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Fabrication of slippery lubricant-infused porous surface with high underwater transparency for the control of marine biofouling Peng Wang, Dun Zhang, Shimei Sun, Tianping Li, and Yan Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09117 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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ACS Applied Materials & Interfaces

Fabrication of Slippery Lubricant-infused Porous Surface with High Underwater Transparency for the Control of Marine Biofouling Peng Wanga,c, Dun Zhanga,*, Shimei Suna,b, Tianping Lia,b, Yan Suna a

Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of

Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, China b

University of the Chinese Academy of Sciences, 19 (Jia) Yuquan Road. Beijing 100039, China.

c

State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266101, China.

Abstract: Marine optical instruments are bearing serious biofouling problem, which affects the accuracy of data collected. To solve the biofouling problem of marine optical instruments, a novel instance of slippery lubricant-infused porous surface (SLIPS) with high underwater-transparency was designed over glass substrate via infusing lubricant into its porous micro-structure fabricated with hydrothermal method. The advantage of SLIPS as anti-biofouling strategy to marine optical instruments was proven by comparing its underwater optical and anti-biofouling performances with three kinds of samples (hydrophilic glass sample, textured hydrophilic glass sample, and superhydrophobic glass sample). The modification of SLIPS enhances the underwatertransparency of glass sample within the wavelength of 500-800 nm, for the infusion of

*

Corresponding author. Tel./Fax:+86 532 82898960. E-mail address: [email protected](Dun Zhang). 1

ACS Paragon Plus Environment


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lubricant with lower refractive index than glass substrate. In contrast with hydrophilic surface, textured hydrophilic surface and super-hydrophobic surface, SLIPS can significantly inhibit bacterial and algal settlements, thereby maintaining high underwater-transparency in both dynamic and static seawater. The inhibition of bacterial and algal settlements over SLIPS results from its liquid-like property. The contact angle hysteresis of water over SLIPS increases with immersion time in seawater under different conditions (static, dynamic, and vibration conditions). Both dynamic and vibration conditions accelerate the failure of SLIPS exposed in seawater. This research provides valuable information for solving biofouling problem of marine optical instruments with SLIPS.

Keywords: Slippery lubricant-infused porous surfaces; Anti-biofouling; Settlement; Under-water transparency; Marine optical windows.

1. Introduction The concerning to marine resources exploitation is leading to a huge demand to long-term marine optical instruments, such as water quality monitoring instruments, underwater cameras, and so forth 1. Biofouling is a common problem to the performance of marine optical instruments during their service periods 2. Organisms, including microscopic bacteria, algae, diatoms, and even larger larvae of invertebrates, can rapidly settle onto their optical windows/sensors in marine environment 3. Biofouling on optical windows can degrade data quality of marine monitoring

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instruments, and it can also make picture blurred for the covering of biofilm over the optical windows of underwater cameras 4. Thus, the development of strategies for biofouling control of marine optical instruments is urgently required. To solve the biofouling problem of marine optical instruments in marine environment, numerous strategies have been proposed, such as mechanical method, chemical method, and photocatalytic methods. Mechanical wipers method, a typical mechanical technique, has been used to reduce biofouling of optical instruments in coastal environment 5. It is effective to keep optical windows clean, but the mechanical complexity is generally regarded as its weakness. Biocide generation systems, which release cupric ions or tributyltin, are the commercial chemical strategies for biofouling control of marine optical instruments 4. However, their negative impact to the environment blocks the practical application. Photocatalytic materials have been widely reported as a potential method for biofouling control, but these materials are only effective within one-meter depth 6. It can be concluded that biofouling control for marine optical instrument is a difficult task. To exploit novel biofouling control strategy for marine optical instrument, the following two important factors should be considered: underwater transparency and anti-biofouling property. Biofouling is a phenomenon that occurs on the interface between substrate and solution. Wettability, the typical surface property, can affect the biofouling process. For instance, hydrophilic surfaces have shown great capability to reduce bacterial attachment and biofilm formation for the surface hydration layer 7-9. The recent report indicated that textured hydrophilic surface can significantly decrease the adherence of

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Gram-positive bacteria (Micromonospora purpurea). The bound water can significantly reduce the H-bonding interaction between surfaces and peptidoglycan, which acts as an effective factor for regulating adhesion of Gram-positive bacterial cell

7,10

. Recently,

non-wetting surface has been regarded as a potential method in marine anti-biofouling. Super-hydrophobic surface, a typical non-wetting surface, was proven to be effective in inhibition of organism adherence for the existence of air layer in its microstructure, which can minimize the contact area between substrate and organism 11. However, the air layer is easily collapsed for organics adherence or microstructure damage. Slippery lubricant-infused porous surface (SLIPS), inspired from pitcher plant in nature, was firstly designed as an alternative non-wetting surface by the research group of J. Aizenberg

