Rapid Formation of Coordination Multilayers Using Accelerated Self

Feb 17, 2010 - ... Tao Zhang , Hongqiang Yan , Mao Peng , Zhengping Fang. Journal of Applied Polymer Science 2013 129 (10.1002/app.v129.5), 2986-2997 ...
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Rapid Formation of Coordination Multilayers Using Accelerated Self-Assembly Procedure (ASAP) Miryam Greenstein, Rivka Ben Ishay, Ben M. Maoz, Haim Leader, Alexander Vaskevich,* and Israel Rubinstein* Department of Materials & Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Received November 22, 2009. Revised Manuscript Received January 31, 2010 Layer-by-layer (LbL) assembly of multilayers on surfaces using metal-organic coordination between consecutive layers is a well-established method for multilayer construction. The basic scheme includes self-assembly of a ligand (anchor) monolayer on the surface, followed by alternate binding of metal ions and multifunctional ligand layers to form a coordination multilayer. Binding of the ligand repeat unit to form a new layer is commonly a slow process, taking typically overnight to complete. This renders the process of multilayer preparation exceedingly slow and, in many cases, impractical. Here we describe a method for LbL synthesis of self-assembled coordination multilayers denoted accelerated self-assembly procedure (ASAP), where binding of a full organic ligand layer occurs in ca. 1 min. In the new protocol a small volume of a dilute ligand solution is spread on the substrate surface and evaporated under natural convection conditions, leaving the surface covered with excess ligand. Extensive rinsing in pure solvent results in complete removal of unbound molecules from the surface, leaving only the new coordinated layer. ASAP is demonstrated here by the construction of two kinds of coordination multilayers, comprising mercaptoundecanoic acid-Cu(II) and bishydroxamate-Zr(IV). Multilayers prepared by ASAP and by the standard (overnight adsorption) procedure are compared using ellipsometry, contact-angle, and FTIR data, showing regular multilayer growth in both cases. However, the rapid binding associated with ASAP may lead to a different structure than the one reached after prolonged assembly. Study of the ASAP mechanism suggests that the fast ligand binding kinetics are attributed to a large increase of the local ligand concentration at the moving liquid front when the solvent evaporates on the surface.

Introduction Molecular self-assembly (SA) has been a leading area in science and technology for three decades, encompassing SA in solution as well as at interfaces.1 The latter is of particular interest for device applications and has been the subject of intense research. While self-assembled monolayers (SAMs) are commonly obtained in a single assembly step and provide two-dimensional structures with a certain organization of the molecular components,2 use of molecular SA for obtaining three-dimensional structures on surfaces usually requires more elaborate methods involving multistep procedures. The most widely used multistep scheme is layerby-layer (LbL) SA, where an assembly step is carried out repeatedly to produce a stable multilayer structure with specific interactions between successive monolayers. The latter may involve electrostatic interactions, covalent bonding, metal-organic coordination, hydrogen bonding, π-stacking, or biological interactions.3 Of these, polyelectrolyte LbL assembly based on electrostatic interactions between oppositely charged polymer layers4 has been the most widely studied scheme. LbL assembly of multilayers on surfaces, where monolayers of molecular components possessing ligand groups are held together by metal-ion coordination between adjacent layers, has been *Corresponding authors: e-mail [email protected] (A.V.), [email protected] (I.R.); Tel þ972-8-9342678, þ9728-9342574; Fax þ972-8-9344137.

(1) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (3) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (4) Decher, G. Science 1997, 277, 1232–1237. (5) Haga, M.; Kobayashi, K.; Terada, K. Coord. Chem. Rev. 2007, 251, 2688– 2701.

