The chemical parameter most investigated with optical fibers is doubtless pH. The first pH optical fiber sensor was described in 1980. Since then, more than one hundred and twenty original papers describing different pH sensors have been published, based on absorption-based indicators on fluorophores. Such interest is perfectly justified, since pH detection is essential in many fields of application, ranging from the environment and medicine to industry and process control. Moreover, pH transduction can be used for measuring different chemical species, such as carbon dioxide, ammonia and pesticides. Notwithstanding the great number of prototypes realized in different laboratories all over the world, only a few products are available on the market. A critical analysis of the state of art in pH sensing using optical fibers is described, outlining the advantages and disadvantages of an optical approach.

A new fiber optic sensor for monitoring pH made by doping of fluorescent dyes in a sol-gel matrix is demonstrated. The indicator, 5-(and6)-carboxy-2'7'-dichlorofluorescein (CDCF), has a lower average pK_a than fluorescein due to a chloride functional modification. The absorption and fluorescence spectra of the immobilized dye at various pH levels show that the indicator is sensitive over a wide pH range. Porous sol-gel coatings are used to make the probes, which are incorporated into a bifurcated fiber optic sensor. The entire absorption and fluorescence spectra are continuously monitored using a miniature fiber optic spectrophotometer. The most responsive area of the fluorescence spectrum is selected and is referenced to a point which is insensitive to pH, so that any changes due to environmental effects and fluctuations in the light source are taken into account. Two approaches for referencing are shown, one involving the back-reflected light from the excitation source, and another utilizing the co-doping of the sol-gel with a second fluorescent dye with the same absorption characteristic as CDCF but which is insensitive to pH and fluoresces at a different wavelength.

In this paper we report the use of phase sensitive fluorometry to obtain preliminary results from micrometer- sized optical fiber oxygen sensors and a pH sensitive dye. Micrometer-sized (100 micrometers and 10 - 20 micrometers ) oxygen sensors were prepared on the distal tip of an optical fiber using a dip-coating procedure to immobilize Tris(4,7- Diphenyl-1,10-Phenanthroline)Ru(II) Chloride (Ru(dpp)) within a sol-gel matrix. These optodes exhibit excellent performance towards both gaseous and dissolved oxygen. The sensor's intensity based excitation and emission spectra, as well as phase fluorometric measurements, were collected utilizing a back-detection set-up. These micro-optodes have many desirable features for minimally invasive bio-medical applications arising from the potential of miniaturization. Carboxynaphthofluorescein solutions are shown to exhibit pH- dependent fluorescence decay times and analysis of the phase and the modulation values indicate a strong pH dependence in the physiological range, thus identifying this water-soluble dye as an attractive candidate for optical pH measurements in biomedical samples.

The present work is concerned with the optical characterization of an evanescent wave sensor for pH detection. First, the interaction between the solution containing the acid-base indicator and the fiber core was investigated. Then, the acid-base indicator, thymol blue, was covalently immobilized on the core of a 200/380 micrometers fiber by means of a silylation process of the glass surface. The fiber core surface was modified along a section of 8 mm. A comparison was made using both bare and tapered fibers, with a tapering ratio (fiber diameter/waist diameter) of 2.3. An enhancement in sensitivity of a factor 6 was observed with tapered fibers in the 1/2.5 range, and a sensitivity of 0.05 pH units was attained.

