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

Abstract

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.

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…A very different use of PiFM was demonstrated by Zhao et al studying Eu 3+ nanostructures as a potential system for biometric recognition. 61 The Eu 3+ /oleic acid complexes were placed under a protein skin and their chemical composition and morphology studied with PiFM using the amide II band at 1540 cm À1 , and the OEu 3+ Phen complex carboxyl band at 1663 cm À1 , Fig. 11.…”

Effective Thermal Diffusion of Eu(III) and F Ions into Hydroxyapatite Nanoparticles by Citric Acid Coordinative Mediation

“…According to previous reports, the coordination environment of Eu 3+ ions in Eu:HA may be changed through the interaction with organic molecules. − Citric acid (Cit) is an organic acid containing three carboxyl groups with a strong coordination ability (i.e., formation of the chelates) with various metal ions, − which can effectively coordinate with lead­(II) and cadmium­(II) ions , and can also coordinate with calcium­(II), terbium­(III), and europium­(III) ions for immobilizing various crystal surfaces. − In addition, the inhibition of crystal growth and the enhancement of crystal solubility occurred through the coordination effect, − and the dissolution–reprecipitation processes of the crystals are strongly related to ion replacement. , Moreover, it was suggested that Cit coordination could promote the replacement state change of Eu 3+ ions in HA. Furthermore, our group has elucidated the interactions between Cit and Eu:HA, which suppressed the photoluminescence (PL) deactivation caused by the adsorption of surface water molecules. − Thus, the control of the Cit coordination behavior can change the PL properties of the Eu 3+ ions in HA.…”