Magnetic nanoparticles are nanomaterials consist of magnetic elements, such as iron, nickel, cobalt, chromium, manganese, gadolinium, and their chemical compounds. Magnetic nanoparticles are superparamagnetic because of their nanoscale size, offering great potentials in a variety of applications in their bare form or coated with a surface coating and functional groups chosen for specific uses. Especially, ferrite nanoparticles are the most explored magnetic nanoparticles, which can be greatly increased by clustering of a number of individual superparamagnetic nanoparticles into clusters to form magnetic beads.
Magnetic nanoparticles can be selective attached to a functional molecules and allow transportation to a targeted location under an external magnetic field from an electromagnet or permanent magnet. In order to prevent aggregation and minimize the interaction of the particles with the system environment, surface coating may be required. The surface of ferrite nanoparticles is often modified by surfactants, silica, silicones, or phosphoric acid derivatives to increase their stability in solution. In general, coated magnetic nanoparticles have been widely used in several medical applications, such as cell isolation, immunoassay, diagnostic testing and drug delivery.
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1. Magnetic Property
The properties of magnetic nanoparticles depend on the synthesis method and chemical structure. In most cases, the magnetic nanoparticles range from 1 to 100 nm in size and can display superparamagnetism. Superparamagnetism is caused by thermal effects that the thermal fluctuations are strong enough to spontaneously demagnetize a previously saturated assembly; therefore, these particles have zero coercivity and have no hysteresis. In this state, an external magnetic field is able to magnetize the nanoparticles with much larger magnetic susceptibility. When the field is removed, magnetic nanoparticles exhibit no magnetization. This property can be useful for controlled therapy and targeted drug delivery.
2. Magnetocaloric Effect
Some magnetic materials heat up when they are placed in a magnetic field and cool down when they are removed from a magnetic field, which is defined as the magnetocaloric effect (MCE). Magnetic nanoparticles provide a promising alternative to conventional bulk materials because of their particle size-dependent superparamagnetic features. In addition, the large surface area in magnetic nanoparticles has the potential to provide better heat exchange with the surrounding environment. By careful design of core-shell structures, it would be possible to control the heat exchange between the magnetic nanoparticles and the surrounding matrix, which provide a possible way for improving therapy technologies, such as hyperthermia.
1. Magnetic separation
In a biomedical study, Isolation and separation of specific molecules including DNAs, proteins, and cells are prerequisites in most fields of biosciences and biotechnology. Among various bioseparation methods, magnetic nanoparticles based bioseparation is mostly documented and widely used due to its unique magnetic separation mood and promising efficiency. In the process, the biological molecules are labeled by magnetic nanoparticles colloids and then subjected to separation by an external magnetic field, which may be applied for cell isolation, protein purification, RNA/DNA extraction, and immunoprecipitation.
Magnetic nanoparticles particles such as beads have been extensively used for separation and purification of cells and biomolecules, due to their small size, promising separation mood, and good dispersibility. One of the trends in this subject area is the magnetic separation using antibodies conjugated with beads to provide highly accurate antibodies that can specifically bind to their matching antigens on the surface of the targeted sites.
Non-invasive imaging methods have been developed by labeling stem cells using magnetic nanoparticles. Among them, Magnetic Resonance Imaging (MRI) is widely used as diagnostic tools to present a high spatial resolution and great anatomical detail to visualize the structure and function of tissues. Several kinds of magnetic nanoparticles have been developed to improve contrast agents in MRI imaging, with significant benefits of improved sensitivity, good biocompatibility and ready detection at moderate concentrations.
Many types of magnetic nanoparticles-based biosensors have been surface functionalized to recognize specific molecular targets, due to their unique magnetic properties which are not found in biological systems. Due to different composition, size and magnetic properties, magnetic nanoparticles can be used in a variety of instruments and formats for biosensing with an enhancement of sensitivity and the stability.
4. Drug delivery
Magnetic nanoparticles have been developed and applied in localized drug delivery to tumors. The magnetic nanoparticles first act as a carrier of the drug, which are attached to its outer surface or dissolve in the coating. Once the drug coated particles have been introduced into the bloodstream of the patient, a magnetic field gradient is created by strong permanent magnet to retain the particles at the targeted region. Moreover, magnetic nanoparticles coated with a drug could be injected intravenously, transported, and retained at targeted sites, which make them highly promising system for drug delivery.
Magnetic nanoparticles with various shells
Magnetic nanoparticles have currently been explored as a technique for targeted therapeutic heating of tumors, which is called hyperthermia. Various types of superparamagnetic nanoparticles with different coatings and targeting agents are used for specific tumor sites. Magnetic particle heating can be accomplished at depths necessary for treatment of tumors located virtually anywhere in the human body. In addition, magnetic nanoparticle hyperthermia can also be used as an adjuvant to conventional chemotherapy and radiation therapy, which shows great potential.
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