English

• Chronic kidney disease (CKD) is a global problem that threatens human health [1]. Early detection and intervention can significantly reduce complications and improve survival in patients with CKD. Under pathological conditions, tyrosine residues are converted to 3-nitrotyrosine (3-NT) by overexpressed peroxynitrite in the kidney [2]. Furthermore, renal insufficiency decreases the metabolism of 3-NT in the bloodstream. Therefore, a high concentration of 3-NT in the blood is considered to be an important biomarker of kidney injury [3,4]. Currently, 3-NT can be detected by immunoassay, and chromatography-mass spectrometry [5–7]. These methods either require expensive equipment or complex preprocessing, making it difficult to achieve rapid detection and analysis of the target [8]. Luminescent detection is a simple, inexpensive and sensitive detection method, which is widely used for the monitoring of relevant biomarkers in aqueous or biological fluids [9–14]. Conventional 3-NT fluorescence sensors are usually quench-type [15,16], moreover, single fluorescence changes are easily disturbed by background fluorescence, severely limiting their ability to accurately monitor the target.

  Purely organic room-temperature phosphorescent (RTP) materials with fluorescence-phosphorescence dual-emission have received much attention, especially in the field of ratiometric luminescent detection due to their outstanding advantages [17–19]. Firstly, the absence of precious metals endows the materials with low cost and low biotoxicity [20]. Secondly, sensitive triplet excitons generate ultra-high sensitivity [21]. Thirdly, fluorescence-phosphorescence dual-signal ratiometric output can effectively avoid self and background fluorescence interference, resulting in higher signal-to-noise ratio, etc. [22]. Therefore, the development of a fluorescence-phosphorescence dual-emission ratiometric luminescence probe that specifically identify biomarkers is highly desirable. However, the weak spin-orbit coupling (SOC) and spin-forbidden intersystem crossing (ISC) processes make it difficult to achieve RTP for organic molecules [23–25]. On the other hand, triplet state excitons are easily quenched by water molecules and oxygen, limits their usefulness in aqueous solution [26]. It is well known that the excited singlet and triplet states are in competition [27]. Therefore, it is a great challenge to modulate the fluorescence-phosphorescence dual emission in aqueous solution.

  The introduction of heavy atoms and heteroatoms with lone-pair electrons can effectively enhance the SOC and ISC processes [28,29]. Meanwhile, the construction of relatively rigid microenvironments can effectively reduce the non-radiative transition of phosphorescent molecules in water, which is an important strategy for promoting RTP in aqueous phase [30,31]. Two simple and effective methods for achieving aqueous-phase RTP are nanoparticulation and macrocyclic encapsulation of phosphorescent molecules [32]. Among them, the 1:2 host-guest complex formed by the self-assembly of cucurbit[8]uril (CB[8]) and 4-(4-bromophenyl)pyridinium derivatives has been shown to effectively inhibit the non-radiative transition and prevent quenching from solution [33–35].

  Herein, we report a florescence and RTP dual-emission host-guest complex formed by supramolecular self-assembly of CB[8] and florescent 4-(4-bromophenyl)pyridinium carboxylic acid derivative (BPPA), which can be used as a luminescent probe for ratiometric monitoring of 3-NT in water (Scheme 1). Macrocyclic confinement of the hydrophobic cavity of CB[8] and inclusion-induced dimeric stacking of guest could efficiently inhibit the molecular motion of the guest, prevent the quenching of triplet state excitons from solution, thus achieving florescence and RTP dual-emission in water. The obtained host-guest complex exhibits ratiometric luminescent detection towards 3-NT with high sensitivity and excellent selectivity. Moreover, this host-guest ratiometric sensor was employed as a reporter in a lateral flow immunoassay (LFIA) platform for the rapid on-site visual detection of 3-NT. This macrocyclic-confinement-induced RTP material with dual signal output provides an alternative candidate for biomarker monitoring under physiological environment, which will broaden the biological application of purely organic RTP.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the construction of RTP supramolecular self-assembly and its ratiometric luminescent detection behavior to 3-NT.

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  Water-soluble BPPA was obtained by a simple one-step reaction (Scheme S1 in Supporting information), and the characterizations were shown in Figs. S1–S4 (Supporting information). Then, ¹H NMR titration experiment was conducted to study the host-guest complexation between CB[8] and BPPA. Upon gradual addition of CB[8], the proton signals of H₁—H₄ on 4-(4-bromophenyl)pyridinium (BP) showed upfield shifts (Δδ = 1.18, 1.12, 0.68, and 0.05 ppm, respectively), while the signal for proton H₆ showed a downfield shift (Δδ = 0.16 ppm) (Fig. 1A), revealing that 4-bromophenylpyridine unit was included in the cavity of CB[8], while the methylene moiety was located outside the cavity of CB[8] [36]. In addition, the proton signals of free BPPA were completely disappeared upon the addition of 0.5 equiv. CB[8] and no split of proton signals on CB[8] was observed, indicating that BPPA⊂CB[8] was a "head to tail" 1:2 host-guest complex (Scheme 1) [37]. Meanwhile, the ¹H-¹H rotating-frame Overhauser effect spectroscopy (ROESY) demonstrated the presence of strong correlated peaks between H₁—H₃, providing further evidence for the "head to tail" dimeric mode of BPPA in the CB[8] cavity (Fig. 1C). UV–vis titration measurements showed that the absorbance of the BPPA at 305 nm gradually decreased with the addition of CB[8], and a slight red shift from 305 nm to 310 nm was observed (Fig. 1B). As shown in Fig. S5 (Supporting information), a maximum value was observed at [BPPA]/([BPPA] + [CB[8]) = 0.67 in Job's plot, further revealing the 1:2 host-guest stoichiometry. The corresponding binding constants calculated from the UV–vis titration are K₁ = 3.62 × 10⁶ L/mol and K₂ = 7.67 × 10⁵ L/mol (Fig. 1D). Meanwhile, electrospray ionization mass spectrometry (ESI-MS) showed an ion peak at m/z = 971.2121, well corresponding to the theoretical calculated value of [CB[8] + 2BPPA-2Br]²⁺ (971.2077), and two neighboring peaks are separated by 0.5 (Fig. 1E and Fig. S6 in Supporting information). The above results jointly confirmed the formation of a stable 1:2 host-guest complex in water.

  Figure 1

  

  Figure 1.  The host-guest self-assembly behavior. (A) ¹H NMR titration spectra (400 MHz, D₂O, 298 K) of BPPA upon addition of (Ⅰ) 0, (Ⅱ) 0.25, (Ⅲ) 0.50 and (Ⅳ) 1.0 equiv. of CB[8] ([BPPA] = 1.2 mmol/L). (B) UV–vis absorption spectra of BPPA (20 µmol/L) upon the addition of CB[8] (0–1.3 equiv.) and (D) the corresponding binding constants. (C) ¹H-¹H ROESY spectrum of BPPA⊂CB[8] ([BPPA] = 2 [CB[8]] = 1.2 mmol/L). (E) ESI-MS spectrum of BPPA⊂CB[8].

