Recent progress in intrinsic fiber-optic chemical sensing II

Sensorsand Actuators B, 11 (1993)43-55

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Recent progress in intrinsic fiber-optic chemical sensing II* R. A. Lieberman Physical Optics Corporation, 20600 Gramercy Place, Bldg. 100,

Torrance,CA 90501 (USA)

Abstract Intrinsic fiber-opticchemicalsensors,in whichan optical fiber, or a section of fiber, plays a role in the transduction of chemicalconcentration information into optical information, have been reported in the scientific literature since 1946. This paper presents a very brief summary of early work in the field, and reviews the progress that has taken place in both passive (evanescent-field refractometric or spectroscopic) and active (chemically modified cores or cladding) sensing since 1989. Intrinsic fiber-optic refractometry continues to attract attention because of its simple geometry and its compatibility with a variety of hostile sensing environments. Evanescent-wave spectroscopic sensors have gained acceptance in some of the same fields as refractometric sensors, but are restricted to use in special environments because of stiff competition from plain ‘end-coupled’ spectroscopic approaches. Fluorimetry and surface-enhanced Raman spectroscopy (SERS) are two subfields in which direct evanescent spectroscopy remains strong. Coating-based sensors have made the most rapid progress. Recent reports include the development of hydrocarbon sensors based on refractive-index-changing polymer coatings, and numerous examples of coatings doped with fluorescent and absorptive sensor dyes. Core-based sensors, particularly those based on porous optical fibers, have undergone a great deal of development over the past few years. The first reports of intrinsic fiber-optic chemical sensor products involve these sensors. In general, intrinsic sensors have attracted increased attention over the past few years, primarily as a result of the interest in the sensing community at large in distributed and multiplexed sensing methodologies. These sensors offer the promise of true distributed measurement, in-line multiplexing, and continuous large-scale production.

Intrinsic fiber-optic sensors differ from extrinsic sensors in that, in intrinsic sensors, optical energy does not have to leave the lightguide to perform its sensing function (see Fig. 1). The intrinsic technique is well known in the realm of physical sensing: fiber-optic gyroscopes, hydrophones, and several types of sensors for vibration, magnetic and electric fields, and even temperature, have been extensively discussed and reviewed by many authors. In chemical sensing, on the other hand, most efforts have been geared toward the

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V-9 Fig. 1. Intrinsic vs. extrinsic fiber-optic chemical sensors: (a) extrinsic optrode; (b) intrinsic optrode.

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creation of sensitive structures that are then ‘read out’ by completely passive, generally multimode, fiber-optic ‘light pipes’. Although a surprising number of these ‘optrodes’ actually employ intrinsically sensitive sections of optical fiber, for the most part chemical fiberoptic sensor researchers have tended to ignore the almost unique opportunities afforded by the transmission medium they have chosen to employ for routing their optical signals between source and detector. Intrinsic fiber-optic chemical sensors can comprise either relatively short ‘optrode-style’transducer sections or long pieces of chemically sensitive fiber with potential uses in distributed monitoring. In most reported cases, fibers are made sensitive to particular chemicals or classes of chemicals after being manufactured in relatively conventional ways. In a few cases, however, fibers have been specially manufactured using designs that make them intrinsically sensitive to chemical concentration from the moment they are drawn [ 11. The clear advantages of this technique for some fiber-optic chemical sensing applications have led a number of groups to work on intrinsic fiber-optic chemical sensors. In particular, ‘long’ intrinsic sensors are uniquely suited to distributed sensing applications. For example, if it is desired to measure the average chemical concentration over a large area, a long (several meters) extrinsic sensor covering the area could be a very

@ 1993- ElsevierSequoia.All rights reserved

economical alternative to the use of many point sensors and subsequent multipoint averaging. ‘Short’ intrinsic sensors are potentially compatible with large-scale production techniques. In fact, given the very low cost of telecommunications-grade fiber, projected costs of a few cents per optrode are completely within reason for sensor elements that are cut a few centimeters at a time from a multi-kilometer spool of intrinsically sensitive fiber drawn on a single day. For many specific applications, technological considerations also favor the use of intrinsic sensing techniques. One very commercially important example is in immunological sensors, where the intrinsic fiber-optic technique carries with it many of the advantages that have made its bulk-optical analog, the total internal reflection fluorescence (TIRF) technique, the method of choice in many biochemical analysis laboratories. Whereas several authors have reviewed individual classes of intrinsic chemical sensor, and others have reviewed the entire field of fiber-optic chemical sensing, very few works have appeared that review the intersection of these two fields. One comprehensive review appears in a book by Wolfbeis [2] and has already been updated once [3]. The present article is the third in this series, and covers developments in intrinsic fiber-optic chemical sensing between mid-1990 and early 1992, while including some background material and some references previously excluded from ref. 4. As before, planar and channel waveguide sensors are omitted from consideration for simplicity. Broad trends in intrinsic fiber-optic chemical sensor development are noted, but the main goal of this review is to provide a concise summary of recent work in this important branch of fiber-optic chemical sensor technology.

