Bulk Nanobubbles Fabricated by Repeated Compression of

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Bulk Nanobubbles Fabricated By Repeatedly Compression of Microbubbles Juan Jin, Zhenqiang Feng, Fang Yang, and Ning Gu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04314 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Bulk Nanobubbles Fabricated By Repeatedly Compression of Microbubbles Juan Jin, Zhenqiang Feng, Fang Yang*, Ning Gu* State Key Laboratory of Bioelectronics, Jiangsu Key Laboratory for Biomaterials and Devices, School of Biological Sciences and Medical Engineering, Southeast University, Sipailou 2, Nanjing, Jiangsu, 210009, P. R. China Graphic Abstract Bulk nanobubbles were generated after shrinkage of microbubbles and stronger hydrogen bond was detected by ATR-FTIR spectroscopy during the formation.

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Bulk Nanobubbles Fabricated By Repeatedly Compression of

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Microbubbles

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Juan Jin, Zhenqiang Feng, Fang Yang*, Ning Gu*

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State Key Laboratory of Bioelectronics, Jiangsu Key Laboratory for Biomaterials and

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Devices, School of Biological Sciences and Medical Engineering, Southeast University,

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Sipailou 2, Nanjing, Jiangsu, 210009, P. R. China

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ABSTRACT

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Nanobubbles (NBs), given its extraordinary properties, have drawn keen attention in the

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field of nano-technology worldwide. However, compared to the prevailing method in

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obtaining surface nanobubbles, to obtain the stable bulk nanobubbles remains an

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arduous task. In this study, we developed a pressure-driven method to prepare bulk

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nanobubbles by repeatedly compressing sulfur hexafluoride (SF6) gas into water. The

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results show that NBs with mean diameter of 240 ± 9 nm and PDI of 0.25 were

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successfully prepared. The generated NBs had a high negative zeta potential (- 40 ± 2

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mV) with stability of more than 48 h. Under the condition of 600 times repeated

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compression, the nanobubble concentration could reach about 1.92×1010 bubbles/mL.

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Furthermore, we examine the possible formation mechanism involved in NBs generation

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by virtue of optical microscope and attenuated total reflectance infrared spectroscopy


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(ATR-FTIR). The microscopic results showed that microbubble about 10~50 μm firstly

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formed and then decreased to be nanoscaled size. The stronger hydrogen bond was

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detected by ATR-FTIR spectroscopy during the shrinking of microbubbles into

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nanobubbles. It is speculated that the disappearance of microbubbles contributes to the

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formation of nanobubbles, and the strong hydrogen-bond at gas-water interface prompts

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the stability of NBs. Therefore, by the repeated compression of pressed gas in aqueous

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solution, it could be a new method to prepare stable nanosized bubbles for wide

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applications in the future.

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Keywords:

nanobubbles,

microbubbles,

stability,

hydrogen

bond


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INTRODUCTION

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Recently, free nanobubbles (NBs) have drawn great attention and been studied for decades due to

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their unique physicochemical properties. Nanobubbles can be divided into two categories: surface

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NBs and bulk NBs. The bulk NBs are gaseous domains surrounded by water that are typically tens

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to hundreds of nanometers in radius.1 The bulk NBs have many excellent properties including

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small size, large surface area, rapid attachment to hydrophobic surfaces and high stability, etc.2

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These particularities make bulk NBs have great potential in applications of many areas such as

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water treatment3 and cleaning4, acceleration of metabolism in vegetable/animal species5,

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biomedical diagnosis6 and treatment7, 8.

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Many experimental observations have shown that NBs (both surface and bulk NBs) are quite

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stable9-11. Evidences such as atomic force microscope (AFM)12,13, cryo-scanning electron

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microscopy (SEM)14, transmission electron microscopy (TEM)15 and total internal reflection

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fluorescence microscopy (TIRFM)16, 17 have been reported to demonstrate the existence of NBs.

