Synthesis and properties of new N, N′ - phenyltetrazole podand

Article abstract of DOI:10.1515/chempap-2015-0209

The synthesis of a new chromogenic N,N′-phenyltetrazole receptor is reported here. The cationbinding properties of this receptor in solution were investigated by naked-eye colour change, electrochemical methods and UV-Vis spectroscopy in various solvents

Full text of DOI:10.1515/chempap-2015-0209

Chemical Papers 70 (3) 253–264 (2016)
DOI: 10.1515/chempap-2015-0209
ORIGINAL PAPER
ꢀ
Synthesis and properties of new , -phenyltetrazole podand
a
a
b
aAgnieszka Pazik* Beata Kamin´ska, Anna Skwierawska, Sandra Nakonieczna,
bTadeusz Ossowski
aDepartment of Chemistry and Technology of Functional Materials, Gdan´sk University of Technology,
Narutowicza St. 11/12, 80-233 Gdan´sk, Poland
bDepartment of Analytical Chemistry, Faculty of Chemistry, University of Gdan´sk, Wita Stwosza St. 63, 80-952 Gdansk, Poland
Received 1 April 2015; Revised 9 August 2015; Accepted 20 August 2015
The synthesis of a new chromogenic N,Nꢀ-phenyltetrazole receptor is reported here. The cation-
binding properties of this receptor in solution were investigated by naked-eye colour change, elec-
trochemical methods and UV-Vis spectroscopy in various solvents (CH3CN, dimethylsulphoxide
(DMSO), DMSO/H2O, CH3CN/H2O and CH3CN/MeOH). In addition, the receptor was used as
a sensing material in ion-selective membrane electrodes. The selectivity and sensitivity of these
electrodes towards alkali, alkali-earth, transition and heavy metal cations in aqueous solution were
tested. The relation between the carrier structure and membrane plasticisers was investigated.
c
ꢀ 2015 Institute of Chemistry, Slovak Academy of Sciences
Keywords: UV-Vis spectroscopy, bis-tetrazoles, ionophores, copper sensor, naked-eye detection
Introduction
Accordingly, the detection of Cu2+ ions is significant
in terms of the environment, medicine and biology. A
number of detection methods are time-consuming and
require high-quality analytical instrumentation, hence
it is important to design and synthesise a sensitive
chromogenic sensor for Cu2+ ions. Sensors that change
colour upon host–guest interaction can be used for
simple naked-eye applications without exploiting any
expensive equipment. Accordingly, in recent research,
a number of highly selective fluorescent chemosensors
(Suganya et al., 2014; Wang et al., 2014; Goswami et
al., 2013; Yu et al., 2013) and colorimetric sensors for
Cu2+ ion (Miao et al., 2013; Xu et al., 2011; Wang
et al., 2008; Huang et al., 2009) have been reported;
especially interesting are those reports that describe
ligands capable of working properly in an aqueous
medium (Noh et al., 2014; Zhang & Zhang, 2014; Huo
et al., 2014; Hrishikesan & Kannan, 2013; Kumar et
al., 2013; Chao et al., 2013).
The behaviour of chromogenic tetrazole derivatives
as potential sensing materials for transition metal ions
has not received attention in the literature to date.
This is due to the high instability (due to a dan-
gerously high nitrogen content) of tetrazole coupled,
e.g. with azo units. The design of new, environmen-
tally friendly, non-toxic and readily obtainable chro-
mogenic ligands applicable, for instance, to the visual
detection of targets such as lead, cadmium, zinc and
copper ions is a very important issue. Among soft
transition metal ions, Cu2+ even at low concentra-
tions, plays an essential role in enzymatic catalysis
and gene expression (Barceloux, 1999; Stern, 2010). A
high level of Cu2+ is toxic for microorganisms, plants
and humans (Georgopoulos et al., 2001). An excess of
Cu2+ causes Wilson’s disease (Ala et al., 2007) and
brain diseases such as Alzheimer’s (Gaggelli et al.,
2006; Brewer, 2007), Parkinson’s and prion (Millhus-
ter, 2004). Furthermore, the delivery of large doses of
Cu2+ ions by plants reduces chlorophyll biosynthesis.
This study sought a detailed synthesis of a new 4-
[1,5-bis[2-(1H-tetrazol-1-yl)phenyl(thio)]-3-azapent-
ane]-3ꢀ-nitroazobenzene (III ) receptor and an evalua-
*Corresponding author, e-mail: mikaga20@wp.pl
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
tion of the properties of III as a colorimetric sensor for
Cu2+ and a cation carrier in a membrane electrode.
