Nanozymes, as an attractive class of biomimetic materials, have advanced catalyst technology by artificially replacing natural enzymes.^1–3 Scrimin and colleagues first coined the term "nanozyme" to illustrate special nanomaterials that exhibit enzyme-like behavior inspired by natural enzymes.⁴ These artificial enzymes are also called synthetic enzymes or "synzymes." However, the profound and pioneering work of Yan et. al the use of ferromagnetic nanoparticles as peroxidase mimics seriously stimulated research and development in this research.⁵ Compared to natural species, nanozymes are easier to synthesize, various activities, more cost-effective, more stable, and reusable. They have widespread potential applications in the biomedical domain.⁶ Likewise, synthetic enzymes are less sensitive to environmental changes such as temperature⁷ and pH,⁸ so that not many changes are observed in their selectivity and reaction rate.^9–13

To date, a significant number of nanomaterials based on metal nanoparticles, metal oxides, metal chalcogenides, metal-organic frameworks, and layered double hydroxides have been investigated for their intrinsic enzyme-like activity.^{9, 12, 14, 15} However, some metal or metal oxide nanoparticles have not performed well with environmental changes. Moreover, the in vivo use of metal based nanozymes is limited mainly due to their cell toxicity.^{16, 17}

Moreover, the family of carbon nanomaterials such as graphene oxide (GO),^{18, 19} carbon nanotubes (CNT)²⁰ and carbon nanospheres (CNS)^{21, 22} have been investigated for their intrinsic enzyme-like activity,^{23, 24} although it has been found that their catalytic efficiency is relatively low or they do not have suitable fluorescence properties.²⁵ However, they have shown remarkable performance in biomedical applications such as sensing and imaging and drug delivery.^{26, 27} Among them, the catalytic potential of carbon quantum dots (CQDs) has enabled their application in biosensors, environmental detection and medical diagnostics.^{28, 29} In addition, their optical properties allow for real-time monitoring and imaging, increasing their application in biomedical and environmental fields. The CQDs are usually synthesized by carbonization of various organic compounds such as citric acid, aniline derivatives, glucose or plant extracts via hydrothermal or microwave thermal methods.^{30, 31} However, the use of recycled carbon source compounds such as polyethylene terephthalate (PET) is also a cheap and available source.³² In this regard, there is significant interest in the surface engineering of quantum dots through the immobilization of metal ions or heteroatoms and decoration of organic molecules, which enhances their catalytic efficiency and selectivity, thereby establishing nanozyme quantum dots as versatile tools for next-generation nanobiotechnologies. However, the challenge of not using metal ions (metal free) remains.³³

Alternatively, the performance of nanozymes is dependent on size, surface modification, and modification, and they exhibit different functions.³⁴ Furthermore, nanozymes often lack a dynamic mechanism for turning on and off the catalytic activity in their synthetic behavior. Their performance is also affected by external factors or changes such as pH,⁸ temperature,⁷ or light exposure.³⁵ However, these nanozymes usually operate in a single functional and continuous state. However, the activities of nanozymes are always in the on state (single enzyme activity), which prevents the performance of other activities for further applications in biological systems. ³⁶ For example, peroxidase is involved in innate immunity and participates in vital physiological processes such as programmed cell death and cell communication.^{37, 38} The use of peroxidase-like nanomaterials such as magnetite nanoparticle (Fe₃O₄) can directly increase the production of various endogenous reactive oxygen species (ROS) in cells and affect biological activity. However, their continuous activity (open activity) produces a large amount of ROS, which leads to damage to cells and tissues and causes numerous diseases such as cardiovascular conditions, neurodegenerative disorders, and kidney disease.^{39, 40} Therefore, it is very important to design and study the dynamic regulation of multi-enzyme activities to prevent oxidative damage.

