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A novel polydentate ligand chromophore for simultaneously colorimetric detection of trace Ag + and Fe3 +
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 186 (2017) 17–22
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A novel polydentate ligand chromophore for simultaneously colorimetric detection of trace Ag + and Fe3 + Zhengquan Yan ⁎, Qi Zhao, Meijun Wen, Lei Hu ⁎, Xuezhong Zhang, Jinmao You ⁎ School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China Shandong Key Laboratory of Life-Organic Analysis, Key Laboratory of Pharmaceutical Intermediates and Analysis of Natural Medicine, Qufu Normal University, Qufu 273165, China
a r t i c l e
i n f o
Article history: Received 4 May 2017 Received in revised form 31 May 2017 Accepted 5 June 2017 Available online 06 June 2017 Keywords: 3,6-diamino acridine Azo derivative Colorimetric probe Simultaneous detection Ag+ Fe3+
a b s t r a c t A novel polydentate ligand chromophore, 3,6-di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA), was identified and synthesized. After its structure was characterized by FTIR, 1H NMR, mass spectra and element analyses, it was noted to find that there was a simultaneously colorimetric response to Ag+ and Fe3+ accompanying with different color changes, i.e., from brown to light purple for Ag+ and further to purple-red for Fe3+, respectively. Their different action mechanisms, a 1:2 complex mode for EEPA-Ag+ and 1:1 for EEPA-Fe3+, were investigated and confirmed by means of Job's plot and theoretical calculation. EEPA would be a potential colorimetric chemo-dosimeter for simultaneous detection of Ag+ and Fe3+ with the detection limits of 1.6 nmol·L−1 and 69 nmol·L−1, respectively. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Transition and heavy metal ions, which can be found in nature, have made great influence throughout the ecosystems and so been received attractive attention in the fields of chemical, biological and environmental sciences [1–7]. Taking Fe3+ and Ag+ as examples, Fe3+, the second most abundant metal element in the earth's crust, is indispensable for most organisms to form hemoglobin for the storage and transport of oxygen. Deficient iron will result in fatigue, poor work performance, and weaken immunity, while excess one will deteriorate lipids, nucleic acids and proteins [8–13]. For Ag+, an important antibiotic material for wound dressings to treat external infection possesses a variety of industrial, healthcare, and domestic applications. On the other hand, excess Ag+ will result in enzymes/proteins inactivation and bioaccumulation owing to the strong bonding ability with amine, imidazole, carboxyl and thiol groups in enzymes/proteins [14–17]. Therefore, great efforts have been devoted for selective and sensitive detection of Fe3 + and Ag+ in environmental samples, ranging from atomic spectrometry [18], mass spectrometry [19], chromatography [20], electrochemistry [21,22], colorimetric spectrophotometry [12,23, 24], fluorescent spectrophotometry [25–28], and so on. However, most of them focus on single metal ion detection, although these assays are respectable innovation and high sensitivity. It will need different ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Yan), [email protected] (L. Hu), [email protected] (J. You).
http://dx.doi.org/10.1016/j.saa.2017.06.007 1386-1425/© 2017 Elsevier B.V. All rights reserved.
detection methods and experimental conditions to detect some coexisting metal ions respectively, which was more complex and inconvenient. For the time being, single sensor for multiple analytes has been paid much attention and widely applied in heavy metal detection by virtue of its lower cost and higher efficiency [22,29–32]. For example, Napaporn, et al. [33], developed a multi-reverse flow injection analysis for the simultaneous determination of Mn(II), Fe(II), Cu(II) and Fe(III) in water samples with high sample throughput and low reagent consumption, which satisfied the criteria of Green Analytical Chemistry and its goals. Miao, et al. [29], for the first time demonstrated a DNA modified Fe3O4@Au magnetic nanoparticle-based electrochemical biosensor for simultaneous detection of Ag+ and Hg2+ to meet the requirements of U.S. Environmental Protection Agency. Gao, et al. [34], reported a Schiff-base fluorescence probe for simultaneous detection of Hg2+ and Cu2+ by two different mechanisms, indicating its promising application in living cells. However, colorimetric detection of Ag+ and Fe3+ simultaneously using a single probe under the same condition has no reports so far. With this aim in mind, in the work, a novel polydentate ligand of 3,6di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA) combining azo and heterocycle acridine as both chromophore and bonding group will be designed and synthesized. It is expected EEPA could simultaneously detect Fe3+ and Ag+ by virtue of the unique colorimetric and visible color changes, which will provide a basis insight for Ag+ and Fe3+ determination over other competing metal ions with effective discrimination and high contrast.