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In the case of surface nanobubbles, different mechanisms including contact line pinning18-20, local

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supersaturation of the dissolved gas21, permeability of the gas/water interface22 and so on have

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been proposed to explain their unexpected long lifetime. However, there is relatively fewer

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investigation on the long lifetime of bulk NBs. Theoretically, according to Laplace and Young

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equation, Δ P = 2γ/r, when the radius of bubble decreases to nanoscale, the pressure inside

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nanobubble is too high to last more than a second. Thus, it seems that it is not possible that the

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nanobubbles could be remained in the solution. The sparked debate whether nanobubbles were

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stable enough to be observed lasts for about a decade. Several models for the stability of a bulk

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nanobubble have been proposed as reviewed by Yasui et al. 23 The dynamic equilibrium model is

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believed to be the most promising one due to the direct TEM images evidence. However, until

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now, there is no widely accepted explanation for their existence and stability.

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In recent years, various techniques such as sonication24,25, electrolysis26-28, gas mixing and

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supersaturation29 for bulk NBs fabrication have been proposed. However, most of the reported

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methods produce mixed micro- and nanoscale bubbles with multimodal size distribution. It needs

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further separation process to obtain uniform nanobubbles. Oh et al10 fabricated uniformly sized

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NBs by mixing carbon dioxide (CO2) gas with distilled water, but the concentration of NBs is

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relatively low. In order to prepare the concentrated and uniform bulk NBs, in this study, a novel


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and general approach to generate stable bulk nanobubbles was developed. The bulk NBs were

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generated by repeatedly compressing sulfur hexafluoride (SF6) gas into the water under high

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pressure and then returning to the standard atmosphere. Influence factors involved in NBs

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generation and the stability of the generated NBs were investigated in detail. The formation process

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of nanobubbles was monitored by optical microscope and attenuated total reflectance Fourier

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transform infrared (ATR-FTIR) spectroscopy. The possible stabilizing mechanism of NBs was

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proposed which may provide a promising approach to fabricate the shell encapsulated nanobubble

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for the future biomedical applications.

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EXPERIMENTAL SECTION

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Materials. Ultra-pure water with a conductivity of 18.25 Ω/cm was prepared using a de-ioned

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(DI) water purification system. SF6 with the purity of 99.99% was purchased from Anhui

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Qiangyuan Gas Co., Ltd. (Wuhu, China). 3 mL glass vials with open screw caps containing Teflon-

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covered silicon rubber septa and 5 mL syringe fitted with a 22G needle were used to produce

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nanobubbles. All of them were repeatedly cleaned to eliminate nanosized impurities.

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Generation of nanobubbles in water. DI water (2 mL) was added into a glass vial (3 mL) that

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was then capped with open screw caps containing Teflon-covered silicon rubber septa. The above

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air in the vial was replaced with SF6 gas, which was connected with a 5 mL syringe to remain

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additional 3 mL SF6 gas. The tip of the needle was kept 3 mm below the water surface. The septa

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and syringe were tested for leakage at required pressure to maintain no detectable release of gas

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from the glass vial. Then, nanobubbles were generated through repeated change of pressure in the

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vial. As shown in Figure 1, the pressure can be changed by the up-and-down movements of the

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plunger as calculated from the vapor phase volume in the vial using Boyle’ s law.

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The typical process mainly includes three procedures: 1) The compression: The gas was injected

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into water through needle, and the maximum pressure was reached at this time; 2) The mixing

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under pressure: Repeated small volume (1 mL) changes by driving the fluids (gas and water) pass

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through needle resulting in dissolution and mixing of gas in the water above atmospheric pressure;

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3) The decompression to atmosphere pressure: Local supersaturated gas in water was nucleated to

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form bubbles. The up-and-down movements of plunger can be driven by an electric motor and the


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whole process was recorded as one compression cycle time. The operation was conducted at room

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temperature with different compression cycles.

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Figure 1. Schematic diagram of nanobubbles generation.

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Influence factors on NBs generation and stability. Factors such as maximum pressure in vial

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and moving speed of plunger that may affect the generation of bulk NBs were further investigated.

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The maximum pressure was regulated at 0.23, 0.30, 0.37 and 0.44 MPa by changing gas volume,

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all of which were the gas oversaturation under pressure (SF6 is hydrophobic gas, and solubility in

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water is 5.4 mL/L at 25°C). Moving speed of plunger was adjusted by the motor motion, and the

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effect of different speeds at 10, 20 and 30 mm/s on NBs generation was investigated. Moreover, it

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would be necessary to verify the effect of pH on the generation and stability of NBs because the

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pH change may directly affect the adsorption of ions at bubble interface. Thus, water with the

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preadjusting pH value from 1 to 12 was prepared by drop wise addition of HCl (4 M) or NaOH (4

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M) to the distilled water at room temperature, respectively, which was monitored by a pH meter

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(FiveEasy, Mettler Toledo, Switzerland).

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Size, zeta potential and concentration measurements of NBs. The size distribution and zeta

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potential of NBs were analyzed by Zeta-sizer (Nano ZS90, Malvern Instrument, United Kingdom).

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The refractive index of the material was set to 1.0 corresponding to the air. All samples were

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measured without dilution. The visualization and concentration measurements of the generated

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NBs in solution were performed via a nanoparticle tracking analysis (NTA) instrument (NanoSight

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LM10 & NTA 2.0 Analytical Software, Malvern Instrument, United Kingdom). NTA technique


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can monitor the motion of individual nanoparticle and provide size and concentration of the

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samples due to its particle-by-particle measurement.10, 30 The obtained values of the mean size,

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zeta potential and number of NBs are the arithmetically calculated values for a triplicate

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experiment.

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To quantitatively detect the change of freshly generated micro-sized bubbles, a Multisizer (Multi

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4e, Beckman, USA) was also used to measure the size distribution and concentration of

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microbubbles. Bubble solution at different time (0, 5 and 30 min) after preparation was measured.

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Size detection range is from 1.34 - 30 μm. 200 μL of bubble dispersion was added in 100 mL

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electrolysis solution for the measurements.

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Real-time monitoring of microbubbles by optical microscopy. Optical microphotographs of

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bulk bubbles were captured by a real-time live cell optical imaging system (IX71, Olympus Co.

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Ltd., Japan). The freshly prepared nanobubbles dispersion was transferred immediately to the glass

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slide and monitored in real time at room temperature. The optical images of microbubbles were

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observed and recorded over time. Video images were captured as digital images at 5 s intervals.

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Detection of water molecules by ATR-FTIR. To investigate the structure change of water in

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the presence of NBs, IR broad bands for the O-H stretching vibration of the water molecule were

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detected by an attenuated total reflection spectrometer (IRAffinity-1, Shimadzu, Japan) with a

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horizontal ATR plate containing ZnSe crystal. When the bubble dispersion was prepared, the fresh

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sample was immediately added to ATR plate and detected by ATR-FTIR at different time (0, 5,

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10, 15, 20, 25, 30 min). Infrared spectra were recorded with a resolution of 0.5 cm−1, and a total of

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20 scans were measured in the range of 450-4000 cm−1. The analysis was performed isothermally

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at 298 K.

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RESULTS

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Generation of nanobubbles in water. Aqueous NBs dispersion was fabricated by continued

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and repeated compression and mixing of SF6 gas into the water and decompression to atmosphere

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pressure. To examine the presence of nanosized impurities in the DI water, DLS was performed

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and no impurities were found (data not shown). After NBs generation, Tyndall phenomenon was

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observed when a laser beam irradiated the aqueous nanobubble solutions, while there was no light

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path in the DI water (Figure 2a). The observed Tyndall phenomenon suggests stable nano-sized


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particles were formed in water. It is very important to verify the generated nanoparticles are actual

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gas filled nanobubbles. Alheshibri et al. reported the methods to distinguish gas filled nanobubbles

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from nanoparticles by evaluation their compressibility and density of nanoparticles.31,32 Here, we

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further studied the effect of freezing and thawing on NBs dispersion by keeping samples at a

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temperature of -20 °C for 24 h and left to thaw at room temperature. Very weak light path was

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observed in NBs dispersion after freezing and thawing as shown in Figure 2a. DLS measurement

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also showed low detectable number of particles (count rate changed from 280-300 to about 10

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after freezing and thawing). Nirmalkar et al verified that most contaminant of surfactants could

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preserve against the effects of freezing and thawing.30 Thus, the disappearance of majority of

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“nanoparticles” during freezing and thawing implies the rupture of the gas filled nanobubbles. We

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also prepared the infrared active CO2 NBs by this method and the measured IR spectrum proved

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the existence CO2 gas in NBs (further details are described in the Supporting Information and

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Figure S1). These results further provide the evidence of gas filled nanobubbles.

