A new Schiff's base ligand immobilized agarose membrane optical sensor for selective monitoring of mercury ion.

Abstract

A highly selective optical sensor was developed for the Hg(2+) determination by chemical immobilization of 2-[(2-sulfanylphenyl)ethanimidoyl]phenol (L), on an agarose membrane. Spectrophotometric studies of complex formation between the Schiff's base ligand L and Hg(2+), Sr(2+), Mn(2+), Cu(2+), Al(3+), Cd(2+), Zn(2+), Co(2+) and Ag(+) metal ions in methanol solution indicated a substantially larger stability constant for the mercury ion complex. Consequently, the Schiff's base L was used as an appropriate ionophore for the preparation of a selective Hg(2+) optical sensor, by its immobilization on a transparent agarose film. A distinct color change, from yellow to green-blue, was observed by contacting the sensing membrane with Hg(2+) ions at pH 4.5. The effects of pH, ionophore concentration, ionic strength and reaction time on the immobilization of L were studied. A linear relationship was observed between the membrane absorbance at 650 nm and Hg(2+) concentrations in a range from 1×10(-2) to 1×10(-5) mol L(-1) with a detection limit (3σ) of 1×10(-6) mol L(-1). No significant interference from 100 times concentrations of a number of potentially interfering ions was detected for the mercury ion determination. The optical sensor was successfully applied to the determination of mercury in amalgam alloy and spiked water samples.

Full text

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Journal of Hazardous Materials 186 (2011) 1794–1800
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
A new Schiff’s base ligand immobilized agarose membrane optical sensor for
selective monitoring of mercury ion
Kamal Alizadeha,∗, Razieh Parooia, Payman Hashemia, Behrooz Rezaeia, Mohammad Reza Ganjalib,c
aDepartment of Chemistry, Lorestan University, Falakolaflak St., Khorramabad, Lorestan 6813717133, Iran
bCenter of Excellence in Electrochemistry, University of Tehran, Tehran, Iran
cEndocrinology & Metabolism Research Center, Tehran University of Medical Sciences, Tehran, Iran
a r t i c l ei n f o
Article history:
Received 9 September 2010
Received in revised form
14 December 2010
Accepted 15 December 2010
Available online 21 December 2010
Keywords:
Optical sensor
Mercury ion
Agarose membrane
Spectrophotometric
a b s t r a c t
A highly selective optical sensor was developed for the Hg2+determination by chemical immobilization
of 2-[(2-sulfanylphenyl)ethanimidoyl]phenol (L), on an agarose membrane. Spectrophotometric studies
of complex formation between the Schiff’s base ligand L and Hg2+, Sr2+, Mn2+, Cu2+, Al3+, Cd2+, Zn2+,
Co2+and Ag+metal ions in methanol solution indicated a substantially larger stability constant for the
mercury ion complex. Consequently, the Schiff’s base L was used as an appropriate ionophore for the
preparation of a selective Hg2+optical sensor, by its immobilization on a transparent agarose film. A
distinctcolorchange,fromyellowtogreen-blue,wasobservedbycontactingthesensingmembranewith
Hg2+ions at pH 4.5. The effects of pH, ionophore concentration, ionic strength and reaction time on the
immobilizationofLwerestudied.Alinearrelationshipwasobservedbetweenthemembraneabsorbance
at650nmandHg2+concentrationsinarangefrom1×10−2to1×10−5molL−1withadetectionlimit(3?)
of 1×10−6molL−1. No significant interference from 100 times concentrations of a number of potentially
interfering ions was detected for the mercury ion determination. The optical sensor was successfully
applied to the determination of mercury in amalgam alloy and spiked water samples.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Mercury is one of the most toxic elements in the world and
represents a major toxicity to microorganisms and environment
even in low concentrations. Inorganic mercury has been reported
to produce harmful effects at concentrations as low as 5?g/L [1].
Inorganic mercury may be converted to methylmercury in marine
environment that is even more toxic than the inorganic mercury
[2]. Methylmercury triggers several serious disorders for humans
including allergic reactions and brain and neurological damage.
If ingested by a pregnant woman, it can cause developmental
delays in children [3]. Thus, the development of simple methods
forselectivedeterminationofmercuryintraceamountsindifferent
matrices is critical.
Atomic spectroscopic methods are powerful analytical tech-
niques for the determination of elements in a great number of
samples. However, these techniques are rather expensive and time
consuming and may not be available in all the laboratories. Opti-
cal chemical sensors (optodes), on the other hand, are simple and
selective tools for the determination of heavy metal ions that have
been extensively developed in recent years [4–6]. Optodes are gen-
∗Corresponding author. Tel.: +98 661 2200185; fax: +98 661 2200185.
E-mail addresses: Alizadehk@yahoo.com, Alizadeh.k@lu.ac.ir (K. Alizadeh).
erallyusedincombinationwithinexpensivespectrophotometricor
spectroflourometrictechniquestoprovidesimpleandfastdetermi-
nation methods with enhanced selectivity and low detection limits
[7–9].
Immobilization of ionophores on transparent membranes is
one of the essential steps in fabrication of optical sensors. The
ionophore immobilized is accomplished by either physical entrap-
ment [10], sol–gel [11,12], multilayered films [13] or chemical
(covalent) bonding methods [14]. Physical entrapment and mul-
tilayered films methods suffer the problem of slow dye leakage
and sensors of these type have relatively short life times [15]. Sen-
sors with covalent bonding, on the other hand, have reproducible
responsesandlonglifetimes[16].Differentapproachestocovalent
immobilization can be found in the literature [17].
Inconstructionofopticalsensors,ionophoresplayanimportant
role. Schiff’s base ligands have been frequently used as ionophores
in construction of membrane sensors because of their ability to
form stable complexes with transition metal ions. They produce
remarkable selectivity, sensitivity and stability for a specific ion
[18–20].
In a previous report, an optical sensor for Cu2+determination
basedoncovalentimmobilizationofcalmagiteonanagarosemem-
brane, was introduced [5]. In the present study, a Schiff’s base
ligand 2-[(2-sulfanylphenyl)ethanimidoyl]phenol, L, is covalently
immobilized on an agarose membrane to be used as an effective
0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2010.12.067

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K. Alizadeh et al. / Journal of Hazardous Materials 186 (2011) 1794–1800
1795
ionophore with S, N and O donor atoms for construction of a selec-
tive optical sensor for the spectrophotometric determination of
Hg2+in aqueous solutions.
HS
CH3
OH
N
L
2. Experimental
2.1. Materials and instruments
All reagents were of analytical reagent grade and were used
as received. Double-distilled water was used throughout and
test solutions were buffered in a 0.02molL−1solution of acetic
acid/sodium acetate and pH adjusted with dropwise addition of
1molL−1solution of HCl or NaOH. For fabrication and prepara-
tionofmembrane,agaroseandepichlorohydrinesuppliedbyMerck
Chemical Company. The Schiff’s base L, with the chemical name of
2-[(2-sulfanylphenyl)ethanimidoyl]phenol, was synthesized and
purified as reported elsewhere [21,22] from the reaction between
2-aminothiophenol and 2-hydroxy acetophenone.
A Jenway (USA) model 3020 pH meter with a combined glass
electrodewasusedaftercalibrationagainststandardMerckbuffers
for pH determinations. A Shimadzu (Japan) model 1650PC double-
beam spectrophotometer was used for running the electronic
absorption spectra (controlled to ±0.1◦C). A home-made polyacry-
lamide holder was used for holding agarose membranes inside the
quartz cells of the spectrophotometer. A totally glass Fisons (UK)
double distiller was used for preparation of doubly distilled water.
2.2. Procedures
For spectrophotometric titrations, 2mL of the ionophore solu-
tion (1×10−4molL−1) in methanol was transferred into a quartz
cell of the spectrometer. Microliter amounts of 1×10−2molL−1
solution of each metal ion were transferred to the ionophore
solution by a 50?L microsyringe. Each spectrum was recorded
immediately after vigorous mixing of the solution. All the
absorbances data were corrected for the dilution.
A method described elsewhere [5,23] was used for preparation
and epoxy activation of agarose membranes with a thickness of
0.2mm from 4% agarose aqueous solution. The transparent-viscose
solution was dispersed between two 20cm×20cm dust-free glass
plates, applying a gentle pressure. Borders of one of the glass plates
were already lined with a 0.20mm thickness tape to adjust the
thickness of the membrane. For the epoxy activation, an epichloro-
hydrine method was used [24]. The membranes were cut into
1cm×3cm pieces and stored at 4◦C under 50% methanol solution
before the use.
ForimmobilizationoftheSchiff’sbaseL,theagarosemembranes
were treated with a 1×10−2molL−1ligand solution, in an alkaline
medium,withamethoddescribedelsewhere[5].Theresultingyel-
lowcolormembraneswerethoroughlywashedwith50%methanol
on a glass filter, soaked in 50% methanol overnight, and washed
vigorously by water to displace any non-bond dye.
A 1cm×2cm piece of the fabricated membrane sensor was cut
and mounted in a polyacrylamide holder and placed inside the
quartz cell of the spectrophotometer. The cell was then used as
usual for the absorbance determinations. All the measurements on
the agarose membranes were performed in aqueous medium.
Fig. 1. Absorbance spectra of L (1×10−4molL−1) in methanol in the presence of
increasing concentration of Hg2+ion (A) and the color change of solution upon
complexation of L with the mercury ions (B). The arrows show the direction of
absorbance changes by increasing the metal ion concentration. The inset shows the
spectra for L, at varying Cu2+concentrations in methanol.
An amalgam alloy sample was obtained from a dentistry office
in Khorramabad, Iran.
The exactly weighed sample was dissolved in 1:1 HNO3and
the mixture was carefully boiled until complete dissolution of the
amalgam. The solution obtained was transferred into a 100mL cal-
ibrated flask and diluted to volume with doubly distilled water.
Working solutions of the amalgam test sample were prepared by
appropriate dilution of the initial solution and pH adjusted on 4.5
for the analysis [25]. Tap water samples were collected from the
watertapinAnalyticalChemistrylaboratoryofLorestanUniversity,
Khorramabad, Iran. The tap water samples were spiked with dif-
ferent amounts of Hg2+and pH adjusted on 4.5 before analysis. For
analysis, about 2.5mL of the samples were transferred into a 1cm
quartz cell equipped with the membrane sensor. The absorbances
were then measured at 650nm and subtracted from an absorbance
readingforabuffersolutionatthesamewavelength.TheHg2+con-
centration was then derived using an ordinary calibration curve
method.
3. Results and discussion
3.1. Spectrophotometric studies
Hg2+displays great affinity for soft coordination centers like
sulfur [26,27]. Among the various types of the detection systems
used in mercury optodes, those based on sulfur-containing ligands,
generally,haveshownbetterselectivityandsensitivity[28].There-
fore, ligand L with a sulfur-containing site was investigated as an
active ionophore for highly selective and sensitive determination
of mercury ion.
The complexation of Hg2+with L in methanol solution was
investigated by spectrophotometric titration. The absorbance
curves are shown in Fig. 1A, with a maximum at about 347nm. By
additionofHg2+,adecreaseinabsorbanceisobservedatthiswave-
length. By decreasing the absorbance at 347nm, a peak at about
551nm is formed that corresponds to the formation of a Hg2+–L
complex. The distinct color change of the solution from yellow to
blue is shown in Fig. 1B. (For interpretation of the references to

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K. Alizadeh et al. / Journal of Hazardous Materials 186 (2011) 1794–1800
Fig. 2. Absorbance of L (1×10−4molL−1) in methanol at 551nm, as a function of
[Mn+]/[L] mole ratio for different metal ions.
colorintext,thereaderisreferredtothewebversionofthearticle.)
For the other studied metal ions (Sr2+, Mn2+, Cu2+, Al3+, Cd2+, Zn2+,
Co2+and Ag+) no effective complexion is detected. The absorbance
curvesfortheformationofpossibleCu2+complexwithL,havebeen
shown in the inset of Fig. 1A as an example.
The resulting plots of the absorbance (at 551nm) against metal
ion/ligandmoleratiosareshowninFig.2.Fromthesharpinflection
point observed for Hg2+at a mole ratio of 0.5, it can be imme-
diately concluded that a 2:1 complex of [L2Hg]2+is formed in
methanol solution. The formation constant of the resulting com-
plex between Hg2+and L was evaluated to be equal to 6.58, from
theabsorbanceversus[Hg2+]/[L]moleratiodatausingknownequa-
tions and utilizing a nonlinear least-squares curve-fitting program,
KINFIT [29–31]. Evidently, the curves for the other metal ions in
Fig. 2, indicate their negligible complex formation by L. Thus, based
on the results obtained from solution studies, L was expected to act
as a suitable and highly selective ionophore in the preparation of
an agarose-based ion-selective optode for mercury ion determina-
tions.
3.2. Theoretical studies
To obtain more information about the conformational changes
of the ionophore upon complexation with mercury ion, the molec-
ular structures of the uncomplexed ionophore (L) and its complex
with the metal ion were simulated with the Hyperchem program
[32]. The structure of the free ligand was optimized using the 6-
31G*basis set at restricted Hartree–Fock (RHF) level of theory. The
optimized structure of the ligand was then used to find out the ini-
tial structure of its mercury complex. Finally, the structure of the
resulting 2:1 complex was optimized using Lanl2mb basis set at
restricted Hartree–Fock (RHF) level of theory. No molecular sym-
metry constraint was applied; rather, full optimization of all band
lengths, angles and torsion angles was carried out using the Gaus-
sian 98 program [33]. The optimized structures are shown in Fig. 3.
It should be noted that, only real frequency values were observed,
indicating that the structural geometries shown in Fig. 3 are quite
acceptable [34–39]. As it is obvious from Fig. 3A, the ligand struc-
ture is more or less planar, while, in the optimized structure of the
2:1 complex with Hg2+(Fig. 3B) the mercury ion coordinates to
all six donating atoms (oxygen, nitrogen and sulfur) of the two L
molecules to form a nice octahedral structure in which the sulfur
groups are now situated in line with the central mercury ion.
Fig. 3. Optimized structures of free ligand (A) and its 2:1 complex (B) with Hg2+ion
in an octahedral geometry.
3.3. Ligand immobilization on agarose membranes
Inagreementwithpreviousstudies[5,23],immobilizationoflig-
and L on an agarose membrane changed, in some extent, its optical
properties. The absorbance maximum of L showed a red shift from
347.5nm to about 359nm upon the immobilization (Fig. 4A curves
1 and 2), probably due to a more flat structure of the immobilized
Schiff’s base ligand that may cause an easier electron resonance. A
similar shift (from 551 to 650nm) was observed for an absorbance
maximum of the Hg2+complex after the ligand immobilization on
theagarosemembrane(Fig.4Bcurves1and2).Duringthetitration,
no measurable spectral shift was observed, which is typical for an
absorption process involving a strong complex formation.
An appropriate L loading on the agarose membrane depends
on the concentration of L as an ionophore, pH and tempera-
ture. In this case, a temperature of 45◦C was used due to the
destructive effects of higher temperatures on the membrane and
inappropriate immobilization at lower ones. The influence of the
1 2
0
0.2
0.4
0.6
0.8
750
650
550
λ(nm)
450
350
A
B
λ(nm)
Absorbance
1
2
0
0.3
0.6
0.9
1.2
550
450
350
250
Absorbance
Fig. 4. The red shift observed for the ligand L (A) and its Hg2+complex (B) upon
immobilization on the agarose membrane. The curves 1 and 2 show the absorbance
curves in methanol solution and after immobilization on an agarose membrane,
respectively.

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1797
0
0.4
0.8
1.2
1.6
0.01
0.001
0.0001
[Ligand]
0.00001
0.000001
Absorbance
Fig. 5. Influence of ligand concentration on its loading on the agarose membrane.
Experimental conditions: Solution pH, 10; wavelength, 359nm; temperature 45◦C.
0
0.3
0.6
0.9
1.2
12
11
10
9
8
7
pH
Absorbance
Fig. 6. Influence of pH of the reaction solutions on the immobilization of
L on the agarose membrane. Experimental conditions: ligand concentration,
2.0×10−3molL−1; wavelength, 359nm.
ionophore concentration was studied in the range of 1×10−2to
1×10−6molL−1at ?=359nm. As it is obvious from Fig. 5, a con-
tinuous increase in the membrane absorbance was monitored by
increasing the ionophore concentration. High loadings, however,
may unacceptably decrease the transmittance of the membrane.
Thusaconcentrationof2.0×10−3molL−1oftheligandwaschosen
as an appropriate concentration in subsequent experiments.
Fig. 6 shows the influence of pH of the reaction solution on the
maximumabsorbanceofthemembrane.ThepHofthesolutionwas
adjusted by addition of appropriate amounts of sodium hydrox-
ide. It was found that a maximum immobilization of the ligand is
achieved in a solution of pH 10.
3.4. Effect of pH of test solution on the sensor response
TheinfluenceoftestsolutionpHontheresponseoftheproposed
Hg2+ion-selectiveopticalsensorisillustratedinFig.7.Theresponse
characteristic of the membrane sensor was highly dependent on
pH. As it is obvious from figure, the absorbance increased rapidly
by changing the pH from 3 to about 4, while it was decreased at pH
valueshigherthan6.ThediminishedresponseatthelowpHregion
maybeexplainedbytheextractionofH+fromthetestsolutioninto
the membrane, via protonation of the nitrogen atom of L, resulting
inanexpectedchangeintheformationofaHg2+–Lcomplex.Onthe
0
0.1
0.2
0.3
0.4
7
6
5
4
pH
3
2
1
Absorbance
Fig. 7. Effect of measurement pH of the test solution on the optical response of the
mercury ion-selective sensor in the presence of 5×10−3molL−1Hg2+.
Fig. 8. The absorption spectra resulting from spectrophotometric titration of the
proposed sensor, under the optimal experimental conditions, the arrows show the
directions of absorbance changes by increasing the Hg2+ions concentration (A).
The color change of the sensor upon complexation with the mercury ions (B). The
calibration curve plot for the mercury membrane sensor at 650nm (C).
other hand, the reduced optical response of the proposed sensor at
pH >6 could be due to a possible hydroxide formation of mercury
ions [28]. Thus, in subsequent experiments, pH 4.5 was used for
absorbance measurements.
3.5. Calibration curve of the sensor
The absorption spectra of the proposed membrane sensor at
pH 4.5 and in the presence of increasing concentrations of mer-
cury ion are depicted in Fig. 8A. The distinct color change of the
membrane sensor from yellow to green-blue is shown in Fig. 8B.
(For interpretation of the references to color in text, the reader is
referred to the web version of the article.) The color change cor-
responds to the formation of a Hg2+–L complex in the membrane
phase with a ?maxof 650nm. Therefore the colorimetric sensor is
appropriate for the determination of mercury ion. The correspond-
ing absorbance-concentration plot is shown in Fig. 8C. It is evident

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K. Alizadeh et al. / Journal of Hazardous Materials 186 (2011) 1794–1800
0
0.03
0.06
0.09
0.12
300
250
200
150
100
50
0
Time(s)
Absorbance
Fig. 9. A typical response curve of the sensor at 650nm as a function of time for
Hg2+concentration level of 2×10−3molL−1.
that the membrane absorbance at 650nm increases linearly by
increasing Hg2+concentration from 1×10−5to 1×10−2molL−1.
A line was fitted to the experimental points shown in Fig. 8C, with
a regression equation of:
Abs. = 43.65[Hg2+] + 0.01
The detection limit based on 3? of the blank was calculated to be
1.0×10−6molL−1Hg2+.
(R2= 0.997)(1)
3.6. The sensor response time
Fig. 9 shows the profile of the response of Hg2+optical sensor at
650nm with time. It can be seen that the time taken to achieve to
the 95% steady state response is within 3min for 2×10−3molL−1
Hg2+at pH 4.5. The response time of this type of optode mem-
branes depends upon a number of variables such as the membrane
thickness, the concentration of the analyte ion, diffusion in the
membrane, the rate of complex formation between the metal ion
and ligand and the rate of complex dissociation. At high concentra-
tions of Hg2+ions a rapid response was achieved, which resulted
in a large change in response (2min for 0.01M). At low concen-
trations of Hg2+ions a slower response time was observed for the
optode membrane (4min for 1×10−5M). In general, the response
time decreased by increasing the analyte concentration and varied
between 2 and 4min. A substantially longer time was needed for a
lower Hg2+concentration with a thicker membrane [40,41].
3.7. Reproducibility, short-term stability and lifetime
The reproducibility and stability are two important character-
istics of an ion-selective optical sensor in evaluating its suitability
forselectivedeterminationofanioninterest.Thereproducibilityof
thesensor’sresponsewasexaminedbyitsmultipleusagesforHg2+
monitoring in test solutions at two concentration levels of 5×10−5
and 2×10−3molL−1. After each absorbance measurement, the
Fig. 10. Reproducibility of the proposed optical sensor signal at two Hg2+concen-
tration levels under optimal experimental conditions.
Fig. 11. Interferences from different metal ions to the spectrophotometric determi-
nation of Hg2+ion with the proposed membrane sensor at pH 4.5. Concentration of
Hg2+and the diverse ions were 5.0×10−5and 5.0×10−3molL−1, respectively.
membrane was regenerated in less than about 2min by soaking
in a 0.02molL−1EDTA solution, pure water and a 0.02molL−1
acetate buffer solution, respectively. As shown in Fig. 10, good
reproducibilities were obtained at both Hg2+concentration levels.
The calculated relative standard deviation (RSD) values from the
results were 2.6% and 1.5%, respectively, which are satisfactory and
highly comparable to those in the literature [42–44].
The short-term stability of the optode membrane was investi-
gated by monitoring its absorbance values during its contact with a
1.0×10−4MsolutionofHg2+atpH4.5overaperiodof6h.Fromthe
absorbancemeasurementsin30minintervals(n=12),itwasfound
that the response was almost unchanged with only 1.0% increase
in absorbance at 650nm after the 6h period.
Meanwhile, it was found that the membrane sensor could be
stored in 20% methanol at 4◦C, without any measurable change in
its absorbance value at 650nm over a period of 4 months. There-
fore,thereisnoionophoreleachingfromthemembraneduringthis
period and it is quite stable over at least 4 months.
3.8. Selectivity
Perhaps the most important characteristics of an ion-selective
optodeareitsselectivity,whichreflectsitsrelativeresponseforpri-
mary ion over divers ions present in solution. Thus, the influence of
anumberofcommonmetalionsontheabsorbanceoftheproposed
Hg2+optical sensor was carried out. To investigate the selectivity
of the proposed Hg2+optical membrane sensor, the absorbance of
a fixed concentration of mercury ion, at 5.0×10−5molL−1level, in
a solution of pH 4.5 was recorded before (Ao) and after (A) addition
of some potentially interfering ions such as Co2+, Ba2+, Na+, Mg2+,
Al3+, K+, Cu2+, Sr2+, Zn2+, Pb2+, Ag+, Cd2+, Mn2+, Ca2+, Fe3+, Cr3+,
Ni2+and Tl+at concentrations up to 100 times of the analyte ion.
The resulting relative error is defined as RE(%)=[(A−Ao)/Ao]×100.
The results of the selectivity studies are summarized in Fig. 11. The
data clearly indicate that, for all the studied metal ions, the rel-
ative error is less than 4%, which is considered as tolerable [28].
Further investigation showed that anions such as C2O42−, NO3−,
Table 1
Application of the optical sensor to the determination of mercury in different
samples.
Sample Hg2+concentration (molL−1)
AddedFounda
Tap water 1
Tap water 2
Tap water 3
5.0×10−3
5.0×10−4
5.0×10−5
(5.1±0.2)×10−3
(4.9±0.2)×10−4
(5.0±0.3)×10−5
SampleMercury content (%)a
Optical sensor AAS
Amalgam alloy 8.1±0.17.9±0.1
aMean of three determinations.

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K. Alizadeh et al. / Journal of Hazardous Materials 186 (2011) 1794–1800
1799
Table 2
Comparison of some previously reported optical sensors for determination of Hg2+with the sensor represented in this work.
Compared parameter
Ref. [8]
Ref. [9]
Ref. [44]
Ref. [45]
This work
Sensing material
Tetra(p-dimethylaminophenyl)porphyrin
Quinolin-8-ol
p-[10’,15’,20’-triphenyl-5’-porphyrinyl]benzoate
5,10,15-Tris(pentafluorophenyl)corrole
(H3(tpfc))
4-(2-Pyridylazo)-resorcinol
(PAR)
2-[(2-Sulfanylphenyl)ethanimidoyl]phenol
Detection method
Fluorescence quenching
Ratiometric, based on
fluorescence quenching
and enhancement
Fluorescence quenching
Spectrophotometric
determination
Spectrophotometric determination
Linear range
4×10−6–4×10−8M
2×10−5–3×10−7M
1×10−4–1.2×10−7M
5.6×10−3–3×10−6M
1×10−2–1×10−5M
Detection limit
8.0×10−9M
2.2×10−8M
NMa
1.51×10−6M
1×10−6M
RSD(%)
5
NM
1.6
3.7
2.6 and 1.5
Response time
Fast, but not mentioned in numerical
Fast, but not mentioned in
numerical
≈5min
20min
≈3min
Interferences
No interference
Cu(II) shows interfering
effect at equal molar
concentration of Hg(II)
Ag(I) above 5.0×10−4M
Pb(II), Ni(II), Co(II), Cu(II)
No interference
Qualitative
–
–
–
–
Yellow to green-blue
Type of sensor
PVC matrix
Ethanolic solution
PVC matrix
Triacetylcellulose membrane
Agarose membrane
aNot mentioned.
NO2−, CO32−, IO3−and CH3COO−present at high concentrations up
to 1.0×10−2M also do not show obvious interfering effect on the
Hg2+assay. The results obtained indicate that the proposed optical
sensorishighlyselectivetowardsHg2+ionandcanbeappliedtothe
determination of traces of mercury ion in natural samples, in the
presence of several other co-existing cationic and anionic species.
3.9. Analytical applications
The proposed Hg2+-selective optical sensor was successfully
applied to the determination of Hg2+ion in three spiked tap water
samples.Theresultsrevealedagoodagreementbetweentheadded
and found concentrations.
The proposed sensor was also applied to the determination of
mercury content of amalgam alloy [25] and the result was com-
paredwiththedataobtainedfromatomicabsorptionspectrometry
(AAS) measurements (Table 1). It is readily seen that the results
obtained by the proposed Hg2+-sensor is in satisfactory agreement
with those obtained by the AAS measurement.
3.10. Comparison with other works
Table 2 compares the main analytical characteristics (i.e., linear
range, detection limit, response time, qualitative analysis, interfer-
ing effect and RSD) of the represented optical sensor with those of
some of the previously reported sensors for the determination of
Hg2+[8,9,45,46].
As it can be seen from the data in Table 2, the proposed sen-
sor is significantly improved compared to the previously reported
Hg2+sensors in terms of linear range, response time, RSD, qual-
itative diagnostic test, effects of various species and methods of
preparation.
4. Conclusion
Insummary, immobilizationof2-[(2-
sulfanylphenyl)ethanimidoyl]phenol (L) on an agarose membrane
resulted in a highly selective optical sensor for the Hg2+deter-
mination. The proposed sensor showed very favorable optical
properties for its use as an optical sensor, such as high selectivity,
adequate life time, fast and reproducible regeneration, low cost
and simple fabrication and handling. In addition, the sensor
showed short response time, appropriate linear calibration curve
and low detection limits. By the means of an easy and low-cost
methodology, satisfactory experimental results were obtained
for the determination of Hg2+ion by the prepared sensor. The
optical sensor can be used for the quantitative and qualitative
determination of mercury(II) ions in different samples without
any significant interference from other metal ions.
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