An amino-functionalized lanthanide-organic framework for ratiometric detection of ONOO⁻

Abstract

Peroxynitrite (ONOO⁻), a common reactive nitrogen, can serve as a potential indicator for the early diagnosis of drug-induced liver injury (DILI). In this work, a lanthanide–organic framework (EuTPTC-NH₂) is constructed for ratiometric luminescence detection of ONOO⁻ with excellent sensitivity and selectivity. The presence of ONOO⁻ leads to an increase in ligand emission at 410 nm and a decrease in Eu³⁺ at 616 nm, accompanied by an emission color change from red to blue, due to the oxidation of the amino group to a nitroso group by ONOO⁻. EuTPTC-NH₂ exhibits a low detection limit (LOD) of 0.0053 μM, a fast response time of 45 s, and excellent selectivity toward ONOO⁻. EuTPTC-NH₂ can also be successfully used for the ratiometric detection of ONOO⁻ in saline, and subsequently employed for the detection of the ONOO⁻ concentration level in serum samples from DILI mice and clinical patients with liver injury, providing a potential biological tool for the early detection of DILI.

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New concepts

We construct a lanthanide–organic framework (EuTPTC-NH₂) for ratiometric luminescence detection of a drug-induced liver injury (DILI) biomarker. Upon the addition of ONOO⁻, the EuTPTC-NH₂ probe exhibited an increase in the luminescence intensity of the ligand, due to the oxidation of the amino group in the ligand to nitroso group. Meanwhile, the characteristic emission peak of Eu³⁺ decreased, because the oxidation of the amino group to a nitroso group also changed the singlet and triplet energy levels of the ligand, thereby reducing the proportion of intersystem crossing process and efficiency of ligand-to-metal energy transfer. The EuTPTC-NH₂ probe demonstrated high sensitivity (LOD = 0.0053 μM) and rapid luminescence response (45 s) to ONOO⁻. Furthermore, the EuTPTC-NH₂ probe was found to be an effective tool for the ratiometric detection of ONOO⁻ in serum samples from an APAP-induced liver injury mouse model and clinical patients with liver injury.

1. Introduction

The liver is the vital organ for drug metabolism, thus rendering it a major target for drug-induced liver injury (DILI) due to adverse drug reactions and overdose.¹ At an early stage of liver injury, drug metabolites produce large amounts of reactive species that react with the biomacromolecules in hepatocytes, leading to a disruption of the redox balance.² For example, excessive use of acetaminophen (APAP), a common pain and fever reliever, can produce large amounts of reactive nitrogen species (RNS) in human mitochondria and blood, leading to severe liver damage.^3–5 ONOO⁻, as a common RNS, has been identified as a sensitive biomarker of DILI, since the concentration of ONOO⁻ is less than 1 μM per minute in normal humans and close to 50–100 μM per minute in liver injury conditions.^6–8

There are many methods that have been developed to identify ONOO⁻, including spectrophotometry,⁹ electrochemical analysis,¹⁰ chromatography,¹¹ chemiluminescence,¹² and so on. Nevertheless, these methods require expensive equipment, complicated operation, or lengthy detection time.¹³ Recently, small-molecule fluorescent probes have been extensively employed in ONOO⁻ detection due to their high selectivity, and excellent spatial and temporal resolution.¹⁴ However, most of these fluorescent probes display single emission intensity variations, which are easily affected by external factors, such as instrument parameters, test environment, photobleaching, etc.^15,16 Furthermore, some existing ratiometric fluorescence probes usually have low sensitivity and non-obvious emission color change, which may not be effective for ONOO⁻ detection in living systems, or may give false signals.¹⁷ Therefore, a ratiometric luminescence probe with high sensitivity and obvious emission color change is urgently needed for accurate ONOO⁻ detection.