Analytical characterization of environmental sites has become a DLR1 to DLR2 billion dollar industry, and field methods to provide lower cost, more rapid analyses are being developed and commercialized. Antibody kits and fieldable gas chromatographs are examples of this work, but they still require extensive manual sample collection and preparation. As a further advance, we are developing a fieldable environmental monitor based on antibodies, fiber optic probes, and compact optoelectronics. Antibodies are immobilized to the surface of etched optical fiber tips, and a displacement reaction between the target molecules and fluor-labeled target analogs is monitored remotely. For extended, autonomous operation, fluor-labeled reagents are contained within a size-selective membrane that surrounds the fiber tip. This creates a small probe head that easily fits within a standard 2' diameter groundwater well. A hand- sized instrument with laser diode, 20-bit A/D, 12 MHz microcontroller, and associated components has also been assembled. Detection limits of 1 ppb for BTEX compounds (benzene, toluene, ethylbenzene, xylene) have been measured. Molecular imprints as lower cost and potentially most durable biomimetic analogs to the antibodies are also being investigated. Preliminary results with toluene and trichloroethylene targets have been obtained with molecularly imprinted fiber optic sensors.

All immunosensors currently described in literature are irreversible. Intelligent Optical Systems, Inc. has developed a revolutionary method for producing reversible immunosensors. In this method, the antibody and a labeled analog (structurally and functionally similar to the antigen) are coimmobilized on the sensor surface. Under equilibrium conditions, the labeled analog interacts with immobilized antibody to produce a sensor response. However, in the presence of antigen (analyte), the equilibrium is disturbed as the analyte competes for the binding sites of the immobilized antibody. This produces a measurable sensor response. The equilibrium is shifted back by washing the analyte away with a wash buffer, and the bound analog interacts with the immobilized antibody. Polarization and intensity based measurements are used to design the analog. Photoinduced electron transfer is used to create fluorescent analogs that provide enhancements in fluorescence intensity that can be measured. This principle can be extended to the detection of bacteria.

The analysis of the composition of liquefied petroleum gases is currently carried out by gas chromatography. This method is accurate, but is expensive and it is not practical for use in a low cost, field portable instrument. It has been shown that sufficient information can be obtained from the near infrared absorption spectra for the components of LPG to be distinguished. Two instruments are described: a novel device based on a number of low cost infrared light emitting diodes with a single detector, and a more conventional filter wheel based device. Experimental results indicate that an infrared system may be used to provide a robust and cost effective method of quality control of LPG products.

An instrument has been developed which combines a portable Fourier Transform Infrared spectrometer with a hollow waveguide infrared gas cell. This instrument has been applied to the multi-component analysis of 5 volatile organic compounds (VOCs), that are common contaminants in soil. The instrument is designed for rapid field screening of the VOC compounds in soil or water. The instrument employs headspace sampling in small sample vials to measure the VOC content of soil or water samples. The instrument employs either a straight or coiled hollow waveguide gas cell to achieve a long infrared absorption pathlength of 1 - 2 m with a very low cell volume in the range of 2 to 5 cc. These cell dimensions permit high detection sensitivity along with a rapid system response time. A theoretical model has been developed to predict the infrared light attenuation in the coiled waveguide cell as a function of coil radius and waveguide bore size. Incoherent infrared light transmission losses associated with coiling of waveguides with a 0.197 m coil radius have been found to have an average value of 0.312 dB/m. Calibration experiments have been performed with a series of 5 component VOC gas mixtures with concentrations in the range of 20 - 200 ppm (volume). Measurements of the partitioning of the VOCs between the soil and gas phase have also been conducted.

The imidization reaction of PMR-15 is monitored in-situ using optical grade sapphire fibers utilizing evanescent wave spectroscopy. To follow the curing reaction, the sapphire fibers are coated with uncured PMR-15 and heated to temperatures up to 200 degree(s)C for varying lengths of time. IR spectra are collected during the cure cycle. The bands that can be used to monitor the reaction are the primary amine bands at 3363 cm^-1 and 3462 cm^-1, and the secondary amide band at 3478 cm^-1. Also used are the ester acid OH band at 4440 cm^-1 and the primary amine band at 5050 cm^-1. These bands are ratioed with the constant CH band at 4050 cm^-1 in order to eliminate uncontrolled experiment-to-experiment variations. The IR spectra show the imidization reaction occurs, very slowly, at temperatures as low as 60 degree(s)C. It also shows that even after 2 hours at 200 degree(s)C, the recommended cure cycle, the reaction is not complete. The focus of this research, to date, is to improve quality control of the cure process to reduce part to part variations in mechanical properties. To this end, an attempt will be made to correlate the absorbance ratio of these bands to mechanical properties of the cured part.

