K2hpo4

All chemicals were of analytical reagent grade and were used as received.
Distilled water (18.2 MΩ cm -1 ) was used throughout the current study. Oxytetracycline (OTC), tetracycline (TC), hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4• 4H2O), and streptavidin (SA) were purchased from Heowns Biochem Technology Co., Ltd. (Tianjin, China). Doxycycline (DC) was purchased from Meryer Chemical Technology Co., Ltd. (Shanghai, China). Hydroxylammonium chloride (NH2OH• HCl) was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). NaOH, K2HPO4, and metal salts including NaCl, KCl, MgCl2, CuCl2, MnCl2, FeCl3, and CaCl2 were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Phenylalanine (Phe), histidine (His), arginine (Arg), valine (Val), and isoleucine (Ile) were purchased from Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Bovine serum albumin (BSA) and calmodulin (CaM) were purchased from Sigma-Aldrich (Beijing, China). Other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).
A stock solution of HAuCl4 (1%, w/v) was prepared by dissolving 1 g HAuCl4 in 100 mL of ultrapure water and stored at 4°C. The preparation of 0.1 M of Mcllvaine-EDTA buffer (pH 4) was carried out by dissolving 5.9 g trisodium citrate dihydrate, 13.8 g Na2HPO4•12H2O, and 18.6 g Na2EDTA•12H2O in 500 mL of ultrapure water.