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Electrogenerated chemiluminescence behavior of Tb complex and its application in sensitive sensing Cd2+
Electrochimica Acta 228 (2017) 1–8
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrogenerated chemiluminescence behavior of Tb complex and its application in sensitive sensing Cd2+ Yinggui Zhu* , Min Zhao, Xiaojuan Hu, Xiaofang Wang, Ling Wang College of Chemistry and Material Science, Anhui Normal University, No. 1 Beijing Road, Wuhu 241000, PR China
A R T I C L E I N F O
Article history: Received 1 September 2016 Received in revised form 3 January 2017 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Electrogenerated Chemiluminescence(ECL) Tb complex pyridine-3-sulfonic acid Cd2+ assay ECL sensor
A B S T R A C T
In this paper, we report a novel rare earth metal complex with the weak ligand of aromatic sulphonic acid (pyridine-3-sulfonic acid, 3-pSO3H), and characterized by FT-IR, UV–vis, energy-dispersive X-ray spectroscopy (EDX), electrochemiluminescence spectra, etc. Then an excellent electrochemiluminescence (ECL) signal was observed with K2S2O8 as the coreactant in NaAc-HAc buffer solution. For another thing, the electrochemical properties of the compound have been thoroughly investigated in acetonitrile solution, the possible ECL reaction mechanism was proposed as well. Furthermore, a simple and straightforward ECL platform was reported for sensitive and selective detection of Cd2+ due to the effective quenching after addition of Cd2+. Other heavy/transition metal ions do not interfere with the sensing. The limit of detection is determined as 0.13 nM, the results suggested that as-prepared complex could be a promising material for developing ECL senors to detect the Cd2+ rapidly indwell in environmental and practical samples. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that toxic heavy metal ions contamination could not only lead to many serious environmental and health problems, but also affects the normal life of people seriously. Cadmium ion has been recognized as a highly toxic heavy metal ion and listed as the seventh on the Top 20 Hazardous Substances Priority List by the Agency for Toxic Substances and Disease Registry and US Environmental Protection Agency (EPA). However, it has been widely used in many fields as well, such as agriculture, metallurgy, war industries, etc. The increasing level in water, soil or food will bring about severe injury to the human body. For instance, metabolism disorders, renal dysfunction, and even certain cancers, which has close relationship with the heath of people [1–8]. Up to now, numerous monitor avenues based on mass spectrometry, atomic absorption spectrometry, atomic fluorescence spectrometry, electrochemical stripping analysis have been reported but these powerful methods are timeconsuming, high cost, complicated preparation and instrumentation [9]. Therefore, developing a reliable, highly sensitive, fastresponse and easy-operation method for Cd2+ assay is of great importance [10–16].
* Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y. Zhu). http://dx.doi.org/10.1016/j.electacta.2017.01.049 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Electrogenerated chemiluminescence (ECL) is a highly sensitive analytical method, which has been applied in all kinds of fields. The ECL signal is usually produced by light-emitting substance at a proper potential, and the electrochemical reactions [17–25] involves the generation of oxidized and reduced species at the electrode surface that undergo an electron-transfer reaction and emit light [26–30]. In comparison to the conventional electrochemical technology, ECL assay possesses many unique advantages such as high sensitivity, good stability, low background, operational simplicity, good temporal and spatial controllability [31–33]. With the rapid developments of ECL assay, it has been combined with many analytical techniques such as high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), mass spectrometry (MS), etc. Meanwhile, ECL sensors have attracted wide attention in recent years owing to its good biocompatibility, electrocatalytic activity and a wide range of applications in DNA analysis, clinical diagnosis and immunoassay. In addition, it was revealed that metal complex with the excellent electrochemical property and outstanding ECL performance has become a promising material in fabricating ECL sensors [34–40]. However, up to now, a widespread investigation mainly focused on [Ru (bpy)3]2+ because of its high ECL sensitivity and high stability. As such, it is our goal to develop other novel high-efficiency ECL systems and expand their potential applications in the analytical chemistry fields [41–45].
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Sulfonates have inherent coordinative pliancy and broad applications in various areas. The common features of sulfonate ligands lie in the following aspects, (1) Sulfonic acids present flexible and diverse coordination patterns, due to that oxygen atoms could connect metal atoms toward different directions thus forming high dimensional structures. (2) Sulfo-group is preferable hydrogen bond acceptor and it is more likely to obtain divergent hydrogen bond. (iii) Because of its intrinsic strong electrophilicity and fine water solubility, that not only promote the photoelectric property of complex but also help to explore water-soluble catalyst. Despite of the report that organosulfonate (RSO3) is regarded as poor ligand and has been employed as ‘non coordinating’ anion [46]. Indeed, a series of organosulfonate coordination polymers that have been reported in majority of papers constructed by transition metal, alkalions, alkaline earth ions and silver ion, the coordination chemistry of arenedisulfonates with other metal ions especially rare earth metal is lack of systematic investigation [47]. Taking into consideration these facts, we initiated our research work which will be an attractive research subject. In this study, we designed and synthesized a Tb complex successfully with the ligand of pyridine-3-sulfonic acid (3-pSO3H) by a simple synthetic method [48]. Under the optimized conditions, a simple ECL senor was established based on the effective quenching of ECL signal by the Cd2+ for sensitive and specific detection of Cd2+ with the concentration over the range from 2.0 1010 to 3.8 105 M with a low detection limit of 0.13 nM. Furthermore, the possible mechanism of ECL reaction based on Tb complex-K2S2O8 system and ECL quenching of Cd2+ was proposed.
2.3. Synthesis of Tb complex The Tb complex was synthesized according to our previous work by a simple method [49]. Briefly, 3-pSO3H (0.0796 g) was dissolved in 10 mL of ultrapure water, a freshly prepared solution of Tb(NO3)36H2O (1.1326 g) in 10 mL of ultrapure water was added dropwise into above solution with stirring for 30 minutes. Then, reaction of the mixture was maintained for 24 h with heating in water bath under a constant temperature of 70 C. After that, the resulting transparent solution was concentrated to remove residual water and dissolved in acetonitrile, followed by filtration to purify. Finally, the obtained complex acetonitrile solution stored in the refrigerate, and over a period of time, the solid particles precipitated from the solvent for further use. 2.4. Procedures for Cd2+ Sensing The GCE electrode was successively polished with 0.05 mm Al2O3 powder until the mirror-like surface appeared, followed by sonication in the deionized water and ethanol, respectively. The ECL behavior of complex were investigated in NaAc-HAc (0.05 M, 2.0 mL) buffer solution (pH = 4.7) containing Tb(C5H4NO3S)2n (H2O) (5 103 M, 250 mL) and K2S2O8 (0.10 M, 250 mL) served as coreactant, over a scanning range of 1.8 V to 0 V at a photomultiplier tube voltage of 800 V and a scanning rate of 0.1 V/s. Then, a series of different amounts of Cd2+ were added in the ECL cells to measure the ECL signals. All the mixtures were diluted to 2.6 mL with ultrapure water and mixed thoroughly. 2.5. Procedures for Cd2+ Sensing in Real Samples
2. Experimental Section 2.1. Materials Pyridine-3-sulfonic acid (3-pSO3H) was purchased from J&K (Beijing, China). Rare earth metal salts was pursed from Aladdin. AgNO3, Pb(NO3)2, CuSO45H2O, MnCl24H2O, NiCl26H2O, BaCl22H2O, Zn(CH3COO)22H2O, CoCl26H2O, FeCl36H2O, CdCl22.5H2O, Al(NO3)39H2O, CaCl2, NaCl, Mg(NO3)26H2O, KCl, HgCl2 were acquired from Shanghai Chemical Reagent Co. All other regants employed were of analytical grade, all solutions were prepared with ultrapure water.