This manuscript summarizes the effort to develop a field- portable Fourier Transform Infrared instrument that can perform a quick and accurate chemical analysis of unknown waste materials at Air Force bases without removing a sample for analysis. We report that devices containing a novel infrared fiber optic sensor can remotely detect and quantify the range of liquid hazardous waste typically found at Air Force bases.

In order to analyze the aqueous ammonia, we developed a simple and an inexpensive spectrometer, which consisted of a diode laser, an optical fiber with a dipping probe and a photodiode detector. Preliminary results are described here on the performances of the system in terms of the stability of output intensity, sensitivity, and reproducibility. Data on the comparisons of the system with the conventional UV- VIS spectrometer were also given. With the standard indophenol method, the detection limit for the system is 39 (mu) g/L which affords enough sensitivity for the determination of ammonia down to (mu) g/L level. The method was applied to the determination of ammonia in environmental samples including rain and snow. The results shown here indicate a potential for developing a new portable spectrometer useful for the on-site monitoring of environmental samples.

Truly distributed sensing systems for nonpolar hydrocarbons are described that are built from a chemically sensitive polymer-clad silica fiber adapted to different optical time domain reflectometer (OTDR) setups. OTDR measurements allow to locate and detect chemicals by measuring time delay between short light pulses entering the fiber and discrete changes in the backscatter signals that are 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. Such arrangements are developed to monitor hydrocarbon leakage from spatially extended technical installations or contaminated areas. Data are presented on the distributed sensing of fluorescent polynuclear aromatic hydrocarbons (PAH) that can be located by combining the fiber with an OTDR setup and a pulsed UV laser light source. This setup allows spatially resolved sensing of PAHs, e.g. fluoranthene, in the low micrograms-per-liter concentration range. However, due to the strong attenuation of the UV excitation light in the fiber, the maximum fiber length is limited to about 100 m. Much longer sensing lengths are possible if OTDR measurements are performed in the near- infrared spectral range. First data on the distributed sensing of chlorinated hydrocarbons (CHCs) with a commercially available mini-OTDR adapted to a sensing fiber of nearly one kilometer length are described. Here, a laser diode emitting at the 850-nm telecommunication wavelength was applied to locate the CHCs by analyzing the step drop (light loss) in the backscatter signal that is caused by refractive index changes in the silicone cladding induced by analyte enrichment.

The ability to collect broadband spectroscopic information about chemical analytes is highly desirable. We report on a technique that combines chemically selective coatings and optical spectroscopy. A 1-meter fiber 150 micrometers in diameter has approximately 5 cm² surface area. This entire surface is used by incorporating selective moieties into the fiber cladding. The Large-Area Chemical Sensor concept for chemical sensing and measurement is based on a combination of three techniques. Specifically, it uses: (1) optical waveguides as the sensor substrate, (2) selectively adsorbing or absorbing materials to concentrate the target materials, and (3) spectroscopic interrogation for verification and quantification. The concept has been demonstrated for an iodine sensor by co-polymerizing methyl, phenyl siloxane into di-methyl siloxane. The phenyl group forms a charge-transfer complex with iodine which has an absorption at ca. 500 nm. Fused silica is the waveguide core. This system provides sensitivities in the 10-ppm range. The concept has been implemented into a prototype field iodine sensor unit. Work on the sensor concept continues with the goal of improving the sensitivity by allowing each photon multiple opportunities to interact with a target molecule.

