The natural compound chrysosplenol-D is a novel, ultrasensitive optical sensor for detection of Cu(II)
Graphical abstract
Introduction
Copper ions are essential to a variety of biological systems including the production of energy in living cells, transportation of oxygen and transduction of nerve signals [1]. However, high concentrations of Cu(II) in cells can lead to severe stress and induce degenerative nerve diseases, including Menke's, Alzheimer's and Wilson's diseases [2,3]. Thus, methods that enable selective and sensitive detection of Cu(II) ions have attracted considerable attention due to the important biological functions of this essential ion, which include anti-inflammatory, anti-proliferative and biocidal activities [4]. With respect to routine analytical methods, techniques based on fluorescent chromophores (chemical probes) or dyes have been extensively utilized to determine various biological species, anions and cations. Several probes have been utilized to detect Cu(II), including fluorescein [5], rhodamine [[6], [7], [8]], coumarin [[9], [10], [11]] and quinolone [12].
Numerous techniques have been developed to detect a variety of molecules or ions individually in the aqueous or gas phases. In aqueous medium, fluorescent nanomaterials or chemical probes present in the same phase as the species to be detected can be excited with light, and the subsequent photoluminescence emission is recorded as the analytical signal of a specific chemical reaction [13,14]. Such events usually occur on the nanosecond timescale. Emission of photoluminescence by the sensor may be accompanied by alterations to the emission wavelength or fluorescence intensity when the probe is bound to the target molecule/ion. The emission wavelengths of sensor materials can be red or blue shifted [[15], [16], [17], [18]]. Alternatively, the fluorescence intensity of the sensor can be quenched or enhanced when bound to the target molecule/ion [[19], [20], [21], [22]]. In all cases, the target molecule/ion modulates and/or phase-shifts the emission peak of the sensor compared to the emission curve of the free sensor in the absence of the ion/molecule under same excitation wavelength. Overall, each method has advantages for measurement under certain conditions.
In addition to the changes in fluorescence intensity or wavelengths induced by metal-ligand binding, some chemical probes or molecules for metal ions alter their electronic structure when bound to the specific target ligand. Such sensor molecules can be utilized in both colorimetric and fluorometric sensors. Colorimetric sensing mechanisms offer the advantages of being easier to use in field detection, while fluorescent sensors hold more potential for imaging of biological systems [[23], [24], [25]]. In addition to emitting visible absorbance signals, fluorometric detection in the visible range could help to improve our ability to detect metal ions compared to methods based on a single type of optical detection. Furthermore, fluorescent sensors are widely applied as they are extremely selective, highly sensitive and exhibit rapid, reversible responses that can be easily monitored [[26], [27], [28]].
With respect to the detection of copper ions, the multiple non-emissive de-excitation pathway, which includes electron transfer or excited state energy transfer, characteristically quenches the fluorescence intensity of sensor molecules [29]. Thus, many techniques exist to detect Cu(II) ions based on quenching of the fluorescence intensity (turn-off) of optical sensors in the presence of the target metal ions [[30], [31], [32], [33], [34], [35]]. These sensor molecules exhibit sensing capability in aqueous media and at physiological pH, which enables the techniques to be employed for in vivo applications. As Cu(II) metal ions are significantly solvated in aqueous solution, the enthalpy of the solvation process presents a challenge when designing and synthesizing appropriate receptors to detect Cu(II) ions in aqueous media. Furthermore, as the chelation mechanism of optical sensors could be attributed to substitution of the hydrogen protons of the optical sensor probe through chelation to Cu(II) ions, assays involving CTC have been extensively applied to determine the optical interaction of many optical sensors [[36], [37], [38], [39]].
Flavonoids are a broad class of natural products that are added to functional foods and natural supplements, used as dye products and employed in sensitized solar cell biosensors and the synthesis of nanomaterials, among other applications. Flavonoids can act as antioxidants, antimicrobials, photoreceptors, visual attractors, feeding repellants and light screening molecules. Flavonoids exhibit a wide range of biological activities, including antiallergenic, antiviral, anti-inflammatory and vasodilating actions [40]. The antioxidant and chelating properties of flavonoids are related to the presence of multiple hydroxyl groups that confer substantial antioxidant, chelating and pro-oxidant activities, methoxy groups that increase lipophilicity and membrane partitioning, and the presence of a CC double bond and carbonyl function, which is suggested to increase activity by generating more stable flavonoid radicals through conjugation and electron delocalization [41]. Flavonoids also exhibit a wide range of fluorescence characteristics depending on the pattern of substitution of the aromatic nucleus [42].
Chiliadenus montanus (Vahl.) Brullo (Family Asteraceae), is a flowering plant common throughout the Sinai Peninsula and popularly known as Haneida. C. montanus is used as a traditional medicine for the treatment of renal problems and some of its chemical components have been shown to exert antimicrobial, anti-diabetic, antioxidant, antiatherogenic, antibacterial, antifungal and/or anti-obesity activities [[43], [44], [45]]. Previous phytochemical research identified the presence of active constituents in the aerial parts of the plant, including phenolic compounds that give C. montanus its natural properties and biological activities [46,47].
Herein, we present a novel optical chemosensor for trace detection of Cu(II) using chrysosplenol-D (Chp-D) isolated from C. montanus. The sensing mechanism is based on metal-ligand chelation between the natural compound Chp-D and Cu(II) ions to form a metal complex. Formation of this complex is accompanied by establishment of a charge transfer complex (CTC) [48,49]. The mechanism by which Chp-D senses Cu(II) was investigated via absorbance and fluorometric measurements. We demonstrate this novel optical chemosensor is highly selective for Cu(II) metal ions and has a low limit of detection (LOD), high sensitivity and rapid reversibility.
Section snippets
Chemicals
All chemicals were purchased from Sigma-Aldrich. MeOH was HPLC grade; all other chemical compounds and solvents were analytical grade and utilized as received. The natural sensor Chp-D was extracted from Chiliadenus montanus (Vahl.) Brullo. All solutions were freshly prepared with deionized water before each experiment and utilized immediately.
Instruments and materials
TLC analyses were carried out on pre-coated silica gel plates (Kieselgel 60 F254, 0.25 mm; Merck, Darmstadt, Germany). The collected spots were detected
Results and discussion
CI8H16O8, 1H NMR (600 MHz, CDCl3): 7.62 (1H, d, J = 2.0 Hz, H-2′), 7.52 (1H, dd, J = 8.1, 2.0 Hz, H-6′), 6.90 (1H, d, J = 8.2 Hz, H-5′), 6.54 (1H, s, H-8), 3.81, 3.75 and 3.70 (9H, singlets, 3 OCH3). 13C NMR (125 MHz, in CDCl3): 152.8 (C-2), 137.7 (C-3), 178.6 (C-4), 152.0 (C-5). 131.6 (C-6), 157.7 (C-7), 93.6 (C-8), 156.0 (C-9), 105.1 (C-10), 122.6(C-l′), 116.1 (C-2′), 148.0 (C-3′), 150.2(C-4′), 112.6 (C-5′), 121.2 (C-6′) 60.4, 60.1 (3-OCH3 and 6-OCH3) and 56.1 (7-OCH3), and ESI-MS m/z 361.1
Conclusion
The natural compound chrysosplenol-D (Chp-D) was extracted and utilized as a highly selective optical chemosensor for detection of Cu(II) ions. The UV absorbance and fluorescence behavior of Chp-D in the presence of Cu(II) ions were studied in 1:99 (v/v) methanol–HEPES buffer solution at pH 7.4. Chp-D exhibits extreme selectivity and sensitivity towards Cu(II) ions, without interference by other various cations. Addition of Cu(II) ions to aqueous solution immediately quenches the intensity of
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