12

. The SLIPS presents self-healing property and withstands high

external pressure, and it shows advantages in industrial application

13-18

. The recent

reports indicated that SLIPS is effective for anti-biofouling in marine environment 22

19-

. J. Aizenberg and co-workers firstly reported the exceptional effect of SLIPS in

preventing biofilm attachment, and anticipated its great potential as an anti-biofilm solution in industry

19

. Pavel A. Levkin and co-workers extended the application of

SLIPS to marine biofouling control, and demonstrated that SLIPS fabricated over microporous butyl methacrylate−ethylene dimethacrylate can effective inhibit the fouling organisms settlement

20

. To solve its limitations in application (e.g. poor

adhesion to substrate, mechanical weakness), J. Aizenberg and co-workers further designed mechanically durable tungsten oxide-SLIPS surface over steel substrate, and demonstrated its potential in bio-fouling control of steel in marine environment 21 It can

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be suspected that designing SLIPS with high underwater transparency may provide a novel solution to biofouling control of marine optical instruments, but its feasibility should be verified. Regarding to the application in biofouling control of marine optical instruments, the following two points should be considered for designing SLIPS. Firstly, it has been reported that transparent SLIPS can be designed over glass surface, and it presents comparable transparency with bare glass in air condition

23-26

. Different with these

reports, underwater transparency should be considered for utilizing SLIPS to solve biofouling problem of marine optical instruments. However, there is no reference about the designing of SLIPS with high underwater transparency until now. Secondly, it is important to consider the longevity for designing SLIPS that is applied in marine environment. It was reported that the infused lubricant layer, which is responsible for the unique properties of SLIPS, can diffuse into water under both dynamic and static conditions

27

. Especially, when SLIPS is exposed to a dynamic environment, the

dynamic flow can shear away the lubricant layer 28. The research from Howard A. Stone and co-workers proved that creating sacrificial regions with different wettability can be utilized to prevent the drainage of lubricant layer under shear force 29. The techniques applied for surface treatment were facile and easily scalable, but hard to apply for optical glass surface treatment. The research from J. Aizenberg and co-workers studied the effect of substrate topography characters to the SLIPS stability under shearing condition

30

. It was indicated that the SLIPS fabricated over nanoscale textured

substrates showed better shear-tolerant performance, in comparison with these

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fabricated over the microscale, and hierarchically textured substrates. This research provides a basic instruction for constructing stable SLIPS. In this research, SLIPS with high underwater-transparency was fabricated on glass substrate via infusing lubricant into its porous micro-structure fabricated with hydrothermal method. Nano-textured structure constructed over glass substrate with hydrothermal treatment can be expected to assist the stabilization of the infused lubricant and thus enhance the stability of SLIPS. The advantage of SLIPS as an antibiofouling strategy to marine optical instruments was evaluated by comparing its underwater optical and anti-biofouling performances with other three kinds of glass samples (hydrophilic glass sample, textured hydrophilic glass sample, and superhydrophobic glass sample). The underwater optical performance of these four glass samples was characterized based on UV-vis spectra, and the difference of optical performance among these samples was discussed by considering the refractive indices of different phases (substrate, environmental media, and surface layer). The typical marine algae (Chlorella vulgaris) and microorganism (Desulfovibrio sp.) were utilized as models for evaluation of anti-biofouling performance of glass samples. The underwater-transparency and anti-biofouling property of SLIPS were evaluated after immersion in dynamic and static seawater medium inoculated with Chlorella vulgaris or Desulfovibrio sp.. The stability of SLIPS was evaluated through monitoring its water contact angle hysteresis after immersion in seawater under static, dynamic, and vibration conditions. Finally, the anti-biofouling mechanism of SLIPS was proposed based on the comparison of its anti-biofouling performance with hydrophobized silicon

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wafer and nano-textured glass sample. This research will provide a novel route to solve biofouling problem of marine optical instruments. 2. Experimental 2.1 Materials and reagents Slide glasses were purchased from Yancheng Feizhou Bosu Co., Ltd. (China). The Perfluoropolyether (PFPE, NascentTM FX-5200), 1H,1H,2H,2Hperfluorodecyl-triethoxysilane (PFTEOS, 97%, Sigma-Aldrich), and other chemical reagents were used for experiments without further purification. 2.2 Modification of glass sample Textured hydrophilic surface (THIS) was fabricated over glass with modified hydrothermal method 31. In details, glass sample was firstly cleaned in the solution mixed with water, acetone and ethanol under ultrasonic vibration, and dried in heating oven with temperature of 50 ℃. The sample was then immersed into a solution of 3 g/L NaOH, and heated at 85℃ for 3 h. After hydrothermal process, the sample was then cleaned with water, and dried with nitrogen for further characterization or use. Super-hydrophobic

surface

(SHOS)

was

designed

by

modifying

hydrophobic layer over THIS. In details, the THIS sample was transferred into the Teflon-lined autoclave with 15 μL PFTEOS, and heated at 120℃ for 20 min. The samples were then taken out of autoclave, and heated at 150℃ for 60 min to remove the PFTEOS that physically adhered on glass surface

32

.

