Stabilization of chlorophyll a in mesoporous silica and its pore size dependence

Chlorophyll a extracted from natural Spirulina was adsorbed by mesoporous silica, FSM. The amount adsorbed depended on the pore size. Few chlorophyll molecules were adsorbed by a usual silica gel and FSMs with pore sizes of less than 2 nm. However, more than 10 wt% of chlorophyll was adsorbed by FSMs with larger pore sizes. Chlorophyll dissolved in benzene was markedly discolored. On the other hand, a chlorophyll–FSM conjugate in water was much more photostable. The stability increases with increasing pore diameter of the FSM in the chlorophyll–FSM conjugate. The absorption maximum is 665 nm for the isolated chlorophyll a in benzene. The adsorption on the outer surface of silica gel and the mesoporous silica shifts the absorption maximum to 673 nm, and the adsorption in the mesopores of diameters of more than 2.5 nm shifts to 677 nm, which resembles the case of chlorophyll in natural leaves. We concluded that the photostability is attributable not only to the chlorophyll–FSM interaction but also to chlorophyll–chlorophyll interactions in the nanometer scale spaces.