Acrylic copolymers with pendant 1,2,4-triazole moieties as colorimetric sensory materials and solid phases for the removal and sensing of cations from aqueous media

Article abstract of DOI:10.1002/pola.24820

Linear water-soluble polymers and nets with gel-like behavior were prepared for use as colorimetric sensors and selective extracting agents for naked eye detection and solid-liquid removal of heavy-metal cations from aqueous environments. Triazole is a we

Full text of DOI:10.1002/pola.24820

Acrylic Copolymers with Pendant 1,2,4-Triazole Moieties as Colorimetric
Sensory Materials and Solid Phases for the Removal and Sensing of
Cations from Aqueous Media
´
´
´
ANA GOMEZ-VALDEMORO, MIRIAM TRIGO, SATURNINO IBEAS, FELIX CLEMENTE GARCIA, FELIPE SERNA,
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JOSE MIGUEL GARCIA
Departamento de Quı´mica, Facultad de Ciencias, Universidad de Burgos, Plaza de Misael Ban˜ uelos s/n, 09001 Burgos, Spain
Received 25 April 2011; accepted 1 June 2011
DOI: 10.1002/pola.24820
Published online 28 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Linear water-soluble polymers and nets with gel-
like behavior were prepared for use as colorimetric sensors
and selective extracting agents for naked eye detection and
solid–liquid removal of heavy-metal cations from aqueous
environments. Triazole is a well-known, heavy-metal cation
host. A water-insoluble methacrylamide containing a 1,2,4-tri-
azole receptor derivative was copolymerized with highly
was soaked in a yellow aqueous solution of Fe3þ, the former
colorless, transparent membrane became reddish (but
retained its transparency), and the solution became colorless.
Aqueous conductivity studies confirmed that both the
extraction and the sensing phenomena arose from the
C
V
interaction of the cations with the triazole moieties.
2011
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49:
3817–3825, 2011
hydrophilic and water-soluble methacrylamides to yield
a
water-soluble polymer or a cross-linked network as a powder
or a dense membrane. In powder form, the material selec-
tively removed Hg2þ, Pb2þ, and Agþ from aqueous media. In
the form of swelled dense membranes or films, the material
acted as colorimetric sensor for Fe3þ. When the membrane
KEYWORDS: acrylic polymers; cation elimination; cation sens-
ing; copolymers; membranes; metal-polymer complexes; poly-
methacrylamides; sensory materials; solid–liquid extraction;
supramolecular structures; water-soluble polymers
INTRODUCTION The selective and reversible interaction of
chemical species in aqueous media is a key event in biologi-
cal systems. The recognition processes, which arise from
these selective, highly directional, and weak interactions play
a major role in the activity of enzymes, the identification of
an antigen by an antibody, the discovery of a drug for a spe-
cific biological target, and many other applications.1–5
water-insoluble methacrylamide containing a triazole recep-
tor and an acid group that we have copolymerized with
highly hydrophilic comonomers to yield either a linear,
water-soluble, polymer or cross-linked water-swellable gels
in two form: as a powder and as a dense membrane or film.
The triazole heterocycle is a well-known cationic recep-
tor.14,34–48 Our materials interact selectively with heavy-
metal cations, allowing them to function as (a) excellent
agents for the removal of poisonous Hg2þ and Pb2þ from
effluent water via solid–liquid extraction and (b) colorimetric
sensory materials for the naked-eye detection and titration
The development of organic and organometallic receptors, in
an effort to mimic Nature, gave rise to host–guest or supra-
molecular chemistry.6–9 Within this broader area of study,
the field of chemosensing is focused on selective interactions
that produce a measurable macroscopic signal. Chemosen-
sors have promising possible applications to the fast, cheap
detection of chemicals, but their uses are often limited to so-
lution systems in organic media, because most of the sensory
molecules have very low aqueous solubility, and they have a
lack of mechanical properties that impair the preparation of
finished sensory devices.10
of aqueous Fe3þ
.
EXPERIMENTAL
All materials and solvents were commercially available and
were used as received, unless otherwise indicated. N-Methyl-
2-pyrrolidone (NMP, Merck) was vacuum distilled over phos-
phorus pentoxide twice, then stored over 4 Å molecular
sieves. 2-Ethoxyethyl methacrylate (EEMA; 99%, Aldrich)
was vacuum distilled. Pyridine (Merck) was dried under
reflux over sodium hydroxide for 24 h and distilled over 4 Å
molecular sieves. Azo-bis-isobutyronitrile (AIBN, Aldrich)
was recrystallized twice from methanol.
