Stability enhancement and color retention of blue anthocyanin incorporated in the mesoporous silica

Although blue color is commonly found in nature, natural blue pigments suitable for practical applications are scarce. Blue butterfly pea anthocyanin (b-AN) is a sparse natural blue dye. However, its blue color is unstable because anthocyanins are generally susceptible to color fading and pH-dependent color changes. In this study, to improve the stability and color retention of b-AN, it was incorporated into hexagonal mesoporous silica (HMS) type mesoporous silica, which was synthesized using a neutral amphiphilic detergent as a structure-directing agent. Upon the incorporation of b-AN into the HMS mesopore, the resulting b-AN/HMS material exhibited a reddish hue because of the acidity within the HMS pores. Additionally, b-AN/HMS demonstrated higher photostability than b-AN adsorbed on conventional silica owing to protection from ambient oxygen in air. To preserve the characteristic blue color, HMS was treated with a weakly alkaline sodium carbonate solution. The resulting composite of b-AN and alkaline-treated HMS maintained the blue color of b-AN and exhibited enhanced photostability compared with b-AN/silica. Although the acidic environment of untreated HMS provided marginally better inherent stability for anthocyanins, the alkaline treatment successfully achieved a balance between color retention and stability improvement. The stability of the system was further enhanced via the addition of gallic acid, which is a natural phenolic antioxidant, to the composite of b-AN and alkaline-treated HMS. This addition did not alter the blue color, whereas it substantially increased absorption retention to 85% after 3 h of visible irradiation, compared with the gallic acid–free material that exhibited a corresponding value of 40%, demonstrating a synergistic effect between the antioxidant and mesoporous structure. The as-synthesized novel composite, which is composed of butterfly pea dye, alkaline-treated HMS, and gallic acid, is a promising material for use as a safe, stable, and eco-friendly blue colorant for food and cosmetic applications. • Blue anthocyanin extracted from butterfly peas is incorporated into mesoporous silica. • Photostability of butterfly pea dye is improved via its incorporation into mesopores. • Characteristic blue color is maintained via the alkaline treatment of mesoporous silica.