Hongliang Kang

Self-Assembly and Dual-Stimuli Sensitivities of Hydroxypropylcellulose-graft-poly(N,N-dimethyl aminoethyl methacrylate) Copolymers in Aqueous Solution

The self-assembly and pH- and thermo-sensitivities properties of hydroxypropyl cellulose-graft-poly(N,N-dimethyl aminoethyl methacrylate) (HPC-g-PDMAEMA) copolymers in aqueous solutions were investigated by transmittance, dynamic light scattering (DLS), and (1)H NMR spectroscopy. Micelles with different structure can be formed by varying either pH value or temperature. At low pH, e.g., 3.0, the HPC backbone of the copolymer collapse to form the core of micelles stabilized with protonated PDMAEMA side chains on the surface of the micelles upon heating. At the medium pH, e.g., 8.1, both HPC backbone and PDMAEMA side chains collapse upon heating to form unstable aggregates. At high pH, e.g., 12.3, PDMAEMA side chains collapse first to form the core of micelles stabilized with HPC chains upon heating. Further heating the copolymer solution at this pH leads to the aggregation of the micelles due to the collapse of the shell HPC chains. The thermal sensitivity of the HPC-g-PDMAEMA copolymers is reversible.

Dissolution mechanism of cellulose in N,N-dimethylacetamide/lithium chloride: revisiting through molecular interactions.

Understanding the interactions between solvent molecules and cellulose at a molecular level is still not fully achieved in cellulose/N,N-dimethylacetamide (DMAc)/LiCl system. In this paper, cellobiose was used as the model compound of cellulose to investigate the interactions in cellulose/DMAc/LiCl solution by using Fourier transform infrared spectroscopy (FTIR), (13)C, (35)Cl, and (7)Li nuclear magnetic resonance (NMR) spectroscopy and conductivity measurements. It was found that when cellulose is dissolved in DMAc/LiCl cosolvent system, the hydroxyl protons of cellulose form strong hydrogen bonds with the Cl(-), during which the intermolecular hydrogen bonding networks of cellulose is broken with simultaneous splitting of the Li(+)-Cl(-) ion pairs. Simultaneously, the Li(+) cations are further solvated by free DMAc molecules, which accompany the hydrogen-bonded Cl(-) to meet electric balance. Thereafter, the cellulose chains are dispersed in molecular level in the solvent system to form homogeneous solution. This work clarifies the interactions in the cellulose/DMAc/LiCl solution at molecular level and the dissolution mechanism of cellulose in DMAc/LiCl, which is important for understanding the principle for selecting and designing new cellulose solvent systems.

Smart Assembly Behaviors of Hydroxypropylcellulose-graft-poly(4-vinyl pyridine) Copolymers in Aqueous Solution by Thermo and pH Stimuli

Thermo- and pH-sensitive graft copolymers, hydroxypropylcellulose-graft-poly(4-vinyl pyridine) (HPC-g-P4VP), were synthesized via atom transfer radical polymerization (ATRP) and characterized. The thermo- and pH-induced micellization and stimuli-responsive properties of HPC-g-P4VP graft copolymers in aqueous solution were investigated by transmittance, (1)H NMR, dynamic light scattering (DLS), and so on. For the pH-induced micellization, the P4VP side chains collapse to form the core of the micelles, and the HPC backbones stay in the shell to stabilize the micelles. In the case of thermoinduced micellization, the HPC backbones collapse to form the core of the micelles that was stabilized by the P4VP side chains in the shell upon heating. Whats more, the cloud point of the HPC-g-P4VP copolymers in the aqueous solution could be finely tuned by changing the length of P4VP side chains or the pH values. In acidic water, the longer the side chains, the higher the cloud point. For those HPC-g-P4VP copolymers with short side chains, for example, HPC0.05-g-P4VP(3), the lower pH correlates a higher cloud point. The thermo- or pH-induced micelles also have the pH- or thermosensitivity due to their P4VP or HPC shells.