can be used to transfer energy out of the core to absorbing species in the surrounding medium (evanescent absorption, the fiber-optic version of attenuated total reflection (ATR) spectroscopy), to create fluorescence in the region outside the core (evanescent excitation), or to couple fluorescence from the surrounding medium into the fiber core (evanescent collection). The extent of the evanescent field and the proportion of electromagnetic energy carried by it depend strongly on the refractive-index difference between the fiber core and the medium around it (the local cladding). In addition to the evanescent field, other mechanisms have been used to create intrinsic fiber-optic chemical sensors. For example, thermal [S] and stress-induced [6] changes in optical pathlength, induced in fibers by sensory coatings, have been used as transduction methods in interferometric sensors. Core-based optical sensors, discussed below, provide another alternative to evanescent-field-based sensing. Intrinsic fiber-optic chemical sensors can be divided into four broad classifications (see Fig. 2): (1) refractometric sensors; (2) evanescent-wave spectroscopic sensors; (3) coating-based sensors; and (4) core-based sensors. Recent scientific and technological progress in each of these classes is presented in the following Sections. For those readers unfamiliar with this field, a very short synopsis of the differences, and some highlights of earlier work, are presented below. Intrinsic refractometric sensors are those in which the optical-fiber transmission properties depend on the refractive index of the medium they are in. Such sensors

Intrinsic fiberqtic chemical sensors In an intrinsic fiber-optic chemical sensor, the fiber itself is involved in the conversion of chemical concentration into optically encoded information. Transduction principles used by these sensors include most of the techniques used in extrinsic sensors, but the features of intrinsic sensing sometimes give rise to unusual variations on these techniques. The ‘workhorse’ of intrinsic sensing, used in many different sensing schemes, is the evanescent field associated with the propagation of light in dielectric waveguides. Because intrinsic sensors require the fiber to interact directly with the medium in which the chemical measurement is being made, while the light continues to be guided by the fiber, it is natural that this non-oscillatory electromagnetic field, which extends beyond the boundaries of the core, is often employed to couple the optical field of the fiber to the chemical milieu in which it rests. Evanescent fields

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(4 Fig. 2. The four types of intrinsic fiber-optic chemicalsensors: (a) refractometer; (b) evanescentspectroscopic;(c) active coating; (d) active core.

can either be very short segments of fiber from which the jacket (and often the cladding itself) has been removed [7], or they can be relatively long pieces of optical fiber that have been specially created, or treated [S], to allow the analyte access to the evanescent field of the light travelling in the fiber core. In either case it is the change in the fiber’s physical waveguiding properties, rather than optical absorbance, that causes the fiber to react to the presence of the analyte. In effect, a change in the refractive index of the cladding causes a change in the ‘total internal reflection angle’ at the surface of the core, and so the efficiency of optical transmission is changed. To enhance this effect, sensors that make use of multimode fiber often have incorporated a means of emphasizing the ‘rays near cutoff (actually higher-order waveguide modes) in the lightlaunching apparatus [91, Alternatively, single-mode cores and tapered fibers have been used to restrict the propagation of light to only the most cladding-index sensitive modes. The most common method, by far, to enhance the higher-order mode content is to bend the fiber, either in a single ‘u’ or in serpentine [lo] or other more complex curves. In practical applications, intrinsic refractometric sensors have primarily been used to measure liquid level, and the concentration of dissolved solids (such as sugar) ot liquids (such as organochlorides) in clear solutions. Evanescent-wave spectroscopic sensors, as the name implies, differ from refractometric sensors in that the change in fiber-transmission properties is caused by the optical absorbance [ 11, 121 of the sample, rather than its refractive index (i.e., the real part of the complex dielectric permittivity rather than the imaginary part), or by some other spectroscopic aspect of the sample, such as intrinsic fluorescence. These sensors do, however, require that the analyte either replaces the cladding or permeates it to an extent that puts significant amounts of it within a few microns of the fiber core. For evanescent-wave spectroscopic sensors, optical waveguiding conditions are somewhat less critical than for refractometric sensors, but for peak efficiency, designs that emphasize higher-order modes [13] or incorporate single-mode [14] or few-mode [15] structures have been used. Evanescent-wave spectroscopic sensors have been tested for use in a wide variety of applications, from monitoring the state of cure of various thermoplastic or thermosetting materials [16] to measuring gas concentrations [ 171. Coating-based sensors represent the largest class of intrinsic fiber-optic chemical sensors. Many can be considered as straightforward extensions of evanescentspectroscopic sensors, where the spectroscopy is now performed on a coating placed in the region of high evanescent field near the core. In this case, the analyte

