8

Optical Electronic Noses

Todd A. Dickinson, David R. Walt

8.1

Introduction

A tremendous amount of technical infrastructure and scientific development has ta-ken place in the area of optics, optical communications, and optical hardware over thelast several decades. These developments have led to new light sources, such as solid-state lasers, laser diodes, and light-emitting diodes (LEDs). Improved materials forconducting light, such as optical fibers and optical fiber arrays, have been devel-oped. Revolutions in detector technology have also taken place; high sensitivity detec-tors, such as avalanche photodiodes, have been developed with the ability to detectsingle photons. Array detectors, such as charge coupled device (CCD) cameras, inten-sified CCD cameras (ICCD), and CMOS (complementarymetal oxide semi-conductor)detectors are in widespread use for such applications as digital photography and as-tronomy. Color versions of these array detectors are also being introduced commer-cially. In addition to these components, significant advances in materials science haveled to new types of filters, dichroics, light-directing components such as micromirrorarrays, and infinity optics. Most of these devices and components have been developedto advance the telecommunications, entertainment, and computer industries for suchapplications as fiber-optic communications, digital music, projection devices, and op-tical information storage. With the advent of these new capabilities, a parallel devel-opment has been taking place in the field of optical sensing.

8.1.1

Optical Sensors

Optical sensors are devices that measure the modulation of a light property. Examplesinclude changes in absorbance, fluorescence, polarization, refractive index, interfer-ence, scattering, and reflectance. Optical sensors are comprised of four basic compo-nents: 1) a light source to interrogate the sensor; 2) suitable optics for directing light toand from the sensor; 3) a detector for detecting the light signal coming from the sensor;

Handbook of Machine Olfaction: Electronic Nose Technology.Edited by T.C. Pearce, S.S. Schiffman, H.T. Nagle, J.W. GardnerCopyright ª 2003 WILEY-VCH Verlag GmbH Co. KGaA, WeinheimISBN: 3-527-30358-8

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and 4) the sensor itself. In the simplest type of sensor, referred to as an intrinsic sensor,the chemical species being measured carries its own signal. For example, some or-ganic molecules absorb light at specific wavelengths, or fluoresce and thereby emitlight at particular wavelengths. These molecules can be detected directly by measuringchanges in absorbance or fluorescence at their absorption or emission wavelengths,respectively. In these systems, the ‘sensors’ are the molecules themselves. Thus, onlythe three instrument components are required as the sensor transduction mechanismis intrinsic to the molecule or molecules being detected.In the more common type of optical sensor, an indicating species is employed.

These types of sensors are referred to as extrinsic sensors. Indicators can be dyes,polymers, or other materials that interact with the chemical species of interest, theanalyte, to produce signal modulation. For example, an optical sensing materialcan be prepared by attaching a chemically sensitive dye to a substrate. When an ana-lyte interacts with the sensing material, an absorbance or fluorescence change occurs,which is monitored by the optical instrumentation. A variety of substrates can be em-ployed for optical sensors. Polymeric films can be used as supports to attach indicators.Glass slides can be used both as vehicles for attaching materials to their surface as wellas for coupling light to the detection system. Optical fibers, also called fiber optics, canbe used to carry light both to and from a sensing material attached to its surface, eitherat its tip or surrounding the fiber along its annulus.

8.1.2

Advantages and Disadvantages of Optical Transduction

Optical sensors have a number of advantages over other sensor transduction mechan-isms. As described above, most of the supporting optical instrumentation has beendeveloped for other applications and can be brought to bear on the optical sensingfield. The ready availability of inexpensive instrumentation, and the promise of im-proved performance with new developments in light sources, optics, and detectors,will continue to enable major advances in optical sensing technologies. The continuedmovement toward fully integrated optical communication and computation bodes wellfor the field. In addition to the ready availability of instrumentation, there is a largeknowledge base, as well as commercial accessibility to a multitude of indicators thatare suitable for optical sensing. Optical signals are not susceptible to electromagneticinterferences. Light is fast. Light attenuation is extremely low through modern fiberoptics, which enables remote sensing over long distances with no need for repeaters oramplifiers. Optical measurements, in particular fluorescence, are extremely sensitiveand can be used to detect single molecules. Optical sensing can be readily multiplexedbecause different optical signals can be carried and detected simultaneously. There arealso several disadvantages of optical sensing compared to other sensing methods. Ingeneral, optical instrumentation tends to be more expensive, materials intensive, andmore complex than sensors based on mass or electrical transduction. These latter twomethods employ instrumentation that can be largely designed as integrated circuits,making them simpler and less expensive. In addition, optical methods are sometimes

8 Optical Electronic Noses182

susceptible to interference by stray light. Finally, optical approaches that utilize fluor-escent indicators suffer from eventual photodegradation of the dye molecules.A variety of electronic noses have been developed using a diversity of optical trans-

duction mechanisms [1]. In most cases, these systems employ cross-reactive sensors(discussed below) combined with smart signal processing (described in other chap-ters). Optical sensor arrays have a much shorter history than electronic noses. Con-sequently, there is the hope that these systems will develop rapidly over the next fewyears.

