Fiber-Optic Chemical Sensors and Biosensors - Analytical Chemistry

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Anal. Chem. 2000, 72, 81R-89R

Fiber-Optic Chemical Sensors and Biosensors Otto S. Wolfbeis

Institute of Analytical Chemistry, University of Regensburg, D-93040 Regensburg, Germany Review Contents Books and Reviews Sensors for Gases, Vapors, and Humidity Ion Sensors Sensors for Organic Compounds Biosensors Applications Sensing Schemes Materials for Fiber-Optic Chemical Sensors Literature Cited

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This biannual review is based on a search based on the following query:

It covers the time period from October 1997 to January 2000. The search resulted in 677 hits. Since the number of citations in this review is limited to 200, a stringent selection had to be made. Priority was given to fiber-optic sensors (FOS) for defined chemical, environmental, and biochemical significance and to new schemes and materials. The review does not include (a) FOS that obviously have been re-discovered; (b) FOS for nonchemical species such as temperature, current and voltage, stress, strain, and displacement, for structural integrity (e.g., of constructions), liquid level, and radiation; and (c) FOS for monitoring processes such as composite curing, injection molding and extrusion, or oil drillling, even though these are important applications of optical fiber technology. Fiber optics serve analytical sciences in several ways. First, they enable optical spectroscopy to be performed on sites inaccessible to conventional spectroscopy, over large distances, or even on several spots along the fiber. Second, fiber optics, in being waveguides, enable less common methods of interrogation, in particular evanescent wave spectroscopy. Fibers are available now with transmissions over a wide spectral range. However, the transmission capabilities of most fibers are optimized for the “telecom range”, i.e., 800-1600 nm. FOS are based on either direct and indirect (indicator-based) sensing schemes. In the first, the intrinsic optical properties of the analyte are measured, while in the second the “color” of an immobilized indicator dye, label, or optically detectable bioprobe is monitored. Particularly active areas of research include advanced methods of interrogation such as time-resolved or spatially resolved spectroscopy, evanescent wave and laser-assisted spectroscopy, surface plasmon resonance, and multidimensional data acquisition. Fiber bundles are more and more being used for imaging purposes. 10.1021/a1000013k CCC: $19.00 Published on Web 05/04/2000

© 2000 American Chemical Society

Another active area of research is the design of chemistries for indicator-based sensing of clinical parameters, the “holy grail” still being sensing of blood gases (oxygen, CO2), pH, electrolytes (Na, K, Ca, Cl), and the enzyme substrates glucose, urea, and creatinine. Since numerous “chemistries” for these species have been described but lack temporal stability, schemes for referencing signals are sought. The same is true for the second major area of application, which is environmental sensing or probing. (Note: In this article, sensing refers to a continuous process, while probing refers to single-shot testing.) Other major fields of applications are in industrial production monitoring, (bio)process control, and the automotive industry. This review is divided into sections on books and reviews (A), on specific sensors for gases and vapors (B), ions (C), and organic species (D), respectively, followed by sections on biosensors (E) and by chapters on application-oriented sensor types (F), sensing schemes (G), and sensor materials (H), respectively. BOOKS AND REVIEWS Dakin and Culshaw have edited volumes 3 and 4 of their book on optical fiber sensors (A1, A2). Volume 3 is on Components and Subsystems and contains chapters on fiber grating evancescently coupled components, fiber lasers and amplifiers, OTDR detection techniques, and spectral measurement techniques. Volume 4 (A2) is on Application, Analysis, and Future Trends and contains chapters on chemical sensing via direct spectroscopy, chemical sensing using indicator dyes, in vivo medical sensors, sensors in industrial systems, distributed sensors, multiplexing techniques, “smart” fiber systems, and commercial sensors, among others. Gauglitz (A3) has updated (458 refs) the area of optochemical and optoimmunosensors including optical fiber sensors. Other reviews that have appeared cover the use of chalcogenide glass fibers for IR applications (A4; 65 refs), fiber-optic imaging sensors (A5; 46 refs), fiber-optic sensors in health care (A6; 32 refs), fiber Bragg gratings (A7; 147 refs), micro-optical sensors using solgels (A8; 12 refs), sol-gels in fiber optics, and integrated optical chemical sensors for environmental monitoring and process control (A9; 7 refs), on ion-selective materials (A10; 32 refs) for use in flow devices, waveguides, film-type sensors, and fiber optics, respectively; on measurement of pH using fibers (A11; 44 refs), mainly for environmental and medical applications but also as transducers for sensing acidic or basic gases as well as enzymatic processes during which pH is changed, and on luminescence decay time-based chemical sensors for clinical applications (A12), in essence pH, pCO2, pO2, Na+, K+, Ca2+, Cl-, NH3, glucose, and urea. Reviews also cover the significance of sensing chloride in civil constructions where chloride causes adverse effects in terms Analytical Chemistry, Vol. 72, No. 12, June 15, 2000 81R

of stability (A13), methods for sensing methane in mines (A14), and surface refractive sensors using evanescent waves (A15). SENSORS FOR GASES, VAPORS, AND HUMIDITY This section covers sensor for gases (including dissolved gases) and humidity. Due to their intrinsic safety, optical hydrogen sensors experience substantial attention. Typical articles describe the design and development of low-cost hydrogen detectors relying on surface plasmon resonant light absorption by a thin film of tungsten oxide (B1), of sensors using Pd-coated fibers with exposed cores and evancescent field interactions over typically 1.5 cm and detecting 0.2-0.6% gas within 20-30 s (B2), and of another Pd-based system with a Bragg grating, the Bragg wavelength being shifted by the stress of the Pd film caused by hydrogen gas (B3). Sensors for methane and related hydrocarbons are another subject of interest, mainly in the context with mine safety and leakage of pipelines. Methane has been detected by direct spectroscopy at between 3.43 and 3.73 µm by use of guided-wave diode-pumped frequency generation (B4). A fiber-optic system for methane detection was reported (B5), also operating in the IR (rather than mid-IR) and capable of detecting methane up to the lower explosion limit. Similarly, hydrocarbons have been sensed with a narrow-line width, tunable Tm3+-doped fluoride fiber laser as the light source (B6) or by evanescent wave spectroscopy of fuels using poly(dimethylsiloxane)-coated silica fibers (B7). A fiber-optic open-path sensor for methane and related hydrocarbons was reported which applies an InP/GaInAsP superluminescent diode (B8). Both gaseous and dissolved molecular oxygen are mostly sensed via the quenching effect it exerts on fluorescent probes such as certain metal-ligand complexes. Thus, the Al3+-ferroin complex in a sol-gel matrix was used as a phosphorescent probe for decay time-based sensing of very low oxygen levels (B9). The details of a commercially available fiber sensor with a 10-µm tip and an indicator chemistry composed of a luminescent Ru3+ complex in an organically modified sol-gel have been outlined (B10). The sensor is based on measurement of luminescence decay time (in the frequency domain) and is sterilizable. Microsensors in which luminescence intensity is measured (rather than decay time) also have been reported (B11). The sensor is based on the oxygen probe tetraphenylporphyrin which was embedded into the plastic clad of a low-cost silica fiber. Fluorescence was excited by the evanescent field of light from a yellow LED. Since the decay time of the probe is only 10 ns, it may be suited for distributed sensing with high spatial resolution (B12). The ketoporphyrins form another class of long-wave-absorbing oxygen probes which in the form of the metal ion complexes have decay times in the microsecond range. If applied on a microporous and strongly scattering support, viable materials are obtained for sensing oxygen but also for monitoring the activity of oxidases such as glucose oxidase (B13). Oxygen sensing based on quenching of luminescence has been extended to fiber imaging (B14) to study the oxygen consumption by the perfused mouse heart. The observation that tetraphenylporphyrin can undergoe an increase in quantum yield in the presence of oxygen (B15) has been applied to design a FOS with the dye contained in the clad; 100 ppm to 1% oxygen are 82R

