An optical sensor for measuring the partial pressure of dissolved oxygen in water is based on the effect of oxygen quenching on the fluorescence lifetime of an optically excited ruthenium complex immobilized in a recently developed polymer. In the operation of this sensor, the fluorescence lifetime and thus the degree of quenching and partial pressure of oxygen are measured by a phase-sensitive detection method described below. This sensor is a prototype of improved oxygen-concentration transducers, which are needed for monitoring critical oxygen concentrations in bioreactors and chemical plants.
The predecessors of this sensor include electrical (galvanic and polarographic) sensors and similar fluorescence-quenching sensors. The electrical sensors are subject to spurious responses due to ambient electrical noise, calibration drift arising from electrolyte depletion and/or fouling followed by reductive oxygen consumption, and poor sensitivity at low oxygen partial pressures. Among the older fluorescence-quenching sensors are versions in which the immobilizing polymers are made of silicone rubber and in the operation of which oxygen levels are deduced from measurements of fluorescent intensity. These older fluorescence-quenching sensors are subject to long-term calibration drift resulting from indicator photobleaching, and frequently require complicated multipoint calibrations because of their nonlinear Stern-Volmer response.
The deviation from linearity with the older optical sensors is partly attributable to electrostatic binding of the indicator molecules - particularly cationic ruthenium complexes - to anionic silanol groups on the surface of silica particles that are added to the silicone polymers to increase tear resistance. This binding reduces the degree of quenching by oxygen for a portion of the polymer-immobilized fluorophores, resulting in a negative deviation from a linear response. Tests of the solubility of the ruthenium complex in traditional silicones lacking silica particles showed the indicator material to be insoluble and, therefore, poorly suited for the construction of oxygen sensors.
The recently developed polymer used in the present sensor is a highly oxygen permeable fluoropolymer possessing slightly polar aromatic chain segments. These polar groups were found to solubilize the indicator complex in the polymer without affecting the degree of accessibility by oxygen. The hydrophobic nature of the fluoropolymer imparts a degree of selectivity in the sensor response by excluding nongaseous water-borne quenchers from the sensing membrane. The polymer is also inherently tear resistant by virtue of microscopic crystalline domains formed from aromatic polymer chain segments.
Unlike the response of older sensors made with silicone rubber, sensors made with the new membrane polymer exhibit a linear Stern-Volmer response that has been attributed to the greater homogeneity of the oxygen-quenching environment created around the ruthenium complex within the sensing membrane. This linear Stern-Volmer response to oxygen should make it possible to use a one- or two-point calibration procedure.
In operation, oxygen from solution diffuses freely into the polymer, where it efficiently quenches the photoexcited ruthenium complex, reducing the lifetime of the fluorescence process. Excitation light energy is provided by a light-emitting diode operating in sinusoidally driven amplitude-modulation mode at a fixed angular frequency, ω. The resulting fluorescent signal is amplitude-modulated at the same frequency, but phase shifted relative to that of the excitation. The average fluorescence lifetime, τ, of the indicator complex is calculated from the ob-served phase shift, θ, occurring to the expression τ = tan(θ)/ω. Fluorescence lifetimes for the polymer-immobilized indicator were shown to be inversely proportional to the solution partial pressure of oxygen over the range 0 to 400 mm Hg.
This work was done by James A. Kane of Polestar Technologies, Inc., forKennedy Space Center.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
James A. Kane
Polestar Technologies, Inc.
220 Reservoir Road
Needham Heights, MA 02194
Tel No.: (781) 449-2284
Refer to KSC-11998