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Optical Sensing Volatile Organic
Compounds Using Porphyrins
Elizaveta Ermakova * and Alla Bessmertnykh-Lemeune *
Posted Date: 16 October 2024
doi: 10.20944/preprints202410.1313.v1
Keywords: optical sensors; porphyrins; volatile organic compounds; sensing materials sensor arrays
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Review
Optical Sensing Volatile Organic Compounds Using
Porphyrins
Elizaveta Ermakova 1,* and Alla Bessmertnykh-Lemeune 2,*
1 Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninsky Pr.
31-4, Moscow 119071, Russia; dr.evermakova@phyche.ac.ru
2 CNRS, ENS de Lyon, LCH, UMR 5182, 69342, Lyon Cedex 07
* Correspondence: dr.evermakova@phyche.ac.ru (E.V.E.); alla.lemeune@ens‐lyon.fr (A.B.‐L.)
Abstract: Detection of volatile organic compounds (VOCs) is a rapidly growing research area due
to importance of VOCs in environmental pollution, human health assessment, food quality control,
and homeland security. Optical sensing materials based on porphyrins are particularly appealing
for VOCs detection, owing to availability of porphyrins, their exceptional optical and binding
properties, as well as their photo- and chemical stability. As research and technology continue to
advance, optical sensors involving these materials are expected to play an increasing role in various
applications. This article presents a comprehensive overview of porphyrin-based sensing materials
developed for use as optical sensors for VOCs. First, sensing films composed exclusively of
porphyrin molecules are discussed followed by materials obtained by grafting or incorporation of
porphyrins into organic and inorganic polymer matrices. Considering growing interest to
multianalyte analysis with porphyrin-based sensor arrays, special section is devoted to this area.
Keywords: optical sensors; porphyrins; volatile organic compounds
1. Introduction
Volatile organic compounds (VOCs) are a group of diverse organic chemicals (including
alcohols, amines, carboxylic acids, halogenated compounds, and nitroaromatics) that exhibit low
boiling points and readily evaporate at room temperature. They are commonly found in indoor air
as pollutants, as by-products of waste management and agriculture, as fuel vapors formed during its
storage and transport, in industrial emissions or released by bacteria and other living organisms
including humans. Many of these compounds pose significant risks to the environment and human
health. [1,2] Monitoring VOCs is also crucial in many everyday social tasks, such as human health
assessment or food control, as many products and living organisms emit specific odors. [3,4] Thus,
there is a growing interest in developing efficient sensors for VOCs. [5–7]
Detection of various chemical compounds has developed within a vast and interdisciplinary
field of knowledge, playing a crucial role in daily human life and applied to different types of
analyses, ranging from qualitative recognition of target analytes to quantification and real-time
monitoring. Many analytical laboratory instruments, such as Atomic Absorption Spectroscopy
(AAS), Inductively Coupled Plasma Spectrometry (ICP), and Gas Chromatography-Mass
Spectrometry (GC-MS), enable complex analyses to be performed within a reasonable timeframe.
While these instruments are indispensable and irreplaceable for our daily comfort, they are also
expensive, require experienced operators, and cannot be used in the field. In many cases, the target
analysis does not necessitate all the capabilities these methods offer but instead requires a precise
assessment of the presence or quantity of a target analyte in a routine manner. This has prompted
numerous studies on simplified sensing devices based on various signal transduction schemes, such
as chemiresistors, mass transducers, electrochemical, photoelectrochemical and optical sensors.
Modern sensors are generally electronic devices that generate signals in response to the presence
of an analyte. Such devices logically consist of two main components: a sensing compound or
material (the receptor) and a transducer which can involve complicated signal-processing software.
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2
Optical sensors utilize light to gather information about the analyte.[8] The most of these devices
deliver these data due to the change of light absorption or luminescence, but sensors employing other
spectroscopies as well as other optical parameters such as refractive index and reflectivity, have also
been developed.[9] Optimized non-invasive optical sensors can achieve high sensitivity and
selectivity, providing rapid response and enabling remote sensing using portable devices. This allows
for real-time monitoring and on-site measurements in various conditions.
Sensing materials for these devices vary widely and can include metal oxides, polymers, or
organic-inorganic hybrid materials. Molecular chemistry contributes to this field by developing
efficient receptors and sensitive transducers, which can sometimes be part of a single molecule.
Although molecular compounds can be dissolved and used for detection as chemosensors in solution,
the solid-state sensors are more convenient due to their high robustness and reliability, and ease of
integration into electronic devices.
Among organic compounds suitable for fabrication of optical sensors, porphyrins are of great
value due to their specific structural and optical properties providing highly sensitive detection,
albite porphyrin-based chemosensors exhibits in general rather low selectivity. Porphyrins absorb
light in different regions of the visible spectrum, primarily due to electronic transitions within their
conjugated π-electron systems. Two distinct types of bands are typically observed: the Soret band
(400–450 nm), corresponding to S0 → S2 transitions, and the Q-bands (500–700 nm), corresponding to
S0 → S1 transitions. Additionally, charge-transfer bands are permitted in complexes where electronic
interactions occur between the d-orbitals of metal atoms (such as Fe and Mn) and the conjugated π-
electron system of the porphyrin ring. This spectral diversity enables multichannel transduction
schemes. High sensitivity of these chemosensors results from their exceptional molar absorptivity in
particular in Soret region and their moderate fluorescence. This enable to increase the detection limits
that is particular important for optic sensors which are typically less sensitive compared to
electrochemical devices. Moreover, many free-base porphyrins and their metal complexes (such as
Pd, Sn, and In) produce long-lived triplet states through an excited-state process known as
intersystem crossing (ISC) being irradiated by visible light. The triplet state of porphyrins is prone to
react with reagents (such as oxygen), which can be utilized in the sensing of these compounds.
There are also numerous ways in which free base porphyrins in the ground state can interact
with an analyte primary due to the presence of a large aromatic macrocycle in these molecules. These
macrocycles often engage in π-π stacking interactions, as well as other weak interactions such as
hydrogen bonding, van der Waals forces, and dipole interactions with the analyte. The periphery of
the macrocycle can be easily modified with functional groups or more complex binding residues,
allowing for specific responses to target analytes.[10] Unfortunately, the common synthetic strategy
for developing chemosensors based on the covalent binding of a receptor and signaling units is
relatively inefficient for porphyrin derivatives. Porphyrin conjugates are typically prepared by
functionalizing meso-positions of the macrocycle using 1,4-phenylene linkers,[11] which are tilted
with respect to the macrocycle's main plane. As a result, the degree of conjugation in such molecules
is relatively low, leading to a reduced optical response to analyte binding compared to other
chromogenic chemosensors. For this reason, many researches in sensing focuses on
metalloporphyrins, which can bind analytes via axial coordination to metal centers located in the
macrocycle cavity. The binding of a ligand or ligand exchange results in a change in symmetry of
molecules that induces the spectral changes observed. This approach is effective for the development
of VOC sensors, as many VOCs are Lewis bases that readily bind to metal ions.
Another interesting feature of bulky porphyrins is that when organized into supramolecular
aggregates in solution or on solid supports, they do not lose their high absorptivity and often remain
emissive. While non-structured aggregates generally absorb and emit less light compared to their
molecular counterparts and partially lose their ability to bind an analyte, structurally ordered
porphyrin aggregates obtained under specific conditions can display intriguing sensing properties
not observed in their molecular precursors. [12–14]
The use of porphyrins and related compounds for chemical sensor applications has been
reviewed in the past. [15–17] The aim of this review is to draw attention to the use of porphyrin
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molecules in VOCs detection in gaseous phase. We describe optical chemical sensors that use these
derivatives as sensing materials emphasizing various strategies for their immobilization. This is a key
step in the fabrication of optical sensors. First, sensing films composed exclusively of porphyrin
molecules are discussed followed by materials obtained by their incorporation into organic and
inorganic polymer matrices. Considering growing interest to multianalyte analysis with array-based
porphyrin sensors, special section is devoted to this area. The detection of other gaseous compounds,
such as oxygen, nitrogen oxides, and ammonia, in which porphyrins also play a significant role, is
beyond the scope of this review. Readers interested in these topics are encouraged to consult
numerous other reviews for a comprehensive understanding. [15,18–25]
2. Sensing Materials Based on Single Porphyrin Derivatives
Porphyrins have gathering significant interest in the detection of VOCs partly because they are
highly sensitive transductors and partly because many metalloporphyrins can axially coordinate
Lewis bases yielding stable complexes. Among VOCs, Lewis bases are frequent and
metalloporphyrins are excellent host for these compounds.
In sensing applications, thin films of porphyrin are often fabricated to increase the number of
binding sites available for interaction with analytes. These films exhibit varying degrees of structural
order, depending on the preparation method. Supramolecular aggregation of porphyrin molecules is
commonly observed in these materials, regardless of the synthesis strategy employed. The optical
and coordination properties of these supramolecular aggregates are influenced by their structure.
Consequently, the selectivity and sensitivity of optical sensors are highly dependent on the method
used to prepare the sensing material. Unfortunately, comparative studies where the same host
molecules are immobilized according different synthetic methods are scarce. Most research focuses
on sensor sensitivity to one or a few VOCs; however, some studies do report multi-analyte analyses,
offering varying levels of insight into the selectivity of these sensors.
