Iodide modified electrodes for the electrochemical detection of environmentally hazardous materials

Article abstract of DOI:10.1149/1.2917900

Electrochemical deposition and chemisorption studies of iodide film have been done on the glassy carbon electrode (GCE) and polycrystalline gold electrode (poly GE). The iodide film preparation under different conditions and its stability were examined. Further, the redox activity of the iodide modified film on GCE and poly GE was examined in different pH solutions. Here, the electrochemically deposited iodide film on GCE showed good stability and was electrochemically active at pH 1.5 and 4 solutions, while the chemisorbed iodide film on poly GE was found active and stable at pH 9 and 13 solutions. The iodide modified GCEs exhibits a good electrocatalytic activity for the oxidation of As (III) and the reduction of Cr (VI), respectively. A detailed study for As (III) oxidation has been done by using the rotating ring-disk electrode technique. Meanwhile, chemisorbed iodide film on poly GE shows electrocatalytic reduction reactions of IO 3- by using different techniques like cyclic voltammetry and differential pulse voltammetry. To examine the practical analytical utility of the proposed electrode, the real sample analyses for arsenic and chromate detection have been examined and the results were found satisfactory.

Full text of DOI:10.1149/1.2917900

Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
F157
0
013-4651/2008/155͑7͒/F157/8/$23.00 © The Electrochemical Society
Iodide Modified Electrodes for the Electrochemical Detection of
Environmentally Hazardous Materials
,
z
Buo-Wei Su, Soundappan Thiagarajan,* and Shen-Ming Chen**
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology,
Taipei 106, Taiwan
Electrochemical deposition and chemisorption studies of iodide film have been done on the glassy carbon electrode ͑GCE͒ and
polycrystalline gold electrode ͑poly GE͒. The iodide film preparation under different conditions and its stability were examined.
Further, the redox activity of the iodide modified film on GCE and poly GE was examined in different pH solutions. Here, the
electrochemically deposited iodide film on GCE showed good stability and was electrochemically active at pH 1.5 and 4 solutions,
while the chemisorbed iodide film on poly GE was found active and stable at pH 9 and 13 solutions. The iodide modified GCEs
exhibits a good electrocatalytic activity for the oxidation of As ͑III͒ and the reduction of Cr ͑VI͒, respectively. A detailed study for
As ͑III͒ oxidation has been done by using the rotating ring-disk electrode technique. Meanwhile, chemisorbed iodide film on poly
−
3
GE shows electrocatalytic reduction reactions of IO by using different techniques like cyclic voltammetry and differential pulse
voltammetry. To examine the practical analytical utility of the proposed electrode, the real sample analyses for arsenic and
chromate detection have been examined and the results were found satisfactory.
©
2008 The Electrochemical Society. ͓DOI: 10.1149/1.2917900͔ All rights reserved.
Manuscript submitted November 12, 2007; revised manuscript received March 17, 2008. Available electronically May 14, 2008.
The development of a good analytical methodology for the de-
tection of environmentally hazardous materials like chromium and
arsenic is vital in analytical science. Generally, arsenic exists in
different chemical forms in nature. Many arsenic compounds are
known to be toxic. Particularly in ground water, they are found
almost exclusively as arsenite and arsenate. Millions of people
throughout the world use drinking water that contains both arsenite
and arsenate in carcinogenic concentrations. Such toxic pollutants
have been associated with many health problems such as skin le-
͑hexavalent chromium oxygenic compounds͒ in the industrial sector
has led to serious concern about workers’ exposure to Cr ͑VI͒,
which has been classified by the U.S. EPA as a human carcinogen in
1
0,11
Group A.
In the human being, the molecular debris associated
with the process of Cr ͑VI͒ to Cr ͑III͒ reduction induces the critical
changes in DNA. Evidence supporting that idea came from in vitro
studies showing that chromate alone does not damage DNA in the
1
2
absence of reducing agents. Based on the above discussion, there
is a need to develop a sensor for the detection of chromate. Several
methodologies including spectral and chromatographic techniques
have been developed for the detection of Cr ͑VI͒. In electroanalyti-
1
sions, keratosis ͑skin hardening͒, lung cancer, and bladder cancer.
Most people in the developing countries are still at risk for cancer
due to chronic arsenic poisoning. Their direct exposure to humans
and animals and the side effects to the ecosystem will persist as an
international problem. According to the U.S. Environmental Protec-
tion Agency ͑EPA͒ and World Health Organization arsenic guide-
lines, the maximum contaminant level of arsenic in a community
water system is 10 ␮g/L. The electrochemical oxidation of arsenic
at various metal electrodes like Pt and Au is of great interest because
As ͑III͒ is an important compound in waste water analysis. There-
fore, it is important to develop a sensor to detect and monitor the
arsenic level in the ecosystem. There are few electrochemical meth-
1
3-16
cal methods, carbon paste electrodes,
titrations,
potentiometric
1
7,18
19
and chitin modified electrodes are some examples
which have been reported already. All these methods demonstrate
the advantages of the direct modification of electrode material which
was capable of differentiating between Cr ͑VI͒ and Cr ͑III͒, which
enabled their usage for analysis in selected samples and have their
specific disadvantages.
