Natural enzymes have a broad range of bioanalytical and biomedical applications^[1,2]. However, intrinsic shortcomings, i.e., ease of denaturation, laborious preparation, and high cost, have limited the widespread applicability of enzymes^[3]. To circumvent these issues, considerable attention has been paid to the exploration of artificial enzymes. Nanomaterials that mimic enzyme properties, i.e., nanozymes, have recently garnered interest^[4-7]. In the past decade, various nanozymes have been developed. Among these nanozymes, carbon-based nanozymes, including graphene, fullerenes, carbon dots, graphene quantum dots, carbon nanospheres, and carbon nanotubes, have received considerable attention due to their unique physiochemical properties, low cost, and high stability^[8-10]. Carbide-derived carbons (CDCs) have attracted interest due to their large surface areas, electric properties, and high energy densities^[11,12]. Atomic-level control of the carbon structure of CDCs can be derived from three-dimensional structured metal carbides. M_n+1AX_n phases are a family of inherently ternary carbides and nitrides with hexagonal crystal structures, which are excellent precursors of two-dimensional materials synthesized through selective extraction. Here, M is an early transition metal, A is typically a group 13 or 14 element, and X represents C and/or N, with n = 1, 2, or 3. To date, CDCs, including porous carbon and carbon/sulfur nanolaminates, have been prepared through the electrochemical (EC) etching of MAX phase carbides^[13,14]. However, the exfoliation of MAX phase carbides typically employs high concentration hydrogen fluoride (HF)^[15], a mixture of hydrochloric acid (HCl) and lithium fluoride (LiF)^[16], or high temperatures^[17,18], which can be safety hazards and dangerous to the environment. The synthesis of CDCs with bienzymatic activities via environmentally conscious reagents and mild reaction conditions is rarely reported.

Here, CDCs were synthesized via a one-step EC method directly from MAX precursors using an ammonium bifluoride (NH₄HF₂) electrolyte at ambient conditions. As-synthesized CDCs showed efficient bienzymatic activities similar to that of peroxidase and superoxide dismutase (SOD). We studied the dependence of the enzyme-like activity of CDCs on different electrolytes and electrolysis times of the Ti₃AlC₂ MAX. We discussed the electrochemical exfoliation mechanism of the CDCs and tested the practicality of the CDC nanozymes via cholesterol detection.

Bulk Ti₃AlC₂ (MAX) was obtained from Beijing Jinhezhi Materials Co., Ltd. NH₄HF₂, NaHF₂, NH₄F, acetic acid, sodium acetate, hydrogen peroxide (H₂O₂), sodium hydroxide (NaOH), HCl, riboflavin, methionine, nitrotetrazolium blue chloride (NBT), and 3,3',5,5'-Tetramethylbenzidine (TMB) were obtained from Sigma-Aldrich Chemical Co. (Milwaukee, WI). 3-(N-morpholino)propanesulfonic acid (MOPS) sodium salt (98%), peroxidase from horseradish (HRP), cholesterol oxidase (ChOx), cholesterol, tetracycline hydrochloride, and europium (III) chloride (EuCl₃·6 H₂O) were purchased from Shanghai Macklin Biochemical Co. Ltd., China. All reagents were analytical grade and used without further purification. Ultrapure water (18.2 MΩ/cm) was obtained from a MilliQ ultrapure system (Qingdao, China).

UV-vis absorbance spectra were measured on a Mapada UV 6300 spectrophotometer (Shanghai, China). Generated O₂ was measured using a JPB portable oxygen-sensitive electrode (Hangzhou Qiwei Instrument Power Equipment Research Institute, China). Atom force microscopy (AFM) was carried out on a SPI3800N microscope (Seiko Instruments Inc.) operating in tapping mode. Powder X-ray diffraction (XRD) patterns were measured by a Bruker D2 PHASER diffractometer using Cu-Kα radiation (λ = 0.1542 nm). The morphology of as-prepared CDCs was characterized via transmission electron microscopy (TEM, JEOL JEM-2100F, Japan) with an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra were measured on a Nicolet 5700 FTIR spectrometer (Thermo Electron Scientific Instruments Corp., USA). X-ray photoelectron spectroscopy (XPS) was carried out using a surface analysis system with monochromatic Al-Kα (1486.6 eV) radiation (Kratos AXIS Ultra^DLD, Manchester, UK).

