Chemosensitive nanocomposite for conductometric detection of hydrazine and NADH

Article abstract of DOI:10.1016/j.electacta.2010.08.092

A new chemosensitive material based on palladium nanoparticles and PEDOT-PSS is described. The composite was characterized by transmission electron microscopy, cyclic voltammetry and in situ resistance measurements. The material was applied for conductometric detection of hydrazine and NADH. Upon exposure to these analytes PEDOT is reduced leading to an increase in its conductance. This process is catalyzed by palladium. A model for description of the potential dependence of polymer conductivity was suggested, tested and applied for the development of new calibration procedure of chemiresistors based on electroactive polymers.

Full text of DOI:10.1016/j.electacta.2010.08.092

Electrochimica Acta 56 (2011) 3679–3684
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Chemosensitive nanocomposite for conductometric detection of
hydrazine and NADH
a
b,∗
Ulrich Lange , Vladimir M. Mirsky
a
Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, 93047 Regensburg, Germany
Department of Nanobiotechnology, Lausitz University of Applied Sciences, 01968 Senftenberg, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2010
Received in revised form 15 August 2010
Accepted 17 August 2010
Available online 16 September 2010
A new chemosensitive material based on palladium nanoparticles and PEDOT-PSS is described. The com-
posite was characterized by transmission electron microscopy, cyclic voltammetry and in situ resistance
measurements. The material was applied for conductometric detection of hydrazine and NADH. Upon
exposure to these analytes PEDOT is reduced leading to an increase in its conductance. This process is
catalyzed by palladium. A model for description of the potential dependence of polymer conductivity
was suggested, tested and applied for the development of new calibration procedure of chemiresistors
based on electroactive polymers.
Keywords:
Chemosensor
Chemiresisor
Nanoparticles
Palladium
PEDOT
©
2010 Elsevier Ltd. All rights reserved.
Hydrazine
NADH
1
. Introduction
nia [14], methanol [15], hydrogen, oxygen and pH [16] and in
biosensors for detection of glucose via hydrogen peroxide oxidation
[17,18]. The composites from conducting polymers and metallic
nanoparticles are typically used in amperometric sensors while
there are only few publications dealing with the application of
such composites in chemiresistors [16]. In most cases they were
used as gas sensors [14,15,19]. The nanoparticles in chemiresis-
tors serve as reaction sites for redoxactive analytes. The resulting
charge is transferred to the conducting polymer thus changing its
conductivity.
Palladium nanoparticles have been successfully used in the elec-
trooxidation of hydrazine [20–22] and NADH [23]. In a comparative
study it was further demonstrated that palladium nanoparticles
are a unique catalytic material for hydrazine oxidation [24]. Con-
ductometric detection of gaseous hydrazine was reported [25–27],
however to our knowledge there was no report on hydrazine detec-
tion in aqueous solutions. Polyaniline doped with polyanions which
extend its electroactive pH range towards pH 7 can be used in
conductometric sensors for NADH [28,29], however polyaniline’s
conductance is strongly pH dependent, which can influence the
measurements.
In the last years conducting polymer-based nanocomposite
materials have attracted enormous interest [1–4]. Within this field
composites of conducting polymers and metal nanoparticles were
shown to be useful materials in chemical sensors [1,5]. Such com-
posites can be prepared by electrochemical deposition of metallic
nanoparticles on a surface or immobilization into a conducting
polymer matrix [6], by electroless reduction of metal salt pre-
cursors by the conducting polymer [6,7], by reduction of metal
salt precursors by monomers resulting in polymerisation of the
monomers [8], by electropolymerisation of conducting polymers
in presence of nanoparticles in solution [9] and by electrostati-
cally driven layer by layer deposition of metal nanoparticles and
conducting polymers [10,11]. There are only few publications deal-
ing with the preparation of dispersions from conducting polymers
and pre-synthesized nanoparticles [12]. However such dispersions
have the advantage that they can be easily transferred to almost
any surface by ink-jet printing, spin- or spray-coating [13]. Pal-
ladium or platinum nanoparticle conducting polymer composites
were used in chemical sensors for detection of hydrazine, ammo-
In this work we present a characterization of a nanocompos-
ite consisting of PEDOT-PSS and palladium nanoparticles and its
application for conductometric detection of hydrazine and NADH.
Furthermore, a new approach to calibrate such sensors is intro-
duced.
∗ Corresponding author. Tel.: +49 3573 85 917; fax: +49 3573 85 809.