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Figure 2. Characterization of the generated nanobubbles. (a) Tyndall phenomenon of NBs

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dispersion before and after freezing and thawing. (b) Size distribution of NBs. (c) Zeta potential

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distribution of NBs.

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Unlike the pure DI water, NBs can be clearly detected by DLS, which is shown in Figure 2b.

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The NBs size distribution showed a regular shape of a mono-modal log-normal curve and the

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measurements had good repeatability. The size distribution by intensity is from 100 to 600

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nanometers, and the average size of NBs is 240 ± 9 nm. The polydispersity index (PDI) of the NBs

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is 0.25, indicating uniform size of the produced bubbles. As a control, SF6 gas was directly mixed

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with DI water in the same setup but just under the atmosphere pressure. It showed no particle

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detected.

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An important property of NBs in solution is the negative zeta potential. Zeta potential of the

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solution was measured before and after NBs generation. The average zeta potential value of water

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before compression is around zero. After the NBs production, the average zeta potential value as

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shown in Figure 2c is - 40 ± 2 mV with a typical single peak. It is reported that the surface of

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microbubbles is negative charged and the zeta potential increases with the decrease of size due to

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the nano gas-water interface.33 Therefore, the relative high zeta potential of the solution further

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confirms the presence of nanobubbles.

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Influence factors of NBs generation. Factors that may affect the generation of NBs were

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further investigated and results were displayed in Figure 3. Figure 3a shows the average sizes and

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zeta potentials of NBs generated under different maximum pressure. With the increase of the

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maximum pressure from 0.23 to 0.37 MPa, the size of bubbles slightly decreases and the absolute

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value of zeta potential gradually increases. When the pressure is high enough (0.44 MPa), the

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average size becomes larger (312 ± 8 nm) and average zeta potential decreases to be – 31 ± 3 mV.

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Also, the detectable number of particles by DLS is very low (count rate showed small values about

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30). These results indicate that only a few large bubbles are retained after generation under

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maximum pressure of 0.44 MPa. The high pressure may lead to high oversaturation to generate

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big bubbles that finally exploded.


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Figure 3. The effects of (a) maximum pressure in vial, (b) moving speed of plunger, and (c, d)

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preadjusting pH in water on the sizes and zeta potentials of NBs.

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Figure 3b shows the effect of moving speed of plunger on the generation of NBs. The average

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size of NBs is 283 ± 10, 265 ± 7 and 240 ± 9 nm when moving speed is 10, 20 and 30 mm/s

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respectively. With the increase of the moving speed, the absolute value of bubble zeta potential

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becomes larger and larger (- 33 ± 2, - 35 ± 2 and - 40 ± 3 mV, respectively) accompanied by the

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decrease of size. High moving speed means fast and complete mixing of gas and water, which

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results in generation of smaller bubbles with high zeta potential.

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The average size and zeta potential of the generated NBs in water with different pH of 1 to 12

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were shown in Figure 3c-d. The average size decreases as the water pH increases from 1 to 12.

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Monodispersed nanobubbles can be generated at pH 6-12 (PDI 0.17-0.25, Table S1). However,

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bubble suspension has a large PDI value greater than 0.4 under strong acidic conditions (pH 1-5),

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indicating the formation of different sized bubbles. As indicated in Figure 3d, the generated

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bubbles have negative zeta potential under a wide range of pH conditions (pH 3-12). Bubbles show

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positive zeta potential only in strong-acid water below pH 3. The negative value increases with the

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increase of pH and reaches at -72 mV at pH 12. High zeta potential means high colloidal stability,


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suggesting that nanobubbles should be much more stable in alkaline solutions. These results are

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consistent with those obtained by other preparation methods in the literatures.30, 33

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Based on the above results, preparation parameters were chosen for further study as follows:

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maximum pressure of 0.3 MPa, moving speed of 30 mm/s, water pH of 7 (DI water without ions

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addition).

