Colorimetric anion sensing by polyamide models containing urea-binding sites

Article abstract of DOI:10.1080/10610270903531549

Novel colorimetric polyamide model compounds for anion-sensing applications are described. Light beige solutions of one of the models (M3) in DMSO develop a red to yellow-reddish colour upon addition of some anions, including OH^-, F^-

Full text of DOI:10.1080/10610270903531549

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Colorimetric anion sensing by polyamide models
containing urea-binding sites
Noelia San-José a , Ana Gómez-Valdemoro a , Saturnino Ibeas a , Félix Clemente García a ,
Felipe Serna a & José Miguel García a
a Departamento de Química, Facultad de Ciencias , Universidad de Burgos , Plaza de Misael
Bañuelos s/n, E-09001, Burgos, Spain
Published online: 05 May 2010.
To cite this article: Noelia San-José , Ana Gómez-Valdemoro , Saturnino Ibeas , Félix Clemente García , Felipe Serna &
José Miguel García (2010) Colorimetric anion sensing by polyamide models containing urea-binding sites, Supramolecular
Chemistry, 22:6, 325-338, DOI: 10.1080/10610270903531549
To link to this article: http://dx.doi.org/10.1080/10610270903531549
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Supramolecular Chemistry
Vol. 22, No. 6, June 2010, 325–338
Colorimetric anion sensing by polyamide models containing urea-binding sites
´ ´ ´ ´ ´ ´
Noelia San-Jose, Ana Gomez-Valdemoro, Saturnino Ibeas, Felix Clemente Garcıa, Felipe Serna and Jose Miguel Garcıa*
´
Departamento de Quımica, Facultad de Ciencias, Universidad de Burgos, Plaza de Misael Ban˜uelos s/n, E-09001 Burgos, Spain
(Received 10 September 2009; final version received 2 December 2009)
Novel colorimetric polyamide model compounds for anion-sensing applications are described. Light beige solutions of one
of the models (M3) in DMSO develop a red to yellow-reddish colour upon addition of some anions, including OH2, F2,
HCO23 , PO432, AcO2, benzoate2, oxalate22 and p-toluenesulphonate2. In contrast, they remain unchanged in the presence
of CH2ClCO22 , H2PO42, CF3CO22, Br2, methanesulphonate2, Cl2 or I2. The reddish colour caused by the addition of basic
anions is due to the deprotonation of the urea groups, while the yellowish colour corresponds to the modification of the
electron density surrounding the urea group due to complexation. The development of the yellow colour of DMSO solutions
of M3 upon addition of acetate ions and the disappearance of the red colour of solutions of M3/fluoride by the displacement
of acetate ions can serve as a novel colorimetric assay for the titration of the acetate anion, with detection limits close to
1 ppb. The host:guest stoichiometry (polyamide model compound:anion complexes) and the stability constants of complexes
were determined by 1H NMR.
Keywords: urea-binding sites; anion sensing; polyamide model compounds
1. Introduction
The protein behaves as a hydrophilic macromolecule,
surrounded by water, with partially hydrophobic binding
sites where hydrogen bonds, or other feeble interactions,
between these sites and the substrates can be formed in the
absence of competition with water (8–12).
The development of selective and sensitive materials for
the colorimetric detection of anionic, cationic and neutral
molecules is a topic of current scientific interest (1–7).
In particular, the sensing of anions has drawn considerable
attention because charged chemical species are ubiquitous
and play major roles in many chemical and biological
processes. Detection is often based on the design and
synthesis of receptors (host molecules) capable of
selectively binding anionic species (guest molecules)
with a concomitant change in a measurable macroscopic
property, such as fluorescence or UV–vis spectra.
The field of synthetic anion receptor chemistry is one of
the fastest growing disciplines within supramolecular
chemistry.
The complexation properties of such receptor proteins
can be mimicked by chemically sophisticated hosts with
well-organised binding sites or, alternatively, by slightly
hydrophilic synthetic macromolecules with a chemically
simple receptor moiety. The latter approach has been
scarcely explored, though it has shown promising results
in the selective recognition of anionic species in water
solutions (6, 8–11, 13–18). For example, rectangular
strips were prepared with polymethacrylate with a
pendant pyrylium substructure, showing excellent colori-
metric selectivity towards hydrogen carbonate in water.
Remarkably, the colour changes in the presence of the
anion were observed only in the slightly water-swelled
polymeric matrix, and there was no change upon addition
of the anion to an aqueous solution of the pyrylium
monomer (14, 18). Polymers with host moieties are not
only excellent supports but also allow fine alterations of
their hydrophilic/hydrophobic characters, allowing for the
tuning of their sensing ability.
Methodologies based on colorimetric changes of host–
guest interactions are especially attractive for their
potential use in naked-eye screening applications,
especially in situations where conventional techniques
are not appropriate or are expensive, or in the development
of screening applications that can be used by non-scientist
personnel.
The design and syntheses of molecules with selective
anionic receptor sites are complex, due mainly to the
variety of geometric shapes of anions. The recognition of
anions in aqueous environments by biological systems is
achieved via hydrogen bonding by highly organised
proteins with partially hydrophobic and sterically well-
defined complexation sites in the interior of the protein.
Our work focused on the development of polymeric
optical transducers for analyses. In previous articles, we
reported on the syntheses and characterisations of polymer
materials with the following host units: crown ether
(19–21), urea (22–25) or pyrylium (13, 18), along with
*Corresponding author. Email: jmiguel@ubu.es
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610270903531549
http://www.informaworld.com

