Fiber-Optic Chemical Sensors and Biosensors

Anal. Chem.2008, 80, 4269–4283 Fiber-Optic Chemical Sensors and Biosensors Otto S. Wolfbeis Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany Review Contents Books and Reviews Sensors for Gases, Vapors, and Humidity Hydrogen Hydrocarbons Oxygen Other Gases Vapors Humidity Sensors for pH and Ions Sensors for Organic Chemicals Organics Biosensors Enzymatic Biosensors Immunosensors DNA Biosensors Bacterial Biosensors Applications Sensing Schemes and Spectroscopies Fiber Optics Capillary Waveguides Microsystems and Microstructures Refractive Index Spectroscopies Literature Cited 4269 4271 4271 4271 4272 4272 4274 4274 4275 4276 4276 4276 4276 4277 4278 4278 4279 4279 4279 4280 4280 4280 4280 4281 This review covers the time period from January 2006 to January 2008 and is written in continuation of previous reviews (1–3). Data were electronically searched in SciFinder and MedLine. Additionally, references from (sensor) journals were collected by the author over the past 2 years. The number of citations in this review is limited, and a stringent selection had to be made therefore. Priority was given to fiber-optic sensors (FOS) of defined chemical, environmental, or biochemical significance and to new schemes. The review does not include the following: (a) FOS that obviously have been rediscovered; (b) FOS for nonchemical species such as temperature, current and voltage, stress, strain, displacement, structural integrity (e.g., of constructions), liquid level, and radiation; and (c) FOS for monitoring purely technical processes such as injection molding, extrusion, or oil drilling, even though these are important applications of optical fiber technology. Unfortunately, certain journals publish articles that represent marginal modifications of prior art, and it is mentioned here explicitly that the (nonpeer-reviewed) Proceedings of the SPIE are particularly uncritical in that respect. Fiber optics serve analytical sciences in several ways. First, they enable optical spectroscopy to be performed at sites inaccessible to conventional spectroscopy, over large distances, or even at several spots along the fiber. Second, in being optical waveguides, fiber optics enable less common methods of interrogation, in particular evanescent wave spectroscopy. Fibers are 10.1021/ac800473b CCC: $40.75  2008 American Chemical Society Published on Web 05/08/2008 available now with transmissions over a wide spectral range. Current limitations are not so much in the transmissivity but in the (usually shortwave) background fluorescence of most of the materials fibers are made from, in particular plastic. There is an obvious trend toward longwave sensing where background signals are weaker. Major fields of applications are in sensing gases and vapors, in medical and chemical analysis, molecular biotechnology, marine and environmental analysis, industrial production monitoring, bioprocess control, and the automotive industry. Note: In this article, sensing refers to a continuous process, while probing refers to single-shot testing. Both have their fields of applications. FOS are based on either direct or indirect (indicator-based) sensing schemes. In the first, the intrinsic optical properties of the analyte are measured, for example its refractive index, absorption, or emission. In the second, the color or fluorescence of an immobilized indicator, label, or any other optically detectable bioprobe is monitored. Aside from the design of label and probes, 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 (SPR), and multidimensional data acquisition. In recent years, fiber bundles also have been employed for purposes of imaging, for biosensor arrays (along with encoding), or as arrays of nonspecific sensors whose individual signals may be processed via artificial neural networks. The success of SPR in general, and in the form of FOS in particular, is impressive. Following the recent sensor hype of (mainly organic) chemists that tend to refer to optical molecular probes as “sensors”, the literature has become more difficult to sort. This review covers literature on methods that enable sensing of (bio)chemical species as opposed to conventional types of optical assays. Unfortunately, there is a tendency to even refer to conventional indicators (such as for pH or calcium) as biosensors if only used in vivo. Similarly, optical analysis of a solution by adding an appropriate indicator probe is now referred to as “sensing” (to the surprise of the sensor community). I have outlined the situation in more detail in my previous review (3). BOOKS AND REVIEWS It is obvious that fiber optic chemical sensors (FOCS) have had a particular success in areas related to sensing gases and vapors. Many of the systems implemented are based on direct spectroscopies that range from UV to IR, and from absorbance to fluorescence and surface plasmon resonance. Optical chemical sensors have been comprehensively reviewed by McDonagh et al. quite recently (4). The article covers sensing platforms, direct (spectroscopic) sensors, reagent-mediated sensors and discusses trends and future perspectives. Fiber-optic UV systems for gas Analytical Chemistry, Vol. 80, No. 12, June 15, 2008 4269 In combination with laser based detection. materials. Sensitivity. and (fiber) optical methods based on the use of glucose oxidase and transduction via oxygen consumption are reviewed in one chapter (22). and spectroscopies have been reviewe (12). Borisov et al. bitumen analysis. 80. Raman spectroscopy. or SPR. No. surface plasmon resonance. and catalytic rates of DNAzymes. by preconcentration. (8). TFOBS are often used with transduction mechanisms such as changes in refractive index. Given its complexity. Practical examples include sensing schemes for oxygenates in fuels. absorption. a liquid multibatch pipeline and a gas pipeline. oxidation. while optical nanobiosensors and nanoprobes were reviewed by Vo-Dinh (24) with respect to the principles. Applicability is demonstrated for two examples. fluorescence. and in environmental sensing by making use of DNAzymes (20). and new means (such as color changes of chiral nematic layers) of converting analyte recognition into optical signals are described. Significant strides have been made in terms of selectivity. arrays. Flowers et al. Buchanan has reviewed recent advances in the use of near-IR (NIR) spectroscopy in the petrochemical industry (6) and points out NIR is particularly attractive in this field because it measures the overtone and combination bands predominately of the C-H stretches. June 15. Mohr (11) has reviewed recent developments in chromogenic and fluorogenic reagents and sensors for neutral and ionic analytes based on covalent bond formation. (10). The state of the art in leak detection and localization and respective legal regulations have been reviewed by Geiger (9). The state of the art in continuous glucose sensing. SPR (which is label-free) possibly can sense proteins with a resolution similar to that of luminescence. Vol. the composition of fuels. DNAzymes can selectively identify charged organic and inorganic compounds at ultratrace levels in waste and emissions. as well as for saccharides also are described. and robustness. Theoretical sensitivities of interferometry and SPR are detailed along with parameters affecting these sensitivities. the detection of food-borne pathogens (19) (based on either SPR. A rather wide and shallow review covers advances in fiberoptic sensing in medicine and biology (14). Nanostructure-based optical fiber sensor systems and examples of their application have been reviewed by Willsch et al. and resolution were calculated and compared from a range of data from the literature. determination of octane numbers. part of the fiber is tapered so that the evanescent field of the lightwave can interact with samples. 12.and vapor analysis have been reviewed (5). Aspects of multiple optical chemical sensing with respect to parameters. and optical detection. amino acids. Adherent cells may be attached to the fiber substrate to provide a method for observing cell migration and for screening antimigratory . One focus is on fiber-optic probes for UV resonance Raman spectroscopy that offer several advantages over conventional excitation/collection methods. resonant mirrors. FOCS for volatile organic compounds have been reviewed by Elosua et al. fiber-optic systems. while interferometry is said to be most suitable for low-molecular weight chemical liquids and vapors if selectivity is not a critical issue. In these. and light-addressable potentiometry). the tapered fiber-optic biosensors (TFOBS) were reviewed (25). lightweight. and optical nanowires are discussed. discussed aspects of fiber-optic spectro-electrochemical sensing for in situ determination of metal ions. It is stated (somewhat overoptimistically) that chemical sensing can “simply” be achieved by transporting light to and from a measurement site with a plain fiber light guide for spectrophotometry.g. nitrogen oxide and nitrite. gene chips) in cellular analyses ranging from medical diagnostics to genomics.. Luminescence is said to offer the best resolutions for sensing of protein and DNA. cyanide.g. dynamic range. summarize their research on plastic microparticles and nanoparticles for fluorescent chemical sensing and encoding (13). DNAzymes enable accurate quantification of such compounds and thus represent an attractive alternative to stateof-the-art affinity sensors. accuracy. and luminescence as biosensors and chemosensors have been critically compared (16). sensitivity. Tools for life science research based on fiber-optic-linked Raman and resonance Raman spectroscopy were also reviewed (7). 