Development of Ultra-Sensitive Fluidic Sensors and Molecular Dynamics Studies of Ion and Water Distribution in Nanochannels

Abstract

This dissertation presents the development of ultra-sensitive fluidic sensors and molecular dynamics studies of ion and water distribution in highly confined nanochannels. The fluidic sensor is based on resistive pulse sensing, i.e., when a non-conducting particle flows through an electrolyte-filled aperture, it displaces a volume of the electrolyte equal to its own volume, which results in an increase in the electrical resistance of the aperture. This resistance increase can be measured as current or voltage pulses to detect the translocation of particles. Instead of direct measurement of the ionic current or electrical potential modulation, the new ultra-sensitive fluidic sensor integrates the fluidic circuit with a MOSFET and monitors the MOSFET drain current to detect particles. The minimum volume ratio of the particle to the sensing channel detected is 0.006%, about ten times smaller than the lowest detected volume ratio previously reported in the literature. To understand the complex system of the electrolyte in highly confined nanochannels with charged surfaces, molecular dynamics studies of ion distribution in a nanochannel were performed using a three-region simulation domain to include two bulk regions on each side of the nanochannel. Results show that both the concentrations of counter-ions and co-ions inside the nanochannel could be significantly different from the bulk concentration, challenging the common practice in the literature of regarding the co-ion concentration as that of the bulk electrolyte. In addition, the ion and water distribution near charged (100) and (111) silicon surfaces were examined. It was shown that, under high surface charge densities, the water molecules within ~ 5 Å from the charged (100) silicon surface can be split from one layer into two layers because of the strong electrostatic interactions between surface charges and water molecules. This phenomenon was not observed for the (111) silicon surface, consistent with results in the literature, which indicates that in addition to the surface charge density, the surface atom density may also affect the near-wall water distribution.