Regulating red-shifted phosphorescence of carbon dots via introducing magnesium chloride

Abstract

Achieving the multicolor phosphorescence of carbon dots (CDs), particularly with controllable wavelengths, remains a challenging yet desirable goal. Herein, we designed and prepared phosphorescent CDs (CDs@Al₂O₃ and CDs@MgAl₂O₄) using an in situ synthesis strategy. The oxygen vacancies in CDs stabilized the triplet excitons, thus enabling the long-lived and strong phosphorescence of these CDs. Importantly, CDs@MgAl₂O₄ exhibited obviously red-shifted phosphorescence (552 nm) compared to CDs@Al₂O₃ (525 nm) owing to increased conjugation and a narrowed bandgap, with the narrowed gap derived from coordination between magnesium and C=O. These CDs showed excitation-dependent phosphorescence, which was tunable from cyan to red by varying excitation wavelengths. Additionally, we synthesized a series of phosphorescent CDs by employing different carbon sources, among which the magnesium-doped CDs consistently displayed red-shifted phosphorescence, thus proving the universality of this strategy and providing an innovative approach for regulating the phosphorescence of CDs.

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1. Introduction

Room-temperature phosphorescence (RTP) materials have garnered significant research interest due to their distinctive optical characteristics, such as a large Stokes shift, high signal-to-noise ratios and persistent luminescence after excitation cessation.^1–3 These properties have enabled their extensive utilization across diverse fields including optoelectronic devices, advanced anti-counterfeiting, high-resolution bioimaging, multifunctional sensing platforms and beyond.^4–7 Nevertheless, conventional RTP materials, encompassing pure organic compounds, metal–organic complexes and rare-earth-doped phosphors, are plagued by multiple disadvantages such as inherent cytotoxicity restricting biomedical applications,^1,8 complex preparation process,⁹ and vulnerability of triplet excitons to oxygen-induced quenching or thermal deactivation mechanisms,¹⁰ all of which undoubtedly hinder their practical applications. Consequently, acquiring RTP materials with cost-effectiveness, simple preparation procedures and high environmental tolerance remains challenging, and thus the precise regulation of the phosphorescent emissions of RTP materials is highly valuable.

Carbon dots (CDs), as a class of zero-dimensional carbon-based luminescent nanomaterials, have attracted much attention due to their diverse optical characteristics, such as fluorescence,¹¹ phosphorescence¹² and thermally activated delayed fluorescence.¹³ These properties originate from their unique electronic and surface structures. Compared with traditional room-temperature phosphorescent materials, CDs offer distinct advantages including easy preparation,¹⁴ economic viability,¹⁵ low biological toxicity¹⁶ and no reliance on rare-earth elements, which not only enhance the practicality of CD-based RTP materials but also highlight their potential to serve as promising phosphorescent alternatives.

Currently, there are two major strategies to realize RTP emission in CD-based materials. The first strategy is rigid matrix engineering, such as polymer chain entanglement¹⁷ and introduction of matrices (SiO₂,¹⁸ B₂O₃,^19,20 urea²¹ or PVA¹³), which suppress the rotation and vibration of CDs or non-radiative transitions, thus enabling RTP emission.²² The second strategy involves doping heteroatoms such as B, N, P or S to regulate the electronic cloud.^23,24 These dopants could enhance spin–orbit coupling (SOC) through heavy-atom effects, which also facilitates efficient intersystem crossing (ISC) by breaking spin-forbidden and increasing the overlap between singlet and triplet energy levels.²⁵ Nitrogen doping, for instance, could suppress the spin-forbidden transition and promote the effective filling of triplet excitons, thereby enhancing the long-lived RTP of CDs.²⁶ Although numerous methods have been developed to achieve RTP in CD-based materials, their RTP is mainly concentrated in the short-wavelength range from blue to green.²⁷ It is still a challenge to obtain CD-based RTP materials with controllable emissions.

