Molecular materials for gas sensors and sensor arrays☆

CHAPTER 3

Molecular materials for gas sensors and sensor arrays☆
Antonio de Sajaa Mariluz Rodriguez-Mendeza,b, Jose a

Group UVaSens, Engineers School, University of Valladolid, Valladolid, Spain BioecoUVA Institute, Engineers School, University of Valladolid, Valladolid, Spain

b

3.1 Introduction An electronic nose (e-nose) is a multisensor system, which consists of an array of lowselective sensors combined with advanced mathematical procedures for signal processing based on pattern recognition and/or multivariate data analysis [1–6]. In all cases, the sensing elements have partial specificity. So, they respond to a range of compounds, rather than to a specific chemical species. The sensors are at the hearth of the e-noses. This is the reason why many efforts have been dedicated to the development of sensors with improved specifications mainly in terms of selectivity, reproducibility, and lifetime. A gas sensor is formed by two main parts: The main element is a sensing material that reacts with the volatile molecules causing a change in a certain property; the second part is a transducer, which detects those changes and transforms them in an electronic signal. The nature of the sensing layer is responsible of the selectivity and sensitivity of the sensors, and a large variety of sensing materials have been used in e-noses. The most commonly used sensing materials are catalytic metals and metal oxide semiconductors. However, organic thin layers have also attracted considerable attention as sensing materials because the interaction between some reactive gases and organic thin layers can cause variations in the physical properties of the reactive sensing layers. Organic materials have the advantage of their versatility, and many different families of organic materials such as polymers, porphyrins, or phthalocyanines can be used to obtain sensing layers. In addition, their reactivity can be tuned by modifying chemically their structures or by doping the sensing layers with a variety of materials. An additional advantage is that organic layers can be deposited by simple methods such as drop casting, spraying, spin coating, and printing or by more sophisticated methods such as self-assembling, layer by layer, or Langmuir-Blodgett. Each technique produces films with different structures or porosity, and these differences can also be used ☆

This chapter was written in the memory of Prof. Jos
e Antonio de Saja who passed away in November 2017. He left an immense legacy.

Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00003-7

Copyright © 2020 Elsevier Inc. All rights reserved.

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to modulate the responses. All these techniques can be applied to deposit layers on different types of substrates (conductive, transparent, piezoelectric, flexible, etc.) and used to develop sensors based on different transduction principles [7]. In most of the cases, measures are carried out at room temperature, and organic sensors can be classified as cold sensors. As mentioned before, the transducer transforms the physical changes occurring in the sensing material into an electronic signal. Transduction mechanisms include measures of resistance, mass, and optical properties. In the next paragraphs, the organic materials used in different types of sensors employed in e-noses will be revised.

3.2 Resistive sensors Resistive sensors are usually metal oxide semiconductors (MOS), which are based on inorganic materials. Tin oxide sensors doped with Pt or Pd are the most commonly used resistive sensors in e-noses. In spite of their advantages, their selectivity is low, and they operate at high temperatures. Many organic materials are well known for their conductive properties. A part from the versatility and the flexibility, they have the advantage of being conductors or semiconductors at room temperature. For this reason, the search of molecular semiconducting materials suitable for being used in e-noses is very active. Among the most interesting organic materials exploited in e-noses, we will mention conducting polymers, phthalocyanines and porphyrins, molecular imprinted polymers (MIPs), nanotubes, and their composites. These materials are deposited as thin films on the surface of interdigitated electrodes, and the changes in the resistivity when exposed to gases are measured at room temperature.

3.2.1 Polymers The first polymeric sensors were obtained by dispersing conducting nanofillers (metal or carbon) into insulating polymers. These sensors are called conductive polymer composites (CPC) and have demonstrated their effectiveness for vapor sensing. Arrays of sensing elements are prepared from inexpensive, commercially available polymers, such as polystyrene, polysulfone, polyvinylbutyl, polycaprolactone, polyvinylacetate, polyethyleneimine, and polymethylmethacrylate. Each CPC sensor has a different response depending on the partition coefficient of the analyte. Some examples of this approach can be found in the literature [8–11]. In the last years, nanomaterials such as carbon nanotubes (CNT) or graphene have been incorporated in polymeric nanocomposites with excellent results [12]. Conducting polymers are among the most popular molecular materials used in e-noses due to their unique conducting properties. The changes in the electrical conductance upon exposure to volatile compounds are the basis of their use in e-noses, and many

