A Session-Lasting Biostable and Biocompatible Hydrogel Microneedle Patch for Nanozyme-Mediated On-Site and Smartphone-Based Quantification of Skin H₂O₂

Hydrogen peroxide (H₂O₂) in skin tissue serves as a critical biomarker for oxidative stress and physiological status, yet its precise in situ monitoring remains challenging due to the lack of non-invasive, real-time sensing platforms. Herein, we presented a smartphone-assisted biosensing system based on a nanozyme-integrated hydrogel microneedle patch for on-site and quantitative detection of cutaneous H₂O₂. The sensing platform incorporated a palladium-embedded metal-organic framework (Pd-PCN) with enhanced peroxidase-like activity, which was co-immobilized with a chromogenic substrate in a polymeric composite hydrogel microneedle array. This composite matrix maintained structural integrity and biocompatibility during short-term skin application while providing an aqueous microenvironment favorable for nanozyme-catalyzed reactions. Upon insertion into the skin, H₂O₂ from interstitial fluid would diffuse into the microneedle and trigger a colorimetric reaction. The resulting color intensity could be quantitatively captured via smartphone-based RGB analysis. The system demonstrated a wide linear detection range (5~100   µM), a low detection limit (1.53   µM), high reproducibility (RSD < 5%), and negligible interference from common biological substances. Confocal imaging and mechanical testing further confirm the structural stability and skin-penetration capability of the microneedle under operational conditions. This work established a reliable, minimally invasive, and user-friendly platform for continuous monitoring of oxidative biomarkers in skin, paving the way for personalized dermatological care and point-of-care diagnostics.