To overcome these problems with discrete organic or orga-
nometallic chemosensors, we propose to use polymers with
receptor moieties chemically anchored to the polymer struc-
ture.10–22 Previously, this approach has been undertaken by
several other research groups.10,23–33 We have prepared a
Correspondence to: J. M. Garcı´a (E-mail: jmiguel@ubu.es)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3817–3825 (2011) 2011 Wiley Periodicals, Inc.
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REMOVAL AND SENSING OF CATIONS FROM AQUEOUS MEDIA, GOMEZ-VALDEMORO ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Monomer synthesis.
Intermediates and Monomers
2-(2-Aminoethoxyethanol) Methacrylamide (4)49
The intermediates and monomers were prepared as depicted
in Scheme 1. Ethyl 4-(3-(4-nitrobenzoyl)thioureido)benzoate
(1) and 4-((5-(4-aminophenyl)-1H-1,2,4-triazol-3-yl)amino)-
benzoic acid (2) were prepared according to the procedures
that are described previously.17
A sample of 25 mmol of methacryloyl chloride was diluted
in 20 mL of tetrahydrofuran (THF) under a nitrogen blanket,
then slowly added to 20 mL of an ice-cold, methanolic solu-
tion of 2-(2-aminoethoxyethanol) (24 mmol). A 1 M aqueous
solution of KOH was added to maintain a pH of 8–9 through-
out the reaction. The mixture was warmed to room tempera-
ture over 4 h. Afterward, it was quenched by addition of HCl
to a final pH of 5, and the product was concentrated by dis-
tillation. NaCl was added to the crude residue, and the mix-
ture was extracted with dichloromethane. The organic layer
was dried with magnesium sulfate, filtered, and concentrated
by solvent distillation under vacuum at room temperature,
and the final traces of solvent were eliminated by bubbling
dry nitrogen to yield the product.
4-((5-(4-(Methacrylamidophenyl)-1H-1,2,4-triazol-3-yl)-
amino)Benzoic Acid (3)
A double-neck, 50-mL-round flask was fitted with a magnetic
stir bar, and a condenser was loaded with 5 mmol of 4-((5-
(4-aminophenyl)-1H-1,2,4-triazol-3-yl)amino)benzoic
acid
and 15 mL of pyridine. Then, 7.5 mmol of methacryloyl chlo-
ride was added dropwise, and the reaction mixture was
stirred for 3 h at room temperature (rt). Afterward, the solu-
tion was added dropwise to 5% aqueous HCl solution in an
ice bath to precipitate the product. The solid was collected
by filtration, washed thoroughly with water, and dried in a
Yield: 97%. 1H-NMR dH (400 MHz, DMSO-d6, Me4Si): 8.04
(1H, s, NH); 5.66 (1H, s, CH2); 5.26 (1H, s, CH2); 4.48 (1H, s,
OH); 1.88 (3H, s, CH3). 13C-NMR, dC (100.6 MHz, DMSO-d6,
Me4Si): 169.34, 139.72, 120.37, 72.42, 69.85, 61.61, 53.65,
39.84. EI-LRMS m/z: 174 ([MþH]þ, 10), 113 (12), 112 (26),
111 (21), 98 (25), 69 (100), 45 (30), 44 (17), 41 (54), FT-IR
[wavenumbers (cmꢁ1)]: mNAH, OAH: broadband (3503, 2489)
ꢀ
vacuum oven at 80 C overnight.
Yield: 97%. M.p.: 285 ꢀC (d). 1H-NMR dH (400 MHz, DMSO-
d6, Me4Si): 13.78 and 12.47 (each signal: 1H, br s, COOH/
NH); 10.05 (1H, s, NH); 9.83 (1H, s, NH); 7.96 (2H, d, J 8.4,
Ph); 7.90–7.88 (4H, m, Ph); 7.69 (2H, d, J 8.0 , Ph), 5.88 (1H,
s, CH2), 5.60 (1H, s, CH2) 2.01 (3H, s, CH3). 13C-NMR, dC
(100.6 MHz, DMSO-d6, Me4Si): 168.20, 167.94, 160.74,
153.55, 147.01, 141.50, 141.20, 131.61, 127.26, 123.18,
121.75, 121.27, 121.07, 115.83, 19.64. EI-LRMS m/z: 363
(Mþ, 100), 319 (48), 294 (12), 277 (18), 259 (5), 187 (6),
118 (6), 77 (15), 69 (59). FT-IR [wavenumbers (cmꢁ1)]:
mC¼O:1704; dNAH:1606.
