Microarray sensors for detecting airborne explosives

Abstract

Due to the enhanced level of national security currently required due to the possibility of terrorist attack, monitoring devices for trace levels of explosive materials are now of the upmost importance. One such method that offers a possible route towards the development of a system for the detection of such analytes is via an electrochemical regime, coupled to the use of disposable sensor technology. Within this study, the use of modified carbon screen-printed sensors for the detection and analysis of such analytes of importance has been investigated. The modification of the base carbon substrate has been undertaken in a two-fold manner; firstly the incorporation of an enhanced electroactive mediator (Cobalt Phthalocyanine) has been investigated as an aid to facilitate the signal response and secondly the use of a novel surface modification technique to produce microelectrode arrays upon the carbon has also been employed. Microelectrodes hold intrinsic advantages over planar electrodes, such as stir independence, low detection limits and increased sensitivity due to their hemispherical diffusional profile. An array of microelectrodes can retain these properties whilst including the added advantage of enhancing the current response. The integration of these two approaches, the microelectrode array coupled with the mediated electrodes, has been developed with the ultimate objective to develop an accurate and sensitive detection system for trace quantities of explosives, namely 2,4,6-trinitrotoluene (TNT). This thesis describes work focussed towards the optimisation of each of the individual components involved in the formation of a sensing device for the detection and measurement of trace levels of explosive materials. In particular, factors and techniques that may facilitate the enhanced sensitivity of the measurement device are described. At every stage, each modification step was also undertaken with a suitable redox probe, ferrocenemonocarboxylic acid to allow for a quantitative assessment to be made. The use of unmediated and mediated carbon ink has been assessed in terms of suitability as a host material for the detection of TNT, with concentrations of 400 nM being measured on these base substrates. Further to this, microelectrode arrays were then formed upon these planar carbon surfaces via insulation with poly(phenylenediamine) coating and subsequent ultrasonic ablation. These thin film microelectrode arrays (~40 nm, pore population ~7.0 x 104 cm- 2 ) were also investigated in terms of response to TNT and were seen to offer an enhanced response in terms of signal differentiation. A final stage was then applied where the microelectrode array was further modified to incorporate a conductively grown polymer from the pore areas. Within this conductive growth, an enzyme/co-factor matrix specific to TNT was deposited which was seen to further increase signal responses, although displaying a lack of sensitivity at lower concentrations. As a final step the developed sensor methodologies were then used in conjunction with an airsampling system, the Coriolis®µ cyclone, to mimic the use of the sensors in realistic environments for practical employment. The sensors were used to successfully measure TNT samples from a concentrated stock sample of 4.4 mM collected via the cyclone technique.