Morphological and mechanical properties of hybrid matrices of polysiloxane–polyvinyl alcohol prepared by sol–gel technique and their potential for immobilizing enzyme
Introduction
Recent technologies in enzyme immobilization required materials that combine properties that cannot be found in conventional matrices. In this case, suitable techniques to produce a designed material that can fulfill all requirements to suit the enzyme and its application are needed to be developed. Among these techniques, the sol–gel process appears to be the best choice. This process involves the transition of a system from a liquid ‘sol’ (mostly colloidal) into a solid ‘gel’ phase. By applying this methodology, it is possible to fabricate ceramic or glass materials in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials [1].
The sol–gel technique is also an excellent method to prepare hybrid material [1], [2]. The low temperature synthesis enables organic or inorganic species to be incorporated into rigid silicon oxide matrices without degradation. The resulting composite combines the chemical and physical properties of the guest with the excellent optic, thermal, and chemical stability of the host silicon oxide matrices [3]. These matrices have also large surface areas, high porosity, inertness and stability in the presence of chemical and physical agents [3], [4], rendering them adequate as support for immobilizing enzyme or other biological molecules [4], [5], [6], [7].
Among the available enzymes, lipases (triacylglycerol alkylhydrolases, EC 3.1.1.3) are versatile biocatalysts used in several industrial processes under aqueous or organic media [8]. To take full advantage of this versatility, efficient methods for immobilizing lipases are needed since immobilization may protect the enzyme to some extent from solvent denaturation and allows enzyme reuse and thus reduces overall process costs [9]. In addition to the ease of handling, immobilized enzymes are well suited for use in continuous packed-bed or fluidized-bed reactors.
Recently, a hybrid matrix of polysiloxane–polyvinyl alcohol (POS–PVA) has been related as a successful support for immobilizing lipases from several sources to be used in different purposes [10], [11], [12]. The procedure used tetraethoxysilane and polyvinylalcohol for the matrix formation, in which reactants are used at fixed proportions following a standard methodology [10], [11]. To use these immobilized derivatives in different bioreactor configurations, other support properties need to be analyzed before hand, particularly in relation to its mechanical strength, porosity and hardness. However, it appears that no attempts have been made to quantify those properties for POS–PVA particles in the context of their use for lipase immobilization.
Thus the present work deals with the preparation of POS–PVA matrices using different molar ratios between the reactants in the matrix formation. The goal was to evaluate physical properties (porosity and hardness) and potential of these matrices to be used as support for immobilizing lipase. Candida rugosa lipase (CRL) was chosen as study model in order to compare the results obtained here with our previous studies [10], [11], [12]. To render functional groups for covalent binding the lipase, the matrices were previously activated with sodium metaperiodate.
Section snippets
Materials
Commercial C. rugosa lipase (Lipomod™ 34P) was kindly provided by Biocatalysts (Cardiff, England). Tetraethoxysilane (TEOS) was acquired from Sigma–Aldrich Co. (St. Louis, MO, USA); commercial hydrochloric acid (36%), ethanol (95%), polyvinyl alcohol (MW 72 000, 97.5–99.5 mol% hydrolysis degree; viscosity of 24–32 mPa s, 4% in H2O under 20 °C), polyethylene glycol (MW 1500) and arabic gum were purchased from Reagen Chemical Co (São Paulo, Brazil). Sodium metaperiodate was from Nuclear (São Paulo,
Results and discussion
The BET method was used to evaluate the surface area, the pores size and volume for the different matrices produced. These analyses were performed before and after the matrix activation with sodium metaperiodate, and the results are shown in Table 1. No data was reported for assays carried out with the highest PVA concentration (2.09 × 10−4 M) as no homogeneous mixture was formed, which failed the transition of the colloidal into a solid ‘gel’ phase.
As can be seen in Table 1 for non activated
Conclusion
Varying the PVA concentration in the synthesis of a hybrid matrix of POS–PVA strongly affected the morphological and mechanical properties of the resulting particles. Moreover, the matrix activation with sodium metaperiodate also modified the material properties. For the activated matrices, decreasing the PVA concentration resulted in lower specific surface area and pore volume values, with an average pore size of about 30–50 Å. The microhardness of the particles was inversely proportional to
Acknowledgments
The authors acknowledge the financial assistance from Fundação de Apoio a Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). They also acknowledge the Biocatalysts for the lipase sample.
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