A terbium-based time-resolved luminescent probe for sulfide ions mediated by copper in aqueous solution

A terbium-based time-resolved luminescent probe for sulfide ions mediated by copper in aqueous solution

Inorganic Chemistry Communications 50 (2014) 31–34 Contents lists available at ScienceDirect Inorganic Chemistry Communications journal homepage: ww...

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Inorganic Chemistry Communications 50 (2014) 31–34

Contents lists available at ScienceDirect

Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche

A terbium-based time-resolved luminescent probe for sulfide ions mediated by copper in aqueous solution Yanmeng Xiao a,b, Yanqin Yang a,b, Guiyan Zhao a,b, Xinxiu Fang a,b, Yongxia Zhao a,b, Pengran Guo c, Wei Yang a,⁎, Jingwei Xu a,⁎ a b c

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Changchun 130022, China University of Chinese Academy of Sciences, Beijing 100039, China Guangdong Provincial Public Laboratory of Analysis and Testing Technology, Guangdong Institute of Analysis, Guangzhou 510070, China

a r t i c l e

i n f o

Article history: Received 18 September 2014 Received in revised form 12 October 2014 Accepted 13 October 2014 Available online 22 October 2014 Keywords: Time-resolved BBPNA-(DO3A-Tb3 +)2 Luminescence lifetime Signal-to-noise

a b s t r a c t A novel Tb(III) complex-based time-resolved luminescent probe, BBPNA-(DO3A-Tb3+)2 (L), was designed and synthesized for the recognition of Cu2+ and sulfide ions in aqueous solutions. The luminescent probe L was efficiently quenched by Cu2+ and displayed “on–off” type luminescence change towards Cu2+. Once combined with Cu2+ to form the non-luminescent L–Cu complex, the L–Cu complex could serve as an “off–on” probe for sulfide ions due to the high affinity of sulfide ions towards Cu2+. In the presence of S2−, the L–Cu complex reacted with S2− to cause the release of the Cu2+ and turn on the luminescence of L again. The rebuilding of luminescence was highly selective to S2− over other common anions, which allowed L function as an “on–off–on” probe for Cu2+ and S2− alternately. Especially, the luminescence lifetime of the probe was about 1.91 ms, much longer than most organic fluorophores. Due to the long luminescence lifetime of the probe L, the probe could be used for time-resolved luminescence measurements to eliminate the interferences from autofluorescence and scattering lights and significantly to improve the signal-to-noise for detection. © 2014 Published by Elsevier B.V.

Besides iron and zinc ions, copper ion (Cu2+) is the third most abundant essential transition metal ion in the human body. It is involved in various vital physiological processes [1,2]. On the other hand, taking in excess amount of copper may lead to liver damage and kidney damage, gastrointestinal disturbance, and a series of neurological problems [3,4]. In addition, copper ion is also a significant environment pollutant which is widely spread in soil and water. In view of the critical role of copper ion in the biological systems and environment, development of rapid and sensitive probes for the detection of Cu2+ in biological and environment systems becomes very important. Due to the paramagnetic nature of Cu2 +, the binding of Cu2 + could induce intrinsic fluorescence quenching and lead to a turn-off signal [5,6]. Interestingly, the resultant non-fluorescence ensemble Cu2 + complex could be explored as an anion selective probe. As a toxic traditional pollutant, sulfide anion is not only generated from industrial processes but also from biological metabolism. To our knowledge, high concentration of sulfide ions can damage mucous membranes of living organisms and cause various biological issues in living organisms. The protonated HS− or H2S forms are even more toxic and caustic than the sulfide ion itself. However, recent studies have recognized H2S as the third gaseous transmitter, in addition to

⁎ Corresponding authors. E-mail addresses: [email protected] (W. Yang), [email protected] (J. Xu).

http://dx.doi.org/10.1016/j.inoche.2014.10.016 1387-7003/© 2014 Published by Elsevier B.V.

nitric oxide (NO) and carbon monoxide (CO) [7,8]. It is produced endogenously in mammalian systems by enzymes, including pyridoxal-5′phosphate(PLP)-dependent enzymes and pyridoxal-5′-phosphateindependent enzymes [9,10]. The concentration of H2S in blood is about 10–100 μM [11]. H2S is involved in a number of physiological and pathological processes, including inhibition of insulin signaling [12], relaxation of vascular smooth muscles [13], regulation of inflammation and O2 sensing [14,15]. Otherwise, its deregulation has been correlated with the symptoms of Alzheimer's disease [16], Down syndrome [17], diabetes [18], chronic kidney disease [19], and liver cirrhosis [20]. Accordingly, the development of sensitive and selective techniques for its detection and measurement is very important. Other than the traditional destructive methods including colorimetric [21], electrochemical [22,23], and gas chromatography assays [24], fluorescence has received great attention recently due to its high selectivity and sensitivity, cell permeability, and offering real-time monitoring in vivo. Several fluorescent probes for H2S have been reported since 2009, mainly through three mechanisms [25] (Table S1): H2S-mediated reduction of azides to amines, trapping of H2S via nucleophilic addition, and copper sulfide precipitation. The former two types of “reactive” probes based on irreversible sulfide-specific chemical reactions are sensitive but limited by relatively long reaction time, poor selectivity over reactive oxygen species (ROS) [26], and relatively strict reaction conditions. The latter takes advantage of the competition of Cu2+ between the probe and S2−, which reacts with Cu2+ to form a low-solubility CuS