2.2. Instrumentation The ECL measurements were measured using an MPI-E ECL analytical system (Xi’an Remax Electronic Science and Technology Co. Ltd., Xi’an, China) with the voltage of 800 V, and the potential was set from 1.8 V to 0 V at a scan rate of 0.1 V s1. The ECL spectrum was measured using a BPCL-GPZ-TIC ultraweak luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences) in conjunction with a CHI660C electrochemical workstation. Cyclic voltammetric (CV) was performed with a CHI 660C Electrochemical Analyzer (Shanghai Chenhua Instrument Co., China). All experiments were performed with the conventional three-electrode system including a glass carbon electrode (GCE, K = 3.0 mm) as working electrode, Ag/AgCl (saturated KCl) as reference electrode and a Pt wire counter electrode, respectively. The Fourier transform infrared spectroscopy (FT-IR) was recoded on a FT-IR spectrophotometer using KBr (FT-IR 8900, Hitachi, Japan). Fluorescence spectra were obtained by Hitachi F-4500 spectrofluorimeter (Tokyo, Japan) in this work. The energydispersive X-ray spectroscopy (EDX) mapping were carried out on Hitachi S-4800 under the accelerating voltage of 5 kV.
Practical samples we measured involving Tap water (from Lab), pond water (from Jinghu Lake, Wuhu). These water samples were measured after two times filtration using 0.22 mm filters. For the detection, series of pure spiked water samples were prepared, then the obtained solutions were introduced to ECL system. Respectively, the mixtures were diluted to 2.6 mL before electrochemical measurements. 3. Result and discussion 3.1. Characterization of the Tb complex Based on EDS (Figs. S1 in Supporting Information), Tb element was detected in the complex which demonstrates that the rare earth metal Tb has reacted with the ligands and the element analysis data (Table 1) were determined to confirm compound composition. According to the results, a possible molecular formula was predicted as Tb(C5H4NO3S)2n(H2O). In order to investigate the successful synthesis and optical performance of the compound, FTIR spectra was used to characterize the complex (as shown in Fig. 1). Comparing the ligand with the obtained complex (Fig. 1, part a and b), the absorption peaks at 1620 cm1, 1466 cm1 could be ascribed to the stretching vibration of pyridyl ring, and after coordinated with metal ion, it moved to 1630 cm1 and 1487 cm1. Besides, as revealed in parts a and b, the absorption peaks of sulfo group have a slight shift after coordination. In
Table 1 Element assay data of complex. Element
Tb
C
O
N
S
Average (%)
3.23
27.41
43.33
19.61
6.44
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in our later work we employ second ligand to promote the structure of complex. 3.2. ECL behavior of the Tb complex
Fig. 1. FTIR spectra of ligand (a) and complex (b).
addition, the broad bands absorption peak at 3381 cm1 shows the presence of crystal water molecules in the complex, UV–vis absorption spectra was explored further to confirm the formation of compound (Fig. S2 in the supporting information). All the results above indicates Tb(C5H4NO3S)2n(H2O) was synthesized successfully. Because of the weak coordination strength of sulfonate toward other metal ions, most well-studied crystal structures were constructed by transition metal ions [50]. It has been reported that by tailoring the chemical environment of the metal ions, organic ligands can compete with water molecules to cordinate with the metal ions. However, the crystal of complex could not be obtained so that accurate structure of the product can’t be sure as well. Thus,
As shown in Fig. 2A, the ECL and CV curves of Tb(C5H4NO3S)2n (H2O) were displayed, an intense ECL emission is obtained in NaAcHAc buffer solution (0.05 M, pH = 4.7) containing Tb(C5H4NO3S)2n (H2O) (5 103 M, 250 mL) in the presence of K2S2O8 (0.10 M, 250 mL) by scanning the potential between 0 and 1.8 V at 100 mV/ s. The emission window was placed in front of the photomultiplier tube, which was biased at 800 V. In order to investigate the ECL properties and ECL reaction mechanism, CV curves in Fig. 2B were carried out. It could be observed that there no reduction peak in blank buffer solution(line a), but a slight reduction peak could be observed when S2O82 were added in the above buffer solution (line b). In contrast, with the addition of complex, apart from the reduction peak of S2O82 at about 0.85 V [51], an obvious reduction peak at about 1.4 V(line d) in Fig. 2B was observed, corresponding to ECL curve (Fig. 2C), suggesting this weak peak might be assigned to the reduction of Tb(C5H4NO3S)2n(H2O). Obviously, there is no ECL response of the blank buffer solution (line a), K2S2O8(line b) or complex(line c), but an obvious ECL signal appeared after addition of K2S2O8 together with complex in the buffer saline(line d) in Fig. 2C. On the other hand, only complex and buffer solution were involved, no ECL response could be observed, after addition of K2S2O8 into the cell, an anticipated ECL signal was appeared. All these results show that the coreactant K2S2O8 was essential in the ECL reaction process and the ECL efficiency has a strong dependence on S2O82. In addition, the controlled trials were performed to prove the fact that ECL signal is attributed to the as-prepared complex but not ligands (as shown in Fig. 2D). As for the reaction mechanisms, mainly two ECL mechanisms, i.e., ion
Fig. 2. (A) ECL-potential and corresponding CV curves of the complex (B) CV curves and (C) ECL-potential curves of simplex buffer saline, K2S2O8, complex and (d) Tb complex (D) ECL-potential curves of the complex (c) and ligand (d) in NaAc-HAc buffer solution (0.05 M, pH = 4.7) containing 0.10 M K2S2O8. (scan range: 2.0 to 0 V, vs Ag/AgCl and scan rate: 100 mV/s, respectively.).
