Carbon Dots: Bottom-Up Syntheses, Properties, and Light-Harvesting Applications

Portable sensing methods based on carbon dots for food analysis

Food analysis is significantly important in monitoring food quality and safety for human health. Traditional methods for food detection mainly rely on benchtop instruments and require a certain amount of analysis time, which promotes the development of portable sensors. Portable sensing methods own many advantages over traditional techniques such as flexibility and accessibility in diverse environments, real-time monitoring, cost-effectiveness, and rapid deployment. This review focuses on the portable approaches based on carbon dots (CDs) for food analysis. CDs are zero-dimensional carbon-based material with a size of less than 10 nm. In the manner of sensing, CDs exhibit rich functional groups, low biotoxicity, good biocompatibility, and excellent optical properties. Furthermore, there are many methods for the synthesis of CDs using various precursor materials. The incorporation of CDs into food science and engineering for enhancing food safety control and risk assessment shows promising prospects.

Carbon Dots Modified Multi Dopants Nitrogen and Boron for an Early Detection of Lead in the Environment

The remarkable optical properties and strong biocompatibility of carbon dots make them highly promising for applications in biochemical sensing and environmental testing. These carbon dots possess a surface that is easily modifiable. In this study, carbon dots have been successfully synthesized and modified by the addition of N and B dopants using the microwave method, along with the functionalization of their surface functional groups with bovine serum albumin (BSA). The maximum fluorescence intensity of N, B-CDs is observed at 462 nm when excited at a wavelength of 352 nm. N, B-CDs have a spherical size with a diameter ranging from 2 to 6 nm, confirmed by UV-Vis absorption spectra and the presence of functional groups in the FT-IR absorption patterns. BSA-functionalized N, B-CDs as the fluorescent probe demonstrate great potential as a sensor for Pb(II) ions in water, with a very low detection limit of 1.05 µg/L. This research could contribute to the development of fluorescence nanosensors.

Carbon Dots Anchoring Single-Atom Pt on C3N4 Boosting Photocatalytic Hydrogen Evolution

Carbon nitride (C3N4) has gained considerable attention and has been regarded as an ideal candidate for photocatalytic hydrogen evolution. However, its photocatalytic efficiency is still unsatisfactory due to the rapid recombination rate of photo-generated carriers and restricted surface area with few active sites. Herein, we successfully synthesized a single-atom Pt cocatalyst-loaded photocatalyst by utilizing the anchoring effect of carbon dots (CDs) on C3N4. The introduction of CDs onto the porous C3N4 matrix can greatly enhance the specific surface area of C3N4 to provide more surface-active sites, increase light absorption capabilities, as well as improve the charge separation efficiency. Notably, the functional groups of CDs can efficiently anchor the single-atom Pt, thus improving the atomic utilization efficiency of Pt cocatalysts. A strong interaction is formed via the connection of Pt-N bonds, which enhances the efficiency of photogenerated electron separation. This unique structure remarkably improves its H2 evolution performance under visible light irradiation with a rate of 15.09 mmol h-1 g-1. This work provides a new approach to constructing efficient photocatalysts by using CDs for sustainable hydrogen generation, offering a practical approach to utilizing solar energy for clean fuel production.

Electrodeposited histidine-(CuCr)layered double hydroxides/carbon dots for in-tube solid-phase microextraction of chlorophenols from water, juice, and honey samples followed by HPLC-UV

A novel adsorbent consisting of a composition of carbon dots and CuCr-layered double hydroxides intercalated with l-histidine (C-dots@His/LDHs) was introduced. This adsorbent was electrochemically deposited on the inner surface of a capillary copper tube. It was used as an adsorbent for in-tube solid-phase microextraction of chlorophenols (CPs). Separation and measurement of CPs were done by high-performance liquid chromatography-ultraviolet detector. The main parameters which had the most impact on the extraction efficiency and time such as extraction time and flow rate, desorption time and flow rate, ionic strength (salt concentration) and pH were optimized. Calibration curves (0.5-1000 μg L-1) were plotted in real sample (tap water) under optimal conditions which coefficients of determination better than 0.9893 and relative recoveries in the range of 88-120 % were obtained. The limits of detection (S/N = 3) and limits of quantification (S/N = 10) were obtained in the range of 0.1-1.0 μg L-1 and 0.3-3.0 μg L-1, respectively. The intra- and inter-assay precisions (RSD%, n = 3) were better than 5.9 and 8.8 %, respectively.

Comprehensive advances in the synthesis, fluorescence mechanism and multifunctional applications of red-emitting carbon nanomaterials

Red emitting fluorescent carbon nanomaterials have drawn significant scientific interest in recent years due to their high quantum yield, water-dispersibility, photostability, biocompatibility, ease of surface functionalization, low cost and eco-friendliness. The red emissive characteristics of fluorescent carbon nanomaterials generally depend on the carbon source, reaction time, synthetic approach/methodology, surface functional groups, average size, and other reaction environments, which directly or indirectly help to achieve red emission. The importance of several factors to achieve red fluorescent carbon nanomaterials is highlighted in this review. Numerous plausible theories have been explained in detail to understand the origin of red fluorescence and tunable emission in these carbon-based nanostructures. The above advantages and fluorescence in the red region make them a potential candidate for multifunctional applications in various current fields. Therefore, this review focused on the recent advances in the synthesis approach, mechanism of fluorescence, and electronic and optical properties of red-emitting fluorescent carbon nanomaterials. This review also explains the several innovative applications of red-emitting fluorescent carbon nanomaterials such as biomedicine, light-emitting devices, sensing, photocatalysis, energy, anticounterfeiting, fluorescent silk, artificial photosynthesis, etc. It is hoped that by choosing appropriate methods, the present review can inspire and guide future research on the design of red emissive fluorescent carbon nanomaterials for potential advancements in multifunctional applications.

