A novel polythiophene derivative as a sensitive colorimetric and fluorescent sensor for anionic surfactants in water

Article abstract of DOI:10.1039/c2nj40460e

A novel cationic polythiophene derivative, poly[N,N,N-trimethyl-4- (thiophen-3-ylmethylene)cyclohexanaminium chloride] (PTCA-Cl), has been prepared successfully and applied as a colorimetric and fluorometric probe for the detection of anionic surfactants. In particular, a new method for the polymerization of cationic polythiophene derivatives is found, which has not been reported previously. The colorimetric and fluorescent response of PTCA-Cl to anionic surfactants have been investigated by absorption and emission spectroscopy. The sensing interaction can be apparently observed by the naked eye, which presents a great advantage for its practical applications. Moreover, the sensor (PTCA-Cl) shows high selectivity towards anionic surfactants in the presence of other negatively charged analytes, as well as high sensitivity with a detection limit at 10^-9 M by using fluorimetry, which is the lowest ever achieved among all synthetic anionic surfactants sensors available. Therefore, the water soluble polythiophene derivative can be applied as a colorimetric and fluorescent probe for the detection of anionic surfactants with high selectivity and sensitivity.

Full text of DOI:10.1039/c2nj40460e

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A Novel Polythiophene Derivative as a Sensitive Colorimetric and Fluorescent
Sensor for Anionic Surfactants in Water
Lin Wang, Qilong Feng, Xiangyong Wang, Meishan Pei, Guangyou Zhang*
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China
*: Corresponding author. Eꢀmail: chem_zhanggy@hotmail.com. Telephone: (+86)ꢀ13296449182
Abstract
A
novel
cationic
polythiophene
derivative,
poly[N,N,Nꢀtrimethylꢀ4ꢀ(thiophenꢀ3ꢀylmethylene)cyclohexanaminium chloride] (PTCAꢀCl), has
been prepared successfully and applied as a colorimetric and fluorometric probe for the detection of
anionic surfactants. In paticular, a new method for the polymerization of cationic polythiophene
derivatives is found, which has not been reported previously. The colorimetric and fluorescent of
PTCAꢀCl response to anionic surfactants have been investigated by absorption and emission
spectroscopy. The sensing interaction can be apparently observed by naked eyes, which presents a
great advantage for its practical applications. Moreover, the sensor (PTCAꢀCl) shows a high
selectivity to anionic surfactants in the presence of other negatively charged analytes and as well an
high sensitivity with the detection limit at 10ꢀ9 M by using fluorometry, which is the lowest ever
achieved among all synthetic anionic surfactants sensors available by far. Therefore, the water
soluble polythiophene derivative can be applied as a colorimetric and fluorescent probe for the
detecting anionic surfactants with high selectivity and sensitivity.
Keywords: conjugated polymers; sensors; anionic surfactants; colorimetric sensing; fluorometric
method
1

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Introduction
Surfactants are widely used in household cleaning detergents, personal care products and industries
like textiles, paints, pesticide formulations, pharmaceuticals, mining, oil recovery and pulp and
paper.1 As a result, large amounts of surfactants are commonly discharged in large quantities to
sewage treatment plants or directly to the aquatic environment in areas where there is no sewage
treatment. In addition, anionic surfactants are the largest group of the surfactants and represent 70%
of the annual surfactant production. Anionic surfactants deserve special interest since they impact on
aquatic/terrestrial ecosystems and are used in large quantities in consumer products, which are
discharged into the wastewaters after use. Thus they are becoming of the major components of
environmental pollutants.2ꢀ6 Therefore, it is becoming important to monitor easily the concentrations
of anionic surfactants in water. Many wellꢀknown surfactant analysis techniques, such as the
methylene blue method,7 ionꢀselective electrodes,8ꢀ10 gas chromatography/mass spectrometry,11ꢀ13
highꢀperformance liquid chromatography14,15 and optical detection based on synthetic receptors,16ꢀ21
capillary electrophoresis,22 and so forth, have limitations in their applicability, owing tedious
procedures, large necessary quantites of toxic solvents, irreproducibility, signal instability and
unsatisfactory sensitivity and selectivity.23ꢀ28 Thus, it is essential to develop new methods that could
improve the simplicity, selectivity, and sensitivity of anionic surfactant detection in water, such as
chromoꢀ or fluorogenic methods. However, there are few reports on the use of UV or fluorescence
changes to detect anionic surfactants.
Recently, a new sensory technology based on conjugated polymers has been developed in view of
their signal amplification effect, and thus is sensitive to very minor perturbations.29,30 Among the
conjugated polymers, waterꢀsoluble conjugated polythiophenes with charged substituents have
2

