Surface-Density-Controlled Spreading-Packing Competition in Antibody Monolayers Revealed by PM-IRRAS and 2D...

Antibody adsorption onto solid surfaces underpins numerous biosensing and bioelectronic platforms, yet quantitative descriptors linking adsorption conditions to interfacial structure remain limited. Here, we quantify the structural evolution of anti-immunoglobulin M (anti-IgM) monolayers physisorbed on a gold substrate. Our investigation spans a wide range of solution concentrations (0.5 to 1000 μg mL^-1) and deposition times (1 to 330 min), encompassing both dilute and densely packed regimes. Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) quantifies secondary-structure fractions. Concurrently, asynchronous two-dimensional (2D) correlation spectroscopy resolves the sequential order of conformational changes. A strong anticorrelation between β-sheet and unordered chains content emerges: at low surface density, adsorbed antibodies adopt conformationally flexible, partially disordered states, whereas increasing concentration and incubation time drive compact, β-sheet-rich assemblies. To unify concentration- and time-dependent effects, we introduce a normalized surface-density (SD) parameter derived from PM-IRRAS intensity, which collapses adsorption pathways onto a single structural coordinate. Expressing secondary-structure fractions as a function of SD quantitatively maps the spreading-packing competition governing antibody monolayers. This surface-density-based framework provides a predictive descriptor for adsorption-induced structural heterogeneity in antibody interfaces.