Carbon Nanoparticles as Chromophores for Photon Harvesting and Photoconversion

High-performance photocatalytic peroxymonosulfate activation by carbon quantum dots via precise surface chemistry regulation: Insight into the structure-function relations

Carbon quantum dots (CQDs) are considered promising metal-free green catalysts for the activation of persulfates, but direct experimental evidence to identify the true active sites on the surface of CQDs is still lacking. We prepared CQDs with different oxygen contents by controlling the carbonisation temperature, using a simple pyrolysis method. Photocatalytic activity experiments show that CQDs₂₀₀ exhibits the best PMS activation performance. By investigating the relationship between the oxygen functional groups on CQDs surface and photocatalytic activity, it was postulated that the C=O groups might be the predominant active site, which was confirmed by selective chemical titrations of the C=O, C-OH and COOH groups. Furthermore, limited to the weak photocatalytic properties of the pristine CQDs, ammonia and phenylhydrazine were used to precisely nitrogen-modify the o-CQD surface. We found that phenylhydrazine-modified o-CQDs-PH promoted the absorption of visible light and the separation of photocarriers, thus enhancing the activation of PMS. Theoretical calculations provide more insights from different levels of the pollutant, fine-tuned CQDs, and their interactions.

Cooccurrence of pH-sensitive shifting blue and immobile green triple surface-state fluorescence in ultrasmall super body-centered cubic carbon quantum dots

Carbon quantum dots are widely used in various fields owing to excellent optical properties and outstanding biocompatibility. We synthesize rare super body-centered cubic (C₈) structured carbon quantum dots by using cheap source materials and simple preparation method. They exhibit one shifting blue emission band and two close immobile green bands. They have large Stokes shifts ranging from 0.68 to 1.01 eV and large quantum yields as high as 60%. The three types of emissions are competitive and their intensities vary sensitively and differently with pH. Moreover, their emission intensity versus excitation power curves followI(P)∝Pkwithkvalues significantly smaller than unity. The blue emission follows the stretched exponential decay law with an intermediate lifetime of ∼3.9 ns and a lifetime-dispersion factor of ∼0.85 whereas the two green emissions exhibit faster and slower decays with respective lifetimes of around 2.0 and 13.0 ns. The results reveal that the blue emission originates from an ensemble of emission sites exhibiting quantum confinement-like effect and two green emissions stem from pH-sensitive surface functional groups-associated fluorophores.

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.

Simple and facile carbon dots based electrochemical biosensor for TNF-α targeting in cancer patient's sample

Tumour Necrosis Factor (TNF-α) is a pro-inflammatory cytokine having key roles in cell death, differentiation, survival, proliferation, migration and is a modulator of immune system. Therefore, TNF-α is an ideal biomarker for several disease diagnosis including cancer. However, out of all the biomarkers of cancer, TNF-α) is less explored for cancer detection. Only a few reports are available of developing biosensors for TNF-α targeting in human serum samples. Also, Carbon Dots (CDs) remains less explored in biosensor application. In this regard, for the first time, a sensitive and low-cost electrochemical biosensor based on CDs has developed. CDs were synthesized by simple yet facile microwave pyrolysis. Poly methyl methacrylate (PMMA) was selected as the matrix to hold CDs to fabricate the biosensing platform. This novel CD-PMMA nanocomposite featuring excellent biocompatibility, exceptional electrocatalytic conductivity, and large surface area. CD-PMMA was applied as transducing material to efficiently conjugate antibodies specific towards TNF-α and fabricate electrochemical immunosensor for specific detection of TNF-α. The fabricated immunosensor was used for the detection of TNF-α within a wide dynamic range of 0.05-160 pg mL^-1 with a lower detection limit of 0.05 pg mL^-1 and sensitivity of 5.56 pg mL^-1 cm^-2. Furthermore, this CDs based immunosensor retains high sensitivity, selectivity, and stability. This immunosensor demonstrated a high correlation with the conventional technique, Enzyme-Linked Immunosorbent Assay for early screening of cancer patient serum samples.

