Abstract:Aggregation processes of colloidal particles are of broad scientific and technological relevance. The earliest stage of aggregation, when dimers appear in an ensemble of single particles, is very important to characterize because it opens routes for further aggregation processes. Furthermore, it represents the most sensitive phase of diagnostic aggregation assays. Here, we characterize dimers by rotating them in a magnetic field and by recording the angle dependence of light scattering. At small scattering angles, the scattering cross section can be approximated by the total cross-sectional area of the dimer. In contrast, at scattering angles around 90 degrees, we reveal that the dependence of the scattering cross section on the dimer angle shows a series of peaks per single 2π rotation of the dimers. These characteristics originate from optical interactions between the two particles, as we have verified with two-particle Mie scattering simulations. We have studied in detail the angular positions of the peaks. It appears from simulations that the influence of particle size polydispersity, Brownian rotation and refractive index on the angular positions of the peaks is relatively small. However, the angular positions of the peaks strongly depend on the distance between the particles. We find a good correspondence between measured data and calculations for a gap of 180 nm between particles having a diameter of 1 micrometer. The experiment and simulations pave the way for extracting distance-specific data from ensembles of dimerizing colloidal particles, with application for sensitive diagnostic aggregation assays.
We
demonstrate a novel approach to quantify the interparticle distance
in colloidal dimers using Mie scattering. The interparticle distance
is varied in a controlled way by changing the ionic strength of the
solution and the magnetic attraction between the particles. The measured
scaling behavior is interpreted using an energy–distance model
that includes the repulsive electrostatic and attractive magnetic
interactions. The center-to-center distances of particles with a 525
nm radius can be determined with a root-mean-square accuracy of 12
nm. The data show that the center-to-center distance is larger by
83 nm compared to perfect spheres. The underlying distance offset
can be attributed to repulsion by charged protrusions caused by particle
surface roughness. The measurement method accurately quantifies interparticle
distances that can be used to study cluster formation and colloid
aggregation in complex systems, e.g., in biosensing applications.
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