As a result of the similarity of tetrazoles to carboxylic
acids (Herr, 2002; Holland & Pereira, 1967) reflected
in applications (Xue et al., 2005; Klapo¨tke et al., 2005;
Ogihara et al., 2004; Aromí et al., 2011), they could
form stable metal complexes with high thermal stabil-
ity. The binding ability of this receptor with Cu2+ ions
was studied using UV-Vis, IR spectroscopy, voltam-
perometric methods and naked-eye colour change.
Synthesis of 4-[1,5-bis[2-aminophenyl(thio)]-
3-azapentane]- 3ꢀ-nitroazobenzene hydro-
chloride (II)
A mixture of 2-aminothiophenol (0.21 mL, 2 mmol),
I (0.62 g, 1 mmol) and anhydrous potassium car-
bonate (0.27 g, 2 mmol) in dry dimethylformamide
(5 mL) was stirred at 100◦C. After 24 h the sol-
vent was removed under reduced pressure, the re-
sulting crude product was dissolved in methylene
chloride then washed with water. The extract was
dried with MgSO4 and concentrated under reduced
pressure. Bisamine(4-[1,5-bis[2-aminophenyl(thio)]-3-
azapentane]-3ꢀ-nitroazobenzene) was then dissolved in
methanol and hydrochloric acid (37 %, 0.1 mL) was
added to afford a red solid of II (0.61 g, 65 %), m.p.
178–182◦C.
Experimental
1H NMR spectra were recorded at 200 MHz and
500 MHz on a Varian (USA) instrument. UV-Vis mea-
surements were carried out with the use of a Unicam
UV 300 (UK) series spectrophotometer. The IR spec-
tra were recorded on a Genesis II FT-IR (Mattson,
USA) instrument. Elemental analyses were performed
on an Eager 200 (USA) apparatus. Electromagnetic
radional (EMF) measurements were carried out us-
ing a 16-channel Lawson Lab potentiometer (USA).
Thin layer chromatography (TLC) analyses were per-
formed on an aluminium sheet covered with silica gel
60 F254 (Merck, Germany). All the reagents and sol-
vents used were of analytical reagent grade and were
purchased from Aldrich (USA) and Merck. Aqueous
solutions were prepared from salts of the highest com-
mercial quality using deionised water (conductivity ≤
0.03 S cm−1), obtained from the Hydrolab (Poland)
purification system. All measurements were carried
out at ambient temperature. The melting points are
uncorrected.
Synthesis of 4-[1,5-bis[2-(1H-tetrazol-1-
yl)phenyl(thio)]-3-azapentane]-3ꢀ-
nitroazobenzene (III)
Compound II (0.6 g, 1 mmol), sodium azide
(0.19 g, 3 mmol) and triethyl orthoformate (0.49 mL,
3 mmol) were dissolved in acetic acid (100 %, 3 mL)
and the mixture was stirred at 80◦C for 5 h. Next, it
was cooled to ambient temperature and the resulting
solution was poured into water (20 mL) and brought
to pH 2 with HCl (37 %) to give the crude product.
Recrystallisation from water afforded pure compound
III as an orange solid (m.p. 65–70◦C, 0.54 g, 83 %
yield).
Membrane preparation
Synthesis of 4-[1,5-di[4-toluenesulphonyloxy]-
3-azapentane]-3ꢀ-nitroazobenzene (I)
The membrane components were: ionophore III
(3 mg, 0.004 mmol), poly(vinyl chloride) (PVC;
50 mg), tetradodecyloammonium tetrakis(4-chloro-
phenyl)borate (KTpClPB; 1 mg, 0.0008 mmol), 2-
nitrophenyl octyl ether (o-NPOE; 0.1 mL, 0.4 mmol)
or bis(n-octyl)sebacate (DOS; 0.13 mL, 0.3 mmol).
The membrane components (174 mg in total) were
dissolved in freshly distilled tetrahydrofuran (1.2 mL).
The solution was placed in a glass ring over a glass
plate. After evaporation of the solvent overnight, the
resulting membrane was peeled from the glass mould
and discs of 5 mm i.d. were cut from it. The membrane
was incorporated into Ag/AgCl electrode bodies of
IS type (Moeller S.A., Zurich, Switzerland). The elec-
trode bodies were filled with an internal filling solution
KCl (0.01 M) and conditioned in the same electrolyte
for 24 h. A double-junction Ag/AgCl, KCl 1 M refer-
ence electrode was used with 1 M CH3COOLi solution
in the bridge cell. The measurements were carried out
at 20◦C, using the following cell: Ag/AgCl/internal
electrolyte / membrane / sample / CH3COOLi / KCl/
AgCl/Ag and 16-channel potentiometer (16 EMF).
