We have measured for the first time the size of pores in the natural wet state during the formation of silica solgel ceramics using the quenching of fluorescent dyes which depends on the distance between the fluorescent dye and a quenching molecule. Our approach makes few assumptions, can be performed continuously in-situ and offers nanoscale resolution. The method is non-destructive and uses a pulsed source to stimulate fluorescence which then decays on the nanosecond timescale with a profile determined by the distribution of quenchers in the pore, thus reflecting the pore size. The pores are formed by the binding of silica nanoparticles and we have discovered a method of calibrating the particle size using standards and measuring it simply by means of the change in pH as the particles grow in size. The size of pores is found to typically have a radius in the region of a few nanometre (nm). The techniques we have developed are generic and can potentially be used on other porous solids as well as ceramics. Our findings are important stepping stones towards the goal of tracking and controlling the fundamental processes whereby a sol-gel ceramic is formed. This could well lead to the creation of new types materials with new physical and optical properties. The research contributed to the setting up of the joint Physics-Chemistry Centre for Molecular Nanometrology in Glasgow in 2005and subsequently helped attract the investment of a £5M Science and Innovation Award in nanometrology for research into molecular science, medicine and manufacture.
The project will provide a generic benefit by offering an improved approach to probing the in-situ morphology of wet porous solids in general, not just silica sol-gels, but also other ceramics such as clays, aluminosilicas, zeolites, shales, titania, sandstones and porous polymer resin supports etc. Specific benefits should accrue to the silica industry by better monitoring and control of the pore size in its products, enhancing product quality and reliability and to fluorescence lifetime instrument companies by the demonstration of a new application for such systems. Our research might also assist the production of new ceramics e.g. bio-compatible materials, photonic components, nano-powders etc where performance ultimately depends on measuring and controlling the nm morphology. Moreover, the determination of donor-acceptor distribution functions for fluorescence resonance energy transfer (FRET) in sol-gels should widen our understanding of this new approach to nano-structural determination and open up applications in other areas e.g. soft-solids such as liposomes, micelles and tissue and important bio-molecules such as proteins, indeed wherever FRET is used. Finally, there is a growing interest in understanding the photophysics of dyes in sol-gels in its own right for use in sensor systems.