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A Metal-Ion Coordinated Hybrid Multilayer Anat Hatzor,† Tamar van der Boom-Moav,‡ Shira Yochelis,† Alexander Vaskevich,† Abraham Shanzer,*,‡ and Israel Rubinstein*,† Departments of Materials and Interfaces and Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel Received February 11, 2000. In Final Form: February 28, 2000 Metal-organic coordination is an attractive means for constructing supramolecular systems, providing versatility, simple synthesis, and a defined geometry. The convenience of changing “building blocks” during multilayer assembly is exploited for the fabrication of novel ion-coordinated hybrid multilayers on gold. Two bifunctional linkers are used, a tetrahydroxamate and an organic diphosphonate, while the connection between layers is accomplished through Zr(IV) coordination, to form a well-defined hybrid multilayer. The two ion binders are compatible with respect to multilayer assembly, allowing the change of linkers during construction while maintaining the film structural integrity and organization. The different chemical reactivity of the binders enables rational structural manipulation of the multilayer, by selective dissolution of the acid-sensitive hydroxamate layers while keeping the acid-resistant phosphonates (and underlying hydroxamates) intact. The process demonstrates the multilayer structural quality, where two diphosphonate monolayers are capable of effectively blocking proton penetration to underlying hydroxamate layers. This allows nanometer-scale reshaping of the molecular film according to a scheme introduced during its construction.
Preparation and study of supramolecular assemblies have been a leading direction in the scientific and technological quest to develop molecular-based optoelectronic elements and devices. Accomplishing these goals requires (i) a high degree of sophistication and control of the systems under study and (ii) the development of strategies for the construction and modification of such systems on solid (particularly metal) surfaces. The possibility to rationally manipulate, by chemical means, the structure and properties of supramolecular assemblies, is of particular interest and potentially useful. Various methodologies have been used for the construction of supramolecular structures, including covalent bonding, hydrogen bonding, and coordination bonding. The latter is of particular interest, as it provides substantial versatility, relatively simple synthesis, and a defined geometry. Metal-organic coordination has, therefore, been used extensively to construct supramolecular architectures of defined composition and three-dimensional structure.1,2 Here we show a metal ion coordinated self-assembled multilayer3,4 comprising two different, yet complimentary, organic layers that can be alternated while maintaining multilayer integrity and organization. When assembled in a certain sequence, the hybrid multilayer may be chemically manipulated, namely, upon pH change it undergoes a predetermined structural reshaping that follows the specific construction scheme. The multilayers were prepared in a step-by-step procedure, by alternate adsorption of organic ligand and metal * To whom correspondence should be addressed. Email:
[email protected] or abraham.shanzer@ weizmann.ac.il. † Department of Materials and Interfaces. ‡ Department of Organic Chemistry. (1) Boxter, P. N. W.; Lehn, J.-M.; Kneisel, B. O.; Baum, G.; Fenske, D. Chem. Eur. J. 1999, 5, 113 and references therein. (2) Takeda, N.; Umemoto, K.; Yamaguchi, K.; Fujita, M. Nature 1999, 398, 794 and references therein. (3) Anell, M. A.; Cogan, E. B.; Neff, G. A.; von Roeschlaub, R.; Page, C. J. Supramolec. Sci. 1997, 4, 21-26. (4) Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256-260.
Figure 1. Schematic presentation of hybrid bishydroxamate/ diphosphonate multilayers on gold, before and after HCl treatment. For simplicity, only one diphosphonate layer is drawn. Inset: schematic presentation of the molecular building blocks.
ion layers onto evaporated, {111} textured gold surfaces.5 The base layer was a disulfide-bishydroxamate molecule (1, Figure 1), which forms a densely packed monolayer on (5) Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 1346913477.
10.1021/la0001979 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
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Table 1. Average δ∆ Values Calculated for the Individual Steps in the Assembly of the Three Hybrid Multilayers in Figure 2a
-δ∆ a
tetrahydroxamate on Zr(IV)
Zr(IV) on tetrahydroxamate
diphosphonate on Zr(IV)
Zr(IV) on diphosphonate
0.74 (0.19)
0.57 (0.22)
0.84 (0.10)
0.57 (0.19)
The standard deviation is given in parentheses.
Figure 2. Ellipsometric results measured during hybrid multilayer construction on gold, presented as -δ∆ vs the number of steps. The bars mark the -δ∆ values obtained for the respective diphosphonate layers.