The selectivity coefficients were determined using
A diazonium salt of 3-nitroaniline was prepared by
dissolving 5.5 g (0.04 mol) of amine in hydrochloric
acid (37 %, 20 mL) and ice-cold water (80 mL). To
this, an ice-cooled solution of sodium nitrite (3.3 g,
0.048 mol) in water (10 mL) was added dropwise. The
mixture was stirred at 0◦C for 20 min then the di-
azonium salt solution was added dropwise to a solu-
tion of N-phenyldiethanoloamine (7.25 g, 0.04 mol)
in the presence of sodium hydroxide to pH 10. The
yellow precipitate (11.85 g, 89.7 %) was separated
by filtration and dried under vacuum. A suspension
of 4-diethanoloamine-3ꢀ-nitroazobenzene (4.95 g, 15
mmol) was prepared in dry pyridine (60 mL) and
cooled to 0◦C. To the stirred mixture, three portions of
4-toluenesulphonyl chloride (total 11.44 g, 60 mmol)
were added over a period of 30 min. The mixture was
maintained at 5◦C for 24 h. After the addition of ice,
the crude product precipitated. The pure product was
obtained by crystallisation from petroleum ether, with
a yield of 5.05 g (52.7 %) of orange solid, m.p. 148–
149◦C.
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255
the separate solution method (SSM) (Baker, 1997)
Voltammetry studies
and were calculated according to Eq.(1) at an ionic
strength of 0.01 M:
The electrochemical measurements were carried
out in a single-compartment three- electrode cell.
The potential was applied with an Autolab potentio-
stat/galvanostat PGSTAT30 (Eco Chemie B.V., The
Netherlands) controlled with General Purpose Elec-
trochemical System (GPES 4.9) software (The Nether-
lands). Platinum wire served as the counter electrode
and Ag/AgCl was used as the reference electrode and
gold (d = 2 mm) was used as the working electrode. A
solution of tetrabutylammonium perchlorate (TBAP;
0.1 M) in acetonitrile was used as the supporting elec-
trolyte. All the electrolyte solutions were purged with
argon prior to removing oxygen from the solutions.
The experiments were performed at ambient temper-
ature (20◦C).
EM0 − EC0 u
pot
Cu,M
log K
=
(1)
SCu
pot
Cu,M
where K
is potentiometric selectivity coefficient,
EM0 and EC0 u are the measured membrane potentials,
SCuis Nernstian electrode slope.
UV-Vis studies of metal cation complexation
All reagents and salts: Zn(ClO4)2 ·6H2O, Cu
(ClO4)2 ·6H2O, Ni(ClO4)2 ·6H2O, Pb(ClO4)2 · 6H2O,
Co(ClO4)2 · 6H2O, AgClO4 ·H2O and FeCl2 were pur-
chased from Aldrich and dried under vacuum. UV-
Vis titration was carried out by the addition of a
metal ion to solution III with an appropriate solvent:
CH3CN, dimethylsulphoxide (DMSO), DMSO/H2O,
CH3CN/H2O and CH3CN/MeOH. Titrations were
carried out in 1 cm path length quartz cuvette main-
taining the volume of the ligand solution constant
(2 mL). Titration step was 0.01 mL.
IR studies
An equimolar amount of III and CuCl2 (1 mmol)
was dissolved separately in small amounts of chloro-
form and methanol, respectively. The solutions were
then combined, stirred at ambient temperature for
Fig. 1. Synthetic route of receptor III.
