This paper discusses the theoretical and experimental implications of tapering a multimode optical fiber with a view to its use in evanescent-wave absorption-spectroscopy. Good experimental results are obtained, showing the possibility of quadruplicating the absorbance efficiency. This easy and reproducible technique for taper fabrication is suitable for the implementation of both probes for spectroscopy and chemically-assisted fiber optic sensors.

We present a sensor for the continuous detection of fluorescent emissions from fluids. The sensor utilizes a patented optical fiber detection system to allow separation of the excitation and emission light without the need for optical filters, has a wide working range and has applications in process control, flow tracing and monitoring. The construction, principles and experimental results of the fiber sensor will be given. The authors have also developed a novel test for total toxicity of aquatic systems, the product of which is fluorescent, enabling the proposal of a rapid, continuous and low cost toxicity measurement system. The biochemistry of the test is easily adapted for all types of aquatic environment and expected pollution levels. Results for several heavy metal and organic contaminants, performed under laboratory conditions, are also presented.

A truly distributed sensing system for nonpolar organic chemicals is described which is built from a chemically sensitive polymer-clad silica fiber adapted to an optical time domain reflectometer (OTDR) set-up. This arrangement allows to measure the time delay between a short light pulse entering the fiber and the discrete signals of backscattered light caused by chemical effects in the fiber cladding. The light guiding properties of the fiber are affected by the enrichment of chemicals in the cladding through the evanescent wave. Changes in the refractive index (RI) of the cladding were produced by contacting the fiber with different solvents (e.g. dichloromethane, 1,1,1-trichloroethane or tetrachloroethene). Hydrocarbon compounds with a higher RI than the fiber cladding penetrating into the polysiloxane layer will increase the refractive index of the cladding and lead to a distinct step decrease in the OTDR response signal of the fiber at the position of enrichment. The size of the step decrease can be quantitatively correlated to the concentration of the hydrocarbon compound. Furthermore, the intensity of the OTDR response signal is dependent on the power of the light source and on the RI of the compound. By using a 5-W laser diode backscatter signals from tetrachloroethene in aqueous solution could be measured even at concentrations in the ppm range. The width of the step drop is linearly dependent on the interaction length between chemical and sensing fiber.

Submicron optical sensors can be prepared by immobilization of fluorescent indicators on tapered fiber tips. However, fluorescence intensity based sensing depends on many parameters (e.g. light source, collection geometry, quenching effects, etc.) and therefore quantification is usually complicated. Ratio measurements are established as a common method to quantify fluorescence signals using a sensing and a reference dye. The sensors described in this work are based on a new immobilization concept which consists of the encapsulation of dextran-linked fluorescence indicators in an organic hydrogel. This concept allows co-immobilization and stable encapsulation of different indicators. The calcium- and pH-sensors presented contain dextran-coupled fluorescein- derivatives as indicators (Calcium Green^TM and fluorescein) and a rhodamine-derivative (Texas Red^R) as reference dye, co-immobilized in PolyHEMA. These sensors exhibit a signal stability of several weeks (when stored in buffer solution), fast response times and calibration curves which are not affected by immobilization. Due to the ratio measurement signal reproducibility was less than or equal to 5%. The working lifetime of submicron sensors was limited only by photobleaching of the indicators which can be minimized by reduction of the laser power. The dynamic range and short response times of these sensors suggest applications in physiological fluids, cell cultures or micro-bioreactors.

Organically modified silicate (ormosil) sol-gel thin films have many advantages over their inorganic sol-gel and polymeric counterparts for sensing applications. The addition of methyltrimethoxysilane (MTMS) to tetraethyl orthosilicate (TEOS)-based gels creates a film with much greater hydrophobicity and less cracking due to replacement of hydroxyl groups by non-hydrolyzable methyl groups. The more hydrophobic thin film is advantageous in oxygen sensing applications, since it allows only gaseous interaction with the sensing element, and liquid infiltration into the gel is minimized. Organic modification of the gels is found to increase the degree of fluorescence quenching in dip-coated films, as evidenced by fluorescence lifetime measurements, due to the more open structure of the ormosil. However, hydrophilicity can still be obtained in the ormosil thin films by adding smaller amounts of MTMS and greater amounts of TEOS. This creates a partially hydrophilic film which still maintains a low degree of cracking due to the MTMS addition. Hydrophilic films are much desired in hydrogen sulfide and carbon dioxide sensing applications, where liquid interaction with the gel matrix itself is necessary for proper protonation and deprotonation reactions. While TEOS-based spin-coated thin films have been shown to quench more poorly with additions of MTMS, it is found that low levels of organic modification will prevent cracking of the spun films while still maintaining a very high degree of fluorescence quenching. Hence, ormosil thin films have strong potential in a wide array of chemical, biochemical, and environmental sensing applications.

