Nanoparticle metrology of silicates using time-resolved multiplexed dye fluorescence anisotropy

Sodium silicates are versatile inorganic chemicals, manufactured by combining silica sand and soda ash under high temperatures. They are often used in coating and bonding applications when in aqueous solution. Additionally, they exhibit various attractive characteristics, such as being odourless, non-toxic, high strength and rigidity, resistant to high temperatures and overall low cost.

An important characteristic of silicates is the relationship between the ratio of silica to soda concentrations and the size of the species. A gold standard used to identify the presence of different oligomer species in sodium silicates is 29Si nuclear magnetic resonance spectroscopy, however, this method does not provide any quantitative data regarding the size of different species1 . Recently, there has been some promising development in determining the size and shape of different oligomer species using small-angle X-ray scattering (SAXS) and dynamic light scattering2 . Unfortunately, both of these methods are far from ideal and have significant drawbacks such as being inaccurate for particles under 10 nm size and being expensive and complex.

The alternative approach developed originally in our laboratory is based on the measurements of time-resolved fluorescence anisotropy of fluorescent probes, which utilizes the relationship between the silica particle size and the rate of the probe’s rotational diffusion when bound to the silica particle. This fluorescence technique offers high sensitivity and has an appropriate timescale due to the high variety of fluorescent probes available. This technique can be adapted to the specification of the medium to be researched3 . In this work we investigate the nanometrology of particle sizes in sodium silicate liquors at high pH using timeresolved fluorescence anisotropy. Instead of using a single dye label as in the previous approach, we investigate the advantages and limitations of multiplexing two fluorescent dye labels. Rotational times of the non-binding rhodamine B and adsorbing rhodamine 6G dyes are used to independently determine the medium microviscosity and the silicate particle radius, respectively. The recovered average particle size has an upper limit of 7.0±1.2Å, consistent with the SAXS measurement.