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Research Advance of Carbon-Dots Based Hydrophilic Room Temperature Phosphorescent Composites

Zixuan Liao , Yuhui Wang , Jianping Zheng

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (2) : 263 -373.

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Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (2) : 263-373 DOI: 10.7536/PC220720
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Research Advance of Carbon-Dots Based Hydrophilic Room Temperature Phosphorescent Composites
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Abstract

Due to the unique merits of long luminescence lifetime and environmental sensitivity, room temperature phosphorescence (RTP) has demonstrated great potential in many fields, e.g., chemo/biosensing, bioimaging, biomedicine, and advanced optical anti-counterfeiting and encryption. In recent years, non-metal doped solid state room temperature phosphorescent (RTP) carbon-dots (CDs) have attracted broad attention because of their facile preparation, chemical inertness, low toxicity, easy surface modification, and so on. However, in an aqueous environment, their RTP emissions suffer from serious triplet quenching that induced by dissolved oxygen and water molecule. How to stabilize the triplet state in aqueous phase is the key to establish their RTP emission, and thus it is necessary to look back the state-of-the-art knowledge of CDs-based water-soluble RTP materials. In this review, in light of the recent advance, we summarize and discuss their synthesis strategies (e.g., inorganic salt melting, SiO2 coating, polymer combination, and hydrogen bond network stabilization), and relevant applications in sensing, imaging and anti-counterfeiting, and finally propose the challenging and future prospects. To the best of our knowledge, this is the first review on the synthesis and applications of hydrophilic RTP materials based on CDs. We hope that this review will provide inspiration for the further fabrication and versatile biological uses of CDs-based RTP materials.

Contents

1 Introduction

2 Synthetic strategies

2.1 Inorganic salt melting method

2.2 SiO2coating

2.3 Polymer-based hydrogen stabilization

2.4 Rigid hydrogen bond network stabilization

3 Applications

3.1 Anti-counterfeiting and information encryption

3.2 Detections of metal ions and small molecules

3.3 Biological detection and imaging

3.4 Other applications

4 Conclusion and outlook

Graphical abstract

Keywords

room temperature phosphorescence / carbon-dots / aqueous phase / synthesis / sensing / bioimaging / optical anti-counterfeiting

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Zixuan Liao, Yuhui Wang, Jianping Zheng. Research Advance of Carbon-Dots Based Hydrophilic Room Temperature Phosphorescent Composites. Progress in Chemistry, 2023, 35(2): 263-373 DOI:10.7536/PC220720

