A liquid permeable metallic coating is utilized in conjunction with a fluorescence based optical sensor. The metallic coating is deposited directly on, and is in physical contact with, the sensing membrane. The metallic coating does not require an intervening substrate layer or other components. When light from a light source is shone through the substantially light transmissive substrate onto the sensing membrane, the metallic overcoating reflects back the excitation light as well as the fluorescence light generated by the sensor such that substantially no light reaches the sample where the light may be scattered and/or absorbed by the sample. Accordingly, the accuracy and repeatability of the sensor is improved while the cost and production times associated with manufacturing the sensor are minimized.