Highly N,P-doped carbon dots: Rational design, photoluminescence and cellular imaging

Oxidative Cleavage of α-O-4, β-O-4, and 4-O-5 Linkages in Lignin Model Compounds Over P, N Co-Doped Carbon Catalyst: A Metal-Free Approach

Developing efficient metal-free catalysts for lignin valorization is essential but challenging. In this study, a cost-effective strategy is employed to synthesize a P, N co-doped carbon catalyst through hydrothermal and carbonization processes. This catalyst effectively cleaved α-O-4, β-O-4, and 4-O-5 lignin linkages, as demonstrated with model compounds. Various catalysts were prepared at different carbonization temperatures and thoroughly characterized using techniques such as XRD, RAMAN, FTIR, XPS, NH3-TPD, and HRTEM. Attributed to higher acidity, the P5NC-500 catalyst exhibited the best catalytic activity, employing H2O2 as the oxidant in water. Additionally, this metal-free technique efficiently converted simulated lignin bio-oil, containing all three linkages, into valuable monomers. Density Functional Theory calculations provided insight into the reaction mechanism, suggesting substrate and oxidant activation by P-O-H sites in the P5NC-500, and by N-C-O-H in the CN catalyst. Moreover, the catalyst's recyclability and water utilization enhance its environmental compatibility, offering a highly sustainable approach to lignin valorization with potential applications in various industries.

Nitrogen and Sulfur co-doped Carbon dots as an "on-off-on" Fluorescent Sensor for the Detection of Hg2+ and Ampicillin

Herein, a fluorescent "on-off-on" nanosensor based on N,S-CDs was developed for highly precise and sensitive recognition of Hg2+ and ampicillin (AMP). Nitrogen and sulfur co-doped carbon dots with blue fluorescence were synthesized by one-pot hydrothermal method using ammonium citrate and DL-methionine as precursors. N,S-CDs exhibited a surface abundant in -OH, -COOH, and -NH2 groups, aiding in creating non-fluorescent ground state complexes when combined with Hg2+, leading to the suppression of N,S-CDs' fluorescence. Subsequent to additional AMP application, the mixed system's fluorescence was restored. Based on this N,S-CDs sensing system, the thresholds for detection for AMP and Hg2+ were discovered to be 0.121 µM and 0.493 µM, respectively. Furthermore, this methodology proved effective in identifying AMP in real samples of tap and lake water, yielding satisfactory results. Consequently, in the area of bioanalysis in intricate environmental sample work, the sensing system showed tremendous promise.

One-pot synthesis of full-color carbon dots with N, O surface-state energy-gap-controlled emission

An easy and effective route for preparing full-color (blue, green, yellow and red) carbon dots (CDs) with brilliant and stable photoluminescent (PL) was developed by using gallic acid and o-Phenylenediamine as precursors. The resulting CDs were then separated by silica-gel column chromatography. The prepared four types of CDs owned high quantum yields (up to 11.2%) and exhibited attractive features such as excellent solubility, high stability, excitation wavelength-independent PL, mono-exponential PL lifetimes, low cell toxicity and homogeneous optical properties. Then, the mechanism of full-color CDs PL was explored. The results suggested that the surface-states of prepared CDs, especially the pyridine N (C = N) and C = O functional groups, were primarily responsible for the full-color PL emission. The blue CDs had the largest content of C = O while the CDs turned to be green as the C = O decreased and the C = N content increased. The surface state regulation of C = N bond was dominant in yellow CDs while red CDs possessed highest content of C = N and new -COOH appeared. The gradual substitution of C = N to C = O made the band gap of CDs narrowed, which is the main reason for the redshift of CDs PL emission peak. Importantly, the easy preparation and unique optical features of these CDs made them potentially useful in numerous applications such as full-color cell labeling, drug delivery and optoelectronic technologies.

Preparation and Evaluation of Aminomalononitrile-Coated Ca-Sr Metal-Organic Frameworks as Drug Delivery Carriers for Antibacterial Applications

After orthopedic surgery, antibiotics are usually employed to reduce the risk of infection. If it is possible to enhance antimicrobial functionality and incorporate antimicrobial agents into the bone-filling matrix, not only it can promote bone tissue regeneration, but it can also enable localized administration of medication to elevate antibacterial efficacy. Meanwhile, previous studies have shown that calcium and strontium can support the growth of osteoblastic cells and diminish bone resorption or deterioration. In the past few years, metal-organic frameworks (MOFs) have been widely used as drug carriers owing to their characteristic advantages. In this study, a MOF was prepared in an aqueous solution by a simple coprecipitation method with the organic ligand 1,3,5-tricarboxylic benzene (H3BTC) as a linker to form Ca-Sr-MOF. Furthermore, the Ca-Sr-MOF was coated with aminomalononitrile (AMN), which adhered through the electrostatic interactions between H3BTC and AMN. With this MOF (Ca-Sr-AMN-MOF), AMN polymerization reactions can occur in aqueous environments, and a polymer layer was observed on the MOF surface with moderate hydrophilicity. The prepared Ca-Sr-MOF and Ca-Sr-AMN-MOF were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and UV-visible spectroscopy. Finally, tetracycline (TC) was selected as the model drug to measure the drug loading efficiency, release profile, and antibiotic activity. The percent cumulative drug release of TC from Ca-Sr-MOF and Ca-Sr-AMN-MOF was 55.15 and 9.1%, respectively. The antibacterial effectiveness of TC-loaded MOF against Gram-negative Escherichia coli bacteria was evaluated, revealing the remarkable antimicrobial performance of these substances.