This work evaluates the usefulness of an intracore long period grating (LPG) structure on optical fiber for constructing a fiber-optic chemical sensor. The sensor response relies on the evanescent field interaction of core- guided light with volatile organic compounds (VOCs) surrounding the long period gratings. The LPGs were coated with proprietary chemical indicators having strong affinities for VOCs. The feasibility of this approach was tested by using representative hydrocarbons and halohydrocarbons in parts per thousand to parts per million concentrations. Test results demonstrate that LPGs offer the promise for sensitively detecting VOCs in air, water, and soil matrices.

We report recent progress on development of evanescent wave fiber optic sensors for hydrazine (HZ), monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine (UDMH). Chemically reversible evanescent sensors capable of detection below 10 ppb were prepared by removing cladding from commercial multimode fiber and coating the exposed core with a hydrazine-sensitive triphenylmethane dye immobilized in an inert polymer matrix, typically poly(vinylchloride). Triphenylmethane dyes bleach reversibly in the presence of hydrazines, enabling colorimetric sensing. The linear dynamic range was typically 0 - 300 ppb and overall dynamic range up to ca. 5 ppm. Sensors optimized for HZ were as much as a factor of 45 less sensitive to MMH and UDMH, suggesting that the sensor film would require optimization for each analyte. Saturation response and relaxation times were on the order of 5 - 8 min, but measurable signals for 10 ppb HZ could be obtained in under 30 s. These sensors demonstrate a novel route to reversible sensing of these highly toxic compounds.

A novel, generic configuration for absorption-based optical sensors is presented. This technique utilizes embossed sol- gel glass derived gratings to couple LED light into and out of a planar waveguide. The power of the guided light is monitored as its evanescent field interrogates a dye-doped sol-gel sensor layer. This waveguiding arrangement is easy to manufacture and shows great potential for use in disposable, mass-produced sensor chips for a range of absorption-based sensing tasks. The sensor is then applied towards ammonia sensing in the ppm region.

We have developed an efficient means of coupling light through a thin mid-IR-transmitting Ge waveguide. A commercial tool for grinding concave cylindrical lenses is used to grind and polish the waveguide, allowing it to be tapered from a thickness of 1 mm at the ends to a minimum of 20 - 100 micrometers at the center. This tapering improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide. Instead, it is possible to obtain extremely efficient coupling with an detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-micrometers X 1 mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000 - 5000 cm^-1 range, with just 2 min of scanning at 8 cm^-1 resolution. The small (0.02 mm²) cross section of the waveguide nevertheless yields great sensitivity to small numbers of IR-absorbing molecules near its surface.

Planar waveguides have evanescent fields sensitive to index of refraction changes in the volume immediately above the waveguide surface. Optically combining one guided sensing beam with a reference beam in an interferometric configuration generates measurable signals. Applying a chemically selective film over the sensing arm of the interferometer provides the basis for a chemical sensor. Tailored chemistries can be passive (e.g.; inducing swelling or dissolution in a film) or active (e.g.; containing reactive or binding sites). Fast and reversible chemistries are the goal, in most cases for both gaseous and liquid applications. Passive mechanisms are used when the target analyte is relatively inert, i.e. aromatic and chlorinated hydrocarbons. Active chemistries developed include tailoring the acid-base strength of the sensing film for pH or ammonia response, and antibody-antigen binding. Currently the integrated optic waveguide platform consists of thirteen interferometers on a 1 X 2-cm glass substrate. A different sensing film deposited on each channel allows for multiple analyte sensing, interferant cancellation, patterned outputs for analyte identification, or extended dynamic range. Sensitivities range from the low ppm to low ppb for both vapor and aqueous applications, 0.01 pH units and ng/mL for biologicals.

