Evolution and fabrication of carbon dot-based room temperature phosphorescence materials

Traditional room temperature phosphorescence (RTP) materials usually include organometallic composites and pure organic compounds, which generally possess the disadvantages of high toxicity, high cost and complicated preparation. Carbon dots (CDs) are a new kind of luminescent material and have attracted widespread attention due to their benefits of excellent tunable emission, nice biocompatibility, cost-effectiveness, facile preparation and environmental friendliness. Since photoluminescence is an important luminescent property of carbon-based fluorescent nanomaterials, CD-based RTP materials have sparked a new research wave due to the properties of extremely long phosphorescence lifetime, large Stokes shift and high environmental sensitivity. In order to construct excellent CD-based RTP materials, many attempts have been made, and the corresponding progress has been achieved. Herein, we summarize the progress in CD-based RTP materials in recent years, mainly focusing on the outstanding contributions over the years, phosphorescence emission, phosphorescence lifetime, preparation and application of CD-based RTP materials. In particular, this review provides a comprehensive summary and analyze the outstanding contributions in the fields of the phosphorescence emission and phosphorescence lifetime of CD-based RTP materials over the years. Finally, several existing challenges and the future outlook of RTP materials based on CDs have been put forward.


Introduction
Since the discovery of carbon dots (CDs) in 2004 1 and aer nearly twenty years of development, researchers have put many efforts towards understanding them. As a new class of zerodimensional uorescent nanomaterials, CDs have attracted widespread concern and great research interest from a wide range of researchers owing to their distinguished properties such as adjustable photoluminescence (PL), excellent stability, facile surface functionalization, environmental friendliness, low toxicity and good biocompatibility. [2][3][4] According to previous reports, 5 CDs can be classied into four types, namely carbon nanodots (CNDs), carbon quantum dots (CQDs), graphene quantum dots (GQDs) and carbonized polymer dots (CPDs). Specically, GQDs are small graphene fragments less than 20 nm in lateral dimension and less than ve graphene akes (∼2.5 nm) in height, exhibiting pre-existing graphitic domains (sp 2 domains) and edge-rich chemical groups. CQDs are morphologically quasi-spherical carbon nanoparticles with a distinct lattice and chemical groups on their surface, and they have a crystalline core based on a mixture of sp 2 and sp 3 domains. CNDs are also dened as quasi-spherical carbon nanoparticles, which consist mainly of cores with an amorphous structure, i.e., CNDs have a certain degree of carbonization. CPDs have a hybrid polymer/carbon structure formed through the aggregation or cross-linking of linear polymers or monomers, with a surface consisting of abundant functional groups/polymer chains and carbon cores. Up to now, some notable achievements have been obtained in CD-related research, including the enhancement of photoluminescence quantum yield (PLQY), the regulation of multicolor uorescence, the study of the luminescence mechanism and potential multifunctional applications. [6][7][8][9][10][11][12][13][14] More importantly, the selection of different carbon source precursors and synthesis methods will lead to different uorescence properties. The uorescence of CDs is only one of the most common phenomena in their luminescent properties. In addition, CDs also possess other types of excellent luminescent properties, such as room temperature phosphorescence (RTP), thermally activated delayed uorescence (TADF), up-conversion luminescence (UCL), chemiluminescence (CL), electrochemical luminescence (ECL), mechanoluminescence and so on. 15 These luminescent properties have shown signicant application prospects in biomedicine, optoelectronic devices, energy storage, sensing, anticounterfeiting, catalysis, imaging and other elds. [16][17][18][19] Particularly, the RTP phenomenon of CDs has aerglow emission performance which is very attractive because of its prolonged emission lifetime, larger Stokes shi and minimized interference from short-lived auto-uorescence and scattered light. Now it has shown a broad application prospect in anticounterfeiting, information encryption, sensing and other advanced elds.
In the last twenty years, the research on CDs has been widely carried out, and people have also reviewed and summarized their uorescence in various aspects, such as the classication, synthesis methods, uorescence regulation, uorescence mechanism, different precursor selection, and applications in various elds. 6,8,10,14,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] At the same time, the corresponding prospects for the uorescence development of CDs are also proposed. Although there are some reviews providing a brief introduction of the aerglow phenomenon (i.e., RTP and TADF) of CDs, [15][16][17][18][19][35][36][37][38] there are few articles specically describing the aerglow phenomenon related performance in detail. 39,40 Based on the results previously reported, the radiative transition of triplet excitons is the key factor of the long aerglow phenomenon produced by CDs. The phosphorescence of CDs needs to be generated and enhanced by heteroatom doping or by embedding CDs into host matrices (e.g., boric acid, polyvinyl alcohol, inorganic salts, layered double hydroxides, and zeolites) or by immobilizing CDs on a substrate. These matrices are essential for stabilizing the long-lived ternary state of CD uorescence by effectively isolating and rigidifying CDs, minimizing the non-radiative leap of the triplet exciton and avoiding collisions of CDs with oxygen. In order to better distinguish the principal difference between uorescence and phosphorescence, as shown in Fig. 1, the ground state (S 0 ) transits to the lowest singlet excited state (S 1 ) aer being absorbed by ultraviolet and visible light, and then re-radiates back to S 0 and emits uorescence. The generated excitons rst enter S 1 from S 0 , and then enter the lowest triplet state (T 1 ) through the intersystem crossing (ISC) process, and the excitons return to S 0 again and generate phosphorescence through the process of radiation transition. Or the exciton rst goes through the process of reverse intersystem crossing (RISC), returns to S 1 from T 1 , and then returns to S 0 to generate delayed uorescence. Obviously, since the energy of T 1 is lower than that of the singlet state S 1 , the phosphorescent emission peak will be signicantly red shied compared with uorescence. It is worth noting that the uorescence phenomenon of CDs is easy to realize, but the RTP of CDs is much harder to achieve. This is mainly