J. Phys. Chem. B 2007, 111, 8547-8553
8547
Hollow Polyelectrolyte Multilayer Tubes: Mechanical Properties and Shape Changes Renate Mueller,† Lars Daehne,‡ and Andreas Fery*,†,§ Max Planck Institute of Colloids and Interfaces, Wissenschaftspark Golm, 14424 Potsdam, Germany, Capsulution Nanoscience AG, Volmer Str. 7b, 12 489 Berlin, Germany, and Department of Physical Chemistry, UniVersity Bayreuth, D95449 Bayreuth, Germany ReceiVed: December 20, 2006; In Final Form: February 26, 2007
In this paper, novel hollow polyelectrolyte multilayer tubes from poly(diallyldimethylammonium chloride) (PDADMAC), poly(styrene sulfonate) (PSS), and poly(allylamine hydrochloride) (PAH) were prepared: Readily available glass fiber templates are coated with polyelectrolytes using the layer-by-layer technique, followed by subsequent fiber dissolution. Depending on the composition of the polymeric multilayer, stable hollow tubes or tubes showing a pearling instability are observed. This instability corresponds to the Rayleigh instability and is a consequence of an increased mobility of the polyelectrolyte chains within the multilayer. The well-defined stable tubes were characterized with fluorescence microscopy, confocal laser scanning microscopy, and atomic force microscopy (AFM). The tubes were found to be remarkably free of defects, which results in an impermeable tube wall for even low molecular weight molecules. The mechanical properties of the tubes were determined with AFM force spectroscopy in water, and because continuum mechanical models apply, the Young’s modulus of the wall material was determined. Additionally, scaling relations for the dependency of tube stiffness on diameter and wall thickness were validated. Because both parameters can be experimentally controlled by our approach, the deformability of the tubes can be varied over a broad range and adjusted for the particular needs.
Introduction Hollow tubes of micron or submicron diameter and high aspect ratio are interesting for various applications. They possess a large specific surface area, making them appealing for catalysis,1 but can still be handled easily when assembled into meshes or fleeces. In this form, they have further potential for filtration2 and sensing,3,4 or they can serve as scaffolding for growing tissue.5,6 Hollow tubes could be used for encapsulation purposes and find applications in fields like drug delivery. In the field of microfluidics, they could act as channels for transport7 or as actuators (like stirrers), since they can be oriented in external fields due to their anisotropic shape.8 In biology, tubes of submicron dimensions like protein nanotubes or microtubules are commonly found. They play an essential role in providing mechanical reinforcement9 and in the structure of cells. In this article, we will focus on artificial tubes, whose dimensions can be tailored and which have well-defined mechanical and permeation properties. Previously, artificial tubes have been formed using different strategies. Pure selfassembly allows producing tubes from carbon,10,11 boronitrate,12 lipid surfactants,13 or polypeptides.14 More recently, tubes were prepared on the basis of template assisted self-assembly. That technique allows influencing the dimensions of the tube as well as their wall thickness. Mesoporous materials and fibers are suitable as templates. They are coated and subsequently dissolved under conditions that do not destroy the coating. Thus, only tubelike objects remain after the dissolution step and they * To whom correspondence should be addressed. Phone: +49 331 567 9204. Fax: +49 331 567 9102. E-mail:
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Capsulution Nanoscience AG. § University Bayreuth.
