A new turn on coumarin-based fluorescence probe for Ga3 + detection in aqueous solution

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 155 (2016) 116–124

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A new turn on coumarin-based fluorescence probe for Ga3+ detection in aqueous solution Liqiang Yan, Yan Zhou, Wenqi Du, Zhineng Kong, Zhengjian Qi ⁎ College of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, PR China

a r t i c l e

i n f o

Article history: Received 8 August 2015 Received in revised form 7 November 2015 Accepted 12 November 2015 Available online 14 November 2015 Keywords: Coumarin derivative Fluorescent chemosensor L-Threonine Ga3 +

a b s t r a c t The probe CT was synthesized and investigated as a novel label-free chemosensor for Ga3+ detection in water. Probe CT showed remarkable selectivity and sensitivity for Ga3+ in Tris–HCl aqueous buffer solution (pH 7.0). The chemosensor responded rapidly to Ga3+ with a 1:1 stoichiometry. Meanwhile, the unapparent changes of fluorescence lifetime decays suggest the turn-on process of probe CT by Ga3+ which appears to be a static mechanism. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Gallium as a trace element has an important influence on our daily life. Gallium and its derivatives are widely applied in energy storage of solar cells, catalysis, and microelectronic and optoelectronic devices [1–10] on account of their unique electrical, optical, and mechanical properties [11–15]. With gallium consumption increase, excess of gallium compound can lead to environmental pollution because they are considered to be health hazardous [16–17]. In clinical practice, radioactive gallium and stable gallium nitrate are used as diagnostic and therapeutic agents in cancer and disorders of calcium and bone metabolism [18]. Additionally, some reports stated that gallium compounds may be utilized as antimicrobial agents against certain pathogens [19]. Absolutely, it is very important to develop reliable methods for Ga3 + detection. So far some methods including ion exchange, adsorption, titrimetric method, and atomic absorption spectrometry, have been developed for Ga3 + determination [20–25]. However, these methods need for expensive instruments or complex operation steps. Therefore, the development of a novel and convenient method to detect Ga3 + for environmental and biological studies is still highly desirable. Fluorescent probe technology has attracted significant focus because of the advantages including capability of visualization, easy operational procedure, rapid response time, cost effective equipment, convenience afforded, and high sensitivity and selectivity [26–30]. The method has been applied for the determination of medicine [31], biomolecules [32], metal ions [33–42], anions [43,44], and pH [45]. ⁎ Corresponding author. E-mail address: [email protected] (Z. Qi).

http://dx.doi.org/10.1016/j.saa.2015.11.012 1386-1425/© 2016 Elsevier B.V. All rights reserved.

In recent years, although a few fluorescent probes for Ga3 + have been reported [18,46–47], they were operated in organic solvent or were severely disturbed by Cu2+. In this work, we designed a turn-on fluorescence probe (CT) for Ga3 + based on coumarin schiff-base derivatives incorporating with L-threonine. By virtue of their excellent properties, such as large Stokes shift, less toxicity, ease of modification and visible wavelength emission, coumarin and its derivatives have an ideal skeleton structure to construct various fluorescent probes [48]. L-Threonine not only can combine with metal ions but also greatly enhances the water solubility of coumarin derivative. Hence, probe CT has been proven to be a simple and convenient sensor that shows excellent water solubility, high selectivity, and visual fluorescent switching behavior toward Ga3+. Probe CT is shown in Scheme 1. 2. Results and discussion 2.1. Metal ion selectivity and competition experiments In order to investigate the effect of various metal ions on the fluorescence spectra of CT, the ions Li+, Na+, K+, Mg2 +, Ca2 +, Ba2 +, Cr3 +, Fe3 +, Al3 +, In3 +, Ni2 +, Cu2 +, Zn2 +, Ag+, Pb2 +, Ga3 +, Sn2 +, Mn2 +, Co2 +, and Hg2 + (as their nitrate salts) were used to evaluate the metal ion binding properties of CT in aqueous solution (Tris–HCl, 0.1 mmol·L−1, pH 7.0). As shown in Fig. 1, compound CT had a large effect only with Ga3 + among the metal ions examined. In the presence of Ga3+, CT showed fluorescence enhancement that is so strong and a dramatic change from low-fluorescent to blue fluorescent which could easily be identified by the naked eye under UV lamp during the detection process. Meanwhile, the maximum of CT–Ga3+ emission wavelength was red-shifted from 450 to 475 nm. Other metal ions gave no distinct

