Thermal Induced Polymerization of l-Lysine forms Branched Particles with Blue Fluorescence

The polycondensation of amino acids can originate complex polymers that display fascinating structural and optical properties. Thermally induced amidation of l-lysine allows forming a branched polymer without the support of any catalyst. The polycondensation is completed at 240–250 °C; at higher temperatures, the amino acid degrades. The obtained polylysine particles have been studied by transmission electron microscopy (TEM), nuclear magnetic resonance, and infrared spectroscopy that allow for investigating the different steps of the synthesis. The resulting structure is characterized by peculiar optical properties, e.g., excitation-dependent blue fluorescence and good quantum efficiency. Hydrogen bonds and the interactions of the amino acids are considered responsible for the optical properties of both l-lysine monomer solutions at high concentrations and the branched nanopolymers.