Langmuir 2008, 24, 10625-10628
10625
Bioinspired Synthesis of Superhydrophobic Coatings Mikael Ja¨rn,† Mikko Heikkila¨,‡ and Mika Linde´n*,† Center for Functional Materials, Department of Physical Chemistry, Åbo Akademi UniVersity, Porthansgatan 3-5, FI-20500, Turku, Finland, and Laboratory of Inorganic Chemistry, Department of Chemistry, UniVersity of Helsinki, P.O. Box 55, FI-00014 UniVersity of Helsinki, Finland ReceiVed July 8, 2008. ReVised Manuscript ReceiVed August 20, 2008 A superhydrophobic material prepared by precipitating calcium phosphate on TiO2 films under in vitro conditions is described. Crystalline calcium phosphate is very porous with octacalcium phosphate as the main phase. The films are made hydrophobic by the surface grafting of a perfluorophosphate surfactant (Zonyl FSE). The as-prepared coatings were strongly hydrophobic, with advancing contact angles exceeding 165° and receding angles exceeding 150°. The formation of the calcium phosphate layer is self-organizing, and the coating is easily functionalized. The material was characterized with dynamic contact angle measurements, SEM, XRD, and XPS. The strong water repellency is explained by the open porous morphology of the calcium phosphate coating together with the successful attachment of the hydrophobic function.
1. Introduction The concept of superhydrophobicity has been known since the 1940s, after the novel work of Cassie and Baxter.1 Interest in this field has expanded greatly during the past decade. It is well known that superhydrophobicity (strong water repellency) is achieved through a combination of low surface polarity and roughness. Ever since the reasons for the superhydrophobic, self-cleaning properties of the Lotus leaf were reported in the late 1990s,2 there have been many attempts to produce materials with similar water-repelling properties synthetically, employing methods such as the solidification of alkylketene dimers,3 phase separation,4,5 plasma polymerization,6 photolithography,7 electrochemical deposition,8 and chemical vapor deposition.9 In addition to a high static contact angle, small contact angle hysteresis is essential for a surface to be truly superhydrophobic because a small hysteresis leads to a water droplet being able to roll off of a surface easily. Hysteresis is measured as the difference between the advancing and receding contact angles. Recently, Gao and McCarthy reported a strongly hydrophobic surface with virtually no contact angle hysteresis, which was prepared by treating silicon wafers with methyltrichlorosilane.10 The wetting of rough, hydrophobic materials is usually described by the Cassie-Baxter equation, where the apparent contact angle is given by
cos θA ) f1cos θ1 - f2
(1)
where f1 is the fraction of solid material, f2 is the fraction of air (f1 + f2 ) 1), and θ1 is the contact angle of the pure solid material.1 Hence, in this model air is trapped in the voids of the rough * Corresponding author. Tel. +358 2 215 4297. Fax. +358 2 215 4706. E-mail:
[email protected]. † Åbo Akademi University. ‡ University of Helsinki.
