Effect of surface modification on the peroxidase-like behaviors of carbon dots

Polymer-enhanced peroxidase activity of ceria nanozyme for highly sensitive detection of alkaline phosphatase

Nanoceria have demonstrated a wide array of catalytic activity similar to natural enzymes, holding considerable significance in the colorimetric detection of alkaline phosphatase (ALP), which is a biomarker of various biological disorders. However, the issues of physiological stability and formation of protein corona, which are strongly related to their surface chemistry, limit their practical application. In this work, CeO2 nanoparticles characterized by enhanced dimensional uniformity and specific surface area were synthesized, followed by encapsulation with various polymers to further increase catalytic activity and physiological stability. Notably, the CeO2 nanoparticles encapsulated within each polymer exhibited improved catalytic characteristics, with PAA-capped CeO2 exhibiting the highest performance. We further demonstrated that the PAA-CeO2 obtained with enhanced catalytic activity was attributed to an increase in surface negative charge. PAA-CeO2 enabled the quantitative assessment of AA activity within a wide concentration range of 10 to 60 μM, with a detection limit of 0.111 μM. Similarly, it allowed for the evaluation of alkaline phosphatase activity throughout a broad range of 10 to 80 U/L, with a detection limit of 0.12 U/L. These detection limits provided adequate sensitivity for the practical detection of ALP in human serum.

Breaking through the pH Limitation of Fe1-xS Nanozymes Using Component-Modulated Coupled Nanoclay

Overcoming the intrinsic low activity of most peroxidase mimics under neutral pH is crucial but still extremely challenging for the detection of disease markers in biological samples. Here, we chose nanoclay (i.e., montmorillonite K10, MK10) as a carrier to modulate the structure of Fe1-xS nanozyme components through an interfacial modulation strategy, aiming at breaking the neutral pH limitation of Fe1-xS. MK10 with abundant hydroxyl groups on its surface acts as a carrier to increase the ratio of Fe(II) and S(II-) content in surface Fe1-xS. We verify that Fe(II)-promoted surface hydroxyl radical generation and S(II-)-promoted regeneration of Fe(II) play key roles in endowing peroxidase-like activity to Fe1-xS at neutral pH. As expected, Fe1-xS/MK10 exhibited 11-fold higher Vmax and 52-fold higher catalytic efficiency than bare Fe1-xS. As a proof of concept, the sensor constructed based on Fe1-xS/MK10 achieved colorimetric detection of xanthine under neutral conditions with a linear range of 5-300 μM and a limit of detection of 2.49 μM. Finally, we achieved highly sensitive detection of xanthine in serum using the constructed biosensor. Our contribution is the novel use of a nanoclay-mediated interfacial modulation strategy for boosting the peroxidase-mimicking activity and breaking the pH limitation, which contributes to the in situ detection of disease markers by nanozymes under physiological conditions.

PEG mediated NiMn2O4 nanomaterials as a nano catalyst for peroxidase mimetic activity and photocatalytic degradation

Artificial peroxidases have garnered a lot of attention owing to their tremendous superiority over their natural counterparts. Here, NiMn2O4 nanoparticles have been successfully prepared through PEG assisted hydrothermal method. The varied PEG concentrations significantly altered the morphology and particle size of the synthesizedmaterials. We demonstrate the improved peroxide-like assay of different NiMn2O4 nanoparticles for the first time. Among them, Ni4 nanoparticles exhibit good peroxidase-like activity by generating the oxidation of chromogenic substrate 3, 3', 5, 5'-tetramethylbenzidine (TMB) in the presence of H2O2 and a blue color charge transfer product with an absorption maximum is positioned at 652 nm. These observations led to the development of a method for assessingH2O2 that can be read visually and photometrically. The Ni4 nanoparticles show enhanced kinetics compared to the natural enzyme horse radish peroxidase (HRP) with a lower Km (0.168 mM) value. Additionally, this Ni4 nanosphere applies as a visible light photocatalyst for the degradation of methylene blue (MB) and rhodamine B (Rh B) dyes under visible-light irradiation. Under optimized conditions, the degradationrates of MB and Rh B are 68 and 80.7 %, respectively, after 210 min, and recyclable efficiency is about 99 % for Rh B photocatalytic degradation in the first test and 98 % for five cycles, and about 98 % for MB photocatalytic degradation in the first test and 97 % for five cycles.

