Salicylaldehyde based colorimetric and “turn on” fluorescent sensors for fluoride anion sensing employing hydrogen bonding

Sensors and Actuators B 158 (2011) 427–431

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Salicylaldehyde based colorimetric and “turn on” fluorescent sensors for fluoride anion sensing employing hydrogen bonding Qian Li a,b , Yong Guo a , Jian Xu a , Shijun Shao a,∗ a Key Laboratory of chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China

a r t i c l e

i n f o

Article history: Received 15 February 2011 Received in revised form 14 May 2011 Accepted 1 June 2011 Available online 13 June 2011

a b s t r a c t Two salicylaldehyde based colorimetric and fluorescent chemosensors 1 and 2 were developed. Both receptors 1 and 2 showed unique selectivity for the fluoride anions over other anions in DMSO solution. [TBA] OH and 1 H NMR titration experiments revealed that the F− -induced colorimetric and “turn on” fluorescence response were driven by hydrogen bonding interaction between the OH protons and F− . © 2011 Elsevier B.V. All rights reserved.

Keywords: Salicylaldehyde Colorimetric “Turn on” fluorescence Hydrogen bonding

1. Introduction The design and development of selective and sensitive optical sensors for anions have gained considerable attentions, as anions play major roles in a wide range of chemical and biological processes [1–5]. Recently, much effort has been devoted to developing fluorescent anion sensors, due to their simplicity, high degree of specificity and low detection limit, however, only few of them are “turn on” fluorescent sensors [6–15]. In terms of sensitivity concerns, sensors exhibiting fluorescence enhancement (fluorescence “turn on”) in sensing process are favored over those showing fluorescence quenching (fluorescence “turn off”), because fluorescence “turn off” sensors may report false results caused by other quenchers in practical samples [16]. Despite the higher acidity of the hydroxyl group, far less attention has been given to the OH based anions receptors and sensors [17–20]. As part of our ongoing studies on simple and easy-to-make anions receptors and sensors [21–24], here we presented Salicylaldehyde based anions receptors 1 and 2 (Scheme 1), synthesized by one facile step condensation, behaved as a highly selective F− sensor in both colorimetric and fluorometric analysis. An earlier reported on the possibility of the analogous bisazine-type ligand [25–28] to act as a bischelating ligand toward transition metal ions has led us to synthesize the receptors 1 and 2 for recognition of

∗ Corresponding author. E-mail address: sjshao [email protected] (S. Shao). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.06.007

anions. In the bisazine-type ligand, the electron density are located on the C N-N C moiety, the binding of metal ions to the ligand would affect the electron density and thus influence the fluorescence of the ligand. In our paper, the strategy for the design of the receptors was based upon the idea that the binding of anions to the receptors may trigger the conformational switching of C N and influence the charge distribution of entire conjugate system of the receptors, which would modulate the spectral properties of the receptors. 2. Experimental 2.1. Materials All reagents for synthesis obtained commercially were used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Sigma–Aldrich Chemical, stored in avacuum desiccator. 2.2. Synthesis of receptor 1 1.22 g salicylaldehyde was dissolved in ethanol (50 ml) and 0.25 ml of hydrazine hydrate (99%) was added at room temperature. The reaction mixture was stirred at room temperature overnight. After completion of the reaction, the obtained yellow precipitate was filtered and washed several times with cold ethanol to yield the pure 1. Yield 85%.

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Scheme 1. Structure of receptor 1 and 2.

Fig. 1. Color changes of receptor 1 (a) and 2 (b) in DMSO solution with the addition of F− .

EI-MS: m/z 241.2 (M+H+ ). 1 H NMR (400 Hz, DMSO-d ), ı: 11.122 (s, 2H, OH), 8.999 (s, 2H, 6 N CH), 7.676–7.699 (m, 2H, Ar–CH), 7.372–7.414 (m, 2H, Ar–CH), 6.943–6.982 (m, 4H, Ar–CH). Anal. calcd. for C14 H12 N2 O2 : C, 69.99; H, 5.03; N, 11.66. Found: C, 69.74; H, 4.86; N, 11.64. Melting point: 219–220 ◦ C.

