Studies of Electrochemical Polymerization of Thionine Using a Wavelength-interrogated Surface Plasmon Resonance Sensor

The electrochemical polymerization of thionine on gold layer was investigated by using a wavelength-interrogated surface plasmon resonance(SPR) sensor that operates at a fixed angle of incidence. The experimental results show that the resonant wavelength of SPR sensor linearly increases with increasing the number of voltammetry scanning cycles and the total redshift of resonance wavelength is 96.6 nm after 100 scanning cycles. The findings indicate that the film growth rate in every scanning cycle is constant and the poly(thionine) film formed after 100 cycles is ca. 71 nm thick based on the best fit of simulation results to the measured data. The poly(thionine) film in acidic buffer solution exhibits high electrochemical activity and its electrochemical reaction is diffusion-controlled. The resonant wavelength of SPR sensor reversibly changes in a voltammetry scanning cycle, indicating the complete reversibility of electrochemical oxidation-reduction reactions for the poly(thionine) film. Compared with the reduced poly(thionine) film, the oxidization of poly(thionine) film leads to a redshift of resonant wavelength, giving a sign that the refractive index of oxidized poly(thionine) film is higher than that of the reduced one.