Rhodamine B-based “turn-on” fluorescent and colorimetric chemosensors for highly sensitive and selective detection of mercury (II) ions

Journal of Luminescence 132 (2012) 35–40

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Rhodamine B-based ‘‘turn-on’’ fluorescent and colorimetric chemosensors for highly sensitive and selective detection of mercury (II) ions Nantanit Wanichacheva a,n, Krit Setthakarn a, Narupon Prapawattanapol a, Oranual Hanmeng a, Vannajan Sanghiran Lee b,c, Kate Grudpan c a

Department of Chemistry, Faculty of Science, Silpakorn University, Nakorn Pathom 73000, Thailand Thailand Center of Excellence in Physics, Commission on Higher Education, Bangkok 10400, Thailand c Department of Chemistry, Center for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b

a r t i c l e i n f o

abstract

Article history: Received 23 May 2011 Accepted 7 July 2011 Available online 18 July 2011

Two novel macromolecules based on 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine covalently bound to one and two units of rhodamine-B moieties, 1 and 2, were prepared and utilized as fluoroionophores and chromophores for the optical detection of Hg2 þ ions. The sensors were readily prepared by a conventional two-step synthesis. Especially, sensor 1 exhibits high sensitivity and selective OFF–ON fluorescence enhancement and chromogenic change upon binding to Hg2 þ , which served as a ‘‘naked-eye’’ indicator by a noticeable color change of the solution (from colorless to pink– red color). 1 is shown to discriminate various competing metal ions, particularly Ag þ and Cu2 þ , as well as Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ , with a detection limit of 10 ppb. & 2011 Elsevier B.V. All rights reserved.

Keywords: Mercury sensor Fluoroionophore Hg2 þ -selectivity Rhodamine-B probe

1. Introduction The design and synthesis of fluorescent chemosensors or fluoroionophores have received much attention because they allow nondestructive, rapid, highly selective and sensitive detections of metal ions. Mercury is one of the most highly toxic and hazardous contaminants in the environment and biota with recognized accumulative and persistent characters [1–3]. Mercury can cause serious human health problems since it can easily pass through the skin, respiratory and cell membrane, resulting in permanent damages of the central nervous and endocrine systems [4]. Accordingly, continuous monitoring and simple, rapid and prompt detections or even ‘‘naked-eye’’ detection for Hg2 þ ions on-site are of great importance. Fluorescent chemosensors for Hg2 þ have been extensively explored, including cyclen [5], hydroxyquinoline [6], azine [7], cyclams [8–11], diazatetrathia crown ethers [12] and calixarene [13]. However, many of these sensors have some limitations in terms of synthetic difficulty, high cost of starting materials, high detection limits or lack of selectivity towards potential competitors such as copper (Cu2 þ ) and silver (Ag þ ) due to their close chemical behaviors to Hg2 þ [6–16]. In addition, most of the reported Hg2 þ fluorescent chemosensors demonstrated a fluorescent quenching

n

Corresponding author. Tel.: þ66 34 255 797; fax: þ66 34 271 356. E-mail addresses: [email protected], [email protected] (N. Wanichacheva). 0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.07.015

‘‘turn-off’’ mechanism due to the quenching characteristic of Hg2þ ions. However, there have been limited reports of fluorescent enhancement ‘‘turn-on’’ Hg2þ -sensors [5,17,18]. In this study, the major motivation of our work is the design and synthesis of Hg2 þ sensors with high sensitivity and selectivity but with a significantly reduced synthetic effort using inexpensive starting materials. In addition, we also focus on fluorescent enhancement ‘‘turn-on’’ Hg2 þ -sensors and ‘‘nakedeye’’ indicators due to the possible potential uses. We have focused on utilizing the readily accessible synthetic ligand, 2-[3-(2-aminoethyl sulfanyl)propylsulfanyl]ethanamine, which provided appropriately located sulfur and nitrogen atoms as the donor atoms that can self assemble around the Hg2 þ ions due to the favorable electrostatic interactions [19,20]. In this study, we have also focused on the effect of utilizing a rhodamine B fluorophore aiming to increase the sensitivity of the sensor system due to its large molar extinction coefficient, high fluorescence quantum yield, long absorption and emission wavelengths ( 4500 nm) in the visible regions. Rhodamine B derivatives have been used as fluorescent chemosensors for mercury ions detections, however, many of them have drawbacks in terms of selectivity, sensitivity or high detection limits [21–25]. The utilization of rhodamine B spirolactam (non-fluorescent) to the ring opened amide (fluorescent) process upon ion binding could provide valuable information for both fluorescent enhancement and colorimetric change. In this work, we now report two new sensors based on 2-[3-(2aminoethylsulfanyl) propylsulfanyl]ethanamine ligand covalently