E-mail address: vmirsky@hs-lausitz.de (V.M. Mirsky).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.092

3
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U. Lange, V.M. Mirsky / Electrochimica Acta 56 (2011) 3679–3684
Fig. 2. TEM images of palladium particles in the PEDOT-PSS matrix.
Fig. 1. Interdigitated electrode for simultaneous two- and four-point measure-
ments.
2.4. Measurements
2
. Experimental
Electrochemical measurements were done using a CHI-660A
electrochemical analyzer. Either Ag/AgCl electrodes or saturated
calomel electrodes were used as a reference electrode. The type
of the electrode used is indicated in the corresponding figures. A
platinum wire was used as a counter electrode.
2
.1. Chemicals
PEDOT-PSS (conductive grade), Pd(NO ) , ascorbate (Na-salt)
3
2
and hydrazinehydroxide were purchased from Sigma Aldrich.
Dimethylsulfoxide (DMSO) and sulfuric acid (98%) were from
Merck. Buffers were prepared from equimolar amounts (50 mM)
of Na HPO and NaH PO4 and were adjusted to pH 7 by NaOH.
Simultaneous two- and four-point measurements (S24) were
used as described in [30,31]. Shortly, potential difference pulses of
alternating polarity were applied between the outer two electrodes
and the resulting current was measured. Simultaneously the poten-
tial drop between the two inner electrodes was measured. To avoid
effects due to a potential difference between the outer electrodes,
only a small potential difference (10 mV) was applied. Potential
control during simultaneous two- and four-point measurements
was performed by programmed a Keithley 617 connected to a refer-
ence electrode (SCE) [31]. Phosphate buffer solution (pH 7, 100 mM)
was used in all experiments. Hydrazine hydrate solutions were
made by dilution of a 1 M stock solution in buffer. To measure the
effect of hydrazine the sensor was placed in a flow cell. Each con-
centration was pumped through the flow cell till the response of the
sensor reached the steady-state value. In case of NADH additions of
NADH from a 10 mM stock solution do the pH 7 buffer containing
the sensor were performed.
2
4
2
Deionized water additionally purified was used in all experiments.
.2. PdNPs synthesis
0 mL of a 1 mM solution of Pd(NO3)2 in water was heated
2
1
until boiling. Then 1 mL of a 0.1 M solution of sodium ascorbate
in water was quickly added. The solution was further boiled for
about 3 min and then allowed to cool down at room tempera-
ture [22]. The nanoparticles were characterized by transmission
electron microscopy (TEM) on a LEO 912 TEM; the samples were
prepared by adsorption of the nanoparticle containing dispersion
on a carbon coated copper grid.
2.3. Sensor fabrication
3. Results and discussion
2
50 ␮L of the PEDOT-PSS dispersion was diluted with 9.75 mL
of water. 500 ␮L of this solution was mixed with the PdNP solu-
tion resulting in a stable dispersion of the PdNPs in the PEDOT-PSS
solution. 1% of DMSO was added to this solution before deposition
onto the electrodes. An addition of DMSO to PEDOT-PSS is known
to enhance the conductivity of PEDOT-PSS layers. The chemire-
sistors were prepared by drop-coating of 0.5 ␮L of this dispersion
on electrodes with a working area of 0.64 mm2 consisting of four
interdigitated gold strips separated by an 8 ␮m gap (Fig. 1). Then
3
.1. Material characterization
TEM images of the PEDOT-PSS/PdNP composite obtained by
adsorption of the composite on a carbon coated copper grid, show
–10 nm big palladium particles (Fig. 2). A characterization of
5
these nanoparticles including TEM and light scattering data was
presented elsewhere [22]. It seems that the particles are stacked
together in a loose random structure. It is known that PEDOT-PSS
consists of 30–200 nm particles [32,33], but they scatter the elec-
trons much smaller than the palladium nanoparticles and therefore
are not observed in the TEM image.
Optical microscopy of a film of the composite on an inter-
digitated electrode shows homogeneously distributed small black
particles with the size of around 1 ␮m along with some bigger
aggregates within a thin film of PEDOT-PSS. It has to be noted that
◦
the chemiresistors were dried at about 80 C for 10 min. For the
NADH sensor, 0.2 ␮L instead of 0.5 ␮L was used. Before coating the
electrodes were cleaned subsequently by acetone, ethanol, piranha
solution and water.
