Yun Yan

Fabrication of Propeller-Shaped Supra-amphiphile for Construction of Enzyme-Responsive Fluorescent Vesicles

Propeller-shaped molecules have been recognized to display fantastic AIE (aggregation induced emission), but they can hardly self-assemble into nanostructures. Herein, we for the first time report that ionic complexation between a water-soluble tetrapheneyl derivative and an enzyme substrate in aqueous media produces a propeller-shaped supra-amphiphile that self-assembles into enzyme responsive fluorescent vesicles. The supra-amphiphile was fabricated upon complexation between a water-soluble propeller-shaped AIE luminogen TPE-BPA and myristoylcholine chloride (MChCl) in aqueous media. MChCl filled in the intramolecular voids of propeller-shaped TPE-BPA upon supra-amphiphile formation, which endows the supra-amphiphile superior self-assembling ability to the component molecules thus leading to the formation of fluorescent vesicles. Because MChCl is the substrate of cholinesterases, the vesicles dissemble in the presence of cholinesterases, and the fluorescent intensity can be correlated to the level of enzymes. The resulting fluorescent vesicles may be used to recognize the site of Alzheimers disease, to encapsulate the enzyme inhibitor, and to release the inhibitor at the disease site.

Out-of-Plane Coordinated Porphyrin Nanotubes with Enhanced Singlet Oxygen Generation Efficiency

A supramolecular porphyrin nanotube displaying J-aggregation feature was constructed by out-of-plane coordinated bismuth-porphyrin. Significantly, compared to traditional J-aggregated porphyrin suffering from fluorescence and singlet oxygen quenching, the nanotube exhibits excellent bio-imaging ability and enhanced production efficiency of singlet oxygen. The out-of-plane structure of bismuth to porphyrin makes the aggregation an appropriate material for theranostics. Furthermore, it is also a potential radio-therapeutic drug owing to the presence of radio-active bismuth. Thus, the self-assembly of out-of-plane coordinated porphyrin can be a facile approach toward effective therapy of tumors and other diseases.

White emission thin films based on rationally designed supramolecular coordination polymers

Fabrication of solid white luminescent materials is a challenging topic. In this work we report that upon combining the advantages of aggregation induced emission and reversible coordination polymers, solid white luminescent films with the CIE coordinates of (0.335, 0.347) and external quantum efficiencies of up to 11.74% can be achieved via layer-by-layer assembly. In particular, the ratio of the R(red) G(green) B(blue) elements can be rationally controlled via concentration, demonstrating the advantage of reversible coordination polymers in the fabrication of functional materials. The white emission of the solid thin film displays excellent stability against temperature, pH, and explosion materials, but exhibits specific detection for Cl2, suggesting its great potential for application as both a robust luminescent material and a chemical sensor.

Dye-sensitized photoelectrochemical water oxidation through a buried junction

Significance Artificial photosynthetic systems use molecular light absorbers to convert sunlight into useful chemical fuels such as hydrogen, methanol, or ammonia. A key bottleneck in these systems is the oxidation of water to produce O2. This anodic reaction occurs under strongly oxidizing conditions that result in damage to organic dyes, electron relays, and catalyst molecules. Here we show that encapsulation of the molecular components of a solid-state dye-sensitized solar cell by a 2-nm-thick coating of TiO2 dramatically improves cell stability under water-splitting conditions. The physical separation of the dye from the solution in which the water-splitting reaction takes place enables the use of dyes that efficiently absorb in the visible and allows optimization of the pH of catalytic water oxidation. Water oxidation has long been a challenge in artificial photosynthetic devices that convert solar energy into fuels. Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) provide a modular approach for integrating light-harvesting molecules with water-oxidation catalysts on metal-oxide electrodes. Despite recent progress in improving the efficiency of these devices by introducing good molecular water-oxidation catalysts, WS-DSPECs have poor stability, owing to the oxidation of molecular components at very positive electrode potentials. Here we demonstrate that a solid-state dye-sensitized solar cell (ss-DSSC) can be used as a buried junction for stable photoelectrochemical water splitting. A thin protecting layer of TiO2 grown by atomic layer deposition (ALD) stabilizes the operation of the photoanode in aqueous solution, although as a solar cell there is a performance loss due to increased series resistance after the coating. With an electrodeposited iridium oxide layer, a photocurrent density of 1.43 mA cm−2 was observed in 0.1 M pH 6.7 phosphate solution at 1.23 V versus reversible hydrogen electrode, with good stability over 1 h. We measured an incident photon-to-current efficiency of 22% at 540 nm and a Faradaic efficiency of 43% for oxygen evolution. While the potential profile of the catalyst layer suggested otherwise, we confirmed the formation of a buried junction in the as-prepared photoelectrode. The buried junction design of ss-DSSs adds to our understanding of semiconductor–electrocatalyst junction behaviors in the presence of a poor semiconducting material.

Self-Assembly-Triggered Cis-to-Trans Conversion of Azobenzene Compounds

Cis-to-trans transition of azobenzene compounds usually occurs under appropriate light irradiation or slow thermal relaxation, and one can hardly obtain complete cis-to-trans transition of azos due to the overlap of the n-π* transition of the trans and the cis isomers. We show that by viewing the photostationary state as a chemical equilibrium between the cis and trans isomers, triggered self-assembly of the trans isomers can promote the cis-to-trans transition, and trans azos with spectrum-grade purity can even be achieved using an elegantly designed coordinating azo. This work establishes a new paradigm for manipulating the cis-to-trans transition of azo compounds, which may inspire designs for various azo-based advanced materials.

