Yanjun Gong

Gels and Lyotropic Liquid Crystals: Using an Imidazolium-Based Catanionic Surfactant in Binary Solvents

The self-assembly behavior of an imidazolium-based catanionic surfactant, 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]), was investigated in water-ethylammonium nitrate (EAN) mixed solvents with different volume ratios. It is particular interesting that this simple surfactant could not only form lyotropic liquid crystals (LLC) with multimesophases, i.e., normal hexagonal (H1), lamellar liquid crystal (Lα), and reverse bicontinuous cubic phase (V2), in the water-rich environment but also act as an efficient low-molecular-weight gelator (LMWG) which gelated EAN-abundant binary media in a broad concentration range. The peculiar nanodisk cluster morphology of gels composed of similar bilayer units was first observed. FT-IR spectra and density functional theory (DFT) calculations reveal that strong H bonding and electrostatic interactions between EAN and the headgroups of [C4mim][C12H25SO4] are primarily responsible for gelation. The self-assembled gels displayed excellent mechanical strength and a thermoreversible sol-gel transition. It is for the first time that a rich variety of controllable ordered aggregates could be observed only by simply modulating the concentration of a single imidazolium-based catanionic surfactant or the ratio of mixed solvents. This environmentally friendly system is expected to have broad applications in various fields, such as materials science, drug delivery systems, and supramolecular chemistry.

Wormlike Micelles with Photoresponsive Viscoelastic Behavior Formed by Surface Active Ionic Liquid/Azobenzene Derivative Mixed Solution

The UV-light-stimulated self-assembly behavior of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), with an azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa), was investigated in aqueous solution. The properties and structures of the aggregates, formed at a concentration ratio equal to 2:1 ([C16mimBr]:[AzoCOONa]), were comprehensively characterized by rheometer and cryogenic transmission electron microscopy. Initially, viscoelastic wormlike micelles with a viscosity of 0.65 Pa·s were constructed in the C16mimBr/AzoCOONa system. Upon irradiation by UV light (365 nm), particularly fascinating is that the wormlike micelles become much longer and more entangled, exhibiting a high viscosity of 6.9 Pa·s. This can be attributed to photoisomerization of the AzoCOONa molecule from trans to cis form. It is the first time that, with exposure to UV or visible light, the aggregate type of the photoresponsive system has remained unchanged, with only a change of internal property parameters. The cation-π interaction prevailing over the hydrophobic interaction and electrostatic interaction between C16mimBr and AzoCOONa molecules is supposed to be responsible for this peculiar phase behavior. The wormlike micelles constructed with the SAIL and photosensitive additive exhibit controllable viscoelastic behavior in the photoresponsive process. In addition, the average contour length of wormlike micelles was found to slightly decrease with the increase of temperature. We expect this system will receive particular attention due to its unique properties and potential applications in drug delivery, biochemistry, and materials science, etc.

Fabrication of pH- and temperature-directed supramolecular materials from 1D fibers to exclusively 2D planar structures using an ionic self-assembly approach

Constructing multiple-response smart materials is a very interesting and challenging task in materials science. Here we employed an ionic self-assembly (ISA) strategy to fabricate pH- and temperature-responsive supramolecular materials with controllable fluorescence emission properties by using charged Congo red (CR) and an oppositely charged COOH-functionalized imidazolium-based surface active ionic liquid (SAIL), N-decyl-N′-carboxymethyl imidazolium bromide ([N-C10, N′-COOH-Im]Br). One-dimensional (1D) slender fibers were obtained in aqueous solution at pH 3.2 by the self-assembly of CR/[N-C10, N′-COOH-Im]Br (molar ratio = 1 : 2) at room temperature. Noteworthy is that two-dimensional (2D) planar structures, viz. bamboo leaf-like, spindly, discoid and rectangular structures, were then formed only by further changing the pH of the solution. Of particular interest is that the transition between 1D and 2D structures is pH reversible. We also found that the slender fibers could aggregate into fiber bundle structures with increasing temperature. In addition, fluorescence emission of the obtained 1D and 2D materials can be controlled by adjusting the morphologies of the aggregates. The electrostatic and hydrophobic interactions, in concert with π–π stacking between Congo red and [N-C10, N′-COOH-Im]Br molecules, were regarded as the main driving forces. The dimer-type π–π stacking existing among CR molecules was testified by DFT calculations.

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.

A polyoxometalate-based supramolecular chemosensor for rapid detection of hydrogen sulfide with dual signals

Hydrogen sulfide (H2S) has been verified as an important biological mediator in human physiological activities, but its rapid and accurate detection is remaining a challenge. Based on our early work, Eu-containing polyoxometalate/ionic liquid-type gemini surfactant hybrid nanoparticles fabricated by EuW10O36·32H2O (Eu-POM) and 1,2-bis(3-hexadecylimidazolium-1-yl) ethane bromide ([C16-2-C16im]Br2) via ionic self-assembly (ISA) strategy, we modified the hybrids with copper (II) ion and used them as a novel turn-off supramolecular fluorescence probe for H2S immediate response. Although copper (II) ions can cause decrease of the fluorescence intensity, the probe with moderate amount of copper (II) still has a high performance in emission property. The copper (II) ion-modified supramolecular sensor (CSS) shows dual signals in the fluorescence intensity and absorbance for H2S detection, and the detection limit is about1.25μM. Furthermore, CSS displays high selectivity for H2S in the presence of other anions and species (e.g. Cl-, Br-, I-, SO42-, SO32-, S2O32-, AC-, H2O2, HCO3-, l-cysteine, homocysteine and l-glutathione), and also have potential for preferential imaging in vivo. Besides, the fluorescence quenching mechanism of CSS in the presence of H2S was explored. CuS generated by the reaction between Cu2+ and H2S was testified to act as a quencher, and the nonradiative resonance energy transfer mechanism was speculated to be responsible for fluorescence quenching. It is anticipated that the as-prepared CSS will be used as an efficient chemosensor for the rapid detection of H2S, which is critical for the diagnosis of some diseases, e.g. Alzhermers disease, Downs syndrome, and diabetes, etc.

Photoluminescent Eu-containing polyoxometalate/gemini surfactant hybrid nanoparticles for biological applications

In water, Eu-containing polyoxometalate/gemini surfactant hybrid spheres with long emission timescale (3.758 ms) and high fluorescence quantum yield (25.17%) behaviors were synthesized. The hybrid spheres can be used as a bioprobe to image living cells.

Supramolecular structures ranging from nano- to macro-scale with fluorescent and organic semiconducting properties

Supramolecular structures ranging from nano- to macro-scale are prepared by an ionic self-assembly (ISA) strategy with commercially available, low-cost dyes and surfactants, viz. Rhodamine 6G (R6G) and sodium bis(2-ethylhexylhexyl) sulfosuccinate (NaAOT). The organic small compounds are employed to fabricate fibers with a length up to two centimeters via ISA. They exhibit strong strength and high flexibility and could be bent without damaging the original shape. It is noteworthy that this supramolecular complex also displays both remarkable solid-state red light emission and organic semiconducting behavior. It is extremely rare that the looped nanochains comprised of organic nanoparticles are found during the formation process of the supramolecular structures. A possible mechanism for the formation of the macroscopic fibers is proposed. The meso- and macro-scale fibers can be widely used in the development of multi-scale materials with high optoelectronic performance.