Metal-Organic Frameworks as Single-Particle Contrast Agents for Localization Optoacoustic Tomography

Localization optoacoustic tomography (LOT) resolves microvascular networks beyond the acoustic diffraction limit by detecting and tracking individual absorbing particles in the bloodstream. Single-particle detection in LOT remains challenging for clinically acceptable contrast materials, typically requiring particle diameters of ~5 μm to overcome the strong absorption background of blood. Here, we show that metal-organic framework (MOF) microparticles overcome this limitation. Rods (3.43 × 0.36 μm) and ellipsoids (1.03 × 0.53 μm) of the porphyrinic zirconium MOF PCN-222 take up 16 to 18 wt% indocyanine green (ICG), which is retained in the framework. Both morphotypes are resolved as isolated optoacoustic (OA) point sources in vitro, with single-particle signal-to-noise ratios of 13 to 82, which would still allow the detection of particles with 22 to 42% of the present volume, down to ~0.4 μm in equivalent diameter. The combined emission of framework and dye allows each particle to be counted by flow cytometry, which sets absolute concentrations for dosing. The particles retain ≥96% of their ICG in water and ≥88% in undiluted serum, keep their per-particle ICG signal in serum, and leave the calcein AM retention of murine dendritic cells at 97 to 103% of the untreated control after 48 h at up to 75.9 (PCN-222-R@ICG) and 15.9 (PCN-222-E@ICG) particles per cell. After intravenous injection at 800 nm excitation and ~35 mJ cm−2, both morphotypes were individually localized in the mouse brain, yielding super-resolved cortical microangiograms and blood flow velocity maps. By enabling LOT with particles one to two orders of magnitude smaller in volume than previous ICG-based agents, PCN-222@ICG microparticles, to our knowledge the first MOFbased contrast agents for LOT, extend single-particle optoacoustic imaging to smaller particle sizes.