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Significant improvement and mechanism of ultrasonic inactivation to E. coli with piezoelectric effect of hydrothermally synthesized t-BaTiO

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Jinxi Feng, Yao Fu, Xiaosheng Liu, Shuanghong Tian, Shenyu Lan, and Ya Xiong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04666 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018

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ACS Sustainable Chemistry & Engineering

Significant improvement and mechanism of ultrasonic inactivation to E. coli with piezoelectric effect of hydrothermally synthesized t-BaTiO3 Jinxi Feng a,b,1, Yao Fu a,b,c,1, Xiaosheng Liu a,b, Shuanghong Tiana,b, Shenyu Lana,b,*, Ya Xionga,b,*

a

School of Environmental Science and Engineering, Sun Yat-Sen University, No. 135, Xingang Xi Road, Guangzhou 510275, P. R. China

b

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, No. 135, Xingang Xi Road, Guangzhou 510275, P. R. China

c

Nuclear safety and radiation environment management station, No 15, Yingui Road, Xianning City 437000, P. R. China

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Dual first authorship

* Corresponding author: Tel.: +86 20 84115556; fax: +86 20 39332690. E-mail address: [email protected] Mailing address: School of Environmental Science and Engineering, Sun Yat-Sen University, No. 135, Xingang Xi Road, Guangzhou 510275, PR China

Abstract Piezoelectric effect was first used to disinfection. In this process, hydrothermally synthesized nano/micrometer tetragonal-BaTiO3 was selected as the piezo-catalyst which was stressed by ultrasound to generate piezoelectric potential. It was found that the inactivation number of the nano/micrometer tetragonal BaTiO3 exposure or sonication alone within 180 min was 0.30 log and 0.70 log for E. coli in deionized water, respectively, while addition of 2 g tetragonal-BaTiO3 with piezo-activity in the ultrasound system resulted in a 2.72 log 1

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inactivation without the input of any additional energy, only by original ultrasound. The latter was 2.72 folds as great as the sum of the two former inactivation numbers. However, addition of cubic-BaTiO3 without piezo-activity only led to a 1.02 log inactivation. And moreover, in the process, the serious physical damages and chemical oxidations of cell, substantial leakages of intracellular materials were observed. These great differences not only clearly indicated that the combination of sonication and piezocatalysis possessed a great inactivation potential, but also suggested that there was a significant synergistic inactivation effect between piezocatalytic oxidation and sono-mechanical destruction.

Keywords: Piezoelectricity; piezo-catalysis; inactivation; sonication; BaTiO3.

Introduction Disinfection is one of important processes in wastewater treatment, such as domestic sewage and hospital effluent etc., in order to minimize microbial contamination of the receiving waters. Chlorination and UV-radiation are two of the most popular disinfection technologies at present. However, the former suffers from the formation of disinfection by-products that may exhibit increased toxicity for aquatic organisms and humans, particularly at prolonged exposure times. The effectiveness of the latter can be adversely affected by turbidity and solid particle etc. in water, and moreover microorganisms are capable of self-repair. Therefore, many new advanced oxidation processes have been investigated in order to seek for greener and more effective disinfection technology in the past years. In these processes, sonication inactivation attracted attention as early as 1920s since the inactivation could be simply achieved under 2


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ambient condition without any chemical compound. And moreover, researches of application and mechanism for sonic inactivation have been continuously in progress.1,2 However, it is found that sonic inactivation alone needs high intensity, especially for flow system, to achieve an advanced inactivation of microorganisms3,4, and moreover it is not effective for all the microorganisms, such as spores.5,6 These economic and technic defects limit its application in water and wastewater treatment on large scale.7 Fortunately, recent investigations have suggested that the combined technology of sonication and other techniques is more effective than the single sonication for food and juice

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, such as, the combination of ultrasound with

mild heat (thermos-sonication) or low hydrostatic pressure (manothermosonication).10-12 The piezoelectric effect is a kind of physical phenomenon in which piezoelectric materials can mediate the transformation between mechanical energy and electric energy through their deformation from the stress. Although this phenomenon was found as early as 100 years ago, the piezoelectric effect of nano/micrometer materials has been investigated only recently.13,14 A little vibration energy, even muscle movement, can drive the deformation of nano/micrometer materials and hence generate piezoelectric potential. This discovery arouses much research interests in harnessing low-frequency vibration energy in nature because these low-frequency vibration energies, as green energy resources, universally exist in our living environment and is available almost anywhere and anytime.15,16 So far, the related researches have mainly been focused on the design and application of piezoelectric nano-generators and self-powered nano-systems.17,18 More recently, it has been found that the piezoelectric potential could be directly converted into the piezo-chemical potential in-situ in aqueous solution and catalyze the decomposition of H2O to generate

