Electropolymerization of cobalt tetraamino-phthalocyanine at reduced graphene oxide for electrochemical determination of cysteine and hydrazine

Article abstract of DOI:10.1039/c6ra01851c

We describe a simple and elegant electropolymerization method to prepare highly stable tetraamino functionalized cobalt phthalocyanine (pTACoPc) at electrochemically reduced graphene oxide (RGO). The described method efficiently bridges the excellent physicochemical properties of RGO with the rich redox chemistry of TACoPc. Graphene oxide was electrochemically reduced to RGO at the electrode surface along with concominent electropolymerization of TACoPc. The electrochemical studies showed that RGO on pTACoP/GCE increased effective surface area, reduced charge transfer resistance and enhanced electrochemical signal. The RGO-pTACoPc film modified electrode exhibits excellent electrocatalytic ability to oxidize cysteine and hydrazine. To determine cysteine, the RGO-pTACoPc sensor displayed a linear concentration range of 50 nM to 2.0 μM, detection limit of 18.5 nM and sensitivity of 10.19 nA nM^-1 cm^-2. Besides, the sensor displayed a linear concentration range of 50 nM to 2.6 μM, detection limit of 10 nM and sensitivity of 1.62 nA nM^-1 cm^-2 to determine hydrazine. The electrocatalytic ability of RGO-pTACoPc shows better performance over other cobalt phthalocyanine derivatives. Furthermore, the described sensor exhibited long-term storage stability, good repeatability and reproducibility. The practical applicability of the sensor has been assessed in biological and water samples.

Full text of DOI:10.1039/c6ra01851c

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Electropolymerization of cobalt tetraamino-phthalocyanine at
reduced graphene oxide for electrochemical determination of
cysteine and hydrazine
Received 00th January 20xx,
Accepted 00th January 20xx
Veerappan Mania,b, Sheng-Tung Huanga,b*, Rajkumar Devasenathipathyc, Thomas C.K. Yanga
DOI: 10.1039/x0xx00000x
We described a simple and elegant electropolymerization method to prepare highly stable tetraamino functionalized
cobalt phthalocyanine (pTACoPc) at electrochemically reduced graphene oxide (RGO). The described method efficiently
bridges the excellent physicochemical properties of RGO with rich redox chemistry of TACoPc. Graphene oxide was
electrochemically reduced to RGO at electrode surface along with concominent electropolymerization of TACoPc. The
electrochemical studies showed that RGO on the pTACoP/GCE increased the effective surface area, reduced the charge
transfer resistance of the electrode and enhanced the electrochemical signal. The RGO-pTACoPc film modified electrode
exhibits excellent electrocatalytic ability to oxidize cysteine and hydrazine. To determine cysteine, RGO-pTACoPc sensor
displayed linear concentration range of 50 nM–2.0 µM, detection limit of 18.5 nM and sensitivity of 10.19 nAnM–1 cm-2.
Besides, the sensor displayed linear concentration range of 50 nM–2.6 μM, detection limit of 10 nM and sensitivity of 1.62
nAnM–1 cm-2 to determine hydrazine. The electrocatalytic ability of RGO-pTACoPc shows better performance over other
cobalt phthalocyanine derivatives reported by other methods. Furthermore, the described sensor exhibited long-term
storage stability, good repeatability and reproducibility. Practical applicability of the sensor has been assessed in biological
www.rsc.org/
and
water
samples.
large surface area, high conductivity, porosity and good
electrocatalytic ability.6 The use of RGO as conducting support
to stabilize electrocatalysts is widely acclaimed.7 A handful of
reports are commenced to stabilize metal phthalocyanines at
RGO, such as layer by layer,8 self-assembly,9 and chemical
methods.2 Electrochemical polymerization is an elegant,
attractive and easy strategy to prepare stable and ordered
films at electrode surface.10,11 Here, we have designed a single
step electrochemical polymerization strategy to prepare highly
stable electropolymerized TACoPc (pTACoPc) at RGO (Scheme
1).
1. Introduction
Metal phthalocyanines are organic macrocyclic molecular
catalysts with metal atoms at the center and possess rich
redox chemistry.1,2 The central metal ion of phthalocyanines
can reversibly bind with reactants and hence they exhibited
good electrocatalytic activity to many important reactions.
