Fig. 3. The photoacoustic response of the Au10s, Au25s, Au40s, and Au50s were measured with a multispectral optoacoustic tomographic (MSOT) system at equivalent mass (100 μg ml⁻¹) and number of particles (1 × 10¹¹ NP ml⁻¹). (a) A MSOT image of the four AuNRs, reconstructed from the raw signal data captured with the MSOT system with a logarithmic colorbar, and (b) the maximum photoacoustic amplitude of the same four AuNRs (purple = 1 × 10¹¹ NP ml⁻¹, orange = 100 μg ml⁻¹).