Heteroatom-doped carbon dots: synthesis, characterization, properties, photoluminescence mechanism and biological applications

Synthesis of the Cu-Doped Dual-Emission Fluorescent Carbon Dots and Its Analytical Application

New Cu-doped dual-emission carbon dots (D-CDs) were synthesized rapidly and simply via a one-pot solvothermal method, and its special photoluminescence mechanism was studied. D-CDs have two fluorescence (FL) emission peaks under one-wavelength excitation and can be used as dual-signal sensor which is usually designed with two or more substances. The prepared CDs show excellent water solubility, photostability, salt tolerance, oxidation resistance, and special optical properties. The raw material ratio, solvent, pH, time, and synthesis temperature were optimized. The characterizations of CDs including transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma spectroscopic analysis, X-ray diffraction assignation of phases, thermogravimetric analysis and differential scanning calorimetry, Fourier transform infrared (FTIR) spectroscopy, FL spectrum, and ultraviolet-visible spectrum (UV-vis) were conducted. The investigation on mechanism indicates that the unique dual-emissive property is mainly caused by the energy-level gaps generated by the surface defects of CDs. The prepared D-CDs have good potential in dual-signal analysis and visualization sensing. To demonstrate the practical application, ferric ions, vitamin A acetate, and pH have been determined successfully.

Hydrothermal synthesis of N,S co-doped carbon nanodots for highly selective detection of living cancer cells

This study presents a facile synthesis method for the preparation of positively charged N,S co-doped carbon nanodots with excellent optical properties, and it develops a selective method for fluorescent detection of living cancer cells. The specific recognition is due to the application of an aptamer sequence, which shows high affinity and specificity to target cells. The aptamer is firstly labeled with BHQ and wraps around the carbon nanodots, then it finally quenches the fluorescence emission of the carbon nanodots. For the sensitive and selective analysis of target cells, the cells are simply mixed with the carbon nanodot-aptamer nanoconjugates, which are then centrifuged at a low speed. The recognition reaction between aptamer and target cells releases the quencher from the surface of the carbon nanodots and the centrifugation process enables the recovery of fluorescence intensity of the suspension, which reflects the level of initial cancer cells. The developed method is simple, highly selective and cost-effective, thus, it may be further exploited in clinical applications in the future.

Nitrogen and sulfur co-doped carbon nanodots toward bovine hemoglobin: A fluorescence quenching mechanism investigation

A deep understanding of the molecular interactions of carbon nanodots with biomacromolecules is essential for wider applications of carbon nanodots both in vitro and in vivo. Herein, nitrogen and sulfur co-doped carbon dots (N,S-CDs) with a quantum yield of 16% were synthesized by a 1-step hydrothermal method. The N,S-CDs exhibited a good dispersion, with a graphite-like structure, along with the fluorescence lifetime of approximately 7.50 ns. Findings showed that the fluorescence of the N,S-CDs was effectively quenched by bovine hemoglobin as a result of the static fluorescence quenching. The mentioned quenching mechanism was investigated by the Stern-Volmer equation, temperature-dependent quenching, and fluorescence lifetime measurements. The binding constants, number of binding sites, and the binding average distance between the energy donor N,S-CDs and acceptor bovine hemoglobin were calculated as well. These findings will provide for valuable insights on the future bioapplications of N,S-CDs.

Heteroatom-Doped Carbon Dots (CDs) as a Class of Metal-Free Photocatalysts for PET-RAFT Polymerization under Visible Light and Sunlight

A key challenge of photoregulated living radical polymerization is developing efficient and robust photocatalysts. Now carbon dots (CDs) have been exploited for the first time as metal-free photocatalysts for visible-light-regulated reversible addition-fragmentation chain-transfer (RAFT) polymerization. Screening of diverse heteroatom-doped CDs suggested that the P- and S-doped CDs were effective photocatalysts for RAFT polymerization under mild visible light following a photoinduced electron transfer (PET) involved oxidative quenching mechanism. PET-RAFT polymerization of various monomers with temporal control, narrow dispersity (Đ≈1.04), and chain-end fidelity was achieved. Besides, it was demonstrated that the CD-catalyzed PET-RAFT polymerization was effectively performed under natural solar irradiation.

