A luminescent Eu(III)-MOF for selective sensing of Ag+ in aqueous solution

Journal Pre-proof + A luminescent Eu(III)-MOF for selective sensing of Ag in aqueous solution Kai-Ming Ge, Da Wang, Zhi-Jun Xu, Rui-Qing Chu PII:

S0022-2860(20)30186-1

DOI:

https://doi.org/10.1016/j.molstruc.2020.127862

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MOLSTR 127862

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Journal of Molecular Structure

Received Date: 4 August 2019 Revised Date:

4 February 2020

Accepted Date: 4 February 2020

Please cite this article as: K.-M. Ge, D. Wang, Z.-J. Xu, R.-Q. Chu, A luminescent Eu(III)-MOF for + selective sensing of Ag in aqueous solution, Journal of Molecular Structure (2020), doi: https:// doi.org/10.1016/j.molstruc.2020.127862. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.

A luminescent Eu(III)-MOF for selective sensing of Ag+ in aqueous solution Kai-Ming Ge a, Da Wang b, Zhi-Jun Xu c and Rui-Qing Chu * E-mail: [email protected] Address: School of Environmental and Material Engineering, Yantai University, No. 30 Qingquan Road, Laishan District, Yantai City, Shandong Province, China Abstract A 3D open structure FeIII(OH)[C6H2(CO2)2(CO2H)2]·xH2O(MIL-82) with carboxyl-rich groups is designed as a parent metal-organic framework (MOF). A stable lanthanide fluorescent MOF was successfully constructed by doping the Eu3+ cation on the coordination site of MIL-82. The intense luminescence of Eu3+ incorporated MIL-82 indicates that uncoordinated carboxyl group is an efficient framework for accommodating and sensitizing Eu3+ cations. The luminescent MOF can be developed as a chemical sensor for Ag+ in aqueous solution. The sensor also displays high selectivity towards Ag+, excellent sensitivity with a relatively low detection limit (0.09µM) and fast response time within 3 min. In addition, the framework has excellent water stability and thermal stability, which provides a rare method for detecting Ag in daily water samples. 1. Introduction With the rapid development of industry, heavy metal ions are increasingly harmful to the environment and human health [1-4]. Therefore, the detection of heavy metal ions plays an indispensable role in life sciences, biomedicine and environmental science. Silver(Ⅰ) is one of the important ions that has a vital impact on the environment, human body and public health [5]. Silver ions can accumulate in the organism through biological chain and water circulation system, causing silver ion poisoning to lead cell damage and organ failure [6,7]. Thus, high selectivity and high sensitivity Ag+ detection methods are very important for environmental safety and human health. Until now, various analytical techniques, such as atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma mass spectrometry (ICPMS), X-ray fluorescence spectroscopy (XRF) have been used for determination of heavy metal ions [8-11]. However, complicated sample preparation, expensive instruments and high cost make these analytical methods inconvenient. Therefore, the detection of Ag+ in aqueous solutions is significant for human health and ecological environmental by a simple method. Metal–organic frameworks (MOFs), as a new class of porous crystalline materials, constructed by inorganic building blocks and organic linkers through coordination bonds, have shown a variety of potential applications, such as in gas absorption, storage, separation, catalysis, sensing, magnetic and biomedical fields due to their porosity, tunability of structure and size, and excellent thermal and chemical stability [12-16]. Luminescent MOFs have attracted much attention due to their