Molybdenum Phosphide Quantum Dots Encapsulated by P/N-Doped Carbon for Hydrogen Evolution Reaction in Acid and Alkaline Electrolytes

A facile and eco-friendly strategy is presented for synthesizing novel nanocomposites, with MoP quantum dots (QDs) as cores and graphitic carbon as shells, these nanoparticles are dispersed in a nitrogen and phosphorus-doped porous carbon and carbon nanotubes (CNTs) substrates (MoP@NPC/CNT). The synthesis involves self-assembling reactions to form single-source precursors (SSPs), followed by pyrolysis at 900 °C in an inert atmosphere to obtain MoP@NPC/CNT-900. The presence of carbon layers on the MoP QDs effectively prevents particle aggregation, enhancing the utilization of active MoP species. The optimized sample, MoP@NPC/CNT-900, exhibits remarkable electrocatalytic activity and durability for the hydrogen evolution reaction (HER). It demonstrates a low overpotential of 155 mV at 10 mA cm-2 , a small Tafel slope of 76 mV dec-1 , and sustained performance over 20 hours in 0.5 M H2 SO4 . Furthermore, the catalyst shows excellent activity in 1 M KOH, with a relatively low overpotential of 131 mV and long-term durability under constant current input. The exceptional HER activity can be attributed to several factors: the superior performance of MoP QDs, the large surface area and good conductivity of the carbon substrates, and the synergistic effect between MoP and carbon species.

Hydrothermal Synthesis of Functionalized Carbon Nanodots and Their Clusters as Ionic Probe for High Sensitivity and Selectivity for Sulfate Anions with Excellent Detection Level

Nitrogen-doped carbon nanodots (CNDs) were synthesized and utilized as sensing probes to detect different anions and metallic ions within aqueous solutions. The pristine CNDs were developed through a one-pot hydrothermal synthesis. o-Phenylenediamine was used as the precursor. A similar hydrothermal synthesis technique in the presence of polyethylene glycol (PEG) was adopted to form the PEG-coated CND clusters (CND-100k). Through photoluminescence (PL) quenching, both CND and PEG-coated CND suspensions display ultra-high sensitivity and selectivity towards HSO₄^- anions (Stern-Volmer quenching constant (K_SV) value: 0.021 ppm^-1 for CND and 0.062 ppm^-1 for CND-100k) with an ultra-low detection limit (LOD value: 0.57 ppm for the CND and 0.19 ppm for CND-100k) in the liquid phase. The quenching mechanism of N-doped CNDs towards HSO₄^- ions involves forming the bidentate as well as the monodentate hydrogen bonding with the sulfate anionic moieties. The detection mechanism of metallic ions analyzed through the Stern-Volmer formulation reveals that the CND suspension is well suited for the detection of Fe³⁺ (K_SV value: 0.043 ppm^-1) and Fe²⁺ (K_SV value: 0.0191 ppm^-1) ions, whereas Hg²⁺ (K_SV value: 0.078 ppm^-1) sensing can be precisely performed by the PEG-coated CND clusters. Accordingly, the CND suspensions developed in this work can be employed as high-performance PL probes for detecting various anions and metallic ions in the liquid phase.

Role of Noncovalent Interactions in N,P-Functionalized Luminescent Carbon Dots for Ultrasensitive Detection of Moisture in D2O: Boosting Visible-NIR Light Sensitivity

It is highly desirable to design cost-efficient and eco-friendly fluorometric sensors that can efficiently detect water contamination in D₂O and other expensive organic solvents. Herein, we have synthesized N,P-codoped carbon dots (N,P-CDs) from o-phenylene diamine (o-PDA) and H₃PO₄ through the bottom-up carbonization method. Heteroatom co-doping increases the absorption cross section in the visible-NIR range, followed by the formation of stable emissive states in longer-wavelength regions. We have critically investigated the noncovalent interactions (especially H-bonding interactions) of various surface functional groups with surrounding solvent media through a detailed structure-property correlation. Based on the sensitivity of noncovalent H-bonding interactions to the stability of longer-wavelength emissive domains, we have utilized these N,P-CDs as cost-effective fluorometric sensors of water/moisture contamination in D₂O especially under visible-NIR light; the optical sensitivity reaches up to 0.1 volume (%) level. The detailed sensing mechanism has been further supported by a computational study through a simple visualization approach by mapping and analyzing all possible noncovalent interactions between the CDs and the solvent medium.