ideally resemble the shape of the cylindrical objects (cylindrical holes or fibers) of which they were templated. For the purpose of coating, wetting of thin polymer films can be used, if interfacial interactions are suitable.15 Fibers can be coated using chemical vapor deposition.16,17 Recently, polyelectrolyte multilayer coating using the layer-by-layer approach has been used for this purpose.18 The advantage of the latter approach is its relative simplicity and its versatility in terms of composition. The layer-by-layer technique was introduced by Decher and co-workers19,20 and is now commonly used to prepare flat films: polyelectrolyte multilayers are formed by alternating adsorption of polyanions and polycations from aqueous solution onto a charged template. In each step, the sign of surface charge is reversed, provided suitable polyelectrolytes are used. The thickness increase per adsorption step is only on the order of nanometers: Electrostatic interactions cause a strong self-repulsion, which in turn limits film growth. Thus, this method allows controlling the layer thickness within nanometers. Apart from this accuracy in adjusting film thickness, the layerby-layer technique is an easy way to functionalize surfaces21 and various substances can be incorporated into the layers, which make the layers biocompatible,5,22 magnetically functional,23 or optically functional.24,25 Multiple functionalities can be combined due to the stepwise buildup of the coating. Details on the method and its applications are given in the recent reviews.21,26,27 A good overview on the potential of the layer-by-layer technique can be found in ref 28. In the past, template assisted selfassembly of polyelectrolyte multilayers has been used with great success for the production of spherical hollow capsules.29 They were produced by coating of colloidal template particles and subsequent dissolution under conditions that do not destabilize the multilayer. This scheme is not limited to spherical templates, but fibroid templates should be usable with equal success.16-18
10.1021/jp068762p CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
8548 J. Phys. Chem. B, Vol. 111, No. 29, 2007 It would be particularly advantageous to combine the versatility of polyelectrolyte multilayer coating with the advantages and scope of applications of tubes mentioned in the first paragraph. Here, both a novel route to prepare tubelike structures using layer-by-layer self-assembly on readily available glass fibers is presented and it is shown that the geometry of the tubes can be designed accurately (diameter, wall thickness, length, and aspect ratio). Depending on the composition of the multilayer, one could either find stable hollow tubes after the dissolution of the template or tubes that show a pearling instability. For the stable tubes, the geometry of the tubes was characterized and for the first time the permeability and the mechanical properties of these tubes were investigated. Materials and Experimental Methods (1) Materials. All chemicals were purchased from SigmaAldrich. As polyelectrolytes, we used poly(allylamine hydrochloride) (PAH, Mw ) 15 000), poly(styrene sulfonate) (PSS, Mw ) 70 000), poly(diallyldimethylammonium chloride) (PDADMAC, Mw ) 200 000-350 000). The concentration of polyelectrolytes in the 0.2 M NaCl solution was 1 g/L, and the pH was adjusted to 5.6 using a 0.02 M acetate buffer. The 0.02 M acetate buffer was produced from acetic acid. In order to monitor the multilayer buildup, PAH and PSS were labeled with rhodamine B. Additionally, the dye rhodamine B (Mw ) 479.01) as well as high molecular weight molecules, fluorescein isothiocyanate (FITC) labeled dextran (Mw ) 500 000, concentration 3.25 g/L) and fluorescein labeled PAH (Mw > 15 000, concentration 3 g/L), were used to monitor the permeability of the wall of the tubes. As templates for the layer-by-layer process, glass fibers (diameter 5-10 µm, URALITA, Spain) were used. Prior to coating, the glass fiber templates were cut with a homogenizer (Ultra Turrax/DIAX 900, Buddeberg, Germany) and activated by means of a basic RCA treatment (caution: highly corrosiVe!): the tubes are immersed into a solution containing 5 parts H2O, 1 part NH3, and 1 part H2O2, heated to 70 °C for 10 min, and afterward thoroughly rinsed with pure Milli-Q water. After coating the templates, they were dissolved in 1 M hydrofluoric acid (HF) (caution: extremely hazardous liquid and Vapor, toxic and highly harmful!). In order to adhere tubes to a glass slide for microscopy, the glass was coated with poly(allylamine hydrochloride) (PAH, Mw ) 70 000, concentration 1 g/L). (2) Experimental Methods. (2.1) Fluorescence Microscopy. The fluorescence microscope Axoivert 200 (Zeiss, Germany) was used in order to obtain optical images, using a 63×/1.4 oil immersion objective. A drop of the sample solution was put on a clean glass slide, which was previously coated with a layer of poly(ethylenimine) (PEI, Mw ) 2000, concentration 1 g/L). The radii of at least 35 tubes were analyzed. (2.2) Confocal Laser Scanning Microscopy. To visualize the tubes in water, confocal laser scanning microscopy (Leica TCS SP and Leica CLSM SP1, Germany) was used with a 40×, 63×, and 100×/1.4 oil immersion objective. The tubes could be easily detected due to the rhodamine labeled PAH or respectively PSS layers in the wall. To the sample solution, a solution of fluorescein labeled PAH and FITC labeled dextran was added in order to test the stability as well as the permeability of the wall. (2.3) Scanning Force Microscopy (SFM). The topography measurements were performed at room temperature and in air with a Nanoscope III Multimode scanning force microscope (Digital Instruments Inc.). The sample was prepared by placing a drop of the sample solution onto a silicon wafer. The silicon
Mueller et al. SCHEME 1: Schematic Display of the Sample Preparationa
a The coating of a glass fiber is followed by the dissolution of the template. Depending on the composition of the multilayer, we observed a pearling instability or stable hollow tubes (scheme not to scale).
