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2025

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Journal of Luminescence - Invited Review | Progress in Aqueous Room Temperature Phosphorescent Carbon Dots

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Introduction

Carbon dots (CDs), as a novel room-temperature phosphorescent (RTP) material, possess excellent optical properties, biocompatibility, and water solubility. Their simple preparation process makes them promising for applications in optics and biomedicine. In recent years, the realization of aqueous room-temperature phosphorescent carbon dots has expanded their applications to liquid environments, providing new tools and methods for biomedical research. However, challenges remain, including short lifetimes and fixed wavelengths. Therefore, exploring the luminescence regulation mechanism of room-temperature phosphorescent carbon dots and summarizing the latest research progress is particularly important. Recently,Liang Yachuan from Zhengzhou University of Light Industry and Professor Liu Kaike from Zhengzhou University collaboratedand publisheda paper titled《水相室温磷光碳点进展》inthe authoritative journal《发光学报》This article,with Zhang Sifan, a master's student,as the first author, comprehensively summarizes the latest research results in this field and looks forward to future development directions.

 

This review first starts from the basic principles of triplet exciton luminescence, systematically expounds the key photophysical processes related to triplets, and in-depth analyzes the influence mechanism of these processes on phosphorescence efficiency and lifetime. Based on this, the synthesis strategies and performance regulation methods of solid-phase and aqueous carbon dots are discussed in detail. Then, the application progress of room-temperature phosphorescent carbon dots in information encryption, optical sensing, illumination display, and biomedicine is summarized. Finally, the future development trends and potential application directions of aqueous room-temperature phosphorescent carbon dots are prospected based on the current research status.

Introduction

Afterglow materials, with their long-lifetime luminescence characteristics, show application potential in multiple fields. Traditional room-temperature phosphorescent (RTP) materials, such as rare earth inorganic phosphors and transition metal complexes, have limitations such as high cost and high toxicity. Carbon dots (CDs), as novel RTP materials, combine excellent optical properties, biocompatibility, and a simple preparation process, making them promising in optical and biomedical Applications. In recent years, research hotspots have focused on the regulation of carbon dot phosphorescent properties. Strategies such as matrix composites, structural design, and energy level regulation have significantly improved their luminescence lifetime and efficiency. Aqueous phosphorescent carbon dots have unique advantages in bioimaging due to their lack of background interference, but their development still faces many challenges. Therefore, exploring the luminescence regulation mechanism of room-temperature phosphorescent carbon dots, optimizing the performance of aqueous room-temperature phosphorescent carbon dots, and expanding their Applications have become current research trends.
 
 

Mechanism of Carbon Dot Phosphorescent Emission

Under photoexcitation, molecules in the ground state (S0) are excited to the singlet state (Sn， n≥1). According to the Kasha rule, singlet electrons relax to the lowest singlet state (S1) through rapid internal conversion (IC). Then, the singlet electrons undergo three competing processes: intersystem crossing (ISC) to the triplet state, non-radiative decay to S0， and radiative decay toS0。IfS1and Tnenergy gaps are sufficiently small, triplet excitons accumulated at Tnmay transfer back to S1。through the thermally activated reverse intersystem crossing (RISC) process. Otherwise, higher triplet excitons decay rapidly to the lowest triplet state (T1) through rapid IC. Finally, T1electrons deactivate to S0。

through phosphorescent transitions, non-radiative transitions, or diffusion quenching processes caused by interactions with oxygen and other substances.

The interaction between electron spin and electron orbit, called spin-orbit coupling (SOC), causes electron spin-flip transitions and is the main driving force for ISC, which has a great influence on the generation and radiative annihilation of triplet excitons. As shown in Figure 2(b),from 1 (n, π*) to 3 (n, π*) or 1 (π, π*) to 3 (π, π*) transitions, ISC has poor orbital overlap, resulting in low efficiency SOC. With3(n, π*) configuration T1due to the allowed spin flip transition from 1 (n, π*) to 1 n 2 , it easily becomes S0with a high phosphorescence decay rate, while with 3 (π, π*) configuration T1due to the forbidden spin flip transition from 3 (π, π*) to 1 π2, its decay rate is extremely slow.
 
Therefore, to obtain highly efficient phosphorescent carbon dots, two requirements must be met: increase the intersystem crossing (ISC) rate to fill a sufficient number of triplet excitons; suppress or reduce the non-radiative transition and quenching processes of T 1 .
 
