter the food
chain and cause severe allergic reactions to human. Hence, it is
important to develop a highly sensitive and efficient method to
detect SAs in environmental and food samples.
In view of this, Zhang et al. developed porphyrin-based
MagCOF (Fe
3
O
4
@COF) for the extraction and analysis of six
SAs (sulfamerazine (SMR), sulfadiazine (SDZ), sulfamethazine
(SMZ), sulfamethoxazole (STZ), sulfamonomethoxine (SMX) and
sulfadimethoxine (SDM)).
114
For the synthesis, firstly, amino-
functionalized MNPs were formed via facile one-pot strategy.
Further, COF shells were grown on the surface of amino-
functionalized nanospheres using dehydration reaction between
two organic ligands namely 4,40-biphenyldicarboxaldehyde and
TAP(Fig.13a).Thepresenceofaminogroupsonthesurface
of Fe
3
O
4
nanospheres facilitated the in situ COF growth via
Schiff base reaction. Moreover, the presence of nitrogen contain-
ing heterocyclic ring in porphyrin-based COF offers specific
multiple-interactions i.e.,p–pinteraction, hydrogen bonding,
hydrophobic and electrostatic interactions with target analytes.
Fig. 13b and c showed that the as-obtained Fe
3
O
4
@COF
(10 2 nm thickness) nanospheres were spherical in shape with
average size of 100 14 nm.
Thereafter, to check the superiority of the Fe
3
O
4
@COF
nanospheres, the extraction of six SAs were evaluated system-
atically using MSPE coupled with HPLC. In the extraction
process, initially, the target analyte SAs were pre-treated via
sample pre-treatment technique (MSPE) and further subjected
to quantitative analysis via HPLC. The chromatograms of
six SAs with extraction, without extraction and by using Fe
3
O
4
Fig. 12 (a) Synthesis of Mag-COF-1 and extraction of paclitaxel, TEM
images of (b) M-PEI and (c) Mag-COF-1 and (d) chromatograms of rat
plasma samples (A) with extraction and (B) without extraction. Reproduced
with permission from ref. 41. Copyright 2016 Elsevier B.V.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1443
nanospheres are shown in Fig. 13d–f, which illustrates the high
extraction efficiency of core–shell Fe
3
O
4
@COF nanospheres.
The results revealed that out of the six SAs, SMX and SDM with
methoxy groups substituted on pyrimidine ring displayed
higher extraction efficiency instead of methyl substitution.
The reason stated that the methoxy group could form conjuga-
tion which lowered the basicity of pyrimidine ring, thereby
enhancing the electrostatic and p–pinteraction with porphyrin
ring than that of methyl substitution. From Fig. 13f, it is
observed that in comparison to Fe
3
O
4
@COF, bare Fe
3
O
4
showed
much lower peak area of SAs extraction.Thisisduetomultiple
interactions between COF layer and SAs. Also, Fe
3
O
4
@COF
showed strong adsorption affinity for SAs due to their high surface
area and porosity, which also leads to enhancement of extraction
efficiency. The method presented a wide linear range from
1 to 500 ng mL
1
. The LOD for six SAs were ranged from and
0.2–1 ng mL
1
with R
2
higher than 0.99. Further, the estab-
lished method was applied in real samples including milk,
pork, chicken, lake water and shrimp for the detection of SAs.
The MSPE-HPLC method showed excellent recoveries (65.3–
107.3%) in environmental water and food with RSD r6.7.
Hence, specific surface area, high stability, good porosity and
large p-conjugated system endowed Fe
3
O
4
@COF nanospheres
as an excellent sorbent for the analysis of SAs.
Likewise, Zhou and co-workers fabricated thiolated-b-cyclo-
dextrin-functionalized MagCOFs for the extraction of trace SAs
from meat samples (Fig. 14).
115
The approach involved amino-
modified MNPs as the support material. In this case, b-CD
could form inclusion complexes via capturing targeted analytes
into its cavity through host–guest relationship. It was prepared
by direct solvothermal method by taking FeCl
3
as the iron
source and ((3-aminopropyl)triethoxysilane) (APTES) as –NH
2
source (to facilitate the Schiff base reaction between COF
monomers and magnetic core). After its formation, the mono-
mers Tp and Bd were covalently grafted on the surface of
Fig. 14 (a) Schematic illustration for the synthesis for Fe
3
O
4
@COF@Au-b-
CD and (b) the MSPE procedure for the determination of SAs in meat.
Reproduced with permission from ref. 114 Copyright 2020 Springer Nature
Publishing AG.
Fig. 13 (a) Synthesis of Fe
3
O
4
@COFnanospheres, TEM images of (b)
Fe
3
O
4
,(c)Fe
3
O
4
@COF, and chromatograms of six SAs standard solutions
(d) before extraction, (e) after extraction by core–shell structured Fe
3
O
4
@
COFs nanospheres and (f) after extraction on bare Fe
3
O
4
nanospheres
under optimum condition. Peak identity: 1, SDZ; 2, SMR; 3, SMZ; 4, SMX; 5,
STZ; 6, SDM. Reproduced with permission from ref. 114. Copyright 2019
Elsevier B.V.
Review Materials Advances

1444 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
Fe
3
O
4
-NH
2
to form Fe
3
O
4
@COF. Further, Fe
3
O
4
@COF@Au was
obtained by initial immobilization of AuCl
4
4H
2
OonFe
3
O
4
@
COF by electrostatic interaction between amine group and
AuCl
4

, followed by reduction of Au(III) to Au NPs using NaBH
4
as the reductant. Finally, b-CD functionalized COF (Fe
3
O
4
@
COF@Au-b-CD) was formed by Au-S bonding between Fe
3
O
4
@
COF@Au and thiolated-b-CD.
The final material Fe
3
O
4
@COF@Au-b-CD (with COF thick-
ness being 25 nm) was exploited for the extraction of nine SAs.
The extraction of SAs increased due to hydrogen bonding
between hydroxyl groups on the surface of MagCOF and
amino groups of SAs. Additionally, the presence of pinter-
action between phenyl rings, uniform pore structure and the
large specific surface area also enhances the extraction effi-
ciency. Besides, hydrophilicity and polarity also played a vital
role in extraction process. To check the applicability of this
adsorbent, authors studied linearity, LOD, precision and RSDs
under optimized conditions. It was found that the proposed
method showed good linearity in the range of 2–100 mgkg
1
and the LODs were calculated to be 0.8–1.6 mgkg
1
(S/N = 3).
Moreover, the authors performed extraction process at two
kinetic models: pseudo-first-order and pseudo-second-order and
it was found that pseudo-second-order kinetic was better fitted
with the adsorption kinetics of SAs with R
2
greater than 0.99.
Admiringly, the proposed method was magnificently applied for
quantification of SAs from real samples. The recoveries were
ranged from 78.9 to 112.0%. The fabricated magnetic adsorbent
furnished high extraction efficiency of trace SAs under mild
conditions.
3.8 Glycopeptides extraction
Glycosylation is considered as one of the most important and
complex post-translational modifications.
116
Protein glycosyla-
tion plays a vital role in biological activity and bio-synthesis
including molecular recognition, protein folding, intracellular
transport, fertilization morphogenesis, endocytosis and many
others.
117
Meanwhile, both structural and compositional changes
in glycosylation are allied with a variety of human diseases, such
as autoimmune, breast cancer, hepatocellular carcinoma, prostate
cancer and genetic diseases.