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attracted increasing interest in recent years due to their unique chemical structures and optical
properties.31ꢀ35
Among the various types of anion receptors, ammoniumꢀbased receptors have been studied
extensively. Moreover, there have been many significant results from the application of new
ammonium receptors to many anionic targets. Chun Li et al. recently reported a colorimetric and
fluorescent method for the selective determination of anionic surfactants using
ammoniumꢀcontaining polythiophene derivative.36,37 However, these systems involve a twoꢀstep
process for the detection of anionic surfactants and an additional material is necessary for detection.
Herein, we report the first example of an anionic surfactantꢀselective sensor only based on water
soluble cationic polythiophene derivative (PTCAꢀCl) (Scheme 1) without any additional material,
which shows colorimetric changes and fluorescence enhancement in the presence of various of
anionic surfactants. Compared to previous reports, this system is simpler and more effective in
sensing anionic surfactants.
The colorimetric method provides unique advantages including visual detection and realꢀtime in
situ responses over fluorometric method. However, the limit of detection of the colorimetric method
is only at 10ꢀ6 M level due to the low sensitivity of absorption spectrometer relative to fluorometric
detection. Thus, we report a selective and sensitive colorimetric and fluorescent dual method for the
detection of anionic surfactants in water based on a cationic polythiophene derivative. The detection
limit can be extended to be as low as of the order of 10ꢀ9 M based on fluorometric method. In this
paper, the simple, rapid, sensitive and selective method is based on conformationally flexible
polythiophene derivative and complementary electrostatic interactions and does not require any
additional material or chemical modification on the probe or target. In addition, a new method for the
3

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polymerization of cationic polythiophene derivatives is found, which is supplementary for FeCl3
oxidative polymerization.
EtO
OEt
O
P
NH2
N
N
2
Na2Cr2O7
HCOOH
HCHO
S
OH
OH
+
O
1
Br-
+
N
N
N
Cl-
CH3Br
(1) SLS (2) FeCl3 / CHCl3
(3) Bu4NCl
CH3CN
S
S
n
S
3
4
PTCA-Cl
Scheme 1. Synthetic route of PTCAꢀCl
Experimental
Materials
All chemical reagents were common commercial level and used as received without further
purification, (thiopheneꢀ3ꢀylmethyl) phosphonic acid diethyl ester (Scheme 1. Compound 2)38 and
4ꢀ(dimethylamino) cyclohexanone (Scheme 1. Compound 1)39 were prepared as reported previously.
Measurement
UVꢀvis spectra was recorded on a Shimadzu 3100 spectrometer Absorption and Fluorescence
measurements were carried out on a Edinburgh Instruments LtdꢀFLS920 fluorescence
spectrophotometer. 1H NMR spectra were recorded on a Bruker AV III 400MHz NMR spectrometer.
Infrared spectra were recorded using Bruker Vertex 70 FTꢀIR spectrometer with KBr pellets. Gel
permeation chromatography (GPC) measurements were performed with a Waters 2410
o
refractiveꢀindex detector at 35 C, and THF was used as eluent at a flow rate of 1.0 mL/min. GPC
4