8.2

Optical Vapor Sensing

Given the wonderfully diverse nature of optical signals, the past 25 years have bornewitness to the development of a wide range of light-based chemical vapor sensors.Although the ‘artificial nose’ approach to designing sensing systems was first con-ceived in the early 1980s [2], only in the last few years has this concept been extendedto the optical arena. An increasing number of research groups are now beginning toexplore the utility of employing optical sensors in cross-reactive arrays for improvingsensing capacity and performance. This section provides a general overview of some ofthe key approaches to building optical vapor sensors that have been developed over thepast two decades, and the transition of some of these approaches into ‘optical electro-nic noses’.

8.2.1

Waveguides

Central to many optical chemical sensors is the use of waveguides in one of severaldifferent formats. Fiber optics, capillary tubes, and planar waveguides all exploit thephenomenon of total internal reflection. Optical fibers, for example, are strands ofglass or plastic in which a central ‘core’ is surrounded by a ‘clad’ with a slightly lowerrefractive index. Light introduced into the fiber core is reflected at the clad/core inter-face and is thereby conducted via total internal reflection to the distal tip of the fiber.Hollow capillary tubes or planar substrates comprised of two or more materials withdiffering refractive indices can also be made to guide light extremely efficiently fromone end to the other. A wide range of creative ways to exploit the properties of wave-guides for chemical sensing have been explored.

8.2.2

Luminescent Methods

Fluorescence methods continue to be among the most popular optical sensing andgeneral spectroscopy approaches for a wide range of applications, usually becauseof high quantum yields, well-separated excitation and emission spectra, and intrinsic

8.2 Optical Vapor Sensing 183183

sensitivity. For a detailed review of fluorescence spectroscopy the reader is referred toLakowicz [3]. Briefly, fluorophores are molecules that absorb light at one wavelengthand emit light at a longer wavelength. This difference in wavelength, and thus energy,is referred to as the Stoke’s shift and represents vibrational relaxation and other energylosses experienced by the molecule following light absorption. How well a fluorophoreconverts absorbed photons to emitted photons is called its quantum yield or quantumefficiency.Walt [4] and co-workers first combined fiber-optic waveguides with fluorescent dyes

for the measurement of organic vapors in 1991 using the polarity-sensitive, solvato-chromic dye, Nile Red. Following this initial work, the approach was extended to high-er-level arrays of solvatochromic sensors and, finally, to its current configuration ashigh-density microsphere arrays. This work and its evolution are described in moredetail in the final section of this chapter.A number of other groups have also begun to explore fluorescence-based methods

for vapor sensing. Fluorescent dyes can exhibit spectral changes based on several me-chanisms. One such mechanism is the twisted intramolecular charge transfer (TICT)excited state. Molecules such as the one designed and synthesized by Orellana et al. [5],shown in Fig. 8.1, can assume a number of different, highly polar configurations intheir excited state. These excited states will be stabilized when solvated in polar en-vironments such as alcohol vapors and lead to red-shifts in their emission spectra.The degree of these shifts will depend on the particular solvation environment andthus can be used to detect specific vapors. By adsorbing these dyes to silica geland immobilizing the resulting gel at the tip of an optical fiber, Orellana has beenable to demonstrate the reversible measurement of various alcohols.Reichardt’s dye, a betaine fluorophore, is another example of a solvatochromic dye

that exhibits high sensitivity to polarity changes, and has been used to create the ET(30)polarity measurement scale for solvents. An increasing number of groups have begunto incorporate betaine dyes onto the ends of optical fibers in various ways to preparechemical sensors. One group modified the dye molecule and covalently attached it to aMerrifield peptide resin via a five-step synthesis. Following immobilization to a fiber,the resulting sensor was successfully used to measure polar octane improvers in ga-solines [6]. In a similar study, Rose-Pehrrson et al. [7] entrapped Reichardt’s dye withina series of different polymer films and studied the responses resulting from the vary-ing absorption of analytes.A number of groups have begun to explore the potential for exploiting host-guest

supramolecular chemistry for sensing. For example, host compounds that form crys-talline inclusions, or clathrates, by temporarily trapping guest molecules within theirlattice structures have been utilized for detecting solvent vapors [8]. By incorporating a

Fig. 8.1 A polyaromatic-substituted 1,3-oxazole (or 1,3-thiazole)

fluorescent indicator that displays polarity-sensitive TICT excited

states [5]

