The one-step method based on chirality transfer strategy is a method for the synthesis of CCDs by one-step hydrothermal carbonization of carbon source and chiral ligand in the same reaction system. The reaction can form larger nanoparticles through condensation between chiral precursors or between chiral precursors and carbon sources, which are finally carbonized to form small CCDs^[65]. By controlling the degree of carbonization, CCDs retain the "chiral structural memory" of the chiral compounds used as building blocks. The one-step carbonization method is simple to operate, but the reaction process is more complex, and the chiral ligand structure is retained in the form of chiral residues in the product, so this method of obtaining chirality is classified as a chirality transfer strategy. Zhang et al. First reported the synthesis of CCDs by hydrothermal carbonization in 2016^[46]. They placed citric acid and L-/D-cysteine in a Teflon autoclave for hydrothermal reaction at 180 ℃ for 1 H (Fig. 1A) through a "one-step method", and the CD spectrum showed that the chirality of the raw materials was well inherited and transferred in CCDs. This method requires strict control of the reaction conditions to avoid the destruction of chirality.

Chiral amino acids contain carbon element, which can participate in the formation of carbon nucleus while providing chiral structure, so some researchers use chiral ligands as a single raw material to obtain CCDs as both carbon source and chiral source. Hu et al. And Wei et al. Respectively used L-/D-cysteine and L-/D-tryptophan as the sole starting materials and NaOH as the additive to prepare CCDs (Fig. 1B, C) by a one-step method at 120 ℃ for 16 H, and the products showed highly symmetrical CD signals and strong fluorescence emission^[29][31]. Other researchers have successfully synthesized CCDs using L-/D-cysteine as a single starting material^[39,40,55]. Recently, Lu Siyu's team at Zhengzhou University used L-/D-serine as the sole raw material to obtain CCDs by hydrothermal carbonization at 140 ℃ for 8 H. Full-color circularly polarized light (CPL) -emitting CCDs-CsPbBr₃ was synthesized in situ by simple ligand-assisted coprecipitation at room temperature (Figure 1 D)^[59]. At present, the raw materials used for the synthesis of CCD's from chiral amino acids are relatively single, and more available chiral raw materials need to be explored. The one-step synthesis of CCDs is summarized in Table 1.

The two-step method refers to the modification of chiral ligands on achiral carbon dots to form CCDs. The starting material is first carbonized to form an achiral carbon core, around which chiral molecules are then modified to form CCDs with chiral features. Hydroxyl, carboxyl or amino groups on the surface of CDs obtained by hydrothermal carbonization are used as active sites to interact with chiral ligands to passivate the surface and form new fluorescence emission centers, thus enhancing the fluorescence emission intensity. The chiral ligand is bound to the carbon skeleton through covalent bonding, electrostatic interaction or hydrogen bonding, and the final CCDs are relatively stable. In this process, the chiral ligand structure is not destroyed, and the CCDs completely inherit the chiral structure of the chiral source. The two-step process is relatively complex, but it can be used to directionally prepare functionalized CCDs to meet the needs of specific tasks.

Amino acid precursors have both amino and carboxyl functional groups, which can undergo amidation or dehydration reactions with the abundant oxygen-containing functional groups on the surface of hydrothermally carbonized CDs to form stable covalent structures, and are often used as chiral ligands. Cysteine is the most common chiral ligand, which can react with CDs through amino or sulfhydryl groups to form CCDs with different chemical structures^[62]. EDC/NHS (1-ethyl- (3-dimethylaminopropyl) carbodiimide hydrochloride/N-hydroxysuccinimide) is a commonly used crosslinking agent, which can modify the chiral precursor on the surface of CDs. Copur et al. Covalently bound L-/D-cysteine to the surface of an achiral carbon dot via EDC/NHS to obtain CCD (fig. 2)^[9]. The increase in intensity of the peak at 1632 cm^-1 and the decrease in intensity of the peak at 1715 cm^-1 in the IR spectrum confirm the successful formation of amide bond^[59]. The two-step method for the synthesis of CCDs is summarized in Table 2.

The results show that the preparation method of CCDs has a certain influence on their size and optical properties. Under the same reaction conditions, the size of CCDs synthesized by one-step method is slightly smaller than that synthesized by two-step method, and the fluorescence emission intensity is higher. Das et al. used cysteine as the chiral source and urea and citric acid as the carbon source to synthesize CCD's by one-step and two-step methods, respectively (Fig. 3A), and found that the size of CCDs synthesized by two-step method (7.0 nm) was slightly larger than that synthesized by one-step method (5.0 nm) (Fig. 3B), and the chemical structure of cysteine was retained on the surface of CCD's^[49]. Calculation of the asymmetry factor (g_lum) shows that the CCDs obtained by the two-step method are one order of magnitude higher than those obtained by the one-step method. It has also been reported that chiral compounds can form CCDs with higher chiral response values in a one-step carbonization process^[41]. In the one-step reaction process, the raw materials are fully reacted, resulting in the formation of a large number of emission centers in CCD's, and the fluorescence quantum yield is large. However, in the two-step formation process of CCDs, the chiral ligand is wrapped on the surface of CDs, resulting in a large number of residual carboxyl and hydroxyl functional groups on the surface of CCDs, resulting in surface defects and low fluorescence quantum yield.

