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Spectrochimica Acta Part A 77 (2010) 625–629

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

selective fluorescence probe for mercury ion based on the fluorescenceuenching of terbium(III)-doped cadmium sulfide composite nanoparticles

ie Fu, Lun Wang ∗, Hongqi Chen, Ling Bo, Cailing Zhou, Jingguo Chennhui Key Laboratory of Chemo-Biosensing, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China

r t i c l e i n f o

rticle history:eceived 21 March 2010eceived in revised form 11 June 2010ccepted 24 June 2010

eywords:luorescence probeerbium(III)

a b s t r a c t

A fluorescent probe for mercury(II) ions, based on the quenching of fluorescence of terbium(III) ions dopedin CdS nanoparticles, has been developed. The terbium(III)-doped cadmium sulfide composite nanopar-ticles were successfully synthesized through a straightforward one-pot process, with the biomoleculeglutathione (GSH) as a capping ligand. In addition, the terbium(III) ions were observed an enhance-ment of emission intensity, owing to fluorescence energy transfer from the excited CdS particles to theemitting terbium(III). Because of a specific interaction, the fluorescence intensity of terbium(III)-dopedCdS particles is obviously reduced in the presence of mercury(II) ions. The fluorescence quenching phe-

ercury(II)uantificationadmium sulfide nanoparticles

nomenon of terbium(III) can be attributed to the fact that the energy transfer system was destroyed bycombining with mercury(II). Under the optimal conditions, the fluorescent intensity of terbium(III) ionsat 491 nm decreased linearly with the concentration of mercury(II) ions ranging from 4.5 nmol L−1 to550 nmol L−1. The limit of detection for mercury(II) was 0.1 nmol L−1. This method is simple, practical,relatively free of interference from coexisting substances and can be successfully applied to the determi-nation of mercury(II) ions in real water samples. In addition, the probable mechanism of reaction betweenterbium(III)-doped CdS composite nanoparticles and mercury(II) was also discussed.

. Introduction

Heavy metals, as one of the most hazardous classes of pollutantsn water sources due to their nonbiodegradability, have caused

idespread water endangerment, contamination of fish, and seri-us health problems [1]. Mercury is one of the most dangerous andidespread global pollutant. Moreover, mercury(II) ions deriva-

ives can accumulate in the organs of living things through foodhain, doing huge harm to human being and the nature [2,3]. Theases in Minamata Bay in Japan in 1953 [4] were particularly disas-rous. Concerns over toxic exposure to mercury provide motivationo explore new methods for monitoring aqueous Hg2+ from biologi-al and environmental samples [5]. Current techniques for mercuryetermination have been established, such as spectrophotometry6], atomic absorption/emission spetroscopy [7,8], inductively cou-led plasma-mass spectroscopy (ICP-MS) [9], inductively coupledlasma-atomic emission spectrometry (ICP-AES) [10] and so on.

hese methods have low limits of detection and wide linear ranges.owever, these analytical methods need expensive and sophisti-ated instrumentation or complicated sample preparation process.herefore, there is a need for analytical methods for the selective,

∗ Corresponding author. Tel.: +86 553 5910008; fax: +86 553 5910008.E-mail address: wanglun@mail.ahnu.edu.cn (L. Wang).

386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2010.06.038

© 2010 Elsevier B.V. All rights reserved.

sensitive, and rapid detection of Hg(II) ions. Recently, fluorescencedetection with Hg2+-responsive chemosensors offers a promisingapproach for simple and rapid tracking of mercury ions for biolog-ical, toxicological, and environmental monitoring [11–13]. Theseprobes and sensors with small molecules [14], DNAzymes [15] andprotein [16] or oligonucleotide platforms [17] for mercury reportedso far generally exhibits long response time, narrow working con-centration range or moderate selectivity. Therefore, searching fornew fluorescence probe with high selectivity and good photochem-ical property is still a challenge for the analytical chemistry researchefforts [18].