The samples for cyclic voltammetry were prepared by drop-
coating of 6 ␮L of the composite dispersion on a glassy carbon
2
◦
electrode (3.14 mm ) and drying at 80 C for 10 min.

U. Lange, V.M. Mirsky / Electrochimica Acta 56 (2011) 3679–3684
3681
ison, the voltammogram for pure PEDOT-PSS on the glassy carbon
electrode is shown in Fig. 4b. The currents of the PEDOT oxida-
tion are much smaller than the currents observed for hydrogen
adsorption/desorption on the palladium nanoparticles. At the start-
ing potential of −0.35 V versus Ag/AgCl PEDOT is already partially
oxidized, therefore only partial oxidation and reduction of PEDOT
takes place during this voltammetric experiments.
The catalytically active palladium nanoparticles are electrically
connected by PEDOT-PSS. This is proved by in situ conduc-
tance measurements. The potential dependent conductance of the
nanocomposite is very similar to that of PEDOT-PSS without pal-
ladium nanoparticles and seems therefore not influenced by the
presence of the PdNPs (Fig. 5). This is in contrast to the PANI-PdNP
composite obtained by layer by layer deposition from diluted PANI
solutions in [22], where PdNPs are assumed to build conducting
pathways in the PANI matrix.
The potential dependent conductance of PEDOT can be
described with a simple model assuming that the polymer compos-
ite has two states: one conducting state A and one non-conducting
state B. In equilibrium the potential influence on the concentrations
of these states can be described using the Nernst equation.
Fig. 3. Picture of a thin film of the composite on an interdigitated electrode, acquired
by optical microscopy.
the film covers the whole surface (Fig. 3). Furthermore no connec-
tions of the microelectrodes by big agglomerates of palladium were
found in all experiments.
RT
cB
0
AB
E = E
+
ln
(1)
nAB
F
cA
Cyclic voltammograms were recorded to evaluate the presence,
electroactivity and stability of palladium particles in the layers.
The voltammogram shows the typical features of palladium; e.g.
the currents corresponding to the hydrogen adsorption (<−0.2 V)
and desorption (−0.05 V) and oxide formation (>0.7 V) and reduc-
tion (0.5 V) (Fig. 4a). The voltammogram is very similar to that
of a PEDOT-PdNP composite obtained by electroless deposition
of palladium in previously reduced PEDOT films [7]. For compar-
Let’s indicate the molar conductivities of the corresponding
states of the polymer as g , where the index x is either A and B.
x
The behaviour of different states of the polymer is assumed to be
independent. The total conductivity of the polymer is the sum of
conductivities of all polymer states:
G = cAg + cBgB
(2)
A
b
6
4
2
0
0
0
0
a
6
4
2
0
0
0
0
-
-
-
-
20
40
60
80
-
-
-
-
20
40
60
80
-
100
-
-
100
-
0,4 -0,2 0,0
0,2
0,4
0,6
0,8
1,0
0,4 -0,2 0,0
0,2
0,4
0,6
0,8
1,0
E vs. (Ag/AgCl) / V
E vs. (Ag/AgCl) / V
Fig. 4. Voltammogram of the nanocomposite of PdNP with PEDOT-PSS (a) and voltammogram of PEDOT-PSS (b). Electrolyte: 0.5 M H2SO4. Sweep rate: 0.1 V/s.
0
0
0
0
0
0
,05
,04
,03
,02
,01
,00
a
0
0
0
,10 b
,08
,06
0,04
0,02
0,00
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
-0,8 -0,6 -0,4 -0,2
0,0
0,2
0,4
E vs. SCE / V
E vs. SCE / V
Fig. 5. Potential influence on the conductance of the nanocomposite from PdNP with PEDOT-PSS (a) and on PEDOT-PSS without palladium nanoparticles (b). Electrolyte:
00 mM phosphate, pH 7.
1

3
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U. Lange, V.M. Mirsky / Electrochimica Acta 56 (2011) 3679–3684
-
-
0,04
0,06
3
2
1
0
0
0
0
-0,08
-
-
0,10
0,12
-0,14
-
-
0,16
0,18
-
-
-
10
20
30
-0,20
-
-
0,22
0,24
-
0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1E-5
1E-4
1E-3
0,01
0,1
E vs. (Ag/AgCl) / V
-1
c (N H ) / mol l
2
4
Fig. 6. Increased concentrations of hydrazine lead to increase of the current on
voltammograms of the nanocomposite of PdNP and PEDOT-PSS deposited on a glassy
carbon electrode. The hydrazine concentrations are: 0, 20, 100, 200, 400 ␮mol/L.
