Chemistry of the silica magnetic beads
The center of the silica magnetic microbead is typically magnetite Fe3O4 or maghemite γ-Fe2O3, giving the bead superparamagnetic properties. In the presence of a magnetic field the beads will become magnetized in the direction of the magnetic field. When the magnet is removed, the beads lose their magnetization completely, reverting back to a fully demagnetized state (standard magnetic materials, as standard iron, have always some remnant magnetization). The beads can be coated in silica, silicon dioxide. This is typically done using a version of the Stöber method, using tetraethyl orthosilicate (TEOS) to create a SiO2 layer around the magnetic bead.
Uses of the silica magnetic bead
The silica magnetic bead can be used to isolate or purify nucleic acids from a solution. The beads are now a common and prefered technique for separating genomic DNA, plasmid DNA, or RNA. Silica magnetic beads are used to avoid contaminations and loss of material throughout the purification process, which involves various buffers, salts, and alcohol. The silica magnetic bead binds nucleic acid, but not irreversibly, and can be retained by a permanent magnet device to perform each wash or liquid removal step by simply removing unwanted material from a container while the beads with bound nucleic acid are held by magnetic force to the edge of the container.
How to bind nucleic acid to silica magnetic beads
The backbone of DNA is negative due to its phosphate backbone. DNA is propelled to bind more efficiently to silica magnetic beads by the presence of a chaotrope. A standard chaotrope called guanidinium hydrochloride is used for this process. A chaotrope will disrupt the bonds of the DNA, particularly to water in a solution. This will allow the DNA to bind elsewhere, such as to the silica coating on the magnetic bead. RNA is purified similarly, but degradation by RNAses is always a concern. For RNA, buffers and solutions are optimized to reduce degradation activity. The main procedure is to put your solution with beads in the presence of a magnet, and remove the contents while keeping the beads. Then the beads can be resuspended in a new desired solution.
Silica magnetic bead for the laboratory
There are multiple methods for nucleic acid separation that have been used before the innovation of the silica magnetic bead. One can do a spin column purification, which involves silica coated columns which capture nucleic acids in the presence of a chaotropic buffer. RNA or DNA can be extracted doing a phenol-chloroform extraction, with extra precautions taken for RNA to avoid degradation. Switching to magnetic beads purifications involved purchasing the magnets specific to biological separation, and the magnetic beads. Some laboratories will make their own beads and pre-coat their own beads. Silica magnetic beads simplify the purification process by having consistent and easy steps. When a solution is in the presence of a magnetic force, the beads will stay in the container while the rest can be completely removed. This reduces the amount of unwanted residual buffer while keeping the yield of the protocol high.
