Selective fluorescence sensor for Cu²⁺ with a novel triazole Schiff-base derivative with coumarin units

Introduction

Selective recognition and sensing of transition metal ions have become a focus of numerous studies in supramolecular chemistry owing to their importance in chemical, biological, and environmental processes [1–4]. Among the essential heavy metal ions in the human body, Cu²⁺ is third in abundance after Fe³⁺ and Zn²⁺, and it plays important roles in many fundamental physiological processes. Either deficiency or overdose of copper can induce some serious diseases [5–7]. Consequently, much attention has been given to the development of selective Cu²⁺ sensors [8–10]. However, owing to the low concentrations at which this metal ion is present in biosystems, high-sensitivity probes are necessary for practical applications.

Schiff bases are known to form stable complexes with transition metal ions and to display different optical properties from the ligand itself. Recently, there have been some excellent studies on chemosensors and molecular logic gates based on Schiff bases [11–13], the nitrogen atoms which easily coordinate metal ions. Owing to their intrinsic high fluorescence quantum yield, good water solubility, and viability for chemical transformations, coumarin derivatives have attracted much attention as some of the most popular fluorophores amenable to a novel sensor design [14, 15]. In this paper, a novel Schiff-base derivative bearing a coumarin unit 3 was synthesized, characterized, and its recognition ability for Cu²⁺ was studied. The results showed that probe 3 is an excellent selective probe for Cu²⁺ in the presence of other metal ions.

Results and discussion

The target product 3 was synthesized using a three-step procedure as shown in Scheme 1. The preparation of the intermediate products 1 and 2 has been reported previously [16, 17]. The desired triazole Schiff-base derivative 3 was obtained by condensation of 1 with 2. The target compound was characterized by IR, MS, ¹H NMR, ¹³C NMR, and elemental analysis.

Scheme 1

Synthesis of the triazole Schiff-base derivative 3.

UV-vis spectroscopic changes of the prepared chemosensing ensemble upon addition of various cations (Ca²⁺, Cd²⁺, Cu²⁺, Hg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Na⁺, Al³⁺, Fe³⁺, Ag⁺) are shown in Figure 1. A significant change in the absorption intensity is observed upon addition of Cu²⁺. No influence on the ultraviolet absorption of 3 is observed in the presence of other tested ions. Thus, it can be concluded that compound 3 selectively recognizes Cu²⁺.

Figure 1

UV-vis spectra of compound 3 (10 μm) upon addition of various metal ions (20 μm) in a mixture of CH₃ OH/DMF (9:1).

As shown in Figure 2, compound 3 exhibits a weak fluorescence emission at 551 nm in CH₃ OH/DMF medium. After addition of Cu²⁺ to the solution of compound 3, the fluorescence intensity is enhanced remarkably, that is, the emission intensity is increased 12-fold. Fluorescence enhancement observed for 3 is attributed to the formation of the 3-Cu²⁺ complex, as a result of which the electron transfer from nitrogen atoms to the coumarin fluorophore is suppressed, resulting in fluorescent enhancement. Importantly, no change in fluorescence occurs in the presence of the other cations including Ca²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Na⁺, Al³⁺, Fe³⁺, Hg²⁺, and Ag⁺ (not shown).

Figure 2

Fluorescence emission spectra of compound 3 upon addition of cupric ion (10 μm) in a mixture of CH₃ OH/DMF (9:1).

The sensitivity of the fluorescent probe 3 was estimated by changing amounts of Cu²⁺ from 0 equiv to 3 equiv with regard to the ligand and quantitatively assessing the fluorescence emission intensity change (Figure 3). As already mentioned, before addition of Cu²⁺, the ligand is almost nonemissive. After the gradual addition of Cu²⁺ to the ligand, a significant increase in fluorescence intensity can be observed. From these titrations, the association constant of compound 3 for Cu²⁺, K_a =4.62×10⁶, and the limit of detection, LOD =3.72×10^-7m, were obtained.

Figure 3

Fluorescence emission spectra of compound 3 (10 μm) for Cu²⁺ ion titration in a mixture of CH₃ OH/DMF (9:1).

Job’s plot was used to analyze the complexation ratio between compound 3 and Cu²⁺ ion. As shown in Figure 4, the maximum point at the mole fraction of 0.33 indicates the complexation ratio of compound 3 and Cu²⁺ as 2:1.

Figure 4

Job’s plot for determining the stoichiometry of receptor 3 and Cu²⁺ ion in CH₃ OH/DMF (9:1). I and I₀ are the fluorescence intensity of 3 in the presence and absence of Cu²⁺, respectively. The total concentration of 3 and Cu²⁺ ion is 0.1 mm, λ_ex=392 nm.

Conclusion

A new triazole Schiff-base derivative was designed and synthesized. Compound 3 displays selective UV-vis absorption and fluorescence changes upon the addition of cupric ion. The design of Schiff-base hosts for selective binding of other ions is currently under investigation.

Experimental

Coumarin aldehyde 1 [17] and 3,5-diphenyl-4-amino-1,2,4-triazole 2 [16] were prepared as previously described. Other chemicals were obtained from Aladdin and used without further purification.