Well‐Designed Highly Conjugated Covalent Organic Frameworks as Light Responsive Oxidase Mimic for Effective Detection of Uric Acid

Covalent organic frameworks (COFs) are widely used in photocatalysis due to their periodic π–π arrays, high crystallinity, and adjustable bandgap. Herein, a new strategy for integrating polyphenyl building blocks in COFs is presented to improve the photocatalytic efficiency. To implement this strategy, a series of COFs with different numbers of phenyl groups are successfully designed and synthesized. By varying the number of phenyl units in the precursor, the COFs exhibit different bandgaps, band‐edge positions, carrier mobilities, and interfacial transfer resistances. The corresponding characterization reveals that the photocatalytic capacity of COFs increases with the number of phenyls in the basic structural unit. Further, under visible light irradiation, the COFs prepared from 1,3,5‐tris [4‐amino(1,1‐biphenyl‐4‐yl)] benzene (TABB) and 1,4‐benzenedicarboxaldehyde (BDB) (named TABB‐BDB COF) exhibit superior light‐responsive oxidase‐mimicking characteristic, which can catalyze the oxidation of 2,2′‐azino‐bis (3‐ethylbenzothiazoline‐6‐sulfonic acid) (ABTS). Based on the aforementioned characteristics, TABB‐BDB COF is designed as a robust colorimetric probe for the inexpensive, highly sensitive, and rapid detection of uric acid (UA) with a linear range of 5–160 mg L−1. This study not only demonstrates COFs‐based light‐response oxidase mimicking for efficient UA detection but also provides an intelligent tactic for boosting the photocatalytic competence of COFs.


