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A polyoxometalate-based supramolecular chemosensor for rapid detection of hydrogen sulfide with dual signals
Journal of Colloid and Interface Science 485 (2017) 280–287
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Regular Article
A polyoxometalate-based supramolecular chemosensor for rapid detection of hydrogen sulfide with dual signals Yongxian Guo a, Yanjun Gong a, Lubin Qi a, Yan’an Gao b, Li Yu a,⇑ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A polyoxometalate-based
A supramolecular fluorescence chemsensor used for H2S detection was built by modifying Cu2+ on the surface of supramolecular particles, and the quenching mechanism of generated CuS was explored.
supramolecular chemsensor CSS has been synthesized. The supramolecular chemsensor CSS can selectively detect H2S with negligible responsive time (second level). The chemsensor response to H2S with dual signals: fluorescence and absorption spectra.
a r t i c l e
i n f o
Article history: Received 1 July 2016 Revised 12 August 2016 Accepted 21 September 2016 Available online 22 September 2016 Keywords: Polyoxometalate Supramolecular chemosensor Rapid detection Hydrogen sulfide Dual signals
⇑ Corresponding author. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jcis.2016.09.047 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
a b s t r a c t Hydrogen sulfide (H2S) has been verified as an important biological mediator in human physiological activities, but its rapid and accurate detection is remaining a challenge. Based on our early work, Eu-containing polyoxometalate/ionic liquid-type gemini surfactant hybrid nanoparticles fabricated by EuW10O3632H2O (Eu-POM) and 1,2-bis(3-hexadecylimidazolium-1-yl) ethane bromide ([C16-2-C16im] Br2) via ionic self-assembly (ISA) strategy, we modified the hybrids with copper (II) ion and used them as a novel turn-off supramolecular fluorescence probe for H2S immediate response. Although copper (II) ions can cause decrease of the fluorescence intensity, the probe with moderate amount of copper (II) still has a high performance in emission property. The copper (II) ion-modified supramolecular sensor (CSS) shows dual signals in the fluorescence intensity and absorbance for H2S detection, and the detection limit is about1.25 lM. Furthermore, CSS displays high selectivity for H2S in the presence of other anions 2 2 and species (e.g. Cl, Br, I, SO2 4 , SO3 , S2O3 , AC , H2O2, HCO3 , L-cysteine, homocysteine and L-glutathione), and also have potential for preferential imaging in vivo. Besides, the fluorescence quenching mechanism of CSS in the presence of H2S was explored. CuS generated by the reaction between Cu2+ and H2S was testified to act as a quencher, and the nonradiative resonance energy transfer mechanism was speculated to be responsible for fluorescence quenching. It is anticipated that the as-prepared CSS will be used as an efficient chemosensor for the rapid detection of H2S, which is critical for the diagnosis of some diseases, e.g. Alzhermer’s disease, Down’s syndrome, and diabetes, etc. Ó 2016 Elsevier Inc. All rights reserved.
Y. Guo et al. / Journal of Colloid and Interface Science 485 (2017) 280–287
1. Introduction Hydrogen sulfide (H2S) with a special smell of rotten eggs has been regarded as the third endogenous gaseous signaling molecule, like nitric oxide (NO) and carbon monoxide (CO) [1,2]. Studies have shown that H2S is mainly produced by three pathways, including cystathionine b-synthase (CBS), cystathionine c-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3MPST)/cysteine aminotransferase (CAT) [3–5]. And H2S plays a vital role in regulating intracellular physiological processes, such as vasodilation, antioxidation, anti-apoptosis and anti-inflammation [6–9]. The abnormal level of H2S is considered as a reflection of some diseases such as Alzhermer’s disease, Down’s syndrome, diabetes and liver cirrhosis [10–12]. In particular, for a certain purpose, it is highly demanding to quickly, facilely and precisely detect the concentration of H2S in living biological systems. The traditional methods for H2S detection are electrochemical assay, colorimetry and gas chromatography [13–15]. However, these methods are destructive and inappropriate for real-time detection [16]. The method with fluorescence probes has attracted much attention with their advantages in real-time detection with superior designability, sensitivity and selectivity [17–19]. Beyond that, fluorescence imaging accomplished through fluorescence probe is becoming the most attractive imaging technique for in vivo detection [20–22]. So far, there are many fluorescence probes for H2S detection have been reported. Chang’s group proposed a new approach to detect biological H2S through the reaction between azide-based fluorescent probes and H2S [23]. Guo and his partners reported a ratiometric fluorescence probe for rapid detection of H2S in mitochondria according to the selective nucleophilic addition of HS to a specific merocyanine derivative in medium of near neutral pH value [24]. However, the detection mechanism of these fluorescence probes is majorly developed by the reduction reaction and nucleophilic reaction, which all need a relatively long responsive time (typically >20 min) [16]. And these probes are generally derivatives of dye molecules, whose synthetic processes are complicated and maybe cause some damages to the environment. Metal sulfide precipitation-based method has the advantages of short responsive time and high detection limit with an inorganic reaction, and there are some correlated reports [1,16,25]. Nagano et al. developed a novel fluorescent probe based on azamacrocyclic Cu2+ complex chemistry and applied it to biological H2S detection [1]. Tang et al. constructed a fluorescence probe with Cu(II)metalated 3D heterogeneous nano metal-organic framework for detection of H2S in living cells with high selectivity and sensitivity [16]. Rational designed supramolecular chemosensors, fabricated by noncovalent bonds and alleviated a time-consuming synthesis process in some extent, are promising substitute to overcome the limitations of traditional fluorescence probes. Among these supramolecular chemosensors, hybrid supramolecular systems, especially constructed by lanthanide-containing polyoxometalate and organic molecules, have inspired much attention in detection field with variable luminescence properties, i.e. narrow emission band, large Stokes shifts, long lifetimes, photostability and sensitive to ambient environment [26,27]. Wan et al. developed a recyclable polyoxometalate-based supramolecular chemosensor with dysprosium-containing POM and a block copolymer for efficient detecting carbon dioxide [28]. In our previous work [29], Eucontaining polyoxometalate/gemini surfactant hybrid nanoparticles with high fluorescence quantum yield (25.17%), long emission timescale (3.758 ms) and superior biocompatibility were fabricated by Na9EuW10O3632H2O (Eu-POM) and 1, 2-bis(3-hexadecyli midazolium-1-yl) ethane bromide ([C16-2-C16im]Br2) through ionic self-assembly strategy. In the present work, a supramolecular
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chemosensor probe was constructed with Eu-POM/[C16-2-C16im] Br2 supramolecular complex as a pre-probe and Cu2+ as a modifier (see Scheme 1). To the best of our knowledge, there are rarely reports on polyoxometalate-based hybrid supramolecules used for H2S detection [30]. Particularly, fluorescence intensity of Cu2+-modified supramolecular sensor (CSS) is quenched slightly by the electron- and/or energy-transfer processes, but still maintains a high emission intensity. The CSS response to H2S is speedy (second level), and the fluorescence measurement was carried out without any time interval. The CSS shows dual responses: fluorescent intensity decreases and the absorbance increases evidently with the elevating H2S concentration due to the generation of copper sulfide (CuS). Additionally, the CSS also exhibits superior selectivity for H2S, with good interference tolerance of other biologically 2 2 relevant anions and species, such as Cl, Br, I, SO2 4 , SO3 , S2O3 , AC, H2O2, HCO , L -cysteine, homocysteine and L -glutathione. This 3 probe not only could rapidly and specifically respond to H2S, but also provide a new method to the quantitative analysis of H2S. It has the potential application for preferential imaging of H2S in living cells.
