Anodic stripping voltammetric determination of silver(I) in water using a 4-tert-butyl-1(ethoxycarbonylmethoxy)thiacalix[4]arene modified glassy carbon electrode

Abstract

A glassy carbon electrode coated with film of 4-tert-butyl-1-(ethoxycarbonylmethoxy)thiacalix[4]arene is designed for the
determination of trace amounts of Ag+. Compared with bare glassy carbon electrode, the modified electrode can greatly improve the measuring sensitivity for Ag+. Under the optimum experimental conditions, the modified electrode in B-R buffer solution (pH 4.5) shows a linear voltammetric
response in the range of 5.0 × 10−8–3.0 × 10−6 M with detection limit 1.0 × 10−8 M for Ag+. The high sensitivity, selectivity, and stability of modified electrode also demonstrate its practical application for a
simple, rapid and economical determination of Ag+ in water samples.

Keywordsanodic stripping voltammetric–silver(I)–glassy carbon electrode

Full text

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Anodic Stripping Voltammetric Determination of Mercury(II) in Water
Using a 4-tert-Butyl-1-(ethoxycarbonylmethoxy)thiacalix[4]arene Modified
Glassy Carbon Electrode
Fei Wanga* (
Yu-Mei Gaoa(
aDepartment of Material and Chemistry Engineering, Henan Institute of Engineering,
Zhengzhou 450007, P. R. China
bZhengzhou Environmental Monitoring Centre, Zhengzhou 450007, P. R. China
), Jian Liua(), Yan-Ju Wub(),
) and Xu-Feng Huangb()
A glassy carbon electrode coated the film of 4-tert-butyl-1-(ethoxycarbonylmethoxy)thiacalix[4]arene
is designed for the determination of trace amounts of Hg2+. Compared with bare glassy carbon electrode,
the modified electrode can improve the measuring sensitivity of Hg2+. Under the optimum experimental
condition, the modified electrode in 0.1 mol L-1H2SO4+ 0.01 mol L-1KCl solution shows a linear voltam-
metric response in the range of 8.0 ? 10-9~ 3.0 ? 10-6mol L-1with detection limit 5.0 ? 10-9mol L-1for
Hg2+. The high sensitivity, selectivity, and stability of modified electrode also prove its practical applica-
tion for a simple, rapid and economical determination of Hg2+in water samples.
Keywords: 4-tert-Butyl-1-(ethoxycarbonylmethoxy)thiacalix[4]arene; Differential pulse anodic
stripping voltammetry; Mercury(II) ion.
INTRODUCTION
Mercury is one of the most toxic of the environment,
because of its high reactivity, extreme volatility and rela-
tive solubility in water and living tissues. It can have sev-
eral effects on human health and organomercury com-
pounds even at low concentrations. Therefore, the determi-
nation of mercury is necessary and urgent at trace levels.1
Some analytical techniques have been employed in metal
analysis, such as atomic absorption spectrometry (AAS),2
X-ray fluorescence3or UV spectrophotometry.4But com-
plicated preconcentrations or multisolvent extraction tech-
niques are also coupled with these techniques because of
the complexity of the real samples and the low concentra-
tion of the analyte. Electrochemical methods, in particular
differential pulse anodic stripping voltammetry (DPASV),
are the most favourable techniques for the determination of
metalionsbecauseofitslowcost,highsensitivity,easyand
the ability for carry out speciation analysis.5,6
Chemically modified electrodes (CMEs) have re-
ceived increasing attentions which improve the sensitivity
and selectivity of electrochemical analysis techniques in
the past decades.7A few articles discuss determination of
mercury(II) ion by CMEs. The modifiers used include or-
ganic chelating groups,8,9polymer,10-14sol–gel and nano-
particles.15-18In recent years, calixarenes, cyclic oligomers
of phenol-formaldehyde condensates, have received many
attention as a basis for molecular and ionic recognition be-
cause of their conformational and flexibility.19Several
calixarenes derivations have been successfully employed
in modified electrode for determination of metals ions.20-22
Thiacalixarenes have recently emerged as new members of
the calixarene family. The presence of four sulfur atoms
(which have lone pairs and vacant 3d orbitals) in the place
of methylene groups tells many novel features and proper-
ties compared with conventional calixarenes. The specific
metal-binding property should be one of the most notable
features of these thiacalix class ligands.23Xiaojun Hu and
co-workers found out the molecular recognition of thia-
calixarenes to Cu2+, Zn2+, Cd2+, Pb2+and Ag+by UV spec-
troscopy and solvent extraction.24We rencently reported
that GCE modified with p-tert-butylthiacalix[4]arene was
used for determination for Cu2+, Cd2+and Pb2+,25,26which
showed better sensing characteristics in comparison with
those based on conventional calix[4]arenes.
778
Journal of the Chinese Chemical Society, 2009, 56, 778-784
* Corresponding author. Tel: +86-0371-67718913; Fax: +86-0371-67763654; E-mail: wf2008_1978@hotmail.com