Efficient imaging based on P - and N-codoped carbon dots for tracking division and viability assessment of lactic acid bacteria

Assessment of lactic acid bacteria (LAB) activity plays a key role in the fermented food industry. Fluorescence imaging method based on dye is facile to detect LAB viability. However, it is difficult to obtain stable fluorescence, non-toxic and low-cost dyes. In this study, we prepare P- and N-doped carbon dots (PN-CDs) via microwave-assisted hydrothermal synthesis. The properties of high quantum yield (60.36%) and excitation dependence allowed for multicolor imaging of LAB (Lactobacillus plantarum [L.p] and Streptococcus thermophilus [S.t]). The abundant functional groups and positive charges (+2.34 mV) on the surface of PN-CDs facilitated their quickly integrated into cell wall of live LAB with obvious fluorescence or into dead cells. As a result, PN-CDs can not only be used to rapidly and efficiently monitor bacterial viability (one minute), but can also be used to visualize LAB division using fluorescence imaging. Importantly, the PN-CDs have potential to rapidly detect LAB activity in LAB-fermented juices.

Microwave synthesis of blue emissive carbon dots from 5-sulpho anthranilic acid and 1,5-diphenyl carbazide for sensing of levocetirizine and niflumic acid

In this work, water soluble carbon dots (CDs) were synthesized by using 5- sulpho anthranilic acid (SAA) and 1,5-diphenylycarbazide (DPC) as precursors via microwave-assisted method and named as "SD-CDs". We studied the effect of SAA and DPC molar ratio (1:3, 2:2 and 3:1) for the preparation of blue fluorescent CDs, showing the best emission properties at molar 3:1 ratio of SAA and DPC. The as-prepared SD-CDs emit bright blue color under UV light at 365 nm, and exhibit emission peak at 392 nm when excited at 319 nm. The as-synthesized SD-CDs act as a fluorescent sensor for detection of levocetirizine and niflumic acid through the fluorescence "turn-on-off" mechanism. The developed probe exhibited good linearity in the concentrations (levocetirizine - 1.0-100 µM and niflumic acid - 0.5-100 µM) with detection limits of 3.92 nM and 0.19 µM for levocetirizine and niflumic acid, respectively. Importantly, the developed analytical method was successfully used for the detection of levocetirizine in tablets and niflumic acid in biofluids of human (serum, plasma and urine), showing good recoveries from 97 to 99 %. Thus, this SD-CDs-based fluorescence method has the potential for levocetirizine and niflumic acid assays in biological and pharmaceutical samples.

Multiple heteroatom dopant carbon dots as a novel photoluminescent probe for the sensitive detection of Cu2+ and Fe3+ ions in living cells and environmental sample analysis

Heavy metal ion pollution harms human health and the environment and continues to worsen. Here, we report the synthesis of boron (B), phosphorous (P), nitrogen (N), and sulfur (S) co-doped carbon dots (BP/NS-CDs) by a one-step facile hydrothermal process. The optimum synthetic parameters are of 180 °C temperature, 12 h reaction time and 15% of PBA mass. The as-synthesized BP/NS-CDs exhibits excellent water solubility, strong green photoluminescence (PL) at 510 nm, and a high quantum yield of 22.4%. Moreover, BP/NS-CDs presented high monodispersity (7.2 ± 0.45 nm), excitation-dependent emission, PL stability over large pH, and high ionic strength. FTIR, XRD, and XPS are used to confirm the successful B and P doping of BP/NS-CDs. BP/NS-CD photoluminescent probes are selectively quenched by Cu²⁺ and Fe³⁺ ions but showed no response to the presence of other metal cations. The PL emission of BP/NS-CDs exhibited a good linear correlation with Cu²⁺ and Fe³⁺ concentrations with detection limits of 0.18 μM and 0.27 μM for Cu²⁺ and Fe³⁺, respectively. Furthermore, the HCT116 survival cells kept at 99.4 ± 1.3% and cell imaging capability, when the BP/NS-CDs concentration is up to 300 μg/mL by MTT assay. The proposed sensor is potential applications for the detection of Cu²⁺ and Fe³⁺ ions in environmental water samples.