Green Synthesis of Multifunctional Carbon Dots with Antibacterial Activities

Carbon dots (CDs) were obtained from medicinal turmeric leaves (Curcuma longa) by a facile one-step hydrothermal method and evaluated for their bactericidal activities against two gram-negative; Escherichia coli, Klebsiella pneumoniae, and two gram-positive counterparts; Staphylococcus aureus, S. epidermidis. The CDs exhibited spherical shapes with a mean size of 2.6 nm. The fluorescence spectra of CDs revealed intense fluorescence at λex/em = 362/429 nm with a bright blue color in an aqueous solution. The CDs showed strong photostability under various environmental conditions (pH, salt, and UV-radiation). The complete bactericidal potency of CDs was 0.25 mg/mL for E.coli and S. aureus after 8 h of exposure, while for K. pneumoniae, and S. epidermidis, the CDs at 0.5 mg/mL good antibacterial effect within 8 h and complete eradication after 24 h of exposure is observed. The release of reactive oxygen species played a crucial role in the death of the bacterial cell. The present study provides a strategy for the preparation of CDs from a medicinal plant and their potential antibacterial activities against four common contagious pathogens.

A Novel Fluorescent Test Papers Based on Carbon Dots for Selective and Sensitive Detection of Cr (VI)

In recent years, carbon dots (CDs) are promising fluorescence probes for ions detection. In this paper, the CDs which are with an average diameter of 5.5 nm were synthesized through a simple one-step hydrothermal carbonization of ethylene diamine tetraacetic acid (EDTA) salt. The CDs have strong yellow photoluminescence (PL) with a maximum emission intensity at 550 nm under an excitation wavelength of 450 nm. As the electron transfer will occur between Cr (VI) and the CDs, yellow fluorescence was quenched after adding the Cr (VI) ions. The CDs probe allows the detection of Cr (VI) ions over a concentration range from 0 to 0.1 M (R² = 0.987) and the lower detection limit is 10⁻⁵ M. Simultaneously, the CDs show highly selectivity and stability toward the detection of Cr (VI) ions.

Group IV nanodots: synthesis, surface engineering and application in bioimaging and biotherapy

Group IV nanodots (NDs) mainly including carbon (C), silicon (Si), germanium (Ge) have aroused much attention as one type of important nanomaterials that are widely studied in optoelectronics, semiconductors, sensors and biomedicine-related fields owing to the low cost of synthesis, good stability, excellent biocompatibility, and some attractive newly emerged properties. In this review, the synthesis, surface engineering and application in bioimaging and biotherapy of group IV NDs are summarized and discussed. The recent progress in the rational synthesis and functionalization, specific therapy-related properties, together with in vivo and in vitro bioimaging are highlighted. Their new applications in biotherapy such as photothermal therapy (PTT) and photodynamic therapy (PDT) are illustrated with respect to C, Si and Ge NDs. The current challenges and future applications of these emerging materials in bioimaging and biotherapy are presented. This review provides readers with a distinct perspective of the group IV NDs nanomaterials for synthesis and surface engineering, and newly emerging properties related to applications in biomedicine.

Fluorescent Carbon Quantum Dots-Synthesis,Functionalization and Sensing Application in FoodAnalysis

Carbon quantum dots (CQDs) with stable physicochemical properties are one of theemerging carbon nanomaterials that have been studied in recent years. In addition to the excellentoptical properties such as photoluminescence, photobleaching resistance and light stability, thismaterial also has favorable advantages of good biocompatibility and easy functionalization, whichmake it an ideal raw material for constructing sensing equipment. In addition, CQDs can combinedwith other kinds of materials to form the nanostructured composites with unique properties, whichprovides new insights and ideas for the research of many fields. In the field of food analysis,emerging CQDs have been deeply studied in food composition analysis, detection and monitoringtrace harmful substances and made remarkable research progress. This article introduces andcompares the various methods for CQDs preparation and reviews its related sensing applicationsas a new material in food components analysis and food safety inspection in recent years. It isexpected to provide a significant guidance for the further study of CQDs in the field of foodanalysis and detection. CQDs; synthesis; fluorescent sensing; food analysis.