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The job's plot [49] also provided additional evidence for the formation of a 1:1 complex of CT–Ga3 +. By keeping total concentration of Ga3+ and CT at 10.0 μmol·L−1, and changing the molar ratio of CT and Ga3+ from 0.1 to 0.9, the fluorescence intensities of CT in the absence (F0) and presence (F) of Ga3+ were determined respectively. A plot of (F–F0) versus [CT] / ([CT] + [Ga3+]) shows that the value goes through a maximum at a molar fraction of about 0.5, indicating a 1:1 stoichiometry complex formation exactly (Fig. 4).

2.3. Emission spectra and association constant K Scheme 1. Synthesis route of probe CT.

response to CT in the fluorescence spectra. This obvious feature reveals that compound CT has high selectivity for Ga3+. To further investigate the interference of other metal ions to the detection of Ga3 +, competition experiments were performed in which various metal ions (50.0 μmol·L− 1) were added to a solution of CT (10.0 μmol·L−1) in the presence of Ga3+ (50.0 μmol·L−1). As shown in Fig. 2, the addition of interference ions resulted in hardly any changes of the fluorescence intensity of CT in the presence of Ga3+. Although the binding ability of Fe3+ with CT is slightly less than Ga3+, and generates a little interference with detection toward Ga3+, the fluorescence emission of CT–Ga3+ was observed clearly in the presence of Fe3+. The result suggested that CT could be an effective probe for the detection of Ga3+ in aqueous solution. 2.2. UV–vis experiments and stoichiometry complexation The binding properties of CT with Ga3+ were measured by a UV–vis titration study in the first place in aqueous solution (Fig. 3). Upon the addition of Ga3+, the absorbance bands at 276 nm, 310 nm enhanced and at 344 nm decreased in the UV–vis spectra. Moreover, the absorbance at 344 nm gradually reduced in the range of 0–1.0 μmol·L−1 of Ga3 +, but no longer decreased when the concentration of Ga3 + is above 1.0 μmol·L−1; such absorbance changes of CT in the presence of 0–1.1 μmol·L−1 of Ga3+ were ascribed to a 1:1 complex formation.

As shown in Fig. 5, the free ligand CT (10.0 μmol·L−1) alone displayed weak fluorescence intensity at 450 nm when it was excited at 344 nm in aqueous solution (Tris–HCl, 0.1 mmol·L−1, pH 7.0). Upon the addition of Ga3 + into the aqueous solution of CT (10.0 μmol·L−1), the fluorescence intensity was distinctly risen. When the concentration of Ga3+ arrived at 1.1 equiv., the fluorescence intensity reached the maximum value (about 11-fold enhancement). This phenomenon can be understood as the sensing mechanism of C_N isomerization [50]. The C_N isomerization of compound CT quenched fluorescence emission of CT. Then the quenched fluorescence could relapse dramatically due to the suppression of C_N isomerization by chelating metal ion (Fig. 6). As a result, the fluorescence intensity of CT (ΦF = 0.65%) increased remarkably at 475 nm (ΦF = 8.98%) upon the addition of Ga3 + ions. Thus we can conclude that CT may serve as a ˝turn-on˝ sensor for Ga3+. In order to further investigate the sensitivity of CT to Ga3+, the detection limits of CT for Ga3+ were tested upon excitation at 344 nm in aqueous solution. As shown in Fig. 5 inset, the plots of F−F0 vs. Ga3+ concentration (0–11.0 μmol·L− 1) fit good linear Stern–Volmer relationship [51]: F−F0 = 8.14E5[Al3 +] + 0.79 (R2 = 0.9838), and the relative standard deviation for five repeated measurements of 0 μM of Ga3 + was 1.57%. The detection limit (DL = 3σ/k) for Ga3 + was 5.79 × 10−8 mol·L−1 when the concentration of CT was 10.0 μmol·L−1, which illustrated that the detections of CT toward Ga3 + were highly sensitive. The association constant K of the complex CT–Ga3+ was then calculated to be 3.7 × 104 M−1 with a linear relationship (Fig. 7) by Benesi–

