Flexible Self-Supporting Nanofibers Thin Films Showing Reversible Photochromic Fluorescence

Highly sensitive stimuli-responsive fluorescent films play an important role in smart sensors and readable optical devices. However, systems involving light-driven fluorescence changes are still limited compared with photochromic materials that simply change color upon photostimulation. Herein, by incorporation of stilbene-based molecules into a poly­(vinyl alcohol) host, we have developed new flexible self-supporting nanofiber films that exhibited fast and obvious photochromic fluorescence (PCF). The reversible transfer between two fluorescent states can be easily recycled. Fluorescence microscopy and atomic force microscopy images supplied in situ evidence of changes in fluorescence and surface morphology, respectively. Density functional theoretical calculations showed that the PCF can be attributed to photoisomerization of the stilbene-based molecules. Therefore, based on the combination of experimental and theoretical studies, this work not only supplies new stilbene-based systems with light-induced fluorescence change, but also gives detailed understanding on the photoisomerization and PCF processes of the nanofibers systems. We anticipate that these PCF films can be applied in erasable memory devices and antiforgery materials, and that our strategy may be extended to other systems to fabricate multistimuli-responsive fluorescent materials

Stereochemically Active Lone Pairs and Nonlinear Optical Properties of Two-Dimensional Multilayered Tin and Germanium Iodide Perovskites

Two-dimensional (2D) metal halide perovskites are promising tunable semiconductors. Previous studies have focused on Pb-based structures, whereas the multilayered Sn- and Ge-based analogues are largely unexplored, even though they potentially exhibit more diverse structural chemistry and properties associated with the more polarizable ns2 lone-pair electrons. Herein, we report the synthesis and structures of 2D tin iodide perovskites (BA)2(A)Sn2I7, where BA = n-butylammonium and A = methylammonium, formamidinium, dimethylammonium, guanidinium, or acetamidinium, and those of 2D germanium iodide perovskites (BA)2(A)Ge2I7, where A = methylammonium or formamidinium. By comparing these structures along with their Pb counterparts, we establish correlations between the effect of group IV-cation’s lone-pair stereochemical activity on the perovskite crystal structures and the resulting semiconducting properties such as bandgaps and carrier–phonon interactions and nonlinear optical properties. We find that the strength of carrier–phonon interaction increases with increasing lone-pair activity, leading to a more prominent photoluminescence tail on the low-energy side. Moreover, (BA)2(A)Ge2I7 exhibit strong second harmonic generation with second-order nonlinear coefficients of ∼10 pm V–1 that are at least 10 times those of Sn counterparts and 100 times those of Pb counterparts. We also report the third-order two-photon absorption coefficients of (BA)2(A)Sn2I7 to be ∼10 cm MW–1, which are one order of magnitude larger than those of the Pb counterparts and traditional inorganic semiconductors. These results not only highlight the role of lone-pair activity in linking the compositions and physical properties of 2D halide perovskites but also demonstrate 2D tin and germanium iodide perovskites as promising lead-free alternatives for nonlinear optoelectronic devices

Fast and Reversible Humidity-Responsive Luminescent Thin Films

Highly sensitive stimuli-responsive fluorescent films are playing an increasingly important role in the development of smart sensors and erasable optical devices. However, systems involving humidity-responsive fluorescence (HRF) are still very limited compared to those responsive to other common environmental stimuli (e.g., light, heat, pressure, or pH). Herein, by incorporating the 4-[4-(dimethylamino)­styryl]­pyridine chromophore into a polyvinylpyrrolidone host, we have developed new flexible self-supporting nanofiber films that exhibit fast and obvious HRF. The reversible transformation between two fluorescence states can be easily observed and recycled at least 200 times. Fluorescence microscopy images provided in situ evidence of changes in both fluorescence and morphology. This work therefore offers an alternative to conventional humidity sensors based on changes in color and electrical properties. Furthermore, we anticipate that these HRF films can also be employed as optical antiforgery materials

Study of Small-Molecule–Membrane Protein Binding Kinetics with Nanodisc and Charge-Sensitive Optical Detection

Nanodisc technology provides membrane proteins with a nativelike lipid bilayer and much-needed solubility and enables in vitro quantification of membrane protein binding with ligands. However, it has been a challenge to measure interaction between small-molecule ligands and nanodisc-encapsulated membrane proteins, because the responses of traditional mass-based detection methods scale with the mass of the ligands. We have developed a charge-sensitive optical detection (CSOD) method for label-free measurement of the binding kinetics of low molecular mass ligands with nanodisc-encapsulated membrane proteins. This microplate-compatible method is sensitive to the charge instead of the mass of a ligand and is able to measure both large and small molecules in a potentially high-throughput format. Using CSOD, we measured the binding kinetics between peptide and small-molecule ligands and a nanodisc-encapsulated potassium ion channel protein, KcsA-Kv1.3. Both association and dissociation rate constants for these ligands are obtained for the first time. The CSOD results were validated by the consistency of the values with reported binding affinities. In addition, we found that CSOD can tolerate up to 3.9% dimethyl sulfoxide (DMSO) and up to 10% serum, which shows its compatibility with realistic sample conditions

