Silver ion-selective electrodes using π-coordinate calix[4]arene derivatives as soft neutral carriers

Article abstract of DOI:10.1021/ac000490d

Calix[4]arene derivatives incorporating π-coordinate substituents such as allyl, benzyl, and propargyl groups were designed as soft neutral carriers for silver ion sensors. Most of all, tert-butylcalix[4]arene tetra(allyl ether) is an excellent neutral carrier for plasticized poly(vinyl chloride)-membrane silver ion-selective electrodes. The ion sensors showed high silver ion selectivity over alkali metal ions and also good selectivity against other soft metal ions such as lead and mercury(II) ions. The electrode potential response was as rapid as that for neutral-carrier-type alkali metal ion electrodes due to the soft interaction between π-coordinate substituants and silver ion, which was elucidated by ¹H NMR spectroscopy.

Full text of DOI:10.1021/ac000490d

Anal. Chem. 2000, 72, 5290-5294
Silver Ion-Selective Electrodes Using π-Coordinate
Calix[4]arene Derivatives as Soft Neutral Carriers
Keiichi Kimura,*,† Setsuko Yajima,† Kenta Tatsumi,‡ Masaaki Yokoyama,‡ and Masatoshi Oue§
Department of Applied Chemistry, Faculty of Systems Engineering, Wakayama University, Sakae-dani,
Wakayama 640-8510 Japan
sulfur atoms, so-called thiacrown ethers, were designed for neutral
carriers for Ag+-selective electrodes. Coordination bonding of Ag+
with the crown-ring sulfur atoms is, however, powerful, as
compared with ion-dipole interaction between metal ions and
oxygen atoms (high complex formation constants). This often
brings about slow metal-ion exchange equilibria in the membrane
interface, resulting in some disadvantage of the thiacrown ether-
based ion sensors over those based on oxygen-atom-containing
neutral carriers, i.e., slow sensor response and poor sensitivity.
The drawback for thiacrown ether-based Ag+ sensors may be
alleviated more or less by the decreased number of the crown-
ring sulfur atoms.5,6
A soft acid, Ag+, is apt to interact with π electrons and the π
coordination is quite selective and relatively weak.7-12 We envisage
that neutral carriers incorporating π-coordinate substituents may
be useful to realize quick response and high sensitivity for neutral-
carrier-type Ag+ sensors. It occurred to us that the incorporation
of π-coordinate groups such as allyl, benzyl, and propargyl groups
into a rigid skeleton may afford excellent π-coordinate neutral
carriers. Thus, we have synthesized calix[4]arene derivatives
containing π-coordinate substituents at the lower rim.13 Here we
report the designing of the π-coordinate calix[4]arene neutral
carriers and the sensor property of plasticized-poly(vinyl chloride)
(PVC) membrane Ag+-selective electrodes based on them.
Calix[4 ]arene derivatives incorporating π-coordinate sub-
stituents such as allyl, benzyl, and propargyl groups were
designed as soft neutral carriers for silver ion sensors.
Most of all, ter t-butylcalix[4 ]arene tetra(allyl ether) is an
excellent neutral carrier for plasticized poly(vinyl chloride)-
membrane silver ion-selective electrodes. The ion sensors
showed high silver ion selectivity over alkali metal ions
and also good selectivity against other soft metal ions such
as lead and mercury(II) ions. The electrode potential
response was as rapid as that for neutral-carrier-type
alkali metal ion electrodes due to the soft interaction
between π-coordinate substituents and silver ion, which
1
was elucidated by H NMR spectroscopy.
A most attractive potentiometric sensor is a neutral-carrier-
type one that realizes excellent ion selectivities. The neutral
carriers are dissolved or dispersed in the ion-sensing membranes
that are generally a liquid membrane or a plasticized polymeric
membrane. The original ion selectivity of the neutral carriers
themselves is reflected in the resulting ion-sensing membranes
and therefore the ion sensors in many cases. Naturally occurring
ionophores such as valinomycin were applied as the neutral carrier
for alkali-metal ion-selective electrodes in the early stages.
