Polydiacetylene-based sensor for highly sensitive and selective Pb2+ detection

Dyes and Pigments 120 (2015) 307e313

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Polydiacetylene-based sensor for highly sensitive and selective Pb2þ detection Minwu Wang, Fang Wang, Yong Wang, Wei Zhang, Xiaoqiang Chen* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2015 Received in revised form 23 April 2015 Accepted 25 April 2015 Available online 5 May 2015

Polydiacetylenes (PDAs), as smart materials with very unique properties, are widely used for sensing various analytes. Derived from the monomers 1-(1,4,7,10,13-pentaoxa-16-aza-cyclooctadec-16-yl)-pentacosa-10,12-diynamide (PCDA-L) and 10,12-pentacosadiynoic acid (PCDA), we report a conjugated polymer (poly-PDA-L) for lead ions detection. The PDA sensing system showed rapid fluorescent and colorimetric response and high sensitivity to Pb2þ in 100% aqueous solution, as well as the color change was obvious to the naked eye. Excellent fluorescence response change and a good linear relationship between fluorescent PDA intensity and Pb2þ amount were observed based on fluorescence microscope images, with a detection limit of 1 mM. The fluorescent and colorimetric responses were attributed to the binding between the moiety 1-aza-18-crown-6-ether of poly-PDA-L and Pb2þ. This interaction further disturbs the backbone of the PDA polymer, allowing the release of the strain energy imposed on the alkyl side chains generated during polymerization, leading to the observed change in optical properties. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Colorimetric sensor Fluorescent sensor Lead ion sensor Polydiacetylenes Fluorescent microscopy Conjugated polymer

1. Introduction Conjugated polymers are receiving considerable attention as optical platforms for detecting various species. Polydiacetylenes (PDAs), as a family of conjugated polymer sensors, have been intensively studied because their absorption, emission, and redox characteristics are sensitive to environmental perturbations [1e3]. Upon environmental stimulation, blue PDAs can undergo a color shift to red phase, accompanied by fluorescence enhancement. Hence, certain systems based on PDAs have been constructed to detect heat [4,5], metal ions [6e12], organic solvents [13e16], pH [17], cationic surfactants [18], biological agents [19e22], and others. Pb2þ, one of the most toxic heavy metal cations, causes several health problems to multiple organs and physiological systems of the human body through the digestive or respiratory tract. The harm for the human nervous system is particularly serious. Lead ion that flows with the blood into the brain can damage the cerebellum and cerebral cortex cell and interfere with the metabolic activity of nerve cells, thereby resulting in certain problems such as memory loss, anemia, and cerebral edema. It can also cause lesions and

* Corresponding author. Tel./fax: þ86 025 83587856. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.dyepig.2015.04.035 0143-7208/© 2015 Elsevier Ltd. All rights reserved.

atrophy of the liver and kidney systems. As a toxic heavy metal ion, Pb2þ particularly affects children. Problems such as inattention, learning difficulties, and aggressive behavior have a significant relationship with the high level of children's blood lead. Children's physical growth is also affected by lead [23]. In 1996, the World Health Organization established guidelines for drinking water quality with a maximal lead value of 10 mg L1 [24]. Pb2þ is an important raw material applied in batteries, gasoline, pigments, and others. With rapid industrial development, Pb2þ has become a serious contaminant in water, air, food, medicine, and our living environment. Thus, lead ion monitoring is increasingly important. The traditional detection methods for lead such as atomic absorption spectrometry [25] and inductively coupled plasma mass spectrometry [26] are costly and time-demanding processes. Therefore, a simple and quick method for detecting lead ion is urgently needed. Many research groups have committed to develop sensitive and selective fluorescent chemosensors for lead ion. Pb2þselective probes based on DNAzyme [27e29] peptides [30], proteins [31], polymers [32], and small molecules [33e37] have been reported. However, most of small molecules based probes work cannot display a good detection for lead ion in 100% aqueous solution. DNAzymes and protein based probes require a complicated process. Besides, a naked-eye detection method is preferable for environmental purposes and for the simplicity of the process.

