Enhancing Gas Solubility in Water via Femtosecond Laser-Induced Plasma

The generation of laser-induced plasma at the gas–liquid interface provides many fundamental and interesting scientific phenomena such as ionization, sharp explosion, shock wave radiation, bubble creation, and water splitting. However, despite the extensive research in this area, there is no reference on the effect of the surrounding environment on the chemical processes that occur during the laser-induced plasma–water interaction. In this work, we investigate the effect of the surrounding gas environment on femtosecond laser-induced plasma when generated at the pure water–gas interface. Ultrashort laser pulses were applied to water in the presence of air and N2 and Ar gas environments. Formation of a significant number of nitrate-based species in water was observed after exposure to femtosecond laser-induced plasma in air and N2 environments. The detected NO3 ions formed in the laser-treated water led to the appearance of an absorption peak in the UV range, a significant decrease in the water pH value, and a significant increase in water’s electrical conductivity. All induced properties of water were stable for 3 months of monitoring after laser treatment. Our work shows that the generation of laser-induced plasma in water propagating into a gaseous medium facilitates the interaction between the two media, as a result of which the compositions of substances present in the gaseous medium can be dissolved in water without increasing the gas pressure. The presented approach may find applications in areas such as water purification, material synthesis, and environmental stewardship.


INTRODUCTION
Ultrafast lasers are used in many applications to modify the chemical, structural, and optical properties of materials. 1−4 The ultrafast nature and nonlinear interaction of a high-intensity laser with matter lead to a decrease in the zone of thermal influence with minimal collateral damage compared to the results achieved using continuous-wave or long-pulse lasers. 5 These properties allow us to precisely modify materials and to investigate ultrafast mechanisms during phase transitions or biochemical and chemical reactions. 1,6−9 Laser irradiation of aqueous media plays an important role in many chemical 10,11 and atmospheric phenomena. 12 Photolysis of water is of paramount importance as it is one of the possible sources of fuel that can be obtained by the dissociation of water into hydrogen and oxygen. 13 It has been shown that plasma generated by ultrashort laser pulses, especially femtosecond pulses, can be used for the production of hydrogen. 14 Although the production of hydrogen by this method is yet uneconomical, femtosecond lasers have recently received considerable interest for their potential use in biological and medical applications with many exciting and promising results. 15,16 Laser-generated plasma in liquids opens exciting applications including reagent-free characterization of biological objects 17 and decontamination of polluted water. 18 These applications contributed significantly toward the development of research on the complex interactions between laser-induced plasma and liquid water. However, most of these studies were mainly interpreted on the basis of physical phenomena, and only few reports were devoted to the chemical processes that occur during the generation of femtosecond laser-induced plasma in pure water. Hence, in most of these studies, the generated products of such interactions were ignored despite their importance in biology and medicine.
Several comprehensive reviews have recently appeared in the literature on the interaction of nonequilibrium plasma with liquids. 19,20 A number of complex phenomena related to the plasma−water interaction have been developed in these reviews. It has been accepted that the production of reactive oxygen and closely related reactive nitrogen radicals plays a dominant role in plasma−water interaction processes. However, those previous studies were focused on the properties of plasma generated by four often used discharges: a pulsed direct discharge in a liquid, a DC air glow discharge with a water electrode, a pulsed plasma jet (nontouching), and a filamentary dielectric barrier discharge, and there was no reference related to laser-induced plasma interactions. 19 Strong laser irradiation of water can cause intriguing phenomena such as plasma generation, shock wave radiation, splitting, and sharp explosions. 14,21,22 In addition, ionization of water leads to the generation of O + , H + , OH + , and H 2 O + radicals. 22 The thermalized radical and ionic species react with water molecules and other species in the solution, forming a variety of species such as H 2 , H 2 O 2 , O 2 , and so forth. 14,22,23 However, despite the extensive research in this area, there is no reference on the effect of the surrounding environment on the chemical processes that occur during the laser-induced plasma−water interaction.
The actual effect of a plasma−air−water interaction is ultimately determined by the generated chemical species and their diffusions, which are in turn determined by the laser (electric field and photons), ions, and electron fluxes and energies. It is clear that a more fundamental understanding of the processes occurring both in water and at its boundary with the atmosphere, when exposed to a laser, will eventually lead to a wider use of femtosecond lasers in water purification as well as other fields.
Recently, significant efforts have been made to develop new methods of water purification from hazardous organic waste. Methods such as the irradiation of waste or the use of supercritical oxidation of water have been proposed. 24 OH is a strong oxidant, so the generation of OH in the treatment process can rapidly oxidize various organic compounds in wastewater. Generating laser-induced plasma in the air−water interface area can involve both physical (ionization, shock waves, cavitation effects, and photolysis) and chemical (formation of reactive species and oxidation of organic contaminants) effects that play significant roles in the decomposition of organic contaminants. Thus, the effect of the laser-induced plasma, on the one hand, can create a large number of radicals directly in water and, on the other hand, can introduce reactive species from the environment, the amounts of which can be controlled by adjusting the laser treatment parameters.
In this work, we study the influence of the environment on the processes occurring in water during the generation of laserinduced plasma near the gas−water interface. Laser-induced plasma is generated in pure water in the presence of various gaseous media, such as air, N 2 , and argon. To characterize and quantify the changes occurring in water after the interaction with plasma, UV−vis spectroscopy and ion chromatography (IC) were employed. Significant changes in the optical absorption, pH, and electrical conductivity were detected in water after treatment with laser-induced plasma in the presence of air or nitrogen gas. IC analysis showed the presence of a large number of nitrate ions, which were formed in water due to the interaction of gaseous nitrogen molecules with plasma at the water surface. We found that the formation process of nitrate ions is significantly affected by the position of the laser beam focus and the laser exposure time. We demonstrate that the generation of laser-induced plasma in water near its surface contributes to a significant dissolution of substances present in the gaseous medium just above the water surface while not requiring (according to Henry's law) an increase in gas pressure. Improving the understanding of the physical and chemical processes in the field of the plasma−liquid−gas interaction will find applications in a wide range of fields, including the science of aerosols, atmospheric and colloidal chemistry, phase equilibrium and gas/liquid solubility, biology, water purification, and environmental protection.

EXPERIMENTAL SECTION
Femtosecond laser pulses were generated using a Yb-doped fiber laser system (UFFL-300-2000-1030-300, Active Fiber Systems GmbH) with a central wavelength of 1030 nm and a repetition rate of 50 kHz. An average power of 10 W and a pulse duration of 40 fs were used for the irradiation of liquid water. 25 The laser beam was directed to a computer-driven galvanometric scanning head (FARO tech., Xtreme-20) that controlled the movement of the beam at a speed of 300 mm/s and a scanning interval of 50 μm. Laser radiation was focused using a 160 mm F-Theta lens, which provided a spot diameter of approximately 100 μm and a laser-scanned area of 20 × 20 mm 2 . The scanning beam was directed through a quartz window inside a glass beaker (40 mm in diameter) that contained 30 mL of deionized water. Water purified in a Millipore Elix5/Milli-Q system (a specific resistance > 18.2 MΩ·cm and a pH of 6.4) was used in all experiments. The position of the glass beaker was fixed in a special holder during the irradiation experiments. The focus position of the laser beam was varied by up−down movements of the holder. Air and N 2 and Ar gases with 99.95% purity flowed continuously through a cap with a flow rate of 2 L/min. Figure 1 shows a sketch of the experimental setup.
