Paper-based sensing of cinnamic ortho-diphenols in food samples using fluorescent laser-induced aluminum nanostructures

In a balanced diet and nutraceutical foods, antioxidants exert preventive and beneficial roles. European Food Safety Authority defined phenolic compounds (PCs) as functional molecules, recommending their daily intake; moreover, PCs can be considered also 'food' quality markers. Ortho-diphenolic compounds (o-PCs) result in the most antioxidants Among PCs, due to their ability to stabilize free radicals and directly or indirectly limit oxidative stress in living organisms. However, the rapid and selective analysis of o-PCs content in food samples is still an open issue due to the need for time and ad-hoc extraction procedures and complex analysis. In this framework, the manufacturing of easy-to-use paper-based analytical devices (PAD) offers captivating opportunities to overcome analytical limitations, increasing also the overall sustainable Herein, a CO2-laser plotter-based innovative and versatile strategy to synthesize on paper photoluminescent aluminum nanostructures, named ‘laser-induced aluminum nanostructures’ (L-Al), will be presented [1]. The L-Al demonstrates photoluminescent features under UV lamp irradiation (λex= 365 nm) and can be produced on paper substrates with the required geometries. The L-Al optical properties were deeply investigated and the laser-induced synthesis was carefully optimized to tailor the photoluminescent effect for the sensing purpose. The L-Al were synthesized on paper in an Elisa-Plate format (n= 117 wells) enabling simultaneous analysis, and have been integrated into a lab-made PAD to facilitate photoluminescent results-readout via smartphone. The L-Al photoluminescent PAD (L-Al3xP) was successfully employed to determine cinnamic o-PCs in food samples. The cinnamic o-PCs induce an L-Al fluorescence turn-on which results in a green color brightness increase, that was exploited as an analytical signal. The L-Al3xP reaction mechanisms and selectivity were carefully studied towards 23 PCs belonging to different phenolic classes. The figures of merit of the device were assessed using caffeic, rosmarinic and chlorogenic acids as representative cinnamic o-PCs. Limit of detections ≤ 3.0 μM were obtained along with linear ranges suitable for food analysis (2.5-15 μM caffeic acid; 2.5-25 μM rosmarinic acid; 10-50 μM chlorogenic acid. R2≥ 0.990). The exploitability of the L-Al3xP was proved via cinnamic o-PCs determination in 15 food samples including vegetables, spices, juices, purees, and infuses. The data obtained with the L-Al3xP correlate with the ones obtained with HPLC-MS/MS (r= 0.993) and Folin Ciocâlteu assay (r= 0.930), proving the reliability and class selectivity of the proposed platform for food analysis.