Fiber-optic sensors have been developed that incorporate multi-layer organically modified silica sol-gel membranes. pH sensors use a single layer hydrophilic organo-silica sol-gel membrane with a covalently attached pH sensitive dye, hydroxypyrene trisulfonic acid. The hydrophilic coating is made by copolymerizing silanol-terminated polydimethylsiloxane and tetraethylorthosilicate with 3-aminopropyltriethoxysilane. Unlike previous methods, which use acid as a catalyst, we have found that a base catalyst produces optically transparent gels. The sol-gel coated sensors are simple to make and require drying and aging times of as little as one day. Sensors made using these gels exhibit very good long-term stability, fast response times and no dye leaching. pCO₂ sensors were fabricated using the same pH sensitive sol-gel layer overcoated with a hydrophobic high organic content sol- gel membrane. The response of the pH and pCO₂ sensors is very fast due to the high porosity of the sol-gel membranes. Although in-situ sensors have been described for a number of organic and inorganic species, many volatile organochloride compounds (VOCs), such as perchloroethylene (PCE) and trichloroethylene (TCE), have been difficult to measure using current fiber-optic sensor transduction schemes. One of the optical sensors described here is a multilayer (3-4) sol-gel system that incorporates a TiO₂/SiO₂ membrane to degrade VOCs into smaller, detectable products. Upon exposure to UV light, TiO₂, a semiconductor with a bandgap of 3.2 eV, produces highly reactive electron-hole pairs that are capable of photodegrading most organic compounds. The VOCs mentioned above are sensitive to degradative oxidation on TiO₂ surfaces. During photodegradation of VOCs a number of products are formed including H⁺, HCl, CO₂ and a number of smaller hydrocarbons. These products are produced in the TiO₂ membrane and on TiO₂ surfaces and the products diffuse into the nearby indicator membrane where they are detected. Carbon dioxide and protons produced are detected by the pH sensitive indicator layer described above. pH and CO₂ sensors, and preliminary data for the measurement of VOCs will be presented.

Integrated optics is becoming one of the most widely researched areas in optical physics. In this paper we describe a simple technique for fabrication of integrated optic units which contain planar waveguides and embossed surface relief grating couplers which may be used as optical chemical sensor platforms. The entire IO unit is fabricated via the sol-gel process. Characterization of the sensor platform and modal considerations of the waveguide structure and sensor performance will also be presented.

A single probe containing three hybrid integrated-circuit, optical waveguide, chemical-biochemical sensors (chip sensors) has been developed. Each chip sensor contains two hybrid waveguides -- one for sensing and one for reference. The sense waveguide is coated with a species-specific or group-specific chemistry or biochemistry. The reference waveguide is coated with a version of the sense chemistry or biochemistry, which is not sensitive to the analyte. The integrated structure is encapsulated and contains a single fixed light source, two detectors (reference and sense), and an optical train. The design is amenable to fluorescence, absorption, and refraction measurements. The three chip sensors are individually mounted in a probe that contains all of the electronics and computing capability necessary to collect and process the output information from each chip sensor. Only the surface of the individual chips are exposed to the target analytes. The probe is rugged, intrinsically safe, and can operate under 75 m (250 ft) of water.

