pubs.acs.org/Langmuir © 2010 American Chemical Society
Substrate Effects on the Wettability of Electrospun Titania-Poly(vinylpyrrolidone) Fiber Mats Jamie M. F. Jabal, Laurel McGarry, Abigail Sobczyk, and D. Eric Aston* Department of Chemical & Materials Engineering, P.O. Box 441021, University of Idaho, Moscow, Idaho 83844-1021 Received April 30, 2010. Revised Manuscript Received July 1, 2010 Titania-poly(vinylpyrrolidone) (PVP) core-shell nano/microfibers are electrospun on substrates of differing hydrophilicity and conductivity in order to investigate the connection between these substrate properties and the apparent water contact angles against the fiber mats. The focus of this study compares current data from silicon- and aluminum foil-supported mats to extant data from ITO and glass-supported fibers to detail the complexities of apparent contact angle dependence on mat structure related to substrate properties. Electrospinning time and collection distance were controlled parameters for producing thicker and denser mats. In all cases, contact angles increased with collection time for a given substrate series. A morphological wettability study of the fiber mat surface was conducted by applying Rhodamine B dye solution droplets. Using fluorescence microscopy, the stained fibers indicate the extent of true wetting contact and the lack of penetration into the fiber layers. Image comparisons with bright-field illumination confirms that even some fibers of the top layers are not wetted.
Introduction Electrospinning—a century-old, versatile method—has been used and modified by various researchers to produce fine to ultrafine fibers of various polymers and oxides.1,2 Although it is possible to electrospin onto insulating materials, conductive substrates are most effective for electrospinning, such as gold electrodes,3,4 stainless steel,5 alumina plates,6 aluminum foil,7,8 and indium-tin oxide (ITO) films on glass9—all exhibiting loopy or chaotically coiled fibers. Of specific importance to this work, titania nano- and microfibers can be encouraged to form into straighter and more aligned fiber formations when directed across copper grids,10,11 onto silicon nitride membranes,12,13 and insulating glass backed with aluminum foil exposed around its perimeter.9 Electrospun titania fibers have typical batch diameters ranging from 20 to 800 nm and lengths up to several centimeters, which can span the collecting substrates, when poly(vinylpyrrolidone) (PVP) is used to promote fiber formation. Fibers are usually baked to remove volatile and/or unreacted chemicals and to further fuse the titania-PVP (core-shell) fibers together into a mechanically robust network8,9 or to decompose and eliminate *To whom correspondence should be addressed. (1) Teo, W. E.; Ramakrishna, S. Nanotechnology 2006, 17, 89–106. (2) Ramakrishna, S.; Fujihara, K.; Teo, W. E.; Yong, T.; Ma, Z.; Ramaseshan, R. Mater. Today 2006, 9, 40–50. (3) Li, D.; Ouyang, G.; McCann, J. T.; Xia, Y. Nano Lett. 2005, 5, 913–916. (4) Yang, Y.; Jia, Z.; Hou, L.; Li, Q.; Wang, L.; Guan, Z. IEEE Trans. Dielectr. Electr. Insul. 2008, 15, 269–276. (5) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555–560. (6) Chandrasekar, R.; Zhang, L.; Howe, J. Y.; Hedin, N. E.; Zhang, Y.; Fong, H. J. Mater. Sci. 2009, 44, 1198–1205. (7) McCann, J. T.; Li, D.; Xia, Y. J. Mater. Chem. 2005, 15, 735–738. (8) Li, D.; McCann, J. T.; Xia, Y. J. Am. Ceram. Soc. 2006, 89, 1861–1869. (9) Jabal, J. M. F.; McGarry, L.; Sobczyk, A.; Aston, D. E. ACS Appl. Mater. Interface 2009, 10, 2325–2331. (10) Yuh, J.; Perez, L.; Sigmund, W. M.; Nino, J. C. Physica E 2007, 37, 254– 259. (11) Srivastava, Y.; Loscertales, I.; Marquez, M.; Thorsen, T. Microfluid. Nanofluid. 2008, 4, 245–250. (12) Li, D.; Wang, Y.; Xia, Y. Nano Lett. 2003, 3, 1167–1171. (13) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Macromolecules 2002, 35, 8456–8466. (14) Lee, S. H.; Tekmen, C.; Sigmund, W. M. Mater. Sci. Eng., A 2005, 398, 77–81.
13550 DOI: 10.1021/la1017399
the polymer altogether and anneal the titania14,15 without destroying the nanostructures. The large surface-to-volume ratio of nanofibers and microfibers combined with the relatively easy fabrication and scale-up of electrospinning techniques creates a natural impetus toward applied research in areas of chemical and biological sensors, tissue engineering, protective clothing manufacture, and wastewater treatment,16-19 which take advantage of particular wettability and strength of the fibers and their networks. The ceramic-polymer core-shell composite fiber can be an attractive system for easy incorporation of various functional components, such as nanotubes, carbon, titania, silver, and iron oxide nanoparticles, to further enhance nanofiber mechanical strength, electrical conductivity, and other properties,20-24 to encapsulate biological molecules such as proteins or DNA to improve biocompatibility25-28 and to embed active enzymes29-31 for producing sensor (15) Jia, C. W.; Xie, E. Q.; Zhao, J. G.; Duan, H. G. J. Appl. Phys. 2007, 101, 1–4. (16) Sawicka, K. M.; Prasad, A. K.; Gouma, P. I. Sensor Lett. 2005, 3, 1–5. (17) Bartl, M. H.; Boettcher, S. W.; Frindell, K. L.; Stucky, G. D. Acc. Chem. Res. 2005, 38, 263–271. (18) Xu, Y.; Inai, R.; Kotaki, M.; Ramakrishna, S. Biomaterials 2004, 25, 877– 886. (19) Xie, Y.; Castracane, J. IEEE Eng. Med. Biol. Mag. 2009, 23–30. (20) Watthanaarun, J.; Pavarajarn, V.; Supaphol, P. Sci. Tech. Adv. Mater. 2005, 6, 240–245. (21) Krishnappa, R. V. N.; Desai, K.; Sung, C. J. Mater. Sci. 2003, 38, 2357– 2365. (22) Borras, A.; Barranco, A.; Gonzalez-Elipe, A. R. Langmuir 2008, 24, 8021– 8026. (23) Fischer, T.; Hampp, N. A. IEEE Trans. Nanobiosci. 2004, 3, 118–120. (24) Sonehara, M.; Sato, T.; Takasaki, M.; Konishi, H.; Yamasawa, K.; Miura, Y. IEEE Trans. Magn. 2008, 44, 3107–3110. (25) Liu, Y.; Chen, J.; Misoska, V.; Wallace, G. G. React. Funct. Polym. 2007, 67, 461–467. (26) Yang, Y.; Jia, Z.; Li, Q.; Guan, Z. IEEE Trans. Dielectr. Electr. Insul. 2006, 13, 580–585. (27) Tsuji, H.; Nakano, M.; Hashimoto, M.; Takashima, K.; Katsura, S.; Mizuno, A. Biomacromolecules 2006, 7, 3316–3320. (28) Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549–5554. (29) Herricks, T. E.; Kim, S.-H.; Kim, J.; Li, D.; Kwak, J. H.; Grate, J. W.; Kim, S. H.; Xia, Y. J. Mater. Chem. 2005, 15, 3241–3245. (30) Rojas, R.; Pinto, N. J. IEEE Sens. J. 2008, 8, 951–953. (31) Haynes, A. S.; Gouma, P. I. IEEE Sens. J. 2008, 8, 701–705.
Published on Web 07/20/2010
Langmuir 2010, 26(16), 13550–13555
Jabal et al.
