A ratiometric type of fluorescent nanoparticle was prepared via an encapsulation-reprecipitation method. By introducing an alkoxysilanized dye as a reference, the nanoparticles (NPs) give both a green and a red fluorescence under one single-wavelength excitation. The resulted ratiometric fluorescence is found to be highly temperature-dependent in the physiological range (25-45 A degrees C), with an intensity temperature sensitivity of -4.0%/A degrees C. Given the small size (20-30 nm in diameter) and biocompatible nature (silica out layer), such kind of NPs were very promising as temperature nanosensors for cellular sensing and imaging.