Molecules or polymer precursors are used as building blocks that can be polymerized and carbonized to form CPDs. The atomic connectivity in CPDs is thus derived from the precursors. Typically, chiral information can be accurately transferred from the precursors to the CPDs (Fig. 2a)44. Prato et al. synthesized chiral CPDs based on arginine (Arg) and (R, R)- or (S, S)-1, 2-cyclohexanediamine (CHDA) by using a microwave-assisted hydrothermal method. The circular dichroism (CD) spectral profiles of Arg-RCHDA CPDs and Arg-SCHDA CPDs showed peaks corresponding to negative and positive Cotton effects, respectively. This observation confirms that it is feasible to directly synthesize chiral CPDs from chiral precursor molecules, indicating the genetic relationship between the CPDs and the precursors. These precursor-dependent features allow the precursor molecules to control the properties of the CPD structures. In this section, we highlight the precursor-dominated structural and functional diversity observed in CPD-based materials.
Fig. 2 Precursor-dependent emission.a Example of the direct synthesis of chiral CPDs from chiral diamine precursors. b Reaction of EDA with CA and TA to form CA-EDA CPDs and TA-EDA CPDs. c Precursor-dependent RTP property (top) and schematic representation of crosslinking sites (bottom) present in PAA-EDA, PAA-EA, and PAA-EG CPDs. d Preparation of RGB PL CPDs from three different phenylenediamine isomers. Reprinted with permission from the literature reports13, 44, 51, 52. Copyright 2019 Nature Publishing Group, 2018 American Chemical Society, 2018 Wiley–VCH, 2015 Wiley–VCH.
CPDs are emerging as bright and biocompatible fluorescent nanomaterials. The PL emission of CPDs in the visible and near-infrared regions can be effectively tuned. The complex optical properties of CPDs have been summarized in a few review papers published recently20, 21, 45. It has been reported that the emission originates from four types of PL centers (molecular-state, crosslink-enhanced-emission (CEE)-effect-related state, sp2 subdomains (carbon core state), and surface state)45. The chemical structures of these PL centers, which determine the emissive properties of CPDs, depend on the molecular structures of the precursors.
The molecular-state luminescence and CEE effect significantly influence the PL emission properties of the slightly carbonized CPDs synthesized by the polymerization–carbonization process. Structural fragments of the precursors are retained in the CPD products. Thus, a variation in the precursor structure directly affects the structure and PL properties of the CPDs. For example, condensation reactions between CA and amine-containing compounds result in the formation of fluorescent molecules during the synthesis of incompletely carbonized CA-based CPDs33, 34, 46-49. Molecular fluorophores can potentially be embedded in a carbon/polymer network to produce building blocks of CPDs50. These can function as molecular-state PL emissive centers. As shown in Table 1, the molecular fluorophores exhibit different absorption peak positions, emission wavelengths, QY, and PL lifetimes, depending on the amine precursors used. These differences highlight the influences exerted by the different atomic species (N, O, and S) and substituent groups present in the precursors on the structure and optical properties of CPDs.
Table 1. Chemical structure and optical properties of molecular fluorophores in CA-based CPDs.
Precursor-dependent structures and emissive properties have also been observed in CEE effect-related CPDs. The CA-EDA CPDs exhibited a high QY (64%), which was significantly higher than those of tricarballylic acid (TA; 7%) and EDA CPDs, although the structures of the copolymers present in both were similar (Fig. 2b)51. The strong PL exhibited by CA-EDA CPDs was attributed to the rigid entanglement of the polymer chains. The hydroxyl-induced supramolecular H-bond interactions, which could promote charge transfer between the entangled polyamide chains (effectively enhancing the PL in polymer-cluster-type CPDs), were responsible for the observed entanglement properties. The bright PL emission could be attributed to the CEE effects. The CEE effects also resulted in the generation of unique room-temperature phosphorescence (RTP), which was significantly influenced by the molecular structure of the precursors. Tao et al. investigated the influences of the precursor structures on the RTP in polyacrylic acid (PAA)-based CPDs13. PAA and a series of EDA analogs were used to synthesize different types of CPDs (Fig. 2c). Their group confirmed that the crosslinked amide/imide structures function as luminescent centers, inducing strong RTP in PAA-EDA CPDs. It was also reported that less dense and weak crosslinking resulted in the generation of weak RTP in PAA-EA CPDs. RTP was not observed in PAA-EG CPDs. The analysis of the theoretically obtained results revealed that the coupled amide/imide units resulted in the formation of a narrow energy gap, which facilitated intersystem crossing and the generation of the triplet state13. This work highlights the inherent relationship (atomic connectivity) between the target CPDs and the molecular precursors, which directly influence the RTP properties.
