Development of coordination complexes as fluorescence probes for analyte applications

Abstract

Organophosphates and nitro explosive compounds are highly toxic and caused various environmental and human health issues. Organophosphates based pesticides are primarily used in agriculture to remove pest, weeds, and insects from the crops. However, the remained residues in the environment can enter in food and drinking water by various source and cause several human health concern. Chemical warfare agents are chemical substances used for a chemical weapon on the battlefield. They are identified as the most dangerous organophosphates that covalently bind with the acetylcholinesterase enzyme and inhibit their regular activity. Acetylcholinesterase is an crucial nervous system enzyme that can catalyze acetylcholine (neurotransmitter) to choline and acetic acid. Nitro explosive compounds are also toxic and widely used in explosive materials for the military and explosive purpose. Thus, the detection of these analytes in the environment is one of the global health and national security concern. Various analytical tools such as GC, HPLC, Mass, and NMR spectroscopy are used for detection of these analytes. However, the aforementioned techniques are costly, required a longer time for calibrations and cannot be accessible for small laboratory condition. Therefore, some economic and straightforward method would be needed for the sensing of toxic analytes. In this context, optical-based detections are a promising strategy and have numerous advantages such as simplicity, sensitivity, quick output response, easily transportable and economically viable. Therefore, absorbance and fluorescence detection are the main emphasis of recently developed optical sensors. Generally, fluorescencebased detection is more sensitive and has broad linear ranges compared to absorbance based detection. Additionally, the fluorescence methods could be easily incorporated in the handheld device for field monitoring of analytes. Thus, the fluorescence-based detection has become one of indispensable tools for sensitive and selective detection of environmentally and biologically important analytes. Recently, considerable efforts have been made to develop the chromogenic and fluorogenic sensor for pesticides, chemical warfare agents and nitro explosive compounds. An efficient strategy has been designed for fluorescence sensor includes the arrangement of binding sites and signaling sites, which response through modulation in emission profile upon binding with a particular analyte. This change in emission could be arises from various phenomenon depending upon the type of host and host-guest interaction. Photoinduced electron transfer (PET), Intramolecular charge transfer (ICT), Forester resonance energy transfer (FRET), aggregation-induced emission (AIE), - stacking interaction are common mechanisms whichgovern the tuning of fluorescence intensity. Based upon these mechanisms, various fluorescence probes such as small organic molecules, polymers, coordination complexes, metal nanoparticles, and quantum dots exist in literature for detection of contaminating analytes. However, the major challenge involves the aqueous medium sensing because most of contaminating species occurred in the environment. Therefore, it is highly desirable to develop a strategy in an aqueous medium. For this purpose, hydrophobic nature coordination complexes were prepared. The coordination complexes were solubilized in an aqueous medium by the reprecipitation method. The present approach involves an injection of a particular sensor (1 mL pure DMSO/THF solution) to 99 mL of double distilled pure water. There is a great disparity between the solubility of the sensor in the organic solvent (good solubility) and water (highly insolubility). However, the mutual solubility compatibility of two solvents is the governing features of the methods. This approach finds a solution of sensing in 99% aqueous medium. The outline of this thesis can be summarized in the following chapter. The contents of each chapter are briefly discussed as: Chapter 1: Introduction This chapter begins with a brief introduction of optical methods and their sensing mechanism. Further, the chemosensory materials for organophosphates and nitro explosive analytes are explored with systematic ways, and particular focus is given to recent year’s research. The broad sensor array such as fluorescent organic molecules, polymeric materials, supramolecular assembly and metal complexes, their detection ability, and detection mechanism is the main spotlight of this chapter. Finally, the conclusion and future outlook of the literature work is given. Chapter 2: Synthesis, Crystal Structure and Photophysical Properties of mixed S, N, O Donor Sites Imine linked Nickel Complexes for Pesticides Detection In this chapter, the author describes the design and synthesis of disulfide based imine linked dipodal receptors. The structural model of dipodal receptors was constructed in such a way that they offer sp2 nitrogen atom (C=N), hydroxyl group and sulphur atom (disulfide linkage) for coordination with particular metal ions. The dipodal receptors were tested for binding with a series of metal ions. However, UV-Vis absorption spectra show a maximum shift with Ni2+ ions. Further, coordination complexes of dipodal receptors were synthesized with nickel ions and iii characterized with single X-ray crystallography. Among all prepared nickel complexes, some of complexes are dinuclear, and some are mononuclear with octahedral geometry. Due to excellent photophysical and structural properties, the prepared nickel complexes were explored for pesticides detection in an aqueous medium. For aqueous sensing studies, the complexes were dissolved in 1 mL of pure DMSO and which is further suspended in 99 mL of double distilled pure water. The fluorescence spectra of dinuclear nickel complexes show tunability in emission profile with phosmet pesticides and could sense it with the lowest detection limit of 44 nM. The mononuclear nickel complexes are exhibiting sensing with chlorpyrifos pesticides with a detection limit of 62 to 71 nM. We compared our sensing result with literature reported pesticides sensor, and we discovered that our sensor has several advantages than existing sensor (discussed in Chapter 2). Chapter 3: Synthesis Crystal Engineering and Photophysical Properties of Naphthalimide based Cu (II), Zn (II), Co (II), Pb (II) Complexes for Molecular Recognition of Organophosphates and Nitro explosive Compounds Naphthalimide skeleton is employed as one of the functional fluorophore and widely used in supramolecular chemistry, due to its synthetic diversity and excellent photophysical properties. Naphthalimide skeleton has the ability to form aggregation by many interactions such as π-π, CH….