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(Ksp = 1.27 × 10 −36) [27]. Utilizing this strategy, several fluorescencebased copper complex sensors have been designed. Compared to organic fluorescence dyes, luminescent lanthanide complex-based probes possess unique luminescent properties including long luminescence lifetime, large Stokes shift, nice photo stability, and sharp emission profile [28,29]. Considering these advantages, luminescent lanthanide probes have been demonstrated as a powerful tool to eliminate background noise for the time-resolved luminescence detection of various biological and bioactive molecules in biological environments [30–37]. Now, we report a time-resolved probe for the detection of S2−. So far as we know, this kind of probe should be the first reported. In this study, we designed and synthesized a binuclear Tb3+ complex according to the previous method [38], in which two DO3A rings (1,4,7,10-tetraazacyclododecane 1,4,7-triacetic acid) [39] for Tb3+ chelating were connected with a 2,6-bis(pyrazol-1-yl) pyridine scaffold that functioned as a receptor for Cu2+ coordination. The luminescence probe L was synthesized by the nucleophilic substitution reaction of compound 1 with compound 2 by three steps (Scheme 1), the detailed synthesis steps were described in the support information (Scheme S1). Once chelating with Cu2+ to form the L–Cu complex, the luminescence of L was quenched by the paramagnetic Cu2+ center [40]. In the presence of S2−, Cu2+ was taken off from the non-luminescence L–Cu complex to form CuS precipitation, resulting in luminescence recovery of L. Thus, complex L exhibited “on–off–on” luminescence response along with the addition of Cu2+ and S2− alternately. The UV–Vis spectrum of L was selectively affected by Cu2+ among various metal ions (Na+, K+, Ca2 +, Mg2 +, Cu2 +, Zn2 +, Al3 +, Mn2 +, Fe3 +, Fe2 +, Cd2 +, Ni2 +) (Fig. S7). Upon addition of Cu2+, the absorbance at 247 and 295 nm decreased gradually and three new peaks at 272, 280, and 340 nm appeared which indicated the interaction between Cu2+ and L. To further confirm the selectivity of L towards Cu2+ over other metal ions, the luminescence emission spectra were carried out. Both the steady-state and time-resolved luminescence emission spectra of L displayed the Tb3 + emission pattern with a main emission peak at 545 nm when excited at 295 nm, showing a large Stokes shift of approximately 250 nm. Besides, the steady-state spectrum also included fluorescence around 367 nm derived from the connector. With the addition of different 1 equiv. metal ions (Fig. 1), the luminescence intensities were not significantly altered, except with 1 equiv. Cu2 +, which effectively quenched the luminescence (Fig. 1). According to the luminescence turn-off behavior shown in Fig. 1, the probe L could recognize Cu2+ ions among the test metal ions. Job's plot analysis (Fig. S8) and titration studies were carried out to investigate the binding property of probe L towards Cu2+, which indicated that L responded to Cu2 + in 1:1 stoichiometry (Fig. 2a, inset). The association constant of Cu2 + was obtained as 3.37 × 104 M− 1 from the titration data (Fig. S9). Moreover, the detection limit of the L for Cu2+ was estimated to be 1.7 × 10−5 M (Fig. S10). To evaluate the selectivity of the L–Cu complex towards S2−, the non-luminescence L–Cu complex was incubated with a series of physiological and environmental important anions, including F−, Cl−, Br−, I−,

Fig. 1. (a) Steady-state and (b) time-resolved (delay time, 20 μs; gate time, 2.0 ms) luminescence emission spectra of L (10 μM, 295 nm) with the addition of various ions (10 equiv. for Na+, K+, Ca2+, Mg2+, Zn2+, Al3+, Mn2+, Fe3+, Fe2+, Cd2+, and Ni2+; 1 equiv. for Cu2+) in HEPES-buffered (pH 7.4, 100 mM) aqueous solutions. 4− 3− − 2− and sulfates, such SCN−, NO3−, CO2− 3 , P2O7 , PO4 , HCO3 , and C2O4 2− 2− 2− 2− as SO4 , SO3 , S2O3 , S2O8 , and sulfoacetic acid disodium salt (SAS), which resulted in negligible luminescence regeneration. Only S2 − restored the luminescence completely (Fig. S11). In addition, some competitive biothiols such as L-cysteine (L-Cys), reduced glutathione (GSH) and dithiothreitol (DTT) were also tested to evaluate the selectivity and the result revealed that these competitive biothiols did not induce luminescence regeneration either. Interestingly, C2O2− 4 scarcely influenced the signal at 545 nm, but increased the luminescence intensity at 367 nm (Fig. S11a). In addition, competition experiments were performed in the presence of 10 equivalents of interfering anions, as

Scheme 1. Synthesis of L.

