AIE active turn-off fluorescent probe for the detection of Cu2 + ions

Accepted Manuscript AIE active turn-off fluorescent probe for the detection of Cu2+ ions

Mehboobali Pannipara, Abdullah G. Al-Sehemi, Abul Kalam, Abdullah M. Asiri, Muhammad Nadeem Arshad PII: DOI: Reference:

S1386-1425(17)30309-8 doi: 10.1016/j.saa.2017.04.045 SAA 15098

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

29 November 2016 22 March 2017 18 April 2017

Please cite this article as: Mehboobali Pannipara, Abdullah G. Al-Sehemi, Abul Kalam, Abdullah M. Asiri, Muhammad Nadeem Arshad , AIE active turn-off fluorescent probe for the detection of Cu2+ ions. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi: 10.1016/ j.saa.2017.04.045

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ACCEPTED MANUSCRIPT AIE active turn-off fluorescent probe for the detection of Cu2+ ions a

Mehboobali Pannipara *, Abdullah G. Al-Sehemi a,b, Abul Kalam a,b , Abdullah M. Asiri c ,d, Muhammad Nadeem Arshad c ,d

Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box

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9004, Abha 61413, Saudi Arabia.

Department of Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha

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61413, Saudi Arabia.

Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203,

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Jeddah 21589, Saudi Arabia

Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box

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80203, Jeddah 21589, Saudi Arabia

* Corresponding Author Address: E-mail: [email protected] (Mehboobali Pannipara) Tel.: +966 553956503 Fax: 0096672418426

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Abstract A novel AIE active Schiff base compound (receptor 1) has been designed, synthesized and characterized spectroscopically. Receptor 1 show weak emission in solution state but emit strongly in solid state. The investigation on the AIE behavior of receptor 1 using solvent-non-

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solvent (THF-Water) interaction reveals a maximum PL intensity on reaching the water fraction

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90% (fw =90) due to nanoaggregation. Furthermore, demonstration of metal ion sensing

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application of receptor 1 unravel that it could act as selective and sensitive sensor for Cu2+ ions in aqueous solution via fluorescence “turn-off” manner. The quenching behavior of receptor 1 in

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presence of Cu2+ ions were evaluated using Stern-Volmer plot.

Keywords: Fluorescent probe; Aggregation-induced emission; Schiff base, Cu2+

ACCEPTED MANUSCRIPT Introduction Luminescent materials derived from organic compounds have attracted much attention in the past decades owing to their potential applications in thefields of organic electronics, optoelectronics, sensors and informational displays [1-4]. The luminescent property of these architectures arises from the extended π-electron conjugation and hence altering the π-extended system can be exploited for various functions such charge carrier transport, nonlinear optical

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response and security features [5, 6].

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Research on fluorescence based molecular probes for the selective and sensitive detection of

transition metal ions has witnessed a tremendous growth over the last decades [7, 8]. Metals are

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essential for the human body that takes part in a variety of metabolic activities such as gene transcription, DNA-binding proteins and neural signal transmission of living organisms [9, 10].

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Copper is the third abundant transition metal in the human body that comes after Fe and Zn. Despite being an essential element in biological system as it is considered as one of the vital

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nutrients to plants and animals, copper has a toxic impact on living organisms being associated with several diseases at higher concentrations [10]. Hence, it is extremely preferred to spot them

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in the related systems by means of easy and affordable methods. Compared with the procedures based on expensive equipment, sensing of metals with the

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help of easily synthesized receptor and nominal instrumental support with the help of small organic molecule-derived probes established on color or fluorescence changes has the benefits of

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low cost, high sensitivity and appropriate operation. A colorimetric probe will stimulate color changes on addition of the target ion which can be observed by the naked eyes. Fluorescent

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changes imparted by fluorescent probes can be analyzed using a fluorometer or even with a portable UV lamp [11]. Designing and developing such a probe involves linking a recognition unit to signal reporting chromophore that can act as efficient binding site [12].

