Selwayan Saini

Organic phase enzyme electrodes.

Abstract Organic phase enzyme electrodes (OPEEs) offer new possibilities both for the design of biosensors for use in organic solvents and in the construction of devices incorporating an organic phase for use with aqueous samples. The advantages that accrue from performing biocatalytic and electrochemical reactions in non-aqueous systems are discussed in relation to possible applications in medicine, pharmaceuticals, petrochemicals, the food industry, environmental monitoring and defence. This critical but speculative review considers the detailed implications of operating enzyme electrodes in organic solvents and briefly examines the broader issues of optical, calorimetric and piezoelectric transduction coupled with both catalytic and affinity systems, such as nucleic acid probes and antibodies.

Mediated amperometric enzyme electrode incorporating peroxidase for the determination of hydrogen peroxide in organic solvents

An amperometric enzyme electrode incorporating horseradish peroxidase is described for the determination of hydrogen peroxide in organic solvents. The enzyme was co-adsorbed with an electron mediator, potassium hexacyanoferrate(II), on the surface of a graphite foil electrode, making reagentless measurement possible. The electrochemical reduction of the enzymatically oxidized mediator was utilized as the analytical signal. Studies in different solvent systems revealed that the electrode could be operated in dioxane, chloroform and chlorobenzene, the last two providing approximately double the sensitivity of the former. The presence of a small amount of aqueous buffer was essential for sensor activity. During 2 weeks of intermittent use, the sensitivity of the electrode decreased to 40% of its initial value. At least 50 assays could be performed with a single sensor.

Multivariate Data Analysis in Electroanalytical Chemistry

Data analysis is becoming an increasingly important aspect of electroanalytical chemistry, as voltammetric techniques and electrode arrays become ever more popular as diagnostic tools. Modern data analysis techniques promise to help us make full use of the large amounts of data collected, allowing electroanalytical chemists to get more out of their existing instruments, and paving the way for new measurement approaches. This article provides an overview of the most widely used multivariate techniques in electroanalysis, citing specific examples of how they have been applied, and looking at their relative merits. As in other areas of analytical science, no single technique is applicable to all applications and the running of controls and appreciation of the applications and limitations of each technique is essential.

Organic phase enzyme electrodes for the determination of hydrogen peroxide and phenol

An amperometric horseradish peroxidase electrode is described for the determination of hydrogen peroxide in organic solvents. The enzyme was co-adsorbed with an electron mediator, hexacyanoferrate(II), to the surface of a graphite foil electrode making reagentless measurement possible. The electrochemical reduction of the enzymatically oxidized mediator was utilized as the analytical signal. The electrode can be operated in dioxane, chloroform and chlorobenzene, the presence of a small quantity of aqueous buffer being essential for activity. On this basis a small, probe-type sensor has been assembled the response of which is linearly related to hydrogen peroxide concentration between 0.05 and 1 mM. A tyrosinase sensor has been constructed by combining a Clark-type oxygen electrode with a membrane bearing adsorbed enzyme. The sensor is capable of measuring between 0.1 and 5 mM phenol in chloroform saturated with aqueous buffer.

Optimisation of a neural network model for calibration of voltammetric data

Abstract Neural networks are powerful tools for the calibration of multivariate analytical data, but the large number of network parameters make it difficult to obtain an optimum calibration model. Often, network parameters are guessed or chosen according to postulated ‘rules of thumb.’ In this paper, we perform a thorough network optimisation for the calibration of electrochemical data from tertiary aliphatic mixtures, with the dual aims of achieving the best possible calibration model, and identifying the most significant network parameters. Two optimisation methods are used: one involves testing networks over the available parameter space, while the other employs a genetic algorithm to perform a more focussed parameter search. The calibration accuracy achieved using the two methods is found to be similar, but the genetic algorithm is considerably more efficient. The number of network inputs and initial weights are found to have the greatest impact on network accuracy, while the number of training epochs and hidden layer neurons are seen to be much less important.

Peroxidase enzyme sensor for on-line monitoring of disinfection processes in the food industry.

For desirable environmental reasons, peroxides have replaced halogenated substances for disinfection purposes in the food and beverage industry. However, cost issues and the requirement to remove these agents completely after disinfection necessitate simple, low-cost and sensitive test methods with a wide dynamic range and on-line capability. The development and performance of such a method is detailed here. Low-cost peroxide sensors were fabricated using a single deposition procedure, in which horseradish peroxidase enzyme and dimethylferrocene mediator were entrapped within a cellulose acetate membrane, over the working electrode area of a screen-printed three-electrode assembly. Optimum performance was obtained using HRP and DMFc loadings of 25 U and 0.03 micromol per electrode, respectively, and a mean cellulose acetate molecular weight of 37,000. The device had a detection limit of 49.5 microM hydrogen peroxide and mean RSD values of 21% across the concentration range 49.5-368 microM. In laboratory studies the sensor was shown to have a stability of > or = 4 d in continuous flow-mode maintaining an accuracy of +/- 16% that was considered acceptable for the intended on-line monitoring of the disinfection process. In a field study, it was successfully used on-line within a flow-cell to measure peroxide levels during disinfection of an industrial fermentation vessel.