In most of the countries, the table salt is iodized with potassium
iodate or iodine. The iodized salt is accepted as the way of choice
and the most successful strategy for the prevention of iodide defi-
ciency disorders. An excess of iodine or iodate can produce goiter
2-4
ods reported for the arsenic detection. For example, in metal elec-
trodes the gold and platinum or modified glassy carbon ͑GC͒ and
pyrolytic graphite with such metal films have been used for the
detection of arsenic by using anodic stripping potentiometry and
voltammetry. However, there are still problems that make routine
analysis inconvenient, such as limited sensitivity, poor precision,
low electron-transfer reaction, high overvoltages, and low stability
2
0,21
and hypothyroidism as well as hyperthyroidism.
Also, the iodate
−
͑IO ͒ is reported as a toxic anion and it may form during the ozo-
3
2
2
nization process. Several techniques have been applied for the
determination of iodate, including chromatography coupled with
spectrophotometry, mass spectrometry, chemiluminescence, spec-
2
3-26
trofluorometry, and amperometry.
In modified electrodes, ruthe-
nium oxide/hexacyanoferrate/silicomolybdate hybrid films, poly-
5
27
29
for the electrode surface between each measurement.
2
8
Chromium is an important target analyte in biological and envi-
ronmental systems. In inorganic oxides, chromium occurs with sev-
eral coordination numbers, different oxidation states, and different
merized new fuchsin film, and polymerized neutral red film were
reported for the IO −3 reduction. Although these methods were of
interest for iodate detection, they need several reaction steps involv-
ing the number of reagents and time-consuming processes. Hence,
there is a need for simple and easier films for the electrode modifi-
cation process to detect the iodate.
The environmentally friendly and technologically suitable elec-
trode, its simplification, and fabrication method for various purposes
have become imperative in contemporary research in analytical
chemistry. For example, in halogens, the iodine has added an inno-
vation in the electrode modification technology, as it is chemically
stable and economically low cost, environmentally green, and tech-
nologically perfect to fix onto electrodes. The chemisorption species
derived from the iodide ion at typical solid–metal electrodes pro-
vides an interesting case study because the strong surface activity of
this ligand brings about profound changes in the redox chemistry of
the iodine/iodide couple in the surface-bound state. Further, this sys-
tem is important because: ͑i͒ the iodide ion can be used as a model
6,7
molecular structures. The different toxicities of Cr ͑III͒ and Cr
8
͑
VI͒ are well known. In practice, Cr ͑IV͒ and Cr ͑V͒ ions exist, but
Cr ͑III͒ and Cr ͑VI͒ are the most common oxidation states in aque-
9
ous chemistry. Further, chromium ͑III͒ is an essential trace element
in the metabolism of lipids and proteins, and vital for the mainte-
nance of a normal glucose tolerance factor. In particular, chromium
͑
VI͒ was more biologically mobile than Cr ͑III͒ and known to be
present in certain water systems due to industrial activities. Also, Cr
VI͒ is reported to be the second most inorganic contaminant in
groundwater at hazardous waste sites. Further, the use of chromates
͑
*
Electrochemical Society Student Member.
*
* Electrochemical Society Active Member.
z
E-mail: smchen78@ms15.hinet.net
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F158
Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
for a strong surface active electrolyte, and ͑ii͒ the kinetics of redox
processes of unadsorbed species can be altered when the working-
3
0
electrode surface is pretreated with a full monolayer of iodine. The
−
structure of I layers has been investigated by many techniques such
3
1,32
as electrochemical quartz crystal microbalance,
scanning tunnel-
atomic force microscopy, and low X-ray pho-
33-36
37
ing microscopy,
toelectron spectroscopy.
become remarkably viewed at different metallic electrodes,
pecially in the synthesis of nanoparticles rich in expected facets.
The underpotential deposition of metallic adlayers of a desired
38,39
−
The I adlayer modified electrodes have
3
0,38
es-
4
0
4
1,42
structure,
the formation of highly ordered arrays of organic
the protection of highly sensitive surfaces of metal
4
3,44
molecules,
single crystals,
dation of H O using an iodine modified electrode
4
5,46
cathodic detection, and the electrochemical oxi-
4
7,48
have been
2
2
reported.
