Si-doped carbon quantum dots: a facile and general preparation strategy, bioimaging application, and multifunctional sensor

Silicon-hybrid carbon dots derived from rice husk: promising fluorescent probes for trivalent rare earth element ions in aqueous media

As a novel fluorescence probe, Si–CDs could not only be applied to distinguish four groups of different rare earth element ions (REEs) but also exhibit a rapid and sensitive response towards individual Tb3+,Eu3+ and Dy3+.

Metal-doped and hybrid carbon dots: A comprehensive review on their synthesis and biomedical applications

Carbon dots (CDs) are the most promising candidates of the carbon family with superior properties like ultra-small size, high aqueous solubility, low cytotoxicity, and inherent photoluminescence which makes them suitable for diverse biomedical applications. Methods have been developed to enhance their applications. Doping/surface passivation of CDs improves their physicochemical properties, visible light absorption probability, and quantum yield by controlling their size, morphology, structure, and band-gap energy. Recently, metal-doped CDs have emerged as an important class of nanomaterials with numerous biomedical applications. Additionally, the conjugation of CDs with semiconductor metal-oxide nanoparticles (NPs) enhances their free radical production rates under visible light irradiation. Conjugation of fluorescent CDs with magnetic NPs leads to the development of multimodal imaging platforms. Similarly, ternary conjugates composed of fluorescent CDs, near-infrared (NIR) responsive, and magnetic NPs are useful for multi-modal imaging-guided, and NIR-responsive synergistic chemo-phototherapy. However, no comprehensive review is published yet which covers metal-doped and hybrid CDs. Therefore, herein we provide detailed information about their synthesis and important biomedical applications. Firstly, we have covered various synthesis methods for CD conjugation including the critical analysis of the effects of the reaction conditions and doping/conjugation on the structure and properties of the CDs. Then we have extensively reviewed their biomedical applications as antimicrobial, antioxidant, and bioimaging agents, and in the field of cancer phototherapy with special emphasis on their mechanisms of actions. Finally, the future directions of research and the applications of the metal-doped and hybrid CDs have been discussed. We believe that this review article will enrich the understanding of different synthetic routes of CD-nanocomposites and their biomedical applications.

Carbon Dots as Nanotherapeutics for Biomedical Application

Carbon nanodots are zero-dimensional spherical allotropes of carbon and are less than 10nm in size (ranging from 2-8nm). Based on their biocompatibility, remarkable water solubility, eco- friendliness, conductivity, desirable optical properties and low toxicity, carbon dots have revolutionized the biomedical field. In addition, they have intrinsic photo-luminesce to facilitate bio-imaging, bio-sensing and theranostics. Carbon dots are also ideal for targeted drug delivery. Through functionalization of their surfaces for attachment of receptor-specific ligands, they ultimately result in improved drug efficacy and a decrease in side-effects. This feature may be ideal for effective chemo-, gene- and antibiotic-therapy. Carbon dots also comply with green chemistry principles with regard to their safe, rapid and eco-friendly synthesis. Carbon dots thus, have significantly enhanced drug delivery and exhibit much promise for future biomedical applications. The purpose of this review is to elucidate the various applications of carbon dots in biomedical fields. In doing so, this review highlights the synthesis, surface functionalization and applicability of biodegradable polymers for the synthesis of carbon dots. It further highlights a myriad of biodegradable, biocompatible and cost-effective polymers that can be utilized for the fabrication of carbon dots. The limitations of these polymers are illustrated as well. Additionally, this review discusses the application of carbon dots in theranostics, chemo-sensing and targeted drug delivery systems. This review also serves to discuss the various properties of carbon dots which allow chemotherapy and gene therapy to be safer and more target-specific, resulting in the reduction of side effects experienced by patients and also the overall increase in patient compliance and quality of life.