(1) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1948, 3, 11. (2) Barthlott, W.; Neinhaus, C. Planta 1997, 202, 1. (3) Onda, T.; Shibuichi, S.; Satoh, N.; Tsuji, K. Langmuir 1996, 12, 2125. (4) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (5) Han, J. T.; Xu, X.; Cho, K. Langmuir 2005, 21, 6662. ¨ ner, D.; Youngblood, J.; McCarthy, (6) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O T. J. Langmuir 1999, 15, 3395. ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (7) O
surface, forming a composite surface of solid-vapor below the droplet. In the following text, we describe a simple bioinspired approach to synthesizing superhydrophobic surfaces on the basis of selforganized calcium phosphate grown in vitro on a sol-gel-derived TiO2 film. We also show that the method could be easily extended to the preparation of hydrophobic CaCO3 surfaces. Sol-gelderived TiO2 coatings and oxidized Ti metal surfaces are well known to nucleate calcium phosphate (CaP) on their surface under in vitro conditions,11,12 and this is the basis for the bioactivity of TiO2 films in vitro and in vivo. It is well known that a number of calcium phosphate phases, such as amorphous calcium phosphate (ACP), dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP), and hydroxyapatite (HA), may be involved in these reactions. The presence and the relative amounts of these phases are constrained by environmental parameters such as the composition of the initial solution, pH, temperature, and so on. The thermodynamically most stable phase, HA, is typically formed through repeated dissolution-reprecipitation reactions if aged in the same solution (refs 13 and 14 and references therein). To create a hydrophobic surface, the initially hydrophilic calcium phosphate has to be chemically modified with a hydrophobic molecule. Organophosphates have previously been shown to bind successfully to metal- and transition-metalcontaining supports (oxide, hydroxides, phosphates, and carbonates).15-19 In this work, we used a perfluorophosphate surfactant (8) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2005, 21, 937. (9) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (10) Gao, L.; McCarthy, T. J. J. Am. Chem. Soc. 2006, 128, 9052. (11) Li, P. Ph.D. Thesis. University of Leiden, Leiden, The Netherlands, 1993. (12) Kim, H.-M.; Miyaji, F.; Kokubo, T.; Nishiguchi, S.; Nakamura, T. J. Biomed. Mater. Res. 1999, 45, 100. (13) Areva, S.; Peltola, T.; Sa¨ilynoja, E.; Laajalehto, K.; Linde´n, M.; Rosenholm, J. B. Chem. Mater. 2002, 14, 1614. (14) Andersson, J.; Johannessen, E.; Areva, S.; Baccile, N.; Azaı¨s, T.; Linde´n, M. J. Mater. Chem. 2007, 17, 463. (15) Fisher, A.; Kuemmel, M.; Ja¨rn, M.; Linde´n, M.; Boissie`re, C.; Nicole, L.; Sanchez, C.; Grosso, D. Small 2006, 2, 569. (16) Ja¨rn, M.; Brieler, F. J.; Kuemmel, M.; Grosso, D.; Linde´n, M. Chem. Mater. 2008, 20, 1476. (17) D’Andrea, S. C.; Fadeev, A. Y. Langmuir 2003, 19, 7904. (18) Tanaka, H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Langmuir 1997, 13, 821.
10.1021/la802160a CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
10626 Langmuir, Vol. 24, No. 19, 2008
Letters
Figure 1. SEM image of a titania coating immersed in a supersaturated calcium phosphate solution for 5 days.
(Zonyl FSE) to functionalize the calcium phosphate surface. Zonyl was recently used to functionalize TiO2 selectively on nanopatterned surfaces.15,16
2. Experimental Section Sol-gel-derived TiO2 coatings were used as substrate materials. The coatings were prepared as described previously.20 Supersaturated calcium phosphate solution was prepared by dissolving reagent-grade NaCl, CaCl2 · 2H2O (Fluka), and Na2HPO4 (Fluka) into Milli Q purified water. The molar concentration of calcium was 3.75 mM; phosphate, 1.5 mM; and sodium chloride, 141 mM (i.e., 1.5 times the concentration of calcium and phosphate in simulated body fluid (SBF)). The pH was buffered at 7.4 with tris(hydroxymethyl)aminomethane and 1 M HCl. The sol-gel-prepared titania coatings were irradiated with a lowpressure UV lamp for about 1 h before immersion in the supersaturated calcium phosphate solution. The solutions were kept in tightly closed polyethylene bottles at 40 °C for 5 days, after which an even layer of calcium phosphate had precipitated on the film. At the end of the experiment, the coatings were washed with Milli Q water and dried at 40 °C. Functionalization of the CaP layer was carried out by immersing the coating in a 0.1 wt % solution of a commercial biodegradable perfluorophosphate surfactant Zonyl FSE (SigmaAldrich/Du Pont) in absolute ethanol for 24 h at room temperature. According to the manufacturer, the chemical structure of zonyl is (F(CF2)n(CH2)2)2PO4NH4, where n ) 3-8. The concentration of the perfluorophosphate surfactant was kept low in order to get surfactant attachment only on the CaP surface without the formation of intercalated, lamellar structures.17,18 A CAM 200 contact angle goniometer (KSV Instruments Ltd., Helsinki, Finland) was used for the determination of water contact angles. The advancing and receding contact angles were determined by adding and removing liquid from the droplet. The contact angles were calculated using the software supplied with the instrument. Images of the coatings were taken using a scanning electron microscope, SEM (Jeol JSM-6335F, Jeol Ltd., Japan), equipped with a Link Inca 300 (Oxford Instruments) EDS unit, which was used for the elemental analysis. The XPS measurements were performed with a Physical Electronics Quantum 2000 instrument equipped with a monochromatic Al KR X-ray source. An operating power of 25 W was used with a spot diameter of 100 µm. An electron flood gun and a low-energy ion gun were used for charge compensation. The detector position was at an angle of 45° in relation to the sample surface. A PANAlytical X’Pert Pro MPD equipped with a PIXcel detector was used in θ-2θ geometry to record X-ray diffractograms.
