Surface Molecular Imprinting on Mn-Doped ZnS ... - ACS Publications

Anal. Chem. 2009, 81, 1615–1621

Surface Molecular Imprinting on Mn-Doped ZnS Quantum Dots for Room-Temperature Phosphorescence Optosensing of Pentachlorophenol in Water He-Fang Wang, Yu He, Tian-Rong Ji, and Xiu-Ping Yan* Research Center for Analytical Sciences, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China A new type of molecularly imprinted polymer (MIP)-based room-temperature phosphorescence (RTP) optosensor was developed by anchoring the MIP layer on the surface of Mn-doped ZnS quantum dots (QDs) via a surface molecular imprinting process. The synergetic combination of the RTP property of the Mn-doped ZnS QDs and the merits of the surface imprinting polymers not only improves the RTP selectivity of the Mn-doped ZnS QDs but also makes the MIP-based RTP optosensor also applicable to selective detecting of those nonphosphorescent analytes without the need for any inducers and derivatization. The new MIP-based RTP sensing protocol was applied to detect trace pentachlorophenol (PCP) in water samples without the interference of autofluorescence and scattering light of matrixes. The detection limit for PCP was 86 nM, and the precision for five replicate detections of 0.4 µM PCP was 2.8% (relative standard deviation). The recovery of spiked PCP in river water samples ranged from 93% to 106%. Room-temperature phosphorescence (RTP) has gained significance as a very useful mode of detection for optical sensing applications as it offers many advantages over fluorescence.1,2 The triplet excited state of phosphorescence provides several merits, such as longer emission lifetime, wider gap between the excitation and emission spectra, and minimum interference from the shortlived autofluorescence and scattering light.1,2 The long lifetime of phosphorescence allows an appropriate delay time so that any fluorescent emission and scattering light can be easily avoided. The selectivity of phosphorescence is enhanced because it is a less usual phenomenon than fluorescence.1-5 However, the selectivity of conventional RTP optosensing systems is still limited when the analyte and its coexisting substances exhibit similar luminescence response. * To whom correspondence should be addressed. Fax: (86) 22-23506075. E-mail: [email protected]. (1) Kuijt, J.; Ariese, F.; Brinkman, U. A. T.; Gooijer, C. Anal. Chim. Acta 2003, 488, 135–171. (2) Sańchez-Barragań, I.; Costa-Fernańdez, J. M.; Valledor, M.; Campo, J. C.; Sanz-Medel, A. Trends Anal. Chem. 2006, 25, 958–967. (3) Salinas-Castillo, A.; Sańchez-Barragań, I.; Costa-Fernańdez, J. M.; Pereiro, R.; Ballesteros, A.; Gonza´lez, J. M.; Segura-Carretero, A.; FernańdezGutie´rrez, A.; Sanz-Medel, A. Chem. Commun. 2005, 3224–3226. 10.1021/ac802375a CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

