Oxygen Reducing Enzymes Encapsulated in Silica Matrix By Chemical Vapor Deposition
University of New Mexico developed a technique for silica encapsulation - chemical vapor deposition (CVD). This technique relies on the extremely high volatility of the silica precursor (tetramethyl orthosilicate or TMOS), which interacts with water containing solution, carries out a hydrolysis and condensation creating silica gel (Fig. 1). This silica gel matrix is permeable to the electrolyte keeping the biological species hydrated.
In this study, the influence of silica encapsulation on the performance, activity, and stability of three different oxygen reducing enzymes (bilirubin oxidase, laccase and ascorbate oxidase) were evaluated. Ink composites consisting of carbon nanotubes, tetrabutylammonium bromide (TBAB) modified Nafion, 1-pyrenebutanoic acid succinimidyl ester (PBSE) and the enzymes were prepared. The inks were deposited on Rotating Disk Electrode (RDE)4. These electrodes were exposed to TMOS vapors to perform the encapsulation. The time of exposure was varied as a way to control the thickness of the created silica layer. Increased time of exposure leads to increased silica gel thickness and density. This was confirmed by the decrease of the generated current with the increased encapsulation time (Fig. 2). A thicker silicon layer introduces diffusional barrier that causes diffusional losses leading to decreased current production. Therefore the time of TMOS exposure is an important parameter that has to be carefully controlled.
Although a linear dependence of the current recorded during the RDE measurements and the square root of the rotation rate (Fig. 3) was observed we cannot use the Koutecky-Levich equation to determine the number of electrons exchanged during the oxygen reduction reaction. An alternative approach was applied.
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