Antifouling and antibacterial behaviors of capsaicin-based pH responsive smart coatings in marine environments

Journal Pre-proof Antifouling and antibacterial behaviors of capsaicin-based pH responsive smart coatings in marine environments Xiangping Hao, Shougang Chen, Dong Qin, Mutian Zhang, Wen Li, Jincheng Fan, Chao Wang, Mengyao Dong, Jiaoxia Zhang, Frank Cheng, Zhanhu Guo PII:

S0928-4931(19)33201-1

DOI:

https://doi.org/10.1016/j.msec.2019.110361

Reference:

MSC 110361

To appear in:

Materials Science & Engineering C

Received Date: 28 August 2019 Revised Date:

12 October 2019

Accepted Date: 22 October 2019

Please cite this article as: X. Hao, S. Chen, D. Qin, M. Zhang, W. Li, J. Fan, C. Wang, M. Dong, J. Zhang, F. Cheng, Z. Guo, Antifouling and antibacterial behaviors of capsaicin-based pH responsive smart coatings in marine environments, Materials Science & Engineering C (2019), doi: https:// doi.org/10.1016/j.msec.2019.110361. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

Antifouling and Antibacterial Behaviors of Capsaicin-based pH Responsive Smart Coatings in Marine Environments Xiangping Hao,a, b Shougang Chen,a* Dong Qin,a Mutian Zhang,a Wen Li,a Jincheng Fan,c,g Chao Wang,d,g Mengyao Dong,e,g, Jiaoxia Zhang,f,g Frank Cheng,a, b and Zhanhu Guog*

a. School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China. b. Department of Mechanical Engineering, University of Calgary, Calgary, Alberta, T2N 1N4, Canada c.

College of Materials Science and Engineering, Changsha University of Science and Technology, Changsha 410114, China

d. School of Materials Science and Engineering, North University of China, Taiyuan 030051, China e. Key Laboratory of Materials Processing and Mold (Zhengzhou University), Ministry of Education; National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450002, China f. School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China g. Integrated Composites Laboratory (ICL), Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996, USA * Correspondence authors Email: [email protected]; [email protected]

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Abstract: Antifouling biocides releasing restricts the longevity of antifouling coatings. Compared with the anchoring state, the releasing behavior of agents is much faster on the voyage, while the biofouling process is tougher. In this work, a series of capsaicin-based pH-triggered polyethylene glycol/capsaicin@chitosan (PEG/CAP@CS), polyvinyl alcohol (PVA)/CAP@CS and alginate (ALG)/CAP@CS multilayer films are prepared with controlling antimicrobial properties in marine environments. There are 23.70, 23.35 and 22.06 ppb CAP releasing from (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 films after immersing in pH 4 solutions for 60 days, while only 13.07, 12.95 and 11.55 ppb CAP have been found in alkaline solutions after immersing for the same time, respectively. All these three types of films exhibit extraordinary pH responsive properties. They can control the CAP release at a low level in alkaline solutions, and make the CAP release fast in acid solutions. Moreover, the antibacterial properties against P.aeruginosa are outstanding about 95.84%, 95.0% and 96.91% for (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 films, respectively. The bacteriostasis of (ALG/CAP@CS)20 film keeps 92.73% after 60 days in alkaline solution, which means it is steadily controlled in the marine environment. Although with similar antibacterial properties to those of (PEG/CAP@CS)20 film, (PVA/CAP@CS)20 film displays the maximum decrease with about 92% in acid solution after 60 days. The ALG/CAP@CS film with the best-controlled release performance and long-term antibacterial properties provides novel guidance for developing new antifouling coatings application in the marine environment.

KEYWORDS: Layer-by-layer; multifunctional; pH-responsive; eco-friendly; Antifouling. 2

1. Introduction Environmentally friendly smart coatings have been used for antibacterial and antifouling purposes in marine engineering [1-3]. The conventional technique is to add biocides in the coatings to make them possess antibacterial and antifouling capabilities. With the prohibition of trisubstituted organotin compounds (TOCs) [4, 5] and the restriction of antifouling agents containing heavy metal substance copper and zinc [6-8], using alternatives like synthetic and natural biocides [9-14] have been searched to prepare coatings with outstanding antifouling properties while mitigating their impact on the environments. Generally, the biocide agents can be released from the coatings more rapidly when the ships are in sailing than in anchoring. However, the microorganism colonization tends to be developed on the ships earlier at anchoring. Obviously, the high-speed releasing of the biocide agents during the shipping will result in the waste of agent, while the slow releasing of the agents at anchoring is not able to effectively inhibit the biofouling. A controlled releasing of the biocides from the antifouling coatings on demand is critical to the functional sustainability of the antifouling coatings, which facilitates the development of smart coating technology to meet the immediate needs.

The main methods to build smart antifouling coatings include the preparation of unique film structures such as super-hydrophobic coatings and non-stick coatings [15, 16] and the addition of self-responsive agents in the coating matrix [16-18]. The biofouling starts with the settlement and growth of bacteria to generate a biofilm [19]. The bacterial reproduction would change the local environment. Thus, the design of bacterial triggering materials (e.g., enzyme- or 3

pH-triggered coatings) has been accepted for controlled release of the biocide agents [20]. As the enzyme is usually selective to the bacteria [21], the enzyme-responsive releasing of agents is limited to certain situations. Instead, the pH-triggered approach is based on the generation of acidic substances by bacterial metabolism, such as lactic and acetic acid, where the environmental pH drops immediately [22, 23]. Therefore, the pH-responsive smart coatings for controlled release of biocides have been on demand urgently [17, 24].

Recently, pH-responsive nanocapsules were fabricated by capsaicin and chitosan, and showed outstanding controlled release of capsaicin in alkaline solutions [25]. Polyelectrolyte multilayer films prepared by layer-by-layer (LBL) self-assembling method can possess pH-responsive surfaces [26]. For example, the capsaicin@chitosan (CAP@CS) nanocapsule as a polycationic electrolyte has excellent pH-responsive antibacterial and antifouling properties , and can be used in the preparation of smart coatings [17]. Polyethylene glycol (PEG) as a biocompatible polyanionic material has widely been used to modify films in medical, sensing and intelligent material areas [27-30]. At the same time, the abundant hydroxy bonds in polyvinyl alcohol (PVA) make it appropriate to prepare various functional films by layer-by-layer method, such as room-temperature-phosphorescence thin films, super-compatible functional thin films and sensor films [31-33] as well as in medical applications [34, 35]. Sodium alginate (ALG) as a non-toxic polyanionic electrolyte possesses the hydroxy and carboxylate radicals [36], and has been used as drug carrier for treatment of mucosal tissue [37]. Hence, PEG, PVA and ALG provide three kinds of polyanionic electrolyte, which can be used to prepare 4

smart coating films to meet the actual needs.

