Dual-Mode Plasmonic and Paper-Based Colorimetric Assays for the Determination of Riboflavin in Green Leafy Vegetables and Whole Grains

We developed plasmonic and paper-based colorimetric assays using poly(vinyl alcohol)-capped silver nanoparticles (AgNPs) to detect vitamin B₂ in food samples. Plasmonic colorimetry shifts the plasmonic band from 405 to 445 nm in the presence of B₂, while the paper-based method changes the color of AgNPs from yellow to reddish yellow. B₂ levels were quantified with a smartphone-assisted RGB color detector app. Experimental and density functional theory investigations validated the selective riboflavin–AgNP interaction. The plasmonic assay detected B₂ in the range of 0.01–1.0 μg mL^–1 with a detection limit of 0.008 μg mL^–1, while the paper-based assay detected in the range of 0.2–2.0 μg mL^–1 with a detection limit of 0.09 μg mL^–1. Both methods had low relative standard deviations of 3.5% (plasmonic) and 4.7% (paper-based). The cost-effective, user-friendly, paper-based assay is particularly suitable for resource-limited settings, validated against high-performance liquid chromatography and Student’s t-test for accuracy.