Cytotoxicity of nickel oxide nanoparticles(NiO NPs) with average diameter of 20 nm were investigated on cultured Chlorella vulgaris. Alga growth-inhibition tests were taken and ultrastructure changes of the microalgae were characterized with transmission electron microscopy(TEM). The biological interface conversion effect between NiO nanoparticles and Chlorella vulgaris were studied by X-ray diffraction(XRD), high-resolution transmission electron microscopy(HRTEM) and X-ray photoelectron spectroscopy(XPS). The results indicated that the NiO nanoparticles had severe inhibitory effect on the growth of microalgae, with a 96 h EC50 value of 31.4 mg/L. Under the exposure to NiO NPs suspensions, Chlorella vulgaris cells showed plasmolysis with a shriveled cell shape, disrupted plasma mem-brane, leaked cytosol and disordered thylakoid grana lamella. The NiO NPs were aggregated and partially reduced to Ni0 inside the Chlorella vulgaris. The bioaccumulation and bio-reduction ability of Chlorella vulgaris provide us with a possible strategy of remediation of aquatic pollution conducted by toxic metal oxide nanoparticles.

•Novel phosphomolybdic acid-based composite film was synthesized by entrapping PMoA particles into a silica sol–gel matrix and introducing ZnFe2O4 nanoparticles as visible light sensitizers.•Resulted composite film exhibited high visible light photochromic responsiveness as well as reversible photochromism.•Photo-generated electron transfer mechanism was identified due to the appearance of Mo5+ species during the photochromic process.By introducing ZnFe2O4 nanoparticles as light sensitizer into the silica sol–gel matrix, a novel phosphomolybdic acid (PMoA)-based composite film with high visible light photochromism was successfully prepared. The resulted thin film was denoted as PMoA/SiO2/ZnFe2O4. The microstructure, visible-light photochromic behaviors and mechanism were fully investigated. Characterizations revealed that the Keggin geometry of PMoA particles and structure of tetrahedral zinc ions were well retained in the film. With visible light irradiation, the colorless transparent PMoA/SiO2/ZnFe2O4 composite film turned to blue and exhibited high photochromic responsiveness as well as reversible photochromism. According to the XPS analysis, the appearance of Mo5+ species indicated the photo-reduction reaction occurred between PMoA naoparticles and ZnFe2O4 sensitizers suggesting the photo-generated electron transfer mechanism during the photochromic process.Novel phosphomolybdic acid-based composite film (denoted as PMoA/SiO2/ZnFe2O4) was successfully synthesized by entrapping PMoA particles into a silica sol–gel matrix and introducing ZnFe2O4 nanoparticles as visible light sensitizers. Remarkably, the resulted composite film exhibited high visible light photochromic responsiveness as well as reversible photochromism. Meanwhile, the photo-generated electron transfer was identified due to the appearance of Mo5+ species during the photochromic process.

•Ethyl cellulose was used as matrix to construct polyoxometalate-based hybrid film.•Reversible visible-light photochromism with high responsiveness was presented.•Proton transfer mechanism was identified during the photochromic process.Novel phosphomolybdic acid/ethyl cellulose hybrid film (denoted as PMoA/EC) was synthesized by entrapping PMoA particles into an EC matrix. The microstructure, visible-light photochromic behaviors and mechanism were fully investigated. The characterizations revealed that spherical PMoA particles dispersed uniformly in the hybrid film and their Keggin geometry were well preserved. The structure of EC was also maintained in the hybrid film after a composite process. Under visible light irradiation, the colorless transparent PMoA/EC hybrid film turned to blue and exhibited high photochromic responsiveness together with reversible photochromism in the presence of oxygen. The appearance of Mo5 + species in XPS spectra indicated that photo-reduction reaction has occurred between PMoA particles and the EC matrix and suggested proton transfer mechanism during the photochromic process. To the best of our knowledge, EC was used to construct polyoxometalate-based hybrid film with visible light photochromism for the first time.Novel phosphomolybdic acid/ethyl cellulose hybrid film (denoted as PMoA/EC) was successfully synthesized by entrapping PMoA particles into an EC matrix. Remarkably, the resulted hybrid film exhibited high visible light photochromic responsiveness together with reversible photochromism in the presence of oxygen. Meanwhile, the proton transfer mechanism was identified due to the appearance of Mo5 + species during the photochromic process.

The visible-light photochromic hybrid film was constructed by entrapping phosphomolybdic acid(PMoA) into polyvinylpyrrolidone(PVPd) networks. The microstructure, photochromic properties and mechanism were investigated with transmission electron microscopy(TEM), atomic force microscopy(AFM), Fourier transform infrared spectroscopy(FTIR), ultraviolet-visible spectra(UV-Vis) and X-ray photoelectron spectroscopy(XPS). The results indicate that the Keggin geometry of PMoA and the basic structure of PVPd are not destroyed during the composite process. Irradiated with visible light, the transparent PMoA/PVPd film changes color from colorless to blue and exhibits reversible photochromism in the presence of oxygen. According to the XPS analysis, the charge-transfer bridge of N-H-O has been built between PMoA and PVPd matrix via non-covalent bonding, and the appearance of Mo5+ species indicates that the photo-reduction process is in accordance with the proton transfer mechanism.

