Nanoporous metal-phenolic particles are fabricated through the nanostructural replication of dense FeIII -TA complexes in nanoporous CaCO3 template particles. The particles have potential for the diagnostic detection of endogenous levels of H2 O2 ex vivo and in vivo by ultrasound imaging, which is based on the catalytic activity of the coordinated Fe3+ in the particles to break down H2 O2 to O2 microbubbles. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.