Synthetic ionophores such as crown ether derivatives, polyether-
amides, and calixarene ionophores have been so far designed as
the neutral carriers. Sophisticated designing of neutral carriers
can afford extremely high ion selectivities that cannot be attained
in “ionic” ion-exchanger-type sensors. Since most of the neutral
carriers contain oxygen or nitrogen atoms as the heteroatoms for
metal-ion binding, the resulting neutral-carrier-type ion sensors
are mainly for alkali and alkaline-earth metal ions.
EXPERIMENTAL SECTION
Synthesis of Calix[4 ]arene Neutral Carriers. The general
procedure for the synthesis is as follows. tert-Butylcalix[4]arene
(1.35 mmol) was dispersed in dry tetrahydrofuran (THF) (50 cm3)
containing dry N,N-dimethylformamide (DMF) (5 cm3) and
treated with NaH (25 mmol) followed by alkyl bromide (25 mmol).
The synthetic ionophores, when some of their oxygen atoms
are replaced with sulfur atoms, exhibit high affinity toward soft
metal ions such as Ag+.1-5 Thus, several crown ethers containing
(5) Oue, M.; Akama, K.; Kimura, K.; Tanaka, M.; Shono, T. J. Chem. Soc., Perkin
Trans. 1 1 9 8 9 , 1675-1678.
(6) Teikidor, F.; Flores, M. A.; Escriche, L.; Vinas, C.; Casabo, J. J. Chem. Soc.,
Chem. Commun. 1 9 9 4 , 963-964.
(7) Ikeda, A.; Tsuzuki, H.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1 9 9 4 ,
2073-2080.
(8) Lhotak, P.; Nakamura, R.; Shinkai, S. Supramol. Chem. 1 9 9 7 , 8, 333-344.
(9) Futterer, T.; Merz, A.; Lex, J. Angew. Chem., Int. Ed. Engl. 1 9 9 7 , 6, 611-
613.
† Department of Applied Chemistry, Faculty of Systems Engineering,
Wakayama University, Sakae-dani, Wakayama 640-8510, Japan.
‡ Department of Material and Life Science, Graduate School of Engineering,
Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan.
§ Department of Chemical Engineering, Nara National College of Technology,
Yata, Yamato-koriyama, Nara 639-1080, Japan.
(10) Dondoni, A.; Ghiglione, C.; Marra, A.; Scoponi, M. Chem. Commun. 1 9 9 7 ,
(1) Lai, M.-T.; Shih, J.-S. Analyst (Cambridge, U. K.) 1 9 8 6 , 111, 891-895.
(2) Casabo, J.; Mestres, L.; Escriche, L.; Teixidor, F.; Perez-Jimenez, C. J. Chem.
Soc., Dalton Trans. 1 9 9 1 , 1969-1971.
673-674.
(11) Delaigue, X.; Harrowfield, J. M.; Hosseini, M. W.; Mocerino, M.; Skelton,
B. W.; White, A. H. Aust. J. Chem. 1 9 9 8 , 51, 111-121.
(12) Choi, H. S.; Suh, S. B.; Cho, S. J.; Kim, K. S. Proc. Natl. Acad. Sci. U. S. A.
1 9 9 8 , 95, 12094-12099.
(3) Chung, S.; Kim, W.; Park, S. B.; Yoon, I.; Lee, S. S.; Sung, D. D. Chem.
Commun. 1 9 9 7 , 965-966.
(4) Malinowska, E.; Brzozka, Z.; Kasiura, K.; Egberink, R. J. M.; Reinhoudt, D.
