The Effect of Gold Nanoparticles Concentration and Laser Fluence on

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The Effect of Gold Nanoparticles Concentration and Laser Fluence on the Laser-Induced Water Decomposition Aleksander V. Simakin, Maxim E. Astashev, Ilya V. Baimler, Oleg V. Uvarov, Valery V. Voronov, Maria V. Vedunova, Mikhail A. Sevost’yanov, Konstantin N Belosludtsev, and Sergey Vladimirovich Gudkov J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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The Journal of Physical Chemistry

The Effect of Gold Nanoparticles Concentration and Laser Fluence on the Laser-Induced Water Decomposition

Aleksander V. Simakin a, Maxim E. Astashev b, Ilya V. Baimler a,c, Oleg V. Uvarov a, Valery V. Voronov a, Maria V. Vedunova d, Mikhail A. Sevost’yanov e, Konstantin N. Belosludtsev f, Sergey V. Gudkov a,d,g,*

a. Prokhorov

General Physics Institute of the Russian Academy of Sciences, 38 Vavilova

st., Moscow 119991, Russia b. Institute

of Cell Biophysics of the Russian Academy of Sciences, 3 Institutskaya st.,

Pushchino, Moscow region 119991, Russia c. Moscow

Institute of Physics and Technology, Institutsky lane 9, Dolgoprudny, Moscow

region, 141700 Russia d.Institute

of Biology and Biomedicine, Lobachevsky State University of Nizhny

Novgorod, 23 Gagarin Ave., Nizhny Novgorod 603950, Russia e. Baikov

Institute of Metallurgy and Materials Science of the Russian Academy of

Sciences, 49 Leninskiy ave., Moscow 119334, Russia f. Mari

State University, 1 Lenina pl., Yoshkar-Ola, Mari El, 424001, Russia

g. Moscow

Regional Research and Clinical Institute (MONIKI), 61/2 Shchepkina St.,

Moscow 129110 Russia * - Corresponding author: Tel: +7915 153 0850; e-mail: [email protected]

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Abstract The article covers the influence of concentration of gold nanoparticles on laser-induced wated decomposition. It was established that addition of gold nanoparticles intensifies laser-induced water decomposition by almost two orders of magnitude. Water decomposition rate was shown to be maximal at nanoparticles concentration around 1010 NP/ml, whereas decrease or increase of nanoparticles concentration leads to decrease of water decomposition rate. It was demonstrated that if concentration of nanoparticles in water-based colloid was less than 1010 NP/ml, laser irradiation of the colloid caused formation of molecular hydrogen, hydrogen peroxide and molecular oxygen. If concentration of nanoparticles exceeded 1011 NP/ml, only two products, molecular hydrogen and hydrogen peroxide, were formed. Correlations between water decomposition rate and main optical and acoustic parameters of optical breakdown-generated plasma were investigated. Variants of laser-induced decomposition of colloidal solutions of nanoparticles based on organic solvents (ethanol, propanol-2, butanol-2, diethyl ether) were also analyzed.

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1. Introduction The first studies of optical breakdown of different media were carried out right after the invention of powerful optical pulse lasers.

1

It is known that optical breakdown of condensed

matter generates plasma 2, UV-, visible and IR radiation 3, acoustic waves 4, and leads to intensive chemical conversion of the matter itself, particularly, its decomposition. 5 The efficacy of laser-induced breakdown of condensed media, for example, organic compounds or water, is known to be increased significantly by addition of small amoutns of nanoparticles, especially metals. 6 Nanoparticles serve as initiators of the breakdown, and the optical breakdown leads to chemical7 and physical8 modification of the nanoparticle/medium properties, including the decomposition of water.9-11 It was shown recently, that decomposition of the medium in which optical breakdown is observed occurs to a greater extent on nanoparticles, and not on a solid target. 12 The search in Google Scholar (https://scholar.google.com) by search tags «interaction of individual nanoparticles with laser irradiation, optical breakdown, liquids» finds almost 18,5 thousand articles. Around 9000 of them are dedicated to fragmentation of nanoparticles (if the word “fragmentation” is added to the search tag), around 8000 cover ablation (+ “ablation” to the tag), and around thousand more are dedicated to various optical effects, including non-linear ones. Around one thousand works are focused directly on interaction of laser radiation with nanoparticles. The major part of them describes plasma formation on individual nanoparticles and chemical transformations of nanoparticle or its surface. The data on other processes occuring along with optical breakdown of colloidal solutions of nanoparticles is often scarce. We will try to list the main known facts related to the planned study below. First, a large amount of redox equivalents is formed under optical breakdown of water. The main reduction equivalents are free electrons, their local concentration can be supposed to reach 1020-1022 cm-1 .13,14 A part of these electrons is hydrated and forms solvated electrons, their concentration being up to 0.1 M 3



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(solvated electron concentration is measured in moles per litre).15 Optical breakdown plasma allows to reduce metal ions effectively even in solutions saturated with molecular oxygen, where it works as electron scavenger

16

From the other hand, as it was shown also in our works,

hydrogen peroxide and a number of radical compounds formed during optical breakdown can play a role of powerful oxidants.17,18 In such conditions, the slightest changes would be observed in nanoparticles from the least reactive metals such as gold and, in lower degree, iridium, platinum and palladium.