Green synthesis of multifunctional carbon quantum dots: An approach in cancer theranostics

Carbon quantum dots (CQDs) have gained significant growing attention in the recent past due to their peculiar characteristics including smaller size, high surface area, photoluminescence, chemical stability, facile synthesis and functionalization possibilities. They are carbon nanostructures having less than 10 nm size with fluorescent properties. In recent years, the scientific community is curiously adopting biomass precursors for the preparation of CQDs over the chemical compounds. These biomass sources are sustainable, eco-friendly, inexpensive, widely available and convert waste into valuable materials. Hence in our work the fundamental understating of diverse fabrication methodologies of CQDs, and the types of raw materials employed in recent times, are all examined and correlated comprehensively. Their unique combination of remarkable properties, together with the ease with which they can be fabricated, makes CQDs as promising materials for applications in diverse biomedical fields, in particular for bio-imaging, targeted drug delivery and phototherapy for cancer treatment. The mechanism for luminescence is of considerable significance for leading the synthesis of CQDs with tunable fluorescence emission. Therefore, it is aimed to explore and provide an updated review on (i) the recent progress on the different synthesis methods of biomass-derived CQDs, (ii) the contribution of surface states or functional groups on the luminescence origin and (iii) its potential application for cancer theranostics, concentrating on their fluorescence properties. Finally, we explored the challenges in modification for the synthesis of CQDs from biomass derivatives and the future scope of CQDs in phototherapy for cancer theranostics.

Broadened optical absorption, enhanced photoelectric conversion and ultrafast carrier dynamics of N, P co-doped carbon dots

Carbon dots (CDs) have attracted extensive attention for their unique properties and promising applications in many fields. Many efforts have been made to improve the optical and physicochemical properties of CDs using an atomic doping strategy; however, the photoelectric properties of CD-based devices have been less studied and the photocurrent density is far from satisfactory for practical operation. Deep understanding of the doping effects on the electronic structure and photophysical properties of CDs is fundamental and essential for effectively improving the optical and photoelectrical performance of CD-based devices. Here, we have synthesized nitrogen (N) and phosphorus (P) co-doped CDs (N, P-CDs) through a one-step hydrothermal approach, and systematically investigated the effects of P-dopants on the improved optical and photoelectric properties of N, P-CDs. The introduction of P atoms into N-CDs significantly changes the electronic structure and extends the absorption spectral region, enhancing the light-harvesting ability of N, P-CDs. Meanwhile, the regulated carrier dynamics have been investigated using time-resolved fluorescence and transient absorption spectroscopy. We found that the carrier recombination was decreased with introducing P atoms, and the photogenerated electrons in the higher excited states could be efficiently transferred to the lowest excited state. Moreover, the photocurrent density of N, P-CDs was increased by twelve times compared with that of N-CDs. Therefore, the effective doping of P atoms can significantly regulate the electronic structure, optical properties, carrier dynamics and photoelectric conversion of N, P-CDs. The achieved broadband light-harvesting, good photoelectric properties and photostability of the as-prepared N, P-CDs demonstrate an important example of P-doping to improve the optical and photoelectrical properties of CD-based devices.

Fluorescent Carbon Dot-Supported Imaging-Based Biomedicine: A Comprehensive Review

Carbon dots (CDs) provide distinctive advantages of strong fluorescence, good photostability, high water solubility, and outstanding biocompatibility, and thus are widely exploited as potential imaging agents for in vitro and in vivo bioimaging. Imaging is absolutely necessary when discovering the structure and function of cells, detecting biomarkers in diagnosis, tracking the progress of ongoing disease, treating various tumors, and monitoring therapeutic efficacy, making it an important approach in modern biomedicine. Numerous investigations of CDs have been intensively studied for utilization in bioimaging-supported medical sciences. However, there is still no article highlighting the potential importance of CD-based bioimaging to support various biomedical applications. Herein, we summarize the development of CDs as fluorescence (FL) nanoprobes with different FL colors for potential bioimaging-based applications in living cells, tissue, and organisms, including the bioimaging of various cell types and targets, bioimaging-supported sensing of metal ions and biomolecules, and FL imaging-guided tumor therapy. Current CD-based microscopic techniques and their advantages are also highlighted. This review discusses the significance of advanced CD-supported imaging-based in vitro and in vivo investigations, suggests the potential of CD-based imaging for biomedicine, and encourages the effective selection and development of superior probes and platforms for further biomedical applications.

A label-free fluorescence nanosensor based on nitrogen and phosphorus co-doped carbon quantum dots for ultra-sensitive detection of new coccine in food samples

In this paper, an innovative method for the sensitive detection of new coccine using N, P-doped carbon quantum dots (N,P-CQDs) as fluorescent nanosensor is reported for the first time. The sensing mechanism is based on the fluorescence quenching of N,P-CQDs by new coccine through inner filter effect (IFE). N,P-CQDs were prepared by simple hydrothermal treatment of citric acid, phosphoric acid and ethylenediamine. Under the optimal conditions, the new coccine has two good linear responses in the concentration range of 0.2-100 and 100-200 μM, and the detection limits are as low as 24.8 and 9.4 nM, respectively. Our developed nanosensor has been successfully used for the determination of new coccine in food samples with good precision and high accuracy. This work highlights the economic, rapid, simple, selective and ultra-sensitive for new coccine detection, and opens up a new way for the monitoring of new coccine in actual food samples.

Immobilization of Heteroatom-Doped Carbon Dots onto Nonpolar Plastics for Antifogging, Antioxidant, and Food Monitoring Applications

This work presents the facile synthesis of heteroatom-doped fluorescent carbon quantum dots (C-dots), which could serve as an antioxidant. Herein, nitrogen, phosphorous, and sulfur codoped carbon dots (NPSC-dots) have been synthesized by a single-step hydrothermal strategy. Owing to the radical scavenging activity of the NPSC-dots, they were tested against several methods as well as in practical applications. The antioxidant ability of the NPSC-dots was efficiently utilized on plastic films by coating with these NPSC-dots. For the very first time, NPSC-dots were immobilized onto nonpolar plastic films (polypropylene) via photochemical covalent grafting to extend the shelf life of food items or storage without affecting the quality of plastic films. The NPSC-dot-coated PP film with negligible deterioration of transparency was extensively studied using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) analysis, X-ray photoelectron spectroscopy (XPS), contact angle measurement, and thermogravimetric analysis (TGA). The fluorescent character, antioxidant ability, and durability under different solvent systems of the coated film were examined. Also, the coated films were extensively and rigorously evaluated against simulated drastic environmental conditions to ensure the durability and antifogging application.

High Contrast Surface Enhanced Fluorescence of Carbon Dot Labeled Bacteria Cells on Aluminum Foil

Surface enhanced fluorescence (SEF) is observed with very high contrast (100-200) from single E. coli bacteria cells labeled with Carbon nanodots (CDs), on aluminum foil and aluminum film. Likely, it is the first application of organic CDs in SEF. SEF with 633 nm excitation delivered a much higher contrast than SEF with 532 nm excitation. Contrast is the ratio of the fluorescent intensities of labeled CDs to unlabeled (control) cells. High contrast with CDs is also observed on the gold film, silicon, and glass. Enhancement factor (EF) is the ratio of the signal on the metal substrate to the signal on the glass. Single E. coli cells, labeled with commercial graphene quantum dots (GCDs), demonstrated higher EFs (44 on gold, 35 on Al film), but at least one order of magnitude lower contrast (7-10 on aluminum and gold) than cells labeled with organic CDs. Therefore, organic CDs can be a good choice for cell imaging/labeling, capable of achieving a signal to noise (standard deviation of the control) as high as 700 on Al film. Overall, aluminum foil and film are highlighted as inexpensive but efficient substrates for Metal Enhanced Fluorescence, particularly MEF of bacterial cells stained with CDs.

Bipyridine- and Copper-Functionalized N-doped Carbon Dots for Fluorescence Turn Off-On Detection of Ciprofloxacin

Herein, a fluorescence turn off-on nanosensor has been successfully developed using functionalized N-doped carbon dots (N-CDs) as the label-free sensing probe for the ultrasensitive detection of Cu²⁺ ions first and then ciprofloxacin (CIP), one of the most commonly used antibiotics for disease control, in the presence of bipyridine. The homogeneous and narrowly distributed N-CDs with a mean size of 5.7 nm and a high quantum yield of 84% are fabricated via the hydrothermal method in the presence of citric acid and ethylenediamine as the carbon and nitrogen sources, respectively. The Cu²⁺ ions serve as both analyte and fluorescence quenchers in the sensing platform of N-CDs, and a good linear response to Cu²⁺ in the range of 0.01-0.35 μM with a limit of detection (LOD) of 0.076 nM is observed. Then, 0.35 μM Cu²⁺ is used as the fluorescence quencher of N-CDs to build up the fluorescence turn off-on sensing probe for the detection of CIP using bipyridine (bipy) as the linker for CIP and Cu²⁺ ions. The addition of CIP to the bipy-Cu@N-CD composites triggers the formation of CIP-bipy-Cu conjugate as well as the release of N-CDs, resulting in the recovery of fluorescence intensity after 6 min of incubation. The sensing probe exhibits a two-phase linear response to CIP in the concentration range of 0.05-1 and 1-50 μM with a LOD of 0.4 nM. In addition, the bipy-Cu@N-CD probe shows high sensitivity toward CIP over the 19 other interferences. Good recovery of 96-110% is also observed when 0.1-0.9 μM CIP is spiked into the real samples. Results obtained in this study clearly demonstrate a newly developed sensing platform with rapid detection of metal ions and antibiotics, which can open an avenue to develop highly efficient and robust sensing probes for the detection of metal ions, organic metabolites, and biomarkers in biological applications.