Introduction
Nanomaterial-based enzyme mimics possess formidable application potential because they combine the considerable catalytic activity of natural enzymes with low cost, high stability, a simple synthesis process, and controllable activity of nanomaterials. [1,2]Plentiful enzyme mimics with excellent catalytic capacity have been developed since Fe 3 O 4 was reported to have peroxidase-like properties by Yan's group. [3,4]6] Therefore, the design of ideal enzyme mimics with outstanding catalytic activity and selectivity requires attention. [7]The catalytic activity of enzyme mimics can be developed by controlling typical nanoscale factors (size, morphology), surface modification and valence, using external stimuli, etc. [4,8] Among them, light stimulation is a promising approach for addressing energy and environmental challenges based on the preponderance of environmental friendliness, efficiency, and sustainability. [9]Much progress has been made in light-responsive oxidase mimics over the past few decades, such as NiO, [10] CMP-PQx, [11] BP QDs, [12] etc. [13,14] However, the amorphous features or absence of long-range order structures can lead to unsatisfactory photocatalytic efficiency.Recent research has shown that the catalytic activities of covalent organic frameworks (COFs) possessing a π-π array framework, high crystallinity, tunable bandgap, and attractive optoelectronic performance can be adjusted by light irradiation. [1,15]It is envisaged that photoactive and metal-free COFs can provide a replaceable, environmentally friendly, inexpensive, and prospective tactic for the development of oxidase mimicking. [16]s a new type of organic porous crystal material, COFs are composed of light elements and connected by covalent bonds between the organic building units. [15,17]The combination of applicable building blocks or linkages into COFs can endow COFs with photoactive peculiarity, which would give a possibility DOI: 10.1002/sstr.202200321Covalent organic frameworks (COFs) are widely used in photocatalysis due to their periodic π-π arrays, high crystallinity, and adjustable bandgap.Herein, a new strategy for integrating polyphenyl building blocks in COFs is presented to improve the photocatalytic efficiency.To implement this strategy, a series of COFs with different numbers of phenyl groups are successfully designed and synthesized.By varying the number of phenyl units in the precursor, the COFs exhibit different bandgaps, band-edge positions, carrier mobilities, and interfacial transfer resistances.The corresponding characterization reveals that the photocatalytic capacity of COFs increases with the number of phenyls in the basic structural unit.Further, under visible light irradiation, the COFs prepared from 1,3,5-tris [4-amino(1,1-biphenyl-4-yl)] benzene (TABB) and 1,4-benzenedicarboxaldehyde (BDB) (named TABB-BDB COF) exhibit superior light-responsive oxidase-mimicking characteristic, which can catalyze the oxidation of 2,2 0azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS).Based on the aforementioned characteristics, TABB-BDB COF is designed as a robust colorimetric probe for the inexpensive, highly sensitive, and rapid detection of uric acid (UA) with a linear range of 5-160 mg L À1 .This study not only demonstrates COFs-based light-response oxidase mimicking for efficient UA detection but also provides an intelligent tactic for boosting the photocatalytic competence of COFs.
to design COF-based metal-free photoresponse oxidase mimicking. [16]However, some of the reported COFs faced a non-negligible drawback of relatively feeble photocatalytic activity, and relevant studies of COF-based oxidase mimics have been scarce until now. [18]To overcome the disadvantage of feeble oxidase-mimicking activity of COFs caused by their inferior photocatalytic activity, numerous methods have been explored.This includes the introduction of special structures (donoracceptor, [1,19] triazine ring, [20] porphyrin, [21] etc. [22][23][24] ) or building heterojunctions (Z-scheme, [25] Type-II Scheme, [26] S-scheme [27] ) to accelerate the charge separation efficiency and heighten the stability of the photocatalytic capability, which could significantly change the intrinsic characteristics of COFs (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) positions, energy bandgap, etc.) to improve photocatalytic performance.However, the higher costs and requirements for the synthesis of COFs need to be considered.For COFs without special functional groups or no postmodification, it is particularly important to effectively improve their photocatalytic activity.It is well known that the ultraviolet absorption of molecules increases with the degree of conjugation.Bhadra et al. proved that the visible light absorption range of COFs is significantly greater than that of the precursors due to the dependence of the intensity of light absorption and absorption range on the spatial overlap degree of π-orbitals and the length of the π-conjugated system. [28,29]herefore, increasing the number of benzene rings in a single cell of COFs to aggrandize the skeleton-conjugated structures could obtain COFs with improved photoresponse oxidasemimicking activity that originally did not contain special functional groups.
Research shows that as the ultimate product of purine metabolism, uric acid (UA) plays an irreplaceable role in maintaining normal physiological activities in vivo because of its significant antioxidant activity. [30]Abundant means have been used for the diagnosis of UA, such as liquid chromatography, [31] electrochemical method, [32] colorimetric method, [33] etc. [34,35] Among these methods, colorimetric detection offers costeffectiveness, straightforward operation, celerity inspection, and routine analysis. [36,37]However, there are no reports on the efficient detection of UA using COF-based mimicking enzymes.
In this study, a COF-based light-responsive oxidase mimicking was synthesized by 1,3,5-tris [4-amino(1,1-biphenyl-4-yl)] benzene (TABB) and 1,4-Benzenedicarboxaldehyd (BDB) (named TABB-BDB COF) (Figure 1).The prepared TABB-BDB COF has a narrower bandgap, higher carrier mobility, and smaller barriers of electrons transfer, which are advantageous for the improvement of light capture capability and accelerated charge separation and transportation to efficiently enhance photocatalytic activity.The TABB-BDB COF with eminent photoresponse oxidase-mimicking activity could efficiently catalyze chromogenic substrates 2,2 0 -azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS).Utilizing these advantages, for the first time, a photoactivated COF-based light-responsive oxidase mimicking was used for the high sensitivity and selectivity detection of UA, and its potential application in complex biological samples was further explored.

Synthesis and Characterization of TABB-BDB COF
The TABB-BDB COF was synthesized by the solvothermal reaction of TABB and BDB with a mixed solvent of mesitylene/1,4dioxane/HAc (Figure 2a).The Fourier-transform infrared (FT-IR) spectrum of TABB-BDB COF (Figure 2b) shows a C═N bond characteristic peak at 1621 cm À1 .The X-ray photoelectron spectroscopy (XPS) of TABB-BDB COF (Figure S1, Supporting Information) showed peaks at 399.5 eV in the N 1s spectrum and at 284.6 eV in the C 1s spectrum that belonged to the N and C elements in the ─C═N bond. [2]After testing a variety of solvent compositions, low-crystallinity TABB-BDB COF was prepared, and the crystallinity was evaluated by powder X-ray diffraction (PXRD) measurements (Figure 2c and S2, Supporting Information).The PXRD pattern exhibits a medium-intensity diffraction peak at 1.9°and other weak peaks at 3.9°.Probably because of the agglomeration effect in this solvent composition, only an irregular morphology formed by the aggregation of many small spherical structures could be observed with scanning electron microscopy (SEM) (Figure 2d).The Brunauer-Emmett-Teller (BET) surface area of TABB-BDB COF was estimated to be 689.82m 2 g À1 (Figure S3, Supporting Information).Furthermore, the thermal stability of TABB-BDB COF was studied by thermogravimetric analysis (Figure S4, Supporting Information), and the results demonstrated that TABB-BDB COF was thermally stable up to 180 °C.

