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A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions
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A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions Yuhao Wu a, Xinjian Cheng b, ∗∗, Chaoyi Xie a, Kang Du a, Xianghong Li a,c, ∗, Dingguo Tang a a Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education, Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan, 430074, China b School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, 430073, China c Key Laboratory of Analytical Chemistry of State Ethnic Affairs Commission, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan, 430074, China
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Article history: Received 3 August 2019 Received in revised form 13 September 2019 Accepted 16 September 2019 Available online xxxx Keywords: Cyclometallated ruthenium complex 2-(2-Thienyl)pyridine Polymer membrane Mercury(II) ions
a b s t r a c t A new cyclometallated ruthenium complex (Ru1) involving a 2-(2-thienyl)pyridine and a benzo[e]indolium block connected with a hexanoic acid was successfully synthesized and characterized, which exhibited the high sensitivity and selectivity to Hg2+ over other common metal ions with the detection limit of as low as 0.053 μM in aqueous system. Then, it was grafted onto a polymer membrane to afford a Hg2+-sensitive membrane (sensor 1), which was characterized by FT-IR, SEM and XPS spectra, respectively. When sensor 1 was dipped into the aqueous solution of Hg2+ ions, the color of the membrane changed from dark-red to yellow, which could be observed by naked eyes easily. It should be noted that the membrane can absorb Hg2+ ions well in aqueous solution and the adsorption capacity of this polymer membrane for Hg2+ ions was determined by atomic absorption spectroscopy, indicating that it also could be used as a potential material for removal of Hg2 + ions. © 2019 Published by Elsevier B.V.
1. Introduction In recent years, detections of heavy metal ions have attracted much attention due to the water pollution [1]. Among those metal ions, mercury ion is the most toxic [2–4]. Mercury ions are usually accumulated from the environment, and ultimately ingested by animals and humans. They can easily pass through the biological membranes and cause serious damage to the central nervous system and endocrine system [3,5,6]. In general, detections of mercury ions require large instruments such as atomic fluorescence spectrometry (AFS) [5], atomic absorption spectroscopy (AAS) [7] and inductively coupled plasma atomic mass spectrometry (ICP-MS) [8]. However, these methods are laborious, time-consuming, and elaboratelyoperated with complicated instruments. Optical chemo-sensors based on color and spectral changes in absorptions or emissions have been widely concerned in detections of Hg 2+ ions due to their simplicity, rapid response and visualization without aids of
∗ Corresponding author. Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education& Hubei Key Laboratory of Catalysis and Materials Science, SouthCentral University for Nationalities, Wuhan, 430074, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Cheng), [email protected] (X. Li).
instruments [4,9–16]. Among them, heavy-metal polypyridyl complexes are favored because they have much larger stoke-shifts and longer luminescence lifetime than organic small molecules [12–16]. Recently, cyclometallated ruthenium complexes have drawn much attention, which replace a nitrogen atom in a pyridine of a ruthenium polypyridyl complex with an anionic carbon center [17–20]. By virtue of the formation of Ru–C, their MLCT absorptions shift to long-wavelength range in comparison with ruthenium polypyridyl complexes [17]. The absorption spectral characteristics allow them exhibit great spectral changes when they utilize specific recognition units to interact with analytes [20–23]. Therefore, the development of a new cyclometallated complex to detect Hg2+ is attractive. As is well known, water is a relatively poor Lewis base and a strong hydrogen bonding agent, which exhibits such strong competitions with chemo-sensors that the sensitivity of chemo-sensors to Hg2+ ions is reduced. Therefore, some biologically toxic solvents are needed to make these chemo-sensors work well, such as acetonitrile, ethanol and dimethyl sulfoxide (DMSO) [9–11,14–16,24,25]. To improve the sensitivity of chemo-sensors in water, numerous efforts have been made on synthesizing new molecules with good water-solubility [6,12,23,26–28] and designing some hybrid-materials [29–31]. To the best of our knowledge, it is difficult to afford water-soluble molecules with hydrophilic blocks during purification. So many works turn to
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Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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Scheme 1. Synthetic route of Ru1 and sensor 1.
focus on the modified gold nanoparticles [32], silver nanoparticles [33], inorganic-organic hybrid nanomaterials [30,31,34,35], and polymers [27,36–40]. These materials have more or less problems such as high cost, complicated synthesis steps and harsh conditions, which limit their popularity in practical applications. Herein, a new cylometallated complex with a hexanoic acid (Ru1, see Scheme 1) has been successfully synthesized and used as a colorimetric sensor to detect Hg2+ ions with high sensitivity and selectivity in water. Furthermore, the complex was grafted onto an easilyprepared polymer membrane to afford a solid-state sensor 1 by virtue of the hexanoic acid in the complex (Scheme 1). The sensor 1 can promptly recognize Hg2+ with the membrane color changing from dark-red to yellow as well as efficiently absorb Hg2+ ions, which display a potential application in detection and removal of Hg2+.
2. Experimental section 2.1. Materials and characterization All solvents were commercially available and used after distillation. [Ru(cymene)Cl2]2 (dichloro(p-cymene)ruthenium(II) dimer), and 2-(5-formyl-2-thienyl)pyridine (Hfthpy) were purchased from Changcheng reagent company. Methyl acrylate, and glycidyl methacrylate stabilized with MEHQ were purchased from J&K China Chemical Ltd. Methyl acrylate was used after being treated with NaOH aqueous solution (5%) to remove the inhibitor MEHQ. Ethyl acetate was dried by anhydrous calcium chloride overnight. 3-(5-Carboxyl)pentyl-1,1,2-
2
Fig. 1. Absorption intensity ratio (A410 nm/A503 nm) of Ru1 in the presence or absence of Hg at different pH values. Herein, each spectrum of absorption was recorded at room temperature after a 5 min delay.
