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A simple and sensitive method for the determination of vitamin B6 was developed based on the fluorescence quenching of l-cysteine capped CdS/ZnS quantum dots (QDs). The synthesised QDs were characterised using UV–vis absorption spectroscopy, fluorescence spectroscopy, FT-IR spectroscopy and transmission electron microscopy. The influence factors on the determination of vitamin B6 were investigated and the optimum conditions were determined. Under optimum conditions, the fluorescence intensity of l-cysteine capped CdS/ZnS QDs was linearly proportional to vitamin B6 over a concentration range from 0.05 mg L−1 to 6.5 mg L−1 with a correlation coefficient of 0.9946. The detection limit of this system was 0.015 mg L−1. The quenching mechanism was studied using the UV–vis absorption and fluorescence spectroscopy. The fluorescence of these core–shell QDs is quenched effectively by the vitamin B6 without obvious shift in maximum photoluminescence wavelength. Similarly, there is no obvious shift observed in the absorption spectra of l-cysteine capped CdS/ZnS QDs before and after adding the different concentrations of vitamin B6. This confirmed that quenching phenomenon in this system is possibly attributed to the effective electron transfer from QDs to vitamin B6. The proposed method was employed to detect vitamin B6 in the pharmaceutical sample with satisfactory results.

A novel oxygen (O2) and sulphur dioxide (SO2) gas sensor was produced by incorporating fluorophores within silica nanoparticles (SiNPs). The fluorophore rhodamine B isothiocyanate (RBITC) was encapsulated within SiNPs (208 ± 9 nm) to produce a nanosensor that is sensitive to SO2, while ruthenium-tris(4,7-diphenyl-1,10-phenanthroline) dichloride (Ru(dpp)3) was encapsulated within silica nanoparticles (192 ± 8 nm) to sense O2. The sensor utilises the fluorescence quenching phenomena as a detection mechanism. Core–shell SiNPs were immobilised in organically modified sol–gel (ormosil) composed of octyltriethoxysilane (OTEOS) and methyltriethoxysilane (MTEOS). The surfaces of SiNPs were modified with carboxylic acid groups in order to bind the SiNPs within the ormosil matrix. A film was produced where Ru(dpp)3 encapsulated silica nanoparticles were combined with RBITC encapsulated silica nanoparticles within a sol–gel matrix. The fluorescence of Ru(dpp)3 was highly quenched by O2 whilst RBITC was highly quenchable by SO2. Furthermore, the nanosensor was capable of detecting oxygen levels as low as 0.018% and SO2 gas at 50%. These novel nanosensors demonstrate potential to develop multisensor systems capable of detecting multiple gasses simultaneously in a single matrix for a magnitude of future applications that require small robust gas sensing.

A molecularly imprinted beta-cyclodextrin polymer (MI beta-CDP) was synthesised using beta-cycloclextrin (beta-CD) as monomer that is cross-linked using toluene 2,4-diisocyanate (TDI); N-phenyl-1-naphthylamine (NPN) Was used as the template molecule. The MI beta-CDP was fluorometrically characterised using a fibre optic cable attached to a self-designed flow-cell. The fluorescence emission spectrum of the MI beta-CDP was found to be associated with the activity of binding to NPN through batch rebinding analysis. Hererogeneous binding models (bi-Langmuir and Freundlich isotherms) that yield information on binding sites affinity distribution and heterogeneity index were employed to characterise this process. Analytical Studies demonstrated that the fluorescence intensity was linear in the analyte concentration range up to 1.6 x 10(-4) M with a limit of detection (LOD) of 1.38 mu M. The non-linear response range was successfully modelled using a power relation that is similar to the Freundlich isotherm and the dynamic response was Successfully extended to 2.0 x 10(-3) M. The response time of the system was determined to be 90s with an Optimum flow of 0.02 mLs(-1) and methanol as analyte solvent. Molecular imprinting efficiently promoted a better sensing signal by increasing the binding-affinity and substrate-selectivity towards the template molecule, compared with the control polymer prepared in its absence. The sensitivity was enhanced by about 16% as measured with three different concentrations of analyte. The sensing receptor was successfully regenerated using acetonitrile and can be reused with no significant decay in intensity with a relative standard deviation (RSD) value of 2.24% (n = 13). The sensor developed was successfully tested for analytical determination of NPN. (C) 2008 Elsevier B.V. All rights reserved.

