Europium Luminescence Used for Logic Gate and Ions Sensing with

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Europium luminescence used for logic gate and ions sensing with enoxacin as the antenna Lixia Lu, Chuanxia Chen, Dan Zhao, Jian Sun, and Xiurong Yang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03593 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Analytical Chemistry

Europium luminescence used for logic gate and ions sensing with enoxacin as the antenna Lixia Lu†‡, Chuanxia Chen†‡, Dan Zhao†‡, Jian Sun*,†, and Xiurong Yang*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China * Fax: +86 431 85689278. E-mail: [email protected] ABSTRACT: Luminescent lanthanide ion complexes have received increasing attention because of their unique optical properties. Herein, we discovered that the luminescence of Europium (Ⅲ) (Eu3+) could be regulated by Ag+ and SCN- in seconds with enoxacin (ENX) as the antenna. Under given conditions, only the simultaneous introduction of Ag+ and SCN- could remarkably enhance the luminescence intensity of Eu3+-ENX complexes. This phenomenon has been exploited to design an “AND” logic gate and specific luminescence turn-on assays for sensitively sensing Ag+ and SCN- for the first time. Furthermore, the addition of S2- resulted in efficient luminescence quenching of the Eu3+/ENX/Ag+/SCN- system due to the strong affinity between Ag+ and S2-. Thus, a new luminescent sensing platform for S2- was established, which exhibited excellent selectivity and high sensitivity. S2- could be detected within the concentration range of 100 nM to 12.5 µM with a detection limit of 60 nM. Such sensing system features simplicity, rapidity and flexibility. Moreover, this proposed Eu3+-based luminescent assay could be successfully applied in the real environmental water sample analysis.

INTRODUCTION The exploration of systems capable of recognizing inorganic ions has attracted considerable interests since the inorganic ions not only play crucial rules in many biological and chemical processes, but also are implicated in plenty of diseases.1, 2 Most intriguing of all though, is the eventual goal of quantitatively determining ions faster and more accurate. Among the various detection means, spectrofluorometric technique stands out due to the involvement of inexpensive instruments, easy operation, fast response as well as high sensitivity. Recently, lanthanide ions-based luminescent materials have drawn extensive attention owing to their unique luminescence properties such as the large-stoke shift, long lifetimes and narrow emission bands.3 They have been exploited in physical and biological contexts,4 which contribute to the broad applications in display materials,5 laser,6 fiber amplifiers,7 biomedicine8 and bioimaging9. However, the luminescence of trivalent lanthanide ions (Ln3+) originating from 4f electron transitions is too weak to be observed because of the Laporte-forbidden transitions and the resultant low absorption coefficients. In general, this problem can be well resolved by the chelation of Ln3+ with strongly absorbing ligands for the sensitization of Ln3+ luminescence, which is called “antenna effect”.10 The efficient intermolecular energy transfer from the excited triplet state of the antenna ligand to the emitting electronic level of Ln3+ would give rise to the subsequent lanthanide emission. Hence, the selection of suitable antenna is the key to constructing highly efficient luminescent complexes. Up to now, a wealth of antenna ligands including nucleotides,11 DNA,12 anti-bacterial agents,13 quinoxaline14 and phthalic acid15 have been reported to form sensitized luminescent complexes with

europium (Ⅲ) (Eu3+) or terbium (Ⅲ) (Tb3+), the two best studied Ln3+ with appropriate energy gaps and strong luminescence intensity.16 The excellent properties of Ln3+ made these complexes well suited to serve as luminescent probes for various analytes3, by offering significantly reduced background signals and improved sensitivity. Especially, they have found the largest applications in bioanalytical field, and some of them have been even successfully commercialized in immunoassays, molecular diagnostics and drug discovery.17 Except for the flourishing biological fields, many efforts have been devoted to the inorganic ions sensing by using Ln3+based luminescent systems, especially metal ions sensing.18-22 Besides the antenna ligands, there are several other factors affecting the luminescence of Ln3+. Usually, the antennas have a high binding affinity toward either Ln3+ or other metal ions.3 Therefore, the introduction of certain metal ions (typically transition metal ions) may have a significant impact on the energy transfer from antenna to Ln3+ and the corresponding luminescence of antenna-sensitized Ln3+, which could be used for sensing metal ions. For instance, based on the fact that Hg2+ has a high affinity to N atoms, Tan et al. constructed a luminescent sensor for the sensitive detection of Hg2+ utilizing Tb3+-adenine complex.18 Additionally, they found that Ag+ was able to greatly enhance the luminescence of Tb3+-adenosine monophosphate complex, which has been applied in the Ag+ sensing.19 Besides, Yan et al. have fabricated various kinds of lanthanide functionalized metal-organic frameworks (MOFs) for the detection of Cd2+20 and Ag+21, 22, in spite of the tedious and time-consuming synthesis process of coordination polymers or MOFs in such analytical systems. Speaking of anions sensing, although some lanthanide complexes have been developed, the antenna ligands usually demand special design

