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

## TL;DR

A new sensor detects how proteins stick to surfaces in real time, revealing hidden tension oscillations caused by competing proteins.

## Contribution

The first sensor to measure unlabeled protein adsorption with high-resolution interfacial tension detection and reveal oscillatory behavior.

## Key findings

- The sensor detects high-frequency, low-amplitude oscillations in interfacial tension during protein adsorption.
- Competitive adsorption amplifies these oscillations, which are intrinsic to colloidal systems.
- Langmuir adsorption characteristics are observed alongside the oscillatory modulation.

## Abstract

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.

An interferometric nanostrain sensor is developed to investigate unlabeled protein adsorption by directly measuring real‐time interfacial tensions at unprecedented resolution (<0.25 mN m−1). A high‐frequency, low‐amplitude oscillation of the interfacial tension is found to be superimposed over the Langmuir adsorption. While this oscillation is intrinsic to any colloidal system, its amplitude is found to be regulated by competitive adsorption.