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An acid-fluorescence and alkali-colorimetric dual channels sensor for Hg2+ selective detection by different coordination manners in aqueous media
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 12–19
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
An acid-fluorescence and alkali-colorimetric dual channels sensor for Hg2+ selective detection by different coordination manners in aqueous media ⁎
Xiulan Zhanga, Yu Wanga, Huihui Yuanb, Xuhong Guoa,c, Bin Daia, , Xin Jiaa,
T
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a School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region, Engineering Research Center of Materials-Oriented Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832003, People’s Republic of China b Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China c State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorescence Colorimetric Mercury ion Dual-channel
Due to the advantages of high selectivity, accuracy and mutual calibration, dual-channel sensors have drawn much attention. In this work, a water-soluble fluorescence and colorimetric dual-channel chemosensor (AAT) with both azomonardine and aminothiazole units has been designed and synthesized. AreOH and ]Ne groups, which respectively belonged to azomonardine and thiazole unit, exhibited pH dependent ionization, which leaded AAT exhibiting strong naked-eye visible fluorescence and color in acidic and alkaline solution, respectively. When ]Ne was ionized in acidic solution, fluorescence was turned on. The colorimetry occurred while the ionization of Ar−OH in alkaline solution. Moreover, the solution pH had a significant impact on the binding efficiency between AAT and Hg2+, resulted in different coordination manners and stoichiometry. At pH 5.0, in the fluorescence channel, the stoichiometry of AAT for Hg2+ was calculated as 1:2 and the linear detection concentration was ranged from 0 to 35 μM. It was in a range of 0–12 μM through colorimetric method with the stoichiometry of 1:1 at pH 7.5. Furthermore, neither in fluorescence or in colorimetric channel, AAT showed high specificity toward Hg2+ among numerous cationic and anion ions. Also, AAT realized the Hg2+ identification and determination in river water samples.
1. Introduction Mercury is a highly toxic and widespread global pollutant. It is oxidized to soluble inorganic ion (Hg2+), provides a pathway for contamination vast amounts of water and soil [1]. Hg2+ has high affinity toward thiols and amino groups in peptides and nucleic acids, and leads to a variety of irreversible damages even in a low concentration, for instance, neurological abnormalities, gingivitis, and tumor formation [2–4]. Therefore, it is of considerable importance to develop a reliable, convenient, selective and sensitive method for monitoring the level of Hg2+. Comparison with some traditional Hg2+ determination techniques, such as inductively coupled plasma [5], atomic absorption spectrometry [6], atomic fluorescence spectrometry [7,8] and surface-enhanced Raman scattering spectroscopy [4,9], fluorescence or colorimetric based approaches are more promising because of their convenience, specificity and sensitivity [10–12]. Recently, fluorescence and colorimetric dual-channel response method, which possesses ⁎
advantages of high selectivity, accuracy, and mutual calibration, has attracted lots of attentions [13–15]. Due to the advantages of polytropical optical properties and ligand groups, a series of organic molecules based dual-channel sensors are developed for heavy metal ions determination. For instance, quinoline appended naphthalene derivant for Al3+ (fluorescent method) and Cu2+ (colorimetric method) determination [16], cyclic steroid-rhodamine conjugated probe for Fe3+ detection [17], 3-hydroxy-2- naphthoylhydrazide and 6-Hydroxyl-3formyl chromone based dual-channel sensor for Al3+ test [18], benzildihydrazone derived colorimetric and fluorescent probe for Cu2+ detection [19], and coumarin-based chemosensor for determination of Pd2+ [20], et al. However, rarely dual channels sensors have been designed for Hg2+ detection. Withal, although these developed organic sensors exhibit splendid selectivity and sensitivity toward the targets, they are limited to organic or organic-aqueous mixtures system. Because of their poor water solubility, they are restricted for further utilization in environmental samples. Azomonardine, an organic molecule, with excellent water solubility,
Corresponding authors. E-mail addresses: [email protected] (B. Dai), [email protected] (X. Jia).
https://doi.org/10.1016/j.jphotochem.2018.12.009 Received 10 October 2018; Received in revised form 5 December 2018; Accepted 5 December 2018 Available online 31 December 2018 1010-6030/ © 2018 Published by Elsevier B.V.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 12–19
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yield of 26.7% was obtained. 1H-NMR: (400 MHz, d6-DMSO) δ 6.35 (d, J = 2.2 Hz, 1H, H-2), 6.44 (dd, J = 8.6, 2.2 Hz, 1H, H-4), 7.55 (d, J = 3.5 Hz, 1H, H-12), 7.65 (dd, J = 7.6, 6.1 Hz, 2H, H-5, 11), 9.13 (s, 1H, H-9), 10.53 (s, 1H, H-8), 12.00 (s, 1H, H-7); 13C-NMR: (400 MHz, d6-DMSO) δ C 102.58 (CH, C-2), 109.12 (CH, C-4), 111.97 (C, C-6), 118.42 (CH, C-12), 134.46 (CH, C-5), 141.33 (CH, C-11), 163.02 (C, C14), 163.56 (C, C-3), 164.26 (C, C-1), 171.30 (CH, C-9); ESI-Mass (m/z): [1 + H]+ calcd for C10H9N2O2S 221.0385, found 221.0385.
can be readily obtained by one-pot reaction of dopamine and resorcinol in aqueous solution [21,22]. The AreOH group of azomonardine is facile to ionize into AreO− in alkalescence aqueous solution and incurs strong fluorescence. The ionized azomonardine owns high fluorescence quantum yield. For these reasons, azomonardine can be used as an ideal fluorophore. On the other hand, thiazole has been declared as a ligand with high affinity for Hg2+ [23,24]. Meanwhile, ]Ne of thiazole can be ionized as =+NH- in acidic solution. On these basis, in this work, by taking advantages of pH dependent ionization of AreO− and ]Ne groups, we have synthesized a water soluble acidic-fluorescence and alkaline-colorimetric dual-channel responses chemosensor (AAT) for Hg2+ highly selective detection via the integrate of azomonardine and amino-thiazole. Of which, azomonardine and amino-thiazole are employed as reporter and receptor, respectively. The synthesized sensor, AAT, is colorless but with strong and naked-eye visible blue fluorescence in acidic solution (pH 3–7), and it exhibits the color of yellow with non-fluorescence in alkaline solution (pH 7–10). It is other than the majority of reported dual-channel sensors which the fluorescence and colorimetric are synchronously occurring, they are asynchronous for AAT. On this benefit, AAT has capable of determination Hg2+ through fluorescence and colorimetric methods respectively in acidic and alkaline solution. Meanwhile, the probe has satisfactory recovery for Hg2+ determination in real samples. Therefore, this proposed sensor has highly promising for monitoring the Hg2+ level in aqueous samples at different pH values.
