Preparation of Surface Imprinting Polymer Capped Mn-Doped ZnS

Aug 11, 2010 - In this paper, Mn-doped ZnS quantum dots (QDs) capped by a molecularly imprinted polymer (MIP) were synthesized. The results showed a h...
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Anal. Chem. 2010, 82, 7380–7386

Preparation of Surface Imprinting Polymer Capped Mn-Doped ZnS Quantum Dots and Their Application for Chemiluminescence Detection of 4-Nitrophenol in Tap Water Junxiao Liu, Hui Chen, Zhen Lin, and Jin-Ming Lin* The Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China In this paper, Mn-doped ZnS quantum dots (QDs) capped by a molecularly imprinted polymer (MIP) were synthesized. The results showed a high selectivity of the MIPcapped Mn-doped ZnS QDs toward the template molecule (4-nitrophenol) by QD fluorescence quenching. The application of MIP-capped Mn-doped ZnS QDs to the chemiluminescence (CL) system was also studied using a KIO4-H2O2 system. This application combines the good selectivity of MIP with the high sensitivity of CL. The linear range of this CL system is from 0.1 to 40 µM, and the detection limit (DL) for 4-nitrophenol in the water can reach 76 nM. The method was also used in the real water samples, and the recoveries can fall in the range of 91-96%. Over the past decade, the monodispersed colloidal semiconductor nanoparticles, known as quantum dots (QDs), have attracted intensive research interest in scientific and technological applications because of their size-dependent novel optical properties, unique large surface-to-volume ratios, and quantum-size effects.1-4 QDs have some advantages, such as great photostability, high photoluminescence efficiency, size-dependent emission wavelengths, and sharp emission profile,5,6 and thus, they are used for sensing and recognizing the organic and inorganic compounds in the challenging environments. There are various reports on the chemical sensors for ions,7-9 biomacromolecules,10-13 and * To whom correspondence should be addressed. E-mail: jmlin@ mail.tsinghua.edu.cn. Fax/Tel: +86 10 62792343. (1) Alivisatos, A. P. Science 1996, 271, 933–937. (2) Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398–401. (3) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (4) Xia, Y. N.; Yang, P. D. Adv. Mater. 2003, 15, 353–389. (5) Tu, R.; Liu, B.; Wang, Z.; Gao, D.; Wang, F.; Fang, Q.; Zhang, Z. Anal. Chem. 2008, 80, 3458–3465. (6) He, Y.; Wang, H.-F.; Yan, X.-P. Anal. Chem. 2008, 80, 3832–3837. (7) Jin, W. J.; Ferna´ndez-Argu ¨ elles, M. T.; Costa-Ferna´ndez, J. M.; Pereiro, R.; Sanz-Medel, A. Chem. Commun. 2005, 7, 883–885. (8) Li, H.; Zhang, Y.; Wang, X.; Xiong, D.; Bai, Y. Mater. Lett. 2007, 61, 1474– 1477. (9) Ferna´ndez-Argu ¨ elles, M. T.; Jin, W. J.; Costa-Ferna´ndez, J. M.; Pereiro, R.; Sanz-Medel, A. Anal. Chim. Acta 2005, 549, 20–25. (10) Chen, X.; Dong, Y.; Fan, L.; Yang, D. Anal. Chim. Acta 2007, 582, 281– 287.

