Researchers
from the NIST Center for Nanoscale Science and Technology have used
three-dimensional single-particle tracking to measure the dynamic
behavior of individual nanoparticles adsorbed at the surface of
micrometer-scale oil droplets in water.
The
results revealed that the diffusion of the particles depends on their
size, with smaller particles diffusing much more slowly than expected. A
detailed understanding of how colloidal nanoparticles interact with
interfaces is essential for designing them for specific applications in
fields ranging from drug delivery to oil exploration and recovery.
The
researchers developed a feedback control system with real-time control
electronics to actuate a piezoelectric stage, moving the sample in order
to lock the moving nanoparticle in the observation volume of an optical
microscope.
The
technique, which triggers off of photons collected in situ from an
individual fluorescing nanoparticle, provides high resolution
three-dimensional position information with excellent time resolution
and with the added benefit of sensitivity to chemical activity.
Particles ranging in size from 20 nm to 2000 nm were followed in
real-time as they diffused freely in water and over the curved surfaces
of variously-sized oil droplets.
As
expected, the diffusion coefficients scaled with particle size for the
freely diffusing particles. However, there was a significant and
unexpected decrease in the diffusion coefficients for smaller (<200
nm) nanoparticles when they diffused at the oil-water interface.
Furthermore,
for a given particle size, the researchers observed a large spread in
the diffusion coefficients measured at the interface, while no such
effect was observed for the freely diffusing particles. In order to
better fit the measurements, the basic model that works well for larger
particles diffusing at a fluid-fluid interface needed to be modified to
account for line tension (the one-dimensional analogue of surface
tension) at the interface between the smaller nanoparticles, the oil,
and the water.
The
researchers believe that the variability in the diffusion coefficients
of the particles adsorbed at the interface is most probably a reflection
of subtle variations in the surface chemistry of the particles,
suggesting that diffusion measurements may provide a new way to compare
particle surface chemistries. Whereas following the dynamics of isolated
particles provides many useful insights into their behavior, typical
man-made and natural systems are usually far more complex, with
heterogeneous fluids, crowded environments, and strong particle-particle
interactions.
The
researchers believe that using real-time, three-dimensional particle
tracking to observe intentionally inserted, single tracer particles may
provide an ideal tool to probe complicated fluid systems, such as the
interior of cells, or oil/water mixtures trapped inside porous rock.
Three-dimensional real-time tracking of nanoparticles at an oil–water interface
Source: NIST
