Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 10. Preparation and Properties of Chemically Sensitive Dendrimer Surfaces

This report is the first account of covalent surface confinement of dendrimers1,2 and the characterization of the resulting monolayer films as chemically sensitive interfaces for chemicalsensing applications. Dendrimer monolayer films3-5 represent a new type of interfacial architecture: voids within the dendrimer superstructure serve as endoreceptors,6-8 and the terminal functional groups of dendrimers serve as exoreceptors.9,10 The vapor-phase dosing experiments discussed here illustrate that this dendrimer architecture responds to vapor-phase probe molecules in a chemically logical way that depends on both the chemical nature of the dendrimer-probe interaction and the dendrimer size. Scheme 1 illustrates our procedure for linking poly(amidoamine) (PAMAM) dendrimers11 to a mercaptoundecanoic acid (MUA) self-assembled monolayer (SAM) Via amide bond formation. To confirm covalent attachment, we obtained FTIRERS spectra of surfaces modified with five different generations (sizes) of PAMAM dendrimers (G0, G2, G4, G6, and G8). Figure 1 shows the amide and carbonyl region of these spectra. There are two important points. First, we do not observe residual infrared absorbance from the MUA carbonyl groups, which would be present in the region between 1720 and 1740 cm-1 (indicated by the arrow in Figure 1), after linking. This indicates complete or nearly complete reaction between all five dendrimers and the MUA surface. Second, the principal infrared-active functionality of PAMAM dendrimers are amides, and the total area of the amide I and II bands increases with increasing dendrimer generation. Further, the inset of Figure 1 shows that both the dendrimer diameters11 and the measured ellipsometric thicknesses12 of the dendrimer monolayers scale linearly with the amide I and II peak area. Since the twodimensional projection of the dendrimer monolayer onto the surface is the same for all five generations, these results demonstrate how the anticipated three-dimensional film structure scales with dendrimer size. Because dendrimers of different size and chemical composition may prove suitable for integration into array-based chemical sensors,13 it is essential that we be able to prepare surfaces ofvarying chemical composition. To provide an illustrative example that such transformations are straightforward, we prepared methyl ester-terminated dendrimers using the Michael addition of methyl acrylate to the primary amine terminal groups of surface-confined dendrimers.14 Following conversion, we observe the characteristic signature of the methyl ester in the carbonyl region of the infrared spectrum (1720-1740 cm-1). Figure 2 shows the relationship between surface reactivity, gauged by the area of this carbonyl band, and the square of the dendrimer radius (rd). The graph illustrates two important points. First, below the threshold at which PAMAM dendrimers adopt a globular geometry (G0 and G2),1,8 the dendrimer films show no reactivity because all or most of the outer functional groups react with the surface during attachment. Second, above the threshold at which PAMAM dendrimers adopt a globular geometry (G4-G8),1,8 the number of dendrimer film reactive * Author to whom correspondence should be addressed. Voice: 409845-5629. Fax: 409-845-1399. E-mail: Crooks@chemvx.tamu.edu. (1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138 and references therein. (2) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638. (3) Alonso, B.; Moran, M.; Casado, C. M.; Lobete, F.; Losada, J.; Cuadrado, I. Chem. Mater. 1995, 7, 1440. (4) Saville, P. M.; Reynolds, P. A.; White, J. W.; Hawker, C. J.; Frechet, J. M. J.; Wooley, K. L.; Penfold, J.; Webster, J. R. P. J. Phys. Chem. 1995, 99, 8283. (5) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (6) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (7) Jansen, J. F. G. A.; Meijer, E. W.; de Brabander-van den Berg, E. M. M. J. Am. Chem. Soc. 1995, 117, 4417. (8) Ottaviani, M. F.; Bossmann, S.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1994, 116, 661. (9) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (10) Turro, C.; Niu, S.; Bossmann, S. H.; Tomalia, D. A.; Turro, N. J. J. Phys. Chem. 1995, 99, 5512. (11) Dendrimers and related information supplied by Dendritech, Inc., Midland, MI. See ref 1. (12) The dendrimer overlayer has a corrugated profile. Thus the average ellipsometric thicknesses of the dendrimer monolayers are smaller than the corresponding bulk-phase dendrimer diameters. Also, we anticipate some flattening of the dendrimer structure after linking. (13) Ricco, A. J. Electrochem. Soc. Interface 1994, 3, 38. (14) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466. Figure 1. FTIR-ERS and ellipsometric results from analysis of five generations of dendrimer films. The inset illustrates how both the average ellipsometric film thickness and dendrimer diameter scale linearly with the total amide peak area. Data in the inset represent a mean of five or more separate experiments.