Fascinatingly, nanozymes with multi-enzyme activities have occupied an interesting topic in the field of science. In a biocatalytic cycle, enzymes are expected to act individually and in concert to maintain the metabolic relationships of the organism.⁴¹ In other words, scenario of cascade activity is formed.⁴² For instance, to regulate cellular energy supply from starch and protection against redox reactions, enzymes such as glucoamylase (GA), glycooxidase (GOD), peroxidase (POD), and other enzymes cooperate for intracellular reactive species (ROS) metabolism characteristics.⁴³ Nanozymes with multi-enzyme activities can mimic the complex systems of natural organisms. This behavior holds promise for biomedical applications. However, the rational design and precise control of complex multi-enzyme functions remains a challenging task.⁴⁴

Recent advances in nanozyme research have revealed that carefully engineered carbon-based nanomaterials can mimic the catalytic behavior of natural peroxidases, with their activity being critically dependent on specific oxygen-containing functional groups.^{23, 45, 46} Through systematic investigations, researchers have identified that carbonyl moieties serve as the primary active sites for peroxidase-like catalysis, while carboxyl groups facilitate substrate binding. Interestingly, hydroxyl groups appear to play an inhibitory role in this process - a finding consistently observed across various carbon allotropes including carbon nanotubes, graphene quantum dots, and related materials.⁴⁵ Therefore, we chose this approach to precisely control the surface chemistry and optimize the distribution of critical functional groups, enhancing the nanozyme's catalytic performance.

In continuation of our research work^{19, 47–49} and considering the potential and attractiveness of CQDs, the QD was synthesized from PET as a recycled material and then modified with dopamine on their surface (PD@CQD). In which it exhibited oxidase-like catalytic behavior due to presence of oxygen and nitrogen of decorated quinone and pyrrole moieties on surface. The last reports have mostly reported only an enzymatic activity. However, the peroxidase-like activity of the PDA@CQD can be reversibly switched to catalase-like activity upon exposure to visible light. These dual photo-induced enzymatic activities were attributed to the polarization effect of the quinoid groups present on the surface of CQDs, which significantly increased the consumption of ROS to produce oxygen (O₂) and water (H₂O). As a result, the process allowed for automatic switching between ROS production and removal, providing a therapeutic strategy that dynamically adapts to improving conditions, as illustrated in Scheme 1.

The CQDs were synthesized from PET waste using a two-step carbonization–hydrothermal approach as shown in scheme1. Then, oxidative polymerization of dopamine was precisely controlled to deposit a conformal PDA layer onto PET-derived CQDs, with the resulting nanostructure was prepared as PDA@CQDs. Through rigorous optimization and triplicate validation, we achieved a consistent product yield of 31.09.%, demonstrating the reliability of our approach.⁵⁰ Where traditional methods for preparing oxygen-functionalized CQDs from stable carbon materials necessitate aggressive conditions such as concentrated acids and high temperatures,⁵¹ but in this approach, polydopamine was decorated on the surface CQDs under milder condition.⁵²

The process of preparation monitored and investigated step by step. The functional groups of structure of CQDs and PDA@CQDs were investigated by FT-IR technique. As displayed in Fig. 1, the FT-IR spectrum of the pristine CQDs shows a broad absorption extending from ~ 2300 to 3300 cm^− 1, characteristic of strongly hydrogen-bonded O–H stretching modes associated with carboxylic acid and surface hydroxyl groups.⁵³ This region merges into the 3400–3200 cm^− 1 domain, where a weak N–H stretching component may appear; however, after surface modification, a clearer N–H stretching band near ~ 3400 cm^− 1 becomes evident,⁵⁴ which originates from the amine groups present in the PDA structure. A distinct C = O stretching band near ~ 1715–1720 cm^− 1 confirms the presence of surface carboxyl and carbonyl groups.⁵⁵ Following polydopamine deposition, the broad hydrogen-bonded envelope decreases markedly, indicating that part of the surface –COOH/–OH groups enter new interactions with dopamine units or undergo structural reorganization. New absorptions emerge, including the quinoid C = N stretch at ~ 1630 cm^− 1 and the N–H bending mode around ~ 1550 cm^− 1, both typical of PDA’s oxidized and amine-containing motifs. The carbonyl region shifts toward ~ 1650–1640 cm^− 1, consistent with altered electronic environments and possible formation of imine- or amide-like linkages.⁵⁶ Enhanced signals in the 1390–1220 cm^− 1 region further support the appearance of C–N and phenolic C–O vibrations introduced by PDA. In total, these spectral changes confirm a clear chemical transformation of the CQD surface upon PDA coating.

