Ratiometric Fluorescent Mercuric Sensor Based on Thiourea

Apr 13, 2009 - A novel “naked-eye” and ratiometric fluorescent Hg2+ probe based on thiourea−thiadiazole−pyridine linked (TTP) fluorescent orga...
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Ratiometric Fluorescent Mercuric Sensor Based on Thiourea-Thiadiazole-Pyridine Linked Organic Nanoparticles Haibing Li* and Huijuan Yan Key Laboratory of Pesticide and Chemical Biology (CCNU), Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: December 03, 2008; ReVised Manuscript ReceiVed: February 11, 2009

A novel “naked-eye” and ratiometric fluorescent Hg2+ probe based on thiourea-thiadiazole-pyridine linked (TTP) fluorescent organic nanoparticles (FONs) was designed and synthesized. TTP FONs show good water solubility; a 46-fold increase in fluorescence quantum yield, high selectivity for sensing, and a 98 nm red shift of the fluorescence emission upon binding Hg2+ in aqueous media are observed. Under the optimum conditions, the relative fluorescence intensity ratio, I501/I403, is increased linearly with the increasing logarithmic concentration of Hg2+ in the range of 2.5-40 µM with a detection limit of 1.13 × 10-7 M (22.6 ppb), whereas the fluorescence intensity of TTP FONs to other metal ions, including Na+, Ca2+, Ni2+, Cd2+, Cu2+, Ag+, Pb2+, Mn2+, and Mg2+, is negligible. The possible mechanism is discussed. Introduction The development of fluorescent sensors with high selectivity and sensitivity for heavy and transition metal ions has always been of particular interest.1 As mercury and its derivatives are widely used in industry and they have inherent high toxicity, the detection of Hg2+ is attracting particular attention.2,3 Some fluorescent Hg2+ sensors have been explored,4 including polymers, 5 foldamers,6 biomolecules,7-9 and small molecules.10-12 However, as is well-known, a majority of the reported mercuric sensors have poor water solubility, and recognition of metal ions is accomplished via measuring the metal-induced changes in fluorescence intensity, which may be influenced by many factors, such as the excitation intensity, dye concentration, and environment around the dye (pH, polarity, temperature, and so forth). Thus, these sensors are inclined to be disturbed in quantitative detection.13 A ratiometric fluorescence measurement14 can increase the selectivity and sensitivity of a measurement and eliminate most or all of the possible variability because the ratio of the fluorescent intensities at two wavelengths is independent of the concentration of the sensor, the fluctuation of source light intensity, and the sensitivity of the instrument. Therefore, the development of water-soluble ratiometric fluorescent sensors for Hg2+ is of great current interest. Fluorescent organic nanoparticles (FONs), as a result of their large diversity in molecular structure and optical properties that are of potential use in optoelectronics and biologics, have become the subject of ever-increasing attention in recent years.15-18 This research has, to date, principally focused on the development of new technology to control their sizes, morphologies, and dispersion. For example, Yao and co-workers have successfully fabricated organic core/diffuse-shell 1,5diphenyl-3-(naphthalene-4-yl)-1H-pyrazoline (DPNP) nanorods by reprecipitation.19 Debuigne and co-workers have reported the preparation of cholesterol, Rhovanil, and Rhodiarome nanoparticles by microemulsions.20 The use of the sol-gel method21 and laser ablation22 to prepare FONs also has been reported. In the above methods, the reprecipitation is the simplest synthetic approach, which is performed by rapid injection of a * Corresponding author. E-mail: [email protected].