2. Experimental 2.1. Materials The reagents listed as follows were obtained commercially and used without further purification. NaCl, NaBr, NaI, Na2S2O3, Na2SO3, Na2SO4, NaAC, NaHCO3, H2O2 (30% aqueous solution), L-cysteine, homocysteine and L-glutathione were purchased from J&K Chemical Technology, China. CuBr2 and Na2S9H2O were provided by Energy Chemical Technology, China. Both Europium nitrate hexahydrate (99%) and sodium tungstate dehydrate (99%) were obtained from J&K Chemical Technology, China. PBS buffer solution (10 mM, pH = 7.4) was purchased from Sigma-Aldrich. The triply distilled water used in this experiment was obtained by a SZ-97 automatic triple water distiller (Shanghai YR BioChem CO., Ltd. China).
2.2. Synthesis of [C16-2-C16im]Br2 and Eu-POM The synthesis processes of 1,2-bis(3-hexadecylimidazolium-1yl) ethane bromide ([C16-2-C16im]Br2) and Na9EuW10O3632H2O (Eu-POM) were carried out according to our previous work [29,31]. 2.3. Preparation of Cu2+-modified supramolecular sensor (CSS) The hybrid supramolecular pre-sensor was constructed by an one-step synthesis method (through ionic self-assembly strategy), mixing the solutions of [C16-2-C16im]Br2 (0.1 mM, 9 ml) and Eu-POM (0.1 mM, 2 ml) in a 50 ml flask under ultrasound. The complex solution shows Tyndall Effect apparently. The Cu2+-modified supramolecular sensor was obtained by adding CuBr2 solution (1.4 mM, 0.5 ml) to 3 ml complex solution mentioned above, and placed in ultrasound environment for 5 min.
2.4. Reaction with H2S Experiments of the CSS response to H2S were performed as follows: adding 0.5 ml prepared Na2S solution (10, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, and 1400 lM) into the CSS solution (3.5 ml), and the mixing solution was used to measure the fluorescent property and UV–vis absorption spectra.
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Scheme 1. The structures of polyoxometalate (left, top) and Gemini surfactant (left, bottom) used in this work. The schematic diagram of the construction process of CSS and its detection mechanism for H2S. The red ball means the supramolecular nanoparticles produced by ionic self-assembly strategy, the blue ball represents Cu2+, and the brown ball shows the generated CuS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.5. Characterization Zeta potential was measured by Malvern Zeta Sizer Nano (Malvern Instruments) in aqueous solution at 25 °C. Absorption spectra were measured by using a U-4100 instrument (Hitachi, Japan) in a quartz cell (1 cm). Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer with the excitation wavelength of 280 nm. The excitation and emission slit widths were 5.0 and 10.0 nm, respectively. For this system, we focused on the fluorescence spectra with the wavelength range from 570 to 670 nm. Time-resolved fluorescence measurements were taken on a FLS920 (Edinburgh) fluorometer, with the excitation at 280 nm and the maximum emission wavelength at 620 nm. The elemental mapping analysis images were taken using a field emissionscanning electron microscope (FE-SEM) instrument. The sample was made by dropping method: picked up a drop of complex solution and transferred onto a silicon substrate, and dried by exposure to air. The sample was coated by a layer of gold. FTIR spectra were measured by a VERTEX-70/70v FTIR spectrometer (Bruker Optics, Germany) from 400 to 4000 cm1 on pressed thin KBr disks.
3. Results and discussion 3.1. Construction of Cu (II)-modified supramolecular sensor (CSS) The preparation process of CSS was divided into two steps: (i). Mixing the solutions of [C16-2-C16im]Br2 and Eu-POM with a certain volume ratio to form supramolecular nanoparticles; (ii). Adding Cu2+ (for minimize the effect of counter ions, CuBr2 as substitution) solution with moderate concentration to form the supramolecular sensor (see Scheme 1). There was no perceivable coagulation of the colloid nanoparticles when the Eu-POM/[C162-C16im]Br2 supramolecular solution was kept standing for more than 3 days. To a certain extent, stability of the resulting CSS solution after adding Cu2+ was weaker than the supramolecular solution without Cu2+, however it could exist as colloidal solution instead of coagulation for many hours (5 h at least), which was enough to carry out the detection process.
Both inorganic cluster Eu-POM and cationic surfactant [C16-2C16im]Br2 are essential parts of the supramolecular system investigated here. Eu-POM has the advantages over other Eu-containing complexes: i) it has superior photoluminescent property, such as narrow emission bands, large Stokes shift, long life time and tunable emission, ii) Eu-POM supports binding sites to accept cationic surfactant to generate surfactants-encapsulated supramolecular structures. As for the key role played by the Gemini cationic surfactant in the detection of H2S, several contrast experiments were carried out. Other cationic surfactants, namely [C12-2-C12im]Br2, [C16im]Br and CTAB, were also used to fabricate supramolecular structures with Eu-POM according to the same synthetic method. But the fluorescence property (e.g. quantum yield and lifetimes [29]) of the supramolecular structures involved in these cationic surfactants was inferior to that of Eu-POM/[C16-2-C16im]Br2. In Table S1, the data convey that Eu-POM/[C16-2-C16im]Br2 has the highest quantum yield and the longest lifetimes among these supramolecular structures. Especially, the quantum yield of the surfactants-encapsulated Eu-POM/[C16-2-C16im]Br2 supramolecular structures is 0.25, more than 27 times of the quantum yield of Eu-POM aqueous solution, 0.0091. Although compared to some substances with ultrahigh quantum yield, this quantum yield value is not extremely high. Yet the fluorescence property is favorable based on the dual consideration of lifetimes and quantum yield. Based on the photoluminescent properties, we try to construct supramolecular chemsensor with Cu2+. It is well-known that Cu (II) as a heavy metal ion is a good quencher through electronand/or energy-transfer processes [32]. In this system, we also observed that Cu2+ could quench the fluorescence intensity of supramolecular structures. Fortunately, the quenching of fluorescent intensity could be tuned by regulating the content of Cu2+ in supramolecular sensor solution. As illustrated in Fig. 1, when we added different concentration of Cu2+ into the supramolecular complex solution, the emission intensity reduces from 197 to 135 at 592 nm and from 547 to 380 at 622 nm under 280 nm excitation. The inset scatter diagram displays that a negative linear relationship exists between the fluorescence intensity at 622 nm and the Cu2+ concentration. This suggests that although the fluorescent intensity is quenched to a certain degree, the chemosensor
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supramolecular structures with moderate doses of Cu2+ to form a sensor for the detection of H2S. Therefore Cu (II) was selected to modify the supramolecular sensor. 3.2. Detection of H2S
Fig. 1. Fluorescence spectra of supramolecular chemosensor solution at room temperature upon adding different concentration of Cu2+ (bulk concentration: 0– 200 lM; excitation wavelength: 280 nm). The inset shows a linear relationship between the fluorescence intensity at 622 nm and Cu2+ concentration.