Herein, three types of polyelectrolytic films were prepared by layer-by-layer assembly method. The pH responsive CAP@CS nanocapsules were synthesized by microemulsion. The nanocapsules, as polycationic electrolytes, were immersed alternately into different polyanionic electrolytes, i.e., PEG, PVA and ALG, respectively. The morphology and structure of the prepared films were characterized. The antifouling and antimicrobial performance of the prepared PEG/CAP@CS, PVA/CAP@CS and ALG/CAP@CS films for P.aeruginosa were investigated by the plate colony counting method and Live/dead® BacLight TM Bacterial Viability Kit (L13152). The pH responsive properties of the prepared films were evaluated by measuring the concentration of released CAP and the plate colony method in solutions with different pH values. Moreover, the pH-responsive properties and the antibacterial longevity of the films in acid and alkaline solutions were compared to give guidance for designing the pH-responsive intelligent antifouling and antibacterial coatings to be potentially deployed in the marine environment.

2. Experimental Section

2.1. Materials Chitosan with a low viscosity < 200 mPa. S was purchased from Aladdin Chemistry Co. Ltd (China). Capsaicin and lecithin with a purity > 98 % were bought from Shanghai Macklin Biochemical Co. Ltd (China). Dopamine hydrochloride was purchased from Sigma-Aldrich (Switzerland). Polyvinyl alcohol with a purity ≥ 99.0 % (1750± 50), polyethylene glycol 2000 5

(average molecular weight 1900~ 2200), sodium alginate and acetic acid with a purity ≥99.8 % were obtained from Sinopharm Chemical Reagent Co. Ltd (China). Tris (hydroxymethyl) aminomethane hydrochloride with a purity 99+ % was purchased from Alfa Aesar. The Live/dead® BacLight TM Bacterial Viability Kit (L13152) was bought from Life Technologies (America). Phosphate-buffered saline (PBS, pH 7.4) solutions were stored at 4 oC after high pressure steam sterilization. P. aeruginosa (ATCC27853) were purchased from Rishui Biotech Co. Ltd. (Qingdao, China). 2.2. Preparation of pH responsive CAP@CS nanocapsules The pH-response CAP@CS nanocapules were fabricated by microemulsion method [17, 25] Amount of CAP was added in 400 µL ethyl alcohol with some lecithin. 10 mg CS was mixed with 1 % v/v acetic acid. The two solutions were mixed and stirred at room temperature. Finally, the CAP@CS nanocapsules were acquired by dialysis eliminating the free CAP. 2.3. Fabrication of (PEG/CAP@CS)m, (PVA/CAP@CS)m and (ALG/CAP@CS)m PEMs films The glass slide substrates were cleaned by acetone, ethanol and distilled water, and dried by high-purity N2. The substrates were then immersed in dopamine hydrochloride (2 mg/mL) with Tris buffer (100 nM, pH 8.5) for 15 min at room temperature in a dark place to colonize polyanionic electrolyte. After that, the glass slides were washed by distilled water for 1 min. The substrates modified by polydopamine were immersed in PEG, PVA and ALG (2 mg/mL) aqueous solutions, respectively, for 15 min, then washed with distilled water, and immersed in CAP@CS solution (1 mg/mL) for another 15 min. By repeating the procedures for the preparation of PEG/CAP@CS, PVA/CAP@CS and ALG/CAP@CS for 10 or 20 times, the 10 or 20 bilayers of (PEG/CAP@CS)10,20, (PVA/CAP@CS)10,20 and (ALG/CAP@CS)10,20 films were fabricated on the glass substrates, which were then dried with high-purity N2.

6

2.4. Antifouling and antibacterial analysis The antifouling performance of the prepared (PEG/CAP@CS)10, 20, (PVA/CAP@CS)10, 20 and (ALG/CAP@CS)10, 20 films against P.aeruginosa was evaluated by the colony counting method and BacLight dead/live kit assay. Briefly, a suspension of P.aeruginosa (20 µL) was inoculated in 10 mL LB sterile tubes containing the filmed glass substrates [38]. The suspension was rotated at 121 rpm and incubated at a constant temperature of 37 oC for 18 h. After that, each 20 µL bacterial suspension was under colony counting by spreading the diluted samples on a solid medium plate and incubating overnight at 37 oC. Each filmed glass substrate was placed in 6-well plate and dyed by a Live/dead® BacLight TM Bacterial Viability Kit for 20 minutes before characterizing by a confocal laser scanning microscope. 2.5. Measurements of pH response properties The prepared (PEG/CAP@CS)10,20, (PVA/CAP@CS)10,20 and (ALG/CAP@CS)10,20 films coated on the glass substrates were immersed in 0.1 M PBS with various pH values of 4 to 8.5 at a 0.5 interval and kept shaking at 37 oC for different periods of time. The glass substrates were taken out after 1, 5, 14, 30 and 60 days of immersion. The concentration of CAP was analyzed by an ultraviolet-visible spectrophotometer at 280 nm after shaking for different times [39]. 2.6. Long-term antibacterial performance of the prepared films The filmed glass substrates taken out from different pH PBS solutions at different times were put in sterile tubes containing 20 µL P. aeruginosa bacterial suspension mixed with 10 mL LB medium. The suspensions were shaken at 120 rpm and incubated at 37 oC for 18 h before spreading uniformly on the solid medium plate. All samples were incubated overnight at 37 oC. The bacteriostasis for each filmed samples against P.aeruginosa was calculated by Equation (1) [38]: 7

BR= (A-B)/A×100%

(1)

where BR is the bacteriostatic percentage of P.aeruginosa, A is the number of viable P.aeruginosa bacteria of the control group, and B is the number of viable P.aeruginosa bacteria of the experimental group.

