A rhodamine-based sensor for Hg²⁺ and resultant complex as a fluorescence sensor for I⁻

Abstract

The recognition and sensing of ions have attracted growing attention because of the important role played by ions in biological, industrial and environmental processes. However, many sensors can only detect a single ion. We report here a rhodamine-based sensor with functions of dual ion detection. It displays a quick colorimetric and fluorometric sensing ability on 1 : 1 binding to Hg²⁺, with a change of color of solution (colorless to pink) and fluorescence color (dark to orange) due to Hg²⁺-induced opening of the spirolactam ring in the rhodamine structure. The detection limits were found to be 32 nM and 44 nM by absorption and fluorescence methods, respectively. More importantly, the resulting complex 1–Hg²⁺ can be used as a reversible fluorescence sensor for I⁻. With the addition of I⁻, the fluorescence intensity was quenched because Hg²⁺ in the complex was grabbed by I⁻ attributed to the stronger binding force between Hg²⁺ and I⁻. Based on the “OFF–ON–OFF” behavior, a molecular device with “INHIBIT” logic gate function was constructed.

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Introduction

As is well known, many metal ions play an indispensable role in sustaining life, and they are precisely controlled in ecology and biology. Nevertheless, the existence of heavy metal ions is dangerous to human health and the environment due to their hazardous nature.¹ Hg²⁺ is considered as one of the most hazardous and ubiquitous pollutants; it is not biodegradable, and can accumulate throughout the food chain in vital organs and tissues, consequently inducing a number of diseases, such as brain damage, kidney failure, Minamata disease, and cognitive and emotional disorders.^2–4 Thus, it has become an important task to develop rapid and efficient methods for monitoring the level of Hg²⁺. Currently, the traditional methods for detecting Hg²⁺ include atomic absorption spectrometry⁵ and inductively coupled plasma mass spectrometry;⁶ however, these methods require expensive, sophisticated instrumentation, and are not good for on-site analysis. Therefore, a simple and inexpensive method is desirable for real-time monitoring of Hg²⁺ from environmental, biological, and industrial samples.

The fluorescence approach is simple, sensitive, and rapid for the detection of Hg²⁺ in biological, toxicological and other environments.⁷ To date, several fluorescence methods using organic fluorophores^8–11 and nanoparticles^12–15 as prototype tools have been developed for simple and rapid detection of Hg²⁺. Additionally, many kinds of fluorescence chemosensors also have been frequently used to detect Hg²⁺.^16–19 These small molecule-based sensors were developed by covalently attaching binding sites with different signaling moieties like phenothiazine,²⁰ BODIPY,²¹ quinoline,²² coumarin,²³ rhodamine,²⁴ etc. Among all these, rhodamine appears to be one of the ideal structures due to its excellent photophysical properties, such as large molar extinction coefficient, long excitation and emission wavelengths and high fluorescence quantum yields.²⁵ The sensing principle is generally based on the reversible molecular structural change between the spirocycle (non-fluorescent) and the open-ring isomers (strongly fluorescent).²⁶ Many rhodamine-based chemosensors have been developed for metal ions such as Al³⁺,²⁷ Cu²⁺,²⁸ Fe³⁺,²⁹ Cr³⁺,³⁰ and Zn²⁺.³¹

Meanwhile, anions also have an important influence in many biological, chemical, and environmental processes.³² I⁻, a trace essential element in human beings, is one of the most closely linked to the human body, an abnormal level of which causes various diseases. For example, its deficiency in the human body causes diseases such as goiter, hypothyroidism and cretinism; however, its excess may lead to disorders in thyroid functioning and autoimmune diseases.^33–35 Therefore, the determination of I⁻ has important significance. To solve this problem, optical-based sensors have been developed using small molecules for the sensing of I⁻. Recently, a few fluorescence sensors for I⁻ have been prepared based on metal coordination compounds, such as Cu²⁺,³⁶ Ag⁺,³⁷ and Hg²⁺, which work due to the stronger binding force between metal ions and I⁻ than between metal ions and sensors.^38,39 Hg²⁺ complexes can be disrupted by the formation of HgI₂, which has an extremely low solubility constant (K_sp = 3.2 × 10⁻²⁹),⁴⁰ meaning that Hg²⁺ complexes have potential use to detect I⁻ via fluorescence response.