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2.4. Synthesis: compound 1

bound to one and two units of rhodamine B moieties, 1 and 2. The sensors were prepared by a conventional two-step synthesis using inexpensive starting materials. Especially, sensor 1 shows highly sensitive and selective fluorescence ‘‘turn-on’’ behaviors toward Hg2 þ in solutions as well as the remarkable colorimetric changes from colorless to pink, which can be easily noticeable. The synthetic simplicity of the sensor, the high degree of sensitivity, selectivity and dual signaling behaviors of the sensor to mercury ions make it attractive as a tagging material and commercial Hg2 þ indicator, such as paper test strips, that can be visualized by the naked eye.

The synthesis of compound 1 was performed according to the synthetic steps outlined in Scheme 1. In a round bottom flask, rhodamine B hydrochloride (0.20 g, 0.42 mmol) was added to a solution of 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine (0.20 g, 1.05 mmol) in 6.0 mL dry ethanol. The solution was then stirred where upon dry triethylamine (0.75 mL, 12.8 mmol) was added. This solution was then refluxed for 30 h under an argon atmosphere. The mixture was filtered and the solvent was removed under vacuum. Dichloromethane (20 mL) was added to the residue and the solution was extracted three times each with 20 mL of deionized water. The organic phase was collected and dried over anhydrous Na2SO4. The solvent was removed under vacuum. The crude product was purified by preparative thin layer chromatography using CH2Cl2:MeOH:NEt3 ¼90:10:0.5 (Rf ¼ 0.68) to give 95.6 mg of a pale pink oil, 37%. 1H NMR (300 MHz, CDCl3): d 1.19 (t, J¼6.9 Hz, 12 H), 1.73–1.78 (m, 2 H), 2.22–2.26 (m, 2 H), 2.35 (s, 2 H, NH2), 2.46–2.56 (m, 4 H), 2.65 (t, J¼6.6 Hz, 2 H), 2.92 (t, J¼6.6 Hz, 2 H), 3.25–3.37 (m, 10 H, CH2–N), 6.27 (dd, J¼ 8.7, 2.7 Hz, 2 H), 6.37–6.44 (m, 4 H), 7.09–7.10 (m, 1 H), 7.42–7.45 (m, 2 H), 7.85–7.95 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): d 11.6, 28.1, 28.3, 29.2, 29.3, 33.9, 39.1, 39.8, 43.3, 63.8, 96.6, 104.3, 107.1, 121.7, 122.7, 127.1, 127.8, 130.1, 131.4, 147.8, 152.3, 166.8 ppm. HRMS (ESI) calcd for C35H46N4O2S2 (MþH) þ 618.3062, found 619.3242.

2. Experimental 2.1. Materials All reagents and solvents were purchased from Fluka Chemical Corporation and were used as received. All the metal salts used in this study were perchlorate salts and were purchased from Strem chemicals, Inc. 2.2. Methods NMR spectra were obtained with a Bruker Avance 300 spectrometer operating at 300 MHz for 1H and 75 MHz for 13C. All NMR spectra were obtained in CDCl3 solutions with TMS as the internal standard. Mass spectra were performed by a ThermoElectron LCQDECA-XP, electrospray ionization ion trap mass spectrometer. IRspectra were measured on a Perkin Elmer Spectrum GX FT-IR System in the range of 400–4000 cm  1. Fluorescence measurements were performed on a Perkin Elmer Luminescence spectrometer LB50. The excitation and emission slit widths were 5.0 nm. Absorption spectra were determined on a single beam Hewlett Packard 8453 spectrophotometer. Molecular modeling was performed with the Material Studio 4.4 program package.

2.5. Synthesis: compound 2 In a similar manner, compound 2 was synthesized in 10% yield as a pale pink oil after preparative thin layer chromatography using CH2Cl2:MeOH¼90:10 (Rf ¼0.83). 1H NMR (300 MHz, CDCl3): d 1.15 (t, J¼ 7.2 Hz, 24 H), 1.52–1.59 (m, 2 H), 2.17–2.22 (m, 4 H), 2.35 (t, J¼7.2 Hz, 4 H), 3.22–3.36 (m, 20 H), 6.25 (d, J¼7.5 Hz, 4 H), 6.41 (t, J¼8.7, 8 H), 7.06–7.09 (m, 2 H), 7.40–7.46 (m, 4 H), 7.87– 7.90 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): d 11.6, 28.0, 28.6, 29.4, 39.2, 43.3, 44.7, 63.7, 96.7, 104.4, 107.1, 121.8, 122.8, 127.1, 127.9, 131.4, 132.4, 147.8, 152.3, 166.7 ppm. HRMS (ESI) calcd for C63H74N6O4S2 (MþH) þ 1042.5213, found 1043.5440.

2.3. Synthesis: 2-[3-(2-aminoethylsulfanyl)propylsulfanyl] ethanamine

2.6. Binding studies The synthesis of the title compound was performed in the same manner as described previously [20] and the synthetic steps are outlined in Scheme 1. Cl H3 N

SH

Br

Br

The binding studies of compounds 1 and 2 were carried out in dichloromethane. The perchlorate salts solutions (1.0  10  2 M)

NaOMe/MeOH 10h, 40 o C

H2 N

S

quantitative yield

NH2

S O

O

S

N

S

NH2

O N N

O

N

O

N

1 O

NEt 3 / EtOH reflux

O S

N

S

N N

N O

O N

N

2 Scheme 1. Synthesis of 1 and 2.

N. Wanichacheva et al. / Journal of Luminescence 132 (2012) 35–40

3.1. Synthesis and molecular design of 1 and 2 The synthesis of 1 and 2 was performed according to the synthetic steps outlined in Scheme 1. 2-[3-(2-Aminoethylsulfanyl)propylsulfanyl]ethanamine was prepared by alkylation of cysteamine hydrochloride with 1,3-dibromopropane. Then, compounds 1 and 2 were obtained by reaction of rhodamine B hydrochloride with 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine. Compounds 1 and 2 are podants, acyclic hosts with pendant binding sites [26], containing two sulfur atoms and two nitrogen atoms for the binding sites, which are covalently bound to rhodamine B subunit(s). We expect that the selective binding can take place through favorable electrostatic interactions between the carbonyl oxygen, sulfur and nitrogen atoms of the sensors and Hg2 þ ions. 3.2. Sensitivity studies In this study, the sensing properties of fluoroionophores 1 and 2 were investigated by UV–visible and fluorescence measurements in several solvent systems. It was found that 1 and 2 provided colorless and non-fluorescent solutions in both common organic solvents and aqueous–miscible organic solvent systems such as 5% DMSO/water, methanol, acetonitrile and dichloromethane. The addition of mercury ions to the solutions of 1 and 2 resulted in pink color and strong fluorescence signals. Fig. 1 shows detailed absorption of 1 upon gradual titration of Hg2 þ ions. The addition of a small amount of Hg2 þ ions elevated a new absorption band at approximately 550 nm and induced a visual color change from colorless to pink, which could be easily detected by the naked eye. After a preliminary survey with various solvent systems, it was found that fluoroionophores 1 and 2 provided good sensitivity to Hg2 þ in DMSO/water, methanol and acetonitrile [supplementary data] and offered high sensitivity in dichloromethane. Based on this observation, we therefore focused on the fluorescence behaviors of 1 and 2 in response to various metal ions in dichloromethane. To elucidate the quantitative binding affinity of 1 and 2, fluorescence titrations of 1 and 2 with Hg2 þ ions were carried out. Figs. 2 and 3 show the fluorescence spectra of 1 and 2, respectively, in the presence and absence of different concentrations of Hg2 þ , which exhibited fluorescence emission maximum at 575 nm when excited at 550 nm.

Absorbance (a.u.)

