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Optical devices
CHAPTER 8
Optical devices Francisco J. Arregui, Jesus M. Corres, Cesar Elosua, Ignacio R. Matias Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain
8.1 Introduction As it has been already commented all along this book, gas sensing receives great attention due to their use in different fields such as in biomedical applications for breath analysis, in automotive industry for the detection of polluting gases and to efficiently control combustion engines, in industrial production to identify potentially hazardous gas leaks and for indoor air quality supervision, or in environment safety [1–3]. The most straightforward sensing mechanisms of these optical gas sensors are the direct methods, approaches that involve the detection of gases using light and photodetectors without using any other help or interaction with additional materials. Most of these direct methods are based on optical spectroscopy, more exactly the measurement of optical absorption; the main reason is that specific gases to be monitored absorb light at precise wavelengths. The very much used and cited high-resolution transmission (HITRAN) database reports these precise wavelengths for most of the gases of interest with a great detail [4]. The optical absorption techniques of these direct methods include nondispersive infrared (NDIR), differential optical absorption spectroscopy (DOAS), Fourier-transform infrared (FTIR), tunable diode laser absorption spectroscopy (TDLAS), UV absorption, photoacoustic spectroscopy (PAS), cavity ring-down spectroscopy (CRDS), and LIDAR [3]. There are other direct methods, nonabsorptionbased techniques, such as gas chemiluminescence or photoionization detector (PID). Usually, these direct methods are very accurate, highly selective, and sensitive; they also enjoy a higher stability and a much longer lifetime compared with nonoptical methods; in addition, their performance is not deteriorated by the mutable environment. Unfortunately, their use is restricted due to their relative high cost [1]. Indirect optical methods that involve the utilization of a chemical indicator or sensing material perhaps can suffer other issues, such as the poisoning by specific gases of the sensing material. Still, they can enjoy other advantages such as capability of working on convenient wavelengths where the gas has no absorption lines, being highly specific, and additionally, if a proper nonreversible indicator is chosen, they can directly measure dosimetry (total exposure over time). In other words, optical indirect methods can be a good approach to implement low-cost gas sensors. Moreover, if instead of using bulky Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00008-6
Copyright © 2020 Elsevier Inc. All rights reserved.
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sensing materials, advanced nanomaterials are utilized, then, sensing properties can be tuned and enhanced, for instance, the response time of the devices and the selectivity. In this chapter, a short review of optical gas sensors based on nanomaterials will be presented. Some of the main optical sensing mechanisms will be described in the next section. Then, some examples of gas sensing will be also illustrated to end with some concluding remarks.
8.2 Sensing mechanisms Optical gas sensing schemes that make use of sensing materials imply the monitoring of changes of the optical properties of these materials such as refractive index [5–11], color, or luminescence. Changes in refractive index can be monitored by means of evanescent field sensors such as Fabry-Perot and Mach-Zehnder interferometers [5, 7, 9], long-period gratings [10, 11], or other devices where the evanescent field of the core of an optical waveguide interacts with the surrounding sensing film [6, 8]. Since the refractive index of the sensing materials can be also affected by many other causes different to gas concentration, such as temperature, humidity, or pH, one of the difficulties is to find a selective material with no cross sensitivity to other parameters different to the target gas. Changes in color are much easier to observe because if these changes are in the visible range, they can be appreciated even by the naked eye or they just can be recorded by devices massively extended as digital cameras or smartphones. There are many indicators that change color with the presence of a target gas. Unfortunately, sensors based on changes of optical absorption are affected by undesired effects such as fluctuations of the intensity of the light source due to external sources, long-term fading of the indicator, or just even the aging of the light source. To overcome these issues, ratiometric measurements at different wavelengths can be used, at one reference wavelength where optical absorption does not change and at another wavelength where absorption changes due to changes of the target concentration. On the other hand, if changes in luminescence can be applied for optical gas sensing, this is usually the preferred technique because this arrangement overcomes some of the issues presented by the other techniques. For instance, changes in the luminescence of appropriate indicators based on the quenching by oxygen are the most common type of optical oxygen sensors [12]. In the sensing schemes based on luminescence, the sensor head has a light source, typically a blue LED light, that excites the luminescent sensing coating that is in physical contact with the medium where the target gas has to be monitored; see Fig. 8.1. The photodetector collects the reflected excitation light from the sensing coating and the emission light from the luminescent indicator at higher wavelengths, typically in the visible red band. Depending on the gas concentration (oxygen
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Fig. 8.1 On top, sensor head of an optical oxygen sensor; at the bottom, the optical spectra collected by the photodetector.
as an example), the luminescence will be quenched as it is indicated at the bottom of Fig. 8.1. This would be the basic configuration, with just one LED and one photo diode. The sensor head can also incorporate an additional LED at higher wavelengths that does not excite the luminescence emission and acts as a reference for measuring the reflectance from the sensing coating, which can be useful to achieve a more stable long-term reading. Unfortunately, this luminescence intensity-based scheme suffers most of the issues mentioned earlier for colorimetric devices, and it is necessary to apply luminescence lifetime and luminescence phase-modulation techniques [12–16]. The lifetime is the average amount of time a luminophore remains in the excited state following excitation, and it can be measured either in time domain or in frequency domain [16]. In time domain, the sample is excited with a pulse of light, and luminescence decay is observed after the
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Exciting light pulse
l0
Luminescence decay curve Intensity
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l=l0 exp[-(t-t0)/t 2] l=l0 exp[-(t-t0)/t 1] (1) (2)
t0 Time
Fig. 8.2 Luminescence decay curves for two different situations: without the presence of the quencher (1) and with the presence of the quencher (2).
removal of the excitation light, when the light pulse is off; see Fig. 8.2. The fluorescence lifetime is related to the target gas (quencher). This decaying fluorescence follows Eq. (8.1): I ¼ I0 exp ½ðt t0 Þ=τ
(8.1)
where I0 is the maximum fluorescence intensity, just before the light excitation is off, that maximum occurs at time t0. The time variable is t, and τ is the time constant. The parameter τ can be used as a measurement of the fluorescence lifetime, and it can be monitored with low-cost electronics. In addition, these lifetime-based sensors display decisive advantages because undesired effects, such as losses of indicator, do not affect the measurement since the decay time is independent of the luminophore concentration. In fact, luminescence lifetime is an intrinsic property of a luminophore, and it does not depend on luminophore concentration, optical absorption by the sample, sample thickness, fluorescence intensity, and photobleaching: it is affected by the presence of fluorescence quenchers and some other external factors such as temperature [17]. In addition, there are no drifts from long-term light source fluctuations or photodetector sensitivity changes, because what is measured here is the decay time with respect to a relative maximum intensity, I0. Luminescence phase-modulation schemes can be another arrangement for measuring the fluorescence lifetime [16]. Here, instead of measuring the decay time in a direct way, this is measured indirectly: from the phase shift between the excitation light and the fluorescence emission; see Fig. 8.3. In this scheme, the excitation light is generally modulated
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Δf
Intensity
Excitation light
Luminescence emission
Time
Fig. 8.3 Phase modulation technique for measuring luminescence lifetime.