2008 spectro-electrochemical sensing does not relate to electroluminescence but to electrochemical conversion of an analyte prior to its spectroscopic detection. and for environmental monitoring using nanoporous thin-film Fabry-Perot transducer elements or intrinsic fiber Bragg grating sensor networks for structural health monitoring. and applications of fiber-optic nanobiosensor systems using antibodybased probes. Such sensors are minimally invasive. development. organophosphates. Selected examples of advanced optical fiber sensor systems based on subwavelength structured components are presented. Applications range from the use of fibers acting as plain light pipes to complex chemical sensors. hydrogen peroxide. formaldehyde. Femtoliter wells can be loaded with individual beads to create such arrays for multiplexed screening and bioanalysis. Applications of optical fiber (bio)sensors (FOBS) also include areas such as high-throughput screening of drugs (18). The term 4270 Analytical Chemistry. A more general review covers recent developments in FOBS (26). Challenges remain in the development of efficient signal transduction technology for in situ applications. fluorometry. peptides and proteins. for application in the gas industry. and environmental analysis. complexation. The strong absorbance of vapors and gases in the UV region is advantageous and resulted in a compact detection system of good accuracy. New indicator dyes for amines and diamines. reduction. Nanoscale optical biosensors and biochips for cellular diagnostics have been reviewed by Cullum (23). and SPR. it does not come as a surprise that the authors point to “the need for further refinement of the sensor’s design and the experimental protocol to improve the method’s sensitivity. photonic crystal fibers. whereas Walt (27) describes fiberoptic biosensor arrays for creating high-density sensing arrays. Specific aspects include sensor reliability. mainly copper(II) (15). has been summarized in a book (21). sensitivity. Borisov and Wolfbeis (17) have presented what appears to be the most comprehensive review on optical biosensors so far.” The performances of interferometry. another on novel probes based on hollow-core photonic band gap fibers that virtually eliminate the generation of silica Raman scattering within the excitation optical fiber. The devices were classified according to their mechanism of operation and in terms of sensing materials. a kind of holy grail in FOBS. new concepts for sensing based on the use of plasmonic metal nanoparticles.. passive and can be multiplexed. e. Additionally. The trend toward miniaturization is obvious. These include sensor for humidity and hydrocarbons. specifically with respect to achievements in employing nanosensors and biochips (e. One specific class of FOBS. Experiments carried out at 113 K revealed the potential of sensing <5% of gaseous hydrogen with good reversibility and fast response time. and emission spectra. Concepts include fluorescence spectrophotometer design geometry and the correlation of color with emission wavelength. A gasochromic TiO2based sensing film was used for hydrogen detection by means of a fiber-optic Fabry-Perot interferometric sensor (44). have reported on the effect of hydrogen on a thick film Pd-Au alloy (41). The resulting sensor consists of a multimode fiber in which a short section of single mode fiber is coated with the Pd-Au film. 2008 4271 . Improved response times were reported for a similar sensor based on the same scheme (36). concentrations. Aspects of process monitoring of fiber reinforced composites using optical fiber sensors were summarized (32). The gasochromic properties of nanostructured tungsten oxide films coated with a palladium catalyst were used to sense hydrogen gas via the change in the optical transmittance at 645 nm.compounds. it is very likely that chemical sensor and biosensor development may benefit from such research. The same material was used to design a hydrogen sensor based on core diameter mismatch and annealed Pd-Au thin films (42). AND HUMIDITY This section covers all room-temperature gaseous species including their solutions in liquids. One more fiber-optic Raman sensor was reported for sensing ethanol and methanol in gasoline (48). In another type of FBG sensor. and regenerates quickly (at room temperature). It was applied to monitor hydrogen gas in air below the lower explosion limit. (30) review optical sensors and biosensors based on sol-gel films. Active and passive coatings. in contrast to hydrogen or oxygen. and even individual enzyme molecules can be loaded into the wells. Luna-Moreno et al. these may be covered with nanostructured coatings (29). lecture demonstrations can be made of various concepts in molecular fluorescence spectroscopy. Hydrogen. The history of research on FOCS and FOBS has been summarized (34). deposited via the Langmuir-Blodgett and electrostatic self-assembly techniques. 80. Heating the palladium layer with an auxiliary laser diode improves the response time at low temperatures. a hydrogen sensor was reported based on palladium coated side-polished single mode fiber (35). can be detected via their intrinsic absorption in the (near) infrared-albeit cross sections (molar absorbances) remains small or via Raman scatter. Vol. a 10 ppm sensitivity for hydrogen was reported (along with cross sensitivity to environmental conditions) (46). The use of FOCS for on-site monitoring and analysis of industrial pollutants with respect to detecting the identity. The FBG sensors appear to be pure strain sensors. The review highlights promising microarray techniques either making use of labels or label-free. The interactions result in both spectral changes (an effect sometimes called gasochromism) and in an expansion of the materials. provide a powerful tool for the simultaneous analyses of thousands of parameters. The nanomorphology of the surface considerably improves the gasochromic properties. Two rather similar articles have been published by this group (39. the time-response. thus enabling single molecule detection via enzymecatalyzed signal amplification. SWCNTs were deposited by the Langmuir-Blodgett technique at the distal end of the fibers. ionic species and solvents. No. be they DNA or proteins. Fiber gratings coated with Pd metal were reported to enable sensing of hydrogen gas (45). gases. The authors have studied the transmission. VAPORS.2 dB (32%) with a risetime of 100 s. and the initial response velocity in the range from -30 to 80 °C. 12. the optimal change output power obtained in this experiment was 1. as well as biosensors. When exposed to 4% hydrogen gas. Palladium also was incorporated into silica nano- composites of the sol-gel type and used in a reversible hydrogen FOCS (37). If exposed to hydrogen. Jeronimo et al. and extent of toxic chemical contamination was overviewed (31). These. typically caused by 1% hydrogen in argon gas (38). Fiber Bragg gratings (FBG) and long period gratings (LPG). along with flammable alkanes. Optical fibers coated with single-walled carbon nanotubes (SWCNTs) were shown to enable determination of hydrogen at cryogenic temperatures (47). Hydrogen interacts strongly with metallic palladium and platinum films and with tungsten oxide. Hydrocarbons. The sensor is capable of monitoring methanol and ethanol in water and gasoline Analytical Chemistry. both coated with Pd nanolayers. remains to be the analyte where safety considerations have led to a substantial amount of research in terms of fiber-optic sensing. with a focus on thermosetting resins and on spectroscopy-based techniques that can be used to monitor the processing of these materials A classroom demonstration was described for a portable fiberoptic probe multichannel spectrophotometer (33). Rather than miniaturizing the optical fiber. excitation. the latter has superior optical and switching properties. 40). has a short response time. Optical microarray biosensing techniques (28). were investigated. Since hydrogen gas does not display intrinsic absorptions/ emissions that could be used for purposes of simple optical sensing. Thus. Palladium-capped magnesium hydride was used as an alternative sensing material in a fiber-optic hydrogen detector (43). While such sensor elements are mainly aimed for use in telecommunications systems. it is always detected indirectly. and LPG sensors are mainly based on the coupling between the cladding modes and evanescent or surface plasmon waves. It employs a frequency doubled 532 nm Nd:YAG laser and a specially designed fiber-optic Raman probe and enables online determination of sample constituents without employing an expensive IR fiber. The Pd-Au film can detect 4% hydrogen with a response time of 15 s. With the use of this instrument. The sensitivity of the LPG sensor is better by a factor of ∼500. SENSORS FOR GASES. Hydrogen. the refractive index of the Pd-Au layer becomes smaller and causes attenuation on the transmitted light. Applications include sensors for pH. in turn. Comparing Mg-Ni and Mg-Ti based alloys. A drop in the reflectance of this material by a factor of 10 is demonstrated at hydrogen levels as low as 15% of the lower explosion limit. One major research focus is on hydrogen and methane because both are highly explosive when mixed with air and may be sensed more safely with FOS than with electrical devices. may be utilized to affect the transmission of optical fibers. June 15. The sensor was used to monitor the aging of certain materials. Methane and other hydrocarbons are almost exclusively sensed via their intrinsic absorption in the near-infrared (NIR). The response occurs within a few seconds. A detection limit of 0. Benzene was quantified down to 500 ppm using a PVC polymer coating. Highperformance fiber-optic oxygen sensors based on fluorinated xerogels doped with quenchable Pt(II) complexes were reported by Chu et al. No.6% and 20. The permeability of contact lenses for oxygen can be determined with a fiber-optic luminescent sensor system (56). evanescent wave fiber-optic sensor for the detection of hydrocarbon pollutants in water was constructed and tested (50). June 15. 12. Most oxygen sensors display nonlinear Stern-Volmer relationship. A luminescent dual sensor for time-resolved imaging of pCO2 and pO2 in aquatic systems was reported by Schroeder et al. have lifetimes on the order of 8-13 µs in the unquenched state. Their surfaces were modified by xerogel layers. The sensors yield linear Stern-Volmer plots. Surface-enhanced Raman spectroscopy and surfaceenhanced IR spectroscopy were applied to selective determination of polycyclic aromatic hydrocarbons (49).solutions.3 °C was achieved. have quantum yields between 0. was immobilized in the cladding using a sol-gel process. respirometry. Arain et al. Thin hydrophilic sensing films consisting of an analyte-sensitive indicator and a reference fluorophore were deposited on the bottom of the MTPs. reduces the risk of fluorescence energy transfer between indicator dyes. Both the UV absorption at 254 nm and the visible absorption at 600 nm were studied. methylene blue. Borisov et al. Capillary silica fibers were also fabricated. The use of microbeads enables the ratio of the signals to be easily varied. The most sensitive sensor for oxygen known so far exploits the efficient 4272 Analytical Chemistry. was copolymerized with a polymerizable monomer. (52).3 and 1. The method is based on kinetic measurements of the oxygen partial pressure inside a chamber sealed by the sample contact lens. Hence. Molecular oxygen in concentrations as low as 200-800 ng/L can be determined by this method either at single sensor spots or spatially resolved over a temperature range of more than 100 °C. and the spectral width of their luminescence. The luminescent indicator. a cross-linking reagent. The probes are less cross-sensitive to temperature. The tip of the fiber was covered with sensor chemistries based on luminescent microbeads that respond to the respective parameters by a change in the decay time. A porous plastic sensor was developed for the determination of dissolved oxygen in seawater (55). This is of interest to characterize the oxidative metabolism in coffee cherries during maturation as it appears to be regulated by the timely expression of redox enzymes such as catalase (CAT). A toxicological assay also is reported that enables monitoring the respiratory activity of P. Oxygen and carbon dioxide are clinically highly significant blood gases. Fiber optic microsensors with a tip diameter of ∼140 µm were reported for simultaneous sensing of oxygen and pH and of oxygen and temperature (62). Other Gases. and toxicological assays. Sensing in the UV region . The assay allows for the screening of green coffee samples for CAT activities. Molecular recognition is accomplished by functionalization of the silver nanoparticles with appropriate host molecules. Sensing is based on dynamic quenching of the fluorescence of the ruthenium indicator. a ruthenium(II)diimine complex. Vol. Oxygen sensing remains another area where FOCS are quite successful. 