In a typical experiment, bulk MAX was applied as the anode (Supplementary Figure S1, available in www.besjournal.com) and a Pt electrode was used as the cathode. A constant 3 V DC potential was applied to the system with varying times and electrolytes (NH₄HF₂, NaHF₂, NH₄F) at room temperature. The suspension was collected by centrifuging at 10,000 rpm for 10 min, and the precipitate was freeze-dried without post-washing. The suspension was then dialyzed in a 1,000 Da dialysis bag for 6 h and stored at room temperature for further characterization and application.

Figure S1.  The installation of positive electrode.

The enzyme kinetics of CDC samples were monitored at 652 nm at 40 °C after 10 min incubation. Experiments were carried out using 200 μL of CDCs in 50 mmol/L NaAc buffer (pH 5.0) containing different concentrations of H₂O₂ and TMB as the substrates. The apparent kinetic parameters were obtained via the Michaelis-Menten equation:

where V stands for the initial velocity, V_max represents the maximal reaction velocity, [S] is the substrate concentration, and K_m stands for the Michaelis constant. K_m and V_max were obtained by the Lineweaver-Burk plot method.

The SOD behavior of CDCs were measured via NBT inhibition. Mixtures containing 20 μmol/L riboflavin, 0.013 mol/L methionine, 75 μmol/L NBT, and different volumes of CDCs were prepared in 50 mmol/L NaAc buffer at pH 7.0 and 25 °C. Upon UV illumination for 10 min, reduced NBT was detected at 560 nm. For control experiments, the reaction mixture was kept in the dark before the absorbance measurement. Detailed information is presented in the Supporting Information.

To confirm SOD-like catalytic activity, europium (III) chloride hexahydrate, MOPS sodium salt, tetracycline hydrochloride, and H₂O₂ were used. The weakly fluorescent Eu³⁺-tetracycline complex (EuTc) was used as a fluorescent probe for H₂O₂ detection. Various volumes of CDC-NH₄HF₂-16h, ranging from 10 to 150 μL, were added to a 3 mL EuTc solution. A dissolved oxygen meter was used to monitor the concentration of O₂ generated during the catalytic process. Different volumes of CDC-NH₄HF₂-16h were added to a 10 mL 50 mmol/L NaAc buffer solution (pH 5).

To investigate H₂O₂ catalyzed by CDC-NH₄HF₂-16h, the following solutions were used:

A) MOPS buffer solution (10 mmol/L): MOPS sodium salt was dissolved in 800 mL of MilliQ water, and the pH was set to 6.9. A volumetric flask was filled with 1 L with MilliQ water, and the solution was homogenized.

B) EuCl₃ solution (6.3 mmol/L of Eu³⁺): 115.3 mg of EuCl₃·6 H₂O was dissolved in 50 mL of solution (A).

C) Tetracycline hydrochloride solution (2.1 mmol/L): 50.5 mg of tetracycline hydrochloride was dissolved in 50 mL of solution (A).

D) EuTc solution: 5 mL of solution (B) and (C) were mixed and with solution (A) for a final volume of 100 mL.

10 μL ChOx (0.5 mg/mL) and 200 μL of different cholesterol concentrations were added to NaAc buffer (800 μL, 0.5 mmol/L, pH 7.0) and incubated at 37 ℃ for 120 min. Then, 500 μL of CDC and 200 μL of TMB (5 mmol/L) were added to the reaction mixture, followed by incubation at 40 °C for 5 min. The maximum absorbance of TMB^•+ was measured at 652 nm.

Optimization calculations were performed using B97X-D/Def2-TZVP^[19,20]. Molecular surface evaluation and cation size measurements were carried out via the Multiwfn program (version 3.6) ^[21] using DFT-ωB97X-D/def2-TZVP.

The EC exfoliation of MAX was conducted using bulk Ti₃AlC₂ MAX and a Pt electrode as the anode and cathode, at ambient conditions, respectively (Figure 1). An aqueous solution of 0.5 mol/L NH₄HF₂ was used as the electrolyte, which contained the necessary cations for interplanar intercalation as well as fluoro-containing groups for Al layer etching in the MAX exfoliation. This process avoids the utilization of concentrated HF. To confirm the role of the electrolyte in the EC exfoliation process, two other inorganic salts, i.e., ammonium fluoride (NH₄F) and sodium acid fluoride (NaHF₂), which possess either the same cation or anion as NH₄HF₂, respectively, were applied as electrolytes for comparison.

