Self-assembled monolayers of sulfonate-terminated alkanethiols investigated by frequency modulation atomic force microscopy in liquid

Interstitially Mixed Self-Assembled Monolayers Enhance Electrical Stability of Molecular Junctions

Electrical breakdown is a critical problem in electronics. In molecular electronics, it becomes more problematic because ultrathin molecular monolayers have delicate and defective structures and exhibit intrinsically low breakdown voltages, which limit device performances. Here, we show that interstitially mixed self-assembled monolayers (imSAMs) remarkably enhance electrical stability of molecular-scale electronic devices without deteriorating function and reliability. The SAM of the sterically bulky matrix (SC₁₁BIPY rectifier) molecule is diluted with a skinny reinforcement (SC_n) molecule via the new approach, so-called repeated surface exchange of molecules (ReSEM). Combined experiments and simulations reveal that the ReSEM yields imSAMs wherein interstices between the matrix molecules are filled with the reinforcement molecules and leads to significantly enhanced breakdown voltage inaccessible by traditional pure or mixed SAMs. Thanks to this, bias-driven disappearance and inversion of rectification is unprecedentedly observed. Our work may help to overcome the shortcoming of SAM's instability and expand the functionalities.

Toward a Quantitative Relationship between Nanoscale Spatial Organization and Hybridization Kinetics of Surface Immobilized Hairpin DNA Probes

Hybridization of DNA probes immobilized on a solid support is a key process for DNA biosensors and microarrays. Although the surface environment is known to influence the kinetics of DNA hybridization, so far it has not been possible to quantitatively predict how hybridization kinetics is influenced by the complex interactions of the surface environment. Using spatial statistical analysis of probes and hybridized target molecules on a few electrochemical DNA (E-DNA) sensors, functioning through hybridization-induced conformational change of redox-tagged hairpin probes, we developed a phenomenological model that describes how the hybridization rates for single probe molecules are determined by the local environment. The predicted single-molecule rate constants, upon incorporation into numerical simulation, reproduced the overall kinetics of E-DNA sensor surfaces at different probe densities and different degrees of probe clustering. Our study showed that the nanoscale spatial organization is a major factor behind the counterintuitive trends in hybridization kinetics. It also highlights the importance of models that can account for heterogeneity in surface hybridization. The molecular level understanding of hybridization at surfaces and accurate prediction of hybridization kinetics may lead to new opportunities in development of more sensitive and reproducible DNA biosensors and microarrays.

Dynamic Light Scattering Measurements for Soft Materials on Solid Substrates: Employing Evanescent-wave Illumination and Dark-field Collection with a High Numerical Aperture Microscope Objective

We developed an instrument that allows us to measure dynamic light scattering from soft materials on solid substrates by avoiding strong background due to the reflection light from substrates. In the instrument, samples on substrates are illuminated by evanescent-light field and the resultant scattered light from the samples is collected with a dark-field optical configuration by employing a high numerical aperture microscope objective. We applied the instrument to measure the dynamic properties of supported lipid bilayers (SLBs), which have been widely utilized in industries as functional materials such as biosensors. From the time course of the scattered light from the SLBs, the power spectrum with the broad peak ranging from 10 to 20 kHz is observed. The use of the microscope objectives enables us to apply the instrument to future light scattering imaging for dynamic properties of soft materials supported on various substrates by combining with conventional microscope systems.

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.

Direct quantification of cancerous exosomes via surface plasmon resonance with dual gold nanoparticle-assisted signal amplification

Sensitive detection of cancerous exosomes is critical to early diseases diagnosis and prognosis. Herein, a sensitive aptasensor was demonstrated for exosomes detection by surface plasmon resonance (SPR) with dual gold nanoparticle (AuNP)-assisted signal amplification. Dual nanoparticle amplification was achieved by controlled hybridization attachment of AuNPs resulting from electronic coupling between the Au film and AuNPs, as well as coupling effects in plasmonic nanostructures. By blocking the Au film surface with 11-Mercapto-1 -undecanol (MCU), nonspecific adsorption of AuNPs onto the SPR chip surface was suppressed and regeneration of the SPR sensor was realized. This method was highly sensitive and we have achieved the limit of detection (LOD) down to 5 × 10³ exosomes/mL, which showed a 10⁴-fold improvement in LOD compared to commercial ELISA. Moreover, the SPR sensor had the capability to differentiate the exosomes secreted by MCF-7 breast cancer cells and MCF-10A normal breast cells. Furthermore, the SPR sensor could effectively detect the exosomes in 30% fetal bovine serum. The work provides a sensitive and efficient quantification approach to detect cancerous exosomes and offers an avenue toward future diagnosis and comprehensive studies of exosomes.