Microwave-Synthesized Polysaccharide-Derived Carbon Dots as Therapeutic Cargoes and Toughening Agents for Elastomeric Gels

Fluorescent carbon dots (CDs) play a versatile role in materials science. Herein, we have developed alginate-derived nitrogen-doped CDs as a drug carrier and a toughening agent for hydrogels by a microwave-assisted method. In the first phase of work, we carried out covalent conjugation of the drug onto the CD surface for controlled delivery of drug molecules, and in the second phase of work, we demonstrated how CDs could act as a toughening agent as well as a viscosity modifier for poly(acrylic acid-co-methacrylamide) copolymer hydrogels. The hydrogels were evaluated by Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, and solid-state nuclear magnetic resonance. The hybrid hydrogels have been tested to be mechanically robust with extraordinary stretchability (∼1200% elongation at break), recoverable to the original position (low permanent set), tunable water uptake, and thixotropic character in dynamic stress. The crosslinked structure has been evaluated through void calculation revealing gradual densification of the network with increasing CD content. Exceptional gel strength (ratio of elastic modulus to loss modulus; G'/G″) has been achieved from analogous crosslinking made by CDs. The delayed network rupturing and superstretchability could make this material a good choice for soft biomaterials and soft robotics.

N,S-co-doped carbon dots for rapid acid test paper and bioimaging

Fluorescent N,S-CDs with a quantum yield of 37.8% were synthesized via a one-pot solvothermal method. Detailed characterizations on physical, chemical and optical properties have been investigated. N,S-CDs demonstrated remarkably quenched and enhanced fluorescence in acidic and basic media. Direct qualitative analysis in pH sensor and cell imaging were preliminarily studied.

Ultra-stable tellurium-doped carbon quantum dots for cell protection and near-infrared photodynamic application

It is important to regulate the concentration of reactive oxygen species (ROS) in cells since they play important roles in metabolism. Thus, developing nanoreagents to control the ROS is critical. Herein, tellurium-doped carbon quantum dots (Te-CDs) were developed by a simple and efficient hydrothermal method, which can scavenge H₂O₂ to protect cells under ambient condition, but generate ·OH under 808 nm irradiation as photodynamic application. This contribution presented a kind of novel CDs with dual-functions, which can potentially regulate ROS under different conditions.

Heteroatom doped carbon dots with nanoenzyme like properties as theranostic platforms for free radical scavenging, imaging, and chemotherapy

Carbon-based artificial nanoenzymes have gained increasing interest as emerging and promising nanotheranostic agents due to their biocompatibility, low cost, and straightforward production. Herein, a multifunctional Mn, N, and S incorporated carbon dots (MnNS:CDs) nanoenzyme exhibiting scavenging activity against reactive oxygen species (ROS) and reactive nitrogen species (RNS), photoluminescence quantum yield of 17.7%, and magnetic resonance imaging (MRI) contrast was explored. The optical, magnetic, and antioxidant properties of MnNS:CDs were then regulated by control over Mn incorporation to achieve higher photostability and antioxidant properties. Furthermore, conjugation of MnNS:CDs with hyaluronic acid (HA) (denoted as MnNS:CDs@HA) endowed them with high biocompatibility, which is validated by in vivo studies on zebrafish, and the ability to specifically target cluster determinant 44 (CD44)-overexpressing B16F1 cells, as verified by in vitro confocal and MRI studies. The MnNS:CDs@HA probe with therapeutic antioxidant and dual-modal imaging capability was further assessed for non-covalent binding of doxorubicin (DOX) as a model chemotherapeutic cancer drug. Results showed that targeted delivery and pH-dependent release of DOX elicited apparent cell toxicity (90%) toward B16F1 cancer cells when compared to free DOX treatment group (60%). Benefiting from their intrinsic antioxidant properties, and dual-modal imaging ability, the MnNS:CDs@HA nanocarrier is projected to improve non-invasive targeted diagnosis and therapy. STATEMENT OF SIGNIFICANCE: Carbon dots (CDs) have gained increasing interest as emerging and promising artificial functional nanomaterials that mimic the structures and functions of natural enzymes. In this work, Mn, N, and S incorporated CDs (MnNS:CDs) were synthesized using a one-pot microwave hydrothermal method to serve as fluorescent and magnetic resonance imaging probes, and catalase mimics in the reduction of the oxidative-stress related damage. Further conjugation of the probes with hyaluronic acid endows them with a good in vitro and in vivo biocompatibility as well as the capability to selectively target CD44-overexpressing cancer cells, as investigated by in vitro fluorescence, and magnetic resonance imaging. The dual-modal nanoprobe was then used to carry on doxorubicin through a non-covalent association. Favorably, targeted delivery, and pH-responsive release of doxorubicin enhanced cell killing efficiency by 50% as opposed to the free doxorubicin treatment group. The presented theranostic heteroatom doped CDs hold great promise for dual-modal imaging enabling accurate diagnosis coupled with therapeutic effect through free radical scavenging and chemotherapy.