solution of the monomer into a solvent in which the monomer is insoluble.23,24 This can not only make the organic compounds form FONs in water but also increase the fluorescent quantum yield (QY).25 Herein, we reported an effective ratiometric fluorescent sensor for Hg2+ ions based on organic nanoparticles of the thioureathiadiazole-pyridine linked molecule (TTP). TTP, 1-(4-chlorobenzoyl)-3-(5-(pyrid-4-yl)-1,3,4-thiadiazol-2-yl)thiourea, was synthesized in 80% yield by condensing 2-amino-5-(pyrid-4yl)-1,3,4-thiadiazole (1)26 with 4-chlorobenzoyl isocyanate (2) in DMF at 50 °C for 3 h (Scheme 1a). It is reasonable to believe that transferring TTP to form FONs in an aqueous system can probe metal ions in aqueous media due to the fact that thiourea and thiadiazole groups of TTP aligned in parallel, which may effectively make a complex with metal ions. Materials Materials. Salts of the different cations studied (NaNO3, Ca(NO3)2, Ni(NO3)2, Cd(NO3)2, Cu(NO3)2, AgNO3, Pb(NO3)2, Mn(NO3)2, Mg(NO3)2, and Hg(NO3)2) were obtained from Shanghai Chemical Reagents Company (China) and used as received without further purification. All solutions were prepared with double-distilled, deionized water. 2-Amino-5-(pyrid-4-yl)1,3,4-thiadiazole26 was synthesized following the method described previously. Apparatus. The morphologies and sizes of the as-prepared TTP FONs were examined on a scanning electron microscope (SEM, JEOL-JEM 2010) at an accelerating voltage of 20 kV. The average particle size (z average size) and size distribution of TTP FONs are measured by photon correlation spectroscopy (PCS) (Nano ZS90 zetasizer, Malvern Instruments Corp., U.K.) at 25 °C under a fixed angle of 90° in disposable polystyrene cuvettes. The measurements are obtained using a HeNe laser of 633 nm. No multiscattering phenomenon is observed. The UV-visible absorption spectra of the aqueous dispersion of TTP FONs were measured on a TU-1901 UV-vis spectrometer. The steady-state emission fluorescence spectra of the aqueous dispersion of TTP FONs were performed on a Fluoromax-P luminescence spectrometer. Slits were set to provide widths of 4 nm for the emission monochromaters in all cases.

10.1021/jp810622a CCC: $40.75  2009 American Chemical Society Published on Web 04/13/2009

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SCHEME 1: (a) Synthesis of TTP and (b) Representation for the Formation Processes of Nanoscale Materials of TTP

The emission intensity ratios of TTP FONs were determined by comparing fluorescence intensities at the wavelengths of 501 and 403 nm and were uncorrected for differences in instrument response at the two wavelengths. Methods Synthesis of TTP. As shown in Scheme 1, to a solution of 1 (9 mmol) in dry DMF (10 mL) was added 2 (9.9 mmol) at room temperature. After the reaction mixture was stirred and heated at 50 °C for 3 h under argon atmosphere, the mixture was concentrated and purified by column chromatography on silica gel to give the 1-(4-chlorobenzoyl)-3-(5-(pyrid-4-yl)-1,3,4thiadiazol-2-yl)thiourea (TTP) as a white powder in a yield of 80%. Mp > 300 °C. 1H NMR (400 MHz, DMSO-d6): δ 7.70 (d, 2H, J ) 5.6 Hz,), 8.07 (s, 2H), 8.20 (d, 2H, J ) 5.6 Hz), 8.87 (s, 2H), 3.51(s, 1H, NH), 13.51 (s, 1H, NH). IR (KBr, ν): 3085, 2914 (N-H), 1682, 1652 (CdO, CdS) cm-1. ESI-MS: m/z ) 375 (M+). Anal. Calcd for C15H10ClN5OS2: C, 47.93; H, 2.68; N, 18.63. Found: C, 47.90; H, 2.65; N, 18.58. Preparation of TTP FONs. The nanoparticle dispersion of 1-(4-chlorobenzoyl)-3-(5-(pyrid-4-yl)-1,3,4-thiadiazol-2-yl)thiourea was prepared by a simple reprecipitation method.27 In a typical preparation, a THF solution of TTP (10-3 M) was prepared and stored in the dark. To the 99 mL of H2O/THF mixed solution (v/v ) 2:1), 1 mL of the TTP in THF solution was rapidly injected. The above solution was then sonicated for 12 min, making sure that the temperature of the H2O/THF nanoparticles did not rise above 10 °C. This was accomplished by sonicating in 4 min intervals and then pausing and letting the solution cool to 5 °C before starting the next sonication period. Finally, 10-5 M TTP FONs in H2O/THF mixed solution (v/v ) 2:1) was obtained and kept in the dark.

