Gold nanoparticle-based colorimetric analysis of protein content in milk

An easy-to-practice colorimetric analysis of milk protein content has been developed. The assay relies on stabilization power of protein that can inhibit gold nanoparticles (AuNPs) aggregation induced by hydrochloric acid. In the presence of milk protein, the assay gave explicit red to blue color depending on protein concentration. Since the appeared colors, which emerge from localized surface plasmon resonance phenomenon, can be observed by naked-eye sensing or UV-visible spectroscopy, protein concentration of the sample could be evaluated. With the optimized condition, the assay is so sensitive that it can distinguish nearby protein concentrations in the same order of magnitude by naked eye sensing with adjustable sensitive range in 2.93x10⁻¹ to 2.93x10⁻³ mg/mL. In the assay, extra accuracy can be achieved using UV-visible spectroscopy to measure λ[subscript max] or intensity ratio at maxima and at 520 nm. Interfering substance study showed that starch, lactose, table sugar, melamine, and sodium chloride cannot deceive the assay. The developed assay is very simple, requires only 6 minutes analysis time and does not need extraction process.