![Sketch of the energy levels for four low-lying states of the oxygen molecule. The pairs of determinants which have to combine to yield the correct adiabatic states are denoted asT OS 0 ; S OS 1 ; S CS 1 and S CS 2 , respectively. [27]](https://www.researchgate.net/profile/Ibukun-Oluwoye/publication/347821777/figure/fig3/AS:1020268109635584@1620262281333/Sketch-of-the-energy-levels-for-four-low-lying-states-of-the-oxygen-molecule-The-pairs_Q320.jpg)
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Review of Chemical Reactivity of Singlet
Oxygen with Organic Fuels and
Contaminants
Jomana Al-Nu’airat,[a] Ibukun Oluwoye,[a] Nassim Zeinali,[a] Mohammednoor Altarawneh,*[b]
and Bogdan Z. Dlugogorski[c]
Abstract: Singlet oxygen represents a form of reactive oxygen species (ROS), produced by
electronic excitation of molecular triplet oxygen. In general, highly reactive oxygen-bearing
molecules remain the backbone of diverse ground-breaking technologies, driving the waves of
scientific development in environmental, biotechnology, materials, medical and defence sciences.
Singlet oxygen has a relatively high energy of about 94 kJ/mol compared to the ground state
molecular O2and therefore initiates low-temperature oxidation of electron-rich hydrocarbons.
Such reactivity of singlet oxygen has inspired a wide array of emerging applications in chemical,
biochemical and combustion phenomena. This paper reviews the intrinsic properties of singlet
oxygen, emphasising the physical aspects of its natural occurrences, production techniques, as
well as chemical reactivity with organic fuels and contaminants. The review assembles critical
scientific studies on the implications of singlet oxygen in initiating chemical reactions,
identifying, and quantitating the consequential effects on combustion, fire safety, as well as on
the low-temperature treatment of organic wastes and contaminants. Moreover, the content of
this review appraises computational efforts, such as DFT quantum mechanical modelling, in
developing mechanistic (i.e., both thermodynamic and kinetic) insights into the reaction of
singlet oxygen with hydrocarbons.
Keywords: Singlet Oxygen, Reaction Mechanism, Combustion Fuels, Wastes and Contami-
nants, Advanced Oxidation
1. Introductory Background on Singlet Oxygen
Singlet oxygen represents an example of endogenous, highly
reactive, oxygen-bearing molecules, popular known as reactive
oxygen species (ROS). The pioneering milestone regarding the
molecular activation of triplet (i.e., the common atmospheric)
molecular oxygen dates back to 1930s in a work conducted by
Kautsky[1] on photo-oxidation triggered by fluorescent dye.
This novel concept gained scientific interest when Seliger[2]
detected a faint-red light emitted from a mixture of sodium
hypochlorite and hydrogen peroxide with a low-resolution
photomultiplier in 1960. In support of this observation, Khan
and Kasha[3] photographed the chemiluminescence (i.e., the
faint-red light), and interpreted it as an indication of an
excited state of molecular oxygen. After that, this form of
oxygen, later termed as singlet oxygen, has provoked a great
deal of interest in the research community. Numerous
applications of singlet oxygen have transpired in strategic areas
ranging from photodynamic therapy (PDT),[4,5] water purifica-
tion and wastewater treatment,[6,7] lipid peroxidation,[8,9]
pharmaceutical delivery to photo-degradation of polymer.[10,11]
Singlet oxygen forms as a result of molecular activation of
triplet oxygen (i.e., dioxygen), usually by photosensitisation,[12]
electrical discharge[13] or via surface-mediated reactions on
[a] J. Al-Nu’airat, I. Oluwoye, N. Zeinali
Murdoch University, Discipline of Chemistry and Physics, College of
Science, Health, Engineering and Education, 90 South Street,
Murdoch, WA 6150, Australia
[b] M. Altarawneh
United Arab Emirates University, Chemical and Petroleum Engineer-
ing Department, Sheikh Khalifa bin Zayed St, Al-Ain 15551, United
Arab Emirates
E-mail: mn.altarawneh@uaeu.ac.ae
[c] B. Z. Dlugogorski
Charles Darwin University, Energy and Resources Institute, Ellengo-
wan Drive, Darwin, NT 0909, Australia
Personal Account
THE
CHEMICAL
RECORD
DOI: 10.1002/tcr.202000143
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particles such as silicon dioxide, aluminium oxide, and
transition metal oxides.[14,15] It resides 95 kJ/mol (ΔGHo298)
above the ground state triplet oxygen molecule,[16,17] making it
kinetically unstable at ambient conditions and highly reactive
against a large variety of electron-rich substrates. As illustrated
in Figure 1, the ground state of O2is a triplet holding two
parallel (unpaired) electrons in its outer orbital while its two
higher energy state species, 1Δgand 1Σg+, encompass a couple
of paired electrons in their antibonding π* orbital, resembling
singlet electronic states. This astounding non-ionic and non-
Dr Jomana Al-Nu’airat received her BSc
in Chemical Engineering from Jordan
University of Science and Technology. She
obtained her PhD in Chemical Engineer-
ing from Murdoch University (Australia)
in 2019, working on the implications of
reactive oxygen species (ROS) in initiating
chemical reactions in Coal spontaneous
combustions and wastewater treatment.
She has held a Systems processing engi-
neering roles at Synergy and Salt Lake
Potash (Australia). Currently, she is a
laboratory supervisor, and research and
development officer at Kalium Lakes
Limited (Australia).
Dr Ibukun Oluwoye completed his PhD
at Murdoch University, Australia, in 2017.
He currently serves as a postdoctoral
research scientist at the same university,
conducting specialised multidisciplinary
research in chemical process kinetics,
heterogenous reactions and atmospheric
environment. His work targets developing
robust solutions for industrial processes
within the focal context of sustainable
developments.
Dr Nassim Zeinali received her PhD in
Chemical and Metallurgical Engineering
from Murdoch University (Australia) in
2020. Her M.Sc. and B.Sc. degrees are in
the field of process design, followed by five
years of working experience in the oil and
gas industry.
Mohammednoor Altarawneh is an Asso-
ciate Professor at the United Arab Emi-
rates University (UAEU). He completed
his PhD in Physical Chemistry in 2008
from the University of Newcastle, Austral-
ia. He worked before at Murdoch Univer-
sity (Perth, Australia) and in Jordan. He is
a recipient of the Bernard-Lewis fellowship
from the Combustion Institute in 2010.
He serves in the editorial board of the
Journal of Computational Biophysics and
Chemistry. His research generally focuses
on formulating reaction mechanisms and
kinetic models on topics spanning thermal
decomposition of halogenated polymers,
emission of nitrogen-nearing pollutants,
low-temperature combustion, and catalysis
by transition metal oxides. He utilizes
tools from quantum chemistry and collab-
orates closely with experimentalists to
underpin reactive systems that govern
chemical phenomena of interest.
Professor Bogdan Dlugogorski is the
Deputy Vice-Chancellor Research and
Innovation at Charles Darwin University,
Australia. He holds a DSc in Fire Safety
Science and Engineering (Newcastle, Aus-
tralia), PhD and MEng in Chemical
Engineering (Montreal, McGill), and
undergraduate degrees in Chemical Engi-
neering and Geophysics (Calgary). His
research combines experiments with quan-
tum-chemical and reaction-kinetics model-
ling, especially, with applications to proc-
ess safety and environment protection. He
is a Fellow of Australian Academy of
Technology and Engineering, Combustion
Institute, Society of Fire Protection Engi-
neers, Engineers Australia and Royal Aus-
tralian Chemical Institute. Professor Bog-
dan Dlugogorski is Immediate Past
Chairman of International Association for
Fire Safety Science, and a Chartered
Engineer and a Chartered Chemist.
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radical form of oxygen can diffuse through packed systems
very quickly with lifetime varying from seconds (in the gas
phase) to a few microseconds (in water).[18]
The 1Δgand 1Σg+singlet states have the energy levels of
about 95 kJ/mol and 158 kJ/mol, respectively, higher than its
ground triplet state (3Σgwith spin-1).[16] The latter state being
extremely short-lived (<1 ns),[19] preventing its distinct con-
tribution to reaction cascades. Thus, singlet oxygen O2(1Δg)
remains the only excited species capable of diffusing in the
reaction media long enough (varying from millimetres in
gaseous[20] to nanometres in condensed phase and living
cells[21]) to affect chemical reactions. The spin-forbidden
transition from O2(1Δg) state to O2(3Σg) state and the spin-
allowed nature of O2(1Σg+) to O2(3Σg) transition verifies the
fact that O2(1Δg) has a relatively long lifetime.[13] In this paper,
the term “singlet oxygen” refers to the O2(1Δg), interchange-
ably written as (1O2).
This review paper seeks to expand the vision of critical
reactions and implications of singlet oxygen in combustion
systems. One of the snubbed reactions remains the kinetic
details of the activation of dioxygen (i.e., the normal allotrope
of oxygen) to singlet oxygen. Figure 2. exemplifies the
application of singlet oxygen, depicting the focus topics in
enhancing internal combustion (IC) engines, the formation of
free radicals and biogenic aerosol, fire safety, surface oxidation
Figure 1. Possible electron configurations for antibonding π-orbitals of the
ground and excited states of molecular oxygen.
