Preparation of Surface Imprinting Polymer CappedMn-Doped ZnS Quantum Dots and Their Applicationfor Chemiluminescence Detection of 4-Nitrophenolin 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) cappedby a molecularly imprinted polymer (MIP) were synthe-sized. The results showed a high selectivity of the MIP-capped Mn-doped ZnS QDs toward the template molecule(4-nitrophenol) by QD fluorescence quenching. The ap-plication of MIP-capped Mn-doped ZnS QDs to thechemiluminescence (CL) system was also studied usinga KIO4-H2O2 system. This application combines thegood selectivity of MIP with the high sensitivity of CL.The linear range of this CL system is from 0.1 to 40µM, and the detection limit (DL) for 4-nitrophenol inthe water can reach 76 nM. The method was also usedin the real water samples, and the recoveries can fallin the range of 91-96%.

Over the past decade, the monodispersed colloidal semicon-ductor nanoparticles, known as quantum dots (QDs), haveattracted intensive research interest in scientific and technologicalapplications because of their size-dependent novel optical proper-ties, unique large surface-to-volume ratios, and quantum-sizeeffects.1-4 QDs have some advantages, such as great photosta-bility, high photoluminescence efficiency, size-dependent emissionwavelengths, and sharp emission profile,5,6 and thus, they are usedfor sensing and recognizing the organic and inorganic compoundsin the challenging environments. There are various reports onthe chemical sensors for ions,7-9 biomacromolecules,10-13 and

small molecules.14-16 However, the coexisting compounds withsimilar luminescence response to the aimed analytes limited theextensive 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 whichcan quench only the fluorescence of the aimed analytes. Themolecule imprinting technique (MIT) is a promising way to tailorthe selectivity due to the separation of the analytes by the polymermaterial.17 In this process, the functional monomers and cross-linkers are copolymerized in the presence of the target analytewhich acts as a molecular template.18 After the template isremoved using proper solvent, there is a predetermined arrange-ment of ligands and a tailored binding pocket formed on themolecularly imprinted polymers (MIPs).19 Such imprinted poly-mers have a stronger affinity to the template molecule thananything else. Therefore, the introduction of the QDs capped byMIPs to the fluorescence detecting systems can obviously enhancethe selectivity and the sensitivity. Recently, some researchers havereported the application of MIPs to the chemical sensors. Forexample, Lakshmi et al. synthesized molecularly imprintedpolymers that had a direct path for the conduction of electronsfrom the active sites to the electrode and applied the polymers tothe electrochemical sensor for detecting catechol and dopamine.20

Gao’s group reported a flow injection chemiluminescence (CL)

* 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.; Fernandez-Arguelles, M. T.; Costa-Fernandez, 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) Fernandez-Arguelles, M. T.; Jin, W. J.; Costa-Fernandez, 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.

(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.

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sensor for the determination of maleic hydrazide using MIP.21

The sensor is reusable and has a great improvement in sensitivityand selectivity for CL analysis.

The other way of enhancing the selectivity of QDs is to improvethe optical properties of the QDs to eliminate the interferenceof the other fluorescent emissions. Mn-doped QDs have attractedconsiderable attention because of the excellent optical properties.The doping ions act as recombination centers for the excitedelectron-hole pairs and result in strong and characteristicluminescence.22 Upon Mn2+ doping, an orange emission banddevelops around 590 nm, for the well-known 4T1-6A1 d-dtransition of Mn2+ ions on Zn2+ sites, where the Mn2+ iscoordinated by S2-.23 Compared with the traditional QDs suchas CdS, Mn-doped ZnS QDs have a longer luminescent lifetime(ca. 1 ms). Hence, it is easy to make the luminescence fromMn-doped ZnS QDs readily distinguishable from the back-ground luminescence in which luminescent lifetime is shorter,and the absence of Cd2+ in the quantum dots can eliminatetoxicity of the cadmium in the process of the experiment. Theseadvantages make them ideal materials as fluorescent labelingagents.