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demonstrated by several groups.5 The basic scheme usually includes construction of an anchor (ligand) monolayer on the surface, followed by alternate binding of metal ions and multifunctional ligand layers to form a coordination multilayer. Published systems of this kind include Zr(IV) phosphonate,6 Cu(II) mercaptocarboxylate,7 Co(II) 1,4-diisocyanobenzene,8,9 Zr(IV) and Ce(IV) bishydroxamate,10 Cu(II), Ni(II), Fe(III), Ce(IV), and Zr(IV) salene,11,12 Zn(II), Cu(II), and Ni(II) terpyridine,13 Cu(II) dicarboxylate,14 Zn(II) bisquinoline,15 and Zr(IV) terephthalic acid.16 We have concentrated on the formation and study of LbL multilayers based on metal-organic coordination, using various bishydroxamate ligand repeat units and Zr4þ as the coordinated ion (in some cases Ce4þ and Hf4þ were also used10,17). In this (6) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618–620. (7) Evans, S. D.; Ulman, A.; Goppertberarducci, K. E.; Gerenser, L. J. J. Am. Chem. Soc. 1991, 113, 5866–5868. (8) Ansell, M. A.; Cogan, E. B.; Page, C. J. Langmuir 2000, 16, 1172–1179. (9) Ansell, M. A.; Zeppenfeld, A. C.; Yoshimoto, K.; Cogan, E. B.; Page, C. J. Chem. Mater. 1996, 8, 591–594. (10) Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469–13477. (11) Byrd, H.; Holloway, C. E.; Pogue, J.; Kircus, S.; Advincula, R. C.; Knoll, W. Langmuir 2000, 16, 10322–10328. (12) Belghoul, B.; Welterlich, I.; Maier, A.; Toutianoush, A.; Rabindranath, A. R.; Tieke, B. Langmuir 2007, 23, 5062–5069. (13) Maier, A.; Rabindranath, A. R.; Tieke, B. Chem. Mater. 2009, 21, 3668– 3676. (14) Soto, E.; MacDonald, J. C.; Cooper, C. G. F.; McGimpsey, W. G. J. Am. Chem. Soc. 2003, 125, 2838–2839. (15) Thomsen, D. L.; Phely-Bobin, T.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 1998, 120, 6177–6178. (16) Yonezawa, H.; Lee, K. H.; Murase, K.; Sugimura, H. Chem. Lett. 2006, 35, 1392–1393. (17) Doron-Mor, H.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer, A.; Rubinstein, I.; Cohen, H. Nature 2000, 406, 382–385.

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manner multilayer structures comprising bifunctional10,18,19 and branched20 ligand molecules, as well as coordination dendrimers21 and Au nanoparticle multilayers,22 were constructed on Au and oxide23 surfaces. A basic characteristic of LbL construction schemes is the serial nature of the process; namely, the structure is obtained by many repeated steps. The LbL process is therefore inherently timeconsuming, presenting a possible barrier to scale-up and wide application of such systems. This difficulty has been recognized in the case of polyelectrolyte LbL assembly, leading to several suggested solutions for speeding up the process. These include LbL assembly by spray-coating,24,25 dewetting,26 and spin-coating.27-30 The issue of adsorption time of the repeated step is even more acute for LbL multilayers prepared by metal-organic coordination. In general, the time of adsorption is dependent on the type of ligand, the metal ions, and the solvent used. While in a few cases short adsorption times were reported,13,15 in the vast majority of reports formation of each ligand layer takes hours to complete.6-9,11,14 In the case of the Zr(IV)-bishydroxamate system it was shown using kinetic measurements that while binding of the metal ion takes only a few minutes, the organic ligand requires overnight adsorption to achieve a complete bound layer.20 The need to spend days to weeks on the preparation of each multilayer may be prohibitive for various research and application purposes. Here we report a LbL assembly scheme that enables rapid preparation of coordination-based multilayers on surfaces, denoted accelerated self-assembly procedure (ASAP). The method is based on evaporation/dewetting and decreases the time required for assembly of a full organic ligand layer to minutes, thus reducing the multilayer construction time by more than 2 orders of magnitude. The ASAP scheme is demonstrated by the formation of LbL multilayers based on tetra- and hexahydroxamateZr(IV)19,20 and mercaptoundecanoic acid (MUA)-Cu(II)7 systems on gold surfaces. In the new protocol a small volume of a dilute ligand-containing solution is spread on a surface terminated with the metal-ion layer. The solvent is evaporated under natural convection conditions, producing a surface covered with excess ligand. In the case of ethanol as the solvent, complete evaporation occurs within ca. 1 min. Extensive rinsing in pure solvent results in complete removal of the physisorbed molecules from the surface, leaving only the new coordinated layer. The process, combined with alternate metal-ion binding, can be repeated to achieve rapid formation of a multilayer. In the present work the ASAP scheme is introduced (18) Moav, T.; Hatzor, A.; Cohen, H.; Libman, J.; Rubinstein, I.; Shanzer, A. Chem.;Eur. J. 1998, 4, 502–507. (19) Doron-Mor, I.; Cohen, H.; Cohen, S. R.; Popovitz-Biro, R.; Shanzer, A.; Vaskevich, A.; Rubinstein, I. Langmuir 2004, 20, 10727–10733. (20) Wanunu, M.; Vaskevich, A.; Cohen, S. R.; Cohen, H.; Arad-Yellin, R.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 2005, 127, 17877–17887. (21) Wanunu, M.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 2006, 128, 8341–8349. (22) Wanunu, M.; Popovitz-Biro, R.; Cohen, H.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2005, 127, 9207–9215. (23) Wanunu, M.; Livne, S.; Vaskevich, A.; Rubinstein, I. Langmuir 2006, 22, 2130–2135. (24) Schlenoff, J. B.; Dubas, S. T.; Farhat, T. Langmuir 2000, 16, 9968–9969. (25) Izquierdo, A.; Ono, S. S.; Voegel, J. C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558–7567. (26) Shim, B. S.; Podsiadlo, P.; Lilly, D. G.; Agarwal, A.; Leet, J.; Tang, Z.; Ho, S.; Ingle, P.; Paterson, D.; Lu, W.; Kotov, N. A. Nano Lett. 2007, 7, 3266–3273. (27) Lee, S. S.; Hong, J. D.; Kim, C. H.; Kim, K.; Koo, J. P.; Lee, K. B. Macromolecules 2001, 34, 5358–5360. (28) Cho, J.; Char, K.; Hong, J. D.; Lee, K. B. Adv. Mater. 2001, 13, 1076–1078. (29) Chiarelli, P. A.; Johal, M. S.; Casson, J. L.; Roberts, J. B.; Robinson, J. M.; Wang, H. L. Adv. Mater. 2001, 13, 1167–1171. (30) Vertlib, V.; Dietiker, M.; Plotze, M.; Yezek, L.; Spolenak, R.; Puzrin, A. M. J. Mater. Res. 2008, 23, 1026–1035.