2.3. Synthesis of receptor 2 A solution of salicylaldehyde (1.22 g, 10 mmol) and 1,2phenylenediamine (0.54 g, 5 mmol) in 50 mL of ethanol was stirred at room temperature overnight. After completion of the reaction, the obtained yellow precipitate was filtered and washed several times with cold ethanol to yield the pure 2. Yield 82%. EI-MS: m/z 317.2 (M+H+ ). 1 H NMR (400 Hz, DMSO-d ), ı: 12.937 (s, 2H, OH), 8.932 (s, 2H, 6 N CH), 7.648–7.671 (m, 2H, Ar–CH), 7.387–7.474 (m, 6H, Ar–CH), 6.944–6.987 (m, 4H, Ar–CH). Anal. calcd. for C20 H16 N2 O2 : C, 75.93; H, 5.10; N, 8.86. Found: C, 75.36; H, 4.91; N, 8.78. Melting point: >300 ◦ C.

The UV–vis spectrum of receptor 1 in DMSO exhibited two absorption bands with maxima at 293 and 354 nm. Titration of receptor 1 with fluoride anions resulted in a bathochromic shift absorption band with a maxima at 461 nm (Fig. 2). Three isosbestic points at 270, 310 and 390 nm were observed during the titration, indicating a single component was produced in response to the interaction between receptor 1 and fluoride anions. No obvious changes in absorption spectrum were observed on the addition of other anions (Supplementary data, Fig. S1). The corresponding fluorescence titrations were carried out in DMSO solution. As shown in Fig. 3, receptor 1 showed very weak fluorescence, with an emission band at 520 nm when excited at 396 nm. Addition of fluoride anions to the solution of receptor 1 induced an increas-

3. Results and discussion The anion binding properties of receptors 1 and 2 were investigated by UV–vis, fluorescence and 1 H NMR spectroscopy. The result of receptor 1 treated with various anions (F− , Cl− , Br− , − I , AcO− , ClO4 − , HSO4 − , H2 PO4 − , tetrabutylammonium salts, TBA) in DMSO solvent was shown in Fig. 1(a). The color of the solution changes from colorless to yellow upon addition of F− . In contrast, no obvious changes in color were observed upon addition of other anions.

Fig. 2. Changes in the UV–vis absorption spectrum of receptor 1 (1.5 × 10−5 M) in DMSO upon addition of F− anions in TBA salts form (0–20 equiv.).

Q. Li et al. / Sensors and Actuators B 158 (2011) 427–431

Fig. 3. Changes in the emission spectrum of receptor 1 (1.5 × 10−5 M) in DMSO upon addition of F− anions in TBA salts form (0–5 equiv.). Inset: the plot of emission intensity at 525 nm versus F− concentration.

ing fluorescence emission together with a red shift (525 nm). The intensity increase was almost saturated upon addition of 3 equiv. of fluoride anions. Addition of AcO− caused a slight enhancement in fluorescence emission. As with the UV–vis titrations, none of the other anions caused obvious changes in fluorescence emission (Supplementary data, Fig. S2). These data clearly suggested that the receptor 1 behaved as a sensitive and selective colorimetric and “turn on” fluorescence sensor for fluoride anions. The UV–vis and fluorescence emission titrations were repeated with the receptor 2. Receptor 2 exhibited two absorption bands at 269 and 331 nm in DMSO solvent, addition of this receptor with fluoride anions resulted in a new absorption band at 435 nm with an isosbestic point at 370 nm (Fig. 4). Accordingly, the color of the solution changes from colorless to yellow (Fig. 1(b)). Receptor 2 displayed weak fluorescence at 420 nm (excitation at 390 nm). Addition of fluoride anions to the solution of receptor 2 induced a red shift emission band at 505 nm (Fig. 5). The emission intensity at 505 nm increased significantly upon the addition of fluoride, however, the fluorescence response of receptor 2 were not as sensitive as receptor 1, the fluorescence intensity at 505 nm began to increase upon addition of 10 equiv. of fluoride anions. In the same conditions, no significant changes in absorption or emission were

Fig. 4. Changes in the UV–vis absorption spectrum of receptor 2 (1.5 × 10−5 M) in DMSO upon addition of F− anions in TBA salts form (0–20 equiv.).