Figure 1.  Schematic illustration of the exfoliation process of Ti₃AlC₂ with NH₄HF₂ as the electrolyte using the one-step EC method.

The influence of a constant 3V potential during the EC process at different time points (0, 1, 2, 4, 5, 6, 8, 10, 16, 20, 22, and 24 h) on the preparation of CDCs was investigated. The color of the NH₄HF₂ electrolyte changed from colorless (Supplementary Figure S2A available in www.besjournal.com) to muddy after 1 h (Supplementary Figure S2B). The color then became yellow (Supplementary Figure S2C) after 2 h, and brown (Supplementary Figure S2D) after 4 h. Finally, the color of the mixture became dark brown (Supplementary Figure S2E) after 5 h, and then black after 6 h (Supplementary Figure S2F), indicating the exfoliation of Ti₃AlC₂ MAX to Ti₃C₂T_x MXene and CDCs. The color of the suspension became more brown until 16 h, after which the color faded when the electrolysis time was extended to 20 h (Supplementary Figure S2G). Gas bubbles were observed surrounding the anode throughout the entire experiment. We note that reaction times of 6 and 16 h were the two critical time points during the electrolysis process. The obtained products from these two time points, denoted as CDCs-NH₄HF₂-6h and CDCs-NH₄HF₂-16h, respectively, were collected for further characterization. When NH₄F or NaHF₂ were used as electrolytes at the same concentration as NH₄HF (0.5 mol/L), the electrolyte solution also changed color over time. However, the color changes were not as obvious as that of NH₄HF₂, and only a gray color was observed for NH₄F, even after 24 h. This phenomenon demonstrated the superior efficiency of NH₄HF₂ for the exfoliation of MAX to MXene. Three obtained products at the 6 h time point, termed CDCs-NH₄HF₂-6h, CDCs-NH₄F-6h, and CDCs-NaHF₂-6h, were selected for further characterization.

Figure S2.  Photographs of the reaction mixtures from the EC generation process at different time periods: (A) 0 h, (B) 1 h, (C) 2 h, (D) 4 h, (E) 5 h, and (F) 6 h, respectively; (G) The photographs of the supernatant upon different electrolytic time.

XRD patterns of CDCs-NH₄HF₂-6h, CDCs-NaHF₂-6h, and CDCs-NH₄F-6h are shown in Supplementary Figure S3 (available in www.besjournal.com). The (002) reflection of Ti₃C₂T_x at ca. 8.84° from CDCs-NH₄HF₂-6h was more obvious compared with CDCs-NaHF₂-6h, indicating that NH₄HF₂ had a greater MAX exfoliation than that of NaHF₂. However, the characteristic MXene peak was faintly detected for NH₄F, indicating that HF⁻ is more favorable for the EC exfoliation of MAX to MXene. XPS spectra of CDCs-NH₄HF₂-6h, CDCs-NaHF₂-6h, and CDCs-NH₄F-6h are shown in Supplementary Figure S4, available in www.besjournal.com. Ti 2p and O 1s peak positions of the samples are shown in Supplementary Table S1, available in www.besjournal.com. The atomic ratios of Ti from the XPS survey spectra were 8.75%, 1.59%, and 0.71% in CDCs-NH₄HF₂-6h, CDCs-NaHF₂-6h, and CDCs-NH₄F-6h, respectively. These results were consistent with the XRD patterns. Na⁺ and NH₄⁺ cations could be inserted into the interspace of Ti₃C₂T_x nanosheets, thus enlarging interplanar spacing during the etching process (Supplementary Figure S5, available in www.besjournal.com). The diameters of Na⁺ and NH₄⁺ were the farthest spacings between all surface points. The cation surfaces were defined as isosurfaces with an electron density of ρ = 0.002 a.u.^[22] Since the radius of NH₄⁺ (1.8685 Å) is much larger than that of Na⁺ (0.934 Å), the geometries of these two cations were calculated via theoretical optimization using Gaussian 16 (version B01)^[23]. A more effective intercalation effect from NH₄HF₂ compared with NaHF₂ is expected for the Ti₃C₂T_x MXene nanosheets^[24]. The better etching by NH₄HF₂ and NaHF₂ compared with NH₄F was due to volatile H₂ production from HF₂⁻ at the anode during the EC process. Thus, NH₄HF₂ was confirmed to be the optimal electrolyte for the efficient exfoliation of MAX into CDCs through EC oxidative cleavage.

