A Hydrogen Peroxide Sensor Prepared by Electropolymerization of Pyrrole Based on Screen-Printed Carbon Paste Electrodes

Abstract

A disposable amperometric biosensor for commercial use to detect hydrogen peroxide has been developed. The sensor is based on screen-printed carbon paste electrodes modified by electropolymerization of pyrrole with horseradish peroxidase (HRP) entrapped. The facture techniques of fabricating the enzyme electrodes are suitable for mass production and quality control. The biosensor shows a linear amperometric response to H2O2 from 0.1 to 2.0 mM, with a sensitivity of 33.24 µA mM⁻¹ cm⁻². Different operational parameters of electropolymerization are evaluated and optimized.

Full text

Sensors 2007, 7, 239-250
sensors
ISSN 1424-8220
© 2007 by MDPI
www.mdpi.org/sensors
Full Paper
A Hydrogen Peroxide Sensor Prepared by
Electropolymerization of Pyrrole Based on Screen-Printed
Carbon Paste Electrodes
Guang Li
1,*
, You Wang
2
and Hui Xu
1
1 National Laboratory of Industrial Control Technology, Zhejiang University, Hangzhou 310027, P.R.
China
Tel: +86-13558068126, Fax: +86-571-87952233-8228, E-mail: guangli@zju.edu.cn (Guang Li).
E-mail: hxu2@iipc.zju.edu.cn (Hui Xu)
2 Department of Biomedical Engineering, Zhejiang University, Hangzhou 310027, P.R. China
E-mail: king_wy@zjuem.zju.edu.cn
* Author to whom correspondence should be addressed.
Received: 5 January 2007 / Accepted: 28 February 2007 / Published: 5 March 2007
Abstract: A disposable amperometric biosensor for commercial use to detect hydrogen
peroxide has been developed. The sensor is based on screen-printed carbon paste electrodes
modified by electropolymerization of pyrrole with horseradish peroxidase (HRP) entrapped.
The facture techniques of fabricating the enzyme electrodes are suitable for mass production
and quality control. The biosensor shows a linear amperometric response to H
2
O
2
from 0.1
to 2.0 mM, with a sensitivity of 33.24 µA mM
-1
cm
-2
. Different operational parameters of
electropolymerization are evaluated and optimized.
Keywords: Electropolymerization; Biosensor; Polypyrrole; Horseradish Peroxidase; Screen-
Printed Electrodes
1. Introduction
The determination of hydrogen peroxide is of considerable interest, because hydrogen peroxide is
not only an important analyte in food, pharmaceutical, clinical, industrial and environmental analyses
but also playing a key role as the product of the enzymatic reaction in coupled enzyme systems [1].
Several analytical techniques have been employed for this determination, such as titrimetry [2],

Sensors 2007, 7
240
spectrometry [3], chemiluminescene [4, 5], but these techniques suffer from interferences, long
analysis time and use of expensive reagents. Electroanalytical methods [6-11] have also been found
suitable since they achieve low detection limits and rapid response time. Coupled with enzymatic
reactions, it is promising for fabrication of simple and low-cost enzyme sensors. Until now,
horseradish peroxidase (HRP) has become a commonly used enzyme to construction hydrogen
peroxide biosensors.
It is well known that direct electron transfer from the reduced enzyme to a distant electrode is
negligible. To improve it, redox enzyme catalysis can be mediated by a variety of organic and
inorganic species which act efficiently in competition with the natural acceptors (or donors) of
electrons [12]. The most widely used mediators are ferrocene derivatives [9, 13], although other types
are employed [14].
The technique to immobilize the enzyme is one of the key issues in developing a reliable biosensor.
Many strategies have been used including direct adsorption, crosslinking with glutaraldehyde, covalent
binding, and entrapment in polymerized films or gels [15]. As carrier material in immobilizing
enzyme, conducting polymers have been receiving great and broad interests. The immobilization of an
enzyme into an electropolymerized film offers many attractive features since the process is
instrumentally controlled and an enzyme electrode can be easily prepared in a rapid one-step
procedure. The most popular conducting polymers for the immobilization of enzyme are polypyrrole
(PPy) [16, 17], polyaniline [18] and polythiophene Especially, PPy and its derivatives are most widely
used for entrapping enzyme, because PPy can be easily electrodeposited onto an electrode surface from
aqueous solutions, which are compatible with most biological elements [15, 19].
Many researches indicated that the enzyme can be entrapped by PPy matrix immobilized on the
surface of Pt, Au and glassy carbon (GC) electrodes. But electropolymerization on the surface of
screen-printed carbon paste electrodes has been few reported. Screen-printing technology is a low-cost
technology which allows depositing thick films (a few to hundreds of micrometers) and is well suited
for mass production and portable devices [20]. They allow fast and easy monitoring. For example,
disposable screen-printed enzyme strips are widely used by diabetic patients for self-monitoring of
their blood glucose levels [21]. Such a microfabrication route offers high-volume production of
extremely inexpensive and yet highly reproducible disposable enzyme electrodes.
In this paper, the fabrication of a disposable hydrogen peroxide amperometric biosensor based on
screen-printing technology was described. Carbon and silver ink were used to form electrodes while
silver was employed as conductive lead. HRP was immobilized in a PPy film electropolymerized on
the surface of screen-printed carbon paste electrodes. Potassium ferrocyanide was deposited as electron
transfer mediator to facilitate efficient electron transfer. The biosensor can determine hydrogen
peroxide concentration using only 1 µL sample. Different operational parameters influencing the
biosensor response, i.e. monomer, electrolyte and HRP concentrations, PPy film growth rate and
thickness were evaluated and optimized.

