The technique developed by Minteer et al. [20-22] involves modifying Nafion® with quaternary ammonium bromide salts. This technique provides an ideal environment for enzyme immobilization due to the biocompatibility and structure of the micellar pores. This method helps to keep the enzyme at the electrode surface as well as maintain high enzymatic activity and protect the enzyme from the surrounding environment. Previous studies by Schrenk et al. have shown that mixture-cast films of quaternary ammonium bromide salts and Nafion® have increased the mass transport of small analytes through the films and decreased the selectivity of the membrane against anions [21].

Enzyme entrapped within a micellar polymer

Metal based electrode

FIGURE 12.5 Enzymes immobilized by entrapment.

Quaternary ammonium bromide salts have a higher affinity to the sulfonic acid side chain than the proton; therefore, they can be utilized to modify the polymer and extend the enzymes lifetime because protons are less likely to exchange back into the membrane and reacidify it. A much higher preference to the quaternary ammonium bromide salts than to the proton has been shown by titrating the number of available exchange sites to protons in the membranes [21]. Due to the fact that quaternary ammounium bromide salts are larger in size than protons, the micellar structure will also be enlarged to facilitate enzyme entrapment.

Immobilizing enzymes in micellar pores will eliminate the issue of covalent bonding, which “wired” techniques are plagued with, because the process can buffer the pH of the membrane for optimal enzyme catalytic activity. In addition, the pore structure also provides a protective and restrictive 3D pore, unlike “wired” techniques where an enzyme is freely subjected to the surroundings and can be easily denatured if introduced to a harsh environment. Also, quaternary ammonium bromide salts have similar hydrophobicity as the enzymes. Nafion® modified with quaternary ammonium bromide salts will not only provide a buff­ered micellar structure for easier enzyme immobilization, but will also retain the electrical properties of unmodified Nafion® as well as increase the mass transport

2NAD

FIGURE 12.6 Oxidation schemes for methanol (top) and ethanol (bottom).

flux of ions and neutral species through the membrane minimizing problems associated with slow diffusion [20].