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fluorescent anthracene moiety as well as a few key functional groups to impart selec-tivity for vapors, the authors created a class of compounds they call ‘fluoroclathrands’.When vapors are introduced into a hydrogel layer containing these compounds, thehost molecules surround the guest vapor molecules and form inclusion complexeswith specific crystal structures and characteristic fluorescence behavior. Dependingon the guest molecule, the complexes exhibit both wavelength shifts and quantumefficiency (intensity) changes in their emission spectra. The authors speculate thatthe bathochromic shifts are due to energy losses associated with increased packingdensity in the inclusion compound, while the intensity changes are most likely a resultof self-quenching that varies as a function of the distance between the fluorophores inthe crystal.Unlike fluorophores, which require an excitation source to generate the emission

signals, chemiluminescence-based sensors employ chemically reactive species capableof directly emitting photons following oxidation. This approach offers the advantage ofsimplified instrumentation, by circumventing the need for excitation light sources, aswell as high sensitivity since signals arise from initially dark backgrounds. While che-miluminescence has frequently been employed for oxygen and metal-ion sensors, themethod has recently been extended to detecting organic vapors such as chlorinatedhydrocarbons, hydrazine, and ammonia [9]. The commonly-used reagent luminolwas used to detect oxidants while a Ru(bpy)3

3þ complex was used for reductants. Lu-minol sensing capacity was expanded to halogenated hydrocarbons by the addition ofan inline heated platinum filament used as a pre-oxidative step.

8.2.3

Colorimetric Methods

Sensors that measure changes in absorbance (i.e., color), or local refractive indexchanges resulting from indicator color changes, have also been developed for vaporsensing. Some of the earliest work in this area was done by Wohltjen and colleagues[10], who developed a reversible capillary tube-based sensor for ammonia, hydrazine,and pyridine by coating a glass capillary with an oxazine perchlorate dye film. Colorchanges experienced by the dye upon exposure to these vapors from 60 to 1000 ppmcaused proportional changes in transmission through the tube and were detected by asimple phototransistor. Similarly, Stetter, Maclay and Ballantine [11] used a coating ofbromothymol blue suspended in a Nafion polymer layer to detect and quantify H2Sand HCl acid vapors down to 10 ppb levels.Even commercially available thermal printer papers have been shown to exhibit

reversible interactions with solvent vapors and may be useful in solvent vapor sen-sing. Wolfbeis and colleagues [12] demonstrated that thermal papers could be im-mersed in an ether atmosphere to produce a dark blue or black color. The treatedpaper was found to decolorize to varying extents upon exposure to different polarsolvent vapors. By incorporating these papers into various optical devices and mon-itoring light absorption at 605 nm, sensors were prepared that were capable of mid-ppm to high-ppm detection levels for typical laboratory solvents such as alcohols and

8.2 Optical Vapor Sensing 185185

acetates. Response times of these sensors ranged from 30 seconds to 3 minutes, withrecovery times of up to 7 minutes for certain analytes.Polymers are frequently employed in a large number of optical sensor constructs for

their differential vapor sorption or binding properties as well as their emissive proper-ties. For example, the color changes exhibited by amine-containing poly(vinylchloride)membranes when interacting with polynitroaromatics have been used to detect 2,4-dinitrotoluene (DNT), a compound commonly present in landmines [13, 14]. Absorp-tion into the polymer generates a complex with an absorbance at 430 nm that can bemonitored over time to characterize DNT levels in an area of interest.Sensor materials play a central role in all of these various optical approaches, and

their study and development has become a major field of exploration in its own right[15]. All of the above vapor-sensing techniques rely on changes in color of an organicsensing material. Inorganic compounds that exhibit environmental sensitivity in boththeir absorptive and emissive properties are another exciting class of sensing materi-als. At the University of Minnesota, Mann et al. [16] have shown substantial shifts inmaximum absorption and emission wavelengths of platinum and palladium isocya-nide complexes resulting from exposure to volatile organic compounds (VOCs). ThePt-Pt compound [Pt(p-C10H21PhNC)4][Pt(CN)4], for example, was found to exhibit ab-sorption and emission maxima shifts as large as 91 nm and 74 nm, respectively, whenexposed to vapor environments ranging from air to CHCl3. The researchers believethat the incorporation of VOCs into the lattice (which appears to be fully reversi-ble) causes a perturbation in the stacking of the anion and cation complexes that leadsto the observed color changes. In the case of polar VOCs, dipole-dipole and/or H-bond-ing interactions with the Pt(CN)4

2� anion are thought to be involved; for nonpolarcompounds, however, the ‘vapochromism’ is explained by lypophilic interactionswith the isocyanide complexes. Photostability and an insensitivity to water vapormake these materials particularly attractive for incorporation into an opto-electronicnose sensing device.Metalloporphyrins (Fig. 8.2) represent another class of inorganic materials that are

particularly good indicators for sensing as they are stable, well characterized, and easilymodified with a wide range of substituents.

Fig. 8.2 General structure of a metalloporphyrin.

