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The concentration of trace gases in the atmospheric environment is extremely low, but it has a great impact on the living environment of organisms. Photoacoustic spectroscopy has attracted extensive attention in the field of trace gas detection because of its high sensitivity, good selectivity, and fast response. As the core of a photoacoustic detection setup, the photoacoustic cell has a significant impact on detection performance. To improve detection sensitivity, a sphere-tube coupled photoacoustic cell (STPAC) was developed, which was mainly composed of a diffuse-reflective sphere and an acoustic resonance tube. Modulated light was reflected multiple times in the sphere to increase optical path, and photoacoustic (PA) signals were further amplified by the tube. Based on STPAC, a PA gas detection setup was built with a laser diode (LD) at 450 nm as the light source. The experimental results showed that the minimum detection limit (noise equivalent concentration, NEC) of NO2 was ~0.7 parts per billion (ppb). Compared with the T-type PA cell (TPAC) in which the modulated light passed through the sphere, the signal-to-noise ratio of STPAC was increased by an order of magnitude at the same concentration of the NO2 sample.

An alternative photoacoustic cell configuration for the determination of the thermal diffusivity (α), at room temperature, for solid materials is presented. The method is based on the use of two identical photoacoustic chambers, inside both of them, a metallic foil thermally thin is used to transform the light energy to heat energy. A Reference material placed parallel to a study material allows to relate the thermal properties of the materials used as support in the photoacoustic chambers of the experimental arrangement presented here. The ratio between experimental and theoretical photoacoustic amplitudes is realized to validate a proposed mathematical model.

The concentration of trace gases in the atmospheric environment is extremely low, but it has a great impact on the living environment of organisms. Photoacoustic spectroscopy has attracted extensive attention in the field of trace gas detection because of its high sensitivity, good selectivity, and fast response. As the core of a photoacoustic detection setup, the photoacoustic cell has a significant impact on detection performance. To improve detection sensitivity, a sphere-tube coupled photoacoustic cell (STPAC) was developed, which was mainly composed of a diffuse-reflective sphere and an acoustic resonance tube. Modulated light was reflected multiple times in the sphere to increase optical path, and photoacoustic (PA) signals were further amplified by the tube. Based on STPAC, a PA gas detection setup was built with a laser diode (LD) at 450 nm as the light source. The experimental results showed that the minimum detection limit (noise equivalent concentration, NEC) of NO2 was ~0.7 parts per billion (ppb). Compared with the T-type PA cell (TPAC) in which the modulated light passed through the sphere, the signal-to-noise ratio of STPAC was increased by an order of magnitude at the same concentration of the NO2 sample.

In photoacoustic spectroscopy, the signal is inversely proportional to the resonant cell volume. Photoacoustic spectroscopy (PAS) is an absorption spectroscopy technique that is suitable for detecting gases at low concentrations. This desirable feature has created a growing interest in miniaturizing PA cells in recent years. In this paper, a simulation of a miniaturized H-type photoacoustic cell consisting of two buffer holes and a resonator was performed in order to detect CO, NH3, NO, and CH4 pollutants. These gases are the main components of the air pollutants that are produced by the automotive industry. The linear forms of the continuity, Navier–Stokes equations, and the energy equation were solved using the finite element method in a gaseous medium. The generated pressure could be measured by a MEMS sensor. Photoacoustic spectroscopy has proven to be a sensitive method for detecting pollutant gases. The objectives of the measurements were: determining the proper position of the pressure gauge sensor; measuring the frequency response; measuring the frequency response changes at different temperatures; studying the local velocity at the resonant frequency; and calculating the quality factor. The acoustic quality coefficient, acoustic response (pressure), local velocity, frequency response, and the effect of different temperatures on the frequency response were investigated. A frequency response measurement represents the fact that different gases have different resonance frequencies, for which CO and NO gases had values of 23.131 kHz and 23.329 kHz, respectively. The difference between these gases was 200 Hz. NH3 and CH4 gases with values of 21.206 kHz and 21.106 kHz were separable with a difference of 100 Hz. In addition, CO and NO gases had a difference of 2000 Hz compared to NH3 and CH4, which indicates the characteristic fingerprint of the designed cell in the detection of different gases. Better access to high-frequency acoustic signals was the goal of the presented model in this paper.