Dual-stimuli sensitive nanogels fabricated by self-association of thiolated hydroxypropyl cellulose

A new type of cellulose derivative was synthesized by means of conjugating cysteamine to hydroxypropyl cellulose (HPC) and the degree of thiol groups could be controlled by the feed ratio of the reactants. The thiolated HPC (HPC–SH) maintains the thermosensitivity of HPC and the thiol groups on the HPC chain can be oxidized to disulfide bonds. Cytotoxity tests performed on MG-63 cells proved that HPC–SH is not harmful to the cells. Nanogels can be fabricated by the self-association of HPC–HS in the solution at 45 °C and then oxidation of thiol groups to disulfide bonds occurs to stabilize the associated structure. The crosslinking degree of the nanogels could be controlled by the substitution degree of thiol groups (–SH) in the thiolated HPC. The hydrodynamic radius of the nanogels can be tuned by adjusting the degree of crosslinking and the concentration of the initial thiolated HPC solution in the self-association process. The hydrodynamic radius of the nanogels can be changed with the temperature and the dissociation process can happen by adding the reducing agent dithiothreitol (DTT). The dual-stimuli sensitive nanogels may have potential applications in controlled drug release, transfer switch device and sensors.

Intermolecular interactions and 3D structure in cellulose-NaOH-urea aqueous system.

The dissolution of cellulose in NaOH/urea aqueous solution at low temperature is a key finding in cellulose science and technology. In this paper, (15)N and (23)Na NMR experiments were carried out to clarify the intermolecular interactions in cellulose/NaOH/urea aqueous solution. It was found that there are direct interactions between OH(-) anions and amino groups of urea through hydrogen bonds and no direct interaction between urea and cellulose. Moreover, Na(+) ions can interact with both cellulose and urea in an aqueous system. These interactions lead to the formation of cellulose-NaOH-urea-H2O inclusion complexes (ICs). (23)Na relaxation results confirmed that the formation of urea-OH(-) clusters can effectively enhance the stability of Na(+) ions that attracted to cellulose chains. Low temperature can enhance the hydrogen bonding interaction between OH(-) ions and urea and improve the binding ability of the NaOH/urea/H2O clusters that attached to cellulose chains. Cryo-TEM observation confirmed the formation of cellulose-NaOH-urea-H2O ICs, which is in extended conformation with mean diameter of about 3.6 nm and mean length of about 300 nm. Possible 3D structure of the ICs was proposed by the M06-2X/6-31+G(d) theoretical calculation, revealing the O3H···O5 intramolecular hydrogen bonds could remain in the ICs. This work clarified the interactions in cellulose/NaOH/urea aqueous solution and the 3D structure of the cellulose chain in dilute cellulose/NaOH/urea aqueous solution.

Osmium Bipyridine-Containing Redox Polymers Based on Cellulose and Their Reversible Redox Activity

Thermo-, pH-, and electrochemical-sensitive cellulose graft copolymers, hydroxypropyl cellulose-g-poly(4-vinylpyridine)-Os(bipyridine) (HPC-g-P4VP-Os(bpy)), were synthesized and characterized. The electrochemical properties of the resulting material were investigated via cyclic voltammetry by coating the graft copolymers on the platinized carbon electrode. The results indicated that the electrochemical properties of the graft copolymer modified electrode were responsive to the pH values of the electrolyte solution. The reversible transformation between the active and inactive state originated from the changes in the architecture of the HPC-g-P4VP-Os(bpy) graft copolymer at different pH values. At high pH (e.g., above the pK(a) of P4VP), the chains of P4VP collapsed, and the electrochemical activity of the electrode was reduced. With immobilization of glucose oxidase (GOx) on the graft copolymer decorated electrode, a biosensor for glucose detection was prepared. The current of the biosensor depended on the glucose concentration in the detected solution and increased with the successive addition of glucose.