Semiconductor nanoparticles have attracted a lot of attentionsin the past few decades due to their unique optical propertiessuch as broad excitation band, size- and composition-tunableemission wavelength and excellent anti-photobleaching [19–21].Recently, quantitative detection of heavy metal ions with semi-conductor nanoparticles via spectra changes in photoluminescencehas been widely reported. With regard to Hg(II), Chen et al. [22]reported mercaptoacetic acid (MAA)-coated CdTe quantum dotsas luminescent probes for mercury ions. The first practical uses

of CdS quantum dots capped with different organic ligands wereemployed as chemical sensor to determine zinc and copper ions inaqueous media [23]. Recently, Yan et al. [24] developed functional-ized CdS nanoparticles as a fluorescence probe for the detection ofHg2+ with high sensitivity. More recently, a novel fluorogenic sen-

6 Acta

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or to probe mercury ions based on l-cysteine-capped CdS quantumots [25] was reported. These reports reveal that CdS nanoparticlestilized as luminescent probes and sensors were widely studied.owever, searching for new fluorophores with high selectivity asell as good photochemical property including excellent photosta-

ility, large Stoke’s shifts, high quantum yield and long fluorescenceifetime is still a challenge.

Recently, numerous papers on the luminescence of semiconduc-or nanoparticles doped with rare earth (RE) ions have appeared,uch as Tb3+-doped functionalized CdS and ZnS nanocrystals26,27], Sm2+-doped ZnS nanocrystals [28], Eu3+-doped ZnS anddS nanocrystalline [29] or Er3+-doped glass matrix contain-

ng CdS QDs [30]. It is known that the absorption bands ofanoparticles are broader than those of RE which ensures theccordability in wavelength of the pump sources for the achieve-ent of optical components such as amplifiers. Direct or indirect

and-gap semiconductors could be good sensitizing centers sinceheir excitation cross sections are very high due to the efficientand-to-band absorptions [31]. Compared to the surface-modifiedemiconductors nanoparticles, the RE-doped II–VI semiconduc-ors nanocrystals exhibit a very narrow emission line, a largexcitation–emission separation and a long fluorescence lifetime. Inarious articles it was concluded that the RE-doped II–VI semicon-uctors nanocrystals “form a new class of luminescent materials”29]. Considering this, the Tb3+-doped cadmium sulfide compos-te nanoparticles used as a fluorescence probe for determination of

ercury ions was researched.In our work, we report a new and simple method for creating

ater soluble Tb3+-doped CdS composite nanoparticles with GSHs the capping ligand. This method is simple, sensitive, low costnd requires only room temperature conditions. Under the optimalonditions, the fluorescent intensity of Tb3+ with maximum band at91 nm doped in CdS nanoparticles can be quenched gradually byercury ions and the quenching of fluorescence fluorescent inten-

ity is proportional to mercury ions concentration. This quenchingan be attributed to the fact that the energy transfer system withdS nanoparticles as donor and Tb3+ ions as accepter was destroyedpon combining with Hg2+. Based on the quenching of fluores-ence of Tb3+ ions, a sensitive and identified fluorescence probeor mercury(II) ions in aqueous solution was developed. The pre-iminary research reports showed that this method will enable uso construct efficient RE-doped CdS nanoparticles for heavy metalsetection based on fluorescence quenching effect.

. Experiment

.1. Apparatus and reagents

The fluorescence spectra were performed using a Hitachi F-500 spectrofluorometer (Hitachi, Japan) equipped with a plotternit and a quartz cell (1 cm × 1 cm). The transmission electronicroscopy (TEM) images were obtained using a JEM-2100 trans-ission electron microscope (JEOL, Japan). The UV spectra were

cquired on a U-3010 spectrofluorometer (Hitachi). All pH valuesere measured with a pHS-3C digital pH meter (Analytical Instru-ents Co., Tianda, Shanghai, China).All chemicals were of analytical-reagent grade or better.

he stock solutions of CdCl2·2.5H2O (Alfa, USA), thioacetamideCH3CSNH2, Alfa), NaOH (Alfa), HgCl2 (Shanghai Reagent Company,hina), sodium hexametaphosphate (Alfa), Tb(NO3)3·6H2O (Sigma)

nd glutathione (GSH, Alfa) were prepared by dissolving themn ultra pure water without further purification. The phosphoricuffer solutions (PBS) were prepared by adjusting 0.067 mol L−1

H2PO4 with 0.067 mol L−1 Na2HPO4. Ultra pure water was usedhroughout.