The determination of candidate drug metabolism is a vital stage in drug development in the pharmaceutical industry. In our previous investigation, a surface plasmon biosensor has been developed with self-assembly monolayer immobilised microsomes containing CYP1A2, NADPH reductase and Cytochrome b(5). The basis of the methodology was proven using NADPH, which allows enzyme to complete the catalytic cycle, rather than simply evaluate association and dissociation constants. The purpose of this investigation is to demonstrate the capacity of the sensor to be used with CYP2D6 microsomes and define drug-drug interactions, pseudo-catalytic rate, identify unreactive substrates and inhibition using a range of substrates. By demonstrating differentiation between each of these conditions, it is possible to suggest that this sensor offers a complete alternative to the present LC-MS and fluorescent methods utilised for metabolism assay investigations. The assay method is now proven to be suitable for trialling against potential candidate drugs. (C) 2008 Elsevier B.V. All rights reserved.

Water soluble CdS quantum dots (QDs) have been synthesised using mercaptoacetic acid (MAA) as surface modifying agent through a one step process by using safe and low cost materials. These MAA capped CdS QDs are highly stable in aqueous solution and have strong affinity for mercury ion as a result of interaction with the functional group present in the capping agent. This probe is based on the characteristic of fluorescence quenching of functionalised CdS QDs. Under the optimum conditions, the fluorescence intensity of CdS QDs is linearly proportional to the mercury ion concentration in the range 0.05 x 10(-7) to 4.0 x 10(-7) M with a detection limit of 4.2 x 10(-9) M. There is no significant wavelength shift on the fluorescence-quenched signals in the presence of mercury ion at the pH 7.4. Compare to the organic dyes, these nanoparticles are brighter and more stable against photobleaching. The effect of common foreign cations oil the fluorescence of the QDs was examined to evaluate the selectivity and the results showed a high selectivity of the MAA capped CdS QDs towards Hg(II) ions, The method presented here is simple, rapid, inexpensive, sensitive and suitable for practical application. (C) 2008 Published by Elsevier B.V.

A surface plasmon resonance sensor for the detection of the common cough suppressant, dextromethorphan is demonstrated. Dextromethorphan is a pharmacological important marker drug used to identify the activity of the CYP2D6 class of p450 monoxygenases. Detection and quantification is achieved by measuring the refractive index changes of multiple surface plasmons resulting from the binding to template pockets within the thin layer imprinted β-cyclodextrin polymer. Initial investigations of the molecular imprinted polymer were conducted using reflectance spectroscopy. A detection limit of 0.035 μM and a dynamic range of 0.035 μM to 6.00 mM dependent upon instrumental setup have been demonstrated and the data are comparable with liquid chromatography-mass spectroscopy and commercial fluorescence based methodologies. The sensor could potentially be used to measure the consumption of dextromethorphan by CYP enzymes.

This paper describes the investigation of surface modified quantum dots (QDs) as a sensing receptor for Cu 2+ ion detection by optical approach. Water-soluble l -cysteine-capped ZnS QDs have been synthesised in aqueous medium. These functionalised nanoparticles were used as a fluorescence sensor for Cu (II) ion, involved in the fluorescence quenching. The optimum fluorescence intensity was found to be at pH 5.0 with a nanoparticle concentration of 2.5 mg L −1 . The effect of foreign ions on the intensity of ZnS QDs showed a low interference response towards other metal ions except Ag + and Fe 3+ ions. The quenching mechanism was studied and the results show the existence of both static and dynamic quenching processes. However, static quenching is more prominent of the two. The limit of detection of this system was found to be 7.1 × 10 −6 M. When compared with single organic fluorophores, l -cysteine-capped ZnS QDs are brighter and more stable against photobleaching. This method is not only simple, sensitive and low cost, but also reliable for practical applications.

An alternative to liquid chromatography-mass spectroscopic analysis of CYP catalysis of candidate drugs is proposed by a Surface plasmon resonance method. By covalently linking microsomes containing human recombinant CYP1A2. NADPH reductase and cytochrome b5 to a dithiodipropionic acid self-assembly monolayer formed on a gold-coated glass slide, a surface plasmon resonance (SPR) waveguide sensor is investigated. Previously only the CYP monoxygenases have been immobilised for SPR experiments, which bring into question the accuracy of association and dissociation constants determined in the absence of the other essential components (NADPH, NADPH reductase, cytochrome b5) for CYP activity. Caffeine was used as a standard Substrate of CYP1A2 and comparisons in the presence and the absence of NADPH were performed to investigate the influence of enabling catalysis upon kinetic constants. This has consequences for future CYP bioassay design as an alternative measure for CYP enzyme kinetics which is demonstrated. The effect of concentration on response time and angular change in the plasmon resonances was also observed. with potential applications for determining metabolism of compounds by this method. (C) 2008 Elsevier B.V. All rights reserved.