Polymer Synthesis
Three different copolymers (ICOOH, IICOOH, and IIICOOH) were
prepared by radical copolymerization of mixtures of the
three different comonomers (see Table 1) using ethylene gly-
col dimethacrylate as a cross-linking agent to obtain the
cross-linked materials (IICOOH and IIICOOH) and AIBN (1.5
mol %) as a radical thermal initiator.
m
d
NAH: 3306; macid OAH: broadband (3096, 2871); mC¼O:1676;
NAH:1604; mAr C¼C: 1516; mC¼¼N: 1547.
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TABLE 1 Chemical Structure of Copolymers
Copolymers
Type
Physical Appearance
R1(x)a
R2(y)a
R3(z)a
CLAb
WSPc
d
ICOOH
Linear, water soluble
Cross-linked
Powder
–
95
95
69
5
5
1
–
5
1
–
IICOOH
IIICOOH
a
Powder
–
–
Cross-linked
Membrane or film
30
170
d
Molar composition ratio of the monomers used for the preparation of
Mn ¼ 3.6 ꢂ 104 g molꢁ1, Mw/Mn ¼ 1.71 [number–average molecular
the copolymers.
weight (Mn), and weight-average molecular weight (Mw) were deter-
mined by gel permeation chromatography using DMF (1% LiBr, 70 ꢀC)
as the eluent on the basis of standard poly(methyl methacrylate)
calibration].
b
molar
percentage
of
cross-linking
agent
(1,2-ethanediol-
dimethacrylate).
c
Water-swelling percentage: weight percentage of water uptake by the
films upon soaking in pure water until equilibrium.
Linear copolymethacrylamide (ICOOH) was prepared by radi-
cal polymerization of the hydrophilic monomer 2-(2-aminoe-
thoxyethanol) methacrylamide (4) and the heterocycle-con-
taining methacrylamide monomer (3) in a 95/5 molar ratio
as follows: 0.15 mmol of methacrylamide (3), 2.85 mmol of
methacrylamide (4), and 0.075 mmol of AIBN were dissolved
in 1.5 mL of dry NMP in an oxygen-free atmosphere and
kept at 65 ꢀC for 5 h. The solution was then precipitated in
cold acetone. The polymer thus obtained was collected by fil-
tration and purified by dissolved in methanol and reprecipi-
tated in cold acetone, after which it was dried in a vacuum
thacrylate). This film exhibited excellent mechanical proper-
ties, even after swelling with water. To prepare the dense
membrane, the reaction was carried out in a 100-lm thick
silanized glass mold in an oxygen-free atmosphere at 65 ꢀC
for 5 h.
To study the influence of the electronic environment of the
triazol moiety on the effectiveness of extraction, the carbox-
ylic acid moiety, which is chemically linked to the hetero-
cycle through a benzene ring, was deprotonated. Cross-linked
copolymer IICOOH was stirred in 15% aqueous Na2CO3 for 30
min, filtered off, and repeatedly washed with distilled water
until the pH of the washes was neutral. Then, it was dried in
a vacuum oven at 60 ꢀC overnight and crushed to obtain a
powder (IICOO-). The deprotonation of the film IICOOH was
accomplished following a similar procedure, without the final
crushing, to yield the membrane IIICOO-. Linear polymetha-
crylamide ICOOH was dissolved in 15% aqueous Na2CO3, and
the solution was added dropwise to a cold water/acetone
mixture (30:70 v/v) to precipitate the deprotonated polymer,
ꢀ
oven at 40 C overnight.
Cross-linked copolymethacrylamide IICOOH, used as a solid
extraction phase, as described later, was prepared as follows:
0.15 mmol of methacrylamide (3) was dissolved in 0.5 mL of
dry NMP in a cylindrical mold, then 2.850 mmol of metha-
crylamide (4), 0.150 mmol of ethylene glycol dimethacrylate,
and 0.075 mmol of AIBN were added in an oxygen-free
atmosphere and kept at 65 ꢀC for 5 h. The solid thus
obtained was extracted with methanol for 24 h in a Soxhlet
apparatus, dried in a vacuum oven at 60 ꢀC overnight, and
crushed to obtain a powdered material.
