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

Abstract

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 µm, and the relative time intervals 0.5÷5tc and (5÷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 μm) provide longer times to attain a thermal equilibrium and, thus, the better precision of measurements of thermal diffusivity.

Composition of the Cytochrome c Complex with Cardiolipin by Thermal Lens Spectrometry

“…For thermal lens measurements of dispersed media, the spatial distribution of the thermal properties of the medium is important. It has previously been shown that for dispersed systems, such as aqueous dispersions of fullerenes and nanodiamonds [ 31 , 32 ], as well as complex compounds and proteins (hemoglobin) [ 33 ], there is a decrease in the thermal lens signal compared to true solutions with the same solvent and the same light absorption. This is because the absorption of laser radiation leads to rapid local overheating of dispersed particles, but the temperature distribution profile caused by thermooptical phenomena throughout the sample has a smaller amplitude due to slower heat transfer from the heated dispersed particles to the dispersion medium ( Figure 3 a,b).…”

“…This is because the absorption of laser radiation leads to rapid local overheating of dispersed particles, but the temperature distribution profile caused by thermooptical phenomena throughout the sample has a smaller amplitude due to slower heat transfer from the heated dispersed particles to the dispersion medium ( Figure 3 a,b). If we compare two dispersed systems, with an increase in particle size (e.g., the formation of clusters of nanodiamonds [ 32 ] or with the formation of an albumin complex with an iron(II) complex with 1,10-phenanthroline [ 33 ]), the signal decreases.…”

“…However, it should be noted that the situation with the formation of the Cyt c –TOCL complex is different from the cases of the formation of the albumin–ferroin complex or clusters in nanodiamond or fullerene solutions [ 33 ]. While in those cases we have chromophore particles of different sizes, here we have either a standalone cytochrome c molecule, which absorbs the photon ( Figure 3 b) or a Cyt c molecule surrounded by a shell of heat-insulating entities, hydrophobic fatty-acid chains of cardiolipin ( Figure 3 c).…”

“…For thermal lens experiments, a coaxial two-laser spectrometer was used [ 33 , 64 ]. The thermal lens is induced in the cell by the radiation from an Innova 90-6 argon ion laser (Coherent, Palo Alto, CA, USA) with generation wavelength of 514.5 nm (TEM 00 mode; maximum output radiation power, 1.5 W).…”

“…A measurement of the transient (time-resolved) thermal lens signal ϑ ( t ) for an excitation on/off cycle of a modulated continuous-wave (cw) excitation beam was calculated as a relative change in the probe beam intensity at a far-field detector plane at the moment t [ 24 , 33 ] where is the intensity of the probe beam at the photodetector plane in the central part of the beam at the time t = 0, and is the intensity of the probe beam at the time t from the excitation on/off cycle start. The signal is determined by where P e is the excitation laser power, is the linear light absorption coefficient of the sample, l is sample path length, ε is molar absorptivity, and c is molar concentration of the absorbing substance in the sample.…”