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A. Wong et al. / Electrochimica Acta 146 (2014) 830–837
Table 1
Parameters optimized for the proposed biomimetic sensor for carbofuran detection
and quantification.
Amount of complex in the paste / % (w/ 7*
w)
9
13
23
Amount of graphene oxide in the paste / 0.02
% (w/w)
0.04 0.11* 0.15
pH
6.0
Phosphate* BR
0.01
6.5
7.0* 7.5
Buffer
TRIS TRISMAN
Phosphate / mol Lꢁ1
0.05 0.075 0.1*
*Optimized value. BR: Britton-Robson buffer.
carbofuran–OH + hemin–Fe(III) ! carbofuran = O +
hemin–Fe(II)
(1)
eꢁ
hemin–Fe(II) ! hemin–Fe(III) +
(2)
Based on these reactions, a plausible mechanism is suggested
regarding the sensor response of carbofuran-phenol (analyte) as
shown in Fig. 2. In the first step is initiated reaction between
carbofuran-OH and hemin-Fe (III) enables the quantitative
oxidation of the analyte to carbofuran = O and promotes the
simultaneous reduction of iron (II) in the hemin complex. In
second step, occur the electrochemical oxidation of hemin-Fe (II)
to hemin-Fe (III) through a reversible process (redox couple in
Figs. 1B and S2). These two steps could be said to be describing a
typical CE (chemical-electrochemical) system [34–36], which
was evident in our experimental results, where an increase was
observed in the oxidation current likewise the regeneration of
hemin-Fe (IV) to hemin-Fe (III) close to 160 mV through electro-
catalytic reaction.
Fig. 4. Response profile in square wave voltammetry (SWV) obtained for varying
concentrations of carbofuran-phenol. Measurements carried out in 0.1 mol Lꢁ1
phosphate buffer (pH 7.0), f = 10 Hz, A = 100 mV,
DEs = 4 mV, Eacc = 0 V and tacc = 50 s.
a!l: [carbofuran-phenol] = 5.0 ꢀ10ꢁ6, 1.5 ꢀ10ꢁ5, 2.4 ꢀ10ꢁ5, 3.4 ꢀ10ꢁ5, 4.3 ꢀ10ꢁ5
,
5.2 ꢀ10ꢁ5, 6.1 ꢀ10ꢁ5, 7.0 ꢀ10ꢁ5, 7.8 ꢀ 10ꢁ5, 8.8 ꢀ 10ꢁ5, 9.5 ꢀ10ꢁ5 mol Lꢁ1
.
The effect of modifiers on the sensor response using SWV is
shown in Fig. 3. Figs. 3A and 3B show the response of the
unmodified carbon paste electrode in the absence (A) and presence
(B) of carbofuran-phenol. Fig. 3C shows the sensor response for a
paste modified by only hemin while Fig. 3D depicts the response of
the proposed sensor. From these results, it is clearly evident that
the modification of the paste with hemin and graphene oxide
promoted a noticeable gain in the sensor response, enabling
the analyte quantification at low potential (110 mV), and
thus contributing towards the improvement of selectivity and
sensitivity of the proposed biomimetic sensor.
3.3. Optimization of sensor response
The results obtained of all optimization parameters are shown
in Table 1.
Fig. 4 shows a typical graph of square wave voltammetry (SWV)
in the optimized conditions of analysis. An analytical curve is
The optimization of the proposed sensor was carried out by
means of square wave voltammetry (SWV). The effect of the buffer
solution on the sensor response was tested in four different buffer
solutions (phosphate, Britton-Robinson, TRIS and Trisman) with
the concentration of 0.1 mol Lꢁ1. When the phosphate buffer
solution was used the baseline was found to be steady, thus
allowing us to obtain better repeatability in the electrochemical
measurements. In this sense, the phosphate buffer solution was
chosen. The effect of pH on the oxidation of carbofuran-phenol was
investigated using SWV in the pH range of 5.0 to 8.0. The oxidation
peak current of carbofuran-phenol was found to be best at the pH
of 7.0 and as such this pH was chosen for the subsequent analytical
experiments (FIGURE S3 of Supplementary Data).
In the study of the parameters of square wave voltammetry,
the optimal values obtained for this analysis were: frequency of
10 Hz, amplitude of 100 mV, potential increment of 4 mV,
accumulation potential of 0 V and accumulation time of 50 s.
These values were chosen based on the highest current value
obtained in SWV.
observed with
a
R2 of 0.999 for carbofuran-phenol in the
concentration range of 5.0 ꢀ10ꢁ6 and 9.5 ꢀ10ꢁ5 mol Lꢁ1
, a
sensitivity of 1.1 ꢀ105 (ꢂ1.4 ꢀ103)
m
A L molꢁ1, detection limit of
9.0 ꢀ10ꢁ9 mol Lꢁ1 (3x
s) [37]. The reproducibility of sensor
response was evaluated by analyzing three replicates of carbo-
furan-phenol, being found a RSD 2.1%.
The results obtained with the proposed biomimetic sensor
when compared to the others described in literature (Table 2)
showed a similar linear response range and a low analysis
potential, which allows us to obtain good selectivity of the sensor,
especially when applied to complex samples. Another feature
which makes this device relatively more advantageous is direct
detection and ability to renew the surface by a simple polishing of
the surface of the carbon paste electrode, making the use of the
proposed sensor feasible in numerous electrochemical analyses.
Table 2
Electrochemistry sensors used for the detection of carbofuran.
Electrode
Dynamic range (mol Lꢁ1
)
Detection limit (mol Lꢁ1
)
ip / mV
Technique
References
ECV (CoO/GO)a
Screen-printed of carbono ink
CNPPEb
0.5 ꢀ10ꢁ6 - 2.0 ꢀ10ꢁ4
0.4 ꢀ10ꢁ6 - 4.0 ꢀ10ꢁ4
0.5 ꢀ10ꢁ7 - 4.4 ꢀ10ꢁ7
5.0 ꢀ10ꢁ9 - 9.0 ꢀ10ꢁ8
5.0 ꢀ10ꢁ6–9.5 ꢀ10ꢁ5
2.0 ꢀ10ꢁ8
5.0 ꢀ10ꢁ8
0.5 ꢀ10ꢁ7
3.6 ꢀ10ꢁ9
9.0 ꢀ10ꢁ9
400
250
750
600
110
DPV
DPV
Amperometry
SWV
SWV
[38]
[32]
[39]
[40]
––
ECV (AChE/Fe3O4)c
This work
a
Glassy carbon electrode modified with CoO and graphene oxide.
Alkaline phosphatase immobilized on a carbon nano-powder paste electrode
Glassy carbon electrode modified with acetylcholinesterase and iron oxide nanocomposite.
b
c