The multispectral modality and technique for optically dense samples of optoacoustic spectroscopy were applied to measure spectra and high absorbances of concentrated aqueous dispersions of undoped nanodiamonds. The data from optoacoustic and optical transmission measurements and DSC data of the mean particle size by the Gibbs–Kelvin equation are compared to estimate the difference in composition of various nanodiamond trademarks. Optoacoustic spectra confirm the contribution of surface dimer chains into the absorption of nanodiamonds in the long wavelength range. Optoacoustic and conventional absorption spectra of aqueous solutions of nanodiamond fractions after centrifugation (15300g) and ultracentrifugation (130000g) revealed a separation of a highly absorbing non-diamond sp2 phase. The two-step separation by ultracentrifugation followed by extra centrifugation made it possible to isolate a highly absorbing and soluble nanodiamond phase with the particle size of 3.6 nm, showing a change in spectra compared to the starting nanodiamond material.

An approach is proposed for assessment of the thermal properties of aqueous pristine fullerene C60 and C70 dispersions (AFDs) at the level of 10–7–10–5 mol L–1 by photothermal (thermal-lens) spectroscopy for their application in medicine and technology. Along with relevant size-characterization techniquesdifferential scanning calorimetry (DSC) with the Gibbs–Kelvin equation and dynamic light scattering (DLS) techniquesthis approach provides an estimation of the size-dependent thermal properties of disperse solutionsthermal diffusivity, thermal effusivity [thermal inertia], and thermal conductivity. The values for AFDs under the conditions of the attained thermal equilibrium show good precision, and the cluster size estimations agree with the reference methods. The reconstruction of the thermal-lens characteristic time over the course of the blooming of the thermal-lens effect reveals a short increase in the apparent thermal diffusivity. This is accounted for by nonequilibrium heat transfer within fullerene clusters upon initial laser heating, which is supported by the independent estimations from widespread methods like high-resolution transmission electron microscopy, DLS, and DSC. Thermophysical parameters of the disperse phase estimated from transient thermal lensing are as follows: thermal diffusivity, 1.6–2.0 × 10–7 m2 s–1 depending on fullerene concentration and up to 3.5 × 10–7 m2 s–1, more than 2-fold higher than for water; thermal effusivity, 6.7 × 102 J m–2 K–1 s–1/2, three times lower than for water.

Graphical abstract

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.

In recent years, the demand for more efficient cooling circuits has resulted in active studies of nanofluids, two-component liquids consisting of a base fluid, and dispersed nanoparticles with high thermal conductivity. From the viewpoint of both physics and chemistry, nanofluids are systems that require the characterization of many interconnected thermal and chemical properties. This perspective article sums up the state of the art and recent trends in the development and applications of nanofluids and especially carbon nanofluids. A focus of the paper is the possibilities of photothermal and photoacoustic methods—as techniques combining molecular spectroscopy and thermal characterization—for the assessment of thermal conductivity and thermal diffusivity of nanofluids. The possibilities of photothermal spectroscopy for wider characterization of nanofluids and related materials are discussed and compared with other techniques. As nanofluids are one of the examples of complex objects dedicated to photothermal spectroscopy, more general outlooks of phototermics are also discussed.