with a sinusoidal signal whose modulation frequency is f and the fluorescence signal generated at the sensor head is measured at the photodetector. To cancel the reflected excitation light from the sensor head, a high-pass optical filter must be included in the photodetector. The phase shift between both signals Δϕ can be detected with low-cost electronics because it is related to the time constant, τ, as follows [16]: tan Δϕ ¼ 2πf τ
(8.2)
In other words, photoluminescence in its different forms, as fluorescence or as phosphorescence, and time-resolved interrogation techniques are usually the most spread techniques for optical gas sensing based on nanomaterials.
8.3 Oxygen sensors Optical oxygen sensors are perhaps the paradigmatic example of sensors based on optical indirect methods with a great commercial success. It is possible to find different schemes of these sensors, but generally, they have in common that the sensing mechanism relays on the changes of the luminescence of a sensing coating that incorporates a luminophore. Although in most of the cases the manufacturers do not provide details of their sensors, these are usually fabricated with organometallic molecules, such as ruthenium or palladium complexes, whose luminescence can be quenched in the presence of molecular oxygen. These devices, depending on the setup, can measure gaseous optical oxygen and also dissolved oxygen (DO), being this last application where these optical sensors are more competitive with respect to the electrochemical alternatives: polarographic and galvanic
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sensors. Both electrochemical methods use electrodes where the dissolved oxygen reacts with the cathode to produce a current. Therefore, these sensors consume oxygen. Moreover, the electrodes have a semipermeable membrane, watertight and gas permeable at the same time, that isolates the electrodes from the environment but allowing the oxygen to pass through. In other words, in order to measure properly, it is necessary a certain flow of liquid across the membrane and for low values of DO in small volume samples, electrochemical sensors could consume a high percentage of the DO in the sample altering the real value of DO. In opposition to this, manufacturers of optical DO sensors claim as one of the best advantages that their sensors do not consume oxygen. A review with the advantages of optical DO sensors with respect to Clark electrode sensors and a list of manufactures of optical DO sensors by Wolfbeis can be found in [18] Optical DO sensors can be presented in different configurations and formats such as rugged probe sensors [19–24], as needle-type sensors [25, 26], as “noninvasive” sensor foils or sensor spots [26–29], and even as multipoint sensor spots [30]. The noninvasive sensor foils are spots or stickers that contain the luminophore and that are placed in contact with the medium (gas or liquid) to be measured, for instance, in the inner surface of a glass vessel or beaker or in the inner part of a plastic package and in general in the inner part of any transparent container. Externally, an optical reader with a light source generates the exciting light that passes through the transparent wall, and the photodetector captures the emitted fluorescent light. This can be achieved using the flash light of a smartphone and its digital camera as is proposed in [31]. There are three issues that can be suffered in these sensing films: leaching of the photoluminescent indicator, photobleaching, and self-quenching. The leaching of the indicator implies that the luminescent intensity can decrease not due to a difference in DO but due to the loss of the indicator when it is immersed in the samples and this obviously compromises the reliability of the sensor. This dye leaking can be solved with an appropriate embedding matrix, and this matrix can be fabricated with, for instance, conventional sol-gel methods. Other possible issue as photobleaching can be due to long-term exposure to high-intensity illumination; this can cause a photochemical modification of the luminophore, preventing the transition of electrons from the ground state to the excited states of the luminophore and backwards and, thus, avoiding the emission of photons. In other words, this provokes a gradual decreasing of the luminescence in the long term. This effect is more complex to solve, and sometimes, the incorporation of antifading agents in the sensing matrix [32] or the utilization of sensing techniques such as the measurement of the luminescence lifetime of luminophores (time-resolved techniques), which were already mentioned earlier [13–16], is necessary. The last of these issues is self-quenching, which is perhaps the most difficult to solve. Quenching consists of the relaxation of electrons to the ground state without emitting photons, which is a decrease of the luminescence, and it can be caused by the presence of the target molecule to be measured (quencher), as in the case of DO sensors. Also, quenching can be due to
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high concentrations of the luminophore, which promote the luminescent particles to collide each other, with the concomitant quenching. In this case, the phenomenon is termed self-quenching. Therefore, in the fabrication of luminescent sensing films, there has to be an optimal distance between the luminophore molecules to achieve the maximum luminescent intensity, which also determines an optimal concentration of the luminophore in the sensing matrix. It is here where nanomaterials and fabrication techniques that allows the control of the fabrication on the nanoscale play a relevant role. For instance, De Acha et al. use the layer-by-layer nanoassembly technique (LbL) to minimize self-quenching [33]. In this work, platinum tetrakis pentafluorophenylporphine (Pt-TFFP), the sensing material, was combined with three different cationic polyectrolytes: poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), and poly(allylamine hydrochloride) (PAH). The spacing between the Pt-TFFP fluorophore films was tuned, therefore minimizing self-quenching, by introducing poly(acrylic acid) (PAA) between the cationic layers. A 700% enhancement was achieved in terms of the relative luminescence intensity value, I/I0, where I0 represents the intensity in the absence of the quencher and I is the emitted intensity at a certain concentration. In fact, the utilization of materials ordered on the nanoscale allows the fabrication of engineered materials that can incorporate several indicators to different targets. For instance, in Ref. [34], the authors report the immobilization in chitosan of three different dyes: platinum tetrakis pentafluorophenyl porphin, a sensitive dye toward oxygen with red color emission; a sensitive pH sensing dye, fluorescein isothiocyanate with green fluorescence emission; and, additionally, 4,40-bis(2-benzoxazolyl) stilbene with blue color emission, which plays the role of an optical reference. In this work [34], a smartphone with camera was used to simultaneous monitoring of both pH and oxygen in a noninvasive way, as the sensor spots mentioned earlier. See, Fig. 8.4, for more details about this work. Since oxygen is one of the most usual materials in biochemical reactions in nature, these oxygen sensors can be used indirectly for additional applications. A good example would be a device to monitor such an important parameter as glucose levels in blood. A classical approach is to combine an oxygen indicator with the glucose oxidase enzyme (GOx) [12], which acts as a catalyzer to oxidize glucose to gluconolactone; therefore, monitoring the difference in oxygen levels, it is possible to monitor indirectly glucose. As an example, in Ref. [35], the authors present a wireless glucose system that uses a smartphone combined with an optical transducer that incorporates oxygen-sensitive polymer dots (Pdots) with glucose oxidase. Real-time in vivo dynamic glucose monitoring in live mice was demonstrated using the smartphone and the implanted Pdot transducer. The optical images of subcutaneous glucose level obtained with the smartphone camera can be utilized to distinguish between euglycemia (5-mM glucose) and hyperglycemia (20-mM glucose) [35]. See Fig. 8.5 for a graphic representation of this system.
