Design and synthesis of a novel rhodamine-based chemosensor and recognition study to Fe³⁺

Introduction

Iron is one of the most essential trace elements in biological systems being involved in enzymatic reactions, muscle contraction, nerve conduction, electron transfer and transport of oxygen through heme [1–3]. Iron can also promote the process of cellular metabolism and the synthesis of DNA and RNA [4–6]. There are many evidences indicating that either deficiency or overload of Fe³⁺ can induce biological disorders in living body. For example, lack of iron in the blood can cause serious anemia. On the other hand, overload of iron ion has also been associated with certain cancers and dysfunction of liver, heart and nervous system [7, 8]. Thus far, many methods such as atomic absorption spectroscopy (AAS) [9, 10], inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [11, 12], voltammetry [13] and inductively coupled plasma-mass spectroscopy (ICP-MS) [14] have been used to determinate Fe³⁺. Although these methods are both quantitative and highly-sensitive, they are not suitable for using in real-time and on-site detection of Fe³⁺. Fluorescent methods have gained much attention of researchers due to their good selectivity and recognition, rapid response and application in the environment of organisms [15–17]. Many excellent fluorescent chemosensors for Fe³⁺ have been reported in the past few years [18–23]. Although most of them have superior detection performance, some of them suffer drawbacks including low tolerance of Cu²⁺ and Cr³⁺, low sensitivity and inconspicuous change in color, which restrict the application of these sensors in biological systems. Additionally, many Fe³⁺-selective fluorescent probes are usually based on fluorescence quenching mechanism [24–26]. These probes are easily influenced by background and other factors. Therefore, it is important to design ‘off-on’ fluorescent chemosensors for Fe³⁺ with color and fluorescence changes in aqueous solution.

Rhodamine skeleton, which has excellent photophysical properties like long absorption and emission wavelength, high fluorescence quantum yield, high molar extinction coefficient and good photostability, has attracted great interest. More importantly, rhodamine derivatives exist in a spirolactam form with weak fluorescence under the neutral or alkaline conditions but can undergo ring opening in the presence of certain metal ions, and the opened form is strongly fluorescent. Therefore, rhodamine framework has become an ideal model to construct ‘off-on’ fluorescent chemosensors. Furthermore, electron donors (such as N, S) are usually introduced to the probes to improve the chelator’s affinity toward metal ions [27]. Thiazoles and their derivatives have been considered as an important class of S and N-containing heterocycles [28]. They can form bi- or multinuclear complexes with transition metal ions [29]. Herein, on the basis of the photo-induced electron transfer (PET) mechanism, we designed a new fluorescent chemosensor Rh1 by using rhodamine B as a fluorophore and an aminothiazole moiety as a Fe³⁺ chelator (Scheme 1). The structure of Rh1 was confirmed by X-ray crystallographic analysis in addition to NMR data. The sensitive chemosensor Rh1 can selectively detect Fe³⁺ in CH₃CN-H₂O (1:1, v/v) solution and can be used in the presence of Cr³⁺ and Cu²⁺.

Scheme 1

Synthesis of Rh1.

Results and discussion

The given structure of Rh1 was confirmed by X-ray crystallographic analysis (Figure 1). White single crystal of Rh1 was obtained by slow concentration at room temperature of the dichloromethane–acetonitrile solution. Single-crystal X-ray diffraction structural analysis indicates that Rh1 crystallizes in orthorhombic system and the space group is Pbca without a symmetric center. Two spiro planes of the rhodamine frame-work are coordinated in mutually vertical positions. Additional crystallographic data and structural refinements for Rh1 are shown in the Supporting Information.

Figure 1

Crystal structure of sensor Rh1.

In order to examine the selectivity of Rh1, spectroscopic studies in the presence of various metal cations were conducted in CH₃CN-H₂O (1:1, v/v) solution. As shown in Figure 2, in the absence of Fe³⁺ ion, the sensor Rh1 (20 μm) shows a very weak fluorescence which can be ascribed to the probe mainly existing in the form of spirolactam. Upon binding with Fe³⁺, Rh1 exhibits an intense fluorescent emission at 580 nm. Moreover, the colorless solution develops a pink color (Figure 3). Under similar conditions, the addition of other metal ions including K⁺, Na⁺, Li⁺, Ag⁺, Ca²⁺, Ba²⁺, Mg²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Hg²⁺, Fe²⁺, Al³⁺ and Cr³⁺ does not induce any visible fluorescence or color changes in the absence of ferric ion. This specific feature demonstrates that probe Rh1 shows excellent recognition toward Fe³⁺ ion in the presence of other cations and can be used for naked-eye detection. In addition, the interference experiments of Rh1 were also conducted (Figure S4 in the Supporting Information). In these experiments, the fluorescence changes of Rh1 in CH₃CN-H₂O (1:1, v/v) solution were measured by the treatment of 20 μm Fe³⁺ ion in the presence of 100 equiv. of other cations. In the presence of other metal ions, the fluorescence intensity of the Rh1- Fe³⁺ complex had no significant variation regardless of whether there were other metal ions.

Figure 2

Fluorescence response of probe Rh1 (20 μmol/L) to different metal ions (20 μmol/L).

Figure 3

The colorimetric (top) and fluorometric (bottom) changes of Rh1 (20 μm) upon addition of Fe³⁺ and other cations in CH₃CN-H₂O (1:1, v/v) solution (20 μm for Fe³⁺ and other cations). The two photos were obtained in sunlight (top) and upon irradiation with a 365-nm UV lamp (bottom).

