Sensitive coatings are described for a novel enzyme-based optical sensor for in-situ continuous monitoring of reactants, such as glucose, in biotechnological production processes. Glucose oxidase, incorporated into suitable coating materials that are applied on lenses or optical fibers, is used to catalyze oxidization of glucose to gluconic acid in the presence of oxygen. The presence and consumption of oxygen is determined by measuring the fluorescence signal of incorporated metal organic ruthenium complexes, which is quenched by oxygen. Inorganic-organic hybrid polymers, synthesized via sol-gel processing, were used as coating material. Due to the hybrid character of the coating, good adhesion is achieved on both glass and polymer surfaces. Good compatibility is also given with enzymes and ruthenium complexes. The sensitive optical coating was built up as double-layer and single-layer structures. The double layer comprised a primary coating containing the oxygen-sensitive ruthenium complex, and a secondary coating containing the enzyme. The single layer comprised a single coating containing both the ruthenium complex and the enzyme. \circledC 2008 FSCT and OCCA.