1 Introduction

Nanozymes, a class of desirable nanomaterial with intrinsic enzyme-like activity, have become spotlight in the catalysis, energy and environmental protection fields [1,2,3]. Compared with natural enzymes, nanozymes hold the advantages including excellent environmental stability, controllable composition, low-cost and ease of synthesis [4,5,6]. Yan’s group found that Fe3O4 nanoparticles possess excellent peroxidase-like activity in 2007 [7]. Subsequently, various nanomaterials have been reported to exhibit enzymes-like activities, such as noble metals, metal oxides [8,9,10], carbon-based nanomaterials [11], metal–organic frameworks [12, 13], single-atom materials [14, 15]. Among them, noble metal nanomaterials (Au, Pt, Pd, Ag) are favored by researchers due to their outstanding catalytic activity, adjustable composition and morphology [16, 17]. However, the uncontrolled size caused by aggregation and the insufficient adsorption sites for substrate still limit their catalytic activity. The controllable growth of noble metal nanoparticles on nanomaterials is an effective strategy to improve the catalytic performance [18, 19], which not only anchors nanoparticles to reduce aggregation, but also increases the adsorption sites of substrate to promote the catalytic reaction.

Covalent organic frameworks (COFs) are a novel type of organic porous polymer composed of organic units in the form of covalent interactions [20, 21]. Due to their good stability, low density, tunable pore size and large specific surface areas [22, 23], COFs have demonstrated to be outstanding nanocarriers for improving the dispersion and catalytic activity of metal nanoparticles [24,25,26]. Meanwhile, COFs hold unique molecular structure with pore space confinement effect, providing abundant anchor sites for limiting the size of noble metal nanoparticles. The strong interactions between the confined metal nanoparticles and COFs constructed by imine bonds can not only improve the stability of the active metal centers, but also enhance their catalytic activity [27]. Therefore, integrating nanoparticles with COFs provides a new paradigm to develop multifunctional nanozyme with designated performance.

Hydrogen peroxide (H2O2) is an important biomarker in the enzyme-catalyzed biochemical reactions, which plays a vital role in maintaining life activities. The abnormal changes of H2O2 concentration are closely related to the occurrence of different diseases including cardiovascular diseases and cancer [28]. Glutathione (GSH) is one of the most abundant sulfhydryl compounds in cells, which plays an essential role in maintaining physiological functions in human body. GSH can act as an antioxidant and free radical removal agent by binding to free radicals, facilitating the conversion of harmful toxins into harmless substrates and promoting their elimination from the body [29]. Therefore, it is crucial to monitor H2O2 and GSH levels for early diagnosis of disease. Currently, various methods have been proposed for GSH and H2O2 detection, including photoelectrochemistry [30], fluorescence [31], colorimetry [32, 33] and electrochemical biosensor [34]. Among these methods, colorimetric sensing platform possesses the advantages of simplicity, fast response and low-cost [35, 36], which facilitates visual detection by the naked eyes and can be used as a powerful tool for the detection of H2O2 and GSH.

In this work, we prepared a novel Pt–Pd/COFs nanozyme through a simple aldimine condensation reaction and in-situ reduction method. The COFs was synthesized with 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,5-dimethoxyterephaldehyde (DMTP) as monomer, which provided abundant anchor sites for Pt–Pd bimetallic nanoparticles to enhance the catalytic activity and stability. With the synergistic effect of Pt–Pd bimetallic nanoparticles, the prepared multifunctional Pt–Pd/COFs nanozyme held superior peroxidase (POD)-like activity and oxidase (OXD)-like activity, which was used to construct colorimetric sensing platform for H2O2 and GSH detection (Scheme 1). The turn-on mechanism in the 3,3′,5,5′-tetramethylbenzidine (TMB)–H2O2 system was confirmed to the generated ⋅OH triggered TMB oxidation process, which realized the detection of H2O2. In addition, the turn-off mechanism in the O2–TMB system was ascribed to the presence of GSH, leading to the reduction of oxidized-TMB and achieving quantitative detection of GSH. Importantly, the Pt–Pd/COFs nanozyme achieved efficient detection of H2O2 and GSH in complex samples, holding promising applications in disease diagnosis and environmental monitoring.

