Turn-On Fluorescence Sensing of Oxygen with Dendrimer-Encapsulated Platinum Nanoparticles as Tunable Oxidase Mimics for Spatially Resolved Measurement of Oxygen Gradient in a Human Gut-on-a-Chip

Turn-on type fluorescence sensing of O₂ is considered a promising approach to developing ways to measure O₂ in microenvironments with spatially distributed O₂ levels. As a class of nanomaterials with a high degree of control over composition and structure, dendrimer-encapsulated nanoparticles (DENs) are promising candidates to mimic biological enzymes. Here, we report a strategy to monitor spatially distributed O₂ across a three-dimensional (3D) human intestinal epithelial layer in a gut-on-a-chip in a turn-on fluorescence sensing manner. The strategy is based on the oxidase-mimetic activity of Pt DENs for catalytic oxidation of nonfluorescent Amplex Red to highly fluorescent resorufin in the presence of O₂. We synthesized Pt DENs using two different types of dendrimers (i.e., amine-terminated or hydroxyl-terminated generation 6 polyamidoamine (PAMAM) dendrimers) with six different Pt²⁺/dendrimer ratios (i.e., 55, 200, 220, 550, 880, and 1320). After clarifying the intrinsic oxidase-mimetic activity of Pt DENs, we determined tunable oxidase-mimetic activity of Pt DENs, especially with fine-tuning the ratios of the Pt precursor ions and dendrimers. Particularly, the optimal Pt DENs having a Pt²⁺/dendrimer ratio of 1320 exhibited an ∼117-fold increase in the oxidase-mimetic activity for catalyzing the aerobic oxidation of Amplex Red to resorufin compared to one having a Pt²⁺/dendrimer ratio of 200. This study exemplified a simple yet effective approach for spatially resolved imaging of O₂ using metal nanoparticle-based oxidase mimics in microphysiological environments like a human gut-on-a-chip.