Cobalt phthalocyanine and its derivatives (CoPcs) are the most
widely studied phthalocyanines as mediator/electrode
modifiers attributed to their widespread electrochemical
applications.3 However, there are two major limitations in
using CoPcs at electrode surface: (1) physically adsorbed CoPcs
on electrode surface is not stable and usually peel off from
electrode surface; (2) They own low conductivity which
reduces electron transfer rate and results in poor
electrochemical activity.4 In order to overcome these issues,
they require suitable support which should offer good
conductivity, large surface area and good adhesion ability.5
Recently, reduced graphene oxide (RGO) attracted many
interests due to its unique physicochemical properties such as
Cysteine (CySH), an important biothiol plays crucial role in
biological systems maintaining plenteous redox-related
physiological process.12 CySH is part of an endogenous
antioxidant system that maintains cells’ normal redox
potential and it also a prominent biomarker.13 However, its
deficiency is associated with slowed growth, hair
depigmentation, edema, lethargy, liver damage, skin lesions
and weakness and hence sensitive determination of CySH is
important. Electrochemical methods are most preferable
method to detect CySH attributed to its short reaction time,
high sensitivity, and portability.14 From literature studies and
our own previous reports; we have realized that CoPc
derivatives are the most suitable electrocatalysts for the
determination of CySH. Herein, we have developed a modified
electrode based on pTACoPc at RGO for the sensitive
determination of CySH. Besides, hydrazine is known for its
widespread industrial applications.15 Despite its extensive
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applications, it is highly toxic and hence its determination is of Subsequently, the electropolymerized TACoPc at RGO (RGO-
great importance.16 Electrochemical detection of hydrazine at pTACoPc/GCE) was carefully washed in DMSO and dried. As
bare electrodes suffers from high overpotential, low sensitivity control, RGO/GCE and pTACoPc/GCE were also prepared.
and surface fouling issues. In the present work, we described a
sensitive amperometric sensor based on pTACoPc decorated
RGO to determine hydrazine. The main objective of the work is
to describe a simple electropolymerization method to prepare
stable pTACoPc at RGO for electrochemical sensing of CySH
and hydrazine. The nanocomposite has excellent
electrochemical properties such as large electrochemically
active surface area, high capacitance current, wide potential
window, high conductivity and large porosity and shown great
electrocatalytic ability to CySH and hydrazine.
2. Materials and methods
2.1 chemicals and reagents
Graphite (powder, <20 μm), CySH and hydrazine were
purchased from sigma-Aldrich and used as received. 4, 4’, 4,
4”’-tetra-amino cobalt phthalocyanine (TACoPc) was
synthesized according to established literature procedure.17 All
Scheme 1 Preparation of graphene oxide and electropolymerization of TACoPc on RGO
for electrocatalytic determination of CySH and hydrazine
other reagents were purchased from sigma-Aldrich and used
without
further
purification.
All
electrochemical
measurements were carried out using CHI 612D
electrochemical work station at ambient temperature.
Electrochemical studies were performed in a conventional
three electrode cell using BAS GCE as a working electrode
(area 0.071 cm2), saturated Ag|AgCl (saturated KCl) as a
reference electrode and Pt wire as a counter electrode. 0.1 M
tetrabutylammonium perchlorate (TBAP) dissolved in DMSO
was used as supporting electrolyte for electrodeposition
process, while phosphate buffer (PB) (pH 7) was used as
supporting electrolyte for electrocatalytic applications. Prior to
each electrochemical experiment, the electrolyte solutions
were deoxygenated with pre-purified nitrogen for 10 min
unless otherwise specified. Scanning electron microscopy
(SEM) and Energy-dispersive X-ray (EDX) spectra were
3. Results and Discussions
3.1 Electrodeposition of TACoPc and simultaneous electrochemical
reduction of GO
Fig. 1A displays the cyclic voltammograms (CVs) obtained for
electrodeposition of TACoPc. The growth of peaks with
increasing peak currents indicates the electropolymerization of
TACoPc. The polymerization of TACoPc involves oxidation of
amino group to form radicals which initiate condensation by
attacking phenyl rings of neighbouring molecules. The CV
obtained at RGO-pTACoPc/GCE features with three important
redox couples at formal potentials (E0’) of 0.002 V (1, 2), –
0.709 V (3, 4) and – 1.323 V (5, 6) and the redox process can be
represented as follows (Fig. 1B),19
performed using Hitachi S-3000
H scanning electron
microscope and HORIBA EMAX X-ACT (Sensor + 24V=16 W,
resolution at 5.9 keV) respectively. Electrochemical impedance
spectroscopy (EIS) studies were carried out using EIM6ex
ZAHNER (Kroanch, Germany).
Co3+/Co2+: [Co3+, Pc2–]+ + e– ↔ [Co2+, Pc2–], E0’ = + 0.002 V (1, 2)
Co2+/Co+: [Co2+, Pc2–] + e– ↔ [Co+, Pc2–]–, E0’ = – 0.709 V (3, 4)
Pc2–/Pc3–: [Co+, Pc2–]– + e– ↔ [Co+, Pc3–]2–, E0’ = – 1.323 V (5, 6)
2.2 Electrodeposition of TACoPc at RGO
Graphite oxide was prepared from graphite by Hummers
method.18 It was exfoliated to graphene oxide (GO) via
ultrasonic agitation for 1 h with subsequent centrifugation at
3000 RPM for 30 min. Next, GCE surface was cleaned by
polishing it with 0.05 µm alumina slurry using Buehler
polishing kit. About 5 µl dispersion of GO (0.5 mg mL-1) was
dropped on pre-cleaned GCE and dried at ambient conditions.