At present, the preparation of CCDs by hydrothermal carbonization is mostly focused on the one-step method with simple operation, but the harsh experimental conditions limit the realization of long wavelength emission. There are few reports on the preparation of multicolor luminescent CCDs, and the realization of long wavelength emission of CCDs by modifying chiral precursors on multicolor luminescent CDs through a two-step method still needs further study.

During hydrothermal carbonization, the carbon source is mainly involved in the formation of carbon nuclei. Like CDs, CCDs are synthesized from a wide range of carbon sources. In theory, any substance containing carbon can be used as a carbon source. The emission wavelength of CCDs can be controlled by selecting appropriate surface chemical structure and conjugated carbon source. Citric acid is a tricarboxylic acid compound, which is easy to form rings in the hydrothermal reaction process and has excellent water solubility. The product can be widely used in the field of biology and is one of the most common carbon sources for the synthesis of CCDs^[50].

The precursor with aromatic structure provides favorable conditions for the formation of CCDs with a large degree of conjugation, the fluorescence quantum yield is high, and the emission wavelength can jump to the deep red and near-infrared spectral regions to form CCDs with red emission^[48,32]. Therefore, precursors containing amino groups, such as o-phenylenediamine, m-phenylenediamine and ethylenediamine, are often used as carbon sources to prepare CCDs^{[32,43][52][57,67]}. The nitrogen-containing functional groups on the surface of CCDs can regulate the fluorescence emission of CCDs. The optical properties of CCDs obtained from the precursor containing both amino and carboxyl functional groups are excellent. CCDs prepared by mixing citric acid with nitrogen-containing substances as carbon sources have high fluorescence quantum yield and excellent water solubility^{[48][9,33,68]}. In general, CCDs synthesized with amino acids as precursors can emit fluorescence without surface passivation and have good biocompatibility. Therefore, some researchers used amino acid enantiomers as carbon source and chiral source to prepare CCQDs, which showed high optical activity^[29,31,39].

Most of the dye molecules show strong absorption at near-infrared excitation, and their inherent conjugated electronic structure can be retained in the product. The obtained CCDs have strong stability and can be used as a carbon source to control the electronic energy level structure to obtain CCDs with long-wavelength excitation and emission. Wei et al. Used L-cysteine and neutral red as raw materials to obtain orange-emitting CCDs by one-step method, which realized the retention of the conjugated electronic structure of cysteine residues and neutral red precursors in the product, and showed excellent photostability and Golgi targeting imaging performance^[75].

In order to meet the needs of green and sustainable development, biomass carbon sources with abundant reserves, various types, wide sources and green and environmental protection have attracted people's attention. The abundant elements of CCDs can also be self-doped to regulate the surface chemical structure of CCDs. Fan et al. Synthesized CCDs from natural waste sugarcane molasses by a two-step method^[61]. A large number of functional groups on the surface of sugarcane molasses provide active sites for the modification of chiral ligand cysteine. The fluorescence emission intensity of CCDs is much higher than that of achiral CDs. They also constructed fluorescent sensors and CD sensors based on CCDs for heavy metal ion detection. Wang et al. Selected Tengcha, which is rich in plant active ingredients such as alkaloids and flavonoids, as a precursor, and a deep eutectic solvent (NADES) composed of biomass-derived components such as amino acids, sugars, and organic acids as a chiral dopant to develop a green and sustainable multifunctional CCDs preparation method (Fig. 4A) for chiral recognition^[35]. The optical properties of CCDs can be changed by doping other elements and surface functionalization according to the emission wavelength control strategy of CDs, which can broaden the application prospects.

The structure of the chiral ligand is the most important factor affecting the chiral characteristics of CCDs. Most of the studies on the synthesis of CCDs use amino acid enantiomers as the chiral source. Due to the presence of both amino and carboxyl functional groups on the surface, the obtained CCDs have excellent water solubility and high fluorescence quantum yield. Among them, cysteine is the most widely used chiral source for the synthesis of CCD's. The co-doping of nitrogen and sulfur makes the P orbital of carbon atom conjugate with the lone pair of electrons of heteroatom, which not only accelerates the electron transfer, but also endows CCDs with unique photoelectric properties and very high fluorescence emission ability^[29][71]. However, cysteine is unstable in aqueous solution, so researchers often choose to react at a lower temperature for a longer time to obtain CCDs with stronger chiral signals^[39,55].