Tuning the Helicity of Self-Assembled Structure of a Sugar-Based Organogelator by the Proper Choice of Cooling Rate

A novel sugar-appended low-molecular-mass gelator, 4′′-butoxy-4-hydroxy-p-terphenyl-β-d-glucoside (BHTG), was synthesized. It formed thermally reversible gels in a variety of aqueous and organic solvents. Three-dimensional networks made up of helical ribbons were observed in the mixture of H2O/1,4-dioxane (40/60 v/v). The handedness of the ribbons depended on the rate of gel formation. Fast-cooling process led to right-handed ribbons, while slow-cooling process led to left-handed ones. A combinatory analyses of microscopic, spectroscopic, and diffraction techniques revealed that BHTG formed a twisted interdigitated bilayer structure with a d spacing of 3.1 nm in gels through a kinetically controlled nucleation−growth process. There were two kinds of molecular orientations of BHTG in the nuclei, clockwise and anticlockwise, which dictated the growth of ribbons. One was metastable and formed first during the cooling process of gel formation. It was able to gradually transform into the more stable latter one with further decreasing temperature. Fast-cooling process did not leave enough time for the nuclei to evolve from metastable to stable state and the ribbons grown from them exhibited right-handedness. However, the metastable nuclei transformed into the stable one when cooled slowly and directed the molecules of BHTG to grow into left-handed aggregates

Imaging Local Heating and Thermal Diffusion of Nanomaterials with Plasmonic Thermal Microscopy

Measuring local heat generation and dissipation in nanomaterials is critical for understanding the basic properties and developing applications of nanomaterials, including photothermal therapy and joule heating of nanoelectronics. Several technologies have been developed to probe local temperature distributions in nanomaterials, but a sensitive thermal imaging technology with high temporal and spatial resolution is still lacking. Here, we describe plasmonic thermal microscopy (PTM) to image local heat generation and diffusion from nanostructures in biologically relevant aqueous solutions. We demonstrate that PTM can detect local temperature change as small as 6 mK with temporal resolution of 10 μs and spatial resolution of submicrons (diffraction limit). With PTM, we have successfully imaged photothermal generation from single nanoparticles and graphene pieces, studied spatiotemporal distribution of temperature surrounding a heated nanoparticle, and observed heating at defect sites in graphene. We further show that the PTM images are in quantitative agreement with theoretical simulations based on heat transport theories

Detection of Charges and Molecules with Self-Assembled Nano-Oscillators

Detection of a single or small amount of charges and molecules in biologically relevant aqueous solutions is a long-standing goal in analytical science and detection technology. Here we report on self-assembled nano-oscillators for charge and molecular binding detections in aqueous solutions. Each nano-oscillator consists of a nanoparticle linked to a solid surface via a molecular tether. By applying an oscillating electric field normal to the surface, the nanoparticles oscillate, which is detected individually with ∼0.1 nm accuracy by a plasmonic imaging technique. From the oscillation amplitude and phase, the charge of the nanoparticles is determined with a detection limit of ∼0.18 electron charges along with the charge polarity. We further demonstrate the detection of molecular binding with the self-assembled nano-oscillators

Dual-Mode White Light Emissions from a Single Copolymer with an Ultralong Phosphorescence Lifetime

Polymeric white-light-emitting materials have received extensive attention due to their excellent flexibility, easy processing, and good thermal stability. However, to develop a kind of polymer with dual-mode white light emissions, especially those accompanying ultralong afterglows, remains greatly challenging. In this work, two copolymers with different feeding ratios of carbazole-dibenzofuran, 4-bromo-1,8-naphthalimide, and N-isopropylacrylamide are designed and synthesized through photo-polymerization upon ultraviolet (UV) irradiation. The obtained polymer P1 films exhibit an ultralong phosphorescence lifetime of 1.86 s with decent quantum efficiency. Notably, cold white light emissions are obtained under UV irradiation with different excitation wavelengths. After removing the irradiation source, excitation-dependent and time-dependent afterglows are engendered, including warm white afterglows with Commission Internationale de l’éclairage (CIE) coordinates of (0.31, 0.37). Interestingly, after introducing fluorine (F) atoms into the polymer films, standard white afterglows with CIE coordinates of (0.34, 0.33) are achieved, which originate from the blue delayed fluorescence and yellow phosphorescence. The dual-mode multicolor emission properties of the polymers hold great promise for advanced anti-counterfeiting applications

Detection of Charges and Molecules with Self-Assembled Nano-Oscillators

Detection of a single or small amount of charges and molecules in biologically relevant aqueous solutions is a long-standing goal in analytical science and detection technology. Here we report on self-assembled nano-oscillators for charge and molecular binding detections in aqueous solutions. Each nano-oscillator consists of a nanoparticle linked to a solid surface via a molecular tether. By applying an oscillating electric field normal to the surface, the nanoparticles oscillate, which is detected individually with ∼0.1 nm accuracy by a plasmonic imaging technique. From the oscillation amplitude and phase, the charge of the nanoparticles is determined with a detection limit of ∼0.18 electron charges along with the charge polarity. We further demonstrate the detection of molecular binding with the self-assembled nano-oscillators