Analytical and bioanalytical applications of carbon dots

Conjugated systems of porphyrin–carbon nanoallotropes: a review

This review summarizes the synthesis and applications of various porphyrin–carbon nanoallotrope conjugates.

Hexamethylenetetramine: an effective and universal nitrogen-doping reagent to enhance the photoluminescence of carbon nanodots

Guidelines of how to select nitrogen-doping reagents for enhancing photoluminescence of carbon nanodots are proposed by taking hexamethylenetetramine as a typical example.

Carbon nanodot aqueous binding phase-based diffusive gradients in thin films device for measurement of dissolved copper and lead species in the aquatic environment

A reliable method for the determination of dissolved Cu²⁺and Pb²⁺species in waterviaa diffusive gradient in thin films (DGT) device using water-soluble carbon nanodots (CD) as the binding agent was developed (CD-DGT).

Bamboo leaf-based carbon dots for efficient tumor imaging and therapy

In this study, carbon dots synthesized from bamboo leaf cellulose were used simultaneously as a staining agent and for doxorubicin delivery to target cancer cells.

Functional Carbon Quantum Dots: A Versatile Platform for Chemosensing and Biosensing

Carbon quantum dot has emerged as a new promising fluorescent nanomaterial due to its excellent optical properties, outstanding biocompatibility and accessible fabrication methods, and has shown huge application perspective in a variety of areas, especially in chemosensing and biosensing applications. In this personal account, we give a brief overview of carbon quantum dots from its origin and preparation methods, present some advance on fluorescence origin of carbon quantum dots, and focus on development of chemosensors and biosensors based on functional carbon quantum dots. Comprehensive advances on functional carbon quantum dots as a versatile platform for sensing from our group are included and summarized as well as some typical examples from the other groups. The biosensing applications of functional carbon quantum dots are highlighted from selective assays of enzyme activity to fluorescent identification of cancer cells and bacteria.

Multicolour nitrogen-doped carbon dots: tunable photoluminescence and sandwich fluorescent glass-based light-emitting diodes

The first use of the combination of ammonium citrate (AC) and ethylenediamine tetraacetic acid (EDTA) as coordinating precursors for the synthesis of highly fluorescent (quantum yield = 67%) multicolour nitrogen-doped carbon dots (CDs) is reported. Under UV light, these CDs emitted outstanding luminescence in colours from dark blue to red. Interestingly, a single component white-light CD point with high fluorescence efficiency was obtained by surface control. Alterations of the photoluminescence (PL) emission of these full-colour CDs were tentatively proposed to benefit from surface functional groups, such as C[double bond, length as m-dash]O and C[double bond, length as m-dash]N. An energy-level model was proposed to explain the continuously adjustable full-colour emission. The white light may be attributed to the overlap of diverse light emission induced by electron transitions between the energy levels. Subsequently, to avoid aggregation-induced solid-state fluorescence quenching, multicolour CD-based sandwich glasses with various colour emission was fabricated, which is anticipated to be compatible with the all-optical light-emitting diodes (LEDs). The facile preparation and outstanding optical features are believed to provide an alternative synthesis route and inspire more research into applications and CD-based materials of multicolour CDs.

A Ratiometric Fluorescence Universal Platform Based on N, Cu Codoped Carbon Dots to Detect Metabolites Participating in H2O2-Generation Reactions

In this work, a new kind of N, Cu codoped carbon dots (N/Cu-CDs) was prepared via a facile one-pot hydrothermal method by using citric acid monohydrate, copper acetate monohydrate and diethylenetriamine. The prepared N/Cu-CDs with a high quantum yield (50.1%) showed excitation-independent emission at 460 nm. The structure and fluorescence properties of N/Cu-CDs were characterized by high-resolution transmission electron microscopy, fluorescence spectrofluorometer, FT-IR spectrometer, UV-visible spectrophotometer and X-ray photoelectron spectroscopy. N/Cu-CDs were applied to establishing a ratiometric fluorescence probe toward H₂O₂ based on the inner filter effect (IFE) between N/Cu-CDs and DAP (2,3-diaminophenazine, the oxidative product of o-phenylenediamine (OPD)), and provided a ratiometric fluorescence universal platform for detection of the metabolites participating in H₂O₂-generation reactions (cholesterol and xanthine). The proposed method was demonstrated to be ultrasensitive and highly selective for cholesterol and xanthine assay with detection limits of 0.03 and 0.10 μM, respectively. The fluorescence probe built was applied to the determination of cholesterol and xanthine in human serum with satisfactory results.