+
trimethylbenzo[e]indolium was prepared according to reference [41] and characterized by 1H NMR spectrum. 1 H NMR spectra and 13C NMR spectra were measured in DMSO‑d6 on a TCI IIITM 600 MHz (Bruker) spectrometer and MS spectra were measured on a MALDI-TOF/TOF 5800 (AB SCIEX) spectrometer, respectively. Fourier transform infrared spectra were taken on a Nicolet NEXUS 470 spectrometer with the sample dried and pressed into KBr pellets. Xray photoelectron spectroscopy (XPS) was performed on a XPS VG Multilab 2000X and the sample was tested under ultra-high vacuum conditions. A scanning electron microscope (SEM, HITACHI SU8010) was used to obtain the surface topography of the sample operating at 25 kV. After the surface of the sample was plated with platinum to enhance its conductivity, it was placed on an aluminum-based sample stage for SEM observation and EDS-mapping. Differential Scanning Calorimeter (DSC) was carried out on a DSC 200F3, NETZSCH (Germany) under a stream of nitrogen. The vacuum dried samples were heated from 30 to 210 °C at a scan speed of 10 K min−1. Atomic absorption spectroscopy was recorded by using an AAS ICE 3500 (Thermo Fisher Scientific) spectrophotometer. Absorption spectra were obtained on a Cary Series UV–Vis–NIR spectrophotometer. 2.2. Synthesis of Ru1 Synthesis of [Ru(bpy)2(fthpy)]PF6: A mixture of 2-(5-formyl-2thienyl)pyridine (Hfthpy) (0.15 g, 0.78 mmol), KPF6 (0.2937 g, 1.6 mmol), acetonitrile (12 mL), triethylamine (0.35 mL) were added
Fig. 2. The absorption changes of Ru1 (10 μM) in the presence of different concentrations of Hg2+ in HEPES (5% DMSO, pH 7.0, 10 mM). Insets: a) The photographs of solution color before and after the addition of Hg2+ in the solution of Ru1; b) The titration curve of Ru1 and Hg2+. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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to a 100 mL three-necked flask. The reaction system was deoxygenated with argon at room temperature for 10 min and then [Ru(cymene)Cl2]2 (0.3 g, 0.5 mmol) was added. After the mixture was refluxed at 55 °C for 16 h under argon, the solvent was evaporated to obtain orange-red solid. Then, 2,2′-bipyridine (0.25 g, 1.57 mmol) and methanol (12 mL) were added subsequently to the above mixture. After the resulting mixture was refluxed at 70 °C for another 12 h under argon, the solvent was evaporated and the residue was purified by silica column chromatography by using CH2Cl2/MeCN (10/1, v/v) as the eluent. Yield: 0.28 g, 59%. 1H NMR (400 MHz, CD3CN) δ: 9.80 (s, 1H), 8.48 (d, J = 8.2 Hz, 1H), 8.41 (d, J = 8.2 Hz, 1H), 8.35 (t, J = 7.7 Hz, 2H), 8.07–7.97 (m, 2H), 7.93–7.77 (m, 7H), 7.71 (t, J = 7.8 Hz,1H), 7.55 (d, J = 5.7 Hz, 1H), 7.45 (t, J = 7.4 Hz, 1H), 7.28 (m, J = 6.5 Hz, 3H), 7.12 (s, 1H), 6.94 (t, J = 6.5 Hz, 1H). 13C NMR (100 MHz, CD3CN) δ: 184.12, 162.22, 157.66, 157.17, 156.87, 155.54, 154.71, 151.06, 150.98, 150.61, 149.42, 146.21, 143.84, 136.62, 136.32, 135.34, 134.49, 134.30, 127.08, 126.53, 126.51, 126.39, 126.26, 123.44, 123.29, 123.05, 122.93, 122.17, 119.69, 117.27. MS m/z: Calculated for 602.06 (M+), Found: 602.04. Synthesis of Ru1: [Ru(bpy)2(fthpy)]PF6 (0.06 g, 0.11 mmol) and 3-(5carboxyl)pentyl-1,1,2-trimethylbenzo[e]indolium (0.04 g, 0.13 mmol) were added into ethanol (10 mL). The mixture was stirred and heated at 80 °C in the dark for 12 h. After the solvent was evaporated, the residue was dissolved in CH2Cl2 and chromatographed by using MeCN/ MeOH/saturated potassium nitrate/H2O as the eluent. Then, the reddish brown solid was collected. Yield: 0.04 g, 40%. 1H NMR (600 MHz, DMSO‑d6) δ: 12.04 (s, 1H), 8.81 (d, J = 8.2 Hz, 1H), 8.78–8.65 (m, 4H), 8.34 (d, J = 8.6 Hz, 1H), 8.25 (d, J = 8.9 Hz, 1H), 8.19 (d, J = 8.2 Hz, 1H), 8.13 (t, J = 7.9 Hz, 1H), 8.08 (d, J = 9.0 Hz, 1H), 8.00 (m, 4H), 7.92 (d, J = 7.9 Hz, 1H), 7.84–7.67 (m, 6H), 7.59 (t, J = 6.5 Hz, 1H), 7.53–7.42 (m, 4H), 7.35 (s, 1H), 7.11–7.03 (m, 2H), 4.65 (t, J = 6.0 Hz,2H), 2.23 (t, J = 8.0 Hz, 2H), 1.95 (s, 3H), 1.91 (s, 3H), 1.86 (t, J = 7.7 Hz, 2H), 1.59 (q, J = 7.5 Hz, 2H), 1.45 (q, J = 7.8 Hz, 2H); 13C NMR (151 MHz, DMSO‑d6) δ: 196.88, 181.16, 174.80, 161.26, 157.64, 157.13, 156.79, 155.46, 154.62, 151.39, 150.85, 150.57, 149.28, 149.02, 147.66, 145.76, 144.97, 139.03, 138.10, 137.48, 137.11, 136.25, 135.40, 135.21, 133.33, 131.49, 130.52, 128.85, 128.07, 127.47, 127.41, 127.35, 127.26, 124.40, 124.18, 123.93, 123.88, 123.47, 123.40, 121.09, 113.46, 109.31, 53.72, 46.32, 40.49, 33.84, 28.45, 26.17, 26.14, 25.85, 24.52; MS (Maldi-TOF, CHCA) m/z: Calculated for 907.24 (M-H+), Found: 907.22. 2.3. Preparationof sensor 1 Preparation of membrane: Under nitrogen atmosphere, methyl acrylate (7.74 g, 0.09 mol) was ultrasonically dispersed in ethyl acetate (45 mL) and heated to 60 °C. Azodiisobutyronitrile (AIBN, 0.1832 g) dispersed in ethyl acetate (5 mL) was then added. After stirring at 60 °C for 1 h, glycidyl methacrylate (1.42 g, 0.05 mol) was added. The mixture was stirred overnight to afford a viscous liquid, which was transferred to a flat plate of polytetrafluoroethylene (PTFE) and dried in air. The unreacted monomers were removed by Soxhlet extraction with ethanol and the remaining solid was dissolved in chloroform and spread out to obtain a transparent membrane. Grafting Ru1 onto the membrane to prepare sensor 1: The preprepared membrane was dispersed in 10 mL of DMF and transferred into a 50 mL three-necked flask, and then 20 mg of Ru1 was added. The mixture was deoxygenated with argon at room temperature for
3
Fig. 3. UV–visible responses of Ru1 (10 μM) to various metal ions in HEPES (5% DMSO, pH 7.0, 10 mM). Bars represent the absorption intensity A410nm/A503nm.
10 min and PPh3 (1 mg) was added into the mixture as a catalyst. The reaction was heated to 120 °C and kept at the temperature for 12 h. After DMF was evaporated under reduced pressure, resultant product was collected and then washed with ethanol for 24 h in a Soxhlet extractor to remove the unreacted complexes to afford sensor 1. 3. Results and discussion 3.1. Characterization of Ru1 and its sensitive to Hg2+ Herein, Ru1 was synthesized from Ru(bpy)2(fthpy)+ and characterized by 1H NMR, C NMR and MS spectra (Fig. S1–S3). As shown in Fig. S1, a single peak at δ 12.04 ppm could be attributed to proton of carboxyl. In the aliphatic region, the proton signals at δ 4.65 ppm are assigned to N+CH2−. In Fig. S2, it can be found that the peak at δ 196.88 ppm could be attributed to the carbon atom connected to the central Ru(II), while the peak of the carbon connected to the N+ ion appeared at δ 181.16 ppm. The signal located at δ 174.80 ppm is assigned to the carbon on the carboxyl group. Moreover, Ru1 was further characterized by MS spectrum in which the peak occurring at m/z 907.22 (calculated m/z 907.24) corresponds to [M−H+]+. Then, the pH effect on the interactions between Hg2+ and Ru1 was investigated (Fig. 1). The results showed that Ru1 is relatively stable at the range of pH 1.0–12.0 when it was incubated for 5 min. Upon the addition of Hg2+ to the above solutions of different pH values, Ru1 displayed a good response to Hg2+ between 6 and 7. As far as we know, carboxylic acids prefer to being converted into carboxylates at pH 6–7, which provide good water-solubility and better chances for Ru1 to interact with Hg2+ by coordination with oxygen from carboxylate. Moreover, the sulfur atom from the thienyl group could also be inclined to combine with Hg2+ due to HSAB theory [14,16,23]. As a result, the HEPES buffer at pH 7.0 was chosen as the sensing system. The absorption spectra of Ru1 in HEPES buffer (5% DMSO) were investigated. It can be seen from Fig. 2 that Ru1 displays several absorption bands in the UV–Vis–NIR regions. The bands occurring at 295 and 352 nm are assigned as ligand-centered (LC) charge transfer transitions, while the strong band with a maximum at 503 nm (ε = 1.64 × 104 M−1 cm−1) is attributed to dπ(Ru)→π*CˆN MLCT transition [17–20]. It should be noted that there was a broad absorption band centered at 750 nm (ε = 2.05 × 103 M−1 cm−1) in the range from 600 to 1000 nm, which can be assigned as dπ(Ru)→π*NˆN MLCT absorption. Obviously, these are significantly red-shifted compared 2+ to Ru(bpy)2+ (Hg(NO3)2, HNO3), the intensity of the absorp3 . Upon the addition of Hg tion band between 450 and 1000 nm was gradually weakened. Meanwhile, the absorption band centered at 503 nm shifted from 503 to 484 nm along with a new peak appearing at 410 nm, which resulted in the solution color change from red to yellow (Fig. 1, inset a). It can be seen that when the amount of Hg2+ reached at about 50 μM, the interaction between Ru1 and Hg2+ reach equilibrium (Fig. 2, inset b). The Job's plot experiment was 13
Table 1 Physiochemical properties of some optical chemo-sensors for Hg2+.
Ru1 Rhodamine B hydrazide derivative 9 Ir–S16 [Ir(TPQ)2(4-EO2-pic)]14 Ferrocene derivative15 Ru1–B23
Change of λabs,max/nm
Change of λem,max/nm
Medium
LOD/method
503 → 410 565↑ 420↑ 483 → 442 322↓ 560 → 440
– 588↑ 590 → 540 620↓ 437↑ –
DMSO/HEPES (5:95, v/v, 10 mM, pH 7.0) DMSO/H2O (1:9, v/v) CH3CN–H2O (98:2, v/v) CH3CN/H2O (1:1, v/v) CH3CN/H2O solution (2:8, v/v). HEPES (10 mM, pH 7.14)
5.3 × 10−8 M (A412 nm/A503 nm) 2.71 × 10−7 M (I588 nm) 4.1 × 10−8 (I540 nm/I590 nm) 1.78 × 10−8 M (I620 nm) 1.56 × 10−8 M (I437 nm) 5.9 × 10−7 M (A560 nm)
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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Fig. 4. FT-IR spectra of membrane, Ru1 and sensor 1, respectively.
Fig. 6. DSC curves of membrane (a) and sensor 1 (b), respectively.
then carried out to identify that the binding stoichiometry between Ru1 and Hg2+ in solution is close to 1:1 (Fig. S4). According to these results, it can be speculated that Hg2+ can interact with Ru1 by coordination with the oxygen of carboxylate and the sulfur of thienyl in the complex. And this coordination could lead to a change in the electron distribution in the Ru center, which may result in the shift and decrease of the MLCT maximum. Then, the detection limit of Ru1 for Hg2+ in water was calculated to be as low as 0.053 μM (Fig. S5), which was comparable to some excellent optical sensors [42]. Herein, the sensing performance of Ru1 and some reported optical sensors for Hg2+ were summarized in Table 1. In addition, the selectivity of Ru1 in water toward Hg2+ over other common metal ions was investigated, respectively. Herein, K+, Na+, Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Cu2+, Ag+, Pb2+, Co2+, Ni2+, and Al3+ were added to the aqueous solution of Ru1. As shown in Fig. 3 and Fig. S6, the presence of these metal ions didn't cause any significant absorption changes. However, when Hg2+ was subsequently added into the abovementioned solutions, the great absorption changes were observed along with the solution colors changing from red to yellow (Fig. 3 and Fig. S7). The results indicate that the sensing of Hg2+ by Ru1 is hardly affected by these commonly coexistent ions. Therefore, Ru1 can be used as a highly selective colorimetric chemo-sensor for Hg2+.
