Small Carbon Nanoparticles Research Articles

Nanoscale Carbon Allotrope at Zero-Dimension: From Small Carbon Nanoparticles to Carbon Dots and Their Organic Hybrids

In the vast research field on carbon nanostructures, a seemingly broadly accepted family configuration of nanoscale carbon allotropes is such that fullerenes take the zero-dimensional dot spot, in perfect harmony with one-dimensional carbon nanotubes and two-dimensional graphene nanosheets. More recently, however, a threat to the nice “family value” has been the emergence of small carbon nanoparticles, which are carbon particles of a few nanometers in size without any defined crystal structures or largely amorphous and populated with abundant surface and other defects. The growing experimental evidence has revealed the unique and/or advantageous optical and photoexcited state properties and photoinduced redox characteristics of the small carbon nanoparticles, especially the spectacular performance enhancements upon their being effectively surface passivated by organic species for the hard-core/soft-shell nanostructures defined as carbon dots. Thus, the ugly and dirty dot-like carbon particles have been elevated to the position of being able to compete with pretty and perfect fullerenes, which by the way are nano-sized molecules strictly speaking, for the title of nanoscale carbon allotrope at the zero-dimension. Nevertheless, carbon dots are not short of controversies, which are due largely to the fact that samples sold or labeled as “carbon dots” in store or literature reports, respectively, are often not what they are advertized, but complex mixtures of varying structures and compositions. Some of these will be discussed in the presentation, so are challenges and opportunities in this seemingly rapid expanding research field. Figure 1

Photoexcited state properties of N-ethylcarbazole-functionalized carbon dots in solution and in PVK polymer matrix

The adducts of N-ethylcarbazole (NEC) to small carbon nanoparticles prepared by microwave-assisted thermal processing are carbon dots (CDots) with NEC for the dot surface functionalization, thus NEC-CDots, which were shown previously for their high stability in the dot structure and properties. In this study, the optical absorptions and photoexcited state properties of NEC-CDots in solution and in poly(N-vinylcarbazole) (PVK) film matrix were evaluated by using optical spectroscopy methods, including those for fluorescence decays and lifetimes. The results suggest that the properties are largely unchanged from solution phase to the polymer matrix environment, very positive to their expected optoelectronic applications.

Divergence in Antiviral Activities of Carbon Dots versus Nano-Carbon/Organic Hybrids and Implications

Carbon dots (CDots) are generally defined as small carbon nanoparticles (CNPs) with effective surface passivation, for which the classical synthesis is the functionalization of pre-existing CNPs with organic molecules. However, “dot” samples produced by “one-pot” thermal carbonization of organic precursors are also popular in the literature. These carbonization-produced samples may contain nano-carbon domains embedded in organic matters from the precursors that survived the thermal processing, which may be considered and denoted as “nano-carbon/organic hybrids”. Recent experimental evidence indicated that the two different kinds of dot samples are largely divergent in their photo-induced antibacterial functions. In this work, three representative carbonization-produced samples from the precursor of citric acid–oligomeric polyethylenimine mixture with processing conditions of 200 °C for 3 h (CS200), 330 °C for 6 h (CS330), and microwave heating (CSMT) were compared with the classically synthesized CDots on their photo-induced antiviral activities. The results suggest major divergences in the activities between the different samples. Interestingly, CSMT also exhibited significant differences between antibacterial and antiviral activities. The mechanistic origins of the divergences were explored, with the results of different antimicrobial activities among the hybrid samples rationalized in terms of the degree of carbonization in the sample production and the different sample structural and morphological characteristics.

Carbon Dots: Classically Defined versus Organic Hybrids on Shared Properties, Divergences, and Myths.

Carbon dots are defined as small carbon nanoparticles with effective surface passivation via organic functionalization. The definition is literally a description of what carbon dots are originally found for the functionalized carbon nanoparticles displaying bright and colorful fluorescence emissions, mirroring those from similarly functionalized defects in carbon nanotubes. In literature more popular than classical carbon dots are the diverse variety of dot samples from "one-pot" carbonization of organic precursors. On the two different kinds of samples from the different synthetic approaches, namely, the classical carbon dots versus those from the carbonization method, highlighted in this article are their shared properties and apparent divergences, including also explorations of the relevant sample structural and mechanistic origins for the shared properties and divergences. Echoing the growing evidence and concerns in the carbon dots research community on the major presence of organic molecular dyes/chromophores in carbonization produced dot samples, demonstrated and discussed in this article are some representative cases of dominating spectroscopic interferences due to the organic dye contamination that have led to unfound claims and erroneous conclusions. Mitigation strategies to address the contamination issues, including especially the use of more vigorous processing conditions in the carbonization synthesis, are proposed and justified.

