Optochemical sensors using electrodeposited polyaniline films: Electrical bias enhancement of reflectance response

• Electrodeposited PANI films were produced over FTO substrates.
• PANI pH reflectance optochemical sensors were developed.
• Protonation/deprotonation as a function of pH was monitored in the visible region.
• The optimal bias-enhanced sensitivity and linearity working point were achieved.

A bias-enhanced reflective optochemical pH sensor with electrochromic polyaniline is presented. Polyaniline films were prepared by electrodeposition process, controlling the total amount of deposited charge by varying the deposition time. The electrochromic property of polyaniline was the main parameter for sensor development. A quick and easy reflective spectra measurement process was obtained using a handset spectrophotometer in the visible region. A film deposited for 120 min presented a linear response (sensitivity of 25/pH and linearity of 97%) for the integrated reflectance in the green band of the spectrum after insertion in buffer solutions with pH ranging from 2 to 8. The reflectance intensity increased by 92% for the applied bias voltage of 1000 mV. Sensitivity of the sensors increased by about 28% (32/pH) with applied bias voltage of 700 mV, and decreased by about 9% (29/pH) with applied bias voltage of 1000 mV. A corresponding slight decrease in linearity by about 4% (93%) was observed with applied bias voltage of 700 mV, which preserves the linearity value within a good range for use. The linearity decreases by about 20% (78%) with applied bias voltage of 1000 mV. Both facts together indicate the presence of an optimal operation point. The improvement in sensor performance is related to the bias effect over polyaniline protonation/deprotonation. The electrochromic property of the polyaniline films were investigated using the CIE L*a*b* color scale. The data indicated changes from the emeraldine base oxidation state of polyaniline, which is blue, in alkali medium, to the emeraldine salt conducting, which is green, in acid medium. Handset spectrophotometers can be used with PANI optochemical pH sensors working with reflective spectra.