2.3.2. Synthesis of compound 2 and AAT Compound 1 (3 mmol, 0.66 g) was dissolved in 30 mL methanol. And then, NaBH4 (3.4 mmol, 0.13 g) was slowly added under the ice protection. The reaction of the mixture was maintained for 2 h at room temperature before it was terminated by 20% HCl. After removing the methanol, about 300 mL H2O was added, and adjusted the solution pH to 12.0 by NaOH (2.0 M). Then, added 3 mmol (0.57 g) of dopamine hydrochloride into the mixture. After stirring for 5 h at room temperature, the solution was neutralized by HCl (2.0 M). Meantime, yellow precipitate product was formed. The lyophilized solid was purified by column chromatography on silica gel (ethyl acetate: ethanol = 9:1), and yield the orange-yellow solid product AAT. 1HNMR: (400 MHz, d6-DMSO) δ 1.4 (d, J = 12.2 Hz, 1H, H-12), 1.9 (m, 1H, H-15), 2.22 (dt, J = 10.4, 6.1 Hz, 2H, H-12, 15), 2.75 (td, J = 12.6, 3.6 Hz, 1H, H-16), 2.86 (dd, J = 13.0, 5.1 Hz, 1H, H-16), 4.32 (d, J = 2.6 Hz, 2H, H-20), 5.66 (s, 1H, H-19), 6.22 (s, 1H, H-6), 6.48 (s, 1H, H-9), 6.61 (d, J = 3.7 Hz, 1H, H-23), 7.02 (d, J = 3.7 Hz, 1H, H-22), 7.6 (s, 1H, H-2), 7.88 (t, J = 5.0 Hz, 1H, H-26), 11.16 (s, 1H, H-5); 13CNMR: (400 MHz, d6-DMSO) δ C 32.01 (CH2, C-16), 37.07 (CH2, C-15), 43.17 (CH2, C-20), 46.86 (CH2, C-12), 82.85 (C, C-13), 89.54 (C, C-14), 98.01 (CH, C-6), 106.61 (CH, C-9), 111.96 (CH, C-23), 112.62 (C, C-4), 121.41 (C, C-1), 125.34 (CH, C-2), 138.32 (CH, C-22), 162.78 (C, C-11), 163.17 (C, C-3), 164.77 (C, C-7), 169.32 (C, C-25), 196.45 (C, C-10); ESI-Mass (m/z): [AAT + H]+ calcd for C18H18N3O4S 372.1018, found 372.1017.
2. Experimental 2.1. Reagents Dopamine hydrochloride was purchased from Sigma-Aldrich (U.S.). 2,4-dihydroxybenzaldehyde, 2-aminothiazole and Hydroxyethyl piperazine ethyl sulfonic acid (HEPES) were obtained from Adamas reagent Co. Ltd. All the metal nitrate, metal chloride salts, and anion ions including (K+, Na+, Cd2+, Ca2+, Hg2+, Cu2+, Ba2+, Zn2+, Ni2+, Fe3+, Pb2+, Li+, Co2+, Al3+, Mg2+, Cr6+, Fe2+, Cr3+, SO42−, NO3-, CO32−, PO43−, SO32−, S2O82−, ClO2−, CH3COO−, OH−, Cl−, F−, I−, S2−) with high purity were purchased from Xiya Co. Ltd. All the other reagents were of analytic grade and were used as received. The DI-water was used to prepare aqueous solutions.
2.4. Hg2+ detection A series of aqueous solutions with different Hg2+ concentrations were prepared by dissolving the HgCl2 in 10 mM HEPES solution (pH = 7.5) or HAC/NaAC buffer solution (pH = 5.0). For Hg2+ detection, 5 μL of AAT (4 mM) was added to Hg2+ solution with the total volume of 2 mL. The mixture was incubated for 1 h. Then, the fluorescence intensity of detection aqueous system was collected at λex/ λem = 435/475 nm. The absorption intensity of that was collected at 425 nm.
2.2. Apparatus and characterization Fourier transform near-infrared spectrometer (Nicolet iS10, USA) was used for recording FT-IR spectra. 1H NMR and 13C NMR spectra were collected on an inova-400 M (Varian) operating at 400 MHZ. Electrospray ionization mass spectrometry (ESI-MS) analysis was performed in the positive ion mode on a liquid chromatography-with mass spectrometry (Micromass LCT, WATERS). UV–vis absorption spectra were recorded on a UV-6100 s (Mapada) UV–vis spectrophotometer at room temperature. The fluorescence measurements were performed on a G9800 A (Agilent) fluorescence spectrophotometer (the excitation and emission slits were both set to 5.0 nm) equipped with a plotter unit and a 1 cm quartz cell. The solution pH was monitored by a PHS-2 F (Leici) pH meter.
2.5. Real samples test To investigate the Hg2+ determination in real samples, river water samples which were collected from mingzhu river (Shihezi, Xinjiang) were employed. The collected river water was firstly filtered by 0.45 μm membrane to remove the large particles and gels, and then straightly utilized for preparation of 10 mM HEPES solution (pH = 7.5) and HAC/ NaAC solution (pH = 5.0). The spiking experiment was carried out by spiking known amounts of Hg2+ into the buffer solution in different known concentrations. Other parameters were as the same with Hg2+ detection experiments.
2.3. Synthesis 2.3.1. Synthesis of compound 1 2,4-Dihydroxybenzaldehyde (10 mmol, 1.38 g) and 2-aminothiazole (12 mmol, 1.2 g) were dissolved in absolute ethyl alcohol and refluxed for 4 h. Then, the major amount of ethyl alcohol was removed by rotary evaporators. It was followed by addition of DI-water to form the yellow precipitate, and then filtered and air-dried. The residue solid was purified by column chromatography on silica gel (petroleum ether: ethyl acetate = 6:4), and bright yellow crystalline solid compound 1 in a
3. Results and discussion 3.1. Optical properties of AAT In pH 5.0 aqueous solution, AAT exhibits a strong fluorescence absorption and emission band centered at 437 nm and 474 nm (Fig. S1A), respectively, and shows a characteristic absorption at 392 nm (Fig. S1A). Fluorescence intensity of AAT shows excitation wavelength 13
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Fig. 1. (A) UV–vis absorption spectra, (B) fluorescence intensity (435 nm) and absorption (425 nm), and (C) the apparent color of AAT (10 μM) in aqueous solution (10 mM phosphate buffer) with different pH values.
Scheme 1. Synthesis of AAT.
intensity of AAT aqueous solution is weakened (Fig. S2B), however, the color maintains lipochromous (Fig. S2A), indicating that Hg2+ distinctly quenches the fluorescence but has indistinctive effect on absorption. Furthermore, as illustration in Fig. 2A, fluorescence spectra are downsides which depend on the Hg2+ concentration. The relative fluorescence intensity ((I0−I)/I0) of AAT reflects a wide range of Hg2+ detection concentration from 0 to 100 μM (Fig. 2B). And it shows a good linear relationship (R2 = 0.9956) in a range of 0–35 μM (Fig. 2B inset). The calculated Low of Detection (LOD) is 146.3 ± 0.5 nM (S/ N = 3) which is below the allowed concentration level (248.8 nM) in industrial wastewater. Moreover, AAT can be coated on paper to prepare test paper for Hg2+ detection by fluorescence method (Fig. S3). With increasing Hg2+ concentration (0, 0.5, 2, 5, 10 μM), the fluorescence of the detection area demonstrates sensitive changes from dark blue to nattier blue under UV light. When the solution pH is equal to or higher than 7.0, a preponderant absorption of AAT at 425 nm appears, and it is enhanced with increasing pH values (pH < 9.0) (Fig. 1A). Based on the calculated (A0A) values, from pH 7.0 to pH 9.0, Hg2+ quenches the absorption intensity with pH dependent, and the optimized solution pH for Hg2+ detection by colorimetric method is 7.5 (Fig. S4). At pH 7.5, the specific absorption at 267 nm and 425 nm are quenched by higher Hg2+ concentration (Fig. 2C). Accordingly, the relative absorption intensity at
dependent. The optimal excitation wavelength is recorded at 435 nm (Fig. S1B). As showed in Fig. 1A, B, solution pH has a significant effect on the intensity of absorption and fluorescence. The absorption at 425 nm is obviously enhanced with raised pH values when the solution pH is lower than 9.0. As the solution pH is higher than 6.5, the absorption is stronger than that of at 392 nm, which induces the solution of AAT converting from colorless to yellow (Fig. 1C). It is because of that the Ar−OH on azomonardine unit tends to ionize into AreO− at higher pH and enhances the absorption of 425 nm [22,25]. When the solution pH is of 4.5–7.5, the fluorescence intensity at 474 nm is increased with decreasing pH values (Fig. 1B), which owns to that ]N− (on thiazole) ionized into ]+NH− at lower pH [26]. As a result, the optimal solution pH for generation fluorescence is about 5.0, and the relative quantum yield of AAT is calculated to be 26.12% by using fluorescein as reference [27–29] (Scheme 1). 3.2. Hg2+ detection For AAT, the ionization of both Ar−OH and ]Ne displays pH dependent, resulting in diverse optical properties in acidic and alkaline solution. On this basis, AAT owns the ability of determination Hg2+ concentration through both fluorescence and colorimetric channels. As the results, at pH 5.0, upon the addition of Hg2+, the fluorescence 14
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Fig. 2. (A) Fluorescence spectra (pH 5.0) changes upon the addition of Hg2+, (B) The relative fluorescence intensity ((I0-I)/I0, where I0 and I are the fluorescence intensity of AAT in the absence and in the presence of Hg2+, respectively) of AAT responses to the concentrations of Hg2+, inset is the linear calibration plot for Hg2+ sensitive detection, (C) UV–vis spectra (pH 7.5) changes upon the addition of Hg2+, (D) The relative absorption intensity ((A0-A)/A0, where A0 and A are the absorption intensity of AAT in the absence and in the presence of Hg2+, respectively) of AAT responses to the concentrations of Hg2+, inset is the linear calibration plot for Hg2+ sensitive detection.
