Nanoscaling Laws of Magnetic Nanoparticles and Their Applicabilities in Biomedical Sciences

Abstract

Magnetic nanoparticles, which exhibit a variety of unique magnetic phenomena that are drastically different from those of their bulk counterparts, are garnering significant interest since these properties can be advantageous for utilization in a variety of applications ranging from storage media for magnetic memory devices to probes and vectors in the biomedical sciences. In this Account, we discuss the nanoscaling laws of magnetic nanoparticles including metals, metal ferrites, and metal alloys, while focusing on their size, shape, and composition effects. Their fundamental magnetic properties such as blocking temperature (T_b), spin life time (τ), coercivity (H_c), and susceptibility (χ) are strongly influenced by the nanoscaling laws, and as a result, these scaling relationships can be leveraged to control magnetism from the ferromagnetic to the superparamagnetic regimes. At the same time, they can be used in order to tune magnetic values including H_c, χ, and remanence (M_r). For example, life time of magnetic spin is directly related to the magnetic anisotropy energy (K_uV) and also the size and volume of nanoparticles. The blocking temperature (T_b) changes from room temperature to 10 K as the size of cobalt nanoparticles is reduced from 13 to 2 nm. Similarly, H_c is highly susceptible to the anisotropy of nanoparticles, while saturation magnetization is directly related to the canting effects of the disordered surface magnetic spins and follows a linear relationship upon plotting of m_s^1/3 vs r⁻¹. Therefore, the nanoscaling laws of magnetic nanoparticles are important not only for understanding the behavior of existing materials but also for developing novel nanomaterials with superior properties. Since magnetic nanoparticles can be easily conjugated with biologically important constituents such as DNA, peptides, and antibodies, it is possible to construct versatile nano−bio hybrid particles, which simultaneously possess magnetic and biological functions for biomedical diagnostics and therapeutics. As demonstrated in this Account, nanoscaling laws for magnetic components are found to be critical to the design of optimized magnetic characteristics of hybrid nanoparticles and their enhanced applicability in the biomedical sciences including their utilizations as contrast enhancement agents for magnetic resonance imaging (MRI), ferromagnetic components for nano−bio hybrid structures, and translational vectors for magnetophoretic sensing of biological species. In particular, systematic modulation of saturation magnetization of nanoparticle probes is important to maximize MR contrast effects and magnetic separation of biological targets.