Investigation of the thermal effects in nanofluidic transport and their application in gating

Abstract

Scaling down to nanometer size fluidic conduits has opened a new window into the world of sensing and manipulation of nanoscale species. Thousands of publications and hundreds of patents in this field are only a starting point for exploring and manipulating at the small-scale. Even these starting studies have offered promising applications in sensing and manipulation of molecules of different types such as DNAs, proteins and viruses as well as small ions. The present thesis focuses on the study and control of the ionic transport through nanometer-size channels, as one of the main applications of nanofluidic features inspired from the protein ion channels present in cell membranes. In the first part of this thesis, the latest developments in the field of nanofluidics are surveyed and a particular attention is given to the methods allowing gating of nanofluidic transport. Different methods of gating the nanofluidic transport are compared and some possible directions for future developments are suggested. Then, a pH-regulated multi-ion model for the electric conductance of nanochannels is introduced. The electrical conductance measurement is a widely used technique for the characterization of nanofluidic devices. Many research groups measured or modeled the electric conductance of nanochannels. Theoretical analysis and experimental investigations imply that the nanochannel conductance does not follow the macro-scale models. It is generally accepted that the conductance of nanochannels deviates from the bulk and tends to a constant value at low ionic concentrations. a new model is presented, which takes into account the surface chemistry of the nanochannel wall and describes the nanochannel conductance at low ionic concentrations in a more realistic way. The electrical conductivity of electrolytes is known to be dependent on temperature. However, the similarity of the temperature sensitivity of the electrical conductivity for bulk and nanochannels has not been validated. In order to examine this dependency, the ionic transport inside the nanochannel was studied. The results from the experimental measurements as well as the analytical modeling show the significant difference between the bulk and the nanoscale. The temperature sensitivity of the electrical conductance of nanochannels is higher at low ionic concentrations where the nanofluidic transport is governed by the electrostatic effects from the wall. Neglecting this effect can result in significant errors for high temperature measurements. Based on the results from temperature sensitivity measurements of the electric conductance of nanochannels, a new nanofluidic gating mechanism is introduced that uses the thermal effect for modulating the ionic transport inside nanofluidic channels. The thermal gate controls the ionic transport more effectively than most other gating mechanisms previously described in scientific literature. Gating in both bulk and overlapping electric double layer regimes is obtained. The response time of the thermal gate is studied and compared with the one of other gating methods. The relatively short time response of the opening and closing processes makes it a good candidate for manipulating small molecules in micro- and nanoscale devices.