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Composites of surface imprinting polymer capped Mn-doped ZnS quantum dots for room-temperature phosphorescence probing of 2,4,5-trichlorophenol
Journal of Luminescence 155 (2014) 298–304
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Composites of surface imprinting polymer capped Mn-doped ZnS quantum dots for room-temperature phosphorescence probing of 2,4,5-trichlorophenol Xiao Wei a,b, Zhiping Zhou a,n, Jiangdong Dai a, Tongfan Hao c, Hongji Li b, Yeqing Xu b, Lin Gao b, Jianming Pan b, Chunxiang Li b, Yongsheng Yan b a b c
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China School of Computer Science, Jilin Normal University, Siping 136000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2014 Received in revised form 28 June 2014 Accepted 1 July 2014 Available online 11 July 2014
In this paper, a simple procedure for the determination of 2,4,5-trichlorophenol (2,4,5-TCP) is reported. Mn-doped ZnS quantum dots (QDs) capped by molecularly imprinted polymers (MIPs) were prepared. MIPs were characterized by spectrofluorometer, UV–vis spectrophotometer, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and scanning electron microscope (SEM). Meanwhile, spectrofluorometer was used to the evaluation of optical stability, the effect of pH, and the selective and sensitive determination of 2,4,5-TCP. Under optical conditions, MIPs-capped Mn-doped ZnS QDs were successfully applied to the detection of 2,4,5-TCP in water selectively and sensitively, and a linear relationship was obtained to cover the concentration range of 5.0–50 μmol L 1 with a correlation coefficient of 0.9913. Moreover, 2,4,5-TCP could quench the room temperature phosphorescence of MIPs-capped Mn-doped ZnS QDs in a concentration-dependent manner, which was best described by a Stern–Volmer-type equation. & 2014 Published by Elsevier B.V.
Keywords: Mn-doped ZnS quantum dots Room temperature phosphorescence Molecularly imprinted polymers 2,4,5-Trichlorophenol Selective recognition
1. Introduction Semiconductor nanocrystals, known as quantum dots (QDs), have attracted much attention in recent years because of their unique optical and electronic properties, larger surface-to-volume ratios, and quantum-size effects [1–3]. Compared with traditional organic fluorophores, QDs show excellent photostability, narrow and symmetric emission band and broad absorption spectra [4,5]. Indeed, the QDs have fully developed, and they have been applied in many fields [6–8]. For example, Tu et al. have composed aminecapped ZnS-Mn2 þ nanocrystals for fluorescence detection of trace TNT explosive [9] and Geszke-Moritz et al. prepared copper- or manganese-doped ZnS quantum dots as fluorescent probes for detecting folic acid in aqueous media [10]. One of the QDs’ properties is room temperature phosphorescence (RTP). In recent years, RTP has become an effective detection mode thanks to its fascinating advantages over fluorescence [11,12]. The RTP shows a number of advantages, such as its longer emission lifetime, making the space between the excitation and the emission spectra wider, and its short-lived autofluorescence and scattering light with the
n
Corresponding author. Tel.: þ 86 511 88790683; fax: þ86 511 88791800. E-mail address: [email protected] (Z. Zhou).
http://dx.doi.org/10.1016/j.jlumin.2014.07.001 0022-2313/& 2014 Published by Elsevier B.V.
minimum interference [13–15]. Wang et al. have reported 3aminopropyltriethoxysilane-functionalized Mn doped ZnS QDs for room-temperature phosphorescence sensing ultratrace 2,4,6trinitrotoluene in aqueous solution [16]. However, traditional RTP detections always suffer from the interference of the coexisting substances. As far as we know, a promising way to achieve the tailored selectivity of RTP detection is to combine the RTP optosensing with the molecular imprinting technology (MIT) [17,18]. Molecular imprinting is a simple but well-established technique for synthesizing cross-linked polymers in the threedimensional space with specific molecular recognition properties, namely molecularly imprinted polymers (MIPs) [19]. Because of their desired predetermination, high specific selectivity and practicability, MIPs are being increasingly used in some significant application areas, such as chemical sensor [20], recognition and separation [21], drug delivery and controlled release [22], and catalysis [23]. Traditionally, MIPs have the shortcomings of incomplete template, poor binding capacity and low binding kinetics, because making the extraction of template molecules embedded inside the thick polymer network is quite difficult [24]. To overcome those shortcomings, the surface molecular imprinting technique has been developed in recent years, which fabricated and situated specific binding sites at the surface or in the proximity of
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materials surface [25]. By coating the MIPs film onto a solid support, the surface-imprinting technique provides an alternative way to improve mass transfer and reduce permanent entrapment of the template [26]. Several protocols have been developed to construct the MIPsbased optical materials (QDs). For example, Zhang et al. have composed CdTe quantum dots and molecularly imprinted polymer as a sensing material for cytochrome c and demonstrated that the MIPs anchored on the surface of the dBSA modified CdTe QDs could be used as selective materials for recognition of target protein [27,28]. Li et al. has reported that molecularly imprinted silica nanospheres are embedded CdSe QDs for highly selective and sensitive optosensing of pyrethroids [29]. As a kind of trend, The Mn-doped ZnS QDs will have more important significance due to their phosphorescence property. Recently, the optosensing systems of MIPs-based RTP have attracted considerable interest. Wang et al. composed MIPs-based Mn-Doped ZnS QDs via a surface molecular imprinting process for RTP optosensing of pentachlorophenol in water [15]. It is expected that the marriage of the RTP of Mn-doped ZnS QDs with MIPs will make a significant contribution to further improvement in the selectivity of Mndoped ZnS QDs based on RTP detection [15]. Chlorophenols are very common and have been listed as the priority pollutants into the aquatic environment [30]. They are primarily produced by chemical industries, such as petroleum refineries, plastics, rubbers, pharmaceuticals, wood-preserving and steel industries. The majority of these chlorophenols are known or suspected as human carcinogens. They are mild acids which permeate human skin in vitro and are ready to be absorbed by gastro-intestinal tract [31,32]. The discharge of chlorophenol contaminated wastewater into aquatic environment without adequate treatment can have negative effect on the water quality, and pose a serious ecological problem as mutagenic effects on the living [33]. Especially, 2,4,5-trichlorophenol (2,4,5-TCP) is considered to have significant toxicological effects and potential carcinogenicity. 2,4,5-TCP has been included in the “persistent, bioaccumulative, toxic chemical list” by the US EPA, posing a serious threat to human health and natural ecosystems. It is resistant to biodegradation in aerobic and anaerobic systems, thereby it tends to bioaccumulate in the environment [34]. Thus, simple, rapid and selective RTP detection of trace 2,4,5-TCP from complex matrix is of great importance. In this work, we report the formation of MIPs-based RTP sensor by anchoring a MIPs layer on the surface of Mn-doped ZnS QDs via a surface molecular imprinting process. 2,4,5-TCP was chosen as a target molecule and the tetraethoxysilane (TEOS) was chosen as cross-linker. The 3-aminopropyltriethoxysilane (APTS) was used as a functional monomer which had a noncovalent interaction with the template 2,4,5-TCP. After the sol-gel process, the artificial receptor was synthesized. When the template was removed by solvent extraction, the MIPs-based RTP sensor is capable of selectively rebinding the target molecule 2,4,5-TCP.
