The catalytic activity of the PtNP-containing APCN microcapsules was measured indirectly by a competition assay with myoglobin (Mb) that acts as a peroxidase and that was added to the outside of the capsules (Figure ). Mb uses hydrogen peroxide to oxidize Amplex Red (AR) to the fluorescent resorufin. The presence of Mb in the suspension of NP-loaded capsules meant that H2O2 would be either used to oxidize AR or decomposed by the PtNPs without production of fluorescence. The NPloaded capsules partially quenched the activity of myoglobin (Figure ), indicating that the PtNPs decomposed hydrogen peroxide. Interestingly, this happened even though the enzyme had easier access to the substrates because hydrogen peroxide did not have to cross a polymer shell before reaching the enzyme. In the absence of Mb, the catalyst-loaded capsules were even able to counteract the noncatalyzed oxidation of AR with H2O2. Overall, it can be concluded that H2O2 can diffuse through the shell of the microcapsules and that the PtNPs inside of the APCN microcapsules are catalytically active in decomposing H2O2. This demonstrates that the capsules act as microreactors for the nanoparticles. While some molecules like fluorescein and hydrogen peroxide diffuse through the microcapsule shell and others, such as the lipophilic dye Nile Red homogeneously distribute throughout the whole thickness of the APCN wall, the outside of the shell might be selectively functionalized with a hydrophobic moiety attached to a hydrophilic macromolecule. The hydrophobic moiety should act as an anchor that enters into the hydrophobic domains on the surface of the APCN, while the hydrophilic macromolecule hinders deep penetration into the APCN due to its bulkiness. To illustrate such a functionalization strategy, APCN microcapsules were incubated in a Cy5-PEG3500-cholesterol (CyPC) solution. This molecule contains a hydrophobic cholesterol anchor that is usually used for insertion into lipid and block copolymer membranes. The PEG is hydrophilic and acts as a spacer to ensure water solubility, and the Cy5 is a fluorescent moiety.