Fluorescence Quenching of Single-Walled Carbon Nanotubes with Transition-Metal Ions

The ability of cobalt, copper, and nickel salts to quench SDBS surfacted single-walled carbon nanotubes (SWNTs) with a concentration between 0.5 and 5 mM per 15 mg.L-1 of SWNT has been investigated. The following metal salts show a decrease in fluorescence of the SWNTs: CuSO4, Cu(OAc)(2), CuCl2, CoSO4, Co(OAc)(2), COCl2, NiSO4, Ni(OAc)(2), and NiCl2. The Stern-Volmer quenching constants are found to depend on the identity of the metal ion and the anion but not on the chirality (similar to other ions of the same size) of the SWNT. As with Group 2 and 12 metal ions, the SWNT exciton formed from light absorption is sensitive to its local environment, and the field around the metal ions has a significant effect on the exciton facilitating nonradiative decay paths. Increased quenching is observed with transition-metal ions as compared to their Group 2 and 12 analogs, and this differs from the established charge versus ionic-volume trend observed with the latter. Some of this apparent increase is due to absorption by aggregates formed between the metal ions and the surfactant; however, these effects can be mitigated by centrifugation. Despite the removal of any aggregates, the transition metals still show greater quenching efficiency than their main group homologues. This observed effect is proposed to be due to a strong M2+...SDBS attractive interaction causing a large concentration of ions near the nanotube surface compared to those of their Group 2 and 12 counterparts. With regard to the counterion, the quenching efficiency follows the trend of Cl- approximate to SO42- > OAc-. The nondissociating transition-metal acetate complexes are less efficient in an environment where charge interactions dominate over the ionic volume-based quenching found in Group 2 and 12 salts.