Figure S3.  XRD patterns of (a) CDCs-NH₄HF2-6h, (b) CDCs-NaHF2-6h, (c) CDCs-NH₄F-6h, respectively.

Figure S4.  The survey XPS spectra (A, D, and G), the narrow scan spectra of Ti 2p (B, E, and H) and O 1s (C, F, and I) of the CDCs-NH₄HF2-6h, CDCs-NaHF2-6h, and CDCs-NH₄F-6h, respectively.

Figure S5.  The cation radius of NH⁺₄ and Na⁺.

To further investigate the electrochemical exfoliation process, precipitates prepared in 0.5 mol/L NH₄HF₂, NaHF_2, and NH₄F after 6 h of EC oxidation (termed CDCsP-NH₄HF₂-6h, CDCsP-NaHF₂-6h, and CDCsP-NH₄F-6h, respectively) were collected for XRD characterization after centrifugation and freeze-drying without post-washing. As shown in Figure 2, the XRD patterns of CDCs-NH₄HF₂-6h and CDCs-NH₄F-6h showed three main byproducts, i.e., (NH₄)₃AlF₆, NH₄AlF₄, and AlF₃∙3H₂O. Additionally, Na₃AlF₆, NaAlF₄, and AlF₃∙3H₂O were the main byproducts of CDCs-NaHF₂-6h. The XRD patterns of CDCs-NH₄HF₂-6h and CDCs-NaHF₂-6h (Supplementary Figure S3) showed a (002) peak of Ti₃C₂T_x in the electrolyte, with the exfoliation mechanism of this process as：

Figure 2.  XRD patterns of (A) CDCs-NH₄HF₂-6h, (B) CDCs-NaHF₂-6h, and (C) CDCs-NH₄F-6h.

NH₄/NaHF₂ →NH⁺₄/Na + HF⁻₂

HF⁻₂ → HF + F⁻

Ti₃AlC₂ + HF + NH⁺₄/Na → (NH₄)_a/NaAlF_b + AlF₃ + H₂↑ + Ti₃C₂

AlF₃ + cH₂O → AlF₃∙cH₂O

Ti₃C₂ + NH₄/NaHF₂ + H₂O → Ti₃C₂Fx(OH)_yX_z (MXene)

${\text{T}}{{\text{i}}_{\text{3}}}{{\text{C}}_{\text{2}}}{{\text{F}}_{\text{x}}}{{\text{(OH)}}_{\text{y}}}{{\text{(N}}{{\text{H}}_{\text{4}}}{\text{)}}_{\text{z}}}\xrightarrow{{{\text{EC}}}}{\text{C}}$

where a, b, c, x, y, and z represent different numbers, while Ti₃C₂F_x(OH)_yX_z is the accurate chemical formula of Ti₃C₂T_x.

Metallic M‒A bonds are relatively weak compared with M‒X bonds; thus, “A” layers in MAX phases are removed. Based on these results, we conclude that the EC etching of Ti₃AlC₂ MAX into Ti₃C₂T_x and CDCs occurs in two steps. First, HF₂⁻ is ionized to HF and F⁻. The HF attacks the Al layer, which is successfully etched, and –F functional groups as well as –OH and –O are formed on the Ti₃C₂ MXene due to the dissolved oxygen in the aqueous medium. Second, the outside layer of the MXene (Ti₃C₂F_x(OH)_yX_z) is further etched, resulting in the formation of amorphous carbon with –Cl, –OH, and –O terminal groups via the simultaneous removal of both M and A atoms. More efficient etching agents and longer electrolysis times should result in higher CDC concentrations. TEM and HRTEM characterization was carried out to confirm the morphology and crystallinity of CDCs-NH₄HF₂-6h and CDCs-NH₄HF₂-16h.