Modifications can occur at each R and R’ position,

and a wide range of metals can be incorporated at the

core of the complex

8 Optical Electronic Noses186

These compounds can both form coordination complexes with analytes as well asadsorb them via van der Waal’s and H-bonding interactions, giving rise to broad se-lectivity particularly suitable for electronic-nose applications. As a result of their aro-matic p-systems, porphyrins exhibit unique absorption and luminescence propertiesdepending on themetal centers and peripheral substituents involved. D’Amico and co-workers [17] were able to distinguish between six different liquors by monitoring ab-sorbance changes with a simple LED and photodetector system. The researchers rea-soned that the optical changes were caused by competitive interaction of the VOCswith aggregated porphyrin complexes that lead to broadening and shifting of spectralbands.Rakow and Suslick [18] used metalloporphyrins to construct a colorimetric array

detector for vapor-phase ligands. An array was assembled by spotting a series of dif-ferentially metalated porphyrins onto silica thin-layer chromatography plates. Imagingthe array with a common office scanner before and after vapor exposure revealed aunique pattern of response for each of the various analytes (Fig. 8.3). The degreeof spectral shift is thought to be a function of the degree of polarizability of the li-gand. Thus, by incorporating a range of metal centers of varying ligand-binding affi-nity, an array can be made to discriminate between several different analytes. Theauthors report good reversibility as well as linearity of the sensors. A cobalt-basedsensor, for example, responded linearly to binary mixtures of trimethylphosphiteand 2-methylpyridine, and could therefore be used to predict the composition of thesesolvent mixtures. Typically, 15-minute exposures were used with the arrays to ensuremaximum array response, although the authors showed that these times could bereduced to 30 seconds for at least one of the sensors. The work employed hydrophobicsubstrates for the array such as reverse phase silica or Teflon films, which had theadvantage of limiting interference from water vapor (one of the most formidable chal-lenges that plague electronic noses). Colorimetric techniques, such as these porphyrinarrays, generally employ simple instrumentation. Sensor reproducibility with sensi-tivity below the ppm level are presumably among the areas targeted for furtherwork with this approach.

8.2.4

Surface Plasmon Resonance (SPR)

In other work, coordination polymers were used as sensing layers in a SPR setup todetect benzene, ethanol, toluene, acetonitrile, and water [19]. Langmuir-Blodgett filmswere created using poly(CuMBSH) (MBSH þ 5,5’-methylenebis (N-hexadecylsalicyli-deneamine), which were found to be excellent sensing materials due to their rapid andreversible interaction with vapor-phase analyte molecules. The SPR technique exploitsthe delocalized conducting electron clouds found at the surface of metal films such assilver and gold. The electron clouds maintain a collective wave vector parallel to theinterface. Light of a particular wavelength and polarization incident at the interface at aprecise, ‘resonant’ angle will couple to these electromagnetic modes, resulting in asharp decrease in the measured reflected intensity of the excitation beam. The mo-

8.2 Optical Vapor Sensing 187187

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mentum matching condition, and thus the resonant angle, is dependent upon therefractive index of the dielectric medium. Therefore any changes in refractive indexat the surface, such as that caused by the sorption of vapor molecules into a polymernetwork at the surface, can be closely measured in real time by monitoring the illu-mination angle needed to give a minimum in the measured reflected light. Alterna-tively, since the resonant angle is also a function of the wavelength of the incident light,a white light source can be used in place of a laser to monitor the wavelength at whichthe surface plasmon resonance occurs [20, 21]. Although the sensitivity was relativelylow in this study, responses to high ppm levels of benzene were demonstrated. TheSPR signals are thought to be directly related to refractive index changes at the surfacedue to swelling of the polymer and/or increased density upon absorption of the analytevapor.In related work, Abdelghani et al. [22] have applied the SPR technique to optical

fibers by coating a 50 nm thick layer of silver onto the core of a silica fiber. To protectagainst oxidation, alkanethiol layers were assembled onto the silver layers. A fluori-nated siloxane was selected to serve as the final cladding layer due to its appropriaterefractive index, surface tension, and gas permeability properties. Although the result-ing sensor responses appear to have improved reproducibility and signal-to-noise ra-tios, the detection limits reported were in the high ppm level for both the aromatic andchlorinated compounds tested, and the cumulative response and recovery times wereof the order of several minutes.