CL is an excellent analytical method used in many fields forit’s high sensitivity, wide linear range, simple instrumentation, andlack of background scattering light interference.24 However, thedevelopment of CL was limited in some CL reaction systemsbecause the intensity of many reactions was not strong enoughfor detecting demand. The introduction of nanomaterials broughta broad prospect for the application of the CL method.

In our work, a method that surface imprinting polymers weremodified onto the Mn-doped QDs was established in order tocombine the excellent selectivity of MIPs with the stable lumi-nescence of the Mn-doped QDs. To illustrate the usefulness ofthe new chemsensor, 4-nitrophenol (4-NP) was chosen as a targetmolecule which is an organic compound used widely in our lifeand chemical industry. However, 4-NP has high toxicity andcarcinogenicity at very low concentration, and it can remain inthe environment for a long time due to it’s stability and bioaccu-mulation.25 In our experiment, 4-NP can be detected easily andrapidly by the fluorescent system. The H2O2-NaIO4 CL system,a popular CL reaction model, has been widely used in thedetection of the phenols, and the mechanism of the reactionhas been elucidated.26 The application of the MIPs to the CLsystems has been reported by the author to improve thedetection limit of dns-L-Phe.27 In this research, the introductionof the new type of surface imprinting material capped Mn-dopedZnS QDs to the H2O2-NaIO4 CL system demonstrates thatthe proposed method has good sensitivity and selectivity andis capable of being used in the determination of 4-NP in theenvironmental samples.

EXPERIMENTAL SECTIONReagents and Materials. ZnSO4 · 7H2O, MnCl2 · 4H2O,

Na2S ·9H2O, H2O2 (30%), NaIO4, 4-NP, and tetraethoxysilicane(TEOS) were obtained from Beijing Chemical Reagent Com-pany (Beijing, China). 3-Mercaptopropyltriethoxysilane (MPTS)and 3-aminopropyltriethoxysilane (APTES) were purchasedfrom Fisher Scientific (Fisher, USA). The stock solutions ofZnSO4 ·7H2O and MnCl2 ·4H2O were prepared in pure waterand diluted as required. The solutions of Na2S ·9H2O and H2O2

(30%) were prepared fresh daily in pure water. All the otherreagents not mentioned above were of analytical grade.

Apparatus. The X-ray diffraction (XRD) spectra were obtainedon a D8 Advance (Bruker, Germany) X-ray diffractometer (CuKR). Fourier transform infrared (FT-IR) measurement was carriedout with a PerkinElmer 100 FT-IR spectrometer (Massachusetts,USA). The transmission electron microscopy (TEM) images wererecorded by a JEM-1200EX electron microscope operating at 100kV (JEOL, Japan). All the fluorescence measurements wereperformed using a FL-7000 spectrofluorometer (Hitachi, Japan)combined with a plotter unit and a quartz cell. The UV spectrawere acquired by a UV-3900 spectraphotometer (Hitachi, Japan).The TEM samples were dispersed in ethanol and dropped on theCu grid coated with a lacey carbon film. The batch experimentwas performed with an BPCL ultraweak chemiluminescenceanalyzer (Institute of Biophysics, Chinese Academy of Science,Beijing, China) using a 3 mL glass cuvette. The luminescencegenerated by CL reaction was detected by a LumiFlow LF-800detector (NITI-ON, Funabashi, Japan), and the solution waspumped with two peristaltic pumps (SJ-1211, Atto, Tokyo, Japan).