Optical Properties
To explore the effect of different degrees of conjunction on photocatalytic activity, a series of 2D COFs (TAPB-OMe COF, TABB-OMe COF, TAPB-BDB COF, TAPB-BTA COF, TAPB-TFPB COF, and TABB-BTA COF) containing different numbers of phenyls in single cells were prepared and characterized (Figure 2e and S5-S8, Supporting Information).The optical absorption properties of COFs were determined by ultraviolet/ visible diffuse reflection spectroscopy.As shown in Figure 3a, COFs with more phenyls (TABB-BDB COF, TABB-OMe COF,   TABB-BTA COF, and TAPB-TFPB COF) show an obvious redshift in optical absorption, which can be attributed to a higher degree of conjugation resulting from delocalization along as well as across the plane in the extended frameworks. [20]The bathochromic shift of the adsorption boundary represents a wider adsorption range and stronger adsorption ability, which can improve the visible light-driven photocatalytic efficiency. [16]he color of these solid powders changed from light to dark (inset of Figure 3a), indicating that there were significant differences in the bandgap electronic energies of these COFs.The bandgap energy (E g ) values were estimated by Tauc plots through calculating the intercept of the tangents of (Ahv) 2 versus photon energy.The E g were 2.59, 2.50, 2.63, 2.56, 2.97, 2.85, and 2.87 eV for TAPB-OMe COF, TABB-OMe COF, TAPB-BDB COF, TABB-BDB COF, TAPB-BTA COF, TAPB-TFPB COF, and TABB-BTA COF, respectively.It can be observed that the bandgap of COFs with more benzene rings was narrower under the same substituents (Figure 3b-d).The photocatalytic performance of semiconductors depends on the regulation of the bandgap.Generally, a narrow bandgap can facilitate charge carrier generation and transport. [16]The conduction band (CB) potential of the COFs was further characterized by the Mott-Schottky curve (Figure S9-S11, Supporting Information).Obviously, all the aforementioned COFs show the characteristics of n-type semiconductor structures.Thus, their CB positions approached the corresponding measured flat-band potential. [38]Therefore, the CB potentials of TAPB-OMe COF, TABB-OMe COF, TAPB-BDB COF, TABB-BDB COF, TAPB-BTA COF, TAPB-TFPB COF, and TABB-BTA COF were À1.08, À1.11, À0.90, À1.07, À0.70, À0.78, and À0.71 V (vs.normal hydrogen electrode, NHE), respectively; further, according to the equation E CB = E VB -E g , their valence band (VB) potentials were 1.51, 1.39, 1.73, 1.49, 2.27, 2.07, and 2.16 V (vs.NHE), respectively.The corresponding band structure alignments of the COFs are shown schematically in Figure 3e.Generally, the more the negative CB, the higher the energy of the excited electrons carried, possessing a stronger reducing ability.The strategy of enlarging the conjugation structures by increasing the number of benzene rings in COF structural units not only leads to an outstanding light-trapping capacity of COFs but also makes the CB more negative, allowing electrons to be transferred to dissolved oxygen more easily, facilitating the production of reactive oxygen species (ROS) to participate in subsequent photocatalytic reactions.
Generally, the photocatalytic ability was affected by the efficiency of generation, migration, and separation of photoinduced carriers.The photocurrent transient response tests demonstrated that the photocurrent density of COFs with more benzene rings was higher than it with less (Figure 4a-c).The result suggests that under the same conditions, the COFs with larger conjugated structures were more likely to be excited by light to generate carriers.In addition, the electrons transfer barrier of COFs was further measured by electrochemical impedance spectroscopy (EIS).As shown in Figure 4d-f, the COFs prepared by the precursor of more benzene rings exhibit a smaller semicircular diameter of the Nyquist plot that represented a smaller interfacial charge transfer resistance, [19] which contributes valid charge carrier migration to reinforce photocatalytic activity.Furthermore, the cyclic voltammetry (CV) study of COFs (Figure S12, Supporting Information) indicated that the redox peak of COFs with more phenyl had a greater response to current.The smaller difference in redox potentials (Table S1, Supporting Information) indicated that the barriers of the COFs with more benzene rings were smaller and the electrons transfer rate was faster. [39]This is consistent with the photocurrent transient response and EIS results.The photoluminescence (PL) emission spectrum (Figure S13, Supporting Information) at an excitation wavelength of 375 nm manifested that the COFs with more phenyl showed a weaker peak intensity, indicating the reduced recombination rate of photogenerated electrons and holes upon introduction of more phenyl structures into the structural unit of COFs. [40]These results clearly suggest that the introduction of more benzene rings in the 2D COFs skeleton could enhance the lightharvesting ability and carrier mobility, and reduce the barrier of photogenerated electrons transfer, resulting in superior photocatalytic performance.

