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Brightening Quinolineimines by Al3+ and Subsequent Quenching by PPi/PA in Aqueous Medium: Synthesis, Crystal Structures, Binding Behavior, Theoretical and Cell Imaging Studies Sharad Kumar Asthana,† Ajit Kumar,‡ Neeraj,† Shweta,† Sumit Kumar Hira,§ Partha Pratim Manna,∥ and K. K. Upadhyay*,†,⊥ †

Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi-221005, India Department of Applied Sciences & Humanities, National Institute of Foundry & Forge Technology, Ranchi-834003, Jharkhand, India § Department of Zoology, Burdwan University, Burdwan-713104, India ∥ Immunobiology Laboratory, Department of Zoology, Institute of Science, Banaras Hindu University, Varanasi-221005, India ‡

S Supporting Information *

ABSTRACT: Recent years have witnessed an upsurge of Al3+ selective optical sensors involving simple Schiff bases to other complex organic frameworks. However, more than ∼95% of such reports lack crystallographic evidence, and proposals of binding sites for Al3+ are based upon spectroscopic evidence only. We herein synthesized and fully characterized a quinolineimine derivative (CMO) and explored its potential toward efficient detection of Al3+ with crystallographic evidence. The ongoing nonradiative photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) processes in CMO got inhibited via the chelation enhanced fluorescence (CHEF) effects induced by Al3+, and consequently turn-on fluorescence response was observed with 18-fold emission enhancements. The theoretical calculations performed were in good consonance with experimental results. We also explored further the applicability of the CMO· Al3+ complex toward highly sensitive and selective detection of inorganic phosphate (PPi) and an explosive picric acid (PA) via fluorescence quenching processes through two different chemical routes. The bioimaging of Al3+ and PPi were carried out in the living human cancer cells (MCF-7).

commonly encountered limitations, viz., tedious synthetic protocol,10 poor detection limit,11 use of organic media for sensing studies, interferences from commonly occurring analytes, etc.12 Hence, there is a great demand for developing low cost and real-time monitoring systems that can effectively determine the Al3+ in natural environment/living organisms successfully. It is worth mentioning here that in the past we have also successfully designed and synthesized a couple of worthy Al3+ sensitive optical sensors.7a−c In continuation of our previous efforts and to conquer the above shortcomings of the current Al3+ probes, herein we have devised a quinolineimine derivative as a fluorogenic “turn-on” probe CMO (Figure 1) for expeditious detection of Al3+ in 100% aqueous solution. The CMO itself is nonfluorescent due to ongoing nonradiative PET and ESIPT processes which were inhibited via the CHEF effects induced through Al3+ complexation, and consequently a turn on fluorescence response was observed. The correspond-

The aluminum ion has been marked as a notorious ion due to its definitive roles in a number of physiological/biological disorders in human beings, e.g., encephalopathy,1 neuronal disorder leading to dementia,2 myopathy,3 Alzheimer’s disease, etc.4 Pure aluminum does not occur in nature as such. The wide uses of aluminum in anthropogenic activities cause the release of free aluminum ions (Al3+) into the environment, leading to the accumulation of Al3+ in food, water, air, etc.5 The effective detection of Al3+ has attracted extensive attention of researchers in the past few decades.6 The fluorescence chemosensors have come up as very efficient probes for the detection of Al3+ in a variety of samples in the recent past.7 The clarity regarding the binding modes of the chemosensor and analyte is an important aspect of any sensing phenomenon. However, more than ∼95% of the probes reported to date involve spectroscopic evidence with respect to chemical structure receptor Al3+, and there is a lack of crystallographic evidence.7,8 One of the probable reasons behind the same may be the strong hydration ability of Al3+ in aqueous solution, which further leads to its weak coordination ability toward most of the sensors.9 Moreover, most of these sensors are also associated with one or other © 2017 American Chemical Society

Received: November 18, 2016 Published: March 2, 2017 3315

DOI: 10.1021/acs.inorgchem.6b02752 Inorg. Chem. 2017, 56, 3315−3323

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Inorganic Chemistry

of 5.6 × 10−10 and 1.4 × 10−8, respectively. The efficient detection of PA through a CMO·Al3+ complex in an aqueous medium with a lowest detection limit of 2.29 × 10−10 M further extends its utility.

