Coumarin Derivative and Gold Nanoparticle Conjugate as a Selective Fluorescent Sensor for Mercury Ion in Real Sample

A biphenyl based coumarin fluorescent molecule, N,N’-bis(7-diethylamino-2-oxo-2 H-chromen-3-yl)methylene)biphenyl-2-2’-dicarbohydrazide (molecule 1) has been synthesized and characterised. Photophysical studies of 1 exhibit solvent polarity dependent absorption and emission maxima. Citrate capped gold nanoparticles (AuNPs) have been mixed with molecule 1 for the preparation of AuNPs/1 conjugate. The association constant of the AuNPs/1 conjugate has been calculated to 4.54 × 10⁴ M^− 1. The AuNPs/1 conjugate has been found to detect Hg²⁺ ion selectively by fluorescence enhancement. While addition of molecule 1 into the solution of AuNPs, fluorescence intensity of 1 quenched. On addition of several monovalent, divalent and trivalent metal ion into the solution of AuNPs/1 conjugate separately, there was no change in fluorescence intensity of 1 has been observed. However, upon addition of Hg²⁺ ion into the solution of AuNPs/1 conjugate, the fluorescence intensity enhancement occurred, indicating released of 1 from the surface of AuNPs and probably aggregation of AuNPs took place in presence of Hg²⁺ ion. The AuNPs/1 conjugate has been found to have a detection limit of 2.3 × 10^− 9 M for Hg²⁺ ion in aqueous solvent. Meanwhile, the AuNPs/1 conjugate have also been successfully applied for the determination of Hg²⁺ in real water samples.