Synthesis of MIP-Capped Mn-Doped ZnS QDs. Theprocess of synthesis of MIP-capped Mn-doped ZnS QDs involvestwo major steps: the first step is the synthesis of the Mn-dopedZnS QDs, and the second one is modifying the surface imprintingpolymers onto the Mn-doped ZnS QDs. The synthesis method ofMn-doped ZnS QDs was shown as follows. To a 200 mL three-necked glass, 25 mmol of ZnSO4, 2 mmol of MnCl2, and 80 mLof water were added. Under the protection of nitrogen gas, themixture was kept stirring for 20 min. Then, a 10 mL, 25 mmolNa2S solution was added dropwisely into the mixture, and afterbeing stirred for 30 min, a 10 mL solution of 1.25 mmol MPTSin ethanol was added. The mixture was kept stirring overnight.Finally, the Mn-doped ZnS QDs were obtained followingcentrifugation, washing with pure water and ethanol threetimes, and drying in vacuum successively.

The process of the surface imprinting onto the Mn-doped ZnSQDs was based on the report, except reducing the amount of thereaction compounds in order to achieve a thinner film.28 A 10mL solution of 100 mg of 4-nitrophenol (template molecule) inabsolute ethanol and 250 µL of APTES (functional monomer) wereadded into a 20 mL flask. After the mixture was stirred for 30min, 1.0 mL of TEOS (crossing linker) was injected, and themixture was kept stirring another 5 min. Then, 500 mg of MPTS-capped QDs and 2.5 mL of 5% NH3 ·H2O (the catalyst) wereadded to the above mixture and kept stirring for 20 h. Thenonimprinted polymers (NIPs) were synthesized in the sameprocess without adding template molecules. The solutions of

(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, ChemicalIndustry 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.

(28) Han, D.-M.; Fang, G.-Z.; Yan, X.-P. J. Chromatogr., A 2005, 1100, 131–136.

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MIP- and NIP-capped Mn-doped ZnS QDs were centrifugedand washed with anhydrous ethanol until the fluorescenceintensity of MIP-capped Mn-doped ZnS QDs was similar to thatof NIP-capped ones.

Application of MIPs to CL System. The static chemilumi-nescence reaction was carried out in the cuvette, and the CLprofile and intensity were displayed and integrated for a 0.1 sinterval while the voltage of PMT was set at 1.2 kV. In a typicalexperiment, a 100 µL mixture of QDs and NaIO4 (1:1, v/v) wasadded to the cuvette, and then 50 µL of H2O2 was injected bya microliter syringe from the upper injection port. Then, wechanged the addition orders of the solution and compared theCL intensity of different orders to design the chemiluminescentflow injection analysis (CL-FIA) system.

A diagram of the flow system is shown in Figure 1. Thesolutions of NaIO4, H2O2, and water were pumped into the flowcell by the peristaltic pumps at different rates. The colloidsolution of 20 mg/L MIP-capped ZnS QDs was injected by avalve injector, and the injection volume was 100 µL. Theluminescence generated by the CL reaction was recorded bya LF-800 luminescent detector. The determination of certainanalytes was based on the data of changes in CL intensity, ∆I) I0 - Is, where I0 and Is were the CL intensity of the blanksolutions and sample, respectively.

RESULTS AND DISCUSSIONSynthesis and Characterization of MIP- and NIP-Capped

Mn-Doped ZnS QDs. Silica coating is proven to be an idealmethod to protect the fluorescent QDs (such as Mn-doped ZnSQDs) since the silica shell is chemically inert and opticallytransparent.29 In the traditional methods of coating silica shell onthe QDs, TEOS or MPTS are mostly used as the silica cross-linking agent.23,30 In our work, we used MPTS as the cross-linkingagent to coat Mn-doped ZnS QDs and Na2S as a catalyst toaccelerate the process of the reaction.

The process of synthesis is similar to the previous methodwith little modification.17 In our experiment, ATPES was used asa functional monomer which had a strong noncovalent interaction

with 4-NP, the template molecule, which is a basic requirementin the molecular imprinting process. The NH3 ·H2O was used asa catalyst instead of acetic acid used in the traditional methodbecause the MPTS-capped QDs were unstable in the acidenvironment. The amount of all the reaction reagents wasreduced compared with that in the traditional method in orderto obtain a polymer layer as thin as possible. This change willaccelerate the binding kinetics and the mass transfer but lowerthe capacity of the polymers. The fluorescent intensity of theMIPs before being washed was 51.82% of that of NIPs, but afterthe polymers were washed three times by ethanol, thefluorescent intensity of MIPs increased to 96.21% of that ofNIPs. This result indicated that the MIPs were successfullycapped on the quantum dots and the template molecule wasbinding on the MIPs with a noncovalent interaction.

