Brownian motion of aggregating nanoparticles studied by photon correlation spectroscopy and measurements of dynamic magnetic properties

We have investigated colloidal stability of magnetic nanoparticle suspensions in different buffer systems and NaCl concentrations commonly used for biological applications. We have also investigated how conjugation of proteins to magnetic nanoparticles affects colloidal stability. Two different techniques, giving complementary information on the state of the particle system studied, have been used and compared. We have monitored the rotational Brownian motion of particles using measurements of dynamic magnetic susceptibility in the frequency domain. The results were processed using an algorithm that enables us to quantify changes of particle size distribution for particle suspensions subjected to various buffer conditions. The measurements were compared to results obtained for the translational Brownian motion of the same nanoparticles using photon correlation spectroscopy (PCS). We demonstrate that the complementarity of the two techniques enables more precise characterization of particles in suspension, particularly for suspensions of particles with a wide distribution in size and shape, or systems that are close to the onset of agglomeration.