Probing Carbon Nanotube–Surfactant Interactions with Two-Dimensional DOSY NMR

Two-dimensional diffusion ordered spectroscopy (2D DOSY) NMR was used to probe the micellar structure of sodium dodecyl sulfate (SDS) and sodium cholate (SC) in aqueous solutions with and without semi­conducting and metallic single-walled carbon nanotubes (SWCNTs). The solutions contain SDS and SC at weight ratios of 1:4 and 3:2, the ratios commonly used to isolate semi­conducting and metallic SWCNTs through density gradient ultra­centrifug­ation (DGU). These results show that the coverage of surfactant on the semi­conducting and metallic SWCNTs is nearly identical in the 1:4 surfactant mixture, and a lower degree of bundling is responsible for the greater buoyancy of semi­conducting SWCNTs. In the 3:2 surfactant mixture, the metallic SWCNTs are only encapsulated in SC while the semi­conducting SWCNTs remain encapsulated in a poorly packed two-surfactant micelle, leading to a large buoyant density difference between the electronic species. This work provides insight into future directions to increase the purity of semi­conducting and metallic SWCNTs sorted through DGU and demonstrates the utility of 2D DOSY NMR in probing SWCNT–surfactant complexes

Model for Adsorption of Ligands to Colloidal Quantum Dots with Concentration-Dependent Surface Structure

A study of the adsorption equilibrium of solution-phase CdS quantum dots (QDs) and acid-derivatized viologen ligands (<i>N</i>-[1-heptyl],<i>N</i>′-[3-carboxypropyl]-4,4′-bipyridinium dihexafluorophosphate, V²⁺) reveals that the structure of the surfaces of the QDs depends on their concentration. This adsorption equilibrium is monitored through quenching of the photoluminescence of the QDs by V²⁺ upon photoinduced electron transfer. When modeled with a simple Langmuir isotherm, the equilibrium constant for QD–V²⁺ adsorption, <i>K</i>_a, increases from 6.7 × 10⁵ to 8.6 × 10⁶ M^–1 upon decreasing the absolute concentration of the QDs from 1.4 × 10^–6 to 5.1 × 10^–8 M. The apparent increase in <i>K</i>_a upon dilution results from an increase in the mean number of available adsorption sites per QD from 1.1 (for [QD] = 1.4 × 10^–6 M) to 37 (for [QD] = 5.1 × 10^–8 M) through desorption of native ligands from the surfaces of the QDs and through disaggregation of soluble QD clusters. A new model based on the Langmuir isotherm that treats both the number of adsorbed ligands per QD and the number of available binding sites per QD as binomially distributed quantities is described. This model yields a concentration-independent value for <i>K</i>_a of 8.7 × 10⁵ M^–1 for the QD–V²⁺ system and provides a convenient means for quantitative analysis of QD–ligand adsorption in the presence of competing surface processes

Evidence for a Through-Space Pathway for Electron Transfer from Quantum Dots to Carboxylate-Functionalized Viologens

Ultrafast transient absorption measurements reveal that the rate constant for photoinduced electron transfer (PET) from colloidal CdS quantum dots (QDs) to alkylcarboxylate-functionalized viologens is independent of the number of methylene groups in the alkyl chain (<i>n</i>). The rate constant for PET is (1.2 ± 0.3) × 10¹⁰ s^–1 for <i>n</i> = 1, 2, and 3, and for <i>n</i> = 0 (methylviologen). The insensitivity of the electron transfer rate constant to the length of the functional groups on the viologen suggests that a “through-space” pathway, where the electron bypasses the alkylcarboxylate and tunnels instead through only the orbitals of the QD and of the bipyridinium core, is the dominant PET pathway

Organic Surfactant-Controlled Composition of the Surfaces of CdSe Quantum Dots

The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots (QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinating solvent increases from 1.2:1 for QDs with radius <i>R</i> ≥ 3.3 nm to 6.5:1 for <i>R</i> = 1.9 nm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. The dependence of Cd/Se on <i>R</i> fits a geometric model that describes the QDs as CdSe cores with Cd/Se = 1:1 encapsulated by a shell of Cd−organic complexes. Use of 99% TOPO as the coordinating solvent produces QDs with Cd/Se ≈ 1:1 for all values of <i>R</i>, and use of 99% TOPO “doped” with <i>n</i>-octylphosphonic acid (OPA), an impurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. These results imply that Cd enrichment of the QDs is driven by tight-binding Cd²⁺−alkylphosphonate complexes that stabilize the interface between the polar CdSe core and the organic medium