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and studied. Multilayers assembled using ASAP are characterized and compared with respective multilayers prepared by the standard procedure, i.e., overnight dipping in the ligand solution.20 It is shown that ASAP effects regular growth of coordinated multilayers in an exceedingly rapid manner.

Experimental Section Chemicals and Materials. Anchor ligand 1 and extension ligands 2 (tetrahydroxamate) and 3 (hexahydroxamate) (Figure 1) were synthesized as previously described. 4,9 11-Mercaptoundecanoic acid (MUA) (95%, Aldrich), Zr(IV) acetylacetonate (Zr(acac)4) (99%, Strem), Cu(ClO4)2 3 6H2O (>98%, Fluka), 3-aminopropyltrimethoxysilane (APTS) (97%, Aldrich), chloroform (BioLab), ethanol (AR, Baker), and methanol (AR, Mallinckrodt) were used as received. Water was triply distilled. The inert gas used was household nitrogen (from liquid N2). Preparation of Au Substrates. Microscope glass cover slides (Menzel-Glaser, No.3) were cut to 22  9 mm2. The glass slides were cleaned by immersion in freshly prepared hot piranha solution (1:3 H2O2:H2SO4) for 1 h followed by rinsing with triply distilled water, rinsing in ethanol three times in an ultrasonic bath (Cole-Parmer 8890), and drying under a stream of nitrogen. (Caution: piranha solution reacts violently with organic materials and should be handled with extreme care.) The gold substrates used in all experiments were prepared by evaporation of a continuous, 100 nm thick Au film onto APTS-treated glass slides.31,32 The silanized slides were mounted in a cryo-HV evaporator (Key High Vacuum) equipped with a Maxtek TM-100 thickness monitor for evaporation of the Au films. Homogeneous deposition was obtained by moderate rotation of the substrate plate. Au (99.99%, Holland-Moran, Israel) was resistively evaporated from a tungsten boat at (1-3)  10-6 Torr and a deposition rate of 0.1 nm s-1. The Au-covered slides were annealed in air 20 h at 220 °C and left to cool to ambient temperature.33 Preparation of Self-Assembled Monolayers (SAMs). Au substrates were pretreated immediately before use by 10 min exposure to UV-ozone (UVOCS model T10x10/OES/E) followed by 20 min dipping in pure ethanol to remove the Au oxide formed.34,35 Anchor SAMs of 1 and MUA were assembled respectively by overnight immersion of the pretreated Au substrates in a 3 mM solution of 1 in 1:1 ethanol-chloroform or in a 1 mM solution of MUA in ethanol, followed by rinsing 5 min in ethanol and drying under N2.21