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Scheme 1. Schematic representation of the ENX-based sensitization of Eu3+ luminescence regulated by Ag+ and SCN- used as an AND logic gate and a S2- sensor.

and synthesis. Few reports are related to anions detection with simplicity, sensitivity and facility heretofore.23 Therefore, we expected to devise a more convenient luminescent probe utilizing Ln3+ complexes for rapid and sensitive inorganic ions sensing, especially anions sensing. Enoxacin (ENX), a broad-spectrum fluoroquinolone antibacterial agent which contains active groups in the form of carboxyl (-COOH) or carbonyl (C=O) in the structure, has been demonstrated to be able to effectively sensitize the lumi nescence of Eu3+ in the previous reports.24 Herein, we developed a simple and facile strategy for regulating the luminescence of Eu3+ sensitized by ENX, during which Ag+ and SCNacted as activators. With a certain amount of ENX, only the simultaneous presence of Ag+ and SCN- could remarkably enhance the luminescence intensity of Eu3+. Inspired by these findings, we designed an “AND” logic gate. Further studies showed that the addition of S2- could effectively quench the luminescence of Eu3+/ENX/Ag+/SCN- complex, thus a sensitive sensor for S2- was constructed. In our opinion, S2- has ultrastrong affinity to Ag+, which might destroy the sensitizing effect. Once the ingredients mixed, the luminescence enhancement or quenching process occurred instantaneously, which allowed the whole detection procedures quick and simple.

EXPERIMENTAL SECTION Chemicals and apparatus. Eu(NO3)3•6H2O (99.99%) and AgNO3 were purchased from Sigma-Aldrich (St. Louis, MO,

USA). Enoxacin (ENX), N-2-hydroxyethyl piperazine-N’-2ethanesulfonic acid (HEPES), cysteine (Cys) and glutathione (GSH) were all purchased from Sangon Biotechnology Co. Ltd. (Shanghai, China). KSCN, Na2S and other inorganic salts were obtained from Beijing Chemical Reagent Co. Ltd (Beijing, China). HEPES buffer (100 mM, pH 7.4) was prepared by dissolving HEPES in ultrapure water; 10 M NaOH was used to adjust pH to 7.4. Unless otherwise stated, all the chemicals are of analytical reagent grade and were used as received without further purification. Ultrapure water (18.2 MΩ•cm; Milli-Q, Millipore) was used throughout. All of the measurements and experiments were conducted at room temperature (20 °C). UV/Vis absorption spectra were recorded with a Cary 50 UV/Vis spectrophotometer (Varian, USA). WFH 201B ultraviolet transmission reflector (Shanghai precision scientific instruments factory, China) was used to provide the 365 nm UV irradiation. Photograghs were taken with a digital camera. The photoemission spectra were recorded on F-4600 FL spectrophotometer (Hitachi, Japan). The excitation wavelength used was 347 nm for the spectra and the slit widths of the excitation and emission were both 5 nm. Fourier transform infrared spectra (FTIR) spectra were recorded on a Bruker Optics VERTEX 70 FTIR spectrometer in the transmission mode. The X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB VG Scientific 250 equipped with monochromatized Al Kα excitation and the binding energies were calibrated with C 1s (284.6 eV).

