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Title:Electrostatic control of microfluidic systems for enhancement of nanoparticle separations and FET nanobiosensors
Author(s):Vilasur Swaminathan, Vikhram
Director of Research:Bashir, Rashid
Doctoral Committee Chair(s):Saif, Taher
Doctoral Committee Member(s):Ferreira, Placid; Hilgenfeldt, Sascha; Fischer, Andrew
Department / Program:Mechanical Science & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Field Effect Transistor (FET)
Abstract:An abundance of ions in saline media or physiological buffers causes extensive shielding of charged surfaces and potentials. As a result, electrostatic forces cannot penetrate beyond the electrochemical double layer (EDL); conversely, charged molecules outside the EDL cannot transduce a sensor surface and are rendered effectively invisible. In this thesis, we discuss a separation problem and a sensing problem that are both constrained by this fundamental limitation. Our goal has been to explore novel electrical pathways for circumventing the loss of electrostatic effects: towards improving the dielectrophoretic separation of sub-micron colloids, or the mitigation of EDL shielding for improved label-free electrical detection of analyte molecules using Field Effect Biosensors. Dielectrophoresis is a favored method for numerous microfluidic applications for the capture of biological species, as well as, separation of organic and inorganic contaminants from water. However, given that ionic mobility (by far) exceed those of the suspended matter, the ions redistribute rapidly to overwhelm any electrophoretic effects within the system. Any attempt at capturing sub-micron particles by electrostatic means is drastically limited due to the dual effect of weak electric fields in the media, as well as poor volumetric scaling of separation forces with respect to particle size. In order to improve the capture of colloids, we have designed a scheme based on multiple interdigitated electrode (IDE) arrays under mixed AC/DC bias. Through AC electroosmotic micromixing that breaks the EDL shielding, as well as a transverse DC component, the separation field penetrates across a microchannel to capture particles from the bulk. We analytically determine favorable biasing conditions for field enhancement through circuit analysis and demonstrate the enhanced electric field through computational simulations. Under these conditions, we experimentally demonstrate improved capture of 0.75 µm diameter colloids over conventional AC-DEP methods. Depending on the salinity of the suspension, the method can be used to design species collection zones in microfluidic systems. Biosensors based on the field-effect transistor (FET) are capable of detecting molecular binding or hybridization events through direct coupling of analyte charge with conduction through the FET. This label-free approach can be used to realize ultraportable and point-of-care (POC) diagnostic devices. However, the charge of biomolecules is inevitably screened by surrounding ions, thus reducing the overall sensitivity in any practical solution. To address this fundamental problem, we have examined the possibility of modifying the salt background around a sensor. With the help of on-chip polarizable electrodes, we demonstrate a method for localized electronic desalting of a FET biosensor. By locally removing the excess ions that shield the device from analytes, the Debye length around the senor can be sufficiently increased to encompass the molecular layers. Hence, the apparent charge of the target molecules can be maximized and the sensitivity of the device increased. We have visited the 100-year-old Gouy-Chapman theory that is the most widely used to model EDLs, and incorporated theoretical advancements from Modified Poisson-Boltzmann models, to accurately predict the regimes in which desalting of highly saline solutions can be effective. Furthermore, we have designed a biasing scheme in which the on-chip microelectrodes can gate bias the FETs during the desalting process itself, thereby circumventing the back diffusion of ions to allow simultaneous sensing experiments. In order to overcome the adverse scaling of surface capacitance against ionic concentration, and desalting limitations arising out of non-zero ion-sizes and ion crowding in EDLs, it is necessary to reduce the scale of the biosensor to that of a microdroplet as well as incorporate surface-area enhancements on the desalting electrodes. Although challenging to implement such a construct, sub-nanoliter droplets around FET(s) can serve as isolated chambers for parallel reactions. We have achieved favorable desalting capacity in the ~10mM salinity regime, without loss of molecular stability for which salt is known to play a crucial role. Additionally, we have also leveraged the scalability of semiconductor electronics to incorporate large scale arrays of FETs to overcome device-to-device variations and also enable parallel detection experiments. Finally, we have adapted and characterized well-known functionalization chemistries for optimal capture of analyte monolayers on the devices. We believe that our study uniquely addresses the fundamental phenomenological limitation associated with excess salt as a key determinant of sensitivity. Our approach can pave the way for multiplexed label-free electronic detection in physiological solutions with improved sensitivity.
Issue Date:2015-08-20
Rights Information:Copyright 2015 Vikhram Vilasur Swaminathan
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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