DESIGN, SYNTHESIS AND EVALUATION OF SOME NOVEL FLUORESCENT CHEMOSENSORS FOR MERCURY (II) ION DETECTION VIA DIFFERENT MECHANISMS

Abstract

Increasing demand for fast and accurate environmental pollution monitoring requires new sensing techniques with outstanding performance, high sensitivity, selectivity and reliability and fluorescence chemosensors offer this distinctive reward over other analytical methods and have been considered as an ultimate tool for monitoring trace toxicants. As one of the most toxic heavy metals, Hg2+ ion has become an ultimate goal for its prolonged effects and rigorous terrorization to the environment and biological system. Novel organic molecular scaffolds for recognition and sensing of environmentally and biologically toxic Hg2+ ions with high selectivity and sensitivity detection in aqueous solution are persistently important for practical research in different fields of science. The central theme of this present thesis entitled “DESIGN, SYNTHESIS AND EVALUATION OF SOME NOVEL FLUORESCENT CHEMOSENSORS FOR MERCURY (II) ION DETECTION VIA DIFFERENT MECHANISMS” discuss about the design, synthesis, characterization and photophysical properties studies of some Hg2+ ion receptors. All chemosensors were characterized by different spectroscopic methods and the photophysical behaviors of these chemosensors towards Hg2+ ion were observed through UV-visible and fluorescence spectroscopy. In addition to that the binding modes of chemosensors with Hg2+ ion were examined by using IR, NMR titration, HRMS and fluorescence lifetime. This thesis includes five chapters, the first chapter deals with “General Introduction”, which is providing brief discussions about principle of optical chemosensors, some common photophysical mechanisms of fluorescence chemosensors and review of some recognition mechanisms Hg2+ ion. Chapter 2 presents about design and synthesis of 1,4-benzothiazine hydrazide as selective and sensitive colorimetric and Turn-On fluorometric sensor for Hg2+ detection in aqueous medium. A highly colorimetric and fluorimetric chemosensor, 3-oxo-[1,4]-benzothiazin-2-ylidene acetohydrazide (L2) is reported for the mercury detection. In HEPESIn HEPESIn HEPESIn HEPES In HEPESIn HEPES -buffered solution (CH buffered solution (CH buffered solution (CHbuffered solution (CHbuffered solution (CH buffered solution (CH buffered solution (CH buffered solution (CH 3CN:HCN:H 2O, 1:2, O, 1:2, v/v, pH 7.2), v/v, pH 7.2), v/v, pH 7.2), v/v, pH 7.2), v/v, pH 7.2), v/v, pH 7.2), L2 showed a characteristic absorption peak at 340 nm, addition of Hg2+ induced colour change from light yellow to purple with significant enhancement in absorbance at 340 nm and a new band centered at 550 nm with red-shift of 110 nm. Furthermore, the L2 exhibited high sensitivity and selectivity with overall emission change of more than 100-fold fluorescence intensity enhancement towards Hg2+ ion with a 1:1 binding ii stoichiometry and 1.938x103 M−1 binding constant within detection limit as low as 5.4x10-8 M. More importantly, it has good practicability in real water sample. Apart from this, theoretical elucidation of the experimental outcome has also been supported by applying density functional theory (DFT) to the ligand and the complex. Moreover, the solution of in situ generated L2+Hg2+ complex displayed high reversibility by I- through Hg2+ displacement approach. This reversibility in fluorescence suggested that the promising applicability of chemosensor as “off–on–off” naked eye sensor and practical applicability of L2 as an INHIBIT logic gate based on the emission changes with the inputs of Hg2+ and I- was investigated. Chapter 3 describers about a novel 1,8-naphthalimide as highly selective naked-eye and ratiometric fluorescent sensor for detection of Hg2+ ion. in this chapter a novel chemosensor 2-(2-mercaptophenyl)-1H-benzo[de]isoquinoline-1,3-(2H)-dione (L3) is reported for Hg2+ ion detection. The selective binding of Hg2+ to L3 afforded new absorbance and fluorescence peaks at 438 nm and 480 nm respectively, in addition to the existing bands of L3 at 332 nm and 376 nm. It also showed apparent colour change from colourless to yellowish green and weak fluorescent to bright yellowish green strong fluorescent due to selective binding of Hg2+. This sensor forms a 1:1 stoichiometry with high binding constant (3.89 x 104 mol-1L) and limit of detection 1.74 x10-8 M with high selectivity and sensitivity towards Hg2+ in the presence of other interfering metal ions. More importantly, it has good practicability in environmental water sample. The outcome of photo-physical experiment has also been in good accordance with the time -resolved resolved resolved resolved fluorescence lifetime decay decay decay decay decay and the density functional theory (DFT) estimations. Moreover, the L3-Hg2+ solution displayed high colorimetric and fluorimetric reversibility via the addition of KI. This reversibility in iii fluorescence showed potential applicability of chemosensor L3 as “on–off–on” naked eye sensor. Chapter 4 illustrates naphthalimide-amino acid conjugates chemosensors for Hg2+ detection based on chelation mediated emission enhancement in aqueous solution.1,8–Naphthalimide-Amino acid conjugates (L4 L4 and and L5 ) were designed, synthesized via straight forward reaction as Hg+2 chemosensors. These chemosensors were found to display aggregation-induced emission (AIE) property along with increment in quantum yield upon changing the medium from methanol to CH 3OH: H2O (1:99, v/v). They exhibited a quick response, splendid selectivity and sensitivity toward Hg2+ ion over other interfering metal ions through sharp selective Turn-On fluorescence via chelation mediated aggregation-induced emission(AIE) in CH 3OH /H2O (1:99, v/v) medium. Job’s plot revealed the formation of 2:1 ligand to metal stoichometric complex. The chemosensors L4 L4 and and L5 L5 could detect Hg2+ ion as low as 22 nM and 5.6 nM respectively. Intriguingly, the fluorescence reversibility study showed good reversibility by adding Hg2+ salts and KI sequentially. iv Chapter 5 presents synthesis and evaluation of AIEE active sulfamethizole functionalized 1,8-naphthalimide for ratiometric fluorescence sensing of Hg2+ ion in aqueous media and anti-microbial activity. A sulfamethizole functionalized 1,8- naphthalimide molecule (L6) has been designed and synthesized which exhibited AIEE active, fluorescence sensing of Hg2+ ion and Ag+ complex as anti-microbial activity. The hydrophobic nature of naphthalimide fluorogenic moiety instigated the aggregation induced emission (AIE) in aqueous medium, which led to excimer emission upon intramolecular excimer formation via metal ion-induced assembly. The high selectivity of the sensor was due to quenching of the monomeric emission of L6 at 390 nm with red shift to gradual enhancement in new peak at 483 nm and 478 nm for Hg2+ and Ag+ ions respectively, which resulted in ratiometric detection. The competitive experiment revealed that the chemosensor is selective to Hg2+ over Ag+ ion and selective and sensitive ratiometric response to Hg2+ without interference of any other tested metal ions. The limits of detection of Hg2+ ion is 14.7 nM in 99% aqueous solution. Meanwhile, visual fluorescence color change of L6 from blue to green was observed under Uv-lamp upon addition of Hg2+ ion to L6 solution. Antimicrobial activities were tested against Gram negative strain. Under similar concentrations, L6 and L6-Ag+ compounds have shown much better anti-microbial activities than the most of the tested antibiotics available in the market.