Separation and purification of fluorescent carbon dots - an unmet challenge

Literature reports demonstrate versatile optical applications of fluorescent carbon dots (CDs) in biological imaging, full-color solid-state lighting, optoelectronics, sensing, anticounterfeiting and so on. The fluorescence associated with CDs may originate significantly from byproducts generated during their synthesis, which need to be eliminated to achieve error-free results. The significance of purification, specifically for luminescence-based characterizations, is highly critical and imperative. Thus, there is a pressing demand to implement consistent and adequate purification strategies to reduce sample complexity and thereby realize reliable results that can provide a tactical steppingstone towards the advancement of CDs as next-generation optical materials. The article focuses on the mechanism of origin of fluorescence from CDs and further demonstrates the different purification approaches including dialysis, centrifugation, filtration, solvent extraction, chromatography, and electrophoresis that have been adopted by various researchers. Furthermore, the fundamental separation mechanism, as well as the advantages and limitations of each of these purification techniques are discussed. The article finally provides the critical challenges of these purification techniques that need to be overcome to obtain homogeneous CD fractions that demonstrate coherent and reliable optical features for suitable applications.

Carbon dots using a household cleaning liquid as a dopant for iron detection in hydroponic systems

Iron (Fe) is a required micronutrient in plants for the production of chlorophyll and transport of oxygen. A commonly used surrogate for measuring nutrient levels is the measurement of electrical conductivity or total dissolved solids, but this technique is not selective towards any particular dissolved ion. In this study, using a conventional microwave, fluorescent carbon dots (CDs) are produced from glucose and a household cleaning product and applied towards monitoring dissolved ferric iron levels in hydroponic systems through fluorescent quenching. The produced particles have an average size of 3.19 ± 0.76 nm with a relatively high degree of oxygen surface groups. When using an excitation of 405 nm, a broad emission peak is centered at approximately 500 nm. A limit-of-detection of 0.196 ± 0.067 ppm (3.51 ± 1.21 μM) with minimal interference from common heavy metal quenchers and ions found in hydroponic systems was determined. Butterhead lettuce was grown while discretely monitoring iron levels via the CDs for three separate weeks of growth. The CDs displayed a non-significant difference (p > 0.05) in performance when compared to a standard method. These results along with a simple and relatively low-cost production method make the CDs in this study a promising tool for monitoring iron levels in hydroponic systems.

Hydrophobic Carbon Dots Derived from Organic Pollutants and Applications in NIR Anticounterfeiting and Bioimaging

In an effort to fulfill the strategy of sustainable development, Rhodamine B, a common and toxic organic pollutant in the textile industry, was reported for the first time as a single precursor to develop a kind of novel hydrophobic nitrogen-doped carbon dot (HNCD) through a green and facile one-pot solvothermal method. The HNCDs with an average size of 3.6 nm possess left and right water contact angles of 109.56° and 110.34°, respectively. The HNCDs manifest excitation wavelength-tunable and upconverted fluorescence from the ultraviolet (UV) to the near-infrared (NIR) range. Furthermore, the PEGylation of HNCDs enables them to be used as an optical marker for cell and in vivo imaging. Notably, the HNCDs with solvent-dependent fluorescence can be used for invisible inks with a wide range of light responses from UV-vis-NIR spectra. This work not only provides an innovative way to recycle chemical waste but also expands the potential application of HNCDs in NIR security printing and bioimaging.

Lights and Dots toward Therapy-Carbon-Based Quantum Dots as New Agents for Photodynamic Therapy

The large number of deaths induced by carcinoma and infections indicates that the need for new, better, targeted therapy is higher than ever. Apart from classical treatments and medication, photodynamic therapy (PDT) is one of the possible approaches to cure these clinical conditions. This strategy offers several advantages, such as lower toxicity, selective treatment, faster recovery time, avoidance of systemic toxic effects, and others. Unfortunately, there is a small number of agents that are approved for usage in clinical PDT. Novel, efficient, biocompatible PDT agents are, thus, highly desired. One of the most promising candidates is represented by the broad family of carbon-based quantum dots, such as graphene quantum dots (GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs), and carbonized polymer dots (CPDs). In this review paper, these new smart nanomaterials are discussed as potential PDT agents, detailing their toxicity in the dark, and when they are exposed to light, as well as their effects on carcinoma and bacterial cells. The photoinduced effects of carbon-based quantum dots on bacteria and viruses are particularly interesting, since dots usually generate several highly toxic reactive oxygen species under blue light. These species are acting as bombs on pathogen cells, causing various devastating and toxic effects on those targets.

