The size of magnetic nanoparticles is very important. The behavior of the magnetic particle in a magnetic field gradient is dependent upon the particle’s diameter. The ability of the particle to overcome drag force and move in the direction of a magnetic field gradient is dependent on the size of the particle.
Therefore, it is important to select a nanoparticle of a suitable size for your application and separation protocol. It may be important touse a pure population of particles with similar diameters. You don’t want to purchase a mixed bag of magnetic nanoparticles with a wide distribution in size.
However, if you do happen to purchase a mixed bag of particles with a wide distribution in size then all is not lost. A chemical engineering group at Auburn University has developed a way to sort those magnetic nanoparticles into subpopulations with narrower size distributions. In a sense, they are putting the “big” particles with the big particles and the small particles with the small particles.
They achieved this by creating what they call a MagCoil. This is composed of tubing wrapped around a 2” long grade N42 diametrically magnetized neodymium cylinder. The original suspension of nanoparticles is put in through the top and the sorted samples are collected at the bottom. The idea of this apparatus is to create a battle between drag force and magnetic force.
The size of the nanoparticle determines the point at which it can no longer stay magnetically attached to the inside wall of the tubing and instead succumbs to the drag force of the fluid flowing through the tube. Three major groups were collected. The smallest particles (96nm) eluted out at the slowest flow rate of 10mL/min. The particles that eluted out at 40 mL/min were larger (123nm), and the left-over particles collected a the end were the largest (140nm). The size distribution of each of the individual groups was much narrower than that of the original sample.
Exploiting Size-Dependent Drag and Magnetic Forces for Size-Specific Separation of Magnetic Nanoparticles. Hunter B. Rogers, TareqAnani, Young Suk Choi, Ronald J. Beyers, and Allan E. David. Int. J. Mol. Sci.2015, 16(8).
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