Slippery lubricant-infused porous surface (SLIPS) was fabricated on slide glasses by pouring lubricant PFPE over the SHOS. After that, the samples were inclined with an angle (around 15°) until the excess PFPE flow away from the 7



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sample 32. 2.3 Characterization X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250) was utilized to determine surface composition of samples. Field emission scanning electron microscope (FE-SEM, Hitachi S4800) was utilized to characterize micromorphology of glass after treatment. Fluorescence microscope (Olympus BX-51) was used to observe the bacterial and algal settlements over samples. Contact angle meter (Powereach JC2000C1) was utilized to measure the water contact angles over samples, including static and dynamic contact angles. The measurement of contact angle was carried out under ambient condition. A syringe was utilized to place a water droplet with volume of 3 μL onto the test sample. The image of water droplet was then taken with digital camera attached to the instrument. The water contact angles were all determined from three droplets at three different locations per sample, and the mean value ± standard deviation of three replicates was reported in this research. The topography of samples was characterized with atomic force microscope (AFM, Agilent 5400). The transmittance of glass samples was measured with spectrophotometer (Hitachi U2900) in the spectral range of 300 to 800 nm. 2.4 Bacteria and algae settlement analysis 2.4.1 Bacteria settlement analysis The model bacteria (Desulfovibrio sp.) samples were isolated from Bo Sea of China. They were firstly cultured in a culture solution that was used in our

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previous report

33

. After growth for 3 days, the separated bacteria by

centrifugation were transferred to 150 mL sterilized seawater with addition of Na2SO4 (0.5 g/L) and MgSO4 (2 g/L). The seawater inoculated with Desulfovibrio sp. was then used for the test medium of the following bacterial settlement experiment. Bare slide glass (BG), THIS, SHOS and SLIPS samples were sterilized under ultraviolet radiation for 30 min, and then vertically immersed into the inoculated seawater to start bacterial settlement experiment. The temperature of test medium was set as 30 ± 2 ℃. During immersion period, 75 mL sterile seawater with addition of Na2SO4 (0.5 g/L) and MgSO4 (2 g/L) was injected into the system every 3 days to replace the same volume of old seawater with Desulfovibrio sp.. To simulate the dynamic condition, the Erlenmeyer flask was shaken with an orbital shaker. The water flow speed can be calculated by multiplying rotation speed by the perimeter of Erlenmeyer flask. Through controlling the rotation speed, the seawater flowing speed over test samples was set as 0.4 m/s, which is closed to the tidal speed in Bo Sea 34. Samples after bacterial settlement experiment were used for the following optical performance test and fluorescence microscope observation. Before the fluorescence microscope observation, the samples were stained with sterilized seawater containing 1% DAPI, and then washed with sterilized seawater

35

.

According to fluorescence microscope results, bacterial coverage over samples was determined with Image J software.

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2.4.2 Algae settlement analysis The algae Chlorella vulgaris was cultured in f/2 culture medium with temperature of 30 ℃ 36. After 10 days of growth, the culture medium with Chlorella vulgaris was diluted with fresh culture medium to get a test medium with Chlorella vulgaris cell concentration of around 8.5×105 CFU/L, which was used for the following algal settlement test. Similar with the bacterial settlement experiment procedure, the sterilized samples (SLIPS, BG, THIS and SHOS) were immersed in the inoculated culture medium (150 mL). During experiment process, fresh culture medium with a volume of 40 ml was injected into the system every 3 days to replace the same volume of the old culture medium. An orbital shaker was used to shake the test medium in Erlenmeyer flask to simulate a dynamic condition. The seawater flowing speed over sample is set as 0.4 m/s by controlling the rotation speed. Samples after settlement experiments were taken out of test media, and then used for the optical performance test and fluorescence microscope observation. The algal coverage over samples was determined with Image J software according to the fluorescence microscope results. 2.5 The stability of SLIPS exposed to various environments The stability of SLIPS was evaluated in sterile seawater under different conditions, including static, dynamic and vibration conditions. To simulate dynamic environment, SLIPS was adhered onto the inner surface of the Erlenmeyer flask, which was shaken in an orbital shaker. The seawater flow speed over

