Responsive Lanthanide Coordination Polymer for Hydrogen Sulfide

Article pubs.acs.org/ac

Responsive Lanthanide Coordination Polymer for Hydrogen Sulfide Baoxia Liu and Yang Chen* State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing, 210096, People’s Republic of China S Supporting Information *

ABSTRACT: Metal organic coordination polymers have received great attention because of their flexible compositions and architecture. Here, we report the design and synthesis of a responsive lanthanide coordination polymer (LCP) for hydrogen sulfide (H2S), utilizing self-assembling of biomolecule nucleotide with luminescent terbium ion (Tb3+) and sensitizing silver ion (Ag+) in aqueous solution. LCP is highly fluorescent due to the inclusion of Ag+ ions, which sensitized the fluorescence of Tb3+ ions. H2S can strongly quench the fluorescence of LCP through its high affinity for Ag+ ions. Such configurated LCP material from initial building blocks showed high sensitivity and selectivity for H2S and was applied to the determination of H2S in human serum. LCP with Tb3+ ions also has a long fluorescence lifetime, which allows for timeresolved fluorescence assays, possessing particular advantages to probing H2S in biological systems with autofluorescence.

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particularly advantageous to the bioassays with autofluorescence. Although lanthanide ions based luminescence analysis has been widely applied in medical diagnostics, analytical sensors, and cell imaging, most of them have been limited to either molecular lanthanide compounds or inorganic nanocrystals.7 The species of these lanthanide compounds and nanocrystals are few, and their luminescent properties are not easy to regulate. MOCPs provide an interesting platform for assembling and modulating lanthanide ions and their ligands in a solid framework. Their luminescent properties can be perturbed by a small molecule or ion, thereby providing new chemical sensing ways. Hydrogen sulfide (H2S) as a toxic gas has been known for centuries. However, today the reputation of H2S is changing with being identified as the third biologically gaseous messenger (gasotransmitter) after nitric oxide (NO) and carbon monoxide (CO).8 More and more evidence is showing that H2S modulates a wide range of biological processes including the regulation of blood pressure, neurotransmission, and antiinflammation.9 Recently, the abnormal levels of H2S have been found to be related to Alzheimer’s disease,10 diabetes,11 and cancers.12 Glycolytic metabolism is markedly accelerated in cancer cells, causing the accumulation of glucose and methionine, which can react and form gaseous sulfur-containing compounds including H2S in tumor tissue.13 Therefore, cancer patients can produce certain odorous compounds, which can be smelled by dogs.14 The determination of H2S is associated with the diagnosis of cancers. Despite the rising interest in H2S biochemistry, fundamental questions regarding its production, mechanism of action, and its destruction remain. One

etal organic coordination polymers (MOCPs), constructed from metal ions and organic bridging ligands, have received great attention because of their diverse compositions and architecture1 and have shown great promise in a number of applications, including gas storage and separation, heterogeneous catalysis, molecular sensing, luminescent materials, and nanomedicine.2 One of the most interesting behaviors of MOCPs is their luminescence properties. Because of component and structural flexibility, the luminescent properties of MOCPs can be tailored for a particular application by varying the lumophore type, ligands, and their relative spatial arrangements in the lattices. The luminescence of MOCPs can also be perturbed by a small guest molecule or ion through a host−guest interaction, leading to wavelength shifts, intensity changes, or even new emission as a result of excimer or exciplex formation.3 The rich variety of MOCPs has caused much speculation that MOCPs could be used as sensing materials with specific recognition ability for molecules or ions. Recently, some fluorescent coordination polymers were shown to detect high explosives in the gas phase,4 whereas lanthanide-based luminescent coordination polymers were shown to respond to the presence of a metal ion or an anion in solution.5 However, these sensings of MOCPs are mainly conducted in organic solution or gaseous phase; less work in aqueous solution has been reported. In aqueous solution, the structure of crystal MOCPs may be changed or collapsed. Trivalent lanthanide ions are of tremendous interest in the past decades because of their unique luminescence features, such as large Stokes shift (>150 nm), sharp emission (3:1). There is a good linear correlation between the fluorescence intensity of AMP/Tb/Ag CP and the concentration of sulfide in the ranges of 1−120 μM (Figure 3, inset). The detection limit is 0.3 μM on the basis of a signal-to-noise ratio of 3:1. Nevertheless, sulfide ions with higher concentrations (>170 μM) cannot quench the fluorescence of AMP/ Tb/Ag CP totally. This could be because Ag+ ions within AMP/Tb/Ag CP were not transformed into Ag2S even if sulfide had been added to a saturation level. In addition, the fluorescence of AMP/Tb/Ag CP is pHdependent. At pH 7.4, the fluorescence intensity of AMP/Tb/ Ag CP reached the maximum (Supporting Information Figure S7). A very acidic or basic medium weakened the fluorescence intensity of AMP/Tb/Ag CP because of the dissociation of the coordination polymer caused by the protonation of the ligands or the formation of terbium hydrate. However, around the neutral pH, AMP/Tb/Ag CP is photostable even if stored for a month (Supporting Information Figure S8). To gain more insight into the Ag+-enhanced fluorescence of Tb3+, we investigated the fluorescence lifetime of AMP/Tb and AMP/Tb/Ag CP. Figure 4 shows that the emission lifetime of AMP/Tb after the incorporation of Ag+ increased from 1.01 to