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2. Experimental Section 2.1. Reagents and Apparatus All chemicals in this work were of AR grade and used as received from Sinopharm Chemical Reagent Co. Ltd. Water used throughout was doubly deionized. A fresh 1.0 mmol·L−1 Fe3+ and Ag+ standard solutions for testing were prepared in doubly deionized water at room temperature and diluted to appropriate concentration when used. 3,6-di-(N-ethyl-Nethoxyl phenylazo) acridine (EEPA) was synthesized according to our previous work [35] and a 8.0 × 10−5 mol/L EEPA stock solution was prepared in N,N-dimethyl formamide (DMF) at room temperature and stored at 4 °C. Phosphate buffers (PB) were prepared by mixing a 0.01 mol/L H3PO4 solution, a 0.01 mol/L K2HPO4 solution, a 0.01 mol/L KH2PO4 solution or a 0.01 mol/L KOH solution in a proper ratio to acquire the desired pH (pH = 4.0, 5.0, 5.5, 6.0, 6.5, 7.0, 8.0, 9.0, 10.0). FTIR spectra of HPNSA with KBr disc were acquired using a Nicolet NEXUS 870 FTIR spectrophotometer at room temperature from 4000– 500 cm− 1. 1H NMR spectra were recorded using a Bruker AMX-500 spectrometer operating at 400 MHz, with tetramethyl-silane (TMS) used as the reference and DMSO-d6 as solvent. Mass spectral analysis was recorded on a GCMS-QP2010 mass spectrometer (Shimadzu Biotech, Japan). Elemental analysis was conducted using an Elemental Vario EL-III apparatus. UV–vis spectra were recorded on a Lambda 35 UV/vis spectrometer using a 1-cm square quartz cell. pH of the solutions was measured by a PHS-25pH meter. 2.2. Preparation of EEPA 0.06143 g (0.25 mmol) 3,6-diamino acridine was dissolved in an icewater solution of 15% sodium nitrite (0.03623 g, 0.525 mmol). After cooling to 0 °C, pH of the solution was adjusted to 1–2 by addition of concentrated hydrochloric acid and kept stirring for 30 min. The excess nitrous acid was destroyed by addition of 6 mg urea. Then, 10 mL buffer solution (KH2PO4/Na2HPO4, pH = 6) containing 78.9 μL N-ethyl-Nethoxyl aniline (0.5 mmol) was dropwise added into and stirred for another 2 h at 0–5 °C. The resultant precipitate was filtered and purified by recrystallizing three times from ethanol to provide purple-red crystal EEPA in the yield of 89.2%. IR (KBr), υ (cm−1): 3312 (\\OH), 3022 (Ar\\H), 1608, 1593, 1555 (Ar ring), 1343 (Ar-N(R1R)), 1197 cm−1(C\\O). 1H NMR (400 MHz, DMSO-d6, δ): 8.82 (d, J = 7.5 Hz, 2H, Ar\\H), 8.40 (d, J = 7.7 Hz, 2H, Ar\\H), 8.22 (d, J = 7.9 Hz, 4H, Ar\\H), 7.91 (s, 1H, Ar\\H), 7.65 (s, 2H, Ar\\H), 6.92 (d, J = 7.9 Hz, 4H, Ar\\H), 4.82 (t, J = 4.8 Hz, 4H, CH2), 3.61 (s, 2H, \\OH), 3.32 (t, J = 4.7 Hz, 4H, CH2), 2.60 (m, J = 3.6 Hz, 4H, CH2), 1.15 (t, J = 3.6Hz, 6 H, CH3). MS: m/z [M + H]+ 383.4091 (theoretical value: 383.3980). Anal. Calcd for C20H20N3O5 (%): C 62.81, H 5.27, N 10.99, O 20.92; Found: C 62.55, H 5.35, N 10.87, O 21.67.
(A600/A505) of the absorption intensity at 600 nm to that at 505 nm was used for Fe3+ quantitative analysis, where A600 and A505 were the absorption intensities of the systems at 600 nm and 505 nm in the presence of Fe3+, respectively. 3. Results and Discussion 3.1. UV–vis Spectral Properties To investigate the possibility for the detection of Ag+ and Fe3+ using EEPA as a colorimetric probe, the UV–vis absorption spectra of EEPA accompanying with its color change in the absence and presence of Ag+ and Fe3 + were recorded first (Fig. 1). From Fig. 1, it is easy to find that, without Ag+ and Fe3+ in DMF/H2O (2/3, v/v), EEPA possesses a strong absorption peak at 505 nm with ε = 3.83 × 104 L·mol−1·cm−1, attributed to the electron transferor of the huge conjugated molecules. With addition of Ag+, the absorption intensity at 505 nm decreased with the color changed into light purple from brown, while the absorption peak red shifted to 528 nm with the color change from brown to purple-red in the presence of Fe3+, respectively, which make it possible to detect Ag+ and Fe3+ simultaneously even in naked-eyes. The reasons might be attributed to the formation of EEPA-metal complex between EEPA and Ag+ and Fe3+ with different reaction mechanisms. 3.2. Optimization of Experimental Conditions It is well known pH is one of the most important factors for metal ion detection for pH influences greatly on the interaction between sensing materials and metal ions [7,36,37]. To make sure of its efficient application in simultaneously colorimetric detection of Ag+ and Fe3+, the effect of pH was investigated over pH 3.0 to 9.0 (Fig. 2). Both the ratio (A600/A505) in the presence of Fe3+ (Fig. 2a) and the difference (ΔA) at 505 nm in the presence of Ag+ (Fig. 2b) vary gradually with the increase of pH and reach the maximum when pH is 6.0. The reason may be that at pH b 6.0, nitrogen atoms in azo and acridine-ring groups are easily protonated, which reduces their coordinated interaction with the recognition ions Ag+ and Fe3 +. While pH is N 6.0, the solubility of Ag+ and Fe3 + in aqueous solution decreases owing to their hydrolyses. They both deteriorate the ability between EEPA and the recognition ions Ag+ and Fe3+. Accordingly, pH 6.0 was selected for simultaneous colorimetric detection Ag+ and Fe3+ for all the subsequent experiments. To illustrate the response rate and stability of EEPA to Ag+ and Fe3+, the difference (ΔA) at 505 nm in the presence of Ag+ (Fig. 2a) and the ratio (A600/A505) in the presence of Fe3+ was measured and calculated at different times after adding Ag+ or Fe3+. The results suggested that the
2.3. Ag+/Fe3+Detection Procedure For Ag+ determination, 1.0 mL PB (pH 6.0), 1.0 mL 8.0 × 10−5 mol/L EEPA and 1.0 mL of the appropriate Ag+ solution or sample were transferred into a 10 mL volumetric flask. The mixture was stirred thoroughly and finally diluted to 10 mL with doubly deionized water. The absorption spectra were measured from 300 nm to 800 nm and the band-slit was set as 2.0 nm. The absorption intensity difference (ΔA) at 505 nm was used for Ag+ quantitative analysis, where Δ A was equal to A0 − A, and A0 and A were the absorption intensities of the systems in the absence and presence of Ag+, respectively. For Fe3+ determination, 1.0 mL PB (pH 6.0), 1.0 mL 8.0 × 10−5 mol/L EEPA and 1.0 mL of the appropriate Fe3+ solution or sample were transferred into a 10 mL volumetric flask. The mixture was stirred thoroughly and finally diluted to 10 mL with doubly deionized water. The ratio
Fig. 1. a) UV–vis spectra of EEPA without and in the presence of Ag+ and Fe3+ in DMF-H2O (V/V = 2:3), respectively; b) UV–vis spectra of EEPA in different solvents.
