Environmental Engineering Reference
In-Depth Information
by pretreatments such as polishing, heating, and acid soaking. This yields a surface
with apolar sites, polar groups, electroactive sites (quinol/quinone type), and, in
particular, carboxylates, and these functional groups provide multiple docking sites
for enzymes. The surface functionalities are also accessible to organic coupling
chemistry, allowing tailoring of the surface properties and covalent immobilization
of enzymes.
Owing to the presence of the many carboxylate groups, a pyrolytic graphite edge
surface has an estimated pK of 5.6 [Armstrong et al., 1987]. The resulting overall nega-
tive charge at physiological pH is optimal for direct adsorption of positively charged
proteins, and, in the presence of multivalent positive ions or positively charged co-
adsorbates, PGE electrodes are also suitable for direct electrochemistry of negatively
charged proteins [Hagen, 1989; Armstrong et al., 1987]. Although specific inter-
actions (hydrogen bonding, salt bridging, hydrophobic, and coordinative) may play
a role, these are not required, because multivalent counter-ions (
j
Z
j
3) tend to
form a spatially correlated Stern layer that can overcompensate the original charge,
resulting in charge inversion even at submillimolar concentrations [Shklovskii,
1999; Besteman et al., 2004].
17.2 ENZYMATIC CATHODES FOR O
2
REDUCTION
Ambient temperature catalysis of O
2
reduction at low overpotentials is a challenge
in development of conventional proton exchange membrane fuel cells ( polymer
electrolyte membrane fuel cells, PEMFCs) [Ralph and Hogarth, 2002]. In this chapter,
we discuss two classes of enzymes that catalyze the complete reduction of O
2
to H
2
O:
multi-copper oxidases and heme iron-containing quinol oxidases.
As discussed above, fuel oxidation in biology is coupled not only to respiratory O
2
reduction but also to reduction of a range of inorganic and organic oxidants, including
fumarate, nitrate, nitrite, dimethylsulfoxide (DMSO), and peroxide. Direct electroca-
talytic reduction of many of these substrates at enzyme-modified graphite electrodes
has been reported (some examples are reviewed in L´ger et al. [2003]). These electro-
des would provide anaerobic alternatives to an O
2
-reducing cathode system for an
enzyme fuel cell, but the oxidants are significantly less convenient to supply than
O
2
(air), and, with the exception of peroxide, give lower cell voltages. In this chapter,
we therefore focus on enzymatic O
2
reduction as the cathode reaction for fuel cells.
17.2.1 Laccase and Bilirubin Oxidase
Laccase and bilirubin oxidase belong to a family of multi-copper oxidases that cata-
lyze the four-electron reduction of O
2
directly to water (without releasing reactive
oxygen intermediates) coupled to oxidation of an organic substrate [Solomon et al.,
1996, 2001; Nakamura and Go, 2005]. Laccase and bilirubin oxidase both contain
four Cu atoms (Fig. 17.5) (Plate 17.2). The mononuclear “blue” Cu center serves as
the entry point for electrons donated by the organic substrate. In its oxidized state,
this center is associated with a strong ligand-to-metal charge transfer (LMCT)
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