Article
elements or microreactor substrates. In particular, for biomedical applications, the natural photocatalytic properties of titania nanofiber mats have been used for antimicrobial purposes.32 The structure of electrospun fibers allows easy access for drug delivery that can be accomplished either by injection, transdermally (i.e., tissue engineering), or by implantation.18,19,33 Electrospun nanofibers have also shown a general compatibility for cell growth where they tend to proliferate in the direction of the fiber orientation;31,34 this is of particular interest toward future applications for titania-PVP fiber mats, as the PVP coating encourages cell attachment and growth while titania has been reported to be biocompatible as well as being a potential photo/electroactive material for active or passive sensor applications.2,5,32 The objective of this paper is to elucidate the effects of the collecting substrate conductivity and wettability on the morphology and apparent contact angle of the titania-PVP composite fiber mats electrospun onto the substrates. The selection of silicon and aluminum foil as the substrates here was based on similarities to the previously studied materials (viz., glass and ITO) in surface roughness, conductivity, and wettability for comparative analysis. This investigation considers the direct and indirect effects of substrate properties on the static contact angles of water on the fiber mats as a function of electrode separation and electrospinning time. Optical microscopy and staining are used to facilitate a deeper understanding of the length scales and morphological features connected with observed contact angles and the substrate type.
Materials and Methods Electrospinning System. Titania-PVP core-shell fibers were fabricated by electrospinning from a metallic needle to a conductive substrate, depositing into a chaotic fiber mat.1-10 The simple setup requires a syringe pump (Advance Series 1200) to deliver the precursor fluid very slowly (10 μL/min) from a 1 mL syringe with 27G 11/4 in. steel needle (BD Medical-Becton, Dickinson, and Co.), where the fluid jet for micro- and nanofibers is formed with a high-voltage dc unit (HVPS, EMCO High Voltage Corp.) that applies a large potential difference (here, 8 kV) between needle and substrate. All fiber mats were deposited from a dual-syringe application with 1.5 cm between the parallel needles. Using two syringes decreases collection time and thereby facilitates more practical production of thicker mats. Substrates. The substrates under investigation are test-grade semiconductor silicon wafers (oriented Æ100æ, p-doped; Wafer World, Inc.) and consumer-grade aluminum foil directly off the roll. Both substrates were used as received without additional rinsing or cleaning. Silicon wafers are cut in the lab to a measurement of approximately 13 10 0.3 mm3; aluminum foil is cut in the lab to a measurement of 13 10 0.006 mm3 pieces. Both silicon and aluminum foil are grounded as the collection electrodes for fiber spinning. Chemicals and Fabrication of Fiber Mats. The polymer/ sol-gel solution is made by combining acetic acid, ethanol, PVP, and titanium precursor.13,25 A PVP solution (1.3 MDa average molecular weight, Sigma-Aldrich, CAS 9003-39-8) is made by adding 0.15 g of PVP to 2.5 mL of pure ethanol (AAPER Alcohol & Chemical Co.) in a small vial. The vial is sonicated for 30 min to allow the PVP to disperse well. Then, the precursor solution is made in another small vial by stirring together 0.52 mL of titanium(IV) isopropoxide (Fluka, Sigma-Aldrich), 1 mL of glacial acetic (32) Azad, A.-M.; McKelvey, S. L.; Al-Firdaus, Z. AMMTIAC Q. 2008, 3, 3–7. (33) Hong, Y.; Ma, Z.; Wang, C.; Ma, L.; Su, M. ACS Appl. Mater. Interface 2009, 1, 251–256. (34) Yang, C.; Ja, Z.; Liu, J.; Wang, K.; Guan, Z.; Wang, L.; Xu, Z. In Electrical Insulation and Dielectric Phenomena, 2008 IEEE Annual Report Conference; Quebec City, Canada, Oct 26-29, 2008; pp 180-183.