10


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sample was set by controlling the rotation speed. To study the effect of seawater flow speed to the stability of SLIPS, the flow speeds were set as 0.8 m/s, 0.4 m/s and 0.1 m/s, respectively. Vibration condition was simulated with the assist of ultrasonic vibration instrument with high vibration energy (20 KHz, 150 W). After exposed to simulated environment for different periods (ranging from 1 h to 20 d), water contact angle hysteresis of SLIPS was measured and utilized as the index for characterizing the stability of SLIPS. 3. Results and discussion 3.1 Wettability, morphology and composition Figure 1 represents the photographs of water droplets on four different kinds of samples (BG, THIS, SHOS, and SLIPS). The BG is hydrophilic for the low static water contact angle (26.1±3.7°, Figure 1a). After hydrothermal treatment, the hydrophilicity of glass sample is enhanced, and the water contact angle over THIS decreases to 8.3±2.9° (Figure 1b). After hydrophobic modification, the static water contact angle over glass sample is 158.0±1.9° (Figure 1c), and the water contact angle hysteresis is 4.9±0.5°, indicating that the glass sample after modification presents super-hydrophobic property 37-39. After the infusion of lubricant, the static water contact angle on the as-fabricated SLIPS is 111.3° ± 2.3°, and water droplet can slip over its surface with a low titled angle of 4.4±0.4° (Figure 1d). It was indicated that a continuous lubricant layer forms over glass substrate, and it is hardly replaced by water. Figure 2 represents the micro-morphology of glass sample after hydrothermal treatment. A layer with thickness of around 170 nm forms over glass sample (Figure 2a). The layer is composed of vertically grown nano-sheets

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with thickness of around 20 nm (Figure 2b, c). Since the glass substrate presents hydrophilic property (Figure 1a), the rough micro-structure can further enhance the hydrophilic property of glass surface according to Wenzel model

40,41

. After

hydrophobic modification procedure, air can be trapped into the grooves among nano-sheets. According to Cassie model

42,43

, water droplet contacts with the

glass sample through the interface which contains both air and solid fractions. The presence of air layer enhances the hydrophobic property of surface, and endows the surface super-hydrophobic property. Regarding to SLIPS, for the chemical affinity between silane layer over substrate and the lubricant, the rough micro-structure over substrate assists the infusion of lubricant layer under capillary effect, thereby forming a stable lubricant layer over rough substrate 32. Because water is immiscible to lubricant, water droplets could slide over a titled SLIPS. To reveal the formation mechanism of rough micro-structure over glass sample, the surface composition of glass sample before and after hydrothermal treatment was characterized with XPS (Table 1). XPS results demonstrated that the contents of Ca, Al, and Mg increase, and the contents of Na and Si decrease after the hydrothermal procedure. It can be expected that ion exchange and etching occur during hydrothermal process 44. Water molecules could diffuse into vitreous network of glass during hydrothermal process, and lead to hydration of glass 44. Because the bonding energy of Na to oxygen is lower than that of other elements in glass (including Ca, Al, and so forth) to oxygen

45

, ion-exchange

process between Na and protons easily occur under hydrothermal condition. Simultaneously, hydroxyl ions can attack siloxane bonds in glass under hydrothermal condition, resulting in the breaking of bridging oxygen bond.

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During this process, a gel layer which contains insoluble network siliceous composite and soluble molecules siliceous composite forms on the glass/solution interface. With the increase of reaction time, the soluble composite can gradually enter into solution, leaving the insoluble composite with a porous structure over the glass surface. 3.2 Underwater optical performance Underwater-transparency of sensors/windows is important to the performance of marine optical instruments

46

. It was known that the transmittance of natural

seawater changes with preserving time for the growth and metabolism of organisms 47. To avoid the test error from the test medium, the underwater-optical performance of glass samples (BG, THIS, SHOS, and SLIPS) was evaluated in pure water. The BG sample presents high transparency with the maximum transmittance of 98.4% in pure water (Figure 3a), which is around 7 % higher than that in air environment (Figure S1). This results from the lower refractive index difference between water and glass than that between air and glass according to Fresnel Formula. After modification of THIS, the underwater-transmittance of glass sample in pure water is enhanced within the visible light range (400-800 nm), and the maximum transmittance increases from 98.4% (BG) to 98.9 %. It is well-known that transmission of glass can be enhanced by construction of layer, and the maximum transmittance can be achieved in case that the layer materials involved satisfy the following criterion (Eq.1): 𝑛𝑐 = (𝑛𝑚 𝑛𝑠 )0.5

(1)

where 𝑛𝑐 , 𝑛𝑚 and 𝑛𝑠 are the refractive indices of layer, environmental medium and substrate, respectively)