Nitrogen/sulfur-co-doped carbon quantum dots: a biocompatible material for the selective detection of picric acid in aqueous solution and living cells

Here, a fast and eco-friendly one-pot hydrothermal technique is utilized for the synthesis of nitrogen/sulfur-co-doped fluorescent carbon quantum dots (NS-CQDs) from a simple precursor of citric acid (CA) and thiosemicarbazide (TSC). The obtained NS-CQDs exhibited strong blue emission under UV light, with fluorescence quantum yield (QY) of ~37.8%. The Commission internationale de l'eclairage (CIE) coordinates originated at (0.15, 0.07), which confirmed the blue fluorescence of the synthesized NS-CQDs. Interestingly, the prepared NS-CQDs were successfully used as a selective nanoprobe for the monitoring of environmentally hazardous explosive picric acid (PA) in different nitro- and non-nitro-aromatic derivatives of PA. The mechanism of the NS-CQDs was also explored, and was posited to occur via the fluorescence resonance electron transfer (FRET) process and non-fluorescent complex formation. Importantly, this system possesses excellent biocompatibility and low cytotoxicity in HeLa cervical cancer cells; hence, it can potentially be used for PA detection in analytical, environmental, and pathological applications. Furthermore, the practical applicability of the proposed sensing system to pond water demonstrated the feasibility of our system along with good recovery. Graphical abstract.

Carbon Dots as Potent Antimicrobial Agents

Carbon dots (CDots) have emerged to represent a highly promising new platform for visible/natural light-activated microbicidal agents. In this article, the syntheses, structures, and properties of CDots are highlighted, representative studies on their activities against bacteria, fungi, and viruses reviewed, and the related mechanistic insights discussed. Also highlighted and discussed are the excellent opportunities for potentially extremely broad applications of this new platform, including theranostics uses.

Yellow-emitting carbon dots for selective detecting 4-NP in aqueous media and living biological imaging

A facile and economical hydrothermal approach is reported for preparing fluorescent nitrogen-doped carbon dots (N-CDs) via using o-phenylenediamine and dicyandiamine. Herein, the bright-yellow-emissive N-CDs are uniformly dispersed spherical nanoparticles with favorable solubility, superior photoluminescence and photobleaching resistance. The fluorescence intensity of N-CDs is linearly quenched by 4-nitrophenol (4-NP) over a concentration range of 0.1-39 μM, corresponding to a detection limit (LOD) of 0.05 μM. Based on this phenomenon, a 4-NP-detection method is exploited and applied to real samples analysis. The synthesized N-CDs are highly biocompatible and capable of biological imaging. Therefore, they are excellent candidates for live biological imaging.

Green Preparation of Fluorescent Carbon Quantum Dots from Cyanobacteria for Biological Imaging

Biomass-based carbon quantum dots (CQDs) have become a significant carbon materials by their virtues of being cost-effective, easy to fabricate and low in environmental impact. However, there are few reports regarding using cyanobacteria as a carbon source for the synthesis of fluorescent CQDs. In this study, the low-cost biomass of cyanobacteria was used as the sole carbon source to synthesize water-soluble CQDs by a simple hydrothermal method. The synthesized CQDs were mono-dispersed with an average diameter of 2.48 nm and exhibited excitation-dependent emission performance with a quantum yield of 9.24%. Furthermore, the cyanobacteria-derived CQDs had almost no photobleaching under long-time UV irradiation, and exhibited high photostability in the solutions with a wide range of pH and salinity. Since no chemical reagent was involved in the synthesis of CQDs, the as-prepared CQDs were confirmed to have low cytotoxicity for PC12 cells even at a high concentration. Additionally, the CQDs could be efficiently taken up by cells to illuminate the whole cell and create a clear distinction between cytoplasm and nucleus. The combined advantages of green synthesis, cost-effectiveness and low cytotoxicity make synthesized CQDs a significant carbon source and broaden the application of cyanobacteria and provide an economical route to fabricate CQDs on a large scale.

A review on nanostructured carbon quantum dots and their applications in biotechnology, sensors, and chemiluminescence

Carbon quantum dots (CQDs) are a member of carbon nanostructures family which have received increasing attention for their photoluminescence (PL), physical and chemical stability and low toxicity. The classical semiconductor quantum dots (QDs) are semiconductor particles that are able to emit fluorescence by excitation. The CQDs is mainly referred to photoluminescent carbon nanoparticles less than 10 nm, with surface modification or functionalization. Contrary to other carbon nanostructures, CQDs can be synthesized and functionalized fast and easily. The fluorescence origin of the CQDs is a controversial issue which depends on carbon source, experimental conditions, and functional groups. However, PL emissions originated from conjugated π-domains and surface defects have been proposed for the PL emission mechanisms of the CQDs. These nanostructures have been used as nontoxic alternatives to the classical heavy metals containing semiconductor QDs in some applications such as in-vivo and in-vitro bio-imaging, drug delivery, photosensors, chemiluminescence (CL), and etc. This paper will introduce CQDs, their structure, and PL characteristics. Recent advances of the application of CQDs in biotechnology, sensors, and CL is comprehensively discussed.

