Aqueous Redox Reaction of SDS-Encased Carbon Nanotubes with Mercuric Ions for Optical Sensing

The redox reaction and the pseudo first-order reaction kinetics of single-walled carbon nanotubes (SWNTs) dispersed in solution of a surfactant sodium dodecyl sulfate (SOS) have been studied with Hg2+ ions. The reaction is monitored by using the unique near-infrared optical absorption properties of SWNTs. Various factors including SOS and SWNT concentrations, different pH levels, and buffer constituents have been systematically examined. It is found that the reactivity is sensitive to the concentrations of SDS and SWNTs, with an optimum at 1 wt % SOS and 0.1 mg/mL SWNTs. The reaction is pH sensitive as well, and the reactivity is higher at lower pHs. The suppressed SWNT spectral intensity by the redox reaction can be recovered by adjusting pH, enabling the reusability of SWNTs. SWNTs show high selectivity and sensitivity on He over eleven other metal ions. The determined detection limit is 0.6 nM, lower than 10 nM, the maximum contaminant level for mercuric ions in drinking water. This work may provide insights for developing new SWNT-based optical sensors for mercuric ion detection.