A promising way to achieve the tailored selectivity of analytes is the use of molecularly imprinted polymers (MIPs). Molecular imprinting is usually a process of the copolymerization of functional and cross-linking monomers in the presence of a template molecule. Removal of the template molecules leaves a predetermined arrangement of ligands and a tailored binding pocket.6 Such imprinted polymer shows an affinity for the template molecule over other structurally related compounds. The MIPbased optosensors are successful to enhance the selectivity of a luminescence optosensing system.7-11 Recently, an elegant synergetic use of MIPs as solid supports with RTP detection mode has been proposed, providing an interesting approach to highly sensitive and selective optical sensors.3-5,12-15 The approach reported by Sanz-Medel’s group involved the synthesis of an iodinated imprinted polymer to ensure the presence of a heavy atom (iodine) in the structure of the MIPs for inducing RTP so that there was no need to add iodine in solution.2-5 Thus, MIPbased RTP sensors were developed for selective detecting of fluoranthene,3,4 and benzo[a]pyrene.5 In the works from DıázGarcıá’s group, nafcillin imprinted sol-gels were used as specific binding materials for the RTP recognition of nafcillin, and a carrier solution of KI was introduced to induce the RTP emission using a flow injection system.12-14 Very recently, a method based on the combination of nitrocellulose membrane-poly(vinyl alcohol) (4) Sańchez-Barragań, I.; Costa-Fernańdez, J. M.; Pereiro, R.; Sanz-Medel, A.; Salinas, A.; Segura, A.; Fernańdez-Gutie´rrez, A.; Ballesteros, A.; Gonza´lez, J. M. Anal. Chem. 2005, 77, 7005–7011. (5) Traviesa-Alvarez, J. M.; Sańchez-Barragań, I.; Costa-Fernańdez, J. M.; Pereiro, R.; Sanz-Medel, A. Analyst 2007, 132, 218–223. (6) 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. (7) Holthoff, E. L.; Bright, F. V. Anal. Chim. Acta 2007, 594, 147–161. (8) Holthoff, E. L.; Bright, F. V. Acc. Chem. Res. 2007, 40, 756–767. (9) Lin, C. I.; Joseph, A. K.; Chang, C. K.; Lee, Y. D. Biosens. Bioelectron. 2004, 20, 127–131. (10) Lin, C. I.; Joseph, A. K.; Chang, C. K.; Lee, Y. D. J. Chromatogr., A 2004, 1027, 259–262. (11) Diltemiz, S. E.; Say, R.; Bu ¨ yu ¨ ktiryaki, S.; Hu ¨ r, D.; Denizli, A.; Erso ¨z, A. Talanta 2008, 75, 890–896. (12) Fernańdez-Gonza´lez, A.; Badıa´ La´ıin õ, R.; Dıáz-Garcıa´, M. E.; Guardia, L.; Viale, A. J. Chromatogr., B 2004, 804, 247–254. (13) Guardia, L.; Badıá, R.; Dıáz-Garcıá, M. E. Biosens. Bioelectron. 2006, 21, 1822–1829. (14) Guardia, L.; Badıá, R.; Dıáz-Garcıá, M. E. J. Agric. Food Chem. 2007, 55, 566–570. (15) Li, Z.-M.; Liu, J.-M.; Liu, Z.-B.; Liu, Q.-Y.; Lin, X.; Li, F.-M.; Yang, M.-L.; Zhu, G.-H.; Huang, X.-M. Anal. Chim. Acta 2007, 589, 44–50.

Analytical Chemistry, Vol. 81, No. 4, February 15, 2009

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ionic imprinting as solid support with RTP detection was reported for the determination of trace copper.15 Strong and stable RTP on the MIP was produced from the ionic association formed between the Cu2+ in the cavity of the MIP and the anion of fluorescein outside the cavity via electrostatic effect.15 While previous approaches to the MIP-based RTP sensors are good for selective optosensing of the analyte whose strong and stable RTP signal can be induced by a heavy atom,3-5,12-14 or can be produced by converting the analyte into a phosphorescent substance,15 new protocols are still required to accomplish the MIPbased RTP optosensing of those nonphosphorescent analytes without the need for any inducers and derivatization. Recently, the RTP of quantum dots (QDs) has attracted considerable interest.16-18 The phosphorescence property of Mndoped ZnS QDs has been explored to develop a novel RTP sensor for enoxacin without the need for deoxidants and other inducers, allowing detecting enoxacin in biological fluids without interference from autofluorescence and scattering light of matrixes.17 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 Mn-doped ZnS QDs based RTP detection. Herein, we report a new type of MIP-based RTP sensor by anchoring an MIP layer on the surface of Mn-doped ZnS QDs via a surface molecular imprinting process not only to provide the source of RTP but also to improve the RTP selectivity of the Mndoped ZnS QDs. Surface molecular imprinting enables the template-imprinting sites to situate at the surface or in the proximity of material’s surfaces, providing the advantages of high selectivity, more accessible sites, and fast association/dissociation kinetics.19-27 The new MIP-based RTP optosensing protocol can be applied to those nonphosphorescent analytes without the need for any inducers and derivatization since the RTP originates from Mn-doped ZnS QDs. Any environmental changes of the QDs can result in diverse RTP intensity of the sensors. To the best of our knowledge, no such MIP-based RTP sensor has been reported to date. To illustrate the usefulness of the new protocol for MIP-based RTP sensor, pentachlorophenol (PCP) was chosen as the target. PCP is a kind of conspicuous environmental pollutant, being widely used as wood preservative, pesticide, disinfectant, and also bactericide.28,29 Public and scientific concerns about PCP have (16) Costa-Fernańdez, J. M.; Pereiro, R.; Sanz-Medel, A. Trends Anal. Chem. 2006, 25, 207–218. (17) He, Y.; Wang, H.-F.; Yan, X.-P. Anal. Chem. 2008, 80, 3832–3837. (18) Thakar, R.; Chen, Y.; Snee, P. T. Nano Lett. 2007, 7, 3429–3432. (19) Shi, H.; Tsai, W.; Garrison, M. D.; Ferrari, S.; Ratner, B. D. Nature 1999, 398, 593–597. (20) Hayden, O.; Mann, K. J.; Krassnig, S.; Dickert, F. L. Angew. Chem., Int. Ed. 2006, 45, 2626–2629. (21) Hayden, O.; Dickert, F. L. Adv. Mater. 2001, 13, 1480–1483. (22) Hayden, O.; Lieberzeit, P. A.; Blaas, D.; Dickert, F. L. Adv. Funct. Mater. 2006, 16, 1269–1278. (23) Yilmaz, E.; Haupt, K.; Mosbach, K. Angew. Chem., Int. Ed. 2000, 39, 2115– 2118. (24) Bossi, A.; Piletsky, S. A.; Piletska, E. V.; Righetti, P. G.; Turner, A. P. F. Anal. Chem. 2001, 73, 5281–5286. (25) Yang, H. H.; Zhang, S. Q.; Tan, F.; Zhuang, Z. X.; Wang, X. R. J. Am. Chem. Soc. 2005, 127, 1378–1379. (26) Li, Y.; Yang, H. H.; You, Q. H.; Zhuang, Z. X.; Wang, X. R. Anal. Chem. 2006, 78, 317–320. (27) Fang, G.-Z.; Tan, J.; Yan, X.-P. Anal. Chem. 2005, 77, 1734–1739. (28) Gao, J.; Liu, L.; Liu, X.; Zhou, H.; Huang, S.; Wang, Z. Chemosphere 2008, 71, 1181–1187.

