John Alderman

A DNA diagnostic biosensor: development, characterisation and performance

Abstract The aim of this work is to develop a sensor for specific DNA sequences, using non-complex synthetic single-stranded oligonucleotides as a model system. A capacitance-based sensor for the direct detection of DNA sequences is described. Hybridisation of analyte DNA with immobilised DNA on the silicon surface induces charge effects, altering the dielectric properties of the biolayer, and can be detected by the associated change in the measured capacitance. DNA has been immobilised on a silicon electrode either by passive adsorption or covalent coupling via 4-aminobutyldimethylmethoxysilane (4-ABDMMS). The work presented here introduces a colourimetric immunodetection technique for the evaluation of the immobilisation process and describes the electrical characterisation and performance of three silicon-based sequence-specific DNA sensors. These sensors consisted of a standard electrolyte-insulator-semiconductor (EIS) structure with covalently bound probe DNA, a mechanically degraded structure with passively adsorbed probe DNA and a mechanically degraded structure with covalently bound probe DNA. The last device had an improved signal to noise ratio and was, therefore, used to construct a standard curve, revealing a detection limit of 100 pmol DNA. On addition of analyte DNA, there was a decrease in measured capacitance. This response was fast, specific and required no addition of mediators to enhance or amplify the signal. This device can be optimised for the detection of complex sequences.

Application of magnetohydrodynamic actuation to continuous flow chemistryElectronic supplementary information (ESI) available: figures depicting a silicon MHD microreactor, finite element solution for velocity profile in the silicon microreactor annulus, and the effect of MHD actuation conditions on the PCR product previously generated by conventional amplification methods and on the PCR reagents prior to thermocycling by conventional methods. See http://www.rsc.org/suppdata/lc/b2/b206756k/

Continuous flow microreactors with an annular microchannel for cyclical chemical reactions were fabricated by either bulk micromachining in silicon or by rapid prototyping using EPON SU-8. Fluid propulsion in these unusual microchannels was achieved using AC magnetohydrodynamic (MHD) actuation. This integrated micropumping mechanism obviates the use of moving parts by acting locally on the electrolyte, exploiting its inherent conductive nature. Both silicon and SU-8 microreactors were capable of MHD actuation, attaining fluid velocities of the order of 300 microm s(-1) when using a 500 mM KCl electrolyte. The polymerase chain reaction (PCR), a thermocycling process, was chosen as an illustrative example of a cyclical chemistry. Accordingly, temperature zones were provided to enable a thermal cycle during each revolution. With this approach, fluid velocity determines cycle duration. Here, we report device fabrication and performance, a model to accurately describe fluid circulation by MHD actuation, and compatibility issues relating to this approach to chemistry.

Voltammetric characterisation of silicon-based microelectrode arrays and their application to mercury-free stripping voltammetry of copper ions.

This paper describes the electrochemical characterisation of a range of gold and platinum microelectrode arrays (MEAs) fabricated by standard photolithographic methods. The inter-electrode spacing, geometry, numbers and dimensions of the electrodes in the arrays were found to influence the voltammetric behaviours obtained. Excellent correlation was found between experimental data and theoretical predictions employing published models of microelectrode behaviour. Gold MEAs were evaluated for their applicability to copper determination in a soil extract sample, where agreement was found between the standard analytical method and a method based on underpotential deposition-anodic stripping voltammetry (UPD-ASV) at the MEAs, offering a mercury-free alternative for copper sensing.

An environmental monitoring system for trace metals using stripping voltammetry

Abstract There is increasing pressure on industry and regulatory bodies to monitor the discharge of trace metals into the aquatic environment. The determination of trace metals can be achieved to parts per billion (ppb) levels using anodic stripping voltammetry (ASV) at screen printed electrodes, controlled using an NMRC potentiostat coupled with software control. The disposable testheads consist of inexpensive materials and allow for low-cost production in batch processes. Voltammetric (the measurement of current as a function of potential) methods of analysis are attractive for the determination of copper, cadmium, lead and zinc. A three electrode set-up is used both in the preparation of the mercury film on a carbon electrode and in the subsequent anodic stripping voltammetric detection step. The performances of the reference electrodes, the screen-printed carbon and the FPGA based unit have been investigated in this paper.

Ion chromatography on-chip.

On-chip separation of inorganic anions by ion-exchange chromatography was realized. Micro separation channels were fabricated on a silicon wafer and sealed with a Pyrex cover plate using standard photolithography, wet and dry chemical etching, and anodic bonding techniques. Quaternary ammonium latex particles were employed for the first time to coat the separation channels on-chip. Owing to the narrow depths of the channels on the chip, 0.5-10 microm, there were more interactions of the analytes with the stationary phase on the chip than in a 50-microm I.D. capillary. With off-chip injection (20 nl) and UV detection, NO2-, NO3-, I-, and thiourea were separated using 1 mM KCl as the eluent. The linear ranges for NO2- and NO3- are from 5 to 1000 microM with the detection limits of 0.5 microM.