Many small CDCs were uniformly observed for the two samples (Figure 3A and 3B). The average diameter of CDCs-NH₄HF₂-6h and CDCs-NH₄HF₂-16h was calculated to be 230 ± 30 and 37 ± 5 nm, from counting 100 particles, respectively (Figure 3A and 3B insets). Additionally, an average in-plane lattice spacing of 0.34 nm was observed for the crystalline structure of CDCs-NH₄HF₂-6h, which indicated the (002) facet as carbon (Figure 3C). An interplanar space of 0.23 nm was also observed, which resulted from the (103) crystal face of the Ti₃C₂T_x MXene (Figure 3C)^[25]. Less lattice fringe, corresponding to a lower crystallinity, was obtained for CDCs-NH₄HF₂-16h, indicative of the generation of amorphous CDCs (Figure 3D) upon extended electrochemical oxidation. The HRTEM results were consistent with the XRD data. There was also a broad peak around 2θ = 20 to 30° in the XRD pattern of CDCs-NH₄HF₂-6h, suggesting the appearance of a (002) facet of carbon with a low degree of graphitization (Supplementary Figure S3, red line)^[26]. This graphitic peak from CDCs-NH₄HF₂-16h was much weaker and broader than that of CDCs-NH₄HF₂-6h, suggesting that CDCs-NH₄HF₂-16h was more disordered due to the further electrochemical oxidation of carbon^[27] (Figure 3E). Elemental mapping characterization showed that C, F, and O were homogenously dispersed (Figure 3F). AFM measurements (Figure 3G) and corresponding height analyses (Figure 3H) revealed that nanosheets were well dispersed in the aqueous suspension, with topographic heights ranging from 2.7 to 4.5 nm, and that the majority of CDCs-NH₄HF₂-16h were 3-5-layer structures. FTIR spectra indicated that both CDCs-NH₄HF₂-6h and CDCs-NH₄HF₂-16h exhibited identical stretching vibrations, including –OH at 3,430 cm⁻¹, C=O at 1,630 cm⁻¹, O–H at around 1,380 cm⁻¹, C–F at around 1,050 cm⁻¹, and Ti-O at around 713 cm^−1[28] (Figure 3I).

Figure 3.  (A, B) TEM and (C, D) HRTEM images of (A, C) CDCs-NH₄HF₂-6h and (B, D) CDCs-NH₄HF₂-16h. (E) XRD pattern of CDCs-NH₄HF₂-16h; (F) TEM and corresponding elemental mapping images of C, O, and F in CDCs-NH₄HF₂-16h. (G) AFM topography of CDCs-NH₄HF₂-16h. (H) Height profiles from the white line section in the AFM image. (I) FTIR spectra of CDCs-NH₄HF₂-6h and CDCs-NH₄HF₂-16h. Insets (A, B) are particle size distributions obtained from TEM.

To further confirm the effects of electrolysis time, samples at two other reaction time points, i.e., CDCs-NH₄HF₂-10h and CDCs-NH₄HF₂-20h, were characterized via XPS. C, Ti, O, and F were observed in the survey spectra for all four samples (Supplementary Figure S6A, C, E, and G, available in www.besjournal.com). C 1s, Ti 2p, and O 1s peak positions of the samples are shown in Table S2. Peaks at 281.75 (282) eV, 284.4 (284.55, 284.41, or 284.3) eV, and 284.85 (285, 285, or 284.99) eV correspond to Ti–C, C–C, and C–O, respectively (Supplementary Figure S6B, D, F, and H). The intensity of the Ti–C peak gradually decreased, demonstrating that MAX was transformed into MXene, and then oxidized to C. For the Ti 2p region, peaks at 466 (466, 465.79, or 465.8) eV and 460.25 (460.3, 460.15, or 460.03) eV were attributed to Ti–O, peaks at 464.9 (465) eV and 459.15 (459.4) eV were attributed to C-Ti-Fx, and the peak at 462.9 eV corresponds to Ti^3+[25] (Supplementary Figure S7A, C, E, and G, available in www.besjournal.com). The intensity of the C-Ti-Fx and Ti³⁺ peaks gradually decreased when the electrolysis time was extended to 16 h, consistent with the O 1s spectra. Detailed information about the O-containing groups, including Ti-O (530.03, 530.9, 530.5, or 531 eV), C-Ti-O_x (531 or 531.6 eV), C-Ti-(OH)_x (531.78 or 532.1 eV), and H₂O (532.6, 532.8, 532.13, or 532.1 eV) were derived from the O 1s spectra. The bond length of C-Ti-(OH)_x decreased, further proving the conversion of Ti₃C₂T_x MXene (Supplementary Figure S7B, D, F, and H). The atomic ratios of C from XPS survey spectra were 32.6%, 41.96%, 63.08%, and 32.48% from CDCs-NH₄HF₂-6h, CDCs-NH₄HF₂-10h, CDCs-NH₄HF₂-16h, and CDCs-NH₄HF₂-20h, respectively. The carbon content reached a maximum value at an electrolysis time of 16 h, indicating that Ti₃C₂T_x was consumed and transformed to carbon. The carbon content in CDCs-NH₄HF₂-20h decreased with ongoing time, which was attributed to further carbon oxidation in the presence of dissolved oxygen from the aqueous medium^[29]. These results are consistent with the color of CDCs fading after 16 h (Supplementary Figure S2G). Based on these results, CDCs-NH₄HF₂-16h was determined as the optimal CDC for this carbon-nanozyme study.