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hydrogen and the degradation of organic pollutants (i.e., piezocatalytic effect). For example, Hong et al. reported that, by simple vibration, microsize ZnO fibers could directly split water into H2 with an ultrahigh energy conversion efficiency of ∼18%.19 Lin et al. reported that the vibration of Pb(Zr0.52Ti0.48)O3 fibers could piezoelectrically degrade about 80% acid orange 7 in solutions (30 µmol L−1) within 50 min.20 Especially, Wu et al. discovered that the single- and few-layer MoS2 nanoflowers possessed an ultrahigh piezocatalytic activity, and the kinetic rate constant reached an amazing 40336 ppmL mol−1 s−1 for the degradation of Rhodamine B dye.21 The piezoelectric potential has been still found to be able to improve the photocatalytic performance. For example, Xue et al. reported that the photocatalytic activity of ZnO nanowires for the degradation of Rhodamine B was considerably enhanced by the piezoelectric process.22 In the same year (2015), Lo et al. also demonstrated that the piezoelectric effect of ZnSnO3 nanowires substantially enhanced its photocatalytic activity (by approximately 27%).23 Li et al. found the piezoelectric potential could not only increase the photocatalytic activity of Ag2O−BaTiO3 hybrid nano-cubes but also endow it a well cyclic property by improvement of Ag2O stability that is lacking for Ag(I)-containing photocatalysts.24 It is noted that in these degradation process, the deformation of piezo-materials is caused all by ultrasound, in other words, the piezoelectric catalytic processes can be driven all by ultrasound. Therefore, the combination of the piezoelectric catalytic process and sonic inactivation is easily carried out because they are both motivated by ultrasound. Moreover, it can be expected that piezoelectric process possible improve efficiency of sonic inactivation because piezoelectric catalytic process can generate many active oxygen species.25 As a new application of piezoelectric effect, this work is mainly devoted to investigating the

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coupling inactivation effect of piezocatalysis and sonication in water or wastewater. These investigations include the improvement of piezocatalysis to sonic inactivation, their synergistic inactivation effect, especially mechanism of the synergistic inactivation etc. In the work, nano/micro tetragonal BaTiO3 (t-BaTiO3) is used as the piezo-material, and E. coli is selected as the target bacterium because it is a typical biological indicator of inactivation number in water systems. The aim of the project is to develop a green advanced inactivation technique for water or wastewater in the future.

Experimental Materials Tetrabutyl titanate (Ti(OC2H5)4), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), trichloroacetic acid (TCA, C2HCl3O2), and thiobarbituric acid (TBA, C4H4N2O2S) were purchased from Aladdin Chemistry Co., Ltd., P. R. China. Malondialdehyde (MDA), Bariu, P. R. China. 2,2,6,6-tetramethylpiperidine (TEMP), AgNO3, Ba(OH)2.8H2O, Dimethyl sulfoxide (DMSO), was afforded by Sinophram chemical reagent Co., Ltd., P.R. China. LB Agar, peptone, yeast extract, and the like were purchased from Guangdong Huankai microbial technology Co. Ltd., China. All chemicals were used without further purification and the solutions for experiments were prepared using deionized water. All glassware and culture medium were autoclaved at 121 °C for 20 min before used.

Preparation of t-BaTiO3 and Ag/t-BaTiO3 Nano/micro t-BaTiO3 was synthesized by a hydrothermal method. The precursor Ti(OH)4

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was prepared by hydrolysis of 50 mL tetrabutyl titanate Ti(OC2H5)4 in 500 mL 60.05 g L−1 acetic acid for 72 h. The resultant precipitation was rinsed with deionized water and then dried for 48 h at 60 oC. Then, 0.21 g L-1 Ti(OH)4 and commercially available Ba(OH)2.8H2O (Ti/Ba=1:1 in molar ratio) were added into 10.00 g L−1 NaOH (30 mL) in a beaker. The mixture containing 6.95 g L−1 Ti(OH)4 and 16.89 g L−1 Ba(OH)2.8H2O was stirred for 1h. The mixture was transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 mL and heated at 200 oC for 68 h, and then cooled naturally to room temperature. The resultant white precipitate was washed extensively with deionized water to remove adsorbed impurities and dried at 105 oC for 24 h. Finally, the t-BaTiO3 powders were grinded and screened over 200 meshes for further use. Ag/t-BaTiO3 was prepared by a photo-reduction method.26 1 g the as-prepared t-BaTiO3 was loaded into a beaker holding 25 mL of 1.70 g L-1 AgNO3 solution. The beaker was placed under a UV illumination source (Philips low pressure UV lamp, 6 W, 254 nm). The distance between the bottom of the beaker and the light source was fixed at 5.5 cm, and the powder was irradiated for 30 min under constant stirring. The powders were then separated from the solution using a centrifuge (4472 g, 10 min), followed by washing with deionized water and absolute ethanol, respectively, and drying at 60 oC. Finally, the resulting powders were grinded and screened over 200 meshes for further use. For comparison, different BaTiO3 samples were obtained by calcining the commercial BaTiO3 at 200 oC, 400 oC, 600 oC, and 800 oC for 2h in muffle oven, respectively.

