Bifunctional Fluoroionphore-Ionic Liquid Hybrid for Toxic Heavy Metal

Apr 24, 2012 - As a proof-of-concept, Hg2+ was chosen as a model HMI, and a rhodamine thiospirolactam was chosen as a model fluoroionphore to construc...
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Bifunctional Fluoroionphore-Ionic Liquid Hybrid for Toxic Heavy Metal Ions: Improving Its Performance via the Synergistic Extraction Strategy Zhen Jin,† De-Xun Xie,† Xiao-Bing Zhang,*,† Yi-Jun Gong,† and Weihong Tan*,†,‡ †

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China ‡ Department of Chemistry and Shands Cancer Center, UF Genetics Institute and McKnight Brain Institute, University of Florida, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: Several heavy metal ions (HMIs), such as Cd2+, Pb2+, and Hg2+, are highly toxic even at very low concentrations. Although a large number of fluoroionphores have been synthesized for HMIs, only a few of them show detection limits that are below the maximum contamination levels in drinking water (usually in the nM range), and few of them can simultaneously detect and remove HMIs. In this work, we report a new fluoroionphore-ionic liquid hybrid-based strategy to improve the performance of classic fluoroionphores via a synergistic extraction effect and realize simultaneous instrument-free detection and removal of HMIs. As a proof-ofconcept, Hg2+ was chosen as a model HMI, and a rhodamine thiospirolactam was chosen as a model fluoroionphore to construct bifunctional fluoroionphore-ionic liquid hybrid 1. The new sensing system could provide obviously improved sensitivity by simply increasing the aqueous-to-ionic liquid phase volume ratio to 10:1, resulting in a detection limit of 800 pM for Hg2+, and afford extraction efficiencies larger than 99% for Hg2+. The novel strategy provides a general platform for highly sensitive detection and removal of various HMIs in aqueous samples and holds promise for environmental and biomedical applications. usage of toxic, flammable, volatile organic compounds. Ionic liquids are nonvolatile, nonflammable, and thermally stable solvents and are very promising replacements for the traditional volatile organic solvents. Ionic liquid-based extractions seem to be more effective methods to remove HMIs from aqueous solution. Modification of ionic liquid with fluoroionphore to form a task-specific ionic liquid (TSIL) should be a simple and efficient way for one to simultaneously detect and remove HMIs. Moreover, it is well established that some ionic liquids alone,8 or appended with a ligand,9 can actively extract HMIs from the aqueous phase. We envisioned that such a process could afford target enrichment to improve the detection sensitivity of fluoroionphore-based TSIL in it for target HMI via the synergistic extraction effect. To verify our hypothesis, we have designed a rhodamine spirolactam-based TSIL 1 for simultaneous detection and removal of HMIs (Scheme 1). Rhodamine spirolactam was chosen as a model fluoroionphore, since it exhibits a HMI-triggered “turn-on” visual and fluorescence signal which can be easily distinguished even by the naked eye.10,11 Hg2+ was chosen as a model HMI, as it is

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everal heavy metal ions (HMIs), such as Cd2+, Pb2+, and Hg2+, are proven to be highly toxic even at very low concentrations, and bioaccumulation of these HMIs in many organisms can cause serious health and physiological problems.1 Therefore, new methods are required for both sensitive detection and effective removal of these HMIs in water. Due to its high sensitivity and fast analysis with spatial resolution and it not destructing the sample, the fluorescent method has attracted much attention and has been applied widely for the detection of various analytes.2 Although quite a few of the fluoroionphores have been developed for toxic HMIs,3−5 only a few of them show detection limits for target HMI that are below the maximum contamination levels in drinking water (usually in the nM range, defined by the U.S. Environmental Protection Agency). Moreover, few of them can simultaneously detect and remove of HMIs.6 The search for a new strategy to improve the sensitivity of classic fluoroionphores and in the meantime realize the function of removal of HMIs is of considerable significance for environmental protection and human health. Traditional liquid−liquid extraction has been proved to be a promising technique for separation of HMIs and has attracted great attention for selective recovery of HMIs from industrial wastes.7 However, such organic solvent-based extractions are expensive and environmentally unfriendly due to their high © 2012 American Chemical Society

