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SPIMSCREEN Report Summary

Project ID: 7808
Funded under: FP6-MOBILITY
Country: United Kingdom

Final Activity Report Summary - SPIMSCREEN (Transducer technology based on impedance imaging for inflammation monitoring)

Inflammation and bacterial infection are frequently accompanied by increased levels of host and bacterial proteases. The aim of the project was to develop a new sensor platform that was capable of detecting several enzymes simultaneously.

The project focus was on the development of the transducer technology required for interrogating sensor arrays for the detection of multiple enzymes. The transducer technology was derived from an impedance imaging technique, namely Scanning photo-induced impedance microscopy (SPIM). SPIM was based on photocurrent measurements at silicon, insulator and electrolyte field-effect structures. The structure was biased towards inversion. An intensity modulated laser beam focussed onto the semiconductor generated a photocurrent at the frequency of modulation that was confined to the illuminated area and contained information about the local impedance of any material deposited onto the insulator.

The sensitivity of SPIM was pushed to its limit by optimising the silicon and insulator substrate. It depended strongly on the electrical impedance of the insulator. Traditionally thermally grown silicon dioxide was used as the insulator. However, the thinner the silicon dioxide the more defects there would be in the oxide film leading to high leakage currents and failure of the devices. This problem was overcome by developing a method for repairing defects in thermal silicon dioxide using anodic oxidation. The sensitivity of SPIM was increased by more than a factor of 10 by replacing the traditionally used thermally grown silicon dioxide by ultrathin repaired thermal oxides or anodically grown silicon dioxide.

A significant improvement of the resolution of SPIM was achieved using silicon-on-sapphire substrates with anodically grown silicon dioxide as the insulator, by improving the optical setup for focussing laser light into the semiconductor substrate. The effects of spherical aberration were eliminated using a microscope objective with correction ring. Sub-micrometer resolution was achieved for the first time using a two-photon effect in silicon-on-sapphire substrates.

Apart from silicon and insulator structures, p-i-n photodiode structures in hydrogenated amorphous silicon (a-Si:H) were explored as substrates for SPIM. The use of a-Si:H was advantageous as it was predicted to provide good lateral resolution for SPIM because of the short diffusion length of charge carriers in this material. SPIM images with a resolution of 10 micrometers were recorded using n-i-p a-Si:H and SiO2 structures. This resolution was deemed sufficient for interrogating biosensor arrays. Another advantage of photodiode structures as substrates for SPIM was that there was no need to apply a Direct current (DC) voltage to the structure as the photocurrent was generated in the depletion layer formed between n-type and p-type materials. Hence, a good quality insulator with a small number of defects was not required to exclude leakage currents.

Furthermore, n-i-p a-Si:H and SiO2 structures were used to construct simple biosensor arrays based on the enzymatic degradation of thin films. A four-dot array consisting of a polymer suitable for the detection of a-chymotrypsin and the inert polymer cellulose acetate was deposited onto the n-i-p a-Si:H and SiO2 substrates. It was shown that SPIM was suitable for the interrogation of biosensor arrays. This sensor technology would be adapted in the future for the detection of multiple enzymes.

SPIM was not only useful for sensor applications but also as a research tool for the characterisation of new materials and biological samples. The increase in the sensitivity and resolution of the technique opened up a range of new areas of applications, such as high-throughput screening of the dielectric properties of new materials.


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