The ¹³C NMR spectrum of PDA@CQDs reveals a heterogeneous carbon environment consistent with polydopamine-passivated, oxygen- and nitrogen-rich carbon dots. A strong resonance at 169.7 ppm indicates conjugated carbonyls such as quinone- or ketone-type C = O groups, while a signal near 164.6 ppm confirms the presence of carboxyl functionalities on the particle surface. Additional peaks in the 148.4–159.6 ppm range correspond to C = C and heteroatom-substituted sp² carbons (= C–O/=C–N) within PDA-derived aromatic or quinoid domains.³³ Notably, signals around 90–100 ppm arise from oxygen- or nitrogen-stabilized sp² carbons and highly deshielded C–O/C–N centers formed during surface oxidation, reflecting the complexity of the CQD core–shell structure. The absence of any significant signals above 200 ppm rules out free aldehydes, supporting that carbonyl chemistry is dominated by conjugated ketones, quinones, and carboxyls—functionalities⁵⁷ known to enhance redox activity and correlating well with the peroxidase-like catalytic behavior observed for PDA@CQDs.

Powder X-ray diffraction (PXRD) analysis revealed that both pristine CQDs and dopamine-modified CQDs (PDA@CQDs) exhibited amorphous structural characteristics in Fig. 99. The PXRD patterns of CQDs showed a broad diffraction halo centered around 20–25° (2θ), which is typical for prepared PET-derived CQDs with disordered atomic arrangements.⁵⁸ After surface functionalization with polydopamine, PDA@CQDs maintained this amorphous nature, as evidenced by a similar broad diffraction feature, though with slightly increased intensity and a small shift in the peak position. This observation confirms that the core carbon structure remains amorphous following PDA coating, while the modification introduces subtle changes in the medium-range ordering of surface functional groups. The persistence of amorphous structure in both samples suggests that the surface modification process preserves the fundamental structural integrity of the CQDs while successfully introducing the desired surface functionalities through polydopamine conjugation.

Moreover, DLS and zeta potential measurements revealed key differences between pristine CQDs and PDA-coated CQDs. The CQDs showed an average hydrodynamic diameter of ~ 2.7 nm with moderate polydispersity (PDI ~ 0.2), while PDA@CQDs presented a larger size (~ 8.1 nm) and narrower size distribution (PDI < 0.1), confirming successful PDA encapsulation. Zeta potential analysis indicated that CQDs had a highly negative surface charge (− 15.23 mV) due to carboxyl/carbonyl groups, whereas PDA@CQDs displayed a reduced negative potential (− 8.15 mV), attributed to PDA's amine groups partially neutralizing the surface. This charge modulation confirms PDA functionalization and suggests improved colloidal stability for both systems, with PDA@CQDs offering enhanced biocompatibility for biological applications, too.

XPS analysis definitively verifies the chemical structure of the PDA@CQDs composite. The high-resolution C 1s spectrum, with its distinct peaks at 284.5 eV (C-C/C = C), 285.2 eV (C-O), 286.3 eV (C = O), and 289.3 eV (-COO), confirms the co-existence of the graphitic CQD core and oxygen-containing surface functionalities.⁵⁹ The N 1s spectrum, deconvoluted into components at 398.3 eV (-C = N-), 399.4 eV (-C = N-C), and 400.4 eV (-C-NH-), serves as a definitive fingerprint for the successful formation of the polydopamine shell.⁶⁰ This is further corroborated by the O 1s spectrum, where the peaks at 530.6 eV (C = O, quinone), 531.2 eV (O-C = O), and 531.8 eV (C-OH) reflect the characteristic catechol/quinone chemistry of PDA.⁶¹ The precise B.E. values of all identified species collectively confirm a hybrid architecture where CQDs are effectively coated by a complex PDA network, endowing the composite with a highly functionalized surface ideal for subsequent applications.

