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Gate Potential Control of Nanofluidic Devices LIBRARIES Gate Potential Control of Nanofluidic Devices by Arnaud Le Coguic Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Masters of Science in Electrical Engineering and Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2005 @ Massachusetts Institute of Technology 2005. All rights reserved. Author............ Department of Electrical Egineering and Computer Science May 6, 2005 Certified by...... ......... .... ngyon Han Assistant Professor Thesis Supervisor Accepted by Arthur C. Smith Chairman, Department Committee on Graduate Students MASSACHUSETTS INS E OF TECHNOLOGY BARKER OCT 2 1 2005 LIBRARIES 2 Gate Potential Control of Nanofluidic Devices by Arnaud Le Coguic Submitted to the Department of Electrical Engineering and Computer Science on May 6, 2005, in partial fulfillment of the requirements for the degree of Masters of Science in Electrical Engineering and Computer Science Abstract The effect of an external gate potential control on the nanofluidic nanochannels was experimentally investigated in this work. Like in the field effect transistors (FET) in microelectronics, molecular transport in micro/nanofluidic channels can be controlled by applying external potentials on the wall of the fluidic channel. In nanofluidic de- vices, this type of control is expected to be more efficient due to its high surface to charge ratio. We focused on a nanofluidic concentrator to study this effect. We could increase or decrease the concentration rate of the device by increasing or decreas- ing the surface charge potential ((-potential) on the walls of the nanochannels. An increased (-potential enhances the electrokinetic effects caused by electrical double layer. Which in turn accelerates the creation of a charge polarization region and improves the concentration capabilities of the device. We also have demonstrated concentration polarization effect, caused by pressure-driven flow in the nanofluidic channel, and showed that this phenomena can also be modulated by changing the gate potential of the nanofluidic devices. The gate potential effect opens the door for closed-loop real-time control of nanofluidic concentrators. Thesis Supervisor: Jongyoon Han Title: Assistant Professor 3 4 Acknowledgments I would like to thank Professor Han for trusting me, giving me the privilege of collab- orating with him and for allowing me to benefit from his drive and cleverness. I am thankful for the numerous lessons he taught me. Sincere thanks also go to the funding sources that made it possible for me to do this research work: the MIT Rosenblith Presidential Fellowship, the National Science Foundation CTS division through its grants CTS-0347348 and CTS-0304106. I would also like to thank all the professors that influenced me along the way, at Lyc6e Louis-le-Grand, Ecole Polytechnique and MIT; their sharpness of mind in- spired me to make all the possible efforts to achieve both precision and clarity, depth and perspective. I would also like to thank all the labmembers-turned-friends who greatly contributed to the learning experience that this thesis proved to be. Yong-Ak Song, Jianping Fu, Hansen Bow, Pan Mao, Wendy Gu, Aleem Siddiqui, Salil Desai and all the others. I thank you for being the nice and interesting people that you are. Let me also thank Ying-Chih Wang for fabricating the silicon devices used in this thesis. I would like to thank my close family for instilling in me values that still guide me today. Finally, some goals are better set and achieved when seen through the eyes of shared enthusiasm, happiness and aspirations. And I thank you, Hanh, for bringing me all this. 5 6 Contents 1 Introduction 19 2 Background 23 2.1 Theory on electrokinetics and double layers ..... .......... 23 2.1.1 Electrical double layer ............. ......... 23 2.1.2 Electroosm osis ... ....................... 26 2.1.3 Electrophoresis ............. ............. 26 2.1.4 Electrical resistance and capacitance of the channel filled with buffer .. ..................... ........ 27 2.1.5 EOF flow vs. pressure driven flow ................ 28 2.2 Relevant prior research on nanofluidics ................. 31 2.2.1 Experimental observation of externally applied potential on EOF 31 2.2.2 Theoretical models for externally applied voltages ....... 32 2.2.3 Debye layer thickness greater than the channel's thickness . 33 2.2.4 Nanofluidic filter Preconcentrator ................ 34 2.3 Our proposal ....... ...................... ....... 34 3 Study of gate-potential control of nanofluidic channels in PDMS nanochannels 37 3.1 Design and working principle ...................... 37 3.2 Experimental setup .................. .......... 39 3.2.1 Properties of the sample and buffer solutions .......... 