Dispersion and Exfoliation of Nanotubes with Synthetic Oligonucleotides: Variation of Dispersion Efficiency and Oligo-Nanotube Interaction with Base Type

Single walled carbon nanotubes (SWNTs) were dispersed and exfoliated in four different homopolymer oligonucleotides (dA15, dG15, dC15, and dT15). The dispersed nanotube concentration and degree of exfoliation were measured for each nucleobase. The nanotubes were more highly exfoliated and more temporally stable in dC15 and dT15. While the degree of exfoliation was relatively time independent, absorption and photoluminescence spectra showed definite changes over time after the initial sample preparation. In particular, photoluminescence signals appeared at well-defined times, consistent with previous evidence of time-dependent DNA wrapping followed by oxide removal. Analysis of the nanotubes’ optical properties, including circular dichroism, suggests that all bases except adenine stack onto the nanotube surface. In contrast, dA15 is unstable on the nanotube surface and eventually returns to a self-stacked arrangement. The order of the dispersion efficiencies was found to be T > C > G ≫ A, where thymine produced the most intense NT optical signals and cytosine was seen to wrap SWNTs the fastest

Role of Solubility Parameters in Understanding the Steric Stabilization of Exfoliated Two-Dimensional Nanosheets by Adsorbed Polymers

In this paper we show that graphene, hexagonal boron nitride, and molybdenum disulfide can all be exfoliated and stabilized against aggregation in solvents that cannot alone exfoliate these materials, provided that dissolved polymers are present. In each case we demonstrate this steric stabilization for a range of polymers. To understand this, we have derived an expression for the free energy of adsorption of polymer chains onto the surface of nanosheets in a solvent environment. Critically, we express all energetic interactions in terms of the Hildebrand solubility parameters of solvent, polymer, and nanosheet. This allows us to predict the dispersed nanosheet concentration to display a Gaussian peak when plotted against polymer Hildebrand parameter. This is borne out by experimental data. The model correctly (within ∼2 MPa^1/2) predicts the peak to occur when polymer and solvent solubility parameters match. In addition, the model describes both the peak width and the dependence of nanosheet concentration on polymer molecular weight. Because of the wide availability of solubility parameters for solvents, polymers, and many nanomaterials, this work is of practical importance for the production of polymer–nanosheet composite dispersions. However, more importantly it extends our understanding of the conditions required for steric stabilization and provides simple rules which define the required combination of solvent and polymer to best stabilize a given type of nanomaterial or colloid

Ordered DNA Wrapping Switches on Luminescence in Single-Walled Nanotube Dispersions

An extensive study of the time dependence of DNA wrapping in single-walled nanotube (SWNT) dispersions has been carried out, revealing a number of unusual phenomena. SWNTs were dispersed in water with salmon testes DNA and monitored over a three-month period. Between 20 and 50 days after the sample was first prepared, the SWNT photoluminescence (PL) intensity was observed to increase by a factor of 50. This increase was accompanied by a considerable sharpening of the van Hove absorption peaks. High-resolution transmission electron microscopy (HRTEM) images showed the progressive formation of a coating of DNA on the walls of the nanotubes over the three-month period. HRTEM and circular dichroism spectroscopy studies showed that the improvement in both the NIR PL intensity and the van Hove absorption peaks coincided with the completion of a monolayer coating of DNA on the SWNT walls. HRTEM images clearly showed the DNA wrapping helically around the SWNTs in a surprisingly ordered fashion. We suggest that the initial quenching of NIR photoluminescence and broadening of absorption peaks is related to the presence of protonated surface oxides on the nanotubes. The presence of an ordered DNA coating on the nanotube walls mediates both deprotonation and removal of the surface oxides. An extensive DNA coating is required to substantially restore the photoluminescence, and thus, the luminescence switch-on and subsequent saturation indicate the completion of the DNA-wrapping process. The temperature dependence of the PL switch-on, and thus of the wrapping process, was investigated by measuring as functions of temperature both the time before PL switch-on and the time required for the PL intensity to saturate. This allowed the calculation of the activation energies for both the process preceding PL switch-on and the process limiting the rise of PL intensity, which were found to be 31 and 41 kJ mol−1, respectively. The associated entropies of activation were −263 and −225 J mol−1 K−1, respectively. These negative activation entropies suggest that the rate-limiting step is characterized by a change in the system from a less-ordered to a more-ordered state, consistent with the formation of an ordered DNA coating

High Quality Dispersions of Hexabenzocoronene in Organic Solvents

We have studied the exfoliation and dispersion of hexabenzocoronene (HBC) in 28 different solvents. We see a wide range of dispersed concentrations and aggregation states, all of which can be related to the solvent properties. To a first approximation, the dispersed concentration is maximized for solvents with Hildebrand solubility parameter close to 21 MPa^1/2, similar to graphitic materials such as nanotubes and graphene. We have also studied the concentration dependence of the absorbance and photoluminescence of HBC for both a good solvent, cyclohexyl pyrrolidone (CHP), and a poor solvent, tetrahydrofuran (THF). In both cases, we observe features that can be associated with either individual molecules or aggregates, allowing us to establish metrics both for aggregate and individual molecule content. While the aggregate content always increases with concentration, good solvents disperse individual molecules at relatively high concentrations while poor solvents display aggregation even at low concentrations. Using these metrics, we determine that large populations of individual molecules are present at low concentrations in certain solvents with Hildebrand solubility parameters close to 21 MPa^1/2. However, the aggregation state of HBC is considerably more sensitive to solvent Hildebrand parameter for halogenated solvents than for amide solvents. We find a combination of high overall concentrations and large populations of individual molecules in four solvents: cyclohexyl pyrrolidone, 1-chloronaphthalene, 1-bromonaphthalene, and 1,2,4-trichlorobenzene. Scanning tunnelling microscopy (STM) measurements show the formation of self-assembled monolayers at the interface between a HBC–solvent dispersion and a highly oriented pyrolytic graphite (HOPG) substrate. Similar structures were observed on ultrathin supports by aberration-corrected transmission electron microscopy (TEM). Also observed were graphitic objects of size ∼1 nm consistent with monomers or aggregated stacks of very few monomers. We believe this is strong evidence of the presence of individual molecules in dispersions prepared with appropriate solvents