Hydrodynamic Properties of Magnetic Nanoparticles with Tunable Shape Anisotropy: Prediction and Experimental Verification

Abstract

We describe the characterization of the hydrodynamic properties of anisotropic magnetic nanoparticles using a combination of transmission electron microscopy (TEM) and dynamic as well as depolarized dynamic light scattering (DLS/DDLS). The particles used are nearly monodisperse hematite spindles with an average length of 280 nm and a minor axis of 57 nm, coated with a layer of silica of variable thickness that allows us to tune the particle aspect ratio between 5 and 2. Their geometrical dimensions can thus be determined easily and quantitatively from TEM. Moreover, their size is ideal to employ DLS and DDLS to measure the translational and rotational diffusion coefficients D(T) and D(R), while the presence of a magnetic core opens a plethora of opportunities for future studies and applications. We demonstrate that we can successfully predict the hydrodynamic properties of the different particles based on a TEM characterization of their size distribution and using established theoretical models for the hydrodynamic properties of anisotropic particles. When compared with the theoretical predictions, our light scattering measurements are in quantitative agreement. This agreement between theory and experiment is achieved without having to invoke any adjustable free parameter, as the TEM results are used to calculate the corresponding diffusion coefficients on an absolute scale. We demonstrate that this is achieved due to a new and simple method for the statistical weighting of the TEM information, and the use of the correct hydrodynamic models for the observed particle shape. In addition, we also demonstrate an enhanced sensitivity of the rotational diffusion for the surface properties of ellipsoidal nanoparticles, and point out that this may serve as an ideal tool toward characterizing functionalized surfaces.

Direction Dependent Diffusion of Aligned Magnetic Rods by Means of X-Ray Photon Correlation Spectroscopy

“…week ending 25 JANUARY 2013 048301-3 similar to the ones for ellipsoidal particles of comparable size found in the literature [14,15].…”

“…While most of the existing studies address structural properties, less is known about the dynamic behavior of anisotropic particles. Several studies combining polarized and depolarized light scattering are published that give access both to the averaged translational and to rotational diffusion coefficients [11][12][13][14][15]. For ellipsoids and spherocylinders, with the averaged translational and rotational diffusion coefficients, the principal components D k and D ?…”

Anisotropic magnetic particles in a magnetic field

“…Their characteristic dimensions were determined from a random selection of 440 particles, and found to be moderately polydisperse and well described by Gaussian distributions. 19 The resulting values for the different structural elements are: the hematite core (full long axis L C = 227 ± 42 nm, full short axis d C = 51 ± 9 nm), the silica shell (polar thickness 30 ± 4 nm, equatorial thickness 27 ± 3 nm), while the PAA layer is invisible for the beam of electrons. The overall size distributions of the solid core–shell particles were: length or long axis L P = 287 ± 41 nm and diameter or short axis d P = 105 ± 8 nm, with an aspect ratio of 2.7 ± 0.3.…”

Using the discrete dipole approximation and holographic microscopy to measure rotational dynamics of non-spherical colloidal particles

“…For clarity, we denote all predicted values of diffusion coefficients with a prime symbol ( ). The diffusion coefficients of a spherocylinder can be expressed as power series in the aspect ratio ω = b/a, as discussed in the appendix of reference [27], which is adapted from references [28] and [29]:…”