8.2.5

Interference and Reflection-Based Methods

Another area of recent activity for sensor development has been the use of interferencespectroscopy. Having demonstrated that analyte-swelled polymer films experiencemuch larger changes in optical thickness than refractive index [23], Gauglitz andothers have pursued reflectometric interference spectroscopy (RIfS) methods for op-tical vapor sensing. In this approach, light incident at the interface between two planaroptical layers can be reflected from both the top and bottom of a polymer sensing film,setting up an interference pattern that is very sensitive to changes in the optical thick-ness of the polymer layer. Gauglitz suggested that the method offers two primaryadvantages over non-optical techniques: 1) the ability to use strictly inert materials(glass and siloxane polymer films) in contact with the vapor samples; and 2) abuilt-in control for checking the condition of the sensing layer. One of the challengesassociated with measuring changes in the interference spectrum has been the require-ment for relatively bulky and expensive light delivery and detection equipment. Im-provements to this approach have been pursued through simpler and less expensiveoptical components [24]. Recent work using four inexpensive LEDs and a single photo-diode demonstrated that despite the lower-resolution, four-point spectrum, the sim-plified RIfS system yields comparable sensitivity and linearity to its more costly pre-cursor [25].The RIfS technique has also been extended to enantiomer discrimination. By de-

positing polymer solutions containing chiral peptide residues from the ‘Chirasil-

8.2 Optical Vapor Sensing 189189

Val’ chromatographic stationary phase, Gopel and colleagues [26] studied the re-sponses of their sensors to several mixtures of (R) and (S)-methyl lactate in varyingproportions. A direct correlation was found: as the concentration of the (S) enantiomerrose in the analyte mixture, the amplitude of the (S)-Octyl-Chirasil-Val sensor rosewhile the (R)-sensor fell.Interference measurements have also been applied to porous silicon chips (PSi).

Sailor and coworkers [27] have developed simple chemical etching methods for gen-erating porous silicon films that display both interferometric and photoluminescenceproperties. In the case of photoluminescence, the group proposed that quenching canbe induced via energy transfer by the adsorption of analyte molecules in the pores ofthe silicon. Thus, by monitoring emission at a specific wavelength (670 nm in thiscase), one can observe sharp decreases in intensity as the interaction with analytevapors takes place. Likewise, adsorption events give rise to refractive index changesthat lead to shifts in Fabry-Perot interference fringes, measured as changes in reflec-tivity. Both of these optical attributes were recently used to measure a range of per-fumes and solvent vapors. When compared side-by-side to a commercial electronicnose containing metal-oxide sensors, the PSi chips displayed comparable discrimina-tion ability for a few standard solvents, ethyl esters, and perfumes. At the saturatedvapor conditions used, the silicon sensors showed significantly faster recovery timesthan their metal-oxide counterparts (30 s versus 15 min). The ability to create a diversearray with high sensitivity and broad selectivity with this approach, however, remainsto be proven.Another absorbance type of vapor sensor is based on simple transmission attenua-

tion through a fiber. Microbending caused by the vapor-induced swelling of siloxanelayers adjacent to the fiber results in transmission attenuation [28].Yet another creative reflection-based approach to chemical sensing has been the use

of resonating microcantilevers such as those used in atomic force microscopy (AFM)for atomic-level imaging. Based on the mass-sensing concepts of resonating piezoelec-tric crystals (e.g., quartz crystal microbalances), the approach uses 180 lm long can-tilevers micromachined into silicon that are sensitive to changes in mass occurring attheir surfaces. Several groups have explored coating polymer films onto these canti-levers and measuring small changes in mass loading. The technique uses optical de-tection by measuring the deflection of an incident laser beam as analyte vapors areadsorbed to the surface. In one study, Thundat et al. [29] showed that such sensorscould be modified to possess desired selectivities, for example by employing hygro-scopic coatings to improve sensitivity to water vapor.A group in Switzerland recently proposed that arrays of differentially coated canti-

levers could be used as a new form of chemical nose [30]. Working in their own mi-crofabrication facility, the group constructed an eight-cantilever sensor array fromsilicon. The individual cantilever coatings included platinum thin films, alkythiolself-assembled monolayers (SAMs), zeolites, and poly(methylmethacrylate). Theauthors studied detection of water vapor, alcohols, and several natural flavors.Although the array was read out sequentially due to the use of a single laser andphoto-sensitive device, one can envision ways of multiplexing through beam-splittersand larger, higher-resolution two-dimensional detector arrays. Detection limits were

8 Optical Electronic Noses190

not calculated in this study, making it difficult to compare the sensitivity of the ap-proach to other methods. In addition, despite the small size of the devices, reportedcycle times were of the order of several minutes. Other challenges with the cantileverapproach include interference from pressure changes during sampling, loss of signaldue to severe bending of the cantilever, laser heating of the cantilever, and limiteddynamic range [29]. Nevertheless, as they continue to be developed and improved,cantilever arrays may prove to be a promising opto-electronic nose format capableof simple integration into silicon-based microelectronic devices.