Metal–organic frameworks (MOFs), a class of solid crystalline materials, are formed by metal ions or metal clusters connected with organic ligands through coordination bonds.^18–20 MOFs have attracted extensive attention due to their size-rich pore structure, unparalleled tunability, and easy functionalization.^21–24 Lanthanide–organic frameworks (Ln-MOFs), as a potential branch of MOFs, combine the intrinsic features of MOFs with the unique luminescence properties of Ln³⁺ ions, including large Stokes shifts, sharp and abundant emission lines, long luminescence lifetimes, and high quantum yields.^25–30 These distinctive properties render Ln-MOFs promising candidates in the field of luminescent sensing.^31–33

In this study, we prepared a water-stable Ln-MOF (EuTPTC-NH₂) by the assembly of an organic ligand (H₄TPTC-NH₂) and Eu³⁺ ions for the rapid detection of ONOO⁻ (Scheme 1). Upon the addition of ONOO⁻, the EuTPTC-NH₂ probe exhibited an increase in the luminescence intensity of the ligand, due to the oxidation of the amino group in the ligand to a nitroso group. Meanwhile, the characteristic emission peak of Eu³⁺ decreased, because the oxidation of an amino group to a nitroso group also changed the singlet and triplet energy levels of the ligand, thereby reducing the proportion of intersystem crossing process and efficiency of ligand-to-metal energy transfer. The EuTPTC-NH₂ probe demonstrated high sensitivity (LOD = 0.0053 μM) and rapid luminescence response (45 s) to ONOO⁻. Furthermore, the EuTPTC-NH₂ probe was found to be an effective tool for the ratiometric detection of ONOO⁻ in serum samples from an APAP-induced liver injury mouse model and clinical patients with liver injury.

Scheme 1 Schematic representation of the ratiometric detection of ONOO⁻ by EuTPTC-NH₂.

2. Results and discussion

2.1. Characterization of EuTPTC-NH₂

EuTPTC-NH₂ was synthesized by the solvothermal reaction of 2′-amino-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid (H₄TPTC-NH₂) and Eu(NO₃)₃·6H₂O according to the previous work (Fig. S1, ESI†).³⁴ As shown in Fig. 1a, the successful preparation of EuTPTC-NH₂ was confirmed by the well-matched powder X-ray diffraction (PXRD) patterns between the as-prepared EuTPTC-NH₂ and the simulated single-crystal X-ray diffraction pattern. Dynamic light scattering (DLS) results showed that the EuTPTC-NH₂ suspension had a hydrodynamic radius of 230 nm (Fig. 1b). Meanwhile, uniform spheres with an average diameter of 200 nm were observed by transmission electron microscopy (TEM) (Fig. 1c). We then investigated the photoluminescence behavior of the EuTPTC-NH₂ suspension in saline. As shown in the UV-Vis absorption spectrum (Fig. S2, ESI†), EuTPTC-NH₂ had an obvious absorption peak from 240 to 400 nm. As illustrated in Fig. 1d, the excitation spectrum of the EuTPTC-NH₂ suspension displayed a broad peak from 250 to 380 nm, attributed to the absorption of H₄TPTC-NH₂, which means that Eu³⁺ was successful coordinated and sensitized by H₄TPTC-NH₂ (Fig. S3, ESI†). The emission spectrum exhibited five sharp peaks at 579, 592, 616, 653, and 698 nm, referred to as the ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺, respectively.^35,36 Among them, the ⁵D₀ → ⁷F₂ transition at 616 nm is dominant, resulting in red emission (Fig. 1d, inset). Meanwhile, the emission spectrum also showed a broad emission peak from 360 to 500 nm, which was ascribed to the intermolecular π–π* transition in the ligand (Fig. S3, ESI†). Furthermore, we also assessed the pH stability by immersing them in aqueous solutions of pH 0–14 for 6 h. The almost identical PXRD and emission spectra demonstrated that EuTPTC-NH₂ has good stability in the pH range of 4–10 (Fig. S4 and S5, ESI†).