Molecular imaging with nanoparticles: the dwarf actors revisited 10 years later

We explore present-day trends and challenges in nanomedicine. Creativity in the laboratories continues: the published literature on novel nanoparticles is now vast. Nanoagents are discussed here which are composed entirely of strongly photoluminescent materials, tunable to desired optical properties and of inherently low toxicity. We focus on "quantum nanoparticles" prepared from allotropes of carbon. The principles behind strong, tunable photoluminescence are quantum mechanical: we present them in simple outline. The major industries racing to develop these materials can offer significant technical guidance to nanomedicine, which could help to custom-design strongly signalling nanoagents specifically for stated clinical applications. Since such agents are small, they can be targeted easily, making active targeting possible. We consider it timely now to study the interactions nanoparticles undergo with tissue components in living animals and to learn to understand and overcome the numerous barriers the organism interposes between the blood and targets in or on parenchymal cells. As the near infra-red spectrum opens up, detection of glowing nanoparticles several centimeters deep in a living human subject becomes calculable and we present a simple way to do this. Finally, we discuss the slow-fuse and resource-inefficient entry of nanoparticles into clinical application. A first possible reason is failure to target across the body's barriers, see above. Second, in the sparse translational landscape funding and support gaps yawn widely between academic research and subsequent development. We consider the agendas of the numerous "stakeholders" participating in this sad landscape and point to some faint glimmers of hope for the future.

Haemostatic bioactivity of novel Schizonepetae Spica Carbonisata-derived carbon dots via platelet counts elevation

Schizonepetae Spica Carbonisata (SSC) has pronounced haemostatic effects for hundreds of years and has been acknowledged in the 2015 Pharmacopoeia of the People's Republic of China (PPRC) as a haemostatic charcoal drug. However, after years of efforts, the underlying mechanism and the material basis is still less defined. In this research, we developed a novel CDs derived from SSC (SSC-CDs) with an average diameter of 1.29-6.87 nm and a quantum yield of 6.31%. SSC was prepared using a modified pyrolysis method and no further modification and external surface passivation agent is required. With abundant surface groups, SSC-CDs showed distinct solubility and bioactivity. In this study, we innovatively used the Deinagkistrodon acutus (D. acutus) venom model as well as the classical haemorrhagic animal model to evaluate the haemostatic bioactivity of SSC-CDs. The results indicated that SSC-CDs had outstanding haemostatic bioactivity and might inhibit the haemorrhagic activity via PLT elevation. According to the results of this study and our previous work, we discovered that CDs derived from different kinds of charcoal drugs presented similarities and differences in the structural feature, physicochemical property and bioactivity. In order to further explore the self-bioactivities, we first named this kind of CDs as "Chinese Medicine charcoal drug nanoparticles" (CMNP). These results may not only provide evidence for further researches of self-bioactivities of CDs but give new insights into potential biomedical and healthcare applications of CDs, therefore, make contributions to future drug discovery.

Zero-Dimensional Carbon Allotropes-Carbon Nanoparticles Versus Fullerenes in Functionalization by Electronic Polymers for Different Optical and Redox Properties

Fullerene cages are known as being able to participate in radical initiated copolymerization reactions with vinyl monomers for polymer-functionalized fullerenes. In this work, poly(N-vinylcarbazole) (PVK) was selected as a representative of electronic polymers in the functionalization of fullerene C₆₀ by the same copolymerization reaction to yield the PVK-C₆₀. Similarly found was that small carbon nanoparticles could also participate in the same copolymerization reaction for the nanoparticles to be surface-functionalized and -passivated by the attached PVK polymers, which are structurally adhering to the general definition on carbon dots (CDots), thus PVK-CDots. In the comparison between PVK-CDots and PVK-C₆₀, the former was found to be more absorptive and therefore more effective in photon harvesting across the visible spectral region and also brightly fluorescent, orders of magnitude more so than the latter. Similar to the PVK-C₆₀ and C₆₀ cages in general, the PVK-CDots exhibited significant photoinduced electron accepting characteristics and, at the same time, also extraordinary electron donating abilities that are not available to fullerenes. Because fullerene-based composites with electronic polymers including PVK have found significant applications in optoelectronic devices and systems, the prospect of CDots represented by the PVK-CDots for similar purposes is discussed.