Evaluation of miniaturised solid state reference electrodes on a silicon based component

Abstract A study of two different types of micro reference electrodes with respect to preparation and performance is described. The first,Type 1 Ag:AgCl, consists of a silver:silver chloride ink screen printed onto platinum on silicon. The second, Type 2 Ag:AgCl, usesthe screen printed reference electrode just described, Type 1 Ag:AgCl, covered with a chloride doped vinyl ester resin. The lattereliminates some of the difficulties and drawbacks associated with Type 1 Ag:AgCl because the requirement for a constant chlorideconcentration in the analyte solution is removed when using Type 2 Ag:AgCl. Type 1 Ag:AgCl acted as a chloride selectiveelectrode with a slope of 55.83 mV per decade for chloride activity and Type 2 Ag:AgCl potential versus SCE varied from ˝40.1to ˝20.3 mV for KCl concentrations of 10 ˝7 and 1 M, respectively. The performance of a commercially available macro solidstate reference electrode (called MacroRefex in this work), fabricated from the same materials as Type 2 Ag:AgCl, was alsoinvestigated.

On-chip definition of picolitre sample injection plugs for miniaturised liquid chromatography.

The fabrication of components for a miniaturised liquid chromatography system on silicon has recently been reported by our research group [J. Cap. Electrophoresis Microchip Technol. 6 (1999) 33; Analyst 125 (2000) 25]. To date, inlet and outlet connection ports, separation micro-channels (20-200 microm in width, 0.5-10 microm in depth, 15-60 cm in length), and an intersection for picolitre injection have been etched on a silicon wafer and then sealed with a Pyrex cover plate on which platinum electrodes for on-chip detection have been patterned. The platinum electrodes have been used for the amperometric detection of phenol, using 20 nl off-chip injection. In this work we present our latest results obtained with on-chip pressure driven picolitre injection, designed to realize the full capabilities of this micro-LC system. The injection volume is dependent on the micro-channel depth, width, and also on the intersection length, allowing injection in the low nanolitre to picolitre range.

Structuring laminar flows using annular magnetohydrodynamic actuation

Laminar flow behaviour is typically observed when transporting fluids in micron-scale channels. Here, cross channel mass transport occurs only by molecular diffusion and mixing adjacent fluid streams becomes problematic. A parabolic velocity profile is also observed with pressure-driven laminar flow in a conduit. This property can be exploited by circulating fluid in an annulus such that two initially separated liquids are forced to pass through each other resulting in massive increases in the interfacial area to promote conditions for mass transfer. Miniature machined and micromachined prototypes with an integrated magnetohydrodynamic (MHD) micropump for fluid circulation were fabricated and tested. Annular MHD micromixing was characterised using fluorescein, bromophenol blue and hydrogen ion solutes for a range of velocities and modelled to include both diffusive and convective components. Furthermore, a lateral partitioning mechanism was identified and examined.

A comparison of calibration methods based on calibration data size and robustness

Abstract Least squares (LS) regression, ridge regression (RR) and partial least squares (PLS) regression have been widely used in statistical calibration of near infrared (NIR) instruments. Comparison of these methods has attracted lots of interest in literature. However, most papers compare calibration methods on the basis of a single experiment and focus on “accuracy” rather than “robustness”. In “real life”, the average accuracy level of various calibration methods may not make that much of a difference, but having an extremely bad prediction may be unacceptable. As well, a calibration set may be very expensive to acquire and methods that work well on small calibration sets may be preferred. In this paper, we compare least squares regression, ridge regression and partial least squares regression in the context of the varying calibration data size. Three data sets used in this study are all NIR-based measurements of fat or protein in milk. For a given calibration data size, 100 simulation experiments are carried out, the average value and 95 percentile of the root mean squared prediction errors of each method are compared. We found that relative performance of the calibration methods depends on data set and calibration data size as well.

Issues in development of NIR micro spectrometer system for on-line process monitoring of milk product

Abstract The development of a near-infrared (NIR) spectrometer system for on line monitoring of fat in milk processing is outlined. This system uses microsystems optical components fabricated using the LIGA technique. This technique is used in microelectronics to fabricate high definition structures in silicon. For production of large numbers of components significant cost reductions can be realised over conventional methods. To date these LIGA fabricated spectrometers have found applications in colour analysis and in quality analysis in diamonds, but have not been integrated into on-line process applications. This work outlines initial installation and test of such a system in a milk processing environment. The possibility of low cost optical systems being introduced for process control may occur with future work. This technology, operating in the near infra red (800–1100 nm), enables a number of applications to be investigated. The system hardware is outlined and initial tests with the first field installation are discussed. The system has been shown to be sensitive to required changes in fat content.