Fig. 1 (a) PXRD patterns of EuTPTC-NH₂, EuTPTC, and EuTPTC-NH₂ treated with various conditions for 6 h. (b) DLS size distribution of the EuTPTC-NH₂ suspension. (c) TEM image of EuTPTC-NH₂. (d) Normalized excitation (λ_mon = 616 nm) (left) and emission (λ_ex = 313 nm) (right) spectra of the EuTPTC-NH₂ suspension; inset: the corresponding photographic image under 365 nm UV light.

2.2. Sensing behavior of EuTPTC-NH₂ towards ONOO⁻

Photoluminescence titration was performed in saline to mimic the internal environment of the organism (Fig. 1a). The emission spectra of the EuTPTC-NH₂ suspension in saline showed no significant change within 30 days (Fig. S6, ESI†), confirming its high luminescence stability. As shown in Fig. 2a, with the addition of ONOO⁻ (0–112 μM), the emission band of Eu³⁺ decreased, whilst that of the ligand at 410 nm increased. The emission intensity ratio (I₄₁₀/I₆₁₆) showed an exponential relationship with the ONOO⁻ concentration within the range of 0–112 μM, as evidenced by the exponential eqn (1):

I₄₁₀/I₆₁₆ = −10.59 × exp(−[C]/21.31) + 10.76 (R² = 0.996) (1)

Fig. 2 (a) Emission spectra of the EuTPTC-NH₂ suspension (0.02 mg mL⁻¹) upon addition of ONOO⁻ (0–112 μM) in saline (λ_ex = 313 nm); inset: the corresponding color change under 365 nm UV light. (b) Emission intensity ratio (I₄₁₀/I₆₁₆) of EuTPTC-NH₂ as a function of ONOO⁻ concentration; the error bars represent the standard deviations of triplicate measurements. (c) The CIE 1931 chromaticity diagram after adding different concentrations of ONOO⁻ (λ_ex = 313 nm). (d) Comparison of LOD and among representative probes.

The correlation coefficient (R²) is 0.996. It is worth noting that a linear relationship is found in the profile of intensity ratio (I₄₁₀/I₆₁₆) versus ONOO⁻ concentration ranging from 0.0053 to 11 μM, with R² = 0.997 (Fig. 2b). The limit of detection (LOD) for ONOO⁻ was calculated to be 0.0053 μM, which is much lower than that of in patients with liver injury (50–100 μM), as calculated by the 3σ/K method³⁷ (where σ is the standard deviation of the 11 blank samples and K is the slope of the linear curve). Therefore, EuTPTC-NH₂ can quantify ONOO⁻ in a physiologically relevant range below the risk level for patients with liver injury. Interestingly, after the addition of ONOO⁻, the emission color of the EuTPTC-NH₂ suspension changed from red to blue (Fig. 2a, inset), which is directly readable from the Commission Internacionale d’Eclairage (CIE) chromaticity diagram with the change of coordinates from (0.3629, 0.2015) to (0.1913, 0.1251) (Fig. 2c). The visualization capability of this probe was tested by the Lab color space. As illustrated in Table S1 (ESI†), the Lab values (L1*, a1*, b1*, L2*, a2*, and b2*) were obtained by means of the eyedropper tool. Chromatism , which represents the difference between two colors, is defined as follows:^38,39

As presented in Table S2 (ESI†), the value was calculated to be 100, exceeding most of the best performing ratiometric probes. As shown in Fig. 2d, combining LOD and in account, the EuTPTC-NH₂ probe has high sensitivity and nearly optimal visualization capability.

