Increasing Solution Viscosity in Dynamic Light Scattering to Lower the Detection Limit for the Determination of Nanoparticle Size

We report on the development of homebuilt Dynamic Light Scattering (DLS) instrumentation to measure the size of monodisperse (MD), spherical nanoparticles (NPs) of gold. HeNe and Ar-ion lasers constitute the excitation sources for the scattering experiment, while an avalanche photodiode detects the scattered light, and an autocorrelation card analyzes the resulting signal to provide a measurement of the translational diffusion coefficient, which allows for the determination of NP diameter. We characterized our instrumentation using commercially-produced gold NPs with diameters ranging from 50nm to 200nm in aqueous solution. Given the strong temperature-dependence of the solution viscosity, periodic ambient temperature measurements were used to produce dynamic values for viscosity and minimize uncertainty in the determination of NP size. Increasing the liquid viscosity slows down the Brownian motion of the NPs and affords measurement of smaller-sized particles that otherwise diffuse too fast for detection. Currently we are suspending NPs in higher viscosity solutions in an effort to lower our size detection floor from approximately 50nm to 10nm.