Open AccessArticle

Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands

Xuemei Wang

^1,2,

Mudassir Iqbal

^1,3,

Jurriaan Huskens

¹ and

Willem Verboom

^1,*

Laboratory of Molecular Nanofabrication, MESA+ Institute for Nanotechnology, University of Twente, Enschede 7500 AE, The Netherlands

Department of Chemistry and Material Engineering, Logistic Engineering University, Chongqing 401311, China

Department of Chemistry, University of Sargodha, Punjab 40100, Pakistan

Author to whom correspondence should be addressed.

Int. J. Mol. Sci. 2012, 13(12), 16822-16832; https://doi.org/10.3390/ijms131216822

Submission received: 22 November 2012 / Accepted: 4 December 2012 / Published: 7 December 2012

(This article belongs to the Special Issue Molecular Self-Assembly 2012)

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Fluorescence (left) and absorption (right) spectra of 1 (a) and 2 (b) (5 μM) in CH3CN/H2O (3/1, v/v) with different metal ions (400 μM), respectively. ">
Photos of chemosensor 1 (5 μM) in CH3CN/H2O (3/1, v/v) upon addition of 80 equiv of Cu2+, Fe3+, and Hg2+ ions, respectively. ">
Changes of the fluorescence spectra of (a) chemosensor 1 and (b) chemosensor 2 (5 μM, λex = 510 nm) in CH3CN/H2O (3/1, v/v) upon addition of increasing amounts of Fe3+ (0–370 μM, left) and Hg2+ (0–400 μM, right), respectively. Inset: Spectrofluorimetric titration curves ((a) λem =580 nm and (b) λem = 590 nm) for a 1:1 complex according to <a href="#FD1" class="html-disp-formula">Equation 1</a>. The data are fitted to a curve with a correlation coefficient of (a) R2 = 0.9952, R2 = 0.9972 and (b) R2 = 0.9945, R2 = 0.9983. ">
Fluorescence response of chemosensor 1 (5.0 μM, λex = 510 nm and λem = 580 nm) (a) in CH3CN/buffer (pH = 7.0) (95/5, v/v) upon addition of 40 equiv of Fe3+, Hg2+, and Cu2+ ions, respectively, and (b) in different fractions of HEPES buffer (0.02 M, pH = 7.0) upon addition of 40 equiv of Fe3+, Hg2+, and Cu2+ ions, respectively. The responses for Fe3+ are below 0.05 in the different fractions of HEPES buffer. ">
Changes of the fluorescence spectra of chemosensor 1 (5 μM, λex = 510 nm) in CH3CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) upon addition of increasing amounts of Hg2+ (0–50 μM). Inset: Fluorescence intensity of 1 at 580 nm (5 μM, λex = 510 nm) in CH3CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) vs the concentration of Hg2+ ions. ">
Bar profiles of fluorescence intensity for chemosensor 1 (5 μM, λex = 510 nm) in CH3CN/HEPES (95/5, v/v) upon addition of 10 equiv of various metal ions as perchlorates. ">
Fluorescence intensity of 1 at 580 nm (5 μM, λex = 510 nm) in CH3CN/H2O (95/5, v/v) (a) with and without Hg2+ ion (50 μM) as a function of pH and (b) upon addition of Hg2+ ion (50 μM) over time. ">
Synthesis of chemosensors 1 and 2. ">
Structure of chemosensors 1 and 2. ">

Versions Notes

Abstract

Rhodamine-based fluorescent chemosensors 1 and 2 exhibit selective fluorescence enhancement to Fe³⁺ and Hg²⁺ over other metal ions at 580 nm in CH₃CN/H₂O (3/1, v/v) solution. Bis(rhodamine) chemosensor 1, under optimized conditions (CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v)), shows a high selectivity and sensitivity to Hg²⁺, with a linear working range of 0–50 μM, a wide pH span of 4–10, and a detection limit of 0.4 μM Hg²⁺.