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been incessant due to its high toxicity, long persistence, and recalcitrance to degrade.30 Therefore, development of new methods for the determination of PCP in environmental samples has captured great interest.31-39 In this work, the proposed Mn-doped ZnS QDs based MIP-RTP optosensing protocol was demonstrated for simple, rapid, and selective RTP detecting of trace PCP in water. EXPERIMENTAL SECTION Reagents. All reagents used were of at least analytical grade. ZnSO4 · 7H2O, MnCl2 · 4H2O, and Na2S · 9H2O were from Tianjin Kaitong Chemicals Co. (Tianjin, China), the Second Chemicals Co. of Shenyang (Shenyang, China), and Tianjin Sitong Chemicals Co. (Tianjin, China), respectively. Tetraethoxysilane (TEOS), 3-mercaptopropyltriethoxysilane (MPTS), and 3-aminopropyltriethoxysilane (APTES) were from Wuhan University Silicone New Materials Co., Ltd. (Wuhan, China). The HAc-NaAc (0.1 mol L-1, pH 4.0-5.0) and NaH2PO4-Na2HPO4 buffer solutions (0.1 mol L-1, pH 5.5-9.0) were used in the experiments. Doubly deionized water (DDW, 18.2 MΩ cm) was obtained from a Water Pro water purification system (Labconco Corporation, Kansas City, MO). PCP, 2,4-dichlorophenol (DCP), 2,4-dinitrophenol (DNP), and phenol (Phe) were from Guangfu Fine Chemical Research Institute (Tianjin, China). The solutions (6 mg L-1) of individual phenols were daily prepared by dissolving an appropriate amount of the solid in DDW. Synthesis of MPTS-Capped Mn-Doped ZnS QDs. To a three-necked flask, 12.5 mmol of ZnSO4, 1 mmol of MnCl2, and 40 mL of DDW were added. After the mixture was stirred under argon at room temperature for 10 min, 10 mL of aqueous solution containing 12.5 mmol of Na2S was added dropwise, and the mixture was kept stirring for 30 min. Then 10 mL of an ethanol solution of 0.625 mmol of MPTS was added, and the mixture was kept stirring for 20 h. The resultant MPTScapped Mn-doped ZnS QDs were centrifuged and washed with DDW and absolute ethanol three times and dried in vacuum. Synthesis of MIP-Capped Mn-Doped ZnS QDs. To a 50 mL flask, 10 mL of an absolute ethanol solution of 125 mg of PCP (template) and 250 µL of APTES (functional monomer) were added and stirred for 30 min. To the resultant mixture, 1 mL of TEOS (cross-linking monomer) was added, and the mixture was kept stirring for 5 min. Then 500 mg of MPTS-capped Mn-doped ZnS QDs and 2.5 mL of 6% NH3 · H2O were added and stirred for 16 h. The nonimprinted polymer (NIP) was synthesized in parallel but without addition of PCP. The resultant MIP- and NIP-capped Mn-doped ZnS QDs were centrifuged and washed (29) Yi, H.; Ao, X.; Ho, Y.-S. Scientometrics 2008, 75, 67–80. (30) Estevinho, B. N.; Ribeiro, E.; Alves, A.; Santos, L. Chem. Eng. J. 2008, 136, 188–194. (31) Leblanc, Y. G.; Gilbert, R.; Hubert, J. Anal. Chem. 1999, 71, 78–85. (32) Becker, R.; Buge, H. G.; Win, T. Chemosphere 2002, 47, 1001–1006. (33) Domeno, C.; Munizza, G.; Nerin, C. J. Chromatogr., A 2005, 1095, 8–15. (34) Liu, Y.; Wen, B.; Shan, X. Q. Talanta 2006, 69, 1254–1259. (35) Mardones, C.; Palma, J.; Sepffllveda, C.; Berg, A.; von Baer, D. J. Sep. Sci. 2003, 26, 923–926. (36) Zhou, Y.; Jiang, Q. W.; Peng, Q.; Xuan, D. L.; Qu, W. D. Chemosphere 2007, 70, 256–262. (37) Fan, J.; Guo, H. Q.; Feng, S. L. J. Fluoresc. 2007, 17, 257–264. (38) Guo, H. Q.; Fan, J.; Guo, Y. H. Luminescence 2007, 22, 407–414. (39) Han, D.-M.; Fang, G.-Z.; Yan, X.-P. J. Chromatogr., A 2005, 1100, 131– 136.

