See also “controlled pore glass” as an example of silica in the outline

A well described matrix for the purificaiton of proteins is silica. It is commercially available in various particle sizes and different pore radii to enable the user to adapt the adsorbent to the product needs. The glass surface consists of silanol groups that are easily derivatized by silanization reagents, offering a wide variety of possible ligands. Thommes et al. Biotechnology and Bioengineering, 45, pp. 205-211 (1995). 

SiO2 (silica gel or silica) is the most abundant material in the earth’s crust and has been widely used by manufacturers fo chromatographic resins due to its high mechanical strenght, high column efficiency, easily controlled particle size and porosity and low cost.

Types of Silica

Controlled pore glass (CPG), like silica, is another media used as a backbone matrix for chromatographic resins. It is generally produced form a borosilicate base material that is heated to separate the borates and the silicates. The borates are leached out from the material, leaving the silica glass with uniform controlled pores. Thus, CPG is very similar to silica with respect to its surface chemistry. (Ghose, “Preparative Protein Purificaiton on underivatized silica” Biotechnology and Bioengineering, 87(3), 2004.) Interactions with underivatized silica are largely dominated by a combination of hydrophobic and some ionic interactions. The integration of these two modes of interaction confer silica with a unique selectivity that can be effectively exploited for downstream bioprocessing. This combination of interactions makes it possible to employ silica to bind proteins in both high and low ionic strenght environments, something which is not possible in conventional hydrophobic interaciton chromatography. Ghose, “Silica and its application in downstream processing – a blast from the past. Immunex Corporation. 

CPG, which is a silicate containing support material chemically similar to silica for use in liquid chromatography, is commercially available with average particle diameter of 37-177 microns and average pore size of 40-1000 Angstroms. (Crane, US 4,606,825).

Typical silica materials include silicate containing clay minerals including talc, kaolinite, pyrophyllite, serpentine, smectite, montmorillonit, mica. Synthetic silicates include silica gels, powders, porous glass and those prepared by hydrolysis of calcium silicide or sodium silicate. (Hou, US 4,639,513). Packing materials comprising silica as a base material, which are typical chromatography packing materials, are excellent in high flow rate cahracteristics. However, they materia is unstable under alkali conditions (US 13/201,647). 

Bare silica, which is underivatized silica particles is primarily sold as a desiccation agent. Silica is a popular backbone for the production of chromatography media, since it is inexpensive and is sufficiently chemically active to accept the addition of a wide range of substituent groups. Underivatized silica is not generally used as a chromatography medium, although it is used as a filter to remove cellular debris and highly hydrophobic impurities. Silica particles are available in a variety of forms, with different sizes of particle and pore size within the particle. (Builder US 5,451,660).