LbL Preparation of Coordination-Based Multilayers. Following assembly of the first monolayer as described above, coordination-based multilayers were prepared by alternate binding of layers of the metal ions and extension ligands. Binding of Zr4þ and Cu2þ ions was carried out respectively by 30 min immersion in 1.0 mM ethanolic solution of Zr(acac)4 or 10 min immersion in 1.0 mM ethanolic solution of Cu(ClO4)2, followed by rinsing in ethanol and drying under a stream of N2.20,36 Binding of extension ligand layers was accomplished using two procedures: Overnight Adsorption (Standard Procedure). Binding of ligands 2 and 3 (Figure 1) was performed by overnight immersion of the metal-ion-terminated substrate in a 1 mM ethanolic or methanolic solution of 2 and 3, respectively, followed by rinsing 10 min in ethanol and drying under a stream of N2. Extension (31) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735–743. (32) Karakouz, T.; Vaskevich, A.; Rubinstein, I. J. Phys. Chem. B 2008, 112, 14530–14538. (33) Wanunu, M.; Vaskevich, A.; Rubinstein, I. J. Am. Chem. Soc. 2004, 126, 5569–5576. (34) Ron, H.; Rubinstein, I. Langmuir 1994, 10, 4566–4573. (35) Ron, H.; Matlis, S.; Rubinstein, I. Langmuir 1998, 14, 1116–1121. (36) Freeman, T. L.; Evans, S. D.; Ulman, A. Thin Solid Films 1994, 244, 784–788.

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Figure 1. Schematic presentations of ligand molecules 1-3. layers of MUA were prepared by overnight immersion of the metal-ion-terminated substrate in a 1 mM solution of MUA in ethanol. Accelerated Self-Assembly Procedure (ASAP). Unless otherwise specified, all ligand solutions used for ASAP were 0.1 mM ligand in ethanol. The slide was positioned horizontally on a hard surface. A drop of 5 μL ligand solution was carefully placed on the metal-ion-terminated surface using a Hamilton syringe; the solution spreads to cover an area of ca. 1.5 cm2. A process of evaporation, dewetting and formation of small drops, and final drying follows and is completed within ca. 1 min. The substrate was then rinsed 5 min in pure ethanol and dried under a stream of N2. ASAP binding was carried out in an open laboratory atmosphere. Ellipsometry. Ellipsometric measurements were carried out using a Rudolph research Auto EL-IV null ellipsometer, at an angle of incidence of 70° and a wavelength λ = 632.8 nm. Unless otherwise stated, four points were measured on each sample. The same points were measured before and immediately after layer formation, with a typical precision of (0.1° in ψ and Δ. δΔ was obtained by subtracting the value of Δ for the bare gold. The results are presented as δΔ values; using a refractive index of nf = 1.50, kf = 0,20 a decrease of 1° in Δ corresponds to ca. 1 nm in layer thickness. Theoretical Layer Thickness. Molecular length of the ligands with complexed ions, oriented perpendicularly to the surface, was calculated using the software CS Chem3D Ultra 7.0.0. Contact Angle (CA) Measurements. Water CAs were measured within 10 min after layer formation, taking the average of three measurements performed at different spots on the sample. A computerized CA meter (CAM 100, KSV Instruments, Finland) was used; data collection and analysis were carried out using the manufacturer0 s software. The standard deviation was ca. 0.7° for the first layer and ca. 2° for subsequent layers.

Polarization Modulation Fourier-Transform Infrared Reflection-Absorption Spectroscopy (PM-IRRAS). PM-IR-

RAS was performed with a Bruker Tensor 27 þ PMA50 spectrophotometer operating in the reflection mode at an angle of incidence of 85° using a liquid-nitrogen-cooled mercury-cadmiumtelluride (LN-MCT) detector. Spectra were collected at 2 cm-1 resolution, and the measurement time was 10 min (355 scans). X-ray Photoelectron Spectroscopy (XPS). XPS measurements were carried out with a Kratos AXIS-HS system, using a monochromatized Al (KR) X-ray source (hυ = 1486.6 eV) at a takeoff angle of 90°. Reflection Measurements. The reflectivity was measured during ASAP experiments carried out using ethanolic solution of ligand 2 or pure ethanol, producing the same reflectivity results. Reflection measurements were carried out in a homemade dark box using an Ocean Optics reflection probe model R00-7 VIS/ NIR and MobiLight quartz tungsten halogen (QTH) lamp. The data were collected using an Ocean Optics Red Tide 650 diodearray spectrophotometer (spectral range: 380-800 nm) at an Langmuir 2010, 26(10), 7277–7284

