Analyte-induced aggregation of conjugated polyelectrolytes: Role of the charged moieties and its sensing application

Article abstract of DOI:10.1039/c002188a

The vital role of the charged moieties of monovalent aromatic anions in regulating the aggregation of a cationic conjugated polyelectrolyte was exploited in terms of the hard-soft acid-base principle and its application to colorimetric sensing of taurine was examined. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002188a

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COMMUNICATION
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Analyte-induced aggregation of conjugated polyelectrolytes: role of the
charged moieties and its sensing applicationw
Zhiyi Yao, Hua Bai, Chun Li* and Gaoquan Shi*
Received 2nd February 2010, Accepted 25th May 2010
First published as an Advance Article on the web 16th June 2010
DOI: 10.1039/c002188a
The vital role of the charged moieties of monovalent aromatic
anions in regulating the aggregation of a cationic conjugated
polyelectrolyte was exploited in terms of the hard–soft acid–base
principle and its application to colorimetric sensing of taurine
was examined.
To investigate the influence of the charged moieties of
the analytes on the aggregation of PMTPA, 2-naphthalene-
sulfonic acid (NSA), 2-naphthalenecarboxylic acid (NCA) and
2-naphthylphosphoric acid (NPA) (Scheme 1a) were chosen as
the model analytes. Fig. 1 compares the absorption spectra of
PMTPA in the presence of different amount of NSA, NCA
and NPA. As shown in Fig. 1, the absorption maximum of
PMTPA in 20 mM borate buffer (pH = 9.0) appears at
407 nm, being attributed to a random-coiled conformation
of the PT backbone.8 Upon adding increasing amounts of
NSA, the absorption maximum is gradually red-shifted and
finally featured with two major peaks at 543 and 589 nm, and a
broad shoulder around 504 nm, being characteristic of the
formation of a p-stacked supramolecular complex.8b Simul-
taneously, a distinct solution color change from yellow to
purple was observed. In comparison, the introduction of NCA
and NPA into aqueous PMTPA solution leads to different
spectral features, in which the absorption maxima are blue-
shifted to 376 and 380 nm, respectively, and the PMTPA
solutions remain yellow. These observations indicated that
NSA is a stronger aggregation-inducing molecule relative to
NCA and NPA, which can be interpreted qualitatively by
Pearson’s hard–soft acid–base (HSAB) principle.9 It is known
that the quaternary ammonium group in PMTPA acts as a
soft acid and sulfonate is a soft base, whereas both carboxylate
and phosphate are hard bases. According to the HSAB principle,
there is an extra stabilization in a hard–hard combination,
or a soft–soft one. Therefore, one can conclude that the
Conjugated polyelectrolytes (CPEs) have attracted increasing
interest during the last decade because of their potential
for applications in sensing bioanalytes.1–4 Hitherto, most
CPE-based biosensors rely on a change in the emission inten-
sity upon binding an oppositely charged analyte, and few
CPEs can be applied to colorimetric detection.2,3,5 Cationic
poly(3-alkoxy-4-methylthiophene)s (P3RO-4MeT), a kind of
water-soluble polythiophenes (PTs) with chain conformation
sensitive to external stimuli, have been confirmed to be sensitive
colorimetric probes for the detection of various bioanalytes.3,6,7
The sensing mechanism for colorimetric detection has been
attributed to the formation of CPE–analyte ion-pair complex,
resulting in the conformation change and/or aggregation of
PT backbones. For sensing small bioanions, analyte-induced
aggregation of cationic P3RO-4MeT through electrostatic,
hydrophobic and aromatic stacking cooperative interactions
has been proposed to account for the remarkable solution
color change.7 There is also strong evidence that polyvalent
bioanions with hydrophobic aromatic moieties facilitate the
aggregation of cationic P3RO-4MeT and thus the colorimetric
effect is further enhanced.7 Although this sensing mechanism
is based on specific lines of experimental evidence, some
important issues remain unsolved.
