Nanometre resolution of silica hydrogel formation using time-resolved fluorescence anisotropy

We report a new approach to particle size measurement in sol–gels which is based on fluorescence anisotropy decay. The effect of siloxane polymerisation on silica particle size and microviscosity as a function pH and sodium silicate concentration is described from initial mixing with sulphuric acid to beyond the time to the macroscopic gel point, tg. The decay of near infra-red fluorescence anisotropy of a dye probe molecule is interpreted in terms of two rotational correlation times corresponding to solvated unbound dye and dye bound to silica particles. As polymerisation proceeds, increasing take-up of the dye from the liquid phase onto silica particles occurs. Silica primary particles of maximum mean hydrodynamic radius ≈1.5 nm are found to be present within the first 20 min of mixing. At pH between 1 and 0.8, irrespective of tg, the particle size increases to a maximum radius of ≈4.5 nm after ≈2000 min. Lowering the pH in the range 0.8–0.15 produces a maximum radius of ≈3.5 nm. For all the sols studied, intra-particle syneresis then reduces the final particle radius by ≈0.5 nm within ≈6000 min. Although the bulk properties of the sol–gel change drastically at tg, the probe detects little change in the microviscosity.