In our laboratory we are investigating new sensors based on composite sol-gel membranes. We have demonstrated sensors with up to four adjacent SiO₂ and TiO₂ layers. pH and CO₂ sensors have been demonstrated that use single and dual layers, respectively. The CO₂ sensor is all solid state and is stable even after being stored dry. The use of TiO₂ membranes allows the design of sensors for very unreactive analytes such as perchlorinated species like CCl₄ and perchloroethylene (PCE). In this paper we show preliminary data for a PCE sensor that is based on the photooxidative power of a TiO₂/SiO₂ membrane. Most of our recent work has been determining the physical and chemical properties of the TiO₂/SiO₂ sol-gel materials and relating these properties to the photoefficiency of PCE photodegradation on these sol-gel materials.

Optical fiber sensors for detecting leakage of gasoline have been studied. When exposed to gasoline vapor, certain rubber-type polymers such as polyisoprene cause swelling and their refractive indexes decrease depending on the vapor pressure of gasoline. Based on this effect, the fiber-type sensor heads were fabricated by coating the swelling polymer as a cladding layer on the fiber core with slightly lower refractive index than that of cladding. This sensor head changes its fiber structure from leaky to guided, and then, change in the transmitted light intensity by leakage of gasoline can be observed. The improvement of sensitivity in a wide range of gasoline vapor pressure was attempted.

We tested seventy-two hydrazine fuel fiber optic dosimeters for periods up to three months or Cape Canaveral in order to determine the effect of the local environment on its lifetime and sensitivity. The dosimeters were deployed at a diverse group of sites including fuel, oxidizer, and hydrocarbon fuel storage and transfer locations, a salt spray corrosion test facility, a satellite processing area, an estuarine marsh, a paint storage locker, and several indoor locations including chemical laboratory fume hoods and bathrooms. In addition, a group were set aside in a sealed enclosure for control purposes. The dosimeters were retrieved at monthly intervals and exposed to measured doses of hydrazine vapor to determine the effects of the field exposure on their hydrazine response. Our analysis indicated that 90% of the exposed dosimeters were able to sense hydrazine at a dose detectivity of less than 15 ppb-hr, a value that meets the current hydrazine sensing requirement. Consequently, we are planning to deploy a full scale, continuously operating fiber optic system for detecting potential hydrazine leaks during launch operations at Cape Canaveral.

The current paper describes a novel absorption-based optode for mercury (II) detection. The dye 2-(5-amino-3,4-dicyano- 2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (L') is a specific ligand for Hg²⁺ in water, undergoing a color change from violet to blue on the formation of the corresponding monochelated complex HgL. The dye L' was fixed on different supports by means of anionic exchange. Controlled pore glasses, cross-linked styrene and cellulose paper were used as supports and two different optode configurations were developed in order to couple them to the optical fibers. Photodegradation and leakage of the dye were thoroughly investigated by means of an accurate spectrophotometric analysis in the presence of mercury acetate [Hg(AcO)₂] solutions. In order to reduce the photodegradation of the dye, an optoelectronic unit was developed using light-emitting diodes as sources and a suitable electronic circuit as the detecting system.

The development of a submicron optical fiber `supertip' has provided advantages over previously produced submicron tips, such as facilitating insertion of these sensors into cells while minimizing damage to the cell membrane. Fiber optic ion correlation-based nanosensors for sodium, potassium and chloride employing these `supertips' have been applied to the monitoring of ion concentrations in single mouse oocytes. These sensors have also been used to monitor the effect of an ion channel-blocking agent. In order to address the challenge associated with single-cell simultaneous measurement of multiple analytes, the use of submicron optical fiber multiprobes has been explored.

We report initial experimental results on the fabrication of multicore near-field optical (NFO) fiber probes after investigating several multicore NFO probe designs. Fluorescence measurement data demonstrating optical distinction between individual adjacent channels of a three- core NFO is presented. Our results demonstrate that simultaneous multiparameter sensing in side single living cells shall soon be a reality.

