Silica nanoparticle metrology using Ursa Blue™ and colloidal Ludox solutions

• Ursa Blue™, a new fluorophore for nanoparticle metrology.
• Fluorescent probe strongly electrostatically binds to negatively charged nanoparticles.
• Long lifetime enables size to be determined using fluorescence depolarization.
• Ursa Blue is photostable, water soluble and has a high quantum yield.

We report spectroscopic properties of a new nanoparticle sizing fluorophore and demonstrate its applicability to characterize silica particles in colloidal solutions using steady-state anisotropy measurements and Perrin graph analysis. The strong electrostatic interaction between the cationic particle sizing fluorophore and the SiO2 surface is demonstrated by comparing data to that recorded for the anionic fluorophore fluorescein. The Perrin analysis reports a mean particle radius of 6.35 nm, which is larger than the manufacture specified radius of 3.5 nm, possibly indicating a degree of aggregation of the particles. The anionic fluorophore fluorescein shows no interaction with the SiO2 surface, and reports a hydrodynamic radius of 0.40 nm. We thus speculate that fluorescein can be used as an internal microviscosity probe for colloidal silica systems. The intensity averaged fluorescence lifetime of the particle sizing fluorophore and fluorescein are approximately 16 ns and 4 ns, respectively.