Aggregation induced emission (AIE) is relatively new phenomena discovered by Tang and group in 2001, in which compounds do not emit light in solution, but exhibit strong luminescence in the aggregation state/ in poor solvents [13]. This phenomenon has efficiently resolved aggregation quenching processes (ACQ) in order to increase the fluorescence quantum

ACCEPTED MANUSCRIPT yield that has troubled researchers for a long time. Recently, AIE active compounds contribute widespread interest and brought renaissance to the application of organic luminescent materials where the optoelectronic properties of these materials can be tuned and tailored for the useful applications in optoelectronic devices [14], fluorescence based sensor systems [15] and optical waveguides [16]. In the present contribution we have successfully designed and synthesized a novel Schiff base derivative having AIE properties. The photophysical properties of the designed

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molecule have been investigated in detail and demonstrated the sensing property towards Cu2+

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ions in a turn-off manner by exploiting AIE properties. 2. Experimental 2.1. Materials and methods

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Analytical grade chemicals and solvents used in this study were used without further purification. Reaction was monitored by TLC with the aid of UV light. 2-hydroxy-1-

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naphthaldehyde, 3-Amino-5-phenylpyrazole and metal salts were purchased from Sigma– Aldrich and Merck. Stock solution of the title compound and its dilutions were prepared for UV–

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visible and fluorescent study at different concentrations. Stock solutions of the salts of Ag+, Fe3+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mg2+, Mn2+, Cd2+and Pb2+were prepared in distilled water and test

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solutions for metal ion detection were prepared by diluting appropriate aliquot of each metal ion stock and compound stock solution to desired concentration. 10 μM solutions of receptor 1 in

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THF or water/THF mixtures with different water fractions were prepared to analyze the AIE

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characteristics.

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2.2 Spectral measurements

Gallenkamp melting point apparatus was used to determine the melting point and the infrared (IR) spectra were recorded on Shimadzu FT-IR 8400S infrared spectrophotometer using KBr pellets. The NMR (1H and

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C) spectra were recorded on a Bruker DPX-600 at 600 MHz

and 150 MHz, respectively, using TMS as the internal standard. The chemical shift values are documented on δ scale and coupling constants (J) in Hertz; Splitting patterns were entitled as follows: s: singlet; d: doublet;m: multiplet. PG UV-160A spectrophotometer was used to record the UV-Vis electronic absorption spectra, and the steady-state fluorescence spectra were

ACCEPTED MANUSCRIPT measured using Varain Cary Eclipse spectrofluorophotometer using a rectangular quartz cell of dimensions 0.2 cm x 1 cm.

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2.3. Synthetic procedure for 1-[(5-Phenyl-1H-pyrazol-3-ylimino)-methyl]-naphthalen-2-ol (Receptor 1)

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Receptor 1 was synthesized by the reaction of 2-Hydroxy-1-naphthaldehyde and 5Amino-3-phenylpyrazole in ethanol by adding catalytic amount of acetic acid [17]. The synthetic

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pathway is shown in Scheme 1. Briefly, two drops of acetic acid were slowly added to the stirred solution of 2-Hydroxy-1-naphthaldehyde (172 mg, 1 mmol) in 20 mL ethanol followed by

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5-Amino-3-phenylpyrazole (160 mg, 1 mmol). The mixture was stirred and heated to reflux for 4 hours. After the completion of the reaction (checked by TLC), the reaction mass was allowed to

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stand overnight. The solid product thus obtained was collected by filtration, dried and recrystallized from ethanol as bright yellow solid (80–85 % yield). The structure of the 13

C NMR and X-ray crystallography. Melting

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compound was confirmed by IR, 1HNMR and

point: 192-1940 C;1H-NMR (600 MHz, d6-DMSO) δ: 5.18 (1H, OH), 8.24 (1H, S, CH=N), 7.60

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(2H, d), 7.5 (1H, dd), 7.65 (1H, dq), 8.2 (1H, d), 7.40 (1H, d), 7.28 (1H, NH), 6.6 (1H, S)6.726.80 (5H, m); 13C-NMR (150 MHz, d6-DMSO) δ: 125.70, 126.72, 128.30, 129.33, 133.6, 154.42,

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112.35, 153.74, 117.8, 174.49, 148.56, 122.62, 122.72, 125.74; FT-IR (KBr) (cm-1); 2900-3200,

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1030.2, 1139.9, 1197.1, 1250.9, 1289.82, 1365.58, 1448.9, 1541.5.