Keywords:

rhodamine; mercury; chemosensor; fluorescence

1. Introduction

Mercury is considered to be a highly dangerous element by the United States Environmental Protection Agency due to its special properties, such as migration through cell membranes and bioaccumulation within living tissues [1,2]. Therefore, there is a high demand for the determination of the Hg²⁺ ion in environmental analysis.

In recent years, rhodamine-based fluorescent chemosensors have received considerable attention for the detection of Hg²⁺[3–14], Cu²⁺[15–17], Pb²⁺[18], Cr³⁺[19], and Fe³⁺[20], because their special structural properties provide an ideal mode to construct off-on fluorescent switch chemosensors. Rhodamine having a spirolactam structure is non-fluorescent, whereas ring-opening of the spirolactam gives rise to a strong fluorescence emission. Moreover, they have a longer emission wavelength (about 550 nm), which is often preferred to serve as reporting group for analytes to avoid the influence of the background fluorescence (below 500 nm) [21–23]. However, most of them have shortcomings in practical application, such as cross-sensitivities toward other metal cations, low water solubility, a narrow pH span, and delayed response, etc. Accordingly, quantitative practical Hg²⁺ detection requires a linear fluorescence response, uniform fluorescence output at a broad pH range, compatibility with aqueous medium, high selectivity, sensitivity, and a fast response, while easy synthetic procedures for the sensors are of utmost importance.

This study deals with new rhodamine-based CHEF (chelation-enhanced fluorescence) chemosensors 1 and 2 (Chart 1) for the detection of Hg²⁺ ions showing that, compared to related rhodamine-based chemosensors, small structural changes give rise to improved selectivity and sensitivity. Chemosensor 1 is a bis(rhodamine) in which the two units are connected via amide groups meta substituted to a benzene ring. In order to study the influence of a third functionalized rhodamine on the Hg²⁺ complexation, tris(rhodamine) chemosensor 2 was prepared and evaluated for comparison.

2. Results and Discussion

Rhodamine derivatives 1 and 2, possessing two or three rhodamine moieties, respectively, were prepared by reacting rhodamine B hydrazide (4) with isophthaloyl dichloride (3) or benzene-1,3,5-tricarbonyl trichloride (5) in THF as a solvent (Scheme 1). The formation of 1 and 2 followed from the ¹H NMR spectra as the doublets at 6.42 and 6.46 ppm in rhodamine B hydrazide (4) shifted to 6.61–6.75 ppm and 6.59–6.76 ppm as multiplets for 1 and 2, respectively. In the ESI-MS mass spectra the [M+H] peaks were found at m/z 1043.5 and 1525.7 for 1 and 2, respectively.

The perchlorate salts of Na⁺, K⁺, Pb²⁺, Co²⁺, Cd²⁺, Cs⁺, Ag⁺, Cu²⁺, Mg²⁺, Zn²⁺, Hg²⁺, Fe²⁺, and Fe³⁺ ions were used to evaluate the metal ion binding properties of chemosensors 1 and 2 in CH₃CN/H₂O (3/1, v/v). The fluorescence spectra were obtained by excitation of the rhodamine fluorophore at 510 nm. Among these metal ions (80 equiv), chemosensors 1 and 2 both showed large chelation enhanced fluorescence (CHEF) effects with Hg²⁺, Fe³⁺ and smaller CHEF effects with Cu²⁺ (Figure 1). The addition of 400 μM (80 equiv) of Fe³⁺ and Hg²⁺ immediately yielded a pink solution with a absorption signal at 561 nm [24] and a strong fluorescence signal at 580 and 590 nm, respectively (Figure 2). For chemosensor 1, there was 35-fold enhancement with Fe³⁺ and 84-fold enhancement with Hg²⁺, while chemosensor 2 yielded a 27-fold enhancement with Fe³⁺ and 33-fold with Hg²⁺. The results can be attributed to a similar binding behavior of 1 and 2 both containing rhodamine moieties. In addition, a very weak fluorescence signal for free 1 and 2 was observed at 580 and 590 nm, respectively, upon excitation at 510 nm, confirming the presence of a ring-closed spirolactam structure, whereas with the addition of Fe³⁺ or Hg²⁺ ions, ring-opening of the spirolactam occurs and gives rise to a strong fluorescence emission at 580 and 590 nm, respectively. Though Cu²⁺ gave a small color change and a very small fluorescence enhancement, the spectroscopy and interaction of chemosensors 1 and 2 with Cu²⁺ are completely different from those of 1 and 2 with Fe³⁺ and Hg²⁺ as recently reported by others [20].

