Photoacoustic Effect of Ethene: Sound Generation due to Plant Hormone Gases

The Photoacoustic (PA) effect, discovered by Alexander Graham Bell in 1880, was first implemented in the use of his invention, the photophone. Later, the PA effect was employed in PA spectroscopy in research by L.B. Kreuzer in 1971 with trace gas detection.¹ The PA effect is created when a sample goes through the process of cyclic heating and cooling of a gas, solid, or liquid substance.¹ This cyclic heating and cooling is produced by the modulation of light of a particular wavelength, depending on the sample used, which causes pressure oscillations within the sample, produced by exposure to a light source.² Similar to other spectroscopy methods, the strength of the acoustic signal produced by the sample is proportional to the light absorbed.³ However, since PA spectroscopy utilizes the production of acoustic waves, light scattering is not an aspect that needs to be taken into account in this system and has no influence on the results.^2,3 For detection of PA signal throughout the entire electromagnetic spectrum, a single microphone can be used.³

Ethene (C₂H₄) is used in many free radical reactions. The main free radical reaction that C₂H₄ is involved in, is the making of polyethylene, the most widely used plastic in the world. C₂H₄ is the byproduct of the aging process in plants, as well. This ripening process is called the methionine cycle and also produces CO₂, HCN, and H₂O. Also, C₂H₄ has aesthetic properties. The main problem with C₂H₄ is that when the gas is produced and released into the atmosphere it becomes flammable at 2.7 vol %. If humans are shipping any sort of plants, imported or domestically, the risk of fires or explosions increases greatly as fruits or vegetables start to ripen.⁴ Negative effects of ethene often include reduced storage life, increased oxidative browning, and quickened senescence. More specific adverse effects of ethene include the formation of bitter-tasting chemicals in carrots, russet spotting on lettuce, and inhibited blooming of carnations.⁵ Due to its large absorption coefficient, C₂H₄ is a strongly absorbing gas. Subsequently, PA spectroscopy is the ideal method for detection. One of the largest absorbance peaks of C₂H₄ is found near 10.6 μm; therefore in order to detect C₂H₄, the laser has to have the wavelength in the same range. A CO₂ laser was used in the experiment because it satisfies these requirements.⁶ It has recently been shown that a PA laser spectrometer with CO₂ emission in the infrared range could detect C₂H₄. An acoustic signal is produced at the resonance frequency of about 2,400 Hz of 67 mm long and 18 mm of diameter resonant cell.⁷

Here, we investigate the detection of C₂H₄ from fruits. Temperature of C₂H₄ in the cell, concentration of C₂H₄ in the cell, length of the gas cell, and power of the CO₂ laser, are the parameters of the experimental setup that were varied to determine the effect on the PA signal produced from C₂H₄, detection limit using trace concentrations of C₂H₄ in an inert gas, N₂, and finally, detection of concentration of C₂H₄ from an avocado and banana in the process of ripening.⁸

The simplified diagram of the experimental setup is pictured in Fig. 1. The light emitted from the CO₂ laser (Access Laser Company Model L3) was aligned through an optical chopper (Thorlabs, Inc.) to chop the continuous laser irradiation, and directed to a gas cell (RJ Spectroscopy Company). Maximum laser power (200 mW) was used in experimentation, unless the power had to be varied. Germanium broadband precision windows (Thorlabs, Inc.) were used on the gas cell. A sensitive microphone (PCB Piezotronics, Inc. Model 130E20) was connected to the outside of the cell at a connection point on the edge of the gas cell. A diode laser (635 nm, Thorlabs, Inc. CPS182) was used as a reference laser and was detected using a Si photo detector (Thorlabs, Inc. DET36A). The microphone and Si photo detector were connected to a digital oscilloscope (Tektronix TBS 1202B). Each data point represented the average of 128 samples. To heat the cell during the experiments that involved temperature, a heating pad connected to a temperature controller (Thorlabs TC200) was placed inside the gas cell. All other experimentation was done at room temperature. During C₂H₄ detection in the fruit, an avocado and banana were left to ripen, separately, in one gallon airtight Ziploc bags for three days. These bags were then connected to the gas cell through a rubber tube and voltages were tested at certain data points over 128 samples using the oscilloscope.

Fig. 2 displays the PA signal from the reference laser and sample as seen on oscilloscope.

The results of experimentation with cell length were used to compare the relationships with resonance frequency using Eq. 1:⁸

where f_res is the resonance frequency (Hz), C_s the sound velocity (cm s⁻¹), L is the length of the gas cell (cm), T is the temperature (K), and the M is the molar mass (g mol⁻¹).

In experiments with varying the lengths of the gas cell, lengths of 20, 35, 50, and 65 cm were used. After finding the resonance frequency for 100% C₂H₄ in each of these gas cells, a calibration curve was created, shown in Fig. 3. The linearity of the calibration curve show the relationship between the resonance frequency of C₂H₄ and the corresponding gas cell length.⁸ The plot of resonance frequency versus cavity length gives the sound velocity. The experimental and theoretical values Cs of C₂H₄ are 327 m/s and 262 m/s. The percent error between these two values is 20%.⁹

The results from experimentation with laser power and temperature were used to compare the relationships with PA signal using Eq. (2).¹⁰

The amplitude of the PA signal for an optically thin gas is given by Eq. (2):

where p_o is the acoustic pressure amplitude (N m⁻²), P is the power of the laser (mW), and α is the optical absorption coefficient (m⁻¹). From Eq. 2, it can be observed that the amplitude is proportional to the power of the laser. Fig. 4 displays the linear dependence between PA intensity and power of the laser as given by Eq. 2.

C₂H₄ was also tested varying the temperature against the PA signal. Using amplitude at four different temperatures (22, 30, 45, 50 °C), the PA signal in the 50 cm cell containing 100% C₂H₄ was recorded. Since PA signal is proportional to the square root of the temperature of the system, the temperatures at which C₂H₄ was tested were plotted in a calibration curve against the corresponding PA signal.⁸ Fig. 5 displays this relationship between the square root of the temperature and the PA signal, resulting in a linear dependence.

Detection of trace amounts of C₂H₄ was one of the main goals of this experimentation. A 10 mL syringe was used in the dilutions of the C₂H₄ gas. The syringe was filled with 1 mL of C₂H₄ and 9 mL of N₂, an inert gas. Out of 10 mL of gas mixture, 9 mL was released, and the syringe was filled again with 9 mL N₂, resulting in a 0.1 mL volume of C₂H₄. This dilution process was repeated and tested for a signal until a signal was no longer distinguishable from the noise. Each point was plotted and a calibration curve was constructed against the PA signal. This experiment was conducted at room temperature, using a gas cell length of 50 cm and laser power setting of about 100 mW (recorded temperatures from 89 to 130 mW). The detection limit obtained from this experiment on C₂H₄ was 10 ppm. Fig. 6 displays the calibration curve constructed from the trace concentration amounts of C₂H₄ and the corresponding PA signal.

After detecting trace amounts of gas concentration, the next question was how much C₂H₄ by volume a banana and avocado emit. During ripening, fruits produce CO₂, HCN, H₂O and C₂H₄, however only C₂H₄ has absorption peaks around 10.6 μm and generates the PA effect.¹¹ When there is no spectral overlap from other buffer gases, they will not contribute to the PA signal; the light passes the detection cell unattenuated. Thus, a single component could be detected out of gas mixtures. The experimentation started with 100% N₂ gas in the cell. After calculating the volume of the gas cell, various amounts, in mL (% of cell) of C₂H₄ were added to the cell and plotted against the PA signal to create a calibration curve for concentration of C₂H₄. After letting the avocado and banana ripen separately in bags for 3 days each, the bags were then connected via an attachment hose to the gas cell and squeezed. These signals were tested and recorded against the concentration curve. The avocado and banana produced a 5% and 13% concentration of gas in the bag, respectively. Fig. 7 shows the calibration curve plotted with volume fraction against PA signal.

The danger about C₂H₄ is that this gas is flammable when its concentration in the air is above 2.7 vol%. At these levels, it is impossible for humans to detect C₂H₄ without special equipment. This is important because the economy has fruit food supply lines that can be in danger of possible fires and explosions. There are multiple documented fires at warehouses that store these fruits. If businesses implemented C₂H₄ trace gas detectors in workplaces, it would keep workers safe, whether it be on ships, trucks, ware-houses, or all three. This application to the real world could save people’s lives. PA spectroscopy is the most sensitive for C₂H₄ trace gas detection. Although the instruments used in this application are not yet cost-effective for the business world, getting those costs down would keep people safer and ultimately make the world safer.⁴

In this experiment, C₂H₄ gas was irradiated using a CO₂ laser, which resulted in thermal expansion and release of heat. This thermal expansion of the sample results in the generation of pressure waves that can be detected with microphone. The amplitude of the PA signal is linearly proportional to the power of the excitation laser light. This linearity has been investigated and the measurements done on C₂H₄ gas have given consistent results. The experimental results have demonstrated that the resonance frequency of PA is related to the cavity length. The experimental results have also demonstrated that PA signal is related to laser power and temperature. The C₂H₄ concentrations can be detected at levels as low as 10 ppm using this experimental setup. It is possible to improve the detection limit by using more powerful and stable CO₂ laser with less power fluctuations. The experimental setup also detects natural C₂H₄ from fruits, which could be improved with the use of a more airtight seal to stop leakage of the gas. This detection measured 5% and 13% by volume of air, of the avocado and banana, respectively.