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small molecules.14-16 However, the coexisting compounds with similar luminescence response to the aimed analytes limited the extensive application of the fluorescence detection systems. There are mainly two ways to improve the selectivity of QDs. One way is to introduce a substance with a good selectivity which can quench only the fluorescence of the aimed analytes. The molecule imprinting technique (MIT) is a promising way to tailor the selectivity due to the separation of the analytes by the polymer material.17 In this process, the functional monomers and crosslinkers are copolymerized in the presence of the target analyte which acts as a molecular template.18 After the template is removed using proper solvent, there is a predetermined arrangement of ligands and a tailored binding pocket formed on the molecularly imprinted polymers (MIPs).19 Such imprinted polymers have a stronger affinity to the template molecule than anything else. Therefore, the introduction of the QDs capped by MIPs to the fluorescence detecting systems can obviously enhance the selectivity and the sensitivity. Recently, some researchers have reported the application of MIPs to the chemical sensors. For example, Lakshmi et al. synthesized molecularly imprinted polymers that had a direct path for the conduction of electrons from the active sites to the electrode and applied the polymers to the electrochemical sensor for detecting catechol and dopamine.20 Gao’s group reported a flow injection chemiluminescence (CL) (11) Goldman, E. R.; Balighian, E. D.; Mattoussi, H.; Kuno, M. K.; Mauro, J. M.; Tran, P. T.; Anderson, G. P. J. Am. Chem. Soc. 2002, 124, 6378–6382. (12) Yao, H.; Zhang, Y.; Xiao, F.; Xia, Z.; Rao, J. H. Angew. Chem., Int. Ed. 2007, 46, 4346–4349. (13) Goldman, E. R.; Anderson, G. P.; Tran, P. T.; Mattoussi, H.; Charles, P. T.; Mauro, J. M. Anal. Chem. 2002, 74, 841–847. (14) Huang, C.-P.; Li, Y.-K.; Chen, T.-M. Biosens. Bioelectron. 2007, 22, 1835– 1838. (15) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.; Clapp, A. R.; Uyeda, H. T.; Deschamps, J. R.; Lassman, M. E.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 6744. (16) Liang, J.; Huang, S.; Zeng, D.; He, Z.; Ji, X.; Ai, X.; Yang, H. Talanta 2006, 69, 126–130. (17) Wang, H.-F.; He, Y.; Yan, X.-P. Anal. Chem. 2009, 81, 1615–1621. (18) Haupt, K.; Mosbach, K. Chem. Rev. 2000, 100, 2495–2504. (19) Alexander, C.; Andersson, H. S.; Andersson, L. I.; Ansell, R. J.; Kirsch, N.; Nicholls, I. A.; O’Mahony, J.; Whitcombe, M. J. J. Mol. Recognit. 2006, 19, 106–180. (20) Lakshmi, D.; Bossi, A.; Whitcombe, M. J.; Chianella, I.; Fowler, S. A.; Subrahmanyam, S.; Piletska, E. V.; Pietsky, S. A. Anal. Chem. 2009, 81, 3579–3584. 10.1021/ac101510b  2010 American Chemical Society Published on Web 08/11/2010

sensor for the determination of maleic hydrazide using MIP.21 The sensor is reusable and has a great improvement in sensitivity and selectivity for CL analysis. The other way of enhancing the selectivity of QDs is to improve the optical properties of the QDs to eliminate the interference of the other fluorescent emissions. Mn-doped QDs have attracted considerable attention because of the excellent optical properties. The doping ions act as recombination centers for the excited electron-hole pairs and result in strong and characteristic luminescence.22 Upon Mn2+ doping, an orange emission band develops around 590 nm, for the well-known 4T1-6A1 d-d transition of Mn2+ ions on Zn2+ sites, where the Mn2+ is coordinated by S2-.23 Compared with the traditional QDs such as CdS, Mn-doped ZnS QDs have a longer luminescent lifetime (ca. 1 ms). Hence, it is easy to make the luminescence from Mn-doped ZnS QDs readily distinguishable from the background luminescence in which luminescent lifetime is shorter, and the absence of Cd2+ in the quantum dots can eliminate toxicity of the cadmium in the process of the experiment. These advantages make them ideal materials as fluorescent labeling agents. CL is an excellent analytical method used in many fields for it’s high sensitivity, wide linear range, simple instrumentation, and lack of background scattering light interference.24 However, the development of CL was limited in some CL reaction systems because the intensity of many reactions was not strong enough for detecting demand. The introduction of nanomaterials brought a broad prospect for the application of the CL method. In our work, a method that surface imprinting polymers were modified onto the Mn-doped QDs was established in order to combine the excellent selectivity of MIPs with the stable luminescence of the Mn-doped QDs. To illustrate the usefulness of the new chemsensor, 4-nitrophenol (4-NP) was chosen as a target molecule which is an organic compound used widely in our life and chemical industry. However, 4-NP has high toxicity and carcinogenicity at very low concentration, and it can remain in the environment for a long time due to it’s stability and bioaccumulation.25 In our experiment, 4-NP can be detected easily and rapidly by the fluorescent system. The H2O2-NaIO4 CL system, a popular CL reaction model, has been widely used in the detection of the phenols, and the mechanism of the reaction has been elucidated.26 The application of the MIPs to the CL systems has been reported by the author to improve the detection limit of dns-L-Phe.27 In this research, the introduction of the new type of surface imprinting material capped Mn-doped ZnS QDs to the H2O2-NaIO4 CL system demonstrates that the proposed method has good sensitivity and selectivity and is capable of being used in the determination of 4-NP in the environmental samples. (21) Fang, Y.; Yan, S.; Ning, B.; Liu, N.; Gao, Z.; Chao, F. Biosens. Bioelectron. 2009, 24, 2323–2327. (22) Zhuang, J.; Zhang, X.; Wang, G.; Li, D.; Yang, W.; Wensheng Yang, Li, T. J. Mater. Chem. 2003, 13, 1953–1857. (23) Steitz, B.; Axmann, Y.; Hofmann, H.; Pe tri-Fink, A. J. Lumin. 2008, 128, 92–98. (24) Lin, J.-M., Ed. Chemiluminescence-Principle and Application, Chemical Industry Press: Beijing, 2004. (25) Shen, X.; Zhu, L.; Liu, G.; Yu, H.; Tang, H. Environ. Sci. Technol. 2008, 42, 1687–1692. (26) Lin, J.-M.; Yamada, M. Anal. Chem. 1999, 71, 1760–1766. (27) Lin, J.-M.; Yamada, M. Anal. Chem. 2000, 72, 1148–1155.