Received: March 8, 2012 Accepted: April 24, 2012 Published: April 24, 2012 4253

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Scheme 1. Structure, Binding Mechanism, and Schematics of the TSIL 1-Based Bifunctional System for Removal and Highly Sensitive Detection of HMIs via the Synergistic Extraction Effect

one of the most toxic HMIs.12 Due to the synergistic extraction effect of hybrid 1 and 1-butyl-3-methylimidazolium hexafluorophosphate, the new liquid−liquid extraction sensing system provides high Hg2+ removal efficiency, and it simultaneously produces a visual color and fluorescence signal for instrumentfree detection of Hg2+. The new bifunctional sensing system could provide obviously improved sensitivity by simply increasing the aqueous-to-ionic liquid phase volume ratio and can be easily regenerated using a simple ethylenediaminetetraacetic acid (EDTA) treatment. It was used successfully for direct detection and removal of Hg2+ in spiked samples of Xiang River water. A quinoline-appended rhodamine thiohydrazide was chosen as both Hg2+ receptor and signal reporter for grafting onto an imidazolium ionium salt for the design of the bifunctional TSIL. TSIL 1 was synthesized through multistep reations and confirmed by MS data and NMR spectra (see the Supporting Information, Experimental details and Scheme S1). Similar to other rhodamine spirolactam derivatives, probe 1 shows a Hg2+induced “off-on” visual signal (Figure 1a), and both fluorescence and color intensities are increased with increasing concentrations of Hg2+ (Figure 1c), indicating that TSIL 1 can really serve as a “naked-eye” probe for instrument-free detection of Hg2+. It is well established that [BMIM]PF6 alone can actively extract Hg2+ from the aqueous phase.11a Therefore, unlike most sensing systems whose signal responses are related only to the concentration of analyte, the fluorescent responses of our proposed bifunctional system should also be related to the aqueous phase volume, which means that the sensitivity can be improved by simply increasing the aqueous-to-ionic liquid phase volume ratio due to the synergistic extraction effect. To test this hypothesis, Hg2+-triggered colormetric and fluorescent responses of TSIL 1 under different aqueous phase/ionic liquid phase volume ratios were then recorded. As shown in Figure 1b, with the volume ratio changing from 1:1 to 10:1, both the color and the fluorescence intensities are increased remarkably, indicating that the increase of the aqueous-to-ionic liquid phase volume ratio can improve the system’s sensitivity. The sensitivity might be further increased using even more water. However, extraction of Hg2+ from aqueous phase to ionic liquid phase would take longer time. A ratio of 10:1 was therefore chosen for further investigations. Figure 2 shows the fluorescence spectra of 1 in 0.5 mL of ionic liquid [BMIM]PF6 after treating with 5 mL of buffered

Figure 1. Change in color (top) and fluorescence (bottom) of (a) 1 (1.0 × 10−5 M) in [BMIM]PF6 treated with buffered (Tris-HNO3, pH = 7.2) aqueous solution (10: 1, v/v) in the absence (left) and presence (right) of Hg2+ (1.0 × 10−6 M); (b) 1 in [BMIM]PF6 treated with Hg2+ at different volume ratios of aqueous phase to ionic liquid phase (1:1, 5:1, 10:1, v/v from left to right); (c) 1 in [BMIM]PF6 treated with different concentrations of Hg2+ (0, 1.0 × 10−8, 5.0 × 10−8, 1.0 × 10−7, 5.0 × 10−7, 1.0 × 10−6, and 2.0 × 10−6 M, from left to right); (d) 1 in [BMIM]PF6 treated with different metal ions at 1.0 × 10−6 M (Ag+, Cr3+, Co2+, Cu2+, Mn2+, Ni2+, Pb2+, Zn2+ and Hg2+, from left to right); (e) extraction experiment (left: [BMIM]PF6 + 1; middle: [BMIM]PF6 + 1 + Hg2+ (1.0 × 10−6 M); right: extracted aqueous phase treated with [BMIM]PF6 + 1 again).