carboxyl group disappears. The results are in accordance with the successful graft of Ru1 onto membrane. Then, the SEM morphology and elements mapping of membrane sensor 1 were investigated. As shown in Fig. S8, it can be seen that elements Ru and S are distributed in the membrane, though the quantity is rather subtle. Moreover, the atomic ratio of ruthenium and sulfur in the sample is close to 1:1 (Table S1), which is basically consistent with the atomic ratio of the two elements in sensor 1. To further determine the elemental composition and identify their chemical states of sensor 1, XPS measurements were performed. As shown in Fig. 5 and Fig. S9, the binding energy of 169 eV is assigned as S2p of thiophene group in Ru1 molecule. Herein, a state of N 1s with binding energy of 399.7 eV should be attributed to pyridine species, while the peak at 402.6 ev could be assigned as the positively charged nitrogen. In Fig. S9, the peak at 284.8eV is attributed to C1s of C–C, C_C or C–H species. The peaks at 288.7and 286.4eV can be assigned as C1s from−C_O and C–OH, respectively. All these results indicated that the dye Ru1 has been grafted onto membrane. Fig. 6 displays the DSC curves of polymer membrane (curve a) and sensor 1 (curve b). The curves demonstrate that the glass transition temperature (Tg) of the membrane is 151 °C. However, the Tg of sensor 1 is 90 °C. This decrease suggests that the introduction of Ru1 might play a role in internal plasticization, increasing the distance between the molecular chains, which is beneficial to the movement of the segments.
3.2. Synthesis and characterization of sensor 1 Herein, methyl acrylate and glycidyl methacrylate was maintained at different ratios. It was finally determined that the optimal molar ratio of the two monomers was 1:9, which afforded a transparent and colorless membrane on flat plate of polytetrafluoroethylene. Then, Ru1 was grafted to the polymer chains of membrane by ring opening reaction of epoxy group and carboxyl group to afford a macromolecular sensor 1, which was characterized by FT-IR, SEM, and XPS spectra. Fig. 4 depicts the FT-IR spectra of membrane, Ru1 and sensor 1, respectively. The characteristic peaks of Ru1 appearing at 3417.2 and 1730.1 cm−1 are attributed to hydroxyl vibration and carbonyl vibration in the carboxyl group. In addition, peaks at 2924.5 cm−1 belong to the stretching vibration of the alkyl group. When Ru1 is grafted onto the membrane, the broad peak at 3417.2 cm−1 attributed to hydroxyl vibration in the
3.3. Sensitivity and absorption of sensor 1 to Hg2+ ions in aqueous solution Sensor 1 was placed in a container, ultrasonically treated with 1 mL of a strong polar solvent (such as acetonitrile) for 30 s. Subsequently, the sensor 1 was changed from a sheet-like solid to a thin membrane, and taken out. As shown in Fig. 7, after the dye Ru1 being grafted, the clear, colorless membrane turns to dark-red. However, when Hg2+ ions were added to the surface of sensor 1, it turned from deep red to yellow immediately. This apparent color change of sensor 1 demonstrates that it can be used for recognition of Hg2+ ions by naked eyes. In general, small molecular sensors only can detect heavy metal ions, but cannot remove them due to their solubility. Herein, several pieces of membranes of sensor 1 with
Fig. 5. XPS spectra of N1s and S2p for sensor 1, respectively.
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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Fig. 7. The color changes of membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Concentration changes of mercury ions treated by sensor 1. Sample Hg2+ concentration C0 (mg/L)
Hg2+ concentration after adsorption C (mg/L)
Adsorption efficiency (%)
1 2 3 4 5
24.35 10.84 77.50 227.05 488.32
51.30 89.16 61.25 43.24 38.96
50 100 200 400 800
uniform length and width were dipped into mercury ions solutions with different gradient concentrations. After being kept in Hg2+ solutions for 24 h, the membranes of sensor 1 are taken out and followed by measuring concentrations of mercury ion solutions on atomic absorption spectrometer (AAS). As shown in Table 2, when initial concentration of mercury ions is 100 mg/L, as high as 89.16% of Hg2+ ions were adsorbed by the membrane. Apparently, sensor 1 could be used to remove mercury ion from solutions.
4. Conclusion In summary, a new cyclometallated ruthenium complex with good absorptions in UV–Vis–NIR region was synthesized and characterized, which displayed a high sensitivity and selectivity to Hg2+. After being grafted onto an easily-prepared membrane, the resulting membrane can be used as Hg2+-sensitive sensor, which can recognize Hg2+ by color change from red to yellow when the membrane was dipped into Hg2+ solution. Moreover, the membrane exhibited a good adsorbing ability to Hg2+, demonstrating that this membrane may be used as a mercury ion filter. Acknowledgement We are grateful for sponsor-id="https://doi.org/10.13039/ 501100001809" xlink:role="http://www.elsevier.com/xml/linkingroles/grant-sponsor" xlink:type="simple">National Science Foundation of China (21971258), the sponsor-id="https://doi.org/10.13039/ 501100012226" xlink:role="http://www.elsevier.com/xml/linkingroles/grant-sponsor" xlink:type="simple">Fundamental Research Funds for the Central Universities, South-Central University for Nationalities, China (CZY18009, CZY18011). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.117541. References [1] J.O. Duruibe, M.O. COgwuegbu, J.N. Egwurugwu, Heavy metal pollution and human biotoxic effects, Int. J. Phys. Sci. 2 (5) (2007) 112–118. [2] P.B. Tchounwou, C.G. Yedjou, A.K. Patlolla, D.J. Sutton, Heavy metal toxicity and the environment, EXS 101 (2012) 133–164. [3] L. Järup, Hazards of heavy metal contamination, Br. Med. Bull. 68 (1) (2003) 167–182. [4] M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang, D. Zhu, Visible near-infrared chemosensor for mercury ion, Org. Lett. 10 (7) (2008) 1481–1484. [5] X. Jia, D. Gong, Y. Han, C. Wei, T. Duan, H. Chen, Fast speciation of mercury in seawater by short-column high-performance liquid chromatography hyphenated to inductively coupled plasma spectrometry after on-line cation exchange column preconcentration, Talanta 88 (2012) 724–729. [6] E.M. Nolan, S.J. Lippard, Tools and tactics for the optical detection of mercuric ion, Chem. Rev. 108 (9) (2008) 3443–3480.