A variety of modelling schemes have been applied to the response of Ru.dpp:ormosil optrode. Artificial neural network model more accurately approximates the optrode response to oxygen than traditional two-site modelling and other standard models. Although there are a few neural networks that have been applied to optical sensor responses, comparison of the method with other modelling techniques has not previously been reported. This paper aims to re-investigate the artificial neural network concept for optical sensors, while explaining a clear methodology and development behind its application. The ANN model is simple when compered to other common modelling techniques. ANN methods present an alternative to numerical modelling systems that attempt to strictly quantify the influence of variables linked to physical parameters such as distribution of luminophores, rate of non-radiative energy transfers, analyte solubility and porosity of matrix. While networks can be designed to predict these physical parameters they are not restricted by them. The network accounts for variables not foreseen in numerical modelling which introduce error in approximating sensor response. High accuracy in predicting response versus actual experimental data is reflected in excellent R2 values. The ANN method applied was found to be superior to two-site modelling, linear regression, multi-linear regression and the oxygen solubility model. While this model applies strictly to the luminophore:matrix combination used here the methodology can be applied to other luminescent based sensor systems.

Sensitivity, dynamic range and resolution have been calculated and compared from a range of analytes sensed in the literature using the techniques of interferometry, surface plasmon resonance (SPR) and luminescence. A detailed explanation of the physical and chemical/biological properties required of optical sensors is included along with the principle of operation of the sensors. Theoretical sensitivities of interferometry and SPR are also detailed along with parameters affecting these sensitivities. In the literature discussed in this review paper, the technique of luminescence, which relies intrinsically on ‘labelling’, offers the best resolutions for sensing of biomolecules (protein and DNA). Interference techniques offer the best resolutions for low molecular weight chemical liquids/vapours. Techniques which are ‘label-free’ are often desirable and it is demonstrated here that by combining the techniques of SPR with interferometry, it is possible to sense proteins with a resolution similar to that of luminescence. The future of chemo- and bio-sensing is discussed in terms of potential for multi-channel analysis, their continuous miniaturisation and their impending nanotechnology revolution.

The reaction of Cu(dmp)22+ with NO has been investigated for potential development into an optical fibre chemical sensor. In aqueous solutions, a linear calibration (r = 0.9950) was obtained between 0 and 2.5 μM NO with a detection limit of 0.32 μM (∼0.0096 ppm) (s/n = 3). The complex electrostatically immobilised in nafion produced a highly linear calibration (r = 0.999) between 0 and 3 ppm in NO gas phase with a detection limit of 0.072 ppm (∼2.38 μM in solution). The response was shown to be fully reversible by immersing the films in hydroxylamine hydrochloride solutions and a steady state response was achieved within 100 s exposure time. The nafion films were also shown to be more selective to NO over O2 (183:1), NO2 (16:1) and CO (795:1). In solution nitrite, H2O2 and peroxynitrite anion did not interfere.

Studies on the use of cholesteric liquid crystals (CLCs) as sensing phase for detection of organic vapours in air are described. Stock solutions of 1.0% (w/v) cholesteryl nonanoate (CN) and cholesteryl chloride (CC) were prepared in tetrahydrofuran. Binary mixtures, with compositions ranging from 0.18 to 0.25% of CC and 0.82–0.75% of CN, respectively, were prepared by appropriate mixing of the stock solutions. Films were cast by pipetting three 10 μl aliquots of the CLC solution mixture onto a glass disk, whose reverse side was made black to absorb unscattered light. The glass disk was adapted to the common end of a bifurcated optical fibre bundle and placed in a glass vial, which provided a headspace of organic vapours. Measurements were carried out at 27±1 °C, a temperature in which the CLC mixtures maintain their liquid crystalline properties. The responses of the CLC mixtures to vapours of ethanol, acetone, benzene, pyridine and hexane were investigated. The colour of the sensing phases depended on their compositions and exposure to organic vapours gives rise to a change in the optical characteristics of liquid crystals. It was found that the CLC layers containing 0.23–0.25% of CC had no significant change in optical properties when exposed to organic vapours and that ethanol did not cause any optical changes in the liquid crystal layers. Benzene as well as hexane always turned all the coloured liquid crystalline layers to colourless. The CLC layers exhibited different behaviours to vapours of acetone and pyridine. For example, the wavelengths of maximum scattering for the 0.19% CC layer were 530 nm in air, 545 nm in pyridine and 580 nm in acetone. The CLC layers showed reversibility. The lifetimes of these layers (interval of time in which the liquid crystalline phase exists, before crystallisation) were investigated by employing acetone and n -hexane vapours. Average lifetimes of 14–15 min were found for films in contact with these vapours, while a lifetime of 205 min was possible when the CLC film was exposed to air.