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In this work, a sensing system for lead ion based on PDAs, which can easily monitor Pb2þ via colorimetric and fluorescent detection in 100% aqueous solution, was studied. The PDA system showed rapid response to Pb2þ, which was attributed to the binding between the head group (1-aza-18-crown-6-ether) of 1-(1,4,7,10,13pentaoxa-16-aza-cyclooctadec-16-yl)-pentacosa-10,12-diynamide (PCDA-L) and Pb2þ (Fig. 1). This interaction of Pb2þ with a PDA unit further disturbs the backbone of the PDA polymer, allowing the release of the strain energy imposed on the alkyl side chains generated during polymerization, leading to the observed change in optical properties. 2. Experimental 2.1. Materials Unless otherwise noted, all reagents, starting materials, and solvents were obtained from commercial suppliers and used without further purification. A buffer solution (HEPES 10 mM, pH 7.4) was prepared using deionized water.

thionyl chloride was added dropwise to the solution. After stirring at room temperature for 12 h under N2, the solvent was evaporated in a vacuum. The crude residue, 1-aza-18-crown 6ether (0.264 g, 1 mmol), and triethylamine (500 mL) were added to MeCN (40 mL), and the solution was refluxed for 12 h under N2 (Scheme S1). The resultant was concentrated by evaporation and purified by silica gel column chromatography (CH2Cl2/MeOH, 100/2 v/v), affording PCDA-L as a yellow oil (360.0 mg, 62.5%). The NMR and HR-mass spectra were provided in Figs. S1eS3. 1H NMR (CDC13, 300 MHz) d (ppm): d 0.85e0.88 (t, 3H, J ¼ 12.6 Hz), 1.25 (m, 26H), 1.48e1.53 (t, 4H, J ¼ 18.78 Hz), 1.60e1.63 (t, 2H, J ¼ 8.9 Hz), 2.20e2.26 (t, 4H, J ¼ 16.22 Hz), 2.30e2.38 (t, 2H, J ¼ 7.61), 3.50e3.70 (m, 24H); 13C NMR (CDC13, 75 MHz) d (ppm): 14.11, 19.22, 19.22, 22.69, 25.36, 28.38, 28.42, 28.84, 28.88, 28.99, 29.11, 29.34, 29.35, 29.47, 29.49, 29.62, 29.64, 29.66, 31.92, 33.12, 46.97, 49.06, 65.28, 65.31, 69.61, 70.08, 70.43, 70.63, 70.73, 70.77, 70.84, 70.88, 70.91, 76.60, 77.03, 77.45, 173.56; HRMS (EI) m/z ¼ 642.4704 [M þ Na]þ, calculated for C37H65NO6Na ¼ 642.4710. 2.4. Preparation of liposome

2.2. Measurements Column chromatography was performed on 230e400 mesh silica gel. 1H NMR spectra were recorded in the solvent stated using an Avance Bruker DRX 300 (300 MHz) instrument, and 13C NMR spectra were recorded at 75 MHz. UV absorption spectra were obtained on a-1860A UVevis spectrometer. Fluorescence emission spectra were obtained using RF-5301/PC spectrofluorophotometer. pH measurements were carried out with a Sartorius PB-10 meter. Fluorescence microscopy images were obtained on Olympus IX73. Raman and FTIR spectra were obtained on LabRam HR800 and FTIR Nexus 670, respectively. SEM image was obtained on a Hitachi S4800 field emission scanning electron microscope. 2.3. Synthesis of PCDA-L 10,12-Pentacosadiynoic acid (0.375 g, 1 mmol) was dissolved in 20 mL of anhydrous dichloromethane, and then 2 mL of

A mixture of PCDA-L and PCDA (3:7 mol ratio) was dissolved in a small amount of DMSO (1 mL). The organic solution was injected into 19 mL of HEPES buffer (10 mM, pH 7.4) with shaking in a water bath kettle at 80  C to yield a total monomer concentration of 1 mM. The sample was then sonicated at 80  C for 1 h. The solution was filtered through 0.8 mm cellulose syringe filter to remove liposomes of undesired size, and stored at 4  C for 12 h. Polymerization was carried out at 4  C by irradiating the solution with 254 nm UV light (1 mW/cm2) for 3 min.

