Silver Thin Films for Ascorbic Acid Sensing

Ascorbic Acid (Vitamin C) is present in a large variety of commercial fruit juices. Their presence can cause changes in the chemical characteristics of the food, such as pH, total acidity and their levels change with storage. Hence, there is a need for having an adequate control and quantification of ascorbic acid to verify the authenticity and quality of the fruits used in food manufacturing. In our project, we suggest a thin film resistor based sensing of Ascorbic acid for such purposes. Copper Hexacyanoferrate (III) (Cu3[Fe(CN)6]2),a Prussian blue analogue is used as the sensing element. A thin film resistor is fabricated by thermally evaporating silver onto a cleaned glass substrate. The sensing element is then spin coated on the thin film. The dimensions of the resistor being 2cm*1.5cm*1-5µm with 1 cm gap between the leads kept by silver paste. The resistor was tested for the change in resistance with respect to different concentrations of Ascorbic acid in open circuit and closed circuit conditions using an electrometer. The working range of the sensor was found to be 0-0.5mg/ml and resistance drop from GΩ to MΩ was observed when biased with 50 mV. The drop in the resistance was correlated with spectrophotometric results. Reduction in absorbance with different concentrations of Ascorbic acid was observed at 473nm. The basis of detection is mainly the oxidation of Ascorbic acid to Dehydroascorbate in the presence of Copper Hexacyanoferrate, with the latter getting reduced to Cuprous Hexacyanoferrate (II).