still remains a high emission intensity. Therefore, a potential way to receive some chemical substance to further quench the fluorescence may be provided. In sum, the modified sensor could be used for detecting some chemicals. Besides, to study the evolution of surface charge on the EuPOM/[C16-2-C16im]Br2 nanoparticles upon addition of Cu2+, we performed the zeta potential measurements for the complex solution. Fig. 2 (black points and line) illustrates that the zeta potential value increases obviously with the concentration of Cu2+, which testifies that Cu2+ may be located on the surface of supramolecular particles in a absorption manner, and can offer some binding sites to receive H2S. As we all know, Cu element is an essential substance in many biological approaches. Although superfluous Cu2+ will cause liver or kidney damage, addition of a small amount of Cu2+ is harmless to living body. Moreover, in our previous work [29], we incubated hela cells with the Eu-POM/[C16-2-C16im]Br2 nanostructures, and found their good biocompatibility. These results give us an evidence that it is feasible to decorate Eu-POM/[C16-2-C16im]Br2
Fig. 2. Zeta potential of supramolecular chemosensor solution after adding different concentration of Cu2+ aqueous solution (black points and black line), and H2S (red points and red line) (Cu2+ with a fixed concentration, 200 lM). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It is reported that the Eu-containing polyoxometalates (Na9EuW10O3632H2O) show red luminescence upon UV excitation, and display the characteristic 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions of Eu3+ [33,34]. Especially, the transitions of 5D0 ? 7F2 observed at 614 and 622 nm are attributed to an electric dipole transition, and their intensities are extremely sensitive to the chemical microenvironments around Eu3+ ions [30]. Inversely, the 5D0 ? 7F1 transitions observed at 590 and 594 nm are assigned to a magnetic dipole transition, and the intensities are almost independent of the nearby micro-environmental changes for Eu3+. Therefore, the intensity ratio I(5D0 ? 7F2)/I(5D0 ? 7F1) is widely used to study changes in the chemical surroundings of Eu3+ ions [31]. So in this work, we concentrate on the changes of these two transitions, with the emission wavelength range from 570 to 670 nm excited by 280 nm. To monitor the CSS’s response to H2S, we chose Na2S as an equivalent due to the following three reasons i). Since the existence state of H2S is gas, it is difficult to determine the concentration or amount of H2S when adding H2S into the constructed supramolecular sensor solution; ii). Na2S solid could be hydrolyzed and release H2S in water according to the chemical equations as follows:
S2 þ H2 O ¼ HS þ OH HS þ H2 O ¼ H2 S þ OH iii). The choice of Na2S solution as the equivalent of H2S has also been reported by some other research groups [1,4]. Then, we investigated dual signals of the fluorescence spectra and UV–vis absorption spectra of CSS in PBS buffer solution (10 mM, pH = 7.4). As can be seen from Fig. 3a, the CSS (0.1 mM, in the presence of 200 lM Cu2+) always shows high fluorescence intensity in PBS buffer solution under 280 nm excitation. But upon addition of H2S, the emission bands are obviously quenched around 594 and 622 nm (Fig. 3a). Fig. 3b is the fitting line bansed on the points, the correlation coefficient (R) value is 0.99103, illustrates that there is a negative linear relationship between the fluorescence intensity at 622 nm and H2S concentration. The detection limit for H2S was found to be 1.25 lM. Then we compared the linear detection range, detection limit and responsive time of CSS prepared in this work with other detection methods reported by previous work. As shown in Table 1, the detection limit of CSS is at a favorable level and the responsive time is tremendously lower than the other methods. Moreover, the UV–vis absorption spectra of CSS without and with H2S in PBS buffer solution are depicted in Fig. 3c. The shoulder band at 250 nm of a tail absorption band, assigned to the characteristic band of Eu-POM, increased with the addition of H2S. Besides, the tail absorption band exhibits concentration-dependent increase in the UV–vis absorbance after treatment with H2S. Meanwhile, by naked eyes, the color of samples can be obviously observed to change from colorless to brown with the increase of H2S concentration (Fig. S1). Fig. 3d shows that there exists a linear growth correlation between UV–vis absorbance signal (at 400 nm) and H2S concentration, the correlation coefficient (R) value is 0.99875. Compared with Fig. 3b and d, we can conclude that the Fluorescence intensity is inversely proportional to H2S concentration, while the absorbance is in direct proportion to H2S concentration. It means that the higher fluorescence intensity or the lower absorbance, the lower concentration of H2S is. Additionally, we can pre-
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Fig. 3. Fluorescence spectra (a, excitation wavelength at 280 nm) and UV–vis absorption spectra (c) upon addition of Na2S (0–175 lM) into CSS solution (0.1 mM, in the presence of 200 lM Cu2+) at room temperature. The points and line show the relationship between the fluorescence intensity at 622 nm (b), absorption intensity at 400 nm (d) and H2S concentration, respectively.
Table 1 Comparison on the linear detection range, detection limit and responsive time of CSS in this work and various H2S detection methods reported previously. Sensor type
Linear detection range (lM)
Detection limit (lM)
Responsive time
Reference
HSip-1 Rhodamine GCTPOC Sulfidefluor-1 CouMC HBT derivate CSS
– 0–125 3.02–170 – 10–200 2.4–100 1.25–175
10 0.38 3.02 5 10 2.4 1.25
– 20 min 40 min 60 min 30 s 30 min Seconds
[1] [4] [21] [23] [24] [35] Present work
liminarily speculate that the quenching of fluorescence is caused by the production of CuS. To examine the chemoselectivity of CSS, various biologically relevant anions and species (12.5 mM) were used to incubate with the sensor in PBS buffer solution (10 mM, pH = 7.4) and their fluorescence response was investigated separately (Fig. 4). It should be noted that the concentration of the interfering anions and species used here is much higher than that in living cells. The anions we used as interference ions including halide ions (Cl, Br, I), sulfur 3 2 2 oxides ions (SO2 ), 4 , SO3 , S2O3 ), acetate (Ac ), bicarbonate (HCO and reactive oxygen species (H2O2) did not trigger turn-off fluorescence responsiveness. In addition, fluorescence spectra of CSS in the presence some interfering molecules (L-cysteine, homocysteine and L-glutathione, 125 lm) were measured, respectively (Fig. S2I). The experimental results indicate that they could affect the fluorescence intensity of the sensor, yet the color of supramolecular chemsensor solutions containing these molecules was obviously different from Na2S (Fig. S2II). This can help us to easily distinguish Na2S from L-cysteine, homocysteine and L-glutathione with the
naked eyes. These results illustrate that the CSS could be applied to the detection of H2S. 3.3. Quenching mechanism of Cu2+ and CuS To research the quenching mechanism, firstly, we wonder if individual H2S could quench the fluorescence of CSS without Cu2+. As illustrated in Fig. S3, for the supramolecular structures without Cu2+, the emission intensity at 622 nm is almost constant in the H2S concentration of 0–175 lM. This conveys a message that single H2S could not quench the fluorescence of CSS, and only co-existence of Cu2+ and H2S can contribute to the fluorescent quenching. In other word, the generation of CuS may be liable for fluorescent quenching. As stated above, we have proved that Cu2+ in CSS is located on the surface of the nanoparticles (the black plot in Fig. 2). Meanwhile, within the investigated concentration range, zeta potential values display obvious decline from 30.7 mV to 8.81 mV upon addition of various concentration of H2S into the CSS solution (with
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Fig. 4. Fluorescence response to various anions and species (12.5 mM) in PBS 2 (10 mM, pH = 7.4) at 622 nm. 1, probe alone; 2, Cl; 3, Br; 4, I; 5, SO2 4 ; 6, SO3 ; 7, S2O2 3 ; 8, AC ; 9, H2O2; 10, HCO3 .