118
Therefore, there is a need for the
development of a stable and sensitive method for the enrichment
of glycopeptides.
Considering the benefits of MagCOFs, Qian and co-workers
fabricated Fe
3
O
4
@TpPa-1 via a two-step solvothermal reaction
and investigated the efficacy of the material for the hydrophilic
enrichment of N-glycopeptides from standard glycoproteins
and human serum digests by Hydrophilic interaction chroma-
tography (HILIC) (Fig. 15).
119
To synthesize crystalline keto-
enamine linked Fe
3
O
4
@TpPa-1, initially, Fe
3
O
4
(of size 40 nm)
were obtained by hydrothermal method. Subsequently, the
monomers Tp and Pa were grafted over MNPs through in situ
solvothermal reaction to acquire MagCOF. The highly ordered 2D
topology with nitrogen rich frameworks and amino-terminals
enhances the enrichment efficiency for N-glycopeptides. The
TEM and SEM images (Fig. 15b and c) show sea-urchin type
morphology of the adsorbent, which could provide increased
specific surface area and available active sites which are highly
advantageous for mass transfer and separation processes.
Further, the feasibility of Fe
3
O
4
@TpPa-1 was checked in
the enrichment of N-glycopeptides using tryptic digests of a
standard glycoprotein (human IgG). Fig. 15d–f represents that
without enrichment, only six peaks of glycopeptides with lower
intensities were observed. However, after enrichment with
Fe
3
O
4
@TpPa-1, 37 glycopeptide peaks were analyzed with high
MS signal intensities and increased S/N ratio which was higher
than previously reported literature. Further the activity of bare
MNPs was also tested: only 6 glycopeptide peaks were detected
along with the detection sensitivity of Fe
3
O
4
@TpPa-1 at
different concentrations. It was found that upon a decrease in
concentration, the number of glycopeptides was decreased.
Moreover, the selectivity and specificity test of Fe
3
O
4
@TpPa-1
was also evaluated using digest mixture of IgG and bovine
serum albumin. Results indicated that Fe
3
O
4
@TpPa-1 showed
fast, efficient and selective enrichment of N-glycopeptides.
Inspired by the results, the authors also investigated the cap-
ability of Fe
3
O
4
@TpPa-1 in tryptic digests of human serum,
which greatly showed identification of 228 glycopeptides
corresponding to 114 glycoproteins within three independent
replicates which is better than commercial HILIC. The LOD was
found to be low (28 fmol) with recoveries of 94.3% and 86.3%
for two deglycopeptides.
On a similar note, Deng et al. presented a promising
approach for the enrichment of endogenous glycopeptides in
human saliva by designing hydrophilic MagCOF (mCTpBd) as
shown in Fig. 16.
120
The mCTpBd with inherent hydrophilicity
has been synthesized by the following procedure: first of all,
Fe
3
O
4
were synthesized by solvothermal method and to facil-
itate the growth of COF shell, MNPs were coated with a layer of
silica via sol–gel approach. Thereafter, to enhance the inter-
action, amine groups were introduced by surface modification
of the silica encapsulated MNPs using APTES (Fe
3
O
4
-NH
2
;
336 nm). Furthermore, Fe
3
O
4
-NH
2
surface was pre-modified
using a monomer, carboxyl-modified 1,3,5-triformylphloro-
glucinol, CTp via amidation reaction (Fe
3
O
4
-NH
2
-CTp), to boost
Fig. 15 Schematic illustration of the (a) synthesis of Fe
3
O
4
@TpPa-1,
(b) SEM, (c) TEM image of Fe
3
O
4
@TpPa-1, MALDI-TOF MS spectra of
human IgG digest (70 fmol mL
1
) (d) direct analysis, (e) after enrichment
by Fe
3
O
4
and (f) after enrichment by Fe
3
O
4
@TpPa-1. Reproduced with
permission from ref. 119. Copyright 2017 Royal Society of Chemistry.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1445
the interaction among inorganic magnetic core and organic
moieties. Finally, mCTpBd (398 nm; 31 nm COF thickness) was
obtained by the reaction between Fe
3
O
4
-NH
2
-CTp and another
monomer Bd through interface deposition method with reflux
conditions under argon atmosphere. mTpBd was also synthe-
sized in a similar manner.
In order to study the enrichment performance of mCTpBd
for glycopeptides, it was first tested for HRP tryptic digest.
It was observed that the introduction of mCTpBd greatly
enhanced enrichment performance. Before enrichment, only
a few of the glycopeptides were observed. Results showed that
after enrichment with mTpBd and mCTpBd, the number of
glycopeptide peaks were increased. But, enrichment perfor-
mance of mCTpBd was vastly improved in comparison with
mTpBd, revealing the enhanced hydrophilicity obtained from
the carboxyl-modified monomer. Further, the adsorption capacity
remained nearly intact even after continuous adsorption–
desorption cycles. Inspired by the outstanding results of
mCTpBd, it was also deployed for the enrichment of glyco-
peptides from biological samples (saliva of healthy people and
patients with inflammatory bowel disease). Noteworthy, the
magnetic adsorbent possessed excellent enrichment capacity
in both the cases. Subsequently, mCTpBd with inherent hydro-
philicity is a favourable candidate for the enrichment of endo-
genous glycopeptides.
Another kind of guanidyl-functionalized MagCOF has been
synthesized by Wu et al. via post synthetic modification using
stable precursors including 2,5-divinylterephthalaldehyde and
TAPB.
121
These two precursors were reacted along with Super-
paramagnetic iron oxide nanoparticles (SPIO) to form grafted
COF, i.e. SPIO@COF. To provide NH
2
functionality, cysteamine
was introduced on SPIO@COF by thiol–ene ‘‘click’’ reaction to
obtain SPIO@COF@NH
2
nanospheres. As guanidyl is a group
having efficient affinity for specific phosphopeptide enrichment,
guanidinylation of SPIO@COF@NH
2
nanospheres was carried
out using 2-ethyl-2-thiopseudourea hydrobromide to form SPIO@
COF-guanidyl nanospheres (20 nm COF thickness) (Fig. 17). The
so-formed magnetic adsorbent was used for the selective enrich-
ment of endogenous phosphopeptides from human saliva and
analyzed by MALDI-TOF MS. Impressively, due to large surface
area, abundant affinity groups, mesoporous structure and excel-
lent magnetic response, SPIO@COF-guanidyl exhibited excellent
performance for the selective enrichment of phosphopeptides.
Likewise, Jia et al. developed guanidyl-based magnetic ionic
COFs (Fe
3
O
4
@iCOFs) using guanidyl as ionic ligand instead of
post synthetic modifications.
122
Fe
3
O
4
@iCOF was fabricated by
immobilizing benzene-1,3,5-tris-carbaldehyde and 1,3-diamino-
guanidino hydrochloride on the surface of MNPs (of size
150 nm)through hydrogen bond interactions (Fig. 18). The fabri-
cated magnetic adsorbent was then utilized for selective enrich-
ment of phosphopeptides. The reason behind phosphopeptides
enrichment is electrostatic and hydrogen bonding interactions
between phosphate and guanidyl groups. By combining with
MALDI-TOF MS determinations, Fe
3
O
4
@iCOFs exhibited good
enrichment capacity for phosphopeptides. The developed proto-
colofferedseveralkeyadvantagessuchassuperparamagnetism,
Fig. 17 (a) Synthetic illustration of the SPIO@COF-guanidyl nanospheres
and (b) workflow of the enrichment strategy for phosphopeptides by
SPIO@COF-guanidyl nanospheres. Reproduced with permission from
ref. 121 Copyright 2020 Elsevier B.V.
