(72b) Highly Stable and Luminescent Individual Swnt in Acidic Media
Aqueous suspensions of single-walled carbon nanotubes (SWNT) in anionic surfactants (SDBS, SDS) readily undergo fluorescence bleaching in acidic conditions, with faster bleaching rates for small band gap tubes. Here we show that, when individual SWNT suspensions are prepared in a mixture of anionic surfactant and polymer (SDBS and PVP), the fluorescence of the entire spectrum becomes enhanced below neutral conditions. These suspensions remain highly luminescent at pH 1 and are stable for weeks, as evidenced by their spectroscopic properties studied by fluorescence, liquid-phase Raman, UV absorbance, and near-infrared luminescence. Microscopy images obtained in both neutral and acidic conditions with Cryo-TEM and AFM corroborate the spectroscopic observations that the SWNT are suspended as individuals. The mechanism of luminescence enhancement is studied using time resolved fluorescence measurements at pH 2. It was found that after acidification the system undergoes a fast process in which the PVP behaves as a buffer between the acid and the tubes and becomes protonated. Protonation of PVP induces swelling which increases the interstitial spacing and allows the tubes to behave more as individuals. These findings open the possibly to study the behavior and properties of SWNT in fields like cell biology in which acidic media is necessary for the stability of cell culture.