425 nm ((A0−A)/A0) is linearly enhanced by increasing Hg2+ concentration from 0 to 12 μM (Fig. 2D) with a good linear relationship (Fig. 2D inset). The LOD is calculated as 149.2 ± 1.2 nM (S/N = 3) through colorimetric method. Comparatively, the highest linear detection concentration by fluorescence method is 3 times of that by a colorimetric method, demonstrating that it can be regulated by adjusting the solution pH. Comparison with some reported small organic molecule sensors for Hg2+ detection [30–34] (Table S1), the achieved LOD values by fluorescence and colorimetric methods were both higher than that of them.
benzene) are decreased with elevatory Hg2+ concentration (Fig. S7A), indicating Hg2+ bonds to AAT via both azamonardine and aminothiazole units [25]. According to our previous research [25], groups on azamonardine unit are languid to bind to Hg2+. However, with the introduction of aminothiazole group, AreOH becomes possible to coordinate with Hg2+, and induces the weaken absorption at 392 nm. At pH 7.5, after AAT incubation with different concentrations of Hg2+, similar UV–vis absorption change rule is obtained (Fig. 2C). Based on these results, combination with diverse stoichiometry, suggesting that AAT captures Hg2+ in a different manner at pH 5.0 and pH 7.5. To further study the binding mode between Hg2+ and AAT at pH 5.0 and pH 7.5, 1H NMR titration experiment is carried out. As shown in Fig. 4, upon the addition of incremental amounts of Hg2+ to d6-DMSO, the protons of type e and type f are decreased. However, a new singlet appears at 6.93 ppm and isometrically increases with the decreasing doublet (proton of type e). But there is none new peak for the proton of type f. It supports that the proton of type f is expropriated, and AAT chelates with Hg2+ via carbanion after deprotonation (Scheme 2) [23]. The ESI-MS spectrum shows a peak at m/z 608.0, which is assigned to the [AAT + HgCl+ + H+] (Fig. S7B). Therefore, AAT-HgCl, as a new species, induces the protons of type a, type c, type d and type r split to two independent singlets. And the integrals of the appeared new singlets increases with the primary singlets reducing. Besides, eNHe (r) and −CHe (c) protons of the new species are downfield shifted and eOH (b) proton is faded away, indicating the coordination of Hg2+ with both amino (r) and phenolic hydroxyl (b) [38]. However, the ESIMS spectrum can’t show the 1:2 stoichiometry of AAT to Hg2+, which may on account of the weak interaction of amino (r) and phenolic hydroxyl to Hg2+. In summary, all these data strongly support the
3.3. Binding mode investigation The Job’s plot [35] for the binding activity between AAT and Hg2+ is investigated for studying the stoichiometric nature of AAT to Hg2+. The fitting results are illustrated in Fig. 3, 2:1 and 1:1 stoichiometry are calculated for Hg2+ bonding to AAT at pH 5.0 and pH 7.5, respectively. Through Benesi-Hilderbrand plot [36,37], the association constants of AAT-Hg2+ are counted to be (3.81 ± 0.32) × 109 M−2 and (2.00 ± 0.23) × 104 M−1 at pH 5.0 and pH 7.5, respectively (Fig. S5). Reaction products of AAT+Hg2+ at pH 5.0 and pH 7.5 are characterized by FT-IR (Fig. S6). The absorption at 3295 cm−1, 1670 cm−1, 1585 cm−1 and 1150 cm−1and 850 cm−1 which belong to νN-H, νC=N, νC=C, νC-O/C–N/C-S-C and γN-H, are impaired (Fig. S6) by comparison with that of AAT. This means that the combination of Hg2+ and AAT might concern to these functional groups. Moreover, at pH 5.0, after AAT incubation with Hg2+, UV–vis absorption at 392 nm (n-π∗ of azamonardine), 305 nm (π-π∗ of ]Ne of thiazole) and 253 nm (π–π∗ of 15
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Fig. 3. The Job’s plot for AAT to Hg2+ in (A) pH = 5.0 (10 mM acetic acid - sodium acetate buffer solution, the total concentration of AAT and Hg2+ is set as 40 μM) and (B) pH = 7.5 (10 mM HEPES buffer solution, the total concentration of AAT and Hg2+ is set as 20 μM).
Hg2+ determination. Therefore, it is indispensable for a sensor with high selectivity. In this study, the optical properties of AAT toward various metal ions and anion ions are investigated. At pH 5.0, among numerous metal ions and anion ions, only Hg2+ has superior efficiency to quench the fluorescence intensity of AAT (Figs. 5A, S10A). The recorded fluorescence spectra (Fig. 5B) and calculated (I0-I)/I0 values (Fig. 5C) also describe AAT has highly selective toward Hg2+ in the solution of pH 5.0. Moreover, in the mixture solution of 25 μM Hg2+ and 50 μM any other metal ions, other metal ions also has inappreciable interference for Hg2+ quantification (Fig. 5C), implying AAT has great potential for Hg2+ detection in real samples. On the other hand, in the solution of pH 7.5, when the buffer solution is prepared by HEPES, except for Hg2+ (20 μM), both Cu2+ and Fe3+ with concentration of 50 μM show some impacts on the absorption at 425 nm (Fig. S8). However, in PBS buffer (10 mM, pH = 7.5), the interference of Cu2+ and Fe3+ is eliminated (Fig. 5D, E). It owes to the abundant of PO42− which has capacity to coordinate with various metal ions (Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, et al) and form as stable complexes [25,40].
proposed bonding mode as showed in Scheme 2. As soon as AAT bonds to Hg2+, the intramolecular electron transfer from azomonardine fluorophore to Hg2+ ion is occurred and induces the fluorescence quenching [39]. On the other hand, at pH 7.5, according to the previous consequence, AreOH is ionized as AreO− which enhances the electron density of phenolic moiety and depress the activation of C-atom toward Hg2+ [23]. AreO− with an electron induction effect causes the protons (a, c, d, r) move to upfield (Fig. S7). The proton of type r is split into two peaks, indicating eNH-thiazole bind to Hg2+ (Fig. S8). Moreover, addition of Hg2+ distinctly quenches the absorption at 253 nm and 425 nm, which demonstrates the interaction between AreO− and Hg2+ [25]. The proposed bonding mode between AAT and Hg2+ at pH 7.5 is shown in Scheme 2. 3.4. Selectivity In practical utilization, complicated constituents contain various of concomitant metal ions and anion ions may cause some interference on
Fig. 4. 1H NMR spectra of AAT in d6-DMSO by addition of (A) 0 equiv, (B) 0.5 equiv, (C) 1.0 equiv, (D) 1.5 equiv, (E) 2.0 equiv and (F) 3.0 equiv Hg2+. 16
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Scheme 2. The proposed binding model for AAT-Hg2+ at pH 5.0 and pH 7.5.
