Fiber-Optic Chemical Sensors and Biosensors

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 4269Sensors for Gases, Vapors, and Humidity 4271

Hydrogen 4271Hydrocarbons 4271Oxygen 4272Other Gases 4272Vapors 4274Humidity 4274

Sensors for pH and Ions 4275Sensors for Organic Chemicals 4276

Organics 4276Biosensors 4276

Enzymatic Biosensors 4276Immunosensors 4277DNA Biosensors 4278Bacterial Biosensors 4278

Applications 4279Sensing Schemes and Spectroscopies 4279

Fiber Optics 4279Capillary Waveguides 4280Microsystems and Microstructures 4280Refractive Index 4280Spectroscopies 4280

Literature Cited 4281

This review covers the time period from January 2006 toJanuary2008andiswritten incontinuationofpreviousreviews(1–3).Data were electronically searched in SciFinder and MedLine.Additionally, references from (sensor) journals were collected bythe author over the past 2 years. The number of citations in thisreview is limited, and a stringent selection had to be madetherefore. Priority was given to fiber-optic sensors (FOS) ofdefined chemical, environmental, or biochemical significance andto new schemes.

The review does not include the following: (a) FOS thatobviously have been rediscovered; (b) FOS for nonchemicalspecies such as temperature, current and voltage, stress, strain,displacement, structural integrity (e.g., of constructions), liquidlevel, and radiation; and (c) FOS for monitoring purely technicalprocesses such as injection molding, extrusion, or oil drilling, eventhough these are important applications of optical fiber technology.Unfortunately, certain journals publish articles that representmarginal modifications of prior art, and it is mentioned hereexplicitly that the (nonpeer-reviewed) Proceedings of the SPIE areparticularly uncritical in that respect.

Fiber optics serve analytical sciences in several ways. First,they enable optical spectroscopy to be performed at sites inac-cessible to conventional spectroscopy, over large distances, oreven at several spots along the fiber. Second, in being opticalwaveguides, fiber optics enable less common methods of inter-rogation, in particular evanescent wave spectroscopy. Fibers are

available now with transmissions over a wide spectral range.Current limitations are not so much in the transmissivity but inthe (usually shortwave) background fluorescence of most of thematerials fibers are made from, in particular plastic. There is anobvious trend toward longwave sensing where background signalsare weaker. Major fields of applications are in sensing gases andvapors, in medical and chemical analysis, molecular biotechnology,marine and environmental analysis, industrial production monitor-ing, bioprocess control, and the automotive industry. Note: In thisarticle, sensing refers to a continuous process, while probing refersto 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 ofthe analyte are measured, for example its refractive index,absorption, or emission. In the second, the color or fluorescenceof an immobilized indicator, label, or any other optically detectablebioprobe is monitored. Aside from the design of label and probes,active areas of research include advanced methods of interrogationsuch as time-resolved or spatially-resolved spectroscopy, evanes-cent wave and laser-assisted spectroscopy, surface plasmonresonance (SPR), and multidimensional data acquisition. In recentyears, fiber bundles also have been employed for purposes ofimaging, for biosensor arrays (along with encoding), or as arraysof nonspecific sensors whose individual signals may be processedvia artificial neural networks. The success of SPR in general, andin the form of FOS in particular, is impressive.

Following the recent sensor hype of (mainly organic) chemiststhat tend to refer to optical molecular probes as “sensors”, theliterature has become more difficult to sort. This review coversliterature on methods that enable sensing of (bio)chemical speciesas opposed to conventional types of optical assays. Unfortunately,there is a tendency to even refer to conventional indicators (suchas for pH or calcium) as biosensors if only used in vivo. Similarly,optical analysis of a solution by adding an appropriate indicatorprobe is now referred to as “sensing” (to the surprise of the sensorcommunity). I have outlined the situation in more detail in myprevious review (3).

BOOKS AND REVIEWSIt is obvious that fiber optic chemical sensors (FOCS) have

had a particular success in areas related to sensing gases andvapors. Many of the systems implemented are based on directspectroscopies that range from UV to IR, and from absorbanceto fluorescence and surface plasmon resonance. Optical chemicalsensors have been comprehensively reviewed by McDonagh etal. quite recently (4). The article covers sensing platforms, direct(spectroscopic) sensors, reagent-mediated sensors and discussestrends and future perspectives. Fiber-optic UV systems for gas

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10.1021/ac800473b CCC: $40.75 2008 American Chemical Society 4269Analytical Chemistry, Vol. 80, No. 12, June 15, 2008Published on Web 05/08/2008

and vapor analysis have been reviewed (5). The strong absorbanceof vapors and gases in the UV region is advantageous and resultedin a compact detection system of good accuracy. Buchanan hasreviewed recent advances in the use of near-IR (NIR) spectroscopyin the petrochemical industry (6) and points out NIR is particularlyattractive in this field because it measures the overtone andcombination bands predominately of the C-H stretches. Practicalexamples include sensing schemes for oxygenates in fuels,determination of octane numbers, the composition of fuels,bitumen analysis, and environmental analysis. Tools for life scienceresearch based on fiber-optic-linked Raman and resonance Ramanspectroscopy were also reviewed (7). One focus is on fiber-opticprobes for UV resonance Raman spectroscopy that offer severaladvantages over conventional excitation/collection methods, an-other on novel probes based on hollow-core photonic band gapfibers that virtually eliminate the generation of silica Ramanscattering within the excitation optical fiber. FOCS for volatileorganic compounds have been reviewed by Elosua et al. (8). Suchsensors are minimally invasive, lightweight, passive and can bemultiplexed. The devices were classified according to theirmechanism of operation and in terms of sensing materials. Thestate of the art in leak detection and localization and respectivelegal regulations have been reviewed by Geiger (9). Specificaspects include sensor reliability, sensitivity, accuracy, and robust-ness. Applicability is demonstrated for two examples, a liquidmultibatch pipeline and a gas pipeline. Nanostructure-based opticalfiber sensor systems and examples of their application have beenreviewed by Willsch et al. (10). Selected examples of advancedoptical fiber sensor systems based on subwavelength structuredcomponents are presented. These include sensor for humidity andhydrocarbons, for application in the gas industry, and for envi-ronmental monitoring using nanoporous thin-film Fabry-Perottransducer elements or intrinsic fiber Bragg grating sensornetworks for structural health monitoring. Additionally, newconcepts for sensing based on the use of plasmonic metalnanoparticles, photonic crystal fibers, and optical nanowires arediscussed.

Mohr (11) has reviewed recent developments in chromogenicand fluorogenic reagents and sensors for neutral and ionic analytesbased on covalent bond formation. New indicator dyes for aminesand diamines, amino acids, cyanide, formaldehyde, hydrogenperoxide, organophosphates, nitrogen oxide and nitrite, peptidesand proteins, as well as for saccharides also are described, andnew means (such as color changes of chiral nematic layers) ofconverting analyte recognition into optical signals are described.Aspects of multiple optical chemical sensing with respect toparameters, materials, and spectroscopies have been reviewe (12).Borisov et al. summarize their research on plastic microparticlesand nanoparticles for fluorescent chemical sensing and encoding(13). A rather wide and shallow review covers advances in fiber-optic sensing in medicine and biology (14). Applications rangefrom the use of fibers acting as plain light pipes to complexchemical sensors. It is stated (somewhat overoptimistically) thatchemical sensing can “simply” be achieved by transporting lightto and from a measurement site with a plain fiber light guide forspectrophotometry, fluorometry, or SPR. Flowers et al. discussedaspects of fiber-optic spectro-electrochemical sensing for in situdetermination of metal ions, mainly copper(II) (15). The term

spectro-electrochemical sensing does not relate to electrolumi-nescence but to electrochemical conversion of an analyte prior toits spectroscopic detection, e.g., by preconcentration, oxidation,reduction, complexation, and optical detection. Given its complex-ity, it does not come as a surprise that the authors point to “theneed for further refinement of the sensor’s design and theexperimental protocol to improve the method’s sensitivity.”The performances of interferometry, surface plasmon resonance,and luminescence as biosensors and chemosensors have beencritically compared (16). Sensitivity, dynamic range, and resolutionwere calculated and compared from a range of data from theliterature. Theoretical sensitivities of interferometry and SPR aredetailed along with parameters affecting these sensitivities.Luminescence is said to offer the best resolutions for sensing ofprotein and DNA, while interferometry is said to be most suitablefor low-molecular weight chemical liquids and vapors if selectivityis not a critical issue. SPR (which is label-free) possibly can senseproteins with a resolution similar to that of luminescence.

Borisov and Wolfbeis (17) have presented what appears to bethe most comprehensive review on optical biosensors so far.Applications of optical fiber (bio)sensors (FOBS) also includeareas such as high-throughput screening of drugs (18), thedetection of food-borne pathogens (19) (based on either SPR,resonant mirrors, fiber-optic systems, arrays, Raman spectroscopy,and light-addressable potentiometry), and in environmental sens-ing by making use of DNAzymes (20). DNAzymes can selectivelyidentify charged organic and inorganic compounds at ultratracelevels in waste and emissions. In combination with laser baseddetection, DNAzymes enable accurate quantification of suchcompounds and thus represent an attractive alternative to state-of-the-art affinity sensors. Significant strides have been made interms of selectivity, sensitivity, and catalytic rates of DNAzymes.Challenges remain in the development of efficient signal trans-duction technology for in situ applications. The state of the art incontinuous glucose sensing, a kind of holy grail in FOBS, hasbeen summarized in a book (21), and (fiber) optical methodsbased on the use of glucose oxidase and transduction via oxygenconsumption are reviewed in one chapter (22).

