Reagent-Free Ion Sensing through Interfacial Processes

Abstract

The reliable detection of dissolved ions is essential in environmental monitoring, healthcare, agriculture, and industrial process control. Conventional electrochemical ion sensors often rely on selective recognition layers or Faradaic reactions, which can limit stability, simplicity, and broader applicability. This dissertation explores an alternative approach: reagent-free ion sensing, in which analytical information is extracted directly from the intrinsic electrical response of the electrode–electrolyte interface.
The central premise of this work is that ions actively reshape the structure of the electric double layer (EDL), and that these ion-dependent interfacial changes influence measurable electrochemical observables such as capacitance and impedance. The dissertation therefore treats ion sensing as an interfacial-physics problem, linking electrolyte composition to electrical response through the physicochemical organization of the interface. To establish this perspective, the thesis first reviews the interfacial electrochemical origins of reagent-free sensing, with emphasis on ion-dependent EDL structure, solvent organization, adsorption, crowding, diffuse screening, and nanoconfinement.
A continuum modeling framework is then developed to connect interfacial structure to measurable impedance response. This framework provides a physically grounded description of how ion properties, interfacial permittivity, ion distribution, and local conductivity shape the frequency-dependent electrochemical response, and it is later extended to include temperature-dependent behavior. Within this framework, impedance non-ideality is interpreted not as a mere fitting artifact, but as a physically meaningful consequence of distributed interfacial processes. In particular, constant phase element behavior and the transition-frequency regime are shown to carry ion-specific information that can be used for sensing.
Building on this physical interpretation, the dissertation further demonstrates that full-spectrum impedance data can be used for ion detection and quantification through machine learning-assisted analysis. Rather than relying on isolated scalar features alone, the work shows that the broader spectral response contains structured information related to electrolyte composition, enabling data-driven interpretation of reagent-free electrochemical measurements.
The dissertation also introduces temperature as an active perturbation of the interface and demonstrates that thermal modulation provides an additional source of compositional information. By analyzing how temperature alters capacitance, impedance non-ideality, and transition-frequency behavior, the work shows that controlled thermal variation can improve the interpretability and discriminatory power of reagent-free sensing.
In addition, the thesis includes in situ XPS characterization of the electric double layer, providing direct experimental access to the buried electrode–electrolyte interface. This part of the work supports the physical picture developed in the main chapters by offering interfacial evidence complementary to the electrochemical and modeling results, and by demonstrating the value of direct characterization for understanding ion-dependent interfacial organization.
Taken together, this dissertation advances reagent-free electrochemical ion sensing as a physically grounded strategy that combines interfacial theory, continuum modeling, impedance spectroscopy, data-driven interpretation, temperature modulation, and direct interfacial characterization. In doing so, it contributes both to the fundamental understanding of electrode–electrolyte interfaces and to the development of broadly applicable sensing concepts for complex aqueous systems.