Although the solution contains other metal ions, the relative absorption of AAT exhibits reliable quantitative analysis of Hg2+ (Fig. 5E). Also, various of anion ions have inappreciable influence on AAT (Fig. S10B). This result also demonstrates that AAT has the great potential for Hg2+ determination in real samples at pH 7.5 in the existence of PBS.
determination in real samples, water samples collected from mingzhu river were tested. The real samples were filtered with 0.45 μm membrane and spiked with various concentrations of Hg2+ prior to incubating with AAT. The results are shown in Table 1, the obtained recoveries are in the range of 96.3%–100.5% and 86.8%–92.3% at pH 5.0 and pH 7.5, respectively, suggesting AAT can be applied to detect Hg2+ in real samples through both fluorescence and colorimetric methods, and the fluorescence method is more accurate.
3.5. Real sample analysis To
evaluate
the
practical
application
of
AAT
for
Hg2+
Fig. 5. (A) The fluorescence photographs of AAT in the presence of various metal ions under the irradiation of UV lamp (365 nm). (B) The fluorescence emission spectra of AAT in acetic acid - sodium acetate buffer solution (pH = 5.0) upon the addition of various metal ions (AAT, 10 μM; Hg2+, 25 μM; other metal ions, 50 μM; slit: E x 5 nm, Em 2.5 nm). (C) The relative fluorescence intensity of AAT towards 17 kinds of metal ions in the absence and in the presence of Hg2+ (pH = 5.0; AAT, 10 μM; Hg2+, 25 μM; other metal ions, 50 μM). (D) The absorption spectra of AAT in PBS buffer solution (pH = 7.5) upon the addition of various metal ions (AAT, 10 μM; Hg2+, 20 μM; metal ions, 50 μM). (E) The relative absorption of AAT towards various metal ions in the absence and in the presence of Hg2+ (pH 7.5; AAT, 10 μM; Hg2+, 20 μM; other metal ions, 50 μM). 17
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Table 1 The result for Hg2+ determination in river water samples. Spiked (μM)
Found (μM)
Recovery (%)
RSD (n = 3, %)
Acid 10 15 20
9.63 ± 0.86 15.08 ± 0.09 19.57 ± 1.36
96.3 100.5 97.9
8.5 0.6 6.8
Alkali 6 8 10
5.21 ± 0.33 7.16 ± 0.26 9.23 ± 0.57
86.8 89.5 92.3
6.5 3.4 6.2
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Elango, Highly selective colorimetric sensing of Hg(ii) ions in aqueous medium and in the solid state via formation of a novel MeC bond, Dalton Trans. 44 (2015) 3259–3264. [24] B. Gu, L. Huang, W. Su, X. Duan, H. Li, S. Yao, A benzothiazole-based fluorescent probe for distinguishing and bioimaging of Hg2+ and Cu2+, Anal. Chim. Acta 954 (2017) 97–104. [25] X. Zhang, X. Guo, H. Yuan, X. Jia, B. Dai, One-pot synthesis of a natural phenol derived fluorescence sensor for Cu(II) and Hg(II) detection, Dye. Pigments 155 (2018) (2018) 100–106. [26] E. Buncel, I. Onyido, Proton and metal-ion activation of C-H exchange in fivemembered azoles, J. Labell. Compd. Rad. 45 (2002) 291–306. [27] L.S. Forster, R. Livingston, The absolute quantum yields of the fluorescence of chlorophyll solutions, J. Chem. Phys. 20 (1952) 1315. [28] C.A. Parker, W.T. Rees, Correction of fluorescence spectra and measurement of fluorescence quantum efficiency, Analyst 85 (1960) 587–600. [29] W.R. Dawson, M.W. Windsor, Fluorescence yields of aromatic compounds, J. Phys. Chem. 72 (1968) 3251–3260. [30] T. Leng, Y. Ma, G. Chen, A novel ratiometric fluorescence and colorimetric probe with a large stokes shift for Hg2+ sensing, J. Photochem. Photobiol. A-Chem. 353 (2018) 143–149. [31] J. Dong, Y. Liu, J. Hu, H. Baigude, H. Zhang, A novel ferrocenyl-based multichannel probe for colorimetric detection of Cu(II) and reversible fluorescent “turn-on” recognition of Hg(II) in aqueous environment and living cells, Sens. Actuators BChem. 255 (2018) 952–962. [32] Y. Long, M. Yang, B. Yang, Development and applications of two colorimetric and fluorescent indicators for Hg2+ detection, J. Inorg. Biochem. 172 (2017) 23–33. [33] J. Choi, S.K. Lee, J. Bae, S.K. Chang, Colorimetric signaling of Hg2+ ions by a nitrobenzoxadiazole- appended cyclen-triester, Tetrahedron Lett. 55 (2014) 5294–5297. [34] Z. Yan, L. Hu, L. Nie, H. Lv, Preparation of 4,4’-bis-(carboxyl phenylazo)-dibenzo18- crown-6 dye and its application on ratiometric colorimetric recognition to Hg2+, Spectrochim. Acta Part A 79 (2011) 661–665. [35] H. Xiao, J. Li, K. Wu, G. Yin, Y. Quan, R. Wang, A turn-on BODIPY-based fluorescent probe for Hg(II) and its biological applications, Sens. Actuators B-Chem. 213 (2015) 343–350. [36] Y. Shiraishi, S. Sumiya, Y. Kohno, T. Hirai, A rhodamine-cyclen conjugate as a highly sensitive and selective fluorescent chemosensor for Hg(II), J. Org. Chem. 73
4. Conclusion In conclusion, we have designed a new water-soluble acidic-fluorescence and alkaline-colorimetric dual-channel chemosensor, AAT, which owns azomonardine unit as reporting group and aminothiazole as receptor. At pH 5.0 and pH 7.5, AAT could determine the concentration of Hg2+ in fluorescence and colorimetric method, respectively. Due to the different stoichiometry of AAT-Hg2+ at pH 7.5 and pH 5.0, the linear detection concentration could be regulated from 0 to 12 μM to 0–35 μM by adjusting the solution pH. The binding modes between AAT and Hg2+ were changed in the acid and alkali environment. At pH 5.0, the thiazole of AAT combined with one Hg2+ ion through carbanion, and the other Hg2+ ion was boned on AAT through N (near the thiazole) and phenol hydroxyl. Nevertheless, at pH 7.5, an AAT molecule just bonded one Hg2+ ion through -NH-thiazole and AreO− groups. Moreover, AAT exhibited highly selective to Hg2+ among 16 metal ions and 13 anion ions through both fluorescence and colorimetric methods. Also, AAT realized the Hg2+ monitoring in real samples and gained a satisfactory recovery. The excellent optical properties and high selectivity of AAT illustrates it is a great potential for Hg2+ detection in practical utilization. Acknowledgments This study was supported by the National Natural Science Foundation of China (U1703351, 51663021), Bingtuan Excellent Young Scholars, Bingtuan Innovation Team in Key Areas (2015BD003) and Yangtze River scholar research project of Shihezi University (No. CJXZ201401). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.12. 009. References [1] S. Yoon, A.E. Albers, A.P. Wong, C.J. Chang, Screening mercury levels in fish with a selective fluorescent chemosensor, J. Am. Chem. Soc. 9 (2005) 16030–16031. [2] M. Vedamalai, D. Kedaria, R. Vasita, S. Mori, I. Gupta, Design and synthesis of BODIPY-clickate based Hg2+ sensors: the effect of triazole binding mode with Hg2+ on signal transduction, Dalton Trans. 45 (2016) 2700–2708. [3] G. Zhu, Y. Li, C.Y. Zhang, Simultaneous detection of mercury(II) and silver(I) ions with picomolar sensitivity, Chem. Commun. 50 (2014) 572–574. [4] P. Makam, R. Shilpa, A.E. Kandjani, S.R. Periasamy, Y.M. Sabri, C. Madhu, et al., SERS and fluorescence-based ultrasensitive detection of mercury in water, Biosens. Bioelectron. 100 (2018) 556–564. [5] C.Y. Tai, S.J. Jiang, A.C. Sahayam, Determination of As, Hg and Pb in herbs using slurry sampling flow injection chemical vapor generation inductively coupled plasma mass spectrometry, Food Chem. 192 (2016) 274–279. [6] L. López-García, Y. Vicente-Martínez, M. Hernández-Córdoba, Determination of ultratraces of mercury species using separation with magnetic core-modified silver nanoparticles and electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 30 (2015) 1980–1987. [7] W. Zu, Z. Wang, Ultra-trace determination of methylmercuy in seafood by atomic fluorescence spectrometry coupled with electrochemical cold vapor generation, J.
