The improved PA contrast with DNDs compared to AuNRs may be due in part to both the optical absorbance and the high thermal conductivity of DNDs. Diamond has the highest thermal conductivity of all known natural materials. For example, the thermal conductivity for single crystal diamond is , which is over three times greater than that of copper at 20°C.33 The interfacial thermal conductance is the inverse of the interfacial thermal resistance, which is a measure of an interface’s resistance to thermal flow. Therefore, lower interfacial thermal resistance or higher interfacial conductance of nanoparticles leads to faster heat transfer from the particles to the ambient medium, which is DI water or biological fluids in this study, and this may result in stronger PA signals. However, this explanation does not fully explain the improved signal compared to SWNTs, which have larger on-axis thermal conductivities, in excess of , despite a low off-axis conductivity of ca. .34 Also, the effects of thermal diffusion are generally thought to be negligible over the short excitement pulse length used for PA imaging.35 Hydrogen bonding also could play a role in the improved PA amplitude of DNDs. Chen et al. found that silica-coated AuNRs produced three-fold higher PA signals than uncoated AuNRs, because the hydroxyl groups of the silica surface were able to form hydrogen bonds with water and lowered the interfacial thermal resistance.36 The surfaces of SWNTs and unmodified nanodiamonds are very hydrophobic, and the hydrogen bonding interactions between the carbon atoms and the water are very weak.37 However, the surface of our DNDs was prepared to have carboxyl groups by strong oxidative acid treatment;38 carboxylic acids are strong hydrogen bond acceptors in water. As a result, the interfacial thermal conductance of DNDs may be higher due to the very hydrophilic surface exposed to the aqueous milieu.