A valence-state-defect–mediated copper nanocluster nanozyme with high peroxidase-like activity for multimodal colorimetric detection of mercury(II)

Abstract

Nanozyme-based colorimetric analysis has potential applications in environmental monitoring and food safety. Here, we report a novel copper nanocluster (CuNC) nanozyme with high peroxidase-like activity. The results demonstrate exceptional catalytic activity of up to 114.29 U mg⁻¹ in the presence of high oxidation state Cu²⁺ species. Accordingly, a CuNC-based colorimetric platform was established for high selectivity Hg²⁺ monitoring with a limit of detection of 4.21 nM. Excitingly, the color change induced by Hg²⁺ can be detected not only with the naked eye but also using a smartphone, without the need for sophisticated instruments or trained personnel. This work not only provides an economical and reliable multimodal detection method for Hg²⁺ but also enriches the existing armory of nanozymes in colorimetric studies and beyond.

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Mercury(II) (Hg²⁺), as a well-known heavy metal ion and highly poisonous pollutant, poses serious hazards to both aquatic and terrestrial organisms, even at extremely low concentrations. To protect human health, the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO) set maximum acceptable Hg²⁺ amounts in drinking water of 10.0 and 30.0 nM, respectively.¹ Currently, many common methods are used to monitor Hg²⁺ amounts, including liquid/gas chromatography-mass spectrometry, atomic absorption/emission spectroscopy, electrochemistry, Raman spectroscopy, chemiluminescence and fluorescence techniques.² Although much progress has been made in Hg²⁺ analysis based on these methods, many challenges still occur in terms of complicated pretreatment processes, expensive instrumentation, real-time monitoring capabilities, accuracy and selectivity. Therefore, considerable efforts are needed for technological improvements and innovations.

Nanozyme-based colorimetric analysis has been extensively applied in various fields such as the assessment of food quality and security, and environmental pollutant monitoring,³ due to its clear advantages of simple and portable operation, and visualization. However, common nanozymes such as metal oxides (Au, Pt, Ag, Fe and Mn) and graphene-based nanozymes have limitations in terms of expensive raw materials, complex preparation and limited active sites, seriously hindering their development and application.^4–6 Recently, Xu et al. reported a single-atom iridium-doped carbon-based nanozyme with oxidase-like activity as a colorimetric sensing platform for sensitive and multimodal Hg²⁺ analysis.⁷ Unfortunately, owing to the scarcity of iridium raw materials, the stringent synthesis conditions, and an unclear catalytic mechanism, its development and applications in sensing are limited. Therefore, avoiding the use of precious metals and complex preparation processes and concurrently ensuring the catalytic activity of the nanozymes are pivotal to improving their effective utilization for aqueous-based matrix sensing applications.

Copper (Cu), a transition metal, is highly abundant in nature, relatively inexpensive, and possesses rich surface defects. Recently, Cu nanoclusters (CuNCs) with vacancy defects have received much attention in the fields of photocatalysis and electrocatalysis because of their excellent catalytic properties.^8,9 However, CuNC-based nanozymes have rarely been reported because of their low oxidation–reduction potential and tendency to form a bulk aggregate state. Inspired by these achievements and limitations, we are interested in developing the investigation of the catalytic activity of CuNC-based nanozymes. We recently achieved high peroxidase (POD)-like activity of a CuNC-based nanozyme and noted a key effect of superficial valence states. The CuNC nanozyme exhibited high catalytic activity in the surface oxidized state, and can rapidly convert the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) into the blue oxidized product oxTMB in the presence of hydrogen peroxide (H₂O₂). Considering the potent antioxidant capacity of cysteine (Cys), the generation of blue oxTMB could be prevented. Nevertheless, in the presence of Hg²⁺, a strong affinity with the thiol/amino group (–SH/–NH₂) of Cys occurred to form Hg²⁺-Cys complexes, thereby blocking the antioxidant capacity, and ultimately restoring the blue color of the reaction system. The degree of blue color development was positively correlated with the concentration of Hg²⁺. Importantly, our constructed CuNC-based colorimetric sensing platform was applied to Hg²⁺ analysis in actual samples through multimodal routes, including ultraviolet-visible (UV-vis) spectroscopy, naked-eye observation and smartphone observation (Fig. 1A). Overall, this study presents a novel route for preparing a highly-efficient CuNC-based nanozyme and successfully exploits an innovative, portable, rapid and cost-effective colorimetric detection system for Hg²⁺.

