Synergistic Strain and Ligand Effects Boosting the Activity of Pt-Based Peroxidase Nanozymes for Femtomolar-Level Colorimetric Immunoassay of Protein Biomarkers

Sensitive detection of protein biomarkers is crucial for advancing biomedical research and clinical management. Although colorimetric enzyme-linked immunosorbent assays (CELISAs) have been widely recognized as a benchmark technique for protein biomarker detection, their sensitivity is fundamentally constrained by the intrinsic catalytic limitations of conventional enzyme labels. In this study, we present the engineering of high-performance Pt-based peroxidase nanozymes leveraging the synergistic effects of strain and ligand interactions. This advancement enables the development of an ultrasensitive CELISA platform capable of detecting protein biomarkers at femtomolar levels, providing a promising solution to address the existing sensitivity limitations. These Pt-based peroxidase nanozymes are precisely engineered by conformally coating Pd nanocubes with uniform, ultrathin Pt shells consisting of just four atomic layers (Pd@Pt_4L nanocubes). The atomic-level Pt shells endow the Pd@Pt_4L nanocubes with the strain and ligand effects, resulting in a ∼2000-fold enhancement in peroxidase-like catalytic activity compared to traditional horseradish peroxidase (HRP), and thus making them highly efficient as catalytic labels for enhancing the sensitivity of CELISAs. Taking interleukin-6 (IL-6) detection as an example, we demonstrate that the Pd@Pt_4L nanocube-based CELISA enables quantitative analysis within a dynamic range of 0.05-5 pg mL^-1 and achieves an impressive limit of detection (LOD) of 0.046 pg mL^-1 (1.8 fM), representing a ∼20-fold enhancement in sensitivity over the conventional HRP-based CELISA. These discoveries underscore the impact of strain and ligand modulation on enhancing the catalytic activity of nanozymes and highlight their potential as catalytic labels for advancing ultrasensitive bioassay technologies.