Salicylic acid and its derivatives are used in the pharmaceutical industry to synthesise medical drugs, and its intermediates are used to manufacture dyes, agrochemicals and perfumery products. Salicylic acid is also commonly encountered in the food industry as a preservative in milk and milk products to increase shelf life. However, it is illegal to do so since it may have adverse effects on human health. Salicylic acid impairs the digestibility of milk or may cause gastric irritation, bleeding, diarrhea and even death, especially in young infants, who are extremely sensitive to such irritants. The addition of small quantities of salicylic acid to milk either to improve its keeping quality or to delay its spoilage has been a problem for regulatory bodies from the early history of dairying [1]. Therefore, creating a valid, quick and easy-to-use system that could help detect and quantify this substance in various food samples is essential. A molecularly imprinted biosensor fits this criteria and could be an excellent tool for salicylic acid detection in milk samples. This study presents some aspects of molecularly imprinted biosensor development.
The typical molecular imprinting procedure consists of a few steps: 1) self-assembly of monomer, crosslinker, and template molecules to create pre-polymerization complexes; 2) chemical or electrochemical polymerisation; 3) removal of the template molecules from the obtained polymer, which generates the binding sites in the structure of the polymer, which are specific or complementary to the template molecules [2,3]. This characteristic is helpful for the development of sensors for specific detection.
Two methods have been used to synthesise the polymer required for the sensor: chemical and electrochemical. During chemical synthesis, the polymerisation of aniline was initiated with ammonium persulphate. The synthesised aniline polymer with molecularly imprinted salicylic acid (MIP) was used for spectrophotometric measurements to determine the interaction of the polymer with the template. The electrochemical synthesis was carried out in a three-electrode electrochemical cell (working electrode (WE) - indium tin oxide coated glass (ITO), counter electrode (CE) - platinum grid, reference electrode (RE) - Ag/AgCl \(_{3 M KCl}\)) by electrochemical pulse technique. To improve the adhesion of the polymer to the surface of the working electrode, ITO was activated with trisodium citrate before polymer deposition. The interaction of the MIP-coated ITO with salicylic acid was further investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV).