Xiaomin Liu

Covalently assembled NIR nanoplatform for simultaneous fluorescence imaging and photodynamic therapy of cancer cells.

A highly efficient multifunctional nanoplatform for simultaneous upconversion luminescence (UCL) imaging and photodynamic therapy has been developed on the basis of selective energy transfer from multicolor luminescent NaYF(4):Yb(3+),Er(3+) upconversion nanoparticles (UCNPs) to photosensitizers (PS). Different from popular approaches based on electrostatic or hydrophobic interactions, over 100 photosensitizing molecules were covalently bonded to every 20 nm UCNP, which significantly strengthened the UCNP-PS linkage and reduced the probability of leakage/desorption of the PS. Over 80% UCL was transferred to PS, and the singlet oxygen production was readily detected by its feature emission at 1270 nm. Tests performed on JAR choriocarcinoma and NIH 3T3 fibroblast cells verified the efficient endocytosis and photodynamic effect of the nanoplatform with 980 nm irradiation specific to JAR cancer cells. Our work highlights the promise of using UCNPs for potential image-guided cancer photodynamic therapy.

Understanding Structures and Hydrogen Bonds of Ionic Liquids at the Electronic Level

Due to their unique properties, ionic liquids (ILs) have attracted the academic and industrial attentions. However, recent controversies have focused on what are the main forces to determine the behaviors of ILs. In this work, a detailed DFT calculation was carried out to investigate the intermolecular interactions in two typical ILs, [Emim][BF(4)] and [Bmim][PF(6)]. The results indicate that hydrogen bonds (H-bonds) are the major intermolecular structural feature between cations and anions. Although the electrostatic force remains the major noncovalent force (70% of the total energy by energy decomposition calculation), the interaction energies calculated at different theoretical levels indicate that H-bond and van der Waals interactions cannot be ignored. However, the H-bonded capacities from natural bond orbital (NBO) delocalization energies do not show the consistent changes in the total interaction energies and number of H-bonds. Based on the canonical orbitals analysis, it is found that the σ-type orbital overlap and the partial charges transfer between anion and cation, finally, result in the significant energy reduction and rationalize the preferable location of anion, which is an essential understanding for the interaction and structure in the ion pair. Additionally, the strong agreement between the experimental IR spectra and the calculated vibrations implies that the structures of the larger ion clusters provide a reasonable depiction for bulk ILs at room temperature condition.

Insight into the Cosolvent Effect of Cellulose Dissolution in Imidazolium-Based Ionic Liquid Systems

Recently, it has been reported that addition of a cosolvent significantly influences solubility of cellulose in ionic liquids (ILs), but little is known about the influence mechanism of the cosolvent on the molecular level. In this work, four kinds of typical molecular solvents (dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), CH₃OH, and H₂O) were used to investigate the effect of cosolvents on cellulose dissolution in [C₄mim][CH₃COO] by molecular dynamics simulations and quantum chemistry calculations. It was found that dissolution of cellulose in IL/cosolvent systems is mainly determined by the hydrogen bond interactions between [CH₃COO](-) anions and the hydroxyl protons of cellulose. The effect of cosolvents on the solubility of cellulose is indirectly achieved by influencing such hydrogen bond interactions. The strong preferential solvation of [CH₃COO](-) by the protic solvents (CH₃OH and H₂O) can compete with the cellulose-[CH₃COO](-) interaction in the dissolution process, resulting in decreased cellulose solubility. On the other hand, the aprotic solvents (DMSO and DMF) can partially break down the ionic association of [C₄mim][CH₃COO] by solvation of the cation and anion, but no preferential solvation was observed. The dissociated [CH₃COO](-) would readily interact with cellulose to improve the dissolution of cellulose. Furthermore, the effect of the aprotic solvent-to-IL molar ratio on the dissolution of cellulose in [C₄mim][CH₃COO]/DMSO systems was investigated, and a possible mechanism is proposed. These simulation results provide insight into how a cosolvent affects the dissolution of cellulose in ILs and may motivate further experimental studies in related fields.

Effects of Cationic Structure on Cellulose Dissolution in Ionic Liquids: A Molecular Dynamics Study

In recent years, great progress has been made in the dissolution of cellulose with ionic liquids (ILs). However, the mechanism of cellulose dissolution, especially the role the IL cation played in the dissolution process, has not been clearly understood. Herein, the mixtures of cellulose with a series of imidazolium-based chloride ionic liquids and 1-butyl-3-methyl pyridinium chloride ([C(4)mpy]Cl) were simulated to study the effect that varying the heterocyclic structure and alkyl chain length of the IL cation has on the dissolution of cellulose. It was shown that the dissolution of cellulose in [C(4)mpy]Cl is better than that in [C(4)mim]Cl. For imidazolium-based ILs, the shorter the alkyl chain is, the higher the solubility will be. In addition, an all-atom force field for 1-allyl-3-methyl imidazolium cation ([Amim](+)) was developed, for the first time, to investigate the effect the electron-withdrawing group within the alkyl chain of the IL cation has on the dissolution of cellulose. It was found that the interaction energy between [Amim](+) and cellulose was greater than that between [C(3)mim](+) and cellulose, indicating that the presence of electron-withdrawing group in alkyl chain of the cation enhanced the interaction between the cation and cellulose due to the increase of electronegativity of the cations. These findings are used to assess the cationic effect on the dissolution of cellulose in ILs. They are also expected to be important for rational design of novel ILs for efficient dissolution of cellulose.

Multiscale Studies on Ionic Liquids

Ionic liquids (ILs) offer a wide range of promising applications because of their much enhanced properties. However, further development of such materials depends on the fundamental understanding of their hierarchical structures and behaviors, which requires multiscale strategies to provide coupling among various length scales. In this review, we first introduce the structures and properties of these typical ILs. Then, we introduce the multiscale modeling methods that have been applied to the ILs, covering from molecular scale (QM/MM), to mesoscale (CG, DPD), to macroscale (CFD for unit scale and thermodynamics COSMO-RS model and environmental assessment GD method for process scale). In the following section, we discuss in some detail their applications to the four scales of ILs, including molecular scale structures, mesoscale aggregates and dynamics, and unit scale reactor design and process design and optimization of typical IL applications. Finally, we address the concluding remarks of multiscale strategies in the understanding and predictive capabilities of ILs. The present review aims to summarize the recent advances in the fundamental and application understanding of ILs.

Effects of anionic structure on the dissolution of cellulose in ionic liquids revealed by molecular simulation

Although ionic liquids (ILs) have shown promise in the pretreatment of cellulose and lignocellulosic biomass, there is no established rule to guide the rational design of such ILs up to date. In this work, the mixtures of cellulose with a series of ILs having the same cation [C2mim](+) but different anions have been simulated to study the effect of anionic nature on the dissolution of cellulose. It was shown that hydrogen bonds (HBs) were formed between anions of the ILs and hydroxyl protons of cellulose. Cl(-) anion and O atom of [CH3COO](-) and [(CH3O)2PO2](-) are better HB acceptors. Furthermore, the effects of electronegativity of HB acceptor atoms, steric effect of alkyl chain length and electron-withdrawing group of the anions on their HB acceptor ability have been investigated. The obtained results are expected to be important for the rational design of novel ILs for efficient dissolution of cellulose.

High conductive gate leakage current channels induced by In segregation around screw- and mixed-type threading dislocations in lattice-matched InxAl1−xN/GaN heterostructures

A correlation between microstructures and high gate leakage current density of Schottky contacts on lattice-matched InxAl1−xN/GaN heterostructures has been investigated by means of current-voltage measurements, conductive atom force microscopy (C-AFM), and transmission electron microscopy (TEM) investigations. It is shown that the reverse-bias gate leakage current density of Ni/Au Schottky contacts on InxAl1−xN/GaN heterostructures is more than two orders of magnitude larger than that on AlxGa1−xN/GaN ones. C-AFM and TEM observations indicate that screw- and mixed-type threading dislocations (S/M-TDs) are efficient leakage current channels in InxAl1−xN barrier and In segregation is formed around S/M-TDs. It is believed that In segregation around S/M-TDs reduces local Schottky barrier height to form conductive channels and leads to high leakage current density of Schottky contacts on InxAl1−xN/GaN heterostructures.

Extractive desulfurization of fuel using N-butylpyridinium-based ionic liquids

Sulfur compounds in fuels have become one of the sources of serious environmental problems. The extractive desulfurization using ionic liquids (ILs) has attracted great attention in recent years. In this work, the pyridinium-based ionic liquids (ILs) N-butylpyridinium thiocyanate ([C4Py][SCN]), N-butylpyridinium bis(trifluoromethylsulfonyl)imide ([C4Py][NTf2]), and N-butylpyridinium dicyanamide ([C4Py][N(CN)2]) were used as extractants for desulfurization of model fuels. The results demonstrate that the structure of the anion influences the extractive performance of ILs, following the order of [NTf2] benzothiophene (BT) > 4,6-dimethyldibenzothiophene (4,6-DMDBT). Moreover, the [C4Py][N(CN)2] can be recycled at least 4 times with little decrease in the desulfurization activity.

The Behavior of Ionic Liquids under High Pressure: A Molecular Dynamics Simulation

The effect of pressure on the structure, interionic interactions, and properties of the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF(6)]) was studied using an all-atom molecular dynamics simulation. A distinct conformational transition from anti (a) to gauche (g) form based on the deformation of the first C-C bond of the butyl chain was observed under high pressure, and the ratio of the a conformation that changed into the g conformation was 5.5% at 6000 bar. Under high pressure, the configuration of the a and g conformer for [C(4)mim](+) tends to make the alkyl chain distorted to the inside of the ring. Results on the density changes indicate a small increase from 5000 to 6000 bar, which could be attributed to the writhing of the reducing end of the alkyl chain in the cation at higher pressure. These simulation results are well agreed with the experimental results. Transport properties were also calculated at different pressures. The results show that diffusion of the ions is reduced under high pressure, and the viscosity is dramatically enhanced.

Molecular simulations of phosphonium-based ionic liquid

Compared with imidazolium-based ionic liquids (ILs), phosphonium-based ILs have been proven to be more stable in thermodynamics and less expensive to manufacture. In this work, a kind of phosphonium-based IL, [PC6C6C6C14][Tf2N], was studied under several conditions using molecular dynamics simulations based on both the all-atom force field (AAFF) and the united-atom force field. Liquid density was calculated to validate the force field. Compared with experimental data, good agreement was obtained for the simulated density based on the AAFF. Heat capacities at constant pressure were calculated at several temperatures, and good linear relationships were observed. Self-diffusion coefficients, viscosities and conductivities were also calculated to study the dynamics properties of this IL. The viscosity of this IL at 293 K was also compared with experimental data, and the error was in a reasonable range. In order to depict the microstructures of the IL, centre-of-mass and site-to-site radial distribution functions were employed. In addition, spatial distribution functions were investigated to present the more intuitive features.