N-Confused Porphyrin - A Unique "Turn-On" Chemosensor for CN- and F- ions and "Turn-Off" Sensor for ClO4 - ions

N-Confused meso-tetrakis(4-carbomethoxyphenyl)porphyrin (1) and its Ni(II) complex (1 a) have been synthesized and utilized for anion sensing studies, and the results are compared with N-confused meso-tetraphenylporphyrin (NCTPP). Anion susceptibilities of 1 and 1 a were investigated using spectroscopic, electrochemical, and DFT studies. Porphyrins 1 and 1 a were able to detect CN^- , F^- , and ClO₄ ^- ions selectively over the tested set of anions even at ppm level. Interestingly, the addition of ClO₄ ^- ions resulted in fluorescence quenching (turn off) whereas the addition of F^- or CN^- resulted in fluorescence enhancement (turn on). Notably, the TFA addition resulted in fluorescence quenching, whereas the fluorescence enhancement was observed while adding TBAOH. The higher association constant (K_a ) values with anions, lower detection limit, and shifts in redox potentials are due to the electron-withdrawing effect of the -COOCH₃ group at the para-position of the meso-phenyl ring. This electron-withdrawing nature is crucial for the higher affinity towards anions. The anion sensing description in this article may not only unveil the built-in nature of N-confused porphyrins, but may also provide a general proposal for the development of novel anion sensors based on porphyrinoids. The electron-deficient porphyrin framework, large polarisable π-system, and anion binding through the outer NH or a combination of the above factors serve as a foundation for N-confused porphyrin to act as an anion sensor.

A review: recent advances in preparations and applications of heteroatom-doped carbon quantum dots

Carbon quantum dots (CQDs) are widely used in optoelectronic catalysis, biological imaging, and ion probes owing to their low toxicity, stable photoluminescence, and ease of chemical modification. However, the low fluorescence yield and monochromatic fluorescence of CQDs limit their practical applications. This review summarizes the commonly used approaches for improving the fluorescence efficiency of CQDs doped with non-metallic (heteroatom) elements. Herein, three types of heteroatom-doped CQDs have been investigated: (1) CQDs doped with a single heteroatom; (2) CQDs doped with two heteroatoms; and (3) CQDs doped with three heteroatoms. The limitations and future perspectives of doped CQDs from the viewpoint of producing CQDs for specific applications, especially for bioimaging and light emitting diodes, have also been discussed herein.

Multifaceted applications of green carbon dots synthesized from renewable sources

Fluorescent carbon dots (CDs) are an emerging class of nanomaterials in the carbon family. There are various inexpensive and renewable resources that can be used to synthesize green CDs, which have received immense attention from researchers because of their improved aqueous solubility, high biocompatibility, and eco-friendly nature compared with chemically derived CDs. Additional surface passivation is not required, as heteroatoms are present on the surface of green CDs in the form of amine, hydroxyl, carboxyl, or thiol functional groups, which can improve their physicochemical properties, quantum yield, and the probability of visible light absorption. Green CDs have potential applications in the fields of bioimaging, drug/gene delivery systems, catalysis, and sensing. Since their discovery, there have been several review articles that describe the synthesis of green CDs and some of their applications. However, there are no review articles describing the synthesis and complete applications of green CDs. Here, we provide detailed information regarding their synthesis and applications based on the available literature. In addition, we discuss some of the less explored applications of green CDs and the challenges that remain to be overcome.

Thermo and flex multi-functional array ionic sensor for a human adaptive device

Recently, electronic skin that mimics human skin in measuring tactile stimuli, temperature, and humidity and having a self-healing function was developed. Furthermore, with the advances in the field of artificial intelligence and health monitoring, various materials and methods have been studied for e-skin. The limitations to work on actual human skin include device flexibility and large-area applications through array structures, and many studies are underway to overcome these problems. Polymeric materials containing ionic liquids can be used to easily fabricate devices in the solid state. They are highly sensitive to both pressure and temperature, making them suitable for multi-sensing devices. Resistive and capacitive sensors have the advantage of having a simple structure, which makes them easy to fabricate. In a single device, both types work well. For resistive sensors, the temperature sensitivity (1.1/°C) is relatively high. Conversely, capacitive sensors have a low temperature sensitivity (0.3/°C). However, they have the advantage of being uniformly variable under each condition and having a smaller error range. In the array structure, independent flex and thermo sensors are arranged repeatedly. The resistive type shows changes in temperature and bending, but in the capacitive type, it is difficult to obtain results from the pixels due to parasitic capacitance.

