Microplasma Synthesis of Luminescent N-Doped Graphene Quantum Dots for pH Sensing

Abstract

Graphene quantum dots (GQDs), the newest type of carbon nanoparticles (CNPs) with size <20 nm, have attracted many research attentions due to the highly tunable photoluminescence (PL) properties, biocompatibility, chemical and photo stability, and low toxicity. Doping GQDs with heteroatoms, such as nitrogen, can increase the PL quantum yield and has strong electron withdrawing ability. Therefore, nitrogen-doped GQDs as fluorescent pH monitor excels in sensitivity, selectivity, rapidity, and applicability comparing with the conventional electrochemical method, especially for bio-related and medical applications. Good biocompatibility and low toxicity also make N-GQDs more preferable to semiconductor quantum dots, organic fluorescent dyes, and noble-metal nanoparticles. As a matter of fact, pH value plays a very important and crucial role in all life forms. A small variations in pH not only endangers the lives of plants and animal but also human beings as it will mainly pollute our drinking water, which may cause some health issues, like physiological dysfunction or diseases. Furthermore, cancer and tumor cells can also be diagnosed through cellular imaging due to the abnormal pH inside human body.Most of the approaches for synthesizing N-GQDs are complex, time-consuming, in need of harsh reaction condition, and costly. Hereby, microplasma emerges as one of the most promising approaches for nanomaterials processing due to its facile, stable, efficient, and relatively low cost. The high electron density provided are very energetic (~10eV), which allows non-thermal dissociation of molecular gases to form high concentrations of reactive radical species, e.g. OH˙, O˙, O2 -˙, and NO˙. These reactive radical species along with electrons are brought into the precursor solutions and will interact with the solution chemical species to form GQDs in a cascaded chemical reactions.Here we report a microplasma synthesis of N-GQDs for pH sensing. N-GQDs with high quantum yield up to 30% have been synthesized via microplasma approach from chitosan. This synthetic method leads to highly crystalline particles with easily tunable properties in high reproducibility from a simple biomass as the sole precursor. Based on protonation and deprotonation of N-GQDs surface functional groups, a PL based pH probe with broad linear range of 1.78-13.56 and high accuracy can be constructed. Moreover, two linear lines relationship between UV absorption and different pH value are also obtained, covering from pH 1.25-10.17 and pH 10.17-13.24. The dual optical sensing characteristics allow for almost covering the entire pH range and better accuracy. Figure 1