Stable Fluorescence of Eu³⁺ Complex Nanostructures Beneath a Protein Skin for Potential Biometric Recognition

We designed and realized highly fluorescent nanostructures composed of Eu³⁺ complexes under a protein coating. The nanostructured material, confirmed by photo-induced force microscopy (PiFM), includes a bottom fluorescent layer and an upper protein layer. The bottom fluorescent layer includes Eu³⁺ that is coordinated by 1,10-phenanthroline (Phen) and oleic acid (O). The complete complexes (OEu³⁺Phen) formed higher-order structures with diameter 40-150 nm. Distinctive nanoscale striations reminiscent of fingerprints were observed with a high-resolution transmission electron microscope (HRTEM). Stable fluorescence was increased by the addition of Eu³⁺ coordinated by Phen and 2-thenoyltrifluoroacetone (TTA), and confirmed by fluorescence spectroscopy. A satisfactory result was the observation of red Eu³⁺ complex emission through a protein coating layer with a fluorescence microscope. Lanthanide nanostructures of these types might ultimately prove useful for biometric applications in the context of human and non-human tissues. The significant innovations of this work include: (1) the structural set-up of the fluorescence image embedded under protein "skin"; and (2) dual confirmations of nanotopography and unique nanofingerprints under PiFM and under TEM, respectively.