New Supramolecular Ion Sensing Probes And Their Application In The Detection Of Environmentally Relevant Ions

Abstract

The thesis entitled “New Supramolecular Ion Sensing Probes and their Application in the Detection of Environmentally Relevant Ions” deals with the design and synthesis of several small molecular probes which can specifically sense environmentally relevant ions of (anion or cation) particularly in aqueous or biological medium. The probes have been designed using four different molecular entities which include anthraquinone, oxidized bis-indolyl system, pyrene and rhodamine. The probes afford naked eye detection of a particular ion in the aqueous medium. This work has been divided into six chapters. Chapter 1. Introduction The first chapter gives a brief idea of ion sensor. It provides the description of various approaches used for designing molecular sensors. The chapter further presents an overview of the four different dyes (anthraquinone, oxidized-bis-indole, pyrene and rhodamine) used for designing probes in this work. The properties of these probes, their advantages and disadvantages to use as a signaling subunit have been discussed. This chapter also describes the use of micellar medium for solubilizing different organic dyes in water. Chapter 2. Colorimetric Probes based on Anthraimidazolediones for Selective Sensing of Fluoride and Cyanide ion via Intramolecular Charge Transfer. The second chapter describes the design and synthesis of four different probes based on anthra [1, 2-d] imidazole-6, 11-dione. The anthraquinone part of each molecule has an acceptor moiety whereas substituted nitrogen linked aromatic unit forms the donor site. Each probe acted as strong colorimetric sensor for fluoride and cyanide ion detection and exhibited intramolecular charge transfer (ICT) band which showed significant red-shifts after addition of either the F¯ or CN¯ ion. One of the probes 2 showed selective colorimetric sensing for both cyanide and fluoride ions. In organic medium 2 showed selective color change with fluoride and cyanide, whereas in aqueous organic medium it showed a selective ratiometric response towards cyanide ion. The effect of anionic charge (on the donor moiety) on ICT has been discussed. Among the various donor moieties, the donor site having negative charges on them was found to disperse greater electron density on them. Figure 1. Molecular structures of the sensors Chapter 3 deals with chemodosimetric detection of cyanide ion in water using various oxidized bis-indole based compounds. Chapter 3A. A Chemodosimetric Probe based on a Conjugated and oxidized Bis¬ indolyl System for Selective Naked Eye Sensing of Cyanide ion in Water. The chapter 3A describes the design and synthesis of a new water-soluble bis-indolyl based probe, 5 which possesses two –COOH groups. This probe specifically reacted with the CN¯ ion in pure water at ambient temperature and produced a remarkable change in color from red to colorless. The mechanism of this process was investigated by NMR (1H, 13C and DEPT-135) spectroscopy, mass spectrometry and kinetic studies. The mechanism investigation showed that the cyanide ion reacts with the probe and removes the conjugation of the bis-indolyl moiety of the probe with that of the 4-substituted aromatic ring which renders the probe colorless. Taken together a plausible mechanism of the reaction was presented which showed to operate via a Michael type adduct formation under ambient conditions of pH and temperature in water. The probe gave a detection limit of 0.38 ppm for detection of cyanide ion in water. Figure 2. Molecular structure of the probe 5. Chapter 3B. Micelle Assisted ppb level Detection of Cyanide ion in Water by Chemodosimetry and Visual detection of the Endogenous Cyanide. The chapter 3B deals with the synthesis of a bis-indole based colorimetric probe 6. The probe showed selective detection of the cyanide ion in water at ppb level and a visible detection of endogenous cyanide from cassava (a major staple food in the developing world) by chemodosimetry. The cyanide ion binds with the probe 6 in a chemodosimetric fashion and follows pseudo first-order kinetics in water under appropriate conditions. It showed a highly sensitive detection of the cyanide ion in water with a detection limit of 0.33 ppm. The use of the micellar medium improved the detection limit drastically and a ppb level detection limit was achieved. The probe also showed the detection of the endogenously bound cyanide in cassava both visually and by spectrophotometer. Figure 3. Molecular structure of the probe 6. Chapter 3C. Ratiometric Cyanide ion probe in Water and for the detection of the Endogenously bound cyanide. Chapter 3C presents the synthesis of two new bis-indolyl (7 and 8) based probes for colorimetric detection of cyanide ion in pure water. Compound 8 showed a ratiometric response with cyanide in water and a visual detection of the endogenously bound cyanide ion in cassava. Using compound 8 the selective detection of the cyanide ion in water was achieved with a detection limit of ~ 17 ppb which is almost 13 times lower than the permitted limit as specified by EPA, United States. 