Fluorescence chemosensor containing 4-methyl-7-coumarinyloxy, acetylhydrazono and N-phenylaza-15-crown-5 moieties for K⁺ and Ba²⁺ ions

Introduction

The design and construction of chemosensors with high selectivity and sensitivity for various metal ions have become the focus of numerous studies due to their importance in chemical, biological and environmental processes [1], [2], [3], [4], [5], [6], [7]. Of various chemosensors known to date, the fluorescence sensors are excellent tools for detecting metal ions due to their high sensitivity and real-time detection [8], [9], [10], [11], [12], [13].

Potassium ion is one of the most abundant physiological metal ions in living organisms. The imbalance in K⁺ in the human body is an early indicator of certain diseases. An excessive concentration of Ba²⁺ in the human body may cause heart rate change, high blood pressure, kidney problem, significant weight loss and nerve palsy. Therefore, the selective detection of K⁺ and Ba²⁺ in the presence of other ions is of practical significance. However, the challenge exists in detecting and accurately measuring the concentrations of these ions in the presence of other ions, such as Na⁺, Ca²⁺ and Mg²⁺ [14], [15], [16], [17], [18], [19], [20], [21]. Few sensors have been developed for K⁺ and Ba²⁺ ions [22], [23]. Sensors for other metal ions are normally composed of crown ether and/or coumarin moieties [24], [25], [26], [27], [28], [29], [30]. On the basis of the information discussed above, a new coumarin derivative 1 was synthesized and characterized (Scheme 1). Compound 1 is capable of selective fluorescent recognition of K⁺ and Ba²⁺ ions, giving rise to a different fluorescent signal for each metal ion under the same conditions.

Scheme 1

The synthetic route to fluorescent sensor 1.

Results and discussion

The straightforward synthesis of compound 1 is shown in Scheme 1. Compound 1 was obtained in an analytically pure form and fully characterized. In order to examine the spectral sensitivity and selectivity of compound 1 as a metal ion sensor, spectroscopic studies in the presence of various metal ions were conducted in ethanol. UV-vis spectra and fluorescence emission spectra changes of 1 upon addition of five equivalents of various metal ions including Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Al³⁺, Fe³⁺, Tb³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Hg²⁺ are shown in Figure S1 and Figure 1, respectively. As can be seen, the addition of these metal ions does not cause significant UV-vis absorption spectral changes of 1. By contrast, a substantial change in fluorescence of 1 is observed in the presence of K⁺ or Ba²⁺. As can be seen from Figure 1, there are 4-fold and 2-fold fluorescence enhancements of 1 at λ_em=406 nm upon addition of K⁺ and Ba²⁺, respectively. Other ions cause a negligible response to fluorescence of 1.

Figure 1

Fluorescence emission spectra of 1 (7 μm) in ethanol in the presence of five equivalents of different metal ions (λ_ex=343 nm).

High selectivity is an important feature in the performance of a fluorescent sensor for the specific analyte over other competitive species. In order to evaluate the selectivity of 1 as a fluorescent chemosensor for the detection of K⁺ and Ba²⁺, the competitive spectral analysis in ethanol in the presence of other metal ions was conducted. Compound 1 (7 μm) was treated with five equivalents of K⁺ or Ba²⁺ and an additional metal ion (five equivalents) and fluorescence at 406 nm was recorded. The results are summarized in Figure 2A and B. As can be seen, the corresponding fluorescence responses of 1 to K⁺ and Ba²⁺ in the absence and presence of other ions are similar. The addition of K⁺ to the mixture of 1 and Ba²⁺ results in an enhancement of fluorescence intensity at 406 nm of 1 compared with the spectrum in the presence of Ba²⁺ alone (Figure 2B). This result is in agreement with the data in Figure 1 and demonstrates that 1 shows a preference to K⁺ relative to any other metal ion.

Figure 2

Panels (A) and (B) show the results of interference tests in ethanol of 1-K⁺ and 1-Ba²⁺ by other metal ions. Blue bar: 1 (7 μm) in the presence of five equivalents of another metal ion. Red bar: 1 (7 μm) in the presence of five equivalents of K⁺ or Ba²⁺ and five equivalents of another metal ion. Response of 1 (7 μm) is included as control. The excitation was at 343 nm and fluorescence at 406 nm was measured in all cases.