 

Main pathways for promoting room temperature phosphorescence of carbon dots

(1) Spin-orbit coupling

Electronic S 1 ↔T 1 or T 1 ↔S 0 transition process is spin forbidden, in T 1 The population of states affects the phosphorescence quantum yield. The main methods for promoting spin-orbit coupling are the heavy atom effect and heteroatom doping. The heavy atom effect mixes singlet and triplet energy levels through spin-orbit coupling (SOC), thus promoting the originally forbidden singlet-triplet transition, increasing the intersystem crossing rate, and improving the triplet population and phosphorescence quantum yield. This effect can be regarded as the mutual mixing of singlet and triplet characteristics, even allowing the triplet to pass through T 1 ←S 0 directly absorbing the population. Heteroatom doping changes the electronic configuration and band structure of the material, breaking the lattice symmetry, leading to redistribution of the electron cloud, and enhancing the SOC effect. The orbital hybridization of heteroatoms with surrounding atoms forms new hybrid orbitals, further strengthening the spin-orbit interaction. In addition, doping may induce charge transfer, forming charge transfer complexes, changing the electron spin and orbital states, and ultimately enhancing the phosphorescence performance.
 

(2) Electron spin-nuclear spin hyperfine coupling

Electron spin-nuclear spin hyperfine coupling (HFC) is an important mechanism for generating T 1 states. Nuclear spin interacts with electron spin by changing the local magnetic field environment. This HFC effect dominates the S→T conversion in weak radical ion pairs (RIPs). Small or zero band gap excitons promote 1 RIP and 3 spin exchange between RIPs, while the nuclear spin magnetic field further induces electron spin conversion, enhancing intersystem crossing (ISC). Heteroatom doping enhances the interaction between electron spin and nuclear spin by changing the local magnetic field environment and nuclear spin orientation. At the same time, doping makes it easier for electron transitions to couple with atomic nuclei, thereby significantly improving the HFC effect and further promoting the ISC process.
 

(3) Adjusting the band gap

Reducing the singlet-triplet energy level difference (ΔE ST) can accelerate intersystem crossing (K ISC ). An effective strategy is to use donor-acceptor type molecules to introduce intermediate states, minimizing ΔE through intramolecular charge transfer statesSTto accelerate K ISC . However, when ΔE ST is too small (such as less than 0.37 eV), it may promote reverse intersystem crossing (RISC), leading to atypical luminescence phenomena such as thermally activated delayed fluorescence, which in turn affects the stability of triplet excitons and has an adverse effect on room temperature phosphorescence (RTP). Studies have shown that RTP and thermally activated delayed fluorescence coexist in many room temperature phosphorescent carbon dots, and their ratio can be adjusted by controlling ΔESTadjustment.
 

(4) Rigidification effect

The rigidification effect effectively suppresses non-radiative transitions by restricting molecular motion or chemical bond vibrations, thereby improving the room temperature phosphorescence (RTP) efficiency. To achieve high-efficiency RTP, various rigid matrices have been developed, including organic matrices (such as urea, melamine), inorganic matrices (such as boric acid, silicon dioxide), and polymers (such as polyvinyl alcohol). Reasonable selection of the rigid matrix is the key to achieving high-efficiency RTP.

Figure 2: (a) Intersystem crossing (ISC) schematic diagram; (b) Hyperfine coupling (HFC) schematic diagram; (c) Band gap adjustment schematic diagram; (d) Rigidification effect schematic diagram.

Main synthesis methods of aqueous room temperature phosphorescent carbon dots

(1) Matrix rigidification

The choice of matrix is crucial for achieving efficient aqueous room temperature phosphorescent carbon dots. SiO₂、Melamine (MA), cyanuric acid (CA), and dendritic mesoporous silica nanoparticles (DMSNs) have all been proven to be ideal carriers for aqueous room temperature phosphorescent carbon dots, as shown in Figure 3. Studies have shown that constructing a hydrogen bond network between water molecules and carbon dots can significantly enhance phosphorescence emission and prolong luminescence lifetime. In addition, covalent bond fixation, due to its higher bond energy, can effectively protect triplet excitons and stabilize the long-lifetime luminescence of carbon dots in aqueous solution. For example, confining carbon dots in a silica encapsulation layer can isolate environmental oxygen and suppress molecular vibrations, thereby significantly improving phosphorescence lifetime and quantum yield. These strategies provide important directions for the performance optimization of aqueous phosphorescent carbon dots.