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A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions Yuhao Wu a, Xinjian Cheng b, ∗∗, Chaoyi Xie a, Kang Du a, Xianghong Li a,c, ∗, Dingguo Tang a a Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education, Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan, 430074, China b School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan, 430073, China c Key Laboratory of Analytical Chemistry of State Ethnic Affairs Commission, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan, 430074, China
a r t i c l e
i n f o
Article history: Received 3 August 2019 Received in revised form 13 September 2019 Accepted 16 September 2019 Available online xxxx Keywords: Cyclometallated ruthenium complex 2-(2-Thienyl)pyridine Polymer membrane Mercury(II) ions
a b s t r a c t A new cyclometallated ruthenium complex (Ru1) involving a 2-(2-thienyl)pyridine and a benzo[e]indolium block connected with a hexanoic acid was successfully synthesized and characterized, which exhibited the high sensitivity and selectivity to Hg2+ over other common metal ions with the detection limit of as low as 0.053 μM in aqueous system. Then, it was grafted onto a polymer membrane to afford a Hg2+-sensitive membrane (sensor 1), which was characterized by FT-IR, SEM and XPS spectra, respectively. When sensor 1 was dipped into the aqueous solution of Hg2+ ions, the color of the membrane changed from dark-red to yellow, which could be observed by naked eyes easily. It should be noted that the membrane can absorb Hg2+ ions well in aqueous solution and the adsorption capacity of this polymer membrane for Hg2+ ions was determined by atomic absorption spectroscopy, indicating that it also could be used as a potential material for removal of Hg2 + ions. © 2019 Published by Elsevier B.V.
1. Introduction In recent years, detections of heavy metal ions have attracted much attention due to the water pollution [1]. Among those metal ions, mercury ion is the most toxic [2–4]. Mercury ions are usually accumulated from the environment, and ultimately ingested by animals and humans. They can easily pass through the biological membranes and cause serious damage to the central nervous system and endocrine system [3,5,6]. In general, detections of mercury ions require large instruments such as atomic fluorescence spectrometry (AFS) [5], atomic absorption spectroscopy (AAS) [7] and inductively coupled plasma atomic mass spectrometry (ICP-MS) [8]. However, these methods are laborious, time-consuming, and elaboratelyoperated with complicated instruments. Optical chemo-sensors based on color and spectral changes in absorptions or emissions have been widely concerned in detections of Hg 2+ ions due to their simplicity, rapid response and visualization without aids of
∗ Corresponding author. Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education& Hubei Key Laboratory of Catalysis and Materials Science, SouthCentral University for Nationalities, Wuhan, 430074, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Cheng), [email protected] (X. Li).
instruments [4,9–16]. Among them, heavy-metal polypyridyl complexes are favored because they have much larger stoke-shifts and longer luminescence lifetime than organic small molecules [12–16]. Recently, cyclometallated ruthenium complexes have drawn much attention, which replace a nitrogen atom in a pyridine of a ruthenium polypyridyl complex with an anionic carbon center [17–20]. By virtue of the formation of Ru–C, their MLCT absorptions shift to long-wavelength range in comparison with ruthenium polypyridyl complexes [17]. The absorption spectral characteristics allow them exhibit great spectral changes when they utilize specific recognition units to interact with analytes [20–23]. Therefore, the development of a new cyclometallated complex to detect Hg2+ is attractive. As is well known, water is a relatively poor Lewis base and a strong hydrogen bonding agent, which exhibits such strong competitions with chemo-sensors that the sensitivity of chemo-sensors to Hg2+ ions is reduced. Therefore, some biologically toxic solvents are needed to make these chemo-sensors work well, such as acetonitrile, ethanol and dimethyl sulfoxide (DMSO) [9–11,14–16,24,25]. To improve the sensitivity of chemo-sensors in water, numerous efforts have been made on synthesizing new molecules with good water-solubility [6,12,23,26–28] and designing some hybrid-materials [29–31]. To the best of our knowledge, it is difficult to afford water-soluble molecules with hydrophilic blocks during purification. So many works turn to
https://doi.org/10.1016/j.saa.2019.117541 1386-1425/© 2019 Published by Elsevier B.V.
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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Scheme 1. Synthetic route of Ru1 and sensor 1.
focus on the modified gold nanoparticles [32], silver nanoparticles [33], inorganic-organic hybrid nanomaterials [30,31,34,35], and polymers [27,36–40]. These materials have more or less problems such as high cost, complicated synthesis steps and harsh conditions, which limit their popularity in practical applications. Herein, a new cylometallated complex with a hexanoic acid (Ru1, see Scheme 1) has been successfully synthesized and used as a colorimetric sensor to detect Hg2+ ions with high sensitivity and selectivity in water. Furthermore, the complex was grafted onto an easilyprepared polymer membrane to afford a solid-state sensor 1 by virtue of the hexanoic acid in the complex (Scheme 1). The sensor 1 can promptly recognize Hg2+ with the membrane color changing from dark-red to yellow as well as efficiently absorb Hg2+ ions, which display a potential application in detection and removal of Hg2+.
2. Experimental section 2.1. Materials and characterization All solvents were commercially available and used after distillation. [Ru(cymene)Cl2]2 (dichloro(p-cymene)ruthenium(II) dimer), and 2-(5-formyl-2-thienyl)pyridine (Hfthpy) were purchased from Changcheng reagent company. Methyl acrylate, and glycidyl methacrylate stabilized with MEHQ were purchased from J&K China Chemical Ltd. Methyl acrylate was used after being treated with NaOH aqueous solution (5%) to remove the inhibitor MEHQ. Ethyl acetate was dried by anhydrous calcium chloride overnight. 3-(5-Carboxyl)pentyl-1,1,2-
2
Fig. 1. Absorption intensity ratio (A410 nm/A503 nm) of Ru1 in the presence or absence of Hg at different pH values. Herein, each spectrum of absorption was recorded at room temperature after a 5 min delay.