Three-Dimensional Electronic Network Assisted by TiN Conductive Pillars and Chemical Adsorption to Boost the Electrochemical Performance of Red Phosphorus

The practical application of red phosphorus (P) for sodium-ion batteries (SIBs) is retarded by its poor reversibility and its unstable cycling life derived from its poor conductivity and huge volume expansion. Graphene is considered as an ideal matrix to remedy these weaknesses due to its excellent conductivity and two-dimensional structure. Its π-π restacking causes spatial collapse, however, meaning that graphene cannot effectively buffer volume expansion. Herein, multifunctional TiN is introduced into a P composite to fix this issue. TiN acts as conductive pillars, electron transfer bridges, and a chemical adsorbent of phosphorus in the composite, to prevent the graphene nanoplates from restacking, to bridge gaps between the graphene nanoplates, and to chemically adsorb the P, resulting in the formation of a three-dimensional electronic network and endowing the pulverized P particles with good contact with the conductive matrix to avoid forming insulating "dead P". Consequently, the P composite presents excellent performance for SIBs.

Fluorescence Intensity Enhancement of Green Carbon Dots: Synthesis, Characterization and Cell Imaging

The hydrothermal treatment of green carbon dots (CDs) is an appropriate fluorescent probe synthesis method. CDs are exploited as biological staining agents, especially for cellular detection and imaging. The nitrogen-doped green carbon dots (N-CDs) formation can improve the fluorescence intensity property in a one-step process. Here, we report two N-CDs from lemon and tomato extraction in the presence of hydroxylamine. Lemon and tomato N-CDs showed the blue fluorescence under ultraviolet radiation of about 360 nm. The characterization of CDs and N-CDs showed the presence of N-H and C-N bonds which enhanced the fluorescence efficiency. The mean size of lemon and tomato N-CDs were about 2 and 3 nm with an increased quantum yield (QY) of 5% and 3.38%, respectively. The CDs and N-CDs cytotoxicity assay exhibited high cell viability approximately 85% and 73%, respectively. N-CDs show superior fluorescent intensity in different solvents and significant stability under long-time UV irradiation, different PH and high ionic strength. Our results indicated that the use of N-CDs in cell imaging can lead to fluorescence intensity enhancement as well as proper biocompatibility. Therefore, the safe and high fluorescence intensity of green N-CDs can be utilized for fluorescent probes in biolabeling and bioimaging applications.

Nitrogen and phosphorous doped graphene quantum dots: Excellent flame retardants and smoke suppressants for polyacrylonitrile nanocomposites

Nitrogen (N-GQD) as well as nitrogen and phosphorous co-doped (NP-GQD) graphene quantum dots were demonstrated as novel, low cost, green and highly effective flame retardants and smoke suppressants for polyacrylonitrile (PAN) nanocomposites. The N-GQD and NP-GQD samples were synthesized by hydrothermal method with citric acid as the main reactant. For the first time, the flame retardant and smoke suppressant properties of the NP-GQD were studied. The GQDs were introduced into PAN by solvent blending route. Subsequently, thermal stability, flame retardancy, fire behavior, fire hazard and structure of the residual char were investigated by thermogravimetric analysis (TGA), UL-94 vertical burning test, cone calorimetry, FE-SEM, and Raman spectroscopy. Results showed that both PAN/N-GQD and PAN/NP-GQD nanocomposites had higher flame retardancy and smoke suppressant behavior in addition to lower fire hazard properties than neat PAN. Furthermore, the residual chars for the nanocomposite samples were increased in comparison to the neat PAN. The improvements were even more significant in case of the PAN/NP-GQD due to the synergistic effect of nitrogen and phosphorous. The improvements were mainly ascribed to the ability of the N-GQD and NP-GQD to provide stronger and larger protective char barrier layers, which was even more pronounced in case of the NP-GQD.

Fluorescence Solvatochromism of Carbon Dot Dispersions Prepared from Phenylenediamine and Optimization of Red Emission

Fluorescent carbon dots (CDs) are of interest as a promising alternative to quantum dots, partly because they do not include heavy metals. However, most CDs exhibit blue or green emission, while red-emitting CDs are required for a variety of applications. In the present work, CDs were synthesized by refluxing three phenylenediamine (PD) isomers with amino groups at different positions (o-PD, m-PD, and p-PD) in diphenyl ether at 250 °C for 4 h. Upon dispersing the resulting CDs in eight solvents with different polarities, emission colors ranging from green to red were observed. Among these CDs, p-PD-derived CDs exhibited both the longest emission wavelength range, from 538 to 635 nm, and the highest absolute red photoluminescence quantum yield (PLQY) of 15%. Herein the results are discussed based on a comparison of the polymerization processes of o-PD, m-PD, and p-PD. This work demonstrated that the optimum reaction time was 2 h, which yields a p-PD-derived CD dispersion in methanol with red emission and an absolute PLQY as high as 18%. Additionally, the use of 1-decanol and deuterated methanol in place of methanol improved the maximum absolute PLQY to 25% and 36%, respectively. These improved values are attributed to reduced concentration quenching by suppression of π-π stacking interactions and inhibition of the nonradiative relaxation process through the vibration of OH groups, respectively.

