AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media

Accepted Manuscript AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media

Mohammed A. Assiri, Abdullah G. Al-Sehemi, Mehboobali Pannipara PII: DOI: Reference:

S1387-7003(18)30894-3 https://doi.org/10.1016/j.inoche.2018.11.001 INOCHE 7165

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

30 September 2018 31 October 2018 1 November 2018

Please cite this article as: Mohammed A. Assiri, Abdullah G. Al-Sehemi, Mehboobali Pannipara , AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media. Inoche (2018), https://doi.org/10.1016/j.inoche.2018.11.001

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ACCEPTED MANUSCRIPT AIE based “on-off” fluorescence probe for the detection of Cu2+ ions in aqueous media

Mohammed A. Assiri a, b, Abdullah G. Al-Sehemi a,b, Mehboobali Pannipara*

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

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61413, Saudi Arabia. b

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

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9004, Abha 61413, Saudi Arabia.

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

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Abstract A simple hydrazine carboxamide derivative with intramolecular charge transfer and aggregartion

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induced emission (AIE) properties have been designed and synthesized. The derivative (Probe 1)

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shows typical AIE characteristic emission in THF-water mixtures and emits yellowish orange

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color on reaching the water fraction 98%. The probe molecule show selective sensing response for Cu2+ ions via fluorescence turn-off mechanism in aqueous media over other metal ions. The

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emission changes towards Cu2+ ions could be clearly observed by the naked eyes under 365 nm UV lamp promote the probe molecule as a promising candidate for practical utilization.

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Furthermore, from the fluorescence titration data, the detection limit for Cu2+ ions was found to

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be 0.44 µM.

Keywords: AIE; Cu2+ ions; visual sensing

ACCEPTED MANUSCRIPT Molecular architectures possessing donor-acceptor group with extended pi-conjugated system attracts massive scientific interest due to intramolecular charge transfer (ICT) characteristics that found applications as fluorescent sensors for biological environments, pH sensors, nonlinear optical (NLO) materials [1-3]. Hence, design and synthesis of ICT compounds have witnessed tremendous growth over past decade the as it can be tuned and tailored by structural modification for their potential applications. The nature of ICT and degrees of charge transfer in the ground

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and excited state in these compounds strongly depend on the polarity of the surrounding

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medium, temperature and nature of the electron donating and accepting substituents [4, 5].

Since after the discovery of the exciting phenomenon termed as aggregation induced emission

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(AIE) by Tang and co-workers, where compounds exhibit strong luminescence in the aggregation state/ in poor solvents and weakly emit in dilute solution, brought renaissance to the

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application of organic luminescent materials [6]. As most of practical applications of the luminescent materials are relied on solid state rather than in solution state, AIE active

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luminescent materials brought a new platform for researchers to explore the photophysical properties associated with solid state luminescent materials and are widely used as fluorescent

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chemosensors, nonlinear optical materials and laser dyes [7-15]. Molecular probes based on fluorescence signaling has witnessed a tremendous growth over the

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last decades for the detection of heavy metal ions owing to its sensitivity, specificity, costeffectiveness and fast response time [16, 17]. Stimuli responsive molecular material are

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considered to be ideal candidates for the use for chemical sensor as the change in molecular configuration brought out by the external stimulus or analyte can be detected either by

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colorimetric or fluorimetric approach [18, 19]. Aggregation-induced emission (AIE) has been found versatile applications for the detection of Cu (II) ions in environmental and biological specimens due to the high sensitivity, ease of manipulation, and visual observation [20-22]. AIE molecules can aggregate and emit strong fluorescence or quench the emission on coordination with metal ions that have great potential for the highly selective and sensitive detection of copper ions in pure and real water, especially in living tissue containing water. Moreover, such kind of AIE probes for detection of Cu (II) ions can easily be prepared on site just by the addition of water to the solution of probe in THF. In case of Cu2+ ion sensing by AIE compounds, the probe molecule mostly work in aqueous media for detection of copper ions as water is

ACCEPTED MANUSCRIPT responsible for aggregation in most of the case and a very low amount of probe can detect copper ions having very less concentration [23-25]. In continuation with our ongoing research in the field of single molecule based metal sensors, the present work report a simple hydrazinecaboxamide derivate (Probe 1) having intramolecular charge transfer and AIE characteristics. The Probe molecule show distinct sensing property towards Cu2+ ions via fluorescence turn-off manner with lower detection limit. The reported molecule enjoy greater practical interest as it

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work in aqueous media for detection of copper ions and with the help of a 365 nm UV lamp,

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fluorescence changes imparted by the metal ion could be visualized easily.

