Analytica Chimica Acta 549 (2005) 10–13

A sensitive and selective “turn on” fluorescent chemosensor forHg(II) ion based on a new pyrene–thymine dyad

Zhuo Wanga,b, Deqing Zhanga,∗, Daoben Zhua

a Organic Solids Laboratory, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, Chinab Graduate School, Chinese Academy of Sciences, Beijing 100080, China

Received 13 April 2005; received in revised form 2 June 2005; accepted 2 June 2005

Abstract

Strong pyrene excimer emission was observed for a new pyrene–thymine dyad that can be easily synthesized in the presence of Hg2+ ion.But, the enhancement of pyrene excimer emission was not detected in the presence of other metal ions. As a result, this new pyrene–thyminedyad is suited for use as a sensitive and selective “turn on” fluorescent chemosensor for Hg(II) ion.© 2005 Elsevier B.V. All rights reserved.

K

1

uecmrrcchdurcHqfldc

eas-esisitived on

sorof

rongtn,

ouldieve

ouldesine

ely.of

andasedhe

0d

eywords:Hg(II) ion; Chemosensor; Pyrene; Thymine; Excimer emission; Fluorescence

. Introduction

Mercury and compounds containing mercury are widelysed in industry, although they have inherent toxicity. Sev-ral diseases have been known to be associated with mer-ury contamination[1–6]. Therefore, design and develop-ent of new sensitive and selective Hg(II) sensors have

eceived a lot of attention. In 1992, Chea and Czarnik[7]eported a chemodosimeter that showed significant fluores-ence alteration after reaction with Hg(II) ion. Fluorescenthemosensors for Hg(II) ion have also been described. Weave recently found that 4-[2-(9-anthryloxy) ethyl]thio-1,3-ithiole-2-thione also displays similar fluorescence changepon reaction with Hg(II) ion[8]. However, most of theeported chemosensors for Hg(II) ion belong to fluores-ence quenching chemosensors, since Hg(II) ion like otherTM (heavy transition metal) ions is known as fluorescenceuenchers[9–16]. Compared with fluorescence quenching,uorescence enhancement is more favorable for sensitiveetection and wide use. By now, only a few Hg(II) ionhemosensors with fluorescence enhancement (“turn on”

fluorescent chemosensor) have been described[17–21]. Inaddition, it is desirable that the chemosensors can beily prepared since complicated or time-consuming synthmay limit their applications. Herein we describe a sensand selective “turn on” chemosensor for Hg(II) ion basea new pyrene–thymine dyad1 (Fig. 1).

The design rationale for this new Hg(II) ion chemosenis illustrated inFig. 1. We want to take the advantagethe fact that a molecule with two pyrene units shows stexcimer emission around 470 nm[22–26]. If a pyrene uniis linked to a ligand that can coordinate with Hg(II) iosuch a coordination compound with two pyrene units wshow strong excimer emission of pyrene. In order to achhigh sensitivity and selectivity, the candidate ligand shdisplay very high affinity to Hg(II) ion. During our studiof thymine derivatives we unexpectedly found that thymand its derivatives can target Hg(II) ion very selectivThe nitrogen or oxygen atoms of the carbonyl groupthymine are possibly the coordination sites. OnoTogashi reported a highly selective oligonuclectide-bsensor for Hg(II) ion[27]. The authors claimed that t

∗ Corresponding author. Tel.: +86 10 62639355.E-mail address:dqzhang@iccas.ac.cn (D. Zhang).

coordination of thymine residue in the oligonucleotide withHg(II) ion played an important role. Encouraged by thisfinding, we design a new pyrene–thymine dyad1 (Fig. 1)

003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.aca.2005.06.031

Z. Wang et al. / Analytica Chimica Acta 549 (2005) 10–13 11

Fig. 1. Coordination mode of two molecules of dyad1 with Hg(II) ion andthe chemical structure of dyad1.

as potential Hg(II) chemosensor with good sensitivity andselectivity.

