Influence of Nanobubbles on Molecular Interactions at a Liquid/Solid Interface

Abstract

The objectives in this thesis are detection of nanobubble formation in situ and investigation of influence of nanobubbles on molecular interactions on solid surface. we used AFM and QCM sensor to achieve the in situ detection of nanobubble formation when nanobubbles form on the surface. We found that nanobubbles were more readily formed at more hydrophobic and porous solid surfaces with lower concentration of buffer solution. We have studied influence of nanobubbles on the molecular interactions on the solid surface using quartz crystal microbalance

to our knowledge, this is the first report of this phenomenon. We found that, for a range of surfaces, namely, quartz surfaces coated with plain gold, DT, or MUA, the presence of nanobubbles on the surface played a key role in the hydrophobic forces driving the adsorption of polystyrene nanoparticles. Also we used a QCM to systematically study the influence of nanobubbles on DNA immobilization and hybridization, as well as thrombin？antithrombin binding on gold-coated surfaces. We found that the presence of nanobubbles on the surface had little effect on surface immobilization of oligonucleotides but played a key role in DNA hybridization and thrombin-antithrombin binding in solution. The differential effect of nanobubbles on immobilization and molecular binding was attributed to the nature of the molecular interactions: covalent bonding for immobilization and noncovalent bonding for hybridization and protein binding. In contrast, a control experiment using a UV？vis spectrometer showed that the DNA hybridization in solution (not on surfaces) occurred regardless of the presence of dissolved air. This study may explain why most biochip experiments performed without controlling for dissolved air are not reproducible. And we have shown that the presence of dissolved air in water plays a key role in the adsorption and stability of SLMs on a range of surfaces, namely quartz surfaces coated with plain gold, MUA, DT, and OT. In the absence of dissolved air, the adsorption of lipid molecules is suppressed and the stability of the SLMs is weakened. Further, we found that the hydrophobic interaction between the lipid layers in the SLMs is stronger than that between the lipid layer and the OT layer than DT layer.Influence of nanobubbles on electrochemical redox reaction and underpotential deposition of lead on gold electrode has been studied using an electrochemical impedance spectroscopy (EIS) and cyclic volatammetry, respectively. We found that changes in electron transfer resistance (Rct) when gold electrode was exposed to degassed electrolyte followed by air-rich electrolyte. As the nanobubbles formed on the surface of gold electrode, the electron transfer resistance (Rct) increased between the electrode and redox probe in solution. The CV transients for UPD always have two reduction peaks

however there is only one reduction peak in degassed condition. The cathodic reductions of O2 in acid solution affect the UPD of lead electrocatalytically. So in degassed solution the lead ions could not reduce to metallic lead. As the degassed solution became air-rich condition, the reduction peak appeared depending on the air exposure time. These findings support the long-standing hypothesis that nanobubbles play a key role in the long-range attractive force between hydrophobic surfaces in aqueous solutions.The presence of nanobubbles on a surface can affect the specific and nonspecific binding to the surface, which degrades the reproducibility of biochips. Moerever this study has revealed the method to improve the performance and reproducibility of biochips by controlling the formation of nanobubbles using photocatalytic nanoparticles. Also this paper reports the first application of photocatalytic nanoparticles in the silver enhancement reaction for gravimetric biosensing.