Cation-controlled aqueous dispersions of alginic-acid-wrapped multi-walled carbon nanotubes.

Carbon nanotubes (CNTs) have attracted a lot of attention in recent years due to their possessing unique electronic, mechanical, and structural properties, as well as having potential applications in electronics, nanodevices, and energy storage. However, exploring the chemistry of CNTs at the molecular level is greatly limited due to their inherently difficult purification and insolubility in water and organic solvents. Therefore, many attempts have been made to improve the solubility of CNTs in common solvents. There are two alternative routes to reach this goal, namely, covalent and noncovalent modification. Direct sidewall attachments of CNTs via oxidation in acidic media, fluorination, nitrene addition, hydrogenation via the Birch reduction, alkylation, arylation, and 1,3-dipolar cycloaddition have been reported. Chemical functionalization can improve the solubility of CNTs, and maintain their unique properties when coupled to other types of materials, but partially damages the p-conjugate system. In contrast, noncovalent coupling can potentially preserve the unique properties of the nanotubes; this strategy has been widely employed via polymer wrapping and adsorption, the adsorption of amines and molecules with large p-systems, and coating the nanotubes with surfactants such as sodium dodecylsulfate (SDS) or benzylalkonium chloride. Compared with other systems, biomacromolecules such as starch, protein, schizophyllan, a-GalNAc residues, and peptides offer considerable advantages due to their biocompatibility. Alginic acid (AA), a natural polysaccharide harvested from brown algae, is an unbranched binary copolymer that is constituted of (1,4)-linked b-dmannuronic acid (M) and a-l-guluronic acid (G; Scheme 1), and is widely used in the food and pharmaceutical industries, macromolecules, and biological cells. In this Communication, we utilize AA as a solubilizing agent to prepare AA-wrapped multi-walled carbon nanotube (MWCNT– AA) complexes. Furthermore, the binding behavior of this complex has been comprehensively investigated by NMR and Raman spectroscopies, thermogravimetric (TG) and differential thermal analysis (DTA), and transmission electron microscopy (TEM). Moreover, preferential precipitation of MWNTs occurs upon the addition of cations, while resolubilization can be achieved when the added cations are trapped using ethylenediaminetetraacetic acid (EDTA). The MWCNT–AA complex was prepared in 61% yield by treating an aqueous solution of AA with MWCNTs under sonication. The solubility of MWCNT–AA in water ( 3.2 mgmL ) is similar to that of starched single-walled carbon nanotubes (SWNTs) (3.0 mgmL ), but the ratio (by weight) of MWCNTs to AA in the complex is approximately 58:42, which is larger than that of SWNTs and amylose (1:5). The observations suggest that the amylose homologues are more applicable for wrapping CNTs. NMR experiments were used to confirm the interaction of the MWCNTs with AA in aqueous solution. As can be seen from Figure 1, the H-2 protons of the a-l-guluronic

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