In the hydrothermal carbonization process, in addition to cysteine, aspartic acid, glutamine, glutathione, phenylglycine, tryptophan, proline, glutamic acid, alanine, lysine and other amino acid precursors used as chiral sources, the doping of some small molecules with chirality, such as ascorbic acid and pentose, can also make carbon dots chiral^{[30][45][48,57][48][31,32,48,69][51][50][66][37][67][35]}. Recently, Wang et al. Used D- (-) -fructose as a chiral source to prepare NADES and obtained CCDs by hydrothermal carbonization. The doping of N and Cl formed more defects on the surface of CCDs, and the product had a high quantum yield (PLQY = 20.42%)^[35].

The chiral precursor with aromatic structure has a narrow band gap and a large conjugated structure. The CCDs obtained from it have a large sp²structure, which can reduce the band gap, thus obtaining CCDs with long wavelength emission. However, at present, most of the common chiral precursors with aromatic structures are water-insoluble, so some additives are often added to the aqueous solution to promote the hydrothermal carbonization reaction. Das et al. Used citric acid and ethylenediamine as carbon sources to synthesize four CCD's (Fig. 4B) by one-step hydrothermal carbonization with four different L-isomers (L-cysteine, L-glutathione, L-phenylglycine, L-tryptophan) under the same conditions.The results show that the CCDs obtained by using L-phenylglycine and L-tryptophan precursors with benzene ring structure have smaller hydrodynamic size, larger surface negative charge and higher fluorescence quantum yield (PLQY > 55%)^[49]. CCDs with smaller size and less surface negative charge groups were formed by using precursors with aliphatic chains such as L-cysteine and L-glutathione.

The dehydration and carbonization process of raw materials can be controlled by adding appropriate additives to the reaction system to change the acidity and polarity, or by adding appropriate dopants to the reaction system to adjust the size of the conjugated domain of CCDs, which has a greater impact on the size, structure and optical properties of CCDs.

The acidity and alkalinity of the solution will affect the size of CCD's, and the alkaline environment will etch the CCDs and form smaller particles^[40]. H₂SO₄ can promote the hydrothermal carbonization to obtain CCDs with larger size^[32]. The emission wavelength changes due to the quantum size effect. In general, the larger the size of CCDs, the larger the conjugated structure, the smaller the band gap, and the greater the red shift of the emission peak. Hu et al., Yang et al., and Liu et al. Synthesized CCDs from L-/D-cysteine as a single raw material by hydrothermal reaction at 120 ℃ for 16 H^[29][42][40]. Hu and Yang added 0.06 G NaOH to 15 mL of water, and the resulting carbon dots were 4 – 5 nm (Fig. 5A) and 2.7 nm (Fig. 5B) in size, respectively, with an emission wavelength of 460 nm. The reason for the small difference may be the difference in the amount of raw materials in the system. However, when Liu added 1 G NaOH to the 10 mL reaction system, the reaction system was more alkaline, and the particle size of the carbon dots obtained was 1.25 nm (Fig. 5C), which was slightly smaller than that of Hu and Yang's study, and the emission wavelength was 410 nm. Subsequently, Ru et al. Reported a fabrication method to realize multicolor luminescent CCDs^[34]. They used L-/D-tryptophan and o-phenylenediamine as raw materials, hydrochloric acid ethanol solution, aqueous solution and sulfuric acid aqueous solution as reaction media, respectively, to obtain blue, yellow and red emission CCD's (Fig. 5D) at 160 ℃ for 7 H, realizing the control of CCDs band gap emission, the particle size gradually increased with the increase of solvent acidity and the decrease of polarity, the conjugated structure of sp² gradually increased, and the emission peak position red-shifted. In this study, CCDs with multicolor emission were prepared from the same raw material for the first time, and it was confirmed that the high acidic environment and low polar solvent were helpful to the dehydration and carbonization of the precursor, and the red emission of CCDs was realized.

The addition of appropriate dopants to the reaction system can enhance the fluorescence emission intensity of CCDs, and also have a certain impact on the chiral signal. Common N atom doping can also change the surface structure of CCDs, form new surface state energy levels, and improve the optical properties. In addition, the experiment shows that the doping of B and S can also improve the emission intensity. Zhang et al. And Das et al. Synthesized CCDs by hydrothermal carbonization of cysteine and citric acid under the same reaction conditions. Das added ethylenediamine on the basis of Zhang's, showing different chiral signals^[46][49]. According to Das, the absorption of CD in the ultraviolet region is attributed to the Cotton effect caused by the hybridization between the electronic state of the high-energy carbon dot and the cysteine energy level, and the peak outside the ultraviolet region comes from the n-π^* transition of the carbon nucleus, which can be applied to two-photon absorption and broaden its application in the biological field.

Hydrothermal carbonization temperature is an important factor controlling the formation of CCDs. Usually, high temperature promotes the formation of graphitized carbon nuclei, resulting in a higher degree of carbonization, the molecular state gradually changes to the carbon nucleus state, and the fluorescence emission peak red-shifts. However, too high temperature will reduce the content of oxygen-containing groups, destroy hydrogen bonds and cause fluorescence quenching. It is worth noting that during the reaction, the temperature should always be kept below the melting point of the chiral raw material, so as to avoid excessive carbonization and destruction of the chiral center. Therefore, the preparation process usually uses a lower temperature, and the degree of graphitization of the formed CCDs is smaller.

Visheratina et al. Fixed the reaction time (4 H) and synthesized CCD's by hydrothermal carbonization of cysteine at 100 ℃, 150 ℃, 200 ℃ and 250 ℃^[79]. CCDs synthesized at lower temperature (100 ℃) have no obvious morphological characteristics, so it is difficult to estimate the particle size. The CCDs formed at high temperature (250 ℃) exhibit elongated geometric morphology with a particle size of 13 nm. With the increase of reaction temperature, the PLQY increases, the chiral structure of chiral precursor decomposes at high temperature, and the g_lum decreases. Pei et al. explored the variation of the properties of CCDs obtained from three chiral amino acid precursors at higher hydrothermal carbonization temperatures (200 ~ 250 ℃), and found that the particle size of CCDs synthesized from precursors containing aliphatic chains (serine and cysteine) decreased gradually with the increase of temperature^[80]. The size variation of CCDs synthesized from the heterocycle-containing precursor (histidine) is slightly different. PLQY decreased with the increase of temperature, which was contrary to the conclusion of most studies.

Wei et al. And Maniappan et al. Explored the effect of temperature on chiral characteristics^[67][60]. Wei prepared CCD's from L-ascorbic acid and ethylenediamine. Circular dichroism spectroscopy (CD spectroscopy) showed that the CD signal and g_lum first increased and then decreased with increasing temperature (90 – 140 ° C) (Fig. 6A). When the temperature reaches 180 ° C, the chiral structure is destroyed. Maniappan recorded the chiral signal change of cysteine and citric acid precursors at 60 ~ 200 ℃, and the CPL signal first increased and then decreased with the increase of hydrothermal carbonization temperature (Fig. 6C), and reached the maximum at 180 ℃, which was consistent with the CD signal change. It can be seen that high temperature can destroy the chiral structure of CCDs, and the selection of appropriate temperature plays a vital role in the control of chiral structure.

The hydrothermal reaction process gradually forms carbon nuclei, and first produces some sub-fluorophores that can emit weak fluorescence. At a certain carbonization temperature, with the increase of reaction time, the raw materials were crosslinked or polymerized to form more stable fluorescent groups, which reduced the fluorescence quenching caused by non-radiative relaxation, the carbonization degree increased, the fluorescence emission intensity increased, and the g_lum gradually decreased to disappear^[45].

Visheratina et al. Reacted cysteine as a single raw material at 150 ℃ for 1 ~ 20 H, and systematically discussed the effect of reaction time on the properties of CCDs^[79]. When the hydrothermal carbonization time is less than 4 H, the dense carbon nucleus can not be formed, and the CCDs purified by dialysis have no chiral signal. With the extension of carbonization time (1 ~ 20 H), the degree of carbonization increased, the average diameter of CCDs increased, and the PLQY increased gradually and tended to be stable after 12 H. New peaks at 7.5 – 6.5 ppm and 4.9 – 1.0 ppm can be observed by hydrogen nuclear magnetic resonance spectroscopy (¹H NMR), indicating the formation of aromatic groups. The g_lum decreased sharply at 4 H of reaction, and the chiral activity disappeared after 12 H. They suggested that with the increase of carbonization degree, the atomic scale chirality gradually disappeared, and the high-symmetry carbon nucleus reduced the nanoscale chirality. In addition, the long-term carbonization process is more conducive to the racemization of CCDs.

Liu et al. And Maniappan et al. Discussed the effect of hydrothermal time on chiral characteristics. Liu used citric acid and D-proline as precursors to carbonize at 180 ℃ for 2, 4, 6, 8 and 10 H in one step^[51][60]. The CD spectrum shows that with the increase of hydrothermal time, the chiral signal of the inherited raw material at 208 nm gradually decreases, and a new chiral signal appears at 220 nm at 6 H, and the intensity increases with the increase of time, reaching the maximum at 8 H (Fig. 6 B). Maniappan used cysteine as the chiral source to synthesize CCDs by changing the reaction time (45, 90, 180, 360 and 540 min). The CPL reached the strongest at 90 min, and then with the increase of reaction time, the chiral groups on the surface of CCDs decomposed, the optical activity decreased, and the signal gradually weakened, which was consistent with the change of CD signal intensity (Fig. 6 D).

In the UV – vis absorption range, the CD spectrum showed that L-CDs and D-CDs exhibited opposite Cotton effects, proving their chiral character^[46]. It has been reported that the chirality of CCDs comes from two aspects: (1) the spatial structure of the four atoms or functional groups attached to the chiral carbon in the chiral precursor has not changed, and the synthesized CCDs have circular dichroism signals similar to those of the raw materials; (2) Under the induction of chiral environment, raw materials aggregate on the surface or inside of CCDs to form chromophores with low energy levels or polycyclic aromatic hydrocarbons, which interact with each other to distort the carbon core structure and form new chiral centers to make CDs chiral.

Most of the hydrothermally carbonized CCDs can inherit the chirality of the starting material, showing a chirality signal similar to that of the starting material at 200 ~ 250 nm. However, the carbonization process of raw materials will lead to a red shift of the signal compared with the raw materials^[30]. If the concentration of CCDs is too high, aggregation will occur, which will also cause the shift of circular dichroism signal^[73]. CCDs prepared by Chen et al. Using glutamic acid (Glu) enantiomer as raw material by solvent-free thermal reaction showed the same chiral signal, showing chiral retention and chiral inversion (Fig. 7A), combined with XPS and FTIR characterization results.They suggested that the chiral signal of L-CDs was consistent with the raw material L-Glu, while D-CDs formed more pyridine nitrogen structure and rigid structure, and the chirality might be more from the steric effect of carbon nucleus, resulting in chiral inversion^[50].

Depending on the synthesis method, the carbon core state induced by the chiral environment and the surface state of the chiral group on the surface of CCDs will form a new chiral center^[49]. Ma et al. Theoretically concluded that the chirality of CCDs synthesized by hydrothermal carbonization of citric acid and glutamine originated from the stacking of polypeptide carbonyl groups adjacent to surface groups, rather than carbon nuclei^[45]. Wei et al. Analyzed the formation mechanism and chiral origin of CCDs prepared from tryptophan enantiomers by hydrothermal carbonization^[31]. The data show that L-/D-tryptophan decomposes with water under hydrothermal, high-pressure and alkaline conditions to form indole and L-/D-alanine, in which indole is mainly involved in the formation of carbon nuclei, and L-/D- alanine is carbonized by dehydration polymerization to form CCDs (Fig. 7B). According to the CD spectrum, they suggested that in addition to retaining the chirality of the raw material tryptophan, the chiral environment of CCDs induced the achiral group to twist, forming new chiral signals at 240 and 290 nm.

Transmission electron microscopy (TEM) and atomic force microscopy (AFM) showed that most of the CCDs synthesized by hydrothermal carbonization were spherical or flat spheres with good dispersion, and the average size ranged from 1. 25 nm to 20 nm^[9][40][33]. High-resolution transmission electron microscopy (HRTEM) shows that CCDs exhibit amorphous structure or obvious lattice structure.

Ru et al. Synthesized multicolor luminescent CCDs with the same raw materials, the emission peak position red-shifted with the increase of particle size, and the conjugated structure of sp² gradually increased^[32]. The lattice structure of CCDs can be observed by HRTEM, and the crystallinity can be analyzed by X-ray diffraction (XRD). Hu et al. Observed by HRTEM that the lattice spacings of 0.21 and 0.34 nm of CCDs synthesized by hydrothermal carbonization correspond to the [100] and [002] crystal planes of graphene, respectively (Fig. 8 A)^[29]. Gao et al. Characterized CCDs by XRD and observed a broad diffraction peak centered at 21.7 ° corresponding to the [002] plane of graphitic carbon with a lattice spacing of 0.41 nm^[30]. The increase of nitrogen content in graphite can reduce the non-radiative relaxation and enhance the fluorescence emission.

Wei et al. Analyzed the relative intensity (I_G/I_D) of sp² hybrid carbon (G band) and sp³ hybrid carbon (D band) by Raman spectroscopy and concluded that CCDs contained a large number of sp² hybrid structures^[52]. It has also been shown that CCDs synthesized by hydrothermal carbonization are amorphous and have no obvious lattice structure. The CCDs synthesized by Zeng et al. Via one-step hydrothermal carbonization showed good dispersion without obvious aggregation^[57]. XRD shows that a broad diffraction peak can be observed at 22 °, indicating a large amount of amorphous structure in CCDs.