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  The photoluminescence properties of the BPPA⊂CB[8] complex in water were then investigated. Only an emission peak at 387 nm was observed in photoluminescence spectrum of free BPPA (Fig. 2A). The emission intensity at 387 nm gradually decreased with the addition of CB[8] (0–1.0 equiv.), while a new emission peak appeared at around 500 nm and gradually increased, accompanied by a change in emission color from dark blue to green (Fig. 2B). Notably, the luminescence emission intensity at 500 nm significantly enhanced under N₂ atmosphere, while the intensity at 387 nm showed no obvious change (Fig. 2C). Meanwhile, transient spectra (delayed 0.5 ms) showed that free BPPA has no emission, while BPPA⊂CB[8] complex only has a green emission at 500 nm (Fig. 2D). Specially, the intensity of green emission had a significant enhancement (5.2 times) under N₂ compared with that of environmental condition (Fig. 2E). Decay time curves showed that both free BPPA and the BPPA⊂CB[8] complex had nanosecond scale lifetimes at 387 nm (0.390 and 0.392 ns, respectively) (Fig. S7 in Supporting information). In contrast, BPPA⊂CB[8] complex displayed long lifetime of 0.472 and 2.52 ms at 500 nm under environmental condition and N₂ atmosphere, respectively (Fig. 2F). Concurrently, the delay emission at 500 nm showed a quenching phenomenon with increasing temperature, thereby precluding the possibility of thermally activated delayed fluorescence (Fig. S8 in Supporting information). Thus, the blue emission at 387 nm was attributed to florescence emission, while the green emission at 500 nm was attributed to RTP emission. What's more, BPPA⊂CB[8] complex exhibited a high RTP quantum yields of 1.37% in water. Based on these results, the intersystem crossing rate, radiative and non-radiative decay rate constants (k_isc, k_r^Phos and k_nr^Phos) of phosphorescence were calculated as 2.37 × 10⁷ s^-1, 2.9 × 10¹ s^-1 and 2.10 × 10³ s^-1, respectively [34]. A series of control tests based on CB[7] were performed to verified the impact of host-guest binding mode on photoluminescence behaviors. As shown in Fig. S9 (Supporting information), Job's plot showed a maximum value at [BPPA]/([BPPA] + [CB[7]) = 0.5, indicating that the binding ratio between CB[7] and BPPA was 1:1 due to the smaller cavity of CB[7] (Fig. S9A and B). UV–vis titration spectra showed a gradual decrease in absorption with the addition of CB[7], together with a slight red shift from 305 to 308 nm (Fig. S9C). Meanwhile, only an enhancement of the fluorescence emission at 387 nm was observed upon the addition of CB[7] (Fig. S9D), which is different from that of CB[8]. These results revealed that the macrocyclic confinement of CB[8] and the dimeric stacking of BPPA in the hydrophobic cavity of CB[8] could effectively deter the non-radiative transition of guest and reduce the quenching effect of water molecules, thus realizing RTP emission of BPPA in water.

  Figure 2

  

  Figure 2.  The photoluminescence properties of the host-guest complex. (A) The photoluminescence spectra (λ_ex = 315 nm) of BPPA (10 µmol/L) upon the addition of CB[8] (0–1.0 equiv.). (B) The corresponding CIE chromaticity diagram. Inset: The photos of BPPA and BPPA⊂CB[8]. (C) The photoluminescence spectra of BPPA⊂CB[8] under air condition and N₂ atmosphere. (D) The phosphorescence emission of BPPA (10 µmol/L) upon the addition of CB[8] (0–1.0 equiv.). (E, F) The RTP emission and RTP lifetime of BPPA⊂CB[8] under air condition and N₂ atmosphere.

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  BPPA⊂CB[8] complex exhibited stable florescence and phosphorescence dual-emission in aqueous solution even after placing for long time (Fig. S10 in Supporting information). We then investigated the ratiometric photoluminescent detection behavior of BPPA⊂CB[8] complex to 3-NT in aqueous solution. In order to conduct on-site detection, all the analytical detection tests were performed at room temperature. The photoluminescence spectra of BPPA⊂CB[8] at varying concentrations demonstrated a relatively stable profile (Fig. S11A in Supporting information). Once the concentration of BPPA⊂CB[8] was below 10 µmol/L, the photoluminescent intensity ratio of fluorescence to phosphorescence (I₃₈₇/I₅₀₀) exhibited a significant increase (Fig. S11B in Supporting information). Thus, the concentration of the host-guest probe was established to be 10 µmol/L. As shown in Fig. S12 (Supporting information), the luminescence alterations reached equilibrium within 5 s following the injection of 3-NT, indicating that the BPPA⊂CB[8] complex exhibited a rapid response to 3-NT. With the gradual addition of 3-NT, the green emission at 500 nm was quenched, while the blue emission at 387 nm increased significantly (Fig. 3A), and a remarkable emission color change from green to dark blue was observed by the naked eye, with the corresponding CIE coordinate changing from (0.1766, 0.2109) to (0.1712, 0.1165) (Fig. 3C). Remarkably, I₃₈₇/I₅₀₀ showed linearity as a function of 3-NT concentration in the range of 0–15 µmol/L with R² of 0.994 (Fig. 3B). The limit of detection (LOD) was calculated to be 10.7 nmol/L using the 3σ/k method, which is much lower than the concentration of 3-NT in the serum of patients with renal failure (28 µmol/L) [38]. The sensitivity of this probe is comparable to many reported methods (Table S1 in Supporting information), making it a promising candidate for the highly sensitive detection of low concentrations of 3-NT in aqueous solution.