Part A 77 (2010) 625–629

2.2. Synthesis of Tb3+-doped CdS composite nanoparticles

The Tb3+-doped CdS nanoparticles were prepared according tothe scheme reported in literatures [32,33] and our previous workfor mercaptoacetic acid capped CdS nanocrystals reported by Liangand co-workers [34] with a little change. Briefly, 30 mL 0.01 mol L−1

CdCl2·2.5H2O, 45 mL of 0.1 mol L−1 sodium hexametaphosphate,30 mL 0.01 mol L−1 GSH and 75 mL 0.01 mol L−1 Tb3+ were mixed ina 500 mL three-necked round bottomed flask and purged with N2in room temperature. Under vigorous stirring, 120 mL 0.01 mol L−1

CH3CSNH2 were added drop-wise and then 30 mL 0.1 mol L−1 NaOHsolutions were added drop-wise to the flask. After stirring for 1.5 h,the composite nanoparticles were obtained. The as-prepared solu-tions are stable for one month at room temperature, no visiblecoacervation or precipitation was observed.

2.3. Procedure of determination of Hg2+

In a series of 10 mL volumetric flasks, 4 mL of as-prepared Tb3+-doped CdS solution, 2 mL of PBS (pH 5.29) and various amount ofHg2+ were added, then the mixture was diluted to the mark withwater and mixed thoroughly. After incubating for 10 min at roomtemperature, the fluorescence spectrum of the F-2500 spectroflu-orometer was recorded within the wavelength region from 390 to650 nm. The excitation and emission wavelengths were 368 and491 nm, respectively.

3. Result and discussion

3.1. TEM image of nanoparticles

The morphology of the Tb3+-doped CdS nanoparticles was stud-ied by TEM is shown in Fig. 1. The TEM image shows that theobserved diameter of the nanoparticles was about 10 nm or so.

3.2. Spectral characteristics and reaction between nanoparticlesand Hg2+

The fluorescence emission spectra of (1) CdS nanoparticles, (2)Tb3+–CdS, (3) Tb3+–CdS–Hg2+, (4) Tb3+ systems are shown in Fig. 2.As shown in Fig. 2, it can be seen that at the excitation of 368 nm,the Tb3+ system emits the characteristic fluorescence of Tb3+ withthe weak emission peaks of 491 and 546 nm, which correspondto the transition of 5D4 → 7F6 and 5D4 → 7F5. The weak charac-teristic fluorescence may due to the fact that rare-earth ion Tb3+

has low absorption (molar absorption lower than 10). From curve1 in Fig. 2, we can see the strong emission wavelength of CdSnanoparticles without Tb3+ occurs at 515 nm. After doping withTb3+ ions, the fluorescence of Tb3+ ions can be enhanced remark-ably. This indicates that CdS nanoparticles have the enhancementeffect on the luminescence of Tb3+. Chowdhury and Patra [35]founded that upon excitation of the CdS host, the energy from non-radiative recombination of electron–hole pairs can be transferredto the high-lying energy levels of the lanthanide’s ion. Similarly,it may be explained that the electron trapped in the surface lev-els of CdS particles recombines with a valence band free hole andthe energy is non-radiatively transferred to the Tb3+ ions. Accord-ing to this suppose, an efficient energy transfer system with CdSnanoparticles as energy donor and Tb3+ ions as energy accepter wasbuilt.

