The grafting of molecular layers to carbon-based materials provides a way to combine the high chemical and thermal stability of these materials with surface properties such as chemical recognition or reactivity. The functionalization of surfaces with ultraviolet light has emerged as a way to modify difficult-to-functionalize materials, such as diamond. We have performed a combined experimental and computational investigation of the photochemical reaction of terminal alkenes with hydrogen-terminated carbon surfaces. 1-Alkenes carrying various terminal functional groups (-NHCOCF3, -NHCOO(tert-butyl), -COOCH3, -CH3) were grafted from the neat liquids using 254 nm light. These layers were characterized using X-ray Photoelectron Spectroscopy and Infrared Reflectance Absorption Spectroscopy. Pronounced differences in reactivity were observed between the molecules: trifluoroacetamide-terminated alkenes grafted the fastest and yielded self-terminating layers after approximately 4 h. Ultraviolet photoelectron spectroscopy and photocurrent measurements show that the grafting reaction involves photoemission of electrons into the liquid. Density functional calculations show that the reactivities of the four molecules are correlated with their electron affinities, with the trifluoroacetamide group acting as the best electron acceptor and having the highest reactivity. Our results demonstrate that photoejection of electrons from the solid into the acceptor levels of the alkenes initiates the functionalization reaction and controls the overall rate. Finally, marginally reactive n-alkenes were induced to react and form dense monolayers by seeding the carbon surface with small amounts of a good electron acceptor, such as the trifluoroacetamide moiety. This study provides important new mechanistic insights into the use of ultraviolet light to initiate grafting of alkenes onto surfaces.

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.

We have investigated the functionalization of vertically aligned carbon nanofibers with the redox-active protein cytochrome c and have characterized the resulting chemical and electrochemical activity. A comparison of monolayers with different terminal groups shows that those exposing carboxylic acid groups are most effective at binding active cytochrome c to carbon nanofibers. Cyclic voltammetry (CV) measurements reveal redox peaks due to electrochemical activity of the nanofiber-bound protein. CV and chemical measurements of enzymatic activity both show that nanofibers modified with cytochrome c yield approximately 10 times more activity than similarly modified surfaces of glassy carbon and gold. However, cytochrome c-modified nanofibers yield a high capacitive background, reducing the signal-to-noise ratio of the electrical measurements. We attribute this in part to inhomogeneous functionalization of the nanofibers at edge-plane versus basal-plane sites on the nanofiber surface, leading to leaky monolayers that yield increased capacitance. Our results demonstrate the ability to link chemically and electrochemically active proteins to nanofibers in a manner that preserves their activity and provide insight into the nanometer-scale factors that control the resulting chemical and electrochemical properties of biologically modified nanostructured electrodes.

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Recent studies have shown that semiconductor surfaces such as silicon and diamond can be functionalized with organic monolayers, and that these monolayer films can be used to tether biomolecules such as DNA to the surfaces. Electrical measurements of these interfaces show a change in response to DNA hybridization and other biological binding processes, but the fundamental nature of the electrical signal transduction has remained unclear. We have explored the electrical impedance of polycrystalline and single-crystal diamond surfaces modified with an organic monolayer produced by photochemical reaction of diamond with 1-dodecene. Our results show that, by measuring the impedance as a function of frequency and potential, it is possible to dissect the complex interfacial structure into frequency ranges where the total impedance is controlled by the molecular monolayer, by the diamond space-charge region, and by the electrolyte. The results have implications for understanding the ability to use molecularly modified semiconductor surfaces for applications such as chemical and biological sensing.