Photo-Assisted Synthesis of a Pd-Ag@CQD Nanohybrid and Its Catalytic Efficiency in Promoting the Suzuki-Miyaura Cross-Coupling Reaction under Ligand-Free and Ambient Conditions

Supported bimetallic nanoparticles are very promising heterogeneous catalysts for carbon-carbon cross-coupling reactions, though reports focusing on their synergistic activity for promoting such reactions are very limited. In the current study, bimetallic Pd-Ag hybrid nanoparticles supported on carbon quantum dots (CQDs), Pd-Ag@CQDs, were synthesized by a facile and fast UV-light-driven (365 nm) one-pot protocol for the first time to investigate such a synergistic activity. The physico-chemical structural features of the Pd-Ag@CQD nanohybrid were evaluated by UV-vis, Fourier transform infrared, X-ray diffraction, electron-dispersive X-ray, and transmission electron microscopy analyses. The nanohybrid was found to have dimensions in the range of ca. 3-5 nm. The bimetallic Pd-Ag@CQD nanohybrid was utilized as an efficient heterogeneous catalyst for promoting the Suzuki-Miyaura coupling reaction with aryl bromides and aryl chlorides under ligand-free and ambient conditions. The synergistic activity of the components of the nanohybrid induced catalytic enhancement of the cross-coupling reaction in terms of short reaction times (<1 h) and high yields (>90%). The heterogeneous character of the nanohybrid system also enabled easy separation and recyclability (up to six cycles).

Quercetin-mediated synthesis of graphene oxide-silver nanoparticle nanocomposites: a suitable alternative nanotherapy for neuroblastoma

Background

Graphene and graphene-related materials have gained substantial interest from both academia and industry for the development of unique nanomaterials for biomedical applications. Graphene oxide (GO) and silver nanoparticles (AgNPs) are a valuable platform for the development of nanocomposites, permitting the combination of nanomaterials with different physical and chemical properties to generate novel materials with improved and effective functionalities in a single platform. Therefore, this study was conducted to synthesize a graphene oxide–silver nanoparticle (GO-AgNPs) nanocomposite using the biomolecule quercetin and evaluate the potential cytotoxicity and mechanism of GO-AgNPs in human neuroblastoma cancer cells (SH-SY5Y).

Methods

The synthesized GO-AgNPs were characterized using various analytical techniques. The potential toxicities of GO-AgNPs were evaluated using a series of biochemical and cellular assays. The expression of apoptotic and anti-apoptotic genes was measured by quantitative real-time reverse transcription polymerase chain reaction. Further, apoptosis was confirmed by caspase-9/3 activity and a terminal deoxynucleotidyl transferase dUTP nick end labeling assay, and GO-AgNPs-induced autophagy was also confirmed by transmission electron microscopy.

Results

The prepared GO-AgNPs exhibited significantly higher cytotoxicity toward SH-SY5Y cells than GO. GO-AgNPs induced significant cytotoxicity in SH-SY5Y cells by the loss of cell viability, inhibition of cell proliferation, increased leakage of lactate dehydrogenase, decreased level of mitochondrial membrane potential, reduced numbers of mitochondria, enhanced level of reactive oxygen species generation, increased expression of pro-apoptotic genes, and decreased expression of anti-apoptotic genes. GO-AgNPs induced caspase-9/3-dependent apoptosis via DNA fragmentation. Finally, GO-AgNPs induced accumulation of autophagosomes and autophagic vacuoles.

Conclusion

In this study, we developed an environmentally friendly, facile, dependable, and simple method for the synthesis of GO-AgNPs nanocomposites using quercetin. The synthesized GO-AgNPs exhibited enhanced cytotoxicity compared with that of GO at very low concentrations. This study not only elucidates the potential cytotoxicity against neuroblastoma cancer cells, but also reveals the molecular mechanism of toxicity.

Strongly tricolor-emitting carbon dots synthesized by a combined aging–annealing route and their bio-application

A novel way has been established to make the FCDs with a very high QY, high stability, low cell cytotoxicity, and outstanding performance as a fluorescence probe.

