Gold Nanoparticles II: Size and Zeta Potential with the Litesizer 500

Gold nanoparticles are promising materials for drug delivery, especially in gene targeting, and in carrier vehicles for diagnostic purposes. Depending on the procedure, gold nanoparticles can be synthesized in the size range of about 3 nm up to several hundred nm.

Particle size and colloidal stability are highly relevant for colloidal gold solutions; thus a fast and accurate way to analyse these two properties with a single device would be useful. Here we demonstrate that the Litesizer 500 can reliably determine particle size and zeta potential, even for dilute solutions of gold nanoparticles. Particle-size determinations with the Litesizer 500 are based on dynamic light scattering (DLS), which measures the time-dependent fluctuations in the scattered light intensity that occur due to the random movement of the particles. These fluctuations tell us the velocity of the particles, from the hydrodynamic diameter can be calculated using the Stokes–Einstein equation. In solution, the hydrodynamic diameter differs from the "true" diameter of gold nanoparticles, as determined by electron microscopy; in fact, the measured hydrodynamic diameter can be up to twice the diameter of the dry particles, especially when the particles are <20 nm.

The Litesizer 500 includes a patented technology called continuously monitored phase-analysis light scattering (cmPALS). Phase-analysis light scattering (PALS) is a well-known method for evaluating laser doppler electrophoresis measurements. PALS greatly improves the sensitivity of such measurements, which is important when analyzing the zeta potential of very small particles, such as gold nanoparticles. However, conventional PALS has a significant limitation: the modulator is non-linear, which restricts the application of the method to modulators with small movements and short measuring intervals. The new technology, cmPALS, overcomes this limitation, allowing the modulators to make large movements despite their non-linearity. This results in enhanced sensitivity and greater stability of laser doppler electrophoresis measurements. Therefore, measurements with shorter duration and lower applied electric field are now feasible with cmPALS.

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