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Synthesis of the cationic fluorescent probes for the detection of anionic surfactants by electrostatic self-assembly
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117446
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Synthesis of the cationic fluorescent probes for the detection of anionic surfactants by electrostatic self-assembly Aiqing Feng a, Fangru Jiang b, Guiyuan Huang b, Peilian Liu b, * a
Department of Life Science, Luoyang Normal University, Luoyang 471934, PR China School of Chemistry and Chemical Engineering, Key Laboratory of Clean Energy Materials Chemistry of Guangdong Higher Education Institutes, Lingnan Normal University, Zhanjiang 524048, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2019 Received in revised form 10 July 2019 Accepted 30 July 2019 Available online 31 July 2019
Anionic surfactants were widespread used in car cleaning agents, household detergents, agricultural and industrial processes, and considered as a major source of environmental pollutant. Therefore, it is necessary to develop a fast, simple, highly selective and sensitive probe for the detection of anionic surfactants. Here, we synthesized two aggregation induced emission (AIE)-active molecules 4,40 ,400 ,4000 (ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis (1-(4-bromobenzyl)pyridin-1-ium) bromide (TPE-Br) and 4,40 ,400 ,4000 -(ethene-1,1,2,2-trayltetrakis(benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium) iodide (TPE-I), which were then applied as fluorescence probes for detecting sodium dodecyl sulfate (SDS) with high selectivity and sensitivity. In the presence of SDS, a multi-fold fluorescence emission intensity enhancement was observed in both two probes (TPE-Br and TPE-I) due to the electrostatic selfassembly of AIE molecular. The limits of detection are 71.5 and 120 nM for TPE-Br and TPE-I, respectively. This study may provide a new strategy for environmental monitoring by AIE-based fluorescent probe. © 2019 Elsevier B.V. All rights reserved.
Keywords: Aggregation induced emission Fluorescent probe Sodium dodecyl sulfate (SDS)
1. Introduction Sodium dodecyl sulfate (SDS) is one of most studied anionic surfactants, which is widely used in various fields of the chemical industry and daily life, such as washing powder, shampoo, toothpaste, cosmetics, paper making, pharmacy, building materials, etc. [1e5]. Not only due to its high water solubility, good foaming, emulsification, decontamination, infiltration and dispersion performance, but also the low cost makes SDS an attractive choice for application. Nevertheless, it is worth noting that SDS is slightly toxic to the human body and may pollute the ecosystem, and even cause the quality deterioration of drinking water. At present, some methods have been established to detect SDS, such as gas chromatography/mass spectrometry, ion selective electrode, high performance liquid chromatography, spectrophotometry and so on [6e10]. However, most of them have various disadvantages including complicated operation, high cost, low sensitivity and selectivity [11e13]. It is thus necessary to develop a new and simple approach to detect SDS in aqueous solution. Organic fluorophores have received great attention due to their widespread application in the fields of chemical sensors, organic
* Corresponding author. E-mail address: [email protected] (P. Liu). https://doi.org/10.1016/j.saa.2019.117446 1386-1425/© 2019 Elsevier B.V. All rights reserved.
light-emitting diodes and biological probes [14e19]. However, conventional fluorophores are generally weakly emissive or even no emissive in aggregated states such as solid state or concentrated solutions. This phenomenon is known as aggregation caused quenching (ACQ) effect, which will limit their widespread application. Fortunately, Tang and co-workers observed a novel class of fluorophores which exhibited opposite properties to the ACQ effect. These specific chromophores showed weak or no emission in the dilute solution but luminescent in the aggregation state or in poor solvent. This so-called aggregation-induced emission (AIE) phenomenon provides a new direction for the design of organic fluorescent materials [20e25]. Among the series of AIE molecules, the tetraphenylstyrene (TPE) derivatives are typical examples that can be simply synthesized and easily functionalized. In the past decade, TPE-based molecules have been widely used in various fields such as chemical and biological sensors, and cell imaging [26e31]. In recent years, a variety of substituents have been attached to TPE to endow it sensing properties, and one can find a lot of remarkable works [32e35]. On the basis of cognition of AIE mechanism, in this paper, we reported a kind of highly efficient and selective fluorescent probe for SDS detection based on the electrostatic interactions and hydrophobic interactions. Specifically, two cationic TPE derivatives 4,40 ,400 ,4000 -(ethene-1,1,2,2tetrayltetrakis(benzene-4,1-diyl))tetrakis (1-(4-bromobenzyl)pyridin-1-ium) bromide (TPE-Br) and 4,40 ,400 ,4000 -(ethene-1,1,2,2-
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2. Experimental sections
atmosphere at 120 C for 3 days before cooled to room temperature. The mixture was concentrated by rotary evaporation to obtain the crude product. After washing with CH2Cl2 for several times, the crude product was recrystallized from dichloromethane to form the purified compound (TPE-I). The synthesis procedure and structure of TPE-I were shown in Fig. S4. 1H NMR (400 MHz, dDMSO) d (ppm) 8.97 (8H, d, J ¼ 8 Hz), 8.46 (8H, d, J ¼ 4 Hz), 8.01 (8H, d, J ¼ 8 Hz), 7.38 (8H, d, J ¼ 8 Hz), 4.31 (12H, s) (Fig. S5). 13C NMR (101 MHz, dDMSO) d (ppm) 153.55, 146.34, 146.03, 141.45, 132.58, 132.48, 128.43, 124.26, 47.58 (Fig. S6). HR-MS: m/z: calculated for [M-4I]/4: 175.0891. Found [M-4I]/4: 175.0884.