N. Anal. Chim. Acta 1 9 9 4 , 298, 245-251.
(13) Kimura, K.; Tatsumi, K.; Yajima, S.; Miyake, S.; Sakamoto, H.; Yokoyama,
M. Chem. Lett. 1 9 9 8 , 833-834.
5290 Analytical Chemistry, Vol. 72, No. 21, November 1, 2000
10.1021/ac000490d CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/30/2000

solution was poured into a flat Petri dish with an inner diameter
of 23 mm. Gradual evaporation of the THF at ambient temperature
afforded an elastic, semitransparent membrane with a thickness
of about 0.2 mm. The resulting PVC membranes consisted of 26.9
wt % PVC, 67.2 wt % plasticizer, 5.4 wt % neutral carrier, and 0.5
wt % KTpClPB. A disk of 5-mm diameter was cut out from the
membrane with a cork borer and was then fixed into an electrode
body (DKK Corp. 7904L type) by using a drop of THF. After
drying the THF for 2 h, an internal filling solution (1 × 10-1 mol
cm-3 AgNO3 aqueous solution saturated with AgCl) was set up
into the electrode. Conditioning of the thus resulting electrodes
was done by soaking them in the AgNO3 solution overnight.
Measurements. Emf measurements were carried out at 25
( 0.1 °C by using a pH/ mV meter (Toko Chemical Laboratory,
model TP-1000). The outer reference electrode was a double-
junction-type Ag-AgCl reference electrode. The electrochemical
cell for the emf measurements was Ag | AgCl | 1 × 10-1 mol dm-3
AgNO3 | ion-sensing membrane | sample solution | 1 mol dm-3
CH3CO2Li | 3 mol dm-3 KCl | AgCl | Ag.
After the reaction, the excess of NaH was treated with methanol
(about 20 cm3) and the mixture was distributed to the chloroform/
diluted aqueous HCl (100/ 200 cm3) mixture. The chloroform
extract was washed with water (about 100 cm3) and dried over
MgSO4. The chloroform evaporation afforded a crude product,
which was purified by recrystallization or silica gel column
chromatography.
For the synthesis of tert-butylcalix[4]arene tetra(allyl ether)
1 , the reaction was carried out at room temperature for 1 h to
obtain a main product with cone conformation. The purification
was performed by recrystallization with ethanol to yield a pure
product of 1 with only cone conformation; colorless crystal; mp
213-214 °C; 1H NMR (270 MHz, CDCl3) δ 1.07(s, 36H, t-Bu), 3.12
and 4.38(d, J ) 12.5 Hz, 4H, ArCH2Ar), 4.46(d, J ) 6.3 Hz, 8H,
OCH2), 5.17(d, J ) 9.9 Hz, 4H, dCH2), 5.26(d, J ) 17.2 Hz, 4H,d
CH2), 6.49-6.37(m, 4H, sCHdC), 6.77(s, 8H, ArH); MS, m/ e (%
relative intensity) 808 (M+,100), 613 (75). Anal. Calcd for C56H72-
O4: C, 83.12; H, 8.97; O, 7.91. Found: C, 83.04; H, 8.94.
t-Butylcalix[4]arene tetra(benzyl ether) 2 was prepared by
refluxing for 1 h, according to a procedure in the literature.14
Similarly, tert-butylcalix[4]arene tetra(propargyl ether) 3 was
obtained under the refluxing conditions.15 The crude product was
purified by silica gel chromatography (benzene/ hexane ) 1:1)
to afford a colorless crystal: mp 212-213 °C; 1H NMR (270 MHz,
CDCl3) δ 1.07(s, 36H, t-Bu), 2.48(s, 4H, CtCH), 3.16 and 4.60(d,
J ) 13.0 Hz, 4H, ArCH2Ar), 4.80(d, J ) 1.9 Hz, 4H, CH2CtC),
6.79(s, 8H, ArH); MS, m/ e (% relative intensity) 800 (M+, 100),
613 (45). Anal. Calcd for C56H64O4: C, 83.96; H, 8.05; O, 7.99.
Found: C, 83.72; H, 8.02.
tert-Butylcalix[4]arene tetra(allyl ester) 4 with cone conforma-
tion was synthesized by the reaction of t-butylcalix[4]arene with
allyl 1-bromoacetate in acetone in the presence of K2CO3.16 In a
similar way to 2 and 3 , calix[4]arene tetra(propyl ether) 5 was
obtained.
