pH-dependent Synthesis and Interactions of Fluorescent L-Histidine Capped Copper Nanoclusters with Metal Ions

In this work, L-Histidine-protected copper nanoclusters synthesized by changing the pH levels of precursor solution have been shown to display different emission wavelengths and intensities. As determined by mass spectrometry, nanoclusters Cu₃L₂ synthesized at acidic pH have 3 atoms in their core and emit in the greenish-yellow region, and nanoclusters Cu₂L₂, synthesized in the basic conditions have 2 atoms in their core and emit in the blue-green region. They are expected to have coordination through the carboxylate group and nitrogen of the imidazole ring of histidine ligand, respectively. Metal ions Mg²⁺, Mn²⁺, Zn²⁺, and Pb²⁺ selectively enhance the interaction between carboxylate - copper metal core and increase the emission intensity of Cu₃L₂. These metal ions weaken the interaction between imidazole nitrogen and copper metal core and quench the emission intensity of Cu₂L₂. As synthesized, nanoclusters exhibit good water solubility and photostability, they can act as fluorescent probes to sense the metal ions, therefore, they were utilized for the optical sensing of the mentioned metal ions. Fluorescent nanoclusters were found to sense even a very low concentration of metal ions with a limit of detection (3 σ/slope) in nanomolar range.