Luminescence of carbon quantum dots and their application in biochemistry

Similar to fullerenes, carbon nanotubes and graphene, carbon dots (CDs) are causing a lot of research work in their own right. CDs are a type of surface-passivated quantum dot that contain carbon atoms. Their distinctive characteristics, such as luminescent emission that varies with size and wavelength, resistance to photobleaching, easy biological binding, lack of toxicity, and economical production without the need for intricate synthetic processes, have led to a noteworthy surge in attention within the research community. Different techniques can be utilized to create these CDs, spanning from basic candle burning to laser ablation. This review article delves into the principles of fluorescence technology, providing insights into how different synthesis methods of quantum dots impact their luminescent properties. Additionally, it highlights the latest applications of quantum dots in catalysis and biomedical fields, with special emphasis on the current status of luminescent properties in biology and chemistry. Towards the end, the article discusses the limitations of quantum dots in current practical applications, pointing out that CDs hold promising potential for future applications.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Histamine Recognition by Carbon Dots from Plastic Waste and Development of Cellular Imaging: Experimental and Theoretical Studies

The present work highlights the sustainable approach for the transformation of plastic waste into fluorescent carbon dots (CDs) through carbonization and then they were functionalized with L-cysteine and o-phenylenediamine. CDs which were characterized by different analytical techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are employed to recognize Cu2+, Fe2+, and Hg2+ ions. The results show that the fluorescence emission was considerably quenched, and it is consistent with the interference and Jobs plots. The detection limit was found to be 0.35µM for Cu(II), 1.38 µM for Hg(II), and 0.51µM Fe(III). The interaction of CDs with metal ions enhances the fluorescence intensity detecting histamine successfully. It shows that plastic waste-based CDs can be applied clinically to detect toxic metals and biomolecules. Moreover, the system was employed to develop the cellular images using Saccharomyces cerevisiae cells with the support of a confocal microscope. Furthermore, theoretical studies were performed for the naphthalene layer (AR) as a model for C-dots, then optimized its structure and analyzed by using the molecular orbital. The obtained TD-DFT spectra coincided with experimental spectra for CDs/M2+/histamine systems.

Rationally Designed Matrix-Free Carbon Dots with Wavelength-Tunable Room-Temperature Phosphorescence

We report the rational design of the matrix-free carbon dots (C-dots) with long wavelength and wavelength-tunable room-temperature phosphorescence (RTP). Taking advantage of microwave-assisted heating treatment, three RTP C-dots in boric acid (BA) composites are synthesized by using diethylenetriaminepentakis (methylphosphonic acid) as a multiple-sites crosslink agent, a moderately acid catalyst and P source; phenylenediamines (either o-PD, m-PD, or p-PD, respectively) as building block while BA as a carbonization-retardant matrix. After the water-soluble BA matrix is removed by dialysis, three matrix-free C-dots are obtained with RTP emission at 540, 550 and 570 nm under an excitation wavelength of 365 nm. Alterations of RTP emission of three matrix-free C-dots are ascribed to the difference in their particle size and band gap from n-π* transition. Furthermore, the application of three matrix-free C-dots are successfully performed in information encryption and decryption.

Fluorescence Turns on-off-on Sensing of Ferric Ion and L-Ascorbic Acid by Carbon Quantum Dots

This study used a hydrothermal approach to create a sensitive and focused nanoprobe. Using an “on-off-on” sensing mechanism, the nanoprobe was employed to detect and quantify ferric ions and L-ascorbic acid. Synthesis of the carbon quantum dots was achieved with a single hydrothermal step at 180°C for 24 hours using hot pepper as the starting material. The prepared CQDs showed high fluorescence with a quantum yield of 30% when excited at 350 nm, exhibiting excitation-dependent fluorescence. The emission of the CQDs can be quenched by adding ferric ions, which can be attributed to complex formation leading to nonradiative photoinduced electron transfer (PET). Adding L-ascorbic acid, which can convert ferric ions into ferrous ions, break the complex, and restore the fluorescence of CQD. The linear range and LOD were (10–90) μM and 1 μM for ferric ions, respectively, and L-ascorbic acid’s linear range was (5–100) μM while LOD was 0.1 μM quantification of both substances was accomplished. In addition, orange fruit was used as an actual sample source for ascorbic acid analysis, yielding up to 99% recovery.

Carbon nanodots with a controlled N structure by a solvothermal method for generation of reactive oxygen species under visible light

Carbon nanodots can function as photosensitizers that have the ability to generate reactive oxygen species such as singlet oxygen, hydroxy (OH) radicals, and superoxide ions. However, most of these can only be generated upon ultraviolet light excitation. Additionally, the mechanism of reactive oxygen species generation by carbon nanodots remains unclear. The development of carbon nanodots that can photosensitize under visible light irradiation is desirable for applications such as photodynamic therapy and pollutant decomposition under visible light. Here, we report novel carbon nanodot-based photosensitizers that generate reactive oxygen species under visible light; they were synthesized using a solvothermal method with two solvents (formamide and water) and amidol as the carbon source. Carbon nanodots from the solvothermal synthesis in formamide showed blue fluorescence, while those obtained in water showed green fluorescence. The photo-excited blue-fluorescent carbon nanodots produced OH radicals, superoxide ions, and singlet oxygen, and therefore could function as both type I and type II photosensitizers. In addition, photo-excited green-fluorescent carbon nanodots generated only singlet oxygen, therefore functioning as type II photosensitizers. It is proposed that the two photosensitizers have different origins of reactive oxygen species generation: the enrichment of graphitic N for blue-fluorescent carbon nanodots and molecular fluorophores for green-fluorescent carbon nanodots.