Co(II)-Based Metal-Organic Framework Derived CA-CoNiMn-CLDHs with Peroxidase-like Activity for Colorimetric Detection of Phenol

Given the serious harm of toxic phenol to human health and the ecological environment, it is urgent to develop an efficient, low-cost and sensitive nanoenzyme-based method to monitor phenol. MOF-derived nanozyme has attracted wide interest due to its hollow polyhedra structure and porous micro-nano frameworks. However, it is still a great challenge to synthesize MOF-derived multimetal synergistic catalytic nanoenzymes in large quantities with low cost. Herein, we reported the synthetic strategy of porous hollow CA-CoNiMn-CLDHs with ZIF-67 as templates through a facile solvothermal reaction. The prepared trimetallic catalyst exhibits excellent peroxidase-like activity to trigger the oxidative coupling reaction of 4-AAP and phenol in the presence of H2O2. The visual detection platform for phenol based on CA-CoNiMn-CLDHs is constructed, and satisfactory results are obtained. The Km value for CA-CoNiMn-CLDHs (0.21 mM) is lower than that of HRP (0.43 mM) with TMB as the chromogenic substrate. Because of the synergistic effect of peroxidase-like activity and citric acid functionalization, the built colorimetric sensor displayed a good linear response to phenol from 1 to 100 μM with a detection limit of 0.163 μM (3σ/slope). Additionally, the CA-CoNiMn-CLDHs-based visual detection platform possesses high-chemical stability and excellent reusability, which can greatly improve economic benefits in practical applications.

A Review on the Catalytic Remediation of Dyes by Tailored Carbon Dots

Water polluted with dyes has become a serious global concern during the twenty-first century, especially for developing countries. Such types of environmental contaminant pose a severe threat to biodiversity, ecosystems, and human health globally; therefore, its treatment is an utmost requirement. Advanced technologies including the use of nanomaterials represent a promising water treatment technology with high efficiencies, low production costs, and green synthesis. Among the nanomaterials, carbon dots, as a new class of carbon-based nanoparticles, have attracted attention due to their unique features and advantages over other nanomaterials, which include high water solubility, easy fabrication and surface functionalisation, excellent electron-donating ability, and low toxicity. Such properties make carbon dots potential nanocatalysts for the Fenton-like degradation of environmental pollutants in water. Although recent studies show that carbon dots can successfully catalyse the degradation of dyes, there are still limited and controversial studies on the ecotoxicity and fate of these nanoparticles in the environment. In this review, the authors aim to summarise the recent research advances in water remediation by technologies using carbon dots, discuss important properties and factors for optimised catalytic remediation, and provide critical analysis of ecotoxicity issues and the environmental fate of these nanoparticles.

Quantum dot synthesis from waste biomass and its applications in energy and bioremediation

Quantum dots (QDs) are getting special attention due to their commendable optical properties and applications. Conventional metal-based QDs have toxicity and non-biodegradability issues, thus it becomes necessary to search for renewable precursor molecules for QDs synthesis. In recent years, biomass-based carbon rich QDs (CQDs) have been introduced which are mainly synthesised via carbonization (pyrolysis and hydrothermal treatment). These CQDs offered higher photostability, biocompatibility, low-toxicity, and easy tunability for physicochemical properties. Exceptional optical properties become a point of attraction for its multifaceted applications in various sectors like fabrication of electrodes and solar cells, conversion of solar energy to electricity, detection of pollutants, designing biosensors, etc. In recent years, a lot of work has been done in this field. This article will summarize these advancements along in a special context to biomass-based QDs and their applications in energy and the environment.