As part of an ongoing investigation into the mechanism and
application of aggregation-based CPE colorimetric sensors for
small bioanions, we have an interest in examining the role of
the charged moieties of aromatic anions in regulating aggrega-
tion of a cationic P3RO-4MeT derivative, poly(3-(4-methyl-30-
thienyloxy)propyltrimethylammonium) (PMTPA, Scheme 1a).
It was confirmed that aromatic sulfonate can induce PMTPA
aggregation much more efficiently relative to carboxylate and
phosphate. This finding is applicable to detect sulfonate-
containing bioanalyte, taurine, by combining with an in situ
premodification technique. To the best of our knowledge, this
is the first example of using a colorimetric CPE probe for
detecting a monovalent bioanalyte in aqueous media.
Department of Chemistry and the Key Laboratory of Bio-organic
Phosphorous Chemistry & Chemical Biology, Tsinghua University,
Beijing 100084, China. E-mail: chunli@mail.tsinghua.edu.cn,
gshi@tsinghua.edu.cn; Fax: +86-10-6277-1149;
Tel: +86-10-6279-8909
w Electronic supplementary information (ESI) available: Experimental
procedures and additional figures. See DOI: 10.1039/c002188a
Scheme 1 (a) Chemical structure of PMTPA, NSA, NCA and NPA.
(b) Schematic illustration of the in situ premodification reaction of
taurine with OPA.
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electrostatic interaction between the quaternary ammonium
group and sulfonate group is stronger than that with carboxylate
or phosphate group.10 Moreover, ionic self-assembly is
characteristic of a cooperative binding mechanism.11 Thus,
the stronger primary electrostatic interaction would stimulate
and enhance further secondary interactions including p–p
stacking and hydrophobic interactions, facilitating the forma-
tion of p-stacked PMTPA aggregates.
On the basis of the new insights into the analyte-induced
aggregation mechanism discussed above, colorimetric sensing
of taurine (2-aminoethanesulfonic acid) based on the PMTPA
probe was examined. Taurine, a sulfur-containing semi-
essential amino acid, not only plays a key role in metabolism,
but also can be a medicine to improve therapy of some
diseases.12 So far, several techniques have been devoted to
detecting it, such as high-performance liquid chromatography
(HPLC),12 high-performance anion-exchange chromatography
coupled with electroanalysis13 and capillary electrophoresis
with laser-induced fluorescence detection.14 However, to a
certain extent, there exist some limitations related to these
methods. For example, they need relatively time-consuming
procedures and expensive equipment. Therefore, it is still
necessary to find new approaches that could improve the
simplicity and selectivity of taurine detection.
Fig. 2 Absorption spectra of PMTPA (1.0 ꢂ 10ꢁ4 M) in the absence
and the presence of OPA, taurine and a OPA–taurine mixture in
borate buffer (20 mM, pH = 9.0): [OPA] = 7.5 ꢂ 10ꢁ4 M; [taurine] =
5.0 ꢂ 10ꢁ4 M.
Taurine, having no aromatic moiety, interacts weakly
with PMTPA in borate buffer, and scarcely induces any
spectral change of PMTPA except for slightly weakening the
absorption intensity (Fig. 2 and S1, ESIw). It is known that
reaction of o-phthalaldehyde (OPA) with primary amine
leads to the formation of the phthalimidine (PI) derivative.15
Therefore, taurine can be converted into the sulfonate-
containing PI derivative (PI-taurine) by the reaction with
OPA (Scheme 1b).16 The introduction of an aromatic group
into taurine is expected to enhance interactions with PMTPA.
Indeed, upon addition of OPA–taurine (a mixture of OPA
and taurine pre-reacted for 3 min) into the PMTPA solution
the absorption maximum was red-shifted to 541 nm along with
the appearance of two broad shoulders around 507 and
587 nm (Fig. 2), being characteristic of the aggregation of
PMTPA chains.7 Simultaneously, a distinct color change from
yellow to red–pink was observed, which was not achieved
in the case of using OPA and taurine only. These results
indicated that PMTPA could be a promising probe for the
colorimetric sensing taurine by combining an in situ premodi-
fied approach.