Carbon Nanodots from an In Silico Perspective

Carbon nanodots (CNDs) are the latest and most shining rising stars among photoluminescent (PL) nanomaterials. These carbon-based surface-passivated nanostructures compete with other related PL materials, including traditional semiconductor quantum dots and organic dyes, with a long list of benefits and emerging applications. Advantages of CNDs include tunable inherent optical properties and high photostability, rich possibilities for surface functionalization and doping, dispersibility, low toxicity, and viable synthesis (top-down and bottom-up) from organic materials. CNDs can be applied to biomedicine including imaging and sensing, drug-delivery, photodynamic therapy, photocatalysis but also to energy harvesting in solar cells and as LEDs. More applications are reported continuously, making this already a research field of its own. Understanding of the properties of CNDs requires one to go to the levels of electrons, atoms, molecules, and nanostructures at different scales using modern molecular modeling and to correlate it tightly with experiments. This review highlights different in silico techniques and studies, from quantum chemistry to the mesoscale, with particular reference to carbon nanodots, carbonaceous nanoparticles whose structural and photophysical properties are not fully elucidated. The role of experimental investigation is also presented. Hereby, we hope to encourage the reader to investigate CNDs and to apply virtual chemistry to obtain further insights needed to customize these amazing systems for novel prospective applications.

Transfer of Axial Chirality to the Nanoscale Endows Carbon Nanodots with Circularly Polarized Luminescence

We report the synthesis, purification and characterization of chiral carbon nanodots starting from atropoisomeric precursors. The obtained atropoisomeric carbon nanodots are soluble in organic solvents and have good thermal stability, which are desirable features for technological applications. The synthetic protocol is robust, as it supports a number of variations in terms of molecular doping agents. Remarkably, the combination of axially chiral precursors and 1,4‐benzoquinone as doping agent results in green‐emissive carbon dots displaying circularly polarized luminescence. Dissymmetry factors of |3.5|×10⁻⁴ are obtained in solution, without the need of any additional element of chirality. Introducing axial chirality expands the strategies available to tailor the properties of carbon nanodots, paving the way for carbon nanoparticles that combine good processability in organic solvents with engineered advanced chiroptical properties.

Pyrolysis and Solvothermal Synthesis for Carbon Dots: Role of Purification and Molecular Fluorophores

Over the last decade, the interest in carbon dots, graphene dots, or similar carbon-based nanoparticles has increased considerably. This interest is based on potentially high fluorescent quantum yields, controllable excitation-dependent emission, low toxicity, and convenient reaction conditions. Carbon dots are often seen as a promising alternative to classical semiconductor quantum dots that are typically made from toxic semiconductor materials. Surprisingly, aspects like the atomic structure, composition, mechanism of formation, and precise understanding of the photophysical properties of carbon dots are still mostly unknown. The large number of different precursor systems and the variety in synthesis routes make a direct comparison of different systems difficult. To advance this, we went for a systematic approach and compared the results of four synthesis routes using two different precursor systems. We used different spectroscopy and microscopy methods including fluorescence correlation spectroscopy to characterize the different reaction products. We found that for syntheses solely based on citric acid as the precursor, we obtain particles where the emission wavelength is strongly dependent on the excitation wavelength despite relatively low quantum yields. In comparison, when urea is added as a nitrogen doping reactant, we observe vastly increased quantum yields. By making use of a combination of dialysis and column chromatography, we were able to isolate various luminescent species with high quantum yields and verify the existence of different molecular fluorophores. A detailed and consistent characterization of the reaction products during the course of purification revealed strong interactions between molecular fluorophores and larger reaction products.

Emerging Strategies in Enhancing Singlet Oxygen Generation of Nano-Photosensitizers Toward Advanced Phototherapy

The great promise of photodynamic therapy (PDT) has thrusted the rapid progress of developing highly effective photosensitizers (PS) in killing cancerous cells and bacteria. To mitigate the intrinsic limitations of the classical molecular photosensitizers, researchers have been looking into designing new generation of nanomaterial-based photosensitizers (nano-photosensitizers) with better photostability and higher singlet oxygen generation (SOG) efficiency, and ways of enhancing the performance of existing photosensitizers. In this paper, we review the recent development of nano-photosensitizers and nanoplasmonic strategies to enhance the SOG efficiency for better PDT performance. Firstly, we explain the mechanism of reactive oxygen species generation by classical photosensitizers, followed by a brief discussion on the commercially available photosensitizers and their limitations in PDT. We then introduce three types of new generation nano-photosensitizers that can effectively produce singlet oxygen molecules under visible light illumination, i.e., aggregation-induced emission nanodots, metal nanoclusters (< 2 nm), and carbon dots. Different design approaches to synthesize these nano-photosensitizers were also discussed. To further enhance the SOG rate of nano-photosensitizers, plasmonic strategies on using different types of metal nanoparticles in both colloidal and planar metal-PS systems are reviewed. The key parameters that determine the metal-enhanced SOG (ME-SOG) efficiency and their underlined enhancement mechanism are discussed. Lastly, we highlight the future prospects of these nanoengineering strategies, and discuss how the future development in nanobiotechnology and theoretical simulation could accelerate the design of new photosensitizers and ME-SOG systems for highly effective image-guided photodynamic therapy.

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.