Figure 2. Emission (right) and excitation (left) spectra of AMP/Tb CP (a), AMP/Tb/Ag CP (b), AMP/Tb/Ag CP in the presence of sulfide (c), and AMP/Tb CP in the presence of 500 μM Ag+ ions (d) in HEPES buffer (100 mM, pH 7.4) at an excitation of 293 nm light. Inset is the corresponding color photographs under a common 365 nm UV lamp.

luminescent; the peaks at 488, 545, 584, and 620 nm are the typical emission of Tb3+ ion, arising from its 5D4 excited state to 7 F6, 7F5, 7F4, and 7F3 ground state, respectively.7a After the inclusion of Ag+, AMP/Tb/Ag CP became strongly luminescent (Figure 2, curve b); the emission intensity was increased approximately 15 times when the mole ratio of Ag+ and Tb3+ in AMP/Tb/Ag CP is 1:1. The different amounts of Ag+ in AMP/ Tb/Ag CP have an effect on the fluorescence intensity of AMP/Tb/Ag CP. A Tb3+-to-Ag+ mole ratio of 1:1 produced the strongest fluorescence enhancement (Supporting Informa11022

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Figure 4. Fluorescence lifetime of AMP/Tb CP and AMP/Tb/Ag CP in aqueous solution.

1.29 ms. A longer emission lifetime implies the increase in the rate constant for radiative deactivation. This might be ascribed to (1) the d−f energy transfer of Ag+ (d block) to Tb3+ ion (f block)17a,21 and (2) the reduction of nonradiative deactivation due to the displacement of water molecules.22 Tb3+ ions with a high-energy excited state (5D4, 20 500 cm−1) as an energy acceptor match an energy transfer which lies in blue or shorter wavelength regions. We also found that the UV absorption of Ag+ within AMP/Tb/Ag CP at 203 nm greatly decreased in the presence of sulfide (Supporting Information Figure S9).23 This further suggested that Ag+ ion absorbed UV radiation to sensitize the emission of Tb3+, and the strong interaction of sulfide with Ag+ weakened the sensitization, leading to the fluorescence quenching of Tb3+. To examine whether other anions had such a quenching action, we tested the influence of CO32−, C2O42−, CN−, F−, Cl−, Br−, I−, SO42−, SO32−, PO43−, Ac−, and IO3−, which have a similar activity with Ag+ ion, on the fluorescence intensity of AMP/Tb/Ag CP. In consideration of the interaction of the thiol group with Ag+ ion,24 we also investigated the fluorescence intensity of AMP/Tb/Ag CP in the presence of cysteine (Cys) and glutathione (GSH). As shown in Figure 5, under identical conditions, only sulfide can produce a significant fluorescence quenching and no remarkable fluorescence responses were observed upon the addition of other anions. Even in the presence of CO32−, C2O42−, or SO32− at a concentration of 20-fold higher than that of sulfide (3.4 mM), SO42−, PO43−, Ac−, or F− at a concentration of 80-fold (13.6 mM), or CN−, IO3−, Cl−, Br−, or I− at a concentration of 2-fold (340 μM), the fluorescence of AMP/Tb/Ag CP has not an obvious change. Cys and GSH can slightly decrease the fluorescence of AMP/Tb/Ag CP (3:1). The recoveries of serum samples containing different concentrations of sulfide are 95.00− 103.00%, indicating a good precision and no obvious method error (Supporting Information Table S1). The determinations of serum samples were further validated by the classic methylene blue method (MB)30 (Supporting Information Figure S11). Their results are basically consistent (Supporting Information Table S1). Many literatures reported that the endogenous sulfide levels in serum are approximately in the range of 30−300 μM, although recent studies found that the concentrations of sulfide in serum are probably far lower than this value.27 This level decreased significantly among the patients with some diseases associated with H2S, approximately 20−70% of the normal value.10−12,26 According to this standard, the current method can satisfy the determination of sulfide in the serum. On the other hand, serum samples displayed high background fluorescence in the wavelength range of 300−800 nm, which covered the emissions of most fluorophores (Supporting Information Figure S12, curve a). But the background fluorescence of serum samples can be eliminated completely when a time-delay measure was applied (curve b). This demonstrated that AMP/Tb/Ag CP with a long fluorescence lifetime has particular advantages to detecting sulfide in biological systems with autofluorescence, such as tissue fluids, blood, and urine.

