DETECTION OF HEAVY-METAL IONS AT FEMTOMOLAR LEVELS USING PROTEIN-BASED BIOSENSORS

Sensors based on proteins (GST-SmtA and MerR) with distinct binding si tes for heavy metal ions were developed and characterized, A capacitiv e signal transducer was used to measure the conformational change foll owing binding. The proteins were overexpressed in Escherichia coli, pu rified, and immobilized in different ways to a self-assembled thiol la yer on a gold electrode placed as the working electrode in a potentios tatic arrangement in a flow analysis system. The selectivity and the s ensitivity of the two protein-based biosensors were measured and compa red for copper, cadmium, mercury, and zinc ions, The GST-SmtA electrod es displayed a broader selectivity (sensing all four heavy metal ions) compared with the MerR-based ones, which showed an accentuated select ivity for mercury ions. Metal ions could be detected with both electro de types down to femtomolar concentration. The upper measuring Limits, presumably due to near saturation of the proteins' binding sites, wer e around 10(-10) M. Control electrodes similarly constructed but based on bovine serum albumin or urease did not yield any signals. The elec trodes could be regenerated with EDTA and used for more than 2 weeks w ith about 40% reduction in sensitivity.