Figure 1. Schematic illustration for fabricating MIP-capped Mn-doped ZnS QDs.

with 40 mL of absolute ethanol two times to get nearly the same RTP intensity of MIP and NIP. Finally, MIP- and NIPcapped Mn-doped ZnS QDs were dried in vacuum and stored in a desiccator. Characterization. The X-ray diffraction (XRD) spectra were collected on a Rigaku D/max-2500 X-ray diffractometer (Rigaku, Japan) with Cu KR radiation. Fourier transform infrared (FT-IR) spectra (4000-400 cm-1) in KBr were recorded on a Magna560 spectrometer (Nicolet, Madison, WI). The morphology and microstructure of MIP-capped Mn-doped ZnS QDs and MPTScapped Mn-doped ZnS QDs were characterized by JEOL 100 CXII (JEOL, Japan) transmission electron microscopy (TEM) and a Philips Tecnai G2 F20 (Philips, Holland) field emission high-resolution TEM (HRTEM). The samples for TEM were obtained by drying sample droplets from water dispersion onto a 300-mesh Cu grid coated with a lacey carbon film. The absolute quantum yield was measured on an Edinburgh FLS920 spectrometer. UV spectra were recorded on a UV-3600 spectrometer (Shimadzu, Japan). The phosphorescence measurements were performed on an F-4500 spectrofluorometer (Hitachi, Japan) equipped with a plotter unit and a quartz cell (1 cm × 1 cm). The BET surface area of the prepared QDs was measured on a Micromeritics ASAP 3000 sorptometer using nitrogen adsorption at 77 K. Phosphorescence Measurement. The phosphorescence measurements were carried out with the excitation wavelength of 316 nm when the spectrofluorometer was set in the phosphorescence mode. The slit widths of excitation and emission were 10 and 20 nm, respectively. The photomultiplier tube (PMT) voltage was set at 950 V. All the buffer, DDW, and solutions were ultrasonicated before RTP measurement. To a 10 mL calibrated test tube an appropriate volume of 400 mg L-1 MIP- or NIPcapped Mn-doped ZnS QDs suspension, 1.0 mL of HAc-NaAc or NaH2PO4-Na2HPO4 (0.1 mol L-1) buffer, and a given concentration of analyte standard solution or DDW were sequentially added. The mixture was then diluted to volume with DDW and ultrasonicated for 5 min before measurement. Analysis of Water Samples. 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 PCP in the collected water samples was detectable by the proposed method, a recovery study was carried out on the samples spiked with 1.1-3.9 µM PCP to evaluate the developed method.

