Toward Operando Dynamic Light Scattering During Ultrasonic Dispersion of Nanoparticles

An experimental approach is presented, which enables simultaneous ultrasonic dispersion (USD) and dynamic light scattering (DLS) measurements of colloidal nanoparticle dispersions. This USD-DLS scheme combines sonotrode-based ultrasonication of nanoparticles with time-resolved DLS data collection to study deagglomeration of nanoparticles. The capabilities of this operando approach are demonstrated using two types of nanoparticle systems: monodisperse polystyrene nanospheres and agglomerated titania nanocrystals. The influence of ultrasonication on the DLS signal is investigated using polystyrene particles as probes. During ultrasonication, the recorded autocorrelation functions (ACFs) mainly probe the formation and dynamics of cavitation bubbles in the liquid medium. Upon stopping of ultrasonication, acoustic streaming dominates the particle dynamics for a few seconds (<15 s), as revealed by the transition of the ACFs to equilibrium diffusive dynamics. Then, the particle size as a function of applied ultrasound energy can be determined. The potential of the operando USD-DLS approach to probe nanoparticle deagglomeration is demonstrated for titania nanoparticles of different levels of agglomeration from two synthesis methods. The application of a sequence of ultrasound pulses results in the continuous reduction of particle size as directly probed by DLS. Detailed deagglomeration curves are determined within a single data collection sequence. These data then form the basis for studying agglomerate stability as shown using two analysis models. In addition, it is demonstrated how the USD-DLS approach can be employed to study reagglomeration of nanoparticles under destabilizing conditions in situ.