Dual-mode color-changing pH sensor based on fluorescent MOF embedded photonic crystal hydrogel

In this work, a novel pH-responsive hybrid hydrogel namely MOF/PC hydrogel was prepared by incorporating metal–organic framework CDs@UiO-66(OH)₂ into the matrix of photonic crystal hydrogel. MOF/PC hydrogel could display colorful structure colors under visible light and emit blue fluorescence under UV light owing to the stop-bond effect of photonic crystal and fluorescence emission capacity of CDs@UiO-66(OH)₂, respectively. Benefiting from these two optical phenomena, MOF/PC hydrogel could work as a dual-mode color-changing sensor, in structure color mode and fluorescence mode, for the sensing of pH. When incubating in aqueous solutions with different pH values, MOF/PC hydrogel could adjust its structure color to the corresponding one ranging from green to pink in 10 s, and this visible color-changing can be used to determine the approximate pH values of the sample by naked eyes without any assistance of instruments. As for the fluorescence mode, the fluorescence intensity of MOF/PC hydrogel showed a good linear relationship with pH value in the range of 3.0~7.0, and it can be used to determine the pH value of aqueous solutions quantitatively. Moreover, the as-prepared sensor is reusable and can be applied to the pH sensing of real water samples.