Fluorescence quenching of single-walled carbon nanotubes in SDBS surfactant suspension by metal ions: Quenching efficiency as a function of metal and nanotube identity

The effects of both the metal ion and the counterion on the fluorescence of SDBS-surfacted single-walled carbon nanotubes (SWNTs) have been investigated, for solutions of group 2, 12, and 13 metal salts with a [Mn+] of between 0.5 and 5 mM per 15 mg.L-1 of SWNT. The following metal salts cause a decrease in fluorescence of the SWNTs: MgCl2, Mg(SO4), Mg(OAc)(2), CaCl2, Ca(OAc)(2), SrCl2, BaCl2, Ba(OAc)(2), Zn(SO4), CdCl2, Cd(SO4) Cd(OAc)(2), HgCl2, and Hg(OAc)(2). In contrast, Ga-2(SO4)(3), and Al(NO3)(3) show no reduction in fluorescence over the concentration range studied. The decrease in fluorescence is found to be due to quenching rather than bundling, acid suppression, or spectral bleaching. The Stern-Volmer quenching constants are found to depend on the identity of the metal ion, the anion, and the diameter (related to the n,m value) of the SWNT. For group 2 metals, the quenching efficiency follows the trend of increasing ionic radii (Mg < Ca < Sr < Ba), whereas the relationship between the group 12 metals is more complex (Zn < Cd Hg). Overall there is a dependence on the ionic radius of the metal: ions with a radius less than I A exhibit little quenching, but those with radii greater than 1 A show increasing quenching efficiency with increased size. With regard to the counterion, the quenching efficiency follows the trend of Cl- approximate to SO42- < OAc-. The Stern-Volmer quenching constants for a particular metal/anion combination show a linear correlation with the SWNT band gap and an inverse, but equal, relationship for the diameter of the SWNT. The formation of a nonflorescent ground-state complex or spectral bleaching may be precluded as possible mechanisms for the quenching, but there is clearly a perturbation of the electronic structure of the SWNTs as indicated by a change in the intensity of the radial breathing mode. We propose that the SWNT exciton formed from light absorption is sensitive to its local environment and that the field around the metal ions has a significant effect on the exciton facilitating nonradiative decay paths. The lack of quenching caused by Al3+ and Ga3+ salts is consistent with either their speciation as M(OH)(4)(-) ions under the conditions employed or the effect on the SWNT exciton being related to charge density.