Multicolor colorimetric assay for copper ion detection based on the etching of gold nanorods

An effective, selective, and multicolor colorimetric assay for Cu²⁺ detection based on the regulation of peroxidase-like nanozyme-mediated etching of gold nanorods (Au NRs) is proposed. Cu²⁺-creatinine complex is selected as the nanozyme that exhibits excellent peroxidase-like activity even in the case of low concentration of Cu²⁺, which can catalyze 3,3,5,5-tetramethylbenzidine (TMB) to produce oxidized TMB (TMB⁺) in the presence of hydrogen peroxide, and TMB⁺ is oxidized to generate TMB²⁺ after adding H⁺, and the TMB²⁺ can etch Au NRs. The determination of Cu²⁺ is achieved based on the blue shift of the longitudinal localized surface plasmon resonance peak of Au NRs. Under the optimal conditions, the developed colorimetric assay exhibits high sensitivity for the detection of Cu²⁺ (limit of detection is 0.034 μM) with a wide linear range of 0.05-4.0 μM (R² = 0.987). The solution shows a rainbow-like color in response to the increase of Cu²⁺ concentration, which can realize the semi-quantitative detection of Cu²⁺ by naked eyes. In addition, the developed method exhibits excellent selectivity for Cu²⁺-detection. The established method was used for the determination of Cu²⁺ in lake water, soil, and normal human serum with satisfactory recovery of spiked samples.