Colorimetric and fluorometric detection of cationic surfactants based on conjugated polydiacetylene supramolecules

Article abstract of DOI:10.1039/b904542b

The polydiacetylenes derived from a hydroxybenzaldehyde substituted diacetylene monomer display a unique colorimetric change (blue to red) and large fluorescent enhancement in the presence of cetyl trimethylammonium salt.

Full text of DOI:10.1039/b904542b

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COMMUNICATION
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Colorimetric and fluorometric detection of cationic surfactants based
on conjugated polydiacetylene supramoleculesw
Xiaoqiang Chen,a Jung Lee,b Min Jung Jou,a Jong-Man Kim*b and Juyoung Yoon*a
Received (in Cambridge, UK) 5th March 2009, Accepted 31st March 2009
First published as an Advance Article on the web 1st May 2009
DOI: 10.1039/b904542b
The polydiacetylenes derived from a hydroxybenzaldehyde
substituted diacetylene monomer display a unique colorimetric
change (blue to red) and large fluorescent enhancement in the
presence of cetyl trimethylammonium salt.
reports adopting either fluorescent or UV changes.9 In this
communication, we report the first example of a CS selective
PDA sensor, which can display fluorescent enhancement as
well as colorimetric change in the presence of CS.
It is known that the carbonyl oxygen in the aldehyde group
can form a strong hydrogen bond with its adjacent phenolic
hydrogen.10 Owing to structural differences, hydroxybenzaldehyde
(HBA) containing two DA monomers (PCDA-HBA 1 and
PCDA-HBA 2), shown in Fig. 1, are expected to adopt different
headgroup interactions when they are transformed into PDA
molecules (Fig. 2). Intermolecular-type hydrogen bonding
between the hydroxyl groups and adjacent aldehyde carbonyl
moieties could be developed in PDA supramolecules derived from
PCDA-HBA 1. In contrast, PCDA-HBA 2 can lead to intra-
molecular hydrogen bonds exclusively. Thus, it would be
intriguing to investigate how the PDAs derived from two DA
monomers that have structural similarity with different
headgroup interactions respond to CS.
Polydiacetylenes (PDAs), a family of conjugated polymers, are
very intriguing materials in several ways.1 Firstly, these
polymers are generally prepared by UV irradiation of self-
assembled diacetylene (DA) supramolecules. Secondly, if
PDAs are prepared under optimal conditions, they, in many
cases, display an intense blue color with a maximum absorp-
tion wavelength at ca. 640 nm. Thirdly, the blue PDAs under-
go a color shift to a red phase (ca. 550 nm maximum
absorption wavelength) upon environmental stimulation.
The stimulus-induced apparent blue-to-red transition of the
PDAs has led to the development of a variety of PDA-based
chemosensors.2 Accordingly, colorimetric detection of
biologically, chemically and environmentally important target
molecules such as DNAs, viruses, proteins, metal ions, organic
solvents etc. have been reported.2
The DA monomers PCDA-HBA 1 and PCDA-HBA 2 were
readily prepared from commercially available 10,12-penta-
cosadiynoic acid (PCDA) by coupling the acid with the
corresponding hydroxybenzaldehydes. Experimental details
and NMR spectra of the two monomers are provided in the
ESI (Fig. S1–S4w).
The development of selective optical signalling systems for
ions has received considerable attention for a few decades due
to their important roles in biological and environmental
processes.3 Cationic surfactants (CS) have been reported as
environmental pollutants since they are heavily used as surface
cleaning agents, such as soaps, shampoo, etc.4 Accordingly, it
is necessary to develop convenient methods for the detection
of such quaternary ammonium surfactants. It is especially
difficult to determine non-aromatic cationic surfactants due to
their lack of chromophores. These cationic surfactants can be
monitored by two-phase titration,5 mass spectrometry,6 high-
performance liquid chromatography (HPLC),7 or GC-MS
methods by converting quaternary ammonium salts to their
corresponding tertiary amines.8 Due to the need for simple
methods to detect these surfactants there have been some
In order to test the above hypothesis, the DA monomers
PCDA-HBA 1 and PCDA-HBA 2 were converted to PDA
supramolecules by employing
a routine procedure. The
detailed experimental procedure is explained in the ESIw. Poly-
merization was carried out at room temperature by irradiating the
solution with 254 nm UV light (1 mW cmÀ2). Fortunately, UV
irradiation of the suspensions derived from both DA monomers
resulted in the formation of stable, blue-colored PDA molecules.
