Multi-scale photoacoustic tomography using reversibly switchable bacterial phytochrome as a near-infrared photochromic probe

Photoacoustic tomography (PAT) of genetically encoded probes allows imaging of targeted biological processes with high spatial resolution at depths. Here, we combined multi-scale photoacoustic imaging with, for the first time, a reversibly switchable non-fluorescent bacterial phytochrome BphP1. With a heme-derived biliverdin chromophore, BphP1 has the most red-shifted absorption among reported genetically encoded probes, and is reversibly photoconvertible between its red and near-infrared light absorption states. We combined single-wavelength PAT with efficient BphP1 photoswitching, enabling differential imaging that substantially removed background signals, enhanced detection sensitivity, increased penetration depth, and improved spatial resolution. In doing so, we monitored tumor growth and metastasis with a ~100 µm resolution at depths approaching 10 mm using photoacoustic computed tomography, and imaged individual cancer cells with a sub-optical-diffraction resolution of ~140 nm using photoacoustic microscopy. This technology is promising for biomedical studies at different length scales.