  Figure 3

  

  Figure 3.  The ratiometric luminescent detection behavior. (A) The photoluminescence spectra (λ_ex= 315 nm) of BPPA⊂CB[8] (10 µmol/L) upon the addition of 3-NT in water (0–15 µmol/L). Inset: The images of BPPA⊂CB[8] before and after the addition of 3-NT. (B) Linear plot of I₃₈₇/I₅₀₀ versus the concentration of 3-NT (0–15 µmol/L). (C) The corresponding CIE coordinates of BPPA⊂CB[8] with the addition of 3-NT (0–15 µmol/L). The photoluminescence spectra (D) and I₃₈₇/I₅₀₀ (E) of BPPA⊂CB[8] upon the addition of 3-NT and other interferents (15 µmol/L). (F) The images of LFIA strips for visual detection toward 3-NT under a 302 nm UV lamp.

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  Subsequently, we explored the specific recognition performance of this luminescent probe for 3-NT (Fig. 3D). With the addition of 3-NT and potential interferents in human serum (15 µmol/L), only 3-NT resulted in a remarkable change in I₃₈₇/I₅₀₀ (Figs. 3D and E). In contrast, the interferents (CaCl₂, KCl, MgCl₂, Na₂SO₄, NaCl, NaH₂PO₄, NaHCO₃, Zn(Ac)₂, uric acid, glucose, glutamic acid, arginine, lysine, tyrosine, urea, sucrose) did not cause any obvious change in I₃₈₇/I₅₀₀ of BPPA⊂CB[8] (Fig. 3E), indicating that BPPA⊂CB[8] has a high specificity for 3-NT over the major components in human serum. Meanwhile, as shown in the photoluminescence spectra (Fig. S13 in Supporting information), the host-guest probe emission was almost unchanged in the presence of common cations, anions, and amino acids. Furthermore, BPPA⊂CB[8] was integrated into a LFIA platform as a reporter to investigate the visual detection of 3-NT. In a negative test, BPPA⊂CB[8] exhibited green emission under 302 nm UV illumination in the absence of 3-NT. However, as the concentration of 3-NT increased, the original green color of the reporter changed to blue (Fig. 3F), demonstrating the potential for rapid on-site visual detection of 3-NT.

