2.3. Colorimetric detection
After the prepared tree-shaped strip were dipped in 6 mL of water for 30 min, different intensities of blue color could be observed. The different concentrations of BSA solution could be judged by eyes semiquantitatively or analyzed the pictures taken by camera or cell phone by Quantity One. In our experimental, camera and parameters are as follows: Olympus FE-210 digital camera with CCD sensor (Tokyo, Japan), maximum resolution 3072 × 2304 pixels, programmed auto exposure without flash light, the object distance is 30–40 cm. These photos were imported into the computer and color differences were analyzed by image software Quantity One (Bio-Rad company). Darkness density of each dot was picked to describe color difference among the volume data in Volume Analysis Report.