Other Materials. PVC (an average polymerization degree of
1100) was purified by reprecipitation from THF in methanol twice.
The membrane plasticizer 2-fluoro-2′-nitrodiphenyl ether (FNDPE)
was purchased from Dojindo Laboratory. o-Nitrophenyl octyl ether
(NPOE)17 and bis(2-ethylhexyl sebacate (DOS) were distilled in
vacuo. Potassium tetrakis(p-chlorophenyl)borate (KTpClPB), which
is an anion excluder, was also obtained from Dojindo Laboratory.
Metal and ammonium nitrates employed were of analytical grade.
Water was deionized and distilled.
The selectivity coefficients for Ag+ with respect to interfering
ions were determined by a mixed solution method, that is, the
fixed interference method (FIM).18 The background concentra-
tions of interfering ions were 5 × 10-4 mol dm-3 for Hg2+; 1 ×
10-3 mol dm-3 for Tl+; 1 × 10-1 mol dm-3 for Pb2+ and H+; 5 ×
10-1 mol dm-3 for Ca2+, Mg2+, Li+; 1 mol dm-3 for Na+, K+, and
NH4+. The backgrounds for the tert-butylcalix[4]arene tetra(allyl
ester) 4 system were 1 × 10-3 mol dm-3 for Na+ and 1 × 10-2
mol dm-3 for K+ and Tl+. The selectivity coefficients for Ag+ with
respect to other cations were determined 10 times, using two
different membranes for each of the neutral carriers, and the
values thus obtained were averaged. The activity coefficients (γ)
were calculated according to the Davies equation, log γ ) -0.511-
(I)1/ 2/ (1 + 0.33R (I)1/ 2) - 0.10I, using the values of ionic strength
(I) and ion size parameter (R).19 For the Ag+ assay, model samples
for aqueous solutions containing 1 mol dm-3 NaNO3, 1 mol dm-3
NH4NO3, and appropriate concentrations (1 × 10-3, 7.5 X 10-4
,
5.0 × 10-4, and 1 × 10-4 mol dm-3) of AgNO3 were employed.
For Gran’s plot method20 on the Ag+ assay, the volume for the
sample solution was 10 cm3. The volumes for added solutions were
1 × 10-2 mol dm-3 for 1 × 10-3 - 5 × 10-4 mol dm-3 Ag+ samples
and 1 × 10-3 mol dm-3 for 1 × 10-4 mol dm-3 sample, 0.4 cm3 for
1 × 10-2 and 1 × 10-3 mol dm-3 Ag+ samples, and 0.2 cm3 for 7.5
× 10-3 and 5 × 10-4 mol dm-3 samples, respectively. The addition
was carried out four times for each measurement.
1H NMR spectroscopy of the calixarene neutral carriers for
the study of Ag+-complexing behavior of calixarene derivatives
was measured in CDCl3/ CD3OD(4/ 1) at room temperature, using
silver trifluoromethanesulfonate as the Ag+ salt. The measure-
ments in the presence of Ag+ were made with a metal ion/ neutral
carrier ratio of 1, unless otherwise stated.
(14) Gutsche, C. D.; Dhawan, B.; Levine, J. A.; No, K. H.; Bauer, L. J. Tetrahedron
1 9 8 3 , 39, 409-426.
(15) Xu, W.; Vittal, J. J.; Puddephatt, J. Can. J. Chem. 1 9 9 6 , 74, 766-774.
(16) Iwamoto, K.; Shinkai, S. J. Org. Chem. 1 9 9 2 , 57, 7066-7073.
(17) Allen, C. F. H.; Gates, J. W. Organic Synthesis; Wiley: New York, 1955; pp
140-141.
Electrode Fabrication. The general procedure for casting the
ion-sensing membranes is the following: PVC (50 mg), a
plasticizer (125 mg), a calix[4]arene derivative (10 mg), and
KTpClPB (1 mg) were dissolved in THF (3 cm3). The whole
(18) Recommendations for Nomenclature of Ion-Selective Electrodes. Pure
Appl. Chem. 1 9 7 6 , 48, 127-132.