A multi-functional and array sensor which is important to imitate the real human skin. The ionic thermoplastic polyurethane is deformable and changed electrical characteristics by temperature and pressure.

Microwave-assisted Synthesis of N,S-co-carbon Dots as Switch-on Fluorescent Sensor for Rapid and Sensitive Detection of Ascorbic Acid in Processed Fruit Juice

To achieve a rapid, sensitive, and economical method for the detection of ascorbic acid (AA) in the presence of Fe³⁺, a nitrogen and sulfur co-doped carbon dots (N,S-co-CDs) based fluorescence sensing system was developed. In this work, N,S-co-CDs were successfully synthesized via a one-step microwave-assisted method within 2.5 min using ammonium citrate and L-cysteine as precursors. The fluorescence of N,S-co-CDs was quenched (off ) by Fe³⁺ through a static-quenching mechanism. Subsequently, the fluorescence was recovered (on) after introducing AA into the quenched system, which was attributed to the reduction effect of AA for Fe³⁺. Therefore, a switch-on sensor (N,S-co-CDs/Fe³⁺ system) was developed for AA detection. Under optimal conditions, the limit of detection (LOD) of 2.31 μmol/L for AA was obtained over a linear range from 0 to 150 μmol/L. Furthermore, the proposed sensing method was successfully applied to detect AA in processed fruit juice with satisfactory results. The most important is that the sensor derived from a microwave-assisted method has simple and eco-friendly synthesis processes, is rapid, and has high detection efficiency. Therefore, such a switch-on sensor may be a promising candidate sensor for AA detection in processed fruit samples.

A novel optical fiber glucose biosensor based on carbon quantum dots-glucose oxidase/cellulose acetate complex sensitive film

A novel optical fiber glucose biosensor based on fluorescent carbon quantum dots (CQDs)-glucose oxidase (GOD)/cellulose acetate (CA) complex sensitive film was fabricated, in which the dip-coating method was adopted to immobilize the CQDs-GOD/CA complex sensitive film onto the end face of the optical fiber. The surface morphology, microstructure and optical performances of the sensitive film were characterized by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), Zeiss Axiovert 25 inverted microscope, Fourier transform infrared spectroscopy (FTIR), Ultraviolet-visible spectrophotometer and fluorescence spectrophotometer, respectively. The developed fiber-optic biosensor exhibits high sensitivity and repeatability for continuous online detection of low concentration glucose, allowing visualization of real-time glucose fluctuations over a period of time. The change ratios in fluorescence intensity of the biosensor are linear with glucose concentration in various ranges including micromole and nanomole levels, and the relationship between relative fluorescence intensity ratio and glucose concentration complies well with the modified Stern-Volmer equation in the range of 10-200 μmol/L with the detection limit of 6.43 μM, and in the range of 10-100 nmol/L with the detection limit of 25.79 nM, respectively.

Nitrogen-doped carbon dots as a probe for the detection of Cu2+ and its cellular imaging

Nitrogen-doped carbon dots were synthesized using citric acid monohydrate and glutathione as raw materials. The synthesized nitrogen-doped carbon dots were characterized by multiple analytical techniques, including transmission electron microscopy, Fourier transform infrared spectroscopy, ultraviolet–visible absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffractometry, and fluorescence spectra. The fluorescence intensity of the nitrogen-doped carbon dots gradually quenched with different concentrations of Cu2+ ions. The effect of the pH value, the nitrogen-doped carbon dot concentration, and the reaction time on the fluorescence intensity of the N-CDs-Cu2+ system was investigated, and the experimental conditions were optimized. A rapid and sensitive method for the determination of Cu2+ ions was established that exhibited a good linearity in the concentration range 0.20–200.0 μM with a detection limit of 0.27 nM. Meanwhile, the fluorescence quenching mechanism of the interaction between nitrogen-doped carbon dots and Cu2+ was preliminarily discussed. The method was used to detect trace Cu2+ in tap water and lake water, with recoveries ranging from 98.1% to 102.0%. Furthermore, due to low cytotoxicity and good biocompatibility, nitrogen-doped carbon dots as a probe were also successfully used in bioimaging.

