In this paper we report the use of phase sensitive fluorometry to obtain preliminary results from opto-chemical fluorescent oxygen nanosensors. PEBBLE (Probe Encapsulated By Biologically Localized Embedding) sensors were fabricated by immobilizing tris(4,7-diphenyl-1,10-phenanthroline)Ru(II) chloride and tris(1,10-phenanthroline)Ru(II) chloride within a polyacrylamide matrix. PEBBLEs have diameters of 20 - 200 nm and exhibit excellent performance for dissolved oxygen detection. Their performance is compared with micrometer-sized (10 - 20 micrometer) optical fiber sensors and free dye in solution. Oxygen sensing ability of PEBBLEs was tested in the presence of other quenchers and compared with free dyes in solution. While PEBBLEs have been developed for minimally invasive intracellular chemical analysis they show additional advantages, such as increased dynamic range, compared to microsensors, and an absence of interference (quenching) by heavy ions in contrast to free dye solutions.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.