Figure 2. Snapshot of some practical implications of singlet oxygen. The combustion-related topics covered in this review are shown in bolded font.
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of hydrocarbons, and low-temperature decomposition of
organic pollutants. From a holistic viewpoint, the following
sections (i) lay the foundation knowledge regarding the
physical chemistry of singlet oxygen, (ii) describe the occur-
rences and generation techniques, (iii) resolve the challenges in
detecting singlet oxygen, (iv) explore the impact of singlet
oxygen in combustion (i.e., oxy-fuel systems), centring the
relative effect on the activation energy of the initiation
reactions, the overall chain-branching mechanism, and igni-
tion, (v) provide perspectives on the computational assessment
of singlet oxygen reaction, (vi) and demonstrate ambient
remediation of polluted wastewater and low-temperature
destruction of organic gas-phase contaminants., such as dioxins
and furans.
2. Physical Chemistry of Singlet Oxygen
The chemical and physical nature of O21Δgimposes some
exciting behaviour in various essential processes. For instance,
singlet O2can open-up “unusual” reaction channels during
oxidation of hydrocarbons, thioethers and organometallic
complexes. The electronic configuration of O2(1Δg) in its
excited state is (1σg)2(1σu)2(2σg)2(2σu)2(3σg)2(3σu)2(3πg)4(3πu)2.
The superscript “1” in O2(1Δg) indicates that it corresponds to
a singlet state, the “Δ”, that its orbital angular momentum
(ML) equals 2 and the subscript “g”, that the symmetry of the
molecule is pair (g from the German gerade, meaning
symmetry). In the last century, the physical and chemical
properties of singlet oxygen O2(1Δg) have been the subject of
broad investigations. Starting from 1925, when Robert
Mulliken applied the modern quantum theory to explain the
magnetic property of molecular oxygen. Where such magnetic
behaviour rationalised by the two unpaired electrons located in
the antibonding π* orbitals (i.e., π*2py and π*2pz). The frontier
molecular orbitals of singlet oxygen (HOMO and LUMO)
shown in Figure 3 are both π* orbitals. The molecular orbital
assignments labelled the electronic structures of singlet oxygen
as a pair of closed-shell determinants in which two electrons
occupy the same πx(or πy) orbital.[12,22,23] However, valence
bond theory (VBT) and molecular orbital theory analyses
suggested that the singlet oxygen be characterised by an open-
shell component and a closed-shell component.[13,24–26] The
point group analysis confirmed this premise. A recent work by
Qu Zexing shows that the singlet state representation in the
D1vpoint group degenerates into two different irreducible
representations in its point subgroup of C2v, in which the
open-shell (OS) and the closed-shell (CS) determinants refer
to A2and A1symmetries, respectively.[27] The author
constructed the configuration of the O2(1Δg) state (see Fig-
ure 4) by using the combination of two open-shell determi-
nants ;1
px*;1
py*i þ ;1
px*;1
py*i
and two closed-shell de-
terminants ;2
px*;0
py*i ;0
px*;2
py*i
.[28]
The terms ’singlet oxygen’and ’triplet oxygen’appear from
each form‘s number of electron spins. When the spin
restrictions are overcome in triplet oxygen, these electrons join
into one of these orbitals, forming “singlet oxygen delta” O2
(1Δg), rare chemical species with high chemical reactivity.[13,29]
The presence of an empty π* orbital assigns strong acidic
properties to O2(1Δg) (i.e., accepting a pair of electrons).
Hence, singlet oxygen O2(1Δg), unlike triplet oxygen O23Σg,
acts as a potent electrophile agent and paramagnetic due to a
net orbital (and not spin) electronic angular momentum, as
shown by the observation of an electron paramagnetic
resonance (EPR) spectrum. Some other specific physical
aspects singlet oxygen has been documented by Schweitzer and
Schmidt.[19]
Figure 3. Singlet and triplet oxygen HOMO, LUMO, and LUMO +1
molecular orbital maps along with their respective quadrupole tensor and
Dipole vector as obtained from ADF software applying M062X functional.
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Singlet oxygen should not be mistaken to other forms of
ROS, especially superoxide anion. Table 1 contrasts the
physicochemical parameters of singlet oxygen to typical ROS
compounds. Based on the energy levels of singlet oxygen O2
(1Δg) and O2(1Σ+g), luminescence decays are detected at
1270 nm (1O2!3O2+hυ) and 762 nm (O2(1Σg+)!3O2+
hυ). 1O2relaxes to 3O2by physical quenching (dissipation of
energy as heat or charge transfer) or by chemical quenching
(oxidation reaction) with other substances generating excited/
radical complexes.[30]
Singlet oxygen has a short lifetime in a solution that limits
its reactivity to the proximity of the site where the 1O2was
formed, i. e., 3.1 μ sec in water, and this corresponds to a
diffusion distance of ~220 nm.[30] However, in a vacuum,
singlet oxygen has a relatively long lifetime estimated to be
45 min. In general, the distance dthat 1O2would move in a
time tcan be expressed by d=(6tD)1/2 comprehended from
Fick’s law (i.e., three-dimensional molecule diffusion in a
uniform concentration); where Dis oxygen diffusion
coefficient.[52,53]
Singlet oxygen (1O2) has a varying lifetime in different
media. The lifetime value is commonly determined by means
of evaluating the rate constants for the decay and reaction of
singlet O2with specific acceptor molecules (i.e. 1,3-diphenyli-
sobenzofuran and rubrene).[54] The lifetime of singlet oxygen
changes dramatically in different solvents[22] depending on the
energy-transfer efficiency from electronic to vibrational states.
The closer the vibrational mode of the solvent to that of singlet
oxygen, the higher the effectiveness of the deactivating process.
For instance, OD and OH bonds vibrate at 2550 and
3500 cm1, respectively, whereas singlet oxygen vibrates at
3286 cm1; hence, it quenches far faster in water than
deuterium oxide (heavy water).[32] Rodgers and T. Snowden[55]
estimated the natural lifetime of singlet oxygen to be thirteen
times higher in D2O than in H2O. Table 2 provides quenching
rate constants kqof delta and sigma singlet oxygen in different
media or its reciprocal parameter, the O2(1Δg) lifetime (τΔ), in
Table 3. Both Tables 2 and 3 reveal that strong dependency of
O2(1Δg) decay kinetics on the media of formation.
Subsequent to the excitation, the electronic energy of
singlet oxygen could be diminished by the physical or chemical
quenching processes. While the physical quenching exclusively
involves deactivation of the excited singlet oxygen to its
ground state, the chemical quenching is ensured by the
reaction of the energetically richer form of molecular oxygen
with other reactive species to form oxygenated products. The
mechanism of quenching of singlet oxygen by numerous
substrates has been studied in the literature.[60,61] Amines, for
instance, are potent quenchers of singlet oxygen and the
quenching rate of singlet oxygen by the number of aliphatic
amines have been estimated by Monroe[62] to be inversely
dependent on the ionisation potential and steric inhibition in
their structure due to the substitution on the α-carbons
bonded to the nitrogen atoms.
Figure 4. Sketch of the energy levels for four low-lying states of the oxygen molecule. The pairs of determinants which have to combine to yield the correct
adiabatic states are denoted asTOS
0;SOS
1;SCS
1and SCS
2, respectively.[27]
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Table 1. Comparative summary of typical ROS species.
IUPAC Name
Symbol E0vs. NHE at pH 7.0 Typical lifetime
Generation technique
�max
(absorption)
Occurrence in [C]ss
Notes
Singlet oxygen
O2(1Δg)
[31–35]
0.65
V(1O2/O2)Exists 4 μs in water and 45 min in
vacuum.
Produced by photosensitisation,[12]
electrical discharge,[13] and
thermally via surface-mediated re-
actions on transition metal
oxides.[14,15] Also produced chemi-
cally by decomposition of triethyl-
silyl hydrotrioxide generated in situ
from triethylsilane and ozone.[36]
1,913 nm 1012 to 1013 M in natural water.
Has application in drug formula-
tions, photodynamic therapy and
in military weapons.
Can enhance the initiation of
chemical reactions.
Superoxide
anion
O2
*
[33,37,38]
0.33/0.137 V
(O2/O2),
0.94/0.95 V
(O2/H+, H2O2)
Persistence ranges from 1–
3,000 min.
Produced by the dissolution of the
alkali metals and alkaline earth
metals salts, such as CsO2, RbO2,
KO2, and NaO2) in water.[39]
Biologically, in phagocytes, O2
*is
produced by the enzyme NADPH
oxidase.[40,41]
240 nm 109to 1012 M in natural water.
Predominantly exists in the proto-
nated hydroperoxyl form at neutral
pH.
The alkali salts of O2
*are used on
the space shuttle, submarines and
in firefighters’oxygen tanks.[42]
Hydroxyl
radical
HO*
[33,43,44]
2.18 V
(HO*, H+/H2O) Can last 0.2–40 s.
Forms during decomposition of
hydroperoxide (ROOH) species,
the reaction of excited atomic oxy-
gen with water, photolysis of 1-
hydroxy-2(1H)-pyridinethione.
UV-light dissociation of H2O2,
and Fenton chemistry.[45]
260 nm 1015 to 1018 M in natural water.