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Analytical Chemistry

Assays for SCN- using the Eu3+/ENX/Ag+ system. Typically, 100 µL Eu(NO3)3 aqueous solution (0.1 mM), 100 µL ENX aqueous solution (1 mM) and 100 µL AgNO3 aqueous solution (1 mM) were added to 100 µL HEPES buffer (100 mM, pH 7.4). Then, different volumes of KSCN solution (1 mM) were added to make a final KSCN concentrations of 0, 0.5, 1, 2.5, 5, 7.5, 10, 15, 20, 30, 40, 50, 75, 100, 150 and 200 µM, respectively; the total volume was 1 mL. These solutions were mixed well for several seconds. Then, the luminescence spectra of the solution were recorded under an excitation wavelength of 347 nm. Assays for Ag+ using the Eu3+/ENX/SCN- system. Typically, 100 µL Eu(NO3)3 aqueous solution (0.1 mM), 100 µL ENX aqueous solution (1 mM) and 20 µL KSCN aqueous solution (10 mM) were added to 100 µL HEPES buffer (100 mM, pH 7.4). Then, different volumes of AgNO3 solution (1 mM) were added to make a final AgNO3 concentrations of 0, 0.1, 0.2, 0.5, 2, 5, 8, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400 and 500 µM, respectively; the total volume was 1 mL. These solutions were mixed well for several seconds. Then, the luminescence spectra of the solution were recorded under an excitation wavelength of 347 nm. Assays for S2- using the Eu3+/ENX/Ag+/SCN- system. Typically, 100 µL Eu(NO3)3 aqueous solution (0.1 mM), 100 µL ENX aqueous solution (1 mM), 25 µL AgNO3 aqueous solution (1 mM) and 100 µL KSCN solution (10 mM) were added to 100 µL HEPES buffer (100 mM, pH 7.4). Then, different volumes of Na2S solution (freshly prepared, 1 mM) were added to make a final Na2S concentrations of 0, 0.1, 0.2, 0.5, 2, 5, 7.5, 10, 12.5, 15, 17.5, 20, 25, 30, 40, 50, 75 and 100 µM, respectively; the total volume was 1 mL. These solutions were mixed well for several seconds. Then, the luminescence spectra of the solution were recorded under an excitation wavelength of 347 nm. The determination of S2- in lake water. Lake water was collected from South Lake (Changchun, China) and filtered with 0.22 µm membranes to remove impurities. A series of water samples containing different concentrations of sulfide were prepared by adding different volumes of 10 mM Na2S solution (fresh) in 1 mL of original lake water sample, respectively. To 100 µL HEPES buffer (100 mM, pH 7.4) containing 100 µL Eu(NO3)3 aqueous solution (0.1 mM), 100 µL ENX aqueous solution (1 mM), 25 µL AgNO3 aqueous solution (1 mM) and 100 µL KSCN solution (10 mM), 100 µL lake water samples were added, sequentially. These solutions were mixed thoroughly. Then, the luminescence spectra of the solution were recorded under an excitation wavelength of 347 nm.

boxylate group with Eu3+.27 In addition, the shifts in the stretching mode of carbonyl group (from 1628 cm-1 to 1633 cm-1) suggested the binding of carbonyl group with Eu3+.28 Theoretically, the energy transfer (ET) could easily occur between ENX and Eu3+, as well as the subsequent sensitization of Eu3+ emission. However, we found that the Eu3+-ENX complexes displayed an extremely weak luminescence in aqueous solution (HEPES buffer, Figure 1), which might result from the occupation of unsaturated sites of Eu3+ by the H2O molecules and the subsequent emission quenching.12 In fact, the coordination sites of lanthanide complexes are often unsaturated, which are susceptible to secondary ligands or solvent molecules. The high frequency vibration of O-H band in water molecules might couple with the Eu3+ vibration, resulting in the inactivation of Eu3+ excited states through non-radiation transitions and the luminescence quenching.13 On the other hand, there should be intermolecular hydrogen bonds between uncoordinated carboxylate oxygen atom and piperazine nitrogen between the ENX molecules.29 Under light excitation, the nitrogen atom of piperazine might transfer its electron to the adjacent ENX molecule and such photoinduced electron transfer (PET) process would decrease the potential intramolecular energy transfer from ENX to Eu3+ (Scheme 1).18, 30 Previous reports have showed that certain transition metal ions can coordinate with some groups of the ligands, which result in a more efficient intermolecular energy transfer from ligands to lanthanide ions.19, 20, 31-33 Herein, we did find that Ag+ could enhance the luminescence intensity of Eu3+-ENX complexes in aqueous solution, but with an extremely weak extent (Figure 1). And the luminescence was quenched slowly when exposed to UV light. In our opinion, Ag+ might interact with the uncoordinated carboxylate oxygen site and piperazine nitrogen site in the ENX molecule, which partly interrupted the PET process. 18 In order to confirm this hypothesis, XPS analysis was performed (Figure S2, SI). After the addition of Ag+ to the ENX, the O 1s peak and N 1s peak were both shifted obviously, suggesting the coordination of Ag+ to piperazine nitrogen site and the carboxylate oxygen site.20, 34

RESULTS AND DISCUSSION The sensing scheme. ENX is an ideal antenna ligand and the binding of ENX to Eu3+ has already been applied in the biosensing, 25 not only because of its containment of carbonyl and carboxyl groups that can coordinate with Eu3+, but also due to the strong overlap between the emission spectra of donor (ENX) and the excitation spectra of accepter (Eu3+) 25, 26. In our analytical system, the coordination interaction between ENX and Eu3+ can be further elucidated by the FTIR analysis. As shown in Figure S1 (SI), the two bands at 1404 cm-1 and 1580 cm-1 in ENX were assigned to the antisymmetric and symmetric stretching modes of the carboxylate. After the addition of Eu3+, these two bands were shifted to 1382 cm-1 and 1569 cm-1 respectively, which reflected the interaction of car-

Figure 1. The luminescence excitation (left), emission (right) spectra and corresponding photos (inset) in the presence of different components. Ag+, 100 µM; SCN-, 200 µM; S2-, 200 µM; HEPES, 10 mM, pH 7.4. The photos inset were taken under a 365 nm UV lamp excitation using a digital camera.

