DESIGN AND SYNTHESIS OF FLUORESCENCE TURN-ON CHEMOSENSORS FOR SOME METAL IONS

Abstract

Over the past few years, high sensitive and selective fluorescent chemosensors towards various transition and other toxic metal ions are particularly attractive to current researchers due to its potential applications in the medicinal, clinical and environmental research areas. At present, many techniques are available for qualitative and quantitative analysis of metal ions and found their applications in various food, biological, geological and industrial effluents such as atomic absorption spectroscopy, inductively coupled plasma-mass spectroscopy, inductively coupled plasma emission spectrometry, neutron activation analysis, chromatography and voltammetry. Nevertheless, most of these methods involve tedious sample preparation procedures, sophisticated instruments and high maintenance expenditure. In recent years there has been a growing need for constructing chemical sensors for fast, ontime and cost-effective monitoring of environmental samples. The research and development (R&D) in the sensors area has expanded exponentially in terms of financial investment, numbers of paper published, and the number of active researchers worldwide. Compared with the traditional analysis instruments, chemical sensors are portable, simple to use, in-situ and miniature in size. These features are ideal for real-time on field measurements, thus the errors caused by the sample transportation and storage can be largely reduced. On the other hand, fluorescent chemosensors have drawn attention and offer considerable advantages over other techniques via their simplicity, convenience, low-cost, sensitivity, immediate response, and naked-eye visualization. The thesis is divided into six chapters. General introduction and a survey of fluorescence-based optical sensors reported in the literature are presented in Chapter 1. These chemosensors are typically derived from a core group of well-known fluorophores, such as coumarin, bipyridine, indole, quinoline, calixarene, porphyrin, crown ether, fluorescein, rhodamine, BODIPY and nanoparticle, each emitting in different regions of the electromagnetic spectrum. Chapter 2 describes the theory which involves during the sensing process. Details related to the photoluminescence process will be discussed in the different subsections. The properties of excited states as well as their relaxation processes are explained with the help of Jablonski diagram. Classification of chemosensors according to the nature of the signal ii emitted by the active unit and the different possible mechanisms such as PET, ICT and ET for signal transduction upon analyte binding to chemosensors, and the terms used in the study of fluorescence sensing have also been discussed in this chapter. In Chapter 3, the fluorescent sensors C1 and C2 with 4-aminoantipyrine unit have been prepared and characterized. Their complexation behaviour and binding mode towards Al3+ and other metal ions have been studied by UV–Vis, fluorescence spectrometric and HRMS methods. The free ligands C1 and C2 exhibited a main absorption band at about 345 nm and 380 nm, respectively. On the addition of metal ions to sensors, a new broad absorption band (mainly for Cu2+, Ni2+, Co2+ and Al3+ ions) was observed at 350–480 nm region. Receptor C1 and C2 alone displayed a very weak single fluorescence emission band at 498 nm and 484 nm respectively, with an excitation of 360 nm. On addition of Al3+, receptors C1 and C2 exhibited a prominent fluorescence enhancement accompanied by a blue shift of 32 nm from 498 to 466 nm and 18 nm from 484 to 466 nm, respectively. Indicating that the receptors C1 and C2 exhibit “off-on” mode with high sensitivity towards Al3+ over other metal ions which are used. The 1H NMR titrations were carried out to explore the nature of interaction between receptor and aluminum ion. These sensors are successfully applied in highly acidic and neutral pH medium with the fastest response time (<5 sec). The fluorescence color change could be easily detected by the naked eye under a UV lamp. Fluorescence quenching of complex is observed in the presence of Cu2+, Ni2+ and Co2+ ions due to dissociation of Al3+ complex of receptor. N N O N Ar C1: Ar = 2-hydroxyphenyl C2: Ar = 2-hydroxynaphthyl Chart 1. Structures of the antipyrine based sensors. In Chapter 4, new fluorescence chemosensors, CS1 and CS2 based on flavonol derivatives were