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NBs concentration controlled by compression times. The concentration of NBs can be

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regulated by compression times. The compression times used to generate NBs were 10, 50, and

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100 times. Figure 4a showed the observed Tyndall phenomenon of the NBs dispersion with

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different compression times. The light path showed thicker and brighter in NBs solution when the

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increase of compression times. These results indicate that more nanobubbles were generated with

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the increase of compression times.

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Figure 4. NBs concentration controlled by compression times. (a) Tyndall phenomenon, (b) size

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distributions and (c-e) NTA results of NBs dispersion after 0, 10, 50, and 100 compression times.

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Scare bare: 5 μm.

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The sizes of NBs with different compression times were measured with DLS, which was shown

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in Figure 4b. The intensity peak positions of NBs generated with different compression times

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maintained almost the same value (around 240 nm), but the intensities increased with the increase

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of compression times. The mean sizes of NBs generated with different compression times showed

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no significant differences.


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Particle analysis using the NTA method was further performed to quantitatively investigate the

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concentrations of nanobubbles in water. Figure 4c-e showed particle analysis results of NBs with

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the same dilution factor prepared by different compression times. The measured NBs

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concentrations with compression times of 10, 50 and 100 were 7.62×108, 2.32×109, 3.49×109

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particles/mL, respectively (Table 1). The results showed the concentration of NBs is proportional

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to the compression times. It's worth pointing out that higher concentration of 1.92×1010 can be

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obtained as shown in Table 1 when compression times reached 600 times.

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Table 1. The size and concentration of NBs prepared with different compression times. Times Size (nm) Concentration (particles/ml)

10

50

100

600

195±72

183±68

184±71

175±50

7.62 ×108

2.32 ×109

3.49 ×109

1.92 ×1010

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Stability of bulk nanobubbles in water. In order to investigate the stability of the generated

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nanobubbles, the evolution of the NBs diameter and zeta potential over 48 h was studied, which is

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shown in Figure 5. The average diameter of NBs at the time of 0, 12, 24 and 48 h were 240 ± 9,

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235 ± 5, 242 ± 2, 248 ± 3 nm respectively, showing no significant differences in size during the

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storage period. These results reveal the relatively high stability of NBs in water. It also indicates

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that the fabricated NBs can be maintained in water longer than 48 h, which is consistent with other

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results that NBs can be stable for days or even months.29 On the other hand, zeta potential of the

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NBs in water was about -35 ~ -40 mV within 48 h. The sustaining high zeta potential also indicates

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the stability of NBs in water.

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Figure 5. Stability of bulk nanobubbles in water. (a) Average size and (b) average zeta potential


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of NBs over a period of 48 hours.

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Nanobubbles formation resulting from microbubbles. We further investigate the possible

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formation mechanism of NBs. Milky solution consisting of microbubbles can be observed during

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operation of the nanobubbles generator. The fresh prepared aqueous bubble dispersion was

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immediately dropped on the slide and observed by an optical microscope. Bubble morphology

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change was shown in Figure 6a-b. Lots of bubbles in 10~50 μm size were observed just after

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bubble fabrication at the very beginning. After about 30 min observation, most of the bubbles

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disappeared and only some big bubbles about 50 μm were left.

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Figure 6. Optical image of bubbles in water (a) 0 min and (b) 30 min after generation. (c) Laser-

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illuminated NBs in DI water. (d) Change process of one typical bubble over time. Scare bare: 10

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μm.

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To further investigate the change of bubbles, one typical bubble in size of 30 μm was focused

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and observed. The optical microscopic images of the bubble at time interval of 45 s were captured

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from the dynamics bubble change process (Video S1, Supporting Information). Figure 6d shows


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the typical shrinking process of the bubble. At the beginning of observation (0-135 s), the bubble

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shrinks slowly. With the decrease of bubble size, bubble shrinks faster during the time from 180

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to 225 s. At the end of observation, the bubble disappears almost instantaneously (shown as red

4

arrow). Due to the limitation of resolution of the optical microscope, nanosized bubbles cannot be

5

observed. However, after the disappearance of microbubbles, DLS and NTA results displayed the

6

presence of stable nanobubbles in water (Figure 6c). Bubbles are gas entity surrounded by water,

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and the interior gas is pressurized due to the increased surface tension of gas-water interface.

