Deformation and Hyperfine Structures of Dendrimers Investigated by Scanning Tunneling Microscopy

Abstract

Scanning tunneling microscopy (STM) is known to provide the highest spatial resolution in real space imaging of materials, and its applications are most common among conductive and semiconductive systems. The high tunneling barrier of insulators diminishes the tunneling probability and thus compromises STM's resolution. This work introduces a simple method to approach this problem, by using STM for high-resolution imaging of insulating materials such as the fourth and fifth generations of poly(amidoamine) hydroxyl-terminated dendrimers. The tunneling barrier is lowered by precoordination with Cu(II) or Pt(II) ions, enabling intramolecular hyperfine features be resolved with 0.2 nm resolution. The spatial distribution, size, and overall number of hyperfine features are consistent with the location of dendrimer termini. The immobilization process deforms dendrimers from the spherical geometry in solution phase to asymmetrical domes in ambient. The ultrahigh vacuum (UHV) environment leads to a higher degree of deformation with reduction of volume. The high-resolution images enable the determination of fundamental parameters of individual dendrimers, including axis, height, asymmetry, and volume. From STM spectroscopy and prior knowledge of dendritic systems, the STM imaging mechanism under UHV is consistent with metal(0) nanoparticles encapsulated by dendrimers, while ambient imaging is most likely via metal-ion-facilitated charge transport. The results from this investigation bring us one step closer toward structural characterization at atomistic level and should enable direct comparison of dendrimer structures with simulations, and deepen our understanding of charge transport in dendrimer systems.

High-Resolution Imaging of Dendrimers Used in Drug Delivery via Scanning Probe Microscopy

“…The STM apparent height, or h APP , is obtained by measuring the height from the lowest point in the immediate surrounding matrix to the top of the dendrimer. These cursors indicate that dendrimers loaded with indomethacin adopt an elliptical dome shape similar to the base dendrimers reported previously [28, 29, 47, 48]. The h APP in cursor profiles 1 and 2 is 0.43 and 0.48 nm, respectively.…”

“…Among 20 G4 PAMAM-OH-(Pt 2+ ) n -(Indo) m dendrimers analyzed, indomethacin features have a h APP range = 0.12–0.25 nm with an average h APP = 0.16 nm. In contrast, intramolecular feature h APP measured on indomethacin-loaded and unloaded metal ion-doped G4 PAMAM-OH dendrimers ranged 0.03–0.10 nm, among 40 dendrimers measured previously [28] and in this study. Using the threshold of 0.12 nm, we were able to assign intramolecular and indomethacin features in the STM images, therefore, to count how many indomethacin each dendrimer could carry.…”

“…Detailed protocols for dendrimers have been discussed previously [28, 29]. For indomethacin carrying dendrimers, first, G4 PAMAM-OH dendrimer solutions were made by diluting aliquots of the methanol-based stock solutions to 12.5  μ M aqueous solutions.…”

Electrochemistry at gold nanoparticles deposited on dendrimers assemblies adsorbed onto gold and platinum surfaces

“…So far, the adsorption of dendrimers on various types of support has been essentially investigated by AFM and scanning tunneling microscope (STM) techniques [27][28][29]. The images obtained from these techniques clearly show that dendrimers underwent deformation while adsorbing on surfaces [28].…”

“Size-Independent” Single-Electron Tunneling

“…6,10,22,32 The capacitance, C = C 1 + C 2 , of the NPs can be calculated from the width of the CB (Δ V ) using Δ V = e / C . 7,33 For the P20 NP shown in Figure 2, Δ V = 1.06 V and C = 0.15 aF.…”