(Invited) From Plasma-Functionalized CNTs to Photonics and Plasma-Engineered 2D pH Sensor Platforms: Evolution of Surface-Engineered Solid-State Biosensing

Abstract

As biosensing platforms based on solid-state electronics continue to evolve toward higher sensitivity, selectivity, and integration density, material-level innovation and surface functionalization have emerged as defining technological requirements. Until the early 2020s, carbon nanotubes (CNTs), graphene, nanomaterials, and metal nanoparticles have been extensively explored as promising signal and enzyme transducers due to their excellent electrical conductivity and large specific surface area. However, for practical biosensing applications, room-temperature process strategies that simultaneously enhance surface reactivity and enzyme loading while preserving or improving electrical transport properties remain insufficiently developed. In particular, CNT functionalization has often relied on high-temperature thermal treatments or high-energy chemical processes, resulting in limited efficiency relative to resource input and inherent constraints in improving loading capacity and sensing sensitivity. Motivated by these limitations, our work began with the development of room-temperature CNT surface functionalization using low-power plasma processes compatible with semiconductor fabrication. By employing damage-free remote SF 6 plasma treatment, semi-permanent semi-ionic C–F bonds (physically adsorbed fluorine) were introduced onto CNT surfaces without degrading intrinsic conduction pathways. This approach enabled simultaneous enhancement of surface chemical reactivity and charge-transfer characteristics, without resorting to conventional high-temperature, high-energy, or acid-based treatments. As a result, plasma-functionalized CNTs exhibited a threefold increase in sensing sensitivity, a twofold reduction in charge-transfer resistance, and more than a sixfold enhancement in glucose oxidase (GOx) loading compared to pristine CNTs. These results quantitatively demonstrate how surface chemistry can alleviate charge-transport bottlenecks and how such correlations directly translate into amplified electrical signals in solid-state biosensors. This surface chemistry–charge transport correlation–based strategy is not limited to CNTs and instead provides a conceptual foundation that can be naturally extended to two-dimensional (2D) materials, where more precise control of electronic and interfacial properties is possible. Owing to their atomic-scale thickness, high carrier mobility, and electrical and mechanical properties surpassing those of silicon, 2D materials (transition metal dichalcogenides (TMDs), graphene, boron nitride, etc.) have emerged as strong candidates for next-generation solid-state bio- and chemical sensing platforms. When combined with low-temperature, maskless laser and plasma-based surface engineering, these materials enable sensing functionalities that are intrinsically embedded within the channel. In this context, laser-induced local oxidation and doping profile formation in TMDs such as WSe 2 produce pronounced electrical modulations, including threshold voltage (V TH ) shifts, enhanced ambipolar behavior, and increased interfacial state density, thereby creating active domains that are highly sensitive to external chemical stimuli. In contrast, plasma treatments using nitrogen, oxygen, argon, or SF 6 introduce distinct defect states, adsorption sites, and charge-trapping characteristics, enabling process-dependent surface reactivity patterns even within a single 2D channel. Device-level validation of these surface-engineered domains was achieved through droplet-based pH sensing experiments using sub-microliter volumes on fully fabricated field-effect transistor devices. Laser-oxidized regions exhibited clear and directional V TH shifts and drain current modulation in response to pH variations, whereas plasma-treated regions showed electrical responses with different magnitudes and polarities depending on the induced defect and adsorption characteristics. These observations directly reflect the distinct surface chemistry and interfacial charge dynamics introduced by each process and further indicate that sensitivity, linearity, and response speed can be selectively optimized through laser–plasma process combinations. By demonstrating reproducible electrical responses of laser-oxidized and plasma-functionalized 2D channels to external chemical stimuli such as pH and basic solutions, this research highlights a new direction for solid-state bio- and chemical sensing platforms enabled by the convergence of solid-state electronics with photonic and plasma-based surface engineering.