Coordination-Triggered Hierarchical Folate/Zinc Supramolecular Hydrogels Leading to Printable Biomaterials

Printable hydrogels desired in bioengineering have extremely high demands on biocompatibility and mechanic strength, which can hardly be achieved in conventional hydrogels made with biopolymers. Here, we show that on employment of the strategy of coordination-triggered hierarchical self-assembly of naturally occurring small-molecule folic acid, supramolecular hydrogels with robust mechanical elastic modulus comparable to synthetic double-network polymer gels can be made at concentrations below 1%. A sequence of hierarchical steps are involved in the formation of this extraordinary hydrogel: petrin rings on folate form tetramers through hydrogen bonding, tetramers stack into nanofibers by π-π stacking, and zinc ions cross-link the nanofibers into larger-scale fibrils and further cross-link the fibril network to gel water. These supramolecular qualities endow the hydrogel with shear-thinning and instant healing ability, which makes the robust gel injectable and printable into various three-dimensional structures. Owing to the excellent biocompatibility, the gel can support cells three-dimensionally and can be used as an ideal carrier for imaging agent (Gd3+), as well as chemodrugs. In combination with its easy formation and abundant sources, this newly discovered metallo-folate supramolecular hydrogel is promising in various bioengineering technological applications.

Concentration-tailored self-assembly composition and function of the coordinating self-assembly of perylenetetracarboxylate

Perylene derivatives are excellent n-type dyes that display superior potential in optical and electronic materials. Significant efforts have been endeavored toward structural modification of the perylene skeleton. Herein, we showed that the simple coordinating self-assembly of perylenetetracarboxylate could already generate excellent one-dimensional perylene-based self-assembly, and its structural details could be simply tailored by concentration. Microbelts with high potential energy are formed at concentrations below 1 mM, whereas nanobelts with low potential energy appear at higher concentrations. The coordination stoichiometry was 1u2006:u20061 in the microbelt and 1u2006:u20062 in the nanobelt, which resulted in drastic material properties, such that the microbelt with 1u2006:u20061 coordinated PTC and Ni2+ displayed a conductivity 80 times that of the 1u2006:u20062 coordinated nanobelt, whereas the latter exhibited 4 times higher specific surface area, significant fluorescence, and polarized light transmittance. These findings not only unveil a scenario of the coordinating self-assembly of perylenetetracarboxylate, but also show that concentration can act as a powerful tool to tailor the function of self-assembled materials.

Understanding the Structure of Reversible Coordination Polymers Based on Europium in Electrostatic Assemblies Using Time-Resolved Luminescence

In situ characterization of the structure of reversible coordination polymers remains a challenge because of their dynamic and concentration-responsive nature. It is especially difficult to determine these structures in their self-assemblies where their degree of polymerization responds to the local concentration. In this paper, we report on the structure of reversible lanthanide coordination polymers in electrostatic assemblies using time-resolved luminescence (TRL) measurement. The reversible coordinating system is composed of the bifunctional ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4) and europium ion Eu(3+). Upon mixing with the positively charged diblock copolymer poly(2-vinylpyridine)-b-poly(ethylene oxide) (P2VP41-b-PEO205), electrostatic polyion micelles are formed and the negatively charged L2EO4-Eu coordination complex simultaneously transforms into coordination polymers in the micellar core. By virtue of the water-sensitive luminescence of Eu(3+), we are able to obtain the structural information of the L2EO4-Eu coordination polymers before and after the formation of polyion micelles. Upon analyzing the fluorescence decay curves of Eu(3+) before and after micellization, the fraction of Eu(3+) fully coordinated with L2EO4 is found to increase from 32 to 83%, which verifies the occurrence of chain extension of the L2EO4-Eu coordination polymers in the micellar core. Our work provides a qualitative picture for the structure change of reversible coordination polymers, which allows us to look into these invisible structures.

Combining superior surface enhanced Raman scattering and photothermal conversion on one platform: a strategy of ill-defined gold nanoparticles

We report a facile one-pot strategy of “ill-defined gold nanoparticles (ID-GNPs)” to prepare dual functional gold nanoparticles. The ID-GNPs were prepared in a simple way by tuning the morphology of the regular GNPs with a tiny amount of cetyltrimethyl ammonium bromide (CTAB). The prepared ID-GNPs comprise both quasi-spherical and elongated gold nanoparticles, which allow the presence of coarse surfaces and high density of hot spots. The substrates fabricated with ID-GNPs show dramatic Raman enhancement for p-aminothiophenol (PATP). PATP can be detected even at a concentration of 10−12 mol L−1. Meanwhile, these ID-GNPs can efficiently convert absorbed photons into heat when irradiated with 808 nm laser. The temperature of the ID-GNPs dispersion can be enhanced 26.0 °C within 10 minutes, which is capable of inducing the phase transition of a temperature-sensitive supramolecular system. Our study suggests that with the strategy of ‘ill-defined gold nanoparticles’, these two different functionalities can be achieved on one platform.

Allosteric Self-Assembly of Coordinating Terthiophene Amphiphile for Triggered Light Harvesting

Allosteric regulation is extensively employed by nature to achieve functional control of protein or deoxyribonucleic acid through triggered conformational change at a remote site. We report that a similar strategy can be utilized in artificial self-assembly to control the self-assembled structure and its function. We show that on binding of metal ions to the headgroup of an amphiphile TTC4L, the conformational change may lead to change of the dipole orientation of the energy donor at the chain end. This on the one hand leads to a drastically different self-assembled structure; on the other hand, it enables light harvesting between the donor-acceptor. Because the Forster resonance fluorescence transfer efficiency is gated by metal ions, controlling the feeding of metal ions allows switching on and off of light harvesting. We expect that using allosteric self-assembly, we will be able to create abundant structures with distinct function from limited molecules, which show prominent potential for the postorganic modification of the structure and function of self-assembled materials.