Bioelectronic Nanosensor Devices for Environmental and Biomedical Analysis
Abstract: A new type of Bioelectronic Nanosensor Device with potential applications in medicine,biotechnology and environmental analysis was designed. The nanosensor is based on RISFET (Regional Ion Sensitive Field Effect Transistor) technology. The design of the nanosensor involves use of a set of nano-sized electrodes on the surface of a silicon chip, giving the chip the ability to sense extremely low concentrations of specific analytes by means of an amplifying process. Analyte ions (or enzymatically generated product ions of these) are focused by a charged field to become concentrated in a narrow region – a conducting channel – located between two sensing electrodes. The focusing process leads to significant changes in conductivity, increasing the level of the current between the sensing electrodes. The current, registered by a pico-ammeter produced in-house, is a measure of analyte concentration. To achieve specificity, the RISFET nanosensor is provided with immobilized enzymes in the form of minicolumns within a flow system. Studies were conducted exploring possibilities of simplifying the system, improving its effectiveness and making it more compact. It appears that the enzymes should best be moved to an area of the chip in the vicinity of the sensing electrodes, and that branched nanowire structures (nanotrees) be placed on the chip-surface area located between the sensing electrodes to serve as carriers of the enzymes. The nanotrees would ensure an adequate load of enzymes without either the focusing of the ions or the sensing ability being disturbed. Features of this sort are seen as being particularly important for future high-density RISFET nanosensor arrays. The basic properties of the sensor were investigated using such analytes as glucose, gluconolactone, acetylcholine and carbofuran. Specificity was found to be achieved when the enzymes glucose oxidase and acetylcholine esterase were employed. The inhibition of acetylcholine esterase by carbofuran was detectable down to about 20 ng/L. An automatic online biosensor unit for the neurotoxic organocarbamate carbofuran was constructed and was found to work satisfactorily. A number of chip configurations, involving use of different silicon technologies and different electrode arrangements, were designed and characterized. In addition, a study of a Quartz Crystal Microbalance (QCM) biometric sensor was carried out; the sensor surface was functionalized by use of molecularly imprinted nanoparticles specific for (R)- or (S)-propranolol. Frequent use was made in the work as a whole of Atomic Force Microscopy (AFM), Scanning Kelvin Probe Microscopy (SKM) and Scanning Electron Microscopy (SEM). The usefulness of multifunctional nanobiosensor array systems for the surveillance of liquids regarding the presence of many different toxic and biomedically relevant analytes, and in medical, environmental and biotechnological analyses generally is discussed.
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