Small molecules derived carbon dots: synthesis and applications in sensing, catalysis, imaging, and biomedicine

Carbon dots (CDs) are the new fellow of carbon family having a size less than 10 nm and attracted much attention of researchers since the last decade because of their unique characteristics, such as inexpensive and facile synthesis methods, easy surface modification, excellent photoluminescence, outstanding water solubility, and low toxicity. Due to these unique characteristics, CDs have been extensively applied in different kind of scientific disciplines. For example in the photocatalytic reactions, drug-gene delivery system, in vitro and in vivo bioimaging, chemical and biological sensing as well as photodynamic and photothermal therapies. Mainly two types of methods are available in the literature to synthesize CDs: the top-down approach, which refers to breaking down a more massive carbon structure into nanoscale particles; the bottom-up approach, which refers to the synthesis of CDs from smaller carbon units (small organic molecules). Many review articles are available in the literature regarding the synthesis and applications of CDs. However, there is no such review article describing the synthesis and complete application of CDs derived from small organic molecules together. In this review, we have summarized the progress of research on CDs regarding its synthesis from small organic molecules (bottom-up approach) via hydrothermal/solvothermal treatment, microwave irradiation, ultrasonic treatment, and thermal decomposition techniques as well as applications in the field of bioimaging, drug/gene delivery system, fluorescence-based sensing, photocatalytic reactions, photo-dynamic therapy (PDT) and photo-thermal (PTT) therapy based on the available literature. Finally, the challenges and future direction of CDs are discussed.

Fluorescent carbon dots functionalization

Carbon dots (CDs), as a new type of luminescent zero-dimensional carbon nanomaterial, have been applied in a variety of fields. Currently, functionalization of CDs is an extremely useful method for effectively tuning their intrinsic structure and surface state. Heteroatom doping and surface modification are two functionalization strategies for improving the photophysical performance and broadening the range of applications for fluorescent CDs. Heteroatom doping in CDs can be used to tune their intrinsic properties, which has received significant research interests because of its simplicity. Surface modification can be applied for varying active sites and the functional groups on the CDs surface, which can endow fluorescent CDs with the unique properties resulting from functional ligand. In this review, we summarize the structural and physicochemical properties of functional CDs. We focused our review on the latest developments in functionalization strategies for CDs and discuss the detailed characteristics of different functionalization methods. Ultimately, we hope to inform researchers on the latest progress in functionalization of CDs and provide perspectives on future developments for functionalization of CDs and their potential applications.

Ultrabright Full Color Carbon Dots by Fine-Tuning Crystal Morphology Controllable Synthesis for Multicolor Bioimaging and Sensing

In this paper, two kinds of novel carbon nanocrystals (CNCs) with different crystal morphologies (the branch-chain young sprout form (CM1) and conifer-pine form (CM2)) were obtained in a controllable way. The mechanism of crystal morphological development was explored well. When the two kinds of the CNCs were dissolved in different polar solvents, they voluntarily become "ultrafine crystals" at the moment. After that, the ultrabright full color carbon dots (UBFCCDs) have been preliminarily prepared by fine-controlling. With the evaporation of the solvents, the CNCs crystallized again, which could repeat back and forth many times. After the conditions of preparing for CDs were optimized carefully, the as-prepared CDs exhibit ultrabright effects of multiexcitation and multiemission (from blue to red) and can show unique up-conversion luminescence characteristics under a lower excitation wavelength of 660 nm instead of a near-infrared wavelength of 980 or 808 nm. Significantly, the QY% of the UBFCCDs can reach 78.0%, which is higher than that of the traditional hydrothermal methods of discarding precipitation and carrying out dialysis (QY% = 69.0%). The as-prepared CDs can be used for multicolor biomedical imaging in vivo and in vitro and metal ion sensing and also show their potential value for industrial applications.

Carbon dots derived from algae as H2O2 sensors: the importance of nutrients in biomass

Carbon dots produced hydrothermally from algae were used directly for H₂O₂ sensing. The mineral nutrients in biomass were found be important for the composition, crystallinity, dispersion and photoluminescence (PL) quenching of the carbon dots under reactive oxygen species, which catalysed the oxidation of passivating ligands.