2.1. Reagents and materials
2.5. Fluorescence detection of SDS in aqueous solution
Tetrakis (4-pyridylphenyl) ethylene (98%), iodomethane (CH3I, 99.5%), 4-bromobenzyl bromide (98%), sodium dodecyl sulfate (SDS, 95%), cetyltrimethylammonium bromide (CTAB, 99%) and all metal salts were purchased from Sigma-Aldrich Co. Ltd. All solvents including dichloromethane (CH2Cl2, 99.9%), acetonitrile (CH3CN, 99.9%), N, N-dimethyl formamide (DMF, 99.9%), tetrahydrofuran (THF, 99.5%), dimethyl sulfoxide (DMSO, 99.7%), ethanol (EtOH, 99.5%) were obtained from HWRK Chem. Co., Ltd., Beijing, China.
Given that the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) at 298 K is 0.008 mol/L, we chose to prepare a 0.008 mol/L SDS as the stock solution. Firstly, 0.1394 g of SDS powder was weighed and poured into a beaker. 40 mL of deionized water was added to dissolve SDS with the assistant of oscillation. Then, the aqueous solution was transferred into a 50 mL volumetric flask to constant the volume. Afterwards, a serials of SDS solutions with various concentrations and 1 mL TPE-Br (5 mM) or TPE-I (5 mM) aqueous solutions were added into a 2 mL centrifuge tube. The mixture was then transferred into a spectrophotometer quartz cuvette. For comparison, we explored the fluorescence responses of other interfering analytes such as Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, 2 2 2 Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , Cl , SO3 and CTAB (150 mM) under the same conditions. The fluorescent spectra were obtained with a Hitachi FL-7000 fluorescence spectrophotometer.
trayltetrakis (benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium) iodide (TPE-I) were designed and synthesized. Both of these two AIE-based molecules show selective response to SDS with multifold fluorescence enhancement in aqueous solution even under the interference of other analytes such as Mg2þ, Kþ, Mn2þ, Co2þ, 2 2 Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , 2 Cl , SO3 and cationic surfactant (CTAB). Such performance indicates that they are excellent fluorescent sensors for SDS, and further potential applications in related areas also can be expected.
2.2. Apparatus 1
H NMR spectra were acquired at 293 K on a Bruker Avance 400 MHz NMR Spectrometer (Bruker, Karlsruhe, Germany). UVeVis spectra were obtained with TECHCOMP spectrophotometer in the range of 250e500 nm with a slit of 2 nm (TECHCOMP, Shanghai, China). Fluorescence spectra were measured using a Hitachi FL7000 fluorescence spectrophotometer with a 450 W xenon lamp as a light source (Hitachi, Tokyo, Japan). Zeta potentials and particle size distribution were determined by a Malvern laser particle size analyzer (Malvern, UK). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100HR transmission electron microscope (JEOL, Japan). All error bars represent standard deviations from three repeated experiments. All error bars stand for the standard deviations of three replicate experiments. 2.3. Synthesis and characterization of TPE-Br Tetrakis(4-pyridylphenyl)ethylene (0.312 mmol, 200 mg) and 4bromobenzyl bromide (0.624 mmol, 282.2 mg) were first dissolved in the mixture of CH3CN (50 mL) and CH2Cl2 (50 mL) in a 250 mL flask. Then, the mixture was refluxed under N2 atmosphere at 120 C for 3 days and cooled to room temperature. The mixture was concentrated by rotary evaporation to obtain the crude product. After washing with CH2Cl2 for several times, the crude product was recrystallized from dichloromethane to form the purified compound (TPE-Br). The synthesis procedure and structure of TPE-Br were shown in Fig. S1. 1H NMR (400 MHz, dDMSO) d (ppm) 9.19 (8H, d, J ¼ 12 Hz), 8.48 (8H, d, J ¼ 8 Hz), 7.97 (8H, d, J ¼ 8 Hz), 7.67 (8H, d, J ¼ 8 Hz), 7.51 (8H, d, J ¼ 8 Hz), 7.34 (8H, d, J ¼ 8 Hz), 5.80 (8H, s) (Fig. S2). 13C NMR (101 MHz, dDMSO) d (ppm) 154.43, 146.40, 145.30, 141.61, 134.46, 132.56, 132.42, 131.79, 131.52, 128.63, 125.13, 123.24, 61.80 (Fig. S3). HR-MS: m/z: calculated for [M-4Br]/3 þ [H]: 439.7152; Found [M-4Br]/3 þ [H]: 439.7034.
2.6. Detection of SDS in tap water samples For SDS detection in tap water samples, various volumes of standard SDS solutions were added to the corresponding centrifuge tubes to achieve different final concentrations. Then, the concentrations of SDS in tap water samples were determined using the same procedure as the detection for SDS aqueous solutions. 3. Results and discussion 3.1. Synthesis and optical properties of TPE-Br and TPE-I Two cationic type of fluorescent probes (TPE-Br and TPE-I) were synthesized by the Suzuki coupling reaction as shown in Figs. S1 and S4. The process and mechanism of TPE-Br and TPE-I fluorescent nanoprobes for the quantitative detection of SDS are shown in Scheme 1. To study the photophysical properties of the probes, the fluorescent emission spectra of TPE-Br (5 mM) and TPEI (5 mM) in different solvents (THF, CH2Cl2, DMSO, DMF, EtOH, H2O, CH3CN) were measured. As shown in Fig. 1A and B, both TPE-Br and
2.4. Synthesis and characterization of TPE-I Similar as the synthesis process for TPE-Br as described above, tetrakis(4-pyridylphenyl)ethylene (0.312 mmol, 200 mg) was first dissolved in the mixture solution of CH3CN (50 mL) and CH2Cl2 (50 mL) which was addressed in a 250 mL flask in the beginning, and iodomethane (1.248 mmol, 117.14 mg) was then injected into the above solution. The mixture maintained reflux under N2
Scheme 1. Schematic illustration of TPE-Br and TPE-I for the detection of SDS.
