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A highly selective probe for fluorescence turn-on detection of Fe3+ ion based on a novel spiropyran derivative
Journal Pre-proof A highly selective probe for fluorescence turn-on detection of Fe novel spiropyran derivative
3+
ion based on a
Ruiqing Zhang, Luping Hu, Zhenxiang Xu, Yanxi Song, Hongqi Li, Xin Zhang, Xucheng Gao, Mengxuan Wang, Chunying Xian PII:
S0022-2860(19)31590-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127481
Reference:
MOLSTR 127481
To appear in:
Journal of Molecular Structure
Received Date: 22 October 2019 Revised Date:
21 November 2019
Accepted Date: 25 November 2019
Please cite this article as: R. Zhang, L. Hu, Z. Xu, Y. Song, H. Li, X. Zhang, X. Gao, M. Wang, C. Xian, 3+ A highly selective probe for fluorescence turn-on detection of Fe ion based on a novel spiropyran derivative, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127481. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A highly selective probe for fluorescence turn-on detection of Fe3+ ion based on a novel spiropyran derivative Ruiqing Zhanga, Luping Hua, Zhenxiang Xub, Yanxi Songc, Hongqi Lia,*, Xin Zhanga, Xucheng Gaoa, Mengxuan Wanga, Chunying Xiana a Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, 2999 North Renmin Road, Shanghai 201620, P. R. China. b Penglai Xinguang Pigment Chemical Co., Ltd, Penglai 265601, P. R. China c School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, P. R. China.
ABSTRACT A new spiropyran bearing multiple phenolic hydroxyl groups is synthesized. It acts as a chemosensor for selective detection of Fe3+ ions in both organic and aqueous solution. The optimal pH range for Fe3+ fluorescent detection by the sensor is pH 4.5‒10.5. Upon addition of Fe3+ to the sensor solution the fluorescence emission intensity is enhanced by around 50 fold. Other common competitive metal ions including Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+ and Cr3+ do not substantially interfere with the selective detection of Fe3+ by the sensor. Changes in the fluorescence emission intensity with the equivalent ratio of Fe3+/sensor exhibits an approximate linear relationship, from which the concentration of Fe3+ ion may be estimated. The detection limit of Fe3+ ion by the sensor is measured to be 1.93 × 10–7 M. A plausible sensing mechanism for selective Fe3+ ion detection by the sensor is proposed based on the Job’s plot measurement. Keywords: Fluorescence; probe; spiropyran; Fe3+
1. Introduction As a kind of transitional metal, iron is the metal element of the second highest crust content and is also the essential trace element in human body. It mainly exists in the form of hemoglobin and has great effect in oxygen supply of human body. On the other hand, excessive intake of Fe3+ may result in a disturbance of physiological function and several diseases [1‒4]. Therefore convenient and effective identification and detection of Fe3+ ions in the organism and in the environment is of high significance. In this respect selective and sensitive fluorescent probes for rapid detection of Fe3+ ions aroused much interest from scientists and a number of fluorescent Fe3+ probes have been developed by utilizing various fluorogenic platforms such as coumarin [5‒9], rhodamine [10‒12], 1,8-naphthalimide [13‒16],
pillar[5]arene [17,18], quantum dots [19‒21], and metal-organic frameworks (MOFs) als [22‒24]. Chemosensors for fluorescent or colorimetric detection of Fe3+ ions based on photochromic spiropyran scaffold have also been reported [25‒27]. However, the selectivity of these sensors toward Fe3+ ions is substantially weakened by other trivalent ions Al3+ and Cr3+. In view that spiropyran is an extraordinarily versatile photochrome [28] and spiropyran-based fluorescent and/or colorimetric probes [29] have been developed for highly selective and sensitive detection of various analytes including pH [30‒32], Li+ [33], Mg2+ [34], Ca2+ [35], Co2+ [36], Cu2+ [37], Zn2+ [38], Hg2+ [39], Pb2+ [40], Al3+ [41], Cr3+ [42], La3+ [43], CN‒ [44], F‒ [45], Cl‒ [46], pyrophosphate [47], CO2 [48], H2O2 [49], H2S [50], H2S/SO2 [51], hydrazine [52], cysteine (Cys)/homocysteine (Hcy) [53], γ-glutamyl-cysteinyl-glycine (GSH) [54], β-amyloid peptide oligomers [55], lysozyme [56], DNA G-quadruplexes [57], aniline pollutants [58], acidic food spoilage [59], ultraviolet (UV) light intensity [60], and mechanical stress [61], we envisage to design and synthesize a fluorescent chemosensor which can detect Fe3+ ions selectively and sensitively even in the presence of other trivalent ions Al3+ and Cr3+ based on spiropyran, following our recent work concerning study on a spiropyran-appended polysiloxane which can be used for probing pH changes and Fe3+ ions and sequential detection of Ag+ and Hg2+ ions [62]. Herein we report the synthesis, characterization and recognition behavior of the new spiropyran-based fluorogenic Fe3+ sensor.