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Fig. 5. Changes in the emission spectrum of receptor 2 (1.5 × 10−5 M) in DMSO upon addition of F− anions in TBA salts form (0–20 equiv.). Inset: the plot of emission intensity at 505 nm versus F− concentration.

observed upon addition of other anions (Supplementary data, Figs. S3 and S4). The association constant of receptor 1 with fluoride anions was calculated to be 4.2 × 103 M−1 through the Benesi–Hildebrand [12,29,30] analysis according to a 1:1 stoichiometry (Supplementary data, Fig. S5). The receptor 2 associates with fluoride anions in a 1:1 stoichiometry, with a association constant of 5.5 × 103 M−1 (Supplementary data, Fig. S6). The protic solvents (MeOH, H2 O) were added to the solution of receptors 1 and 2, respectively, to check the reversibility of the responses of the sensors. Interestingly, when a small amount of water (1%, v%) was added, the color of the solution of receptors 1 and 2 changed back to colorless, which suggested that the protic solvents destroyed the hydrogen bonding between receptors 1 and 2 and fluoride anions. The above results indicated that the interactions between receptors 1 and 2 and fluoride anions were hydrogen bonding interaction. In order to further determine the nature of sensing process, titrations of receptors 1 and 2 with [TBA] OH was carried out. The absorption and emission spectral changes of receptor 1 upon addition of [TBA] OH were different with those of upon addition of F− (Supplementary data, Figs. S7 and S8), which further indicating

Fig. 6. Partial 1 H NMR spectra of receptor 1 on addition of 2 equiv. of F− (TBA salts) in DMSO-d6 .

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Scheme 2. Proposed binding mode of receptor 1 and 2 with fluoride anion.

a hydrogen bonding process involved in the interaction between receptor 1 and F− . Again, the [TBA] OH titration experiments suggested the hydrogen bonding interaction between receptor 2 and F− (Supplementary data, Figs. S9 and S10). The recognition properties of receptors 1 and 2 with fluoride anions were also studied by 1 H NMR titration experiments in DMSO-d6 . As shown in Fig. 6, upon addition of 2 equiv. fluoride anions, the OH signal of receptor 1 disappeared completely, indicating that and the F− indeed interacts with phenolic OH protons. In contrast, Other H signals shifted upfield due to hydrogen bonding interaction resulting in an increase in the electron density of the salicylaldehyde moities (through-bond effect) [15,17,30–32]. As for receptor 2, the chemical shift of phenolic OH protons at 12.94 ppm, indicating a hydrogen bond between phenolic OH protons and imine nitrogen fromed. The phenolic OH protons disappeared completely upon addition of fluoride anions, and the other H signal moved upfield significantly due to through-bond electronic propagation effect induced by the hydrogen bonding (Supplementary data, Fig. S11). From the UV–vis and 1 H NMR titrations spectra, possible binding models of receptors 1 and 2 with F− was proposed, which was shown in Scheme 2. Upon binding of fluoride anions, the C N isomerization of receptors 1 and 2 were restricted and the ICT (intramolecular charge transfer) interaction between electron-rich imine (C N–) and electron-deficient fragments were enhanced, which were responsible for the spectral behaviours. The population of ␲-electrons on the whole conjugate of the receptors, and thus resulted in fluorescence enhancement. As for receptor 2, about 10 equiv. of F− needed to destroy the internal hydrogen bond between phenolic OH protons and imine nitrogen and induce the conformational switching of C N, which may resulted in the insensitive fluorescence response of receptor 2. 4. Conclusion In summary, we have developed new colorimetric and “turn on” fluorescent chemosensors 1 and 2 for fluoride anions. The changes in absorption and fluorescence spectra were driven by the hydro-

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Biographies Qian Li received his bachelor degree from Central South University in 2007. He is currently a PhD candidate in Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences. His main research fields are fluorescent probes design and chemical sensors. Yong Guo received his PhD in Analytical Chemistry from Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences in 2005. He is now working in Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences as a professor. His current interest includes supramolecular chemistry and molecular recognition. Jian Xu received his master degree in Analytical Chemistry from Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences in 2007. He is now working in Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences as an Assistant professor. His current interest is molecular recognition. Shijun Shao received his PhD in Organic Chemistry from Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences in 2000. He is now working in Lan Zhou Institute of Chemical Physics, Chinese Academy of Sciences as a professor. His current interest includes supramolecular chemistry and anions recognition.