The trend toward miniaturization is obvious. Nanoscale opticalbiosensors and biochips for cellular diagnostics have beenreviewed by Cullum (23), specifically with respect to achievementsin employing nanosensors and biochips (e.g., gene chips) incellular analyses ranging from medical diagnostics to genomics,while optical nanobiosensors and nanoprobes were reviewed byVo-Dinh (24) with respect to the principles, development, andapplications of fiber-optic nanobiosensor systems using antibody-based probes. One specific class of FOBS, the tapered fiber-opticbiosensors (TFOBS) were reviewed (25). In these, part of thefiber is tapered so that the evanescent field of the lightwave caninteract with samples. TFOBS are often used with transductionmechanisms such as changes in refractive index, absorption,fluorescence, and SPR. A more general review covers recentdevelopments in FOBS (26), whereas Walt (27) describes fiber-optic biosensor arrays for creating high-density sensing arrays.Femtoliter wells can be loaded with individual beads to createsuch arrays for multiplexed screening and bioanalysis. Adherentcells may be attached to the fiber substrate to provide a methodfor observing cell migration and for screening antimigratory

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compounds, and even individual enzyme molecules can be loadedinto the wells, thus enabling single molecule detection via enzyme-catalyzed signal amplification. Optical microarray biosensingtechniques (28), in turn, provide a powerful tool for the simulta-neous analyses of thousands of parameters, be they DNA orproteins. The review highlights promising microarray techniqueseither making use of labels or label-free. Rather than miniaturizingthe optical fiber, these may be covered with nanostructuredcoatings (29). Active and passive coatings, deposited via theLangmuir-Blodgett and electrostatic self-assembly techniques,may be utilized to affect the transmission of optical fibers. Whilesuch sensor elements are mainly aimed for use in telecommunica-tions systems, it is very likely that chemical sensor and biosensordevelopment may benefit from such research.

Jeronimo et al. (30) review optical sensors and biosensorsbased on sol-gel films. Applications include sensors for pH, gases,ionic species and solvents, as well as biosensors. The use of FOCSfor on-site monitoring and analysis of industrial pollutants withrespect to detecting the identity, concentrations, and extent oftoxic chemical contamination was overviewed (31). Aspects ofprocess monitoring of fiber reinforced composites using opticalfiber sensors were summarized (32), with a focus on thermoset-ting resins and on spectroscopy-based techniques that can be usedto monitor the processing of these materials

A classroom demonstration was described for a portable fiber-optic probe multichannel spectrophotometer (33). With the useof this instrument, lecture demonstrations can be made of variousconcepts in molecular fluorescence spectroscopy. Concepts in-clude fluorescence spectrophotometer design geometry and thecorrelation of color with emission wavelength, excitation, andemission spectra. The history of research on FOCS and FOBShas been summarized (34).

SENSORS FOR GASES, VAPORS, AND HUMIDITY

This section covers all room-temperature gaseous speciesincluding their solutions in liquids. One major research focus ison hydrogen and methane because both are highly explosive whenmixed with air and may be sensed more safely with FOS thanwith electrical devices.

Hydrogen. Hydrogen, along with flammable alkanes, remainsto be the analyte where safety considerations have led to asubstantial amount of research in terms of fiber-optic sensing.Since hydrogen gas does not display intrinsic absorptions/emissions that could be used for purposes of simple opticalsensing, it is always detected indirectly. Hydrogen interactsstrongly with metallic palladium and platinum films and withtungsten oxide. The interactions result in both spectral changes(an effect sometimes called gasochromism) and in an expansionof the materials. Thus, a hydrogen sensor was reported based onpalladium coated side-polished single mode fiber (35). Whenexposed to 4% hydrogen gas, the optimal change output powerobtained in this experiment was 1.2 dB (32%) with a risetime of100 s. Improved response times were reported for a similar sensorbased on the same scheme (36). The authors have studied thetransmission, the time-response, and the initial response velocityin the range from -30 to 80 °C. Heating the palladium layer withan auxiliary laser diode improves the response time at lowtemperatures. Palladium also was incorporated into silica nano-

composites of the sol-gel type and used in a reversible hydrogenFOCS (37).

The gasochromic properties of nanostructured tungsten oxidefilms coated with a palladium catalyst were used to sensehydrogen gas via the change in the optical transmittance at 645nm, typically caused by 1% hydrogen in argon gas (38). Thenanomorphology of the surface considerably improves the gaso-chromic properties. Two rather similar articles have been pub-lished by this group (39, 40).

Luna-Moreno et al. have reported on the effect of hydrogenon a thick film Pd-Au alloy (41). The resulting sensor consistsof a multimode fiber in which a short section of single mode fiberis coated with the Pd-Au film. If exposed to hydrogen, therefractive index of the Pd-Au layer becomes smaller and causesattenuation on the transmitted light. The Pd-Au film can detect4% hydrogen with a response time of 15 s. The same materialwas used to design a hydrogen sensor based on core diametermismatch and annealed Pd-Au thin films (42).

Palladium-capped magnesium hydride was used as an alterna-tive sensing material in a fiber-optic hydrogen detector (43). Adrop in the reflectance of this material by a factor of 10 isdemonstrated at hydrogen levels as low as 15% of the lowerexplosion limit. The response occurs within a few seconds.Comparing Mg-Ni and Mg-Ti based alloys, the latter hassuperior optical and switching properties. A gasochromic TiO2-based sensing film was used for hydrogen detection by means ofa fiber-optic Fabry-Perot interferometric sensor (44). It wasapplied to monitor hydrogen gas in air below the lower explosionlimit, has a short response time, and regenerates quickly (at roomtemperature).

Fiber gratings coated with Pd metal were reported to enablesensing of hydrogen gas (45). Fiber Bragg gratings (FBG) andlong period gratings (LPG), both coated with Pd nanolayers, wereinvestigated. The sensitivity of the LPG sensor is better by a factorof ∼500. The FBG sensors appear to be pure strain sensors, andLPG sensors are mainly based on the coupling between thecladding modes and evanescent or surface plasmon waves. Inanother type of FBG sensor, a 10 ppm sensitivity for hydrogenwas reported (along with cross sensitivity to environmentalconditions) (46). The sensor was used to monitor the aging ofcertain materials. Optical fibers coated with single-walled carbonnanotubes (SWCNTs) were shown to enable determination ofhydrogen at cryogenic temperatures (47). SWCNTs were depos-ited by the Langmuir-Blodgett technique at the distal end of thefibers. Experiments carried out at 113 K revealed the potential ofsensing <5% of gaseous hydrogen with good reversibility and fastresponse time.

Hydrocarbons. Methane and other hydrocarbons are almostexclusively sensed via their intrinsic absorption in the near-infrared(NIR). These, in contrast to hydrogen or oxygen, can be detectedvia their intrinsic absorption in the (near) infrared-albeit crosssections (molar absorbances) remains small or via Raman scatter.One more fiber-optic Raman sensor was reported for sensingethanol and methanol in gasoline (48). It employs a frequencydoubled 532 nm Nd:YAG laser and a specially designed fiber-opticRaman probe and enables online determination of sample con-stituents without employing an expensive IR fiber. The sensor iscapable of monitoring methanol and ethanol in water and gasoline

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solutions. Surface-enhanced Raman spectroscopy and surface-enhanced IR spectroscopy were applied to selective determinationof polycyclic aromatic hydrocarbons (49). Molecular recognitionis accomplished by functionalization of the silver nanoparticleswith appropriate host molecules.

A modular, mid-IR, evanescent wave fiber-optic sensor for thedetection of hydrocarbon pollutants in water was constructed andtested (50). The setup uses a broadband light source withbackreflecting optics coupled to a fiber-optic sensing elementcoated with an analyte-enriching polymer that preconcentrates theanalyte. Benzene was quantified down to 500 ppm using a PVCpolymer coating. Three kinds of microstructure fibers (MSFs)for sensing gaseous hydrocarbons were reported (51) that enablethe quantitation of aromatic hydrocarbons. Their surfaces weremodified by xerogel layers. The MSFs with 10-50 µm air holeswere arranged to one sensor. Capillary silica fibers were alsofabricated. The response of the sensors to toluene in nitrogengas results from spectral changes of the output light from thefibers at 1600-1800 nm. A detection limit of 0.007 vol.% of toluenewas achieved.

Oxygen. Oxygen sensing remains another area where FOCSare quite successful. Oxygen almost exclusively is sensed by virtueof the quenching effect it exerts on certain fluorophores. High-performance fiber-optic oxygen sensors based on fluorinatedxerogels doped with quenchable Pt(II) complexes were reportedby Chu et al. (52). The sensors yield linear Stern-Volmer plots,and response times range from 4 to 7 s. Rather similar materialsresulted in even faster responses as reported in a second paperby this group (53). Most oxygen sensors display nonlinearStern-Volmer relationship, and this can be described by the so-called two-site model assuming two quenching constants. It hasbeen shown that an artificial network also may be applied to modelthe dynamic quenching of the fluorescence of a ruthenium-derivedprobe (54).

A porous plastic sensor was developed for the determinationof dissolved oxygen in seawater (55). The luminescent indicator,a ruthenium(II)diimine complex, was copolymerized with apolymerizable monomer, a cross-linking reagent, and a porogen.It did not leach out due to its high hydrophobicity. Sensing isbased on dynamic quenching of the fluorescence of the rutheniumindicator. The permeability of contact lenses for oxygen can bedetermined with a fiber-optic luminescent sensor system (56). Themethod is based on kinetic measurements of the oxygen partialpressure inside a chamber sealed by the sample contact lens,where a thin luminescent O2-sensitive film is being placed. Unlikein electrochemical techniques, the optical sensor is unaffected bythe thickness of the contact lens and other effects.

