Colorimetric detection of anions in aqueous solution using poly(phenylacetylene) with sulfonamide receptors activated by electron withdrawing group

Article abstract of DOI:10.1021/ma301767g

A series of sulfonamide-conjugated poly(phenylacetylene)s with various electron withdrawing and donating substituents were designed and synthesized in order to develop a novel colorimetric probe for anions in water. The UV-vis absorption measurements, which were performed in an organic solvent to provide fundamental insights into the anion detection properties of these polymers, clarified that the colorimetric response ability is highly enhanced by the incorporation of strong electron withdrawing groups (EWG). Sulfonamide- conjugated poly(phenylacetylene) with nitro groups, the strongest EWG, was therefore used for the detection of anions in an aqueous environment. In a solution containing up to 20% water, the polymer exhibited obvious color changes to anions including the biologically important carboxylates. On the basis of the NMR titration analysis, such a colorimetric response was confirmed to be based on the deprotonation event of the sulfonamide binding site, which was suggested to be essential for the detection of aqueous anions.

Full text of DOI:10.1021/ma301767g

Article
pubs.acs.org/Macromolecules
Colorimetric Detection of Anions in Aqueous Solution Using
Poly(phenylacetylene) with Sulfonamide Receptors Activated by
Electron Withdrawing Group
Ryosuke Sakai,† Eric B. Barasa, Naoya Sakai, Shin-ichiro Sato, Toshifumi Satoh, and Toyoji Kakuchi*
Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628,
Japan
S
* Supporting Information
ABSTRACT: A series of sulfonamide-conjugated poly-
(phenylacetylene)s with various electron withdrawing and
donating substituents were designed and synthesized in order
to develop a novel colorimetric probe for anions in water. The
UV−vis absorption measurements, which were performed in
an organic solvent to provide fundamental insights into the
anion detection properties of these polymers, clarified that the
colorimetric response ability is highly enhanced by the
incorporation of strong electron withdrawing groups (EWG).
Sulfonamide-conjugated poly(phenylacetylene) with nitro
groups, the strongest EWG, was therefore used for the
detection of anions in an aqueous environment. In a solution
containing up to 20% water, the polymer exhibited obvious color changes to anions including the biologically important
carboxylates. On the basis of the NMR titration analysis, such a colorimetric response was confirmed to be based on the
deprotonation event of the sulfonamide binding site, which was suggested to be essential for the detection of aqueous anions.
specific sensor design has recently been developed, which
includes the spatial arrangement of multiple binding sites,15−18
the construction of hydrophobic microenvironments on
receptor moieties,19−22 the incorporation of a metal complex
receptor,23−25 and employing highly acidic donor units as the
binding sites.26−28
INTRODUCTION
■
Anions are essential to life, since many biological processes
including osmosis and ATP formation cannot be realized
without them. They are also important for many industrial
processes such as electroplating and are therefore often found
as harmful pollutants in industrial waste. Consequently, the
development of reliable, robust, and practical anion sensors
applicable for use in complex media, such as serum, cells, soil,
and environmental water, is highly desired.1
The fabrication of chemosensors for anion sensing based on
supramolecular concepts naturally requires a means to output
the sensory event in a quantifiable and measurable change.2−14
One of the more appealing approaches involves the
construction of colorimetric anion sensors, since visual
detection without requiring any spectroscopic instrumentation
immediately indicates qualitative information, while absorption
spectroscopy provides quantitative information. Although
considerable effort has therefore been devoted to the
fabrication of colorimetric anion sensors, a direct sensing of
anions in water is still recognized as a remarkably challenging
task because of the difficulty in the anion recognition process in
water due to the intrinsic properties of anions.9 The most
problematic aspect in the sensing of aqueous anions is that
anions usually have higher free energies of solvation than
cations with the same absolute charge and comparable size.