diffuses into the coating to induce changes in the fluorescence [1,18,19], absorbance [20,21], or some other spectroscopic property of the cladding itself. Usually, the cladding is a permeable material ‘doped’ with a reagent that interacts with the analyte. This ‘active’ technique carries with it the same advantages as in extrinsic sensors: many analytes have no prominent spectral or other properties that can be optically addressed at wavelengths convenient for fiber-optic sensing. The added reagent serves as a means for creating a usable optical change for such compounds. Several novel coating-based sensors that do not make use of evanescent-field effects have also been reported. For example, fluorescent coatings may be placed on the outside of the cladding, and still couple energy into the core via secondary fluorescence [22], and coatings that undergo shape changes [ 61 or heat up in the presence of an analyte [23] have been used successfully in interferometric chemical-sensing schemes. Coating-based fiber chemical measurement systems have found enormous applicability, and show great commercial potential in the realm of biosensing. In particular, although some would argue that the singleuse nature of these devices makes them ‘probes’ rather than ‘sensors’, fiber fluorescent immunoassay is being pursued quite actively by a number of commercial groups. The intrinsically sensitive fiber-optic elements would be used as ‘disposable’ detectors, read out by a relatively simple optical system that could potentially be placed on a physician’s desk-top. Such devices could be made sensitive to a practically limitless range of biological substances, drugs, etc. Other applications for such devices include environmental monitoring and the detection of chemical warfare agents. Long coatingbased sensors for oxygen [l] and ammonia [24,25] have been reported, but this seems to bc an area in which few workers have the resources required (essentially, a fiberoptic drawing laboratory) for the creation of reliable samples. Core-based fiber-optic chemical sensors have attracted some attention because of their very high optical signal levels. In contrast to the first three types of intrinsic sensor discussed above, these sensors place analyte (and the sensing reagent) directly in the region of maximum optical energy flux: the core of the fiber. Most of the work on core-based sensors has involved the use of novel fibers ,made of porous glass, which have then had chemically sensitive reagents immobilized physically or chemically on the surfaces of the pores to sensitize the core to the analyte of interest. Early experiments reported sensors for pH [26], humidity [271, and ammonia concentration [ 281,and an irreversible carbon monoxide probe [29]. Although diffusion times can be significantly larger for core-based sensors than for sensors based on thin ( z IO-20 um) coatings, the fact that

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the optical signals can potentially be orders of magnitude larger than for sensors that rely on the evanescent field means that, in applications where very fast response is less important than sensitivity, this technique may find important applications. Overall, the field of fiber-optic chemical sensing has grown remarkably during the past few years. Intrinsic sensors have shared in this growth, and several commercial efforts have been launched around systems that use these devices.

Refractometric sensors Since many fiber-optic refractometer products are now being sold commercially, there has been only a limited discussion of these devices in the recent literature. A critical review presented by Ulrich elsewhere in this volume [30] gives a rather complete treatment of fiber refractometry, including intrinsic fiber-optic refractometers. Ulrich’s paper has a good discussion of some frequently overlooked aspects of fiber refractometer design. A conventional straight stripped-fiber refractometer engineered for use in the processing of edible oils is reported by Cole et al. [31]. This refractometer, in which a bare silica fiber of unspecified diameter and length is suspended within a flow cell and illuminated with a HeNe laser, was capable of measuring refractive indices in the range 1.42 to 1.455 with four-decimalplace reproducibility. Data taken using solutions of corn syrup in water (to produce known refractive indices) fit very simple theoretical predictions surprisingly well over the range 1.43 to 1.45. In another simple refractometer-based design [32] a short length of 1 mm diameter stripped plastic (PMMA) core is ‘striated’ to make it resemble a threaded screw. These striations cause a great increase in scattering loss for the threaded section, but also make it much more sensitive to the refractive index of the surrounding medium. Although no theory is presented, and the reported experimental results are rather minimal, this very inexpensive device, acting as a link between a very low-cost source-detector pair, has its transmission vary by more than a factor of two when placed in soils containing different amounts of water and gasoline. In the proceedings of a recent conference on fiberoptic chemical sensors, Take0 and Hattori [33] present a paper on a relatively conventional U-shaped refractometer used for a rather unconventional purpose: pressed to human skin, the device is able to measure the moisture content of the skin, and hence get a measurement of ‘dermatological health’. The same device,

which was created by stripping the nylon cladding from a silica fiber of unspecified diameter, and heating it with a carbon dioxide laser to ‘set’the bend (of unspecified radius), was tested as a liquid-level meter, a relative humidity sensor, and as a part of an electrically heated ‘dew point’ determination system. An intrinsic fiber-optic refractometric probe almost identical to that described by Cole et al. [ 3 11,composed of a 2 cm long piece of stripped silica fiber (of known but unspecified diameter), was suspended in a flowthrough cuvette by Heideman et al. [34] in a much more sophisticated detection scheme (see Fig. 3(a)). The entire refractometric cell was placed in one arm of a bulk-optic Mach-Zender interferometer, and the changes in the optical pathlength of the refractometer fiber were monitored by measuring intensity changes in the central fringe of the circular fringe output pattern produced at photodetector PDl. By tilting mirror Ml in the interferometer, the numerical aperture (launch angle) of the HeNe beam launched into the fiber was controlled to investigate the dependence of the sensitivity of the system on this parameter. By careful alignment, only meridional rays

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refractometer: (a) apparatus: BS = Fig. 3. Interferometric beamsplitter, PD = detector; M = mirrors; (b) phase change A$ per centimeter interaction length as a function of refractive index n2 at different coupling angles a.