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Fig. 8.4 On top, schematic of the O2 and pH dual sensor using a smartphone. At the bottom, color observation of the sensor in various pH and pO2 levels. (Reprinted from W. Xu, et al., Simultaneous color sensing of O2 and pH using a smartphone, Sensors Actuat. B Chem. 220 (2015) 326–330, Copyright (2015), with permission from Elsevier.)
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Fig. 8.5 On the left, schematic representation of the wireless dynamic glucose monitoring in live mice by using and implanted optical transducer and the smartphone; on the right, magnified regions in the images taken by smartphone camera at different blood glucose levels. (Reprinted from K. Sun, et al., Ultrabright polymer-dot transducer enabled wireless glucose monitoring via a smartphone, ACS Nano 12 (6) (2018) 5176–5184, Copyright (2018), with permission from American Chemical Society.)
8.4 Hydrogen sensors Due to the expanded use of hydrogen gas as an energy carrier for its use with fuel cells and as a chemical reactant, hydrogen monitoring is of increasing importance. Hydrogen is colorless, odorless, and tasteless; therefore, it cannot be detected by human senses but unfortunately is highly flammable and potentially explosive and will burn in air at a very wide range of concentrations between 4% and 75% by volume. Hydrogen gas concentration monitoring is important in diverse industrial processes such as the synthesis of ammonia and methanol, the hydration of hydrocarbons, the desulfurization of petroleum products, and the production of rocket fuels [36]. It is essential in power stations because hydrogen can be formed in different places and processes such as the radioactive waste tanks, during plutonium reprocessing, or via the undesired reaction of water with high-temperature reactor materials. In fact, a hydrogen explosion contributed to the nuclear accident at the Fukushima accident in 2011 [36]. For this particular field, hydrogen optical sensors offer unique advantages with respect to their electronic counterparts, such as negligible electrical interference or no risk of ignition from an electrical spark, which is crucial in this application. For all these reasons, hydrogen optical sensors are largely reported in the literature, and palladium is perhaps the most used material for the fabrication of the sensors because the presence of hydrogen produces a decrease in the palladium refractive index. Actually, palladium can absorb up to 900 times its own weight in hydrogen gas at room temperature [37]. The utilization of
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heaters to promote the absorption of hydrogen by palladium is contraindicated due to the potential explosive atmospheres of the workplace. Consequently, one of the challenges of these sensors is to enhance their response time at room temperature, and this can be achieved using diverse approaches that pursue a decrease of the thickness (or size) of the palladium films (or nanoparticles), as in Ref. [6], where alternating nanolayers of palladium and gold allowed the fabrication of sensors capable of responding in 5 s for a concentration of 4% hydrogen. Platinum is other widely extended material for hydrogen sensing because it acts as a catalyst for hydrogen and it is used to combine with other materials such as tungsten oxide, as in Ref. [38], where the gasochromic response of WO3 nanotextured thin films coated with a 2.5-nm catalytic Pt layer upon exposure to hydrogen (H2) gas was investigated. In this work, the Pt/WO3 thin films exhibited gasochromic characteristics, when measured in the visible-NIR (400–900 nm) range. The total absorbance increased 15% when exposed to 0.06% H2 gas in synthetic air [38]. Graphene (G) and the more affordable in price graphene oxide (GO) are other materials that are attracting much attention for hydrogen sensing. In Ref. [39], an optical H2 gas sensor by preparing a GO film on a monolayer of gold nanoparticles (AuNP) was reported. The AuNP monolayer had an absorption maximum around 540 nm, while the annealed GO did not show a clear maximum in the optical spectrum. The optical absorption peak of AuNP blueshifted on exposure to a reducing gas, H2, and redshifted on exposure to an oxidizing gas, NO2.
8.5 NH3 gas sensors Ammonia (NH3) is one of the major air pollutants emitted by the agricultural industry. Sources of ammonia include manure from animal feeding and fertilizer from cropping systems. Sensors with the capability of continuous monitoring of ammonia concentration in air are needed to qualify emissions from agricultural activities and evaluate human and animal health status [40]. One of the nanomaterials with higher potential for ammonia sensing is zinc oxide (ZnO) for its interesting optic and electronic properties [41–44]. For instance, in Ref. [40], a ZnO planar waveguide was deposited by pulsed laser deposition on a side-polished single-mode fiber. The sensor element operation principle is based on a distributed coupling between the fiber mode and the corresponding mode of the metal oxide planar waveguide, which is now called Lossy mode resonance (LMR) devices [45]. In Ref. [40], the change of the spectral behavior of the as-obtained optical channel-dropping filter (the LMR) under gas exposure allows optical detection of NH3 gas molecules at room temperature. There are multitude of forms to use ZnO for NH3 sensing, just to mention a few: nanocrystalline ZnO [41], aminefunctionalized ZnO nanoflakes [43], ZnO:Eu2+ nanoparticles [44], or zinc oxide nanoparticle-incorporated graphene oxide (GO-ZnO) [42].
