| Abstract
| - Ultrathin (∼2.0 nm) films of cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetatebutyrate (CAB) supported on Si wafers have been prepared by adsorption and characterized by means of ellipsometry,atomic force microscopy (AFM), and contact angle measurements. CA, CAP, and CAB ultrathin films were characterizedin air just after their formation and after annealing under reduced pressure at temperature higher than the correspondingmelt temperature. Upon annealing, CA, CAP, and CAB ultrathin films became smoother and more hydrophobic,evidencing molecular reorientation at the solid−air interface. CA, CAP, and CAB films were used as supports forthe immobilization of lipase. The adsorption of lipase onto annealed films was more pronounced than that ontountreated films, showing the strong affinity of lipase for the more hydrophobic substrates. Enzymatic activity wasevaluated by a standard procedure, namely, (spectrophotometric) measurement of p-nitrophenol, the product formedfrom the hydrolysis of p-nitrophenyl dodecanoate (p-NPD). Lipase immobilized onto hydrophobic films exhibitedhigher activity than that of free lipase and could be recycled three times while retaining relatively high activity (lossof ca. 30% of original enzymatic activity). The effect of storing time on the activity of immobilized lipase was studied.Compared with free lipase, that immobilized onto more hydrophobic films retained 70% activity after 1 month. Moreimportantly, the latter level of activity is similar to that of free lipase. However, lipase immobilized onto morehydrophilic films retained 50% and 30% activity after 20 and 30 days, respectively. These results are explained interms of surface wettability and the contribution of the interactions between the polar residues of lipase and theglucopyranosyl moieties of cellulose ester to maintain the natural conformation of immobilized enzyme.
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