Confined Brownian motion of individual magnetic nanoparticles on a chip: Characterization of magnetic susceptibility

Abstract

An increasing number of biomedical applications requires detailed knowledge of the magnetic susceptibility of individual particles. With conventional techniques it is very difficult to analyze individual particles smaller than 1μm. The authors demonstrate how the susceptibility of individual nanoparticles can be determined in an efficient way by optically analyzing the confined Brownian motion of a nanoparticle trapped in a known magnetic potential well on a chip. A setup is introduced that has a controllable two-dimensional magnetic potential well, which is defined by an integrated microscopic current wire. Susceptibility measurements have been performed on 150–450nm superparamagnetic beads. They found differences in bead susceptibility of an order of magnitude and differences in volumetric susceptibility of more than a factor of 2.

Accurate quantification of magnetic particle properties by intra-pair magnetophoresis for nanobiotechnology

“…10 However, the characterization of magnetic properties remains challenging. The magnetic moments of single particles have been measured by Brownian motion analysis, 11 field sensing, 12,13 and magnetophoresis, [14][15][16] but the reported methods all suffer from large measurement uncertainties (above 30%) because of large uncertainties in the fields and field gradients applied within the reference frames of the particles.…”

Analysis of individual magnetic particle motion near a chip surface

“…However, then one is limited by a short measurement time per particle; one particle can only be analyzed for about 10 s before either diffusing out of the field of view or before an increasing particle concentration on the wire leads to undesired particle-particle interactions. 27 In this section we will present two designs to catch individual particles in a three-dimensional potential well instead of a two-dimensional potential well, thus greatly enhancing the measurement time per particle. The capture in the third ͑y͒ dimension can be generated either sterically or magnetically.…”

“…27 The magnetic susceptibility is calculated from the thermal distribution of particle positions in the direction perpendicular to the wire,…”

Bond characterization by detection and manipulation of particle mobility in an optical evanescent field biosensor

“…The range in forces is probably caused by the fact that the particle diameters vary up to a factor of two, so magnetic gradient forces may vary an order of magnitude. [24,25] Next, we investigated the influence of the upper magnet on the particle mobility. We found that the upper magnet strongly confines the height of the bound particles (within 5 to 30 nm), corresponding to forces between 0.4 and 2.4 pN.…”