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Fig. 1. Fluorescence emission spectra of TPE-Br (A) and TPE-I (B) (5 mM) in different solvents, and fluorescence emission spectra of TPE-Br (C) and TPE-I (D) (5 mM) in mixture of DMSO/water the broadening of water content.
TPE-I exhibited strong fluorescence emission intensity in THF, while the fluorescence intensity became weak in polar solvents such as H2O and CHCN3, which was probably caused by the solvent polarity and the intramolecular charge transfer (ICT) effect between TPE donor and pyridinium acceptor. Furthermore, we explored the fluorescence behaviors of TPE-Br and TPE-I (5 mM) in solution with different water contents (0%, 15%, 30%, 50%, 70%, 85% and 99.9%). As shown in Fig. 1C and D, TPE-Br and TPE-I showed strong yellow emission at 562 nm in pure DMSO and gradually decrease in fluorescence intensity with the increasing concentration of water. Such expected behavior indicates the fluorescence intensity of TPE-Br and TPE-I were significantly affected by the solvent polarity. In order to advance the practical application of probes, pure water taken as the solvent in the following experiments.
The stability of fluorescent sensors is one of the most important factors in their practical application. We thus explored the pH dependence of TPE-Br (5 mM) and TPE-I (5 mM) aqueous solution by the fluorescence emission spectra and UVevisible absorption spectra. As shown in Fig. S7 and S8, there was no significant change in fluorescence emission intensity in the pH range of 2e13, which was consistent with the UVevisible absorption spectra studies. The results confirmed the excellent pH stability of TPE-Br and TPE-I. 3.2. UVevis analysis of TPE-Br and TPE-I towards SDS The UVevisible absorption spectra of TPE-Br (5 mM) and TPE-I (5 mM) were measured in aqueous solution. There are two
Fig. 2. UVevisible absorption spectra of 5 mM TPE-Br (A) and 5 mM TPE-I (B) upon the adding various concentrations of SDS in aqueous solution.
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Fig. 3. (A). Fluorescence emission spectra of TPE-Br (5 mM) in aqueous solution upon the addition of various concentrations of SDS (from 0 to 19 mM). (Inset: Photographic image of TPE-Br (5 mM) solution after the addition of SDS (0 and 19 mM) under the excitation of 365 nm UV light. (B). Linear relationship between the emission intensity of TPE-Br (5 mM) treatment with various concentrations of SDS.
absorption bands centered at 285 and 363 nm of TPE-Br, which are assigned to the p-p* local electron transition of the conjugate system (Fig. 2A). Upon the addition of SDS (0e19 mM), the absorption peak corresponding to the TPE-Br at 285 nm was decreased obviously, and the absorption peak at 365 nm decreased with a slight blue shift. The blue shift in the absorption spectra might be contributed to the formation of tightly packed nanoaggregates in situ, which was caused by the effect of electrostatic interactions and hydrophobic interactions. As shown in Fig. 2B, the UV curves of TPE-I were similar to those of TPE-I towards SDS. 3.3. Fluorescence studies of TPE-Br and TPE-I towards SDS To evaluate the sensitivity of TPE-Br and TPE-I (5 mM) for the quantitative detection of SDS, we studied the fluorescence emission spectra of TPE-Br and TPE-I with different concentrations of SDS under the optimized experimental conditions. As shown in Fig. 3A, in contrast with the weak fluorescence emission intensity at 550 nm in pure water, the fluorescence intensity of TPE-Br continuously enhanced with the increase of SDS concentration from 0 to 19 mM. The fluorescence intensity reached a maximum when the concentration of SDS reached 16 mM, which was 13-folds of its original intensity. Moreover, the fluorescence intensity of TPE-Br has a good linear relationship with the concentration of SDS in the range of 0.5e16 mM and follows the linear function
Y ¼ 0.67X þ 0.53 (R2 ¼ 0.998) (Fig. 3B). The limit of detection (LOD) for SDS is accurately determined to be 71.5 nM according to the equation LOD ¼ 3SD/slope (SD is the standard deviation of the blank sample). Similarly, as shown in Fig. 4A, the fluorescence intensity of TPE-I is continuously enhanced with the increase of SDS concentration (0e40 mM) and presents a good linear relationship (Y ¼ 0.15X þ 0.91, R2 ¼ 0.997) in the rage of 1.0e34 mM (Fig. 4B). LOD for SDS is calculated to be 120 nM by the same procedure for TPE-Br. The experimental results indicate that both TPE-Br and TPE-I can be used for highly sensitive detection of SDS. The limit of detection as well as the detection range in our proposed approach has been compared with other reported methods (Table S1). Although the detection ranges in the first two methods are wider than our case, our probes possess the lower limit of detection value. Furthermore, it still gives rise to the acceptable detectability and the detection linear range as compared with other similar methods (colorimetric and fluorometric). To explore the selectivity of TPE-Br and TPE-I (5 mM) for SDS detection, we measured the fluorescence response of TPE-Br and TPE-I to other interfering analytes (150 mM) such as cations (Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ), anions (PO3 4 , HS , 2 2 2 NO , NO , HSO , SO , CO , Cl , SO ) and cationic surfactant 3 2 3 4 3 3 (CTAB). As shown in Fig. 5A, the fluorescence emission intensity of TPE-Br was significantly enhanced at 550 nm with the addition of SDS, but did not change with other interfering substances in
Fig. 4. (A). Fluorescence emission spectra of TPE-I (5 mM) in aqueous solution upon the addition of various concentrations of SDS (from 0 to 40 mM). (Inset: Photographic image of TPE-I (5 mM) solution after the addition of SDS (0 and 40 mM) under the excitation of 365 nm UV light. (B). Linear relationship between the emission intensity of TPE-I (5 mM) treatment with various concentrations of SDS.