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
Table 1. Data characterising newly prepared compound
wi(calc.)/%
wi(found)/%
Compound
Formula
Mr
C
H
N
S
III
C30H26N12O2S2
650.17
55.37
55.34
4.03
4.09
25.83
25.89
9.85
9.80
Table 2. Spectral data of compounds
Compound
Spectral data
I
IR, ν˜/cm−1: 3095, 3029, 2957, 2923, 1598, 1528, 1513, 1454, 1394, 1353, 1234, 1175, 1147, 1095, 1000, 977, 894, 816,
759, 663
1H NMR (500 MHz, CDCl3), δ: 2.41 (s, 6H, CH3), 3.71 (t, J = 5.9 Hz, 4H, CH2), 4.19 (t, J = 5.9 Hz, 4H, CH2),
6.54 (d, J = 9.3 Hz, 2H, HAr), 7.28 (d, J = 8.3 Hz, 4H, HAr), 7.69 (t, J = 8.3 Hz, 1H, HAr), 7.72 (d, J = 8.3 Hz,
4H, HAr), 7.84 (d, J = 9.3 Hz, 2H, HAr), 8.27 (dd, J1 = 9.3 Hz, J2 = 8.8 Hz, 2H, HAr), 8.7 (s, 1H, HAr
)
II
IR, ν˜/cm−1: 3363, 2949, 2924, 2869, 2853, 1596, 1556, 1512, 1505, 1478, 1472, 1454, 1447, 1392, 1348, 1312, 1279,
1251, 1142, 951, 877, 820, 752
1H NMR (200 MHz, DMSO-d6), δ: 3.05 (t, J = 6.6 Hz, 4H, CH2), 3.57 (t, J = 6.9 Hz, 4H, CH2), 6.49 (d, J =
9.0 Hz, 2H, HAr), 7.03 (t, J = 7.3 Hz, 2H, HAr), 7.19–7.31 (m, 4H, HAr), 7.52 (d, J = 7.4 Hz, 2H, HAr), 7.69 (d,
J = 8.9 Hz, 2H, HAr), 7.82 (t, J = 8.00 Hz, 1H, HAr), 8.23 (dd, J1 = 9.3 Hz, J2 = 8.7 Hz, 2H, HAr), 8.43 (s, 1H,
HAr
)
III
IR, ν˜/cm−1: 3459, 3337, 3130, 3071, 2927, 1693, 1597, 1524, 1511, 1489, 1456, 1393, 1348, 1279, 1245, 1200, 1144,
102, 995, 951, 819, 758, 673
1H NMR (500 MHz, CDCl3), δ: 2.91 (t, J = 7.3 Hz, 4H, CH2), 3.39 (t, J = 7.3 Hz, 4H, CH2), 6.45 (d, J = 9.3 Hz,
2H, HAr), 7.51 (d, J = 3.9 Hz, 4H, HAr), 7.55–7.61 (m, 2H, HAr), 7.63–7.65 (m, 3H, HAr), 7.84 (d, J = 8.8 Hz, 2H,
HAr), 8.22 (dd, J1 = 9.3 Hz, J2 = 7.8 Hz, 2H, HAr), 8.66 (s, 1H, HAr), 8.96 (s, 2H, Htetrazole
)
0.5 h and the solvent was evaporated to dryness. The
residue was dried under vacuum for 24 h and pro-
tected from moisture. The dry solid so obtained and
dry KBr were used in the preparation of tablets for
IR spectroscopy.
Spectrophotometric studies of metal cation
complexation
Previous studies revealed that 1,5-bis[2-(1H-tetra-
zol-1-yl)phenyl(thio)]-3-phenylazapentane forms
a
very unstable complex with Cu2+ ions resulting in
a momentary change in colour in MeOH solution
(Pazik & Skwierawska, 2013). In order to improve
signalling capacity the molecule was equipped with
two N-phenyltetrazole residues which were joined
by a di-(thiaethanol)amine unit bearing a N-(m-
nitroazobenzene) subunit. It is presumed that the
molecule that contains nitrogen, sulphur atoms, azo
group and tetrazole rings can be a selective colorimet-
ric sensor for Cu2+ ions.
Results and discussion
Synthesis
Receptor III was readily obtained in three steps
(Fig. 1). The intermediates I and II, were prepared
using routine procedures.
Compound I was synthesised by the diazo cou-
pling reaction followed by tosylation of the result-
ing product. The reaction of intermediate I with
2-aminothiophenol afforded compound II with a 65 %
yield. The final step was the formation of a tetra-
zole ring in the reaction of bis-amine II with ethyl
orthoformate and sodium azide in acetic acid (100 %).
The reaction occurred under mild conditions. The
yield of III at the final step was 83 %. The struc-
ture of the final product and intermediates were con-
firmed by 1H NMR, IR spectroscopy and elemen-
tal analysis. The composition and properties of the
corresponding products are summarised in Tables 1
and 2.
The cation-binding properties of receptor III were
1
investigated by UV-Vis and H NMR spectrophotom-
etry. No colour and spectral changes were observed in
the presence of alkali and alkaline earth metal cations
in CH3CN and DMSO. Subsequently, receptor III was
treated with transition-metal ions to evaluate the op-
tical response of the receptor in different solvents.
In the presence of metal ions such as Ni2+, Zn2+
,
Pb2+, Ag+, Co2+ and Fe2+ in CH3CN, in the absorp-
tion spectra of receptor III only an increase in the in-
tensity of the main band is observed (Fig. 2). The most
significant changes in the absorption were found in the
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Fig. 3. Naked-eye colour changes observed on III (c = 2.5 ×
10−5 mol L−1) in the presence of metal ions (c = 10−4
mol L−1) in CH3CN.