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detectable. The red luminescence of certain hexanuclear molybdenum chloride clusters was found to be quenched by oxygen, and this can be exploited to monitor oxygen between 0 and 21% in nitrogen (B16), but humidity is likely to interfere. Fiber sensors fairly selective for NO gas have been reported that make use of immobilized cytochromes (which display intrinsic fluorescence) or by fluorescently labeled cytochrome c′ (B17). In another approach, an NO sensor was obtained based on a fluorescein derivative attached to colloidal gold. Fluorescence is quenched by NO, probably because of dye rearrangement (B18). Numerous sensors have been reported for measurement of dissolved carbon dioxide. Thus, sensors for CO2 in seawater usually require extraordinary temporal stability. So-called “calibration-free” ratiometric sensors have been described (B19, B20), as have decay time-based CO2 sensors (B21, B22) and sensors with improved sensor “chemistry” (B23). Alternatively, CO2 in air may been detected via its IR absorption at 4.3 µm (B24). It is obvious that the indicator-based approaches are preferred in case of sensors for water-dissolved CO2. Detection of chemical species at very low vapor pressures was shown to be possible by partitioning of the vapor into a polymer phase contained on an optical waveguide. By using proper polymers, large concentrations of the species will accumulate in the solid phase and greatly enhance the ability to detect the species. Specifically, 2-nitrodiphenylamine was detected in the gas phase at room temperature at the few-ppt level (B25). Humidity sensors have been described that are based on highly different schemes such as a Fabry-Perot interferometric nanocavity (B26), crystal violet films on a Nafion film (B27), cholesteric liquid crystals with a cobalt chloride crystal cube and OTDR interrogation (B28), or via fiber coatings that undergo a length variation as a function of humidity (B29). The latter approach is applicable to pH as well, if a pH-sensitive coating is applied. ION SENSORS This section covers sensors for all kinds of inorganic ions including the proton (“pH”), and salinity. Numerous fiber sensor have been reported for measurement of pH, and some are minor modifications of previous work. Thus, the absorption indicator naphthophthalein in a sol-gel gave a material with an optical response between pH 4 and 11 (C1), while naphthofluoresceins were again employed for measuring physiological pH’s (C2-C4). Other sol-gels with immobilized pH probes have been described (C5, C6), one for solutions of very high acidity (C7). However, even though it is known that pH sensors are sensitive to ionic strength, no data are given. And despite the known instability of Langmuir-Blodgett films, which so far has prevented their application to sensing pH, sensors based on their use are still being reported and claimed to be practical (C8). pH-sensitive fluorescent probes out of the class of the photoinduced electron transfer (PET) probes were immobilized in plasticized poly(vinyl chloride) membranes to result in a material that can be used for fiber-optic pH sensing (C9). Since most fluorescent pH probes have decay times in the order of nanoseconds (which requires modulation frequencies in the megahertz range in the case of decay time-based sensing), probes with longer decay times are desired. Strategies to design pH sensors with luminescence decay times in the microsecond time regime have

been presented (C10). Several papers describe the use of polyanilines with their pH-dependent absorption spectra, which extend far into the near-IR. Thus, polyanilines (with various substitution patterns) were reported to be LED-compatible sensor materials that can be easily deposited on solid support by oxidation of the respective aniline using ferric chloride (C11). Clear blue membranes of polypyrrole doped with Prussian blue also display pHdependent spectra in the red and near-infrared (C12). Thin pHsensitive polymer layers were immobilized at the end of a gradedindex lens to image its far-field surroundings (C13). If placed along the distance of an optical fiber, immobilized fluorescein enables the distributed measurement of pH along the fiber (C14). Sensors for alkaline and earth alkalines are of interest for use in clinical chemistry. A widely used scheme is based on ion exchange (cation in-proton out) as introduced by Charlton in 1981. It has been extended to decay time-based sensing (of potassium) using the ion pair formed between a cationic ruthenium complex and the bromothymol blue anion along with valinomycin as the ion carrier (C15). This approach, while straightforward, suffers from its cross-sensitivity to pH. This was overcome by making use of a coextraction mechanism, a merocyamine dye being coextracted with K+ from one membrane layer into another, where it is much more fluorescent (C16). Micrometersized sodium-selective fiber sensors were obtained by placing ionophores and an anionic dye in plasticized poly(vinyl chloride) solution at the tip of fibers with tip diameters of 1-10 µm (C17). Similarly, a microsensor for calcium ion was obtained by immobilizing a fluorescein-derived probe on dextran adsorbed at the distal end of a fiber (C4). Thin-film silica sol-gels doped with ion-responsive lipid bilayers were coupled to a bifurcated fiber-optic bundle to produce a metal ion probe (C18). Ionic and neutral crown ethers were immobilized on anionic polymer membranes to give sensors for determination of Ba(II) and Cu(II) ions (C19, C20). Similarly, sensors for heavy metals (HMs) were obtained by immobilizing dithizone (C21; for mercury), pyridylazoresorcines (C22; for zinc and other HMs), or copper chelators (C23). Chromate with its intrinsic yellow color can be monitored via fiber optics in industrial processes (C24). Anions such as chloride or nitrate do not display intrinsic absorption and therefore need to be detected indirectly. A chlorideselective optical sensor based on polymer-stabilized emulsions doped with a polarity-sensitive dye was developed (C25) to measure chloride in the 1-80 mM concentration range. In another scheme, a ruthenium metal ligand complex is employed at the 15-µm tip of a fiber (C26). Its luminescence decay time is affected by chloride. Chloride sensors have been embedded in concrete bridge decks in order to measure chloride penetration, which is a major cause for deterioration (C27). Bromide (like iodide) exerts a quenching effect on the fluorescence of fluorescein immobilized on a fresh mutton film membrane (C28). A compact optical fiber refractive index differential sensor for water salinity measurements was described (C29). The accuracy is (0.5% over a wide range. SENSORS FOR ORGANIC COMPOUNDS This chapter covers sensors for (a) pollutants, agrochemicals, and nerve agents; (b) explosives; (c) drugs and pharmaceuticals; and (d) miscellaneous organics. Chloroorganic compounds form

