Photoacoustic Absorption Spectroscopy Research Articles

Mass Accommodation Coefficients of Water on Organics from Complementary Photoacoustic and Light Scattering Measurements on Laser-Trapped Droplets

The mass accommodation coefficient, $\alpha_M$, describes evaporation and condensation kinetics at the liquid-vapor interface. In spite of numerous experimental efforts, reliable values of $\alpha_M$ are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296K, and to decrease by more than an order of magnitude when the temperature increases to 309K. The experimentally observed temperature dependence of $\alpha_M$ shows an Arrheni...

Specifying the light-absorbing properties of aerosol particles in fresh snow samples, collected at the Environmental Research Station Schneefernerhaus (UFS), Zugspitze

Abstract. Atmospheric aerosol particles like mineral dust, volcanic ash and combustion particles can reduce Earth's snow and ice albedo considerably even by very small amounts of deposited particle mass. In this study, a new laboratory method is applied to measure the spectral light absorption coefficient of airborne particles that are released from fresh snow samples by an efficient nebulizing system. Three-wavelength photoacoustic absorption spectroscopy is combined with refractory black carbon (BC) mass analysis to determine the snow mass-specific and BC mass-specific absorption cross sections. Fullerene soot in water suspensions are used for the characterization of the method and for the determination of the mass-specific absorption cross section of this BC reference material. The analysis of 31 snow samples collected after fresh snowfall events at a high-altitude Alpine research station reveals a significant discrepancy between the measured snow mass-specific absorption cross section and the cross section that is expected from the BC mass data, indicating that non-BC light-absorbing particles are present in the snow. Mineral dust and brown carbon (BrC) are identified as possible candidates for the non-BC particle mass based on the wavelength dependence of the measured absorption. For one sample this result is confirmed by environmental scanning electron microscopy and by single-particle fluorescence measurements, which both indicate a high fraction of biogenic and organic particle mass in the sample.

Decontamination and disinfection of wastewater by photocatalysis under UV/visible light using nano-catalysts based on Ca-doped ZnO

Nano-catalysts based on ZnO-Ca x% (with x = 0, 0.1, 0.3, 0.5, 0.7, and 1.0 mol % Ca2+) were synthesized with a bio-friendly adaptation of the sol-gel method using gelatin as template. These materials were characterized by Fourier Transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Micro-Raman, transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 physisorption, photoacoustic absorption spectroscopy (PAS), and photoluminescence spectroscopy (PL). The Raman results indicated that the signal, attributed to an E1(LO) mode at 580 cm−1, was characteristic of oxygen vacancies that decreased with the increased Ca2+ content in doped oxides. This agreed with the PL results, which showed that the green emission centered at 510 nm and attributable to structural defects in ZnO decreased for Ca-doped ZnO. Our oxides are constituted by nanoparticles with rod-like and spherical morphologies. All the nano-catalysts exhibited the band gap characteristics of semiconductor materials around 3.0 eV. ZnO-Ca 1.0% exhibited the best photocatalytic performance for degradation of Methyl Orange (MO) model dye, degrading about 82% after 240 min of UV–Vis irradiation at pH 7.0. The reaction mechanism was influenced by hydroxyl (OH) and superoxide (O2-) radicals and mainly by active holes (h+). This doped oxide also demonstrated efficiency in wastewater disinfection against heterotrophic bacteria and total coliforms, exhibiting a potential use as an antimicrobial agent for the treatment of hospital wastewater. Furthermore, our nanoparticles did not show significant cytotoxic effects on L929 fibroblast cells.

Synthesis and properties of nanostructured GeSb4Te7 prepared by mechanical alloying

A nanostructured GeSb4Te7 (GST147) phase was produced by mechanical alloying (MA), starting from a mixture with nominal GeSb2Te4 composition of high-purity elemental Ge, Sb and Te powders. The effect of milling on z-coordinates of Wyckoff sites 2c and 2d occupied by the Sb and Te atoms was studied using an approach combining the Rietveld method and pair distribution function analysis. The thermal stability of GST147 and the Raman active modes of GST147 were investigated. The results of photoacoustic absorption spectroscopy show that the thermal diffusivity of nanostructured GST147 produced by MA is similar that of its bulk counterpart.

Structural, Thermal, Optical, and Photoacoustic Study of Mechanically Alloyed Nanocrystalline SnTe

A nanostructured SnTe phase was produced by mechanical alloying after 2 h of milling. Part of the as-milled powder was annealed and its X-ray diffraction (XRD) pattern was recorded. The XRD patterns of the as-milled and annealed samples were refined using the Rietveld method. After annealing, partial decomposition of the SnTe phase was observed and corroborated by estimating the mean crystallite size using a Willianson-Hall plot. The Cowley-Warren parameter αSnTeCWfor the first coordination shell was calculated, showing a preference for homopolar pairs. This preference is consistent with the partial decomposition observed. According to the optical absorbance spectra, the band gap energy is inversely proportional to crystallite size, following a decaying exponential function. From the photoacoustic absorption spectroscopy measurements, the thermal diffusivity parameter and the transport properties of as-milled and annealed SnTe powder were determined.

All-Fiber Photoacoustic Absorption Spectroscopy: Detecting Small Amounts of Dissolved Water in Jet Fuel

Small concentrations of dissolved water in hydrocarbons such as lubricating engine oils and jet fuels prove detrimental to engine performance. In particular, high altitude operation can cause dissolved water to freeze, thus disrupting fuel flow and ignition temperatures in the engine. A new method to determine the concentration of dissolved water is explored using a high energy laser to excite the dissolved water in the fluid sample. Past methods include rigorous filtration systems or humidity tests that are time consuming and sometimes extremely temperature sensitive. Using a laser is a safe and non-invasive method that takes a matter of minutes for data collection. This Physics thesis is currently in the stage of establishing the detection of water excited by the high power laser. In the coming weeks, this project will progress to analyzing the collected data to calculate the amount of dissolved water in the fluid samples. The time efficiency and precision of this method provides an opportunity to develop a device that may be extended to industry for engine damage mitigation.

Effects of Al3+ concentration on the optical, structural, photocatalytic and cytotoxic properties of Al-doped ZnO

ZnO-Al x% nanoparticles (NPs), with x = 0, 1, 3, 5 or 10 mol% Al3+, were synthesized by a modified sol-gel method. The effects of Al3+ content on the structure, morphology and optical properties of Al-doped ZnO were investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, photoacoustic absorption spectroscopy (PAS), photoluminescence spectroscopy (PL) and specific surface area determination. A single crystalline phase characteristic of ZnO with wurtzite structure was detected for all synthesized oxides. With increasing dopant content, the average crystallite size of the NPs decreased from 25 to 7 nm. ZnO-Al x% NPs exhibited enhanced optical properties with the absorption slightly shifted to the visible region and with the band gap energy (Eg) decreased from 3.12 (undoped ZnO) to 2.95 eV (ZnO-Al 3%). Undoped ZnO showed intense PL above 430 nm, which was considerably decreased with the insertion of Al3+. Despite the increase of the surface area with Al3+ doping of ZnO NPs, its catalytic activity in the decomposition reaction of Congo red showed a decrease with doping. Research has shown that ZnO NPs can selectively target and kill cancer cells and that its cytotoxicity is dependent of the concentration. The doping can be a method of shape the cytotoxicity of ZnO NPs and their optical and photocatalytic properties. In-vitro cytotoxicity studies showed that both undoped ZnO and Al-doped ZnO NPs synthesized were highly cytotoxic for fibroblast cells, suggesting that these nanomaterials could be promising anticancer agents.

Experimental and ab initio studies of nanostructured Cu2Sb produced by mechanical alloying

Mechanical alloying of elemental Cu and Sb powders yielded nanostructured Cu2Sb. Structural, thermal and optical properties were studied using X-ray diffraction, photoacoustic and Raman spectroscopies. The positions of the expected Raman active modes were determined by first-principles density functional theory and compared with Raman scattering measurements. From photoacoustic absorption spectroscopy, thermal diffusivity and others transport properties were obtained.

Effects of Azole Drugs on Conformational Dynamics and Energetics Associated with Diatomic Ligand Photorelease and Migration within Bacterial Flavohemoglobins

Bacterial flavohemoglobins (fHbs) are a widely distributed class of proteins which are though to protect microbes from nitrosative stress due to the immune response of a host organism via nitric oxide dioxygenase (NOD) activity. There has been interest lately in developing fHbs as targets for antibiotics due to their ubiquity in bacteria and the discovery of inhibition of NOD activity of fHbs by azole-based drugs which bind with high affinity (<12 μM). To determine how azole drugs affect fHbs we have used photoacoustic calorimetry (PAC) and transient absorption spectroscopy (TA) to calculate enthalpy and volume changes (ΔH and ΔV) and kinetic parameters for CO photorelease and rebinding to fHbs from Cupriavidus necator (FHP) and Staphylococcus aureus (HMPSa) in the presence and absence of the drugs ketoconazole and miconazole. We observe that binding of drugs to fHbs results in a more exothermic ΔH for CO photorelease and faster bimolecular rebinding of CO. We also use classical molecular dynamics (cMD) to characterize ligand migration pathways present in FHP. We have conducted cMD simulations of CO-FHP WT and FHP WT containing free CO molecules using a locally enhanced sampling (LES) algorithm. We have carried out a 10 ns simulation of CO-FHP WT and a 30 ns simulation of the LES-enabled CO:FHP WT. We observe dissociation of 18 of 20 LES-enhanced CO molecules from the distal heme pocket. The primary escape pathways are between the CD-loop and heme-propionate groups and through a pathway exiting the protein matrix near the N-terminal end of the A-helix. These pathways are close to the locations of ketoconazole and miconazole binding predicted by AutoDock Vina, suggesting that drug binding inhibits NOD activity by closing migration pathways for O2 and NO.

Structural, thermal, and photoacoustic study of nanocrystalline Cr3Ge produced by mechanical alloying

A thermodynamic analysis of the Cr-Ge system suggested that it was possible to produce a nanostructured Cr3Ge phase by mechanical alloying. The same analysis showed that, due to low activation energies, Cr-poor crystalline and/or amorphous alloy could also be formed. In fact, when the experiment was performed, Cr11Ge19 and amorphous phases were present for small milling times. For milling times larger than 15 h these additional phases decomposed and only the nanostructured Cr3Ge phase remained up to the highest milling time used (32 h). From the differential scanning calorimetry measurements, the Avrami exponent n was obtained, indicating that the nucleation and growth of the nanostructured Cr3Ge phase may be restricted to one or two dimensions, where the Cr and Ge atoms diffuse along the surface and grain boundaries. In addition, contributions from three-dimensional diffusion with a constant nucleation rate may be present. The thermal diffusivity of the nanostructured Cr3Ge phase was determined by photoacoustic absorption spectroscopy measurements.

Short-lived species detection of nitrous acid by external-cavity quantum cascade laser based quartz-enhanced photoacoustic absorption spectroscopy

Spectroscopic detection of short-lived gaseous nitrous acid (HONO) at 1254.85 cm−1 was realized by off-beam coupled quartz-enhanced photoacoustic spectroscopy (QEPAS) in conjunction with an external cavity quantum cascade lasers (EC-QCL). High sensitivity monitoring of HONO was performed within a very small gas-sample volume (of ∼40 mm3) allowing a significant reduction (of about 4 orders of magnitude) of air sampling residence time which is highly desired for accurate quantification of chemically reactive short-lived species. Calibration of the developed QEPAS-based HONO sensor was carried out by means of lab-generated HONO samples whose concentrations were determined by direct absorption spectroscopy involving a ∼109.5 m multipass cell and a distributed feedback QCL. A minimum detection limit (MDL) of 66 ppbv (1 σ) HONO was achieved at 70 mbar using a laser output power of 50 mW and 1 s integration time, which corresponded to a normalized noise equivalent absorption coefficient of 3.6 × 10−8 cm−1 W/Hz1/2. This MDL was down to 7 ppbv at the optimal integration time of 150 s. The corresponding 1σ minimum detected absorption coefficient is ∼1.1 × 10−7 cm−1 (MDL ∼ 3 ppbv) in 1 s and ∼1.1 × 10−8 cm−1 (MDL ∼ 330 pptv) in 150 s, respectively, with 1 W laser power.

Structural, optical and photoacoustic study of milled and melted Bi2Se3

Powder Bi2Se3 was produced through Mechanical Alloying (MA) and Melting Technique (MT) routes. Structural, optical and photoacoustic properties were evaluated by X-ray diffraction (XRD), Raman Spectroscopy (RS) and Photoacoustic Absorption Spectroscopy (PAS). Minority Bi2O2Se phase was nucleated in both samples. PAS measurements showed a thermal diffusivity reduction of 40% for Bi2Se3 produced through MA when compared with Bi2Se3 produced by MT. In addition, thermal diffusivity of Bi2Se3 obtained by MA and MT was a new report in literature.

Co-doped ZnO nanoparticles synthesized by an adapted sol–gel method: effects on the structural, optical, photocatalytic and antibacterial properties

Pure and Co-doped ZnO nanoparticles were synthesized with different cobalt levels (1–10 mol%) via adapted sol–gel method using water as solvent and characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and photoacoustic absorption spectroscopy. The results showed that all the samples have hexagonal wurtzite structure, with no evidence of any secondary phases until 10 mol% of the dopant. The average crystallite size of the samples was in the range of 25–50 nm, do not showing significant differences with the increase of the dopant level. However, the band gap energy of the nanoparticles decreases from 2.98 eV (pure ZnO) to 1.95 eV (10 mol% of Co). The photocatalytic activity of the samples was evaluated on the removal of methylene blue under visible light irradiation, which revealed an efficiency reduction by Co-doping ZnO. The antibacterial property was carried out indicating activity of the prepared samples against gram-positive bacteria.

Atmospheric CH_4 and N_2O measurements near Greater Houston area landfills using a QCL-based QEPAS sensor system during DISCOVER-AQ 2013

A quartz-enhanced photoacoustic absorption spectroscopy (QEPAS)-based gas sensor was developed for methane (CH₄) and nitrous-oxide (N₂O) detection. The QEPAS-based sensor was installed in a mobile laboratory operated by Aerodyne Research, Inc. to perform atmospheric CH₄ and N₂O detection around two urban waste-disposal sites located in the northeastern part of the Greater Houston area, during DISCOVER-AQ, a NASA Earth Venture during September 2013. A continuous wave, thermoelectrically cooled, 158 mW distributed feedback quantum cascade laser emitting at 7.83 μm was used as the excitation source in the QEPAS gas sensor system. Compared to typical ambient atmospheric mixing ratios of CH₄ and N₂O of 1.8 ppmv and 323 ppbv, respectively, significant increases in mixing ratios were observed when the mobile laboratory was circling two waste-disposal sites in Harris County and when waste disposal trucks were encountered.

Volume and Enthalpy Profile of Surfactant-Induced Protein Folding in Ferrous Cytochrome-C, using Co Photo-Dissociation Methods

Numerous studies have shown that under denaturing conditions, reduction of the Cytochrome-c (Cyt-c) heme group in the presence of carbon monoxide results in a disruption of the axial heme Methionine-80 bond ultimately unfolding the protein. CO-photolysis of this ferrous species results in the transient formation of a five-coordinate high-spin heme and favorable folding conditions allowing for the refolding reaction to be photo-triggered. In this study, we have utilized time-resolved photoacoustic calorimetry (PAC) and transient absorption (TA) spectroscopy to probe the folding dynamics of ferrous Cyt-c in the presence of 0.74mM sodium dodecyl sulfate (SDS) at pH∼7. Under these denaturing conditions, SDS binds to Cyt-c resulting in a partial unfolding of the protein and has been suggested to be in a molten globule (MG) conformation. The results from PAC indicate that Cyt-c folding initially occurs in a three-step process subsequent to CO-photodissociation with lifetimes of <50 ns, 626 ns and 2.6 μs and are attributed to CO dissociation from the heme, followed by a Methionine(80 or 65) and a non-native Histidine(23 or 33) residue coordination to the heme. These processes are accompanied by small volume changes of 1.5 mL/mol, 1 mL/mol, and 2 mL/mol and enthalpy changes of 69 kcal/mol, −7 kcal/mol, and 8 kcal/mol. The kinetic model suggest no large-scale protein collapse and consistent with the protein folding occurring from a compact MG state. The kinetics from PAC are similar to those obtained for CO-bound ferrous Cyt-c under high pH conditions and in the presence of low molar concentration of guanidine hydrochloride, suggesting a uniform mechanism for the earliest folding events regardless of the denatured environment. We have also explored PAC kinetic fits utilizing stretched exponential kinetics and have discovered the possibility of heterogeneous kinetics.

Photoacoustic study of nanocrystalline silicon produced by mechanical grinding

Mechanical grinding (MG) was used to produce nanocrystalline silicon and its thermal and transport properties were investigated by photoacoustic absorption spectroscopy (PAS). The experimental results suggest that in as-milled nanocrystalline silicon for 10 h the heat transfer through the crystalline and interfacial components is similar, and after annealed at 470 °C the heat transfer is controlled by crystalline component.