Hence, in the anion binding process, the sensor molecule must
compete with the solvent molecules especially in polar solvents
such as water. To achieve anion sensing in aqueous solutions, a
We have thoroughly investigated anion detection ability of
conjugated polymers for the development of reliable and
sophisticated anion sensor materials.29−36 Based on such
examinations, we discovered that the conjugated polymer-
based sensors possessed characteristic anion sensing abilities
that have never been observed for small molecule-based
sensors, e.g., variable colorimetric response dependent on the
types of anions,29 highly size-specific anion detection,35 and
efficient anion detection based on the positive allosteric
binding.34 These unique and unprecedented anion sensing
abilities, which are considered to be based on the characteristic
feature of the conjugated polymer,37−40 appear to overcome the
present limitation in the development of a practical anion
sensor. However, such superior anion detection abilities of
these polymers are limited in organic solvents. The polymers
hardly exhibit colorimetric responses to anions in a solvent
containing even a small amount of water due to the inhibition
of the polymer−anion interaction by the water molecules. In
Received: August 23, 2012
Revised: September 28, 2012
Published: October 11, 2012
© 2012 American Chemical Society
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Chart 1. Structures of Sulfonamide-Functionalized Poly(phenylacetylene)s with Different Substituents
Scheme 1. Synthesis of a Series of Sulfonamide-Functionalized Poly(phenylacetylene)s
a
Table 1. Synthetic Results for a Series of Sulfonamide-Conjugated Poly(phenylacetylene)s
d
solubility
b
c
c
polymer
yield (%)
Mn × 10−5
Mw/Mn
DMF
DMSO
THF
DCM
Acetone
Poly-1a
Poly-1b
Poly-1c
Poly-1d
Poly-1e
Poly-1f
86
68
84
85
81
82
0.90
4.77
2.44
2.91
1.20
1.64
1.97
5.65
3.54
2.57
3.33
4.99
++
++
++
++
++
++



++











++


+
++
+
+


a
b
Condition: [monomer] = 0.03 M; [monomer]0/[Rh] = 50; catalyst, Rh(nbd)BPh4; solvent, dry DMF; temperature, 25 °C, time = 24 h. Diethyl
ether insoluble part, determined via gravimetry. Determined by SEC in DMF containing LiCl (0.01 M) using polystyrene standards. ++, soluble; +,
c
d
partially soluble; , insoluble at room temperature.
order to develop practical anion sensor materials, the realization
of a conjugated polymer-based sensor capable of directly
detecting aqueous anions is now very desirable.
response to anions was examined in organic solvent. Moreover,
a H NMR measurement was carried out to elucidate the
anion-binding mechanism of the sulfonamide receptors. After
determining the most suitable candidate, we finally applied this
polymeric receptor in the detection of anions in aqueous media.
1
We have designed poly(phenylacetylene)s with sulfonamide
binding sites (Poly-1a-f), which emerge as ideal candidates
because of the expectation that the highly acidic sulfonamide
will operate as an anion receptor even in an aqueous solution
(Chart 1).41−43 A series of substituent groups with varying
electron withdrawing (EWG) and donating (EDG) properties
was incorporated to explore the optimized receptor design.
First, in order to provide a fundamental insight into the anion
detection ability of the synthesized polymers, the colorimetric
RESULTS AND DISCUSSION
■
Polymer Synthesis. The desired polymers were synthe-
sized via a two-step synthetic route as illustrated in Scheme 1.
The coupling reactions between ethynylaniline and a series of
substituted bezenesulfonyl chlorides (2a−f) proceeded in high
yields to produce the various sulfonamide-conjugated novel
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monomers (1a−f). To provide an extended π-conjugated
system for anion signaling, the prepared monomers were
polymerized using Rh(nbd)BPh4 as the catalyst to produce the
corresponding polymers (Poly-1a−f) in good yields (Table 1).
In all cases, the respective SEC profiles showed monodisperse
polymers with high molecular weights. All of the obtained
polymers were soluble in N,N-dimethylformamide (DMF), but
partially soluble or insoluble in dimethyl sulfoxide (DMSO),
tetrahydrofuran (THF), dichloromethane (DCM), and ace-
tone. Figure 1 depicts the assigned 1H NMR spectrum of Poly-
obvious spectral changes including a distinct red shift in the
absorption local maxima were seen upon the addition of
fluoride (F−) to the solution of Poly-1a in DMF. This
absorption spectral change is considered to be the direct
consequence of the change in the main-chain conformation
induced by the interaction between the sulfonamide receptor in
Poly-1a and F−.30 Furthermore, similar bathochromic shifts
were obtained upon the addition of acetate (AcO−), benzoate
−
(BzO−) and dihydrogen phosphate (H2PO4 ) anions, whereas
−
no essential changes were observed for the azide (N3−),
chloride (Cl−), bromide (Br−), iodide (I−), nitrate (NO3 ),
−
−
hydrogen sulfate (HSO4 ), and perchlorate (ClO4 ) anions. As
well as Poly-1a, all the synthesized polymers responded to the
anion addition with clear red shifts. The detailed spectral
changes are summarized in Figure S1 in Supporting
Information. Important to note is that the observed distinct
spectral changes produced specific colorimetric changes that
were easy to visually recognize. Figure 3 shows a typical
Figure 1. 1H NMR spectrum of Poly-1a in DMF-d7 at ambient
temperature.
Figure 3. Color changes in DMF solutions of Poly-1b (1.0 mg mL−1)
obtained upon the addition of 10 equiv of tetra-n-butylammonium
salts of a series of anions (from left to right: blank, F−, AcO−, BzO−,
−
−
−
−
−
H2PO4 , N3 , Cl−, Br−, I−, NO3 , HSO4 , ClO4 ).
1a as a representative example. The 1H NMR spectrum showed
a characteristic signal due to the main chain proton at 5.60
ppm, indicative of the cis-configuration in the polyacetylene
main chain.44 Similar results were also obtained for other
example of such a colorimetric change. These results
demonstrated that the simple sulfonamide-conjugated poly-
(phenylacetylene)s are quite suitable as an optical chemosensor
capable of detecting anion species.
To accurately quantify the anion detection properties of the
synthesized polymers, we carried out UV−vis titration
experiments. For the Poly-1b/F− system, the polymer was
found to detect fluoride anions at concentrations as low as 1.3
× 10−4 M (Figure S2, Supporting Information). Similar results
were obtained for other detectable anions, thus demonstrating
that the polymers can detect a wide range of anions at a
sufficient sensitivity.
1
polymers. Thus, based on the SEC and H NMR results, a
series of poly(phenylacetylene)s with the various sulfonamide
groups were found to be synthesized via a facile two-step
synthetic process.
Anion Detection Properties of Sulfonamide-Conju-
gated Poly(phenylacetylene)s. To provide fundamental
insights into the suitability of the sulfonamide-conjugated
scaffolds as optical anion detectors, the anion recognition
properties of the synthesized polymers were first investigated
by monitoring the UV−vis spectral changes in the respective
polymer solutions upon the addition of different anions. As a
typical example, Figure 2 displays the absorption spectral
changes of Poly-1a in DMF in the presence of various anions,
which were introduced as their tetra-n-butylammonium (TBA)
salts. Poly-1a showed a characteristic absorption peak in the
absorption wavelength ranging from 370 to 550 nm where the
π-conjugation of the polymer backbone typically appears. The
The observed anion-induced spectral and colorimetric
changes in the polymers were dependent on the types of
both the polymers and anions. In order to clarify the
mechanism of such a selectivity, we next tried to establish a
correlation between the sulfonamide receptor and anion. A
summary of the spectroscopic and colorimetric responses of the
polymers is presented in Table 2 in conjunction with the
Hammett constants of the substituent groups in the
polymers.45 As already established elsewhere, the anion binding
affinity of the urea or thiourea receptors generally correlates to
the basicity of the target anions.2,46 Similar to such receptors,
the examined sulfonamide-functionalized polymers showed a
colorimetric change in accordance with the basicity indices of
the introduced anion species. Therefore, a sufficient basicity of
the anions is necessary for this colorimetric detection. Although
all the polymers could response to anions with a relatively high
−
basicity, such as F−, AcO−, BzO−, and H2PO4 , a clear
substituent effect of the polymers was observed in the
41
−
colorimetric detection of N3 . The polymers decorated with
a nitro group of a strong EWG (Poly-1b) and two
trifluoromethyl group of a weak EWG (Poly-1c) are able to
even detect a mildly basic azide anion, while polymers with
EDGs hardly detect the azide anion. Strong EWGs would
Figure 2. UV−vis absorption spectra of DMF solutions of Poly-1a
obtained upon the addition of a series of anions ([Poly-1a] = 0.2 mg
mL−1; [anion]/[monomeric units in Poly-1a] = 10).
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Table 2. Summary of the Anion Detection by Various
a
Poly(phenylacetylene)s
anion detection
NO3−, Cl−,
Br−, I−−,
Hammett
ClO4 ,
b
polymer constants
F− AcO− BzO− H2PO4
N3
HSO4
−
−
−
Poly-1b
Poly-1c
Poly-1a
Poly-1d
Poly-1e
Poly-1f
0.78
0.46
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
○
×
×
×
×
×
×
×
×
×
c
0.00
−0.14
−0.15
−0.28
×
a
Based on the bathochromic shift observed in the absorption spectrum
shown in Figures 2 and S1, Supporting Information. Key: ○, detected;
b
c
× , not detected. Para-substituted parameters, σp. Meta-substituted
parameter, σm.
decrease the electron density at the receptor nitrogen atom and
hence increase the strength of the receptor−anion interaction,
which results in enhanced anion detection properties. In
contrast, the EDGs increase the electron density on the
receptor nitrogen atom, therefore reducing the ability of
receptor−anion interactions by the detector molecules. We
believe that this is the first instance for a polymeric anion−
detector scaffold in which the anion detection properties were
enhanced by manipulation of the substituent groups.
1
Figure 4. H NMR spectra of 1b with various equivalents of tetra-n-
butyl ammonium fluoride (TBAF) in DMF-d7.
Scheme 2. Mechanism of Deprotonation of 1b by the
Addition of F−
After establishing the effect of EDG versus EWG groups on
the anion detection properties of the series of sulfonamide-
conjugated poly(phenylacetylene)s, we next tried to clarify the
mechanism of anion recognition. Recent work by Fabbrizzi and
co-workers revealed that host-anion interactions are classified
into two general types, i.e., hydrogen bonding and deprotona-
tion.47 For determining the exact mechanism of the host-anion
interactions, 1H NMR titration has been known as an insightful
method, since the shifting or broadening of specific peaks in the
spectra are specific to different interaction mechanisms. To this
end, the 1H NMR titration experiments were carried out by the
progressive addition of aliquots of fluoride anion to a solution
of the monomer in DMF-d7. It should be noted here that the
monomer was used as a model compound of the polymer
because the highly broadened signals in the 1H NMR spectrum
of the polymer were expected to provide no useful information.
The results for 1b are shown in Figure 4 as a representative
example. We traced the aromatic protons adjacent to the
sulfonamide. With the increasing addition of fluoride, an upfield
shift was observed in the signals due to all the aromatic protons
of 1b. This upfield shift reflects the generation of the
deprotonated species. The proposed deprotonation mechanism
and the resulting formation of the deprotonated species are
presented in Scheme 2. The sulfonamide proton is considered
to be more acidic than the corresponding amide and urea
protons.41,43 This is precisely why the neat deprotonation of
the sulfonamide-based anion receptor is possible. Thus, the 1H
NMR titration results indicated that the colorimetric response
of a series of sulfonamide-conjugated poly(phenylacetylene)s is
the direct consequence of the anion-induced deprotonation
process of the sulfonamide receptor.
the acidity, i.e., the anion recognition properties, of substituted
benzenesulfonamides studied herein can be manipulated by the
inherent substituent effect. To clarify this, we compared the 1H
NMR titration of 1b with the nitro groups of EWG and 1f with
the methoxy groups of EDG. Furthermore, to extract
meaningful results, the azide anion that has a moderate basicity
was employed for the titration experiments (Figure S3 and S4,
Supporting Information). The result of the titration of 1b with
the azide anion was essentially the same as the result for
fluoride anion as discussed above. In contrast, the result of the
1f/azide system showed that the deprotonation event did not
occur. As a point of note, these results exactly match the
colorimetric responses of Poly-1b and Poly-1f toward the azide
anion. Thus, this simply adds more proof that the anion
recognition properties of the polymers can be rationally and
deliberately manipulated to the target anions by simply
selecting the appropriate functional group as a substitute of
benzenesulfonamide.
We next investigated the effect of EWG versus EDG on this
deprotonation process. Since Hammett and others have
clarified, the acidity values of simple aromatic Brønsted acids,
such as benzoic acid and sulfonic acid, are significantly
influenced by substituent groups, it is logically expected that
Colorimetric Detection of Anions in Aqueous Envi-
ronments. Generally, protic solvents such as water compete
with anions for the host binding sites thereby impeding the
interaction between the host and guest.9 This is the reason why
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the direct sensing of anions in aqueous media has been
recognized to be quite difficult. However, on the basis of the
demonstrated enhanced anion recognition ability of the
sulfonamide-conjugated polymers, we envisioned that this
setback could be overcome by employing a receptor molecule
that leverages a deprotonation event rather than a hydrogen-
bonding mechanism. Hence, we explored the potential of Poly-
1b, which has the strongest electron withdrawing nitro group,
as a simple and novel colorimetric sensor for anions in aqueous
environments.
form in nature. Figure 6 shows the UV−vis spectra of Poly-1b
in the presence of a series of carboxylates in a DMF/H2O (9/1,
We first analyzed the effect of increasing the water content
on the detection of the fluoride anion, as show in Figure 5.
Figure 6. UV−vis absorption spectra of DMF/H2O (9/1, v/v)
solutions of Poly-1b obtained upon the addition of various sodium
salts of carboxylates ([Poly-1b] = 0.67 mM; [sodium carboxylate] =
6.7 mM).
v/v) mixed solvent. Obvious spectral changes were observed for
all the carboxylates in spite of the presence of a considerable
amount of water in the solutions. Moreover, Poly-1b was found
to output this information as a visible color change, even
though 20% of the water exists in the system (Figure 7).
Figure 5. UV−vis absorption spectra of Poly-1b with F− in DMF/
H2O mixed solvents ([Poly-1b] = 0.67 mM; [F−] = 6.7 mM).
Interestingly, the Poly-1b/F− system showed an obvious red
shift even in a mixed solvent containing up to 20% water,
though the spectroscopic response decreased with the
increasing water content. These spectral changes also provided
colorimetric changes from yellow to orange that can be visually
recognized. Therefore, this investigation indicated that the
polymer is applicable for a practical colorimetric sensor capable
of detecting anions in aqueous media. On the contrary, we
obtained negative results when we tested the rest of the
polymers in the library. This can be attributed to the fact that
Poly-1b has the strongest electron withdrawing nitro group,
which would impart an exceptional anion recognition ability to
the polymer.
To further expand this interesting property, we then
examined whether Poly-1b is applicable for the colorimetric
sensing of biologically important carboxylates, such as acetate,
L-lactate, L-malate, and citrate (Chart 2). These four
carboxylates are simple, basic, and particularly known, thus
providing meaningful insights into the applicability of Poly-1b
as a colorimetric probe for various biologically important
carboxylates in an aqueous solution.48 For this demonstration,
carboxylates were employed as sodium salts that are the general
Figure 7. Color changes in DMF/H2O (8/2, v/v) solutions of Poly-
1b (1.0 mg mL−1) obtained upon the addition of 10 equiv. of various
sodium carboxylates (from left to right: blank, sodium acetate, sodium
L-lactate, sodium L-malate, sodium citrate).
Furthermore, the UV−vis titration results of Poly-1b with
citrate showed that a clear red shift in the spectra was observed
upon the addition of only 0.2 equivalents of citrate (Figure S5,
Supporting Information). This result clarified that Poly-1b is a
potent probe that can detect anions including biologically
important ones in aqueous media even at low anion
concentrations.
To elucidate the anion binding mechanism of Poly-1b in
aqueous media, 1H NMR titration experiments were carried out
by the progressive addition of aliquots of sodium acetate to a
solution of 1b in DMF-d7/D2O (9/1, v/v). Similar to the case
1
of 1b in DMF-d7, the H NMR titration results showed a
characteristic upfield shift in the aromatic peaks, indicative of
the direct deprotonation of 1b even in the presence of water
(Figure S6, Supporting Information). Therefore, the deproto-
nation reaction was concluded to play a crucial role in the
colorimetric changes observed for Poly-1b in the presence of
the anions in an aqueous environment. Given the difficulty in
detecting aqueous anions by the previously prepared polymeric
receptors with a hydrogen bonding-based anion recognition
property, the deprotonation mechanism may be necessary for
such an efficient sensing of anions in aqueous media. In
addition, based on the screening of all the sulfonamide-
conjugated poly(phenylacetylene)s synthesized here, the strong
EWG as well as the deprotonatable sulfonamide binding site
Chart 2. Structures of Employed Sodium Carboxylates
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anions.
CONCLUSIONS
■
We synthesized a series of novel poly(phenylacetylene)s with
various sulfonamide groups to evaluate an efficient colorimetric
anion probe that can even operate in aqueous media. All the
prepared polymers showed visible colorimetric responses upon
the addition of anions to an organic solution, which was
clarified to be due to the deprotonation event of sulfonamide
receptors by a 1H NMR titration experiment. Among the
polymers, poly(phenylacetylene)s with sulfonamide receptors
activated by a strong EWG (nitro group) exhibited an
enhanced anion recognition property, thus being applicable
to the colorimetric detection of anions in an aqueous
environment. Considering that the detection of aqueous anions
by colorimetric means is still a highly difficult task, this
demonstration would be valuable in anion recognition
chemistry. Important to note is that the anion sensing ability
was suggested to be simply and straightforwardly tunable with a
substituent group that is located adjacent to the sulfonamide
receptor. Therefore, we believe that this investigation provided
useful insights into molecular design for the realization of a
practical sensor for aqueous anions including biologically and
environmentally important ones.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and Figures S1−S6 (UV-vis spectra and
absorption and NMR titration). This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +81 11 706 6602. Fax: +81 11 706 6602. E-mail:
kakuchi@poly-bm.eng.hokudai.ac.jp.
■
(29) Kakuchi, R.; Nagata, S.; Sakai, R.; Otsuka, I.; Nakade, H.; Satoh,
T.; Kakuchi, T. Chem.Eur. J. 2008, 14, 10259−10266.
(30) Kakuchi, R.; Kodama, T.; Shimada, R.; Tago, Y.; Sakai, R.;
Satoh, T.; Kakuchi, T. Macromolecules 2009, 42, 3892−3897.
(31) Kakuchi, R.; Nagata, S.; Tago, Y.; Sakai, R.; Otsuka, I.; Satoh, T.;
Kakuchi, T. Macromolecules 2009, 42, 1476−1481.
(32) Kakuchi, R.; Tago, Y.; Sakai, R.; Satoh, T.; Kakuchi, T.
Macromolecules 2009, 42, 4430−4435.
Present Address
†Department of Materials Chemistry, Asahikawa National
College of Technology, Asahikawa 071−8142, Japan
Notes
(33) Kakuchi, R.; Shimada, R.; Tago, Y.; Sakai, R.; Satoh, T.; Kakuchi,
T. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1683−1689.
(34) Sakai, R.; Okade, S.; Barasa, E. B.; Kakuchi, R.; Ziabka, M.;
Umeda, S.; Tsuda, K.; Satoh, T.; Kakuchi, T. Macromolecules 2010, 43,
7406−7411.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (B), a Grant-in-Aid for Young Scientists (B), and the
Global COE Program (Catalysis as the Basis for Innovation in
Materials Science) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT), Japan.
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