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.

Morpholine Derivative-Functionalized Carbon Dots-Based Fluorescent Probe for Highly Selective Lysosomal Imaging in Living Cells

The development of a suitable fluorescent probe for the specific labeling and imaging of lysosomes through the direct visual fluorescent signal is extremely important for understanding the dysfunction of lysosomes, which might induce various pathologies, including neurodegenerative diseases, cancer, and Alzheimer's disease. Herein, a new carbon dot-based fluorescent probe (CDs-PEI-ML) was designed and synthesized for highly selective imaging of lysosomes in live cells. In this probe, PEI (polyethylenimine) is introduced to improve water solubility and provide abundant amine groups for the as-prepared CDs-PEI, and the morpholine group (ML) serves as a targeting unit for lysosomes. More importantly, passivation with PEI could dramatically increase the fluorescence quantum yield of CDs-PEI-ML as well as their stability in fluorescence emission under different excitation wavelength. Consequently, experimental data demonstrated that the target probe CDs-PEI-ML has low cytotoxicity and excellent photostability. Additionally, further live cell imaging experiment indicated that CDs-PEI-ML is a highly selective fluorescent probe for lysosomes. We speculate the mechanism for selective staining of lysosomes that CDs-PEI-ML was initially taken up by lysosomes through the endocytic pathway and then accumulated in acidic lysosomes. It is notable that there was less diffusion of CDs-PEI-ML into cytoplasm, which could be ascribed to the presence of lysosome target group morpholine on surface of CDs-PEI-ML. The blue emission wavelength combined with the high photo stability and ability of long-lasting cell imaging makes CDs-PEI-ML become an alternative fluorescent probe for multicolor labeling and long-term tracking of lysosomes in live cells and the potential application in super-resolution imaging. To best of our knowledge, there are still limited carbon dots-based fluorescent probes that have been studied for specific lysosomal imaging in live cells. The concept of surface functionality of carbon dots will also pave a new avenue for developing carbon dots-based fluorescent probes for subcellular labeling.

Green synthesis of highly stable carbon nanodots and their photocatalytic performance

The present study reports a novel, facile, biosynthesis route for the synthesis of carbon nanodots (CDs) with an approximate quantum yield of 38.5%, using Musk melon extract as a naturally derived-precursor material. The synthesis of CDs was established by using ultraviolet-visible (UV-vis) spectroscopy, Dynamic light scattering, photoluminescence spectroscopy, X-ray diffraction, transmission electron microscopy and Fourier transform infrared (FTIR) spectroscopy. The as-prepared CDs possess an eminent fluorescence under UV-light (λ_ex = 365 nm). The size range of CDs was found to be in the range of 5-10 nm. The authors further explored the use of such biosynthesised CDs as a photocatalyst material for removal of industrial dye. Degradation of methylene blue dye was performed in a photocatalytic reactor and monitored using UV-vis spectroscopy. The CDs show excellent dye degradation capability of 37.08% in 60 min and reaction rate of 0.0032 min^-1. This study shows that synthesised CDs are highly stable in nature, and possess potential application in wastewater treatment.

