Abstract: The invention is an optical sensor useful for the detection of ionizing radiation emitted from an analyte in a fluid. The sensor is composed of a permeable scintillator having a high surface area to scintillator volume ratio and an optical waveguide located in working relation to the scintillator to collect light photons generated in response to an ionizing radiation source. The sensor is especially useful for biomedical applications. Increased sensitivity allows for miniaturization and implantation in a blood vessel of a small experimental animal.