EXPERIMENTAL SECTION Reagents and Materials. ZnSO4 · 7H2O, MnCl2 · 4H2O, Na2S · 9H2O, H2O2 (30%), NaIO4, 4-NP, and tetraethoxysilicane (TEOS) were obtained from Beijing Chemical Reagent Company (Beijing, China). 3-Mercaptopropyltriethoxysilane (MPTS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Fisher Scientific (Fisher, USA). The stock solutions of ZnSO4 · 7H2O and MnCl2 · 4H2O were prepared in pure water and diluted as required. The solutions of Na2S · 9H2O and H2O2 (30%) were prepared fresh daily in pure water. All the other reagents not mentioned above were of analytical grade. Apparatus. The X-ray diffraction (XRD) spectra were obtained on a D8 Advance (Bruker, Germany) X-ray diffractometer (Cu KR). Fourier transform infrared (FT-IR) measurement was carried out with a PerkinElmer 100 FT-IR spectrometer (Massachusetts, USA). The transmission electron microscopy (TEM) images were recorded by a JEM-1200EX electron microscope operating at 100 kV (JEOL, Japan). All the fluorescence measurements were performed using a FL-7000 spectrofluorometer (Hitachi, Japan) combined with a plotter unit and a quartz cell. The UV spectra were acquired by a UV-3900 spectraphotometer (Hitachi, Japan). The TEM samples were dispersed in ethanol and dropped on the Cu grid coated with a lacey carbon film. The batch experiment was performed with an BPCL ultraweak chemiluminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China) using a 3 mL glass cuvette. The luminescence generated by CL reaction was detected by a LumiFlow LF-800 detector (NITI-ON, Funabashi, Japan), and the solution was pumped with two peristaltic pumps (SJ-1211, Atto, Tokyo, Japan). Synthesis of MIP-Capped Mn-Doped ZnS QDs. The process of synthesis of MIP-capped Mn-doped ZnS QDs involves two major steps: the first step is the synthesis of the Mn-doped ZnS QDs, and the second one is modifying the surface imprinting polymers onto the Mn-doped ZnS QDs. The synthesis method of Mn-doped ZnS QDs was shown as follows. To a 200 mL threenecked glass, 25 mmol of ZnSO4, 2 mmol of MnCl2, and 80 mL of water were added. Under the protection of nitrogen gas, the mixture was kept stirring for 20 min. Then, a 10 mL, 25 mmol Na2S solution was added dropwisely into the mixture, and after being stirred for 30 min, a 10 mL solution of 1.25 mmol MPTS in ethanol was added. The mixture was kept stirring overnight. Finally, the Mn-doped ZnS QDs were obtained following centrifugation, washing with pure water and ethanol three times, and drying in vacuum successively. The process of the surface imprinting onto the Mn-doped ZnS QDs was based on the report, except reducing the amount of the reaction compounds in order to achieve a thinner film.28 A 10 mL solution of 100 mg of 4-nitrophenol (template molecule) in absolute ethanol and 250 µL of APTES (functional monomer) were added into a 20 mL flask. After the mixture was stirred for 30 min, 1.0 mL of TEOS (crossing linker) was injected, and the mixture was kept stirring another 5 min. Then, 500 mg of MPTScapped QDs and 2.5 mL of 5% NH3 · H2O (the catalyst) were added to the above mixture and kept stirring for 20 h. The nonimprinted polymers (NIPs) were synthesized in the same process without adding template molecules. The solutions of (28) Han, D.-M.; Fang, G.-Z.; Yan, X.-P. J. Chromatogr., A 2005, 1100, 131– 136.