8.2.6

Scanning Light-Pulse Technique

Lundstrom and coworkers have taken an innovative optical approach by employing amethod called the scanning light-pulse technique [31–34]. In this approach, light im-pinges on the surface of a metal-oxide semiconductor field effect transistor (MOSFET)coated with a thin metal film and penetrates the metal to induce a photocapacitivecurrent. To maintain a constant current, the applied gate voltage (V) must be variedto sustain a constant surface potential. Changes in the gate voltage are monitored andresult in a map of the change in voltage (DV ) as a function of position on the sensingsurface. In one demonstration, a MOSFET array was prepared with three continuousstrips of Pt, Pd, and Ir. The sensor surface was divided into a grid, and a temperaturegradient (110–180 8C) was established down the length of the sensor surface. Thistemperature gradient provided a different sensitivity and selectivity at each point ofthe sensor grid. The sensor grid was exposed to hydrogen, ammonia, and ethanol,and DV was determined. In this manner, image maps of the gases were created.These sensor grids can be applied to identifying gas mixtures, rapid and simultaneousscreening of new sensing materials, and mapping spatially inhomogeneous reactions.Light-pulsed sensing combines many types of information, including the catalyticactivity of the gate metals, gas flow turbulence, edge effects, etc. While not an opticaldetection technique, the method demonstrates the utility of employing light combinedwith electrochemical detection.

8.3

The Tufts Artificial Nose

Optical fibers can be used to create fluorescent-based optical sensors. In this approach,a fluorescent indicating species is attached to the fiber’s distal tip using a variety ofimmobilization techniques. Excitation light is introduced into the fiber, which carrieslight efficiently to the fiber’s distal tip. The fluorescent indicator is excited and some ofthe resulting isotropically emitted light is captured by the same fiber, directed throughsuitable optics, filtered and sent to a detector. The modulated light signal returning tothe detector corresponds to the presence and amount of an analyte.In order to design a cross-reactive optical sensing array, it is necessary to find an

appropriate array of sensing materials to respond to a wide variety of analytes. Our

8.3 The Tufts Artificial Nose 191191

laboratory has developed a series of fluorescence-based optical sensors. It was our goalto create a fluorescent-based cross-reactive array. In 1991, we published a paper inwhich we used a solvatochromic indicator, Nile Red, to create a generic optical vaporsensor [4]. The sensor was based on immobilizing Nile Red within a polymer matrixand attaching the resulting material to the distal tip of an optical fiber. As discussedabove, solvatochromic indicators report on the polarity of their local environment, alsocalled the microenvironment. When solvatochromic dyes, such as Nile Red, are em-bedded in polymers, they report on the polarity of the polymer’s microenvironmentindicated by their color, in particular, their absorption and/or emission spectra. Forexample, Nile Red has an emission spectrum that is relatively blue in nonpolar, hydro-phobic environments, and is red in polar, hydrophilic environments. When an organicvapor sensor containing Nile Red, immobilized within a polymer, is in contact with air,it has an emission spectrum that represents the polarity of the polymer. When such apolymer is exposed to an organic vapor, the organic vapor diffuses into the polymerand modifies the microenvironmental polarity, which is signaled by a change in theemission spectrum of Nile Red. The emission spectrum shift is highly predictable. Avapor that is more polar than the polymer will shift the spectrum to a higher wave-length, whereas a less polar polymer will shift the spectrum to a lower wavelength(Fig. 8.4). The extent of the shift depends both on the polarity difference as well asthe partition coefficient of the vapor into the polymer. In this manner, a generic or-ganic vapor sensor was created by simply immobilizing a single solvatochromic dyewithin a dimethylsiloxane polymer. The sensor was used to detect leaks of hydrocarbonliquids from underground storage tanks by detecting the vapors that preceded theliquid leak.The same sensing principle was used to design a cross-reactive vapor-sensing array

[35–37]. In this system, Nile Red was immobilized within a series of polymers. Hun-dreds of polymers were screened empirically. Each polymer defined the initial polarity

Fig. 8.4 The spectra of four sensors made by incorporating Nile Red

into four polymers of differing polarity. The emission max shifts to the

red with increasing polarity of the polymer matrix

8 Optical Electronic Noses192

of the microenvironment as reported by Nile Red. These polymers were dip-coatedonto the ends of individual optical fibers. Nineteen sensors were bundled into an arrayformat. Upon exposure to an organic vapor, each polymer sensor absorbed vapor ac-cording to its partition coefficient for that vapor. The change in each sensor’s fluor-escence spectrum depended on howmuch vapor partitioned into that sensor as well asthe difference between the vapor’s and polymer’s polarities.There are several aspects of the optical sensor array’s operating mechanisms that