Based on all of the above, we designed and synthesized a rhodamine-based fluorescence chemosensor (1). In order to achieve the specific recognition of Hg²⁺, the “O” in amide bond was replaced by “S” of the rhodamine structure due to the high affinity between Hg and S atoms.^41,42 It displayed a fluorescent and colorimetric response toward Hg²⁺ with high selectivity and sensitivity and no interference from competing metal ions. However, the sequential addition of I⁻ can quench the Hg²⁺-induced strong fluorescence, and the resultant Hg²⁺ complex of this chemosensor shows no change in the presence of other anions for I⁻ detection.

Experimental

Materials and methods

All inorganic salts (HgCl₂, KCl, MnCl₂·4H₂O, Al(NO₃)₃·9H₂O, Cd(NO₃)₂·4H₂O, Ba(NO₃)₂, Cu(NO₃)₂·3H₂O, Ni(NO₃)₂·6H₂O, Pb(NO₃)₂, Zn(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, Cr(NO₃)₃·9H₂O, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, Ca(NO₃)₂·4H₂O, and Mg(NO₃)₂·6H₂O) were purchased and used without purification. To get an aqueous solution (0.1 mol L⁻¹) of each inorganic salt, 1 mmol was weighed and dissolved in 10 mL of distilled water. The obtained stock solutions, before use, were diluted to required concentrations when needed. All other reagents and solvents were of analytical grade.

NMR spectra were recorded with a Bruker AV400 (400 MHz) spectrometer with CDCl₃ as the solvent and tetramethylsilane (TMS) as an internal standard. UV-visible spectra were measured with an Agilent 8453 UV/vis spectrophotometer. Fluorescence spectra were recorded with a Hitachi F-4600 fluorescence spectrophotometer. The fluorescence quantum yield was measured with an Absolute PL Quantum Yield Spectrometer QY C11347-11. Infrared (IR) spectra were collected with a Bruker Vertex-70 spectrometer. Elemental analysis was carried out with a PE CHN 2400 analyzer. Melting points were measured using a WRS-1B melting point apparatus.

The UV-visible and fluorescence studies of 1 were performed using a 2 mL CH₃CN–H₂O (1/1, v/v, 2 × 10⁻⁵ mol L⁻¹) solution with appropriate amounts of metal ions. Solutions were shaken for about 10 min before measuring the absorption and fluorescence intensity in order to allow the metal ions to chelate sufficiently with the sensors.

Synthesis of 1

The synthetic route for 1 is shown in Fig. 1. Compounds 3 and 5 were synthesized according to literature procedures.^43,44

Fig. 1 Synthetic route for 1.

Rhodamine B thiohydrazide (5, 0.24 g, 0.50 mmol) and 1-phenyl-3-methyl-4-formylpyrazolone-5 (3, 0.20 g, 1.00 mmol) were dissolved in 5 mL of ethanol. Then, after refluxing for 48 h, the orange powder was collected by filtration and washed with hot ethanol three times. Yield: 42%. Mp: 368–370 K. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 8.56 (s, 1H), 8.02–8.08 (m, 1H), 7.49 (d, J = 8 Hz, 2H), 7.41 (t, J = 8 Hz, 2H), 7.33–7.36 (m, 3H), 7.03–7.09 (m, 1H), 6.72 (d, J = 8 Hz, 2H), 6.25 (s, 3H), 6.23 (s, 1H), 3.23–3.28 (m, 8H), 2.47 (s, 3H), 1.08 (t, J = 8 Hz, 12H). ¹³C NMR (100 MHz, CDCl₃) 154.54, 150.81, 149.80, 149.57, 136.74, 134.10, 131.09, 129.34, 128.26, 128.14, 128.03, 127.37, 126.76, 126.13, 124.17, 123.88, 121.19, 113.00, 109.58, 107.18, 96.43, 61.85, 43.33, 14.17, 11.63 (Fig. S1†). ESI-MS (m/z): 675.2 [M + H₂O + H⁺]⁺ (Fig. S2†). Anal. calcd for C₃₉H₄₀N₆O₂S: C, 71.31; H, 6.14; N, 12.79%. Found: C, 71.22; H, 6.21; N, 12.73%. IR (KBr, ν, cm⁻¹): 1217, 1510, 1612, 3425.