Then, GO modified GCE (GO/GCE) (3.52 µg GO loading) was
transferred to an electrochemical cell containing 1 mM TACoPc
and 0.1 M TBAP in 5 mL DMSO. The electrodeposition was
carried out by cyclic voltammetry at potential range between
1.0 V and – 2.6 V for 20 cycles, while scan rate was 0.1 Vs-1.
In general, the redox pair situated in most positive
potential region is due to Co3+/Co2+, the middle pair is due to
Co2+/Co+ and the pair situated in most negative potential
region is due to ring based redox process.20 Here, the redox
pair located at E0’ of + 0.002 V is assigned to metal based
electron transfer process (Co3+/Co2+) and the middle pair
appeared at E0’ of – 0.709 V is assigned to Co2+/Co+ process.
The redox pair appeared at E0’ of – 1.323 V is assigned to
phthalocyanine ring based redox process (Pc2–/Pc3–).10,19 The
lack of complete reversibility in Co3+/Co2+ redox couple might
be
due
to
surface
adsorption
effects.21
The
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electropolymerization of TACoPc at bare GCE is unable to shape is sharp at lower scan rates, while changed to broader at
produce good peak growth indicating that the polymerization higher scan rates due to slow kinetics at higher scan rates. The
at bare GCE is scarcely formed (inset to Fig. 1A). However, the plots of Ipa and Ipc versus ν for all three redox couples exhibits
peak growth is uniform at GO/GCE which reveals successful linear relationship which are characteristic of surface confined
polymerization of phthalocyanine. In our approach, both processes (Fig. S2). The loading of TACoPc was estimated using
electrochemical reduction of GO and electrodeposition of electrochemical quartz crystal microbalance studies and
TACoPc are occurring simultaneously which assists TACoPc to adopting well-known Sauerbrey equation which relates the
find fresh sites at RGO during each cycles. The pTACoPc acts as change in mass (i.e. loading of electrodeposited film) at the
a spacer between RGO and makes both faces of RGO quartz crystal with change in frequency.24 About 0.312 µg cm–2
accessible. Additionally, a small cathodic peak was observed at of pTACoPc loading was estimated on the electrode. The
the potential of -1.0 V during first cycle and disappeared in loading of GO is 3.52 µg cm–2 and the weight ratio between
subsequent cycles which is characteristic behavior expected RGO and pTACoPc is calculated to be about 11.28. The film
for electrochemical reduction of GO.22 Also, the thickness effect is studied on the electrocatalytic performance
electrochemical reduction of GO/GCE (without CoTAPc) in towards oxidation of CySH (inset to Fig. 3A) and hydrazine
DMSO exhibited typical voltammograms expected for (inset to Fig. 4A). As shown in figures, the film produced at the
reduction of GO (Fig. S1).
electrodeposition cycle of 20 showed optimum catalytic
performances. Further increase in the cycle numbers led to the
decreased response current attributed to the formation of
thick film. Therefore, we have chosen 20 cycles as the
optimum cycle number for preparing pTACoPc film.
Fig. 1 (A) 20 consecutive CVs obtained at GO/GCE in DMSO containing 1 mM
CoTAP and 0.1 M TBAP. Scan rate= 0.1 Vs-1. Inset: Electropolymerization of
TACoPc on unmodified GCE. (B) CV obtained at RGO-pTACoPC/GCE in DMSO
containing 0.1
M
TBAP, scan rate= 0.02 Vs-1. (C) CVs obtained at RGO-
pTACoPc/GCE in DMSO containing 0.1 M TBAP at various scan rates a to j: 0.01
to 0.1 Vs-1. (D) Electrochemical impedance spectra of pTACoPc (a), RGO (b) and
RGO-pTACoPc (c) performed in 0.1
Amplitude: 5 mV, Frequency: 0.1 Hz to 100 kHz.
M KCl containing 5
mM Fe(CN)63-/4-.
Fig. 2 SEM of RGO (A) and RGO-pTACoPc (C). EDX of RGO (B) and RGO-pTACoPc (D)
3.2 Electrochemically active surface area, cyclic and storage
stability and electrode kinetics
3.3 Morphology and elemental analysis
The SEM image of RGO presents characteristic thin sheets like
morphology with wrinkles and crumbles (Fig. 2A), while its EDX
spectrum presents signals for carbon and oxygen with weight
percentages of 89.54 and 10.46 respectively (Fig. 2B and Fig.