With the increasing number of vehicles, it is more and more important to monitor the concentration of automobile exhaust gas. In order to detect the concentration of main gas components in exhaust gas, a set of automobile exhaust simulation detection device based on photoacoustic spectroscopy technology was designed by using laser as the light source, combined with the self-designed photoacoustic cell and gas distribution system. The system adopts wavelength modulation technology, which effectively overcomes the interference and influence of traditional mechanical chopping modulation. The experimental results show that the system can detect different concentrations of CO, NO and NO2 with stable operation, which can meet the requirements of vehicle exhaust detection.

Los fenómenos físicos derivados del efecto fotoacústico resultan de suma importancia en la determinación de propiedades térmicas. En el presente trabajo, se realiza una revisión del fenómeno, su desarrollo, aplicaciones y formas de mejorar la señal fotoacústica. Se fabricó e instrumentó un sistema experimental de lecturas fotoacústicas, con capacidad de hacer pruebas en un rango de frecuencia variable y constante. Primero, se realizó el diseñó, código G y fabricación de la mesa óptica en una maquina CNC de 3 ejes; luego, se estableció la comunicación del sistema experimental, utilizando la herramienta virtual LabView. Finalmente, se realizaron pruebas, los resultados obtenidos son consistentes con lo reportado en la literatura. El prototipo obtenido resulta ser muy compacto y práctico. Esto es posible, gracias a la eliminación de componentes mecánicas utilizadas en los sistemas tradicionales.

Photoacoustic (PA) spectroscopy techniques enable the detection of trace substances. However, lower threshold detection requirements are increasingly common in practical applications. Thus, we propose a systematic geometry topology optimization approach on a PA cell to enhance the intensity of its detection signal. The model of topology optimization and pressure acoustics in the finite element method was exploited to construct a PA cell and then acquire the optimal structure. In the assessment, a thermo-acoustic model was constructed to properly simulate the frequency response over the range of 0–70 kHz and the temperature field distribution. The simulation results revealed that the acoustic gain of the optimized cell was 2.7 and 1.3 times higher than conventional cells near 25 and 52 kHz, respectively. Moreover, the optimized PA cell achieved a lower threshold detection over a wide frequency range. Ultimately, this study paves a new way for designing and optimizing the geometry of multifarious high-sensitivity PA sensors.

The existence of a signal baseline due to a variety of reasons in a photoacoustic (PA) gas measurement system is a common phenomenon. One major component is the absorption of optical windows in an enclosed PA cell. This work explores the relation between the background signal and the thermoelastic effect inside the windows by modelling the pressure and elastic wave field by means of a Green-function based method. The influence of laser incidence location, angle and radius is discussed based on a rigorous three-dimensional solid-to-fluid coupling model. The effects were theoretically demonstrated culminating in the determination of best (minimum background signal) performance using a collimated and expanded incident laser beam. The results were also validated through experiments.

In this paper a photoacoustic senor for carbon dioxide and nitrous oxide detection is described which uses a quantum cascade laser. The sensor relies on a 4.43 μm continuous-wave room temperature quantum-cascade laser source and a homemade photoacoustic cell based on a cylindrical acoustic resonator. Primary laboratory tests have been performed for estimation of the achievable detection limits and possible applications for in situ and real time atmosphere measurements. It is demonstrated that the minimum detectable concentration of 13CO2 and N2O under laboratory conditions is 8 ppbv and 0.45 ppbv, respectively.

Thermal conversion (combustion, co-combustion, gasification and pyrolysis) appears to be the most promising alternative for sewage sludge management in the future. Nevertheless, safe and ecological usage of sewage sludge as a fuel requires information about their contamination. The aim of this paper is to present the photoacoustic spectroscopy (PAS) as a good method for contamination assessments of dried sewage sludge. Two types of granular sewage sludge: Sewage sludge 1 (SS1) taken from Polish wastewater treatment plant operating in the mechanical-biological system and sewage sludge 2 (SS2) taken from mechanical-biological-chemical wastewater treatment plant with phosphorus precipitation were analysed. The spectrophotometer FTIR Nicolet 6700 equipped with photoacoustic cell (Model 300, MTEC, USA) was used. The comparison with the most popular analytical methods (GC-MS) was also done. The results of PAS studies confirm the difference between the SS1 and SS2 which is in agreement with the GC-MS analysis. Higher absorbance was observed at each wavelength characteristics for the oscillator of chemical moieties for the SS1 with respect to the SS2.

Photoacoustic imaging using a closed photoacoustic cell and an open photoacoustic cell with gas- microphone detection scheme was described. R/G/B LED light sources were used for the closed photoacoustic (PA) cell configuration. The colored specimen enclosed in a PA cell was imaged with R/G/B color light sources, and an image restored from the inverted PA images was compared with the original image. For open cell configuration, an open PA cell using a spheroidal acoustic resonator was applied to measure the amount of large-sized colored specimens. A calibration curve for a food red dye was obtained that apparently showed the ability of the present scheme to measure as a spectroscopic measurement tool.