Carbon dots doped with nitrogen and boron as ultrasensitive fluorescent probes for determination of α-glucosidase activity and its inhibitors in water samples and living cells

An ultrasensitive fluorometric assay is described for the determination of the activity of the enzyme α-glucosidase in waters and living cells. Carbon dots doped with nitrogen and boron (N,B-CDs) were prepared that have excitation/emission peaks at 400/510 nm and a fluorescence quantum yield of 47%. 4-Nitrophenylglucoside is added and then hydrolyzed by α-glucosidase to form yellow 4-nitrophenol which screens off fluorescence due to an inner filter effect. The method was applied to the determination of α-glucosidase activity and has a 3 mU mL^-1 detection limit. It was subsequently applied to the determination of the α-glucosidase inhibitor acarbose which can be determined in a concentration as low as 10 nM (at three times the standard deviation versus slope). The method was also applied to the determination of α-glucosidase activity and acarbose in living HeLa cells and MCF-7 cells. The method is simple, sensitive, and excellently selective. Graphical abstract N,B-CDs as ultrasensitive fluorescence probe for α-glucosidase activity and its inhibitor in waters and living cells based on IFE.

Red emission nitrogen, boron, sulfur co-doped carbon dots for "on-off-on" fluorescent mode detection of Ag+ ions and l-cysteine in complex biological fluids and living cells

Herein, a simple and efficient fluorescent assay for Ag⁺ ions and l-cysteine (L-Cys) in complex biological fluids and living cells was first developed based on the fluorescent "on-off-on" mode of red emission nitrogen, boron, sulfur co-doped carbon dots (NBS-CDs). Red emission NBS-CDs were prepared via one-step hydrothermal synthesis by using 3-aminobenzeneboronic acid and 2,5-diaminobenzenesulfonic acid as precursors. Such NBS-CDs exhibited excellent optical properties and relatively high absolute fluorescent quantum yield compared with some reported NBS-CDs. Due to the strong quenching ability of Ag⁺ ions on the fluorescence of NBS-CDs, red emission NBS-CDs were used for the determination of Ag⁺ ions with high sensitivity and excellent selectivity. The fluorescence of NBS-CDs was recovered after the interaction between Ag⁺ ions and L-Cys, which realized the specific determination of L-Cys in human urine samples and human plasma samples. The established NBS-CDs-based fluorescent "on-off-on" sensor offered a relatively low detection limits of 0.35 μM for Ag⁺ ions and 0.045 μM for L-Cys based on three times signal-to-noise criteria. Notably, this strategy was applied for the visual detections of Ag⁺ ions and L-Cys in living human cancer cells (HeLa cells and MCF-7 cells). This method is simple, high sensitive, and excellent selectivity, which provided a new insight on the potential applications of NBS-CDs to develop the biosensor in clinical diagnosis and other biologically related areas.

Sonochemical synthesis of a multi-responsive regenerable water-stable zinc(II) fluorescent probe for highly selective, sensitive and real-time sensing of benzaldehyde, ferric ion and PH

In this work, a novel water-stable coordination polymer with {4⁴} network topology {[Zn(L)₂(NO₃)₂]}_n (1) (L = 4,4'-Bis(triazol-1-ylmethyl)biphenyl) has been synthesized through the hydrothermal and sonochemical approaches. 1 has been characterized by single crystal X-ray diffraction, powder X-ray diffraction (PXRD), Fourier Transform Infrared Spectroscopy, UV-vis absorption spectrum and scanning electron microscopy (SEM). PXRD patterns of the as-synthesized samples 1 have confirmed the purity of the bulky samples. In the sonochemical preparation approaches, different ultrasound irradiation power and ultrasound time were also used in order to investigate the impact factor for morphology and size of nano-structured 1. Photo-luminescence studies have revealed that 1 can efficiently distinguish Fe³⁺ from Fe²⁺ and other metal ions. On the other hand, 1 also can exhibit a highly sensitive, excellently selective and real-time detection of benzaldehyde and pH through photo-luminescence quenching process. As for 1, density functional theory (DFT) and time-dependent DFT (TDDFT) theory has been applied to calculate these spectroscopic data, the result agree with the experimental results for detection of benzaldehyde. Photo-luminescent recyclability results indicated 1 can be reused at least five times in the detection process. To the best of our knowledge, this is the first example of a multi-responsive regenerable luminescent sensor for highly selective, sensitive and real-time sensing of Fe³⁺ over Fe²⁺, benzaldehyde and pH values.

Insights into the role of nanostructure in the sensing properties of carbon nanodots for improved sensitivity to reactive oxygen species in living cells

The surface states of carbon nanodots (CDs) were engineered by controlling the chemical structure on the surface of the CDs, which play an important role in the chemiluminescence sensing properties of CDs towards peroxynitrite. Their application in monitoring exogenous and endogenous release of peroxynitrite in living cells is demonstrated.