The precursor-dependent PL properties of CPDs have also been observed in carbon core state (sp2-conjugated domains) and surface-state-determined systems. Because of the complex crosslinking/polymerization–carbonization process used for the synthesis of CPDs, the precise chemical structures of these two PL centers have still not been clearly defined. The two PL mechanisms often coexist in a single CPD system. Lin and coworkers reported the preparation of multicolor PL CPDs exhibiting the emission of red, green, and blue PL using three different phenylenediamine isomers (Fig. 2d)52. The absorption spectra of m-phenylenediamine (mPDA), o-phenylenediamine (oPDA), and p-phenylenediamine (pPDA) CPDs gradually redshift. The peaks corresponding to the PL of the three CPDs appear at 435, 535, and 604 nm. Increases in the number of sp2-conjugated domains and in the nitrogen-doped content in the related products were observed with the change in the position of the amino group (the least in mPDA and the maximum in pPDA). This increase induced the generation of redshifted optical absorption and emission properties52, 53.
The precursors are the building blocks of CPDs, and their atomic arrangement and reaction behavior influence the polymerization–carbonization process, which influences the chemical structure of the CPD materials. Thus, the chemical structures and optical properties of CPDs can be tuned by tuning the structural properties of the precursors. Environmentally friendly fluorescent CPD materials have been widely used for the development of various optoelectronic devices, bioimaging markers, anti-counterfeit labels, and PL sensors.
CPDs exhibit PL-related properties and have a wide range of applications. CPDs have versatile functions depending on the design of precursors. CPDs can also be used in the field of catalysis54. They can be used for magnetic resonance imaging55 and scavenging free radicals56. They can also exhibit antibacterial57 and antiviral properties58. CPDs have recently shown promise as green photoelectrocatalysts because of their tunable chemical structures and energy levels. Prato et al. prepared a redox library of CPDs using electroactive quinines as co-reactive precursors during the microwave-assisted synthesis of Arg-EDA-based CPDs (Fig. 3a)16. As shown in Fig. 3b, the redox potentials of the CPDs can be effectively controlled by tuning the molecular structure of the quinines. This feature makes the CPDs potential photocatalysts. Zhao et al. recently demonstrated that the energy levels in CPD materials influence the photocatalytic performances of lead-halide perovskite/CPD hybrids17. They found that the energy level of CPDs prepared from p-aminosalicylic acid (pASA) and CA matched well with the energy level of MAPbI3. Thus, pASA-CA CPDs can be used to enhance the charge lifetime of MAPbI3 perovskites through the process of ultrafast hole extraction. A 35-fold enhancement in the reaction rate (compared to the rate observed when pure MAPbI3 was used) of visible-light-driven photocatalytic H2 evolution could be achieved. In addition to the electrochemical energy level, the properties of the catalytic active sites of CPDs can be tuned by rationally designing the precursors. Li and coworkers synthesized single atomically anchored cobalt (Co) (on CPDs) via a facile pyrolysis method using a metal chelate of vitamin B12 as the precursor. The single Co atom was stabilized by a defined Co−N4 structure inherited from VB12 (Fig. 3c)59. The visible-light-driven oxidation reactions could be effectively catalyzed by CPD-supported Co single-atom materials. This work highlights that CPDs can provide a platform for the development of single-atom catalysis.
Fig. 3 Precursor-dominated versatile functions.a Synthesis of CPDs from Arg, EDA, and quinones. b Corresponding redox potentials of CPDs. c Single Co atom anchored in CPDs derived from VB12 for photocatalysis. d Sel-CPDs (exhibiting green fluorescence) synthesized by hydrothermally treating selenocystine, which can be used as an ROS scavenger. e Met CPDs exhibiting blue fluorescence and selective antibacterial activity against Porphyromonas gingivalis. f Curcumin CPDs and their use as antiviral agents. Reprinted with permission from literature reports16, 56-59. Copyright 2018, Wiley–VCH, 2020 American Chemical Society, 2018 Wiley–VCH, 2017 Royal Society of Chemistry, 2019 Wiley–VCH.