2.7. Measurements of electrostatic interaction between polyanionic electrolytes (PEG, PVA and ALG) and CAP@CS nanocapsules

The quantitative analysis of the electron cloud density of O atom in different polyanionic electrolytes was calculated by Materials Studio. A cluster model of three links was established. The structure was optimized using a Dmol3 software package based on the density functional theory (DFT). The cluster model was calculated using the LDA-PWC type exchange correlation function. The electrostatic interaction between polyanionic electrolytes and CAP@CS nanocapsules was evaluated by the zeta potential measurements. Each polyanionic electrolyte sample (PVA, PEG or ALG, 2 mg/mL) and CAP@CS nanocapsules (1 mg/mL) were dissolved in distilled water with stirred for 30 minutes. The CAP@CS nanocapsules solutions and PEG, PVA and ALG solutions were mixed with a volume ratio 1:1 and got the zeta potential data, respectively.

2.8. Characterization

Scanning electron microscopy (SEM) was PHENOM WORLD PW-100-016 made in Netherlands. The confocal laser scanning microscope (ZEISS Scope. A1) was determined to 8

evaluate the fluorescent assay. Ultraviolet and visible spectrophotometer (U-3900H) manufactured by Hitachi Ltd. was used to determine the concentration of capsaicin. Fourier transforms infrared spectroscopy (FITR) IS-50 spectrometer (Nicolet, America) was used for obtaining the FTIR spectra with KBr discs. The Zeta potential of the core-shell nanocapsules was measured by Zetasizer Nano (Malvern Instrument, UK).

3. Results and Discussion

3.1. Morphological and structural characterization of the prepared films Fig. 1 shows the optical and SEM images of the prepared PEM films by the assembled LBL technique. The as-received clean glass slides are shown in Fig. 1a, and the glass slides after filming by 10 layers and 20 layers are shown in Fig. 1b and 1c, respectively. It is seen that the white films are coated on the glass slides. Morphologically, there is no big difference between different films，but it can be found that the films in Fig. 1c are much whiter than Fig. 1b. It means that the thickness of the film in c is thicker than the films in Fig 1b. The preparation process unfolds as Fig. 1d, and the SEM images of three different kind films after immersing 20 times are shown in Fig.1e to f (treated by PEG, PVA and ALG solutions with CAP@CS solutions alternately, respectively). As can be seen in Fig. 1e-g, there are white little spots scattering on the films. Fig. 1e demonstrates the sample treated by PEG and CAP@CS solution alternately and Fig. 1f displays the PVA and CAP@CS counterpart. It is clear that there are more white spots on the films treated by PVA and CAP@CS. Furthermore, the dispersity of white spots on the films in Fig. 1e is much better compared with the films in Fig. f. As regards of Fig. 1g, there are more 9

white spots on the sample treated by ALG and CAP@CS compared with the other two films. Furthermore, the white spots in Fig. 1g have the best dispersity of the white spots. The white spots are CAP@CS nanocapsules. It means that Fig. 1g could load more CAP@CS and could have better antibacterial properties in these three PEMs films.

Fig.1 (a) The optical images of the clean glass with 1 cm in width and 2 cm in length, (b) 10 bilayered PEM films and (c) 20 bilayered PEM films. (d) The schematic diagram of the preparation and structure of the films. (e-g) the SEM images of the prepared (PEG/CAP@CS)20, (PVA/CAP@CS)20 and (ALG/CAP@CS)20 films, respectively. The red arrows point the CAP@CS nanocapsules. The scale bar is 2 µm. Fig. 2 shows the FITR spectra obtained on the prepared various films. Fig. 2A shows the FTIR spectra of CAP@CS nanocapsules (spectrum a), the films (spectrum b) prepared by 10

CAP@CS nanocapsules and PEG, and PEG (spectrum c). In line a (belongs to CAP@CS) [25], the broad peak at around 3200 cm-1 is attributed to N-H and O-H bonds [40]. The peaks at 2923 and 2844 cm-1are assigned to C-H in stretching vibration [41]. The peaks at 1735 and 1650 cm-1 are associated with the hydrogen bond and C=O stretching vibration, respectively [42]. The peaks located at 1735 and 1650 cm-1 in line b indicate that CAP@CS nanocapsules are contained in the films. For spectrum c, the peaks at 2874 cm-1 belongs to the C-H and the peaks between 1100 and 1041 cm-1 belong to the C-O-C stretching vibration [43]. These peaks can also be detected in spectrum b. It means that PEG is contained in the films, and the PEG/CAP@CS PEMs films are prepared successfully.

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Fig. 2 (A) The FTIR spectra of (a) CAP@CS nanocapsules, (b) PEG/CAP@CS film and (c) PEG; (B) FTIR spectra of (a) CAP@CS nanocapsules, (b) PVA/CAP@CS and (c) PVA films; (C) FTIR spectra of (a) CAP@CS nanocapsules, (b) ALG and (c) ALG/CAP@CS films; (D)

FTIR

spectra of (a) (PEG/CAP@CS)10, (b) (PEG/CAP@CS)20, (c) (PVA/CAP@CS)10, (d) (PVA/CAP@CS)20, (e) (ALG/CAP@CS)10, and (f) (ALG/CAP@CS)20 films. Fig. 2B shows the FTIR spectra of CAP@CS nanocapsules (spectrum a), the films (spectrum b) prepared by CAP@CS nanocapsules and PVA, and PVA (spectrum c). In line a, the peaks at 2923 and 2844 cm-1 in spectrum a are assigned to the C-H stretching vibration [43]. The peak at 2844 cm-1 also present in spectrum b means that there is C-H stretching vibration in this 12

spectrum. The peak at 1650 cm-1 in spectra a belongs to the C=O stretching vibration and it can be seen in line b as well. Moreover, the peak at 1735 cm-1 present in spectra a and b is associated with the hydrogen bond in CAP@CS nanocapsules. It means that the CAP@CS nanocapsules exist in the film. However, the peak intensity (in line b) decreases compared with that in spectrum a. It is because the interaction between the hydroxyl groups of PVA and the amino groups of CS decreases the hydrogen bond between CAP and CS. The broad peak at around 3200 cm-1 and the peaks at 2958 and 2897 cm-1 peaks in spectrum c belong to the O-H and C-H stretching vibration, respectively [44, 45]. It can be found that there is also a peak located 2958 cm-1 in line b. The peaks located between 1250 and 800 cm-1 in spectra c are detected in b which are caused by C-O stretching vibration [44, 45]. These results demonstrate that the PVA is contained in the films. Hence, the CAP@CS and PVA are all detected in the films and there are interactions between CAP@CS and PVA.