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data were calibrated with polystyrene standards. Mass spectra were obtained using a PerkinElmer
Clarus500 Mass Spectrometer.
Sample Preparation
All tests described in this paper were carried out at room temperature in deionized water. In the
experiments of titration with various anionic surfactants, the concentration of PTCAꢀCl in colorimetric
experiments and fluorometric experiments was 0.2 mM and 0.1 ꢁM, respectively.
Syntheses
N,N-Dimethyl-4-(thiophen-3-ylmethylene)cyclohexanamine (3)
A mixture of 4ꢀ(dimethylamino)cyclohexanone (1.41 g, 10 mmol), 60% NaH (0.60 g, 15 mol) and
THF (20 ml) was stirred for 30min under iceꢀwater bath, and the flow of dry N2 gas allowed for an
inert atmosphere. Then compound 2 (2.34 g, 10 mol) was slowly dropped at 0 oC. After completion,
the reaction mixture was warmed to room temperature and stirred further for 12 h. Then, the reaction
mixture was poured into cold water and extracted with chloroform. The organic layer was dried over
MgSO4, filtered, and concentrated. The crude product was purified by chromatography on silica gel
with nꢀhexane/acetate/methanol/triethylamine=30:5:2:1 as an eluent, and this afforded the pure
substance as a colorless oil. (1.05 g, Yield: 48%). Ms: m/z = 221[M+]. FTIR (KBr, cmꢀ1): 1639
1
(CH=C). H NMR (CDCl3, ppm, δ): 7.21 (d, 1H, ThꢀC5ꢀH), 6.99 (s, 1H, ThꢀC2ꢀH), 6.98 (d, 1H,
ThꢀC4ꢀH), 6.13 (s, 1H, CH=C), 2.43 (m, 1H, CH), 2.27 (s, 6H, 2×CH3), 1.96ꢀ1.98 (m, 4H, 2×CH2),
1.31ꢀ1.40 (m, 4H, 2×CH2).
N,N,N-Trimethyl-4-(thiophen-3-ylmethylene)cyclohexanaminium bromide (4)
Compound 3 (0.41 g, 18.6 mmol) and methyl bromide (0.35 g, 37.2 mmol) were dissolved in 15 ml
CH
3
CN, and the reaction mixture was stirred at room temperature for 2 h, the solid was filtered and
5

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washed with ether to obtained product 4 (0.42 g, 72%). Ms: m/z = 315[M+]. FTIR (KBr, cmꢀ1): 1639
1
(CH=C). HNMR (CDCl3, ppm, δ): 7.28 (d, 1H, ThꢀC5ꢀH), 7.10 (s, 1H, ThꢀC2ꢀH), 6.97 (d, 1H,
ThꢀC4ꢀH), 6.17 (s, 1H, CH=C), 3.08 (m, 1H, CH), 2.90 (s, 9H, 3×CH3), 2.42 (t, 2H, CH2), 2.17 (t, 2H,
CH2), 1.93 (m, 2H, CH2), 1.42 (m, 2H, CH2).
Poly[N,N,N-trimethyl-4-(thiophen-3-ylmethylene)cyclohexanaminium chloride] (PTCA-Cl)
Compound 4 (0.42 g, 1.33 mmol) and sodium lauryl sulfate (SLS) (0.38 g, 1.33 mmol) were added to
20 ml dry acetone, then the mixture was smashed by the ultrasonic vessel for 30 min. The acetone
was removed by rotary evaporator, and then the white solid was obtained which was compound
4/SLS ionꢀpair complex. This ionꢀpair complex was dissolved in dry chloroform (20 ml) to give a
translucent liquid. Anhydrous ferric chloride (0.86 g, 5.3 mmol) was suspended in 20 ml dry
chloroform and stirred for 30 min under nitrogen, and then the above resulting translucent liquid was
added dropwise. The mixture was stirred at room temperature for 24 hours. Treatment of the reaction
mixture with methanol (25 ml) resulted in the precipitation of the polymer and then filtered. The filter
cake and 1 ml hydrazine were mixed in 50 ml methanol and stirred overnight. The solution was filtered
and evaporated to dryness. The resulting polymer was washed six times with a saturated solution of
tetrabutylammonium chloride in methanol, until the absorbance remained unchanged. Then PTCAꢀCl
(0.17 g, 47%) was obtained after Soxhlet extraction with a mixed solvent (acetone:methanol=4;1) to
1
remove impurity and oligomer. FTIR (KBr, cmꢀ1): 1635 (CH=C). HNMR (CDCl3, ppm, δ): 7.09 (s,
1H, ThꢀC4ꢀH), 6.15 (s, 1H, CH=C), 3.17 (m, 1H, CH), 2.94ꢀ3.02 (s, 9H, 3×CH3), 2.05 (t, 2H, CH2),
1.89 (t, 2H, CH2), 1.09 (m, 2H, CH2), 1.07 (m, 2H, CH2). GPC: M =8310g/mol, PDI=1.73.
n
6

