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Introduction
Carbon quantum dots (CQDs) represent a unique carbon-based nanomaterial class that is quasi-spherical and made of amorphous cores, with chiefly graphitic sp2 or sp3 hybridized carbons [1,2]. Their size falls in a range of <10 nm [3]. Carbon dots have the potential for various applications, including biomedicine [4]. The unique characteristics of CQDs have drawn the attention of research in drug delivery and led to many detailed studies on them [5–7]. Recently, the magnetic characteristics of CQDs and their electronic spectral characteristics have raised keen interest [8–10]. Experimental results in this research field are enhancing quickly, leading to several significant breakthroughs in the last decade [11,12].
CQDs were accidentally discovered in 2004 when an attempt to separate single-walled carbon nanotubes was made [13]. Followed by it, the luminescence characteristics were explored. Subsequently, Sun et al [14] reported a synthetic procedure for preparing CQDs with bright luminescence, naming them ’carbon quantum dots’. The frequently employed procedures to prepare CQDs include a top-down approach, with the primary method centred on breaking down the carbonaceous macromolecules into nanoscale particles of a few nm sizes [15]. CQDs are usually obtained in oxidized forms with surface hydroxyl and carboxyl functionalities [16]. Depending on the synthesis method, the oxygen content by weight varies from 5 to 50% [17]. The excellent aqueous solubility of CQDs is a property typical of these materials, which renders them capable of chemical modification and surface functionalization with various groups and ligands [18–20]. It allows the tuning of the CQDs' spectroscopic, physical, chemical and biological properties [21–23]. In comparison with the other forms of carbon nanomaterials, like carbon nanotubes, diamonds, fullerenes and graphene, CQDs are marked by distinguishable isotropic shapes with low dimensions, chemically modifiable groups and multi-functional surface ready for conversion to bring out a wide variety of applications including bio-imaging [24,25], therapeutic delivery [26] and tumour diagnosis [27].
CQDs have also been synthesized with a focus on replacing semiconductor quantum dots owing to their characteristic absorption and tuneable fluorescence properties, which make them applicable in biomedicine [28]. CQDs have low toxicity, polar groups on the surface, photobleaching resistance, chemical stability and appreciable biocompatibility [29]. Nevertheless, doping foreign elements has become increasingly common to increase the novelty of CQDs’ structure and spectral characteristics....