numerous possibilities for post modifications in the field of chemical sensing. In particular, lanthanide luminescent MOFs (Ln-MOFs) have unique advantages in the field of fluorescence sensing due to their excellent luminescent properties such as large stokes shift, sharp line emission and long fluorescence lifetime [17]. Up to now, luminescent Ln-MOFs have shown recognition abilities for cations [18,19], anions [20,21], organic small molecules [22,23], neurotransmitters [24], nitro explosives [25,26]. Metal-organic frameworks based on trivalent lanthanides are very promising materials for solving the challenges in luminescent probes [ 27 , 28 ]. As a multi-functional probe system, Ln-MOF combines luminescent spectrum with properties such as chemical and temperature sensing. Therefore, the MOFs are often designed as optical sensors based on lanthanide ions. Especially Eu3+ and Tb3+, because they have strong red and green luminescence, respectively [29,30]. However, there are still few researches on the design of Ln-MOFs as fluorescent probes to detect Ag+ in water. Besides, the flexible construction and preparation of Ln-MOFs is still very challenging due to the high coordination number and complex coordination environment typical for these elements. Recently, post-synthesis methods (PSM) have provided the possibility of rational construction of Ln-MOFs and have become an indispensable strategy for designing and developing superior performances [31,32,]. PSM can be defined as a chemical modification method after the formation of MOFs. It mainly refers to the modification through the reaction of functional groups with active sites on the framework, and the introduction of functional groups will not change the topological configuration of the whole framework [33,34]. Thus, MOFs doped with lanthanides were prepared and developed as chemical sensors. In present work, a carboxyl-functional MIL-82 III (Fe (OH)[C6H2(CO2)2(CO2H)2]·xH2O) was obtained under hydrothermal conditions. MIL-82 was selected as the parent matrix for functionalization with Ln3+ cations due to its high chemical and thermal stability and to the presence of two uncoordinated carboxyl groups on the ligand. Therefore, a kind of MOF with strongly red emission was prepared by doping Eu3+ into MIL-82 lattices. Compound Eu3+@MIL-82 shows a three-dimensional hybrid framework, which can be applied as a luminescent sensor for detecting Ag+ in aqueous solution. What’s more, the probe has many advantages, including stable red light emission, simple operation steps, high selectivity and sensitivity and fast detection time. 2. Experimental 2.1 Materials and instrumentation The salts of Eu(NO3)3·6H2O and 1,2,4,5-benzenetetracarboxylic acid (98%) were purchased available from ordinary supplier without further purification. All the other reagents were analytical pure and used as purchased. Powder X-ray diffraction (PXRD) were performed on an XRD-7000 with Cu-Ka radiation. Fourier transform infrared spectra (FTIR) were recorded in the range 4000–400 cm-1 on a Shimadzu IR Prestige-21 using KBr pellets. Thermogravimetric analysis (TG) was measured using a Netzsch STA 449 F5 Jupiter at a heating rate of 10 K min -1 in air, temperature

region of 50Ⅰ-800Ⅰ. Concentrations of Fe and Eu were determined by using a ICP-MS (Thermo Scientific, icapQ, USA) after the nanostructure was decomposed with concentrated nitric acid. XPS experiments were performed on a Thermo Scientifc ESCALAB 250 Xi instrument. Emission spectra of the solid samples were obtained on Edinburgh FLS980 spectrophotometer. Luminescence spectra and luminescence lifetimes were recorded on an Edinburgh FLS 980 phosphorimeter using a xenon lamp as excitation source. All measurements were carried out at room temperature unless otherwise noted. 2.2 Synthetic procedures 2.2.1 Typical synthesis of MIL-82. A mixture of FeCl2·4H2O (0.5522 g), 1,2,4,5-benzenetetracarboxylic acid (H4btec, 0.3431 g), and H2O (5mL) were added to a 25 mL Teflon-lined vessel under stirring at room temperature and then heated at 200 Ⅰ for 48 h. It is worth noting that this compound is in a very acidic medium before and after crystallization, and the pH is maintained at 1. After cooling at room temperature, the solid was collected by centrifugation, washed with acetone, and dried at room temperature under vacuum to obtain the final product. 2.2.2 Eu3+@MIL-82 preparation. Eu3+@MIL-82 was prepared by post-synthetic modification method. The mixture of 0.1g (0.28 mmol)of MIL-82 and 2 mmol Eu(NO3)3·6H2O in 10 mL aqueous solution was stirred at room temperature for 24 h. The resulting brown solid was then separated from the mixed dispersion by centrifugation and washing with aqueous solution to remove redundant Eu3+, the resulted brown powder was dried under vacuum at 60 Ⅰ for 8 h. 2.2.3 Luminescence-sensing experiment. 3 mg powder of Eu3+@MIL-82 were dispersed into aqueous solution (3ml, 1×10 -2 mol L -1) of M(NO3) x (Mn+= Ag+, Pb2+, Ca2+, Zn2+, Hg2+, Ni2+, Mg2+, Co2+, Cd2+, Na+, Cr3+, Al3+, Fe3+) to form an aqueous suspension and the suspension was sonicated for 7 min for luminescent measurements. 3. Results and discussion 3.1 Crystal structure of complex. The original framework (FeIII(OH)[C6H2(CO2)2(CO2H)2]·xH2O) is produced by the reaction of FeCl2·4H2O and H4btec under hydrothermal conditions. The experimental PXRD pattern of MIL-82 synthesized (Figure. 2) matches well with the pattern simulated from single crystal structure [35], confirming that compound MIL-82 with pure phase was successfully synthesized (CCDC reference number 228135). The three-dimensional framework of MIL-82 is built up from the linkage of Fe3+-based chains by organic ligand pyromellitic acid (Figure. 1a). Each Fe3+ ion is bonded to four carboxylate ligands and two OH groups. The connections between

octahedra via bridging OH and carboxylate groups of 1,2,4,5-benzenetetracarboxylic (a)

(b)

(c)

Figure 1 (a) Projection of the 3D structure of MIL-82 showing the linkage of chains of octahedra by H4btec ligands. (b) The 1,2,4,5-benzenetetracarboxylic acid bridges two parallel chains via carboxylate groups whereas the remaining carboxylic functions are noncoordinated. (c) The pyromellitic moiety of MIL-82.

3+

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Eu @MIL-82

MIL-82

Simulated 5

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Figure 2 PXRD patterns of simulated MIL-82, experimental MIL-82 and Eu3+ @MIL-82.

acid to give chains propagating along the c direction and form a 3D framework with cavities. The X-ray photoelectron spectroscopy (XPS) verified the coordination between the uncoordinated carboxyl sites on the MOF and the modified Eu ions. The complete detection of Eu3+@MIL-82 shows peaks at 136.99 eV and 1134.99 eV for Eu 4d and Eu 3d, respectively, indicating the presence of Eu 3+ in the materials (Figure S4). As shown in Figure S1, the O 1s peak from free carboxylic oxygen atoms at 531.5 eV in 1 is shifted to 531.6 eV upon incorporation of Eu3+ in Eu3+@MIL-82. The main cause of the above shift is the formation of Eu-O coordination between the lanthanide ions and the free carboxyl oxygen sites on the ligand, leading to a decrease in the electron density of O and an increase in its the binding energy. In addition, the amount of Eu3+ loaded in Eu3+@MIL-82 was determined by ICP-MS (Table S1). In fact, each 1,2,4,5-benzenetriate species (Figure. 1c) connects two parallel chains through its two carboxylate groups, and two carboxylate arms in the ligand remain uncoordinated. As shown in Figure S2, the band at 1766cm-1 are assigned to the free -COOH functions. IR spectrum indicates that MIL-82 has reactive nature of the uncoordinated carboxylate group in the framework, which provides a platform for coordination of lanthanides. Therefore, Eu3+ ions were loaded into the pores of MIL-82 by PSM, producing Eu3+@MIL-82. The Eu3+ cation is encapsulated into MIL-82, which retains crystalline integrity of the framework, as shown by the X-ray diffraction pattern (Figure.2). The same XRD pattern of Eu3+@MIL-82 and MIL-82 also indicates that the Eu3+ ion is located in the channel of MIL-82 rather than on its skeleton. The thermal stability was measured in the temperature range of 50–800 °C (Figure 3). The TG curve of MIL-82 displays 2 stages of weight loss. A weight loss (4.47%) below 140Ⅰ is attributed to the elimination of the pores water molecules and the weight loss (72.05%) in the range from 300Ⅰ to 382Ⅰ corresponds to the loss of ligand. The TGA of Eu3+@MIL-82 shows good thermal stability similar to MIL-82, which means the incorporation of Eu3+ cations will not affect the thermal stability of the framework.

