Periodic Reporting for period 1 - NanoscAM (Nanoscale active matter to power microstructures)
Periodo di rendicontazione: 2020-12-01 al 2022-11-30
1) adapt a versatile synthesis method to develop nanomotors able to self-propel in a fluid,
2) develop a nano-optical imaging technique to probe the collective motion of nanomotors,
3) create a novel type of micromachine driven by ensemble of nanomotors.
Overall, the project aimed at addressing the physics of active matter at the nanoscale, where the influence of size reduction opened novel questions, while potentially enabling new routes for the bottom-up design of active materials.
Over the course of the project, we have successfully developed synthesis routes of nanomotors, with sizes ranging from 10 to 100 nm. Following, we have implemented nano-optical and optical imaging techniques to further observe the self-propulsion of nanoparticles and their collective motion, based on the analysis of their scattered light on a wide range of spatiotemporal scales. In particular, the development of the project has integrated nanoscale active matter as a central thematic in the host lab, enabling the formation of a research group around important preliminary results.
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.
Due to its fundamental nature, the project is not expected to have any socio-economic impact or wide social implications. The development of asymmetric motors could yet be interesting for industrials working in the field of Pickering emulsion (e.g. emulsion or foam stabilization using nanoparticles), or in the pharmaceutical and medical field, due to the potential of both nanomotors and micromachines to act as carriers for targeted drug delivery.