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Scheme 1outlines the synthetic procedure for Probe 1. Cyano acetohydrazide 1 was obtained by the reaction between ethyl cyanoacetate and hydrazine hydrate in ethanol. Condensation reaction

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of cyano acetohydrazide 1 with salicylaldehyde yielded 2-cyano-2-[(2-hydroxyphenyl) methylene]hydrazide 2. The Knoevenagel condensation of 2 with 4-dimethylamino bezaldehyde

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lead to the target molecule Probe 1. All the compounds were characterized by various spectral methods (see supporting information).

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The photophysical properties of Probe1 (20µM) in solvents of varying polarity has been investigated by applying UV-vis absorption and emission techniques and are displayed in Fig. 1.

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As inferred from Fig. 1a, the electronic absorption spectra of Probe 1 shows broad absorption band for all solvents except for water where the absorption band was found to be more structured

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and red shifted which might be due to the formation of aggregates in water (explained in detail in the forthcoming section). However, the absorption band did not show any significant regularity

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on changing the solvent polarity, the emission profile of Probe 1 show remarkable influence of solvent polarity as seen from Fig. 1b. On exciting at 400 nm, the emission maxima undergo a red

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shift on increasing the polarity of the solvents indicating larger charge transfer in the excited state due to stabilization of a highly dipolar excited state in polar solvents as observed in other related molecules [26-28]. Furthermore, as seen from Fig. 2, correlation of solvent polarity parameter f with fluorescence peak energies (fl in cm-1) follow reasonably good linear relationship with f and the plot between fl and f show large negative slop, indicating that the emission state is strongly dipolar in nature due to strong intramolecular charge transfer (ICT) [29, 30]. However, contrary to the general trend observed for ICT molecules, the emission intensity in ethylene glycol and water were found to be exceptionally high which might arise due to aggregation induced emission enhancement (AIE) [31-33].

ACCEPTED MANUSCRIPT In order to investigate the AIE behavior of Probe 1 in detail, the emission spectra of Probe 1 were obtained in water/ THF mixtures with varying water volume fractions (fw, vol %) from 0 to 98% by fixing the concentration of Probe 1 (20 μM). Fig. 3a and 3b shows the photographic image of Probe 1 and the corresponding emission profile at different water volume fractions. As evident from Fig. 3, on progressive addition of water fraction, the photoluminescence intensity of Probe 1 remains unchanged in spectral change features up to fw = 50%, but starts to emit intense

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yellowish to orange fluorescence on reaching the water fraction fw =70%. The emission intensity

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was found to be continuously intensified on selected increment of water fractions and reaches its

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maximum at fw = 98%. Further, a 78-fold enhancement (expressed as I-I0/I0) in the emission intensity were observed in comparison with pure solvents; verifying the typical AIE

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characteristics of the Probe molecule. Surprisingly, the Stokes shift was found to be exceptionally large as 130 nm for Probe 1 at higher water fraction with strong emission which

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might arise as a result of ESIPT coupled AIE process. At lower water volume fractions, ESIPT process of Probe 1 was suppressed due to non-radiative way of excited states and free rotation

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around C-N bond. However, in high water volume fraction, rotation was restricted due to close packing of the molecule via intramolecular hydrogen bonding and predominate high quantum

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yield of the keto form in the aggregate state which might cause larger stokes-shift in its PL spectrum [34, 35]. Further, to investigate the formation of nanoparticles during aggregation at

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98% water fraction, Scanning electron microscope (SEM) study was exploited. As seen from Fig. 4, SEM image of Probe 1 found morphology of agglomerated spherical shaped

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nanoaggregates with average size of 210 nm.

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Owing to its potential metal ion coordinating site, Probe 1 was screened for the sensing properties towards various metal ions using fluorescence measurements. The sensing ability of Probe 1 (20 μM ) towards metals ions were carried out in THF-water fraction (fw = 98%,), in order to maintain the AIE characteristics, by treating it with 10 equiv. of varying metal ions namely Ag1+, Al3+, Cd2+, Co2+, Cu2+, Ni2+, Hg2+, Zn2+, Mn2+ and Pb2+. As evident from Fig. 5 (a) and (b), the addition of metal ions to Probe 1 resulted in changes in intensity on the emission profile. However, highly efficient fluorescence quenching was observed for Cu2+ ions as compared to other tested metal ions, which could be even visualized with the help of UV lamp, demonstrating the selectivity of Probe 1 towards Cu2+ ions.

