Charge transfer at thin-film modified solid-liquid interfaces in the absence of redox-active moieties

Gupta, Chaitanya

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https://hdl.handle.net/2142/14605 Copy

Description

Title

Charge transfer at thin-film modified solid-liquid interfaces in the absence of redox-active moieties

Author(s)

Gupta, Chaitanya

Issue Date

2010-01-06T16:13:54Z

Director of Research (if dissertation) or Advisor (if thesis)

• Kenis, Paul J.A.
• Shannon, Mark A.

Doctoral Committee Chair(s)

Kenis, Paul J.A.

Committee Member(s)

• Shannon, Mark A.
• Scheeline, Alexander
• Schroeder, Charles M.

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• self-assembled monolayer
• non-adiabatic charge transfer
• adiabatic charge transfer
• charge transport
• redox-active

Abstract

Advanced diagnostic systems that are capable of the early detection and the narrow therapeutic targeting of diseases represent the future for modern medicine. The development of highly sensitive, single molecule detection technologies with high throughput capabilities is a prerequisite to the large scale deployment of these modern diagnostic systems. Affinity based optical labels used to tag the biomarker of interest are the state-of-the art in biological detection technologies. However, the attachment of a large label molecule modifies the in-vivo physiological activity of the molecular marker. Besides, label based detection platforms tend to be low throughput and rather expensive. The overall goal of this dissertation is, therefore, to propose an alternative label-free spectroscopic platform that utilizes low-energy electrons as interrogating particles or waves to probe the non-equilibrium physical and chemical properties at a biased monolayer modified solid-liquid interface. The variation in the electronic flux due to the different physical and chemical interactions occurring at the monolayer modified solid-liquid interface is measured by the change in impedance of the electrochemical system as a function of applied potential and frequency. The characterization of the flux of electrons (“leakage” currents) crossing the electrified monolayer modified solid-liquid interface in the absence of redox active moieties in the electrolyte is the first step in the development of such a label free methodology. An analytical method is presented in this dissertation that enables the quantitative analysis of the leakage current in the absence of specific information about the species in the electrolyte that couples weakly and non-adiabatically to the electronic orbitals of the metallic electrode. Application of the analytical method enables the identification of the different mechanisms by which leakage current flows through the insulating monolayer film. The current density is limited by Ohmic transport and by space charge at low and intermediate anodic potentials respectively. At higher anodic potentials, quantum mechanical tunneling of the electron from the ground-state energy level of the electrolytic ionic species to the electrode Fermi level becomes rate-limiting. On the other hand, for cathodic potentials, the charge flux is limited by the thermal activation of either the transferring electron or the dielectric molecules over a free energy barrier. The terms anodic and cathodic are defined with respect to a characteristic potential where the electric field in the monolayer becomes zero. The electrolytic species that participates in the charge transfer process is also identified. Properties of the monolayer-electrolyte interface that describe physical and chemical interactions between the electrolyte species and the monolayer modified surface are obtained from the analysis, demonstrating the applicability of the electrode-monolayer-electrolyte system as a sensing platform. An extension of the analysis methodology yields a quantitative estimate of the surface charge density at the monolayer-electrolyte interface. A theoretical description of the evolution of surface charge density in the presence of leakage currents is also proposed here.

Graduation Semester

2009-12

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© 2009 Chaitanya Gupta

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