+
trimethylbenzo[e]indolium was prepared according to reference [41] and characterized by 1H NMR spectrum. 1 H NMR spectra and 13C NMR spectra were measured in DMSO‑d6 on a TCI IIITM 600 MHz (Bruker) spectrometer and MS spectra were measured on a MALDI-TOF/TOF 5800 (AB SCIEX) spectrometer, respectively. Fourier transform infrared spectra were taken on a Nicolet NEXUS 470 spectrometer with the sample dried and pressed into KBr pellets. Xray photoelectron spectroscopy (XPS) was performed on a XPS VG Multilab 2000X and the sample was tested under ultra-high vacuum conditions. A scanning electron microscope (SEM, HITACHI SU8010) was used to obtain the surface topography of the sample operating at 25 kV. After the surface of the sample was plated with platinum to enhance its conductivity, it was placed on an aluminum-based sample stage for SEM observation and EDS-mapping. Differential Scanning Calorimeter (DSC) was carried out on a DSC 200F3, NETZSCH (Germany) under a stream of nitrogen. The vacuum dried samples were heated from 30 to 210 °C at a scan speed of 10 K min−1. Atomic absorption spectroscopy was recorded by using an AAS ICE 3500 (Thermo Fisher Scientific) spectrophotometer. Absorption spectra were obtained on a Cary Series UV–Vis–NIR spectrophotometer. 2.2. Synthesis of Ru1 Synthesis of [Ru(bpy)2(fthpy)]PF6: A mixture of 2-(5-formyl-2thienyl)pyridine (Hfthpy) (0.15 g, 0.78 mmol), KPF6 (0.2937 g, 1.6 mmol), acetonitrile (12 mL), triethylamine (0.35 mL) were added
Fig. 2. The absorption changes of Ru1 (10 μM) in the presence of different concentrations of Hg2+ in HEPES (5% DMSO, pH 7.0, 10 mM). Insets: a) The photographs of solution color before and after the addition of Hg2+ in the solution of Ru1; b) The titration curve of Ru1 and Hg2+. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
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to a 100 mL three-necked flask. The reaction system was deoxygenated with argon at room temperature for 10 min and then [Ru(cymene)Cl2]2 (0.3 g, 0.5 mmol) was added. After the mixture was refluxed at 55 °C for 16 h under argon, the solvent was evaporated to obtain orange-red solid. Then, 2,2′-bipyridine (0.25 g, 1.57 mmol) and methanol (12 mL) were added subsequently to the above mixture. After the resulting mixture was refluxed at 70 °C for another 12 h under argon, the solvent was evaporated and the residue was purified by silica column chromatography by using CH2Cl2/MeCN (10/1, v/v) as the eluent. Yield: 0.28 g, 59%. 1H NMR (400 MHz, CD3CN) δ: 9.80 (s, 1H), 8.48 (d, J = 8.2 Hz, 1H), 8.41 (d, J = 8.2 Hz, 1H), 8.35 (t, J = 7.7 Hz, 2H), 8.07–7.97 (m, 2H), 7.93–7.77 (m, 7H), 7.71 (t, J = 7.8 Hz,1H), 7.55 (d, J = 5.7 Hz, 1H), 7.45 (t, J = 7.4 Hz, 1H), 7.28 (m, J = 6.5 Hz, 3H), 7.12 (s, 1H), 6.94 (t, J = 6.5 Hz, 1H). 13C NMR (100 MHz, CD3CN) δ: 184.12, 162.22, 157.66, 157.17, 156.87, 155.54, 154.71, 151.06, 150.98, 150.61, 149.42, 146.21, 143.84, 136.62, 136.32, 135.34, 134.49, 134.30, 127.08, 126.53, 126.51, 126.39, 126.26, 123.44, 123.29, 123.05, 122.93, 122.17, 119.69, 117.27. MS m/z: Calculated for 602.06 (M+), Found: 602.04. Synthesis of Ru1: [Ru(bpy)2(fthpy)]PF6 (0.06 g, 0.11 mmol) and 3-(5carboxyl)pentyl-1,1,2-trimethylbenzo[e]indolium (0.04 g, 0.13 mmol) were added into ethanol (10 mL). The mixture was stirred and heated at 80 °C in the dark for 12 h. After the solvent was evaporated, the residue was dissolved in CH2Cl2 and chromatographed by using MeCN/ MeOH/saturated potassium nitrate/H2O as the eluent. Then, the reddish brown solid was collected. Yield: 0.04 g, 40%. 1H NMR (600 MHz, DMSO‑d6) δ: 12.04 (s, 1H), 8.81 (d, J = 8.2 Hz, 1H), 8.78–8.65 (m, 4H), 8.34 (d, J = 8.6 Hz, 1H), 8.25 (d, J = 8.9 Hz, 1H), 8.19 (d, J = 8.2 Hz, 1H), 8.13 (t, J = 7.9 Hz, 1H), 8.08 (d, J = 9.0 Hz, 1H), 8.00 (m, 4H), 7.92 (d, J = 7.9 Hz, 1H), 7.84–7.67 (m, 6H), 7.59 (t, J = 6.5 Hz, 1H), 7.53–7.42 (m, 4H), 7.35 (s, 1H), 7.11–7.03 (m, 2H), 4.65 (t, J = 6.0 Hz,2H), 2.23 (t, J = 8.0 Hz, 2H), 1.95 (s, 3H), 1.91 (s, 3H), 1.86 (t, J = 7.7 Hz, 2H), 1.59 (q, J = 7.5 Hz, 2H), 1.45 (q, J = 7.8 Hz, 2H); 13C NMR (151 MHz, DMSO‑d6) δ: 196.88, 181.16, 174.80, 161.26, 157.64, 157.13, 156.79, 155.46, 154.62, 151.39, 150.85, 150.57, 149.28, 149.02, 147.66, 145.76, 144.97, 139.03, 138.10, 137.48, 137.11, 136.25, 135.40, 135.21, 133.33, 131.49, 130.52, 128.85, 128.07, 127.47, 127.41, 127.35, 127.26, 124.40, 124.18, 123.93, 123.88, 123.47, 123.40, 121.09, 113.46, 109.31, 53.72, 46.32, 40.49, 33.84, 28.45, 26.17, 26.14, 25.85, 24.52; MS (Maldi-TOF, CHCA) m/z: Calculated for 907.24 (M-H+), Found: 907.22. 2.3. Preparationof sensor 1 Preparation of membrane: Under nitrogen atmosphere, methyl acrylate (7.74 g, 0.09 mol) was ultrasonically dispersed in ethyl acetate (45 mL) and heated to 60 °C. Azodiisobutyronitrile (AIBN, 0.1832 g) dispersed in ethyl acetate (5 mL) was then added. After stirring at 60 °C for 1 h, glycidyl methacrylate (1.42 g, 0.05 mol) was added. The mixture was stirred overnight to afford a viscous liquid, which was transferred to a flat plate of polytetrafluoroethylene (PTFE) and dried in air. The unreacted monomers were removed by Soxhlet extraction with ethanol and the remaining solid was dissolved in chloroform and spread out to obtain a transparent membrane. Grafting Ru1 onto the membrane to prepare sensor 1: The preprepared membrane was dispersed in 10 mL of DMF and transferred into a 50 mL three-necked flask, and then 20 mg of Ru1 was added. The mixture was deoxygenated with argon at room temperature for
3
Fig. 3. UV–visible responses of Ru1 (10 μM) to various metal ions in HEPES (5% DMSO, pH 7.0, 10 mM). Bars represent the absorption intensity A410nm/A503nm.
