4 research outputs found
Molecular Precision at Micrometer Length Scales: Hierarchical Assembly of DNA–Protein Nanostructures
Robust self-assembly across length
scales is a ubiquitous feature of biological systems but remains challenging
for synthetic structures. Taking a cue from biologywhere disparate
molecules work together to produce large, functional assemblieswe
demonstrate how to engineer microscale structures with nanoscale features:
Our self-assembly approach begins by using DNA polymerase to controllably
create double-stranded DNA (dsDNA) sections on a single-stranded template.
The single-stranded DNA (ssDNA) sections are then folded into a mechanically
flexible skeleton by the origami method. This process simultaneously
shapes the structure at the nanoscale and directs the large-scale
geometry. The DNA skeleton guides the assembly of <i>RecA</i> protein filaments, which provides rigidity at the micrometer scale.
We use our modular design strategy to assemble tetrahedral, rectangular,
and linear shapes of defined dimensions. This method enables the robust
construction of complex assemblies, greatly extending the range of
DNA-based self-assembly methods
Fabrication of Scanning Electrochemical Microscopy-Atomic Force Microscopy Probes to Image Surface Topography and Reactivity at the Nanoscale
Concurrent mapping
of chemical reactivity and morphology of heterogeneous
electrocatalysts at the nanoscale allows identification of active
areas (protrusions, flat film surface, or cracks) responsible for
productive chemistry in these materials. Scanning electrochemical
microscopy (SECM) can map surface characteristics, record catalyst
activity, and identify chemical products at solid–liquid electrochemical
interfaces. It lacks, however, the ability to distinguish topographic
features where surface reactivity occurs. Here, we report the design
and fabrication of scanning probe tips that combine SECM with atomic
force microscopy (AFM) to perform measurements at the nanoscale. Our
probes are fabricated by integrating nanoelectrodes with quartz tuning
forks (QTFs). Using a calibration standard fabricated in our lab to
test our probes, we obtain simultaneous topographic and electrochemical
reactivity maps with a lateral resolution of 150 nm
Local Structure and Global Patterning of Cu<sup>2+</sup> Binding in Fibrillar Amyloid‑β [Aβ(1–40)] Protein
The amyloid-β (Aβ) protein forms fibrils
and higher-order
plaque aggegrates in Alzheimer’s disease (AD) brain. The copper
ion, Cu<sup>2+</sup>, is found at high concentrations in plaques,
but its role in AD etiology is unclear. We use high-resolution pulsed
electron paramagnetic resonance spectroscopy to characterize the coordination
structure of Cu<sup>2+</sup> in the fibrillar form of full-length
Aβ(1–40). The results reveal a bis-<i>cis</i>-histidine (His) equatorial Cu<sup>2+</sup> coordination geometry
and participation of all three N-terminal His residues in Cu<sup>2+</sup> binding. A model is proposed in which Cu<sup>2+</sup>-His6/His13
and Cu<sup>2+</sup>-His6/His14 sites alternate along the fibril axis
on opposite sides of the β-sheet fibril structure. The local
intra-β-strand coordination structure is not conducive to Cu<sup>2+</sup>/Cu<sup>+</sup> redox-linked coordination changes, and the
global arrangement of Cu sites precludes facile multielectron and
bridged-metal site reactivity. This indicates that the fibrillar form
of Aβ suppresses Cu redox cycling and reactive oxygen species
production. The configuration suggests application of Cu<sup>2+</sup>-Aβ fibrils as an amyloid architecture for switchable electron
charge/spin coupling and redox reactivity
Effect of Tin Doping on α-Fe<sub>2</sub>O<sub>3</sub> Photoanodes for Water Splitting
Sputter-deposited films of α-Fe<sub>2</sub>O<sub>3</sub> of
thickness 600 nm were investigated as photoanodes for solar water
splitting and found to have photocurrents as high as 0.8 mA/cm<sup>2</sup> at 1.23 V vs the reversible hydrogen electrode (RHE). Sputter-deposited
films, relative to nanostructured samples produced by hydrothermal
synthesis,, permit facile characterization of the role
and placement of dopants. The Sn dopant concentration in the α-Fe<sub>2</sub>O<sub>3</sub> varies as a function of distance from the fluorine-doped
tin oxide (FTO) interface and was quantified using secondary ion mass
spectrometry (SIMS) to give a mole fraction of cations of approximately
0.02% at the electrolyte interface. Additional techniques for determining
dopant density, including energy dispersive X-ray spectroscopy (EDS),
electron energy loss spectroscopy (EELS), electrochemical impedance
spectroscopy (EIS), and conductivity measurements, are compared and
discussed. Based on this multifaceted data set, we conclude that not
all dopants present in the α-Fe<sub>2</sub>O<sub>3</sub> are
active. Dopant activation, rather than just increasing surface area
or dopant concentration, is critical for improving metal oxide performance
in water splitting. A more complete understanding of dopant activation
will lead to further improvements in the design and response of nanostructured
photoanodes