More importantly and impressively, subsequent experimental results indicated that the addition of SCN- could signif-

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icantly enhance the luminescence intensity of Eu3+-ENX-Ag+ system, while the introduction of SCN- alone to the Eu3+-ENX complexes had little effect (Figure 1). Due to the acknowledged high affinity between Ag+ and SCN-, we attributed this unique finding to the fact that their coordination complexes interrupted the aforementioned PET process thoroughly on one hand, and formed a protective barrier that could block the approach and coordination of water molecules more effectively on the other hand, both of which greatly improved the efficiency of energy transfer from ENX to Eu3+ and the stability of the luminescence intensity. (Scheme 1) The corresponding luminescence excitation and emission spectra were shown in Figure 1. It can be seen that the introduction of Ag+ and SCNremarkably enhanced both the luminescence excitation and emission intensity of Eu3+-ENX complexes, accompanying with the observation of a strong red luminescence with naked eyes under a 365 nm UV lamp. Moreover, the luminescence changes of the system were finished instantaneously. Immediately after the solution being mixed thoroughly, it can be subject to luminescence measurement. Longer reaction time has little effect on the luminescence intensity (Figure S3, SI). The emission peaks at 593, 614, 652 and 697 nm are typically the characteristic peaks of Eu3+, and the addition of Ag+ and SCNdid not change the spectroscopic characteristics. Further experimental results showed that the addition of S2- to the system brought about the luminescence quenching again. The reason should be that S2- had a stronger affinity with Ag+ than SCN-.35 Accordingly, S2- could interact with Ag+ to form precipitation, which might destroy the sensitizing effect. The absorption spectra of solutions with different components were shown in Figure S4 (SI). The absorption maxima of Eu3+ were recorded at 264 nm. Apart from the two apparent absorption peaks exhibited by ENX at 262 nm and 340 nm, there was a weak absorption peak at 204 nm. The addition of Eu3+ to ENX caused a slight change in the absorption intensity of the peaks at 262 nm and 340 nm, but the absorption peak at 204 nm disappeared, which apprised the formation of Eu3+ENX complex. The absorption spectra in the presence of Ag+, SCN- or S2- were nearly identical with that of Eu3+-ENX except a slight change in the absorption intensity, indicating that the introduction of these inorganic ions did not change the coordination mode of Eu3+-ENX and the form of energy transfer. In order to gain more insight into the Ag+/SCN- enhanced luminescence of Eu3+ and verify our speculation, we investigated the luminescence lifetime of solutions with different contents and the results were shown in Figure 2. The luminescence lifetime of the Eu3+-ENX complex was 163 µs and the addition of Ag+ made negligible influence on this value, signaling that the introduction of Ag+ did not reduce the energy loss of the triplet state of Eu3+. This was in agreement with the instability of the luminescence of Eu3+-ENX-Ag+ system. However, the emission lifetime changed to 466 µs after the incorporation of both Ag+ and SCN-, which implied the great increase in the rate constant for radiative deactivation. This marked increase in the lifetime indicated the removal of the quenching O-H oscillators, which meant the displacement of the coordinated H2O molecules and a more effective energy transfer from ENX to Eu3+. All these observations confirmed our preceding analysis. The design of “AND” logic gate and the assays for Ag+ and SCN-. Since the first introduction by de Silva et al.,36 molecu-

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lar logic gates which respond to chemical, biological or optical input signals, have provoked great interest and inspired the generation of smart systems for diagnosis and biosensing applications.37-41 However, the majority of logic gates were constructed using biomolecules while small ions were scarcely implemented as inputs, especially inorganic anions. Herein, the sensitization of Eu3+ luminescence enabled the design of an “AND” logic gate.

Figure 2. The luminescence lifetime of different systems.