10 min and PPh3 (1 mg) was added into the mixture as a catalyst. The reaction was heated to 120 °C and kept at the temperature for 12 h. After DMF was evaporated under reduced pressure, resultant product was collected and then washed with ethanol for 24 h in a Soxhlet extractor to remove the unreacted complexes to afford sensor 1. 3. Results and discussion 3.1. Characterization of Ru1 and its sensitive to Hg2+ Herein, Ru1 was synthesized from Ru(bpy)2(fthpy)+ and characterized by 1H NMR, C NMR and MS spectra (Fig. S1–S3). As shown in Fig. S1, a single peak at δ 12.04 ppm could be attributed to proton of carboxyl. In the aliphatic region, the proton signals at δ 4.65 ppm are assigned to N+CH2−. In Fig. S2, it can be found that the peak at δ 196.88 ppm could be attributed to the carbon atom connected to the central Ru(II), while the peak of the carbon connected to the N+ ion appeared at δ 181.16 ppm. The signal located at δ 174.80 ppm is assigned to the carbon on the carboxyl group. Moreover, Ru1 was further characterized by MS spectrum in which the peak occurring at m/z 907.22 (calculated m/z 907.24) corresponds to [M−H+]+. Then, the pH effect on the interactions between Hg2+ and Ru1 was investigated (Fig. 1). The results showed that Ru1 is relatively stable at the range of pH 1.0–12.0 when it was incubated for 5 min. Upon the addition of Hg2+ to the above solutions of different pH values, Ru1 displayed a good response to Hg2+ between 6 and 7. As far as we know, carboxylic acids prefer to being converted into carboxylates at pH 6–7, which provide good water-solubility and better chances for Ru1 to interact with Hg2+ by coordination with oxygen from carboxylate. Moreover, the sulfur atom from the thienyl group could also be inclined to combine with Hg2+ due to HSAB theory [14,16,23]. As a result, the HEPES buffer at pH 7.0 was chosen as the sensing system. The absorption spectra of Ru1 in HEPES buffer (5% DMSO) were investigated. It can be seen from Fig. 2 that Ru1 displays several absorption bands in the UV–Vis–NIR regions. The bands occurring at 295 and 352 nm are assigned as ligand-centered (LC) charge transfer transitions, while the strong band with a maximum at 503 nm (ε = 1.64 × 104 M−1 cm−1) is attributed to dπ(Ru)→π*CˆN MLCT transition [17–20]. It should be noted that there was a broad absorption band centered at 750 nm (ε = 2.05 × 103 M−1 cm−1) in the range from 600 to 1000 nm, which can be assigned as dπ(Ru)→π*NˆN MLCT absorption. Obviously, these are significantly red-shifted compared 2+ to Ru(bpy)2+ (Hg(NO3)2, HNO3), the intensity of the absorp3 . Upon the addition of Hg tion band between 450 and 1000 nm was gradually weakened. Meanwhile, the absorption band centered at 503 nm shifted from 503 to 484 nm along with a new peak appearing at 410 nm, which resulted in the solution color change from red to yellow (Fig. 1, inset a). It can be seen that when the amount of Hg2+ reached at about 50 μM, the interaction between Ru1 and Hg2+ reach equilibrium (Fig. 2, inset b). The Job's plot experiment was 13
Table 1 Physiochemical properties of some optical chemo-sensors for Hg2+.
Ru1 Rhodamine B hydrazide derivative 9 Ir–S16 [Ir(TPQ)2(4-EO2-pic)]14 Ferrocene derivative15 Ru1–B23
Change of λabs,max/nm
Change of λem,max/nm
Medium
LOD/method
503 → 410 565↑ 420↑ 483 → 442 322↓ 560 → 440
– 588↑ 590 → 540 620↓ 437↑ –
DMSO/HEPES (5:95, v/v, 10 mM, pH 7.0) DMSO/H2O (1:9, v/v) CH3CN–H2O (98:2, v/v) CH3CN/H2O (1:1, v/v) CH3CN/H2O solution (2:8, v/v). HEPES (10 mM, pH 7.14)
5.3 × 10−8 M (A412 nm/A503 nm) 2.71 × 10−7 M (I588 nm) 4.1 × 10−8 (I540 nm/I590 nm) 1.78 × 10−8 M (I620 nm) 1.56 × 10−8 M (I437 nm) 5.9 × 10−7 M (A560 nm)
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Fig. 4. FT-IR spectra of membrane, Ru1 and sensor 1, respectively.
Fig. 6. DSC curves of membrane (a) and sensor 1 (b), respectively.
then carried out to identify that the binding stoichiometry between Ru1 and Hg2+ in solution is close to 1:1 (Fig. S4). According to these results, it can be speculated that Hg2+ can interact with Ru1 by coordination with the oxygen of carboxylate and the sulfur of thienyl in the complex. And this coordination could lead to a change in the electron distribution in the Ru center, which may result in the shift and decrease of the MLCT maximum. Then, the detection limit of Ru1 for Hg2+ in water was calculated to be as low as 0.053 μM (Fig. S5), which was comparable to some excellent optical sensors [42]. Herein, the sensing performance of Ru1 and some reported optical sensors for Hg2+ were summarized in Table 1. In addition, the selectivity of Ru1 in water toward Hg2+ over other common metal ions was investigated, respectively. Herein, K+, Na+, Mg2+, Ba2+, Zn2+, Cd2+, Mn2+, Cu2+, Ag+, Pb2+, Co2+, Ni2+, and Al3+ were added to the aqueous solution of Ru1. As shown in Fig. 3 and Fig. S6, the presence of these metal ions didn't cause any significant absorption changes. However, when Hg2+ was subsequently added into the abovementioned solutions, the great absorption changes were observed along with the solution colors changing from red to yellow (Fig. 3 and Fig. S7). The results indicate that the sensing of Hg2+ by Ru1 is hardly affected by these commonly coexistent ions. Therefore, Ru1 can be used as a highly selective colorimetric chemo-sensor for Hg2+.