Recent Advancements in Doped/Co-Doped Carbon Quantum Dots for Multi-Potential Applications

Carbon quantum dots (CQDs)/carbon nanodots are a new class of fluorescent carbon nanomaterials having an approximate size in the range of 2–10 nm. The majority of the reported review articles have discussed about the development of the CQDs (via simple and cost-effective synthesis methods) for use in bio-imaging and chemical-/biological-sensing applications. However, there is a severe lack of consolidated studies on the recently developed CQDs (especially doped/co-doped) that are utilized in different areas of application. Hence, in this review, we have extensively discussed about the recent development in doped and co-doped CQDs (using elements/heteroatoms—e.g., boron (B), fluorine (F), nitrogen (N), sulphur (S), and phosphorous (P)), along with their synthesis method, reaction conditions, and/or quantum yield (QY), and their emerging multi-potential applications including electrical/electronics (such as light emitting diode (LED) and solar cells), fluorescent ink for anti-counterfeiting, optical sensors (for detection of metal ions, drugs, and pesticides/fungicides), gene delivery, and temperature probing.

High drug-loading system of hollow carbon dots-doxorubicin: preparation, in vitro release and pH-targeted research

Hollow carbon dots (HCDs), as drug carriers, and doxorubicin (DOX), as a model drug, were selected to prepare a HCDs-DOX-loading system. First, HCDs were prepared by a hydrothermal method and characterized by transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and nuclear magnetic resonance (¹³C NMR), UV-vis absorption, Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopies (XPS). The HCDs were then used to load DOX. The drug-loading system of HCDs-DOX was characterized by zeta potential measurements, and UV-vis absorption and fluorescence spectroscopies. We then studied the drug loading, formation mechanism, cytotoxicity, in vitro release and pH-targeted properties. HCDs-DOX was found to have a high drug (DOX)-loading ratio (∼42.9%) and better sustained pH targeted-release and lower cytotoxicity than those of DOX. In the HCDs-DOX system, interactions between the HCDs and DOX were electrostatic resulting in the formation of -N[double bond, length as m-dash]C-via the coupling of -NH₂ (on HCDs) and -C[double bond, length as m-dash]O (on DOX). In vitro release of HCDs-DOX conformed to the Weibull model and Fick diffusion, consistent with that of free DOX. We report, for the first time, that the: (i) functional groups on the HCD surfaces (not their hollow structure) play a key role in drug loading; (ii) the carrier (HCDs) did not change the in vitro release model or mechanism of DOX before and after loading by the HCDs.

Hydrothermal synthesis of carbon dots codoped with nitrogen and phosphorus as a turn-on fluorescent probe for cadmium(II)

Nitrogen and phosphorus co-doped carbon dots (N,P-CDs) have been synthesized via hydrothermal method starting from o-phosphorylethanolamine and citric acid. The blue-green fluorescence of the N,P-CDs (with excitation/emission peaks at 325/435 nm) is gradually enhanced on sequential addition of Cd(II) ions. This fluorometric assay works in the 0.5 μM to 12.5 μM Cd(II) concentration range and has a 0.16 μM detection limit. The phenomenon may be attributed to chelation enhanced fluorescence that is induced by the formation of Cd(II)-N,P-CDs complex with functional groups present on the surface. The method has applied to the detection of Cd(II) in spiked serum and urine samples and gave satisfying results. Graphical abstract Schematic presentation of the synthesis of nitrogen and phosphorus co-doped carbon dots (N,P-CDs) and their application as a turn-on fluorescence probe for Cd(II) detection.

Preparation of N-doped yellow carbon dots and N, P co-doped red carbon dots for bioimaging and photodynamic therapy of tumors

We report on the preparation of N, P co-doped red carbon dots and their applications in bioimaging and photodynamic therapy of tumors.

Orange, yellow and blue luminescent carbon dots controlled by surface state for multicolor cellular imaging, light emission and illumination

Three kinds of carbon dots (CDs) with different photoluminescence (PL) (blue, yellow or orange) were synthesized by microwave heating. They display wavelength-independent excitation wavelengths (in the range from 444 to 574 nm), similar average particle size (from 3.7 to 4.2 nm), and fluorescence lifetimes (from 2.7 to 3.1 ns). Color and quantum yields (from 8 to 45% in ethanol) are related to the oxidation degree and the number of N-functional groups on their surface. The CDs are shown to be viable nanoprobes for multicolor imaging of cells. Three composite phosphors were obtained by coating the various CDs on starch particle. The resulting nanomaterials emit solid-state fluorescence with a quantum yield of ≥16%. They were used to fabricate luminescent blocks and light-emitting diodes with controllable color temperature. Graphical abstract (a) The synthesis process of the three carbon dots (CDs). The application in cell imaging (b), starch/CD phosphors (c), starch/CD phosphors-based luminescent blocks (d) and light-emitting diodes (e). (λex: excitation wavelength).

