Surface characterisation of indium-tin oxide thin electrode films for use as a conducting substrate in DNA sensor development

Abstract

Indium tin oxide (ITO) films have over the past few years become a very important material for use as a conducting substrate in sensor development. Surface characterisation with strict control of deposition and surface chemistry of ITO films is essential for achieving reproducible conductive substrates. Usually a combination of different analytical techniques is used to monitor surface modification of these thin electrode films. ITO was deposited using an e-beam evaporator. Scanning Electron Microscopy and Atomic Force Microscopy have been applied to examine surface topography and determine surface roughness (0.5455 nm in root mean square, relative standard deviation 6%). The film is plasma treated in order to activate the surface further by creating a greater density of hydroxyl groups. Silanisation of the ITO surface using self-assembled monolayers is investigated with the aim of providing a platform for further covalent attachment chemistry. Quality control of the modified ITO films is verified based on sheet resistance (34 Ω/sq), % transmittance (>90% at wavelength >580nm) and contact angle measurements. The optimum conditions reported in the preparation of ITO thin films involved e-beam evaporation at a relatively slow rate and under relatively high pressure. Post annealing of the ITO film is detrimental to the surface uniformity Fourier Transform Infra-Red Spectroscopy and Cyclic Voltammetry are also described for analysis of the alkyl silane monolayers. Subsequent covalent attachment of a cross-linker and successful hybridisation of Cy3 labelled 2-deoxy-5-ribonucleic acid (DNA) to the immobilised complimentary DNA clearly demonstrates that surface chemistry modifications undertaken were successful. It also verified that ITO can be used as a substrate in the development of DNA sensors if the surface is firstly activated (plasma treatment) for attachment of surface chemistry. The development of such ITO based sensors has potential benefits as it can be used to bridge the gap between optical and electrochemical sensors, in that it offers the capability of dual detection, i.e. optical and electrical.