Byung-Keun Oh

Biocatalytic conversion of CO/CO2 syngas to formate provides a route for C1 gas valorization, but purified CODH–FDH enzyme cascades require enzyme purification and immobilization steps that increase catalyst-preparation complexity. As a practical alternative, this study developed an ethyl viologen (EV)-mediated intercellular whole-cell cascade by coupling CODH-expressing Escherichia coli with FDH-expressing Methylobacterium extorquens AM1. In this cascade, the two whole-cell biocatalysts performed the oxidative and reductive half-reactions, respectively, and were functionally coupled through EV redox cycling. The native whole-cell cascade enabled syngas-to-formate conversion but showed lower initial productivity than the immobilized enzyme benchmark under low-EV conditions, while maintaining activity and productivity more effectively during multi-day operation. Module-level activity analysis, EV-dependent FDH kinetics, and apparent Damköhler number analysis indicated that the initial productivity penalty was mainly associated with restricted mediator accessibility in AM1 whole cells. Mild Triton X-100 permeabilization produced envelope-associated morphological changes, increased FDH activity at 0.1 mM EV by 7.9-fold, and enhanced cascade productivity by approximately 7-fold while maintaining multi-day productivity retention. In a pH-controlled gas-fed bubble-column reactor, the permeabilized cascade outperformed the native cascade under both low- and high-cell-loading conditions and approached the theoretical CO-transfer upper bound when sufficient catalyst loading was supplied. These results demonstrate that pre-established enzyme-producing hosts can be assembled into a robust EV-mediated intercellular whole-cell cascade, providing a practical catalyst format for syngas-to-formate bioconversion without constructing a single multifunctional strain or relying on purified and immobilized enzyme catalysts.

Efficient biocatalytic CO-to-formate conversion links C1 gas utilization with sustainable chemical production, but carbon monoxide dehydrogenase (CODH)-formate dehydrogenase (FDH) cascades have typically relied on purified and immobilized enzymes, increasing catalyst-preparation complexity and motivating development of a robust, practical biocatalyst format. Here, an ethyl viologen (EV)-wired intercellular whole-cell CO-to-formate cascade was established by coupling CODH-expressing Escherichia coli with FDH-expressing Methylobacterium extorquens AM1. Using this system, a diagnose-relieve-demonstrate workflow was established for synthetic-mediator-wired whole-cell cascades. Although the native EV-wired cascade was feasible, it exhibited a productivity penalty relative to the immobilized benchmark. Module-resolved activity comparisons, EV-dependent FDH kinetics, and Damköhler number analysis showed that this penalty originated predominantly from restricted mediator access in AM1 whole cells, indicating that the FDH side operated in a transport-controlled regime. Guided by this diagnosis, mild Triton X-100 permeabilization partially altered the AM1 envelope while preserving cell morphology, increased FDH activity at 0.1 mM EV by 7.9-fold, and enhanced whole-cell cascade productivity by approximately 7.0-fold relative to the native whole-cell cascade without a detectable penalty in storage or operational stability. Implemented in a lab-scale bubble-column reactor operated under CO sparging and pH control, the permeabilized whole-cell cascade showed 2.7-fold higher volumetric productivity than the native whole-cell cascade and operated close to the CO-transfer upper bound, whereas the native cascade remained transport-limited. Collectively, these results establish synthetic-mediator wiring as a practical route to construct modular intercellular whole-cell redox cascades and identify host- and envelope-level engineering as an effective design handle for relieving mediator-transport limitations in whole-cell gas-conversion bioprocesses.

Recently, gold nanoclusters (AuNCs) have been widely used in biological applications due to their ultrasmall size, ranging within a few nanometers; large specific surface area; easy functionalization; unique fluorescence properties; and excellent conductivity. However, because they are unstable in solution, AuNCs require stabilization by using ligands such as dendrimers, peptides, DNA, and proteins. As a result, the properties of AuNCs and their formation are determined by the ligand, so the selection of the ligand is important. Of the many ligands implemented, enzyme-stabilized gold nanoclusters (enzyme–AuNCs) have attracted increasing attention for biosensor applications because of the excellent optical/electrochemical properties of AuNCs and the highly target-specific reactions of enzymes. In this review, we explore how enzyme–AuNCs are prepared, their properties, and the various types of enzyme–AuNC-based biosensors that use optical and electrochemical detection techniques. Finally, we discuss the current challenges and prospects of enzyme–AuNCs in biosensing applications. We expect this review to provide interdisciplinary knowledge about the application of enzyme–AuNC-based materials within the biomedical and environmental fields.

Lateral Flow Assays (LFAs) are cost‐effective and widely utilized for rapid diagnostics, yet they often suffer from limited sensitivity. This study introduces a straightforward yet highly effective method to enhance LFAs performance by integrating a poly(ethylene glycol) diacrylate (PEGDA) hydrogel to create a “reaction trap.” This hydrogel reaction trap optimizes the flow rate and reaction time at the sensing components, substantially improving assay performance. By applying various hydrogel concentrations (6%, 9%, 12%, 15%, and 18% w/v), significant enhancements across a range of detection systems are achieved. An optimized 18% hydrogel concentration shows a 1.5 times increase in sensitivity in the tested commercial pregnancy test. Additionally, 12% hydrogel concentration is tested in pregnancy, ovulation, and Coronavirus disease 2019 (COVID‐19) commercial kits, and the improvement reached up to a sevenfold increase in signal intensity. The enhancement in detection illustrates the profound impact of this simple modification and shows the importance of hydrogel concentration optimization to maximize detection improvement. These results demonstrate that hydrogel‐coated LFAs offer a scalable and highly effective solution for boosting the reliability and sensitivity of rapid diagnostics across different healthcare settings, with broad potential for global health diagnostics applications.