OPTIMIZATION OF AN ELECTROCHEMICAL SENSOR FOR DETECTION OF BACTERIAL POPULATION M. Berrettoni 1 , I. Carpani 1 , P. Conti 2 , G. Fumarola 1 , S. Lanteri 3 , E. Leoni 4 , P. Legnani 4 , D. Tonelli 1 1 Dipartimento di Chimica Fisica ed Inorganica e UdR INSTM, Università di Bologna, V.le Risorgimento 4, 40136 Bologna, Italy 2 Dipartimento di Scienze Chimiche, Università di Camerino, Via S. Agostino I-62032 Camerino (MC) Italy 3 Dipartimento di Chimica e Tecnologie Farmaceutiche e Alimentari, Università di Genova, Via Brigata Salerno (s/n), 16147 Genova, Italy 4 Dipartimento di Medicina e Sanità Pubblica, Università di Bologna, Via S. Giacomo 12, 40126 Bologna, Italy The estimation of microbial load is critical for food safety. Conventional detection procedures take more than 24-48 hours to obtain results, a time that is incompatible with on line quality control in food industry [1]. Rapid methods have been developed in recent years but they have some drawbacks, including lack of sensitivity, accuracy and/or specificity, and often require high cost instruments. In a previous paper [2] we described a home-made ultra-microelectrodes array which was used to record Normal Pulse Voltammetry (NPV) signals on suspensions containing different loads of Escherichia Coli and Pseudomonas Aeruginosa chosen as test microorganisms. The voltammetric measurements were coupled with a biased regression and variable selection method [3] (Partial Least Squares with Iterative Prediction Weighting, IPW-PLS) enabling us to rapidly correlate the aerobic plate microbial cell counting with the current signals. In the present work we describe the preliminary results obtained using Square Wave Voltammetry (SWV) in place of NPV measurements in order to verify if this technique allows for lower detection limits. At first, the method has been tested on Escherichia Coli ATCC 13706. The measurements were performed in a conventional three electrodes cell (SCE as reference and platinum wire as counter electrodes) using a CHI 620 electrochemical workstation. An ultra-microelectrodes array consisting of Reticulated Vitreous Carbon (RVC) filled with epoxy resin [2], was the working electrode. A parallel plate counting was performed in order to obtain the bacterial load expressed as colony forming units (CFU) ml -1 . Before each series of samples the working electrode was conditioned in a sterile solution with the same composition as that used for preparing the bacterial suspensions, by repeating the potential scan until almost superimposable curves were obtained (blank signal). The blank signal was also recorded at the end and during the measurements. SWV and NPV measurements were performed in a potential range –1.6 V to 1.6 V in anodic direction. The suspensions at different bacterial loads were prepared by serial dilutions in phosphate buffer of a liquid culture of Escherichia Coli in tryptic soy broth. The results obtained after the data treatment procedure were good and comparable for both electrochemical techniques; cross-validated prediction ability, with the leave-one out validation method, resulting greater than 95%. In particular SWV seems able to detect lower bacterial loads. SWV parameters i.e., potential increment (V), square wave amplitude (V) and square wave frequency (Hz) were then optimised with a full factorial experimental design 2 3 , 3 factors and two levels, plus the triplicate experiment at the central point. Each of the 11 experiments of