2. Experimental

Melting points were measured with an XT4-100Xmicroscope apparatus and uncorrected.1H NMR and 13CNMR spectra were recorded on BRUCK300 MHz. TOF–MSspectra were determined with BEFLEX III. Elementalanalyses were performed on a Carlo-Erba-1106 instrument.Fluorescence measurements were carried out with a Hitachi(model F-4500) spectrophotometer in a 1 cm quartz cell.1-Pyrenemethanol was purchased from Acros.

2.1. Synthesis of compound2

To a solution of 1-pyrenemethanol (0.47 g, 2.0 mmol) indry THF was added NaH (52%, 2.77 g, 60.0 mmol) in N2atmosphere at room temperature. The mixture was stirredovernight. Then, 1,5-dibromopentane was added and the mix-ture was stirred for 48 h. The solvents were removed byvacuum evaporation, and water was carefully added to thereaction mixture to quench the unreacted NaH. The solu-tion was extracted with CH2Cl2 (3× 50 mL). The combinedorganic layer was washed successively with 5% aqueousH r( SOa siduew ther( ofc

2 H),5 (d,1 3,6 24.5,1 29.8,16

2

l)i hem hen,2 the

mixture was stirred for 48 h. The reaction mixture waspoured into water. The solution was extracted with EtOAc(3× 50 mL). The combined organic layer was washedsuccessively with 5% aqueous HCl (50 mL), 10% aqueousNa2CO3 (50 mL) and water (50 mL). Then, the solutionwas dried over anhydrous MgSO4, and the solvents wereevaporated in vacuum. The residue was chromatographedon silica gel with petroleum ether (60–90◦C)/EtOAc (1:2,v/v) as eluent to give 0.05 g (23%) of dyad1 as whitesolid.

Mp.: 142–144◦C; 1H NMR (300 MHz, CDCl3): 1.43 (m,2H), 1.59 (m, 2H), 1.67 (m, 2H), 1.77 (s, 3H), 3.59 (m, 4H),5.22 (s, 2H), 6.73 (s, 1H), 8.00–8.07 (m, 4H), 8.14–8.23 (m,4H), 8.38 (d, 1H);13C NMR (75.5 MHz, CDCl3): 12.7, 23.5,29.1, 29.7, 29.8, 47.9, 70.4, 71.2, 109.2, 124.4, 124.7, 124.9,125.4, 126.1, 127.1, 127.8, 127.9, 128.1, 128.2, 128.3, 129.5,131.2, 131.4, 131.6, 133.0, 142.2, 151.6, 151.7, 163.0, 165.2;MALDI-TOF:, 425.1 (M-1); anal. calcd. for C27H26N2O30.35EtOAc: C, 74.58; H, 6.35; N, 6.13. Found: C, 74.30; H,6.63; N, 6.46.

3. Results and discussions

The synthesis of dyad1 started from the reaction of 1-pyrenemethanol and 1,5-dibromopentane in two steps ass eas-i(1 a1w itatec umo untso i-t of4 ffecto fi ofH ion( singt ul-t ound3 l vari-a edt ithd aflib res-c ducet l con-d li n ofd siono

Cl (50 mL), 10% aqueous Na2CO3 (50 mL) and wate50 mL). Then, the solution was dried over anhydrous Mg4nd the solvents were evaporated in vacuum. The reas chromatographed on silica gel with petroleum e

60–90◦C)/Et2O (50:1, v/v) as eluent to give 0.34 g (45%)ompound2 as white solid.

Mp.: 83–85◦C; 1H NMR (300 MHz, CDCl3): 1.55 (m,H), 1.68 (m, 2H), 1.87 (m, 2H), 3.38 (t, 2H), 3.63 (t, 2.23 (s, 2H), 8.03–8.08 (m, 4H), 8.15–8.23 (m, 4H), 8.38H); 13C NMR (75.5 MHz, CDCl3): 23.6, 27.6, 31.1, 32.8.7, 70.2, 122.0, 123.0, 123.3, 123.5, 123.8, 123.9, 125.5, 125.6, 125.8, 125.9, 126.0, 126.2, 127.9, 129.4, 130.2; EI–MS:380/382 (M+); anal. calcd. for C22H21OBr: C,9.30; H, 5.55. Found: C, 69.24; H, 5.61.

