Plasma Nanoengineering of Bioresource-Derived Graphene Quantum Dots as Ultrasensitive Environmental Nanoprobes

Environmental contamination and energy shortage are among the most critical global issues that require urgent solutions to ensure sustainable ecological balance. Rapid and ultrasensitive monitoring of water quality against pollutant contaminations using a low-cost, easy-to-operate, and environmentally friendly technology is a promising yet not commonly available solution. Here, we demonstrate the effective use of plasma-converted natural bioresources for environmental monitoring. The energy-efficient microplasmas operated at ambient conditions are used to convert diverse bioresources, including fructose, chitosan, citric acid, lignin, cellulose, and starch, into heteroatom-doped graphene quantum dots (GQDs) with controlled structures and functionalities for applications as fluorescence-based environmental nanoprobes. The simple structure of citric acid enables the production of monodispersed 3.6 nm averaged-size GQDs with excitation-independent emissions, while the saccharides including fructose, chitosan, lignin, cellulose, and starch allow the synthesis of GQDs with excitation-dependent emissions due to broader size distribution. Moreover, the presence of heteroatoms such as N and/or S in the chemical structures of chitosan and lignin coupled with the highly reactive species generated by the plasma facilitates the one-step synthesis of N, S-codoped GQDs, which offer selective detection of toxic environmental contaminants with a low limit of detection of 7.4 nM. Our work provides an insight into the rapid and green fabrication of GQDs with tunable emissions from natural resources in a scalable and sustainable manner, which is expected to generate impact in the environmental safety, energy conversion and storage, nanocatalysis, and nanomedicine fields.

Microplasma-Tunable Graphene Quantum Dots for Ultrasensitive and Selective Detection of Cancer and Neurotransmitter Biomarkers

The effective and precise detection of cancer and neurotransmitter biomarkers including folic acid (FA), dopamine (DA), and epinephrine (EP) are essential for early detection and diagnosis of cancer and neurological disorders and for the development of new drugs. However, it remains challenging to detect FA, DA, and EP with high selectivity and sensitivity with a single material. Herein, we report a photoluminescence (PL)-based selective sensing of FA, DA, and EP with nitrogen-doped graphene quantum dots (NGQDs) synthesized from biocompatible chitosan under ambient conditions using atmospheric pressure microplasmas. By regulating the pH, the selective detection is achieved in broad ranges from 0.8 to 80 μM for FA and 0.4 to 100 μM for both DA and EP with the very low limits of detections of 81.7, 57.8, and 16.7 nM for FA, DA, and EP, respectively. The developed PL sensing method shows the high throughput of 5000 detections per hour. Moreover, highly stable colloidal NGQD dispersion with 100 μg/mL concentration for at least 100 PL detections is produced in 1 h by a single microplasma, and the process is scalable. The mechanisms of the outstanding performance are related to the enhanced, size-dependent π-π stacking attraction between the NGQDs and the pH-regulated chemical states of the analytes and the associated pH-specific photo-induced electron transfer and PL.

Microplasmas for Advanced Materials and Devices

Microplasmas are low-temperature plasmas that feature microscale dimensions and a unique high-energy-density and a nonequilibrium reactive environment, which makes them promising for the fabrication of advanced nanomaterials and devices for diverse applications. Here, recent microplasma applications are examined, spanning from high-throughput, printing-technology-compatible synthesis of nanocrystalline particles of common materials types, to water purification and optoelectronic devices. Microplasmas combined with gaseous and/or liquid media at low temperatures and atmospheric pressure open new ways to form advanced functional materials and devices. Specific examples include gas-phase, substrate-free, plasma-liquid, and surface-supported synthesis of metallic, semiconducting, metal oxide, and carbon-based nanomaterials. Representative applications of microplasmas of particular importance to materials science and technology include light sources for multipurpose, efficient VUV/UV light sources for photochemical materials processing and spectroscopic materials analysis, surface disinfection, water purification, active electromagnetic devices based on artificial microplasma optical materials, and other devices and systems including the plasma transistor. The current limitations and future opportunities for microplasma applications in materials related fields are highlighted.