The use of artificial neural networks (ANNs) as a tool for simultaneous determination of Zn(II), Cd(II) and Hg(II) in water has been investigated, by employing an optode based on the 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAP) complex immobilised on Amberlite XAD-4 resin. Studies were performed with binary and ternary mixtures of Zn(II), Cd(II) and Hg(II) ions in the 0-3.0 mg l(-1) range (prepared in pH 8.0 NH4Cl solution), in order to avoid sensing phase saturation. A 0.1 mol l(-1) HCl solution was employed to regenerate the optode after each measurement. Reflectance measurements were recorded in the 400-800 nm region after pumping metal ion solution for 1, 1.5, 2.0, 2.5 and 3.0 min. Spectra were smoothed by employing a weighted sliding average of three reflectance intensities (weights of 1:3:1) and normalised to the reflectance intensities of the immobilised reagent in the NH4Cl solution. A feed-forward ANN with error back-propagation training algorithm was employed for data treatment. The ANN was initially fed through seven inputs neurons, corresponding to reflectance intensities at 558, 568, 583, 590, 605, 615 and 623 nm, and optimised with respect to the number of hidden neurons, momentum and learning rate. Binary mixtures of Zn(II) and Cd(II), and Hg(II) and Cd(II) were employed to evaluate the capability of prediction by the ANN. Measurements were carried out in triplicate leading to standard deviations (expressed in mg l(-1)) better than 0.29 for Cd(II), 0.38 for Hg(II) and 0.35 for Zn(II) and absolute errors better than 0.14 for Cd(II), 0.31 for Hg(II) and 0.36 for Zn(II). Similar results were obtained when solution mixtures containing all three metal ions were employed in the studies. These results indicate that ANN can be employed for simultaneous determination of metal ions in water. However, attention must be given to the leaching and to the saturation of the sensing phase, which limit the lifetime and analytical range of the sensor, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.

α-Benzoinoxime, a highly selective reagent used for the gravimetric and colorimetric determination of copper, was successfully immobilised on XAD-2 and coupled with optical fibres to investigate a sensor-based approach for determining copper. Reflectance measurements were used to quantify sensor response. Optimum response was obtained at a wavelength of 560 nm and at a solution pH of 7. Using a kinetic approach in which the rate of change of reflectance is measured over a 4-min period, the response was found to be linear in the range 5–127 ppm. After measurement, the sensor can be regenerated for reuse by immersing the probe in 0.1 M HCl. With continuous use, the sensing layer was found to be stable for at least 3 days. The advantages of using α-benzoinoxime in this way are compared with its use in the traditional gravimetric and colorimetric way.

Based on the reaction between the Cu(II) complex of Eriochrome cyanine R (ECR) and nitric oxide (NO) in phosphate buffer (pH 7.4), a new colorimetric method for the determination of NO concentration has been developed. The linear calibration range for NO was 0-60 muM with a detection limit of 1.24 muM. This reaction was then used as the basis in the development of a fibre optic chemical sensor for NO gas. The copper complex was incorporated into silicone rubber membranes and exposed to NO gas after incubating the films in phosphate buffer (pH 7.4). A linear calibration for NO gas between 0 and 6 ppm was obtained with a detection limit of 0.227 ppm (1 muM similar or equal to 0.031 ppm NO in solution). The sensor response was shown to be reproducible and reversible (2.77%, R.S.D., n = 4) upon exposure to aqueous phosphate buffer (pH 7.4). The sensor response was also found to be pH independent between 7 and 10. (C) 2003 Elsevier Science B.V. All rights reserved.

An optical Al(III) sensor based on the use of purpurin immobilised on XAD 4 (styrene-divinylbenzene cross-linked copolymer) and solid surface fluorescence measurement has been developed. An optical fibre attachment for the spectrofluorometer and the probe have also been designed in this study. The plot of log[Al(III)] against the sensor response was linear for Al(III) concentration in the range of 5.0×10−5 to 3.2×10−3 M, with a limit of detection of 3.0×10−5 M. Fluoride solution was shown not to quench the complex during measurement and was used as regenerating agent to regenerate the probe. A good repeatability (1%) and reproducibility (4%) of measurement were obtained with this probe.

