C O MMU N I C A T I O N S
9
cystamine monolayer was used to compare it with the PQQ-
functionalized magnetic particles. Curves (a) and (b), Figure 1A,
inset, show the theoretically calculated current densities using the
Levich equation,10 eq 1, (where DNADH ) 2.4 × 10 cm ‚s is
-6
2
-1
the NADH diffusion coefficient,1 and ν ) 0.01 cm ‚s is the
1
2
-1
kinematic viscosity of water) and the experimental electrocatalytic
current densities, icat, of the Au-RDE in the presence of NADH,
0 mM, as a function of rotation speed (ω1/2 rad‚s ), respectively.
-1
5
Figure 1A inset, curve (c), shows the experimental icat of the PQQ-
functionalized magnetic particles in the presence of NADH, 50 mM.
Note that the amount of the immobilized PQQ on the Au-RDE
and on the magnetic particles (10 mg) was the same and that the
currents observed for the PQQ-magnetic particles depicted in Figure
Figure 2. (A) Cyclic voltammograms of a Au-electrode with the magneti-
cally attracted (2)-functionalized magnetic particles (6 mg) in the presence
-
5
of GOx), 1 × 10 M, and glucose, 50 mM upon rotation of the magnet
-1
(
rpm): (a) 0, (b) 10, (c) 100, (d) 400. Potential scan rate, 5 mV‚s . (B)
1A inset, curve (c), are normalized in respect to the current of the
Calibration plots for the amperometric detection of glucose (E ) 0.5 V)
upon rotation of the magnet (rpm): (a) 0, (b) 100, (c) 400. The data were
recorded in 0.1 M phosphate buffer, pH 7.0.
PQQ-functionalized Au-RDE, at low rotation speeds. This allows
extracting the effective surface area of the magnetic particles and
the effective surface coverage of PQQ on the magnetic particles
-
11
-2
experiments reveal that the electrocatalytic anodic currents are
observed only in the presence of glucose oxidase and glucose, and
that no effect of the rotation speed of the external magnet on the
electrocatalytic anodic currents generated by ferrocene-function-
(4.7 × 10
mol‚cm ).
2
/3
-1/6
1/2
icat ) 0.62‚n‚F‚D
‚ν ‚[NADH]‚ω
(1)
NADH
2
alized SiO particles is observed.
Replotting the curves (b) and (c) shown in Figure 1A, inset, in
To summarize, we demonstrate the novel application of redox-
functionalized magnetic particles and an external magnetic rotor
as a coupled system for enhancing electrocatalytic and bioelectro-
catalytic processes. The rotating magnetic particles enhance the
mass transport of substrates to the electrode and hence enhance
the electrochemical transformations at the conducting support. The
coupling of the enhanced currents, observed upon the rotation of
electrocatalytically active magnetic particles, with biomaterial
recognition events at the particles represents a novel amplification
means in biosensors and bioelectronic devices.
the form of the Koutecky-Levich plot (see the Supporting
Information) enabled us to determine the overall electrochemical
rate constants, koverall, for the oxidation of NADH by the PQQ-
4
functionalized particles and by the PQQ-modified RDE, 5.5 × 10
4
-1 -1
and 4.5 × 10
M ‚s , respectively. Following the scheme
12
suggested by Gorton for the electrocatalyzed oxidation of NADH,
eq 2, koverall is given by eq 3. By the determination of the koverall
values for the PQQ-functionalized magnetic particles in the presence
of different concentrations of NADH (data given in the Supporting
-
1
Information), we find that k+2 ) 20 s and K ) 0.6 mM. These
M
values are very similar to those derived for the electrocatalyzed
oxidation of NADH by a PQQ-monolayer associated with the RDE.
Acknowledgment. This research is supported by the German-
Israel Program (DIP).
k
+1
(
Fe O )-PQQ + NADH { }
Supporting Information Available: Koutecky-Levich plots cor-
responding to the experimental data (b) and (c) shown in Figure 1,
inset (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
3
4
k-1
H+, k+2
[
(Fe O )-PQQ‚‚‚NADH]
8
3
4
+
(Fe
O
3 4
)-PQQH
2
+ NAD (2)
References
(
koverall)- ) K /k + [NADH]/k+2
1
(3)
M
+2
(
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(
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M
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+2
(
The magnetic-field stimulated enhancement of the electrocatalytic
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(
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(
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427-439.
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(
9) Katz, E.; L o¨ tzbeyer, T.; Schlereth, D. D.; Schuhmann, W.; Schmidt, H.-
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-
5
10
M, and glucose, 50 mM, at different rotation rates of the
external magnet. Figure 2B shows the calibration curves corre-
sponding to the amperometric responses of the system at different
concentrations of glucose and variable speeds of rotation of the
external magnet. The electrocatalytic anodic currents increase as
the external rotation speed of the magnet is elevated. Control
(10) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and
Applications, Wiley: New York, 1980.
(
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(
JA0264145
J. AM. CHEM. SOC.
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