Molecular materials for gas sensors and sensor arrays

different instruments (including commercial devices) have been developed based on these versatile materials [13]. Conducting polymers are heterocyclic compounds with alternating single and double bonds along the backbone (polypyrrole, polyaniline, poly 3-methyl thiophene, PEDOT, POSS, etc.). They can be n- or p-doped, and this doping generates charge carriers and alters the band structure, thus increasing the mobility. Different types of counterions can be used as dopant agents to obtain polymeric films with diverse physicochemical properties such as conductive or redox properties. In addition, they can be deposited as thin films onto interdigitated electrodes using inkjet deposition, electrodeposition, or electrospinning (among many others) giving rise to films with different structures, hydrophobicity, thickness, and roughness. The electrochemical techniques are very popular to obtain conducting polymers because monomers can be polymerized by applying a certain voltage or a certain current by means of chronoamperometry, chronopotentiometry, or cyclic voltammetry. The experimental conditions used in electrodeposition (time, voltage, and current intensity) can produce films with different porosity and sensitivity. Finally, a variety of doping agents can be introduced in the conducting polymer films during the electropolymerization process [14]. Inject printing is also widely used due to the simplicity, but it has to be taken into account that various factors can affect the quality of the films (materials, substrate treatments, viscosity, etc.) [15]. Other methods such as electrospinning can produce nanofibers with high surface-to-volume ratio [16]. The variety of monomers, counterions, and deposition techniques facilitate to obtain polymeric sensors with cross sensitivity. This versatility has been used to develop e-noses formed by an array of different gas sensors that work at room temperature. They have two disadvantages: first, that the chemoresistive response is strongly influenced by the temperature [17] and, second, their strong response to humidity. In the presence of water vapors, polymeric layers suffer swelling and the consequent change in resistivity. E-noses based on conducting polymers must take into account these facts. Conducting polymer-based e-noses have been used to analyze complex odors. Many works have been devoted to the analysis of foods. For instance, a homemade electronic nose based on Ppy, PANI, and 3-MET deposited by electropolymerization was able to analyze the quality of olive oils [18] or to discriminate olive oils from different varieties of olives [19]. Discrimination of wines using an array of conducting polymer sensors requires the use of solid-phase microextraction (SPME) techniques to eliminate water and alcohol [20]. Polymeric e-noses have been used for many other applications such as the detection of off-odors in automobiles [21]. Gas sensors can detect combustible, explosive, and toxic gases and have been widely used in safety monitoring and process control in residential buildings, industries and mines [11], etc. The great success of conducting polymers in e-noses leads to the development of commercial e-noses. A pioneer e-nose was AromaScan A32S, a conducting polymer electronic nose formed by a 32-sensor array, designed for general-use applications.

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Neotronics was developed by the University of Warwick and was based on 12 conducting polymers [22, 23]. Cyranose, based on conducting polymers, has been the most successful e-nose in the market. It has been used in a variety of medical applications such as the detection of respiratory diseases (lung cancer, asthma, etc.) or digestive diseases [24, 25]. Many works have been dedicated to quality and food control (incoming inspection and verification of bulk chemicals, confirmation of raw materials, ingredients, batch confirmation, contamination, organoleptic analysis, monitoring production, aging, etc.). Studies included fruit juices, wines, and honeys [26–28]. A detailed list of papers published using Cyranose e-nose can be found in [29]. Sensor array platforms based on cross-reactive conducting polymeric sensors can be improved using nanostructured conducting polymers. For instance, a polyaniline polymer nanowire-based chemiresistive sensor array combined with a pattern recognition algorithm was applied for the simultaneous classification and quantification of three chemical species: ascorbic acid, dopamine, and hydrogen peroxide [30]. Similarly, an array of nano-/microstructured-conducting polypyrrole sensors prepared by means of




amperometry and formed by doping with ClO
4 , pTs , Cl , TCA , DS , and DBS was able to detect acetone, methanol, ethanol, 1-propanol, 2-propanol, nitromethane, propylamine, pyridine, and gas mixtures of aliphatic alcohols. Quantitative determinations of the composition of gas mixtures were also successfully achieved [31]. In a very interesting work, a bio-inspired nanofibrous artificial epithelium was combined with the e-nose principles. An array of nine microchemoresistors covered with electrospun nanofibrous structures was prepared by blending doped polyemeraldine (a form of polyaniline) with three different polymers (polyethylene oxide, polyvinylpyrrolidone, and polystyrene), which acted as carriers for the conducting polymer. Such e-nose included a plurality of nanofibers whose electrical parameters were dependent on the tested gases (NO2 and NH3) and on the spatial distribution of the electrospun fibers [32].