RESULTS AND DISCUSSION Preparation and Characterization of MIP- and NIPCapped Mn-Doped ZnS QDs. Silica is an ideal coating for protecting luminescent QDs (such as Mn-doped ZnS) since silica is optically transparent and inert.40 The most common routes for coating silica shell on Mn-doped QDs are sol-gel reaction of TEOS and/or MPTS.41-44 We synthesized MPTS-capped Mndoped ZnS QDs in one step by adding MPTS directly into the reaction mixture of ZnSO4, MnCl2, and Na2S. The sol-gel reaction of MPTS was catalyzed by the alkaline Na2S (Figure 1). MIP-capped Mn-doped ZnS QDs was prepared via a surface molecular imprinting process similar to a previously reported procedure,39 but using NH3 · H2O instead of HCl as the sol-gel catalyst for the purpose to protect Mn-doped ZnS QDs (Figure 1). The concentrations of the reactants were reduced to obtain a thin MIP layer and to minimize the homogeneous self-condensation of TEOS and APTES. Prior to the removal of the template, the RTP intensity of MIP-capped Mn-doped ZnS QDs was about 52.4% of that of NIP-capped Mn-doped ZnS QDs. After two cycles of washing with the absolute ethanol, the RTP intensity of MIPcapped Mn-doped ZnS QDs increased to 99.3% of that of NIPcapped Mn-doped ZnS QDs. These results indicate the facile and effective removal of the template (PCP) from the surface imprinted polymers. Another remarkable advantage of surface imprinting on Mn-doped ZnS QDs was the direct RTP detection of the removal effect of the template, which was more convenient and sensitive than indirect UV detection. The XRD pattern of MPTS-capped Mn-doped ZnS QDs and MIP-capped Mn-doped ZnS QDs (Figure 2a) exhibited a cubic structure (or zinc blende) with peaks for (111), (220), and (311) planes. The size of the Mn-doped ZnS (the core of the QDs) was estimated as 12.1 nm from the Debye-Scherrer equation.45 The intensity of the ZnS(111) diffraction peak of MIP-capped Mn-doped (40) Sun, J.; Zhuang, J.; Guan, S.; Yang, W. J. Nanopart. Res. 2008, 10, 653– 658. (41) Hattoria, Y.; Isobea, T.; Takahashib, H.; Itoh, S. J. Lumin. 2005, 113, 69– 78. (42) Steitz, B.; Axmann, Y.; Hofmann, H.; Petri-Fink, A. J. Lumin. 2008, 128, 92–98. (43) Ethiraj, A. S.; Hebalkar, N.; Kulkarni, S. K.; Pasricha, R.; Urban, J.; Dem, C.; Schmitt, M.; Kiefer, W.; Weinhardt, L.; Joshi, S.; Fink, R.; Heske, C.; Kumpf, C.; Umbach, E. J. Chem. Phys. 2003, 118, 8945–8953. (44) Nien, Y.-T.; Hwang, K.-H.; Chen, I.-G.; Yu, K. J. Alloys Compd. 2008, 455, 519–523. (45) Cullity, B. D.; Stock, S. R. Elements of X-ray Diffraction; Prentice-Hall: Upper Saddle River, NJ, 2001.


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Figure 3. (a) TEM images of MPTS-capped Mn-doped ZnS QDs; (b) TEM, (c) SAED patterns, and (d) HRTEM images of MIP-capped Mn-doped ZnS QDs.

Figure 2. (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 MIPcapped Mn-doped ZnS QDs (curve 2).

ZnS QDs (curve 2 in Figure 2a) was weaker than that of MPTScapped Mn-doped ZnS QDs (curve 1 in Figure 2a). These results can be explained as more amorphous materials (silica) were present in MIP-capped Mn-doped ZnS QDs than MPTS-capped Mn-doped ZnS QDs. To further ascertain the presence of MIP on MPTS-capped Mndoped ZnS QDs, FT-IR spectra of MPTS-capped Mn-doped ZnS QDs and MIP-capped Mn-doped ZnS QDs were compared (Figure 2b). The strong and broad peak around 1066 cm-1 indicates the Si-O-Si asymmetric stretching. Other observed bands about 790 and 459 cm-1 also show the Si-O vibrations. The presence of bands around 2929 cm-1 (aliphatic C-H stretching band) and 3421 and 1541 cm-1 (N-H band) suggests the existence of the aminopropyl group (Figure 2b, curve 2). All these bands show that the MIP generated from sol-gel condensation of APTES and TEOS was grafted on the surface of MPTS-capped Mn-doped ZnS QDs. The HRTEM image (Figure 3d) displays the Mn-doped ZnS (the core of the QDs) of about 12 nm in size (in accordance with XRD results), which was embedded in a larger particle with the size of 30-50 nm as revealed by the TEM images (Figure 3, parts a and b). The size of MPTS-capped Mn-doped ZnS QDs (∼40 nm) was a little smaller than that of MIP-capped Mn-doped ZnS QDs (∼50 nm), also indicating MIP coatings on the MPTS-capped Mndoped ZnS QDs, as shown from the results of XRD and FT-IR in 1618