Luminescent CPDs exhibiting varying functions can be used to fabricate novel multifunctional materials by introducing specific heteroatoms or functional groups into the scaffolds. Zboril et al. reported that Gd-doped CPDs exhibited magnetic resonance imaging properties55, while Huang et al. reported that Tb-doped CPDs could selectively detect 2, 4, 6-trinitrophenol (TNT)60. Xu et al. reported that selenium atoms endow selenocysteine-derived CPDs (Ser CPDs) with redox-dependent reversible fluorescence properties. Ser CPDs also exhibited the ability to scavenge free radicals, thus protecting the biosystem from the damage caused by an excess of reactive oxygen species (Fig. 3d)56. Liu et al. observed that metronidazole-derived CPDs (Met CPDs) emitted strong PL (blue). The Met CPDs also exhibited higher water solubility than metronidazole. The antibacterial activity exhibited by the Met CPDs against Porphyromonas gingivalis was similar to that exhibited by metronidazole (Fig. 3e)57. The antibacterial activity could be attributed to the nitro groups (inherited from the metronidazole precursor) in the Met CPDs. The therapeutic properties of drug CPDs were different from those of the precursors. Curcumin (Cur) exhibits antimicrobial, anticancer, anti-inflammatory, and antioxidant properties, whereas Cur CPDs were found to be effective antiviral agents against enterovirus 71 (EV71). It was believed that the antiviral property originated from the newly formed pyrolytic curcumin polymers (Fig. 3f)58. CPD materials exhibiting various novel functions can be fabricated by selecting the appropriate precursors. CPD materials can be used as lubricants61, anticorrosive agents62, and antioxidants63. They can also be used for the upregulation of glycolysis64.
The abovementioned examples indicate that structural information can be transferred from precursors to the CPDs via the polymerization–carbonization reaction. The synthesis can more systematically and logically control the structures of CPDs during the polymerization–carbonization process. Given that a large library of CPDs is being created, there is no nomenclature to index the library of CPD materials. This motivates us to establish a system of nomenclature for labeling CPDs. Such a system for deciding the nomenclature of materials can help manage this wide and diverse material system.
Precursor-dominated versatile functions
CPDs formed via the polymerization–carbonization process (analogous to polymer materials) are structurally diverse and exhibit the property of polydispersity. The structural features vary within a sample set. Therefore, CPD is better identified as a material and not a precise molecular or nanoscale entity21. This is also one of the reasons why most CPD materials exhibit excitation-dependent PL properties. Given the structural features of CPD materials and the structural relationship between CPDs and precursors, the material system can be indexed according to the precursors. Information on the local micro–nanostructure, properties, functions, and application directions of CPDs can be determined from the CPD names. The development of a system to name the CPDs can help classify and index this large family. CPDs will emerge as valuable functional materials in the future because they exhibit low toxicity and excellent optical properties. We believe that the use of a standardized system of nomenclature can be applied in various fields, such as manufacturing and trade.
To better establish the nomenclature, CPDs are divided into six groups based on their elemental composition and chemical structure: N (nonaromatic), A (aromatic), X (nonmetal-doped), M (metal-doped), P (polymer), and B (biomass) groups (Fig. 4a). The elemental composition of the members of groups N and A CPDs is the same (containing C, H, O, or N). Group N consists of CPDs prepared from nonaromatic (aliphatic) small molecules, such as EDA, CA, EA, and acrylamide. Group A consists of CPDs prepared from aromatic molecules and a mixture of aromatic molecules and nonconjugated precursors. Group X consists of CPDs containing nonmetallic elements other than C, H, O, and N (such as B, Si, S, Se, P, and/or halides). Similarly, groups M, P, and B represent metal-doped CPDs, polymer-derived CPDs, and biomass-based CPDs, respectively.
Fig. 4 Classification and nomenclature of CPDs.a Classification of CPDs based on the precursor structure and elemental composition. b Priority order of the nonconjugated precursors based on the functional groups. c Priority order of the aromatic precursors based on the conjugated rings. d Priority order of the nonmetal-doped precursors. e Priority order of the metal-containing precursors.
CPDs are generally synthesized from multiple (one to three) molecular precursors. When multiple precursors participate in the formation of CPDs, the precursor names should be hyphenated (e.g., citric-acid-ethanediamine CPDs). The priority order of the precursors should be determined for the nomenclature of each CPD material. Based on the chemical structure of the precursors, the following seven rules have been proposed to determine the priority order of the precursors:
1. Biomass (B) < polymer (P) < metal-doped (M) < nonmetal-doped (X) < aromatic (A) < nonaromatic (N). This order can be implemented to identify the priority order of different types of precursors used. For example, CPDs prepared from nonaromatic CA (N-group precursor) and aromatic pASA (A-group precursor) should be identified as pASA-CA CPDs and not as CA-pASA CPDs. In the shorter version of the name, the hyphen (-) should be used to connect each precursor. For another example, PAA and EDA-derived CPDs should be identified as PAA-EDA CPDs.
2. Carboxyl (−COOH) < ester (−COOR) < amide (−CONH−) < aldehyde (−CHO) < carbonyl (−C=O) < hydroxyl (−OH) < amino (−NH2) < alkyne (−C≡C) < alkene (−C=C) < nitryl (−NO2)
(Ⅰ) For naming the N-group CPDs, the priority order of the nonaromatic precursors is determined on the basis of the functional groups present in the scaffold (Fig. 4b). This order is similar to the order followed for naming organic compounds. For example, CA is higher on the priority list than EDA. Thus, the corresponding CPDs should be identified as CA-EDA CPDs.