3.2.2 Phthalocyanines and porphyrins Phthalocyanines (Pc) and porphyrins (Ppy) are tetrapyrrolic compounds where an aromatic ring is coordinated with transition metals or rare earth metals. They are among the most important organic sensing materials due to their amazing semiconducting, optical, and redox properties. These properties are sensitive to the presence of gas molecules, and changes can be monitored by different transduction methods [33–36]. The possible application of Pc (and in lesser extent PPy) as the sensitive layers in resistive sensors is due to their pi-type semiconducting properties (10
10–10
12 S/cm at 300 K) [37]. Lanthanide bisphthalocyanines (LnPc2) are particularly interesting members of the family of Pcs because they show high intrinsic conductivities (10
6–10
3 S/cm at T ¼ 300 K) [38, 39]. The exposure to gaseous pollutants with strong electron-acceptor properties (O2, NOx, halogens, ozone, etc.) causes an increase in the phthalocyanine

Molecular materials for gas sensors and sensor arrays

conductivity, whereas electron-acceptor gases produce a decrease in the conductivity. Volatile organic compounds (VOCs) such as alcohols, aldehydes, and aromatic compounds, which do not possess a strong electron-donor or electron-acceptor character, can also be detected using phthalocyanines [40, 41]. These excellent sensing properties toward gases and VOCs can be tuned by changing the central metal ion or by introducing substituents in the aromatic ring. In addition, different processing methods can be used to obtain well-controlled structures and nanostructures (drop casting, spin coating, evaporation, or Langmuir-Blodgett among other techniques). Both approaches can help to improve the sensibility and reproducibility of the sensors [41–43]. Using these interesting and varied sensing properties, Pcs have been successfully employed in resistive e-noses dedicated to the analysis of wines [44] or olive oils [45, 46] among other applications.

3.2.3 CNT and graphene resistive sensors In the last years, carbon nanotubes (CNTs) or graphene have been introduced in the formulations of gas-sensing materials [47]. CNTs and graphene are interesting active materials for chemical sensors due to their high carrier mobility, their unique geometry, and their capability to adsorb gases. CNT and graphene solutions can be easily deposited by drop casting, spin coating, or printing producing uniform films with easy scalability [12]. These compounds have a high affinity toward a variety of gases that can be adsorbed at their surface. In addition, CNTs and graphene can establish electrostatic interactions with biomolecules (enzymes, DNA, antibodies, etc.) making sensitive and selective biosensors. As mentioned previously, CNTs or graphene can be combined with polymeric matrices such as poly(caprolactone) (PCL), poly(lactic acid) (PLA), poly(carbonate) (PC), poly(methyl methacrylate) (PMMA), and a biobased polyester to obtain CPC arrays of resistive sensors with enhanced properties. The electrical response of these chemical sensors can be explained by the behavior of polymer swelling upon penetration of volatiles or gases into the subsurface of the CNT/polymer film. These arrays of CNT-CPC transducers have been combined with classical pattern recognition methods, producing interesting properties for vapor recognition [48, 49]. For instance, an e-nose system based on polymer/carboxylic-functionalized single-walled carbon nanotubes (SWNT-COOH) was developed and used to detect volatile amines and sun-dried fish odors. Polymers included polyvinyl chloride (PVC), cumene terminated polystyrene-co-maleic anhydride (cumene-PSMA), poly(styrenecomaleic acid) partial isobutyl/methyl mixed ester (PSE), and polyvinylpyrrolidon (PVP) [50]. An additional advantage of CPC sensors is that they can be deposited on flexible substrates or in fabrics. This has allowed developing an innovative wearable electronic nose based on CNT-polymer composites integrated in a Sigsbee wireless system that was able to analyze armpit odor [51, 52].

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CNTs have been combined with other matrices such as chitosan (CHI). These systems have great potential in the e-nose field because chitosan matrix is sensitive not only to polar vapors such as water and methanol (difficult to obtain with synthetic polymers being mostly nonpolar) but also to nonpolar ones like toluene [53]. E-noses based on carbon nanostructures are treated in depth in Chapter 4.