P-doped carbon nano-powders for fingerprint imaging

A simple, fast, and laboratory efficient doped P carbon nanoparticles synthesis is developed for fingerprint imaging, using 1,3-dihydroxyacetone and di-phosphorous pentoxide. Fluorescence nanoparticles, with an average size of 230 nm were obtained, without additional energy input or external heating. ATR, solid NMR, XPS and fluorescence spectroscopy revealed their surface functionalization; a reaction mechanism is proposed. Fluorescence measurements exhibited a maximum emission band at ca. 495 nm, when excited at 385 nm. The images obtained, on different surfaces such as mobile telephone screen, magnetic band and metallic surface of a credit card and a Euro banknote treated with the obtained nano-powders allows us to record positive matches, confirming that the experimental results illustrate the effectiveness of proposed method.

MnO nanoparticles with unique excitation-dependent fluorescence for multicolor cellular imaging and MR imaging of brain glioma

The authors describe MnO nanoparticles (NPs) with unique excitation-dependent fluorescence across the entire visible spectrum. These NPs are shown to be efficient optical nanoprobe for multicolor cellular imaging. Synthesis of the NPs is accomplished by a thermal decomposition method. The MnO NPs exhibit a high r₁ relaxivity of 4.68 mM^-1 s^-1 and therefore give an enhanced contrast effect in magnetic resonance (MR) studies of brain glioma. The cytotoxicity assay, hemolysis analysis, and hematoxylin and eosin (H&E) staining tests verify that the MnO NPs are biocompatible. In the authors' perception, the simultaneous attributes of multicolor fluorescence and excellent MR functionality make this material a promising dual-modal nanoprobe for use in bio-imaging. Graphical abstract A direct method to synthesize fluorescent MnO NPs is reported. The NPs are biocompatible and have been successfully applied for multicolor cellular imaging and MR detection of brain glioma.

Hollow carbon dots labeled with FITC or TRITC for use in fluorescent cellular imaging

Hollow carbon dots (HCDs) were prepared by a solvothermal method and conjugated to either tetramethyl rhodamine isothiocyanate (TRITC) or fluorescein-5-isothiocyanate (FITC). This resulted in HCDs with bright red or green fluorescence, with excitation/emission peaks at 550/580 and 491/520 nm, respectively. The nanocomposites are well water-soluble, remarkably photostable and biocompatible. In addition, the fluorescence of the composites is more stable in a reactive oxygen environment than the free dyes. Confocal images indicate that the nanoparticles quickly enter A549 cells and mainly accumulate in the cytoplasm. The wavelength of functionalized HCDs can be regulating via coupling the HCDs to different dyes. These results demonstrate that these composite materials can be very promising reagents for biological labeling and imaging. Graphical abstract Schematic of the preparation of hollow carbon dots conjugated to tetramethyl rhodamine isothiocyanate (RHCDs) by solvothermal method. The material is water-soluble, remarkably photostable and biocompatible. It was applied to cellular labeling and imaging.

Microwave-assisted synthesis of highly luminescent N- and S-co-doped carbon dots as a ratiometric fluorescent probe for levofloxacin

Uniform N- and S-co-doped carbon dots (NSCDs) with fluorescence quantum yields of up to 64% were synthesized via a one-step microwave-assisted method. Ammonium citrate and L-cysteine act as precursors, and synthesis is completed in 2.5 min using a 750 W microwave oven to give a 62% yield. The NSCDs show bright blue fluorescence (with excitation/emission peaks at 353/426 nm) and have narrow size distribution. On exposure to levofloxacin (LEV), the emission maximum shifts to 499 nm. This effect was used to design ratiometric (2-wavelength) assays for LEV. The fluorometric method (based on measurement of the fluorescence intensity ratio at 499 and 426 nm) has a detection limit of 5.1 μg·L^-1 (3σ/k) and a linear range that extends from 0.01 to 70 mg·L^-1. The method was applied to the determination of LEV in three kinds of spiked water samples and has recoveries in the range from 98.6 to 106.8%. The fluorescent probe described here is highly selective and sensitive. Graphical Abstract Highly luminescent N- and S-co-doped carbon dots were synthesized using AC (ammonium citrate) and Cys (L-cysteine) by microwave-assisted method, and were applied to the visual and ratiometric fluorescence determination of LEV (levofloxacin).

One-pot synthesis of highly fluorescent Fe2+-doped carbon dots for a dual-emissive nanohybrid for the detection of zinc ions and histidine

Fe²⁺ was confirmed to be the only definitive one of the common metal ions to synthesize the highly fluorescent carbon dots with proline as the carbon resource at 80 °C for visual fluorescence sensing Zn2+ and histidine, respectively.