Determination of dihydralazine based on chemiluminescence resonance energy transfer of hollow carbon nanodots

The famous weak chemiluminescence (CL) system of potassium permanganate and sodium bisulfite (KMnO₄-HSO₃^-) was enhanced by the hollow fluorescent carbon nanodots (HCNs). The investigation of mechanism revealed that the enhanced CL was induced by the excited-state HCNs (HCNs^⁎), which could be produced from the electron-transfer annihilation of positively charged HCNs (HCNs⁺) and negatively charged HCNs (HCNs^-) as well as by CL resonance energy transfer (CRET) from excited SO₂ (SO₂^⁎)/¹O₂ to HCNs. The dihydralazine sulfate (DHZS) had a diminishing effect on the CL of HCNs-KMnO₄-HSO₃^- system due to the competitive consumption of O₂^-. Under the optimal conditions, the reduced CL signal with the concentration of DHZS was linear in the range of 1.0×10^-7-7.0×10^-5mol/L with a detection limit of 3.0×10^-8mol/L. The relative standard deviation for seven repeated determination of 5.0×10^-6mol/L DHZS was 2.1%. The established method was applied to the determination of DHZS in pharmaceutical preparations, human urine and plasma samples with good precision and accuracy.

Harmful impact on presynaptic glutamate and GABA transport by carbon dots synthesized from sulfur-containing carbohydrate precursor

Carbon nanoparticles that may be potent air pollutants with adverse effects on human health often contain heteroatoms including sulfur. In order to study in detail their effects on different physiological and biochemical processes, artificially produced carbon dots (CDs) with well-controlled composition that allows fluorescence detection may be of great use. Having been prepared from different types of organic precursors, CDs expose different atoms at their surface suggesting a broad variation of functional groups. Recently, we demonstrated neurotoxic properties of CDs synthesized from the amino acid β-alanine, and it is of importance to analyze whether CDs obtained from different precursors and particularly those exposing sulfur atoms induce similar neurotoxic effects. This study focused on synthesis of CDs from the sulfur-containing precursor thiourea-CDs (TU-CDs) with a size less than 10 nm, their characterization, and neuroactivity assessment. Neuroactive properties of TU-CDs were analyzed based on their effects on the key characteristics of glutamatergic and γ-aminobutyric acid (GABA) neurotransmission in isolated rat brain nerve terminals. It was observed that TU-CDs (0.5-1.0 mg/ml) attenuated the initial velocity of Na⁺-dependent transporter-mediated uptake and accumulation of L-[¹⁴C]glutamate and [³H]GABA by nerve terminals in a dose-dependent manner and increased the ambient level of the neurotransmitters. Starting from the concentration of 0.2 mg/ml, TU-CDs evoked a gradual dose-dependent depolarization of the plasma membrane of nerve terminals measured with the cationic potentiometric dye rhodamine 6G. Within the concentration range of 0.1-0.5 mg/ml, TU-CDs caused an "unphysiological" step-like increase in fluorescence intensity of the рН-sensitive fluorescent dye acridine orange accumulated by synaptic vesicles. Therefore, despite different surface properties and fluorescent features of CDs prepared from different starting materials (thiourea and β-alanine), their principal neurotoxic effects are analogous but displayed at a different level of efficiency. Sulfur-containing TU-CDs exhibit lower effects (by ~30%) on glutamate and GABA transport in the nerve terminals in comparison with sulfur-free β-alanine CDs. Our results suggest considering that an uncontrolled presence of carbon-containing particulate matter in the human environment may pose a toxicity risk for the central nervous system.

Carbon Nanodots-Based Fluorescent Turn-On Sensor Array for Biothiols

Biothiols play important roles in biological processes. In this study, a novel sensor array-based method was proposed to detect and differentiate biothiols. The sensor array was constructed using three kinds of Ag⁺-sensitive carbon nanodots (CDs). The CDs were synthesized with amino acids and urea as carbon sources via a simple microwave method. Results revealed that Ag⁺ can bind with CDs and depress the fluorescence of CDs, while the subsequently joined biothiols can take Ag⁺ away from CDs and recover the fluorescence of CDs. Due to the different binding ability between Ag⁺ and various CDs, as well as Ag⁺ and various biothiols, the CD-Ag⁺ array exhibits a unique pattern of fluorescence variations when interacting with six biothiol samples (cysteamine, dithiothreitol, mercaptosuccinic acid, glutathione, mercaptoacetic acid, and mercaptoethanol). Principal component analysis (PCA) was applied to analyze the pattern and generate a clustering map for a clearer identification of these biothiols. PCA can also be employed to simplify the established three-sensor array into a two-sensor array. Both the three- and two-sensor arrays can identify these biothiols in a wide biothiol concentration range (>10 μM).