gold upon binding via the disulfide group.6,7 Two organic linkers were used: a tetrahydroxamate ligand (2, Figure 1)5 and a diphosphonate ligand (3, Figure 1),8 both capable of strongly binding tetravalent ions such as Zr(IV), and therefore compatible with respect to multilayer construction. Two kinds of linkages are formed upon multilayer assembly, namely, a Zr(IV)-bishydroxamate complex5,7 and a Zr(IV)-phosphonate bond,8 as shown in Figure 1. Hybrid multilayers comprising both molecules were formed by self-assembly using the procedures described previously,5,7,8 with a skeleton of Zr-tetrahydroxamates. During the construction of each Zr-tetrahydroxamate multilayer a bilayer of Zr-diphosphonates was introduced at a certain depth, i.e., after one (multilayer denoted P2,3), four (P5,6), or seven (P8,9) tetrahydroxamate layers. Following adsorption of two diphosphonate layers, the multilayers were completed by assembly of additional tetrahydroxamate layers. The result was three Zr(IV)bound hybrid multilayers, each comprising a total of 11 ligand layers (including the base layer), out of which two are adjacent diphosphonate layers, introduced at a different depth in each multilayer (Figure 1, left; only one layer of diphosphonates is shown). Ellipsometric measurements carried out after each step in the multilayer construction (Figure 2) show highly regular multilayer growth, indicating excellent compatibility of the two organic linkers. This leads to a practically identical ellipsometric thickness (expressed by δ∆) of the three hybrid multilayers, having diphosphonate bilayers (6) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (7) Moav, T.; Hatzor, A.; Cohen, H.; Libman, J.; Rubinstein, I.; Shanzer, A. Chem. Eur. J. 1998, 4, 502-507. (8) Yang, J. C.; Aoki, K.; Hong, H.-J.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855-11862 and references therein.
Figure 3. Advancing water contact angles of hybrid multilayers on gold, measured during multilayer construction (circles). Triangles indicate the contact angle values measured after 56 h of immersion in 1 M HCl solution.
at different positions. The values given in Table 1 show, as expected, a systematic difference between δ∆ for the binding of Zr(IV) ion and the organic linkers. The average values for Zr(IV) binding are independent of the organic linker, while those for the binding of the two linkers are relatively close. The theoretical thickness of the tetrahydroxamate and diphosphonate units (oriented perpendicularly) is quite similar; since hydroxamate multilayers have been found experimantally to be rather perpendicularly oriented,5 the difference in δ∆ may be reasonably attributed to a difference in the refractive index. Contact angle (CA) measurements (Figure 3) show an oscillatory behavior of the Zr-tetrahydroxamate water CAs during multilayer construction, as previously reported.5,9 Diphosphonate bilayer assembly is evident by a marked increase in the CAs. Upon further addition of tetrahydroxamate layers the existence of the diphos-
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Table 2. XPS Atomic Concentration % for Hybrid Multilayers before and after HCl Treatment atom % multilayer type
Au
C
O
N
Zr
P
Cla
P2,3, before HCl P5,6, before HCl P8,9, before HCl P2,3, 38 h in HCl P5,6, 38 h in HCl P8,9, 38 h in HCl P2,3, 56 h in HCl P5,6, 56 h in HCl
1.2 1.5 1.4 13.2 5.5 14.0 18.8 14.6
45.3 44.2 45.2 43.0 46.6 45.2 47.0 47.2
37.8 37.7 36.8 32.2 33.9 28.3 23.3 26.2
8.6 7.6 6.7 3.6 4.1 4.0 2.9 2.5
5.2 5.7 5.4 5.0 5.4 3.9 4.3 4.3
0.8 2.0 3.6 2.5 3.7 4.1 3.6 4.5
1.1 1.3 1.0 0.6 0.8 0.6 0.1 0.7
a Cl impurities may originate from the Zr(Cl) salt used in 4 multilayer construction.
phonate bilayers is silenced, while tetrahydroxamate regular construction (as indicated by the characteristic CAs) is continued to the desired number of layers. Multilayer composition was also confirmed by X-ray photoelectron spectroscopy (XPS) (Table 2, first three rows). The spectra show all the expected elements at percent atomic concentrations resembling their relative abundance in the multilayers. The phosphorus atomic concentration (0.8, 2.0, and 3.6 for P2,3, P5,6, and P8,9, respectively) is consistent with its position in the multilayer, i.e., the deeper the P atom, the weaker is its signal. The two types of linkages are different in their sensitivity to pH variation: While the Zr(IV)-tetrahydroxamate complexes decompose in acidic media due to competition between protons and Zr(IV) ions, Zr(IV)phosphonates are stable and unaffected by high proton concentration.8 The different acid susceptibilities10 allow rational structural manipulation of the hybrid multilayers. The reshaping process was carried out by immersion of the three completed multilayers in an aqueous 1.0 M HCl solution (Figure 1). The samples were then dipped for 30 min in a 1:1 ethanol/water mixture11 to remove unbound residue. Successful reshaping means complete dissolution of the tetrahydroxamate layers above the diphosphonate bilayer, without further collapse of the system. This requires superior structural integrity and perfect blocking of the diphosphonate bilayer toward acid attack on underlying tetrahydroxamate layers. Upon immersion of the hybrid multilayers in 1 M HCl the thickness decreases with time (Figure 4) as a result of selective tetrahydroxamate decomposition. After 56 h the thickness of multilayers P2,3 and P5,6 reaches the thickness measured during multilayer construction after binding of the diphosphonate bilayers (bars on the right side of Figure 4). For multilayer P8,9 the thickness stabilizes at the diphosphonate level up to ca. 22 h, after which time collapse begins, presumably due to dissolution of the underlying Zr-tetrahydroxamate multilayer resulting from acid penetration through pinholes. CA measurements provide additional evidence for successful completion of the reshaping process. After immersion in 1 M HCl for 56 h, the CAs of the multilayers (9) The CA oscillatory behavior exhibits an intriguing reversal, i.e., in certain sets of measurements the CAs measured after Zr4+ binding are higher than those measured after tetrahydroxamate binding (as in ref 5), while in other sets of measurements the opposite trend is observed (as in Figure 3). The origin of this phenomenon, which may indicate a high sensitivity of the CAs to environmental conditions, is now being investigated. (10) Blank experiments with multilayers comprising only Zrtetrahydroxamates or only Zr-diphosphonates (using 1 as the base layer), immersed in 1 M HCl overnight, show essentially no change in δ∆ for the diphosphonate multilayers, while δ∆ of the tetrahydroxamate multilayers decreases to that of a single monolayer, exhibiting the different susceptibilities to acid treatment. (11) Ethanol was added to prevent gold peeling.
