Graded nitro-engineering strategy: Tuning surface states and sp2 conjugated domains of carbon quantum dots for full-color emission
Chunyan Wang , Chen Wei , Hongyang Niu , Ligang Xu , Xue Liu
The development of straightforward methods to regulate the luminescence performance of carbon quantum dots (CQDs) [1,2] from single-color to multi-color emissions can significantly advance their applications in biomedical imaging [3–8] and information display [9–12]. The conjugation effect and surface states are considered key factors in controlling the luminescence color of fluorescent CQDs [7,13–16]. By combining these two critical factors, the luminescence color of CQDs can be tuned across the entire visible spectrum [17]. When both factors are present, it is essential to determine which factor predominates in influencing the luminescence of CQDs and how their interaction affects this luminescence. These studies not only enhance our understanding of the luminescence mechanisms of CQDs but also hold great significance for the precise design and regulation of their photoluminescence (PL) characteristics.
The increase in the particle size of CQDs can enhance the size of the sp2 conjugated domain to a certain extent, which is advantageous for their luminescence in the long-wavelength region [18–21]. An enlarged sp2 conjugated domain improves delocalization of π-electron [22], resulting in a more uniform electron cloud distribution, a reduced energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and a subsequent red-shift in the emission of CQDs. Zhao et al. synthesized water-soluble CQDs through a hydrothermal reaction using glucose as a precursor. By varying the amount of hydrochloric acid, they were able to modify the particle size of the CQDs and the size of the sp2 conjugated domain, achieving tunable fluorescence from blue to orange [19]. Miao et al. produced CQDs with full-color luminescence via controlled pyrolysis of citric acid (CA) and urea. X-ray diffraction and Raman spectroscopy analysis indicated that the red-shift in the emission wavelength of CQDs was attributed to the increase in the sp2 conjugated domain [14]. Chen's group employed density functional theory (DFT) to construct a model of the CQDs structure and confirmed the relationship between the emission shift of CQDs and the size of the sp2 conjugated domain through calculations [23]. In summary, increasing particle size enhances sp2 conjugated domains, which is essential for achieving red-shifted luminescence in CQDs.
In addition, the surface states of CQDs can significantly influence their bandgap structure and serve as a crucial regulatory variable for luminescence modulation. The surface states of CQDs are closely associated with the types of surface functional groups. Notably, certain oxygen-containing and nitrogen-containing functional groups can alter the luminescence color of CQDs by introducing multi-level bandgaps [24]. Ding et al. synthesized CQDs in a single-pot reaction and separated them via column chromatography, obtaining CQDs with varying luminescent colors. These CQDs exhibited similar particle sizes. As the emission peak shifted from 440 nm to 625 nm, the oxygen functional group content on the surface of the CQDs increased. This finding strongly suggests that the extent of surface oxidation of CQDs leads to the observed red-shift in emission [25]. Jeon's group employed DFT to simulate how the number of surface amino groups (-NH2) influences the bandgap energy levels of CQDs. The results revealed that as the number of -NH2 on the surface of CQDs increased, the bandgap gradually decreased, and the fluorescence emission exhibited a consistent red-shift [26]. Zhang et al. demonstrated an environmentally friendly approach to synthesize full-color luminescent CQDs by precisely regulating surface states. Utilizing 4,4-bipyridine and p-phenylenediamine as precursors in glycol and water solvents, the researchers achieved full-spectrum luminescence ranging from purple (441 nm) to red (627 nm). The underlying mechanism involves the modulation of surface states, with C-N bonds playing a significant role in long-wavelength emission, while C-O-C and O-H contribute to the blue shift [27]. In conclusion, functional group modulation of surface states is critical for tuning the luminescent properties of CQDs.
Nitro groups (-NO2) are oxygen-containing functional groups that can enhance the polarity and dipole moment of molecules and materials [28,29]. Due to their relatively large size, -NO2 can significantly increase the rigidity and steric hindrance of compounds [30,31]. The photophysical properties of carbon-based materials can be effectively modified through -NO2 substitution [32]. In this study, we employed a combination of experimental and theoretical methods to investigate the synergistic regulation of the fluorescence color of CQDs by -NO2 and sp2 conjugated domains. Using o-phenylenediamine (o-PD) as the sole carbon source, tricolor-emitting nitro-functionalized CQDs (NO2-CQDs) were synthesized by adjusting the concentration of nitric acid (HNO3) under hydrothermal conditions. Our results demonstrate that varying the number of -NO2 can significantly influence luminescence. We constructed atomic models of CQDs with different quantities of -NO2 and further validated our findings through DFT calculations, evaluations of sp2 domain size, and assessments of the HOMO-LUMO bandgap. At lower numbers of -NO2 (particularly at 0, 1 and 4), luminescence is primarily influenced by surface states. The strong electrophilic effect of -NO2 enhances the delocalization of π electrons, stabilizes the sp2 conjugation, and lower the LUMO energy level, resulting in a red-shift in luminescence. Conversely, when the number of -NO2 increases to 8 and 12, the rigidity and steric hindrance introduced by -NO2 compromise the stability of the sp2 domain, leading to a blue-shift in fluorescence. Ultimately, in light-emitting diode (LED) and cellular imaging applications, the NO2-CQDs exhibit consistent luminescent colors, demonstrating stability across a variety of materials and environmental conditions. This work highlights the potential of a graded nitro-engineering strategy for precise tuning of CQDs luminescence across the visible spectrum.