19

We used gold nanoparticles because gold is a rather inert material, it

possesses well-known physicochemical properties and has a number of important applications in related fields.20-26 Water is a solvent used most often for production of colloidal solutions of nanoparticles.

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Water consists only of hydrogen and oxygen isotopes, so that optical breakdown and further ionization generate predominantly oxygen-hydrogen plasma. 27 Of course, atmospheric gases are often present in water, which can make some contribution into the processes of interest. 28,29 The main short-living products of water ionization are ОН (hydroxyl radical), Н (atomic hydrogen) and solvated electrons.

30

There are only three stable products, Н2, О2 and Н2О2; sometimes,

quasi-stable ozon is also mentioned.18 For comparison, optical breakdown of any organic solvent, for example, ethyl alcohol, which consists of carbon, hydrogen and oxygen, leads to formation of several dozens of products with low yields. Moreover, optical breakdown of organic solvents does not lead to generation of solvated electrons, they are only present in the medium as free particles for very short times.30 Thus, the processes occurring during optical breakdown are basically known in present time. In this work, the influence of gold nanoparticle concentration on laser-induced water decomposition was investigated. Some physical and chemical processes occurring during optical breakdown and their dependences on concentration of nanoparticles were shown in strictly controlled conditions. Physical and chemical properties of plasma (spatial distribution, time 4


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distribution, optical characteristics), ultrasound, generation rate of hydroxyl radicals, molecular hydrogen, molecular oxygen and hydrogen peroxide influenced by optical breakdown were studied. 2. Experimental methods 2.1 The process of obtaining gold nanoparticles Gold nanoparticles were obtained by laser ablation of a solid target in deionized water. Au bulk was located at the bottom of a glass cuvette under a thin layer of working fluid (several millimeters) and irradiated with radiation of Ytterbium fiber laser (90 J/cm2 at 1060-1070 nm, a pulse width of 90 ns, a repetition rate of 20 kHz). Morphology of the generated nanoparticles was investigated using Libra 200 FE HR transmission electron microscope (Carl Zeiss). The size of the nanoparticles was determined using an analytical centrifuge DC24000 (CPS Instruments).31

Fig. 1. Scheme of the experimental setup. The laser pulses were focused by a fused silica lens F1 (ƒ = 9 cm) into water or aqueous colloids of gold nanoparticles. Filament inside the solution was imaged by a CCD camera with bandpass filters in front. Acoustic vibrations induced by laser radiation were recorded by an accurate film sensor. The recovered gases were detected by sensors for molecular oxygen and molecular hydrogen. 5



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2.2 Experimental Setup The experimental setup is shown in Fig. 1. Irradiation of NPs colloidal solutions in the absence of the solid target diluted in required proportion was carried out using the radiation of the Nd:YAG laser at a wavelength of 1064 nm and pulse duration of about 4 ns (FWHM). Laser radiation was focused inside the liquid by an F-Theta objective with focal distance of 90 mm. Laser beam was scanned across the window along linear trajectory about 20 mm in length at the velocity of 3000 mm/s by means of galvanometer mirror system. Laser exposure of 20 ml portions of colloids was carried out at 2 mJ energy per pulse and repetition rate of laser pulses of 8 kHz. Estimated diameter of the laser beam waist was 40 µm, which corresponds to laser fluence in the liquid of 140 J/cm2. Bright line of plasma appeared 2-3 mm above the window inner surface. Amperometric sensors for molecular oxygen and molecular hydrogen were integrated into the experimental setup. Information from the sensors was analyzed with the help of oxygen and hydrogen analyzers АКPМ-1-02 and АVP-02 (Alfa Bassens, Russian Federation). Before the measurements, the sensors were calibrated. All procedures have been previously described in detail.32 To register acoustic oscillations induced by laser radiation of colloidal solutions, a highly sensitive film sensor was used. The sensor was connected to a 300 MHz digital oscilloscope GW Instek GDS-72204, which served as a recorder and primary digital processor. The images of the visible range of plasma were taken using the digital reflex camera Canon EOS 450D, lens Tamron SP Di Macro 90 mm 1: 2.8 lens, macro 1: 1 (macro mode, grayscale, exposure time 10 ms, ISO 800). In order to block the scattered IR laser radiation, two IR dielectric mirrors (Thorlabs), one above the other, were placed on the camera lens. Typical photos are shown in Figure 2. Each photo represents a time interval during which 80 laser pulses pass parallel to each other through a visible region in the photo. First photo shows that optical breakdown was induced in deionized water; in this case, single breakdowns of about 1 breakdown per 100 laser pulses were observed. In the second photo of fig. 2 the optical 6


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breakdown was induced already in an aqueous colloid containing 109 NP / ml, as one can see, the number of optical breakdowns increases noticeably. In the third photo (Fig. 2), one optical breakdown is observed for each laser pulse, in rare cases there is no breakdown or two breakdowns are observed. In the fourth photo a series of breakdowns is observed for each laser pulse when the concentration of nanoparticles is 1011 NP / ml. On the right side of Fig. 2, there is a scheme that represents a case where several optical breakdowns have been recorded per a single laser pulse in a colloid of nanoparticles. In this case, using our software, one can calculate the number of optical breakdowns per laser pulse, the distance between them, the color intensity and the area occupied by the optical breakdown in the photo, etc.