2. Experimental
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2.2. Instrument The X-ray diffraction (XRD) spectra were collected on a XRD6100Lab X-ray diffractometer (Shimadzu, Japan) with Cu Kα radiation over the 2θ range of 10–801. Infrared spectra (4000–400 cm 1) in KBr were recorded to use a Nicolet NEXUS-470 FTIR apparatus (USA). The morphologies of prepared composites were observed by a transmission electron microscope (TEM, JEOL, JEM-2100) and a scanning electron microscope (SEM, JEOL, JSM-7001F). UV–vis adsorption spectra were obtained with a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan). The phosphorescence measurements were performed on a spectrofluorometer (Cary Eclipse) equipped with a plotter unit and a quartz cell (1.0 cm 1.0 cm). 2.3. Synthesis of MPTS-capped Mn-doped ZnS QDS and MIPs-capped Mn-doped ZnS QDs The preparation of MPTS-capped Mn-doped ZnS QDs and MIPscapped Mn-doped ZnS QDs have been described in detail in a previously published procedure [15]. Briefly, to a three-necked flask, 6.25 mmol of ZnSO4, 0.5 mmol of MnCl2, and 20 mL of water was added. Under the protection of nitrogen gas, the mixture was kept stirring at room temperature for 20 min. A 5.0 mL, 6.25 mmol of Na2S solution was added dropwise, and the mixture was kept stirring for 30 min. Then 5.0 mL of an ethanol solution of 0.318 mmol of MPTS was added, and the mixture was kept stirring overnight. Finally the resultant MPTS-capped Mn-doped ZnS QDs was obtained following centrifugation, washing with DDW and absolute ethanol three times and drying in vacuum. To a 100 mL flask, 10 mL of an absolute ethanol solution of 100 mg of 2,4,5-TCP (template) and 250 mL of APTS (functional precursor) were added and stirred for 30 min. Then, 1.0 mL of TEOS (cross-linker) was added, and the mixture was kept stirring for 10 min. Then 500 mg of MPTS-capped Mn-doped ZnS QDs and 2.5 mL of 6% NH3 H2O were added and stirred overnight. The resultants were centrifuged and washed with absolute ethanol. The templates in the MIPs were extracted for 30 min with a mixture solvent of ethanol/acetonitrile (v/v, 8:2) repeatedly. Finally, MIPs-capped Mn-doped ZnS QDs were dried in vacuum. In comparison, the non imprinted polymers (NIPs) were also prepared with the same procedure but without addition of 2,4,5TCP. 2.4. Measurement procedure In the experiments, all the RTP detections were performed under the same conditions: the slit widths of the excitation and emission were both 10 nm, and the excitation wavelength was set at 324 nm with a recording emission range of 450–650 nm. The photomultiplier tube voltage was set at 800 V. MIPs and NIPs were dissolved in DDW to get the fresh-made stock solution (100 mg L 1). 2,4,5-TCP stock solution (200 mg L 1, in DDW) was stocked at 4.0 1C. An appropriate quantity of MIPs and NIPs was added to a 10 mL colorimetric tube and a given concentration of analyte standard solution was added sequentially. The mixture was gently ultrasonicated for 5.0 min before RTP measurement.
2.1. Reagents and chemicals All chemicals were of analytical grade reagents. MnCl2 4H2O, Na2S 9H2O, tetraethoxysilane (TEOS), 3-mercaptopropyltriethoxysilane (MPTS), 3-aminopropyltriethoxysilane (APTS), 2,4,5-trichloro phenol (2,4,5-TCP), 2,4-Dichlorophenol (DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 3-Chlorophenol (3-CP), ZnSO4 7H2O and ammonia solution (25.0–28.0%) were all purchased from Aladdin reagent Co., Ltd. (Shanghai, China). Double distilled water (DDW) was used throughout the experimental procedures.