Fig. 18 Synthetic scheme of Fe
3
O
4
@iCOFs for selective enrichment of
phosphopeptides with Fe
3
O
4
@iCOFs. Reproduced with permission from
ref. 122. Copyright 2020 Elsevier B.V.
Fig. 16 Synthetic route of mCTpBD and enrichment procedure of glyco-
peptides by mCTpBD. Reproduced with permission from ref. 120. Copyright
2020 American Chemical Society.
Review Materials Advances

1446 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
high sensitivity and selectivity along with recycling up to seven
times.
On a similar note, another magnetically recyclable core–
shell structure featuring Ti
4+
functionalized microporous COF
was synthesized by Zhang et al. The synthesis of Fe
3
O
4
@
SiO
2
@TpPa-Ti
4+
(40 nm COF thickness) is depicted in
Fig. 19a wherein MNPs were used as magnetic core and
functionalized with silica layer via Sto
¨ber sol–gel approach.
123
Surface modification of Fe
3
O
4
@SiO
2
was carried out by intro-
ducing amine groups on its surface using APTES. The magne-
tically retrievable adsorbent could be easily recovered from the
reaction mixture and hence, prevented the loss and separation
time of adsorbent. Subsequently, Tp, a monomer of COF was
immobilized on amino functionalized MNPs to form Fe
3
O
4
@
SiO
2
-Tp. In addition, to obtain Fe
3
O
4
@SiO
2
-TpPa-NO
2
,reversible
growth of microporous TpPa-NO
2
was performed on Fe
3
O
4
@SiO
2
-
Tp. Fe
3
O
4
@SiO
2
-TpPa-NO
2
further underwent a reduction in the
presence of NaBH
4
to convert nitro groups into amino groups for
the further phosphate-linking reaction. Also, during the reduction
process the imine linkages of COFs were converted into secondary
amines, which could provide good hydrophilicity to capture
phosphopeptides. Finally, due to strong affinity between phos-
phates and titanium ions, titanium ions were chelated to phos-
phates. As shown in Fig. 19b, Fe
3
O
4
nanoparticles displayed
nearly spherical shape with the average particle size of 220 nm.
Fig. 19c shows the TEM image of Fe
3
O
4
@SiO
2
with dark MNP core
coated by grey silica shell of B20 nm. This silica layer not only
protect Fe
3
O
4
from aggregation but also provides sites for the
functionalization of amino groups which is necessary for the
reversible assembly of covalent organic frameworks. Besides, after
in situ Schiff-base condensation reaction, a porous covalent
organic layer with thickness of 40 nm over Fe
3
O
4
@SiO
2
@TpPa-
NO
2
nanoparticles was observed (Fig. 19d). Further, TEM images
of Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
and EDX-mapping image of Fe, Si
and Ti showed uniform spherical morphology and smooth sur-
face of the synthesized composites and reveals that Ti
4+
groups
were densely grafted into inner channels and surface of COFs
(Fig. 19e and f).
The as-synthesized magnetic adsorbent was utilized for
selective enrichment of phosphopeptides from bio samples.
In this case, tryptic standard phosphoprotein (a-casein) digests
are used as model samples to evaluate the phosphopeptide
enrichment performance of Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
compo-
site. Fig. 19g and h demonstrates MALDI-TOF mass spectra of
a-casein before enrichment and after enrichment with Fe
3
O
4
@
SiO
2
@TpPa-Ti
4+
. The spectra suggested that only two phospho-
peptide peaks (m/z= 1466.7 and 1927.7) were observed before
enrichment at very low signal-to-noise ratio and after enrich-
ment with magnetic adsorbent, thirty peaks of phospho-
peptides were detected, indicating an excellent performance
of so-formed magnetic adsorbent. The selectivity of Fe
3
O
4
@
SiO
2
@TpPa-Ti
4+
was also checked in presence of other samples.
The proposed method gave high precision with low LOD, which
could be attributed to abundant affinity sites. Also, Fe
3
O
4
@
SiO
2
@TpPa-Ti
4+
showed admirable performance in rat brain
lysates with a high specificity of 91.8%.
3.9 Marine biotoxins extraction
Marine biotoxins are produced by aquatic micro-organisms and
accumulate in filter-feeding shellfish mussels, clams, oysters
or scallops, creating a public health risk.
124
These toxins enter
the human body through inhalation and by consumption of
contaminated seafood. However, okadaic acid (OA) and
dinophysistoxin-1 (DTX-1), a group of marine biotoxin, are
lipophilic heat-stable toxins which are produced by marine
dinoflagellates.
125
They pose a serious threat to human health,
e.g. gastrointestinal syndrome, such as diarrhetic shellfish
poisoning (DSP). In addition, after monitoring the concen-
tration of these biotoxins, it was found that their concentration
level is very low in seawater (rng L
1
). Therefore, pre-
concentration of OA and DTX-1 is required prior to detection.
75
In view of this, Salonen et al. performed extraction of OA and
DTX-1 from sea-water in the presence of magnetically retrievable
crystalline COF (mTpBd-Me
2
).
75
To obtain mTpBd-Me
2
, a controll-
able simplified three-step procedure was carried out. Firstly,
amino-functionalized MNPs were obtained by co-precipitation
method by introducing dopamine as capping agent (Fe
3
O
4
@
DOPA; 5 nm). Secondly, a monomer Tp was pre-functionalized
on the surface of Fe
3
O
4
@DOPA (Fe
3
O
4
@DOPA-Tp); pre-grafting
facilitates the nucleation and growth of COF shells around
Fig. 19 (a) Synthetic route of Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
nanoparticles, TEM
images of (b) Fe
3
O
4
, (c) Fe
3
O
4
@SiO
2
,(d)Fe
3
O
4
@SiO
2
@TpPa-NO
2
,
(e) Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
, (f) elemental mapping of Fe
3
O
4
@SiO
2
@TpPa-
Ti
4+
(the first image without elemental labelling is high magnification TEM
image), MALDI-TOF mass spectra of a-casein (g) before enrichment and
(h) after enrichment with Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
.Reproducedwithpermis-
sion from ref. 123 Copyright 2020 Royal Society of Chemistry.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1447
dopamine functionalized MNPs. Lastly, to grow mTpBd-Me
2
,
another COF monomer, di-methyl benzidine (Bd-Me
2
)andTp/o-
tolidine in a 1 : 1.5 molar ratio was exposed on the pre-
functionalized MNPs in the presence of acetic acid as catalyst
(Fig. 20). Noteworthy, non-pre-grafted Fe
3
O
4
@DOPA led to
amorphous COFs.
The optimal concentration for the synthesis of mTpBd-Me
2
was tested with the amount of Tp ranging from 0.005 to
0.4 mmol. Further, 0.1, 0.2 and 0.4 mmol yielded crystalline
composites such as 0.1-mTpBd-Me
2
, 0.2-mTpBd-Me
2
and 0.4-
mTpBd-Me
2
. In addition, 0.2-mTpBd-Me
2
was selected for the
extraction of OA and DTX-1 due to their high organic content
and stability up to 380 1C. The kinetic studies showed fast
adsorption due to presence of large surface of the nanocompo-
site. Moreover, the adsorption isotherm for both the biotoxins
displayed n41, demonstrating favourable adsorption.