3.2.4 Combinations of materials in the same layer One of the most fruitfully strategies in the development of sensors with improved characteristics is to combine to more sensing materials in the same layer. In previous sections, we have already presented the idea of introducing nonconducting materials in a layer of a conductive material that can form easily layers on interdigitated electrodes. Examples of conjugated polymer composite (CPC) were presented and mentioned as combinations of CNTs with nonconducting polymers. In other works, it has been established that the association of two conducting materials can induce new sensing properties due to synergistic effects [54]. This strategy has been widely used to develop new improved sensors, and in some cases, these sensors have been included in e-noses. Some examples are shown in next paragraphs. Selectivity and sensitivity in gas detection can be enhanced by using CNT or graphene-based hybrids, where nanocarbons are functionalized covalently or noncovalently with other conducting materials such as conducting polymers, phthalocyanines, or metal nanoparticles to improve the sensitivity or selectivity for a specific analyte. For instance, CNTs have been easily functionalized with different oligomeric silsesquioxanes (POSS) and used to detect lung cancer VOC biomarkers [55]. CNTs can also be functionalized by means of noncovalent interactions, often through π-π interactions with phthalocyanines and porphyrins. These weak interactions allow for facile functionalization with minimal reduction of the CNT conductivity that usually accompanies covalent functionalization. A chemoresistive sensor array fabricated from SWCNTs noncovalently functionalized with metalloporphyrins combined with statistical analyses could accurately classify VOCs [12]. Graphene-based hybrids with noble metals, metal oxides, and conducting polymers have been widely investigated as chemoresistive gas sensors with high sensitivity and selectivity. These systems have not been used in multisensor systems yet, but they have a promising future in e-noses [56]. Reduced graphene oxide (rGO) is an interesting platform for highly sensitive gas sensors. However, the poor selectivity of rGO-based gas sensors remains a major problem. One attempt of developing e-noses using an array of reduced graphene oxide (rGO)-based integrated sensors has been published recently. Each rGO-based device in such an array has a unique sensor response due to the irregular structure of rGO films at different levels of organization, ranging from nanoscale to macroscale [57]. Hybrids of silver nanoparticle-decorated reduced graphene oxide

Molecular materials for gas sensors and sensor arrays

(Ag-RGO) have been prepared with the use of poly(ionic liquid) (PIL) as a versatile capping agent to develop volatile organic compound (VOC) sensors. These results are promising to design e-noses based on high stability chemoresistive sensors for emerging applications such as anticipated diagnostic of food degradation or diseases by the analysis of VOCs considered as biomarkers [58]. Composite sensors for e-nose applications have also been described consisting on combinations of other conductive sensing materials. Conducting polymers doped with different porphyrin derivatives led to a huge variation in response to VOCs [59]. Tobacco types and cigarette brands were discriminated using an e-nose formed by only three sensors based on a novel derivative of thiophene conducting polymer doped with different porphyrins [60].

3.3 Field effect transistors (FET) FET-based sensors have been used for the fabrication of cross-reactive sensor arrays. There are many different FET sensor structures and sensing materials that could result in a myriad of different sensor system combinations [61]. The most common FET sensor arrays are formed by MOSFET sensors using catalytic metal oxide materials [62] other inorganic materials, and nanomaterials such as metal or silicon nanowires are also quite common [63]. Organic field-effect transistors (OFETs) are the focus of increasing attention in organic electronics. This interest is stimulated by the qualities of organic semiconductors including the variety of molecular structures and functionalizations, morphology of the sensing layers, and solution processability. Organic thin-film transistor (OTFT) are a subset of OFETs fabricated using thin films as the sensing layer that is deposited by techniques such as layer by layer. The discrimination capability of OFET (or OTFT) gas sensors can be improved by combining a number of transistors and/or measurement variables in an array. The combinatorial responses of the whole array provide a unique fingerprint pattern to discriminate analytes [64]. Advanced systems for gas discrimination based on OFETs sensors arrays have been described [65]. Recently, OFET transistors using different polycyclic aromatic hydrocarbon (PAH) derivatives as the semiconductor layers were combined to form OFET gas sensor arrays. Individual OFET sensors displayed unique responses to volatile organic compounds (VOCs) of different polarity and aromaticity in terms of a couple of variables such as IDS, μ, VTh, and Ion/Ioff. Thus, the combination of sensors generated a fingerprint for each analyte [66]. A ChemFET sensor array that used three different polymer composite films—poly(ethylene-co-vinyl acetate), poly(styrene-co-butadiene), and poly(9-vinylcarbazole) each