Fig. 2. Examples of images of optical breakdowns in the visible region of spectrum and a scheme of processing. On the left, there are representative images of optical breakdowns in the visible region of spectrum depending on the concentration of nanoparticles in the cuvette. On the right, a schematic drawing of plasma bursts that remain after passage of the laser pulse track is shown.

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If one knows the number of optical breakdowns per one laser pulse in one photo sequence, the average number of optical breakdowns on laser pulse and probability of optical breakdown (the number of laser pulses, which include at least one optical breakdown over a total number of laser pulses in sequence) could be calculated. In current work we also calculated an average intensity of individual optical breakdown and a mean distance between breakdowns. Detailed program description is given below. 2.3 Signal processing For image processing, we developed LaserPhotoImage special software. To download LaserImage

program

click

(https://drive.google.com/drive/folders/1YRNF2p7qpejlGP55QBiqM108LSGAseaE).

link A

computer program algorithm consists of the following steps: 1. Construction of distribution histograms of the fluorescence intensity of tracks in the space of motions of the laser beam using the single-lens reflex optical system with the possibility to correct probable matrix rotation of the camera relative to the optical system axis of the motion of the laser beam according to the formula: S ( x )= ∑ I ( x,y ) y

where I(x,y) are the values of the fluorescence intensity which were corrected based on the coordinate grid (transformed through image rotation), x is the coordinate along the motion of the laser beam, y is the coordinate along the optical breakdown track, S(x) are the values of the histogram of the distribution of the fluorescence intensity of tracks. 2. Determination of the position of individual laser pulse tracks by the peaks of the histogram obtained in the previous step. 3. Calculation of the dependence of the fluorescence intensity on the position along the track for every individual tack by the formula:

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x +Δx

i

S i ( y )=

∑

I ( x,y )

x=x i− Δx

where xi is the position of a laser track, Δx is a halfwidth of the fluorescence intensity of the track. 4. Determination of the position of optical breakdown within the limit of every individual track by peaks of the dependency of the fluorescence intensity on the position along the track obtained in the previous step. 5. Determination of the characteristics for each breakdown (the distance between neighboring breakdowns, light energy of burst in breakdown) and characteristics for each individual track (the number of breakdowns in the track, total light energy in the track , mean light energy in the breakdown in the track and so forth). 6. Establishment of the statistical characteristics for every experiment. Analyzing images with this program, the user gets comprehensive qualitative description of the experiment. The program can analyze images in automated mode and calculate a great number of parameters (Fig. 2). It should be noted that not less than 500 tracks of laser radiation were analyzed per one experimental point. The program may calculate the probabilities of the breakdown under given conditions, the number of optical breakdowns formed in response to one laser pulse (a track), the fluorescent intensity of each breakdown, the distance between breakdowns in one laser track, the square of the track cross sectional area etc. The program also allows one to take images in terms of probable vibration load and the absence of the alignment between axes of a digital matrix of the camera and laser system of beam motion. The program includes algorithms of the automated image rotation and color correction. 2.4 Detection of OH-radical production Detection of OH-radical production was performed using coumarin-3-carboxylic acid (3CCA). The product of 3-CCA hydroxylation, 7-hydroxycoumarin-3-carboxylic acid, is a suitable 9



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fluorescent probe for the detection of hydroxyl radicals.33 A 0,5 мМ solution of 3-CCA was prepared in phosphate-buffered saline (Sigma, USA) at рН 7.4. Nanoparticles were added into saline directly before exposure to laser radiation. Fluorescence of 7-ОН-CCA generated by reaction of 3-CCA with hydroxyl radical was measured using SRG laser spectrofluorometer where λex = 405 nm, λem = 470 nm. Due to the sensitivity of the method, it was possible to detect concentration of 7-ОН-CCA of the order of 1 nМ. Calibration was carried out using commercial 7-ОН-CCA (Sigma, USA) and after exposure to ionizing radiation at a dose of 0-100 Gy30, and the results of calibrations were compared.34 2.5 Detection of hydrogen peroxide production A highly sensitive enhanced chemiluminescent technique based on luminol-p-iodophenolhorseradish peroxidase was used for detection and quantification of hydrogen peroxide in aqueous solutions. The luminescence was detected by Biotoks-7АM chemiluminometer (Econ, Russia). The concentration of the produced hydrogen peroxide was calculated from the calibration curves plotted by the measured values of chemiluminescence intensity of the templates containing the added hydrogen peroxide in a particular concentration.35 The initial concentration of Н2О2 used for calibration was determined spectrophotometrically at a wavelength of 240 nm with the molar extinction coefficient of 43,6 (М-1  cm-1).36 Samples (3 ml) were put in polypropylene vials (Beckman, США) and supplied with 0,15 ml of “counting solution” containing 10 mM Tris-HCl buffer solution, рН 8.5, p-iodophenol, 50 μМ, luminol, horse radish peroxidase, 10 nМ when nanomolar concentrations of Н2О2 were detected. “Counting solution” was prepared directly before the measurement. Due to the sensitivity of the method it was possible to determine Н2О2 in a concentration of 0.1 nМ.37 In a number of experiments, we removed hydrogen peroxide from a colloidal solution of nanoparticles using enzymes of the peroxiredoxin family. 38,39 2.6 High resolution gas chromatography of organic solvents 10


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Colloidal solutions of gold nanoparticles based on organic solvents were investigated (ethanol, propanol-2, butanol-2, diethyl ether). Laser irradiation was carried out in standard conditions. Optimal irradiation time for the samples comprised 30 min. At shorter irradiation times, concentration of some products did not reach the sensitivity threshold. The samples were sealed and brought to high resolution gas chromatography right after the irradiation. Analysis of the product produced in organic solvents after their laser ablation was performed using Chromatec-Crystal 5000.2 gas chromatograph with flame-ionization detector (FID). The capillary column used was Agilent DB-FFAP, 50m  0,32mm  0,5 µm. Hydrogen was used as gas-carrier in constant flow mode. Injection of the sample with the separation of the flow was automated (autosampler DAJ 2M 3D), the program Chromatec-Analytic 2.6 was used for processing. The algorithm of the processing was described previously.40

Fig. 3. Characteristics of gold nanoparticles. А – Absorption Spectrum of a Water Colloid of Nanoparticles; B – Nanoparticle size distribution by weight determined by an analytical centrifuge; C – Size distribution of nanoparticles from TEM images; D – ТЕМ image of gold nanoparticles. Scale bar, 20 nm. 11



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3. Results 3.1 Characterization of gold nanoparticles The generated gold nanoparticles were characterized by size and morphology (Fig. 3). As it can be noticed, absorption spectrum of Au nanoparticles has a maximum at 520 nm (Fig. 3a) that corresponds to the transverse plasmon resonance of spherical nanoparticles with size of 1020 nm.41 This data is confirmed by TEM imaging and size distribution of Au NPs (Fig. 3b-d). As can be seen, nanoparticles have a size of less than 30 nm. Remarkably, the portion of nanoparticles with average sizes of 15-20 nm amounts to more than 96% of all the gold weight in colloid while the number of nanoparticles with average sizes of 6-10 nm is less than 4% (Fig. 3B). The data obtained by disc centrifuge correlated with TEM image (Fig 3С). As can be seen, all nanoparticles have a spherical shape (Fig. 3D). Zeta potential of gold nanoparticles equals 43 mV.

Fig. 4. Influence of the concentration of gold nanoparticles on characteristics of plasma luminescence of laser-induced optical breakdown in visible region of the spectrum at laser fluence of 140 J/cm2. A – dependence of the probability of optical breakdown on the 12


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concentration of nanoparticles. B - dependence of average number of breakdowns per one laser pulse on the concentration of nanoparticles. C - average distance between the breakdowns in a track depending on the concentration of nanoparticles. D - average intensity of spark depending on the concentration of nanoparticles.

3.2 Influence of gold nanoparticles on characteristics of plasma luminescence of optical breakdown in the visible region of the spectrum Fig.4 shows the influence of the concentration of gold nanoparticles on characteristics of plasma luminescence of optical breakdown in the visible region of spectrum. It has been shown that the probability of laser-induced optical breakdown depends, to a great extent, on concentration of nanoparticles (Fig. 4A). In deionized water without any nanoparticles, the probability of optical breakdown approaches 1%. The probability of optical breakdown per one laser pulse increases exponentially with the increase of concentration of nanoparticles up to 21010 NP/ml. The probability of optical breakdown at such concentration of nanoparticles is close to 90%. Further increase of the concentration of nanoparticles up to 2x1011 NP/ml resulted in a continuous decrease of the probability of laser-induced optical breakdown to 80%. The dependence of the average number of breakdowns per one laser pulse on the concentration of nanoparticles was studied (Fig.4B). The average number of breakdowns per laser pulse in pure water containing no nanoparticles was about 0.01. At the concentration values about 109 Np/ml the optical breakdown occured approximately with every fifth laser pulse. If the concentration of the nanoparticles reached 1010 Np/ml, there was only one optical breakdown per one laser pulse. With the increase of the concentration of the nanoparticles up to 1011 Np/ml, each laser pulse induced nearly 5 optical breakdowns on average. The dependence of the average distance between the breakdowns in a track on the concentration of nanoparticles was studied (Fig. 4C). It has been shown that the average distance between the breakdowns in a track changed non-monotonously with an increase in the 13



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concentration of nanoparticles. Maximum distances between the breakdowns were seen at the concentration of nanoparticles of 0.5-2.01010 NP/ml. When concentration of nanoparticles was 109 NP/ml and less, it was rather difficult to estimate the average distance between the breakdowns, the number of laser pulses leading to the formation of two and more optical breakdowns was not sufficient. It should be noted that the change in the distance between the breakdowns was the most variable value which had the widest distribution in the samplings. The influence of concentration of nanoparticles on the average intensity of the light flow in optical breakdown was studied (Fig.4D). The integral intensity of one optical breakdown decreased monotonously with the increase of concentration of nanoparticles. The brightest spark in optical breakdown was observed at small concentrations. This dependency was quite obvious (Fig.2).