To investigate the binding mode and the affinity, Job’s plots (see Figure S1) were determined and fluorescence titration experiments were carried out for chemosensors 1 and 2 with Fe³⁺ and Hg²⁺ (Figure 3). The Job’s plots show that in all cases 1:1 complexes were formed. The resulting titrations also fitted to a 1:1 binding model, and the association constant (K_s) can be gained using Equation 1 [25,26].

\frac{(I - I_{0})}{I_{0}} = \frac{α \times [M]}{(1 / K_{s}) + [M]}

(1)

Where I₀ is the fluorescence intensity of the chemosensors 1 and 2 in the absence of metal ions and I is the fluorescence intensity upon the addition of metal ions. α is the maximum specific binding, [M] is the metal ion concentration, K_s is the association constant.

The association constants for 1 with Fe³⁺ and Hg²⁺ were found to be 7.99 × 10³ M⁻¹ and 8.62 × 10³ M⁻¹, while those for 2 with Fe³⁺ and Hg²⁺ were 6.18 × 10³ M⁻¹ and 6.08 × 10³ M⁻¹, respectively, which are close to those of a related bis(rhodamine) chemosensor [20]. The K_s values of chemosensors 1 and 2 only marginally differ, those of tris(rhodamine) 2 even being slightly smaller than those of bis(rhodamine) 1. In addition to the Job’s plot determination, this also demonstrates that two rhodamines are sufficient for optimal metal ion binding. In chemosensor 1, two carbonyl oxygens as well as two amide oxygens can provide a stable binding pocket for metal ions.

To obtain a high selectivity and sensitivity for Hg²⁺ under aqueous conditions, HEPES buffer (0.02 M, pH = 7.0), MES buffer (0.01 M, pH = 7.0), PES buffer (0.01 M, pH = 7.0), and Tris HCl buffer (0.01 M, pH = 7.0) were used, respectively (Figure 4a). The fluorescence intensities upon addition of 40 equiv of Fe³⁺, Hg²⁺, and Cu²⁺ ions show the effect of the different buffer systems. In this case, HEPES effectively inhibits the interference of Fe³⁺ and Cu²⁺ ions during the detection of Hg²⁺.

The fraction of HEPES buffer used in CH₃CN played an important role in the affinity of 1 toward Hg²⁺. Because of the strong interaction between buffer anions and Fe³⁺, a small fraction of HEPES buffer in organic solvent was already beneficial to inhibit the binding of Fe³⁺. However, a high fraction of HEPES buffer caused a decrease of the fluorescence emission for 1·Hg²⁺ and an increase of that of the complex with Cu²⁺, which has to be avoided for Hg²⁺ analysis. To determine the optimal analysis condition, 5 μM chemosensor 1 in CH₃CN containing different fractions (5%, 10%, 15%, and 25% (v/v)) of 0.02 M HEPES buffer at pH 7.0 were used for the detection of Fe³⁺, Hg²⁺, and Cu²⁺ (Figure 4b). No significant fluorescence enhancement could be observed at 580 nm for Fe³⁺ compared to that of Hg²⁺ and Cu²⁺ at the same concentration. The results already show that 5% HEPES/CH₃CN was already sufficient for efficient monitoring of Hg²⁺.

