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, t(g). 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 approximate to 1.5 nm are found to be present within the first 20 min of mixing. At pH between 1 and 0.8, irrespective of t(g), the particle size increases to a maximum radius of approximate to 4.5 nm after approximate to 2000 min. Lowering the pH in the range 0.8-0.15 produces a maximum radius of approximate to 3.5 nm. For all the sols studied, intra-particle syneresis then reduces the final particle radius by approximate to 0.5 nm within approximate to 6000 min. Although the bulk properties of the sol-gel change drastically at t(g), the probe detects little change in the microviscosity. (C) 2000 Elsevier Science B.V. All rights reserved.