Synthesis of luminescent chitosan-based carbon dots for Candida albicans bioimaging

In this work, we used chitosan as a raw material to synthesize carbon dots using fast microwave carbonization. We studied the influence of the synthesis time, doping agent, and the molar ratio between the reactants on the quantum yield of carbon dots. Chitosan-based carbon dots displayed stable blue fluorescence emission with excitation-dependent behavior and quantum yield values ranging from 1.16 to 7.07 %. ANOVA results showed that the interaction factor between the doping agent and the molar ratio of the reactants was a significant combination to produce carbon dots with higher quantum yield. The presence of the doping agent improved the carbon dots optical properties by obtaining higher fluorescence intensity values. Confocal laser microscope images showed that the carbon dots internalized in the Candida albicans cellular membrane, exhibiting blue, green, and red emissions, acting as a promising agent for bioimaging.

Biogenic synthesis of carbon quantum dots from garlic peel bio-waste for use as a fluorescent probe for sensing of quercetin

Highly fluorescent and water-soluble carbon quantum dots (CQDs) were synthesized from the bio-waste source of garlic peels (renovation of bio-waste into bio-asset) using a controlled carbonization method. Synthesized CQDs were characterized by various analytical methods and explored as a fluorogenic probe for the recognition of quercetin (QT). UV-Vis result shows an absorption maximum at 275 nm attributed to the conjugation of C=C and C=O of CQDs and demonstrates a blue emission in the range of 330-410 nm. Selectivity was performed with various biomolecules, except for QT, all other do not exhibit any considerable change in the fluorescence of CQDs. On the interaction with QT, emission was completely quenched due to FET, confirming the high selective to QT. Effect of pH, sensitivity, and stability studies displayed excellent results under optimized conditions. The LOD fluorescent probe was found to be 6.73 μM. Our approach may suggest a new platform for the development of quick and low-cost CQDs-based sensors for environmental and biological purposes.

Fluorescent Carbon Dots and their Applications in Sensing of Small Organic Molecules

Background:

Fluorescence-based sensing is considered highly sensitive and fluorescent probes with improved properties are always desired. Fluorescent carbon dots (CDs) are newly emerging quasi-spherical nanoparticles of less than 10 nm in size and belong to the carbon nano-material’s family. CDs have great potential as fluorescent probes and currently are under open deliberation by the researchers due to their striking properties such as low environmental hazard, high selectivity, greater sensitivity, good biocompatibility, tunable fluorescent properties and excitation dependent multicolor emission behavior.

Introduction:

This review demonstrates various available methods for fabrication of fluorescent CDs, capping of CDs and characterization with various techniques including UV-visible, FT-IR, and TEM. Analytical applications using CDs for the sensing of small organic molecules, specifically nitroaromatic compounds in the environmental samples are complied.

Methods:

The review covers literature related to synthesis and characterization of carbon dots. It includes around 171 research articles in this field.

Results:

Carbon dots can be synthesized using numerous routes. In all cases CDs possess spectral properties with little variation in wavelength maxima. Optical properties of CDs can be tuned by compositing these with metallic quantum dots or by modifying their surface with desired functionalities. HR-TEM is needed to see the morphology and size of particles whereas UV-Visible and FTIR are indispensable tools for this kind of research. These particles are successfully applied to sense small molecules in some matrices.

Conclusion:

Carbon dots are bright stars in fluorescent sensing of small molecules. However, more research is needed to determine small organic molecules in diversified areas of analysis.

Solvent effect on the absorption and emission spectra of carbon dots: evaluation of ground and excited state dipole moment

Background

Carbon dots (C-dots) are photoluminescent nanoparticles with less than 10 nm in size. Today, many studies are performed to exploit the photoluminescence (PL) property of carbon dots, and our focus in this study is to estimate the dipole moment of carbon dots. For reaching our aims, C-dots were synthesized and dissolved in the different solvents.

Results

Carbon dots with intense photoluminescence properties have been synthesized by a one-step hydrothermal method from a carbon bio-source. In this research, we report on the effect of aprotic solvents on absorption and fluorescence spectra and dipole moments of C-dots dispersed in a range of many aprotic solvents with various polarity and dielectric constant at room temperature. The change in the value of dipole moment was estimated by using the Stokes shifts. The difference between the dipole moment of the excited state and the ground state was shown using an extended form of Lippert equations by Kawski and co-workers.

Conclusions

The values found for μ_g = 1.077 D, and μ_e = 3.157 D, as well as the change in the dipole moments. The results showed that the dipole moment of the excited state is more than the ground state, indicating a high density and redistribution of electrons in the excited state. Finally, the quantum yield of C-dots in the eclectic aprotic solvents was communicated and discussed.