´
N. San-Jose et al.
326
O
C
O
C
HOOC
COOH
NH2
LiCl, Pyridine
NMP, TPP
N
H
N
H
+
2
115°C, N2
NH
O
NH
k
l
NH
Ar
O
NH
Ar
q
NH
NH
Ar =
O
r
NO2
Diacid: (1)
Polyamide model: M1
(2)
M2
(3)
(4)
M4
M3
Scheme 1. Synthesis, chemical structure and codes of the polyamide models.
their interaction with cations (26–31), anions (13, 18) or
gas molecules (32). Herein, we report four polyamide
model compounds (Scheme 1) that exhibit sensing
selectivities towards acetate ions; one of these showed
colorimetric sensing capabilities towards acetate anions.
The models probably mimic polyamides (Scheme 2,
P1–P4), polymers derived from diacid monomers (1–4),
which are intermediates in the synthesis of the models.
The polymeric materials are supposed to retain the ability
to interact selectively with these diamines, such that cheap
future sensing devices might be user-friendly naked-eye
polyamide film sensors or incorporate a polyamide coating
at the end of an optic fibre connected to a portable UV–vis
diode-array detector. Moreover, the aromatic polyamides
are a class of polymers with great technological
importance, due mainly to their mechanical and electrical
properties and outstanding thermal resistances (13, 33).
Thus, sensing devices prepared using these materials could
be utilised in a wide range of thermal and mechanical
conditions. These properties make them unique
O
C
O
C
HOOC
COOH
NH
HN
LiCl, Pyridine
NMP, TPP
H2N
NH2
x
+
NH
115°C, N2
O
NH
NH
Ar1
O
NH
Ar1
NH
NH
Ar1 =
O
NO2
Polymer code:
P1
P2
P3
P4
Scheme 2. Synthesis, chemical structure and identification of the polyamides.

Supramolecular Chemistry
327
components in the control of industrial chemical
concentrations, such as in air, gases, reactors or waste
products. They also have potential in environmental
protection and occupational safety and health, through the
determination of chemical concentrations.
and 20 ml of NMP. The solution was stirred and heated at
1108C under a dry nitrogen blanket for 4 h. Then, the
system was cooled at room temperature and the solution
was precipitated in 300 ml of methanol. The compound
obtained was filtered and washed with distilled water and
acetone. Then, it was extracted with acetone for 24 h in a
Soxhlet, and dried in a vacuum oven at 808C overnight
(yield, 94%). It was obtained as a semi-crystalline solid
(55% crystallinity) that crystallised at 1698C. Mp: 2568C.
Tg: 1228C. 1H NMR (400 MHz, DMSO-d6): d (ppm),
10.48 (s, 2H); 9.16 (s, 1H); 8.85 (s, 1H); 8.23 (s, 2H); 8.17
(s, 1H); 7.86 (d, J ¼ 8.3 Hz, 4H); 7.55 (d, J ¼ 7.9 Hz, 2H);
7.42 (m, 4H); 7.35 (m, 2H); 7.17 (t, J ¼ 7.7 Hz, 2H); 7.04
(t, J ¼ 7.3 Hz, 1H). 13C NMR (100.6 MHz, DMSO-d6): d
(ppm), 166.21; 153.51; 141.11; 140.39; 140.03; 136.94;
129.79; 129.66; 124.77; 123.11; 121.28; 120.93; 119.40.
EI-LR-MS m/z: 331 (55), 239 (100), 185 (11), 119 (60), 93
(62), 65 (16). FT-IR [wavenumbers (cm21)]: nNZH: 3290;
2. Experimental
All materials and solvents were commercially available
and used as received, unless otherwise indicated. N-
Methyl-2-pyrrolidone (NMP) was vacuum-distilled twice
over phosphorous pentoxide, and then stored in the
˚
presence of 4 A molecular sieves. Lithium chloride was
dried at 4008C for 12 h prior to use. Triphenylphosphite
(TPP) was vacuum-distilled twice over calcium hydride,
˚
and then stored in the presence of 4 A molecular
sieves. Pyridine was dried under reflux over sodium
˚
hydroxide for 24 h, and distilled over 4 A molecular
n
CvO: 1663, 1651; dNZH: 1598; nArCvC: 1541.
5-(3-Naphthalen-1-yl-ureido)-N,N0-diphenyl-isophtha-
sieves. 5-(3-Phenylureido)isophthalic acid (1), 5-(3-
naphthylureido)isophthalic acid (2), 5-(3-(4-nitrophenyl)-
ureido)isophthalic acid (3) and 5-(3-(40-ureidophenyl)-
ureidophenyl)isopthalic acid (4) were prepared by
previously described procedures (22).
lamide (M2), 5-[3-(4-nitro-phenyl)-ureido]-N,N0-diphenyl-
isophthalamide (M3) and N,N0-diphenyl-5-{3-[4-(3-phenyl-
ureido)-phenyl]-ureido}isophthalamide (M4) were prepared
and purified in a similar manner to that of M1.
The following salts were used as received in the
complexation studies: sodium hydroxide (98%; Panreac,
Barcelona, Spain), tetrabutylammonium fluoride hydrate
(98%; Sigma-Aldrich, St Louis, MO, USA), sodium
bicarbonate (99.5%; Sigma-Aldrich), lithium phosphate
(99%; Sigma-Aldrich), lithium acetate (99.99%; Sigma-
Aldrich), lithium benzoate (99%; Sigma-Aldrich), potass-
M2 was obtained as an amorphous solid that
crystallised at 2048C. Yield: 92%. Mp: 2568C. Tg:
1
1308C. H NMR (400 MHz, DMSO-d6): d (ppm), 10.50
(s, 2H); 9.56 (s, 1H); 8.94 (s, 1H); 8.26 (s, 2H); 8.17 (s,
2H); 8.05 (d, J ¼ 7.5 Hz, 1H); 7.99 (d, J ¼ 8.1 Hz, 1H);
7.84 (d, J ¼ 7.8 Hz, 4H); 7.75–7.50 (m, 4H); 7.42 (m,
4H); 7.17 (t, J ¼ 7.5, 2H). 13C NMR (100.6 MHz, DMSO-
d6): d (ppm), 166.18; 153.94; 141.15; 140.02; 136.99;
134.96; 134.67; 129.63; 129.39; 127.15; 126.91; 126.80;
126.78; 124.74; 124.36; 122.32; 121.27; 121.19; 120.96;
118.92. EI-LR-MS m/z: 331 (56), 239 (100), 185 (19), 169
(37), 143 (75), 115 (33), 93 (48), 65 (30). FT-IR
[wavenumbers (cm21)]: nNZH: 3272; nCvO: 1683, 1649;
¨
ium oxalate monohydrate (puriss.; Riedel-de Haen, Seelze,
Germany), sodium p-toluenesulphonate (95%; Sigma-
Aldrich), sodium chloroacetate (98%; Sigma-Aldrich),
lithium dihydrogenphosphate (99%; Sigma-Aldrich),
lithium trifluoroacetate (95%; Sigma-Aldrich), lithium
¨
bromide (puriss.; Riedel-de Haen), sodium methane-
sulphonate (98%; Sigma-Aldrich), lithium chloride
(anhydrous puriss.; Sigma-Aldrich) and tetrabutylammo-
nium iodide (99%; Sigma-Aldrich). Thus, the anion-
binding studies were carried out using different counter-
ions (lithium, sodium, potassium and tetrabutylammo-
nium). The solvents used in the complexation studies were
dry dimethyl sulphoxide (DMSO, 99%; Merck, Darm-
stadt, Germany) or dry DMSO-d6 (deuteration degree min.
99.8%; Merck).
d
NZH: 1598; nArCvC: 1537.
M3 was obtained as a semi-crystalline solid (50%
crystallinity) that crystallised at 2018C. Yield: 87%. Mp:
2718C. Tg: 1578C. 1H NMR (400 MHz, DMSO-d6): d
(ppm), 10.49 (s, 2H); 9.63 (s, 1H); 9.42 (s, 1H); 8.28–8.23
(m, 5H); 7.86–7.78 (m, 6H); 7.42 (m, 4H); 7.17 (t,
J ¼ 7.5 Hz, 2H). 13C NMR (100.6 MHz, DMSO-d6): d
(ppm), 166.06; 153.05; 147.10; 142.19; 140.45; 139.99;
136.97; 129.66; 126.12; 124.81; 121.82; 121.58; 121.29;
118.72. EI-LR-MS m/z: 331 (37), 239 (61), 185 (11), 164
(35), 138 (85), 119 (57), 108 (51), 91 (56), 78 (42). FT-IR
[wavenumbers (cm21)]: nNZH: 3425; nCvO: 1721, 1668;
2.1 Polyamide model synthesis
2.1.1 N,N0-Diphenyl-5-(3-phenyl-ureido)-
isophthalamide model (M1)
d
NZH: 1597; nArCvC: 1540; nNO (As,S): 1552, 1329.
2
M4 was obtained as a crystalline solid. Yield: 91%.
In a 50 ml three-necked flask fitted with a mechanical
stirrer, 20 mmol of aniline, 10 mmol of 5-(3-phenyl-
ureido)isophthalic acid and 1.4 g of lithium chloride were
dissolved in a mixture of 6 ml of pyridine, 22 mmol of TPP
Mp: 2918C. Tg: not observed. 1H NMR (400 MHz,
DMSO-d6): d (ppm), 10.48 (s, 2H); 9.11 (s, 1H); 8.73
(s, 1H); 8.66 (s, 1H); 8.61 (s, 1H); 8.23 (s, 2H); 8.16
(s, 1H); 7.85 (d, J ¼ 8.0 Hz, 1H); 7.52–7.39 (m, 10H);

´
N. San-Jose et al.
328
7.32 (m, 2H); 7.17 (t, J ¼ 7.3 Hz, 2H); 6.99 (t, J ¼ 7.2 Hz,
1H). 13C NMR (100.6 MHz, DMSO-d6): d (ppm), 166.23;
153.60; 153.58; 141.26; 140.79; 140.04; 136.93; 135.34;
134.70; 129.73; 129.66; 124.76; 122.63; 121.27; 121.23;
120.79; 120.28; 119.92; 119.06. EI-LR-MS m/z: 357 (19),
331 (56), 265 (31), 239 (100), 227 (28), 185 (12), 134 (42),
119 (57), 108 (41), 93 (61). FT-IR [wavenumbers (cm21)]:
Differential scanning calorimetry data were
recorded on a Perkin-Elmer Pyris I analyser from
10 mg of sample under a nitrogen atmosphere at a scan
rate of 208C/min.
The membrane cross-section was observed by means
of scanning electron microscopy (SEM). The SEM images
were obtained with a JEOL JSM-6460LV, using an
accelerating voltage of 20 kV. The polymer samples were
cryofractured from film membranes and coated with gold.
Stock solutions of polyamide model compounds and of
concentrated solutions of the salts were prepared in DMSO
n
n
NZH: 3342; nCvO: 1669, 1648, 1636; dNZH: 1599;
ArCvC: 1561.
The polyamide model syntheses and codes are
depicted in Scheme 1.
1
or DMSO-d6 for the UV–vis and H NMR experiments,
respectively. Samples were prepared directly in UV–vis
cuvettes or in NMR tubes by adding the proper polyamide
model compound in the DMSO solution and mixing with
small variable volumes of the concentrated salt solutions.
Although the total volume variation was negligible, it was
considered in the analysis of the UV–vis spectra.
Potassium oxalate and lithium trifluoroacetate were not
soluble in DMSO, so water solutions were used instead of
DMSO solutions. This situation is troublesome because
water competes effectively with the urea groups, giving
rise to solvated anions, which are unfavourable for
creating host:guest complexes. Therefore, the results with
these anions may be questionable.
2.2 Polyamide synthesis, characterisation and
membrane preparation
The reaction of the diacids (1–3; Scheme 1) with
m-phenylenediamine yielded polyamides, as depicted in
Scheme 2. The polymerisation and the polymer charac-
terisation were performed following previously described
procedures (22).
The dense polyamide membranes were prepared by
casting an N,N-dimethylformamide (DMF) solution
containing about 8 wt% of the corresponding polymer on
glass plates. The membranes were then dried in an oven at
708C for 4 h. After removing the membranes from the glass
plates by distilled water, they were vacuum-dried for 24 h
at room temperature and reduced pressure. The resulting
yellowish transparent dense membranes had a thickness of
20–50 mm.
The skinned asymmetric membranes were prepared
by the phase inversion precipitation method. The
polymeric solutions consisted of 15–20 wt% of cellulose
acetate (39.8 wt% acetyl content, average Mn ,30,000;
Aldrich, St Louis, MO, USA), 10 wt% of the
corresponding polyamide and 70–75 wt% of DMF as
the solvent. The polymer solutions were cast onto glass
plates and subsequently immersed in a 25^18C
water/DMF coagulation bath (up to 30% v/v of DMF).
The membrane was peeled off from the glass plate,
washed for 12 h with water to remove all solvents, dried
at room temperature for 24 h and dried further at reduced
pressure for 48 h.
3. Results and discussion
Four polyamide model compounds were prepared
following simple and common synthetic procedures
(Scheme 1). The new polyamide model compounds
contain one or two urea groups and two aromatic amide
linkages, in which the carbonyl groups of the amides act as
slightly electron-withdrawing groups and the urea moieties
act as feeble electron-donating subunits. The urea group is
electron-donating because of its resonance effect and is
conversely electron-withdrawing due to its field effect.
The resonance effect is predominantly observed in the
nitration of N,N0-diphenylurea, during which the urea
group directs the nitration mainly to the para-positions
and slightly to the ortho-positions but not to the
meta-positions (34). As an illustrative example, Figure 1
shows the characterisation of M3.
Due to their strong hydrogen-bonding ability (35–54),
urea groups have been widely exploited in compounds that
exhibit sensing behaviour towards anions, both in solution
and the solid state. However, their possibilities in anion-
sensing applications have not been completely analysed,
partly because of the complex nature of the intermolecular
interactions of the urea-containing organic molecules.
Aromatic urea derivatives have shown the ability to form
oligomeric assemblies, or capsules, in which guests can be
encapsulated in solution. These complex capsules persist
even in the solid state, as demonstrated by the X-ray
2.3 Measurements and instrumentation
The 1H and 13C NMR spectra were recorded using a
Varian Inova 400 spectrometer operating at 399.92 and
100.57 MHz, respectively, with DMSO-d6 as the solvent.
Fast atom bombardment and low-resolution electron
impact mass spectra (EI-LR-MS, 70 eV) were measured
on a Micromass AutoSpec Waters mass spectrometer.
Infrared spectra (FT-IR) were recorded using a Nicolet
Impact spectrometer.
The UV–vis spectra were recorded using a Varian
Cary3-Bio UV–vis spectrophotometer.
´
analysis by Alajarın et al. (55–57).