100

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Temperature / C Figure 3 Thermal gravimetric analysis of MIL-82 (black line) and Eu3+ @MIL-82 (red line)

3.2 Luminescence properties of Eu3+@MIL-82 The photoluminescence spectrum of MIL-82 and Eu3+@MIL-82 in the solid state were examined at room temperature and recorded in Figure 4a and 4b, respectively. Under excitation at 278 nm, the MIL-82 displays emission peak centered around 400 nm. After incorporating Eu3+ cations, the emission spectrum of the Eu3+@MIL-82 exhibits characteristic emission of Eu3+ when excited at 290 nm. The sharp lines located at about 593, 613, 651 and 702nm, corresponding to the 5D0→7FJ (J = 1−4) transitions, respectively. The disappearance of ligand emission indicates that an "antenna effect" has occurred in the system. In this process, the organic ligand absorbs the excitation energy, and the absorbed excitation energy is transferred to Eu3+ through the intersystem transition S1 T1 and the energy transfer T1 f process, causing Eu3+ to generate a strong f-f transition emission. In addition, Eu3+@MIL-82 exhibits a good long lifetime (0.258 ms) which are attributed to the efficient energy transfer from ligand to Eu3+ cations. As can be seen from the above results, Eu3+@MIL-82 can be an ideal choice for luminescent sensors. (b)

λex=278nm

MIL-82

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D0→ F2

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Figure 4 (a) The solid state excitation and emission spectra of MIL-82. (b) The solid state emission spectra of Eu3+@MIL-82.

3.3 Sensing properties of cations To explore the potential of Eu3+@MIL-82 for detection of metal ions, selective experiments were performed. A series of sample’s suspensions in water with 0.01mol/L concentration of M(NO3)x (Mn+= Ag+, Pb2+, Ca2+, Zn2+, Hg2+, Ni2+, Mg2+, Co2+, Cd2+, Na+, Cr3+, Al3+, Fe3+) is prepared by ultrasonic treatment for 5 min. Their emission spectra are recorded in Figure 5 under an excitation peak of 290 nm. The emission intensity at 613 nm shows that various metal ions have a significantly different effect on the luminescence of Eu3+ ions. As shown in Figure 5, the luminescence intensity at 613 nm is decreased when Ni2+, Mg2+, Co2+, Cd2+, Cr3+, Al3+, Fe3+ are introduced. On the contrary, the addition of Ag+ greatly increases the luminescence intensity, and the maximum value is 5.0 times the luminescence intensity of the aqueous solution. The other metal ions (Pb2+, Ca2+, Zn2+, Hg2+, Na+)

tested did not cause significant changes in luminescence intensity. The above results indicate that the Eu3+@MIL-82 can selectively detect Ag+ ions by fluorescence enhancement. The effect of different metal ions on the fluorescence intensity can be more clearly seen in Figure S3. In the presence of Ag+, the emission lifetime of Eu3+@MIL-82 increased significantly from 0.258 ms to 0.991 ms, which is in agreement with the enhancement of the luminescence intensities and indicating presence a more efficient energy transfer between Ag+ and ligand. A concentration-dependent study in the luminescence intensities of 3+ Eu @MIL-82 when concentration of Ag+ ions increased was carried out. The luminescent spectrum was recorded by adding Eu3+@MIL-82 solid samples in a series of different concentrations of Ag+ solution (0-150 µM). When the content of Ag+ is gradually increased, the degree of fluorescence enhancement is stronger (Figure 6a).

Figure 5 Luminescence spectra of Eu3+@MIL-82 dispersed in different metal ions aqueous solutions (10 mM) when excited at 280 nm.

In order to clearly describe the fluorescence enhancement phenomenon of Ag+ to Eu3+@MIL-82, the intensity ratio is plotted against the Ag+ concentration. When the Eu3+@MIL-82 are monitored concentration at a range of 0-150 µM, the intensity ratio and concentration shows a good linear correlation. In addition, the linear relationship between 5D0→7F2 intensity ratio (Y defined as I/I0) and Ag+ concentration (x defined as CAg+) can be used as a function fitting of equation (1), where I and I0 are fluorescence intensities of suspensions with or without Ag+, respectively. Y= 0.0206x + 12.368 (1) 2 From the fitting linear equation, we can get the correlation coefficient (R =0.9984), indicating that Eu3+@MIL-82 can also be used for quantitative analysis of Ag+. The limit of detection (LOD), as calculated by 3Sb/S (Sb is the standard deviation for 20 replicating fluorescence measurements of blank solutions, and S is the slope of the calibration curve), was determined to be 0.09 µM for Ag+. This result met a 50 µg L-1