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In fact, in addition to the earlier mentioned G and GO, reduced graphene oxide (rGO) is also a good compromise between the interesting properties of graphene and the synthesis price and complexity [46]. In Ref. [40], the surface plasmon resonance (SPR) phenomenon was used for the fabrication of an optical fiber sensor, which makes use of a nanocomposite film based on rGO and poly(methyl methacrylate). The sensing probes were prepared by previously depositing copper film onto an unclad portion of the optical fiber followed by coating of PMMA, PMMA/rGO nanocomposite overlayer. Great changes in the optical spectrum were achieved for 10 ppm of NH3 with a sensitivity of almost 1 nanometer (wavelength shift) per ppm, which suggests that the limit of detection is much lower [40]. Korposh et al. [47] report in a highly sensitive optical fiber ammonia gas sensors fabricated via layer-by-layer (LbL) nanoassembly of poly(diallyldimethylammonium chloride) (PDDA) and tetrakis(4-sulfophenyl)porphine (TSPP) onto the surface of the core of a hard-clad multimode fiber that was stripped of its polymer cladding; see Fig. 8.6. The sensitivity of the fiber-optic sensor to ammonia was studied in the
Fig. 8.6 Chemical structures of TSPP and PDDA and a schematic of the LbL deposition of TSPP and PDDA on a multimode optical fiber, which achieves a limit of detection of 0.5 ppm of ammonia. (Reprinted from S. Korposh, S. Kodaira, R. Selyanchyn, F.H. Ledezma, S.W. James, S.-W. Lee, Porphyrinnanoassembled fiber-optic gas sensor fabrication: optimization of parameters for sensitive ammonia gas detection, Opt. Laser Technol. 101 (2018) 1–10, 2018, Copyright (2017), with permission from Elsevier.)
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concentration range of 0.5–50 ppm, and the response and recovery times were less than 3 min, with a limit of detection of 0.5 ppm [47].
8.6 Volatile organic compounds The presence of volatile organic compounds (VOCs) is diverse; they can be biologically generated, even by the human beings. Some foods and beverages emit VOCs; in fact, most scents or odors are VOCs, and the characteristic aroma of a wine, fruit, or oil is due to VOCs. They can be very useful to classify or verify if some specific food is in good condition [48]. The analysis of VOCs in exhaled breath samples represents a new frontier in medical diagnostics because it is a noninvasive and potentially inexpensive way to detect health issues or illnesses such as asthma or lung cancer [49, 50]. On the other hand, many VOCs can also cause adverse health effects although they are commonly used as liquid ingredients in many industrial processes or even in household products where they normally get vaporized at room temperature and can be breathed. VOCs are also present in some workplaces, especially in the chemical industries where synthetic products as paints, wax, or fuels can release toxic vapors. In these cases, it is important to monitor the concentration of the vapors to safeguard the health of the workers and also to keep atmospheric emissions under control to avoid environmental hazards [51]. The accurate measurement of VOCs, which are usually present at low concentrations (parts per million or subparts per million), is usually made by bulky and expensive equipment. Solid-phase microextraction (SPME) techniques are used to collect VOCs at low concentrations for analysis, and then, mass spectrometry techniques are used as protontransfer-reaction mass spectrometry (PTR-MS) and time-of-flight mass spectrometers. Sensors with high selectivity toward a specific VOC are still a technical challenge, and usually, VOC sensors are sensitive to several VOCs, and a higher selectivity is achieved with an array of nonselective sensors. The electronic signal that comes from these sensors can be processed with techniques such as principal component analysis (PCA), artificial neural networks (ANN), or hierarchical cluster analysis (HCA) to achieve the individual and separate analysis of different VOCs [52–56]. These systems based on electronic sensor arrays are named as electronic noses (e-noses) because they try to mimic the human olfaction that functions as a nonseparative mechanism and that detects odors as a global fingerprint; electronic noses are nowadays very relevant, and commercial devices based on electronic noses are increasingly appearing in the market for the detection of chemicals; classification of foods; monitoring of diseases and other medical conditions, such as COPD; or even the detection of explosives. A whole chapter of this book is dedicated to this topic. If electronic noses are based on electronic sensors, arrays of optical sensors that share this same sensing scheme are named as optoelectronic noses [57–60]. Usually, optoelectronic noses are based on colorimetric and fluorometric sensor arrays. These optical
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arrays, based on chemoresponsive dyes and nanoporous pigments, probe the chemical reactivity of analytes, rather than their physical properties. These systems can achieve high sensitivity (often down to parts per billion levels), discrimination among very similar analytes, and fingerprinting of similar mixtures over a wide range of analyte types [61]. Due to the high number of potential applications, there are multitude of examples in the literature of optoelectronic noses [57–74]. A good example of optoelectronic noses are the handheld portable devices designed and studied by Suslick and others [59, 58, 66] to classify food, identify liquors, and detect toxic gases among or even for the detection of explosives among other applications. In Ref. [66], Suslick et al. presents a disposable colorimetric sensor array made from printing various chemically responsive dyes. This cartridge is introduced in a handheld device for on-site assessment and monitoring of the freshness of five meat products: beef, chicken, fish, pork, and shrimp. A commercial CMOS camera enables the real-time collection of colorimetric data. The sensor array shows sensitivity to gaseous analytes, especially amines and sulfides at low parts per billion levels, and achieves discrimination among meat volatiles in terms of meat type and storage time of meat, which enables meat product inspection. In this work, several chemometric approaches are applied including principle component analysis, HCA, and support vector machine analysis. See Fig. 8.7.
Fig. 8.7 Sensing device assembled from a colorimetric sensor array inside a handheld analyzer. (A) Gas sampling from a meat sample into the handheld analyzer (5.0 3.7 1.6 in.3). (B) Top view of the 20-element colorimetric sensor array mounted in a polycarbonate cartridge (3.1 1.1 0.4 in.3). (C) Side view of the cartridge. (D) Sensor array response to five meats. (Adapted with permission from Z. Li, K.S. Suslick, Portable optoelectronic nose for monitoring meat freshness, ACS Sensors 1 (11) (2016) 1330–1335, Copyright (2016) American Chemical Society.)
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8.7 Some other gases It is not intended in this chapter to report all the gases that can be monitored by optical sensors based on nanomaterials, and just some of them will be mentioned here, such as carbon monoxide [75, 76], carbon dioxide [77], hydrocarbons [78–80], hydrogen sulfide [81–83], acetylene [84], nitrogen dioxide [85–87], or nitric oxide [88, 89].
8.8 Concluding remarks Optical gas sensors based on nanomaterials are not so present in the market as their electronic counterparts. Oxygen optical sensors are the dominant technology in dissolved oxygen measurements. They are based on luminescent films, and the most used measurement techniques are time-resolved luminescence. Hydrogen, ammonia, and VOC sensors are devices with a high potential, especially the last ones because VOC sensors can play an important role in the development of optoelectronic noses and they are already present in the market. Since smartphones include light sources (LEDs) and photodetectors (CCD array digital cameras), the number of applications that take advantage of this ubiquitous present devices is rapidly increasing.
Acknowledgements The authors want to acknowledge the support of the Spanish Agencia Estatal de Investigacio´n (AEI) and European Regional Development Fund (FEDER) (TEC2016-79367-C2-2-R).