48

. Based on this criterion, constructing

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porous structure, which can reduce the apparent refractive index of material by trapping substance with low refractive index, is generally utilized to enhance transmission of glass sample

49

. It is known that water can infuse into porous

structure of THIS under capillary effect after it is immersed into water (Figure 3b). The infusion of water can reduce the apparent refractive index of THIS, since the refractive index of water (1.333) is lower than glass (around 1.5), and thus enhances the underwater-transparency of THIS. In contrast with THIS, the underwater-transmission of SHOS is significantly reduced to less than 90 %, and this phenomenon resulted from the existence of air layer between glass and water (Figure 3b). To further prove this point, a procedure was designed to remove the air layer on the SHOS sample immersed in water. SHOS sample was firstly immersed into ethanol, which can infuse into its porous microstructure of SHOS and wet the surface for its low surface energy. The SHOS wetted by ethanol was then transferred into water, which can gradually replace the ethanol for the intersolubility with each other. After this procedure, the air layer on SHOS can be replaced by water. It can be found that the transmission of deaerated SHOS (DSHOS) almost coincides with the THIS, proving that the low underwater-transmission of SHOS is attributed to the air layer between substrate and water. Different with SHOS, the underwater-transmission of glass sample is enhanced after modification of SLIPS within the range 500-800 nm (Figure 3a). It was known that the refractive index of lubricant (1.303) is lower than that of glass substrate (around 1.5), the infusion of lubricant can decrease the apparent refractive index of SLIPS layer, and thus enhances its underwater-transmission. 3.3 Anti-biofouling performance

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To investigate the advantage of applying SLIPS as anti-biofouling strategy for marine optical instruments, the anti-biofouling property of SLIPS was studied, and compared with the other three kinds of glass samples (BG, THIS and SHOS). The typical marine microorganism (Desulfovibrio sp.) and algae (Chlorella vulgaris) were utilized as models for evaluation of anti-biofouling performance of glass samples. In the static environment, bacteria (Desulfovibrio sp.) can easily settle onto BG after immersion of 3 and 10 days (Figures S2), and the average bacteria coverages are 12.569% (3 days) and 21.921% (10 days), respectively (Figure 4a). The bacteria coverages on THIS are 13.564% and 25.320% after immersion of 3 and 10 days, respectively (Figure 4a). It can be speculated that the increase of bacterial settlement over THIS is attributed to its rough microstructure, which increases the surface area for bacteria settlement. It was generally reported that super-hydrophobic surface can inhibit the bacterial and algal settlement for the existence of air layer on substrate 50. In contrast with this report, the bacteria coverages on SHOS are 12.664% and 23.126% after immersion of 3 and 10 days, respectively (Figure 4a), which are closed to these of BG. This result can be attributed to the collapsing of air layer for the adherence of organic matter, and it also points out the weakness of SHOS in marine antibiofouling application. In contrast with these three kinds of samples (BG, THIS, and SHOS), bacteria are seldom found on the SLIPS (Figures S2d1, d2). The statistical bacteria coverage on SLIPS is 1.56% after immersion of 10 days, and it is much lower than these of other three kinds of samples. This result demonstrated that SLIPS could significantly suppress the bacterial settlement in static environment. In ocean environment, tidal movement results in a dynamic stress on sample

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surface, thereby affecting the settlement of micro-organism. To demonstrate the potential application of SLIPS in real marine condition, the anti-biofouling property of SLIPS was studied, and compared with other samples in simulated dynamic condition (the water flowing speed was set as 0.4 m/s). The parameter Reynolds number (Re) was used to determine the flow regime, and it is calculated with the following equation: 𝑅𝑒 = 𝐿𝑣𝜌/𝜇

(2)

where L is a characteristic linear dimension, and it is sample length in flow direction in this research, 𝑣 is the maximum velocity of the object relative to the fluid, 𝜌 is the fluid density, 𝜇 is dynamic viscosity of fluid 32. Base on the experimental parameters in our research, the Re value can be calculated as 5.99×105. The high value of Re proves that the flow regime over test samples is turbulent under dynamic condition. Similar with the results under static condition, bacteria can easily settle onto BG, THIS, and SHOS, and the bacteria coverages on the three kinds of samples increase with immersion time under dynamic condition (Figure S3). Noticeably, the bacteria coverage values on the three samples (17.921% for BG, 18.325% for THIS, 17.362% for SHOS) are closed with each other after immersion of 10 days (Figure 4b), indicating that THIS and SHOS are not effective in biofouling control in simulated dynamic condition. In contrast, bacteria are seldom observed on SLIPS in simulated dynamic condition (Figures S3d1,d2). The bacterial coverage on SLIPS is lower than 1 % after immersion for 10 days (Figure 4b), demostrating that the SLIPS presents exceptional inhibition performance to bacterial settlement under dynamic condition. Furthermore, the bacteria coverage on the four kinds of samples (BG, THIS, SHOS, and SLIPS) in dynamic condition are all apparently lower in comparison with these in static condition. It