Langmuir 2010, 26(16), 13550–13555
acid (Fisher Scientific), and 1 mL of ethanol for 10 min. The precursor solution is slowly poured into the PVP-ethanol solution and stirred for 1 h. The resultant titania-spinning solution is loaded into syringes for synthesis. Prepared solutions may be used within 2-3 days if continuously stirred to prevent solidification and still result in consistent electrospun fibers.9 The syringe pump is first set to deliver fluid, and then the HVPS is activated, producing electrified liquid jets from both needles simultaneously and almost immediately.1,9 The electric field was adjusted for different runs by placing the collection electrode (viz., substrate) at distances of 5, 10, 15, and 20 cm and sets of fiber mat samples were collected over continuous intervals of 3 s, 5 s, 10 s, 20 s, 1 min, 5 min, and 10 min in order to make a direct comparison to our previous work.9 Three seconds is the shortest feasible time for which fibers can be observed to form by electrospinning and can be collected reproducibly for inspection. Fiber mats are baked in a standard lab oven (VWR International) at 200 C for 24 h to drive off the remaining solvents and encourage robust fusion of the fiber network.9,11 Rhodamine B (CAS 81-88-9) in a 10 mM aqueous solution is used to stain titania-PVP fibers for visualization of the wetted contacts via fluorescence microscopy. All dark-field, bright-field, and fluorescence images were captured with a 3.3 MP CCD camera attached to an optical microscope (Olympus BX51) under auto or manual exposure settings with various objective magnifications; fluorescence imaging exposure time was manually optimized for best visualization of the dyed fiber mats. Wettability by Contact Angle. A Rame-Hart goniometer was used to measure the contact angles of NANOPure water (Barnstead NANOPure Infinity System) and Dulbecco’s Modified Eagle Media (DMEM) with the substrates and electrospun fiber mats before and after baking. Five samples of each parameter set (electrospinning time and electrode distance) were measured at five different sample locations, approximately at the sample center and each quadrant center.9 The experimental error reported is the standard deviation of the replicated measurements, and there is an instrument error of (0.5. The substrates used were tested as-received without any additional cleaning procedure. Hydrophobic surfaces are partially nonwetting and considered to have contact angles (θ) between 90 and 180, while hydrophilic surfaces are partially wetting between 0 and 90.35 It is common to make at least four distinct but qualitative refinements to these ranges, where ultrahydrophobic materials (θ . 90) are discriminated from more nominal hydrophobic surfaces (greater than but near 90), and highly wetting surfaces (from low contact angles to completely wetted out at θ = 0) are distinguished from partially wetting interfaces of higher contact angles.
Results and Discussion The majority of the fibers collected on all substrates are well under 1 μm in diameter with a significant fraction of less than 200 nm and a very small number being as large as a few micrometers for the particular method described above. There are no apparent differences in fiber diameter distributions as a function of their collection time or distance. The diameter distribution within a given sample would most likely be related to the balance between electrospinning rate and oscillations in the fluid jet due to the influences of changing surface tension, rheology, mass transfer, and chemical reaction, which are known to govern capillary liquid jet stability and breakup.36 Electrospun titania-PVP fiber mats on aluminum (Al) foil showed significantly lower contact angles compared to those on silicon wafers on a basis of collection time and distance for all but the thickest mats (Figure 1). This was not expected since the (35) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 4th ed.; Butterworth-Heinemann: Oxford, 1992; p 151. (36) Berg, J. Can. Metall. Q. 1982, 21, 121–136.
DOI: 10.1021/la1017399
13551
Article
Jabal et al.
Figure 1. Contact angles of water against titania-PVP fiber mats supported on aluminum (Al) foil and silicon (Si) plotted against electrospinning time for each series of collection distances. Table 1. Contact Angles against As-Received Silicon (Si) Wafer and Aluminum (Al) Foil Measured with Water and DMEM Cell Media contact angle (deg) substrate
H2O
media
Si wafer Al foil
17.1 ( 0.9 59.3 ( 0.7
15.4 ( 0.5 49.2 ( 0.8
wettability of bare silicon wafers is much greater than bare Al foil (Table 1), which was expected to impact contact angles directly for sparse fiber coverage conditions. If the electrospinning time and distance were the main determining factors for fiber mat coverage, then the less hydrophilic substrate should yield higher contact angles for the same conditions when the wetting liquid retains access to the substrate. However, the substrate conductivity is also an important factor, though not a limiting one, in fiber collection and resultant morphology of the mat,1,20,21 as demonstrated here and by our earlier work. In the present study, the silicon substrate was itself sufficiently conductive to act directly as an electrode, though Al foil is much more conductive. All data series in Figure 1 start with the bare substrate contact angle plotted arbitrarily at 1 s for easy comparison. Large increases in contact angles from the bare substrate condition to a 3 s fiber mat demonstrate that even a very sparse coverage is significant to wetting behavior that might affect aqueous applications of these products. The Al foil shows the smallest initial effect on wettability but the highest ultimate contact angles for the thickest mats. Overall, the contact angles for both Si- and Alsupported fibers increased with deposition time and decreased with electrode spacing but with distinct and consistent variations between them. The largest collection distance of 20 cm was observed to be more susceptible to inconsistent air currents under laboratory conditions and gravitational settling of larger diameter fibers. These effects may contribute to slightly skewed results in contact angle trends with distance and/or time; however, the measurements indicate consistent results. Contact angles within a given experimental set of substrate type and collection distance were always very similar for the 3 and 5 s intervals, which becomes overly exaggerated (to little significance) when presented on a log scale. After 1 min of electrospinning, both substrate types had collected fiber mats to full effective coverage as determined by optical microscopy, except for the longest distance series of 20 cm (Tables SI1 and SI2). A practical yet qualitative definition for full effective coverage here would be the condition that no newly deposited fibers could contact the underlying substrate at any location. Full effective coverage might 13552 DOI: 10.1021/la1017399
Figure 2. Contact angles of water against titania-PVP fiber mats supported on glass (G) and silicon (Si) plotted against electrospinning time for each series of collection distances. Glass data plotted from information previously reported in ref 9.