In this work, we successfully developed an in situ strategy to prepare multicolor RTP CDs by introducing AlCl₃ and MgCl₂, denoted as CDs@Al₂O₃ and CDs@MgAl₂O₄. The rigid framework and abundant oxygen vacancies effectively stabilized the triplet states of CDs, thus resulting in persistent phosphorescence even under extremely acidic or basic conditions. To be specific, magnesium incorporation in CDs@MgAl₂O₄ leads to narrowing of the optical band gap, further resulting in red-shifted phosphorescence compared to CDs@Al₂O₃, which lacks magnesium. Ultimately, a series of CDs with multicolor RTP were synthesized by varying the carbon source, thus proving the universality of this proposed approach (Scheme 1).

Scheme 1 Illustration of the synthesis and phosphorescence properties of CDs@Al₂O₃ and CDs@MgAl₂O₄.

2. Results and discussion

2.1. Preparation and optical properties of CDs@Al₂O₃ and CDs@MgAl₂O₄

Boric acid has been employed to prepare room-temperature phosphorescent CDs due to its unique electronic properties. Nevertheless, its structural instability with fluctuated humidity and temperature presents obvious limitations. In contrast, aluminum has a similar electronic structure to boron, and its structure is more stable and exhibits a higher environmental tolerance than boron. Therefore, we here selected folic acid and aluminum chloride as the raw materials to prepare CDs@Al₂O₃. Subsequently, to optimize the performance of CDs@Al₂O₃, we systematically investigated the potential effects of raw material ratios, calcination temperature, and calcination time on its phosphorescence properties. The experimental results indicated that these factors obviously affected the phosphorescence intensity of CDs@Al₂O₃ (Fig. S1), while scarcely affecting the wavelength (Fig. S2). In order to regulate the RTP emission wavelength of CDs, we attempted to change the elemental composition and the electronic cloud density of CDs by introducing magnesium, an alkaline-earth metal element with high charge density, and further synthesized CDs@MgAl₂O₄ using the same way as preparing CDs@Al₂O₃.

The fluorescence and phosphorescence spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄ were initially recorded, and both the corresponding powders exhibited strong fluorescence and RTP. The maximal fluorescence emissions of CDs@Al₂O₃ and CDs@MgAl₂O₄ were separately measured at 415 nm and 427 nm (Fig. 1a and b) as well as their CIE coordinates shown in Fig. 1c. Subsequently, we recorded the phosphorescence spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄ excited with light of 365 nm and 420 nm (Fig. 1d and e), respectively. Particularly, CDs@Al₂O₃ exhibited green phosphorescence at 525 nm under 365 nm excitation and was visible to the naked eye for approximately 10 s, while CDs@MgAl₂O₄ emitted yellow phosphorescence at 552 nm, persisting for around 8 s (Fig. 1h). Under 420 nm excitation, CDs@Al₂O₃ showed visible yellow phosphorescence at 565 nm lasting for 5 s, while CDs@MgAl₂O₄ emitted orange phosphorescence at 583 nm that was visible for around 6 s under the same excitation (Fig. 1i). Fig. 1f shows the CIE coordinates of the phosphorescence emission of CDs@Al₂O₃ and CDs@MgAl₂O₄ as the excitation wavelengths were 365 nm and 420 nm, indicating that the phosphorescent emission wavelengths of these two CDs were obviously affected by the excitation wavelengths. Notably, both CDs@Al₂O₃ and CDs@MgAl₂O₄ exhibited phosphorescence emissions under visible light excitation.

Fig. 1 Fluorescence excitation and emission spectra of CDs@Al₂O₃ (a) and CDs@MgAl₂O₄ (d). (c) Fluorescent CIE coordinates of CDs@Al₂O₃ and CDs@MgAl₂O₄. Phosphorescence emission spectra of CDs@Al₂O₃ (b) and CDs@MgAl₂O₄ (e) under excitation at 365 nm and 420 nm. (f) Phosphorescent CIE coordinates of CDs@Al₂O₃ and CDs@MgAl₂O₄. (g) Images of CDs@Al₂O₃ under 310 nm UV irradiation and after the UV light was turned off. (h) Images of CDs@Al₂O₃ and CDs@MgAl₂O₄ under daylight, 365 nm UV irradiation and after the UV light was turned off. (i) Images of CDs@Al₂O₃ and CDs@MgAl₂O₄ under 420 nm excitation and after the light was turned off. (j) Images of CDs@MgAl₂O₄ under WLED irradiation and after the WLED was turned off. Phosphorescence spectra of CDs@Al₂O₃ (k) and CDs@MgAl₂O₄ (l) under excitation at various wavelengths.

The previous phenomena indicated that the phosphorescence emission wavelengths of CDs@Al₂O₃ and CDs@MgAl₂O₄ were strongly excitation-dependent. To systematically explore this excitation dependency, we further recorded the phosphorescence spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄ under excitation at varied wavelengths (Fig. 1k and l). The results showed that the phosphorescence emission wavelength of CDs@Al₂O₃ showed a red-shift as the excitation wavelength increased from 280 nm to 420 nm. We selected 310 nm as the representative excitation wavelength, under which CDs@Al₂O₃ exhibited obvious cyan phosphorescence at 480 nm (Fig. 1g and S3). Similarly, the phosphorescence emission of CDs@MgAl₂O₄ exhibited a red-shift when the excitation wavelength increased from 320 nm to 460 nm (Fig. 1l). Importantly, the phosphorescence of CDs@MgAl₂O₄ exhibited a distinct red shift compared to that of CDs@Al₂O₃ under the same excitation, and both of them could emit phosphorescence under visible light excitation. Furthermore, CDs@MgAl₂O₄ uniquely exhibited red phosphorescence under white light-emitting diode (WLED) excitation without requiring ultraviolet light, thus highlighting its great potential for visible light activated phosphorescence (Fig. 1j). Therefore, CDs@Al₂O₃ and CDs@MgAl₂O₄ with successive and tunable room-temperature phosphorescence (RTP) were successfully achieved via modulating the excitation wavelengths.

2.2. Structural characterization

To elucidate the luminescence mechanism of CDs@Al₂O₃ and CDs@MgAl₂O₄, their systematic structural characterization was conducted through multiple analytical techniques. The morphologies of the two CDs were initially observed using HR-TEM, followed by obtaining their size distributions. The TEM images revealed that the formation of CDs and the average particle sizes of CDs@Al₂O₃ and CDs@MgAl₂O₄ were determined to be 2.83 nm and 3.50 nm, respectively (Fig. 2a and b). Meanwhile, the HR-TEM images showed distinct lattice fringes in both CDs with the same lattice spacing of 0.21 nm (Fig. 2a and b), and this spacing aligned with the (100) crystal plane of graphite,²⁸ indicating that these CDs retained graphitic-like sp² carbon cores. The increased size of CDs@MgAl₂O₄ corresponded to the extended sp² carbon core, which enhanced the conjugation effects, thus leading to a red shift of the phosphorescence emission. The X-ray diffraction (XRD) patterns of CDs@Al₂O₃ exhibited obvious peaks at 46° and 67°, which were consistent with the characteristic peaks of Al₂O₃ (JCPDS No. 10-0425),²⁹ while the peaks of CDs@MgAl₂O₄ corresponded to the characteristic peaks of MgAl₂O₄ (JCPDS No. 21-1152) (Fig. 2c).³⁰ These findings demonstrated the formation of Al₂O₃ for CDs@Al₂O₃ as well as MgAl₂O₄ for CDs@MgAl₂O₄, whose intrinsic crystalline frameworks were possibly responsible for their phosphorescence properties.

Fig. 2 TEM images of CDs@Al₂O₃ (a) and CDs@MgAl₂O₄ (b). Inset: HR-TEM images and particle size distributions. (c) XRD patterns of CDs@Al₂O₃ and CDs@MgAl₂O₄. (d) FTIR spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄. (e) XPS surveys of CDs@Al₂O₃ and CDs@MgAl₂O₄. High-resolution XPS spectra of (f) C 1s, (g) O 1s, (h) Al 2p and (i) Mg 1s.

