Doping is an efficient way to alter the essential properties of CNDs. By presenting heteroatoms/metal ions doped into CNDs, their electronic structures can be altered, promoting n- or p-type carriers. Therefore, their optical and electronic characteristics are linked to the HOMO–LUMO band gaps and they are able to change by using dopants. Additionally, current progress in research has shown that doping could largely improve the quantum yield of CNDs [40]. Commonly, there are two types of methods that can be used to prepare the CNDs, namely top-down and bottom-up methods. Recent methodologies for the synthesis of doped CNDs are summarized in Table 1. In the top-down process, heavy carbon resources (graphite, coal, ash) are decomposed into CNDs, whereas in the bottom-up process, organic matter is converted and polymerized into CNDs [25].

In the top-down process, sulfuric and nitric (strong) acids are employed to oxidize the carbon bulk materials so the surface of CNDs is surrounded by carboxyl and hydroxyl groups [41,42,43]. The amount of these oxygen-comprising functional groups can be quickly controlled by tuning the pH range of the acids. Moreover, it is very hard to dope heteroatoms/metal ions into CNDs. Additionally, using strong acids may destroy the π-conjugated forms in the CNDs, as it creates low absorption and low-emission wavelengths. Thus, the top-down process is not adaptable for the large-scale preparation of CNDs. However, in the bottom-up process, CNDs are synthesized using the solvothermal method for the organic substances. Commonly, this is an eco-friendly method using non-toxic solvents and produces a high yield compared to the top-down approach; thus, it is fit for bulky synthesis. The surface functional groups on CNDs are majorly dependent on the chemical formula of carbon materials, pH value, pressure, solvents, and temperature. Different kinds of heteroatoms/metal ions can be quickly doped into CNDs [44]. Furthermore, the doping amount and sites are also hard to regularize. The photoluminescence properties of doped CNDs, particularly their emission and absorption wavelengths, are vital for their photodynamic application in cancer therapies. The doped CNDs with strong NIR fluorescence and absorption can be utilized for successful diagnoses and the destruction of tumors in deep tissue [45].

CNDs as 0D carbon group materials have excellent biocompatibility and their lateral dimensions are typically less than 10 nm [64]. The preparation techniques for doped CNDs are entirely dependent on the approach used (top-down or bottom-up), which have been systemically described [64,65]. Apart from these old-style methods for preparing doped CNDs using carbon sources, currently, many eco-friendly green preparation tactics have been utilized. Well-organized heteroatom/metal-doped CNDs with good hydrophilic properties were prepared by an eco-friendly approach using natural polysaccharides as a precursor [66]. In addition, well-formed heteroatom/metal-doped CNDs have been synthesized with exceptional photoluminescence (optical) and biocompatibility (biological) properties. Furthermore, various advanced techniques in the characterization process have provided a strong underpinning for the usage of doped CNDs in the field of biomedical science.

Most reports confirm that heteroatom/metal doping of CNDs could disturb the pristine electronic structure of CNDs. Thus, the PL properties of doped CNDs are linked to the HOMO–LUMO band gaps that have been changed due to the different types of heteroatom/metal-doped CNDs. The high quantum yield in the doped CNDs enhances the PL properties and also introduces some novel physicochemical properties, namely the magnetic resonance imaging (MRI) relativity rate and catalytic performances. In this section, these physicochemical properties will be exposed and debated.

The most popular and attractive property of CNDs is its optical (PL) properties since its clear mechanism is still unpredictable. Many proposed mechanisms for CNDs’ PL are based on their surface effect, quantum confinement effect, and molecule-like state. In CNDs, a smaller particle size leads to quantum confinement, and many researchers have shown that the energy/band gap of the carbon core energy level increases with the decreasing size of CNDs, subsequently in an emission wavelength with the blue shift of CNDs [67,68]. However, the PL properties arise from the surface effect of the CNDs and develop an improved quantum yield, decay lifetime, and emission wavelength. Many functional groups on CNDs’ surface can develop various surface effect energies, which can offer emissive energy levels and create several PL colors [69,70]. Many reported CNDs show blue and green PL emission wavelengths in the range of 400–500 nm; moreover, some red PL emissions have also been observed [71,72,73]. It is important to note that CNDs with good quantum yields and lifetimes are required in cancer theranostics, particularly to enhance image-guided photodynamic therapy [74].

Heteroatom/metal-doped CNDs showed improved PL properties due to the surface defects via the increasing number of energy traps. Due to this effect, the energy gap of CNDs decreases and the non-radioactive electron transfer has less of an impact and consequently leads to the enhancement of the quantum yield of CNDs [37,70]. In heteroatom doping, the N atom is predominantly used as a dopant and causes a red shift in the emission wavelength of CNDs and an increase in the quantum yield. It can be allocated to the surface energy states created by doping the N onto CNDs, developing a radioactive reunion and reducing the chance of the non-radioactive reunion of excited electrons. Moreover, electron-releasing functional groups such as the amino groups can also increase the excited energy states’ stability by improving the pi electrons’ conjugation system in the CNDs skeleton. This can improve the electron transition from the ground state to the excited state so the N atom can contribute to the quantum yield of CNDs.