synthesized and characterized. Complexation behaviour of sensors towards zinc and other tested metals have been studied using UV–Vis and fluorescence spectrometric methods. The free ligands CS1 and CS2 exhibited a main absorption band centred at 343 nm iii and 356 nm, respectively. On the addition of metal ions to sensors, the absorption band of free receptors CS1 and CS2 is shifted to low intensity, while a new broad absorption band (mainly for Cu2+, Ni2+, Zn2+, Pb2+, Al3+, Co2+, Cd2+, Mn2+ and Mg2+ ions) was observed at about 350– 480 nm region. Chemosensors CS1 and CS2 alone displayed a weak single fluorescence emission band at 530 nm and a couple of emission bands at 425 and 530 nm, respectively, with an excitation of 340 nm. On addition of Zn2+, receptors CS1 and CS2 exhibited a prominent fluorescence enhancement accompanied by a blue shift of 54 nm from 530 to 476 nm and 52 nm from 530 to 478 nm, respectively. These reveal selective detection towards Zn2+ ion, along with fluorometric response. Also, those serve as a highly selective chemodosimeter for Zn2+ at neutral pH with naked-eye detection and successfully examined the reversibility of Chemosensor–Zn(II) complexation. O O OH Ar CS1: Ar = Phenyl CS2: Ar = 2-furyl Chart 2. Structures of the flavonol based sensors. In Chapter 5, a simple 4-Methyl-7-hydroxy-8-formyl Coumarin serves as a selective chemosensor for Mg2+ in the presence of alkali and alkaline earth metal ions. The free ligand CS exhibited a single absorption band at about 343 nm, hyperchromic shift was observed when added to Co2+, Cu2+, Gd3+, Mg2+, Mn2+, Nd3+, Ni2+ and Zn2+ ions. It showed a blue shift accompanied by a hyperchromic shift in the presence of Cr3+ and Al3+ metal ions. Chemosensor alone showed a single emission band at 473 nm with an excitation of 350 nm. CS showed a chelation enhanced fluorescence (CHEF) only with Mg2+, even though there was a relatively chelation enhanced fluorescent quenching (CHEQ) effect with Al3+, Co2+, Cr3+, HO O O CHO 4-Methyl-7-hydroxy-8-formyl Coumarin Chart 3. Structure of the Coumarin based sensor. iv Cu2+, Fe2+, Gd3+, Mn2+, Nd3+, Ni2+ and Zn2+. It showed a significant fluorescence enhancement and provides naked-eye detection towards Mg2+. The receptor exhibited a good binding constant and lowest detection limit for Mg2+. The variation of emission signal exists via of reversible chelation enhanced fluorescence (CHEF) with this inherent quenching metal ion. In Chapter 6, a series of rhodamine derivatives have been prepared and characterized by FT-IR, 1H NMR, 13C NMR and ESI-MS, and their colorimetric and fluorescence responses toward various metal ions were explored. Ligand L1–L4 shows fluorescence response to Al3+ in the presence of other competing metal ions in methanol. The detection limit of Al(III) was estimated based on the fluorescence titration profile as 6.0×10−7 M (for L1), 5.8×10−7 M (for L2), 5.0×10−7 M (for L3) and 1.4 × 10−7 M (for L4). The resultant Al3+ complex of the sensor L4 is evaluated for anion recognition properties. The metal complex is highly selective for the determination of AcO− and F− with a detection limit of 0.4 μM in same solvent. The sensors, RS1 and RS2 exhibited highly selective and sensitive “turn-ON” fluorescent and colorimetric response toward Cr3+. The detection limit of Cr(III) was calculated for RS1 and RS2 as 4.9×10−8 M and 2.4×10−7 M, respectively. Receptor RH exhibited strong colorimetric response toward Cu(II), Al(III) and Fe(III) and specific fluorometric response to Fe(III) in semi aqueous medium. The formation of RH–Al3+ complex is fully reversible and can sense to AcO− and F− via dissociation. Thus, the sensor RH provides fluorescence “off-on-off” strategy for the sequential detection of Al3+ and AcO−/F−. All these rhodamine-derived sensors works on the basis of structure change from spirocyclic form (fluorescence “OFF”) to ring-opened amide form (fluorescence “ON”) induced by a specific chemical species such as ionic metal at room temperature. Upon the addition of metal ion, the spiro ring was opened and the complex was formed in a 1 : 1 stoichiometry, and it was further confirmed by ESI-MS spectra. N O N N O N Ar RS1: Ar = 2-hydroxynaphthyl RS2: Ar = 2,4-dihydroxyphenyl O N N N O N O O N N O N N O R L1: R = Ethylene L2: R = 1,3-propylene L3: R = Oxydiethylene N O N N O N L4: Ar = 4-hydroxy-3-methoxyphenyl RH: Ar = 2,5-dihydroxyphenyl Ar Chart 4. Structures of the rhodamine based chemosensors