Development and application of an optical biosensor immunoassay for α-lactalbumin in bovine milk

An automated, label-free biosensor-based immunoassay for α-lactalbumin in bovine milk utilising surface plasmon resonance (SPR) detection is described. α-Lactalbumin content was estimated from the specific interaction with an anti-bovine α-lactalbumin antibody immobilised on the sensor surface in a direct-binding assay format, although an alternative inhibition assay format is also described. Samples were prepared for analysis by direct dilution into buffer. Ligand selection and analysis conditions are defined, and non-specific binding considerations evaluated. Performance parameters include a working range of 10–1000 ng mL−1, a method detection limit of 0.12 mg mL−1 in milk, an overall instrumental reproducibility relative standard deviation (RSDR) of 5.71%, a mean inter-assay RSDR of 7.61% for an infant formula control sample and a surface stability of approximately 500 cycles. Accuracy was confirmed by comparison against an independent liquid chromatographic method. The technique was applied to the measurement of the α-lactalbumin content of consumer milk, colostrum, whey protein concentrates and infant formulae, the temporal change during early bovine lactation and a preliminary study of thermal denaturation.