Development of a Oxygen Sensor for Marine ... - DTU Nanotech

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8 CHAPTER 2. OPTODES AND ISFET only permeable to oxygen. The oxygen flow through the membrane being induced by the consumption of O2 at this electrode. With the electrical flow between the electrodes being proportional to this flow of oxygen. Problem is that this flow is highly dependant on the diffusion though the membrane, and in seawater these can change due to temperature and biological fouling. Especially the biological fouling can be a problem, not only because they can cause a change in the diffusion characteristics of the membrane. But also because biological processes quite often involves the production or the consumption of oxygen, and as such will influence the amount measured.[11] Optodes (also know as optoelectrodes or optrodes[12]) are the optical equivalent of electrodes, and represent a theoretical solution to this. These are usually based on the quenching of a fluorescent indicator by oxygen, and described by the Stern-Volmer equation[13]: I0 I = 1 + KSV [O2] (2.1) Where I and I0 are the fluorescence intensities with and without oxygen present, KSV the Stern-Volmer quenching constant and [O2] the concentration of dissolved oxygen. The sensitivity depends on the value of KSV , which determines the change in the fluorescent intensity with various oxygen concentrations.[14] Figure 2.2: Sketch of the optical system of the optode.[10] The way the optical sensor, as shown on figure 2.2, works is: 1. The blue LED emits light, at 475 nm, to an optical fiber.
2.2. ISFET (ION SELECTIVE FIELD EFFECT TRANSISTOR) 9 2. The optical fiber then carries the light to the probe. The end of the probe tip consists of a thin layer of a hydrophobic material, with ruthenium inside it, protected from the liquid measured on. 3. The light from the LED excites the ruthenium at the probe tip. 4. The excited ruthenium then fluoresces, emitting energy at 600 nm. 5. If the excited ruthenium hits an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence). The degree of quenching corresponds to the level of oxygen concentration or to oxygen partial pressure in the film, which is in dynamic equilibrium with oxygen in the liquid. 6. The energy is collected by the probe and carried through the optical fiber to the data storage/PC. Which in turn translates the data into a graph or a similar output, from which the oxygen level can be read. Some of the advantages of the optodes is that unlike traditional electrodes the optodes doesn’t consume oxygen, and is therefore not sensitive to stirring. Also the partial pressure of the oxygen in the probe is in equilibrium with the partial pressure outside, therefore it is less effected by pressure and the pressure behavior becomes easier to predict. Finally some of the commercially available optodes comes with warranties with up to two years, which allows for long term measurements.[10] Disadvantages of the optodes however is that they tend to be relatively large (the optode itself is small, but the electronics and power supply adds considerably to the size), and is therefor often attached to ships or buoys. It also have a relatively large power consumption in comparison to a micro Clark sensor. 2.2 ISFET (Ion Selective Field Effect Transistor) Electrochemical sensors are one of the older and larger groups of chemical sensor, the sensor type includes those based on electrochemical and charge transfer reactions, meaning the transfer of charge from an electrode to a solid or liquid phase, or the other way around. The chemical process takes place at the electrodes or in the probed volume, and the current from this
Page 1 and 2: Development of a Oxygen Sensor for
Page 3: Abstract This thesis described the
Page 7 and 8: Contents 1 Introduction 1 1.1 Disso
Page 9 and 10: CONTENTS vii B Fabrication Process
Page 11 and 12: Chapter 1 Introduction Roughly 70 %
Page 13 and 14: 1.1. DISSOLVED OXYGEN LEVEL 3 K = a
Page 15 and 16: 1.2. THESIS OUTLINE 5 In Chapter 8
Page 17: Chapter 2 Optodes and ISFET There a
Page 21 and 22: 2.3. SUMMARY 11 of oxygen molecules
Page 23 and 24: Chapter 3 Theory of the Clark Senso
Page 25 and 26: 3.1. CHEMISTRY OF THE CLARK SENSOR
Page 33 and 34: 3.2. THE POTENTIOSTAT 23 Reference
Page 35 and 36: 3.2. THE POTENTIOSTAT 25 Working El
Page 37 and 38: Chapter 4 System Design The sensor
Page 39 and 40: 4.2. ACTUAL DESIGN 29 seen on figur
Page 41 and 42: 4.2. ACTUAL DESIGN 31 The third des
Page 43 and 44: 4.3. TEMPERATURE SENSOR 33 Figure 4
Page 45 and 46: 4.3. TEMPERATURE SENSOR 35 a Pt300,
Page 47 and 48: 4.4. PACKAGING 37 Figure 4.11: Clos
Page 49 and 50: 4.4. PACKAGING 39 90 mm 15 mm 6 mm
Page 51 and 52: Chapter 5 Fabrication The fabricati
Page 53 and 54: 5.1. PROCESSING 43 Figure 5.4: Alig
Page 55 and 56: 5.2. BACK-END PROCESSING 45 under e
Page 57 and 58: 5.2. BACK-END PROCESSING 47 5.2.2 A
Page 59 and 60: Chapter 6 Problems and Solutions In
Page 61 and 62: 6.1. WIREBONDING 51 make a line bet
Page 63 and 64: 6.2. THE FLEX PRINT 53 The reason b
Page 65 and 66: 6.3. THE ’O’-RING 55 Figure 6.8
Page 67 and 68: Chapter 7 Evaluation of Results and
Page 69 and 70:
7.2. TEMPERATURE SENSOR 59 Figure 7
Page 71 and 72:
7.4. SUMMARY 61 Figure 7.5: Polarog
Page 73 and 74:
Chapter 8 Conclusion The primary go
Page 75 and 76:
Bibliography [1] http://earthobserv
Page 77 and 78:
BIBLIOGRAPHY 67 [26] W.E. Morf. The
Page 79 and 80:
Appendix A Silicide Recipe The foll
Page 81 and 82:
Appendix B Fabrication Process 71
Page 84 and 85:
74 APPENDIX B. FABRICATION PROCESS
Page 86:
76 APPENDIX C. PROCESS SEQUENCE
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