Novel methods for in situ characterization of individual micro- and nanoscale magnetic particles
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Introduction With increasing emphasis on the development of magnetic particles for imaging and assay applications, new instrumentation must be developed to help us understand their properties under various biologically relevant conditions: suspended in solution, lodged in tissue, or trapped in living cells. Important characteristics include particle size distribution, magnetic moment distribution, Brownian translation and rotation, dynamic translation or accumulation into a given volume, agglomeration, chaining dynamics, and in general the influence of a particle on its surroundings and vice versa. Each of these phenomena can be used as the basis to measure bio activity or as a magnetic imaging contrast agent or possibly as both. Measurements performed ex situ (often on dried powders) may not be particularly relevant, especially if the magnetic properties of the particle are a function of the local biological environment. In addition, measurements of large numbers of particles probe the ensemble average of the sample and not the properties of the individual particles as they may interact with their local surroundings. Thus, it is important to develop new instrumentation to measure the physical characteristics of individual particles in situ (i.e., in environments similar to
the in vivo or in vitro conditions where they might be used for medical applications or biological research). Several approaches for quantitative measurements of the microscopic magnetic properties of individual particles have been taken recently. For example, optical methods, aided by the availability of high-resolution imaging systems with particle tracking software, can be used to measure particle motion in microfluidic chambers.1,2 Optical methods are limited by the dynamic depth of field of the microscope and are challenging for smaller particles due to the diffraction limit. In addition, they are difficult to implement noninvasively in vivo. In situ magnetic susceptibility measurements of single particles are also possible with micro-coil detectors or microfabricated thin-film sensors.3 The particle must be in very close proximity to the detector (less than a micrometer in some cases) to ensure strong magnetic coupling. In addition, magnetic susceptibility measurements of individual particles can be difficult, especially at low frequencies ( f ) where Johnson noise, 1/f noise, and spurious electromagnetic interference obscure the magnetic induction signal coming from a single particle. In this article, we aim to briefly discuss some novel measurement techniques sensitive enough to characterize individual
John Moreland, National Institute of Standards and Technology, Boulder, CO; [email protected] Yoshihiro Nakashima, National Institute of Standards and Technology, Boulder, CO; [email protected] Jacob W. Alldredge, National Institute of Standards and Technology, Boulder, CO; [email protected] Gary Zabow, National Institute of Neurological Disorders and Stroke, Bethesda, MD; [email protected] DOI: 10.1557/mrs.2013.258
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