detection of heavy metals by using a composite sensor ... - Lirmm

DETECTION OF HEAVY METALS BY USING A COMPOSITE SENSOR
BASED ON A BUILT-IN BISMUTH PRECURSOR
M.T. Castañeda, 1, * B. Pérez, M. Pumera, M. del Valle, A.Merkoçi, S. Alegret
Grup de Sensors i Biosensors, Departament de Química, Universitat Autònoma de Barcelona, 08193
Bellaterra, Catalonia, Spain,
1 On leave from: Departamento de Ciencias Básicas, Universidad Autónoma Metropolitana-Azcapotzalco, Av.
San Pablo 180, Col. Reynosa Tamaulipas 022000, México, D. F., MÉXICO.
* Corresponding author; email: tcb@correo.azc.uam.mx, tel: 34-93581 2118, fax: 34-93581 2379
Abstract: A new graphite-epoxy composite electrode (GECE) containing Bi(NO3)3 as built-in bismuth precursor
for simultaneous and individual anodic stripping analysis of heavy trace metals is reported. The developed
Bi(NO3)3-GECE is compatible with bismuth film electrodes reported previously, including the composite
electrodes (Bi-GECE) recently reported by our group. The sensitive response, combined with the minimal
toxicity of the Bi(NO3)3 allow its utilization in environmental quality monitoring as well as other applications.
Keywords: bismuth nitrate, graphite-epoxy composite electrode, heavy metals.
INTRODUCTION
Mercury-modified electrodes coupled with stripping
techniques have been recognised as the most
sensitive methods for determination of heavy
metals [1]. However, the potential dangers
associated with mercury have led to developing
mercury-free sensors. Unmodified electrodes like
bare carbon, gold or iridium [2-4], graphite-epoxi
composites [5-7], recordable CD [8] or silver-plated
rotograved carbon electrodes [9] have been used
as an alternative to mercury based electrodes.
One of the excited alternatives to mercury based
electrodes is that based on bismuth [10]. Our group
have configured Bi-GECE [11], based on graphiteepoxy
composite electrode (GECE) without
modification but bismuth film formation due to the
presence of bismuth in the measuring solution. In
the present work we present a novel configuration,
Bi(NO3)3-GECE that represents GECE modified
internally with bismuth nitrate salt which serves as
built-in bismuth precursor for bismuth film
formation. This represents an integrated
configuration of bismuth based GECEs for stripping
analysis. The low toxicity of bismuth makes it an
alternative material to mercury in terms of trace
metal determination.
EXPERIMENTAL
The lead and cadmium stock solutions were
prepared by dissolving the corresponding nitrates
in water obtained from an ion-exchange system
Milli-Q (Millipore). Acetate buffer (0.1 M, pH 4.5) or
HCl 0.5 M were used as supporting electrolyte. The
Bi(NO3)3GECE were prepared using graphite
powder with a particle size of 50 µm (BDH, UK),
Epotek H77 (epoxy resin), hardener (both from
Epoxy Technology, USA) and Bi(NO3)3 (Aldrich).
Graphite powder and Bi(NO3)3 salt were first mixed
together. The obtained dried mixture was mixed
well with epoxy resin (mixed with hardener) in a
ratio of 1:4 (w/w) as described in a previous work
[12,13]. The percentage of Bi(NO3)3 in the prepared
paste was varied being 0.1, 0.5, and 2.0 % (w/w).
The resulting Bi(NO3)3 containing graphite-epoxy
paste was placed into a PVC cylindrical sleeve
body (6 mm i. d.), which has an inner electrical
copper contact, to a depth of 3 mm. The conducting
composite material glued to the copper contact was
cured at 40 ºC during one week. Before each use,
the surface of the electrode was wet with doubly
distilled water and then thoroughly smoothed, first
with abrasive paper and then with alumina paper
(polishing strips 301044-001, Orion).