ICOO-. This material was used to tune and test the deprotona-
tion procedure.
Films, or dense membranes, having the composition of
IICOOH prepared by bulk polymerization did not display good
mechanical properties after swelling with water. To over-
come this problem, copolymer film IIICOOH was prepared by
polymerization of a mixture of the hydrophobic monomer
EEMA and the hydrophilic monomer 4 (30:69 mol), using
1% (mol) of the cross-linking agent (ethylene glycol dime-
Measurements
1H- and 13C-NMR spectra were recorded with a Varian Inova
400 spectrometer operating at 399.92 and 100.57 MHz, with
deuterated dimethylsulfoxide (DMSO-d6) as solvent.
Infrared spectra (FT-IR) were recorded with a Nicolet Impact
spectrometer.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
RESULTS AND DISCUSSION
Monomer and Polymer Synthesis and Characterization
A straightforward synthesis following common organic pro-
cedures yielded the methacrylamide comonomers 3 and 4 in
high purity. The NMR spectra of the monomers, shown in
Figure 1, display a clean pattern with signals corresponding
to the expected chemical structure of the monomers.
The polymeric materials were prepared in two different
architectures: as linear, water-soluble polymers and as cross-
linked polymer networks. The cross-linked materials, in turn,
had two forms: a powder and a film or dense membrane,
both of which displayed gel-like behavior.
The linear copolymer ICOOH was obtained by conventional radi-
cal polymerization in solution, followed by precipitation in a
nonsolvent. It was purified by solvation/reprecipitation to yield
1
a water-soluble powder. Figure 2 shows the H-NMR spectrum
of the copolymer; the signals of the monomer containing the tri-
azole core are amplified and assigned. Integration of the spec-
trum confirmed that the molar ratio of monomer 3 to monomer
4 within the copolymers corresponded to the feed ratio used in
the synthesis (5:95). The feed molar ratio was chosen to pro-
duce a water-soluble polymer with a high content of the water-
insoluble triazole-containing monomer 3, but an increase in the
content of 3 beyond this value gave rise to water-insoluble ma-
terial. Treatment of ICOOH with a base to deprotonate the acid
functionality produced the polymer ICOO-. Deprotonation caused
the polymer to become insoluble in organic solvents, such as
DMSO, but obviously did not affect the solubility in water.
The cross-linked polymers were prepared by bulk radical po-
lymerization using 1,2-ethanedioldimethacrylate as the cross-
linking agent. To obtain the powder form of the material,
IICOOH, the cross-linking agent was added to a final molar
content of 5%, along with a small amount of NMP to pro-
mote solubility, keeping the same 5:95 ratio of comonomers
3–4 as in the linear copolymer I (Table 1). Figure 3 shows
the IR-FT spectrum of IICOOH with assignments of relevant
FIGURE 1 1H-NMR spectra of monomers 3 (top) and 4 (bottom)
in DMSO-d6 (* ¼ solvent signals).
Thermogravimetric analysis (TGA) of a 5 mg sample was
performed under a nitrogen or oxygen atmosphere on aꢁT1A
ꢀ
Instrument Q50 TGA analyzer at a scan rate of 10 C min
.
The solid–liquid extraction of Cr3þ, Fe3þ, Co2þ, Ni2þ, Cu2þ
,
Zn2þ, Agþ, Cd2þ, Hg2þ, and Pb2þ as the nitrate or chloride
salt was carried out as follows: ꢃ10 mg of the copolymetha-
crylamide IICOOH or deprotonated polymethacrylamide IICOO-
was shaken with 1 mL of an aqueous solution of the metal
salt for a week at 25 ꢀC. The molar ratio of each cation to
the triazol heterocycle subunits was 1:1. After filtration, the
concentration of each cation in the liquid phase was deter-
mined by inductively coupled plasma mass spectrometry
(Agilient 7500 i), and direct information regarding the per-
centage of extraction of the metal ions by the polyamides
was obtained. Consecutive dilutions of sample aliquots with
ultra pure water/nitric acid (5% v/v) were performed to
reach concentrations in the range of the calibration curve
(0–40 ppb).