In the present work, we report the iodide layers electrodeposition
and chemisorption process on glassy carbon electrode ͑GCE͒ and
polycrystalline gold electrode ͑poly GE͒ in the presence of KI solu-
tion. The electrochemical properties of the iodide films on two dif-
ferent electrodes have been examined in different pH solutions. The
two proposed types of iodide modified electrodes show an attractive
performance toward the electrochemical oxidation of As ͑III͒ and
the reduction of Cr ͑VI͒ and iodate by using the cyclic voltammetry
Figure 1. CVs of a GCE modified with an iodide film synthesized from
H2SO4 solution ͑at pH 1.5͒ containing 1 ϫ 10 M KI. Switching potential
between −0.75 and 1.2 V.
−
3
͑
CV͒, differential pulse voltammetry ͑DPV͒, and rotating ring-disk
electrode ͑RRDE͒ techniques. To validate the practical usage of the
proposed electrode, the real sample analyses for Cr ͑VI͒ and As ͑III͒
in real drinking water sources such as mineral water and tap water
have been examined.
Results and Discussion
Experimental
Characterization of iodide film on GCE and poly GE.— Figure
1
shows the repetitive CV response of GCE in pH 1.5 H SO solu-
Reagents and apparatus.— Sodium meta arsenite was purchased
2 4
−
3
tions containing 1 ϫ 10 M KI. From Fig. 1, the cathodic peaks at
.42 and 0.90 V and the anodic peaks at 1.05 and 0.50 V were
from Sigma-Aldrich, USA. KI and KIO were obtained from Wako
3
0
Chemicals ͑USA͒. Other reagents were of analytical grade. All the
solutions were prepared with double-distilled water. All the experi-
mental results were obtained at room temperature. The buffer solu-
tions for electrocatalysis were pH 1.5 H SO and KOH ͑pH 13͒
observed for the iodide film growth process on GCE. In subsequent
cycles, all the peaks were found growing. Further, the significant
growth of these two obvious redox couples clearly suggests the suc-
cessive formation of the iodide film on GCE. To examine the elec-
trochemical properties of the iodide film modified GCE, it was
2
4
solutions containing 1 ϫ 10−3 M KI. The electrochemical measure-
ments such as CV and DPV were carried out using CHI 410a and
CHI 900 potentiostats ͑CH Instruments, Austin, TX͒. A conventional
three-electrode system was used throughout the experiments. The
Bioanalytical Systems, Inc. ͑BAS, USA͒ GCEs ͑␸ = 0.3 cm diam-
eter͒ GE ͑␸ = 0.16 cm diameter͒ were in the form of disks sealed in
a Teflon jacket having an exposed geometric surface area of 0.07
−
3
transferred to pH 1.5 H SO solutions containing 1 ϫ 10 M KI
for different scan rate studies. Figure 2A represents the CVs of
iodide film at different scan rates from 50–300 mV s . Here, the
anodic and cathodic peak currents of the iodide redox system lin-
early increased with the increasing scan rates up to 300 mV s
͑Fig. 2A͒. Further, Fig. 2B and C shows the cathodic and anodic
peak currents vs the scan rate plot for the iodide film modified GCE.
These results show that the cathodic and anodic peak currents of the
iodide film were linearly increasing, and that the charge transfer
within the iodide was fast compared with the time scale of the po-
tential scan, respectively.
2
4
−
1
−
1
2
and 0.02 cm , respectively. The working electrode was a bare or
iodide film modified GCE or GE, the auxiliary electrode was a plati-
num wire, and an Ag/AgCl electrode was used as a reference. All the
potentials mentioned in this paper were referred to this reference
electrode. The RRDE experiments were performed by a Pine Instru-
ments Co. ͑USA͒ electrode in conjunction with a CHI-750 poten-
tiostat that was connected to a model AFMSRX analytical rotator.
The RRDE containing a GC disk electrode and a GC ring electrode
was used for hydrodynamic studies.
To modify the poly GE with chemisorptions of iodide film, it was
−
3
immersed in KOH solution ͑pH 13͒ containing 1 ϫ 10 M KI.
Then, the chemisorbed iodide film modified GE was examined for
−
1
different scan rate studies ͑50–300 mV s ͒ in the same solution.