  To understand the response mechanism of BPPA⊂CB[8] toward 3-NT, transient spectra (delayed 0.5 ms) were performed. As illustrated in Fig. S14 (Supporting information), the RTP emission of BPPA⊂CB[8] at 500 nm was quenched by 3-NT. Besides, UV–vis absorption showed that the absorption of BPPA⊂CB[8] was shifted from 310 nm to 307 nm with an obvious enhancement after the addition of 0–2.0 equiv. 3-NT (Fig. S15 in Supporting information). We therefore speculate that the BPPA⊂CB[8] host-guest inclusion complex dissociated in the presence of 3-NT, as 3-NT may compete the BPPA out of the CB[8] cavity. ¹H NMR experiments were then measured to verify the possibility of inclusion between CB[8] and 3-NT. After adding CB[8] to 3-NT, the aromatic protons signals (H_a-H_c) and methylene protons (H_d-H_e) of 3-NT underwent upfield shifts, while the proton H_f remained unchanged (Fig. S16 in Supporting information), indicating that the aromatic ring of 3-NT was buried in the cavity of CB[8], and the methylene moiety was located at the portals of CB[8] [39]. The ESI-MS spectrum further demonstrated a 1:2 host-guest assembly of 3-NT⊂CB[8] with an ion peak at m/z 891.2690, well corresponding to the theoretical calculated value of [CB[8] + 2(3-NT) + 2H]²⁺ (891.2622) (Fig. S17 in Supporting information). These results reveal that 3-NT also can form a stable 1:2 host-guest inclusion complex with CB[8]. When 1.0 equiv. 3-NT was added to BPPA⊂CB[8], half of the proton signals of H_1'-H_3' in BPPA showed significant downfield shifts (Δδ = 1.13 ppm, 1.06 ppm, and 0.64 ppm, respectively), while half of the signal for proton H_6' showed an upfield shift (Δδ = 0.13 ppm) (Fig. 4A and Fig. S18 in Supporting information), demonstrating that half of BPPA was squeezed out of the cavity of CB[8]. Notably, the extruded BPPA proton signals did not revert to the original state of free BPPA (Fig. 4A). Meanwhile, the H_1'-H_3' proton signals of the BPPA that were still encapsulated within the host-guest cavity exhibited a slight upfield shift, while the H_6' proton signals exhibited a significant downfield shift (Δδ = 0.12 ppm) (Fig. 4A and Fig. S18). Furthermore, the proton signals of 3-NT exhibited slight downfield shifts (Δδ = 0.06, 0.05, 0.04, 0.02, 0.03, and 0.02, respectively) in comparison to the free 3-NT, which is different from that of 3-NT⊂CB[8] (Fig. 4A). In the control experiment, the ¹H NMR spectrum of the mixture of 3-NT and BPPA showed that all the proton signals of 3-NT displayed downfield shifts (Fig. S19 in Supporting information). Besides, the proton signals of H₁—H₄ on BPPA showed upfield shifts (Δδ = 0.05, 0.05, 0.04, and 0.02 ppm, respectively), while the signal for proton H₆ exhibited a downfield shift (Δδ = 0.10 ppm) (Fig. S19), indicating the existence of hydrogen bonding interactions between the -NH₂ of 3-NT and the -COOH of BPPA [40]. In addition, strong correlated peaks between H_3nullH_a were observed in the ¹H-¹H ROESY spectrum of BPPA⊂CB[8] in presence of 3-NT (Fig. 4B). Thus, ¹H NMR and ¹H-¹H ROESY results preliminary indicate that one 3-NT molecule squeezed one BPPA out of the cavity of CB[8] to form a 1:1:1 ternary host-guest complex. And there are hydrogen bonding interactions between BPPA and 3-NT. Upon the addition of 2.0 equiv. 3-NT, the ESI-MS spectrum of BPPA⊂CB[8] displayed an dominant ion peak at m/z = 930.7381, well corresponding to the theoretical calculated value of [CB[8] + BPPA + 3-NT-Br + H]²⁺ (930.7322) (Figs. 4C and D), which directly confirms the formation of 1:1:1 ternary host-guest complex. The binding constants of 3-NT⊂CB[8] were determined by UV–vis absorption titration, resulting in K₁ = 1.19 × 10⁶ L/mol and K₂ = 5.25 × 10³ L/mol, respectively (Fig. 4E and Fig. S20 in Supporting information). The K₁ of 3-NT⊂CB[8] is higher than the K₂ of BPPA⊂CB[8] (7.67 × 10⁵ L/mol), but lower than the K₁ of BPPA⊂CB[8] (3.62 × 10⁶ L/mol). This is the reason why 3-NT can only squeeze one BPPA guest molecule out the cavity of CB[8]. In addition, the photoluminescence spectrum of BPPA⊂CB[8] even in presence of excess 3-NT did not recover to the original state of free BPPA (Fig. S21 in Supporting information), which is consistent with the analysis of ¹H NMR and ESI-MS tests. As a result, 3-NT resulted in the dissociation of dimer of BPPA in the cavity of CB[8], thus led to the quenching of RTP and recovery of fluorescence.

  Figure 4

  

  Figure 4.  The detection mechanism investigation. (A) ¹H NMR spectra (D₂O, 400 MHz, 298 K) of 3-NT (Ⅰ), BPPA⊂CB[8]−3-NT (Ⅱ), BPPA⊂CB[8] (Ⅲ) and BPPA (Ⅳ) ([BPPA] = [3-NT]= 2[CB[8]] = 1.2 mmol/L). (B) ¹H-¹H ROESY spectrum of BPPA⊂CB[8] with the addition of 3-NT ([BPPA] = [3-NT] = 2[CB[8]] = 1.2 mmol/L). (C, D) ESI-MS spectrum of the BPPA⊂CB[8] with the addition of 2.0 equiv. 3-NT. (E) The binding constants between 3-NT and CB[8].

