Synthesis,characterization and sensing applications of novel schiff base ligands

Abstract

Metals/metal ions are necessary for vital life functions as they play key roles in a variety of essential biological processes. Apart from these, overuse of metals will also act as pollutants that affect human life and environment resulting serious health issues. Even though different analytical techniques have been proposed and used for metal ion monitoring, most of which needs sophisticated instrumentations, lacks on-site real-time monitoring and high cost. Since colourimetric and fluorimetric methods can provide naked eye detection without the use of very expensive equipment, the development of colourimetric and fluorimetric sensors is becoming a growing priority and has great significance in the field of sensing. Therefore, the focus of this research work will be on the design and development of cost-effective, highly sensitive, and selective chemosensors for selected metal ions and picric acid derived from different Schiff bases. In this piece of work, three Schiff base receptors PMB3, BBHN, and AHN have been designed and synthesized, and their chemosensing activity has been investigated using colourimetric and fluorometric techniques in the presence of different analytes. The efficient and simultaneous detection of multiple target ions with a single sensor molecule having different characteristics would be more attractive and less expensive than a one-to-one analysis. This novel feature is the highlight of this work. The Schiff bases synthesized are possessing different characteristics and thus can be utilized for the detection of various analytes. The Schiff base PMB3 acts as an organo-fluorescent sensor for successive detection of bivalent Zinc and Picric acid through an “OFF-ON-OFF” response. The PMB3 exhibits a significant emission enhancement in intensity with Zn2+ with a limit of detection of 11.12×10-7M, however, the intensity of emission of the in-situ produced complex PMB3-Zn2+ ensemble is quenched selectively upon the progressive addition of PA with a detection limit of 42.4×10-15M. PMB3 has shown AIEE characteristics which are applied for the detection of Cu2+ with a detection limit of 16.08 fM and picric acid (PA) with a detection limit of 2.43μM. PMB3, exhibited a sensitive colourimetric response to Cu2+ and Ni2+ ions too among other competing metal ions with a detection limits of 4.56μM and 2.68μM. Schiff base BBHN possesses AIEE characteristics and is applied for the selective fluorescence “Turn off” sensing studies for copper and PA with a detection limit of 35.52 nM and 4.04μM. The fluorescence quenching behaviour of BBHN in the presence of Cu2+ ions take place through dynamic quenching whereas the fluorescence “Turn off” response in the presence of PA is mainly due to π-π interactions, and non-covalent hydrogen bonding interactions. Schiff base AHN, exhibiting multiple analytical responses comprising AIEE and colourimetric activity towards distinct analytes. The probe AHN with AIEE property acts as a fluorescent sensor for the selective detection of PA through fluorescence switch-off response with a detection limit of 2.45μM and as a colourimetric sensor for Cu2+ in aqueous medium with a detection limit of 3.16μM through a change in colour from colourless to yellow.