Extracellular matrix (ECM) proteins, such as fibronectin, laminin, and collagen IV, play important roles in many cellular behaviors, including cell adhesion and spreading. Understanding their adsorption behavior on surfaces with different natures is helpful for studying the cellular responses to environments. By tailoring the chemical composition in binary acidic (anionic) and basic (cationic) functionalized self-assembled monolayer (SAM)-modified gold substrates, variable surface potentials can be generated. To examine how surface potential affects the interaction between ECM proteins and substrates, a quartz crystal microbalance with dissipation detection (QCM-D) was used. To study the interaction under physiological conditions, the ionic strength and pH were controlled using phosphate-buffered saline at 37 °C, and the ζ potentials of the SAM-modified Au and protein were determined using an electrokinetic analyzer and phase analysis light scattering, respectively. During adsorption processes, the shifts in resonant frequency (f) and energy dissipation (D) were acquired simultaneously, and the weight change was calculated using the Kelvin-Voigt model. The results reveal that slightly charged protein can be adsorbed on a highly charged SAM, even where both surfaces are negatively charged. This behavior is attributed to the highly charged SAM, which polarizes the protein microscopically, and the Debye interaction, as well as other short-range interactions such as steric force, hydrogen bonding, direct bonding, charged domains within the protein structure, etc., that allow adsorption, although the macroscopic electrostatic interaction discourages adsorption. For surfaces with a moderate potential, proteins are not significantly polarized by the surface, and the interaction can be predicted through simple electrostatic attraction. Furthermore, surface-induced self-assembly of protein molecules also affects the adsorbed structures and kinetics. The adsorbed layer properties, such as rigidity and packing behaviors, were further investigated using the D-f plot and phase detection microscopy (PDM) imaging.

Cell adhesion is central to many cell behaviors including survival, differentiation, and motility. With the recent development of biomaterials and medical instrumentation, cell behaviors on artificial biosurfaces have gained attention from the research community. Self-assembled monolayers (SAMs) are known for their excellent ability to modify surfaces. To achieve more precise control of surface properties, mixed 6-amino-1-hexanethiol and 6-mercaptohexanonic acid were deposited on a gold substrate, and in a physiological environment, arbitrary zeta potentials between −187 and +6 mV were obtained. This binary SAM system elucidated the effect of surface potential on the adhesion and proliferation of NIH3T3 cells cultured on these surfaces. Cell adhesion, density, morphology, and proliferation were investigated by optical, fluorescence, and scanning electron microscopes. It was found that increased surface potential promoted cell attachment; hence, the initial cell density increased. However, the apparent proliferation rate decreased with increasing surface potential due to contact inhibition between adjacent NIH3T3 cells at higher density. When the initial density was low and cells did not contact each other, surface potential had little or no effect on proliferation. A more positive surface potential also changed the cell shape from bipolar to spreading and allowed more cell–cell and cell–substrate interactions due to the enhanced cell adhesion.

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 6mercaptohexanoic 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.

The authors demonstrate that complex hydrofluorocarbon (HFC) precursors offer significant advantages relative to gas mixtures of comparable elemental ratios for plasma-based selective atomic layer etching (ALE). This work compares mixtures of a fluorocarbon precursor and H2 with an HFC precursor, i.e., mixtures of octafluorocyclobutane (C4F8) with H2 and 3,3,3-trifluoropropene (C3H3F3), for SiO2 ALE and etching of SiO2 selective to Si3N4 or Si. For continuous plasma etching, process gas mixtures, e.g., C4F8/H2, have been employed and enable highly selective material removal based on reduction of the fluorine content of deposited steady-state HFC films; however this approach is not successful for ALE since hydrogen-induced etching reduces the thickness of the ultrathin HFC passivation layer which is required for both etching of SiO2 and passivation of the Si3N4 and Si underlayers, leading to lower materials etching selectivity. Conversely, the experimental results show that C3H3F3-based ALE enables ultrahigh ALE selectivity of SiO2 over Si3N4 and Si. The hydrogen in the precursor structure allows to reduce the fluorine content of the deposited HFC film without suppressing the formation of the passivation layer on the surface. Gas pulsing of complex reactive precursors in ALE provides the prospect of utilizing the precursor chemical structure for achieving high materials selectivity in ALE.

Cell adhesion plays a key role in biomaterial development. Self-assembled monolayers (SAMs) provide a convenient and versatile means of modifying surface properties to study how environmental cues affect the cell adhesion process. Serial ζ-potential surfaces can be realized by introducing various ratios of oppositely charged functional groups on a gold surface. A quartz crystal microbalance with dissipation monitoring (QCM-D) has advantages for examining real-time viscoelastic changes on surfaces. This surfacesensitive technique can be applied in cell adhesion studies to investigate the cell−surface interactions. In this work, HEK293T epithelial cells were used to examine the adhesion kinetics of semiadherent cells on NH 2 −COOH binary SAMmodified surfaces with serial surface potential. Immunofluorescence staining was used to examine focal adhesion sites after a 4-h cell adhesion process. Combined with optical microscopy, QCM-D was used to record in situ and real-time viscoelastic and morphological changes. It was found that HEK293T cells were prone to spread and form more focal adhesion sites on surfaces with more positive charge (more NH 2 groups) but aggregated and remained highly mobile on surfaces with more negative charge (more COOH groups). On NH 2 -rich surfaces, cells underwent three-phase kinetics during the adhesion process. Initially, cells adhered and spread quickly on the NH 2 -rich surfaces with little or no extracellular matrix (ECM) by the attractive interaction between the positively charged amine groups and negatively charged cell membrane. The epithelial cells then shrank their filopodia in the second phase to normalize their size. In the final phase, cells underwent ECM remodeling and formed matured ECM. On COOH-rich surfaces, four phases were identified during the cell adhesion process. Initially, due to electrostatic repulsion between the negatively charged cell membrane and surfaces, direct cell adhesion and spreading were restricted. However, ECM was quickly deposited. In the second phase, cells adhered on and interacted with the surface through the ECM layer. In the third phase, cells underwent ECM remodeling, and additional ECM was deposited on the surfaces. Finally, instead of cell−surface interactions, the cells aggregated to form cell−cell junctions. In summary, the cell adhesion process shifted from direct cell−surface interaction to cell−ECM−surface interaction and cell−cell junctions when the surface potential shifted from positive to negative.