39 3.2.2 Microscope for fluorescence measurements ........... 40 7 3.2.3 Electrical setup ................. ......... 41 3.3 Results ............... .................... 42 3.3.1 Flow reversion .................. ......... 42 3.3.2 Ion selectivity ....................... .... 42 4 Characterization of nanofluidic preconcentrator 43 4.1 Design and working principle ................. ..... 43 4.2 Fabrication ....... ......................... 44 4.3 Experimental setup ........ .................... 44 4.3.1 Properties of the sample and buffer solutions .......... 44 4.3.2 Microscope for fluorescence measurements ........... 44 4.3.3 Electrical setup ........ .................. 44 4.4 An experimental issue: flushing ..................... 44 4.5 Concentration experiments performed on a A-DNA sample solution 46 4.5.1 Control of A-DNA flow through the gate ......... ... 46 4.5.2 Transient behavior ........................ 48 4.6 Pre-concentration of GFP ........................ 51 4.6.1 Experimental setup .................... .... 51 4.6.2 R esults .... ....................... .... 51 4.6.3 Interpretation ....................... .... 52 4.7 Modification of depletion region width with the buffer ionic strength . 58 4.7.1 Experimental setup ............... ......... 58 4.7.2 R esults ....... ............ ........... 58 4.7.3 Interpretation .................. ......... 58 5 External gate potential control of pre-concentration 61 5.1 Modification of the depletion region width with Vgate . ........ 61 5.1.1 Experimental conditions ............ ......... 61 5.1.2 R esults ....... ............ ........... 63 5.1.3 Interpretation ........................... 63 5.2 Influence of Vgate on the pre-concentration rate ........ ..... 64 8 5.2.1 Experimental conditions . 64 5.2.2 Results .... ..... .. 65 5.2.3 Interpretation ....... 67 5.3 Electroosmosis of the second kind 67 5.3.1 Experimental conditions . 67 5.3.2 Results ... ........ 68 5.3.3 Interpretation ....... 68 6 Characterization of pressure-induced concentration polarization in nanofluidic channels 69 6.1 D evice .......................... 69 6.2 Experimental setup ................... 70 6.2.1 Solutions, fluorescence measurement, electrical setting 70 6.2.2 Pressure setup .................. 70 6.3 Pre-concentration of GFP ............... 72 6.3.1 Experimental setup ............... 72 6.3.2 R esults ...................... 73 6.3.3 Interpretation .................. 74 6.4 Influence of buffer ionic strength on pre-concentration rate . 75 6.4.1 Experimental conditions ............ 75 6.4.2 R esults ...................... 75 6.4.3 Interpretation ......... ......... 75 6.5 Influence of pressure on pre-concentration rate .. .. 76 6.5.1 Experimental conditions ............ 76 6.5.2 R esults ...................... 76 6.5.3 Interpretation .................. 76 7 External gate potential control of concentration polarization 85 7.1 Modification of the depletion region width with the Vate .. 85 7.1.1 Experimental conditions .................. 85 7.1.2 R esults ............................ 85 9 7.1.3 Interpretation ........................... 86 7.2 Influence of Vgate on the pre-concentration rate ....... ...... 87 7.2.1 Experimental conditions .. ........... ........ 87 7.2.2 Results ............ ........... ........ 87 7.2.3 Interpretation . .......... .......... ...... 89 8 Conclusion 91 A PDMS device fabrication 93 A.1 Photolithography masks ......................... 93 A.2 Photolithography to fabricate silicon masters ........ ...... 93 A.3 Electrodes fabrication ...... ............. ........ 96 A.4 PDMS Fabrication ..... ............ ........... 97 A.5 PDMS to glass bounding .......... .......... ..... 98 B Silicon device fabrication process flow 99 10 List of Figures 2-1 Helmholtz double layer. White disks represent free ions in solution. 24 2-2 Comparison of the flow profiles for an electroosmotic flow (left) and a pressure-induced flow (right). ...... ..... ...... ..... 31 2-3 Top view of the silicon device. ....... ......... ...... 35 2-4 Cross-section of the silicon device. ..... ...... ...... ... 35 2-5 Bottom view of the silicon device conceived by Ying-Chih Wang, cen- tered on the nanofilters region. ............ .......... 36 3-1 Top view (top image) and cross-section view (bottom image) of the filter region of the PDMS device. The thicknesses featured in the cross-section are t 1 =120nm, t 2=250nm, t 3 =130nm. ..... .... 38 3-2 Top-view picture of the PDMS device showing the mesh of pillars, the shallow gates and the electrode underneath. ....... ...... 39 3-3 M icroscope's filters setup. ... ............. ........ 41 3-4 Microscope's filters transmittance plots. Blue: exciter; red: emitter; green: dichroic mirror. ........ ........ ......
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