Metal-mediated electrochemical oxidation of DNA-wrapped carbon nanotubes.

As part of the ongoing effort to describe electron transfer reactions of carbon nanotubes (CNTs), we studied the mediated electrochemical oxidation of CNTs solubilized by wrapping with a T(60) deoxyribooligonucleotide. Cyclic voltammetry revealed that the oxidation of this CNT-DNA material by electrogenerated ML(3)(3+) mediators completes a catalytic cycle that increases the oxidative current compared to that obtained by voltammetry of the mediator alone (M = Fe(III), Ru(III), or Os(III); L = 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine). We observed a greater increase in current at higher nanotube concentration, slower experimental scan rate, and higher mediator redox potential (E(0)'). Using computer simulation, these observations were shown to be consistent with CNT oxidation involving the removal of multiple electrons from each CNT-DNA moiety (the T(60) oligonucleotide was chosen because it is not oxidized by any of the mediators). The data are well-described by a simulation model based on the classical catalytic mechanism (EC') with the following embellishment: three populations of CNT-DNA redox-active sites with different E(0)' and therefore different oxidation rates are employed to represent the varying redox potentials of different valence band electrons within one CNT chiral type and within the distribution of CNT types present in our sample. This modeling suggests the number of CNT-DNA sites available for oxidation increases with the E(0)' of the mediator. This result can be qualitatively interpreted in terms of CNT band theory.

Effect of ancillary ligands on the DNA interaction of [Cr(diimine)3]3+ complexes containing the intercalating dipyridophenazine ligand.

The synthesis of photoluminescent Cr(III) complexes of the type [Cr(diimine)(2)(DPPZ)](3+) are described, where DPPZ is the intercalating dipyridophenazine ligand, and diimine corresponds to the ancillary ligands bpy, phen, DMP, and TMP (where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, DMP = 5,6-dimethyl-1,10-phenanthroline, and TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline). For TMP, DMP, and phen as ancillary ligands, the complexes have also been resolved into their Lambda and Delta optical isomers. A comparison of the photophysical and electrochemical properties reveal similar (2)E(g) --> (4)A(2g) (O(h)) emission wavelengths and lifetimes, and a variation of 110 mV in the (2)E(g) excited state oxidizing power. A detailed investigation has been undertaken of ancillary ligand effects on the DNA binding of these complexes with a range of polynucleotides. For all four complexes, emission is quenched by the addition of calf thymus B-DNA, with the emission lifetime data yielding bimolecular quenching rate constants close to the diffusion controlled limit. Equilibrium dialysis studies have established a general predilection for AT base binding sites, while companion experiments with added distamycin (a selective minor groove binder) provide evidence for a minor groove binding preference. For the case of [Cr(TMP)(2)(DPPZ)](3+), concomitant equilibrium dialysis and circular dichroism measurements have demonstrated very strong enantioselective binding by the Lambda optical isomer. The thermodynamics of DNA binding have also been explored via isothermal titration calorimetry (ITC). The ITC data establish that the primary binding mode for all four Cr(III) complexes is entropically driven, a result that is attributed to the highly favorable free energy contribution associated with the hydrophobic transfer of the Cr(III) complexes from solution into the DNA binding site.

The quasicatalytic mechanism: a variation of the catalytic (EC') mechanism.

The classic electrochemical catalytic mechanism, often referred to as the EC' mechanism, is traditionally represented by the two reactions A + e <==> B (E(A/B)(0), k(A/B)(0), alpha(A/B)) and B + P <==> A + Q (K(eq), k(f), k(b)). Implicit in this mechanism is the additional heterogeneous electron transfer P + e <==> Q (E(P/Q)(0), k(P/Q)(0), alpha(P/Q)). To observe EC' behavior, the following conditions must be met (we focus on cyclic voltammetric responses): (1) E(P/Q)(0) > E(A/B)(0) (ensuring that K(eq) > 1), (2) k(P/Q)(0)c(P) exp[-alpha(P/Q)(F/RT)(E - E(P/Q)(0))]/(0.446c(A)(FD(A)|v|/RT)(1/2)) << 1 over the potential range of interest (ensuring that the reaction P + e <==> Q does not occur to any significant extent relative to the peak current for reaction A + e <==> B alone), (3) k(f)c(P)RT/F|v| > 1 (ensuring that the catalytic effect is significant). We offer arguments based on Marcus theory that when condition 2 is met, fulfilling condition 3 will be difficult. This could explain why EC' behavior is rare. In the present work we show that EC'-like cyclic voltammetric responses can be obtained even when P + e <==> Q is facile if D(P,Q) (the diffusion coefficient for the substrate-couple species P and Q) is much smaller than D(A,B) (the diffusion coefficient for the mediator-couple species A and B). When D(P,Q)/D(A,B) is sufficiently small, the system behavior becomes identical to that seen for the classical EC' system. We suggest that this "quasicatalytic" behavior should be considered when EC'-like behavior is observed and when the electrochemical system involves a substrate couple whose diffusion coefficients are much smaller than those of the mediator couple. As has been known for some time, when the diffusion coefficients of species A, B, P, and Q are identical (an assumption commonly made to simplify theoretical analysis) and when both heterogeneous electron transfers are reversible, the homogeneous kinetics have no effect on the cyclic voltammetric response--even though the distribution of species in the diffusion layer is dramatically altered.

Atomic force microscopy studies of DNA-wrapped carbon nanotube structure and binding to quantum dots.

Single-stranded DNA is an effective noncovalent dispersant for individual single-walled carbon nanotubes (CNTs) in aqueous solution, forming a CNT-DNA hybrid material that has advantages for CNT separations and applications. Atomic force microscopy (AFM) reveals a regular pattern on the surface of CNT-DNA. We found this pattern to be independent of the length and sequence of the wrapping DNA, yet different from the structures observed for CNTs dispersed with sodium dodecyl sulfate in the absence of DNA. We wrapped CNTs with thiol-modified DNA to form stable conjugates of CNT-DNA and core/shell CdSe/ZnS quantum dots; AFM imaging of these conjugates identified for the first time the location of DNA on the CNT-DNA nanomaterial. Our results suggest that the AFM pattern of CNT-DNA is formed by helical turns (approximately 14-nm pitch) of wrapped DNA strands that are closely arranged end-to-end in a single layer along the CNT. This work demonstrates the useful functionalization of CNTs with quantum dots in a manner that avoids direct, destructive modification of the CNT surface and suggests nearly complete surface coverage of the nanotubes with DNA.