1. Introduction

The noble metal gold (Au) and its salts have attracted intense attention owing to their potentials in catalysis [1, 2] and biomedical applications [3-5]. Gold ions are known to activate alkynes for nucleophilic addition, and a series of organic transformations has been carried out according to the characteristic alkynophilicity of gold ions [6-10]. Also, gold-based drugs have been widely used in the treatment of rheumatoid arthritis and tuberculosis [11-13]. However, gold salts such as gold chloride are also highly toxic because they can damage the liver, kidneys, and nervous system [14-16]. Furthermore, Au³⁺ ions have strong affinity to DNA, which could lead to the serious damage of DNA via the catalytic cleavage [17-19]. Therefore, it is highly desirable to develop effective methods for the specific, rapid, and real-time monitoring of Au³⁺ ions in the environment, agrochemicals, and potentially living systems.

Fluorescence sensing has become a preferable approach for Au³⁺ detection because of its high sensitivity and selectivity, versatility, and ease of use [20, 21]. Alkynes are often used as the recognition moietyfor gold ions, due totheirwell-acknowledgedalkynophilicity. Therefore, introducing a reactive alkyne moiety to the fluorophore could provide an efficient route to developing Au³⁺-selective fluorescent probes. Representative examples are Au³⁺-induced intramolecular cyclization of the rhodamine-based alkynes to yield oxazolecarbaldehyde derivatives [22-25]. Additionally, several alkyne-containing compounds as fluorescent probes for Au³⁺ ions based on fluorescein [26, 27], BODIPY [28], 1, 8-naphthalimide [29], and coumarin [30, 31] fluorophores are also successfully designed and reported in the literature. However, the most prominent drawback of this sensing mechanism is the cross affinity with other alkynophilic metal species such as Au⁺, Ag⁺, Pd²⁺, Ni²⁺, Cu²⁺ and Hg²⁺, which interferes with the detection of Au³⁺ ions. Hence, there is an urgent need to develop a fluorescent probe for Au³⁺ free from the cross-sensitivity of other alkynophilic metal species.

Herein, we present a colorimetric and fluorescent probe (NC7-AL) for the detection of Au³⁺ utilizing the characteristic alkynophilicity of gold ions with low detection limit, large Stokes shift, and rapid response. Probe NC7-AL shows a remarkable fluorescence "turn-on" response only toward Au³⁺ ions over Au⁺, Ag⁺, Pd²⁺, Ni²⁺, Cu²⁺, and Hg²⁺. Upon addition of Au³⁺ ions, color changes from light yellow to colorless could be observed with the naked eye, and the NC7-AL-based modified TLC plates is an easy and convenient testing method that can be used for Au³⁺ detection.

2. Experimental 2.1. Materials and equipments

All solvents were purchased and dried according to standard procedures before use. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker Avance Ⅱ spectrometer at 400 MHz and 100 MHz, respectively. ESI-TOF spectra were recorded over a Waters xevo G2 QT mass spectrometer. UV-vis absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer. Fluorescence spectra were obtained using a Hitachi F-7000 Fluorescence spectrometer. The fluorescence quantum yields were measured with HORIBA FluoroMax-4P.

2.2. Synthesis of 7-((4-(dimethylamino)phenyl)ethynyl)-2-oxoN-(prop-2-yn-1-yl)-2H-chromene-3-carboxamide (NC7-AL)

A solution of NC7 (0.333 g, 1 mmol), 1-hydroxybenzotriazole (HOBT, 0.203 g, 1.5 mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl, 0.211 g, 1.1 mmol), ethyldiisopropylamine (DIPEA, 0.496 mL, 3 mmol), and propargylamine (0.192 mL, 3 mmol) in anhydrous DMF (10 mL) was stirred at r.t. under N₂ atmosphere for 24 h. The crude product was extracted with CH₂Cl₂, washed with brine and purified by column chromatography on silica gel (CH₂Cl₂:petroleum ether=1:1, v/v) to afford 0.136 g of NC7-AL as a yellow solid in a 37% yield. ¹H NMR (400 MHz, DMSO-d₆): δ 8.91 (t, 1H, J=5.6 Hz), 8.87 (s, 1H), 7.98 (d, 1H, J=8.1 Hz), 7.58 (s, 1H), 7.50 (d, 1H, J=7.8 Hz), 7.42 (d, 2H, J=8.6 Hz), 6.74 (d, 2H, J=8.7 Hz), 4.13 (d, 2H, J=3.2 Hz), 3.16 (s, 1H), 2.98 (s, 6H). ¹³C NMR (100 MHz, DMSO-d₆): δ 160.97, 160.08, 153.95, 150.60, 147.16, 132.94, 130.47, 128.92, 127.45, 118.06, 117.81, 117.52, 111.79, 107.12, 96.39, 86.99, 80.70, 73.26, 39.61, 28.70. HRMS (ESI-TOF): m/z: calcd. For [C₂₃H₁₈N₂O₃ + H]⁺: 371.1317, found: 371.1396.