A continuous flow fluorescence based immunosensor has been developed at the Naval Research Laboratory as an inexpensive, field portable device to detect environmental pollutants. Detection of environmental pollutants such as explosives [e.g. trinitrotoluene (TNT) and hexahydro-1,3,5 trinitro- 1,3,5-triazine (RDX)[ and polychlorinated biphenyls (PCBs) have been achieved at low level concentrations. The continuous flow immunosensor (CFI) employs antibodies as recognition elements for specific antigens. Antibodies specific for the environmental pollutants of interest are covalently immobilized on a solid support matrix. Subsequent saturation of the antibody-support complex with a fluorescence analog (i.e. cyanine dye) of the pollutant completes the sensor matrix. The derivatized matrix is prepacked into a micro column with a continuous flow stream of buffer that removes nonspecifically bound fluorescent analog. After a stable baseline is obtained sample injections of the desired pollutant (PCBs, TNT, RDX, etc.) into the flow stream displaces the fluorescence analog from the immobilized antibody on the solid support. A signal response over background from the displaced fluorescence analog is measured and integrated by an in-line fluorometer. Dose response curves reveal the lowest limit of detection for TNT and RDX is 20 ppb (parts-per-billion). Detection limits for PCBs is slightly higher at 1.0 ppm (part-per-million). Results from field trials conducted at two military bases, Umatilla Army Depot (Hermiston, Ore.) and Site F and A at Naval SUBASE Bangor (Bangor, Wash.) demonstrated the capabilities of the immunosensor in performing on-site field analysis in groundwater and soil leachate matrices.

The continuing demand for non-invasive tools for use in clinical diagnosis has created the need for flexible and innovative optical systems which satisfy current requirements. We report the development of a microspectrophotometer system for use on mitochondrial respiratory pigments. This novel optical fiber set-up uses visible spectrophotometry to monitor the reduction of mitochondrial electron carriers. Preliminary data is presented for the reduction of cytochrome-c by two methods. In the first, cytochrome-c was reduced in isolation using sodium dithionite. The second was an in-vivo simulation of the reduction of cytochrome-c using the mitochondrial extract from rat liver. The key features of the system are; front end adaptability, high sensitivity and fast scanning capabilities which are essential for the rapid biological reactions which are observed.

Time-resolved laser-induced fluorescence spectroscopy is one of the most sensitive optical methods which is well suited for on-line in situ analysis. Here, three examples for the steady- state and time-resolved fluorescence analysis of environmentally important analytes, the fluorescent monoaromatic hydrocarbons benzene, toluene, and xylene as well as non fluorescent heavy metal ions forming a fluorescent complex with a cation coordinating fluorescence probe, are presented and the potential of both methods is discussed. For BTX, various mixtures of the spectrally similar compounds B, T, and X showing different fluorescence lifetimes were studied with both methods. As an example for fluorometric metal ion analysis, the fluorescence probe BP(OH)₂ (2,2'-bipyridyl- 3,3'-diol) was employed for the determination of d¹⁰ metal ions in water and the newly developed fluorescence probe APTA for the detection of Cu(II). Cation complexation of BP(OH₂ yields spectrally very similar complexes which differ in their fluorescence lifetimes. Complexation of APTA to Cu(II) leads to small spectral changes and a strong increase in fluorescence quantum yield and lifetime. For the analytes studied, a comparison of the detection limits, standard deviations, and linear dynamic range of both methods clearly demonstrates the analytical potential of time-resolved fluorometry.

Recently a new fluorescence method has been introduced applying two different signals: (1) the conventional fluorescence CF occurring at short path-lengths of the exciting radiation in the fluids, and (2) the multi-path saturation fluorescence MPSF originating at long path-lengths, which can be achieved by multi-path reflection cells, where the exciting radiation is fully absorbed by the fluid. This method considers both fluorescent as well as nonfluorescent substances. The ratio between CF and MPSF yields the total absorption. First experiments were performed to investigate this method for in-situ monitoring of aromatic substances in waste water. For this purpose samples of a waste water plant near Berlin were extracted with cyclohexane within 5 minutes in a first step yielding extracts with a waste water matrix. Then polycyclic aromatic substances were added directly to the cyclohexane extracts. The fluorescence CF and MPSF were measured at different excitation wavelengths in the UV part of the spectrum using a lab-standing experimental setup as well as a portable device. Synchronously, the conventional absorption of each sample was determined by an optical multi- channel analyzer. Regression analyses between the fluorescence ratio MPSF((lambda) ₁)/MPSF((lambda) ₂) and substance concentration yielded squared correlation coefficients r² higher than these of conventional absorption spectrometry. Thus, the MPSF method is more accurate than the conventional absorption leading to lower detection limits. The MPSF method is obviously able to detect fluorescent and nonfluorescent aromatic hydrocarbons in cyclohexane extracts of waste water without further sample pre-treatment. Thus dangerous and harmless samples could be discriminated in field measurements.