Figure 4. Ellipsometric results for hybrid multilayers on gold, obtained during treatment in 1 M HCl, presented as -δ∆ vs immersion time in the acid solution. The bars mark the -δ∆ values measured for the respective diphosphonate layers during multilayer construction; the bar length corresponds to the values after the second phosphonate layer and after the added Zr4+ ion.
change to those characteristic of a phosphonate surface (triangles in Figure 3; the values suggest exposure of the free phosphonates rather than the salt), indicating the existence of phosphonate layers at the multilayerambient interface following acid treatment. Interestingly, while the ellipsometry of multilayer P8,9 shows a substantial thickness decrease after 56 h in HCl (Figure 4), the CAs suggest an outer diphosphonate layer. This can be explained by the low solubility of Zr-diphosphonates in aqueous solutions; when the acid consumes the inner tetrahydroxamates, diphosphonates might drop and coordinate to lower Zr-tetrahydroxamate layers. This observation is supported by XPS measurements, as described below. XPS results for the reshaping process are summarized in Table 2. After 38 h in HCl the P atomic concentration has increased for all three multilayers. The values (2.5, 3.7, and 4.1 for P2,3, P5,6, and P8,9, respectively) indicate dissolution of the upper tetrahydroxamate layers during the reshaping process. For multilayer P8,9 this observation agrees with the CA measurements, showing the exposure of a phosphonate layer after multilayer collapse. For multilayers P2,3 and P5,6 the P atomic concentration reaches its maximal value only after 56 h, in agreement with the ellipsometric results (Figure 4). The acid effect is also evident in the increase of the gold signal, attributed to partial removal of organic layers. As the diphosphonate layers mostly comprise C atoms, its percentage remains high after HCl treatment. In contrast, the O, N, and Zr signals are reduced, as these atoms are found primarily under the diphosphonate bilayer following HCl treatment. In conclusion, the use of two types of molecular linkers, compatible with respect to multilayer formation, enables construction of metal ion coordinated hybrid multilayers, where the two organic ion-binding building blocks can be conveniently alternated to form desired compositions. This provides a particularly convenient protocol for the formation of metal-organic superlattices, comprising a variety of metal ions and organic linker elements. The difference in the sensitivity to acidic environment12 enables, upon exposure to acidic conditions, reshaping the completed
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multilayer with nanometer vertical resolution according to a scheme introduced during multilayer construction. Further development of the approach described here may present intriguing possibilities, including storage of information or latent images, surface patterning, nanometer-scale protective layers, imprinting of chemical or biological entities for sensing applications, and mimicking pH-activated biological systems13,14 for acid-triggered controlled release. (12) Preliminary experiments indicate that even a single diphosphonate monolayer effectively protects underlying Zr-tetrahydroxamate multilayers from acid attack. This implies that the protection cannot be solely attributed to the different acid susceptibilities of the two types of complexations, but also to the difference in the permeability of the organic layers. Hence, the hydrophobicity of a compact diphosphonate hydrocarbon layer is substantially higher than that of a hydroxamate (multi)layer. When combined with the stability exerted by the different complexation, this results in the exceedingly effective proton exclusion by the diphosphonate layers.
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Acknowledgment. A. Hatzor and T. van der BoomMoav were supported by fellowships from the G.M.J. Schmidt Minerva Center on Supramolecular Architectures. We wish to thank Dr. H. Cohen for assisting with the XPS measurements. Support of this work by the Israel Science Foundation and the Israel Ministry of Science (Tashtiot Infrastructure Program), is gratefully acknowledged. A. Shanzer holds the Siegfried and Irma Ullmann Professional Chair. LA0001979
(13) Skehel, J. J.; Bayley, P. M.; Brown, E. B.; Martin, S. R.; Waterfield, M. D.; White, J. M.; Wilson, I. A.; Wiley: D. C. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 968-972. (14) Ramalho-Santos, J.; Nir, S.; Duzgunes, N.; Carvalho, A. P.; Lima, M. C. P. Biochemistry 1993, 32, 2771-2779.
