Magnetic nanoparticles are leading to the development of more sensitive, rapid, and cost-effective biological sensors. These sensors can be used for a variety of applications including the detection of medical conditions and bioterrorist threats.

Recently, there has been an increased interest in magnetic nanoparticles with biologically relevant ligand coatings. These nanoparticles can have many biological and medical uses including targeted drug delivery, magnetic separation, hyperthermal treatment, and biosensors.

Researchers have developed a magnetic-optic biosensor that uses biofunctionalized magnetic nanoparticles. The long-range interaction between magnetic nanoparticles and an external magnetic field enables manipulation and sensitive detection of those particles for improved biosensors. The process involves applying a time-varying external magnetic field and a linearly polarized incident laser light to a suspension of magnetic nanoparticles. The resulting transmitted or reflected light and its polarization are recorded with a photodetector. This allows for the determination of the Brownian relaxation time, which may indicate hydro-dynamic radius changes upon chemical binding of the target to the magnetic nanoparticles.

The biosensor can be used to measure either a material's local rheological properties (i.e., properties related to flow or deformation), given that the Brownian relaxation time is proportional to the viscosity, or chemical binding events that result in an increase of the hydrodynamic radius. The latter can be used as a research tool for investigating binding kinetics in real time or as a custom-designed sensor for specific target molecules. By optimizing the magnetic nanoparticle with respect to the desired target molecule, this approach can be adapted to a wide variety of targets such as specific molecules, proteins, and disease markers. At the same time, the ability to investigate reaction kinetics directly enables the examination of how different chemical/biological environments influence the binding process.

The biosensor is unique because of its ability to enable more rapid and sensitive detection. Magnetic modulation of ferromagnetic particles in liquid can increase the signal sensitivity by several orders of magnitude. In addition to improvements in speed and sensitivity, the biosensor is potentially more cost-effective than existing technologies. lts simplicity also allows relatively untrained personnel to operate the sensor.

The shelf-life of magnetic nanoparticles can essentially be infinite, which is beneficial compared to other biosensing methods that use fluorescent and radioactive materials. The magneto-optic biosensor displays many other advantages including the simplicity of mix-and-measure assembly, the ability to obtain information beyond the biochemical affinity, and straightforward integration with microfluidics.

Biosensors have many potential commercial applications. For bioterrorism and environmental needs, they can be used for remote sensing of airborne bacteria and detecting water toxins and contaminants. ln the medical field, biosensors are used in drug discovery and evaluation and glucose monitoring in diabetes patients. This technology can be used to detect and monitor DNA hybridization, as well as DNA/RNA-protein, protein-protein, and protein-small molecule interactions. With these capabilities, the biosensor could be attractive for diagnostic applications such as detecting disease-causing bacteria and viruses.

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