Fluorescence titrations of Hg²⁺ by 1 were performed, under the optimized conditions of CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) (Figure 5). For the quantitative detection of Hg²⁺ ions, under the optimized conditions, a calibration curve was generated by determining the fluorescence intensity of 1 (5 μM) at 580 nm upon addition of Hg²⁺ ions with different concentrations, ranging from 0 to 50 μM [27]. Figure 5 exhibits over the entire Hg²⁺ concentration range an almost perfect linearity (I₅₈₀ = 1.43 × [Hg²⁺] + 9.34, R² = 0.9962) between the fluorescence intensity of 1 and the Hg²⁺ concentration, indicating a linear detection range for Hg²⁺ determination. The detection limit, defined as three times the standard deviation of the blank signals [28], was found to be 0.4 μM from 10 blank solutions. In addition, Figure 6 shows that chemosensor 1 detected Hg²⁺ ions with high selectivity under these conditions.

For practical applicability of this new chemosensor, a proper pH range of 4–10 was determined. Figure 7a shows variations of the fluorescence intensity of 1 with pH in the absence and presence of the Hg²⁺ ion in CH₃CN/H₂O solution (95/5, v/v). In this region, free 1 has a weak fluorescence emission due to the presence of the ring-closed spirolactam structure, while addition of the Hg²⁺ ion leads to ring-opening of the spirolactam ring, resulting in a remarkable increase of the fluorescence.

The ring-opening of the spirolactam in chemosensor 1 produced a time-dependent dosimetric response, controlled by the reaction kinetics. Under the optimized conditions less than 4 min were required to complete the reaction (Figure 7b).

Chemosensor 1 in CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) has a low detection limit (0.4 μM), a large linear detection range (0–50 μM), a wide pH span (4–10), and a rapid response time (4 min), exhibiting higher sensitivity and selectivity than most other previously reported rhodamine-based chemosensors [3–14].

3. Experimental Section

3.1. General

Absolute acetonitrile of analytical grade and deionized water were used throughout the experiments. All chemicals needed for the synthesis were purchased from known suppliers and used without further purification. The known rhodamine B hydrazide (4) was prepared according to a literature procedure [29]. The metal ion solutions were prepared from their analytical grade perchlorate salts. HEPES buffer, MES buffer, PES buffer, Tris HCl buffer solutions and different pH solutions were prepared using proper amounts of HEPES, MES, PES, Tris, 1.0 M HCl, and 1.0 M NaOH (all of analytical grade) under adjustment by a pH meter.

3.2. Equipment

Absorption spectra were determined on a Perkin Elmer Lambda 850 UV-vis spectrophotometer. Fluorescence spectroscopy measurements were performed on a Perkin Elmer LS55 spectrofluorimeter equipped with a xenon discharge lamp and using 1 cm quartz cells. All pH measurements were made with a Mettler Toledo SevenEasy pH meter. ¹H NMR and ¹³C NMR spectra were recorded on a Varian Unity INOVA (300 MHz) spectrometer in CDCl₃. ¹H NMR (300 MHz) and ¹³C NMR (75 MHz) chemical shift values are reported as δ using the residual solvent signal as an internal standard. Electrospray Ionization (positive mode) mass spectra were recorded on a WATERS LCT mass spectrometer.

3.3. Synthesis of 1 and 2

3.3.1. General Procedure for the Synthesis of 1 and 2

To a solution of rhodamine B hydrazide (4) and triethylamine in THF a solution of isophthaloyl dichloride (3) or benzene-1,3,5-tricarbonyl trichloride (5) in THF was added dropwise at 0 °C. The reaction mixture was brought to room temperature in 1 h, followed by stirring overnight at room temperature. The solvent was evaporated and the residue was dissolved in dichloromethane (50 mL), washed with 10% NaHCO₃ solution (3 × 50 mL) and water (3 × 50 mL). The organic layer was concentrated under reduced pressure to afford crude products 1 or 2.