Photoluminescent Hybrids of Cellulose Nanocrystals and Carbon Quantum Dots as Cytocompatible Probes for in Vitro Bioimaging

We present an approach to construct biocompatible and photoluminescent hybrid materials comprised of carbon quantum dots (CQDs) and TEMPO-oxidized cellulose nanocrystals (TO-CNCs). First, the amino-functionalized carbon quantum dots (NH₂-CQDs) were synthesized using a simple microwave method, and the TO-CNCs were prepared by hydrochloric acid (HCl) hydrolysis followed by TEMPO-mediated oxidation. The conjugation of NH₂-CQDs and TO-CNCs was conducted via carbodiimide-assisted coupling chemistry. The synthesized TO-CNC@CQD hybrid nanomaterials were characterized using X-ray photoelectron spectroscopy, cryo-transmittance electron microscopy, confocal microscopy, and fluorescence spectroscopy. Finally, the interactions of TO-CNC@CQD hybrids with HeLa and RAW 264.7 macrophage cells were investigated in vitro. Cell viability tests suggest the surface conjugation with NH₂-CQDs not only improved the cytocompatibility of TO-CNCs, but also enhanced their cellular association and internalization on both HeLa and RAW 264.7 cells after 4 and 24 h incubation.

Red Emissive Sulfur, Nitrogen Codoped Carbon Dots and Their Application in Ion Detection and Theraonostics

It is highly desirable and a great challenge for red light emission of carbon dots under long wavelength excitation. Here, we developed a facile route to synthesize carbon dots with red emission due to the doping effect of S and N elements, borrowing from the concept of the semiconductor. The maximum emission locates at 594 nm under 560 nm excitation. The absolute photoluminescence (PL) quantum yield (QY) is as high as 29% and 22% in ethanol and water, respectively. XPS and FTIR spectra illustrated that there exist -SCN and -COOH groups on the surface of the carbon dots. They endow the carbon dots with high sensitivity for ion detection of Fe³⁺. The quenched PL emission of Fe³⁺-S,N-CDs can be recovered by adding ascorbic acid to release the -COOH and -SCN group due to Fe²⁺ formation in the presence of ascorbic acid. High PL QY of red emission is beneficial to application in bioimaging. Doxorubicin was loaded onto carbon dots through π-π stacking to form a theranostic agent. When the CD-Dox was injected into the tumor site, a strong PL emission was observed. The PL intensity indicates the concentration of the theranostic agent. After 7 times injection, both the tumor size and weight clearly decrease. The results demonstrate that the S,N-CDs are a potentially excellent bioimaging component in the theranostic field.

Nucleobase chemosensor based on carbon nanodots

A facile and sensitive fluorescence protocol for nucleobase detection was developed based on carbon nanodot (CD) chemosensors. The novel fluorescent CDs were prepared using four kinds of nucleobases (including adenine, guanine, thymine and cytosine) as separate carbon sources via simple hydrothermal strategy. The quantum yield of adenine CDs (A-CDs), guanine CDs (G-CDs), thymine CDs (T-CDs) and cytosine CDs (C-CDs) was checked as 15.1%, 28.3%, 10.6% and 11.7%, respectively. Four CDs can recognize their complementary nucleobases based on the principle of complementary base pairing. Their fluorescence was linearly quenched with the increase of nucleobase concentrations under optimal conditions. Combining the calibration curve, quantitative assay of nucleobase in solution can be realized. For example, A-CDs could determine thymine in the concentration range of 2-20mM with a detection limit of ca. 0.053mM, and the linear equation is fitting as (I₀-I) / I = 0.01961 × C_T(mM) + 0.01756 (R² = 0.994). Thymine can induce the fluorescence lifetime of A-CDs decreasing from 5.58 to 3.34ns, indicating a dynamic quenching mechanism. The novel nucleobase sensors were also evaluated in specific solution environment. A-CDs showed a relatively minor relative standard deviation (< 4.0%) in fetal calf serum solution, indicating a high accuracy and credibility of the sensing system. In view of the excellent sensitivity, preferable biocompatibility as well as simple constructing method, the sensing platform derived from the nucleobase-based CDs present great potential in biological sensing applications.