.2. Synthesis of dyad1

To a solution of anhydrous K2CO3 (0.35 g, 2.50 mmon dry DMF was added thymine (0.13 g, 1.00 mmol). T

ixture was stirred for 0.5 h at room temperature. T(0.19 g, 0.50 mmol) was added to the solution and

hown inFig. 2. The synthesis and separation can bely performed. The solubility of dyad1 in H2O/CH3CN20:1, v/v) is small, but a 1.0× 10−5 M solution of dyad

in H2O/CH3CN (20:1, v/v) can be prepared. When.0× 10−5 M solution of dyad1 in H2O/CH3CN (20:1, v/v)as kept at room temperature for 1 week, no precipan be detected.Fig. 3 shows the fluorescence spectrf dyad 1 and those in the presence of different amof Hg(ClO4)2 in H2O/CH3CN (20:1, v/v). Before add

ion of Hg(II) ion, there is weak emission in the range25–600 nm, which may be due to the aggregation ef dyad 1 in H2O/CH3CN, leading to the formation o

ntermolecular excimer of pyrene units. After additiong(II) ion, the intensity of the pyrene excimer emiss

λmax= 470 nm) is enhanced and it is higher with increahe amounts of Hg(II) ion added to the solution, and simaneously the fluorescence intensities for the bands ar75 nm and 394 nm decrease. This fluorescence spectration of dyad1upon addition of Hg(II) ion should be ascrib

o the formation of the Hg(II) coordination compound wyad1 as shown inFig. 1. It can be ruled out that suchuorescence change of dyad1 after addition of Hg(II) ion

s due to the aggregation effect of dyad1 in H2O/CH3CNased on the following considerations: (1) control fluoence measurements with 1-pyrenemethanol did not inhe enhancement of the excimer emission under identicaitions (see the inset ofFig. 3); (2) addition of other meta

ons of the same amounts as Hg(II) ion to the solutioyad1 did not lead to the increase of the excimer emisf pyrene (see below).

12 Z. Wang et al. / Analytica Chimica Acta 549 (2005) 10–13

Fig. 2. Synthetic route of dyad1.

Variation of the fluorescence intensity ratio(I470 nm/I394 nm) between the excimer emission at 470 nmand that at 394 nm vs. the concentration of Hg(II) ion in thesolution of dyad1 (1.0× 10−5 M) is displayed in the inset ofFig. 3. Interestingly, a nearly linear plot ofI470 nm/I394 nmver-sus the concentration of Hg(II) ion is resulted (y= 13 + 112x,r = 0.99,n= 10), when the concentration of Hg(II) ion waslower than 5.0× 10−6 M. But, if the concentration of Hg(II)ion is larger than 5.0× 10−6 M, namely when the molarratio of Hg(II) ion to dyad1 is greater than 1/2, the valueof I470 nm/I394 nm stops growing and stays in a plateau. Thisresult supports the hypothesized model that each Hg(II) ionis coordinated to two molecules of dyad1 (seeFig. 1). Thedetection limit of dyad1 (1.0× 10−5 M) towards Hg(II) ionwas measured to be 0.1�M (K= 3), indicating that dyad1 isa sensitive chemosensor for Hg(II) ion.

The sensing behavior of dyad1 to Hg(II) ion was alsoinvestigated in solutions of different pH values. As an exam-ple, variation of the fluorescence intensity ratioI470 nm/I394 nm

FHt en thepv H7

for the solution of dyad1 (1.0× 10−5 M) containing Hg(II)ion (5.0× 10−6 M) versus pH values is shown inFig. 4.When the pH of the solution was lower than 5 or higher than9, I470 nm/I394 nm reflecting the enhancement of the pyreneexcimer emission, was rather small. Only when the pH wasin the range of 6–8, doesI470 nm/I394 nmbecame larger. Thisresult indicates that the sensing property of dyad1 to Hg(II)ion is maximized at pH 6–8. This property of dyad1 can beexplained as follows: when the pH of the solution is lowerthan 6, the nitrogen/oxygen atoms of thymine unit in dyad1are protonated to some extent so that the coordination abilityof dyad1 is reduced; by contrast when the pH of the solutionis larger than 8, the competitive coordination of OH− withHg(II) ion limits the corresponding coordination of dyad1with Hg(II) ion. These results are in agreement with the coor-dination model of dyad1 with Hg(II) ion as shown inFig. 1.