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2.4 X-ray crystal structure of Receptor 1

Receptor 1 was crystallized under slow evaporation in ethanol-chloroform solution. This effort was done to endorse our spectroscopic data sets and understand the geometrical behavior of the molecule. Finally, the suitable crystal was picked up under microscope and glued over fiber glass. A hollow copper tube with magnetic base was used to support to glass fiber. One crystal was mounted on Agilent SuperNova (Dual source) Agilent Technologies Diffractometer, equipped with micro focus Cu/Mo Kα radiation for data collection. Mo Kα radiations were used for these samples. The data collection was accomplished using CrysAlisPro software [18] at 296 K under the Cu Kα radiation. The structure solution was performed using SHELXS–97 methods

ACCEPTED MANUSCRIPT and refined by full–matrix least–squares methods on F2 using SHELXL–97 in-built with WinGX [19, 20]. All non–hydrogen atoms were refined anisotropically by full–matrix least squares methods. The Figures were generated through PLATON, ORTEP in built with WinGX and Mercury [21-23]. The aromatic hydrogen atoms were positioned geometrically and treated as riding atoms with C–H = 0.93 Å and Uiso(H) = 1.2 Ueq (C) for all aromatic carbon atoms. The O-H hydrogen atoms was also positioned geometrically and treated as riding model with O-H =

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0.82Å with Uiso (H) = 1.5 Ueq for O atom on the other hand N-H hydrogen atom located

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through fourier map and refined with N-H = 0.89(4) Å with Uiso (H) = 1.2 Ueq for N atom. The

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Crystal data were deposited at the Cambridge Crystallographic Data Centre and deposition number 1538189 has been assigned which is known as CCDC number for title compound. The

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ORTEP diagram of receptor 1 is shown in Fig. 1 and the corresponding crystallographic data collected are displayed in Table. 1. Crystal data can be received free of charge on application to

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CCDC 12 Union Road, Cambridge CB21 EZ, UK. (Fax: (+44) 1223 336-033; e-mail:

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[email protected]).

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Result and Discussion

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Photophysical and Aggregation-induced emission properties

The electronic absorption spectra of 1 have been recorded in solvents of different polarity

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(Fig. 2a). The compound shows two main absorption peaks at ~ 328 nm and 380-385 nm with shoulder peaks at around ~ 456 nm in all solvents studied. As seen in Fig. 2, solvent polarity has

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only slight or negligible effect on the absorption spectra along with relatively high molar extinction coefficient indicating that these could be π-π* type of transition as was found in other related molecules [24]. The fine structure with shoulder peak observed in the absorption band indicates that n-π* transitions also play an important role irrespective of the nature of the solvents. The main absorption band of receptor 1 at~380-385 nm could be assigned to S1←S0 electronic transition (π-π*) and the one centered at higher energy level ~328 nm is probably due to higher order electronic transition of the type Sn←S0. As shown in Fig. 2b, emission spectra of 1 is found to be weakly fluorescent with more structured in aprotic solvents due to attainment of more coplanar structure during excitation.

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Receptor 1 has high solubility in almost all organic solvents used except in water. Mostly, a solvent-non-solvent mixture system is introduced to investigate the AIE behavior of the compounds in aggregated state. Here we used water/ THF mixtures with different water volume fractions (fw, vol %) from 0 to 100 % to explore AIE property of 1 by keeping the concentration fixed at 10 μM. Fig.3 depicts the absorption and fluorescence of behavior receptor 1 in THF

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(solvent)-water (non-solvent) mixture systems and Fig. 4 represents luminescent images of

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receptor 1 in different water fraction (fw= 0 and 90%) with their solid state image under 365 nm

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UV lamp, which clearly illustrate the AIE property of receptor 1. While the absorption spectra of receptor 1 in THF solvent show dispersed and displayed structured absorption spectra with

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almost non-fluorescence emission (Fig. 3), it undergoes a decrease in maximum absorption on increasing the water volume fractions in THF (solvent)-water (non-solvent) systems.

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Furthermore, an abrupt change in the absorption and emission profile was observed on reaching the water fractions 90 and 95%. As seen from Fig. 3a, the maximum absorption wavelength has

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been red shifted to ~ 424 nm with an appearance of level-off tails in the visible region at this fraction. It is evident from the new absorption band in the absorption spectra and red shifted PL

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spectra that the molecular aggregation occurs in the ground state. Moreover, receptor 1 display intense bluish–green fluorescence with unusual fluorescence amplification at this stage (fw=90%)

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at ~ 484 nm, which is about 300-fold higher than in pure solvent (Fig. 3b), indicating the AIE characteristic of the probe molecule. Subsequently, a downward trend on the emission intensity

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was observed on further increasing of the water concentration, i.e at 100% fw. Weak fluorescence emission of receptor 1 in solution state could be attributed to the rapid

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isomerization of the C=N imine bond and the rotation of aromatic rotors around the C-C single bonds that lead to the nonradioactive deactivation of the excited state effectively [25]. On increasing the water fraction will trigger the aggregation of receptor 1 due to intermolecular and intramolecular hydrogen bonding between various H-donors and H-acceptors in the receptor which in turn lock the intramolecular motions (RIM) of aromatic rotors around C-C single bonds and inhibit the C=N imine bond isomerization leading to enhancement of fluorescence by blocking the non-radiative pathway [26, 27].