0.06 0.05

Hg2+

g

0.04 0.03 0.02 a

0.01 0 480

520

560

600

Wavelength (nm) Fig. 1. Absorption spectra of 1 (1.6 mM) in dichloromethane as a function of [Hg2 þ ]. a: 0 mM, b: 0.5 mM, c: 0.7 mM, d: 1.1 mM, e: 1.5 mM, f: 1.8 mM, g: 2.0 mM.

Fluorescence Intensity (a.u.)

3. Results and discussion

1000 800

j

600 400

a

200 0 560

no ions

580

600 Wavelength (nm)

620

640

Fig. 2. Fluorescence emission spectra (lex ¼550 nm) of 1 (1.6 mM) in dichloromethane as a function of [Hg2 þ ]. a: 0 mM, b: 0.5 mM, c: 0.7 mM, d: 1.1 mM, e: 1.5 mM, f: 1.8 mM, g: 2.0 mM, h: 2.1 mM, i: 2.2 mM, j: 2.3 mM.

1000 Fluorescence Intensity (a.u.)

were prepared by dissolving the desired amount of perchlorate salts in tetrahydrofuran. The fluorescence titration was performed with solutions of 1 and 2 (1.6  10  6 M) and measured as a function of metal ions concentration over a fixed wavelength range (560–640 nm) with the excitation wavelength (lex)¼550 nm for compounds 1 and 2.

37

800

g

600 400

a no ions

200 0 560

580

600 Wavelength (nm)

620

640

Fig. 3. Fluorescence emission spectra (lex ¼550 nm) of 2 (1.6 mM) in dichloromethane as a function of [Hg2 þ ]. a: 0 mM, b: 0.5 mM, c: 0.8 mM, d: 0.9 mM, e: 1.1 mM, f: 1.2 mM, g: 1.3 mM.

When an ion-complexation was operative, the fluorescence behavior of 1 clearly demonstrated the OFF–ON switching mechanism that occured in response to Hg2 þ ion complexation, as illustrated in Fig. 2. In the absence of Hg2 þ ions, the fluorescence response was at a minimum and the fluorescence ‘‘turn-on’’ as the Hg2 þ concentration was increased. When the added mercury perchlorate attained a concentration 1.4 times higher than that of 1, the fluorescence response evoked approximately 4330 folds fluorescence turn-on response. The fluorescence quantum yield (ff) of 1 with 7 equivalents of Hg2 þ was determined to be 0.37, using rhodamine B standard with a ff of 0.69 in ethanol as a reference [5]. The association constant, Kassoc, was determined by Benesi–Hildebrand plot of the signal changes in the fluorescence titration results [5,17,18]. It was found to be 3.84  1011 M  2 and the 1:2 complex formation of 1-Hg2 þ was suggested. The 1:2 complex formation was consistent with Job’s plot analysis and molecular modeling experiments. The detection limit of 1 as a fluorescent sensor for the analysis of Hg2 þ was determined from the plot of the fluorescence intensity as a function of Hg2 þ concentrations [27]. It was found that 1 had a detection limit of 5  10  8 M or 10 ppb for Hg2 þ ions, which is a lower value or in the same range compared with the recently reported rhodamine B-based Hg2 þ sensors [21–24,28,29] and is sufficient for the detection of sub-micromolar concentrations of Hg2 þ ions found in many environmental systems such as edible fish [30]. In addition, the sensors offered the change in fluorescence signals

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in the visible regions, which can be employed to fabricate an economical Hg2 þ testing tool. In a similar study, the fluorescence titrations of 2 with Hg2 þ were carried out and 2 acted as an OFF–ON fluorescence switch upon Hg2 þ binding as illustrated in Fig. 3. The sensor showed a high Hg2 þ -sensitivity and the emission intensity of 2 was effectively enhanced upon the addition of Hg2 þ ions. However, compound 2 was found to be a slightly inferior fluoroionophore as compared to compound 1 in terms of sensitivity. It was found that 2 provided a detection limit of 7.4  10–8 mM or 15 ppb for Hg2 þ ions. The fluorescence quantum yield (ff) of 2 with 7 equivalents. of Hg2 þ was found to be 0.48, based on rhodamine B standard [5].

interactions of the sulfur and nitrogen atoms with the second Hg2 þ (ion–dipole interactions) to form a helical wrapping structure. The lowest complexation energy of the 1:2 complex formation of 1:Hg2 þ ( 202.64 kcal mol  1) was a good indication of the stability of this complex. The distances to indicate the binding sites of Hg2 þ bound to compound 1 are illustrated in Fig. 5. From the optimization using DFT, the first Hg2 þ was coordinated between the carbonyl oxygen and sulfur atoms with the distances of 2.50 and 2.97 A˚ while the second Hg2 þ was bound with the ˚ sulfur and nitrogen atoms with the distances of 2.90 and 3.70 A, respectively.

