Effect of Surface Potential on the Adhesion Behavior of NIH3T3 Cells Revealed by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)

Cell adhesion is crucial to cell behaviors including survival, growth, and differentiation. In recent years, quartz crystal microbalance with dissipation monitoring (QCM-D) has exhibited advantages in examining real-time viscoelastic changes of surface interactions. Self-assembled monolayers (SAMs) are known for their convenience and versatility in modifying surfaces. A series of ζ-potentials can be obtained by introducing two functional groups of opposite charge to gold surfaces, namely, 6-amino-1-hexanethiol and 6-mercapto­hexanoic acid. In this work, NIH3T3 mouse embryonic fibroblasts were chosen for examining the cell–surface, extracellular matrix (ECM)–surface, and cell–ECM interactions of these binary SAM-modified surfaces of serial surface potentials. The effect of surface potential on focal adhesion was also characterized by immunofluorescence staining. Combining an optical microscope with the QCM-D system, in-situ and real-time cell morphology and corresponding viscoelastic changes were obtained in order to understand how the surface potential affected the cell adhesion process. After 4 h of the cell adhesion process, cells were also fixed and then dehydrated for scanning electron microscope observation. The morphological results indicated that cells were prone to spread on surfaces of more positive potential, while more negative potentials led to more cell movement on the surface. The QCM-D results indicated that with more positive charge on the surface, soft and elastic cell bodies can adhere to the surface with little or no ECM layer and spread more quickly owing to electrostatic attraction. The shift in resonant frequency and energy dissipation of the quartz substrate can be described using a film resonance model, and a single-phase adhesion process was observed. On the other hand, for surfaces of more negative potential, round cells were observed and behave similarly to coupled oscillators on the QCM-D sensor. Furthermore, three phases were observed during the cell adhesion process. Initially, round cells interact with the surface weakly with a point contact due to the repulsive interaction between negatively charged cell membranes and the surface. Because the higher magnitude of surface charge also promoted the adsorption of ECM proteins, a more rigid ECM layer was quickly deposited on the surface in the second phase of cell adhesion. Finally, cells then adhered on the surface through the ECM layer. In other words, the mechanism of cell adhesion changed from an electrostatic cell–surface interaction to a cell–ECM–surface composite.