2.3. Preparation of test solution

In this paper, a solution of NC7-AL at 5 μmol/L in CH₃CN was used for testing. The solutions of various testing species were prepared in doubly distilled water, and CH₃CN-doubly distilled water (1:1, v/v) was used for Au³⁺ due to solubility problems. Various ions were prepared from AuCl, AuCl₃, Ce(NO₃)₃·6H₂O, Cs₂CO₃, NaCl, ZnCl₂, NiSO₄·6H₂O, CrCl₃·6H₂O, CoCl₂, MnCl₂·4H₂O, CaCl₂, MgSO₄, BaCl₂, KCl, CuCl₂, CuI, FeCl₂·4H₂O, FeCl₃, AgNO₃, HgCl₂, PdCl₂, and n-Bu₄N⁺CN^-.

3. Results and discussion 3.1. Synthesis of probe NC7-AL

Compound NC7 was synthesized from salicylaldehyde following literature procedures [32, 33]. Condensation of NC7 with propargylamine afforded NC7-AL as a yellow powder in a 37% yield (Scheme 1). The structure of NC7-AL was confirmed by ¹H NMR, ¹³C NMR, and HR-MS.

3.2. Spectroscopic properties

The UV-vis and fluorescence spectra of NC7-AL (5 μmol/L) were investigated in CH₃CN. As shown in Fig. 1, the NC7-AL solution displayed almost no fluorescence. Addition of Au³⁺ to this solution resulted in enhancement of an emission band at 420 nm upon excitation at 360 nm. The emission intensity reached its maximum with ca. 6 equivalent of Au³⁺. An approximate 200-fold fluorescence enhancement could be observed accompanied by a quantum yield increase from < 0.1% to 18.5%. The UV-vis spectra of NC7-AL displayed a similar sensing behavior toward the addition of Au³⁺. NC7-AL showed two absorption bands at 300 nm and 422 nm. With the addition of Au³⁺ (10 equivalent), the absorption band at 422 nm diminished, and a new absorption band with a maximum at 330 nm gradually increased. Concomitantly, the solution color changed from light yellow to colorless, facilitating direct observation by the naked eyes. A well-defined isosbestic point at 378 nm was found indicating a desired one-to-one type clean conversion. The Stokes shift was as large as 90 nm, which could minimize self-quenching.

We studied the kinetics of the fluorescence enhancement resulting from the addition of Au³⁺ (6 equivalent) to the solution of probe NC7-AL. As shown in Fig. S1 in Supporting information, the fluorescence intensity reached its maximum in ca. 20 s. Thus, all testing of UV-vis absorption and fluorescence were performed after 1 min. Moreover, a good linear relationship between the dosage of Au³⁺ (from 0 to 2 equivalent) and the emission intensity was observed (Fig. S2 in Supporting information), and the detection limit (3σ/k) [34] was as low as 3.58 nmol/L. These results show that probe NC7-AL has a rapid response and good detection sensitivity toward Au³⁺.

