The authors describe a colorimetric method for the determination of copper ions (Cu2+) based on anti-etching of gold nanorods (AuNRs) induced by cuprous ions (Cu+). The formation of Cu+ from the reaction between Cu2+ and sodium ascorbate causes the deactivation of the horseradish peroxidase (HRP), which is used as the catalyst for the reaction of 3,3′,5,5’-Tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2), resulting in a decrease in the amount of TMB2+ product that can etch AuNRs, which is accompanied by the color changes from yellow to light orange. Thus, determination of Cu2+ is achieved on the basis of AuNR absorbance at 450 nm. A determination limit as low as 0.28 nM at the 3Sblank level, and the detection of 0.4 nM Cu2+ in solution can be achieved with naked eyes, and a dynamic range of 0 to 1.6 nM toward Cu2+ was obtained. Graphical abstractSchematic representation of a colorimetric method for determination of Cu2+ based on the anti-etching of gold nanorods induced by Cu+, which is from the reaction of Cu2+ with sodium ascorbate.