In order to assess the detection performance of EuTPTC-NH₂ in biological systems, we evaluated the response of the EuTPTC-NH₂ suspension in the presence of biologically relevant RNS, peroxide, and reactive oxygen species, including ClO⁻, H₂O₂, NO₂⁻, ¹O₂, ˙OH, and tert-butyl hydroperoxide (TBHP). The emission intensity ratio (I₄₁₀/I₆₁₆) increased significantly with the addition of ONOO⁻, whereas it changed slightly with the addition of other interfering analytes (Fig. 3a and Fig. S7, ESI†), indicating that EuTPTC-NH₂ has excellent selectivity for ONOO⁻. Besides, the emission color change caused by ONOO⁻ can be easily seen with the naked eye under 365 nm UV light (Fig. 3b). The anti-interference test was performed to evaluate the potential interference of other serum components in the detection of ONOO⁻. Notably, the emission intensity ratio (I₄₁₀/I₆₁₆) of the EuTPTC-NH₂ changed slightly with the addition of these interfering substances (urea, glucose, L-serine, L-threonine, glycine, NH₄Cl, NaCl, MgCl₂, CaCl₂, KCl, MgSO₄, Na₂SO₄, Na₂HCO₃, NaH₂CO₃ and Na₂HPO₄)^40,41 compared to the addition of individual ONOO⁻ (Fig. 3c). These results jointly indicated the good selectivity and anti-interference capability of EuTPTC-NH₂ in ONOO⁻ detection. Notably, the emission changes of EuTPTC-NH₂ at 410 and 616 nm reached a plateau within 45 s after the addition of ONOO⁻, demonstrating its rapid response capability (Fig. 3d).

Fig. 3 (a) Emission intensity ratio (I₄₁₀/I₆₁₆) of the EuTPTC-NH₂ suspension (0.02 mg mL⁻¹) in the presence of ONOO⁻ and various analytes (112 μM) in saline (λ_ex = 313 nm), and (b) the corresponding photographs under 365 nm UV light. (c) Emission intensity ratio (I₄₁₀/I₆₁₆) of the EuTPTC-NH₂ suspension (0.02 mg mL⁻¹) toward ONOO⁻ in the presence of potentially interfering substances (112 μM) (λ_ex = 313 nm). (d) Emission intensity changes of EuTPTC-NH₂ at 410 and 616 nm after the addition of 112 μM ONOO⁻ in saline.

2.3. Sensing mechanism

The low LOD, good selectivity, and excellent anti-interference of this probe encouraged us to further investigate the sensing mechanism. The consistent PXRD patterns of EuTPTC-NH₂ before and after adding ONOO⁻ indicated that the changes in luminescence intensity were not caused by the crystalline structure collapse (Fig. 1a).⁴² A control experiment was carried out by using H₄TPTC instead of H₄TPTC-NH₂ to prepare EuTPTC (Fig. 1a and Fig. S8, ESI†). The emission intensity ratio (I₄₁₀/I₆₁₆) was almost unchanged upon the addition of ONOO⁻ (Fig. S9, ESI†), demonstrating that the amino group plays an important role in recognizing ONOO⁻. As reported in previous work,⁴³ the amino group might react with ONOO⁻, resulting in the oxidation of –NH₂ to –NO. As shown in Fig. S10 (ESI†), the luminescence lifetime of EuTPTC-NH₂ in the absence and presence of ONOO⁻ decreased from 0.358 ms to 0.288 ms, suggesting that dynamic quenching might occur in the recognition process of ONOO⁻.⁴⁴ As shown in Fig. S11 (ESI†), upon addition of ONOO⁻, the stretching vibrational peak attributed to N–H of EuTPTC-NH₂ at 3350 cm⁻¹ disappeared and a new peak at 1384 cm⁻¹ attributed to N=O was observed in the Fourier transform infrared (FT-IR) spectra.⁴³ In addition, as illustrated in the X-ray photoelectron spectroscopy (XPS) spectra, after adding ONOO⁻, the peaks of O 1s at 537.36 eV belonging to the N=O bond and N 1s at 407.53 eV ascribed to the N=O bond further confirmed the presence of a nitroso group (Fig. 4a–c).^45,46 In the acid-digested ¹H NMR spectra (Fig. 4d), upon addition of ONOO⁻, the hydrogen protons of EuTPTC-NH₂ showed obvious chemical shifts. A peak at 434.05 (m/z) belonging to H₄TPTC-NO was also observed by the liquid chromatograph-mass spectrometer (LC-MS) in the presence of ONOO⁻ (Fig. 4e). These results confirmed that the amino group has been oxidized to a nitroso group by ONOO⁻.