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mixed with a 20% carbon black loading by weight—sprayed onto the device for the detection of organic vapors was used to detect organic vapors [67]. Using polytriarylamines as the active layer in OFETSs could work as effective vapor sensors with high sensitivity and specificity toward acetone, DMMP, methanol, and propan-1-ol [68]. A back-gated OTFT array using pentacene, poly(3-hexylthiophene) (P3HT), and poly(3-octylthiophene) (P3OT) as the active layer was able to discriminate between milk and water [69]. Arrays based on nanostructured polythiophene OTFTs [70] and on polythiophene films with different side chain length and film thickness [71] have been described. Other materials other than polymers have been used in OTFTs; the best examples are phthalocyanines whose charge transport properties make them suitable for use in organic thin-film transistors (OTFTs). Efforts have been made to modulate the MPc solubility and the surface structure, to obtain self-assembled films [72]. OFET arrays can also be constructed using different semiconductors. For instance, an OFET array was constructed by combining phthalocyanines (which are p-type semiconductors) and naphthalene diimides (which are n-type semiconductors) [73]. As expected, both types of semiconductors exhibited opposite responses toward gases. These multi-OFET-multiparameter-based arrays are promising, but to construct such arrays is complicated. A different strategy to develop e-noses is to use the multiple parameters of the transistors, to develop a multiparametric-but-single-OFET device. This approach has been presented as a proof of concept [74]. In summary, OFET and OTFT sensory arrays have the advantage of the inherent multiparametric feature of the transistors and also by the variety of organic semiconductors. However, multidimensional data are difficult to analyze. Moreover, it is technically difficult to integrate an array of different OFET sensors in the same sensory panel. Indeed, efforts need to be made to combine ad hoc-designed OFET arrays and software able to analyze the data.

3.4 Mass sensors Mass gas sensors measure the variation in mass of a thin film of a sensing material when exposed to volatile molecules. The most common gravimetric e-noses are based on microbalances that consist of piezoelectric crystals (usually quartz) coated with sensing materials that can absorb or adsorb analytes. They belong to the category of bulk acoustic wave (BAW) devices where the wave propagates through the substrate. In quartz crystal microbalances (QCM), the changes in their resonant frequency are recorded during exposure to a certain gas. As analytes adsorb to the sensing layer, the added mass reduces the resonant frequency. A second approach uses arrays of microcantilevers, which are similar to those used in atomic force microscopes, covered with a sensing layer. The system registers changes in oscillations of the microelectromechanical (MEM) devices as a

Molecular materials for gas sensors and sensor arrays

measure of the gas adsorption. The third approach is to work with arrays of surface acoustic wave sensors (SAW). They are composed of a piezoelectric substrate with an input (transmitting) and output (receiving) interdigital transducer deposited on top of the substrate. The sensitive membrane is placed between the transducers, and an AC signal is applied across the input transducer generating an acoustic two-dimensional wave that propagates along the surface of the crystal [75]. The number of sensitive organic materials used in e-noses based on gravimetric sensors is very large.

3.4.1 Polymeric absorbing materials Polymers are widely used as sensing materials because they can absorb vapors producing a swelling effect. Polymeric materials with different polarity include polydimethylsiloxane (PDMS), CarboWax (CW), divinyl benzene (DVB), or their combinations (Car/PDMS or DVB/Car/PDMS). A number of siloxane polymers or polythiophene (PHT) derivatives have also been tested in mass sensors. Polymers have been used in an array of QCM polymeric sensors covered with the regioregular poly (3-hexyl thiophene) (rr-P3HT), which was used to detect VOCs evolved from food spoiled with Salmonella typhimurium [76]. Cantilever arrays have also been successfully used in e-noses. For instance, MEMS coated with polymers could be used to monitor spoilage of fishes due to the modification of the cantilever parameters (mass, stiffness, and surface stress). The number of polymers available is too high, and in this work, a minimal set of polymers from a large list of prospective polymers was selected by means of fuzzy subtractive and fuzzy c-means clustering (FSC and FCM) methods [77]. E-noses based on SAW sensors are quite popular and have been used in a variety of applications such as the detection of explosives and analysis of foods [78, 79]. In an interesting work, Fuzzy c-means clustering algorithm was used to obtain an optimal set of polymers [80]. An array of SAW devices coated with commercially available polymers (PDMS, Car/PDMS, or DVB/Car/PDMS) (SAW) was combined with solid-phase microextraction (SPME). This combination has demonstrated to be a very promising strategy for highly sensitive and selective gas detection in the field of food quality control. Using this system, differentiation between apple varieties or ripe and unripe pineapple was achieved [81].

