A Novel Method to Prepare Superhydrophobic, Self-Cleaning and Transparent Coatings for Biomedical Applications

Yonghao Xiu1,2, Dennis W. Hess1, C. P. Wong2

1Schools of Chemical and Biomolecular Engineering and 2Materials Science and Engineering Georgia Institute of Technology, Atlanta, GA 30332

dennis.hess@chbe.gatech.edu, Telephone: +1 404-894-5922, Fax: +1 404-894-2866 cp.wong@mse.gatech.edu, Telephone: +1 404-894-8391, Fax: +1 404-894-9140

Abstract In this paper, a novel method of preparing nanostructured

ultra-thin films for transparent superhydrophobic coatings is reported for the first time [1]. A sol-gel process that uses a eutectic liquid solvent was employed. This solvent serves as both a low vapor pressure, low melting point liquid and a templating agent in the sol-gel process. UV-Visible spectra indicate that after coating a glass microscope slide, with the transparent superhydrophobic coating, the light transmittance improved relative to that of a bare glass slide. The superhydrophobic surface is biocompatible because of the extremely low surface energy that results from the combination of a fluoroalkyl chain attached to the surface and the surface nanostructure. Preliminary studies suggest that the superhydrophobic coating may effectively prevent surface contamination by bacteria.

1. Introduction Natural superhydrophobicity has recently garnered much

attention and has inspired attempts to create biomimetic surfaces. In many plants, notably the lotus flowers, leaves utilize superhydrophobicity as the basis of a self-cleaning mechanism: nearly spherical water drops can easily roll off of the leaves, carrying undesirable particulates and thereby cleaning the surface. This observation is usually referred to as the lotus effect. Detailed inspection of the leaf surface shows that the surface consists of surface microstructures with wax nano particles on it. Possible technological applications range from nanochips, water repellent coatings, to self-cleaning windows. To convert a hydrophobic surface into a superhydrophobic one, the surface must be roughened. Two criteria are needed to achieve a superhydrophobic surface from a hydrophobic one: the hydrophobic surface must have a high water contact angle (>90˚ on flat surface) and the surface must be micro/nano structured.

The effect of surface roughness on contact angle was modeled by several authors about 50 years ago. Wenzel’s equation[2] was used to account for roughness through r, the ratio of the actual to projected area, as shown in equation 1:

truerough r θθ coscos = (1)

This expression indicates that contact angles less than 90˚ are decreased by roughness, while for θ greater than 90˚, the contact angle increases. If the contact angle is large and the surface sufficiently rough, the liquid may trap air to give a composite surface effect and an apparent contact angle described by equation 2 (Cassie’s equation)[3] :

1coscos 111 −+= frfapparent θθ (2)

where f1and f2 are the surface fraction of each component. These criteria allow a superhydrophobic surface to be achieved.

In addition to excellent water repellency, it would be desirable to develop a robust superhydrophobic coating for self-cleaning, transparent and antifouling applications in biomedical devices, especially for optical detectors. Such a coating will enhance device reliability under conditions where it is exposed to corrosive body fluids[4]. In situations where light detection is required, transparency is also a prerequisite for the successful application of superhydrophobic coatings.

Superhydrophobicity requires both low surface energy and appropriate surface morphology or roughness. Even a material with a low surface energy (6.7 mJ/m2 for a surface with regularly aligned closest-hexagonal-packed -CF3 groups) displays a water contact angle (CA) of only 120° if the surface is smooth. Thus, a major difficulty in developing transparent superhydrophobic surfaces arises from the light scattering that is naturally induced by surface roughness. There are several reports on rough coated or etched surfaces revealing superhydrophobicity with contact angles of 160°. In all cases, however, the scatter losses remained higher than several percent, which is not tolerable for a large variety of applications, such as optical sensors or solar cell panels where signaling or energy transfer is dependent on light. To date, there have been no reports of superhydrophobic surfaces with optical scattering losses below a few percent. Clearly, successful preparation of such layers requires control over nanoscale surface structures to inhibit light scattering.

A eutectic liquid is a combination of salts that decreases the melting temperature for the salt mixture; thus eutectic liquids have similar properties to ionic liquids[5],. Halide salts can form complexes with hydrogen donors and in at least one extreme case, choline chloride and urea (molar ratio of 1:2), form a deep(low melting point) eutectic liquid, with a melting point of 12˚C[6]. For choline chloride, only those compounds that form hydrogen bonds with chloride ions demonstrate the formation of a homogeneous liquid with a significant decrease in the melting point. Compounds with a greater ability to form hydrogen bonds display an enhanced depression in melting point when they are mixed with choline chloride. Compared to ionic liquids, eutectic liquids are sustainable, environmentally friendly and permit liquid mixture properties to be tuned by choosing different readily available materials.

