Supramolecular temperature responsive assembly of polydopamine reduced graphene oxide

Graphene oxide (GO) and reduced graphene oxide (rGO) colloidal systems can directly respond to environmental stimuli such as pH, ionic strength, and light by themselves, but not to temperature. Here we show that surface modification of rGO with polydopamine (PDA) leads to a temperature-responsive composite material, even though neither rGO nor PDA have intrinsic temperature responsiveness. Reducing GO with dopamine results in rGO/PDA flakes with hydrophilic PDA clusters attached to hydrophobic rGO domains, which mimics the amphiphilic structure of temperature responsive poly(N-isopropylacrylamide) (PNIPAM). The rGO/PDA flakes self-assemble at temperature higher than 30 °C, causing flake aggregation and precipitation in suspensions with concentration of 0.05 g L^-1, which is reversible upon cooling, shaking, and re-heating. A solution-to-gelation transition occurs upon heating suspensions with concentration of 10 g L^-1. Nacre-like films and porous monoliths are obtained by drying rGO/PDA suspensions at different concentrations. Films and porous monoliths obtained by drying suspensions that are previously self-assembled through heat have more compact structures compared to those obtained with suspensions that are not heated. Overall, this work introduces the concept of supramolecular temperature responsive assembly of nanomaterials (STRAN), i.e., that temperature response can be introduced in nanomaterials by combining non-responsive components that function cooperatively in supramolecules, whose interactions with solvents can be modulated by temperature changes, mimicking what happens in macromolecular systems such as PNIPAM. STRAN could be applied to nanomaterials beyond GO to develop responsive systems whose self-assembly in suspension and architectural features realized upon drying can be controlled by temperature.

Preparation and Properties of Polyvinylidene Fluoride Nanocomposited Membranes based on Poly(N-Isopropylacrylamide) Modified Graphene Oxide Nanosheets

The nanomaterial of graphene oxide grafting poly (N-isopropylacrylamide) (GO-g-PNIPAAm) was synthesized and PVDF/GO-g-PNIPAAm blended membranes were fabricated by wet phase inversion. In this work, a hydrophilic nanomaterial GO-g-PNIPAAm with poly(N-isopropylacrylamide) (PNIPAAm) grafted on GO, was synthesized by the atom transfer radical polymerization (ATRP) method. The resulting nanomaterial was confirmed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectrum, and X-ray photoelectron spectroscopy (XPS) analysis. The synthesized GO-g-PNIPAAm was incorporated with polyvinylidene fluoride (PVDF) via phase inversion, and investigated for its temperature sensitivity, porosity, contact angle, scanning electron microscopy, and permeate properties. The water contact angle measurements confirmed that GO-g-PNIPAAm nanomaterial-endowed PVDF membranes with better hydrophilicity and thermo-responsive properties compared with those of the pristine PVDF membranes. Bovine serum albumin (BSA) adsorption experiments suggested that excellent antifouling properties of membranes were acquired after adding GO-g-PNIPAAm. The modified membranes showed good performance when the doping amount of GO-g-PNIPAAm was 0.2 wt %.

Environmentally responsive graphene systems

Graphene materials have been attracting significant research interest in the past few years, with the recent focuses on graphene-based electronic devices and smart stimulus-responsive systems that have a certain degree of automatism. Owing to its huge specific surface area, large room-temperature electron mobility, excellent mechanical flexibility, exceptionally high thermal conductivity and environmental stability, graphene is identified as a beneficial additive or an effective responding component by itself to improve the conductivity, flexibility, mechanical strength and/or the overall responsive performance of smart systems. In this review article, we aim to present the recent advances in graphene systems that are of spontaneous responses to external stimulations, such as environmental variation in pH, temperature, electric current, light, moisture and even gas ambient. These smart stimulus-responsive graphene systems are believed to have great theoretical and practical interests to a wide range of device applications including actuators, switches, robots, sensors, drug/gene deliveries, etc.

Straightforward RAFT procedure for the synthesis of heterotelechelic poly(acrylamide)s

Heterotelechelic, hydrophilic polymers with a primary amine and thiol group at the α- and ω-chain end, respectively, are synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization in a straightforward and versatile way and subsequently used for the design of dual-responsive polymer/gold nanohybrids. Therefore, a phthalimido-containing chain transfer agent (CTA) is synthesized and used for the polymerization of the hydrophilic monomers N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA). After polymerization, the trithiocarbonate functionality at the ω-chain end, originating from the CTA, is converted into a thiol upon aminolysis. In the next step, the phthalimido α-chain end is hydrolyzed into a primary amine, resulting in heterotelechelic, hydrophilic polymers. End-group conversions are monitored by (1)H NMR spectroscopy, MALDI-TOF MS analysis, and UV-Vis spectroscopy, confirming that quantitative modifications are obtained during each stage. The amino groups of these heterotelechelic polymer chains are modified with citraconic anhydride, after which the obtained polymers are grafted with the thiol group onto citrate-stabilized gold nanoparticles resulting in the creation of dual-temperature- and pH-responsive gold particles.

Self-assembled graphene-dextran nanohybrid for killing drug-resistant cancer cells

A nanohybrid based on nanoscale graphene oxide (NGO) and dextran has been designed and employed for effectively killing drug-resistant MCF-7/ADR cells. This graphene-based nanohybrid was readily prepared through π-π interaction of NGO and hematin-terminated dextran (HDex), being denoted as NGO-HDex. It revealed an improved stability in physiological conditions as compared to native NGO. Besides, NGO-HDex could efficiently load doxorubicin (DOX), an anticancer drug, with drug loading capacity of 3.4 mg/mg NGO and liberate the drug with a pH-dependent profile. Cell viability assay indicated that the NGO-HDex displayed lower cytotoxicity against MCF-7/ADR cells as compared to native NGO. DOX-loaded NGO-HDex, however, revealed more efficient killing effect in the cells than free DOX because the nanohybrid caused a higher amount of DOX accumulated in the cells. The results of this study highlight that the NGO-HDex has high potential for killing drug-resistant cancer cells.

A graphene-based smart catalytic system with superior catalytic performances and temperature responsive catalytic behaviors

We have successfully developed a unique graphene-based smart catalytic system which consists of the graphene supported Au-Pt bimetallic nanocatalyst with a well-defined core-shell structure and a dextran-based temperature-responsive polymer. The unique catalytic system possesses excellent catalytic performances and the catalytic activities could be readily switched on or off at different temperature windows.