A high-yield and versatile method for the synthesis of carbon dots for bioimaging applications

Enantioselective adsorption of ibuprofen enantiomers using chiral-active carbon nanoparticles induced S-α-methylbenzylamine

The chiral media is crucial to the chiral recognition and separation of enantiomers. In this study, we report the preparation of novel chiral carbon nanoparticles (CCNPs) via surface passivation using glucose as the carbon source and S-(-)-α-methylbenzylamine as the chiral ligand. The structures of the obtained CCNPs are characterized via FT-IR, Raman spectroscopy, DLS, XPS, XRD, TEM, and zeta potential analysis. These CCNPs could be employed as the chiral adsorbent and used for the enantioselective adsorption of the ibuprofen enantiomers. The results demonstrated that the CCNPs could selectively adsorb R-enantiomer from ibuprofen racemate solution and give an enantiomeric excess (e.e.) of about 50% under an optimal adsorption condition. Moreover, the regeneration efficiency of the CCNPs remained above e.e. of 43% after the fifth cycle. The present work confirmed that the prepared CCNPs show great potential in the enantioselective separation of ibuprofen racemate.

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.

Deep insights to explain the mechanism of carbon dot formation at various reaction times using the hydrothermal technique: FT-IR, 13C-NMR, 1H-NMR, and UV-visible spectroscopic approaches

A well-explained mechanism for synthesizing carbon dots (CDs) is not yet explored and is still a subject of great debate and challenge. This study used a one-step hydrothermal method to prepare highly efficient, gram-scale, excellent water solubility, and blue fluorescent nitrogen-doped carbon dots (NCDs) with the particle size average distribution of around 5 nm from 4-aminoantipyrine. The effects of varying synthesis reaction times on the structure and mechanism formation of NCDs were investigated using spectroscopic methods, namely FT-IR, 13C-NMR, 1H-NMR, and UV-visible spectroscopies. The spectroscopic results indicated that increasing the reaction time affects the structure of the NCDs. As the hydrothermal synthesis reaction time is extended, the intensity of the peaks in the aromatic region decreases, and new peaks in the aliphatic and carbonyl group regions are generated, which display enhanced intensity. In addition, the photoluminescent quantum yield increases as the reaction time increases. The presence of a benzene ring in 4-aminoantipyrine is thought to contribute to the observed structural changes in NCDs. This is due to the increased noncovalent π-π stacking interactions of the aromatic ring during the carbon dot core formation. Moreover, the hydrolysis of the pyrazole ring in 4-aminoantipyrine results in polar functional groups attached to aliphatic carbons. As the reaction time prolongs, these functional groups progressively cover a larger portion of the surface of the NCDs. After 21 h of the synthesis process, the XRD spectrum of the produced NCDs illustrates a broad peak at 21.1°, indicating an amorphous turbostratic carbon phase. The d-spacing measured from the HR-TEM image is about 0.26 nm, which agrees with the (100) plane lattice of graphite carbon and confirms the purity of the NCD product with a surface covered by polar functional groups. This investigation will lead to a greater understanding of the effect of hydrothermal reaction time on the mechanism and structure of carbon dot synthesis. Moreover, it offers a simple, low-cost, and gram-scale method for creating high-quality NCDs crucial for various applications.

Applications of Fluorescent Carbon Dots as Photocatalysts: A Review

Carbon dots (CDs) have attracted considerable interest from the scientific community due to their exceptional properties, such as high photoluminescence, broadband absorption, low toxicity, water solubility and (photo)chemical stability. As a result, they have been applied in several fields, such as sensing, bioimaging, artificial lighting and catalysis. In particular, CDs may act as sole photocatalysts or as part of photocatalytic nanocomposites. This study aims to provide a comprehensive review on the use of CDs as sole photocatalysts in the areas of hydrogen production via water splitting, photodegradation of organic pollutants and photoreduction and metal removal from wastewaters. Furthermore, key limitations preventing a wider use of CDs as photocatalysts are pointed out. It is our hope that this review will serve as a basis on which researchers may find useful information to develop sustainable methodologies for the synthesis and use of photocatalytic CDs.

