CTAB-Assisted Formation of Hierarchical Porosity in Cu-BDC-NH₂ Metal–Organic Frameworks and Its Enhanced Peroxidase-like Catalysis for Xanthine Sensing

Abstract

A novel porous metal-organic framework (MOF), pCu-BDC-NH₂, with hierarchical porosity was synthesized using cetyltrimethylammonium bromide (CTAB) as a pore-generation agent. In addition to its common functions including structure-directing ligands or soft micelle templates, the judicious use of CTAB effectively modulated pore architecture in Cu-BDC-NH₂ MOFs. With additional mesopores generated during the synthesis process, the intrinsic MOF scaffolds further obtained pore hierarchies and interconnectivity, enabling efficient substrate access to the active metal centers, and thus significantly facilitated catalytic performance. As a proof of concept, we applied the finely engineered porous MOF pCu-BDC-NH₂ in a cascaded enzymatic system for xanthine sensing. This colorimetric biosensor exhibited a low detection limit of 0.11 μM, and a wide linear range of 1–120 μM. Furthermore, the sensor demonstrated exceptional stability, reproducibility, and was independent of interferences. Our simple yet effective method may find broader applications in tailoring pore architecture, enabling finer engineered structures to improve catalytic activities of nanomaterials.

1. Introduction

Metal-organic frameworks (MOFs) are a class of crystalline materials built up of metal-based secondary building units (SBU, ions or clusters) [1] connected by multivalent organic linkers via coordination interactions [2]. Their unique features including high surface areas, tunable pore sizes, and functional ligand diversities, offer a wide range of applications like gas adsorption, separation, catalysis, sensing and biomedical applications [3,4]. Recently, proprietary MOFs [5,6] exhibited remarkable versatility enabling selective mass transportation, cascaded catalysis and control release [7,8]. Their tunability introduced either through a one-pot synthesis process or post-synthetic modification (PSM) [9] allows for hierarchical and complex nanoscale architectures, thus ensuring a more homogeneous-like catalytic environment due to the efficient exposure of active sites, while maintaining the recyclability like heterogeneous catalysts [10].

The pore sizes of MOFs are typically smaller than 2 nm, inherited from the short organic ligands and arranged topology [11]. As a result, the diffusion of the reactants is hindered, especially in cases of large MOFs crystals. Therefore, not all catalytic sites are adequately exposed, decreasing overall catalytic efficiency and reaction rates [12]. Utilizing longer ligands are limited by commercial availability, undesirable interpenetrated structures or weak stability [13]. Alternatively, the fabrication of mesostructures to interconnect the original chambers or channels of MOFs turns out to be an effective strategy [14,15]. Template-assisted pore engineering is able to tailor the “defects” in MOFs [16]. Sacrificial templates, e.g., surfactants, nanoparticles, and even metal–organic assemblers, are incorporated during the fabrication process of MOFs [17], and then the temporarily occupied cavities are released through the removal of the templates. Recently, Chen et al. used HKUST-1 MOFs as sacrificial templates to prepare a second microporous MOFs of MIL-100(Fe) through the spontaneous ion substitution and hydrolysis process, resulting in better enzyme loading and synergetic degradation capacity of bisphenol A [18]. Tuo et al. synthesized a hierarchically porous Zr-MOF by using Pluronic F127 (PEO₁₀₆PPO₇₀PEO₁₀₆) as a soft template in an emulsion system. After grafting phytic acid, the functionalized MOF exhibited impressive adsorption capacity and kinetics for Uranium [19]. Xu’s group investigated the self-assembly processes of MOF nanocrystals to obtain highly porous morphologies via amino-acid modulation, providing abundant moieties for downstream functionalization with more efficient and selective adsorption abilities [20]. Therefore, employing simple methods to interfere with, regulate nanoscopic processes of crystal growth, or grain assembly, can yield defect-engineered MOFs with hierarchical internal porosity, thereby enhancing overall catalytic performance.