angle of incidence of 90° and a wavelength λ = 525.0 nm. The reflection probe was centrally placed ca. 5 mm from the top edge of the slide, at a vertical distance of 8 mm from the planar substrate. The measured spot was ca. 3 mm in diameter. The experiments included placing 5 μL of 0.1 mM ligand 2 solution in ethanol (or a 5 μL drop of pure ethanol, see above) on the surface comprising Au/1/Zr4þ using a Hamilton syringe and following the reflectivity change during the ASAP process, i.e., solution spreading (to ca. 1.5 cm2), ethanol evaporation, dewetting and formation of small drops, and drying. The process was also filmed using a Sony digital video camera recorder VX2100E and synchronized with reflection measurements taken immediately before and after the video. A sample movie with synchronized reflection data is available as Supporting Information. Reflectivity Calculation. Theoretical reflectivity curves for ASAP were calculated using TFCalc Thin Film Design Software for Windows, Version 3.5 (Software Spectra, Inc.). Reflectance values at 525 nm were calculated for decreasing thickness of the applied liquid. The refractive indexes of gold and glass at 525 nm used for the calculation were provided by the simulation program (gold: n = 0.589, k = 2.035; glass: n = 1.457, k = 0). The refractive index of ethanol at 525 nm was taken as 1.36.37 The layer thicknesses used were 300 μm glass, 100 nm gold, and varying ethanol thickness. The calculations were done for pure ethanol, as the experimental reflectivity results for pure ethanol and for ethanolic solution of ligand 2 are similar (see Reflection Measurements section).

Results and Discussion ASAP Formation of Coordination-Based Multilayers. Construction of a coordination multilayer on Au substrate involves three basic steps: (1) functionalization of the surface with a ligand group, accomplished by preparing a self-assembled monolayer (SAM) of anchor molecules comprised of ligands derivatized with a group that binds to the Au surface (e.g., thiol, disulfide, dialkyl sulfide); (2) coordination of the metal ion to the anchor SAM; (3) coordination binding of the repeat unit, i.e., the bifunctional (or multifunctional) ligand entity that may coordinatively bind two (or more) metal ions. To obtain a multilayer, steps 2 and 3 are alternated repeatedly. In the present work initial functionalization of the surface with a ligand group (step 1), which is carried out once, was perform by the standard procedure (overnight adsorption) to ensure the most reproducible results. Metal ion binding (step 2) was also carried out by regular adsorption, as it is known to be completed in a matter of minutes.20 Binding of the repeat unit (step 3), which is the timeintensive element in multilayer construction, was performed by ASAP, and the resulting multilayers were compared with analogous ones where step 3 was carried out by the standard procedure (overnight adsorption). (37) http://refractiveindex.info/.

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Figure 2. Schematic presentation illustrating the various stages of multilayer construction on gold using ASAP. The anchor layer is not shown.

Figure 3. Change of the ellipsometric parameter Δ during construction of four-layer multilayers comprising Au/(MUA/Cu2þ)4. The first point corresponds to bare Au. Results obtained using the standard procedure (squares) and ASAP (circles) are shown.

The ASAP method is depicted schematically in Figure 2. The procedure is carried out as follows: A 5 μL dilute ligand solution is placed on the Au surface using a syringe. The drop spreads rapidly on the surface to cover an area of ca. 1.5 cm2, and the solvent begins to evaporate under natural convection conditions. As shown below, the evaporation process includes thinning of the liquid layer, receding of the liquid, dewetting and formation of small drops, and final drying. The overall evaporation takes ca. 1 min, after which the surface is covered with coordinated ligand as well as unbound ligand. The sample is then rinsed in pure solvent for 5-10 min to remove the noncoordinated ligand and dried under a nitrogen stream. As the first demonstration of the use of ASAP for rapid preparation of a coordinated multilayer we chose the well-known mercaptoundecanoic acid (MUA)-Cu(II) system.7 In this particular case the long-chain mercaptocarboxylic acid molecule serves as both the anchor and the repeat unit. Although the mechanism of MUA multilayer formation is still unclear,38 the system has been known to produce rather reproducible multilayers. Figure 3 shows ellipsometry results for the formation of a fourlayer multilayer of MUA, comparing data obtained using ASAP and the standard (overnight adsorption) procedure. In both cases a rather regular increase in multilayer thickness (expressed as a (38) Daniel, T. A.; Uppili, S.; McCarty, G.; Allara, D. L. Langmuir 2007, 23, 638–648.