Subsequently, FTIR was employed to explore the surface functional groups of CDs@Al₂O₃ and CDs@MgAl₂O₄ (Fig. 2d). The FTIR spectra of both CDs exhibited a broad and intense stretching vibration band of O–H spanning 3750 cm⁻¹ to 3100 cm⁻¹, indicating the presence of hydroxyls originating from surface functionalization or absorbed water.³¹ The peaks at 1637 cm⁻¹ and 1579–1450 cm⁻¹ corresponded to the stretching vibrations of C=O and C=C in the aromatic ring of CDs@Al₂O₃, while the stretching vibrations of C=O and C=C in the aromatic ring for CDs@MgAl₂O₄ are located at 1618 cm⁻¹ and 1550 cm⁻¹–1435 cm⁻¹, respectively. This red-shift suggested the interaction between magnesium ions and C=O of the aromatic skeleton, thus altering their local electronic cloud.³² The broad bands centered at 840 cm⁻¹ and 576 cm⁻¹ were associated with the Al–O vibrations of CDs@Al₂O₃.²⁶ In the FTIR spectrum of CDs@MgAl₂O₄, the broad peak at 820 cm⁻¹ and the sharp peak at 625 cm⁻¹ corresponded to the vibration of Al–O, while the peak at 487 cm⁻¹ was attributed to the vibration of Mg–O. These findings collectively demonstrated that introducing magnesium affected their structures and magnesium played a critical role in regulating the phosphorescence of CDs@MgAl₂O₄.

XPS was further employed to determine the elemental composition of CDs@Al₂O₃ and CDs@MgAl₂O₄. The XPS survey revealed that CDs@Al₂O₃ was mainly composed of C (21.18%), O (51.36%) and Al (27.46%) elements (Fig. 2e), and the peaks at 531.3 eV, 284.9 eV and 74.2 eV referred to the characteristic peaks of O 1s, C 1s and Al 2p, respectively. In contrast, CDs@MgAl₂O₄ exhibited characteristic peaks at 74.0 eV (Al 2p), 284.6 eV (C 1s), 531.1 eV (O 1s) and 1303.8 eV (Mg 1s), indicating that it was mainly composed of C (22.66%), O (47.73%), Mg (7.73%), Al (21.37%).^26,33 As for the high-resolution C 1s spectrum of CDs@Al₂O₃, the characteristic peaks at 288.3 eV, 286.0 eV and 284.4 eV corresponded to C=O, C–O and C–C/C=C, respectively (Fig. 2f). Similarly, CDs@MgAl₂O₄ showed the same groups of C=O (288.4 eV), C–O (286.2 eV) and C–C/C=C (284.4 eV) (Fig. 2f). Notably, the content of C–C/C=C for CDs@MgAl₂O₄ (78.12%) was higher than that for CDs@Al₂O₃ (67.11%), further indicating that CDs@MgAl₂O₄ showed a larger sp² carbon core. This structural feature resulted in an extended conjugation, thus producing red-shifted phosphorescence. The high-resolution O 1s spectrum of CDs@Al₂O₃ was deconvoluted into three peaks related to C–O (532.8 eV), Al–O (531.5 eV), and C=O (530.5 eV), while the characteristic peaks at 532.8 eV, 531.2 eV and 530.7 eV of CDs@MgAl₂O₄ corresponded to C–O, Al–O/Mg–O and C=O, respectively (Fig. 2g). The Al 2p spectrum of CDs@Al₂O₃ was deconvoluted into three peaks of Al–O–C (75.1 eV), Al–O (74.6 eV) and Al–OH (73.8 eV), and CDs@MgAl₂O₄ also exhibited three similar peaks of Al–O–C (75.1 eV), Al–O (74.1 eV), and Al–OH (73.6 eV) (Fig. 2h). Differently, the Mg 1s spectrum (Fig. 2i) of CDs@MgAl₂O₄ displayed a distinct peak at 1303.6 eV (Mg–O),³⁴ whereas there was no peak in this range for CDs@Al₂O₃.³³ Taken together, these findings suggested the formation of abundant covalent bonds for the two CDs, which effectively inhibited the non-radiative transitions of excitons, thus promoting the emergence of phosphorescence.

2.3. Generation and regulation of phosphorescence

Remarkably, the phosphorescence emissions of CDs@Al₂O₃ and CDs@MgAl₂O₄ were still clearly observed when they were exposed to strong acid (1 M of H₂SO₄) and alkaline (1 M of NaOH) conditions (Fig. 3a and b). The highly tolerant phosphorescence of two CDs was attributed to the rigid bonds of Al–O–C, which not only effectively enhanced ISC efficiency between the singlet and triplet states, but also stabilized the triplet state of CDs.

Fig. 3 Phosphorescence spectra of (a) CDs@Al₂O₃ and (b) CDs@MgAl₂O₄ without and with 1 M H₂SO₄ or 1 M NaOH. Phosphorescence spectra of (c) CDs@Al₂O₃ and AlCl₃ and (d) CDs@MgAl₂O₄ and AlCl₃–MgCl₂. (e) EPR spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄. Time-resolved phosphorescence decay curve of (f) CDs@Al₂O₃ and (g) CDs@MgAl₂O₄. Photoluminescence quantum yield of (h) CDs@Al₂O₃ and (i) CDs@MgAl₂O₄. (j) UV-vis absorption spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄. Diffuse reflectance spectra of (k) CDs@Al₂O₃ and (l) CDs@MgAl₂O₄.

To further validate the role of folic acid in generating phosphorescence, we performed the same experiments by using AlCl₃ or AlCl₃–MgCl₂ without folic acid. At identical excitation wavelengths, distinct phosphorescence emissions were observed for CDs@Al₂O₃ and CDs@MgAl₂O₄, whereas both the products without utilizing folic acid exhibited negligible afterglow (Fig. 3c and d). These results demonstrated that folic acid was necessarily responsible for achieving the phosphorescence.

Meanwhile, electron paramagnetic resonance (EPR) was employed to investigate the presence of defects in the two CDs. In the EPR spectra of CDs@Al₂O₃ and CDs@MgAl₂O₄, both samples exhibited signals at g = 2.006, confirming the existence of oxygen vacancies (Fig. 3e).³⁵ To be specific, these oxygen vacancies served as effective trapping sites for triplet excitons, preventing their immediate transition to the electronic ground state (S₀). During the process of thermal disturbance, the trapped triplet excitons were released from the trap and gradually returned to the electronic ground state (S₀) through a radiative transition, thereby prolonging the lifetime and improving the quantum yield of phosphorescence.

Furthermore, the phosphorescence lifetime of CDs@MgAl₂O₄ was 0.50 s and the phosphorescence lifetime of CDs@Al₂O₃ was 0.69 s (Fig. 3f and g), revealing that both CDs possess relatively long phosphorescent lifetimes. Simultaneously, the photoluminescence (PL) quantum yields of CDs@Al₂O₃ and CDs@MgAl₂O₄ were calculated to be 89.36% and 40.85% under 365 nm excitation (Fig. 3h and i), demonstrating their high quantum yields for both CDs. These findings showed agreement with the data of EPR, indirectly providing evidence that oxygen vacancy-mediated defects were critical for prolonging the lifetime and promoting the quantum yield of phosphorescence. Based on the overlap ratios between photoluminescence and phosphorescence spectra, the phosphorescence quantum yields of the two CDs were also determined to be 9.46% and 6.06%, respectively (Fig. S4 and S5).

In the UV-vis absorption spectra (Fig. 3j), CDs@Al₂O₃ exhibited a characteristic absorption peak at 275 nm, corresponding to the π–π* transition of conjugated C=C within the carbon core. In contrast, CDs@MgAl₂O₄ not only displayed a primary absorption peak at 280 nm, but also showed an obvious shoulder peak at 380 nm that extended into the visible-light region. This shoulder peak was ascribed to the n–π* transition of C=O coordinated with magnesium, thus enhancing the light absorption capacity of C=O by altering the electron cloud through metal coordination.³²

To obtain the optical band gap (E_g) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CDs@Al₂O₃ and CDs@MgAl₂O₄, we further recorded their UV diffuse reflectance spectra. By analyzing the Tauc plot, we calculated their E_g to be 3.679 and 3.131 eV, respectively (Fig. 3k and j). The narrowed band gap of CDs@MgAl₂O₄ (ΔE_g = 0.548 eV) was ascribed to the introduction of magnesium. In brief, the magnesium with a high charge density led to a rearranged electron cloud and a decreased HOMO–LUMO band gap,³⁶ resulting in the red-shifted phosphorescence of CDs@MgAl₂O₄ compared with CDs@Al₂O₃. Furthermore, the DFT calculations further confirmed that the band gap of the magnesium-doped CDs showed a similar narrowing trend, consistent with the experimental results (Fig. S9).