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annihilation ECL and coreactant ECL, have been proposed frequently in the literatures [52–55]. Analogous to our previous work, the probable mechanism of the ECL reaction was proposed. First, the complex was directly reduced on the electrode surface and the ECL is not observed until reaching the potentials where Tb (C5H4NO3S)2n(H2O) is produced. As we know, SO4 plays an important role in ECL processes. There are two possible sources including produced directly on the electrode surface (Eqs. (2), (3)) and from reaction of S2O82 with Tb(C5H4NO3S)2n(H2O) (Eq. (4)). However, an obvious reduction current could be observed at the potentials corresponding to the appearance of ECL signal suggested that (Eq. (4)) is more important. Then the Tb (C5H4NO3S)2n(H2O) reacted with SO4 to emit light and equations were described as follows. The ECL process could be illustrated as Scheme 1. Tb(C5H4NO3S)2n(H2O) + e ! Tb(C5H4NO3S)2n(H2O)
(1)
S2O82 + e ! S2O82
(2)
S2O82 ! SO42 + SO4
(3)
Tb(C5H4NO3S)2n(H2O) + S2O82 ! Tb(C5H4NO3S)2n(H2O) + SO42 + SO4
(4)
Tb(C5H4NO3S)2n(H2O) + SO4 ! Tb*(C5H4NO3S)2n(H2O) + SO42
(5)
Tb*(C5H4NO3S)2n(H2O) ! Tb(C5H4NO3S)2n(H2O) + hv
(6)
Fig. 3A displayed the fluorescence spectra (FL) of Tb complex at an excitation of 320 nm. A broad emission band at ca. 450–650 nm and four characteristic peaks of Tb at 495, 551, 587, 625 nm are observed, respectively corresponding to the transitions 5D4-7FJ (J = 6, 5, 4, 3). The ECL spectrum of complex was measured in 0.10 M K2S2O8 and NaAc-HAc (0.05 M, pH = 4.7) buffer saline by inserting filters at wavelengths of 440, 460, 475, 490, 505, 520, 535, 555, 575, 590, 605 and 620 nm under cycling voltammetry conditions. It can be observed that four peaks were found in the region between 450 nm and 650 nm, and associated with Tb 5D4-7FJ (Fig. 3B), which was in accordance with spectrum of fluorescence[56]. 3.3. Optimization of ECL conditions The ECL signal from Tb(C5H4NO3S)2n(H2O) is found to be affected by several factors such as K2S2O8 concentration, buffer saline pH, and scan rate and types of coreactant. In order to obtain
the excellent ECL response, all these conditions were optimized in this work. Fig. 4A depicts the effect of pH (4.0–6.5) of NaAc-HAc solution on ECL intensity. The ECL intensity increases with the increase in pH, when pH reached at 4.7, the maximum ECL intensity appeared and then the decrease of intensity were observed at higher pH. The result can be explained that the proton could be reduced easily at low pH at the negative potential. In contrast, the ECL intensity decreased due to the consumption of strong oxidant SO4 via scavenging effect of OH [57,58]. Fig. 4B and 4D displays the effect of K2S2O8 concentration (1.0– 10.0 mM) and types of coreactant on ECL intensity. With the increase in K2S2O8 concentration the ECL intensity has a significant improvement, because more radicals were produced from oxidation of negatively charged Tb(C5H4NO3S)2n(H2O) by the electrogenerated SO4. However, the further increase in K2S2O8 concentration causes a decrease in ECL intensity due to that excess S2O82 readily reacted with negatively charged Tb (C5H4NO3S)2n(H2O), which inhibited the formation of radicals. The facts indicates that the ECL efficiency has an intense dependence on S2O82 concentration. Similar phenomenon has been reported before in literatures [59,60]. Hence, 8.0 mM was used as the optimal K2S2O8 concentration throughout this work. Meanwhile, Fig. 4D demonstrates that obvious ECL signal can be observed using S2O82 as the coreactant. Different kinds of coreactants have been employed in coreactant ECL systems, for example, oxidative-reductive coreactants such as TPrA and C2O42, and reductive-oxidative coreactant such as S2O82. It is found that scan rate exerts an effect on ECL intensity, shown in Fig. 4C. With the scan rate increased the ECL intensity increased steadily until the rate is up to a maximum at 350 mV/s then a decrease appeared, which may be explained that ECL efficiency is controled by the formation rate of radicals as well as the diffusion rate of K2S2O8. Proposed in literature, at high rate, the consumption of coreactant would be much faster than the diffusion from the bulk solution to electrode surface resulting a low transition concentration of K2S2O8. In summary, the higher scan rate would decrease the ECL intensity, so in our work, 350 mV/s was selected as the optimal scan rate. Furthermore, the CV curves of different scan rates and the relationship between reduction peak currents of complex and scan rate was explored further to confirm the facts discussed above (Fig. S3A and B). In order to investigate the control reaction such as diffusion-controlled reaction and adsorptioncontrolled reaction during the process, a successive potential scans was conducted. It was found that ECL intensity and peak current began to decrease after the first cycle. However, in the following cycles, a stable CV – ECL process was observed. At the same time, a recovery of ECL intensity and current appeared after several minutes if the electrode were kept in cell without holding the potential. The ECL – potential and ECL – time profiles are shown in Fig. S4B and C. The results are in accordance with continuous CV scans, shown in Fig. S4A. All discussion above indicate a dominant diffusion controlled process. 3.4. Analytical Performances for Cd2+ Sensing
Scheme 1. ECL mechanisms of the Tb(C5H4NO3S)2(H2O)2/K2S2O8 system.
Sensitivity is one of key factors for a sensing system, and the detection limit is particularly significant as well, because the heavy metal Cd2+ is extremely toxic. Therefore, based on the optimized conditions discussed above, the sensitivity of the fabricated ECL sensor was assessed by measuring the variables of ECL intensities in the absence and presence of Cd2+ with different concentrations. As shown in Fig. 5A, the effect of various concentrations of Cd2+ on the ECL intensity was presented. Obviously, with the increase of Cd2+ concentration, the ECL intensity decreased, due to that Cd2+ could effectively quench the ECL signal of the complex. Fig. 5B displayed the relationship of between Cd2+ concentration and
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Fig. 3. (A) Fluorescence spectra and Inset is excitation spectra. (B) ECL spectra of the Tb(C5H4NO3S)2(H2O)4 in NaAc-HAc buffer solution (0.05 M, pH = 4.7) containing 0.10 M K2S2O8 by cycling the potential between 0.00 V and 1.80 V (vs Ag/AgCl).
Fig. 4. Effects of (A) solution pH, (B) K2S2O8 concentration, (C) scan rate and (D) type of coreactant on the ECL intensity of the Tb(C5H4NO3S)2(H2O)4/K2S2O8 system in NaAcHAc buffer solution (0.05 M, pH = 4.7) with 0.10 M K2S2O8. (scan range: 2.0 to 0.0 V, vs Ag/AgCl and scan rate: 100 mV/s, respectively.).
Fig. 5. (A) ECL-potential curves of the ECL sensor in the presence of various concentrations of Cd2+ (3.80 1010 M to 1.93 105 M), the inset in Fig. 1A is the relationship between the ECL intensity and the concentration of Cd2+. (B) Relationship between the (I0-I) and the concentration of Cd2+, where I0 and I are the ECL intensity in the absence and presence of Cd2+, respectively.