require elaboration. First, we decided that, unlike most electronic noses, we would notlook at static headspace measurements but rather would mimic a sniff by observingthe kinetics of the response upon vapor exposure. To this end, we employed a vapordelivery system that was originally designed for delivering odors to animals in olfactoryresearch [38]. The vapor delivery was accomplished by presenting square-wave vaporpulses for a defined period of time to the distal face of the bundled fiber array. Fluor-escence detection was accomplished by using a two-dimensional detector, a CCD cam-era, so that we could acquire fluorescent signals from all the sensors in the array si-multaneously. To simplify signal detection, the fluorescence was collected at a singlewavelength by interposing an emission filter between the fiber and the CCD chip. Theresulting measured fluorescence signals coming from each sensor, upon exposure toorganic vapors, were simply the intensity changes relative to their starting intensity atthat particular emission wavelength. An intensity increase simplymeant that the emis-sion spectrum of the dye in a particular polymer upon exposure to a particular vaporwas shifting closer to the wavelength range defined by the emission filter. Conversely,a decrease in fluorescence intensity indicated that the emission spectrum of the dyewas moving further away from the emission filter range. A final aspect of the responsemechanism resulted from the interaction of the vapor with the polymer. Some of thepolymers exhibited a swelling effect in which the polymer volume increased as vaporpartitioned into it. Polymer swelling causes a dye molecule to increase its averagedistance relative to the fiber surface. As described above, the isotropically emitted lightis captured by the optical fiber. When amolecule moves further from the fiber surface,the capture efficiency for the light decreases because the sine of the half angle of thereturning light is reduced. Therefore, the response of each sensor is due to a combi-nation of vapor partitioning into the polymer, polarity differences between the polymerand the vapor, and polymer swelling. Because the solvatochromic and swelling effectsoperate under different kinetic regimes (i.e., swelling at the bulk polymer surfaceoccurs rapidly while the solvatochromicity requires an intimate slower redistributionof vapor molecules within the polymer matrix), nonlinear effects can be observed. Thefluorescence images are collected before, during, and after a vapor pulse to provide acharacteristic response profile for each sensor in the array.A video image of an array of 19 sensors exposed to a three second pulse of benzene is

shown in Fig. 8.5. The digitized responses of each sensor in the array are shown in thegraph in Fig. 8.6. These complex temporal responses are characteristic of a benzenepulse at a particular concentration and can be used to train a computational classifica-tion program. Both parametric (e.g., intensities, slopes) and nonparametric methodscan be used to train the responses. One of the major challenges in the field of electro-nic noses/cross-reactive arrays is array-to-array variability. This lack of reproducibility

8.3 The Tufts Artificial Nose 193193

results from the inability to prepare polymeric materials identically. When polymersare put onto optical substrates or other surfaces by dip coating, liquid dispensing,photopolymerization, or electropolymerization, slight volume differences, initiatorconditions, or minor heterogeneities can cause significant differences in materialcomposition. These differences, even if the variation is only a few percent, canlead to loss in training fidelity. To address this problem, we have switched to a dif-ferent array platform. Instead of using individual single-core optical fibers we nowemploy optical-imaging fiber arrays. These arrays are comprised of thousands of in-dividual optical fibers, each of which is surrounded by a clad material (Fig. 8.7). Thearrays are fabricated such that they are coherent in nature meaning that the position ofan individual optical fiber within the array retains its position from one end to theother. In this manner, such arrays can be used to carry images, an application thatis being pursued for medical endoscopy. These arrays are fused unitary bundlesrather than mechanically fixed strands of individual fibers. Thus, they maintain theirflexibility and can be handled similarly to single core fibers. A typical optical arraycontains between 10 000 and 50 000 individual fibers in a diameter of a few hundredmicrons with the individual fibers having diameters on the order of 3–5microns each.The difference in materials composition between clads and cores provides a method

for selectively etching the cores. When the polished distal tip of a custom optical ima-ging fiber array is placed into an acid etchant, the cores etch at a faster rate than theclads leading to an array of wells. At the bottom of each well is the distal face of anoptical fiber (Fig. 8.8A). In this manner, each well is ‘optically wired’ to its own indi-vidual optical fiber. We discovered that latex or silica beads, matched in size to thedimensions of the individual wells, would spontaneously assemble into each well

Fig. 8.5 A sequence of images depicting the fluorescence response of a 19-fiber sensor array to a pulse of

benzene vapor

8 Optical Electronic Noses194

in a highly efficient self-organizing fashion. This approach could be used to createsensor arrays based on polymeric microspheres.Microsphere sensors can be created by taking monodisperse polymeric micro-

spheres and swelling them in a suitable organic solvent containing dissolved NileRed [39]. Upon removal from the solvent, evaporation of residual solvent occurs re-sulting in Nile Red being trapped within the polymeric matrix. Another class of beadsensors uses surface modified silica beads to which Nile Red is adsorbed (Fig. 8.9).Many different bead types can be prepared out of a variety of polymers and surface

Fig. 8.6 Temporal plots from 19-fiber array response to benzene vapor pulse

Fig. 8.7 Components of a fiber-optic imaging bundle

8.3 The Tufts Artificial Nose 195195

functional groups. As discussed above, in each of these sensors, the Nile Red reportson the polarity of its local environment. A library of bead types is created containing adiversity of responses to vapors. To create a sensing array, the desired individual beadtypes are mixed. 100 milligrams of beads contains approximately 10 billion beads. Thebeads are randomly distributed onto the distal face of an etched imaging fiber such thatone bead occupies each well (Fig. 8.8B). In order to register the position of each bead inthe array after fabrication, the fiber is connected to the optical imaging system and avapor is pulsed onto the fiber’s sensor end. Because each different type of bead pro-duces a unique and characteristic response profile when exposed to a particular vapor,the responses to the vapor pulse enable the image-processing program to register thebead type occupying each well. We refer to this registration protocol as ‘self-encoding’;that is, the sensor is identified by its response profile to a particular vapor [36]. In thismanner, a library of beads can be used to create hundreds to thousands of individualsensing arrays with each array having the same bead types but located in differentpositions. The bead registration task involves exposing each array to a particular re-

