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Talanta
journal homepage: www.elsevier.com/locate/talanta
art ic l e i nf o a b s t r a c t
Article history: A simple and sensitive electroanalytical method was developed for determination of nanomolar levels of
Received 15 March 2015 Pb(II) based on the voltammetric stripping response at a carbon paste electrode modified with biochar (a
Received in revised form special charcoal) and bismuth nanostructures (nBi-BchCPE). The proposed methodology was based on
16 April 2015
spontaneous interactions between the highly functionalized biochar surface and Pb(II) ions followed by
Accepted 17 April 2015
Available online 27 April 2015
reduction of these ions into bismuth nanodots which promote an improvement on the stripping anodic
current. The experimental procedure could be summarized in three steps: including an open circuit pre-
Keywords: concentration, reduction of accumulated lead ions at the electrode surface and stripping step under
Bismuth nanostructures differential pulse voltammetric conditions (DPAdSV). SEM images revealed dimensions of bismuth
Electrochemical sensor
nanodots ranging from 20 nm to 70 nm. The effects of main parameters related to biochar, bismuth and
Lead
operational parameters were examined in detail. Under the optimal conditions, the proposed sensor has
Ceramic dishes
exhibited linear range from 5.0 to 1000 nmol L 1 and detection limit of 1.41 nmol L 1 for Pb(II). The
optimized method was successfully applied for determination of Pb(II) released from overglaze-deco-
rated ceramic dishes. Results obtained were compared with those given by inductively coupled plasma
optical emission spectroscopy (ICP-OES) and they are in agreement at 99% of confidence level.
& 2015 Published by Elsevier B.V.
1. Introduction techniques are very sensitive, they have the disadvantages such as
high cost, complicated and inadequate instrumentation for field
Lead is one of the most useful metals used since ancient civi- application.
lizations because of its abundance, low cost and ease in their An interesting alternative method for determination of Pb(II)
applications [1]. However, it is characterized by serious effects on and other species at trace and ultra-trace levels is stripping vol-
human health being absorbed and accumulated in the body, tammetry, especially those involving adsorptive steps [8–10]. This
affecting heart, bones, intestines, kidneys, reproductive and ner- is one of the most accessible and widely used techniques [11]
vous systems [2], lag in learning and low neurological develop- characterized by a remarkable sensitivity, low cost, simple equip-
ment in children [3]. Although several efforts have been made to ment, rapid analysis and the possibility of analyses in colored
reduce the exposure to this metal, a large number of people con- samples or with dispersed solid particles [12]. Regarding electrode
tinue being polluted by lead due to many sources of exposure like materials, mercury has been widely used in stripping analysis of
lead due to its ability of yield amalgam with the analyte, wide
paints, water, food, dust, soil, kitchen utensils and leaded gasoline
potential window, high sensitivity and reproducibility [13,14].
[4]. For these reasons, the determination of traces of lead in different
High toxicity, difficulties with storage and disposal of mercury has
matrices has a great importance. The certified standard techniques
stimulated the research for other materials with similar electro-
include electrothermal atomic absorption spectrometry (ET-AAS),
analytical characteristics and low toxicity [15–17].
flame atomic absorption spectrometry (FAAS), inductively coupled
Bismuth modified electrodes provided an alternative material
plasma mass spectrometry (ICP-MS) and inductively coupled plasma
for electrochemical stripping analysis. This metal was introduced
optical emission spectrometry (ICP-OES) [5–7]. Although these by Wang and coworkers in 2000 [18] and it has excellent chemical
and electrochemical characteristics such as low toxicity, ability to
n
Corresponding author. Tel.: þ 55 41 3361 3177; fax: þ55 41 3361 3186. form alloys with different metals, wide potential ranges and low
E-mail address: luiz1berto@ufpr.br (L.H. Marcolino-Junior). sensitivity to dissolved oxygen [19]. Bismuth modified electrodes
http://dx.doi.org/10.1016/j.talanta.2015.04.052
0039-9140/& 2015 Published by Elsevier B.V.