Taken together, once CDs@Al₂O₃ and CDs@MgAl₂O₄ were irradiated with a light source, the electrons of the ground state (S₀) were excited to higher singlet states (S_n) followed by non-radiative transitions to S₁, and subsequently underwent ISC to the triplet state T₁. The triplet state electrons were captured by oxygen vacancies, and the trapped triplet excitons were then released from these oxygen vacancies, and this process prolonged their phosphorescence lifetime and improved their quantum yields, thus emitting long-lived and strong phosphorescence (Fig. 4a). Furthermore, the phosphorescence red-shift of CDs@MgAl₂O₄ was mainly due to the increased conjugation extent of sp² carbon and the narrowed optical band gap. These synergistic effects of the two aspects resulted in the pronounced phosphorescence red-shift of CDs@MgAl₂O₄ compared with that of CDs@Al₂O₃ (Fig. 4b).

Fig. 4 (a) Mechanism of achieving pronounced phosphorescence for CDs@Al₂O₃ and CDs@MgAl₂O₄. (b) Aspects of regulating the phosphorescence red-shift for CDs@MgAl₂O₄.

To prove the universality of this strategy, we selected more carbon sources including L-glutamic acid, glycine, succinic acid, phthalimide and tartaric acid, all of which contained abundant C=O, to synthesize a series of CDs by the same approach of preparing CDs@Al₂O₃ and CDs@MgAl₂O₄. These CDs were named according to their raw materials, which included L-Glu-CDs@Al₂O₃ and L-Glu-CDs@MgAl₂O₄ (Fig. 5a and f), Gly-CDs@Al₂O₃ and Gly-CDs@MgAl₂O₄ (Fig. 5b and g), SA-CDs@Al₂O₃ and SA-CDs@MgAl₂O₄ (Fig. 5c and h), Phth-CDs@Al₂O₃ and Phth-CDs@MgAl₂O₄ (Fig. 5d and i), and TA-CDs@Al₂O₃ and TA-CDs@MgAl₂O₄ (Fig. 5e and j). Under identical conditions, all these CDs exhibited intense phosphorescence under 365 nm excitation. Similarly, the magnesium-doped CDs demonstrated an obvious red-shift in their phosphorescence emission spectra compared with the CDs without magnesium, which suggested the broad universality of this strategy.

Fig. 5 Phosphorescence emission spectra of (a) L-Glu-CDs@Al₂O₃ and L-Glu-CDs@MgAl₂O₄, (b) Gly-CDs@Al₂O₃ and Gly-CDs@MgAl₂O₄, (c) SA-CDs@Al₂O₃ and SA-CDs@MgAl₂O₄, (d) Phth-CDs@Al₂O₃ and Phth-CDs@MgAl₂O₄, and (e) TA-CDs@Al₂O₃ and TA-CDs@MgAl₂O₄. (f–j) The corresponding images under daylight, 365 nm UV irradiation and after the UV light was turned off.

3. Conclusions

In summary, we have successfully designed and prepared CDs@Al₂O₃ and CDs@MgAl₂O₄ with multiple-color phosphorescence and excellent pH tolerance by an in situ preparation strategy. Owing to the oxygen vacancy defects formed, both CDs@Al₂O₃ and CDs@MgAl₂O₄ achieved long-lived and high quantum-yield phosphorescence. Significantly, the incorporation of magnesium in CDs@MgAl₂O₄ significantly enhanced its red-shifted phosphorescence. Additionally, the phosphorescence of CDs@Al₂O₃ and CDs@MgAl₂O₄ was excitation-dependent, and multiple-color phosphorescence could be realized by regulating the excitation wavelength. Through changing the carbon sources, we further synthesized a series of CDs with distinct phosphorescence, demonstrating the universality of this strategy and providing an innovative approach for designing CD-based multicolor phosphorescent materials.

