Water-soluble and optically pH-sensitive single-walled carbon nanotubes from surface modification

There is great interest in using single-walled carbon nanotubes (SWNTs) as nanoscale probes and sensors in biological electronics and optical devices because the electronic and optical properties of SWNTs are extremely sensitive to the surrounding environments. A well-controlled modification of SWNT surfaces may provide unique interfaces that are sensitive to the biological variables such as pH, glucose, various ions and proteins. In this paper, we report a facile chemical routine to prepare water-soluble SWNTs that still retain their van Hove singularities after acid oxidative treatment. The aqueous solutions (0.03-0.15 mg/mL) are stable for more than a month. The solubility in water for as-treated SWNTs with surfaces modified by carboxylate groups provides us with a unique opportunity to reveal the relationship of the SWNT electronic and optical properties with pH. Here we present the first observation that after surface modification with carboxylate groups, the optical absorption of the first interband transition of as-treated water-soluble semiconducting SWNTs reversibly responds to the pH change in aqueous solutions. Our results indicate that surface modification of SWNTs is a promising way for preparing chemically selective SWNT interfaces, which may open new exciting opportunities for various applications.