(19) Davies, C. W. J. Chem. Soc. 1 9 3 8 , 2093-2098.
(20) Gran, G. Analyst (Cambridge, U. K.) 1 9 5 2 , 77, 661-671.
Analytical Chemistry, Vol. 72, No. 21, November 1, 2000 5291

Table 1. Slopes and Linear Ranges in the Calibration
Graph for Ag+-Selective Electrodes Based on
Calixarene Neutral Carriers
neutral carrier
slope (mV decade-1
)
linear range (pAg)
1
2
3
4
5
58.3 ( 0.7
51.1 ( 1.1
51.8 ( 0.5
57.4 ( 0.5
57.1 ( 0.7
1-4
1-4
2-4
1-4
1-4
(Table 1). This is also the case with the electrode of tert-butylcalix-
[4]arene tetra(allyl ester) 4 . However, the electrodes based on
tert-butylcalix[4]arene carrying benzyl and propargyl groups at
the lower rim, 2 and 3 , did not show very good sensitivity, i.e.,
they showed comparably low slopes of the calibration graph (51
mV decade-1) and narrow linear ranges. The modest sensitivities
for Ag+-selective electrodes of 2 and 3 are in contract to the high
sensitivity for the electrode of 1 . tert-Butylcalix[4]arene tetra(allyl
ether) 1 can probably complex Ag+ effectively by the cooperate
π-coordination with the four allyl substituents, which is discussed
later in the 1H NMR measurements. The relatively low sensitivities
in the sensor systems of 2 and 3 , on the other hand, might be
attributed to the poor solubility of the neutral carriers and their
Ag+ complexes, which in turn leads to the inefficient cation
exchange in the membrane interface.
For comparison, calix[4]arene tetra(propyl ether) 5 was also
tested as the Ag+ neutral carrier to elucidate the importance of
the rigid calix[4]arene skeleton and the π-coordinate substituents.
The calix[4]arene derivative without any special π-coordinate
group, 5 , also works as the Ag+ neutral carrier with a near-
Nernstian response to the Ag+ activity changes, although the Ag+
selectivity for the electrodes based on 5 is significantly different
from that for the electrodes of 1 , as discussed later. It is probably
because the aromatic rings themselves of the calix[4]arene
skeleton for 5 can undergo π coordination toward Ag+, as
described in the later discussion by NMR experiments.
Figure 1. Potential response to Ag+ activity changes of Ag+-
selective electrodes based on π-coordinate tert-butylcalix[4]arene
neutral carriers 1-4. Neutral carrier: 1 (b), 2 (4), 3 (0), and 4 (O).
RESULTS AND DISCUSSION
Design of π-Coordinate Calixarene Neutral Carriers. It is
well-known that carbon-carbon double and triple bonds exhibit
π-coordinate property. Appropriate assembly of π-coordinate
groups can afford the coordination atmosphere for Ag+. As-
sembling the π-coordinate groups requires a rigid and stable
skeleton, for which we chose tert-butylcalix[4]arene here.
We decided to use tert-butylcalix[4]arene derivatives incorpo-
rating allyl, benzyl, and propargyl groups at the lower rim. They
are easily accessible from commercially available tert-butylcalix-
[4]arene by a simple one-step reaction of tert-butylcalix[4]arene
and alkyl bromide in the presence of NaH. In the reaction for the
preparation of 1 under reflux conditions,14 mixture products of 1
with cone and partial-cone conformations were formed which were
very hard to separate by simple recrystallization from ethanol.
The reaction for preparation of 1 was, therefore, carried out at
room temperature to obtain a pure product with cone conforma-
tion. For preparation of 2 and 3 , the reflux reaction condition
even afforded the predominant cone conformation.