80. Ozone gas was sensed via its UV and visible absorption (68). Three kinds of microstructure fibers (MSFs) for sensing gaseous hydrocarbons were reported (51) that enable the quantitation of aromatic hydrocarbons.007 vol. Quantum dots undergo temperature-dependent changes of the intensity. or both. coli and Pseudomonas putida. The beads contain Fe3O4 and can be magnetically fixed at the bottom of a microbioreactor and enable contactless monitoring of oxygen in cultures of Escherichia coli using fiber optics. and response times range from 4 to 7 s. and polyphenoloxidase. putida. In order to compensate for the rather strong effects of temperature on oxygen sensors based on dynamic quenching of luminescence. 2008 quenching of the long-lived delayed fluorescence of fullerene C70 incorporated in thin films of ethyl cellulose or organosilica (59). The setup uses a broadband light source with backreflecting optics coupled to a fiber-optic sensing element coated with an analyte-enriching polymer that preconcentrates the analyte. It has been shown that an artificial network also may be applied to model the dynamic quenching of the fluorescence of a ruthenium-derived probe (54). It relies on the measurement of the phase shift of the luminescence decay time of a material composed of microbeadcontained indicators (with well-separated excitation and emission wavelengths) and polymers with excellent permeation selectivities as well as favorable optical and adhesive properties.0. A composite fluorescent material was described that enables simultaneous sensing and imaging of oxygen and carbon dioxide (66). where a thin luminescent O2-sensitive film is being placed. The response of the sensors to toluene in nitrogen gas results from spectral changes of the output light from the fibers at 1600-1800 nm. Activity screening was demonstrated for glucose oxidase and for monitoring the growth of E. The group of Klimant (60) has presented magnetically separable optical sensor beads for oxygen. The sensing dye. and this can be described by the socalled two-site model assuming two quenching constants. An evanescent wave fiber sensor was used to determine oxygen deficiency (58). Oxygen. The same group has presented new and ultrabright fluorescent probes for oxygen that are based on cyclometalated iridium(III) coumarin complexes (61). Effects are almost linear and fully reversible. Oxygen sensors were used to transduce the activity of catalase (57). (67). Oxygen almost exclusively is sensed by virtue of the quenching effect it exerts on certain fluorophores. they can be used as probes to compensate for effects of temperature in FOCS (65). The sensor is said to respond logarithmically linear between 0. the peak wavelength. and reduces the adverse effect of singlet oxygen that is produced in the oxygen-sensitive beads. mid-IR. peroxidase. the optical sensor is unaffected by the thickness of the contact lens and other effects. but are less photostable than ruthenium-based probes for oxygen. A modular. The MSFs with 10-50 µm air holes were arranged to one sensor. where temperature can be measured optically and used to calculate temperature-corrected data for pO2. A resolution of 0. (64) have developed a composite luminescent material for dual sensing of oxygen and temperature.9% oxygen. (63) have characterized microtiterplates (MTPs) with integrated optical sensors for oxygen and pH and have applied them to enzyme activity screening. or the intensity of their luminescence. It did not leach out due to its high hydrophobicity.% of toluene was achieved. Rather similar materials resulted in even faster responses as reported in a second paper by this group (53). and a porogen. Unlike in electrochemical techniques. allows for highly sensitive detectors due to its high absorptivity in that region. A new sensing scheme for ammonia is based on monitoring the optical changes of the Q-band of tetrakis(4sulfophenyl)-porphine (TSPP) at 700 nm. Signal generation is based on the spectroscopic changes of a reagent contained in the nanopores of a sol-gel element that undergoes an irreversible chromogenic reaction. while the ionic liquid contains a pH probe (in its base form) that undergoes a distinct color change after its (reversible) reaction with CO2. Sensors for carbon dioxide and. This is comparable to previously reported probes exploiting the same effect. The silicone matrix acts as a permeation-selective material for CO2. (69). The sensor was regenerated by rinsing with water. (70). to a lesser extent. Most ammonia sensors reported so far make use of its effect on appropriate pH probes. The sensor is not very stable Analytical Chemistry. Three thin film samples with different thicknesses were prepared layer-by-layer. The resulting ammonium ion is stabilized by a cation trap. Optical data are collected for a range of experimental conditions in a flowing glow discharge of N2-O2 mixtures. Ammonia also was sensed with the help of silica nanocomposites doped with silver nanoparticles and deposited on an optical fiber waveguide (78). ammonia can be sensed in watery samples. Because the gel is partially fluorinated. Two fiber-optic designs are described. The microsensors were used to establish ammonia microprofiles of high spatial resolution. No. It also is the prefered one in that it is applicable to aqueous solutions (unless extractive phases are employed in IR sensing schemes). The response to aliphatic amines is linear up to 2 ppm. Nitrogen is a species not easily sensed by optical means except by Raman spectroscopy. In combination with a silicone protection coating. The material was deposited onto the surface of a bent silica fiber by the dip-coating technique. The Raman peak intensity ratios for mixtures of liquid nitrogen and liquid oxygen were analyzed for their applicability to impurity sensing using different excitation light sources. the response is faster. Spontaneous Raman scattering was used to monitor of the purity of liquid oxygen in the oxidizer feed line during ground testing of rocket engines. Cross-sensitivities toward protons and alkali ions were prevented by coating the sensor with a layer of Teflon. The scheme was extended to sensing ammonia via fiber-optic microsensors at 2-100 µg/L levels that are known to be highly toxic to fish and other organisms (77). Exposure to gas containing ammonia enhances the attenuation of light power guided through the fiber probe. Sub-ppm levels of NH3 can be continuously monitored by this technique. It can detect organic amine vapors down to 1-2 ppb levels. 80. reagents typically employed in sensors for CO2 (74). A practical sensor for online sensing of atomic nitrogen in direct current glow discharges was reported by Popovic et al. all showing a linear sensitivity to NH3 in the 0-100 ppm range and a response time of around 30 s. Nanoporous matrixes also may be used to host pH-sensitive phenolic dyes and an alkaline phase transfer agent. Molecular recognition is accomplished by cholesteric liquid crystals combined with molecules carrying a trifluoroacetyl group. Vol. This is also true for one more sensor that makes use of a sol-gel matrix (76). As conventional automotive pollution sensors fail to meet monitoring requirements as specified by the European Community. The detection limit is reported to be 0. 2008 4273 . for various reasons. to record the desired spectral lines. The microsphere sensor is said to be more sensitive than other optical amine sensor described in the literature but heavily interfered by ammonia. The group of Mohr reports (80) that functional liquid crystal films can selectively recognize amine vapors and thereby undergo a change in their color. The UV based sensor can detect 0-1 mg/L of ozone and the longwave sensor 25-126 mg/L. Oberg et al. and a miniaturized sensing system was developed that responds within a few seconds. Ammonia in the gas phase may be sensed either via its NIR absorption or (in being a weak base) via the weak pH changes it can induce in immobilized pH indicators. Carbon dioxide emissions from a diesel engine can be monitored with the help of a midinfrared optical fiber sensor (71). ammonia remain another active area of research. A respective FOCS to monitor the concentration ratio of nitrogen and oxygen in a cryogenic mixture was reported by Tiwari et al. The second approach uses narrow bandwidth optical filters to detect the emission intensity. Both NH3 vapor and trace NH3 dissolved in water can be detected. One more fiber-optic probe was reported for the selective determination of NO2 in air samples (81). Note: sensors for amines in fluid phases are treated in section D on sensors for organic molecules. The latter approach is more sensitive. Interference by trimethylamine was minimized using an 18-crown-6 ether as a cation trap and by the permeability properties of the polymer matrix. A novel kind of optical sensor for carbon dioxide makes use of silicone encapsulated ionic liquids (72). A fluorescent pH indicator placed in a cellulose ester matrix at the tip of an optical microfiber is deprotonated by ammonia. The first approach uses a fiberoptic spectrometer. as induced by ammonia in the electrostatic interaction between TSPP and poly(diallyldimethylammonium) chloride (75). June 15. The visible region has a significantly lower absorption coefficient but enables monitoring high ozone levels. Measurements were performed using two methods. a low-cost sensor employing compact mid-IR components is presented that was used to measure CO2 in the exhaust of a commercial diesel engine. Another FOCS for NO2 makes use of poly(3-octylthiophene) as a sensing material (82). Sensing is accomplished by monitoring the intensities of the atomic nitrogen spectral line at 822 nm and the bandhead at 337 nm. This material undergoes large spectral changes at 543 nm if exposed to NO2. 12. calibrated with a standard source. and one more type of organically modified sol-gel was used to develop one more modification of this kind of sensor (73). relative to the oxygen line at 845 nm. A sulfonated hydroxypyrene named HPTS has been widely used as a pH probe in such CO2 sensors in the past two decades. thereby undergoing a large change in fluorescence intensity. The indicator was immobilized in porous SiO2 which then was deposited on the surface of a bent optical fiber core.5 µg/L. (79) have designed a simple optical sensor for vapors of organic amines based on silica microspheres dyed with pH indicators such as bromocresol green (a probe reported to be useful for sensing ammonia by others several times before). The SiO2 nanocomposite was prepared by the sol-gel technique in the presence of (3-mercaptopropyl)trimethoxysilane) and doped with 25 µm silver nanoparticles. The limit of detection is 13 ppb in gas samples and 5 ppb in water samples. Dissolved CO2 usually is sensed via the effect it exerts on the pH of a buffer immobilized in a matrix. 