Analysis and characterization

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X-ray diffraction (XRD) patterns of BaTiO3 powders were obtained with a D-MAX 2200 VPC diffractometer (Rigaku Corporation, Japan) with Cu Kα radiation at 40 kV and 30 mA. The morphology and elemental mapping image of the powders were observed using a scanning electron microscope (SEM, JEOL JSM-6330 F) with an energy dispersive X-ray spectrometer (EDS) (Inca300, Oxford).27 X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo-VG Scientific system (ESCALAB 250, UK) with advantage software for data acquisition and analysis. The BET surface area of the powders was measured by physical adsorption of N2 at 77 K on an auto-adsorption system (Autosorb-6, Quanta chrome) and calculated using the Brunauer-Emmet-Teller equation. Particle sizes were measured with the laser particle size analyzer (Malvern Mastersizer 3000). Reactive oxygen species (ROS) like •OH, •O2− and 1O2 were detected by the electron spin resonance (ESR) technique at ambient temperature (Bruker A300-10-12 spectrometer, Germany) after they were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), DMPO/DMSO and 2,2,6,6-tetramethylpiperidine (TEMP), respectively. The concentration of H2O2 was determined by a typical spectrophotometrical method.28 The formation of malondialdehyde (MDA) was confirmed and analyzed by the thiobarbituric acid method.29 Briefly, 1.0 ml of sample solution was mixed with 2.0 ml of 10% (w/v) trichloroacetic acid (TCA). The cells and precipitated proteins in the solution were removed by centrifugation at 8000 rpm for 10 min. 3.0 ml of the freshly prepared 0.67% (w/v) TBA solution was added to the supernatant. The samples were incubated in a boiling water bath for 10 min and cooled, and then the absorbance at 532 nm was measured. The concentrations of the MDA formed were calculated based on a standard curve for the MDA. The protein leakage was measured by the Bradford method.29 Before the measurement, the

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samples were centrifuged at 12,000 rpm and 4 oC for 10 min, and the supernatant was used for assays.

Inactivation procedure E. coli strain FSCC 149002 was purchased from Guangdong Huankai microbial technology Co. Ltd., China. The E. coli was maintained on a LB agar slant in the dark at 4 °C before use. The bacteria was inoculated in sterilized LB broth and allowed to grow under 160 rpm shaking at 37 °C. After 24 h of propagation, the bacterial cells were separated from the broth by centrifugation (4472 g, 10 min). After proper dilution with deionized water, E. coli suspension with initial cell concentration of about 108 CFU/mL was ready for use. The inactivation investigation was carried out in a glass reactor (h = 500 mm, Φin = 26 mm). For each experiment, 200 mL of E. coli suspensions and 2 g L-1 t-BaTiO3 powders with a stacking density of 0.9421 g/cm3 were added into the reactor, apart from the experiment related to dosage effect. Then the reactor was fixed with air agitation in Branson 3800-CPXH Ultrasonic Cleaner (40 kHz, 110 W，36.7 W/L ). The reaction solutions in all the experiments were maintained around 25 °C in dark. Samples were withdrawn at different intervals of time. The numbers of E. coli cells were obtained using the plate count method. Briefly, the samples were subjected to a series of 10-fold dilutions, and 0.1 mL of each dilution was coated on a LB agar plate and incubated at 37 °C for 24 h before colony counting. The disinfection efficiency is evaluated with inactivation number of cell, eg., the reduction of log-cell number, as shown in equation (1): Inactivation number = logN0 - logNt

(1)

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Where N0 is the initial cell number (CFU) of E. coli, and Nt is the cell number (CFU) of E. coli in water at a given time(t) in the treatment process.

Results and Discussion Characterization of piezocatalysts BaTiO3 and Ag/ BaTiO3 Figure 1 presents XRD spectra of commercial BaTiO3 after subjecting to calcination for 2 h at various temperatures. It can be seen from these figures that six main diffraction peaks appear at 2θ = 22.1°, 31.6°, 38.9°, 45.2°, 56.2° and 65.9°, respectively, for the virgin commercial BaTiO3. The positions of these peaks are consistent with those of cubic BaTiO3 (c-BaTiO3) (JCPDF 31-0714). With rise of the calcination temperature to 800 0C, the peak at 45.2◦ is slowly getting asymmetric and finally presents an apparent shoulder appearance (the inset of Figure 1). The shoulder peak is generally regarded as the partial overlap of the two small peaks at 2θ = 45.1° for (002) and 2θ = 45.3° for (200) (JCPDF 05-0626). Simultaneously, the crystal cell parameters a and b are synchronously decreased from 4.01842 nm to 4.00967 nm, and c is instead increased from 4.01842 nm to 4.05126 nm, as shown in Table 1, leading to increasing of c/a ratio from 1.000 to 1.010. These changes suggest that the calcination treatment could efficiently convert the commercial BaTiO3 from cubic to tetragonal phase.30,31 Calculated through their lattice parameters (a = b = 4.0096 Å and c = 4.05126 Å) (800 °C), the content of its tetragonal crystal increases from ca. 1% to 90.5%，or the content of its cubic crystal decreases from ca. 99% to 9.5%. The calcination process could not only tune the crystal phase but also significantly change its other physical features. As shown in Table 1 and Figure 2, the original commercial BaTiO3 is

