Photothermal and Heat-Transfer Properties of Aqueous Detonation Nanodiamonds by Photothermal Microscopy and Transient Spectroscopy

Characteristic size and heat-transfer parameters of aqueous detonation-nanodiamond dispersions of various brands as carbon-based nanomaterials for nanofluidic tasks were assessed by confocal photothermal microscopy and transient (time-resolved) photothermal-lens modalities. It was shown that only the part of the transient thermal-lens heating curve of the dispersed sample with light-absorbing particles when the photothermally induced thermal field becomes homogeneous should be used to evaluate the bulk thermal diffusivity. For nanodiamond concentrations of 1–4 mg/mL, a 1–5% increase in thermal diffusivity and thermal conductivity compared with water is observed. A slowdown in the dissipation of the thermal lens effect due to prolonged heat accumulation by nanodiamonds is shown, which is confirmed by significant photothermal signals from nanodiamond clusters according to confocal photothermal microscopy. The size estimation of separate nanodiamond clusters in solution by nonlinear far-field confocal photothermal microscopy is shown; for the selected nanodiamond brands, photothermal microscopy reveals local signals (200–1000 nm) from nanodiamonds that can be assigned to aggregates of ca. 40–70 nm correlated with calculations from the Gibbs–Kelvin equation by differential scanning calorimetry. The discrimination of nonaggregated and aggregated nanodiamonds by photothermal microscopy is demonstrated.