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Metal sensing studies The sensing behavior of receptor 1 (20 μM) towards various metal ions was investigated by treating it with 10 equiv of varying metal ions namely Ag1+, Fe3+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mg2+, Mn2+ and Pb2+ by measuring the changes in emission intensity. All the experiments

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were done by keeping water fraction 90% (THF: H2O (1:9)) in order to maintain the AIE

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behavior. As seen from Fig. 5, no considerable change in the emission spectra was observed on

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addition of ten equivalents different metal ions, except for Cu2+, Ag2+ and Hg2+. In case of Cu2+, the emission intensity suffered significantly and resulted in almost quenching of fluorescence intensity compared to other tested metal ions which clearly demonstrates that receptor 1 have

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higher selectivity towards Cu2+ ions over other metal ions. The quenching efficiency observed by the addition Cu2+ is about 99.77% (expressed in (I0-I0)/I0×100) with a considerable decrease in

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quantum yield (Фf) as shown in Fig. 6. The observed quenching of fluorescence by Cu2+ ions may be ascribed due to the paramagnetic nature of Cu2+ which result in chelation-enhanced

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quenching (CHEQ) by metal to ligand charge transfer reaction between Cu2+ and receptor 1 [28].

In order to obtain more insight into the sensing properties of receptor 1 towards Cu2+,

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fluorescence titration experiment was carried out by varying the concentration of Cu2+ ions. As inferred from the Fig. 7, the intense fluorescence of receptor 1 around 484 nm was gradually

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decreased on increasing the concentration of Cu2+ ions on exciting at 365 nm. In fact, treatment of even with less than 1 equiv of Cu2+ ions (0.1 µM) resulted in the quenching fluorescence

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effectively by 56% of the original value and the complete quenching of emission intensity was obtained with 1.5 equiv of metal ions suggesting highly selective sensing ability of receptor 1 towards the detection of Cu2+ ions. Further, compared to other fluorescent probes that are available in the literature for Cu2+ sensing mostly work only in organic solvents whereas the receptor 1 work in aqueous media for detection of copper ions as water is responsible for aggregation and a very low amount of receptor (20 μM) can detect copper ions having less than 1 μM concentration. To understand the nature of quenching mechanism and the extent of quenching, SternVolmer plot was constructed by applying Stern-Volmer equation [29],

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where KSV is the Stern–Volmer quenching constant, Io and I are the original fluorescence intensity and quenched intensity in presence of the quencher concentration [Cu]. The KSV value obtained by plotting I0/I values versus the concentrations of Cu2+ ions were found to be 3.1 x 106

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M-1 with correlation coefficient (R2) equal to 0.79 (Fig. 8). The higher values for Ksv designate

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the efficient interaction between the Cu metal ions and fluorophore. The deviation from the linearity of the plot with an upward curvature indicates that combination of static and dynamic

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quenching mechanism for receptor 1[23]. The changes in the absorption spectra of receptor 1 on addition Cu metal ions also support the static mode of quenching with ground state complex formation

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(Fig. 9) [30].

Conclusion

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In summary, a novel Schiff base derivative have been designed and synthesized. The probe molecule shows weak emission in solution but emit strongly in solid state. AIE behavior of

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this molecule has been elucidated using water/THF mixtures with different water volume fractions and found that the molecule emits strongly on reaching the water fraction 90%. The

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sensing ability of the probe molecule towards various metal ions revealed that it could act as a selective and sensitive sensor for Cu2+ ions through a turn-off fluorescence signaling mechanism.

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The complete quenching of the emission intensity was observed with very low concentration of metal ions (1.5 eq) unravel the efficacy of the probe molecule to quantify trace amounts of Cu2+

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ions in aqueous samples.

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Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through General Research Project under grant number (G. R. P-217-38).

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[23] Macrae, C.F., Edgington, P.R., McCabe, P., Pidcock, E., Shields, G.P., Taylor, R., Towler, M. & van de Streek, J. (2006). J. Appl. Cryst. 39, 453–457.

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[26] Y. Hong, J.W.Y. Lam, B.Z. Tang, Chem. Soc. Rev.40 (2011) 5361–5388. [27] A. Gogoi, S. Mukherjee, A. Ramesh, G. Das, Anal. Chem. 87 (2015) 6974−6979. [28] S. Zhang, Q. Niu, L. Lan, T. Li Sensors and Actuators B: Chemical 240 (2017) 793-800. [29] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third ed., Springer, New

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[30] M. F. Acquavella, M. E. Evans, S. W. Farraher, C. J. Névoret, C. J. Abelt J. Chem.

