Multifunctional Janus microswimmers and case study of functional coating in self-thermophoresis

Abstract

Synthetic active matters are perfect model systems for non-equilibrium thermodynamics and of great potential for novel biomedical and environmental applications. However, most applications are limited by the complicated and low-yield preparation, while a scalable synthesis for highly functional microswimmers is highly desired. An all-solution synthesis method is developed where the gold-loaded titania-silica nanotree can be produced as a multi-functional self-propulsion microswimmer. By applying light, heat, and electric field, the Janus nanotree demonstrates multi-mode self-propulsion, including photochemical self-electrophoresis by UV and visible light radiation, thermophoresis by near-infrared light radiation, and induced-charge electrophoresis under AC electric field. Due to the scalable synthesis, the Janus nanotree is further demonstrated as a high-efficiency, low-cost, active adsorbent for water decontamination, where the toxic mercury ions can be reclaimed with enhanced efficiency. On the other hand, we investigate the mechanism of self-thermophoresis under NIR radiation by using the solution synthesized Black-TiO2@SiO2 matchstick as the template microswimmers. The thermophoretic motion could be manipulated by the tunable temperature gradient attributing to the black titanium radius and the NIR light intensity in combination with the feasible surface modification by silane chemistry. By applying functional polymer coating, the negative-thermophoresis motion of Black TiO2@SiO2 matchstick by coating crosslinked polyacrylic acid is observed and analyzed, which contradicts the generally studied electric double layer (EDL) mechanism accounting for the positive thermophoresis. The magnitude change of the thermophobic motion is also achieved after coating with polyethylene glycol amine (PEG-NH2) and poly(sodium-4-styrene sulfonate) (PSSNa). The sign and magnitude change of the thermophoretic motion by functional polymer coating are supported by the mechanism model of enthalpic contribution from the system and the measured energy variance (∆H) of the polymer and water including the electrostatic interaction by microcalorimetry. By tuning the environment temperature, the temperature dependence of the thermophoretic motion and the polymer-water interaction energy further support the proposed enthalpy mechanism. Our work of the case study of functional coating on self-thermophoresis would shed light on understanding and utilizing the complex but important thermal behaviours aimed for biomedical and nanotechnological application.