Fig. 2. Changes in absorption spectra of sensor III (c
=
2.5 × 10−5 mol L−1) (line 1) in CH3CN upon addi-
tion of Ag+, Co2+, Fe2+, Pb2+, Zn2+, Ni2+ (line 2)
and Cu2+ ions (c = 10−4 mol L−1) (line 3).
gradually increases then decreases with the concomi-
tant appearance of a red-shifted peak at around
515 nm (Fig. 4a). By increasing the amounts of Cu2+
,
presence of Cu2+ ions. The addition of Cu2+ ions to
the CH3CN solution of III causes a bathochromic shift
from 431 nm to 515 nm, followed by a solution colour
change from yellow to pink. When mixed solvents were
used, receptor III showed only a hypsochromic effect
toward Cu2+ with a larger extinction coefficient. The
naked-eye colour change in the receptor was only ob-
served in the case of the addition of Cu2+ ions to the
ligand solution in CH3CN (Fig. 3).
The absorption spectra of receptor III in the
presence of different concentrations of Cu2+ ions in
CH3CN are presented in Fig. 4. As the concentration
of Cu2+ increases (c = 0.00–4.28 × 10−5 mol L−1), the
absorption maximum of the free receptor at 431 nm
the colour of solution III changed from yellow to pink;
hence the receptor can be used as a naked-eye sensor
for Cu2+ ions. To determine the stoichiometry of re-
ceptor III and Cu2+ ions in the complex, Job’s method
was employed by keeping the total concentration of
metal ions and III (c = 5.0 × 10−5 mol L−1). The
Job plot and a molar ratio plot under titration ex-
periments (Fig. 4b) confirmed the 1 : 1 stoichiometry
of the complex. The association constant of III · Cu2+
in CH3CN was determined as (3.5 0.28) × 102 M−1
using the Benesi–Hildebrand plot (Benesi & Hilde-
brand, 1949).
Changing the solvent from CH3CN to dipolar apro-
tic DMSO caused an adverse influence on the cation-
Fig. 4. Changes in absorption spectra upon titration of III (c = 2.5 × 10−5 mol L−1) with Cu(ClO4)2 (c = 0.00–4.28 × 10−5
mol L−1) (lines 1–18) in CH3CN (a). Titration plot to determine stability constant of the complex between III and Cu2+
(1 : 1) at 515 nm (b).
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
Table 3. Absorption spectroscopic data of receptor III and its Cu2+ complex in various solvents
Solvent
Complex stoichiometry (L : M)
λmax [nm]
∆λa [nm]
CH3CN
DMSO
DMSO/H2O (7 : 3)
MeOH/CH3CN (7 : 3)
CH3CN/H2O (7 : 3)
1 : 1
1 : 1
1 : 1 and 1 : 2
2 : 1
515
450
448
427
430
84
–
–
–
–
1 : 1 and 2 : 1
a) ∆λ = λcomplex – λhost
.
Fig. 5. UV-Vis titration of III (c = 2.5 × 10−5 mol L−1) with Cu(ClO4)2 (c = 0–4.59 × 10−5 mol L−1) (lines 1–18) in DMSO
(a). Titration plot to determine stability constant of the complex between III and Cu2+ (1 : 1) at 455 nm (b).
binding properties. In DMSO, the spectral changes
upon spectrophotometric titration with Cu2+ perchlo-
rate with receptor III are exemplified in Fig. 5. During
the titration, the colour of the receptor solution deep-
ened with the strengthening band at 450 nm. Analysis
of the Job’s plot and a molar ratio plot suggests the
formation of a 1 : 1 III complex with Cu2+ ions. The
association constant of the receptor with metal ions in
DMSO derived from the Benesi–Hildebrand equation
gest the solvent to be essential for complexation prop-
erties. These mixed systems may be involved in hy-
dration of the ligand or cis–trans isomerisation of the
azo unit. Photo-induced isomerisation generates struc-
tural changes which could force a different conforma-
tion that enhances or worsens the properties of sensor.
In order to confirm this assumption, the 1H NMR and
UV-Vis spectra of ligand III under the influence of
light were recorded in CH3CN. A sample of the lig-
and was illuminated at λmax = 360 nm for one hour.
The changes in the UV-Vis spectrum show a continu-
ous increase in the main band of ligand III and a slow
deepening of the colour of the solution. Moreover, no
colour and chemical shifts (δ) for the ligand and their
changes after the illumination were observed in 1H
NMR spectra. These indicate that the isomerisation
of the azo unit was not engaged in the complexation
of Cu2+ ions and change in colour of III solution from
yellow to pink.
is (2.3
0.10) × 102 M−1 which is lower than that
obtained for the CH3CN solution.