Herein, we systematically explore a strategy to tune the hierarchical porosity in Cu-BDC-NH₂ MOFs by controlling the synthesis process with CTAB. During the crystal growth, CTAB, a cationic surfactant, occupies certain copper sites, while its long-chain molecular structure fills the adjacent spaces [21]. This results in the formation of a primary mesoporous network once CTAB is removed. Coupled with the inherent microporosity of the MOFs and their high-density, uniformly distributed copper paddle-wheel catalytic centers, the carefully engineered hierarchical pore structure optimizes accessibility of active sites while promoting substrate diffusion. This strategy addresses the common limitations observed in larger MOF crystals, thereby enhancing the catalytic performance. Based on the well-tuned hierarchical porous MOF pCu-BDC-NH₂, a model xanthine sensor is evaluated for free radical transfer, detection sensitivity, and potential for advanced sensing applications.

2. Materials and Methods

2.1. Instrumentation and Methods

The graphical morphologies of the synthesized materials were obtained using an FEI Quanta250 Field Emission Scanning Electron Microscope (FESEM) (FEI, Houston, TX, USA). FT-IR spectra were measured using a TENSOR Model 27 spectrometer (Bruker Co., Ltd., Natick, MA, USA). X-ray diffraction (XRD) patterns were collected using a Bruker D8 X-ray diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250XI (Thermo Electron Corporation, Waltham, MA, USA) equipped with a monochromatic Al Kα X-ray source. The surface area of the samples was determined by nitrogen adsorption–desorption isotherms at 77 K using an Autosorb-iQ-C instrument (Quantachrome Instruments, Boynton, FL, USA), and the BET surface area was calculated from the data. Ultraviolet-visible (UV-Vis) absorption spectra were collected with an Allsheng Feyond-A300 spectrophotometer (Allsheng, Hangzhou, China).

2.2. Materials

Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, AR, 99%), 2-aminobenzene-1,4-dicarboxylic acid (BDC-NH₂, C₈H₇NO₄), dimethylformamide (DMF, C₃H₇NO), ethanol (C₂H₅OH, AR, 99%), cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN) were purchased from Sinopharm Chemical Reagent Co., Ltd. All other chemicals were of analytical grade and used without further purification. Deionized water (DI) was used throughout the experiment.

2.3. Synthesis of the Porous MOFs

During MOF synthesis, varying concentrations of CTAB were added to the system to investigate their effect on pore formation. At first, 0.0289 g (10 mmol) Cu(NO₃)₂·3H₂O was dissolved in 4 mL of DMF solution, then separately, 0.0065 g (3 mmol) BDC-NH₂ and different amounts of CTAB (0 mmol, 1 mmol, 2 mmol, 3 mmol, 4 mmol) were added to 8 mL of 1:1 (v/v) DMF and ethanol, and ultrasonicated for 30 min to facilitate ionic pre-organization. Thereafter, the aforementioned two solutions were mixed, stirred and transferred to a 20 mL PTFE reaction vessel with a tightened stainless steel liner. The solvothermal reaction was carried out at 120 °C for 8 h to promote crystal growth. At the end of the reaction, the mixture was centrifuged (13,000 rpm, 5 min) to separate out the solidified product. The product was then washed three times with ultra-pure water to remove solvents and impurities. Finally, the rinsed solids were freeze-dried under vacuum at −50 °C for 10 h. The various collected Cu-BDC-NH₂ MOFs were respectively named Cu-BDC-NH₂, Cu-BDC-NH₂-C1, pCu-BDC-NH₂, Cu-BDC-NH₂-C3, and Cu-BDC-NH₂-C4, depending on the amount of CTAB added.

2.4. Peroxidase-like Activity of Cu-BDC-NH₂ MOFs

MOF-mediated oxidation of TMB was performed by mixing 120 μL of 1 mg/mL Cu-BDC-NH₂ MOF suspension (dispersed in 100 mM NaAc-HAc buffer via ultrasound), 1 mL of TMB solution (4 mg TMB dissolved in 1 mL DMF, and then diluted with 9 mL NaAc-HAc buffer), and 50 μL of H₂O₂ solution with concentrations ranging from 1 nM to 200 μM. The different mixtures were incubated at 40 °C for 9 min. The maximum absorbance of the TMB oxidation products was measured at 652 nm.

2.5. Xanthine Concentration Sensing

At room temperature, 50 μL of xanthine solutions at different concentrations (1–600 μM) were mixed with 150 μL of xanthine oxidase solution (1.0 U/mL) and incubated for 30 min. The incubated solution was then mixed with 120 μL of 1 mg/mL Cu-BDC-NH₂ MOF suspension, and added to 1 mL of TMB solution. After mixing, the reaction was incubated at 40 °C for 9 min, with the resulting oxTMB measured at 652 nm absorbance.

