English

• Luminescent materials capable of generating organic afterglow at room temperature have garnered significant interest in the scientific research community in recent years [1-7]. Typically, this organic afterglow phenomenon arises from ultra-long phosphorescence [1,8]. Room-temperature phosphorescence (RTP) material exhibits a distinctive triplet exciton relaxation process, offering potential applications in sensing, anti-counterfeiting, biological imaging, and information encryption. This is due to their ultra-long luminescence lifetime, significant Stokes shift, and efficient exciton utilization [9]. Nevertheless, the phosphorescence characteristics of pure organic compounds encounter obstacles originating from their weak spin-orbit coupling (SOC) and rapid non-radiative decay of triplet exciton caused by thermal molecular motions and collisions. Additionally, the presence of surrounding oxygen and water leads to a quenching effect, significantly restricting their practical applications [10]. Hence, it becomes crucial to augment the intersystem crossing (ISC) and stability of the triplet excited states of organic molecules while mitigating the quenching impact of oxygen on their triplet exciton [11-14]. To achieve this objective, researchers have proposed various strategies in recent years, which include incorporating heavy atoms [15-17], heteroatoms [18-20], and aromatic carbonyls [11,21] to amplify SOC. Heavy atoms play a crucial role in facilitating the mixing of singlet and triplet states of the chromophore, thereby enhancing ISC. Similarly, the carbonyl oxygen within aromatic carbonyl possesses a certain level of SOC, which can generate more intrinsic triplet exciton. Furthermore, the non-radiative transition of triplet exciton can be mitigated through techniques such as host-guest doping method [22-25], crystallization [26-28] or organic frameworks [29-31]. For example, embedding the fluorescent substance within a rigid matrix serves to isolate it from oxygen and stabilize its structure, thereby restricting non-radiative transitions and enabling long-life RTP.

  Macrocyclic compounds prove highly conducive as hosts for long-life RTP through the host-guest doping method. This is because they create a rigid environment that effectively suppresses the non-radiative decay of phosphors and safeguards the triplet excited exciton of luminophores. Cucurbit[n]urils (Q[n]s), recognized as a notably significant macrocyclic host, possess the capability to create stable supramolecular assemblies with diverse guest molecules. This is attributed to the hydrophobic nature of its internal cavity, as well as the ion-dipole and hydrogen bond interactions between the carbonyl groups located on the port or outer surface and the guest molecules [32-40]. In particular, a simpler and more efficient method for constructing supramolecular self-assembly is based on the outer surface interactions of Q[n]s [41-43], whose driving force is derived from the outer surface of Q[n]s with positive electrostatic potential, making Q[n]s supramolecular self-assembly more possible. The self-assembly of Q[n]s and its guest offers several advantages to organic RTP materials. Firstly, the host molecule offers a stable environment for the guest, effectively minimizing the phosphor vibration and thereby reducing the non-radiative relaxation of the triplet state. Secondly, the hydrophobic cavity serves as protection for the phosphor, shielding it from quenching by oxygen and other solvent molecules [44-48]. In this study, the supramolecular assembly was built using ACQ compounds, utilizing the outer surface interaction of cucurbit[n]urils as the primary driving force. The guest molecules are attached to the positively charged outer surface of the cucurbit[n]urils through weak interactions such as hydrogen bonds. This reduces the aggregation degree of the guest molecules, allowing the ACQ compound to exhibit maximum luminescence performance. This method offers a significant advantage over traditional phosphorescent materials by streamlining the synthesis process and broadening the practical application of ACQ molecules.

  In this study, the primary driving force utilized was the outer surface interaction of cucurbit[n]urils, and the host-guest doping method was employed. Specifically, Q[6] was chosen as the host compound, and lanthanum ion was introduced as the inducer for coordination. Fluorescein sodium (FluNa) and calcein sodium (CalNa) are recognized as classical ACQ light sources, characterized by chromophores containing oxygen, nitrogen atoms, and carbonyl groups, which are conducive to the ISC process. Thus, in order to prove the feasibility of the strategy of outer surface interaction in improving the optical properties of ACQ compounds, we opted for FluNa and CalNa as the guest compounds and effectively created color-tunable organic long-afterglow materials, namely Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa based on typical ACQ compounds (Fig. 1). These materials exhibit persistent luminescence upon UV excitation. Notably, the persistent luminescence color of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa can be easily modified by adjusting the concentration of the guest compound at room temperature. This finding opens up a new research avenue for the traditional ACQ compounds to unlock their significant luminescence potential. Additionally, this study also presents novel ideas and methodologies for the practical utilization of organic long-afterglow materials.

  Figure 1

  

  Figure 1.  Construction flow chart of Q[6]-La(NO₃)₃-FluNa/CalNa assemblies.

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  Fluorescein sodium (FluNa) and calcein sodium (CalNa) demonstrate high emission when dissolved in deionized water, emitting comparable green light due to their similar chromophores, highlighting their significant luminescence potential. However, their fluorescence is completely quenched in the solid state, displaying characteristic ACQ properties. Liang et al. [49] determined the decomposition temperature (T_{d, 1%}) of FluNa and CalNa in the air to be 280 ℃, indicating their high thermal stability. Consequently, we introduced varying masses of FluNa and CalNa into a mixed aqueous solution of Q[6] and La(NO₃)₃, respectively. The solvent was then evaporated at approximately 140–160 ℃ to yield crystalline powders of Q[6]-La(NO₃)₃-FluNa-x wt% (x = 5 × 10^–5~5) and Q[6]-La(NO₃)₃-CalNa-y wt% (y = 5 × 10^–5~5). Interestingly, all crystal samples exhibited strong luminescence under 365 nm UV irradiation, suggesting successful dispersion of FluNa and CalNa within the Q[6]-La assemblies. Furthermore, as the doping concentration decreases, the UV absorption intensity also decreases (Fig. S1 in Supporting information). The steady-state photoluminescence maximum (PL) of Q[6]-La(NO₃)₃-FluNa shifts from 599 nm to 483 nm (Fig. 2b). The fluorescence color of the powder gradually transitions from yellow to blue (as illustrated in Fig. S2 in Supporting information), the fluorescence spectrum also shows corresponding changes (Fig. S5 in Supporting information), while the phosphorescence color gradually shifts from orange to yellow, ultimately transforming into green over time (Fig. 2a). Similarly, the PL maximum of Q[6]-La(NO₃)₃-CalNa shifts from 607 nm to 454 nm (Fig. S4c in Supporting information), indicating a blue shift accompanied by a change in the fluorescence color from yellow to blue (Fig. S2), and a phosphorescence color change from orange to yellow, ultimately turning green (Fig. S3 in Supporting information). These findings offer crucial insights for the advancement and application of these materials.

  Figure 2

  

  Figure 2.  (a) Solid-state luminescence images of Q[6]-La(NO₃)₃-FluNa under 365 nm UV excitation at different doping concentrations. (b) Phosphorescence spectra of Q[6]-La(NO₃)₃-FluNa at various doping concentrations. (c) Phosphorescent CIE_{x, y} chromaticity coordinates of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa at different doping concentrations. The materials used for testing are all in solid state.

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  It is noteworthy that upon removal of the UV excitation light source, the powders of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa exhibit prominent afterglow, which a camera can capture for up to 2s (Fig. 2a and Fig. S3, Videos S1 and S12 in Supporting information). Similar to steady-state luminescence, the afterglow color is dependent on the concentration of FluNa and CalNa. We collected the phosphorescence spectra of the samples and transformed them into CIE_{x, y} chromaticity coordinates (Fig. 2c). The findings indicated that as the doping concentration of FluNa and CalNa decreased, the points in the CIE_{x, y} chromaticity coordinates gradually transitioned from the orange region to the blue region. Notably, there was minimal change in the doping concentration range of 5 × 10^–5~5 × 10^–3 wt%. Most significantly, as the doping concentration decreases and over time, the afterglow color can transition from orange to yellow, ultimately reaching the green. This observation aligns with the results of CIE_{x, y} chromaticity coordinates, suggesting that the afterglow color of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa can be readily adjusted across a broad range of regions by controlling the concentration of guest compounds. To validate this conclusion, we compared the phosphorescence spectra of Q[6]-La, FluNa-La, and CalNa-La at a doping concentration of 5 × 10^–1 wt%. As shown in Fig. S6 (Supporting information), FluNa-La and CalNa-La exhibit no phosphorescence emission, whereas Q[6]-La displays strong phosphorescence emission. When considering the data in Fig. S4 (Supporting information), it becomes clear that the addition of FluNa and CalNa can enhances the phosphorescence intensity and easily adjust the afterglow color of the material. These findings establish the groundwork for further research and application of these materials.