Investigation of Light-Responsive Oxidase-Mimicking Activity
To investigate the light-responsive oxidase-mimicking capability of the COFs, ABTS was used as the chromogenic substrate.Under visible light irradiation (460 nm, 28 W blue LED lamp), these COFs immediately oxidized ABTS to produce blue-green products with characteristic absorption peaks at 418 and 730 nm, respectively (Figure S14a, Supporting Information).Obviously, the COFs composed of more phenyl groups exhibited a stronger catalytic ability to generate oxABTS under visible light irradiation (Figure S14b, Supporting Information).Moreover, the concentration of oxABTS and photocatalytic time were in accordance with the first-order kinetic equation, and the reaction rates are listed in Table S2, Supporting Information.Under the same substituents, the reaction rate of COFs with more phenyl was higher than that of the rest, and the difference in the reaction rate was between 1.5 times and 5 times, especially for TABB-BDB COF with low crystallinity, which was 4 times that of TAPB-BDB COF.These results indicate that the photocatalytic activity of the COFs can be effectively improved by introducing more benzene ring structures into the basic repeating unit.Surprisingly, TABB-BDB COF exhibited an extremely high photocatalytic performance compared to other COFs with identical numbers of phenyl groups, which may be due to its smaller interfacial transfer resistance to render more carriers migrating to the surface to participate in the reaction.It is reasonable to predict that the photocatalytic activity of TABB-BDB COF will be further improved when it possesses higher crystallinity.It is undeniable that these COFs have oxidase-like properties under visible light irradiation.Compared with the COFs with fewer benzene rings, highly conjugated COFs with more benzene rings exhibit stronger oxidation performance.To gauge the photocontrolled catalyst capacity of the COFs, their photocatalyst properties were tested under continuous light and shading conditions.As shown in Figure 5a and S15, Supporting Information, with the continuing on and off states of visible light irradiation, these COFs showed a staircase-like behavior, manifesting its light-control catalytic capacity.
Additionally, the effects of pH, light source, and dosage were studied (Figure S16, Supporting Information).The lower pH of the PBS buffer could result in more ABTS being catalyzed by TABB-BDB COF under the same conditions.However, considering the operational safety of subsequent UA detection and the convenience of future test kit design, pH = 5 was selected as the optimal condition.The optimized light source and dosage experiments showed that the blue light of 460 nm and the amount of 5 mg were the best photocatalytic conditions.Moreover, the apparent steady-state kinetic parameter for TABB-BDB COF was determined.The typical Michaelis-Menten curve was acquired for TABB-BDB COF by altering the ABTS concentration (Figure 5b).As shown in Figure 5c, the apparent values of K m and v max were calculated as 6.54 Â 10 À2 mM and 3.75 Â À7 M s À1 , respectively.Table S3, Supporting Information, shows a comparison of the apparent kinetic parameters with some published literatures.Obviously, with ABTS as the substrate, the value of K m of TABB-BDB COF was significantly lower than that of natural horseradish peroxidase (HRP, K m = 0.53 mM), and even comparable to those of noble metal-doped nanozymes, illustrating that the oxidase mimicking of TABB-BDB COF has a strong affinity toward ABTS.After several catalytic cycles, the photocatalytic activity was retained (Figure 5d), indicating that TABB-BDB COF exhibited superior catalytic stability.