Long-term effects of impurities on the particle size and optical emission of carbon dots

Carbon dots (CDs) are fluorescent nanoparticles that exhibit strong photoluminescence (PL) emission throughout the visible range of the electromagnetic spectrum. Recent studies highlight the presence of fluorescent impurities in CD dispersions. Here, the long-term impact of these impurities on the stability of the physical and optical properties of CDs synthesized by the solvothermal method is studied. A significant increase in particle size is observed as a function of time after synthesis from transmission electron microscopy analysis of CDs. Furthermore, the quantum yield of blue PL emission, which is mostly caused by impurities that contain carboxyl groups, gradually decays from 30% to ∼3% over 13 weeks. The reduction in quantum yield is attributed to decomposition of impurities that, consequently, deposit on the particles and increase particle size. Finally, it is observed that the blue emission decreases considerably when CDs are properly purified and a solvent-dependent yellow emission arises. The yellow emission is almost negligible when CDs are dispersed in water; however, the intensity of yellow emission increases significantly when the concentration of ethanol is increased.

Yellow-Emitting Hydrophobic Carbon Dots via Solid-Phase Synthesis and Their Applications

The preparation and application of hydrophobic carbon dots (HCDs) are now the hotspots in the field of nanomaterials. This paper reports the fast synthesis of long-wavelength-emitting HCDs (yellow-emitting, λ_em = 541 nm) through a solid-phase route, with l-cysteine hydrochloride anhydrous and citric acid as carbon sources and dicyclohexylcarbodiimide as a dehydrating agent, reacting at 180 °C for 40 min, with a quantum yield of 30%. The solid-phase route avoids the usage of organic reagents during the synthesis process and is thus environmentally friendly. The obtained HCDs can be simply separated into HCDs-L (less density) and HCDs-W (higher density) with differences in physical (polarity, density), optical, and chemical properties. The differences in HCDs-L, HCDs-W, and water-soluble CDs (WCDs) were compared through various characterization methods, and the synthesis and luminescence mechanisms of HCDs were investigated. Meanwhile, HCDs were employed in the fields of LED lamp production and solid fluorescent shaping material. The prepared HCDs were then modified into WCDs through the liposomal embedding method. The HCDs prepared by the new solid-phase route exhibit stable and highly efficient photoluminescence ability and will have a promising outlook in their applications in various fields.

Sandalwood-derived carbon quantum dots as bioimaging tools to investigate the toxicological effects of malachite green in model organisms

Malachite green is an N-methylated diaminophenylmethane dye that has generated much concern over its suggestive carcinogenic nature. After its excessive use in aquaculture industry as an effective ectoparasitide, much debate was raised over its toxicological effects leading to scientific studies conducted on animal models. Even after several bans, malachite green is still easily available in many parts of the world and unscrupulously even used to give green vegetables a fresher look. This study aims to address this concern by systematically studying the toxicological effects of malachite green through bioimaging in plant and animal cell and tissue. Sandalwood-derived carbon quantum dots have been used as a bioimaging tool since they are non-cytotoxic and show excellent fluorescence properties. Onion tissues demonstrate the translocation of the dye inside cells having high affinity for the nuclei and cell walls. Toxicological effects on the growth of Vigna radiata (mung beans) have been studied methodically. Bioimaging of the transverse cross-section of the dye-treated plant root shows a significant difference from the control. In animal cells, dose-dependent decrease in cell viability of MG-63 cells was observed with MG. CQD showed good fluorescence in both cytoplasm and nucleus of MG63 cells. In addition, CQDs were employed as a great tool for bioimaging of the histopathologically adverse effects of MG in Golden hamster animal model. This study showed CQDs could be used as an alternative non-site specific fluorescent probe for cell and tissue imaging for better visualization of cell and tissue architectural changes.

Recent advancements in the applications of carbon nanodots: exploring the rising star of nanotechnology

Nanoparticles possess fascinating properties and applications, and there has been increasing critical consideration of their use. Because carbon is a component with immaterial cytotoxicity and extensive biocompatibility with different components, carbon nanomaterials have a wide scope of potential uses. Carbon nanodots are a type of carbon nanoparticle that is increasingly being researched because of their astounding properties such as extraordinary luminescence, simplicity of amalgamation and surface functionalization, and biocompatibility. Because of these properties, carbon nanodots can be used as material sensors, as indicators in fluorescent tests, and as nanomaterials for biomedical applications. In this review, we report on the ongoing and noteworthy utilization of carbon quantum dots such as bioimaging tests and photocatalytic applications. In addition, the extension and future components of these materials, which can be investigated for new potential applications, are discussed.