Oxygen sensors were used to transduce the activity of catalase(57). This is of interest to characterize the oxidative metabolismin coffee cherries during maturation as it appears to be regulatedby the timely expression of redox enzymes such as catalase (CAT),peroxidase, and polyphenoloxidase. The assay allows for thescreening of green coffee samples for CAT activities. An evanes-cent wave fiber sensor was used to determine oxygen deficiency(58). The sensing dye, methylene blue, was immobilized in thecladding using a sol-gel process. The sensor is said to respondlogarithmically linear between 0.6% and 20.9% oxygen. The mostsensitive sensor for oxygen known so far exploits the efficient

quenching of the long-lived delayed fluorescence of fullerene C70

incorporated in thin films of ethyl cellulose or organosilica (59).Molecular oxygen in concentrations as low as 200-800 ng/L canbe determined by this method either at single sensor spots orspatially resolved over a temperature range of more than 100 °C.The group of Klimant (60) has presented magnetically separableoptical sensor beads for oxygen. The beads contain Fe3O4 andcan be magnetically fixed at the bottom of a microbioreactor andenable contactless monitoring of oxygen in cultures of Escherichiacoli using fiber optics. The same group has presented new andultrabright fluorescent probes for oxygen that are based oncyclometalated iridium(III) coumarin complexes (61). The probesare less cross-sensitive to temperature, have lifetimes on the orderof 8-13 µs in the unquenched state, have quantum yields between0.3 and 1.0, but are less photostable than ruthenium-based probesfor oxygen.

Fiber optic microsensors with a tip diameter of ∼140 µm werereported for simultaneous sensing of oxygen and pH and ofoxygen and temperature (62). The tip of the fiber was coveredwith sensor chemistries based on luminescent microbeads thatrespond to the respective parameters by a change in the decaytime, or the intensity of their luminescence, or both. The use ofmicrobeads enables the ratio of the signals to be easily varied,reduces the risk of fluorescence energy transfer between indicatordyes, and reduces the adverse effect of singlet oxygen that isproduced in the oxygen-sensitive beads. Arain et al. (63) havecharacterized microtiterplates (MTPs) with integrated opticalsensors for oxygen and pH and have applied them to enzymeactivity screening, respirometry, and toxicological assays. Thinhydrophilic sensing films consisting of an analyte-sensitive indica-tor and a reference fluorophore were deposited on the bottom ofthe MTPs. Activity screening was demonstrated for glucoseoxidase and for monitoring the growth of E. coli and Pseudomonasputida. A toxicological assay also is reported that enables monitor-ing the respiratory activity of P. putida. In order to compensatefor the rather strong effects of temperature on oxygen sensorsbased on dynamic quenching of luminescence, Borisov et al. (64)have developed a composite luminescent material for dual sensingof oxygen and temperature, where temperature can be measuredoptically and used to calculate temperature-corrected data for pO2.Quantum dots undergo temperature-dependent changes of theintensity, the peak wavelength, and the spectral width of theirluminescence. Hence, they can be used as probes to compensatefor effects of temperature in FOCS (65). Effects are almost linearand fully reversible. A resolution of 0.3 °C was achieved.

Oxygen and carbon dioxide are clinically highly significantblood gases. A composite fluorescent material was described thatenables simultaneous sensing and imaging of oxygen and carbondioxide (66). It relies on the measurement of the phase shift ofthe luminescence decay time of a material composed of microbead-contained indicators (with well-separated excitation and emissionwavelengths) and polymers with excellent permeation selectivitiesas well as favorable optical and adhesive properties. A luminescentdual sensor for time-resolved imaging of pCO2 and pO2 in aquaticsystems was reported by Schroeder et al. (67).

Other Gases. Ozone gas was sensed via its UV and visibleabsorption (68). Both the UV absorption at 254 nm and the visibleabsorption at 600 nm were studied. Sensing in the UV region


allows for highly sensitive detectors due to its high absorptivityin that region. The visible region has a significantly lowerabsorption coefficient but enables monitoring high ozone levels.The UV based sensor can detect 0-1 mg/L of ozone and thelongwave sensor 25-126 mg/L.

Nitrogen is a species not easily sensed by optical means exceptby Raman spectroscopy. A respective FOCS to monitor theconcentration ratio of nitrogen and oxygen in a cryogenic mixturewas reported by Tiwari et al. (69). Spontaneous Raman scatteringwas used to monitor of the purity of liquid oxygen in the oxidizerfeed line during ground testing of rocket engines. The Ramanpeak intensity ratios for mixtures of liquid nitrogen and liquidoxygen were analyzed for their applicability to impurity sensingusing different excitation light sources, and a miniaturized sensingsystem was developed that responds within a few seconds. Apractical sensor for online sensing of atomic nitrogen in directcurrent glow discharges was reported by Popovic et al. (70).Sensing is accomplished by monitoring the intensities of theatomic nitrogen spectral line at 822 nm and the bandhead at 337nm, relative to the oxygen line at 845 nm. Measurements wereperformed using two methods. The first approach uses a fiber-optic spectrometer, calibrated with a standard source, to recordthe desired spectral lines. The second approach uses narrowbandwidth optical filters to detect the emission intensity. Opticaldata are collected for a range of experimental conditions in aflowing glow discharge of N2-O2 mixtures.

Sensors for carbon dioxide and, to a lesser extent, ammoniaremain another active area of research. Carbon dioxide emissionsfrom a diesel engine can be monitored with the help of a mid-infrared optical fiber sensor (71). As conventional automotivepollution sensors fail to meet monitoring requirements as specifiedby the European Community, for various reasons, a low-costsensor employing compact mid-IR components is presented thatwas used to measure CO2 in the exhaust of a commercial dieselengine. A novel kind of optical sensor for carbon dioxide makesuse of silicone encapsulated ionic liquids (72). The silicone matrixacts as a permeation-selective material for CO2, while the ionicliquid contains a pH probe (in its base form) that undergoes adistinct color change after its (reversible) reaction with CO2.Dissolved CO2 usually is sensed via the effect it exerts on the pHof a buffer immobilized in a matrix. A sulfonated hydroxypyrenenamed HPTS has been widely used as a pH probe in such CO2

sensors in the past two decades, and one more type of organicallymodified sol-gel was used to develop one more modification ofthis kind of sensor (73). Because the gel is partially fluorinated,the response is faster. Nanoporous matrixes also may be used tohost pH-sensitive phenolic dyes and an alkaline phase transferagent, reagents typically employed in sensors for CO2 (74).

Ammonia in the gas phase may be sensed either via its NIRabsorption or (in being a weak base) via the weak pH changes itcan induce in immobilized pH indicators. The latter approach ismore sensitive. It also is the prefered one in that it is applicableto aqueous solutions (unless extractive phases are employed inIR sensing schemes). A new sensing scheme for ammonia is basedon monitoring the optical changes of the Q-band of tetrakis(4-sulfophenyl)-porphine (TSPP) at 700 nm, as induced by ammoniain the electrostatic interaction between TSPP and poly(diallyldim-ethylammonium) chloride (75). Three thin film samples with

different thicknesses were prepared layer-by-layer, all showing alinear sensitivity to NH3 in the 0-100 ppm range and a responsetime of around 30 s. The sensor was regenerated by rinsing withwater.

Most ammonia sensors reported so far make use of its effecton appropriate pH probes. This is also true for one more sensorthat makes use of a sol-gel matrix (76). Both NH3 vapor andtrace NH3 dissolved in water can be detected. The indicator wasimmobilized in porous SiO2 which then was deposited on thesurface of a bent optical fiber core. In combination with a siliconeprotection coating, ammonia can be sensed in watery samples.The limit of detection is 13 ppb in gas samples and 5 ppb in watersamples. This is comparable to previously reported probesexploiting the same effect. The scheme was extended to sensingammonia via fiber-optic microsensors at 2-100 µg/L levels thatare known to be highly toxic to fish and other organisms (77). Afluorescent pH indicator placed in a cellulose ester matrix at thetip of an optical microfiber is deprotonated by ammonia, therebyundergoing a large change in fluorescence intensity. The resultingammonium ion is stabilized by a cation trap. The detection limitis reported to be 0.5 µg/L. The microsensors were used toestablish ammonia microprofiles of high spatial resolution. Inter-ference by trimethylamine was minimized using an 18-crown-6ether as a cation trap and by the permeability properties of thepolymer matrix. Cross-sensitivities toward protons and alkali ionswere prevented by coating the sensor with a layer of Teflon.

Ammonia also was sensed with the help of silica nanocom-posites doped with silver nanoparticles and deposited on an opticalfiber waveguide (78). The SiO2 nanocomposite was prepared bythe sol-gel technique in the presence of (3-mercaptopropyl)tri-methoxysilane) and doped with 25 µm silver nanoparticles. Thematerial was deposited onto the surface of a bent silica fiber bythe dip-coating technique. Exposure to gas containing ammoniaenhances the attenuation of light power guided through the fiberprobe. Sub-ppm levels of NH3 can be continuously monitored bythis technique.

Oberg et al. (79) have designed a simple optical sensor forvapors of organic amines based on silica microspheres dyed withpH indicators such as bromocresol green (a probe reported tobe useful for sensing ammonia by others several times before).It can detect organic amine vapors down to 1-2 ppb levels. Theresponse to aliphatic amines is linear up to 2 ppm. The micro-sphere sensor is said to be more sensitive than other optical aminesensor described in the literature but heavily interfered byammonia. The group of Mohr reports (80) that functional liquidcrystal films can selectively recognize amine vapors and therebyundergo a change in their color. Molecular recognition is ac-complished by cholesteric liquid crystals combined with moleculescarrying a trifluoroacetyl group. Note: sensors for amines in fluidphases are treated in section D on sensors for organic molecules.