the result of the intrinsic properties of the triplet excitons in relation to the spin-forbidden transition. More importantly, the triplet exciton is readily exhausted by vibrational deactivation and oxygen quenching under room temperature or higher. Thus, it can be seen that in order to obtain effective RTP, two important conditions need to be met, namely to improve the rate of ISC by enhancing the spin-orbit coupling of excitons and to stabilize the triplet-excited states via using structural connement. For the rst condition, it is generally possible to enhance the n-p* transition by incorporating transition metals, halogens or heterocycle groups to promote the ISC process and further generate triplet excitons. The second condition can be well achieved by encapsulating CDs in various substrates or creating self-protective structures (without substrate immobilization). Up to now, some efforts have been made to realize CD-based RTP materials to further explore their potential applications. Although some achievements have been made in the synthesis, construction and application of CD-based RTP materials, so far no article has systematically and completely summarized and analyzed them. And also, there have been several reviews on CD-based RTP materials in recent years, but they are only summarized and analyzed unilaterally for the synthesis, application or construction of CD-based RTP materials. Therefore, this review

Synthesis of CD-based RTP materials
Since the rst report on CDs in 2004, 1 a variety of preparation methods have been reported successively, which can be mainly divided into two categories through summary analysis, namely, top-down approach and bottom-up approach. 8,10,21,25-29, 41 As a special characteristic of CD-based luminescent materials, RTP has also been developed and reported. A variety of methods have also been reported for the synthesis of CD-based RTP materials, and they can also be mainly divided into two categories, namely matrix-assisted method (Table 1) and selfprotection method ( Table 2). The matrix-assisted method refers to the generation and enhancement of CD RTP by embedding them in a host matrix (e.g. boric acid, polyvinyl alcohol, layered double hydroxides, zeolites, etc.) or by immobilizing them on a substrate. This method has a wide range of practicality and can achieve the RTP properties of most CDs, but most of this method requires the use of multiple steps and harsh experimental conditions, oen requiring high temperatures to achieve. The self-protection method means that the RTP properties of CDs are achieved by self-doping. This method is convenient, simple and highly scalable, but it is not suitable for large-scale preparation and it is difficult to nd suitable precursors. At present, the most popular method is matrix-assisted synthesis. To date, in the preparation of metal free RTP materials, especially carbon-based RTP materials (mainly represented by CDs), more and more attention has been paid. The number of papers published is also increasing year by year, as shown in Fig. 2. As of August 31, 2022, a total of 166 articles related to CD-based RTP had been published through the keyword search of the paper. It is worth affirming that there may be omissions in the process of literature retrieval. We are sorry if some literature has not been retrieved.

Matrix-assisted synthesis of CD-based RTP materials
The matrix-assisted approach involves embedding CDs into appropriate matrix structures, such as polymeric matrices, crystalline structures, mesoporous structures, inorganic layer structures, etc. These matrices can provide a strong rigid environment, dense hydrogen bonding sites or strong covalent bonds that can lock their excited triplet states and suppress their non-radiative relaxation, resulting in RTP emission. To date, most CDs-based RTP has been achieved by embedding a number of matrices (Table 1), and many RTP CD-based matrix composites have been discovered by combining the outstanding PL properties of CDs with the effective connement effects of various matrices. Polymers are considered ideal matrices because of their rich functional groups and long regular chains, which not only provide stable chemical bonds with CDs, but also have the effect of separating solvent and oxygen and thus avoiding aggregation-induced uorescence quenching. In particular, polyvinyl alcohol (PVA) was the rst matrix material utilized to achieve CD-based RTP, which is currently the most commonly employed matrix support material. [42][43][44][45][111][112][113][114][115][116][117] In 2013, Deng et al. 42 rst reported the observation of phosphorescence emission by compounding CDs with the PVA matrix (Fig. 3A). It is found that the PVA matrix plays a key role in protecting triplet states with long life from being effectively quenched by nonradiative processes, because it has a large number of hydrogen bonds that can effectively lock the emitting species and inhibit their intramolecular motions, i.e., a non-radiative relaxation channel. In addition, PVA molecules also have a large number of hydroxyl groups, which can effectively form hydrogen bonds, making the C]O bond on the surface of CDs rigid, limiting the intramolecular motions and preventing nonradiative relaxation. Besides, oxygen is a strong quencher of the triplet state, but PVA has excellent oxygen barrier properties. Therefore, another potential function of PVA is to effectively prevent the direct collision between aromatic carbonyls and oxygen molecules, thereby promoting phosphorescence. Mesoporous materials are also a suitable class of matrixassisted materials for achieving the RTP properties of CDs, which are a class of amorphous, ordered or crystalline materials with pore sizes of 2-50 nm that are good host matrices, and capable of accommodating luminescent species. As such, it is capable of accommodating CDs to achieve RTP, particularly silica, and is widely applied for the generation of CD RTP. [53][54][55][56][57][58][59][60]107,118 For example, Li et al. 54 proposed a strategy for achieving ultra-long RTP in air-saturated aqueous media based on carbon dot-based silica composites (CDs@SiO 2 ) (Fig. 3B). The formation of Si-O-C bonds between CDs and silica during TEOS hydrolysis acts as a scaffold for the nucleation and the growth of the silica framework. The CDs are able to covalently attach to the silica network and the silica acts as a matrix that contributes to the dispersion of the CDs, providing protection from environmental bursts such as water and oxygen. More importantly, the abundance of silanol groups on the surface of the composites gives the whole hybridized system a good hydrophilic character. And also, Sun et al. 57 designed and developed a strategy to fabricate metal-free multi-conned CDs (CDs@SiO 2 ) within SiO 2 by generating an effective multi-connement effect (MCE) to develop room temperature phosphorescent materials with simultaneous ultra-long lifetime, high phosphorescence quantum efficiency and excellent stability (Fig. 3C). In addition, zeolites are a class of matrix-assisted materials widely utilized to achieve ultra-long phosphorescence lifetimes, and they are crystalline aluminosilicates or aluminophosphates with a threedimensional 4-connected structure and a uniform pore size of less than 2 nm. [63][64][65][66][67][68]119 For instance, Wang et al. 64 have achieved a facile strategy to modulate the RTP properties of CDs through donor-acceptor energy transfer in the CDs-zeolite system by introducing heteroatoms into the aluminium phosphate zeolite backbone to construct an efficient donor-acceptor system that promotes exchange coupling between the CD exciton and the dopant in the matrix (Fig. 3D). Furthermore, the connement effect of the crystal structure not only immobilizes T 1 but also prevents non-radiative relaxation of T 1 and is also a promising method for generating CD-based RTP. Crystal structures can be divided into two categories: organic crystal structures and inorganic crystal structures. The organic crystal structures include urea, cyanuric acid and melamine, while inorganic crystal structures include sodium chloride, sodium hydroxide, boric acid and so on. [46][47][48]70,[72][73][74][75][76]78,79,82,[120][121][122][123][124][125][126][127][128][129][130] For example, Wang et al. 124 proposed a molten salt method for the preparation of CD-based RTP materials by selecting high charge density magnesium salts and phosphates as doping salts, and calcining the carbon source directly in the presence of inorganic salts (Fig. 3E). During the melting and recrystallization process, CDs are formed and incorporated into a matrix with a special crystal structure. As magnesium phosphate is insoluble in water, the solid matrix provides rigid protection for the CDs. As a result, the CDs are non-phosphorescent as monomers, but the RTP  phenomenon is initiated and enhanced by the aggregation of the salt matrix. Besides this, structural connement displays a unique advantage by reducing the rate of the radiative relaxation process. A number of special structural matrices have also been used for the realization of CD RTP, such as twodimensional (2D) layered double hydroxides (LDHs) which have attracted signicant interest in the available connement matrix owing to their versatile chemical composition and layer charge. Signicantly, LDHs have a nanoscale space that provides a constrained and rigid environment for a number of chromophores that exhibit enhanced uorescence properties (intensity, efficiency, lifetime, etc.) by suppressing the nonradiative relaxation of single-linear excitons. 52 Therefore, exploring the relationship between the relaxation paths of triplet-state excitons and the rigidity of LDHs will help to achieve optimal luminescence efficiency and to develop a new class of RTP materials. Kong et al. 52 employed LDHs as a matrix and proposed the design principle of activation to achieve RTP on CDs through three synergistic effects (structure-bound effect, heavy atom effect, and chemical bonding) (Fig. 3F). The conned and rigid environment of LDHs suppressed the nonradiative deactivation of CD triplet excitons and improved the luminescence efficiency. In conclusion, in addition to the above matrix-assisted materials, there are also other appropriate matrix-assisted materials suitable for achieving RTP of CDs. Therefore, the appropriate selection of matrices is key to achieving efficient and ultra-long RTP CDs.
The earliest use of self-protection methods to achieve the RTP of CDs dates back to 2014, when Yan et al. 86 employed citric acid and ethylenediamine as carbon and dopant sources to achieve the RTP of CDs, but the phosphorescence lifetime at that time was only 160 ms, and thus this result did not attract widespread attention. Until 2016, Chen et al. 87 achieved the RTP of CDs in one step using PVA and ethylenediamine as reaction precursors, with a phosphorescence lifetime of up to 13.4 ms. At this time, the RTP of CDs gradually came into the limelight, and the research frenzy gradually surged. Later, Jiang et al. 131 by simple heat treatment of ethylenediamine and phosphoric acid could produce unexpectedly long room temperature phosphorescence with a duration of about 10 s and a lifetime of 1.39 s (Fig. 4A). It is suggested that the doping of N and P elements is the primary factor in achieving the RTP of CDs. And then, Long et al. 91 reported a new type of uorine-nitrogen codoped CDs, which were obtained by a one-step solvent heat treatment, exhibiting excellent water solubility and blue uorescence in solution or on lter paper, together with pHresponsive green self-protective RTP (Fig. 4B). The spatial protection of C-F bonds and hydrogen bonds was found to reduce the quenching of RTP by oxygen at room temperature, which is key to achieving RTP in CDs. In addition, Wang et al. 94 have successfully engineered a one-step approach for the synthesis of gram-scale CDs with up to 41% total QY by utilizing the polymerization, deamination and dehydration reactions of urea and phosphoric acid in an aqueous environment (Fig. 4C). It was similarly demonstrated that doping with N and P was responsible for achieving white light emission. Also, Jiang et al. 89 reported a facile, rapid, gram-scale preparation of ultralong RTP CDs by employing microwave-assisted heating of aqueous solutions of ethanolamine and phosphoric acid (Fig. 4D). Further studies showed that the amorphous polymer-like structure of the CDs, intraparticle hydrogen bonding and the presence of doping elements N and P were the major factors responsible for their ultra-long RTP. Furthermore, Wang et al. 103 proposed a thermally driven amorphouscrystalline phase transition-based strategy to achieve multicolor CPDs with emission colors adjustable from green to orange-red. Further studies have shown that self-protected covalent crosslinking framework formation as well as the codoping of multiple heteroatoms play a crucial role in the generation of RTP CDs. The color tunability of RTPs can be attributed to the different crystalline contents of conjugated p-domains within the CPDs (Fig. 4E). Similarly, Zhao et al. 98 demonstrated that efficient blue-green uorescentphosphorescent double-emitting CDs could be easily obtained via solvent-free pyrolysis of hydroxyurea (Fig. 4F), and it was demonstrated that the efficient double-emitting properties of the uorescent-phosphorescent CDs were derived from the aromatic carbonyl group at their edges. In summary, co-doping (e.g. N and P co-doping or N and F co-doping) strategies are one of the most effective means applied to achieve ultra-long and efficient synthesis of RTPs with self-protected CDs. In addition, the rational use of edge groups (carbonyl groups) is also expected to achieve self-protected RTP CDs.

Annual representative studies on CD-based RTP materials
According to the synthetic methods described earlier, the synthesis of CDs RTP has been a positive achievement, with progress being made every year. In each year of work, different matrices or methodologies have been employed to fabricate high performance RTP CDs, and the corresponding structures of the resulting RTP CDs varied. Examples include the use of doping to form covalent bonds to achieve RTP of CDs by selfprotection 91 (Fig. 5A), the use of cyanuric acid 74 (Fig. 5B) and boric acid 77 (Fig. 5C) as substrate-assisted methods to form rigid structures to encapsulate immobilized CDs to achieve RTP, the use of silica 55 (Fig. 5D) and zeolites 63 (Fig. 5E) as mesoporous media materials to encapsulate immobilized CDs to achieve RTP, and the use of layered double hydroxides 51 (Fig. 5F) as media to immobilize CDs by intercalation to achieve RTP. So far, some signicant achievements about RTP CDs have been realized and are shown in Fig. 5G. All these studies were outstanding contributions and prominent representatives in that year, but they were not ranked in the same year. In 2013, Deng et al. 42 employed PVA for the rst time to reveal the phenomenon of RTP in CDs, indicating that the hydrogen bonding of the PVA matrix and the C]O bonding on the surface of CDs can achieve cross-linking to form a stable rigid structure, which in turn facilitates the achievement of RTP. In 2014, Yan et al. 86 rst observed the RTP of water-soluble CDs without a deoxidizer and other inducers in pure aqueous solution. They found that due to non-radiative electron transfer, phosphorescent emission can be quenched in the presence of iron ions (Fe 3+ ), and then turned on by phosphate ions through strong interaction. In the presence of CD-based RTP, it is possible to easily avoid the interference of deoxidizers and other inducers necessary in conventional RTP detection, as well as self-uorescence and composite matrix scattering light encountered in uorescence spectrometry. In 2015, Gui et al. 133 for the rst time reported RTP logic gates involving RTP emission based on carbon materials. They used capture ssDNA (cDNA) modied CDs and graphene oxide (GO). That is, rstly, the cDNA was combined with carboxyl group surface modied CDs (HOOC-CDs) by carbodiimide chemistry, and then the cDNA-CD conjugates were adsorbed on the surface of GO through p-p stacking interactions to form a cDNA-CD/GO complex. They veried the feasibility of the RTP logic gate based on the cDNA-CD/GO system. The logic gate is simple, easy to operate, and low-cost, does not require complex labeling and modication, and can be efficiently used in actual samples. In 2016, Jiang et al. 111 reported for the rst time triple-mode emission (i.e., photoluminescence (PL), up-conversion PL and RTP) on luminescent CDs prepared by compounding mphenylenediamine and PVA. It is worth noting that this work is the rst time to realize simultaneously triple-mode emission with a single material and put forward the advanced anticounterfeiting of a triple authentication concept. And Li et al. 47 synthesized a highly efficient CD-based phosphorescent material by heating a mixture of urea and nitrogen doped CDs (NCDs) using one pot. It was found for the rst time that new energy structures can be generated by C]N bonds on the NCD surface, and evidence that they are the origin of phosphorescence is proposed. Then, Tan et al. 49 prepared simple and large-scale nitrogen doped carbon quantum dots (N-CQDs) under microwave irradiation which was a new strategy to synthesize N-CQDs via using isophorone diisocyanate (IPDI) as a single carbon source. N-CQDs were dispersed in the polyurethane (PU) matrix, under the excitation of ultraviolet (UV) light, and they emitted not only FL, but also phosphorescence and DF at room temperature. In the following year, amphiphilic carbon quantum dots (ACDs) were also successfully prepared by Tan et al. via treating oil-soluble Ndoped carbon quantum dots by a one pot hydrothermal method. 134 The obtained ACDs may be homogeneously dispersed in PVA and the PU matrix. Furthermore, RTP can be recognized in these ACD based composites. This is the rst time that ACDs realize phosphorescent emission in polymers, which greatly broadens the research and application scope of CQDs. In 2017, Chen et al. 87 reported for the rst time the aggregationinduced RTP of self-quenching-resistant nitrogen doped CD powder by structure design via using PVA-chains, and the potential application of the temperature sensor is preliminarily prospected. Jiang et al. 53 also reported a new method for preparing room temperature long aerglow materials by covalently xing CDs onto colloidal nanosilica (nSiO 2 ). A CDbased long aerglow material (i.e., m-CDs@nSiO 2 ) is reported for the rst time, and the material is suitable for room temperature, and can even be directly observed in an airsaturated aqueous medium. The results show that the long aerglow materials of m-CDs@nSiO 2 have mainly delayed uorescence properties and mixed partial phosphorescence. In addition, Joseph et al. 62 also showed the observation of RTP emitted by CDs embedded in a silica gel matrix at room temperature. CDs in silica gel showed a long phosphorescence lifetime of 1.8 s, which is the highest value of CDs in solid state matrices, and the phosphorescent emission is displayed in the white gamut region in the chromaticity diagram. As a type of inorganic porous material, zeolite has emerged as one of the most desirable materials for loading and encapsulating metal nanoparticles and luminescent quantum dots because of its three-dimensional ordered structure and strong thermal stability. In the same year, Liu et al. 63 embedded CDs in a family of zeolitic crystalline matrices in situ under solvothermal/ hydrothermal conditions to prepare a novel category of CDbased TADF materials with ultra-long lifetimes and achieved high quantum yields (QYs) up to 52.14%. This work offers a new "dots-in-zeolites" scheme for the design and synthesis of innovative TADF materials, which may open the application of various delayed uorescence in various elds. In 2018, He et al. 43 used electrospinning technology to incorporate CDs into PVA, and achieved the two goals of RTP and TADF. It was found for the rst example of the CDs/polymer system that the ordered mesoporous structure of electrospun CDs/PVA nanobers allows effective stabilization of the triplet state of CDs, thus realizing the RISC process of TADF. To date, this is the rst time that TADF has been found in the CDs/polymer system. Jiang et al. 131 reported the rst example of conversion of a uorescence material to RTP with an external heat stimulus, that is using ethylenediamine and phosphoric acid as raw materials, uorescent emissive polymer CDs were prepared by simple heat treatment and can produce unexpected ultra-long RTP. It was found that the doping of N and P elements may be critical to the RTP production of CDs. Similarly, Jiang et al. 89 again reported an ultra-long CD-based RTP material by microwave-assisted heating of an aqueous solution of ethanolamine and phosphoric acid which showed the longest RTP lifetime (1.46 s) for CD-based materials to date. Long et al. 91 reported for the rst time uorine-nitrogen co-doped CDs (FNCDs) possessing long-lived triple excited states which emit pH-stabilized blue uorescence and pH-responsive green selfprotected RTP. In 2019, Li et al. 54 proposed a reasonable strategy for a kind of silica-based composite using CDs (CDs@SiO 2 ) in air saturated aqueous media for realizing ultralong RTP. More importantly, they reported the rst application in practical imaging of biological samples for CD-based RTP materials. Also, through one-step heat treatment of nitrogen doped CDs and boric acid (BA) (N-CDs/BA), a universal method for activating RTP of both heteroatom-free and heteroatomcontaining CDs was achieved. 120 N-CDs/BA exhibits the best phosphorescence lifetime of 2.26 s and a PQY of 17.5%, representing the most advanced record for CD-based RTP materials to date. In addition, Su et al. 92 proposed a method of microwave synthesis of nitrogen and phosphorus co-doped CDs (P-CDs) via using triethanolamine as the carbon source and phosphoric acid as the dopant. The prepared P-CDs showed not only bright-blue uorescence in aqueous solution, but also obvious green phosphorescence on lter paper. More importantly, this study successfully realized a dual-channel signal for the detection and analysis of the pH value by using P-CDs for the rst time, and found that the RTP signal is more sensitive than the uorescence signal, and thus may provide a wider linear range. In the same year, Yuan et al. 135 also demonstrated for the rst time that the synthesized singlecomponent white carbon nitride quantum dots (W-CNQDs) exhibit a double emission of blue-yellow uorescencephosphorescence. The W-CNQDs offered an overall photoluminescence quantum efficiency (PLQY) of 25%, which is the largest of any white light-emitting material reported so far. In 2020, using CD modied amorphous silica (CDs-SiO 2 ) as the raw material, He et al. 55 realized RTP and TADF in the solid state and aqueous solution through a one-pot sol-gel methodology without the addition of any heavy atomic scramblers and the removal of dissolved oxygen. Importantly, the thermoluminescence spectra of the CD-based matrix were for the rst time visualized and evaluated. Jiang et al. 136 reported for the rst time a simple method to prepare CDs (i.e., TA-CDs) with double emission, robustness and aggregation induced RTP properties. The study showed that the yellow RTP of TA-CD powder may be due to its aggregation. Liang et al. 58 used silica to conne water-soluble phosphorescent carbon nanodots (WSP-CNDs@silica) in their nanospace and realized the ultra-long and efficient phosphorescence of CNDs. The phosphorescence lifetime and quantum yield (QY) reached 1.86 s and 11.6%, respectively, which is the best value of water-soluble phosphorescence nanoparticles reported so far. It is shown that the silica shell outside of CDs restricts the rotation and vibration of the bonds in CDs, resulting in the long life and high efficiency phosphorescence of CDs. Park et al. 76 reported a new engineering approach to manage singlet-triplet energy splitting (DE ST ) in graphene quantum dots (GQDs)/graphene oxide quantum dots (GOQDs) through varying the ratio of oxygencontaining carbon to sp 2 carbon (g OC ). This is the rst time that GQDs are used as a long-life RTP and TADF material to demonstrate anti-counterfeiting and multi-level information security. Wang et al. 94 developed a one-step synthesis method with low cost, rapid processing and environmental protection for the fabrication of single-component white luminescence carbonated polymer dots (SW-CPDs) on a gram scale with high efficiency. The overall QY was as high as 41%, and that was the largest value ever recorded for solid-state uorescent CDs. The results showed that the hybrid uorescent/phosphorescent components promoted the emergence of white light emission. In addition, Zhang et al. 66 successfully achieved high-efficiency aerglow CDs@zeolite composite materials by simply grinding the solid raw material zeolite and precursor CDs at room temperature and then performing thermal crystallization of CDs@zeolite by a solvent-free thermal synthesis strategy. In this method, CDs are embedded into the growing zeolite crystals with the maximum extent, and the non-radiative transition of CDs is surpassed by the strong host-guest interaction, and thus composite materials with ultra-long double emission of TADF and RTP are prepared, and have a lifetime of 1.7 s and 2.1 s, respectively, as well as a QY of 90.7% and a PQY of 24.4%, respectively. These values are at the top superiority level among CD-based PL materials, and thus, represent the majority of organic aerglow materials. What's more, in 2021, Li et al. 79 reported a method to prepare highly efficient RTP materials from crystalline heat-annealed CDs and BA composites which can induce amorphous to crystalline transition by grinding. This method can enable CDs to be uniformly embedded in BA crystals to the maximum extent, reduce the non-radiation attenuation of CDs, and promote the cross-connections between systems by suppressing the free vibration of CDs, thus generating strong RTP materials. The reported PQY is the highest (48%). It is well known that water-soluble red aerglow imaging agents have a great penetration depth and nondurable excitation characteristics which have potential application prospects in time-gated aerglow bioimaging. Thus, Liang et al. 59 reported a red aerglow imaging agent composed of Rhodamine B (RhB) and CNDs, which were conned in a hydrophilic silica shell to form CNDs-RhB@silica nanocomposites.
CNDs-RhB@silica can achieve a luminescence lifetime and aerglow QY of 0.91 seconds and 3.56%, respectively, which is the best result for the red aerglow region. Liang et al. 60 also reported a time division duplex technology based on environmentally friendly CNDs with controllable luminescence lifetime. It was the rst time they demonstrated that the time-division duplexing technique for CNDs and CNDs@silica is independent of the emission color and intensity depending on the phosphorescence lifetime under control. Water has been demonstrated to play an essential role in modulating the luminescence lifetime of CNDs by quenching triple excitons. In addition, Tan et al. 104 also reported a method to realize time-dependent phosphorescence colors in CDs, which were synthesized through a one pot hydrothermal method by using levooxacin as the raw material. They realized a new type of time-dependent phosphorescence color that changed from orange to green in a very short time (1 s). In 2021, Zheng et al. 137 also reported a general approach in which effective radiative energy transfer can be applied to support the reabsorption of upconversion materials (UMs) into CD-based room temperature near-infrared excited multicolor aerglow materials (CDAMs). Please note that this is the rst report on the multicolor aerglow of near-infrared excited in CDAMs. More importantly, this work provides a general route for constructing novel room-temperature aerglow materials with tunable excitation wavelengths. For the rst time, Zhou et al. 138 proposed an approach for efficient energy transfer mediation to boost the RTP of CDs by incorporating pure phosphorescent CDs into the aerglow matrix. In this system of design, there is a signicant increase in the emission intensity, RTP lifetime and emission time of CDs, and all CD-based materials emit visible phosphorescence for more than 20 seconds aer UV excitation. In addition, Mo et al. 139 provided the rst example of visible light excited TADF in aqueous solution by conning the co-doping of CDs with uorine and nitrogen in silica nanoparticles (F, NCDs@SiO 2 ). Although there have been many reports on RTP materials, it is still a challenging task to realize ultra-long RTP in aqueous media, especially for CD-based materials. In 2022, Jiang et al. 73 developed a robust organic long persistent luminescence (OLPL) system with hour-level aerglow emissions through simple microwaveassisted heating of a mixture of m-CDs and urea. It is a very uncommon case of an OLPL system displaying hourly aerglow under ambient conditions, even for aqueous media. Further studies showed that the generation of covalent bonds between cyanuric acid and CDs played a key role in the aerglow presentation. Green preparation has always been the synthetic route pursued by everyone. At present, RTP materials are greatly developed. Nevertheless, it is a huge priority to achieve both multicolor and long wavelength RTP emission with favorable stability in CD-based RTP materials. Liang et al. 140 proposed a new and general "CDs-in-YOHF" scheme to yield multicolor and long wavelength RTP by conning different CDs to a Y(OH) x F 3−x (YOHF) matrix. It should be noted that the RTP lifetime of the orange emissive CDs-o@YOHF is the longest in the reported single-CD-matrix composite materials with emission above 570 nm. Compared with the common representative matrices, the YOHF matrix is also proved to be more effective in protecting the long-wavelength triplet emission of CDs-o. Similarly, Mo et al. 61 reported a newly developed strategy to incorporate phosphorescent CD and uorescent dyes into monodisperse silica nanoparticles below 20 nm to achieve multi-color long aerglow in aqueous solution. For the rst time, CD-based multicolor long aerglow systems (green, yellow, orange and red) were fabricated in aqueous solution by cascaded Förster resonance energy transfer. Especially, under UV excitation, a prolonged red aerglow with a Stokes shi of 255 nm was developed. So far, there have been few studies on thermal stimulated response photoluminescence of CD-based materials. Xu et al. 141 reported for the rst time a polymeric nanocomposite incorporating uorinated CDs (FCDs) that can be efficiently synthesized in large quantities through the utilization of commercial water-soluble polymer sodium carboxymethylcellulose (CMCNa) as a stable matrix. The synthesized FCDs-CMCNa has bimodal emission, i.e., both solid state uorescence and in-room RTP. It is more interesting to note that FCDs-CMCNa exhibits special temperature-sensitive optical properties when the temperature is reduced from 300 K to 90 K. It exhibits an increase in uorescence/ phosphorescence intensity with decreasing temperature up to the switching point of 150 K, which then gradually decreases with decreasing temperature to 90 K. Later, Liu et al. 142 proposed a series of exible dynamic ultra-long RTP polymer composites, which are illuminated by halogen doped CDs and have fully programmable emission. They had produced a transparent, exible and fully programmable dynamic ultralong RTP composite lm with reliable gray-scale display ability from the synthesized RTP material for the rst time. As can be seen, each year's contribution has grown year on year, indicating a steady stream of scholars working on CD-based RTP materials with notable achievements. In each year of the prominent work represented, RTP of CDs was slowly developed in the initial stage, however, three years later, it was springing up. This indicates that as research progresses, a steady stream of ndings is discovered that continue to advance the development of RTP for CDs.
To sum up, the outstanding work of CD-based RTP materials is growing every year. According to Fig. 2, we conducted statistical analysis on all the 166 retrieved papers and summarized the corresponding RTP emission wavelengths (it is worth noting that the number of wavelengths counted here is more than the number of papers published because some articles reported multicolor wavelengths), as shown in Fig. 4. According to the statistical results, the emission of CD-based RTP materials is mainly concentrated in the range of 500-550 nm (Fig. 6a), accounting for 40.68% of the total (Fig. 6b). In other words, the current CD-based RTP materials mainly emit green light. The second is blue to green emission with emission wavelengths concentrated between 450 and 500 nm, followed by yellow-orange emission with emission wavelengths concentrated between 550 and 600 nm. It can be seen that the achievable emission of CDbased RTP materials is mainly concentrated in the short wavelength (450-600 nm) region of visible light, while reports of long wavelength emission (>600 nm) are very rare. Certainly, the realization of long-wavelength emission from CD-based RTP materials, even extending into the near-infrared region, is the focus of current research and the key to enhancing and broadening their application areas. This is a challenging research problem which needs to be solved.

Annual representative studies of phosphorescence lifetime in CD-based RTP materials
RTP materials have attracted much attention because of their great optical application potential. With the deepening of research, researchers have made breakthroughs in CD-based RTP materials, such as multi-mode emission RTP materials, 72,111 multi-color RTP materials, 48,75,120,140,143 aqueous solution RTP materials, 55,59,61,86 multifunctional detection application RTP materials, 43,92,134 biocompatible RTP materials 54 and high PQY 79,94,135 and ultra-long phosphorescence life RTP materials. 46,57,58,73,89,131 However, it is still difficult to obtain RTP materials with simultaneous long-lifetime and high PQY. In addition, phosphorescence lifetime is one of the important parameters to measure CD-based RTP materials. Phosphorescence lifetime is a key feature of RTP materials, and ultra-long phosphorescence lifetime is central to the excellent performance of RTP materials. The key to achieving long phosphorescence lifetimes is the choice of the RTP material structure or the implementation of methods, such as the use of melt co-crystallisation 125 to immobilize CDs in the form of wrapping to achieve RTP (Fig. 7A), the utilization of rigid structural networks and the coexistence of covalent bonds 57 to immobilize CDs in a three-dimensional spatially conned way to achieve RTP (Fig. 7B), the adoption of porous space crystalline materials 66 to immobilize CDs by adsorption and intercalation to achieve RTP (Fig. 7C), the employment of polymers to covalently cross-link CDs 45 to achieve RTP (Fig. 7D), the employment of metal-organic frameworks with porous structures providing active sites 80 to immobilize CDs to achieve RTP (Fig. 7E), and the utilization of high strength rigid crystal structures 72 to immobilize CDs to achieve RTP (Fig. 7F). Each year's work has made corresponding outstanding contributions, from the initial millisecond level to the later second level, and then to the current hour level, which are major breakthroughs one aer another. As shown in Fig. 7G, the best phosphorescent life of CD-based phosphorescent materials is shown in each year from the rst discovery to the present. In 2013, Deng et al. 42 reported for the rst time that the phosphorescence phenomenon of CDs in the PVA matrix was observed, and its average phosphorescence life was as high as ∼380 ms. Then, in 2014, Yan et al. 86 reported for the rst time  47 prepared an efficient CDbased phosphorescent material, that is, NCDs were heated in one pot by treating a mixture of urea and NCDs, and then they were incorporated into the composite matrix. The resulting material had an ultra-long phosphorescent life of 1.06 s under the excitation of 280 nm. In 2017, Joseph et al. 62 embedded CDs in a silica gel matrix to obtain an RTP material. The CDs showed strong blue uorescence in aqueous dispersion and showed green aerglow when combined with silica gel. Under the excitation of 380 nm, the phosphorescent emission lifetime of the CDs is about 1.8 s, which is the highest value of CDs in a solid matrix. In 2018, He et al. 43 combined CDs with PVA via the help of electrospinning technology, and realized RTP and TADF at the same time. In addition, these nanobers showed a longer average aerglow lifetime of 1.61 s and a visual recognition period of 9 s. In 2019, Li et al. 43 realized the preparation of RTP of CDs with or without heteroatom doping through a single-step thermal treatment of CDs and boric acid. This composite exhibits the highest phosphorescence lifetime of 2.26 s and a PQY of 17.5%, and this is the maximum record for CD-based RTP materials to date. In 2020, Sun et al. 57 obtained RTP materials with ultra-long life, high-level PQY and superior stability by rational design and fabrication of multiconstrained CDs. The designed multi-constrained CDs possess an ultra-long lifetime of 5.72 seconds, 26.36% PQY, and outstanding stability to strong oxidants, acids, bases, and polar solvents. In 2021, Yu et al. 68 systematically adapted the reaction parameters and host-guest interactions to achieve nine green RTP CDs-in-zeolite (CDs@zeolite) composites with engineering lifetimes ranging from 0.38 to 2.1 s under solvent-free conditions. In 2022, Jiang et al. 73 developed the rst CD-based organic long persistent luminescence system with a duration of more than 1 h. A system based on CDs (named m-CDs@CA) is reported, which can be conveniently and efficiently fabricated through the utilization of a household microwave oven. What is even more impressive is that its long-term sustained luminescence may be noticed under ambient conditions, even in aqueous media. In summary, the extension of phosphorescence lifetimes has always been at the heart of scholars' pursuits, and shows that phosphorescence lifetimes occupy an irreplaceable and central position in the RTP of CDs, and are a sign that CDs exhibit RTP properties. Progress in the eld of research can be seen in the representative work each year, and in the advancement of scientic capabilities. Following on from the previous research, the phosphorescence lifetime of CD RTP has made a qualitative breakthrough, extending from the initial millisecond level to the hourly level, which provides favorable preconditions for the practical application of CDs RTP.
Phosphorescence lifetime is an important index to measure RTP materials, and the phosphorescence lifetime of CD-based RTP materials is also constantly developing. According to Fig. 2, the phosphorescence lifetime of the published CD-based RTP materials was statistically analyzed. As shown in Fig. 8a, CDbased RTP materials with different phosphorescence lifetimes have been reported one aer another, but the main phosphorescence lifetimes are between 1 and 1000 ms. More obviously, the phosphorescence lifetime of most CD-based RTP materials is still at the millisecond level. Of course, there are also some outstanding performances, such as the existence of phosphorescence lifetime at the level of hours, but this is rare. According to the analysis of statistical data, the ratio of phosphorescence lifetime between 1 and 1000 ms accounts for 67.76%, which is a very high percentage, while the ratio between 1000 and 5000 ms reaches 25.66% (Fig. 8b). It can be seen that the preparation of CD-based phosphor materials with second phosphorescence lifetime is still in the stage of development. It is worth noting that CD-based RTP materials with all second phosphorescence lifetimes account for less than half of all reported CD-based RTP materials. This is a very terrible existence. Therefore, developing CD-based RTP materials with ultra-long phosphorescence lifetime is a hot spot and a difficult problem at present.

Applications
Realizing the nal application is the value of a material. As new luminescent materials, CD-based RTP materials are no  exception. First of all, some CD-based materials have the characteristics of DF, which have the property of long life and can be used in the eld of information security without prompt excitation. Secondly, some CD-based materials with DF characteristics have also been accompanied by RTP performances, and such multiple aerglow emission offers additional possibilities for information encryption and anti-counterfeiting applications. In addition, RTP materials based on CDs have strong sensitivity characteristics in some special detection, and are also used for response detection of ions, pH, temperature, time, ngerprint and so on. It is worth noting that CD-based RTP materials also have low toxicity and good biocompatibility, and also have great potential and development in the eld of biological imaging. More importantly, CD-based RTP materials fully embody the characteristics of light-emitting materials, have good tunability, luminescence, stability and other optical properties, and have been greatly developed in the eld of lightemitting diodes (LEDs). Table 3 summarizes some main applications of CD-based RTP materials. In addition, the special properties of CD based-RTP materials, such as excitationrelated, temperature-related and aerglow color adjustable properties, enable them to build multi-level security defense in the eld of information encryption and anti-counterfeiting. The long aerglow life of CD based-RTP provides new possibilities for easier identication of authenticity and information content. It allows signicantly increased time to visualize cell states, enables speedy detection, and also facilitates bioimaging and detection. Generally speaking, the RTP material based on CDs possesses huge application prospects in the eld of anti-counterfeiting, 110 116,117,122,[185][186][187][188][189][190][191][192][193][194][195] with the properties of DF, TADF, long aerglow life and adjustable aerglow color.
However, the application of CD RTP is also limited by its own deciencies. First, the clarity in the luminescence mechanism of CDs is lacking. Different types of CDs have been reported one aer another since the development of CDs, but equally inconsistent types of luminescence mechanisms have emerged, which seriously affects the unied denition and future application of CDs and prevents a systematic unication. Secondly, the process for the preparation of RTPs for CDs is too harsh. The current implementation of RTP for CDs requires the assistance of a matrix, high temperature or multi-step synthesis, and the process requirements are too demanding and costly, limiting their preparation and application for large scale production. Then, the RTP emission wavelengths of CDs are too short-term. According to the reported articles, most of the RTP emission wavelengths of CDs are concentrated in the range of 480-550 nm, while there is relatively little RTP emission for long wavelengths (>550 nm), which severely limits the application of RTP characteristics of CDs for advanced information security and encryption. Finally, the PLQYs for CDs RTP are relatively low. There is an urgent need for green preparation of RTP CDs with high quality and high PLQY, especially with long wavelength emission. In the eld of optoelectronic devices, especially for WLED applications, red emission is the key to achieving efficient WLEDs and strong luminescence is also the key factor to enhance device

Challenges and prospects
Although signicant achievements in the synthesis and diversied application investigations of CD-based RTP composites have been made, there remain enormous challenges to face. The specic aspects are as follows: First, since the discovery and development of CDs, the luminescence mechanism of CDs has always been questioned since it does not have a unied structure. At present, we can accept two main categories, namely, carbon core luminescence and surface state luminescence. Similarly, the luminescence of CD-based RTP materials, as a special property of CD luminescence, has not yet been clearly explained and dened. For the luminescence mechanism of CD-based RTP, there are different opinions, each with its own words. For example, it has been reported that CD-based RTP originates from C]N, C]O, C]F, hydrogen bonds, polymer matrices and so on. However, these remarks are all unilateral. Thus, it is essential to address systematically their mutual and distinctive features, as well as their structural evolution patterns during CD synthesis. At the same time, the formation process and luminescence mechanism of CD-based RTP are explored and summarized by combining some more advanced characterization technologies and theoretical calculations, so that future generations can better understand the luminescence mechanism of CD-based RTP materials.
Second, luminescence quantum efficiency is one of the important indicators to measure luminescent materials. As a new emerging carbon based uorescent nanomaterial, the PLQY of CDs has always been a hot and difficult research topic, and improving the PLQYs of CDs has always been our goal. More importantly, the PQY of CD-based RTP materials is much lower than the PLQY of CD uorescence, which makes improving the PQY of CD-based RTP materials an urgent need. In addition, compared with CD-based RTP materials, the DF characteristics of CDs require a smaller energy difference (between the excited singlet state and the excited triplet state), which will make the preparation of CD-based DF materials more difficult. Because this also requires that the spatial overlap between the HOMO and LOMO be very small, which in turn reduces the radiation transition rate and ultimately leads to a low PQY. However, reasonable structural design, purication and appropriate matrix selection are expected to solve the pressing problem of low PQY of CD-based RTP materials.
Third, the recognition ability of human eyes is limited. In the visible light region, the luminescence of CDs is mainly concentrated in the short wavelength region (<600 nm), and even the luminescence of CD-based RTP materials is mainly concentrated in the blue to green light range. These short wavelength lights are harmful to the human body. In addition, the luminescence of CD-based RTP materials is mainly concentrated in the solid state, and the luminescence in solution is relatively less. These limitations have seriously hindered the application of CD-based RTP materials in multicolor displays and bio-imaging. Therefore, the development of CD-based RTP materials with a long wavelength and liquid phase luminescence has also become a hot spot and an unsolved problem in current research. But, it is expected to solve the problem of luminescence at long wavelengths and in the liquid phase by choosing suitable hydrophilic matrices and surface modication.
Fourth, the phosphorescence lifetime is also one of the important indicators to measure the performance of CD-based RTP materials. Currently, the phosphorescence lifetime of CDbased RTP materials is mainly concentrated at the second level, and it is only a few seconds to several tens of seconds. Regarding the long-lifetime research, there are still few reports on minute-level, and even hour-level phosphorescence lifetime researches. In addition, CD-based RTP materials with an ultralong phosphorescence lifetime are expected to show application value in new elds, such as solar cells. In order to realize CDbased RTP materials with ultra-long phosphorescence lifetime, an important matrix selection is the key to solving the problem.
Finally, advanced synthesis methods are one of the key technologies for preparing high-quality CD-based RTP materials. At present, the synthesis route of CD-based RTP materials mainly relies on a two-step method, that is, a matrixassisted synthesis route is adopted, CDs are synthesized rst, and then encapsulated into a matrix to realize RTP. This synthesis method is complicated and inefficient. In addition, the stability of the synthesized CD-based RTP material is also not good. More importantly, in addition to cumbersome synthesis methods, complicated purication processes are also required in the process of realizing CD-based RTP, and the low repetition rate of these synthesis routes greatly limits its largescale commercial application. Therefore, in order to solve a series of problems in the preparation process, one kind of excellent matrix with a simple and fast preparation method and high stability properties is the only choice.
In summary, CD-based RTP materials have many excellent properties such as low cost, low toxicity and tunable luminescence. It has some advantages unmatched by other RTP materials, and has become a rising star in the elds of anticounterfeiting, information security, LEDs, sensor detection and so on. However, there are still some problems, such as unclear luminescence mechanism, low PQY, short luminescence wavelength, insufficient phosphorescence lifetime, and simple and convenient preparation methods. Therefore, as a new CD-based RTP material in the future, it will have advantages such as economy, liquid phase stability, long wavelength emission, multi-color emission, high PQY and environmental friendliness. At the same time, CD-based RTP materials can also be combined with other high-performance materials to obtain new composite materials and realize application value in new elds. It is hoped that through the continuous attempts and efforts of the broad masses of people, in modern society where opportunities and challenges coexist, CD-based RTP materials are expected to popularize people's