41

were allowed to traverse the fiber, and the resulting data (see Fig. 3(b)) agreed very well with theoretical calculations developed in the paper. The authors reported the ability to measure refractive index with 0.001 accuracy, but the technique would be capable of far better performance if more advanced interferometric readout techniques were used. He&man et al. plan to use a similar device in a refractive-index-based immunosensor. It is even possible that, with proper optical engineering, such an approach could achieve sensitivities comparable to those obtained using surface plasmon resonance to detect refractive-index changes. In summary, work continues on the development of intrinsic fiber-optic refractometers for a number of specific applications. The use of intrinsic sensors, which tend to average refractive-index variations over their length, is advantageous in situations where variations in refractive index take place on a scale comparable to the size of smaller (optrode-style) fiber refractometers, and so these devices will remain an important type of intrinsic fiber-optic chemical sensor. Evanescent-wave spectroscopic sensors

Recent research on evanescent-wave spectroscopic fiber-optic sensors has concentrated mainly on pushing the wavelength range of the technique into the midinfrared, or ‘chemical fingerprint’, region of the electromagnetic spectrum. Several groups are active in the use of novel non-silica fibers to achieve long-wavelength (3-8 urn) transmission over significant lengths. Others have even achieved a measure of success in ‘short-haul ( < 20 cm) ’ applications by using high-brightness sources to illuminate silica fiber at wavelengths near 3 um. One of the objectives of these experiments has been to develop intrinsic sensors that can follow the course of chemical reactions, particularly in the fabrication of polymer/fiber composite materials. The other, and perhaps the main, application for which fiber evanescent-wave probes are being developed continues to be the detection of low levels of chemical substances in gas and liquid solutions. It should be noted, however, that virtually all work on fiber-optic evanescent fluorescence during the past two years has been directed toward the detection of signals from fluorescent ‘chemical recognition’ (active) compounds placed on the surface of the core or in cladding around the core. For this reason, this Section discusses only absorbance-based evanescent-wave spectroscopic sensors. One prolific group in the field of fiber evanescentwave spectroscopy during the past few years has been active at Dublin City University, Ireland, since 1989. An excellent paper in this volume by a leading member of this group, MacCraith [35], presents an excellent overview of the field of evanescent-wave techniques, including material on both passive (evanescent-wave

spectroscopic sensors) and active (coated fiber sensors) methods. MacCraith also delves into some of the more practical questions that must be answered before evanescent-field sensors become widely used outside laboratory or other ‘clean’ environments. After performing a study of the evanescent-field detection of methylene blue in water using a stripped silica fiber [36], the group moved rapidly into the field of mid-IR evanescent fiber spectroscopy. The initial study uncovered the major difficulty experienced by evanescent-field sensors: ‘fouling’, the adsorbance of analyte molecules onto the surface of the core, causing nonlinear response. In particular, data collected at Dublin, as well as data taken from other work, were used to show that, in many cases, the transmission loss of a fiber at a particular wavelength increases, not linearly as one would expect, but with the square root of the concentration of analyte molecules. Although it may not be the only cause of this effect, fouling is an important problem that plagues evanescent-field sensors more than other types of optical sensors. This is because it is essentially a surface effect, confined to precisely the spatial region where the evanescent field is the highest. For simple end-illumination style optrodes, the bulk absorbance or fluorescence is measured, thus greatly mitigating the effects of surface contamination on system response. Two subsequent papers by Ruddy and coworkers [37, 381 describe the use of a 300/320 urn Teflonclad fluoride glass fiber to sense propane through the evanescent-field absorbance associated with gas that diffused into the cladding. Experiments were performed with 12 cm and I m sections of fiber, and both yielded the extremely long response times one might expect from diffusion into such a hard material as Teflon. The response time to achieve significant signal levels in mixtures containing the lower explosive limit (LEL) of propane (2%) was projected to be about 4 min. Because of these long diffusion times and the relatively low sensitivity of multimode fibers, an unclad fluoride fiber, prepared by etching 2 or 3 cm of its cladding away in place in the test cell, was also subsequently tested in these experiments. The response time to methane at its LEL (5%) was projected to be 2 min for this stripped fiber. A contemporaneous paper [39] by Ruddy discusses theoretical considerations in evanescent-wave absorbance spectroscopy, and offers a partial explanation of the fact that the stripped-section sensor does not give a great increase in sensitivity over the clad fiber. In this paper, the foundation is laid for a full model of the cladding loss phenomenon as it applies to evanescentwave spectroscopy in multimode fibers. Although the paper restricts itself to TE modes in the fiber, and limits its actual calculations to meridional rays, it does deal

48

10

20

30

40

50

60

70 @c

Fig. 4. Ratio of y to rti for 0: = 75” as a function of 0. Effective absorbance coefficient for meridional TE rays. ‘Stripped tip’ in n < nclad medium.

