Expanded porphyrins: functional photoacoustic imaging agents that operate in the NIR-II region

Photoacoustic imaging (PAI) relies on the use of contrast agents with high molar absorptivity in the NIR-I/NIR-II region. Expanded porphyrins, synthetic analogues of natural tetrapyrrolic pigments (e.g. heme and chlorophyll), constitute as potentially attractive platforms due to their NIR-II absorptivity and their ability to respond to stimuli. Here, we evaluate two expanded porphyrins, naphthorosarin (1) and octaphyrin (4), as stimuli responsive PA contrast agents for functional PAI. Both undergo proton-coupled electron transfer to produce species that absorb well in the NIR-II region. Octaphyrin (4) was successfully encapsulated into 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-poly(ethylene glycol) (DSPE-PEG(2000)) nanoparticles to afford OctaNPs. In combination with PAI, OctaNPs allowed changes in the acidic environment of the stomach to be visualized and cancerous versus healthy tissues to be discriminated.

Automated summary

The study evaluates two expanded porphyrins as contrast agents for photoacoustic imaging, which have good absorption in the NIR-II region and can respond to stimuli. The use of OctaNPs allows visualization of changes in the acidic environment of the stomach and differentiation between cancerous and healthy tissues.