Known as the troposphere “deter-
gent” by decomposing pollutants
and eliminating some greenhouse
gases (methane and ozone).[46]
Hydrogen
peroxide
H2O2
[47,48]
1.8 V
(H2O2/H2O) Stable ROS.
Prepared industrially by hydrolysis
of the ammonium peroxydisulfate,
in anthraquinone processes.
Direct synthesis from the elements
using finely dispersed metal
catalysts.[49]
Seawater contains 0.5 to 14 μg/L,
freshwater 1 to 30 μg/L and air 0.1
to 1 ppb.[50]
Functions as strong oxidising agent
in industries. About 60% of the
world‘s production of hydrogen
peroxide is used for pulp- and
paper-bleaching.[51]
Table 2. Rate constant kqfor the deactivation of O2(1Δg) and O2(1Σ+g) (M1s1).[19,56,57]
Medium O2(1Δg) O2(1Σ+g) Medium O2(1Σ+g)
O21.4×1032.7–9.0×108CH45.0×107
H2O 9×1033.2–20×108C2H62.3×108
CO22.3×1032.3–26×107C3H82.7×108
CO NA 1.5–2.6×106C4H10 3.8×108
N2<6.0×10 2.1–12×105C5H12 4.5×108
Ar <1.2×1021.9×106–3.5×108C6H14 5.5× 108
D2O NA 3.7×108C7H16 6.0×108
NH3NA 8.3×108C6H63.9×108
H2S NA 2.6×108CS21.7×106
NO2NA 1.5×107CCl42.7×105
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3. Occurrence, Artificial Generation and Detection
of O2(1Δg)
3.1. Natural Occurrences
Singlet oxygen exists naturally in trace amount in the upper
atmosphere, polluted urban atmosphere, as well as in ozone-
generating conditions (e.g., photo-degradation of turpentine)
in pine forestations.[63–65] The biomolecular reaction of ozone
with proteins, methionine, thiols, ascorbic acids and triphenyl
phosphate also contributes to the atmospheric concentration of
singlet oxygen.[66–69] Generally, the excitation energy source for
the formation of singlet oxygen in the atmosphere originates
from the sunlight. The solar radiation energy could be
absorbed, either directly by triplet oxygen or indirectly by
atmospheric contaminants (e.g., particulate surfaces) that then
transfer the excitation to the ground state O2to form the
excited state 1O2.[70] Furthermore, singlet oxygen arises and
plays a crucial role in biological systems, including
animals[71–73] and plants.[74–77] The paradoxical part of singlet
oxygen lies in being a key participant in cancer
phototherapy.[78,79]
3.2. Artificial Generation of Singlet Oxygen
Artificial generation of singlet oxygen mimics the natural
means of producing it in the environment. These involve
techniques such as photosensitisation and photochemical
reactions,[12,80,81] ozone photolysis,[68] microwave/radiofre-
quency discharge,[82,83] electrical discharge,[13] thermal decom-
position of dioxetanes,[84] decomposition of hydrogen
peroxide,[85] transition metal activation[14,15,86,87] and chemical
synthesis.[88,89] While each method exhibits favourable advan-
tages and disadvantages, the photochemical method (otherwise
termed as dye-sensitised oxidation) has gained wide popularity
in the (bio)chemical and health sectors.[90–92] However, for
combustion applications, laser, electric discharge, and chemical
(including surface-mediated reactions) routes remain relatively
favourable. The following summarises the commonly em-
ployed methods for generating singlet oxygen for specialised
applications.
3.2.1. Photosensitisation Method
This technique, otherwise knowns as the dye-sensitised photo-
oxidation relies on the response of a substance to visible and
ultraviolet (UV) radiations,[93,94] based on the first and second
laws of photochemistry. Typically, the number of activated
molecules, for instance, singlet oxygen is equivalent to the light
absorbed (number of photons).[12] A pioneering study by
Kautsky[95] proposed that singlet oxygen might exist as a
reaction intermediate in dye sensitised photo oxygenations.
This was later confirmed in 1964 by comparing the
physiochemical features of the ROS (i. e., singlet oxygen)
generated by the photosensitisation method to that obtained
from chemical alternatives,[95,96] and radiofrequency.[36] Photo-
sensitisation technique is based on the concept that the ground
state photosensitiser (S0) receives energy from photons of light
to reach to their electronically excited singlet state (S1)
followed by a conversion to the excited triplet form (T1)
through an intersystem crossing (ISC) transition. As shown in
Figure 5, the excited triplet state sensitiser could directly react
with the organic substrate (RH) to produce radicals (R., via
Type-I reaction); or would energetically interact with ground-
state molecular oxygen to produce singlet oxygen that in the
Type-II reaction mechanism.[97]
The abundance of oxygen, light and natural sensitisers on
earth entitles this method as the most demanding methodology
of singlet oxygen production. However, the industrial applica-
tion employs other light sources, including, monochromatic
lamps such as LED, Xenon arc, and medium pressure Hg
lamp,[98] or polychromatic laser.[99] Loponov et al.[100] studied
four different spectral compositions of light (Actinic fluores-
cent, LED, Xenon arc, and medium pressure mercury lamp)
and their overall efficiencies on visible-light sensitised produc-
tion of 1O2. Among all, LED lamps have proven to be the
most efficient source, with nearly 70% of the emitted light
being absorbed in the photoreaction. Figure 6 depicts a a
typical apparatus for the production of singlet oxygen via an
electrical discharge method.
It worth mention that, molecular oxygen can be excited
into its singlet state directly, without the need of the sensitiser
by microwave or radiofrequency discharge generator where
Table 3. Lifetime of singlet oxygen in different solvents.[19,58,59]
Solvent Lifetime Solvent Lifetime Solvent Lifetime
H2O 3.1 μs CH2Cl299.0 μs CDCl37.0 ms
D2O 68 μs CH3CN 77.1 μs C6F13I 25 ms
C6H14 23.4 μs C5H12 34.7 μs CS245 ms
(CH3)2CO 51.2 μs CH3OH 9.5 μs CCl459 ms
(CD3)2CO 992 μs C2HCl3247 μs C6F14 68 ms
C6H630.0 μs CHCl3229 μs C10F18 59 ms
C6D6681 μs C6F621 ms C2Cl3F372 ms
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low-pressure oxygen electrically discharged to produce singlet
oxygen of efficiency around 10–20%.[101] However, the micro-
wave or radiofrequency discharge generator cannot produce
singlet oxygen in pure form it will always be contaminated
with other oxygen forms such as oxygen atoms and ozone that
can quench singlet oxygen violently.
The selection of a suitable sensitiser is mandatory for
efficient singlet oxygen production. The properties of such
photosensitiser should include a high light absorption at the
wavelength of study (i.e., a high extinction coefficient of
light), a high quantum yield ϕΔ, and resistance to oxidation by
singlet oxygen or other oxidants present in the system (i. e.,
long lifetime for the excited triplet state). Some typical dyes
and their structures, Absorbance range, and quantum yield in
different solution are illustrated in Table 4.
3.2.2. Electric Discharge Technique
Foner and Hudson[83] reported the first successful implementa-
tion of electric discharge to produce electronically excited O2
(1Δg) from pure gaseous oxygen with approximately 10–20%
conversion efficiency. In the early 1970s, Cook and Miller[105]
observed that the formation of singlet oxygen is not exclusively
limited to the discharged O2stream; instead, CO2and NO2
gaseous streams could also be exposed to the microwave
discharge to generate singlet oxygen with about 0.5–3.5 %
conversion efficiencies, respectively. The electrical source of
singlet oxygen is inefficient due to the limited overall yield of
singlet oxygen production. Another disadvantage of the
electrical discharge systems is the production of oxygen atoms
and ozone molecules along with singlet oxygen during gas flow
through the discharge cavity, specifically at high-pressure
discharge systems (PO2 >10 Torr).[106] Elias et al.[107] investi-
Figure 5. Mechanisms of singlet oxygen generation in the presence of light, photosensitiser and ground-state molecular oxygen O2(3Σg). Substrate and oxidation
products can differ as they either follow a Type I or Type II process.
Figure 6. A conventional apparatus for generation of singlet oxygen by electrical discharge method.[108]
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Table 4. Singlet oxygen generation from typical photosensitisers in various solvents.[12,59,102–104]
Sensitisers Structure Absorbance Region �max (absorption) Quantum yield ϕΔ
Rose Bengal (RB) 490–575 nm 510 nm ϕΔ(CH3OH)=0.80
ϕΔ(H2O)=0.76
Erythrosin B 470–530 nm 510 nm ϕΔ(H2O)=71
Methylene Blue (MB) 55–700 nm 670 nm ϕΔ(CH3OH)=0.51
Benzophenone 240–380 nm 250 nm ϕΔ(C6H6)=0.36
2-Acetonaphthone 280–320 nm 293 nm ϕΔ(C6H6)=0.71
Acridine 200–400 nm 250 nm ϕΔ(C6H6)=0.83
Buckminsterfullerene (C60) NA NA ϕΔ(CH3OH)=0.90
Tetraphenylporphine (TPP) NA NA ϕΔ(C6H6)=0.66
9,10-Dicyanoanthrance (DCA) NA NA ϕΔ(C6H6)=1.66
ϕΔ(CH3CN)=2.03
Eosin Y 200-560 nm 517 nm ϕΔ(H2O)=0.61
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gated the introduction of a mercury mirror ahead of the
discharge tube and found out that the atomic oxygen was
eliminated from the system. However, the introduction of
mercury, itself, could result in external contamination.