be expected to mark a transition between wettability regimes, where a “fully covered” substrate can no longer directly affect wetting phenomena, which would fit well with the change in slope of the contact angle versus time/distance/coverage trends of this and previous studies.9 The findings of Figure 1 were further intriguing because the silicon data exceeded the glass data reported in earlier work.9 The surface of silicon in stable ambient conditions is a thin native oxide layer and thus similar to glass as a smooth, hydrophilic, low-conductivity silicate. Figure 2 shows the comparison of the wettability of titania-PVP core-shell fiber mats on silicon to the previous data reported on glass-supported mats. The similarity in curve shapes is more striking when contrasted with the Al data. The cause for greater fiber accumulation on silicon compared to glass9 for the same time and distance parameters (Table SI1) is most likely its larger conductivity, even though silicon is a semiconductor with a thin insulating native oxide. This is further demonstrated by the significant curved or spiraled fiber fraction on silicon, while the fibers on the glass samples are nearly all straight. Fiber mats on Al foil exhibited contact angles intermediate to those of glass and indium-tin oxide (ITO) supported mats.9 Similar to ITO, Al foil presents an oxide surface to the wetting fluids that is conductive, and yet the trend in contact angle is much slower to rise than for both ITO (hydrophobic) and silicon (hydrophilic). The Al foil contact angle falls intermediate to these two substrates. The Al foil substrate has an end-to-end resistance (as measured with a simple two-probe voltmeter) of 5 Ω while the ITO has a resistance of 200 Ω. These observations rule out arguments based on the direct effects of the underlying substrate wettability for higher coverage mats. An initial optical comparison of fiber mats (Table SI2) indicates that the effective electrospinning rate on Al foil was substantially higher than on ITO.9 Aluminum substrates appear more densely covered than ITO plates for the same collection times at the 5 and 10 cm collection distances as well as at 1, 5, and 10 min for 15 and 20 cm; relative fiber coverage at shorter times and longer distances than these are indeterminate due to extremely sparse fiber segment densities and the qualitative nature of the optical comparisons. Aluminum has a significantly higher surface conductivity (0.035 S/m) than ITO (0.010-0.014 S/m), which is the most likely cause for the fiber density differences.17,37 The general morphology of both sets of fiber mats is consistent in composition of straight and tortuous (37) Fukushi, Y.; Kominami, H.; Nakanishi, Y.; Hatanaka, Y. Appl. Surf. Sci. 2005, 244, 537–540.
Langmuir 2010, 26(16), 13550–13555
Jabal et al.
