Light-Activated Self-thermophoretic Janus Nanopropellers

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.

💡 Research Summary

The authors present a fuel‑free, reversible, and tunable nanoscale propulsion system based on light‑activated self‑thermophoresis of gold‑silica (Au‑SiO₂) Janus nanoparticles with a radius of approximately 33 nm. The particles consist of a spherical gold core partially coated with a porous silica shell on one hemisphere, creating an asymmetric thermal conductivity profile. Upon illumination with a 532 nm continuous‑wave laser, the gold core strongly absorbs light, generating localized heating. Because the silica cap impedes heat flow, a temperature gradient forms across the particle surface. This gradient drives the particle to move via self‑thermophoresis, i.e., the particle migrates along its own temperature field without any external chemical fuel.

The synthesis follows a two‑step protocol: (i) citrate‑reduced gold nanospheres (average diameter 40 nm) are prepared, then functionalized with polyacrylic acid (PAA) and 4‑mercaptophenylacetic acid (MPAA) to create chemically distinct hemispheres; (ii) tetraethyl orthosilicate (TEOS) is added to nucleate silica selectively on the MPAA‑rich side, yielding Janus dimers with a silica shell thickness of ~25 nm and an overall major axis of ~66 nm. Transmission electron microscopy confirms the morphology, and UV‑Vis spectroscopy shows a red‑shifted plasmon peak (530 nm → 540 nm) due to the silica coating and surface chemistry.

For propulsion experiments, the authors use a dark‑field microscope equipped with a high‑NA oil‑immersion objective (NA 0.6–1.3) and a 532 nm laser that is expanded and homogenized to illuminate a ~130 µm spot uniformly across the field of view. The laser power at the sample is varied from 0 to 81 mW. Particle concentrations are kept at ~3.6 × 10⁹ particles mL⁻¹, ensuring an average inter‑particle distance of ~6 µm, which eliminates collective effects. Single‑particle tracking (SPT) is performed at 476 frames s⁻¹; despite the particles being below the diffraction limit, sub‑pixel localization is achieved by fitting the diffraction pattern with a 2‑D Gaussian, yielding nanometer‑scale precision of the center‑of‑mass positions.

Trajectories are recorded as 2‑D projections of the full 3‑D motion. The authors compute the mean‑squared displacement (MSD) for each lag time τ and average over ~100 trajectories per laser power. In the absence of illumination, both Janus particles and bare gold nanoparticles exhibit similar diffusion coefficients (D_T ≈ 7.3 µm² s⁻¹), confirming that the experimental setup does not introduce spurious drift. With increasing laser power, the MSD slopes increase, indicating an enhanced effective diffusion coefficient D_eff. The authors attribute this to two contributions: (1) Hot Brownian Motion (HBM), where the particle’s elevated temperature reduces the surrounding fluid viscosity, and (2) a genuine self‑thermophoretic drift arising from the asymmetric temperature field.

To model HBM, they adopt a modified Stokes‑Einstein relation: D_HBM = k_B T_eff / (6π η_eff R), where T_eff ≈ T₀(1 + 5ΔT/12) and η_eff is the temperature‑dependent viscosity derived analytically (see supplementary information). Using the measured laser‑induced temperature rise ΔT (estimated from the absorbed power and thermal conductivity), they calculate D_HBM and find that it accounts for only part of the observed increase in D_eff. The residual enhancement is interpreted as the active self‑thermophoretic contribution.

The authors quantify the active propulsion speed v by fitting the MSD to MSD = 2n D_HBM τ + (v²)τ² (for 2‑D motion, n = 2) and extract v for each laser power. From v they compute the Péclet number Pe = v √(D_T D_R), where D_R is the rotational diffusion coefficient (D_R = k_B T / 8π η R³). Across the power range, Pe approaches unity (Pe ≈ 1), indicating that active and passive contributions are comparable—a regime rarely achieved for particles of this size. Importantly, Pe scales smoothly with laser intensity, demonstrating that the propulsion can be finely tuned by simply adjusting the illumination power.

The study provides several key advances: (i) a robust synthesis of sub‑100 nm Janus particles with well‑defined asymmetry; (ii) a clear experimental separation of hot‑Brownian and self‑thermophoretic effects using bare gold particles as controls; (iii) quantitative validation that light‑induced temperature gradients can generate sufficient mechanical work to overcome Brownian diffusion at the nanoscale; and (iv) a platform that is completely fuel‑free, reversible, and compatible with biological environments because it relies only on harmless visible light.

Potential applications include targeted drug delivery, where particles could be guided to specific tissue regions by focused light; nanoscale manipulation and assembly, exploiting the ability to switch propulsion on and off; and fundamental studies of non‑equilibrium statistical physics in the regime where thermal fluctuations and active forces are of comparable magnitude. Future work may explore different core‑shell materials, alternative wavelengths to optimize absorption, and in‑vivo experiments to assess biocompatibility and heating effects in complex media. Overall, this work establishes a minimal yet powerful photothermal nanomotor that bridges the gap between micrometer‑scale active colloids and truly nanoscale swimmers.