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application in living cells, Talanta 170 (2017) 103–110. [39] B. Kirthika Rani, S. Abraham John, Fluorogenic mercury ion sensor based on pyrene-amino mercapto thiadiazole unit, J. Hazard. Mater. 343 (2018) 98–106. [40] C.W. Liu, C.C. Huang, H.T. Chang, Highly selective DNA-based sensor for lead(II) and mercury(II) ions, Anal. Chem. 81 (2009) 2383–2387.
(2008) 8571–8574. [37] Z. Cheng, G. Li, M. Liu, A metal-enhanced fluorescence sensing platform based on new mercapto rhodamine derivatives for reversible Hg2+ detection, J. Hazard. Mater. 287 (2015) 402–411. [38] Y. Feng, Z. Kuai, Y. Song, J. Guo, Q. Yang, Y. Shan, et al., A novel “turn-on” thiooxofluorescein-based colorimetric and fluorescent sensor for Hg2+ and its
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Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
An acid-fluorescence and alkali-colorimetric dual channels sensor for Hg2+ selective detection by different coordination manners in aqueous media ⁎
Xiulan Zhanga, Yu Wanga, Huihui Yuanb, Xuhong Guoa,c, Bin Daia, , Xin Jiaa,
T
⁎
a School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region, Engineering Research Center of Materials-Oriented Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832003, People’s Republic of China b Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China c State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorescence Colorimetric Mercury ion Dual-channel
Due to the advantages of high selectivity, accuracy and mutual calibration, dual-channel sensors have drawn much attention. In this work, a water-soluble fluorescence and colorimetric dual-channel chemosensor (AAT) with both azomonardine and aminothiazole units has been designed and synthesized. AreOH and ]Ne groups, which respectively belonged to azomonardine and thiazole unit, exhibited pH dependent ionization, which leaded AAT exhibiting strong naked-eye visible fluorescence and color in acidic and alkaline solution, respectively. When ]Ne was ionized in acidic solution, fluorescence was turned on. The colorimetry occurred while the ionization of Ar−OH in alkaline solution. Moreover, the solution pH had a significant impact on the binding efficiency between AAT and Hg2+, resulted in different coordination manners and stoichiometry. At pH 5.0, in the fluorescence channel, the stoichiometry of AAT for Hg2+ was calculated as 1:2 and the linear detection concentration was ranged from 0 to 35 μM. It was in a range of 0–12 μM through colorimetric method with the stoichiometry of 1:1 at pH 7.5. Furthermore, neither in fluorescence or in colorimetric channel, AAT showed high specificity toward Hg2+ among numerous cationic and anion ions. Also, AAT realized the Hg2+ identification and determination in river water samples.
1. Introduction Mercury is a highly toxic and widespread global pollutant. It is oxidized to soluble inorganic ion (Hg2+), provides a pathway for contamination vast amounts of water and soil [1]. Hg2+ has high affinity toward thiols and amino groups in peptides and nucleic acids, and leads to a variety of irreversible damages even in a low concentration, for instance, neurological abnormalities, gingivitis, and tumor formation [2–4]. Therefore, it is of considerable importance to develop a reliable, convenient, selective and sensitive method for monitoring the level of Hg2+. Comparison with some traditional Hg2+ determination techniques, such as inductively coupled plasma [5], atomic absorption spectrometry [6], atomic fluorescence spectrometry [7,8] and surface-enhanced Raman scattering spectroscopy [4,9], fluorescence or colorimetric based approaches are more promising because of their convenience, specificity and sensitivity [10–12]. Recently, fluorescence and colorimetric dual-channel response method, which possesses ⁎
advantages of high selectivity, accuracy, and mutual calibration, has attracted lots of attentions [13–15]. Due to the advantages of polytropical optical properties and ligand groups, a series of organic molecules based dual-channel sensors are developed for heavy metal ions determination. For instance, quinoline appended naphthalene derivant for Al3+ (fluorescent method) and Cu2+ (colorimetric method) determination [16], cyclic steroid-rhodamine conjugated probe for Fe3+ detection [17], 3-hydroxy-2- naphthoylhydrazide and 6-Hydroxyl-3formyl chromone based dual-channel sensor for Al3+ test [18], benzildihydrazone derived colorimetric and fluorescent probe for Cu2+ detection [19], and coumarin-based chemosensor for determination of Pd2+ [20], et al. However, rarely dual channels sensors have been designed for Hg2+ detection. Withal, although these developed organic sensors exhibit splendid selectivity and sensitivity toward the targets, they are limited to organic or organic-aqueous mixtures system. Because of their poor water solubility, they are restricted for further utilization in environmental samples. Azomonardine, an organic molecule, with excellent water solubility,
Corresponding authors. E-mail addresses: [email protected] (B. Dai), [email protected] (X. Jia).
https://doi.org/10.1016/j.jphotochem.2018.12.009 Received 10 October 2018; Received in revised form 5 December 2018; Accepted 5 December 2018 Available online 31 December 2018 1010-6030/ © 2018 Published by Elsevier B.V.
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yield of 26.7% was obtained. 1H-NMR: (400 MHz, d6-DMSO) δ 6.35 (d, J = 2.2 Hz, 1H, H-2), 6.44 (dd, J = 8.6, 2.2 Hz, 1H, H-4), 7.55 (d, J = 3.5 Hz, 1H, H-12), 7.65 (dd, J = 7.6, 6.1 Hz, 2H, H-5, 11), 9.13 (s, 1H, H-9), 10.53 (s, 1H, H-8), 12.00 (s, 1H, H-7); 13C-NMR: (400 MHz, d6-DMSO) δ C 102.58 (CH, C-2), 109.12 (CH, C-4), 111.97 (C, C-6), 118.42 (CH, C-12), 134.46 (CH, C-5), 141.33 (CH, C-11), 163.02 (C, C14), 163.56 (C, C-3), 164.26 (C, C-1), 171.30 (CH, C-9); ESI-Mass (m/z): [1 + H]+ calcd for C10H9N2O2S 221.0385, found 221.0385.
can be readily obtained by one-pot reaction of dopamine and resorcinol in aqueous solution [21,22]. The AreOH group of azomonardine is facile to ionize into AreO− in alkalescence aqueous solution and incurs strong fluorescence. The ionized azomonardine owns high fluorescence quantum yield. For these reasons, azomonardine can be used as an ideal fluorophore. On the other hand, thiazole has been declared as a ligand with high affinity for Hg2+ [23,24]. Meanwhile, ]Ne of thiazole can be ionized as =+NH- in acidic solution. On these basis, in this work, by taking advantages of pH dependent ionization of AreO− and ]Ne groups, we have synthesized a water soluble acidic-fluorescence and alkaline-colorimetric dual-channel responses chemosensor (AAT) for Hg2+ highly selective detection via the integrate of azomonardine and amino-thiazole. Of which, azomonardine and amino-thiazole are employed as reporter and receptor, respectively. The synthesized sensor, AAT, is colorless but with strong and naked-eye visible blue fluorescence in acidic solution (pH 3–7), and it exhibits the color of yellow with non-fluorescence in alkaline solution (pH 7–10). It is other than the majority of reported dual-channel sensors which the fluorescence and colorimetric are synchronously occurring, they are asynchronous for AAT. On this benefit, AAT has capable of determination Hg2+ through fluorescence and colorimetric methods respectively in acidic and alkaline solution. Meanwhile, the probe has satisfactory recovery for Hg2+ determination in real samples. Therefore, this proposed sensor has highly promising for monitoring the Hg2+ level in aqueous samples at different pH values.
