Fig. 4. Biocompatibility and labeling conditions of SiCNP620 in MSCs. (a) No significant decrease in cell viability was seen for labeling concentrations smaller than 400 μg mL⁻¹. (b) Cells labeled with SiCNP620 at different concentrations showed similar growth rates to unlabeled cells. Error bars are standard deviations of 8 replicates. (c) A linear relationship is seen between the labeling concentration and photoacoustic signal for four hours of labeling. (d) For a labeling concentration of 50 μg mL⁻¹, a labeling time of four hours was optimal. (e) Photoacoustic intensity was linearly dependent on the concentration of SiCNP620-labeled cells (50 μg mL⁻¹, 4 hours of incubation), and the limit of detection was 37 cells per μL. Error bars are standard deviations of 5 measurements.