In this paper,a simple and highly sensitive colorimetric probe was constructed for the determination of melatonin based on the etching of spherical Ag nanoparticles(AgNPs) by HAuCl4.The AgNPs could be oxidized and etched into Ag+ by HAuCl4,and the reduced Au was deposited on the surface of the etched AgNPs,resulting in the decrease of absorbance and the variation of solution color from yellow to orange.However,in the presence of melatonin,HAuCl4 was reduced by melatonin,inhibiting the etching of AgNPs,and resulting in an increase in absorbance and a decrease in color.Results showed that there existed a good linear relationship between the change values of absorbance(ΔA) and the logarithm values of melatonin concentration(lgC) in the range of 0.1 nmol/L-1.0 mmol/L,with a linear equation of ΔA=0.049 8+0.516lgC,a correlation coefficient(R2) of 0.996 4 and a detection limit of 0.09 nmol/L.This method was successfully applied to the determination of melatonin in human urine and grape with satisfactory results.