In situ AFM observation of BSA adsorption on HOPG with nanobubble

Abstract

Nanobubbles have been proven existent at the liquid/solid interface, and become a focus of research on varied interfacial processes. In the present work, by observing in situ with atomic force microscope (AFM), we found that nanobubbles could influence the adsorption process of bovine serum albumin (BSA) on hydrophobic surface of highly ordered pyrolytic graphite (HOPG). BSA could adsorb evenly, and coexist with nanobubbles at water/HOPG interface. After removing nanobubbles by injecting ethanol, some hollows were found in the BSA layers at the same positions of nanobubbles existing previously. These hollows were about 8 nm in depth and dozens of nanometers in diameter. The correlation coefficient between the areas of nanobubbles and that of the corresponding hollows reached 0.88-0.94, which strongly supported the assumption that the hollows were indeed caused by the nanobubbles. Moreover, the BSA molecules formed rings around the nanobubbles, suggesting the preference of BSA adsorption onto the contact line at water/HOPG interface.nanobubble, adsorption, bovine serum albumin (BSA), highly ordered pyrolytic graphite (HOPG), atomic force microscope (AFM)The water/solid interface is of special interest in terms of its ubiquitous presence and its important role in biological and colloidal systems [1] . Although bubbles at the nanoscale were believed impossible or unstable in classical theory [2] , and experimental studies have shown that nanobubbles do exist at water/solid interfaces [3][4][5] . In the last few years, more and more evidence, including atomic force microscope (AFM) observation, is found supporting the stable existence of nanobubbles [6][7][8][9][10][11][12][13][14][15][16][17][18] . Moreover, Switkes et al. [19] and Zhang et al. [20] explored the effect of degassing on nanobubbles to find that formation of nanobubbles was strongly related with dissolved gas, indicating that there was gas contained in nanobubble. Although the mechanism of nanobubbles at interface remains unclear, its effect on some classic scientific problems and processes was taken into account by the scholars, such as the origin of long distance attraction between hydrophobic surfaces [21] , slippage of simple fluids near a wall [22,23] , microboiling behavior [24] , mineral flotation [25] , and the rupture of a wetting film [26] .As known, protein adsorption is an important phenomenon scientifically and practically, concerning artificial organs, issue engineering, biochip, biosensor, and so on [27] . Castner et al. [28] drew the conclusion that to understand the biological response to a material, especially in vitro, one must fully understand the nature of the adsorbed protein film that formed on that material. There are vast literatures focusing on varied proteins on kinds of materials extensively, for example, protein C hordein and protein A on silicon surface, bovine serum albumin (BSA) onto titanium, fibrinogen assembles on both highly oriented pyrolytic graphite (HOPG) and PHYSICAL CHEMISTRY

Cleaning using nanobubbles: Defouling by electrochemical generation of bubbles

“…As previously observed, BSA adsorption results in a complete and even film on the HOPG surface [15]. This was defined as 100% coverage.…”

“…As in previous reports [14,15], we focus on proteins adsorbed directly to the surface as the formation of this layer is crucial in the fouling process.…”

“…Moreover, it may prove to be an excellent method to prevent bacterial growth when combined with other electrochemical methods [13]. Recently, we found that nanobubbles can inhibit protein adsorption on a variety of surfaces [14,15]. In order to make use of this, control over the production of nanobubbles is required.…”

“…Nanobubbles are formed on the surface with a regular density and the size of the nanobubbles can be controlled by the applied current. We have previously demonstrated that the areal density of nanobubbles on highly oriented pyrolytic graphite (HOPG) surfaces can be controlled electrochemically [18] and using an atomic force microscopy (AFM), it was demonstrated that the presence of nanobubbles on a surface decreases the adsorption of protein dramatically [14,15], that is the nanobubbles act as antifouling agents. Here, we demonstrate that nanobubbles cannot only be used to prevent the nonspecific adsorption of protein but also can be used to remove protein that is already adsorbed and therefore can be used as cleaning (or defouling) agents.…”

On the measurement of slip length for liquid flow over super-hydrophobic surface

“…Most researchers think that there is some gas phase (the nanobubbles [15] or a layer of low density of fluid [16] ) at the interface between the liquid and the surface leading to liquid slippage over these surfaces. Moreover, the nanobubble has been detected by many methods [17][18][19] recently. Inspired by these findings, the super-hydrophobic surface, which is covered with micro-or nanostructures, can enhance the slippage of liquid because of the entrapped gas in these microstructures.…”