One more fiber-optic probe was reported for the selectivedetermination of NO2 in air samples (81). Signal generation isbased on the spectroscopic changes of a reagent contained in thenanopores of a sol-gel element that undergoes an irreversiblechromogenic reaction. Two fiber-optic designs are described.Another FOCS for NO2 makes use of poly(3-octylthiophene) as asensing material (82). This material undergoes large spectralchanges at 543 nm if exposed to NO2. The sensor is not very stable


in that the response decreases after each exposure to NO2 but israpid, highly selective, and sensitive. Vehicle exhaust emissionssuch as NO2, NO, SO2 were detected using an ultraviolet opticalfiber based sensor (83). It is used to simultaneously measure theirconcentrations, with lower limits of detection of 2 ppm for NO2, 2ppm for SO2, and 20 ppm for NO. Response times are <4 s. Opticalsensitivity to NO2, carbon monoxide, and hydrogen (all in dryair) was observed for a layer of a metal oxide multilayer of thetype InxOyNz covered with gold nanoclusters (84). The incorpora-tion of the gold nanoclusters leads to a broad absorption peak inthe visible (purple) due to the excitation of localized surfaceplasmons. The maximum of the peak is shifted on exposure tooxidizing or reducing gases.

An irreversible active core fiber-optic probe was developed fordetermination of trace H2S at high temperature using a cadmiumoxide doped porous silica fiber as a transducer (85). If exposedto a gas sample at high temperatures, trace H2S in the samplediffuses into the porous fiber and reacts with cadmium oxide toform cadmium sulfide (CdS), and this is detected at around 370nm with a fiber-optic spectrometer. The CdS formed also emitsstrong fluorescence, with a peak emission at around 500 nm, butthis signal is quenched at 450 °C, so that fluorescence can onlybe used for probing trace H2S at low temperature. Fuel gases suchas H2, CH4, and CO were found not to interfere. The toxicindustrial chemicals hydrogen cyanide, hydrogen sulfide, andchlorine can be sensed with waveguides coated with dopedpolymer materials in the form of arrays (86). The polymers arepatterned on glass substrates and undergo color changes that canbe interrogated at different wavelengths. Miniature waveguidechannels result in enhanced sensitivity owing to the increasedpath length. The sensors have response times (t90) of less than20 s. Certain cross interferences observed can be eliminated byapplying a signal processing algorithm that also reduces falsealarms. Elemental iodine, unlike chlorine, exerts a quenchingeffect on a derivative of aminobenzanthrone, and this effect wasused in a respective fiber-optic sensor (87). It can detect iodinewith a rather high concentration detection limit of 6 µM.

An optical biosniffer for methyl mercaptan, one of the smellingprinciples in halitosis, was reported (88). Monoamine oxidase(MAO) was immobilized at the tip of a fiber-optic oxygen sensorwith a luminescent oxygen probe (excitation at 470 nm, emissionpeaking at 600 nm). The sensing scheme is based on the detectionof the oxygen consumed as a result of enzymatic oxidation ofmethyl mercaptan (MM) by MAO. The output of the sniffer wasamplified by substrate regeneration via reduction with ascorbicacid. MM levels between 9 and 11 000 ppb were detectable. Thepathological threshold is 200 ppb, the limits of human perceptionare on the order of 10 ppb.

A rather complex method for optical sensing of sulfur dioxidein wines (89) employs a dinuclear palladium ligand compleximmobilized in a PVC membrane plasticized with o-nitrophenyl-octylether (o-NPOE). Both free and total SO2 can be determined.Linear responses up to 50 and 150 mg/L were obtained for freeand total SO2, with detection limits of 0.37 and 0.70 mg L-1,respectively.

Vapors. Vapors, often referred to as volatile organic com-pounds (VOCs), often are sensed by optical means in order toreduce the risk of explosion. An FOCS for VOCs was reported

(90) that uses birefringent porous glass oriented between twocrossed polarizers and serving as the basis for this broad-spectrumsensor. VOCs such as acetonitrile vapors can be detected atconcentrations of >50 ppm. The optical effects resulting fromexposure to various VOCs are reversible and may result fromadsorption of VOCs with attendant reduction of anisotropy. Thesensor can make use of ambient light as a light source and theeye as a detector to register the resulting color changes and thusis capable of real time monitoring of VOCs. Also see ref 91.

A fiber-optic nanosensor for volatile alcoholic compounds hasbeen described by Elosua et al. (92). It is based on a newvapochromic red powder consisting of a silver metal organiccompound. Its color and refractive index change when exposedto vapors of VOCs. The process is fully reversible. The sensorconsists of a nanometer-scale Fizeau interferometer doped withthe vapochromic material and placed at the cleaved end of amultimode fiber-optic pigtail. The response of the material towarddifferent alcohols was measured at 850 nm, and changes up to 3dB in the reflected optical power were registered. The authorshave used the same material in a Fabry-Perot interferometer witha nanosized cavity along with a optical fiber pigtail system (93).

The performance of a carbon nanotube (CNT) based thin filmsfiber-optic for VOCs was described (94). Single-walled CNTmultilayer coatings were used that cause a response towardtoluene and xylene vapors. The Langmuir-Blodgett technique wasapplied to deposit the CNTs directly onto the optical and acousticsensors substrates. The results demonstrate ppm to ppb sensitiv-ity, fast response, and high repeatability. In a related paper, theincorporation of CNTs into hollow core fiber optics is reported(95).

King et al. (96) report on a microsensor for VOCs where aphotonic crystal of porous silicone is attached to the distal end ofan optical fiber. Activated carbon is used as a prefilter, and anybreakthrough of the VOC through the carbon results a largechange in the reflectivity of the porous silica.

Humidity. Optical humidity sensors are a kind of evergreengiven their highly different applications. Numerous materialsincluding Nafion films doped with crystal violet responding tohumidity by a change in their reflectance (97). Relative humidity(RH) between 0 and 0.25% can be determined with the respectivesensor, with a detection limit as low as 0.018% RH (equal to ∼4ppm). The response is fully reversible (with some hysteresis) indry nitrogen. Reversal times depend on exposure time and % RH.The sensor can detect moisture in process gases such as nitrogenand HCl. In another kind of RH sensor, Ru(II) complexes wereemployed since their luminescence decay time depends on RH.The analytical information is obtained via phase-sensitive deter-mination of the decay time (98). The ruthenium probe wasimmobilized on a Teflon support. The operational range is from4 to 100% RH at 20 °C. Its response time is shorter than 2 min(recovery time <1.2 min). Signal stability was verified for >2.5years of discontinuous measurement. The sensor was applied tomeasure RH in food and in a weather station.

Certain porphyrins deposited in Nafion films undergo water-induced tautomerism. This forms the basis of a novel type of fiber-optic RH sensor (99). It exhibits long-term stability and a linearresponse over the humidity range from 0 to 4000 ppm. Anotherfiber-optic RH sensor reported (100) is based on large-core


quartz/polymer optical fiber pairs. Examples of vapors thatinterfere (or may be sensed with this device) include those ofacetone and ethanol. In other words: it is not specific for RH.

The sensitivity of a tapered optical fiber relative humidity (RH)sensor was optimized by means of tuning the thickness ofnanostructured sensitive coatings (101). A single mode taperedfiber was coated with a specific nanostructured polymer whosethickness was controlled so to optimize sensitivity to RH. Thesensor displays a 27 times better sensitivity to RH than acomparable overlay, this enabling monitoring human breathing.Huang et al. report (102) that a thermoplastic polyimide whendeposited on a fiber-optic Bragg grating can act as a materialsensitive to RH in that it undergoes reversible expansion andshrinkage.

SENSORS FOR pH AND IONSThis section covers sensors for all kinds of inorganic ions

including the proton (i.e., pH), cations, and anions. Optical sensingof pH remains to be of greatest interest even though all opticalsensors suffer from cross sensitivity to ionic strength. The numberof articles on pH sensors is decreasing, though, which does notcome as a surprise in view of the state of the art and the fact thatcertain pH FOS are commercially available. On the other side,sensing of pH values below 1 and above 12 still represents asubstantial challenge in terms of material sciences. Optical pHsensors respond over a limited range of pH only (in most cases).It also needs to be stated that many pH sensors presented in thepast few years are modifications only of existing sensing schemesand materials, and that authors often do not properly cite previouswork on the subject. In the worst case, sensors are presentedthat are inferior to others described before. Such sensors are nottreated here.

The group of Scheper has introduced a scheme for referencedsensing of pH that is based on spectral analysis of fluorescence(103). pH is calculated by chemometric means from selected datapoints of the fluorescence spectra of aminofluorescein at highconcentration so that a strong inner filter is operative. Martin etal. describe a new organic pH indicator dye (mercurochrome)that was immobilized in a sol-gel matrix placed at the end of afiber optic and enables measurement of pH in the pH 4-8 range(104). The sensor is constructed from low cost fiber optic andoptoelectronic components including a blue light emitting diodeand a photodiode. The sensing scheme relies on the measurementof the fluorescence intensity of the pH indicator that is related tothe intensity of the blue excitation light reflected by the sensingphase. The ratio between the two signals is proportional to pHbut independent of excitation light intensity. The applicability ofthis sensor was tested for its performance in pH analysis in tapand bottled mineral water (105). The same group also hasdeveloped sol-gel matrixes by controlled hydrolysis of a titaniumtetraalkoxide (106). The resulting sol-gel (TiO2) is said to bemore resistant and to have a longer lifetime than SiO2 films. Thematrix was doped with various pH indicator dyes, each beingsensitive to different pH ranges.