Electrolyte: 100 mM phosphate, pH 7. Sweep rate: 0.1 V/s.
Fig. 8. Open circuit potential of the interdigitated gold electrode coated by the
nanocomposite of PdNP and PEDOT-PSS as a function of hydrazine concentration.
0,05
By combining Eqs. (1) and (2) one and substitution of c g and
cBgB with gA and gB one gets an equation for the potential depen-
dent conductivity of the polymer.
A
A
ꢀ
ꢀ
0,04
0,03
0,02
ꢀ
ꢀ
+ (g 10
B
(E0 PEDOT−E)/a
g
)
A
G =
(3)
(
E
PEDOT−E)/a
0
1
+ 10
where a = 2.3(RT/nABF)
As one can see in Fig. 5, Eq. (3) describes the potential dependent
conductance of the film quite well, especially in the conducting
region. The model is similar to that proposed by Amemiya et al. to
describe the spectroelectrochemical behaviour of polypyrrole [34].
It has to be stated that a logarithmic plot of the conductance versus
the potential would give a better view of the potential dependence
of the composite as with the linear plot the small changes in the
highly oxidized state are overestimated [35], however in this work
the high conducting region is of more interest as even with high
concentrations of hydrazine we do not reach potentials less than
0
,01
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
E vs. SCE / V
Fig. 9. Comparison of the dependencies of the conductance of nanocomposite from
PdNP and PEDOT-PSS on the applied electrode potential (squares) and on the open
circuit potential defined by hydrazine additions (circles). The line presents theoret-
ical curve according to Eq. (3).
−
0.25 V.
The electroactivity of the nanocomposite from PdNP and
PEDOT-PSS towards hydrazine oxidation was studied by cyclic
voltammetry in a pH 7 buffer. A set of cyclic voltammograms
registered by using a glassy carbon electrode coated by the
composite at various concentrations of hydrazine in the range
seems to start at about −0.3 V and has a maximum at about
0.15 V.
3.2. Conductometric detection of hydrazine and NADH
4
0–800 ␮M is presented in Fig. 6. The figure shows the stationary
voltammograms obtained after three cycles carried out at each con-
centration of hydrazine in the solution. The oxidation of hydrazine
As expected, an exposure of the nanocomposite of PdNP with
PEDOT-PSS to hydrazine or NADH leads to the reduction of PEDOT
a
0.5 µM
b
1
0 µM
20 µM
40 µM
0 µM
160 µM
1
µM
1
0
0
0
0
0
0
0
,0
,9
,8
,7
,6
,5
,4
,3
5 µM
0
,0040
1
0 µM
50 µM
0,0038
8
1
00 µM
00 µM
1mM
5
0,0036
0,0034
1
1
0
0
.02
.00
.98
.96
0.1 mM
320 µM
640 µM
5 mM
10 mM
100 mM
0.5 mM
1 mM
mM
5
0.94
.92
0.90
0
0
,0032
,0030
0
10 mM
1.28mM
0
0
.88
.86
0,0028
0,0026
0
5
10 15 20 25 30
t / min
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
t / min
t / min
Fig. 7. Influence of hydrazine (a) and NADH (b) on the conductance of the nanocomposite from PdNP with PEDOT-PSS and pure PEDOT-PSS (inset a). Electrolyte: 100 mM
phosphate, pH 7.

U. Lange, V.M. Mirsky / Electrochimica Acta 56 (2011) 3679–3684
3683
a
b
0
0
0
0
0
,035
,030
,025
,020
0
,0036
0,0034
0
0
0
0
,0032
,0030
,0028
,0026
,015
1
E-7 1E-6 1E-5 1E-4 1E-3 0,01
0,1
10
100
1000
-
1
c (N H ) / mol l
2
4
c (NADH) / µmol l-1
Fig. 10. Calibration curve of the conductometric hydrazine (a) and NADH (b) chemiresistors based on the theoretical model describing redox conversions of PEDOT. The
squares indicate the measured conductance, whereas the lines represent theoretical dependencies obtained from Eq. (5) with corresponding parameters for hydrazine and
NADH.
