Graphitic carbon nitride nanosheets as promising candidates for the detection of hazardous contaminants of environmental and biological concern in aqueous matrices

Abstract

Monitoring of pollutant and toxic substances is essential for cleaner environment and healthy life. Sensing of various environmental contaminants and biomolecules such as heavy metals, pharmaceutics, toxic gases, volatile organic compounds, food toxins, and pathogens is of high importance to guaranty the good health and sustainable environment to community. In recent years, graphitic carbon nitride (g-CN) has drawn a significant amount of interest as a sensor due to its large surface area and unique electrochemical properties, low bandgap energy, high thermal and chemical stability, facile synthesis, nontoxicity, and electron rich property. Furthermore, the binary and ternary nanocomposites of graphitic carbon nitride further enhance their performance as a sensor making it a cost effective, fast, and reliable gadget for the purpose, and opens a wide area of research. Numerous reviews addressing a variety of applications including photocatalytic energy conversion, photoelectrochemical detection, and hydrogen evolution of graphitic carbon nitride have been documented to date. But a lesser attention has been devoted to the mechanistic approaches towards sensing of variety of pollutants concerned with environmental and biological aspects. Herein, we present the sensing features of graphitic carbon nitride towards the detection of various analytes including toxic heavy metals, pharmaceuticals, phenolic compounds, nitroaromatic compounds, volatile organic molecules, toxic gases, and foodborne pathogens. This review will undoubtedly provide future insights for researchers working in the field of sensors, allowing them to investigate the intriguing graphitic carbon nitride material as a sensing platform that is comparable to several other nanomaterials documented in the literature. Therefore, we hope that this study could reveal some intriguing sensing properties of graphitic carbon nitride, which may help researchers better understand how it interacts with contaminants of environmental and biological concern.

Graphical abstract
Graphitic carbon nitride Nanosheets as Promising Analytical Tool for Environmental and Biological Monitoring of Hazardous Substances.

Full text

Vol.:(0123456789)
1 3
Microchimica Acta (2022) 189:426
https://doi.org/10.1007/s00604-022-05516-x
REVIEW ARTICLE
Graphitic carbon nitride nanosheets aspromising candidates
forthedetection ofhazardous contaminants ofenvironmental
andbiological concern inaqueous matrices
TauqirAhmad1· SardarazKhan1· TahirRasheed2· NisarUllah1
Received: 8 June 2022 / Accepted: 28 September 2022
© The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature 2022
Abstract
Monitoring of pollutant and toxic substances is essential for cleaner environment and healthy life. Sensing of various envi-
ronmental contaminants and biomolecules such as heavy metals, pharmaceutics, toxic gases, volatile organic compounds,
food toxins, and pathogens is of high importance to guaranty the good health and sustainable environment to community.
In recent years, graphitic carbon nitride (g-CN) has drawn a significantamount of interest as a sensor due to its large sur-
face area and unique electrochemical properties, low bandgap energy, high thermal and chemical stability, facile synthesis,
nontoxicity, and electron rich property. Furthermore, the binary and ternary nanocomposites of graphitic carbon nitride
further enhance their performance as a sensor making it a cost effective, fast, and reliable gadget for the purpose, and opens
a wide area of research. Numerous reviews addressing a variety of applications including photocatalytic energy conversion,
photoelectrochemicaldetection, and hydrogen evolution of graphitic carbon nitride have been documented to date. But a
lesser attention has been devoted to the mechanistic approaches towards sensing of variety of pollutants concerned with
environmental and biological aspects. Herein, we present the sensing features of graphitic carbon nitride towards the detec-
tion of various analytes including toxic heavy metals, pharmaceuticals, phenolic compounds, nitroaromatic compounds,
volatile organic molecules, toxic gases, and foodborne pathogens. This review will undoubtedly provide future insights for
researchers working in the field of sensors, allowing them to investigate the intriguing graphitic carbon nitride material as
a sensing platform that is comparable to several other nanomaterials documented in the literature. Therefore, we hope that
this study could reveal some intriguing sensing properties of graphitic carbon nitride, which may help researchers better
understand how it interacts with contaminants of environmental and biological concern.
Keywords Graphitic carbon nitride· Sensors· Environmental pollutants· Biosensing· Food toxins· Humidity sensors
Introduction
Today, one of the main leading global problems is the pres-
ence of different pollutants in the environment. These pol-
lutants not only affect human health but also overshadow
the life of other creatures [1–3]. The industrial development,
urbanization, and rapid population growth have become the
main causes of contamination of freshwater bodies over
the past two decades [4–6]. The complex effluents released
from a wide range of industries include leather tanning, hair
coloring, textile, paper production, light harvesting array,
agricultural research, photomechanical cells, antibiotics, and
food technology [7–11].
Monitoring of dangerous and toxic molecules is manda-
tory for general well-being of human healthcare and their
environment. Especially real-time monitoring requires easy
synthesis materials with high sensitivity and fast display of
results [12–18]. Graphitic carbon nitride (g-C3N4) with a
general formula of (C3N3H)n is one of the oldest reported
polymers in the literature and history of it starts from 1834
when a linear CN was obtained by Berzelius and called it
* Tahir Rasheed
tahir.rasheed@kfupm.edu.sa
* Nisar Ullah
nullah@kfupm.edu.sa
1 Chemistry Department, King Fahd University ofPetroleum
andMinerals, Dhahran31261, SaudiArabia
2 Interdisciplinary Research Center forAdvanced Materials,
King Fahd University ofPetroleum andMinerals (KFUPM),
Dhahran31261, SaudiArabia

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“Melon” [19]. Carbon nitride has several allotropic forms
like, α-C3N4, β-C3N4, g-h triazine, cubic C3N4, pseudocu-
bic C3N4, and g-C3N4 [20]. g-C3N4 is regarded as a most
stable allotrope at an ambient condition. It is composed of
tri-s-triazine units connected through planer amino groups,
which makes a layered structure. The staking layers (cova-
lent C-N bonds) are hold together via van der Waals force.
High thermal stability and chemical stability of g-C3N4
originate from this tri-s-triazine structures [21]. g-C3N4 is
a typical semiconductor with a band gap of 2.7eV. It is
considered a sp2-hybridized nitrogen–substituted graphene.
It has unique surface properties, large surface area, extreme
chemical stability, and good light absorption ability. In addi-
tion, unique surface properties make it a good support for
various applications (Fig.1) [22]. Graphitic carbon nitride
(g-C3N4) becomes an important candidate for sensing due
its inherent photoluminescence, chemiluminescent catalytic,
ability of electricity and light conversion, stability, biocom-
patibility, and electrochemical properties [23]. However, due
to low specific surface area with scarcity of active site on
the surface, low conductivity, fast rate of the electron–hole
recombination, high valance band, low quantum yield, and
poor water solubility, the bulk g-C3N4 is not perfectly fit
for sensing applications. Low conductivity of the pristine
g-C3N4 arises due to the presence of excessive nitrogen con-
tents in the structure. Low conductivity of the g-C3N4 results
in low-efficient charge transfer during the electrochemical
processes. This demands suitable modifications of the pris-
tine g-C3N4 to enhance their efficiency for various appli-
cations. Various modification modes are developed which
includes functionalization, structure engineering, incorpora-
tion of nanomaterials, doping with metals, non-metals, and
heteroatoms, co-polymerization with organic compounds,
size reduction, controlling surface morphology, protonation,
and pore texture-tailoring. Conductivity of g-C3N4 can also
be enhanced via self-doping strategy (addition of C) which
changes electronic structure and surface properties. It is well
documented through both experimental and theoretical stud-
ies that replacing N with C leads to formation of delocalized
π bonds, enhancing electrical properties of g-C3N4. In addi-
tion, carbon self-doping improves the visible light-driven
absorption by narrowing bandgap of g-C3N4 and increasing
its surface area by 4.25 times [24]. In addition [15, 25–30],
fabrication of g-C3N4 not only enhances its physiochemical
properties but also provides additional properties, opening
a wide field of research for using g-C3N4 as a sensor. Addi-
tionally, fabrication of the pristine g-C3N4 is also important
to achieve selectivity for the specific analytes. Depending
upon the requirements, the structure, surface functionalities,
and their properties can be altered through functionalization
or elemental doping in a controlled way. Various reports
are published in last few years which are engineering the
structure and controlling properties of g-C3N4 according to
the requirements of the specific analyte [31, 32]. Based on
this properties, g-C3N4 were retained as a sympathetic mate-
rial for the synthesis of various sensor for the detection of
various analytes such as glucose [33, 34], dopamine [35],
uric acid, [36], ascorbic acid [37], cholesterol [38], folic
acid, [39], nucleotide [40], ochratoxin [41], tryptophan [42],
glutathione, [43], bio thiols, hormones [44], vitamins [45],
enzyme [44], biomarkers, [46], antigens [47], proteins, [48],
drugs [49], pesticides [25], and antibiotics [45, 50].
Although some reviews on the sensing of g-C3N4 have
already been published which mainly focused on the applica-
tion of g-C3N4 for sensing heavy metals ions [51]. Magesa
etal. reported the electrochemical sensing of food colorants
and toxic substances in environment [52]. Most recently,
Gupta and coworkers explored the sensing applications
of g-C3N4 in biosensors [53]. All these reviews are well
presented and highlight the applications of g-C3N4 in the
sensing of pollutants. However, they are focusing either on
sensing only through one of the mechanisms (like electro-
chemical sensing or fluorescent sensing) or they are focusing
very small portions of the pollutants, e.g., only heavy met-
als, or only organic pollutants. However, there is a lack of
a comprehensive review which can fill this gap and present
latest development of g-C3N4 as broad sensing tool for vari-
ous applications. The contribution of this work is to give
a description of the four-sensing mechanism of g-C3N4,
i.e., fluorescence, electrochemical, electrochemilumines-
cence, and photoelectrochemical. Furthermore, the recent
progress of g-C3N4-based sensing system for the sensing of
environmental pollutants, food samples, sensing of relative
humidity (%RH), and biosensors following any of the four
sensing mechanism. Finally, a brief comparison of g-C3N4-
based sensing system with various graphene-based materi-
als like reduced graphene’s, graphene oxides, and GQDs is
provided.
2
Fig. 1 Structure of g-C3N4 with representation of various functionali-
ties laying on its surface redrawn from [26]

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Sensing mechanisms ofg‑C3N4
Fluorescence‑based sensors ofg‑C3N4
Graphitic carbon nitride quantum dots (g-C3N4 QDs) due
to their unique physiochemical properties such as non-
toxicity, bright fluorescence, bio-compatibility, and water
solubility work as a novel fluorescent sensor towards
sensing of various analytes. Fluorescent-based sensors of
g-C3N4 consist of monitoring any change in fluorescent
intensity, wavelength, and lifetime due to the interactions
of g-C3N4-based nanomaterials with the analytes. Depend-
ing on the photochemical and photophysical changes hap-
pening due to interactions of analytes and g-C3N4, various
mechanisms like photoinduced electron transfer (PET),
inner filter effect (IFE), and fluorescence resonance energy
transfer (FRET) are followed during the detection of the
analytes [54]. Although the change in various parameters
gives signal for the detection for the analytes, however,
the change in the fluorescent intensity is the simplest
parameter to measure and is commonly used. For exam-
ple, double positively charged mercury ions can bind to
g-C3N4 QDs surface due to the presence of carboxyl and
hydroxyl groups. Easy electron transfer due to these elec-
trostatic interactions completely quenches the fluorescence
of g-C3N4 QDs. Fluorescent can be recovered by expos-
ing the g-C3N4 QDs/Hg2+ complex to glutathione (GSH)
which leads to the development of coordination of Hg2+
to GSH (Fig.2a) [43]. Key merits of the fluorescent-based
sensors are their remarkable fluorescence sensitivity and
selectivity, fast response time, good reproducibility, and
their capability in real-time measurements. However, it
requires photostability of the samples and it can be applied
only to small molecules which limits their applications.
Electrochemiluminescence‑based sensors ofg‑C3N4
In electrochemiluminescence (ECL) mechanism, normally,
a co-reactant like K2H2O8 is used. This mechanism of ECL
using (K2S2O8, H2O2, O2 as a co-reactant) is well explained
by L. Chen etal. [55] (Fig.2b). Mechanism consists of
Fig. 2 a Schematic representation of fluorescent quenching and
restoring of g-C3N4 QDs [43]. b ECL reaction mechanism for
g-C3N4-co-reactant systems taken from [55]. c Schematic illustration
of possible photoelectrochemical mechanism for detection of methyl-
ene blue (MB). d Schematic of the Hg2+ detection based on the fluo-
rescence quenching of graphite carbon nitride [61]