When trace amounts of Hg2+ were added to the Tb3+-doped CdSnanoparticles solution, the wavelength of excitation and emissionwere unchanged, but the intensity of the peak decreased (curve3). The experimental phenomenon can be explained in the termsof strong affinity of mercury onto the surface of CdS nanoparti-

J. Fu et al. / Spectrochimica Acta Part A 77 (2010) 625–629 627

cadmium sulfide composite nanoparticles.

cldisc[qfipapiwttrrscc

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Fig. 1. TEM image of the Tb3+-doped

les. As the Ksp constant of HgS (Ksp(HgS) = 6.4 × 10−53) is muchower than that of CdS (Ksp(CdS)= 8.0 × 10−27), the mercury ionsisplaced the Cd2+ in the CdS nanoparticles due to its higher bind-

ng affinity to S. In addition, Isarov and Chysochoos found that ultramall CuxS particles formed on the surface of CdS nanoparticlesan quench the recombination luminescence of CdS nanoparticles36,37]. Therefore, the formation of ultra small HgxS particles woulduench the recombination luminescence of CdS nanoparticles byacilitating non-radiative recombination of excited electrons (e−)n the conduction band and holes (h+) in the valence band. But thehenomenon of the fluorescence quenching effect of Tb3+ with theddition of trace amounts of Hg2+ attracted much more interest. Theossible mechanism for the fluorescence quenching effect of Tb3+

s that the energy transfer system was destroyed upon combiningith Hg2+, thus reducing the fluorescence quantum efficiency of

he composite nanoparticles. The experimental results indicate thathe reaction between composite nanoparticles and mercury ions at

oom temperature occurs rapidly. The fluorescence spectra of theeaction of Tb3+-doped CdS composite nanoparticles with Hg2+ arehown in Fig. 3(A). The fluorescence intensity of the CdS nanoparti-les and Tb3+ both decreased gradually with the increment in addedoncentrations of Hg2+, it is very interesting to note that the sen-

ig. 2. Emission spectra of (1) CdS nanoparticles, (2) Tb3+–CdS, (3) Tb3+–CdS–Hg,4) Tb3+. Experiment conditions: CdS: 3.6 × 10−4 mol L−1; Tb3+: 9.0 × 10−4 mol L−1;g2+: 1.0 × 10−7 mol L−1, pH 5.29, excitation spectra (�ex = 368 nm).

Fig. 3. (A) Fluorescence spectra of the Tb3+-doped CdS composite nanoparticles(3.6 × 10−4 mol L−1) in the presence of Hg2+ (�ex/�em = 368/491 nm); concentrationof Hg2+ from curve a to curve g (nmol L−1): 0.0; 30.0; 90.0; 180.0; 300.0; 400.0; 500.0.(B) Plot of �F against the concentration of Hg2+.

sitivity and detection limit of the method using the characteristicfluorescence of Tb3+ for Hg2+ detection is better than that of CdSnanoparticles. Moreover, the Tb complex has a very large Stokes

shift that permit more sensitive fluorescence detection. From theplot of �F against the concentration of Hg2+ (Fig. 3(B)), there is agood linear relation among the changes of fluorescence intensity ofTb3+ ions, �F (�F = F0 − F, the difference between the fluorescenceintensity in the absence (F0) and presence (F) of Hg2+) and the con-

628 J. Fu et al. / Spectrochimica Acta Part A 77 (2010) 625–629

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Table 1Interference of different species to the fluorescent determination of Hg2+ with theTb3+-doped CdS composite nanoparticles.

Coexisting substance Coexsiting concentrationa

(mol L−1)Relative error (%)

Ca2+ 5.0 × 10−4 −1.5Mg2+ 5.0 × 10−4 1.2Al3+ 5.0 × 10−5 2.3Fe3+ 5.0 × 10−5 1.9Ba2+ 5.0 × 10−5 2.6Cr3+ 5.0 × 10−5 −2.4Co2+ 1.0 × 10−5 −1.8Pb2+ 1.0 × 10−5 2.1Zn2+ 1.0 × 10−5 3.2Ag+ 1.0 × 10−5 1.5Cd2+ 5.0 × 10−6 2.7Mn2+ 5.0 × 10−6 −2.2Cu2+ 1.0 × 10−6 3.4

2+ −6

ig. 4. Effect of pH on the relative fluorescence intensity. Composite nanoclusters:.6 × 10−4 mol L−1; Hg(II): 50 nmol L−1.

entration of Hg2+. Finally, we chose the fluorescence of Tb3+ ionsor the detection of Hg2+.