200 lM Cu2+) (the red points in Fig. 2). It indicates the formation of new quenching agent, CuS, on the surface of CSS. Meanwhile, field emission-scanning electron microscope (FE-SEM) energy dispersive spectroscopy (EDS) and elemental mapping analyses (Fig. S4) also evince that there are four elements (W, C, Cu and S) almost evenly distributing on the surface of the particles. The abundant element W comes from Na9EuW10O36 which was used in the assembly process of supramolecular sensor. And the element C comes from the organic molecules in CSS. The existence of elements Cu and S provides a direct evidence of the production of CuS. According to the evidences of zeta potential and FE-SEM elemental mapping images, both Cu and S elements are in a uniform distribution on the surface of the supramolecular sensors, which also implies the location of generated CuS. To make clear the structural transformation of CSS caused by the modifier (Cu2+) and analyte (H2S), FTIR spectra were measured and the characteristic bands were summarized in Fig. S5. Apparently, the bands of hydrophobic chains appear at 2920 and 2850 cm1, which suggests that their arrangement is highly ordered [36], insusceptible to the injection of Cu2+ and H2S. This means that the supramolecular structures were hardly disaggregated upon addition of Cu2+ or H2S. The surfactants could encapsulate Eu-POM no matter whether Cu2+ or H2S was added. Furthermore, the FTIR spectra of the supramolecular sensor powder also exhibit four characteristic vibration bands of Eu-POM: 937 and 850 cm1 ascribed to m (W = Od) and m (W-Ob-W), respectively; 792 and 718 cm1 assigned to m (W-Oc-W) [37]. Here Ob and Oc represent the bridge oxygen atoms of two octahedrons sharing a corner and an edge, respectively, and Od is the terminal oxygen [37]. Both CSS without and with H2S show the corresponding bands in Fig. S5, which illustrates that Eu-POM in the supramolecular sensor was well maintained. However there are slight shifts emerged, which can be ascribed to the internal interaction of Cu2+ and CuS with Eu-POM. The emission mechanism of Eu-POM upon excitation has been discussed in many reports [29,33,34,38]. The strong red emission of Eu-POM needs three steps: first, photoexcitation of O ? W ligand to metal charge transfer (LMCT) bands induces d1 electron hopping, and the hole in the lattice releases energy by radiation; second, the energy transfers from O ? W LMCT state to the 5D0 emitting state of Eu3+; third, electron induced by 5D0 emitting state relaxes to the 7Fj ground state, and then Eu-POM generates fluorescence [39]. In this system, the fluorescence quenching caused by Cu2+ may occur in the second step, the excitation energy transfers from O ? W ligand
Fig. 5. Time-resolved luminescence decay lines of Eu-POM/[C16-2-C16im]Br2 supramolecular complex (a), CSS (b), CSS with 62.5 lM (c), 125 lM (d), 175 lM (e) H2S.
Table 2 Summary of lifetimes s1 and s2 and the total lifetimes. Sample
s1 (ms)
Rel (%)
s2 (ms)
Rel (%)
Total (ms)
a b c d e
1.01 1.01 0.91 0.83 0.78
32.97 33.39 44.43 48.51 49.23
3.70 3.26 2.96 2.95 2.92
67.03 66.61 55.57 51.49 50.77
2.83 2.51 2.05 1.92 1.86
to the metal d-orbital and/or O ? W ligand to metal charge transfer (LMCT). And the quenching mechanism was confirmed by timeresolved fluorescence measurement, the fluorescent lifetime of CSS decayed compared with the supramolecules without Cu2+ [40]. So far, the quenching mechanism of CuS has rarely been reported. For the as-prepared CSS with H2S, the absorption bands exhibit a beneficial overlap with the emission band of Eu-POM (Fig. S6), which implies the occurrence of intermolecular efficient energy transfer process [34,41]. Besides, the overlapped region enlarged with the amount of CuS generated, and the fluorescence intensity following quenched acuter. This maybe transfer a message that there is a relationship between the CuS and the fluorescence intensity. To determine the main energy transfer way, we measured the fluorescent lifetime of supramolecular structures for CSS and CSS with different concentration of H2S. As shown in Fig. 5 and Table 2, the fluorescent lifetime at 620 nm decays with the increasing H2S concentration. If radiative energy transfer occurs, the fluorescent lifetime will be unchanged [42]. Therefore we speculate that the intermolecular nonradiative resonance energy transfer between the excitation states of Eu3+ and CuS may be the predominant luminescence quenching mechanism. Additionally, as the method described [43], the energy transfer efficiency, E, can be calculated by the equation as follows.
E¼1
F DA sDA ¼1 FD sD
ð1Þ
where sD and sDA are the lifetime of CSS in the absence and presence of acceptor (CuS), and FDA and FD are the luminescent intensity of CSS without and with acceptor. The E value evaluated from luminescent lifetime is 0.34. Based on the above analysis, the luminescence quenched by CuS may arise from the nonradiative resonance energy transfer mechanism (see Fig. 6). Meanwhile, it has been proved that CuS is a p-type semiconductor, and possesses superior properties as thermoelectric conversion material, solar cell
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Fig. 6. An illustration of quenching mechanism, the red, blue, gray and brown balls represent O, W, Eu elements and CuS, respectively. LRET is the abbreviation of luminescence resonant energy transfer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and optical filter [44]. Chang’s group fabricated CuS/CoS counter electrode and found CuS electrode had a broad absorption band between 400 and 650 nm, as a result of an electron-acceptor state lying with the bandgap [45–47]. The bandgap of bulk CuS is around 2 eV [45,47,48], and the emission photo energy of CSS is about 2–2.1 eV (590–620 nm). So the superior adaptability between the bandgaps and the emission energy provides a possibility that CuS could accept the energy from the excited CSS. And this is in accordance with the energy transfer mechanism between the excitation states of Eu3+ with CuS. 4. Conclusions In summary, we have constructed a novel copper (II)-modified supramolecular sensor (CSS) based on lanthanide-containing polyoxometalate and an ionic liquid-type gemini surfactant for rapid detection of H2S. The CSS chemosensor shows dual responses to H2S: fluorescence intensity linearly decreases with the adding of H2S, while the absorbance is proportional to the elevating H2S concentration. Additionally, the sensor also displays a high sensitivity and superior selectivity for H2S detection among various relevant anions and species. It is speculated that the fluorescence quenching is caused by the generation of CuS with a nonradiative resonance energy transfer mechanism. The new probe prepared here is expected to appropriate for the detection of H2S, which is significant in the diagnosis of some diseases. Acknowledgement This work was supported by Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2014GSF117001), and the Natural Science Foundation of Shandong Province of China (No. ZR2011BM017). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.047. References [1] K. Sasakura, K. Hanaoka, N. Shibuya, Y. Mikami, Y. Kimura, T. Komatsu, T. Ueno, T. Terai, H. Kimura, T. Nagano, Development of a highly selective fluorescence probe for hydrogen sulfide, J. Am. Chem. Soc. 133 (45) (2011) 18003–18005. [2] E. Culotta, D. Koshland, NO news is good news, Science 258 (5090) (1992) 1862–1865. [3] B.L. Predmore, D.J. Lefer, G. Gojon, Hydrogen sulfide in biochemistry and medicine, Antioxid. Redox Sign. 17 (1) (2012) 119–140.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
A polyoxometalate-based supramolecular chemosensor for rapid detection of hydrogen sulfide with dual signals Yongxian Guo a, Yanjun Gong a, Lubin Qi a, Yan’an Gao b, Li Yu a,⇑ a b
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A polyoxometalate-based
A supramolecular fluorescence chemsensor used for H2S detection was built by modifying Cu2+ on the surface of supramolecular particles, and the quenching mechanism of generated CuS was explored.