3. Results and discussion 3.1. Preparation of MIPs-capped Mn-doped ZnS QDs Fig. 1 illustrate the schematic procedure for preparation of MIPs-coated Mn-doped ZnS QDs sensors. In the first step, the Mndoped ZnS QDs-MPTS was synthesized. In the second step, MIPscapped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs
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Fig. 1. Schematic procedures for preparation of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs.
diameter of the MIPs-capped Mn-doped ZnS QDs. It could be observed that the SEM result was the same as that of the TEM. Fig. 5 showed the X-ray diffraction patterns of the Mn-doped ZnS QDs, Mn-doped ZnS QDs-MPTS and MIPs-capped Mn-doped ZnS QDs. The crystal structures of the samples exhibited cubic zinc blende with peaks indexed as (1 1 1), (2 2 0), and (3 1 1) planes. The average size (D) of the Mn-doped ZnS QDs could be calculated according to the Scherrer’s equation: D ¼ kðλ=β cos θÞ
Fig. 2. Excitation (curve 1) and emission (curve 2) spectra of the MIPs-capped Mn-doped ZnS QDs.
were obtained (Fig. 1). 2,4,5-TCP (Fig. S1) was used as template molecule to involve in the formation of MIPs film on the surface of QDs. The 3-aminopropyltriethoxysilane (APTS) was used as a functional monomer which had a noncovalent interaction with the template 2,4,5-TCP. The concentrations of the reactants were reduced to obtain a thin MIP layer and to minimize the homogeneous self-condensation of TEOS and APTS [13]. After the templates were extracted by using the solvent, the RTP intensity of MIPs-capped Mn-doped ZnS QDs could get nearly the same with NIPs-capped Mn-doped ZnS QDs.
ð1Þ
where k is a constant equal to 0.89, λ is the X-ray wavelength and equals to 0.154 nm, β is the full width at half maximum and θ is the half diffraction angle (47.3). The calculated result indicated that the average size (D) was 4.0 nm approximately. DLS analysis in Fig. S2 demonstrated that the average particle size of the ZnS QDs in water is 4.7 nm, which was well consistent with the TEM and XRD results (Table S1). The intensities of the (1 1 1), (2 2 0), and (3 1 1) diffraction peaks of MIPs-capped Mn-doped ZnS QDs (curve 3 in Fig. 5) were weaker than that of MPTS-capped Mn-doped ZnS QDs (curve 2 in Fig. 5) and Mn-doped ZnS QDs (curve 1 in Fig. 5) probably due to the silica shell around the dots. These results could be explained as amorphous materials (silica) which were presented in MIPscapped Mn-doped ZnS QDs and MPTS-capped Mn-doped ZnS QDs. The FT-IR spectra of Mn-doped ZnS QDs-MPTS, MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were measured by KBr disks and shown in Fig. S3. The results illustrated that the MIPs generated from sol-gel condensation of APTS and TEOS were successfully grafted on the surface of MPTS-capped Mn-doped ZnS QDs.
3.2. Optical properties of MIPs-capped Mn-doped ZnS QDs 3.4. Effect of pH The UV absorption spectra of the MIPs-capped Mn-doped ZnS QDs is shown in Fig. S1. The prepared MIPs-capped Mn-doped ZnS QDs had a RTP emission at 596 nm when excited at 324 nm (Fig. 2). As shown in Fig. 3, right, well-dispersed aqueous MIPscapped Mn-doped ZnS QDs particles emitted orange light originating from the characteristic emission of ZnS upon UV light irradiation. 3.3. Characterization The TEM images of original Mn-doped ZnS QDs, Mn-doped ZnS QDs-MPTS and MIPs-capped Mn-doped ZnS QDs are shown in Fig. 4a–c, respectively. The TEM images displayed the Mn-doped ZnS of about 4.4 nm. We could see that the Mn-doped ZnS were coated by MPTS successfully from the TEM images (Fig. 4b). The size of Mn-doped ZnS QDs-MPTS was smaller than that of MIPscapped Mn-doped ZnS QDs, and it also indicates that MIPs were synthesized successfully on the Mn-doped ZnS QDs-MPTS. The SEM images (see Fig. 4d) showed the morphology and the
The pH value has a significant effect on the RTP intensities of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs. As we know, in a pH medium below 4.0, the ZnS QDs were instable, which was evidenced by the smelly H2S generated. As pH increased from 11 to 13, the RTP intensity decreased quickly. The reason is that silica shell could be ionized at high pH, and OH could nucleophilically attack the surface and create surface defects [26]. Finally, the effect of pH in a range between 4.0 and 11 was researched for MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs in Fig. 6. As could be seen from Fig. 6, the RTP intensity of MIPs-capped Mn-doped ZnS QDs in the interval 5.0– 8.0 was considerably stable. At last, a pH of 7.0 was selected for further experiments. 3.5. Stabilities The stabilities of MIPs-capped Mn-doped ZnS QDs and NIPscapped Mn-doped ZnS QDs in water at pH 7.0 were estimated by
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Fig. 3. Photographs of MIPs-capped Mn-doped ZnS QDs in water solution (1) and blank aqueous solution (2) taken under daylight lamp (a) and 365 nm UV light (b).
Fig. 4. TEM images of Mn-doped ZnS QDs (a); Mn-doped ZnS QDs-MPTS (b) and MIPs-capped Mn-doped ZnS QDs (c). SEM images of MIPs-capped Mn-doped ZnS QDs (d).
RTP intensity as a function of time at room temperature, as shown in Fig. 7. The RTP intensities of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were stable for 13 times measurements within 1.0 h. This phenomenon showed that it was the silica shell that made the composite have an excellent stability.
