Periodic Reporting for period 1 - NanoscAM (Nanoscale active matter to power microstructures) Reporting period: 2020-12-01 to 2022-11-30 Summary of the context and overall objectives of the project Living systems are prototypical examples of active matter, which convert energy available in their environment into mechanical motion. This enables them to accomplish tasks well beyond the capabilities of our man-made machines: cells consume energy at the molecular scale to organize collectively at much larger scale. Such active matter has inspired the development of similar artificial units, in the will to both better understand the physical principles underlying the dynamics of living systems and to develop novel materials with life-like properties. While colloidal science has enabled progress in the field, the realization of nanomachineries from nanomotors remains a challenge. It would yet offers opportunities for the building of hierarchical materials from nanoscale components, in a cell-like fashion. Such development has been hindered by experimental challenges. First, it requires robust nanoscale building blocks synthesized in large quantities. Second, the motion of nanoparticles cannot be resolved with standard optical microscopy method and is hardly addressed due to their small size, well beyond the diffraction limit. In this context, the aim of NanoscAM was to tackle these challenges, with the following objective: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. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far 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. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) The realization of the project has enabled the creation of a new research group at the LOMA, and in France, on synthetic active matter driven at the nanoscale. The groundbreaking aspect of NanoscAM is the development of novel tools to guide organization across scale. In the short term, we expect first results on dense samples to be obtained, a preliminary base for the design of complex nano-machineries to drive larger microstructures. Harnessing nanoscale forces to program interactions from the bottom-up, NanoscAM will provide, in the longer term, new opportunities for the development of reconfigurable soft materials, made of micromachines that are made of nanomachines. Fundamentally, the aim of the newly formed group form at the LOMA is to study collective interaction between deformable and internally driven mesostructures. They indeed form model systems to better understand how active stresses are able to drive flows or shape deformations on macroscopic scales, a process akin to biological morphogenesis.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. Image of nano-heterodimers, scheme of their propulsion and analysis by light scattering.