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1 Introduction

Photoluminescence usually refers to the photon radiation produced in the process of returning from the excited state of atoms or molecules to the ground state after the ground state electrons are excited by an external light source. Fluorescence, phosphorescence, and delayed fluorescence are three common photoluminescence (Fig. 1). Among them, phosphorescence has been reported very early, and the first natural phosphor was a luminescent diamond discovered by Cellini in 1568[1]. In 1944, Lewis and Kasha proposed and proved that phosphorescence in organic molecules is a radiative transition between the lowest triplet state and the singlet ground state[2]. However, the excited triplet state needs to be stable at low temperature and in an inert atmosphere, which makes room-temperature-phosphorescence (RTP) difficult to achieve[3,4]. In 1967, Roth discovered the room temperature phosphorescence of small organic molecules in paper-based and cellulose polymer films, which opened a new era in the research and application of RTP materials[5].
The existing room temperature phosphorescent materials mainly include rare earth based luminescent materials, noble metal complexes and pure organic compounds[6,7][8,9][10⇓~12]. However, most of the materials often have the disadvantages of poor processability, high cost and metal toxicity, which limit their synthesis and application. To overcome these drawbacks, a number of pure organic RTP molecules have been recently reported, which exhibit many advantages, such as low cost, good stability, easy processing, and biocompatibility[11⇓⇓~14]. However, most pure organic materials have weak spin-coupled orbitals and high probability of non-radiative transitions, and the triplet excitons of organic emitters are easily inactivated by non-radiative decay processes of thermodynamic vibrations and external quenching, making efficient RTP materials still very rare[15,16].
carbon-dots (CDs) are a new type of carbon-based luminescent materials with a diameter of less than 10 nm and a quasi-spherical shape, which have the advantages of simple preparation, low cost, low toxicity, and unique photoelectric properties, and have shown broad application potential in the fields of chemical sensing, biological imaging, disease diagnosis and treatment, photoelectrocatalysis, and display devices[17⇓⇓⇓⇓⇓~23]. The photoluminescence properties of carbon dots include fluorescence, phosphorescence and delayed fluorescence, and their electroluminescence properties have also been reported[24]. The luminescence mechanism of carbon dots is complex due to the diversity of their structure and surface composition. Generally speaking, the fluorescence sources are mainly attributed to the band gap transition in the conjugated π region, the surface defect state, the molecular state luminescence, and the crosslinking enhanced luminescence, and the mechanisms can be divided into three categories: (1) the fluorescence emission dominated by internal factors including size effect, conjugation effect, surface state, and cooperative factors, which is applicable to most carbon dots[25]; (2) External factors, including surface molecular state luminescence and environmental effects[26]; (3) Cross-linking enhanced emission (CEE) mechanism[27].
The RTP phosphorescence emission of carbon dots is mainly concentrated in the green region. However, recently, researchers have realized the regulation of multi-color long afterglow emission based on the principles of aggregation-induced luminescence, multiple emission centers of carbon dots and energy transfer[28][19][29⇓~31]. In addition, because dissolved oxygen can significantly quench the excited triplet state of carbon dots, most of RTP carbon dots and their applications are limited to solid substrates[32⇓⇓⇓⇓⇓~38]. Therefore, how to effectively construct carbon dot-based aqueous RTP materials is the key to expand its application, especially in the field of biomedical analysis and imaging[39,40].
At present, RTP inorganic materials in aqueous phase mainly focus on rare earth based luminescent materials and long afterglow nanoparticles[41][42]. Organic RTP materials are mainly used to improve their water solubility by means of micelle and host-guest recognition, so as to carry out application research in water environment[43,44]. Although the above two kinds of materials have made positive progress, there are some problems such as harsh preparation conditions (such as high temperature calcination) and cumbersome surface modification process. Comparatively speaking, carbon dot-based aqueous RTP materials are simple in synthesis, low in cost, low in biological toxicity and longer in life, and have made some positive progress recently, but they are still in the initial stage. As shown in Fig. 2, the researchers obtained a series of carbon dots based aqueous RTP composites through construction strategies such as polymer composite, hydrogen bond fixation, and silica (SiO2) coating[45⇓⇓⇓⇓⇓~51]. At present, there is no review on the construction and application of carbon dots based aqueous RTP composites.In this paper, the research progress of carbon dots based aqueous RTP composites is summarized, the construction strategy and related applications are introduced, and the challenges and development trends are prospected.

2 Synthetic strategy

The phosphorescence lifetime and efficiency of CDs are determined by the rates of radiative and non-radiative transitions, and the rate of non-radiative transition is much higher than that of radiative transition. Therefore, how to avoid the non-radiative transition of the triplet state is the key to achieve phosphorescence emission, which depends on whether the excited triplet state of the material can be stabilized[47,52]. The reported synthesis strategies of carbon dots based aqueous RTP materials can be divided into four categories: inorganic salt melting method, SiO2 coating method, polymer composite method and rigid hydrogen bond network immobilization method.

