Elizabeth C Landis

University of Wisconsin, Madison, Madison, MS, United States

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Publications (10)48.18 Total impact

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    ABSTRACT: We demonstrate a modular "click"-based functionalization scheme that allows inexpensive conductive diamond samples to serve as an ultrastable platform for surface-tethered electrochemically active molecules stable out to ∼1.3 V vs Ag/AgCl. We have cycled surface-tethered Ru(tpy)(2) to this potential more than 1 million times with little or no degradation in propylene carbonate and only slightly reduced stability in water and acetonitrile.
    Journal of the American Chemical Society 03/2011; 133(15):5692-4. · 10.68 Impact Factor
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    ChemSusChem 10/2010; 3(10):1176-9. · 7.48 Impact Factor
  • Xiaoyu Wang, Elizabeth C Landis, Ryan Franking, Robert J Hamers
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    ABSTRACT: Many emerging fields such as biotechnology and renewable energy require functionalized surfaces that are "smart" and highly stable. Surface modification schemes developed previously have often been limited to simple molecules or have been based on weakly bound layers that have limited stability. In this Account, we report on recent developments enabling the preparation of molecular and biomolecular interfaces that exhibit high selectivity and unprecedented stability on a range of covalent materials including diamond, vertically aligned carbon nanofibers, silicon, and metal oxides. One particularly successful pathway to ultrastable interfaces involves the photochemical grafting of organic alkenes to the surfaces. Bifunctional alkenes with a suitable functional group at the distal end can directly impart functionality and can serve as attachment points for linking complex structures such as DNA and proteins. The successful application of photochemical grafting to a surprisingly wide range of materials has motivated researchers to better understand the underlying photochemical reaction mechanisms. The resulting studies using experimental and computational methods have provided fundamental insights into the electronic structure of the molecules and the surface control photochemical reactivity. Such investigations have revealed the important role of a previously unrecognized process, photoelectron emission, in initiating photochemical grafting of alkenes to surfaces. Molecular and biomolecular interfaces formed on diamond and other covalent materials are leading to novel types of molecular electronic interfaces. For example, electrical, optical, or electromechanical structures that convert biological information directly into analytical signals allow for direct label-free detection of DNA and proteins. Because of the preferential adherence of molecules to graphitic edge-plane sites, the grafting of redox-active species to vertically aligned carbon nanofibers leads to good electrochemical activity. Therefore researchers could graft electrocatalytic materials to carbon nanofibers to develop new types of selective electrocatalytic interfaces. Extending this chemistry to include metal oxides such as TiO(2) may lead to highly specific and efficient chemical reactions and new materials with useful applications in photovoltaic and photocatalytic energy conversion.
    Accounts of Chemical Research 09/2010; 43(9):1205-15. · 20.83 Impact Factor
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    ABSTRACT: Previous work has shown that organic alkenes will graft to covalent semiconductors such as diamond and silicon. Here, we demonstrate that organic alkenes can be grafted to the surfaces of metal oxide semiconductors, including TiO2 in nanocrystalline thin films and as single-crystal anatase(001) epitaxial films grown on SrTiO3(001) substrates. The resulting layers can be used as a starting point for linking biomolecules such as DNA to metal oxide surfaces. Initial results are presented showing that this chemistry can also be applied to graft molecular layers to zirconium oxide thin films. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 01/2010; 7(2):200 - 205.
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    ABSTRACT: The use of covalently bonded molecular layers provides a way to combine the outstanding stability and electrochemical properties of carbon-based structures with the unique properties of molecular structures for applications such as electrocatalysis and solar conversion. The functionalization of vertically aligned carbon nanofibers (VACNFs) with 1-alkenes, using ultraviolet light, was investigated as a potential way to impart a variety of different functional groups onto the nanofiber sidewalls. We report how variations in the nanofiber growth rate impact both the amount of exposed edge-plane sites and the resulting electrochemical activity toward Ru(NH3)(6)(3+/2+) and Fe(CN)(6)(3-/4-) redox couples. Measurements of the distribution of surface oxides show that surface oxides are unaffected by the grafting of alkenes to the nanofibers. Carbon nanofiber reactivity was also compared to multiwalled and single-walled carbon nanotubes. Our results demonstrate that edge-plane sites are preferred sites for photochemical grafting, but that the grafting of molecular layers only slightly reduces the overall electrochemical activity of the nanofibers toward the Ru(NH3)(6)(3+/2+) couple. These results provide new insights into the relationships between the chemical reactivity and electrochemical properties of nanostructured carbon materials and highlight the crucial role that exposed edge-plane sites play in the electrochemical properties of carbon nanotubes and nanofibers.