Furthermore, a good adsorption capacity of the sorbent was
found to be 26.363 mg
0.219
g
1
L
0.781
and 22.508 mg
0.181
g
1
L
0.819
for OA and DTX-1, respectively, which was indicated by
K
F
(indicator of adsorption capacity in Freundlich theory) value.
Additionally, the maximum adsorption capacity was calculated
to be 812 mg g
1
for OA and 830 mg g
1
for DTX-1, which was
three-fold higher for OA as compared to bulk TpBd-Me
2
. Also,
mTpBd-Me
2
exhibited higher calculated adsorption kinetics,
i.e.,B500-fold increase for OA and B300-fold increase for DTX-
1 as compared with literature precedents. This increase can be
attributed to the ordered pore structure of MagCOFs which
allows higher diffusion rates for bio-toxins instead of non-
magnetic bulk COFs which are difficult to separate from sample
matrix. Besides, the recyclability efficiency of the adsorbent was
also examined and was found that the adsorbent could be
reused for at least five consecutive cycles. Consequently,
mTpBd-Me
2
had good potential for rapid extraction of marine
biotoxins from sea water. The adsorption and concentration of
other pollutants such as pesticides and detoxification of other
bio-toxins can also be studied using mTpBd-Me
2
as adsorbent.
Moreover, routine monitoring of OA and DTX-1 in sea water is
essential to avoid shellfish contamination.
3.10 Metal ions extraction
Rapid pace of industrialization, population expansion and urbani-
zation have significantly deteriorated the quality of water.
126,127
Heavy metal ions, which are discharged into the water bodies
through various effluents, are very fatal to the human health and
surrounding environment as they tend to bioaccumulate through
food chain.
128,129
Amongst all, mercury has been identified as one
of the most hazardous environmental pollutants, exposure to
which causes severe adverse health effects.
130
Hence, its periodic
monitoring is imperative. In this respect, Shuai and co-workers
reported thiol-functionalized MagCOFs, namely M-DAPS-
50
-COF-
SH, via salt-mediated crystallization approach and cutting strategy
(cutting of disulfide linkages) for selective and efficient removal of
Hg
2+
(Fig. 21).
131
Subsequently, three M-DAPS-COFs with three different
molar ratios (0%, 50% and 100%) of DAPS were synthesized.
Remarkably, amongst all, M-DAPS
50
-COF was obtained in high
yield (B95%). It contains densely distributed thioether side
chains as the Hg
2+
receptor due to strong soft-soft interaction
between sulfide and mercury. Impressively, Mag-COF-SH
showed maximum adsorption capacity of Hg
2+
(383 mg g
1
)
which can be attributed to the abundant thiol functional
groups and ultra-porous structure of the framework. The selec-
tivity towards Hg
2+
detection was also checked in presence
of other competitive metal ions and satisfactory results were
obtained. Moreover, resulting composite exhibited rapid kinetics,
i.e., adsorption equilibrium can be achieved within 10 min, which
was attributed to high specific surface area of MagCOF composite
and strong affinity between thiol (–SH) moieties and Hg
2+
.
Additionally, to evaluate the disconnection of disulfide linkages,
integration of MNPs over COF and formation of imine bonds,
XPS, XRD and FT-IR spectra were studied, which illustrates
satisfactory results. Besides, Mag-COF-SH could be reused up to
five adsorption cycles which proposed its use in large scale metal
sensing and removal applications.
Chromium is extensively used in various industries includ-
ing electroplating, leather tanning, metal finishing, pigment
synthesis, petroleum refining, etc. Two stable forms of chro-
mium exist in environment, trivalent Cr
3+
and hexavalent Cr
6+
.
Fig. 20 The three-step synthesis of mTpBD-Me
2
. Reproduced with
permission from ref. 75. Copyright 2019 Royal Society of Chemistry.
Fig. 21 Schematic diagram for the synthesis of M-COF-SH. Reproduced
with permission from ref. 131. Copyright 2020 Elsevier B.V.
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1448 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
However, Cr
6+
is considered as more hazardous due to its high
mobility and solubility in water and soil. Owing to its greater
penetration, it can severely affect central nervous system and is
a classified carcinogen due to its oxidizing nature.
132
Besides
chromium, BPA is also a known pollutant which was discussed
thoroughly in the previous section.
In view of this, Hu et al. performed removal of Cr
6+
and BPA
from aqueous solution using magnetically retrievable COF
(Fe
3
O
4
@COF; TpPa-1 thickness being 10 nm) with b-keto-
enamine linkage under solvothermal reaction conditions
(Fig. 22a).
133
The as-synthesized magnetic adsorbent reflected
numerous advantages such as simple preparation method,
excellent adsorption performance, high stability and good
recoverability (19.5 emu g
1
). TEM images of TpPa-1 and
Fe
3
O
4
@TpPa-1 illustrate sea-urchin type and spherical mor-
phology with an average diameter of B80–100 nm, respectively
(Fig. 22b and c). The effect of pH, ionic strength and adsorption
isotherms are the key factors in the extraction process. The
thermodynamic as well as kinetic studies validate to the
pseudo-second-order and Langmuir isotherm indicating spon-
taneous, endothermic and chemisorption process. The mecha-
nism of BPA extraction follows p–pstacking interaction,
hydrogen-bond and porosity induced by hydrophobicity. More-
over, for Cr
6+
adsorption, firstly anionic Cr
6+
was combined
with the surface protonated functional groups of TpPa-1.
Secondly, Cr
6+
was reduced to Cr
3+
with the aid of adjacent
electron donor groups. Finally, reduced Cr
3+
was returned into
solution due to electronic repulsion. The adsorption capacities
for Cr
6+
and BPA were measured to be 245.45 mg g
1
and
1220.97 mg g
1
, respectively. Additionally, magnetic adsorbent
was reused for five adsorption–desorption cycles. Nevertheless,
there exists certain fundamental aspects which needs to be
focused. In our view, influence of different coexisting anions in
adsorption behaviours of Cr
3+
and Cr
6+
should be studied to
improve the effectiveness of the process. Further, present
strategies mainly target the batch extraction, however, contin-
ual removal of Cr
6+
from industrial effluents is more useful as
effluents mainly originate from industries involved in produc-
tion of large amounts of waste.
3.11 Nitro explosive extraction
Recently, several researchers have begun exploring the
potential of MagCOFs as chemo-sensory materials due to their
exceptional structural tunability and properties. In that context,
selective and sensitive detection of nitroaromatic explosives, in
particular 2,4,6 trinitrophenol (TNP), has gained tremendous
attention due to its detrimental effects on humans, wildlife
and environmental health.
134
On similar account, Gao et al.
designed and fabricated a fluorescent sensor which was
composed of MagCOFs, Molecularly imprinted polymers
(MIP) and carbon dots (CDs).
135
Fig. 23 illustrates the synthetic
protocol of magnetic sensor (MagCOF@MIP@CD) wherein
2,3,5,6-tetrafluoroterephthalaldehyde and TAPB were covalently
immobilized over MNPs (of size 200 nm) to form MagCOF. After
its successful fabrication, MIP (as selective sorbent) and CDs
(as fluorescent sensor) were grafted over MagCOF via reverse
microemulsion method using APTES as –NH
2
provider and
TEOS as cross-linker.
