Carbon dots: An innovative luminescent nanomaterial

In recent years, carbon dots (CDs), including carbon nanodots, carbonized polymer dots, carbon quantum dots, and graphene quantum dots have attracted a mounting interest as readily accessible, nontoxic, and relatively inexpensive carbon‐based nanomaterials. Yet, despite intense research for a number of years, a unifying picture is still lacking to clarify the exact definition, clear chemical structure, and unique optical properties of this family of nanomaterials. In this review, we systematically summarize the recent development of CDs from molecular design to related properties of excited states as well as their applications in optoelectronic devices and biology. We point out the current challenges, including exploring precise synthesis, clarifying the structure‐property relationship, and regulating singlet and triplet states of fluorescence, phosphorescence, and delayed fluorescence. Moreover, the structural optimization of optoelectronic devices, tumor targeting mechanism, selective imaging, and drug delivery of CDs are also highlighted. We hope that the information provided in this review will inspire more exciting research on CDs from a brand‐new perspective and promote practical application of CDs in multiple directions of current and future research.


INTRODUCTION
Carbon dots (CDs) are collectively referred to as a new type of luminescent carbon-based nanomaterials, including carbon nanodots (CNDs), carbonized polymer dots (CPDs), carbon quantum dots (CQDs), graphene quantum dots (GQDs), and other nanoscale carbon particles having typical dimensions below and around 10 nm. [1][2][3] Owing to the low toxicity, great solution-processability, ease of synthesis, and tunable photoluminescence (PL), CDs have attracted mounting interest among scientists in many research fields, especially in terms of optoelectronic devices and biological applications. [4][5][6][7][8][9][10] Importantly, the uniqueness of optical properties and practical applications of CDs often depend on their chemical structure. The CDs consist of the internal carbon core, and the external carbon shell modified with various polymer chains or surface functional groups (e.g., C-OH, C=O, C=N, and O-C=O), showing unique quantum confining effect (QCE), edge effect or surface effect. [11][12][13] There are two main types: one is the F I G U R E 1 Illustration of representative types of CDs, the precursors, synthetic conditions, and structure diagram are presented here and narrow full width at half maximum (FWHM). [19,[20][21][22][23] Besides fluorescence emission, excited states of CDs give rise to several optical processes with room temperature phosphorescence (RTP) and delayed fluorescence (DF) from triplet state, which are uncommon features in molecular fluorophores and semiconductor quantum dots (QDs). [24] Noticeably, structural regulation of the external carbon shell is also crucial, mainly including surface defects (e.g., functional groups, organic small molecules, and polymer), [25][26][27][28][29] surface interaction (e.g., hydrogen bond, metal coordination, electrostatic interaction, and π-π interaction), [25][26] and structural control (e.g., size, shape, and framework). [12,19,25] Apart from the carbon core related optical properties, regulating the carbon shell can not only effectively decrease the nonradiative transition of the triplet state to enhance the intersystem crossing (ISC), but also promote the separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to change the singlettriplet energy splitting (∆E ST ), thus achieving strong RTP and DF with adjustable energy level and lifetime. [16,[25][26][27] Since solvothermal cutting graphene to obtain blue CDs in 2010, [28] applications of carbon-based materials in optoelectronic devices have been expanding continuously, such as light-emitting diodes (LEDs), [29,30] solar cells, [31,32] photodetectors, [33,34] and catalysis. [10,28,35] Considering that the surface defect of excitation-dependent emission as the capture center of excitons greatly limits the injection of carriers, the bandgap emission of CQDs is crucial for the devel-opment of optoelectronic devices. [36] So far, by controlling the sp 2 conjugated domain with high crystallinity and different sizes and shapes, high QY (87%) and narrow-bandwidth (∼30 nm) CQDs covering from blue to red and even white light have been synthesized, achieving full-color and white LEDs (WLEDs) with high performance. [19,36] Compared with the fluorescence emission that only captures 25% singlet excitons, RTP and DF are expected to break the theoretical limit of external quantum efficiency (EQE) based on fluorescent materials (EQE = 5%) by utilizing 75% triplet excitons lost through nonradiative decay. [37,38] At present, CDs usually need to be embedded into a variety of organic and inorganic matrixes to stabilize triplet transition. [26,39] Otherwise, single-component CQDs with intrinsic RTP properties have also been reported, indicating that alternative CQDs-based RTP and DF materials hold great potential for the development of high-efficiency optoelectronic devices. [40,41] CDs are considered to be a kind of candidate in biomedicine, [42][43][44] due to their advantages of good photostability, high water-solubility, biocompatibility, low cost, and facile modification. [45][46][47] CDs have been extensively used as bioimaging probes, [48][49][50] drug/gene carriers, [51,52] promising agents for photodynamic therapy (PDT), [53][54][55] and photothermal therapy (PTT) [56,57] in recent years. Among them, tumor targeting is crucial for both effective tumor diagnosis and treatment. [58,59] It is an effective strategy of tumor-targeted drug delivery to utilize specific vector transporters overexpressed on the surface of tumor cells, including glucose transporters and large neutral amino acid transporters 1 (LAT1). [60] For example, the application of large amino acid mimicking tumor targeted carbon quantum dots (LAAM TC CQDs) is a great progress in tumor drug therapy. As a targeted carrier, it can accurately deliver drugs to various types of tumor cells. [59] Therefore, effective targeting can be achieved by modification of specific molecules at the edges of CDs.
In this review, we update the latest achievements on CDs, mainly including their structure, optical properties, and multiple directions of current and future research. First, there is a clear classification of CDs. And then, a detailed description of structure of internal carbon core and external carbon shell is proposed, followed by several special optical processes closely related to the structure, including fluorescence, RTP, and DF. Finally, we summarize the practical applications of CDs in the field of optoelectronic devices and biology. We hope that this view will advance the exciting research progress and a deeper understanding of CDs in the scientific community.