Fig. 1. Fluorescence spectra of CT (10.0 μmol·L−1, λex = 344 nm) with addition of various metal ions (50.0 μM) in aqueous solution (Tris–HCl, 0.1 mM, pH 7.0).

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Fig. 2. Fluorescence responses of CT (10.0 μmol·L−1, λex = 344 nm) to various metal ions (50.0 μmol·L−1) (blue column), and upon the subsequent addition of Ga3+ (red column) in aqueous solution (Tris–HCl, 0.1 mmol·L−1, pH 7.0). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.4. Structure of the CT–Ga3+ complex

Hildebrand method, Eq. (1) [52]. 1 1 1 ¼ : þ F− F0 Kð F max − F min Þ½Ga3þ  F max −F min

ð1Þ

The reasonable evidence was provided by the structure of CT complexed with Ga3+, which can better explain the fluorescence sensing

Fig. 3. Absorbance titration spectra of CT (1.0 μmol·L−1) in water (Tris–HCl, pH 7.0) upon the addition of increasing amounts of Ga3+ (0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, and 1.1 μmol·L−1).

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Fig. 4. Job's plot of the complexation between CT and Ga3+, the total concentration of CT and Ga3+ is 10.0 μmol·L−1 (Tris–HCl, pH 7.0).

mechanism. The structure unambiguously demonstrates that the sensor CT, acting as a quadridentate ligand via its N and O atoms, forms a 1:1 complex with Ga3+. The bond formation between CT and Ga3+ is further supported by 1H NMR. In Fig. 8, with the addition of 2.0 equiv. Ga(NO3)3, the CH_N proton (4) shifted to up-field 8.95 ppm from 10.41 because of the N atom (Schiff-base) complexing with Ga3+. Two peaks diminished at 8.11 and 11.91 ppm, which results from the inexistence of the OH proton (1 and 3). In addition, the proton (2) peak still exists, which is attributed to the fact that the O atom of C_O in COOH

coordinates to Ga3 +. The plausible binding mode of CT with Ga3 + is shown in Fig. 8. 2.5. Responsiveness and adaptability Adaptability and responsiveness were determined using time– response plot. Time-dependence for binding of the sensor CT with Ga3+ is given in Fig. 9. Following the addition of 1.0 equiv. Ga3+ ions to 10.0 μmol·L−1 sensor CT, the fluorescence intensity of CT was turn

Fig. 5. Fluorescence spectra of CT (10.0 μmol·L−1, λex = 344 nm) upon the titration of Ga3+ (0.1–1.1 equiv.), inset: the Stern–Volmer relationship plot of F–F0 vs. [Ga3+] in aqueous solution (Tris–HCl, pH = 7.0), DL = 5.79 × 10−8 mol·L−1; [CT] = 10.0 μmol·L−1; λex = 344 nm.

