Li Qun Xu

Reduction of graphene oxide by aniline with its concomitant oxidative polymerization.

Graphene oxide (GO) nanosheets are readily reduced by aniline above room temperature in an aqueous acid medium, with the aniline simultaneously undergoing oxidative polymerization to produce the reduced graphene oxide-polyaniline nanofiber (RGO-PANi) composites. The resulting RGO-PANi composites and RGO (after dissolution of PANi) were characterized by XPS, XRD analysis, TGA, UV-visible absorption spectroscopy, and TEM. It was also found that the RGO-PANi composites exhibit good specific capacitance during galvanostatic charging-discharging when used as capacitor electrodes.

Surface Modification of Silicone for Biomedical Applications Requiring Long-Term Antibacterial, Antifouling, and Hemocompatible Properties

Silicone has been used for peritoneal dialysis (PD) catheters for several decades. However, bacteria, platelets, proteins, and other biomolecules tend to adhere to its hydrophobic surface, which may lead to PD outflow failure, serious infection, or even death. In this work, a cross-linked poly(poly(ethylene glycol) dimethacrylate) (P(PEGDMA)) polymer layer was covalently grafted on medical-grade silicone surface to improve its antibacterial and antifouling properties. The P(PEGDMA)-grafted silicone (Silicone-g-P(PEGDMA)) substrate reduced the adhesion of Staphylococcus aureus , Escherichia coli , and Staphylococcus epidermidis , as well as 3T3 fibroblast cells by ≥90%. The antibacterial and antifouling properties were preserved after the modified substrate was aged for 30 days in phosphate buffer saline. Further immobilization of a polysulfobetaine polymer, poly((2-(methacryloyloxy)ethyl)dimethyl-(3-sulfopropyl)ammonium hydroxide) (P(DMAPS)), on the Silicone-g-P(PEGDMA) substrate via thiol-ene click reaction leads to enhanced antifouling efficacy and improved hemocompatibility with the preservation of the antibacterial property. Compared to pristine silicone, the so-obtained Silicone-g-P(PEGDMA)-P(DMAPS) substrate reduced the absorption of bovine serum albumin and bovine plasma fibrinogen by ≥80%. It also reduced the number of adherent platelets by ≥90% and significantly prolonged plasma recalcification time. The results indicate that surface grafting with P(PEGDMA) and P(DMAPS) can be potentially useful for the modification of silicone-based PD catheters for long-term applications.

Tea stains-inspired initiator primer for surface grafting of antifouling and antimicrobial polymer brush coatings.

Inspired by tea stains, plant polyphenolic tannic acid (TA) was beneficially employed as the primer anchor for functional polymer brushes. The brominated TA (TABr) initiator primer was synthesized by partial modification of TA with alkyl bromide functionalities. TABr with trihydroxyphenyl moieties can readily anchor on a wide range of substrates, including metal, metal oxide, polymer, glass, and silicon. Concomitantly, the alkyl bromide terminals serve as initiation sites for atom transfer radical polymerization (ATRP). Cationic [2-(methacryloyloxy)ethyl]trimethylammonium chloride (META) and zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) and N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine (SBMA) were graft-polymerized from the TABr-anchored stainless steel (SS) surface. The cationic polymer brushes on the modified surfaces are bactericidal, while the zwitterionic coatings exhibit resistance against bacterial adhesion. In addition, microalgal attachment (microfouling) and barnacle cyprid settlement (macrofouling) on the functional polymer-grafted surfaces were significantly reduced, in comparison to the pristine SS surface. Thus, the bifunctional TABr initiator primer provides a unique surface anchor for the preparation of functional polymer brushes for inhibiting both microfouling and macrofouling.