3. Results and Discussion

3.1. Characterization of Cu-BDC-NH₂ MOFs

Cu-BDC-NH₂ MOFs, featuring Cu(II) as the metal core and BDC-NH₂ as the bridging ligand, are anticipated to be composed of the specific structural motifs in which two 5-coordinate Cu(II) ions are connected in a paddle-wheel type configuration. The porous Cu-BDC-NH₂ MOFs could be synthesized through defect engineering using CTAB as a cationic surfactant to electrostatically interact with BDC-NH₂ during the crystallization process [22,23]. After the removal of CTAB, its occupied space would be released, resulting in the formation of hierarchical porosity in the MOF crystals, as illustrated in Scheme 1. This hypothesis relies on the stability of the primary MOF in the presence of the different amount of pore-generating agents.

Therefore, The SEM images revealed the morphological features of three selected MOFs under different CTAB conditions. In the absence of CTAB (Figure 1a), Cu-BDC-NH₂ exhibited smooth, defect-free cubic crystals with diameters of approximately 2 μm. This is attributed to the regular hydrogen bonding and the π-π stacking between the two-dimensional planes of Cu-BDC-NH₂, which promotes the stacking of MOF layers along the [201] direction [24], and the degree of stacking is closely related to synthesis conditions [25]. Compared to Cu-BDC (CCDC 687690), the amino groups on the ligand BDC-NH₂ enhanced the tendency for 2D MOF stacking [26,27]. SEM images of pCu-BDC-NH₂ (CTAB 0.02 mmol, Figure 1b) clearly showed pore formation after eluting CTAB, with minimal impact on the overall morphology. On the other hand, when excessive CTAB (0.04 mmol) was added, the resulting product Cu-BDC-NH₂-C4 (Figure 1c) experienced collapse of the crystalline blocks, generating large amounts of fragments.

XRD data for 5 samples with different CTAB contents during the synthesis process was depicted in Figure 1d. Diffraction peaks were present at 10.34°, 11.9°, 13.16°, 16.8°, 18.18°, 20.8°, 24.8°, 28.4°, 33.8° and 41.7°, corresponding to the Miller indices 110, 020, 11 − 1, −201, 111, 220, 131, 31 − 2, −402, and 51 − 2, respectively. These peaks closely aligned with the theoretical PXRD pattern of Cu-BDC as documented in the Cambridge Crystallographic Data Centre with the reference number 687690 and were consistent with the work of Gupta et al. [28]. Some lattice parameters of Cu-BDC-NH₂ were found to differ slightly from those previously published for Cu-BDC, which was attributed to the substitution of BDC with BDC-NH₂. Additionally, the peak at 36.3° was attributed to Cu₂O and its broad pattern indicating the nanoscale degree of crystallization, which was a byproduct derived from the synthesis of Cu MOFs [29].

Based on the SEM images and XRD results, we concluded that addition of CTAB did not affect crystal growth at the nanoscopic scale. However, at a more macroscopic scale, the morphology and pore structure of the products were influenced by the CTAB-assisted modulation: (1) without CTAB, MOF crystals grew stably and aged, forming a characteristic cubic structure; (2) with a moderate amount of CTAB, distinct cavities appeared within the cubic particles, but the integrity of the particle structure was maintained; and (3) with an excessive amount of CTAB, significant collapse of the MOF particles occurred, resulting in smaller crystal grains, and remaining particles adopted increased irregularity.

3.2. Hierarchical Porosity in pCu-BDC-NH₂

According to XRD spectra, the composition of the main chemical bonds in the Cu-BDC-NH₂ MOF samples should be maintained. In the wavenumber region from 4000 to 500 cm⁻¹ (Figure 1e), the presence of a Cu-O stretching vibration was indicated by the distinctive and strong absorption band at 582 cm⁻¹ [30]. This observation served as strong evidence for the successful formation of a metal-oxygen bond between the carboxylate group of BDC-NH₂ and the Cu(II) ion, aligning with the proposed theoretical framework [28]. The characteristic peaks observed at 1104, 1150, 1491, and 1570 cm⁻¹ correspond to the aromatic ring vibration signal, and the bands at 1609 cm⁻¹ and 1372 cm⁻¹ were attributed to the carboxylate ligand (−COO⁻) in BDC-NH₂. Two distinct peaks at 3478 cm⁻¹ (asymmetric) and 3350 cm⁻¹ (symmetric) were attributed to the N-H stretching vibrations of the NH₂-BDC ligands, accompanied by the band at 1255 cm⁻¹ for the aromatic C-N stretching [31], whereas the band at 1670 cm⁻¹ was assigned to the stretching vibration of the carbonyl group in DMF, which weakly coordinated along the z-axis of the copper-based paddlewheel secondary building units (SBU) and could be detached during the subsequent catalytic studies.