Stable Carbon Dots from Microwave-Heated Carbon Nanoparticles Generating Organic Radicals for In Situ Additions

Carbon dots (CDots) are small carbon nanoparticles with effective surface passivation by organic functionalization. In the reported work, the surface functionalization of preexisting small carbon nanoparticles with N-ethylcarbazole (NEC) was achieved by the NEC radical addition. Due to the major difference in microwave absorption between the carbon nanoparticles and organic species such as NEC, the nanoparticles could be selectively heated via microwave irradiation to enable the hydrogen abstraction in NEC to generate NEC radicals, followed by in situ additions of the radicals to the nanoparticles. The resulting NEC-CDots were characterized by microscopy and spectroscopy techniques including quantitative proton and 13C NMR methods. The optical spectroscopic properties of the dot sample were found to be largely the same as those of CDots from other organic functionalization schemes. The high structural stability of NEC-CDots benefiting from the radical addition functionalization is highlighted and discussed.

Carbon Dots versus Nano-Carbon/Organic Hybrids—Divergence between Optical Properties and Photoinduced Antimicrobial Activities

Carbon dots (CDots) are generally defined as small-carbon nanoparticles with surface organic functionalization and their classical synthesis is literally the functionalization of preexisting carbon nanoparticles. Other than these “classically defined CDots”, however, the majority of the dot samples reported in the literature were prepared by thermal carbonization of organic precursors in mostly “one-pot” processing. In this work, thermal processing of the selected precursors intended for carbonization was performed with conditions of 200 °C for 3 h, 330 °C for 6 h, and heating by microwave irradiation, yielding samples denoted as CS200, CS330, and CSMT, respectively. These samples are structurally different from the classical CDots and should be considered as “nano-carbon/organic hybrids”. Their optical spectroscopic properties were found comparable to those of the classical CDots, but very different in the related photoinduced antibacterial activities. Mechanistic origins of the divergence were explored, with the results suggesting major factors associated with the structural and morphological characteristics of the hybrids.

Inactivation of Vesicular Stomatitis Virus with Light-Activated Carbon Dots and Mechanistic Implications

The prevention of viral transmission is an important step to address the spread of viral infections. Using the enveloped vesicular stomatitis virus (VSV) as a model, this study explored the antiviral functions of the specifically designed and prepared carbon dots (CDots). The CDots were prepared using small carbon nanoparticles with surface functionalization-passivation by oligomeric polyethylenimine (PEI). The results indicated that the PEI-CDots were readily activated by visible light to effectively and efficiently inactivate VSVs under various combinations of experimental conditions (viral titer, dot concentration, and treatment time). The photodynamically induced viral structural protein degradation and genomic RNA degradation were observed, suggesting the mechanistic origins, leading to the inactivation of virus. The results suggested CDots as a class of promising broad-spectrum antiviral agents for disinfection of viruses.

(Invited) Nanoscale Carbon Allotrope at Zero-Dimension – Small Carbon Nanoparticles Versus Molecular Fullerenes

In the vast research field on carbon nanostructures, a seemingly broadly accepted family configuration of nanoscale carbon allotropes is such that fullerenes take the zero-dimensional dot spot, in perfect harmony with one-dimensional carbon nanotubes and two-dimensional graphene nanosheets. More recently, however, a threat to the nice “family value” has been the emergence of small carbon nanoparticles, which are carbon particles of a few nanometers in size without any defined crystal structures or largely amorphous and populated with abundant surface and other defects. The growing experimental evidence has revealed the unique and/or advantageous optical and photoexcited state properties and photoinduced redox characteristics of the small carbon nanoparticles, elevating them to the position of being able to compete with fullerenes for the title of nanoscale carbon allotrope at the zero-dimension. Moreover, a related supporting argument is that fullerenes are molecules, nanoscale in their sizes but not necessarily “nanomaterials” strictly speaking. In this presentation, relevant experimental results of the nanoparticles and fullerenes will be compared for a fair assessment and judgment of their title competition, and more importantly their shared structural features and electronic properties and also their enormous potential in the development of materials for a variety of technological applications will be highlighted.