Nitrogen and phosphorus dual-doped carbon dots as a label-free sensor for Curcumin determination in real sample and cellular imaging

A nitrogen and phosphorus dual-doped carbon dots (NP-Cdots) was fastly synthesized with glucose as the carbon source, 1,2-ethylenediamine as N-dopant and concentrated phosphoric acid as P-dopant. The as-synthesized NP-Cdots was utilized as a label-free sensor for determination of Curcumin (Cur). The proposed NP-Cdots-based fluorescence sensor was applied for sensitive detection of Cur in aqueous solution, achieving a linear range of 0.5-20 µmol/L and a detection limit of 58 nmol/L (21.37 ng/mL). The common amino acids and other drugs do not interfere with the detection of Cur, providing good selectivity. The constructed sensor was successfully applied to the determination of Cur in drinking water and the food samples with satisfactory results and the RSDs and recoveries were 0.08-5.39% and 95.2-105.2%, respectively. More importantly, the as-prepared NP-Cdots was used as effective fluorescent agent for cellular imaging without noticeable cytotoxicity. The proposed sensor is simple and practical, illustrating that the potential application of NP-Cdots for biosensing, food monitoring and cellular labeling and imaging.

Applications and perspectives of nanomaterials in novel vaccine development

Vaccines show great potential for both prophylactic and therapeutic use in infections, cancer, and other diseases. With the rapid development of bio-technologies and materials sciences, nanomaterials are playing essential roles in novel vaccine formulations and can boost antigen effectiveness by operating as delivery systems to enhance antigen processing and/or as immune-potentiating adjuvants to induce or potentiate immune responses. The effect of nanoparticles in vaccinology showed enhanced antigen stability and immunogenicity as well as targeted delivery and slow release. However, obstacles remain due to the lack of fundamental knowledge on the detailed molecular working mechanism and in vivo bio-effects of nanoparticles. This review provides a broad overview of the current improvements in nanoparticles in vaccinology. Modern nanoparticle vaccines are classified by the nanoparticles' action based on either delivery system or immune potentiator approaches. The mechanisms of interaction of nanoparticles with the antigens and the immune system are discussed. Nanoparticle vaccines approved for use are also listed. A fundamental understanding of the in vivo bio-distribution and the fate of nanoparticles will accelerate the rational design of new nanoparticles comprising vaccines in the future.

A green synthesis of highly luminescent carbon dots from itaconic acid and their application as an efficient sensor for Fe3+ ions in aqueous medium

The green synthesis of carbon dots (CDs) is one of the hot research areas in the present-day context.

Photoluminescence mechanism and applications of Zn-doped carbon dots

Heteroatom-doped carbon dots (CDs) with excellent optical characteristics and negligible toxicity have emerged in many applications including bioimaging, biosensing, photocatalysis, and photothermal therapy. The metal-doping of CDs using various heteroatoms results in an enhancement of the photophysics but also imparts them with multifunctionality. However, unlike nonmetal doping, typical metal doping results in low fluorescence quantum yields (QYs), and an unclear photoluminescence mechanism. In this contribution, we detail results concerning zinc doped CDs (Zn-CDs) with QYs of up to 35%. The zinc ion charges serve as a surface passivating agent and prevent the aggregation of graphene π–π stacking, leading to an increase in the QY of the Zn-CDs. Structural and chemical investigations using spectroscopic and first principle simulations further revealed the effects of zinc doping on the CDs. The robust Zn-CDs were used for the ultra-trace detection of Hg²⁺ with a detection limit of 0.1 μM, and a quench mechanism was proposed. The unique optical properties of the Zn-CDs have promise for use in applications such as in vivo sensing and future phototherapy applications.

Zinc ions, acting as a surface passivating agent, prevented the aggregation of graphene π–π stacking and increased the quantum yield of Zn-carbon dots.

DNA-templated copper nanoparticles for voltammetric analysis of endonuclease activity

A voltammetric sensor for the detection of endonuclease activity is constructed based on DNA-templated copper nanoparticles.

Novel polymerization of aniline and pyrrole by carbon dots

This work reports on the synthesis of polymers (polyaniline, polypyrrole, and co-polymerization of polyaniline and polypyrrole) from monomers using carbon dots and UV light as initiators.

Multicolor carbon nanodots from food waste and their heavy metal ion detection application

Multicolor carbon dots (C-dots) have excellent performance characteristics, high photoluminescence efficiency, ease of fabrication and low toxicity. C-dots have been used in a wide variety of fields including bioimaging, biomedicine, photocatalysis and environmental monitoring. The mass production of multicolor CDs using low-cost, facile methods is an important issue for future industrial applications. In this article, we reported a simple and highly effective way to prepare the multicolor C-dots and use them to detect heavy metal iron ions. Hydrochar acquired from food waste processed with hydrothermal carbonization (HTC) was used as the carbonaceous material for this process. Four colors of C-dots were obtained and included blue, green, yellow and red. These multicolor C-dots could be used as fluorescence probes with unique selectivity to detect the Fe³⁺ ion. The luminescence response ranged from 1 to 50 μM with a correlation coefficient of 0.9968. This discovery not only shows the high value-added products which can be obtained from food waste but can also lead to new developments in carbonaceous materials which can be used as “green resources”.

Multicolor carbon dots produced from green carbonaceous materials by disposing of food waste through the HTC process could be used as fluorescent probes to detect iron ions.

Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots

Carbon dots (CDs) are a stable and highly biocompatible fluorescent material offering great application potential in cell labeling, optical imaging, LED diodes, and optoelectronic technologies. Because their emission wavelengths provide the best tissue penetration, red-emitting CDs are of particular interest for applications in biomedical technologies. Current synthetic strategies enabling red-shifted emission include increasing the CD particle size (sp² domain) by a proper synthetic strategy and tuning the surface chemistry of CDs with suitable functional groups (e.g., carboxyl). Here we present an elegant route for preparing full-color CDs with well-controllable fluorescence at blue, green, yellow, or red wavelengths. The two-step procedure involves the synthesis of a full-color-emitting mixture of CDs from citric acid and urea in formamide followed by separation of the individual fluorescent fractions by column chromatography based on differences in CD charge. Red-emitting CDs, which had the most negative charge, were separated as the last fraction. The trend in the separation, surface charge, and red-shift of photoluminescence was caused by increasing amount of graphitic nitrogen in the CD structure, as was clearly proved by XPS, FT-IR, Raman spectroscopy, and DFT calculations. Importantly, graphitic nitrogen generates midgap states within the HOMO-LUMO gap of the undoped systems, resulting in significantly red-shifted light absorption that in turn gives rise to fluorescence at the low-energy end of the visible spectrum. The presented findings identify graphitic nitrogen as another crucial factor that can red-shift the CD photoluminescence.

Pulse laser-induced fragmentation of carbon quantum dots: a structural analysis

Carbon quantum dots (CQDs) have attracted enormous interest in recent years owing to their low cytotoxicity, excellent biocompatibility and strong fluorescence. They have been successfully employed in sensor, bio-imaging, and drug carrier applications. A complete understanding of their core-surface structure is essential for tuning their physical and chemical properties for various applications. Conventional characterizations of CQDs are conducted with electron microscopy or spectroscopy, such as transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. However, these techniques cannot fully resolve the core-surface structure of CQDs. In this study, we attempt to analyze the structures of CQDs by laser desorption/ionization mass spectrometry (LDI-MS) using three model CQDs synthesized from citric acid (CA-CQDs), diammonium citrate (AC-CQDs) and spermidine trihydrochloride (Spd-CQDs). Both CA-CQDs and AC-CQDs produced anionic carbon cluster ions ([C_n]^-, n = 4-9) during the laser desorption/ionization process. Additionally, AC-CQDs produced fragments containing C, N, and O that appeared at m/z values of 41.999, 91.015, and 107.008, which were identified by ¹⁵N isotopes as [CNO]^-, [CH₃N₂O₃]^-, and [CH₃N₂O₄]^-, respectively. By contrast, subjecting Spd-CQDs to the same analysis did not yield carbon cluster ions ([C_n]^-); instead, strong chlorine-associated ions with a unique isotopic pattern were observed, strongly implying that Spd-CQDs contain chlorine. The lack of carbon cluster ion formation in nitrogen- and chlorine-doped Spd-CQDs indicates that nitrogen and chlorine are abundantly and homogenously doped in the CQDs. We also found a shot-dependent fragmentation behavior for AC-CQDs that produces nitrogen- and oxygen-containing ions and carbon cluster ions ([C_n]^-) during initial fragmentation of the surface, with a gradual destruction of the nanocrystalline carbon core after additional shots. These results suggest that LDI-MS can be used as a tool for analyzing the core-surface structure of CQDs, particularly when it contains a heteroatom doped carbon core with various surface functional groups containing nitrogen, oxygen and halogens.