accompanied by the decrease in the conductance (Fig. 6). It was
possible to observe an effect on addition of so low concentrations
as 0.5 ␮M of hydrazine or 10 ␮M of NADH. At such concentrations
the signal changes were at least three times higher than typical
fluctuations of the base line, therefore it can be considered as the
detection limit. Although the simple and crude sensor fabrication
technique without precise control of the layer thickness led to some
variation of absolute values of the sensor response from electrode to
electrode, the values of the detection limit where about the same
for all sensors. It has to be noted that for NADH a thinner layer
of the composite was used resulting in a less conducting layer. In
comparison to hydrazine the reduction of the composite by NADH
was rather slow and the conductance changes where lower. One
can assume that hydrazine can diffuse into the composite whereas
NADH oxidation probably takes place only on the surface. Probably
the PdNPs immobilized in the polymer matrix serve as reaction sites
for hydrazine/NADH oxidation and the released electrons reduce
PEDOT. This assumption is supported by the fact that the pure
PEDOT-PSS covered electrode showed only weak changes upon
exposure to hydrazine (14% change at 10 mM in comparison to 45%;
see inset Fig. 7).
of hydrazine in solution.
ꢀ
ꢀ
+ (g 10
B
(E0 PEDOT−E0 N H −0.039 log(c(N2H4
))/a)
g
2 4
A
G =
(5)
(
E
PEDOT−E0 N H −0.039 log(c(N2H4))/a
0
2 4
1
+ 10
Fig. 10 shows that Eq. (5) describes the concentration depen-
dence of the conductance of the layer. It has to be noted that the
all parameters used for this description were determined from
the potential dependent conductivity of the composite and the
potentiometric response of the composite on increasing hydrazine
concentrations. No fitting procedure was used to describe the
dependence. The same approach was used for NADH, however in
this case the sensitivity of the conductance was 59 mV per decade.
At high conductance but before saturation (Fig. 9) Eq. (5) predicts
linearisation of the concentration dependence in semilogarithmic
scale what is observed in Fig. 10.
Another approach suggested in literature to calibrate chemire-
sistors is based on the response time of the sensor after analyte
addition [28,36,37]. However this approach requires that the sensor
is switched to its initial potential before each concentration deter-
mination and is therefore not suitable for continuous monitoring
of analyte concentration.
Alternatively, the analytical information can be obtained from
potentiometric measurements. The open circuit potential of
the composite decreased by 39 mV per decade with increased
hydrazine concentration (Fig. 8). This value can be used to analyze
stoichiometry of the reaction, however in this work we consider it
as an empiric parameter which can be used for quantitative anal-
ysis of the hydrazine concentration. The equation for the potential
of the nanocomposite coated electrode is:
4. Conclusion
A new chemosensitive nanocomposite material was obtained
from palladium nanoparticles and PEDOT-PSS. The composite
forms a stable dispersion in aqueous media which can be used for
deposition on various surfaces via drop-, spray- or spin-coating.
The composite was found to be chemosensitive towards reducing
agents like hydrazine or NADH and belongs therefore to the class of
broad-selective chemosensitive materials for detection of reducing
compounds. Conductometric or potentiometric transducing was
used. The chemosensitivity is probably based on electrocatalytical
activity of palladium nanoparticles corresponding by the reduction
of PEDOT. A simple model describing redox conversion of PEDOT
provided quantitative description of this system and was used for
the development of a new calibration approach.
E = E − 0.039 log(c(N H ))
(4)
0
2
4
where E = −0.26 V.
0
Taking into account this dependence and the dependence of the
conductance of the composite on applied electrical potential it is
clear why the response is not linear with increasing concentration,
but almost linear in the logarithmic concentration scale. By assign-
ing the open circuit potential values to the conductance values
for each hydrazine concentration and plotting these data together
with the potential dependent conductance of the nanocomposite
of PdNP and PEDOT-PSS, one can see that the data match each other
perfectly (Fig. 9). Thus chemical and electrical control of the redox
potential of the film yield quantitatively the same result.
Acknowledgements
The work of U.L. was supported by DFG-graduate school. The
authors are grateful to Prof. V. Tsakova, Dr. S. Ivanov and V. Lyu-
tov for initiation of investigations of nanocomposites of conducting
polymers with palladium nanoparticles and fruitful discussions.
Combining the equation used for the description of the potential
dependent conductance of the composite (Eq. (3)) with the equa-
tion used for description of the electrode potential in dependence
of the hydrazine concentration (Eq. (4)), one gets an equation which
describes the dependence of the conductance on the concentration
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