2008 (90) that uses birefringent porous glass oriented between two crossed polarizers and serving as the basis for this broad-spectrum sensor. fast response. Also see ref 91. The CdS formed also emits strong fluorescence. unlike chlorine. The polymers are patterned on glass substrates and undergo color changes that can be interrogated at different wavelengths. The incorporation of the gold nanoclusters leads to a broad absorption peak in the visible (purple) due to the excitation of localized surface plasmons. The process is fully reversible. A fiber-optic nanosensor for volatile alcoholic compounds has been described by Elosua et al. Another fiber-optic RH sensor reported (100) is based on large-core . with a detection limit as low as 0. Single-walled CNT multilayer coatings were used that cause a response toward toluene and xylene vapors. The sensor can make use of ambient light as a light source and the eye as a detector to register the resulting color changes and thus is capable of real time monitoring of VOCs. The response of the material toward different alcohols was measured at 850 nm. Optical sensitivity to NO2. Elemental iodine. The operational range is from 4 to 100% RH at 20 °C. Linear responses up to 50 and 150 mg/L were obtained for free and total SO2. Its color and refractive index change when exposed to vapors of VOCs. No. The ruthenium probe was immobilized on a Teflon support. A rather complex method for optical sensing of sulfur dioxide in wines (89) employs a dinuclear palladium ligand complex immobilized in a PVC membrane plasticized with o-nitrophenyloctylether (o-NPOE). and any breakthrough of the VOC through the carbon results a large change in the reflectivity of the porous silica. often are sensed by optical means in order to reduce the risk of explosion. Certain porphyrins deposited in Nafion films undergo waterinduced tautomerism.25% can be determined with the respective sensor. The maximum of the peak is shifted on exposure to oxidizing or reducing gases. often referred to as volatile organic compounds (VOCs). The sensing scheme is based on the detection of the oxygen consumed as a result of enzymatic oxidation of methyl mercaptan (MM) by MAO. This forms the basis of a novel type of fiberoptic RH sensor (99). 2 ppm for SO2. VOCs such as acetonitrile vapors can be detected at concentrations of >50 ppm. with lower limits of detection of 2 ppm for NO2. The authors have used the same material in a Fabry-Perot interferometer with a nanosized cavity along with a optical fiber pigtail system (93). trace H2S in the sample diffuses into the porous fiber and reacts with cadmium oxide to form cadmium sulfide (CdS). Certain cross interferences observed can be eliminated by applying a signal processing algorithm that also reduces false alarms. with detection limits of 0. King et al. It is used to simultaneously measure their concentrations. SO2 were detected using an ultraviolet optical fiber based sensor (83). Activated carbon is used as a prefilter. The results demonstrate ppm to ppb sensitivity. The sensor consists of a nanometer-scale Fizeau interferometer doped with the vapochromic material and placed at the cleaved end of a multimode fiber-optic pigtail. The performance of a carbon nanotube (CNT) based thin films fiber-optic for VOCs was described (94).5 years of discontinuous measurement. 80. The Langmuir-Blodgett technique was applied to deposit the CNTs directly onto the optical and acoustic sensors substrates. emission peaking at 600 nm). The analytical information is obtained via phase-sensitive determination of the decay time (98). Reversal times depend on exposure time and % RH. one of the smelling principles in halitosis. It exhibits long-term stability and a linear response over the humidity range from 0 to 4000 ppm. Numerous materials including Nafion films doped with crystal violet responding to humidity by a change in their reflectance (97). An irreversible active core fiber-optic probe was developed for determination of trace H2S at high temperature using a cadmium oxide doped porous silica fiber as a transducer (85). respectively.in that the response decreases after each exposure to NO2 but is rapid. The response is fully reversible (with some hysteresis) in dry nitrogen. The sensor was applied to measure RH in food and in a weather station. The output of the sniffer was amplified by substrate regeneration via reduction with ascorbic acid. was reported (88). CH4. with a peak emission at around 500 nm. It is based on a new vapochromic red powder consisting of a silver metal organic compound. An FOCS for VOCs was reported 4274 Analytical Chemistry. It can detect iodine with a rather high concentration detection limit of 6 µM. Vapors. Relative humidity (RH) between 0 and 0. The optical effects resulting from exposure to various VOCs are reversible and may result from adsorption of VOCs with attendant reduction of anisotropy. Vapors. Vehicle exhaust emissions such as NO2. Fuel gases such as H2. and high repeatability. Vol.018% RH (equal to ∼4 ppm). NO. Its response time is shorter than 2 min (recovery time <1. and 20 ppm for NO. so that fluorescence can only be used for probing trace H2S at low temperature. In another kind of RH sensor. The sensors have response times (t90) of less than 20 s. Humidity. If exposed to a gas sample at high temperatures. and changes up to 3 dB in the reflected optical power were registered. exerts a quenching effect on a derivative of aminobenzanthrone. hydrogen sulfide. The toxic industrial chemicals hydrogen cyanide. and this effect was used in a respective fiber-optic sensor (87). (92). highly selective. Monoamine oxidase (MAO) was immobilized at the tip of a fiber-optic oxygen sensor with a luminescent oxygen probe (excitation at 470 nm. An optical biosniffer for methyl mercaptan. but this signal is quenched at 450 °C. and sensitive. and chlorine can be sensed with waveguides coated with doped polymer materials in the form of arrays (86). Response times are <4 s.37 and 0. MM levels between 9 and 11 000 ppb were detectable. The sensor can detect moisture in process gases such as nitrogen and HCl. carbon monoxide. In a related paper.2 min). The pathological threshold is 200 ppb. Signal stability was verified for >2.70 mg L-1. Ru(II) complexes were employed since their luminescence decay time depends on RH. June 15. the limits of human perception are on the order of 10 ppb. Both free and total SO2 can be determined. and CO were found not to interfere. and this is detected at around 370 nm with a fiber-optic spectrometer. (96) report on a microsensor for VOCs where a photonic crystal of porous silicone is attached to the distal end of an optical fiber. Optical humidity sensors are a kind of evergreen given their highly different applications. and hydrogen (all in dry air) was observed for a layer of a metal oxide multilayer of the type InxOyNz covered with gold nanoclusters (84). 12. Miniature waveguide channels result in enhanced sensitivity owing to the increased path length. the incorporation of CNTs into hollow core fiber optics is reported (95). SENSORS FOR pH AND IONS This section covers sensors for all kinds of inorganic ions including the proton (i. respectively.quartz/polymer optical fiber pairs. describe a new organic pH indicator dye (mercurochrome) that was immobilized in a sol-gel matrix placed at the end of a fiber optic and enables measurement of pH in the pH 4-8 range (104). The number of articles on pH sensors is decreasing. fluores- cence lifetimes. Seki et al.e. respectively. The matrix was doped with various pH indicator dyes. each being sensitive to different pH ranges. The sensitivity of a tapered optical fiber relative humidity (RH) sensor was optimized by means of tuning the thickness of nanostructured sensitive coatings (101). A scheme for measurement of very high pH values as they occur in chemical processing was presented by Gotou et al. respectively. The same group also has developed sol-gel matrixes by controlled hydrolysis of a titanium tetraalkoxide (106). In other words: it is not specific for RH. sensing of pH values below 1 and above 12 still represents a substantial challenge in terms of material sciences. Optical pH sensors respond over a limited range of pH only (in most cases). An amino modified fluorescent aminophenylcorrole immobilized in a sol-gel SiO2 matrix undergoes large changes in fluorescence intensity owing to multiple steps of protonation and deprotonation. report (102) that a thermoplastic polyimide when deposited on a fiber-optic Bragg grating can act as a material sensitive to RH in that it undergoes reversible expansion and shrinkage. and that authors often do not properly cite previous work on the subject. thereby allowing larger pH ranges (1-11) to be covered than via respective tetraphenylporphyrins. pH). In related work but using different materials (for optoelectronic reasons). Dong et al. respectively (107). This is rarely addressed by authors of respective work. this enabling monitoring human breathing.0 and 7. (111). or both. sensors are presented that are inferior to others described before. In the worst case. viz. quantum yields. Martin et al. The microsensors with a tip diameter of ∼140 µm make use of luminescent microbeads that respond to the respective parameters by a change in the decay time. describe a pH sensor based on a heterocore structured fiber optic (112). Vol. The ratio between the two signals is proportional to pH but independent of excitation light intensity.3 and 4. A phase-modulated blue-green LED served as the light source for exciting luminescence whose average decay times or phase shifts served as the analytical information. It is used to establish a health monitoring method for chemically exposed fiber reinforced plastic structures in that it can detect the penetration of corrosive solutions into plastics. Analytical Chemistry. 80. The sensor displays a 27 times better sensitivity to RH than a comparable overlay. cations. The sensors have a dynamic range from pH 4. A single mode tapered fiber was coated with a specific nanostructured polymer whose thickness was controlled so to optimize sensitivity to RH. The pH indicator phenol red was immobilized in a sol-gel matrix at the surface of the heterocore portion. Examples of vapors that interfere (or may be sensed with this device) include those of acetone and ethanol. 