2.3.2. Synthesis of compound 2 and AAT Compound 1 (3 mmol, 0.66 g) was dissolved in 30 mL methanol. And then, NaBH4 (3.4 mmol, 0.13 g) was slowly added under the ice protection. The reaction of the mixture was maintained for 2 h at room temperature before it was terminated by 20% HCl. After removing the methanol, about 300 mL H2O was added, and adjusted the solution pH to 12.0 by NaOH (2.0 M). Then, added 3 mmol (0.57 g) of dopamine hydrochloride into the mixture. After stirring for 5 h at room temperature, the solution was neutralized by HCl (2.0 M). Meantime, yellow precipitate product was formed. The lyophilized solid was purified by column chromatography on silica gel (ethyl acetate: ethanol = 9:1), and yield the orange-yellow solid product AAT. 1HNMR: (400 MHz, d6-DMSO) δ 1.4 (d, J = 12.2 Hz, 1H, H-12), 1.9 (m, 1H, H-15), 2.22 (dt, J = 10.4, 6.1 Hz, 2H, H-12, 15), 2.75 (td, J = 12.6, 3.6 Hz, 1H, H-16), 2.86 (dd, J = 13.0, 5.1 Hz, 1H, H-16), 4.32 (d, J = 2.6 Hz, 2H, H-20), 5.66 (s, 1H, H-19), 6.22 (s, 1H, H-6), 6.48 (s, 1H, H-9), 6.61 (d, J = 3.7 Hz, 1H, H-23), 7.02 (d, J = 3.7 Hz, 1H, H-22), 7.6 (s, 1H, H-2), 7.88 (t, J = 5.0 Hz, 1H, H-26), 11.16 (s, 1H, H-5); 13CNMR: (400 MHz, d6-DMSO) δ C 32.01 (CH2, C-16), 37.07 (CH2, C-15), 43.17 (CH2, C-20), 46.86 (CH2, C-12), 82.85 (C, C-13), 89.54 (C, C-14), 98.01 (CH, C-6), 106.61 (CH, C-9), 111.96 (CH, C-23), 112.62 (C, C-4), 121.41 (C, C-1), 125.34 (CH, C-2), 138.32 (CH, C-22), 162.78 (C, C-11), 163.17 (C, C-3), 164.77 (C, C-7), 169.32 (C, C-25), 196.45 (C, C-10); ESI-Mass (m/z): [AAT + H]+ calcd for C18H18N3O4S 372.1018, found 372.1017.
2. Experimental 2.1. Reagents Dopamine hydrochloride was purchased from Sigma-Aldrich (U.S.). 2,4-dihydroxybenzaldehyde, 2-aminothiazole and Hydroxyethyl piperazine ethyl sulfonic acid (HEPES) were obtained from Adamas reagent Co. Ltd. All the metal nitrate, metal chloride salts, and anion ions including (K+, Na+, Cd2+, Ca2+, Hg2+, Cu2+, Ba2+, Zn2+, Ni2+, Fe3+, Pb2+, Li+, Co2+, Al3+, Mg2+, Cr6+, Fe2+, Cr3+, SO42−, NO3-, CO32−, PO43−, SO32−, S2O82−, ClO2−, CH3COO−, OH−, Cl−, F−, I−, S2−) with high purity were purchased from Xiya Co. Ltd. All the other reagents were of analytic grade and were used as received. The DI-water was used to prepare aqueous solutions.
2.4. Hg2+ detection A series of aqueous solutions with different Hg2+ concentrations were prepared by dissolving the HgCl2 in 10 mM HEPES solution (pH = 7.5) or HAC/NaAC buffer solution (pH = 5.0). For Hg2+ detection, 5 μL of AAT (4 mM) was added to Hg2+ solution with the total volume of 2 mL. The mixture was incubated for 1 h. Then, the fluorescence intensity of detection aqueous system was collected at λex/ λem = 435/475 nm. The absorption intensity of that was collected at 425 nm.
2.2. Apparatus and characterization Fourier transform near-infrared spectrometer (Nicolet iS10, USA) was used for recording FT-IR spectra. 1H NMR and 13C NMR spectra were collected on an inova-400 M (Varian) operating at 400 MHZ. Electrospray ionization mass spectrometry (ESI-MS) analysis was performed in the positive ion mode on a liquid chromatography-with mass spectrometry (Micromass LCT, WATERS). UV–vis absorption spectra were recorded on a UV-6100 s (Mapada) UV–vis spectrophotometer at room temperature. The fluorescence measurements were performed on a G9800 A (Agilent) fluorescence spectrophotometer (the excitation and emission slits were both set to 5.0 nm) equipped with a plotter unit and a 1 cm quartz cell. The solution pH was monitored by a PHS-2 F (Leici) pH meter.
2.5. Real samples test To investigate the Hg2+ determination in real samples, river water samples which were collected from mingzhu river (Shihezi, Xinjiang) were employed. The collected river water was firstly filtered by 0.45 μm membrane to remove the large particles and gels, and then straightly utilized for preparation of 10 mM HEPES solution (pH = 7.5) and HAC/ NaAC solution (pH = 5.0). The spiking experiment was carried out by spiking known amounts of Hg2+ into the buffer solution in different known concentrations. Other parameters were as the same with Hg2+ detection experiments.
2.3. Synthesis 2.3.1. Synthesis of compound 1 2,4-Dihydroxybenzaldehyde (10 mmol, 1.38 g) and 2-aminothiazole (12 mmol, 1.2 g) were dissolved in absolute ethyl alcohol and refluxed for 4 h. Then, the major amount of ethyl alcohol was removed by rotary evaporators. It was followed by addition of DI-water to form the yellow precipitate, and then filtered and air-dried. The residue solid was purified by column chromatography on silica gel (petroleum ether: ethyl acetate = 6:4), and bright yellow crystalline solid compound 1 in a
3. Results and discussion 3.1. Optical properties of AAT In pH 5.0 aqueous solution, AAT exhibits a strong fluorescence absorption and emission band centered at 437 nm and 474 nm (Fig. S1A), respectively, and shows a characteristic absorption at 392 nm (Fig. S1A). Fluorescence intensity of AAT shows excitation wavelength 13
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Fig. 1. (A) UV–vis absorption spectra, (B) fluorescence intensity (435 nm) and absorption (425 nm), and (C) the apparent color of AAT (10 μM) in aqueous solution (10 mM phosphate buffer) with different pH values.