222 D. Agustini et al. / Talanta 142 (2015) 221–227
are usually built on different substrates: gold, platinum, glassy was prepared by dissolving Bi(NO3)3 5H2O in 0.04 mol L 1 HNO3.
carbon, screen-printed ink, carbon fiber or carbon pastes [20]. The Acetate buffer solutions (0.01 mol L 1) were made by mixing
latter has been widely used due to its inexpensive and easy pre- appropriated volumes of 0.05 mol L 1 sodium acetate solution with
paration, renewable surface and stability in different solvents [21]. 0.05 mol L 1 acetic acid solution; the required pH was adjusted
The most common procedures used for preparing bismuth-film with a sodium hydroxide solution. Potassium nitrate solution
modified electrodes are the in situ [22] or ex situ [23,24]. Some (0.1 mol L 1) was prepared by dilution of KNO3 in purified water.
other procedures include carbon paste containing bismuth pre-
cursor or powder such as the incorporation of Bi2O3 with sub- 2.2. Apparatus
sequent reduction to metallic bismuth [25]. In the last years, the
electrode modification with bismuth nanostructures has been Voltammetric measurements were performed in a mAUTOLAB
proposed to combine the electrochemical bismuth characteristics Type III (Metrohm) connected to a microcomputer controlled by
with the inherent advantages of nanomaterials such as enhanced software (NOVA 1.7) for data acquisition and instrumental control.
mass transport, high surface area and improved signal-to-noise All electrochemical experiments were carried out in a three-elec-
ratio [26,27]. However, the fabrication of nano-bismuth is char- trode configuration employing a nBi-BchCPE as the working
acterized for complicated synthesis, complex instrumentation or electrode, platinum wire as counter electrode and Ag/AgCl
long time reduction [28–30]. An alternative route for simple and (3.0 mol L 1 KCl) as reference electrode. All the experiments were
rapid construction of bismuth nanostructures at an electrode performed in a 25 mL glass cell at room temperature and without
surface could be achieved by using functionalized materials to removing oxygen.
support the bismuth ions before the electrochemical reduction. A Metrohm 780 pH meter was used to determine the pH of
Biochar is a kind of carbonaceous material easily obtained by solutions. Scanning Electron Microscopy (SEM) images were
thermal degradation (pyrolysis) of biomass (such as crop residues, obtained from a Quanta 450 ESEM FEG and Energy Dispersive
wood scraps, municipal and industrial solid waste and animal X-ray Spectroscopy (EDS) analysis was performed from an EDAX
manure) in the absence of air [31]. The production of biochar can microanalysis. For the ICP-OES Pb(II) measurements a Thermo
be considered as low-cost and environmental friendly and its Scientific, model 6000 Series equipment was used.
applications include soil amendment, power source conversion
processes, carbon sequestration and as sorbent material for the 2.3. Preparation of the modified carbon paste electrode
removal of organic and inorganic contaminants in soil and water
[32–34]. In these works, biochar has exhibited high removal The carbon paste electrode modified with biochar (BchCPE)
capacity, especially for metallic ions, in a relatively short time was prepared by carefully mixing the dispersed graphite powder
when compared with other sorbent materials. This adsorption (45–75% (w/w)) (Fischer) with biochar (0–30% (w/w)) and mineral
capacity of biochar was exploited in electrochemical stripping oil (25% (w/w)). The components were mixed manually in a
measurements only in two works previously reported by Sugui- mortar and pestle for at least 10 min to obtain an appropriate
hiro et al. [35] for Pb(II) and Cd(II) determination in water samples homogenization. Subsequently, the modified carbon paste was
and Oliveira et al. [36] for Cu (II) determination on spirit drinks. Its packed into piston-driven electrode holder (PVC cylindrical tube,
use as platform for growth of metallic nanostructures has not been i.d. 3.0 mm) and arranged with a copper wire serving as an
explored yet. external electric contact. The electrode surface was smoothed by
In this paper, we report for the first time the development of polishing on a piece of paper. For the carbon paste electrode
easy and sensitive nBi-BchCPE for in-field pre-concentration and modified with biochar and bismuth nanostructures (nBi-BchCPE)
fast quantification of Pb(II) by differential pulse adsorptive stripping preparation, the carbon paste electrode modified only with bio-
voltammetry (DPAdSV). The main novelty of this work is the use of char (BchCPE) was placed in 10 mL of Bi(III) ions solution for 30 s
biochar simultaneously as platform for growth of Bi nanostructures under no applied potential and stirring conditions.
