Metal Nanoparticles Laser Writing on Cellulosic Substrates for Affordable Colorimetric Paper-Based Analytical Device Development

Nowadays, the manufacturing of easy-to-use and sustainable point-of-need devices is a hot topic. In this framework, the colorimetric approaches, which employ optical variation easily monitorable by naked eye or smartphone, can offer captivating opportunities. Due to its high availability, 3D structure, capillarity, mechanical resistance, flexibility, and biodegradability, the paper represents an excellent substrate for the fabrication of paper-based analytical devices (PAD). Paper's color, porosity, and intrinsic chemical features, make it a straightforward building block for developing colorimetric PAD, allowing also the anchoring/storing of sensing materials and reagents, and the realization of microfluidics set-up. Among sensing materials, metal nanoparticles (MNP) are useful colorimetric probes/tags due to their peculiar chemistry and optical features (i.e., localized surface plasmon resonance/ LSPR, and luminescence). Properly synthesized MNP returns visible colors offering infinite opportunities for developing optical and colorimetric PAD; however, paper decoration with MNP with high resolution and defined patterns is still a challenge. In this presentation, a CO2-laser plotter-based innovative and versatile strategy to synthesize on paper plasmonic active gold (Au), silver (Ag), platinum (Pt), copper (Cu), cerium (Ce), nickel (Ni), and aluminum (Al) nanostructures will be presented [1]; this approach allows to form metal-nanostructures on different paper substrates including Whatman paper, office paper, recycled office paper, rice paper, etc. The laser-induced metal nanoparticles (LIMs) result well anchored onto paper, allowing on-demand designs. LIMs possess analytical useful features i.e., (i) are plasmonic active, (ii) possess catalytic features and (iii) show photoluminescent; these features are related to the LIM chemistry and morphology. Therefore, LIMs have been integrated into different lab-made PADs coupled with smartphone-based readouts. The proposed PADs were conceived to integrate analysis steps and facilitate result-readout and were manufactured using low-cost benchtop technologies (i.e., laser/cutter-plotter, thermal-laminator, etc.) and office-grade substrates (i.e., polymeric-sheets, cellulosic substrates, etc.). In this presentation will be presented different PADs enclosing LIMs. (i) A treble lateral-flow device (LF3) for direct milk analysis aimed at the determination of sodium hypochlorite related to bleaching frauds will be shown. The LF3 is equipped with Ag, Au, and Ce-LIMs that act as colorimetric tags enabling different levels of contamination evaluation; the sensing strategy relies on the analyte-mediated LIM etching resulting in paper-substrate discoloration. The LF3 is designed to work directly with milk exploiting the paper fluidics to filter/clean the samples. (ii) A flip paper-based device (Flip-PAD) for the selective determination of ascorbic acid in food and supplements will be presented. The Flip-PAD exploits Pt-LIM oxidase-like activity to obtain a dye-based colorimetric readout, without needing external reagents and allows to obtain accurate analytical results in 1 minute. (iii) Eventually, a fluorometric PAD for the selective detection of o-diphenols in food samples will be also presented. In this case, the Al-LIM fluorescence turn-on is exploited for the detection of the analytes. Interestingly, this PAD allows obtaining qualitative and quantitative results thanks to the LIM's unique features. Summing up, the proposed laser writing strategy can be considered a new sustainable nanopatterning technique, particularly prone to generate optical sensing zones useful to fabricate within everyone's reach paper-based colorimetric devices, conceived to solve analytical issues reducing the use of chemicals and solvents.