Furukawa et al.[108] applied the conventional flow apparatus
(depicted in Figure6) to study the reaction of olefins and
amines with discharge-generated O2(1Δg) and managed to
minimise the mercury contamination problem by cooling the
tube after electrical discharge.
3.2.3. Chemical Reaction Approach
As illustrated in Figure 7, there are varieties of chemically
derived recipes for producing singlet oxygen.[33,69,109]
Half a centenary ago, Seliger,[110] using a low-resolution
photomultiplier, photographed an unexplained red light-
emitting with low intensity after mixing hydrogen peroxide
(H2O2) with sodium hypochlorite NaOCl. A decade later,
Khan and Kasha[111] explained that such observation is
associated with the decay of singlet oxygen generated at a
single chemiluminescence band of 634.8 nm. Moreover, the
authors successful recorded the emission of singlet oxygen (in
solution) at some other wavelength, as shown in Equations 1
and 2.[111]
1O2þ1O2!2ð3O2Þ þ hv
l¼762 and 634 nm (1)
1O2!3O2þhv
l¼1270 nm (2)
The decomposition rate of hydrogen peroxide into water
and singlet oxygen (Equation 3) is considerably slow at room
temperatures; thus, the presence of catalysts is inevitable to
acquire rational decomposition rate in aqueous solutions.
2H2O2!2H2Oþ1O2
l¼762 and 634 nm (3)
The progress in improving the rate of decomposition of
hydrogen peroxide had elucidated mineral compounds like
MoO42, Ca(OH)2and NaOCl as potent catalysts.[85,87,112,113]
Singlet oxygen 1O2can also be generated chemically by the
decomposition of ozonides,[114] endoperoxides[115,116] and super-
oxide ion,[37,117] see Figure 5. Among all, the thermal decom-
position of endoperoxides is the cleanest. Equation 4 describes
the decomposition of hydrogen peroxide, where M represents
an atom of an alkali metal (K, Na, Li).
H2O2þCl2þ2MOH !1O2þ2MClþ2H2O(4)
The decomposition rate of H2O2in the presence of
heterogeneous catalysis such as silver powder (Ag), platinum
black (Pt) or MnO2to be negligible.[118] A drawback of
chemical synthesis of singlet oxygen is the possibility of
quenching in the condensed phase. Besides, these reactions
endure considerable side reactions leading to the formation of
other strong oxidising agents.[119–121] Therefore, chemical
methods are only applied in situations where the use of light is
prohibited or restricted.[17]
One of the practical uses for the chemically generated
singlet oxygen lies in chemical oxygen-iodine lasers (COIL).
Advances in chemical singlet oxygen generators (Figures 8 and
9)[109] has led to the production of higher concentration of
Figure 7. Most efficient chemical sources of singlet oxygen. Figure 8. Methods used to achieve gas-liquid reaction in a singlet oxygen
generator: a- wetted wall; b- aerosol; c-jet; d- sparger.
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singlet oxygen in gas streams, suitable for the operation of
COIL at output power up to 100 kilowatts.
3.2.4. Surface-Mediated Reaction Methods
Singlet oxygen forms via surface-mediated reactions on
particles such as silicon dioxide, aluminium oxide, and
transition metal oxides.[14,15] A recent review by Nosaka et al.
describes the photocatalytic method of generating ROS
generation on TiO2, as well as the supplementary
techniques.[104] The catalytic surfaces can be activated by heat
(thermally) or during light exposure (photons). In the latter
case, the induction of photos (on the surface of the catalyst)
generates electrons (e) and holes (h+) responsible for the
formation of ROS through oxidative and reductive reactions.
The methods allowing quantitative detection of singlet oxygen
in heterogeneous systems appeared have been reported in the
open literature.[104,122–125] There exists a hypothesis blaming
metal oxides in initiating self-heating and spontaneous fires of
coal mines.[126] Metal oxides, namely SiO2, Al2O3, CaO and
Fe2O3, are the fundamental constituents of coal inorganics[127]
and could result in the formation of singlet oxygen,[128]
enhancing the oxidation of hydrocarbons by lowering the
activation energy barriers. Similarly, the formation of singlet
oxygen in the dark has been facilitated by horseradish
peroxidise (HRP) enzyme (Figure 10a) comprising of transi-
tion metals that enact as a catalyst and fixes the oxidation
exothermicity.[129,130] The iron atom‘s sixth octahedral position
is considered as the active site of the enzyme.[131] As shown in
Figure 10b, oxidation of isobutanal in the presence of HRP
enzyme results in the formation of triplet excited acetone, that
subsequently its energy to molecular oxygen to trigger the
formation of O2(1~g).
3.3. Detection of Singlet Oxygen: Methods and Challenges
Detection of singlet oxygen can be classified as direct (electron
magnetic resonance (EPR), chemiluminescence, and
fluorescence probe techniques) or indirect methods (chemical
or spin traps probe molecules). Wherein the direct detection
Figure 9. Different types of chemical singlet oxygen generator: a- wetted wall, with rotating disks; b- aerosol; c- sparger; d- jet.[109]
Figure 10. Chemical structure of iron heme group in horseradish peroxidise (a), and the mechanism of enzymatic formation of singlet oxygen from excited
acetone (b).
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including the EPR,[104,132] and germanium photodiode (or
photomultipliers), requires a singlet oxygen lifetime of a sub-
millisecond timescale.[104] On the other hand, in the indirect
methods, singlet oxygen reacts with probe molecules (i. e.,
chemical or spin traps) to yield a more stable, long-lived
analyte. Therefore, the detection timescale of indirect methods
can scale up hours. Technically, the two classes of techniques
require thorough consideration of sensitivity, selectivity, and
sufficiently fast time resolution,[33] otherwise termed as the
“triple S”.
3.3.1. Photodiode Techniques
Photodiodes are semiconductors typically made from materials
such as silicon and germanium that turn light into electrical
current. In this direct detection method, when 1O2decays it
emits light that can be detected by the photodiode, its energy
will be transferred to surface electrons that pass through a
depletion layer-leaving behind a charge difference between
electrodes proportional to the amount of light received.[104] On
the other hand, photomultipliers are vacuum tubes with high
photosensitivity in ultraviolet, visible, and near-infrared re-
gions, and it can detect weak emissions because for each
dynode electron double photoelectrons is released at the
surface by that the signal is magnified by 100 % with faster
response and lower noise.[133,134] For instance, the chemilumi-
nescence method measures the light that emitted through the
transition of 1O2to its ground state (dimol light emission and
monomol light emission).[135] Such emission in solutions can
also be detected by a liquid nitrogen cooled germanium
photodiode detector[115] or by using photomultiplier tubes
(PMTs) where the later more efficient and can identify both
forms of singlet oxygen (1Δgand 1Σg+) separately.[135] However,
in solutions, the detection is more challenging due to the short
lifetime of 1O2and the low emission (quantum yields <108)
so photomultiplier tubes (PMTs) is applied if the concen-
tration of singlet oxygen is quite significant.[136–138] One of the
common drawbacks of the photocatalytic means (fluorescence)
is that under light illumination the singlet oxygen sensor may
demonstrate problems in photosensitisation, generation and
photodecomposition of 1O2. For instance, Sensor green and
similar probe reagents showed a notable reaction to *OH or
*O2.[104,139]
3.3.2. Time-resolved EPR Technique
Since dioxygen exists as a paramagnetic species that have
unpaired electrons in its electronic structures, EPR spectrom-
eter remains the most reliable technique for detecting singlet
oxygen (as well as other radicals and paramagnetic compounds)
in any phase.[140] EPR technique accurately predicts the
structures of radicals and can further be applied for
quantification via standard calibration. Generally, the EPR can
be sub-grouped into the direct and indirect (using spin trap)
methods. The major drawback is the short lifetime of radicals.
Hence, EPR detector should be used in situ, with adequate
consideration of the pressure (vacuum) and temperature
(cryogenic condition) in order to increase the lifetime and
achieve strong EPR signals.[141] As a result, Kearns et al.[142]
placed a reactor, containing oxygen stream saturated with a
sensitiser (naphthalene), directly inside the EPR cavity and
irradiated it using a mercury lamp. Wasserman et al.[143] also
applied the same idea but extended it to study the other
derivatives of naphthalene as photosensitisers such as octaflur-
onaphthalene and perdeuterated naphthalene vapours at low
pressures in the range of 0.1 to 1 Torr. Where 70% of triplet
oxygen excited to singlet oxygen. Snelling[138] reported that at
about the same pressure limit (1 Torr), the photochemical
generation of 1O2would reach its maximum. Even though the
use of EPR has a pivotal role in the direct detection of singlet
oxygen, up to now, little attention has been paid to its
application, resulting in a limited amount of published data.
Figure 11 illustrates the setup for EPR direct detection of
singlet oxygen and the characteristic signals.