Due to the low solubility of receptor III in water
and MeOH, further measurements were carried out
in mixed solvents (DMSO/H2O, CH3CN/H2O and
CH3CN/MeOH). Changes in the absorption spectra
of receptor III and its Cu2+ complex in various sol-
vents are collated in Table 3. A mole ratio plot re-
vealed that under titration experiments in the case
of mixed solvents with water: CH3CN/H2O (7 :3)
and DMSO/H2O (7 : 3), two pairs of complexes
(III/Cu2+); 1 : 1 and 2 : 1, and 1 : 1 and 1 : 2, re-
spectively, exist. A different model of equilibrium for
III was found in MeOH/CH3CN. During titration of
III with Cu2+, an increase in the main band and only
the formation of (III )2 ·Cu2+ complex were observed.
The results from the UV-Vis titrations may sug-
Voltammetry studies
It is recognised that copper ions Cu2+ in ace-
tonitrile solution are relatively poorly solvated in
comparison with cuprous ions Cu+ (Marcus, 1985).
It was observed that Cu2+ ions in the acetonitrile
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
259
cess. For a high ligand III/Cu2+ > 3 concentration no
electrochemical process on gold electrode in acetoni-
trile solution was effectively observed.
The electrochemical inactivity of copper in the
presence of III may indicate more complicated equi-
libria polymeric forms with a relatively high stability.
IR spectroscopic studies
To support the Cu2+ ions complexation model, the
IR spectroscopy studies were performed. Fig. 7 shows
part of the IR spectrum of receptor III before and after
the addition of Cu2+ ions. Complexation can be seen
as the loss of a stretching vibration at 1693 cm−1 and
new signals were found at 1434 cm−1 and 1259 cm−1
which are the characteristic combinations of the bond-
stretching vibrations typical of coordinated tetrazole
rings. Upon the addition of metal ions, the IR peaks
Fig. 6. Cyclic voltammograms of cuprous acetonitrile solution
(c = 2.0 × 10−3 mol L−1) in 0.1 M TBAP at Au elec-
trode and with III (line 1), mole ratio III/Cu2+ 1 : 10
(line 2) and III/Cu2+ 3 : 1 (line 3).
—
corresponding to the N N units, NO group and
—
2
—
C
N bonds were shifted. The strong absorptions at
—
solution showed two single-electron reduction pro-
cesses on the gold electrode (Fig. 6). The first elec-
trode reaction Cu2+ → Cu+ appeared as a reversible
electron reduction–oxidation process at the potential
E[(pc+pa)/2] = 0.973 V obtained from the well- devel-
oped (Epa) and cathodic (Epc) peak potentials. The
second electrochemical process Cu+ → Cu(0) was ob-
served as more complicated process with the reduction
peak at Epc = –0.55 V and oxidation Epa = –0.30 V.
The complexity of this process is associated with the
deposition of copper in the zero-oxidation state which
implies the final electrochemical step.
The introduction of small amounts of the ligand
to the copper solution resulted in a gradual disap-
pearance of the current peaks of the Cu2+ → Cu+
process up to the mole ratio of III/Cu = 0.1. The sec-
ond reduction/oxidation process Cu+ → Cu(0) varies
slightly with the presence of the ligand. A further in-
crease in the concentration of the ligands leads to dis-
appearance of the second reduction and oxidation pro-
1511 cm−1, 1393 cm−1 and 1143 cm−1 were shifted to
1536 cm−1, 1384 cm−1 and 1181 cm−1, respectively.
Molecule rotations due to the strong intermolecular
interaction of the metal ions cause the appearance of
the additional large vibration at 562 cm−1 character-
istic of the deformation of the substituted tetrazole
rings and the flexible part of the molecule, which is a
chromogenic unit. Complexation can be observed also
as the loss of the stretching vibration at 3130 cm−1
and 3078 cm−1 and the new large peak can be found
below 3381 cm−1. The occurrence of the new peak at
3170 cm−1 confirms the formation of complex bonds
(Fig. 8).
The most probable structure of complex III with
Cu2+ ions is presented in Fig. 9. The incorporation of
a chromophoric N-(m-nitroazobenzene) subunit and
tetrazole rings into aliphatic chains makes it possible
to create a potential sensitive and complexing com-
pound. Additionally, the presence of nitrogen and sul-
phur atoms pre-organises the molecule into a cavity
Fig. 7. IR spectra (KBr) of pure ligand III (a) compared with complex III · Cu2+ (b).
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
Fig. 8. MIR spectra of free III (a) and its complex with Cu2+ ions (b).