Scheme 1
scheme 1

a Synthesis of Pt–Pd/COFs nanozyme b Colorimetric sensing platform based on multifunctional Pt–Pd/COFs nanozyme: POD-like activity for H2O2 detection in TMB-H2O2 system; OXD-like activity for GSH detection in O2–TMB system

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2 Experimental

2.1 Chemical Reagents and Instruments

The information of chemical reagents and instrumentations were shown in the supporting information.

2.2 Synthesis of Pt–Pd/COFs Nanozyme

The COFs were synthesized according to the reported literature [37] and the details were shown in the supporting information. The obtained COFs (4.0 mg) were dissolved in 3.0 mL methanol with the addition of 1.0 mL H2PtCl6⋅H2O (6.0 mmol/L) and K2PdCl4 (6.0 mmol/L) to deposit bimetallic precursors into COFs by freeze-drying method [38], and then redispersed in 5 mL MeOH/H2O solution (3:2, V/V) under vigorous stirring. Subsequently, 2.0 mL NaBH4 (0.25 mol/L) was introduced into the above solution and reacted for 24 h. After centrifugation and washed with ethanol and ultrapure water, the products were redispersed in 2 mL ultrapure water for further utilization. Pt/COFs and Pd/COFs were prepared without adding platinum precursor or palladium precursor under the same reaction conditions.

2.3 Kinetic Assay of Pt–Pd/COFs Nanozyme

The steady-state dynamics assay of Pt–Pd/COFs nanozyme was examined with various concentrations of TMB (0.2–0.7 mmol/L) or H2O2 (0.2–0.7 mmol/L). Total volume of final solution was 3.0 mL. The absorbance at 652 nm (A652) was recorded through the kinetic mode of UV–vis spectrometer. The kinetic data were fitted with a typical Michaelis–Menten curve. The Michaelis–Menten equation is as follows:

$$\frac{1}{V}=(\frac{K_{m}}{V_{max}})\times(\frac{1}{[S]})+\frac{1}{V_{max}}$$
(1)

where [S] is the substrate concentration (TMB or H2O2), Vmax is the maximum reaction velocity speed, Km represents the Michaelis constant.

2.4 Evaluation of Enzyme-Like Activities of Pt–Pd/COFs Nanozyme

The catalytic mechanism of multifunctional Pt–Pd/COFs nanozyme was also discussed. Firstly, the 1,4-benzoquinone (PBQ) was selected as trapping agent of O2⋅−. Pt–Pd/COFs (20 μg/mL), TMB (0.3 mmol/L) and PBQ (1 mmol/L) were added into NaAc–HAc buffer, and the final volume of reaction solution was 3.0 mL. Then the A652 of the solution was recorded. Furthermore, the formation of ⋅OH was explored with the assistance of terephthalic acid (PTA), which can react with ⋅OH and generate the product with strongly fluorescence at 425 nm. In which, Pt–Pd/COFs (2.5 µg/mL), TMB (0.3 mmol/L) and PTA (0.5 mmol/L) were added into NaAc-HAc buffer and the total volume of solution was 3.0 mL, and the absorbance values at 425 nm were recorded.

2.5 Colorimetric Detection of H2O2 and GSH

Pt–Pd/COFs nanozyme exhibited catalytic activity towards H2O2–TMB system and O2–TMB system, due to excellent POD-like activity and OXD-like activity. Therefore, a turn-on colorimetric assay based on TMB–H2O2–Pt–Pd/COFs nanozyme system was developed for the detection of H2O2. For H2O2 sensing, 30 µL TMB (0.3 mmol/L), 10 µL Pt–Pd/COFs nanozyme (2.5 µg/mL) and 100 µL H2O2 with various concentrations were added into NaAc-HAc buffer with the final reaction volume of 3.0 mL. After incubation for 5 min, the final reaction system was measured by a UV–Vis spectrometer.

Taking advantage of the reducibility of GSH, a turn-off colorimetric platform based on TMB–O2–Pt–Pd/COFs nanozyme system was constructed for GSH detection. In brief, 50 µL GSH solution with different concentrations, 0.3 mmol/L TMB and 20 µg/mL Pt–Pd/COFs nanozyme were added into NaAc-HAc buffer and incubated for 20 min. Then, A652 of the solution was recorded.

2.6 Colorimetric Detection of H2O2 and GSH in Practical Samples

To evaluate the practical application capability of the Pt–Pd/COFs nanozyme, the recovery experiments in serum sample and environmental water were conducted. The serum was diluted 100-fold with NaAc–HAc buffer and the GSH with different concentrations (20 µmol/L, 30 µmol/L, 40 µmol/L) was added. Similarly, the standard H2O2 with different concentrations (200 µmol/L, 500 µmol/L, 1000 µmol/L) was added into environmental water. Finally, the prepared samples were detected by the colorimetric platform based on Pt–Pd/COFs nanozyme.

