Light-Activated Self-thermophoretic Janus Nanopropellers

TL;DR

The nanoscale propulsion problem is framed around Brownian diffusion hindering directed motion; the authors demonstrate fuel-free, optically driven self-thermophoresis in $Au-SiO_2$ Janus nanoparticles of radius $R \approx 33$ nm. Using single-particle tracking, they quantify active propulsion and report a Péclet number around $Pe \approx 1$, corresponding to a propulsion speed of approximately $v \approx 35$ µm/s under visible light. The long-time MSD follows $MSD(\Delta t) \approx [4 D_{eff}^{HBM} + v^2 \tau_R] \Delta t$ for $\Delta t \gg \tau_R$, enabling separation of hot Brownian motion from propulsion, and revealing an activity contribution up to about 50% of diffusion at high illumination. This minimal, fuel-free photothermal system provides a robust platform for studying nanoscale active matter and could impact nanomanipulation, nanomedicine, and fundamental nonequilibrium research.

Abstract

Achieving controlled and directed motion of artificial nanoscale systems in three-dimensional fluid environments remains a key-challenge in active matter, primarily due to the prevailing thermal fluctuations that rapidly randomize the particle trajectories. While significant progress has been made with micrometer-sized particles, imparting sufficient mechanical energy, or self-propulsion, to nanometer-sized particles to overcome Brownian diffusion and enable controlled transport remains a major issue for emerging applications in nanoscience and nanomedicine. Here, we address this challenge by demonstrating the fuel-free, reversible, and tunable active behavior of gold-silica (Au-SiO2) Janus nanoparticles (radius R=33 nm) induced by optical excitation. Using single particle tracking, we provide direct experimental evidence of self-thermophoresis, clearly distinguishing active motion from thermal noise. These light-driven Janus nanoparticles constitute a minimal yet robust photothermal system for investigating active matter and its manipulation at the nanoscale.

Figures (9)

• Figure 1: Transmission electron microscopy (TEM) image of Au-SiO$_2$ Janus nanoparticles. The inset displays a high-magnification view of an individual Janus nanoparticle, revealing the distinct contrast between the silica shell (light gray) and the Au nanobead (black).
• Figure 2: Nanoparticle size distribution determined by transmission electron microscopy (TEM). (a) Bare gold nanobeads (N = 116) and (b) Janus nanoparticles (N = 90). The gold nanobeads are nearly spherical, characterized by a diameter $d_{Au}$, while $d_{Janus}$ refers to the major axis of the Janus nanoparticles (see inset). Solid lines represent Gaussian fits to the measured size distributions; error bars on the particle diameter correspond to the standard deviation of these fits: $d_{Au} = 40.3 \pm$ 5.2 nm and $d_{Janus} = 66.4 \pm 7.2$ nm.
• Figure 3: Visible absorption spectra of Janus nanoparticles (blue line) and bare gold nanobeads (orange line). For comparison purposes, the optical density of each spectrum has been normalized to its respective maximum absorption value to account for the difference in sample concentration. The corresponding maximum optical densities are OD$_{max,Au}$ = 3.56 and OD$_{max,Janus}$ = 2.99.
• Figure 4: Schematic representation of the optical setup enabling both direct visualization of the sample via dark-field microscopy and its excitation using green laser illumination.
• Figure 5: Dark-field microscopy image of the Janus nanoparticles. A representative frame acquired during the experiments (see Supplementary Movie S1 in the SI), showing Janus nanoparticles in the absence of optical excitation. Although individual nanoparticles are subject to optical resolution limits, this does not prevent the precise determination of their center-of-mass position via single particle tracking (see Material and methods). The sample concentration is set in the dilute regime at c $\approx$ 3.6$\times$10$^9$ particles/mL.