2. Experimental section 2.1. Materials and instrumentation All materials and solvents employed in this study were of analytical grade. All chemical reagents were purchased from commercial sources and used as received unless other statements. 1,2,3,3-Tetramethylindolindolenium iodide was synthesized according to literature method [63]. Melting point was determined on a WRS-2A capillary melting apparatus and the quoted temperatures were uncorrected. 1H NMR and 13C NMR spectra were recorded on a Bruker AM 400 spectrometer. DMSO-d6 or CDCl3 was used as solvent and chemical shifts recorded were internally referenced to Me4Si (0 ppm). IR spectra were obtained on a Thermo Electron Corporation Nicolet 380 FT-IR spectrophotometer from Thermo Fisher Scienific Company. Mass spectra were obtained on an Agilent 6000 LC-MS instrument (ESI, pos mode, 70 to 1000 amu). Fluorescence spectra were recorded on a FS5 spectrofluorometer from Edinburgh
Instruments Co., Ltd. UV-Vis spectra were recorded on a UV-1800 spectrophotometer from Shimadzu Instruments (Suzhou) Co., Ltd.. 2.2. Synthesis of sensor SP 1,2,3,3-Tetramethylindolindolenium iodide (6.35 g, 21 mmol) was dissolved in anhydrous methanol (100 mL) and heated to reflux. Then triethylamine (1 mL) was added and the solution was stirred for 15 min. 5-Nitrosalicylaldehyde (3.34 g, 20 mmol) was added and the mixture was refluxed for 2 h. After reaction the mixture was allowed to cool to room temperature and methanol was removed by evaporation. The formed orange solid was recrystallized with dichloromethane/ethanol. Yellow solid (5.88 g) of the target product 1,3,3-trimethyl-6´-nitrospiro[chromene-2,2´indoline] was obtained in 91% yield after filtration and drying. Mp 178–180 °C (literature [64] value: 178–179 °C). 1H NMR (400 MHz, CDCl3) δ 8.05–8.03 (m, 2H), 7.25–7.23 (m, 1H), 7.12 (dd, J = 7.3, 1.3 Hz, 1H), 6.99–6.88 (m, 2H), 6.80 (d, J = 8.6 Hz, 1H), 6.59 (d, J = 7.7 Hz, 1H), 5.89 (d, J = 10.3 Hz, 1H), 2.77 (s, 3H), 1.32 (s, 3H), 1.22 (s, 3H). 1,3,3-Trimethyl-6´-nitrospiro[chromene-2,2´-indoline] (6.45 g, 20 mmol) and SnCl2 (18.96 g, 0.10 mol) were added to a flask under N2 atmosphere. Anhydrous ethanol (100 mL) was added and the reaction mixture was heated to reflux for 2 h. After reaction the mixture was cooled to room temperature and filtered. The filtrate was concentrated and 5% aqueous NaOH was added to the residue. The organic phase was extracted with dichloromethane for three times, washed with water and dried. After filtration the solvent was removed by evaporation and the residue was purified by column chromatography with petroleum ether/ethyl acetate (3:1, v/v) as eluent to give the reduction product 1,3,3-trimethyl-6´-aminospiro[chromene-2,2´-indoline] (3.18 g) as brown solid in 54% yield. Mp 139–140 °C. 1H NMR (400 MHz, CDCl3) δ 7.16–7.14 (m, 1H), 7.05 (d, J = 8.3 Hz, 1H), 6.81–6.79 (m, 1H), 6.74 (d, J = 8.8 Hz, 1H), 6.55–6.44 (m, 3H), 6.41 (d, J = 7.9 Hz, 1H), 5.67 (d, J = 10.9 Hz, 1H), 2.85 (s, 3H), 1.45 (s, 3H), 1.30 (s, 3H). 1,3,3-Trimethyl-6´-aminospiro[chromene-2,2´-indoline] (0.73 g, 2.5 mmol) and 2,5-dihydroxybenzaldehyde (0.33 g, 2.4 mmol) were dissolved in dichloromethane (20 mL). To the solution was added 5 drops of acetic acid and the mixture was stirred at room temperature for 1 h. After reaction the mixture was washed with water and the organic layer was separated then dried. The filtration was concentrated and the residue was purified by column chromatography with petroleum ether/ethyl acetate
(5:1, v/v) as eluent to afford the target sensor SP (0.30 g) as red-brown solid in 30% yield. Mp 146–148 °C. IR (KBr pellet): ν 3318, 1644, 1609, 1481, 1450, 1270, 953 cm–1. 1H NMR (400 MHz, CDCl3) δ 8.43 (s, 1H), 7.19 (s, 1H), 7.14–7.12 (m, 2H), 7.03–7.00 (m, 3H), 6.97 (d, J = 2.5 Hz, 1H), 6.85–6.76 (m, 7H), 6.69 (d, J = 8.6 Hz, 1H), 6.47 (d, J = 7.7 Hz, 1H), 5.69 (d, J = 10.2 Hz, 1H), 2.68 (s, 3H), 1.25 (s, 3H), 1.11 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 158.49, 154.06, 152.88, 147.06, 146.78, 139.68, 135.58, 127.98, 126.62, 121.17, 120.49, 119.49, 118.55, 118.27, 118.22, 118.18, 116.85, 116.09, 114.79, 105.83, 103.63, 50.85, 27.91, 24.85, 19.12. MS (ESI): m/z 413 (M+ + 1). 2.3. General UV-vis and fluorescence spectra measurements All spectral analyses were accomplished at room temperature in 0.1 M Tris-HCl buffer solution (DMF/H2O = 9:1, v/v, pH = 7.10). The concentration of sensor SP and metal ions was 4 × 10–5 M for UV-vis and fluorescence measurement. Sensor SP was dissolved in DMF and solutions of metal ions were prepared with nitrate or chloride salts in water. Each sample was added to a 10 mL volumetric flask by using a 20‒200 µL liquid transfer gun and the solution was well mixed before the spectra were measured. Fluorescence titration experiments were performed in 2 mL cuvette by successive addition of prepared solutions at an excitation wavelength of 378 nm with both excitation and emission slits of 5.0 nm. UV-vis absorption spectra were measured in the range of 290‒700 nm.
3. Results and discussions 3.1. Synthesis and structural characterization of sensor SP The novel spiropyran-based sensor SP was synthesized by a three-step protocol from 1,2,3,3-tetramethylindolindolenium iodide and 5-nitrosalicylaldehyde, as showed in Scheme 1. Condensation between 1,2,3,3-tetramethylindolindolenium iodide and 5-nitrosalicylaldehyde in the presence of triethylamine produced the target product 1,3,3-trimethyl-6´-nitrospiro[chromene-2,2´-indoline] in 91% yield. The nitrospiropyran was characterized by both melting point measurement and 1H NMR spectrum. The data were in good accordance with those reported in literature [64]. Reduction of the nitrospiropyran to the corresponding aminospiropyran by SnCl2 afforded a 54% yield. The structure of the obtained aminospiropyran compound was characterized by melting point measurement and 1H NMR spectrum and the spectral
data were in good accordance with those reported in literature [65]. Imination reaction between 2,5-dihydroxybenzaldehyde and the aminospiropyran catalyzed by acetic acid in dichloromethane afforded sensor SP in 30% yield. The chemical structure of the sensor SP was fully characterized by 1H NMR, 13C NMR, IR and mass spectrum. All of the data in the spectra were in good accordance with the structure. Scheme 1 3.2. Effect of medium on fluorescent detection of Fe3+ ions by sensor SP Fluorescence spectra of sensor SP and Fe3+ solution (4 × 10‒5 M) in different solvents including EtOH, MeOH, MeCN, DMF and DMSO at an excitation wavelength of 378 nm were measured and showed in Fig. 1. It was found that the solution exhibited the maximum fluorescence emission at around 450 nm and the fluorescence intensities in DMF and DMSO were much higher that those in other solvents EtOH, MeOH and MeCN. Based on the results DMF was chosen as the solvent in the following determination, considering that DMSO has a higher viscosity and strongly irritating odor though DMSO shows a little higher fluorescence intensity that that of DMF. Fig. 1 It is important for a practical metal ion probe to be usable in an aqueous medium. Therefore the fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in DMF/H2O of different ratios (DMF/H2O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v) were determined at an excitation wavelength of 378 nm and were showed in Fig. 2. It could be observed from Fig. 2 that the solution exhibited the highest fluorescence intensity in 9:1 DMF/H2O. With the increase in the ratio of water, the fluorescence emission intensity decreased sharply. The solution in 7:3 DMF/H2O was fairly fluorescent and further increase in the water content led to very weak fluorescence of the solution. Thus the optimal sensing medium was 9:1 DMF/H2O, which was used in the following determination. Fig. 2 The fluorescence spectra of sensor SP before and after addition of common metal ions (Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+, Cr3+, Fe3+) in DMF/H2O (9:1, v/v) were measured and showed in Fig. 3. The very weak fluorescence emission of the probe SP appeared at 450 nm when it was