Figure 2. Moreover, the particles were nearly uniform in size. The selected area electron diffraction (SAED) patterns of the MIPcapped Mn-doped ZnS QDs (Figure 3c) show the outer diffraction rings of the cubic structure of these Mn-doped ZnS polycrystallites, whereas the strong central scattering displays the amorphous silica of the MIP layer, as revealed by XRD (Figure 2). The BET surface area of MIP-capped Mn-doped ZnS QDs was 11.00 ± 0.14 m2 g-1, whereas that of NIP-capped Mn-doped ZnS QDs was 8.47 ± 0.37 m2 g-1. The difference in the BET surface area between the MIP-capped QDs and NIP-capped QDs (2.53 ± 0.51 m2 g-1) results from the imprinted cavity of the template (PCP). RTP Optosensing of PCP by MIP-Capped Mn-Doped ZnS QDs. The prepared MIP-capped Mn-doped ZnS QDs have a symmetric RTP emission at 600 nm when excited at 316 nm (Figure 4a), with the phosphorescent lifetime of 1.3 ms. The long decay time of 1.3 ms of the orange phosphorescence at 600 nm originates from the transition from the triplet state (4T1) to the ground state (6A1) of the Mn2+ incorporated into the ZnS host lattice.46 This emission represents typical characteristics of a RTP emission. The MIP layer has little effect on the wavelength of RTP emission, as well as the absolute quantum yield (∼15%) of Mn-doped ZnS (the core of the QDs). The effect of the dissolved oxygen, a well-known phosphorescence quencher, on the RTP emission of the solution of MIP- and NIP-capped Mn-doped ZnS QDs is not significant ( 5.0, the interaction between PCP anion and the -(CH2)3NH3+ cation was stronger, giving higher KSV,NIP and lower IF. In contrast, the PCP was in its neutral form at pH < 5.0, and the interaction between PCP molecule and the -(CH2)3NH3+ cation was weaker, so the KSV,NIP was lower but the IF was higher. In the latter case, the nonspecific adsorption of PCP on the MIP-capped Mn-doped ZnS QDs was less likely to occur and the specific recognition by the imprinted cavities was dominative. Several kinds of phenols, namely, DNP (pKa 4.0), DCP (pKa 7.9), and Phe (pKa 10.0), were involved to evaluate the selectivity of the MIP-capped Mn-doped ZnS QDs. The linear Stern-Volmer relationships are also observed for these phenols (Figure 7, parts a and b). Owing to the same -(CH2)3NH2 receptor to bind phenols through the acid-base pairing interaction, MIP- and NIP-capped Mn-doped ZnS QDs also show RTP quenching by the phenols examined. The imprinting cavity left by the removal of the template (as shown by BET measurement) gave more chance for the analytes to access the receptor sites, so for a certain phenol the MIP-capped Mndoped ZnS QDs exhibit larger KSV than the NIP-capped Mndoped ZnS QDs. However, the KSV of PCP was much higher than the other phenols, and the ratio of the KSV,MIP and KSV,NIP was the highest for PCP (2.5), indicating an efficient imprinting 1620


Figure 7. Quenching constant of MIP- and NIP-capped Mn-doped ZnS QDs by different kinds of phenols at pH 5.0 (HAc-NaAc buffer, 10 mM). The concentration of MIP- and NIP-capped Mn-doped ZnS QDs was 16 mg L-1.

effect (Figure 7c). Usually, the stronger acid-base pairing interaction occurs between the stronger acid and the amino group;47 thus, the phenols with lower pKa (such as DNP) should have larger KSV if no imprinting effect is active. However, we observed that the KSV,MIP of DNP was much lower than that of PCP, also indicating an efficient imprinting effect responsible for the high selectivity of the MIP-capped Mn-doped ZnS QDs toward PCP. Both imprinting effect and weaker acid-base pairing interaction can be the reasons for the lower KSV of the