a hardly biodegradable class of pollutants for which numerous sensing schemes have been developed. The limits of detection are a most critical feature. Chlorinated hydrocarbons can be detected in the gas phase with optical fibers coated with porous silica (D1). Direct spectroscopy via fiber optics also enabled detection of aromatic compounds via evanescent wave absorption spectroscopy (D2). The aromatics are partitioned from a 10-mL sample into the polymer clad of a 10-cm fiber. Detection limits typically are 1-18 ppm. A mid-infrared fiber system for hazardous waste identification via Fourier transform has been described (D3). Alternatively, volatile organic compounds (VOCs) may be detected in air, water, or soil by coating a long period grating structure with chemical indicators having strong affinity for VOCs. The sensor response relies on evanescent field interaction (D4). Fiberoptic surface-enhanced Raman scattering (SERS) was coupled to fiber optics to result in a sensor for environmental monitoring of typical pollutants (D5). The SERS-active medium consisted of a permeation-selective polymer on metal-coated nanoparticles or on planar layers. Alternatively, pollutants such as chlorinated hydrocarbons may be sensed by fiber-optic surface plasmon resonance using alkanethiol-coated silver films, onto which a thin layer of a siloxane polymer is deposited (D6). The limit of detection is moderate. A 266-nm microlaser was employed (D7) to excite the fluorescence of aromatic hydrocarbons in soil that can be identified by their unique fluorescence signatures. Alternatively, antibodies can be raised to benzene, to other aromatics, or to trichloroethylene, to result in respective fluorescent immunosensors (D8). A Langmuir-Blodgett type of fiber-optic organophosphorus biosensor was claimed to be useful for monitoring paraoxon in contaminated water (D9), but interferences by humic acids and detergents have not been studied. Detection is based on the inhibition of the enzyme acetylcholinesterase. Organophosphate nerve agents also have been quantified via the enzyme organophosphorus hydrolase immobilized on a nylon membrane placed at the end of a fiber/bundle (D10), and the products of the hydrolysis of the nerve agent Soman can be detected by a combination of molecular imprinting and fiber-optic lanthanidesensitized luminescence (D11). Explosives and explosive-like materials may be detected with optical fiber arrays containing differentially reactive sensor tips. Neural network analysis was employed to discriminate between organic volatiles using pattern recognition, and it was predicted that such sensors may be useful for identifying polynitro compounds at low concentrations (D12). Another immunosensor for common explosives and using a fluorescent antigen in a competitive immunoassay was used in multianalyte assays, with detection limits of typically 5 mg‚L-1 (D13). Cocaine and its metabolites in urine have been quantified using a four-channel fiber-optic biosensor. Binding of the antibody to the antigen-coated fiber is inhibited by cocaine (D14). Three hundred nanograms of cocaine is detectable in a 1-mL sample. Wang et al. have presented a number of sensors for drugs, the sensing scheme being based on dynamic quenching of luminescence by the respective analyte. Thus, berberine quenches the fluorescence of a phenyloxazole (D15), as does tetracycline (D16). Nitrofurantoin (a drug) quenches anthracene (D17), as do tetracycline (D18) and oxytetracycline (D19). Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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Alcohols usually are assayed via their (near-) infrared absorption or enzymatically via oxidases orspreferablysthe more stable dehydrogenases. Recently, alcohol analysis has been reported (D20) using a gold-coated unclad fiber sensor system that can detect alcohols in water in concentrations between 0.5 and 40% (v/v). BIOSENSORS In biosensors, a biological component is used in the recognition process. Typical components include enzymes, antibodies, oligonucleotides, and whole cells. Despite their many limitations in terms of pH sensitivity, ionic strength dependence, stability, and sensitivity to inhibitors, enzymes are widely used (and useful!) for sensing or probing enzyme substrates. Thus, bilirubin has been detected via bilirubin oxidase (BilOx) and the decrease in the oxygen partial pressure it causes when degrading bilirubin (E1). As little as 0.1 µM bilirubin is detectable. However, BilOx is not a very stable enzyme. A dual-enzyme fiber sensor for pyruvate has been reported (E2) comprising an enzyme layer placed at a fiber tip and containing both lactate oxidase and lactate dehydrogenase. The reaction is monitored via the formation of fluorescent NADH. Similarly, glutamate can be quantified with a micrometersized fiber sensor containing the enzyme glutamate dehydrogenase at its tip (E3). The limit of detection is 0.2 µM glutamate, the mass detection limit is 3 amol. New types of oxygen transducers were used to follow the kinetics of the oxidation of glucose by glucose oxidase (GOx) in a flow system (E4). The GOx was immobilized either directly on an oxygen sensor membrane or on controlled-pore glass in a microcolumn reactor. The consumption of oxygen can been monitored via changes in the intensity (E4) or decay time (A12) of an oxygen probe. Glucose and lactate also have been monitored (E5) via the electroluminescence of luminol which is induced by the hydrogen peroxide produced by the respective oxidases and diffusing to a glassy carbon electrode. Picomole quantities of hydrogen peroxide are detectable. In an optical biosensor for choline-derived phospholipids (E6), the substrates are hydrolyzed by the enzyme phospholipase D to give choline which, in a second enzymic reaction, is oxidized by choline oxidase. The consumption of oxygen is a measure for the activity of the enzyme which, in turn, is governed by the concentration of the phospholipid. The dynamic range is from 0.08 to 3 mg of phosphatidylcholine/mL. The area of optical fiber immunosensors (better: immunoprobes) is growing fast. Probes have been described for detection of antibodies to Leishmania donovani via evanescent wave fluorescence excitation at a declad and tapered length of a fiber optic (E7) and for serum anti-plague antibody (E8) based on a competitive immunoformat originally devised for detection of protein toxins or bacterial cells. Immunoprobes also find environmental application such as detection of BTEX compounds (benzene, toluene) if antibodies can be raised. The limit of detection (LOD) for benzene is 1 ppb (E9). In practically all cases, fluorescence is the preferred method for detection or quantitation. The use of heterobifunctional cross-links for immobilizing antibodies to glass cover slips has been described in detail (E10). Bacteria and bacterial toxins are mainly detected by immunological methods. Fiber probes have been described for the highly toxic protein ricin (E11) with an LOD of 1 mg/mL in river 84R

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water; for the staphylococcal enterotoxin B using a man-portable identification system (E12), and for air-borne pathogens (staphylococcus and streptococcus species) in the hospital environment (E13). A fiber-optic genosensor for specific determination of femtomolar concentrations of DNA oligomers was demonstrated (E14). A 13-mer oligonucleotide was attached to the core of a multimode fiber and the complimentary sequence detected via a ds-specific intercalator. Evanescent field excitation assisted in discriminating between bound and unbound species. Hybridization also was detected by evanescent wave fluorescence excitation using tapered fibers and NIR fluorophores (E15). A 20-mer oligonucleotide bound to the surface of a fiber could detect subnanomolar quantitites of the complementary sequence, e.g., that of a fragment of the Heliobacter pylori DNA. A fiber probe for fluorometric detection of T/AT triple helical DNA formation (E16) relies on the immobilization of decaadenylic acid on fused-silica fibers and the formation of a reverse Hoogsteen T/AT triplex at a comparably low temperature. DNA hybridization was detected by chemiluminescence (E17) after labeling the complementary strand with HRP and detecting the enhanced chemiluminescence after binding to the first strand, which was covalently bound to a fiber optic. ATP, ADP, and AMP can be detected (E18) via a compartmentalized three-enzyme system comprising adenylate kinase, creatine kinase, and firefly luciferase, respectively, and monitoring the bioluminescence in a flow injection type of arrangement using optical fibers as light carriers. Typically, 10-2500 pM nucleoside is detectable. The scheme was extended in that luciferin was incorporated into acrylic microspheres and the of co-reactands were added by controlled release (E19). APPLICATIONS This section covers typical applications of FOS to areas such as water analysis, biological and medical research, and industrial (bio)processes corrosion and combustion. An evanescent wave near-IR sensor was applied to analysis of chlorinated hydrocarbons and toluene in water (F1). Chemometric algorithms (PCR and PLS) were applied to the data sets obtained for modeling the responses of the sensors to give mean errors of prediction as little as a few ppm for several species. A prototype fiber-optic sensor for monitoring apolar (chlorinated) hydrocarbons in groundwater was described (F2). The scheme is based on extraction and enrichment of analytes into the hydrophobic coating of the quartz fiber and evanescent wave absorptiometry. Mid-IR evanescent wave sensors were applied to monitor chlorinated hydrocarbons in seawater on a ppb level, and effects of salinity and turbidity were studied (F3). A miniaturized laser-induced fluorometer with a fiber-optic probe for analysis of aromatic fluorophores such as PAHs and BTXE-type pollutants makes use of time-resolved emission spectroscopy and an all-solid-state laser operated alternatively at 266 and 355 nm (F4). A mechanically robust sensor that enables optical detection of the sediment-water interface at the sea floor exploits the increased backscatter of 780-nm light at the surface of the sediment (F5). Carbon dioxide was measured in situ in pore waters of sea floor sediments in 3300-m depth via an absorbance-based CO2 sensor (F6).

