Structural Distortion of Dendrimers on Gold Surfaces:  A Tapping-Mode AFM Investigation

Abstract

We report an atomic force microscopy (AFM) investigation of generation 4 and 8 (G4, G8) polyamidoamine (PAMAM) starburst dendrimers adsorbed on Au (111) surfaces. Heights measured for isolated, adsorbed dendrimers indicate they are substantially more oblate than expected from their spherical shapes in solution. By controlling dendrimer concentration and exposure time during adsorption, modified surfaces ranging from isolated molecules to near-monolayer coverages were obtained. Exposure of surfaces bearing adsorbed isolated dendrimers to hexadecanethiol solutions changed their conformation from oblate to prolate as more stable thiol-Au bonds replaced some of the amine-Au bonds. For surfaces of near-monolayer coverage, exposure to hexadecanethiol caused the dendrimers to gradually agglomerate, forming dendrimer "pillars" up to 30-nm high.There is a vast literature pertaining to dendrimers, 1 but only a few reports of individual dendrimer visualization using TEM, 2 STM, 3 and AFM, 4,5 or showing the grainy structure of a dendrimer monolayer using AFM. 6 For our study, dendrimers 7 were adsorbed onto atomically flat Au (111) facets by dipping the substrate 8 into either a 10 -7 M ethanolic solution (for monolayers) or a 10 -9 M solution (for isolated molecules) for 45 s. Au substrates were then alternately rinsed with ethanol and water. To alter the shape of isolated dendrimers, samples were soaked for 4 h in a 1 mM ethanolic solution of hexadecanethiol and then rinsed as described above. To induce agglomeration, dendrimer monolayers were soaked in hexadecanethiol solution for 24-110 h. Measurements made in a minimum of three different areas within each of five well-separated 10 × 10 µm sites on each of two identically prepared Au substrates (30 areas altogether) yielded consistent results. Tapping-mode AFM measurements (topographical data only) in air were performed using a Nanoscope III STM with an E-type scanner. 9 We investigated two different sizes of dendrimers: the soft and deformable G4 (ideal sphere diameter, 4.5 nm) and the larger G8 having a harder exterior (ideal sphere diameter, 9.7 nm). 7 The results for G8 are displayed in parts a-c of Figure 1. Figure 1a shows the topography of the Au surface covered with isolated G8 dendrimers. In the upper right corner, an Au step edge is visible, its height (0.24 nm) providing a vertical reference scale. The measured diameter of the adsorbed G8 dendrimer is approximately 20 nm, but as discussed below, the lateral dimensions are convoluted with the AFM tip shape. Nevertheless, the height data in Figure 1b are reliable. Within experimental error (0.1-0.2 nm) and on the basis of data from more than 100 single dendrimers, the height of the G8 dendrimers on a naked Au surface ranges from 3.5 to 4.0 nm, or about 60% less than the ideal-sphere diameter of 9.7 nm. The variation in height and lateral size of the surface-confined dendrimers may arise from a distribution of molecular sizes resulting from the synthesis, 2b tipinduced deformation of the dendrimers, 6,10-12 ...

PAMAM Dendrimers‐Based DNA Biosensors for Electrochemical Detection of DNA Hybridization

“…Tsukruk et al [28 -30] systematically studied assembled films of dendrimers in monolayers or multilayers on a solid surface. Crooks et al recently published a short communication reporting individual G4 and G8 PAMAM dendrimers adsorbed on Au (111) substrate visualized by AFM [31]. Li et al [32] have also obtained AFM images of PAMAM dendrimers ranging from G5 to G 10 ethylenediamine (EDA).…”

Dendrimers in Nanoscale Confinement: The Interplay between Conformational Change and Nanopore Entrance

“…The simulations, third, provide a reason for the generation-dependent entrance mode in terms of structural flexibility and compressibility of the outermost dendrimer shell that decreases with higher generation. 51 The different degrees of flexibilities are apparent in the computationally derived dendrimers' terminal relative mobilities which represent the size-dependent change in the angle spanned from the dendrimer center by two terminal branches (Supporting Information Figures S-9 and S-10). As a further illustration that rigidity increases with dendrimer size, we show in Supporting Information Figures S-11 and S-12 that the intramolecular potential energy required to push the dendrimer into the pore scales with generation.…”

Hourglass‐Shaped Dendrimers on Surfaces: A Comparison of Different Scanning‐Tunneling‐Microscopy Approaches

“…SPM Studies in a dry environment explored, among others, self-assembled monolayers [8], assembled layer-by-layer films [34], mono-or multilayer films [31], molecular stacks [36], or single molecules [37] [38]. Contrary to SOMs, the number of layers 7 ) in molecular films is difficult to control, and the packing arrangement is hard to characterize.…”