Physical Chemistry - Lab Report Example

Construction and testing of solid oxygen sensors Advancement in gas sensor technologyhas led to the progress in more research in the field of sensor technology. The construction of an oxygen gas sensor that works on the principles of fluorescence and phosphorescence and dependent on the energy absorption and dissipation is one of the new technologies. There have been studies on performance of solid state oxygen sensors before with emphasis on other metals and materials in construction and calibration. This paper, however, focuses on the use of silicon rubber as the main material in construction and use of different ratios and volumes of oxygen, air and nitrogen in the calibration of the sensor. Introduction Chemical electronic transitions that are usually induced through the absorption of visible light spectrum result in excited state molecules. In the process energy is absorbed, dissipated or both. The absorbed energy must be released to return the excited species back to the original ground state. Typically, on absorption of light at their absorption maxima, dye molecules exhibit long excited state lifetimes and emit radiation at longer wavelengths (fluorescence or phosphorescence). The excited state of the luminescent dye can be quenched by an energy transfer mechanism upon collision with oxygen molecules [1] Fluorescence: Quenching: As a result, the intensity of luminescence is reduced along with the lifetime and the degree of quenching is proportional to the oxygen concentration. Fig. 1 shows a schematic diagram of an optical oxygen sensor. The active components of the sensor are the luminescent dye encapsulated in a polymer medium, a light source (commonly a LED or laser) for exciting the dye at a particular wavelength, (550–800 nm) [2], a photodiode to detect the fluorescent radiation and an optical fiber for the transmission of light. The quenching of the luminescence can be characterized by the Stern-Volmer eqn. [3] Where I0 and I are the luminescent intensities in the absence and presence of oxygen, PO2 in Torr and KSV is the quenching constant which determines the sensitivity of the optical oxygen sensor. Figure 1 Schematics of an optical oxygen sensor. (1) Gas or liquid path, (2) Lumophore dispersed on oxygen permeable membrane, (3) lens and filter, (4) exciting radiation, (5) fluorescent radiation, (6) optical fiber, (7) LED/Laser, (8) photodiode, and (9) display In a typical ground electronic state is a singlet (all electrons paired). Electronic excitation results in an excited state singlet that may undergo internal conversion to a triplet state. Due to the forbidden nature of a triplet singlet transition, the triplet state may exist for a substantial length of time. The emission from this state is known as the phosphorescence. Fluorescence refers to the emission from singlet to singlet transition. Molecular oxygen in the ground state is a triplet species. Energy transfer is allowed when the energy acceptor have the same spin multiplicity. For this reason oxygen is a good energy acceptor for compounds which form excited state triplets efficiently. One class of compounds that has been extensively studied is the inorganic dye molecule ruthenium (II) tri-bypyridine [Ru (bpy) 3]2+ and various analogs. In the presence of oxygen the Ru (bpy) 3 2+ emission is efficiently quenched. The main objective of this experiment is to construct, test and calibrate a solid state oxygen sensor using available materials and to understand the working mechanism of the constructed sensor. Experimental method Materials and reagents Silicon rubber (aquarium sealant), methylene chloride, ruthenium material Procedure A thin film of silicon was prepared (one week before to allow the silicon time to cure) to act as a support. A drop (about 1 ml) of silicon rubber was pressed between two pieces of parafilm. The parafilm was removed from the silicone rubber and a rectangle of 0.75cm by 1.5cm material cut. The film was soaked in a beaker of ruthenium saturated with methylene chloride for several minutes (The film swore several times its original size) The film was removed using forceps and the solvent allowed to evaporate. (The film shrank to its original size). The dye was left embedded in the silicon matrix and the sensor element was constructed from a cuvette. The apparatus were assembled as instructed by the TA. The emission intensity was monitored by a photodiode connected to a lock- in amplifier. A stream of nitrogen gas was passed over the film and changes in emission intensity observed. After the signal stabilized, a stream of air was passed over the sample. Oxygen was also passed over and the changes in emission observed. The time constant and sensitivity of the lock in amplifier was controlled using the LabView interface. Calibration The sensor film was calibrated by passing known mixtures of nitrogen, and oxygen over the film. Mass flow meters were used to measure the volume flow of nitrogen and oxygen. The cuvette was purged with gas mixtures ranging from 0-35% oxygen. The cuvette was filled with room air by pulling air through the cell using house vacuum. The sensor responds to the partial pressure of oxygen and will be lower than expected if the overall pressure in the cell is low. Results Table 1: Data for Sensor Testing Nitrogen Oxygen Oxygen Ratio Signal (intensity) S0/S 101.7 0 0 0.0218 1 101.7 6.5 0.060073937 0.0212 1.02830189 101.6 12.5 0.109553024 0.0211 1.03317536 101.6 18.5 0.154038301 0.0204 1.06862745 101.6 24.9 0.196837945 0.0199 1.09547739 101.7 31.3 0.235338346 0.0194 1.12371134 Figure 2: calibration curve Discussion The silicon rubber (aquarium sealant) used in this experiment is a clear, pliable and inexpensive material to work with. A working sensor is typically characterized by three parameters: sensitivity, selectivity and response time. Sensitivity is the ability of the sensor to quantitatively measure the required parameter under given conditions. The sensitivity of a luminescence based oxygen sensor is influenced by the properties of both the luminescent material and the oxygen permeable encapsulating medium. The natural lifetime of the excited state, τ0, also depends on the encapsulating medium. It is governed by the inherent physical and chemical properties of the materials used. Selectivity of a sensor is its ability to sense a radiation free from interference. Response time is a measure of how quickly the maximum signal change is achieved with UV changes. In addition, reversibility, long term stability and size are other factors influencing the overall performance of a UV oxygen sensor. It is important to note is the downward curvature in the Stern-Volmer (sensitivity) plot as seen in Fig. 2 [6]. This shows problems in calibrating these optical sensors. Though there are models based on theoretical fitting procedures e.g., power law model, two site model, and Gaussian distribution of τ0 and k Q [3,4, 5, 7], the physical understanding of the nonlinearity is still not adequate. Conclusion The construction and testing of the solid state oxygen sensor was successful. The silicon material used produced splendid results. The calibration was also excellent. Further experiments on construction, testing and application of solid state gas sensors will provide a better understanding of the concepts of UV radiation tests and other gas sensors. In future experiments, a variety of materials should be used in the construction of the sensors in order to understand the variability in the sensing techniques. The test using house vacuum proved to be tough. In the future more precise and concise methods would be better to ensure accurate calibration. Acknowledgements References [1]. B. R. Eggins, In “Chemical Sensors and Biosensors” (John Wiley & Sons, England, 2002) [2]. D. G. Fresnadillo, M. D. Marazuela, M. C. M. Bondi and G. Orellana, Langmuir 15 (1999) [3]. A. Mills, Sensors and Actuators B 51 (1998) [4]. I. Bergman, Nature 218 (1968). [5]. E. R. Carraway, J. N. Demas, B. A. Degraff and J. R. Bacon, Anal. Chem. 63 (1991). [6]. P. Douglas and K. Eaton, Ibid. B 82 (2002). [7]. H. Chuang and M. A. Arnold, Anal. Chim. Acta 368 (1998) Read More