(1) Host-guest complexation

Unlike hydrogen bonds and covalent bonds, host-guest complexation is a special type of non-covalent bond, involving the synergistic action of various non-covalent bonds, such as electrostatic interactions, hydrogen bonds, donor-acceptor electron interactions, and hydrophilic-hydrophobic interactions. In the host-guest complex, the host molecule selectively recognizes and accommodates guest molecules or ions through its cavity structure. This interaction is usually reversible and specific. Host-guest complexation not only provides a rigid environment for guest phosphors but also avoids energy transfer and self-quenching caused by high concentration through dispersion. Usually, a host matrix with porosity or cavity structure and carbon dots with matching size, shape, and surface functional groups can form a stable host-guest complex under appropriate conditions, thereby achieving efficient phosphorescence emission.
 

(2) Strong crosslinking

Crosslinkable reactants are used as precursors to generate carbon dots through carbonization and crosslinking processes. Their crosslinked polymers or carbon frameworks can effectively immobilize triplets. Crosslinking enhanced emission (CEE) is the key mechanism for achieving room temperature phosphorescence in such carbon dots. Its effect is mainly reflected in three aspects: first, the hydroxyl, amino, and carboxyl groups of multifunctional precursors condense to form a covalently crosslinked skeleton, suppressing non-radiative transitions; second, crosslinking shortens the distance between functional groups, leading to electron cloud overlap and the formation of new luminescent structures; third, hydrogen bonds enhance the conformational rigidity of the polymer chains, restricting molecular rotation and vibration and promoting radiative transitions. In addition, carbon dots can form a compact three-dimensional crosslinked network framework through intermolecular interactions, exerting spatial constraints on the phosphorescence emission center, isolating external quenchers, and preventing phosphorescence quenching.
 
 

Performance Regulation of Aqueous Room Temperature Phosphorescent Carbon Dots

(1) Wavelength Regulation

The emission wavelength of room temperature phosphorescence is a key parameter that determines its application range. Therefore, the development of long-wavelength aqueous room temperature phosphorescent carbon dots is of great significance for high-sensitivity and high-resolution biomedical analysis. Förster resonance energy transfer (FRET) is a commonly used strategy to achieve long-wavelength RTP. By combining carbon dots as energy donors with suitable energy acceptors, the phosphorescent emission wavelength can be effectively regulated. As shown in Figure 4, combining carbon dots with Rhodamine B (RhB) and utilizing the FRET process to transfer the phosphorescent energy of the carbon dots to RhB can achieve red phosphorescent emission in an aqueous solution. In addition, embedding carbon dots with different photoluminescence characteristics into the same matrix is a universal strategy to achieve a wide range of RTP emission. By regulating the precursor or solvent, multicolor phosphorescent emission from blue to red can be obtained in the same system, providing an important basis for practical applications. These strategies provide important directions for the development of long-wavelength, high-efficiency aqueous RTP carbon dots.

Figure 4: (a) Afterglow mechanism of CNDs and CNDs-RhB@silica; (b) Images of samples I.~X. under sunlight and ultraviolet irradiation.

(2) Lifetime Regulation

The rational selection of a rigid matrix is crucial for achieving long-lifetime phosphorescence. By constructing materials with multiple constraint effects, such as embedding carbon dots in a silica matrix, the phosphorescence lifetime and quantum yield can be significantly improved. For example, controlling the carbonization time can obtain carbon dots with different lifetimes, and their lifetimes vary with the carbonization time. In addition, a one-step hydrothermal method to confine polymer carbon dots in silica nanospheres can effectively promote room temperature phosphorescent emission in the aqueous phase, achieving ultra-long lifetime and high stability.
 
 

Applications of Aqueous Room Temperature Phosphorescent Carbon Dots

(1) Sensing

Room temperature phosphorescent carbon dots, especially in aqueous systems, have wide applications in the sensing field due to their high environmental sensitivity. When in contact with the substance to be measured, the luminescent properties (such as intensity, spectrum, or lifetime) of the carbon dots will change due to interactions such as chemical bonding, energy transfer, or electron transfer, thereby enabling quantitative or qualitative analysis. Phosphorescent detection avoids interference from short-lived background light, improving the signal-to-noise ratio and sensitivity. As shown in Figure 5, carbon dots can be used to detect Fe³⁺, biomarkers (such as 5-hydroxyindole-3-acetic acid and dopamine), and pH values, demonstrating their potential in environmental monitoring and biosensing.