FIGURE 2 1H-NMR spectrum of polymer ICOOH in DMSO-d6 (* ¼
solvent signal).
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FIGURE 4 TGA of linear polymethacrylamide ICOOH (straight
line) and cross-linked polymethacrylamide IICOOH (dashed line)
in a nitrogen atmosphere (w.l.: weight loss percentage).
FIGURE 3 Normalized FT-IR spectra of polymers II:IICOOH (red
line) and IICOO- (black line). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
resistance of polymers ICOOH, IICOOH, and IIICOOH was eval-
uated by thermogravimetry, and the decomposition tempera-
stretching and bending bands corresponding to the func-
tional groups of the copolymer structure. Treatment of
IICOOH with a base deprotonated the acid functionality to
give IICOO-, and a clear change in the IR-FT spectrum was
observed. A new shoulder corresponding to the asymmetric
stretching of the carboxylate groups was detected.
tures that resulted in 5% and 10% weight loss, T5 and T10
,
are shown in Table 2. The results indicate that the materials
have from moderate to good thermal stability, in both inert
and oxidizing atmospheres. The cross-linked materials exhib-
ited higher degradation temperatures, because the thermal
breakage of a bond is less probable to produce a volatile res-
idue in a network than in a linear structure. The different
thermal behaviors of the linear and cross-linked copolymers
ICOOH and IICOOH are shown in Figure 4. The first degrada-
tion step corresponds broadly with the loss of part of the
pendant aliphatic residue.
The film, or dense membrane, forms of the material, IIICOOH
,
were prepared with a lower content of triazole-containing
monomer 3 to obtain films that could be analyzed by UV/vis
spectroscopy. The hydrophobic monomer EEMA was used as
comonomer to give tractable films following swelling with
water. A lower content of the cross-linking agent was used
so as to not excessively reduce the hydrophilic character, or
gel behavior, of the membrane. The water uptake of the
membrane, following soaking in pure water at rt until uptake
equilibrium was reached, was 170% by weight (Table 1).14
This result shows that water-solvated species, such as anions
and cations, can access and interact with the isolated hydro-
phobic sensing motifs within the polymer structure via diffu-
sion into the predominantly hydrophilic swelled membrane.
Hydrophilicity of the Polymers
Polymer structures need to have a highly hydrophilic charac-
ter to be useful for solid–liquid extraction from aqueous
TABLE 3 Solid–Liquid Extraction of Metal Cations from
Aqueous Solution by Solid-Phase Polymethacrylamide IICOOH
and Its Deprotonated Derivative IICOO- Extraction Percentage
(% E), Distribution Coefficient (Kd), and Selectivity (a) Under
Noncompetitive Conditions
Properties of the Polymers
Thermal Properties
IICOOH
IICOO-
Thermal resistance is a key parameter of the suitability of
organic materials for technological applications. The thermal
%
Kd
(L molꢁ1
a
%
Kd
(L molꢁ1
a
Salt
E
)
(Kd /Kd
)
E
)
(Kd /Kd
)
i
Pb
i
Pb
TABLE 2 Thermal TGA Data of the Polymers in an Inert (N2)
Cr(NO3)3 22 69
0.02
0.03
0.04
0.01
0.01
0.01
37 14 ꢂ 10
33 12 ꢂ 10
44 19 ꢂ 10
43 18 ꢂ 10
74 69 ꢂ 10
37 14 ꢂ 10
97 78 ꢂ 102
47 21 ꢂ 10
85 14 ꢂ 102
97 78 ꢂ 102
0.02
0.02
0.02
0.02
0.09
0.02
1.00
0.03
0.18
1.00
Atmosphere
Fe(NO3)3 30 10 ꢂ 10
Co(NO3)2 34 13 ꢂ 10
Ni(NO3)2 10 28
b
c
Polymer
T5 (ꢀC)
T10 (ꢀC)
Char Yield (%)
a
ICOOH
273 (262)
298
301 (293)
321
6.2 (0)
0.5
Cu(NO3)2 25 22
IICOOH
IIICOOH
a
ZnCl2
12 33
305
331
0.0
AgNO3
88 17 ꢂ 102 0.56
Data shown in parentheses correspond to results in an oxidizing (O2)
atmosphere.
Cd(NO3)2 14 38
0.01
Hg(NO3)2 90 23 ꢂ 102 0.75
b
5% weight loss.
