Soluble Oligomerization Is Involved in Fluorescence of Citrate-Copper Complexes

ABSTRACT: Fluorescence of citrate-Cu²⁺ was observed here. Citrate aqueous solution by itself showed weak fluorescence, and the fluorescence intensified about eighth fold (quantum yield increased over eighty fold) with the presence of appropriate Cu²⁺ ion concentrations at two different pH conditions. Only a certain specific ratio of citrate-Cu²⁺ generated homogenous particles of a particular size, which showed intensified fluorescence. Intensified fluorescence of citrate-Cu²⁺ complex was depressed by the presence of EDTA. The coordination between Cu²⁺ ion and citrate was probably through electrostatic chelation via the carboxylate group of citrate, because the required amount of Cu²⁺ ion decreased to obtain the fluorescent citrate-Cu²⁺ complex species, with the increase of pH; and the presence of ethanol disrupted the formation of this strong fluorescent citrate-Cu²⁺ complex species. This provides fresh insight into the molecular basis of fluorescence characters of citrate-Cu²⁺ complex, and into the microscopic mechanisms of charges’ and polarity’s effect on the interaction between solutes within the multiple component solution. This study reveals a fresh theoretical understanding of citrate for Cu²⁺ ion detection, stabilization, and elimination.