3.4.2 Molecular imprinted polymers (MIPs) Molecular imprinted polymers (MIP) are one of the most promising recognition materials for e-noses using mass sensors. MIPs are highly selective receptors with specific binding sites for a molecule. They are prepared by cross-linking polymers in the presence of the molecule that is used as a template. In this way, the polymeric 3-D cavities are

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complementary to the size and shape of the target analyte. MIPs also provide interaction points around the template molecule. Despite the interest of these compounds, MIPs have some limitations—mainly related to manufacture—that need to be overcome before they spread in analytical market [82]. MIP-QCM sensor arrays have been applied to a large variety of analytical problems such as sensing terpenes in fresh and dried herbs [83] and monitoring composting processes [84].

3.4.3 Mass sensors based on porphyrins and phthalocyanines Porphyrins and phthalocyanines have been successfully introduced as coating materials in mass sensors. This is due to their flexible synthesis and ability to interact with a large number of organic vapors. These interactions are related to the coordination capabilities of the central metal ion and to the establishment of π interactions between aromatic rings. The main feature of such sensors is the dependence of the sensing properties (in terms of selectivity and sensitivity) on the nature of the central metal and on the peripheral substituents. Efforts have been carried out to exploit the chemical properties of phthalocyanine or porphyrin films to develop QCM e-noses [85]. Sensitivity and selectivity can be modulated by the nature of the Pc or Ppy (metal ion and peripheral substituents) and by the physical properties of the sensitive films (structure, morphology, porosity, or thickness) [86]. A QCM e-nose based on porphyrins was developed by the University of Rome Tor Vergata. LibraNose 2.1 sensor array consists of eight 20-MHz AT-cut quartz crystal microbalance sensors coated with either metalloporphyrines or polypyrrole polymer films. This e-nose has been used in many different applications such as in food analysis including musts from off-vine dried grapes [87], chocolate [88], or strawberry flavors [89] and in biomedical applications including diagnosis of cancer [90]. Phthalocyanines have also proven to be promising recognition elements in QCMbased sensor arrays due to properties afforded by this class of tunable materials [91]. In spite of their interesting properties, they have been studied in lesser extent as QCM elements than porphyrins. In an interesting work, the anionic sulfonate copper phthalocyanine was combined with different cations to obtain different sensors that were used to discriminate VOCs [92].

3.4.4 Alkanethiol self-assembled monolayers Self-assembled monolayers (SAM) are promising materials for thin-film-based sensors. Alkanethiol-based SAMs provide reproducible and ordered thin films to support a range of chemical tail groups. The affinities and kinetics of VOC adsorption on diverse functionalized films have been analyzed using BAWs [93]. An innovative e-nose was developed by modifying microcantilevers with SAMs of 4-mercaptobenzoic acid (4-MBA), 6-mercaptonicotonic acid (6-MNA), and 2-mercaptonicotonic acid (2-MNA) that was able to detect explosives [94]. One of the main interest of SAMs modified with

Molecular materials for gas sensors and sensor arrays

different tail groups is that they are ideal supports to immobilize biologic probes (peptides, enzymes, DNA, etc.) [95].

