Paulo Roxo Barja

Open photoacoustic cell : Applications in plant photosynthesis studies

Abstract 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 measurements makes this technique particularly suitable for monitoring photosynthesis of plants subjected to specific treatments. Therefore, our objective in the present article is to describe, in more detail, the principles of the OPC technique and to follow the historical path of its applications related to the study of plants and photosynthesis, including recent results. It will be shown that the technique has been used to observe spectroscopic responses to herbicide application and to soil toxicity, in the study of photosynthesis induction, gas exchange, photochemical loss, photoinhibition, photosynthesis saturation, and photosynthetic responses to changes in parameters such as temperature, atmospheric conditions, and nutrient availability. Recent results have demonstrated its usefulness in the determination of the action spectrum directly from oxygen evolution measurements.

Photosynthesis in eucalyptus studied by the open photoacoustic technique: Effects of irradiance and temperature

This work demonstrates the usefulness of the Open Photoacoustic Cell Technique to study the effects of irradiance and temperature on photosynthesis. In vivo and in situ photosynthetic induction measurements were performed in three different species of eucalyptus plants (E. grandis, E. urophylla, and E. urograndis) previously dark-adapted at different temperatures. Photosynthetic activity curves were built as a function of light intensity, indicating the occurrence of photosynthesis saturation. E. urograndis presented higher photosynthetic activity than the other species, especially at low temperature, indicating its tolerance to stress conditions. The incidence of background saturation light of various intensities allowed the in situ study of photoinhibition in eucalyptus plants through open photoacoustics.

Characterization of human skin through photoacoustic spectroscopy

The photoacoustic technique is based on the absorption of modulated light by a sample and subsequent heat generation. This generates thermal waves that propagate in the surrounding media. According to the Rosencwaig-Gersho Model, such waves produce the pressure oscillation detected as the photoacoustic signal. This technique allows the spectroscopic characterization of multilayer systems: as the thermal diffusion length varies with the modulation frequency of the absorbed light, the depth profile of a sample can be studied by the analysis of the photoacoustic signal at different modulation frequencies. In this work, photoacoustic spectroscopy was used to characterize different human skin samples. Measurements were performed at 70Hz and 17Hz, using a 1000W Xe arc lamp as the light source, for wavelengths between 240nm and 700nm. Skin samples were about 0,5cm diameter. It was possible to obtain the photoacoustic absorption spectra of the stratum corneum and of a deeper layer of epidermis; when the lower modulation frequency is utilized, photoacoustic spectroscopy characterizes the absorption of the whole epidermis, because in this case the thermal diffusion length is thicker than that of the stratum corneum. Photoacoustic spectroscopy was also employed to monitor the drying kinetics of the skin. This was done by analyzing the time evolution of the photoacoustic spectra of skin samples. Pre-treatment of the samples included different periods in a drying chamber. Measurements show that the photoacoustic spectra changes according to the humidity of the skin. Future work includes detailed monitoring of skin hydration.

Photosynthetic Induction in Eucalyptus Urograndis Seedlings and Cuttings Measured by an Open Photoacoustic Cell

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.

Photoacoustics as a tool for the diagnosis of radicular stress: Measurements in eucalyptus seedlings

In reforesting companies (cellulose industry), eucalyptus is usually cultivated in small plastic containers (50 mL). As seedlings remain for about 120 days in these containers—until transplantation—their roots become space restricted, with consequent limitations in water and nutrient absorption. These restrictions may lead to plant stress, decreasing productivity. In this work, we used the photoacoustic technique to evaluate the photosynthetic activity of Eucalyptus grandis, E. urophylla and E. urograndis seedlings subjected to this limited space availability, seeking a correlation with morphological parameters and fluorescence measurements in these seedlings. Photoacoustic, fluorescence, and morphological analysis were conducted every 15 days, from 45 to 120 days after sowing. Fluorescence and photosynthetic rate were evaluated in vivo and in situ, the latter one using the open photoacoustic technique. Data show that root dry matter diminished markedly at 90 and 120 days after sowing; this behavior showe...

Transport of oxygen and heat in leaves during photosynthesis studied photoacoustically on in vivo samples at low modulation frequencies

Amplitude and phase angle of the photobaric and photothermal signal were measured on in vivo samples of Jequitiba and Copuifera trapezifilia varying the modulation frequency between about 10 and 55 Hz. For both plant species we found that the phase angle of the photobaric signal decreases with increasing modulation frequency whereas the phase angle of the thermal signal increases. The results are interpreted with models recently developed.

Sunscreen effects in skin analyzed by photoacoustic spectroscopy

In the photoacoustic technique, the signal is proportional to the heat produced in a sample as a consequence of modulated light absorption. This technique allows the spectroscopic characterization of multilayer systems: as the thermal diffusion length varies with the light modulation frequency, one can obtain the depth profile of the sample by analyzing the frequency-dependence of the signal. As the photoacoustic signal depends on thermal and optical properties of the sample, structural changes in the system under analysis account for signal variations in time. In this work, photoacoustic spectroscopy was used to characterize samples of sunscreen and the system formed by sunscreen plus skin. We used photoacoustic spectroscopy to monitor the absorption kinetics of sunscreen applied to samples of human skin, characterizing alterations in the human skin after application of sunscreen. Measurements used 250W Xe arc lamp as light source, for wavelengths between 240nm and 400nm. This range corresponds to most of the UV radiation that reaches Earth. Skin samples were about 0,5cm diameter. The absorption spectra of sunscreen was obtained. Finally, photoacoustics was employed to monitor the absorption kinetics of the sunscreen applied to skin samples. This was done by applying sunscreen in a skin sample and recording the photoacoustic spectra in regular time intervals, up to 90 minutes after application. According to measurements, light absorption by the system sunscreen plus skin stabilizes between 25 and 45 minutes after sunscreen application. Results show that this technique can be utilized to monitor drug delivery and farmacokinetics in skin samples.

Study of the degree of photoactivation of the Z250 resin by photoacoustics

The aim of this work was to characterize the degree of photoactivation of the Z250 resin through photoacoustics. In this technique, the signal detected is proportional to the heat produced in a sample as a consequence of light absorption. This technique has been used for more than 20 years as a work tool in diverse fields of biological and biomedical sciences. Through photoacoustic measurements, it is possible to study optical and thermal properties of samples, and to obtain information on the characteristic times involved in photoinduced processes, as the photoactivation of composed resins. After application on the surface, the Z250 resin is photoactivated by incidence of continuous light (λ = 475 ± 15 nm) coming from a photodiode. This leads to the polymerization of the resin, modifying its thermal properties. The experimental method employed in this work was the following: a) the resin was applied on an aluminum sheet placed in contact with the photoacoustic cell (front incidence); b) modulated white light was applied in the lower surface of the aluminum sheet, black-painted to increase the light absorption; c) the photothermal signal was observed. Polymerization was evaluated through the alteration of the photoacoustic signal caused by the activation of the resin promoted by the incidence of the continuous light, for different activation times. The results show that the polymerization of the resin substantially modifies the photoacoustic signal, indicating that the degree of photoactivation can be evaluated through photoacoustic measurements.

Photoacoustics study of the degree of polimerization of resins and restoration materials

In this work is presented the use of Photoacoustic as an alternative technique to monitor the curing process of odontological materials, emphasizing the resins chemically activated (RCA). Through photoacoustic measurements, it is possible to study optical and thermal properties of samples, and to obtain information on the characteristic times involved in the curing processes. For this study the samples were analyzed to evaluate the polymerization of the RCA for different temperature. The results obtained show the viability of applying the Photoacoustic Techinques to monitor the polyermization kinetic of odontological resins, allowing for a qualitative and quantitative interpretation.