Fig. 2

Photoacoustic characterization of the BphPs in cultured cells. a LIR image of bovine blood, U87 cells expressing either RpBphP1 or DrBphP-PCM. Scale bar, 2 mm. b PA signal decays and their fits from bovine blood, U87 cells expressing either RpBphP1 or DrBphP-PCM during 780 nm light illumination. c Decay constant encoded image showing different photoswitching rates of U87 cells expressing either RpBphP1 or DrBphP-PCM and the non-switchable bovine blood. Scale bar, 2 mm. d The computed decay constants of the three types of cells; error bars are s.e.m. (n = 40), calculated based on the pixel values from regions of interest. e The contrast-to-noise ratio (CNR) of LIR signals from bovine blood and from U87 cells expressing either RpBphP1 or DrBphP-PCM in a clear medium (0 mm in depth) and a scattering medium (15 mm in depth). f PA signal decays and their fits for the two types of cells—HEK-293 cells expressing both DrBphP-PCM and RpBphP1, and U87 cells expressing only DrBphP-PCM—under different illumination fluences. g LIR images (top row) of U87 cells (left) and HEK-293 cells (right) and the images of computed coefficients of b (middle row) and c (bottom row) under different illumination fluences. The signal decays can be modeled in the form of \(g(t) = a + b \cdot e^{( {\frac{{ - t}}{{T_1}}})} + c \cdot e^{( {\frac{{ - t}}{{T_2}}})}\), where T₁ > T₂. The signals from HEK-293 cells were fitted with two similar coefficients b ≈ c ≈ 0.5, while the signals from U87 cells were fitted with very different coefficients b ≈ 1, c ≈ 0. Scale bar, 2 mm. h The computed coefficients k, defined as \(k = \frac{{{\rm{max}}\{ b,c\} }}{{{\rm{min}}\{ b,c\} }}\), under different light fluences, showing a reliable separation of the two types of cells in f, g. Independent of the light fluence, the coefficient k for HEK-293 cells is ~1, and the coefficient k for U87 cells is much larger (>10); error bars are s.e.m. (n = 120), calculated based on the pixel values from regions of interest