Congo red and neutral red on a cellulose acetate support havebeen reinvented as a material for sensing the 4.2-6.3 and 4.1-9.0pH range, respectively (107). Two aromatic Schiff bases werestudied for their suitability in terms of sensing alkaline pH values(108). Absorption and emission spectra, quantum yields, fluores-

cence lifetimes, photostabilities, and acidity constants weredetermined in organic solvents and in PVC. The Schiff bases canbe photoexcited at 556 and 570 nm, respectively, and respond topH in the range from 8.0-12.0 and 7.0-12.0, respectively.

A novel wide pH range sensing system was described that isbased on a sol-gel encapsulated amino-functionalized corrole(109). An amino modified fluorescent aminophenylcorrole im-mobilized in a sol-gel SiO2 matrix undergoes large changes influorescence intensity owing to multiple steps of protonation anddeprotonation, thereby allowing larger pH ranges (1-11) to becovered than via respective tetraphenylporphyrins. Dong et al.(110) have obtained wide-range pH optical sensor by immobilizingthree indicators in a sol-gel matrix that was deposited in a fiberoptic waveguide and optically interrogated by evanescent waveabsorptiometry. The sensors have a dynamic range from pH 4.5to 13.0.

A scheme for measurement of very high pH values as theyoccur in chemical processing was presented by Gotou et al. (111).It is used to establish a health monitoring method for chemicallyexposed fiber reinforced plastic structures in that it can detectthe penetration of corrosive solutions into plastics. A high-pHindicator is used along with an optical fiber connected to aspectrophotometer.

Seki et al. describe a pH sensor based on a heterocorestructured fiber optic (112). It consists of multimode fibers anda short piece of single mode fiber which was inserted into themultimode fibers. The pH indicator phenol red was immobilizedin a sol-gel matrix at the surface of the heterocore portion. pHchanges were detected by measuring the loss spectra at 575 and545 nm, respectively. It is noted at this point by the author of thisreview that sol-gels and other condensates of that kind are knownnot to provide a temporally stable matrix but rather to changetheir microstructure over periods of typically a few months. Thisis rarely addressed by authors of respective work.

Indicator loaded microbeads that are permeation selective foreither protons or dissolved oxygen were used in a respectivedually sensing membrane and a single fiber-optic sensor (113).The pH probe (a carboxyfluorescein) and the oxygen probe (aruthenium complex) were incorporated into two kinds of micro-particles, viz. an amino-modified poly(hydroxyethyl methacrylate)and an organically modified sol-gel, respectively. Both kinds ofbeads were then dispersed into a hydrogel matrix and placed atthe distal end of an optical fiber waveguide for optical interroga-tion. A phase-modulated blue-green LED served as the light sourcefor exciting luminescence whose average decay times or phaseshifts served as the analytical information. Data are evaluated bya modified dual luminophore referencing method which relatesthe phase shift (as measured at two different frequencies) to pHand to O partial pressure.

In related work but using different materials (for optoelectronicreasons), microsensors were described for simultaneous sensingof pH and oxygen (114). This is important in clinical chemistryand if minute sample volumes are only available. The microsensorswith a tip diameter of ∼140 µm make use of luminescentmicrobeads that respond to the respective parameters by a changein the decay time, intensity, or both. A complex algorithm enablesprecise calculation of pH even if the spectra of the indicator dyes(for pH and oxygen) overlap.


An evanescent wave direct spectroscopic sensor for chromateanion reported by Tao & Sarma (115) uses a flexible fused silicalight guiding capillary as a transducer. The capillary has a claddinglayer and a protective polymer coating on its outside surface. Thecladding layer ensures the ability of the capillary to guide light,while the protective coating increases its mechanical strength. Likesimilar sensors described before, the sensor is based on theintrinsic evanescent wave absorption by chromate ions in a watersample inside the capillary. A 30 m long capillary has the capabilityof detecting as little as 31 ppm of chromate.

A fluorescence-based calcium nanosensor was described (116)that exploits silica nanoparticles (prepared by inverse microemul-sion polymerization) doped with the calcium(II) probe calcein asboth a recognition and transduction element for optical determi-nation of calcium in blood serum. Traces of vanadium(V) ion canbeen determined by using an irreversible chromogenic reactionbetween vanadium ion and a hydroxamic acid immobilized in apoly(vinyl chloride) (PVC) membrane (117). A quenchablefluorescent benzofurane derivative in the plasticized PVC matrixserved as the indicator dye in an FOCS for ferric ion (118).Response time, reversibility, limits of detection (poor), dynamicrange, and interferences were studied. The same group hassynthesized a fluorescent semicarbazone and demonstrated itsapplicability as a selective probe in an FOCS for copper(II) (119).PVC and ethyl cellulose acted as sensor matrixes, and bothabsorption and emission spectrometry can be applied. Aluminumion in aqueous media can be probed with a regenerable sensorthat uses a molecularly imprinted polymer (MIP) as the recogni-tion receptor (120). The MIP was prepared with Al(III) ion actingas the template, and 8-hydroxyquinoline sulfonate acted as afluorogenic ligand. The MIP was synthesized from acrylamide,2-hydroxyethyl methacrylate, and ethylene glycol dimethylacrylateas a cross-linker. At pH 5, Cu(II) and Zn(II) interfere to someextent. The dynamic range at pH 5 is linear up to 0.1 mM, thelimit of detection is 4 µM. Mercury(II) ions were optically probedby Li et al. (121). They report on a luminescent nanosensor forHg(II) where the fluorescence of carnitine capped quantum dotsmade from CdSe/ZnS is quenched by Hg(II) ions with anefficiency that resulted in a detection limit of 0.2 µM. In relatedwork it is reported (122) that Pb(II) ions can be (irreversibly)probed by a similar method but using CdTe quantum dots cappedwith thiols. The detection limit is virtually the same.

SENSORS FOR ORGANIC CHEMICALSThis chapter covers sensors for organic species (mainly

saccharides), pollutants, agrochemicals, nerve agents, drugs andpharmaceuticals, and miscellaneous other organics. Mid-IR laserspectroscopy was performed by either using cryogenically cooledlead salt lasers or quantum cascade lasers operating at roomtemperature and applied to reagentless (enzymeless) sensing ofglucose (123). Aqueous solutions of glucose were analyzed byfiber based attenuated total reflection spectroscopy and bytransmission spectroscopy. Both methods have the potential tobe utilized in small fiber sensors that may be inserted intosubcutaneous tissue. Concentrations as low as 0.1 mg/mL weredetectable.

Mohr et al. (124) describe fluoro-reactands for the enzymelessdetermination of saccharides. They are based on hemicyaninedyes containing a boronic acid receptor and are capable of forming

a covalent bond between their boronic acid moiety and the diolmoiety of saccharides. This causes their fluorescence to increase.The probe can be photoexcited at around 460 nm, emits with apeak at 600 nm, and can be used at near neutral pHs. For a review,on probes for saccharides and glycosylated biomolecules see ref125.

Tri-n-propylamine and the drugs benzhexol and procyclidinewere determined via electrochemiluminescence (ECL) in a sensorconstructed by the screen-print technique (126). Ruthenium(II)-tris(bipyridine) on Nafion was immobilized on a carbon electrode,and its ECL resulting from the interaction with the analytes wasmeasured. Limits of detection are as small as 30 nM. Polymermembranes containing a chromogenic functional azo dye undergocolor changes on (reversible) interactions with amines in organicsolvents (127). The dye (covalently linked to the polymer matrix)recognizes the analyte via covalent binding. Binding constants ofthe various amines depend on the kind of solvent and are highlydifferent, this resulting in strongly varying limits of detection.Dissolved amines also can be sensed by incorporating an amine-carrying chromoionophore into a sol-gel matrix (128). Both acid-and base-catalyzed sol-gel processes were studied, but the formerwere found not to respond at all. Stable ormosil layers wereobtained using various fractions of organically modified sol-gelprecursors, e.g., methyl triethoxysilane. Base-catalyzed sensorlayers underwent large signal changes, with response times ofaround 1-2 min, and rather high detection limits (0.1 mM).

Organics. A fluorescent dosimeter for formaldehyde deter-mination in water utilizes the Nash reagent incorporated into silicagel beads (129). On reaction with formaldehyde, a fluorescentproduct is formed that can be detected instrumentally or visually.The dosimeter does not respond to primary alcohols, ketones,and other common substances. The detection limit is reported tobe 30 µg/L. Dissolved organics in water samples can be sensedwith a nanoporous zeolite thin film-based fiber sensor (130). AFabry-Perot interferometric system was developed containing athin layer of nanoporous zeolite synthesized on the cleaved endface of a single mode fiber. The sensor is operated by monitoringchanges in the thickness of the thin film caused by the adsorptionof organic molecules by white light interferometry. Methanol,2-propanol, and toluene were detected with high sensitivity. AFOCS for toluene in water was described by Consales et al. (131).It makes use of single-walled carbon nanotubes whose reflectivitychanges in the presence of toluene. System features include goodstability of the steady-state signal, sensitivity, and rapid response.

BIOSENSORSThis section covers biosensors based on the use of enzymes,

antibodies, nucleic acids, and whole cells. Biosensors make useof biological components in order to sense a species of interest(which by itself need not be a “biospecies”). On the other side,chemical sensors not using a biological component but placed ina biological matrix are not biosensors by definition. It should benoted that some of the biosensors can be found in other chaptersif this was deemed to be more appropriate.