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Figure 1. Schematic diagram of the flow injection chemiluminescence detection system. S, 100 µL sample injector; F, flow cell; W, wastewater; A, 0.1 M H2O2 at 1.2 mL/min; B, 0.05 M NaIO4 at 1.2 mL/min; C, carrier (water) at 2.5 mL/min. High voltage: -800 V.

MIP- and NIP-capped Mn-doped ZnS QDs were centrifuged and washed with anhydrous ethanol until the fluorescence intensity of MIP-capped Mn-doped ZnS QDs was similar to that of NIP-capped ones. Application of MIPs to CL System. The static chemiluminescence reaction was carried out in the cuvette, and the CL profile and intensity were displayed and integrated for a 0.1 s interval while the voltage of PMT was set at 1.2 kV. In a typical experiment, a 100 µL mixture of QDs and NaIO4 (1:1, v/v) was added to the cuvette, and then 50 µL of H2O2 was injected by a microliter syringe from the upper injection port. Then, we changed the addition orders of the solution and compared the CL intensity of different orders to design the chemiluminescent flow injection analysis (CL-FIA) system. A diagram of the flow system is shown in Figure 1. The solutions of NaIO4, H2O2, and water were pumped into the flow cell by the peristaltic pumps at different rates. The colloid solution of 20 mg/L MIP-capped ZnS QDs was injected by a valve injector, and the injection volume was 100 µL. The luminescence generated by the CL reaction was recorded by a LF-800 luminescent detector. The determination of certain analytes was based on the data of changes in CL intensity, ∆I ) I0 - Is, where I0 and Is were the CL intensity of the blank solutions and sample, respectively. RESULTS AND DISCUSSION Synthesis and Characterization of MIP- and NIP-Capped Mn-Doped ZnS QDs. Silica coating is proven to be an ideal method to protect the fluorescent QDs (such as Mn-doped ZnS QDs) since the silica shell is chemically inert and optically transparent.29 In the traditional methods of coating silica shell on the QDs, TEOS or MPTS are mostly used as the silica crosslinking agent.23,30 In our work, we used MPTS as the cross-linking agent to coat Mn-doped ZnS QDs and Na2S as a catalyst to accelerate the process of the reaction. The process of synthesis is similar to the previous method with little modification.17 In our experiment, ATPES was used as a functional monomer which had a strong noncovalent interaction (29) Sun, J.; Zhuang, J.; Guan, S.; Yang, W. J. Nanopart. Res. 2008, 10, 653– 658. (30) Graf, C.; Vossen, D. L. J.; Imhof, A.; Blaaderen, A. Langmuir 2003, 19, 6693–6700.

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Figure 2. Characterization of the MPTS-capped Mn-doped ZnS and MIP-capped Mn-doped ZnS QDs: (a) XRD patterns of MPTS-capped Mn-doped ZnS QDs (curve 1) and MIP-capped Mn-doped ZnS QDs (curve 2), (b) FT-IR spectra of MPTS-capped Mn-doped ZnS QDs (curve 1) and MIP-capped Mn-doped ZnS QDs (curve 2).

