Listening to Chemical Materials: Determination of the Photophysical Parameters by Photoacoustic Spectroscopy

Recently, photoacoustic (PA) spectroscopy has emerged as a valuable tool for the study of various kinds of materials. Herein, we present the results of PA spectral studies of chemical materials. First, the PA study on luminescent materials in condensed states is reported. Combining with the luminescence technique, the energy transfer efficiency and the intrinsic luminescence quantum yield are determined for a europium (III) complex in the glassy state, smectic A phase, and the isotropic liquid. Second, neodymium (III) compounds with l-glycine, l-phenylalanine, and l-tryptophan are synthesized and their PA spectra are reported. The nephelauxetic ratio and Sinha parameter are calculated based on the PA spectra. The environmental effect on the f?Cf transitions of the neodymium(III) ion is also studied.     

Photoacoustic Spectroscopy Analysis of Traditional Chinese Medicine

Chinese medicine is a historic cultural legacy of China. It has made a significant contribution to medicine and healthcare for generations. The development of Chinese herbal medicine analysis is emphasized by the Chinese pharmaceutical industry. This study has carried out the experimental analysis of ten kinds of Chinese herbal powder including Fritillaria powder, etc., based on the photoacoustic spectroscopy (PAS) method. First, a photoacoustic spectroscopy system was designed and constructed, especially a highly sensitive solid photoacoustic cell was established. Second, the experimental setup was verified through the characteristic emission spectrum of the light source, obtained by using carbon as a sample in the photoacoustic cell. Finally, as the photoacoustic spectroscopy analysis of Fritillaria, etc., was completed, the specificity of the Chinese herb medicine analysis was verified. This study shows that the PAS can provide a valid, highly sensitive analytical method for the specificity of Chinese herb medicine without preparing and damaging samples.     

Water Transport Monitoring in Calcium Carbonate Stones by Photoacoustic Spectroscopy

Calcium carbonate is the most abundant inorganic material, and it was used to build the ancient Mayan monuments in the peninsula of Yucatán, México. One of the most important challenges that these structures have to confront is related to water and its transport inside the stone that induces serious deterioration. In this study, the photoacoustic (PA) technique is used to monitor the water permeability of two kinds of solid and compacted powdered stones. The analysis of water permeability in stones is performed using a modified Rosencwaig PA cell. When the stones are in contact with the water reservoir, the PA signal amplitude is observed to decay gradually due to the progressive wetting of the sample. Based on this type of experiment, the water diffusion coefficients as well as the time evolution of the thermal effusivity are obtained.     

Measurement of the Optical Properties of Gold Colloids by Photoacoustic Spectroscopy

The particular features of gold have generated widespread interest for applications in different areas of science and technology. Notably, gold nanoparticles can be prepared with different sizes and forms and can be easily functionalized with a wide range of ligands. Developing effective experimental techniques to characterize such properties is thus important. In this work, photoacoustic spectroscopy was used to explore the relationship between the nanoparticle size and the optical absorption coefficient (at 405 nm and 532 nm) of gold colloid solutions, according to the Beer–Lambert’s law. A correlation between this optical parameter and the nanoparticle size was found. In addition, for comparison purposes, conventional UV–visible spectroscopy was used for measuring the absorbance at these two wavelengths. Very good agreement was obtained between the optical properties measured by the two methodologies at 405 nm. However, large discrepancies were obtained when measurements were performed at 532 nm. At the latter wavelength, the extent of radiation dispersion is too large for the Beer–Lambert’s law to be valid when the conventional spectroscopy technique is used. Unlike the UV–visible spectroscopy, the photoacoustic method is minimally affected by radiation dispersion effects. Thus, the photoacoustic method presents fewer limitations in that regard in characterizing the optical properties of metallic colloidal suspensions.     