Sensors 2007, 7
241
2. Experimental
2.1. Materials and apparatus
HRP (>300 U/mg) was purchased from Qiude Biochemical Engineering CO., Ltd., China. Hydrogen
peroxide (30%, w/w), pyrrole (98%), potassium ferrocyanide, lithium perchlorate (LiClO
4
) and other
chemicals were of analytical grade without further purification. Carbon paste (Jelcon CH-10) was
purchased from Jujo Chemical CO., Ltd., Japan. Double-distilled water was used in all experiments.
0.25 M hydrogen peroxide standard solution was prepared every three days and stored at room
temperature without light. Diluted hydrogen peroxide standard solutions were freshly prepared directly
prior to use, carried out in a 0.2 M phosphate buffered saline (PBS) solution (pH 7.0) containing 0.2M
NaCl.
Amperometric measurements were performed by a CHI760B Electrochemical Work Station (CH
Instrument Inc., USA). During the electropolymerization, a platinum disk (2 mm
2
) and Ag/AgCl were
used as counter and reference electrodes, respectively. The electrodes were screen-printed using a MT-
750A screen printing machine (Ming Tai Screen Printing Machine CO., Ltd., Taiwan)
2.2. Electrode preparation
Screen-printed electrodes were fabricated on polypropylene (PP) sheets using the screen-printer.
Three masks were used to form sliver conducting lead wires, carbon paste film for electrochemical
reaction and insulating film, the process has been described in detail previously [22]. The structure of
the biosensor is shown in Figure 1, consisting of an Ag/AgCl pseudo-reference electrode and two
carbon electrodes, which acting as working and counter electrodes, respectively. The function of the
silver leads was to improve electric conductivity of the electrodes. The reaction area was defined by the
insulating film covering on the carbon paste film. The size of working electrode in reaction area was 1
mm×2 mm while the sensor substrate was 35 mm×10 mm.
Figure 1. The structure of the screen-printed electrodes, the sensor substrate was
35 mm×10 mm while the working electrode area was 1 mm×2 mm.

