Ultra-small gold nanoparticle-coupled MOF-808 enabled sensitive detection of bacteria at neutral pH

Metal-organic framework (MOF)-based mimics are considered star materials to replace natural enzymes. However, their activity is generally limited to acidic conditions, which severely restricts their applications in biological systems where neutral pH is commonly required. Herein, a Zr(IV)-based MOF (MOF-808)/gold nanoparticle (AuNP) hybrid (called Hybrid-60) that shows superior peroxidase-like (POD-like) activity in both acidic and neutral media was prepared by in-situ growth of ultra-small AuNPs (UsAuNPs, ∼3.5 nm) on MOF-808. In comparison with the conventional AuNPs and MOF-808 nanozymes, Hybrid-60 demonstrated ∼8.04- and ∼6.74-time enhanced POD-like activities and superior high activity under neutral conditions, which broke the pH limitation. Furthermore, Hybrid-60 exhibited good tolerance to extreme pH value, concentrated salinity, and high-temperature environments. Taking Staphylococcus aureus (S. aureus) as a model analyte, we developed a simple immune sandwich assay using Hybrid-60 as colorimetric nanotags (ISAHC). A dual recognition strategy using anti-S. aureus antibody and concanavalin A-labeled Hybrid-60 was proposed to specifically capture and high-affinity label the target S. aureus. Then, leveraging the high POD-like activity of Hybrid-60, a simple and specific detection of S. aureus at nearly neutral pH was realized with a wide linear range (1 × 10²-1 × 10⁵ CFU/mL) and a low detection limit (32 CFU/mL). Moreover, the ISAHC method enabled one to detect the target S. aureus in human urine and serum with satisfactory recoveries from 93.8 % to 111.0 %, which indicates its clinical applicability. This study provides a new approach to develop neutral nanozymes and facilitate the point-of-care detection of bacteria.