A highly selective and femto-molar sensitive fluorescence ‘turn-on’ chemodosimeter for Hg

Content Provider	Semantic Scholar
Author	Mukhopadhyay, Sujay Biswas, Arnab Pandey, Rampal Gupta, Rakesh Kumar Pandey, Daya Shankar
Copyright Year	2014
Abstract	A selective fluorescence ‘turn-on’ chemodosimeter N,N0-bis-(4-cyanobenzylidene)-2,4,6-trimethylbenzene-1,3-diamine (3) based on a Schiff base for femto-molar detection of the Hg has been described. It presents the highest level of detection limit for Hg through Schiff base hydrolysis. 2014 Elsevier Ltd. All rights reserved. Heavy and transition metal ions like Cd, Hg and Pb are well known for their high toxicity towards the environment and biological systems. Amongst these, mercury is the most lethal for living organisms as it causes neurological and digestive disorders, malnutrition and hereditary dysfunctions. In the environment, it may be present in various forms like elemental (Hg), ionic (Hg) and organic mercury (CH3Hg). Methylmercury (CH3Hg) enters the human body via food chain by ingestion of fishes or sea mammals, wherein it is bioaccumulated by reaction of the organic molecules with Hg. The so called ‘Minamata disease’, is a harsh consequence of methylmercury. Considering these issues, the detection and control of mercury is a major challenge and development of suitable methodologies for its detection at trace level is highly demanding. In this context, fluorescence based molecular sensing is extremely beneficial due to its simplicity and high sensitivity. Although, a large number of fluorescence ‘turn-off’ probes for Hg are well known, those exhibiting ‘turn-on’ are rather scarce. In addition, limit of detection (LOD) for the available fluorescent ‘turn-on’ probes has always been a major concern. Therefore, the development of sensitive fluorescence ‘turn-on’ probes for Hg is highly demanding. In this direction, chelation enhanced fluorescence (CHEF) and Hg triggered chemodosimetric systems have drawn special attention. Further, it has been observed that in some cases the CHEF based systems are not very effective due to the strong quenching nature of Hg; in such cases chemodosimetric routes offer an excellent alternative. Even though numerous chemodosimeters have been reported for the detection of Hg, most of these are associated with either moderate sensitivity and selectivity or poor solubility in aqueous systems. To overcome these problems compounds containing aldimine moieties have been used as excellent chemodosimetric probes. Although, several systems involving facile hydrolysis of an azomethine (–HC@N–) linkage under the influence of metal ions like Fe and Cu have been extensively studied; those exhibiting Hg promoted hydrolysis have scarcely been investigated. Therefore, the development of a highly sensitive chemodosimeter for Hg through fluorescence ‘turn-on’ signaling is highly desirable. The present work deals with three simple non-chelating Schiff bases viz N,N0-bis-(4-bromobenzylidene)-2,4,6-trimethylbenzene1,3-diamine (1), N,N0-bis-(4-nitro-benzylidene)-2,4,6-trimethyl benzene-1,3-diamine (2) and N,N0-bis-(4-cyanobenzylidene)-2,4, 6-trimethylbenzene-1,3-diamine (3) (Scheme 1). Amongst these, 3 acts as an excellent chemodosimeter in the detection of Hg with ‘femto-molar’ (fM; 10 15 M) sensitivity which is superior to the exceeding limit of Hg in drinking water (2 ppb). Further, fM-level detection of Hg using biogenic gold nano particles and protein based biosensors has been reported; but to the best of our knowledge, the present work describes the highest level of Hg detection through chemodosimetric Schiff base hydrolysis. Properties of the compounds 1 3 have been fine tuned by incorporating weakly electron withdrawing and non-coordinating –Br, highly electron withdrawing and rarely coordinating –NO2 Scheme 1. Structures of 1 3 and their fluorogenic response towards (Hg) sensing. Figure 1. (a) UV/vis spectra of 3 (c, 10 lM) with various metal ions (c, 10 mM) and (b) UV/vis titration plot of 3 with Hg (c, 10 mM, 0–10 equiv). Insets showing Job’s plot for 3 with Hg in aqueous methanol (95%). Figure 2. Fluorescence titration spectra of 3 (c, 10 lM) with (a) Hg (c, 10 mM) and (b) Hg (c, 1 fM) in aqueous methanol (95%). 