Chemosensor 1 was synthesized starting from rhodamine B hydrazide (4) (1.3 g, 2.8 mmol), isophthaloyl dichloride (3) (0.29 g, 1.4 mmol) in THF (5 mL) and triethylamine (0.3 g, 3.0 mmol) in THF (70 mL). The crude product was recrystallized from a mixture of diethyl ether and dichloromethane (3:1) to afford the pure product (0.77 g, 52%) as a solid. mp 184 °C–186 °C. ¹H NMR: δ 1.02–1.22 (m, 24H, CH₃), 3.17–3.41 (m, 16H, CH₂), 6.22–6.40 (m, 8H, ArH), 6.61–6.75 (m, 4H, ArH). 7.15 (d, J = 6.0 Hz, 2H, ArH), 7.39–7.47 (m, 2H, ArH), 7.47–7.56 (m, 4H, ArH), 7.61–7.69 (m, 2H, ArH), 7.96 (d, J = 6.0 Hz, 2H, ArH). ¹³C NMR: δ 12.6, 44.3, 97.7, 98.0, 107.9, 123.5, 124.2, 128.3, 129.3, 131.0, 133.1, 149.0, 153.7. ESI MS: m/z 1043.6, calculated: 1043.5 for [M+H]⁺.

Chemosensor 2 was prepared starting from rhodamine B hydrazide (4) (1.5 g, 3.3 mmol), benzene-1,3,5-tricarbonyl trichloride (5) (0.28 g, 1.1 mmol) in THF (5 mL) and triethylamine (0.35 g, 3.5 mmol) in THF (70 mL). The crude product was recrystallized from a mixture of diethyl ether and dichloromethane (3:1) to afford the pure product (1.02 g, 61%) as a solid. mp 204 °C–206 °C. ¹H NMR: δ 1.05–1.24 (m, 36H, CH₃), 3.19–3.39 (m, 24H, CH₂), 6.21–6.41(m, 12H, ArH), 6.59–6.76 (m, 6H, ArH), 7.12 (d, J = 6.0 Hz, 3H, ArH), 7.38–7.56 (m, 6H, ArH), 7.88–8.01 (m, 6H, ArH). ¹³C NMR: δ 12.6, 44.2, 98.0, 99.9, 103.9, 107.9, 123.5, 124.2, 127.9, 129.2, 129.8, 132.9, 149.0, 153.6. ESI MS: m/z 1525.7, calculated: 1525.7 for [M+H]⁺.

3.4. Absorption and Fluorescence Measurements

Absorption and fluorescence titrations were performed in a 1 cm quartz cell by addition of small aliquots of metal ion work solutions to a 3 mL solution of proper amounts of 1 and 2 in CH₃CN/H₂O (3/1, v/v) and a 3 mL solution of a proper amount of 1 in CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v). After thorough mixing, the solutions were allowed to stand at ambient temperature for 5 min, whereupon absorption or fluorescence spectra were recorded. Both the excitation and emission slits were 5 nm.

To determine the optimal conditions for Hg²⁺ detection, small aliquots of Hg²⁺, Cu²⁺, and Fe³⁺ work solutions were respectively added into 5 μM chemosensor 1 solutions which contained different fractions (5%, 10%, 15%, and 20%) of HEPES buffer (0.02 M, pH = 7.0) in CH₃CN and mixed in a 1 cm quartz cell for 5 min. Then the fluorescence measurement was performed at ex/em = 510/580 nm.

4. Conclusions

In conclusion, the rhodamine derivatives 1 and 2 are very good fluorescent chemosensors, with a good selectivity toward Hg²⁺ and Fe³⁺ over other competitive ions in CH₃CN/H₂O (3/1, v/v). For practical Hg²⁺ detection, the experimental conditions were optimized to CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v). Under these conditions, the fluorimetric quantification of Hg²⁺ by 1 was satisfactory in a linear working range of 0–50 μM, with a detection limit of 0.4 μM Hg²⁺ and a pH span of 4–10. These very good features make chemosensor 1 very promising for practical applications.

Supplementary Information

ijms-13-16822-s001.pdf

Acknowledgments

The authors would like to thank Hua Yan and acknowledge the support of the Logistic Engineering University in China.

