We describe a novel class of electrically conductive transparent materials based on multiwalled carbon nanotubes (MWCNTs). Transparent nanocomposites were fabricated by incorporating an aqueous silk fibroin solution into bacterial cellulose membranes. The transparent nanocomposites had a high transmittance in the visible and infrared regions, regardless of the bacterial cellulose fiber content, due to the nanosize effect of the bacterial cellulose nanofibrils. This phenomenon allowed the preparation of a novel electrically conductive transparent paper. The high dispersity of the MWCNTs was realized by utilizing a bacterial cellulose membrane as a template to deposit them uniformly, thereby achieving electrically conductive transparent papers with outstanding optical transparency. The light transmittance and electrical conductivity varied according to the concentration of the MWCNT dispersion. Good optimal transparency and electrical properties were obtained with a light transmittance of 70.3% at 550 nm and electrical conductivity of 2.1 × 10−3 S/cm when the electrically conductive transparent paper was fabricated from a 0.02 wt % aqueous MWCNT dispersion. In addition, the electrically conductive transparent papers showed remarkable flexibility without any loss of their initial properties. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1235–1242, 2008

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Multiwalled carbon nanotubes (MWCNTs) are considered to be ideal multifunctional materials for biorelated applications, but there is still some controversy regarding their toxicity. In addition, the poor dispersibility of MWCNTs in either water or organic solvents has limited their practical applications. Therefore, obtaining a good dispersion is one of the key issues in their applications. In this study, MWCNTs were dispersed in an aqueous solution using silk fibroin without chemical modification. An optical analyzer, Turbiscan, was used to confirm the stability of the MWCNT aqueous dispersion. Transmission electron microscopy showed individual MWCNTs coated with silk fibroin molecules. Silk fibroin in the sol state can interact with nanotubes through hydrophobic interactions. Therefore, silk fibroin molecules coat the nanotubes, which allow their dispersion in water. Under basic conditions (pH 12.0), an aqueous dispersion of MWCNTs with silk fibroin was stable without sedimentation or gelation. However, the MWCNT/silk fibroin dispersion was unstable under acidic conditions (pH 4.0). In addition, the MWCNT dispersion showed reversible changes with variations in pH. Under acidic conditions, the MWCNTs settled due to conformation changes in the silk fibroin. However, the stability of the MWCNTs had recovered fully under basic conditions. It is believed that silk fibroin has sol-like behavior under basic conditions and gel-like behavior under acidic conditions. Ultraviolet circular dichroism was used to determine the conformation of the silk fibroin molecules with pH. Overall, the pH-sensitive properties of the carbon nanotubes dispersed with silk fibroin can lead to a new class of novel biomaterials for cancer detection and treatment.

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