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decrease in the ellipsometric parameter Δ) with the addition of ligand layers is seen. The thickness of the ASAP derived multilayer is smaller by ca. 10%-20% than that of the multilayer prepared by the common procedure (see below) and is comparable to literature values.7 The thickness is in agreement with the theoretically calculated value of 1.4 nm per MUA-Cu(II) layer. Coordination multilayers based on anchor ligand 1 and repeated binding of bifunctional ligand 2 or trifunctional ligand 3 via Zr4þ, Ce4þ or Hf4þ ions, have been studied extensively.10,17,19,20,23 Achieving full coverage of the ligand repeat unit using the standard adsorption method requires overnight immersion in the ligand solution, as shown previously.10,20 Figure 4 presents ellipsometry and advancing water contact angle (CA) results for the construction of six-layer coordinated multilayers comprising molecules 2 and 3 with Zr4þ as the bridging ions, comparing ASAP to the standard overnight adsorption. The results for binding of ligand 2 (Figure 4a,c) are similar for the two preparation schemes. The thickness increase with added layers is nearly identical for the two techniques and is similar to previously published values.19 The multilayer thickness is lower by ca. 18% than the theoretical thickness calculated using the values 0.87 and 0.80 nm for complexed, perpendicularly oriented ligands 1 and 2, respectively. The oscillatory behavior of the water CAs is qualitatively similar for the two techniques and comparable to earlier reports.19 To test whether the surface concentration of 2 reaches saturation after a single ASAP step, a SAM of 1 was prepared on the Au substrate, Zr4þ was bound to the anchor layer, and ASAP of 2 was repeated several times without additional Zr4þ binding. No further decrease in the ellipsometric Δ was observed after the first ASAP binding of molecule 2, indicating coordination of a complete layer of 2 in a single ASAP step. The results for binding of ligand 3 show a qualitatively similar behavior of the CAs for the two assembly schemes (Figure 4d), reminiscent of the previous report.20 The ellipsometric thickness (Figure 4b) shows a regular increase with the addition of ligand layers in both schemes. The thickness increment per added layer achieved with the standard procedure is generally similar to the previously published data20 while the thickness obtained using ASAP is consistently smaller by ca. 30%. The measured thickness of the ASAP-constructed film is smaller by ca. 17% than the theoretical thickness calculated using 1.28 nm for perpendicularly oriented ligand 3.20 To understand the different results obtained for ligand 3 by the two assembly methods, the following experiment was carried out: Two similar samples, denoted a and b, were prepared by adsorption of a SAM of 1 followed by binding of Zr4þ, ligand 3 using Langmuir 2010, 26(10), 7277–7284

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Figure 4. Change of the ellipsometric parameter Δ (a, b) and water contact angles (CAs) (c, d) during construction of six-layer multilayers comprising Au/1/(Zr4þ/2)5 (a, c) and Au/1/(Zr4þ/3)5 (b, d) on gold. The first point in (a, b) corresponds to bare Au and in (c, d) to Au/1. Results obtained using the standard procedure (squares) and ASAP (circles) are shown.

ASAP, and Zr4þ. Slide a was then treated with ligand 3 using ASAP again, while slide b was dipped in ligand 3 solution overnight (standard method). The ellipsometric thickness measured for the last step followed the preparation procedure; i.e., slide a showed a smaller thickness increment than slide b, in agreement with the corresponding values in Figure 4b. It has been postulated that the mechanism of molecular selfassembly generally includes two stages; i.e., a rapid initial adsorption of an incomplete, disordered monolayer, followed by a much slower stage in which adsorption of additional molecules requires structural rearrangement of the layer, to reach the final density and thickness.39-41 The experiment described above suggests that the lower ellipsometric thickness obtained for ASAP of ligand 3 is due to inability of the newly formed layer to complete the reorganization and reach equilibrium density and structure during the rapid ASAP. This behavior is also observed for MUA (Figure 3) but not for ligand 2 (Figure 4a). We speculate that multilayers of MUA and ligand 3 prepared by overnight adsorption are more rigid and better organized than respective multilayers of ligand 2 and therefore show lower structural similarity with multilayers obtained by ASAP. Hence, while ASAP provides regular growth of coordination multilayers, the resulting structure largely depends on kinetics and is not necessarily the same as that obtained by the standard procedure. The chemical composition of multilayers prepared by the two construction schemes are similar, as evidenced by FTIR (Figure 5) and XPS (Table 1) measurements of multilayers of 2 and 3. The PM-IRRAS results in Figure 5 show similarity of the spectra for five-layer multilayers prepared by the two assembly methods, in the carbonyl and the fingerprint spectral regions. The XPS data in Table 1 also show good correspondence of the relative atomic concentrations of six-layer multilayers prepared by the two construction schemes. (39) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (40) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978–987. (41) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074– 1087.

