Photoacoustic imaging of the smart probe accumulation in cells. HT1080 cells were incubated with 150 μL of 10 μM solution of B-PP-A, B-APP-A or B-CP for 10 minutes and embedded in triplicate in an agar phantom. Fluorescence image (λex 675 nm, ICG emission filter) of the agar phantom shows location of the cells as well as the uptake of B-PP-A and B-APP-A (a). The uptake of B-PP-A is higher than the one of B-APP-A, because B-PP-A lacks the polyglutamic acid part that diminishes the cell membrane transduction efficiency of the polyarginine chain. Photoacoustic images of the agar phantom with embedded cells taken at two wavelengths: 675 nm (b) and 750 nm (c). Subtraction of the images taken at 675 nm and 750 nm resulted in an image with distinct signal coming from the cells incubated with the cleaved probe (d). The accumulation of different probes in the cells was quantified from the subtraction image using mean photoacoustic values for each well (e). Error bars represent the standard error of the mean of triplicates. Accumulation of BHQ3-CPP probe was significantly different (p<0.05) from the accumulation of both B-APP-A and B-PP-A. Color bars represent relative photoacoustic signal intensity. x and y represent vertical and horizontal length of the area of the agar that was scanned.