Fabrication of fluorescent carbon dots-linked isophorone diisocyanate and β-cyclodextrin for detection of chromium ions

A water-soluble fluorescent carbon dots (FCDs) from cellulose was prepared using one-pot simple hydrothermal method. In this work, a novel fluorescent probe material, fluorescent carbon dots-linked isophorone diisocyanate and β-cyclodextrin (FCDs-IPDI-CD), was prepared with FCDs, isophorone diisocyanate (IPDI) and β-cyclodextrin (β-CD) as raw materials. The structure and morphology of FCDs-IPDI-CD were characterized using the Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). The as-prepared FCDs-IPDI-CD exhibits excellent emission property and high stability. The fluorescence of the FCDs-IPDI-CD could be quenched by Cr(VI) ions, and the results indicate that FCDs-IPDI-CD can be used as an effective fluorescent probe for the detection of Cr(VI) ions with good selectivity and sensitivity in an aqueous solution. The influences of environment factors (such as pH, reaction time) on relative fluorescence intensity were studied. According to the optimum conditions, a new sensitive method detecting Cr(VI) ions was established. The method has been successfully applied to detect Cr(VI) ions in water and soil samples with satisfactory results.

Experimental Investigations on Fluorescence Excitation and Depletion of Carbon Dots

Carbon dots (CDs) can be readily synthesized and utilized as attractive fluorescent probes for a variety of applications. In this study, we have synthesized CDs using a previously published method and characterized their photo-physical properties. The resultant CDs possess prominent photo-stability and short emission wavelength in the violet region. Our study reveals that CDs, with weak photo-bleaching, enable them to be employed to achieve high spatial resolution in stimulated emission depletion (STED) microscopy. The depletion efficiency can reach 60%. More importantly, the shorter excitation wavelength of CDs contributes to further improvement of resolution for STED microscopy. An excellent candidate for fluorophores, these CDs have potential to be used in super-resolution imaging for STED microscopy.

Mimicking Horseradish Peroxidase Functions Using Cu2+-Modified Carbon Nitride Nanoparticles or Cu2+-Modified Carbon Dots as Heterogeneous Catalysts

Cu²⁺-functionalized carbon nitride nanoparticles (Cu²⁺-g-C₃N₄ NPs), ∼200 nm, and Cu²⁺-carbon dots (Cu²⁺-C-dots), ∼8 nm, act as horseradish peroxidase-mimicking catalysts. The nanoparticles catalyze the generation of chemiluminescence in the presence of luminol/H₂O₂ and catalyze the oxidation of dopamine by H₂O₂ to form aminochrome. The Cu²⁺-g-C₃N₄-driven generation of chemiluminescence is used to develop a H₂O₂ sensor and is implemented to develop a glucose detection platform and a sensor for probing glucose oxidase. Also, the Cu²⁺-C-dots are functionalized with the β-cyclodextrin (β-CD) receptor units. The concentration of dopamine, at the Cu²⁺-C-dots' surface, by means of the β-CD receptor sites, leads to a 4-fold enhancement in the oxidation of dopamine by H₂O₂ to yield aminochrome compared to that of the unmodified C-dots.