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Fig. 2. Effect of pH a) on A600/A505 in the presence of Fe3+ and b) ΔA at 505 nm in the presence of Ag+ for EEPA, respectively (pH 6.0, cEEPA = 8.0 × 10−5 mol/L).
proposed EEPA-based sensing system possesses a fast response time of b10.0 min and keeps stable in the following 1 h. The influence of ionic strength was also investigated by adding serious NaCl solutions ranging from 2.0 × 10− 2 to 10−7 mol/L, no obvious change observed hinting that the present sensing system is reasonably stable and can be applied in various kinds of surroundings.
3.3. Special Simultaneous Response to Ag+ and Fe3+ To demonstrate the specially simultaneous response of EEPA to Ag+ and Fe3+, the UV–vis absorption spectra of the proposed sensing system were recorded in the presence of some environment-relative metal ions, i.e., Ca2 +, Cd2 +, Hg2 +, Co3 +, Mg2 +, Mn2 +, Na+, Cr3 +, Ni2 +, Fe2+, Cu+, Zn2+, Cu2+, Pb2+, Ag+ and Fe3+ in 1.1 × 10−5 M, respectively and the results were shown as in Fig. 3. From Fig. 3, we could find that addition of the common metal ions except Ag+ and Fe3+, no obvious changes were observed in the absorption spectra between 300 nm and 800 nm for the proposed EEPA-based sensing system, whose absorption intensities at 505 nm were much weaker and b 5% relative intensity comparing with these in the presence of Ag+ and Fe3+. Importantly, the visible color of the EEPA system exclusively changes from brown to light purple upon the addition of Ag+ and from brown to purple-red in the presence of Fe3+, respectively (Fig. 4a). All the results indicate that the present EEPA system possesses an excellent
selectivity for colorimetric detection of Ag+ and Fe3+ simultaneously, even in naked-eyes. 3.4. Analytical Parameters for Simultaneous Detection of Fe3+ and Ag+ In order to disclose the rule of EEPA to detect Ag+ and Fe3+ simultaneously, series of colorimetric titration experiments were carried out with different Ag+ or Fe3 + concentrations under the given optimized experimental conditions and the results were shown in Fig. 4. From Fig. 4, the detection limits and the calibration graphs for simultaneous detection of Ag+ and Fe3+ could be obtained accordingly. For colorimetric detection of Ag+, a linear relationship between the difference (Δ A) at 505 nm of EEPA and Ag+ concentration was exhibited in the range of 2.0–130 × 10−8 mol/L with a correlation coefficient (R) of 0.9961 and the regression equation ΔA = 2.33 × 10−3 + 8.65 × 10−4c (10−7 mol/L) was obtained (Fig. 4b and c). For Fe3+ detection, a linear relationship between the ratio (A600/A505) of EEPA and Fe3+ concentration was exhibited in the range of 3.5–110 × 10−7 mol/L with R of 0.9786. The regression equation A600/A505 = 1.93 + 1.84 × 10−6c (10−7 mol/L) was obtained (Fig. 4d and e). Based on the definition of detection limit, three times of average deviation of ΔA or A600/A505 in 20 blank samples without Ag+ or Fe3+ utilized here, the limits of detection (LOD) for Ag+ and Fe3+ were up to 1.6 nmol·L−1 and 69 nmol·L−1, respectively. 3.5. Application for Simultaneous Detection of Ag+ and Fe3+ To further demonstrate its application, the present sensor was applied to determine Ag+ and Fe3+ concentrations in 3 environmental water samples obtained respectively from the Yi River, underground water and tap water (Tables 1 and 2). All the samples were tested after filtering several times and being concentrated 100 times by evaporation. For recovery studies, a known concentration of Ag+ and Fe3+ (400 nM) was added to the environmental water samples and the total Ag+ and Fe3+ concentrations were determined accordingly. To access the reproducibility of the method, the measurements were carried out 5 times repeatedly and the recoveries of different known amounts of Ag+ and Fe3+ spiked were determined to be from 98.2% to 107.8% % with standard deviations (RSD) ≤ 4.5% for Ag+ and from 98.2% to 107.8% with RSD ≤ 5.6% for Fe3+, respectively. These validate the reliability and practicality of the proposed dual-mode colorimetric method for simultaneous Ag+ and Fe3+ detection in practice. 3.6. Sensing Mechanism
Fig. 3. Effects of different metal ions on the UV–vis spectra of G-AuNPs (from top to bottom: Ca2+, Cd2+, Hg2+, Co3+, Mg2+, Mn2+, Na+, blank, Cr3+, Ni2+, Fe2+, Cu+, Zn2+, Cu2+, Pb2+, Fe3+ and Ag+, respectively. pH 6.0, cEEPA = 8.0 × 10−5 mol/L, cions = 1.1 × 10−5 mol/L).