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Results and discussion
Scheme 1 displays the synthetic route of polymer PTCAꢀCl. 1 was coupled with phosphonate 2 using
a wittingꢀHorner reaction to afford 3. Originally, we wanted to prepare the water soluble polymer
PTCAꢀCl by using the usual FeCl3 polymerisation of monomer 4, but it failed. However, anionic
surfactant SLS was introduced into the mixture of compound 4 and acetone which led to the
formation of a compound 4 / SLS ionꢀpair complex. Then removed the solvent acetone to give white
solid ionꢀcomplex. The resulting ionꢀpair complex as the monomer was polymerized using FeCl3 in
chloroform and then by anion exchange to afford PTCAꢀCl. In this method, the ionꢀpair complex
possesses a good solubility in organic phase, therefore, it can be easily dissolved in chloroform and
readily polymerized. What's more, it is noted here that dodecyl sulfate ion can be easily exchanged
with chloride ion from tetrabutylammonium chloride because the maximum absorption of the
polymer solution tends to be almost unchanged after four times of exchange. PTCAꢀCl displayed
maximum absorption peak at 489 nm in UVꢀvisible spectra and maximum emission peak at 589 nm
in water (Fig. S1).
Fig. 1a illustrates absorption spectra of water soluble PTCAꢀCl upon addition of different amounts
of sodium dodecylbenzenesulfonate (SDBS). It is clearly seen that upon the addition of increasing
amounts of SDBS caused a decrease in absorption at 489nm with a concomitant increase at 434nm.
Simultaneously, a nakedꢀeye color change from red to yellow was observed (Fig. 1a, S2), signifying
that SDBS could cause the dissociation of PTCAꢀCl aggregates. The same phenomenon was also
observed upon sodium dodecyl sulfonate (SDS), sodium lauryl sulfate (SLS) and sodium dodecyl
carboxylate (SDC) (Fig. S2, S3). And the ratio of absorbance measured at 489 nm and 434 nm
(A489/A434) versus the increasing amounts of SDBS was all gradually decreased (Fig. 1b), and the
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detection limit is evaluated to be 10ꢀ6 M. The blue shift caused by addition of SDBS should be
attributed to a decrease in conjugation length,40 arising from the dissociation of the PTCAꢀCl
aggregates and the conformation change of PTCAꢀCl. The electrostatic interaction between the
negatively charged sulfonate groups of the surfactants and the positively charged PTCAꢀCl would be
the driving force and the synergistic effect due to the hydrophobic interaction between alkyl chains in
surfactants plays an important part in promoting the dissociation of PTCAꢀCl aggregates. The
PTCAꢀCl aggregates were dissociated into nonaggregated PTCAꢀCl chains with randomꢀcoiled
conformation.33 These interactions shift the πꢀπ* transition to shorter wavelengths and lead to a color
change of the solution from red to light yellow. To further prove that the transition to shorter
wavelengths and the color change is due to the disassembly of PTCAꢀCl, temperature dependent
absorption spectra of PTCAꢀCl were also measured (Figure S4, Supporting Information). The results
o
declared clearly that after the temperature was up to 60 C, the maximum absorption of PTCAꢀCl
was blueꢀshifted gradually, signifying the PTCAꢀCl aggregates were being dissociated into
nonꢀaggregated PTCAꢀCl chains with randomꢀcoiled conformation. The spectral characteristic
supported the standpoint that the addition of anionic surfactants promoted the disassembly of
PTCAꢀCl aggregates.
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0
0.1mM
(a)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
SDBS/mM
0
0.001
0.005
0.0075
0.01
0.025
0.05
0.075
0.1
300
400
500
600
700
Wavelength/nm
(b)