(about 0.46 µM) standard of U.S. Environmental Protection Agency (EPA) for the maximum allowable levels of Ag+ in drinking water [36]. The function between the immersion time of Eu3+@MIL-82 in 10 mm Ag+ aqueous solution and the luminescence intensity at 613 nm were measured to study the time response characteristics of Eu3+@MIL-82 sensor to Ag+. Figure 7 shows the time-response fluorescence spectra within 45 minutes (a) and the fluorescence intensity after adding Ag+ for 3 minutes (b), respectively. With increasing interaction time between Ag+ aqueous solution and Eu3+@MIL-82 powder sample, the luminescence intensity of the sample is continuously increased (Figure S5). After the suspension of Eu3+@MIL-82 was added to Ag+ for 3 minutes, the luminescence intensity at 613 nm increased to nearly 2.0 times that of the original suspension, (a)

(b)

Figure 6 (a) PL spectra of Eu3+@MIL-82 upon adding different Ag + concentration. (λex = 280 nm). (b) The linear relationship between luminescence intensity ratio and Ag + concentration. 60

45min 35min 25min 15min 5min 3min Origin

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Figure 7 The emission intensity at 613 nm as a function of response time to Ag+, λex = 280 nm.

indicating that the Ag+-induced fluorescence enhancement response time is rapid (Figure S6).

To further understand the mechanism of Ag+ enhanced Eu3+ luminescence, the structural data of Eu3+@MIL-82 and Ag+/Eu3+@MIL-82 were collected by PXRD (Figure S7). By comparison, the PXRD pattern of Ag+/Eu3+@MIL-82 is similar to

Figure 8 Simplified schematic diagram of ligand-metal energy transfer and energy enhancement process in Eu3+ @MIL-82 with Ag+.

that of Eu3+@MIL-82, indicating that the basic skeleton in the compound remains unchanged and the presence of Ag+ does not affect Eu3+@MIL-82 structure. The ICP-MS data of Ag+/Eu3+@MIL-82 indicates that there is strong interaction of the MOF with Ag+, which explains the significant alteration of the luminescence of the MOF in the presence of Ag+(Table S2). As an excellent luminescent material, the advantages of Ln-MOFs are mainly reflected in the “antenna effect” of organic ligands absorbing excitation energy sensitizing lanthanide ion luminescence, and can be manifested by the characteristic emission of lanthanide ions [37]. If there is an “antenna effect” of effective energy transfer, the fluorescence of the lanthanide ions can be enhanced more effectively [38-40]. Therefore, this enhancement effect is attributed to a more efficient energy transfer from ligand to Eu3+ ions induced by Ag+ (Figure 8). As shown in Figure S8 shows the excitation spectrum(λem=613nm) Eu3+@MIL-82 in the presence and absence of Ag+. Both spectra show a broadband emission of 290 nm with a stronger emission in the presence of Ag+, indicating that the addition of Ag+ results in a more efficient energy transfer from the organic ligand to the rare earth ion. As shown in Figure 9, the emission spectrum of the Eu3+@MIL-82 suspension exhibits two positions of luminescence at 278 nm excitation: one is Eu3+ characteristic emission (500-750 nm) and the other is ligand-centered broadband emission (350 nm). The strong emission of the ligand-centered in the aqueous solution clarifies the lower energy transfer efficiency between the organic ligand and the lanthanide ion (IEu(613)/ILigand=1.31). In contrast, the luminescence of ligand-centered in the presence of Ag+ is diminished and the luminescence enhancement of Eu3+ ions indicates a more efficient energy transfer

(IEu(613)/ILigand = 13.49). As a result, the addition of Ag+ significantly enhanced the luminescence of Eu3+@MIL-82. 200

Origin (I(Eu613)/I(ligand) = 1.31) +

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Ag (I(Eu613)/I(ligand) = 13.49) 150

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Figure 9 The emission spectra of the Eu3+@MIL-82 in aqueous solutions (black line) and upon addition of Ag+ in aqueous solution (red line).

Since the pH in each test environment was different, the pH stability of the Eu @MIL-82 sample was investigated (Figure 10). The Eu3+@MIL-82 powder was immersed in a series of aqueous solutions of different pH values. The result shows that the luminescence intensity was no significant change in the pH range of 4-11. The excellent pH-independent stability of Eu3+@MIL-82 allows it to identify Ag+ in ordinary water samples. 3+

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Figure 10 The emission spectra (a) and emission intensities (b) of Eu3+ @MIL-82 in aqueous solutions with different pH values (2-12).