The fluorescence behavior of dyad1was also studied in thepresence of other metal ions under identical condition, suchas Zn2+, Pb2+, Ba2+, Mg2+, K+, Cu2+, Mn2+, Co2+, Ni2+ andFe2+. As illustrated inFig. 5, addition of these metal ionsexcept Hg(II) ion did not lead to the enhancement of the

Fav

ig. 3. Fluorescence spectra of a solution of dyad1 (1.0× 10−5 M) in

2O/CH3CN (20:1, v/v) at pH 7 with the addition of Hg(ClO4)2 (from 0.05o 0.5 equiv.); the inset shows the fluorescence intensities ratio betweyrene excimer at 470 nm and monomer emission at 394 nm (I470 nm/I394 nm)s. the concentration of Hg(ClO4)2 for dyad1 and 1-pyrenemethanol at p.

ig. 4. The profile ofI470 nm/I394 nmvs. pH for dyad1 (1.0× 10−5 M) in thebsence and presence of Hg(ClO4)2 (5.0× 10−6 M) in H2O/CH3CN (20:1,/v).

Z. Wang et al. / Analytica Chimica Acta 549 (2005) 10–13 13

Fig. 5. The fluorescence intensity ratio between the pyrene excimer at470 nm and monomer emission at 394 nm (I470 nm/I394 nm) of dyad 1(1.0× 10−5 M) in the presence of various metal cations (the concentrationof each metal ion is 5.0× 10−6 M) in H2O/CH3CN (20:1, v/v) at pH 7.

pyrene excimer emission. The interference from these metalions for the detection of Hg(II) ion with dyad1 was alsostudied in experiments when all these metals were present.As shown inFig. 6, the fluorescence spectra of dyad1 werealmost not altered in the presence of Hg(II) ion together withother metal ions compared with those only in the presenceof Hg(II) ion. This result indicates that the interference fromother metal ions can be neglected. Therefore, based on thefluorescence spectral studies of dyad1, it can be concludedthat dyad1 is a new “turn on” chemosensor for Hg(II) ionnot only showing good sensitivity but also displaying highselectivity. Preliminary studies indicated that the dyad withpyrene and uracil units showed similar fluorescence changein the presence of Hg(II) ion. But, enhancement of pyreneexcimer emission was not observed for the dyad with pyreneand adenine units upon the addition of Hg(II) ion.

In summary, by combining the high affinity of thyminetowards Hg(II) ion with the excimer emisson of pyrene units,dyad1, which can be easily synthesized in two steps, wasdesigned as a potential “turn on” chemosensor for Hg(II)ion with good sensitivity and high selectivity, which was

FaMFa

demonstrated by the fluorescent spectral studies of dyad1under different conditions. Further studies will address itsreal application to real samples.

Acknowledgements

The present research was financially supported by NSFC,Chinese Academy of Sciences and State Key Basic ResearchProgram (G2000077505). D. Zhang thanks National ScienceFund for Distinguished Young Scholars.

References

[1] G.E. McKeown-Eyssen, J. Ruedy, A. Neims, Am. J. Epidemiol. 118(1983) 470.

[2] P.W. Davidson, G.J. Myers, C. Cox, C.F. Shamlaye, D.O. Marsh,M.A. Tanner, M. Berlin, J. Sloane-Reeves, E. Cernichiari, O. Choisy,A. Choi, T.W. Clarkson, Neurotoxicology 16 (1995) 677.

[3] P. Grandjean, P. Heihe, R.F. White, F. Debes, Environ. Res. 77 (1998)165.