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.00
0.02
0.04
0.06
0.08
0.10
SDBS/mM
Figure. 1 (a) Variation in the absorption spectra and colorimetric response of PTCAꢀCl (0.2 mM) in water with
increasing concentrations of SDBS as indicated. (b) The relationship between A489/A434 and the concentration of
SDBS from 0.001 to 0.1 mM.
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It is well known that fluorometric method has an advantage in extending the detection limit over
colorimetric method due to the high sensitivity of fluromrtric methods relative to absorption
spectroscopy. And PTCAꢀCl examined here can give a strong emission in dilute solution and
excellent signal. Fig. 2 displays emission spectra of PTCAꢀCl in aqueous solution in the absence and
in the presence of different amounts of SDBS. Upon addition of increasing amounts of SDBS, the
emission intensity at 589 nm enhanced gradually, indicating that the fluorometric detection of SDBS
is possible. The similar results also happened on SDS, SLS and SDC (Fig. S5). Though the specific
reason for this phenomenon was unclear, according to our current knowledge, the electrostatic and
hydrophobic cooperative interaction between anionic surfactants and cationic PTCAꢀCl could lead to
increase in vibrational structure, probably due to the backbone being more rigid.41 In addition, the
polymer chains are aggregated in the absence of surfactant, but that the addition of surfactant results
in a disassembly of the aggregates, it is the reason for the blueꢀshift in absorbance, which could also
explain the increase in emission intensity. This approach shows very high sensitivity due to the
extremely low background noise. The detection limit can be extended to the of order of 10ꢀ9 M, being
103 times lower than that based on colorimetric method.
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1600
1400
1200
1000
800
600
400
200
0
SDBS/µM
0.1
0.8
0.06
0.04
0.025
0.015
0.01
0.008
0.006
0.004
0.0025
0.0015
0.001
0
450
500
550
600
650
700
750
800
850
Wavelength/nm
Figure. 2 Emission spectra of the PTCAꢀCl in the absence and the presence of increasing amounts of SDBS.
[PMTPA] =0.1ꢁM, Excitation wavelength: 450 nm.
To address the specificity of the cationic polythiophene PTCAꢀCl toward anionic surfactants,
absorption spectra and colorimetric response of the PTCAꢀCl probe upon addition of SDBS, sodium
dodecyl sulfonate (SDS), sodium lauryl sulfate (SLS), sodium dodecyl carboxylate (SDC),
cetyltrimethylammonium bromide (CTAB), OPꢀ10, and simple ions including K+, Na+, Ca2+, Mg2+,
2ꢀ
2ꢀ
ꢀ
2ꢀ
Fꢀ, Clꢀ, Brꢀ, Iꢀ, SO4 , CO3 , NO3 , PO43ꢀ and HPO4 , were examined under identical conditions. It
was found that the absorbance of PTCAꢀCl at 489 nm was changed distinctly only for anionic
surfactants, whereas upon addition of cationic, nonꢀionic surfactants and simple ions, the absorbance
remained basically unchanged (Fig. 3a). What's more, most of the solutions remained red except for
those containing anionic surfactants, which gave light yellow (SDBS), yellow(SLS) and light orange
(SDS) solutions (Fig. 4). It is also confirmed that only anionic surfactants induced distinct
enhancement in emission intensity by fluorometric approach (Fig. 3b). Moreover, it is found that the
response concentration of SDC is higher than that of SDBS, SLS and SDS (see Fig. S3), which
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should be attributed to the difference of the charged moieties in surfactants. These results are in good
agreement with the previous observations that sulfonate has a strong affinity for quaternary
ammonium group in PTCAꢀCl with respect to carboxylate and phosphate.42 Moreover, the effect of