The chemical structure of CCDs can be studied by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Based on the position and intensity of the peak, the connection mode between the raw materials can be judged^[46]. Hydrothermally carbonized CCD's are mainly composed of C, N, and O elements. Depending on the precursor, CCDs will show similar elemental States to the raw material. Yang et al. Analyzed the chemical structure and element composition of CCDs by XPS, and the results showed that CCDs contained functional groups such as $\mathrm{C}=\mathrm{O}$, C — O, N — H, COO —, C — N, S — H and C — S^[42]. The contents of C, N, O and S were 60%, 22.53%, 8.05% and 9.06% respectively, which were consistent with the raw materials.

FTIR can confirm that the abundant oxygen-containing functional groups on the surface of hydrothermal carbonized CCDs endow them with excellent water solubility and unique biological activity, which promotes their practical applications in photodynamic therapy, drug delivery, biomedical imaging and therapy. In addition, the carboxyl and hydroxyl functional groups on the surface of CCDs can generate excitation energy traps, which are essential for their photoluminescence properties. FTIR of CCDs prepared by Das et al. Using different hydrothermal carbonization methods showed that CCDs synthesized by one-step carbonization method had more $\mathrm{C}=\mathrm{O}$ functional groups on the surface^[49]. The surface of CCDs obtained by surface modification retained the structure of the raw cysteine more, and the peaks formed by C — O, C — N, and C — H observed in the 900~1200 cm^-1 region were more obvious (Fig. 8 B). Visheratina et al. Collected the ¹H NMR spectra of CCDs synthesized at different hydrothermal reaction times, and found that new peaks appeared in the ranges of 7.5 – 6.5 ppm and 4.9 – 1.0 ppm with time, proving the formation of Ar-H and Ar-NH₂ structures, which can be used to analyze the formation process of CCDs^[79].

The chemical composition and structure of CCDs affect their optical properties. In the common study of N-doped CCD's, the doping of pyridine and pyrrole can lead to the blue shift of ultraviolet absorption and fluorescence emission of CCD's, while some amine groups can lead to the red shift of ultraviolet absorption and fluorescence emission^[69].

The optical properties of CCDs are usually reflected in both ultraviolet absorption and fluorescence emission. CCDs obtained by hydrothermal carbonization have a large number of oxygen-containing functional groups on the surface, which are easy to interact with the surrounding environment and change the optical properties, showing pH dependence and solvent effect. The characteristic absorption peak at 240 – 270 nm in the ultraviolet absorption spectrum (UV-vis) is usually due to the π-π^* transition of $\mathrm{C}=\mathrm{C}$, while the other typical absorption band at 300 – 350 nm is caused by the transition of unbonded lone pair electrons such as $\mathrm{N}=\mathrm{H}$, $\mathrm{C}=\mathrm{O}$, — SH on the surface of CCDs to π^* orbital^[76]. The absorption peak at > 400 nm is due to the induction of surface molecular groups^[32]. The position of the peak is shifted due to the carbonization of the raw material and the size effect^[46].

In order to avoid the destruction of the chiral structure of CCDs by harsh hydrothermal conditions, most of the CCDs prepared by hydrothermal carbonization reported so far show blue emission. The sulfur element in the commonly used chiral precursor cysteine can capture more excitons and make the emission peak blue shift^[9]. During hydrothermal carbonization, the product gradually changes from molecular state to carbon nucleus state. The emission wavelength can be controlled by controlling the carbonization process. Wei et al. Used cysteine as a chiral source, added m-phenylenediamine containing a benzene ring conjugated structure as a carbon source and dopant to the reaction system, and synthesized N and S co-doped CCD s with a large graphitized structure. The fluorescence emission peak was at 510 nm, showing green fluorescence emission and being used for cell imaging^[52].

Affected by the surface state or dispersed solvent, the emission wavelength of most CCDs changes with the excitation wavelength, showing excitation dependence. It has also been pointed out that N and S co-doped carbon dots show non-excitation dependence. By comparing the fluorescence spectra of CCDs synthesized by different hydrothermal methods, Das et al. Inferred that a large number of emission centers could be formed in the one-step carbonization process, resulting in an increase in absorption and a smooth extension to the red region, which was excitation-dependent^[49]. The surface modification process can improve the emission by passivating the surface state.

The electrical properties of CCDs, including conductivity, charge transport and electronic state structure, are another important property for the study of their applications in electronic devices and energy conversion.