(Ⅱ) When precursors bear the same highest-priority functional groups, the precursors with a greater number of highest-priority groups should be listed first. For example, CPDs prepared from ethylenediaminetetraacetic acid (EDTA; 4 carboxyl groups) and ammonium citrate (AC; 3 carboxyl groups) should be identified as EDTA-AC CPDs.
(Ⅲ) When precursors bear the same type and the same number of highest-priority functional groups, the priority order of other functional groups should be considered while naming the CPDs.
3. Furan < porphyrin < triazine < aphen < imidazole < benzopyrrole < pyridine < pyrrole < naphthalene < benzene (Ⅰ) When precursors are aromatic in nature, the priority order is first determined by studying the conjugated structure. There are four rules to follow:
(ⅰ) The larger the atomic number of heteroatoms in the aromatic cycle, the higher the priority. For example, furan (O atom) < pyrrole (N atom) < benzene (C atom).
(ⅱ) The greater the number of heteroatoms, the higher the priority. For example, porphyrin < triazine < aphen < benzopyrrole.
(ⅲ) The greater the number of aromatic cycles, the higher the priority. For example, anthracene < naphthalene < benzene.
(ⅳ) The larger the aromatic ring size is, the higher the priority. For example, pyridine < pyrrole.
Based on the above four rules, the priority order of precursors with common conjugated units is summarized in Rule 3 (Fig. 4c). For example, CPDs derived from L-tryptophan (LTry) and oPDA would be named LTry-oPDA CPDs. When the conjugated ring structures of the precursors are the same, the priority order of the precursors is determined on the basis of the substituent groups present (Rule 2). For example, CPDs prepared from phthalic acid (PHA) and oPDA should be named PHA-oPDA CPDs.
4. For nonmetal-doped precursors, the priority order is determined based on the heteroatoms present (Fig. 4d): I < Br < Cl < F < Se < S < P < Si < B. When precursors bear the same heteroatoms in their scaffold, the priority order is determined by Rule 3. Rule 3 is followed by Rule 2.
5. For metal-containing precursors, the priority order is determined by the atomic number: the larger the atomic number is, the higher the priority order (Fig. 4e). When precursors bear the same metallic atoms, the priority order is determined by Rule 4, which is followed by Rule 3 (or Rule 2).
6. When multiple polymers are used as precursors, the priority order is determined by studying the structure of the monomer. The five previously outlined rules are followed.
7. Some special rules:
(Ⅰ) Monomers are given a higher priority than crosslinking agents when naming CPDs prepared by the hydrothermal–addition–polymerization–carbonization (HAPC) method because they are generally used for the construction of the main structure of CPDs. For example, CPDs prepared from acrylamide (AM; monomer) and N, N'-methylenediacrylamide (MBA; crosslinking agent) should be named AM-MBA CPDs. When various monomers are used, their priority order is determined following the six rules outlined previously.
To easily understand the nomenclature, the names of the abovementioned typical CPDs are summarized in Table 2. In addition, we named most of the previously reported classical CPD materials following the rules of the nomenclature outlined above. The properties, functions, and fields of application of the CPDs are presented in Table S1. It has been observed that the chemical structures of the precursors are relatively simple and are easy to identify. The majority of the CPD materials can be named following Rules 1 and 2. During the preparation of CPD materials via solvothermal routes, organic solvents (e.g., EDA or glycerol65, 66, which assuredly act as precursors to form CPDs) should appear in the name of CPDs. Organic solvents such as DMF, DMSO, ethanol, acetonitrile, or formamide are solvents that can be activated or decomposed at high temperature and pressure. These activated solvents or their catabolites may be used as potential precursors for the construction of CPDs. However, the reaction processes are not explicit, so the names of the CPDs do not currently reflect these organic solvents used during synthesis.
Precursor structure Priority order CPD name short name pASA < CA (Rule 1) p-aminosalicylic acid-citric-acid CPDs pASA-CA CPDs PAA < EDA (Rule 1) Polyacrylic-acid-ethylenediamine CPDs PAA-EDA CPDs CA < EDA (Rule 2) Citric-acid-ethanediamine CPDs CA-EDA CPDs EDTA < AC (Rule 2) Ethylenediaminetetraacetic-acid-ammonium-citrate CPDs EDTA-AC CPDs LTry < oPDA (Rule 3) L-tryptophan-o-phenylenediamine CPDs LTry-oPDA CPDs PHA < oPDA (Rule 3) Phthalic-acid-o-phenylenediamine CPDs PHA-oPDA CPDs AM < MBA (Rule 7) Acrylamide-N, N'-methylenediacrylamide CPDs AM-MBA CPDs
Table 2. Name of typical CPDs based on the outlined nomenclature.