To investigate the bonding nature between EEPA and Ag+ or Fe3+, the binding stoichiometry of EEPA with Ag+ or Fe3+ was performed by using Job's plot [38]. For the Job's plot analyses, a series of solutions
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Fig. 4. a) The color change and the absorption spectra of EEPA-based sensing system in the presence of b) Ag+ and d) Fe3+ with different concentrations; the linear relationship between c) −5 3+ mol/L). the ΔA and c+ Ag, and e) between A600/A505 and cFe (pH 6.0, cEEPA = 8.0 × 10
with varying mole fraction of Ag+ or Fe3+ were prepared respectively by maintaining the total EEPA and Ag+ or EEPA and Fe3+ concentration constant (6.0 × 10−5 mol/L). The ratio (A600/A505) in the presence of Fe3 + (Fig. 5a) and the difference (Δ A) at 505 nm in the presence of Ag+ were measured (Fig. 5b), respectively. A 1:2 stoichiometry for the complex between EEPA and Ag+ and 1:1 stoichiometry for the complex between EEPA and Fe3+ could be drawn from their corresponding Job's plots, suggesting that Fe3+ might coordinate with the N atom in acridine ring and Ag+ coordinate with π-electron in N_N bonds by virtue of the strength of their electrophilic ability.
Theoretical calculations were carried out to further confirm the bonding nature between EEPA and Ag+ or Fe3+. Their optimized structures of EEPA before and after coordinating with Ag+ or Fe3 + were shown in Fig. 6, conducted using the B3LYP/631G level of theory and method implemented in the Gaussian 09 suite of program [39]. In the optimized structures, the basic unit of EEPA is rigid and planar with a molecular dipole moment of 1.7571 D (Fig. 6a), hinting electrons are even delocalized in the whole molecule. For EEPA-Fe3+ complex (Fig. 6b), the distance between Fe3 + and N atom in acridine ring was 1.8074 Å with a molecular dipole moment of 1.8235 D. An efficient
Table 1 Fe3+ colorimetric determination results for environmental samples (n = 5)a.
Table 2 Ag+ colorimetric determination results for environmental samples (n = 5)a.
Samplesb
b C3+ Fe in sample (10−7 mol/L)
Spiked (10−7 mol/L)
Found (nM)
Recovery R.S.D. (%) (%)
Samplesb
b C+ Agin sample (10−8 mol/L)
Spiked (10−8 mol/L)
Found (nM)
Recovery R.S.D. (%) (%)
1 (the Yi River) 2 (under-ground) 3 (tap water)
10.5 32.6 0.0
40.0 40.0 40.0
48.6 73.2 43.8
96.2 100.8 109.5
1 (the Yi River) 2 (under-ground) 3 (tap water)
0.0 0.0 0.0
60.0 60.0 60.0
58.9 64.7 60.9
98.2 107.8 101.5
5.6 2.2 4.3
PB, pH 6.0, cEEPA = 8.0 × 10−5 mol/L. The environmental water Fe3+ concentration determined using the proposed EEPAbased sensing system. The real values are the table values × 10−2 nmol·L−1 for the detected water samples were concentrated 100 times. a
b
3.9 1.4 4.5
PB, pH 6.0, cEEPA = 8.0 × 10−5 mol/L. The environmental water Fe2+ concentration determined using the proposed HPNSAbased sensing system. The real values are the table values × 10−2 nmol·L−1 for the detected water samples were concentrated 100 times. a
b
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Fig. 5. Job's plots for a) EEPA and Ag+, b) EEPA and Fe3+ (The total concentration of EEPA and Ag+ or Fe3+ kept at 60 μM).
coordination bond formed between Fe3+ and N atom in acridine and so electron cloud will be concentrated on the core of Fe3+, which accounts for the absorption peak red shifts from 505 nm to 528 nm in the presence of Fe3+. Once EEPA coordinates with Ag+ (Fig. 6c), we can find that the terminal phenyl rings are greatly distorted, because of the binding of Ag+ with π-electron in N_N bonds in EEPA (the distance between Fe3+ and N atom in N_N bond is 1.8074 Å), which resulted in the original conjugated system being destroyed and so the absorption at 505 nm was reduced, accordingly. 4. Conclusion
developed for colorimetric detection of Ag+ and Fe3+ simultaneously. EEPA showed a decrease in absorption at 505 nm in the presence of Ag+ with the visual color change from brown to light purple, and a red-shifted absorption at 528 nm in the presence of Fe3+ with the visual color change from brown to purple-red. Under the optimized conditions, EEPA could be applied for colorimetric determination of Ag+ and Fe3 + in practice with a standard deviation (RSD) ≤ 4.5% for Ag+ and RSD ≤ 5.6% for Fe3+, respectively. The interaction mechanisms between EEPA and Ag+ or EEPA and Fe3+ were confirmed to form different complexes with different stoichiometry ratios. An important molecular probe has been presented for multi-ions simultaneous recognition with excellent selectivity and sensitivity.