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Study of the ASAP Scheme. To elucidate the effect of the ligand concentration in the deposition solution on multilayers prepared by ASAP, samples comprising Au/1/Zr4þ were used as substrates for ASAP of a layer of ligand 2, using varying concentrations of ligand 2 solution, from 1.0 to 0.01 mM. The results presented in Figure 6 indicate that below 0.1 mM the film ellipsometric thickness gradually decays. The decrease in the thickness with concentration may be due to kinetic effects; however, the fact that the behavior shows a threshold transition around 0.1 mM (Figure 6) suggests a different explanation. At 0.1 mM the total amount of molecules in the deposited drop is ca. 3.3  10-10 mol/cm2. This value is of the same order of magnitude as the full surface coverage of common organized monolayers (e.g., for alkanethiols the value is 4.8  10-10 mol/cm2).42,17 Although molecules 2 are more bulky, it seems that the threshold concentration of 0.1 mM is mostly related to the amount of molecules deposited. To further test the latter assumption, the same experiment was carried out using a concentration of 0.02 mM ligand 2, repeating the ASAP deposition several times without adding Zr4þ ions. Figure 7 shows that the coverage increases with the number of times the process was repeated, reaching a constant value close to that obtained by a single ASAP step using g0.1 mM solution of 2 (Figure 6). The maximal coverage is observed after repeating the procedure six times at the low concentration, at which point the number of molecules deposited is close to that obtained by a single deposition of a 0.1 mM solution. In order to monitor the time evolution of the surface coverage during ASAP deposition, the following experiment was carried out: A set of similar slides, each comprising Au/1/Zr4þ, was subjected to an ASAP experiment by placing a 5 μL drop of 0.1 mM ligand 2 solution on the slide. For different slides the adsorption was quenched at different times after deposition of the drop, from 10 to 60 s, by flushing the surface with ethanol, after which the surface was left to dry. The results are presented in Figure 8 as change in the ellipsometric parameter Δ vs time of (42) Finklea, H. O. Electroanal. Chem. 1996, 19, 109-335.

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Figure 5. PM-IRRAS results for five-layer multilayers comprising Au/1/(Zr4þ/2)4 (a, c) and 1/(Zr4þ/3)4 (b, d) on gold, showing the carbonyl region (a, b) and the ether region (c, d). Red and blue lines correspond to preparation by ASAP and by the standard procedure, respectively. Table 1. Relative Atomic Concentrations from XPS Measurements of Six-Layer Multilayers Comprising Au/1/(Zr 4þ/2)5 and Au/1/ (Zr 4þ/3)5 , Constructed by ASAP and by the Standard Method repeat unit

method

C

O

N

S

Zr

ligand 2 ligand 2 ligand 3 ligand 3

standard scheme ASAP standard scheme ASAP

42.7 43.7 38.0 38.4

28.3 28.8 30.4 32.6

7.1 7.5 1.3 1.3

0.7 1.0 5.3 4.3

3.3 3.5 3.1 3.7

Figure 7. Change of the ellipsometric parameter Δ with the number of repeating ASAP experiments using 0.02 mM solution of ligand 2, deposited repeatedly on Au/1/Zr4þ with no addition of Zr4þ.

Figure 6. Change of the ellipsometric parameter Δ for the pre-

paration of a coordinated layer of 2 on Au/1/Zr4þ using ASAP, as a function of ligand concentration in the deposited solution.

adsorption. The error bars in the δΔ values correspond to eight measurements performed at different spots on each slide. The following conclusions can be drawn from the data in Figure 8: (i) The increase in the apparent layer thickness (i.e., surface coverage) shows a general sigmoidal shape, where most of the ligand binding occurs between 30 and 50 s. (ii) The scatter of the ellipsometric data for individual slides is greatest around 40 s. (iii) Full coverage is attained within ca. 60 s. These observations are discussed below. 7282 DOI: 10.1021/la904421n

It may be intuitively assumed that the substantial increase in the ligand binding kinetics compared to the standard procedure is attributed to the increase in ligand concentration as the solvent (ethanol) evaporates. To evaluate this assumption, the evaporation rate of ethanol was measured under conditions similar to an ASAP experiment, i.e., placing a 5 μL drop of 0.1 mM ligand 2 solution on a surface comprising Au/1/Zr4þ. The weight of the ethanol vs time was recorded, and the results are presented in Figure 9 as nominal thickness of the ethanol layer vs evaporation time. Figure 9 shows that after 30 and 40 s the ethanol volume decreases to ca. one-half and one-third of its initial volume, respectively. This simple result rules out the possibility that the 2 orders of magnitude increase in ligand binding kinetics is due to increase in the average ligand concentration during evaporation, as a 3-fold increase in concentration has little or no effect on the binding rate in the standard procedure.20 To elucidate the mechanism and the various stages of the ASAP scheme, as well as the underlying driving force for the large increase in ligand binding rate, a series of reflectivity Langmuir 2010, 26(10), 7277–7284

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Article

Figure 10. Normalized reflectivity as a function of time during an Figure 8. Change of the ellipsometric parameter Δ as a function of time during an ASAP experiment with ligand 2 deposited on Au/1/ Zr4þ. Zero time represents deposition of the drop.