Previous studies have described the use of aerogel films to achieve superhydrophobicity by supercritical solvent extraction[7-9]. However, this approach required high pressure/high temperature equipment and processing

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conditions. In order to circumvent this limitation, ionic liquids have been used as a solvent to synthesize silica aerogels with reduced shrinkage[10], although these liquids were reported to be ecotoxic[11]. In the current paper, we propose the use of eutectic liquids to form silica aerogel films with optical transparency and superhydrophobic characteristics. Indeed, by careful control of film thickness, surface roughness, and a fluoroalkylsilane surface treatment, superhydrophobicity and transparency can be achieved with the use of a eutectic liquid composed of urea and choline chloride (2:1 molar ratio) and a melting point of 12 ˚C.

2. Experimental

Silica film formation The general formulation for solution and film formation from the eutectic liquid is: tetraethoxysilane (TEOS): 0.6 g, choline chloride-urea (C-U): 1.2-2.4 g, ethanol: 1.5-3 g, 1M HCl aqueous solution: 0.3g. Hydrolysis and condensation occurred after addition of HCl to the mixture, and stirring for 3 hrs. The solution was then spincoated onto one square inch glass microscope slides at 3000-6000 rpm to form uniform films of thicknesses between 100 and 500 nm. The coated glass slide was placed in a desiccator with a container of 1 ml ammonia (29%), to promote gelation. After 2 weeks, the glass slide was removed from the desiccator and extracted with acetonitrile for 3 hrs to remove the eutectic liquid in the film and thus yield a porous thin film.

Surface fluoroalkylsilane treatment Substrates with spin-cast silica films were placed in a fluoroalkylsilane (trichloro(1H,1H,2H,2H-perfluorooctyl) silane, PFOS)/n-hexane solution (10mM) for 30 min to allow adsorption of a PFOS layer onto the SiO2 surface; subsequently the samples were heated to 150˚C in air for 1hr and at 220˚C for 5 min to promote silane hydrolysis and condensation, thereby forming a stable fluorosilanated layer on the silica surfaces.

Cell growth conditions and test of cell adhesion on superhydrophobic surfaces

DH5 αpro E. coli from American Type Culture Collection (ATCC) as a parental cell was maintained in Luria-Bertani (LB) medium (5ml, 10g/L tryptone, 10g/L yeast extract, 5g/L NaCl), in a sterilized test tube, with superhydrophobic silica coated glass slide samples (0.5cm by 1cm each) sitting at the bottom of the test tube. The culture together with the samples were incubated at 37˚C for 15 hours using a shaking rate of 200 rpm. Then the samples were taken out of the test tube, blow dried for SEM observation of cell adhesion.

Surface characterization Contact angle measurements, Scanning Electron

Microscopy (SEM), and UV-Visible spectroscopy (UV-Vis) were used to characterize the superhydrophobic silica films. For contact angle measurements, water droplets (4µl size) were introduced using a microsyringe and static images were recorded from which the contact angle of the droplet on the solid surface was determined. For SEM inspection, the samples were attached to aluminum stubs with conductive tape before being coated with ~20nm of gold in an Ernest

Fullam sputter coater. The samples were then imaged using high resolution field-emission scanning electron microscopy (FESEM; LEO 1530 FEG).

3. Results and Discussion It is well-known that sol-gel processes permit the

preparation of dense, nonporous, microporous or mesoporous films[12]. The pore sizes generated generally range from several nanometers to tens of nanometers. Water contact angles of films prepared through traditional two-step acid base catalyzed sol-gel spincoating processes followed by a 550 ˚C calcination were 10.3 ± 0.8˚ and 116 ± 1.1˚, before and after fluoroalkylsilane coating, respectively. This process used a sol-gel composed of tetraethoxysilane (TEOS), water, ethanol and hydrochloric acid as the catalyst. After hydrolysis and condensation polymerization of TEOS, a sol resulted, from which a thin film was formed on a glass slide by spincoating. During this process, ethanol evaporated yielding a dense film. In addition, on this silica film, no surface structure was detected by scanning electron microscopy (SEM) (Figure 1. a). Such results indicate that films prepared from traditional sol-gel processes display smooth surfaces, at least insofar as SEM and contact angle measurements(~115˚) are concerned. However, films prepared from a silica sol containing a eutectic liquid composed of tetraethoxysilane and choline chloride/urea (TEOS:C-U = 1:4) have a contact angle of 171.8 ± 1.2˚, and hysteresis of 4.1 ± 0.4˚, after fluoroalkylsilane coating. The average surface roughness (Ra) of the films after fluoroalkylsilane coating was 84.2 nm, while the corresponding roughness for films prepared from a sol containing only traditional chemical mixtures was ~20 nm. The shapes of water droplets on the different surfaces after fluoroalkylsilane coating are shown in Figures 2a and b. Clearly, the contact angle was higher when the eutectic liquid was used as a solvent. Figure 1 shows SEM micrographs of various sol-gel films before fluoroalkylsilane coating. The films prepared from a sol that used a eutectic liquid as a solvent/templating agent had rough microstructures (Fig. 1. b-d), whereas the surface of the film prepared from the sol without a eutectic liquid was much smoother (Figure 1. a).