Poly(dopamine acrylamide)-co-poly(propargyl acrylamide)-modified titanium surfaces for ‘click’ functionalization

Poly(dopamine acrylamide)-co-poly(propargyl acrylamide) (PDA-co-PPA) copolymers, containing alkyne- and catechol-functionalities, were prepared from reactive poly(pentafluorophenyl acrylate) (PPFA) via functionalization of the pentafluorophenyl-activated ester groups with dopamine hydrochloride and propargylamine. The bifunctional PDA-co-PPA copolymers were then coupled to the titanium (Ti) surface via coordination interaction of the catechol moieties, yielding a functionalizable Ti platform containing ‘clickable’ alkyne groups on the surface for the copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) of azide-labeled fluorescein, rhodamine, β-cyclodextrin, poly(ethylene glycol) (PEG), mannose and biotin. The functionalized Ti surfaces from the CuAAC reactions were characterized by fluorescence microscopy (for the rhodamine- and fluorescein-functionalized Ti surfaces), non-specific protein adsorption (for the PEG-grafted Ti surface), specific protein adsorption (for the mannose- and biotin-functionalized Ti surfaces) and host–guest interaction with antibiotics (for the β-cyclodextrin-functionalized Ti surface).

CO2-triggered fluorescence “turn-on” response of perylene diimide-containing poly(N,N-dimethylaminoethyl methacrylate)

A fluorophore–polymer conjugate, PBI–PDMAEMA, consisting of two arms of CO2-responsive N,N-dimethylaminoethyl methacrylate polymer (PDMAEMA) and a center perylene-3,4,9,10-tetracarboxylic acid bisimide (PBI) fluorophore, has been synthesized via atom transfer radical polymerization of N,N-dimethylaminoethyl methacrylate from the N,N′-bis{2-[2-[(2-bromo-2-methylpropanoyl)oxy]ethoxy]ethyl}perylene-3,4,9,10-tetracarboxylic acid bisimide initiators. This fluorophore–polymer conjugate exhibits a colorimetric change and a fluorescent “turn-on” response to the presence of CO2 in aqueous solutions. Upon removal of adsorbed CO2 by N2 degassing, this CO2 chemosensor is completely recoverable. The application of this fluorophore–polymer conjugate in determining the concentration of CO2 in carbonated beverages was demonstrated.

Rhodamine derivative-modified filter papers for colorimetric and fluorescent detection of Hg2+ in aqueous media

Mercury pollution is a widespread problem. In this work, a solid-state sensor based on rhodamine derivative-modified cellulose filter papers for detection of Hg2+ ions in aqueous media is demonstrated. A three-step approach, involving the introduction of atom transfer radical polymerization (ATRP) initiating sites, surface-initiated ATRP (SI-ATRP) of pentafluorophenyl methacrylate, and post-functionalization of the reactive surface with amino-containing spirolactam rhodamine derivatives in ester–amine reaction, was developed for the covalent immobilization of mercury-responsive probes on the cellulose filter paper. This solid-state sensor exhibits highly selective recognition of Hg2+ ions in a Tris–HNO3 buffer solution (pH = 7.24) over various environmentally relevant metal ions with remarkably enhanced fluorescent emission intensity and distinct color change from colorless to pink. The response of a rhodamine derivative-modified cellulose filter paper to Hg2+ ions is fast (<2 min). The positive fluorescence response and color change of this functionalized filter paper thus provide a disposable solid-state sensor for fluorescent and “naked-eye” detection of Hg2+ ions.

Synthesis of catechol and zwitterion-bifunctionalized poly(ethylene glycol) for the construction of antifouling surfaces

The synthesis of catechol-containing small molecules and macromolecules always requires multiple reaction steps, coupling agents, or enzymes. In this study, a simple and scalable strategy for the preparation of catechol-containing poly(ethylene glycol) (CaPEG) by epoxide–amine polymerization of PEG diglycidyl ether with dopamine is described. The as-formed tertiary amine groups in the backbone of CaPEG can be converted into sulfobetaine structures in an alkylsulfonation step, leading to the formation of catechol and zwitterion-bifunctionalized PEG (SBCaPEG). The resulting catechol-containing CaPEG and SBCaPEG can be anchored on various substrate surfaces, including stainless steel (SS), titanium and silicon wafer, under mild conditions. Since SS is susceptible to fouling by a variety of microorganisms, the antifouling properties of the polymer-coated SS surfaces are studied in detail. The CaPEG- and SBCaPEG-coated SS surfaces effectively reduced the adsorption of protein (albumin–fluorescein isothiocyanate conjugate and bovine plasma fibrinogen), as well as the adhesion of bacteria (Pseudomonas sp. and Escherichia coli) and microalgae (Amphora coffeaeformis), as compared to that of the pristine SS surface. In comparison with the CaPEG-coated SS surfaces, the zwitterionic SBCaPEG-coated SS surfaces exhibited even better antifouling efficiencies.

Reactive Graphene Oxide Nanosheets: A Versatile Platform for the Fabrication of Graphene Oxide–Biomolecule/Polymer Nanohybrids

Graphene oxide (GO) nanosheets can be functionalized with reactive pentafluorophenyl ester via esterification of the carboxylic groups. The resulting reactive GO nanosheets provide a versatile platform for grafting of amino-containing polymers or biomolecules via ester-amine coupling. Coupling of poly[(9,9-dioctylfluorene)-alt-(4-amino-phenylcarbazole)] (PFCz-NH(2) ), amino-terminated hyperbranched polyglycerol (HPG-NH(2) ), and lysozyme (Lyz) was illustrated. The Al/GO-g-PFCz/ITO sandwich thin-film device exhibits bistable electrical switching and rewritable memory effects. The GO-g-Lyz nanohybrids exhibit high bactericidal efficacy against S. aureus and E. coli, while the GO-g-HPG nanohybrids exhibit reduced cytotoxicity toward 3T3 fibroblasts.

Layer-by-layer deposition of antifouling coatings on stainless steel via catechol-amine reaction

Stainless steel (SS) has been widely used as a construction material in maritime structures due to its good corrosion resistance. However, bacteria, algae, barnacles and other marine organisms can readily adhere to its surface in the process of biofouling, leading to serious structure failures and economic losses. In this work, layer-by-layer (LBL) deposition of functional polymer coatings on SS surface provides an alternative approach to combating marine fouling. The catechol-containing antifouling copolymer of dopamine methacrylamide and poly(ethylene glycol) methyl ether methacrylate (P(DMA-co-PEGMEMA)), and amino-rich branched poly(ethyleneimine) (PEI) were assembled sequentially on the SS surface via catechol-amine reaction in a LBL manner. The PEI/P(DMA-co-PEGMEMA) multiple bilayer-coated SS surfaces can effectively reduce the adhesion of bacteria and microalgae (microfouling), and settlement of barnacle cyprids (macrofouling), as compared to the pristine SS surface. The antifouling efficiencies of PEI/P(DMA-co-PEGMEMA) bilayer-coated SS surfaces were also significantly higher than that of the P(DMA-co-PEGMEMA) monolayer-coated SS surface.

Photoinduced anchoring and micropatterning of macroinitiators on polyurethane surfaces for graft polymerization of antifouling brush coatings

Poly[3-azido-2-(2-bromo-2-methylpropanoyloxy)propyl methacrylate] (PAzBrMA) was synthesized as the macroinitiator and anchor for a functional polymer brush coating on polyurethane (PU) films. Ring-opening reaction of the epoxide group of poly(glycidyl methacrylate) with sodium azide produced the hydroxyl and azide functional groups. The hydroxyl groups were substituted with 2-bromoisobutyryl bromide to introduce the alkyl halide initiator. For anchoring, ultraviolet irradiation was applied to convert the azide groups of PAzBrMA physically coated on the PU surface into nitrene intermediates. The nitrene groups reacted with hydrocarbon moieties on the PU surface through hydrogen abstraction to form amine linkages. A photomask could then be employed to create a patterned surface during irradiation. Thus, the anchoring of a PAzBrMA macroinitiator can be achieved under mild conditions, without the use of strong solvents and high temperatures, which will swell or degrade the PU substrates. Finally, 2-hydroxyethyl methacrylate (HEMA) and poly(ethylene glycol) methacrylate (PEGMA) are graft-polymerized on the PAzBrMA-anchored PU film by surface-initiated atom transfer radical polymerization. In comparison with the pristine PU surface, the PU surfaces with grafted HEMA and PEGMA brush coatings were effective in reducing bovine serum albumin adsorption (protein fouling), adhesion of Staphylococcus epidermidis and Pseudomonas sp. (microfouling), and barnacle cyprid settlement (macrofouling). The present surface modification approach provides a simple and versatile means for micropatterning and functionalization of the polymer surfaces.