12.1-9..0-12. Huang et al. The sensing scheme relies on the measurement of the fluorescence intensity of the pH indicator that is related to the intensity of the blue excitation light reflected by the sensing phase. A high-pH indicator is used along with an optical fiber connected to a spectrophotometer. microsensors were described for simultaneous sensing of pH and oxygen (114).0 pH range.0. On the other side. photostabilities. No. Optical sensing of pH remains to be of greatest interest even though all optical sensors suffer from cross sensitivity to ionic strength. an amino-modified poly(hydroxyethyl methacrylate) and an organically modified sol-gel. intensity.5 to 13. A novel wide pH range sensing system was described that is based on a sol-gel encapsulated amino-functionalized corrole (109). Data are evaluated by a modified dual luminophore referencing method which relates the phase shift (as measured at two different frequencies) to pH and to O partial pressure. The sensor is constructed from low cost fiber optic and optoelectronic components including a blue light emitting diode and a photodiode. pH is calculated by chemometric means from selected data points of the fluorescence spectra of aminofluorescein at high concentration so that a strong inner filter is operative. The pH probe (a carboxyfluorescein) and the oxygen probe (a ruthenium complex) were incorporated into two kinds of microparticles. Absorption and emission spectra. and anions. This is important in clinical chemistry and if minute sample volumes are only available. It is noted at this point by the author of this review that sol-gels and other condensates of that kind are known not to provide a temporally stable matrix but rather to change their microstructure over periods of typically a few months. respectively. and acidity constants were determined in organic solvents and in PVC. It consists of multimode fibers and a short piece of single mode fiber which was inserted into the multimode fibers. though. Congo red and neutral red on a cellulose acetate support have been reinvented as a material for sensing the 4.0-12. Such sensors are not treated here. Two aromatic Schiff bases were studied for their suitability in terms of sensing alkaline pH values (108). The group of Scheper has introduced a scheme for referenced sensing of pH that is based on spectral analysis of fluorescence (103).2-6. The resulting sol-gel (TiO2) is said to be more resistant and to have a longer lifetime than SiO2 films.0. The Schiff bases can be photoexcited at 556 and 570 nm. which does not come as a surprise in view of the state of the art and the fact that certain pH FOS are commercially available. pH changes were detected by measuring the loss spectra at 575 and 545 nm. Indicator loaded microbeads that are permeation selective for either protons or dissolved oxygen were used in a respective dually sensing membrane and a single fiber-optic sensor (113). and respond to pH in the range from 8. The applicability of this sensor was tested for its performance in pH analysis in tap and bottled mineral water (105). Both kinds of beads were then dispersed into a hydrogel matrix and placed at the distal end of an optical fiber waveguide for optical interrogation. It also needs to be stated that many pH sensors presented in the past few years are modifications only of existing sensing schemes and materials. A complex algorithm enables precise calculation of pH even if the spectra of the indicator dyes (for pH and oxygen) overlap. 2008 4275 . June 15. (110) have obtained wide-range pH optical sensor by immobilizing three indicators in a sol-gel matrix that was deposited in a fiber optic waveguide and optically interrogated by evanescent wave absorptiometry. pollutants. (131). 80. no such approach is possible in optical sensors.1 mM). Concentrations as low as 0. It should be noted that some of the biosensors can be found in other chapters if this was deemed to be more appropriate. Like similar sensors described before. The capillary has a cladding layer and a protective polymer coating on its outside surface. Sensing glucose remains to be an evergreen. Mid-IR laser spectroscopy was performed by either using cryogenically cooled lead salt lasers or quantum cascade lasers operating at room temperature and applied to reagentless (enzymeless) sensing of glucose (123). 2-propanol. No. The dye (covalently linked to the polymer matrix) recognizes the analyte via covalent binding. the limit of detection is 4 µM. The dosimeter does not respond to primary alcohols.An evanescent wave direct spectroscopic sensor for chromate anion reported by Tao & Sarma (115) uses a flexible fused silica light guiding capillary as a transducer.g. Polymer membranes containing a chromogenic functional azo dye undergo color changes on (reversible) interactions with amines in organic solvents (127). System features include good stability of the steady-state signal. The detection limit is reported to be 30 µg/L. A fluorescent dosimeter for formaldehyde determination in water utilizes the Nash reagent incorporated into silica gel beads (129). but the former were found not to respond at all. Methanol. on probes for saccharides and glycosylated biomolecules see ref 125. Dissolved amines also can be sensed by incorporating an aminecarrying chromoionophore into a sol-gel matrix (128). A Fabry-Perot interferometric system was developed containing a thin layer of nanoporous zeolite synthesized on the cleaved end face of a single mode fiber. A 30 m long capillary has the capability of detecting as little as 31 ppm of chromate. the sensor is based on the intrinsic evanescent wave absorption by chromate ions in a water sample inside the capillary. and other common substances. nucleic acids. Organics. Response time. Cu(II) and Zn(II) interfere to some extent. On the other side. Enzymatic Biosensors. Mercury(II) ions were optically probed by Li et al. BIOSENSORS This section covers biosensors based on the use of enzymes. and 8-hydroxyquinoline sulfonate acted as a fluorogenic ligand. At pH 5. Tri-n-propylamine and the drugs benzhexol and procyclidine were determined via electrochemiluminescence (ECL) in a sensor constructed by the screen-print technique (126). 12. and its ECL resulting from the interaction with the analytes was measured.1 mg/mL were detectable. and can be used at near neutral pHs. A quenchable fluorescent benzofurane derivative in the plasticized PVC matrix served as the indicator dye in an FOCS for ferric ion (118). Vol. In related work it is reported (122) that Pb(II) ions can be (irreversibly) probed by a similar method but using CdTe quantum dots capped with thiols. Aqueous solutions of glucose were analyzed by fiber based attenuated total reflection spectroscopy and by transmission spectroscopy. Limits of detection are as small as 30 nM. The same group has synthesized a fluorescent semicarbazone and demonstrated its applicability as a selective probe in an FOCS for copper(II) (119). and whole cells. while the protective coating increases its mechanical strength. SENSORS FOR ORGANIC CHEMICALS This chapter covers sensors for organic species (mainly saccharides). Both acidand base-catalyzed sol-gel processes were studied.1 mM. and toluene were detected with high sensitivity. The probe can be photoexcited at around 460 nm. sensitivity. agrochemicals. A fluorescence-based calcium nanosensor was described (116) that exploits silica nanoparticles (prepared by inverse microemulsion polymerization) doped with the calcium(II) probe calcein as both a recognition and transduction element for optical determination of calcium in blood serum. methyl triethoxysilane. nerve agents. antibodies. Both methods have the potential to be utilized in small fiber sensors that may be inserted into subcutaneous tissue. PVC and ethyl cellulose acted as sensor matrixes. with response times of around 1-2 min. and rather high detection limits (0. drugs and pharmaceuticals. The detection limit is virtually the same. and interferences were studied. reversibility. Unlike in the case of electrochemical schemes where direct electron shuttle from the substrate to the electrode has become possible as a result of enzyme wiring. and miscellaneous other organics. ketones. Stable ormosil layers were obtained using various fractions of organically modified sol-gel precursors. The sensor is operated by monitoring changes in the thickness of the thin film caused by the adsorption of organic molecules by white light interferometry. and both absorption and emission spectrometry can be applied. Traces of vanadium(V) ion can been determined by using an irreversible chromogenic reaction between vanadium ion and a hydroxamic acid immobilized in a poly(vinyl chloride) (PVC) membrane (117). and rapid response. (121). The MIP was prepared with Al(III) ion acting as the template. Dissolved organics in water samples can be sensed with a nanoporous zeolite thin film-based fiber sensor (130). 2008 a covalent bond between their boronic acid moiety and the diol moiety of saccharides. It makes use of single-walled carbon nanotubes whose reflectivity changes in the presence of toluene. Hence. Biosensors make use of biological components in order to sense a species of interest (which by itself need not be a “biospecies”). Mohr et al. For a review. June 15. The dynamic range at pH 5 is linear up to 0. limits of detection (poor). Aluminum ion in aqueous media can be probed with a regenerable sensor that uses a molecularly imprinted polymer (MIP) as the recognition receptor (120).2 µM. a fluorescent product is formed that can be detected instrumentally or visually. Base-catalyzed sensor layers underwent large signal changes. They are based on hemicyanine dyes containing a boronic acid receptor and are capable of forming 4276 Analytical Chemistry. The cladding layer ensures the ability of the capillary to guide light. and ethylene glycol dimethylacrylate as a cross-linker. emits with a peak at 600 nm. chemical sensors not using a biological component but placed in a biological matrix are not biosensors by definition. this resulting in strongly varying limits of detection. On reaction with formaldehyde. They report on a luminescent nanosensor for Hg(II) where the fluorescence of carnitine capped quantum dots made from CdSe/ZnS is quenched by Hg(II) ions with an efficiency that resulted in a detection limit of 0. This causes their fluorescence to increase. (124) describe fluoro-reactands for the enzymeless determination of saccharides. The MIP was synthesized from acrylamide. Ruthenium(II)tris(bipyridine) on Nafion was immobilized on a carbon electrode. 