  Additionally, to gain further insight into the optical properties of the material, we conducted an in-depth investigation. As depicted in Fig. S7 (Supporting information), as the doping concentration decreases, the phosphorescence quantum yield ($ {{\varPhi }_{\text{PL}}}$) of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa gradually increases until the doping concentration decreases to 5 × 10^–4 wt%. The fitting results of the emission decay curves reveal that the maximum lifetime of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa can reach 150 ms (Figs. S8 and S9 in Supporting information). Notably, when the doping concentration is 5 wt%, the lifetime is shorter, consistent with the characteristics of short phosphorescence lifetimes and low phosphorescence yields typically observed in organic RTP materials with long wavelengths [50-53]. However, even under these circumstances, they still exhibit noticeable afterglow, and their persistent luminescence remains clearly observed to the naked eye after removing the 365 nm UV excitation. These experimental findings demonstrate that leveraging the outer surface interaction of Q[6] as the primary driving force, along with the host-guest doping method and metal ion induction, facilitates the effective dispersion of classical ACQ compounds FluNa and CalNa. This transformative process significantly alters their optical properties and successfully converts them into satisfactory RTP materials.

  Based on the preparation method involving solvent volatilization in this study, an in-depth exploration was conducted on the phosphorescence intensity and lifetime of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa at various temperatures. This investigation aimed to demonstrate the impact of water on the phosphorescence intensity of these materials. Q[6]-La(NO₃)₃-FluNa (5 × 10^–1 wt%) and Q[6]-La(NO₃)₃-CalNa (5 × 10^–1 wt%) were selected as examples. As illustrated in Fig. 3a and Fig. S10a (Supporting information), the RTP intensity within the emission band gradually rises with increasing temperature. Additionally, the phosphorescence lifetime exhibits a similar trend, with the RTP emission lifetime gradually increasing as the heating temperature rises and reaching the maximum at 120 ℃. At varying temperatures, the solid phosphorescence images supported the findings of the CIE_{x, y} chromaticity coordinates. This may be due to the continuous loss of water with the increase of temperature, which reduces the quenching effect of water on RTP performance. According to the dual characteristics of the rigid structure and the outer surface interaction of Q[6], FluNa/CalNa and Q[6] produce a stable non-covalent interaction, resulting in a harder environment, further inhibiting the molecular movement of the object, which is conducive to the reduction of non-radiative transition rate and the enhancement of phosphorescence performance.

  Figure 3

  

  Figure 3.  (a) Phosphorescence intensity of Q[6]-La(NO₃)₃-FluNa at different temperatures. (b) Solid-state luminescence images of Q[6]-La(NO₃)₃-FluNa under 365 nm UV excitation at different temperatures. (c) Phosphorescence CIE_{x, y} chromaticity coordinates of Q[6]-La(NO₃)₃-FluNa at different temperatures. The materials used for testing are all in solid state.

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  To explore the response mechanism underlying the RTP effect induced by Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa, we conducted a series of structural studies. Firstly, we employed Fourier transform infrared spectroscopy (FTIR) to determine the differences among Q[6], Q[6]-La, FluNa, FluNa-La, and Q[6]-La(NO₃)₃-FluNa systems, as well as CalNa, CalNa-La, and Q[6]-La(NO₃)₃-CalNa systems. Using FluNa as an example, we initially compared the infrared spectra of FluNa and FluNa-La. We observed a prominent and broad peak at 3505 cm^-1 after the addition of La³⁺, indicating the presence of free O—H stretching vibration [54]. This peak exhibited greater intensity than the O—H peak of FluNa. We speculate that this enhancement is attributable to the coordination of La³⁺ with O. Furthermore, a subtle red shift was noted in the range of 457~1581 cm^-1 (Fig. S11a in Supporting information), suggesting successful doping of FluNa with La³⁺. Similarly, CalNa and CalNa-La displayed analogous alterations (Fig. S11b in Supporting information). Secondly, we compared the infrared spectra of Q[6] with Q[6]-La. The results indicate that upon the addition of La³⁺, a new peak appeared at 2337 cm^-1, while the peak at 1730 cm^-1 was attributed to C=O stretching transformed from a double peak to a single peak (Fig. 4a). This change was attributed to the coordination of La³⁺ with the carbonyl oxygen on Q[6] [55]. Lastly, we observed the alteration in the infrared spectrum by doping varying amounts of FluNa in Q[6]-La. As the doping amount of FluNa decreases, the intensity of the new peak at 2337 cm^-1 gradually strengthens, and the single peak at 1730 cm^-1 gradually reverts to a double peak (Fig. 4b and Fig. S12a in Supporting information). This phenomenon suggests the successful doping of FluNa into Q[6]-La. Similar changes were observed with CalNa (Figs. S12c and d in Supporting information).

  Figure 4

  

  Figure 4.  (a) The infrared spectra of Q[6] and Q[6]-La. (b) The infrared spectra of Q[6]-La with different doping concentrations (0.00005% ~ 0.005%). (c) XPS spectra of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%). (d) XRD spectra of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%) and Q[6]-La(NO₃)₃-CalNa (5 × 10^–2 wt%). The materials used for testing are all in solid state.