The Indirect detection of singlet oxygen in EPR relies on
the use of chemical spin traps (highly reactive chemical agents
in solution) to capture singlet oxygen by forming a stable
compound (endoperoxide). This makes the detection of the
short-lived singlet oxygen easier and more accurate. The
chemical trap must be highly soluble in the solvent of interest,
transparent in the range of excitation wavelength,[144,145] and
react rapidly and selectively with 1O2to produce a distinct and
stable endoperoxide without any side products. One of the
most common singlet oxygen spin traps, 2,2,6,6-tetramethyl-
4-piperidone (TEMP) reacts with singlet oxygen to form
2,2,6,6-tetramethyl-4-piperidone-N-oxyl (TEMPO) that is
Figure 11. An example of direct EPR sampling of singlet oxygen, generated
by photosensitisation (naphthalene) of pure triplet oxygen. The right figure
compares the spectrum of triplet oxygen (a) to singlet oxygen (b).[132]
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easily detectable by EPR spectroscopy,[146] polycyclic aromatic
compounds such as tetrapotassium rubrene-2,3,8,9-tetracar-
boxylate (RTC) and disodium 9,10-anthracenedipro panoate
(ADP) are also considered popular chemical traps even though
they lack the transparency criteria.[147] Moreover, probes such
as 9,10-anthracenedipropionic acid (ADPA), 2,2,6,6-
tetramethylpiperidine (TEMP), furfuryl alcohol (FFA) and
1,3-diphenylisobenzofuran (DPBF) are also widely used for
indirect detection of 1O2.[33] Our recent work resolved one of
the major gaps in the literature, providing improved kinetic
information on TEMP-trapped singlet oxygen against a
theoretical model.[148] The strictly hindered amine (i.e.,
TEMP) reacts selectively with singlet oxygen to form nitroxide
radical (TEMPO) that displays a the EPR signals (Figure 12,
LHS) that enabled the quantitative validation of our POLY-
MATH-based kinetic model in Figure 12, RHS.[148]
4. Initiation Channels of Reaction Singlet Oxygen
with Hydrocarbons
Apart from excitation energy transfer (EET) mechanism,[27]
singlet oxygen undergoes peculiar chemical reactions. As earlier
describes, singlet oxygen is an electrophilic reactant owing to
its low-lying π-antibonding LUMO.[149] Thus, unlike triplet
oxygen, introducing singlet oxygen atoms into olefins, dienes
or aromatic structures will take different reaction pathways
such as Diels-Alder [4+2] and [2+2] cycloadditions, and ene
reactions with isolated double bonds.[121,143,150,151] Figure 13
summarises the reaction pathways of singlet oxygen, detailed
in the following subsections, with hydrocarbons. Determina-
tion of the dominant reaction rests on different factors. For
example, the distance between Cxand Cyatoms, ionisation
potential of the 1,3 diene, spatial alignment of allylic hydro-
gens, and solvent,[152,153] for instance, [4+2]-cycloaddition
reaction is highly solvent-dependent.[154]
4.1. [4 +2]-Cycloaddition
This intermolecular cycloaddition, also referred to as Diels-
Alder reaction,[155] involves the addition of singlet oxygen
(dienophile) to conjugated dienes to form a six-membered
endoperoxide over a σ-π rearrangement of two π-bonds being
replaced by two σ-bonds. The Diels-Alder reactions of singlet
oxygen range from stepwise, proceeding through polarised
diradical intermediates, to highly asynchronous concerted.
When singlet oxygen reacts with electron-rich molecules, the
frontier orbital interactions are responsible for these highly
asynchronous concerted or stepwise paths. These interactions
also influence the properties of these intermediates, causing
significant rotational barriers about single bonds of the
polarised diradical. The significance of photo-induced [4+2]-
Figure 12. Time course signal of TEMPO (product of the reaction of singlet oxygen and TEMP) in H2O (red circles) and D2O (black squares); TEMPO spectra
recorded by in situ EPR after illuminating TEMP sample in dyed D2O solution at pD =6.35, g=2.00062, and centre field =3487.00 G; and EPR concentration
profile of singlet oxygen generated in H2O using 24 V LED. The dash-dotted line represents the corresponding values obtained from the POLYMATH kinetic
solution.[148]
Figure 13. Singlet Oxygen reaction pathways; ene reaction (1), 2+2 cyclo-
addition (2), 4+2 cycloaddition (3), and addition to sulphides (4).
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cycloaddition of singlet oxygen to cyclic and acyclic dienes
affords the possibility for synthesising complex naturally-
occurring bioactive substances, such as taxol[156] and
agarofuran,[157] with promising biological and anticancer
functions.
The classical [4+2]-cycloaddition of singlet oxygen to
cyclic, acyclic and aromatic 1,3-dienes was exclusively assumed
to occur via a concerted mechanism for many years.[158,159]
Such a concerted mechanism occurs between electrophilic
singlet oxygen and cisoid conformation of the diene yielding
endoperoxides via steps A or B (Figure 14) as six-membered
ring transition states.[159] These steps solely differ in the bond
formation sequence as A is a synchronous mechanism due to
the simultaneous formation of the CO bonds and B is a
nonsynchronous reaction as the CO bonds in the transition
state are not equivalent.[152]
A range of stepwise (non-concerted) mechanisms, as
alternative two-stage reaction routes, involve the formation of
diradical (C), open-chain zwitterion (D), or zwitterionic
peroxolane (E) intermediates as illustrated in Figure 15.
Such intermediates could participate in further intercon-
version reactions which are comprehensively explored by
Maranzana et al.[160] According to their study, the diradical
intermediate (Figure 16) could transform to the relevant
peroxirane or dioxetane through the ring closure paths of F or
G, respectively. It has been proven that the diradical to
peroxirane transformation is higher in energy barrier and thus
less attainable in comparison to the diradical to dioxetane
closure channel.
Formation of a Diels-Alder adduct is accelerated as the gap
between HOMO, and LUMO energy levels of diene and
dienophile components are lessened. As a general rule,
electron-donating groups elevate HOMO and LUMO energy
levels in either the diene or the dienophile, while electron-
withdrawing substituents inversely influence such energy
levels.[161] Thus, the Diels-Alder reactivity associated with
singlet oxygen (functioning as an avid dienophile) and
conjugated dienes interactions would be facilitated via sub-
stituting R and R’(Figure 15 and 16) with electron-donating
groups.
4.2. Ene-type Reaction
Ene reactions of singlet oxygen have been the focus of a
significant number of studies ever since Foote and Wexler[162]
discovered the role of such active oxidant in the oxidation of
olefin and dienoid species in the last century. This type of
reaction also referred to as Schenck reaction, has fundamental
applications in the synthesis of a wide array of functionalised
products.[163] For instance, allylic hydrogens in the most
congested side of olefins will undergo ene reaction by the
abstraction of the allylic proton (Figure 17) yielding the allyl
hydroperoxide, ROOH (R=alkyl), that causes a shift in
the double bond[164], which can then be reduced to the
corresponding allylic alcohol.[165]
Figure 14. Concerted mechanism of [4+2]-cycloaddition of singlet oxygen.
Figure 15. Stepwise mechanism of [4+2]-cycloaddition of singlet oxygen. Figure 16. Ring closure of diradicals.
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The ene reaction of singlet oxygen involves either a stepwise
or concerted addition of singlet oxygen to olefins and dienes
and results in the formation of final hydroperoxide product. A
concerted mechanism is argued to evoke the establishment of a
six-membered ring (1) (Figure 18) as the geometrical nature of
the transition state. However, intermediates such as peroxir-
anes or perepoxides (2), zwitterions (3) and diradicals (4) are
expected through the cleavage of a single bond during the
stepwise mechanism in the ene reaction.
A piece of theoretical evidence published by Davies and
Schiesser[166] favours in both of concerted and non-concerted
pathways as they endure analogous energy barriers on the
reaction coordinates. An earlier mechanistic study by Yama-
guchi et al.[167] suggests that perepoxides and zwitterions are
too high in energy level and not achievable as good
intermediates in singlet oxygen ene reaction for simple and
semi-polar alkenes. In contrast, diradicals are more accessible
and stable intermediates. It has also been advised that
unrestricted Hartee-Fock (UHF) method is more efficient in
estimation of characteristics of open-shell species, such as
diradicals, in comparison to the restricted HF. In fact, UHF
method considers each orbital to be occupied by a specific
electron to avoid the overestimation of reagent properties and
stabilities.
4.3. [2 +2]-Cycloaddition
Electron-rich olefins, as well as compounds with geometrically
inaccessible allylic hydrogens, are among the reagents which
are likely to undergo [2+2]-cycloaddition with singlet oxygen
to facilitate the formation of dioxetanes as the four-membered
ring peroxides in Figure 19.[152]
The unstable cyclic dioxetane adduct would either partic-
ipate in a thermal decomposition (path a) to form carbonyl
compounds enabling light emission upon chemiluminescent
processes or would be reduced through the path (b) to the
relevant diol as depicted in Figure 20.[58] The luminescence
consequent to the thermal cleavage of 1,2-dioxetanes was first
evidenced by Kopecky and Mumford,[84] and they were
pioneers in isolating such [2+2] cycloadducts of singlet
oxygen.
This mode of fundamental singlet oxygen reactions has
been recognised in the late 1960s,[168,169] few years after ene
reaction and [4+2]-cycloaddition have come to the focus of
intense studies of singlet oxygen attack to allylic hydrogens
and conjugated dienes. Although [2+2]-cycloaddition is
sterically plausible during photo-oxidation of trans conjugated
dienes, the cycloaddition of singlet oxygen to cis-dienes is
rather energetically favourable through [4+2]-addition than
[2+2]-reaction.[142] Perepoxide (peroxirane. i.e., open zwitter-
ion) and diradicals are mutual intermediates in stepwise
pathways of singlet oxygen insertion to olefinic bonds via ene
and [2+2]-reaction routes. Yet, more theoretical and exper-
imental support is addressed these intermediates to be formed
upon singlet oxygen stepwise ene reaction.[170,171] The forma-
tion of perepoxide intermediate was first reported by Schaap
and Faler[172] from photo-oxidation of adamantylideneadaman-
tane (1), as an alkene with inaccessible allylic hydrogen, by
singlet oxygen via [2+2]-cycloaddition mechanism shown in
Figure 17. Singlet oxygen ene reaction.
Figure 18. Potential intermediates and transition states in the ene reaction.
Figure 19. [2+2]-cycloaddition of singlet oxygen.
Figure 20. Conversion of sensitive dioxetanes as the products of [2+2]-
cycloaddition of singlet oxygen.
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Figure 21. The authors observed the production of relevant
dioxetane (3) and epoxide (4) as the final reaction products in
pinacolone (solvent). Dioxetane formed via rearrangement of
perepoxide intermediate (2) through the channel (a), while the
epoxide developed by solvent trapping and reduction of the
perepoxide, as shown in the channel (b). The stepwise
mechanism of such [2+2]-reaction is depicted in Figure 21.
The abovementioned reaction routes of singlet oxygen have
resulted in diverse applications in the synthesis of pharmaceut-
ical compounds,[173–176] treatment of wastewater[16,177–179] and
initiation of combustion. This paper centres on combustion
and remediation of toxic organic pollutants. However, one
cannot but summarise the role of singlet oxygen in biomedical
and pharmaceutical industries: Photodynamic therapy (PDT)
possesses a unique and well-documented application of photo-
oxidation in which singlet oxygen plays an indisputable role in
destruction of target malignant living cells.[180–183] In the case
of PDT, photosensitiser in abnormal tissue is required to be
efficiently excited to the triplet state by visible light irradiation
in the presence of oxygen, yielding to reactive oxygen species
(ROS) production and stimulating carcinoma destruction
effects. Some sensitisers (i.e., carotenoids, flavonoids and
chlorophyll) occur naturally inside biological cells[184]; however,
for photodynamic therapy of diseased cells, the synthetic non-
toxic photosensitising drugs are intentionally introduced into
the target organism and activated by illumination of the proper
intensity and wavelength of light close to the maximum
absorption peak of the selected sensitiser.[185] Among a variety
of light sources employed in PDT, the fibre optic devices and
laser light sources have been reviewed to be the most typical
and reliable sources for light delivery.[186] An alternative
medicinal applications of singlet oxygen reactions is the photo-
oxidation of oxazole derivatives (i.e. 2-methyl-4,5-diphenylox-
azole) leading to the synthesis of antimycin A3 which is a
useful biological antitumor. Additionally, macrocyclic lactones
and lactams, with their escalating medicinal values, could
mainly be formed upon a simple photo-oxidative rearrange-
ment of oxazole compounds to triamides intermediates,[173,174]
emphasising the key role of singlet oxygen in procuring vital
chemical and natural products.
The following section itemised such effects in different
combustion scenarios and prominent treatments of gas-phase
and wastewater pollutants.
5. Implications of Singlet Oxygen in Combustion
and Advanced Oxidation of Organic Contaminants
5.1. Enhanced Engine Performance and Spontaneous
Combustion
One promising area of research is concerned with studying the
influence of singlet oxygen O21Δgon combustion and
spontaneous ignition of hydrocarbon fuels.[187,188] As earlier
discussed singlet oxygen O21Δgresides 95 kJ/mol1(ΔGHo298)
above the ground state triplet oxygen molecule O23Σg.[16,189]
Therefore, even when injected in minuscule quantities, singlet
oxygen O21Δgaccelerates the chain-branching mechanism,
decreasing the ignition temperature as well as the induction
time.[188,190–196]
Applying the excitation power of singlet oxygen emerged as
one of the most attractive approaches in combustion
enhancement in internal combustion (IC) engines.[188,197–201]
This implies that when singlet oxygen is presented in
combustion systems, such as H2
O2,[194,202–204]
CH4
O2,[193,205,206] and COH2
O2[192] it intensifies the
ignition and enhances the decomposition kinetic. For example,
in the COH2
O2system, singlet oxygen showed an ability to
convert selectively and rapidly CO into CO2at 900 K.[192]
Likewise, linear hydrocarbons (HCs) such as ethene (C2H4)
and propane (C3H8),[207–209] and alkyl benzenes display notable
improvements in combustion parameters with the addition of
singlet O2.[15,210,211] The latter group constitutes a significant
additive fraction in transportation fuels, e.g., gasoline and
diesel.[212,213] To date, many experimental results and computa-
tional simulations proved the considerable impact of singlet
oxygen on accelerating the chain-branching mechanism,
decreasing the ignition temperature as well as the induction
time.[188,190–196] Starik and Titova[200,214] in their computational
studies projected substantial reduction in the induction length
of ignition followed the excitation of oxygen to singlet oxygen.
This enhanced the formation of O, HO, H radicals. Whereas,
Smirnov et al.[197] used a plasma discharge to excite the oxygen
stream right before merging it with hydrogen. They reported a
reduction in the induction length and attributed it to the
generated singlet oxygen molecules. However, the plasma
discharge suffers from contamination of other radicals. There-
Figure 21. Mechanism of [2+2]-cycloaddition of singlet oxygen to adaman-
tylideneadamantane.
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fore, Ombrello et al.[197,208] in their following research adopted
a lifted flame configuration to study the isolated effect of
singlet oxygen. Where they found that singlet oxygen solely is
behind the increment of the flame speed. Lebedev et al.[215]
modelled the reaction of singlet oxygen with methane applying
ab initio functional.
For combustion applications, singlet oxygen can be
generated by photosensitisation and electrical discharge or
thermally via surface-mediated reactions on particles such as
silicon dioxide, aluminium oxide, and transition metal oxides.
Metal-oxide surfaces, which are well-known for their catalytic
effect on hydrocarbon oxidation, serve as excellent generators
of singlet oxygen.[125] Boikov et al.[122,123] and Tomskii
et al.[124,125] discovered that vanadium-molybdenum oxide
system (V2O5
MoO3) synergistically enhances the oxidation of
toluene (i.e., fuel surrogate) via reaction mechanisms initiated
by singlet oxygen O21Δg. The electrophilic nature of singlet
oxygen engenders the rate constant for its reaction with a
substituted benzene ring to increase in the order of H<
C6H5<CH3<OCH3.[216] This means that the electron density
of a hydrocarbon species dictates its reactivity towards singlet
oxygen. Our computational study[217] provided the kinetic
parameters of the reaction of singlet oxygen with toluene, with
the initiation of the para channel (1,4 cycloaddition) following
a concerted mechanism through an enthalpic barrier of
34.5 kJ/mol with a fitted reaction rate coefficient of k(T)=
1.51×1015 exp(34 500/(RT)) cm3/molecule·s.
Moreover, as shown in Figure 22, the relative reactivity of
singlet oxygen, based on the reaction rate constants, follows
the order of OH>H>CH3>1O2>HO2>3O2. These in-
dicate that the presence of singlet oxygen considerably lowers
the activation energy of the initiation channels, resulting in an
energetically improved combustion process. Table 5 expresses
the role of singlet oxygen on reactions of hydrocarbons in term
of governing rate parameters.
The spontaneous reactions, involving singlet O2, have been
reported to lead to enhanced reaction rates; favourable in
improving the performance of internal combustion engines;
and detrimental in causing unwanted (auto-ignited) fires of
particular interest to safety and economics, the reaction of
singlet oxygen with fuels can result in spontaneous fires, e.g.,
of coal.[221] Contrary to non-spontaneous fires of coal that
emerges in the presence of combustible gases and ignition
sources, spontaneous fires in coal mines depend solely on the
self-heating oxidation reactions.[222] In Australian coal mines,
hundreds of spontaneous combustions of coal incidents have
been reported. For instance, in Queensland alone, 51
spontaneous combustion incidents have been identified, three
of which ended up mine closures and 37 fatalities.[223] The
persistence occurrence of spontaneous fires across the globe
necessitates the need for further investigations of the initiation
cause.[224] These initiation reactions operate at room temper-
ature while maintaining an adequate heat rate to generate the
first or main long-living radical. The hypothesis remains that,
the singlet oxygen (1O2) or other surface-generated ROS may
be the initiator of the self-heating chain reactions. In this case,
the electronically-excited species of oxygen should arise from
spontaneously “dark” reactions,[100,109] or surface-assisted reac-
tions in coal,[14] rather than photosensitisation[99,138] due to the
absence of light in the coal bed. A recent publication[126]
elucidate the influence of excited oxygen species formed on the
surfaces of nano-component of coal in initiating low-temper-
ature combustion (i.e., spontaneous fire) of coal, and
identified the primary products of such surface-mediated
reactions.
5.2. Low-temperature Degradation of Organic
Contaminants from Combustion
A comprehensive review on the kinetics of low-temperature
ignition of fuel species[233] features the role of molecular oxygen
in the abstraction of hydrogen atoms from fuel molecules at
the initiation stage of auto-oxidation. However, complemen-
tary to the low-temperature oxidation by triplet oxygen, the
presence of singlet O2can initiate photochemical or enhanced-
thermal reaction pathways. These classes of low-temperature
reaction have found practical applications in remediating
organic pollutants in wastewater[31,148] and atmospheric degra-
dation of the biogenic (naturally occurring) volatile organic
compounds (BVOC),[232,234] as they are kinetically more
favourable than some other oxidative channels.[235–238] More-
over, recent studies have shown the combustion application of
Figure 22. Arrhenius plots for initiation of the gaseous reaction of toluene
with singlet oxygen[217] and some combustion-active species.[218–220]
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Table 5. Documented reactions of singlet of with hydrocarbons.
Singlet oxygen+HC Medium Rate parameters Notes Ref.
A (cm3s) n Ea (kJmol1)
Hydrogen abstraction
CH4-air mixture
CH4+1O2=
CH3+HO2
Gaseous 7.59×1013 0 18.3 Numerically, the presence of 10 % 1O2in
molecular oxygen increases the speed of flame
propagation by a factor of 1.7 in a fuel-lean
(φ=0.45)
[188]
H+1O2=OH+H Gaseous 1.10 × 1014 0 3.2 Evidence that the reaction occurs principally
via abstraction, H+1O2!OH+O, rather
than via physical quenching, H+1O2!H
+3O2.
[225]
N+1O2=NO+O
1O2+NO=O+NO2
Gaseous
Gaseous 6.46×109
1.00×1012 1
02.1
12.5 NOx formation in the reaction of atomic
nitrogen and NO with 1O2
[140]
C2H6-air mixture
C2H6+1O2=
C2H4+H2O2
Gaseous 5.47×1013.66 5.1 At low temperatures (T0<850 K) and high
pressure (P0=10 atm), the photodissociation
of O2by laser photons with λI=193.3 nm was
effective in accelerating the ignition.
[226]
COH2mixture
CO+1O2=CO2+O
COH2O mixture
H2O+1O2=
OH+HO2
Gaseous
Gaseous 6.77×107
2.05×1015 1.6
013.7
25.0
Modelling studies showed that the abundance
of singlet delta oxygen in the COO2,
COH2OO2and COH2
O2mixtures re-
sults in the acceleration of oxidation process
and allows to shorten the induction time and
decrease the ignition temperature even at a
small content of 1O2
[192,
227,
228]
3O2+1O2=O3+O
O3+1O2=
2·O2+O
Gaseous
Gaseous 1.20×1013
3.13×1013 0
039.7
2.8 The presence of vibrationally and electronically
excited 1O2molecules in the discharge-acti-
vated oxygen flow allows to intensify the chain
mechanism and to shorten the induction zone
length significantly at shock-induced combus-
tion.
[229]
H2S+1O2=
HSO+OH Gaseous 2.85×1080.97 11.5 The abundance of 1O2molecules at only 1%
in total oxygen intensify the chain-branching
in the H2S-air mixture.
At φ=0.6, the presence of 5% 1O2molecules
in total oxygen increases the flame speed by
25%.
[230]
CH2=CHCH3+1O2
=CH2=CHCH2
+HO2
CH2=C(CH3)2+1O2
=CH2=C(CH3)CH2
+HO2
C7H8+1O2=
C7H7+HO2
Gaseous
Gaseous
Gaseous
8.38×100
1.08×102
7.42×102
3.11
2.98
3.77
19.1
13.5
53.3
Elimination of a labile hydrogen atom from a
linear or cyclic hydrocarbon. [231]
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singlet oxygen treating halogenated pollutants such as poly-
chlorinated biphenyls (PCBs) polychlorinated dibenzodioxins
(PCDDs) and furans (PCDFs).[239,240] Halogenated contami-
nants remain of particular concern owing to their extensive
toxicity towards health and accumulation in the
environment.[241,242] Atmospheric oxidation of such persistent
environmental pollutants has little to no kinetic feasibility.
Therefore, the current control measures involve energy-
intensive source incineration of contaminated materials at high
temperatures as high as 850°C. Singlet oxygen offers an
alternative low-energy approach of destroying dioxin-like
compounds.
Figure 23 displays the thermodynamic and kinetic feasi-
bility of singlet oxygen-assisted oxidation of polycyclic and
halogenated aromatic hydrocarbons, via the non-concerted
1,2-cycloaddition of O21~g, yielding dioxetane products via
diradical intermediate channels. The enthalpic requirements of
the initial reactions are relatively small at around 100 kJ/mol,
as compared to those overcame in high-temperature inciner-
ation by normal molecular oxygen (�350 kJ/mol, around
800°C). Subsequent decomposition into permanent oxidation
products will follow a similar mechanism described by
Altarawneh et al.[241,243] Chlorination has negligible effects on
the reported enthalpies. Therefore, singlet oxygen can have
practical implication in photo-oxidation and enhanced thermal
incineration of halogenated waste products.
5.3. Advanced Oxidation of Wastewater Organic Pollutants
The considerable steady-state concentration and lifetime of
photochemically-produced singlet 1O2further emphasises its
importance in the environment. Hence, it is of interest to
illustrate, both experimentally and computationally, the
attributes, benefits, and application of singlet oxygen in
industrial processes involving wastewater.[244] Studies on photo-
sensitised reactions of singlet oxygen with hydrocarbons in
water involved the photochemical activation of triplet oxygen
into the singlet state using a typical photosensitiser, to study
the subsequent oxidation (i.e., remediation) of organics, e. g.,
phenolic-based pollutants, that constitute negative impacts on
public health and the environment in wastewater. This has
also been achieved via thermal- and photo-catalyses.[245,246]
Sometimes termed as an advanced oxidation technique, the
use of singlet oxygen in photosensitised disinfection of water
by solar radiation is usually assisted by either photosensitisers
dissolved in the aqueous phase,[177,247] as enumerated in
Table 4, or solid (polymer)-supported photosensitisers.[178] To
this, waterborne bacterial pathogens and other microorganisms
undergo photocatalytic treatment and indirect mechanism by
singlet oxygen generated in solar photo-reactors or they are
damaged by the solar UV light through the direct mechanism.
In an experiment conducted by Kohn and Nelson,[179] the
Table 5. continued
Singlet oxygen+HC Medium Rate parameters Notes Ref.
A (cm3s) n Ea (kJmol1)
Hydrogen abstraction
Cycloaddition
Isoprene+1O2
Isoprene+1O2
Gaseous
Gaseous 2.6×1012
1.6×1012 0
055
105 Cycloaddition via [2+2] route [232]
Toluene+1O2
Toluene+1O2
Gaseous
Gaseous 1.51×1015
8.31×1014 0
034.5
42.6 Cycloaddition via [4+2] route
Ene reaction [217]
Benzene+1O2Hetero-
genous – – – Cycloaddition via [2+2] route/V2O5[210,
211]
Pyrrole+1O2
Pyrrole+1O2
Pyrrole+1O2
Gaseous 5.40×1013
1.87×1013
2.48×101
0
0
0
111
48
158
Ene reaction
Cycloaddition via [4+2] route
Cycloaddition via [2+2] route
[175]
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influence of dissolved oxygen and pH variation (in the range
of 6.5 to 9.3) is negligible in inactivation rate constants of
pathogens, while an increase in the salinity level of water is
observed to adversely affect the inactivation constant. Table 6
lists reaction constants of singlet oxygen with some organic
pollutants in water (as well as other solvents), indicating a
relatively fast degradation of these contaminants.
Our recent studies map the reaction mechanisms and
develop the comprehensive kinetic models for pyrrole,[175]
aniline,[31] and phenol,[148] updating the respective rate con-
Figure 23. Initial routes for the reaction of singlet oxygen with dibenzodioxin (DBD) (a); and dibenzofuran (DBF) (b) based on the calculation level of B3LYP/
6-311+g(d,p). The values in bold and italic signify reaction and activation enthalpies computed at 298.15 K, respectively (in kJ/mol).[240]
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stants in Table 6. Figure 24 exemplify the mechanistic path-
ways for reaction of the singlet oxygen with phenolic-based
pollutants in water, identifying the formation of para-
benzoquinone as the primary initial oxidation product. In a
recent study by Luo et al., a non-radical oxidation process of
bisphenol A under high salinity condition via peroxymonosul-
fate (PMS) activation by metal-free carbon catalyst was
investigated. Wherein singlet oxygen was identified as the
primary reactive species both via the scavenger experiments
and the EPR analyses.[245] Neves et al., studied the photo
treatment of sewage plant water contaminated with micro-
pollutants such as pharmaceuticals like metoprolol applying
Porphyrins as photosensitisers. He illustrated that the photo-
degradation of metoprolol, follows a pseudo-first order kinetics
with ca. 90% metoprolol degradation after 12 h. HPLC
scavengers confirmed the mechanism of degradation is via
singlet oxygen. Moreover, similar results were obtained in a
parallel experiment. In such the porphyrin was immobilised on
silica support and used as heterogeneous photocatalyst.[247]
In a novel approach, Lyubimenko et al., presented a
photocatalytic PdTFPP-PVDF membranes as a potential
technology for water treatment application. The author
combined a poly(vinylidene fluoride) (PVDF) membrane with
a photosensitiser generating a hybrid material for the
production of singlet oxygen to remove methylene blue (MB,
1 mgL1) at 83% efficiency under a flow rate of 0.1 ×
103Lmin1.[248]
Table 6. Rate constants for the interaction of singlet oxygen with
some organic compounds.[16]
Reactant Solvent k(L mol1s1)
2,5-diphenyl-Oxazole H2O/D2O (50:50) 1.6×108
Pyrrole C6H61.5×108
Cysteine D2O 1.0×105
Pyridine CCl42.0×103
Isoprene CHCl33.7×104
Limonene CH3COCH31.7×105
Malonic acid D2O 4.0 × 104
Benzene CCl43.9×103
Aniline MeOH 2.0×109
Phenol D2O 1.3×106
Figure 24. Potential Gibbs energy map for the initial channels in the reaction of O2(1~g) with phenol (a), and enthalpy (bold green) and Gibbs energy (italic red)
map for the formation of para-BQ from the reaction of singlet oxygen with phenol using M062X (solvation) model calculated at 298.15 K, respectively in kJ/
mol.