respectively, by using rhodamine B as a criterion (QY ) 0.89, EtOH)28 at room temperature, which suggests that there is a 46-fold increase in QY. TTP molecules were found to be readily self-assembled into colloidal nanoparticles as a result of reprecipitation when water was added as a nonsolvent to its solution in THF. As shown in the 1H NMR spectra of TTP (Figure S1A, Supporting Information) and TTP FONs (Figure S1B, Supporting Information), the active proton (CONHCO) was shifted to low field and the aromatic protons were shifted to high field, which indicated that there are hydrogen bonds and π-π stacking interactions in TTP FONs. Thus, as shown in Scheme 1b, two monomers of TTP formed a dimer via hydrogen bonds and π-π stacking interactions. These dimers were the actual building blocks for the solid state and are brought together via van der Waals interactions. Compound TTP forms dimers as the preaggregate. The weak and diffuse nature of van der Waals driving forces for the association of dimers leads to the formation of TTP nanoparticles. The sizes and morphology of TTP nanoparticles (10-5 M) were determined by scanning electron microscopy (SEM). It can be seen that TTP nanoparticles were produced with high dispersion (Figure 2A). The average diameter of TTP nanoparticles was measured by photon correlation spectroscopy at about 200 nm, as shown in Figure 2B. The colloidal stability of the TTP nanoparticles was estimated at room temperature (Figure S2, Supporting Information). It also

Results and Discussion Spectra Characterizations of TTP FONs. It was obvious that the fluorescence intensity of TTP increased remarkably in H2O/THF solutions with appropriate fractions of water. The H2O/THF (v/v ) 2:1) is selected for further experiments because the fluorescence intensity reaches to maximal (Figure 1). The observation that the fluorescence intensity increased remarkably in H2O/THF solutions might be attributed to the tendency of TTP molecules to form aggregates when their solubility decreased in the media with higher fractions of water, resulting in molecular complanation and enhancement of the level of conjugation. The quantum yields (QYs) of TTP in THF and TTP in H2O/THF (v/v ) 2:1) are measured as 0.21 and 9.71%,

Figure 1. Fluorescence spectra of the TTP in solutions with different ratios of H2O/THF.

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Figure 2. Typical SEM image (A) and size distribution (B) of TTP nanoparticles prepared in 10-5 M H2O/THF solution with a v/v of 2:1.

Figure 3. Fluorescence emission responses of TTP FONs to different metal ions. Inset: the fluorescence emission changes of the TTP FONs and the TTP FONs in the presence of Hg2+ under illumination by UV light at 365 nm.

can be seen that TTP FONs are quite stable at room temperature for at least 20 days. Ion-Binding Ability of the TTP FONs. To investigate the ion-binding properties of TTP FONs, spectral features and sensor selectivities to metal ions were determined. The results showed that TTP FONs exhibited a high selectivity in sensing Hg2+. As depicted in Figure 3, the spectrum of the free TTP FONs showed a typical emission band at 403 nm (excitation ) 330 nm). When Hg2+ was added to the TTP FONs, we observed a significant increase of the TTP FONs’ emission and a remarkable red-shifted maximum centered at 501 nm. Actually, an obviously blue-green emission of the solution can easily be observed by the naked eye under illumination by UV light at 365 nm (inset). The effects of potentially interfering metal ions, such as Na+, Ca2+, Ni2+, Cd2+, Cu2+, Ag+, Pb2+, Mn2+, and Mg2+, were not very significant in the aqueous solution. In comparison with TTP FONs, the TTP monomer did not appear to be a selective chemosensor for the specific target metal ion of Hg2+. The ratios of the intensities (I501/I403) ranged over a small variation between 0.1 and 0.21 for the surveyed metal ions, including the Hg2+ ions (Figure 4). It is reasonable to believe that the TTP FONs play an important role in the selective luminescence response to Hg2+. To further explore the selectivity of TTP FONs for Hg2+, competition experiments were also performed for TTP FONs in the presence of Hg2+ mixed with background metal cations, such as Na+, Ca2+, Ni2+, Cd2+, Cu2+, Ag+, Pb2+, Mn2+, and Mg2+. As shown in Figure 5, other metal ions resulted in nearly no disturbance to the selective sensing of TTP FONs toward Hg2+. The quantitative behaviors of TTP FONs in the sensing of Hg2+ ions were further characterized by fluorescence titration

Figure 4. Fluorescence intensity ratiometric responses of the TTP FONs (red) and TTP monomer (green) in the presence of tested metal ions. All metal ion concentrations were 100 µM (excitation wavelength ) 330 nm).

Figure 5. Fluorescence spectra of the TTP FONs in 2:1 H2O/THF (v/v) in the presence of the Hg2+ ion and miscellaneous cations, including Na+, Ca2+, Ni2+, Cd2+, Cu2+, Ag+, Pb2+, Mn2+, and Mg2+ (100 µM, excitation wavelength at 330 nm).

with Hg(NO3)2 in aqueous solution at room temperature (Figure 6). The emission intensity ratio, I501/I403, increases with the increase in Hg2+ concentrations, which allows the free Hg2+ concentration to be determined. It was noted that, when the Hg2+ concentration was 40 µM, the emission had little deviation of I501/I403 (inset). It also seemed that the maximum absorbance wavelength of TTP FONs in the presence of Hg2+ was blueshifted about 40 nm (Figure S3, Supporting Information) in comparison with that of TTP FONs. The changes in the optical properties of TTP FONs can also suggest that the TTP FONs play an important role in the selective binding of mercuric ions.