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is indicated that the shear force generated by flowing water can help to remove the bacteria from solid substrate, thereby reducing bacteria settlement on glass samples in dynamic condition. The underwater-transmittance spectra of the glass samples (BG, THIS, SHOS and SLIPS) after immersion in seawater with bacteria for 10 days were characterized in pure water. It is clear that the underwater-transmittance of the glass samples (BG, THIS and SLIPS) are all reduced, and their maximum transmittance after immersion in static inoculated seawater are reduced to 91.5% (BG), 89.2% (THIS), 94.7% (SLIPS), respectively (Figure 4c). Similar with that, the maximum transmittances of BG, THIS and SLIPS after immersion in dynamic inoculated seawater are reduced to 92.6%, 90.5% and 94.9%, respectively (Figure 4d). It is indicated that the settlement of Desulfovibrio sp. degrades the underwater optical performance of all three kinds of samples under dynamic and static conditions. However, the transmittance of SLIPS is still higher than 94.7% within the range of 400-800 nm after immersion in inoculated seawater under both dynamic static and conditions, and it is much higher in comparison with the other three kinds of samples. This result proves the advantage of SLIPS for maintaining high transmittance in inoculated seawater. Different with these three kinds of samples, the SHOS after immersion presents comparable underwater-transmittance with that before immersion. This can be attributed to the fact that the air film, which can induce the transmittance of glass, was collapsed during immersion period. Anyway, the maximum transmittances of SHOS after immersion for 10 days are 89.7% (static condition) and 91.9% (dynamic condition), and they are apparently lower than these of SLIPS. It is indicated that the SHOS is not feasible for biofouling control of marine optical

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instruments. Algae is another typical kind of biofouling organism in marine environment 51

. To prove the broad-spectrum anti-fouling property of SLIPS, it was immersed

in the culture medium with algae Chlorella vulgaris, and its underwater optical and anti-biofouling performances were compared with the other three kinds of sample (BG, THIS, and SHOS). Based on the fluorescence microscope images of samples (SLIPS, BG, THIS and SHOS) after immersion into the culture media with Chlorella vulgaris for 3 and 10 days (Figures S4, S5), the statistical results of algal settlement coverages are obtained and summarized in Figure 5. Similar with the case in bacterial settlement, the algae coverages on all samples increase with immersion time under both dynamic and static conditions. The coverages of algae on THIS (21.115% and 5.451% for static and dynamic conditions, respectively) and SHOS (16.126% and 4.686% for static and dynamic conditions, respectively) are very closed to that of BG (18.421% and 4.805% for static and dynamic conditions, respectively) after immersion in inoculated seawater for 10 days (Figures 5a,b), proving that the modification of THIS or SHOS presents little effect to the algal settlement. In contrast with that, the algae coverages on SLIPS after immersion of 10 days are 1.586% (static condition) and 0.721% (dynamic condition), which are both much lower than these of other three kinds of samples. It is proven that the SLIPS can inhibit the algal settlement under both conditions. In contrast with bacterial settlement result in inoculated seawater, the algae coverage values on samples under dynamic condition are all much lower than these in static condition. It is demonstrated that the contributing degree of dynamic stress to algae settlement is much higher than that to bacteria settlement, and it can be attributed to the settlement mechanism difference between the two

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species. Figures 5c,d show the underwater-transmittance spectra of four different samples after immersed in culture medium with algae for 10 days. The maximum underwater-transmittance of SLIPS is 94.3% within test spectral range, and it is much higher than these of BG (86.7%), THIS (83.4%), and SHOS (88.0%) after immersion in static culture solution with algae (Figure 5c). Similar with that, the maximum underwater-transmittance of SLIPS after immersion in simulated dynamic condition is 96.5%, and it is higher than these of BG (91.7%), THIS (92.6%) and SHOS (92.7%) (Figure 5d). It was demonstrated that modification of SLIPS over glass sample helped to maintain high underwater-transmittance within visible light range for its anti-biofouling effect. It should also be noticed that the underwater-transmittance of samples after immersion in dynamic environment is higher than that in static environment. It can be attributed to the fact that the dynamic stress helps to reduce the algal settlement. 3.4 Anti-biofouling mechanism of SLIPS Our previous reports indicated that the growth curves of marine microorganism (Desulfovibrio sp.) and algae (Chlorella vulgaris) in the culture medium with addition of lubricant (PFPE) nearly coincide with these in the control culture medium, and thus proved that the lubricant is nontoxic to the model microorganism and algae in this research

52,53

. Thus, a conclusion be

drawn that the anti-fouling performance of SLIPS does not result from the lubricant toxicity, but its special surface properties. It can be imagined that the infusion of lubricant into the porous structure of substrate (SHOS) results in the reduction of roughness of SLIPS and endows the SLIPS with liquid-like property. It was generally believed that SLIPS is smooth in ambient environment

19


12

.