3.6. MIPs and NIPs with template 2,4,5-TCP of different concentrations In this test, our aim was to demonstrate the recognition ability of the MIPs-capped Mn-doped ZnS QDs versus that of the NIPscapped Mn-doped ZnS QDs (Fig. 8). Fig. 8a showed the spectral response of MIPs-capped Mn-doped ZnS QDs with template 2,4,5TCP at different concentrations. The relationship between the RTP intensity and the concentration of quenching 2,4,5-TCP could be
described by the Stern-Volmer equation: I max =I ¼ 1 þ K SV ½S
ð2Þ
I and Imax are the RTP intensities of the MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs at a given related 2,4,5-TCP concentration and in a 2,4,5-TCP free solution, respectively. KSV is the Stern-Volmer quenching constant, and [S] is the 2,4,5-TCP concentration. The dependence of Imax/I as function of [S], are shown in Fig. 7. The KSV,MIP was found to be 25160 M 1. The linear range of the calibration curve was 5.0–50 μmol L 1 with a correlation coefficient of 0.9913. To demonstrate the imprinting effect, as a control experiment, the RTP response of NIPs to the template molecule was investigated. As shown in Fig. 8b, the KSV,NIP was 11860 M 1, the linear range of 2,4,5-TCP was also 5.0–50 μmol L 1 but with a correlation coefficient of 0.9981. According to the results we obtained, the MIPs-capped
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Fig. 5. XRD patterns of Mn-doped ZnS QDs (curve 1), Mn-doped ZnS QDs-MPTS (curve 2) and MIPs-capped Mn-doped ZnS QDs (curve 3).
Fig. 7. Stabilities of MIPs-capped Mn-doped ZnS QDs (black squares) and NIPscapped Mn-doped ZnS QDs (red dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
evident from Fig. 9, the quench efficiency [(Imax I)/Imax] of MIPs for the four compounds followed the order 2,4,5-TCP 42,4,6TCP4 2,6-DP 43-CP. By calculation, the difference in the quench efficiency of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were 0.1871, 0.0744, 0.0252, 0.0430 for 2,4,5TCP, 2,4,6-TCP, 2,4-DCP, 3-CP, respectively. The results suggested that MIPs were better specific to 2,4,5-TCP but less specific to other chlorophenols.
3.8. Application to water sample analysis.
Fig. 6. Effect of pH on RTP of MIPs-capped Mn-doped ZnS QDs (black squares) and NIPs-capped Mn-doped ZnS QDs (red dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Mn-doped ZnS QDs had a better selectivity than the NIPscapped ones. 3.7. Selectivity The RTP response of MIPs-capped Mn-doped ZnS QDs to various chlorophenols was conducted to examine the selectivity, and the initial concentration of each chlorophenol was 50 μmol L 1. The selectivity analyses of the MIPs-capped Mndoped ZnS QDs in single solute were exhibited in Fig. 9. With the
Surface river water samples were collected from local rivers. The samples were filtered through 0.45 μm Supor filters and stored in precleaned glass bottles. As no chlorophenols in the collected water samples were detectable by the proposed method, a recovery study was carried out on the samples spiked with 5.0– 15 μmol L 1 2,4,5-TCP to evaluate the developed method, and the corresponding results were listed in Table 1. 2,4,5-TCP solutions were tested by MIPs-capped Mn-doped ZnS QDs, focusing on the linear regime of RTP changes versus concentration permitted the construction of a “standard” curve to which our data were compared (Fig. 8). From this curve and the RTP recovery measured for the unkown samples, we were able to derive estimates of 2,4,5TCP levels in the water sample collected from local river. The recoveries were from 101.3% to 106.4%. The values determined by the MIPs-capped Mn-doped ZnS QDs show the ability of our QD sensing assembly to provide accurate measures of 2,4,5-TCP concentrations on unknown environmental samples. So it can be used for the direct analysis of relevant real samples.
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Table 1 Recovery of 2,4,5-TCP in water samples with 2,4,5-TCP solution at different concentration levels.
2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP
Concentration taken (μmol L 1)
Found (μmol L 1)
Recovery (%)
RSD (%)
5.0 6.0 8.0 10 12 15
5.32 6.21 8.32 10.27 12.16 15.4
106.4 103.5 104.0 102.7 101.3 102.0
3.8 2.7 3.4 1.7 2.1 2.3
Mn-doped ZnS QDs is in progress in our laboratory, which should spark a broad spectrum of interest because of its great versatility and flexibility for future applications.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 21107037, no. 21176107 and no. 21277063), Natural Science Foundation of Jiangsu Province (no. BK2011461, no. BK2011514), National Postdoctoral Science Foundation (no. 2013M530240), Postdoctoral Science Foundation funded Project of Jiangsu Province (no. 1202002B) and Programs of Senior Talent Foundation of Jiangsu University (no. 12JDG090).
Appendix A. Supporting information Fig. 8. RTP emission spectra of MIPs (a) and NIPs (b) (20 mg L 1) with addition of the indicated concentrations of 2,4,5-TCP in water solution (pH 7.0).
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.07. 001. References
Fig. 9. Quenching efficiency of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs by different kinds of 50 μmol L 1 chlorophenols (2,4,5-TCP, 2,4,6-TCP, 2,6-DCP, 3-CP).