Article Table 3. Contact Angles against Titania-PVP Fiber Mats on Si Wafer and Al Foil Baked at 200 C for 24 h
Figure 3. Contact angles of water against titania-PVP fiber mats supported on aluminum (Al) foil and ITO plates plotted against electrospinning time for each series of collection distances. ITO data plotted from information previously reported in ref 9. Table 2. Contact Angles against Solid Flat Films Supported on Two Different Substrates Measured with Water and DMEM Cell Media contact angle (deg) film/substrate
H2O
media
PVP/Si wafer TiOx/Si wafer PVP-TiOx/Si wafer PVP/Al foil TiOx/Al foil PVP-TiOx/Al foil
90.9 ( 0.3 90.3 ( 0.5 97.9 ( 0.3 88.3 ( 0.5 72.4 ( 0.5 90.1 ( 0.3
90.5 ( 0.5 93.7 ( 0.5 99.8 ( 0.4 85.8 ( 0.4 76.3 ( 0.5 88.4 ( 0.5
segments. Fibers of larger curvature are indicative of the electrospinning regime for conducting substrates38,39 and may result in higher ultimate contact angles, that is, for thick mats. Both ITOand Al-supported mats exhibit higher contact angles than either glass or Si for the 1, 5, and 10 min products at 5 and 10 cm (Figures 1-3). These results appear to be independent of substrate wettability, although the low-coverage samples will clearly be affected by substrate surface energy in a direct manner. In contrast, the ITO-supported fibers of previous work exhibited hydrophobic behavior for even the lowest coverage because of the already hydrophobic substrate. Flat films of PVP, titania, and a mix of PVP and titania cast from the same electrospinning precursor solutions onto silicon and Al foil exhibited similar hydrophobic contact angles to as-received ITO, except for the TiOx/Al foil sample (Table 2). The analogy of roughening a smooth hydrophobic surface to increase its apparent contact angle seems generally consistent, where the fiber mats are effectively rough surfaces for this argument. The contact angles measured with DMEM cell media solution showed very similar values in all cases and are not reported hereafter for simplicity; Tables 1 and 2 establish these similarities in foresight of future cell culturing experiments. The more substantial differences between the Al foil and ITO substrates are the surface roughness and wettability. Both are conductive metal oxides, but their surface energies are very different. Al foil directly out of the box is only partially wetting, in contrast to the as-received ITO surface, which is barely hydrophobic by definition. The impact of substrate wettability on contact angles for less dense (laterally) and/or thinner fiber mats is qualitatively straightforward only where the fluid has access to the substrate. Once the water can no longer wick through (38) Simhan, R. G.; Moore, L. L.; Van Gunten, P. R. J. Mater. Sci. 1985, 20, 1748–1752. (39) Reneker, D. H.; Yarin, A. L. Polymer 2008, 49, 2387–2425.
Langmuir 2010, 26(16), 13550–13555
distance (cm) þ time
silicon (deg)
aluminum foil (deg)
5þ3s 5þ5s 5 þ 10 s 5 þ 20 s 5 þ 1 min 5 þ 5 min 5 þ 10 min
118.9 ( 0.6 119.8 ( 0.6 131.3 ( 0.5 134.8 ( 0.4 137.9 ( 0.3 139.2 ( 0.4 140.1 ( 0.3
88.6 ( 0.5 89.1 ( 0.7 117.8 ( 0.8 129.7 ( 0.5 141.9 ( 0.3 144.8 ( 0.4 145.2 ( 0.4
10 þ 3 s 10 þ 5 s 10 þ 10 s 10 þ 20 s 10 þ 1 min 10 þ 5 min 10 þ 10 min
105.4 ( 0.5 106.1 ( 0.9 111.0 (0.8 130.4 ( 0.5 135.3 ( 0.5 137.2 ( 0.4 138.1 ( 0.3
83.4 ( 0.5 84.7 ( 0.5 89.3 ( 0.5 125.7( 0.5 139.2 ( 0.4 143.9 ( 0.3 144.1 ( 0.3
15 þ 3 s 15 þ 5 s 15 þ 10 s 15 þ 20 s 15 þ 1 min 15 þ 5 min 15 þ 10 min
91.7 ( 0.5 93.9 ( 0.7 99.5 ( 0.5 118.2 ( 0.4 124.2 ( 0.4 134.5 (0.5 135.3 ( 0.5
72.6 ( 0.5 72.7 ( 0.5 84.8 ( 0.9 87.7 ( 0.5 129.2 ( 0.4 134.7 ( 0.5 135.8 ( 0.4
20 þ 3 s 20 þ 5 s 20 þ 10 s 20 þ 20 s 20 þ 1 min 20 þ 5 min 20 þ 10 min
89.1 ( 0.7 89.4 ( 0.5 93.4 ( 0.8 97.6 ( 0.5 104.8 ( 0.4 132.3 ( 0.7 133.8 ( 0.4
61.1 ( 0.9 61.3 ( 0.5 64.4 ( 0.8 78.2 ( 0.4 85.8 ( 0.4 120.7 ( 0.5 121.7 ( 0.5
the fiber mat, the effect of substrate surface energy on apparent wettability of such a highly convoluted morphology becomes indirect and tenuous at best. The obvious argument can be made that substrate surface energy should have no impact on contact angle of a supported fiber network where the wetting fluid cannot penetrate to contact the substrate, which is not contradicted by these or previous results. Determining the precise range of surface coverage and morphological classification for which fluid access to the substrate persists is difficult. This is further exacerbated by the much greater surface roughness of the flexible Al foil compared to the other substrates, which is of two distinct scales: microscale striations from extrusion when fabricated and macroscale nonplanarity and