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Optical devices Francisco J. Arregui, Jesus M. Corres, Cesar Elosua, Ignacio R. Matias Department of Electrical, Electronic and Communications Engineering, Institute of Smart Cities, Universidad Pu´blica de Navarra, Pamplona, Spain
8.1 Introduction As it has been already commented all along this book, gas sensing receives great attention due to their use in different fields such as in biomedical applications for breath analysis, in automotive industry for the detection of polluting gases and to efficiently control combustion engines, in industrial production to identify potentially hazardous gas leaks and for indoor air quality supervision, or in environment safety [1–3]. The most straightforward sensing mechanisms of these optical gas sensors are the direct methods, approaches that involve the detection of gases using light and photodetectors without using any other help or interaction with additional materials. Most of these direct methods are based on optical spectroscopy, more exactly the measurement of optical absorption; the main reason is that specific gases to be monitored absorb light at precise wavelengths. The very much used and cited high-resolution transmission (HITRAN) database reports these precise wavelengths for most of the gases of interest with a great detail [4]. The optical absorption techniques of these direct methods include nondispersive infrared (NDIR), differential optical absorption spectroscopy (DOAS), Fourier-transform infrared (FTIR), tunable diode laser absorption spectroscopy (TDLAS), UV absorption, photoacoustic spectroscopy (PAS), cavity ring-down spectroscopy (CRDS), and LIDAR [3]. There are other direct methods, nonabsorptionbased techniques, such as gas chemiluminescence or photoionization detector (PID). Usually, these direct methods are very accurate, highly selective, and sensitive; they also enjoy a higher stability and a much longer lifetime compared with nonoptical methods; in addition, their performance is not deteriorated by the mutable environment. Unfortunately, their use is restricted due to their relative high cost [1]. Indirect optical methods that involve the utilization of a chemical indicator or sensing material perhaps can suffer other issues, such as the poisoning by specific gases of the sensing material. Still, they can enjoy other advantages such as capability of working on convenient wavelengths where the gas has no absorption lines, being highly specific, and additionally, if a proper nonreversible indicator is chosen, they can directly measure dosimetry (total exposure over time). In other words, optical indirect methods can be a good approach to implement low-cost gas sensors. Moreover, if instead of using bulky Advanced Nanomaterials for Inexpensive Gas Microsensors https://doi.org/10.1016/B978-0-12-814827-3.00008-6
Copyright © 2020 Elsevier Inc. All rights reserved.
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sensing materials, advanced nanomaterials are utilized, then, sensing properties can be tuned and enhanced, for instance, the response time of the devices and the selectivity. In this chapter, a short review of optical gas sensors based on nanomaterials will be presented. Some of the main optical sensing mechanisms will be described in the next section. Then, some examples of gas sensing will be also illustrated to end with some concluding remarks.
8.2 Sensing mechanisms Optical gas sensing schemes that make use of sensing materials imply the monitoring of changes of the optical properties of these materials such as refractive index [5–11], color, or luminescence. Changes in refractive index can be monitored by means of evanescent field sensors such as Fabry-Perot and Mach-Zehnder interferometers [5, 7, 9], long-period gratings [10, 11], or other devices where the evanescent field of the core of an optical waveguide interacts with the surrounding sensing film [6, 8]. Since the refractive index of the sensing materials can be also affected by many other causes different to gas concentration, such as temperature, humidity, or pH, one of the difficulties is to find a selective material with no cross sensitivity to other parameters different to the target gas. Changes in color are much easier to observe because if these changes are in the visible range, they can be appreciated even by the naked eye or they just can be recorded by devices massively extended as digital cameras or smartphones. There are many indicators that change color with the presence of a target gas. Unfortunately, sensors based on changes of optical absorption are affected by undesired effects such as fluctuations of the intensity of the light source due to external sources, long-term fading of the indicator, or just even the aging of the light source. To overcome these issues, ratiometric measurements at different wavelengths can be used, at one reference wavelength where optical absorption does not change and at another wavelength where absorption changes due to changes of the target concentration. On the other hand, if changes in luminescence can be applied for optical gas sensing, this is usually the preferred technique because this arrangement overcomes some of the issues presented by the other techniques. For instance, changes in the luminescence of appropriate indicators based on the quenching by oxygen are the most common type of optical oxygen sensors [12]. In the sensing schemes based on luminescence, the sensor head has a light source, typically a blue LED light, that excites the luminescent sensing coating that is in physical contact with the medium where the target gas has to be monitored; see Fig. 8.1. The photodetector collects the reflected excitation light from the sensing coating and the emission light from the luminescent indicator at higher wavelengths, typically in the visible red band. Depending on the gas concentration (oxygen
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Fig. 8.1 On top, sensor head of an optical oxygen sensor; at the bottom, the optical spectra collected by the photodetector.
as an example), the luminescence will be quenched as it is indicated at the bottom of Fig. 8.1. This would be the basic configuration, with just one LED and one photo diode. The sensor head can also incorporate an additional LED at higher wavelengths that does not excite the luminescence emission and acts as a reference for measuring the reflectance from the sensing coating, which can be useful to achieve a more stable long-term reading. Unfortunately, this luminescence intensity-based scheme suffers most of the issues mentioned earlier for colorimetric devices, and it is necessary to apply luminescence lifetime and luminescence phase-modulation techniques [12–16]. The lifetime is the average amount of time a luminophore remains in the excited state following excitation, and it can be measured either in time domain or in frequency domain [16]. In time domain, the sample is excited with a pulse of light, and luminescence decay is observed after the
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Exciting light pulse
l0
Luminescence decay curve Intensity
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l=l0 exp[-(t-t0)/t 2] l=l0 exp[-(t-t0)/t 1] (1) (2)
t0 Time
Fig. 8.2 Luminescence decay curves for two different situations: without the presence of the quencher (1) and with the presence of the quencher (2).
removal of the excitation light, when the light pulse is off; see Fig. 8.2. The fluorescence lifetime is related to the target gas (quencher). This decaying fluorescence follows Eq. (8.1): I ¼ I0 exp ½ðt t0 Þ=τ
(8.1)
where I0 is the maximum fluorescence intensity, just before the light excitation is off, that maximum occurs at time t0. The time variable is t, and τ is the time constant. The parameter τ can be used as a measurement of the fluorescence lifetime, and it can be monitored with low-cost electronics. In addition, these lifetime-based sensors display decisive advantages because undesired effects, such as losses of indicator, do not affect the measurement since the decay time is independent of the luminophore concentration. In fact, luminescence lifetime is an intrinsic property of a luminophore, and it does not depend on luminophore concentration, optical absorption by the sample, sample thickness, fluorescence intensity, and photobleaching: it is affected by the presence of fluorescence quenchers and some other external factors such as temperature [17]. In addition, there are no drifts from long-term light source fluctuations or photodetector sensitivity changes, because what is measured here is the decay time with respect to a relative maximum intensity, I0. Luminescence phase-modulation schemes can be another arrangement for measuring the fluorescence lifetime [16]. Here, instead of measuring the decay time in a direct way, this is measured indirectly: from the phase shift between the excitation light and the fluorescence emission; see Fig. 8.3. In this scheme, the excitation light is generally modulated
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Δf
Intensity
Excitation light
Luminescence emission
Time
Fig. 8.3 Phase modulation technique for measuring luminescence lifetime.