A near-IR laser system was reported (F7) for transcutaneous monitoring of cerebral oxygenation in the newborn infant. Blood and tissue oxygenation were measured at six NIR laser lines between 775 and 850 nm. In contrast to oxygen saturation (SO2) of hemoglobin, the partial pressure of oxygen is usually measured via its quenching effect on certain ruthenium probes contained in a polymer matrix. This scheme was applied in situ to measure the oxygen consumption at the working mouse heart (F8). Fiber optics enable the acquisition of spectra in vivo from tissue, and methods have been developed to suppress background fluorescence from tissue and background Raman scatter of the fibers (F9). Spatially resolved reflectance spectra from 380 to 950 nm and fluorescence excitation/emission matrixes were acquired of the oral cavity mucosa and related tissue (F10). In industrial applications, fiber optics are often used to get access to the site of measurement or to a harsh environment. Thus, sapphire fibers were applied to measure the content of C2 units in an ethylene-propylene copolymer at 200 °C (F11). Similarly, the content of sugar inside sugar canes was directly measured with a fiber-optic refractometer operated with a 670nm laser diode (F12). Methane-air flames contain CO, CO2, CH4, and water vapor in various concentrations. The IR absorption bands were acquired between 6345 and 6660 cm-1 and used for species detection in a fast-flow multipass absorption cell containing flame and combustion gases (F13). SENSING SCHEMES This section reports on improved or novel sensing schemes based on the use of fiber-optic waveguides. Aside from their use as plain waveguides, fibers have been used for evanescent wave excitation of fluorescence or Raman scatter, for imaging and sensor array purposes, in microsensors and nanosensors, in surface plasmon resonance, and for distributed sensing, to mention only the more important ones. Thus, plain microfibers have been applied to measure the fluorescence of analytes separated by micellar electrokinetic capillary chromatography and by exciting fluorescence with a laser, thus combining the separation power of chromatography with the sensitivity of fluorescence (G1). A FOS for NO was designed (G2) based on the finding that fluorescein when immobilized on colloidal gold rearranges as nitric oxide adsorbs onto the gold and thereby causes a decrease in the fluorescence intensity of fluorescein. The microsensor has excellent reversibility and selectivity. A mathematical model has been developed (G3) to model the effects of the variation of fiber parameters such as diameter and numerical aperture, their position in space, and the optical characteristics of other optical components. The model was used in a UV laser fluorometer for trace detection of aromatic pollutants. An improved two-fiber arrangement was applied to in situ measurement of fluorescence or Raman signals with improved rejection of scattered light (G4), thus distinctly improving the ratio of signal to background. A new microscopic technique was demonstrated (G5) that combines attributes from both near-field scanning microscopy and fluorescence resonance energy transfer. If the fiber tip with a first fluorophore approaches the sample, energy transfer occurs from the second fluorophore (in the sample) to the first fluorophore on the fiber tip. The technique was demonstrated by imaging the fluorescence from a Langmuir-

Blodgett multilayer. The method is likely to display excellent spatial resolution. A liquid core waveguide was described (G6) acting both as a reaction cell and as a light collector and conveyor. The fast chemiluminescence reaction of luminol with hypochlorite is inhibited by ammonia and by ammonium ion. This can be used to quantify 100-300 nM concentrations of ammonium ion. Nitrate was detected via its absorption in the deep UV in a liquid core waveguide (G7). Evancescent wave sensing schemes have been reported for the whole spectral range and for luminescence. Thus, a multimode fiber was partially or fully declad and the evanescent field component of 254-nm light used to measure the absorption of ozone (G8). Improved stability was observed with a silica fiber with a gas-permeable silicone cladding. Concentrations of 0.020.35 vol % ozone are detectable. The theoretical and experimental implications of tapering a multimode evanescent wave absorption sensor were discussed (G9). A strong increase was observed in the collected evanescent fluorescence of a fiber with a high refractive index coating made from a titanium sol-gel (G10). A fiber with an inverted-graded index profile was proposed for use in evanescent wave sensing (G11). The core is doped with germanium oxide or boron oxide, and the sensitivity of the evanescent wave to changes cladding refractive index and light absorption was reported. Randomly ordered high-density optical fiber sensor arrays have been described (G12) in which different analyte-responsive “chemistries” are placed on microspheres located at the respective fiber tips. The identity of each sensor is ascertained by the color of the microsphere, and the array is used for multianalyte sensing. Such “microwell” arrays can be used to study biological processes at a cellular level (G13). The sensor components and the fabrication architectures have been described in detail, and the features of spatially resolved sensor sites versus randomly distributed microsphere sensors were discussed (G14). Single-molecule detection was accomplished by exciting the fluorophore at the water-glass interface of an optical fiber using evanescent wave excitation (G15). The scheme was applied to DNA detection after labeling with rhodamine. A cantilevered fiber optic was utilized to image the fluorescence of single molecules via near-field scanning microscopy (G16). Excellent spatial resolution was achieved, and height changes as small as 1.5 nm could be probed. Micrometer-sized chemical sensors have been reported for oxygen (B10, G17) and sodium and potassium ions (C4, C17, G18, G19) as well as chloride. Typically, a fiber tip of 0.1-15 µm diameter is covered with a respective indicator-on-polymer chemistry. In most cases, fluorescence intensities (or fluorescence decay) are the preferred optical parameters to be measured, the decay time information being much less prone to adverse effects such as fluctuations in source intensity, detector sensitivity, and fiber bending. Surface plasmon resonance (SPR) continues to be an extremely useful tool for studying receptor-ligand interactions. It has been combined with fiber-optic technology in past years. For example, a single-mode fiber SPR sensor based on the resonant interaction between a guided mode and a surface plasma was reported (G20), and an analysis of the effect of the variable parameters of the sensing structure on the sensors performance carried out. Analytical Chemistry, Vol. 72, No. 12, June 15, 2000

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Similarly, an SPR fiber sensor based on retroreflection was described (G21), consisting of a one-piece substrate at the fiber tip and displaying a resolution of 3.4 × 10-5 refractive index units. Surface plasmon excitation also can be accomplished via a goldisland type of films (G22). In this case, the sensing part of the fibers consists of a 1-in. portion whose cladding was removed and onto which a thin layer of gold was deposited to form a particulate surface. The spectral distribution of transmitted light undergoes changes due to changes in the medium surrounding, or adsorbed on, the gold islands. The dynamic range and limits of detection of a white light, multimode fiber SPR sensor can be optimized by modifying the geometry of the sensing tip (G23), and multiple regions of SPR activity can be observed simultaneously on the same probe, thus increasing the information content of an SPR spectrum. A 96-channel fiber-optic sensor system for measurement of SPR in microtiter plates can be used for parallel high-throughput screening (G24). White light is introduced to each sensor via a multiplexed fiber bundle, and the transmitted spectral intensity distribution of each sensor detected with an imaging spectrograph. The sensitivity of wavelength-modulated SPR devices can be enhanced by adding to the bulk solution a dye absorbing at the SPR coupling wavelength (G25). Typically, a 4-fold enhancement in the sensitivity to sucrose in solution and a 2-fold enhancement for binding of proteins to the sensor surface is obtained. Distributed chemical sensing, i.e., sensing a chemical species along the distance of an optical fiber, remains another attractive feature of FOS. Obviously, signals need to be time-resolved. A strategy for spatially resolved sensing combines a method for the fabrication of extended-length fibers with time-of-flight (TOF) detection (G26). It enables (a) localization of zones in the fiber where attennation or fluorescence takes place and (b) the quantification of the local concentration of chemicals by measuring the magnitude of signal variations. The scheme was extended (G27) in that an analyte-intensitive fluorophore was added and its intensity monitored as the function of an analyte-sensitive indicator dye, and the signal-to-noise ratios were improved 3-4fold by sending the laser sequentially from both ends of the fiber. The characterization of a fiber-optic system for distributed measurement of leakages in tanks and pipelines was reported (G28). A fiber with a cladding sensitive to nonpolar chemicals was used along with time-domain reflectrometry as the method for detecting changes in the refractive index of the cladding. A reference fluorophore can be added to give a characteristic peak in the response signal. The leakage of methane gas can be detected with multipoint fiber sensors with micro-optic cells. The main limitation in the signal-to-noise ratio of the system is due to interference effects from the cells (G29). MATERIALS FOR FIBER-OPTIC CHEMICAL SENSORS This section summarizes recent work in material sciences as related to FOS. Specifically, it covers aspects such as new polymers (including polymers for molecular imprints), sol-gels and porous glasses, and organic conductive polymers. Organopolysiloxanes remain to be of interest as chemically sensitive coatings for FOS. While usually unselective, they display excellent temporal and thermal stability. Conventional polysiloxanes can be replaced by organically modified sol-gels (starting from precur86R