S3A). The SEM image of RGO-pTACoPc nanocomposite
exhibited similar morphology of RGO (Fig. 2C); however, EDX
analysis indicates presence of cobalt and nitrogen signals
which validated the formation of nanocomposite (Fig. 2D and
Fig. S3B). EIS is an efficient tool to examine the electrical and
interfacial properties of the modified films on the electrode
surface. Fig. 1D shows the EIS obtained at pTACoPc (a), RGO
(b), and RGO-pTACoPc (c) films modified GCEs in 0.1 M KCl
containing 5 mM Fe(CN)63-/4-. Randles equivalent circuit model
has been used to fit the experimental data where, Rs is
electrolyte resistance, Rct is charge transfer resistance, Cdl is
The electrochemically effective surface areas of RGO/GCE,
pTACoPc/GCE and RGO-pTACoPc/GCE have been assessed
using K3[Fe(CN)6] as a model redox mediator, while Randles–
Sevcik equation was used to calculate areas.23 The active
surface areas of RGO/GCE, pTACoPc/GCE and RGO-
pTACoPc/GCE were calculated to be 0.103 cm2, 0.076 cm2 and
0.152 cm2. Furthermore, the RGO-pTACoPc/GCE retained
97.05% of its initial response current even after one month of
its storage period and also retained 96.22% of its initial peak
current even after 300 successive cycles revealing its excellent
storage and cyclic stability respectively. Fig. 1C portrays the
CVs obtained at RGO-pTACoPc/GCE for different scan rates.
The anodic (Ipa) and cathodic peak currents (Ipc) of all three
redox couples were increased with scan rates (ν). The peak
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double layer capacitance and Zw is Warburg impedance (inset calculated as 0.66 which is close to 0.5 means that the
to Fig. 1D). EIS measurements were represented as Nyquist activated complex is around halfway between reactants and
plots, in which semicircles indicates the parallel combination of products on the reaction coordinate.26 The mechanism for the
electron transfer resistance (Rct) and double layer capacitance oxidation of CySH can be represented by eq. (2), (3) and (4).
(Cdl) at electrode surface resulting from electrode impedance,
while the linear portion represents diffusion limited process.
The size of curves (diameter of the semicircle portion)
obtained was in the order of pTACoPc/GCE > RGO/GCE > RGO-
pTACoPc/GCE. Rct values of the electrodes were obtained by
fitting the Nyquist plot results with Randles equivalent circuit
model. The obtained Rct values of pTACoPc/GCE, RGO/GCE and
RGO-pTACoPc/GCE with respect to their effective area are
7.47, 1.94 and 0.3 KΩ cm-2 respectively. The Rct value obtained
Co2+Pc → [Co3+Pc]+ + e–
(2)
(3)
(4)
[Co3+Pc]+ + RSH → Co2+Pc + RS●+ H+
2RS● → RSSR
Here, RSH refers CySH, while RSSR refers cystine.
at RGO-pTACoPc/GCE is 25 and 6.5 times smaller than that of 3.5 Determination of cysteine
pTAcoPc/GCE and RGO/GCE, respectively. Thus, RGO- Fig. 3C displays the CVs obtained at RGO-pTACoPc/GCE in
pTACoPc/GCE has significantly reduced charge transfer absence (curve a) and presence of different concentration of
resistance over controls electrodes (RGO/GCE, pTACoPc/GCE).
CySH ranging from 50 µM to 0.7 mM in PB (pH 7). The value Ipa
increases linearly as the concentration of CySH increases. A
plot between Ipa and [CySH] exhibited linear relationship (inset
3.4 Electrocatalysis of cysteine
The CVs obtained at unmodified GCE (a), RGO/GCE (b), to Fig. 3C) with slope of 0.2106 µA µM-1. The linear
pTACoPc/GCE (c) and RGO-pTACoPc/GCE (d) in PB (pH 7) concentration range was observed from 50 µM to 0.7 mM. Fig.
containing 0.5 mM CySH were presented as Fig. 3A, while their 3D displays the amperometric i-t curves obtained at RGO-
corresponding electrochemical parameters such as, peak pTACoPc film modified rotating disc GCE for sequential
potential (Ep) and peak current (Ip) (with respect to their addition of 100 nM CySH into PB. The rotation speed of
effective surface area) are given as Table S1. The electrode was 1500 RPM, while the applied potential (Eapp) was
electrocatalytic abilities of these electrodes are in the + 0.20 V.
following order:
RGO-pTACoPc/GCE > pTACoPc/GCE >
RGO/GCE > unmodified GCE. The unmodified GCE and
RGO/GCE were scarcely oxidizing CySH. The pTACoPc/GCE
oxidizes CySH at overpotential of 0.30 V, however with smaller
Ip. Among the electrodes, RGO-pTACoPc/GCE exhibited
maximum electrocatalytic ability in terms of both peak current
and overpotential. In comparison with pTACoPc/GCE, RGO-
pTACoPc/GCE exhibited 150 mV less overpotential and 5 fold
increment in peak current which clearly indicate the enormous
role of RGO in facilitating electrocatalytic ability. The large
surface area of RGO provides sufficient anchoring sites to form
electrochemically active pTACoPc film. Besides, the good
conductivity of RGO provides significantly enhanced sensitivity.