Table 2. Analytical Results for the Determination of PCP in River Water Samples added (µM) samples

PCP

DNP

DCP

Phe

determined PCP (mean ± s; n ) 3, µM)

water 1

0 1.1 0 2.9 0 3.4 0 3.9 0 1.1 0 3.9

0 0 0 0 0 0 0 0 0 3.4 0 7.8

0 0 0 0 0 0 0 0 0 3.4 0 7.8

0 0 0 0 0 0 0 0 0 3.4 0 7.8

not detectable 1.02 ± 0.07 not detectable 2.91 ± 0.27 not detectable 3.15 ± 0.30 not detectable 3.69 ± 0.18 not detectable 1.17 ± 0.10 not detectable 3.76 ± 0.17

water 2 water 3 water 4 water 5 water 6 Figure 8. Fluorescence (curve 1) and RTP (curve 2) spectra of a real water sample.

phenols with higher pKa (such as DCP and Phe) as they were less deprotonated at pH 5.0 by the relatively weak basic amino group. Figures of Merit for the RTP Sensor Based on MIPCapped Mn-Doped ZnS QDs. The quenching efficiencies of Mn2+ RTP depends on the amount of MIP-capped Mn-doped ZnS QDs as well as the concentrations of PCP in solution. The MIP-capped Mn-doped ZnS QDs have distinct linearly RTP quenching toward PCP in the concentration range of 0.2-3.9 µM with a correlation coefficient of 0.9910 and a linear regression equation of ∆P ) 29.1C + 1.4 (where C is the concentration of PCP in µM). The precision for five replicate detections of 0.4 µM PCP was 2.8% (RSD). The detection limit (DL), calculated as the concentration of PCP which produced a RTP quenching three times the standard deviation of the blank signal, was 86 nM. For comparison, the DL in the fluorescent mode was also measured (66 nM). The most important advantage of RTP over fluorescence was the minimum interference from the short-lived autofluorescence and scattering light. This is a remarkable merit of the RTP for the analysis of complex biological and environmental samples since tedious sample pretreatment procedures can be avoided. Although the research on the analytical applications of the RTP property of QDs is still in its infancy, and the RTP property of QDs has been employed not as widely as the fluorescence of QDs for analytical applications, the RTP property of QDs will attract more and more analysts for its wide analytical applications in the near future due to the above advantages. Application to Water Sample Analysis. The developed RTP method based on MIP-capped Mn-doped ZnS QDs was applied to determine PCP in river water samples. Figure 8 elucidates the difference in the background of a real water matrix between the two detection modes of fluorescence and phosphorescence. No RTP background (curve 2 in Figure 8) was observed for the real water sample, whereas the fluorescent background (curve 1 in Figure 8) was significant. As no PCP in the collected water

samples was detectable by the proposed method (Table 2), the background in fluorescent mode should originate from the fluorescent matrix and/or scattering light of the sample. However, such fluorescent background interference was completely avoided in the RTP detection mode due to the short lifetime of fluorescent substances. The analytical results for the river samples spiked with 1.1-3.9 µM PCP are given in Table 2. To demonstrate the potential utility of the present approach for real samples, two river water samples (water 5, and water 6) spiked by a mixture of PCP, DNP, DCP, and Phe were also analyzed for PCP. The concentrations of PCP in the spiked river samples determined by the developed method were in good agreement with those of PCP added, along with the quantitative recovery from 93% to 106%, demonstrating the potential applicability of the MIP-RTP optosensor based on Mn-doped ZnS QDs for the quantification of PCP in real samples. CONCLUSIONS We have anchored a MIP layer on the surface of Mn-doped ZnS QDs to produce a new kind of MIP-based RTP optosensor. The combination of the RTP emission of Mn-doped ZnS QDs and the merits of the surface imprinting polymers not only makes the MIP-based RTP optosensor also applicable to selective detecting of nonphosphorescent analytes without the need for any inducers and derivatization but also improves the RTP selectivity of the QDs. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 20775037, 20705013), the National Basic Research Program of China (No. 2006CB705703), and the 863 Program (No. 2007AA10Z432).

Received for review November 10, 2008. Accepted January 2, 2009. AC802375A


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Surface Molecular Imprinting on Mn-Doped ZnS ... - ACS Publications

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