Thermal diffusivity (TD) of p-Si and Ag/p-Si samples were measured by photoacoustic technique using open photoacoustic cell (OPC). The samples were annealed by heating them at 960, 1050, 1200, and 1300 °C for 3 h in air. The thermal diffusivity of Ag-coated samples was obtained by fitting the photoacoustic experimental data to the thermally thick equation for Rosencwaig and Gersho (RG) theory. For the single layer samples, the thermal diffusivity can be obtained by fitting as well as by obtaining the critical frequency f c . In this study, the thermal diffusivity of the p-Si samples increased with increasing the annealing temperature. The thermal diffusivity of the Ag/p-Si samples, after reaching the maximum value of about 2.73 cm2/s at a temperature of 1200 °C, decreased due to the silver complete melt in the surface of the silicon.

Design of non-invasive and accurate novel methods for liver fibrosis diagnosis has gained growing interest. Different stages of liver fibrosis were induced in Wistar rats by intraperitoneally administering different doses of carbon tetrachloride. The liver fibrosis degree was conventionally determined by means of histological examination. An open-photoacoustic-cell (OPC) technique for the assessment of liver fibrosis was developed and is reported here. The OPC technique is based on the fact that the thermal diffusivity can be accurately measured by photoacoustics taking into consideration the photoacoustic signal amplitude versus the modulation frequency. This technique measures directly the heat generated in a sample, due to non-radiative de-excitation processes, following the absorption of light. The thermal diffusivity was measured with a home-made open-photoacoustic-cell system that was specially designed to perform the measurement from ex vivo liver samples. The human liver tissue showed a significant increase in the thermal diffusivity depending on the fibrosis stage. Specifically, liver samples from rats exhibiting hepatic fibrosis showed a significantly higher value of the thermal diffusivity than for control animals.

Localized, non-invasive cell stimulation has many applications. Here we show the initial proof-of-concept in vitro study of the photoacoustic cell stimulation approach, which is amenable to high-throughput screening applications. The proposed method is implemented as follows: 1) the localized excitation using the focused pulsed laser delivery on an absorptive material placed inside the well containing the cells, 2) cell stimulation by the photoacoustic pressure generated, and 3) fluorescence quantification of the membrane potential change over time. The preliminary proof-ofconcept in vitro study is conducted with primary neurons isolated from mouse cerebral cortex. The absorptive rubber media generates the photoacoustic pressure by the pulsed laser excitation. The experimental results show the feasibility of photoacoustic cell stimulation approach by indicating the significant membrane potential chance from the photoacoustically-stimulated primary neurons. Otherwise, the sham control without any photoacoustic stimulation shows minimal membrane potential change. We envisage that the proposed approach can allow broad strategies for noninvasive cell stimulation by using the photoacoustic contrasts situated at inside or outside of the body such as external absorptive materials or intravascularly-injected photoacoustic contrast particles.

A compact resonance photoacoustic cell with a Π-shaped internal cavity and a volume of ~0.2 cm3 is developed. The design of the cell and an experimental setup for testing its operational capabilities are described. Results are presented from tests of the photoacoustic cell in experiments to detect light absorption by ammonia in nitrogen flows using a single-mode diode laser with a wavelength of ~1.53 μm. The amplitude-frequency characteristics of the photoacoustic absorption response with and without ammonia are analyzed. The operation of the cell under conditions of resonance with the second longitudinal acoustic mode of the inner cavity of the cell (resonant frequency 4.38 kHz, Q-factor 13.9) is analyzed. The measured background signal level from the cell windows corresponds to an absorption coefficient of ~2.8·10–7 cm–1. The microphone-noise limited minimum detectable absorption is ~1.44·10–9 cm–1·W·Hz1/2.

The frequency response of a photoacoustic cell can be evaluated by measuring the output amplitudes point-by-point at different modulation frequencies, by measuring the pulse response, or by recording the cell response to a chirp stimulation, in which the light modulation signal sweeps within a given range of frequencies. This paper presents yet another method, in which the stimulation signal consists of a number of sine waves with different frequencies. A theoretical description is presented, and properties of the techniques are discussed and compared with the experimental results.

