A method for detecting glutaraldehyde in a water sample in which Au-BSA nanoclusters have been introduced is provided. In the method, the water sample is mixed with the Au-BSA nanoclusters to form a mixture. The mixture is incubated for 2 to 10 minutes, and the glutaraldehyde present in the mixture reacts with the Au-BSA nanoclusters and causes fluorescence quenching of the Au-BSA nanoclusters. Then a fluorescence intensity of the quenched Au-BSA nanoclusters in the mixture is measured at an emission wavelength of 675 nm. A presence and concentration of glutaraldehyde in the water sample is then determined by comparing the measured fluorescence intensity of the quenched Au-BSA nanoclusters at the emission wavelength of 675 nm with fluorescence intensity values of calibration samples comprising Au-BSA nanoclusters and known glutaraldehyde concentrations.