Nano-diamond, FeCl₃, H₂SO₄ and HNO₃, 3,3′,5,5′-tetramethylbenzidine (TMB), 1,2-diaminobenzene (OPD), 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS), L-γ-glutamyl-L-cysteinylglycine (GSH), thiourea were purchased from Macklin reagent Co.,Ltd. (Shanghai, China), Human serum albumin (HAS), Bovine serum albumin (BSA), ascorbic acid (AA), glycine (Gly), L-lysine (Lys), L-serine (Ser), D/L-cysteine(D-Cys, L-Cys), glucose, sodium chloride, calcium chloride, cupric sulfate, ferric chloride, potassium chloride, zinc sulfate, magnesium sulfate, tartaric acid, choline chloride were purchased from Sinopharm Chemical Reagent Co. Ltd. NaN₃ was purchased fromTianjin Fuchen Chemical Reagent Factory. H₂O₂ was purchased from Xilong Scientific Co., Ltd. sodium chloride, calcium chloride, cupric sulfate, ferric chloride, potassium chloride, zinc sulfate, magnesium sulfate, tartaric acid, choline chloride was purchased fromSinopharm ChemicalReagent Co., Ltd. Sodium acetate buffer (0.1 M, pH = 4.0) were freshly prepared before use. All solutions prepared for purified water are derived from Wahaha purified water (China).

X-ray powder diffraction (XRD) was collected on a PANalytical B.V. Empyrean powder diffractometer, in which data were collected from 5° to 80° at a scan rate of 10°/min. Scanning electron microscopy (SEM) images were captured on a Hitachi FE-SEM S-4800 instrument with an acceleration voltage of 3 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images was carried on a JEM-2100F. Spherical aberration corrected Transmission Electron Microscope (ACTEM) was carried on a JEM-ARM300F. UV-vis spectra were measured on UV-2700 Spectrophotometer (Shimadzu, Japan).

The purchased NDs (0.2 g) were dispersed in a mixture (20 ml) of H₂SO₄ and HNO₃ with a volume ratio of 3:1 and heated to 90°C for 2 h. After the heating, the suspensions were cooled down to room temperature and neutralized by the adding of NaOH solution. After dialysis against water, the above suspensions were dispersed in HCl solution with a final concentration of 0.1 M and heated at 90°C for another 2 h. O-NDs with abundant specific oxygenated groups were obtained after proper dialysis and freeze-drying. Fe in the as-prepared O-NDs were added into 20 ml distilled water. The obtained solution was heated at 90°C for 1 h under stirring, then the temperature was raised to 100°C for complete water evaporation. The resulting mixture was put into an alumina crucible with a cover, and heated to 700°C with the ramping rate of 20°C/min, and kept at that temperature for another 4 h. This process was conducted with 30 ml/min N₂ flow at atmospheric pressure. Nano-diamond modified with single-atom Fe is denoted as Fe-NDs.

Fe-NDs with peroxidase-like activity can directly oxidize substrates in the presence of H₂O₂. The whole reaction system consists of 44 μl of 0.18 mg ml⁻¹ Fe-NDs, 50 μl H₂O₂ (1 mM) and 100 μl of 4 mM, TMB were added to 806 μl of 0.1 M HAC-NaAC buffer solution (pH = 4.0). Finally, the mixed system was reacted at 55°C for 15 min and the UV absorption was measured at 652 nm.

The detective process of oxidase-like activity is similar to that of peroxidase mimics, except that no H₂O₂ is added. To assess the oxidase activity of Fe-NDs, Typically, 100 μl of 0.18 mg ml⁻¹ Fe-NDs and 100 μl of 4 mM TMB were added to 800 μl of 0.1 M HAC-NaAC buffer solution (pH = 4.0). Finally, the mixed system was reacted at 40°C for 20 min and the UV absorption was measured at 652 nm.

The steady-state kinetics experiment of peroxide-like properties was carried out with Fe-NDs suspension (44 μl, 0.18 mg ml⁻¹), H₂O₂ (50 μl, 1 mM), and TMB (100 μl, 4 mM). The mixed system was reacted at 55°C for 10 min before being used directly for UV-vis absorbance measurements. Similarly, kinetic analysis of the oxidase-like properties was carried out with Fe-NDs suspension (100 μl, 0.18 mg ml⁻¹) by varying the concentration of TMB. The mixed system was reacted at 40°C for 10 min before being used directly for UV-vis absorbance measurements.

A typical experimental operation is to determine the reaction rate changes with different concentrations of TMB under optimal conditions. The kinetic parameters are determined by the following equations: 1/ν = K _m/V _max⋅(1/[S] + 1/K _m), where ν is the initial velocity, V _max is the maximal reaction velocity, and [S] is the concentration of the substrate. K _m is the Michaelis–Menten constant, which indicates the enzyme affinity for the substrate.

The working solution for H₂O₂ determination as follows: 44 μl Fe-NDs suspension (0.18 mg ml⁻¹), 100 μl TMB (4 mM) and different concentrations of H₂O₂ (1–60 μM, 50 μl) were added into 806 μl of 0.1 M HAc-NaAc buffer solution (pH = 4.0). Then, the absorbance of the mixed solution at 652 nm was measured after incubation for 15 min at 55°C temperature.

The whole reaction system for GSH determination consisted of 100 μl TMB (4 mM), 500 μl of 0.1 M HAc-NaAc buffer solution (pH = 4.0) and 100 μl of Fe-NDs suspension (0.18 mg ml⁻¹). After 20 min of reaction at 40°C temperature, the 300 μl GSH solution was added, and then the absorbance is recorded on UV-vis spectra at 625 nm after 15 min. GSH concentration is calculated by measuring the change in absorbance (ΔA) of the reaction system after adding GSH. The blank group was given the same amount of ultrapure water instead of GSH.

The glutathione tablets and injections were produced by Chongqing Yaoyou Pharmaceutical Co., LTD. The tablets and injections were prepared with a certain concentration of GSH test solution, followed by the addition of different concentrations of GSH standard solution to 5, 10, and 15 μM, so that the concentrations were detected in a linear range. The detection method of GSH was performed in accordance with 2.6.

Typically, Fe-NDs were well synthesized with the oxidized NDs asprecursors via coordination with iron, and the composite was further annealing at 800°C. The presence of diamond in the sample is confirmed by the XRD pattern Supplementary Figure S1 (JCPDS No. 75-0219) (Chen T. M. et al., 2017). The size of the synthesized nanoparticles were irregular lamellar structure (Supplementary Figure S2). As demonstrated in Figures 1A–D, C, O, and Fe elements coexisted on the surface of NDs. These results suggest that Fe may exist as a single atom. In order to further verify the existence of Fe single atom, Fe atoms in Fe-NDs samples were directly observed by using spherical aberration corrected Transmission Electron Microscope (ACTEM). As shown in Figures 1E,F, oxidized nano-diamond has lattice structure and abundant single Fe atoms are clearly observed as bright dots, indicating that Fe single atoms were successfully single dispersed on NDs.

Several typical substrates {i.e., TMB, OPD (1,2-diaminobenzene), and ABTS [2,2′-azinobis-(3-ethylbenzthiazoline-6-sulphonate)]} were used to investigate the peroxidase activity of the synthesized Fe-NDs. Supplementary Figure S3 demonstrates that in the presence of H₂O₂, the three colorless substrates (TMB, OPD, and ABTS) were oxidized and turned blue, yellow, and green, respectively, indicating the presence of peroxidase activity in Fe-NDs. TMB was selected as the substrate to further confirm the results. Figure 2 demonstrates that in the absence of H₂O₂, TMB alone could not form chromogenic products. However, when Fe-NDs, TMB, and H₂O₂ were present in the reaction system, an increasing absorption was observedat 625 nm, which indicated that Fe-NDs acted as peroxidase-like mimics in the catalytic reaction between TMB and H₂O₂.