Photocatalytic Mechanism of TABB-BDB COF
To probe the mechanism of the photocatalytic activity of TABB-BDB COF, the influence of ROS on the photocatalytic oxidation of ABTS was investigated.Filled with N 2 , isopropanol (IPA), L-histidine (L-his), catalase (CAT), and ethylenediaminetetraacetic acid (EDTA) were used as chemical scavengers to track superoxide radicals (O 2 •À ), hydroxyl radicals (•OH), singlet oxygen ( 1 O 2 ), H 2 O 2 , and photogenerated holes (h þ ), respectively. [1,2]s shown in Figure 5e, bubbling with N 2 and EDTA scavengers demonstrated apparent negative effects on the photocatalytic activity.On the contrary, the effects of IPA, L-his, and CAT can be neglected.Furthermore, electron paramagnetic resonance (EPR) test results of the reaction system demonstrated the presence of O 2 •À (Figure 5f ).This manifests that O 2 •À and h þ are important active species for the photocatalytic oxidation process.Attributed to the narrower bandgap of TABB-BDB COF, it was easier for photogenerated electrons to transfer from HOMO to LUMO under the stimulation of visible light, where h þ remained in HOMO.Then, the electrons are captured by the dissolved oxygen in the system to form O 2 •À .Overall, O 2 •À and h þ acted as active species to oxidize ABTS (Figure S17, Supporting Information).

Visual Detection of UA
In summary, visible light can stimulate TABB-BDB COF to catalyze ABTS to produce blue-green radicals that react with antioxidative substances.As a common antioxidant, UA is widely present in food and biological samples and is an essential factor for maintaining normal human physiological functions. [30]Based on the antioxidant properties of UA and the oxidative properties of oxABTS, the blue-green color of oxABTS could be discolored by UA causing a conspicuous decrease in absorbance at the characteristic peak of 730 nm (Figure S18, Supporting Information).Thus, a colorimetric UA detection system, including TABB-BDB COF, ABTS, and visible illumination, was successfully constructed.As depicted in Figure 6a, the characteristic absorbance of oxABTS at 730 nm decreased gradually with increasing UA concentrations ranging from 5 to 160 mg L À1 (29.74-951.76μM), accompanied by fading of the color of the solution (inset of Figure 6a).In addition, a favorable linear manner subsisted between the absorbance intensity and UA concentrations from 5 to 160 mg L À1 with R 2 = 0.9943 (Figure 6b), and the limit of detection was 3.57 AE 0.067 mg L À1 (21.24 μM).Compared with other studies, this work has a wider detection range (Table 1), indicating that the sensor has a stronger potential for practical UA detection.More importantly, a few possible obstructions in human serum, including some familiar amino acids (L-arginine (Arg), L-threonine (Thr), L-Asparagine (Asn), L-Glycine (Gly), L-leucine (Leu)) and common inorganic ions (Ca 2þ , Mg 2þ , K þ , Na þ , NH 4 þ , HCO 3 À , Cl À ) had negligible impact on the detection of UA using this method.Only several reductive substances including L-cysteine (L-Cys), ascorbic acid (AA), glutathione (GSH), and SO 3 2À had a minor influence (Figure S19, Supporting Information).However, the concentration of these reducing substances in serum is much lower than that of UA, showing that TABB-BDB COF could be applied as a colorimetric sensing platform for UA detection without any significant interference.
Three serum samples were analyzed to explore the practicability of the UA sensors in complicated biological environments.The UA concentration measured by our sensor was in accordance with clinical data, showing that the sensor can effectively detect UA quantitatively (Table 2).To further evaluate the accuracy of this sensor, the recovery research was carried out, and the results showed good recovery with a range of 99.61-99.98%(relative standard deviations (RSD), <4%) (Table 3).The above results indicated that the UA sensor proposed in this study has excellent anti-interference ability and high selectivity in the detection of complex biological environments.