as well as spontaneous pre-concentration and subsequent deter-
mination of Pb(II) ions. Under these conditions, Pb(II) ions can be 2.4. Measurement procedure
pre-concentrated simultaneously using different electrodes (as
passive samplers) which make possible rapid and consecutive The voltammetric measurements were performed by adsorp-
measurements. The proposed sensor has shown a synergic effect tive stripping voltammetry. For pre-concentration step, the nBi-
based on the high adsorption capacity of biochar for Bi(III) and Pb BchCPE was immersed in a stirred 10 mL of 0.01 mol L 1 acetate
(II) ions and the enhancement of the electrochemical response buffer solution (pH 6) containing Pb(II) for 5 min at open circuit
characterized by presence of Bi nanostructures on the electrode potential. The electrode was then removed from the pre-con-
surface. The proposed electrode was applied for trace determination centration cell, gently rinsed with 0.01 mol L 1 acetate buffer,
of Pb(II) released from overglaze-decorated ceramic dishes samples placed in the electrochemical cell containing 10 mL of a quiescent
and the results were compared with ICP-OES. supporting electrolyte (0.01 mol L 1 acetate buffer pH 4.5 and
0.1 mol L 1 KNO3) and applied a potential of 0.8 V (vs. Ag/AgCl,
KCl 3.0 mol L 1) for 30 s. After that, the voltammograms were
2. Experimental recorded by applying a potential scan from 0.8 to þ0.5 V under
differential pulse voltammetry conditions with potential ampli-
2.1. Material and reagents tude of 100 mV, pulse time of 25 ms and step potential of 5 mV.
Finally, the electrode surface was renewed by mechanical polish-
The biochar (with a particle size of 80 meshes) was produced ing in paper.
from castor oil cake by pyrolysis at 300 °C with a heating rate of
10 °C/min (60 min of residence time). All chemicals had analytical 2.5. Analysis of real samples
reagent grade and used without further purification. The solutions
were prepared with purified water in a Millipore Milli-Q system. A The leaching of Pb(II) present in these dishes was performed
standard solution containing 1000 mg L 1 Pb(II) (Merck) was used according to methodology adapted from ANVISA [37]. For the
as stock. Solutions containing different concentrations of Pb(II) ions release test, 0.1 mol L 1 acetic acid solution was used for 2 h at a
were made by dilution in water. Bi(III) ion solution (3.0 mmol L 1) temperature of 80 °C. An aliquot of the resulting solution was
D. Agustini et al. / Talanta 142 (2015) 221–227 223
added to the pre-concentration solution so that the concentration and after pre-concentration in solution containing 10 mmol L 1 of Pb
of Pb(II) stays within the linear range of techniques. Six brands of (II) ions. Individual determination of lead and bismuth was necessary
Chinese (porcelain/ceramic) dishes were evaluated with the to investigate the less intense peaks of the layers Lα and Lβ for these
techniques of DPAdSV (using the nBi-BchCPE) and ICP-OES. metals in order to avoid signal overlap. A good separation of these
layers were obtained, with values of 10.51; 10.80; 12.56 and 12.97 keV
to Pb Lα, Bi Lα, Pb Lβ and Bi Lβ respectively. As can be seen in Fig. 2D,
3. Results and discussion the BchCPE (Fig. 2D-Curve I) before pre-concentration of lead, the EDS
spectrum has not indicated the presence of any element which could
3.1. Voltammetric performance of Bi-BchCPE for Pb(II) determination interfere with the identification of bismuth or lead. After pre-con-
centration of Pb(II) in BchCPE (Fig. 2D-Curve II), it was possible to
The addition of the modifiers (biochar and bismuth) in the car- observe the characteristic peaks of Pb Lα (10.51 keV) and Pb Lβ
bon paste aims to improve the adsorption and, consequently, the (12.56 keV). For nBi-BchCPE (Fig. 2D-Curve III), before the pre-con-
detectability of Pb(II). To evaluate the effect of the modifiers in the centration of lead, it was noted peaks of Bi Lα (10.80 keV) and Bi Lβ
voltammetric response, the CPE, BchCPE and Bi-BchCPE were sub- (12.97 keV) confirming the incorporation of bismuth in the biochar.