excited at a wavelength of 378 nm. The fluorescence intensity of the probe SP was enhanced by addition of Fe3+ and Al3+ while addition of other metal ions failed to cause substantial change in the fluorescence emission. Addition of Fe3+ and Al3+ induced an enhancement in the fluorescence intensity of the probe SP by around 50 fold and 8 fold, respectively. Therefore the probe SP can be used for selective detection of Fe3+ in an aqueous medium. Fig. 3 3.3. Anti-interference ability of sensor SP for the fluorescent detection of Fe3+ ions In order to investigate the interference of competitive metal ions on the sensing selectivity of the probe SP toward Fe3+ ion, changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ ion (1 equivalent) and/or other competitive metal ions (1 equivalent) were recorded and showed in Fig. 4. It was visible that to the fluorescence turn-on probe SP for Fe3+ ion, addition of 1 equivalent of Cu2+ or Al3+ ion reduced the fluorescence intensity by about 20%. Other competitive ions did not cause obvious interference with the selective detection of Fe3+ ion. Considering that competitive metal ions may exist in a large amount and mix with the analyte such as Fe3+ ion, changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ ion (1 equivalent) and other competitive metal ions (5 equivalent) were measured and showed in Fig. 5. In this case the excessive Al3+, Hg2+ and Fe2+ ions interfered with the selective detection of Fe3+ by sensor SP, bringing about decrease in the fluorescence intensity by 40‒43%. While the presence of excessive Cu2+ ion caused serious interference with the selective detection of Fe3+ by sensor SP, reducing the fluorescence intensity by nearly 84%. Other competitive ions caused relatively small interference to the selective detection of Fe3+ ion. Fig. 4 Fig. 5 The real samples especially waste water samples usually contain multiple metal ions. Therefore the anti-interference ability to multiply mixed metal ions of a sensor for a specific metal ion reflects the ‘real’ anti-interference ability of the sensor. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and 5 kinds of other competitive metal ions together (each 1 equivalent) in
DMF/H2O (9:1, v/v) were determined and showed in Fig. 6. Groups containing 5 kinds of competitive metal ions for anti-interference test were picked up at random on the premise to ensure each group contains a monovalent and a trivalent metal ion. The following groups of metal ions were chosen for the test: Li+, Hg2+, Zn2+, Ni2+ and Al3+ in a group; Na+, Cu2+, Ca2+, Mg2+ and Cr3+ in a group; K+, Fe2+, Mn2+, Sr2+ and Al3+ in a group; Ag+, Zn2+, Co2+, Fe2+ and Cr3+ in a group; and Ag+, Cu2+, Sr2+, Mn2+ and Al3+ in a group. It can be seen that each group containing 5 kinds of competitive metal ions exhibits only a little decrease in the fluorescence intensity compared to the sensor SP/Fe3+ system, indicating that the multiply mixed competitive metal ions, containing Cu2+ or Fe2+ alone, or Hg2+/Al3+, Fe2+/Al3+, and Cu2+/Al3+ together, had negligible interference to the selective detection of Fe3+ ion by sensor SP. The paramagnetic property of Cu2+ would bring about a fluorescence quenching effect and Fe2+/Hg2+ ions also undergo electron or energy transfer to result in a certain degree of fluorescence quenching. Generally the fluorescence quenching phenomenon is positively correlated with the concentration of the quenching ions [66‒69]. This is why Cu2+, Hg2+ and Fe2+ ions cause significant decrease in the fluorescence intensity when these ions are present alone in 5-fold equivalent while the mixed metal ions (each having a much lower concentration) only bring about a little interference to the fluorescence intensity. The enhancement in fluorescence intensity of the probe SP by Fe3+ and Al3+ reaches about 50 fold and 8 fold, respectively. When Al3+ exists in large quantities, the response of the sensor SP to Fe3+ is weakened heavily and results in the decrease of the overall fluorescence intensity. However, the total fluorescence intensity is only slightly reduced if Al3+ and Fe3+ ions are present in equal amount. Overall, the sensor SP exhibited good anti-interference ability for specific detection of Fe3+ ion. Fig. 6 3.4. Effect of UV irradiation and pH on sensor SP for the detection of Fe3+ ions Fluorescence spectra of sensor SP (4 × 10‒5 M in DMF/H2O, 9:1, v/v) before and after UV light irradiation at an excitation wavelength of 378 nm were recorded as showed in Fig. 7. Solution of probe SP was orange-yellow. After the solution was irradiated with UV light for 2 h the color did not change obviously. However, the fluorescence emission intensity at around 450 nm was enhanced greatly after the solution of probe SP was irradiated with UV light for 2 h. It indicated that the weakly fluorescent ring-closed spirobenzopyran isomer gradually transformed into the strongly fluorescent ring-opened merocyanine isomer upon irradiation with UV light, in consistence with the previously reported results observed on photochromic spiropyrans
[33‒37]. Addition of 1 equivalent of Fe3+ to probe SP led to obvious enhancement in the fluorescence emission intensity. Irradiation of the solution with UV light for 2 h did not cause much increase in the fluorescence emission intensity, as showed in Fig. 8. It might indicate that the probe SP was readily ring-opened by addition of Fe3+ ion, resulting in intense enhancement of the fluorescence emission. The sensor SP underwent a ring-opening process in the presence of Fe3+ ion then the ring-opened merocyanine isomer complexed with Fe3+ ion [34‒38]. Fig. 7 Fig. 8 Effect of pH on the fluorescent sensing behavior of probe SP toward Fe3+ ion was investigated by measurement of the fluorescence spectra of sensor SP containing 1 equivalent of Fe3+ in DMF/H2O (7:3, v/v) at different pH values (pH = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, 11.5, 12.5, 13.5) and an excitation wavelength of 378 nm, as showed in Fig. 9. It was clearly visible that the solution exhibited high fluorescence emission intensity in the pH range of pH 4.5‒10.5. The fluorescence intensity decreased by 50% more or less when the pH value was below 3.5. Strong alkaline (pH > 11.5) media resulted in fluorescence quenching of the solution. The results reveal that the probe SP can be used to recognize Fe3+ in a wide pH range except strong alkaline conditions, superior to many fluorescent probes for Fe3+ ion [9‒13]. The optimal pH range for Fe3+ fluorescent detection by probe SP is pH 4.5‒10.5. Fig. 9 3.5. Detection limit of sensor SP for sensing Fe3+ ions Fluorescence spectra of sensor SP containing different concentrations of Fe3+ ion (4 µM, 6 µM, 8 µM, 10 µM, 12 µM, 14 µM, 16 µM, 18 µM, 20 µM) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm were measured and the changes in the fluorescence emission intensity of the solution with Fe3+ ion concentration were showed in Fig. 10. It was undoubted that the fluorescence intensity of the solution increased gradually with increase in Fe3+ ion concentration. A line could be drawn based on the intensity of the fluorescence emission band and the concentration of Fe3+ ion within Fe3+ concentration range from 4 µM to 8 µM and from 8 µM to 20 µM, as showed in Fig. 10. The first linear fitting equation can be depicted as Y = 47002X +
