Inorganic Chemistry Communications 22 (2012) 178–181

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Inorganic Chemistry Communications

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A highly selective and ratiometric sensor for Hg2+ based on a phosphorescentiridium (III) complex

Fang Yan a, Qunbo Mei a,⁎, Lianhui Wang a, Bihai Tong b, Zhijie Xu a, Jiena Weng a,Lingxia Wang a, Wei Huang a,⁎⁎a Key Laboratory for Organic Electronics and Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications,Nanjing 210046, Chinab College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, China

⁎ Corresponding author. Fax: +86 25 8586 6396.⁎⁎ Corresponding author.

E-mail addresses: iamqbmei@njupt.edu.cn (Q. Mei),(W. Huang).

1387-7003/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.inoche.2012.05.056

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 May 2012Accepted 31 May 2012Available online 9 June 2012

Keywords:Iridium (III) complex2-thiophen-2-yl-benzothiazoleHg2+

Phosphorescent chemosensor

A new neutral iridium (III) complex Ir(TBT)2(acac) based on 2-thiophen-2-yl-benzothiazole (TBTH) ligandshas been synthesized and characterized. Ir(TBT)2(acac) exhibited color change from red to orange withoutand with Hg2+, at the same time has a blue shift about 23 nm in fluorescent emission spectra. So it canserve as a highly selective chemodosimeter for Hg2+ with ratiometric and naked-eye detection. The sensorallowed determination of Hg2+ in the working range of 3 μM–10 μM with a detection limit of 2.14 μM. Thephosphorescent chemosensor exhibited excellent selectivity and sensitivity for Hg2+ detection.

© 2012 Elsevier B.V. All rights reserved.

Mercury is one of the most dangerous metal elements to humansand the environment because inorganic mercury can be easily trans-formed into methylmercury which can easily enter the food chain andaccumulate in the upper level of the food chain [1–3]. Ingestion ofmeth-ylmercury can seriously affects the sensory, motor, and cognitive disor-ders [4]. Hence, the detection of mercury is very important. For the pastdecades, chromogenic [5], fluorescent [6] and electrochemical [7]chemosensors have been reported for detecting Hg2+. However, thereare very fewmultisignaling chemosensors for Hg2+ [8]. As a result, con-siderable efforts have been devoted to design new multisignalingchemosensors for Hg2+ detection. Luminescent transition metal com-plexes have been successfully applied in the field of chemosensors be-cause of the photophysical properties of transition metal complexes,such as emission wavelength shifts, significant Stokes shifts and rela-tively long lifetimes.While iridium (III) complexes are extremely usefulphosphorescent dyes, because of their relatively short excited state life-time, high photoluminescence efficiency and excellent color tuningfrom blue to the near-infrared region [9–11]. Currently, iridium (III)complexes have been used as chemosensors for detecting Pd2+, Ca2+,Ag+, etc. However, there were few reports of Hg2+-chemosensorbased on iridium (III) complexes [12–14]. In this article, a novel selec-tive phosphorescent Hg2+ chemosensor, Ir(TBT)2(acac) (TBT=2-

iamwhuang@njupt.edu.cn

rights reserved.

thiophen-2-yl-benzothiazole) containing four sulfur atomswas synthe-sized and the initial results including the photophysical properties inthe absence and presence of Hg2+ were reported.

The neutral iridium (III) complex Ir(TBT)2(acac) was obtainedaccording to previous literature method [15]. As shown in Scheme 1,TBT was obtained conveniently from 2-amino-benzenethiol andthiophene-2-carbaldehyde by the ring-closure reaction andIr(TBT)2(acac) was synthesized in 41.8% yield. The structure of Ir(-TBT)2(acac) was confirmed by NMR and ESI mass spectroscopy. Thedetail descriptions of these characterizations for Ir(TBT)2(acac) areavailable in supporting information.

The UV–vis absorption spectra of Ir(TBT)2(acac) in dichloromethanesolution at room temperature was shown in Fig. 1. The intense absorp-tion bands in the region of 250–350 nm are owing to spin-allowedinterligand (π→π*) transitions. The weak absorption in the region of350–550 nm are assigned to the mixed singlet and triplet metal-to-ligand charge-transfer (MLCT) states with the previous photophysicalstudies of related iridium (III) complexes. Selectivity is the very impor-tant parameters of a probe. As shown in Fig. 1, the chemosensor behav-ior in the presence of 2 equiv. of different metal ions (Ag+, Cd2+, Co2+,Cr3+, Cu2+, Fe3+, Hg2+, K+,Mg2+, Na+, Ni2+, Pb2+, and Zn2+) indicat-ed that only Hg2+ promoted a notable response in its absorption spec-tra. Similar fluorescent change was also observed with Hg2+ in thepresence of other ions in this study (Fig. S7).

