The present invention provides an electro-optical sensing device for detecting the presence or concentration of an analyte. More particularly, the invention relates to (but is not in all cases necessarily limited to) optical-based sensing devices which are characterized by being totally self-contained, with a smooth and rounded oblong, oval, or elliptical shape (e.g., a bean- or pharmaceutical capsule-shape) and a size which permits the device to be implanted in humans for in-situ detection of various analytes.