Controlling nanoparticle aggregation in colloidal microwave absorbers via interface chemistry

ABSTRACT Interface chemistry can be implemented to modulate the aggregation and dispersion of nanoparticles in a colloidal solution. In this experimental study, we demonstrate the controlled aggregation of superparamagnetic magnetite nanoparticles in organic and aqueous solutions. With decrease in solution pH, individual nanoparticles (12-14 nm) reproducibly cluster to form ~52 nm monodisperse aggregates in toluene. Spin-spin (T2) proton relaxation measurements of the micellated clusters before and after ag gregation show a change in the molar relaxation rate from 303 sec -1 mol to 368 sec -1 mol -1 for individual and clustered nanoparticles, respectively. DNA-mediated aggregation of micellated nanoparticles in the colloidal solution is also demonstrated where the number of single-stranded DNA per particle determines the ultimate size of the nanoparticle aggregate. Keywords: Nanoparticle, magnetite, colloid, DNA, microwave, relaxation 1. INTRODUCTION Microwave absorbing materials for military applications have been investigated since the advent of radar systems. The majority of these systems, including Salisbury screens, Jaumann absorbers, radar absorbing paints, and particle laden polymeric layers, are passive in nature. There is some work with active surfaces such as va riable impedance surfaces or other tunable microwave absorbers [1] and can be thought of as switched Jaumann absorbers. Furthermore, there is at least one system described in the literature [2] that modifies the change of the permittivity of an absorbing layer by introduction of a high permittivity liquid into a porous matrix of lower permittivity. The electrostatic actuation of conducting polymers can also be utilized to actively modify the absorption characteristics via application of a low strength DC bias field. The work of Wright and coworkers [3] has examined the absorptivity of polyethylene oxide polymer absorber that contains both polyanilinetetraflouroborate and silver. More recent work with poly(3,4-ethylenedioxythiophene) and copper metal in a polyethylene LiBF