Carbon Dot - An Updated Review

These days, carbon dots (CDs) are the rising stars of nanomaterials. Carbon dots (CDs) are small carbon nanoparticles with the same type of surface passivation (less than 10 nm in size). Researchers found C-dots by accident while purifying single-walled carbon nanotubes (SWCNTs) manufactured using the arc-discharge process. Toxic metal-based quantum dots are being replaced with carbon dots (QDs). Carbon dots are currently being prepared from a variety of natural resources in order to obtain self-passivated products at a reasonable cost. Because of their superior photo physical characteristics, biocompatibility, and low toxicity, carbon dots have prospective applications in bio sensing, bio imaging, and drug administration. Different synthetic processes, precursors, salient properties, and applications were reviewed in this review, as well as some future prospects, obstacles, and possible solutions for future development. Because of their tunable optical characteristics and better biocompatibility, luminous carbon-based nanomaterials have sparked a lot of scientific interest. Different light emission properties of carbon are discussed in this review. Distinct synthesis procedures have resulted in different carbon dots (CDs). Summarized here. The optical properties of CDs that haven’t been synthesized yet surface doping and element doping can be used to further control it. CDs are being functionalized for an adjustable band gap. As a result of their luminescent with reduced cytotoxicity and tunable optical characteristics CDs have been thoroughly investigated in terms of their potential uses in biomedicine, such as analytical sensors, and instruments for bioimaging. Fluorescent carbon dots are a new type of nanomaterial from the carbon family. Green CDs, which have attracted a lot of attention from researchers because of their better water solubility, high biocompatibility, and eco-friendly nature when compared to chemically generated CDs, can be made from a variety of inexpensive and renewable resources. The presence of heteroatoms on the surface of green CDs in the form of amine, hydroxyl, carboxyl, or thiol functional groups, which can improve their physicochemical qualities, quantum yield, and likelihood of visible light absorption, eliminates the need for additional surface passivation.

Carbon Dots as Drug Delivery Vehicles for Antimicrobial Applications: A Minireview.

Carbon dots are small carbon nanoparticles with intrinsic photoluminescence. Because of the advantages such as good biocompatibility and water dispersity, many researchers have applied carbon dots as delivery platforms of antimicrobial agents. Compared with the free small-molecule antimicrobial agents alone, the carbon dot-based systems may exhibit enhanced antimicrobial activity, increased stability, improved cellular uptake, and reduced side effects. This review will mainly discuss the antimicrobial agent-loaded carbon dots for various antibacterial, antifungal, and antiviral applications. The current limitations and future research directions in the related field are also proposed. It is hoped that this review will have implications for the future development of functional carbon dots for various practical antimicrobial uses.

Carbon "quantum" dots for bioapplications.

Carbon "quantum" dots or carbon dots (CDots) exploit and enhance the intrinsic photoexcited state properties and processes of small carbon nanoparticles via effective nanoparticle surface passivation by chemical functionalization with organic species. The optical properties and photoinduced redox characteristics of CDots are competitive to those of established conventional semiconductor quantum dots and also fullerenes and other carbon nanomaterials. Highlighted here are major advances in the exploration of CDots for their serving as high-performance yet nontoxic fluorescence probes for one- and multi-photon bioimaging in vitro and in vivo, and for their uniquely potent antimicrobial function to inactivate effectively and efficiently some of the toughest bacterial pathogens and viruses under visible/natural or ambient light conditions. Opportunities and challenges in the further development of the CDots platform and related technologies are discussed.

Visible Light-Activated Carbon Dots for Inhibiting Biofilm Formation and Inactivating Biofilm-Associated Bacterial Cells.

This study aimed to address the significant problems of bacterial biofilms found in medical fields and many industries. It explores the potential of classic photoactive carbon dots (CDots), with 2,2′-(ethylenedioxy)bis (ethylamine) (EDA) for dot surface functionalization (thus, EDA-CDots) for their inhibitory effect on B. subtilis biofilm formation and the inactivation of B. subtilis cells within established biofilm. The EDA-CDots were synthesized by chemical functionalization of selected small carbon nanoparticles with EDA molecules in amidation reactions. The inhibitory efficacy of CDots with visible light against biofilm formation was dependent significantly on the time point when CDots were added; the earlier the CDots were added, the better the inhibitory effect on the biofilm formation. The evaluation of antibacterial action of light-activated EDA-CDots against planktonic B. subtilis cells versus the cells in biofilm indicate that CDots are highly effective for inactivating planktonic cells but barely inactivate cells in established biofilms. However, when coupling with chelating agents (e.g., EDTA) to target the biofilm architecture by breaking or weakening the EPS protection, much enhanced photoinactivation of biofilm-associated cells by CDots was achieved. The study demonstrates the potential of CDots to prevent the initiation of biofilm formation and to inhibit biofilm growth at an early stage. Strategic combination treatment could enhance the effectiveness of photoinactivation by CDots to biofilm-associated cells.