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sphere-like particle. The average size of its crystal grains is 29.1 nm obtained by calculation with the X-ray data and Scherrer formula. With the rise of calcination temperature from 25 oC to 800 oC, the average size of their crystal grains is increased from 29.1 nm to 47.7 nm. The increasing of the crystal grain size indicates that the part tiny primary crystal grains are subjected to a dissolution-recrystallization process to form new big crystal grains because the determined size by X-ray diffraction should be the size of homogenous crystallites. The change of BaTiO3 is similar to that of Pb0.85La0.15TiO3 observed by Mesquita et al..32 It has been noted from Table 1 that the average crystal grain size determined with X-ray diffraction is much different from the powder particle size by laser particle size analyzer. For example, as far as the calcined BaTiO3 at 800 oC is concerned, the latter is 36.4 µm, being 763.1 folds as that of the former. Such great difference between them suggested that, in the calcination process, the crystal grains of BaTiO3 were still aggregated into greater heterogeneous species (as shown in Figure 2d) by the simple fusion of particle interfaces,33-35 in addition to forming homogenous greater single crystals by dissolution-recrystallization process. It is believed that the above conversion of cubic to tetragonal phase is mainly dependent on the increase of single crystal grain size, other than that of the aggregates, because it has been confirmed that big crystal grain size of BaTiO3 is beneficial to the stability of its tetragonal phase, although there are still some disagreements about the specific size value.36 For hydrothermally synthesized BaTiO3, its SEM image is analogous to that of the original commercial BaTiO3, and no obvious agglomeration is observed, while the average crystal grain size, tetragonal content and peak position of XRD diffraction are similar to those of the calcined BaTiO3 at 800 °C (Table 1 and Figure 1), but its diffraction peak at 2θ = ca. 45° possesses more

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obvious split feature than that calcined BaTiO3 (inset of Figure 1). These results indicate that the hydrothermally synthesized BaTiO3 possesses more narrow grain size distribution, than that of the calcined commercial BaTiO3, as shown in Figure 3, although their tetragonality are similar. When the hydrothermally synthesized tetragonal BaTiO3 and commercial cubic BaTiO3 are added in AgNO3 solution and illuminated with UV light, the color of the resulting samples are slowly changed from white to dark (inset of Figure 4). The color changes suggest that silver ion is possible photo-reduced and deposited on surface of BaTiO3. The suggestion is confirmed by the presence of silver element in EDS-elemental spectra (Figure 4). And moreover, its high-resolution XPS spectrum presents two asymmetric peaks at 367.6 and 373.6 eV, respectively, in the Ag 3d region, which are indexed to the 3d5/2 and 3d3/2 binding energies of metallic Ag0 species.37 This result confirms that metal Ag nanoparticles are successfully deposited on the surface of BaTiO3. It is also noted that no silver is observed in their XRD and EDS-elemental spectra when the mole ratio of Ag+/BaTiO3 is below 0.001. This is possibly due to the fact that the amount of Ag are too less to be detected by them.

Enhancement of t-BaTiO3 to sonic damage for E. coli cell Cell membrane is an important barrier between cell and the outer environment to protect the cell from external disturbance. Therefore, inactivation of microorganisms usually begins with physical and/or chemical damages of their cell membranes in many cases. In these damages，the chemical oxidization of lipid on cell membrane frequently occurs in many processes because it contains high levels of polyunsaturated fatty acids to be easily oxidized. MDA is considered as

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an indicative product of the oxidation.38 Figure 5 presents the changes of MDA in three treatment processes, simple exposure of t-BaTiO3, single sonication and t-BaTiO3 sono-piezocatalysis. It can be seen from the figure that the exposure of BaTiO3 can cause slight increase of MDA. Considering that BaTiO3 is not a kind of oxidant, the slight increase is possiblly due to BaTiO3-induced oxidative stress from endogenous substances by impairing cellular defense mechanisms.39 When E. coli cells are treated by ultrasound, the concentration of MDA is rapidly increased and reaches a peak value (2.63 mg L-1) at 90 min and then slowly decreased. Addition of t-BaTiO3 in sonic system presents a similar change trend of MDA, but its MDA concentration is much greater than that of single ultrasound treatment. For example, the peak value of the latter reaches as high as 4.78 mg L-1, being 1.8 folds as that of the former. Moreover, the latter is more rapidly decreased than the former after 90 min, resulting in a smaller MDA concentration than that of single sonication at 180 min. The decrease is considered to be due to further degradation of MDA to monoaldehydes, carboxylic acids and or complete mineralization products like CO2 and H2O.40 These differences of MDA change between the presence and absence of t-BaTiO3 suggest that t-BaTiO3 piezocatalysis can considerably enhance chemical damage of the cell membrane. The serious physical damages for E. coli are also observed in the t-BaTiO3-added sonic process. As shown in Figure 6a, SEM image of E. coli presents a full cell structure with a characteristic rod shape (average length ca. 2 µm) before treatment. After subjecting to a 30-min treatment, some adsorbing t-BaTiO3 and a small number of pits appears on the surface although the surface still remains relatively smooth (Figure 6b). But after 60-min treatment, they are changed to a rough surface with many potholes, indicating that the cell membrane

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ruptures (Figure 6c). With prolong of treatment time to 120 min, the bacterial cell is collapsed, appearing a kind of ‘compressed’ morphology. The ‘compressed’ characteristics is generally attributed to the large leakage of intracellular substances and building block.41,42 Finally, the cell membrane structure is decomposed into some debris and suffered from extreme damage after 180 min (Figure 6f). The physical and chemical damages of the cell membrane in t-BaTiO3-added sonic process are further confirmed by continuous leakage of protein with increasing of treatment time, as shown in Figure 7. Specially, although slight changes in the SEM image are observed for a 30-min t-BaTiO3-added sonic treatment, concentration of protein reaches 4.32 mg L-1. The leakage of so many proteins indicates that E. coli cell has actually been ruptured. The protein leakage (4.32 mg L-1) with sono-piezocatalysis was increased by 88.6% and 282.3%, respectively, compared with that of single sonication and simple exposure of t-BaTiO3. The increase of the leakage suggests that the sono-piezocatalytic process can more effectively damage the cell membrane of E. coli than single sonication and simple exposure of t-BaTiO3.

