The quantitative detection of oxygen is important for a wide variety of fields ranging from power production to
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.
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.
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).
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
National Science Foundation - Center for Sensor Materials
Dept. of Energy - National Energy Technology Lab
State of Michigan - 21st Century Jobs Development Fund