<title>Fiber optic oxygen sensor using metal-halide cluster luminescence</title>

Abstract

We report on a reflection mode fiber optic oxygen sensor based on the <SUP>3</SUP>O<SUB>2</SUB> quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0 - 20% gaseous oxygen environment have been obtained, which is suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0 to 1.0% range. The response time of the fiber probe is less than or equal to 30 s, limited at present by the gas mixing system. The sensor is fabricated by immobilizing the clusters in an oxygen permeable polymer at the end of an UV transmissive optical fiber. For a 1.5 m long fiber pumped at 325 nm, the auto-fluorescence is less than 4% of the reflected luminescence signal from 600 - 850 nm, enabling remote probing of oxygen in harsh environments. The present probe is thermally stable to 200 degrees Celsius, limited by the polymer matrix. Since the clusters are synthesized at 1000 degrees Celsius, immobilization of the clusters in an inert sol-gel matrix is an attractive alternative. This will enable oxygen sensing in gaseous environments at elevated temperatures as well as liquid environments with varying salinity and pH.