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Figure 1. SEM images of silica films from different TEOS:C-U ratios with ethanol:C-U=1:1; a). No eutectic liquid, b). TEOS: C-U = 1:1, c). TEOS: C-U = 1:2, d). TEOS: C-U = 1:4 (weight ratio).

Figure 2. Contact angle images of water droplets on silica films synthesized from different TEOS:C-U ratios with ethanol:C-U=1:1(weight ratio); a). No eutectic liquid, CA: 115.3˚, b). TEOS: C-U = 1:1, CA: 140.3˚, c). TEOS: C-U = 1:2, CA: 155.4˚, hysteresis: 18.6˚, d). TEOS: C-U = 1:4, CA: 171.2˚ , hysteresis: 4.1˚.

The above results indicate that a rough surface that yields a superhydrophobic silica film can be formed by eutectic liquid extraction of a spin-cast and aged film. At the start of the sol-gel process, TEOS is not soluble in the ethanol-eutectic liquid solution, thereby yielding an opaque solution. After the addition of acid catalyst (aqueous HCl, 1 M), the solution changes from opaque to transparent within 2 minutes, demonstrating the rapid hydrolysis of TEOS and the fact that the hydrolysis product was soluble in the ethanol-eutectic liquid solution. During the spincoating process, ethanol evaporated but the eutectic liquid remained in solution, thereby preventing collapse of the porous structure in the film due to capillary forces. After gelation and aging in ammonium hydroxide for 2 weeks, the 3-D network stabilized; the eutectic liquid was then extracted by refluxing the film in acetonitrile for 3 hrs. The SEM micrograph in Figure 1 combined with the AFM surface topography of Figure 4 indicates that the film surface roughness remained after the extraction process. The film morphology can be described as an aerogel, because an aerogel has a similar volume to that of the original alcogel (little shrinkage) whereas a xerogel displays a collapsed structure relative to that of the original alcogel (e.g., a significantly reduced porosity due to capillary forces).

Powder X-ray diffraction (XRD) pattern of the as prepared silica films is shown in Figure 3. The pattern for the as prepared film shows two peaks at 1.18˚ and 1.97˚ (2θ), which corresponds to pore sizes of 7.5 nm and 4.5 nm. It is expected that the mesopores, which result in the reduced density of silica film, may help the improvement of the surface superhydrophobicity.

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Figure 3. XRD of a C-U templated silica film on a silicon surface with a ratio of TEOS:C-U=1:4.

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Figure 4 shows AFM images of the film surfaces; surface roughness (Ra) is 56.3 nm for film a (weight ratio of TEOS:C-U of 1:1). Film b has a higher Ra of 71.1 nm and displays a high contact angle but also high hysteresis due to the open surface structures.

Figure 4. AFM deflection mode images of surfaces formed using eutectic liquids; a). TEOS: C-U = 1:1, b). TEOS: C-U = 1:2, c). TEOS: C-U = 1:4 (weight ratio).

Film c shows a very high contact angle and low hysteresis: Ra is 84.2 nm, with narrow surface features and dense structures relative to films a and b. This dense structure results in the high contact angle and low hysteresis compared to films a and b. Although Ra for c is higher than for films a and b, it is still sufficiently low to allow light transmission without significant scattering. Therefore, this film displays not only lower hysteresis but also less visible light scattering

than films a and b which have larger features. As a result, the optical transmittance of superhydrophobic film c is only 10% less than that of the original glass slide at ~400 nm as shown in Figure 5. The film thickness from SEM is ~500 nm (Figure 6). According to light scattering theory, in order to achieve effective light transmission through an otherwise transparent material without scattering, the roughness should be less than ¼ of the wavelength of light[13]. However, when the surface roughness is too small (below 30 nm), it cannot be effective in enhancing the surface superhydrophobicity[14]. For instance, for silica films composed of an ordered array of 10-20 nm silica particles as shown in Figure 7, the contact angle is 120.3 ± 1.5˚ after a fluoroalkylsilane treatment.

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Figure 5. Transmittance of glass substrate and silica film coated glass substrate from eutectic liquid (C-U) templating with TEOS:C-U=1:4. Film thicknesses are ~500 nm from SEM cross-section examination (Fig. 6).

Figure 6. Cross-sectional SEM for TEOS:C-U=1:4.

Figure 7. SEM of a silica film composed of an ordered array of silica particles of diameter 10-20 nm[14]

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When the surface roughness is low, the Cassie equation[3] does not accurately describe water contact angle behavior due to the molecular interactions present; that is, the surface is considered ‘smooth’ instead of rough. For simple molecules, London dispersion forces nearly always represent the greatest contribution to the intermolecular attractive force. In the case of superhydrophobic film-water droplet interactions, it is primarily the London forces and Debye forces that produce the long-range attraction to approaching atoms due to the nonpolar nature of the fluoroalkyl silane layer. Because of the retardation effect that is intrinsic to dispersion interactions, the minimum roughness should be >30 nm (this is the retardation distance for two atoms in free space)[15] in order to inhibit van der Waals interactions and display adherence to the Cassie equation. Finally, in order to attain an optically transparent film, the maximum roughness should be less than ~100 nm (1/4 of the lower limit of the visible light wavelength range of 400 - 760 nm).

In order to prepare transparent thin films, we used ethanol as a diluent during the spincoating process; excellent transparent superhydrophobic layers resulted after film preparation followed by treatment with fluoroalkyl silane. Figure 8 shows transmittance data in the visible region for films with fluoroalkylsilane coatings; this result can be compared with that of Fig. 5 without fluoroalkyl coating. In Fig. 8, transmittance of the microscope slide coated with fluoroalkyl silane is also included; this film shows a water contact angle of 115.6˚. Clearly, the superhydrophobic films demonstrate good transmittance in the visible wavelength range. As the ethanol/C-U ratio increases, the films become thinner and the transmittance improves. When the ratio is 9, the transmittance is better than the glass slide without an applied coating because the film serves as a light antireflection coating. Shorter wavelengths show lower transmittance due to enhanced light scattering (wavelength dependence) caused by the presence of pores and silica surface structures on/within the film. Figure 9 is a demonstration of the transparency of the superhydrophobic films. However, when films become very thin (e.g. <30 nm), the surface roughness may be insufficient to establish superhydrophobicity. Figure 10 shows the dependence of contact angle and hysteresis on the ethanol/C-U ratio. When the ethanol/C-U ratio was increased, that is, the film was thinner, the contact angle remained relatively constant although the hysteresis increased. Again, this result demonstrates the trade off between transparency and superhydrophobicity for superhydrophobic coatings.

Preliminary studies have been performed to investigate the possible application of superhydrophobic, self-cleaning and optically transparent surfaces to bacteria adhesion. In order to prevent adhesion on the superhydrophobic surface, the scale of the surface roughness must be smaller than the bacteria scale (generally in the range of several microns). For the silica film, the surface roughness measurement from AFM was below 100 nm, which is much smaller than the bacteria size; therefore this superhydrophobic film is expected to prevent adhesion. Indeed, adhesion between the superhydrophobic film surface was demonstrated using single-celled bacterial, E. coli according to the procedure described previously. From

the SEM images in Figure 11, E. coli (~5µm in size) adheres to the control sample surface of a glass surface coated with fluoroalkyl silane. However, on the superhydrophobic silica surface, no bacteria ahere due to the surface roughness and low surface energy. Furthermore, when the sample was removed from the medium solution, no liquid adhered to the surface, thereby demonstrating self-cleaning. This phenomenon indicates that superhydrophobic silica surfaces may effectively prevent bacteria/cell adhesion.

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Figure 8. Improvement of transmittance of silica films from eutectic liquid (C-U) templating with TEOS:C-U:ethanol=1:4:36.

Figure 9. Demonstration of a transparent superhydrophobic silica film.

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Figure 10. Dependence of contact angle and hysteresis on the ethanol/C-U ratio (film thickness for the different ratios ranges from 100 nm to 500 nm according to SEM cross-section examination).

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Figure 11. Demonstration of the self-cleaning properties of a superhydrophobic silica film for E. coli: a. control sample of silica glass coated with fluoroalkyl silane, b. superhydrophobic and transparent silica surface.

4. Conclusions Superhydrophobic, highly optically transparent films were

formed on glass slides by controlling the surface structure length scales to establish superhydrophobicity, but to ensure minimal light scattering. Results indicate that a roughness value can be obtained that inhibits van der Waals interaction between film surfaces and water droplets. Thus, by controlling the surface roughness to a minimum value, both superhydrophobicity (high contact angle and low hysteresis) and optical transparency can be achieved simultaneously. Preliminary bacterial adhesion tests show that the superhydrophobic surface is self-cleaning compared to a smooth surface with the same surface chemistry. This coating may have numerous applications in optical devices, especially for use in biomedical electronics and sensors.

Acknowledgments We would like to acknowledge financial support from the

National Science Foundation and the National Electric Energy Testing Research and Applications Center (NEETRAC) at Georgia Institute of Technology.

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