Results and discussion

Absorption and fluorescence spectral responses of 1 to different metal ions

Various metal ions (Fe³⁺, Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Ba²⁺, Cr³⁺, Hg²⁺, K⁺, Sr²⁺, Mg²⁺, Mn²⁺, Ni⁺, Pb²⁺, and Zn²⁺) were used to evaluate the selectivity of 1 in CH₃CN–H₂O (1/1, v/v, 2 × 10⁻⁵ mol L⁻¹) solution. Fig. 2 shows the absorption and fluorescence spectral changes in the presence of various metal ions (10 eq.).

Fig. 2 Spectroscopic changes of 1 induced by the addition of various metal ions (10 eq.) in CH₃CN–H₂O (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) solutions: (A) absorption spectral changes, and (B) fluorescence emission intensity changes.

Fig. 2A shows the UV-visible absorption spectra of 1 upon the addition of the various metal ions. When 10 eq. of these metal ions were added to the CH₃CN–H₂O (1/1, v/v, 2 × 10⁻⁵ mol L⁻¹) solutions of 1, the results show that Cu²⁺ and Hg²⁺ can induce an absorption spectra change. When Cu²⁺ and Hg²⁺ were added, a strong absorption peak appeared at 562 nm and 558 nm, respectively, which are characteristic of rhodamine B in its opening state. Although Cu²⁺ could induce an obvious change, the change was weaker than that of Hg²⁺. Fig. 2B shows the fluorescence spectra of 1 upon the addition of the various ions. 1 showed no fluorescence upon excitation at 530 nm. Only Hg²⁺ can induce an emission peak to appear and cause 268-fold enhancement in the fluorescence intensity at 617 nm because of the formation of the open-ring amide form of rhodamine B. Cu²⁺ also induced an increase in the fluorescence intensity, with an emission peak at 614 nm, but this effect can be neglected due to the fluorescence intensity being far weaker than that induced by Hg²⁺. Hence, the different fluorescence intensity changes make it possible for 1 to discriminate between Hg²⁺ and Cu²⁺.

Fluorescence competition experiments were also conducted for sensor 1. When 5 eq. of Hg²⁺ was added into a solution of 1 in the presence of 10 eq. other ions, similar fluorescence spectral change was displayed to that with Hg²⁺ ion only; the addition of these metal ions did not result in significant fluorescence intensity changes (Fig. 3), indicating the binding ability of Hg²⁺ with compound 1 is higher than that of other metal ions. The presence of other metal ions does not interfere with the Hg²⁺ detection, making 1 very useful in practical applications.

Fig. 3 Competitive test for the fluorescent responses of 1 to various metal ions in CH₃CN–H₂O (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) solutions. Blue bars represent the addition of 10.0 eq. of various metal ions to the solution of 1. Red bars represent the addition of Hg²⁺ (5.0 eq.) to each solution.

Absorption and fluorescence spectral responses for Hg²⁺

Preliminary metal ion selectivity examination had revealed that 1 can recognize Hg²⁺. To quantitatively analyze the probe's reaction with Hg²⁺, spectroscopic titration experiment was conducted.

Fig. 4A shows the absorption response in aqueous solution. Upon the gradual addition of Hg²⁺, the intensity of the absorption band at 558 nm was found to increase. As a consequence, the solution color changed from colorless to pink upon addition of Hg²⁺, due to the structure change of rhodamine B from spirocycle (colorless) to the open-ring state (pink). This is consistent with the undoubted conversion of free 1 to the 1–Hg²⁺ complex. By plotting the changes of 1 in terms of the absorbance at 558 nm as a function of Hg²⁺ concentration, a curve has been obtained and is shown in the inset of Fig. 4A. The curve was divided into two parts before reaching the titration plateau. The peak at 558 nm slowly increased with the addition of Hg²⁺ before 0.3 eq. When the proportion of mercury ions was ≥0.3 eq., the absorbance at 558 nm rapidly increased. Finally, the absorbance at 558 nm reached plateau when 0.7 eq. of Hg²⁺ was added. There is a good linearity between the concentration of Hg²⁺ and the absorbance in the range of 6–14 µM, indicating that 1 can quantitatively detect Hg²⁺ at the relevant concentration. The linear equation is found to be y = 0.1639x − 0.9165 (R = 0.997) (Fig. S3†). The limit of detection of Hg²⁺ was determined by the 3σ method and found to be 32 nM (Fig. S4†).