An attempt has been made to improve the sensitivity and stability of optical fiber sensor used for the continuous monitoring of gaseous oxygen. It utilizes the quenching phenomena of cladding fluorescence. Two polymers viz. poly cyclohexyl methylacrylate (PCMA) and poly (4-methyl-1-pentene) (PMP) were selected which were doped with 9,10-diphenyl anthracene (DPA) and sensor head was prepared by dipcoating the polymer on a 4.2 cms length of an especially designed ARTON^TM plastic fiber with 1 mm core diameter. For dipcoating, polymer matrix containing 3% wt of fluorescent dye was used. The sensor head thus prepared was tested for various oxygen concentrations varied by mixing nitrogen. D₂lamp was used for UV side pumping and thus the fluorescence generated in the cladding was coupled to another plastic optical fiber at the output end. The change in fluorescence due to oxygen content was recorded using a photo-multiplier at the (lambda) equals 430 nm. The results obtained have been tabulated in the form of comparative studies. The response was found to be fast, reversible and reproducible with recovery time of the order of few seconds in both the cases. The sensors were found to detect a wide range of oxygen concentration ranging from 0.5% to 100% of O₂. In the case of PCMA, better stability in long-term was observed. The effect of cladding thickness on the sensor performance was also explored.

Linear arrays of single photon avalanche detectors (SPADs), fabricated using a novel planar process that is compatible with CMOS technology, are presented here. Their suitability for application in fluorescence correlation spectroscopy (FCS) is investigated by examining characteristics such as the dark counting rate, breakdown voltage and quantum efficiency. The problem of optical crosstalk between pixels in the array is investigated and a trench isolation process is proposed to eliminate crosstalk between adjacent pixels in the array.

Novel charomogenic ligands are reported which reversibly interact with alcohols resulting in a change of both absorbance and fluorescence. When embedded in plasticized PVC membranes together with tridodecylmethylammonium chloride, N,N-dioctylaminophenyl-4'-trifluoroacetyl-azobenzene (ETH^T 4001) shows a significant signal change on exposure to aqueous ethanol solution with a decrease in absorbance at around 490 nm and an increase in absorbance at around 430 nm wavelength. The sensor layer exhibits a dynamic range from 2% to 40% (v/v) ethanol with maximum relative signal changes being as high as 50%. The limit of detection is 1.5% (v/v). The absorbance of the sensor membrane is almost insensitive to changes in pH, however, the magnitude of the relative signal change between plain buffer and buffer containing ethanol is pH dependent. The sensor is selective for primary alcohols. Sensor membranes composed of the fluorescent dye p-N,N-dioctylamino-4'- trifluoroacetylstilbene (ETH^T 4004) and tridodecylmethylammonium chloride show a change in fluorescence at 576 nm when excited at 452 nm. In order to prevent cross-sensitivity to pH, a protective teflon layer has been attached to the sensor membranes. The dynamic range is similar to the membranes composed of ETH^T 4001, however, the relative signal changes are around 30% for 40% (v/v) ethanol. The response time of teflon-coated membranes is 3 - 4 min for forward, and 5 - 10 min for reverse response. The selectivity of the dye in the sensor membranes is mainly affected by the lipophilicity of the alcohols.

A fiber optic sensor prototype to detect nitride and nitrate in water is presented. Its working principle is based on a spectrophotometric technique, and makes use of specific chemical reagents.

A new design of fiber optic chemical sensor is proposed and demonstrated as a working pH sensor. The sensor is based on the evanescent coupling between a side polished single mode optical fiber and a single mode overlay planar waveguide. The stringent tolerances placed on the planar waveguide thickness are met by depositing the overlay, one molecular layer at a time, by Langmuir-Blodgett (LB) deposition. The advantages of being able to design the optical properties of the organic dye material is demonstrated by comparing two different overlay material and their responses over different wavelength ranges. Finally, a Kramers-Kronig based model, relating the absorption spectrum of the overlay material to its material dispersion, is shown to be able to predict the sensor response.

