Colorimetric determination of cysteine and copper based on the peroxidase-like activity of Prussian blue nanocubes

Abstract

Prussian blue nanocubes were synthesized via a hydrothermal method. Significantly, the redox couple Ni³⁺/Ni²⁺ provided rich oxidation and reduction reactions, which enhance catalytic activity. Furthermore, PBNCs mimic peroxidase activity which could oxidise colourless tetramethyl benzidine (TMB) to a blue colour (TMB⁺) in the presence of H₂O₂. Thus, it can be used as a colorimetric sensing platform for detecting cysteine and Cu²⁺. The addition of cysteine to a TMB + PBNCs sensing system decreases the intensity of the blue colour in the solution with a decrease in the absorption peak at 652 nm in the UV visible spectrum. Subsequently, the addition of Cu²⁺ into the TMB + PBNCs + Cys sensing system increases the intensity of the blue colour due to complex formation of Cu and cysteine. Therefore, the change in intensity of the blue colour of TMB is directly proportional to the concentration of Cys and Cu²⁺. As a result, this sensing system is highly sensitive and selective with an effective low detection limit of 0.002 mM for cysteine and 0.0181 mM for Cu²⁺. Furthermore, this method was applied to the detection of cysteine and copper in spiked real samples and gave satisfactory results.