Microwave-assisted synthesis of polyamine-functionalized carbon dots from xylan and their use for the detection of tannic acid

A facile and straightforward microwave-assisted method was used to prepare polyamine-functionalized carbon dots (CDs) from a precursor comprising renewable xylan and branched polyethyleneimine (BPEI). The as-prepared BPEI-CDs were monodispersed sphere particles with an average diameter of about 8.62 nm, and exhibited excellent fluorescent property and high stability, as well as excitation-independent emission behavior. Furthermore, it is attractive that the BPEI-CDs can be used as novel fluorescent probes for detecting tannic acid (TA) sensitively and selectively. At the optimum condition, the TA detection system was established in water solution and ethanol solution with a dynamic range from 0.1 to 5 μM, and their detection limit of 36.8 nM and 44.9 nM were also determined, respectively. Most importantly, the BPEI-CDs-based sensors can be successfully applied to detect TA in real lake water and white wine samples, suggesting the low-cost and excellent BPEI-CDs are potential suitable for TA detection in practical application.

Assembly of shell/core CDs@CaF2 nanocomposites to endow polymers with multifunctional properties

The shell/core structure of CDs@CaF₂ nanocomposites (CCNCs) were prepared by assembling fluorescent carbon dots (CDs) inside the inorganic CaF₂ substrates using co-precipitation interaction. CDs endow CaF₂ with properties of good UV-absorbing behavior and efficient blue light emission instead of rare-earth such as Eu that is expensive and susceptible to polluting the environment during the mining process. Due to the nanometer size and surface effect of nano CaF_2, and the approximate refractive index between CaF₂ and polyethylene (PE), CCNC/PE film exhibits better elongation at the break than pure PE film while maintaining high transparency and visible light transmittance. Simultaneously, the CCNC/PE film was experimentally demonstrated to have outstanding performance of anti-UV and blue light conversion, which shows that CCNCs can be a novel and promising multifunctional additive applied in polymers especially for greenhouse film.

Nitrogen-doped carbon quantum dots as an antimicrobial agent against Staphylococcus for the treatment of infected wounds

Antimicrobial resistance is becoming more and more serious and has become a potential hazard to human life and health. The fabrication of some new antibacterial substances against resistant bacteria is demanded. With the wide application and research of carbon nanomaterials, nitrogen-doped carbon quantum dots (NCQDs) were synthesized by a one-step chemical route herein. The particle size of NCQDs in the range of 2-5 nm were characterized by transmission electron microscopy (TEM), atomic force microscopy, and dynamic light scattering. The functional groups and optical properties of NCQDs were investigated by UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Disk-diffusion tests showed that the NCQDs had specific antibacterial activity against Staphylococcus. TEM showed that the NCQDs could destroy the cell structure of Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) but could not combat Escherichia coli. The antibacterial mechanism may be that positively charged NCQDs firstly interacted with the negatively charged bacteria, and then specifically anchored on some specific sites on the surface of Staphylococcus. The NCQDs were applied to treat wounds infected with MRSA and showed the same therapeutic effect as vancomycin. Photomicrographs of hematoxylin-eosin-stained histological sections showed that the NCQDs at concentrations effectively killing S. aureus and MRSA caused negligible toxicity to the main rat organs, including heart, liver, spleen, lung, and kidney. Thus, the NCQDs can be developed as a promising antibacterial agent for Staphylococcus. And the NCQDs are likely to treat local infections caused by Staphylococcus clinically, especially S. aureus and MRSA.

TDDFT study on aluminum and fluoride dual-sensing mechanism of a Schiff-Base sensor

The aluminum and fluoride dual-sensing mechanism of a previously reported sensor with a Schiff-base moiety (Spectrochim. Acta A, 2017, 183, 267-274) has been investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The present calculations reproduce the photoproperties of the sensor as well as its aluminum and fluoride complexes, which illustrates that DFT and TDDFT constitute a reliable tool for uncovering detailed fluorescence-based sensing mechanisms in diverse electronic states. Theoretical results indicate that there are two OH⋯N hydrogen bonds in the sensor and two OH⋯F hydrogen bonds in its F¯ complex. Different degrees of coplanarity caused by these hydrogen bonds are responsible for their distinct absorption wavelengths. However, excited-state geometry optimization and a scan of the potential-energy surface show that there is twisted intramolecular charge transfer about the CN bond in the sensor molecule and an excited-state proton-transfer process from the fluoride anion to the neighboring N atom in the fluoride-sensor complex, whereby the fluorescence is quenched. A chelation-enhanced fluorescence effect associated with the aluminum-sensor complex shows a different excited-state process. The local excitation and emission occur exclusively within the planar fluorophore, and negligible structural change upon excitation of the aluminum-sensor complex leads to its strong fluorescence. Therefore, it is theoretically explained why the sensor may be successfully used to analyze the fluoride anion by absorption spectroscopy and the aluminum cation by emission spectroscopy.