Having prepared the PDA suspensions, we next investi-
gated the colorimetric response of the conjugated polymers to
cetyltrimethylammonium chloride (CTAC), dodecyltrimethyl-
ammonium bromide (DTAB) and other ions. Fig. 3a and b
a Department of Chemistry and Nano Science and Department of
Bioinspired Science, Ewha Womans University, Seoul, 120-750,
Korea. E-mail: jyoon@ewha.ac.kr; Fax: +82-2-3277-3419;
Tel: +82-2-3277-2400
b Department of Chemical Engineering, Institute of Nano Science and
Technology, Hanyang University, Seoul, 133-791, Korea.
E-mail: jmk@hanyang.ac.kr; Fax: +82-2-2298-4101;
Tel: +82-2-2220-0522
w Electronic supplementary information (ESI) available: Experimental
details, NMR spectra of two monomers PCDA-HBA 1 and PCDA-HBA
2, colorimetric assays of PCDA-HBA 1-derived polymer with anions and
alcohols, fluorescent titrations of PCDA-HBA 1-derived polymers (50
mM), colorimetric assays of PCDA-Acid-derived polymer with various
analytes and various concentration of CTAC, colorimetric responses of
PDAs derived from the mixture of PCDA-HBA 1 and PCDA-HBA
2/PCDA-Acid with different ratios. See DOI: 10.1039/b904542b
Fig. 1 Structures of diacetylene monomers investigated in the current
study.
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promote colorimetric transition of the PDAs prepared from
PCDA-HBA 1 exclusively. Fig. 3c explains the concentration
dependent colorimetric change of polymerized PCDA-HBA 1
(1 mM) in the presence of various amounts of CTAC in
HEPES (20 mM, pH 7.4)–DMSO (9 : 1, v/v). The colorimetric
changes of PCDA-HBA 1 were further examined with NH4Cl,
NH3(CH3)Cl, NH2(CH3)2Cl, NH(CH3)3Cl, N(CH3)4Br,
hexyltrimethylammonium bromide, pyridine, piperidine and
piperazine in HEPES (20 mM, pH 7.4)–DMSO (9 : 1, v/v). As
shown in Fig. 3d, none of these cations induced any
colorimetric change. Non-ionic lipophilic molecules, such as
hexanol, octanol and decanol did not induce a color change
(Fig. S6w) either. Along with these observations, little
colorimetric change in the presence of hexyltrimethylammonium
bromide (Fig. 3d, trap 8) illustrates that long alkyl chains and
ammonium groups are required for this unique sensing.
DTAB displayed similar colorimetric changes to that of
CTAC, which means that this system can be used for the
detection of other CS with long alkyl chains in aqueous
solution (Fig. 3a).
Fig. 2 Proposed structures of PDAs derived from PCDA-HBA 1 and
PCDA-HBA 2.
show the photographs of the PDA suspensions derived from
PCDA-HBA 1 and PCDA-HBA 2 in the presence of various
cations (100 mM), such as Li+, Na+, K+, Cs+, Ag+, Ca2+
,
,
Cd2+, Co2+, Cu2+, Fe2+, Hg2+, Mg2+, Mn2+, Zn2+
CTAC, DTAB and SDS in HEPES (20 mM, pH 7.4)–DMSO
(9 : 1, v/v). Sodium dodecyl sulfate (SDS) was also tested even
though it is an anionic surfactant. Among these analytes, only
CTAC and DTAB induce a clear blue-to-red transition of the
PDAs prepared from PCDA-HBA 1. The effect of the counter
ion is negligible because the addition of 100 mM NaF, NaCl,
NaBr or NaI does not lead to a color change (Fig. S5w).
In contrast, the suspensions containing the PCDA-HBA 2-derived
polymers did not display any color change with these anions.
Thus, it is clear from Fig. 3a and b that CTAC and DTAB
The CTAC-induced phase transition of the PDAs was
further monitored by visible absorption spectroscopy. As
displayed in Fig. 4, addition of CTAC results in the decrease
of the absorption at 620 nm with simultaneous increase of the
absorption at 540 nm, a typical blue-to-red transition of the
PDA sensors.
Since the blue-to-red transition of the PDAs is accompanied
by the generation of fluorescence,11 the CTAC-promoted reac-
tion was also monitored by fluorescence spectroscopy (Fig. 5).
