detection of heavy metals by using a composite sensor ... - Lirmm
detection of heavy metals by using a composite sensor ... - Lirmm
detection of heavy metals by using a composite sensor ... - Lirmm
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DETECTION OF HEAVY METALS BY USING A COMPOSITE SENSOR<br />
BASED ON A BUILT-IN BISMUTH PRECURSOR<br />
M.T. Castañeda, 1, * B. Pérez, M. Pumera, M. del Valle, A.Merkoçi, S. Alegret<br />
Grup de Sensors i Bio<strong>sensor</strong>s, Departament de Química, Universitat Autònoma de Barcelona, 08193<br />
Bellaterra, Catalonia, Spain,<br />
1 On leave from: Departamento de Ciencias Básicas, Universidad Autónoma Metropolitana-Azcapotzalco, Av.<br />
San Pablo 180, Col. Reynosa Tamaulipas 022000, México, D. F., MÉXICO.<br />
* Corresponding author; email: tcb@correo.azc.uam.mx, tel: 34-93581 2118, fax: 34-93581 2379<br />
Abstract: A new graphite-epoxy <strong>composite</strong> electrode (GECE) containing Bi(NO3)3 as built-in bismuth precursor<br />
for simultaneous and individual anodic stripping analysis <strong>of</strong> <strong>heavy</strong> trace <strong>metals</strong> is reported. The developed<br />
Bi(NO3)3-GECE is compatible with bismuth film electrodes reported previously, including the <strong>composite</strong><br />
electrodes (Bi-GECE) recently reported <strong>by</strong> our group. The sensitive response, combined with the minimal<br />
toxicity <strong>of</strong> the Bi(NO3)3 allow its utilization in environmental quality monitoring as well as other applications.<br />
Keywords: bismuth nitrate, graphite-epoxy <strong>composite</strong> electrode, <strong>heavy</strong> <strong>metals</strong>.<br />
INTRODUCTION<br />
Mercury-modified electrodes coupled with stripping<br />
techniques have been recognised as the most<br />
sensitive methods for determination <strong>of</strong> <strong>heavy</strong><br />
<strong>metals</strong> [1]. However, the potential dangers<br />
associated with mercury have led to developing<br />
mercury-free <strong>sensor</strong>s. Unmodified electrodes like<br />
bare carbon, gold or iridium [2-4], graphite-epoxi<br />
<strong>composite</strong>s [5-7], recordable CD [8] or silver-plated<br />
rotograved carbon electrodes [9] have been used<br />
as an alternative to mercury based electrodes.<br />
One <strong>of</strong> the excited alternatives to mercury based<br />
electrodes is that based on bismuth [10]. Our group<br />
have configured Bi-GECE [11], based on graphiteepoxy<br />
<strong>composite</strong> electrode (GECE) without<br />
modification but bismuth film formation due to the<br />
presence <strong>of</strong> bismuth in the measuring solution. In<br />
the present work we present a novel configuration,<br />
Bi(NO3)3-GECE that represents GECE modified<br />
internally with bismuth nitrate salt which serves as<br />
built-in bismuth precursor for bismuth film<br />
formation. This represents an integrated<br />
configuration <strong>of</strong> bismuth based GECEs for stripping<br />
analysis. The low toxicity <strong>of</strong> bismuth makes it an<br />
alternative material to mercury in terms <strong>of</strong> trace<br />
metal determination.<br />
EXPERIMENTAL<br />
The lead and cadmium stock solutions were<br />
prepared <strong>by</strong> dissolving the corresponding nitrates<br />
in water obtained from an ion-exchange system<br />
Milli-Q (Millipore). Acetate buffer (0.1 M, pH 4.5) or<br />
HCl 0.5 M were used as supporting electrolyte. The<br />
Bi(NO3)3GECE were prepared <strong>using</strong> graphite<br />
powder with a particle size <strong>of</strong> 50 µm (BDH, UK),<br />
Epotek H77 (epoxy resin), hardener (both from<br />
Epoxy Technology, USA) and Bi(NO3)3 (Aldrich).<br />
Graphite powder and Bi(NO3)3 salt were first mixed<br />