A selective pool optode for mercury ion sensing has been developed. The selectivity of sensor operation is based on the reagent pool of safranin–iodide solution trapped behind the Nafion membrane, which is highly permeable to cations and excludes all anions electrostatically. Further selectivity towards cations is governed by safranin–iodide reaction, which occurs specifically in the presence of Hg(II) ions as catalyst. In the absence of Hg(II), no reaction occurs between safranin and iodide. However, when trace of Hg(II) ions are present the reaction take place within a few minutes and a colour change from red to colourless occurs, which can be spectroscopically detected via optical fibres. This kind of sensor scheme would be suitable for vast number of irreversible reactions and also shows the possibility of optode development in cases where the immobilisation of reagent is not suitable. Furthermore, for simple measurements the optode can be used as a disposable sensor, since it preparation is simple and inexpensive. Since a membrane is employed in the sensor system where the reagent and sample are separated, the sensor can be applied for in situ measurements, as sample conditioning (e.g. buffer and ionic strength) would not be necessary. Various experimental parameters, such as reagent concentration, pH, temperature and volume have been optimised to obtain high sensitivity and selectivity of measurements. The optode has a dynamic range 0–50 μM with limit of detection of 5 μM toward Hg(II) ions, with only Ag(I) as the main interferent.

A gas sensor based on the incorporation of electrochromic polymers into a fibre-optic device is described. The optical changes that occur when poly(4,4′-diphenylaminep-heptyloxy)benzylidene (PDPAHB) and poly(4,4′-diphenyliminep-heptyloxy)benzylidene (PDPIHB) are exposed to different relative humidites (RH) or various ammonia concentrations (at 23°C) have been investigated. Results of the exposure of the two polymers to 0–100% relative humidity show that the working range is between 50 and 100% RH. Reversal of the films was possible by flushing with dry air. Exposure of the two polymers to increasing ammonia concentrations results in a decrease in absorbance measured at 430.46 nm for PDPAHB and at 575.78 nm for PDPIHB. Reversal of the optical changes that occurred in both films due to the presence of ammonia, was not successful by flushing with dry air or HCl, respectively.

The simultaneous determination of ammonia and relative humidity (RH) in air employing an optical fibre chemical sensor based on a nafion-crystal violet composite and a multivariate calibration based on and artificial neural network (ANN) has been described. Each and every measurement was made with a new film of 5 mm thickness, prepared from a 1:0 10 ˇ3 mol l ˇ1 crystal violet and 1:0 10 ˇ2 mol l ˇ1 nafion (as sulphonate groups) solution. Studies were performed in the ranges of 30‐70% relative humidity and 0‐25 ppm ammonia. A feedforward ANN, with error back propagation training algorithm, was employed for treatment of data. Input neurons corresponding to reflectance intensities measured at 562, 583, 605, 626 and 660 nm were employed. The optimised ANN provided standard errors of prediction (SEP) of 28.4 and 7.3% for NH3 and RH, respectively, when fed with spectral data recorded after 2 min of exposure (5 input neurons). The generalisation capability of the ANN was improved when it was fed with spectra data recorded after time intervals of 30, 60, 90 and 120 s (20 input neurons), providing SEPs equal to 9.9 and 4.5% for NH3 and RH, respectively. This improvement can be explained considering that water vapour reacts faster than NH3 with the film (a time interval

A new measurement system for chemical optrodes based on the use of multi-LEDs and double-photodiodes is presented. A sensing area is illuminated through one branch of a bifurcated fibre optics with blue, yellow, red and IR LEDs of high intensities. The second arm of the fibre optics guides reflected light from the sensing area to the detector. The sensing system is simple as no monochromators or optical filters are employed and also no need for separate optical modulation as LED is amenable to electronic modulation. The light source and detector consisted of only solid-state electronic components. Therefore, the sensor system is simpler, inexpensive, compact and highly suitable for field applications. The optrode was tested for Hg(II) ions using 4-(2-thiazolylazo) resorcinol (TAR) immobilised onto XAD-4 with yellow LED (590 nm) as light source. The sensor showed good sensitivity in the range 0.1–20 ppm of Hg(II) ion and also gave similar responses to many other toxic metal ions. The sensor can be regenerated rapidly using 0.1 M HCl.

