Functionalized gold nanoparticles for quality assessment of extravirgin olive oil

Introduction: Phenolic compounds are presents in low concentration in extravirgin olive oil (EVOO), usually extracted with a methanolic solution. The purpose of this work is to develop a new extraction method using functionalized hydrophilic gold nanoparticles (AuNPs) in alternative to the traditional one. Materials and methods: Chemicals involved were: deionized water by Milli-Q device, methanol and n-hexane by Sigma, hydrogen tetrachloroaurate (III) trihydrate, sodium borohydride and by Aldrich. Functionalized gold nanoparticles were prepared with a single phase Shiffin and Brust’s reaction and characterized with FTIR, UV-Vis spectrometry and dynamic light scattering (DLS) to evaluate size and size distribution. The extraction of phenolic compounds was carried out using EVOO in the presence of AuNPs. After separation an extraction was executed on EVOO with methanol/water mixture. The quantification of phenolic compounds was carried out through Folin-Ciocalteu reagent, using a UV-Vis spectrophotometer. Results: The hydrophilic AuNPs have been obtained with mean size 10-20 nm. The small dimensions of AuNPs, guaranteed an extended interaction area with phenolic compounds contained in EVOO and permitted the extraction of polar fraction. In order to optimize the extraction process, the combination of different experimental parameters on the extraction of phenolic compounds was tested, such as quantity of AuNPs and contact time between AuNPs and EVOO. It was observed that optimal concentrations of polyphenols were extracted using the highest quantity of NPs tested, with longer time of contact. This method was validated in terms of precision and accuracy. Preliminary tests also showed promising results concerning the possible recovery of AuNPs: in fact the treatment with a basic solution allowed the release of phenolic compounds from AuNPs to the aqueous solution; this allow the re-use of the same AuNPs for others extractions. Discussion: The extraction with AuNPs resulted comparable in terms of recovery with the traditional extraction (using methanol/water mixture). Synthesis of gold nanoparticles is quite expensive in terms of costs of reagents, time of reaction and purification. But the possibility of re-use for several consecutive extraction allows to save costs of production, volume of solvents used and chemical wastes.