2-hydroxyethyl methacrylate. Binding constants of the various amines depend on the kind of solvent and are highly different. A FOCS for toluene in water was described by Consales et al.. dynamic range. e. these still rely on (a) . Haloalkane dehalogenase in whole cells of Xanthobacter autotrophicus immobilized in calcium alginate at the tip of a fiber-optic coverts the haloalkanes into acidic products (i. and asymptomatic controls were tested for the presence of IgM anti-GIPC-1 autoantibodies by the two methods. The unclad portion of an optical fiber was covered with selfassembled gold colloids whose surface was functionalized with antigens. describe a needle-type enzyme sensor system for determining glucose levels in fish blood (134). Autoantibodies to ovarian and breast cancer-associated antigens are detectable with high sensitivity by using a chemiluminescence based optical fiber immunosensor (146). This sensing platform is label-free. Conjugated to biotin. One assay was completed within 3 min. Hydrolysis of organophosphates by OPH causes the pH to fall. (b) of coproducts (such as protons. Immunosensors. Quantitative comparisons were made between the five matrixes and between the binding strategies. The group of Klimant has designed a fiber-optic flow through sensor for online monitoring of glucose in patients in intensive care units (133). The data obtained with this sensor roughly correlated with LC-MS/MS analytical results. and this is reported by the pH probe. A reference oxygen sensor is used to detect changes in oxygen supply caused by adverse effects such as bacterial growth. A human monoclonal IgM isolated from breast cancer patients targets the GIPC-1 protein and thus can be detected in concentrations down to 30 pg/mL. A tubing as used in microdialysis contains an integrated fiber-optic sensor. it was attached to a polystyrene waveguide along with the fluorescent pH indicator carboxynaphthofluorescein. Rajan et al. Luminescent yeast cells were entrapped in hydrogels in order to optically detect estrogenic endocrine disrupting chemicals (137).2-dichloroethane (DCA) (142). Results obtained by SPR and optical waveguide measurements correlate excellently. report on the fabrication and characterization of a surface plasmon resonance based enzymatic FOBS for detection of the bittering component naringin (143). lowers the pH). Antibodies against the F1 antigen of Yersinia pestis (the cause for plague and a potential terroristic agent) can be detected by a sandwich immunoassay on the surface of an optical fiber (145).transduction via metabolic coreactants (such as oxygen or NAD+). The group of Nann (135) found that the luminescence of silica coated quantum dots is dynamically quenched by hydrogen peroxide (H2O2). No. The quantum dots have a rather high specific surface area which enables a relatively large amount of GOx to be immobilized. Its sensitivity is higher by at least 1 order of magnitude than that of the ELISA method.e.. the buildup of fluorescent products from single enzyme molecules catalysis over the array of reaction vessels can be observed. The chemicals induce a chemiluminescence to be emitted by the cells. Single molecules of galactosidase were monitored (140) using a 1 mm diameter fiber-optic bundle with individually sealed femtoliter microwell reactors. hydrogen peroxide. which is 50 times lower than the chemiluminescent ELISA and ∼500 times lower than the colorimetric ELISA. Organophosphate pesticides and chemical warfare agents can be sensed with a FOBS that exploits the enzyme organophosphate hydrolase (OPH) as the biorecognition element (139). Sera from 11 ovarian cancer patients. A hollow needle was used that also was comprised of an immobilized enzyme membrane and a optic fiber probe with a quenchable ruthenium-based oxygen indicator. Fiber optic absorbance spectroscopy was compared with surface plasmon optical detection methods for lactamases bound to interfacial structures via biotin-avidin coupling (138). The antibody conjugate was imAnalytical Chemistry. The amount of oxygen liberated is determined via the quenching effect it exerts on the luminescence of a ruthenium(II) ligand complex. or failure of the peristaltic pump. 2008 4277 . and a digital concentration readout can be obtained by application of statistical analysis. The protein GIPC-1 was conjugated to the tip of an optical fiber. 80. A good reproducibility was observed 60 times without exchange of the enzyme membrane. or (c) on affinity binding (such as to concanavalin A). 12. Vol. A similar approach (141) was applied to 24 000 individual reaction chambers to sense DNA and antibodies and again is expected to be applicable to assays that utilize an enzyme label to catalyze the generation of a fluorescent signal. and this was exploited in a optical glucose assay using the enzyme GOx. A fiber-optic enzymatic biosensor was described for determination of 1. or NADH). and does not require a secondary antibody. A waveguide based immunosensor for the food toxins aflatoxin B1 and ochratoxin A uses either the competitive or the direct immunoassay format (147). enables real-time detection. All matrixes were suited. Antinuclear antibodies in serum can be determined quantitatively. with a linear response at ∼65 mg/L. The gas permeable tubing warrants constant air saturation in the flow cell. 22 breast cancer patients. and the results agree well with the accepted ELISA method. Like most cell-based catalytic biosensors. for the binding of the protein. temperature fluctuations. and this is detected via the pH probe fluorescein immobilized at the tip of the FOBS. DCA was quantified at levels as low as 11 mg/L. Antinuclear antibodies can be quantified with an optical fiber biosensor modified with colloidal gold (144). Genetically modified Saccharomyces cerevisiae cells containing the estrogen receptor expression of the luc reporter gene were immobilized in hydrogel matrixes. The dynamic range for determination of paraoxon is from 1 to 800 µmol/L. On the basis of the previous work on sensing H2O2 via the amount of oxygen formed by catalytic decomposition of H2O2. The sensor was evaluated in a 24 h test on a healthy volunteer. in principle. the response time is slow (8-10 min). Unlike in so-called “ensemble” responses in which many analyte molecules give rise to the measured signal. The calibration curve was linear for glucose in fish plasma. Glucose is sensed via oxygen consumption which occurs as a result of the oxidation of glucose catalyzed by immobilized GOx. Endo et al. June 15. This approach should prove useful for single molecule enzymology and ultrasensitive bioassays. The SPR wavelength maximum increases with the concentration of naringin. The probe was stored for 1 month at -80 °C but full activity was attained again at room temperature. Concentrations down to 80 ng/L of the chemicals were detectable. Mills et al. The sensing area was coated over ∼1 cm length with silver and then with the enzyme naringinase. This is one of the few reversible optical methods that are based on the transduction of oxidase based reactions via H2O2. (136) have designed a robust and reversible sensor where ruthenium(IV)dioxide is used as a catalyst. Comprehensive reviews have appeared (132). Real time activity assays of β-lactamase were performed by a detection scheme that combines an affinity test and a catalytic sensor. thereby changing the fluorescence intensity. 141 µg/L of As(V). 2008 Surface plasmon resonance (SPR) spectroscopy was used for detecting target sequences in genomic DNA differing in terms of copies in the relative genome (154). An isothermal amplification method is employed so to detect the viral DNA in a 25 µL reaction volume following several automated handling steps. this approach may be quite powerful. Sulfathiazole and chloramphenicol in shrimps were sensed by this method with detection limits from 0. No. adsorbed on glass fiber filters as a disposable biocomponent (158).6 µg/L of Hg(II). Vol.5 to 1 ng/mL. The flavobacterium contains a hydrolase that hydrolyzes methyl parathion into optically detectable p-nitrophenol. Bioluminescent signatures were delivered from four channels by switching one at once.and disaccharides. In another type of DNA biosensor. A DNA of 48. Bioavailable mercury and arsenic in soil and sediments were determined with fiber-optic bacterial biosensors (157). DNA Biosensors. localized SPR spectroscopy was coupled to interferometry (155). Microbially available dissolved organic carbon was quantified with a microsensor (159) containing microorganisms in a polyurethane hydrogel. 4278 Analytical Chemistry. (148). report on a DNA detection protocol utilizing the opacity of self-assembled nanometallic particles (NPs) and the optical response of a CMOS image sensor (153). The concentration of the analyte is indirectly determined by the decrease in fluorescence intensity. and ecosystems. which acts as capture DNA to bind hexa-histidine (His6)tagged bacterial biosensors. A gold layer was deposited on porous anodic alumina and interrogated by both interferometry and localized SPR. the assay time being 45 min without a separate amplification step. 12. specifically oligonucleotides and hybridized oligonucleotides. The detection range of the competitive detection method was between 0. Antibiotics present in specific samples triggered the dose-dependent release of the capture DNA-biosensor interaction. In another approach.19 V (vs Ag/AgCl). Results are in good agreement with those obtained by ELISA. The 90% response time was 1-5 min. Genetically engineered bioluminescent bacteria were applied to the detection of toxicants in surface waters (161). Wang et al. Link et al. detecting even single-base mismatched DNA targets in concentrations down to 10 pM. streptogramin.5 kb was found to be a suitable sensing nucleic acid. Hence. Bacterial transcriptional regulators are known to be dosedependently released from their operators upon binding of specific classes of antibiotics. while ss-DNA (which is not intercalated) does not give this effect. Dipping the stick into two reagent solutions results in a correlated conversion of a chromogenic substrate by a His6targeted enzyme complex. and 18 µg/L of As(III). regional economies.3 nmol/L concentrations of methyl parathion. The pesticide methyl parathion was shown to be detectable using Flavobacterium sp. The sensor is reusable because the target DNA can be stripped off the grating surface after the assay. (150) use a generic dipstick-based technology for rapid determination of tetracycline. causing a variety of impacts on public health.mobilized on a sensor chip and exposed to the analytes in a flow injection analyzer. An optical biosensor for real-time detection of DNA interactions was demonstrated with a long-period fiber-grating biosensor (152). If ds-DNA binds to intercalators such as doxorubicin or daunorubicin. The dipstick assay consists of a membrane support strip coated with streptavidin and immobilized biotinylated operator DNA. The system is now being implemented to a drinking water reservoir and river for remote sensing as an early warning system. . The multichannel system developed is based on mini-bioreactors containing four kinds of recombinant bioluminescent bacteria and is connected to a luminometer via a fiber-optic cable. The intensity of light reflected by the chip resulted in an optical pattern that was highly sensitive to changes in the effective thickness of the layer on the surface. the interaction of the two strands is found to be specific both with oligonucleotides and with genomic nonamplified DNA. Another optical fiber biosensor for biochemical oxygen demand (BOD) is based on microorganisms coimmobilized in an ormosil matrix (160). June 15. The probe DNA was immobilized on the silanized surface of the grating and then hybridized with the complementary (target) DNA. An elegant diagnostic device for the detection of the hepatitis B virus was presented (149). Several pathogens (including hepatitis virus) were detected by this technique. The sandwich hybridization assay combines fiber-optic array technology with appropriate oligonucleotide probes immobilized on microspheres. Bacterial Biosensors. the electrochemiluminescence (ECL) of the Ru(II) bipyridyl complex (a weak intercalator) is used to generate optical analytical information (156). The BOD values obtained correlate well with those determined by the conventional BOD5 method for seawater samples. Given the sensitivity of ECL-based methods. 