Electro-oxidation of glucose at an increased current density at a reducing potential

Article abstract of DOI:10.1039/b408411j

Glucose was electro-oxidized at -0.1 V vs. Ag/AgCl and at 1.5 mA cm ^-2 current density in a physiological buffer solution at 37 °C.

Full text of DOI:10.1039/b408411j

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Electro-oxidation of glucose at an increased current density at a
reducing potential
Nicolas Mano,*a Fei Maob and Adam Hellera
a Department of Chemical Engineering and Texas Material Institute, The University of Texas at Austin,
Austin 78712, USA. E-mail: mano@mail.utexas.edu
b Abbott Diabetes Care, 1360 South Loop Road, Alameda, CA 94502, USA
Received (in Cambridge, UK) 4th June 2004, Accepted 30th June 2004
First published as an Advance Article on the web 3rd August 2004
Glucose was electro-oxidized at 20.1 V vs. Ag/AgCl and at
polymer and the polyanionic enzyme precipitates. The polymer/
enzyme mass ratio can be increased if the specific activity of the
enzyme is higher and less enzyme mass is required. If the apparent
electron diffusion coefficient of the polymer is higher and less
polymer mass is required, the ratio can be decreased.
1.5 mA cm22 current density in a physiological buffer solu-
tion at 37 uC.
We described earlier a glucose electrooxidizing anode made by
‘‘wiring’’ glucose oxidase (GOx) with poly(4-vinylpyridine)-
[Os(N,N’-dimethyl-2,2’-biimidazole)3]21/31, comprising 10.7 wt%
of osmium.1 The unique features of this ‘‘wire’’ were its 13-atom
long flexible tethers, binding the redox centers to the backbone, and
the reducing redox potential of the dialkylated biimidazole complex
of Os21/31, 20.2 V vs. Ag/AgCl. The long tethers increased the
frequency of effective electron transferring collisions between
reduced and oxidized osmium centers and thereby the apparent
electron diffusion coefficient, Dapp, which reached in the crosslinked
redox hydrogel 5.8 6 1026 cm2 s21, an order of magnitude higher
than in earlier redox hydrogels.2,3 The effective collection of the
electrons from the glucose-reduced GOx allowed poising of the
anode at a potential as reducing as 20.10 V vs. Ag/AgCl, only
0.26 V positive to the redox potential of the FAD/FADH2 cofactor
in GOx at pH 7.2, where it reached a limiting current density of
Blauch and Saveant5,6 developed a bounded diffusion model to
predict Dapp for polymers when the displacement of the redox
centers is rapid and extensive. According to the model
Dapp ~ 1/6 kex (d2 1 3l2) CRT
(1)
where kex is the solution-phase self exchange rate of the redox
species, d is the characteristic electron hopping distance, l is the
distance across which the tethered redox center can actually move
and CRT is the concentration of the redox species. The model
suggested that Dapp could scale in redox hydrogels with the con-
centration of redox species assuming l remains constant over the
CRT range examined. In pratice however, decreases in Dapp with
increasing CRT, or constant Dapp values, regardless of CRT, or more
complicated profiles of Dapp vs. CRT have been observed, especially
in cases where multiply charged redox species were incorporated in
the polymer matrix.6–9
Fig. 1 represents the optimal composition (polymer/enzyme/
crosslinker) obtained for the bioelectrocatalysts made with
polymers I (solid circles), II (open circles) and III (asterisks).
Increasing the osmium wt% in the redox polymer allowed a
decrease in the polymer/enzyme weight ratio from 1.5 for the
polymer loaded with 10.7 wt% of Os, to 1 for the polymer loaded
with 12 wt% of Os, and to 0.5 for the polymer loaded with 20 wt%
glucose electrooxidation at 1.15 mA cm22
.
To build a faster glucose anode and to decrease the overpotential
for glucose electrooxidation, we synthesized two new redox
polymers with respectively 12.8 wt% and 20 wt% of osmium.
The synthetic strategy for the polymers resembled that reported.1
The NH2-spacer modified tris(N,N’-dialkylated 2,2’-biimidazole)-
Os21/31 was condensed with 6-bromohexanoic acid-quaternized
poly(4-vinylpyridine) to form the amide. The (tethered complex/
pyridine/carboxyl-pentylpyridinium) ratios were for polymer I 10.7/
85/4.3, for polymer II 12.8/85/2.2, and for polymer III 20/75/5.
As illustrated in Scheme 1, there are four electron transfer steps
in the ‘‘wired’’-GOx catalyzed electrooxidation of glucose. As
discussed in the case of the O2 cathode,4 at steady state the current
associated with each step is necessarily the same. Thus, the currents
flowing from glucose to the enzyme molecules (1), from the enzyme