10% weight loss.
Pb(NO3)2 93 31 ꢂ 102 1.00
c
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 Left: UV/vis titration curve of film IIICOOH with aqueous Fe3þ. Right: I/I0 versus Fe3þ concentration at 455 nm; inset:
expansion of the I/I0 versus Fe3þ concentration curve. The reference was an aqueous solution of Fe3þ with the same concentration
as that of the sample cuvette. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
ꢀ
ꢁꢀ ꢁ
media and to behave as sensory materials for the detection
of water-solvated species. Because the highly hydrophilic
monomer 4 was known to produce water-soluble linear
polymers, we chose it as the high molar fraction comonomer
for our materials, both linear and cross-linked. Upon increas-
ing the molar content of water-insoluble monomer 3 relative
to that of monomer 4 in the solution polymerization, starting
at 1%, the linear polymer product remained water soluble
until the content of 3 reached 5%.
%E
V
Kd ¼
(1)
100 ꢁ %E
n
The solid–liquid extraction selectivity, aS,L, is the ratio of two
distribution coefficients as shown in eq 2. The cation (Pb2þ
with the highest distribution coefficient (Kd ) is taken as the
)
M2
reference.
Kd
M1
aS;L
¼
(2)
Kd
M2
The comonomer composition for membrane IIICOOH was cho-
sen to produce a material with good tractability after soak-
ing in water, while maintaining a gel behavior. A hydrophobic
comonomer, EEMA, was therefore added to the mixture of
hydrophilic monomer 4 and triazole-containing monomer 3
along with the cross-linker to give the terpolymer. The molar
content of the hydrophobic monomer was chosen to give
films with good mechanical properties after swelling with
water while maintaining the overall hydrophilic character of
the membrane. The water-swelling percentage of IIICOOH was
170% by weight, confirming the desired gel behavior of the
material.
The ability of the cross-linked powder material II to perform
solid–liquid of heavy-metal cations from aqueous media was
tested with both protonated and deprotonated pendant acid
groups (IICOOH and IICOO-). The extraction of the environmen-
tally deleterious cations Hg2þ and Pb2þ by IICOOH was signifi-
cantly high, 90% or higher, and was selective with respect to
other cations, although the extraction of Agþ was also notewor-
thy. The high extraction percentage of Hg2þ was consistent with
the literature data regarding the favorable host–guest interac-
tion between the triazole heterocycle and the mercury cation.
Upon deprotonation of the polymer to give IICOO-, the extraction
percentages of the majority of the cations increased signifi-
cantly, probably because of electron pumping from the carboxy-
late group to the triazole moiety through the benzene ring.
Applications of the Polymer
Solid–Liquid Extraction
A solid–liquid extraction takes place whenever a solution
containing different target or guest molecules is in contact
with a solid phase that can interact selectively with these
guest molecules, thus acting as a solid host. In our materi-
als, the heterocyclic triazole moieties of the solid polyme-
thacrylamide act as host molecules for water-solvated cati-
ons. The selectivity and effectiveness of the extraction can
be described in terms of the extraction percentage (% E)
and the distribution coefficient (Kd). The distribution coef-
ficient is a measure of the capacity of the material to
extract cations under competitive conditions and is here
defined as the ratio of the concentrations of the cation in
both phases, eq 1, where n is the molar amount of
polymer structural units and, V is the solution volume in
liters.17–22
Colorimetric Sensing of Fe3þ in Water Media
During the extraction experiments, the material IIICOOH
appeared to behave as a cation-sensor for Fe3þ. This poly-
meric material was initially colorless, but became reddish
upon soaking in aqueous solutions containing Fe3þ. In con-
trast, the polymer remained completely colorless in the pres-
ence of the other cations tested in the extraction
experiments.
To test the colorimetric cation-sensing ability of the polymer,
increasing quantities of Fe3þ were added to a quartz UV/vis
cell containing the IIICOOH polymer film. As shown in Figure
5, the absorbance of the 455 nm band gradually increased as
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ARTICLE
brownish and the solution to become colorless, a clear indi-
cation that the concentration of Fe3þ in the water had dimin-
ished because of the interaction of the cations with the host
motifs of the membrane.