supramolecular chemsensor CSS has been synthesized. The supramolecular chemsensor CSS can selectively detect H2S with negligible responsive time (second level). The chemsensor response to H2S with dual signals: fluorescence and absorption spectra.
a r t i c l e
i n f o
Article history: Received 1 July 2016 Revised 12 August 2016 Accepted 21 September 2016 Available online 22 September 2016 Keywords: Polyoxometalate Supramolecular chemosensor Rapid detection Hydrogen sulfide Dual signals
⇑ Corresponding author. E-mail address: [email protected] (L. Yu). http://dx.doi.org/10.1016/j.jcis.2016.09.047 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
a b s t r a c t Hydrogen sulfide (H2S) has been verified as an important biological mediator in human physiological activities, but its rapid and accurate detection is remaining a challenge. Based on our early work, Eu-containing polyoxometalate/ionic liquid-type gemini surfactant hybrid nanoparticles fabricated by EuW10O3632H2O (Eu-POM) and 1,2-bis(3-hexadecylimidazolium-1-yl) ethane bromide ([C16-2-C16im] Br2) via ionic self-assembly (ISA) strategy, we modified the hybrids with copper (II) ion and used them as a novel turn-off supramolecular fluorescence probe for H2S immediate response. Although copper (II) ions can cause decrease of the fluorescence intensity, the probe with moderate amount of copper (II) still has a high performance in emission property. The copper (II) ion-modified supramolecular sensor (CSS) shows dual signals in the fluorescence intensity and absorbance for H2S detection, and the detection limit is about1.25 lM. Furthermore, CSS displays high selectivity for H2S in the presence of other anions 2 2 and species (e.g. Cl, Br, I, SO2 4 , SO3 , S2O3 , AC , H2O2, HCO3 , L-cysteine, homocysteine and L-glutathione), and also have potential for preferential imaging in vivo. Besides, the fluorescence quenching mechanism of CSS in the presence of H2S was explored. CuS generated by the reaction between Cu2+ and H2S was testified to act as a quencher, and the nonradiative resonance energy transfer mechanism was speculated to be responsible for fluorescence quenching. It is anticipated that the as-prepared CSS will be used as an efficient chemosensor for the rapid detection of H2S, which is critical for the diagnosis of some diseases, e.g. Alzhermer’s disease, Down’s syndrome, and diabetes, etc. Ó 2016 Elsevier Inc. All rights reserved.
Y. Guo et al. / Journal of Colloid and Interface Science 485 (2017) 280–287
1. Introduction Hydrogen sulfide (H2S) with a special smell of rotten eggs has been regarded as the third endogenous gaseous signaling molecule, like nitric oxide (NO) and carbon monoxide (CO) [1,2]. Studies have shown that H2S is mainly produced by three pathways, including cystathionine b-synthase (CBS), cystathionine c-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3MPST)/cysteine aminotransferase (CAT) [3–5]. And H2S plays a vital role in regulating intracellular physiological processes, such as vasodilation, antioxidation, anti-apoptosis and anti-inflammation [6–9]. The abnormal level of H2S is considered as a reflection of some diseases such as Alzhermer’s disease, Down’s syndrome, diabetes and liver cirrhosis [10–12]. In particular, for a certain purpose, it is highly demanding to quickly, facilely and precisely detect the concentration of H2S in living biological systems. The traditional methods for H2S detection are electrochemical assay, colorimetry and gas chromatography [13–15]. However, these methods are destructive and inappropriate for real-time detection [16]. The method with fluorescence probes has attracted much attention with their advantages in real-time detection with superior designability, sensitivity and selectivity [17–19]. Beyond that, fluorescence imaging accomplished through fluorescence probe is becoming the most attractive imaging technique for in vivo detection [20–22]. So far, there are many fluorescence probes for H2S detection have been reported. Chang’s group proposed a new approach to detect biological H2S through the reaction between azide-based fluorescent probes and H2S [23]. Guo and his partners reported a ratiometric fluorescence probe for rapid detection of H2S in mitochondria according to the selective nucleophilic addition of HS to a specific merocyanine derivative in medium of near neutral pH value [24]. However, the detection mechanism of these fluorescence probes is majorly developed by the reduction reaction and nucleophilic reaction, which all need a relatively long responsive time (typically >20 min) [16]. And these probes are generally derivatives of dye molecules, whose synthetic processes are complicated and maybe cause some damages to the environment. Metal sulfide precipitation-based method has the advantages of short responsive time and high detection limit with an inorganic reaction, and there are some correlated reports [1,16,25]. Nagano et al. developed a novel fluorescent probe based on azamacrocyclic Cu2+ complex chemistry and applied it to biological H2S detection [1]. Tang et al. constructed a fluorescence probe with Cu(II)metalated 3D heterogeneous nano metal-organic framework for detection of H2S in living cells with high selectivity and sensitivity [16]. Rational designed supramolecular chemosensors, fabricated by noncovalent bonds and alleviated a time-consuming synthesis process in some extent, are promising substitute to overcome the limitations of traditional fluorescence probes. Among these supramolecular chemosensors, hybrid supramolecular systems, especially constructed by lanthanide-containing polyoxometalate and organic molecules, have inspired much attention in detection field with variable luminescence properties, i.e. narrow emission band, large Stokes shifts, long lifetimes, photostability and sensitive to ambient environment [26,27]. Wan et al. developed a recyclable polyoxometalate-based supramolecular chemosensor with dysprosium-containing POM and a block copolymer for efficient detecting carbon dioxide [28]. In our previous work [29], Eucontaining polyoxometalate/gemini surfactant hybrid nanoparticles with high fluorescence quantum yield (25.17%), long emission timescale (3.758 ms) and superior biocompatibility were fabricated by Na9EuW10O3632H2O (Eu-POM) and 1, 2-bis(3-hexadecyli midazolium-1-yl) ethane bromide ([C16-2-C16im]Br2) through ionic self-assembly strategy. In the present work, a supramolecular
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chemosensor probe was constructed with Eu-POM/[C16-2-C16im] Br2 supramolecular complex as a pre-probe and Cu2+ as a modifier (see Scheme 1). To the best of our knowledge, there are rarely reports on polyoxometalate-based hybrid supramolecules used for H2S detection [30]. Particularly, fluorescence intensity of Cu2+-modified supramolecular sensor (CSS) is quenched slightly by the electron- and/or energy-transfer processes, but still maintains a high emission intensity. The CSS response to H2S is speedy (second level), and the fluorescence measurement was carried out without any time interval. The CSS shows dual responses: fluorescent intensity decreases and the absorbance increases evidently with the elevating H2S concentration due to the generation of copper sulfide (CuS). Additionally, the CSS also exhibits superior selectivity for H2S, with good interference tolerance of other biologically 2 2 relevant anions and species, such as Cl, Br, I, SO2 4 , SO3 , S2O3 , AC, H2O2, HCO , L -cysteine, homocysteine and L -glutathione. This 3 probe not only could rapidly and specifically respond to H2S, but also provide a new method to the quantitative analysis of H2S. It has the potential application for preferential imaging of H2S in living cells.
