Recently, carbon-based nanomaterials such as graphene [1], fullerenes [2], nanodiamonds [3], carbon nanotubes (CNTs) [4], and carbon quantum dots (CQDs) have attracted great attention because of their distinctive structural dimensions, as well as their outstanding chemical and physical properties [5]. It was found that the preparation and separation of nanodiamonds are complicated, while other nanomaterials such as graphene, fullerenes, and CNTs do not display good water solubility and also do not exhibit strong fluorescence in the visible region. These limitations prevent their applications in different areas [6]. Although semiconductor quantum dots (SQDs) exhibit good fluorescence properties, because of the presence of heavy metals, they are toxic in nature. This prevents their biological applicationin biosensors, bio-imaging, and drug delivery. In contrast, fluorescent CQDs are nontoxic and, thus, have attracted enormous interest over other carbon-based nanomaterials [7]. Xu et al. in 2004 accidentally discovered CQDs using gel electrophoresis during the purification of single-walled carbon nanotubes [8]. However, the name CQDs was given by Sun et al. in 2006 during the synthesis of carbon nanomaterials of different sizes [9]. Subsequently, CQDs became rising stars among various carbon-based nanoparticles and are considered an extremely precious asset of nanotechnology. CQDs are also known as carbon nano-lights because of their strong luminescence properties [10]. CQDs have attractive features such as ease of synthesis, good water solubility, high photostability, high photoresponse, low cytotoxicity, facile surface functionalization, good catalysis properties, and tunable excitation–emission ^{[11][12][13][14][15][16][17]}[11,12,13,14,15,16,17]. Due to these characteristic properties, CQDs are widely utilized in photovoltaic devices, medical diagnosis, sensing, drug delivery, catalysts, photocatalysis, optronic devices, bio-imaging, laser, single electron transistors, solar cells, and LEDs ^{[18][19][20][21][22][23][24][25][26][27][28][29]}[18,19,20,21,22,23,24,25,26,27,28,29]. However, very few reports have been investigated regarding the application of CQDs as a catalyst in photochemical water splitting [30] the preparation of substituted 4H pyran with indole moieties [31], azide-alkyne cycloadditions [32], the degradation of levofloxacin [33], the selective oxidation of alcohols to aldehydes [34], the removal of Rhodamine B [35], the selective oxidation of amines and imine [36], high-efficiency cyclohexane oxidation [37], H-bond catalysis in Aldol condensations [38], intrinsic peroxidase-mimetic enzyme activity [39], and the ring opening of epoxides [40].

2. Synthesis Approach

Since the discovery of carbon quantum dots (CQDs), several convenient, cost-effective, size-controlled, and large-scale production approaches have been developed. For the synthesis of CQDs, two general categories, top-down and bottom-up, approaches are utilized (Figure 1). Even though CQDs synthesis is facile, there are definite challenges related to their synthesis, such as an aggregation of nanomaterials, the tuning of surface properties, and controlling the size and uniformity [41]. To adjust the functional groups present on the surface and achieve better CQDs performance, post-treatment can be conducted in both approaches. Quantum yields (QYs) of CQDs can be enhanced after surface passivation, which eliminates the emissive traps from the surface. CQDs doped with heteroatoms (N and P) or metals such as Au or Mg improve solubility and electrical conductivity [42]. Even though for the synthesis of CQDs, both the top-down and bottom-up approaches have been used, the environmentally and cost-effective bottom-up approach is most commonly used [43].

2.1. Top-Down Approach

In a top-down approach, the larger carbon resources such as carbon nanotubes, fullerene, graphite, graphene, carbon soot, activated carbon, etc., are broken down into smaller constituents with the help of different techniques such as laser ablation and electrochemical and arch discharge ^{[28][44][45][46][47]}[28,44,45,46,47]. Carbon structures with sp² hybridization that lack efficient energy gaps or band gaps are commonly used as starting materials for top-down processes. Although the top-down approach is extremely helpful and suitable for microsystem industries, it has some limitations, such as the fact that pure nanomaterials cannot be obtained from the large carbon precursor; their purification is costly and also unable to accurately control the morphology and size distribution of CQDs [48].

2.2. Bottom-Up Approach

In a bottom-up approach, the smaller carbon resources such as amino acids, polymers, carbohydrates, and waste materials combine to form CQDs by a variety of techniques such as hydrothermal/solvothermal, combustion, pyrolysis, and microwave irradiation. In this method, the size and structure of CQDs depend on a variety of factors such as solvent, precursor molecular structures, and conditions of the reaction (temperature, pressure, reaction time, etc.). The conditions of the reaction are necessary, since they influence the reactants and the extremely casual nucleation and escalation procedure of CQDs. This approach strengthens the material chemistry because of its ease of operation, lower cost, and easier implementation for production in a large scale ^[49][61]. The precursor used for the synthesis of CQDs may be both chemical and biological, i.e., natural. The chemical precursors include glucose, sucrose, citric acid, lactic acid, ascorbic acid, glycerol, ethylene glycol, etc. ^{[50][51][52][53][54][55][56]}[62,63,64,65,66,67,68]. The natural sources include Artocarpous lakoocha seeds, rice husks, Azadirachta indica leaves, pomelo peel, the latex of Ficus benghalensis, aloe vera, etc. (Figure 2) ^{[57][58][59][60]}[69,70,71,72].