Parameters affecting the synthesis of carbon dots for quantitation of copper ions

A simple, eco-friendly, and low-cost electrochemical approach has been applied to the synthesis of carbon dots (C dots) from histidine hydrochloride in the absence or presence of halides (Cl, Br, and I) at various potentials up to 10 V. The as-formed C dots refer to C dots, Cl-C, Br-C, and I-C dots. The time-evolution UV-vis absorption and photoluminescence (PL) spectra provide more detailed information about the formation of C dots. Upon increasing the reaction time from 1 to 120 min, more and more C dots are formed, leading to increased PL intensity. The halides play two important roles in determining the formation of C dots; controlling the reaction rate and surface states. When compared to chloride and bromide, iodide has a greater effect on varying surface states and inducing PL quenching through intersystem crossing. The PL intensities of the four types of C dots all decrease upon increasing Cu²⁺, Hg²⁺, and Ag⁺ concentrations. In the presence of 0.8 mM I^-, I-C dots compared to C dots, Cl-C dots, and Br-C dots are slightly better for quantitation of Cu²⁺. Fourier transform infrared spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy results of I-C dots reveal the interactions of Cu²⁺ with the surface ligands (imidazole and histidine). The I-C dot probe in the presence of 0.8 mM I^- is selective toward Cu²⁺ over the tested metal ions such as Hg²⁺ and Ag⁺. The assay provides a limit of detection of 0.22 μM for Cu²⁺ at a signal-to-noise ratio of 3. Practicality of this probe has been validated by the analyses of tap, lake, and sea water samples, with negligible matrix effects.

Carbon dots: Applications in bioimaging and theranostics

Carbon dots are a carbonaceous nanomaterial that were discovered accidentally and are now drawing significant attention as a new quantum-sized fluorescent nanoparticle. Carbon dots are biocompatible, non-toxic, photostable, and easily functionalized with good photoluminescence and water solubility. Due to these unique properties, they are used broadly in live cell imaging, catalysis, electronics, biosensing, power, targeted drug delivery, and other biomedical applications. Here, we review the recent development of carbon dots in nanomedicine from their use in drug carriers to imaging agents to multifunctional theranostic systems. Finally, we discuss the challenges and views on next-generation carbon dot-based theranostics for clinical applications.

Gamma ray assisted modification of carbon quantum dot/polyurethane nanocomposites: structural, mechanical and photocatalytic study

In recent years, water pollution and contamination had become a major threat to the ecosystem. However, the use of nanostructured materials has been proven as a very promising approach in the treatment of polluted water. The present study reports the results of the gamma ray-assisted modification of hydrophobic carbon quantum dot (hCQD)/polyurethane nanocomposites for photocatalytic degradation of organic dyes. Different characterization methods were applied to investigate the influence of the different doses of gamma irradiation (1, 10 and 200 kGy) on the physical and chemical properties of nanocomposites (morphology, chemical content, mechanical properties, wettability, and potential for singlet oxygen generation). Surface morphology and mechanical properties analyses showed that gamma rays induced insignificant changes in the structure of nanocomposites, but the potential for singlet oxygen generation increased significantly. Here we also explore, in detail, the photocatalytic properties of gamma-ray modified hCQDs/polyurethane nanocomposites. UV-vis analysis showed that the removal efficiency of the rose bengal dye reached up to 97% for the nanocomposite irradiated with the dose of 200 kGy.

The present study reports the results of the gamma ray-assisted modification of hydrophobic carbon quantum dots (hCQDs)/polyurethane nanocomposites for photocatalytic degradation of organic dyes.

Influence of carbon nanodots encapsulated polycarbazole hybrid on the corrosion inhibition performance of polyurethane nanocomposite coatings

Carbon nanodots encapsulated in a polycarbazole hybrid-dispersed polyurethane nanocomposite coating with new exciting perspectives for high-performance anticorrosive coatings are shown.

Carbon Quantum Dots Modified Polyurethane Nanocomposite as Effective Photocatalytic and Antibacterial Agents

Development of new types of antibacterial coatings or nanocomposites is of great importance due to widespread multidrug-resistant infections including bacterial infections. Herein, we investigated biocompatibility as well as structural, photocatalytic, and antibacterial properties of photoactive hydrophobic carbon quantum dots/polyurethane nanocomposite. The swell-encapsulation-shrink method was applied for production of these nanocomposites. Hydrophobic carbon quantum dots/polyurethane nanocomposites were found to be highly effective generator of singlet oxygen upon irradiation by low-power blue light. Analysis of conducted antibacterial tests on Staphyloccocus aureus and Escherichia coli showed 5-log bactericidal effect of these nanocomposites within 60 min of irradiation. Very powerful degradation of dye (rose bengal) was observed within 180 min of blue light irradiation of the nanocomposites. Biocompatibility studies revealed that nanocomposites were not cytotoxic against mouse embryonic fibroblast cell line, whereas they showed moderate cytotoxicity toward adenocarcinomic human epithelial cell line. Minor hemolytic effect of these nanocomposites toward red blood cells was revealed.