crinkling from cutting and mounting the substrate. However, an empirical estimate is feasible as to which fiber mats restrict water from their substrates, based on visual analysis of the fibers and the changes in contact angle. For example, the Al-supported samples—having a partially wetting substrate—do not become hydrophobic until roughly 50% coverage. This occurs after 5 s at 5 cm, 10 s at 10 cm, 20 s at 15 cm, and 1 min at 20 cm, in agreement with both contact angle values and qualitative optical observations (Table 3, Tables SI1 and SI2). However, all fiber mats on silicon showed significantly higher contact angles, with only the two most sparsely covered sample types (3 and 5 s at 20 cm) falling below the hydrophobic definition. This distinction can be understood as primarily controlled by the perfect planarity of the silicon substrate and its propagating effect to the depositing fiber layers, whereas the irregular contours of even the slightly crinkled Al foil would encourage wettability. The extremely hydrophobic contact angles seem to confer a high degree of certainty to the assertion that water does not penetrate the thicker fiber mats. The densely packed fibers would maintain air pockets that could be either deleterious or beneficial to future biological applications, depending on the need for an DOI: 10.1021/la1017399
13553
Article
Jabal et al.
Table 4. Fluorescence and Bright-Field Images of Titania-PVP Fiber Mats (Spun 1 min at 5 cm) on Al Foil Stained with Rhodamine B
Table 6. Fluorescence and Bright-Field Images of Titania-PVP Fiber Mats (Spun 1 min at 5 cm) on Si Wafer Stained with Rhodamine B
Table 5. Fluorescence and Bright-Field Images of Titania-PVP Fiber Mats (Spun 1 min at 5 cm) on ITO Stained with Rhodamine B
Table 7. Fluorescence and Bright-Field Images of Titania-PVP Fiber Mats (Spun 1 min at 5 cm) on Glass Stained with Rhodamine B
air-water interface or a passive regulation for dissolved gases via Laplace pressure effects, which could be of substantial interest to biofilm formation. In order to demonstrate where the water has access to the titania-PVP fibers below the mat surface, droplets of Rhodamine B solution were placed on the samples in precisely the same fashion as that of the contact angle studies and then immediately rinsed off with water to reduce wicking of the lower surface tension solution or other transient effects of surfactant adsorption and solvent evaporation. While pure water will rest indefinitely on the fiber mats, the dye solution was observed to wick into the mat over the course of tens of seconds. Other dyes could be used that exhibit slower wicking; however, this initial study will serve as an upper-end estimate on the amount of water contact and penetration into the fiber networks. Fluorescent images of the dyed samples (Tables 4-7 and Tables SI3-SI10) indicate that the top layers of the fiber mats have been well stained. At 50 (168 μm 134 μm images) and 100 (84 μm 67 μm images) magnifications, it is possible to distinguish nanofibers in fluorescence, though they quickly fall out of the very narrow focal plane at these scales since the fiber layers do not necessarily lie in-plane. The largest views at 10 magnification (840 μm 670 μm images) provide a better sense of the macroscale morphology. Comparing bright-field micrographs at the same image locations reveals dense mats of fibers where the dye did not contact. Even some topmost fibers are kept from being wetted (black regions in green filter images) due to the surrounding morphology holding up the solution by surface tension forces. Note: color variations under bright-field from white to yellow-hued across different samples and magnifications are caused by automatic adjustment of the CCD exposure time (see also Supporting Information); to the naked eye, all fiber samples appear similar in their light yellowish tone after baking. Only under the microscope and often only with the fluorescence stain are the varying length scales apparent that are collectively responsible for the observed wetting phenomena. Both larger and smaller nonwetted regions are observed in all sample types, having dimensions much greater than the average fiber
segment length, that is, the distance between fiber intersections. The dyed samples reveal the larger nonwetted regions to be irregular in shape while the smaller regions are more like pits that extend only a few fiber diameters in depth, such as in Tables 4 and 5 (see also Supporting Information). ITO facilitates higher contact angles than Al foil attributed to the uniform surface of ITO compared to the ridges on aluminum surfaces.