Most commonly employed transduction schemes in porphyrin-based sensors involve
absorption and fluorescence spectroscopy, although altering other physical properties have also been
explored. Surface plasmon resonance (SPR), which assesses changes in thickness and swelling of the
thin film upon exposure to the analyte, has garnered increasing attention. Additionally, change of the
refractive index at a metal-dielectric interface and within waveguide configurations have also been
utilized for analyte detection. [26,27] Crystalline porous materials based on porphyrins enable the
use of specific optical methods for signal transduction, which will be discussed below.
2.1. Thin Film Formed From Only Porphyrin Molecules
Porphyrins can be deposited as thin films on solid supports using various experimental
techniques, which significantly influence their structural organization and sensing efficiency. Many
of these methods do not require specific functionalization of the porphyrin molecules, and
commercially available derivatives are typically investigated.
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Dip-coating is a well-established technique for fabricating thin films, offering some control over
the deposition of porphyrin onto solid substrates to produce films with relatively uniform thickness.
The process involves immersing a solid support into a porphyrin solution and then withdrawing it
at a controlled speed. The thickness of the resulting film depends on factors such as solution
concentration, withdrawal speed, solution viscosity, and temperature. The self-assembly of
porphyrin molecules on the solid surface can occur due to electrostatic or other specific weak
interactions, enhancing the uniformity and overall morphology of the film. In this context, highly
charged meso-tetraaryl or meso-tetramethylpyridylporphyrins (anionic and cationic species,
respectively) are of particular interest for this deposition method.
Salleh and co-corkers obtained a dense self-assembled monolayer (SAM) by immersing a quartz
substrate in an aqueous solution of Cu(II) meso-tetra(4-sulfonatophenyl)porphyrin (CuTSPP) for 30
min, followed by lifting the slide at a constant speed of 15 mm min–1.28 The deposition process was
controlled by electrostatic interactions between the negatively charged sulfonate groups and the
crystalline support. Due to the repulsion among the negatively charged porphyrin molecules, the
formation of multilayers was prevented and a monolayer film was obtained (Figure 2). The sensing
properties of this film were studied in a closed chamber equipped with a gas inlet, a two-arm fiber
reflectance probe, and a green LED light source (514 nm). This sensor demonstrated sensitivity to
ethanol (EtOH), 2-propanol (iPrOH), and cyclohexane vapors, yielding a fast optical response with
good reproducibility, despite only small optical changes being observed upon exposure to the
analytes.
Figure 2. AFM images of (a) CuTSPP film obtained by dip coating. Ref. 28. Reproduced with
permission of Elsevier; (b) ZnTPP thin film prepared by spin-coating. Ref. 29. Reproduced with
permission of Elsevier.
Dip-coating can also be used for the deposition of nanoparticles and the preparation of sol-gel
films. For instance, a thin film composed of TiO2 nanoparticles covered by iron(III) meso-
tetraphenylporphyrin chloride (FeTPP) was obtained using a glass slide coated with Poly-L-Lysine
as the solid substrate.30 In comparison to sensing films prepared using a similar procedure from
nanoparticles formed by only porphyrin molecules, the composite material showed higher sensitivity
to EtOH, iPrOH, and acetone, as investigated by fluorescence spectroscopy.
Dip coating also useful for deposition of porphyrin H2TSPP onto optical fibers when combined
with UV irradiation of the porphyrin solution in dichloromethane (CH2Cl2).31 Such fibers have
potential applications in the remote detection of VOCs.32
A drop of a porphyrin solution is sometimes deposited onto a solid support and allowed to
evaporate in the air, resulting in a thin film material.33 Ding, Peng, and their co-workers used this
deposition technique, commonly known as drop casting, to prepare a fluorescent "ON-OFF" sensor
for the detection of diethylchlorophosphate (DCP), a simulant of the nerve agent Sarin. They
deposited a dichloromethane solution of 5,10-(4-ethynylphenyl)porphyrin (H₂DAcPP) onto silica
plates using a syringe, followed by air-drying for 5 h. The sensor based on this composite material
demonstrated excellent sensitivity, with a detection limit as low as 10 ppt, while also being reversible
and photostable.34
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2.1.2. Spin-Coating
Spin coating is the most widely used method for depositing porphyrins onto solid supports,
thanks to its low-cost equipment and the ability to control film thickness by adjusting spin speed,
duration of the process, and solution concentration. Moreover, spin coating can be applied to a wide
range of solid substrates enabling optimization of sensing properties and the adaptation of sensing
material to various signal transduction methods. Typically, for preparation of optical sensors, diluted
solutions (10–4 – 10–5 M) of porphyrins in chlorinated solvents are deposited onto transparent supports
such as glass, quartz, or Au-covered glass substrates. The resulting films can be dried at elevated
temperatures to remove solvent traces, that may improve molecular packing in the films and
optimize the sensors performance and robustness.35 This technique can be used for the deposition of
molecular compounds, nanoparticles or in preparation of doped polymer matrices and array-based
sensor systems. The sensing materials prepared according this strategy are listed in Table 1 and most
important results are discussed in the corresponding sections.
The thickness of films obtained by spin-coating commonly exceeded 100 nm (Figure 2) and
undesirable porphyrin aggregates often observed in such materials. 29 Their sensing properties can
be improved by tailoring the molecular structure of porphyrins to minimize aggregation within the
films. This can be achieved by using metalloporphyrins with axial ligands (Fe(III), Mn(III), In(III),
Rh(III)), in which π-π stacking is less pronounced. [36,37] Introducing bulky functional groups or
alkyl chains at the macrocycle periphery also diminishes the aggregation of chromophores, providing
improved sensor characteristics. [38,39] Such functionalization of porphyrins can also create specific
binding sites for target VOCs and enhance the solubility of these aromatic compounds in organic
solvents, thereby facilitating material preparation. [40,41]
Another approach was investigated by Roales and co-workers, who synthesized a
triphenylmethane analogue featuring three porphyrinyl residues (ZnTriad, Figure 3).42 In this bulky
molecule, π-π stacking is reduced, leading to better accessibility of Zn centers for guest molecules.
The sensing properties of thin films composed of ZnTPP and ZnTriad were compared studying
volatile amines as analytes. A total of five primary amines were examined: three linear primary
amines of increasing length, one bulky primary amine (tert-butylamine), and one aromatic compound
(Table 1). The exposure of ZnTriad films to these analytes resulted in specific responses toward each
of amine studied, allowing discrimination of analytes due to the difference on their size and basicity.
As it was discussed above, the axial coordination of analytes to the metal centers within the
macrocyclic cavity typically enhances sensor sensitivity, as the resulting complexes are stable and
exhibit specific spectral properties. However, a contrasting situation arises for analytes that are not
strong Lewis bases. Sensors that utilize metalloporphyrins may exhibit lower selectivity for certain
of these analytes compared to those that use free-base porphyrins, as such VOCs can be more easily
bound through weak interactions with the large aromatic tetrapyrrolic macrocycles. This was
observed when acetone and chloroform (CHCl₃) were detected using spin-coated films fabricated
from octaethyl-substituted porphyrins H2OEP and ZnOEP.43 This data explains why both free-base
porphyrins and their metal complexes are employed in sensing materials and in particular array
sensors based on porphyrins.
As shown in Table 1, most reported sensors register changes in light absorbance in the UV-vis
region upon exposure of sensing materials to VOCs. The changes in both the positions and intensities
of the bands are relatively small, and less pronounced compared to studies conducted in solution.
This was attributed to the strong tendency of porphyrin molecules to aggregate within these
materials and the low diffusion rate of gaseous analytes in non-porous films. Consequently,
considerable attention has been directed toward developing efficient methods of signal treatment.
The simplest approach involves the comparative integration of the Soret band before and after
exposure of the sensor to analyte vapors.36 To perform dynamic analyses of alcohols using a
spectrophotometer with spin-coating film of MgTPP, Kerdcharoen’s group developed in-house
software based on Principal Component Analysis (PCA), one of the most widely used pattern
recognition methods for analyzing gas sensors.35 The acquired data were analyzed in real time for the
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identification of methanol (MeOH), EtOH, and iPrOH. This sensor exhibited varying sensitivities for
these analytes, with the highest response observed for MeOH.
Spadavecchia and coworkers quantified analytes by comparing the relative variations of the
absorbance integral within specific wavelength intervals. [29,39] When the spectral data were treated
in this manner, a linear dependence of the ZnTPP-based sensor response on 4-aminophenol
concentrations was observed in the 5–40 ppb concentration range of the analyte.29 The remarkable
sensitivity, reversibility, and reproducibility of this sensor highlight the advantages of spin-coating
for the immobilization of porphyrins.
To increase sensitivity using this signal treatment method, two chromophores (porphyrin
CuPBPP and phthalocyanine ZnPc (Figure 3)) that absorb light at different wavelengths were mixed
into the same film.38 Dynamic responses to the presence of VOCs were recorded using four channels
in the 300–700 nm range, each covering 50–100 nm. This method enabled the identification of organic
compounds within complex matrices of VOCs (Table 1).