The effect of scan rate was examined in PB (pH 7)
containing 0.5 mM CySH at different scan rates ranges (Fig.
3B). The value of Ipa increases as the scan rate increases from
0.1 to 1 Vs-1. A plot between square root of scan rates and Ipa
exhibited linear relationship indicating diffusion controlled
oxidation process (inset to Fig. 3B). Also, a plot of Epa vs. log ν
displayed linear relationship with slope 89 mV/decade which
indicating that two electrons were involved in the rate
determining step (Fig. S4). Epa can be represented by the Tafel
eq. (1),25
Fig. 3 (A) CVs obtained at unmodified (a), RGO (b), pTACoPc (c) and RGO-
pTACoPc composite (d) films modified electrodes in PB (pH 7) containing 0.5 mM
CySH at scan rate 50 mVs-1. Inset: optimization of cycle numbers for
electrodeposition of pTACoPc at RGO/GCE. CVs were carried out at RGO-
pTACoPc/GCE (0, 5, 10, 15, 20, 25 and 30 cycles) in PB (pH 7) containing 0.5 mM
CySH. (B) CVs obtained at RGO-pTACoPc/GCE in PB (pH 7) containing 0.5 mM
CySH at different scan rates (a to j; 0.1 to 1 Vs-1). Inset: Ip vs. ν1/2; Ipa/μA = 603.8
ν1/2 (μA/(Vs-1)1/2) – 116.75, R2=0.987 (C) CVs obtained at RGO-pTACoPc/GCE in PB
(pH 7) containing CySH; a=0, b=0.05, c= 0.1, d= 0.15, e=0.20, f=0.25, g=0.30,
h=0.35, i=0.40, j=0.45, k=0.50, l=0.55, m=0.60, n= 0.65 and o= 0.70 mM. Inset:
[CySH] vs. Ip; Ip/μA= 0.211 [CySH] (μA/µM) + 34.03. (D) Amperometric response
of RGO-pTACoPc modified rotating disc GCE upon each sequential addition of
100 nM CySH into continuously stirred PB (pH 7) at rotation speed of 1500 RPM.
Epa= [2.303RT/ (1-α) na F] log ν + K
(1)
Where, α and na (= 1) are representing electron transfer
coefficient and number of electrons involved in the rate-
determining step. The constants R, T and F have their usual
meanings (R=8.314 J K-1mol-1, T=298 K, F=96485 C mol-1). By
substituting all the parameters in eq. (1), the value of α was
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Eapp = + 0.25 V. Inset: [CySH] vs. Ip; Ip (µA) = 2.14 [CySH] (nA/nM) + 0.0391, R2=
Selectivity study was carried out in order to investigate
anti-interference ability of the sensor. The common interfering
species in CySH analysis, such as, glycine, alanine,
0.999.
The RGO-pTACoPc film exhibited quick and sensitive phenylalanine, lactic acid, L-tyrosine, glucose, uric acid, valin,
amperometric response to each addition of CySH. The and creatinine were tested. As shown in the fig. S5, 500-fold
response current reached 95% steady-state current within 5s excess concentration of these species had no significance
of CySH injection indicating rapid detection. The concentration influences on the sensing of CySH which validated selectivity of
dependent linear plot displayed good linearity with slope of the film.
2.14 nA nM-1 (inset to Fig. 3D). The working concentration
range was linear from 50 nM – 2.0 µM with sensitivity of 10.19 3.7 Real-time application
nAnM–1 cm-2. The limit of detection (LOD) was calculated as Practical feasibility of the sensor has been assessed in human
18.5 nM (S/N=3). The LOD was calculated using the formula, serum sample. Blood sample was collected from a healthy man
LOD= 3 sb/S where, sb is the standard deviation of ten blank and allowed to clot. Next, the clot was removed through
measurements and
S is the sensitivity. The important centrifugation for 15 min at a speed of 2000×g and serum was
parameters of sensor, such as LOD, linear range and sensitivity separated and stored at –20°C. 1 mL serum sample was diluted
were compared with other modified electrodes available in with 50 mL PB. Next, known concentrations of CySH were
literature (Table 1). The analytical performance of the RGO- injected and detection analyses were performed adopting
pTACoPc/GCE film is comparable with the previous reports. similar conditions optimized for lab sample (Table S2). The
Remarkably, LOD of the sensor is much lower than the described sensor has shown acceptable recoveries which
previously reported sensors.
validated practical applicability of the sensor.