Polymer-Based Synthetic Routes to Carbon-Based Metal-Free Catalysts

Carbons are increasingly important as possible alternatives to expensive metal catalysts owing to the wide range of chemical properties they can exhibit and the growing set of synthetic routes available to produce them. This progress report discusses the process of making catalytic carbons from polymeric precursors, focusing on mechanisms of carbonization and how the polymer structures and synthetic procedures affect the resulting carbons. In considering what is necessary to move laboratory catalytic carbons to industrial and commercial applications, the cost and complexity to produce them are a considerable challenge to overcome. Industrially produced carbons are typically made from biopolymers such as lignin while many of the catalytic carbons studied in literature are from synthetic polymers. Thus, studying polymer-derived carbons can provide insights into the carbonization process and the properties of catalytic carbons, which can subsequently be translated to improve biopolymer-derived carbons in an economical way. Aspects of polymer carbonization discussed include carbonization mechanisms, effects of crosslinkers, polymer microstructure, heteroatom control, and effects of nanostructuring.

Luminescent Sol-Gel Glasses from Silicate–Citrate–(Thio)Ureate Precursors

Recent advancements in synthesis and analysis of the composites based on silica and carbon quantum dots have revealed great potential of such systems in bioimaging, sensor, as well as solid-state lighting applications. Most of the synthetic methods for obtaining such materials are still relatively complex and costly. The aim of this work was to study the luminescent properties of silica-based composites prepared by the simple sol-gel method using low-cost silicate–citrate–(thio)ureate precursors. The glassy composites were prepared by acid hydrolysis of ethyl silicate (40%) in aqueous solution of citric acid ureates or thioureates with the citric acid-to-(thio)urea molar ratio of 1:1, 1:1.5 or 1:3. The results of spectrofluorimetric analysis have shown that heat-treated at 270 °C such silica gels upon UV excitation (with an optimum at λexc = 360 nm) emit light in a visible spectrum (400–600 nm). Upon this, photoluminescence efficiency of ureate-derived glasses (quantum yield 70.53% for 1:1.5 sample) appeared to be much higher than that for thioureate-derived glasses (quantum yield 11.25% for 1:3 sample) suggesting that the preparation conditions to obtain the glasses with optimal photoluminescence characteristics are quite different in case of urea and thiourea. Thus, citrate–ureate-derived silica glasses already demonstrate very good potential to be efficient materials for different fluorescence-related applications.

Silicon doped graphene quantum dots combined with ruthenium(iii) ions as a fluorescent probe for turn-on detection of triclosan

In this work, silicon doped graphene quantum dots (Si-GQDs) were prepared and applied for the sensitive and selective fluorescence detection of triclosan (TCS) in combination with Ru³⁺ ions.

Solvent-controlled synthesis of multicolor photoluminescent carbon dots for bioimaging

Multicolor fluorescent carbon dots (CDs) have potential applications in multichannel detection and multicolor imaging. In this study, multicolor fluorescent CDs were synthesized by changing the solvent type and adjusting the reactant ratio. The four prepared CDs emitted bright and stable blue (B-), green (G-), yellow (Y-), and red (R-) fluorescence under a single UV light (λ_ex = 365 nm). The photoluminescence (PL) emission wavelengths changed from 445 nm for B-CDs to 620 nm for R-CDs, and therefore covered the entire visible spectrum. The absolute quantum yields for the B-, G-, Y-, and R-CDs were 27.3%, 31.1%, 22.9%, and 8.8%, respectively. Characterization of the CDs showed that the differences among the optical features of the four prepared CDs arise from the differences among the surface states and nitrogen-derived structures in the carbon core. The four prepared CDs all showed low toxicity and steady PL, and therefore have potential applications in both in vitro and in vivo imaging.

The synthesis and bioimaging of multicolor carbon dots from citric acid and urea.