In conclusion, a novel acridine-based polydentate azo ligand chromophore, 3,6-di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA) was Acknowledgement The authors gratefully acknowledge the financial supports from Shandong Provincial Natural Science Foundation (No: ZR2016BQ13), Anhui Provincial Natural Science Foundation (No: 1308085ME57) and the National Natural Science Foundation of China (No: 21277103). References
Fig. 6. Frontier molecular orbitals optimized at B3LYP/6-31G level of theory, a) EEPA; b) EEPA-Fe3+; and c) EEPA-Ag+. Ag, Fe, O, N, C, and H atoms are represented as green, purple, red, blue, gray, and white-gray, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A novel polydentate ligand chromophore for simultaneously colorimetric detection of trace Ag + and Fe3 + Zhengquan Yan ⁎, Qi Zhao, Meijun Wen, Lei Hu ⁎, Xuezhong Zhang, Jinmao You ⁎ School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China Shandong Key Laboratory of Life-Organic Analysis, Key Laboratory of Pharmaceutical Intermediates and Analysis of Natural Medicine, Qufu Normal University, Qufu 273165, China
a r t i c l e
i n f o
Article history: Received 4 May 2017 Received in revised form 31 May 2017 Accepted 5 June 2017 Available online 06 June 2017 Keywords: 3,6-diamino acridine Azo derivative Colorimetric probe Simultaneous detection Ag+ Fe3+
a b s t r a c t A novel polydentate ligand chromophore, 3,6-di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA), was identified and synthesized. After its structure was characterized by FTIR, 1H NMR, mass spectra and element analyses, it was noted to find that there was a simultaneously colorimetric response to Ag+ and Fe3+ accompanying with different color changes, i.e., from brown to light purple for Ag+ and further to purple-red for Fe3+, respectively. Their different action mechanisms, a 1:2 complex mode for EEPA-Ag+ and 1:1 for EEPA-Fe3+, were investigated and confirmed by means of Job's plot and theoretical calculation. EEPA would be a potential colorimetric chemo-dosimeter for simultaneous detection of Ag+ and Fe3+ with the detection limits of 1.6 nmol·L−1 and 69 nmol·L−1, respectively. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Transition and heavy metal ions, which can be found in nature, have made great influence throughout the ecosystems and so been received attractive attention in the fields of chemical, biological and environmental sciences [1–7]. Taking Fe3+ and Ag+ as examples, Fe3+, the second most abundant metal element in the earth's crust, is indispensable for most organisms to form hemoglobin for the storage and transport of oxygen. Deficient iron will result in fatigue, poor work performance, and weaken immunity, while excess one will deteriorate lipids, nucleic acids and proteins [8–13]. For Ag+, an important antibiotic material for wound dressings to treat external infection possesses a variety of industrial, healthcare, and domestic applications. On the other hand, excess Ag+ will result in enzymes/proteins inactivation and bioaccumulation owing to the strong bonding ability with amine, imidazole, carboxyl and thiol groups in enzymes/proteins [14–17]. Therefore, great efforts have been devoted for selective and sensitive detection of Fe3 + and Ag+ in environmental samples, ranging from atomic spectrometry [18], mass spectrometry [19], chromatography [20], electrochemistry [21,22], colorimetric spectrophotometry [12,23, 24], fluorescent spectrophotometry [25–28], and so on. However, most of them focus on single metal ion detection, although these assays are respectable innovation and high sensitivity. It will need different ⁎ Corresponding authors. E-mail addresses: [email protected] (Z. Yan), [email protected] (L. Hu), [email protected] (J. You).
http://dx.doi.org/10.1016/j.saa.2017.06.007 1386-1425/© 2017 Elsevier B.V. All rights reserved.
detection methods and experimental conditions to detect some coexisting metal ions respectively, which was more complex and inconvenient. For the time being, single sensor for multiple analytes has been paid much attention and widely applied in heavy metal detection by virtue of its lower cost and higher efficiency [22,29–32]. For example, Napaporn, et al. [33], developed a multi-reverse flow injection analysis for the simultaneous determination of Mn(II), Fe(II), Cu(II) and Fe(III) in water samples with high sample throughput and low reagent consumption, which satisfied the criteria of Green Analytical Chemistry and its goals. Miao, et al. [29], for the first time demonstrated a DNA modified Fe3O4@Au magnetic nanoparticle-based electrochemical biosensor for simultaneous detection of Ag+ and Hg2+ to meet the requirements of U.S. Environmental Protection Agency. Gao, et al. [34], reported a Schiff-base fluorescence probe for simultaneous detection of Hg2+ and Cu2+ by two different mechanisms, indicating its promising application in living cells. However, colorimetric detection of Ag+ and Fe3+ simultaneously using a single probe under the same condition has no reports so far. With this aim in mind, in the work, a novel polydentate ligand of 3,6di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA) combining azo and heterocycle acridine as both chromophore and bonding group will be designed and synthesized. It is expected EEPA could simultaneously detect Fe3+ and Ag+ by virtue of the unique colorimetric and visible color changes, which will provide a basis insight for Ag+ and Fe3+ determination over other competing metal ions with effective discrimination and high contrast.