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Bubble shrinks with the dissolution of the gas. According to the Laplace equation,ΔP = 2γ/r, when

9

the radius of bubble decrease, the pressure inside bubble increases and bubble shrinks faster. We

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speculate that the formation of NBs is related to the shrinkage and disappearance of microbubbles.

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ATR-FTIR measurement of NBs from microbubbles. The change of freshly generated

12

bubbles in micrometer scale were analyzed with particle counter. Figure 7a shows the mean

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diameter and concentration of microbubbles at 0, 5 and 30 min after the preparation. A sharp peak

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near the lower limit can be recognized during size measurement (Figure S2), and the peak grew

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up in accordance with the decrease and disappearance of big sized microbubbles. The mean

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diameter of microbubbles decreases from 3.3 ± 0.2 to 2.5 ± 0.1 μm, and the concentration of

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microbubbles increases from (9.3 ± 0.3) to (13.8 ± 0.3) ×106/mL after 30 min later. The results

18

indicate that big bubbles shrink accompanied by the generation of smaller bubbles. Here, we

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speculated that the disappearance of microbubbles prompts the formation of nanobubbles.

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Figure 7. (a) Change of mean diameter and concentration of microbubbles after generation. (b)

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ATR-FTIR spectra for O-H stretching mode in SF6 bubbles dispersions at different time after

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generation. Insert is overall spectra of water and bubbles dispersions.


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IR spectrum was measured to investigate the structure change of water molecules during

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shrinking of microbubbles. The fresh nanobubbles dispersion was detected by ATR-FTIR

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measurement every 5 min, and the spectrum of DI water was measured as control. The results were

5

shown in Figure 7b. The ATR-FTIR spectra of all samples have three main features: band around

6

3400 cm-1, 1640 cm-1 and 600 cm-1 (shown in insert in Figure 7b). These peaks are attributed to

7

the stretching vibration and bending vibration of O-H. The spectra of bubble water are consistent

8

with those of water, confirming that there are no obvious impurities in the water. For SF6 bubbles

9

dispersions, all the spectra showed red shift compared to that of water. Hydrogen-bonded OH

10

originates from two kinds of water: bulk water in liquid and structured water at the interface of

11

bubbles. Hence, the red shift of the peak should come from SF6 bubbles.

12

Broad band around 3400 cm-1 are usually attributed to stretch modes of hydrogen-bonded OH

13

of water. The frequency of the OH stretching vibrations is correlated with the strength of the

14

hydrogen-bond interaction: the OH frequency decreases with the increase of H-bond strength.34 In

15

addition, amplification of OH stretching vibrations of bubble water from 2700 to 3800 cm-1 with

16

the increase of time is shown. The peak position moves toward to low wavenumber direction over

17

time, indicating the formation of more strongly H-bonded water molecules. The strong H-bond at

18

the interface of gas-liquid is beneficial for decreasing the surface tension and maintaining the

19

nanobubbles stable enough. As discussed above, microbubbles are generated at the beginning. As

20

time goes on, microbubbles would gradually shrink and eventually form stable nanobubbles in

21

water. The ATR-IR results indicate that more strongly hydrogen bonds are formed with the

22

shrinking of microbubbles and generation of NBs.

23 24

DISCUSSION

25

In this study, lots of microbubbles less than 50 μm are firstly generated by cyclic compression

26

and decompression. As observed through microscope, the bubbles decrease in size and eventually

27

disappear underwater because of the rapid dissolution of the interior gas. However, DLS and

28

nanosight results show that the stable nanobubbles are maintained in aqueous solutions.

29

Considering these results, it is speculated that the disappearance of microbubbles results in the

30

formation of nanobubbles. After each compression, a certain number of microbubbles are created


14

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Page 16 of 21

1

and eventually disappeared, accompanied by the production of stable nanobubbles. Once the NBs

2

are formed in solution, they do not easily disappear. Therefore, the final concentration of

3

nanobubbles is the cumulative result of cycle compression. The concentration of NBs is

4

proportional to the compression times. The nanobubble concentration can reach about 1.92×1010

5

bubbles/mL after compression of 600 times, which is higher than the concentrations (2.94×108

6

bubbles/mL by mixing CO2 with water10, 109 bubbles/mL by acoustic cavitation30) reported in

7

literature.