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sors such as R-Si(OCH3)3, where R is alkyl or aryl) to give coatings with properties anywhere between sol-gels and silicones (H1, H2). The refractive index can be tailored in the range from 1.46 to 1.56, and sensitivity to gases can be achieved by incorporating functional groups such as the amino group (H1) or indicator dyes responsive to ammonia (H3), oxygen (H4), or pH (H5). Vapors of aromatic and aliphatic hydrocarbons were detected using such fibers, a phenyl-modified porous silica forming the cladding in the sensitive area (H2). Sol-gels also can act as a matrix for zeolite-incorporated fluorescent probes, thus providing stability toward even high temperatures (H6). Even proteins may be incorporated (H7) as shown also for a site-selectively labeled human serum albumin (H8) whose extrinsic fluorescence undergoes changes as a result of denaturation or quenching by iodide, even though a substantial fraction of the protein remains inaccessible to other species. The poor selectivity of plain polymer coatings on fibers can be improved by making use of so-called imprinted polymers (H9), i.e., polymers prepared in the presence of a template which subsequently is removed, leaving behind cavities of the specific shape of the template. These cavities then can be occupied again by the analyte (the former template). Thus, molecular imprints in polymethacrylates have been prepared for assay of theophylline, caffeine, and xanthine (H10) and coupled to SPR detection. The uptake of the water-dissolved drug by the polymer was measured by SPR after drying the polymer interface. Typically, 0.4 mg of theophylline in 1 mL of water is detectable with surprisingly high selectivity. Polyanilines, polypyrroles, and related organic conductive polymers have pH-dependent absorption spectra between 550 and >1000 nm. This has been exploited for near-IR sensing of pH over comparable wide dynamic ranges (H11-H13), in the case of polypyrroles from pH 6 to 12 and of polyanilines from pH 2 to 10 (depending on substituents). By incorporating boronic acids into the polyaniline network, a material with an optical response to saccharides such as fructose and sorbitol is obtained (H14). Clear blue composite films of Prussian Blue and certain polypyrroles were obtained by oxidative polymerization of pyrrole with ferric chloride (H15) and were applied to optical monitoring of pHs between 5 and 9 at wavelengths of typically 720 nm. The rate of the deposition of thin films of polypyrrole by plasma polymerization on the uncladded portion of a multimode fiber can be monitored by evanescent wave absorptiometry (H16). A plastic optical-fiber sensor based on evanescent wave fluorescence spectroscopy has been reported (H17) and characterized in terms of design considerations, method of preparation, and fluorescence response characteristics to organic gases. The new sensors are said to possess higher sensitivities to organic gases and vapors than sensors based on transmission absorption spectroscopy. An array of the evanescent wave fluorescence sensors has been used for discrimination of gases. An unusual material has been reported that responds to chemicals by a change in refractive index rather than color (H18). It is a crystalline colloidal array of 100-nm polymer spheres polymerized within a hydrogel and swells and shrinks in the presence of alkali metal ions or glucose. This gives rise to a shift in the Bragg peak of the diffracted light andshencesa color change.

Otto S. Wolfbeis received a Ph.D. in chemistry in 1972, then spent several years at the Max-Planck Institute for Radiation Chemistry in Muelheim and at the University of Technology at Berlin,, and in 1981 became an Associate Professor of Chemistry at Karl-Franzens University in Graz, Austria. Since 1995 he is Professor of Analytical Chemistry at the University of Regensburg, Germany, heading a team of typically 30 co-workers. He has authored more than 320 papers and reviews on optical (fiber) chemical sensors, fluorescence spectroscopy, and fluorescent probes, has edited books on Fluorescence Spectroscopy: New Methods and Applications and on Fiber Optic Chemical Sensors and Biosensors, and has (co)organized several conferences related to optical chemical sensors and biosensors. His research interests are in the design of novel schemes for optical chemical sensing, in fluorescent probes, in optical sensores and biosensors for biomedical applications, in biosensors based on thin gold films, and in design of advanced materials for use in (bio)chemical sensing.

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(B12) Potyrailo, R. A.; Hieftje, G. M. Anal. Chim. Acta 1998, 370, 1-8; Chem. Abstr. 1998, 129, 89486. (B13) Ovchinnikov, A. N.; Ogurtsov, V. I.; Trettnak, W.; Papkovsky, D. B. Anal. Lett. 1999, 32, 701-716; Chem. Abstr. 1999, 131, 15846. (B14) Zhao, Y.; Richman, A.; Storey, C.; Radford, N. B.; Pantano, P. Anal. Chem. 1999, 71, 3887-3893; Chem. Abstr. 1999, 131, 269082. (B15) Vishnoi, G.; Morisawa, M.; Muto, S. Opt. Rev. 1998, 5, 1315; Chem. Abstr. 1998, 128, 289423. (B16) Ghosh, R. N.; Baker, G. L.; Ruud, C.; Nocera, D. G. Appl. Phys. Lett. 1999, 75, 2885-2887; Chem. Abstr. 1999, 131, 345796. (B17) Barker, S. L. R.; Kopelman, R.; Meyer, T. E.; Cusanovich, M. A. Anal. Chem. 1998, 70, 971-976; Chem. Abstr. 1998, 128, 85956. (B18) Barker, S. L. R.; Kopelman, R. Anal. Chem. 1998, 70, 49024906; Chem. Abstr. 1999, 130, 78207. (B19) DeGrandpre, M. D.; Baehr, M. M.; Hammar, T. R. Anal. Chem. 1999, 71, 1152-1159; Chem. Abstr. 1999, 130, 176766. (B20) Tabacco, M. B.; Uttamlal, M.; McAllister, M.; Walt, D. R. Anal. Chem. 1999, 71, 154-161; Chem. Abstr. 1999, 130, 71111. (B21) Marazuela, M. D.; Moreno-Bondi, M. C.; Orellana, G. Appl. Spectrosc. 1998, 52, 1314-1320; Chem. Abstr.199,1998, 129, 350270. (B22) Neurauter, G.; Klimant, I.; Wolfbeis, O. S. Anal. Chim. Acta 1999, 382, 67-75; Chem. Abstr. 1999, 130, 217187. (B23) Wolfbeis, O. S.; Kovacs, B.; Goswami, K.; Klainer, S. M. Mikrochim. Acta 1998, 129, 181-188; Chem. Abstr. 1998, 129, 48831. (B24) Alayli, Y.; Bendamardji, S. Eur. Phys. J. Appl. Phys. 1998, 1, 353-360. Chem. Abstr. 1998, 129, 47186. (B25) Butler, M. A.; Andrzejewski, W. Proc. Electrochem. Soc. 1999, 99-23, 386-389; Chem. Abstr. 1999, 131, 193464. (B26) Arregui, F. J.; Liu, Y.; Matias, I. R.; Claus, R. O. Sens. Actuators, B 1999, B59, 54-59; Chem. Abstr. 2000, 132, 61120. (B27) Raimundo, I. M., Jr.; Narayanaswamy, R. Analyst (Cambridge, U.K.) 1999, 124, 1623-1627; Chem. Abstr. 2000, 132, 18233. (B28) Kawamura, M.; Takeuchi, T.; Fuad, H.; Sato, S. Jpn. J. Appl. Phys., Part 1 1999, 38, 849-850; Chem. Abstr. 1999, 130, 184206. (B29) Kronenberg, P.; Culshaw, B.; Pierce, S. G. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3670, 480-485; Chem. Abstr. 1999, 131, 159195. ION SENSORS (C1) Ben-David, O.; Shafir, E.; Gilath, I.; Prior, Y.; Avnir, D. Chem. Mater. 1997, 9, 2255-2257; Chem. Abstr. 1997, 127, 287280. (C2) Grant, S. A.; Glass, R. S. Sens. Actuators, B 1997, B45, 3542; Chem. Abstr. 1998, 128, 190010. (C3) Xu, Z.; Rollins, A.; Alcala, R.; Marchant, R. E. J. Biomed. Mater. Res. 1998, 39, 9-15; Chem. Abstr. 1998, 128, 93171. (C4) Ji, J.; Rosenzweig, Z. Anal. Chim. Acta 1999, 397, 93-102; Chem. Abstr. 1999, 131, 306425. (C5) Nivens, D. A.; Zhang, Y.; Angel, S. M. Anal. Chim. Acta 1998, 376, 235-245; Chem. Abstr. 1999, 130, 89667. (C6) Manyam, U. H.; Shahriari, M. R.; Morris, M. J. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3540, 10-18; Chem. Abstr. 1999, 131, 82169. (C7) Noire, M. H.; Bouzon, C.; Couston, L.; Gontier, J.; Marty, P.; Pouyat, D. Sens. Actuators, B 1998, B51, 214-219; Chem. Abstr. 1999, 130, 204286. (C8) Flannery, D.; James, S. W.; Tatam, R. P.; Ashwell, G. J. Inst. Phys. Conf. Ser. 1996, 150, 316-320; Chem. Abstr. 1999, 130, 176748. (C9) Daffy, L. M.; De Silva, A. P.; Gunaratne, H. Q. N.; Huber, C.; Lynch, P. L. M.; Werner, T.; Wolfbeis, O. S. Chem. Eur. J. 1998, 4, 1810-1815; Chem. Abstr. 1998, 129, 297597. (C10) Kosch, U.; Klimant, I.; Werner, T.; Wolfbeis, O. S. Anal. Chem. 1998, 70, 3892-3897; Chem. Abstr. 1998, 129, 197179. (C11) Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247-252; Chem. Abstr. 1998, 128, 123180. (C12) Koncki, R.; Wolfbeis, O. S. Anal. Chem. 1998, 70, 2544-2550; Chem. Abstr. 1998, 128, 316614. (C13) Michael, K. L.; Taylor, L. C.; Walt, D. R. Anal. Chem. 1999, 71, 2766-2773; Chem. Abstr. 1999, 131, 67254. (C14) Wallace, P. A.; Uttamlal, M.; Elliot, N.; Holmes-Smith, A. S.; Campbell, M. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3483, 128131; Chem. Abstr. 1999, 130, 89678. (C15) Krause, C.; Werner, T.; Huber, C.; Klimant, I.; Wolfbeis, O. S. Anal. Chem. 1998, 70, 3983-3985; Chem. Abstr. 1998, 129, 210964. (C16) Krause, C.; Werner, T.; Huber, C.; Wolfbeis, O. S.; Leiner, M. J. P. Anal. Chem. 1999, 71, 1544-1548; Chem. Abstr. 1999, 130, 334780. (C17) Kurihara, K.; Ohtsu, M.; Yoshida, T.; Abe, T.; Hisamoto, H.; Suzuki, K. Anal. Chem. 1999, 71, 3558-3566; Chem. Abstr. 1999, 131, 124557. (C18) Sasaki, D. Y.; Shea, L. E.; Sinclair, M. B. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3602, 275-284; Chem. Abstr. 1999, 131, 237175.