Scheme 1. Synthesis of AAT.
intensity of AAT aqueous solution is weakened (Fig. S2B), however, the color maintains lipochromous (Fig. S2A), indicating that Hg2+ distinctly quenches the fluorescence but has indistinctive effect on absorption. Furthermore, as illustration in Fig. 2A, fluorescence spectra are downsides which depend on the Hg2+ concentration. The relative fluorescence intensity ((I0−I)/I0) of AAT reflects a wide range of Hg2+ detection concentration from 0 to 100 μM (Fig. 2B). And it shows a good linear relationship (R2 = 0.9956) in a range of 0–35 μM (Fig. 2B inset). The calculated Low of Detection (LOD) is 146.3 ± 0.5 nM (S/ N = 3) which is below the allowed concentration level (248.8 nM) in industrial wastewater. Moreover, AAT can be coated on paper to prepare test paper for Hg2+ detection by fluorescence method (Fig. S3). With increasing Hg2+ concentration (0, 0.5, 2, 5, 10 μM), the fluorescence of the detection area demonstrates sensitive changes from dark blue to nattier blue under UV light. When the solution pH is equal to or higher than 7.0, a preponderant absorption of AAT at 425 nm appears, and it is enhanced with increasing pH values (pH < 9.0) (Fig. 1A). Based on the calculated (A0A) values, from pH 7.0 to pH 9.0, Hg2+ quenches the absorption intensity with pH dependent, and the optimized solution pH for Hg2+ detection by colorimetric method is 7.5 (Fig. S4). At pH 7.5, the specific absorption at 267 nm and 425 nm are quenched by higher Hg2+ concentration (Fig. 2C). Accordingly, the relative absorption intensity at
dependent. The optimal excitation wavelength is recorded at 435 nm (Fig. S1B). As showed in Fig. 1A, B, solution pH has a significant effect on the intensity of absorption and fluorescence. The absorption at 425 nm is obviously enhanced with raised pH values when the solution pH is lower than 9.0. As the solution pH is higher than 6.5, the absorption is stronger than that of at 392 nm, which induces the solution of AAT converting from colorless to yellow (Fig. 1C). It is because of that the Ar−OH on azomonardine unit tends to ionize into AreO− at higher pH and enhances the absorption of 425 nm [22,25]. When the solution pH is of 4.5–7.5, the fluorescence intensity at 474 nm is increased with decreasing pH values (Fig. 1B), which owns to that ]N− (on thiazole) ionized into ]+NH− at lower pH [26]. As a result, the optimal solution pH for generation fluorescence is about 5.0, and the relative quantum yield of AAT is calculated to be 26.12% by using fluorescein as reference [27–29] (Scheme 1). 3.2. Hg2+ detection For AAT, the ionization of both Ar−OH and ]Ne displays pH dependent, resulting in diverse optical properties in acidic and alkaline solution. On this basis, AAT owns the ability of determination Hg2+ concentration through both fluorescence and colorimetric channels. As the results, at pH 5.0, upon the addition of Hg2+, the fluorescence 14
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Fig. 2. (A) Fluorescence spectra (pH 5.0) changes upon the addition of Hg2+, (B) The relative fluorescence intensity ((I0-I)/I0, where I0 and I are the fluorescence intensity of AAT in the absence and in the presence of Hg2+, respectively) of AAT responses to the concentrations of Hg2+, inset is the linear calibration plot for Hg2+ sensitive detection, (C) UV–vis spectra (pH 7.5) changes upon the addition of Hg2+, (D) The relative absorption intensity ((A0-A)/A0, where A0 and A are the absorption intensity of AAT in the absence and in the presence of Hg2+, respectively) of AAT responses to the concentrations of Hg2+, inset is the linear calibration plot for Hg2+ sensitive detection.
425 nm ((A0−A)/A0) is linearly enhanced by increasing Hg2+ concentration from 0 to 12 μM (Fig. 2D) with a good linear relationship (Fig. 2D inset). The LOD is calculated as 149.2 ± 1.2 nM (S/N = 3) through colorimetric method. Comparatively, the highest linear detection concentration by fluorescence method is 3 times of that by a colorimetric method, demonstrating that it can be regulated by adjusting the solution pH. Comparison with some reported small organic molecule sensors for Hg2+ detection [30–34] (Table S1), the achieved LOD values by fluorescence and colorimetric methods were both higher than that of them.
benzene) are decreased with elevatory Hg2+ concentration (Fig. S7A), indicating Hg2+ bonds to AAT via both azamonardine and aminothiazole units [25]. According to our previous research [25], groups on azamonardine unit are languid to bind to Hg2+. However, with the introduction of aminothiazole group, AreOH becomes possible to coordinate with Hg2+, and induces the weaken absorption at 392 nm. At pH 7.5, after AAT incubation with different concentrations of Hg2+, similar UV–vis absorption change rule is obtained (Fig. 2C). Based on these results, combination with diverse stoichiometry, suggesting that AAT captures Hg2+ in a different manner at pH 5.0 and pH 7.5. To further study the binding mode between Hg2+ and AAT at pH 5.0 and pH 7.5, 1H NMR titration experiment is carried out. As shown in Fig. 4, upon the addition of incremental amounts of Hg2+ to d6-DMSO, the protons of type e and type f are decreased. However, a new singlet appears at 6.93 ppm and isometrically increases with the decreasing doublet (proton of type e). But there is none new peak for the proton of type f. It supports that the proton of type f is expropriated, and AAT chelates with Hg2+ via carbanion after deprotonation (Scheme 2) [23]. The ESI-MS spectrum shows a peak at m/z 608.0, which is assigned to the [AAT + HgCl+ + H+] (Fig. S7B). Therefore, AAT-HgCl, as a new species, induces the protons of type a, type c, type d and type r split to two independent singlets. And the integrals of the appeared new singlets increases with the primary singlets reducing. Besides, eNHe (r) and −CHe (c) protons of the new species are downfield shifted and eOH (b) proton is faded away, indicating the coordination of Hg2+ with both amino (r) and phenolic hydroxyl (b) [38]. However, the ESIMS spectrum can’t show the 1:2 stoichiometry of AAT to Hg2+, which may on account of the weak interaction of amino (r) and phenolic hydroxyl to Hg2+. In summary, all these data strongly support the
3.3. Binding mode investigation The Job’s plot [35] for the binding activity between AAT and Hg2+ is investigated for studying the stoichiometric nature of AAT to Hg2+. The fitting results are illustrated in Fig. 3, 2:1 and 1:1 stoichiometry are calculated for Hg2+ bonding to AAT at pH 5.0 and pH 7.5, respectively. Through Benesi-Hilderbrand plot [36,37], the association constants of AAT-Hg2+ are counted to be (3.81 ± 0.32) × 109 M−2 and (2.00 ± 0.23) × 104 M−1 at pH 5.0 and pH 7.5, respectively (Fig. S5). Reaction products of AAT+Hg2+ at pH 5.0 and pH 7.5 are characterized by FT-IR (Fig. S6). The absorption at 3295 cm−1, 1670 cm−1, 1585 cm−1 and 1150 cm−1and 850 cm−1 which belong to νN-H, νC=N, νC=C, νC-O/C–N/C-S-C and γN-H, are impaired (Fig. S6) by comparison with that of AAT. This means that the combination of Hg2+ and AAT might concern to these functional groups. Moreover, at pH 5.0, after AAT incubation with Hg2+, UV–vis absorption at 392 nm (n-π∗ of azamonardine), 305 nm (π-π∗ of ]Ne of thiazole) and 253 nm (π–π∗ of 15
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Fig. 3. The Job’s plot for AAT to Hg2+ in (A) pH = 5.0 (10 mM acetic acid - sodium acetate buffer solution, the total concentration of AAT and Hg2+ is set as 40 μM) and (B) pH = 7.5 (10 mM HEPES buffer solution, the total concentration of AAT and Hg2+ is set as 20 μM).