This paper describes the design and optimisation of the photoacoustic (PA) cell for nondestructive PA spectroscopy. The results of numerical calculations of the acoustic properties of the PA cell are compared with experimental measurements with respect to the frequency response and the quality factor of the cell. The method of designing and optimising of the PA cell is presented. Advantages and disadvantages of this study are discussed.

A homemade low-temperature, T shaped photoacoustic (PA) spectrometer is well established. The temperature stability and characteristics of the PA system in the temperature range of −90 to 0 °C were evaluated experimentally. A mathematical model for calculation of resonance frequencies and end-correction are proposed. The results of numerical calculations of the temperature dependent of resonant frequencies and the Q-factor of PA cell are compared with experimental measurements. The measured results show that the PA system has high Q-factor and the theoretical value is very well consistent with the experimental results. The variation of sensitivity of the PA system is very small in temperature range of −90 to 0 °C, which illustrates that the sensitivity of the microphone has little been affected by low temperature. So the system can be used in trace gas detection at low-temperature.

Photo-acoustic spectroscopy gas detection technology has the advantages of high sensitivity, good selectivity, small size and real time monitoring and has been widely used in environmental monitoring, industrial production, medical diagnosis, biological technology and monitoring of power facilities. In this paper, a method to improve the sensitivity of photo-acoustic spectroscopy system is presented, which is combined with the technique of Herriott type multiple pass cell. In this experimental apparatus, the design of the experimental device can make the beam pass the cell 18 times. By comparing the signal of one time pass through the photoacoustic cell and the signal of 18 times passes pass through the photoacoustic cell, we can confirm that the signal is increased and this method is feasible.

We present here the development and the characterization, in laboratory conditions, of our photoacoustic cell associated with QCL’s and specific readout circuit for gas traces sensing. Assessment has been done on CO2.

We describe the design and development of an injection seeded Nd:YVO4 laser for use in a differential fluorescence lidar for measuring atmospheric formaldehyde profiles. A high repetition rate Q-switched laser is modified to accept injection seed input to spectrally narrow and tune the output. The third harmonic output is used to excite formaldehyde (HCHO) fluorescence when tuned to a HCHO absorption line. Spectral confirmation is made with the use of a photoacoustic cell and grating spectrometer.

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.

The determination of the frequency response of a photoacoustic cell is a typical activity during design and adjustment of a photoacoustic setup. The most common method of such measurements is based on point-by-point checking of the amplitude of the photoacoustic signal at different modulation frequencies. Such an approach can be time-consuming, especially if a lot of measurements at low modulation frequencies are to be performed. The article presents a method based on investigation of a photoacoustic response to short square pulse stimulation instead of continuous modulation of the light source. The method allows for measurements of the frequency response to be substantially shortened. A theoretical description and properties of the pulse measurement method are discussed and illustrated with a comparison of experimental results obtained from both frequency response measurement methods.

This paper discusses laser-induced photoacoustic investigations; it concerns a non-destructive technique for the determination of the thermal diffusivity of colon phantom tissue employing open photoacoustic cell configuration.

The use of a Helmholtz resonator as a photoacoustic cell has many advantages. Its main drawback in high-sensitivity photoacoustic measurements is the relatively low values of Q-factors of such cells. One of the important loss sources which limit the quality factor of photoacoustic Helmholtz resonators is the rapid change of the cross section at the cavity–duct boundaries. This paper presents a design of a cell in which the shape of the duct was modified in such a way that the cross-section change is more gradual. A presented comparison of measurements of the Q-factors shows clearly that the Q-factor of the cell can be improved by almost a factor of two by such a modification of the duct shape.

A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype cell with the internal volume ~ 0.5 cm^3. The minimal detectable absorption for the prototype is estimated to be ~ 1.2 10^{-8} cm^{-1} W Hz^{-1/2}.

The microstructural changes in the maize pericarp during the critical lime-cooking step of a traditional nixtamalization process are reported. Scanning Electron Microscopy at low vacuum (SEM-LV) images and Energy Dispersive Spectrometry (EDS) were used to study the evolution of the pericarp transformation during lime-cooking and the CaCO3 formation on the pericarp. Moisture content in cooked maize kernels and calcium content in the pericarp were used as physicochemical criteria in order to establish the end of the cooking step. For the first time, the cooking step was analyzed in situ by using a differential photoacoustic cell (DPC), in order to associate photoacoustic amplitude signals of nixtamalized pericarp with its structural modifications. X-ray diffraction patterns of nixtamalized pericarp showed an increase in the peak intensity of the crystalline fraction compared with the native pericarp. These results were supported by SEM images and amplitude signals obtained through the use of the photoacoustic technique.

Polyaniline in a powder form was chemically prepared and its thermal diffusivity was determined by photoacoustic method in the open cell configuration. The effect of acid doping on heat transport in polyaniline was investigated. The observed increase in thermal diffusivity of acid-doped samples points to a carrier contribution to heat diffusion.

The open photoacoustic cell technique was used to measure tomato leaves from plants with and without mycorrhizal fungi (Glomus fasciculatum). Based on measurement of the photobaric contribution of the photoacoustic signal, the exponential parameter of the diffusion behavior for this contribution was calculated. From this value, the oxygen diffusion coefficient was derived. The changes in the oxygen- diffusion-coefficient-dependent exponential parameter are statistically significant (p < 0.05) and are consistent with the expected benefits of mycorrhizal symbiosis. Potentially similar results obtained from the photothermal contribution are discussed.

Thermal effusivity of virgin coconut oil-methanol mixtures were measured using open photoacoustic cell technique. The samples were prepared by simply mixing virgin coconut oil and methanol using similar procedure applied for preparation of biodisel. Thermal effusivity of the sample was obtained by fitting the experimental data of photoacoustic amplitude signal to the expression of photoacoustic signal as a function of chopping frequency. Thermal effusivity of mixtures decrease between 0.0851 W s1/2 K−1 cm−2 (pure virgin coconut oil) and 0.0644 W s1/2 K−1 cm−2 (pure methanol) with the increasing of methanol in the mixture.

Ultrathin endoscopes, such as transnasal endo-scopes, have been developed to alleviate discomfort during diagnosis and therapy. However, their application to optional diagnostics is limited since many optional diagnostic instruments are designed to fit through larger side channels. The aim of this study was to develop a smart endoscope that can obtain various diagnoses based on photoacoustic spectroscopy. The photoacoustic process comprises complex energy conversions involving optical, thermal, and elastic processes. This work focused on the scaling potential of photoacoustic sensors. Photoacoustic sensors with two different volumes were developed, and the amplitudes and frequency responses of the photoacoustic signals for silicone rubbers with six different Young's moduli were investigated. The results showed that photoacoustic signals can be enhanced by reducing the volumes of the sensors. Embedding a miniaturized photoacoustic sensor in an endoscope was confirmed to improve the sensitivity.

In this article, we review the applications of the open photoacoustic cell (OPC) technique in studies of photosynthetic activity in plant leaves. The ability to perform in vivo and in situ measurem...

In this work the open photoacoustic cell technique (OPC) is applied for measuring the thermal diffusivity (a) of human nail tips. Human nails are natural polymers that receive less attention in clinical analysis than other human body parts, although they are very interesting in giving information about some external diseases like dystrophies. Diagnosis and therapy with topic application of anti-fungal creams could be monitored since thermal properties are known. The OPC experiments in the low frequency range were done and through photoacoustic signal decay, the OPC model were used for fitting data in order to obtain the thermal diffusivity of the human nail in vitro. The average value for the nail tips used was found to be a ∼(8.9 ′ 1.3) x 10 - 4 cm 2 /s, when different light source is used for photothermal heating. This average is of the order of that evaluated for the human skin.

Kinetics of water and calcium ion diffusion into biological layer (pericarp) exposed to different thermal treatments was investigated using the novel concept of differential photoacoustic cell (DPC). The latter enabled the real time measurements of amplitude and phase of PA signals originating from the reference and test samples. The uptake of calcium ion and water into kernel during the cooking of maize occurs predominantly through the pericarp. This process affects the physiochemical properties of kernel causing the corresponding changes of the signal and the phase of PA signal in time. The novel concept of DPC including temperature profiles is an excellent tool to study in-situ kinetics and thermal properties of material.

The applied procedure of thermal effusivity determination for liquid samples taking advantage of photoacoustic signal amplitude measurement with microphone registration is presented. The experimental results obtained for distilled water and ethyl alcohol are shown to illustrate the sensitivity of the applied procedure. The results are in reasonable agreement with the data obtained from the literature.

We report here on the use of the photoacoustic technique for the determination of low concentrations of total iron in corn meal samples. The determination of this element in food is of considerable interest because several foods are currently enriched with it at proper levels in order to increase the resistance of people, after consumation, to several diseases, such as anemia. The proposed technique is based on an open photoacoustic cell configuration in conjunction with a suitable colorimetric method. We applied it to a measurement of the total iron concentration in corn meal samples. The results agree very well with those obtained using a conventional spectrophotometric method, showing the possibilities of new experimental methodologies based on photothermal methods to perform this kind of study, with the advantage of a higher sensitivity and increment of the range of appreciable absorbance.