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Figure 6. Fluorescence spectra of TTP FONs in the presence of different concentrations of Hg2+: 0.5, 1, 2.5, 8.5, 15, 30, 40, 50, 75, 100, 200, and 250 µM. Inset: Ratiometric calibration curve, I501/I403 as a function of Hg2+ concentration.

Czarnik’s Hg2+-selective chemodosimeter that is based on the thioamide derivative of anthracene.30 In this paper, the TTP monomer did not appear to be a selective chemosensor for the specific target metal ion of Hg2+ in THF, due to no hydrolytic conversion in organic solvent. However, the large Hg2+-selective fluorescence enhancement of TTP FONs was caused by the Hg2+-induced transformation of the thiourea function into a urea group, followed by the removal of the HgS, and resulted in the formation of new urea-thiadiazole-pyridine (UTP) FONs, as depicted in Scheme 2. The Hg2+-induced transformation of the thioamide TTP to the amide UTP was evidenced by 1H NMR, IR, UV-vis, and mass spectroscopic measurements. After TTP was treated with 10 equiv of Hg2+ ions in H2O/THF (v/v ) 2:1), the recovered residue was well-characterized to confirm the suggested transformation by showing a prominent peak at m/z ) 359.6 for the UTP (detailed analytical data are shown in the Supporting Information). Conclusions

Figure 7. Relative fluorescence ratio, I501/I403, as a function of log [Hg2+] (M). The curve is fit to the experimental data.

The titration results were analyzed by a nonlinear curve-fitting procedure for sensing Hg2+, and the concentration of mercury ions ranges from 0.5 to 250 µM (Figure 7). When TTP FONs were employed from 2.5 to 40 µM, I501/I403 also increased linearly with the concentration of Hg2+ (inset) and remained linearly dependent (coefficient: R ) 0.9979). On the basis of the 3σ IUPAC criteria, the detection limit of TTP FONs for the sensing of Hg2+ ions was determined to be 1.13 × 10-7 M (22.6 ppb). Compared with other fluorescent Hg2+ ion sensors, most of these sensors, however, require the involvement of organic solvent, show quenched emissions, and suffer from poor selectivity.10-12 Only a few such sensors can detect Hg2+ ions in water with high sensitivity and selectivity.7,8 Herein, we developed a new fluorescent nanosensor for Hg2+ that exhibits a lower detection limit in aqueous media with good sensitivity and selectivity over other interfering cations. The possible ratiometric mechanism is also discussed. The hydrolytic conversion of thiamides into amides catalyzed by certain metal ions has been known to be very efficient.29 In the present case, the conversion was effective exclusively with Hg2+ ions in aqueous THF. This behavior is quite reminiscent of

In summary, we have developed a new class of “naked-eye” and ratiometric fluorescent sensor, TTP FONs, for Hg2+ in aqueous solution. A highly Hg2+-selective fluorescence enhancing property, in conjunction with a remarkable red shift of the fluorescence emission, was observed. It might be the first ratiometric and water-soluble Hg2+ fluorescent sensor derived from thiourea-thiadiazole-pyridine linked FONs, which allowed the development of a new highly selective and sensitive method to detect Hg2+ ion and also the applications of nanotechnology in chemosensor chemistry. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20602015 and 20772038), the Program for Distinguished Young Scientists of Hubei Province (2007ABB017), and the Program for Chenguang Young Scientists of Wuhan (200750731283). Supporting Information Available: 1H NMR spectra, figures, and detailed analytical data for UTP.This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515–1566. (b) Valeur, B.; Leray, I. Coord. Chem. ReV. 2000, 205, 3–40. (c) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem. ReV. 2000, 205, 59–83. (2) (a) Descalzo, A. B.; Martı´nez-Ma´n˜ez, R.; Radeglia, R.; Rurack, K.; Soto, J. J. Am. Chem. Soc. 2003, 125, 3418–3419. (b) Zhang, X.-B.; Guo, C.-C.; Li, Z.-Z.; Shen, G.-L.; Yu, R.-Q. Anal. Chem. 2002, 74, 821–

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