Nanoparticle sizing by focused-beam dynamic ultrasound scattering method.

When nanoparticles in Brownian motion in liquid are irradiated with ultrasonic waves in the megahertz frequency range, scattering from the particles occurs, albeit at a very low intensity. The diffusion coefficient and the corresponding particle size can be calculated by analyzing the time correlation function of the ultrasound pulses. Since ultrasonic waves with long wavelengths in comparison with the particle size are unfavorable for detecting such small particles, increasing the energy of the ultrasonic waves is a primary solution. Increasing the energy, conversely, causes an unexpected acoustic flow. Consequently, a method using a strong ultrasound pulse while suppressing this acoustic flow field is required. In this study, we apply (1) a focused transducer with high ultrasonic energy, (2) a high-frequency sensor enhancing scattering performance, and (3) a short pulse repetition time to achieve high-speed and high-precision nanoparticle measurement while confining the sample in a narrow space to eliminate the acoustic flow. This enables direct tracking of nanoparticle motion by observing diffusive motion without perturbation particle dynamics. We specifically investigated a suspension of silica particles with a hydrodynamic radius of 15 nm and also achieved particle size discrimination in mixtures. The time correlation analysis using megahertz ultrasound pulses also takes advantage of ultrasonic waves' inherent advantages, such as high-speed, high-precision particle size analysis in the submicron range beyond the wavelength of visible light.