Apparatus and method for measuring properties of certain analytes are provided. The apparatus includes a sensor capsule having a processing chamber defined by a wall which in one embodiment has a semi-permeable membrane permeable to the analyte as at least a portion thereof. Receptor material is disposed within the chamber and is capable of chemically interacting with the analyte. In one embodiment, at least a portion of the sensor is translucent. A light source, which may be an optical fiber, causes light to impinge on a translucent portion of the capsule and pass therethrough. Responsive fluorescent light is generated and emitted. This light may be received by detector means and processed in a conventional manner to determine concentration of the analyte. A dye-labelled analog-analyte may be provided within the chamber. Both the receptor material and analog-analyte are of such size that they will not pass through the semi-permeable membrane although the analyte can. The invention is particularly suited to use of the capsule as an implantable element which is physically separated from the light source and detector and can be placed underneath the skin. In another embodiment, a receptor reagent and a receptor are covalently bonded to a stable polymer structure and a semi-permeable membrane is not required.