Fig. 9. One possible structure for complex III in presence of Cu2+ ions.
with at least five possible donor atoms which can inter-
In the preliminary experiments, the potentiomet-
act with Cu2+ ions. Cu2+ ions can be located at differ-
ent sites within the cavity which is formed between the
tetrazole rings and aliphatic chains. The MIR spectra
confirm the coordination of the two tetrazole rings to
Cu2+ ions.
ric response properties of the polymeric membranes
ion-selective electrodes doped with compound III were
investigated towards various cations. The electrode re-
sponse depends mainly on properties and amounts of
ionophore and type of plasticiser. Due to this, two
different plasticisers, o-NPOE and DOS, were used.
The ion-selective membranes consisted of 1.7 mass %
ionophore, 0.6 mass % KTpClPB 69 mass % plasticiser
and 28.9 mass % PVC. The membrane components,
PVC/o-NPOE and PVC/DOS, are 174 mg in total.
Membrane electrodes and potentiometric measure-
ments
Previous studies found that the N,Nꢀ-phenyltetra-
zole podand containing aliphatic chains with sulphur
The electrodes examined responded to the ions (Cu2+
Zn2+, Pb2+, Ni2+, Fe2+, Co2+, Cd2+, Mn2+, Ag+)
in the concentration range of 10−5–10−2 mol L−1
,
and oxygen atoms was Pb2+-selective (logKPb,Cu
=
–2) (Pazik & Skwierawska, 2012). Furthermore, the
incorporation of amide groups in tetrazole dipo-
dal and tripodal ligands enhanced Pb2+ selectivity
(logKPb,Na = –4.39) (Pazik & Skwierawska, 2014).
The current studies were performed to ascertain
whether combining the m-nitroazobenzene subunit
with N,Nꢀ-phenyltetrazole podand had an influence
on the ionophoric properties. The existence of the two
donating sulphur atoms as well as the donating ni-
trogen niche with tetrazole rings was expected to be
selective for Cu2+ ions.
.
The limit of detection of the electrodes was below
10−6 mol L−1. The ion-selective membrane electrodes
doped with III displayed the more sensitive responses
to Cu2+ than to other metal ions. In both membranes,
the electrode slopes for Cu2+ were found close to the
Nernstian value: 28.7 mV per decade (PVC/o-NPOE)
and 29.0 mV per decade (PVC/DOS). The potential
responses of the electrodes to Co2+, Zn2+, Pb2+, Fe2+
,
Mn2+ and Cu2+ are shown in Figs. 10a and 10b. The
selectivity of the potentiometric response of electrode
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A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
261
Fig. 11. Selectivity coefficient, determined as logKCu,M by the
SSM for electrodes with two different plasticisers: o-
NPOE and DOS.
nated. The best value of the selectivity coefficient for
this membrane was for Zn2+ (logKCu,Zn = –2.35). The
electrodes with a polar ether-type plasticiser exhib-
ited high sensitivity and better potentiometric char-
acteristics with a Nernstian slope in a wide linear con-
centration range. Membranes with a ester-type plas-
ticiser exhibit a higher affinity towards Pb2+ than
Cu2+ (logKCu,Pb = 0.42), whereas membranes with
PVC/o-NPOE are more selective towards Cu2+ than
Pb2+ (logKCu,Pb = –0.31). Moreover, membranes with
PVC/DOS exhibit a higher affinity towards Fe2+ than
Cu2+ (logKCu,Fe = 1.19), while a membrane with
Fig. 10. Potential response of III/o-NPOE (a) and III/DOS
(b) membrane ion-selective electrodes to various metal
cations.
III with o-NPOE and with DOS for the cations exam-
ined decreased in the order Cu2+, Pb2+, Fe2+, Co2+
Mn2+, Zn2+ and Fe2+, Pb2+, Cu2+, Zn2+, Co2+
Mn2+, respectively.
,
,
Furthermore, the response time of electrodes was
found to be around 1–3 s, while the recovery time
exceeded 10–20 min.
The selectivity coefficients were determined by
the SSM, by using the EMF values for the highest
recorded ion activities belonging to the linear response
range for a given ion. The calibration of the electrodes
was performed for various cations, starting preferably
from the most discriminating. The values of the se-
lectivity coefficients (logKCu,M) are presented in Ta-
ble 4 and Fig. 11. The results show the transition
metal cations to be well discriminated. A compari-
son of the selectivity of membranes with plasticisers
of different polarity leads to substantial changes in the
complexation properties. The selectivity coefficients
are more favourable when using more polar, PVC/o-
PVC/o-NPOE is more selective for Fe2+ (logKCu,Fe
=
–0.81). This might suggest a stronger affinity of Fe2+
ions than other cations to a less polar sebacate. Both
electrodes suffer from a strong interference from Ag+
ions, which are highly favoured in both cases. The
selectivity coefficients determined for PVC/o-NPOE
with Ag+ (logKCu,Ag = 4.12) are lower than for mem-
branes with PVC/DOS (logKCu,Ag = 6.36).