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  In summary, effective aqueous RTP was realized in a host-guest complex assembled from CB[8] and BPPA. The unique 1:2 host-guest structure and macrocyclic confinement effect effectively restricted the molecular motion of BPPA, thus inhibited its non-radiative transition. Besides, the hydrophobic cavity of CB[8] further reduced the quenching from aqueous solution, thus florescence and RTP dual-emission were realized. Based on competitive binding, the obtained BPPA⊂CB[8] demonstrates the ability of rapid identification and ratiometric luminescent detection of 3-NT in aqueous solution with a LOD of 10.7 nmol/L, much lower than that of concentration in human serum of patients with chronic kidney disease. It is expected that this research will not only provide a safe and sensitive method for the detection of biomarkers, but will also further expand the application areas of RTP materials.

  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

  Qingyu Niu: Investigation. Yulu Zhang: Investigation. Zerong Ge: Investigation. Jiabao Liu: Investigation. Zhiqiang Li: Supervision, Conceptualization. Yong Chen: Supervision. Yu Liu: Supervision.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 22171069 and 21871075), the Educational Committee of Hebei Province (No. JZX2024012) and the Tianjin Natural Science Foundation (No. 23JCYBJC00800).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110935.

  
  
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• 

  Scheme 1  Schematic illustration of the construction of RTP supramolecular self-assembly and its ratiometric luminescent detection behavior to 3-NT.

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  Figure 1  The host-guest self-assembly behavior. (A) ¹H NMR titration spectra (400 MHz, D₂O, 298 K) of BPPA upon addition of (Ⅰ) 0, (Ⅱ) 0.25, (Ⅲ) 0.50 and (Ⅳ) 1.0 equiv. of CB[8] ([BPPA] = 1.2 mmol/L). (B) UV–vis absorption spectra of BPPA (20 µmol/L) upon the addition of CB[8] (0–1.3 equiv.) and (D) the corresponding binding constants. (C) ¹H-¹H ROESY spectrum of BPPA⊂CB[8] ([BPPA] = 2 [CB[8]] = 1.2 mmol/L). (E) ESI-MS spectrum of BPPA⊂CB[8].

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  Figure 2  The photoluminescence properties of the host-guest complex. (A) The photoluminescence spectra (λ_ex = 315 nm) of BPPA (10 µmol/L) upon the addition of CB[8] (0–1.0 equiv.). (B) The corresponding CIE chromaticity diagram. Inset: The photos of BPPA and BPPA⊂CB[8]. (C) The photoluminescence spectra of BPPA⊂CB[8] under air condition and N₂ atmosphere. (D) The phosphorescence emission of BPPA (10 µmol/L) upon the addition of CB[8] (0–1.0 equiv.). (E, F) The RTP emission and RTP lifetime of BPPA⊂CB[8] under air condition and N₂ atmosphere.

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  Figure 3  The ratiometric luminescent detection behavior. (A) The photoluminescence spectra (λ_ex= 315 nm) of BPPA⊂CB[8] (10 µmol/L) upon the addition of 3-NT in water (0–15 µmol/L). Inset: The images of BPPA⊂CB[8] before and after the addition of 3-NT. (B) Linear plot of I₃₈₇/I₅₀₀ versus the concentration of 3-NT (0–15 µmol/L). (C) The corresponding CIE coordinates of BPPA⊂CB[8] with the addition of 3-NT (0–15 µmol/L). The photoluminescence spectra (D) and I₃₈₇/I₅₀₀ (E) of BPPA⊂CB[8] upon the addition of 3-NT and other interferents (15 µmol/L). (F) The images of LFIA strips for visual detection toward 3-NT under a 302 nm UV lamp.

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  Figure 4  The detection mechanism investigation. (A) ¹H NMR spectra (D₂O, 400 MHz, 298 K) of 3-NT (Ⅰ), BPPA⊂CB[8]−3-NT (Ⅱ), BPPA⊂CB[8] (Ⅲ) and BPPA (Ⅳ) ([BPPA] = [3-NT]= 2[CB[8]] = 1.2 mmol/L). (B) ¹H-¹H ROESY spectrum of BPPA⊂CB[8] with the addition of 3-NT ([BPPA] = [3-NT] = 2[CB[8]] = 1.2 mmol/L). (C, D) ESI-MS spectrum of the BPPA⊂CB[8] with the addition of 2.0 equiv. 3-NT. (E) The binding constants between 3-NT and CB[8].

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