3.4.5 Host-guest materials In the last years, many families of adsorbing materials have been introduced in gravimetric e-noses. The interactions between the sensing material and the gas are based in the principles of the host-guest chemistry, where a host molecule with well-defined cavities is immobilized on the device surface. In many cases, these host molecules are deposited in a polymeric uniform film. One interesting family of host molecules is cyclodextrins, which are barrel-shaped ring structures of glucose units with a hydrophobic cavity. Cyclodextrins can be formed by a different number of glucose units, and these units can be functionalized to tune both the size and the polarity of the cavity. Cyclodextrins mixed with polymers have been used as the sensing materials in SAW devices dedicated to the detection of explosives [96]. Calixarenes, cup-shaped cyclic oligomers, can be adequately functionalized to form effective SAMs on Au multiarrayed microcantilevers. Such multisensors can detect a variety of cations, and the binding properties can be modulated by chemically changing the number of units in the oligomer and the nature of the substituents [97]. In a recent work, a microfabricated sensor array has been developed in which each resonator was coated with different supramolecular monolayers including a calixarene, a porphyrin, a cyclodextrin, and a cucurbituril. Supramolecular monolayers fabricated by Langmuir-Blodgett techniques could be used as multiparameter fingerprint patterns for highly selective detection and discrimination of VOCs [98]. Metal-organic frameworks (MOFs) are interesting materials for use in e-noses due to their high surface areas, reproducibility, and tunability. An interesting example of an e-nose formed by SAW sensors covered with MOFs has been recently published [99]. Due to the number of MOFs to choose, it is a challenge to select the right combination of materials for any given sensing application.

3.5 Optical sensors The interaction between some ambient reactive compounds and organic thin layers can cause variations in the optical properties of the sensing materials. Optical gas sensors have several advantages such as the stability and reproducibility of the optical signals, the high signal-to-noise ratio, and the low energy consumption. Sensing materials used in optical e-noses must be optically active, and changes in absorbance, fluorescence, evanescent wave, etc. can be suitable methods to be applied in optical e-noses. One of the pioneer works in this field consisted in analyzing the changes in the optical properties of a surface coated with catalytic metals when exposed to gases. Then, a light pulse scanned the surface transforming the optical data into electrical signals. Next

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breakthrough was the development of a chemical sensor array formed by optical-fiber bundles. Each bundle was coated with fluorescent materials that reacted with gases, emitting light of different wavelengths. The pattern of colored circles could be processed to yield an “olfactory image” of the sample [100].

3.5.1 Porphyrins and phthalocyanines Metalloporphyrins (MP) and metallophthalocyanines (MPc) are one of the most attractive materials for optical detection. The UV-vis-NIR spectra of these compounds are characterized by well-defined peaks with high molar absorptivity in the Q band that appear in the 500–800-nm region [101]. The large changes that occur in these absorption spectra (particularly in the region of the Q band) during oxidation and reduction have been used to develop optical sensors. A simple UV-vis spectrophotometer can be easily modified to be the transducer for the optical e-nose [102]. The large variety of phthalocyanines and porphyrins derivatives is a clear advantage for this type of e-nose because compounds with different colors, reactivity, and color changes can be used to form the array. Moreover, the presence of substituents can change the solubility, and this expands the choice of methods to form films. The molecular structure of the films and the thickness also modulate the color of the devices, the position of the Q band, and the optical response. A pioneer work demonstrated the possibility of using the color changes that occur in porphyrins when exposed to gases in e-noses [103]. Since then, most of the works in optical e-noses were carried out using porphyrins and phthalocyanines as sensing elements. For instance, using a colorimetric array of sensors obtained by printing nine porphyrins and three pH indicators on silica-gel flat plate was used to discriminate Chinese green teas [104]. Similarly, an optical e-nose has been obtained using five sensors where sol-gel films containing phthalocyanines and porphyrins have been deposited by inkjet printing [105]. In a final example, sensors prepared with nanostructured Langmuir-Blodgett films of zinc porphyrin and zinc phthalocyanine deposited on quartz substrates have been used to form an optical e-nose used to discriminate VOCs [106].

3.6 Conclusions Organic materials are an interesting alternative for e-noses. There is a large variety of materials that can be used as sensing materials with different transduction units. The main results have been obtained with porphyrinoid materials (phthalocyanines and porphyrins), with polymers (adsorbing polymers, MIP, or conducting polymers) and with nanocarbons (CNT and graphene). It has been evidenced that mixtures of materials combined with the utilization of organic and inorganic analog in nanocomposites may allow for improvement of the

Molecular materials for gas sensors and sensor arrays

sensor performance due to synergetic/complementary effects. There is still a long way to improve the performance of sensors, including combination with biomaterials.

Acknowledgments Financial support by MINECO and FEDER (AGL2015-67482-R) and the Junta de Castilla y Leo´n-FEDER (VA-032U13) is gratefully acknowledged.

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