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

A facile and versatile molten-salt method was developed to prepare hydrosoluble carbon dots (CDs) from various precursors, in high yields and on a large scale. Citric acid-based CDs (CA-CDs) were obtained in a maximum yield of 39.6% and exhibited a high fluorescence quantum yield of 20.8% without any passivation. The CA-CDs showed little cytotoxicity even at a concentration as high as 800 μg mL^-1. In addition, CA-CDs could be used as multicolour fluorescence imaging agents in vitro with blue, green, and red fluorescence emissions at excitation wavelengths of 405, 488, and 543 nm, respectively. Moreover, the CA-CDs could be chelated with gadolinium ions (Gd³⁺) to construct Gd-CA-CDs for dual-mode magnetic resonance and fluorescence imaging. The Gd-CA-CDs showed good water dispersibility, excellent biocompatibility, a strong fluorescence quantum yield of 13.1%, and a high magnetic resonance relaxivity of 22.45 mM^-1 s^-1. The molten-salt method was demonstrated to be applicable to other precursors, such as sodium lignosulphonate, sucrose, glucose, and p-phenylenediamine, and the maximum yield of the four as-prepared CDs was as high as 66.7%, which is much higher than the value reported in previous studies. This study proves that the molten-salt synthesis is a versatile method to obtain CDs in high yields, which will promote the application of CDs in the field of bioimaging.

Preparation of Yellow-Green-Emissive Carbon Dots and Their Application in Constructing a Fluorescent Turn-On Nanoprobe for Imaging of Selenol in Living Cells

Selenocysteine (Sec) carries out the majority of the functions of the various Se-containing species in vivo. Thus, it is of great importance to develop sensitive and selective assays to detect Sec. Herein, a carbon-dot-based fluorescent turn-on probe for highly selective detection of selenol in living cells is presented. The highly photoluminescent carbon dots that emit yellow-green fluorescence (Y-G-CDs; λ_max = 520 nm in water) were prepared by using m-aminophenol as carbon precursor through a facile solvothermal method. The surface of Y-G-CDs was then covalently functionalized with 2,4-dinitrobenzenesulfonyl chloride (DNS-Cl) to afford the 2,4-dinitrobenzene-functionalized CDs (CD-DNS) as a nanoprobe for selenol. CD-DNS is almost nonfluorescent. However, upon treating with Sec, the DNS moiety of CD-DNS can be readily cleaved by selenolate through a nucleophilic substitution process, resulting in the formation of highly fluorescent Y-G-CDs and hence leads to a dramatic increase in fluorescence intensity. The proposed nanoprobe exhibits high sensitivity and selectivity toward Sec over biothiols and other biological species. A preliminary study shows that CD-DNS can function as a useful tool for fluorescence imaging of exogenous and endogenous selenol in living cells.

Synthesis, properties and biomedical applications of carbon-based quantum dots: An updated review

Carbon-based quantum dots (CQDs) are a newly developed class of carbon nano-materials that have attracted much interest and attention as promising competitors to already available semiconductor quantum dots owing to their un-comparable and unique properties. In addition, controllability of CQDs unique physiochemical properties is as a result of their surface passivation and functionalization. This is an update article (between 2013 and 2016) on the recent progress, characteristics and synthesis methods of CQDs and different advantages in varieties of applications.

Living cell intracellular temperature imaging with biocompatible dye-conjugated carbon dots

Within living cells, the biological functions of subcellular organelles are highly dependent on the distribution of local temperature.

Fluorescent carbon dots with tunable emission by dopamine for sensing of intracellular pH, elementary arithmetic operations and a living cell imaging based INHIBIT logic gate

Luminescent carbon dots with tunable emission were prepared by the pyrolysis of citric acid and dopamine for sensing of intracellular pH, elementary arithmetic operations and a living cell imaging based INHIBIT logic gate.

Aconitic acid derived carbon dots as recyclable “on–off–on” fluorescent nanoprobes for sensitive detection of mercury(ii) ions, cysteine and cellular imaging

Aconitic acid is used as a new precursor for fabricating CDs and developing a sensitive “on–off–on” sensor for Hg²⁺, Cys and cellular imaging.

