Measurement Precision and Thermal and Absorption Properties of Nanostructures in Aqueous Solutions by Transient and Steady-State Thermal-Lens Spectrometry

A simultaneous steady-state and transient photothermal-lens modality was used for both the thermal and optical parameters of aqueous dispersed systems (carbon and silica nanoparticles, metal iodides, surfactants, heme proteins, albumin, and their complexes). Heat-transfer parameters (thermal diffusivity and thermal effusivity), the temperature gradient of the refractive index, light absorption, and concentration parameters were assessed. To simultaneously measure thermal and optical parameters, the time scale of thermal lensing (characteristic time, tc) should correspond to an excitation beam size of 60-300 mu m, and the relative time intervals 0.5 divided by 5tc and (5 divided by 20)tc should be selected for transient and steady-state measurements, respectively. Dual-beam thermal-lens spectrometers in a mode-mismatched optical schematic at various excitation wavelengths were built. The spectrometers implement back-synchronized detection, providing different measurement conditions for the heating and cooling parts of the thermal-lens cycle. By varying the measurement parameters depending on the dispersed system, the conditions providing the suitable precision (replicability, repeatability, and reproducibility) of thermal-lens measurements were found; setups with a broad excitation beam (waist size, 150 and 300 mu m) provide longer times to attain a thermal equilibrium and, thus, the better precision of measurements of thermal diffusivity.