Noncovalent Functionalization of Carbon Nanotubes by Fluorescein−Polyethylene Glycol:  Supramolecular Conjugates with pH-Dependent Absorbance and Fluorescence

We report the use of fluorescein−polyethylene glycol (Fluor-PEG) to noncovalently functionalize single-walled carbon nanotubes (SWNTs) for obtaining water-soluble nanotube conjugates (Fluor-PEG/SWNT) and simultaneously affording fluorescence labels to nanotubes. We find serendipitously that fluorescein, a widely used fluorophore, can strongly adsorb onto the sidewall of the SWNTs, likely via π-stacking, and the hydrophilic PEG chain imparts high aqueous solubility. Interaction between fluorescein and SWNT is pH dependent; it weakens as the pH is increased, causing the Fluor-PEG/SWNT conjugate to be less stable at high pHs. Fluorescein molecules bound to SWNTs exhibit interesting pH-dependent optical absorbance and fluorescence properties that are distinct from free molecules, as a result of pH-dependent interactions with SWNT sidewalls. Fluorescence emission from fluorescein adsorbed on SWNT is quenched by ∼67%, but remains sufficient and useful as a fluorescent label. The utility of Fluor-PEG/SWNT as a simultaneous fluorescent marker and an intracellular transporter is demonstrated by uptake of Fluor-PEG/SWNT by mammalian cells and detection of fluorescence inside the cells. Raman detection of SWNTs in the cells is also carried out and used to prove the co-localization of fluorescein and SWNT.