Carbon nanotube–polymer composites: Chemistry, processing, mechanical and electrical properties

Although biomedical applications of carbon nanotubes have been intensively studied in recent years, its sister, graphene, has been rarely explored in biomedicine. In this work, for the first time we study the in vivo behaviors of nanographene sheets (NGS) with polyethylene glycol (PEG) coating by a fluorescent labeling method. In vivo fluorescence imaging reveals surprisingly high tumor uptake of NGS in several xenograft tumor mouse models. Distinctive from PEGylated carbon nanotubes, PEGylated NGS shows several interesting in vivo behaviors including highly efficient tumor passive targeting and relatively low retention in reticuloendothelial systems. We then utilize the strong optical absorbance of NGS in the near-infrared (NIR) region for in vivo photothermal therapy, achieving ultraefficient tumor ablation after intravenous administration of NGS and low-power NIR laser irradiation on the tumor. Furthermore, no obvious side effect of PEGylated NGS is noted for the injected mice by histology, blood chemi...

Graphene in Mice: Ultrahigh In Vivo Tumor Uptake and Efficient Photothermal Therapy

We investigated the mechanism by which transferrin-coated gold nanoparticles (Au NP) of different sizes and shapes entered mammalian cells. We determined that transferrin-coated Au NP entered the cells via clathrin-mediated endocytosis pathway. The NPs exocytosed out of the cells in a linear relationship to size. This was different than the relationship between uptake and size. Furthermore, we developed a mathematical equation to predict the relationship of size versus exocytosis for different cell lines. These studies will provide guidelines for developing NPs for imaging and drug delivery applications, which will require "controlling" NP accumulation rate. These studies will also have implications in determining nanotoxicity.

Elucidating the Mechanism of Cellular Uptake and Removal of Protein-Coated Gold Nanoparticles of Different Sizes and Shapes

In the last years membrane processes for gas separation are gaining a larger acceptance in industry and in the market are competing with consolidated operations such as pressure swing absorption and cryogenic distillation. The key for new applications of membranes in challenging and harsh environments (e.g., petrochemistry) is the development of new tough, high performance materials. The modular nature of membrane operations is intrinsically fit for process intensification, and this versatility might be a decisive factor to impose membrane processes in most gas separation fields, in a similar way as today membranes represent the main technology for water treatment. This review highlights the most promising areas of research in gas separation, by considering the materials for membranes, the industrial applications of membrane gas separations, and finally the opportunities for the integration of membrane gas separation units in hybrid systems for the intensification of processes.

Membrane Gas Separation: A Review/State of the Art

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Applications of carbon nanotubes in drug delivery

