Optical Tracking of Distinct Protein Transport at Curved Interfaces Leading to Innovative Diagnostic Approaches

We report the distinct adsorption dynamics of albumin and globulin from bulk to curved air-liquid interface of microdroplets under high ammonium sulphate concentration, and integrate this with a microPAD to enable plasma protein sensing. Proteins and salts are the key components of most biofluidic systems, and their adsorption to curved air-liquid interface in high salt microenvironment governs drying, crystallization, and phase separation. Yet, protein transport and self-assembly under such extreme conditions remain poorly understood. Using a custom optical setup, we captured the time-resolved dynamics of albumin and globulin transport under salting-out condition, revealing distinct bulk and interfacial pathways arising from their intrinsic physicochemical properties. Theoretical estimates of transport times for each protein to the droplet apex, obtained by coupling species transport and droplet evaporation equations, showed excellent agreement with the experimental results. Finally, integrating the droplet system with a microPAD enabled a point-of-care protein assay, where blood/plasma protein levels were quantitatively predicted through length-based estimation with more than 94% accuracy as compared to gold standard automated biochemistry analyzer (DiaSYS 400). These findings elucidate the unique protein transport dynamics at droplet interface, highlighting their potential for advancement in interfacial sciences, lab-on-chip platforms, and next-generation POCT.