Figure 3 shows the different scan rate studies of the iodide film
modified GE. Figure 3 shows that the linear increase in the peak
currents of the redox couples at various scan rates confirms the
chemisorbed iodide layer on poly GE. The inset of Fig. 3 shows the
anodic and cathodic peak currents vs the scan rate plot of the iodide
redox couple at −0.4 V. In particular, the redox couples of iodide
film were obtained at around −0.4 and −0.8 V, showing the prefer-
ential chemisorptions of iodide onto the poly GE. Also, the CV
obtained here for the chemisorptions of the iodide adlayer is similar
Preparation of the iodide film on GCE and poly GE.— Prior to
modification, the bare GCE and GE were polished by using a BAS
polishing kit with aqueous slurries of finer alumina powder
͑
0.05 ␮m͒, rinsed, and ultrasonicated in double-distilled deionized
water. All the solutions were deaerated by passing high-purity nitro-
gen before the electrochemical experiments. The iodide film was
electrochemically deposited on GCE from pH 1.5 H SO solution
2
4
containing 1 ϫ 10−3 M KI by cycling the potential between −0.75
−
1
and 1.2 V vs ͑Ag/AgCl͒ at the scan rate of 100 mV s for 30
cycles. Further, the GE was electrochemically pretreated by repeat-
ing the potential scan in the range of −0.2 to 1.5 V vs ͑Ag/AgCl͒ at
3
9
to that obtained by Stickney et al. Further, the sharp peak and
broad peak obtained at −0.4 and −0.8 V proves the cathodic
adsorption/desorption of the iodide layer on the poly GE, respec-
tively. In particular, the sharp peak at −0.4 V is attributed to the
adsorption process, and the broad cathodic peak obtained was due to
more desorption of iodide from the electrode surface and change in
−
1
1
00 mV s for 60 cycles in 0.05 M H SO solution to attain the
2 4
CV characteristic of a clean GE. The electrochemically pretreated
GE is called poly GE, which was immersed in 0.1 M KOH solution
containing 1 ϫ 10−3 M KI to accomplish the chemisorptions of io-
dide adlayers. Finally, the iodide modified poly GE was fabricated.
4
8
the structure. Besides, the chemisorbed iodide adlayers are more
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Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
F159
Figure 2. ͑A͒ CVs of iodide layer on
GCE in the presence of 1 ϫ 10− M KI in
pH 1.5 H SO solution with various scan
3
2
4
−
1
rates: 50, 100, 150, 200, and 300 mV s
.
Inset ͑B and C͒ shows the plot of the ca-
thodic and anodic peaks current vs the
scan rate.
4
7
electronegative and highly polarizable than Au, and the iodide ad-
layers induce a partial positive charge on the poly GE surface and
they become partially negative charged. Finally, the above results
clearly illustrate the successful modification process of iodide layers
by the electrodeposition and chemisorption processes on the GCE
and poly GE surface.
Electrochemical activities of iodide film in different pH
solutions.— The electrochemical responses of the iodide film on
GCE in different pH solutions have been examined. Figure 4 dem-
onstrates the iodide film ͑on GCE͒ in different pH buffer solutions
͑
1.5, 4, 7, and 9͒ in the presence of 1 ϫ 10− M KI. In pH 1.5
3
electrolyte solution, the film exhibited two significant and clear re-
dox couples at 0.4 and 0.9 V, which was compatible with the origi-
nal redox activities of the iodide film. Further, in pH 4 buffer solu-
tions, it shows the two redox couples whose peak separation was
very low when compared with the same film’s property in pH 1.5
solutions. Here, the redox couple at 0.4 V appeared as the obvious
one. Next, in pH 7 buffer solutions, the two redox couples of the
iodide film appeared as quasi-reversible peaks and very close to
each other. Finally, in pH 9 buffer solution, the iodide film exhibits
one irreversible peak. From the above results, it can be seen that, as
the solution pH increases, there is a change in the redox peak po-
tentials and at a higher pH it fails to produce the redox couples of
the iodide film. Generally the redox couples of the iodide film were
not pH dependent. However, in the different species such a pH
change in the film will exhibit different properties. From these pH
studies, it was fournd that pH 1.5 and 4 are the optimized pH con-
ditions for the iodide film modified GCE.
Figure 3. CVs of iodide layer on poly GE in the presence of 1 ϫ 10−3 M KI
in pH 13 KOH solution with various scan rates: ͑a͒ 50, ͑b͒ 100, ͑c͒ 150, ͑d͒
1
2
00, and ͑e͒ 300 mV s− . Inset shows the plot of the cathodic and anodic
At the same time, the cathodic chemisorptions of iodide film at
poly GE also reveals different properties in various pH buffer solu-
peaks current vs the scan rate.
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F160
Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
Figure 5. CVs of iodide film on poly GE in the presence of 1 ϫ 10−3 M KI
in various buffer solutions.
