Synthetic microswimmers are a class of artificial nano- or microscale particle capable of converting external energy into motion. They are similar to natural microswimmers such as bacteria in behavior and are, therefore, of great interest to the study of active matter. Additionally, microswimmers show promise in applications ranging from bioanalytics and environmental monitoring to particle separation and drug delivery. However, since their sizes are on the nano-/microscale and their speeds are in the μm s(-1) range, they fall into a low Reynolds number regime where viscosity dominates. Therefore, new propulsion schemes are needed for these microswimmers to be able to efficiently move. Furthermore, many of the hotly pursued applications call for innovations in the next phase of development of biocompatible microswimmers. In this review, the latest developments of microswimmers powered by ultrasound are presented. Ultrasound, especially at MHz frequencies, does little harm to biological samples and provides an advantageous and well-controlled means to efficiently power microswimmers. By critically reviewing the recent progress in this research field, an introduction of how ultrasound propels colloidal particles into autonomous motion is presented, as well as how this propulsion can be used to achieve preliminary but promising applications.
The Taguchi method was used to optimize the phytochemicals mediated green synthesis parameters of silver (Ag) and gold (Au) nanoparticles. The ratio of respective metal ion precursor to Aegle marmelos leaf extract, plant based surfactants (Reetha, Acacia, and Shikakai), pH, temperature, and colour of the light source were chosen as significant parameters affecting the size of the nanoparticles. The Ag (~13 nm) and Au (~23 nm) nanoaprticles with narrow size distributions were produced after optimization of process parameters by the Taguchi method. Statistical analysis based on experimental results indicates that the pH and temperature have greater influence on the Ag nanoparticles size, whereas it is surfactant media and temperature for the Au nanoparticles. The square plate type nanoplates of Au with edge length of ~630 nm and width ~45 nm were also obtained by just changing the process parameters. This study shows the suitability of the Taguchi methodology in green synthesis and obtaining desired size nanoparticles for simple, one-pot, economical, and large scale production.
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