    Chemistry of Materials. 01/2010; 22(7):2357-2366.
  • Ryan A Franking, Elizabeth C Landis, Robert J Hamers
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    ABSTRACT: Well-defined molecular layers can be formed on the surface of nanocrystalline anatase TiO2 by photochemically grafting organic molecules bearing a terminal vinyl group. The molecular layers produced are shown to have minimal oxidation and are able to be patterned and uniformly grafted through optically thick nanocrystalline films. Stability tests show that the layers have excellent stability in deionized water at 80 degrees C, aqueous solutions at pH=1.0 and pH=10.3 at 65 degrees C, and acetonitrile for time scales approaching 1200 h. Degradation of the films in deionized water occurs using a AM1.5 full-spectrum solar simulator as an illumination source but is partially suppressed by filtering with a 400 nm UV blocking filter which blocks the above-bandgap light. A mechanism is proposed for the grafting reaction in which the surface hydroxyl groups trap photoexcited holes, facilitating reaction with the vinyl group.
    Langmuir 09/2009; 25(18):10676-84. · 4.19 Impact Factor
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    ABSTRACT: TiO2 thin films are highly stable and can be deposited onto a wide variety of substrate materials under moderate conditions. We demonstrate that organic alkenes will graft to the surface of TiO2 when illuminated with UV light at 254 nm and that the resulting layers provide a starting point for the preparation of DNA-modified TiO2 thin films exhibiting excellent stability and biomolecular selectivity. By using alkenes with a protected amino group at the distal end, the grafted layers can be deprotected to yield molecular layers with exposed primary amino groups that can then be used to covalently link DNA oligonucleotides to the TiO2 surface. We demonstrate that the resulting DNA-modified surfaces exhibit excellent selectivity toward complementary versus noncomplementary target sequences in solution and that the surfaces can withstand 25 cycles of hybridization and denaturation in 8.3 M urea with little or no degradation. Furthermore, the use of simple masking methods provides a way to directly control the spatial location of the grafted layers, thereby providing a way to photopattern the spatial distribution of biologically active molecules to the TiO2 surfaces. Using Ti films ranging from 10 to 100 nm in thickness allows the preparation of TiO2 films that range from highly reflective to almost completely transparent; in both cases, the photochemical grafting of alkenes can be used as a starting point for stable surfaces with good biomolecular recognition properties.
    ACS Applied Materials & Interfaces 05/2009; 1(5):1013-22. · 5.01 Impact Factor
  • E. C. Landis, R. J. Hamers
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    ABSTRACT: Electrochemically active ferrocene groups were covalently linked to vertically aligned carbon nanofibers (VACNFs) in a simple and efficient manner via the Cu(I)-catalyzed azide alkyne cycloaddition (CuAAC), one form of "click" chemistry. The VACNFs were terminated with azide groups followed by the attachment of ethynylferrocene through a 1,4-disubstituted 1,2,3-triazole linkage. Our results show that the CuAAC reaction goes to completion in one hour and provides highly stable attachment of electrochemically active ferrocene groups to the nanofibers. X-ray photoelectron spectroscopy measurements of the density of surface-bound ferrocene molecules are in good agreement with those determined by cyclic voltammetry. The rates of electron transfer were found to be slightly faster than those measured previously through alkyl linkages to the VACNF surface. Stability tests show that the covalently grafted ferrocene groups are stable for more than 1500 repeated cyclic voltammograms and over a potential window of > 1.5 V, limited by the solvent. These results suggest that the use of "click" chemistry with VACNFs provides a facile route toward synthesis of high-surface-area electrodes with high stability and tailored electrochemical properties.
    Chemistry of Materials. 01/2009; 21(4):724-730.
  • E. C. Landis, R. J. Hamers
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    ABSTRACT: Ferrocene was used as a model system to understand the electron-transfer properties of redox-active molecules covalently linked to the surface of vertically aligned carbon nanofibers. Ultraviolet-initiated grafting of organic alkenes was used to prepare carboxylic acid-terminated layers, and ferrocene was then linked to these layers via amide groups. The electrical properties of the resulting layers were measured Using cyclic voltammetry and electrochemical impedance spectroscopy. Standard rate constants for electron transfer (k(app)) of approximately 1.0-1.3 s(-1) were found for ferrocene covalently linked to the nanofiber surfaces, compared to about 3 s(-1) on glassy carbon surfaces. Measurements of the electron-transfer rates through molecular layers of different length show no significant change. Similarly, no significant changes were observed in electron-transfer rate constants or peak width upon dilution of the ferrocene-containing molecules. Our results show that molecular layers grafted to carbon nanofibers are sparse and disordered compared with those commonly studied on planar surfaces. A model based on preferential grafting to exposed graphitic edge planes is proposed to explain the results.
    Journal of Physical Chemistry C. 01/2008; 112(43):16910-16918.
  • Chemistry of Materials. 01/2006; 18(23):5398-5400.