Fig. 1 (A) Schematic illustration of the VSD-mediated trimodal colorimetric analysis of Hg²⁺ using a CuNC nanozyme. (B) TEM image of CuNCs. The high-resolution Cu 2p spectra of the ^oxCuNCs (C) and reduced CuNCs (D). (E) UV-vis spectra of TMB + H₂O₂ (curve a), CuNCs + H₂O₂ (curve b), CuNCs + TMB (curve c) and CuNCs + TMB + H₂O₂ (curve d). (F and G) Lineweaver–Burk curves of CuNCs for TMB and H₂O₂.

The reported CuNCs with valence-state defects (VSDs),¹⁰ herein, were selected to study the behavior of copper-based nanozymes. The detailed synthetic steps are described in the Experimental section of the SI. The transmission electron microscopy (TEM) image showed that the CuNCs appear as quasi-spherical and monodispersed nanoparticles with an average diameter of 16 ± 0.5 nm (Fig. 1B). The high-resolution TEM image confirmed that the nanoparticles were aggregates of multiple individual clusters with an average size of 2.5 ± 0.5 nm (Fig. S1). Next, the surface states and components of the CuNCs were investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The XPS full-spectrum of the CuNCs revealed the presence of C 1s, N 1s, O 1s, Cu 2p and Cu 3p core levels (Fig. S2). In the Cu 2p spectrum of the CuNCs (Fig. 1C), two obvious binding energies of 931.7 and 933.8 eV could be attributed to Cu⁰ 2p3/2 and Cu⁺ 2p3/2, respectively, while two other binding peaks at 951.4 and 953.6 eV were attributed to Cu⁰ 2p1/2 and Cu⁺ 2p1/2.¹¹ In addition, two strong satellite peaks were noted at 940.3 and 943.3 eV in the Cu 2p spectrum, indicating the existence of abundant Cu²⁺ in the CuNC surface. The compatibility of Cu⁰ and Cu⁺/Cu²⁺ oxidation states in the CuNCs was further confirmed by the fitting result of the XPS Cu LMM Auger spectrum (Fig. S3). These findings were consistent with our previous reports,¹⁰ and we concluded that the presence of the Cu⁺/Cu²⁺ oxidation states resulted in the formation of superficial VSDs on the CuNC surfaces, that is, the CuNCs were in high oxidation states, ^oxCuNCs. Next, the N 1s spectrum (Fig. S4) revealed that the delocalized CuNCs formed through coordination between Cu atoms and –NH₂ groups, demonstrating the retention of some –NH₂ groups on the NC surfaces. Concomitantly, the appearance of a peak at ∼609 cm⁻¹ in the FT-IR spectrum of the CuNCs (Fig. S5) proved that the 2,4-toluenediamine (2,4-TDA) ligand molecules were anchored to the CuNC surfaces through Cu–N bonds.¹² In addition, the retention of the absorption bands at 3430, 2924–2850, 1630–1510 and 1388 cm⁻¹ in the FT-IR spectrum indicated that the 2,4-TDA ligand retains its basic framework and structure in the process of CuNC preparation. Accordingly, the retention of the conjugated structure of the ligand and high superficial VSDs on the surfaces of the CuNCs should be responsible for the high catalytic activity of the CuNCs as mimetic nanozymes.

The catalytic POD-like activities of the prepared CuNCs were first investigated using TMB as the colorimetric substrate and CuNCs as the nanozyme. The results indicated that only the ^oxCuNCs + TMB + H₂O₂ system exhibited an evident absorption peak at 652 nm (curve d, Fig. 1E), whereas no corresponding characteristic peak was observed in the control experiments (curves a–c, Fig. 1E), including for 2,4-TDA (a, Fig. S6). Note that when the ^oxCuNCs were reduced, and upon confirming Cu²⁺ reduction using the high-resolution Cu 2p spectrum (Fig. 1D), the catalytic performance can be neglected (c, Fig. S6), illustrating the ability of ^oxCuNCs to catalyze the oxidation of colorless TMB to blue oxTMB, and also providing concrete evidence for the superficial VSD-mediated catalytic behavior of the CuNCs. Next, the catalytic conditions related to the POD-like activity of the CuNC nanozymes, including pH, temperature, time and nanozyme concentration, were examined. The experimental results showed that the optimal reaction conditions to analyze the POD-like activity of the CuNCs (Fig. S7–S10) were: 70 μg mL⁻¹ of nanozyme, pH 4.0 and reaction temperature of 50 °C for 15 min. In addition, the absorption spectra of the CuNCs showed nearly no significant change under low temperature conditions (∼4 °C) for two weeks (Fig. S11). Moreover, the CuNCs also exhibited stable POD-like activity during this period (Fig. S12). These findings confirmed the relatively stable and preferable POD-like activity of the prepared CuNCs.

