A highly sensitive optical sensor based on the luminescence quenching of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium (II) complex (Rudpp) immobilized in a sol-gel-based thin silca xerogel was fabricated. Sensor material was prepared by entrapping it in chemically inert silica using the sol-gel process. The sensor showed good linearity from the modified Stern-Volmer plot. The dynamic response result demonstrated the fast response time of the sensor, less than 10 s. Largely enhanced photostability of the material was also one of the sensor's most promising properties.