SYNTHESIS
Currently, the synthetic method of CDs is still dominated by either top-down or bottom-up approach. The top-down method is to break larger carbon materials [28,61] into smaller pieces by means of arc discharge, [62] laser ablation, [63] acid-oxidation exfoliation, [64] or electrochemical oxidation, [61,[65][66][67] providing a key step toward large-scale preparation. However, there are also inevitable shortcomings, such as various surface defects, lack of tunability in the term of particle size and surface functional groups. [19] Therefore, the top-down preparation method is not conducive to the one-pot preparation of CQDs with intact sp 2 lattice, narrow-bandwidth, as well as fluorescence, RTP and DF emission with high QY. In a stark contrast, the synthesis of CDs with the bottomup method can be achieved by fusing organic molecules or carbonizing organic precursors under hydrothermal, [6,68] or solvothermal conditions. [17,48,68] It should be emphasized that the selection of precursors can significantly influence the physical and chemical properties of as-synthesized CQDs, including their size, crystallinity, oxygen/nitrogen content, and luminescence properties. [11] In addition, both theoretical calculations and experimental results have shown that heavy atoms (e.g., N, S, and P) or carbonyl group play an essential role in the process of nπ* transition, which can effectively enhance the spin-orbit coupling (SOC) to improve the efficiency of ISC. [16]

Selection of precursors
Under high temperature and pressure conditions, appropriate precursors are chosen with the assistance of certain solvents and catalysts for aggregation, chemical reactions, carbonation, and nucleation to form CDs with specific structures and optical properties. [6] Small organic molecules with heavy atoms or heteroatoms, either in the form of aromatic rings or single chains, are expected to achieve high QY and strong SOC. [22,52,69,70] For example, triangular CQDs with the narrow-bandwidth emission (∼29 nm) are synthesized by solvothermal reaction of three-fold symmetric phloroglucinol. [19] Moreover, various precursors including polymers, [46] carbon fibers, [65] and natural resource [71] are also widely used in the preparation of CDs, which have attracted extensive attention due to their wide range of sources, simple availability, and unique optical properties. Importantly, the as-obtained CDs are expected to be dissolved in water for use in biological systems [36,72] or in organic solvents (e.g., o-dichlorobenzene, dimethyl sulfoxide, and N, N-dimethylformamide) for fabrication of electroluminescent LEDs. [83] Besides, it is very crucial to prepare CDs with targeting function or intrinsic targeting ability to cancer cells, in which some targeting ligands can be used as carbon sources for cancer cell targeting and detection. Specially, folic acid (FA) [74,75] and hyaluronic acid (HA) [76,77] possess high affinity to the folate and CD44 receptors overexpressed on cancer cells, respectively, and other special targeted molecules [66] can also specifically bind to cancer cells.

Postsynthetic modification
Since CDs contains specific functional groups, it offers a platform for a facile and versatile postsynthetic modification. According to previous experimental works and theoretical calculation results, the as-synthesized CDs through further surface/edge modification can considerably improve the solvent properties either in water or organic solvents. [73,78] Moreover, energy level and SOC of singlet and triplet state are expected to be regulated to achieve multicolor fluorescence, RTP and DF with high QY, which is crucial to the performance improvement of the electroluminescent device. Particularly, in biological system, edge modification of specific molecules can specifically recognize receptors or transporters in cell, resulting in efficient tumor-targeting performance. [59] It should be noted that the resulting RTP or DF of CDs should not only have short lifetime and high QY, but also have the properties of preparing high-performance electroluminescent LEDs, such as good film-forming and high and matched charge mobility, and so on.

Separation and purification
The separation and purification of CDs are of great importance for realizing quality control in both biological system and high-performance electroluminescent LEDs. At present, the main purification methods are achieved by centrifugation, dialysis, and column chromatography. [79,80] Among them, column chromatography is employed to separate and purify target samples according to the different polarity between each component absorbed on the column filler. However, complete separation of some components with similar polarity but different particle size still has certain limitations.
Recently, C 18 reverse column [81] and alumina column [82] are expected to achieve efficient separation of multicomponent of CDs with narrow particle size distribution. [79] The synthesis of CDs is the basis for realizing the novel structure and optical properties. Therefore, rational selection of specific precursors, post-synthetic modification, and efficient separation and purification are of great significance for F I G U R E 2 (A) (a) Energy gap and the number of aromatic rings calculated using DFT. Reprinted with permission: Copyright 2010, John Wiley and Sons. [34] (b) Sp 2 domains of RF-CQDs. Reprinted with permission: Copyright 2014, Royal Society of Chemistry. [74] (c) Dependence of the HOMO and LUMO energy levels on the particle size of CQDs. Reprinted with permission: Copyright 2020, American Association for the Advancement of Science. [100] [101] preparing CDs with great solubility, high QY, tunable luminescence, short lifetime of RTP or DF, high charge mobility, as well as cell targeting for tumor diagnosis and treatment.