with 4-NP, the template molecule, which is a basic requirement in the molecular imprinting process. The NH3 · H2O was used as a catalyst instead of acetic acid used in the traditional method because the MPTS-capped QDs were unstable in the acid environment. The amount of all the reaction reagents was reduced compared with that in the traditional method in order to obtain a polymer layer as thin as possible. This change will accelerate the binding kinetics and the mass transfer but lower the capacity of the polymers. The fluorescent intensity of the MIPs before being washed was 51.82% of that of NIPs, but after the polymers were washed three times by ethanol, the fluorescent intensity of MIPs increased to 96.21% of that of NIPs. This result indicated that the MIPs were successfully capped on the quantum dots and the template molecule was binding on the MIPs with a noncovalent interaction. Figure 2a shows the X-ray power diffraction (XRD) patterns for MIP-capped Mn-doped ZnS QDs and MPTS-capped Mn-doped ZnS QDs. From this figure, it is shown that the powers exhibited a cubic zinc blende structure with peaks for (111), (220), and (311). The mean crystallite size can be estimated by the width of the XRD peaks using Scherrer’s equation:

β1/2 )

0.94λ d cos Θ

(1)

where β1/2 is the full peak width at half-maximum (fwhm), λ is the X-ray wavelength, d is the crystallite size, and Θ represents the Bragg angle. From the diffractogram, the particle size of the MPTS-capped Mn-doped ZnS QDs is estimated to be about 3.15 nm. The diffractogram of the MIP-capped Mn-doped ZnS QDs is a little different from that of the MPTS-capped Mndoped ZnS QDs. The broad silica peak (2θ ) 22.59) appears in the diffractogram of MIP-capped Mn-doped ZnS QDs, and the full peak (2θ ) 29.06) width at half-maximum of MIPcapped Mn-doped ZnS QDs is a little wider than that of MPTScapped ones. The particle size of the MIP-capped Mn-doped ZnS QDs is nearly 3.21 nm which is larger than that of the MPTS-capped ones. All these differences indicate the thinner layer of MIP has been capped onto the Mn-doped ZnS QDs. To further ensure the MIP coating onto the QDs, FT-IR spectra of MIP-capped Mn-doped ZnS QDs and MPTS-capped Mn-doped ZnS QDs were compared in Figure 2b. The strong peak at around 1097 cm-1 indicated Si-O-Si and Si-OH stretching vibrations, respectively, representing the same stretching vibrations as the peak at 1076 cm-1 in the lower curve. The shift of the peak may be caused by the MIP being modified onto the material. The bands around 472 and 797 cm-1 resulted from Si-O vibrations. A characteristic feature of MIP-capped Mn-doped ZnS QDs compared with MPTS-capped Mn-doped ZnS QDs is the N-H band around 1534 cm-1 and C-H band around 2925 cm-1, which suggests that APTES and TEOS have been successfully grafted onto the MPTS-capped Mn-doped ZnS QDs after modification. The TEM images were also taken to characterize the products in our work (data not shown). From the TEM images, the size of the nanomaterials was 10-30 nm larger than the size calculated through the XRD spectra (around 3 nm), which was mainly caused by soft reunion. The size of MPTS-capped Mn-doped ZnS QDs was smaller than the size of MIP-capped Mn-doped ZnS QDs, which also proved the MIPs were synthesized successfully on the basis of the results of XRD patterns and FT-IR spectra. Fluorescence Sensing of 4-NP by MIP-Capped Mn-Doped ZnS QDs. The UV-vis spectra and fluorescence emission spectra were shown in Figure 3a. The MIP-capped Mn-doped ZnS QDs display two fluorescence emission peaks when excited at 310 nm. The weak blue peak around 440 nm is generated by the defect related to the emission of the ZnS QDs. The strong orange peak around 590 nm can be attributed to the 4T1-6A1 transition of the Mn2+ impurity.31 The well-known green emission of Zn vacancies around 480 nm was not observed in the MIP-capped Mndoped ZnS QDs because the emission was quenched by the electron being transferring to the Mn2+ ions.32 The defectrelated emission peak was caused by the doped ions in the semiconductive nanomaterials, which is greatly affected by many factors such as the process of synthesis and environments. Thus, the fluorescence emission of doped ions was chosen for detection because that is more stable and control(31) Sapra, S.; Prakash, A.; Ghangrekar, A.; Periasamy, N.; Sarma, D. D. J. Phys. Chem. B 2005, 109, 1663–1668. (32) Biswas, S.; Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 17526– 17530.

