The first part of the project has focused on the realization of nanomotors able to self-propel in 3D. To this end, we have synthesized nano-heterodimers, having an intrinsic shape asymmetry that enables their propulsion by phoresis, an interfacial phenomena whereby a particle can move in a gradient of temperature (thermophoresis) or chemicals (diffusiophoresis). To this end, we have adapted two synthesis methods of Au/TiO2 and Ag/TiO2, and Au/SiO2 nano-heterodimers.
The first route has been developed in the host lab, and relies on the photodeposition of metallic precursors on TiO2 nanoparticles upon their absorption of UV light. We have studied the influence of physical parameters on the synthesis results, and could obtain nano-heterodimers with a 90% yield in bulk solution (size 50 nm).
The second route relies on the sol-gel synthesis (i.e. formation of solid particles from dissolved chemical species in a solution) of SiO2 particles, grown on metal (Au) substrates. The size of the metal seed was controlled before growing the SiO2, allowing us to reach large sizes of nano-heterodimers, up to 100 nm.
Following the synthesis, we have focused on a case-study of self-propulsion by diffusiophoresis, which uses the photocatalytic ability of TiO2 under UV light absorption to make the dimer propel. We have used a Dynamic Light Scattering setup, which analyzes the temporal fluctuations of light scattered by a sample of particles to obtain information on the dynamics of the particlese. By activating our nanomotors by UV light, we obtained preliminary results suggesting the self-propulsion of TiO2-based nanomotors, at speeds of a few thousands body-lengths per second, well above the ones reported for micromotors.
In the second part of the project, we have focused on designing optical methods to probe and control the motion of nanoparticles. To this end, we have implemented Dynamic Differential Microscopy (DDM), to extract the propulsion speed of nanomotors in the presence of high thermal noise. The work has mainly focused on simulated movies of dense and diffraction-limited nanoparticles. DDM has been shown effective even when time scale of active motion is smaller than the one of fluctuations.
Following, we have developed, from scratch, a state-of-the-art confocal microscope, enabling time-correlated spectroscopy at the single particle level and wide field optical imaging. Our aim was then to avoid the use of diffusiophoresis, which relies on fuel consumption. We thus aimed at developing thermophoretic propulsion, based on the laser heating of the metallic parts of the nano-heterodimers. To this end, we have included a laser heating line on the confocal microscope, made of two counter-propagating beams to control absorption and scattering forces. We probed the dynamics of nanoparticles from their individual, backscattered signal, at time scales ranging from nanoseconds to tens of seconds.
The setup has been tested on various types of nanoparticles following a detailed analysis of the scattering signal. We subjected gold nanoparticles (size 20-100 nm) to photothermal heating, and demonstrated a highly non-equilibrium behavior of the nanoparticles, which exhibited a much faster dynamics when heated than when at equilibrium. This points towards the possibility of having highly efficient nanomotors from nanojanus.
The project has been communicated through local workshop and seminars, despite the impact of the COVID pandemic. It has also been advertised to students on campus, enabling the recruitments of interns and of a PhD student, to pursue on the topic covered by NanoscAM.