NO2-CQDs were synthesized via a hydrothermal reaction using o-PD as the carbon source, and their emission colors were tuned by adjusting HNO3 concentrations (Fig. S1 and Table S1 in Supporting information).
To investigate the origin of tunable PL and the mechanistic role of HNO3 in CQDs formation, three representative CQDs were selected (Scheme 1a). Specifically, GQDs, RQDs, and BQDs were synthesized using HNO3 concentrations of 0, 3.48, and 4.06 mmol/L, respectively. Their morphology, and structure were characterized (Scheme 1b). TEM and HRTEM images revealed that NO2-CQDs exhibited uniformly dispersed spherical nanoparticles with narrow size distributions: BQDs (3.40 ± 0.52 nm), GQDs (3.09 ± 0.48 nm), and RQDs (4.92 ± 0.77 nm) (Scheme 1b). The HRTEM images showed a crystalline carbon lattice structure, with well-resolved lattice fringes of 0.21 nm corresponding to the (100) crystallographic facet of graphitic carbon, indicating high stability [22,24,33–35].
PL analyses were conducted to assess the optimal optical properties of the CQDs. NO2-CQDs ethanol solutions displayed different colors in daylight and exhibited tunable PL emission from blue to red under 365 nm UV light (Scheme 1c). BQDs show a maximum emission wavelength of 462 nm upon excitation from 300 nm to 390 nm (Fig. 1a). GQDs exhibit a peak at 542 nm with excitation between 340 nm and 560 nm (Fig. 1b). RQDs displayed excitation within the range of 470–600 nm, with a maximum emission at 606 nm and a shoulder peak at 652 nm (Fig. 1c). The dual-peak emission of RQDs is attributed to transitions between different vibrational energy levels within the carbon core, driven by electron-phonon coupling [36]. Additionally, in Fig. S2 (Supporting information), the excitation-normalized spectra of the NO2-CQDs show consistent emission profiles, further confirming that NO2-CQDs exhibit excitation-independent PL, attributed to their highly ordered graphitic surface structure and uniform surface states [22,35]. Optimal excitation and emission peaks of NO2-CQDs are shown in Figs. 1d and e, respectively. To determine the spatial coordinates of chromophores, the optimal fluorescence emission spectra were converted into CIE 1931 chromaticity coordinates, establishing values for blue (0.1688, 0.2253), green (0.3867, 0.5853), and red (0.6388, 0.3608) emitters. The transformation of NO2-CQDs emission from blue to red can be observed in the CIE 1931 chromaticity diagram (Fig. 1f). Absolute PL quantum yields (QYs) were measured as 3.98% for BQDs, 8.62% for GQDs, and 19.33% for RQDs. The fluorescence lifetimes of NO2-CQDs were fitted to two exponentials, demonstrating a dual-exponential decay pattern (Fig. S3 and Table S3 in Supporting information) [19,22,33,37]. The results indicate that the formation of full-color CQDs is influenced by both core and surface states. The average fluorescence lifetimes for BQDs, GQDs, and RQDs were calculated to be 10.93, 11.25, and 12.48 ns, respectively, further supporting the role of HNO3 in modulating CQDs PL characteristics.
To quantitatively link the optical properties to structural changes. The optical bandgap (Eg) of NO2-CQDs was estimated using the Tauc plot method, as detailed in Fig. S4 (Supporting information) [12,20,38,39]. The calculated bandgap energies for BQDs, GQDs, and RQDs decreased progressively from 2.28 eV to 2.08 eV to 1.91 eV, respectively. Correspondingly, the emission peaks were observed at 462 nm for BQDs, 542 nm for GQDs, and 606 nm for RQDs. These findings indicate that the bandgaps of CQDs play a pivotal role in regulating their fluorescence, with narrower energy gaps corresponding to longer-wavelength emissions, a trend governed by the expansion of sp2 conjugated domains.
To gain insight into the formation mechanism of NO2-CQDs and their intrinsic connection with core and surface states, a detailed structural characterization was conducted to further elucidate the relationship between color purity and the structure of NO2-CQDs. Theoretical predictions for sp2 conjugated domains were validated using Raman and X-ray diffraction (XRD) analyses. Raman spectra of NO2-CQDs (Fig. 2a) displayed two distinct peaks at 1373 and 1520 cm−1, corresponding to disordered sp3 carbon structures (D band) and graphitic sp2 carbon domains (G band), respectively. The ID/IG ratio is a measure of graphitization, with sp3 to sp2 carbon ratios reflecting carbon hybridization [14,19,35,38]. ID/IG values for BQDs, GQDs, and RQDs are 1.12, 0.98, and 0.86, respectively, indicating increased graphitization and sp2 domain enlargement from BQDs to RQDs. Additionally, the Raman G band of RQDs shows a blue-shift compared to BQDs with increasing HNO3 concentration, likely due to edge defects in the material [40]. Furthermore, the average sp2 domain size was quantitatively assessed using Raman spectroscopy (Fig. S5 and Table S4 in Supporting information) [41]. The calculated sizes of the sp2 conjugated domains in NO2-CQDs were 2.83, 2.93, and 4.03 nm, respectively, suggesting a direct correlation between sp2 domain size and conjugation extent. The XRD pattern (Fig. 2b) exhibits a broad peak at 21°, corresponding to a 0.21 nm spacing of the (100) plane of graphitic carbon, suggesting tightly packed sp2 domains within the CQDs cores, consistent with HRTEM images. From BQDs to RQDs, the intensity and shape of this peak at 21° gradually increase, indicating higher graphitization and sp2 conjugation [14,35,42].
The prediction of the surface state is supported by UV–vis absorption spectra analysis (Fig. 2c). NO2-CQDs demonstrate similar absorption in the UV region (200–350 nm) but exhibit distinct characteristics at longer wavelengths. Notably, the prominent UV absorption peak at 293 nm is primarily due to π-π* transitions in sp2 conjugated domains [43]. In the longer wavelength range (340–700 nm), NO2-CQDs exhibit distinct absorption bands. BQDs display an absorption peak at 344 nm, GQDs at 434 nm, and RQDs at 538 nm, each attributed to n-π* transitions involving O and N group configurations within the sp2 structural domain [22,33]. Furthermore, the UV absorption peaks closely align with the optimal excitation wavelengths of NO2-CQDs fluorescence. This alignment indicates that HNO3 addition modifies CQDs surface states, influencing the fluorescence emission center.
In addition, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to examine the surface functional groups, elemental states, and chemical composition of NO2-CQDs. By comparing the FTIR spectra of o-PD and NO2-CQDs (Fig. 2d), the NO2-CQDs show O-H/N-H stretching vibrations at 3430 cm−1, indicating hydrophilic groups that enhance solubility. Peaks at 2827–2925 cm−1 correspond to C-H (sp2) stretching vibrations, while C=C and C=N stretching vibrations at 1620 cm−1 suggest aromatic and imine structures. The absorption peak near 1125 cm−1 is attributed to the C-N and C-O stretching vibrations. A peak at 1387 cm−1 (C-N-C stretching) indicates phenazine structures, while peaks at 752 and 613 cm−1 correspond to C-H out-of-plane bending in aromatic benzene within the phenazine framework [42]. In o-PD, these peaks are also present, indicating that NO2-CQDs retain intrinsic raw material properties. -NO2 was detected at 1344 cm−1, confirming HNO3 reacts with o-PD through electrophilic substitution under hydrothermal conditions. Comparisons of the FTIR spectra of NO2-CQDs reveal two major trends as the fluorescence emission shifts from blue to red: Increased C=C stretching from BQDs to RQDs, indicating more extensive sp2 conjugation, and a variation in -NO2 content, with an initial decrease followed by an increase, although the overall -NO2 level remains lower. This finding represents the first demonstration of full-color PL tuning in CQDs by modulating -NO2 content.
XPS analysis provided further evidence for these observations [12,14,22,38,42]. XPS spectra of NO2-CQDs (Fig. S6a in Supporting information) confirm the presence of C (285.0 eV), N (398.4 eV), and O (531.8 eV) in NO2-CQDs. High-resolution XPS spectra for C 1s (Fig. 2f), N 1s (Fig. 2g), and O 1s (Fig. S6b in Supporting information) reveal distinct peaks for each element. The C 1s spectrum shows four peaks at 284.5, 285.6, 286.4, and 288.5 eV, corresponding to sp2 C (C=C/C-C), sp3 C (C-N), sp3 C (C-O), and COOH groups, respectively. Peaks at 398.5, 399.3, 400.2, 401.1, and 406.1 eV in the N 1s spectrum are attributed to pyridinic N, amide N, pyrrolic N, graphitic N, and -NO2. The O 1s spectrum reveals peaks at 531.1 and 533.1 eV, corresponding to C=O and C-O bonds, respectively. Analysis of peak areas (Fig. 2e) shows sp2 C (C=C/C-C) ratios of 34.4%, 45.4%, and 56.2% in BQDs, GQDs, and RQDs, respectively, indicating a gradual increase in the size of the sp2 conjugated domains in CQDs synthesized via o-PD carbonylation polymerization reactions, from BQDs to RQDs. Furthermore, it is worth noting that BQDs, GQDs, and RQDs are influenced not only by the sp2 conjugated domains within the carbon core but also by the external surface -NO2, leading to complex structural configurations. This is evident in the N 1s spectra, where -NO2 content gradually increases from 5.6% (GQDs) to 15.7% (RQDs) as HNO3 concentration rises from 0 mmol/L to 3.48 mmol/L, accompanied by a red-shift in emission peaks. With further increases in HNO3 concentration to 4.06 mmol/L, -NO2 content rises to 19.8%, resulting in a blue-shift in CQDs emission (Fig. 2e and Table S5 in Supporting information). In GQDs (0 mmol/L HNO3), the oxidation of -NH2 to -NO2 is likely mediated by ROS generated via dissolved oxygen activation [44–46]. These findings confirm the indispensable role of -NO2 in modulating the PL of NO2-CQDs, marking the first demonstration of NO2-controlled full-color CQDs emission.
Based on the comprehensive characterization and analysis presented above, we propose a hierarchical -NO2 engineering strategy that achieves full-color emission in CQDs through precise modulation of their surface states and sp2-conjugated domains by controlling HNO3 concentrations. This strategy operates via a dual modulation mechanism: (ⅰ) Electron-withdrawing effects dominate at low HNO3 concentrations (0–3.48 mmol/L), enhancing π-delocalization and red-shifted emission [42]. (ⅱ) Steric hindrance prevails at high HNO3 concentrations (≥4.06 mmol/L), disrupting conjugation and inducing blue-shifted emission.
At low HNO3 concentrations (0–3.48 mmol/L), nitric acid plays dual roles in enhancing sp2-conjugated domains of CQDs through both carbon framework modification and surface functionalization. During CQDs synthesis, HNO3 accelerates oxidative polymerization by protonating intermediate products and promoting intermolecular crosslinking, thereby forming larger aromatic frameworks (e.g., RQDs exhibit a maximum sp2 domain size of 4.03 nm). Concurrently, surface-functionalized -NO2 introduce electron-withdrawing effects, further enhancing π-electron delocalization. Structural characterization of CQDs confirms these effects alongside increased -NO2 content. Raman spectroscopy (ID/IG ratio decreases from 0.98 for GQDs to 0.86 for RQDs, Table S4) reveals enhanced graphitization. XPS analysis demonstrates an elevated sp2-carbon content from 45.4% (GQDs) to 56.2% (RQDs) (Figs. 2e and f), providing direct evidence of conjugated domain expansion. Tauc plot analysis (Fig. S4) further shows a narrowed bandgap (from 2.08 eV for GQDs to 1.91 eV for RQDs), consistent with the emission redshift from GQDs (542 nm) to RQDs (606 nm).
When the nitric acid concentration reaches 4.06 mmol/L, the -NO2 content increases. Excessive -NO2 introduce steric hindrance [31], distorting the planarity of the graphitic backbone and reducing sp2 conjugated domain sizes (e.g., BQDs exhibit sp2 domains of 2.83 nm, 1.2 nm smaller than RQDs). Additionally, high HNO3 concentrations promote over-carbonization, shifting hybridization from sp2 to sp3 and further limiting conjugation.
To further support our hypothesis, DFT calculations were conducted [22,35]. Initially, planar macromolecular models with varying numbers of benzene rings were constructed, and the energy levels of the HOMO and the LUMO, along with the corresponding bandgaps, were calculated (Fig. 3a and Table S6 in Supporting information). As the number of benzene rings increased from 0 to 7, 19, and 37, the bandgap of the molecule decreased from 5.19 eV to 1.97 eV. This trend indicates that as the sp2 conjugation domains in CQDs expand, the degree of electron delocalization in the conjugated system increases, which contributes to a decrease in the LUMO energy level and a reduction in the bandgap, thereby enhancing the red-shift phenomenon. Theoretical calculations show that the expansion of the conjugation domain in CQDs leads to a reduced bandgap and fluorescence red-shift, which agrees with the experimental results.
Furthermore, to investigate the effect of -NO2 on the sp2 conjugation domains, a molecular model consisting of nineteen benzene rings was selected to study the impact of functional groups, particularly -NO2, on the molecular bandgap. The number of hydroxyl and carboxyl groups on the surface remained constant, while the number of -NO2 varied from 0 to 1 to 4, the bandgap gradually decreased from 2.82 eV to 2.62 eV and 2.51 eV, respectively. As the number of surface -NO2 increased to 8 and 12, the bandgap widened, reaching 2.57 eV and 2.60 eV (Fig. 3b and Table S7 in Supporting information). Theoretical calculations suggest that a moderate increase in the number of -NO2 on the surface of CQDs enhances delocalization of π electrons due to their strong electron-withdrawing properties. This enhancement leads to an extended conjugated structure, resulting in a reduction of the LUMO energy level and the bandgap. The observed optical red-shift in fluorescence emission is a direct consequence of these structural changes. However, when CQDs are treated with a high concentration of HNO3, the -NO2 content of the resulting BQDs is significantly higher than that of GQDs and RQDs. The calculated results reveal a notable increase in steric hindrance when the -NO2 concentration surpasses a certain threshold. The side view of the calculated results illustrates that this steric hindrance constrains the planarity and degrees of freedom of the molecules, leading to a reduction in the effective length of the conjugated system. Consequently, this increases the difficulty for electrons to transition within the molecules, necessitating higher energy to achieve such transitions. This phenomenon is optically manifested as a blue-shift in the fluorescence emission wavelength. Theoretical calculations indicate that the -NO2 on the surface of CQDs have the potential to regulate the full-color luminescence of CQDs by modulating the size and shape of the conjugated domains. These calculations establish a direct correlation between surface -NO2 content, conjugation-domain size, and emission wavelength, thereby bridging theoretical predictions with experimental observations.
To demonstrate the impact of surface -NO2 on NO2-CQDs PL, the surface structure was meticulously controlled (Fig. S7 in Supporting information) [22,47,48]. Surface -NO2 were reduced to -NH2 using B2(OH)4 and 4, 4′-bipyridine in DMF. The resulting NO2-CQDs, designated as BQDs+, GQDs+, and RQDs+, each showed significant shifts in fluorescence emission wavelengths. The optimal emission wavelength of BQDs shifted from 454 nm to 479 nm (red-shift), while GQDs shifted from 545 nm to 512 nm and RQDs from 605 nm to 594 nm (both blue-shift). These results demonstrate that surface -NO2 are crucial for modulating the synthesis and optical properties of NO2-CQDs, providing direct evidence for the proposed dual mechanism.
In summary, DFT calculations revealed that elevated lower -NO2 content (XPS: from 5.6% to 15.7%) expands sp2-conjugated domains, reduces the LUMO energy (from -2.12 eV to -3.39 eV) and bandgap (from 2.82 eV to 2.51 eV), thereby inducing a fluorescence redshift from 542 nm (GQDs) to 606 nm (RQDs). At higher -NO2 concentrations (XPS: from 15.7% to 19.8%), steric hindrance disrupts conjugation planarity, increases the bandgap to 2.60 eV, and causes a blue shift to 462 nm (BQDs). Reduction of -NO2 experiments validate this hierarchical strategy, confirming dual modulation mechanisms dominated by electronic effects at lower -NO2 content and steric effects at higher -NO2 content.
Given the intriguing full-color PL of CQDs, previous studies have demonstrated that the full-color PL of CQDs can be extended to solid-state films for applications in solid-state lighting [49–52]. In this study, we successfully fabricated full-color emissive NO2-CQDs/polyvinyl alcohol (PVA) composite films, which retained the full-color PL characteristics of the NO2-CQDs. Fig. 4a shows their corresponding fluorescence spectra, with inset photographs of the composite under daylight and UV light. Upon exposure to UV light at 365 nm, the NO2-CQDs/PVA films emit strong light across the spectrum from blue to red. Notably, there is minimal change in the position of the emission peak after combining the NO2-CQDs with PVA, suggesting that the fluorescent properties of the NO2-CQDs remain stable upon integration with PVA. The emission spectra provided chromaticity coordinates of (0.1736, 0.2932) for B-film, (0.3889, 0.5453) for G-film, and (0.6447, 0.3549) for R-film (Fig. 4b). Fluorescence spectra and CIE coordinates at varying concentrations (0.05–2.0 mg/mL, Fig. S8 and Table S8 in Supporting information) demonstrate a gradual decline in intensity and systematic chromatic shifts toward less saturated regions with reduced concentration. This behavior is likely due to decreased emission center density and enhanced non-radiative recombination, confirming the broad blue-to-red gamut of the films for full-spectrum luminescence. Additionally, NO2-CQDs-based LEDs with various color emissions were prepared using the NO2-CQDs/PVA films and InGaN LED chips centered at 365 nm. The CQDs-based monochromatic LEDs displayed emission peaks at 467 nm (blue), 559 nm (green), and 607 nm (red) (Figs. 4d–f), matching their solution-phase PL profiles. Their emission spanned 450–650 nm, covering the visible spectrum from blue to red. The CIE 1931 coordinates for the blue, green, and red devices were (0.2266, 0.2687), (0.4021, 0.4638), and (0.4905, 0.4571), respectively (Fig. 4c). This trichromatic coverage confirms their viability for full-color displays. In conclusion, full-color CQDs have demonstrated significant potential and utility in the development of innovative multicolor films and LEDs.
Considering the outstanding PL of the CQDs, their potential as probes for full-color fluorescent bioimaging was investigated. Figs. S9a, b and c (Supporting information) showed that our CQDs presented intracellular imaging ability. Moreover, the cytotoxicity assessment results demonstrated no significant cytotoxicity (cell viability > 90%) for all CQDs (Figs. S9d and e in Supporting information).
In summary, this study successfully achieved precise regulation of luminescence color by manipulating the content of -NO2 in CQDs. The -NO2 content also influences two key determinants of CQDs luminescence: surface states and conjugated domains. We calculated the HOMO and LUMO levels of CQDs with varying -NO2 contents using DFT. Additionally, we estimated the sp2 domain size of CQDs prepared under different concentrations of HNO3, which aligns with the experimental results. The synthesized NO2-CQDs exhibit excellent color stability in LED and cellular imaging applications, highlighting their significant potential for practical applications.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Chunyan Wang: Writing – review & editing, Writing – original draft, Methodology, Conceptualization. Chen Wei: Writing – review & editing, Writing – original draft. Hongyang Niu: Writing – review & editing. Ligang Xu: Writing – review & editing. Xue Liu: Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