ASAP experiment with ligand 2 deposited on Au/1/Zr4þ. The different stages marked on the figure are identified using the synchronized video movie in the Supporting Information.

Figure 9. Nominal thickness of the liquid layer as a function of time during an ASAP experiment with ligand 2 deposited on Au/1/ Zr4þ. The weight of the liquid was measured vs time and converted to nominal thickness using ethanol density and an area of 1.5 cm2.

measurements were carried out during ASAP experiments using 5 μL drops of 0.1 mM ligand 2 solution deposited on Au/1/Zr4þ, while the process was also video recorded. A typical result is presented in Figure 10, showing the change in reflectivity with time, as well as assignment of the steps in the ASAP process. The different stages were identified using the synchronized video movie (see Supporting Information). The process starts by deposition of the drop and rapid spreading on the surface. This is followed by evaporation and thinning of the liquid layer, showing a characteristic interference pattern of the reflectivity. After ca. 45 s the liquid film undergoes dewetting and formation of small drops, followed by drying of the drops, seen as a stabilization of the reflectivity at a level similar to the initial reflectivity. Figure 11 shows a comparison of the reflectivity measured during the thinning stage (Figure 11a) to a theoretical reflectivity curve calculated as detailed in the Experimental Section (Figure 11b). The time scale in the experimental graph was replaced with an average thickness scale using the data in Figure 9. The theoretical curve in Figure 11b indicates that under the conditions applied in the ASAP experiment the reflectivity during Langmuir 2010, 26(10), 7277–7284

Figure 11. Comparison of the reflectivity measured during the thinning stage of an ASAP experiment as in Figure 10 (a) with a theoretical reflectivity curve calculated as detailed in the Experimental Section (b). The time scale in the experimental graph was converted to average thickness using the data in Figure 9.

the liquid thinning process is expected to show interference fringes of the type seen in Figure 11a. Combining the results in Figure 10 and in Figure 8, it is clear that binding of the new ligand layer occurs mostly during the rapid dewetting and formation of small drops and is completed during drying of the drops. The large standard deviation of the ellipsometric parameter Δ in this stage (Figure 8) is the result of the surface inhomogeneity, comprising dry areas with complete binding of ligand 2 and liquid drops with little binding. DOI: 10.1021/la904421n

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Our conclusion that the rapid ligand binding is associated with dewetting of the initial drop and of the small residual drops suggests that the reaction occurs at the moving metal-liquid-air interface. Solvent drying leads to a very large increase in the local ligand concentration in the vicinity of the moving liquid front, providing the driving force for the fast binding kinetics.

Conclusions Accelerated self-assembly procedure (ASAP) provides an effective means for rapid layer-by-layer (LbL) construction of coordination multilayers. The method is based on evaporation of a ligand solution on the metal-ion-terminated substrate surface under natural convection conditions, followed by rinsing and drying. In this scheme a full organic ligand layer is assembled in about 1 min vs several hours in the common procedure (prolonged immersion). Ellipsometry and contact angle measurements indicate regular growth of a coordination multilayer using the ASAP approach. Study of the different stages of ASAP suggests that the fast ligand binding occurs upon dewetting of the liquid during solvent evaporation and is driven by a large increase in ligand concentration in the vicinity of the moving liquid front. The great advantage of ASAP with respect to the common procedure is a dramatic increase of ca. 2 orders of magnitude in the rate of ligand binding and multilayer formation. However, the high binding rate may lead to a multilayer structure

7284 DOI: 10.1021/la904421n

which is different from that obtained after long adsorption. Therefore, in cases where the detailed organization in the layers is not crucial, ASAP provides an effective and exceedingly fast scheme for the preparation of coordination multilayers. Although not tested here, the quality of multilayers obtained by ASAP may be improved by choosing a slower evaporating solvent, providing more time for molecular organization. The applicability of ASAP to different coordination LbL systems may depend on the characteristics of the specific system, such as the polarity and vapor pressure of the solvent, and has to be tested on a case-by-case basis. Acknowledgment. We are grateful to A. B. Tesler for assisting with the FTIR measurements and to Dr. H. Cohen and Dr. T. Bendikov for performing the XPS analysis. Support of this work by the Israel Science Foundation, Grant No. 672/07, and by the Minerva Foundation with funding from the Federal German Ministry for Education and Research, is gratefully acknowledged. This research is made possible in part by the historic generosity of the Harold Perlman family. Supporting Information Available: Real-time movie showing the different stages of deposition of a ligand layer by ASAP, synchronized with reflection measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(10), 7277–7284