Fig. 2C shows the FTIR spectra of CAP@CS nanocapsules (spectrum a), the films (spectrum b) prepared by CAP@CS nanocapsules and ALG, and ALG (spectrum c). The broad peak at 3200 cm-1 is associated with N-H and O-H [46, 47] in line a. As seen from spectrum a, the peaks located at 2923, 2844 and 1735 cm-1 are attributed to the C-H stretching vibration, and hydrogen bond, respectively. These peaks appear in line b demonstrating that the films are modified by CAP@CS [46]. From spectra b and c, the 1600 and 1406 cm-1 peaks are due to the -COO- symmetrical stretching vibration and asymmetric stretching vibration, respectively, indicating that the ALG is included in the films. From the change of the peak intensity of the 13

hydrogen bond in CAP@CS (line a) and PEMs films (line b), the polyanionic electrolyte can eliminate the interaction between CAP and CS. (PEG/CAP@CS)10,20, (PVA/CAP@CS)10,20 and

Fig. 2D shows the FTIR spectra of

(ALG/CAP@CS)10,20. The interaction between

CAP@CS nanocapsules and a polyanionic electrolyte is not influenced by the number of bilayers in the films. It is thus proven that CAP@CS can be prepared in the PEMs films with different polyanionic electrolytes by layer by layer method.

Fig. 3 The SEM images of the cross section of (PEG/CAP@CS)10 (a), (PEG/CAP@CS)20 (a1), (PVA/CAP@CS)10 (b), (PVA/CAP@CS)20 (b1), (ALG/CAP@CS)10 (c) and (ALG/CAP@CS)20 (c1) films. The red arrows mark the thickness of the films. Note: the scare bar is 1 µm. Fig. 3 shows the SEM images of the cross-section of the prepared films. The thickness of (PEG/CAP@CS)10, (PVA/CAP@CS)10 and (ALG/CAP@CS)10 films is about 1.33, 1.38 and 1.43 µm, respectively, and their 20 bilayered films have the thicknesses of about 2.68, 2.77 and 3.08 14

µm, respectively. It can be found that the thickness of the coating shows a linear growth trend according to the increase of the number of alternate immersion in different polyanionic electrolyte solutions and CAP@CS solutions. Specifically, the thickness of 20 bilayer films is about twice that of the 10 bilayer films. It means that the film thickness increases linearly as the deposition steps and times increase. Moreover, there is the thickest film for (ALG/CAP@CS)m under the identical condition, followed by (PVA/CAP@CS)m films, and the (PEG/CAP@CS)m films are the thinnest. This is attributed to different electrostatic interactions between the polyanionic electrolyte and CAP@CS nanocapsules, as discussed later.

3.2 Antibacterial and antifouling performance of the prepared film

The antibacterial performance of the prepared PEMs films is shown and ranked in Fig. 4. It is seen in Fig. 4a that, after 18 h of cultivation in Luria-Bertani medium containing P.aeruginosa bacteria, the count of colony on the control specimen is 280 CFU, while the colony counts on the (PEG/CAP@CS)m and (PVA/CAP@CS)m films are only around 14 CFU and 13 CUF, respectively. The least number of colony count is on the (ALG/CAP@CS)m films of about 8 CFU only. The antibacterial property of the (ALG/CAP@CS)m film ranks the best of the prepared films, because (ALG/CAP@CS)m films possess the most of CAP@CS (shown in Fig.1). As shown in Fig. 4b, the bacteriostasis of the (PEG/CAP@CS)m and (PVA/CAP@CS)m films with 10 bilayers and 20 bilayers is about 95 %, while those for (ALG/CAP@CS)10 and (ALG/CAP@CS)20 films are up to 96.44% and 96.91%, respectively. The results indicate that all these three types of PEMs films have outstanding antibacterial properties. Meanwhile, with 15

increasing the amount of CAP@CS contained in the films, their antifouling and antibacterial properties become better.

Fig. 4 (a) The number of P.aeruginosa bacterial cells on the controlled specimen and prepared films in Luria-Bertani medium for 18 h; (b) the bacteriostasis of P. aeruginosa on the prepared films. The antibiofouling performance of the filmed glass substrates is also evaluated by BacLight live/dead fluorescent assay and the results are shown in Fig. 5. The green spots in the images indicate live bacteria, while the red spots are for dead bacteria. The results show that all the prepared films possess a remarkable antibacterial effect. The unfilmed glass slide does not have the antibiofouling property, as can be seen in Fig. 5a. For the specimens filmed with the (PEG/CAP@CS)m, (PVA/CAP@CS)m, and (ALG/CAP@CS)m, only red spots are observed, demonstrating that the bacteria are killed by the agents released from the films. Moreover, there is a little difference between the 10 bilayer and 20 bilayer films. Hence, the prepared PEMs films 16

in this work, i.e., (PEG/CAP@CS)m, (PVA/CAP@CS)m and (ALG/CAP@CS)m, possess high-performance antibacterial and antifouling properties. Because of the high antibacterial properties of these three types of films, it can’t figure out which one is the best. Hence, a combination of the results coming from Fig. 4 and Fig. 5, these three kinds of PEMs films, i.e., (PEG/CAP@CS)m, (PVA/CAP@CS)m and (ALG/CAP@CS)m present fantastic antibacterial and antifouling properties, but the performance of (ALG/CAP@CS)m is the best.

Fig. 5 BacLight Live/Dead kit assay staining results of P.aeruginosa colonized on the blank glass slide (a), and the specimens filmed with (PEG/CAP@CS)10 (b), (PEG/CAP@CS)20 (b1), (PVA/CAP@CS)10 (c), (PVA/CAP@CS)20 (c1), (ALG/CAP@CS)10 (d), (ALG/CAP@CS)20 (d1), 17

respectively. Note: the scale bar is 10 µm. 3.3. Electrostatic interaction between the prepared films and CAP@CS nanocapsules In order to compare the electronegativity of the different oxygn-containing functional groups of polyanion electrolytes, we used the Dmol3 software package based on the DFT. The Mulliken charge distributions of O atom on these three types of prepared films are shown in Fig. 6 and Table 1. The O atoms are divided into bridging oxygen (1-2 O) and hydroxyl oxygen (3 O) for PEG, and the profiles are -0.432 and -0.639 Hartree for 1-2 O atoms and 3 O atom. It can be found that as for PEG, the Mulliken charge distributions of O atom on hydroxyl oxygen is more than that on bridging oxygen. Compared with the hydroxyl oxygen of PVA, the average of the Mulliken charge distribution is about -0.644 Hartree, which is slightly higher than that of PEG. It means that the same type of O atom could have a similar Mulliken charge distributions. As seen in Fig. 6c and Table 1, there are four types of O atoms located in the ring (1-6 O), i.e., bridging oxygen (7-11 O), hydroxyl oxygen (12-24 O), carboxyl oxygen linked to sodium atoms (25-30 O), and carboxyl oxygen linked to carbon atoms (31-36 O). The hydroxy oxygen can be divided into 3 types, i.e., that is near sodium atoms (12-14 O), that is similar to the 12-14 oxygen atoms but in the other side of the ring skeleton and far away from sodium atoms (15-16 O), and the other hydroxyl oxygen (17-24 O). The Mulliken atomic charge of 12-14 O atoms is the highest, which is about -0.748 Hartree due to its interaction with the adjacent sodium atom, hydrogen atom and carbon atom. Since it is far away from sodium atoms, the Mulliken atomic charge of oxygen atoms (15-16 O) is around -0.618 Hartree. The average Mulliken atomic charge 18

of the other oxygen atoms is about -0.633 Hartree, which is similar to that of oxygen atoms (3 O, -0.639 Hartree) on PEG. Furthermore, the carboxyl oxygen atoms (25-30 O) adjacent to sodium are around -0.642 Hartree, which is similar to that of 1-3 O on PVA. Compared with these three types of polyanionic electrolyte, according to the Mulliken charge distribution of O from PEG and PVA, the total negative charge of PVA (1750 kDa) could be slightly more than that of PEG (2000 kDa) when the molecular weights of these two polymers are similar. While the hydroxy oxygen atoms on ALG have the highest electronic density. Furthermore, as there are the most O atoms on ALG, the total quantity of Mulliken atomic charge could be the most for ALG polyanionic electrolyte. Thus, the ALG can interact with CAP@CS much easier compared with the other two polyanionic electrolytes.

Fig. 6 The optimized cluster model for PEG (a), PVA (b) and ALG units (c). The white balls refer to H atoms, the gray ones for C atoms, the red one for O atoms and the purple ones for Na atoms. The number of O atoms is marked, and the Mulliken charge distribution of the serial number of O atoms from the three polyanionic electrolytes is listed. Table 1 Mulliken charge distribution of O from PEG, PVA and ALG. Materials Type models

of

Average density

Materials Type

of

Average density

models

19

PEG

1-2 O

-0.432

3

O

PVA ALG

ALG

12-14 O

-0.748

-0.639

15-16 O

-0.618

1-3 O

-0.644

17-24 O

-0.633

1-6 O

-0.457

25-30 O

-0.642

7-11 O

-0.437

31-36 O

-0.532

Furthermore, the zeta potentials are measured to evaluate the electrostatic interaction between the polyanionic electrolytes with CAP@CS, and the results are shown in Tables 2-4. The constitutional unit of PEG is ether bond, and the zeta potential is about 13.60±0.50 mV. Since the zeta potential of CS@CAP is about 33.58 ± 2.63, PEG could generate an electrostatic interaction with CAP@CS through the amino of CS. The zeta potential of PEG/CAP@CS is 26.60±3.25 mV, and thus, the interaction occurs between PEG and CAP@CS. The interaction force can help the CAP@CS nanocapsules settle on the PEMs films to generate the antibacterial effect. Compared with PEG, the PVA has hydroxyl groups in the constitutional unit, providing active sites to form hydrogen bond interactions with the amino groups of CS. Furthermore, in Table 3, the zeta potential of PVA is a representative polyanionic electrolyte of -13.63±0.59 mV. An electrostatic interaction can occur with CS (33.58 ± 2.63) to generate PVA/CAP@CS with the zeta potential of 27.62±2.25 mV. With the -COO- and -OH of the ALG structure, ALG has the highest electronegativity of three different polyanionic electrolytes, which is around -30.97± 1.66 mV, as shown in Table 4. According to the theory of layer by layer self-assembly method,

20

the electrostatic interaction between polyanion electrolyte and CAP@CS can influence the thickness of films and the number of loading CAP@CS. With the electronegativity, the driving force becomes increasing during the preparation process. It means that the thickness of the films and the amount of the CAP@CS loading are increasing with increasing the electronegativity, respectively. Because of the ranking list of electronegativity of these three polymers, the thickness of these films order is (ALG/CAP@CS)m, (PVA/CAP@CS)m and (PEG/CAP@CS)m. Moreover, the loading of CAP@CS in such films exhibits the same ranking list. It can be found that the properties of the film can be judged by the electronegativity of the polyanion electrolyte. Hence, the (ALG/CAP@CS)m film possesses the best antibacterial property against P. aeruginosa due to the strongest interaction between ALG and CAP@CS, resulting in more CAP@CS nanocapsules immobilized on the (ALG/CAP@CS)m films.

In summary, the antibacterial properties of (PEG/CAP@CS)m, (PVA/CAP@CS)m and (ALG/CAP@CS)m films are dependent on the electrophilic effect the sp -hybridized oxygen atom. With the enhanced electrophilic effect, the electrostatic interaction between CAP@CS and the poly anionic electrolyte increases. According to the zeta potential measurements, the ALG possesses the best performance, followed by the PVA and PEG. These results can explain well about the ranking list of antifouling and antibacterial properties of these three types of films, as well as the ranking list of the thickness of them.

3.4. pH responsive response for self-releasing of antibacterial agents The CAP releasing rate from the prepared films determines the long-term antifouling and 21

antibacterial

performance.

The

pH-responsive

properties

of

the

(PEG/CAP@CS)20,

(PVA/CAP@CS)20 and (ALG/CAP@CS)20 films are evaluated by measuring the concentration of the released CAP during the 60-day immersion in PBS solutions with varied pH (i.e., 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, and 8.5).