with a sinusoidal signal whose modulation frequency is f and the fluorescence signal generated at the sensor head is measured at the photodetector. To cancel the reflected excitation light from the sensor head, a high-pass optical filter must be included in the photodetector. The phase shift between both signals Δϕ can be detected with low-cost electronics because it is related to the time constant, τ, as follows [16]: tan Δϕ ¼ 2πf τ
(8.2)
In other words, photoluminescence in its different forms, as fluorescence or as phosphorescence, and time-resolved interrogation techniques are usually the most spread techniques for optical gas sensing based on nanomaterials.
8.3 Oxygen sensors Optical oxygen sensors are perhaps the paradigmatic example of sensors based on optical indirect methods with a great commercial success. It is possible to find different schemes of these sensors, but generally, they have in common that the sensing mechanism relays on the changes of the luminescence of a sensing coating that incorporates a luminophore. Although in most of the cases the manufacturers do not provide details of their sensors, these are usually fabricated with organometallic molecules, such as ruthenium or palladium complexes, whose luminescence can be quenched in the presence of molecular oxygen. These devices, depending on the setup, can measure gaseous optical oxygen and also dissolved oxygen (DO), being this last application where these optical sensors are more competitive with respect to the electrochemical alternatives: polarographic and galvanic
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sensors. Both electrochemical methods use electrodes where the dissolved oxygen reacts with the cathode to produce a current. Therefore, these sensors consume oxygen. Moreover, the electrodes have a semipermeable membrane, watertight and gas permeable at the same time, that isolates the electrodes from the environment but allowing the oxygen to pass through. In other words, in order to measure properly, it is necessary a certain flow of liquid across the membrane and for low values of DO in small volume samples, electrochemical sensors could consume a high percentage of the DO in the sample altering the real value of DO. In opposition to this, manufacturers of optical DO sensors claim as one of the best advantages that their sensors do not consume oxygen. A review with the advantages of optical DO sensors with respect to Clark electrode sensors and a list of manufactures of optical DO sensors by Wolfbeis can be found in [18] Optical DO sensors can be presented in different configurations and formats such as rugged probe sensors [19–24], as needle-type sensors [25, 26], as “noninvasive” sensor foils or sensor spots [26–29], and even as multipoint sensor spots [30]. The noninvasive sensor foils are spots or stickers that contain the luminophore and that are placed in contact with the medium (gas or liquid) to be measured, for instance, in the inner surface of a glass vessel or beaker or in the inner part of a plastic package and in general in the inner part of any transparent container. Externally, an optical reader with a light source generates the exciting light that passes through the transparent wall, and the photodetector captures the emitted fluorescent light. This can be achieved using the flash light of a smartphone and its digital camera as is proposed in [31]. There are three issues that can be suffered in these sensing films: leaching of the photoluminescent indicator, photobleaching, and self-quenching. The leaching of the indicator implies that the luminescent intensity can decrease not due to a difference in DO but due to the loss of the indicator when it is immersed in the samples and this obviously compromises the reliability of the sensor. This dye leaking can be solved with an appropriate embedding matrix, and this matrix can be fabricated with, for instance, conventional sol-gel methods. Other possible issue as photobleaching can be due to long-term exposure to high-intensity illumination; this can cause a photochemical modification of the luminophore, preventing the transition of electrons from the ground state to the excited states of the luminophore and backwards and, thus, avoiding the emission of photons. In other words, this provokes a gradual decreasing of the luminescence in the long term. This effect is more complex to solve, and sometimes, the incorporation of antifading agents in the sensing matrix [32] or the utilization of sensing techniques such as the measurement of the luminescence lifetime of luminophores (time-resolved techniques), which were already mentioned earlier [13–16], is necessary. The last of these issues is self-quenching, which is perhaps the most difficult to solve. Quenching consists of the relaxation of electrons to the ground state without emitting photons, which is a decrease of the luminescence, and it can be caused by the presence of the target molecule to be measured (quencher), as in the case of DO sensors. Also, quenching can be due to
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high concentrations of the luminophore, which promote the luminescent particles to collide each other, with the concomitant quenching. In this case, the phenomenon is termed self-quenching. Therefore, in the fabrication of luminescent sensing films, there has to be an optimal distance between the luminophore molecules to achieve the maximum luminescent intensity, which also determines an optimal concentration of the luminophore in the sensing matrix. It is here where nanomaterials and fabrication techniques that allows the control of the fabrication on the nanoscale play a relevant role. For instance, De Acha et al. use the layer-by-layer nanoassembly technique (LbL) to minimize self-quenching [33]. In this work, platinum tetrakis pentafluorophenylporphine (Pt-TFFP), the sensing material, was combined with three different cationic polyectrolytes: poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), and poly(allylamine hydrochloride) (PAH). The spacing between the Pt-TFFP fluorophore films was tuned, therefore minimizing self-quenching, by introducing poly(acrylic acid) (PAA) between the cationic layers. A 700% enhancement was achieved in terms of the relative luminescence intensity value, I/I0, where I0 represents the intensity in the absence of the quencher and I is the emitted intensity at a certain concentration. In fact, the utilization of materials ordered on the nanoscale allows the fabrication of engineered materials that can incorporate several indicators to different targets. For instance, in Ref. [34], the authors report the immobilization in chitosan of three different dyes: platinum tetrakis pentafluorophenyl porphin, a sensitive dye toward oxygen with red color emission; a sensitive pH sensing dye, fluorescein isothiocyanate with green fluorescence emission; and, additionally, 4,40-bis(2-benzoxazolyl) stilbene with blue color emission, which plays the role of an optical reference. In this work [34], a smartphone with camera was used to simultaneous monitoring of both pH and oxygen in a noninvasive way, as the sensor spots mentioned earlier. See, Fig. 8.4, for more details about this work. Since oxygen is one of the most usual materials in biochemical reactions in nature, these oxygen sensors can be used indirectly for additional applications. A good example would be a device to monitor such an important parameter as glucose levels in blood. A classical approach is to combine an oxygen indicator with the glucose oxidase enzyme (GOx) [12], which acts as a catalyzer to oxidize glucose to gluconolactone; therefore, monitoring the difference in oxygen levels, it is possible to monitor indirectly glucose. As an example, in Ref. [35], the authors present a wireless glucose system that uses a smartphone combined with an optical transducer that incorporates oxygen-sensitive polymer dots (Pdots) with glucose oxidase. Real-time in vivo dynamic glucose monitoring in live mice was demonstrated using the smartphone and the implanted Pdot transducer. The optical images of subcutaneous glucose level obtained with the smartphone camera can be utilized to distinguish between euglycemia (5-mM glucose) and hyperglycemia (20-mM glucose) [35]. See Fig. 8.5 for a graphic representation of this system.