Origin of luminescence
Although the optical properties of CDs have attracted significant attention, the origin of tunable PL remains a source of dispute with the deepening of research. Due to their nanoscale dimensions, the nonzero bandgap and related PL properties of CDs are expected to be readily adjusted by changing the carbon core and surface chemistry. Two popular mechanism models for the origin of luminescence of CDs have been proposed: one is the bandgap emission based on the conjugated π-domains, in which the quantum confinement effect is significant, and the other involves the surface defect-states, mainly manifested as the surface/edge effect ( Figure 2). For carbon quantum dots (CQDs) with an intact sp 2 carbon core, the bandgap emission of conjugated π-domains is generally regarded as the major intrinsic PL center (Figure 2A). [12,19] The bandgap emission is derived from the QCE of the quantum dots, resulting in the energy level splitting when the particle size is smaller than the exciton Bohr radius. [83,84] Therefore, the regulation of intact carbon core has a greater influence on optical properties of CQDs, such as the edge effect [85,86] and elements doping, [87,88] showing size-dependent emission. First, the arrangement of benzene rings in the carbon core leads to the formation of specific edge structures, such as zig-zag and armchair. [64,89] It has been proved that the CDs localization at the zig-zag edge reduces the bandgap by lowering the conduction band, while that of the armchair edge effectively widens the bandgap, causing a blueshift of emission wavelength. In addition, by synthesizing aromatic macromolecules with specific edge structures, Müllen et al. found that specific edge arrangements would lead to a significant change in the molecular vibration signals and polarity, resulting in unique magnetism, catalysis, electron transfer, and optical waveguide properties. [90][91][92] Typically, the in-depth study of edge effect is imperative to explore the optical and electronic properties of bandgap fluorescent CDs, including the emission wavelength, triplet energy level, FWHM, and charge transfer. On the other hand, to further explore the effect of size-dependence, Kang et al. used column chromatography to separate the electrochemically prepared CDs and obtained samples with sizes varying from 1.5 to 3.0 nm, corresponding to fluorescence emission from blue to red. [36] Through theoretical calculation, Wang et al. further confirmed that the PL characteristics of bandgap fluorescent CDs are consistent with their sizes, which is achieved by adjusting the bandgap of HOMO and LUMO. [36,93,100] The other fluorescence mechanism of CDs is characterized by excitation dependent emission from surface-related defect states as exciton capture centers, [94,101] which are structurally represented by sp 3 and sp 2 hybridized carbons and other surface defects of CDs, such as nitrogen-oxygen containing functional groups or connected polymer chains ( Figure 2B). [95,96] Notably, as the degree of surface oxidation becomes higher more surface defects on CDs can be introduced, resulting in obvious lattice distortion and redshift of the emission center. [97,98] Hu et al. synthesized various CDs by adjusting precursors and reaction conditions, concluding that the protonation or deprotonation of surface epoxides or hydroxyl groups results in significant changes of the peak position and fluorescence intensity. [99] In addition, surface interactions between the exposed specific functional groups and surrounding molecules will also affect their solubility and fluorescence properties, such as hydrogen bond, metal coordination, electrostatic interactive, π-π interaction, and so on. For instance, the pH-responsive red fluorescent CDs are formed by transforming the structure from lactone to quinone under strong alkaline conditions. [100] In addition, the fluorescence emission and intensity can be completely changed by the reduction of oxygen-containing functional groups. [94,101] For example, the intrinsic blue emission of CDs is greatly enhanced by passivating of -COOH and epoxy into -CONHR and -CNHR with alkylamines, while the surface defect state related green emission disappears. [101] Therefore, the aminecontaining functional groups can also make contribution to the surface defect states of CDs, in which the unpaired electron of nitrogen acts as an electron donor, adjusting the fluorescence properties of CDs.
Furthermore, for CPDs, the large number of incomplete carbonized polymer chains can divide the internal carbon core into multiple sp 2 -cluster regions, serving as multiple emission centers of fluorescence. [40] Recently, CPDs synthesized by the chemically cross-linked sodium alginate via glutaraldehyde show obvious excitation dependence and fluorescence enhancement, illustrating the existence of different fluorescence centers and complex energy levels. [102] Meanwhile, CPDs exhibit obvious molecular state luminescence and crosslink enhanced emission effect, that is, aggregation-induced emission, which is significantly different from the aggregation-induced quenching of traditional CQDs at high concentrations. It has been proven that the various chromophore and polymer around CPDs can reduce the vibration and heat loss between molecules by highly efficient immobilization and control the emission level, thereby achieving multicolor emission with high QY. [103,104] Recently, the origins of the red/near IR optical absorption observed in some precursor mixtures have been proposed to be from the molecular chromophore of thermally induced chemical reactions and have nothing to do with any nanoscale carbon entities produced by carbonization. [105] It overturns the previous understanding of the carbon core as emission center which is worthy of further discussion.

Fluorescence emission
Due to the difference in size distribution, the fluorescence emission of CDs covers a wide spectrum range from deep UV to NIR regions and has a large Stokes shift. [34,106] In addition, CDs with surface-related defect state exhibit excitationdependent fluorescence emission compared with semiconductor QDs and perovskite. [107] However, surface defects that capture excitons will greatly limit the practical applications of CDs, especially in LEDs, resulting in a low QY and poor luminescence efficiency. Therefore, the development of bandgap emission CDs is the key direction in future optoelectronic device ( Figure 3).
Our group synthesized multicolor bandgap fluorescent emission CQDs (MCBF-CQDs) from blue to red with QY as high as 75% ( Figure 3A). [12] Similar to the band edge transitions of semiconductor QDs with QCE, the fluorescence emission wavelength of MCBF-CQDs is basically independent of excitation in a wide range of wavelengths, and their maximum excitation wavelengths correspond to the exciton absorption peaks. Furthermore, the time-resolved fluorescence spectrum proved that MCBF-CQDs exhibit a single exponential decay, indicating radiative transition of excitons from a single channel. Moreover, transmission electron microscopy (TEM) confirmed the effect of size-dependence of highly crystallized CQDs, in which the fluorescence emission wavelength of MCBF-CQDs gradually redshifts as the particle size increases.
The QY of CQDs varies with the synthetic methods and surface states. It has been proved that the electronwithdrawing groups such as carboxyl and epoxy groups greatly reduce the recombination center of electron-hole and electron cloud density on the surface of CQDs, leading to the nonradiative decay. Therefore, modifying or passivating functional groups to reduce surface defect states is the key to improve QY. [108] By modifying amino molecules with carboxyl and epoxy groups, nonradiative recombination is significantly reduced, thereby enhancing the QY of CQDs. [95,109,110] Further theoretical calculations and experiments indicate that nitrogen/oxygen atoms can effectively regulate the energy band and electron cloud density. [4,14,50] In addition, alkyl groups have also been successfully used for surface passivation as electron-donor groups. For example, our group has synthesized three bright red emission CQDs (R-EGP-CQDs) at 637, 642, and 645 nm, which exhibit great solvent properties with the highest QY reaching 86.0% ( Figure 3B). [111] Theoretical studies show that the rigid πconjugated structure is the origin of red emission, and the passivation of -NMe 2 , -NEt 2 , and -NPr 2 induces efficient charge transfer, thus achieving maximum QY of red fluorescence CQDs so far. Despite extensive research on the electronic and optical properties of CQDs, CQDs generally produce low QY and broad emission with FWHM typically greater than 80 nm. Therefore, it is a challenge to develop narrowbandwidth multicolor CQDs.
By engineering the shape of carbon core, our group synthesized an unprecedented narrow-bandwidth emission (29 nm) CQDs with QY up to 54-72% (NBE-T-CQDs) (Figure 3C). [19] It is the first time that we updated the previous concepts of CQDs with wide emission and poor color purity. Notably, the NBE-T-CQDs had a special triangular structure with high crystallization, which passivated by pure hydroxyl functional groups around the carbon core. Moreover, the femtosecond transient absorption spectra, temperaturedependent PL spectra and theoretical calculations indicate that compared with the amorphous CQDs, the special highly crystalline triangular structure has a better delocalization and stability, significantly reducing the electron-phonon coupling. Therefore, the exciton emission peak with high color-purity and the narrow-bandwidth is achieved. More recently, by controlling the functional groups, a deep blue narrow-bandwidth CDs (35 nm) with QY of 70 ± 10% (HCP-DB-CDs) was synthesized by Sargent and coworkers. [36] Theoretical calculations show that the oxygen-containing functional groups (e.g., carboxyl group and hydroxyl group) at the edges of CDs increase the molecular vibration and distortion, leading to the broadening of FWHM. But the nitrogen-containing functional groups, such as amino group, can effectively reduce the bandgap fluctuation, thus increasing the delocalization of the wavefunction. Combined with time-resolved spectra, femtosecond transient absorption, and temperature-dependent PL spectra, it can be concluded that amino modified HCP-DB-CDs exhibited rapid exciton transfer, single exponential decay, and significantly fluorescence enhancement at low temperature, indicating extremely few defect centers and negligible nonradiation decay. Through modifying the molecular structure to replace the oxygencontaining functional groups with amino groups, narrower bandwidth, and stronger radiative recombination are realized. Besides, preparation of highly efficient rare-earth elementfree red/green/blue solid-state bandgap fluorescent CQDs (R/G/B-SBF-CQRs) with gram-scale has been realized, which provides a new idea for the overcoming aggregationinduced quenching of traditional CQDs ( Figure 3D). [137] At present, various experimental characterizations and theoretical calculations are the effective means to explain the fluorescence mechanism of CDs. However, there are still many obstacles to overcome, such as uneven size distribution, uncontrolled degree of crystallization and various surface defects. Therefore, well-designed synthetic route and clarification of the structure-property relationship of CDs would shed light on the luminescence mechanism.

RTP and DF properties of CQDs
According to the difference of the multiple spin states in the radiative relaxation process, the luminescence mainly involves singlet excitons with the same spin multiplicity and triplet excitons with different spin multiplicity. [112] Different from inorganic luminescence, organic optoelectronic materials with rich and diverse excited state structures lead to complex and various luminescent phenomena, including traditional fluorescence emission (nanosecond), phosphorescence emission (millisecond or second), and DF emission (nanosecond or microsecond), such as thermally activated delayed fluorescence (TADF), [113] and triplet-triplet annihilation (TTA). Importantly, when the energies of triplet excited state (T 1 ) and singlet excited state (S 1 ) are close, that is, ΔE ST is very small, the endothermic reverse intersystem crossing (RISC) process occurs through the thermal motions of the molecule atoms. As a result, the nonradiative triplet excitons are transformed into singlet excitons through RISC (T 1 →S 1 ), resulting in DF emission (S 1 →S 0 ) ( Figure 4A). In electroluminescence (EL), quantum efficiency is the basic parameter that characterizes electroluminescent LEDs, especially for EQE, which directly determines the degree of material utilization and the commercialization of electroluminescent devices. Under electrical excitation, excitons in organic fluorescent materials are typically divided into 25% singlet excitons and 75% triplet excitons, of which only singlet excitons for fluorescence emission can be used (S 1 →S 0 ). Unfortunately, due to the spin-forbidden transition, the electrically generated 75% triplet excitons are decayed as heat rather than light. Even with 100% PLQY for conventional F I G U R E 4 Fundamental mechanism of fluorescence, room-temperature phosphorescence (RTP) and delayed fluorescence (DF) phenomenon (A) in photoluminescence and (B) in electroluminescence fluorescent molecules, theoretically, the maximum EQE is only roughly 5% that is limited by the light out-coupling efficiency (∼20%) of OLED device. [37] Generally, the theoretical maximum external quantum efficiency (η EQE ) for electroluminescence LEDs can be estimated by the following equation [37] : where out is the light-out-coupling efficiency (typically out ≈ 0.2); γ is charge balance of injected holes and electrons (ideally γ = 1); η S/T is efficiency of radiative singlet and triplet exciton; Φ PLQY is the QY of PL in the emitter material. In order to break through the theoretical limit of electroluminescent devices, both singlet and triplet excitons should be used as much as possible ( Figure 4B).
So far, phosphorescence emission generally comes from noble metal complexes, such as iridium and platinum complexes, in which metal ions and complexes have strong SOC to enhance ISC. [114][115][116] Due to the high cost and biotoxicity of noble metals, the competitiveness of metal-free materials is greatly improved; nevertheless, the triplet excited state of RTP from metal-free materials can be easily quenched by oxygen and other nonradiative deactivation. Crystal state has been proven to be an efficient method to reduce molecular vibrations and nonradiative transition, however, the repeatability of sample is greatly reduced due to its dependence on the critical growth environment. [39,117,118] Considering the environment friendly, processability, and easy access, CQDs with abundant energy levels such as singlet and triplet states, offer an alternative approach to achieving the elusive metalfree RTP and DF. [16,37] Surprisingly, novel RTP and DF properties have been developed through embedding CQDs into various matrices, such as zeolites, [119] polyvinyl alcohol, [120] and polyurethane. [15] More recently, CQDs with intrinsic RTP properties without additional matrix doping has also been proven. For example, using urea as a precursor, by introducing plentiful carbonyl groups, blue-yellow fluorescent and phosphorescent CQDs with QY up to 25% were synthesized and realized single-component WLEDs, [16] which demonstrates a promising alternative RTP material based on the CQDs for developing high-efficiency optoelectronic device.

