Enzyme-polymer hybrids for highly stanle functional materials and self-cleaning coatings

Abstract

Being deployed originally for biosynthesis or biodegradation, enzymes have also shown great potentials for development of functional materials. One particular challenge in deriving enzyme-based functional materials is the control of protein-material interactions for best compatibility, activity and stability. The main objectives of this study are to (1) investigate the incorporation of industrial enzymes into general purpose materials like hydrophobic coatings and hydrophilic thin films; (2) probe the fundamental mechanisms that control enzyme functionality and stability; and (3) explore novel applications in the realm of enzyme based functional materials, especially self-cleaning coatings. One major effort was dedicated to the development of enzyme-functionalized polyurethane coatings to afford a self-cleaning functionality with a reasonable lifetime. To act effectively against surface gluing organic stains, the applicability and suitability of hydrolytic enzymes like amylase, protease and lipase have been evaluated. Enzymes were incorporated into polymeric coatings in a manner to ensure high activity and stability. In the final cured coatings, a small portion of enzyme molecules is partially exposed to the outer surface, enabling the desired self-cleaning functionality; whereas the majority of the enzyme is entrapped in the interior, forming a reservoir for the gradual replenishment. This special way of enzyme immobilization enabled a high surface activity that is concomitantly accompanied by longevity as needed in practical applications. The enzyme was entrapped into coatings through direct dispersion of an aqueous enzyme solution into a hydrophobic prepolymer mixture. Enzyme loadings of up to 5% wt (dry mass) have been tested. The polymer-enzyme mixture was applied as a coating solution and subsequently cured for 30 min at 83oC, resulting in bioactive coatings with high surface enzyme activity (~2.0 unit/cm2 for á-amylase and ~0.18 unit/cm2 for protease). In the fully cured coatings the enzymes were entrapped uniformly in form of microspherical particles. Such coatings also demonstrated very good activities against real-life stains in absence of bulk water, including food stains, smashed insects and fingerprints. In comparison to enzyme-free coatings, the rate of stain removal of was accelerated by a factor up to five orders of magnitude. It was further demonstrated that self-cleaning coatings can be reused for more than 50 times. Practical applications of such bioactive coatings also require outstanding durability against weathering and other detrimental effects. Therefore, two predominant weathering factors, heat and water, which tend to inactivate enzymes easily were investigated. Coated enzymes revealed a good thermal stability against both factor and showed half lifetimes in the order of years. However, contacting the surface exposed enzyme with water resulted in a decay of the self-cleaning functionality as part of the enzyme was released from the surface. Strategies for improved enzyme retention were developed by alternating the enzyme distribution and improving crosslinking between enzyme and polymer. Several preparation parameters including pH of the enzyme solution, the post-addition of surfactant and NCO/OH ratio were found effective in manipulating the degree of enzyme-polymer crosslinking, enzyme activity, enzyme particle size and distribution. For instance, an improvement of enzyme retention by a factor of 85% was achieved for á-amylase coating when the NCO/OH ratio was increased from 1.2 to 2.4. A parallel study was performed to explore the advantages and limitations of enzyme entrapment into hydrophilic films. A hydrophilic matrix allows monophasic enzyme entrapment as in contrast to the two-phase PU coating system. Hydrogels made of polyacrylamide are known for being fully compatible with enzymes. The polymeric network is swollen in the water and becomes a rigid glass upon drying. It was assumed that final dry glassy PAG matrix had been in geometrical congruence with the structure of enzyme molecules entrapped inside, providing effective spatial confinement for the enzyme molecules. PAG with enzyme loadings of up to 2%, were prepared and examined in this study. Scanning electron microscope (SEM) characterization confirmed the dimensions of the pores of the annealed gels in the range tens of nanometers, comparable to the dimension of single protein molecules. PAG-enzyme composite showed tremendously improved enzyme stabilities, against thermal or solvent inactivation. The most dramatic stabilization was observed for PAG-glucose oxidase (GOx) composite. The half-life time for PAG-entrapped GOx at 75oC in pure ethanol was about half year, more than 6 orders of magnitude higher than that of it native counterpart, a result not achieved by another types of enzyme stabilization measures. Marked stability was further observed on the PAG-entrapped á-chymotrypsin implying such a spatial confinement method could be general to all the proteins. In summary, this work proved the feasibility of the concept of enzyme-based self-cleaning coatings. It also reveals a generic approach for preparation of a wide array of biofunctional smart coatings. We further discovered that spatial confinement of enzymes into hydrophilic polymer matrix can substantially stabilize enzymes against thermal and chemical inactivation.