Monitoring the chemical production of citrus-derived bioactive 5-demethylnobiletin using surface-enhanced Raman spectroscopy.

To develop an accurate and convenient method for monitoring the production of citrus-derived bioactive 5-demethylnobiletin from the demethylation reaction of nobiletin, we compared surface-enhanced Raman spectroscopy (SERS) methods with a conventional high-performance liquid chromatography (HPLC) method. Our results show that both the substrate- and solution-based SERS methods correlated with the HPLC method very well. The solution method produced lower root-mean-square error of calibration and higher correlation coefficient than the substrate method. The solution method used an "affinity chromatography"-like procedure to separate the reactant nobiletin from the product 5-demthylnobiletin based on their different binding affinities to the silver dendrites. The substrate method was found simpler and faster to collect the SERS "fingerprint" spectra of the samples because no incubation between samples and silver was needed and only a trace amount of samples was required. Our results demonstrated that the SERS methods were superior to the HPLC method in conveniently and rapidly characterizing and quantifying 5-demethylnobiletin production.