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Many redox enzymes require expensive reduced cofactors like NAD(P)H which need to be recycled during catalysis, presenting a major cost and technical barrier to industrial exploitation. An electrochemical biphasic microfluidic setup is presented here, in which these cofactors are replaced by a mediator (methyl viologen) that acts by feeding electrons into the active site of the enzyme pentaerythritol tetranitrate reductase (PETNR). In this microfluidic recirculation setup, both enzyme and mediator remain in the reactor for reuse, allowing easy product recovery. System optimisation studies were performed using 2-cyclohexen-1-one as a model substrate prior to the investigation of a variety of different substrates whose reduction rates were determined to be 15-70% of those obtained when NADPH was used as sole electron donor. Additional data obtained with a thermophilic 'ene' reductase (TOYE) support the potential universality of this device for possible industrial applications.

The purpose of this study is to develop a droplet-based microfluidic device capable of monitoring drug precipitation upon a shift from gastric pH (pH 1.5) to intestinal pH (pH 6.5-7.0). The extent of precipitation occurring in droplets over time was measured using a novel on-chip laser scattering technique specifically developed for this study. The precipitation of ketoconazole, a poorly water-soluble basic drug, was investigated under different concentrations and pH values. It has been shown that the drug precipitates rapidly under supersaturation. Two water-soluble aqueous polymers, namely, polyvinylpyrrolidone (PVP) and hydroxypropylmethylcellulose (HPMC) have been evaluated as precipitation inhibitors. HPMC was shown to be the most potent precipitation inhibitor. It is envisaged that the microfluidic pH-shift method developed in this study would form a proof-of-concept study, towards the development of a high throughput method for screening pharmaceutical excipients/precipitation inhibitors.

A new method has been developed to improve the determination of thiocyanate using isotachophoresis. This method uses complexation with copper(II) as a mechanism for improving the separation of thiocyanate from chlorate and perchlorate. By using a pH of 3.25 the method can also be used to analyse nitrite. Separations were carried out using a miniaturised poly(methyl methacrylate) (PMMA) separation device. Linearity was observed from 1.25 to 75 mg dm−3 with a correlation coefficient of 0.998 for both thiocyanate and nitrite. Limits of detection for these two species were calculated to be 0.8 mg dm−3 and 0.9 mg dm−3 respectively. The method was successfully applied to the analysis of these anions in a range of samples including explosive residues.

A novel continuous recirculating two-phase flow miniaturised bioreactor was developed for biocatalytic transformations with the enzyme pentaerythritol tetranitrate reductase using on-chip spectroscopic detection of the organic and aqueous phases. A phase separation technique is described that uses electrostatic attraction to force charged droplets to merge back into the aqueous phase and thus allow the monitoring of both reaction phases during enzymatic turnover. We report an increased rate of enzyme catalysed reduction of trans-2-(2-nitrovinyl) thiophene, which was used as a model system to demonstrate the principles of the bioreactor design, compared to conventional macroscale experiments. Additional data obtained with ketoisophorone, trans-cinnamaldehyde and 2-methylmaleimide support our findings and provide a basis for improving the chemistry of biocatalysis.

An integrated microfluidic/electrochemical sensor with a dual working electrode configuration was fabricated for voltammetric and electrochemiluminescence (ECL) applications. The fabrication method integrates the two techniques of injection molding and screen printing for the first time. Injection molding is used to fabricate the flow cell with integrated electrical and fluidic connectors. Screen printing was used to print the electrodes directly onto the injection molded base plate. An over molding procedure followed by ultrasonic welding was used to form the complete sensor. The electrochemical and ECL performance of the sensors was assessed by applying different electrochemical methods such as cyclic voltammetry (CV), anodic stripping voltammetry (ASV) and chronoamperometry. The attractive analytical performance demonstrates good capability of the disposable sensors for routine on-line electrochemical and ECL measurements.

This paper presents and fully characterises a novel simplification approach for the development of microsystem based concentration gradient generators with significantly reduced microfluidic networks. Three microreactors are presented; a pair of two-inlet six-outlet (2-6) networks and a two-inlet eleven-outlet (2-11) network design. The mathematical approach has been validated experimentally using a purpose built optical detection system. The experimental results are shown to be in very good agreement with the theoretical predictions from the model. The developed networks are proven to deliver precise linear concentration gradients (R(2) = 0.9973 and 0.9991 for the (2-6) designs) and the simplified networks are shown to provide enhanced performance over conventional designs, overcoming some of the practical issues associated with traditional networks. The optical measurements were precise enough to validate the linearity in each level of the conventional (2-6) networks (R(2) ranged from 0.9999 to 0.9973) compared to R(2) = 1 for the theoretical model. CFD results show that there is an effective upper limit on the operating flow rate. The new simplified (2-11) design was able to maintain a linear outlet profile up to 0.8 mu l/s per inlet (R(2) = 0.9992). The proposed approach is widely applicable for the production of linear and arbitrary concentration profiles, with the potential for high throughput applications that span a wide range of chemical and biological studies.