mitted to measurements after pre-concentration step in solution Finally, for nBi-BchCPE after pre-concentration and reduction of Pb(II)
containing a 10 mmol L 1 of Pb(II) ions. In Fig. 1A, the CPE presented (Fig. 2D-Curve IV) the EDS spectrum taken from a nanodot has
a very low adsorption capacity and negligible response. However, revealed the presence of both Bi and Pb in the same nanostructure.
the signal observed for lead oxidation (Epa ¼ 0.57 V) using the These results confirm not only the pre-concentration of analyte at the
BchCPE (Fig. 1B) is remarkably higher than CPE which can be electrode surface but also they suggest strongly the formation of alloy
attributed to the presence of biochar in the paste due to the phe- between lead with bismuth.
nomena such as metal exchange, complexation with functional
groups, surface precipitation or physical adsorption that can sig- 3.3. Influence of experimental parameters on the voltammetric
nificantly increase the pre-concentration of Pb(II) on the electrode response nBi-BchCPE
surface [38]. Finally Fig. 1C represents the peak current of Pb(II) in
Bi-BchCPE being remarkably greater than BchCPE, which clearly In order to obtain the best voltammetric behavior of the nBi-
confirms the efficiency in the use of proposed sensor to the deter- BchCPE for Pb(II) determination, several parameters related to paste
mination of Pb(II) due to the inherent advantages of the bismuth.
composition, bismuth incorporation, pre-concentration solution and
Bismuth oxidation can be observed at potential of 0.10 V.
operational parameters were examined in detail.
The amount of biochar added in the nBi-BchCPE has a sig-
3.2. SEM and EDS characterization of modified carbon paste nificative influence on the voltammetric responses. Electrodes
electrodes
with different percentages of biochar were prepared and exam-
ined for their voltammetric signals to Pb(II) under identical con-
The size and distribution of bismuth anchored on the BchCPE
ditions. The peak currents for Pb(II) have increased with increasing
can affect significantly the determination of Pb(II). Fig. 2A shows a
the amount of biochar in the nBi-BchCPE due to the increased
homogeneous dispersion of bismuth nanostructures (light gray
adsorption capacity of the electrode. At 25% (w/w) of biochar the
spherical nanodots with some white agglomerates) on the biochar
highest peak current was obtained. For amounts of biochar higher
particles incorporated on the electrode surface which can be
than 25% the response of the nBi-BchCPE decreased due to the
observed in detail in Fig. 2B. Fig. 2C shows the size distribution
reduction in conductivity of the modified electrode [39]. According
graph obtained by manual counting of at least 500 structures,
to these results a carbon paste composition of 25% biochar, 50%
which indicates an average size of 4273 nm. The obtained bis-
graphite and 25% mineral oil was used in further studies.
muth nanostructures were manufactured and deposited on the
The presence of bismuth nanostructures on the electrode surface
electrode surface by using an easier strategy in comparison with
is fundamental to its high sensitivity to the determination of Pb(II).
some other reported [22].