Sensors 2007, 7
242
2.3. Fabrication of enzyme electrode
The hydrogen peroxide biosensor was fabricated by electropolymerization in a 5 mL 0.2 M PBS
solution (pH 7.0) containing 0.075 M pyrrole, 0.075 M LiClO
4
and 0.8 mg/mL HRP. One of the
screen-printed carbon paste electrodes was used as working electrode while the platinum disk and
Ag/AgCl were used as counter and reference electrodes, respectively. Electrochemical polymerization
carried out potentiostatically at +1 V vs. Ag/AgCl until a final optimum charge deposit (50 mC/cm
2
)
was reached. The electrodes were washed and rinsed in double-distilled water to remove the unfixed
enzyme and then were dried at room temperature. Subsequently 1 µL PBS solution (pH 7.0) containing
0.1 M potassium ferrocyanide was coated on reaction area uniformly and dried at room temperature in
dark. For comparison, a biosensor was fabricated in a similar way except the HRP (2 mg/mL) was
immobilized by physical adsorption.
2.4. Measurement
Amperometric measurements were carried out using a CHI760B electrochemical workstation. The
working potential was -300 mV vs. the screen-printed Ag/AgCl pseudo-reference electrode, 1 µL
hydrogen peroxide standard solution was dropped onto reaction area of enzyme electrode uniformly.
Current-time curves of the amperometry were recorded using an IBM PC compatible computer via a
RS232 series port communicating to the electrochemical workstation at room temperature. The
response time of the sensors, time needed to reach a plateau corresponding to the steady state when the
testing sample was added, was 40 seconds. The response current to H
2
O
2
was determined by
subtracting the background current from the observed current. The calibration curve was obtained with
testing samples of different hydrogen peroxide concentration to investigate the characteristics of the
biosensor to determine hydrogen peroxide concentration.
3. Results and discussions
3.1. Optimization of the electropolymerization
The sensitivity of a hydrogen peroxide biosensor depends on the activity of the immobilized HRP
enzyme in the PPy film. The activities of the immobilized HRP enzyme were electrochemically
analyzed.
In the presence of 1 µL 1 mM H
2
O
2
solution, the effects of pyrrole concentration, PPy film
thickness, electropolymerization potential, LiClO
4
concentration and HRP concentration on the
response of the hydrogen peroxide biosensor were evaluated.
3.1.1. Pyrrole concentration
In potentiostatic electropolymerization mode, pyrrole monomer concentration is an important factor
which influences the development of the polymer. A constant potential of +1.0 V vs Ag/AgCl was

Sensors 2007, 7
243
applied to achieve the charge deposit of 50 mC/cm
2
, while various concentrations of pyrrole were used.
Figure 2 shows the response current to 1 mM H
2
O
2
of the sensors with the film electropolymerized
using pyrrole of different concentration. Lower pyrrole concentration did not allow sufficient polymer
formation and HRP entrapment onto the electrode surface. Optimized pyrrole concentration, 0.075 M,
was selected in the experiments.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0. 02 0. 04 0 .0 6 0. 08 0.1 0.12
[pyrrole] (M)
Response current (µA)
Figure 2. Effect of pyrrole concentration on response current to
1 mM H
2
O
2
(1 µL, -300 mV potential applied).
3.1.2. Density of charge deposit
The thickness of the PPy film was determined by the charge deposited during electropolymerization.
A thick PPy film could entrap much enzyme, but retard the analyte diffusion. In the research on a PPy
glucose biosensor, the PPy film has been proven to act as a serious diffusion barrier, both the diffusion
rates of glucose to the immobilized GOx and H
2
O
2
to the platinum electrode were retarded severely by
the polymer film [23]. However on hydrogen peroxide biosensors, Tatsuma indicated that the PPy film
functioned as a conductive material, a part of the electrode material, and didn’t retard the transport of
the mediators [7]. Our experiment results are shown in Figure 3. When the charge deposited less than
25 mC/cm
2
, the response current (curve c) was very small because too little enzyme was entrapped. In
the range from 50 to 150 mC/cm
2
, the response current to H
2
O
2
changed due to the background current
(curve b) increasing significantly though the observed current (curve a) remained almost unchanged. It
was observed that although the thickness of PPy did not retard the transport of the mediators, it
affected the amount of enzyme entrapped within the film and the background current to change the
sensitivity of the sensor. In our experiments, the density of charge deposit of 50 mC/cm
2
was selected.

Sensors 2007, 7
244
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 5 5 0 75 100 12 5 15 0 17 5
Density of charge deposit (mC/cm
2
)
Response current (µA)
Figure 3. Effect of the density of charge deposit on (a) observed current; (b) background current
and (c) response current to 1 H
2
O
2
(1 µL, -300 mV potential applied).
3.1.3. Potential for electropolymerization
The effect of electropolymerization potential on enzyme electrode response was evaluated because
the potential determined the growth rate of polymer film. With the increase of potential from 0.8 V to
1.2 V, the peak current during electropolymerization process increased from 18 to 175 µA to affect the
sensitivity of the sensor, so that the electropolymerization potential should be optimized. When
hydrogen peroxide concentration was 1 mM, the potential dependence of response current was shown
in Figure 4. The results indicated that the maximum response current was obtained when
electropolymerization potential was 1.0 V, which was accordingly used in our experiments.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.7 0.8 0.9 1 1.1 1.2 1.3
Electropolymerization ptential (V)
Response current (µA)
Figure 4. Effect of electropolymerization potential on response current to
1 mM H
2
O
2
(1 µL, -300 mV potential applied).

Sensors 2007, 7
245
3.1.4. LiClO
4
concentration
The electrolyte concentration should be high enough to facilitate the charge transfer in solution, but
it should not compete with the pyrrole monomer cation radicals. Both LiClO
4
and KCl were commonly
used as electrolyte for pyrrole polymerization [6, 8, 24, 25]. Comparing both electrolytes, we used
LiClO
4
in the experiments due to its better catalyze effect besides the effect of
4
-
ClO
doping. When
LiClO
4
concentration was within a range of 0.075 to 0.15 M, the electropolymerized enzyme sensor
performed well, so the LiClO
4
concentration of 0.075 M was selected for our experiments.
3.1.5. Enzyme concentration
The HRP concentration in the pyrrole monomer solution for preparing the enzyme electrode was a
key factor to affect the sensitivity, because the sensor response depended upon the amount of
immobilized HRP. Figure 5 shows the response current to 1 mM H
2
O
2
of the sensors using HRP of
different concentration. With the increase of HRP concentration from 0.2mg/ml to 0.8mg/ml, the
response current was almost doubled. When the HRP concentration was higher than 0.8 mg/ml, the
response current of the sensor did not increase further. So the HRP concentration of 0.8mg/ml was
used in the experiments for the economic consideration.
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
[HRP] (mg/ml)
Response current (µA)
Figure 5. Effect of HRP concentration on response current to
1 mM H
2
O
2
(1 µL, -300 mV potential applied).
3.2. pH and electrolyte influences on biosensor response
The pH influence was investigated by amperometric measurement of 1 mM H
2
O
2
in PBS 0.067 M
at different pH values between 5.0 and 9.0 as shown in Figure 6. The maximum response current was
observed at pH 7.0 in agreement with Ref.[26]. In order to obtain the maximum bioactivity and optimal
sensitivity, PBS solution of pH 7.0 was selected for our experiments.

Sensors 2007, 7
246
0
0.05
0.1
0.15
0.2
0.25
0.3
4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5
pH
Response current(µA)
Figure 6. Effect of pH on response current to 1 mM H
2
O
2
(1 µL, -300 mV potential applied).
Phosphate (0.033, 0.067, 0.1, 0.2 M) was chosen to study the influence of electrolyte concentration.
The results showed that the effect of phosphate concentration on the response was little. So 0.2 M PBS
solution was selected in the subsequent experiments.
3.3.
Effect of ferrocyanide on the response current of the biosensor
The ferrocyanide improves the charge transfer between the redox active site of the enzyme and the
electrode, and should exhibit a fast electron exchange rate, while their electrochemical transformation
usually takes place at low potentials to avoid the interference of other electroactive species normally
present (e.g. ascorbic acid and uric acid). Figure 7 shows the cyclic voltammetry of the electrode in the
absence (B) and presence (A) of ferrocyanide. Comparing Figures 7(A) and 7(B), it is obviously that
with the presence of mediator, the reduce reaction took place at lower potentials (0 V vs. -0.5 V). From
Figure 7(B), it indicated that without the ferrocyanide as mediator, biosensor response was likely
caused by the direct electron transfer between HRP and PPy with subsequent reduction of the oxidized
PPy. It’s similar to those reported in previous literatures [6, 7].

Sensors 2007, 7
247
Figure 7. Cyclic voltammetry of the electrodes in the PBS solution (pH7.0). (A) electrodes
modified with PPy/HRP/ ferrocyanide (B) electrodes modified with PPy/HRP.
(curve a-- with 1 mMH
2
O
2
, curve b-- absence of H
2
O
2
.)
3.4. Sensor characteristics
The calibration curve of the sensor is shown as curve (a) in Figure 8. Eight hydrogen peroxide
samples of different concentrations were measured using the biosensor fabricated under the optimized
condition as described, and each sample of a certain concentration was measured five times. The inset
illustration indicates the linear range of the PPy-HRP electrode response was from 0.1 to 2 mM, and
the sensitivity corresponding to the linear range was about 33.24 µA mM
-1
cm
-2
. The equation of the
response was [Y(µA)=0.6647X(mM)-0.0234, R
2
=0.9953].
The reproducibility between electrodes represented by the relative standard deviation (R.S.D.) was
measured. For all the H
2
O
2
concentrations investigated, the average R.S.D. was 10.24% (n=5).
The calibration curve of a sensor with HRP immobilized by physical adsorption is shown as curve
(b) in Figure 8. Although the response of the electropolymerized sensor was a little lower than the one
of the sensor with physically adsorbed HPR,
the quantity of the HRP fixed in the PPy film can be
relative easily adjusted by quantitatively controlling the electrical charge of polymerization.

Sensors 2007, 7
248
Figure 8. Calibration curve of H
2
O
2
biosensor with HRP immobilized by (a) PPy film and
(b) physical adsorption. The inset indicates calibration curve obtained for the linear range.
Experimental condition: 1 µL H
2
O
2
, -300 mV applied potential.
Different from previous researches reported, we electropolymerized PPy-HRP on a screen-printed
carbon paste electrode rather than Pt or GC electrode [6, 27]. The sensor was developed for disposable
use to allow quick and easy hydrogen peroxide monitor while only 1 µL sample was required. The
sensor exhibited a sensitivity of 33.24 µA mM
-1
cm
-2
which was higher than the hydrogen peroxide
sensors reported in Ref.[6, 27] and allowed to be measured with a handheld meter conveniently.
3.5. Sensor lifetime
The operational stability was examined by measuring the response to 1 mM H
2
O
2
. The sensors were
stored in 0.2 M PBS (pH 7.0) at 5 Celsius degree for 3 weeks, while measurements were conducted
every 2 days during the first week and then once a week subsequently. The response did not change
significantly.
4. Conclusions
A hydrogen peroxide biosensor with PPy-HRP electropolymerized on a screen-printed carbon paste
electrode was studied. When the condition for electropolymerization was optimized, the sensor
exhibited a sensitivity of 33.24 µA mM
-1
cm
-2
to H
2
O
2
with a linear range of 0.1 to 2 mM while only 1
µL sample was required. This method is promising for manufacture of economic disposable hydrogen
peroxide biosensors.

Sensors 2007, 7
249
Acknowledgements
This work has been funded by National Natural Science Foundation of China Grant #60421002.
References and Notes
1.
Ruzgas, T.; Csoregi, E.; Emneus, J.; Gorton, L.; Marko-Varga, G. Peroxidase-modified
electrodes: fundamentals and application. Anal. Chim. Acta 1996, 330, 123-138.
2. Hurdis, E.C.; Hendrik Romeyn, J. Accuracy of determination of hydrogen peroxide by cerate
oxidimetry. Anal. Chem. 1954, 26, 320-325.
3. Matsubara, C.; Kawamoto, N.; Takamura, K. Oxo[5, 10, 15, 20-tetra(4-pyridyl)porphyrinato]
titanium(IV): an ultra-high sensitivity spectrophotometric reagent for hydrogen peroxide.
Analyst 1992, 117, 1781-1784.
4. Aizawa, M.; Ikariyama, Y.; Kun, H. Photovoltaic determination of hydrogen peroxide with a
biophotodiode. Anal. Lett. 1984, 17, 555-564.
5. Nakashima, K.; Maki, K.; Kawaguchi, S.; Akiyama, S.; Tsukamoto, Y.; Imai, K. Peroxyoxalate
chemiluminescence assay of hydrogen peroxide and glucose using 2,4,6,8-tetrathio-
morpholinopyrimido[5,4-d]pyrimidine as a fluorescent component. Anal. Sci. 1991, 7, 709.
6. Serradilla Razola, S.; Lopez Ruiz, B.; Mora Diez, N.; Mark Jr, H.B.; Kauffmann, J.M.
Hydrogen peroxide sensitive amperometric biosensor based on horseradish peroxidase
entrapped in a polypyrrole electrode. Biosens. Bioelectron. 2002, 17, 921-928.
7. Tatsuma, T.; Gondaira, M.; Watanabe, T. Peroxidase-incorporated polypyrrole membrane
electrodes. Anal. Chem. 1992, 64, 1183-1187.
8. Thanachasai, S.; Rokutanzono, S.; Yoshida, S.; Watanabe, T. Novel hydrogen peroxide sensors
based on peroxidase-carrying poly{pyrrole-co-[4-(3-pyrrolyl)butanesulfonate]}copolymer
films. Anal. Sci. 2002, 18, 773-777.
9. Oungpipat, W.; Alexander, P.W.; Southwell-Keely, P. A reagentless amperometric biosensor
for hydrogen peroxide determination based on asparagus tissue and ferrocene mediation. Anal.
Chim. Acta 1995, 309, 35-45.
10. Wang, L.; Wang, E. A novel hydrogen peroxide sensor based on horseradish peroxidase
immobilized on colloidal au modified ITO electrode. Electrochem. Commun. 2004, 6, 225-229.
11. Yang, Y.; Mu, S. Determination of hydrogen peroxide using amperometric sensor of
polyaniline doped with ferrocenesulfonic acid. Biosens. Bioelectron. 2005, 21, 74-78.
12. Vidal, J.-C.; Garcia-Ruiz, E.; Castillo, J.-R. Recent Advances in Electropolymerized
Conducting Polymers in Amperometric Biosensors. Microchim. Acta 2003, 143, 93.
13. Vidal, J.C.; Garcia, E.; Castillo, J.R. Development of a platinized and ferrocene-mediated
cholesterol amperometric biosensor based on electropolymerization of polypyrrole in a flow
system. Anal. Sci 2002, 18, 537-542.
14. Hong, M.Y.; Chang, J.Y.; Yoon, H.C.; Kim, H.S. Development of a screen-printed
amperometric biosensor for the determination of L-lactate dehydrogenase level. Biosens.
Bioelectron. 2002, 17, 13-18.

Sensors 2007, 7
250
15. Cosnier, S. Biosensors based on electropolymerized films: new trends. Anal. Bioanal. Chem.
2003, 377, 507-520.
16. Xue, H.; Mu, S. Bioelectrochemical response of the polypyrrole xanthine oxidase electrode. J.
Electroanal. Chem. 1995, 397, 241.
17. Koopal, C.G.J.; Bos, A.A.C.M.; Nolte, R.J.M. Third-generation glucose biosensor incorporated
in a conducting printing ink. Sens. Actuators B 1994, 18, 166.
18. Xue, H.; Shen, Z.; Li, C. Improved selectivity and stability of glucose biosensor based on in situ
electropolymerized polyaniline-polyacrylonitrile composite film. Biosens. Bioelectron. 2005,
20, 2330-2334.
19. Vidal, J.C.; García, E.; Méndez, S.; Yarnoz, P.; Castillo, J.R. Three approaches to the
development of selective bilayer amperometric biosensors for glucose by in situ
electropolymerization. Analyst 1999, 124, 319-324.
20. Hart, J.P.; Crew, A.; Crouch, E.; Honeychurch, K.C.; Pemberton, R.M. Some recent designs
and developments of screen-printed carbon electrochemical sensors/biosensors for biomedical,
environmental, and Industrial analyses. Anal. Lett. 2004, 37, 789-830.
21. Newman, J.D.; Turner, A.P.F. Home blood glucose biosensors: a commercial perspective.
Biosens. Bioelectron. 2005, 20, 2435-2453.
22. Li, G.; Ma, N.Z.; Wang, Y. A new handheld biosensor for monitoring blood ketones. Sens.
Actuators B 2005, 109, 285-290.
23. Uang, Y.M.; Chou, T.C. Criteria for designing a polypyrrole glucose biosensor by galvanostatic
electropolymerization. Electroanalysis 2002, 14, 1564-1570.
24. Da Silva, S.; Shan, D.; Cosnier, S. Improvement of biosensor performances for nitrate
determination using a new hydrophilic poly(pyrrole-viologen) film. Sens. Actuators B 2004,
103, 397-402.
25. Vela, M.H.; De Jesus, D.S.; M., C.M.C.; Araujo, A.N.; Montenegro, M.C.B.S.M.
Electroimmobilization of MAO into a polypyrrole film and its utilization for amperometric
flow detection of antidepressant drugs. Electroanalysis 2003, 15, 133-138.
26. Harbury, H.A. Oxidation-reduction potentials of horseradish peroxidase. J. Biol. Chem. 1957,
225, 1009.
27. Gaspar, S.; Habermuller, K.; Csoregi, E.; Schuhmann, W. Hydrogen peroxide sensitive
biosensor based on plant peroxidases entrapped in Os-modified polypyrrole films. Sens.
Actuators B 2001, 72, 63-68.
© 2007 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.