4. Conclusion In summary, we have demonstrated that the MIPs anchored on the surface of the MPTS modified Mn-doped ZnS QDs could be used as selective materials for the recognition of target 2,4,5-TCP. The MIPscapped Mn-doped ZnS QDs integrated the advantages of the high selectivity of the molecular imprinting and strong RTP property of the QDs. A series of binding experiments have further demonstrated that the materials had good selectivity for template 2,4,5-TCP over analogues. Under optical conditions, MIPs-capped Mn-doped ZnS QDs was successfully applied to selectively and sensitively detection of 2,4,5TCP in water. Furthermore, molecular recognition of MIPs-capped
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Composites of surface imprinting polymer capped Mn-doped ZnS quantum dots for room-temperature phosphorescence probing of 2,4,5-trichlorophenol Xiao Wei a,b, Zhiping Zhou a,n, Jiangdong Dai a, Tongfan Hao c, Hongji Li b, Yeqing Xu b, Lin Gao b, Jianming Pan b, Chunxiang Li b, Yongsheng Yan b a b c
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China School of Computer Science, Jilin Normal University, Siping 136000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 12 March 2014 Received in revised form 28 June 2014 Accepted 1 July 2014 Available online 11 July 2014
In this paper, a simple procedure for the determination of 2,4,5-trichlorophenol (2,4,5-TCP) is reported. Mn-doped ZnS quantum dots (QDs) capped by molecularly imprinted polymers (MIPs) were prepared. MIPs were characterized by spectrofluorometer, UV–vis spectrophotometer, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and scanning electron microscope (SEM). Meanwhile, spectrofluorometer was used to the evaluation of optical stability, the effect of pH, and the selective and sensitive determination of 2,4,5-TCP. Under optical conditions, MIPs-capped Mn-doped ZnS QDs were successfully applied to the detection of 2,4,5-TCP in water selectively and sensitively, and a linear relationship was obtained to cover the concentration range of 5.0–50 μmol L 1 with a correlation coefficient of 0.9913. Moreover, 2,4,5-TCP could quench the room temperature phosphorescence of MIPs-capped Mn-doped ZnS QDs in a concentration-dependent manner, which was best described by a Stern–Volmer-type equation. & 2014 Published by Elsevier B.V.
Keywords: Mn-doped ZnS quantum dots Room temperature phosphorescence Molecularly imprinted polymers 2,4,5-Trichlorophenol Selective recognition
1. Introduction Semiconductor nanocrystals, known as quantum dots (QDs), have attracted much attention in recent years because of their unique optical and electronic properties, larger surface-to-volume ratios, and quantum-size effects [1–3]. Compared with traditional organic fluorophores, QDs show excellent photostability, narrow and symmetric emission band and broad absorption spectra [4,5]. Indeed, the QDs have fully developed, and they have been applied in many fields [6–8]. For example, Tu et al. have composed aminecapped ZnS-Mn2 þ nanocrystals for fluorescence detection of trace TNT explosive [9] and Geszke-Moritz et al. prepared copper- or manganese-doped ZnS quantum dots as fluorescent probes for detecting folic acid in aqueous media [10]. One of the QDs’ properties is room temperature phosphorescence (RTP). In recent years, RTP has become an effective detection mode thanks to its fascinating advantages over fluorescence [11,12]. The RTP shows a number of advantages, such as its longer emission lifetime, making the space between the excitation and the emission spectra wider, and its short-lived autofluorescence and scattering light with the
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Corresponding author. Tel.: þ 86 511 88790683; fax: þ86 511 88791800. E-mail address: [email protected] (Z. Zhou).
http://dx.doi.org/10.1016/j.jlumin.2014.07.001 0022-2313/& 2014 Published by Elsevier B.V.
minimum interference [13–15]. Wang et al. have reported 3aminopropyltriethoxysilane-functionalized Mn doped ZnS QDs for room-temperature phosphorescence sensing ultratrace 2,4,6trinitrotoluene in aqueous solution [16]. However, traditional RTP detections always suffer from the interference of the coexisting substances. As far as we know, a promising way to achieve the tailored selectivity of RTP detection is to combine the RTP optosensing with the molecular imprinting technology (MIT) [17,18]. Molecular imprinting is a simple but well-established technique for synthesizing cross-linked polymers in the threedimensional space with specific molecular recognition properties, namely molecularly imprinted polymers (MIPs) [19]. Because of their desired predetermination, high specific selectivity and practicability, MIPs are being increasingly used in some significant application areas, such as chemical sensor [20], recognition and separation [21], drug delivery and controlled release [22], and catalysis [23]. Traditionally, MIPs have the shortcomings of incomplete template, poor binding capacity and low binding kinetics, because making the extraction of template molecules embedded inside the thick polymer network is quite difficult [24]. To overcome those shortcomings, the surface molecular imprinting technique has been developed in recent years, which fabricated and situated specific binding sites at the surface or in the proximity of
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materials surface [25]. By coating the MIPs film onto a solid support, the surface-imprinting technique provides an alternative way to improve mass transfer and reduce permanent entrapment of the template [26]. Several protocols have been developed to construct the MIPsbased optical materials (QDs). For example, Zhang et al. have composed CdTe quantum dots and molecularly imprinted polymer as a sensing material for cytochrome c and demonstrated that the MIPs anchored on the surface of the dBSA modified CdTe QDs could be used as selective materials for recognition of target protein [27,28]. Li et al. has reported that molecularly imprinted silica nanospheres are embedded CdSe QDs for highly selective and sensitive optosensing of pyrethroids [29]. As a kind of trend, The Mn-doped ZnS QDs will have more important significance due to their phosphorescence property. Recently, the optosensing systems of MIPs-based RTP have attracted considerable interest. Wang et al. composed MIPs-based Mn-Doped ZnS QDs via a surface molecular imprinting process for RTP optosensing of pentachlorophenol in water [15]. It is expected that the marriage of the RTP of Mn-doped ZnS QDs with MIPs will make a significant contribution to further improvement in the selectivity of Mndoped ZnS QDs based on RTP detection [15]. Chlorophenols are very common and have been listed as the priority pollutants into the aquatic environment [30]. They are primarily produced by chemical industries, such as petroleum refineries, plastics, rubbers, pharmaceuticals, wood-preserving and steel industries. The majority of these chlorophenols are known or suspected as human carcinogens. They are mild acids which permeate human skin in vitro and are ready to be absorbed by gastro-intestinal tract [31,32]. The discharge of chlorophenol contaminated wastewater into aquatic environment without adequate treatment can have negative effect on the water quality, and pose a serious ecological problem as mutagenic effects on the living [33]. Especially, 2,4,5-trichlorophenol (2,4,5-TCP) is considered to have significant toxicological effects and potential carcinogenicity. 2,4,5-TCP has been included in the “persistent, bioaccumulative, toxic chemical list” by the US EPA, posing a serious threat to human health and natural ecosystems. It is resistant to biodegradation in aerobic and anaerobic systems, thereby it tends to bioaccumulate in the environment [34]. Thus, simple, rapid and selective RTP detection of trace 2,4,5-TCP from complex matrix is of great importance. In this work, we report the formation of MIPs-based RTP sensor by anchoring a MIPs layer on the surface of Mn-doped ZnS QDs via a surface molecular imprinting process. 2,4,5-TCP was chosen as a target molecule and the tetraethoxysilane (TEOS) was chosen as cross-linker. The 3-aminopropyltriethoxysilane (APTS) was used as a functional monomer which had a noncovalent interaction with the template 2,4,5-TCP. After the sol-gel process, the artificial receptor was synthesized. When the template was removed by solvent extraction, the MIPs-based RTP sensor is capable of selectively rebinding the target molecule 2,4,5-TCP.