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  The chemical composition of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%) was further determined using X-ray photoelectron spectroscopy (XPS). As depicted in Fig. 4c, the peaks corresponding to C 1s, N 1s, O 1s, and La 3d are located at 287.06, 399.66, 531.69, and 837.81 eV, respectively. This indicates that Q[6]-La(NO₃)₃-FluNa primarily consists of C, N, O, and La. The elemental composition was found to be C, 48.97%, N, 22.95%, O, 26.04%, and La, 2.03%, respectively. The high-resolution spectrum of C 1s is illustrated in Fig. S13a (Supporting information). The absorptions observed at 284.78, 287.58, and 289.28 eV indicate the presence of -C=C/-C-C, -C-N, and -C=O bonds in Q[6]-La(NO₃)₃-FluNa, respectively. In the N 1s spectrum (Fig. S13b in Supporting information), the absorption at 399.58 eV can be attributed to the -C-N bond, while the absorption at 406.38 eV is attributed to the -N-O bond. In the O 1s spectrum (Fig. S13c in Supporting information), the absorption at 532.78 eV corresponds to the -C=O bond, while the lower binding energy at 531.58 eV is attributed to the -La-O bond [54]. Additionally, in the energy level spectrum of La (Fig. S13d in Supporting information), two distinct peaks with binding energies of 835.08 and 851.78 eV can be observed. These peaks are associated with the spin-orbital energy levels of La 3d_5/2 and 3d_3/2, respectively, with an energy gap of 17 eV. Additionally, two additional peaks at 838.38 and 855.18 eV are observed in the La 3d spectrum, which are La 3d satellite peaks. These peaks are attributed to the relocation of electrons from O 2p to an unoccupied La 5f orbital, indicating the oxidation state of La³⁺ [56]. Similarly, Q[6]-La(NO₃)₃-CalNa (5 × 10^–2 wt%) also displays similar characteristics (Fig. S14 in Supporting information).

  Furthermore, Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa were characterized by SEM, and the results are shown in Fig. S15 (Supporting information). Taking Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%), Q[6]-La(NO₃)₃-FluNa (5 × 10^–5 wt%), Q[6]-La(NO₃)₃-CalNa (5 × 10^–2 wt%) and Q[6]-La(NO₃)₃-CalNa (5 × 10^–5 wt%) as examples, the results show that both Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa complexes exhibit irregular block structures. Additionally, the structure of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa were further analyzed by X-ray Diffraction (XRD). The crystalline phases of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%) and Q[6]-La(NO₃)₃-CalNa (5 × 10^–2 wt%) are depicted in Fig. 4d. Both materials exhibit two significant peaks at 2θ values 10° and 15°, respectively, these strong and narrow peaks show the high crystallinity of the materials. And two weaker peaks at 23° and 42.87°, respectively. These findings suggest that these materials are polycrystalline [57]. In summary, FluNa and CalNa have been successfully doped into Q[6]-La.

  The potential of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa materials in multi-level anti-counterfeiting applications were investigated, leveraging their distinctive multi-color afterglow and varying luminescence durations. Initially, we capitalized on the flexible properties of polyvinyl alcohol (PVA) to fabricate films featuring different letter shapes ('G', 'Z', and 'U', representing Guizhou University), as depicted in Fig. 5a. These films demonstrated yellow and blue fluorescence upon UV light irradiation. Subsequently, upon turning off the UV irradiation, the films emitted orange, yellow, and green afterglow (Video S13 in Supporting information). In the future, by employing a wider range of mold shapes, we can produce diverse multi-color luminescent films, thereby achieving first-level anti-counterfeiting measures. Secondly, FluNa and Q[6]-La(NO₃)₃-FluNa were utilized as luminescent materials, mixed with PVA to form a viscous liquid. This liquid was then applied to form numbers and the logo of Guizhou University (Fig. 5b). Under UV light irradiation, the visible number displayed was '8888'. Upon turning off the UV light, the displayed number transitioned from '8888' to '1902' (representing the founding year of Guizhou University), emitting a yellow afterglow resembling the school logo, thus achieving second-level anti-counterfeiting measures (Videos S14 and S15 in Supporting information). Lastly, we utilized varying proportions of luminescent material powder to create different characters and patterns, such as the English letter 'OPEN', the number '8888', and the small flower pattern depicted in Fig. 5c. After the UV light was switched off, leveraging the afterglow color and duration of different luminescent materials, the English letters transitioned from 'OPEN' to 'OFF', the small flower pattern altered from two layers of petals to one layer of petals. The numbers changed from '6898' to '5632', and finally to '32', thus achieving multi-level anti-counterfeiting measures (Videos S16-S18 in Supporting information). These experimental outcomes effectively emphasize the practical application value of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa in multi-level anti-counterfeiting through UV light treatment.

