Ultra-Fine Palladium\nNanoparticle-Encapsulated Graphene\nOxide Framework: A Bifunctional Photonanozyme for Colorimetric Detection\nof Dopamine

The concept of photonanozyme, which refers to enzyme-mimicking\nnanomaterials with a light-harvesting capacity, has recently gained\nsignificant attention. It offers a promising opportunity to use light\nas a “trigger” to initiate or enhance nanozymatic activity.\nIn this study, a nanocomposite (PdNPs@GOF) based on a graphene oxide\nframework (GOF) containing ultra-fine palladium nanoparticles (particle\nsize ∼4 nm) in its gallery space was established as a photonanozyme\ndue to its narrow semiconductor-like bandgap. This nanocomposite exhibits\nexcellent peroxidase-mimic and oxidase-mimic activity toward the oxidation\nof the chromogenic 3,3′,5,5′-tetramethylbenzidine (TMB)\nsubstrate under visible light irradiation. PdNPs@GOF was employed\nto develop a colorimetric assay for the quantitative detection of\nthe important biomarker dopamine (DA) within a wide linear range of\n0.05–20 μM based on its dual photonanozymetic activity\nwith a limit of detection (LOD) as low as 0.18 μM. The developed\ncolorimetric assay manifests excellent selectivity toward DA detection\nwhen compared against several other antioxidant biothiols, biomolecules,\nand salts. The colorimetric assay was successfully used to quantitatively\ndetect different concentrations of DA in spiked fetal bovine serum\nand qualitatively detect DA in mammalian muscle cell lysates. The\nPdNPs@GOF nanocomposite remains stable, maintaining nearly constant\nefficiency to detect DA for up to 4 months. This study shows that\nGOF-based nanocomposites can be used as bifunctional photonanozymes\nfor colorimetric detection of the DA biomolecule with low LOD, good\nselectivity, and long-term stability.