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Fluorescence is one of the techniques adopted for a large number of optical bioassays and chemical sensing probes. The key driving motivation is basically governed by the ease of operational process, simple setup, high sensitivity, online throughput readouts, and most importantly the well understood principles behind fluorescence spectroscopy. Typically, an optical sensor adopting this technique requires sensing receptors that will interact with an analyte, subsequently causing a change that can be correlated to the identity and/or quantity of the analyte of interest. For this instance, various fluorophores are suitable to be used as the receptor and the most recent class is the fluorescent inorganic nanoparticles; portraying similar fluorescence properties to conventional organic dyes, but having special features and nature that are unique by themselves. This paper offers a rational review particularly on the development of these fluorescent inorganic nanoparticles in the area of optical sensing, excluding the coverage on fluorescent nanoparticles made of organic based fluorophores. It will cover the fundamental properties, basic methods of synthesis, engineering features, and the available characterisation options of the nanoparticles. Specifically, the application aspects for use as optical sensing receptors is highlighted with the focus on the possible sensing mechanisms, the crucial optimisation processes during the development of a sensor, and the options available for data analysis on the signals recorded from the optical sensors. Various successful demonstrations of the utilisation of such fluorescent nanoparticles for detecting different analytes will be given in this review. This paper offers a good insight on real practical ways to utilise the unique optical properties of fluorescent inorganic nanoparticles for sensing applications.

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.

Fluorescence is one of the techniques adopted for a large number of optical bioassays and chemical sensing probes. The key driving motivation is basically governed by the ease of operational process, simple setup, high sensitivity, online throughput readouts, and most importantly the well understood principles behind fluorescence spectroscopy. Typically, an optical sensor adopting this technique requires sensing receptors that will interact with an analyte, subsequently causing a change that can be correlated to the identity and/or quantity of the analyte of interest. For this instance, various fluorophores are suitable to be used as the receptor and the most recent class is the fluorescent inorganic nanoparticles; portraying similar fluorescence properties to conventional organic dyes, but having special features and nature that are unique by themselves. This paper offers a rational review particularly on the development of these fluorescent inorganic nanoparticles in the area of optical sensing, excluding the coverage on fluorescent nanoparticles made of organic based fluorophores. It will cover the fundamental properties, basic methods of synthesis, engineering features, and the available characterisation options of the nanoparticles. Specifically, the application aspects for use as optical sensing receptors is highlighted with the focus on the possible sensing mechanisms, the crucial optimisation processes during the development of a sensor, and the options available for data analysis on the signals recorded from the optical sensors. Various successful demonstrations of the utilisation of such fluorescent nanoparticles for detecting different analytes will be given in this review. This paper offers a good insight on real practical ways to utilise the unique optical properties of fluorescent inorganic nanoparticles for sensing applications.

We have synthesised water soluble CdS/ZnS core-shell quantum dots (QDs) capped with mercaptoacetic acid (MAA). They were characterised by UV–vis absorption spectroscopy, fluorescence spectroscopy, FT-IR and transmission electron microscopy. Such QDs can be used as fluorescent probes for the determination of metal ions because they quench the fluorescence of the QDs. The QDs exhibit absorption and emission bands at 345 nm and 475 nm respectively, which is more longer wavelength compared to MAA-capped CdS QDs and obviously is the result of the larger particle size. The fluorescence intensity of CdS-based QDs is strongly enhanced by coating them with a shell of ZnS. In addition, such functionalised QDs are more sensitive to Hg(II) ions. Parameters such as pH, temperature and concentration of the QDs have been optimised. A high selectivity and sensitivity toward Hg(II) ions is obtained at pH 7.4 and a concentration of 12.0 mg of QDs per L. Under optimum conditions, the fluorescence intensity of CdS/ZnS QDs is linearly proportional to the concentration of Hg(II) in the range from 2.5 to 280 nM, with a detection limit of 2.2 nM. The effect of potentially interfering cations was examined and confirmed the high selectivity of this material.

This paper describes a simple, economical and practical optical sensor that has been developed using an ion imprinted polymer for detecting copper(II) ion using reflectance spectrometry. The imprinted polymer was synthesised in the presence of copper(II) ion using 4-vinylpyridine as monomer, 2-hydroxyethyl methacrylate as co-monomer, and ethylene glycol dimethacrylate as cross-linker in methanol via free radical polymerisation. The polymer formed was physically ground into fine particles, fabricated into a sensor probe and attached at the tip of an optical fibre bundle. The sensor operates optimally at pH 5 under constant stirring condition and has a linear dynamic range for copper(II) ion determination of 0.4–4.0 mM. The limit of detection for the sensor was 123 µM. The sensor has high selectivity for monitoring copper (II) ion in a multi-analyte environment, while also exhibiting better sensitivity compared to non-imprinted polymer. The response of the sensor can be regenerated fully without significant loss in its analytical signal for re-use. This reduces the consumable cost and at the same time making the operational process of such sensor simpler and more practical to be employed in real applications.

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 1,2-diaminoanthraquinone (DAA) with NO has been investigated for potential development into an optical fibre chemical sensor. In aqueous solutions a linear calibration was obtained between 0 and 40 µM NO with a calculated detection limit of about 5 µM (s/n = 3). DAA was shown to be pH independent between 6 and 10 with a pKa of ∼5.5 making it a suitable reagent for detection of physiological NO. NO was shown to quench the fluorescence of DAA and this was due to a static quenching mechanism. The effects of nitrite, peroxynitrite, superoxide and hydrogen peroxide were investigated. Peroxynitrite did not react with DAA and nitrite did not interfere with the reaction of NO with DAA. Though attempts to immobilize DAA in silicone rubber and nafion were successful, exposure to NO gas did not result in a significant response.