. Effect of reaction variables

.1. Effect of pH

In order to select the optimum conditions for the determinationf Hg2+, the effect of pH in a range between 4.49 and 9.18 wastudied. As shown in Fig. 4, the pH of the solution has great effectn the fluorescence intensity of the system. In the plot it coulde seen that I�F reached the maximum when the pH was 5.29. Tobtain high fluorescence intensity with good precision, a pH of 5.29as chosen for the further experiments.

.2. Optimum amounts of the Tb3+-doped CdS compositeanoparticles in determination

The effect of the concentration of composite nanoparticles onhe fluorescence intensity of the assay system was also inves-igated. As the concentration of the Tb3+-doped CdS compositeanoparticles increased, the linear range of calibration functionecame wider whereas the sensitivity decreased. The optimal con-entration of the Tb3+-doped CdS composite nanoparticles shouldive attention to both sensitivity and linear range of the calibra-ion function. So 3.6 × 10−4 mol L−1 of the aqueous solution wasmployed for further experiments.

.3. Reaction and mixing sequence

The experiments indicated that the reaction between compositeanoparticles and Hg2+ reached the equilibrium at room tempera-ure within 10 min, and the fluorescent intensity remained stableor at least 1 h. So, there is enough time to conduct the measure-

ents. After investigating several adding sequences, we found thathe best order is to mix nanoparticles solution, buffer solution first

nd then Hg2+. Therefore, all the measurements were made afteranoparticles solution, buffer solution and Hg2+ were completelyixed for 10 min.

Ni 1.0 × 10 −3.0

a The concentration of Hg2+ was fixed at 1.0 × 10−7 mol L−1 (pH 5.29).

5. Analytical application

5.1. Interference of coexisting foreign substances

In order to apply the proposed method to determine mercury(II),the effects of foreign ions on the fluorescence intensity of the sys-tem were tested. A foreign species was considered not to interferewith measurement if a relative error caused by it was less than 5% inthe determination of 1.0 × 10−7 mol L−1 Hg2+. As shown in Table 1,most of the species tested caused no interference when existed inspecific molar excesses. The results indicate that the method hasfairly good selectivity.

5.2. Analytical performance of the Tb3+-doped CdS compositenanoparticles

The fluorescence spectra of the Tb3+-doped CdS compos-ite nanoparticles and its fluorescence titration with Hg2+ wererecorded. Under the optimum experimental conditions, a good lin-earity between the quenched fluorescence intensity (�F = F0 − F)of the Tb3+ and the concentration of Hg2+ was observedfrom 4.5 nmol L−1 to 550 nmol L−1 with a correlation coefficient(�2) of 0.9982 and a linear regression equation is �F = 8.59C(nmol L−1) + 163.24 (Fig. 3(B)). A detection limit of 0.1 nmol L−1 wasdetermined on the basis of three times the standard deviation ofseven replicate measurements of the quenched fluorescence inten-sity due to 50 nmol L−1 of Hg(II). As a comparison, the linear rangeand detection limit of several selected fluorimetric methods forHg(II) detection are summarized in Table 2. It is obvious that thesensitivity and detection limit of the proposed method for Hg2+

detection based on RE-doped semiconductors nanocrystals is betterthan that of most traditional fluorimetric methods.

To assess the utility of the prepared optical sensor, we appliedthem to the determination of mercury in real environmental sam-ples. The real samples were simply filtered and the results showedthat no Hg2+ existed. So different concentrations of Hg2+ wererespectively added in the real samples and then analyzed with theoptical sensor. The results were compared with the data obtainedfrom inductively coupled plasma-atomic emission spectroscopy(ICP-AES) method. The results summarized in Table 3 show that themercury contents of different samples determined by the proposed

Hg2+-sensor are in satisfactory agreement with those obtained byICP-AES measurements. The recovery study of added Hg2+ deter-mined by the proposed sensor exhibited satisfactory results.