Synthesis of amidoxime functionalized cellulose derivatives as a reducing agent and stabilizer for preparing gold nanoparticles

In this paper, amidoxime functionalized cellulose (AOFC) derivatives have been synthesized by a two step approach and characterized. Firstly, cyanoethyl cellulose (CEC) was synthesized by using Michael addition between acrylonitrile and hydroxyl groups of cellulose in the homogeneous cellulose/NaOH/urea solution. The nitrile groups of CEC were then converted into amidoxime groups using NH2OH. The optimal reaction parameters for the amidoximization are at a pH of 7 and a reaction temperature of 75 °C. More than 90% of the nitrile groups of CEC can be converted into amidoxime groups after the reactions were carried out under these conditions for 8 h. The degree of substitution of amidoxime groups per glucose unit of cellulose can be adjusted by varying the feeding molar ratio of the acrylonitrile to the anhydroglucose units in the reaction solution during the synthesis of CEC. The new cellulose derivative AOFC can be used as both a reducing agent and stabilizer for preparing gold nanoparticles (AuNPs). The AOFC stabilized AuNPs have the excellent stability in whole pH range, which may have the promising applications in the fields of catalysis, biotechnology and medicine.

Regulation of the thermal sensitivity of hydroxypropyl cellulose by poly(N-isopropylacryamide) side chains

Hydroxyproyl cellulose graft poly(N-isopropylacryamide) (HPC-g-PNIPAm) copolymers were synthesized by single-electron transfer living radical polymerization (SET-LRP) in water and THF mixture solvent and characterized. The controllability and polymerization rate of SET-LRP can be adjusted by the water/THF ratio in the mixture solvent. The monomer conversion rate is relatively low in the solvent with low water content. The thermal responsive property of HPC-g-PNIPAm copolymers in aqueous solution depends on the length of the graft chains. The relatively short PNIPAm side chains (<150 repeat units) can effectively regulate the low critical solution temperature (LCST) of the HPC-g-PNIPAm copolymers in aqueous solution due to the hydrophilic properties of the short PNIPAm chains. This work provides an approach for the regulation of the LCST to body temperature region by graft copolymerization.

An effect of alginate on the stability of LDH nanosheets in aqueous solution and preparation of alginate/LDH nanocomposites

Nanosheets under 10nm in thickness are obtained by exfoliating layered double hydroxide (LDH) in formamide. The LDH nanosheets are dispersed and stabilized in an alginate aqueous solution after removing formamide by water washing and ultracentrifugation. During the water washing stage LDH nanosheets can be prevented from restacking by electrostatic stabilization of the surface of LDH sheets through the adsorption of alginate. Alginate/LDH nanocomposites can be prepared by drying the dispersion, and sandwich-like structures in the nanocomposites are formed with two alginate layers contained between two LDH sheets. LDH nanosheets in the dried alginate/LDH nanocomposites can be re-dispersed in water. The thermal stability of alginate in the nanocomposite is increased by LDH. Alginate membranes containing this layered nanocomposite can be prepared. The addition of LDH into the alginate matrix leads to an increase in the mechanical properties of the nanocomposite.

Dual-stimuli sensitive keratin graft PHPMA as physiological trigger responsive drug carriers

Keratin graft poly(N-(2-hydroxypropyl)methacrylamide) (K-g-PHPMA) copolymers were synthesized and characterized. On account of the thiol groups of keratin and the amphiphilicity of the graft copolymers, micelles with cleavable cross-links on a keratin core were fabricated in water. The K-g-PHPMA micelles can efficiently encapsulate doxorubicin (DOX) and can be used as a drug carrier. The DOX content in the micelles increases with the keratin content of the graft copolymers. The release of the encapsulated DOX in the micelles is sensitive to the physiological environment. Redox trigger glutathione (GSH), especially at the intracellular level, and trypsin can effectively trigger the release of the encapsulated DOX. In vitro cellular uptake experiments indicate that the DOX released from the DOX-loaded K-g-PHPMA micelles can be efficiently internalized into cells. Under higher GSH condition, the DOX shows a much faster release into the nucleus of the cells. The K-g-PHPMA copolymers have promising applications as drug carriers for enhanced intracellular drug delivery in cancer therapy.