ACCEPTED MANUSCRIPT Furthermore, fluorescence titration were performed with varying concentration of Cu2+ , to get more insight into the quenching efficiency of metal ions. As expected, increasing the concentration of Cu2+ ions resulted in the gradual decrease of the intense fluorescence of Probe 1 as inferred from Fig. 6. It is obvious to note that addition of 10 µM of Cu2+ ions has resulted almost complete quenching of emission intensity, suggesting highly selective sensing ability of Probe 1 towards the detection of Cu2+ ions. The observed quenching of fluorescence by Cu2+ ions

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may be ascribed due to the paramagnetic nature of these ions, which result in chelation-enhanced

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quenching (CHEQ), by metal to ligand charge transfer reaction between metal ions and Probe 1

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[36]. In order to gain understanding of the binding mode, we determined the stoichiometry of the interaction between Probe 1 and Cu2+ ions using Job's plot method and it show 1:1 binding

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stoichiometry (Fig. 7).

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The detection limit of Probe 1 for Cu2+ ion was calculated from the fluorescence titration data was found to be 4.39×10-7 M; using equation (1),

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Detection limit: 3 σ/ k

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where σ is the standard deviation of ten blank measurment and k is the slope of fluorescence

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intensity versus metal ion concentration [37].

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The fluorescence quenching data were analyzed using Stern-Volmer equation (2) in order to

𝐼𝑜 𝐼

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establish the extent and nature of quenching by Cu2+ ions [38]. = 1 + 𝐾𝑠𝑣 [Cu]

(2)

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where Io and I are the original fluorescence intensity and quenched intensity, respectively and KSV denotes the Stern–Volmer quenching constant. The value for KSV obtained from the plot of I0/I values versus the concentrations of Cu2+ ions was found to be 1.67x106 M-1, indicating much significant interaction between the Cu2+ ions fluorophore. However, the positive deviation from the linearity of the plot with an upward curvature indicates that quenching process simultaneously follows combination of static and dynamic quenching mechanism (Fig. 8) [39]. In conclusion, a simple hydrazine carboxamide AIE active fluorescent probe has been successfully designed and synthesized. The probe molecule exhibit AIE emission characteristics

ACCEPTED MANUSCRIPT in THF-water mixtures and intense yellowish orange emission were observed on reaching the water fraction at 98%. The probe molecule exhibited selective and sensitive sensor for Cu2+ ions through a turn-off fluorescence manner on compared to other tested metal ions. The probe molecule could be used as a promising candidate for practical utilization, as it works in aqueous media and the emission changes towards Cu2+ ions could be clearly observed by the naked eyes under 365 nm UV lamp. Moreover, owing to its higher sensitivity and selectivity, the reported

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probe molecule have potential practical interest and could be applied for the intracellular

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detection of Cu (II) ions in living systems.

Acknowledgements

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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-

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294-39).

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Scheme 1. Synthesis of Probe 1. (a) Ethanol, 0 0C, 5 h; (b) Ethanol, 1 h, 70 0C; (c) Ethanol, piperidine, p-Dimethyl aminobezaldehyde, 2 h, reflux.

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Fig. 1 Electronic absorption spectra (a) and emission spectra (b) of Probe1 (20 µM) in different solvents.

Fig. 2 Plot of emission frequency as function of solvent polarity parameter (Δf).

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Fig. 3 Photographic image (a) and corresponding of emission spectra (b) of Probe 1 (20 µM)in THF/ water mixtureswith different water fractions (fw).

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Fig. 4 SEM images of nanoaggregates of Probe 1 in THF/ water (fw = 98%).

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Fig. 5 (a) Photographic image of Probe 1(20 µM) under 365 nm UV lampand the corresponding (b) emission spectra in THF/ water (fw = 98%, λex= 400 nm) after addition of different metal ions.

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Fig. 6 PL spectra of Probe 1 as a function of [Cu2+].[1] =20 µM in THF:water, fw=98% (λex=400 nm).

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Fig. 7. Jobs plot of Probe 1 with Cu 2+ showing maxima at 1:1

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

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Graphical abstract

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A simple caboxamide derivative displaying aggregation induced emission has been synthesized. It can act as fluorescent probe for Cu2+ions.

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The sensing ability of the derivative could be visualized using 365 nm UV lamp.

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