The acidity (pH) reading was directly examined by a digital pH meter (MP 220, Metler Toledo, Greifensee, Switzerland) of film solutions [40 (link)]. The turbidity of the films was diluted 50 times in PBS (0.01 M, pH 7.0) as a reference and evaluated at 600 nm with a UV-2550 ultraviolet-visible spectrophotometer (Shimadzu Co., Shanghai, China) [39 (link)].
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