Effect of nitrogen atom positioning on the trade-off between emissive and photocatalytic properties of carbon dots

Carbon dots (CDs) are a versatile nanomaterial with attractive photoluminescent and photocatalytic properties. Here we show that these two functionalities can be easily tuned through a simple synthetic means, using a microwave irradiation, with citric acid and varying concentrations of nitrogen-containing branched polyethyleneimine (BPEI) as precursors. The amount of BPEI determines the degree of nitrogen incorporation and the different inclusion modes within the CDs. At intermediate levels of BPEI, domains grow containing mainly graphitic nitrogen, producing a high photoluminescence yield. For very high (and very low) BPEI content, the nitrogen atoms are located primarily at the edge sites of the aromatic domains. Accordingly, they attract photogenerated electrons, enabling efficient charge separation and enhanced photocatalytic hydrogen generation from water. The ensuing ability to switch between emissive and photocatalytic behavior of CDs is expected to bring substantial improvements on their efficiency for on-demand light emission or energy conversion applications.

Highly Efficient Red-Emitting Carbon Dots with Gram-Scale Yield for Bioimaging

Carbon dots (CDs) are a new class of photoluminescent (PL), biocompatible, environment-friendly, and low-cost carbon nanomaterials. Synthesis of highly efficient red-emitting carbon dots (R-CDs) on a gram scale is a great challenge at present, which heavily restricts the wide applications of CDs in the bioimaging field. Herein, R-CDs with a high quantum yield (QY) of 53% are produced on a gram scale by heating a formamide solution of citric acid and ethylenediamine. The as-prepared R-CDs have an average size of 4.1 nm and a nitrogen content of about 30%, with an excitation-independent emission at 627 nm. After detailed characterizations, such strong red fluorescence is ascribed to the contribution from the nitrogen- and oxygen-related surface states and the nitrogen-derived structures in the R-CD cores. Our R-CDs show good photostability and low cytotoxicity, and thus they are excellent red fluorescence probes for bioimaging both in vitro and in vivo.

Carbon dots for fluorescent detection of α-glucosidase activity using enzyme activated inner filter effect and its application to anti-diabetic drug discovery

Recently, α-glucosidase inhibitor has been widely used in clinic for diabetic therapy. In the present study, a facile and sensitive fluorescent assay based on enzyme activated inner filter effect (IFE) on nitrogen-doped carbon dots (CDs) was first developed for the detection of α-glucosidase. The N-doped CDs with green emission were prepared by a one-step hydrothermal synthesis and gave the fluorescence quantum yield of 30%, which were used as the signal output. Through α-glucosidase catalysis, 4-nitrophenol was released from 4-nitrophenyl-α-d-glucopyranoside (NGP). Interestingly, the absorption of 4-nitrophenol and the excitation of CDs were completely overlapping. Due to its great molar absorptivity, 4-nitrophenol was capable of acting as a powerful absorber to affect the fluorescent signal of CDs (i.e. IFE). By converting the absorption signals into fluorescence signals, the facile fluorescence assay strategy could be realized for α-glucosidase activity sensing, which effectively avoided the complex modification of the surface of CDs or construction of the nanoprobes. The established IFE-based sensing platform offered a low detection limit of 0.01 U/mL (S/N = 3). This proposed sensing approach has also been expanded to the inhibitor screening and showed excellent applicability. As a typical α-glucosidase inhibitor, acarbose was investigated with a low detection limit of 10^-8 M. This developed method enjoyed many merits including simplicity, lost cost, high sensitivity, good reproducibility and excellent selectivity, which also provided a new insight on the application of CDs to develop the facile and sensitive biosensor.

Hyaluronic acid-based carbon dots for efficient gene delivery and cell imaging

Two polymers were used with no additives to directly construct multifunctional carbon dots by a microwave-assisted method for simultaneous gene delivery and cell imaging.

Full-colour carbon dots: from energy-efficient synthesis to concentration-dependent photoluminescence properties

A facile and energy-efficient method is presented for the large-scale synthesis of fluorescent carbon dots whose fluorescence emission wavelengths shift gradually from 630 to 400 nm with reduction in their concentration.

Sustainable carbon-dots: recent advances in green carbon dots for sensing and bioimaging

This review article highlights recent progress in use of green precursors for synthesis of carbon-dots and their applications in fluorescence-based sensing and bioimaging.

Rapid synthesis of nitrogen doped carbon dots and their application as a label free sensor array for simultaneous discrimination of multiple proteins

A label free sensor array for discriminating various proteins and distinguishing serums from rectal cancer patients, Alzheimer's disease patients and healthy people was developed by utilizing novel nitrogen doped carbon dots.