(a)
(b)
Figure 3. The structure of (A) ZnTriad (B) phthalocyanines used for preparation of drop-casting films
and LB films.
Further development of this approach was reported by Seesaard and coworkers, who developed
optical sensors capable of discriminating among the odors of three pathogenic bacteria.44 This was
achieved by preparing a sensing film composed of two porphyrins (ZnTPP and MgTPP) and
phthalocyanine ZnPC1 (Figure 3). An in-house optical artificial nose system based on light-emitting
diodes and a photodetector was fabricated using commercially available components. It consisted of
a hybrid optical sensor and a data acquisition algorithm involving multichannel signal registration.
Development of optical waveguide sensing is of great importance for performing remote
measurements.45 In a sensor operating according to this principle, a MnTPP film was used, which
was obtained by spin-coating a porphyrin solution onto the surface of a K⁺ exchange glass optical
waveguide with a thickness of 1–2 mm. [46] Linear response to the presence of triethylamine (NEt3)
was observed in the concentration range of 0.1–1000 ppm of the analyte. The response time was only
1.5 s, while the recovery time was 50 s.
The same strategy was employed in the development of sensors for ethylenediamine (EDA)
vapors. [47–49] The optical waveguide was prepared using a spin-coated film of H2TCPP. This sensor
was studied for the detection of eighteen VOCs and exhibited a high response only in the presence
of EDA vapors, with a detection limit of 0.1 ppm.47 When H₂TSPP was immobilized using spin-
coating, the resulting H₂TSPP-magnetite film exhibited higher selectivity compared to the film
containing only porphyrin.50 Recently, H2THPP-based films were prepared by the same method for
further optimization of EDA sensing.49
Surface plasmon resonance (SPR), which has garnered significant attention in the development
of optical sensors due to its high sensitivity,51 is still rarely employed in the development porphyrin-
based sensors.43Tonezzer and co-workers developed planar metal-cladding leaky waveguides, where
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a thin porphyrin film serves as the guiding layer.52 Detection of analytes was achieved by monitoring
changes in the refractive index of the guiding layer upon interaction with the analyte. Using a film
composed of H2TPP molecules, a very fast (t90 < 30 s) linear optical response was observed as a
function of ethanol vapor concentration in air, within the range of 375–3000 ppm concentration range
of the analyte.
Table 1. Spin-coated film for VOCs sensing.
Porphyrin
precursora
Solid Support
Matrix
Optical
response
VOCs
Ref
H2TPP
or CoTPP
or FeTPP
P-doped (100)
silicon wafer
-
UV–vis
spectroscopy
MeOH, EtOH,
iPrOH
37
FeTPP
or MnTPP
Glass
-
UV–vis
spectroscopy
Py, NEt3,
Me2NH
36
CoTPP
silica
-
UV–vis
spectroscopy
EtOH
53
ZnTPP
Quartz
-
UV–vis
spectroscopy
Py, MeOH, ethyl
acetate
40
MgTPP
Glass
-
UV–vis
spectroscopy
MeOH, PrOH,
iPrOH
35
ZnTPP–NO2
Quartz
-
UV–vis
spectroscopy
Py, NEt3,
MeOH, ethyl
acetate
40,41
ZnTriad
Glass
-
UV–vis
spectroscopy
PrNH2, BuNH2,
HexNH2,
PhNH2, tBuNH2
42
MnTPP
K+-exchanged
glass optical
waveguide
-
UV–vis
spectroscopy
NEt3
46
H2TCPP
K+-exchanged
glass optical
waveguide
-
UV–vis
spectroscopy
ethylenediamine
47
H2TMPP
K+-exchanged
glass optical
waveguide
-
UV–vis
spectroscopy
ethylenediamine
48
H2TSPP
glass optical
waveguides
UV–vis
spectroscopy
ethylenediamine
50
H2TSPP
maghemite-
covered glass
optical
waveguides
-
UV–vis
spectroscopy
ethylenediamine
50
H2THPP
TiO2-covered
glass optical
waveguides
-
UV–vis
spectroscopy
ethylenediamine
49
H2OEP
or ZnOEP
Au- covered
glass
-
SPR
CHCl3, acetone
43
H2TPP
Au-covered
glass
-
Reflectance
spectroscopy
EtOH,
52
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H2TPP
or ZnTPP
or CdTPP
Glass
SiO2
Fluorescence
spetroscopy
2,4,6-
trinitrotoluene,
2,4-
dinitrotoluene,
nitrobenzene
54
H2TMPyPP
or
H2DPyPP
Glass
substrate
PMMA
PVP
Fluorescence
spetroscopy
2,4,6-
trinitrotoluene,
2,4-
dinitrotoluene,
nitrobenzene
55
CuTBPP
+ZnPCb
Quartz
-
UV–vis
spectroscopy
MeOH, EtOH,
iPrOH, acetone
39
CuTBPP
+ZnPCb
Quartz
-
UV–vis
spectroscopy
MeOH, EtOH,
iPrOH, PhNEt2,
Py, 2-
bromopyridine,
Hex, acetone,
tBuNH2
38
ZnTPPP+MnT
PP
+ZnPC1b
Glass
-
UV–vis
spectroscopy
EtOH, acetone,
MeCO2H,
acetone, ethyl
acetate,
formaldehyde
44
array of M-
PCN- 222c
M=Ag, Zn, Fe,
Cu, Co
Glass
PDMS
UV–vis
spectroscopy
acetone, CHCl3,
CH2Cl2, EtOH,
hexanal, BuNH2,
tetrahydrofuran,
toluene, 2,4-
dinitrotoluene
56
array of MTPP
M=Mg, Zn
Glass
-
UV–vis
spectroscopy
MeOH, EtOH,
iPrOH, acetone,
MeCO2H,
methyl benzoate
57
a Porphyrin and phthalocyanine structures are shown in Figure 1. b Structures of phthalocyanines are illustrated
in Figure 3. c PCN-222 is Zr-based MOF with 5,10,15,20-tetrakis(4-carboxy- phenyl)porphyrin linkers.
2.1.3. Vacuum Evaporation
In vacuum evaporation technique, solid porphyrin evaporated at high temperature (300 °C) in
a high vacuum (10–4 Pa) and the vapors are deposited on solid substrate enabling the formation of
thin films on various substrates. This ensures that the deposited film maintains high purity and
uniformity. Depending on the application, post-deposition treatments such as annealing may be
performed to improve film quality, crystallinity, or adhesion. This method has rarely been used for
the preparation of VOC sensing materials because it requires expensive equipment and the film
preparation process is time-consuming. Thin films of H2TPP, FeTPP, and CoTPP porphyrins were
deposited on P-doped (100) silicon wafers.37 These three sensors were compared to the corresponding
films obtained by spin coating in the detection of alcohols (MeOH, EtOH, and iPrOH). The vacuum-
evaporated films were significantly more sensitive and exhibited much faster responses for all
alcohols compared to the spin-coated films. This increased sensitivity was attributed to the high
purity of the vacuum-evaporated films, which did not contain traces of solvent (CHCl3), resulting in
greater reactivity toward analyte molecules.
2.1.4. Glow-Discharge-Induced Sublimation
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Glow-discharge-induced sublimation (GDS) is an alternative to vacuum evaporation,
particularly useful for depositing compounds that are difficult to evaporate using traditional thermal
methods. In this process, sublimation is triggered by a glow discharge within a vacuum chamber
evacuated to approximately 10–4 Pa. The glow discharge occurs when a gas, typically an inert gas like
argon or helium, at a pressure of about 20 Pa, is ionized by applying a high voltage, creating a plasma
that can be sustained at low pressure. As the sublimated material interacts with the substrate in the
presence of the glow discharge, a thin film forms on the solid surface. The energy from the plasma
also enhances the adhesion and orientation of molecules in the deposited films. Materials obtained
through GDS are typically very pure and possess a small thickness – two features that are highly
desirable in sensing applications.
Tonezzer and co-workers use this method to deposit CoTPP onto a silica substrate.53 CoTPP was
also immobilized on the silica substrates using spin-coating (CoTPP-SC) and vacuum evaporation
(CoTPP-VE). The three films exhibited very different morphologies, as shown in Figure 4.
Specifically, CoTPP-VE displayed a flat surface, while CoTPP-SC contained shallow holes formed
due to solvent evaporation. In contrast, the CoTPP-GD film was composed of microsized particles
and exhibited a rough surface. Different supramolecular organization of the films was also observed
using UV–vis spectroscopy. Unfortunately, the sensing properties of these materials were
investigated only briefly. The CoTPP-GD film demonstrated significantly higher sensitivity to EtOH
vapor compared to CoTPP-SC and CoTPP-VE, exhibiting an optical response that was both rapid and
reversible.
Figure 4. SEM images of CoTPP thin films formed by spin-coating (a), vacuum evaporation (b) and
GDS (c). Ref. 53. Reproduced with permission of Elsevier.