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variate of ECL intensity, from which DI was found to be linearly dependent on the logarithm of Cd2+ concentration in the range from 3.85 1010 to 1.90 105 M. The regression equation was DI = 6471.142 + 648.323 lg c (c was the value of Cd2+ concentration) with the R of 0.99575. Meanwhile, the limit of detection (LOD) was estimated to be 1.3 nM at S/N = 3. According to US EPA and World Health Organization (WHO) standards, maximum limit for bottled water is about 4 and 40 nM [61]. In addition, the comparison of the sensitivity of different detection methods of Cd2+ are listed in Table S1. It has been found that a wide range of applications almost based on the fluorescence sensors and probes. However, many fluorescence probes work in nonaqueous medium with a high background, except several sensitive systems are fit for living cells and environment samples detection. In contrast, our synthetic ECL sensor is more “practical” and has a preferable sensitivity. The main superiority reflects in relative simple fabrication and operation processes, low cost, and fine water-solubility. The results indicates that this novel ECL system has great potential for Cd2+ determination. To further assess the selectivity and specificity of the present ECL sensor, the effects of various metal ions using as inferences substances like Cu2+, Ag+, Pb2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Ba2+, Al3 + , Fe3+, Na+, Hg2+ at a concentration of 1.0 105 M on the sensing were also examined in the contrast experiments. As shown in Fig. 6, only in the presence of Cd2+, a large ratio of I0 to I was
observed, but Cu2+ ions exhibit a slight effect on ECL signal, whereas other metals ions have very little effects on it comparing with the blank. A possible explanation that interaction between the compound and Cu2+ can be expected to be much weaker than the interaction between the compound and Cd2+. Meanwhile, we use the thiourea as masking agent to decrease the interference of Cu2+ ions during the detection of Cd2+ (Fig. 6B). All these results suggested that the proposed ECL sensor possessed an excellent selective response to Cd2+. It was worth noting that the ECL quenching and enhancement on different metal ions have been reported in literature previously. Therefore, the high quenching efficiency of Cd2+ at the cathodic potential range could attributed to the better energy match between the compound and the redox potential of metal ions. To the best our knowledge, a proper potential difference between energy levels and an efficient charge transfer is important. Because a high potential difference would offer a strong driving force to fast transfer charge. However, an over high potential difference would go against the charge transfer owing to other undesired interaction [62]. On the other hand, the decreased ECL intensity could be due to the fact that the redox potential of Cd2+ (0.403 V vs NHE) lies between the conduction and the valence bands of the compound and the excited electrons were accepted by metal ions instead of transferring to the ground state of Tb(C5H4NO3S)2n(H2O). As regards the exact mechanism, it was not clear, but we observed a distinct increment in electrochemical current and an obvious change on CV curves during the ECL reactions (Fig. S3, A and B). Besides, a majority of fluorescent sensors exhibiting excellent fluorescence response to Cd2+ have already been reported in many literatures. Most originated from the intramolecular chargetransfer (ICT) effect after the coordination of Cd2+ to the compound. With regard to this, a contrast experiment were also conducted (Fig. S3C in the Supporting Information). In other words, this phenomenon might explain the decrease in ECL efficiency to some extent is because of some unwanted combination. 3.5. Application in Sample Assay To further validate the reliability and potential applicability of this method, it’s applied to the real samples determination, various water samples, including tap water from our laboratory and environmental water samples (pond water) were tested, respectively. Pond water samples were filtered before injected into ECL cell. As Cd2+ ions were spiked, the evident decrease of ECL intensity was recorded and compared with a standard calibration curve. Furthermore, for the two water samples, the recovery of spiked samples obtained are 94–105%, as 12.5 nM and 25 nM Cd2+ are spiked, respectively (Table 2). Because of excellent analytical performances (high sensitivity and selectivity), the proposed ECL sensor was applicable for practical samples detection.
Table 2 Cd2+ Assay in Real Water Samples.
Fig. 6. (A) ECL responses of the ECL sensor in the presence of different metal ions and the selectivity to Cd2+ by comparing it to interfering metal ions and (B) use the thiourea as masking agent to decrease the interference of Cu2+ ions during the detection of Cd2+(The concentrations are 5.0 mM, respectively).
samples
Added Cd2+ (nM)
founded Cd2+ (nM)
recovery (%)
Tap water
0 12.5 25.0
not found 12.7 26.2
/ 102 105
Pond water
0 12.5 25.0
not found 11.9 24.8
/ 95 99
Y. Zhu et al. / Electrochimica Acta 228 (2017) 1–8
4. Conclusion In conclusion, a novel complex with poor ligand of pyridine-3sulfonic acid has been synthesized and characterized. The ECL behaviors of Tb complex have also been investigated and a possible reaction mechanism was proposed as well. In this work, an efficient ECL emission was observed using K2S2O8 as coreactant, the unique ECL performances of complex on the surface of GCE offer an ECL sensing platform. The as-prepared ECL sensor can be used for highly sensitive Cd2+ detection with a detection limit of 0.13 nM. This strategy have unique advantages of instrument simplicity, easy operation quick-response versus other analytical techniques, which expands the applications of ECL technology, and predicted the potential value of rare earth complex in the analytical methodology. Acknowledgments This work was financially supported by National Naturual Science Foundation of China (No. 21275007), and the Foundation for Innovation Team of Bioanalytical Chemistry is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.049. References [1] P. Zhuang, M.B. McBride, H. Xia, Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China, Total Environ. 407 (2009) 1551. [2] W. de Vries, P.F. Römkens, G. Schütze, Critical soil concentrations of cadmium, lead, and mercury in view of health effects on humans and animals, Rev. Environ. Contam. Toxicol. 191 (2007) 91. [3] X.J. Jiang, M. Li, H.L. Lu, A Highly Sensitive C3-Symmetric Schiff-Base Fluorescent Probe for Cd2+, Inorg. Chem. 53 (2014) 12665. [4] Q. Zhao, R.F. Li, S.K. Xing, A highly selective on/off fluorescence sensor for cadmium(II), Inorg. Chem. 50 (2011) 10041. [5] J. Yin, T. Wu, J. Song, SERS-active nanoparticles for sensitive and selective detection of cadmium ion, Chem. Mater. 23 (2011) 4756. [6] P. Huang, S. Li, N. Gao, Toward selective, sensitive, and discriminative detection of Hg2+ and Cd2+ via pH-modulated surface chemistry of glutathione-capped gold nanoclusters, Analyst 140 (2015) 7313. [7] H. Li, Y. Yao, C. Han, Triazole-ester modified silver nanoparticles: click synthesis and Cd2+ colorimetric sensing, Chem. Commun. (2009) 4812. [8] S.R. Ostrowski, S. Wilbur, C.H. Chou, Agency for Toxic Substances and Disease Registry's 1997 priority list of hazardous substances. Latent effects— carcinogenesis, neurotoxicology, and developmental deficits in humans and animals, Toxicol. Ind. Health 15 (1999) 602. [9] Z. Chen, Y. Liu, Y. Wang, Dynamic evaluation of cell surface N-glycan expression via an electrogenerated chemiluminescence biosensor based on concanavalin A-integrating gold-nanoparticle-modified Ru (bpy) 32+-doped silica nanoprobe, Anal. Chem. 85 (2013) 4431. [10] L.B. Allen, P.H. Siitonen, H.C. Thompson, Methods for the determination of arsenic, cadmium, copper, lead, and tin in sucrose, corn syrups and highfructose corn syrups by inductively coupled plasma atomic emission spectrometry, J. Agric. Food Chem. 45 (1997) 162. [11] L. Patrick, Toxic metals and antioxidants: part II the role of antioxidants in arsenic and cadmium toxicity, Med. Rev. 8 (2003) 106. [12] L. Pari, P. Murugavel, S.L. Sitasawad, Cytoprotective and antioxidant role of diallyl tetrasulfide on cadmium induced renal injury: an in vivo and in vitro study, Life Sci. 80 (2007) 650. [13] X. Peng, J. Du, J. Fan, A selective fluorescent sensor for imaging Cd2+ in living cells, J. Am. Chem. Soc. 129 (2007) 1500. [14] M. Li, H.Y. Lu, R.L. Liu, Turn-on fluorescent sensor for selective detection of Zn2+ Cd2+, and Hg2+ in water, J. Org. Chem. 77 (2012) 3670. [15] L. Xue, Q. Liu, H. Jiang, Ratiometric Zn2+ fluorescent sensor and new approach for sensing Cd2+ by ratiometric displacement, Org. Lett. 11 (2009) 3454. [16] S. Sarkar, R. Shunmugam, Unusual red shift of the sensor while detecting the presence of Cd2+ in aqueous environment, ACS Appl. Mater. Interfaces 5 (2013) 7379.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrogenerated chemiluminescence behavior of Tb complex and its application in sensitive sensing Cd2+ Yinggui Zhu* , Min Zhao, Xiaojuan Hu, Xiaofang Wang, Ling Wang College of Chemistry and Material Science, Anhui Normal University, No. 1 Beijing Road, Wuhu 241000, PR China