3 Results and Discussion

3.1 Characterization of Materials

The COFs were synthesized by Schiff-base condensation reaction with TAPB and DMTP as the reaction monomers, and Pt–Pd/COFs nanozyme was synthesized through in situ reduction method. The size and morphology of COFs and Pt–Pd/COFs were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Fig. 1a and Fig. S1, the prepared COFs exhibited a spherical shape structure with the particle size about 200 nm. Besides, the uniform distribution of Pt–Pd bimetallic nanoparticles could be observed on Pt–Pd/COFs nanozyme, and the anchor of the bimetallic nanoparticles did not affect the structure of COFs (Fig. 1b and Fig. S2). Furthermore, the elemental mapping of Pt–Pd/COFs was also conducted to reveal the element composition and distribution. Figure 1c illustrated the homogeneous dispersion of Pt–Pd bimetallic nanoparticles on COFs, demonstrating that Pt–Pd bimetallic nanoparticles were successfully deposited on COFs. These results revealed the successful formation of Pt–Pd/COFs nanozyme. In addition, Fourier transform infrared spectroscopy (FT-IR) of DMTP, TAPB, COFs and Pt–Pd/COFs were also carried out to verify the composition of chemical bonds. As shown in Fig. 1d, a distinct peak at 1675 cm−1 was observed, which belonged to the tensile vibration of C=O bonds of DMTP, and the characteristic peak at 2759 cm−1 could be associated with the vibration of C–H bond in the –CHO groups. The peaks of TAPB at 3226 cm−1 and 3343 cm−1 were attributed to the stretching bond of N–H. The disappearance of -NH2 (3343 cm−1) and –CHO (1675 cm−1) peaks for the monomers and the appearance of a new peak (1586 cm−1) for imine bonds suggested the successful occurrence of condensation reaction between –NH2 and –CHO groups. Meanwhile, the crystallinity of COFs was further characterized by powder X-ray diffraction (PXRD). As observed in Fig. 1e, a prominent peak at 2.80° and a series of weak peaks (4.78°, 5.60°, 7.44°, 9.71° were observed, corresponding to the (100), (110), (200), (210), (220) crystal planes respectively, which proved the successfully formation of high crystalline COFs [37]. Then the formation of Pt–Pd/COFs nanozyme was investigated by zeta potential. As shown in Fig. 1f, the zeta potential of COFs changed from − 4.03 to − 23.40 mV after modified with Pt–Pd bimetallic nanoparticles. X-ray photoelectron spectroscopy (XPS) patterns of the COFs and Pt–Pd/COFs were presented in Fig. S3. The peaks of C 1s, O 1s and N 1s displayed in the XPS pattern of COFs. After loaded with Pt–Pd bimetallic nanoparticles, the peaks of Pt 4f and Pd 3d could be observed in the XPS pattern of Pt–Pd/COFs. The peaks at 398.60 eV and 400.30 eV were shown in the N 1s spectrum (Fig. 1g), which were associated with C=N and –NH2 [39]. The appearance of C=N bonds confirmed the successful preparation of TAPB-DMTP COFs by Schiff-based condensation reaction. As shown in Fig. 1h, a pair of peaks at 71.3 eV and 74.4 eV could be assigned to Pt 4f7/2 and Pt 4f5/2, indicating that Pt element was mainly zero-valence state in the Pt–Pd/COFs. As shown in Fig. 1i, the peaks at 337.3 eV and 342.6 eV could be ascribed to the 3d3/2 and 3d5/2 orbitals of Pd, respectively. In addition, the peaks at 341.1 eV and 337.3 eV represented the 3d3/2 and 3d5/2 orbitals of PdO [40]. The experimental results demonstrated the successful preparation of Pt–Pd/COFs nanozyme.