2.1.5. Langmuir-Blodgett/Langmuir-Schäfer films
Langmuir-Blodgett (LB) and Langmuir-Schäfer (LS) techniques, [58,59] rely on the
supramolecular organization of molecules at gas–liquid (generally air–water) interfaces applying
surface pressure and subsequent transfer of the monolayers thus formed onto solid supports oriented
perpendicularly (LB) or parallel (LS) to the liquid surface.60 Both hydrophilic and lipophilic solid
supports can be covered by molecular monolayers, and the thickness of the film thus obtained can be
precisely controlled by changing the number of transferred layers. Recent studies have demonstrated
that these techniques are not only valuable for producing thin films from traditional amphiphilic
molecules with long alkyl chains but also yield excellent results in the deposition of complex
functional molecules and nanoparticles. [61–65] When the LB technique is employed to fabricate thin
films from porphyrins, the π–π stacking of the tetrapyrrolic macrocycles often serves as a key driving
force for the supramolecular organization of the films. This organization is further influenced by
various weak interactions between the peripheral substituents and the macrocycle. The structure of
porphyrin molecules should be tailored because not all of these compounds form stable Langmuir
monolayers on the water surface. Moreover, even when compression isotherm at air–water interface
can be obtained three-dimensional aggregates with ill-defined shapes are usually formed both at
water and on solid surfaces. [66–68] However, LB/LS techniques offer greater control over the
structural and optical characteristics of molecular films compared to spin-coating, leading to
improved sensitivity, selectivity, and functionality of sensors. While complete control over the
monolayer structures is not yet possible, practical guidelines are available, and monitoring their
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structure using compression isotherms and fiber-optic spectroscopy (UV–vis and fluorescence)
simplifies the investigation and structural optimization of the films thus obtained. [59,64,69]
LB/LS porphyrin-based films are widely explored as gas sensors [70] and researches related to
VOC analysis summarized in Table 2. As shown in the table, most sensors were prepared using
vertical deposition (LB film), while the LS method was employed much less frequently. UV–Vis
absorption and reflectance spectroscopy are commonly used in signal transduction schemes due to
the exceptional absorption properties of porphyrin molecules. These methods are compatible with a
fiber setup. [3,71] Reflectance anisotropy spectroscopy, which has recently gained significant interest,
[72–74] allows not only the detection of analytes but also provides valuable insights into the structural
organization of the films before and after analyte binding.
Recent research has also focused on the use of SPR in signal transduction. [75,76] SPR-readout
has been shown to be effective for detecting aromatic compounds.76 Comparative studies on the
binding of acetic acid (MeCO2H) and methyl amine (MeNH2) with LS films composed of H2OPP
molecules, utilizing UV–Vis spectroscopy and SPR, have demonstrated that sensor sensitivity is
largely independent of the signal transduction method in these setups.75
Magneto-optical SPR combines the principles of magneto-optical effects with surface plasmon
resonance, allowing for analyte detection by analyzing changes in the refractive index of films as well
as the magneto-optical properties of materials near the surface. This signal transduction technique
was employed to prepare a sensor using LS films of porphyrin dimer Co-H(OEP)₂ (Figure 1). This
sensor demonstrated remarkable sensitivity to all three investigated alcohols (MeOH, EtOH, and
iPrOH), providing a linear response within the range of 1–14 × 10–4 ppm of the analytes despite its
sensing performance varied depending on the structure of the alcohol.
Porphyrin-based LB/LS films allowing naked-eye detection of VOCs were reported only
recently.77 In this study, a 15-layer LS film of ZnTPP on a glass substrate enables detection of Py
visually, despite a relatively small red-shift of only 5 nm being observed after exposing the film to
the analyte vapors for 1 min. The sensitivity of the film to vapors of other amines was not
investigated.
Preparation of highly ordered LB/LS films is essential for developing optical sensors and
remains a primary focus in this field. Despite understanding that the structure of these films is
influenced by the nature of substituents on the periphery of the tetrapyrrolic macrocycle and the
metal centers within it, preparation of well-structured monolayer films is still challenging. [68,77–80]
Several guidelines have been reported to reduce the aggregation of porphyrin molecules in the
monolayers.
The oldest and simplest method involves preparing mixed porphyrin–fatty acid monolayers.
[3,71,81,82] Fatty acids such as arachidic acid (Figure 5) also facilitate the transfer of the monolayer
onto solid substrates, as fatty acids are well-known for their ability to form stable films on solid
substrates.
Another strategy is based on the preparation of LB films using mixtures of porphyrins with an
amphiphilic calix [8]arene derivative (Figure 5). [78,83,84] Comparative studies of LB films prepared
from MEHO (M = Zn, Mn) with and without calixarene have shown that the separation of porphyrin
molecules by this macrocyclic molecules decreases the size of nanoaggregates in the LB films, which
induces an increase in both the magnitude and rate of the optical sensor response. The binding of
amines by these two sensors is reversible and produced spectral changes which are depended on the
amine structure and temperature.
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Figure 5. The structure of compounds used for preparation mixed LB films.
In the case of free-base porphyrins, the formation of dense aggregates in the Langmuir
monolayer can be decreased by adding carboxylic acids to the water subphase. Partial protonation of
the tetrapyrrolic macrocycle significantly changes the supramolecular organization of the monolayer,
yielding rarified layers in which analytes diffuse easily to.85
Metal centers play a crucial role in sensor design, as clearly demonstrated by Dunbar and co-
workers, who compared the optical response of LB films formed by the free-base porphyrin H2EHO
and six metal complexes with this ligand in the presence of ten VOCs (Table 2). [83,86]
Metalloporphyrins with axial ligands form rarified films due to steric hindrance imposed by the
ligand, which enhances diffusion and improves analyte binding.36 However, films composed of
complexes with very strong axial ligands, such as the two hydroxyl ligands in SnEHO, did not show
any response to the analytes. [83,86] The highest optical responses were observed for the Co(II)
complex, which can alter its oxidation state in the presence of certain analytes, resulting in Co(III)
complexes with different light absorption properties.86 The Mg complex produced the largest optical
response in the presence of MeCO2H among the ten studied VOCs, likely due to its transformation
into the free-base porphyrin H₂EHO under acidic conditions, followed by chemisorption of this
analyte.83
Aggregation of porphyrin molecules in LB/LS films can also be decreased by introducing alkyl
chains at the periphery of this macrocyclic molecule. This approach has been widely investigated and
applied to VOC detection. [72,76,87] For instance, Richardson and co-workers compared the
sensitivity properties of LB films obtained from two N-alkyl substituted derivatives of [5,10,15,20-
tetrakis(3-amidophenyl)porphinato]zinc(II) ZnTmBuPP and ZnTmRPP (Figure 1). The porous film
formed from the bulky compound ZnTmRPP exhibited a faster and higher response compared to its
analogue ZnTmBuPP in the detection of a series of primary, secondary, and tertiary amines.87
Interestingly, the rate of porphyrin deposition on solid substrates influences the morphologies
of films and their sensing properties, due to the increased porosity of non-homogeneous films
obtained at ultra-fast deposition rates (1000 mm min⁻¹).88 This was found application in the
development of portable sensors for 2-methylbutan-2-ol, which is shown in Figure 6 and represents
a prototype of a useful portable toxic gas sensors.89 The device consists of a commercial blue LED that
emits light detected by a phototransistor. The phototransistor is coated with an LS film of MEHO
obtained by the ultra-fact deposition. Upon exposure to 2-methylbutan-2-ol, the film undergoes a
subtle shift in its absorbance characteristics, which varies the light intensity received by the
phototransistor. This results in a change in the voltage across the phototransistor when sensor is
exposed to the analyte. Under dynamic conditions, the optical response of this device stabilizes after
approximately 300 sec. Absorbance changes are initially rapid, reaching about half of the maximum
output in under 30 sec. The highly reproducible response increases linearly with the concentration of
the analyte below 41 ppt concentration of the analyte.
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Figure 6. Schematic representation of portable sensing device using LS film of MgEHO as sensing
material. Ref. 89. Reproduced with permission of Elsevier.
It has been reported that the selectivity of sensing by LB/LS films can be increased by covering
these films with protective thin layers containing specific molecules capable of providing size-
selective diffusion. Evyapan and Dunbar have shown that such a layer can be obtained by performing
the deposition of PMMA and carboxylic acid-substituted calix [8]arene (Figure 5) on top of the
H₂EHO LS film.85 This protective LS film served as a barrier to the diffusion of bulky carboxylic acids,
enabling an increase of MeCO2H selectivity.
Sometimes sensing properties of LB/LS films can be improved by their annelation at 50–100 °C.
[81,90] The surface of the film obtained after annealing was found to be smoother, indicating a
significant structural reorganization of the aggregated species transferred onto the glass surface
during the heating process.
Thus, despite some degree of structural organization in LB/LS films, selective detection of VOCs
has not been yet achieved, although the optical signals of different analytes were distinguishable and
useful for their identification. [78,84]
Table 2. Optical sensors based on porphyrins LB/LS films for VOCs determination.