Figure 2a shows the X-ray power diffraction (XRD) patternsfor MIP-capped Mn-doped ZnS QDs and MPTS-capped Mn-dopedZnS QDs. From this figure, it is shown that the powers exhibiteda cubic zinc blende structure with peaks for (111), (220), and(311). The mean crystallite size can be estimated by the width ofthe XRD peaks using Scherrer’s equation:

(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.

Figure 1. Schematic diagram of the flow injection chemilumines-cence 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 1.2mL/min; C, carrier (water) at 2.5 mL/min. High voltage: -800 V.

Figure 2. Characterization of the MPTS-capped Mn-doped ZnS andMIP-capped Mn-doped ZnS QDs: (a) XRD patterns of MPTS-cappedMn-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).

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�1/2 ) 0.94λdcos Θ (1)

where �1/2 is the full peak width at half-maximum (fwhm), λ isthe X-ray wavelength, d is the crystallite size, and Θ representsthe Bragg angle. From the diffractogram, the particle sizeof the MPTS-capped Mn-doped ZnS QDs is estimated to beabout 3.15 nm. The diffractogram of the MIP-capped Mn-dopedZnS QDs is a little different from that of the MPTS-capped Mn-doped ZnS QDs. The broad silica peak (2θ ) 22.59) appearsin the diffractogram of MIP-capped Mn-doped ZnS QDs, andthe full peak (2θ ) 29.06) width at half-maximum of MIP-capped Mn-doped ZnS QDs is a little wider than that of MPTS-capped ones. The particle size of the MIP-capped Mn-dopedZnS QDs is nearly 3.21 nm which is larger than that of theMPTS-capped ones. All these differences indicate the thinnerlayer of MIP has been capped onto the Mn-doped ZnS QDs.

To further ensure the MIP coating onto the QDs, FT-IR spectraof MIP-capped Mn-doped ZnS QDs and MPTS-capped Mn-dopedZnS QDs were compared in Figure 2b. The strong peak at around1097 cm-1 indicated Si-O-Si and Si-OH stretching vibrations,respectively, representing the same stretching vibrations as thepeak at 1076 cm-1 in the lower curve. The shift of the peakmay be caused by the MIP being modified onto the material.The bands around 472 and 797 cm-1 resulted from Si-Ovibrations. A characteristic feature of MIP-capped Mn-dopedZnS QDs compared with MPTS-capped Mn-doped ZnS QDs isthe N-H band around 1534 cm-1 and C-H band around 2925cm-1, which suggests that APTES and TEOS have beensuccessfully grafted onto the MPTS-capped Mn-doped ZnS QDsafter modification.

The TEM images were also taken to characterize the productsin our work (data not shown). From the TEM images, the size ofthe nanomaterials was 10-30 nm larger than the size calculatedthrough the XRD spectra (around 3 nm), which was mainly causedby soft reunion. The size of MPTS-capped Mn-doped ZnS QDswas smaller than the size of MIP-capped Mn-doped ZnS QDs,which also proved the MIPs were synthesized successfully on thebasis of the results of XRD patterns and FT-IR spectra.