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(C19) Tunoglu, N.; Caglar, P.; Wnek, G. E. J. Macromol. Sci., Pure Appl. Chem. 1998, A35, 637-647; Chem. Abstr. 1998, 128, 295817. (C20) Malcik, N.; Tunoglu, N.; Caglar, P.; Wnek, G. E. Sens. Actuators, B 1998, B53, 204-210; Chem. Abstr. 1999, 131, 13023. (C21) Ahmad, M.; Hamzah, H.; Marsom, E. S. Talanta 1998, 47, 275-283; Chem. Abstr. 1999, 130, 45006. (C22) Vaughan, A. A.; Narayanaswamy, R. Sens. Actuators, B 1998, B51, 368-376; Chem. Abstr. 1999, 130, 162389. (C23) Shimizu, Y.uuzi; Saito, T. Bunseki Kagaku 1999, 48, 429433; Chem. Abstr. 1999, 130, 346455. (C24) D’Emilia, G.; Iaconis, F. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3537, 35-39; Chem. Abstr. 1999, 130, 305755. (C25) Huber, C.; Werner, T.; Krause, C.; Wolfbeis, O. S. Analyst (Cambridge, U.K.) 1999, 124, 1617-1622; Chem. Abstr. 1999, 131, 359589. (C26) Werner, T.; Klimant, I.; Huber, C.; Krause, C.; Wolfbeis, O. S. Mikrochim. Acta 1999, 131, 25-28; Chem. Abstr. 1999, 131, 96484. (C27) Fuhr, P. L.; Huston, D. R.; Maccraith, B. Opt. Eng. Bellingham, Wash.) 1998, 37, 1221-1228; Chem. Abstr. 1998, 128, 303368. (C28) Chen, X.; Tan, J.; Nie, L.; Yao, S. Fenxi Yiqi 1997, 4, 21-23 (Chinese); Chem. Abstr. 1998, 128, 238568. (C29) Domanski, A. W.; Roszko, M.; Swillo, M. IEEE Instrum. Meas. Technol. Conf. 1997, 2, 953-956; Chem. Abstr. 1997, 127, 298366. SENSORS FOR ORGANIC COMPOUNDS (D1) Abdelghani, A.; Chovelon, J. M.; Jaffrezic-Renault, N.; Lacroix, M.; Gagnaire, H.; Veillas, C.; Berkova, B.; Chomat, M.; Matejec, V. Sens. Actuators, B 1997, B44, 495-498; Chem. Abstr. 1998, 128, 175579. (D2) Merschman, S. A.; Tilotta, D. C. Appl. Spectrosc. 1998, 52, 106-111; Chem. Abstr. 1998, 128, 132116. (D3) Chadha, S.; Kyle, W.; Stevenson, C.; Bolduc, R. A.; Druy, M. A. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3540, 83-90; Chem. Abstr. 1999, 131, 35287. (D4) Goswami, K.; Prohaska, J.; Menon, A.; Mendoza, E.; Lieberman, R. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3540, 115-122; Chem. Abstr. 1999, 131, 34912. (D5) Stokes, D. L.; Alarie, J. P.; Ananthanarayanan, V.; Vo-Dinh, T. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3534, 647-654’ Chem. Abstr. 1999, 130, 356763. (D6) Abdelghani, A.; Veillas, C.; Chovelon, J. M.; Jaffrezic-Renault, N.; Gagnaire, H. Synth. Met. 1997, 90, 193-198; Chem. Abstr. 1998, 128, 135894. (D7) Bloch, J.; Johnson, B.; Newbury, N.; Germaine, J.; Hemond, H.; Sinfield, J. Appl. Spectrosc. 1998, 52, 1299-1304; Chem. Abstr. 1999, 130, 24520. (D8) Ives, J. T.; Doss, H. M.; Sullivan, B. J.; Stires, J. C.; Bechtel, J. H. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3540, 36-44; Chem. Abstr. 1999, 131, 35510. (D9) Choi, J.-W.; Min, J.; Jung, J.-W.; Rhee, H.-W.; Lee, W. H. Thin Solid Films 1998, 327-329, 676-680; Chem. Abstr. 1998, 129, 249851. (D10) Mulchandani, A.; Pan, S.; Chen, W. Biotechnol. Prog. 1999, 15, 130-134; Chem. Abstr. 1999, 130, 178438. (D11) Jenkins, A. L.; Uy, O. M.; Murray, G. M. Anal. Chem. 1999, 71, 373-378; Chem. Abstr. 1999, 130, 178419. (D12) Albert, K. J.; Dickinson, T. A.; Walt, D. R.; White, J.; Kauer, J. S. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3392, 426-431; Chem. Abstr. 1999, 130, 97710. (D13) Bakaltcheva, I. B.; Ligler, F. S.; Patterson, C. H.; Shriver-Lake, L. C. Anal. Chim. Acta 1999, 399, 13-20; Chem. Abstr. 1999, 131, 353272. (D14) Nath, N.; Eldefrawi, M.; Wright, J.; Darwin, D.; Huestis, M. J. Anal. Toxicol. 1999, 23, 460-467; Chem. Abstr. 2000, 132, 1018. (D15) Wang, Y.; Liu, W.; Wang, K.; Shen, G.; Yu, R. Fresenius’ J. Anal. Chem. 1998, 361, 827; Chem. Abstr. 1998, 129, 235709. (D16) Wang, Y.; Liu, W.-H.; Wang, K.-M.; Shen, G.-L.; Yu, R.-Q. Talanta 1998, 47, 33-42; Chem. Abstr. 1999, 130, 71637. (D17) Guo, J.; Chen, J. Yaowu Fenxi Zazhi 1997, 17, 228-231; Chem. Abstr. 1998, 129, 72065. (D18) Liu, W.; Wang, Y.; Tang, J.; Shen, G.; Yu, R. Analyst 1998, 123, 365-369; Chem. Abstr. 1998, 128, 225580. (D19) Tang, J.; Liu, W.; Wang, Y.; Shen, G.; Yu, R. Fenxi Huaxue 1998, 26, 797-801; Chem. Abstr. 1998, 129, 140759. (D20) Mitsushio, M.; Kamata, S. Bunseki Kagaku 1999, 48, 757762; Chem. Abstr. 1999, 131, 193459. BIOSENSORS (E1) Li, X.; Rosenzweig, Z. Anal. Chim. Acta 1997, 353, 263-273; Chem. Abstr. 1998, 128, 112491. (E2) Zhang, W.; Chang, H.; Rechnitz, G. Anal. Chim. Acta 1997, 350, 59-65; Chem. Abstr. 1997, 127, 242596. (E3) Cordek, J.; Wang, X.; Tan, W. Anal. Chem. 1999, 71, 15291533; Chem. Abstr. 1999, 130, 349167. 88R

