Strong nanozymatic activity of thiocyanate capped gold nanoparticles: an enzyme–nanozyme cascade reaction based dual mode ethanol detection in saliva

Abstract

This article reports on the strong nanozymatic activity of thiocyanide capped gold nanoparticles (TC-AuNPs). The terephthalic acid test confirmed the capabilities of hydroxyl (˙OH) radical formation by TC-AuNPs and their ability to enhance nanozymatic activity in the presence of 3,3′,5,5′-tetramethylbenzidine (TMB) and H₂O₂. The ˙OH radical scavenging ability of the thiocyanide group might play a vital role in the nanozymatic activity of TC-AuNPs. The synthesized TC-AuNPs had superior nanozymatic activity compared to chitosan-capped AuNPs, toward the TMB substrate. The kinetic nature of TC-Au NP nanozymes followed the classical Michaelis–Menten model, indicating that the TC-Au NPs had a higher affinity toward TMB than horseradish peroxidase (HRP) and other nanozymes. Novel colorimetric and electrochemical assays were also developed to detect ethanol (EtOH) in saliva. The main EtOH sensing mechanism was based on the formation of the ˙OH radical, which further oxidized TMB to form TMB²⁺, changed the colour of the solution due to the oxidation of TMB and enabled the quantification of EtOH spectroscopically. Alternatively, both H₂O₂ and TMB²⁺ were reduced at the electrochemical sensor surface to generate an electrochemical current signal correlated with the concentration of EtOH. Under the optimum conditions, the change in the colour of the solution and the generated current were used to detect the ultra-low level of EtOH, in the range of 0.02–0.3%, in saliva, which corresponds to the legal limit stipulated in many countries. The pioneering approach demonstrated in this study will open a new window in nanozyme-based biosensor and bioimaging applications.