Fiber-optic imaging sensors are being developed for in-situ chemical measurements using microfiber image guides. Applications of particular interest are analyte transport in thin membranes and ion transport in electrochemical boundary layers. We have demonstrated that microfiber image guides can be used for in-situ transport studies of analyte transport in thin membranes and for ion transport at electrode boundary layers in a working electrochemical cell. We have also shown that imaging sensors can be made by coating the image guide with pH sensitive and other ion- sensitive fluorescent indicator molecules, and that these sensors can be used to measure the time development of concentration gradients in-situ. Finally, it is demonstrated that these techniques can be used to obtain fundamental transport information such as diffusion coefficients in-situ at the microscopic level. Image guide sensors are described here along with a discussion of preliminary transport studies.

Oxygen- and carbon dioxide-sensitive fluorescent indicators are incorporated into a polymeric membrane for their detection by ratiometric fluorescence measurements. Oxygen is measured by fluorescent quenching of Ruthenium, and Platinum dyes. Carbon dioxide is monitored by tracking the change in the fluorescence emission spectrum of a pH indicator dye as the gas diffuses through the polymer membrane and interacts with the dye. The detection principle is the pH change induced in the membrane due to the influx of carbon dioxide. The indicator dye studied is HPTS. The polymer membranes are made of silicone or polystyrene.

Fiber-optic chemical sensors (FOCS's) are useful for making remote, in-situ, and microscale measurements. Many intensity-based and lifetime-based FOCS's have been developed for a wide range of properties and analytes. Most of these sensors give only a single-point measurement however, recently a few intensity based imaging FOCS's have been described. We have developed intensity-based fiber- optic imaging systems to measure the transport of water in thin Nafion^TM membranes and to monitor the development of pH gradients at the surface of an electrode during electrolysis of water. However, intensity-based measurements are difficult to calibrate because of the dependence of luminescence intensity on many interfering factors including dye concentration and varying excitation intensity. As a result, we are developing lifetime-based fiber-optic imaging sensors for a variety of applications. At this point we have measured a lifetime image across a sol-gel crack using a fiber-optic image guide to carry the excitation light to the sample and the resulting luminescence image to an ICCD. Currently, we are testing an oxygen imaging FOCS to capture lifetime-based images of at least two different lifetimes. This paper describes the single-point, lifetime-based sensors we have developed as precursors to fiber-optic imaging chemical sensors, the intensity-based imaging studies of water transport in thin Nafion^TM membranes and the development of pH gradients at electrode surfaces. It also discusses the instrumental system and methods used to collect lifetime images of sol-gel cracks with a fiber- optic, and the preliminary results of our imaging oxygen sensor.

Optical fiber sensors are a novel and ideal approach for making chemical and physical measurements in a variety of harsh environments. They do not corrode, are resistant to most chemicals, immune to electromagnetic interference, light weight, inherently small and have a flexible geometry. This paper presents recent test results using optical fiber long-period grating (LPG) sensors to monitor corrosion precursors and by-products. With the appropriate coating, the LPG sensor can be designed to identify a variety of environmental target molecules, such as moisture, pH, sulfates, chlorates, nitrates and metal-ions in otherwise inaccessible regions of metallic structures. Detection of these chemicals can be used to determine if the environment within a particular area of an airplane or infrastructure is becoming conducive to corrosion or whether the corrosion process is active. The LPG sensors offer a clear advantage over similar electrochemical sensors since they can be rendered immune to temperature cross-sensitivity, multiplexed along a single fiber, and can be demodulated using a simple, low-cost spectrum analyzer. By coating the LPG sensor with specially designed affinity coatings that selectively absorb target molecules, selective, real-time monitoring of environmental conditions is possible. This sensing platform shows great promise for corrosion by- product detection in pipe networks, civil infrastructure, process control, and petroleum production operations and can be applied as biological sensors for in-vitro detection of pathogens, and chemical sensors for environmental and industrial process monitoring.

A new fiber optical SPR sensor device will be presented. It works wavelength resolved at a fixed angle. The main advantage of this SPR sensor is the easy adjustment of the measuring tip corresponding to specific application conditions. First results of measurements in different media will be given.