Figure 3. Optical property of MIP-capped Mn-doped ZnS QDs: (a) UV absorption spectra of MIP-capped Mn-doped ZnS QDs (10 mg/ L) in water (curve 1) and fluorescence emission spectra of MIPcapped Mn-doped ZnS QDs (10 mg/L) in water with an excited light at 310 nm (curve 2); (b) stable fluorescence emission measurement of MIP-capped Mn-doped ZnS QDs (10 mg/L) in water. Fluorescence experimental condition: The photomultiplier tube (PMT) voltage was set at 700 V, and the silt widths of excitation and emission were 10 and 20 nm.

lable and has a higher quantum yield than the defect one. The relative standard deviation (RSD) of 3.85% was obtained by 12 repeated detections of the fluorescence insensitivity in the 10 mg/L MIP-capped Mn-doped ZnS QD aqueous solution every 5 min. The result shown in Figure 3b indicates the stable emission of the QDs. The main reason for the stable emission is that the inner Mn2+ is protected by the amorphous silica shell. In the mixed solution, the amino groups (-NH2) in the molecule of APTES can interact with the functional groups (such as hydroxyl group) in the template molecule to form a complex through hydrogen bonding. This interaction between function monomer and template molecule can be confirmed from the UV-vis spectra. Figure 4 shows the UV-vis spectra of the 4-NP solution before and after APTES was added. The great difference of the two spectra indicates that there is a strong interaction between 4-NP and APTES. The amino groups give binding sites on the surface of MIP and NIP-capped Mn-doped ZnS QDs through the hydrogen binding. Therefore, both MIPand NIP-capped Mn-doped ZnS QDs have a response to the template 4-NP, but the quencher constant of MIP-capped MnAnalytical Chemistry, Vol. 82, No. 17, September 1, 2010

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Figure 4. UV-vis spectra of 4-NP (10 ppm) before (curve 1) and after (curve 2) the addition of APTS (20 ppm) in ethanol.

doped ZnS QDs toward 4-NP is two times that of NIP-capped ones, which indicate the good selectivity of the sensors shown in Figure 5. Compared with the NIP-capped ones, the MIP-capped Mn-doped ZnS QDs have more binding sites suitable for the template 4-NP due to an efficient imprinting effect. In our work, the phenol was chosen for the similar structure with that of the template molecule (4-NP). The results were shown in the Table 1, and the great difference of the quenching constant indicated the better selectivity of the materials we synthesized. A simple schematic illustration of the fluorescence quenching process was shown in Figure 6a. When there is no template 4-NP around the MIP-capped Mn-doped ZnS QDs, an orange emission is generated by accepting the excited energy. After adding the template 4-NP, there will be a strong interaction between the template molecule and the amino groups, which is a main reason for the fluorescence quenching. We suggest that a quenching mechanism is the electron transfer from the Mn-doped ZnS QDs to the 4-NP species through the strong binding to the template molecule. As seen in Figure 6b, the electrons were excited from the valence band to the conduction band and transited to the initial condition following the solid arrow to generate the two emissions. The fluorescence quenching of the MIP-capped Mn-doped ZnS QDs is mainly achieved by two pathways because of the addition of 4-NP and the strong interaction between the amino groups and the template 4-NP. In Figure 4, the UV-vis adsorption of 4-NP and 4-NP ion is around 227 nm, 314 and 392 nm, respectively, which is near the band gap of the ZnS QDs shown in Figure 6b. The electrons at the conductive band of the MIP-capped Mn-doped ZnS QDs can directly transfer to the lowest unoccupied molecular orbital (LUMO) of UV and the visible band of the 4-NP molecules and 4-NP ions followed the paths shown as the dashed arrows. Since all the energy bands of the 4-NP molecules and 4-NP ions are higher than the blue emission of the MIP-capped Mn-doped ZnS QDs around 440 nm, the excited electrons tend to go back by the dashed paths and a quenching is generated. The energy mechanism is impossible for the fluorescence quenching because there is no overlap bands between the 4-NP molecules or ions and the emissions of the QDs. According to the mechanism above, the large quenching constant means that there are more suitable binding sites on the Mn-doped ZnS QDs. 7384

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Figure 5. Evolutions of Mn ions fluorescence emission spectra of MIP-capped Mn-doped ZnS QDs: (a) MIP-capped Mn-doped ZnS QDs (10 mg/L), (b) NIP-capped Mn-doped ZnS QDs (10 mg/L) with increasing 4-NP concentrations in the water solution QDs. Inset graphs: Stern-Volmer plots from (a) MIP-capped Mn-doped ZnS QDs and (b) NIP-capped Mn-doped ZnS QDs with the 4-NP. Fluorescence experimental condition: The photomultiplier tube (PMT) voltage was set at 700 V; the excited light was set at 310 nm, and the silt widths of excitation and emission were 10 and 20 nm. Table 1. Summary of Quenching Constants of MIP- and NIP-Capped Mn-Doped ZnS QDs Ksv (MIP) -1

Ksv (NIP)

K′{Ksv (MIP)/Ksv (NIP)}

-1

4-nitrophenol 40015.36M 18936.67M phenol 20149.14M-1 19654.56M-1

2.113 1.025

The fluorescence quenching in this system can be quantified by the Stern-Volmer equation as follows: F/F0 ) 1 + KSVcq

(2)

where F0 is the initial fluorescence intensity in the absence of quencher, F is the fluorescence intensity in the presence of analyte, Ksv is the quenching constant of the quencher, and cq is the concentration of the quencher. Shown in Table 1, different linear Stern-Volmer relationships were observed

Figure 7. CL profiles in batch experiments of different injection orders: H2O2 (0.1 M) injected into NaIO4 (0.05 M) solution (Peaks 1), NaIO4 (0.05 M) injected into the mixture of H2O2 (0.1 M) and MIPcapped QDs (10 mg/L; Peaks 2), H2O2 (0.1 M) injected into the mixture of NaIO4 (0.05 M) and MIP-capped QDs (10 mg/L; Peaks 3). Batch experiment conditions: voltage of PMT was set at 1.2 kV; interval time was set for 0.1 s.

Figure 6. Schematic illustrations for the quenching mechanism: (a) Fluorescence quenching process for 4-NP detection. (b) Fluorescence quenching mechanism of electron transferring from quantum dots to 4-NP species.

between MIPs and NIPs. The ratio of the MIP and NIP’s Ksv was important data to evaluate the selectivity of the materials we obtained. According to the results we obtained, the MIP-capped Mn-doped ZnS QDs have a better selectivity than the NIPcapped ones. Application of MIP-Capped Mn-Doped ZnS QDs to CL System. The application of QDs and MIPs to the CL system has been reported,25,27 but the use of MIP-capped QDs to the CL system has not been concerned. In our work, the product we synthesized was used in the CL system to improve selectivity and sensitivity of CL method. In order to apply MIP-capped Mn-doped ZnS QDs to the FIA system, we carried the batch experiments to evaluate the different orders of injection solutions. There is no CL signal when MIP-capped Mn-doped ZnS QDs were mixed with H2O2 or NaIO4. From the results listed in Figure 7, it can be seen that there is a weak luminescence when H2O2 was injected into NaIO4 solution. The CL intensity followed the addition order as Peak 3 is about five times the CL intensity as Peak 2. This can be explained as decomposition of H2O2 for the surface effect of quantum dots. According to the results we achieved, the MIP-capped Mn-doped ZnS QDs were applied to the H2O2-NaIO4 CL system following the process shown in Figure 1. The MIP-capped Mn-doped ZnS QDs were injected by a valve injector and carried by water. Then, the solution was first mixed with samples because some time is needed for the templates to arrive at the selectivity holes. After that, the solution and NaIO4 was mixed and entered the flow-cell to react with H2O2.

Figure 8. Typical FIA peaks for the determination of 4-NP: (1) not added; (2) 1.0 µM; (3) 10.0 µM; (4) 20.0 µM; (5) 30.0 µM; (6) 40.0 µM; (7) 15.0 µM spiked sample result. Experimental conditions: 0.1 M H2O2 at 1.2 mL/min; 0.05 M NaIO4 at 1.2 mL/min; carrier (water) at 2.5 mL/min. High voltage: -800 V. 100 µL samples injector was used.