Fig. 7 (a) The concentration of CAP released from the (PEG/CAP@CS)20 multilayer films in PBS solutions with varied pH. (b) The number of P.aeruginosa bacteria after 18 h immersion in Luria-Bertani medium of the (PEG/CAP@CS)20 multilayer filmed specimen. (c) The bacteriostasis of the (PEG/CAP@CS)20 multilayer films against P.aeruginosa. 22

Table 2 Zeta potential of PEG, CAP@CS nanocapsules and PEG/CAP@CS mixed solution. Sample

Zeta potential (mV)

PEG

-13.60±0.50

CAP@CS

33.58±2.63

PEG/CAP@CS

26.60±3.25

Fig. 7 shows the pH-responsive properties profiles of (PEG/CAP@CS)m. As seen in Fig. 7a, with increasing the pH, the concentration of CAP released from the (PEG/CAP@CS)20 films decreases. In pH 4 to pH 7.5 solutions, the CAP is released faster during the first 5 days compared with the rest of the time. During the initial time, the pH 4 group’s curve exhibits the most dramatic upward trend from 12.83 ppb to 20.54 ppb within a series of different counterparts, and the CAP concentration of (PEG/CAP@CS)20 modified substrates is up to 23.35 ppb after 60-day immersion in acid PBS solutions. As for immersing in pH 4.5 to pH 6 solutions, the CAP releasing rate exhibits a similar trend to pH 4 group. The concentration of released CAP of (PEG/CAP@CS)20 films after 60 days is 22.23, 22.41, 21.48 and 20.89 ppb, respectively. In regards to pH 6.5 and 7 groups, both increase smoothly within the whole 60 days, which are both

23

from 12.25 ppb climb to 19.37 ppb and 17.62 ppb, respectively. In pH 7.5 to pH 8.5 solutions, the concentration of the released CAP keeps at a low level during the 60 days period. The concentration of the released CAP increases from 12.13, 11.55 and 12.20 ppb to 13.42, 13.07 and 12.95 ppb, respectively. For comparison, the amount of the CAP released in pH 4 and pH 8.5 solutions is increased by 10.52 and 1.75 ppb, respectively. Thus, the (PEG/CAP@CS)20 multilayers films possess pH responsive performance. According to the UV-vis profiles, when the marine environment becomes acid, the CAP can release from the films to kill the bacteria preventing the bioantifouling process. Meanwhile, because of the control ability for CAP releasing in alkaline solutions, the antibacterial performance of this film can be prolonged in the marine environment.

24

Fig. 8 (a) The concentration of the CAP released from (PVA/CAP@CS)20 multilayers films in PBS solutions with varied pH (4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, and 8.5). (b) The number of P. aeruginosa bacteria after 18 h of immersion in Luria-Bertani medium on (PVA/CAP@CS)20 multilayers filmed specimens. (c) The bacteriostasis of different treated (PVA/CAP@CS)20 multilayers films against P.aeruginosa. Table 3 Zeta potential of PVA, CAP@CS nanocapsules and PVA/CAP@CS mixed solution. Sample

Zeta potential (mV)

PVA

-13.63±0.59

CAP@CS

33.58±2.63 25

PVA/CAP@CS

27.62±2.25

To further determine the antibacterial properties of (PEG/CAP@CS)m films, P.aeruginosa bacteria strain is selected as a representative of marine bacteria to evaluate the long-term antibacterial performance of the (PEG/CAP@CS)20 films by the colony counting method, and the results are shown in Fig. 7b. The bacteriostasis results are shown in Fig. 7c. It is seen that a low level of bacteria counts is observed in all solutions in the first day, compared with the control group (near 428 CFU). The corresponding data of bacteriostasis of each sample are 91.52%, 92.06%, 93.54%, 93.62%, 93.62%, 93.85%, 94.16%, 94.24%, 94.71% and 95.64% for pH 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8 and 8.5 groups, respectively (Fig. 7c). With increased the immersing time, the quantity of bacteria increases in pH 4 to 7 solutions, and the upward tendency is increasing with the pH value decreases. The pH 4 solution records 365 CFU after 60 days, while the pH 7 solution records 196 CFU. The percentage of the killing bacteria drops to under 30% for pH 4 to 6 groups and pH 7 group only has 54.24% bacteriostasis after 60 days immersing as shown in Fig. 7c. In pH 7.5, 8 and 8.5 solutions, the bacterial colonies keep at a low number of approximately 40 CFU during 60 days. The (PEG/CAP@CS)m films possess an outstanding stability in alkaline solutions with more than 90% bacteriostasis. With the sensitive pH responsive properties, the CAP can be released from the (PEG/CAP@CS)m films to kill bacteria when the pH of the local environment decreases, which is triggered by the reproduction of bacteria in the environment.

Fig. 8a shows the concentration of the released CAP from (PVA/CAP@CS)20 films after 26

60 days of immersion in PBS solutions with varied pH. In pH 4 to 7 solutions, the amount of each group’s CAP climbs stably from near 12~13 ppb to approximately 23.70, 23.46, 23.11, 21.94, 21.01, 19.72 and 18.56 ppb, respectively, during the whole 60 days. Within 14 days to 60 days, the quantity of released CAP immersing in pH 4 to 6 solutions goes up slowly (increased by 1.05, 1.87, 1.64, 1.28 and 0.82 ppb, respectively) compared with the initial days (increased by 9.69, 8.88, 8.76, 7.94 and 7.48 ppb for pH 4, 4.5, 5, 5.5 and 6, respectively) especially the initial 5 days. In terms of pH 6.5 and 7 groups, the amount of CAP releases closely at a constant speed at around 0.12 ppb and 0.10 ppb per day. It is thus seen that the concentration of CAP released from (PVA/CAP@CS)20 films keeps at a lower level from around 12.13, 11.67 and 11.55 ppb at first day to near 13.53, 13.30 and 13.06 ppb after 60 days. The results show that the (PVA/CAP@CS)20 films have good pH responsive properties, which is similar to the (PEG/CAP@CS)20 films.

27

Fig. 9. (a) The concentration of the CAP released from (ALG/CAP@CS)20 multilayers films in PBS solutions with varied pH (4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, and 8.5). (b) The number of P. aeruginosa bacteria after 18 h of immersion in Luria-Bertani medium on (ALG/CAP@CS)20 multilayers filmed specimens (c) The bacteriostasis of different treated (ALG/CAP@CS)20 multilayers films against P.aeruginosa.

