Role of molecular interactions and structural defects in the efficient fluorescence quenching by carbon nanotubes

Fluorescence quenching effect of single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) on three most common fluorophores fluorescein, rhodamine 6G and quinine sulphate has been studied and compared. Comparative studies of quenching efficiency shows that SWCNTs are more efficient fluorescence quencher than the MWCNTs. Nature of Stern–Volmer plot was found to be highly non-linear indicating combined effect of dynamic and static quenching. The contribution of dynamic quenching component was assessed through the fluorescence lifetime measurements. Studies on vacuum annealed SWCNTs with low defect contents suggest that structural defects primarily contribute to the large quenching. Fluorescence quenching was found to be dominant even in the cases where adsorption was low implying that surface adsorption play a minor role in the quenching, except for rhodamine 6G. Adsorption isotherms have been studied using Langmuir and Freundlich models. Freundlich model was found to be closer in behaviour implying a multilayer adsorption of molecules on the surface. The contributions of metal nanoparticles and carbon impurities present in different allotropic forms to the fluorescence quenching were also assessed. We speculate that defect mediated nonradiative energy transfer through dipole–dipole coupling may be the dominant mechanism of high efficiency quenching by SWCNTs.