## 1 Introduction  
Carbon dots were fortuitously discovered in 2004 [1] as an unexpected fluorescent material within arc-discharge soot. Since then, carbon dots have rapidly emerged as a new, fascinating member of the carbon nanomaterial family alongside carbon nanotubes, fullerenes, and graphene. To date, the term \"carbon dots\" has usually referred to various nanosized materials mainly composed of carbon. These materials can be divided into two main categories, i.e., graphene quantum dots (GQDs) and carbon nanodots (CDs) [2]. GQDs can be described as zero-dimensional (0D) nanomaterials possessing lateral dimensions that are larger than their height and composed of a few layers of graphene sheets bearing functional groups at the edges. According to their graphene-like structure, GQDs usually display intrinsic crystal lattices which can be evidenced by HR-TEM characterizations in the range of 0.15 to 0.25 nm [3]. In contrast to GQDs, the core structure of CDs is quasi-spherical nanoparticles composed of an amorphous lattice structure [4]. Graphene quantum dots have been the subject of many published investigations and reviews [5, 6], owing to their wide range of attributes, e.g., excitation-dependent photoluminescence [7], chemical inertness, solubility in various polar solvents, possible further functionalization, resistance to photobleaching, biocompatibility [8], and low cost. GQDs are nanometer-sized particles (<20 nm) consisting of a partial sp\({}^{2}\)-hybridized graphitic core bearing various oxygen- and nitrogen-containing functional groups, where O- and N- atoms are involved in the formation of carbonyl, hydroxyl, epoxy, and amine moieties, mostly located at the edges. The presence of these functionalities allows GQDs to be highly soluble in many organic solvents, thus making their processability much easier than that of most other carbon-based materials [9; 10]. Since their discovery, it has been demonstrated that GQDs have valuable applications in many domains ranging from anti-counterfeiting [11], photocatalysis [12; 13], batteries [14], supercapacitors [15; 16; 17], solar cells [18; 19; 20], LEDs [21; 22], sensors [23; 24], bioimaging [25], and as drug carriers [26]. Graphene quantum dots can be synthesized either by bottom-up or top-down approaches [27; 28]. Bottom-up synthetic strategies relate to physical or chemical treatments of small organic molecules to promote their graphitization. In comparison, top-down approaches refer to the miniaturization of large carbon-based materials (graphite, graphene oxide, carbon nanotubes, or fullerenes) by either hydrothermal or solvothermal cutting [29], microwave-assisted exfoliation [30], electrochemical methods [31], or oxidation [32]. Alternatively, GQDs can be synthesized from biomass-originating raw materials such as biochar [33] or rice husks [34], thus limiting the overall process cost. Nevertheless, typically, the majority of techniques used to prepare GQDs are limited to laboratory-sized operations with long reaction times, the use of high pressure and high temperature, and expensive materials. Also, ultracentrifugation and/or dialysis remain the main techniques to purify GQDs, while being limited to small quantities. While graphene quantum dots are already commercially available from some suppliers around the world, nowadays, only a few dozen milligrams can be accessed at a price of ca. EUR 5000/g, hampering the development of GQD-based applications at industrial scale. High-quality GQDs have to be synthesized through cost-effective, reproducible, and scalable processes in order to facilitate the applications to reach industry. Herein, we report on the synthesis and characterizations of small GQDs (\(\approx\)1-2 nm) that were prepared from various biomass wastes (orange peel, date stones, and oak acorms) in a straightforward, rapid, and high-yielding chemical transformation performed under fast microwave heating.