Enhanced stability of natural anthocyanin incorporated in Fe-containing mesoporous silica

• Natural anthocyanin was adsorbed on HMS type mesoporous silica containing metal ions.
• Photostability of the anthocyanin was enhanced by the adsorption onto Fe-HMS.
• The iron species in Fe-HMS promotes the adsorption of the anthocyanin in the mesopore.

Anthocyanin is known as a safe coloring material although its poor stability often limits more extended use. In this study, we tried the enhancement of the photostability of the anthocyanin by the incorporation into the pore of HMS type mesoporous silica containing various metal ions. The anthocyanin was adsorbed on the HMS by pouring a highly concentrated anthocyanin solution to the dried HMS, urging the incorporation of the dye into the mesopore by a capillary condensation. Improved stability of the anthocyanin against visible irradiation was achieved by the adsorption onto the HMS containing Fe3+, although other metal ions in the HMS showed less effect on the stabilization. From the UV–vis spectroscopy, the Fe3+ species effective for the stabilization of the anthocyanin was found to be the tetrahedral one highly dispersed in the silica framework. The decrease in the absorptivity of the anthocyanin on the Fe-HMS compared to other metal-containing HMS suggested that the Fe-HMS promoted the aggregate formation of the anthocyanin molecules. The highly dispersed Fe3+ species had an ability to gather the anthocyanin molecule close to itself, resulting in tight incorporation of the dye inside the mesopore. The incorporation and aggregate formation of the anthocyanin was expected to contribute to the improvement of the stability against irradiation.

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