101413 with R = 0.99488 (4 µM‒8 µM), from which the detection limit can be calculated to be 1.93 × 10–7 M. The second linear fitting equation can be depicted as Y = 14055X + 379360 with R = 0.99352 (10 µM‒20 µM). Thus sensor SP may be utilized for quantitative determination of Fe3+ ion within a concentration range of 4‒20 µM. Fig. 10 3.6. Study on Fe3+ ion sensing mechanism by sensor SP Investigation on possible mode of complexation between probe SP and Fe3+ ion was carried out by determination of the Job's plot. The total concentration of probe SP and Fe3+ ion was kept at 0.2 mM and changes in the fluorescence intensity of the solution containing sensor SP and Fe3+ ion with the molar ratio of sensor SP were measured as showed in Fig. 11. It can be seen from the Job’s plot that the maximum fluorescence emission intensity appears at 0.25. Thus it can be inferred that probe SP and Fe3+ ion form a complex in 1:3 ratio. The plausible mode of the complexation between sensor SP and Fe3+ ion is showed in Fig. 12, which reveals the isomeric structure of spiropyran SP (closed, weakly fluorescent spiropyran isomer) and the Fe3+-induced ring-opened Fe3+ complex (opened, strongly fluorescent merocyanine isomer). In other works concerning fluorescent chemosensors for Fe3+ ion, the reported stoichiometry ratio between the sensor and Fe3+ ion is 1:1 generally [12–15]. In our case the formation of 1:3 SP–Fe3+ complexation may be attribute to the specific SP structure bearing multiple phenolic hydroxyl groups which readily complex with Fe3+ ions. Fig. 11 Fig. 12
4. Conclusions A new spiropyran-based fluorescent probe has been developed and investigated. The probe can transform from weakly fluorescent closed loop to strongly fluorescent opened loop upon UV irradiation or addition of Fe3+ ion. The fluorescence emission of the probe solution is intensely enhanced by adding Fe3+ ion in DMF/H2O (9:1, v/v). The fluorescence enhancement is not obviously interfered by addition of one or more other
common competitive metal ions. The probe can be utilized for sensing Fe3+ ion in a wide pH range except strong alkaline conditions and the optimal pH range for Fe3+ fluorescent detection is pH 4.5‒10.5. Investigation on the changes in the fluorescence emission intensity of the solution containing the sensor and Fe3+ ion with the equivalent ratio of Fe3+/sensor reveals an approximate linear relationship. Thus the sensor may be useful for quantitative determination of Fe3+ ion within a concentration range of 4‒20 µM. The detection limit of Fe3+ ion by the sensor is 1.93 × 10–7 M. Job’s plot indicates complexation between the sensor and Fe3+ ion in a stoichiometry ratio of 1:3. It may be attribute to the characteristic sensor structure bearing multiple phenolic hydroxyl groups which readily complex with Fe3+ ions. These findings well demonstrate the versatility of spiropyrans as selective fluorescent probes for various metal ions including Fe3+ ion.
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post-modified
sulfone-metal-organic
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OH CHO N
I
+
Et3N/MeOH reflux 91%
N O
NO2
NO2 SnCl2/HCl EtOH, reflux
2,5-dihydroxybenzaldehyde AcOH/CH2Cl2
54%
N O
NH2
HO N O
N SP
OH
Scheme 1. Synthetic route of chemosensor SP
30%
Fig. 1. Fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in different solvents (EtOH, MeOH, MeCN, DMF, DMSO) at an excitation wavelength of 378 nm.
Fig. 2. Fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in DMF/H2O of different ratios (DMF/H2O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v) at an excitation wavelength of 378 nm.
Fig. 3. Fluorescence emission spectra of sensor SP (4 × 10‒5 M) before and after addition of solution (4 × 10‒5 M) of different metal ions (Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+, Cr3+, Fe3+) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 4. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and/or other competitive metal ions (1 equivalent). Black bars denote responses of individual metal ions while red bars show responses of Fe3+ ion in the presence of other coexisting metal ions (in 9:1 DMF/H2O, at an excitation wavelength of 378 nm).
Fig. 5. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and other competitive metal ions (5 equivalent) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 6. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and 5 kinds of other competitive metal ions together (each 1 equivalent) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm: bar 1. Fe3+; bar 2. Fe3+, Li+, Hg2+, Zn2+, Ni2+, Al3+; bar 3. Fe3+, Na+, Cu2+, Ca2+, Mg2+, Cr3+; bar 4. Fe3+, K+, Fe2+, Mn2+, Sr2+, Al3+; bar 5. Fe3+, Ag+, Zn2+, Co2+, Fe2+, Cr3+; bar 6. Fe3+, Ag+, Cu2+, Sr2+, Mn2+, Al3+.
Fig. 7. Fluorescence spectra of sensor SP (4 × 10‒5 M in DMF/H2O, 9:1, v/v) before and after UV light irradiation for 2 h at an excitation wavelength of 378 nm.
Fig. 8. Fluorescence spectra of sensor SP (4 × 10‒5 M) containing 1 equivalent of Fe3+ in DMF/H2O (9:1, v/v) before and after UV light irradiation for 2 h at an excitation wavelength of 378 nm.
Fig. 9. Fluorescence spectra of sensor SP (4 × 10‒5 M) containing 1 equivalent of Fe3+ in DMF/H2O (7:3, v/v) at different pH values (pH = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, 11.5, 12.5, 13.5) with an excitation wavelength of 378 nm.
Fig. 10. Changes in fluorescence intensity of sensor SP (4 × 10‒5 M) containing different concentrations of Fe3+ (4 µM, 6 µM, 8 µM, 10 µM, 12 µM, 14 µM, 16 µM, 18 µM, 20 µM) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 11. Job’s plot obtained from fluorescence intensity of sensor SP and Fe3+ ion solution ([SP] + [Fe3+] = 200 μM, in 9:1 DMF/H2O, v/v) at an excitation wavelength of 378 nm.
Fe3+
HO Fe3+ N O
N
SP weakly fluorescent
OH
O
N N
O Fe3+
strongly fluorescent
Fig. 12. Plausible mode of complexation between sensor SP and Fe3+ ion.
Fe3+
O
A new spiropyran-based chemosensor was developed. The sensor is selective for fluorescent detection of Fe3+ in aqueous solution. The detection limit of Fe3+ ion by the sensor is measured to be 1.93 × 10–7 M.
