ABSTRACT
The class of elements (iron, nickel, and cobalt) and their chemical compounds that can be manipulated using the magnetic ϐield are called magnetic nanoparticles (MNPs).1 Scaling down the material to the nano-scale range changes the fundamental structure of the material structure. This is because, when the grain size is reduced, the normal macroscopic domain structure transforms into a single domain state at a critical size that typically lies below 100 nm. Once this transformation occurs, the mechanism of magnetization reversal can only be via the rotation of the magnetization vector from one magnetic easy axis to another via a magnetically ϐirm direction. This change of reversal mechanism and the basic underlying physical mechanism governing it was ϐirst discussed by E. C. Stoner.2 These magnetic materials for their splendid features were used in applications of biology and medicine since 1960s. However, for
biomedical applications, magnetite is one of the most commonly used magnetic materials because it has strong magnetic property and low toxicity.3,4 MNPs are of great interest and have been widely used in various disciplines, such as magnetic ϐluid5 catalysis,1 biotechnology/biomedicine,3,6,7 and magnetic resonance imaging (MRI),8 of medical ϐields with a multiskill approach (Fig. 6.1). A number of suitable methods have been deployed for the synthesis of MNPs of various compositions. However, the formed particles perform the best when the size of the nanoparticles is below the critical domain, which is dependent on the material typically around 10-20 nm. So each nanoparticle becomes a single domain and demonstrates superparamagnetic behavior. Superparamagnetism is a form of magnetism that appears in small ferromagnetic or ferrimagnetic nanoparticles. In nanoparticles, magnetization can randomly ϐlip directions under the inϐluence of temperature. The typical time between two ϐlips is called the Neel relaxation time. In the absence of an external magnetic ϐield, when the time used to measure the magnetization of the nanoparticles is much longer than the Neel relaxation time, their magnetization appears to be in average zero; hence, they are said to be in the superparamagnetic state. In this state, an external magnetic ϐield is able to magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than that of paramagnets. Such individual nanoparticle has a large constant magnetic moment and behaves like a giant paramagnetic atom with a fast response to the magnetic ϐield with negligible remnance (residual magnetism) and coercivity (the ϐield required to bring the magnetism to zero). These features make superparamagnetic nanoparticles very attractive for a broad range of biomedical applications because the risk of agglomeration becomes negligible at room temperature. Therefore, magnetic materials have their own advantages that provide many exciting opportunities in biomedical applications. First, their controllable sizes range in nanometers, and their optimization of sizes and properties that enable the site-speciϐic delivery of the therapeutic agent. Second, nanoparticles can be manipulated by an external magnetic force. This “action at a distance” provides a tremendous advantage for many applications. Third, MNPs play an important role as MRI contrast-enhancement agents because the signal of the magnetic moment of a proton around MNPs can be captured by resonant absorption.