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In this approach, we presents 4-tert-butyl-1-(ethoxy-
carbonylmethoxy)thiacalix[4]arene (TCA), the see Fig. 1,
modified glassy electrode as voltammetric sensor for the
detecting Hg2+ions. Compared with a bare GCE, the fabri-
cated electrode can improve the sensitivity of measuring
Hg2+ions. At the same time, the fabricated electrode dis-
played excellent sensitivity and reproducibility, and its
practicalapplication for asimple, rapid and economicalde-
termination of Hg2+ion in a water sample.
EXPERIMENTAL
Apparatus and Reagents
All electrochemical techniques were carried out on
the CHI 650 Electrochemical analyzer (CH Instruments
Company, USA). A traditional three-electrode consisting
of a saturated calomel reference electrode (SCE), a plati-
num wire auxiliary electrode, and a modified GC working
electrode (the geometric diameter of GCE is 3 mm), were
employed. The electrochemical cell consists of a 10 mL
vessel supplied with an electrical spiral stirrer.
All reagents were analytical grade and were used
without further purification. The aqueous solutions were
prepared using redistilled water. The TCA was purchased
from Tokyo Kasei Kogyo Company (Japan). Stock solu-
tions(1.0?10-3molL-1and1.0?10-5molL-1)ofHg(NO3)2
were prepared by dissolving into 0.1 mol L-1HNO3and
stored darkly. The working standard solutions, for optimi-
zation studies, were prepared daily by suitable dilution of
this stock solution. High-grade nitrogen was used for de-
oxygenated of samples and solvents during analysis.
Preparation of Modified Electrode
Before immobilization, the GCE was polished with
0.1 mmaluminumslurry,rinsed thoroughlywith redistilled
water and sonicated successively in ethanol and redistilled
water, each for 3 min. The electrodes were electrochemi-
cally pretreated by cycling the electrode in 0.5 mol L-1
H2SO4. Then, the GCE was inverted upside and TCAsolu-
tion was dropped on the electrode tip with the help of a
micropipette. It was allowed to dry for 30 min at room tem-
perature. This modified electrode was named TCA/GCE.
Analytical Procedure
The working electrode was immersed in 10 mL 0.1
mol L-1H2SO4+ 0.01 mol L-1KCl of sample solution con-
taining a known amount of Hg2+. The accumulation step
proceeded at -0.6 V while stirring solution. Then the cyclic
voltammetry (CV) or differential pulse voltammogram
(DPV) was recorded from -0.2 to +0.6 V after 30 s quies-
cence. After each determination, the fabricated electrode
was held at 0.6 V for at least 1 min, and then dipped into
0.02 mol L-1EDTA for 5 min, then rinsed with water. All
measurementswerecarriedoutatroomtemperature(25?1
?C).
RESULTS AND DISCUSSION
Electrochemical impedance spectroscopy and cyclic
voltammetry characterization of the TCA/GCE
It is well known that electrochemical impedance
spectroscopy (EIS) is an effective tool for studying the
boundary properties of surface-modified electrodes. Ret,
the semicircle diameter at higher frequencies in Nyquist
plot of EIS can be used to describe the boundary properties
of the electrode, because it controls the interfacial electron
transfer rate of the redox probe between the solution and
theelectrode.Itsvaluevarieswhendifferentsubstancesare
adsorbed on the electrode surface.27Fig. 2 shows the re-
sults of ac impedance spectroscopy on bare GCE and dif-
ferent amount TCAsolutions modified GCE in the solution
of equimolar 1.0 ? 10-3mol L-1Fe(CN)6
tively. As the result, the bare GCE shows an almost straight
line (Fig. 2A) that is characteristic of a diffusion limiting
step of the electrochemical. At the same time, the TCA/
GCE displays a semicircle portion (Fig. 2B, C), obeyed at
higher frequencies, which matches to the electron transfer
limited, followed by a linear part, characteristic of the
lower frequencies and attributable to a diffusion limited
electrontransfer.Anditcanbealsoseenthediameterofthe
semicircleatthehighfrequencyincreasesontheincreasing
amount of TCA solutions of the modifier on the electrode
surface. The respective semicircle diameter agrees to the
3-/4-ions, respec-
Determination of Mercury(II) Using Modified Electrode
J. Chin. Chem. Soc., Vol. 56, No. 4, 2009
779
Fig. 1. The chemical structure of 4-tert-Butyl-1-(eth-
oxycarbonylmethoxy)thiacalix[4]arene (TCA).

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electron transfer resistance at the electrode surface.28After
fitting suitable circuit and calculation, Retgot is about 1.35
k? and 4.40 k? for 5 ?l, 10 ?l TCAsolutions, respectively.
This suggested that TCA acted as the blocking layer for
electron and mass transfer that hinders to diffuse ferri-
cyanide toward the electrode surface, which obviously
proves that TCA film is successfully immobilized on the
GCE surface.
The CV of ferricyanide is also a valuable and conve-
nient tool to check the barrier of the modified electrode.
Therefore, it was also chosen as a marker to study the
changes of the electrode behavior after different amount
modifier. The experiment confirmed the TCA was suc-
cessfully assembled on GCE surface. Fig. 3 shows CVs of
different electrodes in a solution of 1.0 ? 10-3mol L-1
K3Fe(CN)6. Well-defined CVs, characteristic of a diffu-
sion-limited redox, are observed at the bare GCE (Fig. 3,
curvea).Aftermodifyingtheelectrode,theredoxpeakcur-
rents decreased and the peak potentials shifted remarkably
(Fig.3,curveb,c).ThereasonistheTCAcanactasaninert
electron and mass transfer blocking layer, and it hinders to
diffuse ferricyanide toward the electrode surface, which
obviously also prove that TCA film is successfully immo-
bilized on the GCE surface. Based on the EIS and CV re-
sults, we can decide that TCA is successfully immobilized
on the GCE surface.
Electrochemical behavior of Hg2+on the TCA/GCE
The ability of the TCA/GCE to preconcentrate Hg2+
was studied. Fig. 4 show the DPVs of bare GCE and GCE
modified after accumulation in 0.1 mol L-1H2SO4+ 0.01
mol L-1KCl solution containing 1.0 ? 10-7mol L-1of Hg2+.
In bare GCE, a small oxidation peak could be discerned
(Fig. 4, curve a), indicating some adsorption of Hg2+could
have occurred at the electrode surface. However, this min-
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J. Chin. Chem. Soc., Vol. 56, No. 4, 2009
Wang et al.
Fig. 2. Electrochemical impedance spectroscopy (EIS)
of a clean, freshly polished bare GCE (A) and
different amount TCA solutions modified GCE
in 1 ? 10-3mol L-1Fe(CN)64-/3-solution; (B) 5
?l, (C) 10 ?l.
Fig. 3. CVs of Fe(CN)63-(1 ? 10-3mol L-1) at a clean,
freshly polished bare GCE (a), and different
amount TCA solutions modified GCE (b, c)
with scan rate ? = 0.10 V s-1; (b) 5 ?l, (c) 10 ?l.

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uscule peak provided evidence for the absence of signifi-
cant preconcentration of Hg2+at the unmodified electrode.
In contrast, when TCA/GCE was applied, a well-defined
and symmetrical anodic stripping peak with peak potential
of +0.12 V appeared after accumulation (Fig. 4, curve b).
And the electrochemical response of TCA/GCE in this me-
dium was a flat curve (Fig. 5, curve a), so this peak was be-
causeofoxidatingHg2+producedbyreducingaccumulated
Hg2+atthenegativepotentials.Ascanbeseen,thereisare-
markable enhancement in Hg2+ion oxidation peak current
when GCE modified with TCA. At the same time, a reduc-
tion smallpeak at-0.1 Vcan beseen (Fig.5,curveb)on the
reverse scan. It implied that majority of Hg2+have been
stripped out from the surface of electrode and diffused out
into solution. Because the oxidation peak current is more
sensitiveto reduction peak current, theanodicpeak of Hg2+
was systematically studied by differential pulse anodic
stripping voltammetry for analytical applications.
Mechanism of the method
From the above observations, under the conditions of
the experiments, the possible mechanism of electrode reac-
tion can be described as follows:
Preconcentration step:
TCA/GCE(surface) +Hg2+(solution)?TCA/GCE-
Hg2+(surface)
Reduction step:
TCA/GCE - Hg2+(surface) + e ? TCA/GCE - Hg0
(surface)
Stripping step:
TCA/GCE - Hg0(surface) ? TCA/GCE (surface) +
Hg2+(solution) + e
Analytical applications and methods validation
Optimum condition of determination
To examine the influences of supporting electrolytes,
including HClO4, H2SO4, HNO3, KCl, HAc-NaAc, phos-
phate buffer solution and B-R buffer solution, voltammet-
ricpeakswereobtained in alltheseelectrolytes.Theresults
showed that voltammetric peaks were observed in all these
electrolytes, but the electrochemical responses in 0.1 mol
L-1H2SO4+ 0.01 mol L-1KCl solutions were the best shape
and largest stripping peak current among these buffers.
Therefore, this supporting electrolyte was employed in the
following experiments.
Accumulation potential (Eacc) and accumulation time
(tacc) are important parameters for stripping techniques and
has nonnegligible influence on the efficiency of precon-
centration and the sensitivity of determination. Eaccbe-
tween -1.4 and -0.4 V were investigated. Preconcentration
of1.0?10-7molL-1Hg2+wasperformedfor90s.ThenHg(0)
was oxidized and recorded by DPV. It was observed that
the anodic peak current was higher in the interval of be-
tween -0.6 and -0.8 V. At more positive potentials a de-
crease in the anodic peak current was caused by an ineffi-
cient reduction of Hg2+to Hg0at the electrode surface.
Thus -0.6 V was chosen as the reduction potential in all ex-
periments. The anodic peak current increases linear as the
Determination of Mercury(II) Using Modified Electrode
J. Chin. Chem. Soc., Vol. 56, No. 4, 2009
781
Fig. 4. CVs of 1.0 ? 10-7mol L-1Hg2+in 0.1 mol L-1
H2SO4+ 0.01 mol L-1KCl solutions at different
GCE: (a) bare GCE; (b) TCA/GCE; accumula-
tion time: 90 s, accumulation potential: -0.6 V.
Fig. 5. CVs of blank solutions (a) and 1.0 ? 10-7mol
L-1Hg2+(b) at TCA/GCE; other experimental
conditions are the same as those described in
Fig. 4.

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preconcentration time increases, and starts to reach a pla-
teau around 210 s with the mercury concentration of 1.0 ?
10-7mol L-1. It takes longer time for the peak current to
level off for a lower detected concentration. Therefore, to
increase the sensitivity of detection, a longer time is neces-
sary for testing in the sample of lower concentration. Con-
sidering the decrease of upper limit of calibration plot in
longer preconcentration time, 90 s is adopted.
Therelationshipsbetweenpeakcurrentsanddifferent
amounts of TCA solutions modified GCE were also inves-
tigated. Films which were too thin resulted in a low effi-
ciencyofpreconcentrationandalowpeakcurrent,whereas
films which were too thick led to a decrease in the stripping
signal of Hg2+due to their high resistance which blocked
electron transfer between Hg2+and electrode. The best re-
sponse was obtained when 5 ?l TCA solutions was used,
thus this film was employed in the following experiment.
Calibration curve, detection limit, reproducibility and
stability
Series concentrations of standard solutions of Hg2+
were detected under the optimized working conditions.
Fig.6 showed thatthepeak currentsincreased linearlywith
increasing concentration of Hg2+. The inserted calibration
plot highlights a linear relationship between peak currents
and increasing concentration with a regression coefficient
of 0.998. The peak currents responded were linearly rela-
tionship with Hg2+concentrations in the range of 8.0 ? 10-9
~ 3.0 ? 10-6mol L-1. The linear equation was:
ip(?A) = 0.05037 + 0.826 ? 107c
(? = 0.998).
Based on the signal-to-noise ratio of 3 (S/N),29the detec-
tion limit was obtained as 5.0 ? 10-9mol L-1. These values
confirmed the sensitivity of the proposed method for the
determination of Hg2+. To estimate the repeatability of the
proposed method, the R.S.D. of five times successful mea-
surement of peak current of 1 ? 10-7mol L-1Hg2+on TCA/
GCE was calculated to be 4.6%, which demonstrates the
good repeatability of the method.
The long-term stability of the TCA film coated GCE
was evaluated by measuring the current responses at con-
centration of 1 ? 10-7mol L-1over a long period. After ev-
ery measurement, the TCA film coated GCE was removed
from the solution, and then rinsed with redistilled water,
and exposed to the air. After that, it was used to determine
Hg2+according to the analytical procedure. After 30 days,
the current responses deviated by 5.7% for Hg2+, revealing
that the TCAfilm coated GCE exhibits long term stability.
Table 1 summarizes characteristic parameters in the
determination of Hg2+with different macrocyclic modified
electrodes and in the literature. Among macrocyclic modi-
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Wang et al.
Fig. 6. Differential pulse anodic stripping voltammograms and their associated calibration plot (insert) for increasing con-
centrationsofHg2+ionatTCA/GCEunderoptimumconditions;Hg2+concentration(molL-1):1)8? 10-9molL-1,2)1
? 10-8mol L-1, 3) 4 ? 10-8mol L-1, 4) 6 ? 10-8mol L-1, 5) 1 ? 10-7mol L-1, 6) 2 ? 10-7mol L-1, 7) 3 ? 10-7mol L-1, 8) 5 ?
10-7mol L-1, 9) 7 ? 10-7mol L-1, 10) 1 ? 10-6mol L-1, 11) 1.5 ? 10-6mol L-1, 12) 2 ? 10-6mol L-1.