Supramolecular Chemistry
329
(a)
a
O
O
h
g
b
f
d
N
N
H
e
c
H
i
j
νC=O
νN–H
k HN
O
m
HN
l
n
o
νAr C-C
νNO
2
p
q
4000
3000
Wavenumber (cm–1
2000
1000
NO2
)
c
b
(b)
e
(c)
i
c
b
o
p
p
g
a
f
o
h
d
q
a
i
m
h
l
j
k
n
10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0
170
160
150
140
δ, ppm
130
120
δ, ppm
1
Figure 1. Characterisation of the polyamide M3 model by (a) FT-IR, (b) H NMR and (c) 13C NMR.
The interactions of the model compounds with
different anions (OH2, F2, HCO23 , PO432, AcO2,
benzoate, oxalate, p-toluenesulphonate, CH2ClCO22 ,
H2PO42, CF3CO22, Br2, methanesulphonate, Cl2 and I2)
in solution were evaluated using NMR and UV–vis
techniques.
as strong bases in organic conditions, causing the
deprotonation of the urea group and giving rise to a charge
transfer complex responsible for the UV–vis absorption
band centred at 478 nm. This is a well-known behaviour
described by Esteban-Gomez et al. (58). As an example, the
UV–vis spectrum of the M3:F2 system in DMSO is shown
in Figure 3. The addition of water causes the disappearance
of the charge transfer absorption band (lmax ¼ 490 nm)
and the development of a new absorption band at
lmax ¼ 478 nm, characteristic of the host–guest inter-
actions between the anion (F2) and the model. The
breakage of the strong charge transfer complex, giving rise
to the host–guest complex upon water addition, can be
monitored by the appearance of an isosbestic point
Figure 2 shows the effect of the addition of anions to a
solution of M3 in DMSO, as seen by the naked eye. Colour
changes are observed upon addition of (a) AcO2,
benzoate, oxalate and p-toluenesulphonate (yellow) and
(b) OH2, F2, HCO32 and PO342 (red–orange).
The reddish colour developed upon addition of OH2,
F2, HCO23 and PO342 is not due to a host–guest interaction,
but to an acid–base reaction. The unsolvated anions behave
Figure 2. Photograph of M3 solutions in DMSO ([M3] ¼ 2 £ 1023 M). (a) Upon addition of complexing anions, from left to right:
control blank, acetate, benzoate, oxalate, p-toluenesulphonate, chloroacetate, dihydrogen phosphate, trifluoroacetate, bromide,
methanesulphonate, chloride and iodide. (b) Upon addition of basic anions producing deprotonation of the urea group, from left to right:
control blank, OH2, F2, HCO32 and PO342
.