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EXPERIMENTAL SECTION

Synthesis of Receptors CMO and CMS. The synthesis of receptors CMO (1, 2-dihydro-7-methoxy-2-oxoquinolene-3-carbaldehyde isonicotino-hydrazide) and CMS (1, 2-dihydro-7-methoxy-2thioxo-quinolene-3-carbaldehyde isonicotino-hydrazide) is shown in Scheme 1. 1,2-Dihydro-7-methoxy-2-oxoquinolene-3-carbaldehyde

Figure 1. Chemical structures of CMO and CMS.

ing host−guest CMO·Al3+ ensemble and its binding intricacies were well explored through single crystal X-ray studies along with spectroscopic studies and theoretical calculations. We also synthesized a controlled compound CMS (Figure 1) by introducing a sulfur atom at the place of the oxygen atom of the quinoline moiety. Interestingly, CMS did not show any fluorescence with Al3+, which may be understood in terms of poor hard−soft interactions between Al3+ and sulfur. Thus, the role of the carbonyl oxygen atom of the quinoline moiety plays a significant role in the binding of Al3+. Recently, the strategies of detecting multiple analytes by a single sensor have gained more attention.13 Along similar lines to the work of Yoon et al.14 published in 2010, we also explored the potential applicabilities of CMO·Al3+ toward subsequent detection of inorganic phosphate (PPi). The same involved stripping of Al3+ from CMO·Al3+, resulting in fluorescence quenching. It is worth mentioning that the pyrophosphate ion (PPi) is a biologically important target as it is the product of ATP hydrolysis under cellular conditions and also plays an important role in energy transduction in organisms and controls metabolic processes by participating in a number of enzymatic reactions.15 Besides PPi, CMO·Al3+ was also used for the highly selective and sensitive detection of an explosive, viz. picric acid (PA), via a fluorescence quenching process. Contrary to PPi, here PA binds with the receptor via hydrogen bonding between pyridine nitrogen and the hydroxyl group of PA instead of decomplexation of metal. Among various nitro explosives of derivative types, the detection of picric acid is important because of its wide usage in the manufacture of rocket fuels, fireworks, deadly explosives, and sensitizers in photographic emulsions and as a component in safety matchboxes etc.16 PA has also been used in medicinal formulations in the treatment of malaria, trichinosis, herpes, smallpox, and antiseptics.17 Further, PA has also been recognized as an environmental contaminant and is harmful to wildlife and humans.18 Therefore, its detection is also very important. Thus, we trust that CMO is a fit case to fulfill the basic characteristics of any sensors, i.e., solubility, selectivity, and sensitivity from its application viewpoint. On the other hand, from the academic viewpoint, CMO and its binding event with Al3+ were fully confirmed through single crystal XRD studies, which is rarely reported in the literature for similar types of sensing ensembles. Moreover the binding behavior is also supplemented through various spectroscopic studies and theoretical calculations. The sensing ability of CMO was also checked in living cells, which clearly indicated that CMO has good cell permeability and shows effective intracellular fluorescence emission through the formation of the CMO· Al3+ complex. The same loses its fluorescence intensity upon treatment with PPi. Hence, CMO has an ability to serve as a working optical sensor for Al3+/PPi detection in biological systems and environment samples with lowest detection limits

Scheme 1. Synthesis of Receptor CMO and CMS

(0.406 g, 2.0 mmol) or 1,2-dihydro-7-methoxy-2-thioxo-quinolene-3carbaldehyde (0.438 g, 2.0 mmol) were added to an equimolar isonicotinic acid hydrazide (0.274 g, 2.0 mmol) in 10 mL of absolute ethanol separately having one drop of HCl followed by constant stirring for 4−5 h (Scheme 1). The yellowish solid precipitate was filtered, washed with ethanol, and finally dried under a vacuum over anhydrous CaCl2. Both CMO and CMS were characterized through various spectroscopic techniques, viz. IR and 1H and 13C NMR spectral studies along with mass determination [Figures S1−S7 in the Supporting Information (SI)] and single crystal X-ray (Figure 2).

Figure 2. Perspective view of single crystal of CMO and CMS with 50% thermal ellipsoid probability. Spectroscopic Characterization Data for Receptor CMO. Yield: 89%. IR/cm−1: 3558, 3457, 3206, 3010, 2924, 2852, 1657, 1627, 1588, 1556, 1410, 1374, 1068, 976, 803. 1H NMR (300 MHz, DMSO-d6, Si(CH3)4): δ 12.11 (s, 1H, NH−), 11.90 (s, 1H, NH−), 8.72 (d, 1H, Ar−H, J = 3.6 Hz), 8.69 (s, 2H, Py-H), 8.43 (s, 1H, CH = N), 7.79 (m, 2H, Py-H, 1H, Ar−H.), 6.83 (d, 2H, Ar−H, J = 8.7 Hz), 3.82 (s, 3H, -OCH3). 13C NMR (75 MHz, DMSO-d6, Si(CH3)4): δ 162.43, 161.90, 151.34, 144.06, 141.27, 139.15, 135.30, 131.37, 129.14, 126.07, 122.39, 121.55, 118.98, 116.16, 55.00. ESI-MS m/z calculated for C17H14N4O3 [M]: 322.1. Found [M+1]+: 323.1. Spectroscopic Characterization Data for Receptor CMS. Yield: 89%. IR/cm−1: 3461, 2929, 2852, 2625, 1606, 1563, 1483, 1434, 1405, 1306, 1260, 1085, 1070, 904. 1H NMR: (300 MHz, DMSO-d6, Si(CH3)4): δ 13.75 (s, 1H, NH), 12.31 (s, 1H, NH), 9.21 (d, 1H, ArH, J = 6.9 Hz), 8.76 (d, 3H, ArH, J = 5.1 Hz), 8.42 (s, 1H, HCN), 7.95−7.84 (m, 3H, ArH), 7.16 (s, 1H, ArH), 7.04 (d, 1H, ArH, J = 8.4 Hz), 3.86(s, 3H, -OCH3). 13C NMR (75 MHz, DMSO-d6, Si(CH3)4): δ 180.02, 150.16, 145.14, 141.50, 137.57, 130.76, 129.44, 124.94, 121.64, 118.68, 114.95, 111.45, 107.95, 98.14, 55.77. 3316

