The methods currently being used for the detection and analysis of drugs of abuse such as cocaine, morphine, heroin, etc., are either instrument-based procedures, including chromatographic (i.e. TLC, GC, HPLC) and spectroscopic (IR, MS, NMR) techniques or immunoassay. The instrumental techniques are usually expensive, not readily amenable to on-site applications, and require pre-treatment and pre-concentration stages before analyte quantification. The immunoassay methods currently used for the analysis of drugs of abuse are mainly designed for analysis of urine samples, and include radioimmunoassay (RIA) and enzyme immunoassay (EIA) procedures. However, RIA involves the handling and disposal of radioactive residues. To make the immunoassay procedure more environmentally friendly, EIA and fluorescence-labelled immumoassay (FLIA) have been used in place of RIA. The drawback of EIA and FLIA is that the procedures usually involve separation steps, and often require secondary labelling and the use of secondary reactants. What is required is a sensor system that can detect as well as quantify the drugs(s) present in a sample in a variety of matrices. Another technique developed for detecting amino-containing drugs, is the use of ion-selective electrodes that employ selective membranes that respond to particular drugs². The problems with this type of potentiometric sensor is that the membranes also respond to various biologically active amines, and it is therefore possible to give false results with these electrodes. Thus there is an urgent need to develop new analytical tools that would be portable, reliable, selective and highly sensitive toward particular analytes. Biosensors are being developed to meet these requirements.

A biosensor consists of a biospecific sensing element that responds to a given property of the substance being sensed. The sensing element is usually in contact with or integrated within a suitable transducer. The biospecific sensing elements may be an enzyme, antibody, bio-membrane component or micro-organism. The transducer then detects the interaction of the biospecific element with the analyte. The electrical or optical signal requires amplification and processing; hence a signal processor is employed to convert this signal into a processable form. The biosensor is generally of small size and capable of continuous measurements. Generally, biosensors are very reliable means of measuring specific analytes faster and at lower cost than traditional methods.

Biosensors that use antibodies as the biospecific sensing elements are termed immunosensors. As an alternative to conventional immunoassay procedures (RIA,EIA and FLIA), considerable attention is now being given to the development of immunosensors³ that can provide continuous, in-situ, and rapid detection and quantification of analytes in samples. The development of electrochemical immunosensors (ECIS^{) 4,5} makes it possible to continuously monitor and detect a wide range of analytes. ECIS is particularly suited for situations where real time monitoring capabilities are required. Wheras the antibody-antigen (Ab-Ag) interaction conventional immunoassay methods may require a duration of several minutes to several hours, such interactions in ECIS are completed within microseconds to minutes.

A practical immunosensor for monitoring drugs of abuse, such as cocaine, morphine and amphetamines, must offer rapid, inexpensive and easy-to-use formats with reversible and continuous sensing capabilities, in order to replace the need for the chemical regeneration normally required at the antibody-containing surface in conventional immunoassay procedures. ECIS that operate in the potentiometric and amperometric modes⁶ have been developed to offer fast responses associated with voltammetric sensors. However, ECIS performance so far has shown that the generation of direct sensitive reproducible signals resulting from the Ab-Ag interaction is difficult. This problem arises from the lack of a faradaic signal and from the essentially irreversible nature of the Ab-Ag binding process. We have shown in a recent study, that the problem associated with the generation of rapid sensitive, and reversible Ab-Ag interactions, can be overcome by the use of antibodies immobilised in conducting electroactive polymer matrices⁷ such as those containing Os-polymers.

In this project, a multichannel sensor will be developed using the conducting electroactive poly(aniline) (PANI), in combination with a pulsed potential wave-form to enable selective molecular recognition and provide a unique solution to the detection of drugs of abuse. This novel signal generation technology will use the PANI film as the sensing electrode coupled with periodic pulsed voltage wave-forms. The voltage wave-form will induce changes in the PANI film such that a detectable interaction with a target analyte can be obtained in a reversible manner. The PANI film is particularly useful because of its high stability over a long period of time in both aqueous and organic phases8, and its ability to be doped with gaseous substances. For simultaneous detection of a variety of illicit drugs, a sensor array design will be constructed, based on thick film transducer screen printed technology, which in principle will enable the analysis of up to 12 different analytes in a single measurement. The principle is to modify each of the 12 sensor arrays with a PANI film which can be accurately deposited on a chosen electrode by electropolymerisation. The electrode surfaces will then be modified with enzyme-labelled monoclonal antibodies that are specific for particular drugs. The combination of the specificity of antibodies with the selective molecular recognitioin brought about by the unique use of pulsed potential wave-form, is designed to produce a reliable, fast responding sensor that is capable of identifying the analytes in a gaseous or liquid samples. The design of a multichannel potentiostat system to control the system has previously been described ^9,10. The electrochemical immunosensor technique that is proposed in based on amperometric detection, and therfore will provide more sensitivity than potentiometric sensors because of the linear relationship between current and concentration, unlike in potentiometric sensors where the potential and concentration have a logarithmic relationship. The multi-channel sensor technique for the detection of drugs of abuse will offer selective detection in a stable and reproducible manner, without electrode fouling or hysteresis effect in the detection data, because each sensor in the array will be individually addressed as the potential is pulsed. This detection method may be used in conjunction with fixed laboratory methods such as flow-injection analysis, liquid and ion chromatography, and capillary electrophoresis. Alternatively, the method may be used for in-situ field measurements when coupled to low-cost battery-operated electronic devices. Relevant interactions between the sensor and the target analytes can include specific adsorption and photometric effects or spectroelectrochemical effects producing colour changes. Thick-film technology utilising screen-printer transducers will be applied in the development of the sensor arrays. This technology is available in NIBEC at the University of Ulster, Newtownabbey, with whom we collaborate in the BEST CENTRE. The reason for using this technology is its low cost of production and the suitability of the method for mass production of the ECIS. The base transducer would be 12 channels of printed carbon or Pt working electrodes and a reference Ag/AgC1 electrode. At the BEST CENTRE of NIBEC, the work will be associated with the design and fabrication of screen –printed working and reference electrode channels, and electrochemical characterisation of the multi-channel electrode system, including impedance studies. Optimisation of mechanical properties of the screen-printed electrodes, such as adhesion, resistivity, surface roughness and wettability will also be performed there.

The electrosynthesis of the poly(aniline) electroactive film and the immobilisation of the relevant antibodies on the screen-printed transducers, would be carried out at DCU BEST CENTRE. The analytical examination of the detection limit, sensitivity, shelf-life, and reproducibility of the screen-printed electrochemical immunosensor and the application to real samples will also be carried out at the BEST CENTRE, DCU.

The time duration of this project is 24 months.