7; R = H 8; R = -(OCH2CH2)3CH3 Figure 4. Molecular structures of the probes 1 and 2. Chapter 4 deals with the colorimetric and ratiometric detection of the Cu2+and Hg2+ions using different small synthetic molecular probes. Chapter 4A. Colorimetric Sensors for Ratiometric Detection of Copper and Mercury ions in Biological media and below ppm level in Water. The chapter 4A deals with the synthesis of two novel colorimetric probes (9, 10) using bispicolyl unit as the binding moiety and anthraimidazolediones and bis-indolyl system as a signaling sub-unit. Using the two sensors, Cu2+ion can be detected below the permitted limit (1.3 ppm) in both drinking water and at physiological pH 7.4. Sensor 9 can detect both Cu2+and Hg2+ in water with very low detection limit. It showed specific binding with Cu2+ at physiological pH 7.4 and in presence of serum albumins. Chemosensor 10 can be used for the specific detection of both Cu2+and Hg2in water as well as for the contamination in microorganisms. Figure 5. Molecular structure of the sensors 9 and 10. Chapter 4B. A New Molecular Probe for the Selective Sensing of Cu2+ and Hg2+ ions in Micellar Media and in Live ells.This chapter describes a synthesis of a novel bispicolyl based sensor 11 which can detect Cu2+ ion specifically in water medium and both Cu2+ and Hg2+ ions selectivelyin Brij-58 micellar medium. In micellar medium both the ions can be detected in the ppb level. Using fluorescence spectroscopy these two metal ions can be discriminated.The probe is also be useful for checking metal ion contamination in cellular samples. Figure 6. Molecular structure of the sensor 11. Chapter 4C. Rhodamine based Sensors for Cu2+ and Hg2+ ions in Water and in Biological media. The chapter 4C presents the synthesis and the sensing properties of the three positional isomers of the pyridine end of the rhodamine-pyridine compounds (12-14). The three isomers only differ in the position of nitrogen of the pyridine moiety. Sensor 12, which contains the pyridine nitrogen at the ortho-position showed selective sensing toward Cu2+ ion in both pure water and in buffered physiological media of pH 7.4. It gave a detection limit of ~13 ppb which is 100 times lesser than the EPA permitted limit. The other two sensors 13 and 14, which possessed the pyridine ends with the nitrogen atom at the meta- and the para- positions respectively showed the selective sensing of Hg2+ ion in water and did not show any interaction with the Cu2+ ion. Probes 2 and 3 showed ‘turn-on’ detection of Hg2+ ion both in the UV-vis and the fluorescence emission spectroscopy. Compound 2 and 3 showed a detection limit of ~ 9 and 4 ppb respectively. The NMR titration showed the change in color was due to the opening of the spirolactam ring of the rhodamine. The sensors can also be used for the detection of Cu2+ and Hg2+ ion in real life water samples and in the live cells. Figure 7. Molecular structure of the sensors 12, 13 and 14. Chapter 5. Ratiometric and ppb level Detection of Toxic Transition Metal ions using a Single Probe in Micellar media. This chapter describes the selective sensing of multiple ions using a single probe 15. The probe incorporates pyrene and pyridine as signaling and interacting moiety respectively. The sensor showed different responses towards different metal ions just by varying the medium of detection. In organic solvent (acetonitrile), the probe showed selective detection of Hg2+ ion. In water the fluorescence quenching was observed with three metal ions, Cu2+, Hg2+ and Ni2+. Further just by varying the surface charge of different micellar media, the probe showed selective interaction with Hg2+ ion in neutral micelles (Brij-58). However, in anionic micellar medium (SDS), the probe showed selective changes with both Cu2+ and Ni2+ in the UV-vis spectroscopy. The discrimination between these two ions was achieved by emission spectroscopy, where it showed selective quenching only with Cu2+. Thus using a single probe all the three metal ions Cu2+, Hg2+ and Ni2+ can be detected and discriminated just by varying the surface charge of the micellar medium. Figure 8. Molecular structure of the sensors 15. Chapter 6. Highly sensitive Rhodamine Based Dual Probes for the Visual detection of F¯ and Hg2+ ions in Water. This chapter deals with the design and synthesis of two new rhodamine based probes (16-17) which act as dual probes for the ppb level selective detection of Hg2+ and F¯ ions in water and at physiological pH 7.4. The two probes were synthesized by coupling tert-butyldiphenylsilyl (TBDPS) protected forms of 4-hydroxybenzaldehyde and 2, 4- dihydroxy benzaldehyde with rhodamine hydrazone. The F¯ ion detection is based on the desilylation of the probe, whereas the spirolactam ring opening leads to the detection of Hg2+ ion. The two probes gave turn-on detection of both Hg2+ and F¯ ion selectively in aqueous medium with the detection limit well below the EPA permitted limits. The probes showed detection of both the ions by dual mode with visibly different color and fluorescence under UV-lamp. The F¯ ion interacts with the silyl bond of probe and the cleavage results into yellow color whereas; the addition of Hg2+ ion to the probe solution opened the spirolactam ring and resulted into appearance of pink color. Figure 9. Molecular structure of the probes 16 and 17. (For structural formula pl see the abstract file)