Deprotonating the IIICOOH membrane produced the IIICOO-
membrane. The colorimetric response of this membrane to
aqueous cations was very similar to that of the protonated
form, albeit somewhat less sensitive toward Fe3þ. The titra-
tion curve shown in Figure 7 shows a measurable colorimet-
ric response above 3 ꢂ 10ꢁ4 M, or 16 ppm. Like the proto-
nated membrane, it displayed a clear range of measurable
Fe3þ concentrations, with one plateau below the detection
limit and another at the highest measurable concentration.
We made a comment regarding the detection limits and
measurable regions of this kind of system. These can be eas-
ily modified and tuned to fit situational needs by increasing
or decreasing the molar ratio of the sensing monomer (3) to
the other comonomers (4 and the cross-linking agent). These
highly interesting parameters may also be modified by
increasing or decreasing the hydrophilic character, or the gel
behavior, of the membrane using other comonomers or by
varying the cross-linking ratio.
FIGURE 6 A cuvette containing 8 ꢂ 10ꢁ3 M aqueous Fe(NO3)3
before (left) and after (right) introducing the IIICOOH membrane,
which is immobilized on a black support.
Conductivity
the concentration of Fe3þ rose. A titration could thus be per-
formed, which resulted in a detection limit below 6 ppb,
much lower than 300 ppb limit for iron established by the
United States Environmetal Protection Agency (EPA) in the
National Secondary Drinking Water Regulation.50 The bind-
ing isotherm plateaued at a Fe3þ concentration of 4 ꢂ 10ꢁ3
Mꢁ1 because of the saturation of the receptor motifs within
the membrane, an upper limit to the concentration of iron
that can be measured. The film remained colorless upon addi-
tion of any of the other cations used in the solid–liquid extrac-
tion experiments, indicating a high selectivity toward Fe3þ in
aqueous solution. The different behavior of Fe3þ, compared
with other cations regarding the host/guest interaction with
hosts containing nitrogen atoms, was attributed by Cui et al.31
to two facts: (a) the smaller size of Fe3þ, resulting in a higher
charge density; therefore, having a higher electron-accepting
capability and consequently leading to more stable complexes
and (b) the d5 structure, where the five electrons may be pres-
ent as two orbitals occupied by pairs of electrons and one hav-
ing single occupancy, so it could form an inner-orbital com-
plex, which is more stable than the other host/metal
complexes. Thomann et al.51 and Kolnaar et al.52 demonstrated
the capability of 1,2,4-triazole-containing compounds to give
stable bonds with Fe3þ by means of characterizing the com-
plexes structures using X-ray diffraction technique. The authors
reported N–Fe3þ distances of about 2.2 Å for the complexes in
the solid state, forming each two nitrogens of each heterocycle
To gain insight about the cation–polymer interaction, conduc-
tivity measurements were performed on solutions containing
ICOOH with a solubilized polymer content corresponding to a 5
ꢂ 10ꢁ4 M concentration of the triazole-containing monomer
(3), increasing concentrations of Hg2þ or Pb2þ salts, and 2-
[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)-1,3-propanediol
(BIS–TRIS) buffer (pH 7.0).
The variation of the molar conductivity, DK ¼ Kexp ꢁ K0, of
aqueous Hg(NO3)2 and Pb(NO3)2 at different salt concentra-
tions in the presence (Kexp) and the absence (K0) of polymer
ICOOH is shown in Figure 8. The negative values of DK can
two bonds with two different Fe3þ
.
The colorimetric detection of aqueous Fe3þ by the IIICOOH
membrane can be seen by the naked eye in two ways, as
shown in Figure 6. Placing the colorless membrane, in a
black support, in a UV–vis cuvette containing the yellow
aqueous Fe3þ solution caused the membrane to become
FIGURE 7 UV/vis titration curve of film IIICOOH with aqueous
Fe3þ. I/I0 versus Fe3þ concentration at 457 nm. The reference
was an aqueous solution of Fe3þ with the same concentration
as that of the sample cuvette.
´
REMOVAL AND SENSING OF CATIONS FROM AQUEOUS MEDIA, GOMEZ-VALDEMORO ET AL.
3823

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
molecules to appropriate polymeric nets, these supramolecu-
lar phenomena can be easily performed in water.
The authors gratefully acknowledge financial support provided
by the Spanish MICINN (Ministerio de Ciencia e Innovacio´n)-
FEDER (MAT2008-00946) and by the Junta de Castilla y Leo´n
(BU001A10-2).
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