4. Conclusions In summary, a new class of lanthanide luminescent MOFs were prepared by post-synthesis modification, encapsulating Eu3+ in the pores of MIL-82. The compound has been developed as an excellent fluorescent sensor for detecting Ag+ in aqueous solution. As one of the important ways of Ag+ luminescence sensors, the

advantages of Eu3+@MIL-82 include simple detection steps, good selectivity, fast response time (<3 min), high sensitivity. The developed sensor has well-defined correlation between the luminescent intensity ratio and the concentration of Ag+. The detection limit for Ag+ was estimated to be 0.09 µM, which is lower than the maximum allowable level of Ag+ in drinking water. This sensing capability shows that high value detection of Ag+ in real environmental water samples. The fluorescence enhancement mechanism can be attributed to Ag+ induction, resulting in more efficient energy transfer from the ligand to Eu3+. Comparing the reported examples of Ln-MOFs for the detection of metal ions, the present work has several advantages, namely working in a pure water medium rather than in an organic solvent, with a lower detection limit, showing a simpler preparation method. What’s more, there is a rare example about Ag+ chemical sensor based on lanthanide luminescence MOFs. Acknowledgements This work was supported by the National Key R&D Program of China (2016YFB0402701), Focus on research and development plan in Shandong province (GG201809190252), the Natural Science Foundation of Shandong Province of China (ZR2016EMM02). Keywords: lanthanide ions, functionalized MOFs, luminescence, sensing Duruibe, Ogwuegbu, and Egwurugwu, Heavy metal pollution and human biotoxic effects,Int. J. Phys. Sci. 2 (2007) 112-118. 3+ [2] Y. Zhou, B. Yan, An Eu post-functionalized nanosized metal–organic framework for cation exchange-based Fe3+-sensing in an aqueous environment, J. Mater. Chem. A. 2 (2014) 13691-13697.– [3] X. Y. Xu, B. Yan, Eu (III)-functionalized MIL-124 as fluorescent probe for highly selectively sensing ions and organic small molecules especially for Fe (III) and Fe (II), ACS. Appl. Mater. Inter. 7 (2015) 721-729. [4] K. Jayaramulu, R. P. Narayanan, S. J. George, T. K. Maji, Luminescent microporous metal-organic framework with functional lewis basic sites on the pore surface: specific sensing and removal of metal ions, Inorg. Chem. 51 (2012) 10089-10091; [5] J. F. Zhang, Y. Zhou, J. Yoon, J. S. Kim, Recent progress in fluorescent and colorimetric chemosensors for detection of precious metal ions (silver, gold and platinum ions), Chem. Soc. Rev. 40 (2011) 3416-3429. [6] J. Wu, G. Q. Chen, F. Z. Cui, T. N. Kim, J. O. Kim, A mechanistic study of theantibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus, Inc. J Biomed Mater Res. 52(2000), 52, 662-668. [7] A.B.G. Lansdown, Silver I: its antibacterial properties and mechanism of action, J. W. C. 11 (2002) 125-130. [8] Z. Fang, J. Růžička, E. H. Hansen, An efficient flow-injection system with on-line ion-exchange preconcentration for the determination of trace amounts of heavy metals by atomic absorption spectrometry, Anal. Chim. Acta. 164 (1984) 164, 23-29. [9] C. B. Zheng, Y. Li, Y. H. He, Q. Ma, X. D. Hou, Photo-induced chemical vapor generation with formic acid for ultrasensitive atomic fluorescence spectrometric determination of mercury: potential application to mercury speciation in water, J. Anal. Atom. Spectrom. 20 (2005) 746-750. [10] D. T. Quang, J. S. Kim, Fluoro- and Chromogenic Chemodosimeters for Heavy Metal Ion [1]

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Author Contributions Section Author 1: Kai-Ming Ge a. Conceived and designed the analysis. b. Collected the data. c. Contributed data and analysis tools. d. Performed the analysis. e. Wrote the paper. Author 2: Da Wang a. Collected the data. b. Performed the analysis. Author 3: Zhi-Jun Xu a. Conceived and designed the analysis. b. Collected the data.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: