Selective detection of F- by chromogenic tetrazole receptor

Article abstract of DOI:10.1080/10610278.2012.745546

A chromogenic anion host 4, containing two amide functionalities linked to azo dye and tetrazole rings, was synthesised and its complexes with various anions were investigated. The results show that receptor 4 can recognise selectively biologically import

Full text of DOI:10.1080/10610278.2012.745546

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Selective detection of F- by chromogenic tetrazole
receptor
Agnieszka Pazik a & Anna Skwierawska a
a Department of Chemical Technology, Gdansk University of Technology, Narutowicza Street
11/12, 80-233, Gdańsk, Poland
Version of record first published: 03 Dec 2012.
To cite this article: Agnieszka Pazik & Anna Skwierawska (2012): Selective detection of F- by chromogenic tetrazole receptor,
Supramolecular Chemistry, DOI:10.1080/10610278.2012.745546
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Supramolecular Chemistry
iFirst article, 2012, 1–11
Selective detection of F2 by chromogenic tetrazole receptor
Agnieszka Pazik* and Anna Skwierawska
´
Department of Chemical Technology, Gdansk University of Technology, Narutowicza Street 11/12, 80-233 Gdansk, Poland
(Received 20 August 2012; final version received 28 October 2012)
A chromogenic anion host 4, containing two amide functionalities linked to azo dye and tetrazole rings, was synthesised and its
complexes with various anions were investigated. The results show that receptor 4 can recognise selectively biologically
important F2 ion. The binding affinity for F2 was investigated by naked-eye colour change, UV–vis and 1H NMR
spectroscopy. Addition of F2 ion in CH3CN and Dimethyl sulfoxide to receptor 4 causes a change in colour of the solution
from colourless to yellow. The stoichiometry for host:anion is 1:1. Furthermore, receptor 4 was used as an ion carrier in ion-
selective membrane electrodes. Selectivity of this membrane was studied towards various anions in water solution. Binding
behaviour of receptor 4 towards several anions (Cl2, F2, Br2, I2) has been investigated using density functional theory
calculations.
Keywords: anion recognition; UV–vis spectroscopy; fluoride sensor; membrane electrodes; bis-tetrazole
Introduction
potential sensing F2 receptor is nowadays very important,
and visual detection of F2 can give immediate qualitative
information, and is becoming increasingly appreciated in
terms of quantitative analysis. Here, we report a neutral
receptor for F2 – chromogenic tetrazole receptor 4,
containing two amide functionalities linked by azobenzene
subunit and tetrazole rings. We assumed that the receptor
with two strong hydrogen bonds and derivative of organic
dye and tetrazole ring allow selectivecolorimetric detection
of anions. The colour changewill occur upon the addition of
anion guest that affects the electronic properties of the
chromogenic units. Also, we found out that the neutral
tetrazole can bind anion tightly in polar solution comparing
to the carboxylic acid analogues (21).
Over the past few years, there has been increasing emphasis
in supramolecular chemistry on the development of new
synthetic sensing receptors for recognising anionic species
(1, 2). It is commonly known that anions play a fundamental
role in a wide range of chemical and biological processes,
and consequently the development of selective anion
receptors is of great interest. Various compounds have been
proposedor utilised asanion receptors, butthe difficulty with
the size and salvation has been observed. Anions such as
fluoride, chloride, phosphate and carboxylate play crucial
roleina range ofbiological phenomena and are implicated in
many diseased states (3–6). Among them, fluoride is the
smallest anion with a high charge density and a hard Lewis
basic nature, which results in unusual chemical properties.
Fluoride ion is biologically important anion because of their
role in, e.g. dental care and the treatment of osteoporosis.
Excess fluoride anions cause serious diseases such as
fluorosis, thyroid activity depression, bone disorders and
immune system disruption (7, 8). In this regard, the sensing
of fluoride ions has attracted growing attention (9–14).
Although a numberofmethods are available for F2 detection
including voltammetry and potentiometry, the procedures
are time consuming which require sophisticated instrumen-
tation (15–19). The design of new environmentally friendly,
non-toxic and easy-making chromogenic ligand for visual
observation is a challenging goal (20).
Results and discussion
Synthesis
Receptor 4 was synthesised through the series of reactions
detailed in Figure 1. Compound 2 has been synthesised by
heating chrysoidine and chloroacetic chloride in chloro-
form. The final step of reaction was to obtain compound
4 by alkylation of 5-substituted tetrazole 3 with
N-arylchloroacetamide 2 in the presence of KOH and
ethanol (22). The reaction occurs under mild conditions,
proceeding regioselectively at position 2 of the tetrazole
ring and with good yield. Different salts and solvents
allowed for obtaining the desired product with large
amount of by-products. The best results were observed
when the reaction was carried out at 608C for 24 h. The
The purpose of our work was to achieve heterocyclic
ligands of required properties, which can be used as general
molecular receptors for determining F2 ions. Design of
1
progress of reactions was monitored by H NMR and IR
*Corresponding author. Email: mikaga20@wp.pl
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2012.745546
http://www.tandfonline.com

2
A. Pazik and A. Skwierawska
Figure 1. Synthesis of receptor 4.
spectroscopies. The 1H NMR spectra confirmed the
formation of CH2Cl groups by the appearance of signals
at 4.25 and 4.31 ppm. However, the major difference in the
spectra was observed in IR spectroscopy by disappearing
C;N band (e.g. 2230 cm21), confirming formation of
tetrazole ring and showing characteristic for tetrazole
bands and amide groups (e.g. 1670 cm21).
1
UV–vis and H NMR spectroscopy
The incorporation of a chromophoric azobenzene subunit
and tetrazole rings into diamide allows to create possible
sensitive and complexing molecule. According to our
knowledge, the ion recognition ability of 4 has not been
examined using UV–vis spectrophotometry and has
received no attention yet in the literature. In the preliminary
Figure 2. Change in the UV–vis absorption spectrum of the
(1.0 £ 1025 M) in the presence of tetra-n-
receptor
4
butylammonium salts of different anions in CH3CN (100-fold
excess).
studies, Naþ, Kþ, Mg2þ, Ca2þ, Co2þ, Cu2þ, Zn2þ, Ni2þ
,
Pb2þ and Fe2þ ions were used for ion complexation. No
colour and spectral change were observed in case of
examined cations in CH3CN and DMSO. Comparing to
previously described bis(phenylhydrazono-1H-tetrazol-5-
yl-acetonitriles) (23), where no spectrophotometric
response towards Cu2þ and Pb2þ ions was found, the
difference in amide and hydrazone bonds with tetrazole
rings can be seen in beneficial effect for anion recognition.
The anion recognition properties of receptor 4 have
been investigated by UV–vis upon addition of tetra-n-
butylammonium salt of Cl2, Br2, AcO2, F2, SCN2, I2,
HSO24 , NO23 and BzCOO2 in CH3CN and DMSO. The
effect of various anions with different shapes and sizes on
4 in CH3CN is presented in Figure 2, and the change in the
Figure 3. Naked-eye colour changes of receptor 4 solution
(1.0 £ 1025 M) upon addition of the tetra-n-butylammonium salt
of a particular anion (100-fold excess) in CH3CN.

Supramolecular Chemistry
colour of the receptor solution upon addition of few anions
3
is shown in Figure 3. The most significant changes in the
absorption spectra were observed only in the presence of
F2 ion. The results, shown in Figure 2, clearly demonstrate
that the addition of F2 ion into CH3CN solution of 4
(1.0 £ 1025 M) causes strong changes in the absorption
band at 369 nm which decreased while a new maximum
appeared at 478 nm, concomitant with a solution colour
change from light yellow to deep yellow.
The examined anions can cause strong hydrogen
bonding with receptor 4 which is observed in visual colour
change. Only in case of F2 ion, a detectable naked-eye
colour change was observed. Addition of other anions to
the solution of receptor and F2 ion did not show any
notable spectral or colour change, indicating no interaction
of these anions with receptor.
Figure 4. Changes in the UV–vis absorption spectrum of
receptor 4 (1.0 £ 1025 M) upon different concentrations of TBAF
in CH3CN.
In further research, we study the UV–vis spectral
changes in the spectrum upon addition of the different
concentrations of F2 ion to receptor 4 (c ¼ 1.0 £ 1025 M).
A titration was carried out with a fixed concentration of
receptor 4 and varying concentrations of tetra-n-butylam-
monium fluoride (TBAF) (1024 to 1023 M). Changes in the
spectrum on addition of F2 are presented in Figures 4 and 5.
As the concentration of F2 ion increased, the absorption
maximum of the free ligand at 369 nm slowly decreased its
intensity and new peak gradually appeared. It is worth
noting that the changes in absorption spectra registered in
100-fold excess of F2 ion are more spectacular than those
obtained during the course of titration (Figure 5). However,
concentration of the F2 ion (1023 M) makes determination
of complexation constant impossible.
receptor. The binding constant of F2 was calculated
according to the Benesi–Hildebrand method. As shown in
Figure 6, the Benesi–Hildebrand plot of 1/(A 2 A0) versus
1/(F2) provides a straight line, with an association
constant of 4.2 £ 102 M21 ^ 0.15.
The effect of solvent on anion binding strength in case
of F2 ions was also investigated. The change of solvent
from CH3CN to dipolar aprotic DMSO caused adverse
influence on anion recognition. Due to the basic nature of
fluorides in DMSO, during the titration conditions,
deprotonation of receptor 4 occurs. In the presence of a
large excess of TBAF (Figure 7), the colour of the ligand
solution changes to orange and is related with the
appearance of a new band at 480 nm in the UV–vis
spectrum. The UV–vis titrations of F2 ions (c ¼ (0–
3.85) £ 1025 M) were performed to estimate the stability
constant value of the complex. Molar ratio plot revealed
that with titration experiments complex of 1:1 stoichi-
ometry was found (Figure 8). The association constant was
To determine the stoichiometric ration of receptor 4
and F2, the Job’s plot was used. The results show that the
host–guest complex concentration approaches a maxi-
mum when the mole fraction of the host is about 0.5,
which indicates that the anion form a 1:1 complex with the
Figure 5. (a) UV–vis titration of receptor 4 (1.0 £ 1025 M) with TBAF (0–0.82 £ 1024 M) in CH3CN solution. (b) Titration plot to
determine the stability constant of the complex between 4 and F2 (1:1) at 490 nm.

4
A. Pazik and A. Skwierawska
Table 1. The chemical shift changes in the 1H NMR spectra of
diamide 4 in the presence of an equimolar amount of TBAF in
DMSO-d6.
Ligand 4
4 þ F2
Dd (ppm)
A
B
C
8.62
7.67
7.80
7.97
7.61
7.55
7.61
7.61
8.44
7.69
7.75
7.90
7.54
7.48
7.54
7.54
20.18
20.02
20.05
20.07
20.07
20.07
20.07
20.07
DD0
EE0
F
GG0
H
Figure 6. Benesi–Hildebrand plot to determine the stability
constant of the complex between receptor 4 and F2.
In order to confirm fluoride complexation theory, the 1H
NMR spectra of diamide 4 in the presence of equimolar
amounts of F2 were recorded in DMSO-d6. The chemical
shifts (ppm) for free ligand and their changes in the
presence of tetra-n-butylammonium salt (1 equiv.) are
summarised in Table 1. The respective spectra are shown in
Figure 9. Upon the addition of 0.5 equiv. F2 ions, the NH
protons shifted downfield with significant decrease. With
1 equiv. amounts of F2, the signal of NH protons almost
completely disappeared due to broadening.
The most significant changes in chemical shift values of
the aromatic protons were observed for CH proton between
NH moieties. In the presence of a large excess of salt, the
CH protons (A) were shifted upfield compared to the free
ligand 4 (Figure 10). Smaller amounts of base showed no
significant changes in aromatic protons. These changes
indicate that fluoride binding to receptor 4 is occurring. In
contrast to the above, completely different spectral changes
were observed in the case with excess of salt (20 equiv.).
Changes suggest that after few hours, TBAF promotes
amide bond cleavage, and as strong base causes elimination
of bifluoride ion HF22 (triplet at 16.1 ppm with J ¼ 164 Hz)
and amine derivatives (Figure 11).
Figure 7. UV–vis titration of receptor 4 (2.5 £ 1025 M) with
TBAF (0–3.75 £ 1024 M) in DMSO.
calculated according to the Benesi–Hildebrand method
and was determined to be 0.62 £ 102 M21 ^ 0.16.
Comparing the results with those which were obtained in
CH3CN, the stability constant values for complex 4 with
F2 ions in DMSO are much lower.
Figure 8. (a) Changes in absorption spectra for 4 (2.5 £ 1025 M) upon titration with TBAF (0–3.846 £ 1025 M) in DMSO. (b) Titration
plot to determine the stability constant of the complex between 4 and F2 (1:1) at 400 nm.

Supramolecular Chemistry
5
Figure 9. 1H NMR spectra of (a) diamide 4; (b) upon addition of 0.5 equiv. and (c) 1 equiv. of TBAF in DMSO-d6.
Molecular modelling
the NZH· · ·X bond lengths in the case of different anions
tend to decrease from I2 . Br2 . Cl2 which can be
explained by different geometry shapes of guest.
Furthermore, the optimised structure of ligand 4 and its
complexes shows that anion is implemented inside the
ligand cavity between amide bonds and tetrazole rings. We
assume that azo moieties are not involved in complexation.
In order to study the binding behaviour of receptor 4, the
chemical calculations at density functional theory (DFT)
level were carried out using Gaussian 03 software with
B3LYP method and were optimised using a basic set:
6 2 31 þ þ G (d,p). Receptor 4 contains two amide
functionalities which are not in the same plane according
to optimised global minima for compound 4 (Figure 12).
The optimised structure of 4 confirms that distances
Membrane electrodes and potentiometric measurements
˚
N1· · ·H1 and N2· · ·H2 are 1.063 A. The binding affinity of
this ligand was determined towards several anions. The
introduction of X2 ions into ligand 4 leads to a
reorganisation structure from flexible conformer to a
more rigged system. Clearly, the NZH moieties in
structure 4 preorganise into a cavity that can hold guest
anions by two strong hydrogen bonds. The distances
between intramolecular hydrogen bonds (NZH· · ·X) and
halogen bonds are presented in Table 2. It is seen that the
calculated intermolecular distances in the complex range
for F2 ion (see Figure 13) are the smallest for all examined
In preliminary experiments, the anion-selective properties
of poly(vinyl chloride) membrane electrode based on
receptor 4 are examined. The potentiometric response
towards various anions was studied, starting from the most
discriminated. The solution of sodium salt of F2, Cl2, I2,
Br2, CH3COO2, NO32, HSO42 and HPO422 (c ¼ 1026 to
1022 M) was examined. The selectivity pattern of an
electrode is presented in Figure 14. Electrode responds to
changes in anion concentration with Nernstian slopes
(255 and 227 mV/dec). The response time of electrode
was found to be about 1 s, while the recovery time exceeds
10–20 min. The electrode is selective for F2 ion because it
exhibited a linear response in the whole examined range of
˚
anions (NZH· · ·F ¼ 2.880–3.722 A), also the interactions
are more linear. Fluorine ion is the smallest one and can fit
inside cavity of 4 and form 1:1 stable complex. Moreover,

6
A. Pazik and A. Skwierawska
1
Figure 10. Changes in H NMR spectrum of free receptor 4 and upon addition of 1 equiv. of TBAF in DMSO-d6.
anion concentration. The selectivity coefficients were
determined using separate solution method (SSM)
according to the procedure described by Baker et al. and
were calculated using Electromotive Force (EMF) values
for the highest measured ion activities that belong to the
linear response range for a given ion (Table 3). For halide
anions, a linear response was observed nearly for a whole
range of anion concentration. Furthermore, it can be seen
that receptor 4 show the highest affinity towards HPO422
( log K2F;J ¼ 1:39). The results also show that I2 and Br2
belong to ions most discriminated by the studied receptor
(log KF,J ¼ 0.10 and log KF,J ¼ 0.12).

Supramolecular Chemistry
7
Figure 11. 1H NMR spectrum upon addition of 20 equiv. of TBAF in DMSO-d6.
The potentiometric response of the electrode towards
different anions was comparable to those used for
traditional ISE. However, screen-printed electrodes have
a longer response time (1 min) while the recovery time
exceeds 30–40 min. The examined membrane can be
considered as potentially useful anion sensing material.
However, the response towards various anions is not as
magnificent as the UV–vis measurements though there are
certain differences in the working mechanism of these two
methods (e.g. different solutions – water/acetonitrile).
Conclusion
In conclusion, chromogenic tetrazole receptor 4 with azo-
and amide moieties built in their structure was successfully
synthesised. The spectroscopy properties have been
1
studied by UV–vis and H NMR spectroscopies. UV–
vis measurements in CH3CN and DMSO showed that
receptor 4 and F2 ion form stable 1:1 complex.
Furthermore, receptor 4 ensures selective recognition of
F2 ion by naked-eye colour change, which is not observed
in addition of other anions. The synthesised receptor has
been characterised potentiometrically in polymeric ion-
selective membranes and by DFT method.
Figure 12. Optimised geometry of receptor 4.
Due to the observed increase in range of miniature
sensors, biosensors and molecular devices, we decided to
use our receptor 4 as ionophores in miniature potentio-
metric sensors. Composition of the membrane was the
same as was used for classic ion-selective electrode (ISE).
Experimental section
General methods
1H NMR spectra were recorded on Varian instrument
(USA) at 200 and 500 MHz. UV–vis measurements were
carried out with the use of UNICAM UV 300 series
spectrophotometer. IR spectra were taken on Genesis II
FT-IR (Mattson, USA) instrument. Elemental analyses
were done on EAGER 200 apparatus. EMF measurements
were carried out using 16-channel LAWSON LAB
potentiometer (16 EMF, USA). The screen-printed
graphite electrodes used were obtained from Institute of
Electronic Materials Technology, Warsaw, Poland (plates
of 18–15 mm with six electrodes, openings area ca.
1 mm2). Gaussian 03 software was used for all theoretical
Table 2. Theoretical parameters of the complexes of 4.
Substrates
4·F2
4·Cl2
4·I2
4·Br2
NZH1· · ·X
NZH2· · ·X
N5· · ·X
N6· · ·X
N7· · ·X
N8· · ·X
N9· · ·X
N10· · ·X
N11· · ·X
N12· · ·X
2.880
3.722
6.428
2.187
4.290
6.222
5.229
6.031
5.645
6.660
3.322
4.547
5.613
6.406
6.685
7.083
6.131
5.294
5.933
7.077
3.508
4.874
5.343
6.125
6.506
6.889
6.355
5.500
6.228
7.441
3.106
4.699
5.236
6.074
6.335
6.749
6.301
5.497
6.241
7.414

8
A. Pazik and A. Skwierawska
Figure 13. Optimised geometry of receptor 4 and F2 ion.
Figure 14. Potentiometric response of membrane ion-selective electrode for anions.
calculations (24). All materials and solvents were of
analytical reagent grade. Thin layer chromatography
(TLC) was done on aluminium plates covered with silica
gel 60 F254 (Merck, Germany). All aqueous solutions
were prepared from salts of analytical grade using
deionised water. All measurements were carried out at
room temperature. The melting points are uncorrected.
Synthesis
Synthesis of 1,3-dichloroacetchrysoidine 2
To the solution of chrysoidine 1 (0.5 g, 2 mmol) and
triethylamine (0.7 ml, 4 mmol) in CHCl3 (20 ml), chlor-
oacetic chloride (1.59 ml, 0.02 mol) in CHCl3 was added
slowly, and the mixture was stirred and reflux for 24 h. The
solvent was evaporated in vacuo yielding the crude
product. Recrystallisation in i-PrOH afforded yellow solid
(mp 2108C, 65% yield). 1H NMR (200 MHz, CDCl3):
d ¼ 4.25 (s, 2H, CH2), 4.31 (s, 2H, CH2), 7.53 (m, 3H,
HAr), 7.92 (m, 3H, HAr), 8.46 (s, 1H, HAr), 8.62 (m, 1H,
HAr), 11.33 (s, 2H, NH) ppm. IR (KBr): 3340, 3285, 3206,
3109, 1670, 1615, 1569, 1545, 1412, 1337, 1252, 1245,
Table 3. Potentiometric selectivity coefficients for various
interfering ions.
Ion NO32 HSO24 HPO242 CH3COO2 Br2 Cl2
I2
L-4 0.83 0.98 1.39 1.22 0.12 1.02 0.10

Supramolecular Chemistry
9
875, 826 and 684 cm21. Anal. calcd for C7H4N4Cl2: C
39.09, H 1.86, N 26.05. Found: C 39.14, H 1.89, N 26.12.
1 mg of tetradodecyloammonium tetrakis(4-chlorophe-
nyl)borate. The membrane components, total 178 mg,
were dissolved in 1.2 ml of freshly distilled tetrahydro-
furan. 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
the discs of 5 mm i.d. were cut out. The membrane was
incorporated into Ag/AgCl electrode bodies of IS 561 type
(Moeller S.A., Zurich, Switzerland). Electrode bodies
were filled with an internal filling solution – KCl
(1022 M). The electrode was conditioned in KCl (1022 M)
for 24 h. A double-junction reference electrode (RAE 112,
Monokrystaly, Turnov, Czech republic) was used with
NH4NO3 (1 M) solution in the bridge cell.
Synthesis of 2,6-dichloro-5-phenyl-1H-tetrazole 3
A mixture of 2,6-dichlorobenzonitrile (0.34 g, 2 mmol),
sodium azide (0.13 g, 2 mmol) and ammonium chloride
(0.11 g, 2 mmol) in dry N,N0-dimethylformamide (DMF).
(7 ml) was stirred and heated at 1208C for 24 h. It was
subsequently cooled to room temperature and the insoluble
salts were filtered, and the solvent was evaporated under
reduced pressure. The resulting solution was poured into
40 ml of water and then was brought to pH 2 with
concentrated. HCl to give crude product that was purified by
recrystallisation using distilled water affording pure 3 as a
brown solid (mp 113–1168C, 88% yield). 1H NMR
(200 MHz, d-MeOH): d ¼ 7.4 (t, J ¼ 8.78Hz, 1H, Htetrazole),
7.62 (m, 3H, HAr) ppm. IR (KBr): 3407, 3089, 2937, 1604,
1570, 1433, 1393, 1298, 1170, 1130, 1106, 1053, 993, 933,
870, 790, 730, 548 and 415 cm21. Anal. calcd for
C16H14N4O2Cl2: C 52.61, H 3.87, N 15.35, Cl 19.42.
Found: C 52.72, H 3.75, N 15.39, Cl 19.47.
Screen-printed electrodes
The solution (1–0.5 ml) of the same composition such as
traditional ISE was applied onto graphite screen-printed
electrodes, and the electrodes were left to dry over 1 day.
Then the electrodes were conditioned in a 1022 M KCl for
24 h. A double-junction Ag/AgCl, KCl 1 M reference
electrode (Monokrystaly RAE 112) was used with 1 M
NH4NO3 solution in the bridge cell. The measurements
were carried out at room temperature, and the selectivity
coefficients were determined using SSM according to the
procedure described by Baker et al. (25) and were
calculated using EMF values for the highest measured ion
activities that belong to the linear response range for a
given ion.
Synthesis of 1,3-bis(2,6-dichlorophenyltetrazolyl)
diacetchrysoidine 4
A solution of compounds 2 (0.37 g, 1 mmol) and 3 (0.42 g,
2 mmol) and KOH (0.11 g, 2 mmol) in ethanol (20 ml) was
stirred at 708C for 24 h. The reaction progress was
monitored by TLC (CH2Cl2:MeOH, 30:1). Upon com-
pletion of the reaction, the solvent was evaporated and the
resulting solid was purified by recrystallisation in ether to
give a yellow solid (mp 187–1908C, 30% yield). 1H NMR
(500 MHz, d-DMSO): d ¼ 4.31 (s, 2H, CH2), 4.53 (s, 2H,
CH2), 7.55 (d, J ¼ 6.83 Hz, 1H, HAr), 7.61 (m, 8H, HAr),
7.67 (d, J ¼ 8.79 Hz, 1H, HAr), 7.80 (d, J ¼ 8.79 Hz, 1H,
HAr), 7.97 (d, J ¼ 7.32 Hz, 2H, HAr), 8.62 (s, 1H, HAr),
10.68 (s, 1H, NH), 10.84 (s, 1H, NH) ppm. IR (KBr): 3363,
3274, 3212, 3112, 1675, 1608, 1567, 1528, 1440, 1407,
1347, 1255, 1243, 870, 831, 766 and 687 cm21. Anal.
calcd for C30H20N12O2Cl4: C 49.88, H 2.79, N 23.27, Cl
19.63. Found: C 49.94, H 2.81, N 23.32, Cl 19.58.
Acknowledgements
Financial support of this work from Gdansk University of
Technology, Grant Nos. BW 014694/038 and BW 014694/039,
is kindly acknowledged.
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Membrane preparation
Classical ISE
The membrane contained 7 mg of ionophore 4, 50 mg of
polyvinyl chloride, 0.1 ml of 2-nitrophenyl octyl ether and

10
A. Pazik and A. Skwierawska
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Supramolecular Chemistry
11
Cl
Cl
Cl
Cl
H
N
H
N
N
N
N
N
F–
N
N
N
N
O
N
O
N
N
N
N
F–
O
N
HN
HN
Cl
O
N
N
N
N
Cl
N
N
Cl
Cl
Agnieszka Pazik and Anna Skwierawska
Selective detection of F2 by chromogenic tetrazole receptor
1–11