Enhanced His@AuNCs oxidase-like activity by reduced graphene oxide and its application for colorimetric and electrochemical detection of nitrite

Enzyme-mimicking (nanozyme)-based biosensors are attractive owing to their unique catalytic efficiency, multifunctionality, and tunable activity, but examples of oxidase-like nanozymes are quite rare. Herein, we demonstrated that histidine-capped gold nanoclusters (His@AuNCs) possessed intrinsic oxidase-like activity, which could directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to blue colored ox-TMB without H₂O₂. The assembly of reduced graphene oxide (RGO) with His@AuNCs could further improve its oxidase-like activity and the His@AuNCs/RGO nanocomposites had a lower Michaelis constant (K_m) and higher catalytic constant (K_cat) for TMB oxidation. Furthermore, compared to other nanomaterials, the as-prepared His@AuNCs/RGO also exhibited enhanced electrocatalytic activity toward TMB. Interestingly, nitrite inhibited the catalytic (chromogenic) and electrocatalytic processes of His@AuNCs/RGO in the oxidation of TMB. The oxidase-like and electrocatalytic activity of His@AuNCs/RGO was evaluated with nitrite and TMB as substrates, and the results indicated that TMB and nitrite might share the same catalytic active sites. On the basis of these findings, a colorimetric and electrochemical sensor was developed with the His@AuNCs/RGO composite as an oxidase mimic for determination of nitrite with linear ranges of 10-500 μM and 2.5-5700 μM, respectively. The developed method was successfully applied to the detection of nitrites in real samples. The present work suggests that the oxidase-like nanozyme is not only suitable for colorimetric assay but also for development of electrochemical sensors in bioanalysis. Graphical abstract The colorimetric and electrochemical detection of nitrite using His@AuNCs/RGO.