Hg2+ determination. Therefore, it is indispensable for a sensor with high selectivity. In this study, the optical properties of AAT toward various metal ions and anion ions are investigated. At pH 5.0, among numerous metal ions and anion ions, only Hg2+ has superior efficiency to quench the fluorescence intensity of AAT (Figs. 5A, S10A). The recorded fluorescence spectra (Fig. 5B) and calculated (I0-I)/I0 values (Fig. 5C) also describe AAT has highly selective toward Hg2+ in the solution of pH 5.0. Moreover, in the mixture solution of 25 μM Hg2+ and 50 μM any other metal ions, other metal ions also has inappreciable interference for Hg2+ quantification (Fig. 5C), implying AAT has great potential for Hg2+ detection in real samples. On the other hand, in the solution of pH 7.5, when the buffer solution is prepared by HEPES, except for Hg2+ (20 μM), both Cu2+ and Fe3+ with concentration of 50 μM show some impacts on the absorption at 425 nm (Fig. S8). However, in PBS buffer (10 mM, pH = 7.5), the interference of Cu2+ and Fe3+ is eliminated (Fig. 5D, E). It owes to the abundant of PO42− which has capacity to coordinate with various metal ions (Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, et al) and form as stable complexes [25,40].
proposed bonding mode as showed in Scheme 2. As soon as AAT bonds to Hg2+, the intramolecular electron transfer from azomonardine fluorophore to Hg2+ ion is occurred and induces the fluorescence quenching [39]. On the other hand, at pH 7.5, according to the previous consequence, AreOH is ionized as AreO− which enhances the electron density of phenolic moiety and depress the activation of C-atom toward Hg2+ [23]. AreO− with an electron induction effect causes the protons (a, c, d, r) move to upfield (Fig. S7). The proton of type r is split into two peaks, indicating eNH-thiazole bind to Hg2+ (Fig. S8). Moreover, addition of Hg2+ distinctly quenches the absorption at 253 nm and 425 nm, which demonstrates the interaction between AreO− and Hg2+ [25]. The proposed bonding mode between AAT and Hg2+ at pH 7.5 is shown in Scheme 2. 3.4. Selectivity In practical utilization, complicated constituents contain various of concomitant metal ions and anion ions may cause some interference on
Fig. 4. 1H NMR spectra of AAT in d6-DMSO by addition of (A) 0 equiv, (B) 0.5 equiv, (C) 1.0 equiv, (D) 1.5 equiv, (E) 2.0 equiv and (F) 3.0 equiv Hg2+. 16
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Scheme 2. The proposed binding model for AAT-Hg2+ at pH 5.0 and pH 7.5.
Although the solution contains other metal ions, the relative absorption of AAT exhibits reliable quantitative analysis of Hg2+ (Fig. 5E). Also, various of anion ions have inappreciable influence on AAT (Fig. S10B). This result also demonstrates that AAT has the great potential for Hg2+ determination in real samples at pH 7.5 in the existence of PBS.
determination in real samples, water samples collected from mingzhu river were tested. The real samples were filtered with 0.45 μm membrane and spiked with various concentrations of Hg2+ prior to incubating with AAT. The results are shown in Table 1, the obtained recoveries are in the range of 96.3%–100.5% and 86.8%–92.3% at pH 5.0 and pH 7.5, respectively, suggesting AAT can be applied to detect Hg2+ in real samples through both fluorescence and colorimetric methods, and the fluorescence method is more accurate.
3.5. Real sample analysis To
evaluate
the
practical
application
of
AAT
for
Hg2+
Fig. 5. (A) The fluorescence photographs of AAT in the presence of various metal ions under the irradiation of UV lamp (365 nm). (B) The fluorescence emission spectra of AAT in acetic acid - sodium acetate buffer solution (pH = 5.0) upon the addition of various metal ions (AAT, 10 μM; Hg2+, 25 μM; other metal ions, 50 μM; slit: E x 5 nm, Em 2.5 nm). (C) The relative fluorescence intensity of AAT towards 17 kinds of metal ions in the absence and in the presence of Hg2+ (pH = 5.0; AAT, 10 μM; Hg2+, 25 μM; other metal ions, 50 μM). (D) The absorption spectra of AAT in PBS buffer solution (pH = 7.5) upon the addition of various metal ions (AAT, 10 μM; Hg2+, 20 μM; metal ions, 50 μM). (E) The relative absorption of AAT towards various metal ions in the absence and in the presence of Hg2+ (pH 7.5; AAT, 10 μM; Hg2+, 20 μM; other metal ions, 50 μM). 17
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Table 1 The result for Hg2+ determination in river water samples. Spiked (μM)
Found (μM)
Recovery (%)
RSD (n = 3, %)
Acid 10 15 20
9.63 ± 0.86 15.08 ± 0.09 19.57 ± 1.36
96.3 100.5 97.9
8.5 0.6 6.8
Alkali 6 8 10
5.21 ± 0.33 7.16 ± 0.26 9.23 ± 0.57
86.8 89.5 92.3
6.5 3.4 6.2
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4. Conclusion In conclusion, we have designed a new water-soluble acidic-fluorescence and alkaline-colorimetric dual-channel chemosensor, AAT, which owns azomonardine unit as reporting group and aminothiazole as receptor. At pH 5.0 and pH 7.5, AAT could determine the concentration of Hg2+ in fluorescence and colorimetric method, respectively. Due to the different stoichiometry of AAT-Hg2+ at pH 7.5 and pH 5.0, the linear detection concentration could be regulated from 0 to 12 μM to 0–35 μM by adjusting the solution pH. The binding modes between AAT and Hg2+ were changed in the acid and alkali environment. At pH 5.0, the thiazole of AAT combined with one Hg2+ ion through carbanion, and the other Hg2+ ion was boned on AAT through N (near the thiazole) and phenol hydroxyl. Nevertheless, at pH 7.5, an AAT molecule just bonded one Hg2+ ion through -NH-thiazole and AreO− groups. Moreover, AAT exhibited highly selective to Hg2+ among 16 metal ions and 13 anion ions through both fluorescence and colorimetric methods. Also, AAT realized the Hg2+ monitoring in real samples and gained a satisfactory recovery. The excellent optical properties and high selectivity of AAT illustrates it is a great potential for Hg2+ detection in practical utilization. Acknowledgments This study was supported by the National Natural Science Foundation of China (U1703351, 51663021), Bingtuan Excellent Young Scholars, Bingtuan Innovation Team in Key Areas (2015BD003) and Yangtze River scholar research project of Shihezi University (No. CJXZ201401). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.12. 009. References [1] S. Yoon, A.E. Albers, A.P. Wong, C.J. Chang, Screening mercury levels in fish with a selective fluorescent chemosensor, J. Am. Chem. Soc. 9 (2005) 16030–16031. [2] M. Vedamalai, D. Kedaria, R. Vasita, S. Mori, I. Gupta, Design and synthesis of BODIPY-clickate based Hg2+ sensors: the effect of triazole binding mode with Hg2+ on signal transduction, Dalton Trans. 45 (2016) 2700–2708. [3] G. Zhu, Y. Li, C.Y. Zhang, Simultaneous detection of mercury(II) and silver(I) ions with picomolar sensitivity, Chem. Commun. 50 (2014) 572–574. [4] P. Makam, R. Shilpa, A.E. Kandjani, S.R. Periasamy, Y.M. Sabri, C. Madhu, et al., SERS and fluorescence-based ultrasensitive detection of mercury in water, Biosens. Bioelectron. 100 (2018) 556–564. [5] C.Y. Tai, S.J. Jiang, A.C. Sahayam, Determination of As, Hg and Pb in herbs using slurry sampling flow injection chemical vapor generation inductively coupled plasma mass spectrometry, Food Chem. 192 (2016) 274–279. [6] L. López-García, Y. Vicente-Martínez, M. Hernández-Córdoba, Determination of ultratraces of mercury species using separation with magnetic core-modified silver nanoparticles and electrothermal atomic absorption spectrometry, J. Anal. At. Spectrom. 30 (2015) 1980–1987. [7] W. Zu, Z. Wang, Ultra-trace determination of methylmercuy in seafood by atomic fluorescence spectrometry coupled with electrochemical cold vapor generation, J.
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