was cleaned with RCA previously. After the evaporation of the water in the sample solution, the tubes collapsed on the silicon and the thickness of the wall was measured at the step collapsed tube - silicon. At least 20 profiles were analyzed. (2.4) Atomic Force Microscopy (AFM) Force Spectroscopy. Force spectroscopy measurements were carried out under water using a commercial AFM NanoWizard setup (JPK Instruments, Germany). In this setup, a stand-alone atomic force microscope is placed on an Axiovert 200 optical microscope (Zeiss, Germany) such that fluorescence microscopy can be used to locate and monitor the rhodamine labeled tubes during the experiment. For the measurement, the conventional pyramidal tip (CSC12/AlBS/50, MicroMash, Estonia, typical radius of curvature of the tip, according to manufacturer: 10 nm) and the colloidal probe technique were used:30,31 a glass bead (according to manufacturer: diameter 30-50 µm, Polyscience Inc.) was attached to a tipless cantilever (CSC12/tipless/AlBS/ 50, MicroMash, Estonia) with a two-component epoxy glue (UHU Plus endfest 300, UHU GmbH & Co.KG, Germany) by using a micromanipulator (Suttner Instruments Co.). According to previous work,32 both tips produce the same force-deformation characteristics as long as the deformations of the shell are on the order of its wall thickness. The spring constant of the cantilevers was determined using the thermal noise method33 or the Sader method.34 Both methods agreed within 10%, and values of the spring constants were also in the range reported by the manufacturer. Clean glass slides were coated with a layer of PEI in order to adhere the tubes onto the surface and immobilize them during the experiment. Then, individual tubes were compressed and both the force and the deformation of the tubes were measured, similar to force-deformation experiments done on capsules, as described in more detail in refs 35 and 36. Deformations were limited on the order of the wall thickness; this avoids plasticity and other effects caused by the permeation of solvent through the tube membrane, as discussed in ref 27. Results and Discussion In this work, commercial glass fiber templates were coated using polyelectrolyte multilayer self-assembly. Two different samples were prepared: in one case, clean glass fibers were coated with six double layers of PDADMAC/PSS, and in the second case, fibers were coated with six double layers of PDADMAC/PSS and one additional double layer of PAH/PSS, both starting with PADADMAC. Subsequently, the fiber substrates were dissolved in a HF solution. For the pure PDADMAC/PSS tubes the dissolution process resulted in a pearling instability; see the upper right corner in Scheme 1. However, for the second case, one ended up with stable hollow tubes after the core dissolution; see the lower right corner in Scheme 1.
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Figure 1. PAH stabilized tubes (a) before annealing and (b) after annealing (pictures were taken of different tubes).
Figure 2. (a) AFM topography image of a microtube and (b) step height analysis in between the two white arrows.
Similar effects as in the first case were found for cylindrical lipid bilayer vesicles;37-39 the instability observed here is called pearling instability. This instability corresponds to the wellknown Rayleigh instability,40,41 in which a liquid cylinder decays into a series of droplets. Like in the case of the Rayleigh instability, surface tension is the most probable cause for the instability we observed here: At room temperature and low ionic strength, the tubes were stable, because the polyelectrolyte multilayer is in a glasslike state36 and the polyelectrolyte chains are immobile. During the template dissolution, the ionic strength was high and the chain mobility within the multilayer increased. This, in turn, strongly deceased the stiffness of the material, as quantified earlier for PADAMAC/PSS complexes42 and for polyelectrolyte multilayer capsules.43,44 As the mobility of the polyelectrolytes increased, the tube adjusted its shape in order to minimize surface tension. This led to the well-known Rayleigh phenomenon resulting for this case in a string of water filled pearls. In order to get stable hollow tubes, an additional layer of PAH was added. PAH/PSS layers are well-known to be stable at high ionic strength,43 and indeed, the instabilities during dissolution could be suppressed; see Figure 1a. Previously, it was found that multilayer softening also occurs upon annealing at high temperatures,36 which offered a possibility to test our earlier hypothesis: we annealed the stabilized tubes for 30 min at 90 °C. This resulted in a pearl-necklace-like structure, see Figure 1b, which is in fact similar to the one observed earlier on pure PDADMAC/PSS multilayers.45 While this effect is interesting from a fundamental perspective, we focus here on the case of tubes resembling the template shape, since template fidelity is a prerequisite for the applications mentioned in the Introduction.
Tubes were characterized by means of fluorescence microscopy and atomic force microscopy. Figure 2a shows an AFM topography image of a dried tube on freshly cleaved mica. The tubes collapsed upon drying and offered the possibility to verify the complete dissolution of the glass fiber material. The wall thickness of the tube was determined by measuring the step height between mica and the tube; see Figure 2b. The average wall thickness was found to be t ) 8.64 ( 0.64 nm per bilayer, which was slightly larger than the typical range for this material combination.46,47 Fluorescence microscopy showed the tubes are exact replicas of the glass fiber templates, with a length of several tens to a few hundred microns; see Figure 3a. The length could be varied by cutting the fibers, allowing an adjustment of the aspect of the tube ratio. Fibers cut by a homogenizer before coating resulted in closed hollow tubes after the template was dissolved, whereas tubes cut after the coating, for example, using a pulsed UV laser (P.A.L.M. Microlaser Technologies, Germany, λ ) 337 nm), showed open ends. The diameter of the tubes was found to range from 5 to 17 µm; see Figure 3b. This large polydispersity originated from the polydispersity of the glass fiber templates. For potential application (e.g., microfluidics, encapsulatation), a crucial property for the performance is the permeability of the wall material. This property was probed with confocal laser scanning microscopy. The tubes were immersed into a solution of fluorescently labeled molecules. Impermeable and closed tubes showed no fluorescence in their interior. It was found that the permeability of the tubes is dependent on the size and the charge of the fluorescent molecules; see Figure 4. The tubes were impermeable for fluorescently labeled dextran molecules
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Figure 3. (a) Fluorescence microscopy image of the tubes and (b) the histogram of the diameter of the tubes.
Figure 4. Images showing that the tubes were (a) impermeable for rhodamine molecules but (b) permeable for fluorescein molecules. (c) With increasing osmotic stress, the tubes collapsed leaving cavities behind, which were free of any dye (the noncollapsed tube in the center showed fluorescence due to defects in the wall).