Multifaceted Interaction Studies between Carbon Dots and Proteins of Clinical Importance for Optical Sensing Signals

Carbon dots (CDs) are emerging as efficient optical probes. However, their application potential for clinical diagnosis has not been adequately explored. Herein, we examined the suitability of pyroglutamate CDs for detecting glucose, cholesterol, and alcohol in blood serum through their peroxidative activity in the respective enzyme-catalyzed reactions following fluorometric and colorimetric approaches. In buffer, the CD's fluorescence intensity (λ_ex 354nm) enhanced over 115% after interaction with the enzyme proteins due to different lifetime components on its surface. The enhancement was also linked to FRET with the proteins (λ_ex 274nm for TRP/TYR). The electrostatic interactions, as revealed from the zeta potential study, generated binding energy (ΔG, kcal/mol) in the range of -5.8 to -6.3 and greatly shifted the protein's secondary structure to β-strand contents. The CD's fluorescence in the blood serum medium was also enhanced where serum's particulate components contributed to the emission. All these subvert fluorescence emissions could be substantially cleaned for detection of peroxide generated in the enzymatic reaction by filtering the serum particulates and redox proteins prior to the addition of CDs to the reaction systems. The CD, however, could complement well in ABTS-based (absorbance at λ_max 414nm) colorimetric reaction in blood serum without introducing protein or particle separation steps for sensitive detection of peroxide. The limit of detection, dynamic range, and sensitivity discerned for peroxide in the glucose oxidase-catalyzed reaction system were 183 μM, 0.02-0.10 mM (R² = 0.98), and 0.2482 AU mM^-1, respectively. Overall, these findings will guide clinical application of the peroxidatic CDs to detect various analytes in blood serum following fluorometric- and colorimetric-based principles.

Detection and Difference Analysis of the Enzyme Activity of Colloidal Gold Nanoparticles With Negatively Charged Surfaces Prepared by Different Reducing Agents

Nanozymes are particles with diameters in the range of 1–100 nm, which has been widely studied due to their biological enzyme-like properties and stability that natural enzymes do not have. In this study, several reducing agents with different structures (catechol (Cc), hydroquinone (Hq), resorcinol (Rs), vitamin C (Vc), pyrogallic acid (Ga), sodium citrate (Sc), sodium malate (Sm), and sodium tartrate (St)) were used to prepare colloidal gold with a negative charge and similar particle size by controlling the temperature and pH. The affinity analysis of the substrate H2O2 and TMB showed that the order of activities of colloidal gold Nanozymes prepared by different reducing agents was Cc, Hq, Rs, Vc, Ga, Sc, Sm, St. It was also found that the enzyme activity of colloidal gold reduced by benzene rings is higher than that of the colloidal gold enzyme reduced by linear chains. Finally, we discussed the activity of the colloidal gold peroxidase based on the number and position of isomers and functional groups; and demonstrated that the nanozymes activity is affected by the surface activity of colloidal gold, the elimination of hydroxyl radicals and the TMB binding efficiency.

Catalytic degradation of methylene blue using iron and nitrogen-containing carbon dots as Fenton-like catalysts

Carbon dots were modified with iron and nitrogen groups to produce specific surface groups and charge which demonstrated high efficiency for the Fenton-like degradation of methylene blue whilst markedly minimising its effluent toxicity.

Analytical application of H 2 O 2 -induced chiroptical graphitic carbon dots

Carbon dots (CDs) have emerged as efficient peroxidase mimics in recent years. However, to further increase its efficiency as peroxidase-mimic, it is also desirable to understand the modification of CD’s geometry during the catalytic reaction. Herein, we focused on the change in material property of the CDs upon their reaction with H2O2 during the peroxidase reaction. D-(+)-glucose was transformed into chiroptical CDs bearing peroxidase-like activity and can be used to detect H2O2 with a limit of detection of 630 μM. The addition of H2O2 to the CDs resulted in its increased molecular orderliness leading to the introduction of polycrystallinity without affecting its peroxidase-like activity.

Nanozyme Catalytic Turnover and Self-Limited Reactions

Enzymes have catalytic turnovers. The field of nanozyme endeavors to engineer nanomaterials as enzyme mimics. However, a discrepancy in the definition of "nanozyme concentration" has led to an unrealistic calculation of nanozyme catalytic turnovers. To date, most of the reported works have considered either the atomic concentration or nanoparticle (NP) concentration as nanozyme concentration. These assumptions can lead to a significant under- or overestimation of the catalytic activity of nanozymes. In this article, we review some classic nanozymes including Fe₃O₄, CeO₂, and gold nanoparticles (AuNPs) with a focus on the reported catalytic activities. We argue that only the surface atoms should be considered as nanozyme active sites, and then the turnover numbers and rates were recalculated based on the surface atoms. According to the calculations, the catalytic turnover of peroxidase Fe₃O₄ NPs is validated. AuNPs are self-limited when performing glucose-oxidase like activity, but they are also true catalysts. For CeO₂ NPs, a self-limited behavior is observed for both oxidase- and phosphatase-like activities due to the adsorption of reaction products. Moreover, the catalytic activity of single-atom nanozymes is discussed. Finally, a few suggestions for future research are proposed.

