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Visual color transition of the MF-C6-Nf membrane at different pHs (pH 2, 3, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, and 9) under sunlight and images of reversible color of the MF-C6-Nf membrane when it was exposed to pH 4.5 and 7.5 under sunlight. Reproduced with permission from [119]. Copyright 2020©, Elsevier.
