2.1. Materials
Cellulose membrane (3 mm, CHR chromatography paper) was purchased from Whatman (Maidstone, England). Bromphenol blue (BPB) was supplied by Nanjing Bookman Biotechnology Ltd. BSA was obtained from Roche Ltd. Citric acid monohydrate was from Nanjing Chemical Reagent Ltd. Other reagents used were of analytical grade.