Avenue to Large-Scale Production of Graphene Quantum Dots from High-Purity Graphene Sheets Using Laboratory-Grade Graphite Electrodes

Abstract

Graphene has unprecedented physical, chemical, and electronic properties, but need of the hour is to develop low-dimensional nanomaterials, such as graphene quantum dots (GQDs), that could be incorporated into nanoscale devices. This article depicts the production of GQDs from ultrafine, thin (0.8–1 nm), bilayer graphene sheets (GSs) possessing large micron-sized lateral dimension, low defect density ( I D / I G : 0.1), and oxidation degree (C/O ratio: 27) of lowest level, in contrast to many other techniques where synthesis of GSs was done using analytical-grade expensive graphite electrodes. This low-cost manufacturing of GSs for industrial-scale applications was achieved by utilizing only 99%-purity graphite electrodes. The variants of such graphite electrodes (graphite rod, film, pencil) are etched in different pH electrolytes (H 2 SO 4 , NaCl, NaOH) via prompt electrochemical exfoliation, each giving more than 50% yield. Nowadays, semiconductor quantum dots (QDs) are utilized in smart device production industries, but their toxicity is a major issue of concern. Therefore, the dimension of this two-dimensional (2D) material is reduced to <10 nm to generate GQDs. A facile and highly reproducible approach has been reported for the large-scale generation of GQDs (size ca. 6–10 nm) with minimal surface defects. The protocol followed in this article to synthesize GQDs involves the use of ethylenediamine (en), which passivates the surface and reduces defects, thereby enhancing the optical properties. We demonstrate the correlation of the electrochemical and hydrothermal parameters with the growth mechanism and morphological, structural, chemical, and optical properties of the graphene nanomaterials. Raman spectroscopy and X-ray diffraction (XRD) reveal the structural configurations of GSs and GQDs to investigate the nature of defects. Field emission scanning electron microscopy (FESEM) confirms the morphological characteristics of the as-prepared GSs and GQDs with energy-dispersive X-ray (EDX) analysis determining the C/O ratio. The optical properties like UV–visible absorption and fluorescence assays show the quantum confinement effect phenomenon in GQDs. The obtained GSs and GQDs display enhanced solution stability in DI water and other solvents due to controllable oxidation degree as elucidated through Fourier transform infrared (FTIR) analysis.

Bioanalytical Applications of Graphene Quantum Dots for Circulating Cell-Free Nucleic Acids: A Review

“…This technology has been thoroughly studied due to its low cost, high production and reproducibility, and simple operating principles. To convert functional GQDs with an average size of 3–5 nm from graphene film, Qu et al established an electrochemical method . The synthesized GQDs showed green fluorescence and could last several months in an aqueous solution without losing stability.…”

“…To convert functional GQDs with an average size of 3−5 nm from graphene film, Qu et al established an electrochemical method. 51 The synthesized GQDs showed green fluorescence and could last several months in an aqueous solution without losing stability. Electrochemical synthesis offers a route for the more accurate synthesis of GQDs by selectively oxidizing the precursor material by the applied electric potential.…”

Preparation, Marriage Chemistry and Applications of Graphene Quantum Dots–Nanocellulose Composite: A Brief Review

“…Both approaches use different parameters to produce GQDs [36]. The top-down method involves direct cutting of graphite or graphene-based materials via acid exfoliation [16], sonochemistry [42], solvothermal synthesis [43], electrochemistry [44][45][46][47], or chemical oxidation [48][49][50]. The advantages of these methods are an abundance of inexpensive precursor (graphite), the high graphitic nature and the formation of GQDs with high oxygen-containing functional groups, which renders good solubility and functionality.…”

Graphene Quantum-Dot-Based Nanophotonic Approach for Targeted Detection of Long Noncoding RNAs in Circulation

“…The GQDs are zero-dimensional in nature and comprise small sheets of graphene with lateral sizes of less than 10 nm. 2 , 3 Thus, apart from the properties of graphene, these NPs also exhibit amazing physicochemical properties that include a quantum confinement effect, edge effects, and a nonzero band gap. 4 − 6 In comparison to QDs and other metallic NPs, GQDs offer steady fluorescence properties, improved biocompatibility, and minimal toxicity and thus are considered superior for various biological applications.…”

“…In this regard, the flexible surface characteristics and optical merits of carbon-based quantum dots, particularly graphene quantum dots (GQDs), have proved to be superior. The GQDs are zero-dimensional in nature and comprise small sheets of graphene with lateral sizes of less than 10 nm. , Thus, apart from the properties of graphene, these NPs also exhibit amazing physicochemical properties that include a quantum confinement effect, edge effects, and a nonzero band gap. − In comparison to QDs and other metallic NPs, GQDs offer steady fluorescence properties, improved biocompatibility, and minimal toxicity and thus are considered superior for various biological applications. , The other remarkable optoelectric features of GQDs, such as the excitation-dependent photoluminescence emission, transfer and redox of photo-induced electrons, and size-tunable optical characteristics, further allow flexibility in the selection of the desired wavelength with minimal spectral overlapping. This has also widened the use of these NPs for various multiplexed, highthroughput diagnostic applications. − …”