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

Abstract

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 environ-ments.1-5 For the applications of SWNTs-based sensors in a biological environment, an immediate question is how the sensors respond to the biological conditions such as pH,5c glucose, various ions, and proteins. This study requires a well-controlled modification of SWNT surfaces to obtain interfaces that are sensitive to these variables.6 The exploration in this exciting area is still not in full blossom, partially due to the difficulty in preparing water-soluble SWNTs while maintaining the SWNT electronic structure intact.4 In light of recent great progress in solubilization of SWNTs in various solvents by polymer wrapping and sidewall functionaliza-tion,3a,4,5b,7-10 a better controlled modification of SWNT surfaces may be realized soon. In this work, we report a facile chemical routine to prepare water-soluble SWNTs that still retain their van Hove singularities after oxidative treatment.7 The solubility in water for as-treated SWNTs with modified surfaces provides us with a unique opportunity to reveal the relationship of their electronic and optical properties with pH. Here we observe that after surface modification with carboxylate groups, the optical absorption of as-prepared water-soluble semiconducting SWNT