Varnishes used for the protection of paintings are also key factors influencing the appearance of paintings, since they are able to provide more gloss and saturated colors. In order to give a stable appearance to a painting, varnishes must have stable or reversible optical characteristics (color, refractive index) even in the presence of different environmental conditions. This work describes how optical fibers can be used to monitor temperature effects on the varnish refractive index.

We present the requirements and analysis of measurements of high frequency low intensity electromagnetic fields with particular focus on field perturbation by the measuring probe and the measurements in the near field region. To this end novel electric and magnetic field measurement systems (probes) utilizing semiconductor and optical technology have been developed, tested and calibrated in the frequency range up to 1.8 GHz. These probes show an outmost advantage over currently available measurement systems in that they are passive, all- dielectric and EMI immune. They also offer the possibility to measure field frequency and phase change if a reference signal is available.

The Brillouin scattering in combination with frequency-domain analysis is a new sensing technique for the distributed measurement of temperature and strain. Comprehensive theoretical examinations and experimental results of distributed temperature and strain measurements demonstrate the feasibility of this new concept. In an experimental demonstration a spatial resolution of 3 m was achieved for an 11-km-long single-mode fiber.

The development of a simulation fluorophore material and a decontamination simulator for the training of personnel associated with the cleaning and decontamination of hazardous particulate materials or solutions is reported. The reported device is based on photoluminescence and is expected to provide a safer substitute for the conventional Geiger counter based training equipment.

For control of space debris of both artificial and natural origin, the passive ground based optical system is proposed. Quantitative estimates of penetrability and resolution of the proposed system testify to the possibility of its use for observation of space debris with apparent brightness up to m equals plus 18^m stellar magnitude and angular sizes (theta) equals 10^-8 rad. The principles of construction and application strategy of the proposed system are considered.

A two step filtration based fluorometric device for continuous measurement of DDT residues is developed. The device which exploits the native fluorescence of DDT can be easily adopted to a commercial spectrofluorometer. The device was tested for its use in measuring DDT contamination in soil, potato peel and orange juice extracts and provides a detection limit approximately 1 (mu) M.

A fiber-optic probe suitable for remote elemental analysis using laser-induced breakdown spectroscopy (LIBS) has been developed and has been used to determine the concentration of lead in samples of dry paint. To develop a suitable fiber- optic sensor for remote measurements using LIBS a number of key issues must be addressed. The issue of coupling high-power laser pulses into optical fibers without fiber damage was first addressed by measuring damage threshold values, and by performing long-term durability tests for several different fiber types and sizes. This study led to the design of a highly flexible fiber-optic LIBS probe that can be used for a variety of sample substrates. To address the issue of reproducibility, we are investigating matrix affects and the dynamics of the laser ablation and laser-induced plasma processes by using a variety of spectroscopic techniques including time-resolved spectroscopic imaging. We have also modified the probe so that laser ablated material can be injected into a mass spectrometer.

Laser-induced plasma spectroscopy (LIPS) was used to study the nature and abundance of heavy metal hydrocolloids with particle diameters between 0.1 micrometer and 1 micrometer in aquifer systems. A miniaturized ultrafiltration system with a 0.1 micrometer membrane filter was employed for on site analysis. For representative heavy metal colloids absolute limits of detection in the ng-range were achieved with good reproducibility. The device is suitable for subsurface sampling under flow conditions, thus minimizing sampling artifacts.

To provide quasi-on-line information on the elemental chemical composition of heavy metal aerosols, laser-induced plasma spectroscopy (LIPS) is employed. The major objective of this work is to develop a transportable and low cost sensor system for an on-site analysis of aerosol filter samples. The presented system is based on a laser unit and a detector unit including the spectrometer with a 0.275 m Czerny-Turner- spectrometer and a gateable intensified CCD-camera. Both unites are connected to a miniaturized sensor head via fiberoptics. A laser plasma is created by focusing the light transmitted through the fiber directly on the surface of a glass fiber filter. Preliminary investigations background limitations due to blank filters and heavy metal aerosol loaded filters are given.

The concept of waste water treatment from microelectronics fab processes is referring to waste water discharged from ultrapure water plant which can't be treated on recovery waste water section. These wastes concentrated contain organic and inorganic acids, alkalis, metals, cyanide, chromium and fluoride effluent from fab processes. They will be canalized on qualities for treatment sections which permit the discharge of treated waste water as neutral with solids removal as compacted sludge. For management of the waste water treatment plant we are using the fiber optic sensors as follows: level and flow control, signalization automatic pumps control and protection, solid control, leak detection a.s.o. The neutral quality of treated water has 'null' impact against the environmental system recommended for all semiconductors and microelectronics fab processes.

An optical device for monitoring of volatile aliphatic chlorocarbons based on the measurement of fluorescence quenching of an indicator molecule that forms part of the sensing element is reported. Chlorocarbon vapors present in air mixture or in the water when in contact with the fluorescent indicator immobilized in a polymer film cause a quantitative decrease in the fluorescence intensity. The sensor offers response time of ca. 100 s, and recovery times, governed by diffusion of the analyte in the polymer film, are less than 100 s for measurement in air. A higher response time was observed for carbon tetrachloride saturated water. Detection limits less than ppm were obtained.

The atmospheric and/or the soil interactions of ¹⁴C- labelled hexachlorobenzene (HCB) with four-year-old Norway spruce cuttings originated from an autochthonous forest stand were analyzed in detail. The developed methodological approach can be applied for environmental investigations in different forest ecosystems polluted by persistent organic compounds.

The need for new instruments to measure the optical properties of natural waters, at higher spectral resolutions than existing commercial instruments, has become apparent in recent years. Such instruments will be required to support the calibration of the next generation of remote sensing platforms by providing in-situ measurements of the optical light field. In addition, high resolution spectral measurements will also support the development of optical closure models which are used to relate the observed optical properties of the water to its biological and geological content. This paper describes the design and construction of a flexible, general purpose, high resolution underwater spectrometer which can be re- configured to perform a variety of optical measurements using interchangeable, optical fiber based sensor heads. Typical results obtained during the first deployment of the instrument at sea are also presented.

Backscatter noise in fiber optic gas sensors is investigated. Interference between signal waves and backscattered waves causes signal fluctuation in the output which limits the performance of the sensing system. Sensor resolutions limited by backscatter noise are calculated for both reflection and transmission type sensors.

This paper describes an optical fiber based sensor for detecting groundwater and for monitoring the concentration of particulate material which may be entrained in that water. The sensing system uses white light from a multi-wavelength spectral source which is transmitted through a 200 micrometer diameter multimode fiber to a sensing head. The infra-red portion of the light acts as a reference while the visible part is transmitted through a sampling area and is reflected back in to a receiving fiber. The reference and modulated spectra are detected by a dual epitaxial diode. Electronic processing results in compression of the information from an intensity versus wavelength format to a single number called dominant wavelength. This number is different for air and water, and for water with and without particulates. The discrimination between air and water is possible because of differences in the absorption spectra and refractive indices, and between water with and without particulates through Mie scattering.

In this study, novel coating materials for MIR fiber-optic sensors have been investigated, in order to assess the possibilities and limitations associated with introducing materials of higher molecular complexity as coating materials. Two materials -- poly(acrylonitrile-co-butadiene) and poly(styrene-co-butadiene) -- could be found that show increased sorption coefficients for substances such as chlorinated phenols and anilines together with only moderately narrowed spectral windows.

The analysis of the composition of liquified petroleum gases (LPG) is currently carried out by gas chromatography. This method is accurate, but is expensive and is not suitable for use in field portable instruments. In this work we show that there is sufficient information in the near infrared absorption spectra to allow the main components of LPG to be distinguished. Classical spectroscopic methods can be used for this, but currently there are no near infrared spectrometers available suitable for use in a low cost portable instrument. Nor is it clear how such detailed spectroscopic data should be used to yield an LPG quality factor. We show how a chromatic system based ont three broad band detectors may be used as a low cost robust method of quality control of LPG products.