[4] T. Takeuchi, N. Morikawa, H. Matsumoto, Y. Shiraishi, Acta Neu-ropathol. 2 (1962) 40.

[5] H. Matsumoto, G. Koya, T. Takeuchi, J. Neuropathol. Exp. Neurol.24 (1965) 563.

[6] M. Harada, Crit. Rev. Toxicol. 25 (1995) 1.[7] M.Y. Chea, A.W. Czarnik, J. Am. Chem. Soc. 114 (1992) 9704.

em.

ik,

[[ al.

[ A.A.

[ to,

[ 69

[[ 04)

[ . Soc.

[ rad-6769.

[ om-

[ 0.[ 04)

[ 54)

[[[ .A.

[ .[

ig. 6. Fluorescence spectra of a solution containing dyad1 (1.0× 10−5 M),mixture of heavy metal ions [mix.: Zn(ClO4)2, Pb(ClO4)2, Ba(ClO4)2,g(ClO4)2, KClO4, Cu(ClO4)2, Mn(ClO4)2, Co(ClO4)2, Ni(ClO4)2,e(ClO4)2 (the concentration of each ion is 1.0× 10−5 M)] and differentmounts of Hg(ClO4)2 in H2O/CH3CN (20:1, v/v) at pH 7.

[8] G. Zhang, D. Zhang, S. Yin, X. Yang, Z. Shuai, D. Zhu, ChCommun. (2005) 2161.

[9] J. Yoon, N.E. Ohler, D.H. Vance, W.D. Aumiller, A.W. CzarnTetrahedron Lett. 38 (1997) 3845.

10] D.Y. Sasaki, B.E. Padilla, Chem. Commun. (1998) 1581.11] X.-B. Zhang, C.-C. Guo, Z.-Z. Li, G.-L. Shen, R.-Q. Yu, An

Chem. 74 (2002) 821.12] M.C. Aragoni, M. Arca, F. Demartin, F.A. Devillanova, F. Isaia,

Garau, V. Lippolis, F. Jalali, U. Papke, M. Shamsipur, L. Tei,Yari, G. Verani, Inorg. Chem. 41 (2002) 6623.

13] A.B. Descalzo, R. Martınez-Manez, R. Radeglia, K. Rurack, J. SaJ. Am. Chem. Soc. 125 (2003) 3418.

14] S.Y. Moon, N.R. Cha, Y.H. Kim, S.-K. Chang, J. Org. Chem.(2004) 181.

15] R. Metivier, I. Leray, B. Valeur, Chem. Eur. J. 10 (2004) 4480.16] J.H. Kim, A.-R. Hwang, S.-K. Chang, Tetrahedron Lett. 45 (20

7557, and references therein.17] G. Hennrich, H. Sonnenschein, U. Resch-Gengar, J. Am. Chem

121 (1999) 5073.18] L. Prodi, C. Bargossi, M. Montalti, N. Zaccheroni, N. Su, J.S. B

shaw, R.M. Izatt, P.B. Savage, J. Am. Chem. Soc. 122 (2000)19] K. Rurack, U. Resch-Genger, J.L. Bricks, M. Spieles, Chem. C

mun. (2000) 2103.20] E.M. Nolan, S.J. Lippard, J. Am. Chem. Soc. 125 (2003) 142721] X.F. Guo, X.H. Qian, L.H. Jia, J. Am. Chem. Soc. 126 (20

2272.22] Th. Forster, K. Kasper, Z. Phys. Chem. (Frankfurt am Main) 1 (19

275.23] Th. Forster, Angew. Chem. 8 (1969) 364.24] Th. Forster, Angew. Chem. 8 (1969) 333.25] H. Irngartinger, R.G.H. Kirrstetter, C. Krieger, H. Rodewald, H

Staab, Tetrahedron Lett. (1977) 1425.26] H.A. Staab, R.G.H. Kirrstetter, Liebigs Ann. Chem. (1979) 88627] A. Ono, H. Togashi, Angew. Chem. Int. Ed. 43 (2004) 4300.