molecules to the polymer (2), through the redox polymer (3), and
from the polymer to the anode (4) are all identical. The limit of each
of these currents is defined by the polymer/enzyme ratio. The limits
of (3) and (4) increase upon increasing the polymer/enzyme mass
ratio, unless the electrostatic adduct between the polycationic
Fig. 1 Dependence of the glucose electrooxidation current density on the
polymer/GOx weight percentage ratio when the bioelectrocatalyst is made
with polymer I (solid circles), polymer II (open circles), and polymer III
(asterisks). Electrode poised at 10 V vs. Ag/AgCl in a 20 mM phosphate
buffer, 37 uC, 1 mV s21 scan rate, 200 rpm, under 1 atm N2, 15 mM
glucose.
Scheme 1 The electrooxidation of glucose. Electrons are transferred from
glucose to the enzyme (1), from the enzyme to the polymer (2), through the
polymer (3) and from the polymer to the electrode (4).
2 1 1 6
C h e m . C o m m u n . , 2 0 0 4 , 2 1 1 6 – 2 1 1 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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The hydrated films of crosslinked polymer II constitute not only
the best electron conducting hydrogel to date, but also the best
‘‘wires’’ of glucose oxidase (GOx). Even though the redox potential
of the hydrogel formed upon crosslinking polymer II with
poly(ethylene glycol) diglycidyl ether (PEGDGE) is as reducing
as 2195 mV vs. Ag/AgCl, the polymer efficiently oxidizes the
FADH2 centers of glucose oxidase.
As earlier shown, the long tethers not only increase the apparent
diffusivity of electrons, but also facilitate electron transfer from the
reduced (FADH2) reaction centers of glucose oxidase to the redox
polymer. Engineering of the polymer and the higher density of
active redox sites also led to an adhering, tough, better electrode-
covering film, with the glucose flux limited current density increased
by 20%.
In combination with a reported cathode, enabling the rapid
four-electron electroreduction of molecular oxygen to water at
pH 7,4,11,12 the described glucose anode is likely to form the basis
for a compartment-less, miniature glucose/O2 biofuel cell, operating
at a voltage exceeding 10.58 V vs. Ag/AgCl, in a physiological
buffer solution at 37 uC.13–15
The study was supported by the Office of Naval Research
(N00014-02-1-0144) and by the Welch Foundation. N.M. thanks
The Oronzio de Nora Industrial Electrochemistry Fellowship of
The Electrochemical Society.
Fig. 2 Polarization of 7 mm diameter, 2 cm long carbon fiber anodes,
modified with polymer I (center), polymer II (top), or polymer III (bottom)
at the GOx/polymers/PEGDGE ratios indicated in the text. Quiescent
solution, under argon, 37.5 uC, PBS buffer, 15 mM glucose, 1 mV s21
.
of Os. The 0.5 ratio could not be exceeded for polymer III, because
of precipitation of the electrostatic adduct. The precipitation,
associated with a low current density, is attributed to formation of
a charge-neutral electrostatic adduct between the polycationic
polymer and the anionic (pI ~ 4.1) enzyme. In polymer II, with the
Os wt% increased by 1.3%, the polymer-to-enzyme ratio could be
reduced by as much as 33%, and the limiting current density
increased by 20% to 1.5 mA cm22. At this time, it is not clear
whether the cause of the increase is tighter electrostatic coupling of
the enzyme to the polymer, or enhanced electron diffusion.
The polarization of the miniature carbon fiber anodes (7 mm
diameter, 2 cm long) in a quiescent pH 7.2, 0.1 M NaCl, 20 mM
phosphate, 15 mM glucose solution under air at 37.5 uC in PBS,
modified with 1 wt% PEGDGE and containing either 45 wt%
glucose oxidase–54 wt% polymer II (top curve), 39.5 wt% glucose
oxidase–59.5 wt% polymer I (center curve) or 69 wt% glucose
oxidase–30 wt% polymer III (bottom curve) is seen in Fig. 2.
Fig. 2 shows that the carbon fiber anodes made by wiring GOx
with polymers I and II reach, respectively, limiting current densities
of 1.15 mA cm22 and 1.3 mA cm22 already at 20.1 V vs. Ag/AgCl,
only 0.26 V positive the redox potential of the FAD/FADH2
cofactor in GOx at pH 7.2.10 For both, the threshold for glucose
electrooxidation is 20.36 V vs. Ag/AgCl, the redox potential of the
FAD/FADH2 centers of GOx. The limiting current density of
glucose electrooxidation of the electrode made with polymer III is,
however, only 0.65 mA cm22 at 10.1 V vs. Ag/AgCl, well below
the 1.30 mA cm22 limiting current density of the electrode made
with polymer II.
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C h e m . C o m m u n . , 2 0 0 4 , 2 1 1 6 – 2 1 1 7
2 1 1 7