Fig. 4 (a) XPS survey spectra of EuTPTC-NH₂ before (i) and after (ii) the addition of ONOO⁻, (b) O 1s, and (c) N 1s spectra. (d) Partial ¹H NMR spectra of EuTPTC-NH₂ before (i) and after (ii) addition of ONOO⁻ (DMSO-d₆/DCl, 5/1, v/v, 25 °C). (e) LC-MS of acid-digested EuTPTC-NH₂ in the presence of ONOO⁻ (H₄TPTC-NO, exact mass = 435.34). (f) Schematic diagram depicting the energy-transfer process in EuTPTC-NH₂ before and after adding ONOO⁻.

As illustrated in Fig. S12 (ESI†), the emission spectra of H₄TPTC-NH₂ exhibited a notable increase upon addition of ONOO⁻ (0–24 μM), suggesting that the emission of the ligand increased following the oxidation of the amino group to a nitroso group. In general, the luminescent properties of Ln-MOFs are influenced by the energy transfer between the singlet/triplet states of the ligand and the lanthanide energy level.⁴⁷ According to the intrinsic energy levels of the ligand and Eu³⁺ ions, Fig. 4f shows the energy transfer process between them. The singlet state energy levels (S₁) of the ligand before and after adding ONOO⁻ obtained from UV-Vis absorption spectra were 28 169 and 30 581 cm⁻¹, respectively (Fig. S13, ESI†). We prepared GdTPTC-NH₂ using Gd³⁺ instead of Eu³⁺ (Fig. S14, ESI†), and the triplet energy levels (T₁) of the ligand H₄TPTC-NH₂ in the absence and presence of ONOO⁻ were 22 883 and 23 923 cm⁻¹, respectively (Fig. S15, ESI†). The energy gaps ΔE(S₁–T₁) were 5286 and 6658 cm⁻¹, respectively, which indicates that the intersystem crossing (ISC) process can occur, in accordance with Reinhoudt's empirical rules.⁴⁸ In addition, according to Latva's empirical rule,⁴⁹ the ligand-to-metal energy transfer is most efficient when the energy gap between the ligand and the Eu³⁺ ΔE(T₁–⁵D₀) located in the range from 2500 to 4000 cm⁻¹, and larger energy gap will cause an energy loss.^50,51 In a nutshell, the addition of ONOO⁻ increased the emission of the ligand, leading to a decrease in the proportion of ISC, while the energy gap ΔE(S₁–T₁) became larger, reducing the efficiency of ligand-to-metal energy transfer, which together led to a decrease in the emission intensity of Eu³⁺.

2.4. Sensing ONOO⁻ in a biological environment

To further evaluate the practicality of EuTPTC-NH₂ for sensing ONOO⁻ in biological environments, an acute liver injury (ALI) model was established by intraperitoneal injection of APAP in C57/BL6 mice. Compared with the control group, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were significantly increased in the ALI groups (Table S3, ESI†). Furthermore, hematoxylin-eosin (H&E) histological staining of liver tissues showed that the hepatocytes of normal control mice were arranged in an orderly manner, with no hepatocyte degeneration or necrosis (Fig. 5a). In contrast, the hepatocytes of the APAP-treated mice were disorganized, with a large number of hepatocytes severely damaged and showing nuclear consolidation (indicated by yellow arrows). These results jointly demonstrated the successful establishment of the liver injury models. As illustrated in Fig. 5b, we further used EuTPTC-NH₂ to determine the content of ONOO⁻ in serum samples from the ALI models (LOD = 0.173 μM). As shown in Table S4 (ESI†), the concentrations of ONOO⁻ in the serum of mice from the ALI groups were determined to be 76.83 ± 0.05 μM, 78.91 ± 0.05 μM, and 87.82 ± 0.06 μM, respectively, which were notably higher than that of the control mouse (0.67 ± 0.01 μM). In order to elucidate the clinical significance of EuTPTC-NH₂, we also applied the probe to monitor the ONOO⁻ level in human serum samples from both a healthy donor and clinical patients with liver injury (Fig. 5c and Table S5 (ESI†), LOD = 0.291 μM, ESI†). As illustrated in Table 1, the initial concentration of ONOO⁻ in the serum of the healthy donor was determined to be 0.31 ± 0.04 μM, which was much lower than that of clinical patients with liver injury (83.76 ± 0.27, 87.60 ± 0.32, and 91.63 ± 0.18 μM). Subsequently, different concentrations of ONOO⁻ were added into the serum samples to further assess the reliability of the EuTPTC-NH₂ probe. The recoveries of ONOO⁻ in mice and human serum samples were in the range of 97.00–103.50% and 94.50–105.50%, respectively, with relative standard deviation (RSD) values of less than 1.50% and 2.21%, respectively. These results demonstrate that our probe can accurately quantify ONOO⁻ in both mouse and human serum, confirming its potential for practical application in clinical sample analysis.