´
N. San-Jose et al.
330
3.1 Titration of the acetate ion in an M3 solution in
DMSO
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
The UV–vis spectrum of M3 in a DMSO solution shows
an absorption maximum around 346 nm, giving rise to a
light beige solution. Upon addition of lithium acetate, the
solution turns golden yellow due to the displacement of the
lmax to higher wavelengths (Figure 5). The magnitude of
the displacement can be correlated with the molar ratio of
M3 to acetate ions, allowing for a titration of acetate ions
with a detection limit of approximately 100 ppb. A more
correct titration curve can be obtained by monitoring the
absorbance at 367 nm and plotting these values as a
function of the acetate concentration in a DMSO solution
of M3 (5 £ 1025 M) (Figure 5). From the curve-fitting
equations, values of the acetate concentration can be
obtained as a function of the lmax or the absorbance at
367 nm. Acetate concentrations between 65 and 610 ppb
could be estimated by both calculations with mean errors
of 8 and 11%, respectively, and with a mean error of 6%
when combining both calculations.
300
350
400
450
500
550
λ (nm)
Figure 3. UV–vis spectra of M3 in DMSO
([M3] ¼ 3 £ 1025 M) with 10 equivalents of F2 without water
(dashed line) and with different water contents (solid lines). The
arrows show the trend of the absorption bands with increasing
quantities of water.
(Figure 3; 305 nm), corresponding to the different
concentration of the M3:F2 complex. The disappearance
of the urea proton signal in the 1H NMR spectra of M3 in
DMSO-d6 solution upon addition of F2 is in agreement
with the deprotonation of urea and the formation of the
strong charge transfer push–pull complex. Figure 4 shows
the UV–vis spectra of M3 solutions with acetate, benzoate,
oxalate and fluoride anions.
3.2 Titration of the acetate ion by the displacement of
fluoride from its binding complex with M3 in DMSO
Efficient displacement of fluoride in the host:guest M3:F2
complexes was observed by titration with acetate. The red
solution of M3:F2 in DMSO turns yellow upon addition of
increasing quantities of acetate (Figure 6), indicating the
displacement of the fluoride anion.
The other model compounds, M1, M2 and M4, do not
show a colorimetric response towards anions, although
they exhibit complexing abilities towards anions, as
detected by UV–vis and 1H NMR spectra. The differences
between these models and M3 arise from the strong
electron-withdrawing nitro group in the 4-nitrophenyl-
ureido moiety in the latter.
Figure 6 shows the UV–vis spectra of a DMSO solution
of M3:F2 (1:1) with increasing concentrations of acetate.
Fitting the curve of the acetate:M3 ratio, which is equal to
the acetate:F2 ratio vs. the absorbance at 364 and 479 nm
(Figure 7) allows for the calculation of the acetate
concentration once the absorbance is known. The
calculated acetate concentrations in the range of
0.6–2.6 ppm had a mean error of 3%, whereas the mean
error for calculated anion concentrations between 5 ppb and
3 ppm was 7%. The latter range of acetate concentrations
corresponds to a ratio of M3:F2:AcO2 of 1:1:1.
The driving force of the process is largely dependent
on the strong acetate:urea interaction (Table 1). The proton
mobility associated with the acid–base reaction of the urea
group and the naked fluorine anion is restricted by the
double hydrogen bond of the acetate ion/urea (see the
chemical structure in Figure 9, inset). The acetate
counterion (Liþ) could also play a role in the stabilisation
of the fluoride anion, giving rise to ion pairs.
0.4
M3:C6H5COO–
0.3
M3
M3:AcO–
M3:Oxalate2–
0.2
M3:F–
0.1
0.0
300
350
400
450
500
550
λ (nm)
Figure 4. UV–vis spectra of M3 solutions in DMSO
([M3] ¼ 2 £ 1025 M) with 10 equivalents of fluoride, acetate,
benzoate and oxalate. The spectra of the M3 solutions with 10
equivalents of p-toluenesulphonate, methanesulphonate,
trifluoroacetate and chloroacetate are indistinguishable from the
initial solution. The solid line represents the spectrum of M3 in
DMSO without salts.
3.3 Analysing the host:guest interactions by stability
constants
The strength of the interaction of the anions (acetate,
benzoate, oxalate, p-toluenesulphonate, trifluoroacetate

Supramolecular Chemistry
331
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
(a)
300
350
400
450
λ (nm)
(b)
(c)
356
0.195
0.190
0.185
0.180
0.175
0.170
0.165
0.160
354
352
350
0
2
4
6
8
10 12
0
2
4
6
8
10 12
[AcO–] x 106
[AcO–] x 106
Figure 5. (a) UV–vis spectra of solutions of M3:acetate ([M3] ¼ 2 £ 1025 M). The arrows indicate the spectra obtained upon
increasing the acetate concentration from a M3:acetate ratio of 1:0 to 1:10. (b) Titration curves of acetate at 367 nm vs. acetate
concentration. (c) lmax vs. acetate concentration.
and chloroacetate) with the polyamide models in DMSO
solution was analysed in terms of their stability constants
1
by H NMR (59–63).
To get insight into the interactions between anions
1
and models in solution, H NMR spectra of the models
addition of increasing quantities of the anions. Upon
addition of the anions, chemical shifts of the protons of
the urea groups to lower magnetic fields were observed
(see Figures 8 and 9 for the M3:acetate system). The
graphical representation of the displacements of the
chemical shifts of the M3 model upon addition of
different quantities of acetate ions (Figure 9) clearly
points to the formation of a 1:1 complex (model:anion).
The same behaviour was observed for models M1 and
in DMSO-d6 (0.05 M) were measured at 208C with the
1.2
364 nm
1.0
0.8
0.6
0.4
0.2
479 nm
0.0
Figure 6. UV–vis spectra of M3:F2 (1:1) solutions in DMSO
upon adding increasing quantities of acetate ions to reach a ratio
of M3:F2:AcO2 of 1:1:4. Inset: photograph depicting titration.
The figure depicts the ratio of fluoride to acetate, with the arrows
indicating increasing concentrations of acetate. The dashed line
corresponds to the initial spectrum of M3:F2 (1:1) without
acetate. The concentrations of M3 and fluoride were 5 £ 1025 M.
0
1
2
3
4
5
[AcO–]/[M3]
Figure 7. Absorbance of DMSO solutions of M3:F2 (1:1) at a
given wavelength upon addition of increasing quantities of
acetate ions, reaching a ratio of M3:F2:AcO2 of 1:1:4. The
concentration of M3 was 5 £ 1025 M.

