Stimuli-Responsive Polyoxometalate/Ionic Liquid Supramolecular Spheres: Fabrication, Characterization, and Biological Applications

We report fabrication, characterization, and potential applications of polyoxometalate (POM)/ionic liquid (IL) supramolecular spheres in water for the first time. These supramolecular spheres have highly ordered structures and show excellent reversible self-assembly and tunable photoluminescence properties, which can be manipulated by adjusting pH of the aqueous solution. Specifically, the formation of POM/IL supramolecular spheres results in quenching of fluorescence emitted by Eu-POM because hopping of the d₁ electron in the POM molecule is blocked by hydrogen bond existing between the oxygen atom of POM and the carboxylic acid group of IL. However, the fluorescence can be completely recovered by gradually increasing pH of the aqueous solution due to the pH-induced deprotonation of the carboxylic acid group of IL, which results in disassembly of the fabricated supramolecular spheres. Applications of these stimuli-responsive photoluminescent POM-based supramolecular materials are demonstrated in biological media. Dual signaling responses of turbidity and fluorescence are observed simultaneously in the detection of urease and heavy metals based on pH-induced disassembly of the supramolecular spheres during the biochemical events in aqueous solution. In addition, guest molecules are encapsulated into the supramolecular spheres, and controlled release of these entrapped molecules is demonstrated in the presence of external stimuli. This study shows potential of stimuli-responsive POM/IL supramolecular materials in biological applications

Removal of Nitric Oxide through Visible Light Photocatalysis by g‑C₃N₄ Modified with Perylene Imides

For photocatalytic removal of nitric oxide (NO), two major issues need to be addressed: incomplete oxidation of NO and deactivation of the photocatalyst. In this study, we aimed to solve these two problems by constructing an all-solid-state Z-scheme heterojunction (PI-<i>g</i>-C₃N₄) consisting of g-C₃N₄ surface modified with perylene imides (PI). PI-<i>g</i>-C₃N₄ exhibits significant enhancement in photocatalytic activity (in comparison to pristine g-C₃N₄) when examined for NO removal. More importantly, the Z-scheme charge separation within PI-<i>g</i>-C₃N₄ populates electrons and holes into the increased energy levels, thereby enabling direct reduction of O₂ to H₂O₂ and direct oxidation of NO to NO₂. H₂O₂ can further oxidize NO₂ to NO₃^– ion at a different location (via diffusion), thus alleviating the deactivation of the catalyst. The results presented may shed light on the design of visible photocatalysts with tunable reactivity for application in solar energy conversion and environmental sustainability

Voltage-Induced Transients in Methylammonium Lead Triiodide Probed by Dynamic Photoluminescence Spectroscopy

In this work, we use time-resolved photoluminescence (PL) spectroscopy, microscopy, and current measurements to characterize the slow transient responses of methylammonium lead triiodide (MAPbI₃) on a lateral interdigitated electrode device. By systematically varying the applied bias magnitude and electrode polarity, we observed distinct reversible and irreversible PL transient responses in the form of spectrally and spatially resolved PL quenching occurring over a range of 0.5–100 s. When the simultaneous current and the PL measurements were correlated, the reversible responses, present under all electric fields, were attributed to charge trapping, whereas the irreversible response, occurring above a nominal electric field between 1 and 5 kV cm^–1, was attributed to ion migration. Thus, these results indicate that the slow transient response, and therefore hysteretic behavior, in MAPbI₃ devices is a complex response with contributions from both charge trapping and ion migration

One-Step Surface Doping of Organic Nanofibers to Achieve High Dark Conductivity and Chemiresistor Sensing of Amines

High dark electrical conductivity was obtained for a p-type organic nanofibril material simply through a one-step surface doping. The nanofibril composite thus fabricated has been proven robust under ambient conditions. The high conductivity, combined with the intrinsic large surface area of the nanofibers, enables development of chemiresistor sensors for trace vapor detection of amines, with detection limit down to sub-parts per billion range

Thermoactivated Electrical Conductivity in Perylene Diimide Nanofiber Materials

Thermoactivated electrical conductivity has been studied on nanofibers fabricated from the derivatives of perylene tetracarboxylic diimide (PTCDI) both in the dark and under visible light illumination. The activation energy obtained for the nanofibers fabricated from donor–acceptor (D–A) PTCDIs are higher than that for symmetric <i>n</i>-dodecyl substituted PTCDI. Such difference originates from the strong dependence of thermoactivated charge hopping on material disorder, which herein is dominated by the D–A charge-transfer and dipole–dipole interactions between stacked molecules. When the nanofibers were heated above the first phase transition temperature (around 85 °C), the activation energy was significantly increased because of the thermally enhanced polaronic effect. Moreover, charge carrier density can be increased in the D–A nanofibers under visible light illumination. Consistent with the theoretical models in the literature, the increased charge carrier density did cause decrease in the activation energy due to the up-shifting of Fermi level closer to the conduction band edge

Single-Molecule Charge Transport and Electrochemical Gating in Redox-Active Perylene Diimide Junctions

A series of redox-active perylene tetracarboxylic diimide (PTCDI) derivatives have been synthesized and studied by electrochemical cyclic voltammetry and electrochemical scanning tunnelling microscopy break junction techniques. These PTCDI molecules feature the substitution of pyrrolidine at the bay (1,7-) position of perylene and are named pyrrolidine-PTCDIs. These moieties exhibit a small bandgap (2.1 eV) compared with the “normal” (unsubstituted) PTCDI molecule (2.5 eV). Pyrrolidine-PTCDIs were functionalized with different anchoring groups (thiol, amine, pyridine) for building metal–molecule–metal (m–M–m) junctions. The single-molecule conductance values of pyrrolidine-PTCDIs have been determined by analyzing a large number of molecular (m–M–m) junctions created between an STM tip and substrate using a statistical method. Furthermore, we studied the gate dependence of the single-molecule conductance by trapping a molecule between the two electrodes and recording the current as a function of electrochemical gate potential. The experimentally determined conductance values for these bay-substituted pyrrolidine-PTCDI molecules are about twice as much as the unsubstituted PTCDI molecules. The present work shows that single-molecule conductance can be tuned by the bandgap of a molecular system without significantly altering the conductance pathway