Figure S6.  The survey XPS spectra (A, C, E, and G) and the narrow scan spectra of C 1 s (B, D, E, and H) of the CDCs-NH₄HF2-6h, CDCs-NH₄HF2-10h, CDCs-NH₄HF2-16h, and CDCs-NH4HF2-20h, respectively.

Figure S7.  The survey XPS spectra of Ti 2p (A, C, E, and G) and O 1s (B, D, F, and H) of the CDCs-NH₄HF₂-6h, CDCs-NH₄HF₂-10h, CDCs-NH₄HF₂-16h, and CDCs-NH₄HF₂-20h, respectively.

A schematic illustration of CDCs-NH₄HF₂-16h-catalyzed oxidation of TMB by H₂O₂ is shown in Figure 4A. The intrinsic peroxidase mimetic activity of CDCs-NH₄HF₂-16h was studied using TMB as a chromogenic substrate for H₂O₂. In the absence of CDCs-NH₄HF₂-16h, the solution of TMB and H₂O₂ was colorless. This phenomenon was also observed for either TMB or H₂O₂ alone with CDCs-NH₄HF₂-16h. However, in the presence of CDCs-NH₄HF₂-16h, TMB reacts with H₂O₂ to generate TMB^•+, which showed a characteristic absorption peak at 652 nm (Supplementary Figure S8, available in www.besjournal.com). An obvious TMB^•+ absorbance peak was observed for the system containing CDCs-NH₄HF₂-16h, H₂O₂, and TMB (Figure 4B). Photographs of the different systems are shown in the inset of Figure 4B, which confirmed the intrinsic peroxidase-like activity of CDCs-NH₄HF₂-16h. The pH and temperature tolerance of the peroxidase-like behavior of CDCs-NH₄HF₂-16h and HRP were compared at various temperatures (5–95 °C) and different pHs (1−12). The optimal pH of CDCs-NH₄HF₂-16h was 5.0, which was similar to the value (pH 4.0) for HRP (Figure 4C). As shown in Figure 4D, the optimal temperature for HRP was 35 °C. The catalytic performance decayed at either increasing or decreasing temperatures. In contrast, the catalytic activity of CDCs-NH₄HF₂-16h increased with increasing temperature (even up to 95 °C), indicating its thermal stability.

Figure 4.  (A) Schematic illustration of CDCs-NH₄HF₂-16h catalyzing the oxidation of TMB in the presence of H₂O₂. (B) Time-dependent absorbance changes at 652 nm of TMB in different reaction systems in 50 mM NaAc (pH = 5) at room temperature. The inset in (B) shows the corresponding photographs: (a) a mixture of TMB and H₂O₂ in the absence of CDCs, (b) a mixture of TMB and CDCs in the absence of H₂O₂, (c) a mixture of CDCs and H₂O₂ in the absence of TMB, and (d) a mixture of TMB, H₂O_2, and CDCs. The (C) pH dependence and (D) temperature dependence of the peroxidase-like activity of nanosheets and HRP in 50 mmol/L NaAc. The H₂O₂ and TMB concentration were 50 mmol/L and 1 mg/mL, respectively.

