Inflammatory biomarker detection in milk using label-free porous SiO₂ interferometer

N-acetyl-β-d-glucosaminidase (NAGase) is an established indicative biomarker released upon damage or necrosis of tubular epithelial cells in both humans and animals, indicating severe nephrological disorders and bovine mastitis (BM), respectively. The latter is the most common and costly disease in dairy cattle associated with production losses, elevated somatic cell counts and deteriorated health status. Herein, we report on a reflective based assay for early diagnosis of BM through the analysis of NAGase inherent content found in whole milk samples using a miniaturized optical transducer. Gelatin functionalized porous Si Fabry-Pérot interferometers are employed for monitoring the lysosomal activity in various stages of the inflammation (healthy, subclinical and clinical). The enzymatic reaction products precipitate and accumulate within the porous nanostructure, thus alter the average refractive index monitored using reflectometric interference spectroscopy. The optical assay is calibrated within the clinically relevant concentrations of BM while presenting a dynamic range of 1.04-16.7 μM min^-1 and the detection limit of 0.49 μM min^-1. The specific optical performance of the biosensor correlates with a gold standard laboratory-based approach, in which escalated somatic cell counts reveal augmented NAGase levels and thus severe pathogenesis. Overall, our study provides new opportunities to develop a convenient bio-diagnostic sensing system for BM detection and classification by addressing the limitations of conventional practices.