Fig.5 Influence of laser fluence on characteristics of luminescence of optical breakdown in the visible region of the spectrum. A –dependence of the probability of optical breakdown on laser fluence. B – dependence of the average number of breakdowns per one laser pulse on laser

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fluence. C – average distance between the breakdowns in a track depending on laser fluence. D – average intensity of spark depending on laser fluence.

Fig. 5 presents the influence of energy in a laser pulse on characteristics of light in optical breakdown in the visible region of the spectrum. It has been shown that the curve that represents the probability of optical breakdown as a result of the laser fluence (Fig.5A), and the curve that represents the average number of optical breakdowns per one laser pulse as a result of laser energy (Fig.5B) are sigmoidal and very similar to each other. The dependence of the average distance between the breakdowns in a track on laser fluence was explored (Fig. 5C). It was shown that the average distance between the breakdowns in a track decreased with the decrease of laser fluence up to 110 J/cm2. Further increase of fluence almost did not change the distance between breakdowns at concentration 1011 NP/ml. At lower concentrations of nanoparticles, the increase of mean distance between the optical breakdowns was observed. It was rather difficult to study the process when fluence was less than 70 J/cm2, whereas the number of laser pulses leading to the formation of two or more optical breakdowns was insufficient. For nanoparticle concentrations less than 4×109 NP/ml, such difficulties occurred even at 100 J/cm2 fluence value. The impact of laser fluence on the average intensity of the light flow in optical breakdown (Fig.5D) was examined.

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Fig. 6. Dependence of average intensity of acoustical oscillations induced by laser radiation on the concentration of gold nanoparticles and laser fluence. 1 – 70 J/cm2; 2 – 85 J/cm2; 3 – 100 J/cm2; 4 – 110 J/cm2; 5 – 125 J/cm2; 6 – 140 J/cm2

3.3 Influence of gold nanoparticles on the intensity of acoustic vibrations induced by laser radiation The influence of the concentration of gold nanoparticles and laser fluence on the intensity of acoustic oscillations induced by laser radiation was studied (Fig.6). It was shown that the intensity of ultrasound vibrations decreased with a decrease of laser fluence. This dependency is valid for all studied range of concentrations of nanoparticles. All used values of laser fluence exhibited clear peaks of the intensity of acoustic vibrations when the concentration of nanoparticles was about 1010 NP/ml. On the whole, there was a tendency to a more rapid decrease in the intensity of acoustic vibrations with a decrease in the values of laser fluence. The contribution of acoustic vibrations measured in the cuvette to total intensity was investigated with complex wavelet transformation and fast Fourier transforms. The main most significant harmonic curve with contribution of about 90% could be seen in the interval of 125 μs. This period agreed well with the frequency of tracking laser pulse of 8 kHz. Several harmonic curves, including high frequency ones, were present in the signal. 16


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Fig.7 Formation of laser-induced hydroxyl radicals. A – influence of concentration of nanoparticles on generation of laser-induced hydroxyl radicals (fluence 140 J/cm2). B – influence of laser fluence on generation of hydroxyl radicals (1010 NP/ml). The insert shows accumulation of hydroxyl radicals in time.

3.4 Influence of gold nanoparticles on generation of reactive oxygen species under the effect of plasma of laser-induced optical breakdown The formation of hydroxyl radicals under the effect of laser-induced optical breakdown is shown in Fig.7. It should be noted that after exposure to laser radiation different polyhydroxylated molecules of our probe were detected for OH-radicals. This indicates that plasma of optical breakdown damages the molecules of the probe. Probably, we can consider analytical measurement of concentration of hydroxyl radicals only at plasma boundary. It has 17



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been shown that concentration of nanoparticles has an influence on generation of hydroxyl radicals under effect of laser-induced optical breakdown (Fig. 7A). When the concentration of nanoparticles grows up to 1010 pieces/ml, a reliable increase of radiation-chemical yield of hydroxyl radicals is observed. Maximum generation of hydroxyl radicals is observed at concentration of gold nanoparticles of 1010 pieces/ml. At further increase of concentration of nanoparticles, the rate of the formation of hydroxyl radicals decreases significantly. The dependence of generation of hydroxyl radicals on laser fluence is close to being sigmoidal (Fig.7B). The rate of generation of hydroxyl radicals in the studied system remains virtually unchanged within 10 minutes (Fig.7B, insert). The formation of hydrogen peroxide under effect of laser-induced optical breakdown is presented in Fig. 8. It was shown that the concentration of nanoparticles had an effect on the formation of hydrogen peroxide under the effect of laser-induced optical breakdown (Fig. 8A). It has been found that maximum rate of hydrogen peroxide formation was observed when concentrations of nanoparticles ranged from 0.5 to 5.0 x 1010 pieces/ml. In general, the rate of hydrogen peroxide formation at concentrations beyond the range mentioned above was 3 times less. The dependence of hydrogen peroxide formation on laser fluence was also close to sigmoid (Fig. 8B). The rate of the formation of hydrogen peroxide in the studied system remained actually the same within 60 minutes (Fig. 8B, insert). The effect of laser radiation of the same power but much lower energy density, which is not able to lead to an optical breakdown of colloidal solutions of nanoparticles, on aqueous solutions was studied. For this purpose, lens F1 was removed from the experimental setup (Fig. 2). It has been shown that under these conditions the formation of hydrogen peroxide with the rate of 200 nmol/min was also observed. The initial amount of oxygen in the irradiated aqueous solution had an influence on the rate of generation of hydrogen peroxide. Solutions from which

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hydrogen peroxide was completely removed by the enzyme peroxiredoxin 2 were used as controls.

