No abstract

Fluorescence enhancement by factors of 5-12 times 8-alkyl thiourido-7-ethoxy-4-methyl coumarin derivatives was observed upon complexation with Hg(2+), Ag(+), and Ag nanoparticles. The study reveals a chelation-enhanced fluorescence (CHEF) mechanism with the formation of 1:2 complexes in Hg(2+)/coumarin derivatives and 1:1 complexes in Ag(+)/coumarin derivatives. The activation parameters of the complexation processes were evaluated with energy of activation values in the case of Ag(+) being nearly twice those in the case of Hg(2+) complexation. Isokinetic studies indicate an enthalpy-controlled mechanism in the Hg(2+)/coumarin derivatives complex formation. No fluorescence enhancement was observed with Fe(3+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), La(3+), and Ce(3+), making the present coumarin thiourea derivatives selective chemosensors of both Hg(2+) and Ag(+) ions with different complexation time scales between these two ions. Fluorescence enhancement of the studied coumarin thiourea derivatives using silver nanomaterials occurs almost instantaneously and can be induced by silver nanoparticles in the picomolar (pM) concentration ranges.

Based on chelation-enhanced fluorescence, a new fluorescent coumarin derivative probe 3(1-(7-hydroxy-4-methylcoumarin)ethylidene)hydrazinecarbodithioate for Hg(2+), Ag(+) and Ag nanoparticles is reported. Fluorescent probe acts as a rapid and highly selective "off-on" fluorescent probe and fluorescence enhancement by factors 5 to 12 times was observed upon selective complexation with Hg(2+), Ag(+) and Ag nanoparticles. The molar ratio plots indicated the formation of 1:1 complexes between Hg(2+) and Ag(+) with the probe. The linear response range covers a concentration range 0.1 × 10(-5) -1.9 × 10(-5) mol/L, 0.1 × 10(-5) -2.3 × 10(-5) mol/L and 0.146 × 10(-12) -2.63 × 10(-12) mol/L for Hg(2+), Ag(+) and Ag nanoparticles, respectively. The interference effect of some anions and cations was also tested.