In the FT-IR spectra, completeness of functional groups in the MOF structure was confirmed, indicating that the dominant material remained was Cu-BDC-NH₂ MOFs. Samples with or without CTAB displayed matched spectra, suggesting the complete removal of washed CTAB. Therefore, the spaces previously occupied by CTAB during synthesis were likely released. To confirm the presence of a mesoporous structure in pCu-BDC-NH₂, N₂-sorption measurements were conducted. Figure 1f depicted the N₂ adsorption-desorption isotherm of pCu-BDC-NH₂ particles, which exhibited characteristics intermediate between type I (typical of microporous materials) and type IV (typical of mesoporous materials). A distinct mesoporous hysteresis loop in the N₂ isotherm was apparent. Pore size distribution of pCu-BDC-NH₂ was different from those of Cu-BDC-NH₂ and Cu-BDC-NH₂-C4, with significantly higher abundance of pores in the 3 to 5 nm range, as shown in Figure 1f and Figure S1. These findings indicated that synthesized pCu-BDC-NH₂ material possessed both mesoporous and microporous structures.

3.3. Synergetic Enhancement of Catalysis

In order to analyze the mechanism and potential enhancement in catalysis, the chemical composition of some crucial atoms should be determined. Full-scan XPS survey spectra of Cu-BDC-NH₂, pCu-BDC-NH₂, and Cu-BDC-NH₂-C4 exhibited similar characteristic peaks to C, O, N, and Cu at their respective binding energies (Figure S2a–c). However, significant differences emerged when the high-resolution spectra of the Cu 2p and O 1s orbitals were analyzed. All the Cu 2p spectra in Figure 2 presented two clear peaks at binding energies of 933 and 960 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 levels, respectively, accompanied by several satellite peaks. The presence of Cu(II) was confirmed by the observation of four shake-up satellite peaks, resulting from p → d hybridization in the d9 configuration, within the energy ranges of 940–945 eV and 958–964 eV. After deconvolution, it can be observed that the peak at 933 eV decomposed into two peaks at 932.7 and 934.7 eV, which correspond to Cu(I) and Cu(II), respectively. Noticeably, a significantly higher proportion of Cu(I) was observed in the pCu-BDC-NH₂ sample (Figure 2a) compared to samples Cu-BDC-NH₂ (Figure 2b) and Cu-BDC-NH₂-C4 (Figure 2c). This could be attributed to the specific heterogeneous interface formed by CTAB and the surrounding solvents environment that facilitate the reduction of Cu(II). In contrast, the excessive concentration of CTAB in Cu-BDC-NH₂-C4 disrupted the MOF structure, reducing the Cu(I) portion. This outcome could affect the enhancement of peroxidase-like activity of the Cu-BDC-NH₂ MOFs, because Cu(I) is known to facilitate the breakdown of H₂O₂ and the production of HO•, which is then followed by the alternative and cyclic transformation between Cu(II) and Cu(I) [32]. Additionally, the O 1s spectrum was deconvoluted into three peaks (Figure 2d), corresponding to C=O, C-O, and Cu-O groups, respectively. The presence of Cu-O bonds is also one of the powerful indications of the successful synthesis of Cu MOF. The N 1s (Figure S3a–c) and C 1s (Figure S3e–f) spectra showed similar states of these elements, regardless of CTAB addition, which was consistent with our expectations.