Enzymatic Biosensors. Sensing glucose remains to be anevergreen. Unlike in the case of electrochemical schemes wheredirect electron shuttle from the substrate to the electrode hasbecome possible as a result of enzyme wiring, no such approachis possible in optical sensors. Hence, these still rely on (a)


transduction via metabolic coreactants (such as oxygen or NAD+);(b) of coproducts (such as protons, hydrogen peroxide, or NADH);or (c) on affinity binding (such as to concanavalin A). Compre-hensive reviews have appeared (132). The group of Klimant hasdesigned a fiber-optic flow through sensor for online monitoringof glucose in patients in intensive care units (133). A tubing asused in microdialysis contains an integrated fiber-optic sensor.Glucose is sensed via oxygen consumption which occurs as aresult of the oxidation of glucose catalyzed by immobilized GOx.The gas permeable tubing warrants constant air saturation in theflow cell. A reference oxygen sensor is used to detect changes inoxygen supply caused by adverse effects such as bacterial growth,temperature fluctuations, or failure of the peristaltic pump. Thesensor was evaluated in a 24 h test on a healthy volunteer. Endoet al. describe a needle-type enzyme sensor system for determin-ing glucose levels in fish blood (134). A hollow needle was usedthat also was comprised of an immobilized enzyme membraneand a optic fiber probe with a quenchable ruthenium-based oxygenindicator. The calibration curve was linear for glucose in fishplasma. One assay was completed within 3 min. A good reproduc-ibility was observed 60 times without exchange of the enzymemembrane. The group of Nann (135) found that the luminescenceof silica coated quantum dots is dynamically quenched byhydrogen peroxide (H2O2), and this was exploited in a opticalglucose assay using the enzyme GOx. This is one of the fewreversible optical methods that are based on the transduction ofoxidase based reactions via H2O2. The quantum dots have a ratherhigh specific surface area which enables a relatively large amountof GOx to be immobilized. On the basis of the previous work onsensing H2O2 via the amount of oxygen formed by catalyticdecomposition of H2O2, Mills et al. (136) have designed a robustand reversible sensor where ruthenium(IV)dioxide is used as acatalyst. The amount of oxygen liberated is determined via thequenching effect it exerts on the luminescence of a ruthenium(II)ligand complex.

Luminescent yeast cells were entrapped in hydrogels in orderto optically detect estrogenic endocrine disrupting chemicals(137). Genetically modified Saccharomyces cerevisiae cells contain-ing the estrogen receptor expression of the luc reporter gene wereimmobilized in hydrogel matrixes. The chemicals induce achemiluminescence to be emitted by the cells. Concentrationsdown to 80 ng/L of the chemicals were detectable. The probewas stored for 1 month at -80 °C but full activity was attainedagain at room temperature. The data obtained with this sensorroughly correlated with LC-MS/MS analytical results.

Fiber optic absorbance spectroscopy was compared withsurface plasmon optical detection methods for lactamases boundto interfacial structures via biotin-avidin coupling (138). Quantita-tive comparisons were made between the five matrixes andbetween the binding strategies. All matrixes were suited, inprinciple, for the binding of the protein. Results obtained by SPRand optical waveguide measurements correlate excellently. Realtime activity assays of �-lactamase were performed by a detectionscheme that combines an affinity test and a catalytic sensor.

Organophosphate pesticides and chemical warfare agents canbe sensed with a FOBS that exploits the enzyme organophosphatehydrolase (OPH) as the biorecognition element (139). Conjugatedto biotin, it was attached to a polystyrene waveguide along with

the fluorescent pH indicator carboxynaphthofluorescein. Hydroly-sis of organophosphates by OPH causes the pH to fall, and thisis reported by the pH probe. The dynamic range for determinationof paraoxon is from 1 to 800 µmol/L.

Single molecules of galactosidase were monitored (140) usinga 1 mm diameter fiber-optic bundle with individually sealedfemtoliter microwell reactors. Unlike in so-called “ensemble”responses in which many analyte molecules give rise to themeasured signal, the buildup of fluorescent products from singleenzyme molecules catalysis over the array of reaction vessels canbe observed, and a digital concentration readout can be obtainedby application of statistical analysis. This approach should proveuseful for single molecule enzymology and ultrasensitive bioas-says. A similar approach (141) was applied to 24 000 individualreaction chambers to sense DNA and antibodies and again isexpected to be applicable to assays that utilize an enzyme labelto catalyze the generation of a fluorescent signal.

A fiber-optic enzymatic biosensor was described for determi-nation of 1,2-dichloroethane (DCA) (142). Haloalkane dehaloge-nase in whole cells of Xanthobacter autotrophicus immobilized incalcium alginate at the tip of a fiber-optic coverts the haloalkanesinto acidic products (i.e., lowers the pH), and this is detected viathe pH probe fluorescein immobilized at the tip of the FOBS. DCAwas quantified at levels as low as 11 mg/L, with a linear responseat ∼65 mg/L. Like most cell-based catalytic biosensors, theresponse time is slow (8-10 min). Rajan et al. report on thefabrication and characterization of a surface plasmon resonancebased enzymatic FOBS for detection of the bittering componentnaringin (143). The sensing area was coated over ∼1 cm lengthwith silver and then with the enzyme naringinase. The SPRwavelength maximum increases with the concentration of naringin.

Immunosensors. Antinuclear antibodies can be quantifiedwith an optical fiber biosensor modified with colloidal gold (144).The unclad portion of an optical fiber was covered with self-assembled gold colloids whose surface was functionalized withantigens. Antinuclear antibodies in serum can be determinedquantitatively, and the results agree well with the accepted ELISAmethod. This sensing platform is label-free, enables real-timedetection, and does not require a secondary antibody. Its sensitiv-ity is higher by at least 1 order of magnitude than that of theELISA method.

Antibodies against the F1 antigen of Yersinia pestis (the causefor plague and a potential terroristic agent) can be detected by asandwich immunoassay on the surface of an optical fiber (145).Autoantibodies to ovarian and breast cancer-associated antigensare detectable with high sensitivity by using a chemiluminescencebased optical fiber immunosensor (146). The protein GIPC-1 wasconjugated to the tip of an optical fiber. A human monoclonal IgMisolated from breast cancer patients targets the GIPC-1 proteinand thus can be detected in concentrations down to 30 pg/mL,which is 50 times lower than the chemiluminescent ELISA and∼500 times lower than the colorimetric ELISA. Sera from 11ovarian cancer patients, 22 breast cancer patients, and asymp-tomatic controls were tested for the presence of IgM anti-GIPC-1autoantibodies by the two methods.

A waveguide based immunosensor for the food toxins aflatoxinB1 and ochratoxin A uses either the competitive or the directimmunoassay format (147). The antibody conjugate was im-


mobilized on a sensor chip and exposed to the analytes in a flowinjection analyzer. The detection range of the competitive detectionmethod was between 0.5 and 10 ng/mL. The indirect method wasused to sense toxins in barley and wheat flour samples. Resultsare in good agreement with those obtained by ELISA.

DNA Biosensors. A fiber-optic DNA microarray for simulta-neous determination of multiple harmful algal bloom species wasreported by Ahn et al. (148). Algal blooms are a serious threat tocoastal resources, causing a variety of impacts on public health,regional economies, and ecosystems. The sandwich hybridizationassay combines fiber-optic array technology with appropriateoligonucleotide probes immobilized on microspheres. Hybridiza-tion was visualized using fluorescently labeled secondary probes.A detection limit as low as 5 cells is achieved, the assay time being45 min without a separate amplification step. An elegant diagnosticdevice for the detection of the hepatitis B virus was presented(149). An isothermal amplification method is employed so todetect the viral DNA in a 25 µL reaction volume following severalautomated handling steps.

Bacterial transcriptional regulators are known to be dose-dependently released from their operators upon binding of specificclasses of antibiotics. In another approach, Link et al. (150) usea generic dipstick-based technology for rapid determination oftetracycline, streptogramin, and macrolide antibiotics in foodsamples. The dipstick assay consists of a membrane support stripcoated with streptavidin and immobilized biotinylated operatorDNA, which acts as capture DNA to bind hexa-histidine (His6)-tagged bacterial biosensors. Antibiotics present in specific samplestriggeredthedose-dependentreleaseof thecaptureDNA-biosensorinteraction. Dipping the stick into two reagent solutions resultsin a correlated conversion of a chromogenic substrate by a His6-targeted enzyme complex. It has detection limits as low as 1/40of the licensed threshold. In a comparable approach, antibioticsin seafood were screened for a fiber-optic fluorometric assay basedon competitive binding of the analyte and an intercalating dye todouble stranded DNA (151). The antibiotics affect the binding ofthe intercalator to the double stranded nucleic acid, therebychanging the fluorescence intensity. The concentration of theanalyte is indirectly determined by the decrease in fluorescenceintensity. A DNA of 48.5 kb was found to be a suitable sensingnucleic acid. Sulfathiazole and chloramphenicol in shrimps weresensed by this method with detection limits from 0.5 to 1 ng/mL.

An optical biosensor for real-time detection of DNA interactionswas demonstrated with a long-period fiber-grating biosensor (152).The probe DNA was immobilized on the silanized surface of thegrating and then hybridized with the complementary (target)DNA. The sensor is reusable because the target DNA can bestripped off the grating surface after the assay. Wang et al. reporton a DNA detection protocol utilizing the opacity of self-assemblednanometallic particles (NPs) and the optical response of a CMOSimage sensor (153). The authors exploit the fact that DNAfragments attached to NPs precipitate them only at locationswhere cDNA strands exist. Hence, the opacity of the chip surfacechanges due to the accumulation of NPs and this can be used todetect targeted DNA fragments. The approach is very sensitive,detecting even single-base mismatched DNA targets in concentra-tions down to 10 pM.

