Fig. 2.

(a) Ultrasound (left column), photoacoustic (middle column), and merged (right column) images of inclusions containing MDA-MB-231 cells incubated in cell culture media containing AuNPs for 1, 2, 3, and 4 h, respectively. GC-AuNPs were taken up by cells (right-hand side inclusion in each image); PEG-AuNPs were used in the (left) phantom as a control. Each cell phantom contained $3.6\times {10}^5\mathrm{cells}$. In photoacoustic images, the laser wavelength was 680 nm. Scale bar is 4 mm. (b) Significant increase in photoacoustic signal from cells incubated with GC-AuNPs appears after 3 h of cellular uptake, compared with PEG-AuNPs [$t$-test, $n=3$ for each group, $p=0.01393$ (3 h), and $p=0.012384$ (4 h)].