together. The obtained dried mixture was mixed<br />
well with epoxy resin (mixed with hardener) in a<br />
ratio <strong>of</strong> 1:4 (w/w) as described in a previous work<br />
[12,13]. The percentage <strong>of</strong> Bi(NO3)3 in the prepared<br />
paste was varied being 0.1, 0.5, and 2.0 % (w/w).<br />
The resulting Bi(NO3)3 containing graphite-epoxy<br />
paste was placed into a PVC cylindrical sleeve<br />
body (6 mm i. d.), which has an inner electrical<br />
copper contact, to a depth <strong>of</strong> 3 mm. The conducting<br />
<strong>composite</strong> material glued to the copper contact was<br />
cured at 40 ºC during one week. Before each use,<br />
the surface <strong>of</strong> the electrode was wet with doubly<br />
distilled water and then thoroughly smoothed, first<br />
with abrasive paper and then with alumina paper<br />
(polishing strips 301044-001, Orion).<br />
A platinum auxiliary electrode (model 52-67 1,<br />
Crison, Spain) and double junction Ag/AgCl<br />
reference electrode (Orion 900200) with 0.1 M KCl<br />
as external reference solution and the Bi(NO3)3-<br />
GECE as working electrode were used. The square<br />
wave anodic stripping voltammetry (SWASV)<br />
experiments were performed <strong>using</strong> an Autolab<br />
PGSTAT 20 System (Eco-chemie, The<br />
Netherlands). A Hitachi S-570, Japan Scanning<br />
Electron Microscope (SEM) was used to observe<br />
the surface <strong>of</strong> the working electrodes.<br />
SWASV measurements were carried out in the<br />
presence <strong>of</strong> dissolved oxygen. The three<br />
electrodes were immersed into the electrochemical<br />
cell containing 25 mL 0.1 mL 0.1 M acetate buffer<br />
(pH 4.5). The deposition potential <strong>of</strong> -1.3 V was<br />
applied to Bi(NO3)3-GECE while the solution was<br />
stirred. Following 120 s deposition step, the stirring<br />
was stopped and after 15 s equilibration, the<br />
voltammogram was recorded <strong>by</strong> applying a squarewave<br />
potential scan between -1.3 and -0.3 V with a<br />
frequency <strong>of</strong> 50 Hz, amplitude <strong>of</strong> 20 mV and<br />
potential step <strong>of</strong> 20 mV.<br />
Aliquots <strong>of</strong> the target metal standard solution were<br />
introduced after recording the background<br />
voltammograms. A 60 s conditioning step at +0.6 V<br />
(with solution stirring) was used to remove the<br />
remaining reduced target <strong>metals</strong> and bismuth, prior<br />
to the next cycle. The electrodes were washed<br />
thoroughly with deionized water between each test.
Measurements in phosphate buffer for the study <strong>of</strong><br />
pH effect as well as in HCl medium were also<br />
performed in the same experimental conditions as<br />
described above.<br />
RESULTS AND DISCUSSION<br />
The surface morphologies <strong>of</strong> Bi(NO3)3-GECEs<br />
(containing different quantities <strong>of</strong> Bi(NO3)3 salt)<br />
before and after the preconcentration step<br />
(electrolysis at -1.3 V during 120 s) were observed<br />
<strong>by</strong> Scanning Electron Microscopy (SEM).<br />
As can be seen, the surfaces <strong>of</strong> Bi(NO3)3-GECE,<br />
with different concentration <strong>of</strong> Bi(NO3)3, before<br />
preconcentration step (Fig. 1 A) appears to have<br />
clusters <strong>of</strong> conducting material gathered in random<br />
areas. This is due to the graphite particles<br />
randomly distributed and randomly oriented in the<br />
epoxy resin [14]. Microcrystalline Bi(NO3)3 particles<br />
should be also distributed randomly but due to the<br />
very low percentage (0.1 to 2.0 %, w/w) were not<br />
visible. The darker coverage <strong>of</strong> the same Bi(NO3)3-<br />