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CONCLUSIONS In summary, we built a responsive lanthanide coordination polymer for H2S from initial molecular and ionic building blocks. The introduction of Ag+ ions was found to be able to strongly sensitize the fluorescence of lanthanide coordination polymer. This lanthanide coordination polymer showed high sensitivity and selectivity for H2S in aqueous solution and human serum without the help of extra molecular recognition 11023

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Tb(NO3)3 aqueous solution (10 mM) and 1 mL of AgNO3 aqueous solution (5 mM) were added to 1 mL of AMP disodium salt water solution (10 mM); white precipitate was formed immediately. After stirring for 3 h, the white precipitate was collected by centrifugation at 10 000 rpm for 10 min. To remove unreacted reactants, we washed the precipitate with ultrapure water for several times. Element ratio by EDX elemental analysis calcd (atom %) for C10H12N5O7P·Tb·Ag: C/ N/O = 10:5:7. Found: C/N/O = 10.35:5.02:6.74. Finally, the obtained AMP/Tb/Ag CP (approximately 2.6 mg, dry) was dispersed in 1 mL of HEPES buffer (100 mM, pH 7.4) to form an AMP/Tb/Ag CP suspension. As a control experiment, AMP/Tb CP was synthesized under the same conditions except replacing AgNO3 by 1 mL of water. Fluorescence Response of AMP/Tb/Ag CP to H2S in Aqueous Solution. To 10 μL of AMP/Tb/Ag CP suspension, different volumes of Na2S solution (freshly prepared, 200 μM) and HEPES buffer (100 mM, pH 7.4) were added to make a final Na2S concentration of 0, 0.3, 0.8, 1, 10, 30, 70, 100, 120, and 170 μM, respectively; the total volume is 100 μL. These solutions were mixed well and stood for 3 min. Then, the variations of fluorescence of AMP/Tb/Ag CP at 544 nm were recorded under an excitation wavelength of 293 nm. Selectivity of the Determination of H2S. For the experiments of selectivity, 10 μL of stock solutions (1.7 mM) of F−, Cl−, Br−, I−, CO32−, C2O42−, CN−, SO42−, SO32− Ac−, PO43−, IO3−, Cys, and GSH were added to 10 μL of AMP/Tb/ Ag CP suspensions, respectively. Then different volumes of HEPES buffer were added to these mixtures until the total volume reached to 100 μL. After incubating for 3 min, the fluorescence intensities of these solutions were measured on the spectrofluorometer under the same conditions as the determination of H2S. The blank solution was composed of 10 μL of AMP/Tb/Ag CP suspension and 90 μL of HEPES buffer (100 mM, pH 7.4). Determination of H2S in Human Serum Samples. Human serum was collected from volunteers (Southeast University Hospital). A series of serum samples containing different concentrations of sulfide were prepared by adding different volumes of 2 mM Na2S solution (fresh) in 1 mL of original serum, respectively. To 80 μL of HEPES buffer (10 mM, pH 7.4) solutions containing 10 μL of Tb/Ag/AMP CP suspension, 20 μL of serum samples were added, respectively, After being incubated for 5 min, the fluorescence intensities at 545 nm were recorded under the same conditions as above. The classic methylene blue method (MB) with a slight modification was used as a control assay for H2S.30 The proteins in serum samples were removed using an ultrafiltration spin column (Sartorius, MWCO 30K Da) by centrifugation at a speed of 5000 rpm (rpm) for 30 min. For the MB method, briefly, 200 μL of zinc acetate solutions (5%, w/v) and 100 μL of pure water were added to 500 μL of serum samples. Then, 100 μL of 10 mM N,N-dimethyl-p-phenylenediamine dihydrochloride in 3.6 M H2SO4 and 100 μL of 30 mM Fe(NH4)(SO4)2 in 0.36 M H2SO4 were added. These mixtures were incubated for 20 min at room temperature; their UV−vis absorption at 666 nm was recorded.

