A novel sensor for electrochemical pH monitoring based on polyfolate

Abstract

• Polyfolate can be successfully employed for pH monitoring in the interval from 6 to 9. • Electrode substrate has an influence on PFA performance and the best one was using pyrolytic graphite electrode. • Surface modification with chitosan extended stability of the sensor. • The on-shelf lifetime of the pH sensor is at least 2 months. A novel conducting polymer polyfolate (PFA) electrochemically synthesised on a pyrolytic graphite electrode (PGE) covered with chitosan (Chit) was used as a base to develop a pH sensor. A few approaches to electrode architecture were tested, and the optimal one has been obtained when Chit was cast on PGE and then PFA was polymerised from folate (FA) monomer in 0.1 M phosphate buffer saline solution of pH 6.0. A microscopic characterisation revealed that PFA is synthesised and forms clusters on Chit coated PGE. Electrochemical investigation with the redox probe demonstrated that diffusion is a limiting process at the bare PGE and Chit/PGE and the Chit coating increases the electrode electroactive area. Since PFA is a redox compound, no additional redox probe was needed for electrochemical characterisation and it revealed that the electrochemical process is adsorption limited. The surface coverage was determined to be from 52 to 183 nmol cm −2 depending on pH. A capacitive behaviour was observed from electrochemical impedance spectra at bare and modified electrodes in the negative potential region while at higher positive potentials, i.e. + 1.0 V, it was resistive due to charge transfer. A novel electrode sensitive to pH in the region from 6.0 to 9.0 finally was developed. PFA/Chit/PGE was sufficiently sensitive in the pH range studied with a sensitivity of 43.5 ± 1.5 mV/pH with the stability of over 70 days.