Fluorescence Sensing of 4-NP by MIP-Capped Mn-DopedZnS QDs. The UV-vis spectra and fluorescence emission spectrawere shown in Figure 3a. The MIP-capped Mn-doped ZnS QDsdisplay two fluorescence emission peaks when excited at 310 nm.The weak blue peak around 440 nm is generated by the defectrelated to the emission of the ZnS QDs. The strong orange peakaround 590 nm can be attributed to the 4T1-6A1 transition of theMn2+ impurity.31 The well-known green emission of Zn vacan-cies around 480 nm was not observed in the MIP-capped Mn-doped ZnS QDs because the emission was quenched by theelectron being transferring to the Mn2+ ions.32 The defect-related emission peak was caused by the doped ions in thesemiconductive nanomaterials, which is greatly affected bymany factors such as the process of synthesis and environ-ments. Thus, the fluorescence emission of doped ions waschosen for detection because that is more stable and control-

lable and has a higher quantum yield than the defect one. Therelative standard deviation (RSD) of 3.85% was obtained by 12repeated detections of the fluorescence insensitivity in the 10mg/L MIP-capped Mn-doped ZnS QD aqueous solution every5 min. The result shown in Figure 3b indicates the stableemission of the QDs. The main reason for the stable emission isthat the inner Mn2+ is protected by the amorphous silica shell.

In the mixed solution, the amino groups (-NH2) in themolecule of APTES can interact with the functional groups(such as hydroxyl group) in the template molecule to form acomplex through hydrogen bonding. This interaction betweenfunction monomer and template molecule can be confirmedfrom the UV-vis spectra. Figure 4 shows the UV-vis spectraof the 4-NP solution before and after APTES was added. The greatdifference of the two spectra indicates that there is a stronginteraction between 4-NP and APTES. The amino groups givebinding sites on the surface of MIP and NIP-capped Mn-dopedZnS QDs through the hydrogen binding. Therefore, both MIP-and NIP-capped Mn-doped ZnS QDs have a response to thetemplate 4-NP, but the quencher constant of MIP-capped Mn-

(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 MIP-capped Mn-doped ZnS QDs (10 mg/L) in water with an excited lightat 310 nm (curve 2); (b) stable fluorescence emission measurementof MIP-capped Mn-doped ZnS QDs (10 mg/L) in water. Fluorescenceexperimental condition: The photomultiplier tube (PMT) voltage wasset at 700 V, and the silt widths of excitation and emission were 10and 20 nm.

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doped ZnS QDs toward 4-NP is two times that of NIP-capped ones,which indicate the good selectivity of the sensors shown in Figure5. Compared with the NIP-capped ones, the MIP-capped Mn-dopedZnS QDs have more binding sites suitable for the template 4-NPdue to an efficient imprinting effect. In our work, the phenol waschosen for the similar structure with that of the template molecule(4-NP). The results were shown in the Table 1, and the greatdifference of the quenching constant indicated the better selectiv-ity of the materials we synthesized.

A simple schematic illustration of the fluorescence quenchingprocess was shown in Figure 6a. When there is no template 4-NParound the MIP-capped Mn-doped ZnS QDs, an orange emissionis generated by accepting the excited energy. After adding thetemplate 4-NP, there will be a strong interaction between thetemplate molecule and the amino groups, which is a main reasonfor the fluorescence quenching. We suggest that a quenchingmechanism is the electron transfer from the Mn-doped ZnS QDsto the 4-NP species through the strong binding to the templatemolecule. As seen in Figure 6b, the electrons were excited fromthe valence band to the conduction band and transited to the initialcondition following the solid arrow to generate the two emissions.The fluorescence quenching of the MIP-capped Mn-doped ZnSQDs is mainly achieved by two pathways because of the additionof 4-NP and the strong interaction between the amino groups andthe template 4-NP. In Figure 4, the UV-vis adsorption of 4-NPand 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-dopedZnS QDs can directly transfer to the lowest unoccupied molecularorbital (LUMO) of UV and the visible band of the 4-NP moleculesand 4-NP ions followed the paths shown as the dashed arrows.Since all the energy bands of the 4-NP molecules and 4-NP ionsare higher than the blue emission of the MIP-capped Mn-dopedZnS QDs around 440 nm, the excited electrons tend to go backby the dashed paths and a quenching is generated. The energymechanism is impossible for the fluorescence quenching becausethere is no overlap bands between the 4-NP molecules or ionsand the emissions of the QDs. According to the mechanism above,the large quenching constant means that there are more suitablebinding sites on the Mn-doped ZnS QDs.