2. Experimental 2.1. Materials The reagents listed as follows were obtained commercially and used without further purification. NaCl, NaBr, NaI, Na2S2O3, Na2SO3, Na2SO4, NaAC, NaHCO3, H2O2 (30% aqueous solution), L-cysteine, homocysteine and L-glutathione were purchased from J&K Chemical Technology, China. CuBr2 and Na2S9H2O were provided by Energy Chemical Technology, China. Both Europium nitrate hexahydrate (99%) and sodium tungstate dehydrate (99%) were obtained from J&K Chemical Technology, China. PBS buffer solution (10 mM, pH = 7.4) was purchased from Sigma-Aldrich. The triply distilled water used in this experiment was obtained by a SZ-97 automatic triple water distiller (Shanghai YR BioChem CO., Ltd. China).
2.2. Synthesis of [C16-2-C16im]Br2 and Eu-POM The synthesis processes of 1,2-bis(3-hexadecylimidazolium-1yl) ethane bromide ([C16-2-C16im]Br2) and Na9EuW10O3632H2O (Eu-POM) were carried out according to our previous work [29,31]. 2.3. Preparation of Cu2+-modified supramolecular sensor (CSS) The hybrid supramolecular pre-sensor was constructed by an one-step synthesis method (through ionic self-assembly strategy), mixing the solutions of [C16-2-C16im]Br2 (0.1 mM, 9 ml) and Eu-POM (0.1 mM, 2 ml) in a 50 ml flask under ultrasound. The complex solution shows Tyndall Effect apparently. The Cu2+-modified supramolecular sensor was obtained by adding CuBr2 solution (1.4 mM, 0.5 ml) to 3 ml complex solution mentioned above, and placed in ultrasound environment for 5 min.
2.4. Reaction with H2S Experiments of the CSS response to H2S were performed as follows: adding 0.5 ml prepared Na2S solution (10, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, and 1400 lM) into the CSS solution (3.5 ml), and the mixing solution was used to measure the fluorescent property and UV–vis absorption spectra.
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Scheme 1. The structures of polyoxometalate (left, top) and Gemini surfactant (left, bottom) used in this work. The schematic diagram of the construction process of CSS and its detection mechanism for H2S. The red ball means the supramolecular nanoparticles produced by ionic self-assembly strategy, the blue ball represents Cu2+, and the brown ball shows the generated CuS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2.5. Characterization Zeta potential was measured by Malvern Zeta Sizer Nano (Malvern Instruments) in aqueous solution at 25 °C. Absorption spectra were measured by using a U-4100 instrument (Hitachi, Japan) in a quartz cell (1 cm). Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer with the excitation wavelength of 280 nm. The excitation and emission slit widths were 5.0 and 10.0 nm, respectively. For this system, we focused on the fluorescence spectra with the wavelength range from 570 to 670 nm. Time-resolved fluorescence measurements were taken on a FLS920 (Edinburgh) fluorometer, with the excitation at 280 nm and the maximum emission wavelength at 620 nm. The elemental mapping analysis images were taken using a field emissionscanning electron microscope (FE-SEM) instrument. The sample was made by dropping method: picked up a drop of complex solution and transferred onto a silicon substrate, and dried by exposure to air. The sample was coated by a layer of gold. FTIR spectra were measured by a VERTEX-70/70v FTIR spectrometer (Bruker Optics, Germany) from 400 to 4000 cm1 on pressed thin KBr disks.
3. Results and discussion 3.1. Construction of Cu (II)-modified supramolecular sensor (CSS) The preparation process of CSS was divided into two steps: (i). Mixing the solutions of [C16-2-C16im]Br2 and Eu-POM with a certain volume ratio to form supramolecular nanoparticles; (ii). Adding Cu2+ (for minimize the effect of counter ions, CuBr2 as substitution) solution with moderate concentration to form the supramolecular sensor (see Scheme 1). There was no perceivable coagulation of the colloid nanoparticles when the Eu-POM/[C162-C16im]Br2 supramolecular solution was kept standing for more than 3 days. To a certain extent, stability of the resulting CSS solution after adding Cu2+ was weaker than the supramolecular solution without Cu2+, however it could exist as colloidal solution instead of coagulation for many hours (5 h at least), which was enough to carry out the detection process.
Both inorganic cluster Eu-POM and cationic surfactant [C16-2C16im]Br2 are essential parts of the supramolecular system investigated here. Eu-POM has the advantages over other Eu-containing complexes: i) it has superior photoluminescent property, such as narrow emission bands, large Stokes shift, long life time and tunable emission, ii) Eu-POM supports binding sites to accept cationic surfactant to generate surfactants-encapsulated supramolecular structures. As for the key role played by the Gemini cationic surfactant in the detection of H2S, several contrast experiments were carried out. Other cationic surfactants, namely [C12-2-C12im]Br2, [C16im]Br and CTAB, were also used to fabricate supramolecular structures with Eu-POM according to the same synthetic method. But the fluorescence property (e.g. quantum yield and lifetimes [29]) of the supramolecular structures involved in these cationic surfactants was inferior to that of Eu-POM/[C16-2-C16im]Br2. In Table S1, the data convey that Eu-POM/[C16-2-C16im]Br2 has the highest quantum yield and the longest lifetimes among these supramolecular structures. Especially, the quantum yield of the surfactants-encapsulated Eu-POM/[C16-2-C16im]Br2 supramolecular structures is 0.25, more than 27 times of the quantum yield of Eu-POM aqueous solution, 0.0091. Although compared to some substances with ultrahigh quantum yield, this quantum yield value is not extremely high. Yet the fluorescence property is favorable based on the dual consideration of lifetimes and quantum yield. Based on the photoluminescent properties, we try to construct supramolecular chemsensor with Cu2+. It is well-known that Cu (II) as a heavy metal ion is a good quencher through electronand/or energy-transfer processes [32]. In this system, we also observed that Cu2+ could quench the fluorescence intensity of supramolecular structures. Fortunately, the quenching of fluorescent intensity could be tuned by regulating the content of Cu2+ in supramolecular sensor solution. As illustrated in Fig. 1, when we added different concentration of Cu2+ into the supramolecular complex solution, the emission intensity reduces from 197 to 135 at 592 nm and from 547 to 380 at 622 nm under 280 nm excitation. The inset scatter diagram displays that a negative linear relationship exists between the fluorescence intensity at 622 nm and the Cu2+ concentration. This suggests that although the fluorescent intensity is quenched to a certain degree, the chemosensor
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supramolecular structures with moderate doses of Cu2+ to form a sensor for the detection of H2S. Therefore Cu (II) was selected to modify the supramolecular sensor. 3.2. Detection of H2S
Fig. 1. Fluorescence spectra of supramolecular chemosensor solution at room temperature upon adding different concentration of Cu2+ (bulk concentration: 0– 200 lM; excitation wavelength: 280 nm). The inset shows a linear relationship between the fluorescence intensity at 622 nm and Cu2+ concentration.