To estimate the catalytic efficiency of the CuNCs, typical Michaelis–Menten curves were measured using TMB and H₂O₂ as substrates (Fig. S13). Additionally, the corresponding Lineweaver–Burk double-reciprocal plots are shown in Fig. 1F and G. According to the Lineweaver–Burk equation,¹³ the Michaelis constant/maximum reaction velocity (K_m/V_max) values of the CuNCs towards TMB and H₂O₂ were 1.94 mM/1.04 × 10⁻⁷ M s⁻¹ and 1.15 mM/0.47 × 10⁻⁷ M s⁻¹, respectively, which compare favourably with the natural enzyme horseradish peroxidase (HRP, 1.39 mM/1.00 × 10⁻⁷ M s⁻¹ and 7.92 mM/0.87 × 10⁻⁷ M s⁻¹),¹⁴ demonstrating the superior substrate affinity and efficient catalytic rate, which underpin the exceptional POD-mimetic behavior of the CuNCs. Furthermore, Fig. 2A shows that the number of catalytically active units was proportional to the mass of the CuNCs, and the specific activity (SA) of the CuNCs was calculated to be 114.29 U mg⁻¹, much higher than that of the traditional Fe–N–C single-atom nanozymes (25.33 U mg⁻¹) and Asp-Fe-CDs (∼0.1 U mg⁻¹),^15,16 demonstrating their exceptional catalytic activity as novel nanozymes.

Fig. 2 (A) The POD-like activity of the CuNCs. (B) The effect of different concentrations of scavengers on the absorbance of the CuNCs + TMB + H₂O₂ system at 652 nm. (C) UV-vis spectra of the CuNCs + TMB + H₂O₂ system (curve a), and in the presence of the scavengers of TU (curve b), p-BQ (curve c), His (curve d) and a mixture of the three scavengers (curve e).

The catalytic mechanism of nanocluster POD-like activity is dependent on the reduction of H₂O₂ to generate reactive oxygen species (ROS) such as singlet oxygen (¹O₂), hydroxyl radicals (˙OH) and/or superoxide anions (˙O₂⁻). The generation of the ROS was investigated by trapping experiments during the catalytic reaction with thiourea (TU) as an ˙OH scavenger, 1,4-benzoquinone (p-BQ) as an ˙O₂⁻ scavenger, and histidine (His) as an ¹O₂ scavenger.¹⁷ As shown in Fig. 2B, when His was added into the CuNCs + TMB + H₂O₂ system, the absorbance intensity exhibited the maximum change (curves a and d, Fig. 2C), and p-BQ came second (curves a and c, Fig. 2C), indicating that ¹O₂ and ˙O₂⁻ were the main reactive species. Interestingly, changing the TU amount caused negligible changes in the absorption intensity (Fig. 2B), showing that no ˙OH was generated and involved in the catalytic process (curves a and b, Fig. 2C). Note that the product ¹O₂ contributes much more to the catalytic behavior than ˙O₂⁻, also revealing that multiple ROS were involved in the catalytic activity of the CuNCs (curve e, Fig. 2C). The above reaction can also be easily monitored through color changes (Fig. S14). Compared with the previously reported metal-doped carbon-based nanozymes,^13,18,19 the significant difference in catalytic mechanism strongly supports the conclusion that the novel CuNC-based nanozyme not only changes the reaction pathway, but also improves the POD-like activity. The possible catalytic behavior by which CuNCs catalyze the production of ¹O₂ and ˙O₂⁻ from H₂O₂ might follow the Russell mechanism,²⁰ as shown in Fig. S15. Because of the presence of superficial VSD Cu⁺/Cu²⁺ species, the ^oxCuNCs might be inductively oxidized to form the intermediate ˙OOR radical group by reaction with H₂O₂. Then, the combination of two ˙OOR groups could form a transition state product, followed by structural rearrangement to generate ¹O₂ and ˙O₂⁻.