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2. Experimental Section 2.1. Reagents and Apparatus All chemicals in this work were of AR grade and used as received from Sinopharm Chemical Reagent Co. Ltd. Water used throughout was doubly deionized. A fresh 1.0 mmol·L−1 Fe3+ and Ag+ standard solutions for testing were prepared in doubly deionized water at room temperature and diluted to appropriate concentration when used. 3,6-di-(N-ethyl-Nethoxyl phenylazo) acridine (EEPA) was synthesized according to our previous work [35] and a 8.0 × 10−5 mol/L EEPA stock solution was prepared in N,N-dimethyl formamide (DMF) at room temperature and stored at 4 °C. Phosphate buffers (PB) were prepared by mixing a 0.01 mol/L H3PO4 solution, a 0.01 mol/L K2HPO4 solution, a 0.01 mol/L KH2PO4 solution or a 0.01 mol/L KOH solution in a proper ratio to acquire the desired pH (pH = 4.0, 5.0, 5.5, 6.0, 6.5, 7.0, 8.0, 9.0, 10.0). FTIR spectra of HPNSA with KBr disc were acquired using a Nicolet NEXUS 870 FTIR spectrophotometer at room temperature from 4000– 500 cm− 1. 1H NMR spectra were recorded using a Bruker AMX-500 spectrometer operating at 400 MHz, with tetramethyl-silane (TMS) used as the reference and DMSO-d6 as solvent. Mass spectral analysis was recorded on a GCMS-QP2010 mass spectrometer (Shimadzu Biotech, Japan). Elemental analysis was conducted using an Elemental Vario EL-III apparatus. UV–vis spectra were recorded on a Lambda 35 UV/vis spectrometer using a 1-cm square quartz cell. pH of the solutions was measured by a PHS-25pH meter. 2.2. Preparation of EEPA 0.06143 g (0.25 mmol) 3,6-diamino acridine was dissolved in an icewater solution of 15% sodium nitrite (0.03623 g, 0.525 mmol). After cooling to 0 °C, pH of the solution was adjusted to 1–2 by addition of concentrated hydrochloric acid and kept stirring for 30 min. The excess nitrous acid was destroyed by addition of 6 mg urea. Then, 10 mL buffer solution (KH2PO4/Na2HPO4, pH = 6) containing 78.9 μL N-ethyl-Nethoxyl aniline (0.5 mmol) was dropwise added into and stirred for another 2 h at 0–5 °C. The resultant precipitate was filtered and purified by recrystallizing three times from ethanol to provide purple-red crystal EEPA in the yield of 89.2%. IR (KBr), υ (cm−1): 3312 (\\OH), 3022 (Ar\\H), 1608, 1593, 1555 (Ar ring), 1343 (Ar-N(R1R)), 1197 cm−1(C\\O). 1H NMR (400 MHz, DMSO-d6, δ): 8.82 (d, J = 7.5 Hz, 2H, Ar\\H), 8.40 (d, J = 7.7 Hz, 2H, Ar\\H), 8.22 (d, J = 7.9 Hz, 4H, Ar\\H), 7.91 (s, 1H, Ar\\H), 7.65 (s, 2H, Ar\\H), 6.92 (d, J = 7.9 Hz, 4H, Ar\\H), 4.82 (t, J = 4.8 Hz, 4H, CH2), 3.61 (s, 2H, \\OH), 3.32 (t, J = 4.7 Hz, 4H, CH2), 2.60 (m, J = 3.6 Hz, 4H, CH2), 1.15 (t, J = 3.6Hz, 6 H, CH3). MS: m/z [M + H]+ 383.4091 (theoretical value: 383.3980). Anal. Calcd for C20H20N3O5 (%): C 62.81, H 5.27, N 10.99, O 20.92; Found: C 62.55, H 5.35, N 10.87, O 21.67.
(A600/A505) of the absorption intensity at 600 nm to that at 505 nm was used for Fe3+ quantitative analysis, where A600 and A505 were the absorption intensities of the systems at 600 nm and 505 nm in the presence of Fe3+, respectively. 3. Results and Discussion 3.1. UV–vis Spectral Properties To investigate the possibility for the detection of Ag+ and Fe3+ using EEPA as a colorimetric probe, the UV–vis absorption spectra of EEPA accompanying with its color change in the absence and presence of Ag+ and Fe3 + were recorded first (Fig. 1). From Fig. 1, it is easy to find that, without Ag+ and Fe3+ in DMF/H2O (2/3, v/v), EEPA possesses a strong absorption peak at 505 nm with ε = 3.83 × 104 L·mol−1·cm−1, attributed to the electron transferor of the huge conjugated molecules. With addition of Ag+, the absorption intensity at 505 nm decreased with the color changed into light purple from brown, while the absorption peak red shifted to 528 nm with the color change from brown to purple-red in the presence of Fe3+, respectively, which make it possible to detect Ag+ and Fe3+ simultaneously even in naked-eyes. The reasons might be attributed to the formation of EEPA-metal complex between EEPA and Ag+ and Fe3+ with different reaction mechanisms. 3.2. Optimization of Experimental Conditions It is well known pH is one of the most important factors for metal ion detection for pH influences greatly on the interaction between sensing materials and metal ions [7,36,37]. To make sure of its efficient application in simultaneously colorimetric detection of Ag+ and Fe3+, the effect of pH was investigated over pH 3.0 to 9.0 (Fig. 2). Both the ratio (A600/A505) in the presence of Fe3+ (Fig. 2a) and the difference (ΔA) at 505 nm in the presence of Ag+ (Fig. 2b) vary gradually with the increase of pH and reach the maximum when pH is 6.0. The reason may be that at pH b 6.0, nitrogen atoms in azo and acridine-ring groups are easily protonated, which reduces their coordinated interaction with the recognition ions Ag+ and Fe3 +. While pH is N 6.0, the solubility of Ag+ and Fe3 + in aqueous solution decreases owing to their hydrolyses. They both deteriorate the ability between EEPA and the recognition ions Ag+ and Fe3+. Accordingly, pH 6.0 was selected for simultaneous colorimetric detection Ag+ and Fe3+ for all the subsequent experiments. To illustrate the response rate and stability of EEPA to Ag+ and Fe3+, the difference (ΔA) at 505 nm in the presence of Ag+ (Fig. 2a) and the ratio (A600/A505) in the presence of Fe3+ was measured and calculated at different times after adding Ag+ or Fe3+. The results suggested that the
2.3. Ag+/Fe3+Detection Procedure For Ag+ determination, 1.0 mL PB (pH 6.0), 1.0 mL 8.0 × 10−5 mol/L EEPA and 1.0 mL of the appropriate Ag+ solution or sample were transferred into a 10 mL volumetric flask. The mixture was stirred thoroughly and finally diluted to 10 mL with doubly deionized water. The absorption spectra were measured from 300 nm to 800 nm and the band-slit was set as 2.0 nm. The absorption intensity difference (ΔA) at 505 nm was used for Ag+ quantitative analysis, where Δ A was equal to A0 − A, and A0 and A were the absorption intensities of the systems in the absence and presence of Ag+, respectively. For Fe3+ determination, 1.0 mL PB (pH 6.0), 1.0 mL 8.0 × 10−5 mol/L EEPA and 1.0 mL of the appropriate Fe3+ solution or sample were transferred into a 10 mL volumetric flask. The mixture was stirred thoroughly and finally diluted to 10 mL with doubly deionized water. The ratio
Fig. 1. a) UV–vis spectra of EEPA without and in the presence of Ag+ and Fe3+ in DMF-H2O (V/V = 2:3), respectively; b) UV–vis spectra of EEPA in different solvents.
