Nanozyme colorimetric sensor array based on monatomic cobalt for the discrimination of sulfur-containing metal salts

The identification of sulfur-containing metal salts (SCMs) is of great interest because they play an important role in many biological processes and diseases. Here, we constructed a ternary channel colorimetric sensor array to detect multiple SCMs simultaneously, relying on monatomic Co embedded in nitrogen-doped graphene nanozyme (CoN₄-G). Due to the unique structure, CoN₄-G exhibits activity similar to native oxidases, capable of catalysing directly the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) by O₂ molecules independent of H₂O₂. Density functional theory (DFT) calculations suggest that CoN₄-G has no potential barrier in the whole reaction route, thus presenting higher oxidase-like catalytic activity. Based on different degrees of TMB oxidation, different colorimetric response changes are obtained as "fingerprints" on the sensor array. The sensor array can discriminate different concentrations of unitary, binary, ternary, and quaternary SCMs and has been successfully applied to detect six real samples (soil, milk, red wine and egg white). To advance the field detection of the above four types of SCMs, we creatively propose a smartphone-based autonomous detection platform with a linear range of 1.6-320 μM and a limit of detection of 0.0778-0.218 μM, which demonstrates the potential use of sensor arrays in the application of disease diagnosis and food and environment monitoring.