Zn-assisted modification of the chemical structure of N-doped carbon dots and their enhanced quantum yield and photostability

This article presents the Zn-assisted synthesis of N-doped carbon dots (N-CDs) with an enhanced quantum yield (QY) and photostability. There have been intensive studies to improve or tune the optical properties of carbon dots (CDs) to meet the demand for luminescent materials in various fields, including energy conversion, photocatalysis, bioimaging, and phototherapy. For these applications, the photostability of the CDs is also a critical factor, but the related studies are relatively less common. The Zn-assisted N-CDs (denoted as Zn:N-CDs) obtained by the addition of Zn(OAc)₂ to the precursors during the synthesis of N-CDs not only exhibited an enhanced quantum yield but also improved photostability compared to those of N-CDs. A comprehensive study of the chemical composition of Zn:N-CD and N-CD using X-ray photoelectron spectroscopy indicated a correlation between their chemical structure and photostability. Zn(OAc)₂, which acts as a catalytic reagent, induced the modification of chemical structures at the edges of carbogenic sp² domains, without being doped in N-CD, and the heteroatom-carbon bonds in Zn:N-CD seemed to be more resistant to light compared to those in N-CDs. The increased QY and photostability of Zn:N-CDs make them more suitable as an optical probe and they could be used in fingerprint identification. With Zn:N-CDs, the microstructure of fingerprints was confirmed clearly for a long duration effectively.

Chiral carbon dots: synthesis, optical properties, and emerging applications

Carbon dots are luminescent carbonaceous nanoparticles that can be endowed with chiral properties, making them particularly interesting for biomedical applications due to their low cytotoxicity and facile synthesis. In recent years, synthetic efforts leading to chiral carbon dots with other attractive optical properties such as two-photon absorption and circularly polarized light emission have flourished. We start this review by introducing examples of molecular chirality and its origins and providing a summary of chiroptical spectroscopy used for its characterization. Then approaches used to induce chirality in nanomaterials are reviewed. In the main part of this review we focus on chiral carbon dots, introducing their fabrication techniques such as bottom-up and top-down chemical syntheses, their morphology, and optical/chiroptical properties. We then consider emerging applications of chiral carbon dots in sensing, bioimaging, and catalysis, and conclude this review with a summary and future challenges.

Heavy metal ion detection using green precursor derived carbon dots

The discovery of carbon dots (CDs) for environmental remediation has gained awareness because of the diverse economically viable and environmental friendly green precursors generated from biowastes and biomass compared to the toxic inorganic quantum dots and CDs prepared from chemical precursors. This review presents the recent progress in green CDs, including their synthesis methods and sensing applications for the detection of heavy metal ions such as Iron (III), Mercury (II), Copper (II), Chromium (VI), Lead (II), Arsenic (III), Cobalt (II), Aluminum (III), Silver (I), and Gold (III) which are prominent environmental pollutants. The comparison based on selectivity, sensitivity, quantum yield, detection limit, linear concentration range, and sensing mechanisms are also reported. This review also covers the performance of doped green CDs using heteroatoms, toward the detection of heavy metal ions. Apart from the future perspectives, this review provides a general guide to use such environmental friendly CDs to detect harmful pollutants.

Carbon-Based Nanomaterials for Sustainable Agriculture: Their Application as Light Converters, Nanosensors, and Delivery Tools

Nano-enabled agriculture is now receiving increasing attentions. Among the used nanomaterials, carbon-based nanomaterials are good candidates for sustainable agriculture. Previous review papers about the role of carbon-based nanomaterials in agriculture are either focused on one type of carbon-based nanomaterial or lack systematic discussion of the potential wide applications in agriculture. In this review, different types of carbon-based nanomaterials and their applications in light converters, nanosensors, and delivery tools in agriculture are summarized. Possible knowledge gaps are discussed. Overall, this review helps to better understand the role and the potential of carbon-based nanomaterials for nano-enabled agriculture.

Citric acid/β-alanine carbon dots as a novel tool for delivery of plasmid DNA into E. coli cells

Successful delivery of plasmid DNA into the microbial cells is fundamental in recombinant DNA technology. Natural bacterial transformation is limited to only certain species due in part to the repulsive forces between negatively charged DNA and bacterial membranes. Most common method of DNA delivery into bacteria is artificial transformation through heat shock and electroporation. These methods require sophisticated instruments and tedious steps in preparation of competent cells. Transformation by conjugation is also not applicable to all plasmids. Nanoparticles have been used successfully in therapeutics for drug delivery into animal cells. They are starting to gain popularity in plant sciences as novel DNA nano carriers. Despite their promise as tool for DNA delivery, their use in microbial cell transformation has not been reported yet. Here we report the synthesis of carbon dots (CDs) from citric acid and β-alanine and their use in DNA delivery into E. coli cells. CDs were fabricated using microwave assisted synthesis. Plasmids carrying RFP reporter and ampicillin resistance genes were transferred to bacterial cells and further confirmed using polymerase chain reaction. Our findings indicate that CDs can be used successfully for delivery of foreign DNA of up to 10 kb into E. coli. We have demonstrated the use of β-alanine/citric acid carbon dots as nanocarriers of DNA into E. coli cells and identified their limitation in terms of the size of plasmid DNA they could carry. Use of these carbon dots is a novel method in foreign DNA delivery into bacterial cells and have a potential for the transformation of resistant organism for which there is still no reliable DNA delivery systems.

Exploring the Role of Surface States in Emissive Carbon Nanodots: Analysis at Single-Particle Level

Fluorescent carbon nanodots (CDs) have been highlighted as promising semiconducting materials due to their outstanding chemical and optical properties. However, the intrinsic heterogeneity of CDs has impeded a clear understanding of the mechanisms behind their photophysical properties. In this study, as-prepared CDs are fractionated via chromatography to reduce their structural and chemical heterogeneity and analyzed through ensemble and single-particle spectroscopies. Many single particles reveal fluorescence intensity fluctuations between two or more discrete levels with bi-exponential decays. While the intrinsic τ₁ components are uniform among single particles, the τ₂ components from molecule-like emissions spans a wider range of lifetimes, reflecting the inhomogeneity of the surface states. Furthermore, it is concluded that the relative population and chemical states of surface functional groups in CDs have a significant impact on emissive states, brightness, blinking, stability, and lifetime distribution of photoluminescence.

Highly Enhanced Enzymatic Activity of Mn-Induced Carbon Dots and Their Application as Colorimetric Sensor Probes

Nanomaterial-based enzyme mimetics (nanozymes) have attracted significant interest because of their lower cost and higher stability compared to natural enzymes. In this study, we focused on improving the enzymatic properties of metal induced N-doped carbon dots (N-CDs), which are nanozymes of interest, and their applications for sensory systems. For this purpose, Mn(acetate)2 was introduced during the synthetic step of N-doped carbon dots, and its influence on the enzymatic properties of Mn-induced N-CDs (Mn:N-CDs) was investigated. Their chemical structure was analyzed through infrared spectroscopy and X-ray photoelectron spectrometry; the results suggest that Mn ions lead to the variation in the population of chemical bonding in Mn:N-CDs, whereas these ions were not incorporated into N-CD frameworks. This structural change improved the enzymatic properties of Mn:N-CDs with respect to those of N-CDs when the color change of a 3,3',5,5'-tetramethylbenzidine/H2O2 solution was examined in the presence of Mn:N-CDs and N-CDs. Based on this enhanced enzymatic property, a simple colorimetric system with Mn:N-CDs was used for the detection of γ-aminobutyric acid, which is an indicator of brain-related disease. Therefore, we believe that Mn:N-CDs will be an excellent enzymatic probe for the colorimetric sensor system.

Antibacterial Properties of Citric Acid/β-Alanine Carbon Dots against Gram-Negative Bacteria

While multi-drug resistance in bacteria is an emerging concern in public health, using carbon dots (CDs) as a new source of antimicrobial activity is gaining popularity due to their antimicrobial and non-toxic properties. Here we prepared carbon dots from citric acid and β-alanine and demonstrated their ability to inhibit the growth of diverse groups of Gram-negative bacteria, including E. coli, Salmonella, Pseudomonas, Agrobacterium, and Pectobacterium species. Carbon dots were prepared using a one-pot, three-minute synthesis process in a commercial microwave oven (700 W). The antibacterial activity of these CDs was studied using the well-diffusion method, and their minimal inhibitory concentration was determined by exposing bacterial cells for 20 h to different concentrations of CDs ranging from 0.5 to 10 mg/mL. Our finding indicates that these CDs can be an effective alternative to commercially available antibiotics. We also demonstrated the minimum incubation time required for complete inhibition of bacterial growth, which varied depending on bacterial species. With 15-min incubation time, A. tumefaciens and P. aeruginosa were the most sensitive strains, whereas E. coli and S. enterica were the most resistant bacterial strains requiring over 20 h incubation with CDs.

One Spot Microwave Synthesis and Characterization of Nitrogen-Doped Carbon Dots with High Oxygen Content for Fluorometric Determination of Banned Sudan II Dye in Spice Samples

A simple microwave-assisted synthesis of nitrogen-doped carbon dots with high oxygen content (O-N-CDs) was carried out with citric acid as a carbon source and 2,4-diamino-6-methyl-1,3,5-triazine as a nitrogen source in triethylene glycol (TEG) media. It was determined by SEM analysis that O-N-CDs consisted of particles of different sizes and shapes. Transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction (XRD) analysis confirmed that O-N-CDs have a graphitic structure. Moreover, they showed a high fluorescence property based on the excitation wavelength. Therefore, a new fluorometric method was developed for the determination of banned food dye Sudan II by using the O-N-CDs. The proposed method was used in the determination of Sudan II in spiked spice samples. The detection limit was 0.6 mg L^-1 and the linear range was 0-8 mg L^-1.

Novel Hg (II) selective fluorescent green sensor based on carbon dots synthesized from starch and functionalized with methimazole

We describe a green new method for the synthesis of water-soluble photoluminescent carbon dots (CDs) that were functionalized with methimazole (MTZ) and applied to determine Hg²⁺ based on the fluorescence extinction. Starch obtained from rice was used as a natural source for the production of CDs by hydrothermal treatment. Also, it was proposed a factorial design to optimize the parameters for CD synthesis and the results showed that the luminescence intensity is a function of temperature and not of the heating time in the hydrothermal process. The synthesized CDs were characterized using fluorescence techniques, Fourier transform infrared spectroscopy (FTIR), and UV-Vis spectroscopy. Through transmission electron microscopy (TEM) and dynamic light scattering (DLS), it was found the formation of CDs on a nanometer scale with an average size of 11 nm. The functionalization with MTZ, eliminated all interferences from other metals, indicating a selective response to Hg²⁺ ions. The method was applied to Hg²⁺ determination in waters. Under optimal conditions, was obtained a limit of detection of 1.8 × 10^-7 mol L^-1 with a linear range from 3.3 × 10^-7 to 50.0 × 10^-6 mol L^-1. Therefore, the proposed method can be considered a simple, selective, and precise alternative that minimizes the number of reagents used for Hg²⁺ determination in natural waters, and can be applied on a large scale in environmental analyzes.

Green Sources Derived Carbon Dots for Multifaceted Applications

For the past decade, the Carbon dots (CDs) a tiny sized carbon nanomaterial are typically much attentive due to their outstanding properties. Nature is a fortune of exciting starting materials that provides many inexpensive and renewable resources which have received the topmost attention of researchers because of non-hazardous and eco-friendly nature that can be used to prepare green CDs by top-down and bottom-up synthesis including hydrothermal carbonization, microwave synthesis, and pyrolysis due to its simple synthetic process, speedy reactions and clear-cut end steps. Compared to chemically derived CDs, green CDs are varied by their properties such as less toxicity, high water dispersibility, superior biocompatibility, good photostability, bright fluorescence, and ease of modification. These nanomaterials are a promising material for sensor and biological fields, especially in electrochemical sensing of toxic and trace elements in ecosystems, metal sensing, diagnosis of diseases through bio-sensing, and detection of cancerous cells by in-vitro and in-vivo bio-imaging applications. In this review, the various synthetic routes, fluorescent mechanisms, and applications of CDs from discovery to the present are briefly discussed. Herein, the latest developments on the synthesis of CDs derived from green carbon materials and their promising applications in sensing, catalysis and bio-imaging were summarized. Moreover, some challenging problems, as well as upcoming perspectives of this powerful and tremendous material, are also discussed.

Paper microfluidic device using carbon dots to detect glucose and lactate in saliva samples

Bioanalyses are commonly performed with blood or serum samples. However, these analyses often require invasive and painful blood collection using a needle or finger pricking. Saliva is an alternative and very attractive biological medium for performing clinical analyses, since it contains many types of clinically relevant biomarkers and compounds. Its collection is straightforward and can be achieved in a non-invasive and stress-free way. However, the analytes are frequently present at low concentrations, while the viscosity of whole saliva hinders its analysis using paper devices, especially those with multiple layers (3D-μPADs). This work explores the use of a simple, fast, and low-cost saliva sample pretreatment using a cotton-paper-syringe filtration system, allowing the analysis of saliva samples using multilayer paper devices. The proposed methodology employs the oxidation of glucose and lactate, catalyzed by specific oxidase enzymes, producing hydrogen peroxide. The detection is based on the fluorescence quenching of carbon dots in the presence of hydrogen peroxidase. The concentrations of the analytes showed good linear correlations with the fluorescence quenching, with LODs of 2.60 × 10^-6 and 8.14 × 10^-7 mol L^-1 for glucose and lactate, respectively. The proposed method presented satisfactory intra-day and inter-day repeatabilities, with %RSD values in the range 3.82-6.61%. The enzymatic systems proved to be specific for the analytes and the matrix had no significant influence on the glucose and lactate determinations. The proposed methodology was successfully applied to saliva and serum samples and was validated using certified material.

Study and Comparison on Purification Methods of Multicolor Emission Carbon Dots

Recent years have witnessed a rapid development of carbon dots (CDs), due to their outstanding luminescence properties and excellent biocompatibility. However, the internal structure and photoluminescent (PL) mechanism of CDs are still the subject of considerable debate, which is due to the fact that reaction products usually contain mixtures of several CD fractions as well as molecular intermediate and side products. Therefore, careful purification of the CDs is significant for analysis of structure and luminescence mechanism. Here, multicolor emission CDs were prepared by a one-pot pyrolysis of citric acid in formamide. Then, the precipitation method, dialysis and gel permeation chromatography (GPC) are successively employed to purify the multicolor emission CDs. This post-treatment allowed us to compare the optical properties of CDs obtained by different separation methods and provide a valuable guidance for the purification of CDs.

Highly sensitive fluorometric method based on nitrogen-doped carbon dot clusters for tartrazine determination in cookies samples

Nitrogen-doped carbon nanodots (CDs) were prepared via the solvothermal method, using urea and triethylene glycol as the starting materials. The as-prepared CDs had individual diameters of approximately 100 nm and were in clusters of different sizes. The surface composition and optical properties of the as-prepared CDs were characterized. They exhibited multicolor emission properties in the visible range when excited with a wide wavelength range. The aqueous solution of the CDs was used in highly sensitive tartrazine determination. The fluorescence quenching of the CDs was in a linear relationship with the concentrations of tartrazine in the range of 0.5–30.0 μM. The detection limit of the assay was 0.18 μM. Acceptable recovery results were obtained via spike-recovery experiments on cookie samples.

Effect of heteroatoms on the optical properties and enzymatic activity of N-doped carbon dots

Carbon dots (CDs) are attractive nanomaterials because of their facile synthesis, biocompatibility, superior physicochemical properties, and low cost of their precursors. Recent advances in CDs have particularly relied on the modulation of their properties by heteroatom doping (e.g., nitrogen). Although nitrogen-doped CDs (N-CDs) have attracted considerable attention owing to their different properties compared to those of the original CDs, the effects of the heteroatom content and types of bonding on the properties of N-doped CDs remain underexplored. In this work, we prepared N-CDs with controlled nitrogen contents, and fully examined their optical properties, enzymatic activity, and toxicity. We demonstrate that (i) the type of carbon–heteroatom bonding (i.e., carbon–oxygen and carbon–nitrogen bonds) can be altered by changing the ratio of carbon to heteroatom sources, and (ii) both the heteroatom content and the heteroatom-bonding character significantly influence the properties of the doped CDs. Notably, N-CDs exhibited higher quantum yields and peroxidase-like activities than the non-doped CDs. Furthermore, the negatively charged N-CDs exhibited negligible cytotoxicity. Such comprehensive investigations on the physicochemical properties of N-CDs are expected to guide the design of N-CDs for targeted applications.

The characteristics of N-CDs suitable for their optical applications or for use as nanozymes were demonstrated by rationalizing the relationship between the dopant content (e.g., the amount of doped N and types of chemical bonding) and physicochemical properties.

Multicolor polymeric carbon dots: synthesis, separation and polyamide-supported molecular fluorescence

Multicolor carbon dots (CDs) have been developed recently and demonstrate great potential in bio-imaging, sensing, and LEDs. However, the fluorescence mechanism of their tunable colors is still under debate, and efficient separation methods are still challenging. Herein, we synthesized multicolor polymeric CDs through solvothermal treatment of citric acid and urea in formamide. Automated reversed-phase column separation was used to achieve fractions with distinct colors, including blue, cyan, green, yellow, orange and red. This work explores the physicochemical properties and fluorescence origins of the red, green, and blue fractions in depth with combined experimental and computational methods. Three dominant fluorescence mechanism hypotheses were evaluated by comparing time-dependent density functional theory and molecular dynamics calculation results to measured characteristics. We find that blue fluorescence likely comes from embedded small molecules trapped in carbonaceous cages, while pyrene analogs are the most likely origin for emission at other wavelengths, especially in the red. Also important, upon interaction with live cells, different CD color fractions are trafficked to different sub-cellular locations. Super-resolution imaging shows that the blue CDs were found in a variety of organelles, such as mitochondria and lysosomes, while the red CDs were primarily localized in lysosomes. These findings significantly advance our understanding of the photoluminescence mechanism of multicolor CDs and help to guide future design and applications of these promising nanomaterials.

Carbon Nanoparticles and Materials on Their Basis

Carbon nanoparticles (CNPs) are novel nanostructures with luminescent properties. The development of CNPs involves the elaboration of various synthetic methods, structure characterization, and different applications. However, the problems associated with the CNP structure definition and properties homogeneity are not solved and barely described in depth. In this feature article, we demonstrate the approaches for the effective separation and purification of CNPs by size and size/charge ratio. We propose a promising way for the synthesis of the uniform-size structures by the application of calcium carbonate porous microparticles as reactors with defined size. Additionally, the application of the CNPs agglomerates for controllable release systems triggered by light and in-situ synthesis of fluorescent conductive carbonaceous films on the base of polyelectrolyte multilayers are under consideration.

Applications of fluorescent materials in the detection of alkaline phosphatase activity

Alkaline phosphatase (ALP) is important in the diagnosis of many diseases. Because ALP is used to detect biomarkers for many diseases, many researchers conduct investigations to develop ALP detection strategies. The use of fluorescent material has attracted attention because of the technique's high sensitivity and the low sample volume required. Herein, we review and discuss the working mechanisms and advantages of four main categories:DNA fluorescent probes, molecular fluorescent probes, chemical coordination-based probes, and nanoparticle probes. Development prospects and trends are also discussed.

Carbon nanodots revised: the thermal citric acid/urea reaction

Luminescent compounds obtained from the thermal reaction of citric acid and urea have been studied and utilized in different applications in the past few years. The identified reaction products range from carbon nitrides over graphitic carbon to distinct molecular fluorophores. On the other hand, the solid, non-fluorescent reaction product produced at higher temperatures has been found to be a valuable precursor for the CO₂-laser-assisted carbonization reaction in carbon laser-patterning. This work addresses the question of structural identification of both, the fluorescent and non-fluorescent reaction products obtained in the thermal reaction of citric acid and urea. The reaction products produced during autoclave-microwave reactions in the melt were thoroughly investigated as a function of the reaction temperature and the reaction products were subsequently separated by a series of solvent extractions and column chromatography. The evolution of a green molecular fluorophore, namely HPPT, was confirmed and a full characterization study on its structure and photophysical properties was conducted. The additional blue fluorescence is attributed to oligomeric ureas, which was confirmed by complementary optical and structural characterization. These two components form strong hydrogen-bond networks which eventually react to form solid, semi-crystalline particles with a size of ∼7 nm and an elemental composition of 46% C, 22% N, and 29% O. The structural features and properties of all three main components were investigated in a comprehensive characterization study.

A minireview on doped carbon dots for photocatalytic and electrocatalytic applications

To date, carbon dots (CDs) or carbon quantum dots (CQDs), considered as alternatives to conventional fluorescent materials such as organic dyes and semiconductor quantum dots (QDs), have drawn significant attention from relevant researchers due to their superior properties, including nontoxicity, biocompatibility, low cost and facile synthesis, and high photoluminescence. In particular, doping heteroatoms with CDs can not only dramatically enhance the fluorescence but also greatly improve the electronic structure and doped CDs have been successfully applied in various technological fields. Herein, this minireview summarizes recent advances on the synthesis and optical properties of doped CDs and their promising applications for photocatalysis and electrocatalysis. Finally, some challenging issues as well as future perspectives of this exciting material are discussed.

Evaluation of Different Bottom-up Routes for the Fabrication of Carbon Dots

Carbon dots (CDs) are carbon-based nanoparticles with very attractive luminescence features. Furthermore, their synthesis by bottom-up strategies is quite flexible, as tuning the reaction precursors and synthesis procedures can lead to an endless number of CDs with distinct properties and applications. However, this complex variability has made the characterization of the structural and optical properties of the nanomaterials difficult. Herein, we performed a systematic evaluation of the effect of three representative bottom-up strategies (hydrothermal, microwave-assisted, and calcination) on the properties of CDs prepared from the same precursors (citric acid and urea). Our results revealed that these synthesis routes led to nanoparticles with similar sizes, identical excitation-dependent blue-to-green emission, and similar surface-functionalization. However, we have also found that microwave and calcination strategies are more efficient towards nitrogen-doping than hydrothermal synthesis, and thus, the former routes are able to generate CDs with significantly higher fluorescence quantum yields than the latter. Furthermore, the different synthesis strategies appear to have a role in the origin of the photoluminescence of the CDs, as hydrothermal-based nanoparticles present an emission more dependent on surface states, while microwave- and calcination-based CDs present an emission with more contributions from core states. Furthermore, calcination and microwave routes are more suitable for high-yield synthesis (~27-29%), while hydrothermal synthesis present almost negligible synthesis yields (~2%). Finally, life cycle assessment (LCA) was performed to investigate the sustainability of these processes and indicated microwave synthesis as the best choice for future studies.

