Temperature dependence of Grüneisen parameter in optically absorbing solutions measured by 2D optoacoustic imaging

A new experimental approach for measurements of temperature dependence of the Gruneisen parameter in optically absorbing solutions is proposed. Two-dimensional optoacoustic (OA) imaging is used to improve accuracy of signal amplitude measurements and spatial localization of the studied samples. We estimated OA response of optically absorbing solutions measuring median intensity of OA images within the region of interest (ROI) as a function of temperature. We showed that when normalized to its value at a particular temperature, OA image intensity becomes an accurate metric reflecting temperature changes of Gruneisen parameter regardless of local optical fluence and absorbance, assuming those remain constant with temperature. Using the proposed method we studied temperature dependence of aqueous solutions of nickel and cupric sulfate in the range from 4 to 40°C. Obtained results were compared with temperature dependence for the Gruneisen parameter of DI-water, which we measured by using carbon ink colloid. We also found that Gruneisen-temperature relationship for nickel sulfate exhibits linear trend with respect to the concentration, and is independent of coupling medium and laser excitation wavelength.In the future, the developed methodology could be adopted for important applications of in vivo optoacoustic temperature monitoring.