2.5. Scanning electron microscopy (SEM) The sample (20 mL) was freshly made and deposited on silicon wafer. Then the silicon wafer deposited with PDAs was dried at least 8 h in an incubator. A SEM image of poly-PDA-L was obtained by a field emission scanning electron microscope.

Fig. 1. The binding of polymer with Pb2þ.

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2.6. Preparation of metal ion solutions for fluorescent study 2þ

309

O

2þ

Stock solutions (10 mM) of the perchlorate salts of Cu , Pb , Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ, and Al3þ in distilled water were prepared. In a typical experiment, test solutions were prepared by placing 150 mL of the probe stock solution into a test tube, adding an appropriate aliquot of each metal stock, and diluting the solution to 3 mL with 0.01 M HEPES (pH 7.4). Normally, excitation was at 492 nm. The excitation and emission slit widths were both 5 nm. Fluorescent spectra were measured after addition of Pb2þ for 5 min. For low-concentration titration of Pb2þ, fluorescence spectra were measured after addition of Pb2þ for 5 min, and the excitation and emission slit widths were 5 and 10 nm, respectively. Fluorescence titration was carried out under the fluorescence microscope to calculate the detection limit, which was then calculated with the equation:

O

O

O

O

O N

HO O

N

HO O

7

O

O

HO O

7

O

O

7

HO

O UV

7

O 7

O 7 *

10

10

10

10

10

10

poly-PDA-L

Fig. 2. Self-assembly and polymerization of poly-PDA-L.

Detection limit ¼ 3sbi =m where sbi is the standard deviation of blank measurements, and m is the slope between intensity and sample concentration. 2.7. UVevis absorption assays UV titration was performed using 50 mM solution of polymer in HEPES (10 mM, pH 7.4). The color change of the polymer from blue to red was calculated by colorimetric response (CR) using the following equation:

CR ¼ ½ðPB0  PB1 Þ=PB0   100 where PB ¼ Ablue/(Ablue þ Ared), A is the absorbance at either the blue or the red component in the UVevis spectrum. PB0 and PB1 are the pre- and post-exposure values, respectively. 2.8. Fluorescent imaging of polymer with Pb2þ through microscopy Stock solutions (8, 16, 32, 40, 50, 100, 200, 400, and 800 mM) of Pb(ClO4)2 in distilled water were prepared. Various concentrations of Pb2þ (0.2 mL) were dropped onto the wells with polymer (1 mM) on glass plates. Fluorescent images were collected by microscopy. The fluorescence brightness of each photo was quantified through cellSens Life Science Imaging Software.

demonstrating the formation of polymer backbone during the polymerization process. 3.2. Fluorescence and absorption performance of poly-PDA-L for sensing Pb2þ Subsequently, the colorimetric responses of poly-PDA-L toward various metal ions were examined. As shown in Fig. 4a, among the various metal cations, including Pb2þ, Cu2þ, Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ, and Al3þ, the PDA suspension exhibited selective and clear phase transition from blue to red in the presence of Pb2þ (Fig. 4a). By contrast, the color of the PDAs polymerized from pure 10,12-pentacosadiynoic acid did not show any change (Fig. 4b). With increasing Pb2þ concentration from 0 to 9 mM, the PDAs displayed a clear blue-to-purple transition (Fig. 4c) (in web version). To further monitor the selectivity of polyPDA-L, the spectral change of the PDAs in contact with the various metal cations, namely, Cu2þ, Pb2þ, Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ and Al3þ, was analyzed. The UVevis and fluorescence data (Fig. 5a and b) show a significant spectral change for Pb2þ only, whereas the other metal ions induced only slight changes in the absorption and fluorescence spectra.