directly with the commonly encountered case where a ‘normal’ (clad) optical fiber is used to deliver light to a ‘strongly guiding’ sensor section. (A ‘strongly guiding’ fiber is one for which the refractive index of the core is much greater than that of the cladding; this case applies for all stripped-fiber evanescent absorbance gas sensors, as well as to many liquid evanescent sensors.) Ruddy found that the sensitivity of such a sensor is less than might be expected from simple considerations. In particular, simply multiplying the optical absorbance of the species being detected by the fraction of power carried by the evanescent field of a strongly guiding fiber in equilibrium grossly overestimates the effective absorbance of such a sensor, unless it is ‘fed’ by a fiber segment having the same numerical aperture. In Fig. 4, the dependence of the loss of the fiber (y) on the critical angle of the launch is plotted. Notice that there is almost a linear dependence of effective absorbance on launch angle for most angles. Since the fraction of power in the evanescent field (r in the Figure) is itself a small number (typically much less than 1% for a strongly guiding multimode structure), this graph shows that very long stripped fibers are needed to create absorbances anywhere near those observed in typical bulk spectroscopic studies. In a subsequent attempt to increase gas sensitivity even more, the group accessed the evanescent field by creating a ‘polished half-coupler’ roughly 3 mm long using a single-mode 15/l 25 pm Teflon-clad fluoride fiber mounted in a plastic block [40]. Unfortunately, preliminary experiments suggest that such a sensor would actually have to be at least 7 cm long to detect 100% methane at 3.3 urn, even if the half-coupler structure were ‘ideal’. A quite acceptable response to liquids such as isopropanol was, however, seen for the 2 mm sensor.

Colin et al. [41] also discuss a technique to increase the evanescent-field spectroscopic sensitivity of a multimode fiber. In this case, a short (12 cm) section of 100/120 pm (low-water silica/down-doped silica) fiber was mounted in a flow cell, and ‘fed’with a conventional FTIR through a longer section of clad fiber. Just before entering the test chamber, the ‘feed’ fiber is placed in a serpentine ‘mode scrambler’ to enhance the higher-order mode content. By observing the absorbance of water at 5000 kaisers, the group was able to show/confirm theoretical predictions of an eight-fold increase in evanescent-field spectroscopic sensitivity when the fiber was subject to periodic lateral displacements on the order of 0.9 mm. A group at Rutgers University recently treated the performance of a chalcogenide-based long-wavelength evanescent-wave spectroscopic fiber for detecting contaminants in liquids [42]. Chalcogenide glasses are transparent in the 6-12 pm region, and so are suitable for monitoring the fundamental infrared absorbance lines of materials such as organic liquids. For a 380 urn diameter section of unclad fiber 15 cm long, experiments showed detection limits on the order of 3% for liquids such as acetone, ethyl alcohol, and sulfuric acid, mixed in water. Another ‘exotic’fiber, sapphire of unspecified diameter, was used to monitor the curing of epoxy resins by Druy et al. [43] a group that is quite familiar with this application [44, 161.This evanescent-field sensor, linked to a conventional FTIR by means of fluoride fiber cables, easily survived a complete cure cycle (which involved heating to temperatures above 350 “F), and demonstrated an incremental improvement in reliability and ruggedness compared with earlier work by the same group. Coating-based sensors

Numerous papers within this volume touch on, or are devoted to, coating based techniques. During the past two years, there has been a surge of interest in this variety of intrinsic fiber-optic chemical sensing. One reason for this increase in interest has been the ‘coming of age’ of fiber-optic-based fluoroimmunoassays. As commercial products begin to creep into the marketplace, several sub rosa research efforts are coming to light. Another reason is that, because of their versatility, fast response times, and relative ease of implementation, coating-based techniques offer the researcher a very attractive set of tools for the design of practical intrinsic or distributed sensing systems. A truly excellent set of papers covering many aspects of fiber-optic fluoroimmunoassay has been collected by Wise and Wingard [45]. In this book, Thompson and Ligler, of the US Naval Research Laboratory, present a brief but well-balanced overview of the main biochemi-

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cal and optical engineering issues faced in the development of practical coated-fiber immunosensors. Of particular interest is a section on the future of fiberoptic immunosensing, which outlines some of the problems and potential solutions still to be encountered in the realization of truly reliable sensing systems. In another recent paper, this group briefly presents some interesting results on the dependence of total (excitation and collection) fluorescence on fiber parameter (‘V number’) [46]. These authors found that, when the V number is varied by putting the active end of a stripped-tip fiber coated with a thin film of fluorophore into media of different refractive indices, an ‘optimum V number’ exists for which the total fluorescence (excitation and collection) is maximized. This interesting result is a consequence of the non-optimal launch and return conditions occasioned by the geometry used (the higher-order modes in the fibers, which really carry and excite most of the fluorescence, are stripped out for the higher V numbers). This also explains why very few modern authors describe evanescent-fluorescence-based sensors that make use of conventional fibers leading to stripped sections. This edition of Sensors and Actuorors contains a paper by this group describing the use of tapered-fiber techniques in immunoassays, building on the earlier work of Villaruel et al. [ 15,471. Also in Wise and Wingard’s book, Love et al. [48] present what is probably the ne plus ultra of ray-opticbased analyses of the excitation and collection of fluorescence from thin coatings on short optical-fiber probes. This thorough in-depth study of the problem, which arises from a longstanding effort [49,50], has resulted in software that can be used as a ‘design tool in the creation of practical immunoprobe designs. Several important insights into coated-fiber fluorimetry are given, and relatively good agreement with experimentally derived results is presented to verify the validity of the theoretical treatment. One aspect of the problem that is treated is the variation of efficiency with launch angle. The difference between signal intensity and background intensity is predicted to depend on the eighth power of the launch angle, up to the cut-off angle of the waveguide. This remarkable result, discussed in earlier work [ 501,is nicely confirmed by experimental data (see Fig. 5). The effect of spot size on efficiency is also treated, and although the result is no surprise (precisely filling the core is the best), the theoretical framework provides a basis for the treatment of the effect of slight mistakes or variations in spot size. The effect of the inherent numerical aperture of the step-index fiber-optic structure may surprise some workers in the field. The lowest numerical apertures actually have the highest signalbackground levels, with the signal levels varying as the inverse fourth power of the ‘material NA’, as shown by