A R T I C L E I N F O
Article history: Received 1 September 2016 Received in revised form 3 January 2017 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Electrogenerated Chemiluminescence(ECL) Tb complex pyridine-3-sulfonic acid Cd2+ assay ECL sensor
A B S T R A C T
In this paper, we report a novel rare earth metal complex with the weak ligand of aromatic sulphonic acid (pyridine-3-sulfonic acid, 3-pSO3H), and characterized by FT-IR, UV–vis, energy-dispersive X-ray spectroscopy (EDX), electrochemiluminescence spectra, etc. Then an excellent electrochemiluminescence (ECL) signal was observed with K2S2O8 as the coreactant in NaAc-HAc buffer solution. For another thing, the electrochemical properties of the compound have been thoroughly investigated in acetonitrile solution, the possible ECL reaction mechanism was proposed as well. Furthermore, a simple and straightforward ECL platform was reported for sensitive and selective detection of Cd2+ due to the effective quenching after addition of Cd2+. Other heavy/transition metal ions do not interfere with the sensing. The limit of detection is determined as 0.13 nM, the results suggested that as-prepared complex could be a promising material for developing ECL senors to detect the Cd2+ rapidly indwell in environmental and practical samples. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that toxic heavy metal ions contamination could not only lead to many serious environmental and health problems, but also affects the normal life of people seriously. Cadmium ion has been recognized as a highly toxic heavy metal ion and listed as the seventh on the Top 20 Hazardous Substances Priority List by the Agency for Toxic Substances and Disease Registry and US Environmental Protection Agency (EPA). However, it has been widely used in many fields as well, such as agriculture, metallurgy, war industries, etc. The increasing level in water, soil or food will bring about severe injury to the human body. For instance, metabolism disorders, renal dysfunction, and even certain cancers, which has close relationship with the heath of people [1–8]. Up to now, numerous monitor avenues based on mass spectrometry, atomic absorption spectrometry, atomic fluorescence spectrometry, electrochemical stripping analysis have been reported but these powerful methods are timeconsuming, high cost, complicated preparation and instrumentation [9]. Therefore, developing a reliable, highly sensitive, fastresponse and easy-operation method for Cd2+ assay is of great importance [10–16].
* Corresponding author. Tel.: +86 553 3869303; fax: +86 553 3869303. E-mail address: [email protected] (Y. Zhu). http://dx.doi.org/10.1016/j.electacta.2017.01.049 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Electrogenerated chemiluminescence (ECL) is a highly sensitive analytical method, which has been applied in all kinds of fields. The ECL signal is usually produced by light-emitting substance at a proper potential, and the electrochemical reactions [17–25] involves the generation of oxidized and reduced species at the electrode surface that undergo an electron-transfer reaction and emit light [26–30]. In comparison to the conventional electrochemical technology, ECL assay possesses many unique advantages such as high sensitivity, good stability, low background, operational simplicity, good temporal and spatial controllability [31–33]. With the rapid developments of ECL assay, it has been combined with many analytical techniques such as high-performance liquid chromatography (HPLC), capillary electrophoresis (CE), mass spectrometry (MS), etc. Meanwhile, ECL sensors have attracted wide attention in recent years owing to its good biocompatibility, electrocatalytic activity and a wide range of applications in DNA analysis, clinical diagnosis and immunoassay. In addition, it was revealed that metal complex with the excellent electrochemical property and outstanding ECL performance has become a promising material in fabricating ECL sensors [34–40]. However, up to now, a widespread investigation mainly focused on [Ru (bpy)3]2+ because of its high ECL sensitivity and high stability. As such, it is our goal to develop other novel high-efficiency ECL systems and expand their potential applications in the analytical chemistry fields [41–45].
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Sulfonates have inherent coordinative pliancy and broad applications in various areas. The common features of sulfonate ligands lie in the following aspects, (1) Sulfonic acids present flexible and diverse coordination patterns, due to that oxygen atoms could connect metal atoms toward different directions thus forming high dimensional structures. (2) Sulfo-group is preferable hydrogen bond acceptor and it is more likely to obtain divergent hydrogen bond. (iii) Because of its intrinsic strong electrophilicity and fine water solubility, that not only promote the photoelectric property of complex but also help to explore water-soluble catalyst. Despite of the report that organosulfonate (RSO3) is regarded as poor ligand and has been employed as ‘non coordinating’ anion [46]. Indeed, a series of organosulfonate coordination polymers that have been reported in majority of papers constructed by transition metal, alkalions, alkaline earth ions and silver ion, the coordination chemistry of arenedisulfonates with other metal ions especially rare earth metal is lack of systematic investigation [47]. Taking into consideration these facts, we initiated our research work which will be an attractive research subject. In this study, we designed and synthesized a Tb complex successfully with the ligand of pyridine-3-sulfonic acid (3-pSO3H) by a simple synthetic method [48]. Under the optimized conditions, a simple ECL senor was established based on the effective quenching of ECL signal by the Cd2+ for sensitive and specific detection of Cd2+ with the concentration over the range from 2.0 1010 to 3.8 105 M with a low detection limit of 0.13 nM. Furthermore, the possible mechanism of ECL reaction based on Tb complex-K2S2O8 system and ECL quenching of Cd2+ was proposed.
2.3. Synthesis of Tb complex The Tb complex was synthesized according to our previous work by a simple method [49]. Briefly, 3-pSO3H (0.0796 g) was dissolved in 10 mL of ultrapure water, a freshly prepared solution of Tb(NO3)36H2O (1.1326 g) in 10 mL of ultrapure water was added dropwise into above solution with stirring for 30 minutes. Then, reaction of the mixture was maintained for 24 h with heating in water bath under a constant temperature of 70 C. After that, the resulting transparent solution was concentrated to remove residual water and dissolved in acetonitrile, followed by filtration to purify. Finally, the obtained complex acetonitrile solution stored in the refrigerate, and over a period of time, the solid particles precipitated from the solvent for further use. 2.4. Procedures for Cd2+ Sensing The GCE electrode was successively polished with 0.05 mm Al2O3 powder until the mirror-like surface appeared, followed by sonication in the deionized water and ethanol, respectively. The ECL behavior of complex were investigated in NaAc-HAc (0.05 M, 2.0 mL) buffer solution (pH = 4.7) containing Tb(C5H4NO3S)2n (H2O) (5 103 M, 250 mL) and K2S2O8 (0.10 M, 250 mL) served as coreactant, over a scanning range of 1.8 V to 0 V at a photomultiplier tube voltage of 800 V and a scanning rate of 0.1 V/s. Then, a series of different amounts of Cd2+ were added in the ECL cells to measure the ECL signals. All the mixtures were diluted to 2.6 mL with ultrapure water and mixed thoroughly. 2.5. Procedures for Cd2+ Sensing in Real Samples
2. Experimental Section 2.1. Materials Pyridine-3-sulfonic acid (3-pSO3H) was purchased from J&K (Beijing, China). Rare earth metal salts was pursed from Aladdin. AgNO3, Pb(NO3)2, CuSO45H2O, MnCl24H2O, NiCl26H2O, BaCl22H2O, Zn(CH3COO)22H2O, CoCl26H2O, FeCl36H2O, CdCl22.5H2O, Al(NO3)39H2O, CaCl2, NaCl, Mg(NO3)26H2O, KCl, HgCl2 were acquired from Shanghai Chemical Reagent Co. All other regants employed were of analytical grade, all solutions were prepared with ultrapure water.
2.2. Instrumentation The ECL measurements were measured using an MPI-E ECL analytical system (Xi’an Remax Electronic Science and Technology Co. Ltd., Xi’an, China) with the voltage of 800 V, and the potential was set from 1.8 V to 0 V at a scan rate of 0.1 V s1. The ECL spectrum was measured using a BPCL-GPZ-TIC ultraweak luminescence analyzer (Institute of Biophysics, Chinese Academy of Sciences) in conjunction with a CHI660C electrochemical workstation. Cyclic voltammetric (CV) was performed with a CHI 660C Electrochemical Analyzer (Shanghai Chenhua Instrument Co., China). All experiments were performed with the conventional three-electrode system including a glass carbon electrode (GCE, K = 3.0 mm) as working electrode, Ag/AgCl (saturated KCl) as reference electrode and a Pt wire counter electrode, respectively. The Fourier transform infrared spectroscopy (FT-IR) was recoded on a FT-IR spectrophotometer using KBr (FT-IR 8900, Hitachi, Japan). Fluorescence spectra were obtained by Hitachi F-4500 spectrofluorimeter (Tokyo, Japan) in this work. The energydispersive X-ray spectroscopy (EDX) mapping were carried out on Hitachi S-4800 under the accelerating voltage of 5 kV.
Practical samples we measured involving Tap water (from Lab), pond water (from Jinghu Lake, Wuhu). These water samples were measured after two times filtration using 0.22 mm filters. For the detection, series of pure spiked water samples were prepared, then the obtained solutions were introduced to ECL system. Respectively, the mixtures were diluted to 2.6 mL before electrochemical measurements. 3. Result and discussion 3.1. Characterization of the Tb complex Based on EDS (Figs. S1 in Supporting Information), Tb element was detected in the complex which demonstrates that the rare earth metal Tb has reacted with the ligands and the element analysis data (Table 1) were determined to confirm compound composition. According to the results, a possible molecular formula was predicted as Tb(C5H4NO3S)2n(H2O). In order to investigate the successful synthesis and optical performance of the compound, FTIR spectra was used to characterize the complex (as shown in Fig. 1). Comparing the ligand with the obtained complex (Fig. 1, part a and b), the absorption peaks at 1620 cm1, 1466 cm1 could be ascribed to the stretching vibration of pyridyl ring, and after coordinated with metal ion, it moved to 1630 cm1 and 1487 cm1. Besides, as revealed in parts a and b, the absorption peaks of sulfo group have a slight shift after coordination. In
Table 1 Element assay data of complex. Element
Tb
C
O
N
S
Average (%)
3.23
27.41
43.33
19.61
6.44
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3
in our later work we employ second ligand to promote the structure of complex. 3.2. ECL behavior of the Tb complex
Fig. 1. FTIR spectra of ligand (a) and complex (b).
addition, the broad bands absorption peak at 3381 cm1 shows the presence of crystal water molecules in the complex, UV–vis absorption spectra was explored further to confirm the formation of compound (Fig. S2 in the supporting information). All the results above indicates Tb(C5H4NO3S)2n(H2O) was synthesized successfully. Because of the weak coordination strength of sulfonate toward other metal ions, most well-studied crystal structures were constructed by transition metal ions [50]. It has been reported that by tailoring the chemical environment of the metal ions, organic ligands can compete with water molecules to cordinate with the metal ions. However, the crystal of complex could not be obtained so that accurate structure of the product can’t be sure as well. Thus,
As shown in Fig. 2A, the ECL and CV curves of Tb(C5H4NO3S)2n (H2O) were displayed, an intense ECL emission is obtained in NaAcHAc buffer solution (0.05 M, pH = 4.7) containing Tb(C5H4NO3S)2n (H2O) (5 103 M, 250 mL) in the presence of K2S2O8 (0.10 M, 250 mL) by scanning the potential between 0 and 1.8 V at 100 mV/ s. The emission window was placed in front of the photomultiplier tube, which was biased at 800 V. In order to investigate the ECL properties and ECL reaction mechanism, CV curves in Fig. 2B were carried out. It could be observed that there no reduction peak in blank buffer solution(line a), but a slight reduction peak could be observed when S2O82 were added in the above buffer solution (line b). In contrast, with the addition of complex, apart from the reduction peak of S2O82 at about 0.85 V [51], an obvious reduction peak at about 1.4 V(line d) in Fig. 2B was observed, corresponding to ECL curve (Fig. 2C), suggesting this weak peak might be assigned to the reduction of Tb(C5H4NO3S)2n(H2O). Obviously, there is no ECL response of the blank buffer solution (line a), K2S2O8(line b) or complex(line c), but an obvious ECL signal appeared after addition of K2S2O8 together with complex in the buffer saline(line d) in Fig. 2C. On the other hand, only complex and buffer solution were involved, no ECL response could be observed, after addition of K2S2O8 into the cell, an anticipated ECL signal was appeared. All these results show that the coreactant K2S2O8 was essential in the ECL reaction process and the ECL efficiency has a strong dependence on S2O82. In addition, the controlled trials were performed to prove the fact that ECL signal is attributed to the as-prepared complex but not ligands (as shown in Fig. 2D). As for the reaction mechanisms, mainly two ECL mechanisms, i.e., ion
Fig. 2. (A) ECL-potential and corresponding CV curves of the complex (B) CV curves and (C) ECL-potential curves of simplex buffer saline, K2S2O8, complex and (d) Tb complex (D) ECL-potential curves of the complex (c) and ligand (d) in NaAc-HAc buffer solution (0.05 M, pH = 4.7) containing 0.10 M K2S2O8. (scan range: 2.0 to 0 V, vs Ag/AgCl and scan rate: 100 mV/s, respectively.).
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annihilation ECL and coreactant ECL, have been proposed frequently in the literatures [52–55]. Analogous to our previous work, the probable mechanism of the ECL reaction was proposed. First, the complex was directly reduced on the electrode surface and the ECL is not observed until reaching the potentials where Tb (C5H4NO3S)2n(H2O) is produced. As we know, SO4 plays an important role in ECL processes. There are two possible sources including produced directly on the electrode surface (Eqs. (2), (3)) and from reaction of S2O82 with Tb(C5H4NO3S)2n(H2O) (Eq. (4)). However, an obvious reduction current could be observed at the potentials corresponding to the appearance of ECL signal suggested that (Eq. (4)) is more important. Then the Tb (C5H4NO3S)2n(H2O) reacted with SO4 to emit light and equations were described as follows. The ECL process could be illustrated as Scheme 1. Tb(C5H4NO3S)2n(H2O) + e ! Tb(C5H4NO3S)2n(H2O)
(1)
S2O82 + e ! S2O82
(2)
S2O82 ! SO42 + SO4
(3)
Tb(C5H4NO3S)2n(H2O) + S2O82 ! Tb(C5H4NO3S)2n(H2O) + SO42 + SO4
(4)
Tb(C5H4NO3S)2n(H2O) + SO4 ! Tb*(C5H4NO3S)2n(H2O) + SO42
(5)
Tb*(C5H4NO3S)2n(H2O) ! Tb(C5H4NO3S)2n(H2O) + hv
(6)
Fig. 3A displayed the fluorescence spectra (FL) of Tb complex at an excitation of 320 nm. A broad emission band at ca. 450–650 nm and four characteristic peaks of Tb at 495, 551, 587, 625 nm are observed, respectively corresponding to the transitions 5D4-7FJ (J = 6, 5, 4, 3). The ECL spectrum of complex was measured in 0.10 M K2S2O8 and NaAc-HAc (0.05 M, pH = 4.7) buffer saline by inserting filters at wavelengths of 440, 460, 475, 490, 505, 520, 535, 555, 575, 590, 605 and 620 nm under cycling voltammetry conditions. It can be observed that four peaks were found in the region between 450 nm and 650 nm, and associated with Tb 5D4-7FJ (Fig. 3B), which was in accordance with spectrum of fluorescence[56]. 3.3. Optimization of ECL conditions The ECL signal from Tb(C5H4NO3S)2n(H2O) is found to be affected by several factors such as K2S2O8 concentration, buffer saline pH, and scan rate and types of coreactant. In order to obtain
the excellent ECL response, all these conditions were optimized in this work. Fig. 4A depicts the effect of pH (4.0–6.5) of NaAc-HAc solution on ECL intensity. The ECL intensity increases with the increase in pH, when pH reached at 4.7, the maximum ECL intensity appeared and then the decrease of intensity were observed at higher pH. The result can be explained that the proton could be reduced easily at low pH at the negative potential. In contrast, the ECL intensity decreased due to the consumption of strong oxidant SO4 via scavenging effect of OH [57,58]. Fig. 4B and 4D displays the effect of K2S2O8 concentration (1.0– 10.0 mM) and types of coreactant on ECL intensity. With the increase in K2S2O8 concentration the ECL intensity has a significant improvement, because more radicals were produced from oxidation of negatively charged Tb(C5H4NO3S)2n(H2O) by the electrogenerated SO4. However, the further increase in K2S2O8 concentration causes a decrease in ECL intensity due to that excess S2O82 readily reacted with negatively charged Tb (C5H4NO3S)2n(H2O), which inhibited the formation of radicals. The facts indicates that the ECL efficiency has an intense dependence on S2O82 concentration. Similar phenomenon has been reported before in literatures [59,60]. Hence, 8.0 mM was used as the optimal K2S2O8 concentration throughout this work. Meanwhile, Fig. 4D demonstrates that obvious ECL signal can be observed using S2O82 as the coreactant. Different kinds of coreactants have been employed in coreactant ECL systems, for example, oxidative-reductive coreactants such as TPrA and C2O42, and reductive-oxidative coreactant such as S2O82. It is found that scan rate exerts an effect on ECL intensity, shown in Fig. 4C. With the scan rate increased the ECL intensity increased steadily until the rate is up to a maximum at 350 mV/s then a decrease appeared, which may be explained that ECL efficiency is controled by the formation rate of radicals as well as the diffusion rate of K2S2O8. Proposed in literature, at high rate, the consumption of coreactant would be much faster than the diffusion from the bulk solution to electrode surface resulting a low transition concentration of K2S2O8. In summary, the higher scan rate would decrease the ECL intensity, so in our work, 350 mV/s was selected as the optimal scan rate. Furthermore, the CV curves of different scan rates and the relationship between reduction peak currents of complex and scan rate was explored further to confirm the facts discussed above (Fig. S3A and B). In order to investigate the control reaction such as diffusion-controlled reaction and adsorptioncontrolled reaction during the process, a successive potential scans was conducted. It was found that ECL intensity and peak current began to decrease after the first cycle. However, in the following cycles, a stable CV – ECL process was observed. At the same time, a recovery of ECL intensity and current appeared after several minutes if the electrode were kept in cell without holding the potential. The ECL – potential and ECL – time profiles are shown in Fig. S4B and C. The results are in accordance with continuous CV scans, shown in Fig. S4A. All discussion above indicate a dominant diffusion controlled process. 3.4. Analytical Performances for Cd2+ Sensing
Scheme 1. ECL mechanisms of the Tb(C5H4NO3S)2(H2O)2/K2S2O8 system.
Sensitivity is one of key factors for a sensing system, and the detection limit is particularly significant as well, because the heavy metal Cd2+ is extremely toxic. Therefore, based on the optimized conditions discussed above, the sensitivity of the fabricated ECL sensor was assessed by measuring the variables of ECL intensities in the absence and presence of Cd2+ with different concentrations. As shown in Fig. 5A, the effect of various concentrations of Cd2+ on the ECL intensity was presented. Obviously, with the increase of Cd2+ concentration, the ECL intensity decreased, due to that Cd2+ could effectively quench the ECL signal of the complex. Fig. 5B displayed the relationship of between Cd2+ concentration and
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Fig. 3. (A) Fluorescence spectra and Inset is excitation spectra. (B) ECL spectra of the Tb(C5H4NO3S)2(H2O)4 in NaAc-HAc buffer solution (0.05 M, pH = 4.7) containing 0.10 M K2S2O8 by cycling the potential between 0.00 V and 1.80 V (vs Ag/AgCl).
Fig. 4. Effects of (A) solution pH, (B) K2S2O8 concentration, (C) scan rate and (D) type of coreactant on the ECL intensity of the Tb(C5H4NO3S)2(H2O)4/K2S2O8 system in NaAcHAc buffer solution (0.05 M, pH = 4.7) with 0.10 M K2S2O8. (scan range: 2.0 to 0.0 V, vs Ag/AgCl and scan rate: 100 mV/s, respectively.).
Fig. 5. (A) ECL-potential curves of the ECL sensor in the presence of various concentrations of Cd2+ (3.80 1010 M to 1.93 105 M), the inset in Fig. 1A is the relationship between the ECL intensity and the concentration of Cd2+. (B) Relationship between the (I0-I) and the concentration of Cd2+, where I0 and I are the ECL intensity in the absence and presence of Cd2+, respectively.
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variate of ECL intensity, from which DI was found to be linearly dependent on the logarithm of Cd2+ concentration in the range from 3.85 1010 to 1.90 105 M. The regression equation was DI = 6471.142 + 648.323 lg c (c was the value of Cd2+ concentration) with the R of 0.99575. Meanwhile, the limit of detection (LOD) was estimated to be 1.3 nM at S/N = 3. According to US EPA and World Health Organization (WHO) standards, maximum limit for bottled water is about 4 and 40 nM [61]. In addition, the comparison of the sensitivity of different detection methods of Cd2+ are listed in Table S1. It has been found that a wide range of applications almost based on the fluorescence sensors and probes. However, many fluorescence probes work in nonaqueous medium with a high background, except several sensitive systems are fit for living cells and environment samples detection. In contrast, our synthetic ECL sensor is more “practical” and has a preferable sensitivity. The main superiority reflects in relative simple fabrication and operation processes, low cost, and fine water-solubility. The results indicates that this novel ECL system has great potential for Cd2+ determination. To further assess the selectivity and specificity of the present ECL sensor, the effects of various metal ions using as inferences substances like Cu2+, Ag+, Pb2+, Cd2+, Ni2+, Co2+, Mn2+, Zn2+, Ba2+, Al3 + , Fe3+, Na+, Hg2+ at a concentration of 1.0 105 M on the sensing were also examined in the contrast experiments. As shown in Fig. 6, only in the presence of Cd2+, a large ratio of I0 to I was
observed, but Cu2+ ions exhibit a slight effect on ECL signal, whereas other metals ions have very little effects on it comparing with the blank. A possible explanation that interaction between the compound and Cu2+ can be expected to be much weaker than the interaction between the compound and Cd2+. Meanwhile, we use the thiourea as masking agent to decrease the interference of Cu2+ ions during the detection of Cd2+ (Fig. 6B). All these results suggested that the proposed ECL sensor possessed an excellent selective response to Cd2+. It was worth noting that the ECL quenching and enhancement on different metal ions have been reported in literature previously. Therefore, the high quenching efficiency of Cd2+ at the cathodic potential range could attributed to the better energy match between the compound and the redox potential of metal ions. To the best our knowledge, a proper potential difference between energy levels and an efficient charge transfer is important. Because a high potential difference would offer a strong driving force to fast transfer charge. However, an over high potential difference would go against the charge transfer owing to other undesired interaction [62]. On the other hand, the decreased ECL intensity could be due to the fact that the redox potential of Cd2+ (0.403 V vs NHE) lies between the conduction and the valence bands of the compound and the excited electrons were accepted by metal ions instead of transferring to the ground state of Tb(C5H4NO3S)2n(H2O). As regards the exact mechanism, it was not clear, but we observed a distinct increment in electrochemical current and an obvious change on CV curves during the ECL reactions (Fig. S3, A and B). Besides, a majority of fluorescent sensors exhibiting excellent fluorescence response to Cd2+ have already been reported in many literatures. Most originated from the intramolecular chargetransfer (ICT) effect after the coordination of Cd2+ to the compound. With regard to this, a contrast experiment were also conducted (Fig. S3C in the Supporting Information). In other words, this phenomenon might explain the decrease in ECL efficiency to some extent is because of some unwanted combination. 3.5. Application in Sample Assay To further validate the reliability and potential applicability of this method, it’s applied to the real samples determination, various water samples, including tap water from our laboratory and environmental water samples (pond water) were tested, respectively. Pond water samples were filtered before injected into ECL cell. As Cd2+ ions were spiked, the evident decrease of ECL intensity was recorded and compared with a standard calibration curve. Furthermore, for the two water samples, the recovery of spiked samples obtained are 94–105%, as 12.5 nM and 25 nM Cd2+ are spiked, respectively (Table 2). Because of excellent analytical performances (high sensitivity and selectivity), the proposed ECL sensor was applicable for practical samples detection.
Table 2 Cd2+ Assay in Real Water Samples.
Fig. 6. (A) ECL responses of the ECL sensor in the presence of different metal ions and the selectivity to Cd2+ by comparing it to interfering metal ions and (B) use the thiourea as masking agent to decrease the interference of Cu2+ ions during the detection of Cd2+(The concentrations are 5.0 mM, respectively).
samples
Added Cd2+ (nM)
founded Cd2+ (nM)
recovery (%)
Tap water
0 12.5 25.0
not found 12.7 26.2
/ 102 105
Pond water
0 12.5 25.0
not found 11.9 24.8
/ 95 99
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4. Conclusion In conclusion, a novel complex with poor ligand of pyridine-3sulfonic acid has been synthesized and characterized. The ECL behaviors of Tb complex have also been investigated and a possible reaction mechanism was proposed as well. In this work, an efficient ECL emission was observed using K2S2O8 as coreactant, the unique ECL performances of complex on the surface of GCE offer an ECL sensing platform. The as-prepared ECL sensor can be used for highly sensitive Cd2+ detection with a detection limit of 0.13 nM. This strategy have unique advantages of instrument simplicity, easy operation quick-response versus other analytical techniques, which expands the applications of ECL technology, and predicted the potential value of rare earth complex in the analytical methodology. Acknowledgments This work was financially supported by National Naturual Science Foundation of China (No. 21275007), and the Foundation for Innovation Team of Bioanalytical Chemistry is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2017.01.049. References [1] P. Zhuang, M.B. McBride, H. Xia, Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China, Total Environ. 407 (2009) 1551. [2] W. de Vries, P.F. Römkens, G. Schütze, Critical soil concentrations of cadmium, lead, and mercury in view of health effects on humans and animals, Rev. Environ. Contam. Toxicol. 191 (2007) 91. [3] X.J. Jiang, M. Li, H.L. Lu, A Highly Sensitive C3-Symmetric Schiff-Base Fluorescent Probe for Cd2+, Inorg. Chem. 53 (2014) 12665. [4] Q. Zhao, R.F. Li, S.K. Xing, A highly selective on/off fluorescence sensor for cadmium(II), Inorg. Chem. 50 (2011) 10041. [5] J. Yin, T. Wu, J. Song, SERS-active nanoparticles for sensitive and selective detection of cadmium ion, Chem. Mater. 23 (2011) 4756. [6] P. Huang, S. Li, N. Gao, Toward selective, sensitive, and discriminative detection of Hg2+ and Cd2+ via pH-modulated surface chemistry of glutathione-capped gold nanoclusters, Analyst 140 (2015) 7313. [7] H. Li, Y. Yao, C. Han, Triazole-ester modified silver nanoparticles: click synthesis and Cd2+ colorimetric sensing, Chem. Commun. (2009) 4812. [8] S.R. Ostrowski, S. Wilbur, C.H. Chou, Agency for Toxic Substances and Disease Registry's 1997 priority list of hazardous substances. Latent effects— carcinogenesis, neurotoxicology, and developmental deficits in humans and animals, Toxicol. Ind. Health 15 (1999) 602. [9] Z. Chen, Y. Liu, Y. Wang, Dynamic evaluation of cell surface N-glycan expression via an electrogenerated chemiluminescence biosensor based on concanavalin A-integrating gold-nanoparticle-modified Ru (bpy) 32+-doped silica nanoprobe, Anal. Chem. 85 (2013) 4431. [10] L.B. Allen, P.H. Siitonen, H.C. Thompson, Methods for the determination of arsenic, cadmium, copper, lead, and tin in sucrose, corn syrups and highfructose corn syrups by inductively coupled plasma atomic emission spectrometry, J. Agric. Food Chem. 45 (1997) 162. [11] L. Patrick, Toxic metals and antioxidants: part II the role of antioxidants in arsenic and cadmium toxicity, Med. Rev. 8 (2003) 106. [12] L. Pari, P. Murugavel, S.L. Sitasawad, Cytoprotective and antioxidant role of diallyl tetrasulfide on cadmium induced renal injury: an in vivo and in vitro study, Life Sci. 80 (2007) 650. [13] X. Peng, J. Du, J. Fan, A selective fluorescent sensor for imaging Cd2+ in living cells, J. Am. Chem. Soc. 129 (2007) 1500. [14] M. Li, H.Y. Lu, R.L. Liu, Turn-on fluorescent sensor for selective detection of Zn2+ Cd2+, and Hg2+ in water, J. Org. Chem. 77 (2012) 3670. [15] L. Xue, Q. Liu, H. Jiang, Ratiometric Zn2+ fluorescent sensor and new approach for sensing Cd2+ by ratiometric displacement, Org. Lett. 11 (2009) 3454. [16] S. Sarkar, R. Shunmugam, Unusual red shift of the sensor while detecting the presence of Cd2+ in aqueous environment, ACS Appl. Mater. Interfaces 5 (2013) 7379.
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