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Fig. 8.4 On top, schematic of the O2 and pH dual sensor using a smartphone. At the bottom, color observation of the sensor in various pH and pO2 levels. (Reprinted from W. Xu, et al., Simultaneous color sensing of O2 and pH using a smartphone, Sensors Actuat. B Chem. 220 (2015) 326–330, Copyright (2015), with permission from Elsevier.)
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Fig. 8.5 On the left, schematic representation of the wireless dynamic glucose monitoring in live mice by using and implanted optical transducer and the smartphone; on the right, magnified regions in the images taken by smartphone camera at different blood glucose levels. (Reprinted from K. Sun, et al., Ultrabright polymer-dot transducer enabled wireless glucose monitoring via a smartphone, ACS Nano 12 (6) (2018) 5176–5184, Copyright (2018), with permission from American Chemical Society.)
8.4 Hydrogen sensors Due to the expanded use of hydrogen gas as an energy carrier for its use with fuel cells and as a chemical reactant, hydrogen monitoring is of increasing importance. Hydrogen is colorless, odorless, and tasteless; therefore, it cannot be detected by human senses but unfortunately is highly flammable and potentially explosive and will burn in air at a very wide range of concentrations between 4% and 75% by volume. Hydrogen gas concentration monitoring is important in diverse industrial processes such as the synthesis of ammonia and methanol, the hydration of hydrocarbons, the desulfurization of petroleum products, and the production of rocket fuels [36]. It is essential in power stations because hydrogen can be formed in different places and processes such as the radioactive waste tanks, during plutonium reprocessing, or via the undesired reaction of water with high-temperature reactor materials. In fact, a hydrogen explosion contributed to the nuclear accident at the Fukushima accident in 2011 [36]. For this particular field, hydrogen optical sensors offer unique advantages with respect to their electronic counterparts, such as negligible electrical interference or no risk of ignition from an electrical spark, which is crucial in this application. For all these reasons, hydrogen optical sensors are largely reported in the literature, and palladium is perhaps the most used material for the fabrication of the sensors because the presence of hydrogen produces a decrease in the palladium refractive index. Actually, palladium can absorb up to 900 times its own weight in hydrogen gas at room temperature [37]. The utilization of
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heaters to promote the absorption of hydrogen by palladium is contraindicated due to the potential explosive atmospheres of the workplace. Consequently, one of the challenges of these sensors is to enhance their response time at room temperature, and this can be achieved using diverse approaches that pursue a decrease of the thickness (or size) of the palladium films (or nanoparticles), as in Ref. [6], where alternating nanolayers of palladium and gold allowed the fabrication of sensors capable of responding in 5 s for a concentration of 4% hydrogen. Platinum is other widely extended material for hydrogen sensing because it acts as a catalyst for hydrogen and it is used to combine with other materials such as tungsten oxide, as in Ref. [38], where the gasochromic response of WO3 nanotextured thin films coated with a 2.5-nm catalytic Pt layer upon exposure to hydrogen (H2) gas was investigated. In this work, the Pt/WO3 thin films exhibited gasochromic characteristics, when measured in the visible-NIR (400–900 nm) range. The total absorbance increased 15% when exposed to 0.06% H2 gas in synthetic air [38]. Graphene (G) and the more affordable in price graphene oxide (GO) are other materials that are attracting much attention for hydrogen sensing. In Ref. [39], an optical H2 gas sensor by preparing a GO film on a monolayer of gold nanoparticles (AuNP) was reported. The AuNP monolayer had an absorption maximum around 540 nm, while the annealed GO did not show a clear maximum in the optical spectrum. The optical absorption peak of AuNP blueshifted on exposure to a reducing gas, H2, and redshifted on exposure to an oxidizing gas, NO2.
8.5 NH3 gas sensors Ammonia (NH3) is one of the major air pollutants emitted by the agricultural industry. Sources of ammonia include manure from animal feeding and fertilizer from cropping systems. Sensors with the capability of continuous monitoring of ammonia concentration in air are needed to qualify emissions from agricultural activities and evaluate human and animal health status [40]. One of the nanomaterials with higher potential for ammonia sensing is zinc oxide (ZnO) for its interesting optic and electronic properties [41–44]. For instance, in Ref. [40], a ZnO planar waveguide was deposited by pulsed laser deposition on a side-polished single-mode fiber. The sensor element operation principle is based on a distributed coupling between the fiber mode and the corresponding mode of the metal oxide planar waveguide, which is now called Lossy mode resonance (LMR) devices [45]. In Ref. [40], the change of the spectral behavior of the as-obtained optical channel-dropping filter (the LMR) under gas exposure allows optical detection of NH3 gas molecules at room temperature. There are multitude of forms to use ZnO for NH3 sensing, just to mention a few: nanocrystalline ZnO [41], aminefunctionalized ZnO nanoflakes [43], ZnO:Eu2+ nanoparticles [44], or zinc oxide nanoparticle-incorporated graphene oxide (GO-ZnO) [42].
