Triazine based fluorescent sensor for sequential detection of Hg2+ and L-Cysteine in real samples and application in logic Gate: A combination of Extensive experimental and theoretical analysis

Abstract

Triazine based fluorescent sensor TBT was rationally designed and synthesized to achieve sequential detection of Hg²⁺ and L-cysteine based on the presence of sulfur moiety and suitable cavity in the molecule. Sensor TBT exhibited excellent sensing potential for the selective detection of Hg²⁺ ions and L-cysteine (Cys) in real samples. Upon addition of Hg²⁺ to sensor TBT, enhancement in emission intensity of sensor TBT was observed which was accredited to the presence of sulfur moiety and size of cavity in the sensor. Upon interaction with Hg²⁺ blockage of intramolecular charge transfer (ICT) along with chelation-enhanced fluorescence (CHEF) resulted in the increase in fluorescence emission intensity of sensor TBT. Further, TBT-Hg²⁺ complex was employed for the selective detection of Cys through fluorescence quenching mechanism. This was attributed to the significantly stronger interaction of Cys with Hg²⁺, which resulted in the formation of Cys-Hg²⁺ complex and subsequently sensor TBT was released from TBT-Hg²⁺ complex. The nature of interaction between TBT-Hg²⁺ and Cys-Hg²⁺ complex was evaluated through ¹H NMR titration experimentations. Extensive DFT studies were also carried out which include thermodynamic stability, frontier molecular orbitals (FMO), density of states (DOS), non-covalent interaction (NCI), quantum theory of atom in molecule (QTAIM), electron density differences (EDD) and natural bond orbital (NBO) analyses. All the studies supported the non-covalent type of interaction between analytes and sensor TBT. The limit of detection for Hg²⁺ ions was found to be as low as 61.9 nM. Sensor TBT was also employed for the quantitative detection of Hg²⁺ and Cys in real samples. Additionally, logic gate was fabricated by using sequential detection strategy.

Keywords: Chelation-enhanced fluorescence; Non-covalent interaction; Sequential detection of Hg(2+) and L-cysteine; Triazine based fluorescent sensor.