Metal ion interactions with single walled carbon nanotubes

Abstract

The optical properties of single walled carbon nanotubes were probed for their various responses in the presence of a wide range of metal ion compounds. The effects of both the metal ion and the counter ion 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--5 mM per 15 mg.L-1 of SWNT. The decrease in fluorescence is found to be due to quenching. 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. Overall there is a dependence on the ionic radius of the metal: ions with a radius less than 1 A exhibit little quenching, but those with radii greater than 1 A show increasing quenching efficiency with increased size. 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. We propose that the SWNT exciton formed from light absorption is sensitive to its local environment, and that the field around metal ions has a significant effect on the exciton facilitating non-radiative decay paths. The ability of cobalt, copper, and nickel salts to quench SDBS surfacted SWNTs has been studied. Increased quenching is observed with transition metal ions as compared to their Group 2 and 12 analogs. 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 removal of any aggregates, the transition metals still show greater quenching efficiency than their main group homologs. 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 their Group 2 and 12 counterparts. To further understand metal ion interactions with SWNTs, a comparative study between the surfactant SDS and SDBS was conducted, which allowed for an indirect observation of the structure of the surfactant-SWNT conjugates. Greater quenching by Group 2 ions (higher KSV) was observed for SDBS-SWNTs as compared to SDS-SWNTs, which is consistent with the metal being held doser to the SWNT surface. Group 12 metal ions cause ground state changes for SDS-SWNT conjugates indicating a rearrangement of the surfactant micelle. The observation of an enhanced quenching mechanism characteristic found in transition metal quenching led to the development of a hydrophobically immobilized surfactant template for the synthesis of ultra-small copper nanoparticles. The particles are produced by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPDA) reduction of aqueous Cu2+ on a hydrophobically immobilized sodium dodecylbenzene sulfonate (SDBS) surfactant template in the presence of sodium citrate at room temperature. Single walled carbon nanotubes (SWNTs) act as a scaffold controlling the size of the SDBS micelle, which in-turn confines a limited number of copper ions near the nanotube surface. TMPDA reduction forms copper nanoparticles that are less than 2 nm. Particles formed in the absence of the SWNT immobilizer range from 2 to 150 nm.