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(E4) Ovchinnikov, A. N.; Ogurtsov, V. I.; Trettnak, W.; Papkovsky, D. B. Anal. Lett. 1999, 32, 701-716; Chem. Abstr. 1999, 131, 15846. (E5) Marquette, C. A.; Blum, L. J. Anal. Chim. Acta 1999, 381, 1-10; Chem. Abstr. 1999, 130, 264176. (E6) Marazuela, M. D.; Moreno-Bondi, M. C. Anal. Chim. Acta 1998, 374, 19-29; Chem. Abstr. 1999, 130, 49309. (E7) Nath, N.; Jain, S. R.; Anand, S. Biosens. Bioelectron. 1997, 12, 491-498; Chem. Abstr. 1997, 127, 204086. (E8) Anderson, G. P.; King, K. D.; Cao, L. K.; Jacoby, M.; Ligler, F. S.; Ezzell, J. Clin. Diagn. Lab. Immunol. 1998, 5, 609-612; Chem. Abstr. 1999, 130, 50995. (E9) Ives, J. T.; Doss, H. M.; Sullivan, B. J.; Stires, J. C.; Bechtel, J. H. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3540, 36-44; Chem. Abstr. 1999, 131, 35510. (E10) Shriver-Lake, L. C.; Donner, B.; Edelstein, R.; Breslin, K.; Bhatia, S. K.; Ligler, F. S. Biosens. Bioelectron. 1997, 12, 11011106; Chem. Abstr. 1998, 128, 152804. (E11) Narang, U.; Anderson, G. P.; Ligler, F. S.; Buranst, J. Biosens. Bioelectron. 1997, 12, 937-945; Chem. Abstr. 1998, 128, 124572. (E12) King, K. D.; Anderson, G. P.; Bullock, K. E.; Regina, M. J.; Saaski, E. W.; Ligler, F. S. Biosens. Bioelectron. 1999, 14, 163170; Chem. Abstr. 1999, 130, 292495. (E13) Ferreira, A. P.; Werneck, M. M.; Ribeiro, R. M. Biotechnol. Technol. 1999, 13, 447-452; Chem. Abstr. 1999, 131, 283494. (E14) Kleinjung, F.; Bier, F. F.; Warsinke, A.; Scheller, F. W. Anal. Chim. Acta 1997, 350, 51-58; Chem. Abstr. 1997, 127, 231447. (E15) Pilevar, S.; Davis, C. C.; Portugal, F. Anal. Chem. 1998, 70, 2031-2037; Chem. Abstr. 1998, 128, 226738. (E16) Uddin, A. H.; Piunno, P. A. E.; Hudson, R. H. E.; Damha, M. J.; Krull, U. J. Nucleic Acids Res. 1997, 25, 4139-4146; Chem. Abstr. 1998, 128, 30762. (E17) Zhang, G.; Zhou, Y.; Yuan, J.; Ren, S. Anal. Lett. 1999, 32, 2725-2736; Chem. Abstr. 2000, 132, 45519. (E18) Michel, P. E.; Gautier-Sauvigne, S. M.; Blum, L. J. Anal. Chim. Acta 1998, 360, 89-99; Chem. Abstr. 1998, 128, 254799. (E19) Michel, P. E.; Gautier-Sauvigne, S. M.; Blum, L. J. Talanta 1998, 47, 169-181; Chem. Abstr. 1999, 130, 78212. APPLICATIONS (F1) Blair, D. S.; Bando, J. Environ. Sci. Technol. 1998, 32, 294298; Chem. Abstr. 1998, 128, 26642. (F2) Burck, J.; Dentes, P.; Mensch, M.; Kramer, K.; Scholz, M. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3534, 222-233; Chem. Abstr. 1999, 130, 328920. (F3) Mizaikoff, B. Meas. Sci. Technol. 1999, 10, 1185-1194; Chem. Abstr. 2000, 132, 40086. (F4) Marowsky, G.; Lewitzka, F.; Karlitschek, P.; Bunting, U.; Niederkruger, M. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3539, 2-9; Chem. Abstr. 1999, 130, 290824. (F5) Klimant, I.; Holst, G.; Kuhl, M. Limnol. Oceanogr. 1997, 42, 1638-1643; Chem. Abstr. 1998, 128, 265409. (F6) Hales, B.; Burgess, L.; Emerson, S. Mar. Chem. 1997, 59, 5162; Chem. Abstr. 1998, 128, 66151. (F7) Hamza, M.; Hamza, M.; Hamza, A. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3259, 130-135; Chem. Abstr. 1998, 129, 172551. (F8) Zhao, Y.; Richman, A.; Storey, C.; Radford, N. B.; Santano, P. Anal. Chem. 1999, 71, 3887-3893; Chem. Abstr. 1999, 131, 269082. (F9) Shim, M. G.; Wilson, B. C.; Marple, E.; Wach, M. Appl. Spectrosc. 1999, 53, 619-627; Chem. Abstr. 1999, 131, 196478. (F10) Zuluaga, A. F.; Utzinger, U.; Durkin, A.; Fuchs, H.; Gillenwater, A.; Jacob, R.; Kemp, B.; Fan, J.; Richards-Kortum, R. Appl. Spectrosc. 1999, 53, 302-311; Chem. Abstr. 1999, 130, 349252. (F11) Gotz, R.; Mizaikoff, B.; Kellner, R. Appl. Spectrosc. 1998, 52, 1248-1252; Chem. Abstr. 1998, 129, 331440. (F12) Pham, V. H.; Bui, H.; Hoang, C. Dz.; Phung, H. A.; Vu, D. T. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3555, 121-126; Chem. Abstr. 1999, 130, 111723. (F13) Mihalcea, R. M.; Baer, D. S.; Hanson, R. K. Appl. Opt. 1997, 36, 8745-8752; Chem. Abstr. 1998, 128, 5420. SENSING SCHEMES (G1) Sepaniak, M. J.; Vo-Dinh, T.; Tropina, V.; Stokes, D. L. Anal. Chem. 1997, 69, 3806-3811; Chem. Abstr. 1997, 127, 242592. (G2) Barker, S.; L. R.; Kopelman, R. Anal. Chem. 1998, 70, 49024906; Chem. Abstr. 1999, 130, 78207. (G3) Bunting, U.; Lewitzka, F.; Karlitschek, P. Appl. Spectrosc. 1999, 53, 49-56; Chem. Abstr. 1999, 130, 196241. (G4) Wright, A. O.; Pepper, J. W.; Kenny, J. E. Anal. Chem. 1999, 71, 2582-2585; Chem. Abstr. 1999, 131, 38949. (G5) Vickery, S. A.; Dunn, R.u C. Biophys. J. 1999, 76, 1812-1818; Chem. Abstr. 1999, 131, 15877. (G6) Li, J.; Dasgupta, P. K. Anal. Chim. Acta 1999, 398, 33-39; Chem. Abstr. 1999, 131, 294850.