3. Results and discussion 3.1. Design synthesis characterization of sensing system The DA monomers (PCDA-L and PCDA) were converted into PDA supramolecules by irradiating the solution with 254 nm UV light at room temperature. Through this method, stable PDA molecules (poly-PDA-L) were obtained (Fig. 2). The responses to Pb2þ were checked using the PDA polymer prepared from various PCDA and PCDA-L ratios (Fig. S4), and the optimized PCDA and PCDA-L ratio was found to be 7:3, indicating that both polymerization and response to Pb2þ were desirable. The SEM image showed liposomes as well as aggregates formed, and the resulting polymer is nearly spherical with about 30 nm diameters (Fig. S5). The Raman spectra of the monomer PCDA-L and the polymer poly-PDA-L were analyzed. As shown in Fig. 3a, the characteristic peak at 2255.91 cm1 ascribed to the C^C bond of monomer PCDA-L was clearly observed. After polymerization, the characteristic peak at 2255.91 cm1 disappeared, and a new broad peak at 1000e2000 cm1 ascribed to C]C stretching emerged (Fig. 3b),

Fig. 3. The Raman spectra of pure PCDA-L (a) and poly-PDA-L (b).

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Fig. 4. (a) Colorimetric responses of poly-PDA-L (500 mM) toward various metal ions (10 mM) in HEPES (10 mM, pH 7.4); (b) Colorimetric responses of PDAs derived from PCDA toward various metal ions; (c) Colorimetric responses of poly-PDA-L (250 mM) toward various amounts of Pb2þ (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 mM).

As is well known, one of the essential requirements for Pb2þ ion detection is minor or no interference from other metal ions, especially from the most common metal ions in the environment. The competition studies of the Pb2þ ion detection has been done, to confirm the sensing system has a good selectivity for Pb2þ in

Fig. 5. Vis absorption spectra (a) and fluorescence titrations (b) of poly-PDA-L (50 mM) in HEPES buffer (10 mM, pH ¼ 7.4) with various metal cations (5 mM), including Cu2þ, Pb2þ, Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ, and Al3þ.

various metal cations. As shown in Fig. 6, the poly-PDA-L vesicles with mixture metal ions (Cu2þ, Pb2þ, Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ and Al3þ, each 5 mM) show a slight spectral change in the absorption spectra, and the CR% value was only 11.6%. When the Pb2þ ions (5 mM) were added to the mixture solution, it induced a remarkable change in the absorption spectra, and the CR% value was 71.2%. To further monitor the phase transition of poly-PDA-L with the concentration of Pb2þ, the absorption spectral changes were investigated by visible absorption spectroscopy. As displayed in Fig. 7a, adding Pb2þ resulted in a decrease in the absorption at 640 nm with a simultaneous increase in the absorption at 540 nm, indicating a typical blue-to-red transition of PDA sensors. The representative percentage of colorimetric response values (CR%) were calculated and are shown in Fig. 7b. Based on the result, the response value reached a maximum when the Pb2þ concentration increased to 6 mM. The blue-to-red transition of the PDAs accompanied by the generation of fluorescence was also monitored by fluorescence spectroscopy. As shown in Fig. 8, the polymer fluorescence of the PDAs was significantly enhanced with increased Pb2þ concentration. The fluorescence spectra of the PDA showed a gradual increase in the presence of 0e6 mM Pb2þ. An approximately 6-fold

Fig. 6. (a) UVevisible spectra and (b) colorimetric responses (CR%) of poly-PDA-L vesicles (50 mM) with a mixture of metal ions (mix: Cu2þ, Naþ, Hg2þ, Fe3þ, Mn2þ, Cr3þ, Co2þ, Zn2þ, Mg2þ, Liþ, Ni2þ, Cd2þ, Agþ and Al3þ, each 5 mM) in the absence and the presence of Pb2þ ions (5 mM) in water at room temperature.

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enhancement in the fluorescence intensity at 565 nm was observed when 6 mM Pb2þ was added to the PDA suspensions. We also calculated the detection limit as 0.05 mM according to the titration curve on fluorescent intensity versus concentration of Pb2þ in solution (Fig. S6). 3.3. Proposed mechanism The changes in the optical spectra of the polymer induced by Pb2þ could be attributed to the interaction of Pb2þ with the PDA unit, which further disturbed the backbone of the PDA polymer, allowing the release of the strain energy imposed on the alkyl side chains generated during polymerization, leading to the color change and fluorescence enhancement. To verify the binding between 1-aza-18-crown 6-ether moiety and Pb2þ, the FTIR spectra of PCDA-L with and without Pb2þ were analyzed. As shown in Fig. 9, upon incubation with Pb2þ, the characteristic band of ketone (the C]O stretching vibration) at 1646 cm1 decreased, implying the interaction between Pb2þ and C]O group. 3.4. Fluorescent imaging of polymer with Pb2þ through microscopy

Fig. 7. a) Vis absorption spectra of poly-PDA-L (50 mM) in HEPES buffer (10 mM, pH ¼ 7.4) upon incubation with various Pb2þ concentrations (0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, and 6 mM); b) Quantitative colorimetric response of poly-PDA-L (50 mM) toward various Pb2þ concentrations in HEPES buffer (10 mM, pH 7.4).