The CuNCs, as a mimetic nanozyme, can catalyze the generation of large amounts of ¹O₂ and ˙O₂⁻ intermediates from H₂O₂, thereby oxidizing colorless TMB to blue oxTMB with an absorption peak at 652 nm (Fig. 1A). This oxidation process can be inhibited by antioxidant species, such as ascorbic acid (AA), dopamine (DA), glutathione (GSH) and Cys, leading to faded color evolution and a decrease in the absorption intensity at 652 nm (Fig. 1A, up). Next, the antioxidant efficacy of the CuNCs + TMB + H₂O₂ system was evaluated using the above four species within 30 min. As shown in Fig. 3A, compared with the limited stability of AA, DA and GSH, Cys possessed relatively good stability under this reaction system. Accordingly, Cys can strongly bind with the transition metal cation Hg²⁺ to form a metal-complex through the effective coordination interaction between the –SH/–NH₂ of Cys and Hg²⁺,²¹ locking in its antioxidant activity, and thus generating a “turn-on” colorimetric signal (Fig. 1A, down). Considering the stability of the antioxidant and the “turn-off” progress within 10 min, Cys was selected as a crucial “on–off” bridge for Hg²⁺ colorimetric monitoring. From Fig. 3B, the absorbance value at 652 nm gradually decreased as the Cys concentration increased (0.1–80 μM), accompanied by the gradual attenuation of the blue color (inset of Fig. 3C). The relative values of [(A₀ − A)/A₀] displayed a linear correlation with Cys concentration in the range of 0.1–80 μM, with a linear regression equation of y = 0.01x + 0.06 (R² = 0.993) (Fig. 3C). A₀ and A are the absorbance in the absence and presence of Cys, respectively. The results demonstrated that the CuNCs can be used as a colorimetric probe to detect antioxidants.

Fig. 3 (A) Antioxidant capacity of AA (80 µM), DA (80 µM), Cys (80 µM) and GSH (80 µM). (B) UV-vis spectra of the CuNCs + TMB + H₂O₂ system with various Cys concentrations. (C) Calibration curve of [(A₀ − A)/A₀] versus the Cys concentrations shown in Fig. (B). (D) UV-vis spectra of the CuNCs + TMB + H₂O₂ system (curve a), and this system in the presence of Cys (80 µM, curve b) and Cys (80 µM) + Hg²⁺ (20 µM, curve c). (E) UV-vis spectra of the CuNCs + TMB + H₂O₂ + Cys system with various Hg²⁺ concentrations (the Hg²⁺ concentrations were (from bottom to top) 0.01, 5, 10, 15, 20, 25 and 30 μM). (F) Calibration curve of [(A − A₀)/A₀] versus the Hg²⁺ concentrations shown in Fig. (E). Insets of Fig. (C), (D), and (F): the color changes of the respective reaction systems. (G) The selectivity of CuNCs for Hg²⁺ detection (the concentrations of Hg²⁺ and Ag⁺ were 30 μM, and the concentrations of the other metal ions were 100 μM). (H) Smartphone-based colorimetric method for the CuNC sensor for the detection of Hg²⁺.

Next, we explored the feasibility of using the CuNC-catalyzed TMB system for Hg²⁺ analysis through colorimetric response. It can be seen that the primitive CuNCs + TMB + H₂O₂ system in the absence of Cys or Hg²⁺ exhibited a strong peak at 652 nm (curve a, Fig. 3D). After the addition of 80 μM Cys, the absorption peak completely disappeared (curve b, Fig. 3D), implying that Cys can completely inhibit TMB oxidation. Interestingly, the peak at 652 nm was dramatically regenerated by 20 μM Hg²⁺ (curve c, Fig. 3D), owing to the effective chelation between Hg²⁺ and Cys, resulting in the primitive state of the CuNC-catalyzed TMB system. Additionally, this “turn-on” behavior to reinstate the blue coloration can be easily observed visually (inset of Fig. 3D). It is conceivable that Hg²⁺ acts as a metal chelator to suppress the antioxidant Cys by forming a metal complex between Cys and Hg²⁺ through Hg²⁺–S/Hg²⁺–N bonding, and the peak at 652 nm gradually increased with increasing Hg²⁺ concentration (Fig. 3E), accompanied by a visible color transition from light to dark blue (inset of Fig. 3F). The Hg²⁺ dependence plot of [(A − A₀)/A₀] (A₀ and A are the values for the CuNCs + TMB + H₂O₂ + Cys system at 652 nm in the absence and presence of Hg²⁺, respectively) showed good linearity with concentrations of Hg²⁺ in the range of 0.01–30 μM (Fig. 3F), as represented by the equation y = 0.033x + 0.01 (R² = 0.991), with a limit of detection (LOD) of 4.21 nM (3σ/k). These findings demonstrated that CuNCs together with an antioxidant can be utilized as a colorimetric sensing platform for the analysis of Hg²⁺. Therefore, benefiting from the high sensitivity of the CuNC-based colorimetric sensor, we can easily achieve real-time analysis of Hg²⁺ through naked-eye observation for field and onsite monitoring without the need for sophisticated equipment and professionals.

