Fig. 4

Quad-wavelength ratiometric PA pH imaging of phantoms. a Measured PA signal amplitude ratios between the three wavelengths and the isosbestic point (i.e., 576 nm/565 nm, 584 nm/565 nm, and 600 nm/565 nm) from pH 5.8–7.7 with 0.1 pH interval (n = 3, error bars represent standard deviations). By performing linear fittings of the scattered measurements in the range of pH 5.8–7.8, three calibration lines were generated. b Quantitative pH images of phantoms containing different concentrations (2, 10 and 20 mg ml⁻¹) of SNARF-PAA NPs buffered at different pH levels (pH 6.6, 7.0 and 7.4). The means and the standard deviations of the pH levels in each PA image were calculated. The measurement accuracy was better than 0.1 pH. The presence of whole blood (1%, w/w) did not affect the quantification of pH levels