Molecular Signature in the Photoluminescence of -Glycine, L-Alanine and L-Asparagine Crystals: Detection, ab initio Calculations, and Bio-sensor Applications

We present the photoluminescence spectra of -glycine, L-alanine, and L-asparagine crystals. They are broad and structured, comprising green to ultraviolet emission in the 1.75-3.60 eV range. Absorption measurements show that the band gap energies of the crystals are of the order of 5.0 eV. Ab initio calculations of their electronic structures allow for the assignment of the observed peaks in the visible region to lattice-related processes of exciton nature associated with polaron levels. The very thin photoluminescence peaks in the ultraviolet region are assigned to intramolecular transitions, being a signature of the weakly interacting amino acid molecules in the crystals.