2. Experimental
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2.2. Instrument The X-ray diffraction (XRD) spectra were collected on a XRD6100Lab X-ray diffractometer (Shimadzu, Japan) with Cu Kα radiation over the 2θ range of 10–801. Infrared spectra (4000–400 cm 1) in KBr were recorded to use a Nicolet NEXUS-470 FTIR apparatus (USA). The morphologies of prepared composites were observed by a transmission electron microscope (TEM, JEOL, JEM-2100) and a scanning electron microscope (SEM, JEOL, JSM-7001F). UV–vis adsorption spectra were obtained with a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan). The phosphorescence measurements were performed on a spectrofluorometer (Cary Eclipse) equipped with a plotter unit and a quartz cell (1.0 cm 1.0 cm). 2.3. Synthesis of MPTS-capped Mn-doped ZnS QDS and MIPs-capped Mn-doped ZnS QDs The preparation of MPTS-capped Mn-doped ZnS QDs and MIPscapped Mn-doped ZnS QDs have been described in detail in a previously published procedure [15]. Briefly, to a three-necked flask, 6.25 mmol of ZnSO4, 0.5 mmol of MnCl2, and 20 mL of water was added. Under the protection of nitrogen gas, the mixture was kept stirring at room temperature for 20 min. A 5.0 mL, 6.25 mmol of Na2S solution was added dropwise, and the mixture was kept stirring for 30 min. Then 5.0 mL of an ethanol solution of 0.318 mmol of MPTS was added, and the mixture was kept stirring overnight. Finally the resultant MPTS-capped Mn-doped ZnS QDs was obtained following centrifugation, washing with DDW and absolute ethanol three times and drying in vacuum. To a 100 mL flask, 10 mL of an absolute ethanol solution of 100 mg of 2,4,5-TCP (template) and 250 mL of APTS (functional precursor) were added and stirred for 30 min. Then, 1.0 mL of TEOS (cross-linker) was added, and the mixture was kept stirring for 10 min. Then 500 mg of MPTS-capped Mn-doped ZnS QDs and 2.5 mL of 6% NH3 H2O were added and stirred overnight. The resultants were centrifuged and washed with absolute ethanol. The templates in the MIPs were extracted for 30 min with a mixture solvent of ethanol/acetonitrile (v/v, 8:2) repeatedly. Finally, MIPs-capped Mn-doped ZnS QDs were dried in vacuum. In comparison, the non imprinted polymers (NIPs) were also prepared with the same procedure but without addition of 2,4,5TCP. 2.4. Measurement procedure In the experiments, all the RTP detections were performed under the same conditions: the slit widths of the excitation and emission were both 10 nm, and the excitation wavelength was set at 324 nm with a recording emission range of 450–650 nm. The photomultiplier tube voltage was set at 800 V. MIPs and NIPs were dissolved in DDW to get the fresh-made stock solution (100 mg L 1). 2,4,5-TCP stock solution (200 mg L 1, in DDW) was stocked at 4.0 1C. An appropriate quantity of MIPs and NIPs was added to a 10 mL colorimetric tube and a given concentration of analyte standard solution was added sequentially. The mixture was gently ultrasonicated for 5.0 min before RTP measurement.
2.1. Reagents and chemicals All chemicals were of analytical grade reagents. MnCl2 4H2O, Na2S 9H2O, tetraethoxysilane (TEOS), 3-mercaptopropyltriethoxysilane (MPTS), 3-aminopropyltriethoxysilane (APTS), 2,4,5-trichloro phenol (2,4,5-TCP), 2,4-Dichlorophenol (DCP), 2,4,6-trichlorophenol (2,4,6-TCP), 3-Chlorophenol (3-CP), ZnSO4 7H2O and ammonia solution (25.0–28.0%) were all purchased from Aladdin reagent Co., Ltd. (Shanghai, China). Double distilled water (DDW) was used throughout the experimental procedures.