Figure 4. CVs of iodide film on GCE in the presence of 1 ϫ 10−3 M KI in
various buffer solutions.
sorbed iodide film exhibits two redox couples at 0.4 and −0.8 V
potentials, respectively. Here, the redox couple at −0.4 V was sharp
and the other one at −0.8 V appeared in the wide range. From these
results, the chemisorbed iodide on poly GE shows its own electro-
chemical activities in different pH solutions. However, during the
pH change, there is no potential shift for the redox couple at
tions. The electrochemical behavior of chemisorbed iodide film on
poly GE was examined in various pH buffer solutions ͑4, 7, 9, and
1
3͒ and the results are shown in Fig. 5. In pH 4 buffer solution, the
chemisorbed iodide film exhibits only one obvious redox couple at
0.4 V. Next, in pH 7 buffer solution, it shows one perfect redox
−
0.4 V. But, there is an interesting broad peak found at −0.8 V in
−
alkaline pH conditions. Finally, we have ascertained the electro-
couple at the same potential like in the pH 4 solution and addition-
ally one broad redox peak at −0.7 V which was not apparent. Fur-
ther, in pH 9 buffer solutions, the iodide film shows two redox
couples at −0.4 and −0.8 V, respectively. In that, the redox couple
at −0.4 V was the obvious one and another couple at −0.8 V ex-
hibits as a broader one. Finally, in pH 13 buffer solutions, the chemi-
chemical activities of the iodide films ͑on GCE, poly GE͒ by exam-
48
ining them in various pH solutions.
Electrocatalytic oxidation of As (III) on iodide modified
GCE.— The fine performance of iodide film modified GCE toward
the detection of As͑III͒ makes it attractive for the fabrication of
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Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
F161
͑
III͔͒, it can be seen that unmodified bare GCE fails to oxidize the
As ͑III͒. Furthermore, the inset in Fig. 6A shows the anodic peak
currents vs concentration plot for As ͑III͒ oxidation. From the cali-
bration plot, the linear regression equation for As ͑III͒ oxidation was
found as y = 0.043x + 3.679, with a correlation coefficient of
0
.9730. Finally, the above results validate the electrocatalytic oxida-
tion of As ͑III͒ on iodide modified GCE.
2
7
Ϫ
Electrocatalytic reduction of Cr O
on iodide film modified
2
2
7
−
GCE.— We have examined the electrocatalytic activity of Cr O
2
reduction on iodide film modified GCE. Figure 6B shows the CVs of
electrocatalytic reduction of Cr ͑VI͒ to Cr ͑III͒ on iodide film modi-
−
3
fied GCE in pH 1.5 H SO containing 1 ϫ 10 M of KI. Here, the
2
4
reaction process on iodide film modified GCE might be rationalized
as the one-step reduction of Cr ͑VI͒ to Cr ͑III͒. Further, the electro-
2
7
−
catalytic reduction peaks of Cr O were obtained at 0.42 V. The
2
iodide film on GCE shows a linear current increase response for the
2
7
−
−5
Cr O
2
reduction in the concentration range of 2 ϫ 10 –1
−
4
2−
ϫ 10 M. When Cr O was added to the solution, the charge
2
7
under the reduction peak was found to increase with a decrease in
the oxidation peak currents. This behavior is typical of that expected
2
−
for a reduction of Cr O at the iodide modified GCE. At the same
2
7
2
−
time, the electrocatalytic reduction of Cr O on unmodified bare
2
7
GCE was examined and it fails to reduce the chromate ͑curve aЈ͒.
Further, by plotting the reduction current vs concentration, the linear
regression equation for chromate reduction was found as y
=
−0.641x − 17.45, with a correlation coefficient of 0.939. Finally,
2
−
the above results confirm the successful reduction of Cr O at io-
2
7
dide modified GCE.
RRDE measurements.— To evaluate the electrocatalytic proper-
ties of iodide modified film on GCE, the electrochemical oxidation
of As ͑III͒ was studied by the hydrodynamic method. Here, the
RRDE technique was the enhanced technique for the electrochemi-
cal oxidation of As ͑III͒ on rotating GC ring disk electrode. Figure
7
A shows the RRDE voltammograms for the redox behavior of As
͑
III͒ at the iodide modified GC ring disk electrode in pH 1.5 H SO
2
4
−
3
aqueous solution ͑containing 1 ϫ 10 M of KI͒ with different con-
centrations of As ͑III͒. Here, the As ͑III͒ was spiked in the linear
range of 1 ϫ 10 –7 ϫ 10 M and the rotation rate of the elec-
trode was fixed at 2500 rpm. Well-defined voltammograms with a
mass transport-limited current were observed at each addition of As
͑III͒. Further, the ring current ͑I ͒ and the disk current ͑I ͒ were
−
4
−4
Figure 6. ͑A͒ CVs of iodide film on GCE in the presence of 1 ϫ 10−3 M KI
in a pH 1.5 H SO with various concentration of ͓As ͑III͔͒= ͑a͒ 0, ͑b͒ 2
2
4
R
D
−
5
−5
−4
−5
−4
ϫ 10 , ͑c͒ 6 ϫ 10 , ͑d͒ 1.0 ϫ 10 , ͑e͒ 1.8 ϫ 10 , and ͑f͒ 2 ϫ 10 M;
found to increase with the increasing concentration of As ͑III͒. The
inset of Fig. 7A shows the linear relationship between the concen-
tration and catalytic currents, which clearly indicates that the iodide
modified electrode catalyzes the As ͑III͒. Meanwhile Fig. 7B repre-
sents the RRDE voltammograms of iodide modified GCE for the
͑
aЈ͒ bare GCE and ͓As ͑III͔͒ = 2 ϫ 10−4 M. Inset is the plot of Ipa vs
concentration. ͑B͒ CVs of iodide film on GCE in the presence of 1
−
3
2−
7
ϫ 10 M KI in a pH 1.5 H SO with various concentrations of ͓Cr O
͔
2
4
2
͑a͒ 0, ͑b͒ 2 ϫ 10−5, ͑c͒ 4 ϫ 10 , ͑d͒ 8 ϫ 10 , and ͑e͒ 1.0 ϫ 10 M;
−5
−5
−4
=
͑
2
−
2
−4
electrocatalytic oxidation of As ͑III͒ ͑7 ϫ 10 M͒ at iodide film in
centration.
−
3
pH 1.5 H SO solution ͑containing 1 ϫ 10 M of KI͒ at different
2
4
rotation rates ͑200–2500 rpm͒. Figure 7B shows that both the oxi-
dation current at the disk and the reduction current at the ring in-
creased with the rotation speed. The linear properties of the Levich
plot show that the catalytic reaction is exclusively controlled by the
mass transfer process in the solution. The collection efficiency was
calculated from the ratio between I and I ͑inset of Fig. 7B͒. Here,
iodide-based sensors. Figure 6A shows the typical CV response of
iodide film modified GCE for the successive additions of As ͑III͒ in
−
5
−4
the range of 2 ϫ 10 –2 ϫ 10 M. Further, the important factor
for the performance of iodide film was the presence of KI in the
buffer solution. According to this behavior of iodide film, all the
catalysis has been done in the presence of KI in the corresponding
R
D
the maximum ratio obtained was 0.18, which slightly decreased with
the rotation speed. The theoretical collection efficiency was calcu-
lated by using the geometry of RRDE ͑r = 0.269 cm, r
buffer solutions. Here, the As ͑III͒ oxidation has been done in the
1
2
presence of 1 ϫ 10−3 M KI in pH 1.5 H SO solution. From Fig.
= 0.313 cm, and r = 0.383 cm͒ and the reversible redox reaction
3
2
4
3
6
−
4−
6
of ͓Fe͑CN͒ ͔/͓Fe͑CN͒ ͔. Here, the RRDE experiments of
6
A, it can be seen that the oxidation peak current increases linearly
3
−
4−
at 0.8 V with the increasing concentrations of As ͑III͒. When As ͑III͒
was added, the anodic peak current increased noticeably while that
of the cathodic current decreased substantially. The decrease of the
cathodic and the enhancement of the anodic currents clearly indi-
cates the electrocatalytic oxidation of As ͑III͒ at iodide modified
GCE. Further comparing with bare GCE ͓for 2.0 ϫ 10− M of As
͓Fe͑CN͒ ͔/͓Fe͑CN͒ ͔ redox reactions were carried out under dif-
6 6
ferent rotation speeds by fixing the ring potential at 0.5 V. In par-
ticular, from the present investigation it was found that the theoret-
ical efficiency calculated was 0.35. Furthermore, the constant value
of the I /I ratio at a different electrode rotation rate points out the
R
D
4
absence of any chemical reaction couple with electron transfer pro-
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F162
Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
modified electrodes. Figure 8A shows the background subtracted
DPVs for the detection of As ͑III͒ obtained from the mineral water.
Here, the catalytic oxidation peaks of As ͑III͒ were detected in min-
eral water by spiking 0.2 nM, 0.02 ␮M, and 0.2 ␮M concentra-
tions. Further, the oxidation peak currents increased linearly with the
increasing concentrations of As ͑III͒. Similar type results were re-
5
,49
ported in the literature.
From these results, it was ascertained that
the iodide modified GCE shows a good electrocatalytic activity for
the detection of As ͑III͒ in the mineral water. Furthermore, Fig. 8B
exhibits the DPVs of As ͑III͒ detection in tap water. Here, the As
͑
III͒ oxidation peaks were detected in the tap water for 0.2 nM,
0
.02 ␮M, and 0.2 ␮M concentrations. In addition, the results ob-
tained from the tap water were significantly higher than the level
obtained for the mineral water. In terms of peak height, the response
of the iodide modified electrode for spiked As ͑III͒ in tap water was
higher than the mineral water. Next, Fig. 8C and D shows the de-
tection of Cr in the mineral water and tap water. Here, the reduction
peaks of Cr ͑Fig. 8C͒ in mineral water were obtained by spiking the
chromate concentration in the range of 0.2–2 ␮M. Further, the lin-
ear increase in the reduction peak currents of Cr reveals the electro-
catalytic nature of the iodide modified GCE. In the case of Cr de-
tection in tap water, it shows obvious reduction peaks for 2 nM and
0
.2 ␮M concentrations ͑Fig. 8D͒. These results validate the mini-
mal possible detection of As and Cr in real drinking water sources
like mineral water and tap water. Finally, these results support the
minimal possibility of using iodide modified electrodes for real
sample analyses in sub-micromolar levels.
Ϫ
Electrocatalytic reduction of IO3 on poly GE.— Generally, the
−
3
electrocatalytic reduction reaction of IO requires higher overpoten-
tials at bare electrodes. To overcome this problem, the modified
−
3
electrodes were applied for the electrocatalytic reduction of IO .
Here, we have utilized the chemisorbed iodide film on poly GE for
−
the electrocatalytic reduction reactions of IO ; the results are shown
3
−
3
in Fig. 9A. The electrocatalytic reduction of IO was carried out on
iodide modified poly GE in pH = 13 KOH buffer solution ͑contain-
ing 1 ϫ 10− M KI͒ with various concentrations of KIO . The
3
3
chemisorbed iodide film on poly GE showed the reduction peak for
−
IO3 at −0.8 V. Here, the linear additions of KIO in the buffer
3
−
5
−4
solution were in the range of 2 ϫ 10 –1 ϫ 10 M. For each ad-
dition of IO , the cathodic reduction peak current increases signifi-
Figure 7. ͑A͒ RRDE voltammograms of iodide film on GCE in the presence
−
3
−
3
of 1 ϫ 10 M KI in a pH 1.5 H SO with various concentrations of
͓
2
4
−
4
−4
−4
−4
cantly and the anodic current concomitantly decreases, showing the
As ͑III͔͒= ͑a͒ 0, ͑b͒ 1 ϫ 10 , ͑c͒ 2 ϫ 10 , ͑d͒ 3 ϫ 10 , ͑e͒ 4 ϫ 10 , ͑f͒
−4 −4 −4
−
electrocatalytic reduction of IO . At the same time, there is no ob-
5
=
ϫ 10 , ͑g͒ 6 ϫ 10 , and ͑h͒ 7 ϫ 10 M; electrode rotation speed
2500 rpm. ER = 0 V. Inset is the plot of I and I vs ͓As ͑III͔͒. ͑B͒ RRDE
3
−
3
vious response at the bare poly GE electrode for the IO reduction in
the same condition, as shown. Thus, it indicates that IO3 was
R
D
−
3
−
voltammograms of iodide film on GCE in the presence of 1 ϫ 10 M KI in
−
4
a pH 1.5 H SO with 7 ϫ 10 M of As ͑III͒ at different rotation speeds: ͑a͒
2
4
strongly reduced by the chemisorbed iodide layer on poly GE. Fur-
2
=
00, ͑b͒ 400, ͑c͒ 600, ͑d͒ 900, ͑e͒ 1200, ͑f͒ 1600, and ͑g͒ 2500 rpm. ER
0 V. Inset is the plot of variation of IR and ID vs rotation rate.
−
ther, the reduction current of IO vs concentration was plotted and
3
the linear regression equation was found as y = −10.88x − 0.318,
with a correlation coefficient of 0.998. Finally, Table I shows the
comparison of the present work with previous reports for the reduc-
cess. Here, the collection efficiency of As ͑III͒ at an iodide modified
electrode was found to be somewhat less than that of the theoretical
value.
−
tion of the IO .
3
DPV studies.— DPV is known to be sufficiently sensitive, fast,
and reliable for iodate determination. Although the peak heights and
sensitivity were slightly higher in DPV, lower background currents
Detection of As (III) and Cr in real samples.— For the real
sample analysis, the As ͑III͒ and chromate detection has been con-
ducted in real-world drinking waters like mineral water and tap wa-
ter ͑Taipei Water Department, Taiwan͒. The drinking mineral water
source used here for the analysis was well characterized and fur-
nished detailed information about the various minerals it contained
and better peak resolution were obtained by DPV. The electrocata-
−
3
lytic reduction of IO at iodide modified poly GE was carried out in
the potential range of 0 to −1.2 V, and well-defined reduction peaks
at −0.8 V were obtained ͑Fig. 9B͒. The linear ranges for the detec-
−
−5
−4
tion of IO using DPV were 2 ϫ 10 –2 ϫ 10 M, respectively.
͑
Ca = 14.8 to 27.6, Mg = 8.8 to 16.4, Fe Ͻ 0.018, F Ͻ 0.1, and
3
−
Here, the reduction peak currents of IO increased linearly with
increasing the concentrations of the IO . Further, the inset of Fig. 9B
shows the calibration plot for the current vs concentration of IO
catalysis. The calibration plot for IO reduction shows the uniform
Na = 2.7–11.3 mg/L; pH 6.0–8.0͒. The certification shows that
there were probably no other toxic metals in the mineral water.