For a fast and stable incorporation of nano-bismuth the pH of the
To investigate the interaction between biochar, bismuth and lead,
solution has great influence. In this way, pH solutions ranging from
EDS measurements were performed in BchCPE and nBi-BchCPE before
0.5 to 2.0 were used to study the grafting of Bi(III) ions on the
electrode surface. The maximum incorporation was achieved at pH
1.5, whereas at low pH values a strong competition occurred
between Bi(III) and H þ ions for adsorption sites of biochar. At pH
above 1.5, there is a spontaneous process of hydrolysis of bismuth
ions according to the reaction [40] Bi3 þ þ3H2O⇆Bi(OH)3 þ3H þ ,
limiting the use of adsorption solutions of Bi(III) at pH values
around 1.5.
The effect of the amount of bismuth added on the biochar was
evaluated by immersion of the electrode in the solution containing
bismuth ions for different intervals of time ranging from 15 to
300 s. After that, the electrode was submitted to pre-concentration
step in solution containing 10 mmol L 1 of Pb(II). As shown in
Fig. 3, the faradaic signals observed for oxidation of bismuth have
increased with increase of time suggesting an increase in the
amount Bi(III) ions incorporated on the electrode surface. By other
side, the stripping peak height of Pb(II) (Fig. 3) increased with
Fig. 1. DPAdSV of 1.0 10 5 mol L 1 Pb(II) at (A) CPE, (B) BchCPE and (C) Bi- increasing time of Bi(III) grafting from 15 to 30 s and it gradually
BchCPE in 0.01 mol L 1 acetate buffer pH 4.5 þ 0.1 mol L 1 KNO3. Reduction
decreases for values above 30 s probably due to the formation of
potential: 0.8 V, reduction time: 30 s, potential scan: 0.8 to þ0.5 V, potential
amplitude: 100 mV, pulse time: 25 ms, step potential: 5 mV. Pre-concentration step large clusters of nano-bismuth with consequent saturation of
in 0.01 mol L 1 acetate buffer pH 6 containing 10 mmol L 1 of Pb(II) ions for 5 min. adsorption sites of biochar. Therefore, the optimum time for Bi(III)
224 D. Agustini et al. / Talanta 142 (2015) 221–227
Fig. 2. (A) SEM micrograph of nBi-BchCPE with magnification of 25k . (B) Detail of bismuth nanostructures in 132k . (C) Size distribution graph of bismuth nanos-
tructures. (D) EDX pattern of BchCPE and nBi-BchCPE before and after Pb(II) pre-concentration.
more negative than 0.8 V there was a gradual and slight inter- 25 ms and potential amplitude of 100 mV) was selected for ana-
ference of H þ ions in the reduction of lead. Reduction time applied lytical applications.
before voltammetric scan was investigated in the range of 0–120 s In order to improve the analytical performance of the proposed
and it has demonstrated a gradual increase in the response sensor calibration curves were constructed using the pre-con-
between 0 and 30 s, followed by stabilization of current values centration time of 20 min under DPV conditions (Fig. 4A and B).
over this time indicating that 30 s is enough for the effective Fig. 4C shows a calibration plot that was linear over the range
reduction of Pb(II) ions previously adsorbed. Based on these concentration from 5.0 to 1000 nmol L 1 for Pb(II). The linear
results, a reduction potential of 0.8 V for 30 s was selected for regression equation found was I (μA)¼ 1.27 þ82.02CPb(II) (where
further optimization studies. CPb(II) in mmol L 1). The LOD obtained was 1.41 nmol L 1 and the
LOQ was 4.70 nmol L 1.