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Fig. 6. Fluorescence sensing mechanism of probe CT for Ga3+.

on rapidly, and reached a stable value within 2 min. The short response time demonstrates that CT possesses good responsiveness and adaptability to Ga3+. 2.6. The pH effect To check the pH effecting on the fluorescence response, the fluorescence spectra of CT with Ga3+ under different pH conditions (from 4 to 10) were investigated. The emission intensity was seen to change depending on pH values. As shown in Fig. 10, the emission intensity of the complex between CT and Ga3+ reached to the maximum at pH 6–8. The acid system (pH b 6) causes the dissociation of the complex and hydrolysis of the CT, therefore the decrease in the emission intensity. At higher pH values (pH N 8), the CT competes with OH− ions for the Ga3+ ion, increasing the formation of the metal ion precipitation, accordingly lowering the emission intensity [53]. Hence, using CT to detect Ga3+ ion is shown to be proper within a pH range (6–8). Additionally, it's very interesting that the fluorescence intensity of CT displays a remarkable enhancement upon the increase of pH (pH N 8), which is attributed to the N atom of C_N possibly formed hydrogen bonds with OH− from solvent and the C_N isomerization was weakened.

2.7. The decay of lifetime The lifetimes of CT in the presence of different Ga3+ are presented in Fig. 11. The fluorescence lifetimes were measured by single photon counting at the excitation 340 nm of NanoLED source. The decay of CT was found to be a good single-exponential decay. With increasing concentration of Ga3+, the lifetimes increase from 4.62 to 4.81 ns, accordingly. The unapparent changes of lifetimes demonstrate that the turn-on process of probe CT by Ga3+ is a static mechanism, in which the fluorescence improvement should originate from complexation of CT and Ga3+. To further confirm the chelation of CT and Ga3+, the reversibility of the probe function was tested by titration of the CT–Ga3+ complex with sodium pyrophosphate (Na4P2O7). Addition of Na4P2O7 into water solution (Tris–HCl, pH 7.0) of the CT–Ga3+ complex induced the opposite trend in the emission spectra to that observed on titration with Ga3+. Upon the addition of 1 equiv. of Na4P2O7, the optical fluorescence intensity returned to the levels observed for the free compound CT. This shows that the process of titrating sensor CT with Ga3+ was reversible, and the reversible process could be repeated several times with little fluorescence efficiency loss (Fig. 12).

Fig. 7. Benesi–Hildebrand plot (λex = 344 nm) of CT, assuming 1:1 stoichiometry for association between CT and Ga3+ in water (Tris–HCl, pH 7.0).

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Fig. 8. The comparative trial of H NMR spectra of CT and CT–Ga3+ (d6-DMSO) and the plausible binding mode of CT with Ga3+.

3. Experimental 3.1. Instrumentation and chemicals Melting point was determined using an SGW X-4 digital melting point apparatus. 1H NMR spectrum was run on a Varian Mercury-Plus 400 MHz NMR spectrometer using TMS as the

internal standard. Elemental analysis was taken with a Vario EL CHNS elemental analyzer. Mass spectrum was recorded with a VG ZAB-HS double focusing mass spectrometer. Fluorescence spectra, fluorescence lifetime and quantum efficiency were measured with a Horiba Fluorolog 3-TSCPC. All chemicals were purchased from Aladdin Industrial Corporation and used without further purification.

Fig. 9. Fluorescence turn-on profile of addition Ga3+ (1.0 equiv.) to CT (10.0 μmol·L−1) in water (Tris–HCl, pH 7.0).

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Fig. 10. Emission of probe CT and CT–Ga3+ complex (10.0 μM) in aqueous solutions at different pH values at 475 nm, λex = 344 nm.

3.2. Synthesis of probe CT 8-Formyl-7-hydroxy-4-methylcoumarin: 8-formyl-7-hydroxy-4methylcoumarin was prepared by the known method [54]. 8-((s-3-carboxyl-2-hydroxy-3-ylimino)methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CT): 8-formyl-7-hydroxy-4-methylcoumarin (0.19 g, 0.1 mmol) and L-threonine (0.12 g, 0.1 mmol) were dissolved in anhydrous ethanol (10 mL). The reaction mixture was refluxed for 6 h and then the mixture was cooled to room temperature. The precipitate

was filtered off, washed with cold ethanol two times and dried in vacuum to give the desired product as yellow solid. Yield 0.20 g (65.6%). m.p. 220.5–222.6 °C. 1H NMR (d6-DMSO, 400 MHz, δ ppm): 1.12 (d, J = 9.0 Hz, 3 H); 2.35 (s, 3H); 4.24 (m, 1H) ; 4.47 (s, 1H); 6.02 (s, 1H); 6.56 (d, J = 9.0 Hz, 1H); 7.63 (d, J = 9.0 Hz, 1H); 8.91 (s, 1H); 14.57 (s, 1H). 13C NMR (d6-DMSO, 100 MHz, δ ppm): 18.93, 20.82, 66.85, 70.37, 104.15, 106.62, 107.83, 119.61, 131.71, 155.03, 156.36, 160.01, 161.80, 170.94, 176.43. Found, %: C, 58.98; H, 5.05; N, 4.56. Calculated for C14H13NO6: C, 59.01; H, 4.95; N, 4.59. FT-IR (KBr, cm−1) 3423 (ν–OH,

Fig. 11. Fluorescence decay curves of CT and CT + Ga3+ in aqueous solution (Tris–HCl, pH 7.0) obtained by NanoLED source 340 nm.

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Fig. 12. Fluorescence intensity showing the reversible complexation between CT and Ga3+ by introduction of Na4P2O7.

alcohol), 2975 (νCH3), 1713 (νC_O), 1649(νC_N,) 1528–1592 (νAr), 1428 (νCOOH), 1348 (νC–O), 1293 (νC–N), 1229 (νAr–OH),1114(νC–O, alcohol). ESI-MS: m/z: 306.1 ([M + H]+). 4. Conclusions In conclusion, a water-soluble fluorescent chemosensor CT for Ga3+ based on coumarin schiff-base conjugate has been designed and synthesized. It showed high sensitivity and selectivity for Ga3+ recognition in comparison to other metal ions in aqueous solution (Tris–HCl, pH 7.0). The sensor CT exhibited 1:1 coordinates with Ga3 + and rapidly response to Ga3+ detection within 2 min. We believe that the fluorescence sensor CT may be utilized for the detection and analysis of various gallium-related issues in environmental, biological and medical areas. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (KYLX15_0125) and National Major Scientific Instruments and Equipment Development Projects (2014YQ060773). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2015.11.012. References [1] J.M. Kikkawa, D.D. Awschalom, Nature 397 (1999) 139–141. [2] H. Li, Y. Yang, J. Liu, Appl. Phys. Lett. 101 (2002) 073511. [3] H.M. Kim, Y.H. Cho, H. Lee, S.I. Kim, S.R. Ryu, D.Y. Kim, T.W. Kang, K.S. Chung, Nano Lett. 4 (2004) 1059–1062. [4] R. Trotta, P. Atkinson, J.D. Plumhof, E. Zallo, R.O. Rezaev, S. Kumar, S. Baunack, J.R. Schröter, A. Rastelli, O.G. Schmidt, Adv. Mater. 24 (2012) 2668–2672. [5] J. Yoon, S. Jo, I.S. Chun, I. Jung, H.S. Kim, M. Meitl, E. Menard, X. Li, J.J. Coleman, U. Paik, J.A. Rogers, Nature 465 (2010) 329–333. [6] M. Yao, N. Huang, S. Cong, C.Y. Chi, M.A. Seyedi, Y.T. Lin, Y. Cao, M.L. Povinelli, P.D. Dapkus, C. Zhou, Nano Lett. 14 (2014) 3293–3303. [7] X.L. Huang, J. Chai, T. Jiang, Y.J. Wei, G. Chen, W.Q. Liu, D.X. Han, L. Niu, L.M. Wang, X.B. Zhang, J. Mater. Chem. 22 (2012) 3404–3410.

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