As displayed in Fig. 5, the polymerized PCDA-HBA 1 (1 mM)
results in a large fluorescent enhancement with CTAC. The
overall emission change upon the addition of CTAC was about
40-fold. Using rhodamine B as a standard, the quantum yield of
the blue PDAs was calculated to be 6.3 Â 10À5, and the addition
of 30 mM CTAC enhanced the quantum yield of the PDAs to
2.5 Â 10À3. The quantum yield of blue PDA, usually estimated
to be o10À5, is attributed to ultra-fast relaxation of the excited
state.12 The quantum yield of a red PDA film at room
temperature is usually estimated to be o0.02.12 The detection
limit was estimated to be less than 1 mM by monitoring the
emission change with different amounts of CTAC using 50 mM
blue PDAs (Fig. S7w). The other ions listed in Fig. 3 were found
to be inefficient in the promotion of the fluorescence
Fig. 3 Colorimetric responses of PDAs (1 mM) in the presence of
analytes in HEPES–DMSO (9 : 1, v/v, 20 mM, pH 7.4). (a) PDAs
derived from PCDA-HBA 1 with various analytes (100 mM). (b) PDAs
derived from PCDA-HBA
2 with various analytes (100 mM).
(c) Colorimetric titrations of PCDA-HBA 1 with various amounts of
CTAC. (d) Colorimetric changes of PCDA-HBA 1 with various
ammonium salts: 1, blank; 2, NH4Cl; 3, NH3(CH3)Cl; 4,
NH2(CH3)2Cl; 5, NH(CH3)3Cl; 6, N(CH3)4Br; 7, CTAC; 8, hexyl-
trimethylammonium bromide; 9, pyridine; 10, piperidine; 11, piperazine.
Fig. 4 Visible spectroscopic monitoring of PCDA-HBA 1-derived
PDAs (1 mM) in the presence of various amounts of CTAC in
HEPES–DMSO (9 : 1, v/v, 20 mM, pH 7.4).
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(Fig. S10w). This result also provides the key proof for the
importance of intermolecular hydrogen bonding. For the
mixed vesicles of PCDA-HBA 1 and PCDA-Acid, only 40%
PCDA-HBA 1 is necessary (Fig. S11w), which might be
attributed to the possible intermolecular hydrogen bonding
between PCDA-HBA 1 and PCDA-Acid.
In summary, we have developed a conceptually new
PDA-based chemosensor system for the detection of CS in
aqueous solution. The colorimetrically and fluorometrically
responses of the conjugated polymers can be attributed to the
disruption of the headgroup hydrogen bonding, for which
both ammonium groups and long alkyl chains are required.
Since this is, to the best of our knowledge, the first example of
a PDA-based CS sensor, the results described above should be
an important addition to PDA sensor systems.
Fig. 5 Fluorescence titrations of PCDA-HBA 1-derived polymers
(1 mM) with CTAC in HEPES–DMSO (9 : 1, v/v, 20 mM, pH 7.4;
excitation at 535 nm, slit: 5 nm/5 nm).
enhancement of the polymer. The colorimetric and fluorescent
changed images are also povided in Fig. S8w.
This research was supported by the Korea Science and
Engineering Foundation (KOSEF) grant (NRL) (R04-2007-
000-2007-0, R04-2008-000-20047-0), the SRC program of
KOSEF/MEST (R11-2005-008-02003-0) and WCU program
(R31-2008-000-10010-0). J.L. and M.J.J. ackknowledge the
BK21 fellowship.
The selectivity of PCDA-HBA 1-derived PDAs for CTAC
could be attributed to the unique head group interactions in
aqueous solution as shown in Fig. 6. The addition of CTAC
can disturb the regularly arrayed hydrogen bonding between
head groups by ionic interactions between the phenolate of the
head group and the ammonium group. The disruption of the
hydrogen bonding could allow the release of the strain energy
imposed on the alkyl side chains generated during polymeriza-
tion.13 The release of the side chain strain can cause partial
distortion of the arrayed p-orbitals, leading to a decrease in
the effective conjugation length of the polymer. On the other
hand, the salicylaldehyde group of the polymerized
PCDA-HBA 2 is involved in intramolecular hydrogen bonding
and the addition of CTAC has a negligible effect on the
colorimetric transition of the PDA supramolecules.
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Fig. 6 Proposed headgroup structures of the PDAs derived from
PCDA-HBA 1 in the presence of CTAC.
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3436 | Chem. Commun., 2009, 3434–3436