A thermal switch-activated variable-speed nanorobot with efficient removal of aqueous heavy metal ions.

Regulating the motion speed of photothermally driven nanorobots remains a significant challenge, as relying solely on a single thermal gradient often fails to provide sufficient driving force differentials for effective speed-switching. Herein, inspired by the thermo-responsive phase-change behavior of organics, we report a thermal switch-activated variable-speed nanorobot based on an aminophenol-formaldehyde concave asymmetric hollow nanobowl. By co-encapsulating ammonium bicarbonate as a chemical fuel and 1-tetradecanol as a phase change material, the designed nanorobot (denoted as AB/AF@TD) possesses a thermo-activated switch for speed control. Under 1.0 W/cm2 NIR irradiation, 1-tetradecanol provides an endothermic buffer that limits the photothermal temperature, resulting in a low-speed level driven primarily by self-thermophoresis. Conversely, higher-power NIR irradiation (2.0 W/cm2) triggers the phase transition of 1-tetradecanol, inducing rapid temperature elevation and the subsequent decomposition of ammonium bicarbonate. The generated microbubbles propel the nanorobot to a high-speed level through a synergistic effect with self-thermophoresis. Accordingly, AB/AF@TD exhibits three distinct motion levels containing Brownian motion, low-speed level (5.4 μm/s), and high-speed level (13.5 μm/s). The high-to-low velocity ratio is 2.5, significantly outperforming the 1.4 ratio of the aminophenol-formaldehyde control nanorobot (denoted as APFC). Benefiting from accelerated mass transfer and the chelating and redox activities of surface phenolic hydroxyl and amino groups, AB/AF@TD achieves 95.6% removal of 20 mg/L aqueous Cr(VI) within 30 min, a removal efficiency 1.2 times that of APFC. These results offer implications for the motion control of nanorobots and emphasize their potential in smart environmental remediation.