The preparation of nearly monodisperse (40 nm), silica-coated LaF3:Ln3+ nanoparticles and their bioconjugation to FITC–avidin (FITC=fluorescein isothiocyanate) is described in this report. Doping of the LaF3 core with selected luminescent Ln3+ ions allows the particles to display a range of emission lines from the visible to the near-infrared region (λ=450–1650 nm). First, the use of Tb3+ and Eu3+ ions resulted in green (λ=541 nm) and red (λ=591 and 612 nm) emissions, respectively, by energy downconversion processes. Second, the use of Nd3+ gave emission lines at λ=870, 1070 and 1350 nm and Er3+ gave an emission line at λ=1540 nm by energy downconversion processes. Additionally, the Er3+ ions gave green and red emissions and Tm3+ ions gave an emission at λ=800 nm by upconversion processes when codoped with Yb3+ (λex=980 nm). Bioconjugation of avidin, which has a bound fluorophore (FITC) as the reporter, was carried out by means of surface modification of the silica particles with 3-aminopropyltrimethoxysilane, followed by reaction with the biotin–N-hydroxysuccinimide activated ester to form an amide bond, imparting biological activity to the particles. A 25-fold or better increase in the FITC signal relative to the non-biotinylated silica particles indicated that there is minimal nonspecific binding of FITC–avidin to the silica particles.