This work was supported by National Natural Science Foundation of China (No. 51873085), Natural Science Foundation of Liaoning Province-Outstanding Youth Foundation (No. 2022-YQ-14), Liaoning Revitalization Talents Program (No. XLYC2007056), China Scholarship Council (CSC Scholarship, No. 202006800009), and the Shenyang Science and Technology Project (No. RC230707).
Supplementary material associated with this article can be found, in the online version, at doi:
Q.J. Ni, S.Y. Zhang, K. Wang, et al., J. Mater. Chem. A 12 (2024) 31253–31261. doi: 10.1039/d4ta06855f
S. Fu, B. J Tang, Z. M Wang, et al., Chem. Eng. J. 500 (2024) 157207. doi: 10.1016/j.cej.2024.157207
B. Zhao, H.Y. Ma, M.Y. Zheng, et al., Carbon Energy 4 (2022) 88–114. doi: 10.1002/cey2.175
S. Pandey, D. Bodas, Adv. Colloid Interface Sci. 278 (2020) 102137. doi: 10.1016/j.cis.2020.102137
Y. Liu, L.L. Zhang, H.J. Cai, et al., Sci. Bull. 69 (2024) 3127–3149. doi: 10.3390/s24103127
J.R. Li, X. Gong, Small 18 (2022) 2205099. doi: 10.1002/smll.202205099
M. Jiang, Y.Z. Sun, M.Y. Chen, et al., Chem. Eng. J. 496 (2024) 153761. doi: 10.1016/j.cej.2024.153761
K. Jiang, S. Sun, L. Zhang, et al., Angew. Chem. Int. Ed. 54 (2015) 5360–5363. doi: 10.1002/anie.201501193
B. Zhao, H.Y. Ma, H.R. Jia, et al., Angew. Chem. Int. Ed. 62 (2023) e202301651. doi: 10.1002/anie.202301651
Z.S. Yan, T. Chen, L.P. Yan, et al., Adv. Sci. 10 (2023) 2206386. doi: 10.1002/advs.202206386
Y.F.S. Wu, X. Chen, W. Wu, Small 19 (2023) 2206709. doi: 10.1002/smll.202206709
R.Y. Dai, X.P. Chen, N. Ouyang, et al., Chem. Eng. J. 431 (2022) 134172. doi: 10.1016/j.cej.2021.134172
B.H. Zhang, B.Z. Wang, E.V. Ushakova, et al., Small 19 (2023) 2204158. doi: 10.1002/smll.202204158
X. Miao, D. Qu, D.X. Yang, et al., Adv. Mater. 30 (2018) 1704740. doi: 10.1002/adma.201704740
L. Ai, Y.S. Yang, B.Y. Wang, et al., Sci. Bull. 66 (2021) 839–856. doi: 10.1016/j.scib.2020.12.015
X. Yang, X. Li, B.Y. Wang, et al., Chin. Chem. Lett. 33 (2022) 613–625. doi: 10.1016/j.cclet.2021.08.077
Y.B. Liu, D.Y. Chao, L. Zhou, et al., Carbon 135 (2018) 253–259. doi: 10.1016/j.carbon.2018.02.004
Y. Zhou, H.L. Duan, K.J. Tan, et al., Nanoscale 16 (2024) 11642–11650. doi: 10.1039/d4nr01111b
Y.N. Zhao, C.L. Ou, J.K. Yu, et al., ACS Appl. Mater. Interfaces 13 (2021) 30098–30105. doi: 10.1021/acsami.1c07444
F.L. Yuan, T. Yuan, L.Z. Sui, et al., Nat. Commun. 9 (2018) 2249. doi: 10.1038/s41467-018-04635-5
H.Y. Bai, X.L. Jin, Z. Cheng, et al., Adv. Compos. Hybrid Mater. 6 (2023) 62. doi: 10.3847/1538-4357/aced94
B.Y. Wang, J.K. Yu, L.Z. Sui, et al., Adv. Sci. 8 (2021) 2001453. doi: 10.1002/advs.202001453
M.A. Sk, A. Ananthanarayanan, L. Huang, et al., J. Mater. Chem. C 2 (2014) 6954–6960. doi: 10.1039/C4TC01191K
F.Y. Yan, H. Zhang, N.H. Yu, et al., Sens. Actuators B: Chem. 329 (2021) 129263. doi: 10.1016/j.snb.2020.129263
H. Ding, S.B. Yu, J.S. Wei, et al., ACS Nano 10 (2016) 484–491. doi: 10.1021/acsnano.5b05406
S.H. Jin, D.H. Kim, G.H. Jun, et al., ACS Nano 7 (2013) 1239–1245. doi: 10.1021/nn304675g
H.Z. Zhang, J.F. Bai, X.L. Chen, et al., J. Colloid Interface Sci. 678 (2025) 77–87. doi: 10.1016/j.jcis.2024.08.073
M.B. Singh, P. Jain, F. Mohammad, et al., ACS Omega 9 (2024) 16458–16468. doi: 10.1021/acsomega.4c00173
T.E. Olalekan, I.A. Adejoro, B. VanBrecht, et al., Spectrochim. Acta A 139 (2015) 385–395. doi: 10.1016/j.saa.2014.12.018
A. Javadi, A. Shockravi, M. Koohgard, et al., Eur. Polym. J. 66 (2015) 328–341. doi: 10.1016/j.eurpolymj.2015.02.032
T. Tong, S. Wang, D.Z. Mo, et al., Electrochim. Acta 470 (2023) 143327. doi: 10.1016/j.electacta.2023.143327
P.L. Gkizis, I. Triandafillidi, C.G. Kokotos, Chem 9 (2023) 3401–3414. doi: 10.1016/j.chempr.2023.09.008
H. Ding, J.S. Wei, P. Zhang, et al., Small 14 (2018) 1800612. doi: 10.1002/smll.201800612
H. J Li, S.S. Han, B.W. Lyu, et al., Chin. Chem. Lett. 32 (2021) 2887–2892. doi: 10.1016/j.cclet.2021.03.051