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However, weak fluctuation was observed over lubricant layer due to the anisotropic pressure when the SLIPS was immersed into water environment

54

.

The lubricant layer of SLIPS can be sheared away by the dynamic flow when it was exposed to a dynamic environment

28

, and it can diffuse into water even

under a static condition 27. Thus, it can be expected that the roughness of SLIPS increases gradually for the loss of lubricant during the immersion period for bacterial and algae settlement analysis (Figure 6). Anyway, the roughness of SLIPS during the immersion period in seawater must be between the values of its solid substrate (SHOS) and an ideally smooth surface. Under this assumption, the effect of roughness to the anti-fouling property of SLIPS can be clarified by comparing its performance with DSHOS and hydrophobized smooth surface. It should be mentioned that the DSHOS, but not SHOS, was used as a reference, because the air layer can be trapped in SHOS after immersion in seawater, and it can affect the settlement of bacteria and algae. It was known that polished silicon wafer presents extremely smooth property with hydrophilic property. To avoid the effect to wettability to the settlement result, PFTEOS was utilized to modify the silicon wafer. After modification with PFTEOS, the polished silicon wafer is hydrophobic with static water contact angle of 109.2°±3°(Figure S6), which is closed to that of SLIPS (111.3°±3°). AFM result (Figure S7) indicated that the hydrophobized silicon wafer shows smooth property with low roughness (around 1 nm). Thus, it can be utilized as a reference with hydrophobized smooth surface property in this research. According to the statistical results of bacterial and algal settlement coverages on the SLIPS, hydrophobized silicon wafer (HS), and DSHOS after immersion in inoculated seawater for 10 days (Figures 7), the bacterial and algal settlement coverages on HS are closed to these on DSHOS under dynamic and 20


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static conditions, implying that the roughness shows negligible effect to the bacterial and algal settlements under the experimental condition in this research. Noticeably, the bacterial and algal settlement coverages on SLIPS are much lower than the two kinds of solid substrates (HS and DSHOS) under all experimental conditions. Thus, it was inferred that the liquid-like property, but not the reduction of roughness, is the essential contributor for inhibition of bacterial and algal settlement on SLIPS. As previous reported

19

, the SLIPS is a

mobile interface, where bacteria and algae can hardly anchor through the mechanism that is applied on solid substrate. Thus, the SLIPS can maintain high underwater-transparency in inoculated seawater, and it can be regarded as a good candidate for biofouling control of marine optical instruments. 3.5 Stability of SLIPS in different environments The stability of SLIPS exposed in seawater under different conditions (static condition, dynamic condition, and vibration condition) was evaluated by monitoring the water contact angle hysteresis over its surface after immersion (Figure 8). To provide valuable data for the application of SLIPS in real ocean, dynamic condition was simulated under different seawater flow speeds (0.8 m/s, 0.4 m/s, and 0.1 m/s). The highest flow speed (0.8 m/s) is much higher that the tidal speed in Bo Sea. It should be noticed that the water contact angle hysteresis increased with immersion time under all conditions. In comparison with static condition, the water contact angle hysteresis increased more rapidly under dynamic and vibration conditions, indicating that dynamic flow can reduce the

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lifetime of SLIPS by shearing away the lubricant layer. As we can imagine, the failure of SLIPS was accelerated with the increase of flow speed from 0.1 m/s to 0.8 m/s, Especially, the water contact angle hysteresis of SLIPS increased to around 110︒ after exposure in seawater under vibration condition for only 1 h, indicating that it can hardly withstand high-energy vibration during practical application. The conclusion can be drawn that the stability of SLIPS under dynamic condition is its weakness for application. Efficient strategy is required for enhancing the stability of SLIPS under dynamic condition.