Many ligands such as 1-nitroso-2-naphthol (NN), 4-(2-pyridylazo)resorcinol (PAR), 2,4-dinitrosoresorcinol (DNR) and 1-(2-pyridylazo)-2-naphthol (PAN) have been investigated in the development of optical sensors for heavy metal ions found in aquatic environments which include Co2+, Cu2+, Ni2+, Fe3+, Cd2+, Zn2+, Pb2+ and Hg2+. The reagents were immobilised by physical adsorption on to polymeric materials such as XAD-4, XAD-7 and Dowex ion-exchange resins, and were found to exhibit chromic characteristics in the absence and presence of metal ions in solution. Reflectance measurements are used here. Parameters such as pH, measurement wavelength and reagent concentration have been optimised for a given metal ion, and the reversibility/regenerability characteristics of the sensing reagent have been studied together with the limit of detection and the concentration range producing linear response for the metal ion.

Polymeric films of silicone rubber and PVC containing organoruthenium complexes and films of Nafion® containing crystal violet have been investigated in the development of optical gas and moisture sensors. The gases included in these studies are chlorine, nitrogen dioxide, and oxygen. The film thickness determines the speed of response to gases and their reversal. The noise in the measured signal in silicone rubber films, to some extent, is determined by their surface topography. While the Ruthenium trisbipyridyl complex (Ru-bipy)/silicone rubber films showed high sensitivity to chlorine, the Ru-dpp/PVC films showed poor response to chlorine but good responses to both nitrogen dioxide and oxygen. Nafion films containing crystal violet showed rapid and reversible response to humidity. This polymer film showed reduced memory effects. All the films studied are simple to construct and exhibited long shelf lifetimes.

Optical fibre based reflectance sensors for the detection of heavy metal ions have been developed based on the use of the immobilised reagent Br-PADAP. Two methods of immobilisation, namely physical adsorption and membrane entrapment, have been employed and their relative merits have been discussed. Br-PADAP immobilised onto XAD-4 shows a reproducible and reversible response to heavy metal ions and a good limit of detection (31 ppb for Zn(II) with an analysis time of 6 min). Br-PADAP/PVC membranes show fairly quick and reproducible responses, but are irreversible. Br-PADAP/XAD-4 is proposed as a suitable reagent phase for use in a multi-metal ion sensor.

A fibre-optic based on immobilized acetylcholinesterase is described and its application in the detection of carbamate and organophosphate pesticides through enzyme inhibition measurements is discussed. The bioactive component of the sensor consists of acetylcholinesterase covalently immobilized on preactivated isothiocyanate glass mixed with thymol blue indicator bound on aminopropyl glass and the sensor was constructed by packing a thin layer of the glass bead mixture at the tip of a bifurcated fibre-optic sensor head, which was then integrated with a flow-through cell. The response of the sensor to acetylcholine was highly reproducible (RSD2%) and readily reversible. The sensor exhibited a linear response to acetylcholine in the concentration range 2.5-25 mM (r(2)=0.992). Inhibition plots obtained for test organophosphate (paraoxon) and carbamate (carbofuran) pesticides exhibited concentration-dependent behaviour and showed linear profiles in the concentration ranges 5x10(-8)-5x10(-7) M for carbofuran and 5x10(-7)-5x10(-6) M for paraoxon. The detection limits, calculated at I(10%), are 1.5x10(-8) M (3.1 ppb) and 1.1x10(-7) M (24.7 ppb) for carbofuran and paraoxon, respectively. The regeneration of paraoxon-inhibited sensor was possible using 2-pyrimidine aldoxime, while repetitive substrate injection was necessary to reactivate the carbofuran-inhibited optrode. The factors affecting the inhibition and reactivation processes were investigated.

A multilayer feed-forward artificial neural network has been utilized to model the input-output data of an optical-fibre pH sensor. An experiment to generate the optical response spectrum and network application is described. Responses from several wavelengths of the spectrum are used as the input data for the neural network, which enables the measurement of pH in the range 1.60–10.18, with maximum error of about 0.3. The neural network tested with noisy data produces an average error of 0.2 pH units for additive noise levels of up to 13% of the input signal.

The sensitivity of quinoline- and acridine-type indicators towards the detection of low levels of chloride ions in aqueous and amine-doped media, by fluorescence quenching, is reported in this paper. Solution studies are based on the fluorescence of quinine sulphate, acridine and 3-(6-methoxyquinolino) propanesulphonate (SPQ) and the fluorescence response of these indicators has been examined at different pH values and at various chloride ion concentrations in aqueous media.

