Soft Interferometric Nanostrain Sensor Reveals Solid‐Liquid Interfacial Tension Oscillation Amplified by Competitive Adsorption

Understanding the adsorption of protein mixtures is significant for engineering biosurface functionalities that affect biomaterial hemocompatibility and infections over medical instruments. However, existing methods cannot detect real-time competitive adsorption events or require fluorophore labelling that may alter the adsorption characteristics. An interferometric nanostrain sensor has been developed to investigate real-time adsorption of unlabeled proteins. The sensor exploits the elastocapillary effect on the substrate nanometer deformation by a sessile protein drop to measure instantaneous interfacial tensions at its three-phase contact line. Using fetal bovine serum (FBS), it is shown that asymptotically, the solid-liquid interfacial tension (γ_SL) decreases with increasing FBS concentrations, and the solid-vapor tension (γ_SV) remains uncorrelated. Results link molecular adsorption events with macroscale surface energy. Dynamically, γ_SL shows Langmuir adsorption characteristics on a coarse timescale. With the sensor's high spatiotemporal resolutions (2.38 nm or <0.25 mN m^-1), it is reported for the first time a high-frequency, low-amplitude oscillatory modulation of γ_SL at a fine timescale over the Langmuir adsorption curve. Compared with other colloids with fewer competing elements of adsorption, this oscillation is intrinsic to any colloidal system but strongly amplified by competitive adsorption. The sensor provides a novel ensemble-averaging technology to quantify competitive adsorption, thereby revealing new mechanisms for protein-surface interactions.