Facile Access to Fabricate Carbon Dots and Perspective of Large-Scale Applications

Carbon dots (CDs), fluorescent carbon nanoparticles with particle sizes < 10 nm, are constantly being developed for potential large-scale applications. Recently, methods allow CD synthesis to be carried out on large-scale preparation in a controlled fashion are potentially important for multiple disciplines, including bottom-up strategy, top-down method. In this review, the recent progresses in the research of the methods for large-scale production of CDs and their functionalization are summarized. Especially, the methods of CD synthesis, such as large-scale preparation, hydrothermal/solvothermal, microwave-assisted, magnetic hyperthermia microfluidic and other methods, along with functionalization of CDs, are summarized in detail. By promising applications of CDs, there are three aspects have been already reported, such as enhancing mechanical properties, flame retardancy, and energy storage. Also, future development of CDs is prospected.

Photocatalytic removal of pharmaceutical water pollutants by TiO2 - Carbon dots nanocomposites: A review

Pharmaceuticals are becoming increasingly more relevant water contaminants, with photocatalysts (such as TiO₂) being a promising approach to remove these compounds from water. However, TiO₂ has poor sunlight-harvesting capacity, low photonic efficiency, and poor adsorption towards organic pollutants. One of the emerging strategies to enhance the photocatalytic performance of TiO₂ is by conjugating it with fluorescent carbon dots. Herein, we performed a critical review of the development of TiO₂ - carbon dots nanocomposites for the photocatalytic removal of pharmaceuticals. We found that carbon dots can improve the photocatalytic efficiency of the resulting nanocomposites, mostly due to increasing the adsorption of organic pollutants and enhancing the absorption in the visible range. However, while this approach shows significant promise, we also identified and discussed several aspects that need to be addressed before this strategy could be more widely used. We hope that this review can guide future studies aiming to the development of enhanced photocatalytic TiO₂ - carbon dots nanocomposites.

Life Cycle Assessment-Based Comparative Study between High-Yield and "Standard" Bottom-Up Procedures for the Fabrication of Carbon Dots

Carbon dots (CDs) are carbon-based nanomaterials with remarkable properties that can be produced from a wide variety of synthesis routes. Given that “standard” bottom-up procedures are typically associated with low synthesis yields, different authors have been trying to devise alternative high-yield fabrication strategies. However, there is a doubt if sustainability-wise, the latter should be really preferred to the former. Herein, we employed a Life Cycle Assessment (LCA) approach to compare and understand the environmental impacts of high-yield and “standard” bottom-up strategies, by applying different life cycle impact assessment (LCIA) methods. These routes were: (1) production of hydrochar, via the hydrothermal treatment of carbon precursors, and its alkaline peroxide treatment into high-yield CDs; (2) microwave treatment of carbon precursors doped with ethylenediamine; (3) and (6) thermal treatment of carbon precursor and urea; (4) hydrothermal treatment of carbon precursor and urea; (5) microwave treatment of carbon precursor and urea. For this LCA, four LCIA methods were used: ReCiPe, Greenhouse Gas Protocol, AWARE, and USEtox. Results identified CD-5 as the most sustainable synthesis in ReCiPe, Greenhouse Gas Protocol, and USEtox. On the other hand, in AWARE, the most sustainable synthesis was CD-1. It was possible to conclude that, in general, high-yield synthesis (CD-1) was not more sustainable than “standard” bottom-up synthesis, such as CD-5 and CD-6 (also with relatively high-yield). More importantly, high-yield synthesis (CD-1) did not generate much lower environmental impacts than “standard” approaches with low yields, which indicates that higher yields come with relevant environmental costs.

Highly fluorescent carbon dots as novel theranostic agents for biomedical applications

As an emerging fluorescent nanomaterial, carbon dots (CDs) exhibit many attractive physicochemical features, including excellent photoluminescence properties, good biocompatibility, low toxicity and the ability to maintain the unique properties of the raw material. Therefore, CDs have been intensively pursued for a wide range of applications, such as bioimaging, drug delivery, biosensors and antibacterial agents. In this review, we systematically summarize the synthesis methods of these CDs, their photoluminescence mechanisms, and the approaches for enhancing their fluorescence properties. Particularly, we summarize the recent research on the synthesis of CDs from drug molecules as raw materials and introduce the representative application aspects of these fascinating CDs. Finally, we look into the future direction of CDs in the biomedical field and discuss the challenges encountered in the current development.