´
N. San-Jose et al.
332
Table 1. Stability constants (K1) corresponding to the interaction of models M1–M3 with selected anions obtained upon application of
Equation (9) (for proton assignment see Scheme 1).
Urea proton k
Urea proton l
Anion
K1
DdAM
K1
DdAM
M1 modela
M2 modela
M3 modela
Benzoate
Acetate
Trifluoroacetate
Oxalate
358^115
6641^4378
218^96
3.19^0.08
3.18^0.03
0.47^0.02
1.25^0.04
344^114
6553^4800
198^80
3.09^0.08
3.11^0.03
0.51^0.02
1.29^0.04
16^2
16^2
Benzoate
Acetate
Trifluoroacetate
Oxalate
127^14
385^192
16^3
2.84^0.04
2.71^0.09
0.71^0.04
0.09b
113^11
340^154
15^3
2.46^0.03
2.45^0.08
0.64^0.04
0.06b
b
b
–
–
Benzoate
Acetate
p-Toluenesulphonate
Oxalate
Chloroacetate
647^245
28889^8317
11.8^0.8
3.63^0.03
3.68^0.01
0.54^0.01
0.48b
691^232
21516^8120
13^1
3.56^0.05
3.59^0.08
0.51^0.02
0.45b
b
b
–
33^3
–
28^3
1.71^0.04
1.45^0.03
a The trifluoroacetate and oxalate anions did not fit well to the mathematical model, because of the small water content of the measurement solutions.
The salts were insoluble in DMSO, therefore requiring them to be added as concentrated water solutions.
b The stability constants are too low to be accurately measured (DdAM values are experimental values).
M2 with all the studied anions. However, the presence of
two urea moieties in the M4 model gave rise to 1:2
complexes.
The urea protons of the M3 model in DMSO-d6
showed sharp 1H NMR signals. Upon addition of
acetate, the signal broadened, indicating a fast exchange
of the acetate between different urea groups when the
M3:acetate ratio is low (lower than 1:0.8). Upon
increasing the acetate concentration to a ratio of 1:1, the
signals became sharp again, indicating a low mobility of
the ion and a strong stability constant, with a negligible
Molar ratio
free acetate concentration in solution.
* *
[M3]/[AcO–]
1/10
l
1/5.0
1/2.0
1/1.5
1/1.0
1/0.8
1/0.5
1/0.3
1/0.1
1/0.0
13
12
11
10
9
k
O
C
O
C
N
H
N
H
k
N
N
H
O
* *
O
CH3
l
H
O
* *
* *
10
* *
III
14
13
12
11
9
8
7
0
2
4
6
8
10
δ, ppm
[AcO–]/[M3]
Figure 8. 1H NMR spectra of the M3:lithium acetate system at
different molar ratios of M3 to acetate ion (solvent, DMSO-d6;
[M3] ¼ 5 £ 1022 M; *, urea proton signals).
Figure 9. Chemical shifts of the urea protons vs. acetate to M3
molar ratio ([M3] ¼ 5 £ 1022 M).

Supramolecular Chemistry
333
The stoichiometry of the anion:metal complexes have Thus, the stability constant is given by Equation (3):
been mathematically determined by studying the chemical
shift displacements of the urea protons (as determined
½MAꢀ
1
from H NMR experiments) vs. the anion concentration
K1 ¼
:
ð3Þ
½Mꢀ½Aꢀ
and their corresponding Job’s plots for the following
anions: acetate, benzoate, trifluoroacetate, chloroacetate,
p-toluenesulphonate and oxalate. Job’s plots of M1–M3
have a maxima appearing at XH ¼ 0.5, clearly indicating
the formation of complexes with a 1:1 stoichiometry with
the different anions, even with the divalent oxalate.
Conversely, the maxima of M4 were found at 0.33,
pointing to complexes with 1:2 stoichiometry (model to
anions). As an illustrative example, Figure 10 depicts the
chemical shift displacements and Job’s plots correspond-
ing to the interaction of the benzoate anion with the
models.
Under fast-exchange conditions (59), the observed
chemical shift for a model can be expressed by
Equation (4):
Ddobs ¼ xMdM þ xMAdMA
;
ð4Þ
where dM and dMA represent the chemical shift of the free
and complexed models, respectively, and xM and xMA are
their molar fractions. From the mass balance, the observed
chemical shifts can be expressed as Ddobs ¼ dobs 2 dM and
DdMA ¼ dMA 2 dM. Equation (5) is deduced from
Equation (4):
With the stoichiometry of the complexes known, the
dimensionless stability constants of the formation of an
n:m complex (MmAn) between the anion (A) and the model
(M) can be determined by
DdMA
Ddobs
¼
½MAꢀ;
ð5Þ
½Mꢀ0
½MmAnꢀ
mM þ nL N MmAn; Knm
¼
:
ð1Þ
m
n
where [M]0 is the initial concentration of model M.
The mass balances of M and A are given by
½Mꢀ ½Lꢀ
We studied the interaction of the models with
benzoate, acetate, trifluoroacetate, chlorobenzoate,
p-toluenesulphonate and oxalate. As previously outlined,
the M1–M3 models have a 1:1 stoichiometry, whereas
M4 is characterised by a 1:2 (M4:anion) stoichiometry.
The study of the complex formation between each
model:anion pair was performed following the chemical
shift variations of the proton signals k, l, q and r (Scheme 1)
of the models in DMSO-d6 at 208C with increasing anion
concentrations.
½Mꢀ0 ¼ ½Mꢀ þ ½MAꢀ
ð6Þ
and
½Aꢀ0 ¼ ½Aꢀ þ ½MAꢀ:
ð7Þ
Considering Equation (3) and the mass balances of M
and A (Equations (6) and (7)), the complex concentration
can be solved as a function of K1 and the initial
concentrations of M and A:
The 1:1 complex formation between the host (model)
and the guest (anion) is represented by Equation (2), where
M represents the model, A represents the anion and MA
represents the complex with a 1:1 stoichiometry.
ꢀ
ꢁ
1
2
½MAꢀ 2 ½Aꢀ0 þ½Mꢀ0 þ
½MAꢀþ½Aꢀ0½Mꢀ0 ¼ 0: ð8Þ
K1
M þ A N MA:
ð2Þ
(a) 4.0
(b)
1.5
1.0
0.5
0.0
M1
M2
M3
M4
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
M1
M2
M3
M4
0.00
0.25
0.50
0.0
0.2
0.4
0.6
0.8
1.0
[A],M
XM
Figure 10. (a) Plot of the observed chemical shift displacements, Dd, of the urea protons vs. benzoate concentration. (b) The
corresponding Job’s plot. Solid lines are the results of fitting and applying the corresponding equations.

´
N. San-Jose et al.
334
From Equations (3) and (8), Equation (9) is deduced as Therefore, gas-phase density functional theory calcu-
lations at the B3LYP/6-31G level of theory were
*
ꢂꢀ
ꢁ
DdMA
1
performed following a previously described procedure
(31, 66). The interaction energies of the acetate ion with
simplified models of M1–M3 (i.e. diphenylurea, 1-
(naphthalen-2-yl)-3-phenylurea and 1-(4-nitrophenyl)-3-
phenylurea) were 2224, 2219 and 2266 kJ/mol,
respectively.
Ddobs
¼
½Aꢀ0 þ ½Mꢀ0 þ
½Mꢀ0
K1
9
=
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð9Þ
ꢀ
ꢁ
2
1
2
½Aꢀ0 þ ½Mꢀ0 þ
24½Aꢀ0½Mꢀ0
;
;
K1
If we consider a 1:2 complex formation between the
host (M4 model) and the guest (anion), the complex
formation can be represented by Equations (10) and (11):
which relates the experimental Ddobs with the [A]0.
Thus, the experimental data can be fitted using a nonlinear
least-squared algorithm (64) to obtain the parameters K1
and DdMA
.
½MAꢀ
M þ A N MA; K1 ¼
ð10Þ
The stability constants of the 1:1 complexes were
calculated for M1–M3 with Equation (9) separately using
the chemical shifts of the two urea protons (k and l;
Scheme 1). The results (Table 1) show that the stability
constantscalculatedusingbothprotonsignalsare equivalent,
as expected, confirming thevalidity of the calculation model.
The host–guest analyses performed by the ‘naked-eye’
and the UV–vis technique confirmed that the addition of
acetate ions to a solution of M3 in DMSO gave rise to the
stronger colour changes. The stability constants for each
model obtained by the 1H NMR analyses revealed that the
acetate anion has the strongest interaction for all of the
models in DMSO. The strength of the different complexes
is related to the electron density surrounding the urea
group. Thus, the high electron-withdrawing capability of
the nitro group through inductive and mesomeric effects
withdraws electron density from the urea group, giving
rise to the higher stability constants of the M3:anion
complexes. Furthermore, the strength of the interaction of
the carboxylate anions with a specific model depends
inversely on the pKa of their conjugated bases in
DMSO (65), thus the association depends inversely on
the localisation of the negative charge over the carboxylic
group because of the mesomeric effect and the electron
density due to inductive effects.
½Mꢀ½Aꢀ
and
½MA2ꢀ
MA þ A N MA2; K2 ¼
;
ð11Þ
½MAꢀ½Aꢀ
where K1 and K2 are stability constants of the complexes.
The mass balances are expressed as
½Mꢀ0 ¼ ½Mꢀ þ ½MAꢀ þ ½MA2ꢀ
ð12Þ
and
½Aꢀ0 ¼ ½Aꢀ þ ½MAꢀ þ 2½MA2ꢀ:
ð13Þ
Considering the stability constant, Equations (10)
and (11) and the mass balances of M and A (Equations (12)
and (13)), Equation (14) can be deduced as
3
2
K1K2½Aꢀ þ ðK1K2ð2½Mꢀ0 2 ½Aꢀ0Þ þ K1Þ½Aꢀ
þ ðK1ð½Mꢀ0 2 ½Aꢀ0Þ þ 1Þ½Aꢀ þ ½Aꢀ0
¼ 0:
ð14Þ
The order of stability constants of the model:acetate
complexes (M3:acetate . M1:acetate . M2:acetate)
is in agreement with theoretical calculations (ab initio).
Given the concentration of [A], the equilibrium
concentrations of M, MA and MA2 can be calculated
Table 2. Stability constants (K1 and K2) corresponding to the interaction of M4 with selected anions (for proton assignment see Scheme 1).
Anions
K1
K2
DdAM
DdMA
K1
K2
DdAM
DdMA
2
2
Urea proton k
Urea proton l
Benzoate
Acetate
p-Toluenesulphonate
379
778
14
85
1.1
2.9
2.3
2.77
0.23
2.99
3.05
0.44
436
774
23
80
0.4
2.2
2.15
2.65
0.21
2.83
3.13
0.56
Urea proton q
Urea proton r
Benzoate
Acetate
p-Toluenesulphonate
424
0.005
27
121
101
1.2
1.19
1.71
0.18
2.66
2.41
0.72
424
0.005
27
121
97
1.1
1.17
1.67
0.18
2.63
2.35
0.71
Note: The stability constants corresponding to the M4:oxalate complex are too low to be accurately measured and are not depicted in the table.

