Probing the Enzymatic Activity of Individual Biocatalytic fd-Viral Particles by Electrochemical-Atomic Force Microscopy

Scanning Contact Force Microscope-Scanning Electrochemical Microscopy: A New Approach for Tip Positioning and Simultaneous Imaging of Interfacial Topography and Activity

Scanning Electrochemical Microscopy (SECM) offers exceptional spatial and temporal resolution while enabling the detection of localized chemical activity. However, the conventional SECM methodology lacks a robust positioning mechanism, leading to the convolution of electrochemical responses with topographical features. This study presents an innovative SECM positioning feedback mechanism, termed as "Scanning Contact Force Microscopy (SCFM)", leveraging microforce servo control principles to precisely determine the "contact zero point" between the tip and the substrate. The core innovation lies in the integration of a low-stiffness, flexible microbeam with a high-precision capacitive displacement sensor (CDS), significantly minimizing the contact force exerted on the tip. Experimental results demonstrate that SCFM exhibits a normal positioning accuracy of less than 8 nm and a contact force below 20 μN. Morphology detection errors relative to commercial AFM systems were under 2%. Through the integration with SECM, the electrochemical signal is decoupled from the morphology of glassy carbon (GC) substrate with complex structure. Fully compatible with laser-pulled SECM glass electrodes, this technique requires no modifications to electrodes or instrumentation, offering a simple, cost-effective, and versatile solution for advancing nanoscale electrochemical investigations.

Exploring scanning electrochemical probe microscopy in single-entity analysis in biology: Past, present, and future

Scanning Electrochemical Probe Microscopy (SEPM) shows significant potential promise for analyzing localized electrochemical activity at biological interfaces of single entities. Utilizing various SEPM probe manipulations allows real-time monitoring of the morphology and physiological activities of single biological entities, offering vital electrochemical insights into biological processes. This review focuses on the application of five SEPM techniques in imaging single biological entities, highlighting their unique advantages in the observation and quantitative evaluation of biological morphology. Specifically, these techniques not only enable high-resolution imaging of single biological structures but also allow for quantitative analysis of their response behavior. Additionally, the integration of Artificial Intelligence (AI) is discussed to improve data processing and image analysis, potentially advancing SEPM technology towards automation. Although still in an early stage, AI integration opens new avenues for deeper single-entity analysis. This review aims to offer an interdisciplinary perspective and encourage advancements in SEPM-based imaging and analytical techniques, contributing to the bioanalytical field.

Nanobody-guided redox and enzymatic functionalization of icosahedral virus particles for enhanced bioelectrocatalysis

Icosahedral, 30 nm diameter, grapevine fanleaf virus (GFLV) virus particles are adsorbed onto electrodes and used as nanoscaffolds for the assembly of an integrated glucose oxidizing system, comprising the enzyme pyrroloquinoline quinone-glucose dehydrogenase (PQQ-GDH) and ferrocenylated polyethylene glycol chains (Fc-PEG) as a redox co-substrate. Two different GFLV-specific nanobodies, either fused to the enzyme, or chemically conjugated to Fc-PEG, are used for the regio-selective immunodecoration of the viral particles. A comprehensive kinetic characterization of the enzymatic function of the particles, initially decorated with the enzyme alone shows that simple immobilization on the GFLV capsid has no effect on the kinetic scheme of the enzyme, nor on its catalytic activity. However, we find that co-immobilization of the enzyme and the Fc-PEG co-substrate on GFLV does induce enzymatic enhancement, by promoting cooperativity between the two subunits of the homodimeric enzyme, via "synchronization" of their redox state. A decrease in inhibition of the enzyme by its substrate (glucose) is also observed.

Single-molecular protein-based bioelectronics via electronic transport: fundamentals, devices and applications

Biomolecular electronics is a rapidly growing multidisciplinary field that combines biology, nanoscience, and engineering to bridge the two important fields of life sciences and molecular electronics. Proteins are remarkable for their ability to recognize molecules and transport electrons, making the integration of proteins into electronic devices a long sought-after goal and leading to the emergence of the field of protein-based bioelectronics, also known as proteotronics. This field seeks to design and create new biomolecular electronic platforms that allow for the understanding and manipulation of protein-mediated electronic charge transport and related functional applications. In recent decades, there have been numerous reports on protein-based bioelectronics using a variety of nano-gapped electrical devices and techniques at the single molecular level, which are not achievable with conventional ensemble approaches. This review focuses on recent advances in physical electron transport mechanisms, device fabrication methodologies, and various applications in protein-based bioelectronics. We discuss the most recent progress of the single or few protein-bridged electrical junction fabrication strategies, summarise the work on fundamental and functional applications of protein bioelectronics that enable high and dynamic electron transport, and highlight future perspectives and challenges that still need to be addressed. We believe that this specific review will stimulate the interdisciplinary research of topics related to protein-related bioelectronics, and open up new possibilities for single-molecule biophysics and biomedicine.

Redox-labelled electrochemical aptasensors with nanosupported cancer cells

The transfer of redox-labelled bioelectrochemical sensors from proteins to cells is not straightforward because of the cell downward force issue on the surface of the sensors. In this paper, 20-nm-thick nanopillars are introduced to overcome this issue, in a well-controlled manner. We show on both molecular dynamics simulations and experiments that suspending cells a few nanometers above an electrode surface enables redox-labelled tethered DNA aptamer probes to move freely, while remaining at an interaction distance from a target membrane protein, i. e. epithelial cell adhesion molecule (EpCAM), which is typically overexpressed in cancer cells. By this nanopillar configuration, the interaction of aptamer with cancer cells is clearly observable, with 13 cells as the lower limit of detection. Nanoconfinement induced by the gap between the electrode surface and the cell membrane appears to improve the limit of detection and to lower the melting temperature of DNA aptamer hairpins, offering an additional degree of freedom to optimize molecular recognition mechanisms. This novel nanosupported electrochemical DNA cell sensor scheme including Brownian-fluctuating redox species opens new opportunities for the design of all-electrical sensors using redox-labelled probes.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Enzymatic activity of individual bioelectrocatalytic viral nanoparticles: dependence of catalysis on the viral scaffold and its length

The enzymatic activity of tobacco mosaic virus (TMV) nanorod particles decorated with an integrated electro-catalytic system, comprising the quinoprotein glucose-dehydrogenase (PQQ-GDH) enzyme and ferrocenylated PEG chains as redox mediators, is probed at the individual virion scale by atomic force microscopy-scanning electrochemical atomic force microscopy (AFM-SECM). A marked dependence of the catalytic activity on the particle length is observed. This finding can be explained by electron propagation along the viral backbone, resulting from electron exchange between ferrocene moieties, coupled with enzymatic catalysis. Thus, the use of a simple 1D diffusion/reaction model allows the determination of the kinetic parameters of the virus-supported enzyme. Comparative analysis of the catalytic behavior of the Fc-PEG/PQQ-GDH system assembled on two differing viral scaffolds, TMV (this work) and bacteriophage-fd (previous work), reveals two distinct kinetic effects of scaffolding: An enhancement of catalysis that does not depend on the virus type and a modulation of substrate inhibition that depends on the virus type. AFM-SECM detection of the enzymatic activity of a few tens of PQQ-GDH molecules, decorating a 40 nm-long viral domain, is also demonstrated, a record in terms of the lowest number of enzyme molecules interrogated by an electrochemical imaging technique.