A novel pH sensor with application to milk based on electrochemical oxidative quinone-functionalization of tryptophan residues

Abstract

The electrochemical oxidative quinone-functionalization of tryptophan (Trp) residues, and the proton-coupled electron transfer properties between quinonized Trp (Trp=O) and its re-reduced species (Trp-OH) are investigated for potentiometric pH sensors. The Trp=O/Trp-OH couples are assembled on a graphite electrode using multiple cyclic voltammetry, and exhibit a pair of well-defined redox peaks at redox potential of 0.090 V (vs. SCE) at pH 7.0 and 298 K. The proton-coupled electron transfer properties of Trp=O/Trp-OH couples are dependent on electrode substrate material, incubation temperature and pH values, and Trp residue sources including amino acid, peptide and protein. The Trp=O/Trp-OH-based surface-confined exothermic electrode reaction rates show a decrease with increasing incubation temperature from 283 to 323 K at pH 5.0. Both of redox peak potential and open-circuit potential under optimum conditions have a good linear pH dependence between 1.0 and 12.0 with sensitivity of more than 52 mV pH −1 and relative standard derivation of 1.5%. The proposed potentiometric pH sensor has significant advantages of low cost, good stability and reproducibility, and strong anti-interference ability. The present study shows a new approach to endow Trp, Trp-containing peptides and non-conjugated proteins with analogous proton-coupled electron transfer performance to prosthetic groups of quinoproteins for monitoring pH changes in milk samples.