Fig.8. Formation of laser-induced hydrogen peroxide. A –influence of concentration of nanoparticles on the formation of laser-induced hydrogen peroxide (fluence 140 J/cm2). B – influence of laser fluence on the formation of hydrogen peroxide (1010 NP/ml). The insert shows accumulation of hydrogen peroxide in time.

3.5 The influence of gold nanoparticles on the formation of molecular oxygen and molecular hydrogen under the effect of plasma of optical breakdown induced by laser radiation The formation of molecular oxygen and molecular hydrogen in terms of laser-induced optical breakdown is presented in Fig. 9A. It was found that the maximum rates of the formation of both molecular oxygen and molecular hydrogen were observed when the concentration of 19



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nanoparticles was 1010 NP/ml. As can be noticed, the rates of the formation of molecular oxygen and molecular hydrogen were approximately 4 times higher than those at concentrations of nanoparticles of 5  108 NP/ml. The dependence of generation rate of molecular hydrogen and molecular oxygen on laser fluence was close to sigmoid (Fig.9B).

Fig. 9. The formation of molecular oxygen and molecular hydrogen under exposure to laser radiation on Au colloidal solution. A –influence of concentration of nanoparticles on the formation of laser-induced molecular oxygen and molecular hydrogen (fluence 140 J/cm2). B – influence of laser fluence on the formation of molecular oxygen and molecular hydrogen (1010 NP/ml).

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3.6 The formation of molecular products under the effect of plasma of laser-induced optical breakdown in organic solvents Probably, the formation of different chemical products in water is restricted by the limited number of combinations of atoms and also because it is impossible to obtain long-chain compounds only with the use of the atoms of oxygen and hydrogen. An attempt was undertaken to estimate the influence of plasma of optical breakdown on the formation of molecular products in organic solvents (ethanol, propanol-2, butanol-2 and diethyl ether). Table 1 represents the main condensed thermostable volatile products produced during the optical breakdown in the mentioned organic compounds. Table 1. The rate of the formation of different chemical products during optical breakdown in ethanol, propanol-2, butanol-2 and diethyl ether.

Substance Acetaldehyde CH3-CH=O Methanol CH3-OН Acetone CH3-СO-CH3 Ethanol CH3- CН2-OН Isobutyl acetate CH3-CO-O-CH2- CH-(СН3)2 Methyl acetate CH3-CO-O-CH3 Propanol-1 CH3- CН2-CН2-OН Propanol-2 CH3-CH(OH)-CH3 Ethyl acetate CH3-CO-O- CH2-CH3 Crotonaldehyde CH3-CH=CH-CHO Benzaldehyde C6H5-CH=O Butanol-1 CH3-(CH2)3-OH Butanol-2 (CH3)2-CH-CH2-OH Butanone-2 CH3-C(O)-CH2-CH3 Isoamylol (СН 3)2-СН-СН2-СН2-ОН Ethyl butyrate CH3-CO-O-CH2-C3H7 * – this compound is the reaction medium.

Rates of formation of chemical products during optical breakdown in the medium, pmol / min Diethyl Ethanol Propanol-2 Butanol-2 ether 8296,6 3128,3 1388,4 2786,1 335,5 2720,5 261,3 82,3 50,8 5212,9 200,6 * 204,6 786,5 8779,5 1,8 12,1 9,7 68,8 36,5 49,1 29,7 277,1 * 1657,6 861,2 30,1 44,1 2,8 29,1 5487,9 14,2 * 14,6 99,2 19,1

21



Page 22 of 48

4. Discussion This study shows that the interaction mechanism of laser beam with the colloid depends considerably on the concentrations of nanoparticles in this colloid (Fig.4,6-9). In deionized water without any nanoparticles the probability of optical breakdown approaches 1%. Apparently, the breakdown in deionized water occurs on bubstons - stable gas bubbles of submicron size. The physicochemical principles of this phenomenon are discussed in papers.42,43 It has been demonstrated that at concentration of nanoparticle up to 1010 NP/ml there was an increase in the probability of breakdown, in the number of breakdowns induced by one laser pulse (Fig.4), and in the intensity of acoustic vibrations induced by the laser radiation (Fig.5). As concentration of nanoparticles increased up to 5 x 1010 NP/ml, the probability of the breakdown reduced (Fig.4), with a decrease in the intensity of acoustic vibrations (Fig.5). Interestingly that when the concentration of nanoparticles was about 1010 NP/ml, maximum distances between optical breakdowns were recorded (Fig.4C). Thus, the interaction mechanism of the laser beam with the colloid was altered significantly if the concentration of nanoparticles was above 5 x 1010 NP/ml. Taking into account that the average intensity of one breakdown continuously decreased with an increase of the suspension concentration, it is possible to conclude that qualitatively the interaction of laser beam with nanoparticle remained unchanged. Clearly, the laser beam intensity became insufficient to stimulate breakdown. This was apparently related to the absorption of radiation before the focal region, as well as with the thermal defocusing inside the focal neck.44 Thus, we can suggest that the highest medium decay rate was observed at nanoparticle concentration around 1010 NP/ml. At such concentration, maximal rates of generation of hydroxyl radicals, hydrogen peroxide, molecular oxygen and hydrogen were observed (Fig. 7-9). The data on medium decomposition correlated well with data on intensity of acoustic 22


