Distance within colloidal dimers probed by rotation-induced oscillations of scattered light

Abstract

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.

Rotating magnetic particles for lab-on-chip applications – a comprehensive review

“…38 With a detection angle of the scattered light at 90 degrees, Ranzoni et al 43 demonstrated that the concentration of PSA in undiluted blood plasma could be measured in the low picomolar range, by recording the amplitude of the 2f signal (at twice the driving frequency). Vliembergen et al 124,125 have extended the optical detection method by analyzing the rotation-angle dependent scattering signals. Sharp peaks in the spectrum allowed them to measure nanometer-scale inter-particle distances within the clusters.…”

Nanoscale Interparticle Distance within Dimers in Solution Measured by Light Scattering

“…For dimers, the scattered light from both individual particles is coupled and results in a spatial distribution of the scattered light that depends on their mutual distance and the dimer orientation with respect to the laser beam. The distance dependence is especially apparent for near-perpendicular scattering angles, 12 which is why a photodiode at 90° is used.…”

“…The values for the parameters are identical to those in a previous theoretical paper. 12 Specifically, the diameter of the particles is 1050 nm, the refractive index of the particles is n = 1.68 + 0.005 i , and the refractive index of the solution is 1.331. In the following, only scattering angles of 90° are considered, as these correspond to the measured PD signals at 90°.…”

“…The size distribution of the particles itself has barely any effect; as for the 3% polydispersity the shape of the signal, and thus the ratio of Fourier components, is almost identical to that for perfectly monodisperse particles. 12 …”

“…Light scattering of rotating particle dimers ( Figure 1 a) has been measured using the setup sketched in Figure 1 b. 12 Briefly, the setup consists of a square cuvette (internal size 1 mm) containing the sample solution, which is surrounded by an electromagnetic quadrupole. A laser (660 nm) is focused into the cuvette with a 150 mm lens.…”