The CL intensity of FIA system was enhanced obviously by adding both MIP- and NIP-capped Mn-doped ZnS QDs. However, the MIP-capped Mn-doped ZnS QDs have a greater enhancement than the NIP ones, and when a 40 µM solution of 4-NP was added, the quenching efficiency of MIP-capped QDs (∆IMIP ) 50.02 mV) is about four times that of NIP-capped QDs (∆INIP ) 13.87 mV). In order to create an analysis method, we also study the linear range of this method. A set of 4-NP samples were added to the system, and the results were shown in Figure 8. We can see that the CL intensity decreased with the increase of the concentration of the 4-NP solutions. When the concentration of the 4-NP solution reached 40 µM, the increase of the concentration could not bring an obvious decrease of CL intensity, which Analytical Chemistry, Vol. 82, No. 17, September 1, 2010

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Table 2. Inhibition Effects of Organic Compounds on the CL System of MIP-Capped QD-NaIO4-H2O2 (Concentration: 10-5 M)

Table 3. Optimum Conditions and Analytical Figures of the FIA-CL System 0.05 M NaIO4 of 1.2 mL/min flow rate optimum conditions 0.1 M H2O2 of 1.2 mL/min flow rate sample carrier (water) of 2.5 mL/min flow rate

Determination of 4-NP in Tap Water samples

spiked (µM)

found (µM)

recoveries (%)

RSD (%)a

tap water tap water

0 15

not detected 14.11

94.07

2.02

a

n ) 3.

CL reaction. In brief, H2O2 directly reacts with NaIO4 to generate •O2- radicals which may produce energy-rich precursors of excited molecules (O2)2*. The excited molecules (O2)2* transfer their energy to the higher fluorescence efficiency QDs which results in a strong light. The process was shown as follows:

a

Quenching ) (1 - I/I0)*100%.

indicated the quenching entered a platform period. The lower right range is mainly because the binding sites on the surface are limited for the thinner shell we obtained. When all the binding sites were used, the effect of quenching was not obvious. The detection limit of 4-NP in water is 76 nM, and the linear range is from 0.1 µM to 40 µM. In our work, we chose six aromatic organic compounds which were applied to the MIP-capped Mn-doped ZnS QD-NaIO4H2O2 CL system. The results were listed in the Table 2 to prove the selectivity of this CL system. In the six aromatic organic compounds, 2-NP has the similar optical and chemical properties and similar structure to 4-NP. However, the quenching of the CL system to the two similar compounds is different. The quenching to template molecule 4-NP (27.2%) is about three times of that to 2-NP (11.1%). This proves the selectivity of this CL system to the compounds which have the similar properties. The CL system has a different response to the other compounds in the experiment which may be caused by the different chemical properties of the compounds. We suggest that these compounds may influence the reaction between NaIO4 and H2O2 and bring the change of the CL intensity. In conclusion, the application of MIP-capped QDs to the NaIO4-H2O2 CL system can improve the selectivity of CL method. This method was also used in real water samples detection. 4-NP cannot be detected in the tap water acquired in the laboratory. Then, the spiked experiment was carried out, and the recoveries reached 94.07% (RSD ) 2.02%, n ) 3). All the results were shown in the Table 3. Possible Mechanism of Chemiluminescent Reaction. The CL mechanism of the H2O2-NaIO4 system has been studied in our previous work.26 We consider that the MIP-capped Mndoped ZnS QDs play a luminophor role in the process of the

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Analytical Chemistry, Vol. 82, No. 17, September 1, 2010

IO4- + H2O2 f IO3 + •O2- + H2O

(3)

O2- f (O2)2*

(4)

(O2)2* + QD f O2 + (QD)*

(5)

(QD)* f QD + hv

(6)

•

When the quencher (4-NP) was added into the CL system, 4-NP was absorbed on the surface of MIP-capped Mn-doped ZnS QDs and the fluorescence emission was quenched. As the possible mechanism shown above, the fluorescence emission of the QDs was the main factor to influence the intensity of the CL after the MIP-capped Mn-doped ZnS QDs were added into the system. Therefore, the CL intensity decreased alomost linearly with the increasing concentrations of quencher. CONCLUSIONS In summary, MIP-capped Mn-doped ZnS QDs were synthesized suceessfully. The products have a specific selectivity for the template molecule by fluorescence quenching which was mainly caused by an electron transmission between QDs and 4-NP. The application of MIP-capped Mn-doped ZnS QDs to the CL system can improve the selectivity of the CL method which is a limit for the further development of CL system as an analytical method and can introduce more CL reactions to use for detection. A possible mechanism was also proposed to explain the process of the CL reaction and the quench of CL intensity. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant Nos. 20935002 and 90813015). Received for review June 8, 2010. Accepted July 27, 2010. AC101510B