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several steps. Cathodic polarization of potential increases
energy of electron on glassy carbon electrode (GCE)
adequately to inject it to the conduction band of g-C3N4
(Eq.(1)). At the same time hole donor is produced from
reductive-oxidative co-reactant like S2O8 (Eq.(2)). Then,
SO4
•− injects a hole into the highest occupied molecular
orbital of g-C3N4
•− and giving excited state of g-C3N4
(g-C3N4*) and emits blue light when go back to ground
state (Eqs. (3) and (4)). Complete process is explained
below as scheme1. This method has high sensitivity, very
good selectivity, flexibility in fabrication procedures with
low background emission, and wide detection range. How-
ever, it is a time-consuming method, which requires com-
plex and expensive optical imaging devices which makes
it a high-cost method with the need of trained peoples. In
addition, frequent electrode fouling is the other drawback
of this method.
Electrochemical sensors ofg‑C3N4
Electrochemical (EL)-based sensors are one of the oldest
reported techniques due to their simple instrumentation
and sensing procedures [27, 51, 54]. In the EL system,
electric signal is produced due to the interaction of ana-
lyte with the electrode surface which is detected with the
help of electrodes. The change in the electric signal may
be conductometric (change of conductance), voltametric
or amperometric (change of current with the applied volt-
age), potentiometric (change of membrane potential), and
impedimetric (change of impedance) [56–60]. For exam-
ple, in case of metal ion sensing through EL mechanism,
initially, an electron–hole pair is generated by g-C3N4-
modified working electrode in an electric field. Metal’s
ions are converted to elemental metal by accepting these
electrons. Electrochemical detection methods like cyclic
voltammetry (CV), differential pulse voltammetry (DPV),
and linear sweep anodic stripping voltammetry (LSASV)
are used for the demonstration of this process. This is a
very important sensing method due to their high selectiv-
ity, fast response time, low limit of detection, less operat-
ing power, and suitability for on-site detection. However, it
has short lifetime and due to degradation of electrode, the
responses decrease with time. In addition, easy contami-
nation and temperature intolerance are the other demerits
of this method.
Photoelectrochemical‑based ion sensors ofg‑C3N4
The photoelectrochemical-based sensing mechanism of
g-C3N4 is explained by [62] when adsorption of MB on sur-
face of g-C3N4 was detected via this system. Visible light
can also excite g-C3N4 to generate electron (e−) and hole
(h+). Compared to energy level of g-C3N4, energy level of
the conduction band (CB) of indium tin oxide (ITO) lies
lower, causing photogenerated electrons in the CB of g-C3N4
to migrate easily to ITO electrode, generating photocurrent.
Photogenerated electrons can now transfer to Pt electrode
from ITO electrode via a wire and then g-C3N4 and recom-
bine with holes, i.e., a cycle is formed. Now when MB is
adsorbed on the surface of g-C3N4, the excitation of g-C3N4
will slow down leading to decrease in photocurrent response
(Fig.2c). In case of metal ion like Cu2+, it can accept elec-
tron to form elemental metal and with increased exposure to
light metal can be deposited on the surface of Pt electrode.
Thus, materials promoting separation of electrons from
holes enhance the performance of photocurrent. Photoelec-
trochemical reaction of g-C3N4 system can be represented as
follows (Scheme2). This method has several merits such as
high sensitivity, simple equipments, facile miniaturization,
low background, and low cost. However, it has the issues of
repeatability and reliability. Table1 shows the comparison
of different sensing mechanisms of g-C3N4 towards differ-
ent analytes.
Scheme1 Mechanism of ECL
adopted from reference [35]
Scheme2 Schematic represen-
tation of Cu2+ detection with
g-C3N4 through photoelectro-
chemical sensing

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Application ofg‑C3N4 towardssensing
ofenvironmental pollutants
Environmental pollutants are one of the most serious
threats affecting human health and aquatic life [11, 77–82].
It can be defined as the contamination of earth components
a chemical that can affect normal environmental process.
Any chemical accumulates over their natural level or when
causes toxicity is considered a pollutant. Air pollution and
water pollution are caused due to excessive use of natural
resources and tremendous increase in industrialization [83,
84].
Sensing ofheavy metal ions
Heavy metals can cause severe problems to human health
even at low concentration. Both human activities and natu-
ral resources can pollute environment (soil, water, and air)
with these metals [85–91]. To avoid pollution and protect
environment, early detection of these heavy metal ions is
mandatory. However, its detection is a crucial challenge due
to low concentration of these heavy metal ions. The need of
costly apparatuses and complex sampling procedures further
adds to this challenge [92–96].
Hg2+ ion sensing
Li etal. developed a fluorescent sensor in which fluores-
cence was quenched upon binding to Hg2+ ion [61]. In this
interesting study, g-C3N4 nanosheets functionalized with
single-stranded DNA aptamer were used as a sensing probe.
The probe under 380nm light excitation gives a strong fluo-
rescence at 440nm in the absence of Hg2+. When Hg2+
is added, it interacts with double-stranded DNA leading
to formation of thymine- Hg2+-thymine complex. Com-
plex formation results in fluorescence quenching (Fig.2d).
Under optimum conditions, low limit of detection (LOD) of
0.17nM was calculated. Thermal polymerization of dicy-
anamide leads to synthesis of g-C3N4 nanosheets which were
used to synthesize carboxyl rich g-C3N4 NPs (Fig.3a) [97].
Carboxylation was done via hydrothermal oxidation using
concentrated HNO3 and refluxing for 48h. Carboxyl-rich
g-C3N4 were used for constructing selective sensor for Fe3+
and Hg2+ detection reaching to LOD of 190 and 12nM,
respectively.
Graphitic carbon nitride nanosheet g-C3N4 NS based
“on–off-on” sensor was developed for the detection of Hg2+
and 6-thioguanine (6-TG) [98] (Fig.3b). When 6-TG was
added to g-C3N4, it quenches its fluorescent properties (turn
off) which were recovered by addition of Hg2+ (turn on).
Table 1 Comparison of g-C3N4 sensing towards various analytes through different mechanisms
Sensor name Sensing mechanism Analytes LOD Linear range Detection system Ref
AgNPs embedded
sulfur-doped g-C3N4
QDs
Fluorescence Hg2+ 0.13µM 0.1–0.6μM water [63]
Multilayered g-C3N4Fluorescence Hg2+ 1.14nM ‒water [64]
MOF/g-C3N4 QDs Photoluminescence Hg2+ 2.4nmol.L–1 ‒water [65]
g-C3N4 QDs Fluorescence Ag(I) and L-Cysteine 8.0 and 10.3nmol.L–1 ‒Tap water [66]
PDA@W-g-C3N4/ITO EL Cu2+ 3.789nmol.L–1 0.033 to 0.933µmol
L−1
water [67]
S,O–g-C3N4 QDs Fluorescence Cu2+ 0.58nM 0.50–15μM water & living cells [68]
g-C3N4 QDs Photoluminescence Fe3+ 0.259µM ‒water [69]
Pd@TiO2/g-C3N4EL ethanol – 50–10,000ppm Gas [70]
g-C3N4/LGS SAW EL NO2⁓158ppb ‒Gas [71]
g-C3N4/Au NWs SERS 4-aminophenol 6.08 × 10–9M _ ‒[72]
Bi2S3/Zn-g-C3N4EL nitric oxide 0.007µM _ water & blood serum [73]
ND-g-CN PEC ciprofloxacin 20ng L−1 60 − 19,090ng L−1 ‒[74]
VOPc/CN PEC diclofenac 0.03nM 0.1 − 500nM ‒[28]
Bi/BiVO4/g-C3N4PEC OTC 0.033nM 0.01 − 1000nM ‒[29]
ZnFe2O4/g-C3N4
(ZFO/g-
C3N4)
EL H2O21μM 5 & 200μM ‒[30]
graphite carbon
nitride nanoparticles
(GCNNs
Fluorescence dopamine, glu-
tathione and ascor-
bic acid
0.064, 0.11 and
0.16µmol L−1 respec-
tively
0.1–3.0, 0.3–10.0 &
0.3–16.0µmol L−1
human blood
serum
[75]
g − C3N4 nanosheets
(NSs)
ECL histamine 4.30 × 10−8mol/L 1.0 × 10 − 7 to
7.5 × 10−4mol/L
‒[76]

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Patir and Gogoi presented synthesis of sensor for Hg2+
detection based on g-C3N4 quantum dots co-doped with
sulfur and oxygen [99]. Thermal method was applied for
the synthesis using thiourea and ethylenediaminetetraacetic
acid disodium salt. Sensor gives low LOD of 0.01nM. Elec-
trochemical-based sensor based on nanoparticles of Hg2+
impregnated polymer was synthesized for the detection of
Hg2+ [100]. Graphitic carbon nitrides together with these
particles were used to fabricate carbon paste electrode and
used for analysis of Hg2+ through several steps. First, Hg2+
was accumulated on modified electrode, followed by the
reduction of Hg2+ to Hg, and in last, electrochemical signals
were generated through a square wave anodic stripping vol-
tammetry (SWASV). Sulfur-doped g-C3N4 assembled with
Fig. 3 a Schematic representation of synthesis and metal sensing
mechanism of carboxyl rich g-C3N4 NPs [97]. b Schematic repre-
sentation of the sensing mechanism of the g-C3N4 nanosheets as an
“on–off-on” fluorescent sensor for the detection of 6-TG and Hg2+;
fluorescence spectra of g-C3N4 nanosheets after addition of 30 µM
6-TG a and 6-TG-g-C3N4 nanosheets upon adding 25µM Hg2+ [98].
c Schematic representation for the determination of [HSBMIM]Br
based on “ON–OFF-ON” sensing platform [105]