π, and dipole-dipole interaction, based on the derivatization with N-heterocyclic or highly polarized aromatic system. These interactions would be explored in structure chemistry to generate robust and functional supramolecular design. N-heterocyclic linked 1,8-naphthalimide based ligands have been synthesized and investigated for metal complex formation with transition metal ions. The coordination chemistry of metal complexes was explored by crystallographic studies. The present chapter is divided into three sections (3.2, 3.3 and 3.4). The first section describes the synthesis of copper complexes of 1,8-naphthalimide based ligands, which was further explored for chemosensor activity of organophosphates. The chemosensor ability of copper complexes was checked with the library of organophosphates/biophosphates, and original emission of the ligand was restored in the presence of azamethiphos. This shows that the cation displacement assay work in the sensing of azamethiphos. The second section describes the development of fluorescence zinc complexes of 1,8-naphthalimide ligands. The supramolecular architecture of fluorescence zinc complexes exhibits a number of intermolecular interactions such as π-π, C-H….π, and hydrogen bonding between naphthalimide moieties. These interactions are highly sensitive to the nature of the microenvironments and tuned the emission attribute of the zinc complexes without changing their electronic structure. The π-π stacking interaction in zinc complexes is explored for selective detection of nitroaromatic compounds in an aqueous medium. The emission spectra of zinc complexes show eight to ten-fold emission quenching upon addition of picric acid which is due to resonance energy transfer from electron rich naphthalimide to electron deficient picric acid. The other nitroaromatic and haloaromatic compounds did not affect the emission intensity of the zinc complexes. The last section of this chapter describes the development of cobalt (II) and lead (II) complexes of naphthalimide ligands. The emission spectra of lead complexes are nonemissive due to heavy metal effect of lead ions; while cobalt complexes are emissive due to the naphthalimide ring. The X-ray crystallography studies show that all complexes exhibit π-π stacking interaction between naphthalimide moieties. Therefore, the emissive cobalt complexes were further utilized for selective detection of nitroaromatic compounds in an aqueous medium. The cobalt complexes show selectivity for picric acid and details mechanistic investigation for sensing analysis was studies with time-resolved and DFT calculation. Thus, the discerning ability and extremely fluorescence quenching response of zinc and cobalt complexes consider them as a potential sensor for picric acid. Chapter 4: Development of “Aggregation Induced Emission” based Luminescent Probes for Biomolecules and Chemical Warfare Agents’ Detection Aggregation-induced emission (AIE) based fluorescence probes are weekly emissive or nonemissive in dilute solution; however, exhibit strong emission in the aggregated state. This can happen by the suppression of non-radiative energy decay caused by the restriction of intramolecular rotation and vibration motion of the fluorescence probes. AIE based fluorescence probes have a feature of high signal to noise ratio, strong photostability, and large Stokes shift. These advantageous characteristics of AIEgens make them potential materials for environmental monitoring, biological research, disease diagnosis and fabrication of optoelectronic devices. This chapter is divided into three sections as 4.2, 4.3 and 4.4. The first section illustrates the design and synthesis of benzimidazole-based ligands. Further, metal complexes of these ligands were constructed with zinc metal ions. Due to the possibility of labile anions displacement and chelate ring formation, the zinc complexes were explored for sensing of biomolecules in an aqueous medium. The zinc complexes exhibit selectivity for pyrophosphate with a detection limit of 25 nM. Further, the work is extended to Hela cell imaging and real-time fluorescence assay for the enzymatic activity of Alkaline Phosphatases (ALP). Section 4.3 and 4.4 explain the synthesis of Naphthalimide based Ion-pair dipodal receptors. The dipodal receptors possess a suitable cavity for particular metal ions, and hydrogen bonding as well as electrostatic interaction for chemical warfare agents. Fluorescent nanoaggregates of the dipodal receptor was prepared and used for the successful recognition of chemical warfare agents. Moreover, the cellulose paper-based sensing stripe of fluorescent nanoaggregates was developed and successfully detect chemical warfare agents in the aqueous medium. Thus, the sensor device provides a straightforward and cost-effective sensing kit for detection of nerve agents. Chapter 5: Synthesis, Structure Evaluation and Photophysical Properties of Ni (II) Complexes of Pentadentate Schiff base Ligands for Insecticides Detection and Degradation Studies Enzymes catalyze hydrolytic reactions of various functional groups such as amides, nitriles, and phosphatases are well known. Several enzymes are recognized which contain metal ions in their catalytical centers. Therefore, it has been a high demand for the development of enzyme models for hydrolysis of various functional groups. In this chapter, the author reported the synthesis of Ni (II) complexes of Schiff base ligands. The structural properties of complexes were explored with X-ray crystallography studies and further nickel complexes were explored for insecticides detection and degradation studies. This chapter is divided into two parts. First part involves: Single crystal to single crystal transformation of dinuclear nickel complex of Schiff base ligand to dimeric-tetrameric nickel complex was achieved through mechanochemical synthesis. Both the complexes were characterized by X-ray crystallography and spectroscopic data of analysis. Single X-ray crystal studies revealed that both the complexes are distorted octahedral in geometry. The second part contain: Dinuclear nickel complexes of pentadentate Schiff base ligands were synthesized and characterized with spectroscopic data of analysis. The synthesized complexes were utilized for ethion sensing studies. The detection limit of the sensor was achieved of 19-21 nM. Further, the complexes were explored for catalytical degradation studies of ethion to less toxic components. After 16 hours of reaction, the ethion was completely hydrolyzed to diethyl thiophosphate and ethion monoxon. Thus, the prepared nickel complexes possess excellent catalytical activity for hydrolysis of phosphorothioate bond. Chapter 6: Conclusion This chapter describes the salient features and the overall conclusion of the thesis.