The development of a simple and low cost electrochemical impedance immunosensor based on screen printed gold electrode for rapid detection of Escherichia coli in water is reported. The immunosensor is fabricated by immobilizing anti-E. coli antibodies onto a gold surface in a covalent way by the photochemical immobilization technique, a simple procedure able to bind antibodies upright onto gold surfaces. Impedance spectra are recorded in 0.01 M phosphate buffer solution (PBS) containing 10 mM Fe(CN)63-/Fe(CN)64- as redox probe. The Nyquist plots can be modelled with a modified Randles circuit, identifying the charge transfer resistance Rct as the relevant parameter after the immobilization of antibodies, the blocking with BSA and the binding of E. coli. The introduction of a standard amplification procedure leads to a significant enhancement of the impedance increase, which allows one to measure E. coli in drinking water with a limit of detection of 3 × 101 CFU mL-1 while preserving the rapidity of the method that requires only 1 h to provide a "yes/no" response. Additionally, by applying the Langmuir adsorption model, we are able to describe the change of Rct in terms of the "effective" electrode, which is modified by the detection of the analyte whose microscopic conducting properties can be quantified.
Keywords: Escherichia coli; antibodies; cyclic voltammetry; electrochemical impedance spectroscopy; immunosensor; photochemical immobilization technique.