Porphyrin
precursor a
Method of
deposition/Substrate
Optical response
VOCs
Ref
ZnTPP
LS/ glass
UV–vis
spectroscopy
Py
77
MnTPPCl
LB/ glass
Reflectance
spectroscopy
EtOH
90
RuTPP+
arachidic acidb
LB/ glass substrate
Reflectance
spectroscopy
MeOH, EtOH,
iPrOH
81
H2THOPP
LB/ oxidized Si(001)
Reflectance
anisotropy
spectroscopy
EtOH
72
ZnTHOPP
LS/ quartz
Reflectance
anisotropy
spectroscopy
EtOH, Hex, NMe3
73
ZnTHOPP
LB/ quartz
Reflectance
anisotropy
spectroscopy
EtOH, Hex
74
ZnEHO
or ZnEHO
LB/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
78
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+calix [8]-
areneb
HexNH2,
HepCHO, OctOH,
OctNH2, NEt3,
P(OMe)3
ZnEHO
LB/ HMDS-covered
glass
UV–vis
spectroscopy
PrNH2, BuNH2,
PeNH2, HexNH2,
HepNH2, OctNH2
78
H2EHO
LSd/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H, PrCO2H,
PeCO2H
85
H2EHO,
or MgEHO
or SnEHO
or ZnEHO
or ZnEHO+calix
[8]-areneb
LB/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
HexNH2,
HepCHO, OctOH,
OctNH2, NEt3,
P(OMe)3
83
MEHO
M=Mg, Sn,
Zn,Au,Co,
Mn
LB/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
HexNH2,
HepCHO, OctOH,
OctNH2, NEt3,
P(OMe)3
86
MnEHO+calix
[8]-areneb
ZnEHO+calix [8]-
areneb
LB/ hydrophobic
glass
UV–vis
spectroscopy
PrNH2, BuNH2,
PeNH2, HexNH2,
HepNH2,
OctNH2,
NonNH2, NHEt2,
NHPr2, NHBu2,
NHHex2, NEt3,
NPr3
84
ZnTmBuPP
ZnTmRPP
LB/ hydrophobic
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
HepCHO, OctOH,
P(OMe)3, PrNH2,
BuNH2, NHPr2,
NHBu2, NPr3,
NBu3
87
MgEHO
LS/ HMDS-covered
glass
Phototransistor
2-methyl-butan-2-
ol
89
H2EHO
LS/ glass or Au
(Au-coated glass
SPR chip)
SPR and UV–vis
spectroscopy
MeCO2H, MeNH2
75
Co-H(OEP)2
LS/ Au
(Au/Co/Au coated
SPR chip)
Magneto-optical
SPR
MeOH, EtOH,
iPrOH, NMe3
91
ZnTDPP
LB/ gold coated SPR
chip
SPR
benzene, toluene,
ethyl benzene,
xylene
76
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array of
ZnTPP,
ZnPc2c
LB/ quartz
UV–vis
spectroscopy
pyridine, methanol
92
array of
FeTPP,
FeOEP,
each mixed with
arachidic acid
LB/ glass
UV–vis
spectroscopy
2-propanol,
acetone,
cyclohexane,
ethanol.
71
array of
MnOEP
FeOEP
CoOEP
RuOEP, each
mixed with
arachidic acid
LB/ glass
UV–vis
spectroscopy
EtOH, iPrOH,
acetone,
cyclohexane
82
array of
FeTPP,
MnTPP,
CoTMPP,
CoOEP, each
mixed with
arachidic acid
LB/ glass
UV–vis
spectroscopy
fresh capsicum
annum, dried
capsicum annum,
fresh capsicum
minimum
3
array of
H2DmRDAPP,
H2TmOPP,
H2EHO,
H2EHO,
H2TmRPP,
H2A2BCP,
H2TZPP
LB/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
HexNH2,
HepCHO, OctOH,
OctNH2, NEt3,
P(OMe)3
93
array of
H2TXPP,
H2TYPP
H2TPPP-Br,
H2A2B2P
LB/ HMDS-covered
glass
UV–vis
spectroscopy
MeCO2H,
butanone, ethyl
acetate, HexSH,
HexNH2,
HepCHO, OctOH,
OctNH2, NEt3,
P(OMe)3
93
ZnTPP
LS/ glass
UV–vis
spectroscopy
Py
77
MnTPPCl
LB/ glass
Reflectance
spectroscopy
EtOH
90
RuTPP+
arachidic acidb
LB/ glass substrate
Reflectance
spectroscopy
MeOH, EtOH,
iPrOH
81
a The structure of porphyrins are illustrated in Figure 1. b The structures of calix [8]arene and arachidic acid are
present in Figure 5. c The structure of this compound is shown in Figure 3. d Langmuir layers were formed after
adding MeCO2H in the water subphase to reduce the aggregation of H₂EHO in the monolayer.
2.2. Materials Based on Oxide Matrices
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Immobilization of chemosensors into transparent silica matrices is a promising strategy for
preparing sensing materials, owing to the versatility of the sol-gel process. When organosils are
prepared, this process involves the hydrolysis and polycondensation of a siloxane solution in the
presence of chemosensors, which may or may not be functionalized with anchoring siloxane groups.
A more or less porous 3D or 2D materials are formed under these conditions, within which
chemosensor molecules are either encapsulated or covalently bonded to a micro- or mesoporous
silica matrix, allowing analyte molecules to diffuse easily through the film. The accessible range of
chemosensor loading and the porosity of the matrix are highly dependent on experimental
conditions, which should be optimized to achieve the best sensing performance of the material thus
prepared. Functionalized optical fibers for remote spectroscopic detection can also be obtained using
this technique. Immobilization of porphyrins through the sol-gel process is quite challenging due to
their low solubility in the polar protic solvents (such as alcohols and water) generally required for
this process.
Silica monolite containing Co(III) porphyrin Co(MSiTPP) have shown low efficiency in the
detection of pyridine (Py) due to low diffusion of the vapors in this monolithic material. 94 This work
highlights particular importance of 2D materials in detection of gaseous analytes.
To prepare transparent mesoporous silica films on glass slides, spin-coating is a convenient
method because this technique allows fine-tuning of structural parameters of sol-gel films. This can
be achieved by adding surfactants to the solution of porphyrin and siloxane, which is then deposited
on the solid surface. Li and coworkers synthesized sensing films from the porphyrin H2TSiPP
functionalized with siloxane anchoring groups (Scheme 1) and tetraethoxysilane (TEOS) in the
absence and presence of small molecular (cetyltrimethylammonium bromide (CTAB)) and polymeric
(Pluronic F127 (EO106PO70EO106) and P123 (EO20PO70EO20)) surfactants.54 The films exhibited different
mesostructures, being respectively non-porous, hexagonal, and worm-like (Scheme 1).
Scheme 1. Synthesis and schematic representation of sensing materials based on porphyrin H2TSiPP.
Ref.54. Reproduced with permission Royal Chemical Society.
The insertion of zinc and cadmium ions into the incorporated tetrapyrrolic macrocycles occurs
rapidly, yielding the corresponding complexes. Only the mesoporous films revealed a fast
fluorescence response to trace amounts of nitro-containing aromatics (2,4,6-trinitrotoluene (TNT) and
2,4-dinitrotoluene (DNT)). The best results (quenching efficiency close to 60% after 10 s of exposure)
were achieved in the detection of TNT with the CdTPPP-based worm-like film, which enables rapid
diffusion of TNT molecules to the metalloporphyrin residues that can bind them due to coordinative
binding of nitro groups to metal atoms and/or π-stacking between the aromatic macrocycle and this
electron-deficient analyte. Finally, the energy-level matching between the porphyrin molecules and
TNT allows for fluorescence quenching in the presence of the analyte.
To overcome the solubility problems associated with the traditional sol-gel process, the
incorporation of porphyrin into a silica matrix using atmospheric pressure dielectric barrier discharge
(AP-DBD) was investigated by Boscher's group.95 In this method, plasma is generated and maintained
at atmospheric pressure using high voltage, eliminating the need for expensive high vacuum systems.
However, controlling film thickness may be more complex compared to the preparation of films
under vacuum. AP-DBD, which generates low-temperature plasmas, offers the opportunity to work
with heat-sensitive compounds and can be easily adapted for industrial production. Using this
technique, a solution of the porphyrin CrTPP and hexamethyldisiloxane (HMDS) in CH2Cl2 was
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sprayed through an ultrasonic atomizing nozzle onto transparent polyethylene terephthalate foils
placed on the moving stage of an AP-DBD reactor. This setup allows for the prompt exposure of the
deposited liquid layer to the plasma discharge, facilitating the polymerization of the siloxane
precursor. SEM images of the resulting material have shown that the film was porous and composed
of agglomerated nano-spheres. Unfortunately, the sensing ability of the film was explored only
briefly despite their promising morphology. A UV–vis response (a 5 nm hypsochromic shift of the
Soret band) was observed in the presence of triethylamine (NEt3), but it was too small to be visible
by the naked eye. The reported experimental data do not allow for a conclusion regarding whether
all metal centers in the film were accessible to the analyte molecules.
This porphyrin was also incorporated into polyvinylsiloxane matrix, replacing HMDS with
vinyltrimethoxysilane.4 The resulting films were tested as sensors for trimethylamine (NMe3), NEt3,
and dimethylamine (NHMe2) under both static and dynamic flow conditions. Notably, the sensor
was able to detect concentrations as low as 10 ppm of NEt3 under dynamic conditions and was
successful in monitoring fish spoilage. However, once again, the color changes were too subtle for
detection by the naked eye.