DOI: 10.1021/acs.inorgchem.6b02752 Inorg. Chem. 2017, 56, 3315−3323

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Inorganic Chemistry Synthesis of Al3+ Complex of CMO. The Al3+ complex of CMO was synthesized by adding a 10.0 mL methanolic solution of AlCl3 (0.066 g, 0.5 mmol) slowly to a magnetically stirred 10.0 mL methanolic solution of CMO (0.161 g, 0.5 mmol). The mixture was further stirred at room temperature for ∼4 h, whereby a yellowish precipitate was formed. The reaction mixture was filtered and washed several times with diethyl ether and finally dried under a vacuum over anhydrous CaCl2. The Al3+ complex was characterized through various spectroscopic techniques, viz., IR and 1H and 13C NMR spectral studies along with mass determination (Figures S8−S11 in the SI) and single crystal X-ray. Spectroscopic Characterization Data for Al3+ Complex. Yield: 89%. IR/cm−1: 3558, 3453, 3208, 3056, 2947, 2852, 1651, 1630, 1608, 1589, 1573, 1553, 1513, 1459, 1265, 1166, 1068, 887. 1H NMR: (300 MHz, DMSO-d6, Si(CH3)4): δ 9.40 (s, 1H, NH−), 8.73 (m, 2H), 8.62 (s, 1H, ArH), 8.45 (s, 1H, ArH), 7.96 (s, 1H, ), 7.80 (t, 3H, ArH), 6.81 (s, 1H, ArH), 3.79 (s, 3H, CH3). 13C NMR (75 MHz, DMSO-d6, Si(CH3)4): δ 162.02, 161.27, 150.16, 144.35, 141.23, 135.32, 130.76, 121.66, 113.30, 111.75, 97.80, 55.47. ESI-MS m/z calculated for C34H26AlN8O6 [M]: 669.1. Found [M-2]+: 669.5. Synthesis of PPi Complex of CMO·Al3+. The PPi complex (inorganic pyrophosphate) with CMO·Al3+ was synthesized by adding a 5.0 mL aqueous solution of PPi (0.346 g, 2.0 mmol) slowly (over a time period of ∼0.5 h) to a magnetically stirred 10.0 mL aqueous ethanolic solution of CMO·Al3+ (0.334 g, 0.5 mmol). The mixture was further stirred at room temperature for approximately 1 h, whereby a yellow precipitate was formed. The reaction mixture was filtered and washed several times with diethyl ether and finally dried under a vacuum over anhydrous CaCl2. The PPi complex was characterized through various spectroscopic techniques, viz., IR and 1H and 13C NMR spectral studies along with mass determination (Figures S12− S15 in the SI). Spectroscopic Characterization Data for [CMO·Al3+·PPi] Complex. Yield: 89%. IR/cm−1: 3444, 2926, 2854, 1789, 1667, 1633, 1552, 1505, 1384, 1122, 1072, 865. 1H NMR: (300 MHz, DMSO-d6, Si(CH3)4): δ 12.10 (s, 1H, NH−), 11.91 (s, 1H, NH−), 8.76 (s, 1H), 8.70 (s, 1H), 8.69 (s, 1H), 8.41 (s, 1H), 7.84−7.77 (m, 3H), 6.81 (d, 2H, J = 9.3 Hz), 3.80 (s, 3H). 13C NMR (75 MHz, DMSO-d6, Si(CH3)4): δ 161.53, 160.98, 151.30, 144.00, 140.27, 139.15, 135.30, 131.37, 129.14, 125.07, 122.39, 121.55, 118.98, 115.15, 55.05. ESI-MS m/z calculated [M]: 322.1. Found: [M-1]: 320.9. Synthesis of PA Adduct of CMO·Al3+. The complex of PA (picric acid) with CMO·Al3+ was synthesized by adding a 5.0 mL aqueous solution of PA (0.458 g, 2.0 mmol) slowly (over a time period of ∼0.5 h) to a magnetically stirred 10.0 mL aqueous ethanolic solution (9:1; v/v) of CMO·Al3+ (0.334 g, 0.5 mmol). The mixture was further stirred at room temperature for ∼1 h, whereby a yellow precipitate was formed. The mixture was filtered and washed several times with diethyl ether and finally dried under a vacuum over anhydrous CaCl2. The PA complex was characterized through various spectroscopic techniques, viz., IR and 1H and 13C NMR spectral studies along with mass determination (Figures S16−S19 in the SI). Spectroscopic Characterization Data for [CMO·Al3+·PA] Complex. Yield: 89%. IR/cm−1: 3415, 3171, 3049, 2830, 2389, 2364, 1652, 1623, 1595, 1548, 1491, 1412, 1337, 1290, 927, 839, 879. 1HNMR: (300 MHz, DMSO-d6, Si(CH3)4): δ 9.82 (s, 1H), 9.51 (s, 2H), 8.98 and 8.79 (m, 2H), 8.65 (s, 1H), 8.36 (s, 1H), 8.02 (s, 1H), 7.94- 7.93 (3H, m), 7.44 (s, 1H), 3.79 (3H, s). 13C NMR (75 MHz, DMSO-d6, Si(CH3)4): δ 162.70, 161.14, 151.82, 145.08, 140.27, 139.16, 135.32, 132.37, 129.14, 125.27, 124.24, 121.56, 119.92, 116.25, 55.89. ESI-MS m/z calculated for C46H32AlN14O20 [M]: 1127.1766. Found [M+1]+: 1128.1772.