3. Results and Discussion An SEM image of a CaP coating is shown in Figure 1. Here, TiO2 had been immersed in a supersaturated calcium phosphate (19) Nakatsuka, T.; Kawasaki, H.; Itadani, K.; Yamashita, S. J. Colloid Interface Sci. 1981, 82, 298.
Figure 2. XRD pattern of a titania coating immersed in a supersaturated calcium phosphate solution for 5 days. (OCP stands for octacalcium phosphate, and HA stands for hydroxyapatite.)
solution for 5 days, where the calcium and phosphate concentrations were 1.5 times higher than those used in biological in vitro experiments in order to enhance the kinetics of CaP formation. The CaP film is very porous and consists of platelets aligned at a fairly steep angle relative to the surface. The thickness of the platelets is roughly 200 nm. The CaP layer thickness in this case is about 20 µm (Supporting Information), but the CaP film thickness is naturally determined by the time allowed for the nucleation and growth of the CaP layer. The observed platelet morphology is comparable to that previously observed for calcium phosphate growth on titania surfaces from solutions with similar compositions and pH. (See, for example, refs 21-23.) If hydrophobized, such a platelet structure should give rise to small solid-liquid contact between a water droplet and the material (i.e., a small f1 value in the Cassie-Baxter equation) and thus should represent a self-organizing superhydrophobic surface. Furthermore, the superhydrophobicity should also be enhanced because a continuous contact line cannot form on this platelike structure, which should lead to small contact angle hysteresis.6 Figure 2 shows thin-film XRD patterns of the precipitated CaP coating. The platelet morphology has previously been observed for octacalcium phosphate (OCP)21 or a mixture of OCP and hydroxyapatite (HA).22,23 The reflection at around 16° (2θ) is characteristic of OCP, whereas the reflections at 26 and 32° could originate from both OCP and HA. The high intensity of some reflections is due to the platelike morphology of the precipitated CaP. OCP will typically convert to the more stable HA upon further aging in solution. Because of the similarities between the structures of OCP and HA epitaxial overgrowths of these phases will easily occur,24,25 we cannot exclude the presence of HA, although the main phase appears to be OCP. The successful attachment of zonyl ((F(CF2)n-CH2CH2O)2POO-, n ) 3-8) to the calcium phosphate coating was confirmed by X-ray photoelectron spectroscopy (XPS). The corresponding surface concentrations were 20.06% C, 33.92% O, 26.99% F, 9.26% P, and 9.77% Ca, where the values are given in atomic (20) Ja¨rn, M.; Areva, S.; Pore, V.; Peltonen, J.; Linde´n, M. Langmuir 2006, 22, 8209. (21) Wu, W.; Nanchollas, G. H. Langmuir 1997, 13, 861. (22) Shi, J.; Ding, C.; Wu, Y. Surf. Coat. Technol. 2001, 137, 97. (23) Wen, H. B.; de Wijn, J. R.; van Blitterswijk, C. A.; de Groot, K. J. Biomed. Mater. Res. 1999, 46, 245. (24) Nanchollas, G. H.; Tomazic, B. J. Phys. Chem. 1974, 78, 2218. (25) Iijima, M.; Tohda, H.; Moriwaki, Y. J. Cryst. Growth 1992, 116, 319.
Letters
Langmuir, Vol. 24, No. 19, 2008 10627
Figure 3. XPS C 1s spectra of zonyl-functionalized calcium phosphate.
