Abstract
Several inorganic materials are potentially suitable for enzymatic covalent immobilization, by means of several different techniques. Such materials must meet stringent criteria to be suitable as solid matrices: complete insolubility in water, reasonable mechanical strength and chemical resistance under the operational conditions, the capability to form manageable particles with high surface area, reactivity towards derivatizing/functionalizing agents. Non-specific protein adsorption should be always considered when planning covalent immobilization on inorganic solids. A huge mass of experimental work has shown that silica, silicates, borosilicates and aluminosilicates, alumina, titania, and other oxides, are the materials of choice when attempting enzyme immobilizations on inorganic supports. More recently, some forms of elemental carbon, silicon, and certain metals have been also proposed for certain applications. With regard to the derivatization/functionalization techniques, the use of organosilanes through silanization is undoubtedly the most studied and the most applied, although inorganic bridge formation and acylation with selected acyl halides have been deeply studied. In the present article, the most common inorganic supports for covalent immobilization of the enzymes are reviewed, with particular focus on their advantages and disadvantages in terms of enzyme loadings, operational stability, undesired adsorption, and costs. Mechanisms and methods for covalent immobilization are also discussed, focusing on the most widespread activating approaches (such as glutaraldehyde, cyanogen bromide, divinylsulfone, carbodiimides, carbonyldiimidazole, sulfonyl chlorides, chlorocarbonates, N-hydroxysuccinimides).
Highlights
The term “enzyme immobilization” encompasses a wide range of laboratory and industrial processes aimed at retaining a fully active enzyme on a solid insoluble support [1,2,3,4,5,6,7]
Magnetite could otherwise be prepared as particles of suitable size, so they could be directly used for covalent enzyme immobilization, or—more frequently—incorporated within particles such as silica-based materials, that are the true reacting support for enzyme immobilization
Described as supports for enzyme immobilization [137], their organic moieties are inherently part of the whole molecule rather than tethers added along functionalization/activation procedures, so they are beyond the scope of the present review
Summary
The term “enzyme immobilization” encompasses a wide range of laboratory and industrial processes aimed at retaining a fully active enzyme on a solid insoluble support [1,2,3,4,5,6,7]. Immobilized enzymes find applications in several fields, such as biosensor production [2,11,12,13], bioproduct synthesis [3,6,14,15], bioethanol and biodiesel synthesis [13,16,17,18,19,20,21], pollutant removal [12,22,23,24], and biofuel cells [12,25,26] Both organic (mainly polysaccharides, polyacrylic and polyvinylic materials) and inorganic supports (mainly silica- or other metal-oxide-based) have been described as efficient carriers for enzyme immobilization [3,6,27,28]. Covalent attachment does not usually interfere with reagents/products mass transfer, and allows the highest enhancement of operational stability (especially towards heat, pH, organic solvents, and regarding the storage) These are crucial features in the feasibility of any industrial process. Polyamides, such as nylon, find applications in this field [40,41]
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