Enhancement of t-BaTiO3 to sonic inactivation Considering that the damage of cell membrane is only used as an indicator of the first step of the bactericidal inactivation,43 the inactivation number of treatment processes are quantitatively evaluated in the term of the reduction of log-cell number, i.e., inactivation number of cell，as described in formula (1). The single sonication possesses certain bactericidal activity, as shown in Figure 8. For example, the number of E. coli is reduced by 0.70 log after subjecting to a 180-min ultrasonic treatment, but the inactivation number (0.70 log) is much less

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than that of industrial requirement (2.0 log) for an antimicrobial agent.8 With addition of t-BaTiO3, the bactericidal efficiency is apparently enhanced. The inactivation number reaches as high as 2.72 log for 2 g L-1 t-BaTiO3-medianted sono-piezocatalysis within 180 min in water. The inactivation number of sono-piezocatalysis is 3.9 folds as that of the single sonication. Particularly, it is noteworthy mention that, for a 30-min sono-pizeocatalysis, the inactivation number reached as high as 1.30 log. In other words, the inactivation efficiency is 95.0%, that is, most of E. coli cells are rapidly killed in the initial phase so that no flat ‘shoulder phase’ with characteristics of slow inactivation is observed in the inactivation curves.44 In the literature,45 for the inactivation of E. coli with the initial concentration of 1.44×105 CFU/L (much lower than 108 used in this paper), the inactivation number is 0.7 log at 350 W by single ultrasonication and only 1.2 log even when the ultrasonic power increased to 1400 W. The result indicates that the sono-piezocatalytic possesses the potential to become a kind of fast and efficient disinfection technology. It can be still seen from Figure 9 that the inactivation of the sono-piezocatalytic process is considerably dependent on ultrasonic power. The inactivation number for 300 W is 2.86 log, being 2.2 times as much as that for 50 W in 180 min. But the difference of the inactivation cell from 100 W to 300 W just is 0.14 log, much less than that between 50 W and 100 W. By overall consideration of the inactivation number and energy saving, the ultrasound with 100 W was used in other investigations of the project. Figure 10 presents inactivation curves of Ag/t-BaTiO3 mediated sono-pizeocatalytic bactericidal process. It can be seen that the deposited Ag can considerably increase sono-pizeocatalytic bactericidal activity of t-BaTiO3. For example, Ag/t-BaTiO3 with silver

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equivalent to 1/1000 of t-BaTiO3 mole dose can cause 4.31 log cell inactivation, much great than that of pure t-BaTiO3 mediated sono-piezocatalytic process (2.72 log). It is well known that metal Ag itself possesses bactericidal activity. After subjecting to a simple exposure treatment with the same dose Ag/t-BaTiO3, E. coli is inactivated by 1.32 log. The inactivation number is 1.02 log higher than that of pure t-BaTiO3 exposure (0.30 log). It is noted that if the difference between cell inactivation of Ag/t-BaTiO3 and t-BaTiO3 exposure is simply regarded as the contribution of Ag exposure to sterilization, the sum (3.72 log) of inactivation number from the metal Ag exposure (1.02 log) and t-BaTiO3 sono-pizeocatalytic process is much lower than the inactivation number of Ag/t-BaTiO3 sono-pizeocatalytic process. The result suggests that there is a significant synergetic bactericidal effect between metal Ag and t-BaTiO3 electric process. It has been reported that the addition of Ag could remarkably increase the dielectric constant of piezoelectric materials by development of a built-in dielectric field, which is an important index characterizing the piezoelectric activity of materials.46,47 Therefore, it is reasonably believed that the enhancement of the sono-piezocatalytic inactivation is partly dependent on the improvement in its piezoelectric property of t-BaTiO3 with the deposition of Ag particles. In addition, in the sono-piezocatalytic process, the piezo-induced negative charges can be rapidly transferred from t-BaTiO3 and temporarily stored to the deposited Ag particles, due to well conductivity of Ag, before deformation recovery of t-BaTiO3. According to the similar principle to photocatalysis,48 the rapid transformation of the piezo-induced electrons can improve their utilization efficiency and also result in the enhancement of the inactivation number. Therefore, Ag/t-BaTiO3 mediated sono-piezocatalytic process is possible a more promise bactericidal technology.

