Concentration of colloidal silica suspensions using fluorescence spectroscopy

A simple detection method based on fluorescence excitation-emission spectroscopy has been developed to quantify the concentration of Ludox colloidal silica nanoparticles having diameters ranging from 7 to 22 nm. The technique works for suspensions of nominally negatively (SM30, LS, and TM40) and positively (CL) charged colloid silica particles. Without pretreatment, the method detection limits are 1.4 mg/L SiO₂ for CL (12 nm) and TM40 (22 nm) suspensions and 12 mg/L SiO₂ for SM30 (7 nm) and LS (12 nm) suspensions at an excitation wavelength of 308 nm and an emission wavelength of 318 nm. The fluorescence intensities of all the colloidal silica particle suspensions are linear (R² > 0.98) with concentration in the range of 0-300 mg/L SiO₂. The fluorescence intensity of the negatively charged particle suspensions is constant between pH 3 and 9. The fluorescence intensity of suspensions of particles that are nominally positively charged is constant above pH 5.5, where the particles become negatively charged.