Firstly, we investigated the effect of the concentration of ENX on the luminescence intensity and the results were shown in Figure S5. Keeping the concentration of Ag+ fixed at 100 µM and SCN- 200 µM respectively, it can be seen that the luminescence intensity at 614 nm increased first with the increase of ENX concentration, whereas it leveled off when the concentration of ENX reached 100 µM. As a control, we studied the correlation between the luminescence intensity and the concentration of ENX in the absence of Ag+ and SCN-. The results suggested negligible changes in luminescence intensity treated with various concentrations of ENX. Therefore, the optimal concentration of ENX was chosen to be 100 µM. Then, the relationship between the luminescence intensity and the concentration of SCN- was studied. As shown in Figure 3a and 3b, the luminescence intensity continuously increased with the gradual increase of SCN- concentration. When SCN- concentration reached 200 µM, the luminescence intensity reached a plateau. A concentration of 0.5 µM SCNcould produce an obvious luminescence enhancement. Additionally, a linear calibration curve could be established in the lower SCN- concentration range of 0.5 µM to 30 µM. The specific response for SCN- was investigated by examining the luminescence signal of the Eu3+/ENX/Ag+ system in the presence of various small ions. It was found in Figure 3c that none of them could induce an obvious luminescence enhancement as SCN- even if their concentrations were 5 times higher than that of SCN-, indicating that the Eu3+/ENX/Ag+ system was suitable to be applied for SCN- detection with high sensitivity and selectivity. SCN- is widely employed in industries such as fabric dying, medicine, photography and prevention of erosion.42 It not only has detrimental effects on the environment due to the generation of highly toxic species, like CN-, CNCl through irradiation and chlorination, but also does harm to human health by a cumulative effect. Therefore, the detection of SCN- is of practical importance.

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Analytical Chemistry

Figure 3. (a) Luminescence spectra of Eu3+/ENX/Ag+ system (with 100 µM Ag+ added) in the presence of different concentrations of SCN- (0, 0.5, 1, 2.5, 5, 7.5, 10, 15, 20, 30, 40, 50, 75, 100, 150, 200 µM) in HEPES buffer, pH 7.4. (b) Plot of luminescence responses to Eu3+/ENX/Ag+ solution at 614 nm to the various concentrations of SCN-. Inset is the linear relationship between the luminescence responses at 614 nm and the concentrations of SCN-. (c) Bars represent the luminescence responses at 614 nm of Eu3+/ENX/Ag+ system to 20 µM SCN- and I-, and 100 µM other ions.

Figure 4. (a) Luminescence spectra of Eu3+/ENX/SCN- system (with 200 µM SCN- added) in the presence of different concentrations of Ag+ (0, 0.1, 0.2, 0.5, 2, 5, 8, 10, 25, 50, 75, 100, 150, 200, 250, 300, 400, 500 µM) in HEPES buffer, pH 7.4. (b) Plot of luminescence responses to Eu3+/ENX/SCN- solution at 614 nm to the various concentrations of Ag+. Inset is the linear relationship between the luminescence responses at 614 nm and the concentrations of Ag+. (c) Bars represent the luminescence responses at 614 nm of Eu3+/ENX/SCNsystem to 20 µM Ag+, and 100 µM other ions.

Next, the effect of Ag+ concentration on the luminescence intensity was studied. As depicted in Figure 4a and 4b, with the addition of an increasing concentration of Ag+ to the Eu3+/ENX/SCN- system, the luminescence intensity was gradually enhanced. Ag+ concentration was proportional to the luminescence intensity in the range of 100 nM to 500 µM, and when the concentration exceeded 500 µM, the luminescence intensity remained unchanged. Thus, this system also enabled the sensitive detection of Ag+. A linear calibration curve could be constructed in the lower Ag+ concentration range of 100 nM to 100 µM. The minimum detectable concentration (100 nM) met the 50 µg/L-1 (about 460 nM) standard of U.S. Environmental Protection Agency (EPA) set for the maximum allowable level of Ag+ in drinking water.43 To validate the selectivity of the Ag+-stimulated luminescence enhancement, a series of foreign ions were added to investigate the luminescence responses. Figure 4c distinctly shows that a remarkable luminescence enhancement was observed only in the case of Ag+, while the addition of other foreign ions brought about nearly negligible changes. Ag+ can accumulate in the human body through drinking water or food chain, resulting in pathological disorders such as cell toxicity and organ failure.44 Hence, it is of considerable importance to construct such a simple and sensitive method for Ag+ detection. The significant luminescence enhancement of Eu3+/ENX system in the simultaneous presence of Ag+ and SCN- inspired us to design an “AND” logic gate. In this logic gate, Ag+ and

SCN- were used as inputs, and the normalization luminescence intensity of the system at a wavelength of 614 nm as output. For output, strong luminescence intensity (>0.2) and weak luminescence intensity (