Author Contributions Ruiqing Zhang has carried out the main experimental work; Luping Hu, Zhenxiang Xu and Yanxi Song have carried out a part of measurement work; Hongqi Li is responsible for the whole work and submission of the paper; Xin Zhang, Xucheng Gao, Mengxuan Wang and Chunying Xian have participated in part of the work.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
3+
ion based on a
Ruiqing Zhang, Luping Hu, Zhenxiang Xu, Yanxi Song, Hongqi Li, Xin Zhang, Xucheng Gao, Mengxuan Wang, Chunying Xian PII:
S0022-2860(19)31590-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.127481
Reference:
MOLSTR 127481
To appear in:
Journal of Molecular Structure
Received Date: 22 October 2019 Revised Date:
21 November 2019
Accepted Date: 25 November 2019
Please cite this article as: R. Zhang, L. Hu, Z. Xu, Y. Song, H. Li, X. Zhang, X. Gao, M. Wang, C. Xian, 3+ A highly selective probe for fluorescence turn-on detection of Fe ion based on a novel spiropyran derivative, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.127481. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
A highly selective probe for fluorescence turn-on detection of Fe3+ ion based on a novel spiropyran derivative Ruiqing Zhanga, Luping Hua, Zhenxiang Xub, Yanxi Songc, Hongqi Lia,*, Xin Zhanga, Xucheng Gaoa, Mengxuan Wanga, Chunying Xiana a Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, College of Chemistry, Chemical Engineering & Biotechnology, Donghua University, 2999 North Renmin Road, Shanghai 201620, P. R. China. b Penglai Xinguang Pigment Chemical Co., Ltd, Penglai 265601, P. R. China c School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, P. R. China.
ABSTRACT A new spiropyran bearing multiple phenolic hydroxyl groups is synthesized. It acts as a chemosensor for selective detection of Fe3+ ions in both organic and aqueous solution. The optimal pH range for Fe3+ fluorescent detection by the sensor is pH 4.5‒10.5. Upon addition of Fe3+ to the sensor solution the fluorescence emission intensity is enhanced by around 50 fold. Other common competitive metal ions including Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+ and Cr3+ do not substantially interfere with the selective detection of Fe3+ by the sensor. Changes in the fluorescence emission intensity with the equivalent ratio of Fe3+/sensor exhibits an approximate linear relationship, from which the concentration of Fe3+ ion may be estimated. The detection limit of Fe3+ ion by the sensor is measured to be 1.93 × 10–7 M. A plausible sensing mechanism for selective Fe3+ ion detection by the sensor is proposed based on the Job’s plot measurement. Keywords: Fluorescence; probe; spiropyran; Fe3+
1. Introduction As a kind of transitional metal, iron is the metal element of the second highest crust content and is also the essential trace element in human body. It mainly exists in the form of hemoglobin and has great effect in oxygen supply of human body. On the other hand, excessive intake of Fe3+ may result in a disturbance of physiological function and several diseases [1‒4]. Therefore convenient and effective identification and detection of Fe3+ ions in the organism and in the environment is of high significance. In this respect selective and sensitive fluorescent probes for rapid detection of Fe3+ ions aroused much interest from scientists and a number of fluorescent Fe3+ probes have been developed by utilizing various fluorogenic platforms such as coumarin [5‒9], rhodamine [10‒12], 1,8-naphthalimide [13‒16],
pillar[5]arene [17,18], quantum dots [19‒21], and metal-organic frameworks (MOFs) als [22‒24]. Chemosensors for fluorescent or colorimetric detection of Fe3+ ions based on photochromic spiropyran scaffold have also been reported [25‒27]. However, the selectivity of these sensors toward Fe3+ ions is substantially weakened by other trivalent ions Al3+ and Cr3+. In view that spiropyran is an extraordinarily versatile photochrome [28] and spiropyran-based fluorescent and/or colorimetric probes [29] have been developed for highly selective and sensitive detection of various analytes including pH [30‒32], Li+ [33], Mg2+ [34], Ca2+ [35], Co2+ [36], Cu2+ [37], Zn2+ [38], Hg2+ [39], Pb2+ [40], Al3+ [41], Cr3+ [42], La3+ [43], CN‒ [44], F‒ [45], Cl‒ [46], pyrophosphate [47], CO2 [48], H2O2 [49], H2S [50], H2S/SO2 [51], hydrazine [52], cysteine (Cys)/homocysteine (Hcy) [53], γ-glutamyl-cysteinyl-glycine (GSH) [54], β-amyloid peptide oligomers [55], lysozyme [56], DNA G-quadruplexes [57], aniline pollutants [58], acidic food spoilage [59], ultraviolet (UV) light intensity [60], and mechanical stress [61], we envisage to design and synthesize a fluorescent chemosensor which can detect Fe3+ ions selectively and sensitively even in the presence of other trivalent ions Al3+ and Cr3+ based on spiropyran, following our recent work concerning study on a spiropyran-appended polysiloxane which can be used for probing pH changes and Fe3+ ions and sequential detection of Ag+ and Hg2+ ions [62]. Herein we report the synthesis, characterization and recognition behavior of the new spiropyran-based fluorogenic Fe3+ sensor.
2. Experimental section 2.1. Materials and instrumentation All materials and solvents employed in this study were of analytical grade. All chemical reagents were purchased from commercial sources and used as received unless other statements. 1,2,3,3-Tetramethylindolindolenium iodide was synthesized according to literature method [63]. Melting point was determined on a WRS-2A capillary melting apparatus and the quoted temperatures were uncorrected. 1H NMR and 13C NMR spectra were recorded on a Bruker AM 400 spectrometer. DMSO-d6 or CDCl3 was used as solvent and chemical shifts recorded were internally referenced to Me4Si (0 ppm). IR spectra were obtained on a Thermo Electron Corporation Nicolet 380 FT-IR spectrophotometer from Thermo Fisher Scienific Company. Mass spectra were obtained on an Agilent 6000 LC-MS instrument (ESI, pos mode, 70 to 1000 amu). Fluorescence spectra were recorded on a FS5 spectrofluorometer from Edinburgh