The fabricated MagCOF@MIP@CD was further utilized as a
fluorescent sensor for environmental pollutant TNP. The maxi-
mum excitation and emission wavelength were recorded to be
370 nm and 470 nm, respectively. MagCOF@MIP@CD without
TNP exhibited strong fluorescence signal which was quenched
in presence of TNP. Amongst various competitive compounds
of TNPs and metal ions, best selectivity was obtained for TNP
due to the effective p–pinteraction and hydrophobic effect
Fig. 22 (a) Synthesis process of core–shell structured Fe
3
O
4
@TpPa-1,
TEM image of (b) TpPa-1 and (c) Fe
3
O
4
@TpPa-1. Reproduced with permis-
sion from ref. 133. Copyright 2020 Elsevier B.V.
Fig. 23 Synthesis and application of MagCOFs@MIPs@CDs as fluorescent
sensor. Reproduced with permission from ref. 135. Copyright 2020
Elsevier B.V.
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1449
between TNP with MagCOF@MIP@CD. Hence, MagCOF@
MIP@CD not only provided excellent magnetism and good
fluorescence but also exhibited good sensitivity with high
imprinting factor (8.4) and shorter detection time (within
1 min). Delightedly, good recoveries (88.7% to 103.4%) were
obtained when the fluorescent sensor was applied in real
samples. Moreover, recyclability of the sensor up to eight
consecutive runs also imparted green credentials for their use
at large scale.
Along with nitroaromatic explosives, selective and sensitive
detection of endocrine disruptor, PAHs and many more organic
pollutants have raised incredible attention due to their adverse
effects on aquatic ecosystems, humans and environmental
health (Fig. 24).
136
Also, cigarette smoke is composed of various
chemicals including gaseous nicotine and tar which have
carcinogenic effect on human health. Inhaling such toxic
chemicals, Bap and phenols, may cause cancer and many other
diseases.
137,138
Therefore, adsorption and removal of these
organic pollutants are of great necessity. To fulfil current needs,
Zhang et al. synthesized novel porous aromatic framework
(PAF) decorated MNPs, PAF-6 MNPs, using cyanuric chloride
for triangular planar basis and piperazine as linker unit via one-
step polymerization process as shown in Fig. 25.
139
Zhang
developed this sample pre-treatment method by coupling of
MSPE with HPLC-UV for the adsorption of TNP, 3-nitro-
chlorobenzene, BPA, naphthalene and naphthol from well-
water, tap-water, waste water and yellow river water.
The presence of aromatic units and nitrogen atoms provides
them multiple recognition sites. The fabricated Porous aro-
matic framework-6 (PAF-6) MNPs showed remarkable adsorp-
tion capability due to presence of p–pinteractions, hydrogen-
bond interactions and inclusion complexation. TEM image of
PAF-6 MNPs showed spherical core–shell structure with narrow
size distribution (Fig. 25b). Moreover, PAF-6 MNPs were also
used to identify toxicants present in cigarette smoke such as
phenolic compounds (o,m,p-cresol, phenol, pyrocatechol, resor-
cinol and hydroquinone), Bap and tar. The results showed that
upon increasing the amount of PAF-6 MNPs from 5 mg to 20 mg,
the removal efficiencies increased rapidly (Fig. 25c and d). The
method showed maximum adsorption capacity (166.85 mg g
1
),
good linearity for different organic pollutants in the range of
0.01–12 mgmL
1
and the R
2
was more than 0.97; the LODs were
between 0.083 and 5.020 according to the three-fold signal to
noise ratio. The extraction efficiency of the method was in
between 84.0% and 96.0% with reusability of at least six cycles.
These magnetic porous composites exhibited high stability,
desirable durability and enhanced magnetic separation. The
results proved that PAF-6 MNPs displayed outstanding adsorp-
tion performance for diverse multi target analytes, nitrosamines,
heavy metal ions and other toxic compounds.
3.12 Phthalate esters extraction
Phthalate Esters Extraction (PAEs) are widely used as plastici-
zers and additives in many plastics, pesticides and paints.
However, they tend to leach into various environmental
matrices and eventually migrate to humans. PAEs cause adverse
effects on human health such as carcinogenicity and endocrine
disruption.
140,141
On this note, MagCOF (COF-(TpBd)/Fe
3
O
4
)was
designed by Pang et al. for efficient extraction of PAEs in
beverage samples.
142
Fig. 26a depicts the magnetic adsorbent
synthetic protocol and SPE of PAEs wherein COF-(TpBd) was
synthesized by conventional solvothermal method through
Schiff base condensation reaction between monomers Tp and
Bd. Further, the obtained COF-(TpBd) was delaminated into COF
nanosheets via mechanochemical grinding. The as-synthesized
COF-(TpBd) nanosheets were further anchored on the surface of
MNPs (of size o50 nm) through co-precipitation method to form
COF-(TpBd)/Fe
3
O
4
. The designed magnetic adsorbent was suc-
cessfully utilized for extraction of 15 PAEs and displayed excel-
lent extraction efficacy (Fig. 26b). The authors also compared the
previously reported methods for PAEs detection. The results
endowed that our developed method showed great significance
in extraction of PAEs due to hydrophobic effect, p–pinteractions
and pore size selectivity between PAEs (8 Å to 17 Å) and COF-
(TpBd)/Fe
3
O
4
(24 Å). Moreover, under optimized conditions, a
Fig. 24 Structures of a few emerging organic micropollutant.
Fig. 25 (a) Preparation procedure of core–shell microspheres (PAF-6
MNPs), (b) TEM image of PAF-6 MNPs and (c) effect of PAF-6 MNPs levels
on the removal efficiency of phenolic compounds from cigarette smoke
and (d) effect of PAF-6 MNPs dosage on the removal efficiency of
Bap from cigarette smoke. Reproduced with permission from ref. 139.
Copyright 2018 Elsevier B.V.
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1450 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
series of analytical parameters were thoroughly studied. The
proposed approach possessed a good linear range (10–1000 mgL
1
)
with R
2
40.9912. The LOD and LOQ were from 0.005 to
2.748 mgL
1
and from 0.018 to 9.151 mgL
1
, respectively with
lower RSDs. In addition, to check the accuracy of the method,
COF-(TpBd)/Fe
3
O
4
was applied in recognition of PAEs from real
samples. The method showed good recoveries in the range of
79.3–121.8% and RSDs values were within 2.1–11.9%. Last but
not the least, magnetic adsorbent was reused for four cycles.
Similarly, Liang and co-workers fabricated MagCOFs by
Schiff base condensation reaction of two monomers Tb and
Bd on PDA functionalized Fe
3
O
4
(Fig. 26c).
143
In this case, PDA
was used as a hydrophilic middle layer which displayed excel-
lent water dispersibility and good biocompatibility. The synthe-
sized adsorbent exhibited several advantages including high
surface area, large number of active sites for PAEs detection (via
p–pand hydrophobic interactions), easy separability and high
aqueous dispersibility. The fabricated magnetic adsorbent was
further deployed for recognition of nine PAEs from a human
plasma sample. Favourably, Fe
3
O
4
@PDA@TbBd unveiled great
potential in PAEs detection with wide linearity (50–8000 ng mL
1
)
and low LOD (0.0025–0.01 ng mL
1
). The proposed strategy
showed good recovery (92.3–98.9%) and a small RSD value (for
intra-day o4.6% and for inter-day o6.8%). To check the super-
iority of the adsorbent it was applied in real sample analysis
and favourable results were obtained. Besides, the adsorbent
was magnetically recovered and used repeatedly for five cycles.
