What are the common enzyme immobilization methods?

Enzyme immobilization methods may be categorized as physical or chemical based on the binding property. Physical enzyme immobilization methods include adsorption, entrapment, and encapsulation. Chemical methods include covalent binding, cross linking, and affinity immobilization. 

• Adsorption involves attaching enzymes onto the surface of a solid support material through forces such as hydrophobic interactions and salt bridges. To achieve this, the enzymes may be dried onto electrode surfaces or the support material may be immersed in the enzyme solution. In this technique, the adsorbed enzymes are protected from factors such as aggregation, proteolysis, and interaction with hydrophobic surfaces. 
• In the Entrapment technique, enzymes are physically entrapped within a porous polymer or gel matrix. There may be covalent or noncovalent bonds between the enzyme and its matrix, which is typically water-soluble. Also known as lattice entrapment, this method immobilizes the enzymes while allowing substrates and products to diffuse freely. 
• Encapsulation involves securing or enclosing enzymes in an aqueous solution within a semi-permeable capsule or membrane. This allows smaller substrates and products to pass through while restricting the migration of larger enzymes. The efficiency of this immobilization technique depends on the enzyme’s stability.   
• In Covalent Binding, enzymes are covalently bonded to support materials such as agarose, porous silica, or polyacrylamide. The binding strength is strong and stable and no enzymes are lost in this method. 
• Cross Linking involves cross-linking enzymes with each other, so the enzymes act as their carrier. No support material is used. This immobilization technique avoids the downsides of using carriers, resulting in a pure enzyme product. 
• In affinity immobilization, enzymes are attached to support materials under varying physiological conditions. This technique uses the specific binding properties of the enzyme.