Molecular-scale surface structures of oligo(ethylene glycol)-terminated self-assembled monolayers investigated by frequency modulation atomic force microscopy in aqueous solution

Self-Assembled Monolayers of Gemini-Type Amphiphilic Hexabenzocoronenes on Gold: Contribution of Their Triethylene Glycol Side Chains to Self-Assembly Formation

We report on a two-dimensional self-assembled structure of a supramolecule with hydrophilic oligoethylene glycol (EG) units, which are capable of stronger electrostatic interactions than van der Waals (vdW) interactions between alkyl chains. For this purpose, hexabenzocoronene (HBC) with two hydrophobic dodecyl chains on one side of the HBC core and two hydrophilic triethylene glycol (TEG) chains on the other side of the HBC core (HBCGemini) and HBCGemini with a trinitrofluorenone (TNF) added to the end of one TEG chain (HBCTNFGemini) were employed. Scanning tunneling microscopy (STM) revealed the presence of multiple two-dimensional self-assembled structures in each of HBCGemini and HBCTNFGemini deposited on the gold substrate in vacuum. The role of polar functional groups in these observations is discussed based on semiempirical molecular orbital simulations. Two types of 2D organized structures of HBC-TEG were observed: one with rectangular and relatively dense unit cells and the other with nearly square and relatively sparse unit cells. In both organized structures, the phenyl group TEG units and alkyl chains were considered to be the main molecular interactions with each other. On the other hand, in HBCTNFGemini, three types of organized structures were observed, which could be explained by the mechanism of interdigitation of the TEG-containing side-chain moieties to form a dimeric core. The EG units are more flexible than the alkyl chains and thus can interact flexibly with the hydrophobic HBC core, and the glycol side chains facilitate the intermolecular interactions as well as the alkyl chains.

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at −0.818, −0.861, and −0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).

Local Cross-Coupling Activity of Azide-Hexa(ethylene glycol)-Terminated Self-Assembled Monolayers Investigated by Atomic Force Microscopy

Azide-oligo(ethylene glycol)-terminated self-assembled monolayers (N₃-OEG-SAMs) are promising interfacial structures for surface functionalization. Its many potential applications include chemical/bio-sensing and construction of surface models owing to its cross-coupling activity that originates from the azide group and oligo(ethylene glycol) (OEG) units for non-specific adsorption resistance. However, there are only a few studies and limited information, particularly on the molecular-scale structures and local cross-coupling activities of N₃-OEG-SAMs, which are vital to understanding its surface properties and interfacial molecular design. In this study, molecular-scale surface structures and cross-coupling activity of azide-hexa(ethylene glycol)-terminated SAMs (N₃-EG₆-SAMs) were investigated using frequency modulation atomic force microscopy (FM-AFM) in liquid. The N₃-EG₆-SAMs were prepared on Au(111) substrates through the self-assembly of 11-azido-hexa(ethylene glycol)-undecane-1-thiol (N₃-EG₆-C₁₁-HS) molecules obtained from a liquid phase. Subnanometer-resolution surface structures were visualized in an aqueous solution using a laboratory-built FM-AFM instrument. The results show a well-ordered molecular arrangement in the N₃-EG₆-SAM and its clean surfaces originating from the adsorption resistance property of the terminal EG₆ units. Surface functionalization by the cross-coupling reaction of copper(I)-catalyzed azide-alkyne cycloaddition was observed, indicating a structural change in the form of fluctuating structures and island-shaped structures depending on the concentration of the alkyne molecules. The FM-AFM imaging enabled to provide information on the relationship between the surface structures and cross-coupling activity. These findings provide molecular-scale information on the functionalization of the N₃-EG₆-SAMs, which is helpful for the interfacial molecular design based on alkanethiol SAMs in many applications.

Dendronized oligoethylene glycols with phosphonate tweezers for cell-repellent coating of oxide surfaces: coarse-scale and nanoscopic interfacial forces

Coarse-scale and nanoscopic interfacial force measurements unraveled how dendronized oligoethylene glycols with phosphonate tweezers prevent non-specific cell adhesion to oxide surfaces.

Improvements in fundamental performance of in-liquid frequency modulation atomic force microscopy

In-liquid frequency modulation atomic force microscopy (FM-AFM) has been used for visualizing subnanometer-scale surface structures of minerals, organic thin films and biological systems. In addition, three-dimensional atomic force microscopy (3D-AFM) has been developed by combining it with a three-dimensional (3D) tip scanning method. This method enabled the visualization of 3D distributions of water (i.e. hydration structures) and flexible molecular chains at subnanometer-scale resolution. While these applications highlighted the unique capabilities of FM-AFM, its force resolution, speed and stability are not necessarily at a satisfactory level for practical applications. Recently, there have been significant advancements in these fundamental performances. The force resolution was dramatically improved by using a small cantilever, which enabled the imaging of a 3D hydration structure even in pure water and made it possible to directly compare experimental results with simulated ones. In addition, the improved force resolution allowed the enhancement of imaging speed without compromising spatial resolution. To achieve this goal, efforts have been made for improving bandwidth, resonance frequency and/or latency of various components, including a high-speed phase-locked loop (PLL) circuit. With these improvements, now atomic-resolution in-liquid FM-AFM imaging can be performed at ∼1 s/frame. Furthermore, a Si-coating method was found to improve stability and reproducibility of atomic-resolution imaging owing to formation of a stable hydration structure on a tip apex. These improvements have opened up new possibilities of atomic-scale studies on solid-liquid interfacial phenomena by in-liquid FM-AFM.