Voltage-Induced Transients in Methylammonium Lead Triiodide Probed by Dynamic Photoluminescence Spectroscopy

In this work, we use time-resolved photoluminescence (PL) spectroscopy, microscopy, and current measurements to characterize the slow transient responses of methylammonium lead triiodide (MAPbI₃) on a lateral interdigitated electrode device. By systematically varying the applied bias magnitude and electrode polarity, we observed distinct reversible and irreversible PL transient responses in the form of spectrally and spatially resolved PL quenching occurring over a range of 0.5–100 s. When the simultaneous current and the PL measurements were correlated, the reversible responses, present under all electric fields, were attributed to charge trapping, whereas the irreversible response, occurring above a nominal electric field between 1 and 5 kV cm^–1, was attributed to ion migration. Thus, these results indicate that the slow transient response, and therefore hysteretic behavior, in MAPbI₃ devices is a complex response with contributions from both charge trapping and ion migration

Fluorescence Ratiometric Sensor for Trace Vapor Detection of Hydrogen Peroxide

Trace vapor detection of hydrogen peroxide (H₂O₂) represents a practical approach to nondestructive detection of peroxide-based explosives, including liquid mixtures of H₂O₂ and fuels and energetic peroxide derivatives, such as triacetone triperoxide (TATP), diacetone diperoxide (DADP), and hexamethylene triperoxide diamine (HMTD). Development of a simple chemical sensor system that responds to H₂O₂ vapor with high reliability and sufficient sensitivity (reactivity) remains a challenge. We report a fluorescence ratiometric sensor molecule, diethyl 2,5-bis­((((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)­benzyl)­oxy)­carbonyl)­amino)­terephthalate (DAT-B), for H₂O₂ that can be fabricated into an expedient, reliable, and sensitive sensor system suitable for trace vapor detection of H₂O₂. DAT-B is fluorescent in the blue region, with an emission maximum at 500 nm in the solid state. Upon reaction with H₂O₂, DAT-B is converted to an electronic “push–pull” structure, diethyl 2,5-diaminoterephthalate (DAT-N), which has an emission peak at a longer wavelength centered at 574 nm. Such H₂O₂-mediated oxidation of aryl boronates can be accelerated through the addition of an organic base such as tetrabutylammonium hydroxide (TBAH), resulting in a response time of less than 0.5 s under 1 ppm of H₂O₂ vapor. The strong overlap between the absorption band of DAT-N and the emission band of DAT-B enables efficient Förster resonance energy transfer (FRET), thus allowing further enhancement of the sensing efficiency of H₂O₂ vapor. The detection limit of a drop-cast DAT-B/TBAH film was projected to be 7.7 ppb. By combining high sensitivity and selectivity, the reported sensor system may find broad application in vapor detection of peroxide-based explosives and relevant chemical reagents through its fabrication into easy-to-use, cost-effective kits

Temperature-Controlled, Reversible, Nanofiber Assembly from an Amphiphilic Macrocycle

One-dimensional nanostructures are self-assembled from an amphiphilic arylene-ethynylene macrocycle (AEM) in solution phase. The morphology and size of the nanostructures are controlled by simply changing the temperature, reversibly switching between monomolecular cross-sectioned nanofibers and large bundles. At elevated temperature in aqueous solutions, the tri­(ethylene glycol) (Tg) side chains of the AEM become effectively more hydrophobic, thus facilitating intermolecular association through side chain interactions. The enhanced intermolecular association causes the ultrathin nanofibers to be bundled, forming an opaque dispersion in solution. The reported observation provides a simple molecular design rule that may be applicable to other macrocycle molecules for use in temperature-controlled assembly regarding both size and morphology

Atomic Scale Imaging of Nucleation and Growth Trajectories of an Interfacial Bismuth Nanodroplet

Because of the lack of experimental evidence, much confusion still exists on the nucleation and growth dynamics of a nanostructure, particularly of metal. The situation is even worse for nanodroplets because it is more difficult to induce the formation of a nanodroplet while imaging the dynamic process with atomic resolution. Here, taking advantage of an electron beam to induce the growth of Bi nanodroplets on a SrBi₂Ta₂O₉ platelet under a high resolution transmission electron microscope (HRTEM), we directly observed the detailed growth pathways of Bi nanodroplets from the earliest stage of nucleation that were previously inaccessible. Atomic scale imaging reveals that the dynamics of nucleation involves a much more complex trajectory than previously predicted based on classical nucleation theory (CNT). The monatomic Bi layer was first formed in the nucleation process, which induced the formation of the prenucleated clusters. Following that, critical nuclei for the nanodroplets formed both directly from the addition of atoms to the prenucleated clusters by the classical growth process and indirectly through transformation of an intermediate liquid film based on the Stranski–Krastanov growth mode, in which the liquid film was induced by the self-assembly of the prenucleated clusters. Finally, the growth of the Bi nanodroplets advanced through the classical pathway and sudden droplet coalescence. This study allows us to visualize the critical steps in the nucleation process of an interfacial nanodroplet, which suggests a revision of the perspective of CNT