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6. Progress in Computational Assessment of Singlet
Oxygen Reactions
The theoretical investigation on kinetics and thermodynamic
properties of singlet oxygen reaction is mostly addressed via
two broad categories of computational approaches, i. e., the ab
initio studies[249,250] and density functional theory (DFT).[251,252]
The drawback of the former is that the wave function is a
hypothetical construct and not a physical parameter to be
specified with sensible approaches.[253–255]
Density functional theory (DFT), on the other hand,
provides a reliable means of predicting the geometries, vibra-
tional frequencies and energies of reaction through deploying
the measurable electron density within simple or complex
systems. Accordingly, there will be a considerable decline in
demanded computational time for the calculations, even
though the precision of calculation is substantially improved.
As a drawback, DFT lacks a suitable strategy in terms of
treatment of singlet oxygen reactions with the broken-
symmetry (open-shell) state due to the spin contamination
effects triggered by its lower-lying triplet state, and therefore
spin projection procedure needs to be applied for energy
correction.[256]
The open-shell electronic structure of singlet oxygen (with
a diradical nature) could best be described by the implementa-
tion of both dynamical and non-dynamical electron
correlation.[149] The DFT treats the dynamical electron
correlations,[257] while wave functions (i.e. CASSCF, HF) are
suited for the static (non-dynamical) relationship. Although
DFT is claimed to be inconvenient in the case of multi-
reference systems with diradical characters,[258] its association
with B3LYP level of theory is proven to be reliable in the
prediction of diradical energetics in concerted and stepwise
pathways.[259] In principle, DFT approach is for treatment of
closed-shell systems;[25] however, Filatov and Shaik[260] have
proposed a spin-restricted open-shell Kohn-Sham method
followed by DFT approach to address this problem with DFT
that would cover open-shell molecules. This methodology
employs a non-interacting reference wave function with both
the closed-shell and open-shell electronic subsystems in which
the electronic orbitals and energies are spin free suitable for
multi-configuration frameworks. The exclusive Kohn-Sham
DFT experiences difficulty in multi-determinant cases in
which the static correlation plays an important role in the total
energy.[261] Therefore, multi-reference methods are needed to
describe the electronic structure of the singlet state correctly.
The multistate density functional theory (MSDFT), as a
generalisation of the KS-DFT, includes the static correction
explicitly and has been successfully applied to treat multi-
reference cases.[262–267] As such, the adiabatic excitation energies
of the four spin-adapted configuration state functions (CSFs)
of oxygen molecules have been obtained using MSDFT
methods.[27]
Kearn[142] published a pioneer theoretical investigation to
examine the reactions of singlet oxygen with mono-olefins and
conjugated dienes in a concerted manner deploying a set of
zero-order singlet- and triplet-state wave functions. His ab-
initio analysis is a shred of evidence to the fact that singlet
oxygen is most likely capable of undergoing [4 +2]-cyclo-
addition to cis-dienes, while [2+2]-addition of singlet oxygen
to olefins might not be the case in systems with high π-
ionisation potential. In accordance to the effectiveness of DFT
approach in reproducing experimental values, Jursic[268] in-
quired into the geometric parameters and energies of small
oxygen-containing molecules deploying both ab initio and
DFT functions and demonstrated that DFT calculation over-
rides MP2 ab initio method in obtaining satisfactory results,
particularly when polarised 6–31 g(d,p) basis set is utilised.
Furthermore, the efficiency of DFT method in comparison to
the ab initio approach in estimating the molecular properties
has been recently investigated by Ess and Cook[269] concluding
that density functional calculations are more authentic in the
prediction of the singlet-triplet gap energies and bond
dissociation energies of various open-shell singlet diradicals
with the inclusion of spin projection.
As an example of the recent theoretical investigations on
photo-oxidative effects of singlet oxygen, Reddy and
Bendikov[252] studied the addition of singlet state molecular
oxygen to the most electron-rich spots of acenes structure,
concluding that the interaction of acenes with singlet oxygen
mainly dominates via Diels-Alder addition of O2(1Δg) to the
central ring (as the most electron-rich regions of the molecule)
through either a concerted or stepwise mechanisms while the
reactivity increases as the number of the rings increase from
benzene through pentacene. A detailed mechanism study of
singlet oxygen reaction with aniline reveals the formation of p-
and o-iminobenzoquinones as principle products via [4+2]-
cycloaddition of O21~gto the benzylic ring at para-ipso
positions.[270] In the study reported by Al-Nu’airat et al.[148],
the mechanistic and kinetic aspects of phenol reaction with
singlet oxygen are experimentally and computationally ad-
dressed for the first time. Singlet oxidation of aniline, O21~gis
attested to have an acute selectivity towards phenol ring at
para position, affording para-benzoquinone through a facile
exothermic [4+2]-cycloaddition reaction. Similarly, a con-
certed [4+2]-cycloaddition energetically leads the initiation
channels of toluene interaction with excited oxygen, yielding
to the production of para- and ortho-quinonemethides as
reactive intermediate species.[217]
In accordance with the abovementioned results, our
previous kinetic study on the mechanism of singlet oxygen
addition to pyrrolic ring[175] justifies the tendency of singlet
oxygen on attacking aromatic and semi-aromatic rings via a
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concerted [4+2]-addition. Furthermore, our obtained results
authenticate the enhancement of pyrrolic ring reactivity
towards singlet oxygen attack via substitution of π-donating
functional groups at beta-carbon (Cβ) position. On the other
hand, a stepwise [2+2]-cycloaddition mechanism energetically
favours isoprene oxidation initiated by singlet oxygen.[232]
Unlike the previously mentioned aromatic species, isoprene
features a conjugated π-system with double electron rich sites
susceptible to singlet oxygen electrophilic attacks. Accordingly,
methyl vinyl ketone and methacrolein, as the two experimen-
tally detected products of isoprene photo-oxidation, are
conceived to be generated upon exceedingly exothermic CC
bond fission within the structure of 1,2-dioxetane intermedi-
ates.
To date, the B3LYP and M062X DFT functionals along
with accurate basis set are the most convenient (i. e., common)
means for theoretical scrutiny of reaction of singlet oxygen
with hydrocarbons. For comparison purposes, quantum
chemistry composite methods such as CBS-QB3 overestimate
the singlet-triplet energy gap of oxygen by 68 kJ/mol (i.e.,
163.0 kJ/mol instead of the reported experimental value of
95 kJ/mol), and cannot perform “unrestricted” calculations; a
necessary procedure for obtaining the spin-contamination
operator S2 (HS) for energy refinements using the spin-
projection scheme (AP) pioneered by Yamaguchi’s
group.[271,272]
7. Conclusions and Perspectives
Like other chemical industries, the practical application of
singlet oxygen is expected to grow further within the
combustion society. Extensive efforts will corroborate new
experimental designs to validate the theoretical model. Future
works should investigate the influence of functional groups on
(electron donors or withdrawals) enhancing/prohibiting the
reaction pathways of organic fuels with singlet oxygen. Study-
ing the impact of functional groups on charge distribution and
electronic population of the molecules is critical to expound
new applications, e.g., in molecular-based electronics. This
will also reinforce the further clarification of surface-generated
ROS.
The current IC systems apply laser as a source of singlet
oxygen, leading to the formation of other ROS like ozone (O3)
and O atoms. Therefore, it would be beneficial to test the
enhancement effect of singlet oxygen in IC selectively. This
improvement could be achieved by applying photosensitisation
gaseous-based techniques. Furthermore, it is interesting to test
the role of singlet oxygen, i.e., kinetically, in breaking down
other persistent organic pollutants. Such work should broaden
the class of contaminants to include brominated flame
retardants (BFR), mixed halogenated dioxins/furans, and
polyfluoroalkyl substances (PFAS), and other forms of
persistent organic pollutants (POP).
Acknowledgement
This study has been supported by funds from the Australian
Research Council (ARC). J. N. and N.Z. thank Murdoch
University for the award of postgraduate scholarships. M. A.
acknowledges a start-up grant from the College of Engineering
at the United Arab Emirates University (UAEU) (grant
number: 31N421).
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Manuscript received: October 29, 2020
Revised manuscript received: November 26, 2020
Version of record online: December 16, 2020
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