Characterization of Maize Grains with Different Pigmentation Investigated by Photoacoustic Spectroscopy

A knowledge of grains’ optical parameters is of great relevance in the maize grain technology practice. Such parameters provide information about its absorption and reflectance, which in turn is related to its color. In the dough and tortilla industries, it is important to characterize this attribute of the corn kernel, as it is one of the attributes that directly affects the quality of the food product. Thus, it is important to have techniques that contribute to the characterization of this raw material. It is traditionally characterized by conventional methods, which usually destroy the grain and involve a laborious preparation of material plus they are expensive. The objective of this study was to determine the optical absorption coefficient \(\beta \) for maize grains (Zea mays L.) with different pigmentations by means of photoacoustic spectroscopy (PAS). The genotype A had bluish coloration and genotype B had yellowish coloration. In addition, the photoacoustic signal obtained by two methods was analyzed mathematically: the standard deviation and the first derivative; both results were compared (Fig. 1). In combination with mathematical analysis, PAS may be considered as a potential diagnostic tool for the characterization of the grains. Fig. 1

PAS experimental setup     

Study on the Phase Transition of Lanthanide-Doped Titania Nanoparticles by Photoacoustic Spectroscopy

In this study, fairly uniform anatase TiO₂ spheres doped with neodymium (III) have been prepared. It is found that the impregnation of neodymium (III) can remarkably inhibit the anatase?Crutile transition. In order to analyze the correlation of the dopant effect with the local environment of neodymium (III) ions, photoacoustic spectra of the f?Cf transitions of neodymium (III) have been studied. The variation of the nephelauxetic parameters can be attributed to the substitutional neodymium (III) ions at low doping level (???1?mol%), and to both the substitutional and interstitial neodymium (III) ions at high doping level (1?mol% to 8?mol%). The phase transformation mechanism of the samples has been interpreted based on the X-ray diffraction and transmission electron microscopy results. The nephelauxetic parameters are the same for all the samples calcined at 1150?°C. This is due to the segregation of neodymium (III) ions in the samples as a separated phase.     

Study of Lanthanide Complexes with BTFA in Silica Gels by Photoacoustic Spectroscopy

In this work, lanthanide \(\beta \)-diketonate complexes Ln(btfa)\({}_{3} \cdot 2\hbox {H}_{2}\)O (Ln\(^{3+}\): Eu\(^{3+}\), Sm\(^{3+ }\), and Tb\(^{3+}\); btfa: 4,4,4-trifluoro-l-phenyl-1,3-butanedione) were incorporated into silica gels by a sol–gel method. Photoacoustic (PA) spectra of these complex-doped silica samples were measured and studied. The PA intensity of the \(\beta \)-diketonate ligand is nearly the same for lanthanide complexes in wet gels. After heat treatment at 150 \(^{\circ }\)C, however, the PA intensity of the ligand increases for Eu\(^{3+}\), Sm\(^{3+}\), and Tb\(^{3+}\) complexes in silica gels, respectively. Different PA intensities of the samples are interpreted by comparison with their luminescence spectra. The luminescence result is consistent with the PA spectra. The result indicates that lanthanide \(\beta \)-diketonate complexes cannot be formed in silica gels without a suitable heat treatment. Moreover, the relaxation process model is proposed based on the PA and luminescence results.     

Photoacoustic Analysis of Blue Corn Pigments in Nixtamalized Flours

Anthocyanins are natural pigments with antioxidant properties, and recently they have received more attention because these pigments are consumed in nutritional diets with therapeutic effects. In addition, anthocyanins are important in the treatment of diseases caused by oxidation of free radicals in live systems, which could be the cause of chronic diseases, like cancer. Anthocyanins, found in flowers and some fruits, are also present in blue Mexican corn (Zea mays L.). In this work, photoacoustic spectroscopic, chemical and spectrophotometric analyses of nixtamalized blue corn flours are presented. Different Ca(OH)₂ concentrations were used in the fractioned nixtamalization process, and the total anthocyanin concentrations of these flours were obtained by chemical extraction of these pigments and compared with relative intensity of optical absorption obtained by photoacoustic spectroscopy.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

Photoacoustic Spectroscopy of Candida albicans Treated with Methylene Blue

In the present work the phototoxic effect of methylene blue (MB) on Candida albicans cultures was studied using the photoacoustic spectroscopy (PAS) technique. An aliquot of 50???L of C. albicans suspension??strain ATCC 10-231??was incubated with 50???L of MB solution (0.5?mg/mL), at room temperature. After the proper incubation time, a colony forming unit (CFU) with approximately 3?mm diameter was chosen in each plate. The CFU selected was irradiated using an InGaAlP laser during 20 s. After irradiation and new incubation, the CFUs were collected and stored at ?70?°C, until spectroscopy analysis. The spectroscopy analysis was performed using an open PAS setup. The study was conducted in different groups: (1) control (non-treated); (2) irradiated with laser light; (3) treated with MB (non-irradiated); and (4) treated with MB and irradiated with laser light. The PAS measurements were performed on C. albicans in a sterile physiological solution. The measurements indicate that the presence of MB and irradiation promotes a change in the redox state of the cells to the reduced state. The absorption spectrum after photodynamic therapy (PDT) was observed 12 h and 36 h later. It was inferred that PDT can be related to structural changes in cytochrome molecules after 36 h. It is concluded that MB can be an efficient photosensitizer in C. albicans through modification of the cytochrome molecule affecting the cell metabolism.     

Pulsed Photoacoustic Spectroscopy of I2 and NO2

A novel approach based on the use of ?? = 532?nm pulsed radiation obtained by the second-harmonic generation from the Q-switched Nd: YAG laser and its application for excitation of acoustic modes in a specially designed multi-resonant high-Q factor photoacoustic (PA) cell is reported. The cell is employed for time-domain spectroscopic studies of gaseous NO₂ and I₂ molecules. The time-resolved spectra of NO₂ and I₂ were recorded up to 8000?Hz and 26?000?Hz, respectively. It has been demonstrated for the first time that excited cavity modes which correspond to different transition lines of the I₂ molecules show different behaviors under different pressure and input laser energy. The lowest limit of detection attained with the proposed PA system for NO₂ buffered in realistic air was 14.6?ppbV.     

Authentication of Concentrated Orange Essential Oils Using Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PS) was used to study the thermal diffusivity and its relation with the composition in folded (concentrated) cold-pressed Mexican orange essential oils. A linear relation between the amplitude (on a semi-log scale) and phase, as functions of the sample thickness, for PS was obtained through a theoretical model to fit the experimental data for thermal-diffusivity measurements in concentrated orange essential oils. Experimental results showed a linear increase in thermal-diffusivity values with the folding degree: 5-fold, 10-fold, 20-fold, and 35-fold due to a decrease in terpenes (mainly D-limonene) related with the folding process that can be correlated with the thermal diffusivity of the orange essential oils. The obtained values in this study and those previously reported (see Int. J. Thermophys. 32, 1066, 2011) showed the possibility of using this thermal property to make distinctions between citrus oils obtained by different extraction processes and also between concentrated citrus oils. This provides the viability of a new complementary method for this purpose, contrasting with the use of density and refraction index, physical properties commonly used in the authentication of citrus essential oils.     

Resonant Absorption in GaAs-Based Nanowires by Means of Photo-Acoustic Spectroscopy

Semiconductor nanowires made of high refractive index materials can couple the incoming light to specific waveguide modes that offer resonant absorption enhancement under the bandgap wavelength, essential for light harvesting, lasing and detection applications. Moreover, the non-trivial ellipticity of such modes can offer near field interactions with chiral molecules, governed by near chiral field. These modes are therefore very important to detect. Here, we present the photo-acoustic spectroscopy as a low-cost, reliable, sensitive and scattering-free tool to measure the spectral position and absorption efficiency of these modes. The investigated samples are hexagonal nanowires with GaAs core; the fabrication by means of lithography-free molecular beam epitaxy provides controllable and uniform dimensions that allow for the excitation of the fundamental resonant mode around 800 nm. We show that the modulation frequency increase leads to the discrimination of the resonant mode absorption from the overall absorption of the substrate. As the experimental data are in great agreement with numerical simulations, the design can be optimized and followed by photo-acoustic characterization for a specific application.     

