We demonstrated transferable and flexible resistive switching (RS) memory devices using a nondestructive water-dissolution method. To satisfy future demands, the free-standing Al/Fe3O4-PMMA/Al devices were transferred onto various nonconventional substrates to demonstrate various features, such as flexibility, 3-D conformality, and biocompatibility. Thanks to the strong van der Waals interaction, the devices can easily conform to these substrates and normally display RS behavior even after undergoing bending tests. In particular, the memory devices with the PET substrate present excellent memory performance as well as high flexibility, including fast switching speed (<50 ns), large ROFF/RON ratio (∼4 × 105), and long retention time (>104 s). No performance degradation occurs after bending the device to different angles and up to 104 times. The RS mechanism can be attributed to the trapping/de-trapping of electrons at the sites of Fe3O4 nanoparticles. This result provides a feasible approach to achieve transferable RS memory device for future conformal and flexible electronics.

Solvent plays a vital role in the syntheses, purifications, and broad applications of upconversion nanoparticles (UCNPs). In this work, the effect of various dispersive solvents, including single solvents and mixed solvents, on the luminescence properties of NaYF:Yb, Er UCNPs was studied systematically. The differences in both upconversion luminescence (UCL) intensities and color outputs of the nanoparticles were observed when dispersing the UCNPs in deuterium oxide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, or water. The attenuation of the excitation and emission light of the UCNPs caused by absorption of the solvents, as well as the high-frequency vibrational groups of the solvents, such as -OH, -CH, and -CH groups, are responsible for the decrease in UCL intensities and increase in the red to green emission intensity ratios (RGR). The changes in water or OH ion contents of ethanol/water mixed solvent triggered similar changes in UCL properties. Interestingly, the quenching of the solvents for the UCL cannot be fully eliminated by changing the dispersive solvents once the UCNPs have touched the solvents containing high-frequency vibrational groups. Our work will facilitate the comprehension of the solvent induced luminescence variations of the nanoparticles and provide guidance for their applications.

Highly sensitive, specific, and selective immunoassays are of great significance for not only clinical diagnostics but also food safety, environmental monitoring, and so on. Enzyme-linked immunosorbent assays and fluorescence-based and electrochemical immunoassays are important intensively investigated immunoassay techniques. However, they might suffer from low sensitivity or false-positive results. In this work, a simple, reliable, and ultrasensitive magnetic-bead-based immunoassay was performed using biofunctionalized ZnS semiconductor nanocrystals as resonant Raman probes. The resonant Raman scattering of ZnS nanocrystals displays evenly spaced multi-phonon resonant Raman lines with narrow bandwidths and has strong resistance to environmental variation due to the nature of the electron-phonon interaction, thus rendering reliable signal readout in the immunoassays. The superparamagnetic Fe3O4 nanoparticles facilitated greatly the separation, purification, and concentration processes. It is beneficial for both reducing the labor intensity and amplifying the detection signals. The immobilization of antibodies on the surface of magnetic beads, the preparation of resonant Raman probes, and the immunological recognition between the antibody and analyte all occurred in the liquid phase, which minimized the diffusion barriers and boundary layer constraints. All these factors contributed to the ultralow detection limit of human IgG, which was determined to be about 0.5 fM (∼0.08 pg/ml). It is nearly the highest sensitivity obtained for IgG detection. This work shall facilitate the design of nanoplatforms for ultrasensitive detections of proteins, DNAs, bacteria, explosives, and so on. Graphical abstract An ultrasensitive magnetic-bead-based immunoassay was performed using multi-phonon resonant Raman lines of ZnS nanoparticles as detection signals.

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The incorporation of magnetic and upconversion luminescent properties into one single nanostructure is highly desirable in nanomedicine for contrast agents and/or nanotheranostic platforms. Current magnetic upconversion luminescent nanocomposites generally suffer from relatively large size and/or low magnetization, which might induce unsatisfactory colloidal stability, reticuloendothelial system clearance, and limit their applications in biolabeling, sensing, imaging, bioseparation, magnetic targeting, and so on. Herein, we constructed multimagnetic-beads-embedded Fe3O4/NaYF4: Yb, Er nanocomposites to overcome these problems. Polyelectrolyte was introduced as an organic intermediate layer to offset the crystal lattice mismatch between Fe3O4 and NaYF4: Yb, Er. It also acted as the ligand to direct the growth of NaYF4: Yb, Er on the surface of Fe3O4. So-prepared nanocomposites exhibited an average size of 33.8 nm, much smaller than those with magnetic nanoparticle clusters as the core. The saturation magnetization of the nanocomposites is 17.8 emu/g, higher than those following current single magnetic nanoparticle embedded approach. To demonstrate their application potential in bioimaging and theranostics, magnetic field-assisted sensitive upconversion luminescence cell imaging is presented.

Magnetic-field-assisted sensitive upconversion luminescence imaging and enhanced photothermal therapeutic efficacy were achieved with multifunctional NaYF4:Yb,Er@PE3@Fe3O4 nanocomposites.

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