Pulsed photoacoustic spectroscopy (PPAS) provides a convenient means for in vivo and in situ monitoring of human skin properties and surface concentrations of locally applied substances, such as drugs and cosmetics. In this article, we applied the one-dimensional theoretical model of Mandelis and Royce (J. Appl. Phys. 50, 6 (1979)) to simulate the photoacoustic response of human skin and diffusion phenomena through it, for various optical absorption coefficients and sample thicknesses. This approach is analyzed through the temperature and pressure variations in the photoacoustic cell.

It is simulated that the photoacoustic effect in three-dimensional model to compare photoacoustic effect with attenuated different Gaussian radii distribution light sources by the finite element method (FEM). We motivate the enclosed photoacoustic cell at a range of frequencies and compare the results of the amplitude of acoustic pressure frequency curve, the distribution of acoustic pressure and the variation of temperature within the gas layer.

We propose an approach to match the resonant characteristics of a photoacoustic cell with that of a microphone in order to enhance the signal-to-noise ratio in the photoacoustic sensor system. The synergetic resonance matching of a photoacoustic cell and a microphone was achieved by observing that photoacoustic cell resonance is merged with microphone resonance, in addition to conducting numerical and analytical simulations. Using this approach, we show that the signal-to-noise ratio was increased 3.5-fold from the optimized to non-optimized cell in the photoacoustic spectroscopy system. The present work is expected to have a broad impact on a number of applications, from improving weak photoacoustic signals in photoacoustic spectroscopy to ameliorating various sensors that use acoustic resonant filters.

Cell-tracking method has an important role in detection of metastatic circulating tumor cells (CTCs) and cell-based therapies. Label-free imaging techniques are desirable for cell-tracking because they enable long time observations without photobleaching in living cells or tissues where labeling is not always possible. Photoacoustic microscopy is a label-free imaging technique that offers rich contrast based on nonfluorescent cellular optical absorption associated with intrinsic chromophores and pigments. We show here that photoacoustic imaging is feasible for detecting very low numbers (x 104) of melanoma cells without labeling because of the strong instinct optical absorption of melanin in near-infrared wavelength. Flowing melanoma cells are imaged with micrometerresolution (40 μm) and penetration depths of centimeters (13 mm) in real-time. Photoacoustic imaging as a new cell-tracking method provides a novel modality for cancer screening and offers insights into the underlying biological process of cancer growth and metastasis and cell therapy.

Photosynthetic induction in leaves of four-month-old Eucalyptus urograndis seedlings and of cuttings obtained from adult trees that were previously dark-adapted was studied by the in vivo and in situ Open Photoacoustic Cell Technique. Results for the gas exchange component of the photoacoustic (PA) signal were interpreted considering that the gas uptake component would have a phase angle nearly opposite to that of the oxygen evolution component. By subtracting the thermal component from the total PA signal, we studied the competition between gas uptake and oxygen evolution during the photosynthetic induction. Seedlings presented a net oxygen evolution prior to cuttings, but cuttings reached a higher steady-state photosynthetic activity. The chlorophyll (Chl) a/b ratio and the Chl fluorescence induction characteristic Fv/Fm were significantly higher for cuttings, while there was no difference between samples in stomata density and leaf thickness. Thus the differences in PA signals of seedlings and cuttings are associated to differences between the photosystem 2 antenna systems of these samples.

In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4′-octyloxybiphenyl (8OCB) and 4-cyano-4′-heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases.

A double-purpose open photoacoustic cell, suitable for optical and thermal characterization of liquid samples is described. An experimental method for the determination of very low concentrations of contaminants in liquid substances is proposed. The proposed technique was used to determine the concentration of Chromium (VI) in water via the measurement of its optical absorption coefficient. To test the suitability of the proposed technique for experimental determination of thermal properties of liquids, studies on ethanol were performed. The agreement between experimentally obtained thermal properties and values reported in the literature is good.

Mathematical formulization of the thermal response in a two-layer photoacoustic cell, where the interference of thermal waves in a sample is compounded by the inter-layer interference effect, has been carried out by employing the thermal-wave interference theory. The physical meanings of the obtained formulae are then discussed in terms of the thermal-wave interference to explain the origin of the thermal phase lag for various cases. In addition, it is found that the inter-layer thermal-wave interference results in an extra phase lag.