A gas sensor using optical spectrometry and dedicated to a specific gas is studied. It should be able to operate out of laboratories with a very long life and a low maintenance requirement. It is based on TLDS (tunable laser diode spectroscopy) and uses a standard Perot-Fabry laser diode already developed for telecommunications. The mode selection is realized by a passband filter and the wavelength tuning is performed via the diode temperature or its injection current. A PIN photodiode is used for detection, however a rough photoacoustic solution is intended for the future. Absorptions as low as 3.10^-3 are detected with this rough system and a limit detection of 10^-3 is available with a signal to noise ratio of unity. Experiments have shown that this system is strongly selective for the specified gas (currently the methane). A simulation has been performed which very well fits the experiment and allows us to extrapolate the performances of the system for other gases.

We give an overview of near IR laser diode based methods for gas sensing discussing both the devices enabling this technology and the techniques utilized. Of particular importance for this technology is development of single frequency laser diodes for laser based gas sensing. We discuss and present results on laser diode based gas sensing using sources such as distributed feedback laser diodes, vertical cavity laser diodes and other novel single frequency devices. Detection limits for particular gases of interest, e.g. methane, oxygen and hydrogen sulphide are discussed.

The exploitation of surface plasmon resonance (SPR) sensing principle for the study of formation of multilayers of human immunoglobulin (hlgG) and monoclonal antibody against human choriogonadotropin (a-hCG) as well as for monitoring immunoreaction between the immobilized a-hCG and human choriogonadotropin (hCG) is reported. It is demonstrated that even a very small attachment of proteins (due to the adsorption or the specific interaction) may be detected by the presented SPR sensor. An immobilization technique is described which allows to form stable multilayers of antibodies on the gold surface.

Surface plasmon resonance sensor based on resonant interaction between a guided mode of a single-mode optical fiber and a surface plasmon wave supported by a thin metal overlayer is presented. Two modes of operation of the sensor based on the measurement of changes in the fiber mode attenuation and on the measurement of changes in the wavelength at which the resonance occurs are described. Theoretical analysis of the proposed sensing structure based on the equivalent planar waveguide approach and the mode expansion and propagation method is carried out. It is shown that the operation range of the sensor may be tuned by a thin dielectric overlayer. Experimental results have shown that in both the modes of operation the sensor is capable of detecting changes in the refractive index of the sensed medium below 2 multiplied by 10^-5.

For biosensors it is very useful to work with enzymes which have a constant activity over a long period of time. For that purpose we tested 3,3',5,5'-tetramethylbenzidine (TMB), luminol, argon saturation of the buffer, bovine serum albumin (BSA) and Tween 20 on their stabilizing effect on the enzyme horseradish peroxidase. We found that TMB and luminol stabilize the enzyme very efficiently. Storing the solutions in the dark, even at stabilizer concentrations below 0.1 mM no significant loss of activity was observed during 12 weeks. At daylight the activity decreased with this stabilizers to 80% of initial activity within six weeks. In the dark no argon saturation of the buffer is necessary. On the other side, if the samples are stored at daylight, deactivation of the enzyme is strongly reduced by saturation of the buffer with argon. No stabilizing effect was observed with additives like BSA or Tween 20, which are often proposed as stabilizers in literature. The stabilized enzyme could be used for colorimetric or chemiluminescent detection independent from the stabilizing reagent used. We are now able to ensure high activity for the enzyme label HRP at room temperature over several weeks up to three months.

Cross-reactivities are one of the most important characteristics of immunoassays. Nevertheless most cross- reactivity studies are only performed with small molecules, which are similar to the target analyte. The complex mechanism of the binding event of an antibody makes it likely that the orientation of the hapten plays a critical role. Therefore cross-reactivities of hapten-derivatives with spacers may be quite different compared to the simple compound, especially when the spacer has been coupled to a large molecule, like a marker enzyme (e.g. peroxidase). We examined the relative affinity patterns of 17 enzyme tracers to 16 monoclonal and polyclonal antibodies for the analysis of triazine herbicides (atrazine, terbuthylazine, etc.). This allows interesting discussions about the structure of the antibody binding site and the comparison of antibodies generated with different immunogens. In addition, some general rules for the selection of immunogen structures could be derived from the data. The figures shown in this paper facilitate to find suitable tracer haptens for one of the tested antibodies and support for instance the optimization of immunosensor regeneration, as tracer affinity is closely correlated to the dissociation rate constant of an antibody-tracer complex.