Upon addition of Hg2+ to the solution of Ir(TBT)2(acac), the absorp-tion band at 480 nm disappears progressively, while the absorptionband at 405 nm gradually increased. The color of the solution changedfrom orange to yellow (inset of Fig. 2), indicating that Ir(TBT)2(acac)

2 2

2 2acacNa3 2

NH2

SH

+S CHO

N

S S

H2O/2-ethoxyethanol C H C l

S

S N

4

Ir Cl

S

S N

2

O

O

Ir

DMF

120

IrC l .3 H O

Ir(TBT)2(acac)

TBT

Scheme 1. Synthesis of phosphorescent sensor Ir(TBT)2(acac).

179F. Yan et al. / Inorganic Chemistry Communications 22 (2012) 178–181

can serve as a sensitive “naked-eye” indicator for Hg2+. The stoichiom-etry of Ir(TBT)2(acac) is given by the variation of A405 nm/A480 nm withrespect to equivalents of Hg2+ added. Effectively, A405 nm/A480 nm de-creases continuously until the addition of 0.5 equiv. of Hg2+; further ad-dition of Hg2+ induces only very minor changes in A405 nm/A480 nm

(inset of Fig. 2). Generally, luminescent emission spectroscopy is moresensitive toward small changes that affect the electronic properties ofmolecular receptors [16]. As shown in Fig. 3, Ir(TBT)2(acac) in dic-hloromethane solution showed an intense emission band at 608 nm.Upon gradual addition of Hg2+ (0–0.7 equiv.) to the solution ofIr(TBT)2(acac) in dichloromethane (c=2×10−5 M), the intensity ofthe emission band centered at 608 nm decreased gradually and a newband at 585 nm appeared and increased. As a result, an obvious changein fluorescent color from red to orangewas observed (inset of Fig. 3). Asdetermined by variations in the fluorescent response of Ir(TBT)2(acac)in the presence of various amounts of Hg2+, the stoichiometry of thecomplex formed between Ir(TBT)2(acac) and Hg2+ is 2:1 (inset ofFig. 3), which is consistent with the result obtained from the UV–vis ab-sorption spectra. According to the literature [17], the detection limitwas also estimated from the titration results and was 2.14×10−6 M.

300 400 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Abs

orba

nce

Wavelength nm

Ag+, Cd2+,Co2+,Cr3+,Cu2+,Fe3+,K+,

Mg2+,Na+, Ni2+,Pb2+,Zn2+ and blank

Hg2+

331 nm

405 nm

480 nm

Fig. 1. UV–vis spectra of Ir(TBT)2(acac) in the presence of 2 equiv. of different metal ions.

To explore practical applicability of Ir(TBT)2(acac) as a Hg2+ selec-tive sensor, a competition experiment was done. As shown in Fig. 4,under the same condition slight fluorescent changes of Ir(TBT)2(acac)were observed in the presence of 1 equiv. of different metal ions(Ag+, Cd2+, Co2+, Cr3+, Fe3+, K+, Mg2+, Na+, Ni2+, Pb2+, and Zn2+).We investigated the selective response of Ir(TBT)2(acac) to variousmetal ions by using the ratio of the phosphorescent emission intensityat 608 and 585 nm (I608 nm/I585 nm) as the output signal. Only additionof Hg2+ resulted in a prominent fluorescent change, whereas additionof 1 equiv. of other competitive cations caused only slight fluorescentchanges. Moreover, a competition experiment was done to further ex-plore the utility of Ir(TBT)2(acac) as an ion-selective probe for Hg2+.The coexistent metal cations had a negligible interfering effect on thefluorescent blue shift of Ir(TBT)2(acac) upon addition of Hg2+. As a re-sult, Ir(TBT)2(acac) displayed a high selectivity in sensing Hg2+.

Herein, to seek the detailed information on the bindingmechanism,1H NMR spectra titration experiments of complex Ir(TBT)2(acac) withdifferent equivalents of Hg2+ were conducted. Upon addition of Hg2+,a variation in the

1H NMR spectra of Ir(TBT)2(acac) especially in the

cyclometalated ligand TBT moiety was observed. However, no1H NMR

250 300 350 400 450 500 5500.0

0.1

0.2

0.3

0.4

0.5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0.0

0.2

0.4

0.6

0.8

1.0

A40

5 nm

/A48

0 nm

[Hg2+]/[Ir(TBT)2(acac)]

Abs

orba

nce

Wavelength nm

0 eq.