13554 DOI: 10.1021/la1017399
Conclusions The combination of hydrophobic contact angles with the evidence from the dyed fiber mats shows clearly that water is unable to penetrate below the uppermost layers of the nano/ microfiber networks created by electrospinning titania-PVP (core-shell) fibers onto conducting or insulating materials. Therefore, the substrate hydrophobicity cannot be directly connected to observed contact angles against thick fiber mats. On the basis of the qualitative microscopic analyses, it is clear that the fiber length scale (diameter and segment length) is not exclusively dictating the wetting behavior, but rather a larger irregularity in the mat morphology also contributes. Moreover, the higher electrospinning rates achieved with collection electrodes of greater conductivity are not alone sufficient to explain the contact angle trends. In growing agreement with previous studies, the most notable qualitative difference in titania-PVP fiber morphology between substrates is the nature of the individual fiber shapes produced. The semiconducting surface of silicon collected mostly straight fibers—similar to glass-supported fibers in the previous study— with some coiled and curved, compared to a greater fraction of high-curvature fibers deposited on aluminum foil, and previously on ITO. The similar nature of the silicon wafer surface to glass in smoothness, planarity, and hydrophilicity indicates that the conductivity at the surface of the collection electrode would be a key factor in controlling the apparent wettability in the electrospun fiber mats. It appears that a smooth and planar substrate is important to induce higher contact angles for the low-density fibers on hydrophilic supports. This is demonstrated by the silicon-supported Langmuir 2010, 26(16), 13550–13555
Jabal et al.
samples yielding higher contact angles than the Al-foil mats, where higher substrate conductivity and larger curved-fiber fractions on the Al foil do not create the conditions expected to exceed the silicon results until very thick mats have been deposited. The two physical features of curved fiber fraction and fiber segment density work together to produce a matted surface of effectively higher curvature on silicon for the thinner mats. However, it is unclear precisely why the ITO-supported mats exhibited the highest apparent contact angles based on these considerations. The comparative outcome from the high-conductivity substrates may indicate a complex structure-property relationship between fiber mat morphology and contact angle, such as a critical surface segment density that sustains a maximum contact angle where densities both higher or lower would yield smaller contact angles. It remains difficult to interpret the cause for the relative values of the ultimate contact angles between substrate types, since full effective coverage has been far exceeded at the longest time and shortest distance. A robust but practical image analysis method may be required to quantify the geometries of the fiber mats
Langmuir 2010, 26(16), 13550–13555
Article
relevant to wettability in order to specify more precise conditions that express particular contact angles. This could be especially important in explaining why contact angles continue to increase slightly with electrospinning time for the thickest fiber mats. Acknowledgment. The authors gratefully acknowledge the support of the NSF-Idaho EPSCoR Program and by the National Science Foundation under Award EPS-0447689 and by the University of Idaho Biological Applications of Nanotechnology (BANTech) Strategic Initiative. Supporting Information Available: Optical images of titania-PVP fiber mats examples for both silicon wafers and aluminum foil with variable collection times and distances; fluorescence image comparisons of the fiber mats on all substrates (silicon, aluminum, ITO, and glass) with longer electrospinning times. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la1017399
13555
