Oxidase-Mimicking MnO₂/Au/Ag Hollow Nanoframes for Smartphone-Assisted Colorimetric Detection of Hydrogen Sulfide

Hydrogen sulfide (H₂S), a volatile sulfur-containing compound, is a key indicator of fish spoilage. However, existing analytical methods often require unstable oxidants or complex instrumentation, limiting their real-time use. Here, we report a smartphone-assisted colorimetric assay using MnO₂/Au/Ag nanoframes (NFs) for sensitive and selective H₂S detection. The hollow Au/Ag scaffold, coated with an amorphous, defect-rich MnO₂ shell, exhibits H₂O₂-independent oxidase-like activity with strong substrate affinity (K_m = 0.086 mM) and high catalytic efficiency. The assay shows a linear range of 2.1–30 μM H₂S, a detection limit of 3.85 μM, excellent selectivity against thiol-type reductants and dimethyl sulfide, good reproducibility (relative standard deviation 3.94%), and stability for up to 4 weeks under refrigeration. Application to fish samples confirmed time-dependent H₂S release, consistent with spoilage levels. This nanozyme-based, smartphone-readable strategy offers a practical approach for on-site freshness monitoring and portable biosensing of volatile biomarkers in food safety.