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Mechanism of inactivation In fact, in the t-BaTiO3-sonic system, there are still two other kinds of possible routes to enhance sonic inactivation, t-BaTiO3 exposure and sonocatalysis, apart from piezoelectric catalysis. The first is not the dominate route as indicated above. The second is mainly carried out by increasing nucleation sites of cavitation bubble on surfaces of BaTiO3 solid particles.49 In order to evaluate contribution of the second route to inactivation, c-BaTiO3 was used as a probe material for sonocatalysis because its feature is much similar to that of t-BaTiO3, except for the absence of piezo-activity.28 As shown in Figure 11, the inactivation number of E. coli for a 180-min sonication in the presence of c-BaTiO3 is 1.02 log, which is similar to the sum of inactivation numbers for single sonication (0.70 log) and c-BaTiO3 exposure (0.22 log). Even though c-BaTiO3 is loaded with 1/1000 Ag, the inactivation number is only 1.85 log for a 180 min-sonication (Figure 11). It is similar to the sum (1.80 log) of inactivation numbers for single sonic process (0.70 log) and Ag/c-BaTiO3 exposure (1.10 log). These facts suggest that uncalcined c-BaTiO3 do not basically possess the activity of sonocatalytic sterilization. In other words, they also confirm that the piezo-effect, rather than the sonocatalysis, should be responsible for the enhancement effect of t-BaTiO3 to ultrasonic inactivation. It can still be seen from Figure 11 that the inactivation number is apparently increased with calcination temperature before 400 0C, although their surface area is reduced. For example, the inactivation number of the calcined BaTiO3 at 400 0C is 2.53 log, being 2.5 times as that of the uncalcined c-BaTiO3, while the surface area of the former is 9.1 m2/g, being 27.7% less than that of the latter. The change seems contradictory to the general catalytic principle (the less

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surface area, the lower catalytic activity), but it is in accordance with that of t-BaTiO3 content (Table 1). One possible interpretation for the contradiction is that the positive effect from the increase of piezo-catalyst t-BaTiO3 exceeds the negative effect from the decrease of surface during the calcination process, in terms of piezoelectric activity. This result confirms again that the enhancement effect of t-BaTiO3 to sonic sterilization mainly stemms from the piezocatalytic inactivation, not others. The piezocatalytic inactivation is mainly dependent on three processes: piezoelectricity, piezoelectric-chemical

conversion

and

cell

damage.

The

first

process,

i.e.,

the

mechanical-electric transformation, has been reported in detail.13,15 The second process was supposed by Hong et al.19 to possible generate active radical •OH through a similar photocatalytic mechanism: t-BaTiO3 (e- + h+)

t-BaTiO3 + vibration →

(2)

e- + H2O

→

OH- + •H

(3)

h+ + OH-

→

•OH

(4)

To confirm Hong et al.s’ hypothesis, ESR spectra of t-BaTO3-sonic system were determined after adding DMPO as trapping agent of radicals. As shown Figure 12, a well-defined 4-fold ESR peak with intensity ratio of 1:2:2:1 ESR characteristics of DMPO•−OH is observed in t-BaTO3-sonic system, but not apparent in sonic system alone and c-BaTO3-sonic system. These observations confirm the occurrence of more hydroxyl radical in the t-BaTO3 -piezo system. Apart from the OH radicals mentioned by Hong et al.,19 another active oxygen specie, H2O2 is also found. As shown in Figure 13, the concentration of H2O2 is rapidly increased with sonic time and reaches 61.5 µmol L-1 at 60 min in the presence of t-BaTiO3, but for sonication alone

17



or c-BaTiO3-sonic system, no significant H2O2 is determined. The difference indicates the H2O2 is derived from t-BaTiO3 piezo-chemical process. Moreover, Figure 13 still shows that Ag deposition can considerably increase generation of H2O2. For example, for a 60-min piezo-process, the concentration of H2O2 reaches 160.6 µmol L-1. The concentration is 2.6 folds as that of H2O2 from pure t-BaTO3. Unexpectedly, it is found by comparison of Figure 10 and Figure 13 that increase of H2O2 is inconsistent with that of the inactivation number. If the inactivation number of Ag/t-BaTiO3 self without ultrasound (1.32 log) is deducted, the difference between inactivation numbers from sono-piezocatalytic processes of Ag/t-BaTiO3 and t-BaTiO3 is small (0.27 log), being much less than difference (99.1 µmol L-1) of their H2O2, although the inactivation number of the former (4.31 log) is much greater than that of the latter (2.72 log). Therefore, the enhancement of piezocatalytic process to sonication inactivation does not mainly originate from the increase of H2O2. The observation can be understood because the bactericidal activity of H2O2 at low concentrations was very low.50-52 Specially, the concentration of H2O2 is determined in pure water, not in E. coli suspension, it will be reduced by the enzymes like catalase leaked from ultrasound damaged cells, which may decompose the piezo-generating H2O2 to O2.53 Therefore, it can be inferred that the enhancement effect of piezocatalytic process to sonic inactivation is possible originated from the above determined OH radicals for it possesses stronger oxidation power. In order to confirm the possibility, 0.5 mmol L-1 isopropanol is added in the t-BaTiO3 sono-piezocatalytic inactivation system because isopropanol has been widely used as a kind of OH radical scavenger in studies aiming to track its effect.54 It is found that isopropanol can indeed reduce the inactivation number to 1.1 log for 180 min. The inactivation number is just