still remains a high emission intensity. Therefore, a potential way to receive some chemical substance to further quench the fluorescence may be provided. In sum, the modified sensor could be used for detecting some chemicals. Besides, to study the evolution of surface charge on the EuPOM/[C16-2-C16im]Br2 nanoparticles upon addition of Cu2+, we performed the zeta potential measurements for the complex solution. Fig. 2 (black points and line) illustrates that the zeta potential value increases obviously with the concentration of Cu2+, which testifies that Cu2+ may be located on the surface of supramolecular particles in a absorption manner, and can offer some binding sites to receive H2S. As we all know, Cu element is an essential substance in many biological approaches. Although superfluous Cu2+ will cause liver or kidney damage, addition of a small amount of Cu2+ is harmless to living body. Moreover, in our previous work [29], we incubated hela cells with the Eu-POM/[C16-2-C16im]Br2 nanostructures, and found their good biocompatibility. These results give us an evidence that it is feasible to decorate Eu-POM/[C16-2-C16im]Br2
Fig. 2. Zeta potential of supramolecular chemosensor solution after adding different concentration of Cu2+ aqueous solution (black points and black line), and H2S (red points and red line) (Cu2+ with a fixed concentration, 200 lM). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
It is reported that the Eu-containing polyoxometalates (Na9EuW10O3632H2O) show red luminescence upon UV excitation, and display the characteristic 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions of Eu3+ [33,34]. Especially, the transitions of 5D0 ? 7F2 observed at 614 and 622 nm are attributed to an electric dipole transition, and their intensities are extremely sensitive to the chemical microenvironments around Eu3+ ions [30]. Inversely, the 5D0 ? 7F1 transitions observed at 590 and 594 nm are assigned to a magnetic dipole transition, and the intensities are almost independent of the nearby micro-environmental changes for Eu3+. Therefore, the intensity ratio I(5D0 ? 7F2)/I(5D0 ? 7F1) is widely used to study changes in the chemical surroundings of Eu3+ ions [31]. So in this work, we concentrate on the changes of these two transitions, with the emission wavelength range from 570 to 670 nm excited by 280 nm. To monitor the CSS’s response to H2S, we chose Na2S as an equivalent due to the following three reasons i). Since the existence state of H2S is gas, it is difficult to determine the concentration or amount of H2S when adding H2S into the constructed supramolecular sensor solution; ii). Na2S solid could be hydrolyzed and release H2S in water according to the chemical equations as follows:
S2 þ H2 O ¼ HS þ OH HS þ H2 O ¼ H2 S þ OH iii). The choice of Na2S solution as the equivalent of H2S has also been reported by some other research groups [1,4]. Then, we investigated dual signals of the fluorescence spectra and UV–vis absorption spectra of CSS in PBS buffer solution (10 mM, pH = 7.4). As can be seen from Fig. 3a, the CSS (0.1 mM, in the presence of 200 lM Cu2+) always shows high fluorescence intensity in PBS buffer solution under 280 nm excitation. But upon addition of H2S, the emission bands are obviously quenched around 594 and 622 nm (Fig. 3a). Fig. 3b is the fitting line bansed on the points, the correlation coefficient (R) value is 0.99103, illustrates that there is a negative linear relationship between the fluorescence intensity at 622 nm and H2S concentration. The detection limit for H2S was found to be 1.25 lM. Then we compared the linear detection range, detection limit and responsive time of CSS prepared in this work with other detection methods reported by previous work. As shown in Table 1, the detection limit of CSS is at a favorable level and the responsive time is tremendously lower than the other methods. Moreover, the UV–vis absorption spectra of CSS without and with H2S in PBS buffer solution are depicted in Fig. 3c. The shoulder band at 250 nm of a tail absorption band, assigned to the characteristic band of Eu-POM, increased with the addition of H2S. Besides, the tail absorption band exhibits concentration-dependent increase in the UV–vis absorbance after treatment with H2S. Meanwhile, by naked eyes, the color of samples can be obviously observed to change from colorless to brown with the increase of H2S concentration (Fig. S1). Fig. 3d shows that there exists a linear growth correlation between UV–vis absorbance signal (at 400 nm) and H2S concentration, the correlation coefficient (R) value is 0.99875. Compared with Fig. 3b and d, we can conclude that the Fluorescence intensity is inversely proportional to H2S concentration, while the absorbance is in direct proportion to H2S concentration. It means that the higher fluorescence intensity or the lower absorbance, the lower concentration of H2S is. Additionally, we can pre-
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Fig. 3. Fluorescence spectra (a, excitation wavelength at 280 nm) and UV–vis absorption spectra (c) upon addition of Na2S (0–175 lM) into CSS solution (0.1 mM, in the presence of 200 lM Cu2+) at room temperature. The points and line show the relationship between the fluorescence intensity at 622 nm (b), absorption intensity at 400 nm (d) and H2S concentration, respectively.
Table 1 Comparison on the linear detection range, detection limit and responsive time of CSS in this work and various H2S detection methods reported previously. Sensor type
Linear detection range (lM)
Detection limit (lM)
Responsive time
Reference
HSip-1 Rhodamine GCTPOC Sulfidefluor-1 CouMC HBT derivate CSS
– 0–125 3.02–170 – 10–200 2.4–100 1.25–175
10 0.38 3.02 5 10 2.4 1.25
– 20 min 40 min 60 min 30 s 30 min Seconds
[1] [4] [21] [23] [24] [35] Present work
liminarily speculate that the quenching of fluorescence is caused by the production of CuS. To examine the chemoselectivity of CSS, various biologically relevant anions and species (12.5 mM) were used to incubate with the sensor in PBS buffer solution (10 mM, pH = 7.4) and their fluorescence response was investigated separately (Fig. 4). It should be noted that the concentration of the interfering anions and species used here is much higher than that in living cells. The anions we used as interference ions including halide ions (Cl, Br, I), sulfur 3 2 2 oxides ions (SO2 ), 4 , SO3 , S2O3 ), acetate (Ac ), bicarbonate (HCO and reactive oxygen species (H2O2) did not trigger turn-off fluorescence responsiveness. In addition, fluorescence spectra of CSS in the presence some interfering molecules (L-cysteine, homocysteine and L-glutathione, 125 lm) were measured, respectively (Fig. S2I). The experimental results indicate that they could affect the fluorescence intensity of the sensor, yet the color of supramolecular chemsensor solutions containing these molecules was obviously different from Na2S (Fig. S2II). This can help us to easily distinguish Na2S from L-cysteine, homocysteine and L-glutathione with the
naked eyes. These results illustrate that the CSS could be applied to the detection of H2S. 3.3. Quenching mechanism of Cu2+ and CuS To research the quenching mechanism, firstly, we wonder if individual H2S could quench the fluorescence of CSS without Cu2+. As illustrated in Fig. S3, for the supramolecular structures without Cu2+, the emission intensity at 622 nm is almost constant in the H2S concentration of 0–175 lM. This conveys a message that single H2S could not quench the fluorescence of CSS, and only co-existence of Cu2+ and H2S can contribute to the fluorescent quenching. In other word, the generation of CuS may be liable for fluorescent quenching. As stated above, we have proved that Cu2+ in CSS is located on the surface of the nanoparticles (the black plot in Fig. 2). Meanwhile, within the investigated concentration range, zeta potential values display obvious decline from 30.7 mV to 8.81 mV upon addition of various concentration of H2S into the CSS solution (with
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Fig. 4. Fluorescence response to various anions and species (12.5 mM) in PBS 2 (10 mM, pH = 7.4) at 622 nm. 1, probe alone; 2, Cl; 3, Br; 4, I; 5, SO2 4 ; 6, SO3 ; 7, S2O2 3 ; 8, AC ; 9, H2O2; 10, HCO3 .
