Naked eye colorimetric quantification of protein content in milk using starch-stabilized gold nanoparticles

An easy-to-perform colorimetric analysis of protein content in milk based on aggregation-induced color change of starch-stabilized gold nanoparticles (AuNPs) has been developed. The synthesized AuNPs were employed without further surface modification. The assay relies on the additional stabilization power of proteins that inhibits the aggregation of AuNPs induced by hydrochloric acid. In the presence of milk proteins, the assay shows red to blue color shift as the total protein concentration decreases. Since the localized surface plasmon resonance (LSPR) expresses correlation with protein concentration, the protein content can be quantified. With an optimized condition, the assay is so sensitive that it can accurately determine protein content from 2.93 × 10−1 to 2.93 × 10−3 mg/mL via naked eye observation based on color change. Moreover, an extra accuracy can be achieved via the corresponding UV–visible spectra. Adulterating 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 min of analysis time without any extraction process. This technique provides a great potential for on-site milk analysis, milk adulteration detection as well as in-house use.