80. The consumption of dissolved oxygen is measured with a fluorescent optical fiber sensor. A fiber-optic DNA microarray for simultaneous determination of multiple harmful algal bloom species was reported by Ahn et al. It has detection limits as low as 1/40 of the licensed threshold. to fatty acids. and macrolide antibiotics in food samples. The indirect method was used to sense toxins in barley and wheat flour samples. A detection limit as low as 5 cells is achieved. Only 75 µL of sample are needed to detect 0. The authors exploit the fact that DNA fragments attached to NPs precipitate them only at locations where cDNA strands exist. the opacity of the chip surface changes due to the accumulation of NPs and this can be used to detect targeted DNA fragments. The approach is very sensitive. In a novel kind of DNA biosensor. In a comparable approach. A strain was used that displays low substrate selectivity and responded to mono. The system can be continuously operated due to the separation of the bacteria culture reactor from the test reactor without system shutdown by abrupt inflows of severe pollutants. Alginate-immobilized recombinant luminescent bacteria were immobilized on optical fibers and enabled luminescent quantification of 2. antibiotics in seafood were screened for a fiber-optic fluorometric assay based on competitive binding of the analyte and an intercalating dye to double stranded DNA (151). and to amino acids. The antibiotics affect the binding of the intercalator to the double stranded nucleic acid. Algal blooms are a serious threat to coastal resources. Hybridization was visualized using fluorescently labeled secondary probes. an easily detectable ECL is generated in the presence of the ruthenium complex at +1. The usual amplification step is found not to be necessary. Following fragmentation with restriction enzymes and denaturation.5 and 10 ng/mL. The method allows immediate analysis of alfalfa without prior sample treatment. Regional differences in the water content of human skin can be studied by diffuse reflectance near-infrared spectroscopy (169). A noninvasive fiber-optic oxygen meter detects oxygen permeability of PET bottles for soft drinks by measuring trace levels of oxygen inside the bottle. K. Graded index optical Analytical Chemistry. Data obtained can give information on the properties of different types of concrete but also on the migration of dissolved salts. Huber et al. pH sensors for concrete are needed to detect any (highly adverse) changes in the rather high pH (>11) of concrete. biotechnological. One large area of application of FOCS is in monitoring the mechanical and chemical integrity of concrete structures. The optical fiber sensors reacted to the ingress of water by detecting the moisture migrating through the sample. Permeation rates are obtained without piercing the package or bottle which is ideally suited for assurance. An fiber-optic pH sensor was developed that can be incorporated into concrete and thus is capable of early detection of the danger of corrosion in steel-reinforced concrete structures (173). oxidized fibers that can release arsenate ions are toxic. Sensitivity was further enhanced by coating a monolayer of colloidal gold nanoparticles as the active material on the grating surface of the LPFG which increase the sensitivity by a factor of 2. fibers have been used for evanescent wave excitation of fluorescence or Raman scatter. A fiber-optic sensor was inserted into muscle for continuous measurement of intramuscular data. The sensor exhibited a linear decrease in the transmission loss and resonance wavelength shift when the chloride was increased. Mn. In combination with robust and flexible fiber-optics. while freshly prepared or washed fibers are not. however. 2008 4279 . Glasses displaying less strong interfering vibrations in the 2-5 µm spectral region were prepared from TeO2-BaO-Bi2O3 mixtures (166). changes in chemical composition are of high interest given the health risk and costs associated with disintegrated structures. and related applications of FOCS and FOBS. Hence. The pH indicator phenol red is covalently immobilized on the inner wall of the capillary. Optical fibers are used to connect the interrogating unit to the sensing capillary. Most regional differences of water content were calculated from the peak height of the 1900 nm water band normalized to the peak height of the 2175 nm amide band. Thus. The biocompatibility of Te-As-Se (TAS) glass fibers for use in infrared sensors was studied by Wilhelm et al. for imaging and sensor array purposes. The glass underneath this layer is. Blood samples were taken from the forearm every minute during each exercise bout. There was a difference in the ratio of the two water bands centered near 1450 and 1900 nm between the contact and noncontact measurements. Chloride ion in concrete can be sensed with a long-period fiber grating (LPFG) (172). (163) reports on the measurement of the ingress of oxygen into PET bottles using oxygen sensor technology. Reflectance spectra in the 1250-2500 nm region for the skin of volunteers reveal large regional differences of water content. to mention only the more important ones. Fe. The LPFG device is sensitive to the refractive index of the medium around the cladding surface of the sensing grating. A fiber-optic structure is presented that maintains the structural integrity of the optical fiber. a newly developed miniaturized diamond ATR probe has been described that displays high chemical and pressure resistance. The data for venous and arterial blood were found to be highly different when exercising. A similar approach was introduced by Yeo et al. (171). Zn. and moisture in alfalfa (164). industrial. such as sodium chloride which is important in view of their deleterious effects on reinforcement bars within concrete. Fiber Optics. 80.07 pH units were obtained. Aside from sensing mechanical integrity (not treated here). The interstitial sample fluid drawn subcutaneously from adipose tissue flows through a microfluidic circuit formed by a microdialysis catheter in series with a glass capillary. Sensors based on the interaction of surface plasmons or evanescent waves with the surrounding environment are usually obtained by tapering an optical fiber. The current success of (fiber optic) surface plasmon resonance (SPR) is obvious. stable in water over several days. Data on pH. TAS fibers undergo oxidation on air to form a water-soluble nanometer-thin layer that is soluble in water. A partial least-squares regression method was employed. A fiber-optic sensor for measurements of interstitial pH was further improved (167). Gaseous or dissolved oxygen can be detected in the ppm to ppb range. P. indicated by a shift in the Bragg wavelength of the sensor. production. and for distributed sensing. The fibers are used for IR direct spectroscopy of cultivated mammalian cells. June 15. The limit of detection for chloride ions is ∼0. Na. in microsensors and nanosensors. The authors measured chloride ions in a typical concrete sample immersed in salt water solutions in concentrations ranging from 0 to 25%. it enables inline chemical reaction monitoring (162). and the temperature dependence of the viscosities of the glasses is reported.03 pH units and an accuracy of 0. A resolution of 0. Preliminary in vivo tests were carried out in pigs with altered respiratory function. Vol. SENSING SCHEMES AND SPECTROSCOPIES This section reports on improved or novel sensing schemes based on the use of fiber optics and related waveguides. Near-infrared reflectance spectroscopy was applied to quantitate Ca. protein. carbon dioxide. Sensing is based on quenching of the fluorescence by oxygen of a sensor spot placed on the inner wall of the transparent bottle.APPLICATIONS This section comprises sensors for environmental. Aside from their use as plain waveguides. The humidity sensors were fabricated using fiber Bragg gratings coated with moisture sensitive polymers and are employed in detecting the movement of moisture through standardized cubes made from samples of different types of structural concrete. 12. IR and Raman spectroscopic studies were carried out. medical. Optical fiber sensors have been used to monitoring the ingress of moisture in structural concrete (170). No. pharmaceutical. thus offering the possibility of detecting a change in chemical concentration. (165). and quality control applications. and oxygen as obtained with fiberoptic sensors on intramuscular and venous blood during rhythmic handgrip exercise were compared (168). A fiber-optic cable is positioned outside and measures changes in luminescence lifetime. food.04%. New in-line fiber-optic structures for environmental sensing applications were described (174). The prediction capacity of the model and the robustness of the method were checked in the external validation in alfalfa samples of unknown compounds. These elements are fused by an optical fiber splicer.. A study of the capabilities of microstructure fibers for evanescent wave vapor sensing is presented in ref 181. Absorbance within the tubing was measured by optically coupling the FSCap to a spectrophotometer. and it is found that that the systems have similar characteristics.g. colorimetric calcium-sensitive films were deposited onto a DVD. The sensor measures the light intensity of the internally reflected light at a fixed wavelength from an optical fiber where the extinction cross-section of self-assembled gold nanoparticles on the unclad portion of the optical fiber changes with the refractive index of a sample near the gold surface. The sensor is based on evanescent wave polarimetric interferometry and is intended for use in gas chromatography. and a coreless optical fiber acts as the interaction area. (176) described the application of a light guiding flexible tubular waveguide in evanescent wave absorption based sensing. vapors of acids (based on immobilized bromophenol blue). This perturbation leads to a wavelength-splitting in two separate peaks: the peak at lower wavelengths corresponds to the thinned region and is dependent on the outer RI and the local temperature. The sensor was characterized in terms of thermal and RI sensitivities. Similarly. 