Fig. 8. Fluorescence titrations of poly-PDA-L (50 mM) in HEPES buffer (10 mM, pH ¼ 7.4) upon incubation with various Pb2þ concentrations (0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, and 6 mM).

The fluorescence responses of the polymer to Pb2þ were further evaluated through the fluorescence microscope images. As shown in Fig. 10a, poly-PDA-L (0.2 mL, 1 mM) were initially dropped onto microwells on the glass, and then 0.2 mL of various Pb2þ concentrations (8, 16, 32, 40, 50, 100, 200, 400, and 800 mM) were added to the microwells and incubated with poly-PDA-L for 30 min. When the solvent completely evaporated in each microwell, the fluorescence images were collected by the fluorescence microscope. Based on Fig. 10b, the polymer fluorescence of the PDAs was clearly enhanced with increasing Pb2þ concentration. Furthermore, the fluorescence intensity of each microwell was quantified through cellSens Life Science Imaging Software. As shown in Fig. 10c, the concentration of Pb2þ and the mean fluorescence intensity were fitted at each Pb2þ concentration. The fluorescence intensity showed a good linear relationship with the amount of Pb2þ. According to the fitted line, the calculated detection limit was about 1 mM. Comparing to the detection limit (0.05 mM) obtained according to the titration in solution (Fig. S6), the higher detection

Fig. 9. FTIR of pure PCDA-L (a) and PCDA-L cooperated with Pb2þ (b).

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Fig. 10. a) Strategy for fluorescent study through microscopy: Step 1, 0.2 mL of poly-PDA-L (1 mM) was dropped to microwells on the glass; Step 2, 0.2 mL of various Pb2þ concentrations (8, 16, 32, 40, 50, 100, 200, 400, and 800 mM) was added to each microwell; Step 3, the fluorescence image of the microwell was collected using the fluorescence microscope, and pictures were taken; Step 4, the fluorescence images were quantified through cellSens Software. b) The fluorescence microscope images of PDA-DPA-L upon incubation with various Pb2þ concentrations (4, 8, 16, 20, 25, 50, 100, 200, and 400 mM). c) Correlation curve between the mean fluorescence intensity and the amount of Pb2þ.

limit was attributed to that the bottom surface of each microwell is unsmooth, which cause more large standard deviation. A similar work recently reported by Ding et al. [38], where the Gly was linked to PDA as the sensing moiety, indicated good response to Pb2þ. The detection limit was determined as 0.24 mM, which relies on a SiO2 nanoparticle decorated polydiacetylene embedded polyacrylonitrile nanofibrous membranes. The previous work is timeconsuming and complicated in preparation process of sample. In contrast, a simple and rapid method based on PDA system for Pb2þ detection is at hand.

colorimetric change from blue to red among various metal ions and exhibited fluorescence enhancement. More importantly, the detection with this PDA-based chemosensor system can be easily monitored with the naked eye after adding Pb2þ in a very short period of time. The detection limit was 1 mM, calculated through microscopy equipped with cellSens software. The polymer polyPDA-L was demonstrated to be a good sensing system for Pb2þ detection.

4. Conclusion

This work was supported by the National Natural Science Foundation of China (21376117, 21406109), the Jiangsu Natural Science Funds for Distinguished Young Scholars (BK20140043), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (14KJA150005), the Qing Lan Project and the

A PDA system polymerized from a mixed liposome comprising PCDA and PCDA-L at 7:3 ratio was developed for the detection of Pb2þ in aqueous solution. The sensing system provided a selective

Acknowledgments

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Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2015.04.035.

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