28

Table 4 Zeta potential of ALG, CAP@CS nanocapsules and ALG/CAP@CS mixed solution. Sample

Zeta potential (mV)

ALG

-30.97±1.66

CAP@CS

33.58±2.63

ALG/CAP@CS

-29.40±0.70

The antibacterial properties of the (PVA/CAP@CS)20 films against P. aeruginosa are shown in Fig. 8b and Fig. 8c. As shown in Fig. 8b, the increasing tendency of profiles is similar to (PEG/CAP@CS)20 films counterpart. The number of bacteria of all samples shows a low value (near 48, 44, 47, 46, 42, 41, 38, 37, 28, 24 CFU for pH 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5 groups, respectively), compared with the control group (around 428 CFU) after 1 day. Correspondingly, the antibacterial rate increases from 88.87% in pH 4 to 94.48% in pH 8.5 solution (Fig. 8c). However, the antibacterial properties drop rapidly in pH 4 to 7 solutions after 5 days. As for the coupon immersed in pH 4 solution, only 5.80% bacteriostasis is left for the filmed (PVA@CAP@CS)20 after 60 days. While, when with the increased of pH, the antibacterial performance unfolds an upward tendency. The antibacterial properties of the (PVA/CAP@CS)20 films decrease by 40.09% after 60 days in pH 7 PBS solution. In pH 7.5 to 8.5 solutions, there is an increasing trend within 5 days to 60 days from 36, 35 and 33 CFU to 44, 42 and 41 CFU, respectively, as shown in Fig. 8b. The bacteriostasis of (PVA/CAP@CS)20 films is 89.72%, 90.12% and 90.35% for pH 7.5, 8 and 8.5 groups, respectively. The percentage of bacterial reproduction increases by less than 4.1% only in the alkaline solutions. It means such PEMs films have

29

noticeable pH-responsive properties and possess good antibacterial longevity for serving in marine environment.

The concentration of the CAP released from the (ALG/CAP@CS)20 films and the antibacterial performance in PBS solutions with varied pH as a function of time are shown in Fig. 9. The CAP resealing rate includes three types in Fig. 9a. The first one is that CAP is releasing fast during the whole 60 days period, but the increasing rate of initial 5 days surpasses that of the rest of the time (pH 4 - 6 solutions). The second one is that the releasing rate climbs sharply at the first 5 days, while the concentration of released CAP turn to become increasing stably and slowly within 5 days to 60 days (pH 6.5 - 7 solutions). The last one is that the concentration of CAP keeps at a low level all the time (pH 7.5 to 8.5 solutions). In the pH 4 solution, the CAP concentration increases from 12.72 ppb at day 1 to 20.54 ppb at day 5, and reaches 22.06 ppb after 60 days. In pH 6.5 and 7 solutions, the CAP concentration increases slowly from 11.55 and 10.50 ppb at day 1 to 16.37 and 15.32 ppb after 60 days. In pH 7.5 to 8.5 solutions, a long-term stability is observed. The CAP concentration increases gradually from 9.91, 9.68 and 9.56 ppb to 12.02, 11.67 and 11.55 ppb in pH 7.5, pH 8 and pH 8.5 solutions, respectively. The concentration of released CAP of these groups are increased by 2.11, 1.99 and 1.99 ppb, respectively. It demonstrated that the amount of released CAP in PBS 8.5 solutions are less near 7.35 ppb than pH 4 PBS solutions counterpart in the 60 days period.

The (ALG/CAP@CS)20 filed specimen is taken from the PBS solutions (pH from 4 to 8.5 at a 0.5 interval) after immersion for 1 day, 5 days, 14 days, 30 days and 60 days. The long-term 30

antibacterial performance of the film is evaluated by the counting method, and the results are shown in Fig. 9b and 9c. As shown in Fig. 9b, briefly, in pH 4 to pH 7 solutions, the amount of colony counts climbs fast during the period of time, while in pH 7.5 to 8.5 solutions, a low number of bacteria is recorded after 60 days shaken. In the initial 5 days, there is no big difference between the P.aeruginosa numbers in all solutions. There are around 35, 33, 32, 30, 25, 24, 25, 25, 22 and 21 CFU for pH 4 to 8.5 groups, respectively, demonstrated in Fig. 9b. Correspondingly, the antibacterial performance are about 94.88%, 95.17%, 95.32%, 95.61%, 96.34%, 96.49%, 96.34%, 96.34%, 96.73% and 96.83%, respectively as revealed in Figure 9c. it gives the greatest increasing including these 10 groups. As for the pH 4.5 to 7 solutions, the number of bacteria rise from 237, 217, 186, 175, 165, 107 CFU to 438, 435, 388, 285, 236 CFU. They are increased by 201, 218, 202, 120, 129 CFU from 5 days to 60 days, respectively. However, the number of bacteria in pH 4 solution increases sharply from 253 to 539 CFU from 5 days to 60 days. These profiles are matched with the Fig. 9a. With the concentration of released CAP goes up rapidly, the antibacterial properties unfold prompt decrease. Fig. 9c shows that the bacteriostasis of the (ALG/CAP@CS)20 film results in the decrease of the bacterial counts by 73.81%, 64.34%, 59.42%, 59.22%, 53.12%, 38.25%, 30.83% after 60 days of immersion in the PBS solutions from pH 4 to 7 values, respectively. Compared with the films in acidic and neutral solutions, the long-term antibacterial effect of the (ALG/CAP@CS)20 film in alkaline solution is impressive, with about 91.37%, 91.56% and 92.73% of bacteriostasis rate after 60 days of immersion.

