9.6.3.5　Silica NPs（SNs）

SNs could confine UV absorbents within a hollow silica matrix or precursors of UV filters to a silica matrix for chemically binding with UV absorbents.Their particle sizes vary from 10 nm to 10 μm.There is a new generation of UV absorbents based on the photochemical transformation of suitable precursors upon exposure to sunlight.The UV absorbents are generated from their precursors on demand，affording protection at the time，place and degree required，increasing their UV blocking ability as UV exposure is increased.The protective ability of these UV absorbents was shown to be further enhanced by encapsulation in SNs（average particle size was 100-1000 nm）as demonstrated for a silyl precursor of BP-3 that was copolymerized with alkoxysilane using sol-gel technology.The encapsulation improved the yield of transformation of the BP-3 precursor by increasing the absorbance of the final product 3-fold compared to non-encapsulated BP-3.From the chemical point of view，encapsulation in a rigid matrix is expected to improve the light conversion efficiency by restricting the mobility of the radicals formed in the initial steps of the reaction.Furthermore，chemical incompatibilities of the UV filter with other components of the sunscreen formulation are prevented by the physical barrier between them.From a biological point of view，confinement in the silica matrix should lead to a reduction in the toxicologic and allergenic potential of the UV filters as well.Finally，as silica is a natural and chemically inert material，a reduction of the environmental impact of such supramolecular sunscreens relative to molecular filters can also be expected，as long as the UV filter is not released.(https://www.daowen.com)