Powering fuel cells through biocatalysis - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 403

exhaustive and other physiological tissues may be considered as well for an eventual
implantation.
The dioxygen partial pressure in blood vessels is about 95 mmHg in arteries and
40 mmHg in veins, which corresponds to a concentration of respectively 2.14

10 4 M and 5.4 10 5 M in free dioxygen. The concentration of glucose in blood ves-

sels is ca. 9 mM in healthy adults. Chloride is the anion present in the highest concentra-
tion in physiological media with chloride concentration in blood about 136-145 mM. It is
an important plasmolyte to take into account as it can be an inhibitor of some enzymes,
for instance via binding to copper sites in some oxidases. Other anions present in appre-
ciable concentration are bicarbonate (

10 mM), phosphate ( 0.6 mM), and sulfate

( 0.2 mM). Sodium is the main cation in blood (132 to 144 mM) followed by potassium

4 mM, calcium 2 mM, and magnesium 1 mM. Other elements such as aluminum, copper,
fl uorine, iron, and zinc are present in low concentration ( M), while manga-

nese, lead, tin, bromine, and iodine are present in very low concentration (

5 to 30 µ

M).
Blood is a buffered solution with a pH of 7.4. Other body fl uids are also close to neutral
pH, like, for example, saliva at the pH of 7.2. A notable exception is acidic gastric juices
(pH 2). The human body is also thermostated at 37ºC, a peculiarity common to all mam-
mals where constant body temperature lies in the range 34 to 40ºC depending on the
species. The velocity of blood in blood vessels is of the order of 1 to 10 cm/s. Other body
fl uids may be considered as well, for instance the pH of tears is ca. 7.5 and contains,
not surprisingly, low levels of glucose (50-500

0.1 to 5 µ

µ M).

12.6.2 Assembled glucose-oxygen biocatalytic fuel cells
Many assembled glucose-O 2 biocatalytic fuel cells have been reported in recent years.
This section aims at giving an overview of the methods that have been used to build
these biocatalytic fuel cells and of the performances obtained by these devices depend-
ing on their different designs.
Katz et al. [17] reported on a biocatalytic fuel cell based on the surface reconstitu-
tion of apo-GOx onto FAD that was previously coupled to a pyrrolo-quinoline qui-
nine (PQQ) relay conjugated to a self-assembled monolayer of cysteamine on gold.
The cathode consisted of a cross-linked affi nity complex between cytochrome c (Cyt. c )
and cytochrome c oxidase (COx) assembled on gold through a thiol modifi ed Cyt. c
promoter. The promoter is a maleimide monolayer that is specifi cally reacted with the
cysteine102 residue of Cyt. c from Saccharomyces cerevisae ensuring proper alignment
between the electrode surface and the heme site of the protein, and consequently elec-
trical communication between the electrode and the protein. Electron transfer between
the biocatalyst and the electrode was particularly effi cient at the anode and thought
not to be perturbed by molecular oxygen, the natural electron donor, or by typical
interfering species like ascorbic acid or uric acid. Comparatively, interfacial electron
transfer was much less effi cient at the cathode. The reduction of O 2 occurred at 0 V vs
SCE at the cathode while oxidation of glucose at the anode occurred at the potential of
PQQ ( 0.12 V vs SCE at pH 7). The maximum reported power for this cell was 4 µ W

Wcm 2 ). at a load of 0.9 k and an electromotive force of 50 mV (power density 5 µ