Supramolecular Chemistry
335
Table 3. Stability constants (K1 and K2) corresponding to the interaction of M4 with acetate calculated from the 1:1 interaction model
for each urea group using Equation (9) (for proton assignment see Scheme 1).
Urea proton k
528^93 (778)
Urea proton l
Urea proton q
97^25 (101)
Urea proton r
98^24 (97)
K1
557^119 (774)
Note: For comparison, we included the values obtained by the 2:1 model (Equation (19)) in parentheses.
as described by Al-Soufi et al. (60):
4. Outlooks
The amide linkages are the key motif of the polyamide
backbones, and are naturally present in the polyamide
models. Nevertheless, the amide groups do not play an
important role in the complexation. The small downfield
displacement of the amide protons upon interaction with
acetate ions, as observed by 1H NMR (Figure 8), arises from
modification of the overall electron density of the molecule
due to the interaction of the urea motif with the anions, as
can be observed in the chemical shift displacements of the
amidic protons vs. acetate to M3 molar ratio (Figure 11),
where it is observed a change in the slope of the curve
corresponding to a M3:acetate molar ratio of 1:1.
½Mꢀ0
½Mꢀ ¼
;
ð15Þ
ð16Þ
2
1 þ K1½Aꢀ þ K1K2½Aꢀ
½MAꢀ ¼ K1½Aꢀ½Mꢀ
and
½MA2ꢀ ¼ K2½MAꢀ½Aꢀ:
ð17Þ
The observed chemical shift of a proton, dobs, is given
by the following equation:
Solutions in DMSO of the polymers prepared upon
polymerisation of the diacids 1–4 with m-phenylenedi-
amine (Scheme 2) retain the ability to interact selectively
Ddobs ¼ xMdM þ xMAdMA þ xMA dMA
;
2
ð18Þ
2
where dM, dMA and dMA represent the chemical shifts of
the free and complexed forms of the model, and xM, xMA
2
1
with anions, as determined by qualitative H NMR and
‘naked-eye’ analyses. However, the addition of water to
the solution gives rise to the solvation of the anions with
the concomitant loss of the host–guest interactions.
Nevertheless, and in agreement with previous results
(13, 18), a membrane with an adequate hydrophilic/
hydrophobic balance can partially inhibit the efficient
water–anion interaction, giving rise to the recognition
event inside the membrane in a solid–liquid interaction,
where the solid is a partially water-swelled matter.
The water-swelled organic membrane must have enough
water content to allow thewater-solvated anions to enter the
membranebydiffusion. Atthesame time, thiswater content
should be lower enough to let lipophilic domains inside the
and xMA are their molar fractions, respectively. From
Equations (12) and (18), the following expression can be
deduced:
2
DdMA
DdMA
2
Ddobs
¼
½MAꢀ þ
½MA2ꢀ:
ð19Þ
½Mꢀ0
½Mꢀ0
As has been previously outlined for a 1:1 stoichio-
metry, Equation (19) can be fitted to experimental data
considering Equations (14)–(17) and using a nonlinear
least-squared algorithm (67) to obtain the parameters K1,
K2, DdMA and DdMA . The results of the stability constants
2
are depicted in Table 2. Upon fitting the data of each urea
proton, the two stability constants are sometimes obtained,
indicating that the interaction of a single model guest
molecule with one of the urea groups can affect the other.
This is true for benzoate and p-toluenesulphonate, but not
for acetate. The distinct behaviour for acetate is reflected
in the results obtained using Equation (19) and is shown in
Table 2. When we tried to obtain the stability constants
with the chemical shifts of one of the urea groups, we
obtained a correct value for the constant of the interaction
with this group but a value near zero for the other urea
group. We believe that this fact is due to the restricted
mobility of the acetate anions because of the stronger
host:acetate interaction compared to those with other
anions. Nevertheless, each constant of M4:acetate can be
estimated by applying the 1:1 model to each urea group by
Equation (9) (Table 3), and the results are similar to those
obtained with the 2:1 model.
0.25
0.20
0.15
0.10
0.05
0.00
0
2
4
6
8
10
[AcO–]/[M3]
Figure 11. Chemical shifts of the amidic protons vs. acetate to
M3 molar ratio ([M3] ¼ 5 £ 1022 M).

´
N. San-Jose et al.
336
Figure 12. (a) SEM cross-section image of a cellulose acetate/P3 asymmetric membrane obtained by casting a solution of cellulose
acetate/P3/DMF (15:10:75, w/w/v) in a water/DMF (70:30, v/v) coagulation bath. (b) Enlargement of a pore.
membrane where the recognition process takes place.
In polyacrylate and polymethacrylate networks, mem-
branes with a water uptake of 50% w/w have an optimum
balance for anion recognition in pure water.
models behave as an effective colorimetric probe for
carboxylic acid salts, specifically for acetate. The light
beige solutions of M3 in DMSO develop a reddish colour
upon addition of basic anions, such as OH2 and F2, and a
yellowish colour upon adding acetate salts. The titration of
acetate can be performed by adding the anion to (a) a
model solution in DMSO, giving rise to the development of
a yellow colour, or (b) a red solution of a mixture of
model/fluoride (1:1), turning the reddish solution to
yellow due to the displacement of fluoride by the acetate.
The host:guest stoichiometry (polyamide model
compound:anion complexes) and the stability constants
Dense membranes, or film strips, prepared with
polyamides P1–P4 (Scheme 2) were hydrophobic [films
prepared with P3 showed a water uptake of 3.6% at 65%
relative humidity (rh)] and remained silent in water media
with increasing quantities of anions. In order to increase
the hydrophilicity, the films were prepared with a blend of
cellulose acetate and P3 (10% of the latter), achieving a
water uptake of 14.5% (at 65% rh).
1
The blend of the polyamides with cellulose acetate
has also been used to obtain asymmetric membranes.
The objective was to increase the surface of the materials to
test the influence on the recognition processes in water
media. The pore control was achieved by means of
varying the DMF content of the water/DMF coagulation
bath. Figure 12 depicts an SEM photograph of an
asymmetric membrane showing large transversal pores
and their porous structures.
of complexes were determined by H NMR. The models
mimic polyamides, the polymers derived from the diacid
monomers (1–4), which are intermediates in the synthesis
of the models. The polymers retain the ability to interact
selectively with anions in organic media, and cheap future
sensing devices might take the form of user-friendly
naked-eye polyamide film sensors or might incorporate a
polyamide coating at the end of an optic fibre connected to
a portable UV–vis diode-array detector. Further work is
needed to achieve a colorimetric response of the polymer
materials in water environments.
Both membranes, dense and asymmetric, based on P3
and cellulose acetate, show a weak ‘naked-eye’ colori-
metric response towards fluoride in water. Thus, white
strips obtained with the membranes turned to pale red
when immersed in water containing the fluoride ion.
Nevertheless, a high concentration of fluoride was needed
to observe the phenomenon. Accordingly, further work is
needed to obtain positive results with much lower guest
concentrations in water environments. One way to achieve
this is to increase the hydrophilicity of the system by
increasing the water affinity of the polymer to be blended
with the polyamides.
Acknowledgements
The financial support provided by the Spanish MICINN
´
´
e Innovacion) – FEDER
(Ministerio de Investigacion
(MAT2008-00946/MAT) and the Caja de Burgos is gratefully
acknowledged.
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