To further investigate the influence of Fe³⁺ on Rh1, a fluorescence titration experiment at different Fe³⁺ concentrations was carried out. As shown in Figure 4, free Rh1 exhibits a very slight fluorescence, which is due to the efficient photoinduced electron transfer (PET) process from the electron-rich receptor aminothiazole moiety to the excited rhodamine fluorophore. Upon addition of Fe³⁺, the PET mechanism is quenched and, as a result, the fluorescent emission peak at 580 nm increases in intensity. The increase of fluorescent emission becomes saturated in the presence of 1.5 equiv. of Fe³⁺. Moreover, a good linear relationship between the fluorescence emission intensity of Rh1 and the concentration of Fe³⁺ in the range of 2 μm ~ 7 μm can be noted (Figure S5). The regression equation is Y = 2264.8X - 92.075, R² = 0.9978. Based on the expression 3 × δ_blank/k (where δ_blank is the standard deviation of the blank solution and k is the slope of the calibration curve), the limit of detection was calculated to be 2.43 × 10^-8m. A Job’s plot for the fluorescent was applied at room temperature to determine the stoichiometry of Rh1 and Fe³⁺. Keeping the sum of the initial concentration of Fe³⁺ and Rh1 at 20 μm, the molar ratio of Fe³⁺ was varied from 0 to 1. As shown in Figure 5, the maximum emission intensity is obtained at a molar ratio of 0.5. This indicated that Rh1 chelates Fe³⁺ with a 1:1 stoichiometry. The association constant (Ka) for Rh1 and Fe³⁺ was also measured using the Benesi–Hildebrand method (Figure 6). The estimated Ka value is 2.26 × 10⁴m^-1.

Figure 4

Fluorescence emission spectra of Rh1 (20 μm) in the presence of increasing concentrations of Fe³⁺. The inset shows emission intensity at λ_em = 580 nm as a function of Fe³⁺ concentration (λ_ex = 545 nm).

Figure 5

Job’s plot of probe Rh1 and Fe³⁺.

Figure 6

Benesi-Hildebrand plot of Rh1 assuming 1:1 binding stoichiometry with Fe³⁺ (λ_ex = 545 nm, λ_em = 580 nm).

Besides, it was interesting to investigate the reversible binding nature of the sensor (Figure 7). Upon addition of ethylenediamine (20 μm) to the solution of Rh1 (20 μm) and Fe³⁺ (20 μm), the fluorescence intensity at 580 nm decreased (A→B) due to the competitive binding of Fe³⁺ from Rh1 by ethylenediamine and formation of the spirolactam. The strong fluorescence was observed again (B→C) upon further addition of 20 equiv. Fe³⁺. As can be seen, this sequence was followed five times without any change in intensity. This results shows that complexation of Rh1 with Fe³⁺ is chemically reversible.

Figure 7

Fluorescence intensity of the system Rh1 + Fe³⁺ as a function of NH(CH₂CH₃)₂ concentration in CH₃CN-H₂O (1:1, v/v) solution. Inset shows reversible complexation/decomplexation cycles carried out with Rh1 and Fe³⁺ (λ_ex = 545 nm, λ_em = 580 nm).

To understand the structural change of Rh1 upon the complexation with ferric ion, molecular modeling was performed using DFT calculations at B3LYP/6-31G level with the Gaussian 09 program [30, 31] (Figure 8). Combined with Figure 8 and PET mechanism [32], it can concluded that in the absence of Fe³⁺, the HOMO of the electron-rich receptor aminothiazole moiety has a higher energy than the half-filled HOMO of the excited rhodamine fluorophore. This energy difference drives a rapid electron transfer from the aminothiazole moiety to the excited-state rhodamine fluorophore, which inhibits the intrinsic fluorescence of the rhodamine and, as a result, the probe exhibits a weak fluorescence. Upon addition of Fe³⁺, this cation interacts with the lone-pair electrons on the nitrogen atoms of the aminothiazole moiety and the rhodamine fluorophore undergoes opening of the spiro-ring. The energy level of the aminothiazole moiety is lower than that of the HOMO of the excited rhodamine fluorophore, and the electron delocalization is not energetically favored. The exited electron of rhodamine fluorophore is returned to the stationary state and the fluorescence is ‘switched on’.

Figure 8

The energy-minimized structure of free Rh1 molecule (A) and the Rh1-Fe³⁺ complex (B).

Conclusion

An ‘off-on’ fluorescent chemosensor based on rhodamine has been synthesized. It exhibits an excellent selectivity and a high sensitivity to the detection of Fe³⁺ in CH₃CN-H₂O (1:1, v/v) solution. The sensor is also suitable for visual detection of ferric ion. The fluorescence intensity and the concentration of Fe³⁺ have a linear relationship in the range of 2 μm ~ 7 μm. The detection limit is 2.43 × 10^-8m and the association constant (Ka) for Rh1 and Fe³⁺ is 2.26 × 10⁴m^-1.

Experimental

The fluorescent spectra were recorded on a HITACHI F-4500 fluorescence spectrophotometer. X-ray crystal data were collected on Bruker Smart APEX II CCD diffractometer. All solvents and reagents (analytical grade) were purchased from commercial suppliers and used without further purification. Double distilled water was used. The solutions of metal ions were prepared from corresponding nitrate and chloride salts. Stock solutions of Rh1 (1 mm) were prepared in acetonitrile. Stock solutions of the metal ions (10 mm) were prepared in double distilled water with the metal salts KCl, NaCl, LiCl, MgCl₂, BaCl₂, AgCl, CaCl₂, ZnCl₂, CdCl₂, Cu(NO)₃, MnCl₂, CoCl₂, NiCl₂, FeCl₂, Al(NO)₃, FeCl₃, HgCl₂ and CrCl₃.

Supporting information

Online-only supplementary data include fluorescence spectra for Rh1.