Using fluorescence quenching of single walled carbon nanotubes with metal ions as a probe of surfactant$\ctdot$SWNT interactions

The treatment of SDS-SWNTs with the Group 2 metal ions Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ causes quenching of the SWNT's fluorescence in the same manner but to a lesser extent (lower K_sv) as they quench SDBS surfacted nanotubes. In contrast, the quenching with the Group 12 metal ions Zn²⁺, Cd²⁺ and Hg²⁺ does not follow a simple Stern-Volmer behavior. Instead, two effects are observed. First, the different chirality nanotube's emission wavelengths are shifted by all Group 12 metals but to varying degrees. Second, the corrected decreases in fluorescence intensity is only affected above a critical concentration of metal ion, [M²⁺]_c, at which there is a steep change in fluorescence intensity, then above this concentration there is no significant further reduction in fluorescence intensity. There is a distinct relationship between the identity of the metal ion and the critical concentration: [Zn²⁺]_c > [Cd²⁺]_c > [Hg²⁺]_c. The differentiation between SDBS- and SDS-surfacted SWNTs can be rationalized by a consideration of the relative hardness of the metal ions and the sulfonate and sulfate “ligands”, in particular, the structure of the SDS-SWNT conjugate is overcome by the addition of the Group 12 metal but not by Group 2 metals.