(Mw ) 500 000 g/mol) (data not shown). These molecules are slightly negatively charged due to the fluorescein label and positively charged rhodamine 6 G molecules (Mw ) 479 g/mol). No fluorescence could be detected within 90% of the tubes, indicating a remarkably defect-free hull. Earlier investigations showed this is most likely due to the glass fiber template. Polymeric templates tended to cause more defects, because their dissolution products were of high molecular weight and applied a high osmotic pressure onto the multilayer.48 In contrast, the tubes were permeable for negatively charged fluorescein molecules (Mw ) 376 g/mol). The outermost layer of the tubes was negatively charged PSS. Different from our expectation, the diffusion into the tubes was larger for positively charged probe molecules (like rhodamine) and lower for homonymous probe molecules49,50 (like fluorescein). Because this was not the case, other interactions like steric or hydrophobic might have played a role. In Figure 4c, a high concentration of labeled PAH molecules (concentration 3 g/L) was added to the solution containing the tubes. PAH creates an osmotic pressure on the membranes, which is proportional to the number of its monomers due to its counterions.51,52 Once the outer pressure exceeded the stability of the tubes, they collapsed (despite tubes showing defects), but there were still cavities left behind, which showed no fluorescence in their interior. Under the assumption that the total elastic energy of the tube is a sum of bending strain energy, compression strain energy, and the product of the outer pressure acting on the cross section area of the tube, a critical pressure under which the tube collapses53 was derived. Sun et al.54 found
that, in the case that the outer pressure (P) exceeded the critical pressure (Pcrit), a hollow cylinder with a circular cross section transformed into a cylinder with an elliptical cross section. The critical pressure decreased with the tube radius (R), as described by Pcrit ∼ 1/R3. Taking the variations in the radii of the tubes (Figure 3b) into account, a critical pressure was predicted, depending on the radii of the tubes: it was maximum for the smallest tube radii (R ) 2.5 µm) with Pcrit,R)2.5µm ) 786 Pa, for average radii (R ) 3.75 µm), it was found to be Pcrit,R)3.75µm ) 190 Pa, and for the biggest radii (R ) 8 µm), Pcrit,R)8µm ) 23 Pa, it showed the lowest value. These predictions fit very well with the results on collapsing tubes presented here, because the tubes were stable at osmotic pressures around 8 Pa (FITCdextran) but collapsed at osmotic pressures of approximately 100 Pa (fluorescein labeled PAH). A deeper systematic analysis of the critical pressure could have been used to determine the mechanical properties of the tubes. Instead, AFM force spectroscopy was used to derive the mechanical properties, because, on one hand, the assumptions on permeability and osmotic pressure for high concentrations are nontrivial. On the other hand, we previously demonstrated the force-deformation characteristics of hollow polyelectrolyte multilayer capsules are independent of permeability properties, provided the deformations are reversible.35 So far, mechanical properties of nanometer-sized tubelike structures have been investigated by GravelandBikker et al. for self-assembled protein nanotubes55 and de Pablo et al. for microtubules.56 Here, all measurements were carried out in water, without added salt. As expected, it was found that the deformation
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Figure 5. (a) Force-extension curves taken from different locations on the tube. The numbers in the graph correspond to positions in the (b) fluorescence microscopy picture.
Figure 6. (a) Force-deformation curves for tubes with varying diameter (black spots d ) 5.54 µm, gray spots d ) 7.88 µm, and light gray triangles d ) 12.49 µm) and (b) histogram of the stiffness of the tubes.
properties of the tubes strongly depended on the probing position on the tube. Figure 5 shows a series of measurements performed on the same tube, with varying probing position. In all locations, a linear force-deformation behavior was found for small deformations. However, the stiffness of the tube varied and increased when the edge of the tube was approached. Close to the edge, instabilities, most likely due to lateral evasion of the tube, were observed. Therefore, in the following, the tubes were probed only close to their “pole region” between positions 1 and 2. For all measurements, the slope of the force-deformation characteristic was reported, which was called the stiffness of the tubes (c). The stiffness of the tubes was determined for tubes with varying diameter; see Figure 6a. Qualitatively, the stiffness of the tubes decreased with increasing tube diameter, which could be expected from continuum mechanical considerations as described in refs 57 and 58. Figure 6b displays a histogram of the stiffness measured on 24 different tubes. The stiffness values varied from a few to almost 100 pN/m. The distribution of the stiffness values originated from the polydispersity in the radius of the coated glass fiber templates, which in turn enabled us to tailor the stiffness of the tubes by simply varying the radii of the template. For comparison with other composite materials, the Young’s modulus was evaluated. It was independent from the geometry of the tubes. The linear deformation of a curved shell combines an out-of-plane bending and an in-plane compression. A long thin-walled cylinder under a point load returns to its original shape at a certain distance from the point
of force application: the softest mode of deformation extends along the axis, roughly for a distance of R(R/t)1/2, with R being the tube radius and t the wall thickness. The deformation perpendicular to the axis is proportional to R. The connection between the deformation (d), the loading force (F), and the elastic constant of the wall material, the Young’s modulus (E) was calculated according to de Pablo et al.:56
c)
t5/2 F = 1.18E 3/2 d R
(1)
This model allowed a test of the scaling predictions; while the thickness of the tubes (t) is assumed to be uniform through the sample, the diameters scatter, as displayed in Figure 3b. Indeed, considering the stiffness values (c), which are 47, 26, and 12 pN/nm, one found according to eq 2 rescaled stiffness values of 0.22, 0.24, and 0.18 nNm1/2, respectively.