The fluorescence quenching in this system can be quantifiedby the Stern-Volmer equation as follows:

F/F0 ) 1 + KSVcq (2)

where F0 is the initial fluorescence intensity in the absence ofquencher, F is the fluorescence intensity in the presence ofanalyte, 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 4. UV-vis spectra of 4-NP (10 ppm) before (curve 1) andafter (curve 2) the addition of APTS (20 ppm) in ethanol.

Figure 5. Evolutions of Mn ions fluorescence emission spectra ofMIP-capped Mn-doped ZnS QDs: (a) MIP-capped Mn-doped ZnSQDs (10 mg/L), (b) NIP-capped Mn-doped ZnS QDs (10 mg/L) withincreasing 4-NP concentrations in the water solution QDs. Insetgraphs: Stern-Volmer plots from (a) MIP-capped Mn-doped ZnS QDsand (b) NIP-capped Mn-doped ZnS QDs with the 4-NP. Fluorescenceexperimental condition: The photomultiplier tube (PMT) voltage wasset at 700 V; the excited light was set at 310 nm, and the silt widthsof excitation and emission were 10 and 20 nm.

Table 1. Summary of Quenching Constants of MIP- andNIP-Capped Mn-Doped ZnS QDs

Ksv (MIP) Ksv (NIP) K′{Ksv (MIP)/Ksv (NIP)}

4-nitrophenol 40015.36M-1 18936.67M-1 2.113phenol 20149.14M-1 19654.56M-1 1.025

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between MIPs and NIPs. The ratio of the MIP and NIP’s Ksv wasimportant data to evaluate the selectivity of the materials weobtained. According to the results we obtained, the MIP-cappedMn-doped ZnS QDs have a better selectivity than the NIP-capped ones.

Application of MIP-Capped Mn-Doped ZnS QDs to CLSystem. The application of QDs and MIPs to the CL system hasbeen reported,25,27 but the use of MIP-capped QDs to the CLsystem has not been concerned. In our work, the product wesynthesized was used in the CL system to improve selectivity andsensitivity of CL method. In order to apply MIP-capped Mn-dopedZnS QDs to the FIA system, we carried the batch experiments toevaluate the different orders of injection solutions. There is noCL signal when MIP-capped Mn-doped ZnS QDs were mixed withH2O2 or NaIO4. From the results listed in Figure 7, it can beseen that there is a weak luminescence when H2O2 was injectedinto NaIO4 solution. The CL intensity followed the additionorder as Peak 3 is about five times the CL intensity as Peak 2.This can be explained as decomposition of H2O2 for the surfaceeffect of quantum dots. According to the results we achieved,the MIP-capped Mn-doped ZnS QDs were applied to theH2O2-NaIO4 CL system following the process shown in Figure1. The MIP-capped Mn-doped ZnS QDs were injected by a valveinjector and carried by water. Then, the solution was first mixedwith samples because some time is needed for the templates toarrive at the selectivity holes. After that, the solution and NaIO4

was mixed and entered the flow-cell to react with H2O2.

The CL intensity of FIA system was enhanced obviously byadding both MIP- and NIP-capped Mn-doped ZnS QDs. However,the MIP-capped Mn-doped ZnS QDs have a greater enhancementthan 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 linearrange of this method. A set of 4-NP samples were added tothe system, and the results were shown in Figure 8. We cansee that the CL intensity decreased with the increase of theconcentration of the 4-NP solutions. When the concentration ofthe 4-NP solution reached 40 µM, the increase of the concentrationcould not bring an obvious decrease of CL intensity, which

Figure 6. Schematic illustrations for the quenching mechanism: (a)Fluorescence quenching process for 4-NP detection. (b) Fluorescencequenching mechanism of electron transferring from quantum dots to4-NP species.