GECEs after preconcentration step (Fig. 1 B)<br />
compared to Bi(NO3)3-GECE before<br />
preconcentration (Fig. 1 A) is clearly visible. This is<br />
due to the bismuth film formation coming from the<br />
Bi(NO3)3 salt inside the <strong>sensor</strong> matrix.<br />
The quantity <strong>of</strong> Bi(NO3)3 didn’t have any visible<br />
effect on the <strong>sensor</strong> surface. The images 1 B (a-c)<br />
have similar darkness. Seems that for the Bi(NO3)3<br />
quantities used the bismuth film has the same<br />
configuration.<br />
For all the Bi(NO3)3-GECEs after the<br />
preconcentration step there can be also seen<br />
dimensional fibril-like networks onto their surfaces<br />
which is in correlation with the early report <strong>of</strong> Bi-film<br />
at carbon surface [10]. The black and thick<br />
appearances <strong>of</strong> bismuth deposit can be attributed<br />
to carbon substrate that has positive effect on the<br />
nucleation and growth <strong>of</strong> the bismuth film. The<br />
same deposition <strong>of</strong> bismuth was clearly observed<br />
for GECE used in connection with bismuth in<br />
measuring solution.<br />
The characteristics <strong>of</strong> the electrodes must be very<br />
dependent on the amounts <strong>of</strong> Bi(NO3)3 used for the<br />
Bi(NO3)3-GECEs preparation.<br />
The effect <strong>of</strong> Bi(NO3)3 loadings (0.1-2.0 %, w/w) in<br />
the SWASV <strong>of</strong> the resulting Bi(NO3)3-GECEs were<br />
studied for a 70 ppb solution <strong>of</strong> Pb 2+ at 0.1 M<br />
acetate buffer pH 4.5. The increase <strong>of</strong> Bi(NO3)3<br />
content in the <strong>composite</strong> electrode increase the<br />
bismuth ion release during the contact with the<br />
measuring solution, and consequently the bismuth<br />
film formation capability. On the other hand, the<br />
higher Bi(NO3)3 content may reduce the<br />
conductivity <strong>of</strong> the Bi(NO3)3–GECE. A maxim<br />
response was observed for Bi(NO3)3-GECE<br />
containing 0.1 % Bi(NO3)3.<br />
The stripping performance <strong>of</strong> Bi(NO3)3-GECE was<br />
tested for lead and cadmium and the resulting<br />
voltammograms are showed in Fig.2. The Figure<br />
demonstrates the square wave stripping<br />
voltammograms for increasing concentration <strong>of</strong><br />
cadmium (A) in 10 µg L -1 steps (b–j) and lead (B) in<br />
10 µg L -1 steps (b–h). Also shown are the<br />
corresponding blank voltammograms (a) and the<br />
calibration plots (right) over the ranges 10 – 90 µg<br />
L -1 cadmium and 10 – 70 µg L -1 lead. The Bi(NO3)3-<br />
GECE displays well-defined and single peaks for<br />
cadmium (Ep = -0.76 V) and lead (Ep = -0.54 V).<br />
Detection limits <strong>of</strong> 7.23 and 11.81 µg L -1 can be<br />
estimated for cadmium and lead respectively based<br />
on the upper limit approach (ULA) [15]. Also in the<br />
concentration ranges mentioned above, the<br />
calibration plots (right) were linear exhibiting the R<br />
values <strong>of</strong> 0.9968 and 0.9953 for cadmium and lead<br />
respectively.<br />
As in the case <strong>of</strong> Bi-GECE the bismuth film<br />
formation onto Bi(NO3)3-GECE is shown to be a<br />
homogenous and uniform one due to the novel<br />
supporting material. The rich microstructure <strong>of</strong><br />
Bi(NO3)3-GECE, composed <strong>of</strong> a mixture <strong>of</strong> carbon<br />
microparticles forming internal microarrays might<br />
have a pr<strong>of</strong>ound effect upon the bismuth film<br />
structural features. The obtained peak widths <strong>of</strong> 20<br />
mV for lead and cadmium were similar to other<br />
bismuth film electrodes reported previously.<br />
The simultaneous measuring <strong>of</strong> lead and cadmium<br />
with Bi(NO3)3-GECE was also performed as shown<br />
at Fig.3. This figure displays square wave stripping<br />
voltammograms for cadmium (Ep= -0.72 V) and<br />
lead (Ep= -0.54 V) for increasing concentrations in<br />
10 µg L -1 steps (Pb) and 20 µg L -1 steps (Cd) (b–e).<br />
The well resolved peaks increase linearly with the<br />
metal concentration. The voltammogram clearly<br />
indicates that these <strong>metals</strong> can be measured<br />
simultaneously following a short deposition time <strong>of</strong><br />
2 min. In the concentration range from 10-40 µg Pb<br />
L -1 and 20-80 µg Cd L -1 the stripping signals<br />
remained undistorted and the resulting calibrating<br />
plots <strong>of</strong> this concentration range are linear<br />
exhibiting the R values <strong>of</strong> 0.9562 and 0.9762<br />
respectively, for lead and cadmium. Detection limits<br />
<strong>of</strong> around 19.1 and 35.8 µg L -1 can be estimated for<br />
lead and cadmium respectively based on the same<br />
method [15].<br />
A more sensitive measurement was observed for<br />
lead at 0.5 M HCl as measuring solution. Fig.4<br />
represent typical subtractive square-wave stripping<br />
voltammograms (removing blanks) for increasing<br />
concentration <strong>of</strong> lead ranging from 1 to 10 µg L -1<br />
steps (a–h). Also the calibration plot (right) over the<br />
studied range, is shown. This highly sensitive<br />
response in HCl medium, as expected also from
a<br />
b<br />
c<br />
the study <strong>of</strong> the pH effect is probably related to an<br />
improved bismuth release and alloy formation in<br />
this medium.<br />
The stability <strong>of</strong> the Bi(NO3)3-GECEs in 10<br />
consecutive measurements for 50 ppb cadmium in<br />
0.1 M acetate buffer <strong>of</strong> pH 4.5 and <strong>using</strong> the same<br />
surface was tested. It was observed that the<br />
reproducibility <strong>of</strong> the obtained current peak was<br />
comparable with that <strong>of</strong> the Bi-GECE [11]<br />
developed previously, that uses bismuth solution. It<br />
seems that the Bi precursor in the Bi(NO3)3-GECEs<br />
surface keeps ensuring the same <strong>heavy</strong> metal<br />
preconcentration. The relative standard deviation <strong>of</strong><br />
this measurement was 9.33 %.<br />
Although the Bi(NO3)3 particles were not uniform in<br />
size they were expected to be exposed in a<br />
reproducible way onto the freshly obtained<br />
Bi(NO3)3-GECE surfaces after each mechanical<br />
polishing procedure. This was confirmed <strong>by</strong><br />
checking the reproducibility <strong>of</strong> the measurements<br />
for a series <strong>of</strong> 10 different surfaces <strong>of</strong> the same<br />
Bi(NO3)3-GECE. The relative standard deviations <strong>of</strong><br />
these measurements performed in the same<br />
experimental conditions as for the stability study<br />
was 10.69% for cadmium measurements.<br />
A B<br />
50 µm<br />
Fig.1 Scanning electron microscopy images for Bi(NO3)3-<br />
GECE before (A) and after (B) the preconcentration step<br />
from solutions <strong>of</strong> 0.1 M acetate buffer (pH 4.5) at -1.3 V<br />
during 120 s. All electrode surfaces have been polished<br />
in the same way as explained in the text. The same<br />
accelerated voltage (10 kV) and resolution (10 µm) were<br />
used. The Bi(NO3)3 concentrations in the prepared<br />
<strong>sensor</strong>s were 0.1 (a), 0.5 (b) and 2.0 % (c) (w/w).<br />
A<br />
B<br />
5µA<br />
Fig2. Square-wave stripping voltammograms for<br />
increasing concentration <strong>of</strong> cadmium (A) in 10 µg L -1<br />
steps (b – h) and lead (B) in 10 µg/L steps (b – j). Also<br />
are shown the corresponding blank voltammograms<br />
(a) and the calibration plots (right) over the ranges 10<br />
– 90 µg L -1 cadmium and 10 – 70 µg L -1 lead.<br />
Solutions 0.1 M acetate buffer (pH 4.5). Square-wave<br />
voltammetric scan with a frequency <strong>of</strong> 50 Hz, potential<br />
step <strong>of</strong> 20 mV and amplitude <strong>of</strong> 20 mV. Deposition<br />
potential <strong>of</strong> -1.3 V during 120 s.<br />
10µA<br />
Cd<br />
Pb<br />
d<br />
c<br />
b<br />
a<br />
e<br />
-0.9 -0.7 -0.5<br />
j<br />
i<br />
h hh<br />
g gg<br />
f<br />
e<br />
d<br />
c<br />
b bb<br />
a<br />
-1.0 -0.8 -0.6 -0.4<br />
10µA<br />
d<br />
c<br />
h<br />
g<br />
f<br />
e<br />
b<br />
b<br />
a<br />
-0.7 -0.6 -0.5 -0.4 -0.3<br />
e<br />
I (µA)<br />
I (µA)<br />
14.0<br />
10.0<br />
40 40.0<br />
20 20.0<br />
00.0<br />
16<br />
12<br />
Cd<br />
10 30 50 70<br />
Fig. 3 Determination <strong>of</strong> cadmium and lead for increasing<br />
concentrations in 10 µg L -1 steps (Pb) and 20 µg L -1<br />
steps (Cd); concentration ranges <strong>of</strong> 10 – 40 (Pb) and 20<br />
– 80 (Cd) µg L -1 . Also is shown the blank (a) and the<br />
corresponding calibration plots. Experimental conditions<br />
as in Fig. 2.<br />
6.0<br />
2.0<br />
50<br />
30<br />
10<br />
8<br />
4<br />
0<br />
0 20 40 60 80 100<br />
E (V) [Cd 2+ ] (µg L -1 )<br />
I (µA)<br />
0 20 40 60<br />
E (V) [Pb 2+ ] (µg L -1 )<br />
Pb<br />
E (V) [Pb 2+ ], [Cd 2+ ] (µg L -1 )
10µA<br />
h<br />
g<br />
f<br />
e<br />
d<br />
c<br />
b<br />
a<br />
I (µA)<br />
-0.7 -0.6 -0.5 -0.4 -0.3<br />
20<br />
15<br />
10<br />
Fig. 4 Square-wave stripping voltammograms for<br />
increasing concentration <strong>of</strong> lead: (a) 1, (b) 2, (c) 3, (d) 4,<br />
(e) 5, (f) 8, (g) 9, (h) 10 µg L -1 . Also is shown the<br />
corresponding calibration plot (right) over the range 1–10<br />
µg L -1 lead. The measuring solution was 0.5 M HCl.<br />
Other experimental conditions as in Fig. 2.<br />
CONCLUSIONS<br />
0 2 4 6 8 10<br />
A novel GECE that incorporates Bi(NO3)3 salt in the<br />
sensing matrix is developed. The resulted Bi(NO3)3-<br />
GECE is compatible with bismuth-film electrodes<br />
for use in stripping analysis <strong>of</strong> <strong>heavy</strong> <strong>metals</strong>. The<br />
built-in bismuth property is the distinctive feature <strong>of</strong><br />
this Bi(NO3)3 modified GECE which can be utilized<br />
for the generation <strong>of</strong> bismuth adjacent to the<br />
electrode surface.<br />
The developed Bi(NO3)3-GECE is related with an<br />
in-situ bismuth ions generation and film formation<br />
without the necessity <strong>of</strong> external addition <strong>of</strong> the<br />
bismuth in the measuring solution. The good<br />
stability (9.33 % for cadmium measurements) <strong>of</strong> the<br />
Bi(NO3)3-GECE is owing to the unique surface<br />
morphology resulting in enhanced contact between<br />
the GECE matrix and the electrochemically<br />
reduced bismuth. Moreover, the surface <strong>of</strong> the<br />
Bi(NO3)3-GECE could be renewed easily <strong>by</strong> simple<br />
polishing so that the utility <strong>of</strong> the <strong>sensor</strong> is<br />
improved.<br />
5<br />
0<br />
E (V) [Pb2+ ] (µg L-1 E (V) [Pb )<br />
2+ ] (µg L-1 )<br />
The convenience <strong>of</strong> this built-in bismuth <strong>sensor</strong> in<br />
voltammetric analysis will be greatly improved if<br />
this novel <strong>composite</strong> should be used <strong>by</strong> screen<br />
printed technology. The utilization <strong>of</strong> the Bi(NO3)3-<br />
GECE for real <strong>heavy</strong> metal samples along with<br />
other applications are underway in our laboratory.<br />
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