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the following equation, which summarizes the key results of this impressive modeling and experimental effort: S,, cc loLa(r~,/a)2 sin800maX(N.4m,,“) where S,,, is the total fluorescent signal obtained from one end of the fiber, 1, is the optical source radiance, L is the fiber length, a is the fiber radius, r,,, is the radius of the launch spot, coma is the maximum (external) angle of the launched light cone, and NA,,, is the material NA, (nkre - n&dding)‘j2. The same group has published several papers detailing practical results and ‘benchmark’ data from various systems designed with the aid of the theoretical foundation presented in ref. 48. For example, a study of the use of a high-yield fluorophore (phycoerythrin) in a sandwich-assay-type sensor [ 5 l] shows that this particular system may have an ultimate sensitivity on the order of 1O-22 moles. This implies that the 1 mm diameter fiber tip theoretically may have the potential of detecting only six molecules of fluorophore! This result, however, is only theoretical, based on extrapolation from data taken with tluorescein-labelled digoxin. Excellent

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agreement between fiber fluoroimmunoassay and radioimmunoassay results is also presented in this paper, as well as in a paper [52] that discusses the binding of fluorescein-labelled ferritin antibodies to ferritin antigens immobilized on the surface of a fiber probe. In this work, which also contains a very good summary of some of the practical problems to be solved in the biochemical development of reliable probes, it is estimated that roughly lo-l6 molecules were bound on the 1 cm long fiber probe. Another paper [53] presents the results of an experimental study on the detection of the cardiac-specific isoenzyme creatine kinase-MB (CKMB) using phycoerythrin. In this study, levels in the 1- 10 picomolar range were measured for sample incubation times of approximately 15 min. A paper in this volume presents some recent findings of this group [ 541. Lackie et al. [55] have continued their work on the engineering of a compact fiber immmunoassay system that makes use of a rather novel ‘button’ design incorporating an immunoprobe fiber, built-in launch optics, and a mounting grommet in a single, very low-cost structure (see Fig. 6). Lackie et al. make the point that multimode fiber probes, when illuminated at high NA, should be much more sensitive than either planar or buried-channel multimode integrated optical waveguides, since, in the ray approximation, the cylindrical geometry essentially allows the sensing chemistry to be coated on all reflecting faces of the waveguide. For buried (or planar) guides, half (or 3/4) of the surface reflections take place in surfaces that cannot be reached by the analyte. Compact portable systems developed by this group have been produced and sold to a number of customers, although mass production of ‘button probes’ has not yet occurred. Krull et al. [56] are continuing their work on fiberoptic evanescent-fluorescence biosensors, and have reported a novel method of immobilizing fluorescentlabelled biomolecules on the surface of fibers. This group

Fig. 6. ‘Button’ immunosensor incorporating mounting seal.

transducer lens and

reports the use of egg phosphatidyl choline and cholesterol lipid membranes to stabilize and attach pyrene-labelled concanavalin A, and, in a separate experiment, to stabilize the receptor monosialoganglioside on the fiber. Interactions of these two probes with, respectively, fluorescein-labelled dextran and concanavalin A led to the conclusion that the target substances did indeed react with the lipid-stabilized species, but that further work is needed to minimize the effects of nonspecific binding of the targets on system performance. One of the most important results of this work is the proof that lipid membranes can be applied to optical fibers and used to immobilize active biomolecules. Many other groups are working on fiber fluorosensors that use biological reagents, but the scope of this review makes it impossible to cover all of their activities. Several interesting papers on non-biological coatedfiber intrinsic chemical sensors have also appeared during the last two years. For example, in an extension of a technique discussed in the previous Section, a thinned-cladding single-mode fiber ‘half-coupler’ was coated with hydroxy pyrenetrisulfonic trisodium salt (HFTS) to create an ‘intrinsic’ coated-fiber pH sensor. Fluorescence from the HF’TS coating was excited and collected by the fiber, and the response of the fiber sensor was defined as the ratio of the fluorescence intensity at two wavelengths corresponding to the emission peaks of two enantiomers of the pa-sensitive dye. Response in the range pH 4- 10 was compared with the response of HPTS in aqueous solution, and was found to agree to within 0.1 pH in the range pH 5.2-6.5. No attempt was made to calculate the fluorescence collection and excitation efficiencies of this sensor. Recent theoretical work by Egalon and Rogowski has shed a great deal of light on the efficiency with which fiber cores collect fluorescence from sensory coatings. Egalon uses a sophisticated wave-theoretic approach to treat the general (neither limited to weakguiding fibers, nor to meridional rays) problem of coupling from sources distributed throughout a cladding to guided modes on the fiber core. Counter to the intuitive notion presented in the earlier paper by Lieberman [4], which argues (as does Marcuse [ 571) that the coupling efficiency will always increase with increasing V number, Egalon finds that V is, in fact, not a good predictor of the efficiency of collection of fluorescence from the cladding. Figure 7, taken from this work, shows that the efficiency can either increase or decrease with increasing V number. In fact, a much better ‘rule of thumb’ is that the efficiency increases with increasing n,,, - ncladding,and with increasing wavelength. A good independent variable for characterizing collection from fluorescent claddings is Ku = 27r/L. Note that Egalon’s results are for true bound modes in the fiber, and do not take into account the transport of

