The Doping of nonmetals in CDS can affect the overall electrical distribution and the relevant levels of electronic energy in the nonmetal-doped CDs
[81]. CDs can be tuned to exhibit their desired fluorescent and other properties by doping foreign atoms
[81]. In order to improve the properties of CDs for further applications, various nonmetals such as boron
[82], nitrogen
[83], phosphorous
[84], sulfur
[85], silicon
[86], and halogens (F, Cl)
[87] have been doped into the CDs, using either the “top-down” or “bottom-up” approaches. B, N, and P- doped CDs were recently synthesized by Kumar et al., using hydrothermal and sonochemical methods
[45]. They synthesized N-doped carbon quantum dots (NCDs) from bovine serum albumin protein in water via a hydrothermal chemical reaction
[15]. The involved the steps of fragmentation, aromatization, polymerization, and carbonization
[88]. During the hydrothermal reaction (>175 °C) carbonization of the BSA polymer occurs leading to the formation of a final NCDs material with a complicated surface structure
[88]. The same hydrothermal method was used to synthesize boron-doped and phosphorus-doped CDs
[45]. A temperature-dependent mechanism for the synthesis of N-doped CDs has been proposed. These NCDs were used for doping/functionalization of hydroxyapatite (NCDs-HA) nanomaterials in water by stirring of hydroxyapatite (HA) with NCDs at room temperature
[89]. A study presents a schematic illustration of doping NCDs with the HA particles. NCDs were used as the stabilizer in three different compositions of HA particles accommodate 1% to 5% NCDs
[89]. Adding P and N did not improve the fluorescence properties of N/P-co-doped CDs, although their aqueous dispersibility increases significantly
[90]. The purpose of S-doping in CDs is typically to enhance metal ion (Fe
3+ ions, other heavy metal ion) detection sensitivity by 0–500 µM
[91]. However, if the S atom is fixed to the ring of polythiophene, strong red-emitting light can be observed when the excitation wavelength is 543 nm
[92]. Further, the B/ N/S-co-doped CDs demonstrate high-efficiency red emission at wavelength near 600 nm
[92]. Even though a variety of nonmetals, such as N, P, S, and B, have been reported, there is still a strong demand for the development of highly efficient CDs in term of real application for the society
[92]. Therefore, further exploration of other nonmetal doping systems is highly desirable. For example, although fluorine is not present in biological systems, the incorporation of F-containing grafts can increase the therapeutic efficacy of many drugs, improve the chemical stability of proteins, and enhance phase separation in both polar and nonpolar conditions
[93]. Thus, by modifying CDs with the F, it may be possible to significantly tune their fluorescence properties including wavelengths and behavior in biological systems or biomedical applications
[93]. Zuo et al., studied the F-doping strategy to dramatically lengthen the emission wavelength of CDs
[94]. Using 1,2-diamino-4,5-difluorobenzene as the fluorine source and tartaric acid to improve aqueous solubility, they synthesized a kind of F-doped CD (F-CD) in a one-pot solvothermal process
[94]. When compared with undoped CDs, the emission wavelength of F-CDs is redshifted by 50 nm
[63][94][95][96]. Using an excitation wavelength of 480 nm, the F-CD emits intense yellow fluorescence, while the emitted red light is found upon excitation at 540 nm
[94]. Additionally, the F-CDs provide highly sensitive detection of intracellular Ag+ as well as high imaging sensitivity of red blood cells in various cell systems
[94]. The hydrothermal method was by Kalaiyarasan to synthesize phosphorus-doped CQDs (PCQDs) from phosphoric acid and Trisodium citrate (TSC)
[97]. They investigated the effect of the reaction temperature, durationand precursor concentrations on the formation of PCQDs and the doping effect of phosphorus in CDs. and found that fluorometric probes for iron detection. Moreover, the P-CDs with high fluorescence QY have been examined for changes in the fluorescence intensities induced by the addition of Fe
3+. These changes are a consequence of the formation of a stable fluorescent inactive complex (P-CDs-Fe
3+), which may be useful for biomedical applications (Such as live cell imaging, Fe
3+ detection in blood, urine)
[97]. In addition, a highly sensitive fluorescent probe based on sulfur-doped carbon dots (S-CDs) was synthesized by Kamali et al., using a microwave irradiation method
[36]. Using this probe, a strong blue emission was observed with 36% QY
[36]. This class of nonmetal-doped CDs (B, N, P, S, F, etc) or co-doping two/three nonmetals was utilized as an effective fluorescent materials for live cell imaging owing to their excellent QY, good water dispersibility, tunable strong fluorescence, better biocompatibility and low cytotoxicity.