4. Conclusions A kind of slippery lubricant-infused porous surface with high underwatertransmittance was fabricated on glass sample via infusing lubricant into its porous micro-structure fabricated with hydrothermal method. The infusion of lubricant can decrease the apparent refractive index of SLIPS, thereby enhancing the underwater-transparency of glass within the wavelength range of 500-800 nm. In contrast with super-hydrophobic surface and

textured hydrophilic surface,

slippery lubricant-infused porous surface presents exceptional inhibition performance to bacterial and algae settlements in inoculated seawater under dynamic and static conditions, thereby maintaining high underwatertransparency. The exceptional anti-biofouling property of SLIPS results from its liquid-like property, which can hard to provide anchor for the settlement of bacteria and algae. In comparison with static condition, the dynamic stress helps

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to release the bacterial and algal settlements over sample, and to maintain high underwater-transmittance in inoculated seawater under dynamic condition. The contributing degree of dynamic stress to the bacteria and algae settlement varies with species for their different settlement mechanisms. The water contact angle hysteresis of SLIPS, which is utilized as an index for evaluating its stability, increases with immersion time in the seawater under different conditions (static, dynamic, and vibration conditions) for the loss of lubricant. Both dynamic and vibration conditions accelerate the failure of SLIPS exposed in seawater. This research will provide a promising anti-biofouling technique for marine optical instruments, and it provides a deep insight to the interactions among marine biofouling, surface property and optical performance of glass.

Supporting Information Available: Transmittance spectra of bare glass exposed to air. The fluorescent images of glass samples after immersion in static seawater with Desulfovibrio sp. for 3 and 10 days. The fluorescent images of glass samples after immersion in dynamic seawater with Desulfovibrio sp. for 3 and 10 days. The fluorescent images of samples after immersion in static culture medium with Chlorella vulgaris. The fluorescent images of samples after immersion in dynamic culture medium with Chlorella vulgaris. Photograph of water droplet on hydrophobized silicon wafers. Water contact angles of hydrophobized silicon wafers.

Acknowledgements

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This work was supported by National Key Basic Research Program of China (2014CB643304), National Natural Science Foundation of China (41576079), Science & Technology Basic Research Program of Qingdao (15-9-1-31-jch) and Nantong (MS12015119), Foundation of State Key Laboratory for Marine Corrosion and Protection (KF1600405).

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Figures

(a)

(b)

(c)

(d) Figure 1. Photograph of the water droplet on glasses: the static water contact angle on (a) bare glass, (b) textured hydrophilic glass, and (c) the glass after modification of super-hydrophobic surface; (d) the dynamic motion of water droplet on the slippery lubricant-infused porous surface with a low titled angle (＜10°).

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(a)

(b)

(c) Figure 2. The morphology of glass after hydrothermal process in NaOH solution: (a) cross-section view, (b, c) top view. 34


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(a)

(b) Figure 3. (a) Underwater-transmittance spectra and (b) schematic of different kinds of glass samples (BG, THIS, SHOS, SLIPS and DSHOS) exposed to water.

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(a)

(b)

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(c)

(d) Figure 4. Statistical results of bacterial settlement density on four kinds of samples (BG, THIS, SHOS and SLIPS) after immersion in (a) static and (b) dynamic seawater with bacteria for 10 days, and underwater-transmittance spectra of four kinds of samples (BG, THIS, SHOS and SLIPS) after immersion in (c) static and (d) dynamic seawater with bacteria for 10 days.

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(a)

(b)

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(c)

(d) Figure 5. (a, b) Statistical results of algal settlement coverage and (c, d) underwater-transmittance spectra of four kinds of samples (BG, THIS, SHOS and SLIPS) after immersion in dynamic and static culture solution with algae: statistical results of SRB settlement coverage on the four kinds of samples after immersion in (a) static and (b) dynamic culture solution with algae for 3 and 10

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days, underwater-transmittance spectra of the four kinds of samples in pure water after immersion in (c) static and (d) dynamic culture solution with algae for 10 days.

Figure 6. Scheme showing the change of existence state of SLIPS with the immersion time in seawater.

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(a)

(b) Figure 7. Statistical results of (a) bacterial and (b) algal settlement coverages on three kinds of samples (HS: hydrophobized silicon wafer with low roughness; DSHOS: dearated super-hydrophobic surface; SLIPS: slippery lubricantinfused porous surface) after immersion in inoculated seawater under static and dynamic conditions for 10 days.

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Figure 8. Water contact angle hysteresis of SLIPS as a function of immersion time in seawater under different conditions (static condition; dynamic conditions with seawater flow speeds of 0.1 m/s, 0.4 m/s and 0.8 m/s, respectively; vibration condition). These values are represented as the means of nine measurements with error bars representing the standard deviations.

Table 1. The element content of bare glass sample and glass sample after hydrothermal treatment. Elements

Si

O

Al

Na

Ca

Mg

Bal.

Bare glass

33.62

39.04

0.74

13.88

7.20%

4.00%

1.52 %

sample

%

%

%

%

Glass after

10.41

34.52

1.35

6.19%

31.93

14.08%

1.52 %

treatment

%

%

%

42


%

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Fabrication of Slippery Lubricant-Infused Porous ... - ACS Publications

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