Conclusion
In summary, we successfully demonstrated a practical strategy to improve the photocatalytic performance of COFs by varying the number of phenyl units in the precursor.For the first time, a COF-based (TABB-BDB COF) oxidase mimicking was constructed for the detection of UA in real biological samples.This method has several advantages, such as high sensitivity, excellent anti-interference ability, convenience, and wide detection range (5-160 mg L À1 ), which can meet the needs of clinical UA detection.
The high catalytic activity is due to the improved optical absorption boundary, narrowed bandgap, and low interfacial transfer resistance of photogenerated electrons as a result of polyphenyl building block integration in COFs, which greatly enhances the light capture ability and carrier mobility.This work provides an effective solution for strengthening the photocatalytic activity of COFs with nonspecial structures and also affords practical and effective methods for the colorimetric detection of UA.With further development of the structural and functional design of COFs, a plethora of COFs-based artificial enzymes can be constructed for diverse applications.

Experimental Section
A mixture of 1,4-Benzenedicarboxaldehyd (BDB, 44 mg, 0.24 mmol) and 1,3,5-tris [4-amino(1,1-biphenyl-4-yl)] benzene (TABB, 92 mg, 0.16 mmol) was taken in a pyrex tube; then, 3 mL of mixed solvent of 1, 4-dioxane and 1,3,5-Trimethylbenzene (1:1, v/v) was added.The mixture was sonicated for 2 mins, followed by the addition of 17.45 M acetic acid (0.2 mL), which was degassed three times.After flame sealing, the mixture was heated at 120 °C for 72 h to yield a tan solid, which was isolated by centrifugation and washed with methanol until the supernatant became colorless.The resulting powder was immersed in anhydrous THF for 24 h (with a Soxhlet extractor) and dried at 50 °C and put in a vacuum for 12 h to afford a tan powder.Other COFs including TAPB-OMe COF, TABB-OMe COF, TAPB-BDB COF, TAPB-BTA COF, TAPB-TFPB COF, and TABB-BTA COF were also synthesized (see detailed synthetic schemes in the Supporting Information).
The measurements were conducted on CHI660E electrochemical analyzer (Shanghai, China) and CS310M electrochemical analyzer (Wuhan, China) in a standard three-electrode system, using a platinum foil as the counter electrode and Ag/AgCl (saturated KCl) as the reference electrode.The working electrodes were prepared as follows: 10 mg of photocatalyst powder was dispersed in 0.5 mL of methanol with 300 μL of Nafion solution (5%) by sonication for 15 min, and then the obtained slurry was dip coated on the surface of F-doped tin oxide (FTO) glass substrate and glassy carbon electrode.The electrolyte was 0.1 M Na 2 SO 4 and mixed solution of KCl (0.5 M), K 3 [Fe (CN) 6 ] (0.005 M), and K 4 [Fe (CN) 6 ] (0.005 M).
UA detection with the TABB-BDB COF was performed as follows: the TABB-BDB COF (5 mg) and ABTS (3 mL, 1 mM prepared with PBS (pH = 5.0)) mixture was irradiated for 15 min by visible light to yield oxABTS.The supernatant was obtained after centrifuging at 10 000 rpm for 5 min.Then, 20 μL of UA of different concentrations was added to 400 μL of supernatant.Finally, the absorption spectra of oxABTS were recorded for quantitative detection of UA.
Statistical Analysis: The experimental results were expressed as mean AE standard deviation (SD).The sample size for each statistical analysis was 3.

Figure 2 .
Figure 2. Synthesis and characterization of the TABB-BDB COF.a) Schematic synthesis of the TABB-BDB COF; b) FTIR spectra of the TABB-BDB COF; c) PXRD patterns of the TABB-BDB COF; d) SEM images of the TABB-BDB COF; e) schematic synthesis of COFs.

Figure 5 .
Figure 5.The properties of light-responsive oxidase-mimicking and photocatalytic mechanisms.a) Staircase-like behavior of the catalytic capacity of TABB-BDB COFs with the successive on and off states of the light source, labeled by blue and red arrows, respectively.b) Steady-state kinetic assay for TABB-BDB COF by varying ABTS concentrations.c) LineweaverÀBurk plot of the catalytic capacity of TABB-BDB COF with ABTS as the substrate.d) Reusability of photocatalytic activity of TABB-BDB COF for ABTS oxidation.e) Effect of radical scavengers ((0.1, 0.2 and 0.5) mg mL À1 or mM) on the light-responsive oxidase-like activity of TABB-BDB COF.f ) EPR spectra of the DMPO-TABB-BDB COF system before and after illumination using a LED lamp (λ = 460 nm, 28 W) for 3 min in DMSO.

Table 1 .
Comparison of detection methods for UA.