4. Experimental section

4.1. Chemicals and materials

Folic Acid (FA) was purchased from Heowns Biochem Technologies LLC (Tianjin, China). AlCl₃·6H₂O, tartaric acid and succinic acid were acquired from Macklin Biochemical Co., Ltd (Shanghai, China), while MgCl₂ was from Bide Pharmatech Co., Ltd (Shanghai, China). NH₃ H₂O was purchased from Chongqing Chuandong Chemical (Group) Co., Ltd (Chongqing, China). Phthalimide was purchased from Aladdin Reagent Co., Ltd (Shanghai, China) and L-glutamic acid and glycine were purchased from Chron Chemicals Co., Ltd (Chengdu, China). In addition, all the commercial reagents were utilized directly without further purification, and ultrapure water produced using an Aquapro AWL-0502-P water system (18.25 MΩ cm) was employed throughout the entire experimental procedure.

4.2. Characterization

The synthesis of CDs@Al₂O₃ and CDs@MgAl₂O₄ was performed in a ZSX 1200 muffle furnace (Beijing, China). The fluorescence and phosphorescence spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan), and UV-vis absorption was recorded with a Shimadzu UV-3600i Plus ultraviolet-visible spectrophotometer (Tokyo, Japan). The morphology was characterized using a JEOL JEM-F200 high-resolution transmission electron microscope (HR-TEM) (Tokyo, Japan) operating at 200 kV. Meanwhile, X-ray photoelectron spectroscopy (XPS) of CDs@Al₂O₃ and CDs@MgAl₂O₄ was performed using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer (New York, America), and X-ray diffraction (XRD) patterns were acquired using a Rigaku Ultima IV X-ray diffraction analyzer. The surface groups of CDs were examined through a Fourier transform infrared (FTIR) spectrometer (Tokyo, Japan). The phosphorescence spectra, quantum yields and lifetime measurements were performed using an Edinburgh FLS1000 photoluminescence spectrophotometer (Edinburgh, Scotland) equipped with a microsecond flash lamp (lF900, 100–0.1 Hz) and a time-gating device (delay time ranging from 0.001 to 9999 ms). Again, the Electron paramagnetic Resonance (EPR) spectra were recorded on an EPR spectrometer (Bruker EMXplus-9.5/12) operating at X-band frequencies (≈9.846 GHz) and room temperature.

4.3. Synthesis of CDs@Al₂O₃ and CDs@MgAl₂O₄

During the synthesis, 0.1 mmol of folic acid (FA) and 8 mmol of AlCl₃·6H₂O were mixed with 3 mL of ultrapure water. Then, 5 mL NH₃ H₂O was slowly added to the mixed solution with stirring, and the mixture was dried at 60 °C for 12 h after the uniform stirring. Finally, the dried raw product was ground into a fine powder and calcined in air in a muffle furnace at 530 °C for 150 min at a rate of 5 °C/min to obtain CDs@Al₂O₃. Similarly, CDs@MgAl₂O₄ was synthesized using 0.1 mmol of folic acid, 4 mmol of MgCl₂ and 8 mmol of AlCl₃·6H₂O through the same way as CDs@Al₂O₃.

4.4. DFT calculations

The DFT method was applied to explore the configuration and mechanism. A finite ring model of graphene was chosen in the present work, where Al³⁺ ions and Mg²⁺ ions presented different coordination. DFT and plane wave basis sets were used to calculate the energy and were implemented in the Vienna ab initio Simulation Package (VASP). The projection enhanced wave (PAW) method was used to deal with the nuclear valence electron interaction. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was used to approximate the electronic changes and correlations. The convergence criterion of the electron self-consistent loop was 10⁻⁵ eV, and the ion relaxation converged to a force tolerance of less than 0.05 eV Å⁻¹.

Author contributions

Y. L. and H. Z. contributed to investigation and formal analysis. Y. C. was responsible for supervision and DFT calculations. X. Y. was responsible for supervision and wrote the original draft. All authors contributed to reviewing and editing the manuscript, and approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this work have been included in the main text and supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5sc09281g.

Acknowledgements

This work was financially supported by Natural Science Foundation of Chongqing, China (CSTB2024NSCQ-MSX0417).

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