Page 23 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oscillations.

For instance, for the data on acoustic oscillations and oxygen concentration

R2=0,97 (Fig. 6). As for luminescence studies, at nanoparticle concentration of 1010 NP/ml order the probability of breakdown under laser irradiation stopped to increase, while the distance between optical breakdowns reached its maximum. It is interesting to note that generation rate of OH-radicals grew the most slowly with nanoparticle concentration, in contrast to hydrogen peroxide and gases (Fig. 7). This fact is in concordance with luminescence data and allows to propose a significant contribution of OH-radicals recombination process into water decomposition under laser irradiation. The curves, that represent the probability of optical breakdown (Fig.5A) and that represent the average number of breakdowns per one laser pulse (Fig.5B) from laser fluence, are likely to be sigmoids. This dependency is conventional for most of lasers and media.45-47 In fact, with these sigmoidal diagrams it is possible to estimate critical laser fluence.48 However, we chose another way. As it is seen from Fig 5C, critical fluence for the studied system is about 110 J/cm2. It is the point from which a decrease in the average intensity of optical breakdowns in the visible region slowed down a little with an increase in concentration of nanoparticles (Fig.5D). It is known that three stable products (molecular oxygen, molecular hydrogen, and hydrogen peroxide) and about ten of short-living products (hydroxyl radical, hydroperoxide radical, superoxide anion radical, etc.) are formed during ionization of pure water.5 Hydrogen peroxide, hydroxyl radical, hydroperoxide radical, superoxide anion radical, etc. are often called reactive oxygen species.49,50 In the current paper, we have shown the process of formation of all three stable products (Fig. 8,9) and also the process of the generation of hydroxyl radical (Fig. 7). It is established that the generation rates of various products depend significantly on the concentration of nanoparticles. Figure 10 demonstrates the ratio between the number of the molecules generated per unit of time under optical breakdown of the colloidal solution containing different concentrations of gold nanoparticles. 23



Page 24 of 48

Fig. 10. The ratio between the number of molecules generated per unit of time under optical breakdown in colloidal solutions containing different concentrations of gold nanoparticles. A –influence of concentration of nanoparticles (fluence 140 J/cm2). B –influence of laser fluence (1010 NP/ml). It was shown that in wide range of concentrations of nanoparticles two molecules of hydrogen peroxide and four molecules of hydrogen were formed per one molecule of oxygen (Fig. 10A). Taking that stoichiometry into account, one can assume that general photolysis equation is as follows: 6H2O  4H2 + 2H2O2 + O2

(1)

24


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This ratio did not change significantly in the fluence range studied by us (Fig. 10B). With an increase of the concentration of nanoparticles in a colloidal solution over 1010 NP/ml, the number of oxygen molecules relative to the number of hydrogen and hydrogen peroxide molecules decreased greatly. When the concentration of gold nanoparticles was about 1011 NP/ml, 8 hydrogen molecules and 6 hydrogen peroxide molecules were formed per one oxygen molecule. The general equation of photolysis of water is as follows: 14H2O  8H2 + 6H2O2 + O2

(2)

Obviously, there was a tendency of the amount of produced oxygen to

decrease.

Thus, the main remaining products were only molecular hydrogen and hydrogen peroxide. Herewith, the general equation of photolysis of water probably tended to the following form: 2H2O H2 + H2O2

(3)

This fact could be explained as follows. Photolysis of water took place due to partial homolytic dissociation (4), ionization (5) and complete homolytic dissociation (6). These processes require 4.8, 12.6 and 20.7 eV, respectively.51 H2O  H + OH

(4)

H2O  H2O+ +e

(5)

H2O  H + O + H

(6)

In addition, a hydroxyl radical (Fig. 7) and a hydrogen atom were formed, during the least energy-consuming process of dissociation (4). It could lead to the formation of molecular hydrogen, hydrogen peroxide, and water as a result of recombination (7).52 H + H H2; HO + OH H2O2; HO + H H2O

(7)

All other molecular and radical products could be formed only in ionization (5) and complete homolytical dissociation (6) processes. Free electron and atomic oxygen are the main agents in these processes. The free electron can interact with all participants of the processes described above. The chain of reaction leading to the formation of superoxide anion radical is the 25