Fig. 1
figure 1

TEM images of a COFs and b Pt–Pd/COFs. c EDS element mapping of Pt–Pd/COFs. d FT-IR spectra of COFs, Pt–Pd/COFs, TAPB and DMTP. e XRD pattern of COFs. f Zeta potentials of COFs and Pt–Pd/COFs. High-resolution XPS spectra of g N 1s, h Pt 4f and i Pd 3d

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3.2 Kinetic Assay of Pt–Pd/COFs Nanozyme

To explore the catalytic efficiency of Pt–Pd/COFs nanozyme, the steady-state kinetic was studied with TMB (Fig. 2a,c) and H2O2 (Fig. 2b,d) as substrates, respectively. Based on the typical Michaelis–Menten kinetics Eq. (1), where Vmax means the maximum velocity and Km is the Michaelis–Menten constant, typically, a smaller Km represents stronger affinity between the enzyme and substrate, the Km and Vmax with TMB as substrate were 1.12 mmol/L and 34.15 × 10–8 (mol/L)/s, respectively, and the Km and Vmax for H2O2 were 0.32 mmol/L and 19.27 × 10–8 (mol/L)/s, respectively, indicating that Pt–Pd/COFs nanozyme had high affinity for H2O2. The novel Pt–Pd/COFs nanozyme had high Vmax and low Km values, which was associated with the outstanding catalytic activity of bimetallic nanoparticles and the high loading capacity of COFs.

Fig. 2
figure 2

Michaelis–Menten fitting curves of Pt–Pd/COFs nanozyme with a TMB and b H2O2 as substrates. c, d Lineweaver–Burk model of a and b

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3.3 The Catalytic Mechanism of Pt–Pd/COFs Nanozyme

To check the catalytic activity of Pt–Pd/COFs nanozyme towards the H2O2–TMB system, POD-like activity of COFs, Pt/COFs, Pd/COFs, Pt–Pd/COFs were investigated, respectively. In Fig. 3a, b, the negligible absorption peak was observed in TMB–H2O2 system when no material was added or only COFs were present. In contrast, the reaction system containing Pt–Pd/COFs appeared stronger absorption peak at 652 nm compared with that containing Pt/COFs and Pd/COFs, owing to the synergistic effect of bimetallic nanoparticles. In addition, compared with bare COFs, the absorbance intensity increased by 7.81-fold. POD-like and OXD-like activities of Pt–Pd/COFs nanozyme were demonstrated through TMB oxidation process. In addition, the absorbance of system continuously enhanced with the concentration of Pt–Pd/COFs increasing (Fig. 3c), indicating that Pt–Pd/COFs nanozyme possessed intrinsic POD-like activity. As shown in Fig. 3d, in the reaction systems contained only TMB, H2O2 or Pt–Pd/COFs, no obvious absorption peaks were observed at 652 nm. In the presence of TMB and Pt–Pd/COFs, an absorption peak at 652 nm was obtained. While a stronger absorption peak was observed in the reaction system that existed both TMB, H2O2 and Pt–Pd/COFs. Therefore, the synthesized Pt–Pd/COFs nanozyme had the dual enzyme properties of POD-like and OXD-like activities. Subsequently, o-phenylenediamine (OPD) and 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonate) (ABTS) were also used as colorimetric substrates to further verify the enzymatic activity of Pt–Pd/COFs nanozyme. As shown in Fig. 3e, f, OPD and ABTS were oxidized by Pt–Pd/COFs nanozyme and the absorption peaks appeared at 448 nm and 416 nm respectively, which further evidenced the POD-like and OXD-like activities of Pt–Pd/COFs nanozyme.

Fig. 3
figure 3

a UV–Vis spectra of various reaction solutions and b the corresponding absorption intensity at 652 nm: (1) TMB + H2O2, (2) TMB + H2O2 + COFs, (3) TMB + H2O2 + Pt/COFs, (4) TMB + H2O2 + Pd/COFs and (5) TMB + H2O2 + Pt–Pd/COFs. c Absorption spectra of TMB-H2O2 system with different concentrations of Pt–Pd/COFs nanozyme. d–f The absorption spectra of various reaction systems with various chromogenic substrates (insets: the relevant physical photos) (1) TMB, OPD, or ABTS; (2) TMB, OPD, or ABTS + Pt–Pd/COFs; (3) TMB, OPD, or ABTS + H2O2; (4) Pt–Pd/COFs; (5) H2O2 + Pt–Pd/COFs; (6) TMB, OPD, or ABTS + H2O2 + Pt–Pd/COFs

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To achieve the optimal catalytic performance of the Pt–Pd/COFs nanozyme, experimental conditions including the pH of the reaction system and the incubation time of GSH were investigated. As shown in Fig. S4, pH 4.5 was the optimal value for TMB oxidation. The rate of catalysis of the Pt–Pd/COFs nanozyme gradually increased with reaction time and reached stable at 20 min, so the optimal incubation time was 20 min for GSH detection (Fig. S5).