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 new sensor based on the use of 2-hydroxybenzaldehyde benzoylhydrazone as a colorimetric reagent immobilized onto styrenedivinylbenzene disks has been carried out for the determination of Cu(II) ions within several minutes. The sensor is designed on a rapid and easy two-step procedure: (1) the extraction of Cu(II) ions onto a disk loaded with the copper-selective colorimetric reagent and (2) the determination of the complexed analyte directly on the surface of the disk using diffuse reflectance measurements at 400 nm. The color of the disk changed from white to green in the presence of Cu(II) ions. The work herein details the optimization of the sensing system employing a fractional factorial design 33-1 considering three variables (pH, immobilization time, and amount of ligand immobilized onto the disk). The Pareto chart and response surfaces in a spherical domain indicated that the optimum conditions for the sensing of copper ions were pH = 7, with a ligand immobilization time of 10 min and 6.25 mg of reagent loaded onto the disk. Under the optimum conditions, the analytical parameters of the proposed method were determined. The calibration graph was linear over the range of 0 to 2.5 mg L−1 of Cu(II) with a detection limit of 0.21 mg L−1. The relative standard deviation for six measurements of 1 mg L−1 of Cu(II) was found to be 4.87%. The interference from inorganic salts and other metals was found not to be of major concern when monitoring copper ions in water samples. The simplicity and rapidity of this technique make it convenient and amenable for on-site and routine analysis.

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

An optical-fiber chemical sensor (optode) is described for the simultaneous determination of Zn2+ and Cd2+ mixtures using 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol immobilized onto XAD-4 (Br-PADAP/XAD-4). A simultaneous analysis was achieved by using a single-wavelength kinetic approach coupled with a regression model. Here, the regression model was derived for the slope response of Br-PADAP/XAD-4 to binary mixtures of the metal ions, Zn2+ and Cd2+, at pH 8. The model was used then to predict the concentration of one metal ion while the other was kept constant. This was found to be reasonably successful for Zn2+ and Cd2+, with deviations of 3% and 10%, respectively. The sensor was also found to provide excellent sensitivity for both metal ions with excellent LODs (< 0.1 ppm) and very short analysis times (< 30 s).

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.

The composition of Nafion–Crystal Violet films and casting processes were studied in order to optimise the performance of an optical relative humidity sensor. The films were immobilised on a glass disc support, by placing a 5.0 µL drop of a methanolic solution of a mixture of various compositions (molar ratio) of Nafion and Crystal Violet. The homogeneity of the films, and also their reversibility to humidity, increased when the molar ratio of Nafion to Crystal Violet was increased. The homogeneity was even further improved when these films were treated with a 5.0 µL drop of methanol, leading to films of higher durability. The durability of films decreased with decrease in the film thickness. A methanol-treated, 5 µm thickness film with a Nafion∶Crystal Violet molar ratio of 10.1 showed a linear response range from 30 to 70% relative humidity (r2 > 0.99), with good stability and reversibility.

A capillary optode for determination of mercury(II) in aqueous solution has been developed. The sensors are composed of the capillary glass tubes with a chemically sensitive coating on the inner surface, consisting of a 4-(2-thiazolylazo) resorcinol (TAR) and Nafion® composite thin film. When a sample solution is passed through a capillary tube, the mercury(II) ions forms complexes with the immobilised TAR resulting in a change of the colour of the inner surface of the capillary which, in turn, can be monitored spectrophotometrically. The characteristics of these sensors including effect of flow-rate and pH, reproducibility, sensitivity, linear range, life time and selectivity are discussed.

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 simple optical flow injection ammonia sensor has been developed, based on the reflectance changes at 620 nm of an indicator fitted with a gas membrane. The sensor uses a commercially available pH indicator strip for a simple and fast assay. The response time to a 1-10 mM change in ammonia concentration is fast (≤10 seconds) and is fully reversible. The reversal time of the sensor is ≤30 seconds for ammonia assay and it can also be used for fast urea assay within the range 2-10 mM.

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.

Despite the increasing number of usage of molecularly imprinted polymers (MIPs) in optical sensor application, the correlation between the analytical signals and the binding isotherms has yet to be fully understood. This work investigates the relationship between the signals generated from MIPs sensors to its respective binding affinity variables generated using binding isotherm models. Two different systems based on the imprinting of metal ion and organic compound have been selected for the study, which employed reflectance and fluorescence sensing schemes, respectively. Batch binding analysis using the standard binding isotherm models was employed to evaluate the affinity of the binding sites. Evaluation using the discrete bi-Langmuir isotherm model found both the MIPs studied have generally two classes of binding sites that was of low and high affinities, while the continuous Freundlich isotherm model has successfully generated a distribution of affinities within the investigated analytical window. When the MIPs were incorporated as sensing receptors, the changes in the analytical signal due to different analyte concentrations were found to have direct correlation with the binding isotherm variables. Further data analyses based on this observation have generated robust models representing the analytical performance of the optical sensors. The best constructed model describing the sensing trend for each of the sensor has been tested and demonstrated to give accurate prediction of concentration for a series of spiked analytes.