18


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40.7% as that of the sono-piezocatalytic system without isopropanol. If the inactivation number (0.7 log) of sonication alone is simply deducted, the inactivation number after OH radical capture is only 14.7% without the radical scavenger. The facts indirectly suggest that other active oxygen species is not significant, except for OH radicals. It is well known that ultrasound possesses mechanical and sonochemical inactivation effects. However, many investigations have shown that ultrasound inactivation, especially with low frequency as-used in the project, is mainly attributed to mechanical effects of acoustic cavitation.55 That is, the cavitation can generate shock waves or liquid Jets with approximately 100 m/s in formation and collapse of cavitation bubble in solution.1 The shearing force of these shock waves or liquid Jets can result in deformation and rupture of cell membranes. Piezo-generating OH radicals to cell inactivation is mainly by oxidation damage of cell external membrane through the initiating lipid oxidation chain reactions as indicated by the main product MDA of OH radicals exposure in Figure 14. It has been confirmed that the oxidation damage can weaken the chemical structure of the bacteria cell membranes.56 It can be reasonably believed that these weakened cell membranes are more susceptible to the mechanical destruction of cell membrane by the sonic shock waves. In turn, the mechanical destruction is beneficial to nano-BaTiO3 and piezo-generating ROS to enter cell, leading to its inner damage, because the destruction can increase permeability of the cell membrane. Therefore, it can be inferred that piezocatalytic oxidation and the cooperative effect between the oxidation and sonic mechanical destruction are responsible for the significant enhancement effect of t-BaTiO3 to sonic inactivation. Based on the above observations and analysis, the main pathways of the sono-piezocatalytic inactivation using piezo-catalyst t-BaTiO3 can be described

19



below： (Please see Figure 14)

Conclusions The peizocatalytic process is an emerging ‘green’ technology. It is found that for the piezocatalysis to increase inactivation number in water without any additional energy and chemical consume, only by the original ultrasound. In the process, the serious physical damages and chemical oxidations of cell, substantial leakages of intracellular materials were observed. These damages, oxidations and leakages finally resulted in a lot of cell inactivation. Its inactivation number reached as high as 2.72-log within 180 min. It was 2.72 folds as the sum of inactivation numbers from t-BaTiO3 exposure and sonication alone. The great increase clearly indicated that there is a significant synergistic effect of inactivation between t-BaTiO3-mediated piezocatalytic oxidation and ultrasonic mechanical destruction. These findings direct that the sono-piezocatalytic process possesses the potential to be developed into a green advanced disinfection technology.

Acknowledgments This research is supported by the Nature Science Foundations of China (21677180), Science and Technology Key Projects of Guangdong Province (2014B020216004, 2015B020237005) and Science and Technology Research Programs of Guangzhou City (201510010083)

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For Table of Content Use Only Figure 1. XRD spectra of the commercial BaTiO3 after subjecting to calcination for 2h at various temperatures. Figure 2. SEM photographs of the BaTiO3 powders: (a) c-BaTiO3; (b) calcined c-BaTiO3 at 200 oC; (c) calcined c-BaTiO3 at 400 oC; (d) calcined c-BaTiO3 at 800 oC, the inset is a local

enlargement;

(e)

hydrothermally

synthesized

BaTiO3 (t-BaTiO3);

(f)

Ag/t-BaTiO3. (All is 50000x, beside (d) is 5000x). Figure 3. Particle size distribution of various BaTiO3 powders. Figure 4. EDS of Ag/BaTiO3 power (Inset：photopicture of Ag/t-BaTiO3 and elemental mapping of Ba, Ti and Ag from left to right). Figure 5. Changes of MDA concentration in various processes (t-BaTiO3: 2.0 g L-1). Figure 6. SEM images of E. coli after piezocatalytic treatment for (a) 0 min, (b) 30 min, (c) 60 min, (d) 120 min, (e) 150 min, (f) 180 min. Figure 7. Leakage of protein in t-BaTiO3 (2.0 g L-1) piezocatalytic process. Figure 8. Dependence of inactivation number on reaction time for different dosage of t-BaTiO3 in sono-piezocatalytic bactericidal process (Inset: inactivation number of the different dosages at 180 min). Figure 9. Dependence of inactivation number on reaction time for ultrasound powers in t-BaTiO3 (2.0 g L-1) sono-piezocatalytic bactericidal process (Inset: inactivation 30


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number for different ultrasound powers at 180 min. Figure 10. Changes of inactivation number with time in nAg/t-BaTiO3-medianted sono-piezocatalytic process. Insert is the comparison of the inactivation number by single nAg/t-BaTiO3 and nAg/t-BaTiO3-medianted sono-piezocatalysis within 180 min. The n value indicates that the molar ratio of silver to t-BaTiO3 (nAg/t-BaTiO3: 2.0 g L-1). Figure 11. Changes of inactivation number with sonic time in the presence of various c-BaTiO3 (A: Ag/c-BaTiO3 (800 oC, 2.0 g L-1) with sonication; B: Ag/c-BaTiO3 (800 oC, 2.0 g L-1); C: Calcined c-BaTiO3 (800 oC, 2.0 g L-1) with sonication; D: Sonication; E: c-BaTiO3 (2.0 g L-1)). Figure 12. ESR spectra of BaTiO3-sonic system at various conditions (BaTiO3: 2.0 g L-1). Figure 13. Piezo-generation of H2O2 in various sonic conditions (BaTiO3: 2.0 g L-1). Figure 14. Schematic mechanism of sono-piezocatalytic inactivation using piezo-catalyst t-BaTiO3.

