BYU Physics and Astronomy

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

We describe a technique which allows for atomic force microscopy to be used to make a physical measurement of the thickness of thin film samples. When dealing with a film which is ultrathin (<100 nm), standard measurement techniques may become difficult to apply successfully. The technique developed involves the fabrication of a distinct, abrupt step on the film surface, using a device we call the Abruptor. This step can be scanned with an atomic force microscope, revealing the height of the step. Films from 6-15 nm are now routinely measured in this way, though it is possible to apply this measurement technique to thinner and thicker films. The thinnest film we measured was 3.6 nm.

Horse spleen ferritin (HoSF) containing 800−1500 cobalt or 250−1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co−HoSF and Mn−HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 ± 0.10 e/M forming a stable M(OH)2 mineral core. At pH 9.0, ascorbic acid (AH2), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH2 shifted the reaction to completion with a M3+/AH2 stoichiometry of 1.9−2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH2 was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M-1 min-1, respectively, for Co− and Mn−HoSF at pH 9.0. From the variation of absorbance with increasing AH2 concentration, equilibrium constants at pH 9.0 of 5.0 ± 1.9 for Co−HoSF and 2.9 ± 0.9 for Mn−HoSF were calculated for 2M(O)OH + AH2 = 2M(OH)2 + D, where AH2 and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)−HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)−HoSF is 27 mV positive relative to AH2. Fe2+ in solution with Co− and Mn−HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron.

Electrical conductivity measurements were performed on single apoferritin and holoferritin molecules by conductive atomic force microscopy. Conductivity of self-assembled monolayer films of ferritin molecules on gold surfaces was also measured. Holoferritin was 5-15 times more conductive than apoferritin, indicating that for holoferritin most electron-transfer goes through the ferrihydrite core. With 1 V applied, the average electrical currents through single holoferritin and apoferritin molecules were 2.6 pA and 0.19 pA, respectively.

Three-branched DNA molecules have been designed and assembled from oligonucleotide components. These nucleic acid constructs contain double- and single-stranded regions that control the hybridization behavior of the assembly. Specific localization of a single streptavidin molecule at the center of the DNA complex has been investigated as a model system for the directed placement of nanostructures. Highly selective silver and copper metallization of the DNA template has also been characterized. Specific hybridization of these DNA complexes to oligonucleotide-coupled nanostructures followed by metallization should provide a bottom-up self-assembly route for the fabrication and characterization of discrete three-terminal nanodevices.