8

DLS and zeta potential measurements show that the generated NBs are stable and their lifespan

9

is longer than 48 h. Other researchers have also demonstrated that NBs can remain in liquid for a

10

long time. Nirmalkar et al reported that bulk nanobubble suspensions were stable over periods of

11

many months30. Oh et al. fabricated NBs using CO2 and showed that the lifespan of generated NBs

12

is longer than 24 h.10 The evolution from freshly generated microbubble to stable nanobubbles was

13

monitored by microscope and IR spectrum. Stronger hydrogen bonds are detected with the

14

decrease of bubble size. Here we presume that stronger hydrogen-bond of water are formed at gas-

15

water interface when the bubble shrinks to nanoscale. Similar interpretation can be inferred from

16

the recent literature. Smolentsev et al reported the interfacial structure of nanosized water droplets

17

(200 nm) in a hydrophobic liquid by vibrational sum frequency scattering measurements.34 The

18

hydrophobic liquid-water interface at nanoscale reveals significantly stronger hydrogen bonds than

19

that at macroscopic planar interfaces. Strong hydrogen-bond may also form at the gas-liquid

20

interface of nanobubbles. Hard interface composed of strongly hydrogen-bonded water can reduce

21

the diffusivity of gas from the nanobubbles and help to maintain a kinetic balance against the high

22

internal pressure. Moreover, it is found that strong acidic condition is not suitable to generate high

23

concentration of monodispersed NBs. Adsorption of OH- at bubble interface in alkaline solutions

24

results in higher zeta potential than that in neutral solutions. Both the arrangement of water

25

molecules in forming hydrogen bonds and the accumulation of OH- ions at the shrinking gas–water

26

interface may attribute to the high negative surface charge on NBs, which creates repulsive forces

27

to stable the nanobubbles.

28 29

CONCLUSION

30

In this study, a simple and novel approach to prepare bulk nanobubbles were developed by


15

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Langmuir

1

repeatedly compressing SF6 gas into water under high pressure and returning to standard

2

atmosphere. Influence factors of NBs generation such as maximum pressure in vial, moving speed

3

of plunger and preadjusting pH in water were investigated. The formation and stability mechanism

4

of NBs were studied. NBs with an average size of (240 ± 9) nm and PDI of 0.25 are prepared with

5

maximum pressure of 0.3 MPa, moving speed of 30 mm/s, water pH of 7. The concentration of

6

the NBs generated in water can be controlled by compression times. The generated NBs have a

7

high negative zeta potential (- 40 ± 2 mV) and can be stable more than 48 h. Freshly prepared

8

microbubbles about 10~50 μm shrinked and disappeared accompanied by the generation of stable

9

monodispersed nanobubbles. IR spectroscopy of bubble dispersion showed red shift compared to

10

water. It’s speculated that stronger hydrogen bond at nanobubbles interface. Therefore, it provides

11

a new and general method to prepared gas nanobubbles such as CO2, O2 or N2 to make a promising

12

platform for basic research and application of nanobubbles.

13 14

AUTHOR INFORMATION

15

Corresponding Author

16

*E-mail: [email protected];

17

[email protected].

18

ORCID

19

Fang Yang: 0000-0001-6922-6348

20

Ning Gu: 0000-0003-0047-337X

21

Notes

22

The authors declare no competing financial interest.

23

ACKNOWLEDGEMENTS


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Page 18 of 21

1

This investigation was financially funded by the project of National Key Research and

2

Development Program of China (2017YFA0104302), National Natural Science Foundation of

3

China (51832001, 61821002, 31370019). And the funding partially also comes from the Six Talent

4

Peaks Project of Jiangsu Province (2017-SWYY-006) and Zhong Ying Young Scholar of

5

Southeast University. The authors also would like to thank the support from Collaborative

6

Innovation Center of Suzhou Nano Science and Technology.

7 8

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Bulk Nanobubbles Fabricated by Repeated Compression of

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