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Table 1. Performance parameter of various BaTiO3.

Lattice parameters a or b c Samples

c/a (Å)

*1

(Å)

Commercial BaTiO3

4.01842

4.01842 ~1.000

BaTiO3 (200°C)

4.01239

4.01239

BaTiO3(400°C)

Content Crystal*1 Powder*2 Inactivation grain of number size particle t-BaTiO3 （log） size (µm) (nm) (%) ~1

29.1

3.89

1.02

1.001

18.8

34.1

4.63

1.74

4.01069 4.04247

1.008

89.8

39.7

5.51

2.53

BaTiO3 (800°C)

4.00967

4.05126

1.010

90.5

47.7

36.4

0.99

Hydrothermal BaTiO3

4.00824

4.0352

1.007

89.7

41.2

6.11

2.72

Ag/t-BaTiO3*3

4.00824

4.00824

1.007

89.7

45.3

//

4.31

Average size of crystal grains is determined by X-ray diffraction.

*2

Average size of powder particles is

determined by laser particle size analyzer. *3The mole ratio of Ag and t-BaTiO3 is 10-3:1.

Synopsis：：The piezo-disinfection is a new green process driven possible by environmental vibration energy, even muscle movement, not chemicals.

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Graphical abstract 29x8mm (600 x 600 DPI)



Figure 1. XRD spectra of the commercial BaTiO3 after subjecting to calcination for 2h at various temperatures. 66x48mm (300 x 300 DPI)


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Figure 2. SEM photographs of the BaTiO3 powders: (a) c-BaTiO3; (b) calcined c-BaTiO3 at 200℃; (c) calcined c-BaTiO3 at 400℃; (d) calcined c-BaTiO3 at 800℃, the inset is a local enlargement; (e) hydrothermally synthesized BaTiO3 (t-BaTiO3); (f) Ag/t-BaTiO3. (All is 50000x, beside (d) is 5000x). 87x55mm (300 x 300 DPI)



Figure 3. Particle size distribution of various BaTiO3 powders. 75x64mm (300 x 300 DPI)


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Figure 4. EDS of Ag/BaTiO3 power (Inset：photopicture of Ag/t-BaTiO3 and elemental mapping of Ba, Ti and Ag from left to right). 49x27mm (300 x 300 DPI)



Figure 5. Changes of MDA concentration in various processes (t-BaTiO3: 2.0 g L-1). 69x54mm (300 x 300 DPI)


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Figure 6. SEM images of E. coli after piezocatalytic treatment for (a) 0 min, (b) 30 min, (c) 60 min, (d) 120 min, (e) 150 min, (f) 180 min. 87x55mm (300 x 300 DPI)



Figure 7. Leakage of protein in t-BaTiO3 (2.0 g L-1) piezocatalytic process. 69x54mm (300 x 300 DPI)


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Figure 8. Dependence of inactivation number on reaction time for different dosage of t-BaTiO3 in sonopiezocatalytic bactericidal process (Inset: inactivation number of the different dosages at 180 min). 69x53mm (300 x 300 DPI)



Figure 9. Dependence of inactivation number on reaction time for ultrasound powers in t-BaTiO3 (2.0 g L-1) sono-piezocatalytic bactericidal process (Inset: inactivation number for different ultrasound powers at 180 min). 69x53mm (300 x 300 DPI)


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Figure 10. Changes of inactivation number with time in nAg/t-BaTiO3-medianted sono-piezocatalytic process. Insert is the comparison of the inactivation number by single nAg/t-BaTiO3 and nAg/t-BaTiO3-medianted sono-piezocatalysis within 180 min. The n value indicates that the molar ratio of silver to t-BaTiO3 (nAg/tBaTiO3: 2.0 g L-1). 68x51mm (300 x 300 DPI)



Figure 11. Changes of inactivation number with sonic time in the presence of various c-BaTiO3 (A: Ag/cBaTiO3 (800 oC, 2.0 g L-1) with sonication; B: Ag/c-BaTiO3 (800 oC, 2.0 g L-1); C: Calcined c-BaTiO3 (800 oC, 2.0 g L-1) with sonication; D: Sonication; E: c-BaTiO3 (2.0 g L-1)). 71x56mm (300 x 300 DPI)


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Figure 12. ESR spectra of BaTiO3-sonic system at various conditions. 76x65mm (300 x 300 DPI)



Figure 13. Piezo-generation of H2O2 in various sonic conditions. 74x62mm (300 x 300 DPI)


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Figure 14. Schematic mechanism of sono-piezocatalytic inactivation using piezo-catalyst t-BaTiO3. 47x11mm (300 x 300 DPI)



Table 1. Performance parameter of various BaTiO3. 59x39mm (300 x 300 DPI)


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Significant Improvement and Mechanism of ... - ACS Publications

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