The fluorescence of tris-bipyridyl ruthenium complex (Ru-TRL) is sensitive to changes in chlorine gas concentration through dynamic quenching processes. However, the use of TRL in conjunction with a fibre-optic-based sensing system depends on finding a suitable immobilization medium for use with such a system. Several physical methods of immobilization have been examined, namely, XAD 4, polyvinyl pyrrolidone (PVP) and silicone rubber as suitable matrices, and it has been shown that the use of both PVP and silicone rubber as matrices for TRL allows the design of an optical sensor with rapid response to measure low chlorine gas concentrations. Limits of detection of 0.675 and 1 ppm have been possible using PVP and silicone rubber, respectively, as immobilization matrices under nitrogen atmospheres. Further studies have shown that the limit of detection for Ru-TRL in silicone rubber increased to 5 ppm under an atmosphere of air.

The acid-base complexation reaction between the metal free porphyrin, H2TPP, and hydrogen chloride gas has been used as the basis for a chemical gas sensor. The porphyrin is incorporated into a thin silicone rubber membrane which provides the hydrophobic conditions needed for the operation of the sensor. Porphyrin fluorescence emission provides an optical probe for the measurement of sorbed hydrogen chloride gas. A kinetic method of operation achieves low ppm (at least 2 ppm) detection. Factors such as long term stability and humidity effects limit the use of the membranes and these are considered in detail. Good agreement has been obtained between the experimental response and the response generated by a diffusion controlled model. The membranes have also been found to show sensitivity to chlorine and nitrogen dioxide but not to sulphur dioxide.

An artificial neural network (ANN) has been applied for the analysis of the response of an optical fibre pH sensor. The optrode was based on a 3,4,5,6-tetrabromophenol sulphonephthalein indicator, which was covalently bound onto aminopropyl glass beads, then packed at the tip of a bifurcated fibre-optic bundle. A three layer feed forward network was used and network training was performed using the recursive prediction error (RPE) algorithm. It was found that an optimised network with 13 hidden neurons was highly accurate in predicting the response of the optical pH sensor, with the worst interpolation error of 0.08 pH for test data set and 0.07 pH for measuring unknown buffer solutions. Overall, the application of ANN enabled the extension of the useful pH response range of the sensor from its narrow linear range (pH 5.0–7.25) to the full calibration response (pH 2.51–9.76).

The response time of an optical-fibre pH sensor with a design discussed in this paper depends upon the matrix quantity and particle size, the indicator solution concentration, direction and magnitude of the pH change, shape of the sensor tip, temperature of the analyte solution and transition interval of the indicator relative to the pH of the solution. This paper reports studies of the response time of optical-fibre pH sensors based on the use of the indicators bromophenol blue and bromothymol blue measured using diffuse reflectance spectrometry.

The factors which influence the pH response range of physically immobilised 3,4,5,6-tetrabromophenolsulfonephthalein (TBPSP) have been studied. The electronic properties of the support matrix cause an increase in the apparent p K a (p K app ) of the immobilised indicator, for electron donating matrices, and a decrease in p K app , for electron withdrawing matrices. A theoretical model, based on the Kubelka-Munk theory, has been formulated and may be used to predict the pH versus reflectance response of TBPSP immobilised on IRA-400 anion-exchange resin. Increasing the immobilised indicator concentration may be used to decrease the p K app and increase the pH sensitivity of the immobilised reagent phase. For TBPSP immobilised on IRA-400 an increase in solution ionic strength causes an increase in p K app which may be described using the Davies equation.

An optosensor for the determination of aluminium based on the use of eriochrome cyanine R (ECR) immobilised on XAD-2 and diffuse reflectance spectrophotometry has been developed. This system consists of a flow-cell containing the reagent phase which stimulates the distal end of the bifurcated fibre, which in turn is connected to the source lamp and detector. The response of the sensor was found to be reproducible with a response time of

An optical Al(III) sensor based on the use of Eriochrome cyanine R (ECR) immobilised on XAD-2 (styrene—divinylbenzene cross-linked copolymer) and diffuse reflectance spectrophotometry has been developed. A kinetic approach was used to quantify sensor response to Al(III) concentration in which the reflectance signal is measured at a fixed time interval of 3 min. Reproducible measurement of Al(III) was possible using the same probe (R.S.D. = 1.8%). Linear response was obtained for Al(III) concentration 1.3 × 10−5−4.0 × 10−4 M with limit of detection of 1.0 × 10−5 M of the metal ion. The sensor was also used for the determination of Al(III) in aqueous samples and the results obtained were comparable to those obtained by graphite furnace atomic absorption spectrometry.