To study the influence of the environment, a cap was placed on the glass beaker through which the corresponding gas flowed. A double-beam UV−vis−NIR spectrophotometer (Unicam Helios Alpha) was used in the optical absorbance measurement mode. The U-4100 model was used for the measurement of a broad range of wavelengths that extended from 200 to 2500 nm; the U-3900 model was used for shorter wavelength measurements up to 190 nm over a narrower range of 800 nm. The spectrophotometer was operated with a fixed spectral resolution of 2 nm. The measurements were performed in quartz cuvettes. Inorganic ions were determined using a Shimadzu Prominence ion chromatograph system (Japan) equipped with a conductivity detector. Sodium carbonate (Na 2 CO 3 ) and sodium hydrogen carbonate (NaHCO 3 ) from Sigma-Aldrich were used for the preparation of eluents (1.8 mmol/l Na 2 CO 3 + 1.7 mmol/l NaHCO 3 ).
, and SO 4 − ) and cations (Na + , K + , Mg 2+ , and Ca 2+ ) were prepared with Sigma-Aldrich reference solutions. Calibration solutions were made by diluting appropriate standard solutions right before their application. All solutions were kept in glass or high-density polyethylene containers at room temperature. Electrical conductivity and pH were measured using an HI 2300 EC/TDS/NaCl meter (HANNA Instruments, USA) and the Hach sensION+ (Thomas Scientific, USA), respectively.

Effect of the Focus Position on UV Absorbance.
The first step in studying the interaction of water with the femtosecond laser beam was to determine how the properties of water changed depending on the position of the laser-beam focus. The irradiation was carried out under an ambient atmosphere with a fixed time of 20 min and a laser intensity of 6 × 10 13 W/cm 2 to produce continuous plasma. The position of the laser focus was determined by the plasma glow at the lowest laser power. The optical absorption of the samples was measured after laser exposure. Figure 2a shows the typical absorption spectra of water where the laser-beam focus position was varied from 0 mm (water surface) to a depth of 20 mm under the water surface. A broad absorbance peak appeared in the UV region of the spectrum, and no signal was detected at the longer wavelength. As can be seen from Figure  2b, the intensity of the absorbance peak increased as the focus moved deeper from the surface, reaching a maximum absorbance value at a position of 5 mm. With further immersion of the focus, the intensity of the absorption signal decreased significantly. These results clearly show that the position of plasma in water is one of the important parameters in the formation of active radicals in water.
3.2. Effect of the Environment on UV Absorbance. In order to investigate the effect of the environment on the femtosecond laser-induced plasma−water interaction, the glass beaker was tightly closed with a cap through which air and N 2 and Ar gases flowed. All samples were irradiated using the same laser parameters and exposure time and a focus position at 5 mm. Figure 3 displays the spectra obtained under each condition.   Figure 3 reveals that a strong absorbance peak is observed when irradiation was carried out in flowing air and N 2 gas. However, a much weaker peak is obtained in the case of Ar gas. Furthermore, irradiation of the water samples under an air or nitrogen flow gave the same absorbance value at λ max ∼ 220 nm . This clearly points out that the most likely candidate responsible for the appearance of the strong peak is nitrogen gas, which could be dissolved in water during the interaction with the femtosecond laser. It is worth mentioning that the obtained absorbance in Figure 3 is significantly higher than that obtained in Figure 2 under static air irradiation. This increase could be explained by the removal of water vapor that occurs during the laser treatment, which tends to reduce the laser energy and hence its effectiveness in water photolysis. Figure 4a shows the spectra of water irradiated for different exposure times at a fixed focus position of 5 mm under flowing air. Inspection of this figure reveals that the intensity of the absorption spectra increased with increasing laser exposure time with a simultaneous increase in λ max from 214 to 221 nm. Upon analyzing the data for the exposure time dependence of the peak areas, it was found that the absorbance increased nonlinearly. It can be seen from Figure 4b that the intensity of the absorbance peak increased nonlinearly with increasing laser exposure time. It should be mentioned that the intensity of the absorbance peaks did not change even after 3 months.

Content Analysis.