A platinum auxiliary electrode (model 52-67 1,
Crison, Spain) and double junction Ag/AgCl
reference electrode (Orion 900200) with 0.1 M KCl
as external reference solution and the Bi(NO3)3-
GECE as working electrode were used. The square
wave anodic stripping voltammetry (SWASV)
experiments were performed using an Autolab
PGSTAT 20 System (Eco-chemie, The
Netherlands). A Hitachi S-570, Japan Scanning
Electron Microscope (SEM) was used to observe
the surface of the working electrodes.
SWASV measurements were carried out in the
presence of dissolved oxygen. The three
electrodes were immersed into the electrochemical
cell containing 25 mL 0.1 mL 0.1 M acetate buffer
(pH 4.5). The deposition potential of -1.3 V was
applied to Bi(NO3)3-GECE while the solution was
stirred. Following 120 s deposition step, the stirring
was stopped and after 15 s equilibration, the
voltammogram was recorded by applying a squarewave
potential scan between -1.3 and -0.3 V with a
frequency of 50 Hz, amplitude of 20 mV and
potential step of 20 mV.
Aliquots of the target metal standard solution were
introduced after recording the background
voltammograms. A 60 s conditioning step at +0.6 V
(with solution stirring) was used to remove the
remaining reduced target metals and bismuth, prior
to the next cycle. The electrodes were washed
thoroughly with deionized water between each test.

Measurements in phosphate buffer for the study of
pH effect as well as in HCl medium were also
performed in the same experimental conditions as
described above.
RESULTS AND DISCUSSION
The surface morphologies of Bi(NO3)3-GECEs
(containing different quantities of Bi(NO3)3 salt)
before and after the preconcentration step
(electrolysis at -1.3 V during 120 s) were observed
by Scanning Electron Microscopy (SEM).
As can be seen, the surfaces of Bi(NO3)3-GECE,
with different concentration of Bi(NO3)3, before
preconcentration step (Fig. 1 A) appears to have
clusters of conducting material gathered in random
areas. This is due to the graphite particles
randomly distributed and randomly oriented in the
epoxy resin [14]. Microcrystalline Bi(NO3)3 particles
should be also distributed randomly but due to the
very low percentage (0.1 to 2.0 %, w/w) were not
visible. The darker coverage of the same Bi(NO3)3-
GECEs after preconcentration step (Fig. 1 B)
compared to Bi(NO3)3-GECE before
preconcentration (Fig. 1 A) is clearly visible. This is
due to the bismuth film formation coming from the
Bi(NO3)3 salt inside the sensor matrix.
The quantity of Bi(NO3)3 didn’t have any visible
effect on the sensor surface. The images 1 B (a-c)
have similar darkness. Seems that for the Bi(NO3)3
quantities used the bismuth film has the same
configuration.
For all the Bi(NO3)3-GECEs after the
preconcentration step there can be also seen
dimensional fibril-like networks onto their surfaces
which is in correlation with the early report of Bi-film
at carbon surface [10]. The black and thick
appearances of bismuth deposit can be attributed
to carbon substrate that has positive effect on the
nucleation and growth of the bismuth film. The
same deposition of bismuth was clearly observed
for GECE used in connection with bismuth in
measuring solution.
The characteristics of the electrodes must be very
dependent on the amounts of Bi(NO3)3 used for the
Bi(NO3)3-GECEs preparation.
The effect of Bi(NO3)3 loadings (0.1-2.0 %, w/w) in
the SWASV of the resulting Bi(NO3)3-GECEs were
studied for a 70 ppb solution of Pb 2+ at 0.1 M
acetate buffer pH 4.5. The increase of Bi(NO3)3
content in the composite electrode increase the
bismuth ion release during the contact with the
measuring solution, and consequently the bismuth
film formation capability. On the other hand, the
higher Bi(NO3)3 content may reduce the
conductivity of the Bi(NO3)3–GECE. A maxim
response was observed for Bi(NO3)3-GECE
containing 0.1 % Bi(NO3)3.