Fig. 8.8 A) wells formed by etching an imaging bundle, and B) beads immobilized in the wells

Fig. 8.9 Silica beads can be modified in a variety of ways before being dyed in order to generate a diverse

library of sensors

8 Optical Electronic Noses196

gistration vapor and using an image-processing program to automatically register theposition of each bead in the array using a lookup scheme.A key advantage of the self-encoding array sensors is that the training can be trans-

ferred from one sensor array to another. All the sensor beads of a particular type givevirtually identical responses because they are all prepared at the same time. Thus,when mixed in a library, each bead type maintains its particular response profile. An-other important feature of these cross-reactive optical arrays is the built-in redundancyof each of the sensors. The small size of the fibers combined with the random dis-tribution of the different microspheres in the array dictates that there will be replicatesof each sensor in every array. The numbers of each sensor type will distribute them-selves according to Poisson statistics. Replicates provide significant advantages interms of signal-to-noise. The signal-to-noise ratio scales as 1/5 n, where n is equalto the number of sensors of each type. By summing or averaging sensor repli-cates, significant signal-to-noise enhancements can be achieved resulting in improveddetection limits due to the ability to make more precise measurements at lower con-centrations (Fig. 8.10) [36]. The microsphere arrays also have several other advantagessuch as flexibility of array types, scalability, and simple manufacturing.The major limitation with fluorescent dyes for optical sensor arrays is photobleach-

ing. Upon exposure to light, any indicating material loses its intensity because ofphotooxidation. Over long periods of exposure, the light intensity degrades consider-ably. In order to avoid this problem, we employ autoscaled response profiles so thattraining is not dependent on absolute signal intensities. Despite this autoscaling pro-cedure, photobleaching eventually degrades the signal-to-noise ratios. At this point,the array must be replaced. Since each array has the identical sensing elements,the training performed on one array is transferable to a second array. We have recentlydemonstrated training transfer of a classifier over a nine-month period with robust-ness of classification.

Fig. 8.10 Signal-to-noise

ratios can be dramatically

improved by averaging over

multiple copies of the same

bead type within an array

8.3 The Tufts Artificial Nose 197197

8.4

Conclusion

Optical electronic noses have a relatively short history relative to conducting polymer ormetal-oxide-based approaches. In the roughly five years since they were first reported,there have been a variety of advances in the types of optical signals employed as wellas the materials used to perform the recognition [18]. The area of molecular recognitionis burgeoning. Many of these receptors have built-in optical transduction. New polymers[14] and nanostructured materials [27] with recognition and optical signaling are beingdeveloped. In addition, the data richness of optical sensor arrays should make them at-tractive as analytical systems. With continued emphasis on new optical materials anddevices development for the telecommunications and computer industries, combinedwith advances in molecular recognition and advanced materials, optical approachesto sensing should continue to improve in sensitivity, selectivity, and performance.

Acknowledgments

The authors wish to thank the ONR and DARPA for research funding, and KeithAlbert and Shannon Stitzel for assistance with figures.

Tab. 8.1 Summary table of optical electronic nose approaches.

Transduction

Mechanism

Description References

Luminescence Fiber-optic sensors using polarity sensitive fluorophores such

as solvatochromic or TICT dyes.

4–7, 35–37

Randomly assembled solvatochromic bead arrays. 39, 40

Host-guest supramolecular chemistry: shifts in wavelength and

intensity of ‘fluoroclathrands’ based on packing density changes

caused by vapors.

8

Chemiluminescence-based detection, using luminol and Rubpy

dyes.

9

Colorimetric Color changes of an oxazine perchlorate dye coated on glass

capillaries.

10

Bromothymol blue in Nafion polymer layers. 11

Thermal printer paper as vapor sensors. 12

Inorganic sensing materials (e. g. Pt-Pt compounds): color

changes caused by perturbation of stacking in charged complexes.

16

Metalloporphyrins: formation of coordination complexes with

analytes, and use of different metals for changing sensing

properties.

17, 18

Surface plasmon

resonance

Method for detecting changes in refractive index at a surface. 20–22

Interference,

reflection

Reflective interferometric ipectroscopy for detecting changes

in optical thickness of polymer layers.

23–26

Interference measurements using chemically etched porous

silicon chips.

27

Mass loading Detecting mass changes on resonating atomic force microscope

microcantilevers.

29, 30

8 Optical Electronic Noses198

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