Fluorescent carbon dots: rational synthesis, tunable optical properties and analytical applications

This review summarizes current advances on the design and the employment of fluorescent carbon dots in sensing applications, especially from the point of analytical view.

Preparation and optical properties of magnetic carbon/iron oxide hybrid dots

Carbon/Fe₃O₄ hybrid dots are prepared for their optical properties in reference to those of neat carbon dots.

Carbon quantum dots and applications in photocatalytic energy conversion

Quantum dots (QDs) generally refer to nanoscale particles of conventional semiconductors that are subject to the quantum-confinement effect, though other nanomaterials of similar optical and redox properties are also named as QDs even in the absence of strictly defined quantum confinement. Among such nanomaterials that have attracted tremendous recent interest are carbon dots, which are small carbon nanoparticles with some form of surface passivation, and graphene quantum dots in various configurations. In this article, we highlight these carbon-based QDs by focusing on their syntheses, on their photoexcited state properties and redox processes, and on their applications as photocatalysts in visible-light carbon dioxide reduction and in water-splitting, as well as on their mechanistic similarities and differences.

Toward structurally defined carbon dots as ultracompact fluorescent probes

There has been much discussion on the need to develop fluorescent quantum dots (QDs) as ultracompact probes, with overall size profiles comparable to those of the genetically encoded fluorescent tags. In the use of conventional semiconductor QDs for such a purpose, the beautifully displayed dependence of fluorescence color on the particle diameter becomes a limitation. More recently, carbon dots have emerged as a new platform of QD-like fluorescent nanomaterials. The optical absorption and fluorescence emissions in carbon dots are not bandgap in origin, different from those in conventional semiconductor QDs. The absence of any theoretically defined fluorescence color-dot size relationships in carbon dots may actually be exploited as a unique advantage in the size reduction toward having carbon dots serve as ultracompact QD-like fluorescence probes. Here we report on carbon dots of less than 5 nm in the overall dot diameter with the use of 2,2'-(ethylenedioxy)bis(ethylamine) (EDA) molecules for the carbon particle surface passivation. The EDA-carbon dots were found to be brightly fluorescent, especially over the spectral range of green fluorescent protein. These aqueous soluble smaller carbon dots also enabled more quantitative characterizations, including the use of solution-phase NMR techniques, and the results suggested that the dot structures were relatively simple and better-defined. The potential for these smaller carbon dots to serve as fluorescence probes of overall sizes comparable to those of fluorescent proteins is discussed.

Functional surface engineering of C-dots for fluorescent biosensing and in vivo bioimaging

Nanoparticles are promising scaffolds for applications such as imaging, chemical sensors and biosensors, diagnostics, drug delivery, catalysis, energy, photonics, medicine, and more. Surface functionalization of nanoparticles introduces an additional dimension in controlling nanoparticle interfacial properties and provides an effective bridge to connect nanoparticles to biological systems. With fascinating photoluminescence properties, carbon dots (C-dots), carbon-containing nanoparticles that are attracting considerable attention as a new type of quantum dot, are becoming both an important class of imaging probes and a versatile platform for engineering multifunctional nanosensors. In order to transfer C-dots from proof-of-concept studies toward real world applications such as in vivo bioimaging and biosensing, careful design and engineering of C-dot probes is becoming increasingly important. A comprehensive knowledge of how C-dot surfaces with various properties behave is essential for engineering C-dots with useful imaging properties such as high quantum yield, stability, and low toxicity, and with desirable biosensing properties such as high selectivity, sensitivity, and accuracy. Several reviews in recent years have reported preparation methods and properties of C-dots and described their application in biosensors, catalysis, photovoltatic cells, and more. However, no one has yet systematically summarized the surface engineering of C-dots, nor the use of C-dots as fluorescent nanosensors or probes for in vivo imaging in cells, tissues, and living organisms. In this Account, we discuss the major design principles and criteria for engineering the surface functionality of C-dots for biological applications. These criteria include brightness, long-term stability, and good biocompatibility. We review recent developments in designing C-dot surfaces with various functionalities for use as nanosensors or as fluorescent probes with fascinating analytical performance, and we emphasize applications in bioimaging and biosensing in live cells, tissues, and animals. In addition, we highlight our work on the design and synthesis of a C-dot ratiometric biosensor for intracellular Cu(2+) detection, and a twophoton fluorescent probe for pH measurement in live cells and tissues. We conclude this Account by outlining future directions in engineering the functional surface of C-dots for a variety of in vivo imaging applications, including dots with combined targeting, imaging and therapeutic-delivery capabilities, or high-resolution multiplexed vascular imaging. With each application C-dots should open new horizons of multiplexed quantitative detection, high-resolution fluorescence imaging, and long-term, real-time monitoring of their target.

