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We have developed a multistep route to the formation of covalently linked adducts of single-wall carbon nanotubes (SWNT) and deoxyribonucleic acid (DNA) oligonucleotides. X-ray photoelectron spectroscopy was used to characterize the initial chemical modification to form amine-terminated SWNTs, which were then covalently linked to DNA. The resulting DNA−SWNT adducts hybridize selectively with complementary sequences, with only minimal interaction with noncomplementary sequences.

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Exposing single-walled carbon nanotubes to room-temperature UV-generated ozone leads to an irreversible increase in their electrical resistance. We demonstrate that the increased resistance is due to ozone oxidation on the sidewalls of the nanotubes rather than at the end caps. Raman and X-ray photoelectron spectroscopies show an increase in the defect density due to the oxidation of the nanotubes. Using ultraviolet photoelectron spectroscopy, we show that these defects represent the removal of pi-conjugated electron states near the Fermi level, leading to the observed increase in electrical resistance. Oxidation of carbon nanotubes is an important first step in many chemical functionalization processes. Because the oxidation rate can be controlled with short exposures, UV-generated ozone offers the potential for use as a low-thermal-budget processing tool.

We compare two different strategies for covalently modifying carbon nanofibers with biological molecules such as DNA. One method begins with a photochemical reaction between the nanofibers and molecules bearing both a terminal olefin group and a protected amine group followed by deprotection to yield the free primary amine. The second method uses a chemical reaction of an aryldiazonium salt with the nanofibers followed by electrochemical reduction to the primary amine. Both methods then link the primary amines to thio-terminated DNA oligonucleotides. Our measurements show that both methods yield DNA-modified carbon nanofibers exhibiting excellent specificity and reversibility in binding to DNA probe molecules in solution having complementary vs noncomplementary sequences. Quantitative measurements show that 2.3 × 10 14 DNA molecules/cm 2 will hybridize to the DNA-modified nanofiber samples, approximately eight times higher than for a flat sample of glassy carbon functionalized in an identical manner. Similar results were obtained comparing the amount of avidin that specifically binds to biotin-modified surfaces of nanofibers and glassy carbon. Our results demonstrate the ability to covalently functionalize nanofibers via two different methods that both provide excellent biomolecular recognition properties. Since the photochemical method uses molecules that are highly insulating while the diazonium method uses molecules bearing aromatic groups that are expected to be conductive, these methods can be used to prepare biologically modified nanofibers with a range of electrical properties that may be useful for electrical sensing of specific biomolecules in solution.

A panel of five zinc-chelated aza-macrocycle ligands and their ability to catalyze the hydration of carbon dioxide to bicarbonate, H(2)O + CO(2) → H(+) + HCO(3)(–), was investigated using quantum-mechanical methods and stopped-flow experiments. The key intermediates in the reaction coordinate were optimized using the M06-2X density functional with aug-cc-pVTZ basis set. Activation energies for the first step in the catalytic cycle, nucleophilic CO(2) addition, were calculated from gas-phase optimized transition-state geometries. The computationally derived trend in activation energies was found to not correspond with the experimentally observed rates. However, activation energies for the second, bicarbonate release step, which were estimated using calculated bond dissociation energies, provided good agreement with the observed trend in rate constants. Thus, the joint theoretical and experimental results provide evidence that bicarbonate release, not CO(2) addition, may be the rate-limiting step in CO(2) hydration by zinc complexes of aza-macrocyclic ligands. pH-independent rate constants were found to increase with decreasing Lewis acidity of the ligand-Zn complex, and the trend in rate constants was correlated with molecular properties of the ligands. It is suggested that tuning catalytic efficiency through the first coordination shell of Zn(2+) ligands is predominantly a balance between increasing charge-donating character of the ligand and maintaining the catalytically relevant pK(a) below the operating pH.

We demonstrate the electrically addressable biomolecular functionalization of single-walled carbon nanotube electrodes and vertically aligned carbon nanofiber electrodes. The method uses an electrochemical reaction in which nitro groups on specific nanostructures are reduced to amino groups and then used to covalently link DNA to only these nanostructures. We demonstrate fabrication of a four-element array of distinct DNA oligonucleotides on carbon nanotube electrodes and the addressable functionalization of submicron bundles of <100 nm diameter vertically aligned carbon nanofibers. DNA hybridization shows that the DNA-modified nanoscale structures have excellent biological selectivity.

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