Y.X. Zheng, K. Arkin, J.W. Hao, et al., Adv. Opt. Mater. 9 (2021) 2100688. doi: 10.1002/adom.202100688
B.Y. Wang, Z.H. Wei, L.Z. Sui, et al., Light Sci. Appl. 11 (2022) 172. doi: 10.1038/s41377-022-00865-x
F. Ehrat, S. Bhattacharyya, J. Schneider, et al., Nano Lett. 17 (2017) 7710–7716. doi: 10.1021/acs.nanolett.7b03863
J. H Xu, Q.J. Liang, Z.J. Li, et al., Adv. Mater. 34 (2022) 2200011. doi: 10.1002/adma.202200011
Y.Y. Wang, Q. Su, X.M. Yang, Chem. Commun. 54 (2018) 11312–11315. doi: 10.1039/c8cc06116e
Y. Dong, S. Zhang, L.L. Shi, et al., Mater. Chem. Phys. 203 (2018) 125–132. doi: 10.1016/j.matchemphys.2017.09.064
J. Kang, Y. Ko, J.P. Kim, et al., Nat. Commun. 14 (2023) 901. doi: 10.1038/s41467-023-36524-x
J.J. Liu, D.W. Li, K. Zhang, et al., Small 14 (2018) 1703919. doi: 10.1002/smll.201703919
Y. Li, C. Liu, H. Sun, et al., Adv. Sci. 10 (2023) 2300543. doi: 10.1002/advs.202300543
S.Y. Li, W. Zhao, L. L Wang, et al., ACS Omega 9 (2024) 39715–39723. doi: 10.1039/d4ra04949g
J.H. Wang, H. Li, Y.H. Cai, et al., Anal. Chem. 91 (2019) 6155–6161. doi: 10.1021/acs.analchem.9b00759
K. Adamic, M. Dunn, K.U. Ingold, Can. J. Chem. 47 (1969) 287–294. doi: 10.1139/v69-040
M. Jang, T. Lim, B.Y. Park, et al., J. Org. Chem. 87 (2022) 910–919. doi: 10.1021/acs.joc.1c01431
C. Liu, L. Bao, M.L. Yang, et al., J. Phys. Chem. Lett. 10 (2019) 3621–3629. doi: 10.1021/acs.jpclett.9b01339
H.R. Jia, Z.B. Wang, T. Yuan, et al., Adv. Sci. 6 (2019) 1900397. doi: 10.1002/advs.201900397
T. Meng, Z.F. Wang, T. Yuan, et al., Angew. Chem. Int. Ed. 60 (2021) 16343–16348. doi: 10.1002/anie.202103361
Y.X. Shi, Y. Zhang, Z.B. Wang, et al., Nat. Commun. 15 (2024) 3043. doi: 10.1038/s41467-024-47372-8
T. Yuan, F.L. Yuan, L.Z. Sui, et al., Angew. Chem. Int. Ed. 62 (2023) e202218568. doi: 10.1002/anie.202218568
Scheme 1 (a) Synthesis route and morphology of NO2-CQDs: BQDs, GQDs, and RQDs were prepared via hydrothermal treatment of o-PD with 4.06, 0, and 3.48 mmol/L HNO3, respectively. (b) Morphology of NO2-CQDs observed by TEM and HRTEM. Upper right inset: Particle count and size distribution; Lower left inset: HRTEM lattice fringe images (0.21 nm for all NO2-CQDs). (c) Photographs of NO2-CQDs in ethanol under daylight (left) and UV light (right).
Figure 1 Optical properties of NO2-CQDs with blue, green, and red emissions. (a-c) PL spectra of NO2-CQDs under different excitation wavelengths. (d) Excitation spectra of NO2-CQDs at maximum emission wavelengths (BQDs: Ex = 320 nm; GQDs: Ex = 440 nm; RQDs: Ex = 540 nm). (e) Emission spectra of NO2-CQDs at optimum excitation wavelengths (BQDs: Em = 462 nm; GQDs: Em = 542 nm; RQDs: Em = 606 nm). (f) CIE 1931 chromaticity coordinates of BQDs, GQDs, and RQDs at their respective optimum excitation wavelengths.
Figure 2 Structural characterization of NO2-CQDs. (a) Raman spectra. (b) XRD patterns. (c) UV–vis absorption spectra. (d) FTIR spectra. (e) Relative contents of functional groups on NO2-CQDs, and high-resolution XPS C 1s (f) and N 1s (g) spectra of BQDs, GQDs, and RQDs.
Figure 3 Theoretical simulation results for NO2-CQDs. (a) HOMO and LUMO orbitals of the computational model with an increasing number of aromatic rings. (b) HOMO and LUMO orbitals of the computational model with an increasing number of -NO2.
Figure 4 Optical properties of NO2-CQDs/PVA films and their LED applications. (a) PL spectra of NO2-CQDs/PVA films. The concentration of NO2-CQDs is 0.2 mg/mL. Inset: Photographs of NO2-CQDs/PVA films under 365 nm UV light and ambient daylight. CIE 1931 chromaticity coordinates of (b) NO2-CQDs/PVA films and (c) LED devices. (d-f) EL spectra of NO2-CQDs-based LED devices. Insets: Photographs of NO2-CQDs/PVA films in a monochromatic LED device.
扫一扫看文章
扫一扫关注我们
DownLoad:
下载:
下载: