Holmium(III)-selective fluorimetric optode based on N,N-bis(salicylidene)- naphthylene-1,8-diamine as a neutral fluorogenic ionophore

Article abstract of DOI:10.1016/j.saa.2013.10.057

For the first time a highly sensitive and selective fluorimetric optode for determination of trace amounts of Ho³⁺ ions was prepared. The sensing system was prepared by incorporating of N,N-bis(salicylidene)- naphthylene-1,8-diamine (L) as a neutral Ho³⁺-selective fluoroionophore, in a plasticized PVC membrane containing sodium tetraphenyl borate as a lipophilic anionic additive. The response of the sensor is based on the strong fluorescence quenching of L by Ho³⁺ ions. At pH 5.4, the proposed sensor displays a calibration curve over a wide concentration range of 1.0 × 10^-10-1.0 × 10^-3 M, with a relatively fast response time of less than 1 min. In addition to high stability, high reproducibility and a relatively long working lifetime, the sensor shows a good selectivity towards Ho³⁺ ion with respect to common coexisting cations. The fluorescence optode was applied to determination of holmium ion contents of water samples.

Full text of DOI:10.1016/j.saa.2013.10.057

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 224–229
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Holmium(III)-selective fluorimetric optode based on N,N-bis
(salicylidene)-naphthylene-1,8-diamine as a neutral fluorogenic
ionophore
c
a
a
Mohammad Reza Ganjali a,b, , Morteza Hosseini , Anahita Karimi , Hedieh Haji-Hashemi ,
⇑
Masoud Salavati-Niasari d, Parviz Norouzi a,b
a Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran
b Biosensor Research Center, Endocrinology and Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran
c Department of Life Science Engineering, Faculty of New Sciences & Technologies, University of Tehran, Tehran, Iran
d Institute of Nano Science and Nano Technology, University of Kashan, Kashan, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
A selective fluorimetric optode for
determination of trace amounts of
Ho3+ was prepared.
N,N-bis(salicylidene)-naphthylene-
1,8-diamine (L) was used as a
fluoroionophore.
The response of the sensor is based on
the strong fluorescence quenching of
L by Ho3+ ions.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2013
Received in revised form 12 October 2013
Accepted 17 October 2013
Available online 26 October 2013
For the first time a highly sensitive and selective fluorimetric optode for determination of trace amounts
of Ho3+ ions was prepared. The sensing system was prepared by incorporating of N,N-bis(salicylidene)-
naphthylene-1,8-diamine (L) as a neutral Ho3+-selective fluoroionophore, in a plasticized PVC membrane
containing sodium tetraphenyl borate as a lipophilic anionic additive. The response of the sensor is based
on the strong fluorescence quenching of L by Ho3+ ions. At pH 5.4, the proposed sensor displays a calibra-
tion curve over a wide concentration range of 1.0 ꢁ 10ꢂ10–1.0 ꢁ 10ꢂ3 M, with a relatively fast response
time of less than 1 min. In addition to high stability, high reproducibility and a relatively long working
lifetime, the sensor shows a good selectivity towards Ho3+ ion with respect to common coexisting cations.
The fluorescence optode was applied to determination of holmium ion contents of water samples.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Optode
PVC membrane
Fluorescence quenching
Ho(III)
Sensor
Introduction
The field of optical chemical sensors has been a growing re-
search area over the last three decades. Chemical sensor technol-
ogy involves the key processes of chemical recognition of the
analytes of interest and subsequent transduction of the analytical
signals. Fluorescent materials are among the mostly used reagents
⇑
Corresponding author at: Center of Excellence in Electrochemistry, Faculty of
Chemistry, University of Tehran, Tehran, Iran. Tel.: +98 21 61112788; fax: +98 21
66495291.
E-mail address: ganjali@khayam.ut.ac.ir (M.R. Ganjali).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.10.057

M.R. Ganjali et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 224–229
225
for a variety of sensing applications [1–4] and there is an increas-
ing current interest in the development of fluorescent sensors for
metal ions [5–16]. In developing such fluorescent ionic sensors, be-
sides specificity towards the ion of interest, a large change in fluo-
rescence intensity in the presence of that ion and/or a spectral
change are required. Moreover, the sensing element must be as
lipophilic as possible in order to minimize its loss through leaching
in the aqueous phase and, consequently, to prolong the lifetime of
the sensor [2,17].
Holmium compounds have specialized uses in ceramic, glass
and phosphor lamps. Holmium can absorb fission-bred neutrons.
It is also used in nuclear reactors to keep the atomic chain reac-
tions from running out of control. There is a growing trend in the
uses of this element, due to the fact that it is studied to produce
catalysts and to polish glass [18]. The bioaccumulation of the hol-
mium ions in the body can be a threat to the liver. Nowadays, it is
known that holmium causes damages to the cell membranes of the
water animals with negative effects on the reproduction and the
nervous systems. This makes the analysis of this element greatly
important. However, conventional methods that can be used for
the analysis of this element, such as inductively coupled plasma
atomic emission spectroscopy (ICP–AES), neutron activation analy-
sis (NAA), are either expensive or difficult to work with. Hence the
ability to analyze this element in different matrices is of great
importance.
chemical shift (d ppm), 14.80 (s, 2H, OAH), 9.56 (s, 2H, CH@N),
6.43–8.12 (m, 14H, H-aromatic). 13C NMR (400 MHz) chemical
shift (d ppm): 163.1 (AC@N), 162.0 (AO phenolic), 119.8, 134.4,
123.4, 134.1, 122.5 (benzene), 153.9, 117.2, 130.6, 128.8, 138.3,
122.6 (naphthalene).
Apparatus
All fluorescence measurements were carried out on a Perkin–
Elmer L50B luminescence spectrometer (USA). A corning ion ana-
lyzer 250-pH/mV meter (USA) was used for the pH measurements.
Membrane preparation
The membrane solutions were prepared by thoroughly dissolv-
ing 25 mg of powdered PVC, 62 mg of plasticizer DBP, 5 mg of addi-
tive NaTPB and 8 mg of fluorogenic ligand L in 3 ml THF. A volume
of 0.2 ml of this solution was then pipetted and spread onto a
7 mm ꢁ 50 mm dust-free quartz plate located in a THF-saturated
desiccators. In this way, a membrane of approximately 2–5 lm
thickness was coated on the plate, and was allowed to stand in
ambient air before use.
Fluorescence measurements
In recent years, we have prepared a number of fluorimetric bulk
optode membranes based on novel fluoroionophores for selective
determination of trace amounts of Ag+ [19], Ni2+ [20], Tb3+ [9],
Er3+ [21], Dy3+ [22] and Hg2+ [23] ions. In this work, we found that
the strong fluorescence of N,N-bis(salicylidene)-naphthylene-1,8-
diamine (L), Scheme 1, in a PVC membrane (kex = 300 nm and
kem = 440 nm) quenched considerably, and in a selective manner,
upon complexation with Ho3+ ion. Thus, we decided to probe the
fluoroioophore L in PVC membranes for selective sensing of traces
of holmium(III) in aqueous solutions. L is a symmetric Schiff’s Base
with semi-cavity which can be a suitable selectophore [24,25].
The membranes were placed diagonally inside the sample cuv-
ette of the instrument containing 3 ml buffer solution of pH 5.4.
The fluorescence intensity, at an excitation wavelength of
300 nm, was measured at 440 nm. Then, the sample was titrated
with standardized holmium ion solutions and the fluorescence
intensity of the system was measured after ꢃ40 s, required to
reach the equilibrium.
Results and discussion
Preliminary studies
To evaluate whether L could be used as a selective fluorescent
chemosensor for Ho(III), we recorded it is fluorescence emission
spectra variations upon addition of increasing amounts of metal
ion in an acetonitrile solution of the ligand (5 ꢁ 10ꢂ6 M) at
25.0 0.1 °C. The shape and position of the fluorescence emission
band (kem = 382 nm) did not change in the presence of other ions
compared to that of the free ligand, whereas emission intensities
sharply decreased as a function of Ho3+/L molar ratio according
to the curve reported in Fig. 1.
A rather pronounced chelation quenching of the fluorescence
(CHEQ effect) is observed for L upon addition of increasing the
quantity of Ho3+ ion, which begins to level off at a ligand/metal
ion molar ratio of 1. From such a sharp inflection point at a molar
ratio of 1, it can be immediately concluded that a 1:1 [HoL]3+
complex cation is formed in acetonitrile solution. The formation
Experimental
Reagents
Reagent grade benzyl acetate (BA), dibutyl phthalate (DBP),
nitrobenzene (NB), sodium tetraphenyl borate (NaTPB), high rela-
tive molecular weight PVC, tetrahydrofuran (THF) and acetonitrile
(AN) were purchased from Merck chemical company (Germany,
purchased from a local company) at the highest purity available
and used without any further purification except for vacuum dry-
ing. Doubly distilled deionized water was used throughout the
experiments. pH was adjusted to 5.4 by a acetate buffer solution
and diluted to mark with double distilled water.
Synthesis of fluoroionophore L
The fluorogenic reagent L was prepared as follows: To an etha-
nolic solution (50 ml) of salicylaldehyde (3.09, 25.28 mmol) was
added an ethanolic solution (20 ml) of 1,8-diaminonaphthalene
(2 g, 12.64 mmol) drop by drop with stirring. A yellow compound
separated out during mixing at room temperature. The mixture
was heated under reflux on a water bath for 30 min and was cooled
to room temperature. The yellow precipitates separated were suc-
tion filtered, washed with ethanol and dried under vacuum.
F.W = 366.33, Yield 54%, color green, mp = 161 °C (decomposed).
Selective IR bands (cmꢂ1), KBr pellets, (OAH, C@N, CAO), 3355,
1607 (s), 1249. Anal. calcd. for C24H18N2O2: C, 78.64; H, 4.93; N,
7.62. Found: C, 78.45; H, 4.84; N, 7.94. 1H NMR (400 MHz)
H
H
N
N
OH
HO
L
Scheme 1. Chemical structure of L.

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700
600
500
400
300
200
100
0
900
800
700
600
500
400
300
200
100
0
10-10
10-3
300
350
400
450
500
550
600
0
0.5
1
1.5
Wavelength(nm)
M
n+/L
Fig. 2. Emission (Em) spectra of the holmium-selective optical sensor (the PVC
Tb
Co
Ce
Zn
Gd
Cu
La
K
Ho
Pr
Er
membrane composition used is that given for membrane No. 4) in the presence of
varying concentration of Ho3+ ion in acetate buffer of pH 5.4: (1) 0.0, (2) 1.0 ꢁ 10ꢂ10
,
Yb
(3) 1.0 ꢁ 10ꢂ9, (4) 1.0 ꢁ 10ꢂ8, (5) 1.0 ꢁ 10ꢂ7, (6) 1.0 ꢁ 10ꢂ6, (7) 1.0 ꢁ 10ꢂ5, (8)
1.0 ꢁ 10ꢂ4, (9) 1.0 ꢁ 10ꢂ3 M.
Fig. 1. Fluorescence intensity vs. [Mn+]/[L] mole ratio plots in AN solution for
different metal ions.
The emission (at kex = 300 nm) spectra of the proposed fluores-
cent sensor in an acetate buffer of pH 5.4 and in the presence of
increasing concentrations of holmium ions are shown in Fig. 2.
As it can be seen from Fig. 2, the intensities are decreased consid-
erably upon increasing concentration of the holmium ion without
no measurable spectral position shifts, as a result of the strong and
selective quenching effect of Ho3+ ions. It is noteworthy that, as ex-
pected, the emission wavelengths observed for the membrane sen-
sor (kem = 440 nm) are quite different from those observed for L in
acetonitrile solution (kem = 382 nm). Similar observations have al-
ready been reported in [26].
constant of the resulting complex was evaluated by fitting the fluo-
rescence intensity/[Ho3+] molar ratio data to a 1:1 model using a
nonlinear least-squares curve-fitting program, as described else-
where [19], and results (Table 1) showed that; log Kf = 6.22 0.32
for Ho3+, 4.31 0.22, for Cu2+, 3.91 0.12 for Zn2+, 2.64 0.32 for
Ce3+ and <2 for other metal ions used. Thus, based on the relative
stabilities of resulting complexes, the L molecule was expected to
act as a selective fluoroionophore in the preparation of a PVC-
based ion-selective optode for Ho3+ion.
Response mechanism
Measuring principle
The proposed fluorescent membrane sensor was prepared by
incorporation of the lipophilic fluoroionophore L, in a plasticized
PVC containing NaTPB as a lipophilic anionic additive. The fluoro-
ionophore possesses unique binding properties to Ho3+ ions and
its fluorescence is strongly and selectively quenched as a result
of the binding event. The lipophilic anionic sites, TPBꢂ, provide
the optode membrane with the necessary ion-exchange properties,
because the fluoroionophore acts as a neutral ligand and therefore,
cannot function as an ion exchanger. Under the experimental
conditions used and considering the formation of a 1:1 complex
between L and Ho3+ ions in the organic phase, the response mech-
anism of the proposed optical system can be explained by the fol-
lowing ion-exchange pathway [17,20,9,21]:
Based on the above mentioned response mechanism of the pro-
posed optical sensor, can be explained as follows. As a result of the
establishment of complexation equilibrium between Ho3+ ion in
the aqueous sample solution (A) and the fluoroionophore L in the
organic membrane phase, a AaLn complex can be formed:
Ke
aAaq þ nLðorgÞ $ AaLnðorgÞ
½AaLnꢄðorgÞ
Ke ¼
ð1Þ
a
n
½AꢄðorgÞ½LꢄðorgÞ
where a and n indicate the complexing ratio between Ho3+, and L
and Ke is the equilibrium constant of the overall reaction. It should
be noted that the extent of ion-pairing of holmium nitrate in aque-
ous solutions over the low concentration ranges studied, is
negligible.
LðorgÞ þ 3NaþðorgÞ þ 3TPBꢂðorgÞ þ Hoþ3ðaqÞ
¼ HoLþ3ðorgÞ þ 3TPBꢂðorgÞ þ 3NaþðaqÞ
By definition, the relative fluorescence intensity
a is the ratio of
uncomplexed L in the membrane phase, [L](org), to its total amount,
[Lt](org), so that:
Table 1
The formation constants of L–Mn+ complexes at 25.0 0.1 °C.
Ion
Log Kf
Ion
Log Kf
Ion
Log Kf
½LꢄðorgÞ
K+
<2.0
<2.0
4.31 0.22
3.91 0.12
6.22 0.31
Pr3+
Er3+
Tb3+
Gd3+
La3+
<2.0
<2.0
<2.0
<2.0
<2.0
Ce3+
2.64 0.32
a
¼
ð2Þ
ð3Þ
½LtꢄðorgÞ
Co2+
Cu2+
Zn2+
Ho3+
1 ꢂ
a
½AaLnꢄðorgÞ ¼ ½LtꢄðorgÞ
n

M.R. Ganjali et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 224–229
227
emphasizing the expected increase in sensitivity of the optical sen-
sor in the presence increased amount of the fluoroionophore in the
membrane phase. It should be noted that the use of 8% L in the
preparation of membrane sensors caused a very intense fluores-
cence, which fell out of the detection range of the instrument used.
In practice,
a
value during titration with Ho3+ ions can be deter-
mined by the fluorescence intensity measurement of the optical
sensor, FI:
FI ꢂ FI0
a
¼
ð4Þ
FIa ꢂ FI0
Effect of pH
where FI0 and FI are the limiting fluorescence intensities of the
a
fluorescent intensities of the fluorescent membrane sensor at
Fig. 3 shows the influence of the pH test solution on the fluores-
cence response of the proposed holmium-selective optical sensor.
a
= 0 (i.e., totally complexed L) and a = 1 (i.e., uncomplexed L),
respectively. The relationship between the a-value and the concen-
The fluorescence intensity measurements were made for
a
tration of Ho3+ ion in aqueous sample solution, [A](aq), can be ob-
10 ꢁ 10ꢂ5 M Ho3+ solutions at different pH values. As it is seen,
the response of the sensor is independent of the pH of the test solu-
tion in a range of 4.0–8.4, adjusted by either HCl or NaOH. Beyond
this pH range, the optical response decreased. The diminished re-
tained by combining Eqs. (2) and (3), as follows:
an
1 ꢂ
1
¼
a
ð5Þ
nꢂ1
a
nKe½LtꢄðorgÞ½AꢄðorgÞ
sponse at pH <
w40 could be due to the extraction of H from the
aqueous solution into the PVC-membrane, via protonation of the
nitrogen atoms of L, resulting in an expected change in the mobil-
Eq. (5) can then be used as a basis for the quantitative determi-
nation of holmium ions, using the proposed optical membrane.
ity of
p-electrons in the system. On the other hand, the diminished
optical response of the sensor at pH >
w
84 could be due to the
hydroxide formation of holmium ions as well as a possible slight
swelling of the polymeric film under alkaline conditions of solu-
tion. Thus, in subsequent experiments, a solution of pH 5.4 ad-
justed by a 0.05 M buffer solution, was used for further studies.
Effect of the membrane composition
The response characteristics and working concentration range
of optical membrane sensors is well known, largely by changing
the membrane composition [17,24].
Measuring range and dynamic response time
In bulk liquid membrane optodes, a mass transfer of analyte
from the sample solution into the membrane is required in order
to facilitate the establishment of a thermodynamic equilibrium be-
tween the membrane and the sample. Thus, in the proposed hol-
mium-selective fluorimetric membrane sensor containing L as a
neutral fluoroionophore, the presence of a lipophilic anionic addi-
tive like NaTPB was found to be necessary to facilitate the corre-
sponding ion-exchange equilibrium. In the optical sensors
prepared, the absence of NaTPB not only affected the working con-
centration range of the sensor (Table 2), but also caused its pro-
longed response time and reduced selectivity [17].
The presence of 61–65% plasticizer in the prepared PVC mem-
branes resulted in optimum physical properties, as well as rela-
tively high motilities of their constituents [17]. It is well-known
that in addition to the amount of plasticizer, its nature will also lar-
gely influence the measuring concentration range of the solvent
polymeric sensors, as well as their selectivity coefficients [17,27–
31]. Table 2 displays among three different solvent mediators used,
NB with higher polarity than DBP and BA exhibits better sensitiv-
ity. This is due to the NB ability as a polar solvent to extract hol-
mium ions with relatively high charge density from the aqueous
solution to the organic membrane phase thus, DBP was chosen as
the solvent mediator.
Fig. 4 displays the optical response of the Ho3+-selective fluores-
cent optode to various concentrations of Ho3+ ion under optimal
experimental conditions at pH 5.4. As it is obvious, this curve can
be suitably used as a calibration curve for the determination of
Ho3+ ions in a concentration range of 1.0 ꢁ 10ꢂ10–1.0 ꢁ 10ꢂ3 M.
The limit of detection (LOD) of the sensor determined from the
intersection of the two extrapolated segments of the calibration
plot was 6.3 ꢁ 10ꢂ11 M [17].
The dynamic response time of the present optode is controlled
by the time required for the analyte to diffuse from the bulk of the
solution toward the membrane interface to associate with ligand.
The response of the membrane was found to reach equilibrium
within 40 s, over the entire calibration concentration of holmium
ion.
Reproducibility, reversibility, short-term stability and life time
The regeneration of the proposed membrane sensor was stud-
ied in by the use of different concentration EDTA. It was found that,
1.2
The data given in Table 2 is also indicative of the pronounced
influence of the amount of fluoroionophore on the working range
of the proposed optical sensor. As is obvious, an increase in the
amount of L from 1% to 8% resulted in an improved lower limit
of the working range from 10ꢂ6 to 10ꢂ10 M, respectively,
(A)
0.9
0.6
Table 2
(B)
Optimization of the membrane ingredients for Ho(III) optode.
Optode no.
Composition (wt%)
Linear range (M)
0.3
0
L
NaTPB
Plasticizer
PVC
1
2
3
4
7
6
7
1
1
4
5
8
8
8
0
1
2
3
5
5
5
NB,69
NB,68
NB,64
NB,62
NB,62
DBP,62
BA,62
30
30
30
30
25
25
25
1.0 ꢁ 10ꢂ3–1.0 ꢁ 10ꢂ5
1.0 ꢁ 10ꢂ2–1.0 ꢁ 10ꢂ5
1.0 ꢁ 10ꢂ2–1.0 ꢁ 10ꢂ6
1.0 ꢁ 10ꢂ2–1.0 ꢁ 10ꢂ8
1.0 ꢁ 10ꢂ3–1.0 ꢁ 10ꢂ10
1.0 ꢁ 10ꢂ3–1.0 ꢁ 10ꢂ5
1.0 ꢁ 10ꢂ2–1.0 ꢁ 10ꢂ6
2.6
4.6
6.6
8.6
pH
Fig. 3. Effect of pH of the test solution on the optical response of the holmium ion-
selective sensor in the absence (A) and presence of 1.0 ꢁ 10ꢂ5 M Ho3+ (B).

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M.R. Ganjali et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 224–229
Table 3
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Interference of different metal ions to the fluorescence determination of Ho3+ ion with
the proposed membrane sensor.
Mn+
Relative error (%)
Mn+
Relative error (%)
La3+
Ce3+
Dy3+
Sm3+
Gd3+
Eu3+
Lu3+
Yb3+
Pr3+
3.1
2.0
1.9
2.2
3.3
2.3
4.0
2.8
2.3
4.0
Cr3+
Ni2+
Co2+
Cd2+
Cu2+
Pb2+
Ca2+
Sr2+
Na+
1.9
2.0
2.8
2.6
4.9
2.8
1.2
2.0
1.1
Nd3+
Table 4
The Ho3+ ions recovery from binary mixtures by the designed Ho3+-selective optode.
Ho3+ (ppm)
Added cation (ppm)
Recovery (%)
11
9
7
5
3
20
20
20
20
20
20
20
20
20
20
20
20
20
Li+, 100
100.2a 0.5
99.8 0.3
99.9 0.4
100.9 0.2
99.5 0.7
100.3 0.2
101.4 0.3
101.1 0.4
98.2 0.3
98.6 0.6
101.3 0.4
101.2 0.1
101.4 0.4
pHo 3+
Na+, 100
Ca2+, 100
Mg2+, 100
Lu3+, 100
Pr3+, 100
Co2+, 100
Zn2+, 100
Cu2+, 100
Tb3+, 100
Sm3+, 100
Ce3+, 100
Er3+, 100
Fig. 4. Relative fluorescence intensity vs. pHo3+. The curve fitting of the experi-
mental data was carried out based on Eq. (5).
in the presence of 0.001 M EDTA, the optode membrane will be
regenerated completely within a short regeneration time period
of 10–20 s. After regeneration and for the next Ho3+ ion concentra-
tion measurement, the membrane should be kept in a buffer solu-
tion of pH 5.4.
The reproducibility and reversibility are two important charac-
teristics in evaluating the suitability of an optical sensor for selec-
tive determination of ion of interest. These characteristics were
studied by fluorescence intensity measurements of the same mem-
brane sensor when it was exposed to repeated concentration step
changes between 1.0 ꢁ 10ꢂ7 and 1.0 ꢁ 10ꢂ6 mol Lꢂ1 at pH 5.4. The
coefficient of variation of 5 determinations for the 1.0 ꢁ 10ꢂ7 and
1.0 ꢁ 10ꢂ6 mol Lꢂ1 holmium were found to be 1.5 and 0.95,
respectively. The results indicated that the proposed sensor pos-
sess good repeatability and reversibility.
The short-term stability of the optode membrane was investi-
gated by its fluorescence intensity measurements in contact with
a 1.0 ꢁ 10ꢂ5 M solution of Ho3+ in a buffer solution of pH 5.4 over
a period of 5 h. From the fluorescence intensities taken every
30 min (n = 10), it was found that the response was almost the
same, with only 1.9% increase in intensity after 5 h monitoring.
In addition, it was found that the membrane sensor could be
stored in wet conditions without any measurable change in its
fluorescent intensity for at least 9 weeks, which implies that the
fluoroionophore L is quite stable in a membrane in contact with
water. Thus, the membrane sensor was immersed in a buffer solu-
tion of pH 5.4 when not in use.
a
Results are based on three measurements.
selectivity coefficients, Ksel, is employed to describe this phenome-
non. To investigate the selectivity of the optical membrane sensor
proposed, the fluorescence intensity of a fixed concentration of
Ho3+ (i.e., 1.0 ꢁ 10ꢂ8 M in a solution buffered at pH 5.4 was re-
corded before (FI0) and after adding the interfering ion at a
1.0 ꢁ 10ꢂ6 M level (FI) into the holmium ion solution. The resulting
relative error is defined as RE(%) = [(FIꢂFI0)/FI0] ꢁ 100. The results
of selectivity studies are summarized in Table 3.
The data given in Table 3 indicate that in the case of all ions
studied, the relative error is less than 5%, which is recognized as
tolerated. The results obtained indicated that the proposed optical
sensor can be applied to the determination of traces of Ho3+ ion in
real samples, in the presence of several other co-existing cationic
species.
Analytical applications
Due to the high selectivity and the very low detection limit of
the constructed Ho3+ sensor, it was applied for the monitoring of
the Ho3+ ions concentration in various binary mixtures and the
resulting data are given in Table 4. As it can been seen from the ta-
ble the recoveries of the Ho3+ ions in all mixtures are in the range
Selectivity
In the ion-selective optical sensors, the existence of the interfer-
ing ions affects their response behavior. For this reason, the term of
Table 5
Determination of holmium ion in tap water and river samples with the proposed optode.
Sample
Added (ppm)
Found optodea (ppm)
Relative error (%)
Tap water
10
5
(9.2a 0.2)
(5.3 0.2)
8.0
6
River water
10
5
(10.7 0.3)
(5.2 0.2)
7
4
a
Results are based on three measurements.

M.R. Ganjali et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 224–229
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mium selectivity exhibited by the proposed optode.
Another analytical application for the proposed optode was low
level monitoring of the holmium ion concentrations in spiked
water samples. The 10.0 ml of each water sample (tap and river
water samples, Tehran, Iran) was taken and the different amounts
of holmium ions (5, 10 ppm) are added to water samples, then di-
luted with distilled water in a 25.0 ml volumetric flask. The differ-
ent amounts of holmium ions (5, 10 ppm) are added to water
samples. The holmium contents of the water samples was deter-
mined by the proposed optode using the calibration method. The
results obtained with the sensors are summarized in Table 5. It
was found that the accuracy of holmium detection in different
solution samples is almost quantitative.
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Conclusions
In this work, a first efficient and selective fluorimetric optode
for holmium ion detection at low concentration levels is described.
It is based on the considerable selective quenching of the strong
fluorescence of N,N-bis(salicylidene)-naphthylene-1,8-diamine
(L), upon complexation with holmium ions. The proposed sensor
revealed good response characteristics in terms of stability, repro-
ducibility, calibration range, detection limit and selectivity. The
proposed fluorescent optode was successfully applied to the deter-
mination of Ho3+ ion in binary mixture and water samples.
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