Discrimination of whiskies using an “add-a-fluorophore” fluorescent fingerprinting strategy

Abstract

This study is concerned with the development of a fluorescent fingerprinting technique using external fluorophores. This approach is based on the quenching/dequenching of emissions by the components in the sample and therefore has wider possibilities than the common fingerprinting technique, which is based on the intrinsic fluorescence of a sample. The procedure involved mixing the sample with fluorophores in 96-well plates and recording emissions by digital photography using a Camag Visualizer 2 instrument. After RGB splitting of the images, the data were normalized and treated by principal component analysis (PCA). The quality of discrimination, characterized by the number of separate groups of points in the scores plot, was essentially improved when the images were contrasted using the bundled software. To enable interactions with a wide range of potential marker compounds, we studied six fluorophores of different chemical origins (rhodamine B on silica nanoparticles, fluorescein attached to polyethyleneimine, a Schiff base obtained using o‑phthalaldehyde and polyethyleneimine, quantum dots of CdSe/CdS/ZnS, a ruthenium(II) complex with 2,2′‑bipyridine (Ru(bpy)32+), and a zinc complex with 8‑hydroxyquinoline‑5‑sulfonate). Interestingly, just one fluorophore, Ru(bpy)32+, sufficed to discriminate between 16 samples of whiskies (Scotch, Irish, Bourbon, and other varieties). The quality of discrimination by RGB images was at least equal to that obtained using the emission spectra measured with a spectrofluorimeter (400–700 nm). Using any of the studied fluorophores is more efficient for discrimination than depending on the intrinsic fluorescence of individual whiskies.