Growth and stabilization of silver nanoparticles on carbon dots and sensing application

Carbon dots (C-dots) have been proven to show the capability for direct reduction of Ag(+) to elemental silver (Ag(0)) without additional reducing agent or external photoirradiation by incubating Ag(+) with C-dots for 5 min in a water bath at 50 °C. Silver nanoparticles (Ag-NPs) are simultaneously formed with an average size of 3.1 ± 1.5 nm and grew on carbon dots. This process involves the oxidation of amine or phenol hydroxyl groups on the aromatic ring of C-dots. Meanwhile C-dots protect and stabilize the Ag-NPs from aggregation in aqueous medium; that is, the Ag-NPs are stable at least for 45 days in aqueous medium. The formed Ag-NPs cause significant resonance light scattering (RLS), which correlates closely with the concentration of silver cation, and this facilitates quantitative detection of silver in aqueous medium.

Photoluminescence properties of graphene versus other carbon nanomaterials

Photoluminescent nanomaterials continue to garner research attention because of their many applications. For many years, researchers have focused on quantum dots (QDs) of semiconductor nanocrystals for their excellent performance and predictable fluorescence color variations that depend on the sizes of the nanocrystals. Even with these advantages, QDs can present some major limitations, such as the use of heavy metals in the high-performance semiconductor QDs. Therefore, researchers continue to be interested in developing new QDs or related nanomaterials. Recently, various nanoscale configurations of carbon have emerged as potential new platforms in the development of brightly photoluminescent materials. As a perfect π-conjugated single sheet, graphene lacks electronic bandgaps and is not photoluminescent. Therefore, researchers have created energy bandgaps within graphene as a strategy to impart fluorescence emissions. Researchers have explored many experimental techniques to introduce bandgaps, such as cutting graphene sheets into small pieces or manipulating the π electronic network to form quantum-confined sp(2) "islands" in a graphene sheet, which apparently involve the formation or exploitation of structural defects. In fact, defects in graphene materials not only play a critical role in the creation of bandgaps for emissive electronic transitions, but also contribute directly to the bright photoluminescence emissions observed in these materials. Researchers have found similar defect-derived photoluminescence in carbon nanotubes and small carbon nanoparticles, dubbed carbon "quantum" dots or "carbon dots". However, they have not systematically examined the emissions properties of these different yet related carbon nanomaterials toward understanding their mechanistic origins. In this Account, we examine the spectroscopic features of the observed photoluminescence emissions in graphene materials. We associate the structural characteristics in the underlying graphene materials with those emission properties as a way of classifying them into two primary categories: emissions that originate from created or induced energy bandgaps in a single graphene sheet and emissions that are associated with defects in single- and/or multiple-layer graphene. We highlight the similarities and differences between the observed photoluminescence properties of graphene materials and those found in other carbon nanomaterials including carbon dots and surface defect-passivated carbon nanotubes, and we discuss their mechanistic implications.

Efficient fluorescence quenching in carbon dots by surface-doped metals--disruption of excited state redox processes and mechanistic implications

The carbon dots in this study were small carbon nanoparticles with the particle surface functionalized by oligomeric poly(ethylene glycol) diamine molecules. Upon photoexcitation, the brightly fluorescent carbon dots in aqueous solution served the function of excellent electron donors to reduce platinum(IV) and gold(III) compounds into their corresponding metals to be deposited on the dot surface. The deposited metals even in very small amounts were found to have dramatic quenching effects on the fluorescence emission intensities, but essentially no effects on the observed fluorescence decays. The obviously exclusive near-neighbor static quenching could be attributed to the disruption of electron-hole radiative recombinations (otherwise responsible for the fluorescence emissions in carbon dots). The results provide important evidence for the availability of photogenerated electrons that could be harvested for productive purposes, which in turn supports the current mechanistic framework on fluorescence emission and photoinduced redox properties of carbon dots.