pH-sensitive multiwalled carbon nanotube dispersion with silk fibroins. 
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.