(a) Normalized optoacoustic spectra (NOAS) of water solutions of DMSO at different concentrations (after water subtraction). The absorption band due to the S=O vibration of the DMSO molecule is clearly recognized above the baseline (BL) down to a concentration of 2.5 mM. (b) Area of NOAS for the S=O absorption band vs concentration of DMSO, the lowest detected concentration with a SNR > 2 is indicates by the red arrow (2.5 mM with a SNR of 6). (c) MiROM micrograph of D2O and carbon tape at 1650 cm−1. The LOD for albumin was tested changing its concentration from 0 to 757 µM while measuring the OA signal at the indicated point S. The OA signal at point R in the carbon tape served as spectral reference. (d) NOAS of D2O solutions of albumin at different concentrations (after D2O subtraction). The amide I absorption band of albumin is clearly recognized above the BL down to a concentration of 1.5 µM. (e) Area of NOAS of the amide I band vs concentration of albumin, the lowest detected concentration with a SNR > 2 is indicates by the red arrow (1.5 µM with a SNR of 4.5). (f) Contrast test of albumin solutions in the area indicated in c (red dashed line) showing that: images of albumin with concentrations as low as 1.5 µM can be obtained and recognized above the contrast level of D2O. (g) Contrast test of the carbon tape (for reference) in the area indicated in c (black dashed line). Data in a-g are representative of three independent experiments. In b,e the error bars represent: area of NOAS ± noise level (see Methods).