The catalytic mechanism of POD-like and OXD-like activities of Pt–Pd/COFs nanozyme were also investigated (Fig. 4a). To confirm the POD-like activity of Pt–Pd/COFs nanozyme, the catalytic mechanism was further investigated in the presence of H2O2. Owing to the strong affinity of the Pt–Pd/COFs nanozyme for H2O2 (Km = 0.32 mmol/L), when H2O2 was added into the reaction system, H2O2 was first adsorbed on the surface of Pt–Pd/COFs nanozyme, after which the O–O bonds in the H2O2 broke and decomposed to form ⋅OH. To confirm the presence of ⋅OH, PTA was as the specific fluorescent probe, which could absorb ⋅OH and produce 2-hydroxyterephthalic acid with the high fluorescence intensity at around 425 nm [41]. As shown in Fig. 4b, the fluorescence intensity at 425 nm increased gradually with the increasing time, indicating that Pt–Pd/COFs nanozyme catalyzed H2O2 to produce ⋅OH intermediate continuously. Besides, isopropanol (⋅OH scavenger agent) was used to validate the formation of ⋅OH. The A652 significantly decreased with the addition of isopropanol into the reaction solution, proving the production of ⋅OH (Fig. S6). Furthermore, ultra-low electron paramagnetic resonance spectroscopy (EPR) was also utilized to study the generation of ⋅OH. During the oxidation process of TMB with H2O2, the quadruple peak signal with a typical intensity ratio of 1:2:2:1 was presented (Fig. 4c), confirming the generation of ⋅OH.

Fig.4
figure 4

a Schematic diagram of POD-like and OXD-like activities of Pt–Pd/COFs nanozyme. b Fluorescence spectra of PTA + H2O2 + Pt–Pd/COFs reaction systems under different reaction time conditions. c EPR spectra of ⋅OH trapped by DMPO. d UV–Vis absorption spectra of Pt–Pd/COFs under different atmospheric conditions: N2, air and O2. e UV–Vis spectra of various reaction systems: (1) Pt–Pd/COFs + TMB + PBQ and (2) Pt–Pd/COFs + TMB. f EPR spectra of O2⋅− trapped by DMPO

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To investigate the effect of O2 on oxidase catalytic activity, the TMB oxidation reactions were carried out in nitrogen, air and oxygen-saturated atmosphere, respectively. The absorbance intensity was that A652(O2) > A652(air) > A652(N2) (Fig. 4d). It showed that O2 participated in the oxidation of TMB. Then, p-benzoquinone (PBQ, O2⋅− scavenger) was used to further evaluate the type of reactive oxygen species during the process of TMB oxidation. As shown in Fig. 4e, after the addition of 1 mmol/L PBQ to the Pt–Pd/COFs system, OXD-like activity was significantly inhibited, which showed that O2⋅− was generated during TMB oxidation process. In addition, in the reaction system contained 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and Pt–Pd/COFs nanozyme, the quadruple peak with a distinct signal of 1:1:1:1 was also displayed in the EPR spectrum (Fig. 4f), indicating that Pt–Pd/COFs nanozyme could catalyze O2 to product O2⋅−. Pt–Pd/COFs nanozyme could capture O2 molecules and catalyze the cleavage of O–O bonds into O2⋅− due to its large specific surface area and porous structure [42]. The dispersed Pt–Pd bimetallic nanoparticles provided enough metal active sites to convert O2 to O2⋅−, resulting in the oxidation of TMB. These experimental results proved the Pt–Pd/COFs nanozyme held excellent OXD-like activity.

3.4 Colorimetric Assay of H2O2

Based on the POD-like activity of Pt–Pd/COFs nanozyme, a turn-on colorimetric assay was constructed for the detection of H2O2 (Fig. 5a). As shown in Fig. 5b, c, with the increasing concentration of H2O2, the absorbance intensity at 652 nm increased gradually, and exhibited a good linear relationship in the range of 5–1000 μmol/L. The linear regression equation was y = 0.0627 Logc‒0.0061 (R2 = 0.9977) and the detection limit was 1.14 μmol/L (3б/k). The assay presented in this paper had a lower detection limit than other reported works (Table S1). In addition, we also explored the selectivity of the assay. The interferences including Na+, K+, Ca2+, glucose, Al3+, Zn2+, Mg2+, urea and uric acid showed inconspicuous absorbance compared with H2O2 (Fig. 5d), which confirmed the good selectivity of the colorimetric assay for the detection of H2O2. Moreover, the reproducibility was also verified by five independent parallel experiments, which showed similar absorbance value, implying that the colorimetric assay based on Pt–Pd/COFs nanozyme had good reproducibility for the detection of H2O2 (Fig. 5e).