space group (Table S1 in the SI). The perspective views of CMO and CMS are shown in Figure 2, while the corresponding crystal data and structural refinement details are listed in Table S1 in the SI. The asymmetric unit of CMO possesses a trans orientation, i.e., O2 and O3 are trans to each other, and attains (E)-configuration about the CN bond (C11−N2), having a bond distance of 1.273 Å. The C10−N1 and C10−O2 bond lengths were found to be 1.36 and 1.24 Å, respectively. The C10−N1 bond length matched with the C−N bond length (1.48 Å), and the CO bond length is very close to normal CO double bond (1.23 Å). These observations clearly indicated the complete dominance of the keto-form in the crystal structure of CMO. The intermolecular hydrogen bondings in CMO knitted nice double-stranded DNA like the supramolecular architecture of the same, as shown in Figure 3.

Figure 3. Asymmetric unit of CMO showing intermolecular hydrogen bondings, H-bonding distances, and a view of the DNA-like doublehelical chain.

X-ray Diffraction Studies of the CMO-Al3+ Complex. We obtained single crystals of the Al3+ complex (Figure 4) by

Figure 4. Crystal structure of CMO·Al3+ complex with 50% thermal ellipsoid probability.

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slow evaporation of a solution of CMO·Al3+ in a mixture of MeOH/DMF solvents (7:3; v/v) at room temperature over 12 to 15 days. The asymmetric unit of the complex crystallizes in a triclinic system with the P1̅ space group (Table S1 in the SI). The perspective view of CMO·Al3+ is shown in Figure 4. The crystal structure of CMO·Al3+ supports 1:2 stoichiometry between Al3+ and CMO and also shows the deprotonation of the amidic (N3) proton from CMO, which is also

RESULTS AND DISCUSSION X-ray Diffraction Studies of CMO and CMS. The single crystals of CMO and CMS were developed from their respective solutions in DMF and methanol (1:1; v/v) over a time period of 10−15 days. The asymmetric units of CMO and CMS got crystallized in a monoclinic system with the P21/n 3317

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Inorganic Chemistry established by the 1H NMR titration and mass spectrum of the complex (expected m/z, 669.1; reported mass, 669.5). The crystal structure of CMO shows a N3−C12 bond length of 1.34 Å, which matched with the N−C single bond upon complexation with Al3+; the N3−C12 bond length reduces to 1.29 Å, i.e., acquired partial double bond character, which supported the deprotonation of the N3 proton (Figure S20 in the SI). The C12−O3 bond length was 1.23 Å, which was well fitted with double bond character, while upon complexation, the C12−O3 bond stretched to 1.28 Å because the π-electron density resonates among N3−C12−O3 bonds. The bond lengths C10−N1 and C10−O2 of CMO are 1.36 and 1.24 Å, respectively, and underwent slight changes to 1.31 and 1.27 Å, respectively, upon complexation. All of these observations regarding bond length indicated the complexation of CMO with Al3+ (Figure S20 in the SI). The 1:2 binding stoichiometry between Al3+ and CMO is also a rare instance7a in the literature due to its strong hydration and weak coordination ability in comparison to transition metal ions.9 Thus, the CMO being presented by us through this paper has an edge over most of the chemosensors reported previously for Al3+. The nonclassical H-bonding in CMO·Al3+ led a nice supramolecular architecture, as shown in Figure 5.

Figure 6. Emission profile of 0.1 μM aqueous solution of receptor CMO upon concomitant additions of Al3+ up to 70 equiv. (λex = 380 nm).

not in a plane with quinoline moiety of CMO. Hence overall it is nonplanar. The same was also confirmed by theoretical studies through DFT (Figure S26 in SI). This nonplanar geometry of CMO (Figure S27 in SI) led to an interruption in conjugation, which makes the lone pair of >CN more available for the PET process, causing the nonfluorescent/very weak fluorescent nature of CMO. The weak fluorescent behavior of CMO can also be understood in terms of water mediated intermolecular H-bonding, which introduces a slight planarity in the same (Figure 3). Moreover, the control compound, i.e., CMS, was also tested for its possible binding with a series of metal ions as mentioned above including Al3+, but they give either no response or a nonselective response toward naked-eye and fluorescence changes. This may be understood in terms of poor hard−soft interactions between Al3+ and sulfur. The fluorescence behavior of CMO was studied upon separate additions of 10 equiv of a variety of metal ions (Figure S28 in SI). A strong blue fluorescence visible to the naked eye was observed selectively with Al3+ as its chloride salt. The corresponding emission spectrum showed dual emission bands at 420 and 443 nm upon excitation at 380 nm. The dual fluorescence emission band (420 and 443 nm) upon concomitant additions of Al3+ was observed due to ESIPT involving conversion of the enol-form to the excited keto-form on a subpicoseconds time scale. The emission band at 420 nm was assigned for the enol form of CMO, while the band at 443 nm was assigned for the keto form of the same in the excited state (Figure 6). The enolic proton (solution state) in an excited state transfers to the nitrogen atom of the imine group of the Schiff base and gets converted into the keto form. The same may be understood in terms of weakening of ICT of CMO upon coordination with Al3+ and inhibiting PET, consequently allowing CMO to fluoresce. In order to have a deeper insight of the binding feature of CMO and Al3+, we carried out a fluorescence titration experiment between CMO (1.0 × 10−6 M) and Al3+ (70 equiv). Upon concomitant additions of different aliquots of Al3+ to the CMO, a dual emission pattern was grown (Figure 6). The same was explained above in terms of an intramolecular shift of the proton from N to O in the binding unit of CMO and activation of ESIPT.

Figure 5. Nonclassical H-bonding in CMO·Al3+.

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PHOTO PHYSICAL STUDIES OF CMO The sensing behavior of CMO with different metal ions, viz., Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Al3+, Cd2+, Hg2+, Pb2+, Li+, Na+, K+, Mg2+, and Ba2+, as its chloride salt was investigated in pure water by UV−visible and fluorescence measurements. A 10 μM solution of CMO shows significant variations of the absorption spectrum in the 300−470 nm range for all the above-mentioned ions along with no categorical naked-eye response against any specific metal ions (Figures S21 and S22 in SI). Upon the addition of an Al3+ ion, no drastic change of the UV−visible spectra was observed, as shown in Figure S23 in the SI. Hence, we re-sorted for the fluorescent measurements. The 1 μM solution of CMO is weakly emitted at 442 nm (λex = 380 nm; quantum yield, Φ = 0.289). However, upon the addition of Al3+ as its chloride salt to the solution of CMO, the same experienced fluorescence switching in the form of dual emission bands at 420 and 443 nm (λex = 380 nm; Φ = 0.571; Figure 6) was observed. Other chosen metal ions were unable to produce similar fluorescent behavior under similar experimental conditions to that for Al3+. The same also shows fluorogenic naked-eye response (Figure S24 in SI). Negligible effects were noticed upon a change of counteranions of Al3+ (Figure S25 in SI). The nonfluorescent nature of CMO is well explained in terms of crystal structure and theoretical studies using density functional theory (DFT). The dihedral angles D1 (177.39°) and D2 (177.69°) (Figure S26 in SI) showed the quinoline moiety and binding unit are coplanar, but the dihedral angle D3 (142.49°) indicates that the pyridine moiety of binding unit is 3318

DOI: 10.1021/acs.inorgchem.6b02752 Inorg. Chem. 2017, 56, 3315−3323

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Inorganic Chemistry

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THE BINDING BEHAVIOR OF AL3+ WITH CMO The meaningful Job’s plot derived from the fluorescence titration experiment shows that CMO binds with Al3+ in a 2:1 binding stoichiometry (Figure S29 in SI). The same was further confirmed by the ESI-MS of the (CMO·Al3+) complex (Figure S11 in the SI) which showed a molecular ion peak m/z at 669.5 (calcd. for [2(CMO)·Al] = 671.62) with the loss of two protons, one from each moiety. The corresponding binding constant of CMO with Al3+ has been determined by the nonlinear fitting of fluorescence titration data (Figure S30 in the SI) using the equation given below.19 F (x ) =

a + b × cx n 1 + cx n

where F(x) is the intensity of the solution during titration, x is the concentration of the metal ion, a is the intensity of CMO without a metal ion, b is the intensity at saturation, c is the association constant, and n is the number of Al3+ ions bound to each CMO (here n = 0.5). The value of the binding constant (c) was found to be 695 M−2 with a satisfactory correlation coefficient value (R2 = 0.993). The 2:1 stoichiometry was worked out for CMO·Al3+, which was further confirmed through the Job’s plot method. The detection limit was calculated to be 5.60 × 10−10 M using fluorescence titration data as per IUPAC definition20 (Figure S31 in the SI). The results of pH study demonstrated that the operational pH range of CMO for Al3+ is 4−8 (Figure S32 in the SI). After successful determination of binding stoichiometry between CMO and Al3+, we further carried out 1H NMR studies in DMSO-d6 (Figure S33 in the SI) to know the binding sites on CMO for Al3+. With the addition of 2.0 equiv of Al3+ (in D2O) to the solution of CMO, the 1NH peak at δ 12.11 ppm shifted upfield at δ 9.40 ppm as a consequence of ESIPT. The other imine proton at δ 11.90 ppm (2NH) disappeared as a result of deprotonation (Figure 7). The other aromatic protons remain

Figure 8. Mechanisms involved in sensing of Al3+ by receptor CMO.

binding behaviors are finally confirmed through single crystal X-ray studies as discussed above (Figure 4). It is worth mentioning that although the XRD studies revealed (Figure 4) complete deprotonation of both −NH protons of CMO during the complexation process, the results of mass spectral data and 1 H NMR studies support deprotonation of only one −NH (quinolone part), and the other one may remain intact with CMO via ESPIT. This may be due the variation of solid and solution states, respectively.

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THEORETICAL STUDIES We also performed DFT calculations on CMO and its Al3+ complex. The same showed that the donor sites in trans orientation of donors underwent cis orientation upon the addition of Al3+ leading to sufficient strain in the system (Figure 9). The bond lengths between C−O1 and C−O2 are quite similar at 1.229 and 1.218 Å (double bond character), respectively.

Figure 9. Optimized structures of CMO and CMO·Al3+.

The optimized configuration of the Al3+ complex indicated that the the bond length between Al3+ and O1 of CMO was 1.9394 Å, while the Al3+ and O2 distance observed was 1.8833 Å and the Al3+−N1 bond length was 1.9963 Å, very close to the Al−N distance (2.065 Å; Figure 9). Thus, after complexation with Al3+, the C−O1 and C−O2 bond lengths increased from 1.2292 to 1.2680 Å and 1.2184 to 1.3016 Å due to the involvement in co-ordination with Al3+. Thus, in addition to the conformational arrest of receptor CMO, the Al3+ also affects the internal molecular geometry substantially. The electron densities of the highest occupied molecular orbital (HOMO; −5.700 eV) of CMO are delocalized only on quinoline (donor part), not on the pyridine moiety (acceptor part), whereas in the lowest unoccupied molecular orbital (LUMO; −2.124 eV), the electron density spread over both the

Figure 7. 1H NMR titration spectra showing metalation and demetalation in DMSO-d6.

virtually unperturbed. Overall changes in the chemical shifts of the various protons clearly indicated the N atom of aldimine along with both oxygen atoms of CMO as the donor atoms in CMO·Al3+ ensemble. The 13C NMR spectrum of the CMO· Al3+ ensemble also exhibited minor changes in comparison to that of CMO (Figures S2 and S9 in the SI). The ongoing processes with and without Al3+ could be easily represented through Figure 8 based upon various spectroscopic studies. The corresponding structural changes in CMO and its 3319

DOI: 10.1021/acs.inorgchem.6b02752 Inorg. Chem. 2017, 56, 3315−3323

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Inorganic Chemistry quinoline and pyridine moieties of the CMO. The energy gap between HOMO and LUMO is 3.576 eV (Figure 10).

Figure 11. Titration profile of 1.0 μM aqueous solution of CMO·Al3+ ensemble in the presence of PPi (0−200 equiv; λex = 380 nm).

and S15 in the SI]. The 1H NMR spectrum of CMO·Al3+ upon the addition of PPi clearly proved the stripping of Al3+ from the above ensemble (Figure S38 in SI). The influence of PPi on the CMO·Al3+ ensemble was checked upon the addition of 2.0 equiv of PPi to the solution of the CMO·Al3+. The two small peaks corresponding to 1NH and 2NH protons reappeared due to pyrophosphate induced decomplexation as explained in Figure 7. To further look into the nature of interaction between the CMO·Al3+ ensemble and PPi, ESI mass determination was carried out. The solated CMO·Al3+ peak at m/z 669.5 corresponding to [2(CMO)·Al3+-2H+] was obtained (Figure S11 in SI). When PPi was added to this complex, the peak at m/z 669.5 disappeared, and a peak at m/z 320.9 corresponding to CMO was observed (Figure S15 in SI). In this way, almost identical results with 1H NMR and mass studies suggest that the binding affinity between PPi and Al3+ is much stronger than that between CMO and Al3+ and ultimately leads to the removal of Al3+ from the CMO·Al3+ by the PPi. Moreover, the ensemble detected PPi with a detection limit of 1.40 × 10−8 M (Figure S39 in SI). The reversibility of CMO with respect to Al3+ and PPi was also tested by respective additions of Al3+ and PPi to the solution of CMO. When Al3+ was added to CMO, it became fluorescent and underwent quenching by subsequent additions of PPi. The same behavior was observed for up to three cycles (Figure S40a,b in SI). This “on−off” fluorescence for several cycles confirms the reversible formation and deformation of the {(CMO·Al3+) + PPi} complex. Detection of Picric Acid (PA). The fluorescence quenching sensors have been used quite often and are proven to be very simple and sensitive tools to detect nitro-based explosives. The explosive power of picric acid (PA) is superior to that of 2,4,6-trinitrotoluene (TNT), yet less attention has been paid to the development of methods for its selective detection. Nevertheless, in the past few years, a number of molecular and chemosensors for PA have been reported.22 The CMO·Al3+ complex is highly fluorescent in solution, we studied the fluorescence behavior of the CMO·Al3+complex toward different nitro derivatives such as picric acid (PA), 2nitrotoluene (2-NT), 2-nitrophenol (2-NP), etc. (Figure S41 in SI). The fluorescence intensity of CMO·Al3+ kept on decreasing with an increase in the concentration of PA (Figure 12). The addition of 50 μM of PA results in the total

Figure 10. HOMO−LUMO orbitals of receptor CMO in the absence and presence of Al3+.

Overall, the complex molecule did not achieve extra stability as exhibited by the relative energies of the HOMOs of CMO and CMO·Al3+ (Figure 10). Furthermore, the HOMO−LUMO energy gap is also reduced by 0.416 eV. However, upon the addition of pyrophosphate to the Al3+ ensemble strain in CMO, it is released due to the demetalation of Al3+ by PPi. This provides relative stability, but the HOMO−LUMO gap in this system is drastically increased as observed in free CMO.

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PHOTOPHYSICAL STUDIES CMO·AL3+ ENSEMBLE Detection of PPi. The Al3+ ensemble of CMO was further checked for any optical response in the presence of different anions, viz., F−, Cl−, Br−, I−, HSO3−, SO32−, HSO4−, Aco−, Bzo−, BF4−, PF6−, ClO4−, S2−, HPO42−, H2PO4−, PPi, and PO43−, and it was observed that only pyrophosphate (PPi) gave a response (Figure S34 and S35 in SI). There was no categorical fluorescent response with any anion except for inorganic pyrophosphate (PPi), which selectively quenches the fluorescence of the CMO·Al3+ ensemble. As we expected, a regular decrease in emission intensity of the CMO·Al3+ ensemble (Φ = 0.121) was observed upon concomitant additions of PPi (0−200 equiv; Figure 11). This quenching of fluorescence further substantiated our assumption regarding stripping of Al3+ from CMO-Al3+ on the basis of 1H NMR experiments. Nevertheless, previous workers have reported a few phosphate responsive probes by utilizing a similar design strategy (demetalation) to ours.14,21 We checked the effect of commonly responsive phosphate derivatives and biomolecules, viz. ATP, AMP, GDP, NADP, etc. toward an ensemble (CMO·Al3+), but to our surprise the same did not affect the PPi responsive characteristics of our probe CMO·Al3+ (Figure S36 in SI). Thus, the selectivity of CMO·Al3+ toward PPi is unique, as the same did not experience any interference from a number of phosphate derivatives and biomolecules. The same selectivity is shown in a fluorescence photograph (Figure S37 in SI). Our speculation regarding the demetalation of the CMO·Al3+ ensemble upon the addition of PPi was further confirmed by 1 H NMR as well as through mass spectral studies [Figures S12 3320

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Inorganic Chemistry fluorescence quenching of the CMO·Al3+ ensemble (Φ = 0.105).

The efficient detection of PA through the CMO·Al3+ complex in an aqueous medium with a lowest detection limit of 2.29 × 10−10 M (Figure S42 in SI) further extends its utility. Logic Gate Studies. The selective “on−off” switching of CMO with Al3+ and PPi/PA inspired us to investigate the complete sensing phenomenon and its potential application in the field of logic gates and molecular switches. Thus, the fluorescence turn “on” (output “1”) happens when Al3+ is the only input. The introduction of PPi/PA to ensemble CMO·Al3+ leads to turn “off” the fluorescence response. Therefore, we expect this phenomenon to act as a more precise inhibition (INH) logic gate. Thus, output “1” is obtained only when the input is Al3+ and output “0” when the input is either PPi or both inputs are absent, viz., Al3+ and PPi/PA. Thus, it is clear from the truth table (Figure 15) that when both inputs are present, fluorescence becomes “off.” This behavior of CMO toward Al3+ and PPi gets fitted with the truth table of the INHIBIT logic gate.

Figure 12. Titration profile of 1.0 μM aqueous solution of the CMO· Al3+ ensemble in the presence of picric acid (0−40 equiv; λex = 380 nm).

The interaction between the CMO·Al3+ ensemble and PA in solution was further studied by 1H NMR titration experiments (Figure 13). The signals of the hydrogen atoms of the aromatic rings were observed to be downfield shifted with a broadening of peaks upon the addition of 3.0 equiv of PA.

Figure 15. Al3+ and PPi as inputs and their corresponding logic read out representation. (A) Truth table for two-input INHIBIT logic gate. (B) The current signals of this logic gate in the presence of different inputs. (C) Logic representation of INHIBIT gate. (D) Fluorescence photograph image showing output of logic gate.

Figure 13. Partial 1H NMR spectra showing the effect of PA (picric acid) on the CMO·Al3+ ensemble.

Cell Imaging Studies. The cellular studies clearly suggest that CMO has good cell permeability and shows effective intracellular fluorescence emission through the formation of the CMO·Al3+ complex, which loses its fluorescence intensity upon treatment with PPi. After incubation of CMO at 37 °C for 6 h, the cells had very weak intracellular fluorescence (Figure S43E,H in SI). However, cells exhibited intense fluorescence when exogenous Al3+ was introduced into the cells (Figure 16) via incubation with aluminum nitrate salt solution shown in Figure 43F,I in the SI. The intensive fluorescence behavior was, however, strongly suppressed when PPi was added to the CMO·Al3+ensemble. Because of the fact that PPi has a strong tendency to bind with Al 3+ , the sensors with a fluorescence property were competitively inhibited; hence the intensive fluorescence intensity decreases by gradually increasing the concentration of PPi (Figure S44E−H in SI). Thus, the fluorescence responses of CMO with added Al3+ are clearly evident from the cellular imaging, and it also supports the demetalation of Al3+ from ensemble CMO·Al3+ upon the addition of PPi (Figure 16). The viability of human PBMC in the presence of the ensemble CMO·Al3+ was assessed by colorimetric XTT assay. CMO did not show significant cytotoxic effects in human PBMC for at least up to 4 h of treatment. This suggests that

The above results suggested that the nitrogen atom of the pyridine ring moiety of CMO·Al3+ is the only reasonable site for protonation by PA, as shown in Figure 14. Mass spectral data supported this proposal because when PA was added to CMO·Al3+, the peak at m/z 669.5 disappeared and a peak at m/ z 1128.1772 corresponding to CMO·Al3+·PA was observed (Figure S19 in SI).

Figure 14. Schematic representation showing the interaction of picric acid with a CMO·Al3+ ensemble. 3321

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Inorganic Chemistry

Figure 16. Representative fluorescence image of MCF-7 cells exposed to CMO or CMO loaded Al(NO3)3 and CMO loaded Al(NO3)3 in the presence or absence of inorganic-pyrophosphate (PPi) at different ratios for 6 h at 37 °C and 5% CO2 in a culture medium. The cells were visualized under a fluorescence microscope, EVOS FL Cell Imaging System equipped with Plan Fluor, 40X, NA 0.75 objective (Life Technologies), USA.

Present Address

CMO can be readily used for cellular application at the indicated dose without causing any damage to the cells. Similar studies were also performed in MCF-7 tumor cells. Lower concentrations of either CMO alone or in the presence of ensemble CMO·Al3+ were tolerated by the MCF-7 cells. A higher concentration (5 mg/mL) of the compounds was moderately toxic for MCF-7 (Figure S45 in SI).

⊥

Banaras Hindu University, Institute of Science, Varanasi221005.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SKA thanks the CSIR, New Delhi for JRF/SRF [09/ 013(0475)/2012-EMR-I]. AK thanks SERB, New Delhi for the Young Scientist award [YSS/2015/000057]. Neeraj acknowledges the UPE-JRF fellowship [chem./2012-13/154]. Shweta acknowledges UGC, New Delhi for JRF/SRF [19-12/ 2010(i) EU-IV].

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CONCLUSIONS In summary, we have successfully prepared a potent fluorescent sensor for Al3+ in 100% aqueous solution. The CMO·Al3+ ensemble proved itself as a nice probe for selective detection of pyrophosphate (PPi) and picric acid. The simplicity, sensitivity, and water compatibility of CMO and CMO·Al3+ ensemble for the detection of Al3+ and PPi/PA, respectively, demonstrate the worth of the same toward environmental and biological systems. The cell imaging results further support their permeability in the cell membrane and the detection of intracellular Al3+ and pyrophosphate within living cells quite efficiently. Moreover, a binding event between CMO and Al3+ was fully confirmed through single crystal XRD studies which are rarely reported in the literature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02752. Materials and methods, X-ray crystallography, and characterization data for the newly synthesized compound and spectroscopic data related to Al3+, PPi/PA sensing (PDF) X-ray crystallographic data of CMO (CIF) X-ray crystallographic data of CMS (CIF) X-ray crystallographic data of CMO·Al3+ (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 542-6702488. E-mail: [email protected] and [email protected]. ORCID

Ajit Kumar: 0000-0001-6321-3335 K. K. Upadhyay: 0000-0002-2438-1675 3322

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