Page 26 of 48

most interesting for us. The superoxide anion radical is protonated in water forming a hydroperoxide radical.53 Molecular oxygen, most likely, is the secondary product, being formed during dismutation of hydroperoxide radicals (8), which are formed at protonation of superoxide anion radicals.54 Also, molecular oxygen in insignificant quantities can be formed due to the decomposition of hydrogen peroxide (9). 55 Probably, there is still a channel for recombination of atomic oxygen (10), which can also arise under the action of plasma.56 Most likely this is the least likely way of generating molecular oxygen. НO2 + O2Н  H2O2 + O2

(8)

2H2O2  2H2O + O2

(9)

O + O

O2

(10)

One can assume that, having the lowest energy, partial homolytic dissociation process must prevail at high concentrations of nanoparticles (4) while ionization processes (5) and complete homolytic dissociation processes (6) are largely inhibited. Likely, it takes place because of partial scattering and defocusing of laser radiation. The situation seems to be undoubtedly more complicated when organic solvents are used during optical breakdown.57 Consider a situation when diethylene ether is employed. Its main products are ethanol and acetaldehyde (Table 1). They are likely to form when the ether bond of diethylene ether is broken, the overall reaction is: CH3- CH2-O-CH2-СН3  CH3-CH2-O + CH2-СН3

(11)

Thereby, acyl and ethyl radicals are formed (11). The future of these compounds is apparently different. Acyl radical can be added to ethanol, acetaldehyde, ethyl acetate and its alkylated derivatives.

58

Ethyl radical can be added to gaseous products, for example ethane or

heavy products of dimerization. 59 Using the products listed in Table 1, there is, at least, one else type of the breakage of a chain from diethyl ether, the overall reaction is: CH3- CH2-O-CH2-СН3  CH3- CH2-O-CH2 + СН3 26


(12)

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This reaction provides the formation of the products with an odd number of carbon atoms (methanol, propanol, methyl acetate). The formation of the products with an odd number of carbon atoms is almost two orders of magnitude less effective as compared to that one with even number of carbon atoms. Herewith, the energy of disruption of such bond is higher only by tens of the percents as compared to the energy of disruption of the ether bond. Also, it can be assumed that the following reactions are occurring in the solution of diethyl ether: CH3- CH2-O-CH2-СН3  H + С4ОН9

(13)

CH3- CH2-O-CH2-СН3 e + С4ОН10

(14)

These reactions provide the reduction potential of all reactions in the system. Molecular oxygen, the concentration of which is maintained in the system by supplying an atmospheric air through convective mixing, is seemed to be an oxidant.60 In ethanol, propanol-2, butanol-2 the same processes are likely to occur.

5. Conclusions 1.

It was shown that optical breakdown in liquids happens more often in two orders of magnitude with adding nanoparticles in the liquid. The dependence of the intensity of chemical processes under the laser radiation on the concentration of nanoparticles had a sigmoidal form. Despite the fact of fragmentation of nanoparticles under the optical breakdown the generation rates of a number of products were constant for a long period of time.

2. Maximal generation rates of the stable chemical products of the optical breakdown in the

liquid were observed if only one optical breakdown per one laser pulse occurred in water colloid. The conditions when there were several optical breakdowns per one laser pulse were not optimal from the point of view of laser chemistry.

27



Page 28 of 48

3. Under the optical breakdown in the liquid with nanoparticles, there were two different

ways of laser-induced water decomposition. The main pathway was the prevailing partial homolytic dissociation process. The molecular hydrogen and hydrogen peroxide were the primary products in this case. The second mode was observed only at a low concentration of nanoparticles, when the generation of molecular oxygen began to be observed. 4. The new original optical technique allowing to explore and find optimal conditions

leading to optical breakdown in liquids with nanoparticles was developed.

Conflicts of interest The authors declare that there are no conflicts of interest.

Acknowledgment This work was supported by a grant of the President of the Russian Federation for the support of young scientists (MD-3811.2018.11) and the Russian Foundation for Basic Research (17-44-500476_r-a, 16-02-01054_a, 18-52-70012 e-Asia_a and partly 19-02-00061_a). The optical part of the work was carried out with the support of R&D program (AAAA-A18118021390190-1). The authors are grateful to the Center for Collective Use of the GPI RAS for the equipment provided. The authors are grateful to Vladimir Maximovich Vozniak, Director of Independent test laboratory for food, feed, water, biomaterial and pharmaceuticals (TestPushchino), for his help with the chromatographic work.

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TOC Graphic

37



Fig. 1 85x63mm (300 x 300 DPI)


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Fig. 2 85x63mm (300 x 300 DPI)



Fig. 3 85x63mm (300 x 300 DPI)


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Fig. 4 85x63mm (300 x 300 DPI)



Fig. 5 85x63mm (300 x 300 DPI)


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Fig. 6 85x73mm (300 x 300 DPI)



Fig. 7 85x125mm (300 x 300 DPI)


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Fig. 8 85x120mm (300 x 300 DPI)



Fig. 9 85x124mm (300 x 300 DPI)


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Fig. 10 85x137mm (300 x 300 DPI)



TOC Graphic 63x47mm (300 x 300 DPI)


Page 48 of 48

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