Low-voltage actuators that operate based on changes in pH are used in biomedical devices and micromechanical systems [07Spi]. A dual mode actuation during pH switching was obtained using chitosan/polyaniline and carbon nanotube fibers. Chitosan fibers were co-spun with polyaniline to combine the reversible swelling ability in acidic/neutral solutions and the good solubility of polyaniline. Carbon nanotubes were added to increase the conductivity of the fiber [06Spi]. It was found that pH strains of about 2 % and electrochemical strains of 0.3 % were obtained, and the addition of carbon nanotubes was necessary to obtain electrochemical actuation, and the fibers were supposed to be useful for micro — and macro actuators [06Spi]. Chitosan fibers were used to self-assemble gold nanoparticles, and the composite fibers were intended for use as biosensors [06Wan]. Fibers (50 nm-5 pm in width) were formed via freeze-drying and partially cross-linked with glutaralde — hyde. These fibers were later dispersed into 0.025 mol/l aqueous tetrachloroauric acid and allowed to stay for 6 days. Nanoparticles formed experienced surface and interface interactions such as adsorption, formation of nuclei, crystal growth, dispersion, self-assembly, and agglomeration on the surface of the chitosan fibers. Gold flakes with thickness of 50 nm were formed, and the size of the flakes increased with increasing time. Spherical gold clusters (200 nm) containing gold nanoparticles of 5 nm diameter self-assembled on the fiber surface. Figure 26.11 shows gold nanoparticles beginning to self-assemble on the fibers (left) and fibers with a continuous gold coating (right) [06Wan]. To overcome the aggregation problems, single-walled carbon nanotubes were functionalized and incorporated into chitosan fibers to be used as actuators [08Oza]. Carbon nanotubes were treated with 70 % nitric acid to oxidize and functionalize the nanotubes by adding carbox­ylic groups. The addition of the nanotubes (0.4 wt%) increased the tenacity of the fibers to 180 MPa compared to 96 MPa for the non-functionalized nanotubes. The fibers also showed significant swelling and deswelling (up to 800 % volume change) with change in pH as seen in Fig. 26.12. The extent of change in the fibers was also dependent on the applied voltage and the concentration of the nanotubes and salt.

Chitosan fibers were considered to be versatile fibers for assembling proteins and provide distinct properties [08Shi]. Two His-tagged proteins (His-GFP) and His-protein G were used to biofunctionalize the fibers. Chitosan fibers were treated with nickel solution (0.01 M NiCl2 in 0.9 % NaOH, pH 5) for 1 h. Later, the fibers were treated with 5 % nonfat milk to block the nonspecific binding of proteins. The fibers were then treated with antibody-containing solutions, and the antigen binding was studied. It was found that the fibers were able to absorb antibodies, and the fibers would have potential protection and detection applications.

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