2.1 Inorganic salt fusion method

molten salt (MS) is a salt that is solid at ambient conditions but enters the liquid phase at high temperatures. Molten salt method refers to the method of growing crystals from molten salt agent at high temperature. The process is similar to the growth of crystals from aqueous solution. By selecting a large number of easily available salts, the melting temperature and different ions can be accurately controlled. Based on this, Wang et al. Developed a MS method to prepare yellow RTP carbon dots in situ by structural confinement of recrystallized molten salt and doping with high charge density metal ions[53]. The method has good generality, and CDs with Aggregation-induced phosphorescence (AIP) can be obtained by screening carbon sources and inorganic salts. On this basis, they prepared CD composite (CDs @ MP) by selecting high charge density magnesium salt (MgCl2) and phosphate (KH2PO4) as doping salts, adding potassium nitrate and mixing with carbon source (1,2,4-triaminobenzene), and the specific process is shown in Fig. 3. During melting and recrystallization, CDs are formed and incorporated into the special crystal structure of the matrix. Since magnesium phosphate is insoluble in water, its solid state matrix provides strict protection for the triplet state of CDs[49]. Under the synergistic effect of AIP and crystallization limitation, the as-prepared CDs @ MP exhibited a strong yellow RTP in aqueous solution with a quantum yield of 26. 4% and a lifetime of 1. 28 s, both of which were at a high level compared with the existing literature[47,48,52,54]. This method is suitable for most carbon sources, and the RTP performance of the obtained CDs can be simply controlled by the experimental factors such as the mass ratio of P/Mg, the concentration of carbon source, and the calcination temperature. Finally, they successfully applied CDs @ MP for phosphorescence visualization sensing of pH and temperature in aqueous media.
The MS method for preparing carbon dot-based aqueous RTP is simple and universal, and has excellent phosphorescence performance. However, the synthesis process needs to be calcined at high temperature (400 ℃) for a long time, and the surface does not have chemical functional groups that can be modified, which limits the wide application of this method to a certain extent.

2.2 SiO2 coating method

SiO2 is a non-toxic and non-pollution non-metallic material.Natural SiO2 includes crystalline and amorphous, while nano-SiO2 is usually amorphous, which is mainly prepared by chemical precipitation, sol-gel method, microemulsion method and so on. The stable physicochemical properties and unique structure make SiO2 suitable as a host matrix to coat various nanomaterials. In addition, the SiO2 has the advantages of simple preparation process, strong processability, good optical transparency and the like[55]. Using SiO2 coating technology to stabilize the excited triplet state is one of the general technical means to prepare carbon dot-based aqueous RTP composites at present. There are three main construction strategies, which are described as follows.

2.2.1 TEOS direct hydrolysis coating

There are many methods to prepare SiO2, such as sol-gel method, chemical vapor deposition method, microemulsion method and so on, among which the sol-gel method is the most widely used because of its simple operation and easy synthesis[56][57][58,59]. The main steps of this scheme are to synthesize fluorescent carbon dots, and then add silicic acid precursor and alkaline catalyst (ammonia) to hydrolyze the corresponding carbon dot complexes. Wang et al. Constructed a nanomixture (CDs-CaTiO3∶Pr3+@SiO2) composed of water-soluble carbon dots and water-dispersible lanthanide-doped inorganic nanoparticles CaTiO3∶Pr3+@SiO2, where CaTiO3∶Pr3+@SiO2 was prepared by forming a microemulsion of CaTiO3∶Pr3+ with cyclohexane and 1-hexanol, and then adding tetraethyl orthosilicate (TEOS) and ammonia to break the system to obtain the nanocomposite[60]. Mo et al. Realized multicolor long afterglow in aqueous solution based on the principle of energy transfer by compounding phosphorescent carbon dots (PCDs) and fluorescent dyes into monodisperse SiO2 nanoparticles[30]. Based on this method, Liang et al. Directly dispersed CDs aqueous solution, Rhodamine B (RhB) and TEOS in aqueous solution, and obtained CDs-RhB@SiO2 composite phosphorescent materials by hydrolysis reaction under alkaline conditions (Figure 4A)[31].

2.2.2 Covalent immobilization

Colloidal silica (nSiO2) has also been introduced into related preparation systems due to its advantages such as high water dispersibility, optical transparency and easy surface functionalization. Based on the in-depth analysis of the previous research, Jiang et al carried out a simple hydrothermal reaction between nSiO2 and m-CDs (prepared by carbonization of m-phenylenediamine), in which a covalent bond was formed between C — N/C = N groups to achieve a long afterglow in the aqueous phase observable by the naked eye, and developed the application of information encryption and anti-counterfeiting[47]. This method is simple, clean, low-toxicity and pollution-free, and is suitable for batch preparation. The lifetime of the carbon dots after recombination is up to 0. 703 s. This strategy provides an effective solution for fixing the triplet excited state of carbon dots by covalent bonds. Inspired by related research, Li et al. Encapsulated CDs in colloidal silica through the traditional St Stöber synthesis route, which is shown in Fig. 4 B. The hydrolysis and condensation of TEOS in the presence of CDs, water, and ammonia allowed CDs to be directly covalently attached to the SiO2 network to achieve aqueous RTP emission with a lifetime of 1.64 s[54].

2.2.3 One-step hydrolysis method

One-step synthesis method, that is, the target product can be obtained within one step without two or more reaction steps. Commonly used methods include hydrothermal method, pyrolysis method and microwave method[61]. Hao et al. Prepared ultra-long room temperature phosphorescent polymer carbon dots (PCDs)-SiO2 nanospheres (SNSs) composites (PCDs-SNSs) by one-step hydrothermal method using TEOS as the sole raw material[62]. In the hydrothermal process, PCDs are formed and encapsulated in SNSs at the same time, and the covalent bonds and hydrogen bonds between them shield the non-radiative process of the triplet state, which is the key to realize RTP. Subsequently, the research group realized the composite with RTP (N-CDs@SiO2) through the one-step hydrothermal reaction of triethylenetetramine and TEOS. Nitrogen-doped carbon dots (N-CDs) and SiO2 are formed simultaneously during the hydrothermal process, and the SiO2 acts as a host matrix to adhere and immobilize the N-CDs[63]. The method is simple in preparation, can realize one-step in-situ generation and SiO2 coating of RTP carbon dots, and the composite system has good stability and shows strong RTP emission performance in an aqueous environment.

2.3 Hydrogen bond immobilization based on polymer recombination

A polymer is a high molecular weight compound composed of many identical, simple structural units connected repeatedly by covalent bonds. Polymer composite technology is often used in RTP carbon dots in the solid state to achieve phosphorescence emission. For example, Lin et al. First reported the three-mode emission characteristics (solid state fluorescence, up-conversion luminescence, RTP) of carbon dot-polyvinyl alcohol (PVA) composite in 2016[64]. Among them, the key to realize RTP is that the abundant amino groups on the surface of carbon dots form strong hydrogen bonds with PVA, thus fixing the excited triplet state of M-CDs.
It is also a common strategy to prepare carbon dot-based aqueous RTP materials by using the abundant polar groups of polymers to form hydrogen bonds with the surface of carbon dots. Polymers, such as PVA and polyurethane (PU), contain abundant polar groups, which can form strong hydrogen bonds with the polar groups on the surface of carbon dots, effectively promote the radiative relaxation from T1 to S0 and the formation of hydrogen bond networks, and realize room temperature phosphorescence emission in aqueous phase. Deng et al. Obtained water-soluble phosphorescent carbon dots by dispersing CDs into PVA matrix, with an emission lifetime of 380 ms[45]. The study shows that phosphorescence originates from the excited triplet state of the aromatic carbonyl group on the surface of the carbon dot, and PVA can stabilize the excited triplet state through hydrogen bonding, effectively protecting its energy from rotational or vibrational losses. Wu et al. Used the natural product Schisandra polysaccharide (SCP) as a carbon source, and then embedded CDs into the PVA matrix to obtain an aqueous RTP CDs-PVA solution with a lifetime of up to 271.2 ms (Figure 5A)[65]. Structural characterization shows that there are abundant C = N and C = O groups on the surface of CDs, which increase the probability of intersystem crossing of electrons in the singlet excited state and can form hydrogen bonds with PVA to suppress non-radiative transitions.

2.4 Rigid hydrogen bond network immobilization

The results show that the hydrogen bond framework can stabilize the triplet state and enhance the inhibition of non-radiative transitions, which plays a key role in the room temperature phosphorescence emission in aqueous phase. Cyanuric acid (CA) is a unique cyclic amide composed of three hydrogen bond donors and three acceptors, which can easily form a hydrogen bond network with polar groups on the surface of nanomaterials. Based on this, Zhou et al. Used water molecules to construct a hydrogen bond network between CDs and CA to prepare aqueous RTP materials (Fig. 5B)[66]. The results show that in the presence of water, the highly ordered and non-frozen bound water attached to the surface of CA particles can construct a strong bridge-like hydrogen bond network between CDs and CA, which effectively hardens the $\mathrm{C}=\mathrm{O}$ bond of CDs and enhances the rigidity of the system, thus enhancing the phosphorescence emission. The CDs-CA suspension exhibited a long phosphorescence lifetime of 687 ms at 373 nm excitation. Furthermore, Zheng et al. Constructed a highly stable and full-color RTP composite (m, p-CDs @ CA) in aqueous phase through CA, with a phosphorescence quantum yield of 23.3% and an emission lifetime of 1.74 s, and carried out its application research in highly sensitive time-resolved biological target detection[67].
In addition, as a small molecule with a similar structure to cyanuric acid, melamine is also a candidate material for building a hydrogen bond network. Inspired by this, Gao et al. Reported a method of using CDs and melamine to construct a hydrogen bond network to achieve a long afterglow emission material of carbon dots in water phase (Figure 5C)[48]. The hydrogen bonding interaction between CDs and melamine forms a supramolecule-like framework, and can capture surrounding water molecules to form bound water, which constructs a rigid environment to suppress non-radiative transitions, thus achieving efficient phosphorescence. At the same time, some melamine can also be connected with CDs through covalent bonds, which can better stabilize the hydrogen-bonded skeleton network structure.

3 Application

Phosphorescent materials have attracted much attention because of their unique luminescence properties such as long lifetime luminescence, large Stokes shift, and high signal-to-noise ratio. Carbon dot-based aqueous RTP materials break through the environmental limitations of phosphorescence applications (no water, no oxygen, low temperature, etc.), and have been applied in many fields such as multi-dimensional optical anti-counterfeiting, information encryption, time-resolved detection and imaging.

3.1 Anti-counterfeiting and Information Security Encryption

The unique properties of RTP materials, which are highly available and have a typical phosphorescence duration, are readily used as a new anti-counterfeiting technology. The development of advanced anti-counterfeiting and information encryption technology is of great significance to combat counterfeit and inferior products and information forgery, among which visual optical means have gained additional attention. The development of new optical performance materials provides the possibility of multiple advanced anti-counterfeiting.
Deng et al. Dispersed carbon dots into PVA matrix and found that obvious phosphorescence could be observed when excited by ultraviolet light at room temperature, and their CDs-PVA composite film could be used as anti-counterfeiting materials and printed on various commodity packages[45]. Since then, the application of RTP carbon dots in information encryption and anti-counterfeiting has gradually increased. In 2017, the m-CDs@n-SiO2 prepared by Jiang et al. emitted blue phosphorescence in aqueous solution under 365 nm ultraviolet irradiation, and subsequent characterization found that it was insensitive to water and oxygen, and proposed and demonstrated a moisture-related information protection strategy (Figure 6a)[47]. When decrypting, the filter paper is thoroughly wetted first, and then the correct information (i.e. "315201") is obtained by observing the afterglow emission. The results show that the m-CDs@nSiO2 composite has long afterglow characteristics, which can be potentially applied to some advanced counterfeiting and information protection systems, such as confidential documents and banknotes. Subsequently, in 2019, the team designed and prepared metal-free, aggregation-induced, dual-emission RTP carbon dots (TA-CDs) by hydrothermal treatment of trimellitic acid for the first time, and produced yellow RTP emission under ultraviolet excitation (365 nm)[28]. Taking advantage of the unique high photostability and aggregation-induced RTP properties of TA-CDs, the TA-CDs dispersion is considered to be used as an encryption security ink in the field of advanced anti-counterfeiting and information encryption, and an advanced anti-counterfeiting and information encryption method (water stimulus response to produce RTP) has been developed. In the same year, Gao et al. Used CDs and melamine to construct a hydrogen bond network to form a complex, and further developed it into an advanced anti-counterfeiting ink, which was used for advanced authentication or hiding of unusually important information[40].

3.2 Metal ion and small molecule detection

Compared with fluorescence spectroscopy, RTP has a longer luminescence lifetime, which can avoid the interference of short-lived background fluorescence and scattered light in the detected sample, and has obvious advantages in the detection of complex sample matrices, which can greatly improve the analytical sensitivity or signal-to-noise ratio, and has been widely used in the highly sensitive sensing detection of various target molecules[68,69][70⇓~72].
In 2018, Li et al. Reported that water molecules were used to construct a hydrogen bond network between CDs and CA particles to achieve RTP emission in water phase[66]. The ion detection of the Fe3+ can be realized based on static quenching caused by the formation of a non-fluorescent complex between the CDs and the Fe3+. The CDs-CA system has high sensitivity and good selectivity, and realizes the accurate and high-sensitivity detection of Fe3+ in lake water and complex biological samples. In 2019, Wu et al. Used Schisandra chinensis polysaccharide as the sole carbon source to green synthesize CDs-based RTP materials in situ, and after compounding with PVA, they prepared a CDs-PVA sensing film (Fig. 6B)[65]. Based on the principle that the triplet electron can be transferred to the Fe3+ orbit by non-radiative electron to cause simultaneous quenching of fluorescence and phosphorescence, the CDs-PVA film can be used for fluorescence and RTP dual-channel detection of Fe3+ with high sensitivity and good specificity. In addition to detecting Fe3+, sensing of other ions has also been reported. For example, Wang et al. Constructed a nanoprobe with CDs-CaTiO3∶Pr3+@SiO2 based on the principle of inner filter effect. In the presence of Hg2+, the phosphorescence of CDs was significantly quenched, while the phosphorescence emission of CaTiO3∶Pr3+@SiO2 nanoparticles was almost unchanged. The ratiometric phosphorescence sensor constructed based on this principle was successfully applied to the highly sensitive analysis of Hg2+ in water and complex biological samples, which fully embodied the advantages of time-resolved detection technology[60].
In addition to ion detection, the aqueous RTP carbon dot system is also widely used in the detection of various small molecules, such as tetracycline, warfarin, pesticide residues and physiologically active small molecules, such as L-tryptophan, cholesterol and glucose[73][74][50][75][76].

3.3 Biological detection and imaging

With excellent biocompatibility and luminescent properties, fluorescent carbon dots have achieved good applications in the field of biological detection and imaging[77⇓⇓⇓⇓⇓⇓⇓~85]. However, under the excitation of ultraviolet or visible light, biological samples will have serious interference such as background fluorescence and scattered light, which greatly affects the sensitivity of detection. Carbon dot-based aqueous RTP materials can effectively avoid the above optical interference by using their long afterglow emission characteristics, which has attracted wide attention in biological analysis. In 2019, Li et al. Reported that the CDs@SiO2 aqueous RTP nanocomposite system could be applied to time-resolved phosphorescence imaging of mouse breast cancer cells, demonstrating extremely high imaging signal-to-noise ratio (Figure 6 C)[54]. In 2020, Liang et al. Prepared water-soluble phosphorescent carbon dots (WSP-CNDs@SiO2) with ultra-long afterglow emission, which were injected into the back of mice, and the phosphorescent signal could be easily detected, demonstrating its in vivo imaging potential[86]. The multicolor long afterglow material (PCDs@SiO2) reported by Mo et al. First showed the potential of bioimaging through human hepatoma cells (HepG2), and further verified the possibility of in vivo imaging through mouse experiments[30]. The above studies show that the water-soluble RTP carbon dot system has excellent biocompatibility and high imaging signal-to-noise ratio, and is a highly competitive and sensitive labeling probe.