80. The disk drives also perform the function of reading and writing digital content to optical media. Sensing is based on the measurement . A FSCap for pH is demonstrated. Toluene vapor in nitrogen gas was investigated. Two optical system designs were compared for fiberoptic chemical sensor applications (175). exposed to different concentrations of Ca(II). chemical concentrations at high pressure conditions (189). Fiber optic (bio)sensors were reported that are based on localized surface plasmon resonance (SPR) (188). This novel gas sensor has the capability of gas detection with a cell volume in the sub-nanoliter range. hollow core microstructured polymer optical fibers can be used for sensing chiral species. A sub-nanoliter spectroscopic gas sensor was described (180) and compared to existing sensors designs. The sensor consists of a Bragg grating where the cladding layer is removed along half of the grating length. A distributed fiber-optic polarimetric sensor was reported by Caron et al. for sensing gaseous chlorine (based on a chromogenic reaction). Three types of splitters were tested. Absorbance measurements in 50. 4280 Analytical Chemistry. Capillary Waveguides. Moreover. (177).fiber elements are used as lenses. Evanescent absorbance was linear with the concentration of a nonpolar dye but nonlinear with ionic dyes due to adsorption to the capillary wall. and peroxides (based on immobilized horseradish peroxidase and a chromogenic substrate). Because of the spatial separation. It causes a change in the refractive index changes of the xerogel fiber cladding at 670 nm. (179). also if used in a colorimetric CO2 sensor. 12. (184) showed that nanophase-separated amphiphilic networks represent versatile matrixes for optical chemical and biochemical sensors. A self-temperature-referenced sensor based on nonuniform thinned fiber Bragg gratings was described (186). A single grating spectrograph with fiber-optic input and photodiodes at three different wavelengths was compared to a system comprising 1-3 fiberoptic splitters and photodiode detectors with integrated interference filters. Hanko et al. and multimode fibers and postsensitized by HF etching treatment (185). Various prototypes of sensors were prepared. 2008 Analog signal acquisition from computer optical disk drives was demonstrated to be useful in chemical sensing (182). Sensing of the Ni(II) ion and label-free detection of streptavidin and staphylococcal enterotoxin B is demonstrated at picomolar levels. Hence. A light guiding flexible fused silica capillary (FSCap) was used that is similar to a conventional silica fiber in that it can guide light in the wavelength region from UV to near IR. Hence. It is demonstrated how the band gaps of such a hollow core polymer optical fiber scale with the refractive index of a liquid introduced into the holes of the microstructure. A related reflection based localized SPR fiber-optic probe was developed to determine refractive indexes and. and quantified in the optical disk drive. and 250 µm inner diameter FSCaps show that greater sensitivity is achieved in thinner walled tubings because of more internal reflections. Tao et al. and the resulting optical rotation is measured. High-sensitivity optical chemosensors were implemented by exploiting fiber Bragg grating structures in D-shape. A liquid-filled hollow core microstructured polymer optical fiber (178) is said to be opening up many possibilities in FOCS. Such a sensor platform is quite universal and can be applied to sensing and combinatorial screening. It allows realtime monitoring of the displacement of a chemical substance along a capillary. The resulting devices were used to measure the concentrations of sugar solutions. Signals were obtained from optical sensor films deposited on conventional CD and DVD optical disks. They consist of nanosized domains of hydrophilic and hydrophobic polymers (comparable to a polyacrylamide-co-polyacrylonitrile copolymer referred to as Hypan and previously introduced by others). the waveguiding properties of a FCSap for chemical sensing applications were investigated by Keller et al. The reproducibility of the microjet printing process was found to be excellent for micrometer-sized sensor spots. coating the inner surface of the FSCap with a polymer. Specifically. The fiber is then filled with an aqueous solution of (L)-fructose. Almost any optical disk can be employed for deposition and readout of sensor films. An array of photopolymerizable sensing elements containing a pH sensitive indicator was deposited on the surface of an optical fiber image guide. Spectroscopies. the intrinsically insensitive Bragg grating became sensitive to refractive index (RI). Sensors for Cu(II). connecting the coated FSCap to a light source and a photodetector. Refractive Index. Vol. e. Ink jet printing technology was applied to fabricating microsized optical fiber imaging sensors (183). thus. and delivering a sample through the FSCap were developed. Techniques were developed for activating the inner surface of an FSCap. June 15. The inner surface of the FSCap capillary was coated with a reagent doped polymer to design a FOCS. single-mode. Microsystems and Microstructures. there are domains in which the indicator reagents are immobilized and domains where diffusive transport of the analyte occurs. The FSCap operated evanescently or as a liquid core waveguide depending upon the refractive index of the sample solution within the capillary. 150. No. toluene in water samples and ammonia in a gas sample were fabricated and tested. while the peak at longer wavelength responds to thermal changes only. A photonic band gap fiber for measurement of RI was described by Sun and Chan (187). specific changes in absorbance due to C-H overtone absorptions of toluene at 1600-1800 nm were exploited. pp 351-375. Schulze.. C. 1201–1214. Eng. M. 2008. 375– 403. Anal. and has acted as the (co)organizer of several conferences related to fluorescence spectroscopy (MAF) and to chemical sensors and biosensors (Europtrode). M.. Araujo. Handbook of Near-Infrared Analysis. 400–422. 2008. C.. DC. S. Anal. (Optical Sensing Technology and Applications) Mohr. Med. Baldini. Schwotzer. 413432. W. A. T. American Chemical Society: Washington. Bosch. J. F. 40. 2007. B. A.. Surf. K. N. The light attenuation caused by the absorption of self-assembled gold nanoparticles on the unclad portion of the optical fiber changes with a different refractive index of the environment near the gold surface. Opt. Eds. M.. 2007.. A. Sensors 2006. C. DC. The analytical information is obtained by measurement of the phase shifts at two modulation frequencies. Borisov. The method was demonstrated to work for the case of dually sensing oxygen and carbon dioxide. 2007. Li. and the instrument can be deployed in-field. (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) Analytical Chemistry. Glucose Sensing.56 µm yield sub-ppb (v/v) sensitivity in the gas phase for CO2. S. Opt. Nagl. Sensors 2007. M. Wolfbeis holds a Ph.. Otto S. T. Halter. D.. V. pp 133-155.525 µm. S. Wolfbeis. low cost. 2007. Practical Spectroscopy. 118. Adriaens. Mater. He has authored around 470 papers and reviews on optical (fiber) chemical sensors. A: Pure Appl. 74. Serra. J. McDonagh. respectively. Turner. Meas. This continuous-wave CRD spectrometer uses a rapidly swept cavity of simple design. 2006. 65850B/1–65850B/8.. 2006. CRC: Boca Raton. A. June 15. serves as the analytical information. Ecke. M. B125. Educ. Leung. Schaeferling. T. Tatam. Buchanan. pp 1-13.. Rev. Flowers. P. Vol. which is an important feature for any kind of FOCS or FOBS. Chapter 2. P. Anal. 72. Acta 2006.-T. R. 2007. The probe was used to detect crystal violet and malachite green at ppb levels. C. 51. I. I. D. in biosensors based on thin metal films using capacitive or SPR interrogation. J. Rev. S. C. M. B... K. R. R. CRC Press: Boca Raton. Sanchez. Grattan. 1440–1465. M. 2006. J. S. The limits of sensitivity were further pushed (191). Jeronimo. Chem. 76. 11.. pp 521-527.. S. 423–461. Geddes. A modified dual lifetime ratiometric (DLR) method was introduced for simultaneous luminescent determination and sensing of two analytes simultaneously (194). Anal. Scopesi. Kocincova. Vannela. 118. Vol. 108. Vol. 2006. 3rd ed. The probe demonstrated a stable and repeatable response for sequential operations of pressurization and depressurization at 0.. LITERATURE CITED BOOKS AND REVIEWS (1) (2) (3) (4) (5) Wolfbeis... 2006. He acts on the board of several journals including Angewandte Chemie and is the Editor-in-Chief of Microchimica Acta. Vol.. and Nanosystems . A. Mayr. 2007. O. B. J. Eckhardt. Eng.S.. G. 80. 386. fluorescent probes. R. Addison. Vo-Dinh. S. P. Spectrosc. Lakowicz. Rojas. M. 797–859.D. M. 10.. 17/1–17/10. F.. Austria. 1442–1458. Wolfbeis. Vol. Opt. Biol. selectivity can be achieved without the need of any functionalization of the surfaces or the use of recognizing elements. 224.. 2007. O. 2006. A. P. 18. A significant variation of the spectral transmittance of the device is produced as a function of the concentration of the analyte by tuning the plasmon resonance to a wavelength for which the outer medium is absorptive. A. 13–27. Arnett. NH3. Chem. 2006. S. Nanotechnol. 2007. A. W. Ed. Lakowicz. Eds. In Hazardous Industrial Waste Treatment. Weidgans. Sheeran. Ojeda. 2004. C. 108. S. Interface Anal. 2002. H2O.) 2008. 3859–3873. D. Technol.. Bally. J.. Duerkop. 529531. N.... J. Y. 505–519. MacCraith.. Arregui. and both emissions are measured simultaneously. A. New Approaches in Biomedical Spectroscopy. Homola. Blitz.. Burke. Gas 2006. FL. Vol.. R. Chem. B. Eds. L.. H. S430–S444. Martelucci. R. 2663–2677.. T. O. F. Fernando.. Topics in Fluorescence Spectroscopy. Vol. Two luminescent indicators are needed in this scheme that have overlapping absorption and emission spectra but largely different decay times.. 2008. O. 758–760. S. Wolfbeis. I.4 MPa at 308 K. J. F. Rev. Oil. Comb. Spangenberg. he became an Associate Professor of Chemistry in 1981 at Karl-Franzens University in Graz... Polymers. Montenegro. C.. J. The geometry of a fiber-optic surfaceenhanced Raman scattering (SERS) sensor was optimized with respect to trace detection (192). Stich... The method is based on the difference between the lifetimes of the phosphorescence and fluorescence emissions of a dually emitting indicator (an aluminum-ferron complex). H.. Ed. A. Sci. J. Wolfbeis. 4.. Opt. In Optical Chemical Sensors.. Chem. K. Crit. 533. Opt. Ser. Sens. O. has (co)edited several books. 132. 32. (Washington. R. in fluorescent probes and labels. 2006. M. SPIE-Int. Ojeda. Springer: Dordrecht. 1683–1688. BioTechniques 2006. Sci.of the intensity of internal light reflection at a fixed wavelength from an optical fiber. Environ. Voros. 1–20. Rojas. 6 (11). Wolfbeis. W. P. The CRD spectrometer therefore is a high performance sensor in a relatively simple.. CAN 148:44768. G.1007/4243_013. Chester. S.. H. Konorov. T. 569. M. Geiger. M. M. H. B. Hung. Narayanaswamy. Taricska. R. and other hydrocarbons. S. J. 11. Geng.. 8. in fluorescence-based analytical formats including imaging. Chem.-F. 12. M. Sanchez. Topics in Fluorescence Spectroscopy. No. N. Sun. A. S. 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