The sensitivity of sensor 1 was estimated by changing amounts of K⁺ and Ba²⁺ from 0–7.5 equivalents and 0–9.5 equivalents, respectively (Figure 3A and B). When 1 was titrated with K⁺ or Ba²⁺, the fluorescence emission intensity of 1 gradually increased. The maximum intensity was obtained for 7.5 equivalents of K⁺ and 9.5 equivalents of Ba²⁺, leading to 4.2-fold and 2.8-fold fluorescence enhancement, respectively, at λ_em=406 nm. This result is consistent with the superior binding ability of 1 towards K⁺, as already discussed. The observed fluorescence enhancement might be attributed to the inhibition of the photo-induced electron transfer (PET) process upon complexation of sensor 1 with K⁺ and Ba²⁺ [31]. The Benesi-Hildebrand equation was used to analyze the complexation ratio and binding constants. The results in Figure 3A and B indicate a 1:1 binding stoichiometry of 1 towards K⁺ and Ba²⁺. The calculated binding constants (lgK_s) for K⁺ and Ba²⁺ are 4.11 and 3.61, respectively.

Figure 3

Fluorescence emission changes of 1 (7 μm) in ethanol upon addition of 0–7.5 equivalents of K⁺ (A) and addition of 0–9.5 equivalents of Ba²⁺ (B) upon excitation with λ_ex=343 nm. Inserts are Benesi-Hildebrand plots of fluorescence emission at 406 nm versus the equivalents of K⁺ (A), Ba²⁺ (B) added.

For practical applications, the detection limit is also an important parameter. The limits of detection of 1 for the analysis of K⁺ and Ba²⁺ were calculated to be 2.28 μm and 5.21 μm, respectively. A good linear relationship between the fluorescence emission intensity at 406 nm of 1 and the concentration of K⁺ and Ba²⁺ in the respective ranges of 2.2 μm–50 μm and 5.0 μm–66 μm were obtained. In both cases, the coefficients of linearity were 0.99. The recognition reversibility of sensor 1 to K⁺ and Ba²⁺ were further verified by adding sodium tetraphenylborate (NaTPB) to the solution of 1-K⁺ complex and Na₂EDTA to the solution of 1-Ba²⁺ complex. The addition of NaTPB to a mixture of 1 with K⁺ or the addition of Na₂EDTA to a mixture of 1 with Ba²⁺ both resulted in the return of the original fluorescence at 406 nm, for free compound 1. These results demonstrate that 1 is as a reversible chemosensor for K⁺ and Ba²⁺ ions.

Conclusion

A new fluorescent chemosensor 1 was synthesized and used to detect K⁺ and Ba²⁺ ions at the respective concentrations in ethanol as low as 2.28 μm and 5.21 μm. Sensor 1 shows a preferential selectivity to K⁺ relative to other metal ions. The 1:1 binding stoichiometry of 1 towards K⁺ and Ba²⁺ was obtained by linear fitting analyses to the Benesi-Hildebrand equation. The binding constants (lgK_s) of 4.11 for 1-K⁺ and 3.61 for 1-Ba²⁺ were obtained.

Experimental

Solutions of metal ions (Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺, Fe³⁺, Co²⁺, Cu²⁺, Ni²⁺, Al³⁺, Cd²⁺, Hg²⁺, Tb³⁺ and Zn²⁺) were prepared from the corresponding metal chloride salts. Proton nuclear magnetic resonance (¹H NMR) spectra (500 MHz) were recorded on a Bruker Advance III 500 spectrometer with tetramethylsilane (TMS) as internal standard. Elemental analyses were done on a Vario micro-elemental analyzer. UV-vis and fluorescence spectra were recorded on Hitachi U-2900 and F-4600 spectrophotometers. Mass spectra were obtained using Thermo LCQ Advantage LC-MS instrument operating in electrospray ionisation (ESI) mode. N-Phenylaza-15-crown-5 (2), N-(4-formylphenyl)-aza-15-crown-5 (3) and 7-hydroxy-4-methylcoumarin (4) have been described in the literature and are commercial products.

Supporting information

Supplementary data for this article can be found in the online version.