Characterization of Individual Magnetic Nanoparticles in Solution by Double Nanohole Optical Tweezers

We study individual superparamagnetic Fe3O4 (magnetite) nanoparticles in solution using a double nanohole optical tweezer with magnetic force setup. By analysis of the trapping optical transmission signal (step size, autocorrelation, the root-mean-square signal, and the distribution with applied magnetic field), we are able to measure the refractive index, magnetic susceptibility, remanence and size of each trapped nanoparticle. The size distribution is found to agree well with scanning electron microscopy measurements, and the permeability, magnetic susceptibility and remanence values are all in agreement with published results. Our approach demonstrates the versatility of the optical tweezer with magnetic field setup to characterize nanoparticles in fluidic mixtures with potential for isolation of desired particles and pick-and-place functionality.