In addition to sensitivity, the specificity is another critical parameter to evaluate the capability of the constructed sensor. To further evaluate the selectivity of the CuNC-based colorimetric system for Hg²⁺, the sensory responses to various other metal ions, including Pb²⁺, Cd²⁺, Cr³⁺, Co²⁺, Ni²⁺, Ba²⁺, Ca²⁺, Zn²⁺, Mg²⁺, Ag⁺, K⁺, Cu²⁺ and Al³⁺, were determined. From Fig. 3G, a significant colorimetric response was only observed upon addition of Hg²⁺. In contrast, no large colorimetric responses were observed upon adding other metal cations, including highly disruptive Ag⁺. This might be due to the inability of Ag⁺ to inhibit the absorption of ROS by Cys in the constructed sensing system. That is, the presence of Hg²⁺ resulted in a significant increase in absorbance, suggesting that this detection system provided robust resistance to interference.

To avoid inaccuracies caused by naked-eye observation, current development progress has led to a smartphone-based colorimetric method for the analysis of various analytes.²² When the colorimetric reaction was finished, the captured true-color images were decomposed into RGB channels and converted into digitized parameters with specific intelligent software. The RGB values refer to the response signals of Hg²⁺ contents. As illustrated in Fig. 3H, the variation of the color intensity depends on the Hg²⁺ amounts, which could be easily observed by the naked eye. Through an image-processing algorithm, the signal of the kit increases with increasing Hg²⁺ amount, with a fitted linear equation of y = −1.74x + 130.9 (R² = 0.997) in the range of 0.01–30 μM. This underscores the ease with which the developed CuNC nanozyme can be integrated with simple color readout devices, facilitating fast and reliable real-time sensing of Hg²⁺ and onsite monitoring in various environments. Moreover, analysis of Hg²⁺ in real sewage samples using the proposed colorimetric method was performed after spiking the real samples with a standard Hg²⁺ concentration. As shown in Table S1, the spiking recoveries for Hg²⁺ using UV-vis spectrometry analysis were in the range of 96.0–103.3%, and the smartphone-based analysis results indicated Hg²⁺ recovery rates ranging from 92.0% to 110.0%. These results demonstrated that the established portable method considerably reduces detection costs and could be applied to the accurate analysis of Hg²⁺ in real samples.

Conclusions

In summary, we report a novel CuNC nanozyme with high POD-like activity, to catalyze the simultaneous production of ¹O₂ and ˙O₂⁻ from H₂O₂via a Russell-like mechanism. By investigating the catalytic behaviors and superficial valence-state composition of the CuNCs, we observed enhanced catalytic performance in the presence of high oxidation state species on the surfaces of the CuNCs. As a result of the robust binding affinity between –SH/–NH₂ of Cys and Hg²⁺, a novel colorimetric platform for Hg²⁺ analysis was developed. The sensing platform achieved highly sensitive and selective monitoring of Hg²⁺ and enabled trimodal detection through UV-vis spectroscopy, naked-eye observation and smartphone use. In addition, the application of this colorimetric detection platform to real water samples for Hg²⁺ analysis further validated its economy, effectiveness and reliability in practical settings. We believe this economical and convenient method has good application prospects in many fields, including water quality assessment, environmental monitoring and public health protection.

Author contributions

Zhong-Xia Wang: writing – original draft, writing – review & editing, funding acquisition and supervision. Peng Shan: investigation, data curation and formal analysis. Ze-Yu Sun: data curation. Weijie Ding: validation. Fen-Ying Kong: formal analysis. Heng-Ye Li: project administration. Wei Wang: supervision and validation.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting the findings of this study are available within the article or its Supplementary Information (SI). The SI file associated with this article contains the experimental section, high-resolution TEM, XPS spectra, FT-IR spectrum, as well as the obtained datasets of the catalytic activity investigation. See DOI: https://doi.org/10.1039/d5an01159k.

