Coloured hybrid materials: exploiting an emergent surface property of fluorinated Al₂O₃ containing anthocyanins and betacyanins

Two fluorinated γ-Al₂O₃ series were synthesized by a sol-gel method with two solvents (2-propanol and 2-butanol), two aluminium sources (ATB and ATP) and one fluorine source (Na₃AlF₆). The resulting inorganic matrixes were evaluated to characterize aluminium and fluorine species ([AlO₄⁵⁻], [AlO₅⁷⁻], [AlO₆⁹⁻], [AlF₄⁻], [AlF₅²⁻] and [AlF₆³⁻]) by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS-NMR) and infrared spectroscopy (IR-ATR). BET and BJH analyses using the nitrogen isotherms of these materials allowed identifying a clear trend in some textural parameters such as specific surface area and fluorine content. These results were confirmed by scanning electron microscopy (SEM). Chemical affinity and acid surface properties were evidenced with colour shifts in two groups of hybrid pigments, prepared with natural anthocyanins (Brassica oleracea) and betacyanins (Bougainvillea glabra).