Comparative life cycle assessment of high-yield synthesis routes for carbon dots

Carbon dots (CDs) are carbon-based nanomaterials with advantageous luminescent properties, making them promising alternatives to other molecular and nanosized fluorophores. However, the development of CDs is impaired by the low synthesis yield of standard fabrication strategies, making high-yield strategies essential. To help future studies to focus on cleaner production strategies, we have employed a Life Cycle Assessment (LCA) to compare and understand the environmental impacts of available routes for the high-yield synthesis of carbon dots. These routes were: (1) production of hydrochar, via hydrothermal treatment of carbon precursors, and its alkaline-peroxide treatment into high-yield carbon dots; (2) thermal treatment of carbon precursors mixed in a eutectic mixture of salts. Results show that the first synthesis route is associated with the lowest environmental impacts. This is attributed to the absence of the mixture of salts in the first synthesis route, which offsets its higher electricity consumption. Sensitivity analysis showed that the most critical parameter in the different synthetic strategies is the identity of the carbon precursor, with electricity being also relevant for the first synthesis route. Nevertheless, the use of some carbon precursors (as citric acid) with higher associated environmental impacts may be justified by their beneficial role in increasing the luminescent performance of carbon dots. Thus, the first synthesis route is indicated to be the most environmental benign and should be used as a basis in future studies aimed to the cleaner and high-yield production of carbon dots.

Carbon dots inhibit root growth by disrupting auxin biosynthesis and transport in Arabidopsis

Carbon dots (CDs) possess considerable potentials in fields like biomarker and cell imaging due to its good fluorescence properties. Nevertheless, the molecular mechanism concerning influences of CDs on plant growth still remains unknown. In this study, the subcellular localization of CDs in Arabidopsis and the molecular mechanism of CDs toxicity to plants were investigated. Results demonstrate that CDs tend to accumulate in meristematic nucleus of root tips. CDs can inhibit growth of meristem zone of primary root (PR) of Arabidopsis seedlings significantly. The transcription level of auxin biosynthesis related genes decreases and the abundance of auxin efflux carriers PIN1 and PIN2 declines after 40 mg/L CDs treatment, thus lowering the auxin level in root tips. Moreover, CDs weaken activity of cell division in meristem zone by disturbing expressions of DNA damage repair genes and cell cycle regulation genes, thus enabling to inhibit growth of the meristem zone. To sum up, CDs inhibit growth of Arabidopsis seedlings through above pathways. These results provide useful information to elaborate potential toxicity mechanism of CDs on terrestrial plants.

Single-Atom Gadolinium Anchored on Graphene Quantum Dots as a Magnetic Resonance Signal Amplifier

A single-atom metal doped on carbonaceous nanomaterials has attracted increasing attention due to its potential applications as high-performance catalysts. However, few studies focus on the applications of such nanomaterials as nanotheranostics for simultaneous bioimaging and cancer therapy. Herein, it is pioneeringly demonstrated that the single-atom Gd anchored onto graphene quantum dots (SAGd-GQDs), with dendrite-like morphology, was successfully prepared. More importantly, the as-fabricated SAGd-GQDs exhibits a robustly enhanced longitudinal relaxivity (r₁ = 86.08 mM^-1 s^-1) at a low Gd³⁺ concentration of 2 μmol kg^-1, which is 25 times higher than the commercial Gd-DTPA (r₁ = 3.44 mM^-1 s^-1). In vitro and in vivo studies suggest that the obtained SAGd-GQDs is a highly potent and contrast agent to obtain high-definition MRI, thereby opening up more opportunities for future precise clinical theranostics.

NIR-laser-triggered gadolinium-doped carbon dots for magnetic resonance imaging, drug delivery and combined photothermal chemotherapy for triple negative breast cancer

BACKGROUND

Owing to high genetic diversities of tumor cells and low response rate of standard chemotherapy, patients with triple negative breast cancer (TNBC) have short progression-free survivals and poor outcomes, which need to explore an effective approach to improve therapeutic efficacy.

METHODS

Novel gadolinium doped carbon dots (Gd@CDs) have been designed and prepared through hydrothermal method with 3,4-dihydroxyhydrocinnamic acid, 2,2'-(ethylenedioxy)bis(ethylamine) and gadolinium chloride. The synthesized nanostructures were characterized. Taking advantage of good biocompatibility of Gd@CDs, a nanoplatform based on Gd@CDs has been developed to co-deliver chemotherapy drug doxorubicin hydrochloride (Dox) and a near-infrared (NIR) photothermal agent, IR825 for magnetic resonance imaging (MRI) guided photothermal chemotherapy for TNBC.

RESULTS

The as-synthesized Dox@IR825@Gd@CDs displayed favorable MRI ability in vivo. Upon NIR laser irradiation, Dox@IR825@Gd@CDs could convert the NIR light to heat and efficiently inhibit tumor growth through photothermal chemotherapy in vitro and in vivo. Additionally, the impact of photothermal chemotherapy on the murine motor coordination was assessed by rotarod test. Dox@IR825@Gd@CDs presented low toxicity and high photothermal chemotherapy efficiency.

CONCLUSION

A noble theranostic nanoplatform (Dox@IR825@Gd@CDs) was developed that could be tailored to achieve loading of Dox and IR825, intracellular delivery, favorable MRI, excellent combination therapy with photothermal therapy and chemotherapy to enhance therapeutic effect against TNBC cells. This study will provide a promising strategy for the development of Gd-based nanomaterials for MRI and combinational therapy for TNBC.

Afterglow Carbon Dots: From Fundamentals to Applications

The ability of carbon dots (CDs) to emit afterglow emission in addition to fluorescence in response to UV-to-visible excitation allows them to be a new class of luminescent materials. When compared with traditional organic or inorganic afterglow materials, CDs have a set of advantages, including small size, ease of synthesis, and absence of highly toxic metal ions. In addition, high dependence of their afterglow color output on temperature, excitation wavelength, and aggregation degrees adds remarkable flexibility in the creation of multimode luminescence of CDs without the need for changing their intrinsic attributes. These characteristics make CDs particularly attractive in the fields of sensing, anticounterfeiting, and data encryption. In this review, we first describe the general attributes of afterglow CDs and their fundamental afterglow mechanism. We then highlight recent strategic advances in the generation or activation of the afterglow luminescence of CDs. Considerable emphasis is placed on the summarization of their emergent afterglow properties in response to external stimulation. We further highlight the emerging applications of afterglow CDs on the basis of their unique optical features and present the key challenges needed to be addressed before the realization of their full practical utility.

A hyperbranched polysiloxane containing carbon dots with near white light emission

In this work, we prepared a novel hyperbranched polysiloxane containing carbon dots, which can emit near white light with the CIE chromaticity coordinates of (0.301, 0.333) via mixing dual emission at 414 and 532 nm excited at λ_ex = 340 nm.

Multicolor Emitting N-Doped Carbon Dots Derived from Ascorbic Acid and Phenylenediamine Precursors

In this research, we report the green, blue, and orange color emitting N-doped carbon dots (CDs), which are being synthesized from ascorbic acid and o-/m-/p-phenylenediamine (o-PDA, m-PDA, and p-PDA, respectively). The effects of the solvent polarity and solution pH on the PL emission properties of the as-synthesized CDs have been systematically investigated. It has been observed that the PL emission of the as-synthesized CDs decreases with the increase in solvent polarity due to the greater agglomeration. The surface charge of CDs also shows prominent effects on the pH-dependent PL emission properties.

Polyol-Mediated Synthesis of Nitrogen-Containing Carbon-Dots from Tetracyanobenzene with Intense Red Fluorescence

Nitrogen-containing C-dots were prepared by heating (160 °C, 1 h) 1,2,4,5-tetracyanobenzene (TCB) in polyethylene glycol 400 (PEG400). The as-prepared monocrystalline C-dots were 2-4 nm in diameter and contained 24.4 wt. % of nitrogen. They showed intense fluorescence under excitation at 400-500 nm as well as under excitation at 600-700 nm. In addition to an excitation-wavelength-depending emission at 400 to 650 nm, the emission spectra exhibited a strong emission peaking at 715 nm, whose position was independent from the wavelength of excitation. For this deep-red emission a remarkable quantum yield of 69% was detected. The synthesis of nitrogen-containing C-dotswas completely performed in the liquid phase. Moreover, the C-dots could be directly dispersed in water. The resulting aqueous suspensions of PEG400-stabilized nitrogen-containing C-dots also showed intense red emission that was visible to the naked eye.

Color tunable room temperature phosphorescent carbon dot based nanocomposites obtainable from multiple carbon sources via a molten salt method

A molten salt (MS) method is designed for the preparation of carbon dot-based room temperature phosphorescent (RTP) materials. Carbon dots (CDs) are in situ formed and confined in inorganic salts during the recrystallization process. The composite materials CDs@MS and their RTP were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (PXRD) and low temperature (77 K) fluorescence and phosphorescence spectroscopy. The as-prepared CDs@MS exhibits long lifetime RTP (up to 886 ms) and excitation dependent phosphorescence, i.e., the emission can be facilely tuned from 510 nm to 573 nm (green to yellow color) by changing the excitation wavelength. The RTP phenomenon is ascribed to the fact that the crystallization of molten salts forms a rigid structure, which preserves the triplet state of CDs and suppresses the nonradiative transition. It was found that the high charge density of metal ions plays a critical role in reducing the energy gap for realizing effective intersystem crossing. CD-based RTP materials with yellow phosphorescent emission are achieved from a variety of carbon sources and a gram-scale synthetic method. The excitation dependent RTP feature of CDs@MS nanocomposites could provide a novel dual security protection strategy in high-level information anticounterfeiting.

Bioimaging Applications of Carbon Nanodots: A Review

Carbon nanodots (CNDs) is the newest member of carbon-based nanomaterials and one of the most promising for the development of new, advanced applications. Owing to their unique and unparalleled physicochemical and photoluminescent properties, they are considered to be a rising star among nanomaterials. During the last decade, many applications have been developed based on CNDs. Among others, they have been used as bioimaging agents to label cells and tissues. In this review, we will discuss the advancements in the applications of CNDs in in the field of imaging, in all types of organisms (i.e., prokaryotes, eukaryotes, and animals). Selective imaging of one type of cells over another, imaging of (bio)molecules inside cells and tumor-targeting imaging are some of the studies that will be discussed hereafter. We hope that this review will assist researchers with obtaining a holistic view of the developed applications and hit on new ideas so that more advanced applications can be developed in the near future.

Recent development of carbon quantum dots regarding their optical properties, photoluminescence mechanism, and core structure

Carbon quantum dots (CDs) are a relatively new class of carbon nanomaterials which have been studied very much in the last fifteen years to improve their already favorable properties. The optical properties of CDs have drawn particular interest as they display the unusual trait of excitation-dependent emission, as well as high fluorescence quantum yields (QY), long photoluminescence (PL) decay lifetimes, and photostability. These qualities naturally lead researchers to apply CDs in the field of imaging (particularly bio-imaging) and sensing. Since the amount of publications regarding CDs has been growing nearly exponentially in the last ten years, many improvements have been made in the optical properties of CDs such as QY and PL lifetime. However, a great deal of confusion remains regarding the PL mechanism of CDs as well as their structural properties. Therefore, presented in this review is a summary and discussion of the QYs and PL lifetimes reported in recent years. The effect of method as well as precursor has been evaluated and discussed appropriately. The current theories regarding the PL mechanism of CDs are discussed, with special attention to the concept of surface state-controlled PL. With this knowledge, the improvement of preparation and applications of CDs related to their optical properties will be easily accomplished. Further improvements can be made to CDs through the understanding of their structural and optical properties.

Facile synthesis of orange fluorescence carbon dots with excitation independent emission for pH sensing and cellular imaging

High-efficient orange fluorescence nitrogen-doped carbon dots (N-CDs) were facilely prepared from P-phenylenediamine as a precursor via hydrothermal method. The as-prepared N-CDs with an average diameter of 3.65 nm displayed excitation-independent emission at 590 nm. The N-CDs demonstrated a remarkable fluorescence enhancement behavior with the increase of pH. A sigmoidal curve was well fitted using BiDoseResp equation with pK_a1 3.57 and pK_a2 6.01, which can be ascribed to the unique surface properties of N-CDs. Two-segment linear ranges of 2.6-4.6 and 5.0-6.8 broaden the response range to pH of the orange-emission N-CDs to some extent. The confocal fluorescent microscopic images of SMMC7721 cells were performed successfully, which demonstrating that N-CDs possess exceptional cell membrane permeability and can implement as biosensing platform to monitor pH fluctuations in living cells.

Carbon nanodot-induced gelation of a histidine-based amphiphile: application as a fluorescent ink, and modulation of gel stiffness

This study demonstrates carbon dots induced hydrogelation of an amino acid based amphiphile and the potential use of this gel as a fluorescent ink.

Physicochemical properties and cytotoxicity of carbon dots in grilled fish

Fluorescent CDs with good biocompatibility and low cytotoxicity are extracted from grilled pike eel, which are used for bio-imaging.