31

3.5. Comparison of pH responsive properties and antibacterial performance of (the prepared films As shown in Fig. 10a, the concentration of the released CAP is 23.70, 23.35 and 22.06 ppb for (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 films after 60 days of immersion in pH 4 PBS solutions, respectively. With respect to the average rate of CAP release in such environment, the number of CAP increased by 10.75, 10.52 and 9.34 ppb for (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 film from 1 day to 60 days, respectively. Meanwhile, the concentration of CAP released from different films in pH 8.5 PBS solutions exhibits 13.07, 12.95 and 11.55 ppb of CAP for (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 films, respectively. It indicates that the (ALG/CAP@CS)20 film is the most stable in both acidic and alkaline environments. With the highest zeta potential of ALG and the Milliken charge distribution of O atom on the hydroxyl and carboxyl groups as shown in Table 1, 4 and Fig. 6, there is the strongest electrostatic interaction force between CAP@CS and ALG for the (ALG/CAP@CS)20 film. Hence, the releasing rate of CAP can be controlled by the -COO- and –OH groups of ALG to some extent. Since there are many amino groups on CS are influenced by interaction with such oxygen-containing functional groups, and cannot achieve protonation and deprotonation totally and adjust the size of channels to control CAP release when the environment’s pH changed. Compared with the ALG, the oxygen-containing functional groups of PEG are –OH and C-O-C. according to the calculation results, the electrostatic interaction between amoin groups on CS and –OH and C-O-C are lower than those of ALG counterparts. It means that the protonation and deprotonation process of CS is easier to complete 32

in the (PEG/CAP@CS)m films. For (PEG/CAP@CS)20 film, the CAP releasing rate is higher than that of the (ALG/CAP@CS)20 film due to the weak interaction between -OH and amino for PEG and CAP@CS. The zeta potential of PEG is about -13.60±0.50 mV (Table 2), which is much lower than that of the ALG counterpart (-30.97±1.66 mV). The Milliken charge distribution of O atom on the PVA backbone and the terminal hydroxyl group is about -0.432 and -0.639 Hartree, respectively, as shown in Fig. 6. As for (PVA/CAP@CS)20 film, although the negative charge is similar for

(PEG/CAP@CS)20 film, the unstable properties of

(PVA/CAP@CS)20 in aqueous solutions accelerate the CAP releasing, resulting in the reduced longevity [48].

Fig. 10 (a) Comparison of the amount of CAP released from the (PEG/CAP@CS)20, (PVA/CAP@CS)20 and (ALG/CAP@CS)20 films in pH 4 and pH 8.5 PBS solutions, respectively. (b) Comparison of the bacteriostasis of (PEG/CAP@CS)20, (PVA/CAP@CS)20 and (ALG/CAP@CS)20 films after 60 days of immersion in pH 4 and pH 8.5 PBS solutions, respectively. To further compare the antibacterial properties of the three types of films, Fig. 10b shows the 33

bacteriostasis of (PVA/CAP@CS)20, (PEG/CAP@CS)20 and (ALG/CAP@CS)20 films. This work shows that the prepared (ALG/CAP@CS)20 film ranks top in terms of antibiofouling performance, followed by the (PEG/CAP@CS)20 and (PVA/CAP@CS)20 films, which are about 92.73%, 91.44% and 90.35%, respectively, after shaken in pH 8.5 PBS solutions for 60 days. However, the bacteriostasis of the three films is only 3.35%, 14.86% and 21.07% after 60 days of immersion in pH 4 PBS solutions, respectively. The antibacterial performance is well consistent with the results of the CAP releasing rate in the PBS solutions with varied pH in Fig. 10a. With the concentration of released CAP increases, the antibacterial property of the films decreases. It means that the antibacterial performance of the films relys on the amount of CAP contained in the films. The contained CAP in the films is stable in alkaline solutions. In acidic solution such as those the marine microorganism (i.e. bacteria) reproduces, the films are in response to the pH change and can kill bacteria through by releasing CAP from the films. In summary, the three prepared films can maintain above 90% of bacteriostasis after 60 days of immersion in alkaline solutions. The (ALG/CAP@CS)20 film possesses the most remarkable long-term antibacterial proprieties, whereas the antibacterial performance of (PVA/CAP@CS)20 film drops the fastest due to water permeability into the PVA films [48].

4. Conclusions

In this work, the capsaicin-based pH responsive multilayer films are prepared by the layer-by-layer self-assemble method. Three types of PEM films, i.e., (PEG/CAP@CS)m, (PVA/CAP@CS)m and (ALG/CAP@CS)m, exhibit outstanding pH-responsive properties and 34

antibacterial performance. In acidic environments, the CAP releases rapidly, while in alkaline environments, it kept in the films. As a result, the films maintain long-term antifouling and antibacterial properties in the marine environment. The (ALG/CAP@CS)m films pre-loaded with the most pH responsive nanocapsules possess the best controlled releasing of CAP. Although the (PVA/CAP@CS)m films possess similar antibacterial properties to the (PEG/CAP@CS)m films, the long-term controlled releasing properties and antimicrobial properties are not comparable. The (ALG/CAP@CS)m films have the best long-term antibacterial performance against bacterial strains present in marine environments. To sum up, capsaicin-based pH responsive multilayers films can be prepared utilizing such capsaicin-based nanocapsules as poly cationic electrolyte and polyanionic electrolyte by layer-by-layer a method. Through comparing the results of these three different PEMs films, it could come up with guide and suggestions for preparing such intelligent antifouling and antibacterial films for being adapt to different environments, especially in the static conditions such as anchoring staus of ships or offshore platform in the marine environments. With the consideration of magnetic field induced drug deliver [49], the combination of natural products or polymers [50-59] and other nanostructrual functional fillers [60-76] can broaden the applications of this system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

35

This work was supported by Joint Funds of the National Natural Science Foundation of China (U1806223), the National Natural Science Foundation (51572249), the Natural Science Foundation for Shandong Province (ZR2014EMM021), the Fundamental Research Funds for the Central Universities (201964009), Key Research Project of Shandong Province (911861230) and Findings of Joint Training of Ph.D. Students in Ocean University of China. References [1] H. Sun, Z. Yang, Y. Pu, W. Dou, C. Wang, W. Wang, X. Hao, S. Chen, Q. Shao, M. Dong, S. Wu, T. Ding, Z. Guo, Zinc oxide/vanadium pentoxide heterostructures with enhanced day-night antibacterial activities, J. Colloid Interface Sci., 2019, 547,40-49. [2] Z.Q. Yang, X.P. Hao, S.G. Chen, Z.Q. Ma, W.H. Wang, C.Y. Wang, L.F. Yue, H.Y. Sun, Q. Shao, V. Murugadoss, Z.H. Guo, Long-term antibacterial stable reduced graphene oxide nanocomposites loaded with cuprous oxide nanoparticles, J. Colloid Interface Sci.,

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Research Highlights

The capsaicin-based pH-triggered multilayer films were prepared by layer-by-layer method. (PEG/CAP@CS)m,

(PVA/CAP@CS)m

and

(ALG/CAP@CS)m,

exhibited outstanding pH-responsive properties and antibacterial performance. The (ALG/CAP@CS)m films showed the best long-term antibacterial performance against bacterial strains for marine environments.

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