Fig. 5
figure 5

a The schematic illustration of detection principle for H2O2. b UV–Vis absorption spectra of TMB and Pt–Pd/COFs nanozyme system with various concentrations of H2O2. c A linear curve at A652 with different concentrations of H2O2 (Inset: the color change with increasing concentrations of H2O2). d, e Selectivity and reproducibility experiments for H2O2 sensing

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3.5 Colorimetric Assay of GSH

Based on the OXD-like activity of Pt–Pd/COFs nanozyme, a turn-off colorimetric platform was constructed for GSH detection (Fig. 6a). As shown in Fig. 6b, the Pt–Pd/COFs nanozyme catalyzed the TMB–O2 system to produce a significant absorbance peak, nevertheless the absorbance intensity decreased obviously with the addition of GSH, which was due to the fact that GSH caused the reduction of oxidized-TMB. The A652 gradually decreased with the GSH increasing concentration, and exhibited a favorable linear relationship in the concentration range of 1–40 μmol/L (Fig. 6c). The linear regression equation was y = − 0.0121x + 0.6818 (R2 = 0.9942) and the detection limit for GSH was calculated to be 0.43 μmol/L (3б/k) (Fig. 6d). The Pt–Pd/COFs nanozyme-based colorimetric platform exhibited a lower detection limit compared with other detection methods listed in Table S2. The selectivity of Pt–Pd/COFs nanozyme for GSH detection was investigated (Fig. 6e). Compared with a variety of interfering substrates, including glucose, glycine (Gly), arginine (Arg), K+, Al3+, Na+, histidine (His), urea and uric acid, the presence of GSH caused a noticeable decrease of the absorbance intensity, confirming the desirable selectivity of the colorimetric platform for GSH detection. In addition, five independent parallel experiments showed similar absorbance value, implying that the colorimetric assay based on Pt–Pd/COFs nanozyme had good reproducibility for GSH detection (Fig. S7). The stability of Pt–Pd/COFs nanozyme was further evaluated, which still showed good catalytic performance for H2O2 and GSH detection after 10 days storage, indicating the good stability of Pt–Pd/COFs nanozyme (Fig. S8).

Fig. 6
figure 6

a The schematic illustration of detection principle for GSH. b UV–Vis absorption spectra of various reaction systems: (1) TMB + Pt–Pd/COFs, (2)TMB + Pt–Pd/COFs + GSH and (3) TMB. c UV–Vis absorption spectra of TMB and Pt–Pd/COFs nanozyme system with various concentrations of GSH. d A linear curve at A652 with different concentrations of GSH (inset: the color change with increasing concentrations of GSH). e Selectivity experiments for GSH sensing

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To verify the feasibility of Pt–Pd/COFs nanozyme in biological applications, the recovery measurements were carried out. As shown in Table S3, the recoveries of H2O2 in environmental water were 94.91–108.0%, with the relative standard deviation (RSD) of 2.1–4.3% (n = 3). As shown in Table S4, the recoveries of GSH in human serum were 93.70–102.8%, with the RSD of 1.3–2.0% for GSH (n = 3). The results indicated the promising applications of the Pt–Pd/COFs nanozyme for H2O2 and GSH detection in complex samples.

4 Conclusions

In this study, a multifunctional Pt–Pd/COFs nanozyme simultaneously with superior POD-like activity and OXD-like activity was designed for colorimetric detection of H2O2 and GSH. Compared with monometallic nanozyme, the Pt–Pd/COFs nanozyme exhibited higher catalytic activity due to the bimetallic synergistic effect. Using the multifunctional nanozyme, the colorimetric platform realized sensitive detection of H2O2 and GSH, with the detection limits of 1.14 μmol/L and 0.43 μmol/L, respectively. The Pt–Pd/COFs nanozyme with dual enzyme-like activities possessed bright prospects in various fields including environmental protection and biosensing.