aqueous solution containing different concentrations of Hg2+. In the absence of Hg2+, 1 in [BMIM]PF6 does not emit the characteristic fluorescence of rhodamine, suggesting that the spirocyclic form is still preferred in this condition. However, with the introduction of Hg2+ and the resulting structural change from the spirocyclic to the ring-open form, a new emission band at 580 nm appears. Moreover, the emission intensity gradually increases with increasing Hg2+ concentration (Figure 2a), and a 44-fold emission increase is observed when the concentration of Hg2+ reaches 2.0 × 10−6 M (Figure 2b). The large signal-to-background ratio is favorable for the fluorescence detection of Hg2+ with high sensitivity: dynamic range from 1.0 × 10−9 to 1.0 × 10−6 M for Hg2+, with a detection limit of 8.0 × 10−10 M (3σ/slope, Figure 2b). 4254

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metal ions show neither color nor fluorescence change after they are added to buffered solution of 1 (Figure 1d and Figure 3a), indicating that our proposed probe exhibits high selectivity to Hg2+. However, compounds that could interact with Hg2+, such as thiol compounds, will affect the fluorescence response of probe 1 to Hg2+. To test its practical applicability, competition experiments were also carried out. Our probe demonstrates nearly unchanged responses to Hg2+ before and after the addition of 10-fold concentration of competing metal ions (Figure 3b). These results indicate that the probe is highly selective. To evaluate the removal ability of the bifunctional TSIL system, liquid/liquid extraction was carried out by treating 0.5 mL of aqueous solutions of Hg2+ with 0.5 mL of 1 in [BMIM]PF6 for 30 min in a vortex mixer and then centrifuging to separate the two phases. The extraction efficiencies (E) were calculated by: E(%) = [(Cb)aq − (Ca)aq]/(Cb)aq × 100; where (Cb)aq and (Ca)aq are the concentrations of Hg2+ in aqueous phase before and after extraction, respectively. Experimental results showed that the E values of the TSIL system for Hg2+ at different concentrations are larger than 99% (Table 1), Table 1. Extraction Efficiency of [BMIM]PF6 and [BMIM]PF6 + 1 for Hg2+ with Different Concentrations [Hg2+] extractant

10−8 M

10−7 M

10−6 M

[BMIM]PF6 only [BMIM]PF6 + 1

71.82 >99

73.23 >99

72.65 >99

indicating an excellent Hg2+ removal capability. Previous studies have reported that the ionic liquid [BMIM]PF6 itself can actively absorb Hg2+. Therefore, to better understand the role of 1 in Hg2+ removal, the extraction experiment for [BMIM]PF6 alone was also carried out, giving E values of about 70% (Table 1). Taken together, these results show that both TSIL 1 and [BMIM]PF6 play important roles in the extraction of Hg2+. The Hg2+ removal can also be easily observed by the naked eye. As shown in Figure 1e, after extraction with TSIL 1 once, the concentration of unextracted Hg2+in the aqueous phase is too low to trigger the color “off-on” response of TSIL 1. To test the reusability of the system, EDTA (10 μM) was added to the TSIL-based biphasic system which was pretreated with 1 μM of Hg2+ (Figure 4). A remarkable decrease of fluorescence intensity was observed, and the visual signal

Figure 2. (a) Fluorescence emission spectra of TSIL 1 exposed to various concentrations of Hg2+ (water/[BMIM]PF6 = 10: 1, v/v): 0, 1.0 × 10−9, 2.0 × 10−9, 3.0 × 10−9, 4.0 × 10−9, 5.0 × 10−9, 1.0 × 10−8, 5.0 × 10−8, 1.0 × 10−7, 5.0 × 10−7, 1.0 × 10−6, and 2.0 × 10−6 M, from bottom to top. (b) Calibration curve of probe 1. λex = 520 nm.