Variable Temperature Synthesis of Tunable Flame-Generated Carbon Nanoparticles

In this study, flame-formed carbon nanoparticles of different nanostructures have been produced by changing the flame temperature. Raman spectroscopy has been used for the characterization of the carbon nanoparticles, while the particle size has been obtained by online measurements made by electrical mobility analysis. The results show that, in agreement with recent literature data, a large variety of carbon nanoparticles, with a different degree of graphitization, can be produced by changing the flame temperature. This methodology allows for the synthesis of very small carbon nanoparticles with a size of about 3–4 nm and with different graphitic orders. Under the perspective of the material synthesis process, the variable-temperature flame-synthesis of carbon nanoparticles appears as an attractive procedure for a cost-effective and easily scalable production of highly tunable carbon nanoparticles.

Carbon dots versus nano-carbon/organic hybrids – dramatically different behaviors in fluorescence sensing of metal cations with structural and mechanistic implications

Carbon dots (CDots) are defined as surface-passivated small carbon nanoparticles, with the effective passivation generally achieved by organic functionalization. Photoexcited CDots are both potent electron donors and acceptors, and their characteristic bright and colorful fluorescence emissions make them excellent fluorescence sensors for organic analytes and metal ions. For the latter extraordinarily low detection limits based on extremely efficient quenching of fluorescence intensities by the targeted metal cations have been observed and reported in the literature. However, all of the dot samples in those reported studies were made from “one-pot” carbonization of organic precursors mostly under rather mild processing conditions, unlikely to be sufficient for the required level of carbonization. Those dot samples should therefore be more appropriately considered as “nano-carbon/organic hybrids”, characterized structurally as being highly porous and spongy, which must be playing a dominating role in the reported sensing results. In this study, we compared the dot samples from carbonization syntheses under similarly mild and also more aggressive processing conditions with the classically defined and structured CDots for the fluorescence sensing of copper(ii) cations in aqueous solutions. The observed dramatic decoupling between quenching results for fluorescence intensities and lifetimes of the carbonization samples, with the former being extraordinary and the latter within the diffusion controlled limit, suggested that the quenching of fluorescence intensities was greatly affected by the higher local quencher concentrations than the bulk associated with the porous and spongy sample structures, especially for the sample from carbonization under too mild processing conditions. The major differences between the classical CDots and the nano-carbon/organic hybrids are highlighted, and the tradeoffs between sensitivity and accuracy or reproducibility in the use of the latter for fluorescence sensing are discussed.

Identification of physicochemical properties that modulate nanoparticle aggregation in blood.

Inorganic materials are receiving significant interest in medicine given their usefulness for therapeutic applications such as targeted drug delivery, active pharmaceutical carriers and medical imaging. However, poor knowledge of the side effects related to their use is an obstacle to clinical translation. For the development of molecular drugs, the concept of safe-by-design has become an efficient pharmaceutical strategy with the aim of reducing costs, which can also accelerate the translation into the market. In the case of materials, the application these approaches is hampered by poor knowledge of how the physical and chemical properties of the material trigger the biological response. Hemocompatibility is a crucial aspect to take into consideration for those materials that are intended for medical applications. The formation of nanoparticle agglomerates can cause severe side effects that may induce occlusion of blood vessels and thrombotic events. Additionally, nanoparticles can interfere with the coagulation cascade causing both pro- and anti-coagulant properties. There is contrasting evidence on how the physicochemical properties of the material modulate these effects. In this work, we developed two sets of tailored carbon and silica nanoparticles with three different diameters in the 100–500 nm range with the purpose of investigating the role of surface curvature and chemistry on platelet aggregation, activation and adhesion. Substantial differences were found in the composition of the protein corona depending on the chemical nature of the nanoparticles, while the surface curvature was found to play a minor role. On the other hand, large carbon nanoparticles (but not small carbon nanoparticles or silica nanoparticles) have a clear tendency to form aggregates both in plasma and blood. This effect was observed both in the presence or absence of platelets and was independent of platelet activation. Overall, the results presented herein suggest the existence of independent modes of action that are differently affected by the physicochemical properties of the materials, potentially leading to vessel occlusion and/or formation of thrombi in vivo.

Carbon Dots: Zero-Dimensional Carbon Allotrope with Unique Photoinduced Redox Characteristics.

Carbon dots (CDots) exploit and enhance the intrinsic properties of small carbon nanoparticles. Their optical absorptions and photoinduced redox characteristics are competitive with those of conventional semiconductor quantum dots at one end and fullerenes and other carbon nanomaterials at the other. Highlighted in this mini review are the effective photon harvesting over a broad spectral range by CDots and their subsequent excited-state charge transfer processes and interactions, which have enabled their use as sensors, for photodynamic effects, and in various energy conversion technologies.

Evaluation of Commercial “Carbon Quantum Dots” Sample on Origins of Red Absorption and Emission Features

The commercially acquired aqueous solution of “carbon quantum dots” sample was evaluated by optical absorption and fluorescence emission methods; in reference to aqueous dispersed small carbon nanoparticles and representative carbon dots prepared from chemical functionalization of the carbon nanoparticles. The results suggest a very low content of carbon that is associated with nanoscale carbon particles/domains in the as-supplied sample; and likely significant contamination by dye-like species/mixtures. In the absence of any information on the synthesis and history of the commercial sample, the possible cause of the contamination was illustrated by an example on similar dye formation in the one-pot carbonization synthesis of “red carbon dots” from citric acid–formamide precursor mixtures under too mild processing conditions that were insufficient for the intended carbonization. The negative impacts to the carbon dots research field by the apparent proliferation and now commercial availability of carbon-deficient or even largely carbon-less “carbon quantum dots”, which are more susceptible to dye contamination or dominance, are discussed.

Photoluminescence of carbon dots prepared by ball milling and their application in Hela cell imaging

Carbon dots are small carbon nanoparticles with various surface passivation schemes. The deliberate functionalization of carbon nanoparticles and carbonization of organic or carbon-rich species are two main synthesis routes for carbon dots. However, these two synthesis routes suffer from drawbacks such as critical reaction conditions and uncontrollable structure and performance. In our present work, a hybrid approach combining the advantageous characteristics of these two synthesis routes was applied to prepare carbon dots. The spherical and hydrophilic carbon dots were prepared by high-energy ball milling of active carbon and potassium carbonate. The carbon dots prepared are disperse and fine with an average size of 3.5 nm and a size distribution of 1.6–6.0 nm. They exhibit a nearly excitation-independent and high photostable blue photoluminescence behavior with the maximal emission wavelength at 430 nm. They also show an excellent photostability under high NaCl concentrations or long UV exposure time and a stable photoluminescence behavior in a wide pH range from 3 to 11. They are nearly not cytotoxic to Hela cells, can be taken up by Hela cells, and applied to Hela cell imaging. Our hybrid approach did not use any strong basic and can be scaled up for large-scale preparation of carbon dots.

Carbon Dots for Sensing and Killing Microorganisms

Carbon dots (or carbon quantum dots) are small (less than 10 nm) and luminescent carbon nanoparticles with some form of surface passivation. As an emerging class of nanomaterials, carbon dots have found wide applications in medicine, bioimaging, sensing, electronic devices, and catalysis. In this review, we focus on the recent advancements of carbon dots for sensing and killing microorganisms, including bacteria, fungi, and viruses. Synthesis, functionalization, and a toxicity profile of these carbon dots are presented. We also discuss the underlying mechanisms of carbon dot-based sensing and killing of microorganisms.

Carbon dots for energy conversion applications

Quantum dots (QDs), generally referring to semiconductor nanocrystals that display the quantum confinement effect, have been widely pursued for many energy conversion applications. More recently, carbon dots (CDots), which are small carbon nanoparticles with various surface passivation schemes, have been found to possess optical properties and photoinduced redox characteristics resembling those of conventional semiconductor QDs and thus are amenable to some of the same uses in energy conversions. Among the various carbon nanomaterials, fullerenes have been extensively investigated for their use as critical components in optoelectronic devices and systems. Carbon nanoparticles, representing a largely ignored nanoscale carbon allotrope, are in fact more effective in some of the same functions, which are materialized and much enhanced upon the surface passivation of the nanoparticles in CDots. In this perspective article on CDots for energy conversion applications, the optical properties and redox characteristics of CDots, including the related mechanistic framework and its relationship to the use of CDots as potent photocatalysts for the conversion of CO2 into small organic molecules, are highlighted. Also highlighted are results from representative studies using CDots in light-emitting diodes and various solar cells to demonstrate their excellent potential for a wide range of roles in optoelectronic devices and systems. Issues and opportunities in the further development of the CDots platform and related technologies are discussed.