0.7 eq.

Fig. 2. Changes in the UV absorption spectra of Ir(TBT)2(acac) on addition of Hg2+.

Blank Ag+ Cd2+ Co2+ Cr3+ Cu2+ Fe3+ Hg2+ K+ Mg2+ Na+ Ni2+ Pb2+ Zn2+

1

2

3

4

5

I 608

nm

/I58

5 nm

Metal ions

Ir(TBT)2(acac)+Mn++Hg2+

Ir(TBT)2(acac)+Mn+

Fig. 4. Fluorescence intensity ratio changes (I608 nm/I585 nm; λex=340 nm) after addi-tion of 1 equiv. of Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mg2+, Na+, Ni2+,Pb2+ and Zn2+, respectively.

Ir(TBT)2(acac)

Ir(TBT)2(acac)-0.3 eq. Hg2+

Ir(TBT)2(acac)-0.6 eq. Hg2+

TBT-2.0 eq Hg2+

TBT

Fig. 5. Partial1H NMR spectral changes of Ir(TBT)2(a

550 600 650 7000

50

100

150

200

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

1

2

3

4

5

6

I 608

nm/I

585

nm

[Hg2+]/[Ir(TBT)2(acac)]

Inte

nsit

y

Wavelength nm

0 eq.

0.7 eq.

Fig. 3. Changes in the emission spectra of Ir(TBT)2(acac) on addition of Hg2+.

180 F. Yan et al. / Inorganic Chemistry Communications 22 (2012) 178–181

characteristic peaks correspond to free TBTH appeared upon addition ofvarious amounts of Hg2+ (Fig. 5), indicating that the ligand TBT stillbound with iridium (III). Interestingly, the characteristic peaks ofHacac at 5.18 ppm disappeared for Ir(TBT)2(acac) in the presence of0.5 equiv. of Hg2+ (Fig. 5), suggesting that the ancillary ligand acacwas removed from complex Ir(TBT)2(acac). Furthermore, the UV–visand fluorescent emission spectra showed that there was no significantdifferent after the addition of 0.5 equiv. of Hg2+ dissolved in THF andCH3CN solution to the dichloromethane solution of Ir(TBT)2(acac)(Fig. S11–S12). It indicated that the complex Ir(TBT)2(acac) with Hg2+

was not formed [Ir(TBT)2(MeCN)2]+ or [Ir(TBT)2(MeCN)2Hg]+. Addi-tionally, we also conducted aMALDI-TOF analysis to confirm the sensingmechanism. When 0.5 equiv of Hg2+ was added to the Ir(TBT)2(acac)solution, two new peak appeared at 626.52 and 861.02 (m/z),which correspond to [Ir(TBT)2]+ and [Ir(TBT)2(H2O)2Hg]+, respec-tively. Therefore, we tentatively summarize the sensing mechanism ofIr(TBT)2(acac) with Hg2+ as follows (Scheme 2): the fast decomposi-tion of Ir(TBT)2(acac) with departure of acac from the complex toform two new complexes [Ir(TBT)2]+ and [Ir(TBT)2(H2O)2Hg]+. Sucha process results in the evident changes of absorption and emissionproperties.

In summary, we synthesized a novel neutral iridium (III) complexIr(TBT)2(acac) with two sulfur atoms, which was a phosphorescentchemosensor with high sensitivity and selectivity for detecting ofHg2+. This chemosensor was easily prepared and found to be possi-ble to detect the Hg2+ ratiometrically. Moreover, the dramatic colorchange of the solution made the detection of Hg2+ possible by thenaked-eye.

1H NMR titration and MALDI-TOF experiments were car-

ried to explain the detection mechanism. The interaction betweenHg2+ and the sulfur atom of the cyclometalated ligand led to a signifi-cant change of the optical signals. The great efforts include furthermechanism studies, structural optimization and bioimaging application.

Acknowledgments

This work was financially supported by the National Basic Re-search Program of China (973 Program, 2009CB930601), the NationalNatural Science Foundation of China (Project no. 50803027, no. 50903001, no. 20905038), and the Natural Science Fund for Colleges and Uni-versities in Jiangsu Province (Grant no. 08KJD430020).

Appendix A. Supplementary material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.05.056.

cac) and TBT in CDCl3 in the presence of Hg2+.

Scheme 2. The possible binding mechanism of Ir(TBT)2(acac) with Hg2+.

181F. Yan et al. / Inorganic Chemistry Communications 22 (2012) 178–181

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