Increased Time-Gated Eu-Centered Emission Intensity from Europium Oxide Nanoparticles Sensitized with Kynurenic Acid and Dipicolinic Acid in Tissue Samples

We isolated Eu2O3 nanoparticles, capped with the sensitizers dipicolinic acid (DPA) and kynurenic acid (KA) and a combination of the two, as imaging agents for biological tissues. The latter system, EuPEGKADPA, combined the advantages of the individual sensitizers and exhibited the longest emission lifetime of 1.244 ms and a quantum yield of 1.7% in Tris-buffer. In the presence of tissue lysate, this system displayed an emission intensity ∼60% higher than that of tissue autofluorescence. With time-gating, an almost 800-fold increase in the signal-to-noise ratio was observed. Eu(III) emission was observed with EuPEGKADPA in dried tissue samples mounted with DPX, ProLong Diamond, and Permount. In the DPX-mounted tissue, EuPEGKADPA demonstrated a 15-fold improvement in detection limit on a concentration basis for the nanoparticles when compared to rhodamine B. The long-lived lifetime, high signal-to-noise ratio, and improved detection limit of the systems studied here validate further studying of ligand-sensitized Eu(III) nanoparticles for bioimaging applications.