A stock solution of sulfide ions (0.1 mM) was prepared in water and stored at 4◦C in the refrigerator. Standard test solutions wereอprepared by appropriate dilution of the stock solution. For the detection of sulfide ions, 100 L of 1.2 mM AgNO3, and 10 L of Na2S solutions with different concentrations, 200 L of 1 mM TMB,was added sequentially in 690 L of 0.1 M NaAc buffer solutions(pH 4.0). After thorough mixing, the solution was incubating for at 30◦C for 5 min. Then, the solution was transferred for UV–vis scanning.