The Ag+ selectivities against other interfering ions were
determined in the electrode systems that afforded Nernstian or
near-Nernstian responses, their selectivity coefficients being
summarized in Figure 2. The electrode based on tert-butylcalix-
[4]arene tetra(allyl ether) 1 is highly Ag+-selective against alkali
and alkaline-earth metal ions, NH4+, H+, and even a soft metal
ion, Hg2+. There is some interference by a monovalent soft metal
ion, Tl+, in this electrode. Conventional Ag2S-based Ag+-selective
Thus, we applied π-coordinate calix[4]arene derivatives 1 , 2 ,
and 3 as neutral carriers for Ag+-selective electrodes. For
comparison, tert-butylcalix[4]arene tetra(allyl ester) 4 and calix-
[4]arene derivative possessing propyl groups instead of allyl ones
5
were also employed. The calix[4]arene derivatives, 1 -4 ,
possess only cone conformation. Calixarene derivative 5 is a
mixture of its cone and partial-cone conformations7 that was used
without time-consuming separation.
electrodes generally suffer from terrible interference by Hg2+
.
P otential Response and Selectivity of Silver Ion Selective
Electrodes. The π-coordinate calixarene derivatives were applied
as neutral carriers for Ag+-selective electrodes. Plasticized-PVC
membranes were adopted as the ion-sensing membranes, and
NPOE, DOS, and FNDPE were tested as the plasticizer. The
solubility of π-coordinate neutral carriers 1 -3 in plasticizer are
in the order FNDPE > NPOE . DOS. So, FNDPE was chosen
as the plasticizer of Ag+-selective PVC membranes. Figure 1 shows
typical potential responses for Ag+-selective PVC-membrane
electrodes based on π-coordinate calix[4]arene derivatives 1 -3 ,
together with that for the electrode of calix[4]arene tetra(allyl
ester) 4 for comparison. The electrode of tert-butylcalix[4]arene
tetra(allyl ether) 1 exhibited a near-Nernstian response to Ag+-
activity changes in the range of 1 × 10-4 - 1 × 10-1 mol dm-3
Definitely, the present calixarene-based Ag+ electrodes are
superior to the Ag2S-based electrodes from this point of view. It
should be noted that the electrode based on the calix[4]arene
tetra(allyl ester) 4 suffers from remarkable interference by Na+,
though the electrode responds to Ag+ activity changes with a near
Nernstian slope. Calix[4]arene tetraester derivatives are well-
known Na+ neutral carriers for the ion-selective electrodes, since
their ester carbonyl groups can bind Na+ powerfully.21,22 The high
affinity of tert-butylcalix[4]arene tetra(allyl ester) 4 to Na+ is the
(21) Diamond, D.; Svehla, G.; Seward, E. M.; McKervey, M. A. Anal. Chim. Acta
1 9 8 8 , 204, 223-231.
(22) Kimura, K.; Miura, T.; Matsuo, M.; Shono, T. Anal. Chem. 1 9 9 0 , 62, 1510-
1513.
5292 Analytical Chemistry, Vol. 72, No. 21, November 1, 2000

Figure 2. Selectivity comparison among Ag+-selective electrodes
based on calix[4]arene neutral carriers 1, 4, and 5.
reason for the terrible interference by Na+ in the Ag+-selective
electrode of 4 , which is a very undesirable factor on practical
applications of Ag+ electrodes. Another noteworthy thing is the
terrible Tl+ interference in the Ag+-selective electrode based on
calix[4]arene tetra(propyl ether) 5 (Figure 2). The high Tl+
interference in the electrode of 5 cannot be neglected on the Ag+
assay using the ion electrode, if the sample solutions contain even
a small quantity of Tl+. This implies that the π-coordinate cavity
formed by the aromatic rings in the calix[4]arene skeleton of 5
is more suitable for complexing Tl+ than Ag+.
1 H NMR Study for Silver Ion Complexing Behavior. 1H
NMR spectroscopy affords important information about the Ag+-
complexing behavior of the present calix[4]arene neutral carriers,
i.e., how they undergo π-coordination toward the metal ion. This
helps us to understand the difference in the Ag+ selectivity against
other metal ions, especially Na+ and Tl+, among the calix[4]arene
neutral carriers. For instance, the 1H NMR spectrum for tert-
butylcalix[4]arene tetra(allyl ether) was changed by addition of
an equimolar amount of Ag+, especially in the chemical shift for
Figure 3. 1H NMR spectral changes for tert-butylcalix[4]arene tetra-
(allyl ether) 1 solution on Ag+ addition. (a) 2.5 × 10-3 mol dm-3 1;
(b) 2.5 × 10-3 mol dm-3 1 and Ag+ in CDCl3/CD3OD (4/1).
Table 2. 1H NMR Chemical Shift Changes (∆δ) for
Protons Assigned to Possible π-Coordinate Groups of
tert-Butylcalix[4]arene Neutral Carriers 1-4 on
Addition of Equimolar Ag+
∆δ (ppm)a
1
calixarene skeletal aromatic ring
π-coordinate group
protons of the potential π-coordinate groups. Typical H NMR-
spectral changes for the 1 -Ag+ system were given in Figure 3,
with respect to the proton peaks assigned to its allyl group and
aromatic ring. The changes (∆δ) in the chemical shift for protons
of the potential π-coordinate groups for the 1 system on the Ag+
addition are also listed in Table 2, together with the data for the
other π-coordinate tert-butylcalix[4]arene neutral carriers. The allyl
proton assigned to dCH2 shifted remarkably to a higher frequency
with ∆δ values of 0.51 and 0.62, while the aromatic protons shifted
slightly with a ∆δ value of 0.18. The significant spectral changes
support the much more extensive contribution of the allyl group
to the Ag+ complexation by π coordination rather than the
aromatic ring. Thus, a plausible structure of the Ag+ complex of
1 is as shown schematically in Scheme 1a.
In the tert-butylcalix[4]arene tetra(benzyl ether) 2 system, the
chemical shift changes are not very remarkable in the aromatic
protons, both for the benzyl and calixarene skeleton. This means
that the π coordination of the benzyl group in the 2 system is
not so efficient as that of the allyl group in the 1 system, probably
because of the steric effect of the bulky benzyl group. A marked
chemical shift change of the propargyl protons with a slight
change of the skeletal aromatic protons was also found in the
1
0.18
0.62 and 0.51 (dCH2)
0.26 (CHd)
0.05 and 0.12 (C6H5 for benzyl)
0.42 (tCH)
0.11 and 0.12 (dCH2)
(CHd)
2
3
4
0.17
0.08
0.40
0.14
∆δ(ppm) ) δ (with an equimolar amount of Ag+) - δ (without
the metal ion).
a
system of tert-butylcalix[4]arene tetra(propargyl ether) 3 in the
presence of Ag+.15 The propargyl-group-containing calixarene
derivative, 3 , must be a powerful π-coordinate ligand for Ag+. This
is also supported by the fact that the addition of an excess amount
(10 times) of Ag+ to the 3 solution for the NMR measurement
results in significant precipitation of its Ag+ complexes. The easy
precipitate formation of Ag+ complexes with 3 brought about the
poor sensitivity of its Ag+ electrode. In the electrode based on 3 ,
a severe drift was observed on the emf measurements, which is
definitely derived from some precipitate formation of the neutral
carrier-Ag+ complexes in the interface between the ion-sensing
membrane and aqueous sample phase.
Analytical Chemistry, Vol. 72, No. 21, November 1, 2000 5293

Scheme 1
Table 3. Silver-Ion Assay in Model Samples for Waste
Fixing Solutions for Photographya
It is of much interest to compare the Ag+-complexing behavior
between tert-butylcalix[4]arene tetra(allyl ether) 1 and its corre-
sponding allyl-ester derivative 4 . In contrast with the significant
chemical shift change of the allyl proton in the 1 system, the
change is considerably small in the 4 system. The chemical-shift
change for the skeletal aromatic proton is, on the other hand,
very remarkable. This suggests that the contribution of the allyl
group of 4 to its Ag+ complexation is not so great as that of 1 .
