Progress in the Application of Carbon Dots-Based Nanozymes

Preparation and Application of Carbon Dots Nanozymes

Carbon dot (CD) nanozymes have enzyme-like activity. Compared with natural enzymes, CD nanozymes offer several advantages, including simple preparation, easy preservation, good stability and recycling, which has made them a popular research topic in various fields. In recent years, researchers have prepared a variety of CD nanozymes for biosensing detection, medicine and tumor therapy, and many of them are based on oxidative stress regulation and reactive oxygen species clearance. Particularly to expand their potential applications, elemental doping has been utilized to enhance the catalytic capabilities and other properties of CD nanozymes. This review discusses the prevalent techniques utilized in the synthesis of CD nanozymes and presents the diverse applications of CD nanozymes based on their doping characteristics. Finally, the challenges encountered in the current utilization of CD nanozymes are presented. The latest research progress of synthesis, application and the challenges outlined in the review can help and encourage the researchers for the future research on preparation, application and other related researches of CD nanozymes.

Emerging enzyme-based nanocomposites for catalytic biomedicine

With the promising advances in nanomedicine, numerous strategies have emerged for the diagnosis and treatment of diseases. Among them, enzyme-based multifunctional nanocomposites have attracted a great deal of attention in the field of catalytic biomedicine. These nanocomposites with high catalytic activity are capable of converting low/non-toxic substances into therapeutic ones, thus realizing highly efficient, site-specific therapy with minimal side effects. Enzyme-based nanocomposites for catalytic biomedicine are mainly divided into three types: (i) natural-enzyme based nanocomposites; (ii) artificial-nanozyme based nanocomposites; and (iii) nanocomposites of natural-enzymes and nanozymes. In this review, we discuss key aspects of enzyme-based catalytic biomedicine, including the construction of enzyme-based nanocomposites, their unique properties and applications in catalytic biomedicine. We also highlight the main challenges faced in this field, and provide relevant guidelines for the rational design and extensive application of enzyme-based nanocomposites from our point of view.

Antibacterial functionalized carbon dots and their application in bacterial infections and inflammation

Bacterial infections and inflammation pose a severe threat to human health and the social economy. The existence of super-bacteria and the increasingly severe phenomenon of antibiotic resistance highlight the development of new antibacterial agents. Due to low cytotoxicity, high biocompatibility, and different antibacterial mechanisms from those for antibiotics, functionalized carbon dots (FCDs) promise a new platform for the treatment of bacterial infectious diseases. However, few articles have systematically sorted out the available antibacterial mechanisms for FCDs and their application in the treatment of bacterial inflammation. This review focuses on the available antibacterial mechanisms for FCDs, including covalent and non-covalent interactions, reactive oxygen species, photothermal therapy, and size effect. Meanwhile, the design of antibacterial FCDs is introduced, including surface modification, doping, and combination with other nanomaterials. Furthermore, this review specifically concentrates on the research advances of antibacterial FCDs in the treatment of bacterial inflammation. Finally, the advantages and challenges of applying FCDs in practical antimicrobial applications are discussed.

Carbon-Based Enzyme Mimetics for Electrochemical Biosensing

Natural enzymes are used as special reagents for the preparation of electrochemical (bio)sensors due to their ability to catalyze processes, improving the selectivity of detection. However, some drawbacks, such as denaturation in harsh experimental conditions and their rapid de- gradation, as well as the high cost and difficulties in recycling them, restrict their practical applications. Nowadays, the use of artificial enzymes, mostly based on nanomaterials, mimicking the functions of natural products, has been growing. These so-called nanozymes present several advantages over natural enzymes, such as enhanced stability, low cost, easy production, and rapid activity. These outstanding features are responsible for their widespread use in areas such as catalysis, energy, imaging, sensing, or biomedicine. These materials can be divided into two main groups: metal and carbon-based nanozymes. The latter provides additional advantages compared to metal nanozymes, i.e., stable and tuneable activity and good biocompatibility, mimicking enzyme activities such as those of peroxidase, catalase, oxidase, superoxide dismutase, nuclease, or phosphatase. In this review article, we have focused on the use of carbon-based nanozymes for the preparation of electrochemical (bio)sensors. The main features of the most recent applications have been revised and illustrated with examples selected from the literature over the last four years (since 2020).

