Ultra-Fine Palladium Nanoparticle-Encapsulated Graphene Oxide Framework: A Bifunctional Photonanozyme for Colorimetric Detection of Dopamine

The concept of photonanozyme, which refers to enzyme-mimicking nanomaterials with a light-harvesting capacity, has recently gained significant attention. It offers a promising opportunity to use light as a “trigger” to initiate or enhance nanozymatic activity. In this study, a nanocomposite (PdNPs@GOF) based on a graphene oxide framework (GOF) containing ultra-fine palladium nanoparticles (particle size ∼4 nm) in its gallery space was established as a photonanozyme due to its narrow semiconductor-like bandgap. This nanocomposite exhibits excellent peroxidase-mimic and oxidase-mimic activity toward the oxidation of the chromogenic 3,3′,5,5′-tetramethylbenzidine (TMB) substrate under visible light irradiation. PdNPs@GOF was employed to develop a colorimetric assay for the quantitative detection of the important biomarker dopamine (DA) within a wide linear range of 0.05–20 μM based on its dual photonanozymetic activity with a limit of detection (LOD) as low as 0.18 μM. The developed colorimetric assay manifests excellent selectivity toward DA detection when compared against several other antioxidant biothiols, biomolecules, and salts. The colorimetric assay was successfully used to quantitatively detect different concentrations of DA in spiked fetal bovine serum and qualitatively detect DA in mammalian muscle cell lysates. The PdNPs@GOF nanocomposite remains stable, maintaining nearly constant efficiency to detect DA for up to 4 months. This study shows that GOF-based nanocomposites can be used as bifunctional photonanozymes for colorimetric detection of the DA biomolecule with low LOD, good selectivity, and long-term stability.