The catalytic centers of Cu-BDC-NH₂ MOFs were located on the paddle-wheel structured SBUs, whereas in defect-free MOF cubic crystals, the majority of these centers were buried within the crystal structure. The hierarchical pore system combines both mesopores and micropores enabling greater access to catalytic sites, with reasonable pore interconnections facilitating reactant diffusion and transport. Moreover, the Cu(I) species in the pCu-BDC-NH₂ sample, either in the MOF scaffold or in the massively generated Cu₂O nanoparticles, were able to provide a parallel route for the generation of reactive oxygen radicals during the catalytic decomposition of H₂O₂, as follows:

$$\mathrm{Main}\mathrm{route}:\mathrm{p}\mathrm{C}\mathrm{u}\left(\mathrm{I}\mathrm{I}\right)-\mathrm{B}\mathrm{D}\mathrm{C}-\mathrm{N}{\mathrm{H}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{p}\mathrm{C}\mathrm{u}\left(\mathrm{I}\right)-\mathrm{B}\mathrm{D}\mathrm{C}-\mathrm{N}{\mathrm{H}}_2+\mathrm{H}\mathrm{O}{\mathrm{O}}^{•}+{\mathrm{H}}^{+}$$

(1)

$$\mathrm{Synergistic}\mathrm{enhancement}:\mathrm{C}\mathrm{u}{(\mathrm{I})}_2\mathrm{O}+2{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{C}\mathrm{u}{(\mathrm{I}\mathrm{I})}_2\mathrm{O}+2\mathrm{H}{\mathrm{O}}^{•}+2\mathrm{O}{\mathrm{H}}^{-}$$

(2)

$$\mathrm{Cyclic}\mathrm{catalysis}:\mathrm{p}\mathrm{C}\mathrm{u}\left(\mathrm{I}\right)-\mathrm{B}\mathrm{D}\mathrm{C}-\mathrm{N}{\mathrm{H}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{p}\mathrm{C}\mathrm{u}\left(\mathrm{I}\mathrm{I}\right)-\mathrm{B}\mathrm{D}\mathrm{C}-\mathrm{N}{\mathrm{H}}_2+\mathrm{H}{\mathrm{O}}^{•}+\mathrm{O}{\mathrm{H}}^{-}$$

(3)

$$\mathrm{C}\mathrm{u}{(\mathrm{I}\mathrm{I})}_2\mathrm{O}+2{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{C}\mathrm{u}{(\mathrm{I})}_2\mathrm{O}+2\mathrm{H}\mathrm{O}{\mathrm{O}}^{•}+2{\mathrm{H}}^{+}$$

(4)

Among them, Equation (1) is the rate-limiting step in the classical Cu-Fenton reaction [33], while the presence of Cu(I) in the samples provides the possibility for rapid reaction in the catalytic process.

3.4. pCu-BDC-NH₂ Enzyme Mimicry

To assess the peroxidase-like activity of pCu-BDC-NH₂, oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ was optically detected by quantifying the characteristic absorption peak of the blue oxTMB located at 652 nm in the UV-vis spectrum [34,35], with a reaction process involving reactive oxygen species (ROS) radicals as follows:

$${\mathrm{TMB}+\mathrm{HO}}^{•}\to {\mathrm{oxTMB}+\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{TMB}+\mathrm{HOO}}^{•}\to {\mathrm{oxTMB}+\mathrm{H}}_2\mathrm{O}$$

(6)

Colorimetric experiments illustrated hierarchically porous MOF pCu-BDC-NH₂, exhibiting superior performance in the TMB-H₂O₂ reaction, yielding a pronounced blue color compared to other experimental groups (Figure 3a). pCu-BDC-NH₂ composite material exhibited enhanced peroxidase mimicry, attributed to the high surface area of the CTAB-mediated hierarchical porous structure inside the MOF crystals and the concomitant active Cu(I) oxide species formed during the synthesis process.

To thoroughly evaluate the performance of pCu-BDC-NH₂ in the enzyme-mimicking catalytic reactions, a H₂O₂-TMB gradient oxidation assay model was employed. As the H₂O₂ concentration increased from 1 nM to 200 μM, the absorption peak intensity at 652 nm gradually intensified. Digital photographs taken during the experiment further demonstrated the progressive deepening of the solution color (from light blue to dark blue), corresponding to the increase in H₂O₂ concentration. This indicated that the system possessed good sensitivity and resolution in detecting low concentrations of H₂O₂.