To evaluate the peroxidase-mimicking properties of PDA@CQDs, we utilized a standard enzymatic assay involving H₂O₂ -mediated oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), a widely used chromogenic peroxidase substrate. Besides, the catalase-like activity was investgated via oxygen evolution from H₂O₂ by a dissolved oxygen meter.

Figure 6 represented evaluation of the optimal operational conditions of the catalytic pathways. The pH and temperature profiles were systematically examined for both the catalase-like and peroxidase-like activities. As can be seen, both pH and temperature exerted strong influences on activity, but with distinct profiles for the catalase-like and peroxidase-like pathways. Under illumination, the catalase-like activity progressively increased from acidic to mildly alkaline media and reached its maximum at pH 8, whereas the peroxidase-like activity peaked under acidic-to-neutral conditions (pH 6–7), showing the characteristic sensitivity expected for peroxidase mimics.

Temperature-dependent analyses further differentiated the two catalytic modes: the catalase-like pathway exhibited its highest activity at 40– 45 ℃, maintaining high efficiency across a broader thermal window, while the peroxidase-like response maximized at 35–40 ℃ and declined more sharply at elevated temperatures. This divergence highlights a functional switching behavior where light not only activates the catalase-like route but also expands its operational robustness relative to the dark-state peroxidase pathway. These results confirmed that PDA@CQDs can be behaved as a dual-mode, stimuli-responsive nanozyme, with each pathway exhibiting its own optimized physicochemical regime, too.

The catalytic kinetics of CQDs and PDA@CQDs were evaluated using both Michaelis–Menten and Lineweaver–Burk models, as shown in Fig. 7. Under light irradiation (365 nm), CQDs exhibited peroxidase-like activity with a V_max of 1.89 × 10^− 7 M·s^− 1 and a high K_m of 13.02 mM, indicating weak substrate affinity and limited efficiency. While, the PDA@CQDs exposed obviously enhanced peroxidase-like activity in the dark, with a significantly higher V_max of 11.6 × 10^− 7 M·s^− 1 and a much lower K_m of 0.14 mM, corresponding to ~ 6-fold higher catalytic rate and ~ 100-fold stronger substrate affinity compared to CQDs.

Likewise, PDA@CQDs exhibited pronounced catalase-like activity toward H₂O₂ decomposition (V_max = 14.5 × 10^− 7 M·s⁻¹, K_m = 0.83 mM), while CQDs showed minor catalase activity. These results clearly demonstrate the synergistic effect between PDA and CQDs, where PDA introduces redox-active sites and CQDs facilitate electron transfer, collectively enabling enhanced catalytic efficiency and dual enzyme-mimicking behavior

It is worth mentioning that under dark conditions, the PDA@CQDs exhibited strong peroxidase-like activity, efficiently oxidizing the TMB substrate. However, upon light exposure, we observed a dramatic suppression of peroxidase function, with complete activity loss occurring after 30 minutes of illumination. Interestingly, while peroxidase function diminished under light, catalase-like activity of PAD coted on CQDs markedly increased, as evidenced by rapid H₂O₂ decomposition and oxygen bubble formation. This light-triggered switch from peroxidase to catalase mimicry suggests a tunable, stimulus-responsive dual-nanozyme system. The complete inversion of catalytic preference demonstrates the system's potential for precisely controlled catalytic applications in fields such as biomedicine and environmental remediation.

The CQDs typically exhibited a relatively wide band gap (≈ 2.6–3.2 eV) due to quantum confinement effects. While, polydopamine-functionalized CQDs showed a reduced band gap of 2.18 eV, as determined from Tauc plot analysis. This band gap narrowing is attributed to the π-conjugated PDA layer and nitrogen-containing functional groups, which introduce additional electronic states and enhance visible-light absorption. As mentioned, under illumination, these CQDs demonstrate dominant catalase-like activity, efficiently decomposing H₂O₂ into water and oxygen.

Table 1 summarizes a comparative analysis of peroxidase-like activity across various artificial catalytic systems and natural peroxidase. From this evaluation, the PDA@CQDs nanozyme demonstrates superior catalytic efficiency and substrate affinity toward both TMB and H₂O₂, outperforming other synthetic nanozymes examined. The lower K_m and of higher V_max of PDA@CQDs compared to HRP indicate superior substrate affinity and catalytic efficiency, respectively.