3.13 Aromatic dyes removal
Aromatic dyes represent a substantial class of pollutants due to
their unfavourable effects on human and wildlife such as
carcinogenicity, teratogenicity and mutagenicity.
144
Among aro-
matic dyes, auramine O (AO) has been used for colouring paper,
textile, leather and rhodamine (RB) have been utilized for
industrial purposes.
145,146
These aromatic dyes are substances
of concern for the environment and human health as they
create gastrointestinal and respiratory tract irritation. Therefore,
their removal is of great significance for researchers.
Recently, Rafiee and co-workers fabricated a melamine-rich
magnetic covalent organic polymer (MCOP) for the removal of
AO and RB.
147
Fig. 27a depicts the fabrication of the MCOP
wherein MNPs (of sizes 20–50 nm) were synthesized by solvo-
thermal technique. Surface modification of MNPs using glu-
cose and polyacrylonitrile provided carboxylate units which
ease the COP coverage easily onto Fe
3
O
4
. The final adsorbent
(Fe
3
O
4
/C-COP) was formed by Schiff base condensation reac-
tion between TPA and melamine on the surface of modified
MNPs. Fig. 27b represents the mechanistic pathway for the
Fig. 27 (a) Schematic representation for the preparation of Fe
3
O
4
/C-
COP, (b) proposed adsorption mechanism of AO/RB onto Fe
3
O
4
/C-COP
and (c) VSM curves of Fe
3
O
4
and Fe
3
O
4
/C-COP before and after AO and
RB adsorption, (d) regeneration studies of magnetic Fe
3
O
4
/C-COP using
ethanol and (e) N
2
adsorption/desorption isotherm of Fe
3
O
4
/C-COP
before (a) and after (b) AO and RB adsorption Reproduced with permission
from ref. 147. Copyright 2020 American Chemical Society.
Fig. 26 Schematic diagram for (a) magnetization of COF-(TpBD),
(b) application of COF-(TpBD)/Fe
3
O
4
as absorbent for MSPE and detection
of PAEs. Reproduced with ref. 142, Copyright 2020 Elsevier B.V.
(c) Synthesis of Fe
3
O
4
@PDA@TbBd.
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© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1451
removal of AO and RB. The presence of free lone pair electron
in various imine groups present on Fe
3
O
4
/C-COP could electro-
statically interact with cationic RB and AO, which enhanced the
adsorption behaviour. Also, due to the planar structure of the dye
it can be readily adsorbed on the adsorbent through hydrogen-
bonding and van der Waals forces. Brunauer–Emmett–Teller
(BET) surface area and Barrett–Joyner–Halenda (BJH) pore-size
analyses confirmed the cavity encapsulation. The surface area and
pore volume of Fe
3
O
4
/C-COP before and after adsorption was
determined by BET analysis (Fig. 27c). The surface area and pore-
volume before adsorption was calculated to be 150.51 m
2
g
1
and
0.4125 cm
3
g
1
, respectively whereas after adsorption of AO and
RB both surface area (38.89 m
2
g
1
) and pore-volume (0.2191 cm
3
g
1
)
decreased.ThedecreaseinvalueswasduetoencapsulationofAOand
RB in the cavity. The authors also compared the results with previously
reported adsorbents, which revealed better results in terms of mass
(12 mg) and contact time (4 min). The maximum adsorption
capacity for AO and RB were 107.11 and 131.23 mg g
1
,respec-
tively. The higher maximum adsorption capacity value for RB is
attributed to the p–pinteractions mechanism between RB and the
material.
From VSM analysis, the saturation magnetization value for
Fe
3
O
4
/C-COP was observed to be 37.5 emu g
1
which is lower than
the magnetization of bare Fe
3
O
4
(81.5 emu g
1
). Fig. 27d shows
that M
s
value of Fe
3
O
4
/C-COP after six cycles was found to be lower
after AO and RB adsorption than the fresh Fe
3
O
4
/C-COP. This
reduction in the saturation magnetization can be attributed to the
fact that with the addition of AO and RB to Fe
3
O
4
/C-COP the
surface magnetic anisotropy of magnetite changes which leads to
an increase in surface spin disorientation. This results in the
decrease of magnetic moment of the adsorbent and hence
decrease in M
s
value of Fe
3
O
4
/C-COP. Besides, due to simple
magnetic recoverability, the adsorbent was reused for nine con-
secutive cycles.
3.14 Fluoroquinolones extraction
Fluoroquinolones (FQs) possess strong anti-bacterial activity for
both pathogenic Gram-positive and Gram-negative bacteria.
156,157
Due to their excellent activity they have been widely used in the
field of farming, clinical practice and aquaculture. However, their
overdose can result in residues in foodstuffs in humans and
animal origin. Also, the extensive usage of FQs led to many toxic
effects such as pathogen resistance and allergic reactions in
human beings. Therefore, there is a need to develop feasible
method to inspect FQs residues.
In this regard, Gao and co-workers fabricated core–shell
structured MagCOFs for the efficient extraction of six FQs
(ciprofloxacin, enrofloxacin, lomefloxacin, gatifloxacin, levo-
floxacin and pefloxacin) from pork, milk and human plasma
through MSPE coupled HPLC technique (Fig. 28a).
158
The
MagCOF was synthesized by immobilizing 4,400-diamino-p-
terphenyl and 1,3,5-tris(p-formylphenyl) benzene onto Fe
3
O
4
by Schiff base condensation reaction. The new magnetic
adsorbent exhibited low density, high surface area, excellent
porosity, high thermal stability and offered effortless magnetic
separability. Moreover, VSM studies revealed that Fe
3
O
4
@COFs
display excellent magnetization (36 emu g
1
) (Fig. 28b).
To check the feasibility of MagCOF, adsorption kinetics and
isotherms were obtained. Fig. 28c depicts the saturation
adsorption capacities of six FQs i.e., as the concentration of
FQ in the solution increases, adsorption of FQ on Fe
3
O
4
@COFs
increases whilst Fig. 28d demonstrates that adsorption capa-
cities were gradually increased in the initial 30 min and after
30 min adsorption kinetic curves are flat. These results suggest
that adsorption equilibria was achieved in 30 min. Furthermore,
from adsorption isotherms it was noted that Freundlich isotherm
was better fitted in FQs adsorption, indicating that the adsorption
processisnotcausedbymonolayer,butbyvariousinteractions.
The adsorption mechanism of MagCOF to FQs was mainly
attributed to hydrogen bonding and p–pinteraction between
phenyl rings. Further, under optimized conditions (sorbents
dosage, extraction time, pH of samples solution, ionic strength,
etc.) analytical performance was investigated. The MSPE coupled
HPLC analysis presented wide linear range (2.5–1500 ng g
1
), low
LOD (0.5 ng g
1
)withR
2
40.9996. The method showed good
recoveries (78.7–103.5%) in milk, pork and human plasma/actual
sampleswithRSDsrangingfrom1.5to4.3%forintra-dayand
2.9% to 6.2% for inter-day. Besides, Fe
3
O
4
@COFs could be reused
for ten adsorption–desorption cycles.
Similarly, Li et al. synthesized sulfonate-functionalized Mag-
COF for the effective extraction of FQs in food samples through
MSPE-HPLC-MS/MS method (Fig. 29).