To identify the species formed in water after laser exposure, the samples were analyzed by the IC method. Figure 5a shows chromatogram examples for the water samples irradiated for different exposure times at a focus position of 5 mm and under flowing air and Ar gas. These measurements revealed that only one signal, which corresponds to the NO 3 anion, was observed from the water treated under flowing air. The intensity of the peak increased with The results of pH and electrical conductivity measurements of the treated samples showed an increase in water acidity and electrical conductivity as laser exposure increases. As can be seen from Table 1, after 5 min of irradiation, water acquires acidic properties. For comparison, the absorbance spectrum and pH/conductivity data of nitric acid with a molarity of 0.001 M are shown in Figure 5b and Table 1, respectively.
3.5. Discussion. The laser parameters used in this work (a laser power of 10 W and a pulse duration of 40 fs) together with a diameter of the focused laser spot of 100 μm yielded to an intensity on the order of 10 13 W/cm 2 . Such a high-intensity ultrashort (fs) laser pulse allows for the generation of plasma through ionization of water by multiphoton absorption and by cascade ionization. The plasma creates a wide range of chemical particles and physical effects, which include radical and reactive particles, ions, electrons, UV radiation, electric fields, heat, and shock waves. The basic chemical reactions that occur during the generation of femtosecond laser-induced plasma in pure liquid water are given in eqs 1−5. 22 (2) On the other hand, the formation of long-lived chemical products (O 3 , H 2 O 2 , NO 3 − , and NO 2 − ) in water was shown to be produced by a gaseous discharge plasma at the gas−liquid interface. 20, 24,28,29 Furthermore, these studies revealed that nitrogen products could be formed in water through the dissolution of nitrogen oxides formed in air plasma by gasphase reactions of the dissociated N 2 and O 2 as given in eqs 6−9. 19 The generated reactive species in the gaseous phase move to the liquid phase by convection and diffusion. The parameters that control the solubility of the gaseous species at the water interface are affected by the ratio of species concentrations in the gas and liquid at the interface and by the Henry's law coefficient (k H ). 19 Species that have low values of k H display low solubility and hence are difficult to be transferred from the gas to the liquid phase.
Plasma generation in water close to the surface can contribute to a greater involvement of particles from the gaseous environment in the interaction of plasma and water, thereby increasing their solubility in water. To realize this process, the plasma generated by the laser in water must also propagate outward. The size of the laser-induced plasma is determined by the Rayleigh length (Z R ), which is defined by   Figure 6. Rayleigh length criterion. Z R is the Raleigh length, w 0 is the waist radius, and f is the focal length.
the waist radius (w 0 ) and the wavelength (λ) of the laser as given in Figure 6. 30 Inserting the values of a waist radius (w 0 ) of 50 μm (see the Experimental Section) and a wavelength of the laser of 1030 nm in the equation yields a Rayleigh length of 8 mm and a confocal parameter, which is twice the Raleigh length, of 16 mm. It is also well known that the generated plasma in the liquid can be expanded and increase in its volume by two processes. 23,31 The first process is the thermal expansion around the breakdown site that yields the formation of water bubbles, and the second one is the continued ionization that leads to the formation of a new plasma up the beam path (in the direction of the laser) from the original breakdown site. The generated water vapor, in turn, contributes to the spread of plasma in the air. The observed dependence of the peak intensity on the focus position of the laser as given in Figure 2 could be explained by the size of the plasma in contact with water. Focusing the laser beam at the water surface led to a slight appearance of nitrogen radicals (low absorbance signal) which indicates small contact between the plasma and water, leading to insufficient ionization of water. A deeper focus of the plasma in water leads to an increase in contact volume of water subjected to the plasma, which results in the formation of more radicals that can react with species dissolving from the gaseous phase in contact with the plasma. Furthermore, the formation of a shock wave by laser plasma in water facilitates the diffusion of the reaction products throughout the entire volume of water. Further increasing the depth of the laser focus in water will result in decreasing its contact with the gaseous phase and hence prevent the formation of gaseous particles that can penetrate into water from air, leading to the observed decrease in peak intensity (Figure 2).
To identify the ions contributing to the observed signal in Figures 2 and 4, the treated water samples were analyzed by IC and are presented in Figure 5. As can be seen from this figure, only one peak appeared in the chromatograph, which corresponds to the NO 3 − anion. The absence of the expected appearance of NO 2 − anions can be explained by their transformation into NO 3 − , which is usually accelerated in acidic conditions. 32 The pH measurements of the treated water showed a decrease in its value with an increase in laser exposure time ( Table 1). The observed increase in the concentration of nitrate ions with a simultaneous decrease in pH suggests the formation of nitric acid by laser-induced plasma. This implies that the concentration of nitrate ions and protons in this solution should be equal. To confirm, we have calculated the concentration of protons from the pH value and the concentration of nitrate ions from IC. For 25 min lasertreated water, the pH value was 2.94. This corresponds to a molar concentration of H + of 1 × 10 −3 mol/L. The measured NO 3 − ion concentration in this solution was 50 mg/L. This corresponds to a nitrate concentration of 0.8 × 10 −3 mol/L. These values are in close agreement with each other and hence lend further support to our claim of the formation of nitric acid in the treated water solution.
In order to identify the ions contributing to the observed UV spectra of the laser-treated water, Gaussian function fitting of the absorbance peak was performed, which showed the existence of three signals centered at 196, 217, and 222 nm (Figure 7). These signals can be ascribed to the optical absorption of different forms of NO 3 ions in water. 28 The peak centered at 222 nm could be ascribed to the presence of conjugation of three or more nitrate ions that leads to the formation of trimers, tetramers, or higher order species. The peaks at 217 nm could be ascribed to the existence of monomers and dimers in the solution. These conjugations lead to the observed red shift in the absorption spectra as compared to the spectra of the dilute solution of nitric acid which occurs at λ max of 211 nm (data not shown). The red shift of the absorbance spectrum with increasing laser exposure time can be explained by the formation of a higher order of conjugation, which results in a decrease in the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital in the NO 3 ions. 33 This proposition is supported by the lack of other ions that could be retrieved from IC measurements. Furthermore, hydrogen peroxide (H 2 O 2 ) is excluded from this analysis since its λ max (160 nm) is located outside the wavelength range of the spectrophotometer. 28 The peak at 196 nm, which appeared even after processing in the Ar gas environment (see Figure 3), did not change its position with increasing laser exposure time and, therefore, was attributed to unknown radicals formed in water during laser irradiation.

CONCLUSIONS
The laser-induced plasma−liquid interface provides many fundamental and interesting scientific phenomena such as ionization, sharp explosion, shock wave radiation, bubble creation, and splitting. The coupling of liquid phase plasma kinetics with gas-phase reactivity remains a challenge and requires a systematic study of plasma−liquid−air interactions and subsequent chemical reactions in the liquid phase. In our study, we focused on the effect of the surrounding environment on the processes that occur in water during the generation of laser-induced plasma at the water−gas interface. Plasma was generated in pure water using a femtosecond laser in the presence of different surrounding environments such as air and N 2 and Ar gases. We observed a large number of species from the environment�in particular, nitrate ions� dissolved in water after laser plasma generation near the water surface. Moreover, the number of these species was found to depend on the position of the laser focus in water and on the laser exposure time. The formation of reactive nitrogen (and possible oxygen) species in water can be used for water cleaning and disinfection, which has high impact and tremendous potential in the fields of laser nanosurgery, environmental protection, colloidal chemistry, and gas/liquid solubility. This laser-driven approach can increase the solubility of hydrophobic gases in water at low pressure, which would allow the use of such modified water as a new solvent for green organic chemistry and biology. Moreover, a high solubility of gases in water offers a new and potentially effective way to produce nanobubbles. The high surface areato-volume ratio of nanobubbles can enhance the rates of gas solution reactions 34 and may play an important role in a host of biological phenomena and processes. 35 ■ AUTHOR INFORMATION