Fig. 4 (A) Changes in absorption spectra of 1 induced by Hg²⁺ in CH₃CN–H₂O (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) solution. The inset shows absorption intensity changes of 1 at 558 nm with different equivalents of Hg²⁺. (B) Changes in fluorescence emission spectra of 1 induced by Hg²⁺ in CH₃CN–H₂O (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) solution. The inset shows emission intensity changes of 1 at 617 nm with different equivalents of Hg²⁺.

The fluorescence titration profile of 1 with Hg²⁺ is shown in Fig. 4B. Free 1 showed very weak fluorescence signal in the range of 540–800 nm. Upon gradual addition of Hg²⁺ to the solution of 1, a significant enhancement in fluorescence intensity at 617 nm was observed following excitation at 530 nm which gradually increased with Hg²⁺ concentration. With a handheld UV lamp (at an excitation of 365 nm), the fluorescence color significant changed from dark to orange. In accord with UV-visible titration spectra, the emission peak at 617 nm slowly increased with the addition of Hg²⁺ before 0.3 eq. When the proportion of mercury ions was 0.3–0.7 eq., the reaction continued and the emission intensity at 617 nm rapidly increased. Fig. S5† also shows that there is a good linearity between the concentration of Hg²⁺ and the fluorescence intensity in the range of 5–12 µM, indicating that sensor 1 can quantitatively detect Hg²⁺ at the relevant concentration. The linear equation is found to be y = 301.46x − 1376.68 (R = 0.992) (Fig. S5†). Meanwhile, the absolute fluorescence quantum yield of 1–Hg²⁺ complex peaked at 0.34 from 0.05. The limit of detection of Hg²⁺ was determined by the 3σ method and found to be as low as 44 nM (Fig. S6†).

In order to understand the binding stoichiometry of 1–Hg²⁺ complexe, Job's plot experiments were carried out. In Fig. 5A, the emission intensity at 617 nm is plotted as a function of the mole fraction of 1 under a constant total concentration. Maximum emission intensity was reached when the mole fraction was 0.5. These results indicate a 1 : 1 ratio for 1–Hg²⁺ complexe, in which one Hg²⁺ ion was bound with one 1 unit. Further, the formation of 1–Hg²⁺ complex was confirmed using ESI-MS in which the peak at m/z 911.1 ([1 + H₂O + Hg²⁺ + Cl⁻]⁺) indicates a 1 : 1 stoichiometry for 1–Hg²⁺ complexes (Fig. 5B).

Fig. 5 (A) Job's plot for the complexation of 1 with Hg²⁺, indicating the formation of a 1 : 1 complex. The total [1] + [Hg²⁺] = 40 µM. (B) Positive-ion electrospray ionization mass spectrum of 1 upon addition of Hg²⁺ in CH₃CN–H₂O (1/1, v/v) solution.

Fluorescence spectral responses for I⁻

Next, we wanted to determine whether I⁻ would react with the Hg²⁺ of complex 1–Hg²⁺, and whether the occurrence of this reaction indicates a linear relationship between the I⁻ concentration and the fluorescence changes of the resultant 1–Hg²⁺ solution. Then, this relationship could be used in designing a sensor for I⁻.

To study the performance of the 1–Hg²⁺ complex in anion sensing, the fluorescence changes of the complex in the presence of a number of anions were systematically examined. Fig. 6 shows the changes in the fluorescence emissions of the complex 1–Hg²⁺ solution upon the addition of a series of anions (10 eq. of F⁻, Cl⁻, Br⁻, NO₃⁻, SCN⁻, CN⁻, HSO₄⁻, CO₃²⁻, HCO₃²⁻, SO₄²⁻, H₂PO₄⁻, SO₃²⁻, ClO₄⁻ and CH₃COO⁻). As expected, among these anions (Fig. 6A), only I⁻ caused a significant decrease in fluorescence. This indicates that the selectivity of the 1–Hg²⁺ complex for I⁻ is high compared with its selectivity for other anions.

