Concentration-dependent photoacoustic signals produced by single walled carbon nanotubes (SWCNTs).

For each concentration, 3 spots were spotted. In each spot, the photoacoustic signal signals of all the trace amounts of SWCNTs are integrated and represented as the total photoacoustic signal. Then the average photoacoustic signal and the standard deviation were obtained from the three spots of each concentration. The noise-equivalent mass sensitivity was calculated based on the quantification of the average noise level per resolution voxel. The photoacoustic signals of all the above concentrations were used to generate the standard curve in Fig. 2.