1. College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072，China
2. Key Laboratory of Opto-Electronics Information Technology (Tianjin University), Ministry of Education, Tianjin 300072，China
3. Key Laboratory of Micro Opto-electro Mechanical System Technology (Tianjin University), Ministry of Education, Tianjin 300072, China
Abstract：The accurate measurement of dissolved oxygen concentration plays a crucial role in medical applications, marine monitoring, industrial and agricultural production, and other fields. A ratiometric optical fiber dissolved oxygen sensor is proposed. Organic modified silicates (ORMOSILs) using tetraethoxysilane (TEOS) and triethoxyoctylsilane (Octyl-triEOS) as precursors as carrier matrices, tris(4,7-diphenyl-1,10-phenanthrolin) ruthenium(II) dichloride complex (Ru(DPP)2+3) as the oxygen-sensitive dye, 7-amino-4-(trifluoromethyl) coumarin (AFC) as the reference dye. The absorption spectrum indicates that the oxygen-sensitivedye and reference dye can be excited by a light source with a central wavelength of 405nm. The emission spectrum indicates no spectral overlap between the emission wavelengths of the oxygen-sensitivedye and the reference dye so that the ratio method can measure the dissolved oxygen concentration. ORMOSILs prepared by -the sol-gel process fix oxygen-sensitive dyes and reference dyes on the end of plastic optical fiber to form composite oxygen-sensitive films. The thickness and hydrophobicity of the sensing film were characterized, with a thickness of 569 μm. The water contact angle is 81°. The sensor was tested in an aqueous solution. Under the excitation of a light source, there were obvious emission peaks at 605 and 409 nm for the oxygen-sensitivedye and reference dye. Oxygen has a quenching effect on the fluorescence of oxygen-sensitive dyes. As the concentration of dissolved oxygen increases, the fluorescence intensity of oxygen-sensitive dye gradually decreases. The fluorescence intensity of the reference dye remains stable at a certain value, and the purpose of detecting oxygen concentration is achieved by measuring the ratio of oxygen-sensitive dyes to the reference dye.The linear relationship between the fluorescence intensity of the ruthenium complex and the concentration of dissolved oxygen is described by the Stern-Volmer equation. The calibration curve of the sensor was 98.22% in the range of 0~20.05 mg·L-1. The sensitivity can reach 0.433 4/unit [O2], and the response time from saturated oxygen solution to saturated nitrogen solution is 144s, and from saturated nitrogen solution to saturated oxygen solution is 12 s, introducing the asymmetric factor ASY to indicate the asymmetry of the sensing film. The optical stability and repeatability of the sensor were characterized. The photostability and repeatability of the sensor were tested, and the ratio type fiber optic sensor can overcome light source fluctuations and has stronger stability compared to single fluorescence intensity sensing.
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