3. Results and discussion 3.1. Preparation of MIPs-capped Mn-doped ZnS QDs Fig. 1 illustrate the schematic procedure for preparation of MIPs-coated Mn-doped ZnS QDs sensors. In the first step, the Mndoped ZnS QDs-MPTS was synthesized. In the second step, MIPscapped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs
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Fig. 1. Schematic procedures for preparation of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs.
diameter of the MIPs-capped Mn-doped ZnS QDs. It could be observed that the SEM result was the same as that of the TEM. Fig. 5 showed the X-ray diffraction patterns of the Mn-doped ZnS QDs, Mn-doped ZnS QDs-MPTS and MIPs-capped Mn-doped ZnS QDs. The crystal structures of the samples exhibited cubic zinc blende with peaks indexed as (1 1 1), (2 2 0), and (3 1 1) planes. The average size (D) of the Mn-doped ZnS QDs could be calculated according to the Scherrer’s equation: D ¼ kðλ=β cos θÞ
Fig. 2. Excitation (curve 1) and emission (curve 2) spectra of the MIPs-capped Mn-doped ZnS QDs.
were obtained (Fig. 1). 2,4,5-TCP (Fig. S1) was used as template molecule to involve in the formation of MIPs film on the surface of QDs. The 3-aminopropyltriethoxysilane (APTS) was used as a functional monomer which had a noncovalent interaction with the template 2,4,5-TCP. The concentrations of the reactants were reduced to obtain a thin MIP layer and to minimize the homogeneous self-condensation of TEOS and APTS [13]. After the templates were extracted by using the solvent, the RTP intensity of MIPs-capped Mn-doped ZnS QDs could get nearly the same with NIPs-capped Mn-doped ZnS QDs.
ð1Þ
where k is a constant equal to 0.89, λ is the X-ray wavelength and equals to 0.154 nm, β is the full width at half maximum and θ is the half diffraction angle (47.3). The calculated result indicated that the average size (D) was 4.0 nm approximately. DLS analysis in Fig. S2 demonstrated that the average particle size of the ZnS QDs in water is 4.7 nm, which was well consistent with the TEM and XRD results (Table S1). The intensities of the (1 1 1), (2 2 0), and (3 1 1) diffraction peaks of MIPs-capped Mn-doped ZnS QDs (curve 3 in Fig. 5) were weaker than that of MPTS-capped Mn-doped ZnS QDs (curve 2 in Fig. 5) and Mn-doped ZnS QDs (curve 1 in Fig. 5) probably due to the silica shell around the dots. These results could be explained as amorphous materials (silica) which were presented in MIPscapped Mn-doped ZnS QDs and MPTS-capped Mn-doped ZnS QDs. The FT-IR spectra of Mn-doped ZnS QDs-MPTS, MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were measured by KBr disks and shown in Fig. S3. The results illustrated that the MIPs generated from sol-gel condensation of APTS and TEOS were successfully grafted on the surface of MPTS-capped Mn-doped ZnS QDs.
3.2. Optical properties of MIPs-capped Mn-doped ZnS QDs 3.4. Effect of pH The UV absorption spectra of the MIPs-capped Mn-doped ZnS QDs is shown in Fig. S1. The prepared MIPs-capped Mn-doped ZnS QDs had a RTP emission at 596 nm when excited at 324 nm (Fig. 2). As shown in Fig. 3, right, well-dispersed aqueous MIPscapped Mn-doped ZnS QDs particles emitted orange light originating from the characteristic emission of ZnS upon UV light irradiation. 3.3. Characterization The TEM images of original Mn-doped ZnS QDs, Mn-doped ZnS QDs-MPTS and MIPs-capped Mn-doped ZnS QDs are shown in Fig. 4a–c, respectively. The TEM images displayed the Mn-doped ZnS of about 4.4 nm. We could see that the Mn-doped ZnS were coated by MPTS successfully from the TEM images (Fig. 4b). The size of Mn-doped ZnS QDs-MPTS was smaller than that of MIPscapped Mn-doped ZnS QDs, and it also indicates that MIPs were synthesized successfully on the Mn-doped ZnS QDs-MPTS. The SEM images (see Fig. 4d) showed the morphology and the
The pH value has a significant effect on the RTP intensities of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs. As we know, in a pH medium below 4.0, the ZnS QDs were instable, which was evidenced by the smelly H2S generated. As pH increased from 11 to 13, the RTP intensity decreased quickly. The reason is that silica shell could be ionized at high pH, and OH could nucleophilically attack the surface and create surface defects [26]. Finally, the effect of pH in a range between 4.0 and 11 was researched for MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs in Fig. 6. As could be seen from Fig. 6, the RTP intensity of MIPs-capped Mn-doped ZnS QDs in the interval 5.0– 8.0 was considerably stable. At last, a pH of 7.0 was selected for further experiments. 3.5. Stabilities The stabilities of MIPs-capped Mn-doped ZnS QDs and NIPscapped Mn-doped ZnS QDs in water at pH 7.0 were estimated by
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Fig. 3. Photographs of MIPs-capped Mn-doped ZnS QDs in water solution (1) and blank aqueous solution (2) taken under daylight lamp (a) and 365 nm UV light (b).
Fig. 4. TEM images of Mn-doped ZnS QDs (a); Mn-doped ZnS QDs-MPTS (b) and MIPs-capped Mn-doped ZnS QDs (c). SEM images of MIPs-capped Mn-doped ZnS QDs (d).
RTP intensity as a function of time at room temperature, as shown in Fig. 7. The RTP intensities of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were stable for 13 times measurements within 1.0 h. This phenomenon showed that it was the silica shell that made the composite have an excellent stability.