Quantitative immunoassay for determining polyaromatic hydrocarbons in electrical insulating oils

Abstract The development and application of a combined sample extraction and immunoassay protocol for the quantification of polyaromatic hydrocarbons (PAHs) in transformer oils is reported. Tests were performed on 12 different used transformer oils from three major manufacturers. The removal of matrix interferents was achieved by loading oil fractions onto silica solid phase extraction cartridges and eluting with non-polar solvent prior to evaporation and reconstitution in a more polar medium. Extracts were immunoassayed using two commercially available PAH test kits either having broad specificity towards priority PAHs or enhanced binding specificity toward more carcinogenic PAHs. The total and carcinogenic PAH test kits yielded PAH levels in the oil extracts 5.86-fold and 126-fold lower than the industry-standard IP346 method. The latter method, widely used by the industry, since it correlates with biological carcinogenicity tests, grossly over-estimates PAH levels in oils since it is a non-specific gravimetric solvent extraction approach. The assay was found to be unaffected by the extract sample matrix and was capable of determining PAHs at the nanogram per millilitre level. The assay protocol was simple, low-cost and rapid (

A liquid handling system for the automated acquisition of data for training, validating and testing calibration models

Multivariate calibration of a single sensor for many mixed analytes is useful, but generating, validating and testing the calibration models requires large data sets which can be time consuming to collect, particularly when the number of analytes is large. In this paper, a solution to this problem is presented, in the form of an automated liquid handling system capable of producing mixtures and triggering an external analytical instrument to analyse them. As a case study, the electrochemical technique of dual pulse staircase voltammetry (DPSV) was used to collect data for three analytes (glucose, fructose and ethanol) mixed in varying concentrations. Artificial neural networks (ANNs), optimised using genetic algorithms were used to create the best possible multivariate calibration model. The liquid handling system performed 1668 experiments used in the study in approximately 60 h, compared to over 2 weeks that would be required to collect perform the experiments manually. The best calibration models produced from the data bettered those produced from previous manually collected data [1,2] for all the analytes concerned.

Field-based supercritical fluid extraction and immunoassay for determination of PAHs in soils

A field-compatible supercritical-fluid extraction (SFE) device and method for extraction of organic contaminants from soil has been developed and combined with field-based immunoassay for on-site PAH (polycyclic aromatic hydrocarbon) determination. The optimised extraction method was tested in field experiments on natural samples with varying water content (0–32% w/w) without any sample pretreatment, yielding an average PAH recovery of 80% versus Soxhlet. The immunoassay functioned in buffer-diluted 10% v/v MeOH SFE extracts, allowing direct determination of PAHs with minimum sample manipulation. Immunoassay served as a reliable semi-quantitative technique for rapid screening of PAHs in SFE preparations of natural samples extracted in the field. Poor performance of the commercial solvent-shake extraction (SSE) method further supported the on-site SFE/immunoassay method.

Preliminary investigation of a bioelectrochemical sensor for the detection of phenol vapours

This work investigates the feasibility of constructing a bioelectrochemical sensor that can operate directly in gases. A series of experiments are described, resulting in a sensor that is responsive to phenol vapours. The sensor was based on ionically conducting films that incorporate a biological redox catalyst at the surface of an array of interdigitated microband electrodes. Exposure to phenol vapour drives the bioelectrochemical reaction, providing a basis for a current signal under constant potential conditions. Ionic materials included Nafion and films based on tetrabutylammonium toluene-4-sulphonate (TBATS). The quasi-reversible electrode reaction of potassium hexacyanoferrate (II) within TBATS was investigated as a function of drying time. Eo′ and K0 were determined at a TBATS modified microdisc electrode under steady-state conditions. Drying time (water loss) from the TBATS film had the effect of increasing the film ionic strength. It was shown that as the film ionic strength increased, E0′ for potassium hexacyanoferrate (II) shifts toward positive potentials (because of ion pairing) and there was a corresponding increase in the heterogeneous rate constant K0. The latter effect was attributed to increasing ion-ion (cation-ferrocyanide ion) interactions as the film dried and the enhancing effect this had on the prevention of surface poisoning reactions at the electrode. These factors are discussed in relation to sensor design.