Novel microfluidic resistive network designs have been evaluated theoretically and experimentally. Such networks are an important component of microreactor technology and are used in analytical and biomedical applications for precisely controlled and evenly stepped dilution. A detailed model and a simplification algorithm have been devised for these networks. Their combination produces novel simplified network designs that exhibit less hierarchical branching over existing network designs. Rapid prediction and optimization of a layout that meets an arbitrary outlet profile is also possible. High levels of linearity were achieved on three microdevice fluidic networks. Good agreement of the experimental results from tested devices with the model was obtained. In addition, CFD simulation of one of the designs gave good agreement with results from the model presented, a good linear outlet profile being obtained.

This work reports the construction and characterization of plastic electrochemical micro-flow-cells with integrated injection-moulded polymer electrodes. The three electrodes (working, auxiliary, and reference) were fabricated by injection-moulding from a conducting grade of polystyrene loaded with carbon fibers. On-chip reference electrodes were prepared by coating one of the conducting polymer electrodes with a Ag/AgCl layer (implemented either by e-beam evaporation of Ag followed by electrochemical formation of AgCl or by applying a Ag/AgCl paste). Working electrodes were either polymer electrodes coated with Au by e-beam evaporation or bare conducting polymer electrodes. The electrodes were integrated into the micro-flow-cells by an over-moulding process followed by ultrasonic welding. The devices were characterized by optical and electrochemical techniques. Studies by cyclic voltammetry (CV), anodic stripping voltammetry (ASV) and electrochemiluminescence (ECL) demonstrate ‘proof–of-principle’ of the micro-flow-cells as electrochemical sensors.

Catalytic adsorptive stripping voltammetry (CAdSV) has been demonstrated at a bismuth film electrode (BiFE) in an injection-moulded electrochemical micro-flow cell. The polystyrene three-electrode flow cell was fabricated with electrodes moulded from a conducting grade of polystyrene containing 40% carbon fibre, one of which was precoated with Ag to enable its use as an on-chip Ag/AgCl reference electrode. CAdSV of Co(II) and Ni(II) in the presence of dimethylglyoxime (DMG) with nitrite employed as the catalyst was performed in order to assess the performance of the flow cell with an in-line plated BiFE. The injection-moulded electrodes were found to be suitable substrates for the formation of BiFEs. Key parameters such as the plating solution matrix, plating flow rate, analysis flow rate, solution composition and square-wave parameters have been characterised and optimal conditions selected for successful and rapid analysis of Co(II) and Ni(II) at the ppb level. The analytical response was linear over the range 1 to 20 ppb and deoxygenation of the sample solution was not required. The successful coupling of a microfluidic flow cell with a BiFE, thereby forming a “mercury-free” AdSV flow analysis sensor, shows promise for industrial and in-the-field applications where inexpensive, compact, and robust instrumentation capable of low-volume analysis is required.

An assay capable of simultaneously measuring both general toxicity and more subtle genotoxicity, in aqueous environmental samples, is described. The assay uses eukaryotic (yeast) cells, genetically modified to express a green fluorescent protein (GFP) whenever DNA damage, as a result of exposure to genotoxic agents, is repaired. A measure of the reduction in cell proliferation is used to characterise general toxicity producing familiar EC50 and LOEC data. The assay protocol has been developed for proposed use in the field and hence employs dedicated, portable instrumentation, the development of which is described. A range of environmentally relevant substances has been evaluated using the assay, including solutions of metal ions, solvents and pesticides. Preliminary data comparing the yeast assay's response to that of a standard Daphnia test in the analysis of the toxicity of 34 varied industrial waste effluents are also presented. The sensitivity to a wide range of substances and effluents suggests the assay should be useful for environmental toxicity monitoring.

The use of miniaturised isotachophoresis, performed on a planar poly(methyl methacrylate) device with integrated platinum conductivity electrodes, for the analysis of the ascorbate content of photographic developer solutions has been investigated. An electrolyte system has been developed which enabled the analysis to be made without interference from any of the other components in the developer solution, a number of which were present at significantly higher concentrations than that of the ascorbate ions. Using this system, the ascorbate content of the developer solutions could be analysed in under 6 min. The limit of detection for ascorbate using miniaturised isotachophoresis was calculated to be 0.011 mmol dm−3.

The work presented here describes the design, fabrication, evaluation and use of miniaturised polymeric devices, for performing capillary isotachophoresis (CITP). CITP is an electrophoretic separation method that provides a simple technique for the analysis of ionic substances. The two principal advantages of CITP over capillary zone electrophoresis (CZE) are that it allows larger volumes of dilute sample solutions to be loaded into the separation channel and the electrolyte system can be chosen to eliminate sample matrix constituents which are not of interest. In addition CITP can be used as a preconcentration technique to provide a narrow sample band for subsequent on-line CZE separation. The devices are fabricated from silicone elastomer with on-chip conductivity detection. The resulting system has been used to separate simple mixtures of ions.