The effects of pH on the fluorescence response to Hg2+ of the new probe 1 were also investigated (see the Supporting Information, Figure S1). Experimental results show that probe 1 shows good fluorescence response toward Hg2+ in the neutral pH range (5.0−8.0), thereby favoring its application in biological and medical samples. Except Hg2+, other heavy

Figure 3. (a) Fluorescence emission spectra of TSIL 1 (1.0 × 10−5 M) exposed to different metal ions (1.0 × 10−6 M); (b) fluorescence response of 1 (1.0 × 10−5 M) to the mixture of 1.0 × 10−7 M of Hg2+ with 1.0 × 10−6 M of other metal ions. 4255

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improved performance via the synergistic extraction effect. By choosing Hg2+ as a model HMI, a rhodamine-based TSIL 1 was designed and synthesized, which can both detect and remove Hg2+ from aqueous samples. This new system could provide obviously improved sensitivity by simply increasing the aqueous-to-ionic liquid phase volume ratio. Such bifunctional TSIL possesses several unique advantages compared to previously reported methods. First, the novel TSIL is endowed with both detection and removal functions, and the two tasks can be completed in a one-step operation, which is simple, economical, convenient, and nontoxic to the environment. Moreover, selectively induced by Hg2+, the TSIL molecule exhibits an easily distinguished “off-on” visual change for instrument-free detection of Hg2+, which is more convenient for the general public than other instrument-based methods. Most importantly, the bifunctional TSIL design provides a general platform for various HMIs, thus holding promise for a broad spectrum of applications.

Figure 4. Fluorescence emission spectra of reusability of the proposed 1 in [BMIM]PF6 with Hg2+. Inset: change in color (top) and fluorescence (down); a: [BMIM]PF6 + 1 only; b: [BMIM]PF6 + 1 + Hg2+(1.0 × 10−6 M); c: [BMIM]PF6 + 1 + Hg2++EDTA(10eq); d: [BMIM]PF6 + 1 + Hg2++EDTA+ Hg2+(1.0 × 10−6 M).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, supplementary figures, and spectral data. This material is available free of charge via the Internet at http://pubs.acs.org.

disappeared. After removal of the EDTA-containing aqueous phase, the TSIL system was retreated with 1 μM Hg2+; both color and fluorescence were recovered, with fluorescence intensity reaching 90% of the original value. This result indicates that the present system can be easily regenerated for repeated use. In order to study the reaction of 1 with Hg2+, the visible absorption spectra of 1 were investigated (see the Supporting Information, Figure S2a). It can be seen that free 1 exhibits almost no absorption. Upon the addition of Hg2+ to the solution, a new absorption peak at 560 nm emerges, which can be ascribed to the change from the spirocyclic form to the ringopened amide form. The above-mentioned EDTA experiment also supports a reversible spiro ring-opening mechanism. To determine the stoichiometry of the 1-Hg2+ complex, the Job’s method for fluorescence intensity was applied by keeping the sum of the concentration of Hg2+ and 1 at 2.0 × 10−5 M with the molar ratio of Hg2+ to 1 changing from 0 to 1 (see the Supporting Information, Figure S2b). The results show that the fluorescence intensity goes through a maximum at a molar fraction of about 0.5, indicating a 1:1 stoichiometry of the Hg2+ to 1. The proposed binding mechanism of 1 with Hg2+ is shown in Scheme 1. The practical application of the designed bifunctional TSIL system was first evaluated by detection of spiked Hg2+ in Xiang River water samples. It can be seen that the results obtained in river water samples show good recovery values (see the Supporting Information, Table S1), confirming that the proposed system is feasible for practical Hg2+ detection. The Hg2+ removal capability of TSIL was also estimated by treating the Hg2+-spiked river water samples with an equal volume of solution of 1 in [BMIM]PF6 for 30 min. The upper aqueous phase was then separated and collected, and the concentrations of Hg2+ were determined by HG-AFS. The E values of TSIL for Hg2+ in river water samples are larger than 99 (data not shown), indicating an excellent Hg2+ removal ability for real water samples. In summary, we have developed a novel fluoroionphore-ionic liquid hybrid-based strategy for design of an efficient bifunctional sensing system for detection and removal of HMIs with

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-B.Z.); [email protected]fl.edu (W.T.). Phone: +86-731-88821894. Fax: +86-731-88821894 (X.-B.Z.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 20975034, 21177036), the National Key Scientific Program of China (2011CB911001, 2011CB911003), the National Key Natural Science Foundation of China (No. 21135001), Program for Changjiang Scholars and Innovative Research Team in University, and Hunan Provincial Natural Science Foundation (Grant 11JJ1002).

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