200 lM Cu2+) (the red points in Fig. 2). It indicates the formation of new quenching agent, CuS, on the surface of CSS. Meanwhile, field emission-scanning electron microscope (FE-SEM) energy dispersive spectroscopy (EDS) and elemental mapping analyses (Fig. S4) also evince that there are four elements (W, C, Cu and S) almost evenly distributing on the surface of the particles. The abundant element W comes from Na9EuW10O36 which was used in the assembly process of supramolecular sensor. And the element C comes from the organic molecules in CSS. The existence of elements Cu and S provides a direct evidence of the production of CuS. According to the evidences of zeta potential and FE-SEM elemental mapping images, both Cu and S elements are in a uniform distribution on the surface of the supramolecular sensors, which also implies the location of generated CuS. To make clear the structural transformation of CSS caused by the modifier (Cu2+) and analyte (H2S), FTIR spectra were measured and the characteristic bands were summarized in Fig. S5. Apparently, the bands of hydrophobic chains appear at 2920 and 2850 cm1, which suggests that their arrangement is highly ordered [36], insusceptible to the injection of Cu2+ and H2S. This means that the supramolecular structures were hardly disaggregated upon addition of Cu2+ or H2S. The surfactants could encapsulate Eu-POM no matter whether Cu2+ or H2S was added. Furthermore, the FTIR spectra of the supramolecular sensor powder also exhibit four characteristic vibration bands of Eu-POM: 937 and 850 cm1 ascribed to m (W = Od) and m (W-Ob-W), respectively; 792 and 718 cm1 assigned to m (W-Oc-W) [37]. Here Ob and Oc represent the bridge oxygen atoms of two octahedrons sharing a corner and an edge, respectively, and Od is the terminal oxygen [37]. Both CSS without and with H2S show the corresponding bands in Fig. S5, which illustrates that Eu-POM in the supramolecular sensor was well maintained. However there are slight shifts emerged, which can be ascribed to the internal interaction of Cu2+ and CuS with Eu-POM. The emission mechanism of Eu-POM upon excitation has been discussed in many reports [29,33,34,38]. The strong red emission of Eu-POM needs three steps: first, photoexcitation of O ? W ligand to metal charge transfer (LMCT) bands induces d1 electron hopping, and the hole in the lattice releases energy by radiation; second, the energy transfers from O ? W LMCT state to the 5D0 emitting state of Eu3+; third, electron induced by 5D0 emitting state relaxes to the 7Fj ground state, and then Eu-POM generates fluorescence [39]. In this system, the fluorescence quenching caused by Cu2+ may occur in the second step, the excitation energy transfers from O ? W ligand
Fig. 5. Time-resolved luminescence decay lines of Eu-POM/[C16-2-C16im]Br2 supramolecular complex (a), CSS (b), CSS with 62.5 lM (c), 125 lM (d), 175 lM (e) H2S.
Table 2 Summary of lifetimes s1 and s2 and the total lifetimes. Sample
s1 (ms)
Rel (%)
s2 (ms)
Rel (%)
Total (ms)
a b c d e
1.01 1.01 0.91 0.83 0.78
32.97 33.39 44.43 48.51 49.23
3.70 3.26 2.96 2.95 2.92
67.03 66.61 55.57 51.49 50.77
2.83 2.51 2.05 1.92 1.86
to the metal d-orbital and/or O ? W ligand to metal charge transfer (LMCT). And the quenching mechanism was confirmed by timeresolved fluorescence measurement, the fluorescent lifetime of CSS decayed compared with the supramolecules without Cu2+ [40]. So far, the quenching mechanism of CuS has rarely been reported. For the as-prepared CSS with H2S, the absorption bands exhibit a beneficial overlap with the emission band of Eu-POM (Fig. S6), which implies the occurrence of intermolecular efficient energy transfer process [34,41]. Besides, the overlapped region enlarged with the amount of CuS generated, and the fluorescence intensity following quenched acuter. This maybe transfer a message that there is a relationship between the CuS and the fluorescence intensity. To determine the main energy transfer way, we measured the fluorescent lifetime of supramolecular structures for CSS and CSS with different concentration of H2S. As shown in Fig. 5 and Table 2, the fluorescent lifetime at 620 nm decays with the increasing H2S concentration. If radiative energy transfer occurs, the fluorescent lifetime will be unchanged [42]. Therefore we speculate that the intermolecular nonradiative resonance energy transfer between the excitation states of Eu3+ and CuS may be the predominant luminescence quenching mechanism. Additionally, as the method described [43], the energy transfer efficiency, E, can be calculated by the equation as follows.
E¼1
F DA sDA ¼1 FD sD
ð1Þ
where sD and sDA are the lifetime of CSS in the absence and presence of acceptor (CuS), and FDA and FD are the luminescent intensity of CSS without and with acceptor. The E value evaluated from luminescent lifetime is 0.34. Based on the above analysis, the luminescence quenched by CuS may arise from the nonradiative resonance energy transfer mechanism (see Fig. 6). Meanwhile, it has been proved that CuS is a p-type semiconductor, and possesses superior properties as thermoelectric conversion material, solar cell
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Fig. 6. An illustration of quenching mechanism, the red, blue, gray and brown balls represent O, W, Eu elements and CuS, respectively. LRET is the abbreviation of luminescence resonant energy transfer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
and optical filter [44]. Chang’s group fabricated CuS/CoS counter electrode and found CuS electrode had a broad absorption band between 400 and 650 nm, as a result of an electron-acceptor state lying with the bandgap [45–47]. The bandgap of bulk CuS is around 2 eV [45,47,48], and the emission photo energy of CSS is about 2–2.1 eV (590–620 nm). So the superior adaptability between the bandgaps and the emission energy provides a possibility that CuS could accept the energy from the excited CSS. And this is in accordance with the energy transfer mechanism between the excitation states of Eu3+ with CuS. 4. Conclusions In summary, we have constructed a novel copper (II)-modified supramolecular sensor (CSS) based on lanthanide-containing polyoxometalate and an ionic liquid-type gemini surfactant for rapid detection of H2S. The CSS chemosensor shows dual responses to H2S: fluorescence intensity linearly decreases with the adding of H2S, while the absorbance is proportional to the elevating H2S concentration. Additionally, the sensor also displays a high sensitivity and superior selectivity for H2S detection among various relevant anions and species. It is speculated that the fluorescence quenching is caused by the generation of CuS with a nonradiative resonance energy transfer mechanism. The new probe prepared here is expected to appropriate for the detection of H2S, which is significant in the diagnosis of some diseases. Acknowledgement This work was supported by Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2014GSF117001), and the Natural Science Foundation of Shandong Province of China (No. ZR2011BM017). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.09.047. References [1] K. Sasakura, K. Hanaoka, N. Shibuya, Y. Mikami, Y. Kimura, T. Komatsu, T. Ueno, T. Terai, H. Kimura, T. Nagano, Development of a highly selective fluorescence probe for hydrogen sulfide, J. Am. Chem. Soc. 133 (45) (2011) 18003–18005. [2] E. Culotta, D. Koshland, NO news is good news, Science 258 (5090) (1992) 1862–1865. [3] B.L. Predmore, D.J. Lefer, G. Gojon, Hydrogen sulfide in biochemistry and medicine, Antioxid. Redox Sign. 17 (1) (2012) 119–140.
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