154
For the preparation,
firstly Fe
3
O
4
NPs were modified by Schiff base reaction between
Tp and Bd. Further, Fe
3
O
4
@TpBd-Au was fabricated by loading
gold nanoparticles on Fe
3
O
4
@TpBd. Furthermore, Fe
3
O
4
@
TpBd-Au was functionalized by immobilization of sodium
3-mercaptopropanesulphonate via Au–S bond formation. The
adsorption of FQs is attributed to the strong p–pstacking,
hydrogen bonding and electrostatic interactions. The proposed
method showed linearity with excellent R
2
value (Z0.9989) and
lower LOD (0.1–1.0 mgkg
1
). Moreover, the method displayed
Fig. 28 (a) Synthesis and application of Fe
3
O
4
@COFs, (b) VSM studies
of Fe
3
O
4
, (c) adsorption isotherms of six FQs on Fe
3
O
4
@COFs and (d)
adsorption kinetics of six FQs on Fe
3
O
4
@COFs. Reproduced with permission
from ref. 158 Copyright 2019 Springer Nature Publishing AG.
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1452 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
satisfactory recoveries (82–110.2%) in food stuffs with lower
RSDs. The as-synthesized magnetic adsorbent showed superior
adsorption ability and magnificent chemical stability. Adsorption
performance and structural parameters of variety of COFs have
been listed in Table 2.
3.15 Estrogen enrichment
Estrogens are widely used in oral contraceptive and hormone
therapy which are chiefly involved in reproduction, growth and
sexual behaviour. Recently, due to potential risk on human
health and environment their widespread use has gained
considerable attention.
162,163
Therefore, determination of estro-
gens in complex samples is very important from both environ-
mental and humanitarian grounds. In this regard, Lin and
co-workers reported the first example of MagCOF nanobeads
(Fe
3
O
4
@TbBd) and explored it as an adsorbent for MSPE of
estrogens from human urine samples followed by HPLC-MS
analysis (Fig. 30).
155
Due to the complex matrix effect and
because of ultra-low concentration of estrogens in biological
samples, pretreatment and enrichment processes are impera-
tive prior to HPLC-MS. The core–shell structured MagCOF
(Fe
3
O
4
@TbBd) nanobeads were synthesized by Schiff base
condensation reaction of monomers Tb and Bd in the presence
of DMSO on the surface of Fe
3
O
4
at room temperature. The
superior utility of Fe
3
O
4
@TbBd was demonstrated by some of
their attractive features such as large specific surface area
(202.18 m
2
g
1
), excellent chemical and thermal stability, high
porosity (2.8 nm) and high magnetic responsivity (41.4 emu g
1
),
which made them an ideal adsorbent for selective separation and
enrichment of estrogens. Moreover, TEM images indicated uni-
form size and monodispersity of bare Fe
3
O
4
(B200 nm) and
distinct core–shell structure of Fe
3
O
4
@TbBd nanobeads with dark
Fe
3
O
4
NPs core and gray COF layer with thickness of 40 nm
(Fig. 30c and d). The as-synthesized Fe
3
O
4
@TbBd@COF was
characterized by various physio-chemical techniques. Moreover,
a simple and sensitive method was developed by combination
of MSPE with HPLC-MS by using Fe
3
O
4
@TpBd nanobeads,
which displayed LOD (0.2–7.7 ng L
1
, (S/N = 3)), good linearity
Fig. 29 Schematic fabrication process of Fe
3
O
4
@COF(TpBD)@Au-MPS
and the MSPE procedure for the determination of FQs. Reproduced with
permission from ref. 154. Copyright 2020 Elsevier B.V.
Table 2 Adsorption performance and structural parameters of MagCOFs
MagCOF Adsorption
time (min) Desorption
time (min) Adsorption
dosage (mg) Elution solvent Surface area
(m
2
g
1
)Pore volume
(cm
3
g
1
) Pore size Ref.
Fe
3
O
4
@COF 30 3 40 Methanol 166.5 0.26 n.d. 148
NiFe
2
O
4
@COFs 5 n.d. 10 Acetonitrile:methanol 169.7 0.276 n.d. 159
Fe
3
O
4
@TpBd 5 1 n.d. Ethanol 272.6 0.457 1.7 nm 67
Fe
3
O
4
@COF 10 2 20 i-Propanol 181.36 0.45 3.6 nm 70
Fe
3
O
4
@COFs-Apt 30 n.d. 30 Hexane 185 n.d. n.d. 36
Magnetic TpPa-1 n.d. n.d. 5 Acetonitrile 247.8 0.40 0.4–2.0 nm 71
COF-LZU1@PEI@Fe
3
O
4
30 3 5 Acetonitrile n.d. n.d. n.d. 160
MOPs 20 10 5 Ethyl acetate 378 n.d. n.d.
Mag-COF-2 20 n.d. 0.1 Methanol 334.9 0.32 3.9 nm 149
MagCOF 50-DCF 80-SMT n.d. 10 Acetonitrile 2245 n.d. 2.5 nm 108
Mag-COF-1 60 min 5 5 Acetonitrile n.d. n.d. 41
Fe
3
O
4
@COF 20 2 10 Acetonitrile 30.6 0.187 2–5 nm 114
Fe
3
O
4
@COF@Au-b-CD 30 10 20 Acetonitrile-acetic acid n.d. n.d. n.d. 150
Fe
3
O
4
@TpPa-1 30 10 0.02 Acetonitrile-H
2
O-TFA 186 0.17 3.6 nm 119
mCTpBd 30 30 0.2 Acetonitrile-H
2
O-TFA 120.41 n.d. 1.3 nm 120
SPIO@COF-guanidyl 30 10 10 Acetonitrile-H
2
O-TFA 149.75 n.d. 3.2 nm 121
Fe
3
O
4
@iCOFs 30 15 5 Acetonitrile-H
2
O 7 n.d. n.d. 122
Fe
3
O
4
@SiO
2
@TpPa-Ti
4+
30 10 1 Acetonitrile-TFA 128 n.d. 1.4 nm 123
mTpBd-Me
2
60 4 h n.d. 2-Propanol 538 0.40 1.1 nm 152
M-DAPS-COF-SH 10 n.d. 5 HCl and thiourea 181.5 0.24 n.d. 131
Fe
3
O
4
@COF(TpPa-1) 10-BPA 100-Cr n.d. 5 Ethanol 485.2 0.34 2 nm 153
MagCOF@MIP@CD 30 2 0.5 Methanol:acetic acid 137.34 n.d. 4.72 nm 151
PAF-6 MNPs 5 5 60 Acetonitrile 120.2 n.d. 2–5 nm 161
COF-(TpBd)/Fe
3
O
4
30 15 30 Methanol n.d. n.d. n.d. 142
Fe
3
O
4
/C-COP 2 4 12 Ethanol 150.51 0.4125 n.d. 147
Fe
3
O
4
@COFs 60 20 14 Methanol 124 0.386 3.1 nm 158
Fe
3
O
4
@COF(TpBD)@Au-MPS 30 25 20 Formic acid:methanol 70.14 0.15 n.d. 154
Fe
3
O
4
@TbBd 30 2 20 Acetonitrile 202.18 0.65 2.8 nm 155
Materials Advances Review

© 2022 The Author(s). Published by the Royal Society of Chemistry Mater. Adv., 2022, 3, 1432–1458 | 1453
(R40.9978), LOQ (0.6–25.6 ng L
1
, S/N = 10), high enrichment
(75–197-fold) as well as excellent recoverability. Also, the
proposed method was used for the analysis of trace estrogens in
urine samples of pregnant women and showed good recoveries
(80.6–11.6%). Fe
3
O
4
@TpBd can be reused up to ten adsorption–
desorption cycles which proposed its use in large scale extraction
and as a novel adsorbent in sample pre-treatment.