We further investigated the selectivity profile of NC7-AL against other metal ions, such as Cu²⁺, Cu⁺, Ba²⁺, Mn²⁺, Ni²⁺, Pd²⁺, Ca²⁺, Cs⁺, Mg²⁺, Co²⁺, Ce³⁺, Cr³⁺, Na⁺, K⁺, Zn²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Ag⁺, and Au⁺. The fluorescence intensity changes of the probe NC7-AL upon addition of various other metal ions are shown in Fig. 2a. It can be seen that the fluorescence of NC7-AL showed no obvious changes after the addition of metal ions except Au³⁺ ions. Remarkably, commonly coexistent alkynophilic metal ions, such as Au⁺, Ag⁺, Pd²⁺, Ni²⁺, Cu²⁺, and Hg²⁺ have no interference on NC7-AL. This is a notable advantage of NC7-AL compared to the existing probes for Au³⁺. As shown in Fig. 2b, bar graphs of NC7-AL toward various other metal ions (60 equivalent) both in the absence and presence of Au³⁺ (6 equivalent) were carried out to investigate the selectivity of NC7-AL to Au³⁺ in the presence of other metal ions. The fluorescence was activated when Au³⁺ ions were added to the solutions of NC7-AL and those metal ions, which indicated that the coexistence of other metal ions had little effect on the detection of Au³⁺. As a result, NC7-AL could efficiently detect Au³⁺ ions in the mixture of other species.

3.3. The sensing mechanism

We propose that the carbon-carbon triple bond on the 3-position of coumarin framework (a terminal ethynyl) in NC7-AL could coordinate with Au³⁺, which initiates a cascade to generate the oxazolecarbaldehyde (NC7-CHO). This mechanism is widely acknowledged in the literature [22, 24, 25]. To further confirm the speculation (Scheme 1), the ESI-TOF mass spectra were used to study the reaction process of NC7-AL by Au³⁺. The peak of [NC7-AL + H]⁺ at m/z=371.1396 was observed (Fig. 3a). When 6 equivalent of Au³⁺ was added to the solution, the peak at m/z=385.1191 appeared, which unambiguously proved the formation of [NC7-CHO + H]⁺ (Fig. 3b). In addition, the carbon-carbon triple bond on the 7-position of coumarin framework (an internal ethynyl) also had an affinity function with Au³⁺, which was confirmed by the divalent peak at m/z=338.3411 corresponding to the [NC7-CHO-Au(H₂O)(CH₃CN)Cl]²⁺ in Fig. 3b. To further demonstrate that Au³⁺ can coordinate with the internal ethynyl in probe NC7-AL, a certain amount of tetrabutylammonium cyanide (n-Bu₄N+CN^-) was added into the solution of NC7-AL pre-incubated with Au³⁺. The intense fluorescence immediately turned off (Fig. S3 in Supporting information), accompanying a distinctive color change from colorless to light yellow. The dramatic "on-off" response indicated that CN^- could sequester Au³⁺ from the complex NC7-AL-CHO-Au, and obtained free NC7-CHO. The result is in accordance with the blue-shift in the UV-vis absorption spectra of NC7-AL after addition of Au³⁺, and a similar mechanism has been reported by Emrullahoğlu and co-workers [35]. In light of the above, we speculated that the high selectivity of NC7-AL to Au³⁺ can be attributed to not only the Au³⁺-induced intramolecular cyclization of terminal ethynyl in the probe, but also the strong affinity of Au³⁺ to the internal ethynyl in NC7-AL. We think it is of great value in the rational design of Au³⁺ probes with high selectivity.

3.4. Applications

Au³⁺ ions are widely used in various industrial processes and do extreme harm to humans. As such, easy and convenient testing methods for practical application are highly demanded. Therefore, we prepared the test strips by immersing TLC plates in the CH₃CN solution of NC7-AL (10 μmol/L). Then we immersed the modified TLC plates into different metal ion solutions, and a distinct color changefromlightyellowtocolorlesswasobservedinthepresenceof Au³⁺, comparing to no color changes for the other metal ions (Fig. 4). Therefore, the NC7-AL-based modified TLC plates can conveniently detect Au³⁺ in solutions without any additional equipment.

4. Conclusion

In summary, a coumarin-based colorimetric and fluorescent probe (NC7-AL) for the detection of Au³⁺ ions was reported with a low detection limit, large Stokes shift, and rapid response. Probe NC7-AL exhibited a prominent "turn-on" fluorescent enhancement response only toward Au³⁺. High selectivity toward Au³⁺ was exhibited, and no cross-sensitivity was observed to other commonly coexistent alkynophilic metal ions. NC7-AL can detect Au³⁺ by the color change from light yellow to colorless without the use of any instrumentation.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Nos. 21102001, 21271035 and 21372005), Anhui Provincial Natural Science Foundation (No. 1608085MB39), Natural Science Foundation of Education Department of Anhui Province (Nos. KJ2015A047, KJ2014A013) and 211 Project of Anhui University.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.02.021.