3.5. pCu-BDC-NH₂ Xanthine Colorimetric Monitoring

H₂O₂ is involved in a wide range of biological functions, and xanthine oxidation by Xanthine oxidase (XOD) to produce H₂O₂ [36] was used as a proof of concept. Our pCu-BDC-NH₂ would catalyze these cascaded reactions into a measurable colored product. Prior to conducting the sensing studies, the experimental conditions were optimized, as shown in Figure 4, taking into account the concentration of TMB, pH, temperature, time, and the amount of pCu-BDC-NH₂, in order to maximize the catalytic activity of pCu-BDC-NH₂. First, the effect of TMB concentration was shown in Figure 4a. The absorbance at 652 nm increased linearly and reached maximum at 0.4 mg/mL. In term of pH, the reaction activity increased from pH 3.0 to pH 5, then decreased (Figure 4b). In Figure 4c, we found that the reaction temperature was also a significant factor, and the optimum temperature was 40 °C. As for the incubation time, 9 min (Figure 4d) was most efficient. The best additive amount for the pCu-BDC-NH₂ catalyst was tested to be 120 μg, as shown in Figure 4e. In summary, the optimized conditions for the colorimetric system were as follows: TMB concentration of 0.4 mg/mL, pH 5.0, reaction temperature of 40 °C, reaction time of 9 min, and catalyst weight of 120 μg. These conditions were employed in subsequent experiments to ensure consistent and maximal catalytic performance.

Xanthine can be successfully quantified based on the serial catalysis strategy, as illustrated in Figure 5a. With the increase of the concentration of xanthine in solution, the concomitant rise in absorbance at 652 nm occurred as shown in Figure 5b. In this experiment, a good linear relationship between absorbance and xanthine concentration is shown (Figure 5c), with two characteristic ranges of 1 to 120 μM, and 120 to 600 μM. Thanks to the enhanced catalytic ability, our experimental results can be identified by the naked eye without the need of instrumental assistance. By employing the 3σ method [37], the limit of detection (LOD) for xanthine by this strategy was calculated to be 0.11 μM, significantly lower than the concentrations reported in previous studies (

Table 1. Comparison of pCu-BDC-NH₂ and other reported enzyme-mimic catalysts for the detection of xanthine.

Nanozyme	Linear Range (μM)	LOD (μM)	Ref.
CuO@g-C3N4XOD	1–120	0.20	[38]
Co-doped-g-C3N4	7–450	5.40	[39]
WO3 nanosheets	25–200	1.24	[40]
MoSe2nanosheets	10–320	1.96	[41]
Fe3Ni7MOF	3–70	1.39	[42]
pCu-BDC-NH2	1–120	0.11	our work

). Therefore, applying the novel pCu-BDC-NH₂ catalyst as the signal transducer, a visual and straightforward detection of the target molecule was achieved.

To further validate the application potential of the pCu-BDC-NH₂ material as a xanthine sensor, we evaluated its selectivity and stability. Experiments were conducted under conditions where various possible interferents, such as alanine, glucose, Mg²⁺, Na⁺, and K⁺, were added to ensure that the response to xanthine was not significantly affected. As shown in Figure 6a, no significant changes in the colorimetric signal were observed, with the absorbance intensity at 652 nm remaining stable, while common ions or small molecules in complex sample environments did not interfere with the accuracy and reliability of the detection. To assess the long-term stability of the sensor, as depicted in Figure 6b, a 60-day dry storage performance at room temperature was done. Samples were taken every 5 days for xanthine detection experiments, recording changes in the colorimetric response. The results showed that throughout the testing period, the catalytic performance and color intensity of the system did not exhibit significant decay, and the absorbance values at 652 nm remained highly consistent. This fully demonstrates the chemical stability and catalytic activity durability of the material under mid-term storage conditions.

4. Conclusions

We investigated a strategy to creat novel MOFs including hierarchical porosities by simply incorporating the surfactant CTAB into the solvothermal synthesis system, followed by its subsequent elution. The physicochemical properties of MOFs at varying concentrations of surfactants were characterized, pointing to pCu-BDC-NH₂ MOF as the one with superior catalytic performance. Thanks to increased accessibility of the active metal centers to substrate molecules, significantly improving the overall catalytic activity. As a proof of concept, a xanthine detection biosensor was subsequently tested, enabling a simple, rapid, and quantitative detection by colorimetry with TMB. This sensor achieved a detection limit of 0.11 μM and exhibited high specificity even in the presence of a high concentration of potential interferents. We believe that this copper MOF material as well as its synthesis strategy and excellent peroxidase-like activity would promote the development of other types of biosensors.