Novel UV-light and visible-light photochromic inorganic–organic multilayers composed of polyoxometalates (phosphomolybdic acid (PMoA)) and poly(acrylamide) (PAM) were prepared by the layer-by-layer (LbL) self-assembly method. The grown process, composition, surface topography, and photochromic properties and mechanism of the multilayer films were investigated by ultraviolet–visible (UV–vis) spectra, Fourier transform infrared spectra (FT-IR), atomic force microscopy (AFM), electron resonance spectra (ESR), and X-ray photoelectron spectra (XPS). Irradiation with UV-light or visible-light, the transparent films changed from colorless to blue and showed reversible photochromism. PMoA/PAM LbL films had higher photochromic efficiency under UV-light irradiation than visible-light irradiation. The bleaching process occurred when the films were in contact with O2 in the dark or heated in air. The photochromic process of PMoA/PAM LbL film was in accordance with radical mechanism.

Phosphomolybdic acid–reduced graphene oxide (PMoA–rGO) nanocomposites are fabricated by a photochemical reduction method. They are characterized by ultraviolet-visible spectra, scanning electron microscope images, Fourier transform infrared spectroscopy, Raman spectra and X-ray photoelectron spectroscopy in order to confirm that oxygen-containing functional groups on GO are replaced by PMoA and that GO is reduced to rGO in the photo-reduction process. The electrochemical properties of PMoA–rGO are investigated by cyclic voltammetry, which shows that the PMoA–rGO modified glassy carbon electrode has high electrocatalytic activity in acid solution via a fast, surface-controlled electron transfer process. The results indicate that the use of rGO not only increases the electroactive surface area, but also facilitates electron transfer due to its high electric conductivity.

•VPMA copolymer was copolymerized with VP and MA monomer.•PWA/VPMA was synthesized by introduced PWA into the VPMA copolymer.•The photo-reduction processes occurred according to the proton transfer mechanism.The composite film (PWA/VPMA) was synthesized by introduced phosphotungstic acid (PWA) into the poly(vinylpyrrolidone and methyl acrylate) (VPMA), which was copolymerized with vinylpyrrolidone (VP) and methyl acrylate (MA) monomer. The microstructure, thermal stability, photochromic properties and mechanism were investigated via atomic force microscopy, Fourier transform infrared spectroscopy, differential thermal analysis, differential scanning calorimetry, ultraviolet–visible spectra and electron resonance spectra. The results indicated that the Keggin geometry of PWA and the basic structure of VPMA were not destroyed in the composite process. After PWA particles were introduced into the VPMA, the thermal stability of composite film was lowered due to the interaction between PWA and VPMA. Irradiated with UV light, PWA/VPMA composite film changed from colorless to blue and showed reversible photochromism. The appearance of W5+ in ESR spectra indicated that the photo-reduction process between PWA and copolymer matrix occurred according to the proton transfer mechanism.

•pH had an effect on the microstructure and photochromic properties of composite films.•The intensity of absorbance was decreased with increase of pH after UV irradiated.•Responsiveness was related to microstructure’s change generated by different pH.A series of nanocomposite films with different pH were constructed by entrapped phosphomolybdic acid (PMoA) into polyacrylamide (PAM) networks. The structure, photochromic properties and mechanism of composite films were studied with transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), ultraviolet–visible spectra (UV–vis) and X-ray photoelectron spectroscopy (XPS). The results indicated that pH had a significant effect on the microstructure and photochromic properties of composite films. The TEM image showed that PMoA particles changed from well dispersed sphere-shape to irregular similar ball-shape with the increase of pH. XRD pattern showed that PMoA changed from crystal phase to amorphous phase with pH increased. The FT-IR results revealed that the octahedral structure of PMoA was destroyed with the increase of pH. PMoA/PAM films with pH 2 had higher photochromic efficiency than those films with other pH value. According to XPS, the different photochromic properties were caused by the microstructure change of the heteropoly compound with different pH value.

Phosphomolybdic acid–reduced graphene oxide (PMoA–rGO) nanocomposites are fabricated by a photochemical reduction method. They are characterized by ultraviolet-visible spectra, scanning electron microscope images, Fourier transform infrared spectroscopy, Raman spectra and X-ray photoelectron spectroscopy in order to confirm that oxygen-containing functional groups on GO are replaced by PMoA and that GO is reduced to rGO in the photo-reduction process. The electrochemical properties of PMoA–rGO are investigated by cyclic voltammetry, which shows that the PMoA–rGO modified glassy carbon electrode has high electrocatalytic activity in acid solution via a fast, surface-controlled electron transfer process. The results indicate that the use of rGO not only increases the electroactive surface area, but also facilitates electron transfer due to its high electric conductivity.