Yttrium oxysulfide upconverting phosphor nanoparticles, doped with Yb as a sensitizer and Er (or Ho, Tm) as an activator, have been prepared via a solid-gas reaction using precursor oxalate particles obtained in an emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system. The resulting Y2O2S:Yb, Er particles, mainly smaller than 50 nm in diameter, demonstrated green upconversion emission under infrared excitation (lambda(ex) = 980 nm) via a two-photon process. Distinct green and blue upconversion emission were also demonstrated under the same infrared excitation from Y2O2S:Yb, Ho and Y2O2S:Yb, Tm nanoparticles, respectively. These upconverting phosphor nanoparticles, together with Y2O3:Yb, Er infrared-to-red upconverting phosphor particles, with different emission under the same infrared excitation may be applied to the luminescent reporter materials for the detection of the targeted analyte in multiplexed assays. (C) 2004 Elsevier Inc. All rights reserved.