Tuning the photoluminescence, magnetism and cytotoxicity of ZnO by tailoring the nanostructures
Authors: Zhang Jian-Hui  , Han Ji-Gang  ,   National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China
Published: December 2020

ZnO nanomaterials have been extensively investigated for its broad applications such as room-temperature UV lasers, light-emitting diodes, solar cells, dilute magnetic semiconductors, bio-labeling, and target medicines. Tuning and optimizing the properties of ZnO nanostructures are urgent for the practical applications. Here, the photoluminescence, magnetism, and cytotoxicity of ZnO nanparticles have been effectively tuned by adjusting the nanostructures. Firstly, by developing the novel polyvinylpyrrolidone(PVP)-directed crystallization route, microwave heating-assisted forced hydrolysis method, and post-treating with surfactants, a series of high pure ZnO nanostructures including spheres, semispheres, rods, tubes, T-type tubes, tripods, wafers, gears, double layers, multilayer, capped pots, and bowls with tunable size and surface component/charge has been successfully prepared. The PVP can greatly promote the ZnO nucleation by binding water, and direct the ZnO growth by forming a variety of soft-templates and/or selectively capping the specific ZnO facet which is confirmed by the infrared absorption spectra. Secondly, the band-edge UV emission of ZnO has been greatly modified in both intensity and peak position by simply changing the sizes, shapes, and surface component of the ZnO nanoparticles. However, changing the surface charge of ZnO nanoparticles can only vary the intensity of the band-edge UV emission of ZnO. Significantly, the fluorescence of fluorescein isothiocyanate (FITC) is increased by up to 90 fold through doping the FITC molecules into the ZnO naoncrystals, which can effectively separate the FITC molelcules and avoid the energy transfer and the resulting fluorescence self-quenching. Thirdly, the room temperature ferromagnetism with tunable intensity is induced in the ZnO nanoparticles by coating them with different surfactants at different concentrations. As confirmed by the x-ray photoemission spectra, the coated surfactant molecules can donate electrons to the ZnO nanoparticles and induce the ferromagnetism. The electron number varies with the surfactant and the surfactant concentration, leading to the fluctuant ferromagnetism. The theoretical calculation further reveal the fluctuant nature of ferromagnetism in the ZnO nanoparticles coated with surfactants. This explains the previously reported seemingly irreconcilable ZnO ferromagnetism induced by capping surfactants, and provides a general chemical approach to tuning the ferromagnetism of ZnO nanoparticles by modifying the capping-surfactant concentration. Finally, it is revealed that the shape, size, surface charge/composition, and band-gap of ZnO nanostructures have different influences on the ZnO-induced cytotoxicity. The surface composition or adsorbed species of NPs may contain the toxic matter such as OH-ions that determine the NP-induced cytotoxicity, and should be detected before cytotoxicity assays are conducted. The rod-like NPs are more toxic than the spherical NPs. The positive surface charge can accelerate the nanoparticle-induced toxic action and enhance the cytotoxicity. Compared with the effects of shape and surface composition/charge, the influence of the nanoparticle-size variation on the nanparticle-induced cytotoxicity is less significant, and can be overwhelmed by other factors. These results will be conducible to the cytotoxicity assay and safe usage of ZnO NPs.