Abstract
Single domain magnetic nanoparticles are increasingly investigated as actuators of biological and chemical processes that respond to externally applied magnetic fields. Although their localized effects are frequently attributed to nanoscale heating, recent experimental evidence casts doubt on the existence of nanoscale temperature gradients in these systems. Here, using the stochastic Landau-Lifshitz-Gilbert equation and finite element modelling, we critically examine an alternative hypothesis that localized effects may be mediated by the induced electric fields arising from the detailed dynamical behavior of individual single domain magnetic particles. We apply our model to two significant case studies of magnetic nanoparticles in alternating magnetic fields: 1) magnetogenetic stimulation of channel proteins associated with ferritin and 2) catalytic enhancement of electrochemical hydrolysis. For the first case, while the local electric fields that ferritin generates are shown to be insufficient to perturb the transmembrane potential, fields on the surface of its mineral core on the order of 102 to 103 V/m may play a role in mass transport or release of iron ions that indirectly lead to stimulation. For the second case, our model indicates electric fields of approximately 300 V/m on the surface of the catalytic particles, with the highest interfacial electric field strengths expected during reversal events. This suggests that the nanoparticles best suited for hysteresis heating would also act as intermittent sources of localized induced electric fields in response to an alternating applied field. Finally, we put the magnitude and timescale of these electric fields in the context of technologically relevant phenomena, showing that they are generally weaker and faster.
Popular Summary The possibility of using magnetic fields to exert wireless control over biological or chemical processes has stimulated vigorous research efforts across disciplines. Magnetic nanoparticles exposed to alternating magnetic fields have repeatedly been found to exert an influence at the nanoscale, for instance triggering biological responses or regulating chemical catalysis. While these effects have been attributed to nanoscale heating, recent experiments have shown that the temperature in the vicinity of magnetic nanoparticles may not differ appreciably from their surroundings. Could another nanoscale phenomenon be at work?
Here, we critically examined the idea that electric fields induced in the immediate vicinity of magnetic nanoparticles might help explain nanoscale effects. The fact that magnetic nanoparticles thermally fluctuate is widely appreciated, but the process that dominates the generation of electric fields is the rapid (typically > 1 GHz) precession that the magnetic moment undergoes during reversal events. Combining a model of the detailed motion of a single magnetic moment with numerical calculation of the induced electric field, we consider the possible role of induced electric fields in two technologically important cases. The first is stimulation of neurons with weakly magnetic ferritin and the second is enhancement of hydrogen production by catalytic magnetic nanoparticles.
Understanding the mechanism by which magnetic nanoparticles act on their surroundings is crucial to designing more optimal materials for triggering chemical and biological processes. The role of electric fields explored here also suggests the possibility of pairing magnetic nanoparticles with resonant stimuli to directly drive precession.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵* michael.christiansen{at}hest.ethz.ch
The manuscript has been extensively revised. This version examines the original hypothesis in greater depth by using finite element methods to calculate the induced electric fields and resulting transmembrane potentials. Ultimately, we find that the inductive signal generated on the neuronal membrane by neighboring ferritin can be ruled out as a mechanism for magnetogenetic stimulation. Additionally, the scope of the paper is broadened to more generally consider electric fields in the vicinity of single domain magnetic nanoparticles undergoing thermal fluctuation and coherent reversal. We consider the electric fields induced at the surface of ferritin's mineral core, which may play a role in the mass transport of iron ions. A case study of superparagmagnetic catalytic iron carbide particles has also been included, showing that the induction of localized electric fields is correlated with reversal events during hysteresis.