Instruments Co., Ltd. UV-Vis spectra were recorded on a UV-1800 spectrophotometer from Shimadzu Instruments (Suzhou) Co., Ltd.. 2.2. Synthesis of sensor SP 1,2,3,3-Tetramethylindolindolenium iodide (6.35 g, 21 mmol) was dissolved in anhydrous methanol (100 mL) and heated to reflux. Then triethylamine (1 mL) was added and the solution was stirred for 15 min. 5-Nitrosalicylaldehyde (3.34 g, 20 mmol) was added and the mixture was refluxed for 2 h. After reaction the mixture was allowed to cool to room temperature and methanol was removed by evaporation. The formed orange solid was recrystallized with dichloromethane/ethanol. Yellow solid (5.88 g) of the target product 1,3,3-trimethyl-6´-nitrospiro[chromene-2,2´indoline] was obtained in 91% yield after filtration and drying. Mp 178–180 °C (literature [64] value: 178–179 °C). 1H NMR (400 MHz, CDCl3) δ 8.05–8.03 (m, 2H), 7.25–7.23 (m, 1H), 7.12 (dd, J = 7.3, 1.3 Hz, 1H), 6.99–6.88 (m, 2H), 6.80 (d, J = 8.6 Hz, 1H), 6.59 (d, J = 7.7 Hz, 1H), 5.89 (d, J = 10.3 Hz, 1H), 2.77 (s, 3H), 1.32 (s, 3H), 1.22 (s, 3H). 1,3,3-Trimethyl-6´-nitrospiro[chromene-2,2´-indoline] (6.45 g, 20 mmol) and SnCl2 (18.96 g, 0.10 mol) were added to a flask under N2 atmosphere. Anhydrous ethanol (100 mL) was added and the reaction mixture was heated to reflux for 2 h. After reaction the mixture was cooled to room temperature and filtered. The filtrate was concentrated and 5% aqueous NaOH was added to the residue. The organic phase was extracted with dichloromethane for three times, washed with water and dried. After filtration the solvent was removed by evaporation and the residue was purified by column chromatography with petroleum ether/ethyl acetate (3:1, v/v) as eluent to give the reduction product 1,3,3-trimethyl-6´-aminospiro[chromene-2,2´-indoline] (3.18 g) as brown solid in 54% yield. Mp 139–140 °C. 1H NMR (400 MHz, CDCl3) δ 7.16–7.14 (m, 1H), 7.05 (d, J = 8.3 Hz, 1H), 6.81–6.79 (m, 1H), 6.74 (d, J = 8.8 Hz, 1H), 6.55–6.44 (m, 3H), 6.41 (d, J = 7.9 Hz, 1H), 5.67 (d, J = 10.9 Hz, 1H), 2.85 (s, 3H), 1.45 (s, 3H), 1.30 (s, 3H). 1,3,3-Trimethyl-6´-aminospiro[chromene-2,2´-indoline] (0.73 g, 2.5 mmol) and 2,5-dihydroxybenzaldehyde (0.33 g, 2.4 mmol) were dissolved in dichloromethane (20 mL). To the solution was added 5 drops of acetic acid and the mixture was stirred at room temperature for 1 h. After reaction the mixture was washed with water and the organic layer was separated then dried. The filtration was concentrated and the residue was purified by column chromatography with petroleum ether/ethyl acetate
(5:1, v/v) as eluent to afford the target sensor SP (0.30 g) as red-brown solid in 30% yield. Mp 146–148 °C. IR (KBr pellet): ν 3318, 1644, 1609, 1481, 1450, 1270, 953 cm–1. 1H NMR (400 MHz, CDCl3) δ 8.43 (s, 1H), 7.19 (s, 1H), 7.14–7.12 (m, 2H), 7.03–7.00 (m, 3H), 6.97 (d, J = 2.5 Hz, 1H), 6.85–6.76 (m, 7H), 6.69 (d, J = 8.6 Hz, 1H), 6.47 (d, J = 7.7 Hz, 1H), 5.69 (d, J = 10.2 Hz, 1H), 2.68 (s, 3H), 1.25 (s, 3H), 1.11 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 158.49, 154.06, 152.88, 147.06, 146.78, 139.68, 135.58, 127.98, 126.62, 121.17, 120.49, 119.49, 118.55, 118.27, 118.22, 118.18, 116.85, 116.09, 114.79, 105.83, 103.63, 50.85, 27.91, 24.85, 19.12. MS (ESI): m/z 413 (M+ + 1). 2.3. General UV-vis and fluorescence spectra measurements All spectral analyses were accomplished at room temperature in 0.1 M Tris-HCl buffer solution (DMF/H2O = 9:1, v/v, pH = 7.10). The concentration of sensor SP and metal ions was 4 × 10–5 M for UV-vis and fluorescence measurement. Sensor SP was dissolved in DMF and solutions of metal ions were prepared with nitrate or chloride salts in water. Each sample was added to a 10 mL volumetric flask by using a 20‒200 µL liquid transfer gun and the solution was well mixed before the spectra were measured. Fluorescence titration experiments were performed in 2 mL cuvette by successive addition of prepared solutions at an excitation wavelength of 378 nm with both excitation and emission slits of 5.0 nm. UV-vis absorption spectra were measured in the range of 290‒700 nm.
3. Results and discussions 3.1. Synthesis and structural characterization of sensor SP The novel spiropyran-based sensor SP was synthesized by a three-step protocol from 1,2,3,3-tetramethylindolindolenium iodide and 5-nitrosalicylaldehyde, as showed in Scheme 1. Condensation between 1,2,3,3-tetramethylindolindolenium iodide and 5-nitrosalicylaldehyde in the presence of triethylamine produced the target product 1,3,3-trimethyl-6´-nitrospiro[chromene-2,2´-indoline] in 91% yield. The nitrospiropyran was characterized by both melting point measurement and 1H NMR spectrum. The data were in good accordance with those reported in literature [64]. Reduction of the nitrospiropyran to the corresponding aminospiropyran by SnCl2 afforded a 54% yield. The structure of the obtained aminospiropyran compound was characterized by melting point measurement and 1H NMR spectrum and the spectral
data were in good accordance with those reported in literature [65]. Imination reaction between 2,5-dihydroxybenzaldehyde and the aminospiropyran catalyzed by acetic acid in dichloromethane afforded sensor SP in 30% yield. The chemical structure of the sensor SP was fully characterized by 1H NMR, 13C NMR, IR and mass spectrum. All of the data in the spectra were in good accordance with the structure. Scheme 1 3.2. Effect of medium on fluorescent detection of Fe3+ ions by sensor SP Fluorescence spectra of sensor SP and Fe3+ solution (4 × 10‒5 M) in different solvents including EtOH, MeOH, MeCN, DMF and DMSO at an excitation wavelength of 378 nm were measured and showed in Fig. 1. It was found that the solution exhibited the maximum fluorescence emission at around 450 nm and the fluorescence intensities in DMF and DMSO were much higher that those in other solvents EtOH, MeOH and MeCN. Based on the results DMF was chosen as the solvent in the following determination, considering that DMSO has a higher viscosity and strongly irritating odor though DMSO shows a little higher fluorescence intensity that that of DMF. Fig. 1 It is important for a practical metal ion probe to be usable in an aqueous medium. Therefore the fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in DMF/H2O of different ratios (DMF/H2O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v) were determined at an excitation wavelength of 378 nm and were showed in Fig. 2. It could be observed from Fig. 2 that the solution exhibited the highest fluorescence intensity in 9:1 DMF/H2O. With the increase in the ratio of water, the fluorescence emission intensity decreased sharply. The solution in 7:3 DMF/H2O was fairly fluorescent and further increase in the water content led to very weak fluorescence of the solution. Thus the optimal sensing medium was 9:1 DMF/H2O, which was used in the following determination. Fig. 2 The fluorescence spectra of sensor SP before and after addition of common metal ions (Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+, Cr3+, Fe3+) in DMF/H2O (9:1, v/v) were measured and showed in Fig. 3. The very weak fluorescence emission of the probe SP appeared at 450 nm when it was