3.3. Binding modes of the sensors The photophysical properties, IR data and molecular modeling results revealed that the binding of the sensors and Hg2 þ ions took place through electrostatic interactions between the carbonyl oxygen, sulfur and nitrogen atoms of the sensor and Hg2 þ . The selective binding resulted in the change in the structures from the spirolactams (non-fluorescent forms) of 1 and 2 to the non-cyclic forms (fluorescent forms) as indicated by the OFF–ON fluorescence signal upon Hg2 þ binding. The operation of the sensor is proposed and shown in Scheme 2. The large fluorescence enhancement could be attributed to the spirolactam ring opening, which was induced by the complexation of Hg2 þ ions. When the sensors coordinated with Hg2 þ ions, the fluoroionophore structure contained more conjugated double bonds from the opening form of the spirolactam ring, which resulted in an increase of the fluorescence intensity. IR titration results clearly supported the proposed ring-opening mechanism. As demonstrated in Fig. 4(a), the carbonyl absorption of 1 at 1688 cm  1 shifted to a lower frequency at 1647 cm  1 upon the addition of Hg2 þ , indicating that the carbonyl oxygen coordinated with Hg2 þ resulting in the spirocycle ring opening. A similar behavior was observed for 2, the addition of Hg2 þ led to a shift of the carbonyl absorption at 1692 cm  1 to a lower frequency at 1651 cm  1, as shown in Fig. 4(b). To clarify the coordination geometry of 1 and Hg2 þ upon binding, the molecular modeling was performed using the Material Studio 4.4 program package. The initial structure of compound 1 was modified from the crystal structure of rhodamine B in the protein databank PDB ID ¼3BR5 and optimized using density functional theory with local density approximation (LDA) of local functional PWC [31]. The complexation energy of the host–guest structure was calculated from the energy of complex – energy of rhodamine – energy of Hg2 þ using density functional theory with local density approximation (LDA) of local functional PWC with implicit distance-dependent dielectrics. The final structure of the host–guest complex is shown in Fig. 5 indicating that ions-recognition of the sensor originated from a self assembly process of compound 1 and 2Hg2 þ from the nucleophilic coordination of carbonyl oxygen to the first Hg2 þ (led to spirocycle opening) and the favorable electrostatic

Fig. 4. IR spectra of 1(a) and 2(b) in the absence of Hg2 þ (i) and in the presence of Hg2 þ (ii).

Fig. 5. Optimized structure of 1:2 1-Hg2 þ complex from molecular dynamic with local density approximation (LDA) of local functional PWC.

O N

N

O

S

N

spirolactam (non-fluorescent form)

S

NH2

O

Hg 2+

N

S

Hg 2+ S

NH2

Hg2+ N

O

N

non-cyclic form (fluorescent form)

Scheme 2. Proposed operation of sensor 1: before binding to Hg2 þ (left) and after binding to Hg2 þ (right).