Investigation of Micromechanical Structures by Photoacoustic Elastic Bending Method

Photoacoustic (PA) and photothermal (PT) science and technology extensively developed new methods for the investigation of micro (nano)-mechanical structures. PA and PT effects can be important also as driven mechanisms for optically excited micromechanical structures. The photoacoustic elastic bending method (PA-EBM) is based on the optical excitation of the micromechanical structure and detection of the acoustic response (PA signal) with a very sensitive PA detection system. The experimental PA elastic bending signals of the whole micromechanical structure were measured by using a special constructed PA cell (the gas-microphone detection technique with transmission configuration). The PA amplitude and phase spectra were measured, as a function of the modulation frequency in a frequency range from 20?Hz to 20?000?Hz, for different samples (Si chip with square membrane). The electronic and thermal elastic PA effects (electronic deformation and thermoelastic mechanisms of elastic wave generation) in a Si simply supported rectangular plate (3D geometry), photogenerated by a uniform and intensity-modulated optical beam, were studied. The theoretical model for the PA elastic bending frequency distribution by using the Green function method was given. The amplitude and phase PA signals were calculated and analyzed, including the thermalization and surface and volume recombination heat sources. The theoretical results were compared with experimental data.     

用光声法测量陶瓷材料的热参量

以固体光声效应的RG理论为依据,用光声法测量了压电陶瓷材料PZT的热参量。实验结果表明:用此方法只需要测出一个量就能同时确定PZT的α、k和C的值,并且实验设备简单,速度快,是一种很有前途的测热参量方法。     

Study of Silicon Cantilevers by the Photoacoustic Elastic Bending Method

Rectangular silicon cantilevers are studied by the photoacoustic (PA) elastic bending method. Experimental signals versus modulation frequency of the excitation optical beam are measured and analyzed in a frequency range from 20 Hz to 50 000 Hz. The procedure for experimental signal correction to eliminate the frequency characteristics of the measuring system is given. The corrected experimental signal shows a good correlation with theoretically calculated PA signal at frequencies below 32 000 Hz. The corrected experimental PA elastic bending signals for cantilevers with different thicknesses are analyzed. The experimental results allow identifying the resonant frequency (the first resonant mode) of the cantilever vibrations. These values are in good agreement with the theoretically computed values. A theoretical model of the optically excited Si cantilever is derived, taking into account plasmaelastic, thermoelastic, and thermodiffusion mechanisms. Dynamic relations for the amplitude and phase of electronic and thermal elastic vibrations in optically excited cantilevers are derived. The theoretical model is compared to the experimental results.     

Hypersensitive Transition of Nd(III) Complexes in Liquids Detected by Pulsed-Laser-Induced Photoacoustic Spectroscopy

Laser-induced photoacoustic (PA) spectroscopy for the spectral measurements of extremely weak absorption such as a forbidden transition of lanthanide ions in liquids has been established. In spectroscopy, a pulsed Nd:YAG laser connected with a MOPO series optical parametric oscillator which emits a broad spectrum covering UV and visible regions is used as the excitation source, and the induced PA signals are detected by an optimized PA piezoelectric transducer. The absorption spectra of trivalent lanthanide ions ( $\text{ Pr}^{3+}, \text{ Ho}^{3+}$ , and $\text{ Nd}^{3+})$ in aqueous solutions have been obtained by the detection system with a detection-limit absorbance of $1.3\times 10^{-5}\,\text{ cm}^{-1}$ at room temperature. In addition, the effects of different binding environments on the band shapes and oscillator strengths of the hypersensitive transitions of $\text{ Nd}^{3+}$ ions, i.e., $\text{ Nd}(\text{ CH}_{3}\text{ COO})_{3}$ $\cdot $ $\text{ H}_{2}\text{ O}$ dissolved in $0.1\,{\text{ mol}} \cdot \text{ l}^{-1}$ acetic acid and $\text{ Nd(3-butanedione)}_{3}{\cdot } 2\text{ H}_{2} \text{ O}$ dissolved in triglycol compared with $\text{ NdCl}_{3}$ in $0.1\,{\text{ mol}}\cdot \text{ l}^{-1}$ hydrochloric acid, are observed. The results show that the chemical environment around the lanthanide ions has great impact on 4f–4f transitions, which is rationalized as the impact in terms of ligand (or solvent) special structures and coordination properties.