Dr. Ruby Ghosh: Optics Research

Fiber Optic Oxygen Sensors

The quantitative detection of oxygen is important for a wide variety of fields ranging from power production to chemical process control and biological applications. In these environments the advantages of fiber-optic chemical sensors are that they: can probe remote locations, are immune to electrical interference and may be miniaturized into small flexible probes. Oxygen quenching of the fluorescence from organic and organometallic compounds has been used to develop a number of fiber sensors. However, a drawback of these indicators is that chromophores often degrade with time an and have a limited operational temperature range.
We are studying the photophysics and physical properties of Mo₆Cl₁₂ and related metal-halide compounds for oxygen sensing schemes. Absorption of ultraviolet (UV) photons through the broad 300 - 400 nm absorption band raised the cluster to an excited electronic state. Emission of red luminescence from the excited state is efficiently quenched by ground state ³O₂.

In this case quenching by a factor of 5.5 is observed upon introduction of 20% oxygen into an inert (<0.005% O₂) environment


In contrast with organic indicators the luminescent properties of the Mo₆Cl₁₂ clusters are largely immune to environmental influences (such as temperature, salinity and pH). This is due to the fact that the photophysics of interest is due to electronic transitions confined to the [Mo₆Cl₈]⁴⁺ core.


We have developed a reflection mode fiber optic sensor by immobilizing the Mo-clusters in an oxygen permeable matrix for gas phase and liquid phase applications. A simple UV light source (such as a 365 nm LED) excites the clusters, and the resultant red luminescence, which depends on the local oxygen concentration, is collected by the detector. Due to the long cluster lifetime (>100 s) and large Stokes shift (300 nm), the luminescence is easily detectable by integrating over the broad emission band.

Real-time Dissolved Oxygen Sensing for Biological and Chemical Process Control Quantitative monitoring of dissolved oxygen (DO) in aqueous media are necessary for a wide range of chemical and biological processes. These applications require sensitive, precise, continuous monitoring, without restrictions on the frequency of measurement or total number of data points. Our completely inorganic DO sensor will enable real-time studies of metabolic activity in biological systems.
Sensor signal in water as a function of oxygen partial pressure (% O₂) and dissolved oxygen concentration (mg/L) showing real-time (1 data point/ 10 sec) continuous (36 hours) data collection.

We are pursuing applications for Aquaculture and Bio-reactors.

Gas Phase Oxygen Sensing Output signal from a fiber optic probe in the 0% to 21% oxygen concentration range.
We are currently designing a high temperature oxygen sensor for power plant applications by immobilizing Mo₆Cl₁₂ in an inorganic sol-gel matrix. To monitor the photophysical parameters of the cluster in-situ at high temperature, we have developed a technique to perform emission spectroscopy in a controlled gas environment.
To probe further: [pdf] R. N. Ghosh, P. A. Askeland, S. Kramer and R. Loloee, "Optical dissolved oxygen sensor utilizing molybdenum chloride cluster phosphorescence", App. Phys. Lett. 98, 221103 (2011) [pdf] R. N. Ghosh, G. L. Baker, C. Ruud and D. G. Nocera, “Fiber optic oxygen sensing using molybdenum chloride cluster luminescence”, Appl. Phys. Lett. 75, p. 2885 - 2887 (1999). [pdf] R. N. Ghosh, D. J. Osborn and G. L. Baker, "Fiber optic sensor for power plant applications", IEEE Sensors 2003, p. 807 - 808 (2003). [pdf] D. J. Osborn, G. L. Baker and R. N. Ghosh, " Mo₆Cl₁₂ - incorporated sol-gel for oxygen sensing applications", ", J. Sol-gel Sci. & Tech., 36 (1), p. 5-10 (2005). [pdf] P. Zhang, D. J. Osborn, G. L. Baker and R. N. Ghosh, "High temperature oxygen sensing using K2Mo6Cl14 luminescence", Sensors, 2005 IEEE, p. 628 - 631 (2005). Acknowledgements Thanks to our collaborators in this project: Prof. Gregory L. Baker, Dept. of Chemistry, Michigan State University Dr. Per Askeland, Composite Material Center, Michigan State University Dr. Christopher Weeks, Dept. of Fisheries & Wildlife, Michigan State University Funding: National Science Foundation - Center for Sensor Materials Dept. of Energy - National Energy Technology Lab State of Michigan - 21st Century Jobs Development Fund