The spatial fixation of bacteria , cells or enzymes in gel particles, capsules or in delimited reaction spaces is called immobilization . Immobilization leads to a shift in the catalytic activity from submicroscopic and microscopic units to macroscopically detectable particles in order to achieve retention.
Various methods of immobilization have been described in the literature. The simplest method is adsorption when the substances have sufficient affinity for one another. However, since adsorbed molecules are constantly being washed out, other methods are usually used. These can be divided into different groups of procedures:
- Covalent bond to the surface of a carrier,
- Cross-linking ,
- Membrane separation and
- Inclusion immobilization .
Binding to a surface
The simplest form of immobilization is the binding of a biocatalyst to a carrier. The adsorption is or the growth on the carrier, although very gentle on the biocatalyst, but this form is not the immobilization for all catalysts suitable and the bonding often quite weak, resulting in a continuous loss of the immobilized molecule by washing. The same applies to the ionic bond . A more stable link is achieved through a covalent bond , but the enzyme activity of the biocatalyst can be impaired . By using very porous materials, a large surface area available for binding can be achieved. However, the biocatalysts further inside are often poorly supplied with substrates due to the lower diffusion .
Cross-linking also produces enlarged particles with high catalytic activity. Either will biocatalyst crosslinked directly with itself ( English crosslinking ) or via a suitable support ( co-crosslinking ). This method is unsuitable for living cells and is used almost exclusively for the immobilization of enzymes or killed cells with an intact enzyme system. Here too, damage to the enzymatic activity cannot be ruled out. However, this method has the advantage of increased stability of the immobilizate, often without a drop in activity.
During membrane separation, the biocatalyst is enveloped by a membrane . These immobilization methods include processes as diverse as membrane reactors or hollow spheres. In membrane reactors , the cells are retained in free form in a more or less large reaction volume and, if necessary, concentrated. This process is very gentle as it leaves the biocatalyst in its usual suspended form. However, these membrane reactors often involve a high outlay in terms of equipment, are therefore expensive and mostly only suitable for high-quality applications.
In the case of inclusion immobilization, biocatalysts are embedded in a matrix that allows substrates to enter and the products formed to flow off. Different shapes of the resulting particles are conceivable. The shape is determined by the requirements of the later application as well as the type of shaping possible with the desired material. By far the most common shape is the spherical shape, which can be created by various methods. Possible processes for this are draining processes (with and without a lateral blow-off flow), vibration processes and the jet cutter process .
- Meriem Nouaimi-Bachmann: Immobilization of enzymes on polyester fleeces and their applications . 2003 ( PDF [accessed on November 10, 2010] Dissertation; Faculty of Chemistry and Pharmacy at the Eberhard-Karls-Universität Tübingen; with theoretical part: “Immobilization of enzymes”).