3.8 Electrocatalysis of hydrazine at RGO-pTACoPc/GCE
Table 1 Comparison of electroanalytical parameters obtained at RGO-pTACoPc/GCE
with previous reports for the determination of CySH
Fig. 4A presents the CVs obtained at unmodified (a), RGO (b),
pTACoPc (c) and RGO-pTACoPc (d) films modified GCEs in
presence of 1 mM hydrazine. The unmodified GCE is unable to
oxidize hydrazine, while RGO/GCE and TACoPc/GCE were
shown poor electrocatalytic ability. However, RGO-
pTACoPc/GCE exhibited a sharp Ipa with significantly improved
peak current. The overpotential obtained at RGO-
pTACoPc/GCE is + 0.17 V, which is significantly smaller than
that obtained at other electrodes. Additionally, Ipa observed at
RGO-pTACoPc is 5.6 fold higher than that observed at
RGO/GCE. The obtained less overpotential and large Ipa
revealed good electrocatalytic ability of the RGO-pTACoPc. The
effect of applied scan rate to hydrazine oxidation was
investigated (Fig. 4B). The Ipa increases as the scan rate
increases from 0.05 to 0.5 Vs-1. A plot between ν1/2 and Ip
exhibits good linearity indicating diffusion controlled process
(inset to Fig. 4B). A plot between Epa and log ν displays good
linearity with slope of 109.4 mV/decade which indicates that
the rate determining step involves one electron process in (Fig.
S6). The value of α was estimated to be 0.67 by substituting na
(= 1) and slope of Epa vs. log ν in eq. (1). The value of α is close
to 0.5 indicating that the activated complex is almost midway
between reactant and product. The number of electrons (n)
involved in the overall hydrazine oxidation process was
calculated using eq. (5),25
Electrodes
LOD/
μM
2
Linear
range/µM
4–100
Sensitivity/
μAμM cm−2
–
Ref.
aFePc-based SAM
bCoTPC/poly-l-lysine
Au NPs–bFePc
28
29
30
31
32
0.15
0.27
0.019
1
0.50–216
50–1000
0.1–1000
10–200
0.157
57.2
Pectin/Au NPs/MWCNTs
cFeTSPc/MWCNTs
0.46
0.176
RGO-pTACoPc/GCE
0.018
0.05–2.0
10.19
This
work
aFePc= Iron phthlocyanine bCoTSPc= Cobalt tetrasulphonated
phthalocyanine; cFeTSPc= Iron Tetrasulfonated Phthalocyanine
3.6 Repeatability, reproducibility, stability and selectivity
In order to calculate repeatability of the sensor, five repeated
amperometric experiments were performed using RGO-
pTACoPc film modified electrode towards determination of
CySH (100 nM). The relative standard deviation (R.S.D)
between five repeatable measurements was calculated to be
2.14%. In order to calculate reproducibility of the sensor, five
different RGO-pTACoPc films modified electrodes were
fabricated and amperometric experiments were performed on
each electrodes towards determination of CySH (100 nM). The
R.S.D between five amperometric measurements was
calculated to be 2.83%. The obtained R.S.D values are in the
acceptable range and hence the sensor validated with good
repeatability and reproducibility. Next, stability of the
described film on the electrode surface was investigated. The
sensor retained 94.28% of its initial response current even
after one month storage period validated good storage
stability. Only 6.4% of initial peak current was loosed even
after 200 cycles indicating the excellent stability and anti-
fouling ability of the modified electrode.
Ip = (2.99 x 105) n [(1-α)na]1/2 A Co Do1/2 ν1/2
(5)
Here, D0 (cm2 s−1) is diffusion coefficient (2.42×10-5 cm2 s-1,
estimated via chronoamperometry), A is electrode area (0.071
cm2) and Co is bulk concentration of hydrazine (1×10-3 mol
cm−3), while other parameters represents their conventional
meanings. By substituting all these parameters in eq. (5), the
value of n was calculated as four which is consistent with
previous reports.16,27 The first step involves slow one-electron
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transfer (eq. 6), while second step involves fast three-electron increases as the concentration of hydrazine increases. A
transfer (eq. 7).
concentration dependent linear plot was obtained (inset to
Fig. 4C). The concentration range was linear over 0.2 mM–2.8
mM with sensitivity of 42.88 μA mM–1. Fig. 4D presents the
amperometric curves obtained at RGO-pTACoPc film modified
rotating disc GCE for sequential addition of 50 nM hydrazine
into PB (pH 7). The rotation speed was 1500 RPM, while Eapp
was + 0.25 V. The concentration dependent current plot
displayed good linearity with slope of 0.341 nA nM-1 (inset to
Fig. 4D). The concentration range was linear from 50 nM to 2.6
µM with sensitivity of 1.624 nA nM–1 cm–2, while LOD was
calculated to be 10 nM (S/N=3). The important sensor
parameters, such as α, Do, LOD and linear range were
compared with other sensors (Table 2). The described sensor
exhibited quite comparable performance with reported
sensors. Notably, LOD of the sensor is 10 nM which surpassed
most of the reported metal phthalocyanines based sensors.