unit, showing the great potential of metal organic coordination polymers in the design of chemical sensors by means of their component and structural flexibility. Such configurated coordination polymer with Tb3+ ions also has a long fluorescence lifetime, which allows for time-resolved fluorescence assays, possessing particular advantages to probing H2S in biological systems with autofluorescence. To the best of our knowledge, this is the first gas-sensitive coordination polymer which works in aqueous solution. We hope that the present strategy constructing sensing function from basic elements can be beneficial to the development and design of various molecular/ionic sensors.

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EXPERIMENTAL SECTION Chemicals and Solutions. Tb(NO3)3·6H2O (99.99%) was purchased from Rewin Rare Earth Metal Materials Co. Ltd. Metal salts (NaNO3, Na2CO3·9H2O, Na2C2O4, K2SO4, Na2SO3, Na2S·9H2O, Na3PO4, CH3COONa (NaAc), NaCN, NaF, NaCl, KBr, KI, KIO3), Zn(CH3COO)2·2H2O, Fe(NH4)(SO4)2·12H2O, cysteine (Cys), and glutathione (GSH) were purchased from Nanjing Chemical Reagents Co. Ltd. Adenosine-5′-monophosphate disodium (AMP) and N-2hydroxyethyl piperazine-N′-2-ethanesulfonic acid (HEPES) were obtained from Sangon Biotech (Shanghai) Co. Ltd. N,N-Dimethyl-p-phenylenediamine dihydrochloride (99.0%) was purchased from Sigma-Aldrich. 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. Ultrapure water (18 MΩ cm; Milli-Q, Millipore) was used for the preparation of all aqueous solutions. The stock solutions of various anions (1.7 mM), Cys (1.7 mM), and GSH (1.7 mM) were prepared by directly dissolving appropriate amounts of reagents in ultrapure water, respectively. These stock solutions were diluted as required. Unless otherwise stated, all chemicals are of analytical reagent grade and were used without further purification. At physiological temperature and pH (7.4), hydrogen sulfide (H2S) exists approximately 18.5% as H2S, 81.5% as HS−, and small amount as S2− in aqueous solution due to the dissociation equilibrium.28 In view of the uncertain content of NaHS reagent, we used Na2S as an H2S source, and “sulfide” to refer to the total concentration of H2S, HS−, and S2−. Instruments and Determinations. The morphology of coordination polymers (CPs) was examined by transmission electron microscopy (TEM) (JEM-2100, Japan). The samples for TEM analysis were prepared by dropping a little AMP/Tb/ Ag CP suspension containing 170 μM sulfide ions onto the copper grids. The elemental analysis was performed by scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometer (EDX, X-Max Oxford, U.K.). Fourier transform infrared (FT-IR) spectra were recorded with an Avatar 360 FT-IR spectrometer (Nicolet, U.S.A.). UV−vis absorption spectra were recorded with a UV-3150 spectrophotometer (Shimadzu, Japan). Fluorescence spectra were recorded on an LS55 luminescence spectrometer (PerkinElmer, U.K.) with a xenon lamp as excitation source. The detection solution was placed in a quartz microcuvette with 100 μL capacity and 2 mm light path. A delay time of 0.05 ms and a gate time of 2 ms were used. All the experiments were performed at room temperature. All error bars represent standard deviations from three repeated experiments. Preparation of AMP/Tb/Ag Coordination Polymer. AMP/Tb/Ag CP was synthesized by a previously reported method with minor modifications.18,29 Typically, l mL of

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR, SAED, EDX, XRD analysis, and additional data. This material is available free of charge via the Internet at http:// pubs.acs.org. 11024

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 25 83790171. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC) with Grant Nos. 60671014 and 20775012.

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