excited at a wavelength of 378 nm. The fluorescence intensity of the probe SP was enhanced by addition of Fe3+ and Al3+ while addition of other metal ions failed to cause substantial change in the fluorescence emission. Addition of Fe3+ and Al3+ induced an enhancement in the fluorescence intensity of the probe SP by around 50 fold and 8 fold, respectively. Therefore the probe SP can be used for selective detection of Fe3+ in an aqueous medium. Fig. 3 3.3. Anti-interference ability of sensor SP for the fluorescent detection of Fe3+ ions In order to investigate the interference of competitive metal ions on the sensing selectivity of the probe SP toward Fe3+ ion, changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ ion (1 equivalent) and/or other competitive metal ions (1 equivalent) were recorded and showed in Fig. 4. It was visible that to the fluorescence turn-on probe SP for Fe3+ ion, addition of 1 equivalent of Cu2+ or Al3+ ion reduced the fluorescence intensity by about 20%. Other competitive ions did not cause obvious interference with the selective detection of Fe3+ ion. Considering that competitive metal ions may exist in a large amount and mix with the analyte such as Fe3+ ion, changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ ion (1 equivalent) and other competitive metal ions (5 equivalent) were measured and showed in Fig. 5. In this case the excessive Al3+, Hg2+ and Fe2+ ions interfered with the selective detection of Fe3+ by sensor SP, bringing about decrease in the fluorescence intensity by 40‒43%. While the presence of excessive Cu2+ ion caused serious interference with the selective detection of Fe3+ by sensor SP, reducing the fluorescence intensity by nearly 84%. Other competitive ions caused relatively small interference to the selective detection of Fe3+ ion. Fig. 4 Fig. 5 The real samples especially waste water samples usually contain multiple metal ions. Therefore the anti-interference ability to multiply mixed metal ions of a sensor for a specific metal ion reflects the ‘real’ anti-interference ability of the sensor. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and 5 kinds of other competitive metal ions together (each 1 equivalent) in
DMF/H2O (9:1, v/v) were determined and showed in Fig. 6. Groups containing 5 kinds of competitive metal ions for anti-interference test were picked up at random on the premise to ensure each group contains a monovalent and a trivalent metal ion. The following groups of metal ions were chosen for the test: Li+, Hg2+, Zn2+, Ni2+ and Al3+ in a group; Na+, Cu2+, Ca2+, Mg2+ and Cr3+ in a group; K+, Fe2+, Mn2+, Sr2+ and Al3+ in a group; Ag+, Zn2+, Co2+, Fe2+ and Cr3+ in a group; and Ag+, Cu2+, Sr2+, Mn2+ and Al3+ in a group. It can be seen that each group containing 5 kinds of competitive metal ions exhibits only a little decrease in the fluorescence intensity compared to the sensor SP/Fe3+ system, indicating that the multiply mixed competitive metal ions, containing Cu2+ or Fe2+ alone, or Hg2+/Al3+, Fe2+/Al3+, and Cu2+/Al3+ together, had negligible interference to the selective detection of Fe3+ ion by sensor SP. The paramagnetic property of Cu2+ would bring about a fluorescence quenching effect and Fe2+/Hg2+ ions also undergo electron or energy transfer to result in a certain degree of fluorescence quenching. Generally the fluorescence quenching phenomenon is positively correlated with the concentration of the quenching ions [66‒69]. This is why Cu2+, Hg2+ and Fe2+ ions cause significant decrease in the fluorescence intensity when these ions are present alone in 5-fold equivalent while the mixed metal ions (each having a much lower concentration) only bring about a little interference to the fluorescence intensity. The enhancement in fluorescence intensity of the probe SP by Fe3+ and Al3+ reaches about 50 fold and 8 fold, respectively. When Al3+ exists in large quantities, the response of the sensor SP to Fe3+ is weakened heavily and results in the decrease of the overall fluorescence intensity. However, the total fluorescence intensity is only slightly reduced if Al3+ and Fe3+ ions are present in equal amount. Overall, the sensor SP exhibited good anti-interference ability for specific detection of Fe3+ ion. Fig. 6 3.4. Effect of UV irradiation and pH on sensor SP for the detection of Fe3+ ions Fluorescence spectra of sensor SP (4 × 10‒5 M in DMF/H2O, 9:1, v/v) before and after UV light irradiation at an excitation wavelength of 378 nm were recorded as showed in Fig. 7. Solution of probe SP was orange-yellow. After the solution was irradiated with UV light for 2 h the color did not change obviously. However, the fluorescence emission intensity at around 450 nm was enhanced greatly after the solution of probe SP was irradiated with UV light for 2 h. It indicated that the weakly fluorescent ring-closed spirobenzopyran isomer gradually transformed into the strongly fluorescent ring-opened merocyanine isomer upon irradiation with UV light, in consistence with the previously reported results observed on photochromic spiropyrans
[33‒37]. Addition of 1 equivalent of Fe3+ to probe SP led to obvious enhancement in the fluorescence emission intensity. Irradiation of the solution with UV light for 2 h did not cause much increase in the fluorescence emission intensity, as showed in Fig. 8. It might indicate that the probe SP was readily ring-opened by addition of Fe3+ ion, resulting in intense enhancement of the fluorescence emission. The sensor SP underwent a ring-opening process in the presence of Fe3+ ion then the ring-opened merocyanine isomer complexed with Fe3+ ion [34‒38]. Fig. 7 Fig. 8 Effect of pH on the fluorescent sensing behavior of probe SP toward Fe3+ ion was investigated by measurement of the fluorescence spectra of sensor SP containing 1 equivalent of Fe3+ in DMF/H2O (7:3, v/v) at different pH values (pH = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, 11.5, 12.5, 13.5) and an excitation wavelength of 378 nm, as showed in Fig. 9. It was clearly visible that the solution exhibited high fluorescence emission intensity in the pH range of pH 4.5‒10.5. The fluorescence intensity decreased by 50% more or less when the pH value was below 3.5. Strong alkaline (pH > 11.5) media resulted in fluorescence quenching of the solution. The results reveal that the probe SP can be used to recognize Fe3+ in a wide pH range except strong alkaline conditions, superior to many fluorescent probes for Fe3+ ion [9‒13]. The optimal pH range for Fe3+ fluorescent detection by probe SP is pH 4.5‒10.5. Fig. 9 3.5. Detection limit of sensor SP for sensing Fe3+ ions Fluorescence spectra of sensor SP containing different concentrations of Fe3+ ion (4 µM, 6 µM, 8 µM, 10 µM, 12 µM, 14 µM, 16 µM, 18 µM, 20 µM) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm were measured and the changes in the fluorescence emission intensity of the solution with Fe3+ ion concentration were showed in Fig. 10. It was undoubted that the fluorescence intensity of the solution increased gradually with increase in Fe3+ ion concentration. A line could be drawn based on the intensity of the fluorescence emission band and the concentration of Fe3+ ion within Fe3+ concentration range from 4 µM to 8 µM and from 8 µM to 20 µM, as showed in Fig. 10. The first linear fitting equation can be depicted as Y = 47002X +