On such a conformation of 1 , the allyl groups may be a little too
far from the rigid calixarene skeleton to assemble the four allyl
groups effectively for the metal-ion complexation in tert-butylcalix-
[4]arene tetra(allyl ester) 4 . Scheme 1b presents a plausible
structure of the Ag+ complex with 4 , in which Ag+ is located
differently from the way it is located in the 1 complex. The Na+
interference in the Ag+ electrode based on 4 might be ascribable
not only to the powerful Na+ binding of the carbonyl groups but
also to the comparatively inefficient π interaction of the allyl groups
with Ag+.
Ag+ concn (mol dm-3
)
coefficient of
variation (%)
actual value
obtained value
1.0 × 10-3
7.5 × 10-4
5.0 × 10-4
1.0 × 10-4
1.0 × 10-3
7.5 × 10-4
4.8 × 10-4
1.1 × 10-4
4.9
6.5
26.6
29.2
a
The measurement was made 10 times by Gran’s plot method. See
the Experimental Section for the details.
seconds on the Ag+ activity change from 1 × 10-3 to 3 × 10-3
mol dm-3. The potential response, which is similar to that for
popular neutral-carrier-based alkali metal ion electrodes, is faster
than that for the Ag+-selective electrodes based on thiacrown ether
derivatives.5 Ag+ assay in aqueous sample solutions with high
background concentrations of Na+ and NH4+ was carried out with
the electrode based on tert-butylcalix[4]arene tetra(allyl ether) 1 ,
to check a possibility for Ag+ assay with the ion-selective
electrodes in waste fixing solutions for silver photography. The
results are summarized in Table 3. The values for coefficient of
variation depend on the Ag+ concentrations. In the Ag+ concentra-
tion of 7.5 × 10-4 mol dm-3 or higher, the metal ion can be
Since there are tiny but clear chemical-shift changes in the
skeletal aromatic protons in the system of the π-coordinate tert-
butylcalix[4]arene neutral carriers, 1 -4 , the skeletal aromatic
rings conceivably contribute to their Ag+ complexation to some
extent. This is evidenced by the fact that the calix[4]arene
derivatives not possessing any special π-coordinate group, 5 ,
determined within several percentage points of the coefficient of
variation even in the presence of 1 mol dm-3 of Na+ and NH4
1
works as the neutral carrier for Ag+-selective electrodes. The H
+
.
Such a Ag+ assay by the electrodes based on tert-butylcalix[4]-
arene tetra(ally ester) 4 , of course, is almost impossible due to
the terrible Na+ interference.
In conclusion, the present Ag+-selective electrode based on
tert-butylcalix[4]arene tetra(ally ether) 1 showed high sensitivity
and selectivity and fast potential response. The present ion sensors
may therefore be promising for applications such as a Ag+ assay
in sample solutions containing high backgrounds of alkali-metal
ions.
NMR data for the systems of 5 was a little difficult to interpret
due to their mixtures of cone and partial-cone conformations.
However, in the calix[4]arene derivatives not possessing any
special π-coordinate group, there seems to be some peak shift
(0.2-0.3 ppm) of the skeletal aromatic protons to a higher
frequency on the Ag+ addition. It is probable in the case of 5 that
the calixarene derivative accommodates Ag+ in the center of a
cylindrical cavity made from the calix[4]arene skeleton (Scheme
1c).7 The Tl+ selectivity over Ag+ in the electrode based on 5
may also suggest that Tl+ can fit into the cylindrical cavity of the
calix[4]arene tetra(propyl ether) 5 more suitably than Ag+ can.23
Application of Calixarene-Based Silver-Ion Electrodes.
The Ag+-selective electrode based on tert-butylcalix[4]arene tetra-
(ally ether) 1 is an excellent Ag+ sensor with high sensitivity and
selectivity. The response time (t90) for the electrode was several
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, and Culture.
Received for review April 28, 2000. Accepted August 7,
2000.
(23) Kimura, K.; Tatsumi, K.; Yokoyama, M.; Ouchi, M.; Mocerino, M. Anal.
Commun. 1 9 9 9 , 36, 229-230.
AC000490D
5294 Analytical Chemistry, Vol. 72, No. 21, November 1, 2000