To optimize the film porosity, a mixture of isomers of sterically bulky CrDMDP porphyrins was
embedded onto HMDS matrix using AP-DBD technique.96 Porous films based on CrDMDP was
sensitive to NEt3; however, the ultimate objective of naked-eye detection has not yet been achieved.
Carboxylate-substituted porphyrins can also be adsorbed on TiO2 support. Although
carboxylate anchoring groups yield less stable materials compared to phosphonate anchors,97 their
stability is adequate for sensing gaseous analytes. Zn(II) 5,10,15,20-tetra(3-carboxyphenyl)porphyrin
(ZnTmCPP) and Zn(II) 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (ZnTCPP) were anchored onto
porous microcolumnar TiO2 thin films (with a thickness of 150−400 nm), yielding structurally
different materials. [98,99] FTIR analysis revealed that ZnTmCPP molecules are bound to the support
by all carboxylic groups, in contrast to those of ZnTCPP, where only one or two of these groups
participate in material formation. As a result, the optical properties of the two materials differ. The
Soret band of the para-substituted derivative in the film is hypsochromically shifted compared to the
solution spectrum, indicating formation of H-aggregates in the film thus obtained. In contrast, grafted
ZnTmCPP remained monomeric due to its planar orientation on the TiO2 surface. Both thin films
exhibited colorimetric responses in the presence of acetone, acetonitrile (MeCN), butylamine
(BuNH2), CHCl3, EtOH, and tetrahydrofuran. The optical response of the ZnTmCPP/TiO₂ was found
to be more pronounced and faster than that of the ZnTCPP/TiO₂ film. The ZnTmCPP/TiO₂ film was
also more sensitive to the nature of the studied analytes.
2.3. Hybrid Materials Based on Organic Polymers
Polymer matrices are widely used to immobilize chemosensors. They offer many advantages
and compare well with sol-gel materials the detection of gaseous analytes. The most commonly used
polymer supports are polyvinyl chloride (PVC), polystyrene, polymethyl methacrylates (PMMA),
and cellulose derivatives. Dyes can be chemisorbed on polymer surfaces, encapsulated within the
polymer matrix, or co-polymerized with inert monomers.
Sensing materials prepared by impregnating WypAll X60 or 100% woven cotton with various
metalloporphyrins were used for the development of portable sensors for EtOH.100 No significant
differences in color were observed between the two supports; however, the nature of the porphyrin
ligand played a key role. Chemisorbed complexes of Deuteroporphyrin IX bis ethyleneglycol (H2DIX)
showed a stronger response compared to those of meso-tetra(4-aminophenyl)porphyrin (H2TAP).
Encapsulation of H2TPP in PVC matrices is performed by dissolving PVC, a plasticizer, the dye,
and sometimes specific additives in THF, then casting this solution onto glass plates. Electrostatic
interactions between H₂TPP and EtOH induce a significant decrease in fluorescence. This process
yields films with a thickness that exceeded 5 μm. H2TPP encapsulated in a PVC matrix is sensitive to
this guest molecule over a wide range of concentrations.101 After optimizing the plasticizer, the doped
polymer enabled a linear response in the range of 1 to 75% of saturated vapor pressure and was
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applied to the determination of EtOH in various types of wines and whisky. The detection limit was
as low as 0.05 vol%, which is rarely achieved with other previously reported optical sensors.
Films of tetramethylpyridinium porphyrin tetrachloride (H2TMPyPP) and 2,3,7,8,12,13,17,18-
octamethyl-5,15-bis- [4-trimethylammoniumylphenyl)porphyrin dichloride (H2DPyPP) in PMMA
and polyvinyl pyrrolidone (PVP) were prepared by spin-coating. The sensing properties of these
films with thickness of ca. 1 mm were investigated with respect to benzene. Membranes containing
encapsulated H2DPyPP were more sensitive but cannot be used for monitoring of benzene in air
where this toxic compound is acceptable in the level of ppb.55
To increase the optical response of sensor, Meyerhoff and co-workers used the monomer – dimer
equilibrium of In(III) complexes of hydroxo(2,3,7,8,12,13,17,18-octaethylporphyrinato)indium(III)
encapsulated in PVC membrane in the presence of sodium tetrakis [3,5-
bis(trifluoromethyl)phenyl]borate.102
Scheme 2. Detection of NEt3 by the dimeric In(III) complex.
This salt with a lipophilic anion enables dimerization of porphyrins within polymers (Scheme
2). The sensing mechanism involves axial coordination of amines to metal centers in the polymeric
matrix, accompanied by the formation of two monomer species. This process induces a red shift of
the Soret band of up to 16 nm due to the significant structural difference between the bridged dimer
and the two monomeric complexes formed after amine binding. The best sensitivity was obtained
using o-nitrophenyl octyl ether as a plasticizer. Depending on the relative partition coefficient into
the polymer film and the ligating properties, different degrees of monomer formation were observed
for eight primary alkylamines and Py studied in this work. With optimized film composition, BuNH2
was detected at a level of 0.1 ppm, while the detection limit for less lipophilic primary amines did not
exceed 10 ppm.
Porphyrin nanotubes prepared by Wang’s method103 were embedded in polydimethylsiloxane
to prepare a thin film containing spatially separated nanotubes suitable for sensing.13 This film was
investigated as a sensing material for NEt3, EtOH, toluene, and acetic acid. The polymeric matrix
demonstrated good permeability to the studied analytes, and the response of the sensing layer was
measured using the differential absorbance method. The analytes were successfully identified;
however, they interacted with the porphyrin molecules through a variety of ways rather than being
incorporated into the inner cavity of the nanotubes.
A serious drawback of the functionalized polymers discussed above is their low porosity. To
increase the surface area of the sensing material, O’Donnell and co-workers prepared crosslinked
polymer membranes by performing the polymerization of free base 5,10,15,20-tetrakis(4-
hydroxyphenyl)porphyrin (H2THPP) and succinyl chloride at THF–CH2Cl2 interfaces, modifying
previously reported methods [104,105] for the preparation of microporous membranes.106
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Unfortunately, the porosity and morphology of the films thus obtained were not characterized, but
their investigation by UV–vis spectroscopy showed a high degree of dye aggregation in this
polymeric matrix. The membranes were then deposited onto glass supports and transformed into
materials containing metalloporphyrin residues by treating them with Zn(II), Cu(II), and Co(II) salts.
These metalloporphyrin membranes were exposed to alcohols (MeOH, EtOH, and iPrOH), ketones
(acetone, butanone, and 2-pentanone), and toxic chlorinated compounds (CH2Cl2, CHCl3, and 1,2-
dichloroethane) vapors to test their efficiency as vapochromic materials using colorimetry. The
spectral changes were rather small, and the absorbance of the films was monitored at two wavelength
(425 nm and 550 nm) before and after exposure to analytes. The percentage difference in absorbance
at each wavelength was calculated and averaged to draw final conclusions. The CuTHPP-based
membrane showed the best discrimination among the three alcohol vapors, while the thin film
containing ZnTHPP enabled efficient detection of MeOH vapors. Sensors based on H2THPP,
CuTHPP, and ZnTHPP exhibited reversible responses toward ketones and membranes containing
H2THPP and ZnTHPP could distinguish between different ketones. In contrast, the CoTHPP-based
membrane was efficient only in the discrimination of chlorinated compounds.
Wang and co-workers prepared a nanoporous membrane by copolymerizing Zn(II) 5,10-bis(4-
aminophenyl)-15,20-diphenylporphyrin (ZnDADPP) with pyromellitic dianhydride (PMDA) and
oxydianiline (ODA) using an electrospinning technique, followed by heating the nanofibers up to 250
°C.107 The structure of the resulting polymer is shown in Figure 7.
Figure 7. Naked-eye sensing of Py using a membrane based on ZnDADPP. Reproduced with
permission from Ref. 107, which is open access article distributed under the Creative Commons
Attribution License (CC BY), MDPI.
The membrane, consisting of these fibers, enables rapid and reversible detection of pyridine
vapor, with a detection limit of 0.041 ppm, achieved due to a color change visible to the naked eye.
Notably, this membrane demonstrated excellent selectivity for Py over common amines (NHEt2,
NEt3, pyrrole, and cyclohexylamine) and other potential gaseous compounds (CO₂ and H₂O). The
selectivity of the membrane for Py was attributed to differences in the basicity of the analytes.
However, it is also important to consider the high stability of the zinc porphyrin complexes with Py,
as well as the low accessibility of the metal centers within this polymeric matrix.
2.4. Supramolecular Assemblies and Metal-Organic Frameworks
Metal-organic frameworks (MOFs) are porous crystalline materials composed of metal ions or
clusters coordinated with organic ligands. These frameworks offer exceptional tunability in their
structural, physical and chemical properties, making them highly promising for various applications,
including gas storage, separation, and catalysis. Their high surface area, abundance of binding sites,
and adjustable pore sizes are particularly advantageous for sensing applications and the removal of
VOCs. MOFs have already found their specific role as optical sensors for gaseous analytes, [108–110]
but their use in the detection of VOCs remains relatively rare. [111–113] Reported studies
demonstrate that these materials hold significant potential for developing optical sensors for VOCs
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with various signal transduction mechanisms, including light absorption, emission, and refractive
index changes. MOFs also show selectivity towards certain VOCs, often based on size exclusion or a
correspondence of hydrophilic-lipophilic balance between MOF and an analyte. However, the main
challenge in this field lies in fabrication of sensing materials that are compatible with the conditions
of target signal transduction schemes, as these materials are often difficult to integrate with existing
devices due to their 3D character, and the technology for growing MOFs on solid supports is still
underdeveloped.
MOFs based on porphyrin ligands not only exhibit high porosity but also demonstrate strong
light absorption in the visible spectrum. Additionally, they efficiently generate singlet oxygen, which
can be utilized for the removal of VOCs through performing photocatalytic oxidation reactions.114
However, porphyrin-based MOFs have been scarcely investigated as VOCs sensors. They have been
reported as sensing elements in optical sensing arrays, which are discussed in the following sections.
Coordination-based supramolecular chemistry offers also a pathway to create a wide variety of
rigid, discrete multi-metal complexes featuring well-defined nanoscale cavities. These compounds
hold significant promise for small-molecule sensing. In optimized structures, analyte retention can
be achieved by adapting the size of the cavity to match the dimensions of the analyte. In such
materials, optical sensing transduction is typically achieved by changes in color or luminescence
upon analyte binding. However, they also enable more sophisticated signal transduction schemes.
Hupp and co-workers reported a proof-of-concept study showing that light diffraction could be used
in sensing of gaseous analytes (the photonic lattice method).114 The self-assembly of the porphyrin
ZnDPyMBP (Figure 8) and [ReCl(CO)5] resulted in the formation of tetra-metallic complex defining
an open-ended nanoscale box with a cavity of approximate dimensions of 18 × 18 × 18 Å . In crystal
form, these cavities align to create one-dimensional channels and this structural organization can be
easily achieved evaporating solutions of this complex. To prepare a sensing material, a
micropatterned elastomeric stamp was covered with this porphyrin-based tetramer. This resulted in
a square-perforated grid covering a few square millimeters and containing several thousand
perforations. The presence of benzene, dioxane, and Py within the pores of this material was detected
and the analytes were distinguished using photonic lattice diffraction measurements.
Figure 8. Schematic representation of molecular squares based on ZnDPyMBP.
3. VOCs Sensing Based on Porphyrin Arrays
The challenge of detecting VOCs arises from the limited number of binding sites in their
molecules, which generally results in their weak interactions with host molecules. These interactions
are predominantly governed by Brønsted or Lewis acid/base forces, making the development of
selective sensors a difficult task. In nature, mammals use around four hundred different receptors to
distinguish among thousands of odorants.115 Inspired by these biological olfactory systems, artificial
array-based sensing strategy has been developed. This strategy employs multiple sensing elements,
each providing a specific yet non-selective response, which work in unison to produce a unique,
combined response for each analyte in a complex mixture. Artificial noses that modulate optical
signals have emerged in this century as promising, low-cost, environmentally friendly, and portable
sensors. Optical arrays based on color changes in dyes, photonic crystals and fluorescence have been
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developed. Recently, sensor arrays utilizing near-infrared reflectance have also gained increasing
interest.116
Colorimetric sensing arrays (CSAs) operate by relying on color changes produced by each
component of the array in response to specific analytes. Commonly sensing arrays include different
classes of dyes, such as Brønsted acidic or basic dyes, Lewis acidic or basic dyes, and chromophores
with large permanent dipoles, which are among the most studied options for colorimetric sensing.
[22,117] This structural diversity of components enhances the differentiation of VOCs. CSAs often
require specific equipment to analyze the color changes, such as flatbed scanners, digital cameras
(including photo cameras), or specialized optical detectors. These devices capture images of the
colorimetric array, allowing for the analysis of color and its intensity, often with the aid of
sophisticated software.
One of the earliest and simplest opto-electronic noses developed by Di Natale and co-workers is
illustrated in Figure 9.118
Figure 9. Schematic representation of opto-electronic noses. Ref. 33. Reproduced with permission of
Elsevier.
This home-made device consisted of a low-cost blue LED and an array of photodiodes. Sensing
films were deposited on one of the internal surfaces of a Plexiglass chamber equipped with gas inlets
and outlets, with each porphyrin film positioned along a specific light path. Four sensing films were
prepared by drop-casting Co(II) and Rh(III) porphyrin complexes (CoTNPP and RhTPP), as well as
two other porphyrinoids: Mn(III) 2,3,7,8,12,13,17,18-octamethylcorrole and 3,17-diethyl-
2,3,7,8,12,18,22,23-octamethylsapphyrin.
In preliminary studies, light absorption changes of sensor components to the presence of various
VOCs such as hexane (Hex), propanal, MeOH, EtOH, acetone, MeCO2H, and NEt3 were evaluated.
The highest optical responses were recorded in the presence of carboxylic acids and amines, although
most of the analytes were detectable with these films. Consequently, these complexes were selected
as working components for the development of an opto-electronic nose, with the output signals
analyzed using the Self-Organizing Map (SOM). Notably, this CSA was composed solely of
porphyrinoids, whereas recent practical colorimetric arrays typically include other organic dyes.
This and other early works, particularly from Suslick's group, [119–122] highlighted both the
interest and complexity of developing opto-electronic noses, prompting further extensive research to
optimize each component of detection systems and analysis methods. These researches have been
recently discussed in excellent reviews. [15,22] Readers interested in these devices are encouraged to
consult these and other reviews, [22,123–125] which detail all the steps involved in developing opto-
electronic noses. In our review, we focus specifically on various strategies for preparing sensing
materials for these devices, as well as highlighting selected remarkable results obtained using sensor
arrays with principally porphyrins as dyes. Porphyrin-based CASs reported recently are summarized
in Table 3.
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Table 3. Recently reported (2018-2024) porphyrin-based CSAs for VOCs sensing.
Porphyrin precursor
Solid support
VOCs
Ref
ZnTPP, MnOEP
BMGa
reverse-phase silica
gel plate
ethanol, ethyl acetate
and acetic acid
116
H2TPP, ZnTPP,
EuTPP, H2TTPS1,
H2TF5PP
Sodium fluoresceina
PMMA plate
p-xylene, isoprene,
styrene, and hexanal
126
H2TPP, CuTPP,
ZnTPP, MnTPP,
PdTPP, H2TPP-Cl,
ZnTPP-Cl, ZnTPP-F,
ZnTMPP
BCGa
reverse-phase silica
gel plate
2,4,5-
Trimethyloxazole
127
FeTPP, NiTPP,
CoTPP, PdTPP,
MnOEP, FeTMPP,
FeIITMPP, MnTSPP,
H2TSPP
Doila, pCarDPMa,
NO2Br2BDPa,
NaiOCH3BDPa, and 4
other dyes
silica gel plate
VOCs formed in the
storage process of
oysters
128
H2TPP, FeTPP,
MnTPP, CuTPP,
VTPP, H2TMPP
FeTMPP,
H2OEP,MnOEP,
PdOEP,
Phenol reda
Bromocresol purplea
reverse-phase silica
gel plate
VOCs formed in the
storage process of
beef
129
PCN-222b, PCN-
222(Ag), PCN-
222(Zn), PCN-
222(Fe), PCN-222(Cu)
PCN-222(Co)
PDMSc membrane
acetone, CHCl3,
CH2Cl2, EtOH,
PenCHO,
BuNH2, THF, toluene,
DNT
56
a The structure of this compounds is illustrated in Figure 10. b PCN-222 – Zr-based MOF with H2TCPP ligands.
PCN-222(M) – Zr-based MOF with MTCPP ligands M = (Ag, Zn, FeCl, Cu, CoCl). c PDMS –
poly(dimethylsiloxane).
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Figure 10. The structures of dyes which was used in CASs. For references see Table 3.
Given that free-base porphyrins and their metal complexes offer specific responses to target
analytes as discussed above, sensor arrays frequently include different metal complexes of the same
tetrapyrrolic ligand, with H₂TPP often being a commonly used choice.
The importance of the substitution pattern of the tetrapyrrolic macrocycle for developing CSAs
was clearly demonstrated by Brittle and co-workers.93 They explored sensing properties of two arrays
based on free-base porphyrins, which were prepared deposited on glass substrates using LB
technique. The analytes they employed for discrimination included octanol (OctOH), hexylamine
(HexNH₂), octylamine (OctNH2), NEt3, octanal, MeCO2H, 1-hexanethiol, 2-butanone, ethyl acetate,
and P(OMe)3. Two arrays, comprising six and four sensing elements respectively, exhibited distinct
sensing behaviors influenced by both the molecular structure of dyes and their packing within the
films. Notably, LB films of porphyrins containing only electron-donating substituents on the
macrocycle periphery produced a significantly stronger response compared to those with both
electron-donating and electron-withdrawing groups, contrasting their performance in solution
detection. This finding underscores the key role of molecular packing in LB films, suggesting that a
porphyrin exhibiting a robust chemosensory response in solution might not perform similarly when
incorporated into a thin film. This study also indicated that fabrication of CSAs based on porphyrins
with electron-donating substituents could be a crucial strategy for developing highly sensitive optical
sensors for VOCs. Nevertheless, this conclusion requires further experimental validation, as the
complexity of these sensor systems and the limited number of examples reported previously may not
completely represent the behavior of all porphyrins in LB/LS sensing films.
