A Label-Free Optical Biosensor Based on Ordered Porous Layer Interferometry for IgG Quantification in Milk

Background Immunoglobulin G (IgG) levels in milk are critical indicators of dairy quality. Conventional measurement methods require labeling and complex sample pretreatment, limiting their practical use for real-time monitoring. Objective This study aimed to develop a label-free optical biosensor based on ordered porous layer interferometry (OPLI) for the rapid quantification of IgG in milk at different processing stages. Methods Test portions were directly applied to the flow tank, where protein A from Staphylococcus aureus was immobilized as a functional ligand via chemical conjugation. Conditions for ligand immobilization, flow rate, contact time, and sensor regeneration were optimized to minimize non-specific binding and enhance accuracy. Results The OPLI biosensor demonstrated a linear quantification range of 0.44 μg/mL to 5 mg/mL, with a limit of quantification of 0.44 μg/mL. This dynamic range effectively covers the IgG content from raw milk to finished milk. The method showed high reproducibility, with an instrumental response relative standard deviation (RSD) of 0.05% and an average RSD of 12.1% across batches. The biosensor was successfully applied throughout raw milk and final milk production, facilitating IgG calibration for consumer milk. Conclusion The OPLI-based biosensor offers a sensitive, real-time and wide-range analytical method for IgG analysis, enabling effective quality control in dairy production. Highlights The study developed a label-free biosensor for IgG detection in milk, with a linear quantification range of 0.44 μg/mL to 5 mg/mL and high reproducibility. The biosensor is applicable across all stages of dairy production for IgG quality assessment.