Fluorescence enhancement by factors of 5-12 times 8-alkyl thiourido-7-ethoxy-4-methyl coumarin derivatives was observed upon complexation with Hg2+, Ag+, and Ag nanoparticles. The study reveals a chelation-enhanced fluorescence (CHEF) mechanism with the formation of 1:2 complexes in Hg2+/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 Hg2+ complexation. Isokinetic studies indicate ail enthalpy-controlled mechanism in the Hg2+/coumarin derivatives complex formation. No fluorescence enhancement was observed with Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, La3+, and Ce3+, making the present coumarin thiourea derivatives selective chemosensors of both Hg2+ 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 call be induced by silver nanoparticles in the picomolar (pM) concentration ranges.