Facile One-Pot Synthesis of Polydopamine Carbon Dots for Photothermal Therapy

Carbon dots (CDs) are a member of fluorescent carbon nanomaterials that are widely applied in bioimaging, photothermal therapy (PTT), and biosensors for its tunable fluorescence, photothermal conversion property, and excellent biocompatibility. Surface passivation and doping especially the doping of N atoms are critical factors to enhance the fluorescent intensity of CDs. Until now, a variety of nitrogen-rich molecules has been applied for the surface passivation of CDs such as L-Dopa, amino acids, and polyethylenimine (PEI). Herein, we report the synthesis of fluorescent polydopamine (PDA)-passivated carbon dots (CD-PDA) via one-pot microwave-assisted pyrolysis within 5 min, dramatically simplifying the reaction process compared with the hydrothermal treatment reported before. DLS, FT-IR, UV-Vis, and fluorescence spectroscopy were used to confirm the components of CD-PDA and to illuminate the mechanism of its tunable photoluminescence (PL). Due to the doping of N atoms by PDA, quantum yield (QY) of the CD-PDA was measured at 5%, which was nearly triple the original CDs without adding PDA. Yield of CD-PDA was about 1.5 times of the CDs on account of the enhancement of nucleation site for the carbon dot formation with the phenolic group provided by PDA. Meanwhile, photothermal conversion efficiency of the CD-PDA was determined to be 35% because of the excellent NIR light-thermal conversion property of PDA. Overall, we provided an extremely efficient approach to fabricate the fluorescent N-doped CD-PDA with stable photothermal conversion efficiency and excellent biocompatibility. More importantly, the passivation of PDA enabled the CD-PDA synthesized in our research compatible for further modification through Michael addition or Schiff base reaction.

Thermal carbonization in nanoscale reactors: controlled formation of carbon nanodots inside porous CaCO3 microparticles

Synthesis of carbon nanodots (CNDs) in confined geometry via incorporation of dextran sulphate into pores of CaCO3 microparticles is demonstrated. The preparation process included three steps: co-precipitation of solutions of inorganic salts and carbon source, thermal treatment and CaCO3 matrix removal. We show that geometric constraints can be used to precisely control the amount of source material and to avoid formation of large carbon particles. Analysis of TEM data shows particle size of ~3.7 nm with narrow size distribution. Furthermore, we found that variation in pore morphology has a clear effect on CNDs structure and optical properties. CNDs with graphene oxide like structure were obtained in the nanoporous outer shell layer of CaCO3 microparticles, while less ordered CNDs with the evidence of complex disordered carbons were extracted from the inner microcavity. These results suggest that confined volume synthesis route in CaCO3 nanopores can be used to precisely control the structure and optical properties of CNDs.

Dispersibility of carbon dots in aqueous and/or organic solvents

Carbon dots have a wide range of applications in biological and medical fields as an alternative to quantum dots because of their low toxicity and excellent luminescence properties. To date, a large number of carbon dots have been prepared and they were consequently reviewed according to their synthetic method, luminescence properties and related applications. The dispersibility of carbon dots in aqueous and/or organic solvents could actually play a significant role in the properties and various application fields, however, such a perspective is ignored by most of the reviewed literature studies. Thus in this minireview, we focus on the surfactant groups of carbon dots which could be classified as hydrophilic, hydrophobic and amphiphilic types. They have accordingly a marked impact on the dispersibility of carbon dots in different solvents as well as the further advantage in those imaging studies in vitro and in vivo.

Carbonization of Human Fingernails: Toward the Sustainable Production of Multifunctional Nitrogen and Sulfur Codoped Carbon Nanodots with Highly Luminescent Probing and Cell Proliferative/Migration Properties

A simple yet effective method is employed to prepare multifunctional fluorescent carbon nanodots (CNDs) from human fingernails. The results demonstrate that the CNDs have excellent optical properties and a quantum yield of 81%, which is attributed to the intrinsic composition of the precursor material itself. The CNDs are used to develop an ultrasensitive fluorescent probe for the detection of hexavalent chromium (limit of detection: 0.3 nM) via a combined inner-filter and static mechanism. Moreover, the toxicity of the CNDs over four epithelial cell lines is assessed. A negligible toxicity is induced on the three of the cell lines, whereas an increase in HEK-293 cell viability is demonstrated, granting cell proliferation properties to the as-synthesized CNDs. According to cell cycle analysis, cell proliferation is achieved by enhancing the transition of cells from the S phase to the G2/M one. Interestingly, CNDs are found to significantly promote cell migration, maybe because of their free-radical scavenging ability, making the CNDs suitable for wound healing applications. In addition, relevant experiments have revealed the blood compatibility of the CNDs. Finally, the CNDs were found suitable for cell imaging applications, and all of the aforementioned merits make it possible for them to be used for extraordinary, more advanced biological applications.

Gram-Scale Synthesis and Kinetic Study of Bright Carbon Dots from Citric Acid and Citrus japonica via a Microwave-Assisted Method

Tracking dynamic cellular processes necessitates fluorescent materials that are photostable, biocompatible, water-soluble, nanosized, and nontoxic. In this study, highly fluorescent carbon dots (CDs) were produced from cheap and readily available sources, citric acid (CA) and Philippine citrus (Citrus japonica Thunb.) or calamansi juice (CJ) via a microwave-assisted method. A number of synthetic conditions were investigated systematically to optimize the preparation of CDs from CA and CJ. The formation mechanism, surface chemistry, and photoluminescence of CA-based CDs (CA-CDs) and CJ-based CDs (CJ-CDs) were evaluated after each stage of pyrolysis in detail using different characterization techniques, such as dynamic light scattering, diffusion-ordered spectroscopy, atomic force microscopy, ζ potential, X-ray diffraction, Fourier transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance spectroscopy, and absorption/emission spectroscopy. Gram-scale pyrolysis of CA with ethylenediamine (EDA) and CJ with EDA were carried out to provide CA-CDs (CA-18) within 18 min total pyrolysis time at 97% yield and CJ-CDs (CJ-14) within 14 min total pyrolysis time at 7% yield. Aqueous suspensions of CA-18 and CJ-14 CDs gave comparable bright blue luminescence at 462 nm. CA-CDs were shown to be nontoxic for mung beans up to 2 mg/mL, whereas CJ-CDs with higher surface negative charges inhibited growth above 0.5 mg/mL. This study demonstrates that bright CA- and CJ-CDs can be produced in gram-scale quantities using inexpensive methods. The size, amount, and extent of EDA incorporation are important in contributing to the formation of highly emissive particles.

Pyridine derivative-induced fluorescence in multifunctional modified carbon dots and their application in thermometers

We present a simple hydrothermal method to fabricate multifunctional modified carbon dots (with -COOH, -OH, -SH, and -NH₂ groups, named NS-Cdots) using citric acid and l-cysteine as raw materials. The functional NS-Cdots exhibit high fluorescent quantum yield (38.9%), low cytotoxicity and good biocompatibility. A reasonable photoluminescence mechanism of the NS-Cdots was proposed in which the pyridine derivatives serve as conjugating units and dominate the main emission behavior. The NS-Cdots can also be used as excellent fluorescent probes for cell imaging in vitro and as effective thermometers with a wide temperature detection range from 20 °C to 95 °C.

One-Step Synthesis and Characterization of N-Doped Carbon Nanodots for Sensing in Organic Media

Photoluminescent nitrogen-doped carbon nanodots (N-doped CNDs) soluble in organic media are synthesized in a one-step synthesis from a single-source precursor (an amphiphilic polymer), which exhibits a very high quantum yield (QY = 78%), excitation wavelength-dependent emission, and upconversion emission properties. The evolution of N-doped CND formation is studied via ultraviolet-visible and photoluminescence spectroscopy. Their analytical application as an effective sensor for the direct determination of nitroaromatic explosives and byproducts is shown based on their selective response via a fluorescence quenching mechanism. The proposed method is validated in soil samples by directly using the sensor in organic media without any further treatment or additional functionalization, which is an interesting aspect for practical applications.

Influence of Inert and Oxidizing Atmospheres on the Physical and Optical Properties of Luminescent Carbon Dots Prepared through Pyrolysis of a Model Molecule

This work used L-tartaric acid as a model molecule to evaluate how the use of inert and oxidizing atmospheres during pyrolysis affected the physical and optical properties of the resulting carbon dots (CDs). Pyrolysis revealed to be a simple procedure that afforded CDs in a single step, dismissed the addition of organic solvents, and involved only one extraction stage that employed water. By X-ray diffraction a dependency between the structure of the CDs and the atmosphere (oxidizing or inert) used during the pyrolysis was found. Potentiometric titration demonstrated that the CDs were largely soluble in water; it also aided characterization of the various groups that contained sp(3) -hybridized carbon atoms on the surface of the dots. Raman spectroscopy suggested that different amounts of sp(2)- and sp(3)-hybridized carbon atoms emerged on the CDs depending on the pyrolysis atmosphere. In conclusion, the pyrolysis atmosphere influenced the physical properties, such as the composition and the final structure.

An overview of nanoparticles commonly used in fluorescent bioimaging

This article gives an overview of the various kinds of nanoparticles (NPs) that are widely used for purposes of fluorescent imaging, mainly of cells and tissues. Following an introduction and a discussion of merits of fluorescent NPs compared to molecular fluorophores, labels and probes, the article assesses the kinds and specific features of nanomaterials often used in bioimaging. These include fluorescently doped silicas and sol-gels, hydrophilic polymers (hydrogels), hydrophobic organic polymers, semiconducting polymer dots, quantum dots, carbon dots, other carbonaceous nanomaterials, upconversion NPs, noble metal NPs (mainly gold and silver), various other nanomaterials, and dendrimers. Another section covers coatings and methods for surface modification of NPs. Specific examples on the use of nanoparticles in (a) plain fluorescence imaging of cells, (b) targeted imaging, (c) imaging of chemical species, and (d) imaging of temperature are given next. A final section covers aspects of multimodal imaging (such as fluorescence/nmr), imaging combined with drug and gene delivery, or imaging combined with therapy or diagnosis. The electronic supplementary information (ESI) gives specific examples for materials and methods used in imaging, sensing, multimodal imaging and theranostics such as imaging combined with drug delivery or photodynamic therapy. The article contains 273 references in the main part, and 157 references in the ESI.

Luminescent colloidal carbon dots: optical properties and effects of doping [Invited]

We review the effect of doping on the optical properties of luminescent colloidal carbon dots. They are considered as a hybrid material featuring both molecular and semiconductor-like characteristics, where doping plays an important role. Starting from the short overview of synthetic strategies, we consider the evolution of carbon dots from molecular precursors to fluorescent nanoparticles, and the relevant structural properties of carbon dots. Choice of the reactant materials, dopant atoms and reaction parameters provide carbon dots with varying optical properties. High chemical stability, bright luminescence and customizable surface functionalization of carbon dots open their use in a broad range of applications, which are exemplary presented at the end of this review.

Bioinspired synthesis of fluorescent calcium carbonate/carbon dot hybrid composites

Herein, we report a novel method to synthesise fluorescent calcium carbonate/carbon dots (CaCO3/CDs) by simply mixing CaCl2 and Na2CO3 solutions in the presence of CDs. There are two roles of CDs in this easy and cost-effective biomimetic strategy, that is as the template to direct the formation and assembly of calcite nanocrystals into hierarchical spheres with diameters in the range of 200-300 nm and simultaneously as the phosphor to enable the CaCO3 to emit blue fluorescence under UV (365 nm) irradiation with a quantum yield of 56.2%. The CaCO3/CD hybrid composites possessing unique fluorescence properties are potentially useful in various applications.

A simple turn on fluorescent sensor for the selective detection of thiamine using coconut water derived luminescent carbon dots

In this study microwave-assisted hydrothermal method was used to prepare highly luminescent carbon dots (1-6 nm size) within a minute from tender coconut (Cocos nucifera) water. The synthesized carbon dots (C-dots) exhibit emission of blue and green lights while excited at 390 and 450 nm wavelengths, respectively. As an application, these C-dots were tested for a simple "turn on" fluorescent sensor for rapid detection of thiamine (vitamin B1). The detection of thiamine in human body is very important to prevent various diseases such as beriberi, neurological disorders, optic neuropathy, etc. The fluorescence emission intensity of C-dots quenches after addition of Cu(2+) ion and then again increases selectively (turn on) after the addition of thiamine. The fluorescence emission intensity enhancement of Cu(2+) ion modified C-dots in the presence of thiamine exhibits a linear relationship within the thiamine concentration range of 10-50 μM. The limit of detection was found to be 280 nM from this study. The selectivity of the detection was also tested in the presence of different organic molecules and inorganic ions (Ca(2+), Mg(2+), Na(+), K(+), Cl(-), SO4(2-), and NO3(-)) which are present in blood serum and urine and found to be almost no interference in the detection. Finally, to see the applicability in real samples a commercial vitamin capsule was tested and found less than 3% error in the detected concentration. The C-dots were also used for bioimaging of fungus and the results show they are also suitable for this application too.

Fluorescent Nanoparticles from Several Commercial Beverages: Their Properties and Potential Application for Bioimaging

The presence of nanoparticles in beverages has raised great concern in terms of potential impacts to consumer health. Herein, carbon dots in beverages kvass, pony malta, pilsner beer, Vivant Storm, and Profit were identified. They were shown to have a strong fluorescence under the excitation of ultraviolet light. The emission peaks shift to longer wavelengths accompanied by a remarkable fluorescence intensity decrease. The carbon dots are in the nanosized range and roughly spherical in appearance. Elemental analysis by X-ray photoelectron spectroscopy demonstrated the composition of Kvass carbon dots to be C 83.17%, O 13.83%, and N 3.00%. No cytotoxicity was found at concentrations up to 20 mg/mL for human tongue squamous carcinoma cells, and they can be directly applied in both carcinoma and onion epidermal cell imaging. This work represents the first report of the carbon dots present in beverages, providing valuable insights into these nanoparticles for future biological imaging.

Synthesis and Modification of Carbon Nanomaterials utilizing Microwave Heating

Microwave-assisted synthesis and processing represents a growing field in materials research and successfully entered the field of carbon nanomaterials during the last decade. Due to the strong interaction of carbon materials with microwave radiation, fast heating rates and localized heating can be achieved. These features enable the acceleration of reaction processes, as well as the formation of nanostructures with special morphologies. A comprehensive overview is provided here on the possibilities and achievements in the field of carbon-nanomaterial research when using microwave-based heating approaches. This includes the synthesis and processing of carbon nanotubes and fibers, graphene materials, carbon nanoparticles, and capsules, as well as porous carbon materials. Additionally, the principles of microwave-heating, in particular of carbon materials, are introduced and important issues, i.e., safety and reproducibility, are discussed.

Green synthesis of fluorescent hydrophobic carbon quantum dots and their use for 2,4,6-trinitrophenol detection

Fluorescent hydrophobic carbon dots are synthesized in a green way and used for determination of 2,4,6-trinitrophenol in a hydrophobic medium for the first time.

A pH-responsive drug delivery system with an aggregation-induced emission feature for cell imaging and intracellular drug delivery

A pH-sensitive fluorescent vehicle based on aggregation-induced emission (AIE) has been developed as a drug delivery system for simultaneous cell imaging and therapeutic treatment.

The regulation of hydrophilicity and hydrophobicity of carbon dots via a one-pot approach

The hydrophilicity or hydrophobicity of carbon dots is regulated by varying the H₃PO₄/ethanol molar ratio, via a hydrothermal process with 1-butyl-3-methylimidazolium hexafluorophosphate as the carbon source in a H₃PO₄–ethanol medium.

An ionic liquid promoted microwave-hydrothermal route towards highly photoluminescent carbon dots for sensitive and selective detection of iron(iii)

Carbon dots with a high photoluminescence efficiency of ∼22.58% are obtained by a facile microwave-hydrothermal treatment of rice straw with the presence of ionic liquid.

Facile synthesis of analogous graphene quantum dots with sp(2) hybridized carbon atom dominant structures and their photovoltaic application

Graphene quantum dot (GQD) is an emerging class of zero-dimensional nanocarbon material with many novel applications. It is of scientific importance to prepare GQDs with more perfect structures, that is, GQDs containing negligible oxygenous defects, for both optimizing their optical properties and helping in their photovoltaic applications. Herein, a new strategy for the facile preparation of "pristine" GQDs is reported. The method we presented is a combination of a bottom-up synthetic and a solvent-induced interface separation process, during which the target products with highly crystalline structure were selected by the organic solvent. The obtained organic soluble GQDs (O-GQDs) showed a significant difference in structure and composition compared with ordinary aqueous soluble GQDs, thus leading to a series of novel properties. Furthermore, O-GQDs were applied as electron-acceptors in a poly(3-hexylthiophene) (P3HT)-based organic photovoltaic device. The performance highlights that O-GQD has potential to be a novel electron-acceptor material due to the sp(2) hybridized carbon atom dominant structure and good solubility in organic solvents.

Multifunctional water-soluble luminescent carbon dots for imaging and Hg2+ sensing

We propose an ingenious method for large-scale fabrication of water-soluble photoluminescent carbon dots (CDs) by a one-step microwave route in the presence of citric acid and ethylenediamine. In contrast to other CDs-based nanomaterials, the CDs prepared exhibit a highly fluorescent quantum yield (QY) and excellent stability in both organic and inorganic phases. After simple post-treatment, the CDs can be used as a new type of fluorescent ink for information storage and nanofiber electrospinning. It should be noted that the CDs are a superior fluorescent bioimaging agent in cells, plants and animals according to their excellent solubility and ultra-low toxicity. In addition, the CDs could be utilized as a modification-free biosensor reagent capable of detecting Hg²⁺ in complex environments. More significantly, environmental friendly "skillful pens" were fabricated that provided an effective platform for portable qualitative-detection of Hg²⁺.