4. Comparison with other magnetic
materials
Till date, a variety of homogenous and heterogeneous adsor-
bents have developed for extraction of various organic and
inorganic pollutants from different solutions. But, now a days,
the use of magnetic adsorbents has gained tremendous atten-
tion due to their efficient recyclability, excellent stability and
enhanced functionality. Table 3 summarizes the comparative
data of MagCOFs with other magnetic adsorbents.
5. Conclusion and future outlook
In the past few years, COFs have gained tremendous attention
in the field of separation science due to their magnificent
Fig. 30 (a) Preparation of Fe
3
O
4
@TbBd nanobeads, (b) MSPE process for
estrogens in urine samples, TEM images of (c) bare Fe
3
O
4
NPs and
(d) Fe
3
O
4
@TbBd nanobeads. Reproduced with permission from ref. 155.
Copyright 2018 Elsevier B.V.
Table 3 Comparison of various MagCOFs with other magnetic materials/adsorbents
Magnetic materials Linear range
(ng mL
1
)Limit of detection
(ng mL
1
) Sample Extraction
time (min) Recoveries
(%) Ref.
Endocrine disrupting chemicals
Fe
3
O
4
@MIP 0.5–100 0.1–0.3 Canned orange/milk 3 72–113 164
magG@PDA@Zr-MOF 50–20 000 0.1–1 Water 20 64.8–92.8 165
Fe
3
O
4
@C 0.01–100 0.017–0.15 Tap water 20 47.9–119 166
Fe
3
O
4
@rGO 2.5–100 1.23 Tap water/river water/waste water n.d. 96.3–112.6 167
Fe
3
O
4
@COFs 0.5–1000 0.08–0.21 Tea drinks 30 81.3–118.0 168
Ni@Fe
2
O
4
@COF 0.1–200 0.019–0.096 Tap/barrelled/river water;
activated beverage; human
urine/serum sample
5 83.4–106.2 159
Fe
3
O
4
@COFs 0.1–50 0.001–0.078 Human serum 10 93.0–107.8 70
Persistent organic pollutants
Graphene/Fe
3
O
4
composite 0.03–80 0.009–0.020 Water 4 83–107 169
Fe
3
O
4
@ZIF-8 1.0–100 0.08–0.24 Water 8 85.6–103.6 170
Magnetic MIL-100(Fe) 0.5–500 0.032–2.11 Water 10 81.4–126.9 171
PFu/Fe
3
O
4
0.02–200 0.005–0.02 Urine samples 15 93.2–99.2 172
Magnetic TpPa-1 0.002–0.2 0.00024–0.00101 Water n.d. 73–110 71
Sulfonamides Extraction
CoFe
2
O
4
-graphene nanocomposite 20–50 000 1.59 Milk 20 62.0–104.3 173
Graphene-Fe
3
O
4
nanoparticles 5–200 0.89–2.31 Milk 15 62.7–104.8 174
Fe
3
O
4
@GO 2.0–100 0.02–0.13 Milk 5 73.4–97.4
Polypyrrole/SiO
2
/Fe
3
O
4
nanoparticles 0.30–200 0.30–1 Water 20 86.7–99.7 175
Fe
3
O
4
@COFs 1–500 0.2–1.0 Water, milk, pork, chicken, shrimp 20 65.3–107.3 114
Fe
3
O
4
@COF@Au-b-CD 2–100 0.8–1.6 Meat 30 78.9–112.0 150
Metal ion extraction
Magnetic materials Maximum adsorption
capacity (mg g
1
)
Fe
3
O
4
@SiO
2
148.8 176
g-Fe
2
O
3
@CTF-1 165.8 177
MPTS-mesoporous Fe
3
O
4
/C@SiO
2
118.6 178
Fe
3
O
4
@SiO
2
-NH
2
27.2 179
M-DAPS50-COF-SH 383 131
Fe
3
O
4
@TpPa-1 246.45 153
Review Materials Advances

1454 | Mater. Adv., 2022, 3, 1432–1458 © 2022 The Author(s). Published by the Royal Society of Chemistry
properties. Although, COFs have achieved remarkable advances
in separation applications but some crucial challenges are still
to be addressed such as high cost, cumbersome filtration and
centrifugation technique for the recovery of the adsorbent.
Immobilization of COFs over MNPs has opened up a new
avenue for the development of green and sustainable magnetic
adsorbents by eliminating the shortcoming of traditionally
used COFs. This review article presents an overview on the
preparation of MagCOFs and their applicability in the extrac-
tion of various chemical pollutants, heavy metal ions, nitro
explosives, etc. The combination of MNPs and COFs offered
improved characteristics such as easy magnetic recovery,
enhanced functionality, cost-effectiveness and high extraction
efficiency. The incorporation of magnetism in COFs has been
proven to be an effective strategy to solve the problem of
recovery of COFs. Adsorption and recyclability are the key
advantages of MagCOF materials. In addition, MagCOF-based
adsorbents are being designed to selectively extract EDCs,
POPs, PAHs, metal ions and even marine biotoxins. Usually,
there are strong p–pstacking interactions, electrostatic inter-
action, hydrophobic effect and hydrogen bonding between
phenyl rings of MagCOFs and benzene rings of the concerned
analyte, thereby allowing efficient extraction/adsorption. Mean-
while, the present review gives new insights into the facile
functional modification of COFs as adsorbent in MSPE. Hence,
MagCOF based adsorbent opens a new door with a plethora of
potential applications.
However, despite the promising results, the full potential of
the MagCOFs composites in certain areas of concerns still need
further improvement Besides, not only the extraction of EDCs,
POPs, PAHs and metal ions, extraction of microorganisms will
also need to be studied to fully open up their application as
versatile adsorbent. Moreover, systematic and continuous
efforts are required for better designing and large-scale appli-
cations of MagCOFs in various fields. In the next step, it is also
necessary to gain insights about the toxicity of the nanomater-
ials for their long-term use. Another important point to con-
sider is the better understanding of adsorption–desorption
mechanism i.e. to avoid the use of non-green and costly
solvents. Other aspect, which is of utmost importance is the
synthetic cost and efficiency of the MagCOF composites. The
use of high cost monomers to synthesize COFs materials is
not economical. Therefore, to enhance manufacturing effi-
ciency by reducing the monomers cost through incorporating
cost-effective functional groups is a significant topic from
future perspective. Also, large-scale magnetic separation of
EDCs, marine bio-toxins, PPCPs, POPs, etc. from large amount
of crude culture media at standard conditions is still required.
In terms of extraction applications, MagCOFs materials
have shown enhanced performance but the use of MagCOFs
in catalysis and other applications is still in its infancy.
Therefore, there is need to expand the scope of application. Also,
duetopresenceofstablep–pstacking between COF layers, it can
greatly improve the photocatalytic reactions. But it is believed that
post-synthetic modification at the pore will provide opportunities
for incorporation of specific active sites for catalysis.
Despite these above-mentioned challenges, undoubtedly,
the progress in recent years have predicted well for the bright
future of this new type of porous materials and we sincerely
hope that the present review will significantly contribute in
facilitating that development in near future.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
The authors, P. Y., R. G., and G. A., gratefully acknowledge the
University Grants Commission and Council of Scientific &
Industrial Research, Delhi, India, for awarding research
fellowships.
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