(G7) Belz, M.; Dress, P.; Klein, K.-F.; Boyle, W. J. O.; Franke, H.; Grattan, K. T. V. Water Sci. Technol. 1998, 37, 279-284; Chem. Abstr. 1999, 130, 56844. (G8) Potyrailo, R. A.; Hobbs, S. E.; Hieftje, G. M. Anal. Chem. 1998, 70, 1639-1645; Chem. Abstr. 1998, 128, 238562. (G9) Mignani, A. G.; Falciai, R.; Ciaccheri, L. Appl. Spectrosc. 1998, 52, 546-551; Chem. Abstr. 1998, 129, 21001. (G10) Kao, H. P.; Yang, N.; Schoeniger, J. S. J. Opt. Soc. Am. A 1998, 15, 2163-2171; Chem. Abstr. 1998, 129, 308019. (G11) Matejec, V.; Chomat, M.; Kasik, I.; Ctyroky, J.; Berkova, D.; Hayer, M. Sens. Actuators, B 1998, B51, 340-347; Chem. Abstr. 1999, 130, 162388. (G12) Michael, K. L.; Taylor, L. C.; Schultz, S. L.; Walt, D. R. Anal. Chem. 1998, 70, 1242-1248; Chem. Abstr. 1998, 128, 212251. (G13) Taylor, L. C.; Walt, D. R. Proc.-Electrochem. Soc. 1998, 98, 168-175; Chem. Abstr. 1999, 130, 78216. (G14) Steemers, F. J.; Walt, D. R. Mikrochim. Acta 1999, 131, 99105; Chem. Abstr. 1999, 131, 88693. (G15) Fang, X.; Tan, W. Anal. Chem. 1999, 71, 3101-3105; Chem. Abstr. 1999, 131, 196549. (G16) Talley, C. E.; Lee, M. A.; Dunn, R. C. Appl. Phys. Lett. 1998, 72, 2954-2956; Chem. Abstr. 1998, 129, 106123. (G17) McCulloch, S.; Uttamchandani, D. IEE Proc.: Sci., Meas. Technol. 1999, 146, 123-127; Chem. Abstr. 1999, 131, 110477. (G18) Kurihara, K.; Ohtsu, M.; Yoshida, T.; Abe, T.; Hisamoto, H.; Suzuki, K. Anal. Chem. 1999, 71, 3558-3566; Chem. Abstr. 1999, 131, 124557. (G19) Werner, T.; Klimant, I.; Huber, C.; Krause, C.; Wolfbeis, O. S. Mikrochim. Acta 1999, 131, 25-28; Chem. Abstr. 1999, 131, 96484. (G20) Slavik, R.; Homola, J.; Ctyroky, J. Sens. Actuators, B 1999, B54, 74-79; Chem. Abstr. 1999, 130, 245746. (G21) Cahill, C. P.; Johnston, K. S.; Yee, S. S. Sens. Actuators, B 1997, B45, 161-166; Chem. Abstr. 1998, 128, 162245. (G22) Merinudeau, F.; Downey, T.; Wig, A.; Passian, A.; Buncick, M.; Ferrell, T. L. Sens. Actuators, B 1999, B54, 106-117; Chem. Abstr. 1999, 130, 275790. (G23) Obando, L. A.; Booksh, K. S. Anal. Chem. 1999, 71, 51165122; Chem. Abstr. 1999, 131, 306464. (G24) Ralph C.; Siegfried, Mark C. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3603, 313-322; Chem. Abstr. 1999, 131, 211117. (G25) Hanning, A.; Roeraade, J.; Delrow, J. J.; Jorgenson, R. C. Sens. Actuators, B 1999, B54, 25-36; Chem. Abstr. 1999, 131, 2301. (G26) Potyrailo, R. A.; Hieftje, G. M. Anal. Chem. 1998, 70, 14531461. Chem. Abstr. 1998, 128, 238564. (G27) Potyrailo, R. A.; Hieftje, G. M. Anal. Chem. 1998, 70, 34073412; Chem. Abstr. 1998, 129, 130618. (G28) Sensfelder, E.; Burck, J.; Ache, H.-J. Appl. Spectrosc. 1998, 52, 1283-1298. Chem. Abstr. 1998, 129, 334723. (G29) Stewart, G.; Tandy, C.; Moodie, D.; Morante, M. A.; Dong, F.; Sens. Actuators, B 1998, B51, 227-232; Chem. Abstr. 1999, 130, 162513.

MATERIALS FOR FIBER-OPTIC CHEMICAL SENSORS (H1) Rose, K.; Matejec, V.; Hayer, M.; Pospisilova, M. J. Sol-Gel Sci. Technol. 1998, 13, 729-733; Chem. Abstr. 1999, 130, 240847. (H2) Abdelmalek, F.; Chovelon, J. M.; Lacroix, M.; Jaffrezic-Renault, N.; Matejec, V. Sens. Actuators, B 1999, B56, 234-242; Chem. Abstr. 1999, 131, 77350. (H3) Lobnik A.; Wolfbeis O. S. Sens. Actuators,B 1998, B51, 203207; Chem. Abstr. 1999, 130, 176747. (H4) Klimant, I.; Ruckruh, F.; Liebsch, G.; Stangelmayer, A.; Wolfbeis, O. S, Mikrochim. Acta 1999, 131, 35-46; Chem. Abstr. 1999, 131, 96485. (H5) Manyam, U. H.; Shahriari, M. R.; Morris, M. J. Proc. SPIEInt. Soc. Opt. Eng. 1999, 3540, 10-18; Chem. Abstr. 1999, 131, 82169. (H6) Remillard, J. T.; Jones, J. R.; Poindexter, B. D.; Narula, C. K.; Weber, W. H. Appl. Opt. 1999, 38, 5306-5309; Chem. Abstr. 1999, 131, 276000. (H7) Wolfbeis, O. S.; Reisfeld, R.; Oehme, I. Optical and Electronic Phenomena in Sol-Gel Glasses; Structure & Bonding85; Springer Verlag: New York, 1996; pp 51-98; Chem. Abstr. 1996, 125, 264392. (H8) Flora, K.; Brennan, J. D. Analyst 1999, 124, 1455-1462; Chem. Abstr. 2000, 132, 32734. (H9) Wolfbeis, O. S.; Terpetschnig, E.; Piletsky, S.; Pringsheim, E. In Applied Fluorescence in Chemistry, Biology and Medicine; Rettig, W., Ed.; Springer: Berlin, 1999; pp 277-295; Chem. Abstr. 1999, 130, 334834. (H10) Lai, E. P. C.; Fafara, A.; Vandernoot, V. A.; Kono, M.; Polsky, B. Can. J. Chem. 1998, 76, 265-273; Chem. Abstr. 1998, 129, 86100. (H11) de Marcos, S.; Wolfbeis, O. S. Sens. Mater. 1997, 9, 253265; Chem. Abstr. 1997, 127, 302538. (H12) Del Pilar T.; Sotomayor, M.; De Paoli, M.-A.; Alves de Oliveira, W. Anal. Chim. Acta 1997, 353, 275-280; Chem. Abstr. 1998, 128, 83681. (H13) Pringsheim E.; Terpetschnig E.; Wolfbeis O. S. Anal. Chim. Acta 1997, 357, 247-252; Chem. Abstr. 1998, 128, 123180. (H14) Pringsheim, E.; Terpetschnig, E.; Piletsky, S. A.; Wolfbeis, O. S. Adv. Mater. (Weinheim, Ger.) 1999, 11, 865-868; Chem. Abstr. 1999, 131, 145219. (H15) Koncki, R.; Wolfbeis, O. S. Anal. Chem. 1998, 70, 2544-2550; Chem. Abstr. 1998, 128, 316614. (H16) Jose, D.; John, M. S.; Radhakrishnan, P.; Nampoori, V. P. N.; Vallabhan, C. P. G. Thin Solid Films 1998, 325, 264-267; Chem. Abstr. 1998, 129, 195938. (H17) Yamakawa, S. Sens. Mater. 1997, 9, 187-196; Chem. Abstr. 1997, 127, 314237. (H18) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832; Chem. Abstr. 1998, 128, 43013.

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