Methods of porphyrin immobilization used in the preparation of colorimetric arrays are
generally similar to those discussed in the previous section, with drop-casting being a commonly
utilized technique. Porphyrins were immobilized onto solid supports using diverse substrates such
as reverse-phase silica gel plates, [40,41,118–121,129–132] silica gel plates, [128] polyethylene
terephthalate (PET) foils,133 polyvinylidene fluoride (PVDF) membrane, [134–136] poly(methyl
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methacrylate) (PMMA) plate,126 and filter paper.137 Efficient CAS were also prepared according LB/LS
technique (Table 2) and spin coating (Table 1).
Dyes can also be embedded in polymeric matrices to serve as sensing materials in CAS. Huo
and co-workers incorporated a mixture of chemosensors and 2,4-dinitrophenylhydrazine into a PEG-
100 matrix. The 16-channel CSA thus prepared demonstrated excellent discrimination of nine
aldehydes in a low concentration range (40 ppb to 10 ppm).138
The importance of porosity of sensing materials was highlighted long ago. Suslick and co-
workers utilized the incorporation of dyes into various silica matrices to obtain porous organosil
materials.133 By combining this sol-gel tecknique with rational molecular design of sensing
components and improving data analysis methods, this group developed numerous advanced CSAs.
For example, they achieved sensitivities below 1 ppm for detecting seven different amines (NHPr₂,
HexNH₂, cyclohexylamine, OctNH₂, pyridine, pyrrolidine, homopiperidine) and DMF, using a CSA
with 24 components, of which eighteen were involved porphyrin derivatives.121 In another study,
they developed a 36 channel system capable of differentiating among 19 toxic industrial chemicals,
including four VOCs (NHEt2, EtNH2, NH2NHMe, NMe3), after just 2 min of exposure to analytes at
concentrations considered hazardous to health.133
Functionalized nanomaterials have recently emerged as promising candidates to improve sensor
arrays. Coating nanoparticles with dyes enhances the surface area of materials, which in turn
improves the accessibility of the receptor sites. Gu and co-workers use colloidal crystalline beads
(CCB) (Figure 11) as a solid support for chromophores in developing fluorescent sensor. [139,140]
Colloidal crystals composed of three-dimensional ordered arrays of submicrometer particles, often
formed by self-assembly. In this work, these species were prepared from nanosized silica synthesized
according to a modified Stöber method in a microfluidic device. The CCBs, with diameters of about
300 µm, were initially hydrophobized using trimethoxy(octadecyl)silane and then functionalized
with a series of porphyrins (ZnTPP, SnTPP, ZnTAP, H2TPP, H2TCPP, and H2TAP) using a dip-coating
process.140 This sensor generated fluorescent responses which was displayed for calculation the
absolute difference between the RGB colors of the porphyrins during exposure to gaseous analytes.
Carboxylic acids, ketones, and amines (cyclohexane, ethyl acetate, MeCO2H, acetone, MeOH, EtOH)
were distinguished by this sensor array. Notably, the fluorescent response differed for MeOH and
EtOH, allowing semi-quantitative analysis of EtOH in the concentration range of 10–60 ppm.
Figure 11. HR SEM images of CCB before (a) and after (b) covering with ZnTPP. SEM images is shown
in the inlets. Ref. 140. Reproduced with permission of American Chemical Society.
Pedrose and co-workers utilized TiO2 thin film prepared by glancing angle physical vapor
deposition (GAPVD), as host material for carboxylate-substituted porphyrins.98 This solid support is
transparent in the visible spectrum, non-dispersive, and porous, making it suitable for gas sensing
via UV–visible and IR spectroscopies. Metalloporphyrins were deposited from ethanol solutions
using impregnation, a technique that did not prevent dye aggregation. Porphyrin/TiO2 films, each
containing one of 11 different porphyrins, were exposed to 12 VOCs. Spectral changes observed in
the Soret band region were explored using imaging spectroscopy, generating a recognition pattern
that enables the easy identification of each VOC. This sensor exhibited a rapid (within a few seconds),
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reversible, and reproducible response, that was attributed to significant porosity of the TiO2 film with
a columnar structure and open pores.
Recently, Pedrose and co-workers developed a rapid and straightforward microwave-assisted
method to synthesize nano-sized Zr-based MOFs incorporating meso-tetra(4-
carboxyphenyl)porphyrin residues (PCN-222).56 The insertion of various metal ions (M = Fe, Co, Cu,
Zn, Ag) into the tetrapyrrolic macrocycles within this matrix proceeded smoothly, resulting in
materials labeled as PCN-222(M) (Figure 12).
Figure 12. A) Photographs of suspensions of PCN-222(M) in methanol. B) PXRD patterns of PCN-222
and PCN-222(M). C) SEM images of PCN-222(M). Reproduced with permission from Ref. 56, which
is open access article distributed under the Creative Commons Attribution License (CC BY), Wiley.
These modified MOFs were then used to prepare a CSA. The nano-sized MOFs were dispersed
into poly(dimethylsiloxane) (PDMS) to prepare transparent and flexible membranes. The sensing
properties of these materials were tested against nine different VOCs, including DNT, acetone,
CHCl3, CH2Cl2, THF, BuNH2, hexanal, toluene, and EtOH, as well as gases like hydrogen sulfide,
hydrogen chloride, and ammonia, using UV–vis spectroscopy. The analytes produced distinct
barcode-like identification patterns following 30 minutes of exposure, enabling easy differentiation
between them. The membranes also demonstrated good stability, maintaining their functional
properties for at least three months after fabrication.
Opto-electronic noses hold significant societal importance across various fields. As discussed
earlier, they can be effectively utilized for the quantification of toxic industrial compounds, playing
a critical role in environmental safety and pollution control. They are also crucial for monitoring
human health. [126] For instance, Huo and co-workers developed a CSA specifically designed to
detect lung cancer-related VOCs. [126,134,138] For instance, reported CSA based on a combination of
free-base porphyrins (H2TPP, H2TF5PP, H2MS5PP), two metalloporphyrins (ZnTPP, EuTPP), and
sodium fluorescein. This CSA successfully discriminated among key VOCs such as p-xylene, styrene,
isoprene, and hexanal, which are associated with lung cancer, when their concentrations were varied
within the 50–500 ppb range. 126 This level of sensitivity is essential for early diagnosis and continuous
monitoring of health conditions.
In the recent years, CSA are becoming increasingly popular for control of food freshness during
storage. [127–129,132,136,137,141] The porphyrin dyes are particularly appealing here because being
incorporated in sensing materials they can detect a wide range of specific VOCs released during food
spoilage, including amines, aldehydes, 2,4,5-trimethyloxazole, and other degradation products, even
at very low concentrations. Since VOCs emitted by food products often exhibit complex
compositions, detecting target analytes in these mixtures is essential. Porphyrin-based CSA facilitate
the analyses of complex gaseous mixtures and are suitable for creating portable devices that are
compatible with smartphone cameras.
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4. Conclusions
Detection of volatile organic compounds (VOCs) is a rapidly expanding research field due to
their significance in environmental monitoring, human health assessment, food quality control, and
homeland security. Expanding our knowledge of biosystems and progress of industry are
significantly broadening the scope and applications of VOC analysis.
Porphyrins and related compounds hold considerable promise for the optical sensing small-
molecules. Porphyrins, known for their exceptional light absorption properties, are widely used as
sensing materials in UV–visible and reflectance spectroscopy-based signal transduction, and these
materials hold great promise for the development of portable devices using photodiodes and
smartphone cameras. Although porphyrins have moderate emission, their high light absorption
afforded finally relatively high brightness, making them valuable for sensor development. However,
most fluorescent porphyrin-based sensors currently operate in an ON-OFF mode, which presents
serious limitations for practical applications.
Porphyrins are also excellent building blocks for preparation of porous nanomaterials that can
be employed in design of devices utilizing diverse optical signal transduction mechanisms, including
methods based on changes in refractive index. Furthermore, many free-base porphyrins and their
metal complexes exhibit outstanding thermal, photochemical, and chemical stability, which
surpasses that of most other organic dyes. This stability provides a solid background for developing
robust devices that deliver reliable and reproducible responses over extended periods of use.
The use of porphyrins in VOC detection has advanced significantly in this century, with key
contributions coming primarily from physicists and physical chemists. Modern miniature sensors
can now feature up to 36 channels, enabling the discrimination and quantification of target VOCs in
complex mixtures. However, many of these sensors still rely on readily available porphyrin
derivatives that are not specifically designed to interact with the target analytes. Enhancing the
sensing materials, both at the molecular level and through improved synthetic techniques, is
anticipated to make significant strides in progress of this field.
Author Contributions: The manuscript was written through contributions of all authors. All authors have read
and agreed to the published version of the manuscript.
Funding: This work was supported by Ministry of Science and Higher Education of Russian Federation, CNRS and ENS
de Lyon.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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