Therefore, the described electropolymerization method has
great advantage over other methods to fabricate highly
sensitive hydrazine sensor.
N2H4 + H2O → N2H3 + H3O+ + e– (slow)
N2H3 + 3H2O → N2 + 3H3O+ + 3e– (fast)
(6)
(7)
The overall reaction can be given as eq. (8) [27],
N2H4 +4H2O → N2 + 4H3O+ +4e–
(8)
3.10 Repeatability, reproducibility, stability, selectivity and
real sample analysis
In order to calculate repeatability of the sensor, five repeated
amperometric experiments were performed using RGO-
pTACoPc film modified electrode towards determination of
hydrazine (100 nM). The R.S.D between five repeatable
measurements was calculated to be 2.55%. In order calculate
reproducibility of the sensor, five different RGO-pTACoPc films
modified electrodes were fabricated and amperometric
experiments were performed on each electrodes towards
determination of CySH (100 nM). The R.S.D between five
Fig. 4 (A) CVs obtained at unmodified (a), RGO (b), pTACoPc (c) and RGO-pTACoPc (d)
film modified GCEs in PB (pH 7) containing 1 mM hydrazine. Scan rate= 50 mVs-1. Inset:
optimization of pTACoPc electrodeposition cycles. CVs were carried out at RGO-
pTACoPc/GCE (0, 5, 10, 15, 20, 25 and 30 cycles) in PB (pH 7) containing 1 mM amperometric measurements was calculated to be 2.47%. The
hydrazine. (B) CVs obtained at RGO-pTACoPc/GCE in PB (pH 7) containing 1 mM
hydrazine at different scan rates from 0.05–0.5 Vs-1. Inset: ν1/2 vs. Ip, Ip/μA= 333.32 ν1/2
)
(μA/(Vs-1 1/2)– 24.522. (C) CVs obtained at RGO-pTACoPc/GCE in PB (pH 7) containing
obtained R.S.D values are in the acceptable range and hence
the sensor validated with good repeatability and
reproducibility.
hydrazine; a=0, b=0.2, c= 0.4, d=0.6, e=0.8, f=1.0, g=1.2, h=1.4, i=1.6, j=1.8, k=2.0, l=2.2,
In order to determine storage stability, the electrocatalytic
m=2.4, n=2.6 and o=2.8 mM. Inset: [hydrazine] vs. Ipa; Ip/μA= 42.88 [hydrazine] (mM) +
6.45. (D) Amperometric response of RGO-pTACoPc film modified rotating disc GCE response of the sensor was monitored every day; 95.45% of
upon each sequential addition of 50 nM hydrazine into continuously stirred PB (pH 7)
initial Ip was retained over one month of its continuous use
at rotation speed of 1500 RPM. Eapp = + 0.20 V. Inset: [hydrazine] vs. Ip; Ip/µA= 0.341×
10-3 [hydrazine] (µA/nM)–0.005.
which revealed good storage stability. Stability of the sensor
under hydrodynamic conditions was investigated. Only 5.4% of
initial response current was decreased even after continuously
operation for 3000 s.
Table 2 Comparison of electroanalytical parameters obtained at RGO-pTACoPc/GCE
with reported works for the determination of hydrazine
Selectivity of the sensor was investigated in presence of
–
Electrode
Detection
limit/µM
Linear
Sensitivity
Ref.
–
likely interferences, such as, F–, Cl–, Br–, I–, CO32–, NO3 , NO2 ,
range/µM
/μAμM−1cm−2
+
Na+, Mg2+, Ca2+, Ba2+, Fe2+ , Co2+, Ni2+, Zn2+, NH4 , glucose and
33
34
Curcumin/MWCNT
ZnO nanonail
1.4
0.2
2–44
0.322
8.56
fructose (1000-fold excess concentration than hydrazine) (Fig.
S7). As shown in figure S7, the senor had not shown any
quantifiable responses to these species; however it shows
quick response to hydrazine which revealed its good anti-
interference ability. Water samples collected from rain and tap
were employed to evaluate practicality of the sensor, (Table
S3). The spiked hydrazine concentrations were 50 mM and 100
nM. The found and recovery values are in acceptable range
validated practical feasibility of the sensor.