Fig. 6 (A) Fluorescence spectra of 1–Hg²⁺ (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) in the presence of various anions. (B) Competitive test for the fluorescent responses of 1 in CH₃CN–H₂O (1/1, v/v) solutions. Red bars represent the addition of 10 eq. of various anions to the solution of 1–Hg²⁺. Green bars represent the addition of I⁻ (5 eq.) to each solution.

A competition experiment was subsequently carried out by adding I⁻ to a 1–Hg²⁺ solution containing other anions, and the results are shown in Fig. 6B. Before the introduction of I⁻, there was almost no fluorescence change at 617 nm in the presence of other anions (10 eq.). When 5 eq. of I⁻ were introduced to the above mentioned solutions, a remarkable quenching in the fluorescence emission intensity at 617 nm was observed. This clearly demonstrated any anions considered in the study did not interfere with the detection of I⁻.

The 1–Hg²⁺ complex shows high specificity for I⁻. Thus, we further investigated the sensitivity of the 1–Hg²⁺ complex to I⁻. A fluorescence titration experiment was performed to determine the interaction between I⁻ and the 1–Hg²⁺ complex. Fig. 7 shows that the fluorescence intensities gradually decreased as the I⁻ concentration was increased, and a highly linear relationship between the fluorescence decrease and the I⁻ concentration was obtained when the concentration of I⁻ varied from 32 to 72 µM (Fig. S7†). Thus, those results prove the analytical platform based on 1–Hg²⁺ complex for I⁻ detection is practicable and sufficiently accurate.

Fig. 7 Fluorescence titration of 1–Hg²⁺ by adding I⁻ in CH₃CN–H₂O (1/1, v/v, 2.0 × 10⁻⁵ mol L⁻¹) solution. The inset shows emission intensity changes of 1–Hg²⁺ at 617 nm with different equivalents of I⁻.

Design of logic gate

It has been demonstrated that Hg²⁺ could conjugate with 1 to form a 1–Hg²⁺ complex, resulting in the fluorescence “on” state. The sequential addition of I⁻ can restore the fluorescence ‘off’ state. Based on this principle, Hg²⁺ and I⁻ were defined as the two inputs for our logic gate, the fluorescence intensity change at 617 nm was defined as the output, and the threshold was set to be 2000. For the input, the presence of Hg²⁺ and I⁻ was defined as 1 (“on” state), and their absence was defined as 0 (“off” state). For the output, the original fluorescence intensity of 1 was considered as 0, and the enhanced fluorescence was defined as 1 (Fig. 8A). The four possible input combinations were (0, 0), (1, 0), (0, 1) and (1, 1), as shown in the truth table (Fig. 8D). With no input, or with I⁻ input alone, the output was 0. With Hg²⁺ input alone the output signal was 1. When the two inputs were introduced together into the system, the strong binding force between Hg²⁺ and I⁻ released the Hg²⁺ from the 1–Hg²⁺ complex and the fluorescence output signal was 0. Therefore, monitoring the fluorescence at 617 nm, upon addition of Hg²⁺ and I⁻ and their combined mixture lead to a satisfactory INHIBIT logic gate function.

Fig. 8 (A) Output signals (E_m = 617 nm) of the logic gate in the presence of different inputs with corresponding gray diagram (B). (C) General representation of the symbol of an INHIBIT logic gate. (D) Corresponding truth table of the logic gate.

Conclusions

In a summary, a rhodamine-based dual sensing ability sensor for the selective detection of Hg²⁺ and I⁻ was reported. It could selectively recognize Hg²⁺ over 15 other metal ions. The Job's plot and ESI-MS reveal that a 1 : 1 stoichiometry was most favourable for the binding mode. The detection limits were found to be 32 nM and 44 nM by absorption and fluorescence methods, respectively. The sensor displays a reversible change in fluorescence upon the sequential addition of I⁻ in complex 1–Hg²⁺ solution with little interference with other anions. Moreover, it can be used for successful fabrication of molecular “INHIBIT” logic gates. This work would offer a reference for the development of sensors with sequential recognition of Hg²⁺ and I⁻.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (21362013, 51373072, 51372051, 51321061), National Basic Science Research Program (2012CB339300), the Science Funds of Natural Science Foundation of Jiangxi Province (20142BAB203005, 20132BAB203005), and State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology (2016TS03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17861h

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