NED solution (1.0gL−1): 0.5g of NED (N-(1-naphthyl) ethy- lene diamine dihydrochloride, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was dissolved in 500mL water, stored in a brown glass bottle and stored at 4 ◦ C in a refrigerator while not in use. This solution was stable for 1 month.