The catalytic behavior of PDA@CQDs toward H₂O₂can be rationalized through an electron-transfer–driven redox cycle involving five key dopamine-derived intermediates (DP1–DP5). Under light irradiation, photoexcited CQDs inject electrons into the PDA surface, stabilizing the semiquinone state (DP1) and promoting its conversion to the hydroperoxo-bound intermediate DP2. This photo-induced reduction facilitates heterolytic cleavage of H₂O₂ and generation of DP3, a transient peroxo adduct that rapidly decomposes into O₂, thereby enabling a catalase-like pathway. Because the photogenerated electrons continuously regenerate DP1 from DP3, the catalytic cycle proceeds efficiently in light, keeping the peroxidase route suppressed (“OFF”). In contrast, in dark conditions where the CQDs cannot supply conduction-band electrons for transformation of the DP1 to DP2 becomes thermodynamically unfavorable. The accumulated DP3 instead shifts toward homolytic peroxide activation, forming DP4 to DP5 intermediate through hydroxylation steps and ultimately generating reactive oxygen intermediates, which constitute the peroxidase-like pathway. The coexistence of reversible redox states (quinone 1 semiquinone 1 hydroquinone) within PDA, combined with light-responsive electron shuttling by CQDs, explains why the system toggles between catalase-like activity in the presence of light and oxygen, and peroxidase-like activity in the dark. This mechanistic cycle can be indicated for the persistent switching behavior and the strong dependence of catalytic function on the photochemical electron flow at the PDA@CQD interface. It worthing to mention that adding hydroquinone (10 mM) or p-tert-phenol (20 mM) as scavengers stop both the peroxidase activity in the dark and catalase activity in light, proving the reactions depend on •OH radicals.

In conclusion, PET-derived PDA@CQDs were successfully engineered as a metal-free, light-responsive nanozyme through precise surface functionalization. PDA coating modulated the electronic structure, reduced the band gap to 2.18 eV, and enabled efficient photoinduced electron transfer. As a result, the catalytic behavior of the nanozyme could be reversibly switched from peroxidase-like activity in the dark to dominant catalase-like activity under light irradiation. This work demonstrates that surface engineering of quantum dots provides a powerful and remarkable strategy to program and switch nanozyme functions, offering new opportunities for biomimetic systems and advanced quantum dot–based material technologies.

The prepared CQDs (30 mg) was dispersed in 10 ml of Tris–HCl buffer (10 mM, pH 8.5) and sonicated to obtain a homogeneous suspension. Dopamine hydrochloride (5 mg) was then added to the CQD dispersion under continuous stirring for 15 min. The reaction mixture was allowed to proceed at room temperature under ambient atmosphere for 3 h. After completion of the reaction, the suspension was centrifuged at 4000 rpm to remove large aggregates and uncoated PDA, and the supernatant containing PDA@CQDs was carefully collected. The collected dispersion was then diluted with 10 mL of deionized water and subsequently freeze-dried, yielding PDA@CQDs as a green powder.

Peroxidase-like activity was evaluated using TMB as the chromogenic substrate.⁷³ A defined amount of the PDA@CQDs or CQDs (50 µg) was introduced into a cuvette containing sodium acetate-acetic acid buffer (NaAc–HAc, 0.2 M) adjusted to the optimal pH. Subsequently, 100 µL of TMB solution (0.1–0.5 mM) was added, and the reaction mixture was equilibrated at the optimal temperature in a thermostated water bath for 1 min under dark conditions. The catalytic reaction was initiated by adding H₂O₂ to reach a final concentration of 1 mM. A control experiment without H₂O₂ was conducted in parallel to serve as a blank reference. After gentle mixing, the reaction was allowed to proceed in the dark, and the time-dependent absorbance at 650 nm was monitored for up to 300 s. The change in optical density at 650 nm as a function of time was used to quantify catalytic activity. The kinetic parameters (K_m and V_max) were obtained from Michaelis–Menten fitting and Lineweaver–Burk plots.