51

011 0

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100

120

140

160

V

Fig. 7. f’,,,, of a bulk distribution of sources vs. the V-number. This graph was obtained by varying the wavelength, 1, from 0.45 to 2.0 pm. The core radius, the cladding radius, and the indices of refraction of the core and cladding are held fixed at a = 5.0 pm, b = 25.0 pm, n,,, = 3.0 and nslad= 1.I.

energy by leaky modes, which probably carry a great deal of the energy in short-fiber coated fluorosensors such as those commonly used by workers in immunosensing. Egalon was able to reproduce precisely the plot of efficiency versus V number published by Marcuse [57], so both are in agreement with the single ‘data point’ available to check these theories [ 11. Egalon has also shown [58] that the same general comments hold true for fluorescent sources that are immobilized in a thin layer at the core/cladding interface. Furthermore, as expected, the collection of fluorescence from sources in this thin layer is at least two orders of magnitude more efficient than collection from sources in the bulk cladding. Lest the reader be mislead by this result: bulk claddings include a thin layer of sources near the core, so the total energy coupled into the fiber will be greater from bulk distributions of cladding sources than from thin layers with nonfluorescent cladding around them, if the concentration of the fluorescent sources is the same in both cases. The main importance of the result is to show that, even for bulk-distributed sources, most of the guided energy is coupled from the sources very near the core. Egalon has also begun to investigate the dependence of coupling efficiency on polarization of the fluorescent emission. Fluorescent molecules generally emit in only one polarization, and some TIRF workers have achieved high degrees of polarization in fluorescence emission from molecules on flat surfaces. Preliminary results [59] show that sources polarized parallel to one another and perpendicular to the axis of the fiber couple fluorescence emission to guided modes approximately 1.2 times as well as do randomly oriented sources, and nearly twice as well as do sources that emit photons with their polarization vectors parallel to the axis of the fiber.

La1 and Yappert [60] have discussed the use of the transparent jacket of a conventional fiber as a ‘secondary waveguide’, since it is bounded by low-index materials, i.e., the cladding on its inside surface and air on its outside. They purport to show the excitation of fluorescence in the jacket by the evanescent field of light being transmitted in the fiber core. The. data clearly show that, when light is launched carefully only into the core, some jacket fluorescence is observed. Furthermore, when the fiber is exposed to iodine vapor, this fluorescence is quenched. It is, however, very doubtful that this fluorescence is excited by the evanescent field of light travelling in the core, since the cladding is 20 urn thick and the step-index geometry of the fiber means that the evanescent field is negligible more than a very few microns away from the surface of the 100 urn diameter core. Tt is more likely that leaky rays created by bends or scattering centers in the fiber are launched across the core/cladding interface and are subsequently absorbed directly by the jacket. A coated-fiber intrinsic sensor with a conventional geometry, but a rather unconventional coating, is reported by MacCraith et al. [61]. A tetraethyl orthosilicate (TEOS) sol-gel, with HPTS dissolved into it during the precursor stage, is dip-coated to a depth of 0.3 urn onto a 3 cm long section of bare fused silica. The doped sol-gel is then oven-dried to produce a hard porous low-density cladding. When the resulting sensor is illuminated with 488 nm laser light, the resulting fluorescence depends very predictably upon the pH of aqueous solution into which the sensor is placed. Furthermore, the pH dependence is remarkably similar to the free-solution spectrum of the same dye. MacCraith et al. hold out the possibility that this process could be automated, resulting in true distributed sensors for pH. Work of a similar kind is reported in the current volume [62]. Muto et al. [63] have continued their earlier work [64] in the application of fluorescent plastic fibers to monitoring chemical concentration. Their latest work presents a humidity sensor based on the quenching of fluorescence of umbelliferone immobilized in a polymethylmethacrylate (PMMA) film on a clear fiber. This sensor was side-illuminated by a halogen lamp, and a simple photodetector was used to record the fluorescence intensity. The sensor was used as a breathing monitor, and when connected to a peak-processing comparator circuit, produced surprisingly good results. Perhaps the most innovative coating-based tiberoptic chemical sensor scheme to have been proposed during the last two years is that of Meltz et al. [65]. In this work, light is directed out of the fiber core by an intrinsic grating, impinges on a fluorescent sensor region, and the resulting fluorescence is captured by another grating in the core. The excitation energy