To evaluate the specificity of PMTPA toward taurine,
absorption spectra of PMTPA in borate buffer upon addition
of OPA and sulfur-containing amino acids, methionine (Met),
cysteine (Cys), homocysteine (Hcy) and cystine (Cyt), were
recorded under identical conditions.17 It was found that upon
addition of these bioanions, all the solutions remained yellow
with lmax o 430 nm (see Fig. S3, ESIw). However, the most
remarkable effect was observed for taurine, which gave a
red–pink solution (Fig. 3). Thus, one would conclude that
the dramatic color change of PMTPA upon addition of
OPA–taurine provides a simple means for visual detection of
taurine in aqueous solutions. The ratio of the absorbance
difference between those at 600 nm (p-stacked aggregates)
and 700 nm (light-scattering from the aggregates) to the
absorbance at 407 nm (PMTPA random coil), (A600 ꢁ A700)/
A
407, was calculated to further estimate the selectivity of
PMTPA toward taurine (Fig. 4). It is clearly seen that the
most striking effect is observed for taurine with the ratio
value of 9.5ꢂ higher at least than that containing the other
amino acids. These results indicate that PMTPA is selectively
responsive to taurine and confirms that the charged moiety
of the analyte plays a key role in controlling the ionic
Fig. 1 Absorption spectra of PMTPA (1.0 ꢂ 10ꢁ4 M) in the presence
of different amounts of NSA (A), NCA (B) and NPA (C) in 20 mM
borate buffer (pH = 9.0). The bands between 300–350 nm are
attributed to naphthalene moieties.
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Fig. 3 Changes in the color of PMTPA solutions (1.0 ꢂ 10ꢁ4 M) in
borate buffer (pH = 9.0) induced by addition of various OPA–amino
acid mixtures: [OPA] = 7.5 ꢂ 10ꢁ4 M; [amino acid] = 5.0 ꢂ 10ꢁ4 M.
self-assembly of PMTPA to form p-stacked aggregates. To
further verify this conclusion, PI-b-alanine (PI-b-Ala, see
Fig. S4, ESIw) induced-aggregation ability towards PMTPA
was examined as a control experiment. It was found that
PI-b-Ala scarcely induced the spectral change of PMTPA
(Fig. S5, ESIw). These results indicated that sulfonate in the
PI derivative does have a strong affinity for the quaternary
ammonium group in PMTPA relative to carboxylate.
To check the performances of PMTPA toward sensing
taurine, variation in the absorption spectra of PMTPA
with increasing amounts of taurine in borate buffer (20 mM,
pH = 9.0) was examined (Fig. 5). It was found that upon
addition of increasing amounts of taurine, the absorption
maximum is gradually red-shifted from 407 to 541 nm along
with a dramatic color change from yellow to red–pink. The
inset of Fig. 5 displays a linear relationship between A541/A407
and the concentration of taurine (R = 0.995 from 0.01 to
0.40 mM), indicating that this approach is applicable to the
ratiometric detection of taurine.18
Fig. 5 Variation in the absorption spectra of PMTPA (1.0 ꢂ 10ꢁ4 M)
in 20 mM borate buffer (pH = 9.0) with increasing concentrations of
taurine as indicated: [OPA] = 1.0 ꢂ 10ꢁ3 M. Inset: the relationship
between (A600 ꢁ A700)/A407 and the concentration of taurine from
0.01 to 0.40 mM.
Notes and references
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monovalent inducent molecules play a crucial role in regulating
the aggregation of cationic PMTPA. According to HSAB
principle, soft–soft combination facilitates the primary electro-
static interaction between PMTPA and the inducent molecule
(aromatic sulfonate) during the ionic self-assembly process,
and further enhance secondary interactions including p–p
stacking and hydrophobic interactions, thus promoting the
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understand the induced-aggregation mechanism of CPE, but
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Fig. 4 Relative absorbance of PMTPA (1.0 ꢂ 10ꢁ4 M) in the
presence of OPA (7.5 ꢂ 10ꢁ4 M) and Met, Cys, Hcy, Cyt and taurine
in borate buffer (pH = 9.0): [amino acid] = 5.0 ꢂ 10ꢁ4 M.
of 10ꢁ8 M (see Fig. S7, ESIw).
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5096 | Chem. Commun., 2010, 46, 5094–5096