hydrophobic anion BF4ꢀ were examined (see supporting information, Fig. S6), the blue shift and
colour change were also observed, however, the sensitivity was not higher than that of anionic
surfactants, but it also supported the standpoint of deaggregation mechanism. Notably, the most
dramatic effects are observed for anionic surfactants from these results, and hence we can clearly
come to the conclusion that PTCAꢀCl probe exhibits satisfied selectivity toward anionic surfactants.
In order to further verify the feasibility of the present method, anionic surfactant in source water was
detected. In this case, a certain known amount of SDBS (0.01 and 0.05 mM) was added to source
water and the concentration was determined by using a suitable calibration curve. The recovery in
percentage was determined to be 93% and 92%, respectively. These relatively good results indicate
that this approach would be useful in detection of anionic surfactants.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
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(b)
1600
1400
1200
1000
800
600
400
200
0
SDBS
SLS
SDS
SDC
OP-10
CTAB
0.00
0.02
0.04
0.06
0.08
0.10
Surfactants/µM
Figure. 3 (a) Relative absorbance of the PTCAꢀCl in water in the presence of various analytes. [PTCAꢀCl] = 0.2
mM; [SDBS] = [SDS] = [SLS] = [CTAB] = [Triton Xꢀ100] = 0.1 mM; [SDC] = [simple ions] = 0.6 mM. (b)
Emission intensity of PTCAꢀCl versus the concentrations of surfactants as indicated. [PTCAꢀCl] = 0.1 µM.
Excitation wavelength: 450 nm.
Figure. 4 Changes in the color of PTCAꢀCl in water induced by the addition of various analytes. [PTCAꢀCl] = 0.2
mM; [SDBS] = [SLS] = [SDS] = [CTAB] = [OPꢀ10] = 0.1mM; [simple ions] = 0.6 mM.
Conclusions
In conclusion, we have developed a simple and convenient colorimetric and fluorescent sensor for
anionic surfactants in water based on a novel cationic polythiophene derivative PTCAꢀCl. This
methodology does not require any chemical modification of the probe or the analytes and is based on
conformational change of the conjugated backbone of cationic PTCAꢀCl. Absorption and emission
spectra can be applied to monitoring the conformational change of PTCAꢀCl, realizing the
colorimetric and fluorometric detection of anionic surfactants. This cationic polymer exhibited
selective and color changes as well as fluorescence changes when exposed to anionic surfactants.
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And the fluorescent detection limit to anionic surfactants can be extended to as low as 10ꢀ9 M.
Moreover, the PTCAꢀCl is a novel cationic polymer, which has not been reported so far. And the new
synthetic method is first reported in this paper, which can be applied in other similar cationic
polyelectrolytes. We believe that PTCAꢀCl will not only provide a simple, convenient and sensitive
approach for sensing anionic surfactants, but also develop various inexpensive means for the rapid
detection and identification of target nucleic acids, proteins or other biological molecules.
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Scheme 1
EtO
O
OEt
P
NH2
OH
N
N
2
Na2Cr2O7
HCOOH
HCHO
S
OH
+
O
1
Br-
Cl-
+
N
N
N
CH3Br
(1) SLS (2) FeCl3 / CHCl3
(3) Bu4NCl
CH3CN
S
S
n
S
3
4
PTCA-Cl

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A Novel Polythiophene Derivative as a Sensitive Colorimetric and Fluorescent
Sensor for Anionic Surfactants in Water
Lin Wang, Qilong Feng, Xiangyong Wang, Meishan Pei, Guangyou Zhang*
we have developed a a sensitive colorimetric and fluorescent sensor for anionic surfactants in water
based on a novel cationic polythiophene derivative, which has not been reported so far, especially, a
new method for the polymerization of cationic polythiophene derivatives is found.