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Soc., Perkin Trans. 2 (1995) 385-388.

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Figures

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Fig. 1 Labelled ORTEP diagram of Receptor 1 where thermal ellipsoids were drawn at 50%

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probability level.

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Fig. 2 Electronic absorption spectra (a) and emission spectra (b) of receptor 1 (10µM) in different solvents

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Fig. 3. Electronic absorption spectra (a) and emission spectra (b) of receptor 1 (10µM) in H2O/THF mixtures with different water fractions (fw).

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Fig. 4. Luminescent images of 1 in different water fraction (fw= 0 and 90%) with the solid state image under 365 nm UV lamp.

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Fig. 5. Emission spectra of 1 (2 × 10−5M) receptor 1 upon addition of Ag1+, Fe3+, Co2+,Cu2+, Ni+2, Pb2+, Mg2+, Hg2+, Zn2+ and Cd2+ (10 equiv.) in (THF: H2O (1:9)) (λex=365 nm).

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Fig. 6. Emission intensity of 1 in presence of different metal ions (λex=365 nm).

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Fig. 7 PL spectra of 1 as a function of [Cu2+]. [1] =2x 10-5 M in THF:water =1:9, fw=90 (λex=365 nm). Fig. 8 Stern-Vollmer plot for 1(2x10-5M) at various concentration of Cu2+ ions. Fig. 9 Absorbance spectrum of receptor 1 (10µM) and 1 with (10 equiv.) of Cu2+ ions in (THF: H2O

(1:9)).

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Scheme 1

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Fig. 1: Labelled ORTEP diagram of Receptor 1, where thermal ellipsoids were drawn at 50%

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probability level.

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Fig. 2 Electronic absorption spectra (a) and emission spectra (b) of receptor 1 (10µM) in different solvents

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Fig. 3. Electronic absorption spectra (a) and emission spectra (b) of receptor 1 (10µM) in H2O/THF mixtures with different water fractions (fw).

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Fig. 4. Luminescent images of 1 in different water fraction (fw= 0 and 90%) with the solid state image under 365 nm UV lamp.

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Fig. 5. Emission spectra of 1 (20µM) receptor 1upon addition of Ag1+, Fe3+, Co2+,Cu2+, Ni+2, Pb2+, Mg2+, Hg2+, Zn2+, Mn2+ and Cd2+ (10 equiv.) in (THF: H2O (1:9)) (λex=365 nm).

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Fig. 6 Emission intensity of 1 in presence of different metal ions (λex=365 nm).

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Fig. 7 PL spectra of 1 as a function of [Cu2+].[1] =20µMin THF:water =1:9, fw=90 (λex=365 nm).

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Fig. 8 Stern-Vollmer plot for 1(20µM) at various concentration of Cu2+ ions.

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Fig. 9 Absorbance spectrum of receptor 1(10µM) and 1 with(10 equiv.) of Cu2+ ions in (THF: H2O

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(1:9)).

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Empirical formula C20H15N3O Formula weight 313.36 Temperature/K 296 Crystal system monoclinic Space group P21/c a/Å 13.8967(18) b/Å 5.8012(8) c/Å 20.684(4) α/° 90 β/° 108.347(17) γ/° 90 3 Volume/Å 1582.7(5) Z 4 3 ρcalcmg/mm 1.3150 -1 μ/mm 0.084 F(000) 656.3 3 Crystal size/mm 0.45 × 0.05 × 0.03 2θ range for data collection 5.9 to 58.84° Index ranges -18 ≤ h ≤ 17, -7 ≤ k ≤ 7, -26 ≤ l ≤ 21 Reflections collected 10626 Independent reflections 3889[R(int) = 0.0757] Data/restraints/parameters 3889/0/216 2 Goodness-of-fit on F 1.050 Final R indexes [I>=2σ (I)] R1 = 0.0774, wR2 = 1676 Final R indexes [all data] R1 = 0.2055, wR2 = 0.2367 -3 Largest diff. peak/hole / e Å 0.52/-0.57

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Table. 1 Crystal data and structure refinement for Receptor 1

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Graphical abstract

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Highlights

The photophysical properties this compound were studied in detail.

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It can act as fluorescent probe for Cu2+ ions

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Aggregation-induced emission enhancement (AIEE) phenomena of a novel Schiff base has been investigated.