Single-molecule detection in electrochemical nanogap devices

This thesis presents results obtained during a research project aimed at realizing electrochemical single-molecule detection in water. By virtue of being inherently electrical in nature, electrochemical sensors are particularly well suited for integration with microelectronics compared to sensors based on other detection principles. They may thus enable cost effective, massively parallel analysis and diagnostics platforms. However, limited sensitivity and selectivity have been bottlenecks for years in the development of a wide range of bio-electrochemical sensors. In this project, a microfabricated nanogap device with significantly enhanced properties based on redox cycling is developed. With the ability to resolve single molecules, sensitivity is pushed to its most fundamental limit and selectivity can potentially be improved by distinguishing species via their single-molecule signatures. The nanogaps are fabricated with optical lithography and standard microfabrication techniques, they consist of two electrodes with a length in the range of 10 to 100 µm and a width of several microns embedded in the ceiling and floor of a nanofluidic channel with a height of ~50 nm. Redox molecules enter the detection region defined by the two electrodes through entrance holes located at the two ends of the channel. With the electrodes biased at high over-potentials, the molecules are oxidized and reduced repeatedly, generating an amplified current. Preliminary work to characterize and understand the behavior of the device, including how redox couples with different diffusion coefficients and reversible adsorption of analyte species influence its performance, are presented in Chapter 3-5. Based on a self-aligned fabrication scheme that minimizes the gap size and dead volume of the device, redox-cycling efficiency was further improved and electrochemical detection in aqueous solution with single-molecule resolution was achieved. This is the first reported electrochemical detection of single-molecules in water solution in a nanofluidic device. Further building on this approach, we also demonstrated the first study ever to characterize mass transport of individual redox molecules using an electrochemical method. Analysis of the distribution of first-passage times of single molecules suggests anomalous surface diffusion of adsorbed analyte molecules at ultra-low (10 pM) concentrations compared to measurements at high (µM to mM) concentrations.