At present, the electrochemical properties of CCDs are rarely analyzed, and most of them are based on the surface charge information of CCDs provided by Zeta potential analysis to speculate their surface chemical structure. The results show that the relatively high absolute value of potential (> 30 mV) of CCDs can prove their good dispersion stability in aqueous solution^[57]. After Wang et al. Modified L-arginine on the surface of CDs, the surface of CDs changed from negative charge (− 42.55 eV) to positive charge (+ 33.42 eV), which confirmed the successful binding of chiral ligands^[44]. Zhang et al. And Hu et al. Used CCDs as the working electrode of a three-electrode system to explore the electrochemical performance of CCDs through electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) tests, and realized electrochemical recognition and catalysis through the different charge transfer rates between L-CDs and D-CDs and the analyte^[46][29].

The abundant oxygen-containing functional groups on the surface of hydrothermally carbonized CCDs endow them with excellent water solubility, good biocompatibility and low toxicity, which can be used for targeted monitoring and treatment of some diseases, and become a new material in the field of biomedicine. Inspired by the impact of chiral nanomaterials on biological activities, researchers have carried out research on hydrothermal carbonized CCD's in the biomedical field.

Enantiomers in nature show different molecular configurations, and CCDs produce different interaction forces with them, thus achieving simple and rapid chiral recognition. In addition, similar to CDs, the interaction between the analyte and the surface functional groups of CCDs leads to electron transfer, resulting in fluorescence quenching, which can also achieve quantitative and qualitative analysis of the sample.

Asymmetric catalytic synthesis refers to the conversion of achiral substrates into optically active asymmetric products with a small amount of chiral catalyst, which is one of the most effective methods to obtain pure chiral substances. The existing catalyst has long preparation period, high cost and strict requirements on reaction conditions. CCDs is used as a small molecule catalyst, which has excellent catalytic performance and stable reaction.

The CCDs obtained by Liu et al. From citric acid and d-proline by a one-step hydrothermal method showed high asymmetric catalytic activity for the aldol reaction of p-nitrobenzaldehyde and cyclohexanone^[51]. When the hydrothermal time was 4 H, the catalytic activity of CCDs was the best. Separation of the product by column chromatography and judgment of the ratio of enantiomers (ee value) by high performance liquid chromatography gave a yield of 98% and an ee value greater than 71%. The D-proline is linked to the carbon nucleus by a C — N covalent bond, and the chiral sites are dispersed on the surface of the CCDs. Subsequently, CCDs were obtained from L-/D-alanine and citric acid, and it was found that both L-CDs and D-CDs could achieve good catalytic activity for the asymmetric aldo-keto condensation. The catalytic performance was also evaluated for the condensation of cyclohexanone and p-nitrobenzaldehyde at 160 ℃, with yields of 86% ~ 95% and ee values of 67% ~ 80%^[66].

Multiphoton absorption excitation is a characteristic of luminescent materials, which can avoid the influence of autofluorescence and be used in the field of biology. Das et al. Collected the two-photon excitation spectra of CCDs synthesized by different methods, and confirmed that the emission signal originated from the two-photon absorption process, indicating that the energy was effectively transferred to the surface of CCDs during the two-photon absorption process.Then five different chiral precursors were selected to synthesize CCD's, which showed two-photon stimulated luminescence (TPL) signals under 800 nm femtosecond laser excitation^[49][10].

In addition to the excellent photoluminescence properties of CCDs, researchers have also turned their attention to their CPL emission properties. This is a phenomenon that enables spontaneous emission of left and right circularly polarized luminescence. CPL emission can play an important role in the fields of chiral recognition, biological imaging, information encryption and anti-counterfeiting. Therefore, how to realize the CPL characteristics of CCDs and broaden its application in the field of CPL has become one of the research hotspots. Most of the current reports are based on template-assisted formation of CCDs composites to achieve circularly polarized emission. Maniappan et al. paid attention to the CPL activity of CCDs themselves, and reported for the first time the preparation method of CCDs that can exhibit CPL emission in both liquid and solid States^[60]. Intense CPL signals were observed in CCDs synthesized with threonine and citric acid as precursors. Similar CPL signals were observed for the subsequent synthesis of CCDs with threonine replaced by glutathione and glutamic acid enantiomers. However, CCDs synthesized from other amino acids (lysine, serine, methionine and cysteine) did not observe CPL peaks and lacked excited state chirality. It shows that the structure of the chiral source can affect the circular polarization activity of CCDs. Recently, Yan et al. Summarized and prospected the application of CCDs in CPL^[81].

In addition to observing the great application value of CCDs themselves, researchers have also prepared composites based on CCDs to broaden the application of CCDs in the fields of CPL emission, anti-counterfeiting and multi-functional identification.