Enhanced Photosensitization by Carbon Dots Co-adsorbing with Dye on p-Type Semiconductor (Nickel Oxide) Solar Cells

In this work, the effect of carbon dots (C-dots) on the performance of NiO-based dye-sensitized solar cells (DSSCs) was explored. NiO nanoparticles (NPs) with a rectangular shape (average size: 11.4 × 16.5 nm²) were mixed with C-dots, which were synthesized from citric acid (CA) and ethylenediamine (EDA). A photocathode consisting of a composite of C-dots with NiO NPs (NiO@C-dots) was then used to measure the photovoltaic performance of a DSSC. A power conversion efficiency (PCE) of 9.85% (430 nm LED@50 mW/cm²) was achieved by a DSSC fabricated via the adsorption of N719 sensitizer with a C-dot content of 12.5 wt % at a 1.5:1 EDA/CA molar ratio. This PCE value was far larger than the PCE value (2.44 or 0.152%) obtained for a NiO DSSC prepared without the addition of C-dots or N719, respectively, indicating the synergetic effect by the co-adsorption of C-dots and N719. This synergetically higher PCE of the NiO@C-dot-based DSSC was due to the larger amount of sensitizer adsorbed onto the composites with a larger specific surface area and the faster charge transfer in the NiO@C-dot working electrode. In addition, the C-dots bound to the NiO NPs shorten the band gap of the NiO NPs due to energy transfer and give rise to faster charge separation in the electrode. The most important fact is that C-dots are the main sensitizer, while N719 tightly adsorbs on C-dots and NiO behaves as an accelerator of a positive electron transfer and a restrainer of the electron-hole recombination. These results reveal that C-dots are a remarkable enhancer for NiO NPs in DSSCs and that NiO@C-dots are promising photovoltaic electrode materials for DSSCs.

Nitrogen-doped carbon dot threads as a "turn-off" fluorescent probe for permanganate ions and its hydrogel hybrid as a naked eye sensor for gold(III) ions

Highly fluorescent nitrogen-doped carbon dot (NCD) threads were synthesized via simple pyrolysis of citric acid, p-hydroxybenzoic acid, and ammonia. The NCDs show excitation-independent behavior with maximum excitation and emission wavelengths of 350 nm and 435 nm, respectively. The developed probe was used as a turn-off fluorescent sensor for the selective and sensitive determination of permanganate ions in aqueous media. The probe's hydrogel hybrid displayed a beautiful purple color demonstrating its potential as a naked eye sensor for gold detection. The ratiometric sensor exhibited excellent selectivity towards permanganate ions over 27 other ions with a linear range of 510 nM to 2 μM, a detection limit of 170 nM, and a linear regression value (R²) of 0.9944. Similarly, the linear range and limit of detection for gold ions was 3.89-20 μM and 1.285 μM, respectively. The synthesized NCDs were also used as a fluorescent ink as well as a naked eye marker in association with a gold solution demonstrating its potential forensic and anti-counterfeiting applications. Graphical abstract.

Carbon Dots for Bacterial Detection and Antibacterial Applications-A Minireview

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The prevention and treatment of various infections caused by microbes through antibiotics are becoming less effective due to antimicrobial resistance. Researches are focused on antimicrobial nanomaterials to inhibit bacterial growth and destroy the cells, to replace conventional antibiotics. Recently, carbon dots (C-Dots) become attractive candidates for a wide range of applications, including the detection and treatment of pathogens. In addition to low toxicity, ease of synthesis and functionalization, and high biocompatibility, C-Dots show excellent optical properties such as multi-emission, high brightness, and photostability. C-Dots have shown great potential in various fields, such as biosensing, nanomedicine, photo-catalysis, and bioimaging. This review focuses on the origin and synthesis of various C-Dots with special emphasis on bacterial detection, the antibacterial effect of CDots, and their mechanism.

Fe(III) doped carbon nanodots with intense green photoluminescence and dispersion medium dependent emission

The preparation and investigation of Fe(III) doped carbon nanodots (CNDs) with intense green photoluminescence and emission dependence on the dispersion medium are reported. Their unusual photoluminescence is especially highlighted in water where the initial blue emission is gradually shifted to intense deep green, while in other common solvents (chloroform, acetone etc.) this behavior has not been observed. Through embedding in a polymer matrix (e.g., PVA) the color transition becomes reversible and dependent on water content, ranging from a full blue emission, when completely dried, to an intense green emission, when wetted. The preparation path of the Fe(III) doped CNDs undergoes two main stages involving the initial obtaining of Fe(III)-N-Hydroxyphthalimide complex and then a thermal processing through controlled pyrolysis. Morphostructural investigations of the prepared Fe(III) doped CNDs were performed through TG, FT-IR, XPS, DLS, TEM and AFM techniques whereas absolute PLQY, steady state and lifetime fluorescence were used to highlight their luminescence properties. The results issued from structural and fluorescence investigations bring new insights on the particular mechanisms involved in CNDs photoluminescence, a topic still open to debate.

Advances in nanomaterial application in enzyme-based electrochemical biosensors: a review

Electrochemical enzyme-based biosensors are one of the largest and commercially successful groups of biosensors. Integration of nanomaterials in the biosensors results in significant improvement of biosensor sensitivity, limit of detection, stability, response rate and other analytical characteristics. Thus, new functional nanomaterials are key components of numerous biosensors. However, due to the great variety of available nanomaterials, they should be carefully selected according to the desired effects. The present review covers the recent applications of various types of nanomaterials in electrochemical enzyme-based biosensors for the detection of small biomolecules, environmental pollutants, food contaminants, and clinical biomarkers. Benefits and limitations of using nanomaterials for analytical purposes are discussed. Furthermore, we highlight specific properties of different nanomaterials, which are relevant to electrochemical biosensors. The review is structured according to the types of nanomaterials. We describe the application of inorganic nanomaterials, such as gold nanoparticles (AuNPs), platinum nanoparticles (PtNPs), silver nanoparticles (AgNPs), and palladium nanoparticles (PdNPs), zeolites, inorganic quantum dots, and organic nanomaterials, such as single-walled carbon nanotubes (SWCNTs), multi-walled carbon nanotubes (MWCNTs), carbon and graphene quantum dots, graphene, fullerenes, and calixarenes. Usage of composite nanomaterials is also presented.