J. Fu et al. / Spectrochimica Acta Part A 77 (2010) 625–629 629

Table 2Comparison of the linear range and detection limit of several selected fluorimetric methods for determination of Hg2+.

Reagent Linear range (10−7 mol l−1) LOD (10−9 mol l−1) Reference

H2tpp-based sensor 0.4–40 40 [38]TDMAPP-based sensor 2.26–452 8.0 [39]Functionalized CdS QDs 0.16–1.12 2.4 [24]dBSA-coated-CdTe 0.12–15 4.0 [12]l-Cysteine-capped CdS QDs 0.2–10 5.0 [25]Tb3+-doped CdS QDs 0.045–5.5 0.1 This work

Table 3Determination of Hg(II) in natural water samples.

Sample Hg(II) added (10−9 mol L−1) Hg(II) found (10−9 mol L−1)a Recovery (%)

This method ICP-AES

Waste waterb 20.0 19.0 19.2 95.0100.0 96.0 98.0 96.0

Lake waterc 20.0 18.8 19.4 94.0100.0 97.5 99.0 97.5

Tap water 20.0 20.7 21.0 103.5100.0 102.0 103.0 102.0

Synthetic Hg2+-polluted waterd 20.0 21.1 20.5 105.5100.0 101 100.8 101.0

a The average of five replicate determinations.

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. Conclusions

In conclusion, a Tb(III)-doped CdS composite nanoparticlesapped with GSH has been developed as optical sensor for highlyelective and sensitive detection and quantification of mercury(II)n aqueous environments with a detection limit of 0.1 nmol L−1.nergy transfer from the CdS particles (energy donor) to Tb(III) ionsenergy acceptor) is also studied. Based on the fact that the energyransfer system can be destroyed by combining with Hg2+, the fluo-escent intensity of both Tb(III) ions and CdS particles are obviouslyeduced in the presence of Hg2+. Among them, Tb(III) ions showedreferable fluorescence response to mercury(II). Thereby, the com-osite nanoparticles design takes advantage of the Tb complex,hich has a very large Stokes shift and long fluorescence lifetime

hat permit more sensitive fluorescence detection of Hg2+. More-ver, this sensor has been used for the determination of Hg2+ inifferent water samples with satisfactory recovery. Hopefully, weelieve that this method may offer the possibility to construct thehemical sensors and fluorescent probes based on these kinds ofE (Eu3+, Tb3+, Er3+, Sm3+, etc.) doped semiconductor nanoparticlesCdS, ZnS, etc.).

cknowledgements

This work was supported financially by the National Naturalcience Foundation of China (20875004, 20905003).

eferences

[1] J.P. Vernet, Heavy Metals in the Environment, Elsevier, New York, 1991.[2] A.H. Stern, R.J.M. Hudson, C.W. Shade, S. Ekino, T. Ninomiya, M. Susa, Science

303 (2004) 763–766.

[3] D.W. Boening, Chemosphere 40 (2000) 1335–1351.[4] T. Tsubaki, K. Irukayama, Minamata Disease: Methylmercury Poisoning, Koden-

sha, Tokyo, 1977.[5] Y. Sungho, E.A. Aaron, P.W. Audrey, J.C. Christopher, J. Am. Chem. Soc. 127

(2005) 16030–16031.[6] M.S. Hosseini, H. Hashemi-Moghaddam, Talanta 67 (2005) 555–559.

[[[

[[

.0 nmol L−1 of Cu(II) and Ni(II), 450 nmol L−1 of Al(III), 15.0 nmol L−1 of Fe(III) and

[7] N. Bloom, W.F. Fitzgerald, Anal. Chim. Acta 208 (1988) 151–161.[8] J.V. Cizdziel, S. Gerstenberger, Talanta 64 (2004) 918–921.[9] E. Curdova, L. Vavruskova, M. Suchanek, P. Baldrian, J. Gabriel, Talanta 62 (2004)

483–487.10] A.N. Anthemidis, G.A. Zachariadis, C.E. Michos, J.A. Stratis, Anal. Bioanal. Chem.

