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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.