3.4 Other Applications

In addition to the above application areas, the application of carbon dot-based aqueous RTP materials in other aspects, such as photosensitive antibacterial, has also been reported. Zhang et al. Found that a class of phosphorescent carbon dots had photosensitization characteristics, and confirmed that their photooxidation activity was related to their phosphorescence quantum yield. Compared with common molecular photosensitizers, the nitrogen-doped carbon dots showed higher activity and could complete the oxidation reaction in a few seconds[87]. The excellent photosensitized oxygen activation property makes this carbon dot promising as a promising nano-oxidase and for photodynamic antibacterial application. Miao et al. Used isophthalic acid (IPA) as a carbon source to prepare aqueous RTP carbon dots, and further combined with polylysine (PLL) to prepare IPA-CDs/PLL composite system (Fig. 6d)[88]. The IPA-CDs/PLL showed significant anti-biofouling performance in static and dynamic anti-biofouling tests. IPA-CDs can use the principle of photosensitization to produce singlet oxygen (1O2) through energy transfer from the excited triplet state to dissolved oxygen, which can be used as a photodynamic antibacterial agent to effectively prevent biological contamination.

4 Conclusion and prospect

To sum up, the key to obtain RTP carbon dots in aqueous phase is how to stabilize the excited triplet state and avoid the quenching effect of dissolved oxygen in the aqueous environment. Although the aqueous RTP emission of carbon dots can be achieved by polymer composite, hydrogen bond network, SiO2 coating and other means, and it has shown significant advantages in sensing, imaging and optical anti-counterfeiting applications, and has made gratifying research progress[88⇓~90]. However, the research of carbon dots based aqueous RTP materials still faces many challenges, especially its controllable preparation and performance regulation, as well as the further expansion of related applications.
Obtaining aqueous RTP carbon dots with uniform size and morphology is the key to application. However, the morphology of the existing aqueous RTP-carbon dot hybrid materials obtained by the composite method is usually not uniform enough and the size is large, which limits the further application. Therefore, the development of new synthetic strategies to obtain carbon dot-based aqueous RTP materials with uniform size and morphology is a topic worthy of further development. The preparation strategies and emission mechanisms of RTP carbon dots, such as carbonization polymerization, crosslinking enhanced emission and aggregation-induced phosphorescence, as well as the research progress of room temperature phosphorescent gel and the breakthrough of ultraviolet phosphorescence emission of carbon dots, will provide valuable reference for the preparation of pure aqueous RTP carbon dots[48,90⇓⇓~93].
In addition, the optical properties of carbon dots based aqueous RTP materials need to be regulated. Firstly, its excitation is mostly in the ultraviolet region (300 ~ 400 nm), and its emission is relatively single (mainly green light), so high-energy ultraviolet radiation can cause photodamage to biological samples. In addition, this band does not cover the excitation light source of fluorescence confocal microscopy, which leads to the need to configure additional ultraviolet light sources for biological imaging. Therefore, the construction of visible or even near-infrared light excited aqueous RTP carbon dots is the key to expand its application in the biomedical field[94]. The selection of carbonization precursors of aromatic compounds, heterogeneous element doping, aggregation-induced luminescence, energy transfer and other technical means can provide the possibility for the construction and optical properties optimization of aqueous RTP carbon dots.
The application fields of carbon dot-based aqueous RTP materials need to be further expanded, and should not be limited to anti-counterfeiting, information encryption, analysis and detection, bioimaging, etc. Their applications in multicolor/multi-channel sensing, biomarkers, super-resolution imaging, efficient photosensitization, optical coding, and other emerging fields are worth exploring.
In a word, the research of carbon dot-based aqueous RTP materials is facing many challenges, and it is particularly important to learn from the design concept of the existing RTP material system. In addition, with the continuous development of carbon dot structure design theory and nanotechnology, more carbon dot-based RTP materials with excellent performance will be designed and developed, which will shine brilliantly in the fields of high-sensitivity and high-resolution biomedical analysis.

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Funding

Zhejiang Provincial National Natural Science Foundation of China(LY20B050003)

Ningbo 3315 Innovation Teams Program(2019A-14-C)

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