Fig. 5 (a) H&E staining images of liver tissues from mice treated with saline and APAP. Emission intensity ratio (I₄₁₀/I₆₁₆) of EuTPTC-NH₂ as a function of ONOO⁻ concentration (b) in mouse serum and (c) in human serum; inset: the corresponding healthy serum samples (red pentagrams) and liver-injured serum samples (orange, brown, and green pentagrams), the error bars represent the standard deviations of triplicate measurements.

Table 1 Quantification of ONOO⁻ in human serum samples with RSD (n = 3)

Samples	Added (μM)	Found (μM)	Recovery (%)	RSD (%)
Healthy donor	0	0.31 ± 0.04	—	2.21
	2	2.32 ± 0.08	100.00	0.31
	4	4.12 ± 0.03	100.50	1.15
	6	6.43 ± 0.06	101.84	0.80
DILI patient-1	0	83.76 ± 0.27	—	0.23
	2	85.62 ± 0.04	94.50	0.24
	4	87.54 ± 0.14	98.75	0.05
	6	89.77 ± 0.03	100.17	0.28
DILI patient-2	0	87.60 ± 0.32	—	0.07
	2	89.70 ± 0.62	105.00	0.02
	4	91.57 ± 0.29	99.25	0.007
	6	93.72 ± 0.35	102.00	0.04
DILI patient-3	0	91.63 ± 0.18	—	0.05
	2	93.74 ± 0.62	105.50	0.06
	4	95.82 ± 0.68	104.75	0.09
	6	97.89 ± 0.09	104.33	0.06

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3. Conclusion

In this work, a dual-emission ratiometric luminescent Ln-MOF has been developed as a DILI biomarker probe. When different concentrations of ONOO⁻ were added, the emission intensity of Eu³⁺ ions decreased, whilst the emission intensity of the ligand increased, and the ratio of I₄₁₀/I₆₁₆ was linearly correlated with the concentration of ONOO⁻ ranging from 0.0053 to 11 μM. Furthermore, the probe had a detection limit as low as 0.0053 μM and was able to respond to ONOO⁻ within 45 s. The probe was also able to identify ONOO⁻ in the serum samples from mice and clinical patients with liver injury. There is no doubt that this work provides a competitive candidate for the medically sensitive and rapid specific identification of biomarkers of liver injury.

Data availability

The data underlying this article will be available on reasonable request from the corresponding author. The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22171069 and 21871075), the Educational Committee of Hebei Province (JZX2024012 and QN2025269), the Tianjin Natural Science Foundation (23JCYBJC00800), the Hebei Province Graduate Student Innovation Program (CXZZSS2024010), the China Postdoctoral Science Foundation (2024M750714) and Postdoctoral Fellowship Program of CPSF (GZC20240368) and Natural Science Foundation of Hebei Province (B2024202066) and the Tianjin Medical University Cancer Institute & Hospital Outstanding Innovation Talent Support Program (2024-1-29).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh00885a

‡ H. Z. and X. L. contributed equally to this work.

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