N-Doped carbon dots: green and efficient synthesis on a large-scale and their application in fluorescent pH sensing

A green and efficient anhydrous method was developed to synthesize nitrogen doped carbon dots (N-CDs) on a large-scale, and the N-CDs exhibited excellent reversible pH-sensitive property.

Functional carbon nanodots for multiscale imaging and therapy

As an emerging class of carbon nanomaterials, carbon dots (CDs) have garnered many researchers' interests in the past decade due to their excellent biocompatibility, replete surface functional groups, water dispersibility, and unique photoluminescence. These extraordinary properties have opened new avenues for their advanced application in cell labeling, bioimaging, drug delivery, sensors, and energy-related devices. In this paper, we critically review recent advances in the synthetic strategies and the application of CDs for biological purposes, specifically, imaging and therapy. Finally, a perspective has been given on the potential challenges facing the translation of these materials from the bench to the market. WIREs Nanomed Nanobiotechnol 2017, 9:e1436. doi: 10.1002/wnan.1436 For further resources related to this article, please visit the WIREs website.

Thermal treatment of hair for the synthesis of sustainable carbon quantum dots and the applications for sensing Hg2

A facile, simple and low-cost approach for synthesizing highly fluorescent carbon quantum dots (CQDs) from thermal treatment of sustainable hair has been developed. The resultant CQDs exhibited strong blue emission with a quantum yield of 10.75%, excellent photostability and high stability in high salt conditions. As the fluorescence of CQDs can be efficiently quenched by Hg2+, the CQDs can be constructed as a nanosensor for Hg2+ with good sensitivity and selectivity. And as low as 10 nM Hg2+ can be successfully detected.

Application of DNA aptamers as sensing layers for detection of carbofuran by electrogenerated chemiluminescence energy transfer

In this study, an electrogenerated chemiluminescence (ECL) sensing platform for carbofuran detection was constructed based on ECL energy transfer (ECRET) and carbon dot (C-dot)-tagged aptamers as the recognition element. Fullerene (C60)-loaded gold nanoparticles (C60-Au) were used as the energy donor, modified on a glassy carbon electrode. C-dot-tagged DNA aptamers were used as the receptor, and ECRET then occurred between C60-Au and C-dots. After accepting the energy, the C-dots acted as a signal indicator and showed decreased signal intensity in the presence of targets, which competitively bound to DNA aptamers and blocked energy transfer. Using this robust, straight-forward strategy, the sensor showed a linear ECL response to carbofuran at concentrations from 2.0 × 10^-11 mol L^-1 to 8.0 × 10^-9 mol L^-1. The detection limit of this assay was shown to be 8.8 × 10^-13 mol L^-1. Thus, the sensing approach described in this study could be adapted for use in the detection of various pesticide residue targets.

The origin of emissive states of carbon nanoparticles derived from ensemble-averaged and single-molecular studies

At present, there is no consensus understanding on the origin of photoluminescence of carbon nanoparticles, particularly the so-called carbon dots. Providing comparative analysis of spectroscopic studies in solution and on a single-molecular level, we demonstrate that these particles behave collectively as fixed single dipoles and probably are the quantum emitter entities. Their spectral and lifetime heterogeneity in solutions is explained by variation of the local chemical environment within and around luminescence centers. Hence, the carbon dots possess a unique hybrid combination of fluorescence properties peculiar to dye molecules, their conjugates and semiconductor nanocrystals. It is proposed that their optical properties are due to generation of H-aggregate-type excitonic states with their coherence spreading over the whole nanoparticles.

Dual-Shell Fluorescent Nanoparticles for Self-Monitoring of pH-Responsive Molecule-Releasing in a Visualized Way

The rational design and controlled synthesis of a smart device with flexibly tailored response ability is all along desirable for bioapplication but long remains a considerable challenge. Here, a pH-stimulated valve system with a visualized "on-off" mode is constructed through a dual-shell fluorescence resonance energy transfer (FRET) strategy. The dual shells refer to carbon dots and fluorescent molecules embedded polymethacrylic acid (F-PMAA) layers successively coating around a SiO2 core (ca. 120 nm), which play the roles as energy donor and acceptor, respectively. The total thickness of the dual-shell in the solid composite is ca. 10 nm. The priorities of this dual-shell FRET nanovalve stem from three facts: (1) the thin shell allows the formation of efficient FRET system without chemical bonding between energy donor and acceptor; (2) the maximum emission wavelength of CD layer is tunable in the range of 400-600 nm, thus providing a flexible energy donor for a wide variety of energy acceptors; (3) the outer F-PMAA shell with a pH-sensitive swelling-shrinking (on-off) behavior functions as a valve for regulating the FRET process. As such, a sensitive and stable pH ratiometric sensor with a working pH range of 3-6 has been built by simply encapsulating pH-responsive fluorescein isothiocyanate (FITC) into PMAA; a pH-dependent swelling-shrinking shuttle carrier with a finely controllable molecule-release behavior has been further fabricated using rhodamine B isothiocyanate (RBITC) as the energy donor and model guest molecule. Significantly, the controlled releasing process is visually self-monitorable.

Fluorescent nanoprobes for sensing and imaging of metal ions: recent advances and future perspectives

Recent advances in nanoscale science and technology have generated nanomaterials with unique optical properties. Over the past decade, numerous fluorescent nanoprobes have been developed for highly sensitive and selective sensing and imaging of metal ions, both in vitro and in vivo. In this review, we provide an overview of the recent development of the design and optical properties of the different classes of fluorescent nanoprobes based on noble metal nanomaterials, upconversion nanoparticles, semiconductor quantum dots, and carbon-based nanomaterials. We further detail their application in the detection and quantification of metal ions for environmental monitoring, food safety, medical diagnostics, as well as their use in biomedical imaging in living cells and animals.

Facile synthesis of water-soluble carbon nano-onions under alkaline conditions

Carbonization of tomatoes at 240 °C using 30% (w/v) NaOH as catalyst produced carbon onions (C-onions), while solely carbon dots (C-dots) were obtained at the same temperature in the absence of the catalyst. Other natural materials, such as carrots and tree leaves (acer saccharum), under the same temperature and alkaline conditions did not produce carbon onions. XRD, FTIR, HRTEM, UV-vis spectroscopy, and photoluminescence analyses were performed to characterize the as-synthesized carbon nanomaterials. Preliminary tests demonstrate a capability of the versatile materials for chemical sensing of metal ions. The high content of lycopene in tomatoes may explain the formation of C-onions in alkaline media and a possible formation mechanism for such structures was outlined.

Exploiting the biological windows: current perspectives on fluorescent bioprobes emitting above 1000 nm

With the goal of developing more accurate, efficient, non-invasive and fast diagnostic tools, the use of near-infrared (NIR) light in the range of the second and third biological windows (NIR-II: 1000-1350 nm, NIR-III: 1550-1870 nm) is growing remarkably as it provides the advantages of deeper penetration depth into biological tissues, better image contrast, reduced phototoxicity and photobleaching. Consequently, NIR-based bioimaging has become a quickly emerging field and manifold new NIR-emitting bioprobes have been reported. Classes of materials suggested as potential probes for NIR-to-NIR bioimaging (using NIR light for the excitation and emission) are quite diverse. These include rare-earth based nanoparticles, Group-IV nanostructures (single-walled carbon nanotubes, carbon nanoparticles and more recently Si- or Ge-based nanostructures) as well as Ag, In and Pb chalcogenide quantum dots. This review summarizes and discusses current trends, material merits, and latest developments in NIR-to-NIR bioimaging taking advantage of the region above 1000 nm (i.e. the second and third biological windows). Further consideration will be given to upcoming probe materials emitting in the NIR-I region (700-950 nm), thus do not possess emissions in these two windows, but have high expectations. Overall, the focus is placed on recent discussions concerning the optimal choice of excitation and emission wavelengths for deep-tissue high-resolution optical bioimaging and on fluorescent bioprobes that have successfully been implemented in in vitro and in vivo applications.