Most of the reports of chiral assembly functional composites mainly transfer the chirality of the template to the carbon dots while retaining the inherent structural characteristics of the template, which is a promising method for the preparation of new nanomaterials. Cellulose nanocrystals (CNCs) are one of the most common templates because of their green color, nontoxicity, and wide source^[69]. For the first time, Lizundia et al. Co-assembled nitrogen-doped carbon dots with CNCs by evaporation-induced self-assembly (EISA) to prepare chiral nematic composites (Fig. 15 A), which showed higher thermal stability and strong iridescent color under natural light^[53]. In our group, N-doped achiral carbon dots and CNC were used to form chiral nematic nanocomposites with independent iridescent luminescence by EISA, which increased the ionic strength of CNC dispersion, reduced the helical spacing of the film, and showed higher thermal stability and strong iridescent color under natural light.The CPL film exhibited a bright blue iridescent color under white light illumination and a strong blue fluorescence under UV light illumination (Fig. 15 B), which realized circularly polarized light emission and was used for chiral photoinduction and photoregulation of azobenzene supramolecular polymers^[56].

Cholesteric liquid crystals (CLCs) have a spontaneous supramolecular helical structure, which can selectively reflect circularly polarized light and form a photonic band gap, and can be used to develop a variety of tunable optical devices^[76]. Gollapelli et al. Self-assembled a certain amount of CDs and CLC to prepare CLC/CDs mixture, and created a dual-mode anti-counterfeiting material^[78]. This CLC composed of CDs has light tonicity. Under the condition of reflection and fluorescence, CLC/CDs droplets show reflection color and bright fluorescence color. The photoresponse material is simple to prepare, has visible reflection and fluorescence colors, and can be applied to the fields of intelligent decoration, dual-mode displays and anti-counterfeiting labels.

Lu Siyu's group reported a solvent-controlled synthesis of multicolor luminescent CCDs, which were doped into chiral organogel by electrostatic interaction, and white light circular polarization luminescence was achieved by adjusting the proportion of blue, yellow and red carbon dots^[32]. Recently, the research team introduced CCDs into perovskite nanocrystals with bright emission, and obtained the first CPL-separated full-color inorganic perovskite (Fig. 16A)^[59].

In addition, the chirality of CCDs has been transferred to achiral receptors to form new composites. Porphyrin-based molecules can be endowed with chirality by self-assembly of chiral materials. Liu et al. Successfully synthesized CCDs by a simple one-step carbonization method, and induced porphyrin to form supramolecular chiral materials (Fig. 16b), where the chirality was transferred to porphyrin by electrostatic interaction, and the chiral signal was amplified, providing a variety of options for the development of chiral composite materials^[40]. Recently, Niu et al. Reported a method for the preparation of chiral template-induced porphyrin-based self-assembled materials for electrochemical chiral sensing^[63]. They modified CDs with tryptophan to obtain CCDs, which induced porphyrin self-assembly through hydrogen bonding, π-π interaction and other forces for electrochemical recognition of phenylalanine enantiomers. The results show that the porphyrin-based composite synthesized by L-CDs can interact with D-phenylalanine, and the porphyrin-based composite synthesized by D-CDs can form a relatively stable structure with L-phenylalanine, which has obvious electrochemical recognition effect and expands the application of CCDs in electrochemistry.

Liu et al. Encapsulated CCDs in ZIF-8 nanoparticles to form a chiral composite CCDs/ZIF-8 as a bifunctional sensing platform for the recognition of folate enantiomers and antibiotic nitrofuranone (Fig. 17 A)^[58]. In the recognition process, folic acid can enhance the fluorescence of CCD's/ZIF-8, and D-FA has a higher fluorescence intensity change than L-FA. The enantioselectivity is proportional to the concentration of the analyte, and the fluorescence enhancement mechanism is attributed to the dynamic enhancement mechanism. In addition, the addition of antibiotic nitrofuranone can destroy the ZIF-8 skeleton and enhance the fluorescence emission.

The team also encapsulated CCDs and fluorescein into ZIF-8 in situ to construct a chiral dual-emission (430 and 513 nm) composite (fluorescein/CCDs @ ZIF-8) to achieve highly selective and sensitive detection of p-phenylenediamine (PD) isomers. The fluorescence emission at 430 nm was significantly enhanced by the addition of p-phenylenediamine (PPD) ethanol solution to the composite, which could be used as a single fluorescent probe or a ratiometric fluorescent probe for the detection of PPD, with a detection limit of 8. 51 μmol/L. In addition, the addition of oxidized m-phenylenediamine (MPD) can be used as a highly efficient ratiometric fluorescence probe for the quantitative analysis of the concentration of oxidized MPD in ethanol solution. When OPD was added to ethanol solution, the emission of fluorescein increased significantly and red-shifted to 550 nm. The composite has multifunctional recognition and micro-detection functions, and can realize the fluorescence response of p-phenylenediamine (Fig. 17B)^[82].