Nowadays, the S atom is also employed as a dopant to improve the quantum yield of CNDs [75,76]. Zhang et al. [77] investigated the electronic structures of carbon dots (CDs), S-doped CDs, and N, S co-doped CDs to describe the high quantum yield of the red and orange emissions of N, S co-doped CDs. The observed results show that the heteroatoms can create more electronic states and in-gap trap states as recombination points. The heteroatoms (N and S) can quickly bind to carbon owing to their similar atomic radii and the accessibility of the 5 and 6 electrons in their valence band. Thus, N and S doping can alter the band gap of CDs and increase the probability of electronic transition from the singlet ground level to the triplet excited level, concluding in a remarkably high quantum yield of CDs. Similar to the above investigation, Dong et al. [74] analyzed the outcome of N and S co-doped CDs and demonstrated a noteworthy improvement in the quantum yield of S,N co-doped CDs of 73% compared to N-doped CDs with 16.9% and CDs with 5.3%. Moreover, the observed lifetime of N and S co-doped CDs (12.11 ns) was higher than that of non-doped CDs (7.45 ns). The results confirmed that a novel surface energy state formed in CDs through the N-doping method, together with the further co-doping of the S element, can remarkably improve the density of this state. Certainly, the S element can remove the O elements from the surface state and become stable and then improve the surface state of the N element. 

In recent years, many metal ions have been doped to enhance the luminescence properties of CNDs. The metal doping agents can enhance the quantum yield due to the presence of the valence electron and its electron-transferring processes, which aid the radioactive recombination of holes and electrons on the externally modified surface of the CNDs [32,78,79]. Generally, metal-doped CNDs exhibit a solid and broad absorption band between 200 nm and 600 nm. Compared with CNDs, the emission color of metal-doped CNDs is greatly improved in the visible space at a similar level of concentration owing to the possibility of a charge transfer between the metal dopants and graphite [80,81]. Moreover, most of the metal-doped CNDs show emissions with various colors of fluorescence in a 360 nm UV-vis region, such as yellow [82], green [63], blue [83], blue-green [84], and so on. When compared with CNDs, metal-doped CNDs exhibit a red shift in the fluorescence emission owing to their excitation-based luminescence properties and the emission peaks also demonstrate a higher wavelength with the intensity slowdown as the excitation wavelength exhibits a red shift [56,61,85,86,87,88]. Moreover, almost all oxygen-holding functional ligands on the metal-doped CNDs decrease owing to their chemical treatment with the doping metal ions but still show high solubility in aqueous and organic solvents. For instance, Wang et al. [89] showed that the emission fluorescence color of Mn-doped carbon dots changed from blue to yellow as the solvent polarity increased, whereas the emission color of the non-doped CDs was blue with similar solvents and denotes the vital role of Mn doping on CNDs. Heteroatom/metal-doped CNDs can synergistically deliver disease imaging and targeting via PDT, which can be utilized as a new approach for forthcoming cancer theranostics.

Even though the possibilities of using CNDs in nanomedicine are vast, it is essential to study the vital biohazards of CNDs due to the interactions between the CNDs’ biomaterials and the body’s organs [90,91,92]. Basically, once CNDs are offered for a particular biomedical use, they must elevate their adequate bio-functions, which can lead to clean renal clearance without side effects [93,94,95]. Therefore, it is vital to systematically evaluate the toxic nature of heteroatom/metal-doped CNDs from the perspective of toxicology studies. To determine the cytotoxicity in vitro, a few biological chemical pointers, namely apoptosis, DNA damage, and reduced oxidative stress, are commonly utilized to fully demonstrate the biocompatibility of heteroatom/metal-doped CNDs with a cell-line medium. Various intravenous processes to introduce CNDs can effectively alter the physiological system and disturb the pharmacokinetics, which depends on the timeline and outcome of the intake, circulation in the bloodstream, metabolic activity, bio-distribution, renal excretion, and bio-interface interaction of superficially introduced CNDs in an organism [96,97,98].

The quantum yield improvement of doped CNDs is noticeable and important due to the heteroatom/metal doping, which sorts doped CNDs into more convenient biomedical applications. Moreover, the related issue of cytotoxicity caused by the doping of heteroatoms/metals should be observed particularly for their vital role in cancer theranostics applications. Evaluating the cytotoxicity of CNDs through in vitro and in vivo assays to allow for their biomedical applications is necessary for their supplementary advancement. To date, many scientists have assessed the in vitro biocompatibility of different heteroatom/metal-doped CNDs. Commonly, in vitro cytotoxicity models expose cell viability measurements via specific assays namely CCK-8, MTT, and WST-1. Due to their nanosize, which is a major factor affecting their toxicities, CNDs indicate fairly small cytotoxicity in many in vitro models.

Surface-modified CNDs with functional groups, namely –NH₂, –OH, –COOH, and heteroatom/metal-doped CNDs, also show low cytotoxicity. Table 2 shows a summary of the biocompatibility experiments of heteroatom/metal-doped CNDs. Many reports have demonstrated the outstanding cell viability of heteroatom-doped CNDs [99,100,101,102]. For instance, Edison et al. [103] showed that the nitrogen-doped carbon dots have low cytotoxicity on HeLa cells. Xu et al. [104] achieved a 100% cell viability assay with HeLa and HepG2 cells on doped carbon dots after 24 and 48 h. The biocompatibility reports showed that metal-doped CNDs had low toxicity [83,84,105,106,107,108,109,110,111]. For instance, Xu et al. [112] placed an in vivo investigation of Gd-doped carbon dots in liquid form (20 mmol/kg) into mice via a tail vein. After a week of observation, the histological differences in the heart, liver, spleen, kidney, and intestine were analyzed through hematoxylin and eosin staining. In this study, the results showed that there were no significant variations between the control and experiment groups and there were no tissue pathological impairments resulting from the intravenous injection of Gd-doped carbon dots.