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Fig. 5. Fluorescence spectra response of TPE-Br (A) and TPE-I (B) (5 mM) in aqueous solution to SDS (40 mM) and various interfering analytes (150 mM). ((Inset: photographic images 2 2 2 of TPE-Br (A) and TPE-I (B) solution upon the addition of various analytes (Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , Cl , SO3 , CTAB).
Fig. 6. Zeta potentials of (A) TPE-Br, SDS, TPE-Br-SDS and (B) TPE-I, SDS, TPE-I-SDS in aqueous solution.
contrast. The visible inset reflects the apparent change of TPE-Br under ultraviolet light (l ¼ 365 nm) (Fig. 5A, inset). Additionally, the relative intensity of TPE-Br at 550 nm was almost no changes by the co-existence of the above-mentioned interfering analytes (Fig. S9). Figs. 5B and S10 show the similar fluorescent emission behavior with different interfering substances for TPE-I. These results demonstrated that TPE-Br and TPE-I can be applied for highly selective of SDS in practical applications. 3.4. Mechanism for the recognition of SDS The detection mechanism for SDS was characterized by zeta potential and transmission electron microscopy (TEM) in sensing process. TPE-Br (5 mM) and TPE-I (5 mM) exhibited positive charges with values of 18.17 ± 3.55 mV and 19.44 ± 2.25 mV, respectively
(Fig. 6). In contrast, the zeta potential of SDS (40 mM) was measured to be 32.77 ± 4.83 mV under the same conditions. After adding SDS to the TPE-Br and TPE-I solution, the zeta potentials of TPE-BrSDS and TPE-I-SDS were reduced to 6.97 ± 1.05 mV and 9.96 ± 4.2 mV, respectively, which further illustrated that TPE-Br and TPE-I were encapsulated by the negatively charged SDS via electrostatic interaction. Furthermore, as shown in Figs. 7 and S11, the morphology of TPE-Br and TPE-I showed that they are in a homogeneous dispersion state before the addition of SDS. We determined that they are nanoparticles with average particle sizes of 6.3 nm and 15 nm, respectively, by TEM characterization of at least 100 samples (Fig. S12). After the addition of SDS, the negatively charged SDS can react with TPE-Br and TPE-I. It can be clearly seen forming largely aggregated nanoparticles according to the TEM images of TPE-Br-
Fig. 7. TEM images of (A) TPE-Br (5 mM) and (B) TPE-Br-SDS (5 mM TPE-Br þ 19 mM SDS).
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References
Table 1 The results of determination of SDS by TPE-Br (5 mM) in tap water (n ¼ 3). Sample number (no.)
SDS added (mM)
SDS found (mM)
Recovery (%)
RSD (%)
1 5 10
0.98 5.08 9.96
98.0 101.6 99.6
1.98 2.03 1.79
No. 1 No. 2 No. 3
Table 2 The results of determination of SDS by TPE-I (5 mM) in tap water (n ¼ 3). Sample number (no.)
SDS added (mM)
SDS found (mM)
Recovery (%)
RSD (%)
5 10 20
4.80 9.92 20.42
96.0 99.2 102.1
2.26 1.04 1.28
No. 1 No. 2 No. 3
SDS and TPE-I-SDS. The dynamic light scattering technology was used for further study in particle size distribution, the particle sizes of TPE-Br and TPE-I are dramatically increased with the increasing concentration of SDS (Fig. S12), which is approximate to the TEM results. It is further manifested that SDS could promote the aggregation of TPE-Br and TPE-I by electrostatic interaction. 3.5. Detection of SDS in tap water samples To promote the probes into practical application, various concentrations of SDS were added to the tap water and analyzed by TPE-Br and TPE-I, respectively. As shown in Table 1, after adding a certain concentration of SDS (1.0, 5.0 and 10.0 mM) to the tap water, the finally measured SDS contents were 0.98, 4.95 and 9.96 mM using TPE-Br probe. The average recoveries of SDS for all spiked samples were between 98.0 and 101.6%, and the lower relative standard deviations (1.79e2.03%) were obtained. As for the probe TPE-I, the similar average recoveries (between 96.0 and 102.1%) and relative standard deviation (1.04e2.26%) of SDS for the three spiked samples also can be obtained (Table 2). These results demonstrated that TPE-Br and TPE-I are capable of meeting the requirements for the quantitative detection of SDS in real samples. 4. Conclusions In conclusion, we have synthesized two “turn-on” fluorescent probes (TPE-Br and TPE-I) for the high sensitivity, selectivity and quantitative detection of SDS in aqueous solution. SDS could promote the formation of TPE-Br and TPE-I nano-aggregates in situ based on synergistic electrostatic and hydrophobic interactions, and enhance the fluorescent emission due to the AIE property of TPE-based molecules. Moreover, this research results are highly considerable for the quantitative detection of SDS level in the real samples. Acknowledgments The work was supported by Science and Technology Planning Project of Henan Province of China (172102110105), Natural Science Foundation of Guangdong Province (2017A030310604; 2018A030307035), Guangdong University Provincial Key Platform and Major Research Projects: Characteristic Innovation Project (2017KTSCX118), Natural Science Foundation of Lingnan Normal University (ZL1802). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117446.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of the cationic fluorescent probes for the detection of anionic surfactants by electrostatic self-assembly Aiqing Feng a, Fangru Jiang b, Guiyuan Huang b, Peilian Liu b, * a
Department of Life Science, Luoyang Normal University, Luoyang 471934, PR China School of Chemistry and Chemical Engineering, Key Laboratory of Clean Energy Materials Chemistry of Guangdong Higher Education Institutes, Lingnan Normal University, Zhanjiang 524048, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 May 2019 Received in revised form 10 July 2019 Accepted 30 July 2019 Available online 31 July 2019
Anionic surfactants were widespread used in car cleaning agents, household detergents, agricultural and industrial processes, and considered as a major source of environmental pollutant. Therefore, it is necessary to develop a fast, simple, highly selective and sensitive probe for the detection of anionic surfactants. Here, we synthesized two aggregation induced emission (AIE)-active molecules 4,40 ,400 ,4000 (ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis (1-(4-bromobenzyl)pyridin-1-ium) bromide (TPE-Br) and 4,40 ,400 ,4000 -(ethene-1,1,2,2-trayltetrakis(benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium) iodide (TPE-I), which were then applied as fluorescence probes for detecting sodium dodecyl sulfate (SDS) with high selectivity and sensitivity. In the presence of SDS, a multi-fold fluorescence emission intensity enhancement was observed in both two probes (TPE-Br and TPE-I) due to the electrostatic selfassembly of AIE molecular. The limits of detection are 71.5 and 120 nM for TPE-Br and TPE-I, respectively. This study may provide a new strategy for environmental monitoring by AIE-based fluorescent probe. © 2019 Elsevier B.V. All rights reserved.