Realizing triplet emission of CDs
Despite numerous works on the RTP and DF properties of CQDs, the research on triplet modulation based on CQDs is still at an early stage compared with the well-developed transition metal complexes, which show extremely low phosphorescence QY and long lifetime (milliseconds or even seconds). [121] To realize highly efficient CQDs-based RTP or DF, there are two prerequisites: one is to stabilize the triplet states through enhanced ISC, and the other is to effectively suppress the nonradiative transition of the triplet states.
According to the first-order perturbation theory, the rate constant of intersystem crossing (k ISC ) is expressed as Equation (2): where 1 Ψ and 3 Ψ are the wavefunctions of the singlet and triplet states,Ĥ SO is the spin-orbit coupled Hamiltonian, ∆E ST is the energy gap between singlet and triplet states. This equation indicates that a larger SOC and a smaller ∆E ST favor the formation of a higher k ISC . Specifically: (a) The introduction of heavy atoms (Br and I) into the CQDs can increase k ISC by enhancing SOC, resulting in stronger RTP or DF ( Figure 5A). [40,113,122,123] For example, Kim et al. achieve bright RTP with QY of up to 55% by directional halogen bonding in cocrystal to activate efficient ISC. [124] In addition, according to the El-Sayed's rule, it is advantageous when the SOC between singlet and triplet states has different electronic configurations (e.g., 1 nπ* and 3 ππ*, 1 ππ*, and 3 nπ*). [125,126] Consequently, incorporation of carbonyl group or heteroatoms (e.g., N, S, and P) with lone pair electrons into CQDs can enhance SOC by inducing nπ* transition. For instance, persistent RTP with lifetime of 0.49 s has been achieved by promoting ISC through intermolecular electronic coupling of n-π in crystal. [126] It needs to be emphasized that in biological detection, RTP or DF with long lifetime is highly needed, which avoids the interference of human fluorescent background and scattering. However, the lifetime of RTP or DF used for electroluminescent LEDs should be less than a few tens of milliseconds. [127] The main reason is that the triplet states with long lifetime leads to the accumulation of excitons at (b) Reducing ∆E ST enhances ISC and reverse ISC, which has been successfully applied in designing RTP and DF ( Figure 5B,C). [58,128] Meanwhile, possible structures with smaller ∆E ST are selected by time-dependent density functional theory (TD-DFT) calculation, providing a new strategy for experimental synthesis of high-efficiency phosphorescent or delayed fluorescent CQDs. In structurally distorted donor-acceptor (D-A) system, the spatial separation of HOMO and LUMO results in a smaller ∆E ST . When 0.30 eV < ∆E ST < 0.80 eV, phosphorescence emission occurs, and when ∆E ST < 0.30 eV, DF emission occurs ( Figure 6B). In addition, by designing the aggregation states, such as H-aggregation and J-aggregation, it is possible to split the energy level and change the orbital gap, which has been extensively studied in aggregation-induced emission (AIE) ( Figure 6C).
The structural design of CDs is expected to achieve the regulation of energy level, [124] which mainly includes internal carbon core and external carbon shell. Among them, the regulation of carbon core includes size, morphology, and functional group. For example, the high crystallinity CQDs with triangular structure can effectively reduce the coupling between electron and phonon, thus enhancing QY, reducing the FWHM, and realizing high-efficiency fluorescence CQDs with multicolor and narrow-bandwidth. [19] Moreover, structural regulation of the external shell includes surface connection or reticulated structure. Theoretical calculations have shown that by connecting regular functional groups and molecules at the edges of CQDs, the distribution of electron cloud and SOC can be effectively regulated, achieving a smaller ∆E ST by adjusting energy level of HOMO and LUMO. [129] Furthermore, when CQDs are surrounded by chain-like polymers [116] or ordered crystals, [26] the rotation and vibration of the molecules can be reduced, thus achieving an efficient ISC ( Figure 6D). Therefore, structure regula-tion of CQDs can effectively adjust ∆E ST , enhance SOC and improve ISC, thus preparing phosphorescent or delayed fluorescent CQDs with high QY, short lifetime and tunable PL.

Light-emitting diodes
LEDs have been regarded as the next-generation lighting devices due to their high stability, low cost, environmentfriendliness, and large-scale production. [47] As a new type of carbon-based QDs, compared with expensive rare earthbased phosphors and toxic metal-based semiconductor QDs, CQDs appear to be one of the most promising alternatives to facilitate the development of electroluminescent full-color displays and lighting. Generally, the LEDs based on CQDs include two categories, phosphor-converted LEDs (pc-LEDs) and electroluminescent LEDs. The pc-LEDs refers to the use of blue/NUV chips as the main light source to optically pump CQD-based phosphors. For electroluminescent LEDs, under electrical excitation, electrons and holes are injected into the CQDs and undergo radiative recombination that acts as an active emission layer. Among them, the development of high efficiency electroluminescent LEDs is of great significance in next-generation display (Figure 7).

Phosphor-converted LEDs
Generally, blue/NUV LED chips and CQD phosphors are usually combined to generate pc-WLEDs. [73,[130][131][132] However, CQD phosphors are confined to self-quenching in the solid state due to direct π-π interactions, which is insufficient for practical applications. In this case, a common way that embedding phosphors into solid matrices (e.g., starch, silica xerogel, and polymers) has been carried out to  [193] (D) (a) Single-crystal structures. (b)The proposed mechanism for organic persistent RTP. (c) The normalized room temperature phosphorescence spectra. (d) Time-resolved PL-decay curves of the six target compounds. Reprinted with permission: Copyright 2018, Springer [194] realize solid-state lighting of CQD-based composites. Due to fewer multicolor CQDs with high QYs in solid states and the difficulty in solution-processability, the realization of high-performance multicolor pc-WLEDs still faces great challenges. In recent years, the research of efficient singlet-component CQDs in solid states has been carried out. For instance, the R/G/B-SBF-CQRs with high QYs up to 30-46% have been successfully applied to LEDs to achieve warm white light emission, which is promising to act as the substitute for traditional rare earth phosphors and realize industrialization in the future ( Figure 7A). [137]

Electroluminescent LEDs
Due to the tunable bandgap emission, great solutionprocessability, low toxicity, and large-scale production of CDs, electroluminescent LEDs based on CQDs have achieved rapid development in recent years. [19,30] However, because the excitation-dependent fluorescence emission caused by the surface-defect states on CDs, the carrier injection efficiency of optoelectronic devices is generally low. Therefore, it is very important to prepare bandgap emission CQDs. Notably, our group first reported the monochrome electroluminescent LEDs from blue to red based on MCBF-CQDs with an L max of 2050 cd m −2 and a current efficiency of 1.1 cd A −1 , which can even be compared with those of semiconductor QD-based LEDs ( Figure 7B). [12] It should be emphasized that electrons and holes are directly injected into the MCBF-CQDs as an active emission layer to generate electroluminescence by radiative recombination. Moreover, by engineering the rigid triangular structure of carbon core, fluorescent CQDs with FWHM of 30 nm yielded the unprecedented narrow-bandwidth electroluminescent LEDs, [19] which breaks the belief that CQDs possess broad emission and poor color-purity due to the bandwidth commonly exceeding 80 nm. The multicolored LEDs based on the NBE-T-CQDs display high color-purity, narrowbandwidth of 30 nm and high-performance with a L max of 1882 to 4762 cd m −2 and a current efficiency of 1.22 to 5.11 cd A −1 . These results lay the foundation for development of next-generation high-performance CQDs-based displays ( Figure 7C). However, compared to semiconductor QDs and perovskite, the EQE of highly efficient fluorescent emitting organic materials is only 5% because of the limit imposed by the statistics of the electron spin state under electrical excitation. [37] To illustrate, by mixing with the host material PVK, the R-EGP-CQDs-NMe 2 , -NEt 2 , and -NPr 2 F I G U R E 7 (A) Photographs and EL spectra of the operational pure (a) and warm (b) WLED lamp. CIE color coordinates (c). EL spectra of the warm WLED lamp (d). Reprinted with permission: Copyright 2021, John Wiley and Sons. [137] (B) (a) The device structure. (b-f) The normalized PL spectra, the corresponding EL spectra and the photographs of the operational LEDs from blue to red. Reprinted with permission: Copyright 2017, John Wiley and Sons. [12] (  [83] with a QY of up to 86% exhibit solution-processed electroluminescent warm-WLEDs with voltage-stable warm white spectrum, and high luminance of 5909 cd m −2 and current efficiency of 3.85 cd A −1 , but demonstrating a relatively low EQE ( Figure 7D). [83] With further optimization of the HTL and the host material, deep-blue LEDs based on HCP-DB-CDs with a QY of 70 ± 10% display high performance with a maximum luminance of 5240 cd m −2 and an EQE of 4%, which is the maximum EQE achieved to date. [36] In contrast, CQD-based LEDs using RTP and DF based on luminescence from the triplet state can achieve an internal quantum efficiency of 100%. However, LEDs based on triplet emission still faces great challenges due to the instability of triplet excitons, the difficulty in solution-processing and poor charge mobility, which is the top priority for the future development of CQDbased optoelectronic devices. From the perspective of device design, the high-efficiency electroluminescent LEDs based on triplet emission of CDs involves the following aspects. The first is the film-forming properties of CDs, which corresponds to the solution processing performance. The active emission layer with smooth and compact surface can greatly suppress nonradiative recombination by reducing the generation of leakage current. [83] The second is the energy level matching between the CDs as the active emission layer and organic transport layer. On the one hand, it can reduce the interface barrier of charge transition, making it easier to inject electrons and holes, and thereby enhancing the transfer and recombination of charges in device. [36] On the other hand, it can improve the balance and matching of electrons and holes  [66] (C) Schematic diagram and the photoresponse mechanism of the GQDs photodetector. Reprinted with permission: Copyright 2014, American Chemical Society. [40] (D) Schematic diagram of a GQDs PD device and band diagrams under different biases. Reprinted with permission: Copyright 2014, Springer. [39] (E) (a) Schematic illustration of the configuration of photodetectors. (b) Semilogarithmic I-V curves. (c) Time response of the devices with symmetric electrodes. Reprinted with permission: copyright 2015, American Chemical Society [146] that are injected from cathode and anode into the active emission layer, thereby promoting exciton recombination, and achieving high-efficiency electroluminescence. In general, in order to further promote the practical application of CDs in electroluminescent LEDs, more efforts are needed to explore.

Solar cells
Due to the wide range of light absorption, CQDs have potential applications in solar cells (SCs), such as dye-sensitized SCs, organic SCs, and silicon based SCs. [134][135][136][137] At present, by inserting a submonolayer layer of CQDs between the perovskite and the mesoporous titanium dioxide layers in a typical perovskite SC, the photocurrent and power conversion efficiency (PCE) of the corresponding SCs (from 8.81% to 10.15%) can be significantly improved. This work proved that due to the slow cooling of the hot electrons, it is possible to collect hot electrons from the photo-excited perovskite ( Figure 8A). [31] Moreover, by introducing CQDs to stabilize MAPbI 3 via passivation of the grain boundaries, the PCE (from 17.59% to 18.81%) was improved and the stability of perovskite SCs was increased ( Figure 8B). [138] Although there is little research on CQDs in the field of SCs, CQDs are still effective substitutes for toxic and unstable inorganic halide perovskites.

Photodetectors
Different from the exciton recombination in electroluminescence, photodetectors are used to collect separated photo-generated carriers. [139] In general, due to the characteristic absorption peak at 250 nm comes from the π-π* transition, CQDs can perform deep-UV photoluminescence and photodetection. For example, by coating N-CQDs onto interdigital gold electrodes, a nitrogen-doped CQD-based broadband photodetector with responsivity of up to 325 V W −1 is achieved ( Figure 8C). [34] Importantly, the CQDbased photodetectors show negative photoresponse under the irradiation of different light sources. Under light illumination, the bound excitons may capture the carriers to form photoinduced charge traps, which prevents the photogenerated electron-hole pair from moving freely. In addition, the photodetectors exhibiting high detectivity (> 10 11 cm Hz 1/2 W −1 ), high responsivity (0.2-0.5 A W −1 ) as well as abroad spectral response ranging from UV to NIR were achieved by combing CQDs with the p-type graphene sheets (Figure 8D). [33] Furthermore, photodetectors based on deep UV CQDs were fabricated, which have asymmetric electrodes with Au as the anode and Ag as the cathode. [140] Due to the high inhibition of the recombination of photonic carriers, even at a light intensity of 8 mW cm −2 , the device still has a high on/off ratio and short time of rise and decay ( Figure 8E).

F I G U R E 9 (A) (a)
The UV-vis absorption spectra and (b) PL emission spectra of GQDs, RBD-GQDs, and RBD-GQDs-Fe 3+ (c), The illustration for RBD-GQDs binding with Fe 3+ . Reproduced with permission: Copyright 2015, American Chemical Society. [169] (B) Schematic illustration of the g-CNQDs with glucose-binding sites for glucose detection. Reproduced with permission: Copyright 2015, Elsevier. [195] (C) Schematic diagrams of pH-responsive-GQDs for tumor imaging. Reproduced with permission: Copyright 2017, Royal Society of Chemistry. [110] (D) Schematic illustration of a fluorescence sensor for detection of CQDs and GO based DNA. Reproduced with permission: Copyright 2014, Royal Society of Chemistry [66] Taking into account the unique absorption and spectral overlap in the visible region, high QY and strong stability, CQDs have been explored and applied to realize efficient photoelectric catalysis [141] and lasers. By further structural design and multicomponent recombination, [142] the potential of CDs in optoelectronic devices is expected to be further studied.

BIOLOGICAL APPLICATIONS
The biocompatibility of CDs is reflected in their good water solubility and low toxicity. Hydrophilicity is one of the most principal properties for the biomedical applications. Modification of the hydrophilic functional groups on the edge of CDs can greatly improve the hydrophilicity of CDs. Watersoluble CDs has been extensively used as drug carriers to improve the effectiveness of cancer therapy. Low toxicity is another important biocompatibility feature of CDs. It has been widely demonstrated that as a kind of carbon nanomaterial, CDs usually show very low toxicity both in vitro and in vivo. [143,144]

Biosensing applications of CDs
CDs possess the specific physical and electrochemical properties, showing excellent sensing capabilities for biosensing. According to the excitation characteristics of CDs, various methods were designed to determine different targets such as metal ions, [145] small molecules [68,146] and other biological substances. [99,147,148] To this end, multiple detection strategies have been developed based on the fluorescent CDs.
Over the past decade, most studies have demonstrated that even very low levels of metal ions (e.g., Hg 2+ , Cu 2+ , Fe 3+ ) can have adversely effect on the human nervous and immune systems. [100,149,150] Therefore, real-time and sensitive detection of low concentration metal ions is an important approach for protecting human health. For instance, g-CNQDs can adsorb Fe 3+ on their surfaces through the electrostatic interactions and thus reduce their blue fluorescence. [151] In addition, graphene quantum dots (RBD-GQDs) with 43% QY can be used as a Fe 3+ turn-on fluorescent nanosensor with a detection limit of 0.02 μmol/L 100 ( Figure 9A). Nucleic acid is the main substance that constitutes the activity form of life. Therefore, quantitative analysis of specific sequences of single stranded DNA plays a key role in the diagnosis of generelated diseases. Correlating detection can also be realized using functional CDs. As shown in Figure 9D, CDs show high selectivity and sensitivity to distinguish the targeted DNA from the DNA with only one-base mismatch. [152]

Bioimaging applications of CDs
Bioimaging technology plays a key role in basic research and clinical settings. [153] Tracking cells in vivo is especially useful for noninvasive and real-time visualization of various biological processes, especially for cell-level therapy. The excellent features of CDs, such as good biocompatibility and penetrability, low cytotoxicity, easy clearance, and easy preparation, make them useful tools for labeling and imaging for diagnostic applications. [2,49,154,155] In truth, CDs can easily penetrate the biological membrane and accumulate in the cytoplasm or nucleus due to their small size and special physical and chemical properties, tumors. Copyright 2020, Springer Nature. [66] (C) (a), Schematic synthesis of the pSCDs and preparation of DOX-Loaded pSCDs (b), Schematic diagram for the drug delivery process of DOX-Loaded pSCDs. Reproduced with permission: Copyright 2019, American Chemical Society. [67] (D) (a), Schematic illustration of the preparation of the amino-rich red emissive carbon dots (RCDs) and Ce6-modified RCDs (Ce6-RCDs) (b), Schematic digram of the Ce6-RCDs for simultaneous in vivo FL, PA, and PT Imaging and PTT/PDT process. Reproduced with permission: Copyright 2019, American Chemical Society [196] thus acting as fluorescent probes. The potential of CDs in biological imaging was first discovered in 2006. [156] From then on, researchers have tried to use CDs to image a variety of cancer cells, such as MCF-7 cells, [157,158] HeLa cells, [158,160] HepG2 cells, [161] and so on. [162] CDs have been proved to emit fluorescence in the NIR region, thus simultaneously improving the signal-to-noise ratio and penetrating deep tissues. [67,163] In particular, some CDs can also reach the nucleus through the nuclear membrane. [58,72,164] For instance, as shown in Figure 10A, CSCNP-RCQDs with a red fluorescence peaking at 620 nm could effectively enter the nucleus of tumor stem cells for nuclear imaging. [165] Furthermore, the possibility of using CDs for various biolog-ical applications in vivo has also been evaluated. Glioma is a common but fatal brain tumor. However, due to the existence of blood-brain barrier (BBB), its imaging has become an important, urgent, and challenging task. [100] LAAM TC-CQDS is a qualified imaging diagnostic reagent for gliomas, which can penetrate the blood-brain barrier and interact with brain tumors through tail vein injection in mice. [59] Although fluorescence imaging plays an important role in intraoperative image-guided surgery, the spatial resolution remains low due to a limited depth of penetration. [166] Besides, fluorescence imaging is still unable to be used in large-scale living body scanning, such as human. A possible way to solve these problems is to develop multimodal imaging technology, which simultaneously utilizes another clinically applicable imaging technology, such as photoacoustic imaging (PAI), [167] positron emission tomography (PET), single photon emission computed tomography (SPECT), [168,169] magnetic resonance imaging (MRI) [170] or computed tomography (CT). [171] Among them, CDs has been widely studied as a multifunctional imaging probe.

Cancer therapy applications of CDs
Cancer therapy mainly depends on the inhibitory drugs, genes, and PTT/PDT drugs entering target organs to inhibit or even cure the tumor. Compared with the existing carrier materials (liposomes, polymers, albumin, etc.), CDs have the advantages of suitable size, large surface area, easy modification, good water solubility, and good biocompatibility, which can load and deliver the above-mentioned drugs to tumor cells. Many chemotherapy drugs usually suffer from poor water solubility, biocompatibility, as well as various side effects, and thus functional drug carriers are needed to avoid these shortcomings. It can be proposed that if CDs can bind to specific anticancer drugs and maintain stable adhesion, then it can load anticancer drugs and deliver them to the tumor site. [17,172,173] Gene therapy usually modifies or suppresses tumor growth by introducing therapeutic genes into tumor cells. It has also been shown that CDs, in addition to being nanocarriers for drug delivery, can also carry genes into the cells. [174][175][176] Besides, some kinds of CDs have therapeutic properties of their own, such as antibacterial and antiviral functions. [177][178][179][180] Vision-activated CDs can reduce the infection of MS2 phage model to host cells, and its antiviral effect is related to spot concentration and treatment duration. [181] In terms of antibacterial, it has been reported that CDs prepared by protamine sulfate exhibit strong antibacterial activity against Staphylococcus aureus, and highly sensitive to the structural differences of gram-negative bacteria. [182] PDT and PTT are promising noninvasive cancer treatment approaches for various types of tumors. [53,56] The combination of a photosensitizing agent and focused irradiation is used to elicit reactive oxygen in a localized area, and eventually destroy the cancer cells. [183] CDs can act as photosensitizer agent carriers owing to their biosecurity, excellent biocompatibility, and good surface functionalization, which greatly improves the efficiency of commercial photosensitizers. [163,184,185] For PTT, photothermal agents generate heat through transforming the energy absorbed by photons to destroy tumor cells. Specifically, CDs have become striking photothermal agents since they contain a large amount of π electrons. [186] In order to achieve more effective tumor treatment, people have conducted extensive researches on the selective administration of anticancer drugs in recent years. Enhanced permeability and retention (EPR)-based tumor therapy has been widely used in pre-clinical and clinical research. [187,188] However, it is not suitable for clinical application due to the poor selectivity. Receptor-mediated therapy strategies can specifically localize to targeted cancerous tissues. Since most of the receptors exist in both tumor and normal cells, very few up-regulated receptors can be found only on the can-cer cell membrane. In addition, each tumor has its own unique biological characteristics, so different types of cancer cells may not be able to target with the same ligand. [189] A promising method for tumor specific therapy is to use specific vector transporters with high differential expression in cancer cells, for example, LAT1 transporter and glucose transporter. [51,190] LAT1 is mainly responsible for mediating the transport of large neutral amino acids. It is overexpressed in a variety of tumors and has attracted much attention in cancer research. [191] Recently, we synthesized tumor-targeted LAAM TC-CQDs by hydrothermal treatment of TAAQ and citric acid. [59] The unique structure of LAAM TC-CQDs makes them mimic large amino acids and internalize into cancer cells through the LAT1-dependent, clathrin-mediated endocytosis. It can act as carriers and delivery chemotherapy drugs to tumor. The chemotherapy effect of TPTC was greatly improved after loaded with LAAM TC-CQDs, which can selectively transport TPTC to tumor cell ( Figure 10B).

CONCLUSION
In this review, latest progress in the synthesis, structure, and optical properties of CDs have been systematically summarized. Moreover, considering the excellent performance, CDs achieved an extensive range of applications, particularly in the fields of optoelectronic devices and biology. Systematic investigation on the relationship of structureproperties and luminescence mechanism of CDs are still one of the key factors promoting their further development. The combination of practice and theoretical calculation is essentially required to overcome these limitations. [192] In addition, precise synthesis and controllable structure, including the arrangement of benzene ring, surface modification, doping of elements, and adjustment of the shape and size of CDs are bottleneck at present. Therefore, multifunctional and high-precision structural characterization methods and comprehensive property analysis are urgently needed as well.
In the field of optoelectronic devices, the high-performance electroluminescent LEDs based on multicolor bandgap emission CQDs with narrow-bandwidth and high QY is of great significance. However, the study of fluorescent CDs needs to be further advanced, and the study of triplet state is still in the initial stage. The following objectives are expected to be pursued, including narrow-bandwidth multicolor fluorescent CQDs with QY of up to 100% and controllable triplet emission (e.g., lifetime, energy level, QY, and solutionprocessability). For electroluminescent devises, by satisfying the energy level matching condition, the efficient recombination of injected electrons and holes, [63] and the modification of heterojunction between active emissive layer and transport layer, it allows to reduce interface barrier and the charge attenuation, thus achieving high quality CQD-based film, low leakage current, and less exciton quenching. In addition, in catalysis, solar cells, lasers, photodetectors, and other fields, although the advantages of CDs have been shown, more indepth exploration is still the direction of future efforts.
In the field of biological application, we discuss the recent development of CDs about their advanced applications in biosensing, bioimaging, and cancer therapy. Although great progress for CDs within a short time has been accomplished, more efforts still need to be made in the future. In the field of cancer treatment, most broad-spectrum anticancer drugs, such as paclitaxel, are insoluble and cause widespread damage. Therefore, CDs as targeted drug carriers can effectively improve the water solubility and targeting of anticancer drugs, thereby achieving efficient tumor diagnosis and treatment. However, considering the clinical needs, more in-depth research on the targeting mechanism of CDs will be a critical battle for future development. In addition, the sudden COVID-19 has made a negative impact on human life in recent years, so the detection of nucleic acid combined with the advantages of CDs will be an urgent need. In addition, immunotherapy, as a new cancer treatment strategy, has attracted much attention owing to its broad clinical potential. With the rapid development and various advantages in biomedical applications, CDs can be used as special adjuvants for tumor suppressant vaccine. For example, CDs-ovalbumin nanocomposites can effectively enhance the expression of costimulatory molecules CD80 and CD86 and product tumor necrosis factor α (TNF-α) from dendritic cells. The first report of CDs as vaccine adjuvants for tumor inhibition is reported recently. [197] We believe that CDs will have a remarkable performance in immunotherapy in the future.
Finally, studies of CDs in these fields are at an early stage, and plenty room still exists for further improvement. With continuous deepening of both theoretical and experimental research, we are looking forward to more breakthroughs of CDs from fundamental research to practical application.