Surface plasmon resonance (SPR) spectroscopy was used fordetecting target sequences in genomic DNA differing in terms ofcopies in the relative genome (154). Following fragmentation withrestriction enzymes and denaturation, the interaction of the twostrands is found to be specific both with oligonucleotides andwith genomic nonamplified DNA. The usual amplification stepis found not to be necessary. In another type of DNA biosensor,localized SPR spectroscopy was coupled to interferometry(155). A gold layer was deposited on porous anodic aluminaand interrogated by both interferometry and localized SPR. Theintensity of light reflected by the chip resulted in an opticalpattern that was highly sensitive to changes in the effectivethickness of the layer on the surface, specifically oligonucle-otides and hybridized oligonucleotides.

In a novel kind of DNA biosensor, the electrochemilumines-cence (ECL) of the Ru(II) bipyridyl complex (a weak intercalator)is used to generate optical analytical information (156). If ds-DNAbinds to intercalators such as doxorubicin or daunorubicin, aneasily detectable ECL is generated in the presence of theruthenium complex at +1.19 V (vs Ag/AgCl), while ss-DNA(which is not intercalated) does not give this effect. Given thesensitivity of ECL-based methods, this approach may be quitepowerful. Several pathogens (including hepatitis virus) weredetected by this technique.

Bacterial Biosensors. Bioavailable mercury and arsenic insoil and sediments were determined with fiber-optic bacterialbiosensors (157). Alginate-immobilized recombinant luminescentbacteria were immobilized on optical fibers and enabled lumines-cent quantification of 2.6 µg/L of Hg(II), 141 µg/L of As(V), and18 µg/L of As(III). The pesticide methyl parathion was shown tobe detectable using Flavobacterium sp. adsorbed on glass fiberfilters as a disposable biocomponent (158). The flavobacteriumcontains a hydrolase that hydrolyzes methyl parathion intooptically detectable p-nitrophenol. Only 75 µL of sample areneeded to detect 0.3 nmol/L concentrations of methyl parathion.

Microbially available dissolved organic carbon was quantifiedwith a microsensor (159) containing microorganisms in a poly-urethane hydrogel. A strain was used that displays low substrateselectivity and responded to mono- and disaccharides, to fattyacids, and to amino acids. The 90% response time was 1-5 min.Another optical fiber biosensor for biochemical oxygen demand(BOD) is based on microorganisms coimmobilized in an ormosilmatrix (160). The consumption of dissolved oxygen is measuredwith a fluorescent optical fiber sensor. The BOD values obtainedcorrelate well with those determined by the conventional BOD5

method for seawater samples.Genetically engineered bioluminescent bacteria were applied

to the detection of toxicants in surface waters (161). Themultichannel system developed is based on mini-bioreactorscontaining four kinds of recombinant bioluminescent bacteria andis connected to a luminometer via a fiber-optic cable. The systemcan be continuously operated due to the separation of the bacteriaculture reactor from the test reactor without system shutdownby abrupt inflows of severe pollutants. Bioluminescent signatureswere delivered from four channels by switching one at once. Thesystem is now being implemented to a drinking water reservoirand river for remote sensing as an early warning system.


APPLICATIONS

This section comprises sensors for environmental, industrial,biotechnological, food, pharmaceutical, medical, and related ap-plications of FOCS and FOBS. Thus, a newly developed miniatur-ized diamond ATR probe has been described that displays highchemical and pressure resistance. In combination with robust andflexible fiber-optics, it enables inline chemical reaction monitoring(162).

Huber et al. (163) reports on the measurement of the ingressof oxygen into PET bottles using oxygen sensor technology. Anoninvasive fiber-optic oxygen meter detects oxygen permeabilityof PET bottles for soft drinks by measuring trace levels of oxygeninside the bottle. Permeation rates are obtained without piercingthe package or bottle which is ideally suited for assurance,production, and quality control applications. Sensing is based onquenching of the fluorescence by oxygen of a sensor spot placedon the inner wall of the transparent bottle. A fiber-optic cable ispositioned outside and measures changes in luminescence life-time. Gaseous or dissolved oxygen can be detected in the ppm toppb range.

Near-infrared reflectance spectroscopy was applied to quanti-tate Ca, K, P, Fe, Mn, Na, Zn, protein, and moisture in alfalfa (164).The method allows immediate analysis of alfalfa without priorsample treatment. A partial least-squares regression method wasemployed. The prediction capacity of the model and the robust-ness of the method were checked in the external validation inalfalfa samples of unknown compounds.

The biocompatibility of Te-As-Se (TAS) glass fibers for usein infrared sensors was studied by Wilhelm et al. (165). The fibersare used for IR direct spectroscopy of cultivated mammalian cells.TAS fibers undergo oxidation on air to form a water-solublenanometer-thin layer that is soluble in water. The glass underneaththis layer is, however, stable in water over several days. Hence,oxidized fibers that can release arsenate ions are toxic, whilefreshly prepared or washed fibers are not. Glasses displaying lessstrong interfering vibrations in the 2-5 µm spectral region wereprepared from TeO2-BaO-Bi2O3 mixtures (166). IR and Ramanspectroscopic studies were carried out, and the temperaturedependence of the viscosities of the glasses is reported.

A fiber-optic sensor for measurements of interstitial pH wasfurther improved (167). The interstitial sample fluid drawnsubcutaneously from adipose tissue flows through a microfluidiccircuit formed by a microdialysis catheter in series with a glasscapillary. The pH indicator phenol red is covalently immobilizedon the inner wall of the capillary. Optical fibers are used to connectthe interrogating unit to the sensing capillary. A resolution of 0.03pH units and an accuracy of 0.07 pH units were obtained.Preliminary in vivo tests were carried out in pigs with alteredrespiratory function.

Data on pH, carbon dioxide, and oxygen as obtained with fiber-optic sensors on intramuscular and venous blood during rhythmichandgrip exercise were compared (168). A fiber-optic sensor wasinserted into muscle for continuous measurement of intramusculardata. Blood samples were taken from the forearm every minuteduring each exercise bout. The data for venous and arterial bloodwere found to be highly different when exercising.

Regional differences in the water content of human skin canbe studied by diffuse reflectance near-infrared spectroscopy (169).

Reflectance spectra in the 1250-2500 nm region for the skin ofvolunteers reveal large regional differences of water content. Therewas a difference in the ratio of the two water bands centered near1450 and 1900 nm between the contact and noncontact measure-ments. Most regional differences of water content were calculatedfrom the peak height of the 1900 nm water band normalized tothe peak height of the 2175 nm amide band.

One large area of application of FOCS is in monitoring themechanical and chemical integrity of concrete structures. Asidefrom sensing mechanical integrity (not treated here), changes inchemical composition are of high interest given the health riskand costs associated with disintegrated structures. Optical fibersensors have been used to monitoring the ingress of moisture instructural concrete (170). The humidity sensors were fabricatedusing fiber Bragg gratings coated with moisture sensitive poly-mers and are employed in detecting the movement of moisturethrough standardized cubes made from samples of different typesof structural concrete. Data obtained can give information on theproperties of different types of concrete but also on the migrationof dissolved salts, such as sodium chloride which is important inview of their deleterious effects on reinforcement bars withinconcrete. The optical fiber sensors reacted to the ingress of waterby detecting the moisture migrating through the sample, indicatedby a shift in the Bragg wavelength of the sensor. A similarapproach was introduced by Yeo et al. (171).

Chloride ion in concrete can be sensed with a long-period fibergrating (LPFG) (172). The LPFG device is sensitive to therefractive index of the medium around the cladding surface ofthe sensing grating, thus offering the possibility of detecting achange in chemical concentration. The authors measured chlorideions in a typical concrete sample immersed in salt water solutionsin concentrations ranging from 0 to 25%. The sensor exhibited alinear decrease in the transmission loss and resonance wavelengthshift when the chloride was increased. The limit of detection forchloride ions is ∼0.04%. Sensitivity was further enhanced bycoating a monolayer of colloidal gold nanoparticles as the activematerial on the grating surface of the LPFG which increase thesensitivity by a factor of 2.

pH sensors for concrete are needed to detect any (highlyadverse) changes in the rather high pH (>11) of concrete. Anfiber-optic pH sensor was developed that can be incorporated intoconcrete and thus is capable of early detection of the danger ofcorrosion in steel-reinforced concrete structures (173).

SENSING SCHEMES AND SPECTROSCOPIESThis section reports on improved or novel sensing schemes

based on the use of fiber optics and related waveguides. Asidefrom their use as plain waveguides, fibers have been used forevanescent wave excitation of fluorescence or Raman scatter, forimaging and sensor array purposes, in microsensors and nanosen-sors, and for distributed sensing, to mention only the moreimportant ones. The current success of (fiber optic) surfaceplasmon resonance (SPR) is obvious.

Fiber Optics. New in-line fiber-optic structures for environ-mental sensing applications were described (174). Sensors basedon the interaction of surface plasmons or evanescent waves withthe surrounding environment are usually obtained by tapering anoptical fiber. A fiber-optic structure is presented that maintainsthe structural integrity of the optical fiber. Graded index optical


fiber elements are used as lenses, and a coreless optical fiber actsas the interaction area. These elements are fused by an opticalfiber splicer. Two optical system designs were compared for fiber-optic chemical sensor applications (175). A single grating spec-trograph with fiber-optic input and photodiodes at three differentwavelengths was compared to a system comprising 1-3 fiber-optic splitters and photodiode detectors with integrated interfer-ence filters. Three types of splitters were tested, and it is foundthat that the systems have similar characteristics, also if used ina colorimetric CO2 sensor.