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In fact, in addition to the earlier mentioned G and GO, reduced graphene oxide (rGO) is also a good compromise between the interesting properties of graphene and the synthesis price and complexity [46]. In Ref. [40], the surface plasmon resonance (SPR) phenomenon was used for the fabrication of an optical fiber sensor, which makes use of a nanocomposite film based on rGO and poly(methyl methacrylate). The sensing probes were prepared by previously depositing copper film onto an unclad portion of the optical fiber followed by coating of PMMA, PMMA/rGO nanocomposite overlayer. Great changes in the optical spectrum were achieved for 10 ppm of NH3 with a sensitivity of almost 1 nanometer (wavelength shift) per ppm, which suggests that the limit of detection is much lower [40]. Korposh et al. [47] report in a highly sensitive optical fiber ammonia gas sensors fabricated via layer-by-layer (LbL) nanoassembly of poly(diallyldimethylammonium chloride) (PDDA) and tetrakis(4-sulfophenyl)porphine (TSPP) onto the surface of the core of a hard-clad multimode fiber that was stripped of its polymer cladding; see Fig. 8.6. The sensitivity of the fiber-optic sensor to ammonia was studied in the
Fig. 8.6 Chemical structures of TSPP and PDDA and a schematic of the LbL deposition of TSPP and PDDA on a multimode optical fiber, which achieves a limit of detection of 0.5 ppm of ammonia. (Reprinted from S. Korposh, S. Kodaira, R. Selyanchyn, F.H. Ledezma, S.W. James, S.-W. Lee, Porphyrinnanoassembled fiber-optic gas sensor fabrication: optimization of parameters for sensitive ammonia gas detection, Opt. Laser Technol. 101 (2018) 1–10, 2018, Copyright (2017), with permission from Elsevier.)
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concentration range of 0.5–50 ppm, and the response and recovery times were less than 3 min, with a limit of detection of 0.5 ppm [47].
8.6 Volatile organic compounds The presence of volatile organic compounds (VOCs) is diverse; they can be biologically generated, even by the human beings. Some foods and beverages emit VOCs; in fact, most scents or odors are VOCs, and the characteristic aroma of a wine, fruit, or oil is due to VOCs. They can be very useful to classify or verify if some specific food is in good condition [48]. The analysis of VOCs in exhaled breath samples represents a new frontier in medical diagnostics because it is a noninvasive and potentially inexpensive way to detect health issues or illnesses such as asthma or lung cancer [49, 50]. On the other hand, many VOCs can also cause adverse health effects although they are commonly used as liquid ingredients in many industrial processes or even in household products where they normally get vaporized at room temperature and can be breathed. VOCs are also present in some workplaces, especially in the chemical industries where synthetic products as paints, wax, or fuels can release toxic vapors. In these cases, it is important to monitor the concentration of the vapors to safeguard the health of the workers and also to keep atmospheric emissions under control to avoid environmental hazards [51]. The accurate measurement of VOCs, which are usually present at low concentrations (parts per million or subparts per million), is usually made by bulky and expensive equipment. Solid-phase microextraction (SPME) techniques are used to collect VOCs at low concentrations for analysis, and then, mass spectrometry techniques are used as protontransfer-reaction mass spectrometry (PTR-MS) and time-of-flight mass spectrometers. Sensors with high selectivity toward a specific VOC are still a technical challenge, and usually, VOC sensors are sensitive to several VOCs, and a higher selectivity is achieved with an array of nonselective sensors. The electronic signal that comes from these sensors can be processed with techniques such as principal component analysis (PCA), artificial neural networks (ANN), or hierarchical cluster analysis (HCA) to achieve the individual and separate analysis of different VOCs [52–56]. These systems based on electronic sensor arrays are named as electronic noses (e-noses) because they try to mimic the human olfaction that functions as a nonseparative mechanism and that detects odors as a global fingerprint; electronic noses are nowadays very relevant, and commercial devices based on electronic noses are increasingly appearing in the market for the detection of chemicals; classification of foods; monitoring of diseases and other medical conditions, such as COPD; or even the detection of explosives. A whole chapter of this book is dedicated to this topic. If electronic noses are based on electronic sensors, arrays of optical sensors that share this same sensing scheme are named as optoelectronic noses [57–60]. Usually, optoelectronic noses are based on colorimetric and fluorometric sensor arrays. These optical
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arrays, based on chemoresponsive dyes and nanoporous pigments, probe the chemical reactivity of analytes, rather than their physical properties. These systems can achieve high sensitivity (often down to parts per billion levels), discrimination among very similar analytes, and fingerprinting of similar mixtures over a wide range of analyte types [61]. Due to the high number of potential applications, there are multitude of examples in the literature of optoelectronic noses [57–74]. A good example of optoelectronic noses are the handheld portable devices designed and studied by Suslick and others [59, 58, 66] to classify food, identify liquors, and detect toxic gases among or even for the detection of explosives among other applications. In Ref. [66], Suslick et al. presents a disposable colorimetric sensor array made from printing various chemically responsive dyes. This cartridge is introduced in a handheld device for on-site assessment and monitoring of the freshness of five meat products: beef, chicken, fish, pork, and shrimp. A commercial CMOS camera enables the real-time collection of colorimetric data. The sensor array shows sensitivity to gaseous analytes, especially amines and sulfides at low parts per billion levels, and achieves discrimination among meat volatiles in terms of meat type and storage time of meat, which enables meat product inspection. In this work, several chemometric approaches are applied including principle component analysis, HCA, and support vector machine analysis. See Fig. 8.7.
Fig. 8.7 Sensing device assembled from a colorimetric sensor array inside a handheld analyzer. (A) Gas sampling from a meat sample into the handheld analyzer (5.0 3.7 1.6 in.3). (B) Top view of the 20-element colorimetric sensor array mounted in a polycarbonate cartridge (3.1 1.1 0.4 in.3). (C) Side view of the cartridge. (D) Sensor array response to five meats. (Adapted with permission from Z. Li, K.S. Suslick, Portable optoelectronic nose for monitoring meat freshness, ACS Sensors 1 (11) (2016) 1330–1335, Copyright (2016) American Chemical Society.)
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8.7 Some other gases It is not intended in this chapter to report all the gases that can be monitored by optical sensors based on nanomaterials, and just some of them will be mentioned here, such as carbon monoxide [75, 76], carbon dioxide [77], hydrocarbons [78–80], hydrogen sulfide [81–83], acetylene [84], nitrogen dioxide [85–87], or nitric oxide [88, 89].
8.8 Concluding remarks Optical gas sensors based on nanomaterials are not so present in the market as their electronic counterparts. Oxygen optical sensors are the dominant technology in dissolved oxygen measurements. They are based on luminescent films, and the most used measurement techniques are time-resolved luminescence. Hydrogen, ammonia, and VOC sensors are devices with a high potential, especially the last ones because VOC sensors can play an important role in the development of optoelectronic noses and they are already present in the market. Since smartphones include light sources (LEDs) and photodetectors (CCD array digital cameras), the number of applications that take advantage of this ubiquitous present devices is rapidly increasing.
Acknowledgements The authors want to acknowledge the support of the Spanish Agencia Estatal de Investigacio´n (AEI) and European Regional Development Fund (FEDER) (TEC2016-79367-C2-2-R).
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