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 CuL synthesized at acidic pH have 3 atoms in their core and emit in the greenish-yellow region, and nanoclusters CuL, 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 Mg, Mn, Zn, and Pb selectively enhance the interaction between carboxylate - copper metal core and increase the emission intensity of CuL. These metal ions weaken the interaction between imidazole nitrogen and copper metal core and quench the emission intensity of CuL. 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.