Similar to the properties of horseradish peroxidase (HRP), the peroxidase-like properties of Fe-NDs depend on pH, temperature, and H₂O₂ concentration. The activity of the material was measured at a pH of 2–9 and a temperature of 25–70°C (Figures 3A–D). Figure 3A demonstrates that the catalytic activity of Fe-NDs was greatly affected by pH, and the highest activity was at a pH of 4, which is similar to the activity of HRP and other reported peroxide-like enzymes (Zhang et al., 2013; Xia et al., 2015; Wang et al., 2017). In addition, Fe-NDs maintained a high catalytic activity in a wide range of temperatures. As demonstrated in Figure 3B, Fe-NDs maintained more than 80% catalytic activity in the temperature range of 35–60°C. The catalytic activity of Fe-NDs increased with an increase in the amount of material and concentration of H₂O₂ (Figure 3D). Eventually, the optimal experimental conditions were determined to be a pH of 4.0, a temperature of 55°C, and a material concentration of 8 μg ml⁻¹.

Oxidases oxidize the peroxidase substrate TMB to produce blue-colored oxTMB. Oxidase activity was found in studying the peroxide-like activity of Fe-NDs, which was confirmed by TMB turned blue directly in the absence of H₂O₂ (Figure 2). To study the oxidase activity of NDs before and after the addition of Fe, the change in absorbance at 652 nm was monitored using a UV-vis spectrometer. As demonstrated in Supplementary Figure S4, Fe-NDs significantly catalyzed TMB to produce a blue-colored reaction without H₂O₂. TMB is oxidized to oxTMB by oxygen in the presence of Fe-NDs, with a concomitant visible colorimetric change that can be observed by the naked eye. However, untreated NDs did not exhibit oxidase activity, indicating that the mixed acid oxidation process of NDs and the introduction of Fe played a key role in the oxidase activity of NDs. Fe-NDs can also oxidise different color-developing substrates (ABTS, which turns green, and OPD, which turns yellow) under certain conditions without adding H₂O₂, as demonstrated in Supplementary Figure S5. Therefore, the results indicated that Fe-NDs exhibited significant oxidase-like catalytic activity and directly catalysed the substrate.

Subsequently, the effects of different reaction conditions on the oxidase activity of Fe-NDs were studied. Parameters such as catalyst concentration, pH, temperature, and TMB concentration were investigated (Figures 4A–D). To analyze the influence of pH on the catalytic activity of Fe-NDs, pH ranging from 2 to 7 was used in the colorimetric experiment. The results revealed that the optimal pH for the catalytic activity of Fe-NDs was 4.0. The temperature range of 25–60°C was used to analyze the influence of temperature on the catalytic activity of Fe-NDs. As demonstrated in Figure 4B, the catalytic activity of Fe-NDs remained above 80%, proving that Fe-NDs exhibited catalytic activity over a wide temperature range. Eventually, 40°C was selected as the optimal temperature for follow-up experiments. In addition, we analyzed the influence of catalyst concentration ranging from 1 to 20 μg ml⁻¹ on the activity of Fe-NDs. The results revealed that the catalytic activity of Fe-NDs increased rapidly when the concentration was 1–10 μg ml⁻¹, and the growth rate was slow when the concentration was greater than 10 μg ml⁻¹ until the activity reached the highest at 18 μg ml⁻¹. Therefore, 18 μg ml⁻¹ was determined as the optimal catalyst concentration. At the same time, the catalytic activity of Fe-NDs remained above 80% when the concentration of TMB was higher than 0.4 mM, so 0.4 mM was selected as the optimal substrate concentration. The optimal experimental conditions were as follows: temperature, 40°C; pH, 4.0; material concentration, 18 μg ml⁻¹ and TMB concentration, 0.4 mM.

To evaluate the peroxidase-like catalytic performance of Fe-NDs, the steady-state kinetic parameters were analyzed by changing the concentration of TMB and H₂O₂ in the reaction system. The absorbance of the TMB oxidation product, ε = 39 000 M⁻¹ cm⁻¹ (652 nm), was used to calculate the concentration of the substance corresponding to the absorbance. A typical Michaelis-Menten curve is shown in Figure 5, and the maximum initial velocity (V _max) and Michaelis–Menten constant (K _m) are provided in Supplementary Table S1. It was observed that Fe-NDs exhibited a stronger affinity in terms of H₂O₂ (K _m = 0.87 mM) compared to horseradish peroxidase (HRP) (K _m = 3.7 mM). Furthermore, the K _m value of Fe-NDs was 0.76 mM when TMB was used as the substrate, suggesting that a higher concentration of TMB is required to achieve maximal enzymatic activity for the prepared nanozyme.

To better understand the catalytic mechanism of POD-like nanozyme, we used some free radical trapping agents. Results as shown in Supplementary Figure S6, p-benzoquinone, NaN₃, and thiourea were captured by the superoxide radicals (O₂·⁻), singlet oxygen molecules (¹O₂), and hydroxyl free radicals (OH·), respectively. In the presence of thiourea in the system, the catalytic activity of Fe-NDs is significantly lower than that of the blank. The addition of p-benzoquinone can reduce the catalytic activity by about 10%. The main active substance produced in the TMB oxidation process is OH· and a little O₂ ⁻· is also produced in this process.

To further evaluate the oxidase-like catalytic performance of Fe-NDs, the kinetic experiment was performed by changing the concentration of TMB under optimal experimental conditions. A typical Michaelis–Menten curve is demonstrated in Figure 6, and the maximum initial velocity V _max and K _m calculated are provided in Supplementary Table S2. The values of K _m and V _max were 0.55 mM and 4.01 × 10⁻⁸ M s⁻¹, respectively, when TMB was used as the substrate. Compared with the classic CeO₂ NPs, Fe-NDs had a higher affinity for TMB, which may be attributed to the uniform dispersion of Fe atoms on the diamond surface enhancing its catalytic properties.

Oxygen plays an important role as an electron acceptor in the catalytic activity of oxidases. The reaction solution was pre-treated with nitrogen and oxygen for half an hour to confirm the role of oxygen in catalysis, and Fe-NDs were subsequently added to catalyze TMB under optimal conditions. As demonstrated in Supplementary Figure S7, under the saturation condition of nitrogen, the catalytic activity of Fe-NDs was significantly inhibited and was only 40%. However, under the saturation condition of oxygen, the catalytic activity was significantly increased to 72% compared with that in the air, which proved that oxygen played an important role in the oxidation of TMB. The oxidase activity of Fe-NDs may be attributed to the reactive oxygen species (ROS) produced during the oxidation of TMB. To analyze the influence of different free radicals on the reaction system, we used different concentrations of p-benzoquinone, NaN_3, and thiourea to scavenge thesuperoxide radicals (O₂·⁻), singlet oxygen molecules (¹O₂), and hydroxyl free radicals (OH·), respectively. The results are demonstrated in Supplementary Figure S8, the three different concentrations of trapping agents can inhibit the catalytic activity of Fe-NDs, indicating that the system produces three kinds of ROS, which are O₂·⁻, ¹O_2, and OH· respectively.

H₂O₂ has been associated with cell damage and several diseases (Song et al., 2010; Fu et al., 2014). Therefore, it is important to establish a simple, highly sensitive, rapid technique for the visual detection of H₂O₂. The experimental results of H₂O₂ detection by Fe-NDs colorimetric method are demonstrated in Figures 7A,B, it demonstrated that the absorbance of TMB increases with an increase in H₂O₂ concentration and exhibits a good linear relationship with H₂O₂ concentration (1–60 μM; R² = 0.9989). According to the 3σ rule the detection limit for H₂O₂ was calculated to be 0.3 μM, indicating that the H₂O₂ sensor had a low detection limit. As demonstrated in Figure 7A, the color of the solution deepened with increasing H₂O₂ concentration, indicating that the sensor exhibited excellent visual detection performance. Compared with other sensors based on peroxide-like activity used for detecting H₂O₂, as mentioned in Supplementary Table S3, the colorimetric method used in this study has a higher sensitivity and lower detection limit.

GSH is a typical reducing agent that can directly reduce blue-colored oxTMB to colorless TMB owing to its rich mercapto functional groups (Liu et al., 2013). Differences in absorbance (ΔA) before and after adding GSH exhibited a good linear relationship with the concentration of GSH in the solution within a certain range. Based on changes in the absorbance value before and after (ΔA) detection, the Fe-NDs colorimetric biosensor was established to quantitatively detect GSH. To determine the optimal conditions for GSH detection, we studied the effects of pH, temperature, and material concentration on the catalytic activity of Fe-NDs in the presence of GSH (20 μM) (Supplementary Figure S9). As demonstrated in Supplementary Figure S9A, pH had an impact on the reaction system. The highest relative activity was achieved by deducting blank when pH was 4.0. Therefore, a pH of 4.0 was considered optimal for subsequent experiments. No significant difference was observed in the value of ΔA in the temperature range of 25–50°C (Supplementary Figure S9B). Considering the optimal detection conditions, 40°C was selected as the optimal temperature. Based on the combined results of detection using different pH, temperature, catalyst dosage, and TMB concentration (Supplementary Figure S9D), the optimal conditions for GSH detection were as follows: pH, 4.0; temperature, 40°C; Fe-ND concentration, 18 μg ml⁻¹ and TMB concentration, 0.4 mM.

A simple, fast, and sensitive visual colorimetric sensor for GSH detection can be established based on the properties of oxidases. Figure 8A demonstrates the UV-vis spectrum curve for detecting the absorption value of GSH in the concentration range of 1–25 μM at 652 nm. The corresponding calibration curve in the range of 1–25 μM (R² = 0.9997) is demonstrated in Figure 8B. A good linear relationship was observed between ΔA and GSH concentration, and the equation was as follows: ΔA = 0.01 [GSH] (μM) + 0.027. The detection limit for GSH was 0.072 μM according to the three-sigma (3σ) rule, which indicates that the colorimetric sensor can be used for reliable detection of GSH in real samples. As demonstrated in Figure 8C, the color of the solution gradually became lighter as the concentration of GSH increased, indicating that the modified sensor had adequate visual detection performance. The Fe-ND-based sensor had a lower detection limit than the previously published nanoenzyme-based colorimetric GSH sensors mentioned in Supplementary Table S4, indicating the high sensitivity of the method.

To evaluate the anti-interference performance of the established colorimetric sensor, various co-existing disturbance species were tested under the same conditions, including various metal ions (Na⁺, K⁺, Ca²⁺, Fe³⁺, Mg²⁺, Cu²⁺, Zn²⁺, and Mn²⁺), amino acids [glycine, lysine, L-serine and D/L(+)-cysteine], HAS, BSA, glucose, tartaric acid, choline chloride, and ascorbic acid. As demonstrated in Figure 9, the concentration of these cationic interfering species and amino acids was 100 times (1 mM) that of GSH, and their influence on the absorbance value of the Fe-NDs/TMB system was negligible. Some biological macromolecules such as BSA (1 mg/mL) and HSA (1 mg/mL) had also little influence on the system. The absorbance values (ΔA) of AA and D/L(+)-cysteine were similar to those of the 10 μM GSH solution; however, the difference in concentration of AA (1 mM) and D/L(+)-cysteine (1 mM) and GSH (10 μM) solution was 100 times, and its effect could be ignored. Therefore, the proposed method has a higher selectivity for GSH detection and can be widely used for the rapid quantification of biological and biomedicine samples.

As demonstrated in Supplementary Figure S10, the stability of the oxidase-like activity of Fe-NDs was investigated. The catalytic activity of Fe-NDs remained above 80% at room temperature for 60 days, indicating that the material has good stability.

We used some drug samples to demonstrate the feasibility of this method to detect GSH in a complex environment and the results are provided in Supplementary Table S5. Statistical analysis indicate that the recovery rates of GSH in tablets and injections were in the range of 94.6–101.5% (RSD, 1.5–1.9%) and 97.2–98.5% (RSD, 1.2–2.0%), respectively (Table 1). All these results demonstrate that the proposed method is reliable for practical applications. At the same time, HPLC-UV was used to verify the accuracy of the Fe-NDs colorimetric sensor (National Pharmacopoeia Committee, 2020). The final results showed that the Fe-NDs colorimetric sensor designed in this paper has good accuracy and sensitivity which can be used to detect GSH in actual samples.