Figure 5: (a) Phosphorescent spectra of CD suspensions with different calculated concentrations of Fe 3+; (b) Phosphorescent emission spectra of m, p-CDs@CA suspensions after incremental addition of HIAA; (c) Stern-Volmer plot of m, p/CDs-ME suspensions at different dopamine concentrations; (d) Phosphorescent spectra of P-CDs coated on paper at different pH values and the relationship between phosphorescent intensity and pH value.

(2) Biomedical

Aqueous room temperature phosphorescent carbon dot materials, due to their excellent biocompatibility, low cytotoxicity, small size, and excellent cell penetration ability, show great potential in biological applications. Their phosphorescent properties can be used for photodynamic antibacterial chemotherapy and bioimaging, especially long-wavelength phosphorescent emission is more suitable for deep tissue imaging. In addition, by designing carbon dots with narrow band gaps and long-lifetime excited states, sonodynamic therapy can also be enhanced, providing new avenues for tumor treatment. These characteristics make room temperature phosphorescent carbon dots have broad application prospects in bioimaging, antibacterial, and cancer treatment.

Figure 6: (a) Treatment of bacterial cultures with CDs and PB under different conditions; (b) In situ activation and detection of WSP-CNDs@silica phosphorescence in live mice; (c) Schematic diagram of in situ activation and afterglow detection of CNDs-RhB@silica in live mice, and fluorescence and phosphorescence images of mice subcutaneously implanted with different concentrations of CNDs-RhB@silica; (d) Ex vivo NIR imaging of p-n-CD@143B main tissues and tumors.

Summary and Outlook

 
Room temperature phosphorescent carbon dots have made significant progress. The breakthrough from solid state to aqueous phase has broken the traditional understanding that triplet excitons cannot emit light in water, and it has shown unique advantages in multiple application scenarios. However, the complexity of the carbon dot structure, the diversity of precursors, and the variability of synthesis conditions have brought many challenges to the development of aqueous room temperature phosphorescent carbon dots. Currently, the luminescent colors of aqueous room temperature phosphorescent carbon dots are mainly concentrated in green and yellow light. Although there have been a few reports on multicolor phosphorescent carbon dots, efficient red and near-infrared phosphorescence are still relatively scarce. In addition, their excitation light is mostly limited to the ultraviolet range, limiting applications in fields such as bioimaging. There is an urgent need to develop phosphorescent carbon dots excited by visible light or X-rays. At the same time, large-scale preparation and the expansion of key application outlets are still bottlenecks. Combining theoretical calculations and machine learning is expected to promote the large-scale synthesis of high-quality phosphorescent carbon dots and their applications in information encryption, light-emitting devices, and bioimaging. Although there have been reports on dynamic phosphorescence and multi-stimulus responsive carbon dots, relevant research is still limited. Developing phosphorescent carbon dots that respond to multiple stimuli such as light, electricity, heat, force, oxygen, solvent, and ultrasound will greatly expand their potential in sensing and detection. In the future, interdisciplinary collaboration is needed to further deepen the systematic research on aqueous room temperature phosphorescent carbon dots to promote their practical applications.
 
 

Paper Information

Zhang Sifan, Liang Yachuan*, Liu Kaike*, Shan Chongxin. Progress in Aqueous Room Temperature Phosphorescent Carbon Dots [J]. Journal of Luminescence, 2025, 46(02): 260-272.DOI: 10.37188/CJL.20240245.
https://cjl.lightpublishing.cn/zh/article/doi/10.37188/CJL.20240245/

 

 

through phosphorescent transitions, non-radiative transitions, or diffusion quenching processes caused by interactions with oxygen and other substances.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6: (a) Treatment of bacterial cultures with CDs and PB under different conditions; (b) In situ activation and detection of WSP-CNDs@silica phosphorescence in live mice; (c) Schematic diagram of in situ activation and afterglow detection of CNDs-RhB@silica in live mice, and fluorescence and phosphorescence images of mice subcutaneously implanted with different concentrations of CNDs-RhB@silica; (d) Ex vivo NIR imaging of p-n-CD@143B main tissues and tumors.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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