A microfluidic device for self-synchronised production of droplets.

The primary requirement for a mixing operation in droplet-based microfluidic devices is an accurate pairing of droplets of reaction fluids over an extended period of time. In this paper, a novel device for self-synchronous production of droplets has been demonstrated. The device uses a change in impedance across a pair of electrodes introduced due to the passage of a pre-formed droplet to generate a second droplet at a second pair of electrodes. The device was characterised using image analysis. Droplets with a volume of ~23.5 ± 3.1 nl (i.e.~93% of the volume of pre-formed droplets) were produced on applying a voltage of 500 V. The synchronisation efficiency of the device was 83%. As the device enables self-synchronised production of droplets, it has a potential to increase the reliability and robustness of mixing operations in droplet-based microfluidic devices.

Determination of the potassium content of explosive residues using miniaturised isotachophoresis.

A new method has been developed to allow the determination of potassium in post-explosion residues to be made using miniaturised isotachophoresis. The method is based on the use of a caesium leading ion with 4.5 mM 18-crown-6 ether added to retard the potassium to allow reliable determinations to be made. With the conditions selected no interference was noted from other small inorganic cations, such as ammonium, barium, calcium, magnesium, sodium or strontium. The method was successfully applied to the analysis of seven samples containing explosive residues obtained from the unconfined burning of several flash powders. The procedure was found to offer good linearity for potassium determinations over the concentration range of 1.25-150 µg/mL with a coefficient of determination of 0.999 achieved.

Toward a microfluidic-based rapid amylase assay system.

This article describes work into a prototype system for the assay of amylase, using microfludic technologies. The new system has a significantly shorter cycle time than the current laboratory methods, which generally use microtitre plates, yet is capable of generating significantly superior results. As such, we have shown that sensitivity is enhanced by a factor of 10 in the standard assay trials, and by a factor of 2 in the real-sample lab trials. In both assays, the use of a microreactor system reduced the reaction time by a factor of 6.2, from 20 min incubation to 3.2 min. Basing the conclusion on the Megazyme Cerealpha Standard Method, and using the Cerealpha units as a measure of assay efficiency, the typical response for the microfluidic assay was shown to be 1.0 x 10(-3) CU/mL (standard deviation [SD] 2.5 x 10(-4) CU/mL), compared to 2.56 x 10(-4) CU/mL (SD 5.94 x 10(-5) CU/mL) for the standard macroassay. It is believed that this improvement in the reaction schematics is due to the inherent advantages of microfluidic devices such as superior mixing, higher thermal efficiency, and enhanced reaction kinetics.

Determination of chlorine containing species in explosive residues using chip-based isotachophoresis.

A new method has been developed to allow the determination of the chlorate, chloride and perchlorate anions in inorganic explosive residues to be made using isotachophoresis (ITP). To enable a good separation of these species to be achieved the method involves the use of two complexing agents. Indium(III) is used to allow the determination of chloride whilst using nitrate as the leading ion and alpha-cyclodextrin is used to allow the separation of chlorate and perchlorate. Separations were carried out using a miniaturised poly(methyl methacrylate) (PMMA) separation device. The method was applied to analysing both model samples and actual inorganic explosive containing residue samples. Successful determinations of these samples were achieved with no interference from other anions typically found in inorganic explosive residues. Limits of detection (LOD) for the species of interest were calculated to be 0.80 mg l(-1) for chloride, 1.75 mg l(-1) for chlorate and 1.40 mg l(-1) for perchlorate.

An integrated optical leaky waveguide sensor with electrically induced concentration system for the detection of bacteria.

An integrated, sensitive and rapid system was developed for the detection of bacteria. The system combined an optical metal-clad leaky waveguide (MCLW) sensor with an electric field. The electric field was used to concentrate Bacillus subtilis var. niger(BG) bacteria spores onto the immobilized anti-BG antibody on the MCLW sensor surface. This sensor combination has been characterised by detecting the scattering from bacterial spores, which are concentrated at the sensor surface, when they are illuminated at the coupling angle; and by detection of fluorescence from labelled antibodies added after the spores had been captured on the surface. The light scattering and fluorescence detection methods gave a detection limit of BG bacterial spores of 1 x 10(3) spores ml(-1) when the electric field was applied for 3 minutes.

Bacteria detection using disposable optical leaky waveguide sensors.

Novel disposable absorbing material clad leaky waveguide sensor devices (LWD) have been developed for the detection of pathogenic particles such as bacteria. These chips are tailored to give the maximum extension of the evanescent field at the sensor surface in order to place the entire volume of the bacteria captured by immobilized antibodies on the chip surface within this field. This in turn increases the interaction of the light with the bacteria's bulk volume. Disposable LWD chips were fabricated at room temperature and without the use of expensive fabrication equipment. These LWDs have been characterised by detecting refractive index (RI) changes, scattering and fluorescence from bacterial spores at the sensor surface when illuminated at the coupling angle. The detection limit of Bacillus subtilis var. niger (BG) bacterial spores was 10(4) spores/ml and the illumination intensity of the spores was found to be three times greater than the illumination intensity generated using the surface plasmon resonance (SPR).

Analysis of amino acids by miniaturised isotachophoresis.

A method allowing the miniaturised isotachophoretic analysis of amino acids has been developed. To overcome the problems of carbonate contamination which occur when performing separations at alkaline pH levels glycolate was used as the leading ion. Addition of magnesium to the leading electrolyte as a counter species was found to improve the separations. The method has been used on a poly(methyl methacrylate) microdevice with integrated on-column conductivity detectors. The behaviour of a range of common amino acids was investigated and successful separations of up to seven amino acids were made. Good linearity was observed with calibration curves for aspartic acid and phenylalanine over the range 0.063-1.0 mM. Limits of detection for these two species were calculated to be 0.060 and 0.018 mM, respectively.

Use of miniaturised isotachophoresis on a planar polymer chip to analyse transition metal ions in solutions from metal processing plants.

An electrolyte system, using malic acid as a complexing agent, has been developed to allow the determination of transition metal cations using miniaturised isotachophoresis. The method allowed the simultaneous determination of Mn2+, Cr3+, Fe2+, Co2+, Zn2+ and Ni2+ to be made without interference from other common ions. Limits of detection were calculated to be in the range 0.5-1.0mg l(-1) for Mn2+, Cr3+ Co2+ and Zn2+ and 2.0 mg l(-1) for Fe2+ and 4.7 mg l(-1) for Ni2+. The successful analysis of five industrial samples, containing a range of these metal ions, obtained from metal processing plants were achieved in under 13 min. The separations were performed on a poly(methyl methacrylate) chip with integrated platinum wire conductivity detection electrodes.

Determination of the ascorbate content of photographic developer solutions using miniaturised isotachophoresis on a planar chip.

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).