Figure 4. Advancing (a) and receding (b) water droplets on zonyl-functionalized calcium phosphate.
percent. The fitted high-resolution spectra of the carbon C 1s signal confirmed the presence of CF2, CF3, C-O, and C-C (Figure 3). The observed CdO signal originates from surface contaminants. The area ratio of the CF2 peak to the CF3 peak of about 5.5 indicates that the mean number of carbon atoms attached to fluoro groups in one chain is n ) 6.5, which is in the higher end of the range of 3-8 given by the manufacturer. This results in a mean fluorine/phosphorus ratio of 28:1 for one zonyl molecule, which allows us to calculate the portion of phosphorus originating from the zonyl function. By subtracting this contribution from the total amount of phosphorus, a Ca/P ratio of about 1.2 was obtained. The value is close to the Ca/P ratio of 1.3 obtained from SEM-EDS measurements. This ratio is lower than the theoretical value for hydroxyapatite (1.67) but is close to the theoretical value of octacalcium phosphate (1.33), again suggesting that OCP is the main calcium phosphate phase. The morphology of the CaP as judged by SEM and XRD remained unchanged upon surface functionalization (results not shown). The superhydrophobic properties of the coatings are demonstrated by the advancing (>165°) and receding (>150°) water contact angles (Figure 4). We do not report an exact value for the contact angle because it gets extremely difficult to measure such high contact angles accurately using the sessile drop technique. However, the contact angle values were significantly higher than those previously reported for fluorocarbon-functionalized hydroxyapatite films for which advancing water contact angles of 115° or lower have been reported.17 We attribute this
difference mainly to the difference in structural properties between our precipitated calcium phosphate film and that of films prepared by solution deposition of hydroxyapatite particles. The low contact angle hysteresis demonstrates the superhydrophobic properties of the coatings, which are also supported by the observation of water droplets that were free falling onto the surface and bounced and finally rolled off of the surface (Supporting Information). Nonfunctionalized CaP surfaces were superhydrophilic, as expected for rough hydrophilic surfaces, and a deposited water droplet fully wetted the coating. The surface functionalization step can also be performed with water, but we noticed that some reorganization of the CaP film occurred during functionalization, most probably because of the dissolution-reprecipitation reactions of the calcium phosphate coating. This leads to larger contact angle hysteresis than in the case of the films functionalized from ethanol, but also in this case the advancing and static contact angles exceeded 160°. Because organophosphonates also show an affinity for carbonates,19 the zonyl function can be used to hydrophobically functionalize calcium carbonate, which is a typical scale in many industrial processes. Because zonyl also is water-soluble, it can also be used to surface modify calcium-derived scales on nonTiO2 surfaces in an early stage of scale formation, thus preventing further scale formation. To test this, we pressed a tablet of CaCO3 powder and immersed it in an ethanolic zonyl solution for 24 h, after which the calcium carbonate showed a static water contact
10628 Langmuir, Vol. 24, No. 19, 2008
Letters
angle of about 120°. Nonfunctionalized CaCO3 was strongly hydrophilic.
methodology can also be extended to CaCO3 surfaces, widening the applicability of the results to industrially relevant surfaces.
Summary
Acknowledgment. The Academy of Finland is acknowledged for financial support. M.J. also acknowledges support from the Graduate School of Materials Research, GSMR.
We have demonstrated a very simple, bioinspired synthesis route for the preparation of a superhydrophobic material from heterogeneously precipitated calcium phosphate. The formation of the CaP layer is self-organizing, and the coating can be easily functionalized with a perfluorophosphate surfactant (zonyl), which is suggested to covalently attach to surface P-OH groups of CaP through the phosphonate end-group. The hydrophobic function, in combination with the morphology of the CaP layer, creates a strongly water-repellent material. The described
Supporting Information Available: A higher-magnification SEM micrograph and a cross-sectional SEM micrograph of the calcium phosphate coating. A video of a water droplet that is free falling onto the grafted calcium phosphate surface demonstrates the superhydrophobic properties of the coating. This material is available free of charge via the Internet at http://pubs.acs.org. LA802160A