Single Molecule Profiling of Molecular Recognition at a Model Electrochemical Biosensor

The spatial arrangement of target and probe molecules on the biosensor is a key aspect of the biointerface structure that ultimately determines the properties of interfacial molecular recognition and the performance of the biosensor. However, the spatial patterns of single molecules on practical biosensors have been unknown, making it difficult to rationally engineer biosensors. Here, we have used high-resolution atomic force microscopy to map closely spaced individual probes as well as discrete hybridization events on a functioning electrochemical DNA sensor surface. We also applied spatial statistical methods to characterize the spatial patterns at the single molecule level. We observed the emergence of heterogeneous spatiotemporal patterns of surface hybridization of hairpin probes. The clustering of target capture suggests that hybridization may be enhanced by proximity of probes and targets that are about 10 nm away. The unexpected enhancement was rationalized by the complex interplay between the nanoscale spatial organization of probe molecules, the conformational changes of the probe molecules, and target binding. Such molecular level knowledge may allow one to tailor the spatial patterns of the biosensor surfaces to improve the sensitivity and reproducibility.

Combined Experimental and Simulation Study of Amplitude Modulation Atomic Force Microscopy Measurements of Self-Assembled Monolayers in Water

Atomic force microscopy (AFM) can be used to measure surface properties at the nanoscale. However, interpretation of measurements from amplitude modulation AFM (AM-AFM) in liquid is not straightforward due to the interactions between the AFM tip, the surface being imaged, and the water. In this work, amplitude-distance measurements and molecular dynamics simulations of AM-AFM were employed to study the effect of surface chemistry on the amplitude of tip oscillation in water. The sample surfaces consisted of self-assembled monolayers where the hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. Analysis showed that surface chemical composition influences the hydration structure near the interface which affects the forces experienced by the tip and in turn changes the amplitude profile. This observation could aid our understanding of AM-AFM measurements of interfacial phenomena on various surfaces in water.

Molecular-scale structures of the surface and hydration shell of bioinert mixed-charged self-assembled monolayers investigated by frequency modulation atomic force microscopy

Water molecules above a bioinert mixed-charged self-assembled monolayer (MC-SAM) surface are highly structured compared to those of bioactive SAM surfaces.

Properties of modified surface for biosensing interface

Properties of modified surface, behavior against salting-out effect, suppressive effect for protein nonspecific adsorption, and wettability were examined using various mercapto compounds bearing methyloligoethylene glycol, oligoethylene glycol, alkyl oligoethylene glycol, alkyl phosphoryl choline, alkyl inverse phosphoryl choline, and alkyl sulfobetaine moieties. The behavior against salting-out effect was examined using gold nanoparticle with PBS and NaCl aqueous solution. The suppressive effect for protein nonspecific adsorption was evaluated by SPR, and the wettability was measured on the SPR chip. The gold nanoparticle modified with 8C3EG, 12C4EG, 12CPC, 6CCP, and 12CCP showed excellent behavior against salting-out effect. The suppression of protein nonspecific adsorption was effective with 6EG, 12C4EG, 12CPC, and 12CS. On the other hand, the modified surface possessed high wettability except for the surface modified with M6EG. The results indicate that incorporation of alkyl group into surface modification materials is effective for the enhancement of behavior against salting-out effect and suppressive effect for protein nonspecific adsorption regardless of wettability. Among the zwitter ionic derivatives, inverse phosphoryl choline derivatives showed intriguing properties, high behavior against salting-out effect with high wettability but low suppressive effect for protein nonspecific adsorption.

Fabrication of electron beam deposited tip for atomic-scale atomic force microscopy in liquid

Recently, possibilities of improving operation speed and force sensitivity in atomic-scale atomic force microscopy (AFM) in liquid using a small cantilever with an electron beam deposited (EBD) tip have been intensively explored. However, the structure and properties of an EBD tip suitable for such an application have not been well-understood and hence its fabrication process has not been established. In this study, we perform atomic-scale AFM measurements with a small cantilever and clarify two major problems: contaminations from a cantilever and tip surface, and insufficient mechanical strength of an EBD tip having a high aspect ratio. To solve these problems, here we propose a fabrication process of an EBD tip, where we attach a 2 μm silica bead at the cantilever end and fabricate a 500-700 nm EBD tip on the bead. The bead height ensures sufficient cantilever-sample distance and enables to suppress long-range interaction between them even with a short EBD tip having high mechanical strength. After the tip fabrication, we coat the whole cantilever and tip surface with Si (30 nm) to prevent the generation of contamination. We perform atomic-scale AFM imaging and hydration force measurements at a mica-water interface using the fabricated tip and demonstrate its applicability to such an atomic-scale application. With a repeated use of the proposed process, we can reuse a small cantilever for atomic-scale measurements for several times. Therefore, the proposed method solves the two major problems and enables the practical use of a small cantilever in atomic-scale studies on various solid-liquid interfacial phenomena.

Spectroscopic ellipsometry meets AFM nanolithography: about hydration of bio-inert oligo(ethylene glycol)-terminated self assembled monolayers on gold

An accurate thickness determination provides insight into the complex vertical morphology of OEG-terminated SAMs.