Synthesis of Nitrogen-Doped Lignin/DES Carbon Quantum Dots as a Fluorescent Probe for the Detection of Fe3+ Ions

Carbon quantum dots (CQDs) as a rising star of carbon nanomaterials have extensive applications due to their excellent characteristics. In this work, we introduce a simple and green method to prepare nitrogen-doped lignin carbon quantum dots (N-L-CQDs) by using alkali lignin carbon sources and deep eutectic solvent (DES) as solution and nitrogen source. The physiochemical characterization results suggested that N-L-CQDs with diameters ranging from 4 to 12 nm were successfully synthesized. The optical properties data indicated that the as-prepared N-L-CQDs with a quantum yield of 7.95% exhibited excellent optoelectronic properties, excitation-dependent and pH stability. After that, we have investigated the N-L-CQDs used as fluorescent probes to detect iron ions, which suggested that the as-prepared N-L-CQDs exhibited excellent sensitivity and selectivity for Fe³⁺ with a detection limit of 0.44 μM. Besides, cytotoxicity of N-L-CQDs was also evaluated by MTT assay. These results demonstrated that the as-prepared N-L-CQDs with excellent properties have potential applications in environment and biomedicine.

Fabrication and photoluminescent properties of Tb3+ doped carbon nanodots

Carbon nanodots (CNDs) doped with Tb ions were synthesized using different synthetic routes: hydrothermal treatment of a solution containing carbon source (sodium dextran sulfate) and TbCl3; mixing of CNDs and TbCl3 solutions; freezing-induced loading of Tb and carbon-containing source into pores of CaCO3 microparticles followed by hydrothermal treatment. Binding of Tb ions to CNDs (Tb-CND coupling) was confirmed using size-exclusion chromatography and manifested itself through a decrease of the Tb photoluminescence lifetime signal. The shortest Tb photoluminescence lifetime was observed for samples obtained by hydrothermal synthesis of CaCO3 microparticles where Tb and carbon source were loaded into pores via the freezing-induced process. The same system displays an increase of Tb photoluminescence via energy transfer with excitation at 320-340 nm. Based on the obtained results, freezing-induced loading of cations into CNDs using porous CaCO3 microparticles as reactors is proposed to be a versatile route for the introduction of active components into CNDs. The obtained CNDs with long-lived emission may be used for time-resolved imaging and visualization in living biological samples where time-resolved and long-lived luminescence microscopy is required.

A chiral responsive carbon dots-gold nanoparticle complex mediated by hydrogen peroxide independent of surface modification with chiral ligands

Chiral recognition of enantiomers is fundamentally important. In this study, a novel strategy for the chiral discrimination of glucose enantiomers was constructed based on the hydrogen peroxide (H₂O₂)-mediated generation of a carbon dots-gold nanoparticle (C-dots@Au NP) complex independent of surface modification with chiral ligands. H₂O₂ is essential as a reductant to promote the growth of Au NPs from gold salts. Besides, the modification of C-dots with sulfhydryl groups is necessary for its anchoring on the surface of Au NPs. Therefore, in the presence of H₂O₂, the C-dots@Au NP complex can be self-generated from a simple mixture containing C-dots and Au salts. It is worth noting that glucose oxidase can selectively catalyze d-glucose but not l-glucose to generate H₂O₂. In this regard, the chiral recognition process can trigger the formation of the C-dots@Au NP complex. Furthermore, based on the production of reddish Au NPs and the reduction of C-dot fluorescence quenched by Au NPs, the resultant C-dots@Au NP complex enables achieving the chiral discrimination of glucose enantiomers by combining colorimetric and fluorometric assays. Compared with the conventional approaches that use chiral ligands to decorate NPs, the generation of the chiral-responsive C-dots@Au NP complex is much simpler and faster. Upon combination with specific enzymatic reactions that produce a reductive product, the current strategy provides a general approach for the identification of chiral enantiomers.

Biogreen Synthesis of Carbon Dots for Biotechnology and Nanomedicine Applications

Over the past decade, carbon dots have ignited a burst of interest in many different fields, including nanomedicine, solar energy, optoelectronics, energy storage, and sensing applications, owing to their excellent photoluminescence properties and the easiness to modify their optical properties through doping and functionalization. In this review, the synthesis, structural and optical properties, as well as photoluminescence mechanisms of carbon dots are first reviewed and summarized. Then, we describe a series of designs for carbon dot-based sensors and the different sensing mechanisms associated with them. Thereafter, we elaborate on recent research advances on carbon dot-based sensors for the selective and sensitive detection of a wide range of analytes, including heavy metals, cations, anions, biomolecules, biomarkers, nitroaromatic explosives, pollutants, vitamins, and drugs. Lastly, we provide a concluding perspective on the overall status, challenges, and future directions for the use of carbon dots in real-life sensing.