Keywords: Aggregation induced emission Fluorescent probe Sodium dodecyl sulfate (SDS)
1. Introduction Sodium dodecyl sulfate (SDS) is one of most studied anionic surfactants, which is widely used in various fields of the chemical industry and daily life, such as washing powder, shampoo, toothpaste, cosmetics, paper making, pharmacy, building materials, etc. [1e5]. Not only due to its high water solubility, good foaming, emulsification, decontamination, infiltration and dispersion performance, but also the low cost makes SDS an attractive choice for application. Nevertheless, it is worth noting that SDS is slightly toxic to the human body and may pollute the ecosystem, and even cause the quality deterioration of drinking water. At present, some methods have been established to detect SDS, such as gas chromatography/mass spectrometry, ion selective electrode, high performance liquid chromatography, spectrophotometry and so on [6e10]. However, most of them have various disadvantages including complicated operation, high cost, low sensitivity and selectivity [11e13]. It is thus necessary to develop a new and simple approach to detect SDS in aqueous solution. Organic fluorophores have received great attention due to their widespread application in the fields of chemical sensors, organic
* Corresponding author. E-mail address: [email protected] (P. Liu). https://doi.org/10.1016/j.saa.2019.117446 1386-1425/© 2019 Elsevier B.V. All rights reserved.
light-emitting diodes and biological probes [14e19]. However, conventional fluorophores are generally weakly emissive or even no emissive in aggregated states such as solid state or concentrated solutions. This phenomenon is known as aggregation caused quenching (ACQ) effect, which will limit their widespread application. Fortunately, Tang and co-workers observed a novel class of fluorophores which exhibited opposite properties to the ACQ effect. These specific chromophores showed weak or no emission in the dilute solution but luminescent in the aggregation state or in poor solvent. This so-called aggregation-induced emission (AIE) phenomenon provides a new direction for the design of organic fluorescent materials [20e25]. Among the series of AIE molecules, the tetraphenylstyrene (TPE) derivatives are typical examples that can be simply synthesized and easily functionalized. In the past decade, TPE-based molecules have been widely used in various fields such as chemical and biological sensors, and cell imaging [26e31]. In recent years, a variety of substituents have been attached to TPE to endow it sensing properties, and one can find a lot of remarkable works [32e35]. On the basis of cognition of AIE mechanism, in this paper, we reported a kind of highly efficient and selective fluorescent probe for SDS detection based on the electrostatic interactions and hydrophobic interactions. Specifically, two cationic TPE derivatives 4,40 ,400 ,4000 -(ethene-1,1,2,2tetrayltetrakis(benzene-4,1-diyl))tetrakis (1-(4-bromobenzyl)pyridin-1-ium) bromide (TPE-Br) and 4,40 ,400 ,4000 -(ethene-1,1,2,2-
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2. Experimental sections
atmosphere at 120 C for 3 days before cooled to room temperature. The mixture was concentrated by rotary evaporation to obtain the crude product. After washing with CH2Cl2 for several times, the crude product was recrystallized from dichloromethane to form the purified compound (TPE-I). The synthesis procedure and structure of TPE-I were shown in Fig. S4. 1H NMR (400 MHz, dDMSO) d (ppm) 8.97 (8H, d, J ¼ 8 Hz), 8.46 (8H, d, J ¼ 4 Hz), 8.01 (8H, d, J ¼ 8 Hz), 7.38 (8H, d, J ¼ 8 Hz), 4.31 (12H, s) (Fig. S5). 13C NMR (101 MHz, dDMSO) d (ppm) 153.55, 146.34, 146.03, 141.45, 132.58, 132.48, 128.43, 124.26, 47.58 (Fig. S6). HR-MS: m/z: calculated for [M-4I]/4: 175.0891. Found [M-4I]/4: 175.0884.
2.1. Reagents and materials
2.5. Fluorescence detection of SDS in aqueous solution
Tetrakis (4-pyridylphenyl) ethylene (98%), iodomethane (CH3I, 99.5%), 4-bromobenzyl bromide (98%), sodium dodecyl sulfate (SDS, 95%), cetyltrimethylammonium bromide (CTAB, 99%) and all metal salts were purchased from Sigma-Aldrich Co. Ltd. All solvents including dichloromethane (CH2Cl2, 99.9%), acetonitrile (CH3CN, 99.9%), N, N-dimethyl formamide (DMF, 99.9%), tetrahydrofuran (THF, 99.5%), dimethyl sulfoxide (DMSO, 99.7%), ethanol (EtOH, 99.5%) were obtained from HWRK Chem. Co., Ltd., Beijing, China.
Given that the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) at 298 K is 0.008 mol/L, we chose to prepare a 0.008 mol/L SDS as the stock solution. Firstly, 0.1394 g of SDS powder was weighed and poured into a beaker. 40 mL of deionized water was added to dissolve SDS with the assistant of oscillation. Then, the aqueous solution was transferred into a 50 mL volumetric flask to constant the volume. Afterwards, a serials of SDS solutions with various concentrations and 1 mL TPE-Br (5 mM) or TPE-I (5 mM) aqueous solutions were added into a 2 mL centrifuge tube. The mixture was then transferred into a spectrophotometer quartz cuvette. For comparison, we explored the fluorescence responses of other interfering analytes such as Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, 2 2 2 Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , Cl , SO3 and CTAB (150 mM) under the same conditions. The fluorescent spectra were obtained with a Hitachi FL-7000 fluorescence spectrophotometer.
trayltetrakis (benzene-4,1-diyl))tetrakis(1-methylpyridin-1-ium) iodide (TPE-I) were designed and synthesized. Both of these two AIE-based molecules show selective response to SDS with multifold fluorescence enhancement in aqueous solution even under the interference of other analytes such as Mg2þ, Kþ, Mn2þ, Co2þ, 2 2 Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , 2 Cl , SO3 and cationic surfactant (CTAB). Such performance indicates that they are excellent fluorescent sensors for SDS, and further potential applications in related areas also can be expected.
2.2. Apparatus 1
H NMR spectra were acquired at 293 K on a Bruker Avance 400 MHz NMR Spectrometer (Bruker, Karlsruhe, Germany). UVeVis spectra were obtained with TECHCOMP spectrophotometer in the range of 250e500 nm with a slit of 2 nm (TECHCOMP, Shanghai, China). Fluorescence spectra were measured using a Hitachi FL7000 fluorescence spectrophotometer with a 450 W xenon lamp as a light source (Hitachi, Tokyo, Japan). Zeta potentials and particle size distribution were determined by a Malvern laser particle size analyzer (Malvern, UK). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100HR transmission electron microscope (JEOL, Japan). All error bars represent standard deviations from three repeated experiments. All error bars stand for the standard deviations of three replicate experiments. 2.3. Synthesis and characterization of TPE-Br Tetrakis(4-pyridylphenyl)ethylene (0.312 mmol, 200 mg) and 4bromobenzyl bromide (0.624 mmol, 282.2 mg) were first dissolved in the mixture of CH3CN (50 mL) and CH2Cl2 (50 mL) in a 250 mL flask. Then, the mixture was refluxed under N2 atmosphere at 120 C for 3 days and cooled to room temperature. The mixture was concentrated by rotary evaporation to obtain the crude product. After washing with CH2Cl2 for several times, the crude product was recrystallized from dichloromethane to form the purified compound (TPE-Br). The synthesis procedure and structure of TPE-Br were shown in Fig. S1. 1H NMR (400 MHz, dDMSO) d (ppm) 9.19 (8H, d, J ¼ 12 Hz), 8.48 (8H, d, J ¼ 8 Hz), 7.97 (8H, d, J ¼ 8 Hz), 7.67 (8H, d, J ¼ 8 Hz), 7.51 (8H, d, J ¼ 8 Hz), 7.34 (8H, d, J ¼ 8 Hz), 5.80 (8H, s) (Fig. S2). 13C NMR (101 MHz, dDMSO) d (ppm) 154.43, 146.40, 145.30, 141.61, 134.46, 132.56, 132.42, 131.79, 131.52, 128.63, 125.13, 123.24, 61.80 (Fig. S3). HR-MS: m/z: calculated for [M-4Br]/3 þ [H]: 439.7152; Found [M-4Br]/3 þ [H]: 439.7034.
2.6. Detection of SDS in tap water samples For SDS detection in tap water samples, various volumes of standard SDS solutions were added to the corresponding centrifuge tubes to achieve different final concentrations. Then, the concentrations of SDS in tap water samples were determined using the same procedure as the detection for SDS aqueous solutions. 3. Results and discussion 3.1. Synthesis and optical properties of TPE-Br and TPE-I Two cationic type of fluorescent probes (TPE-Br and TPE-I) were synthesized by the Suzuki coupling reaction as shown in Figs. S1 and S4. The process and mechanism of TPE-Br and TPE-I fluorescent nanoprobes for the quantitative detection of SDS are shown in Scheme 1. To study the photophysical properties of the probes, the fluorescent emission spectra of TPE-Br (5 mM) and TPEI (5 mM) in different solvents (THF, CH2Cl2, DMSO, DMF, EtOH, H2O, CH3CN) were measured. As shown in Fig. 1A and B, both TPE-Br and
2.4. Synthesis and characterization of TPE-I Similar as the synthesis process for TPE-Br as described above, tetrakis(4-pyridylphenyl)ethylene (0.312 mmol, 200 mg) was first dissolved in the mixture solution of CH3CN (50 mL) and CH2Cl2 (50 mL) which was addressed in a 250 mL flask in the beginning, and iodomethane (1.248 mmol, 117.14 mg) was then injected into the above solution. The mixture maintained reflux under N2
Scheme 1. Schematic illustration of TPE-Br and TPE-I for the detection of SDS.
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Fig. 1. Fluorescence emission spectra of TPE-Br (A) and TPE-I (B) (5 mM) in different solvents, and fluorescence emission spectra of TPE-Br (C) and TPE-I (D) (5 mM) in mixture of DMSO/water the broadening of water content.
TPE-I exhibited strong fluorescence emission intensity in THF, while the fluorescence intensity became weak in polar solvents such as H2O and CHCN3, which was probably caused by the solvent polarity and the intramolecular charge transfer (ICT) effect between TPE donor and pyridinium acceptor. Furthermore, we explored the fluorescence behaviors of TPE-Br and TPE-I (5 mM) in solution with different water contents (0%, 15%, 30%, 50%, 70%, 85% and 99.9%). As shown in Fig. 1C and D, TPE-Br and TPE-I showed strong yellow emission at 562 nm in pure DMSO and gradually decrease in fluorescence intensity with the increasing concentration of water. Such expected behavior indicates the fluorescence intensity of TPE-Br and TPE-I were significantly affected by the solvent polarity. In order to advance the practical application of probes, pure water taken as the solvent in the following experiments.
The stability of fluorescent sensors is one of the most important factors in their practical application. We thus explored the pH dependence of TPE-Br (5 mM) and TPE-I (5 mM) aqueous solution by the fluorescence emission spectra and UVevisible absorption spectra. As shown in Fig. S7 and S8, there was no significant change in fluorescence emission intensity in the pH range of 2e13, which was consistent with the UVevisible absorption spectra studies. The results confirmed the excellent pH stability of TPE-Br and TPE-I. 3.2. UVevis analysis of TPE-Br and TPE-I towards SDS The UVevisible absorption spectra of TPE-Br (5 mM) and TPE-I (5 mM) were measured in aqueous solution. There are two
Fig. 2. UVevisible absorption spectra of 5 mM TPE-Br (A) and 5 mM TPE-I (B) upon the adding various concentrations of SDS in aqueous solution.
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Fig. 3. (A). Fluorescence emission spectra of TPE-Br (5 mM) in aqueous solution upon the addition of various concentrations of SDS (from 0 to 19 mM). (Inset: Photographic image of TPE-Br (5 mM) solution after the addition of SDS (0 and 19 mM) under the excitation of 365 nm UV light. (B). Linear relationship between the emission intensity of TPE-Br (5 mM) treatment with various concentrations of SDS.
absorption bands centered at 285 and 363 nm of TPE-Br, which are assigned to the p-p* local electron transition of the conjugate system (Fig. 2A). Upon the addition of SDS (0e19 mM), the absorption peak corresponding to the TPE-Br at 285 nm was decreased obviously, and the absorption peak at 365 nm decreased with a slight blue shift. The blue shift in the absorption spectra might be contributed to the formation of tightly packed nanoaggregates in situ, which was caused by the effect of electrostatic interactions and hydrophobic interactions. As shown in Fig. 2B, the UV curves of TPE-I were similar to those of TPE-I towards SDS. 3.3. Fluorescence studies of TPE-Br and TPE-I towards SDS To evaluate the sensitivity of TPE-Br and TPE-I (5 mM) for the quantitative detection of SDS, we studied the fluorescence emission spectra of TPE-Br and TPE-I with different concentrations of SDS under the optimized experimental conditions. As shown in Fig. 3A, in contrast with the weak fluorescence emission intensity at 550 nm in pure water, the fluorescence intensity of TPE-Br continuously enhanced with the increase of SDS concentration from 0 to 19 mM. The fluorescence intensity reached a maximum when the concentration of SDS reached 16 mM, which was 13-folds of its original intensity. Moreover, the fluorescence intensity of TPE-Br has a good linear relationship with the concentration of SDS in the range of 0.5e16 mM and follows the linear function
Y ¼ 0.67X þ 0.53 (R2 ¼ 0.998) (Fig. 3B). The limit of detection (LOD) for SDS is accurately determined to be 71.5 nM according to the equation LOD ¼ 3SD/slope (SD is the standard deviation of the blank sample). Similarly, as shown in Fig. 4A, the fluorescence intensity of TPE-I is continuously enhanced with the increase of SDS concentration (0e40 mM) and presents a good linear relationship (Y ¼ 0.15X þ 0.91, R2 ¼ 0.997) in the rage of 1.0e34 mM (Fig. 4B). LOD for SDS is calculated to be 120 nM by the same procedure for TPE-Br. The experimental results indicate that both TPE-Br and TPE-I can be used for highly sensitive detection of SDS. The limit of detection as well as the detection range in our proposed approach has been compared with other reported methods (Table S1). Although the detection ranges in the first two methods are wider than our case, our probes possess the lower limit of detection value. Furthermore, it still gives rise to the acceptable detectability and the detection linear range as compared with other similar methods (colorimetric and fluorometric). To explore the selectivity of TPE-Br and TPE-I (5 mM) for SDS detection, we measured the fluorescence response of TPE-Br and TPE-I to other interfering analytes (150 mM) such as cations (Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ), anions (PO3 4 , HS , 2 2 2 NO , NO , HSO , SO , CO , Cl , SO ) and cationic surfactant 3 2 3 4 3 3 (CTAB). As shown in Fig. 5A, the fluorescence emission intensity of TPE-Br was significantly enhanced at 550 nm with the addition of SDS, but did not change with other interfering substances in
Fig. 4. (A). Fluorescence emission spectra of TPE-I (5 mM) in aqueous solution upon the addition of various concentrations of SDS (from 0 to 40 mM). (Inset: Photographic image of TPE-I (5 mM) solution after the addition of SDS (0 and 40 mM) under the excitation of 365 nm UV light. (B). Linear relationship between the emission intensity of TPE-I (5 mM) treatment with various concentrations of SDS.
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Fig. 5. Fluorescence spectra response of TPE-Br (A) and TPE-I (B) (5 mM) in aqueous solution to SDS (40 mM) and various interfering analytes (150 mM). ((Inset: photographic images 2 2 2 of TPE-Br (A) and TPE-I (B) solution upon the addition of various analytes (Mg2þ, Kþ, Mn2þ, Co2þ, Zn2þ, Ni2þ, Fe3þ, Agþ, Al3þ, PO3 4 , HS , NO3 , NO2 , HSO3 , SO4 , CO3 , Cl , SO3 , CTAB).
Fig. 6. Zeta potentials of (A) TPE-Br, SDS, TPE-Br-SDS and (B) TPE-I, SDS, TPE-I-SDS in aqueous solution.
contrast. The visible inset reflects the apparent change of TPE-Br under ultraviolet light (l ¼ 365 nm) (Fig. 5A, inset). Additionally, the relative intensity of TPE-Br at 550 nm was almost no changes by the co-existence of the above-mentioned interfering analytes (Fig. S9). Figs. 5B and S10 show the similar fluorescent emission behavior with different interfering substances for TPE-I. These results demonstrated that TPE-Br and TPE-I can be applied for highly selective of SDS in practical applications. 3.4. Mechanism for the recognition of SDS The detection mechanism for SDS was characterized by zeta potential and transmission electron microscopy (TEM) in sensing process. TPE-Br (5 mM) and TPE-I (5 mM) exhibited positive charges with values of 18.17 ± 3.55 mV and 19.44 ± 2.25 mV, respectively
(Fig. 6). In contrast, the zeta potential of SDS (40 mM) was measured to be 32.77 ± 4.83 mV under the same conditions. After adding SDS to the TPE-Br and TPE-I solution, the zeta potentials of TPE-BrSDS and TPE-I-SDS were reduced to 6.97 ± 1.05 mV and 9.96 ± 4.2 mV, respectively, which further illustrated that TPE-Br and TPE-I were encapsulated by the negatively charged SDS via electrostatic interaction. Furthermore, as shown in Figs. 7 and S11, the morphology of TPE-Br and TPE-I showed that they are in a homogeneous dispersion state before the addition of SDS. We determined that they are nanoparticles with average particle sizes of 6.3 nm and 15 nm, respectively, by TEM characterization of at least 100 samples (Fig. S12). After the addition of SDS, the negatively charged SDS can react with TPE-Br and TPE-I. It can be clearly seen forming largely aggregated nanoparticles according to the TEM images of TPE-Br-
Fig. 7. TEM images of (A) TPE-Br (5 mM) and (B) TPE-Br-SDS (5 mM TPE-Br þ 19 mM SDS).
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References
Table 1 The results of determination of SDS by TPE-Br (5 mM) in tap water (n ¼ 3). Sample number (no.)
SDS added (mM)
SDS found (mM)
Recovery (%)
RSD (%)
1 5 10
0.98 5.08 9.96
98.0 101.6 99.6
1.98 2.03 1.79
No. 1 No. 2 No. 3
Table 2 The results of determination of SDS by TPE-I (5 mM) in tap water (n ¼ 3). Sample number (no.)
SDS added (mM)
SDS found (mM)
Recovery (%)
RSD (%)
5 10 20
4.80 9.92 20.42
96.0 99.2 102.1
2.26 1.04 1.28
No. 1 No. 2 No. 3
SDS and TPE-I-SDS. The dynamic light scattering technology was used for further study in particle size distribution, the particle sizes of TPE-Br and TPE-I are dramatically increased with the increasing concentration of SDS (Fig. S12), which is approximate to the TEM results. It is further manifested that SDS could promote the aggregation of TPE-Br and TPE-I by electrostatic interaction. 3.5. Detection of SDS in tap water samples To promote the probes into practical application, various concentrations of SDS were added to the tap water and analyzed by TPE-Br and TPE-I, respectively. As shown in Table 1, after adding a certain concentration of SDS (1.0, 5.0 and 10.0 mM) to the tap water, the finally measured SDS contents were 0.98, 4.95 and 9.96 mM using TPE-Br probe. The average recoveries of SDS for all spiked samples were between 98.0 and 101.6%, and the lower relative standard deviations (1.79e2.03%) were obtained. As for the probe TPE-I, the similar average recoveries (between 96.0 and 102.1%) and relative standard deviation (1.04e2.26%) of SDS for the three spiked samples also can be obtained (Table 2). These results demonstrated that TPE-Br and TPE-I are capable of meeting the requirements for the quantitative detection of SDS in real samples. 4. Conclusions In conclusion, we have synthesized two “turn-on” fluorescent probes (TPE-Br and TPE-I) for the high sensitivity, selectivity and quantitative detection of SDS in aqueous solution. SDS could promote the formation of TPE-Br and TPE-I nano-aggregates in situ based on synergistic electrostatic and hydrophobic interactions, and enhance the fluorescent emission due to the AIE property of TPE-based molecules. Moreover, this research results are highly considerable for the quantitative detection of SDS level in the real samples. Acknowledgments The work was supported by Science and Technology Planning Project of Henan Province of China (172102110105), Natural Science Foundation of Guangdong Province (2017A030310604; 2018A030307035), Guangdong University Provincial Key Platform and Major Research Projects: Characteristic Innovation Project (2017KTSCX118), Natural Science Foundation of Lingnan Normal University (ZL1802). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117446.
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