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Fig. 2. Effect of pH a) on A600/A505 in the presence of Fe3+ and b) ΔA at 505 nm in the presence of Ag+ for EEPA, respectively (pH 6.0, cEEPA = 8.0 × 10−5 mol/L).
proposed EEPA-based sensing system possesses a fast response time of b10.0 min and keeps stable in the following 1 h. The influence of ionic strength was also investigated by adding serious NaCl solutions ranging from 2.0 × 10− 2 to 10−7 mol/L, no obvious change observed hinting that the present sensing system is reasonably stable and can be applied in various kinds of surroundings.
3.3. Special Simultaneous Response to Ag+ and Fe3+ To demonstrate the specially simultaneous response of EEPA to Ag+ and Fe3+, the UV–vis absorption spectra of the proposed sensing system were recorded in the presence of some environment-relative metal ions, i.e., Ca2 +, Cd2 +, Hg2 +, Co3 +, Mg2 +, Mn2 +, Na+, Cr3 +, Ni2 +, Fe2+, Cu+, Zn2+, Cu2+, Pb2+, Ag+ and Fe3+ in 1.1 × 10−5 M, respectively and the results were shown as in Fig. 3. From Fig. 3, we could find that addition of the common metal ions except Ag+ and Fe3+, no obvious changes were observed in the absorption spectra between 300 nm and 800 nm for the proposed EEPA-based sensing system, whose absorption intensities at 505 nm were much weaker and b 5% relative intensity comparing with these in the presence of Ag+ and Fe3+. Importantly, the visible color of the EEPA system exclusively changes from brown to light purple upon the addition of Ag+ and from brown to purple-red in the presence of Fe3+, respectively (Fig. 4a). All the results indicate that the present EEPA system possesses an excellent
selectivity for colorimetric detection of Ag+ and Fe3+ simultaneously, even in naked-eyes. 3.4. Analytical Parameters for Simultaneous Detection of Fe3+ and Ag+ In order to disclose the rule of EEPA to detect Ag+ and Fe3+ simultaneously, series of colorimetric titration experiments were carried out with different Ag+ or Fe3 + concentrations under the given optimized experimental conditions and the results were shown in Fig. 4. From Fig. 4, the detection limits and the calibration graphs for simultaneous detection of Ag+ and Fe3+ could be obtained accordingly. For colorimetric detection of Ag+, a linear relationship between the difference (Δ A) at 505 nm of EEPA and Ag+ concentration was exhibited in the range of 2.0–130 × 10−8 mol/L with a correlation coefficient (R) of 0.9961 and the regression equation ΔA = 2.33 × 10−3 + 8.65 × 10−4c (10−7 mol/L) was obtained (Fig. 4b and c). For Fe3+ detection, a linear relationship between the ratio (A600/A505) of EEPA and Fe3+ concentration was exhibited in the range of 3.5–110 × 10−7 mol/L with R of 0.9786. The regression equation A600/A505 = 1.93 + 1.84 × 10−6c (10−7 mol/L) was obtained (Fig. 4d and e). Based on the definition of detection limit, three times of average deviation of ΔA or A600/A505 in 20 blank samples without Ag+ or Fe3+ utilized here, the limits of detection (LOD) for Ag+ and Fe3+ were up to 1.6 nmol·L−1 and 69 nmol·L−1, respectively. 3.5. Application for Simultaneous Detection of Ag+ and Fe3+ To further demonstrate its application, the present sensor was applied to determine Ag+ and Fe3+ concentrations in 3 environmental water samples obtained respectively from the Yi River, underground water and tap water (Tables 1 and 2). All the samples were tested after filtering several times and being concentrated 100 times by evaporation. For recovery studies, a known concentration of Ag+ and Fe3+ (400 nM) was added to the environmental water samples and the total Ag+ and Fe3+ concentrations were determined accordingly. To access the reproducibility of the method, the measurements were carried out 5 times repeatedly and the recoveries of different known amounts of Ag+ and Fe3+ spiked were determined to be from 98.2% to 107.8% % with standard deviations (RSD) ≤ 4.5% for Ag+ and from 98.2% to 107.8% with RSD ≤ 5.6% for Fe3+, respectively. These validate the reliability and practicality of the proposed dual-mode colorimetric method for simultaneous Ag+ and Fe3+ detection in practice. 3.6. Sensing Mechanism
Fig. 3. Effects of different metal ions on the UV–vis spectra of G-AuNPs (from top to bottom: Ca2+, Cd2+, Hg2+, Co3+, Mg2+, Mn2+, Na+, blank, Cr3+, Ni2+, Fe2+, Cu+, Zn2+, Cu2+, Pb2+, Fe3+ and Ag+, respectively. pH 6.0, cEEPA = 8.0 × 10−5 mol/L, cions = 1.1 × 10−5 mol/L).
