A coumarin coupled electron donor-acceptor dyad for cascade detection of aluminium ions and explosive nitroaromatic compounds

Highlights

• Coumarin151 appended “off-on-off” chemosensor (HL) is developed for the cascade detection of Al³⁺ ions (ON) and picric acid (PA, OFF).

• A visual color change from yellow to colorless is observed in the presence of Al³⁺ ions in the naked eye.

• A bluish-green color could be seen under 365 nm UV light irradiation in the presence of Al³⁺ ions.

• The fluorescence of the HL-Al³⁺ complex is quenched in the presence of PA among all the tested nitroaromatic compounds.

Abstract

A coumarin coupled electron donor–acceptor dyad (HL) has been synthesized and characterized by different conventional techniques. Details spectroscopic investigation has been carried out to understand the photophysical behavior of as-prepared HL. The UV–visible spectral behavior of HL is found to be intramolecular charge transfer in nature and shows broadband, having maxima at 432 nm with a shoulder peak around 348 nm in DMF. The emission spectrum of HL also gives broadband, having maxima located at 480 nm in DMF. Further, HL is employed for the cascade detection of Al³⁺ ions (ON) and picric acid (PA, OFF) in DMF-H₂O (7:3, v/v) media. A colorimetric change from yellow to colorless is observed in the presence of Al³⁺ ions. A significant fluorescence enhancement is observed due to the well coordination of Al³⁺ ions with HL. A visual bluish-green color could be seen under 365 nm UV light irradiation, which is also well supported by the color chromaticity diagram. The fluorescence of the HL-Al³⁺ complex is quenched in the presence of PA among all the tested nitroaromatic compounds. The sensitivity of HL and HL-Al³⁺ complex towards detection of Al³⁺ ions and PA can be possible down to 0.638 μM and 0.623 µM, respectively. The developed chemosensor is employed for the practical application for mapping Al³⁺ ions in the living cell. The performance of HL toward Al³⁺ ions proved that it could be exploited as a signal tool for environmental and biological samples.

Introduction

Among the earth's crust metals, aluminium (Al³⁺) is the most plentiful and the 3rd most abundant element (8.3 % by weight). In day-to-day life and industry, Al³⁺ is widely used in packing items, paper making, food additives, electrical equipment, water treatment, medicines, and the production of light alloys [1], [2], [3], [4], [5], [6]. Enzyme-catalyzed reaction and biotechnological transformation like biochemical reaction, Al³⁺ also plays a vital role [7]. Although Al³⁺ is less toxic than the other elements, excess Al³⁺ can cause harmful effects on the human body. Sometimes, human beings suffer from central nervous system damage caused by Alzheimer's disease [8], [9]. Parkinson's diseases [10], [11], osteomalacia [12], and osteoporosis [13] due to excess Al³⁺ consumption. Excess Al³⁺ in the human body is also responsible for impaired lung function, bone softening, chronic renal failure, fibrosis, etc. [14], [15], [16]. Acid rains leach Al³⁺ ions from the soil, and the concentration of Al³⁺ ions increases on the surface water. The water and soil become Al³⁺ contaminants which are highly dangerous for aquatic plants and animals. Al³⁺ contaminant soil also prevents the natural growth of plants [17], [18]. Due to the lousy impacts of Al³⁺, it is crucial to find a way to detect Al³⁺ in both biological and environmental systems efficiently. Aluminate compounds are also frequently used as pharmaceutical drugs in human and veterinary drugs, such as buffered aspirin contains aluminium glycimate, which is generally used as an analgesic [19]. The WHO/ FAO joint expert committee recommended a maximum intake of Al³⁺ up to 3–10 mg per body mass, and more than this amount can cause severe health issues [20], [21].

Several conventional methods such as electrochemistry, atomic absorption spectroscopy, mass spectrometry, graphite furnace atomic absorption spectrometry, NMR technology, and inductively coupled plasma atomic emission spectrometry have been used to identify trace amount of Al³⁺ in a contaminated sample [22], [23], [24], [25]. But, these techniques have severe drawbacks, requiring a large amount of sample, time-consuming, and sophisticated instrumentation, which is generally costly [23], [26]. Over the past few decades, researchers have been extensively attracted by spectrofluorosensor to detect different target analytes because of their massive application in various fields like analytical chemistry, medicinal biology, and the environmental sciences and their specific selectivity, sensitivity, real-time detection, and operational simplicity [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. However, researchers always have to take challenges to develop fluorescence chemosensors for detecting Al³⁺ ions because of their lack of spectroscopic properties, poor coordination, and strong hydration ability [39], [40]. However, some research groups have successfully developed a few highly selective and sensitive Al³⁺ fluorescence sensors and overcome the difficulties regarding detecting trace amounts of Al³⁺ selectively over the other ions [41], [42], [43], [44], [45], [46], [47], [48], [49], [50]. Al³⁺ is widely known as hard-acid, so it preferably binds with the hard-base donor site like N and O [40], [41], [42], [43], [44]. The fluorescence moiety containing Schiff base ligand can provide the hard base N and O environments to detect Al³⁺ selectively [51]. Most of the chemosensors so far developed for Al³⁺ ions are primarily used in non-aqueous solvents [52], [53]. Although few scientific reports are available in the literature that detects Al³⁺ in aqueous solvents selectively over the other ions [54], [55], [56], [57], [58], most of the reported Al³⁺ sensors lack selectivity, sensitivity, and water solubility with low-level detection limits. In these scenarios, for real biomedical and environmental application of chemosensor to detect Al³⁺ ions in the aqueous or semi-aqueous medium is most important. However, literature reports that deal with detailed mechanistic aspects of detection mechanisms and their thermodynamic feasibility are scarce. Understanding the detailed photophysics of the receptors and mechanistic aspects of sensory performance are of paramount importance, which will pave the way for designing and developing new sensorial materials for selective detection of targeting analytes over colonial crowding conditions.

If the sensor shows significantly enhanced photoluminescence upon binding with Al³⁺ ions, in that case, the resultant Al-complex can act itself as an essential device for sensing the different external analytes [59], [60]. Among the electron-deficient nitroaromatic compounds (NACs), picric acid (PA) is primarily typical “turn-off” or fluorescence quenching-based detection element [61]. Among the different NACs, PA is extensively used in leather industries as a yellow pigment, medicines as an antiseptic agent, rocket fuel manufacturing, and the preparation of explosives, propellants, and ammunition [62], [63], [64], [65], [66]. Due to its strong electron-accepting properties, PA is highly water-soluble (14 g L⁻¹ at 20C) that may threaten for human beings as living creatures could consume it with the drinking water. It can also pollute the soils and enter the human body [67], [68], [69]. The tolerable concentration of PA in drinking water is 0.001 mg L⁻¹, as provided by WHO [70]. PA is highly responsible for diseases such as eye irritation, anemia, headache, kidney problems, respiratory organ, and severe liver damage [71], [72], [73], [74], [75]. Therefore, it is of paramount importance to develop a highly selective and sensitive sensor to detect nitroaromatic explosives, especially PA. There are some standard techniques such as surface-enhanced Raman spectroscopy (SERS) [76], ion-mobility spectrometry (IMS) [77], and gas chromatography (GC) [78] for the detection of PA. These techniques may have enormous drawbacks like lack of portability, high cost, and low selectivity. For the detection of PA, the best way is to use fluorescence chemosensors by multi-analyte or sequential detection techniques [79], [80], [81], [82] because the relay recognition approach offers the added benefit of sensing dual analytes using a single molecule [59], [61].

Keeping the above facts in mind, in this report, a coumarin151 (C151) derived Salen type “off-on” Al³⁺ ions fluorescence chemosensor, HL, has been synthesized and characterized by different analytical techniques. We have preferred coumarin 151 (C151) as a signaling unit to develop this Al³⁺ spectrofluorosensor because of its extraordinary photophysical properties like large Stokes shift, excellent light stability, less toxicity, visible excitation-emission wavelength, and high fluorescence quantum yield [83], [84], [85]. On the other hand, N, N-diethylsalicyalimine is chosen as a binding site having hydroxyl and imine groups. Due to having a hard center, N and O environments could bind selectively hard centers such as Al³⁺ ions preferably than the others. Photophysical aspects of HL have been investigated in detail. Further sensorial behavior and mechanistic aspects of different target analytes have been evaluated. The synthesized sensor can act as a colorimetric and “off-on” fluorochemosensor for Al³⁺ ions in 30 % water-DMF solvent with high selectivity and sensitivity (LOD- 0.638 µM), which could be applied in real and biological sample analysis. Subsequently, the resultant HL-Al³⁺ complex could act as an “on-off” fluorosensor for the selective detection of PA with high sensitivity. The need for a dual analyte detection system with such outstanding “off-on” and “on-off” process characteristics led us to consider the prepared HL salen type molecule, a feasible nominee for constructing molecular logic circuits. The optical response of the synthesized salen type molecule towards the accumulation of Al³⁺ ions and subsequently PA as chemical inputs provides an opportunity to construct the INH molecular logic gate. Also, the anticancer activity of as-prepared HL has been investigated and found to be more effective against hepatocellular carcinoma, HepG2 cell but is less potent against breast adenocarcinoma, MCF7 cell. Further, HL is also employed for intercellular detection of Al³⁺ ions in MCF7 breast cancer cells.