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gold nanoparticles were used for the ultra-sensing of Hg2+
based on calorimetric method [101]. Method was applied
for Hg2+ sensing in spiked river water samples and exhibited
low LOD of 0.275nM. Similarly, nanocomposite of gold
nanoparticle-g-C3N4 nanosheets was applied by Wand etal.
for the calorimetric sensing of Hg2+ [102]. Good selectiv-
ity for Hg2+ was observed in the presence of other ions and
shows good results in real water samples. Methylated mer-
cury (CH3Hg+), a bio-magnifier, causes toxic effect on the
brain as it can cross the blood–brain barrier [103]. A GCE
modified with Au nanoparticles-g-C3N4 (AuNPs/g-C3N4)
was synthesized via photochemical method for the electro-
chemical detection of CH3Hg+. Extensive characterized with
XRD (X-ray diffraction), SEM (scanning electron micro-
scope), TEM (transmission electron microscopy), XPS, and
FTIR (fourier transform infrared spectroscopy) were done
to confirm the deposition of AuNPs on g-C3N4. Differential
pulse stripping voltammetry was applied towards the sens-
ing of CH3Hg+. The linear range of detection of 1–25mg/L,
sensitivity of 0.285 µA/µg‒1L, and LOD of 0.103µg/L were
obtained. The interference from Hg2+ was minimized with
the addition of SnCl2 or diethylene triamine pentaacetic acid.
Nanohybrid of silver nanoparticles (NPs), graphene oxide
(GO), and g-C3N4 was combined to methylene blue (MB) for
the detection of Hg2+ ion [104]. Charge transfer to AgNPs
from GO/g-C3N4 and then to MB causes enhancement in
surface enhanced Raman spectroscopy (SERS) intensity.
Furthermore, MB bonded to metal NPs of SERS substrate
work as a Raman tag for sensing of Hg2+ at low concentra-
tion of 0.01986ppm.
Hota etal. reported an efficient method for the prepara-
tion of AgNP-embedded sulfur-doped g-C3N4QDs. The syn-
thesized QDs having pore size of 3.7nm showed remarkable
blue fluorescence and a very high relative quantum yield of
36.5% was obtained. The nanosensors (Ag–S-g-C3N4) dis-
played excellent sensing potential for the selective detection
of Hg2+ in aqueous media at pH 5. The measured limit of
quantification and LOD for these materials was found to be
0.43µM and 0.13µM, respectively, with a linear detection
range of 0.1‒0.6µM. Furthermore, from the time-resolved
decay experiment, a mechanism of static quenching was
suggested which clearly showed that the transfer of elec-
tron occurred from metallic Ag to Hg2+ via redox reaction.
Moreover, the developed sensing system was also applied
to the real water samples and a significant quantity of Hg2+
was recovered with a relative standard deviation of less than
5%. The research study demonstrated that the presence of
AgNPs and sulfur doping in g-C3N4QDs enhanced the sens-
ing capability of Hg2+ due to the formation of active sites
[63]. Due to their toxic nature, fast and precise removal of
Hg2+ is highly desirable. Sun and coworkers reported a facile
strategy for the synthesis of multilayered g-C3N4 through
thermoploymerization treatment. The fluorescence probe
displayed excellent performance for the selective detection
of Hg2+ in lab water. The LOD calculated in the water was
1.14nM. It was observed that the photoluminescence (PL)
exhibited high linear correlation when the concentration of
the Hg2+ is ranging from 0 to 100nM [64].
Sliver ion sensing
Ultrasonic exfoliation g-C3N4 nanosheets working as dual
“On–Off-On” were used for the simultaneous detection of
Ag2+ and thiol-based ionic liquids (THIL) [105]. The addi-
tion of Ag2+ to system causes quenching (turn Off) which
were recovered (turn on) with the addition of THIL (using
[HSBMIM]Br as an example) due to the reaction between
THIL and Ag2+ (Fig.3c). The recovered fluorescent inten-
sity increases with THIL concentration giving s linear range
of 15–360nM and LOD of 4.28nM. Radial polymerization
route was adopted to synthesis g-C3N4 nanosheets/poly-
acrylamide/polyacrylic acid composite hydrogel as a fluo-
rescent probe for the sensing of Ag+ [106]. The fluorescent
intensity of the synthesized probe was dependent on the
concentration of Ag+ in the range of 0–100mM. Limit of
detection of 6.31mM was calculated. Contrary to above flu-
orescent sensors, M. Li etal. used a calorimetric sensor for
the determination of Ag(I) [107]. To mimic the peroxidase
activity, platinum nanoparticles (PtNPs) and g-C3N4 hybrid
materials were used as a low-cost colorimetric sensor for the
detection of Ag(I). The hybrid material (g-C3N4-PtNPs) was
synthesized through reduction of chloroplatinic acid using
sodium borohydride (NaBH4) under ultrasonication in the
presence of g-C3N4. The prepared hybrid material can pro-
duce colored product by catalyzing oxidation 3,3′,5,5′-tetra-
methylbenzidine (TMB) by mimicking peroxidase activity.
The addition of certain amount of Ag(I) in the presence of
citric acid will reduce Ag(I) to Ag(0) through catalysis of
PtNPs. Ag(0) will deposit on the surface of g-C3N4-PtNPs
inhibiting its catalytic activity resulting in the formation
of less blue product (Fig.4a). Under optimum conditions,
Ag(1) LOQ of 0.05–5.0nM and LOD of 22pM resulted
due to high catalytic activity of the hybrid material. Wang
etal. documented one pot easily accessible formation of
g-C3N4QDs via methylamine-based hydrothermal proto-
col. The material was used for the detection of Ag(I) and
cysteine with magnificent fluorescence property. By the
addition of Ag(I), g-C3N4QDs fluorescence intensity was
rapidly quenched. However, the fluorescence resonance light
scattering intensity was enhanced, thus leading to the syner-
gistic mechanism between the photoinduced electron shift
and chelation process. Based on an “on–off-on” and “off–on-
off” synergetic fluorescence switching, the g-C3N4QDs dis-
placed excellent selectivity and sensing performance for
the detection of Ag(I) and cysteine in natural samples. The

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LOD values obtained for Ag(I) and cysteine were 8.0 and
10.3nmol.L–1, respectively [66].
Copper ion sensing
Oxygenated group modified with g-C3N4 nanosheet was
reported for the ultrasensitive voltametric detection of Cu2+
ions [108]. Electrode modified with g-C3N4 nanosheets con-
taining mostly hydroxyl group as an oxygen moiety, greatly
enhanced Cu2+ accumulation which resulted in pico-mol
level detection of Cu2+ ion. Furthermore, Pb2+ and Hg2+
were also detected simultaneously with this sensor at dif-
ferent anodic voltage with very less interference with Cu2+
detection. In another study g-C3N4 nanosheets modified with
oxygenated group were assembled to porous structures on
multiwall carbon nanotubes to use as a voltametric simul-
taneous multi-ion detection platform [109]. Nano frame-
work of modified g-C3N4 nanotubes on multi-walled car-
bon nanotubes was assembled on carbon fiber disk through
drop-casting method (Fig.4b). The aim of the study was
to study the mechanism phytoremediation. Highly sensi-
tive simultaneous detection of Pb2+, Cu2+, and Hg2+ in real
sample containing rice roots was achieved. A dual-emission
ratiometric fluorescence film was synthesized from g-C3N4,
chitosan, and Au nanoclusters (Au NCs) for the selective
detection of Cu2+ [110]. Results have hinted that g-C3N4
Fig. 4 a Colorimetric assay for Ag+ based on the peroxidase-mimic
activity of g-C3N4-PtNPs [107]. b Preparation and characterization
of P-CNT60/MWCNT/CFE (A) Diagrammatic sketch of P-CN_T60/
MWCNT/CFE preparation. SEM image of (B) bare carbon fiber. (C)
carbon fiber disk microelectrode. (D) MWCNT formed framework
on carbon fiber disk microelectrode. (E) P-CNT60 assembled on the
MWCNT formed framework on carbon fiber disk microelectrode. (F)
Cyclic voltammograms of P-CNT60/MWCNT/CFE in 1.0 × 10−3mol
L−1 K3Fe(CN)6 and 1.0 mol L−1 KCl aqueous solution at differ-
ent potential scan rate of 1V s−1, 0.5V s−1, 0.1V s−1, 0.05V s−1,
0.01V s−1, and 0.005V s−1. (G) DPV curves of P-CNT60/MWCNT/
CFE, CNT60/MWCNT/CFE and MWCNT/CFE in the solution con-
taining 1.4 × 10−7 mol L−1 Cu2+, Pb2+ and Hg2+ (Black and white)
[109]. c The preparation process of gold-cluster-based dual-emission
ratiometric fluorescent sensing film and sensing mechanism [110]. d
Schematic illustration of photo electrochemistry for sensing of Cu2+
[111]. e The fluorescence mechanism of GCNS. b The fluorescence
quenching mechanism of GCNS after adding Fe3+ ion [112]

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work as a fluorescence emission source and enhance thermal
and mechanical properties of the films. Binding of Cu2+ ion
results in quenching red shift of Au NCs while blue shift due
to g-C3N4 still continues to occur (Fig.4c). The ratio of fluo-
rescent intensities is dependent on the concentration of Cu2+
ion. Increasing the concentration of Cu2+ ion also changes
the fluorescence color from orange red-yellow-cyan and blue
at the end. Thus, this dual-emission fluorescence paper is a
convenient way of Cu2+ ion detection giving LOD of 10ppb.
Want etal. synthesized fluorescent sensor for the detec-
tion of Cu2+ based on fluorescent oxygen and sulfur co-
doped g-C3N4 quantum dots [113]. Due to improved optical
properties, stability of doped g-C3N4 quantum dots in water,
and better dispersion, the prepared sensor shows enhanced
sensing towards Cu2+. Chen etal. boosted the fluorescence
effect of the nanocomposite by using g-C3N4 with zeolitic
imidazolate framework-8 (ZIF-8) [114]. Due to fine-tuning
in fluorescent quenching effect, a great enhancement in
the sensitivity of composite towards Cu2+ and Ag2+ was
observed. On the other hand, H. Xu applied ECL sensor
for Cu2+ ion detection [115]. GC electrode modified with
g-C3N4 shows good ECL response at a scan rate of 0.1V s−1
when using 10 × 10−3mM S2O8
2‒ as a co-reactant. Addi-
tion of Cu2+ quenched the ECL intensity via photoinduced
electron transfer (PET) mechanism. However, liberation of
Cu2+ from Cu2+-g-C3N4 with the addition of pyrophosphate
anion could recover the ECL emission. A nanohybrid com-
posite of g-C3N4 quantum dots and Bi2MoO6 nanoparticles
was prepared for the photoelectrochemical (PEC) detection
of Cu2+ [116]. In total, 3-folds and 6-folds high photocur-
rent intensities were displayed by fabricated nanohybrids as
compared to g-C3N4 quantum dots and Bi2MoO6 nanopar-
ticles, respectively. This enhancement in photocurrent was
caused by accelerated charge transfer from CB of g-C3N4
quantum dots to Bi2MoO6 nanoparticles. The as-synthesized
PET sensor was applied for the detection of Cu2+ ion and
under optimal conditions good selectivity, and wide linear
range of 3nM to 40µM was displayed. g-C3N4 nanorods and
nanosheets were prepared at low temperature, and selectiv-
ity and sensitivity towards Cu2+ detection were compared
[117]. g-C3N4 on photoexcitation emits blue light which is
quenched when metal ion is added due to complex formation
between lone pairs of nitrogen at the edges of nanorods and
metal ions. However, compared to nanosheets, porous nature
nanorods of g-C3N4 display higher surface area and provide
more nitrogen atoms to edges and display high fluorescent
quenching. Another study revealed that novel photoelec-
trode of 3D-branched crystalline (3DBC) g-C3N4 with 1D
nanoneedles (3DBC-C3N4/FTO) displays good photoelectro-
chemical reduction of Cu2+ due to effective charge separa-
tion, large surface area, and fast charge mobility [118]. The
prepared sensor exhibited low LOD of 0.38nM, with linear
range of 1–100nM when applied for the detection of trace
Cu2+ in aqueous environment. A double-potential ratiomet-
ric ECL sensor based on g-C3N4 nanosheets and graphene
quantum dots was developed for the determination of Cu2+
[111]. g-C3N4 nanosheets were mixed with multi-walled car-
bon nanotubes and immobilized on glassy carbon electrode.
It produces strong cathodic ECL at a potential of − 1.2V
(vs. Ag/AgCl). On the other hand, graphene quantum dots
give anodic ECL at + 2.5V in the solution. The purpose of
MWCNTs mixing with g-C3N4 nanosheets was to amplify
its anodic ECL signals. The addition of Cu2+ causes decline
in cathodic ECL signal while the anodic ECL signal was
unchanged. The ratio of cathodic signal to anodic signals
was related to Cu2+ concentration giving a linear range of
5.0 × 1‒10 to 1.0 × 1‒6 with LOD of 0.37nmol/L.
In 2021, Li etal. reported the synthesis of PDA@W-
g-C3N4 by the combination of polydopamine (PDA) with
g-C3N4 under the basic conditions. It was confirmed through
XRD, SEM, and FTIR techniques that the improvement that
occurred in the adherence and enrichment of metal ions on
the surface of PDA@W-g-C3N4 was due to the successful
coating of PDA on W-g-C3N4. For the detection limit of
Cu2+, differential pulse stripping voltammetry was used.
They found best linear relationship between the dissolution
peak current and concentration of Cu2+ when the concen-
tration of Cu2+ is ranging from 0.033 to 0.933µmol.L–1.
The LOD value was calculated to be 3.789nmol L−1. Addi-
tionally, the synthesized sensor shows high replicability
and greater stability without causing any pollution [67]. In
the following year, Jia and coworkers developed a reliable
method for the synthesis of sulfur and oxygen co-doped
g-C3N4 QDs (S,O–g-C3N4 QDs) by mixing ethylenediami-
netetraacetic acid disodium salt dehydrate and thiourea.
XPS, TEM, FTIR, and XRD techniques were used for the
study of functional groups and morphology of S, O–g-C3N4
QDs. The results showed that S,O–g-C3N4 QDs served as
excellent fluorescence probe for the detection of Cu2+ with
LOD of 0.58nM over a wide linear range of 0.50–15µM.
High recoveries (99.0–110.0%) were achieved when the
fluorescence probe was utilized for the detection of Cu2+
in the samples of lake water, tape water, urine, and human
serum. Furthermore, due to the low toxic nature and greater
cell penetration power, the O–g-C3N4 QDs were efficiently
employed for the monitoring of Cu2+ in the living cells [68].
Iron ion sensing
Graphitic carbon nitrides were synthesized through ther-
mal polycondensation from urea and melamine precur-
sors [119]. Chemical structures and optical properties of
g-C3N4 obtained from both these precursors were com-
pared. Characterization techniques like, TEM, XRD,
elemental analysis, SEM, FTIR, XPS, and differential
reflectance spectroscopy (DRS) show that g-C3N4 sheets

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obtained from melamine were more condensed as com-
pared to urea precursor, favoring δ* to nitrogen lone
pair state transition in the obtained material, involved in
fluorescence sensing of metal ions. On the other hand,
g-C3N4 obtained from urea precursor was exfoliated, less
condensed, and with low photoluminescence emission.
g-C3N4 obtained from melamine was applied towards
sensing of Fe3+, displaying low LOD of 8.7µM and linear
range of concentration 0–100µM. In another study W.
Tang etal. prepared g-C3N4 nanosheets (g-C3N4 NS) for
the fluorometric determination of Fe3+ ion and explained
the fluorescence quenching mechanism by Fe3+ ion [112].
Mechanism of fluorescent quenching between g-C3N4 and
Fe3+ ion was explained which springs from redox poten-
tial and empty d-orbital of Fe3+ ion. Graphitic carbon
nitride is an n-type semiconductor with redox potential of
valence band that is 1.6eV while for conduction band, it
is − 1.1 e V. Redox potential of Fe3+ ion makes it possible
to have an impurity level between conduction band and
valence band. Most important factor is the paramagnetic
and unfilled d orbital of Fe3+ ion. Compared to other metal
ions, Fe3+ ion has maximum lone electrons resulting five
rails for unit with the N unpaired electrons of g-C3N4,
which are encouraged to δ* band (Fig.4e). This leads to
phenomenon of fluorescence quenching. Chemical oxida-
tion and exfoliation method was used for the synthesis
of water-soluble g-C3N4 quantum dots from bulk g-C3N4
[120]. The as-synthesized QDs of varying sizes work as
a fluorescent sensor with excitation/emission wavelength
of 241/368nm. It displays a good selectivity for Fe 3+ in
the presence of competing ions and within-minute detec-
tion of Fe3+ in spiked natural water sample was obtained.
LOD of 23 × 10‒9M and linear response corresponding to
Fe3+ concentration in the range of 0.2 × 10‒6‒60 × 10‒6M
were shown.
Liu etal. reported the gram-scale formation g-C3N4 QDs
with the purity of 99.96 wt%. The result demonstrated that
the synthesized g-C3N4 QDs at a wavelength of 365nm
displayed very high and steady UV-photoluminescence.
Moreover, it was investigated that these materials served as
efficient fluorescent probe for the selective detection of Fe3+
with the linear range of 0–100µM and LOD of 0.259µM,
respectively. It was observed that increasing the concen-
tration of g-C3N4 QDs, the fluorescence response between
Fe3+ and g-C3N4QDs was improved. The best fluorescence
response of g-C3N4 QDs for the detection of Fe3+ (0–10µM)
was achieved when g-C3N4 QDs having concentration of
0.050mg/mL [69].
Sensing ofother ions
In an excellent study, thermal polymerization was used for
the polymerization of urea precursor to get bulk graphitic
carbon nitride (b-g-C3N4) which was activated through
ultrasonic liquid exfoliation and protonation resulting in
formation graphitic carbon nitride nanosheets (g-C3N4 NS)
[121]. Due to high surface area and active site of g-C3N4 NS,
GCE modified with g-C3N4 NS displays tremendous elec-
trochemical performance towards the detection of Cd2+ with
3.9nM LOD and 22.668 µA/µM sensitivity. Interference due
to Hg2+, Cu2+, and Pb2+ was very less. Furthermore, it gives
high spike recoveries when modified electrode was applied
for the determination of Cd2+ in rice samples and natural
water. With green synthesis approach, carbon dots and
g-C3N4 (CDs@g-C3N4) nanocomposite-based “Turn-On”
fluorescent sensor were synthesized for the detection of mul-
tiple ions [122]. Initially excited state non-radiative energy
transfer process from CDs to g-C3N4 causes quenching of
fluorescent intensities of CDs. However, the attachment of
metal to the surface of CDs@g-C3N4 nanocomposites results
in the formation of surface complex formation with g-C3N4
which recovers the fluorescent property of CDs. Fluores-
cence increases with the increase of metal ion concentration
until reach to a saturation point. Studying sensing property
of synthesized sensor at different pH value enhances the
sensing to great extent and at optimal conditions gives low
LOD of 0.2nM, 0.54nM, and 0.18nM for Pb2+, Cr4+, and
Cu2+ respectively. Graphitic carbon nitride quantum dots
and graphene oxide quantum dots were prepared using
readily available materials like urea, trisodium citrate, and
citric acid [123]. The synthesized nanozymes worked as a
redox catalyst, reducing H2O2 and simultaneously oxidiz-
ing colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue
color–oxidized TMB in the presence of H2O2. Based on this,
to mimic the activity of natural enzymes, g-C3N4 QDs were
used for the fluoride ion detection in water samples. Wide
linear range of detection up to 120µM and LOD of 4.06µM
was calculated. Another important transition metal is Zn ion
working as a cofactor for many enzymes in the body [12].
But still, there is no study of using g-C3N4 or its composites
for the sensing of Zn ion despite its good sensing properties
towards transition metals. The use of g-C3N4 or its compos-
ites for the sensing of Zn ion will be a good idea from future
perspective.
Sensing ofpharmaceutical contaminants
Pharmaceutical contaminants are severe threat to environ-
ment due to extensive uses. Dimetridazole, an antiprotozoal
drug because of suspect to carcinogenic, is banned in some
places. Ni–Fe-layered double hydroxide nanosheet/sulfur-
doped graphitic carbon nitride was synthesized through an
environmentally benign protocol. The synthesized sensor
was applied for the determination of dimetridazole, exhib-
iting higher sensitivity, LOD of 1.6nM, and linear range
of response of 0.008–110.77µM with good stability and

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selectivity were calculated [124]. Carbon nitride was pre-
pared via one-step electrochemical synthesis, and C3N4,
polyaniline and CdO were grown on electrode (GCE/mpg-
C3N4/PANI/CdO) in step-wise manner [125]. The as-syn-
thesized sensor was used for the electrochemical determina-
tion of ciprofloxacin (CIP), epinephrine (EPI), mefenamic
acid (MFA), and paracetamol (PAR) (Fig.5a). Two linear
responses in the range of 0.01–20µM and 25–250µM
for CIP, two for EPI in the range of 0.05–80 µM and
100–1000µM, one for MFA in the range of 0.2–400µM and
one for PAR in the range of 0.1–790µM. LOD of 0.005µM,
0.011µM, 0.045µM, and 0.026µM was calculated respec-
tively for CIP, EPI, MFA, and PAR.
Cobalt molybdate nanorods were decorated on boron-
doped graphitic carbon nitride (CoMoO4/BCN) sheets by
sonochemical method for the electrochemical sensing of
furazolidone [126]. Electrochemical detection of furazo-
lidone was favored by lower resistance charge transfer (Rct)
of impedance of CoMoO4/BCN-fabricated screen-printed
carbon electrode. Low limit of detection of 1.6nM was cal-
culated with a linear range of detection of 0.04 to 408.9µM
and sensitivity of 11.6 36.277 µAµM‒1 cm‒2 with differen-
tial pulse voltammetry (DPV) method. Oxygen-doped gra-
phitic carbon nitride was synthesized for the electrochemical
sensing of metronidazole (antimicrobial) via thermal polym-
erization of urea and oxalic acid [127]. The as-synthesized
O–g-C3N4 was characterized with various characteriza-
tion techniques. The doping of oxygen was confirmed by
comparing electrochemical properties of O–g-C3N4 with
pristine g-C3N4 through electrochemical impedance spec-
troscopy (EIS), cyclic voltammetry (CV), and DPV studies
which displays improved surface area, lower impedance,
and enhanced reduction current response after the doping.
Wide linear range of 0.01–2060µM, sensitivity of 7.784 µA
µM‒1 cm‒2, and LOD of 0.005µM was calculated (Fig.5b).
A nanohybrid of g-C3N4 NPs and polyaniline-polypyr-
role (g-C3N4-PANI-PPy) was synthesized through oxi-
dative polymerization reaction for the electrochemical
sensing of mebendazole drug and photocatalytic degrada-
tion of methylene blue [130]. Nanohybrid was thoroughly
characterized with EDAX, UV–Vis, XPS, FESEM, XRD,
Raman, and HRTEM. The limit of detection and limit of
quantification of MBZ drug were calculated to be of 0.1481
µMµA‒1 and 0.4717 µMµA‒1, respectively. Nano compos-
ite of mesoporous graphitic carbon nitride, black phospho-
rous (BP), and gold nanoparticles (mpg-C3N4/BP-Au) was
applied to photocatalytic hydrogen evolution and sensing
of paracetamol [131]. Deposition of mpg-C3N4/BP-Au over
Fig. 5 a Schematic representation of synthesis and sensing proce-
dure of GCE/mpg-C3N4/PANI/CdO [125]. b Graphical representa-
tion of O-gCN preparation and their electrochemical performance
towards sensing of MTZ [127]. c (a) Response values of the sensors
based on SnO2, SnO2/g-C3N4-5, SnO2/g-C3N4-7, and SnO2/g-C3N4-9
to 500 ppm ethanol as a function of the operating temperature. (b)
The responses of sensors (SnO2, SnO2/g-C3N4-5, SnO2/g-C3N4-7, and
SnO2/g-C3N4-9) operated at 300°C versus different concentrations of
ethanol [128]. d Schematic representation of the interaction mecha-
nism between ZnO/rGO/g-C3N4 nanocomposites and ethanol [129]

