Novel MOSFET-Based Fluidic Sensors and Simulations of Thermal Bubble Nucleation in Nanochannels
Traditional particle sensing schemes are based on the resistive-pulse sensing technique. A non-conducting particle displaces a volume of electrolyte, equal to its own volume, from a sensing channel when it flows through. Correspondingly, the resistance of the sensing channel increases and this resistance modulation is measured directly by the resultant ionic current or electrical potential modulation across the sensing channel. The novel MOSFET-based sensing scheme integrates a MOSFET with the fluidic circuit and detects particles by monitoring the MOSFET drain current modulation instead of the direct ionic current modulation. Using this new sensing scheme, we are able to detect a minimum volume ratio of the particle to the sensing channel of 0.006%, which is about ten times lower than the lowest detected volume ratio previously reported in the literature. The new sensing scheme is first tested at the microscale and then extended down to the nanoscale. The fundamental limitation of particle sensors is the amplitude of noise observed with respect to the baseline current measured. It was recently suggested that nanobubble nucleation and transport inside nanopore-based devices could be a source of noise in nanofluidic experiments. This source of noise has not been investigated thoroughly. We carried out molecular dynamics simulations of thermal bubble nucleation to investigate whether nanobubbles can indeed form in nanochannels and thus, be a plausible source of noise in nanofluidic experiments. We investigated thermal bubble nucleation in nano-confined NPT systems of argon and water and found that bubbles did not form for temperatures up to the superheat limit of the fluids when the external pressure on the system ranged from 0.01 to 0.1 MPa. We propose a pressure wave hypothesis to explain our simulation results and show that our results are consistent with this hypothesis. Our initial investigations suggest that it might be difficult to form thermal bubbles in nano-confined systems. This casts doubt on whether nanobubbles can be cited as a source of noise in nanofluidic experiments.