Optical Oxygen Sensor
Oxygen optical sensors work according to the principle of dynamic fluorescence quenching. The sensors contain fluorescent dye that is excited by light of a certain wavelength. Depending on the amount of oxygen molecules present, the luminescence response of the optical sensor varies. A polymer optical fiber transmits the excitation light of the sensor and at the same time also transmits the fluorescence response of the sensor to the measurement device. The oxygen sensitive dye is immobilized in a polymer matrix. This polymer can be applied to carrier material and used as sensor spots or sensor foil. It can also be coated directly onto the optical fiber. Oxygen quenching luminophores have been studied from at least 1939 when Kautsky described quenching of luminescence by oxygen. More recently, as optical sources, detectors, and data processing have become more advanced, the application of luminophores to the measurement of oxygen concentrations in liquids has resulted in bench-top instruments and optodes, with significant advances made in the 1990’s. Recent advances in blue light-emitting diodes and low-powered high-speed electronics have enabled the miniaturization of oxygen sensitive optodes to the point of field-deployable units. The sensors do not consume oxygen and are stable over long deployment periods.
The new REDFLASH technology is based on the unique oxygen-sensitive REDFLASH dyes. In contrast to common techniques using blue-light excitation, the REDFLASH dyes are excitable with orange-red light and show an oxygen-dependent luminescence in the near infrared (NIR). The REDFLASH technology impresses by its high precision, high reliability, low power consumption, low cross-sensitivity, and fast response times. The orange-red light excitation significantly reduces interferences caused by auto-fluorescenced samples. Further, the NIR detection technology significantly reduces interference with ambient light, known from the old blue-light techniques The new REDFLASH technology is based on the unique oxygen-sensitive REDFLASH indicator showing excellent brightness. The measuring principle is based on the quenching of the REDFLASH indicator luminescence caused by collision between oxygen molecules and the REDFLASH indicator immobilized on the sensor tip or surface. The REDFLASH indicators are excitable with red light (more precisely: orange-red at a wavelength of 610-630 nm) and show an oxygen-dependent luminescence in the near infrared (NIR, 760-790 nm).
Principle: Red light excited REDFLASH indicators show luminescence in the near infrared (NIR), which decreases with increasing oxygen (quenching effect). A) high NIR emission at low oxygen and B) low NIR at high oxygen. The measuring principle is based on a sinusoidally modulated red excitation light. This results in a phase-shifted sinusoidally modulated emission in the NIR. The FireSting O2 measures this phase shift (termed “dphi” in the software). The phase shift is then converted into oxygen units based on the Stern-Vollmer-Theory.
The red light excitation significantly reduces interferences caused by autofluorescence and reduces stress in biological systems. The REDFLASH indicators show much higher luminescence brightness than other optical sensor working with blue light excitation. Further, due to the excellent luminescence brightness of the REDFLASH indicator, the actual sensor matrix can be now prepared much thinner, leading to fast response times of the oxygen sensors.
The external part of the Idronaut Blue Cap Oxygen Optical Sensor is a titanium support with a 11,7mm diameter, where at its center is placed a 3 mm fiber optics well sealed to guarantee 700 bar operation. The length of the support is about 44 mm (without the Blue Replaceable Membrane Cap installed); two 2-12 Parker o-rings seal the support onto the probe housing. The measuring membrane cap is simply fitted inside the titanium support till it stops and is provided with a friction system (groovers) to prevent unwanted removal or accidental loss. The membane cap is made of blue plastic to better shield the external light and is very similar to the Idronaut pH watering cap. The only difference is that a hole at its bottom allows the factory installation of the glass window on its inside. The black sensor spot, which allows the oxygen measurements, is centrally placed on the outside of the glass window. The other side of the optical fiber remains inside the CTD probe and is fitted in a unique miniaturized transducer whose optics and electronics transform the optical signal into RS485 output.