The functionalization of carbon nanotubes (CNTs) has been carried out in various ways for numerous applications in biotechnology, including for the preparation of sensors, as scaffolds for cell growth, imaging reagents, and transporters for drug delivery. One way is to immobilize DNA onto the surface of CNTs through noncovalent interactions or covalent bonds. Covalent-bond approaches might compromise and even spoil the functions of DNA owing to chemical reactions and the difficulty in releasing DNA. Nevertheless, noncovalent approaches developed to date may only provide metastable immobilization of DNA onto the surface of CNTs. It was reported that the migration of DNA linked covalently to CNTs was retarded in gel electrophoresis but noncovalent interactions between DNA and CNTs did not completely prevent migration. Polyethylenimine (PEI) is a type of polymer with a high density of amines, thus DNA may be immobilized securely onto the surface of multiwalled carbon nanotubes (MWNTs) that have been functionalized with PEI through strong electrostatic interactions arising from these amines. Hence, we have adopted a grafting-from approach to prepare polyethylenimine-graft multiwalled carbon nanotubes (PEIg-MWNTs). DNA has been immobilized securely onto the surface of PEI-g-MWNTs as demonstrated by the total inhibition of the migration of DNA in gel electrophoresis, and PEI-g-MWNTs showed transfection efficiency for delivery of DNA that was similar to or even several times higher than that of PEI (25 K) and several orders of magnitude higher than that of naked DNA. PEI was grafted onto the surface of MWNTs by performing a cationic polymerization of aziridine in the presence of amine-functionalized MWNTs (NH2–MWNTs). NH2– MWNTs were obtained by introducing carboxylic acid groups onto the surface of MWNTs by heating at reflux in 3.0m nitric acid. The carboxylic acid groups were transformed into acyl chloride groups by treatment with thionyl chloride followed by treatment with ethylenediamine. The grafting of PEI was realized through two mechanisms, the activated monomer mechanism (AMM) or the activated chain mechanism (ACM), by which protonated aziridine monomers or the terminal iminium ion groups of propagation chains, respectively, are transferred to amines on the surface of MWNTs. (see Supporting Information.) The relative amount of PEI grafted onto the surface of MWNTs was investigated by thermogravimetric analysis (TGA) performed under nitrogen. MWNTs were thermally stable up to 600 8C (Figure 1A, curve a) whereas pure PEI degraded completely at about 500 8C (Figure 1A, curve d). At 500 8C, pristine MWNTs, NH2–MWNTs, and PEI-g-MWNTs showed negligible, about 2.3%, and 10.5% weight losses, respectively, thus PEI-g-MWNTs contained about 8.2% PEI. Grafting with PEI made PEI-g-MWNTs easy to disperse in water, and the resulting suspension was still stable after six months. However, NH2–MWNTs dispersed poorly in water and precipitation occurred within several hours (see Supporting Information). Transmission electron microscopy (TEM) provides direct evidence of grafting of PEI onto the surface of MWNTs. Figure 1B shows TEM images of PEI-g-MWNTs on a holey carbon film: individually dispersed MWNTs are separated from others. High-resolution TEM (Figure 1B, inset) indicates that PEI was grafted onto the surface of MWNTs as lumps with different sizes instead of as a uniform coating. This clumping results from the carboxylic acid groups, the ethylenediamine, and the PEI adhering preferably to the defects, which are the most active locations for chemical or physical functionalization; these defects tend to cluster at the bends along the surface of MWNTs grown by chemical vapor deposition (CVD). In Figure 1C, the H NMR spectrum of PEI-g-MWNTs in D2O is compared with that of PEI in D2O (pH 7.0). The signal at about d= 3.1 ppm for PEI-g-MWNTs is attributed to grafted PEI, but the significantly decreased mobility of the PEI chains in PEIg-MWNTs leads to broadening of the resonances. Some of the amine groups of PEI were protonated (pKa of PEI is greater than 8.0); we found that protonation or partial protonation of PEI was necessary for the formation of a stable aqueous suspension of PEI-g-MWNTs and neutralizing PEI by adjusting the pH value to 9 or higher led to precipitation of dispersed PEI-graft-MWNTs within several hours. PEI obtained by cationic polymerization of aziridine has a dendritic structure that contains primary, secondary, and tertiary amines with a molar ratio of about 1:2:1. Grafting PEI onto the surface of MWNTs should have a negligible effect on the chemistry. The migration of DNA was totally inhibited in gel electrophoresis when the weight ratio of PEI[*] Dr. Y. Liu, D.-C. Wu, Dr. W.-D. Zhang, Dr. C.-B. He Institute of Materials Research and Engineering 3 Research Link, Singapore 117602 (Singapore) Fax: (+65)6872-7528 E-mail: ye-liu@imre.a-star.edu.sg

Ye Liu

Papers

Polyethylenimine-grafted multiwalled carbon nanotubes for secure noncovalent immobilization and efficient delivery of DNA.

Enhanced gas separation performance of nanocomposite membranes using MgO nanoparticles

Separation of CO2/CH4 through carbon molecular sieve membranes derived from P84 polyimide

Fabrication of fluoropolyimide/polyethersulfone (PES) dual-layer asymmetric hollow fiber membranes for gas separation

Chemical cross-linking modification of 6FDA-2,6-DAT hollow fiber membranes for natural gas separation