Further, the tap water analysis reports from Taiwan says it contains
various substances which affect the palatability ͑Fe = 0.019, Cu
3
−
3
−
3
−
3
−
=
0.00068, Zn = 0.00243, chloride = 5.63, sulfate = 11.8, pH 7.1͒.
electrocatalytic activity of the chemisorbed iodide film for the IO3
Also, the report was clear that it does not have any toxic metals like
As, Pb, Se, Cr, Cd, Ba, Sb, Ni, and Hg in it. Both the water sources
have been examined for the As ͑III͒ and Cr detection by using iodide
reduction. The linear regression equation obtained from the calibra-
tion plot was found as y = −0.404x − 0.673, with a correlation co-
efficient of 0.998. Finally, from these two different types of tech-
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Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
F163
Figure 8. ͑Color online͒ ͑A͒ DPVs of
As ͑III͒ detection in mineral water
a = 0.2 nM, b = 0.02 ␮M, and c = 0.2
͑
␮
M͒. ͑B͒ As ͑III͒ in tap water ͑a = 0.2
nM, b = 0.02 ␮M, and c = 0.2 ␮M͒. ͑C͒
DPVs of chromate detection in mineral
water ͑a = 0.2 ␮M, b = 2 ␮M͒. ͑D͒
Chromate detection in tap water ͑a = 2
nM, b = 0.2 ␮M͒.
−
3
Figure 9. ͑A͒ CVs of iodide film on poly GE in the presence of 1 ϫ 10 M KI in a pH 13 KOH with various concentration of ͓KIO ͔= ͑a͒ 0, ͑b͒ 2
3
−
5
−5
−5
−5
−4
−4
ϫ 10 , ͑c͒ 4 ϫ 10 , ͑d͒ 6 ϫ 10 , ͑e͒ 8 ϫ 10 , and ͑f͒ 1 ϫ 10 M; ͑aЈ͒ bare poly GE and ͓KIO ͔ = 1 ϫ 10 M. Inset is the plot Ipc vs concentration. ͑B͒
3
−
3
−5
DPVs of iodide film on poly GE in the presence of 1 ϫ 10 M KI in a pH 13 KOH with various concentrations of ͓KIO ͔= ͑a͒ 0, ͑b͒ 2 ϫ 10 , ͑c͒ 4
3
−
5
−5
−5
−4
−4
−4
ϫ 10 , ͑d͒ 6 ϫ 10 , ͑e͒ 8 ϫ 10 , ͑f͒ 1.2 ϫ 10 , ͑g͒ 1.6 ϫ 10 , and ͑h͒ 2 ϫ 10 M. Inset is the plot of Ipc vs concentration.
Table I. Comparison of chemisorbed iodide film/poly GE with other methods for IO −3 reduction.
Detection limit
Electrode
Linear range ͑mol L−1
͒
͑mol L
−1
͒
Epc ͑V͒
Reference
−
3
−3
Poly luminol film
Poly fuchsin film
Poly neutral red film
2.5 ϫ 10 –5.0 ϫ 10
—
—
—
—
−0.2
−0.3
−0.4
0.1
50
28
29
27
−
4
−3
−3
−4
6.0 ϫ 10 –1.8 ϫ 10
−
4
3.3 ϫ 10 –1.0 ϫ 10
3
−
4−
12 40
−4
͑
RuO/Fe͑CN͒ /SiMo O ͒ film
2.0 ϫ 10 –8.0 ϫ 10
6
−
5
−5
−5
Iodide film
2.0 ϫ 10 –10 ϫ 10
1.447 ϫ 10
−0.8
Proposed method
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F164
Journal of The Electrochemical Society, 155 ͑7͒ F157-F164 ͑2008͒
9
. Basic Inorganic Chemistry, 2nd ed., F. A. Cotton, G. Wilkinson, and P. L. Gaus,
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iodide film on poly GE shows satisfactory results for the electrocata-
1
0. IARC Monographs of the Evaluation of the Carcinogenic Risk of Chemicals to
Humans, International Agency for Research on Cancer, Lyon, France ͑1990͒.
−
3
lytic reduction of IO .
1
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GCE and poly GE. The properties of the iodide film for different
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1
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trocatalytic activities for As ͑III͒ oxidation, Cr O7 and IO reduc-
2
1
1
1
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−
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3
2
−
and Cr O by using CV and DPV techniques. The minimal practi-
2
7
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for Cr and As ͑III͒ detection in tap water and mineral water, and the
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film is very easy and eco-friendly for the determination of environ-
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solution for different types of electrodes, economically cheaper than
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Acknowledgment
This work was supported by the National Science Council of
Taiwan.
National Taipei University of Technology assisted in meeting the publi-
cation costs of this article.
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