Fig. 1

OA- and physiological characterization of HGA-derived pigmentation in vitro. a Chemical structure of homogentisic acid (HGA, top). Pelleted Ana-1 cells showing increasing pigmentation after treatment with 0–1 mM HGA for 96 h (bottom). b Normalized optoacoustic signal intensities of Ana-1 cells plotted against their corresponding HGA concentrations (0, 0.3, 0.7, 1, 1.2 mM) after 96 h of treatment. The secondary axis shows the percentage of viable cells for each concentration (black diamond). N = 4. c Polymerization kinetics of the pigment followed by a single dose of 0.3 mM HGA to Ana-1 cells shown as a function of OA signal. N = 2, z = 7. d Hypothetical model of OA-sensitive HDP formation in macrophages. Our data implies that the formation of HDP occurs at an accelerated rate in the presence of intact cells indicated by the reaction taking place inside the box (Cell). We do not exclude reactions additionally occurring at other locations. e–g Comparison of HDP vs. synthetic eumelanin and nanoparticles (NPs; SiAuRods) in macrophages. e Linear increase of OA signal intensities measured in macrophages as a function of concentration of HGA (0, 0.3, 0.5 mM), of the soluble fraction taken from synthetic melanin (OD_700nm of 0, 0.03, 0.05, 0.07, 0.1) and of supplemented NPs (0, 5 × 10¹⁰, 10¹¹ particles). HDP and synthetic eumelanin were recorded at 700 nm and NPs at 780 nm. N = 2–6, z = 6. f Cell viability plotted against the OA signal of macrophages treated with HGA for 96 h, synthetic melanin for 24 h and 96 h, or NPs for 24 h and 48 h. N = 2–9. g In vitro cell motility of primary M1-polarized macrophages comparing untreated (WT) with HPD vs. NPs-labeled cells. Cells were pretreated with HGA for 96 h and NPS for 16 h prior to start and allowed to migrate for 24 h. Motility of NPs-fed cells was significantly decreased compared with HPD and WT cells, the latter two showing no significant (ns) difference. N = 3. Error bars, mean ± SD. N = biological replicates, z = number of positions for data acquisitions within a replicate. Source data are provided as a Source Data file