Conflict of InterestThe authors declare no conflict of interest.

References and Notes

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Figure 1. Fluorescence (left) and absorption (right) spectra of 1 (a) and 2 (b) (5 μM) in CH₃CN/H₂O (3/1, v/v) with different metal ions (400 μM), respectively.

Figure 2. Photos of chemosensor 1 (5 μM) in CH₃CN/H₂O (3/1, v/v) upon addition of 80 equiv of Cu²⁺, Fe³⁺, and Hg²⁺ ions, respectively.

Figure 3. Changes of the fluorescence spectra of (a) chemosensor 1 and (b) chemosensor 2 (5 μM, λ_ex = 510 nm) in CH₃CN/H₂O (3/1, v/v) upon addition of increasing amounts of Fe³⁺ (0–370 μM, left) and Hg²⁺ (0–400 μM, right), respectively. Inset: Spectrofluorimetric titration curves ((a) λ_em =580 nm and (b) λ_em = 590 nm) for a 1:1 complex according to Equation 1. The data are fitted to a curve with a correlation coefficient of (a) R² = 0.9952, R² = 0.9972 and (b) R² = 0.9945, R² = 0.9983.

Figure 4. Fluorescence response of chemosensor 1 (5.0 μM, λ_ex = 510 nm and λ_em = 580 nm) (a) in CH₃CN/buffer (pH = 7.0) (95/5, v/v) upon addition of 40 equiv of Fe³⁺, Hg²⁺, and Cu²⁺ ions, respectively, and (b) in different fractions of HEPES buffer (0.02 M, pH = 7.0) upon addition of 40 equiv of Fe³⁺, Hg²⁺, and Cu²⁺ ions, respectively. The responses for Fe³⁺ are below 0.05 in the different fractions of HEPES buffer.

Figure 5. Changes of the fluorescence spectra of chemosensor 1 (5 μM, λ_ex = 510 nm) in CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) upon addition of increasing amounts of Hg²⁺ (0–50 μM). Inset: Fluorescence intensity of 1 at 580 nm (5 μM, λ_ex = 510 nm) in CH₃CN/HEPES buffer (0.02 M, pH = 7.0) (95/5, v/v) vs the concentration of Hg²⁺ ions.

Figure 6. Bar profiles of fluorescence intensity for chemosensor 1 (5 μM, λ_ex = 510 nm) in CH₃CN/HEPES (95/5, v/v) upon addition of 10 equiv of various metal ions as perchlorates.

Figure 7. Fluorescence intensity of 1 at 580 nm (5 μM, λ_ex = 510 nm) in CH₃CN/H₂O (95/5, v/v) (a) with and without Hg²⁺ ion (50 μM) as a function of pH and (b) upon addition of Hg²⁺ ion (50 μM) over time.

Scheme 1. Synthesis of chemosensors 1 and 2.

Chart 1. Structure of chemosensors 1 and 2.

© 2012 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).

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Wang, X.; Iqbal, M.; Huskens, J.; Verboom, W. Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands. Int. J. Mol. Sci. 2012, 13, 16822-16832. https://doi.org/10.3390/ijms131216822

AMA Style

Wang X, Iqbal M, Huskens J, Verboom W. Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands. International Journal of Molecular Sciences. 2012; 13(12):16822-16832. https://doi.org/10.3390/ijms131216822

Chicago/Turabian Style

Wang, Xuemei, Mudassir Iqbal, Jurriaan Huskens, and Willem Verboom. 2012. "Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands" International Journal of Molecular Sciences 13, no. 12: 16822-16832. https://doi.org/10.3390/ijms131216822

APA Style

Wang, X., Iqbal, M., Huskens, J., & Verboom, W. (2012). Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands. International Journal of Molecular Sciences, 13(12), 16822-16832. https://doi.org/10.3390/ijms131216822

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Turn-On Fluorescent Chemosensor for Hg²⁺ Based on Multivalent Rhodamine Ligands

Abstract

1. Introduction

2. Results and Discussion