To investigate the bonding nature between EEPA and Ag+ or Fe3+, the binding stoichiometry of EEPA with Ag+ or Fe3+ was performed by using Job's plot [38]. For the Job's plot analyses, a series of solutions
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Fig. 4. a) The color change and the absorption spectra of EEPA-based sensing system in the presence of b) Ag+ and d) Fe3+ with different concentrations; the linear relationship between c) −5 3+ mol/L). the ΔA and c+ Ag, and e) between A600/A505 and cFe (pH 6.0, cEEPA = 8.0 × 10
with varying mole fraction of Ag+ or Fe3+ were prepared respectively by maintaining the total EEPA and Ag+ or EEPA and Fe3+ concentration constant (6.0 × 10−5 mol/L). The ratio (A600/A505) in the presence of Fe3 + (Fig. 5a) and the difference (Δ A) at 505 nm in the presence of Ag+ were measured (Fig. 5b), respectively. A 1:2 stoichiometry for the complex between EEPA and Ag+ and 1:1 stoichiometry for the complex between EEPA and Fe3+ could be drawn from their corresponding Job's plots, suggesting that Fe3+ might coordinate with the N atom in acridine ring and Ag+ coordinate with π-electron in N_N bonds by virtue of the strength of their electrophilic ability.
Theoretical calculations were carried out to further confirm the bonding nature between EEPA and Ag+ or Fe3+. Their optimized structures of EEPA before and after coordinating with Ag+ or Fe3 + were shown in Fig. 6, conducted using the B3LYP/631G level of theory and method implemented in the Gaussian 09 suite of program [39]. In the optimized structures, the basic unit of EEPA is rigid and planar with a molecular dipole moment of 1.7571 D (Fig. 6a), hinting electrons are even delocalized in the whole molecule. For EEPA-Fe3+ complex (Fig. 6b), the distance between Fe3 + and N atom in acridine ring was 1.8074 Å with a molecular dipole moment of 1.8235 D. An efficient
Table 1 Fe3+ colorimetric determination results for environmental samples (n = 5)a.
Table 2 Ag+ colorimetric determination results for environmental samples (n = 5)a.
Samplesb
b C3+ Fe in sample (10−7 mol/L)
Spiked (10−7 mol/L)
Found (nM)
Recovery R.S.D. (%) (%)
Samplesb
b C+ Agin sample (10−8 mol/L)
Spiked (10−8 mol/L)
Found (nM)
Recovery R.S.D. (%) (%)
1 (the Yi River) 2 (under-ground) 3 (tap water)
10.5 32.6 0.0
40.0 40.0 40.0
48.6 73.2 43.8
96.2 100.8 109.5
1 (the Yi River) 2 (under-ground) 3 (tap water)
0.0 0.0 0.0
60.0 60.0 60.0
58.9 64.7 60.9
98.2 107.8 101.5
5.6 2.2 4.3
PB, pH 6.0, cEEPA = 8.0 × 10−5 mol/L. The environmental water Fe3+ concentration determined using the proposed EEPAbased sensing system. The real values are the table values × 10−2 nmol·L−1 for the detected water samples were concentrated 100 times. a
b
3.9 1.4 4.5
PB, pH 6.0, cEEPA = 8.0 × 10−5 mol/L. The environmental water Fe2+ concentration determined using the proposed HPNSAbased sensing system. The real values are the table values × 10−2 nmol·L−1 for the detected water samples were concentrated 100 times. a
b
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Fig. 5. Job's plots for a) EEPA and Ag+, b) EEPA and Fe3+ (The total concentration of EEPA and Ag+ or Fe3+ kept at 60 μM).
coordination bond formed between Fe3+ and N atom in acridine and so electron cloud will be concentrated on the core of Fe3+, which accounts for the absorption peak red shifts from 505 nm to 528 nm in the presence of Fe3+. Once EEPA coordinates with Ag+ (Fig. 6c), we can find that the terminal phenyl rings are greatly distorted, because of the binding of Ag+ with π-electron in N_N bonds in EEPA (the distance between Fe3+ and N atom in N_N bond is 1.8074 Å), which resulted in the original conjugated system being destroyed and so the absorption at 505 nm was reduced, accordingly. 4. Conclusion
developed for colorimetric detection of Ag+ and Fe3+ simultaneously. EEPA showed a decrease in absorption at 505 nm in the presence of Ag+ with the visual color change from brown to light purple, and a red-shifted absorption at 528 nm in the presence of Fe3+ with the visual color change from brown to purple-red. Under the optimized conditions, EEPA could be applied for colorimetric determination of Ag+ and Fe3 + in practice with a standard deviation (RSD) ≤ 4.5% for Ag+ and RSD ≤ 5.6% for Fe3+, respectively. The interaction mechanisms between EEPA and Ag+ or EEPA and Fe3+ were confirmed to form different complexes with different stoichiometry ratios. An important molecular probe has been presented for multi-ions simultaneous recognition with excellent selectivity and sensitivity.
In conclusion, a novel acridine-based polydentate azo ligand chromophore, 3,6-di-(N-ethyl-N-ethoxyl phenylazo) acridine (EEPA) was Acknowledgement The authors gratefully acknowledge the financial supports from Shandong Provincial Natural Science Foundation (No: ZR2016BQ13), Anhui Provincial Natural Science Foundation (No: 1308085ME57) and the National Natural Science Foundation of China (No: 21277103). References
Fig. 6. Frontier molecular orbitals optimized at B3LYP/6-31G level of theory, a) EEPA; b) EEPA-Fe3+; and c) EEPA-Ag+. Ag, Fe, O, N, C, and H atoms are represented as green, purple, red, blue, gray, and white-gray, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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