Water-soluble magnetic nanoparticles were synthesized using a method that combines co-precipitation, surface-modification, and decomposition of a hybrid hydrogel. The resulting Fe₃O₄ nanoparticles were analyzed by various analytical methods, including transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), superconducting quantum interference device (SQUID), and dynamic light scattering (DLS). The results showed that the average size of the synthesized Fe₃O₄ nanoparticles was 7.7 nm and their surfaces were grafted with water-soluble polymer chains. The zeta potentials of aqueous Fe₃O₄ solutions were measured as −34.1 mV, −68.3 mV and −60.5 mV at pH 5.3, 7 and 11.3, respectively. Furthermore, the stability of the magnetic nanoparticles was found to be good not only in aqueous media of different pHs but also in mixed solvents of water and hydrophilic organic solvents, including ethanol, methanol, acetone, dimethyl sulphoxide (DMSO), and dimethylformamide (DMF).

Light-responsive hydrogel actuators show attractive biomedical applications for in vivo drug delivery tool, surgical tissue repair operation, and vascular cleaning due to its non-contact, rapid, precise, and remote spatial control of light. Conventional visible–light–responsive hydrogels contain special chemical structure or groups, and the difficulty in synthesis results in that few can be applied to fabricate visible–light–driven hydrogel actuators. In this study, based on photothermal effect, surface-modified Fe₃O₄ nanoparticles were incorporated into poly(N-isopropylacrylamide-acrylamide) hydrogel by UV photopolymerization, which revealed excellent green–light–responsive volume change. Under a laser irradiation of 200 mW at 520 nm, the bending angle deformation of hydrogel strips with 2.62 wt% Fe₃O₄ reached 107.8°. Strip-shaped hydrogel actuators could be applied to transport tiny objects. Furthermore, a boomerang-like hydrogel actuator was designed and fabricated to drive floating foam on water. By 12 cycles of continuous laser on–off irradiation to a hydrogel actuator underwater, a circular returning movement of the float was accomplished. The study on driving a float using visible–light–triggered hydrogel actuators provides a new idea for the design of light-driven biomedical devices and soft robots.