The stripping performance of Bi(NO3)3-GECE was
tested for lead and cadmium and the resulting
voltammograms are showed in Fig.2. The Figure
demonstrates the square wave stripping
voltammograms for increasing concentration of
cadmium (A) in 10 µg L -1 steps (b–j) and lead (B) in
10 µg L -1 steps (b–h). Also shown are the
corresponding blank voltammograms (a) and the
calibration plots (right) over the ranges 10 – 90 µg
L -1 cadmium and 10 – 70 µg L -1 lead. The Bi(NO3)3-
GECE displays well-defined and single peaks for
cadmium (Ep = -0.76 V) and lead (Ep = -0.54 V).
Detection limits of 7.23 and 11.81 µg L -1 can be
estimated for cadmium and lead respectively based
on the upper limit approach (ULA) [15]. Also in the
concentration ranges mentioned above, the
calibration plots (right) were linear exhibiting the R
values of 0.9968 and 0.9953 for cadmium and lead
respectively.
As in the case of Bi-GECE the bismuth film
formation onto Bi(NO3)3-GECE is shown to be a
homogenous and uniform one due to the novel
supporting material. The rich microstructure of
Bi(NO3)3-GECE, composed of a mixture of carbon
microparticles forming internal microarrays might
have a profound effect upon the bismuth film
structural features. The obtained peak widths of 20
mV for lead and cadmium were similar to other
bismuth film electrodes reported previously.
The simultaneous measuring of lead and cadmium
with Bi(NO3)3-GECE was also performed as shown
at Fig.3. This figure displays square wave stripping
voltammograms for cadmium (Ep= -0.72 V) and
lead (Ep= -0.54 V) for increasing concentrations in
10 µg L -1 steps (Pb) and 20 µg L -1 steps (Cd) (b–e).
The well resolved peaks increase linearly with the
metal concentration. The voltammogram clearly
indicates that these metals can be measured
simultaneously following a short deposition time of
2 min. In the concentration range from 10-40 µg Pb
L -1 and 20-80 µg Cd L -1 the stripping signals
remained undistorted and the resulting calibrating
plots of this concentration range are linear
exhibiting the R values of 0.9562 and 0.9762
respectively, for lead and cadmium. Detection limits
of around 19.1 and 35.8 µg L -1 can be estimated for
lead and cadmium respectively based on the same
method [15].
A more sensitive measurement was observed for
lead at 0.5 M HCl as measuring solution. Fig.4
represent typical subtractive square-wave stripping
voltammograms (removing blanks) for increasing
concentration of lead ranging from 1 to 10 µg L -1
steps (a–h). Also the calibration plot (right) over the
studied range, is shown. This highly sensitive
response in HCl medium, as expected also from

a
b
c
the study of the pH effect is probably related to an
improved bismuth release and alloy formation in
this medium.
The stability of the Bi(NO3)3-GECEs in 10
consecutive measurements for 50 ppb cadmium in
0.1 M acetate buffer of pH 4.5 and using the same
surface was tested. It was observed that the
reproducibility of the obtained current peak was
comparable with that of the Bi-GECE [11]
developed previously, that uses bismuth solution. It
seems that the Bi precursor in the Bi(NO3)3-GECEs
surface keeps ensuring the same heavy metal
preconcentration. The relative standard deviation of
this measurement was 9.33 %.
Although the Bi(NO3)3 particles were not uniform in
size they were expected to be exposed in a
reproducible way onto the freshly obtained
Bi(NO3)3-GECE surfaces after each mechanical
polishing procedure. This was confirmed by
checking the reproducibility of the measurements
for a series of 10 different surfaces of the same
Bi(NO3)3-GECE. The relative standard deviations of
these measurements performed in the same
experimental conditions as for the stability study
was 10.69% for cadmium measurements.
A B
50 µm
Fig.1 Scanning electron microscopy images for Bi(NO3)3-
GECE before (A) and after (B) the preconcentration step
from solutions of 0.1 M acetate buffer (pH 4.5) at -1.3 V
during 120 s. All electrode surfaces have been polished
in the same way as explained in the text. The same
accelerated voltage (10 kV) and resolution (10 µm) were
used. The Bi(NO3)3 concentrations in the prepared
sensors were 0.1 (a), 0.5 (b) and 2.0 % (c) (w/w).
A
B
5µA
Fig2. Square-wave stripping voltammograms for
increasing concentration of cadmium (A) in 10 µg L -1
steps (b – h) and lead (B) in 10 µg/L steps (b – j). Also
are shown the corresponding blank voltammograms
(a) and the calibration plots (right) over the ranges 10
– 90 µg L -1 cadmium and 10 – 70 µg L -1 lead.
Solutions 0.1 M acetate buffer (pH 4.5). Square-wave
voltammetric scan with a frequency of 50 Hz, potential
step of 20 mV and amplitude of 20 mV. Deposition
potential of -1.3 V during 120 s.
10µA
Cd
Pb
d
c
b
a
e
-0.9 -0.7 -0.5
j
i
h hh
g gg
f
e
d
c
b bb
a
-1.0 -0.8 -0.6 -0.4
10µA
d
c
h
g
f
e
b
b
a
-0.7 -0.6 -0.5 -0.4 -0.3
e
I (µA)
I (µA)
14.0
10.0
40 40.0
20 20.0
00.0
16
12
Cd
10 30 50 70
Fig. 3 Determination of cadmium and lead for increasing
concentrations in 10 µg L -1 steps (Pb) and 20 µg L -1
steps (Cd); concentration ranges of 10 – 40 (Pb) and 20
– 80 (Cd) µg L -1 . Also is shown the blank (a) and the
corresponding calibration plots. Experimental conditions
as in Fig. 2.
6.0
2.0
50
30
10
8
4
0
0 20 40 60 80 100
E (V) [Cd 2+ ] (µg L -1 )
I (µA)
0 20 40 60
E (V) [Pb 2+ ] (µg L -1 )
Pb
E (V) [Pb 2+ ], [Cd 2+ ] (µg L -1 )

10µA
h
g
f
e
d
c
b
a
I (µA)
-0.7 -0.6 -0.5 -0.4 -0.3
20
15
10
Fig. 4 Square-wave stripping voltammograms for
increasing concentration of lead: (a) 1, (b) 2, (c) 3, (d) 4,
(e) 5, (f) 8, (g) 9, (h) 10 µg L -1 . Also is shown the
corresponding calibration plot (right) over the range 1–10
µg L -1 lead. The measuring solution was 0.5 M HCl.
Other experimental conditions as in Fig. 2.
CONCLUSIONS
0 2 4 6 8 10
A novel GECE that incorporates Bi(NO3)3 salt in the
sensing matrix is developed. The resulted Bi(NO3)3-
GECE is compatible with bismuth-film electrodes
for use in stripping analysis of heavy metals. The
built-in bismuth property is the distinctive feature of
this Bi(NO3)3 modified GECE which can be utilized
for the generation of bismuth adjacent to the
electrode surface.
The developed Bi(NO3)3-GECE is related with an
in-situ bismuth ions generation and film formation
without the necessity of external addition of the
bismuth in the measuring solution. The good
stability (9.33 % for cadmium measurements) of the
Bi(NO3)3-GECE is owing to the unique surface
morphology resulting in enhanced contact between
the GECE matrix and the electrochemically
reduced bismuth. Moreover, the surface of the
Bi(NO3)3-GECE could be renewed easily by simple
polishing so that the utility of the sensor is
improved.
5
0
E (V) [Pb2+ ] (µg L-1 E (V) [Pb )
2+ ] (µg L-1 )
The convenience of this built-in bismuth sensor in
voltammetric analysis will be greatly improved if
this novel composite should be used by screen
printed technology. The utilization of the Bi(NO3)3-
GECE for real heavy metal samples along with
other applications are underway in our laboratory.
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