Laser-manufactured paper-based colorimetric analytical kit for sodium hypochlorite determination in milk

The manufacturing of easy-to-use, sustainable point-of-need devices is a hot topic. In this field, the paper represents an excellent substrate for analytical device implementation given its 3D structure, capillarity, flexibility, and biocompatibility. However, there is still a need for affordable technologies for both precise patterning/cutting paper and integrating functional nanomaterials on/into it. In this presentation, a multiple Laser-Induced Metal nanoparticles (LIMs) based treble lateral-flow paper-based device (LF3) for the analysis of sodium hypochlorite in milk will be presented; the determination of this analyte will be contextualized in the framework of milk bleaching frauds. The LF3 was conceived as an analytical ‘kit’, in which each component (i.e., LF3, sample tray, and dark chamber for smartphone-based readout) was manufactured ‘ad hoc’ using CO2-laser cut coupled to other benchtop technologies (i.e., wax printing). The LF3 is equipped with Ag, Au, and Ce-LIMs that act as colorimetric sensing elements with different sensitivity toward sodium hypochlorite; LIM can be synthesized on different cellulosic substrates and possess tunable features according to their metal source and morphology [1]. LF3 sensing strategy relies on the analyte-mediated LIM etching resulting in paper-substrate discoloration. The colorimetric assay consists of three steps: (i) milk auto sampling via LF3 capillarity, (ii) fluidic sample reaction wsith LIMs, and (iii) colorimetric readout by smartphone. The device exploitability was tested on 9 milk samples belonging to different product categories (i.e., whole, semi-skimmed, skimmed, lactose-free, vitamin-added, and goat-based). LF3-based kit enables sodium hypochlorite reproducible determination in milk at different contamination levels in 10 min (RSD ≤ 17%, n=3), with no matrix effects and recoveries ranging from 85 to 115%.