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Exceptionally Slow Movement of Gold Nanoparticles at a Solid/Liquid Interface Investigated by Scanning Transmission Electron Microscopy
Gold
nanoparticles were observed to move at a liquid/solid interface
3 orders of magnitude slower than expected for the movement in a bulk
liquid by Brownian motion. The nanoscale movement was studied with
scanning transmission electron microscopy (STEM) using a liquid enclosure
consisting of microchips with silicon nitride windows. The experiments
involved a variation of the electron dose, the coating of the nanoparticles,
the surface charge of the enclosing membrane, the viscosity, and the
liquid thickness. The observed slow movement was not a result of hydrodynamic
hindrance near a wall but instead explained by the presence of a layer
of ordered liquid exhibiting a viscosity 5 orders of magnitude larger
than a bulk liquid. The increased viscosity presumably led to a dramatic
slowdown of the movement. The layer was formed as a result of the
surface charge of the silicon nitride windows. The exceptionally slow
motion is a crucial aspect of electron microscopy of specimens in
liquid, enabling a direct observation of the movement and agglomeration
of nanoscale objects in liquid
The Intrinsically Disordered CāRING Biomineralization Protein, AP7, Creates Protein Phases That Introduce Nanopatterning and Nanoporosities into Mineral Crystals
We
report an interesting process whereby the formation of nanoparticle
assemblies on and nanoporosities within calcite crystals is directed
by an intrinsically disordered C-RING mollusk shell nacre protein,
AP7. Under mineralization conditions, AP7 forms protein phases that
direct the nucleation of ordered calcite nanoparticles via a repetitive
protein phase deposition process onto calcite crystals. These organized
nanoparticles are separated by gaps or spaces that become incorporated
into the forming bulk crystal as nanoporosities. This is an unusual
example of organized nanoparticle biosynthesis and mineral modification
directed by a C-RING protein phase
An Oligomeric CāRING Nacre Protein Influences Prenucleation Events and Organizes Mineral Nanoparticles
The
mollusk shell nacre layer integrates mineral phases with macromolecular
components such as intracrystalline proteins. However, the roles performed
by intracrystalline proteins in calcium carbonate nucleation and subsequent
postnucleation events (e.g., organization of mineral deposits) in
the nacre layer are not known. We find that AP7, a nacre intracrystalline
C-RING protein, self-assembles to form amorphous protein oligomers
and films on mica that further assemble into larger aggregates or
phases in the presence of Ca<sup>2+</sup>. Using solution nuclear
magnetic resonance spectroscopy, we determine that the protein assemblies
are stabilized by interdomain interactions involving the aggregation-prone
T31āN66 C-terminal C-RING domain but are destabilized by the
labile nature of the intrinsically disordered D1āT19 AA N-terminal
sequence. Thus, the dynamic, amorphous nature of the AP7 assemblies
can be traced to the molecular behavior of the N-terminal sequence.
Using potentiometric methods, we observe that AP7 protein phases prolong
the time interval for prenucleation cluster formation but neither
stabilize nor destabilize ACC clusters. Time-resolved flow cell scanning
transmission electron microscopy mineralization studies confirm that
AP7 protein phases delay the onset of nucleation and assemble and
organize mineral nanoparticles into ring-shaped branching clusters
in solution. These phenomena are not observed in protein-deficient
assays. We conclude that C-RING AP7 protein phases modulate the time
period for early events in nucleation and form strategic associations
with forming mineral nanoparticles that lead to mineral organization
Formation and Structure of Calcium Carbonate Thin Films and Nanofibers Precipitated in the Presence of Poly(Allylamine Hydrochloride) and Magnesium Ions
That the cationic
polyelectrolyte polyĀ(allylamine hydrochloride)
(PAH) exerts a significant influence on CaCO<sub>3</sub> precipitation
challenges the idea that only anionic additives have this effect.
Here, we show that in common with anionic polyelectrolytes such as
polyĀ(aspartic acid), PAH supports the growth of calcite thin films
and abundant nanofibers. While investigating the formation of these
structures, we also perform the first detailed structural analysis
of the nanofibers by transmission electron microscopy (TEM) and selected
area electron diffraction. The nanofibers are shown to be principally
single crystal, with isolated domains of polycrystallinity, and the
single crystal structure is even preserved in regions where the nanofibers
dramatically change direction. The formation mechanism of the fibers,
which are often hundreds of micrometers long, has been the subject
of intense speculation. Our results suggest that they form by aggregation
of amorphous particles, which are incorporated into the fibers uniquely
at their tips, before crystallizing. Extrusion of polymer during crystallization
may inhibit particle addition at the fiber walls and result in local
variations in the fiber nanostructure. Finally, we investigate the
influence of Mg<sup>2+</sup> on CaCO<sub>3</sub> precipitation in
the presence of PAH, which gives thinner and smoother films, together
with fibers with more polycrystalline, granular structures
Ultrastructure and Crystallography of Nanoscale Calcite Building Blocks in <i>Rhabdosphaera clavigera</i> Coccolith Spines
Coccolithophores create an intricate
exoskeleton from nanoscale
calcite platelets. Shape, size, and crystal orientation are controlled
to a remarkable degree. In this study, the structure of <i>Rhabdosphaera
clavigera</i> is described in detail for the first time through
a combination of electron microscopy techniques, including three-dimensional
electron tomography. The coccolithophore exhibits several micrometer
long 5-fold symmetric spines with diameters of approximately 0.5 Ī¼m.
The nanorystals constituting the spine are arranged radially along
the longitudinal axis, protruding from the almost flat disks that
form the coccosphere. The stem of the spine is shown to consist of
{104} calcite rhombohedra single crystalline platelets, arranged in
five separate spiral āstaircasesā. The spine tip shows
15 structural elements: five large āpanelsā protruding
outward along the lateral plane and five leaf-shaped smaller units
which form the topmost steps of the staircases. The outer tip consists
of five long thin platelets protruding along the length of the spine
axis. This feature extends downward into the spine-core. This core-feature
may serve as a base for crystal nucleation and assembly analogous
to the proto-coccolith ring in the <i>V/R</i> growth model
(Young J. R. et al. Nature, 1992, 356, 516ā518). However, we find significant dissimilarities
of the crystal elements constituting the spine in comparison to that
model