A new on-fluorescent sensor for Ag⁺ based on benzimidazole bearing bis(ethoxycarbonylmethyl)amino groups

Introduction

Some metal ions, such as Ca²⁺, Fe³⁺, Zn²⁺, K⁺, Mg²⁺, Ag⁺, and Na⁺, are playing a key biological role in human body [1], whereas other heavy metal ions, including Al³⁺, Cd²⁺, Cr³⁺, Hg²⁺, and Pb²⁺, can be harmful to organisms [2]. Accordingly, the development of fluorescent probes for the detection of various metal ions is becoming an important research area [3–6]. Over the last decades, the rational design and synthesis of efficient sensors to selectively recognize targets has been a hot topic in molecular recognition studies. Generally, a typical sensor is devised by a covalent linkage of three parts, namely, a receptor unit, a spacer unit, and a signaling unit, although there are some examples of spacer-free probes also. Usually, highly selective probes for transition or heavy elements that give a positive response rather than fluorescent quenching upon the ion binding are preferred to promote the sensitivity factor [7–10]. The design of such turn-on silver ion (Ag⁺) sensors is an intriguing challenge because many transition elements often cause fluorescent quenching [11].

Most of the reported signaling units are mainly based on rhodamine [12] and coumarin fragments [13]. The imidazole unit [14, 15] is a newer fluorescent group that is becoming important in the field of chemosensors given its fluorescence off-on behavior that results from its particular structural properties [16]. Park and coworkers [17] have shown that a proper substitution of a tetraphenylimidazole scaffold can lead to white light-emitting compounds by the combination of excited-state intramolecular proton transfer and restricted energy transfer. In addition, a polymer containing thienoimidazole systems can be used for both colorimetric and ratiometric detections of Hg²⁺ as well as fluorometric detection of Zn²⁺ via fluorescence turn-on response with augmented lifetime via the chelation of metal ions to both S and N heteroatoms [18]. Unfortunately, in the existing reports, the fluorescence enhancement in most cases is small and usually suffers from a high background interference. To our knowledge, few reports have been devoted to fluorescence enhancement of probes for Ag⁺.

Results and discussion

In continuation of our work on imidazole derivatives [19], a new silver ion probe L was synthesized and studied. The synthetic route to the ligand compound-bearing bis(ethoxycarbonylmethyl)amino and benzimidazole functionalities is outlined in Scheme 1.

Scheme 1

The fluorescence properties of ligand L were investigated by UV-vis absorption and fluorescence emission spectra in methanol. The UV absorption spectrum of L in the presence of various cations is presented in Figure 1. The metal-free ligand shows no absorption higher than 350 nm. The addition of 10 equivalents of metal ion, including K⁺, Na⁺, Ca²⁺, Mg²⁺, Ba²⁺+, Zn²⁺, Fe³⁺, Pb²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, and Hg²⁺, results in a little change in absorption (Figure 1), as analyzed by the UV absorption spectrum and the visible absorption observed by the naked eye. Upon the addition of 10 equivalents of Cr³⁺ or Al³⁺, the UV absorption peak exhibits a slightly red shift from 323 to 345 nm. By contrast, in the presence of Ag⁺ ion under otherwise similar conditions, the solution of L undergoes a significant color change from colorless to dark yellow (inset in Figure 1) with an emergence of a new absorption peak at approximately 424 nm. These results show the selectivity of the interaction of compound L with silver ion in the presence of other ions.

Figure 1

Absorption spectra of L(10 μm) in the presence of 10 equivalents of different metal ions in methanol.

During the spectrophotometric titration of the solution of L in methanol by solution of AgNO₃ in the same solvent, the optical density of the ligand absorption band (λ_max=323 nm) decreases, and the characteristic absorption band of the complex Ag-L with λ_max=424 nm appears and grows gradually with the increasing concentration of Ag⁺ (not shown). This process parallels the color change observed by the naked eye.

The insert shows color change to the naked eye upon the addition of Ag+ to the solution of L under otherwise similar conditions.

To further investigate the interaction of Ag⁺ and L, the fluorescence spectra were analyzed (Figure 2). Compound L shows a very weak fluorescence at 424 nm in the absence of metal ions with the fluorescence intensity of approximately 26 a.u. After the addition of metal ion, including Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Na²⁺, Ni²⁺, Pb²⁺, or Zn²⁺, changes in the fluorescence emission are minimal. Even in the presence of Al³⁺ or Cr³⁺ when a slight red shift in UV absorption is observed, the fluorescence spectra are still very similar to that of the free ligand L. By contrast, the addition of Ag⁺ (10 equivalents) causes a remarkable enhancement of fluorescence intensity at 424 nm by a factor of approximately 5.5 (Figure 2A). To validate the selectivity of L toward Ag⁺, competition experiments in the presence of Ag⁺ only (fluorescence intensity F₀) and in the presence of Ag⁺ and other ions (fluorescence intensity F) were conducted. As shown in Figure 2B, the maximum fluorescence intensity ratio F/F₀ is approximately 1.28 for K⁺ and the minimum value of 0.86 is observed in the presence of Pb²⁺. These results demonstrate that the enhancement in fluorescence intensity resulting from the addition of Ag⁺ is not influenced significantly by the addition of the background metal ions. Additional experiments showed that the fluorescence quantum yield of the ligand L at 424 nm is increased from 0.21 to 0.72 in the presence of Ag⁺ in methanol solution.

Figure 2

Selective fluorescent spectrum of the interaction of Ag ion and L. (A) Fluorescence spectra of L (10.0 μm) in the absence and presence of various cations (100.0 μm) in methanol solution (λ_ex=323 nm). (B) Fluorescence intensity ratios F/F₀ of L (5.0×10^-7 M, constant) at 424 nm (λ_ex=323 nm) in methanol in the presence of Ag+ only (F₀) and in the presence of Ag+ and 10 equivalents of additional metal ion (F).

The fluorescence titration experiments of L with Ag⁺ in a methanol solution were also investigated in the range of molar ratios [Ag⁺]/[L]=0–60. The fluorescence intensity increases with the increasing concentration of Ag+ and reaches a plateau with a ratio of [Ag⁺]/[L] >20 (not shown).

To determine the stoichiometry of the Ag-L complex, Job’s method was applied using fluorescence titration experiments (Figure 3). As can be seen, the fluorescence intensity reaches a maximum for the ratio of [Ag⁺]/{[Ag⁺]+[L]} of 0.5, which indicates a 1:1 stoichiometry of Ag⁺ to L in the complex.

Figure 3

A 1:1 stoichiometry of the host-guest relationship realized from the Job’s plot between the probe L and the Ag⁺.

Experimental

¹H NMR (600 MHz) and ¹³C NMR (150 MHz) spectra were measured on the Bruker instrument. UV-vis absorption spectra were recorded on a TU-1901 ultraviolet and visible spectrophotometer (1 cm quartz cell) at 25°C. Fluorescence measurements were conducted on a Perkin Elmer LS55 fluorescence spectrometer using a 1-cm quartz cell at 25°C, with excitation and emission slit widths of 10 and 4 nm, respectively, and excitation wavelength at 323 nm. Starting materials 2 and 3 were synthesized according to the reported procedures [20].