3.4. Analytical performance of nBi-BchCPE In comparison with a previously described Pb(II) sensor with
biochar in the absence of bismuth [35] the proposed sensor
In order to obtain the best analytical performance for the reaches a limit of detection (LOD) almost 10-fold better. The ana-
proposed sensor, calibration curves for Pb(II) were obtained by lytical features of the proposed electrode were compared (Table 2)
using of DPV, SWV and LSV under optimal conditions (not shown). with other bismuth modified electrodes previously reported in the
Table 1 shows the figures of merit of nBi-BchCPE for each vol- literature for Pb(II) determination. Moreover, the main advantage
tammetric technique used; the limit of detection (LOD) was esti- of the proposed sensor is the Pb(II) pre-concentration performed
mated based on three times the blank standard deviation divided at open circuit potential conditions that allows its use as a passive
by slope of the calibration curve and the limit of quantification sampler (especially for environmental applications) as well as the
(LOQ) was calculated based on 10 times the blank standard
deviation divided by slope of the calibration curve. Table 2
Calibration curves obtained using DPV have shown a linear Comparison of present work and other bismuth modified electrodes for the
determination of Pb(II).
response for a wider concentration range of Pb(II) ions in com-
parison with other voltammetric techniques evaluated. From Electrode Technique LOD Linear range Ref.
Table 1, it was observed that LSV had the poorer analytical per- (nmol L 1) (nmol L 1)
formance probably due to the high capacitive current present in
m-NP/BiFE DPAdSV 87 482.6–1930.5 [44]
their measurements, which resulted in a low sensitivity and BiFE SWASV 33 50–482.6 [45]
high LOD and LOQ. The SWV showed the best sensitivity Bi-HA CME SWASV 24 50–965.3 [46]
(151.64 mA L mmol 1) and an elevated noise in the background Bi/poly(p-ABSA) DPASV 3.7 5–627.4 [47]
current. For these reasons, considering the best LOD and LOQ, DPV film electrode
Bismuth modified SWASV 9.6 50–2413.1 [48]
(under optimized conditions: step potential of 5 mV, pulse time of carbon tape
electrode
Table 1 BiFE SWASV 2.9 5–241.3 [49]
Analytical performance for Pb(II) determination by DPV, SWV and LSV using the Bi-CNT electrode SWASV 6.3 9.6–482.6 [50]
proposed sensor. nBi-BchDCPE DPAdSV 1.4 5–1000 This
work
DPV SWV LSV
m-NP/BiFE: micro/nanoparticle bismuth film electrode; DPAdSV: differential pulse
Linear range (nmol L 1) 10–5000 50–1000 500–10,000 adsorptive stripping voltammetry; BiFE: bismuth film electrode; SWASV: square
Sensitivity (mA L mmol 1) 29.61 151.64 3.80 wave anodic stripping voltammetry; Bi-HA CME: bismuth-modified hydroxyapatite
Correlation coefficient 0.9997 0.9991 0.9995 carbon electrode; Bi/poly(p-ABSA) film electrode: bismuth/poly(p-aminobenzene
Limit of detection-LOD (nmol L 1) 2.83 7.00 149 sulfonic acid) film electrode; Bi-CNT electrode: bismuth-modified carbon nanotube
Limit of quantification-LOQ (nmol L 1) 9.43 23.3 495 electrode; nBi-BchDCPE: nano-bismuth modified biochar doped carbon paste
electrode.
100
80
12
75
60
10
50 40
I /µA
Iap /µA
I /µA
8 20
25
0
0 6
-0.8 -0.6 -0.4 -0.8 -0.6 -0.4 0.0 0.5 1.0
-1
E / V vs. Ag/AgCl KCl 3 mol L -1
CPb2+ /µmol L
Fig. 4. DPV measurements under optimized conditions for Pb(II) using nBi-BchCPE. (A) Pb(II) concentration ranging from 0 to 1.0 mmol L 1; Pb(II) concentration ranging
from 0 to 0.1 mmol L 1; (C) calibration plot (n¼ 3).
226 D. Agustini et al. / Talanta 142 (2015) 221–227
4. Conclusions
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