Visible-Light-Activated Carbon Dot Photocatalyst for ROS-Mediated Inhibition of Algae Growth

The growing occurrence of detrimental algal blooms resulting from industrial and agricultural activities emphasizes the urgency of implementing efficient removal strategies. In this study, we have successfully synthesized stable and biocompatible carbon dots (R-CDs) capable of generating reactive oxygen species (ROS) upon exposure to natural light irradiation. Phaeocystis globosa Scherffel (PGS) was selected as a representative model for conducting anti-algal experiments. Remarkably, in the presence of R-CDs, the complete eradication of harmful algae within a simulated light exposure period of 27 h was achieved. Furthermore, fluorescence lifetime imaging microscopy (FLIM) was first employed to study the physiological processes involved in the oxidative stress induced by PGS when subjected to ROS attack. The findings of this study demonstrate the potential of R-CDs as a highly promising anti-algal agent. This elucidation of the mechanism contributes to a comprehensive understanding of the efficacy and effectiveness of such agents in combating algal growth, further inspiring the development of other anti-algal agents.

Sensitive acid phosphatase assay based on light-activated specific oxidase mimic activity

Acid phosphatase (ACP) is a key marker in the diagnosis of many diseases. However, exploiting a simple and sensitive sensor for the real-time quantitative analysis of ACP is still challenging. Herein, we attempted to develop a sensitive colorimetric sensing strategy for the detection of ACP based on light-activated oxidase mimic property of carbon dots (CDs). The synthesized CDs were proved to be capable of intrinsic light-activated oxidase mimic activity, which could generate reactive oxygen species to oxidize chromogenic substrate under ultraviolet light stimulation. Interestingly, this light-activated oxidase mimic behavior would be effectively suppressed by the antioxidant ascorbic acid (AA), a product from the hydrolysis of 2-phospho-L-ascorbic acid trisodium (AAP) mediated by ACP. Based on the above property, a facile and sensitive colorimetric sensing method for ACP was developed. Under the optimal conditions, the linear range for ACP 0.1-5.5 U/L, and the detection limit was 0.056 U/L. Compared with conventional nanozyme based ACP assay systems, the catalytic activity of light-activated nanozyme could be conveniently regulated by switching the light on and off, which made it easier to precisely control the extent of the reaction and ensured the accuracy of the assay. In addition, the proposed sensing system would be readout directly by the naked eye or smartphone-based RGB analysis system, and have been successfully applied to analyze diluted in diluted fetal bovine serum and urine samples spiked with ACP. All these results indicated that this approach holds good promise for future applications in clinical analysis and point-of-care (POC) biosensor platforms.

Enhancing catalytic efficiency of carbon dots by modulating their Mn doping and chemical structure with metal salts

Nanozymes are emerging materials in various fields owing to their advantages over natural enzymes, such as controllable and facile synthesis, tunability in catalytic activities, cost-effectiveness, and high stability under stringent conditions. In this study, the effect of metal salts on the formation and catalytic activity of carbon dots (CDs), a promising nanozyme, is demonstrated. By introducing Mn sources that possess different counter anions, the chemical structure and composition of the CDs produced are affected, thereby influencing their enzymatic activities. The synergistic catalytic effect of the Mn and N-doped CDs (Mn&N-CDs) is induced by effective metal doping in the carbogenic domain and a high proportion of graphitic and pyridinic N. This highly enhanced catalytic effect of Mn&N-CDs allows them to respond sensitively to the interference factors of enzymatic reactions. Consequently, ascorbic acid, which is an essential nutrient for maintaining our health and is a reactive oxygen scavenger, can be successfully monitored using color change by forming oxidized 3,3',5,5'-tetramethylbenzidine with H₂O₂ and Mn&N-CDs. This study provides a basic understanding of the formation of CDs and how their catalytic properties can be controlled by the addition of different metal sources, thereby providing guidelines for the development of CDs for industrial applications.

A critical review on the environmental applications of carbon dots

The discovery of zero-dimensional carbonaceous nanostructures called carbon dots (CDs) and their unique properties associated with fluorescence, quantum confinement and size effects have intrigued researchers. There has been a substantial increase in the amount of research conducted on the lines of synthesis, characterization, modification, and enhancement of properties by doping or design of composite materials, and a diversification of their applications in sensing, catalysis, optoelectronics, photovoltaics, and imaging, among many others. CDs fulfill the need for inexpensive, simple, and continuous environmental monitoring, detection, and remediation of various contaminants such as metals, dyes, pesticides, antibiotics, and other chemicals. The principles of green chemistry have also prompted researchers to rethink novel modes of nanoparticle synthesis by incorporating naturally available carbon precursors or developing micro reactor-based techniques. Photocatalysis using CDs has introduced the possibility of utilizing light to accelerate redox chemical transformations. This comprehensive review aims to provide the reader with a broader perspective of carbon dots by encapsulating the concepts of synthesis, characterization, applications in contaminant detection and photocatalysis, demerits and research gaps, and potential areas of improvement.

Fungus-Based MnO/Porous Carbon Nanohybrid as Efficient Laccase Mimic for Oxygen Reduction Catalysis and Hydroquinone Detection

Developing efficient laccase-mimicking nanozymes via a facile and sustainable strategy is intriguing in environmental sensing and fuel cells. In our work, a MnO/porous carbon (MnO/PC) nanohybrid based on fungus was synthesized via a facile carbonization route. The nanohybrid was found to possess excellent laccase-mimicking activity using 2,2′-azinobis (3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS) as the substrate. Compared with the natural laccase and reported nanozymes, the MnO/PC nanozyme had much lower K_m value. Furthermore, the electrochemical results show that the MnO/PC nanozyme had high electrocatalytic activity toward the oxygen reduction reaction (ORR) when it was modified on the electrode. The hybrid nanozyme could catalyze the four-electron ORR, similar to natural laccase. Moreover, hydroquinone (HQ) induced the reduction of oxABTS and caused the green color to fade, which provided colorimetric detection of HQ. A desirable linear relationship (0–50 μM) and detection limit (0.5 μM) were obtained. Our work opens a simple and sustainable avenue to develop a carbon–metal hybrid nanozyme in environment and energy applications.

Regulation Mechanism of ssDNA Aptamer in Nanozymes and Application of Nanozyme-Based Aptasensors in Food Safety

Food safety issues are a worldwide concern. Pathogens, toxins, pesticides, veterinary drugs, heavy metals, and illegal additives are frequently reported to contaminate food and pose a serious threat to human health. Conventional detection methods have difficulties fulfilling the requirements for food development in a modern society. Therefore, novel rapid detection methods are urgently needed for on-site and rapid screening of massive food samples. Due to the extraordinary properties of nanozymes and aptamers, biosensors composed of both of them provide considerable advantages in analytical performances, including sensitivity, specificity, repeatability, and accuracy. They are considered a promising complementary detection method on top of conventional ones for the rapid and accurate detection of food contaminants. In recent years, we have witnessed a flourishing of analytical strategies based on aptamers and nanozymes for the detection of food contaminants, especially novel detection models based on the regulation by single-stranded DNA (ssDNA) of nanozyme activity. However, the applications of nanozyme-based aptasensors in food safety are seldom reviewed. Thus, this paper aims to provide a comprehensive review on nanozyme-based aptasensors in food safety, which are arranged according to the different interaction modes of ssDNA and nanozymes: aptasensors based on nanozyme activity either inhibited or enhanced by ssDNA, nanozymes as signal tags, and other methods. Before introducing the nanozyme-based aptasensors, the regulation by ssDNA of nanozyme activity via diverse factors is discussed systematically for precisely tailoring nanozyme activity in biosensors. Furthermore, current challenges are emphasized, and future perspectives are discussed.

High sensitivity and rapid detection of hepatitis B virus DNA using lateral flow biosensors based on Au@Pt nanorods in the absence of hydrogen peroxide

Au@Pt nanorods with enhanced oxidase-like activity were designed and used as signal probes to construct LFBs for the high sensitivity detection of hepatitis B virus DNA (HBV-DNA).

Starch-Based Carbon Dots for Nitrite and Sulfite Detection

Nitrite and sulfite play important roles in human health and environmental science, so it is desired to develop a facile and efficient method to evaluate NO2 - and SO3 2- concentrations. In this article, the use of green alternatives with the potential of multi-functionality has been synthesized to detect nitrite and sulfite based on fluorescent probe. The carbon dots (CDs) with starch as only raw materials show fluorescence turn "on-off-on" response towards NO2 - and SO3 2- with the limits of detection of 0.425 and 0.243 μМ, respectively. Once nitrite was present in the solution, the fluorescence of CDs was quenched rapidly due to the charge transfer. When sulfite was introduced, the quenching fluorescence of CDs was effectively recovered because of the redox reaction between NO2 - and SO3 2-, and thus providing a new way for NO2 - and SO3 2- detection. Owing to their excellent analytical characteristics and low cytotoxicity, the "on-off-on" sensor was successfully employed for intracellular bioimaging of NO2 - and SO3 2-.