0.1–1.2
35
36
37
C@ZnO nanorod
CuS/RGO
0.1
0.3
0.1–3.8
1–1000
9.4
0.008
0.562
1.624
Co-graphene
0.1
0.25–370
0.05–2.6
RGO-pTACoPc/GCE
0.01
This
work
3.9 Determination of hydrazine
Fig. 4C shows the CVs obtained at RGO-pTACoPc/GCE in PB (pH
7) containing different concentration of hydrazine. The Ipa
6 | J. Name., 2012, 00, 1-3
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ARTICLE
15 E. W. Schmidt, 2001, Hydrazine and its derivatives
4. Conclusions
16 R. Devasenathipathy, V. Mani, S.-M. Chen, D. Arulraj and V.
Vasantha, Electrochimica Acta, 2014, 135, 260-269.
17 B. Achar, G. Fohlen, J. Parker and J. Keshavayya, Polyhedron,
In
summary,
we
have
developed
a
simple
electropolymerization approach to prepare RGO-pTACoPc film
modified electrode. The polymerized film was characterized by
morphological, elemental and electrochemical methods. The
presence of RGO on the electrode surface greatly improves the
polymerization process and also significantly enhances
conductivity and electrocatalytic ability as revealed by EIS
data. The RGO-pTACoPc film modified electrode exhibited
excellent electrocatalytic ability to CySH and hydrazine. The
effects of film thickness, applied scan rate and analytes
concentration were studied. The electrode detects CySH at
wide linear range of 50 nM–2.0 µM with LOD of 18.5 nM. The
electrode detects hydrazine with wide linear range of 50 nM–
2.6 μM and low LOD of 10 nM. Besides, the electrode showed
appreciable stability, repeatability, reproducibility and
practicality. The easy preparation procedure, high stability and
excellent electrocatalytic ability of RGO-pTACoPc revealed its
great potential in electrochemical sensor applications.
1987, 6, 1463-1467.
18 W. S. Hummers Jr and R. E. Offeman, Journal of the American
Chemical Society, 1958, 80, 1339-1339.
19 K. L. Brown, J. Shaw, M. Ambrose and H. A. Mottola,
Microchemical journal, 2002, 72, 285-298.
20 B. R. Kozub and R. G. Compton, Sensors and Actuators B:
Chemical, 2010, 147, 350-358.
21 L. D. Rollmann and R. T. Iwamoto, Journal of the American
Chemical Society, 1968, 90, 1455-1463.
22 H.-L. Guo, X.-F. Wang, Q.-Y. Qian, F.-B. Wang and X.-H. Xia,
ACS nano, 2009,
23 C. Liu, H. Zhang, Y. Tang and S. Luo, Journal of Materials
Chemistry A, 2014, , 4580-4587.
3, 2653-2659.
2
24 S. -M. Chen, C. –J. Liao, V. S. Vasantha, Journal of
Electroanalytical Chemistry 2006, 589, 15–23.
25 A. J. Bard and L. R. Faulkner, Electrochemical methods:
fundamentals and applications, Wiley New York, 1980.
26 A. A. Ensafi, S. Dadkhah-Tehrani and H. Karimi-Maleh,
Analytical Sciences, 2011, 27, 409.
27 R. Devasenathipathy, V. Mani and S.-M. Chen, Talanta, 2014,
124, 43-51.
28 K. I. Ozoemena and T. Nyokong, Electrochimica Acta, 2006,
51, 2669-2677.
Acknowledgements
29 R. d. C. S. Luz, A. B. Moreira, F. S. Damos, A. A. Tanaka and L.
T. Kubota, Journal of pharmaceutical and biomedical
analysis, 2006, 42, 184-191.
30 M. N. Abbas, A. A. Saeed, B. Singh, A. A. Radowan and E.
Dempsey, Analytical Methods, 2015, 7, 2529-2536.
31 R. Devasenathipathy, C. Karuppiah, S.-M. Chen, V. Mani, V. S.
Vasantha and S. Ramaraj, Microchimica Acta, 2014, 1-9.
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S. Ramaraj, Int. J. Electrochem. Sci, 2015, 10, 682-690.
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2009, 135, 650-655.
This work was supported by the Ministry of Science and
Technology, Taiwan (NSC 103-2811-M-027-002 and 102-2113-
M-027-002-MY3). Veerappan Mani gratefully acknowledges
the Ministry of Science and Technology (MOST), Taiwan for the
postdoctoral fellowship).
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Table of contents
A simple electrodeposition route to prepare tetraamino phthalocyanine polymers on reduced
graphene oxide for electrochemical determination of cysteine and hydrazine