Time-gated luminescence bioimaging based on microsecond-lifetime luminescent biolabels can provide complete background-free conditions for detecting target cells in an autofluorescence biosample matrix. However, a major drawback of the current lanthanide biolabels is the requirement for UV excitation (<370 nm), which leads to damage to many biological systems and greatly affects the improvement of time-gated luminescence instruments. Herein we describe luminescent europium nanoparticles that have an excitation peak around 406 nm with high quantum yield (∼66%) and fine monodispersity in aqueous solutions. The nanoparticles were prepared by copolymerization of a visible-light-sensitized Eu3+ complex 4,4′-bis(1″, 1″,1″,2″,2″,3″,3″-heptafluoro-4″, 6″-hexanedion-6″-yl)chlorosulfo-o-terphenyl-Eu3+-2-(N,N- diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine conjugated with 3-aminopropyl(triethoxy)silane, free 3-aminopropyl(triethoxy)silane and tetraethyl orthosilicate in a water-in-oil reverse microemulsion. Characterization by transmission electron microscopy and luminescence spectroscopy indicates that the nanoparticles are monodisperse, spherical and uniform in size, <50 nm in diameter, and show strong visible-light-sensitized luminescence with a large quantum yield and a long luminescence lifetime. The new nanoparticles were successfully applied to distinguish an environmental pathogen, Giardia lamblia, within a concentrate of environmental water sample using a time-gated luminescence microscope with pulsed visible light excitation. The method resulted in highly specific and sensitive imaging for Giardia lamblia. These results suggest a broad range of potential bioimaging applications where both long time microscopy observation and high signal-to-background ratio are required for samples containing high concentrations of autofluorescence background.