Silicone Oil-Infused Slippery Surfaces Based on Sol–Gel Process

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Silicone Oil-Infused Slippery Surfaces Based on Sol-Gel Process-Induced Nanocomposite Coatings: A Facile Approach to High Stable Bioinspired Surface for Biofouling-Resistance Cunqian Wei, Guangfa Zhang, Qinghua Zhang, Xiaoli Zhan, and Fengqiu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09879 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Silicone Oil-Infused Slippery Surfaces Based on Sol-Gel Process-Induced Nanocomposite Coatings: A Facile Approach to High Stable Bioinspired Surface for Biofouling-Resistance Cunqian Wei,† Guangfa Zhang,† Qinghua Zhang,*† Xiaoli Zhan, † and Fengqiu Chen† †College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China

ABSTRACT: Slippery liquid-infused surfaces (SLIPS) have aroused widespread attention due to their excellent liquid-repellency properties associated with broad applications in various fields. However, the complicated preparation processes and the vulnerable surface lubricant layers severely restrict the practical applications of SLIPS. In this work, robust transparent slippery hybrid coatings (SHCs) were easily fabricated by the infusion of sol-gel-derived nanocomposite coatings in silicone oils of varying viscosity. The prepared silicone oil-infused surfaces exhibited outstanding long-term slippery stability even under extreme operating conditions such as high shear rate, elevated evaporation, and flowing aqueous immersion. Static bacteria culture tests confirmed that the SHCs could significantly inhibit biofilm formation. In addition, bovine serum albumin adhesion experiments were conducted after lubricant loss tests, showing significantly less protein absorption and a long service life of the SLIPS. The unique ultralow bacterial attachment and remarkably long-term protein-resistant performance render the as-prepared SLIPS as a promising candidate for biomedical applications even under harsh environmental conditions.

KEYWORDS: slippery surfaces, transparent, anti-biofouling, stability, sol-gel process

1. INTRODUCTION Biofilm formation has significant negative economic and environmental consequences across a wide range of areas, including in medical devices, ships, food packaging, and storage.1-4 The initial adhesion processes in biofilms are generally related to the protein component of extracellular polymeric substances.5,6 According to the Cassie regime, superhydrophobic surfaces can induce a reduction in protein adsorption due to a decreased solid surface area at the liquid interface. Therefore, these surfaces have attracted wide interest with regards to their potential applications in anti-biofouling.7 Superhydrophobic surfaces with superwetting properties can be easily manipulated

by surface

chemical composition and

micro/nano

topography.8-11

However,

superhydrophobic surfaces generally suffer from inherent limitations that severely restrict their wide applicability. For example, the air cushions formed by air trapped within the micro/nano structures cannot withhold high pressures and temperatures, and therefore organic liquids with a low surface tension will easily 1 ACS Paragon Plus Environment