N. Wanichacheva et al. / Journal of Luminescence 132 (2012) 35–40

3.4. Selectivity studies Selectivity studies of 1 and 2 were performed by recording the fluorescence spectra of the sensors solutions after the addition of each representative metal ion. In the present study, the sensitivity of the sensors was performed by a similar method to the Separate Solution Method (SSM) used in ion-selective electrode applications [32]. This method involved the measurement of a series of separate solutions and each solution contained only a salt of the determined ion. Fig. 6 shows the dependence of the fluorescence intensity of 1 as a function of cation concentrations for Hg2 þ , transition-metal, heavy metal, alkali earth and alkali ions, including Ag þ , Cu2 þ , Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ . The values in the plot were normalized to the fluorescence intensity (575 nm) in the absence of any cations. The selectivity studies clearly demonstrated the high selectivity of 1 for Hg2 þ in comparison with other foreign cations such as Ag þ , Cu2 þ , Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ . The results showed that the fluorescence intensity increased as a function of added Hg2 þ ions. In contrast, the fluorescence responses of the sensor did not cause any significant changes after the addition of foreign ions under identical conditions. It should be noted here that 1 showed very high selectivity for Hg2 þ over Ag þ , Cu2 þ and Cd2 þ , which were potential competitors [7–12,14]. The selectivity of 1 presented here was expected and was due to the favorable electrostatic interactions of Hg2 þ to the sensor. The appropriate locations of the S and N donor atoms of the 2-[3-(2-aminoethylsulfanyl)propylsulfanyl]ethanamine ligand, and the O donor atom of the rhodamine-B moiety to Hg2 þ ions could provide the cation dipole interaction, causing the selective self-assembly of the sensor molecule around the Hg2 þ ions. Sensor 1 provided a selective complexation to Hg2 þ and discriminates other representative metal ions. The selective binding was indicated not only by fluorescence enhancement but also by chromogenic change, as demonstrated in Fig. 7.

Normalized Fluorescence Intensity

70

Hg(II) Cd(II) Na(I) Li(I) K(I) Ag(I) Ba(II) Co(II) Cu(II) Ni(II) Mg(II) Mn(II) Al(III)

60 50 40 30 20 10 0 0

0.5 1 Ion Concentration (µM)

1.5

Fig. 6. Normalized fluorescence intensity (575 nm) of 1 (1.6 mM) versus the concentration of various metal ions Hg2 þ , Ag þ , Cu2 þ , Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ in dichloromethane.

Fig. 7. Chromogenic change of sensor 1 (1.6 mM) in the absence and presence of Hg2 þ (2.3 mM), Ag þ , Cu2 þ , Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ ions (23 mM).

39

The addition of Hg2 þ to the solution of 1 led to the change of color of the solutions from colorless to pink, which could be easily detected by the naked eye. In contrast, the titration of foreign ions ( 4 10 equivalents of Ag þ , Cu2 þ , Cd2 þ , Na þ , Li þ , K þ , Ba2 þ , Co2 þ , Ni2 þ , Mg2 þ , Mn2 þ and Al3 þ ) induced negligible color changes in solutions. In a similar study, 2 was found to be a slightly inferior fluoroionophore as compared to 1 in terms of selectivity to Hg2 þ . Selectivity studies of 2 were performed and it was found that some cations, such as Cu2 þ , Mn2 þ and Al3 þ (in a high concentration) promoted small fluorescence intensity changes at 575 nm. [supplementary data] The lack of selectivity of 2 might be due to the steric effect from two rhodamine B moieties upon ions binding. It should be noted here that increasing another unit of the rhodamine B moiety in the structure of 2, as compared to 1, resulted in a small decrease in Hg2 þ -sensing sensitivity and selectivity.

4. Conclusion In conclusion, we have successfully prepared two novel mercury fluoroionophores, 1 and 2. Sensor 1, based on 2-[3-(2aminoethylsulfanyl)propylsulfanyl]ethanamine ligand covalently bound to one unit of rhodamine-B moiety, exhibits high sensitivity and selectivity for Hg2 þ over a wide range of foreign ions, but with a significantly reduced synthetic effort. The readily accessible synthetic sensor 1 presented here is remarkable in terms of synthetic simplicity, low detection limit for the detection of Hg2 þ and high selectivity with particular discrimination of Cu2 þ and Ag þ . A remarkable feature of the sensor, which involves the change in color of the solution from colorless to pink, is readily observed, and the low cost of starting materials, make it attractive for commercial use. The high degree of selectivity and dual signaling behavior of 1 for Hg2 þ ions makes it attractive to be used as tagging materials for on-site monitoring purposes and for potential uses in molecular level sensor devices.

Acknowledgments This work was supported by Grant MRG 5380093 from the Thailand Research Fund, the Center for Innovation in Chemistry (PERCH-CIC), and the Commission on Higher Education, Ministry of Education of Thailand. The authors would like to express grateful acknowledgement to the Computational Nanoscience Consortium (CNC), Nanotechnology (NANOTEC), Thailand for the access to Material Studio 4.4 program package.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jlumin.2011.07.015.

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