Capillary Waveguides. Tao et al. (176) described the ap-plication of a light guiding flexible tubular waveguide in evanescentwave absorption based sensing. A light guiding flexible fused silicacapillary (FSCap) was used that is similar to a conventional silicafiber in that it can guide light in the wavelength region from UVto near IR. The inner surface of the FSCap capillary was coatedwith a reagent doped polymer to design a FOCS. Techniques weredeveloped for activating the inner surface of an FSCap, coatingthe inner surface of the FSCap with a polymer, connecting thecoated FSCap to a light source and a photodetector, and deliveringa sample through the FSCap were developed. Sensors for Cu(II),toluene in water samples and ammonia in a gas sample werefabricated and tested. Similarly, the waveguiding properties of aFCSap for chemical sensing applications were investigated byKeller et al. (177). Absorbance within the tubing was measuredby optically coupling the FSCap to a spectrophotometer. TheFSCap operated evanescently or as a liquid core waveguidedepending upon the refractive index of the sample solution withinthe capillary. Evanescent absorbance was linear with the concen-tration of a nonpolar dye but nonlinear with ionic dyes due toadsorption to the capillary wall. Absorbance measurements in 50,150, and 250 µm inner diameter FSCaps show that greatersensitivity is achieved in thinner walled tubings because of moreinternal reflections. A FSCap for pH is demonstrated.

A liquid-filled hollow core microstructured polymer optical fiber(178) is said to be opening up many possibilities in FOCS. It isdemonstrated how the band gaps of such a hollow core polymeroptical fiber scale with the refractive index of a liquid introducedinto the holes of the microstructure. The fiber is then filled withan aqueous solution of (L)-fructose, and the resulting opticalrotation is measured. Hence, hollow core microstructured polymeroptical fibers can be used for sensing chiral species. A distributedfiber-optic polarimetric sensor was reported by Caron et al. (179).The sensor is based on evanescent wave polarimetric interferom-etry and is intended for use in gas chromatography. It allows real-time monitoring of the displacement of a chemical substance alonga capillary.

Microsystems and Microstructures. A sub-nanoliter spec-troscopic gas sensor was described (180) and compared toexisting sensors designs. This novel gas sensor has the capabilityof gas detection with a cell volume in the sub-nanoliter range. Astudy of the capabilities of microstructure fibers for evanescentwave vapor sensing is presented in ref 181. Toluene vapor innitrogen gas was investigated. It causes a change in the refractiveindex changes of the xerogel fiber cladding at 670 nm. Moreover,specific changes in absorbance due to C-H overtone absorptionsof toluene at 1600-1800 nm were exploited.

Analog signal acquisition from computer optical disk driveswas demonstrated to be useful in chemical sensing (182). Signalswere obtained from optical sensor films deposited on conventionalCD and DVD optical disks. Almost any optical disk can beemployed for deposition and readout of sensor films. The diskdrives also perform the function of reading and writing digitalcontent to optical media. Such a sensor platform is quite universaland can be applied to sensing and combinatorial screening.Specifically, colorimetric calcium-sensitive films were depositedonto a DVD, exposed to different concentrations of Ca(II), andquantified in the optical disk drive.

Ink jet printing technology was applied to fabricating micro-sized optical fiber imaging sensors (183). An array of photopo-lymerizable sensing elements containing a pH sensitive indicatorwas deposited on the surface of an optical fiber image guide. Thereproducibility of the microjet printing process was found to beexcellent for micrometer-sized sensor spots. Hanko et al. (184)showed that nanophase-separated amphiphilic networks representversatile matrixes for optical chemical and biochemical sensors.They consist of nanosized domains of hydrophilic and hydrophobicpolymers (comparable to a polyacrylamide-co-polyacrylonitrilecopolymer referred to as Hypan and previously introduced byothers). Because of the spatial separation, there are domains inwhich the indicator reagents are immobilized and domains wherediffusive transport of the analyte occurs. Various prototypes ofsensors were prepared, e.g., for sensing gaseous chlorine (basedon a chromogenic reaction), vapors of acids (based on immobilizedbromophenol blue), and peroxides (based on immobilized horse-radish peroxidase and a chromogenic substrate).

Refractive Index. High-sensitivity optical chemosensors wereimplemented by exploiting fiber Bragg grating structures inD-shape, single-mode, and multimode fibers and postsensitizedby HF etching treatment (185). Hence, the intrinsically insensitiveBragg grating became sensitive to refractive index (RI). Theresulting devices were used to measure the concentrations ofsugar solutions. A self-temperature-referenced sensor based onnonuniform thinned fiber Bragg gratings was described (186).The sensor consists of a Bragg grating where the cladding layeris removed along half of the grating length. This perturbation leadsto a wavelength-splitting in two separate peaks: the peak at lowerwavelengths corresponds to the thinned region and is dependenton the outer RI and the local temperature, while the peak at longerwavelength responds to thermal changes only. The sensor wascharacterized in terms of thermal and RI sensitivities. A photonicband gap fiber for measurement of RI was described by Sun andChan (187).

Spectroscopies. Fiber optic (bio)sensors were reported thatare based on localized surface plasmon resonance (SPR) (188).The sensor measures the light intensity of the internally reflectedlight at a fixed wavelength from an optical fiber where theextinction cross-section of self-assembled gold nanoparticles onthe unclad portion of the optical fiber changes with the refractiveindex of a sample near the gold surface. Sensing of the Ni(II) ionand label-free detection of streptavidin and staphylococcal entero-toxin B is demonstrated at picomolar levels. A related reflectionbased localized SPR fiber-optic probe was developed to determinerefractive indexes and, thus, chemical concentrations at highpressure conditions (189). Sensing is based on the measurement


of the intensity of internal light reflection at a fixed wavelengthfrom an optical fiber. The light attenuation caused by theabsorption of self-assembled gold nanoparticles on the uncladportion of the optical fiber changes with a different refractive indexof the environment near the gold surface. The probe demonstrateda stable and repeatable response for sequential operations ofpressurization and depressurization at 0.1-20.4 MPa at 308 K. Anew concept of an SPR fiber-optic sensor was presented (190). Asignificant variation of the spectral transmittance of the device isproduced as a function of the concentration of the analyte bytuning the plasmon resonance to a wavelength for which the outermedium is absorptive. With this mechanism, selectivity can beachieved without the need of any functionalization of the surfacesor the use of recognizing elements, which is an important featurefor any kind of FOCS or FOBS.

Cavity ringdown (CRD) absorption spectroscopy enables spec-troscopic sensing of gases with a high sensitivity and accuracy. Thelimits of sensitivity were further pushed (191). This continuous-waveCRD spectrometer uses a rapidly swept cavity of simple design.Measurements in the near IR from 1.51 to 1.56 µm yield sub-ppb(v/v) sensitivity in the gas phase for CO2, CO, H2O, NH3, C2H2, andother hydrocarbons. By measuring at 1.525 µm, acetylene gas canbe detected at limits as low as 19 nTorr(!). The CRD spectrometertherefore is a high performance sensor in a relatively simple, lowcost, and compact instrument. The geometry of a fiber-optic surface-enhanced Raman scattering (SERS) sensor was optimized withrespect to trace detection (192). As a result, its active surface andthe number of internal reflections at the interface between silica andsilver is largely increased. The probe was used to detect crystal violetand malachite green at ppb levels. Response is fast, and theinstrument can be deployed in-field.

A luminescent ratiometric method in the frequency domainwith dual phase-shift measurements was applied to oxygen sensing(193). The method is based on the difference between thelifetimes of the phosphorescence and fluorescence emissions ofa dually emitting indicator (an aluminum-ferron complex). Theintensity ratio of the long-lived and short-lived emissions, respec-tively, serves as the analytical information. A fiber-optic prototypewas constructed using low-cost optoelectronics including a lightemitting diode and a photodiode detector. A modified dual lifetimeratiometric (DLR) method was introduced for simultaneousluminescent determination and sensing of two analytes simulta-neously (194). Two luminescent indicators are needed in thisscheme that have overlapping absorption and emission spectrabut largely different decay times. They are excited by a singlelight source, and both emissions are measured simultaneously.In the frequency domain m-DLR method, the phase of the short-lived fluorescence of a first indicator is referenced against that ofthe long-lived luminescence of the second indicator. The analyticalinformation is obtained by measurement of the phase shifts attwo modulation frequencies. The method was demonstrated towork for the case of dually sensing oxygen and carbon dioxide.

Otto S. Wolfbeis holds a Ph.D. in chemistry. After having spent severalyears at the Max-Planck Institute of Radiation Chemistry in Muelheimand at the University of Technology at Berlin, he became an AssociateProfessor of Chemistry in 1981 at Karl-Franzens University in Graz,Austria. Since 1995 he is a Full Professor of Analytical and InterfaceChemistry at the University of Regensburg, Germany. He has authoredaround 470 papers and reviews on optical (fiber) chemical sensors,fluorescent probes, and bioassays, has (co)edited several books, and has

acted as the (co)organizer of several conferences related to fluorescencespectroscopy (MAF) and to chemical sensors and biosensors (Europtrode).He acts on the board of several journals including Angewandte Chemieand is the Editor-in-Chief of Microchimica Acta. His research interestsare in optical chemical sensing and biosensing, in fluorescent probes andlabels, in fluorescence-based analytical formats including imaging, inbiosensors based on thin metal films using capacitive or SPR interrogation,and in the design of advanced (nano)materials (including fluorescenceupconverters) for use in (bio)chemical sensing.

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