carboxyl group disappears. The results are in accordance with the successful graft of Ru1 onto membrane. Then, the SEM morphology and elements mapping of membrane sensor 1 were investigated. As shown in Fig. S8, it can be seen that elements Ru and S are distributed in the membrane, though the quantity is rather subtle. Moreover, the atomic ratio of ruthenium and sulfur in the sample is close to 1:1 (Table S1), which is basically consistent with the atomic ratio of the two elements in sensor 1. To further determine the elemental composition and identify their chemical states of sensor 1, XPS measurements were performed. As shown in Fig. 5 and Fig. S9, the binding energy of 169 eV is assigned as S2p of thiophene group in Ru1 molecule. Herein, a state of N 1s with binding energy of 399.7 eV should be attributed to pyridine species, while the peak at 402.6 ev could be assigned as the positively charged nitrogen. In Fig. S9, the peak at 284.8eV is attributed to C1s of C–C, C_C or C–H species. The peaks at 288.7and 286.4eV can be assigned as C1s from−C_O and C–OH, respectively. All these results indicated that the dye Ru1 has been grafted onto membrane. Fig. 6 displays the DSC curves of polymer membrane (curve a) and sensor 1 (curve b). The curves demonstrate that the glass transition temperature (Tg) of the membrane is 151 °C. However, the Tg of sensor 1 is 90 °C. This decrease suggests that the introduction of Ru1 might play a role in internal plasticization, increasing the distance between the molecular chains, which is beneficial to the movement of the segments.
3.2. Synthesis and characterization of sensor 1 Herein, methyl acrylate and glycidyl methacrylate was maintained at different ratios. It was finally determined that the optimal molar ratio of the two monomers was 1:9, which afforded a transparent and colorless membrane on flat plate of polytetrafluoroethylene. Then, Ru1 was grafted to the polymer chains of membrane by ring opening reaction of epoxy group and carboxyl group to afford a macromolecular sensor 1, which was characterized by FT-IR, SEM, and XPS spectra. Fig. 4 depicts the FT-IR spectra of membrane, Ru1 and sensor 1, respectively. The characteristic peaks of Ru1 appearing at 3417.2 and 1730.1 cm−1 are attributed to hydroxyl vibration and carbonyl vibration in the carboxyl group. In addition, peaks at 2924.5 cm−1 belong to the stretching vibration of the alkyl group. When Ru1 is grafted onto the membrane, the broad peak at 3417.2 cm−1 attributed to hydroxyl vibration in the
3.3. Sensitivity and absorption of sensor 1 to Hg2+ ions in aqueous solution Sensor 1 was placed in a container, ultrasonically treated with 1 mL of a strong polar solvent (such as acetonitrile) for 30 s. Subsequently, the sensor 1 was changed from a sheet-like solid to a thin membrane, and taken out. As shown in Fig. 7, after the dye Ru1 being grafted, the clear, colorless membrane turns to dark-red. However, when Hg2+ ions were added to the surface of sensor 1, it turned from deep red to yellow immediately. This apparent color change of sensor 1 demonstrates that it can be used for recognition of Hg2+ ions by naked eyes. In general, small molecular sensors only can detect heavy metal ions, but cannot remove them due to their solubility. Herein, several pieces of membranes of sensor 1 with
Fig. 5. XPS spectra of N1s and S2p for sensor 1, respectively.
Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541
Y. Wu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx
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Fig. 7. The color changes of membranes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 2 Concentration changes of mercury ions treated by sensor 1. Sample Hg2+ concentration C0 (mg/L)
Hg2+ concentration after adsorption C (mg/L)
Adsorption efficiency (%)
1 2 3 4 5
24.35 10.84 77.50 227.05 488.32
51.30 89.16 61.25 43.24 38.96
50 100 200 400 800
uniform length and width were dipped into mercury ions solutions with different gradient concentrations. After being kept in Hg2+ solutions for 24 h, the membranes of sensor 1 are taken out and followed by measuring concentrations of mercury ion solutions on atomic absorption spectrometer (AAS). As shown in Table 2, when initial concentration of mercury ions is 100 mg/L, as high as 89.16% of Hg2+ ions were adsorbed by the membrane. Apparently, sensor 1 could be used to remove mercury ion from solutions.
4. Conclusion In summary, a new cyclometallated ruthenium complex with good absorptions in UV–Vis–NIR region was synthesized and characterized, which displayed a high sensitivity and selectivity to Hg2+. After being grafted onto an easily-prepared membrane, the resulting membrane can be used as Hg2+-sensitive sensor, which can recognize Hg2+ by color change from red to yellow when the membrane was dipped into Hg2+ solution. Moreover, the membrane exhibited a good adsorbing ability to Hg2+, demonstrating that this membrane may be used as a mercury ion filter. Acknowledgement We are grateful for sponsor-id="https://doi.org/10.13039/ 501100001809" xlink:role="http://www.elsevier.com/xml/linkingroles/grant-sponsor" xlink:type="simple">National Science Foundation of China (21971258), the sponsor-id="https://doi.org/10.13039/ 501100012226" xlink:role="http://www.elsevier.com/xml/linkingroles/grant-sponsor" xlink:type="simple">Fundamental Research Funds for the Central Universities, South-Central University for Nationalities, China (CZY18009, CZY18011). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.117541. References [1] J.O. Duruibe, M.O. COgwuegbu, J.N. Egwurugwu, Heavy metal pollution and human biotoxic effects, Int. J. Phys. Sci. 2 (5) (2007) 112–118. [2] P.B. Tchounwou, C.G. Yedjou, A.K. Patlolla, D.J. Sutton, Heavy metal toxicity and the environment, EXS 101 (2012) 133–164. [3] L. Järup, Hazards of heavy metal contamination, Br. Med. Bull. 68 (1) (2003) 167–182. [4] M. Zhu, M. Yuan, X. Liu, J. Xu, J. Lv, C. Huang, H. Liu, Y. Li, S. Wang, D. Zhu, Visible near-infrared chemosensor for mercury ion, Org. Lett. 10 (7) (2008) 1481–1484. [5] X. Jia, D. Gong, Y. Han, C. Wei, T. Duan, H. Chen, Fast speciation of mercury in seawater by short-column high-performance liquid chromatography hyphenated to inductively coupled plasma spectrometry after on-line cation exchange column preconcentration, Talanta 88 (2012) 724–729. [6] E.M. Nolan, S.J. Lippard, Tools and tactics for the optical detection of mercuric ion, Chem. Rev. 108 (9) (2008) 3443–3480.
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Please cite this article as: Y. Wu, X. Cheng, C. Xie, et al., A polymer membrane tethered with a cycloruthenated complex for colorimetric detection of Hg2+ ions, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117541