In order to develop a simple sensing system for taste substances, an optical method using an LB film doped with a voltage-sensitive dye was studied experimentally. The blended LB film of arachidic acid and rhodamine B (RB_C18) dye with a long hydrocarbon chain, which as a mixture ratio of 75:1, was transferred on to the ITO substrate and was used to measure both the fluorescence intensity around (lambda) _f equals 600 nm and the membrane potential of the RB_C18-LB film in various taste substance solutions. As a result, for example, it was found that the fluorescence intensity increased as the NaCl concentration increased but on the other hand the membrane potential decreased. The different patterns were obtained for the different taste substance solutions. These experimental results show the possibility of an optical fiber taste substance sensor using blended LB film of arachidic acid and RB_C18.

A multiparameter fiber optic probe, which consists of pH, temperature and calcium ions sensors, is described. The probe was tested in laboratory conditions with the use of artificial samples. Each of the sensors is based on the absorbance changes of an appropriate reagent. Light emitting diodes (LEDs) are used as light sources and they are matched to maximum absorbance of the reagent.

In December 1992 a station for atmospheric observations has been put into operation at the Institute of Atmospheric Optics within the frameworks of the program of ecological monitoring of Siberia. The station provides for acquiring data on gas and aerosol composition of the atmosphere, on meteorological quantities, and the background of gamma radiation. The station operates day and night and the whole year round. All the measurement procedures are fully automated. Readouts from the measuring devices are performed very hour 10 minutes averaged. In addition, synoptic information is also received at the station. Periodically gas chromatographic analysis is being done to determine concentrations of hydrocarbons from the methane row. Occasionally, chemical composition of suspended matter is determined relative to 39 ingredients. The station is located to the north-east of Tomsk, Akademgorodok. Therefore sometimes it measures air mass coming from Tomsk down town area and sometimes the air mass from rural areas. As a result information obtained at this station should be typical for recreation zones around Tomsk.

We have fabricated IR-transmitting Ge waveguides as supported single crystals 30 - 100 micrometers in thickness. These waveguides are useful as internal reflection elements for evanescent-wave absorption sensing and spectroscopy, when used in conjunction with a Fourier transform infrared microscope. This combination affords great sensitivity to small numbers of IR-absorbing molecules near the waveguide surface, and is especially useful for analyzing thin coatings on small-area substrates. For example, we have selectively observed absorption bands, as high as 20 times the noise level, from the adhesive layer of a 0.07 mm² piece of Scotch tape without observing absorption from the backing. By comparing spectra taken using 70- and 30-micrometer thick waveguides, we demonstrate a clear increase in sensitivity to small samples with decreasing waveguide thickness.

We report the operation of an aerosol analyzer capable of measuring the fluorescence spectra of single micrometer-size bioaerosol particles. Aerosol particles in an air stream initially transverse a cw 488-nm 'trigger' laser beam where their elastic scattering and total fluorescence is measured with photomultipliers. When the elastic scattering and fluorescence signals meet certain criteria, a UV (266 nm) 'probe' laser is triggered and it illuminates the selected particles. The UV laser-excited spectra of particles are measured with the instrument's image-intensified CCD detector, gaged with signals from the trigger laser. We demonstrate the ability of the instrument to capture the fluorescence of single airborne biological particles. The results suggest that it may be possible to differentiate among biological particles based on their single particle fluorescence spectra.

A CO laser emitting radiation between 1300 and 2000 cm^-1 is applied to monitor trace gas emissions using photoacoustic detection. Several biologically interesting gases can be detected by tuning the laser frequency to absorption bands of these species. The sensitivity of the system reaches the ppbv level (acetaldehyde: 0.1 ppbv, ethanol: 3 ppbv, C₂H₆: 1 ppbv) allowing us to study e.g. an individual piece of fruit. The selectivity is enhanced by a cold trap kept at a temperature between 0 and minus 180 degrees Celsius so that unwanted species can be frozen out. The biological sample is placed in a continuous flow system of a few liters per hour which leads the released gases from the sample to the photoacoustic cell, thus creating an on-line and non-intrusive technique.