52

would thus travel with its low-loss equilibrium power density until it reaches the chemical-sensing site. To summarize: the most rapid progress being made in the field of coating-based intrinsic fiber-optic chemical sensing is in the understanding of the basic phenomenon of the excitation and in-coupling of cladding fluorescence through the evanescent field of modes bound in the fiber core. With this knowledge, sensor designs are becoming more rational and more efficient. Several rules of thumb have been put on firm theoretical underpinnings, and a few have been thrown into question as the field begins to mature. Core-based sensors There are still very few groups working on corebased intrinsic fiber-optic chemical sensing. Wei et al. [66] have begun efforts to put the earlier results of workers such as Fuwa et al. [67-691 on a firmer theoretical footing. Using an ensemble average of the behavior of meridional modes, Wei et al. have arrived at explanations of some of the more puzzling features of these types of sensors. For example, it has been observed that the transmission loss of liquid-core fibers does not depend, as expected, in a linear way on either the length of the fiber or the concentration of absorbent species in the liquid. By arriving at a general expression for the power transmitted by the liquid core, and then taking various parameters to their limits, this approximation appears to be able to explain the observation that loss is proportional to concentration for low concentrations, but varies as the square of the concentration in the high-concentration limit. Plans are in progress to take new experimental data to test the validity of this theory further. Egalon ef al. [70f have applied the mode-theoretic approach, mentioned in the Section on coated fibers above, to the inverse of the problems mentioned in that Section: the efficiency of coupling core fluorescence to guided modes. Intuitively, it would seem that all, and only, the photons emitted within the numerical aperture cone of the fiber would be coupled to guided modes. While Egalon et al. show that this is essentially true, they do arrive at one surprising result: for a fixed amount of fluorophore, coupling is actually more efficient if the fluorophore is concentrated at the core/cladding boundary than if it is evenly distributed throughout the core. Although fluorescent-core fibers will still always be capable of being made brighter than fluorescent-skinned fibers, this result means that those sources closest to the cladding actually influence the amount of captured fluorescence disproportionately to sources closer to the core. This has several implications, including a non-exponential time response for sensors based on the permeation of fluorescence-quenching agents into the active core of an intrinsic fiber-optic chemical sensor.

The pioneering work on porous fibers, begun by Macedo [26], Shahriari [28], and Zhou [29], is being carried forward both at Rutgers University and in commercial research and development groups. Workers at GeoCenters, Inc., have reported interesting results on a number of measurands [71,72]. Among the devices under development are a reversible porous-glass pH sensor with response in the range pH 4.0-7.0, and a reversible ammonia sensor that promises responses in the low ppm range. Also recently demonstrated are irreversible indicator-based probes for the ‘one-way’ detection of hydrazine and ethylene, which can respond to ppb (1 part in 10’) levels of these compounds. Work continues at GeoCenters on reversible humidity and moisture sensors and the irreversible CO probes, first demonstrated at Rutgers. Recent reports [71] indicate that permeable polymer fibers may hold more promise as core-based sensors than porous-glass fibers, at least for some applications. In addition to these results, recent work at Physical Optics Corporation has shown that reversible response to carbon monoxide can be obtained using organometallic complexes immobilized on porous fibers. Other work shows that, possibly because of its similarity to silica gel (often used by fluorescence spectroscopists to enhance signals), porous fiber may offer distinct increases in the sensitivity of quench-based oxygen sensors when compared with other immobilization media.

Conclusions As with other types of fiber-optic chemical sensors, intrinsic sensors have, in the past few years, made important strides towards practical realization, but have yet to appear as commercially viable products. The most rapidly growing area of intrinsic chemicalsensing research is in the realm of ‘smart material processing’, where the evanescent-field technique appears to be one of the best means of measuring the state of ‘cure’ of the epoxides used in the manufacture of light-weight fiber-reinforced composite structures. By far the largest body of work on intrinsic fiber-optic chemical sensors has been devoted to research on and development of coated fiber ‘probes’(primarily fluorescence-based) for immunoassay. While the initial products expected to appear soon will be almost exclusively targeted at the medical market, the chemical versatility of immunosystems virtually assures that intrinsic fiberoptic immunoprobes and immunosensors will also find applications in environmental sensing, biotechnology, fermentation, and other, chemical process-control applications. Other types of intrinsic sensors, particularly corebased and evanescent-spectroscopic sensors, appear

53

poised for rapid development. New materials and new immobilization techniques, as well as increased understanding of the optical processes that affect the sensitivity of such sensors, are helping to bring these two types of sensors out of the research laboratory and into the ‘real-world’ environment. Markets for sensors having specific applications, like environmental monitoring, composite material curing, and the distributed measurement of humidity, will stimulate research and development efforts on these sensors, as the medical market has stimulated, and continues to stimulate, work on coatedfiber sensors. The sheer volume of work covered by this review attests to the extremely rapid growth of the field of intrinsic fiber-optic chemical sensing in the past two years.

Acknowledgements The author would like to express his appreciation for the efforts of his colleagues at Physical Optics who have assisted in preparing this article. This work has been supported, in part, by the Gas Research Institute, the New York Gas Group, and by the US National Aeronautics and Space Administration (NASA).

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