By using three equations, viz., the Nernst equation, the acid-base dissociation constant equation and the equation for the Kubelka-Munk function for reflectance, an equation describing the redox behaviour of the immobilised indicator 2,6-dichloroindophenol has been formulated. After determination of the acid dissociation constant of the reduced form of the indicator, theoretical responses were calculated and compared to the experimental responses. The results indicate that the redox equilibrium involves a one-electron transfer in the pH range 2 to 5.

Optical fibre sensor development has been a growing new technology during the past decade. The current and future trends of this potentially important technology combine the use of optical fibres with chemical transducers consisting of specific reagent systems that would interact with the parameters of interest. A variety of spectroscopic techniques including reflectance and fluorescence have been employed in conjunction with fibre optics. Such a construction can result in a series of optical-fibre based chemical transducer systems that can have many advantages in the application to environmental analysis. Based on a simple concept, optical fibre chemical transducers rely on an interdisciplinary approach with expertise from the fields of analytical chemistry, physics (optics) and optoelectronics. It is believed that progress in this new field of chemical measurement is mainly dependent on the ease and the manner in which the chemical transducer can be developed and interfaced with fibre optics. This paper reviews the principles of this technique and some selective applications to environmental analysis.

The effects of immobilisation on the oxidation-reduction equilibria of 2,6-dichloroindophenol (DIP) between pH 2 and 6 have been studied. The formal potentials of the indicator in solution and in the immobilised form were obtained using absorbance and reflectance techniques, respectively, and were deduced from percentage reduction versus redox potential plots. The redox potential of the solution was altered using potentiopoised solutions containing different ratios of potassium hexacyanoferrate(II) and hexacyanoferrate(III). The formal potentials were found to be lowered when DIP was immobilised on XAD-4 resin. This and other observations described in this paper would be useful in the development of optical redox sensors.

It has been found that by conditioning porphyrin films to NOX, a reproducible and reversible interaction with the gas at room temperature can be obtained. The interaction is measured using the technique of fluorescence quenching, which easily detects 9.4 ppm of NOX. Observed variations in the responses between different films may be due to differences in the conditioning process. Conductivity measurements indicate that a dark interaction takes place, which at elevated temperatures is fast and reversible.

Dithizone immobilized on XAD-4 resin has been studied as a sensor element of an optical sensor for lead using a flow-cell. Using this arrangement, lead in solution has been determined in the concentration range 1 x 10(-5)-3 x 10(-7)M with a detection limit of 1 x 10(-8)M(i.e., 2 mug/l.). The standard deviation of the method for the measurement of lead at a concentration of 1 x 10(-6)M was found to be 7%. The response of the sensor was reproducible and can be regenerated using 0.01M hydrochloric acid followed by citrate-hydroxylamine solution.

Chlorine gas concentrations in the range 0–5 ppm can be measured with a limit of detection of 0.043 ppm using an immobilized reagent ( o -tolidine is best) and refiectance spectrophotometry. The system described utilizes a nylon tape dry reagent carrier whose change in reflectance over 10 s is probed in real time by means of an optical fibre.

The optical sensor is based on the formation of a fluoride ternary complex from an immobilized fluorescent binary zirconium-calcein blue chelate at pH 2.2. The enhanced fluorescence enables fluoride to be determined in the range 2.63 × 10−5 −4.21 × 10−4 M (0.5–8.00 mg l−1) with a limit of detection of 2.63 × 10−5 M. The response of the sensor to fluoride is affected by ionic strength and temperature. The response time depends on the particle size and the packing density of the polymer (XAD-4) substrate used.

Infrared spectroscopy of organometallic complexes in polymers films at 12–293 K has been used to investigate reactive species; in the photoreactions of (η5-C5H5)Mo(CO)3(CH3) both CO dissociation and MoCH3 bond homolysisreactions were observed.

The complexing abilities of a series of chromogenic crown ethers for potassium and sodium ions have been investigated, by spectrophotometry for the reactions in solution, and by diffuse reflectance spectroscopy when the crown ethers were immobilized. The binding coefficients of the reagents increased with increasing negative charge in the cation-binding site and with increasing extent of chelation. Centimolar K + concentrations could be determined with the immobilized reagents, with a K + /Na + selectivity ratio of approximately 10.

Reflectance was measured through optical fibres at diffeent pH values for three acidbase indicators immobilised on Amberlite XAD-2 resin. The variation of reflectance with pH suggested the existence of simultaneous equilibria involving dissociation and adsorption. Acidity constants were determined through Kubelka-Munk functions. The results indicate that adsorption of the indicator on a nonpolar surface represses the dissociation of the solute.