Influence of amphiphilic molecules on the peroxidase-like behavior of nanoparticles in an aqueous solution

Carbon dots (CDs) have drawn considerable attention in recent decades due to their outstanding biocompatibility and environmental friendliness. In this study, we synthesized ionic liquid (1-aminopropyl-3-methyl-imidazolium bromide)-modified carbon dots (IL-CDs) showing good peroxidase-like activity. Furthermore, we investigated their enzymic behavior in the presence of two different amphiphilic molecules, namely tert-butanol (TBA, a typical hydrotrope) and sodium bis (2-ethylhexyl) sulfosuccinate (AOT, a typical anionic surfactant). In an aqueous solution of TBA, a microscopic heterogeneous structure was formed in a certain concentration range of TBA, which resulted in an anomaly in the reaction process. However, in the AOT aqueous solution, the situation became more complicated. IL-CDs formed vesicles or precipitation at different concentrations of AOT, which led to different enzymic activities of IL-CDs due to the variance in the structure and the surface electronic density.

Magnetic nanomaterials with unique nanozymes-like characteristics for colorimetric sensors: A review

Colorimetric sensors for the rapid detection of numerous analytes have been widely applied in many fields such as biomedicine, food industry and environmental science due to their highly sensitive and selective response, easy operation and visual identification by naked eyes. In this review, the recent progress of the colorimetric sensors based on the magnetic nanomaterials with unique nanozymes-like catalytic activity (magnetic nanozyme) and their colorimetric sensing applications are presented. Emerging magnetic nanozyme-based colorimetric sensors, such as metal oxide/sulfides-based, metal-based, carbon-based, and aptamer-conjugated magnetic nanomaterials, offer many desirable features for target analytes detection. And due to the unique nanoscale physical-chemical properties, magnetic nanozymes have been used to mimic the catalytic activity of natural enzymes such as peroxidases, oxidases and catalases. This review also highlights the catalytic mechanisms of enzyme-like reactions, and promising colorimetric sensing system for the detection of chemical compounds like H₂O₂, pesticide, ascorbic acid, dopamine, tetracyclines, perfluorooctane sulfonate, phenolic compounds, heavy metal ion and sulfite have been deeply discussed. In addition, the remaining challenges and future directions in utilizing magnetic nanozyme for colorimetric sensors are addressed.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.

Catalase active metal-organic framework synthesized by ligand regulation for the dual detection of glucose and cysteine

Mimic enzymes greatly improve the inherent insufficiencies of natural enzymes. Therefore, mimic enzyme sensors attract increasing research interest. Metal-organic framework (MOF) is emerging in the field of mimic enzyme catalysis due to its remarkable structural properties. In this paper, a colorimetric method is designed for rapid and sensitive detection of glucose and cysteine levels. The MOF Eu-pydc (pydc-2,5-pyridinedicarboxylic acid) is synthesized by a new strategy which is regulated by ligands at room temperature and found to have peroxidase activity. Then, the MOF is used as a mimic enzyme to catalyze chromogenic substrate (3,3',5,5'-tetramethylbenzidine, TMB) for colorimetric sensing of glucose. The developed method can accurately detect glucose in the range of 10 μM-1 mM (R² = 0.9958) with a relatively low detection limit about 6.9 μM. Moreover, a cysteine sensor with a detection limit of 0.28 μM is also established based on the disappearance of the color of oxTMB. Additionally, the proposed glucose sensor exhibits excellent selectivity and is successfully applied to blood glucose detection. At the same time, the detection of cysteine is also highly sensitive. In short, the dual sensor is fast, low cost, and convenient, and has great application potential in the diagnosis of disease.

Fabrication of highly active phosphatase-like fluorescent cerium-doped carbon dots for in situ monitoring the hydrolysis of phosphate diesters

The hydrolytic cleavage of BNPP was catalyzed and monitored by the fluorescent CeCDs.