Figure 7. CL profiles in batch experiments of different injectionorders: 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 MIP-capped QDs (10 mg/L; Peaks 2), H2O2 (0.1 M) injected into themixture 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 8. Typical FIA peaks for the determination of 4-NP: (1) notadded; (2) 1.0 µM; (3) 10.0 µM; (4) 20.0 µM; (5) 30.0 µM; (6) 40.0µM; (7) 15.0 µM spiked sample result. Experimental conditions: 0.1M 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 wasused.

7385Analytical Chemistry, Vol. 82, No. 17, September 1, 2010

indicated the quenching entered a platform period. The lower rightrange is mainly because the binding sites on the surface arelimited for the thinner shell we obtained. When all the bindingsites were used, the effect of quenching was not obvious. Thedetection limit of 4-NP in water is 76 nM, and the linear range isfrom 0.1 µM to 40 µM.

In our work, we chose six aromatic organic compounds whichwere applied to the MIP-capped Mn-doped ZnS QD-NaIO4-H2O2 CL system. The results were listed in the Table 2 to provethe selectivity of this CL system. In the six aromatic organiccompounds, 2-NP has the similar optical and chemical propertiesand similar structure to 4-NP. However, the quenching of the CLsystem to the two similar compounds is different. The quenchingto template molecule 4-NP (27.2%) is about three times of that to2-NP (11.1%). This proves the selectivity of this CL system to thecompounds which have the similar properties. The CL systemhas a different response to the other compounds in the experimentwhich may be caused by the different chemical properties of thecompounds. We suggest that these compounds may influencethe reaction between NaIO4 and H2O2 and bring the change ofthe CL intensity. In conclusion, the application of MIP-cappedQDs to the NaIO4-H2O2 CL system can improve the selectivityof CL method.

This method was also used in real water samples detection.4-NP cannot be detected in the tap water acquired in thelaboratory. Then, the spiked experiment was carried out, and therecoveries reached 94.07% (RSD ) 2.02%, n ) 3). All the resultswere shown in the Table 3.

Possible Mechanism of Chemiluminescent Reaction. TheCL mechanism of the H2O2-NaIO4 system has been studied inour previous work.26 We consider that the MIP-capped Mn-doped ZnS QDs play a luminophor role in the process of the

CL reaction. In brief, H2O2 directly reacts with NaIO4 togenerate •O2

- radicals which may produce energy-rich precur-sors of excited molecules (O2)2*. The excited molecules (O2)2*transfer their energy to the higher fluorescence efficiency QDswhich results in a strong light. The process was shown asfollows:

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 ZnSQDs and the fluorescence emission was quenched. As the possiblemechanism shown above, the fluorescence emission of the QDswas the main factor to influence the intensity of the CL after theMIP-capped Mn-doped ZnS QDs were added into the system.Therefore, the CL intensity decreased alomost linearly with theincreasing concentrations of quencher.

CONCLUSIONSIn summary, MIP-capped Mn-doped ZnS QDs were synthe-

sized suceessfully. The products have a specific selectivity for thetemplate molecule by fluorescence quenching which was mainlycaused by an electron transmission between QDs and 4-NP. Theapplication of MIP-capped Mn-doped ZnS QDs to the CL systemcan improve the selectivity of the CL method which is a limit forthe further development of CL system as an analytical methodand can introduce more CL reactions to use for detection. Apossible mechanism was also proposed to explain the process ofthe CL reaction and the quench of CL intensity.

ACKNOWLEDGMENTThis 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

Table 2. Inhibition Effects of Organic Compounds onthe CL System of MIP-Capped QD-NaIO4-H2O2

(Concentration: 10-5 M)

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

Table 3. Optimum Conditions and Analytical Figures ofthe FIA-CL System

optimum conditions0.05 M NaIO4 of 1.2 mL/min flow rate0.1 M H2O2 of 1.2 mL/min flow ratesample 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 0 not detectedtap water 15 14.11 94.07 2.02

a n ) 3.

7386 Analytical Chemistry, Vol. 82, No. 17, September 1, 2010