2.2. Colorimetric assay
Chromatography paper was firstly cut into tree-shaped sheets, with seven branches (3 mm × 45 mm) and a stem (8 mm × 55 mm), which was then immersed into 250 mM citric buffer solution (pH 1.8). The paper sheet was then allowed to dry at 25 °C for 20 min. After that, 0.5 μL of 9 mM BPB solution in ethanol and 0.5 μL of BSA solution were spotted on the sheet respectively (5 mm from the bottom of the branch for BPB and 20 mm from the bottom of the branch for BSA, a series concentration of BSA in PBS buffer were spotted on the left six branches and a BSA in the artificial urine [10] was spotted on the right one). The sheet was then allowed to be dried at 25 °C for 10 min. The procedure of preparation and detection is shown in Fig. 1.