101413 with R = 0.99488 (4 µM‒8 µM), from which the detection limit can be calculated to be 1.93 × 10–7 M. The second linear fitting equation can be depicted as Y = 14055X + 379360 with R = 0.99352 (10 µM‒20 µM). Thus sensor SP may be utilized for quantitative determination of Fe3+ ion within a concentration range of 4‒20 µM. Fig. 10 3.6. Study on Fe3+ ion sensing mechanism by sensor SP Investigation on possible mode of complexation between probe SP and Fe3+ ion was carried out by determination of the Job's plot. The total concentration of probe SP and Fe3+ ion was kept at 0.2 mM and changes in the fluorescence intensity of the solution containing sensor SP and Fe3+ ion with the molar ratio of sensor SP were measured as showed in Fig. 11. It can be seen from the Job’s plot that the maximum fluorescence emission intensity appears at 0.25. Thus it can be inferred that probe SP and Fe3+ ion form a complex in 1:3 ratio. The plausible mode of the complexation between sensor SP and Fe3+ ion is showed in Fig. 12, which reveals the isomeric structure of spiropyran SP (closed, weakly fluorescent spiropyran isomer) and the Fe3+-induced ring-opened Fe3+ complex (opened, strongly fluorescent merocyanine isomer). In other works concerning fluorescent chemosensors for Fe3+ ion, the reported stoichiometry ratio between the sensor and Fe3+ ion is 1:1 generally [12–15]. In our case the formation of 1:3 SP–Fe3+ complexation may be attribute to the specific SP structure bearing multiple phenolic hydroxyl groups which readily complex with Fe3+ ions. Fig. 11 Fig. 12
4. Conclusions A new spiropyran-based fluorescent probe has been developed and investigated. The probe can transform from weakly fluorescent closed loop to strongly fluorescent opened loop upon UV irradiation or addition of Fe3+ ion. The fluorescence emission of the probe solution is intensely enhanced by adding Fe3+ ion in DMF/H2O (9:1, v/v). The fluorescence enhancement is not obviously interfered by addition of one or more other
common competitive metal ions. The probe can be utilized for sensing Fe3+ ion in a wide pH range except strong alkaline conditions and the optimal pH range for Fe3+ fluorescent detection is pH 4.5‒10.5. Investigation on the changes in the fluorescence emission intensity of the solution containing the sensor and Fe3+ ion with the equivalent ratio of Fe3+/sensor reveals an approximate linear relationship. Thus the sensor may be useful for quantitative determination of Fe3+ ion within a concentration range of 4‒20 µM. The detection limit of Fe3+ ion by the sensor is 1.93 × 10–7 M. Job’s plot indicates complexation between the sensor and Fe3+ ion in a stoichiometry ratio of 1:3. It may be attribute to the characteristic sensor structure bearing multiple phenolic hydroxyl groups which readily complex with Fe3+ ions. These findings well demonstrate the versatility of spiropyrans as selective fluorescent probes for various metal ions including Fe3+ ion.
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OH CHO N
I
+
Et3N/MeOH reflux 91%
N O
NO2
NO2 SnCl2/HCl EtOH, reflux
2,5-dihydroxybenzaldehyde AcOH/CH2Cl2
54%
N O
NH2
HO N O
N SP
OH
Scheme 1. Synthetic route of chemosensor SP
30%
Fig. 1. Fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in different solvents (EtOH, MeOH, MeCN, DMF, DMSO) at an excitation wavelength of 378 nm.
Fig. 2. Fluorescence spectra of solutions containing Fe3+ ion (4 × 10‒5 M) and sensor SP (4 × 10‒5 M) in DMF/H2O of different ratios (DMF/H2O = 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, v/v) at an excitation wavelength of 378 nm.
Fig. 3. Fluorescence emission spectra of sensor SP (4 × 10‒5 M) before and after addition of solution (4 × 10‒5 M) of different metal ions (Li+, Na+, K+, Ag+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+, Mn2+, Sr2+, Hg2+, Ca2+, Mg2+, Al3+, Cr3+, Fe3+) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 4. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and/or other competitive metal ions (1 equivalent). Black bars denote responses of individual metal ions while red bars show responses of Fe3+ ion in the presence of other coexisting metal ions (in 9:1 DMF/H2O, at an excitation wavelength of 378 nm).
Fig. 5. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and other competitive metal ions (5 equivalent) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 6. Changes in fluorescence intensity at 450 nm of sensor SP upon addition of Fe3+ (1 equivalent) and 5 kinds of other competitive metal ions together (each 1 equivalent) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm: bar 1. Fe3+; bar 2. Fe3+, Li+, Hg2+, Zn2+, Ni2+, Al3+; bar 3. Fe3+, Na+, Cu2+, Ca2+, Mg2+, Cr3+; bar 4. Fe3+, K+, Fe2+, Mn2+, Sr2+, Al3+; bar 5. Fe3+, Ag+, Zn2+, Co2+, Fe2+, Cr3+; bar 6. Fe3+, Ag+, Cu2+, Sr2+, Mn2+, Al3+.
Fig. 7. Fluorescence spectra of sensor SP (4 × 10‒5 M in DMF/H2O, 9:1, v/v) before and after UV light irradiation for 2 h at an excitation wavelength of 378 nm.
Fig. 8. Fluorescence spectra of sensor SP (4 × 10‒5 M) containing 1 equivalent of Fe3+ in DMF/H2O (9:1, v/v) before and after UV light irradiation for 2 h at an excitation wavelength of 378 nm.
Fig. 9. Fluorescence spectra of sensor SP (4 × 10‒5 M) containing 1 equivalent of Fe3+ in DMF/H2O (7:3, v/v) at different pH values (pH = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, 10.5, 11.5, 12.5, 13.5) with an excitation wavelength of 378 nm.
Fig. 10. Changes in fluorescence intensity of sensor SP (4 × 10‒5 M) containing different concentrations of Fe3+ (4 µM, 6 µM, 8 µM, 10 µM, 12 µM, 14 µM, 16 µM, 18 µM, 20 µM) in DMF/H2O (9:1, v/v) at an excitation wavelength of 378 nm.
Fig. 11. Job’s plot obtained from fluorescence intensity of sensor SP and Fe3+ ion solution ([SP] + [Fe3+] = 200 μM, in 9:1 DMF/H2O, v/v) at an excitation wavelength of 378 nm.
Fe3+
HO Fe3+ N O
N
SP weakly fluorescent
OH
O
N N
O Fe3+
strongly fluorescent
Fig. 12. Plausible mode of complexation between sensor SP and Fe3+ ion.
Fe3+
O
A new spiropyran-based chemosensor was developed. The sensor is selective for fluorescent detection of Fe3+ in aqueous solution. The detection limit of Fe3+ ion by the sensor is measured to be 1.93 × 10–7 M.
Author Contributions Ruiqing Zhang has carried out the main experimental work; Luping Hu, Zhenxiang Xu and Yanxi Song have carried out a part of measurement work; Hongqi Li is responsible for the whole work and submission of the paper; Xin Zhang, Xucheng Gao, Mengxuan Wang and Chunying Xian have participated in part of the work.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: