Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy–Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1–2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.

The hydrocarbon sensing properties of CdSe semiconductor quantum dots (QDs) tailored with benzoic, phenylacetic, 4-phenylbutanoic, or 6-phenylhexanoic acids as a surface enhancement agent have been studied to determine their dependence on the phenyl group distance from the QD surface. QDs of roughly the same size were used to attach each of the mentioned surface agents with similar surface coverages, as determined by nuclear magnetic resonance technique, followed by drop-coating and drying of the QD solutions onto porous anodic aluminum oxide substrates for sensing tests. Results indicated that normalized photoluminescence (PL) enhancement did not vary noticeably for QDs with longer chained phenyl groups compared to those with shorter length groups upon film exposure to xylenes vapors over the range of 15 to 9400 ppm in a balance of air. The quantum yield of CdSe QDs with the 6-phenylhexanoic acid surface ligand has a higher value, 29.4%, compared to those with benzoic acid, 4.5%. Moreover, the PL sensitivity toward xylenes exposure for QDs with 6-phenylhexanoic acid versus those with the surface-bound benzoic acid was about 8 times and 16 times greater at xylenes concentrations below 2500 ppm and above 4500 ppm, respectively. These results suggest that the QDs with longer length surface-enhancing ligands are more promising for this type of hydrocarbon sensing applications.

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