Quantitative photoacoustic imaging: correcting for heterogeneous light fluence distributions using diffuse optical tomography

The specificity of molecular and functional photoacoustic (PA) images depends on the accuracy of the photoacoustic absorption spectroscopy. The PA signal is proportional to the product of the optical absorption coefficient and local light fluence; quantitative PA measurements of the optical absorption coefficient therefore require an accurate estimation of optical fluence. Light-modeling aided by diffuse optical tomography (DOT) can be used to map the required fluence and to reduce errors in traditional PA spectroscopic analysis. As a proof-of-concept, we designed a tissue-mimicking phantom to demonstrate how fluence-related artifacts in PA images can lead to misrepresentations of tissue properties. To correct for these inaccuracies, the internal fluence in the tissue phantom was estimated by using DOT to reconstruct spatial distributions of the absorption and reduced scattering coefficients of multiple targets within the phantom. The derived fluence map, which only consisted of low spatial frequency components, was used to correct PA images of the phantom. Once calibrated to a known absorber, this method reduced errors in estimated absorption coefficients from 33% to 6%. These results experimentally demonstrate that combining DOT with PA imaging can significantly reduce fluence-related errors in PA images, while producing quantitatively accurate, high-resolution images of the optical absorption coefficient.

Structural instability and photoacoustic study of AlSb prepared by mechanical alloying

High-purity elemental Al and Sb powders were blended with equiatomic composition and submitted to mechanical alloying. For all milling times, the milled powders showed a mixture of AlSb and elemental Sb. The largest amount of AlSb was reached for milling times between 7 and 10 h. For milling times larger than 10 h, decomposition of AlSb was observed. The volume fractions of the crystalline and interfacial components were estimated using the X-ray diffraction pattern of a sample milled for 10 h. Photoacoustic absorption spectroscopy (PAS) was used to determine the thermal diffusivity and other heat transport parameters in the same sample. A combination of XRD and PAS data was used to estimate the thermal diffusivity of the interfacial component, which has a significant contribution to the thermal diffusivity of the sample.

X-ray diffraction, Raman and photoacoustic studies of InSb nanocrystals

Zinc blend InSb nanocrystals were generated by mechanical alloying and X-ray diffraction, Raman spectroscopy and photoacoustic absorption spectroscopy techniques were used to study its structural, optical and thermal properties. Annealed nanocrystals were also studied. Residual amorphous and minority crystalline (Sb and In 2O 3) phases were also observed for mechanical alloyed and thermal annealed samples, respectively. The structural parameters, phase fractions, average crystallite sizes and microstrains of all crystalline phases found in the samples were obtained from Rietveld analyses of their X-ray diffraction patterns. Raman results for both as-milled and annealed samples show the Raman active LO and TO modes of the zinc blend InSb phase and Sb-rich regions. The photoacoustic results of both samples were satisfactorily explained by thermal bending heat transfer mechanism and an increase on effective thermal diffusivity coefficient was observed after annealing.

The Role of the Internal Disulfide Bond in the Conformational Dynamics of Neuroglobin

Neuroglobin (Ngb) is a member of the globin family expressed mainly in brain tissue of mammals and other vertebrates that plays a role in the neuronal response to hypoxia and ischemia. Human Ngb has two cysteine residues (Cys46 and Cys55) within the CD loop and the D helix of the protein that were shown to form an internal disulfide bond, however these two residues are replaced by Gly in rat Ngb and the internal disulphide bond is missing. Therefore, to investigate the impact of the internal disulfide bond on the dynamics and energetics in Ngb we used photoacoustic calorimetry and transient absorption spectroscopy and determined the time-resolved volume and enthalphy changes associated with CO rebinding to human Ngb, rat Ngb and a rat Ngb mutant with and engineered internal disulfide bond (Cys46rNgb). The relaxation of the protein structure associated with the ligand photo-release is fast ( < 50 ns) and involves a decrease in the volume of the protein matrix. The enthalpy change associated to CO photo-dissociation for all samples studied was 19 kcal mol−1, whereas the reaction volume changes for human Ngb and Cys46rNgb were roughly two times higher than for rat Ngb. The reaction volume changes obtained for human and Cys46rNgb were 13.4±0.9 mL mol−1 and 10.3±0.6 mL mol−1, respectively, and 4.6±0.3 mL mol−1 for rat Ngb. These results indicate that the presence of the engineered internal disulphide bond in Cys46rNgb leads to a structural volume change that is similar to that found for human Ngb indicating that the internal disulphide bond control, to some extend, conformational dynamics associated with the ligand binding to deoxy Ngb.