379 (2004) 764–769.11] E.M. Nolan, S.J. Lippard, Chem. Rev. 108 (2008) 3443–3480.12] Y.S. Xia, C.Q. Zhu, Talanta 75 (2008) 215–221.13] S. Yoon, A.E. Albers, A.P. Wong, C.J. Chang, J. Am. Chem. Soc. 127 (2005)

16030–16301.14] Y.K. Yang, K.J. Yook, J. Tae, J. Am. Chem. Soc. 127 (2005) 16760–16761.15] J.M. Thomas, R. Ting, D.M. Perrin, Org. Biomol. Chem. 2 (2004) 307–312.16] P. Chen, C. He, J. Am. Chem. Soc. 126 (2004) 728–729.17] A. Ono, H. Togashi, Angew. Chem. Int. Ed. 43 (2004) 4300–4302.18] C.L. He, F.L. Ren, X.B. Zhang, Z.X. Han, Talanta 70 (2006) 364–369.19] W.C.W. Chan, D.J. Maxwell, X.H. Gao, R.E. Bailey, M.Y. Han, S.M. Nie, Curr. Opin.

Biotechnol. 13 (2002) 40–46.20] M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998)

2013–2016.21] W.C.W. Chan, S.M. Nie, Science 281 (1998) 2016–2018.22] B. Chen, Y. Yu, Z.T. Zhou, P. Zhong, Chem. Lett. 33 (2004) 1608–1609.23] Y.F. Chen, Z. Rosenzweig, Anal. Chem. 74 (2002) 5132–5138.24] Z.X. Cai, H. Yang, Y. Zhang, X.P. Yan, Anal. Chim. Acta 559 (2006) 234–239.25] J.L. Chen, C.Q. Zhu, Anal. Chim. Acta 546 (2005) 147–153.26] C. Tiseanu, R.K. Mehra, R. Kho, J. Photochem. Photobiol. A: Chem. 173 (2) (2005)

169–173.27] C. Tiseanu, R.K. Mehra, R. Kho, M. Kumke, J. Phys. Chem. B 107 (2003)

12153–12160.28] T. Kushida, A. Kurita, M. Watanabe, Y. Kanematsu, K. Hirata, N. Okubo, Y. Kane-

mitsu, J. Lumin. 87–89 (2000) 466–468.29] S.J. Xu, S.J. Chua, B. Liu, L.M. Gan, C.H. Chew, G.Q. Xu, Appl. Phys. Lett. 73 (4)

(1998) 478–480.30] D.Q. Chen, Y.S. Wang, F. Bao, Y.L. Yu, Script. Mater. 55 (2006) 891–894.31] Y.X. Zheng, J. Lin, Y.J. Liang, Q. Lin, Y.N. Yu, S.B. Wang, C. Guo, H.J. Zhang, Mater.

Lett. 54 (2002) 424–429.32] H. Liu, W.Y. Li, H.Z. Yin, X.W. He, L.X. Chen, Acta. Chim. Sinica 63 (2005) 301–

306.33] L. Wang, Y. Liu, H.Q. Chen, A.N. Liang, F.G. Xu, Chin. Chem. Lett. 18 (2007)

369–372.34] L. Wang, A.N. Liang, H.Q. Chen, Y. Liu, B.B. Qian, J. Fu, Anal. Chim. Acta 616 (2008)

170–176.

35] P.S. Chowdhury, A. Patra, Phys. Chem. Chem. Phys. 8 (2006) 1329–1334.36] A.V. Isarov, J. Chrysochoos, Langmuir 13 (1997) 3142–3149.37] H.Y. Xie, J.G. Liang, Z.L. Zhang, Y. Liu, Z.K. He, D.W. Pang, Spectrochim. Acta Part

A 60 (2004) 2527–2530.38] W.H. Chan, R.H. Yang, K.M. Wang, Anal. Chim. Acta 444 (2001) 261–269.39] Y. Yang, J.H. Jiang, G.L. Shen, R.Q. Yu, Anal. Chim. Act 636 (2009) 83–88.