Silica-coated Ln3+-doped LaF3 nanoparticles as robust down- and upconverting biolabels

The preparation of nearly monodisperse (40 nm), silica-coated LaF3:Ln(3+) nanoparticles and their bio-conjugation to FITC-avidin (FITC= fluorescein isothiocyanate) is described in this report. Doping of the LaF3 core with selected luminescent Ln(3+) ions allows the particles to display a range of emission lines from the visible to the near-infrared region (lambda= 450-1650 nm). First, the use of Tb3+ and Eu3+ ions resulted in green (lambda= 541 nm) and red (lambda = 591 and 612 nm) emissions, respectively, by energy downconversion processes. Second, the use of Nd3+ gave emission lines at lambda=870, 1070 and 1350 nm and Er 31 gave an emission line at lambda = 1540 nm by energy downconversion processes. Additionally, the Er 31 ions gave green and fed emissions and TM3+ ions gave an emission at = 800 nm by upconversion processes when codoped with Yb3+ (lambda(ex)= 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.