Graphene-supported Pd/Pt nano-catalysts for enhanced colorimetric detection of dopamine and NADH using paper-based microfluidic devices

Background and purpose: Dopamine and nicotinamide adenine dinucleotide (NADH) are key biomarkers associated with neurological and metabolic disorders. Developing rapid, low-cost, and portable detection platforms of these biomarkers is essential for a point-of-care diagnostic kit. In this work, we report a colorimetric sensing approach using paper-based microfluidic devices (µPADs) modified with graphene-supported palladium (G/Pd) and platinum (G/Pt) nanocatalysts to enhance detection performance. Experimental approach: Monolayer G/Pd and G/Pt nanocomposites were synthesized via a hydrothermal method with precursor concentrations ranging from 0.1 to 10 mM. The catalytic behaviour and metal-graphene interactions were further investigated using spin-polarized density functional theory (DFT) calculation (PHASE/0). Microfluidic paper-based analytical devices (µPADs) were laser-printed on commercial filter paper and folded into 3D origami structures. Colorimetric responses were quantified using red, green, blue (RGB) and hue, saturation, value (HSV) analysis, where time-dependent Euclidean distance in RGB colour space was used to assess the reaction kinetics. Key results: DFT results indicate that Pd and Pt clusters preferentially adopt a top-site configuration on graphene, facilitating interfacial charge redistri­bution and enhancing catalytic activity experimentally. Catalyst-modified µPADs significantly improve reaction kinetics, reducing detection time by up to 3.7× for dopamine and 2.5× for NADH compared to unmodified devices. G/Pt (10 mM) exhibits the best overall performance, achieving limits of detection of 0.16 µM for dopamine and 0.195 µM for NADH with good linearity (R2 = 0.91). G/Pd displays competitive sensitivity, particularly at lower precursor concentration. Conclusion: The findings highlight that optimizing catalyst morphology and interfacial electronic structure is more critical than minimizing activation energy for achieving high-performance colorimetric sensing. The resulting platform shows potential as a cost-effective and portable tool for the detection of clinically relevant biomarkers in point-of-care settings.