The characteristic features of the newly developed
ISE were compared with those of the other Cu2+-ISEs
reported in the literature. The details are given in Ta-
ble 5. The properties of the proposed electrode were
compared with ion-selective electrodes based on cal-
ixarenes, tertiophene, Schiff’s bases, acyclic di-peptide
pyridine derivatives and others.
NPOE membranes. It can be seen that the Zn2+
Mn2+ and Co2+ were among the ions most discrimi-
,
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262
A. Pazik et al./Chemical Papers 70 (3) 253–264 (2016)
Table 4. Characteristics and potentiometric selectivity coefficient as logKCu,M of electrodes studied with ionophore III
o-NPOE
S [mV per decade]
DOS
S [mV per decade]
Ion (Mn+
)
logKCu,M
logKCu,M
Co2+
Zn2+
Ni2+
Pb2+
Fe2+
Cd2+
Mn2+
Ag+
24.5
23.7
27.2
26.9
30.5
29.0
23.6
45.1
54.1
–1.98
–2.35
–1.22
–0.31
–0.81
–1.17
–2.03
4.12
21.5
23.1
22.2
24.4
42.6
30.1
23.3
56.4
51.8
–1.16
–0.98
–0.93
0.42
1.19
–0.89
–1.22
6.36
H+
0.43
–2.08
Table 5. Comparison of properties of Cu2+ ion-selective electrodes based on some ligands
logKCu,M
Ionophore
Limit of
detection
Range
Response
time [s]
Reference
Zn
Cd
Pb
Ag
Fe
Schiff’s bases
3 × 10−8
5 × 10−6
4.7 × 10−6
6 × 10−8 to 10−1
8 × 10−6 to 10−1
5 × 10−6 to 10−1
5
15
< 30
–3.5 –3.6 –3.2 –2.1
–2.24 –2.2 –0.52 –3.0
–
–
–
Ganjali et al. (2001)
Sadeghi et al. (2003)
Singh and Bhatnagar (2004)
–1.09 –1.0
–
–0.26
thiacrown ethers
aza-thioethers crown
acyclic di-peptide
pyridine derivatives
cyclic tetrapeptide
derivatives
1.4 × 10−7
8 × 10−6
9 × 10−6
10−5 to 10−1
10−5 to 10−1
10−5 to 10−2
–
15
–
–1.5 0.1
–
–
–
Brzózka et al. (1988)
Shamsipur et al. (2001)
–2.85 –3.39 –2.69 –2.13
10–20 –2.01 –1.83 –0.72 –0.75 –0.35 Kamel et al. (2014)
7.5 × 10−7
10−6 to 10−2
15–20
–1.7 –2.39 –0.52 –3.09 –2.84 Hassan et al. (2005)
–1.37 –1.43 –1.0 –1.33 –1.55 Kamel et al. (2010)
calix[4]arene derivative 7.5 × 10−6 8.1 × 10−6 to 10−2
< 20
–
calixazacrown ethers
1 × 10−5 3 × 10−5 to 3 × 10−3
–
–1.92
–
–
–
Park et al. (2001)
Conclusions
work from Gdansk University of Technology, grant no. BW
020331/006 DS 020222/003 is gratefully acknowledged. The
authors would like to thank Professor J. Biernat for invaluable
assistance in implementation of the research and preparation
of the manuscript.
The synthesis of chromogenic bis-tetrazole deriva-
tive III sensitive for Cu2+ ions is described. The
spectroscopic properties were studied by UV-Vis and
IR spectroscopies. The spectrophotometric measure-
ments in CH3CN and DMSO solutions revealed that
receptor III and Cu2+ ions formed a stable 1 : 1
complex. However, in the case of the mixed solvents
formation of complexes of 1 : 2 and 2 : 1 stoi-
chiometry (III/Cu2+) was observed. The naked-eye
colour change of a receptor following the addition
of Cu2+ ions was observed only in the CH3CN so-
lution. In addition, the properties of compound III
were characterised potentiometrically using polymeric
ion-selective membranes. Compound III was found to
be a Cu2+ selective ionophore with a theoretical Ner-
stian response slope, a wide linear range and good
selectivity. The potentiometric selectivity coefficients
were determined for all the cations studied and it was
proved that they are well discriminated. Membranes
with o-NPOE plasticiser afforded better potentiomet-
ric characteristics.
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