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Fig. 3. Time-resolved (delay time, 20 μs; gate time, 2.0 ms) luminescence spectrum of L–Cu (10 μM) in HEPES-buffered (pH 7.4, 100 mM) aqueous solutions upon addition of different concentrations of S2− (0–1 equiv.).

Fig. 2. (a) Steady-state and (b) time-resolved (delay time, 20 μs; gate time, 2.0 ms) luminescence spectra of the L (10 μM) upon addition of different concentrations of Cu2+ (0–2 equiv.) in HEPES-buffered (pH 7.4, 100 mM) aqueous solutions. Excitation wavelength: 295 nm. Inset shows the luminescence at 545 nm as a function of Cu2+ concentration.

shown in Fig. S12, these interfering anions did not alter the S2− induced luminescence enhancement significantly, implying the anti-interfering capability of probing S2− in the presence of these anions. The quantitative luminescence titration of S2− was performed, the luminescence of L increased steadily upon the increase of S2− concentration. From the S2− concentration-dependent time-resolved luminescence spectra (Fig. 3), the detection limit [41] for S2− was calculated to be 1.9 × 10−6 M (Fig. S13). The luminescence quantum yield of L was determined by the method described by Haas and Stein, using quinine sulfate in 1 N sulfuric acid as the standard (Ф = 0.546 [42]). As shown in Table 1, the quantum yield of L decreased from 2.26% to 0.77% upon addition 1 equiv. Cu2+ and returned to 2.03% in the presence of S2−. The luminescence lifetime (Fig. S14) of the L was approximately 1.91 ms and became 0.85 ms upon addition of 1 equiv. of Cu2 + (Table S2), which was much longer than those of general organic fluorescent dyes (in the nanosecond region) [43] (Table S1) and enabled it to be used as a time-resolved luminescent probe. Its long luminescence lifetime allowed it eliminate the influence of short-lived background fluorescence and scattering lights, providing high signal-tonoise ratios. In such cases, the fluorescence around 367 nm derived

from the connector in the steady state spectra were completely excluded by performing time-resolved measurements with a delay time of 20 μs (Fig. 1a and b). Compared with other H2S probes (Table S1), complex L provided the largest Stokes shift, about 250 nm, while other probes were all below 200 nm. Moreover, the response time of L was similar to that of probe in exploiting CuS precipitation mechanisms, and was faster than that of probes through azide reduction and H2S trapping mechanisms. These properties made this probe worth for the further developments. The proposed mechanism for S2− detection was supported by luminescence and MALDI-TOF-MS measurements. Upon addition of S2− in the L–Cu complex system, the luminescence of L was recovered, which indicated that S2− turned on the emission of L again by taking off the coordinated Cu2 + to form the low-solubility and stable CuS (Scheme 2). The association and dissociation of Cu2+ were confirmed by MALDI-TOF-MS. In mass spectra, complex L displayed a molecularion peak [M + H]+ at m/z 1284.4 (Fig. S5). In the presence of Cu2+, a peak at m/z 1346.3 was found, corresponding to [M + Cu2 + − H]+ (Fig. S6). When S2− was further added, only the peak at m/z 1284.4 was observed while the peak at m/z 1346.3 disappeared (Fig. S15), suggesting the loss of Cu2+ from the L–Cu complex system. In conclusion, we designed a novel water-soluble Tb3+-based timeresolved luminescence probe for the recognition of S2− anion in aqueous solutions based on the displacement approach. Complex L had a strong emission at 545 nm and selectively responded to Cu2+ to form the L–Cu complex. Once the L–Cu complex formed, the strong emission at 545 nm was quenched by the Cu2+ binding to the pyrazole center. With addition of S2−, S2− reacted with the Cu2+ in the L–Cu complex to liberate complex L and restored the luminescence of L again. This restoration of luminescence was highly selective to S2− over other common anions, which allowed the L–Cu complex function as an “off–on” time-resolved probe for S2−. In addition, the long luminescence lifetime rendered this probe as an excellent potential candidate for time-gated imaging.

Table 1 Luminescence quantum yields. Complex

λex (nm)

λem (nm)

Ф (%)

L L–Cu L–Cu + S2−

340 340 340

545 545 545

2.26 0.77 2.03

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Scheme 2. Graphic of the proposed mechanism of the signaling of S2−.

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