To study the SOD-like activity of CDCs, e.g., CDCs-NH₄HF₂-16h, the decrease of superoxide anion radicals (O₂^•-) and the increase of H₂O₂ and O₂ were measured. SOD strongly enhances the disproportionation of O₂^•- into H₂O₂ and O₂ and thus inhibits the reduction of NBT via the mediation of O₂^•- (Figure 5A). As shown in Figure 5B, no fluorescence signal was obtained without UV irradiation in the presence of riboflavin, methionine, and NBT (curve a). Upon UV irradiation, the fluorescence signal sharply increased, indicating a high level of O₂^•- (curve b)^[4]. After addition of CDCs-NH₄HF₂-16h, the fluorescence signal decreased dramatically, which confirmed that CDCs-NH₄HF₂-16h possessed SOD-like activity and could remove O₂^•- (curve c). As seen in Figure 5C, the fluorescence of the reaction system decreased with increasing CDCs-NH₄HF₂-16h concentration, which indicated that the scavenging efficiency of O₂^•- was directly proportional to the amount of CDCs-NH₄HF₂-16h.

Figure 5.  (A) Schematic illustration of the SOD-like activity of CDCs-NH₄HF₂-16h. (B) Scavenging efficiency of superoxide radicals in different conditions: (a) blank control without ultraviolet radiation, (b) absence, and (c) presence of CDCs-NH₄HF₂-16h after ultraviolet radiation. Inset shows corresponding photographs. (C) Scavenging efficiency of superoxide radicals with different concentrations of CDCs-NH₄HF₂-16h. (D) Photographs of the scavenging efficiency of superoxide radicals (a) before and (b) after ultraviolet radiation. The photographs show different volumes of CDCs ranging from 0 to 450 μL (50 μL increments), from left to right.

The Eu³⁺-tetracycline complex (EuTc) is not fluorescent; however, binding with H₂O₂ forms a fluorescent europium tetracycline hydrogen peroxide complex (EuTc-HP), which results in a strong enhancement in fluorescence intensity^[30]. The fluorescence intensity change of EuTc at 620 nm (with λ_ex = 405 nm) was monitored against H₂O₂ concentration. The fluorescence signal increased with increasing amounts of H₂O₂ (Supplementary Figure S9A available in www.besjournal.com).

Figure S9.  The fluorescence spectra of EuTc solutions in the presence of different volumes of (A) H₂O₂ and (B) CDCs-NH₄HF₂-16h in MOPS buffers, respectively. (C) The fluorescence spectra at different times of reaction with 20 μL CDCs-NH₄HF₂-16h added. (D) The fluorescence intensity changes with the addition of different volumes of CDCs-NH₄HF₂-16h of 10, 15, 20, 40, 60, 80, 100, 150 μL. (E) The O₂ emission catalyzed by CDCs-NH₄HF₂-16h at different volumes range from 1 mL–8 mL (1 mL as a unit) in solution of 10 mL 50 mmol/L NaAc (pH = 5).

The fluorescence intensity of the EuTc system increased with increasing CDCs-NH₄HF₂-16h and reaction time, which confirmed the activity of CDCs-NH₄HF₂-16h as a SOD mimic for generating H₂O₂ (Supplementary Figure S9B–D). Furthermore, an oxygen-sensitive electrode was used to track the concentration of O₂ generated during the reaction. Increasing the amount of CDCs-NH₄HF₂-16h resulted in an increased formation of O₂ (Supplementary Figure S9E). These results indicated the formation of H₂O₂ and O₂ in the presence of CDCs-NH₄HF₂-16h, which demonstrated the SOD-mimic activity of the CDCs. To study the kinetics of the peroxidase-mimic activity of the CDCs, the steady-state kinetics of observed color variations were analyzed by changing TMB and H₂O₂ concentrations (Supplementary Figure S10 available in www.besjournal.com). Maximum initial reaction rates (V_max) were calculated from the double reciprocal of the Michaelis-Menten equation defined in the experimental section (Supplementary Table S3, Supplementary Figure S11 available in www.besjournal.com). The V_max of CDCs was enhanced with increasing electrolysis times, from 6 to 16 h (Supplementary Table S3). However, the V_max decreased with extended reaction times. The UV-vis absorption spectra of different electrolysis times, ranging from 6−24 h, confirmed that CDCs-NH₄HF₂-16h exhibited the highest enzymatic activity (Supplementary Figure S12 available in www.besjournal.com). Supplementary Figure S13 (available in www.besjournal.com) shows corresponding photographs of CDCs with different electrolysis times, ranging from 6–24 h, after the addition of TMB and H₂O₂ for 30 min. The peroxidase-like activity increased with increasing CDC content. These results confirmed that the peroxidase-like catalytic activity of CDCs was a result of the carbon material.

Figure S10.  Time-dependent absorbance changes of TMB^•+, generated upon the oxidation of TMB with variable concentrations of TMB (A, C, E, and G) and H₂O₂ (B, D, F, and H) of CDCs-NH₄HF₂-6h, CDCs-NH₄HF₂-12h, CDCs-NH₄HF₂-16h, and CDCs-NH₄HF₂-20h, respectively.

Figure S11.  Steady-state kinetic assay and catalytic mechanism of CDCs-NH₄HF₂-6h, CDCs-NH₄HF₂-12h, CDCs-NH₄HF₂-16h and CDCs-NH₄HF₂-20h. The double reciprocal plots of activity of CDCs-NH₄HF₂-6h, CDCs-NH₄HF₂-12h, CDCs-NH₄HF₂-16h and CDCs-NH₄HF₂-20h with the concentration of one substrate (H₂O₂ or TMB) fixed and the other varied. The error bars represent the standard deviation for three measurements.

Figure S12.  The UV absorption spectra of different electrolysis time from 6–24 h (2 h as a unit) after adding TMB (1 mmol/L) and H₂O₂ (50 mmol/L) for 30 min.

Figure S13.  The photograph of nanosheets with different electrolysis time from 6–24 h (2 h as a unit) after adding TMB (1 mmol/L) and H₂O₂ (50 mmol/L) for 30 min.

Due to the intrinsic peroxidase-like catalytic activity of CDCs, cholesterol detection was carried out by combining the mimetic enzyme with cholesterol oxidase. As shown in Figure 6A, the absorbance intensity at 652 nm was proportional to the cholesterol concentration, ranging from 7.79 μmol/L to 116.96 μmol/L. A linear regression equation of I = 0.45 × C_cholesterol + 0.01 (unit of C is μmol/L) with R² = 0.9928 is shown in Figure 6B. The detection limit was determined to be 3 μmol/L (S N⁻¹ ≥ 3, Supplementary Figure S14 available in www.besjournal.com), which is superior or comparable to other reported nanozyme based cholesterol detection methods. These results demonstrate the broad applicability of these CDCs in both chemical and biochemical applications.

Figure 6.  (A) UV-vis spectra of CDCs-NH₄HF₂-16h+TMB+ChOx with different concentrations of cholesterol (Inset: images of colored products for different concentrations of cholesterol). (B) Calibration curve of cholesterol detection (Inset: linear calibration plot of cholesterol detection).

In summary, CDCs were synthesized by the one-step electrochemical exfoliation of bulk Ti₃AlC₂ in NH₄HF₂. The CDCs possessed both peroxidase- and SOD-like activity, which increased with increasing carbon content. Thus, we confirmed that the enzymatic activity resulted from the CDC material. The optimal reaction time for the peroxidase-like catalytic activity of CDCs was determined to be 16 h of the EC process. Additionally, the synthetic mechanism of CDCs was demonstrated, and possible reaction equations between Ti₃AlC₂ and bifluoride were proposed. Moreover, the applicability of the nanozymes was demonstrated by the sensitive detection of cholesterol. This synthesis strategy eliminates tedious centrifugation intercalation and delamination steps and is environmentally conscious due to the avoidance of high concentration HF, high temperatures, and halogen gases. This study paves the way for designing two-dimensional material-based nanocatalysts for nanoenzyme and other applications.

We appreciate Dr. Tingting Zhang and Hao Wan for her help in some experiments, who works in the Biology Experimental Teaching Center of Qingdao University.

The authors declare no conflict of interest.

Figure S8.  UV-vis adsorption spectra of different reaction systems in a 50 mmol/L NaAc (pH = 5) at room temperature. (a) Photography of the mixture of TMB and H₂O₂ in the absence of CDCs. (b) Photography of the mixture of TMB and CDCs in the absence of H₂O₂. (c) Photography of the mixture of CDCs and H₂O₂ in the absence of TMB. (d) Photography of the mixture of TMB, H₂O₂ and CDCs. The H₂O₂ and TMB concentration were 50 mmol/L, 1 mg/mL, respectively.

Figure S14.  UV-vis spectra of CDCs-NH₄HF₂-16h+TMB in the absence (the black line) and presence of 3 μmol/L cholesterol (the red line).