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modified GCE gives LOD of 00,425µM and linear range of
0.3–120µM under optimum conditions. In another study,
graphitic carbon nitride was prepared from urea and thiourea
precursor and used for the adsorption of MB from wastewa-
ter [132]. After this, g-C3N4 was decorated with Ag NPs and
used for detection of acetaminophen drug through insitu
surface-enhanced Raman scattering. Good sensing perfor-
mance was displayed by the developed materials towards
the drug.
Sensing oftoxic gases
Carbon nitride–based gas sensor is very effective due to its
higher surface area, improving diffusion and adsorption of
target gas molecules. Hybrid materials based on g-C3N4
attachment with other materials can alter the properties
according to nature of target gas thus leading to its suit-
ability for the sensing of a variety of gas molecules [133].
Sensing ofethanol
Graphitic carbon nitride was decorated with cocoon-like
ZnO through hydrothermal method using PEG400 surfactant
[134]. Different characterization techniques were applied to
characterize the nanocomposite. Characterization revealed
that cocoon-like ZnO nanocrystals with length of 200 to
300nm and width of 30 to 50nm were randomly distributed
on g-C3N4 surface possessing close interface. The as-syn-
thesized sensor displayed improved sensing towards ethanol
at low optimum temperature with good linear range of con-
centration. In another study, carbon nitride was decorated
with Co3O4 again through hydrothermal method for ethanol
sensing and different analytical techniques were used for the
characterization [135]. This sensor exhibited best sensing
at 210 ℃ with good selectivity in presence of other simi-
lar gases. In addition, composite displays 1.6 times higher
response value as compared to pure Co3O4 for 500ppm etha-
nol at optimum temperature. Using calcination method for
the synthesis of SnO2/g-C3N4 composite from SnCl4•5H2O
and urea precursor [128]. Characterization results display
the 2D structure of the sample with highly dispersed SnO2
NPs on the surface of g-C3N4. SnO2/g-C3N4 composite with
7 wt % of g-C3N4 displays the highest response of 360 at 300
℃ towards ethanol, which is higher than pure SnO2 sensor
which exhibited only 95 response value at 320 ℃ (Fig.5c).
Enhanced sensing performance of SnO2/ g-C3N4 could be
attributed to high surface area and improved electronic prop-
erties of the SnO2/g-C3N4. A ternary material (ZnO/rGO/g-
C3N4)-based sensor was developed for the sensing of ethanol
[129]. First, a hybrid of graphene oxide with g-C3N4 (GO/g-
C3N4) was synthesized through ultrasonication and electro-
static self-assembly. Then using hydrothermal process, ZnO
was coated on GO/g-C3N4 resulting in GO reduction (rGO).
Graphitic carbon nitride was working as a sensitization mod-
ifier for ZnO/rGO (Fig.5d). The as-synthesized hybrid was
used for the sensing of ethanol which displays at 300 ℃ the
highest response of 178 towards 100ppm of ethanol vapors
which was many folds higher than ZnO and ZnO/rGO alone.
Furthermore, low LOD of 500ppb was obtained.
In 2022, Nasresfahani etal. described the formation of
Pd@TiO2/g-C3N4 nanosheet sensing materials by employ-
ing both calcination and solvothermal growth methods.
The XRD was used which revealed the formation of Pd@
TiO2 nanoparticles on the surface of g-C3N4. By examin-
ing various mass ratios of Pd, the detection capabilities of
Pd@TiO2/g-C3N4 were checked. The experimental studies
demonstrated that the synthesized 5% Pd@TiO2/g-C3N4
displayed high detection over a wide range of concentration
(50–10,000ppm) with rapid recovery (15s) and minimal
response time (30s), thus leading to the excellent selectivity
of ethanol, and high durability at 125 ℃. The excellent sens-
ing ability of the Pd@TiO2/g-C3N4 was attributed due to the
2D structure of the g-C3N4, the high catalytic performance
of Pd, and its metallic core–shell heterostructure [70].
Sensing of NO2 gas
Synthesis of g-C3N4 via pyrolysis of melamine and for-
mation of composite with rGO and AgNPs was reported
[136]. AgNPs were randomly distributed on the surface of
hybrid to form Ag@rGO/g-C3N4 composite. Both reducing
and oxidizing gases were detected where hybrid and highly
improved response was observed as compared to pristine
materials. When applied for the sensing of NO2 and NH3,
responses of 8% and 95% for 50ppm of NH3 and NO2 at
room temperature was estimated (Fig.6a). Heat treatment
of melem hydrate through polycondensation process leads
to the synthesis of porous g-C3N4 due to release of NH3
and removal of H2O [137]. This porous g-C3N4 having
exposed edges works as an excellent sensor towards sens-
ing of NO2 (Fig.6b). Hybridization of porous g-C3N4 with
AuNPs further enhances their sensing performance. At
constant current, the change in resistance with change in
gas environment was monitored. Hybrid materials of sensor
with AuNPs display LOD of 60ppb at ambient conditions
In another study, adsorption behavior of NO2 on g-C3N4
and g-C3N4 decorated with Ir, Co, and Rh was studied
through first-principle calculations and their performances
were compared [138]. Adsorption energy analysis sug-
gests physisorption of NO2 on pristine g-C3N4 and chem-
isorption on transition metal–decorated g-C3N4. However,
much stronger interactions were calculated for NO2 and
Ir–embedded g-C3N4 as compared to Rh and Co–embed-
ded g-C3N4. This strong interaction reduces the band gap
energy which enhances charge transfer. Recently, Z. Gai and
co-workers enhanced fluorescent wavelength and intensity

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of bulk g-C3N4 by high temperature heating (750°C for 2h)
and hydrolyzed in 4mol/L NaOH solution for 8h [139].
As a result, g-C3N4 nanobelts with emission peaks 19nm
higher than bulk g-C3N4 and quantum yield of 23.6% were
obtained. The as-synthesized nanobelts were used for NO2
sensing at room temperature by designing portable home-
made sensing system. A p-n heterojunction of g-C3N4 NSs
and Co3O4 particles derived from ZIF-67 was developed for
the detection of NO2. g-C3N4 NSs increases the electron
density of Co3O4 which cause enhanced conductivity, while
Co3O4 gives high porosity and surface area. So, in overall,
the formation of heterojunction leads towards increased gas
sensing property. At optimum conditions, Co3O4/g-C3N4
displays excellent response (17.83) to 00ppm of NO2 gas
at room temperature with 1.06s and 26.6s of response and
recovery time, respectively (Fig.6c) [140].
Kim etal. reported 2D g-C3N4 nanosheets coated on the
langasite (LGS) using surface acoustic wave (SAW) device
and used for the detection of NO2. It was observed that the
thickness of the g-C3N4/LGS SAW materials is very crucial
and a thickness of 220nm exhibited comparatively better
detection power. At room temperature, the g-C3N4/LGS
SAW displayed negative frequency shift of ⁓3.1kHz for
100ppm NO2 with a fast recovery (22s) and response time
(42s). Furthermore, the effect of temperature (27,200) was
also investigated, and they found a significant changes in
NO2 sensing which tremendously enhanced from ⁓3.1 to
⁓23.3kHz for 100ppm NO2. Moreover, the g-C3N4/LGS
SAW exhibited high stability, greater sensitivity, and excel-
lent selectivity with LOD of ⁓158ppb [71].
Fig. 6 a Schematic fabrication process of Ag@rGO/g-C3N4 hybrid
nanostructures [136]. b Schematic of the preparation of porous
g-C3N4 fibers used for gas sensing [137]. c Schematic illustration of
Co3O4-g-C3N4 nanocomposite formation from the ZIF-67/g-C3N4
heterostructure [140]. d Sensing mechanism of Pd/g-C3N4-based
resistive hydrogen gas sensor [141]. e Schematic of fabrication of the
ratiometric fluorescence sensor based on FRET (H2BDC: 1,4-benzen-
edicarboxylic acid, TED: 1,4-diazabicyclo [2.2.2] octane) [142]

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Sensing ofother gases
Nickel oxide nanosheets were decorated over two-dimen-
sional g-C3N4 via hydrothermal method to develop three-
dimensional hierarchical nanostructure (g-C3N4/NiO) for the
sensing of NO2 gas [140]. Higher selectivity and low limit of
detection up to 10ppb were calculated. Composite exhibited
good sensitivity of 25.4‒50ppm, quick response (0.53s),
and recovery time (25.06s). Composite-2 (g-C3N4/NiO-2)
obtained at hydrothermal temperature of 140°C revealed
higher selectivity, low limit of detection up to 10ppb, and
12-week stability. Enhanced sensing response of g-C3N4/
NiO-2 was attributed to large surface area, occurrence of
more defect sites, porous nature, and extended ICT between
p-n heterojunction. To the composite of carbon nitride and
reduced graphene oxide was incorporated CuNPs to change
its electronic properties and the resulting composite was
used for the detection of CO2 [143]. Density functional the-
ory (DFT) study reveals enhanced stability of CO2 on CN/
CuNPs@rGO due to H-bonding between Cu chemosorbed
CO2 and carbon nitride. Experimental results proved the
humidity to be an important factor effecting CO2 sensing and
highest response was observed at 90% of relative humidity
as compared to dry environment which gives comparatively
less response. Light (395nm) irradiation was also found to
be important for sensing enhancement. Platinum-dispersed
graphitic carbon nitride (Pt-g-C3N4) nanocomposite was
synthesized from melamine and chloroplatinic acid hexa-
hydrate by wet-chemical approach [144]. The synthesized
sensor was fabricated by jet nebulizer–based spray pyroly-
sis setup. A developed sensor was applied to sensing of H2
gas at room temperature and inert atmosphere. The change
in electrical resistance in the presence and absence of H2
gas was measured. Good sensitivity towards H2 gas was
observed at different concentrations [141]. Pd NPs fabri-
cated g-C3N4 for the sensing of H2 gas using ammonium
tetrachloropalladate with a reducing agent, as a source of Pd
for NP synthesis. While g-C3N4 was used from urea under
a muffled furnace at 550°C, the screen printing technique
was applied to deposit the Pd-dispersed g-C3N4 on an inter-
digited carbon electrode. During analysis of 1eV meas-
urement, clear change is resistant when H2 presence and
absence were observed. Sensing mechanism is displayed in
Fig.6d. Graphitic carbon nitride functionalized with fluorine
(F-g-C3N4) was used a cataluminescence (CTL) sensing sys-
tem for the detection of H2S gas. Results revealed that F-g-
C3N4 was favorable towards the adsorption of O2 and H2S
due to large surface area, ease in H-bonding formation and
modified electronic structure which was supported with DFT
calculations. So, contrary to g-C3N4, F-g-C3N4 displayed 30
times higher cataluminescence response towards H2S gas.
F-g-C3N4 has CTL response in 1.27 to 64.00µg mL−1 and
LOD of 0.07µg mL−1 [145]. Electrochemical sensor based
on bulk g-C3N4, sodium-doped g-C3N4, and potassium-
doped g-C3N4 for the sensing of nitrite was fabricated and
characterized with various characterization techniques [146].
Sodium-doped g-C3N4 displays good sensitivity, selectivity,
and reproducibility towards nitrite detection with LOD of
1.9µM and linear range of 10µM to 2mM. Furthermore, it
was screen-printed on electrode to check its stability for the
formation of disposable electrochemical sensors.
Chen and coworkers used thermal and ultrasonication
strategy for the introduction of the bismuth sulfide (Bi2S3)
nanorods onto zinc-doped g-C3N4. The novel electrode
material (Bi2S3/Zn-g-C3N4) was employed for the electro-
chemical sensing of toxic nitric oxide. Different techniques
such as XPS, FTIR, XRD, TEM, SEM, and Raman were
used for the characterization. Moreover, for the measure-
ment of electrode, kinetics DPV, CV, and EIS were used.
Owing to the greater electrochemically active surface and
lesser charge transfer opposition, the Bi2S3/Zn-g-C3N4 mate-
rial exhibited high selectivity, replicability, and durability.
The excellent sensing for nitric oxide was achieved due to
the facile transfer of electron between Bi2S3 nanorods and
Zn-g-C3N4 via synergistic effect. Consequently, the modified
glassy carbon electrode showed very low LOD of 0.007µM
to the sensing of nitric oxide [73].
Sensing oftoxic nitroaromatic, amino, andvolatile
organic compounds
4-Nitrophenol (4-NP) is considered a dangerous environ-
mental pollutant around the world, which has been widely
used as a precursor to produce pharmaceutical samples,
paper, dyes, and pesticides. It affects lungs and causes can-
cer for living organisms [147, 148]. Jiang etal. developed
the insitu growth facile method for the disposition of gold
nanowires (Au NWs) on 2D g-C3N4 nanosheets. The as-
synthesized senor (g-C3N4/Au NWs) showed high sensitivity
and served as vital reproducible surface-enhanced Raman
scattering sensor (SERS) for the monitoring of gaseous ana-
lytes. The g-C3N4/Au NWs was employed for the detection
of 4-aminophenol which degraded successfully in 60min in
the sunlight leading to the LOD value of 6.08 × 10–9M. To
validate the practical utility of g-C3N4/Au NWs, the SERS
was used in the detection of gaseous 4-aminophenol under
the inference of 2-naphthalenethiol, thus delivering satisfac-
tory recovery rates enhanced from 86.3 to 96.9% [72].
Microwave solvothermal method was used to fabricate
functionalized carbon nitride quantum dots (CNQDs) by
moderate carbonization of L-tartaric acid [149]. The as-
synthesized monodispersed CNQDs displayed excellent
sensitivity and selectivity of fluorescent quenching towards
2,4,6-trinitrophenol (TNP). The probe exhibited response
time within 1min and quenching efficiency coefficient Ksv
of 4.75 × 104 M−1. The linear range of 0.1–15 uM and LOD

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of 87nM were calculated. Novel SnO2@ZIF-8/g-C3N4 nano-
hybrids were synthesized for electrochemical detection of
p-nitrophenol using DPV, chronoamperometry (CA), and CV.
An excellent electrochemical performance was displayed by
nanohybrid with sensitivity of 2.63 µA µM‒1 cm‒2 and LOD
of 0.565µM and consistency of response for up to 30days.
Common interferents like phenol and aminophenol show
negligible interference; however, SnO2@ZIF-8/g-C3N4 fails
to differentiate between the isomers of p-nitrophenol [150].
Nanosheets of g-C3N4 were decorated with NiO synthesized
through hydrothermal method and utilized for triethylamine
(TEA) sensing [151]. Sensing of composites with different
% by wt of g-C3N4 was compared and obtained that compos-
ite with 10% of g-C3N4 displays highest sensing of 20.03 to
500ppm of TEA at 280 ℃ with good selectivity. Enhancement
in gas sensing performance can be correlated to internal charge
in p-n heterojunction. Moreover, more reaction and contact of
TEA with the composite become due to high surface area of
composite. K. Mori and co-workers synthesized Eu3+ coordi-
nated g-C3N4 nanosheets for the determination of acetone and
cyclohexane [152]. Eu3+ was doped in g-C3N4 nanosheets via
pyrolysis of dicyandiamide and EuCl3•6H2O at 550 ℃ under
nitrogen using NH4Cl as a dynamic gas template. Characteri-
zation of complex through XRD, XPS, Eu LIII-edge X-ray
absorption fine structure, and high-angle annular dark-field
scanning transmission electron microscopy reveals the genera-
tion of single-atom Eu3+ which is bonded to six nitrogen atoms
from three heptazine units of the g-C3N4. The characteristic
emissions of the complex were quenched at room tempera-
ture when exposed to volatile organic compounds (VOCs) like
acetone and cyclohexane. FTIR analysis shows that the sens-
ing properties are attributed to ligand–metal energy transfer
between the single atom Eu3+-g-C3N4 complex and VOCs.
Graphitic carbon nitride was loaded with ordered mesoporous
Pt-MoO3 by using nano casting technique for the temperature-
dependent sensing of acetone [153]. Mesoporous silica (KIT-
6) was used as a template for the development of Pt-MoO3/
mpg-C3N4. Good results were displayed by the Pt-MoO3/mpg-
C3N4 even at very low temperature. Furthermore, good selec-
tivity, fast response time (8.6s), wide linear range, reusability,
and stability (for 5weeks at 175°C) were demonstrated.
Sensing applications ofg‑C3N4 towardsfood
samples
The detection of contaminates in food sample is very impor-
tant for taking healthy food; because of these, it has grabbed
the attention of researchers at all the time [154]. A ratio-
metric fluorescent sensor of g-C3N4 quantum dots-Zn-MOF
composite was reported for the determination of riboflavin in
milk and vitamin B2 tablets [142]. The sensor was working
on Forster resonance energy transfer mechanism in which
g-C3N4 quantum dots-Zn-MOF composite was working as a
donor while RF was working as an acceptor (Fig.6e). Limit
of detection of 15nM was obtained with the developed sen-
sor. Graphitic carbon nitride was decorated with bismuth tel-
luride (Bi2Te3) as a modified electrode for the electrochemi-
cal detection of salbutamol (food additive) in meat samples
[155]. The binary nanosheet of Bi2Te3/g-C3N4 displays
excellence performance towards the detection of salbutamol
due to formation of charge assisted interaction of salbuta-
mol with surface of binary nanosheet of Bi2Te3/g-C3N4. The
linear range of 0.01–892.5µM corresponding to salbutamol
concentration, sensitivity of 36.277 µA µM‒1 cm‒2, and
LOD of 1.36nM was calculated in 0.05M phosphate buffer
supporting electrolyte (pH 7.0). Graphitic carbon nitride and
graphene nanoflake composite was synthesized via hydro-
thermal process to form a porous three-dimensional network
[156]. The as-synthesized composite was used for the elec-
trochemical quantitative determination of DNA bases in
beef and chicken liver to judge the quality of food products.
The electrochemical current was established to be a linear
range from 0.3 × 10‒7 to 6.6 × 10‒6 for guanine, 0.3 × 10‒7
to 7.3 × 10‒6 for adenine, and 5.3 × 10‒6 to 63.3 × 10‒4 for
thymine. Similarly, LOD was calculated to be 4.7, 3.5, and
55nM for guanine, adenine, and thymine, respectively. In
another study, g-C3N4 nanosheets were fabricated and char-
acterized with HR-TEM, FT-IR, UV–visible, XRD, and
fluorescent spectroscopy for the detection and degradation
of food colorants, sunset yellow (SY), and tartrazine (Tz)
[157]. Fluorescence of nanosheets was quenched by both
colorants, giving linear range response at low concentra-
tions. Limit of detections were calculated to be 32.5nM for
Tz and 221nM for SY. Moreover, the nanosheets worked
as efficient photocatalyst for the degradation of colorants
which was confirmed with mass and UV–vis spectra analy-
sis. Copper oxide-graphitic carbon nitride nanocomposite
was synthesized directly by insitu growth of CuO on g-C3N4
to construct a sensitive photoelectrochemical sensor [158].
The nanocomposite was further combined with molecular
imprinted polymer (MIP). The MIP works as a sensing part
for the recognition of target aflatoxin B1 (AFB1) while com-
posite part works as a heterojunction to enhance cathodic
photocurrent response. Photogenerated electrons of g-C3N4
are transferred to CuO while under visible light irradiations,
the photogenerated holes of CuO are permitted to transfer to
g-C3N4 (Fig.7a). Under optimal conditions, good selectivity,
wide linear range of 0.02ng mL‒1 to 1µg mL‒1, and LOD
of 6.8pg mL‒1 were calculated towards the detection of
AFB1. MIP-PEC sensor displays satisfactory results when
it was applied to maize solution. Using urea and glucose as
a primary precursor for the one-step thermal polymerization
process to synthesize carbon-rich graphitic carbon nitride
(C-g-C3N4) for the detection of diphenylamine (DPA) [159].

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The as synthesized C-g-C3N4 was characterized with
FT-IR, XRD, atomic force microscope (AFM), TEM,
UV–vis-DRS, SEM, and XPS analysis. The addition of extra
carbon to g-C3N4 enhances its conductivity and increases
the active surface area, which boosts the DPA sensing.
Good linear range of 0.008–682µM and LOD of 0.009µM
were calculated for C-g-C3N4 towards the detection of
DPA. Recovery of 98–107% and RSD % less than 5% were
observed when applied to apple juice sample for the detec-
tion of DPA. Sulfamethazine (SMZ), a sulfonamide found in
animal-derived food, is dangerous to human health. Based
on affinity of antibody and aptamer to analyte, a sandwich-
type electrochemical biosensor was developed for SMZ
determination [161]. AuE was modified with octahedral Au
NPs (AuOct) and poly (ethyleneimine) (PEI) functional-
ized g-C3N4 and sulfamethazine monoclonal antibody (Ab)
was immobilized on modified AuE. AuOct-PEI-g-C3N4 was
working as a sensing platform while Au@Pt by loading MB
and aptamer was working as a signal label. As SMZ can
attach with Ab and aptamer only, an Ab-SMZ-G-quadruplex
complex is formed, and MB at Au@Pt core shell NPs con-
nects to electrode, and MB signal is directly related to SMZ
concentration. Thus, current change in square wave voltam-
metry (SWV) is linearly related to logarithm of SMZ con-
centration. The linear range of SMZ 0.0001–100ng mL‒1
and relative LOD of 0.069pg mL‒1 were calculated. Y. X.
Rui in 2021 developed a sensitive photoelectrochemical
sensor based on g-C3N4/non-integral bismuth chloride/bis-
muth bromide heterojunction and plasma resonance effect
of AuNPs for the sensing of plant hormone 2-chloroethyl
phosphate (ETH) [162]. Different characterization tech-
niques\ were applied to characterize the sensor and sensing
were studied electrochemical impedance spectroscopy, cur-
rent–time curve. Charge density of atoms was calculated
with density functional theory. The synthesized composite
can cooperate with plasma resonance effect of AuNPs to
Fig. 7 a (A) Schematic
illustration of CuO-g-C3N4
nanocomposite synthesis. (B)
Mechanism of charge carrier
separation and transfer in CuO-
g-C3N4 heterojunction under
light illumination. (C) Fabrica-
tion of MIP-PEC sensor [158].
b Humidity-sensing mechanism
diagram of the g-C3N4 film sen-
sor [160]

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enhance the generation and separation of photogenerated
carrier resulting in the acceleration of electron conduction
between analyte and electrode thus resulting in enhanced
photocurrent. However, when ETH is reacted with the com-
posite, redox reaction of ETH with composite decreases the
current density, which is related to concentration of ETH.
Under optimized conditions, linear range of 20.00nmol L‒1
to 63µmol L‒1 and LOD of 6.9nmol L‒1 were obtained.
Application ofg‑C3N4 towardssensing
ofrelative humidity (%RH)
Large surface area of g-C3N4 with ample active sites makes
it easy for water molecules to adsorb and desorb on g-C3N4
[163]. Cerium oxide/graphitic carbon nitride nanocompos-
ite–based flexible wearable humidity sensor was reported
which was self-powered by a motion-driven alternator [164].
The composite was characterized with XPS, XRD, TEM,
and SEM. CeO2 were purposely dispersed on the surface
of g-C3N4 as it plays a major role in interaction with water
molecules and provides large surface area. Based on electro-
magnetic generator, energy acquisition circuit was designed.
The change in humidity was correlated to the capacitance of
CeO2/g-C3N4 sensor. At low relative humidity (RH), adsorp-
tion occurs only as a single layer through chemical interac-
tions with surface. At high RH, further water is adsorbed at
initially chemically adsorbed layers. When water molecule
penetrates to layer between g-C3N4 and CeO2, dielectric
constant is increased resulting in increase of capacitance
value. Measurements of any change and display system
were specially designed for mobile phones and wireless
communication technologies. With change in humidity
from 0 to 97%, the response value of the sensor reaches to
6573. Furthermore, it displays good sensitivity, stability, and
recovery time. A g-C3N4 NS on epoxy substrate with /CuNi
electrode–based humidity sensor was developed and char-
acterized with various characterization techniques. Sensor
working was based on variation in resistance on surface of
g-C3N4 NS [160]. At low RH, H-bonding is formed between
surface shrinks and NH2 on surface of g-C3N4 making film
curl. It will cause hindering of water molecule movement
adsorbing on the surface resulting high resistance. How-
ever, water molecules at high RH are adsorbed on g-C3N4
in the form of multiple layers. These adsorbed water mol-
ecules make H-bonding with surface NH2 groups causing
expansion of surface. More H-bonding will be formed at
high RH and adsorbed water molecules will produce pro-
tons (H3O+ → H2O + H+) while amino group is protonated
(-NH → NH2
+). Proton transfer is also expected between
adjacent water molecules which can lead to Grotthuss chain
reaction (H2O + H3O+ → H3O+ + H2O). Water molecules
will increase sensor conductivity and decrease resistance
when it will be reached to intermediate layers of g-C3N4
(Fig.7b) [160]. Tomer etal. prepared mesoporous g-C3N4
fabricated with Ag NPs. Fabricated sensor displays excep-
tional detection with real-time %RH detection in 11–98%
range of RH. This humidity sensor gives response in just
3s with 1.4-s recovery time for 11–98% range of RH [165].
In another study, a humidity sensor of g-C3N4/ZnO was
developed for the monitoring of respiration [166]. DFT
calculations indicate that adsorption energies towards the
adsorption of water molecules of oxygen vacancy and -OH
groups on surface g-C3N4/ZnO are higher than ZnO which
allows more water molecules to be adsorbed on g-C3N4/
ZnO. Experimental study indicates that the composite for-
mation increases -OH group and oxygen vacancy on sur-
face paving more water molecules to absorb to surface and
accelerate formation of conductive ions from their decom-
position, leading to enhanced performance of sensor. Syn-
thesized humidity sensor shows higher stability in the range
of 11–95% RH at 5% mass ratio of g-C3N4 and ZnO. Fur-
thermore, response rate of (1.05 ± 0.07) × 104 and response/
recovery speed of 22/5s were displayed.
Biosensors based ong‑C3N4
g-C3N4 displays good potentials to be used as a biosensor.
Again, mechanism of biosensor is based on EC, PEC, ECL,
and fluorescent.
Sensing ofglucose
The detection of glucose is very important for human life.
Glucose level in the blood serum increases during malfunc-
tioning of insulin secretion leading to diabetes mellitus DM
2 (type 2) [167]. As an approach to develop reusable non-
enzymatic electrochemical glucose sensor, g-C3N4 was
modified with ZnO and Pt and characterized with various
techniques [168]. Initially, using unmodified gold electrode
(AuE), g-C3N4 did not catalyze glucose while AuE modified
with Pt-g-C3N4 displays glucose catalysis in 0.0–0.4V poten-
tial region, with anodic peak at 0.29V and increasing peak
current (ipa) from 1.8 × 10‒7 to 6 × 10‒7 A. However, deco-
ration with ZnO increases catalysis by 4-folds, as confirmed
with increase in oxidation peak current and 0.2V negative
shift in peak potential as compared to Pt-g-C3N4 (Fig.8a).
ZnO-Pt-g-C3N4 displays fast electrochemical response (5s)
towards glucose with high sensitivity 3.34 µA µM‒1 cm‒2 at
low potential of + 0.20V (vs. Ag/AgCl). Furthermore, low
LOD of 0.1µM and wide linear range of 0.25–110mM were
calculated. Aminoboronic acid functionalized g-C3N4 quan-
tum dot–based photoluminescence sensor was synthesized
using melamine and amino phenylboronic acid in two-step
processes (Fig.8b) [169]. The as-synthesized sensor exhibited

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high quantum yield of 78.5% and work effectively as “On–Off-
On” sensor for the selective detection of glucose. The linear
range of 0 to 10mM and low LOD of 42nM were shown by
the synthesized sensor. Efficiency was comparable to com-
mercial glucometer when applied to real blood samples. In
another study, g-C3N4 QDs functionalized with phenylboronic
acid (g-C3N4 QDs/PBA) was synthesized through hydrother-
mal method as a fluorescence sensor for glucose detection
[170]. The as-synthesized fluorescent sensor was character-
ized with XRD, FT-IR, XPS, and TEM. Quantum yield of
67% was displayed by g-C3N4QDs/PBA using quinine sulfate
as a standard. However, two linear range from 1µM to 1mM
and 25nM to 1µM with LOD of 16nM and good selectivity
were obtained with this fluorescent sensor [170]. Polymeric
g-C3N4 in combination with 3,3′,5,5′-tetramethylbenzidine
(TMB) were used for the detection of glucose [171]. Sen-
sitivity of the polymeric g-C3N4 was greatly enhanced with
the addition by using Cu(II) and Fe(II)–adsorbed polymeric
g-C3N4. Adsorption of glucose oxidase and TMB on catalyst
gives color change from yellow to green when exposed to glu-
cose solution. UV–vis monitoring indicates fast response of
Cu(II) and Fe(II)–adsorbed polymeric g-C3N4 as compared to
original polymeric g-C3N4. Niobium (Nb)-doped g-C3N4 (Nb-
g-C3N4) was synthesized via pyrolysis of urea and used for the
detection of glucose in human blood [168]. Metallic behavior
of the Nb-g-C3N4 was indicated by the presence of two differ-
ent oxidation peaks during forward and reverse potential scan-
ning. Oxidation of glucose to gluconolactone was indicated
Fig. 8 a Reusable non-enzymatic glucose sensing at ZnO-Pt–g-C3N4
nanozyme [168]. b Schematic diagram of g-CNQDs/3APBA synthe-
sis process from melamine [169]. c Schematic representation for the
non-enzymatic detection of hydrogen peroxide (H2O2) using Na,O–
g-C3N4/GCE as sensor [176]. d Principles of the model FRET-based
fluorescence quenching assay: (a) the specific binding between the
ligand and the receptor results in conditions suitable for the energy
transfer and the CNNP emission is quenched. b Addition of free
ligand (IgG) inhibits the binding of the fluorescent-labelled ligand to
the receptor and the probe emission is turned on [177]. e Preparation
of signal probe. (B) Step-by-step assembling process of microRNA
sensor [178]

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by oxidation at 0.39V, while the presence of intense peak in
reverse scan confirms the oxidation of gluconolactone to CO2
and H2O. An extended range of work from 0.5 to 200mM with
good sensitivity of 1.62 µA mM‒1 cm‒2 was observed towards
glucose sensing. Good results and signal recoveries up to 4
times reuse were observed for the developed sensor.
Sensing ofhydrogen peroxide
The detection of H2O2 has attracted great attention of the
researchers due to their biological significance. The presence
of H2O2 plays an import role in cell death, proliferation, and
signal transduction of cell as well as a promising biomarker
for the diagnosis of cancer in their early stages [172, 173].
Ru NPs were synthesized and used for the prepara-
tion of ruthenium/nitride carbon (Ru/CN) via pyrolyzing
tris(2,2′-biphenyl)ruthenium(II) chloride (TBRC) with
carbon [174]. Synthesized Ru/NC-800 exhibited excellent
sensing towards non enzymatic sensing of H2O2 with linear
range of concentration of 0.001–10.000mM, high sensi-
tivity of 698 µA mM-1 cm‒2, and low LOD of 0.468µM.
The high performance of Ru/NC-800 can be correlated to
highly dispersed Ru NPs and N-doping, providing high num-
ber of active sites for H2O2 detection. A new CL system of
g-C3N4 QDs-Cu(II)/H2O2 was reported for the detection of
H2O2 and glucose [175]. Approximately, 75-folds enhance
in chemiluminescence (CL) intensity was observed due to
g-C3N4 QDs. Synthesized CL probe was applied to the sens-
ing of glucose and H2O2 with LOD of 100nM and 10nM,
respectively; Na, O-co-doped-g-C3N4 possessing large sur-
face area, and enhanced optical properties were synthesized
under basic conditions followed by [176]. A GCE modified
with Na,O–g-C3N4 (Na,O–g-C3N4/GCE) was used for elec-
trochemical sensing of H2O2. With cyclic voltammetry, the
modified electrode displays LOD of 0.05µM with sensitivity
of 3.41 µA µM‒1 cm−2 and a wide range of work (1–50µM)
towards the sensing of H2O2. Similarly, using LSV, wide lin-
ear range of response of 1–45µM with low LOD of 0.1µM
and high sensitivity of 17.57 µA µM‒1 cm‒2 was displayed
by Na,O–g-C3N4/GCE (Fig.8c).
Other biosensing applications ofgraphitic carbon
nitride
Although much more literature is available about sensing
of glucose and H2O2 in the recent years, however, graphitic
carbon nitride is used as a versatile non-enzymatic biosen-
sor and used for the detection of various biomolecules. A
fluorescent sensor based on acid-treated oxidized graphitic
carbon nitride nanosheets (g-C3N4 NSs) was synthesized
for the sensing of hemin based on the inner filter effect
[179]. Florescent properties of g-C3N4 NSs were quenched
with the addition of hemin due to overlap of emission
spectrum and absorption spectrum of g-C3N4 NSs and
hemin, respectively. At optimum conditions, liner range of
hemin concentration up to 0.5–25 µM with LOD of
0.15µM was obtained; g-C3N4 were doped with phenyl
groups to make it strong PL [180]. Doping of phenyl group
increases stock effect and efficiency of PL. DFT calcula-
tions indicated that phenyl group changes the electronic
properties of g-C3N4. The as-synthesized material exhib-
ited good applications for the imaging of fingerprints.
Exfoliation and functionalization of carbon nitride
nanosheets (CNNS) with copper phthalocyanine (CNNS-
TsCuPc) via mechanical milling were reported [181]. Due
to π-π interactions and energy level matching, a good
donor acceptor pair with enhanced photocurrent under
irradiation with red light (λ > 630 nm) was developed.
When applied to the detection of dopamine (DA) in blood,
good linear range of concentration, selectivity, and low
LOD was obtained with as-synthesized sensor. H2O2/
NaHSO4 have weak chemiluminescence which was
enhanced with g-C3N4 QDs and used for the sensing of
ascorbic acid (AA) [182]. Chemiluminescence of H2O2/
NaHSO4@g-C3N4QDs is retarded by ascorbic acid. This
property was utilized for the sensing of AA in linear con-
centration range of 2.85 × 10‒7–2.85 × 10‒4mol/L with
LOD of 8.0 × 10‒8mol/L. Furthermore, AA in medicine
was determined with 96.2–106% recoveries. Bright fluo-
rescent and highly hydrophilic carbon nitride nanoparti-
cles (CNNPLys) were synthesized via thermal polymeriza-
tion of urea and lysin [177]. The as-synthesized sensor was
used for the sensing of proteins. Conjugated CNNPLys
after attachment to protein is when excited under UV, it
emits in the 400–450nm of visible range. Gold nanopar-
ticles were attached to staphylococcal protein, which
quenches the fluorescent of CNNPLys via Froster resonant
energy transfer (FRET). However, the addition of free
human IgG recovers the fluorescent property, making it
on–off fluorescent sensor (Fig.8d). In another study, a
versatile fluorescent sensor based on single-stranded DNS
with protonated g-C3N4 was prepared and used for the
sensing of various analytes like Hg2+, biomolecules adeno-
sine triphosphate, and Aflatoxin B1 [183]. A highly sensi-
tive electrochemical biosensor based on g-C3N4 nanosheets
(g-C3N4 NS) functionalized with Au NPs as a sensing
platform and DNA concatemers containing MB as a signal
probe for the sensing of microRNA-21 were synthesized
[178]. The signal probe was synthesized through continu-
ous hybridization chain reaction between two different
strands of DNA in which one strand was labelled at posi-
tion-3 with MB. As a result, a long concatemer containing
numerous MB signal molecules was obtained. On the sur-
face of glassy carbon electrode modified with Au NPs-g-
C3N4 NS nanohybrid was first immobilized thiolated hair-
pin probe (HP) and blocked with MCH and then

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consecutively was hybridized with microRNA-21 and
signal probe, respectively (Fig.8e). Sensing signal was
measured with differential pulse voltammetry (DPV), giv-
ing linear range of 10 fM–500nM and LOD was calcu-
lated to be 0.33 fM. Graphitic carbon nitride was deco-
rated with manganese ferrite nanoparticles (MnFe2O4
NPs) through ultrasonic irradiation method for C (30kHz
and 70W/cm2) [184]. The synthesized nanocomposite was
characterized with energy-dispersive spectroscopy (EDS),
XRD, TEM, and XPS. Nanocomposite displayed enhanced
electrocatalytic response towards the detection of
5-hydroxytratamine [Serotonin (neurotransmitter)]. Wide
linear range of 0.1–522.6µM, LOD of 3.1nM, and sensi-
tivity of 19.377 µA µM‒1 cm‒2 were calculated. Further-
more, good results were displayed by composite when
applied to 5-hydroxytratamine in huma blood serum and
rat brain serum. Novel fluorescent biosensor–based
aptamer/AuNPs/g-C3N4 nanosheets were developed for the
determination of small molecules like digoxin in human
plasma sample without any pretreatment steps [185]. The
fluorescent intensity of the g-C3N4 probe decreases when
interacted with label-free/gold nanoparticles. However,
fluorescent was enhanced when analyte was added due to
analyte-aptamer interactions and aggregation of AuNPs.
Under the optimized conditions, linear range of 10ng to
500ng/L, LOD of 3.2ng/L, % RSD of 2.6, 4.0, and 6.5%
for the analyte concentrations of 25, 100, and 500ng/L
respectively. The surface of graphitic carbon nitride was
decorated with nickel cobalt sulfide, an effective indicator
of chronoamperometry, to make a label-free electrochemi-
cal immunosensor for the early detection of procalcitonin
[186]. In order to make it a dual-mode analysis, CNTs
were hybridized with the composite material to load Ag
NPs which works as an indicator of DPV due to their
excellent oxidizing properties. The synthesized probe
worked on DPV and amperometric i-t curve dual mode
analysis system. A wide linear response of 0.05 to
50ng mL‒1 and 1.00pg mL‒1 to 10.00 ng mL‒1 was
obtained for DPV and i-t respectively. The limit of detec-
tion of 16.70pg mL‒1 for DPV and 0.33pg mL‒1 for i-t
was calculated. Graphitic carbon nitride and nanocompos-
ite of g-C3N4 with copper-coordinated dithiooxamide
metal organic framework (Cu-DTO MOF) were synthe-
sized and characterized with FTIR, XRD, TGA, XPS,
FESEM, EDX, and TEM [187]. Electrochemical response
of the probes towards an endocrine disruptor, i.e., 4-(4-iso-
propoxy-benzenesulfonyl)-phenol (BPSIP) was compared
using CV and DPV techniques. Higher response was
recorded by nanocomposite as compared to pure g-C3N4.
Low limit of detection 0.02µM and sensitivity of 0.5675
µAµM‒1 cm‒2 were calculated for the g-C3N4/Cu-DTO
MOF sensor towards BPSIP. In an interesting study, a
sensing array containing graphitic carbon nitride decorated
with boronic acid (( +)BA-g-C3N4) for the differentiation
of pathogenic bacteria at various acid pHs [178]. Compos-
ite of (+)BA-g-C3N4 attracts pathogenic bacteria either
through electrostatic interactions or affinity of boronic acid
of the sensor and saccharide unit on the surface of patho-
genic bacteria, causing sedimentation of the composite of
bacteria-(+)BA-g-C3N4. This sedimentation causes a
decrease in fluorescent intensity which is different for vari-
ous pathogenic bacteria at different pHs, providing easy
way for the discrimination of pathogenic bacteria. Using
this sensing array, nine types of pathogenic bacteria were
discriminated into three kinds of gram-positive and six
kinds of gram-negative bacteria with linear discrimination
analysis at OD600 = 0.01. Sulfur-doped graphite phase
carbon nitride quantum dots (S-GCN QDs)-based ECL
sensor was synthesized for sensing of K-RAS gene [188].
Sulfur doping produces new elemental vacancies on the
quantum dots which enhances the efficiency by 2.5 times
as compared to pristine graphic carbonitride. At optimum
conditions, ECL sensor displays LOD of 16 fM and LOQ
of 50 fM−1nM. Bisphenol A (BPA), a structure like estro-
gen, is an endocrine disrupting chemical which upon bind-
ing to estrogen receptor causes cancer and reproductive
disorders. A nanocomposite base on polyaniline, ruthe-
nium oxide, and g-C3N4 was synthesized ultrasonically
and characterized with TEM, XRD, FT-IR, EDX, TGA
elemental mapping, and UV analysis [189]. GCE modified
with sensor were used for BPA detection in human and
animals’ urine. In result, linear range of 0.01–1.1µM and
LOD of in pM were obtained. A new electrochemilumi-
nescence (ECL) immunosensor was designed to detect
carbohydrate antigen 19–9 (CA 19–9) and carbohydrate
antigen 24–2 (CA 242) simultaneously [186]. Pt NP-dec-
orated g-C3N4 was used as a cathodic probe while luminol-
AgNPs@ZIF-67 was serving as the cathodic probe. Con-
ductivity of the sensing interface on the dual disk GCE
was enhanced by modifying two spatially resolved areas
with AuNPs film. ECL response was recorded at two dif-
ferent excitation potentials, and LOD of CA 19–9 was cal-
culated to be 31 µU/mL with linear range of 0.0001 to 10
U/mL. For the CA 242, LOD of 0.16mU/mL was calcu-
lated with linear range of 0.0005 to 10 U/mL. Further-
more, both CA 19–9 and CA 242 were detected in real
samples. g-C3N4 QDs were synthesized by solid-state
method at low temperature. K2S2O8 was used as a co-reac-
tant for the cathodic ECL signals of g-C3N4 QDs in phos-
phate buffer [190]. ECL resonance energy transfer between
g-C3N4 QDs as emitter and AuNPs as acceptor was con-
structed. Connecting AuNPs at hairpin DNA terminal
leads to the formation of signal probe. ECL resonance
energy transfer occurs because of ECL quenching of
AuNPs to g-C3N4 QDs leading to decrease in ECL signals
when the probe was connected to g-C3N4QDs. In the

Microchim Acta (2022) 189:426
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presence of target DNA, the AuNPs are disconnected from
g-C3N4 QDs resulting in ECL signal recovery due to stop-
page of ECL resonance energy transfer procedure. Using
this sensor, the linear range of 0.02 fM–0.1pM and LOD
of 0.01 fM were calculated.
Comparison ofg‑C3N4 withgraphene
andgraphene‑based materials
Graphene is made up of a single layer of sp2-hybridized car-
bon atoms arranged in a rigid two-dimensional (2D) hon-
eycomb network. The structure of the graphene working as
a fundamental building block in the of formation different
dimension carbon materials such as zero-dimension (0D)
fullerene, one-dimension (1D) nanotubes, and three-dimen-
sion (3D) graphite [191]. It possesses large surface area and
excellent electron transfer capacity and is mechanically very
strong which gives it an excellent thermal stability. These
properties allow it to be used in various fields including
sensing, energy storage, electronic devices, and catalysis
[192]. In addition, graphene and their derivatives such as
graphene oxide (GO) and reduced graphene oxide (rGO)
are used in sensing of different analytes in different samples.
For example, graphene is used for the electrochemical sens-
ing metal ions [193], 4-nitrophenol [194], and dopamine in
human samples [https:// doi. org/ 10. 1016/j. bios. 2014. 01. 037.
Inspired from the remarkable properties of graphene have
originated special interest in graphene like 2D nanomateri-
als such as g-C3N4. They are called graphene-like as their
structure resembles graphene [195]. However, they have dis-
played remarkable electrical, optical, catalytic, and thermal
properties. Because of these properties, they are widely used
as a chemical and biological sensors and other applications.
The tri-s-triazine ring structure and high degree of conden-
sation make the g-C3N4 very stable as compared to graphene
and their derivatives. It is stable in chemicals like various
organic solvents, acid, and base; also in air, it is stable up to
600°C [196]. These properties make it an ideal choice as a
detection system for a diverse kind of pollutants in various
types of chemical media and in temperature extremes. Con-
trary to the graphene and their derivatives, the presence of
excessive nitrogens in the structure of g-C3N4 gives a strong
electron donor nature which allows it to develop electrostatic
interactions with the various analytes [197]. In addition,
g-C3N4 is defect-rich due to which the nitrogen atoms and
the defects work as an active site for the electron conductiv-
ity [198]. These properties make g-C3N4 an ultrasensitive
with improved selectivity for the detection of ultra-trace
chemicals from the target samples.
Challenges andfuture perspective
Owing to their inexpensive production costs, great chemi-
cal and thermal stability, low toxicity, biocompatibility, and
distinctive optical and electrical properties, g-CN nano-
structures have emerged as attractive signal transduction
platforms. The numerous amine moieties, ease of tailoring,
and triazine ring structure make g-CN a very ideal sens-
ing and bio-sensing platform. Despite the many advantages
that g-CN has over traditional semiconductors, it should be
highlighted that there are still some difficulties that must be
overcome in order to realize the full potential of g-CN-based
sensing platforms. The C/N/H ratios in g-CN nanostructures
greatly depends upon the synthetic methodologies and raw
materials used, which affects the electronic properties and
the applicability for a particular application. Therefore,
effective planning of the synthetic processes is necessary to
produce g-CN with the desired characteristics. Another dif-
ficulty with g-CN-based sensors is acquiring g-CN thin films
that firmly adhere to the substrate. Although methods like
chemical/thermal vapor deposition can offer a suitable level
of homogeneity and adhesion, they are too expensive and
time-consuming to be widely used for the manufacturing of
the g-CN-based sensors. EPD may be a productive, econom-
ical way to get g-CN films on conductive substrates. How-
ever, in this case, the film thickness often varies between a
few millimeters, which can significantly impair the sensing
performance. Additionally, the surface charge of the g-CN
nanostructures plays a significant role in the film production
in EPD.
The visible fluorescence (FL) of the components of
g-CN-based sensors can easily obstruct the blue wavelength
region of the spectrum where the intrinsic FL of most g-CN
nanostructures is transmitted. Therefore, the construction of
FL sensors with little background noise is possible by adjust-
ing the bandgap of g-CN so that its FL is moved from the
UV–vis region towards the near-infrared region (redshift).
Another thing to keep in mind is that g-CN structures still
do not have enough electric conductivity, even after adjust-
ing the size and undergoing other modifications. Although
many efforts have been made to increase its electrical con-
ductivity, results have not yet proven adequate. Combining
computer simulation and experimental-based research data
can make it easier to build more usable models, which are
essential for understanding how g-CN interacts with other
moieties. The acquired knowledge can be used to create
more effective g-CN derivatives and nanocomposites with
improved and adjustable characteristics. To ascertain the
energy, structure, and electronic characteristics of solids,
unit cells, molecules, atoms, and crystals, DFT calculations
are a potent computer modeling method. It can provide
insightful information based on simulations and theoretical

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predictions, which can direct the investigation of the inher-
ent features of materials. The impact of various alterations
on the electronic characteristics of g-CN nanostructures has
been studied using DFT calculations.
Despite the intriguing advancements, g-CN nanostruc-
tures are still in their infancy compared to other semicon-
ducting nanomaterials, making it difficult to create futur-
istic sensing devices that incorporate multianalyte sensing
and miniature lab-on-a-chip platforms. For the creation of
wearable electronic devices, the growth of g-CN nanostruc-
tures on flexible substrates like paper or ITO coated with
polyethylene terephthalate sheets can be investigated. It is
possible to design multimodality sensors that incorporate
various g-CN features (such electrochemical and FL), which
may be more sensitive than the currently used single modal-
ity sensors. Future research should concentrate on creating
g-CN nanostructure-based sensors that can enable real-time
detection in intricate biological contexts and offer a sample-
in-answer-out method.
In this review, we have focused on recent advancement in
groundbreaking achievements of graphitic carbon nitride in
the field of sensing especially environmental sensing, bio-
sensing, humidity sensing, and food toxin sensing. Extensive
applications towards sensing of heavy metal ions, like cop-
per ion, silver ion, and iron ion have covered in a compre-
hensive way. Versatile biosensing application is focused but
glucose and H2O2 are discussed in much detail. Neurotrans-
mitters like serotonin are also focused. Recent development
in sensing of food toxins and additives is illustrated. The
study about detection of toxic chemicals and gases is covered
in a massive way.
Abbreviations g-C3N4:Graphitic carbon nitride; QDs: Quantum
dots; GSH:Glutathione; ECL:Electrochemiluminescence; PL:Pho-
toluminescence; GQDs:Graphene quantum dots; IFE:Inner fil-
ter effect; GCE:Glassy carbon electrode; MB : Methylene blue;
CB:Conduction band; ITO:Indium tin oxide; LOD:Limit of detec-
tion; NPs:Nanoparticles; SWASV:Square wave anodic stripping
voltammetry; GO:Graphene oxide; SERS:Surface enhanced Raman
spectroscopy; PtNPs:Platinum nanoparticles; THIL: Thiol based
ionic liquids; NaBH4:Sodium borohydride; NCs:Nanoclusters; ZIF-
8:Zeolitic imidazolate framework-8; PET:Photoinduced electron
transfer; PEC:Photoelectrochemical; 3DBC:3D branched crystalline;
FTO:Fluorine-doped tin oxide; DRS:Differential reflectance spec-
troscopy; CIP:Ciprofloxacin; EPI:Epinephrine; MFA:Mefenamic
acid; PAR:Paracetamol; BCN:Boron-doped graphitic carbon nitride;
DPV:Differential pulse voltammetry; LSASV:Linear sweep anodic
stripping voltammetry; EIS :Electrochemical impedance spectros-
copy; BP:Black phosphorous; CTL:Cataluminescence; CV:Cyclic
voltammetry; CA:Chronoamperometry; DFT:Density functional
theory; DPA:Diphenylamine; MIP:Molecular imprinted polymer;
AFM:Atomic force microscope; XRD:X-ray diffraction; SEM:Scan-
ning electron microscope; TEM:Transmission electron microscopy;
FTIR:Fourier transform infrared spectroscopy; SMZ:Sulfamethazine;
PEI:Poly (ethyleneimine); SWV:Square wave voltammetry; RH:Rel-
ative humidity; AuE:Gold electrode; PBA: Phenylboronic acid;
TBRC:Tris(2,2′-biphenyl)ruthenium(II) chloride; XPS:X-ray photo-
electron spectroscopy; FRET:Fluorescence resonance energy transfer;
CL:Chemiluminescence; HP:Hairpin probe; EDS:Energy dispersive
spectroscopy; PDA:Polydopamine; LGS:Langasite; SAW:Surface
acoustic wave; Au NWs:gold nanowires; SERS:Surface-enhanced
Raman scattering; AFP:2-Amino fluorine polymer
Funding This work was funded by the Deputyship for Research &
Innovation, Ministry of Education in Saudi Arabia through the project
number DRI 74.
Declarations
Conflict of interest The authors declare no competing interests.
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