water soluble, encapsulation into nanoparticles was necessary to facilitate further analysis under biocompatible aqueous conditions (Figure ). Aggregated organic dots (a-Odots) were prepared using a simple nanoprecipitation method with amphiphilic DSPE-PEG2k as a surfactant to form nanoparticles with a hydrophobic metalloisoporphyrinoid-containing core, and a hydrophilic corona. This method has previously been used by our group and others to prepare water-dispersible nanoparticles containing hydrophobic fluorophores for bioimaging and photothermal applications. The resulting nanomaterials showed hydrodynamic diameters of 72 -99 nm, as determined using nanoparticle tracking analysis (Table , Figure ). The UV-visible absorbance spectra of the metalloisoporphyrinoid nanomaterials all exhibit strong absorbance between 700 -900 nm, which is within the biological transparency window (Figure , Table ). The compounds are accordingly strong candidates for in vivo use, allowing for better targeting of deeper tissues with NIR radiation. Besides facilitating water dispersibility, nanoparticle formation also increases the per-particle absorbance of the materials on account of the high density of the chromophore within the hydrophobic core. The ability of the metalloisoporphyrinoids to convert light into heat was determined using Roper's method for photothermal conversion efficiency measurements. A detailed discussion of the method can be found in our earlier work, and in the Supplementary Information. In brief, 2 mL of 150 µg/mL nanoparticle dispersions in a standard 1 cm x 1 cm x 4 cm quartz cuvette were irradiated with a 785-nm laser (0.4 W) for 20 minutes, and allowed to cool for 30 minutes.