3.6. MIPs and NIPs with template 2,4,5-TCP of different concentrations In this test, our aim was to demonstrate the recognition ability of the MIPs-capped Mn-doped ZnS QDs versus that of the NIPscapped Mn-doped ZnS QDs (Fig. 8). Fig. 8a showed the spectral response of MIPs-capped Mn-doped ZnS QDs with template 2,4,5TCP at different concentrations. The relationship between the RTP intensity and the concentration of quenching 2,4,5-TCP could be
described by the Stern-Volmer equation: I max =I ¼ 1 þ K SV ½S
ð2Þ
I and Imax are the RTP intensities of the MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs at a given related 2,4,5-TCP concentration and in a 2,4,5-TCP free solution, respectively. KSV is the Stern-Volmer quenching constant, and [S] is the 2,4,5-TCP concentration. The dependence of Imax/I as function of [S], are shown in Fig. 7. The KSV,MIP was found to be 25160 M 1. The linear range of the calibration curve was 5.0–50 μmol L 1 with a correlation coefficient of 0.9913. To demonstrate the imprinting effect, as a control experiment, the RTP response of NIPs to the template molecule was investigated. As shown in Fig. 8b, the KSV,NIP was 11860 M 1, the linear range of 2,4,5-TCP was also 5.0–50 μmol L 1 but with a correlation coefficient of 0.9981. According to the results we obtained, the MIPs-capped
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Fig. 5. XRD patterns of Mn-doped ZnS QDs (curve 1), Mn-doped ZnS QDs-MPTS (curve 2) and MIPs-capped Mn-doped ZnS QDs (curve 3).
Fig. 7. Stabilities of MIPs-capped Mn-doped ZnS QDs (black squares) and NIPscapped Mn-doped ZnS QDs (red dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
evident from Fig. 9, the quench efficiency [(Imax I)/Imax] of MIPs for the four compounds followed the order 2,4,5-TCP 42,4,6TCP4 2,6-DP 43-CP. By calculation, the difference in the quench efficiency of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs were 0.1871, 0.0744, 0.0252, 0.0430 for 2,4,5TCP, 2,4,6-TCP, 2,4-DCP, 3-CP, respectively. The results suggested that MIPs were better specific to 2,4,5-TCP but less specific to other chlorophenols.
3.8. Application to water sample analysis.
Fig. 6. Effect of pH on RTP of MIPs-capped Mn-doped ZnS QDs (black squares) and NIPs-capped Mn-doped ZnS QDs (red dots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Mn-doped ZnS QDs had a better selectivity than the NIPscapped ones. 3.7. Selectivity The RTP response of MIPs-capped Mn-doped ZnS QDs to various chlorophenols was conducted to examine the selectivity, and the initial concentration of each chlorophenol was 50 μmol L 1. The selectivity analyses of the MIPs-capped Mndoped ZnS QDs in single solute were exhibited in Fig. 9. With the
Surface river water samples were collected from local rivers. The samples were filtered through 0.45 μm Supor filters and stored in precleaned glass bottles. As no chlorophenols in the collected water samples were detectable by the proposed method, a recovery study was carried out on the samples spiked with 5.0– 15 μmol L 1 2,4,5-TCP to evaluate the developed method, and the corresponding results were listed in Table 1. 2,4,5-TCP solutions were tested by MIPs-capped Mn-doped ZnS QDs, focusing on the linear regime of RTP changes versus concentration permitted the construction of a “standard” curve to which our data were compared (Fig. 8). From this curve and the RTP recovery measured for the unkown samples, we were able to derive estimates of 2,4,5TCP levels in the water sample collected from local river. The recoveries were from 101.3% to 106.4%. The values determined by the MIPs-capped Mn-doped ZnS QDs show the ability of our QD sensing assembly to provide accurate measures of 2,4,5-TCP concentrations on unknown environmental samples. So it can be used for the direct analysis of relevant real samples.
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Table 1 Recovery of 2,4,5-TCP in water samples with 2,4,5-TCP solution at different concentration levels.
2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP 2,4,5-TCP
Concentration taken (μmol L 1)
Found (μmol L 1)
Recovery (%)
RSD (%)
5.0 6.0 8.0 10 12 15
5.32 6.21 8.32 10.27 12.16 15.4
106.4 103.5 104.0 102.7 101.3 102.0
3.8 2.7 3.4 1.7 2.1 2.3
Mn-doped ZnS QDs is in progress in our laboratory, which should spark a broad spectrum of interest because of its great versatility and flexibility for future applications.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (no. 21107037, no. 21176107 and no. 21277063), Natural Science Foundation of Jiangsu Province (no. BK2011461, no. BK2011514), National Postdoctoral Science Foundation (no. 2013M530240), Postdoctoral Science Foundation funded Project of Jiangsu Province (no. 1202002B) and Programs of Senior Talent Foundation of Jiangsu University (no. 12JDG090).
Appendix A. Supporting information Fig. 8. RTP emission spectra of MIPs (a) and NIPs (b) (20 mg L 1) with addition of the indicated concentrations of 2,4,5-TCP in water solution (pH 7.0).
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2014.07. 001. References
Fig. 9. Quenching efficiency of MIPs-capped Mn-doped ZnS QDs and NIPs-capped Mn-doped ZnS QDs by different kinds of 50 μmol L 1 chlorophenols (2,4,5-TCP, 2,4,6-TCP, 2,6-DCP, 3-CP).
4. Conclusion In summary, we have demonstrated that the MIPs anchored on the surface of the MPTS modified Mn-doped ZnS QDs could be used as selective materials for the recognition of target 2,4,5-TCP. The MIPscapped Mn-doped ZnS QDs integrated the advantages of the high selectivity of the molecular imprinting and strong RTP property of the QDs. A series of binding experiments have further demonstrated that the materials had good selectivity for template 2,4,5-TCP over analogues. Under optical conditions, MIPs-capped Mn-doped ZnS QDs was successfully applied to selectively and sensitively detection of 2,4,5TCP in water. Furthermore, molecular recognition of MIPs-capped
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