Acknowledgements

This work was financially supported by the Project of State Key Laboratory of Analytical Chemistry for Life Science (SKLACLS2309) and the Jiangsu Province for the College Graduate Research and Innovation Projects (KYCX23_3470).

References

1. Z. X. Wang and S. N. Ding, Anal. Chem., 2014, 86, 7436–7445.
2. D. Dai, J. Yang, Y. Wang and Y. W. Yang, Adv. Funct. Mater., 2021, 31, 2006168.
3. Z. Chi, Q. Wang and J. Gu, Analyst, 2023, 148, 487–506.
4. Q. Liu, C. Xu, S. Chu, S. Li, F. Wang, Y. Si, G. Mao, C. Wu and H. Wang, J. Mater. Chem. B, 2022, 10, 10075–10082.
5. Z. Wang, R. Zhang, X. Yan and K. Fan, Mater. Today, 2020, 41, 81–119.
6. R. Qin, Y. Feng, D. Ding, L. Chen, S. Li, H. Deng, S. Chen, Z. Han, W. Sun and H. Chen, ACS Appl. Bio Mater., 2021, 4, 5520–5528.
7. T. Li, J. Xia, M. Wu, C. Liu, Y. Sun, W. Zhao, M. Qian, W. Wang, W. Duan and S. Xu, Small, 2025, 21, e2408785.
8. M. Bodiuzzaman, K. Murugesan, P. Yuan, B. Maity, A. Sagadevan, H. N. Malenahalli, S. Wang, P. Maity, M. F. Alotaibi, D. E. Jiang, M. Abulikemu, O. F. Mohammed, L. Cavallo, M. Rueping and O. M. Bakr, J. Am. Chem. Soc., 2024, 146, 26994–27005.
9. B. Zhang, X. Tan, Z. H. Zhang, C. Y. Liu, F. Hu, H. S. Hu, C. Chen and Q. M. Wang, CCS Chem., 2025, 7 DOI:10.31635/ccschem.025.202505606.
10. Z. X. Wang, H. Gao, X. Q. Li, Y. L. Jia, T. Wang, Q. Y. Cheng, B. Kang, H. Y. Chen and J. J. Xu, Adv. Funct. Mater., 2023, 33, 2305209.
11. B. Santiago-Gonzalez, A. Monguzzi, C. Capitani, M. Prato, C. Santambrogio, F. Meinardi and S. Brovelli, Angew. Chem., Int. Ed., 2018, 57, 7051–7055.
12. Z. X. Wang, H. Gao, Y. L. Jia, X. Q. Li, T. Wang, S. N. Ding, H. Y. Chen and J. J. Xu, Small, 2024, 20, e2403842.
13. S. Wu, Z. Yu, J. Zhao, Y. Wang, J. Nan, L. Xu, X. Li, L. Yang and S. Dong, Anal. Chem., 2025, 97, 15320–15328.
14. L. Gao, J. Zhuang, L. Nie, J. Zhang, Y. Zhang, N. Gu, T. Wang, J. Feng, D. Yang, S. Perrett and X. Yan, Nat. Nanotechnol., 2007, 2, 577–583.
15. X. Song, X. Wen, Y. Huang and Z. Fan, Food Chem., 2025, 491, 145224.
16. L. Jiao, J. Wu, H. Zhong, Y. Zhang, W. Xu, Y. Wu, Y. Chen, H. Yan, Q. Zhang, W. Gu, L. Gu, S. P. Beckman, L. Huang and C. Zhu, ACS Catal., 2020, 10, 6422–6429.
17. X. Luo, G. Huang, C. Bai, C. Wang, Y. Yu, Y. Tan, C. Tang, J. Kong, J. Huang and Z. Li, J. Hazard. Mater., 2023, 443, 130277.
18. X. Xie, Y. Wang, X. Zhou, J. Chen, M. Wang and X. Su, Analyst, 2021, 146, 896–903.
19. Q. Liu, A. Zhang, R. Wang, Q. Zhang and D. Cui, Nano-Micro Lett., 2021, 13, 154.
20. C. Wang, F. Cao, Y. Ruan, X. Jia, W. Zhen and X. Jiang, Angew. Chem., Int. Ed., 2019, 58, 9846–9850.
21. F. Jalilehvand, B. O. Leung, M. Izadifard and E. Damian, Inorg. Chem., 2006, 45, 66–73.
22. J. Zhu, K. Q. Liu, P. Shan, F. Y. Kong, F. Li, H. Gao and Z. X. Wang, Dyes Pigm., 2025, 243, 113070.

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