  Figure 5

  

  Figure 5.  Schematic diagram of flexible film application for first-level anti-counterfeiting (a), implementation of PVA doping for second-level anti-counterfeiting (b), and utilization for the third-level anti-counterfeiting (c) based on Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa.

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  In summary, this study harnessed the outer surface interactions of cucurbit[n]urils, with a focus on Q[6] as the primary host molecule. Utilizing the host-guest doping method and metal ion coordination, classical aggregation-caused quenching (ACQ) compounds, FluNa and CalNa, were successfully converted into afterglow luminescent materials, exhibiting a spectrum of colors and varying luminescence durations. Notably, at room temperature, the maximum lifetime of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa can extend up to 150 ms. Even at extremely low doping concentrations (5 × 10^–5 wt%), FluNa and CalNa have significant yellow and green afterglow capabilities. The multi-color afterglow observed in Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa can be attributed to several factors. These include the coordination of lanthanum ions with guest molecules or carbonyl oxygen at the Q[6] port, as well as the self-absorption characteristics inherent to FluNa and CalNa. We conjectured that these factors facilitate the creation of large T₂-T₁ energy gaps within the guest molecules, thereby promoting their intersystem crossing (ISC) process and resulting in the observed multi-color afterglow effects. Leveraging the significant multi-color afterglow and varied luminescence durations exhibited by these materials, we explored their applications in anti-counterfeiting and information encryption. Consequently, the development of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa with long-afterglow characteristics presents a robust supramolecular approach for constructing organic photoluminescence materials with optimized emission and tunable color. This advancement has the potential to greatly broaden the application prospects of multi-color luminescent materials across diverse fields, including sensing, anti-counterfeiting, and optoelectronic device.

  Declaration of competing interest

  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.

  CRediT authorship contribution statement

  Xingyue Yuan: Writing – original draft, Methodology, Formal analysis. Li Wu: Data curation. Qiuyu Peng: Methodology. Yanyan Tang: Writing – review & editing. Mingxu Wang: Writing – review & editing, Methodology. Yuhang Wei: Writing – review & editing. Zhu Tao: Writing – review & editing. Xin Xiao: Writing – review & editing, Resources, Funding acquisition, Conceptualization.

  Acknowledgments

  We acknowledge the support of the National Natural Science Foundation of China (No. 22361011), Guizhou Provincial Science and Technology Projects (No. ZK [2023] General 040) and the Guizhou Provincial Key Laboratory Platform Project (No. ZSYS [2025] 008).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110821.

  
  
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  Figure 1  Construction flow chart of Q[6]-La(NO₃)₃-FluNa/CalNa assemblies.

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  Figure 2  (a) Solid-state luminescence images of Q[6]-La(NO₃)₃-FluNa under 365 nm UV excitation at different doping concentrations. (b) Phosphorescence spectra of Q[6]-La(NO₃)₃-FluNa at various doping concentrations. (c) Phosphorescent CIE_{x, y} chromaticity coordinates of Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa at different doping concentrations. The materials used for testing are all in solid state.

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  Figure 3  (a) Phosphorescence intensity of Q[6]-La(NO₃)₃-FluNa at different temperatures. (b) Solid-state luminescence images of Q[6]-La(NO₃)₃-FluNa under 365 nm UV excitation at different temperatures. (c) Phosphorescence CIE_{x, y} chromaticity coordinates of Q[6]-La(NO₃)₃-FluNa at different temperatures. The materials used for testing are all in solid state.

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  Figure 4  (a) The infrared spectra of Q[6] and Q[6]-La. (b) The infrared spectra of Q[6]-La with different doping concentrations (0.00005% ~ 0.005%). (c) XPS spectra of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%). (d) XRD spectra of Q[6]-La(NO₃)₃-FluNa (5 × 10^–2 wt%) and Q[6]-La(NO₃)₃-CalNa (5 × 10^–2 wt%). The materials used for testing are all in solid state.

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  Figure 5  Schematic diagram of flexible film application for first-level anti-counterfeiting (a), implementation of PVA doping for second-level anti-counterfeiting (b), and utilization for the third-level anti-counterfeiting (c) based on Q[6]-La(NO₃)₃-FluNa and Q[6]-La(NO₃)₃-CalNa.

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