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penetrate the structure under these conditions.12-14 Moreover, precise surface morphologies are prone to destruction by mechanical damage during practical applications,15,16 thus irreversibly losing partial or complete superhydrophobicity. In nature, several animals, insects, and plants have developed liquid-coated substrates with anti-biofouling properties;17 “liquid-infused” surfaces have been recently developed following biomimetic inspiration from such special functional surfaces. Inspired by Nepenthes pitcher plants, Aizenberg et al.18 first reported on the preparation of slippery liquid-infused porous surfaces (SLIPS) through the infusion of a lubricating liquid into rough micro/nanostructured substrates to achieve smooth liquid-infused versatile surfaces. SLIPS lead to the formation of continuous and homogenous overlying liquid interfaces, which provide extremely slippery, low-hysteresis, non-stick surface to a broad range of liquids and solids.19-21 Moreover, the surfaces display excellent liquid repellency, pressure stability, self-healing, and an enhanced optical transparency, all of which surpass the inherent properties of superhydrophobic surfaces. According to the Cassie state,22 liquid droplets can overhang on the topside of superhydrophobic surfaces, condense within the surface texture, impale the vapor pockets, and strongly pin to the surface, resulting in poor droplet mobility and a liquid-repelling performance. In contrast, according to the slippery Wenzel state,23 liquid droplets will float on the smooth dynamic liquid layers of SLIPS, where they can maintain remarkable mobility. Due to their special wetting state, SLIPS can be used in water harvesting and desalination, as well as in heat transmitters,24,25 anti-icing coatings,26,27 and oil-water separations.28,29 SLIPS also exhibit an excellent anti-fouling performance and have broad applications in biomedical materials30-32 and marine anti-fouling coatings33,34 due to the exceptionally low bacterial adhesion and biofilm formation prevention on their stable, extremely slippery interfaces. Thus far, two different methods for SLIPS fabrication have been proposed. A typical strategy involves the infusion of lubricant oils into low surface energy micro/nano-structured substrates. Based on this fabrication route, various methods have been explored to prepare textured substrates, including layer-by-layer assembly,35-37 etching processing,24 electrochemistry,33,34 porous membranes,38,39 and chemical deposition,27 among others. Nevertheless, such an approach requires the fabrication of topographic feature supports and subsequent fluoridation, which lead to increased costs due to the precise structural requirements and the various steps involved. The second strategy involves the use of self-replenishing materials. Crosslinked networks20,40,41 or large free-volume structures42,43 are required to encapsulate or swell the lubricant in the matrix and maintain the fluent surface over a long period. However, a long processing time, often of several days, is required for the crosslinked gelatin to reach equilibrium40 and the lubricant secretion speed is difficult to manipulate.41,44 Thus, the available SLIPS fabrication methods are complex, time-consuming, or substrate specific. Further, the commonly used lubricating oils (perfluoropolyethers) are expensive and the longevity of the 2 ACS Paragon Plus Environment

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lubricant layers is often questioned and not satisfactory given its critical nature in their application as anti-fouling or anti-icing coatings.45 Therefore, a simple, highly efficient, and cost-effective strategy for the fabrication of highly versatile and robust SLIPS is of great significance for their practical application. Herein, novel silicone oil-infused SLIPS were synthesized and characterized with the aim to prepare surfaces capable of preventing biofilm formation. A simple sol-gel process was proposed to fabricate robust nanoscale hybrid coatings (HCs) with abundant internal crosslinked networks. Highly stable and transparent SLIPS were then obtained after immersion in silicone oils. The association between the wettability of the slippery HCs (SHCs) and the surface topography and lubricant viscosity was investigated in detail. Blood and bacterial attachment on the as-fabricated coatings was explored to evaluate their bio-fouling resistance. Finally, accelerated lubricant loss and protein adhesion experiments were performed to estimate the long-service performance of the fabricated SHCs in harsh environments.

2. MATERIALS AND METHODS 2.1 Materials Tetraethyl orthosilicate (TEOS), 2-isopropanol, acetic acid, and other reagents were obtained from Sinopharm Chemical Reagent Co. and used as received. Methyltriethoxysilane (MTES) was purchased from KCCHEM Co., Ltd (Nanjing, China). N-octyltriethoxysilane (OTES) was purchased from Guotai-huarong New Chemical Materials Co., Ltd (Nanjing, China). Silicone oils with a viscosity of 10, 20, 50, 100, 200, and 500 mPa·s were purchased from Sigma-Aldrich.

2.2 Fabrication of SHCs TEOS, MTES, and OTES were dissolved in 2-isopropanol and stirred at 30 °C for 30 min. The sol-gel reaction was initiated by adding an aqueous mixture containing acetic acid and distilled water. The reaction mixture had a molar ratio of n[TEOS]: n[MTES]: n[OTES]: n[2-isopropanol]: n[H2O]: n[acetic acid] of 4 : 7 : 2 : 33 : 46 : 0.7. The mixture was added dropwise into the 2-isopropanol solution and stirred constantly for 2 h. An acidic nano-sol was obtained after ageing for 3 days and then sprayed on glass slides or aluminum sheets by an airbrush (HD 180, No 302, Taiwan) at a pressure of 30 Psi and a distance of 20 cm. All samples were slowly evaporated at ambient temperature for 1 h and then thermally treated at 120 °C for 2 h to obtain the HCs. Subsequently, SHCs were obtained by the following procedures. HCs were immersed in silicone oils of varying viscosities for 2 h and then centrifuged at 1000 rpm for 1 min to remove the excess lubricant. The HCs lubricated with silicone oils of 10, 20, 50, 100, 200, and 500 mPa·s were denoted as SHC-10, SHC-20,

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SHC-50, SHC-100, SHC-200, and SHC-500, respectively. Unless otherwise mentioned, all substrates here are glass slides.

2.3 Characterization Fourier transform infrared (FTIR) spectroscopy of the HCs was performed on a Nicolet 5700 FT-IR spectrometer. The chemical composition was analyzed by XPS (Thermo Scientific, USA) with an Al Kα X-ray source. The X-ray gun was operated at 14 kV and 350 mW, and the analyzer chamber pressure was set at 10–9 to 10–10 Pa. The surface structure of the hybrid surfaces was observed under field emission scanning microscopy (FESEM, Hitachi TM-1000) at an accelerating voltage of 20 kV. The roughness of the HCs and SHCs was analyzed by atomic force microscopy (AFM, Veeco, USA) operated by Multi Mode in tapping and contacting mode, respectively. The scanning range was 3 × 3 µm and the root-mean-square (RMS) roughness values were calculated from the obtained AFM two-dimensional height images. The transmittance was measured using ultraviolet-visible absorption spectrometry (UV−Vis 3802, Unico (Shanghai) Instrument Co. Ltd., China). Contact angles (CAs) were measured through the sessile drop method using a CAM 200 optical contact angle goniometer (KSV Instruments, Helsinki, Finland). Advancing and receding CAs were measured for macroscopic droplets (~7 µL) by using the goniometer at ambient condition by slowly increasing and decreasing the volume of the droplet to induce sliding and then analyzing the obtained images to find the best fitting CAs. The water CA hysteresis (CAH) reported here is the difference between the advancing water and receding water CAs.

2.4 Stability experiments Lubricant Shear Stability Test. In order to measure lubricant retention on the various surfaces, SHCs were prepared on glass slides (2.5 × 7.6 cm2). The samples were spun from 200 to 3500 rpm by low speed centrifuge (SC-3610, Anhui USTC Zonkia Scientific Instruments Co., Ltd) for 1 min. The gradual loss of silicone oil in the SHC samples was calculated by the weight change before and after each shear test and the thickness of the lubricant was then measured.46 Lubricant Thermal Stability Test. Accelerated and uniform removal of silicone oil was assessed by placing the samples (2.5 × 7.6 cm2) in an oven at 70 °C for 1–7 days. The mass of the samples was then measured daily using a BS 224S (Sartorius, Beijing) analytical balance to obtain the lubricant mass loss via evaporation. Flowing Water Stability Test. SHCs (2.5 × 7.6 cm2) were immersed in 200 mL of water in a 250-mL beaker at a stirring speed of 20 rpm for 1–7 days. The samples were then dried at 70 °C for 1 h for further measurement. 4 ACS Paragon Plus Environment

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After each of the three stability experiments, the static, advancing, and receding water CAs of these surfaces were measured.

2.5 Biofouling experiments Bacterial Biofouling Resistance Test. Liquid stock solutions of Escherichia coli (E. coli) strain DH-5α in tryptic soy broth (TSB) medium at a density of approximately 1 × 1011 cells/mL were prepared. E. coli was chosen because of its role as an infection-causing agent in many healthcare settings. Tests against bacterial adhesion were performed by dipping glass, HC, SHC-20, and SHC-100 (2 × 2 cm2) into the E. coli stock solution at 37 °C for 3 and 24 h. After incubation, the substrates were gently removed from the biofilm medium. Samples for biofilm coverage analysis were immediately stained by immersion in a 0.1% (w/v) solution of crystal violet in water for 10 min, gently rinsed in distilled water to remove excess stain, dried at 50 °C for 12 h, and then photographed. For colony forming unit (CFU) analysis, samples immersed for 24 h in bacterial medium were washed in sterile purified water to remove excess bacteria. CFU counts were performed by sonicating the samples in sterile phosphate buffered saline solution (pH 7.4, 10 mL) for 5 min to remove the bacteria from the samples, then diluted and plated out on TSB agar. All the experiments were performed on a clean bench to avoid introduction of other bacteria. For both biofilm coverage and CFU counts, at least six replicates per treatment were prepared. Protein Adsorption Resistance Test. The coated samples (2 × 2 cm2) were immersed into 2 mL of bovine serum albumin (BSA) solution (1 g/L, in PBS buffer solution, pH 7.4) at 25 °C for 24 h and then removed from the solution. The BSA concentration before and after adsorption was determined by UV−Vis spectrophotometry at a wavelength of 280 nm and calculated according to a standard curve (Figure S1). The mean value of six measurements was reported for each sample.

3. RESULTS AND DISCUSSION 3.1 Fabrication of SHCs HCs were synthesized using a simple sol-gel procedure during which, through the addition of water, the ethoxy groups in TEOS, MTES, and OTES were hydrolyzed and subsequently generated hydroxyl groups at acidic condition (approximately pH 5). A polycondensation reaction between hydrolyzed TEOS and the silane coupling agent (MTES, OTES) was then allowed to occur (Figure 1A). Following spraying of the sol onto the substrate, heat treatments were performed in order to allow gel formation. The polycondensation reaction between the hydrolyzed TEOS, MTES, and OTES enhanced the adhesive strength of the coated layer.8 The cross-hatch and tape test confirmed that the hybrid layer adhered to the aluminum and glass substrates firmly 5 ACS Paragon Plus Environment

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(Figure S2). As shown graphically in Figure 1B, the resulting composite possesses nanoscale morphology as well as abundant internal crosslinked networks results in water-repellent slippery surfaces after immersed in silicone oils.

Figure 1. (A) Schematics of the hydrolyzation and the polycondensation of TEOS, MTES, and OTES. (B) Schematic illustration of silicone-infused SHC. The chemical composition of the synthesized HC was verified by FT-IR spectroscopy (Figure 2A). The adsorption peak observed at approximately 440 cm–1 was assigned to the bending vibration of the Si-O group. The 1061 and 1269 cm–1 adsorptions were primarily assigned to the Si-O-Si stretching vibrations, indicating hydrolytic condensation of TEOS, MTES, and OTES. The broad band at 796 cm–1 was assigned to the Si-C grafted aliphatic chain vibrations, which overlapped with the –(CH2)n rocking vibrations. The symmetric stretching vibration bands for CH3 and CH2 bonds for HC appeared at approximately 2900 cm–1. The other two bands, at 1462 and 1408 cm–1, were ascribed to the CH2 and CH3 bending vibrations, respectively. The peak at 3479 cm–1 corresponded to the stretching vibration of the hydroxyl group. The quantitative elemental compositions (Si2p, C1s, and O1s) of the HC were evaluated from XPS data (Figure 2B). There was a relatively higher ratio of C to Si contents in the XPS data compared to the theoretical composition, which could be attributed to a surface enrichment of alkyl chains upon gelation.

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Figure 2. (A) FT-IR spectrum of hybrid coating HC. (B) XPS spectrum of HC. The inset table showed surface elemental concentrations of the sample. a The actual values were determined based on the XPS measurement. b The theoretical values were calculated based on the known composition of HC.

Figure 3. Photographs of silicone oils (approximately 5 µL) with viscosities of (A) 20 mPa·s (B) 100 mPa·s spread on HCs. Six silicone oil viscosities were used as lubricous liquids to evaluate the effect of lubricant viscosity on surface performance and lubricant stability. As shown in Figure 3, all silicone oils were able to quickly wet and permeate within the HC, forming a continuous fluid layer on the HC surface. The low-viscosity silicone oil (20 mPa·s) was able to permeate the HC sample within 10 s, yet the high-viscosity oils (above 100 mPa·s) required 120 s to completely permeate the substrate, indicating that a good compatibility between silicone oil and the designed HC substrate. Following immersion in silicone oil for 2 h, HCs were infused with the lubricant and all superfluous lubricant was removed by spinning at 1000 rpm for 1 min. The coating thickness for the SHC-20 and SHC-100 samples was 5.60 ± 0.95 µm and 8.21 ± 0.34 µm (at 35 °C ambient temperature), respectively. The surface morphologies of HC and SHC were investigated by FESEM and AFM (Figure 4). The HC 7 ACS Paragon Plus Environment

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surface presented with a typical nanoscale structure with a RMS roughness of 9.50 nm (Figure 4C). The nanotextured topography of HC was seen to enlarge the surface area and enhance the capillary action and storage capacity of silicon oil, thus contributing to the high stability of the slippery surface. Following silicone oil infusion, the slippery surface became highly smooth and defect-free, with a RMS roughness of 0.29 nm (Figure 4B, D). The surface topography observed by both the FESEM and AFM images was in good agreement with each other. Thus, the infused silicone oil permeates the sol-gel matrix and forms a molecular-scale smooth lubricant layer over the HCs.

Figure 4. (A-B) FESEM images of HC and SHC-20. The inset image is the high-magnification image for HC. (C-D) 3D AFM height images of HC and SHC-20. The static CA, advancing angle, and receding angle as well as the CAH of water droplets on the HCs before and after silicone oil infusion were investigated (Figure 5A). Compared with oil-infused slippery surfaces (SHC), the bare HC without a lubricant film showed typical hydrophobic properties, with a water CA of 98.5 ± 0.5° and a high CAH (8.70°). Following infusion with silicon oils of various kinematic viscosities, SHC sample surfaces all retained a similar water CA, while the CAH decreased considerably to values below 5°, expect for SHC-500, where the CAH increased to 7.17°, likely due to the viscous dissipation force in the silicone-oil rim around the droplet, which causes a decline in water droplet mobility.47 These ultralow CAH values confirm a lack of pinning and super-slippery surfaces. The prepared slippery surfaces had an excellent 8 ACS Paragon Plus Environment

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transparency performance (roughly 90% transmittance, superior to that of glass; Figure 5B) in the visible region due to the anti-reflection properties afforded by the smaller refractive index of oil compared to that of the non-infused HC surfaces; indeed, the HC surface microtopography often generates multi-diffusion of light, leading to opaqueness ( 0

(1)

where γ1 and γ2 are the surface tension of the pre-suffused liquid and the test liquid, respectively, θ1 and θ2 are their corresponding CAs on the solid (with air around), γ12 is the interfacial tension at the liquid-liquid interface (See Supporting Information for the measurement of γ12). As described in Table 1, the silicon oil CAs on the surface of HC on glass substrates are as low as 0°, but the CA of water on the surface of HC is 99.16 ± 0.66°. ∆γ for the surface of SHC-20 and SHC-100 are 16.74 ± 2.03 and 5.89 ± 1.97 mN/m, respectively, and both of which are positive. For SHC-20- and SHC-100-coated aluminum substrates (Table S2), ∆γ are still positive, indicating that the substrates have little influence on the surface properties. These results further illustrate that the prepared SHCs possess excellent stability and slippery properties for water droplets. Table 1. Surface/interface tensions of silicone oil and water, and their contact angles on HC. Samples

γ1 (mN/m)

γ2 (mN/m)

γ12 (mN/m)

θ1 (°)

θ2 (°)

∆γ (mN/m)

SHC-20

20.07 ± 0.06

70.85 ± 0.15

14.63 ± 2.16

~0

99.16 ± 0.66

16.74 ± 2.03

SHC-100

20.73 ± 0.06

70.85 ± 0.15

26.14 ± 1.83

~0

99.16 ± 0.66

5.89 ± 1.97

3.2 Stability of SHCs Capillary length is the parameter to evaluate the stability of lubricant on substrates. 46 The length scale of the HC nanotextures is much smaller than the capillary length (~1.46mm, see Supporting Information for a detailed calculation) of the silicone oil, such that SHCs can achieve a stable slippery behavior under mild forces associated with gravity (Figure 6). However, in practical applications, even greater shear conditions can be expected, which not only oppose the capillarity but also change the lubricant’s effective capillary length. Therefore, in order to determine the durability and robustness of the fabricated SHC, accelerated lubricant loss tests, such as high centrifugal force, evaporation at high temperature, and immersion in flowing water, were performed. For lubricant shear stability tests, SHCs were then sustained centrifugation by increasing spin rates from 200 to 3500 rpm for 1 min. The thickness of the lubricant layers decreased with increasing spinning rate (Figure 7A, B) and the surface roughness increased slightly (Figure S3A). Less viscous silicone oils (20 mPa·s) require less energy to be pushed along a surface and, therefore, it is more likely that the oil layer will be thinner after the same shear force application. Following centrifugation for 1 min below 3000 rpm, the CAH 10 ACS Paragon Plus Environment

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of SHC-20 and SHC-100 was kept below 5° (Figure 7C, D). However, as the rotating speed was further increased to 3500 rpm, the thickness of the silicone oil layers for SHC-20 and SHC-100 were both below 3 µm (Table S3). As described in Figure 7Bc, the surface nanotexture was slightly exposed and the surface roughness increased to 9.29 nm (Figure S3B). Additionally, there was a slight increase in CAH and a small degradation of the slipperiness. For the SHC-20-coated aluminum substrates, the results were consistent with those of the equivalent glass substrates (Figure S4). The above results indicate that the fabricated silicone oil-infused slippery surfaces possess excellent stability and durable slippery properties even under high shear conditions.

Figure 7. (A) Thickness of the remaining silicone oil on SHCs with increasing shear rate. (B) Schematic representations for the evolution of lubricant thickness. (C-D) Variations of surface wettability for SHC-20 and SHC-100 after spinning for 1 min at each spin rate starting from 200 rpm to 3500 rpm. (CA: static water contact angle, AA: advancing angle, RA: receding angle, CAH: contact angle hysteresis) It has been reported that the evaporation of fluoroliquids on the Nepenthes-inspired slippery surfaces generally affects their surface wettability and self-cleaning properties49. For a more suitable application in marine coatings, evaporation at elevated temperatures should be explored. The evaporation test of silicone oil 11 ACS Paragon Plus Environment

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was performed by placing the SHC samples in an oven at 70 °C for 1–7 days and accurately measuring the sample mass (Table S4) so as to determine the loss of lubricant through evaporation. Due to the intrinsic low volatility of the silicon oil and its good chemical affinity with the nanotextured HC, the volatile loss of the lubricant was suppressed. After 7 days of evaporation at 70 °C, the SHC sample masses showed no apparent change. Additionally, the CAH of water remained below 5° for both SHC-20 and SHC-100 (Figure 8A, B). Both the extremely low volatility of the infused silicone oils and the stable storage of the lubricant within the crosslinked networks of HC lead to a durable slippery performance of the prepared SHCs under the evaluated evaporation conditions.

Figure 8. Variations of CA, AA, RA as well as CAH of water measured on SHCs. (A) SHC-20 and (B) SHC-100 after keeping in 70 °C for 1–7 days. (C) SHC-20 and (D) SHC-100 after immersion in water with the spin rate of 20 rpm for 1–7 days. (CA: static water contact angle, AA: advancing angle, RA: receding angle, CAH: contact angle hysteresis) A major challenge in the application of slippery surfaces is their stability in aqueous systems. Therefore, the surface wettability change under rapidly moving water conditions was investigated in detail. The SHC samples were placed in a beaker containing water for 1–7 days. The movement speed of water over the SHCs can be 12 ACS Paragon Plus Environment

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estimated to be 8 cm/s (see Supporting Information for a detailed calculation). The static and dynamic CAs on the surfaces of SHC-20 and SHC-100 remained nearly constant after immersion in dynamic water for 7 days (Figure 8C, D). On the other hand, a slight increase in CAH (from 3.6 increased to 4.7°) was observed for SHC-20, likely due to the silicone oil being partly removed from the HCs and being dissolved in the water given its relatively low viscosity, making the surface rougher and leading to an increase in CAH. Nevertheless, after 7 days immersion, the CAH for SHC-20 and SHC-100 were both less than 5°, indicating a significant stability of the prepared slippery surface in a flowing water environment.

3.3 Anti-biofouling properties of SHCs To evaluate the biofouling resistance properties of slippery surfaces, the adhesion or adsorption interactions of blood, bacteria, and protein solutions onto the SHCs were also evaluated. The ability to prevent the adhesion of blood and bacteria on a surface is of particular relevance in medical applications such as dialysis, transfusion, analyte detection, and pathogen removal. As shown in Videos 5 and 6, blood (fresh whole human blood in sodium heparin, kindly supplied by Zhejiang University Hospital) was placed on HC and SHC-20, yet only SHC-20 showed a blood-repelling property, while the blood was pinned on HC. The anti-bacterial properties of SHCs were investigated by using a biological model system, consisting of four samples, namely hydrophilic glass, hydrophobic HC, and SHC-20 and SHC-100, to investigate the inhibition behavior of slippery surfaces against E. coli colonization. E. coli easily settled on glass and HC after 3 h of immersion in the static environment (Figure 9A). Both of these control samples failed to resist biofilm attachment, resulting in a statistical coverage of 23.01 ± 6.80% and 31.19 ± 5.38% over glass and HC, respectively. In contrast, the coverage on SHC-20 and SHC-100 was 0.31 ± 0.15% and 0.06 ± 0.05%, respectively. When increasing the immersion time to 24 h, E. coli attachment increased, resulting in biofilm coverage of between 40.80 ± 7.93% and 73.65 ± 9.35% (Figure 9B) and CFU counts of 27.1 (± 7.55) × 105 and 62.07 (± 34.27) × 105 cells/cm2 (Figure 9C) for glass and HC, respectively. The slippery surfaces, in contrast, showed no visible adherent biofilms upon being removed from the culture medium, resulting in surface coverage values of 0.86 (± 0.18) × 105 and 1.35 (± 0.38) × 105 cells/cm2 for SHC-20 and SHC-100, respectively. Baier50 has reported that the minimum biological adhesion to substrates lies on the critical surface energy zone, between 20 and 30 mN/m; as shown in Table S1, HC, SHC-20, and SHC-100 showed a low surface energy within this range, but the biofilm coverage was completely different. Considering these facts, the liquid-like property of SHCs is an essential contributor to the prominent suppression of bacterial colonization. These results demonstrate that, compared to low-adhesion surfaces, such as hydrophilic glass, or low surface energy surfaces such as HC, the slippery coatings are more effective as fouling-release surfaces against E. coli adhesion. 13 ACS Paragon Plus Environment

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Figure 9. Image analysis of E.coli settlement coverages on glass, HC, SHC-20 and SHC-100 after exposure to the solution of E.coli for (A) 3 h and (B) 24 h. The inset images are the representative samples after adherent biofilm with crystal violet. (C) CFU counts of samples in (B). (D) Images of bactericidal tests against E. coli. (diluted to 10-4) It is generally recognized that the biofouling of marine organisms is initiated by the adhesion of proteins (1 min) followed by that of bacteria and diatoms (1–24 h) when a clean surface is immersed in natural sea water.5 Herein, we used BSA as a model contaminant for the protein resistance test. In contrast to glass and HC, the SHC-20 and SHC-100 samples had an obviously decreased BSA adsorption of 1.79 ± 0.29 and 2.09 ± 0.81 µg/cm2, respectively (Figure 10). Then the durability of the slippery surfaces for the protein resistance was investigated. The SHC-20 and SHC-100 samples were treated by spinning at 3000 rpm for 1 min, evaporation at 70 °C for 7 days, and immersion in dynamic water for 7 days. After accelerated lubricant loss treatments, the protein absorption on the SHCs remained low (