crescaled ) cR3/2
(2)
The effective Young’s modulus (E) of the wall material when calculated according to eq 1 was found to be E ) 207 MPa. This value fits well in between the values recorded for a pure PAH/PSS system of 249 MPa59 and a pure PDADMC/PSS (in salt-free environment) multilayer of approximately 100 MPa.36 Still, this calculation was based on the assumption of a homogeneous material, whereas in this case the multilayer combined PDADMAC/PSS and PAH/PSS multilayers. This
8552 J. Phys. Chem. B, Vol. 111, No. 29, 2007 could also explain the slight variation in the rescaled sample stiffness above. It is also worthwhile to mention that as long as the tube is much longer than its diameter the deformation properties of the tubes, like the tube stiffness and the critical pressure for implosion, are decoupled from the internal volume of the tube. While for spherical microcapsules the internal volume is coupled with the deformation properties via the radius of the sphere, tubes of any volume can be easily tailored to show certain deformation properties. Conclusion A novel route to produce hollow cylindrical tubes from polyelectrolyte multilayers using the LbL technique on commercial glass fibers was presented. The use of polyelectrolyte multilayers is particularly appealing, because these layers are highly versatile in terms of composition and can be (multi)functional. Tubes were formed when fibers were dissolved after polyelectrolyte multilayer formation on their surface, yielding hollow structures made from the (undissolved) polyelectrolyte multilayer. After fiber dissolution, depending on the composition of the multilayer, stable hollow tubes or tubes that showed a pearling instability were found. It was also found that increased chain mobility (due to high ionic strength or high temperature) was most likely the reason for the pearling instability. The instability resembles the well-known Rayleigh instability of liquid cylinders and was caused by surface tension effects that became dominant for membranes of high polymer mobility. The instability could be suppressed consequently when layers of low polymer mobility were added. Since template fidelity is a prerequisite for many potential applications of tubes, we focused on the case of tubes where no instability occurred during dissolution for further characterization. The geometry of the tubes was characterized with fluorescence microscopy and confocal laser scanning microscopy (CLSM). The wall thickness of the tubes was measured with AFM topography imaging. The tube diameter was of several microns, resembling the diameter of the glass fiber template, while the wall thickness was on the order of several tens of nanometers. Permeability measurements with CLSM showed that the tubes were impermeable to high molecular weight and small positively charged molecules, which indicated a remarkably dense and defect-free multilayer, whereas the layers were permeable for small negatively charged molecules. Additionally, the mechanical properties of the tubes were determined with AFM force spectroscopy of individual tubes. The measurements showed that the scaling of the tube stiffness with the tube radius was well described by continuum mechanical models. A Young’s modulus was derived, as well, which was compatible with previous measurements. Therefore, these simple models can serve as a guideline for tailoring the tube deformation properties, ensuring suitable stability or release by mechanical stimuli. Acknowledgment. The authors are grateful to Helmuth Mo¨hwald (MPI, Potsdam-Golm) for valuable discussions and comments, Anne Heilig (MPI, Potsdam-Golm) for AFM imaging, and Philippe Carl (University of Mu¨nster) who provided the analysis software PUNIAS. References and Notes (1) Sanchez-Castillo, M. A.; Couto, C.; Kim, W. B.; Dumesic, J. A. Angew. Chem., Int. Ed. 2004, 43, 1140. (2) Dersch, R.; Greiner, A.; Steinhart, M.; Wendorff, J. H. Chem. Unserer Zeit 2005, 39, 26. (3) Wang, X. Y.; Kim, Y. G.; Drew, C.; Ku, B. C.; Kumar, J.; Samuelson, L. A. Nano Lett. 2004, 4, 331.
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