1438 S. Mukhopadhyay et al. / Tetrahedron Letters 55 (2014) 1437–1440 and electron withdrawing and coordinating –CN group (Scheme 1). Characterization of 1 3 has been achieved by satisfactory elemental analyses and spectral (FT-IR, H and C NMR, ESI-MS, UV/vis and emission) studies. Crystal structures of 2 and 3 have been verified by X-ray single crystal analyses (Figs. S29 and S30, Supplementary data). The H NMR spectra of 1, 2 and 3 displayed aldimine protons as singlet at d 8.16, 8.36 and 8.26 ppm and aromatic protons of the central mesitylene core resonated as a singlet at d 6.94 (1), 6.99 (2) and 6.97 (3) ppm. The signals due to phenyl ring protons appeared as two distinct doublets in the range d 7.63– 8.33 ppm, while methyl protons (two sets) resonated as a singlet in the range d 2.13–1.91 ppm (Figs. S1, S4 and S7, Supplementary data). The position and integrated intensity of various signals corroborated well to the respective formulations. C NMR spectra of 1 3 displayed an analogous pattern of resonances and supported their formations (Figs. S2, S5 and S8, Supplementary data). In their ESI-mass spectra 1 3 exhibited molecular ion peaks at m/z 485.0879 (calcd 485.2263) 1; 417.1611 (calcd 417.4293) 2; and 377.2519 (calcd 377.4531) 3 due to [M+1] (Figs. S3, S6 and S9, Supplementary data) and strongly supported their formation. The aqueous stability of 1 3 has been evaluated by increasing the proportion of water in methanol and found that these retain identity up to 50% aqueous methanol (Fig. S15a Supplementary data). Electronic absorption spectral studies for 1 3 (c, 10 lM) have been performed in aqueous methanol (95%). On the basis of their position and intensity the low energy (LE) bands at 355 nm (e, 4.0 10 M 1 cm , 1), 380 nm (e, 4.2 10 M 1 cm , 2) and 369 nm (e, 6.1 10 M 1 cm , 3) have been assigned to n–p⁄ transitions, whereas the bands at 266 (e, 4.3 10 M 1 cm , 1), 280 (e, 3.2 10 M 1 cm , 2) and 264 nm (e, 5.9 10 M 1 cm , 3) (Fig. S10a, Supplementary data) to the high energy (HE) p–p⁄ transitions. Metal ion interaction studies for 1 3 have been investigated by UV/vis spectroscopy (c, 10 lM; 95% aqueous methanol, initial pH 7.13) in the presence of various cations viz Na, K, Ca, Mg, Mn, Fe, Co, Ni, Cu, Ag, Zn, Cd, Hg, Pb (c, 10 mM) (Fig. 1a). Notably, 3 exhibited significant changes, selectively with Hg, while 1 and 2 were silent in the presence of tested metal ions (Fig. S10b and S10c, Supplementary data). To gain a deep insight into the binding affinity and sensitivity of 3 towards Hg, absorption titration studies have been performed. Addition of Hg (4.0 equiv) to a solution of 3 led to a blue shift in the position of HE band (Dk 9 nm) with a small decrease in its optical density. Further, upon gradual increase in the concentration of Hg (4–10 equiv) the blue shift becomes more prominent (243 nm, Dk 21 nm). On the other hand, intensity of the LE band gradually diminished with emergence of a new band at 296 nm characteristic of MDA (2,4,6-trimethyl-1,3-benzene-diamine) which may arise due to the hydrolysis of 3 (Fig. 1b). The presence of an isosbestic point at 250 nm indicated existence of more than two species in the solution. At saturation point the solution turned colourless (from yellow; Fig. 3a) and pH of the solution became slightly acidic ( 6.1). Based on the above results it was realized that 3 may also serve as a fluorescent chemosensor for Hg. The emission spectra of 1–3 acquired in aqueous methanol (95%) exhibited significant fluorescence (Table S1, Supplementary data). To explore the use of these compounds as a fluorescent probe (280–520 nm) their solutions were treated with various metal ions. Notably 1 and 2 did not show any significant change in the presence of the tested metal ions at their respective excitations, (Fig. S17, Supplementary data) however 3 displayed fluorescence ‘turn-on’ (kem, 308 nm) only in the presence of Hg (c, 10 mM), (Fig. 2a). To understand the binding affinity and sensitivity of 3 towards Hg, emission titration studies have been performed. Aliquot addition of Hg (1.0–4.0 equiv) to a solution of 3 leads to an increase both in the fluorescence intensity and quantum yield by 3.5 (308 nm) and 3.6 fold ( U, 0.19) corresponding to MDA (probably arising due to hydrolysis of 3). Surprisingly, addition of an excess of Hg (>4.0 equiv) causes significant fluorescence quenching (probably due to complexation of MDA with Hg, vide supra) (Fig. S16a, Supplementary data). Job’s plot analysis revealed 1:1 stoichiometry between 3 and Hg (Fig. S13a, Supplementary data) and binding constant converged to 3.6 10 M 1 using the Benesi–Hildebrand method (Fig. S14a, Supplementary data). Considering high sensitivity of the fluorescence technique we evaluated the limit of detection (LOD) of 3 towards Hg, starting from fM concentration of Hg (1–10 10 15 M) (Fig. S13b, Supplementary data). Notably it exhibited significant ‘turn-on’ response within short period of time (0.5 min). Upon gradual addition of fM concentrations of Figure 3. (a) Images showing naked eye visible changes of 3 in the presence of Hg (1 equiv) and (b) fluorogenic changes of same solutions at 365 nm. Scheme 2. Plausible mechanism for the hydrolysis of 3 and subsequent complexation to generate 3A. S. Mukhopadhyay et al. / Tetrahedron Letters 55 (2014) 1437–144
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