Water-soluble single-walled carbon nanotubes via noncovalent functionalization by a rigid, planar and conjugated diazo dye

A new noncovalent approach for the dissolution and exfoliation of SWNTs in water by a rigid, planar and conjugated diazo dye, Congo red (CR), is reported. Using a simple physical grinding treatment, the mixture of SWNTs and CR can be dissolved in water with a solubility as high as 3.5 mg/ml for SWNTs. The complete elimination of free CR from the mixture hardly changed this excellent solubility. High-resolution transmission electron microscope images showed that the SWNTs bundles were efficiently exfoliated into individual SWNTs or small ropes, which was further demonstrated by the increased π-plasmon UV–vis absorption and the upshift of the radical breathing mode in Raman scatterings. The π-stacking interaction between adsorbed CR and SWNTs was considered responsible for the high solubility. The uninterrupted structure of SWNTs with CR functionalization was ascertained by the unchanged disorder-induced Raman scattering.