Cyano-determined mercury (II) ion selective fluorescence assay over polymeric carbon nitride

Selective response is the key index to evaluate the performance of polymeric carbon nitride (PCN)-based heavy metal ion fluorescence sensors. Herein, to explore the role of cyano groups on selectivity, four kinds of PCN, including PCN-Cl, PCN-Ac, PCN-B and PCN-K were prepared by the molten salt method of sodium chloride and sodium acetate, the reduction method of sodium borohydride and the etching method of potassium hydroxide, respectively. These PCNs exhibited different surface cyano characteristics, but all of them had significant blue emission under ultraviolet excitation. It is proved that the assistant of sodium chloride or potassium hydroxide is an effective method to prepare PCNs with abundant surface cyano group. A series of fluorescence quenching experiments of metal ions showed that the cyano-rich degree of PCN is closely related to its selective response to mercury (II) ions. PCN-Cl and PCN-K emerged good selective quenching of mercury (II) ions, which may be related to the soft acid-soft base strong interaction between mercury (II) ions and cyano groups. Both PCN-Cl and PCN-K fluorescent probes for mercury (II) ions had a linear range of 5 ∼ 50 μmol L-1, and PCN-Cl exhibited a lower detection limit of 0.38 μmol L-1. This work confirmed the selective fluorescence response of cyano-rich PCN to mercury (II) ions, proposed the mechanism of selective fluorescence quenching response of mercury (II) ions, and provided a new idea for the design of efficient and accurate PCN-based fluorescence probes.

Electrocatalytic efficiency of carbon nitride supported gold nanoparticle based sensor for iodide and cysteine detection

Extensive investigations are being conducted on gold nanoparticles focusing on their applications in biosensors, laser phototherapy, targeted drug delivery and bioimaging utilizing advanced detection techniques. In this work, an electrochemical sensor was developed based on graphite carbon nitride supported gold nanoparticles. Carbon nitride supported gold nanoparticles (Au-CN) was synthesized by applying a deposition-precipitation route followed by a chemical reduction technique. The composite system was characterized by X-ray diffraction and X-ray photo electron spectroscopy methods. Electron microscopy analysis confirmed the formation of gold nanoparticles within the size range of 5-15 nm on the carbon nitride support. Carbon nitride supported gold based sensor was employed for the electrochemical detection of iodide ion and l-cysteine. The limit of detection and sensitivity of the sensor was attained 8.9 μM and 0.96 μAμM⁻1cm⁻2, respectively, for iodide ion, while 0.48 μM and 5.8 μAμM⁻1cm⁻2, respectively, was achieved for the recognition of cysteine. Furthermore, a paper-based electrochemical device was developed using the Au-CN hybrid system that exhibited promising results in detecting iodide ions, highlighting its potential for economic and portable device applications.

Progress and obstacles in employing carbon quantum dots for sustainable wastewater treatment

We explored the potential of carbon quantum dots (CQDs) as novel materials for wastewater treatment and their role towards environmental sustainability. The advantages of CQDs over other carbon-based materials, when synthesized using the same precursor material and for the same contaminant are discussed, enabling future researchers to choose the appropriate material. CQDs have demonstrated exceptional adaptability in various wastewater treatment, acting as efficient adsorbents for contaminants, exhibiting excellent photocatalytic properties for degradation of organic pollutants, and functioning as highly sensitive sensors for water quality monitoring. We found that bottom-up approach has better control over particle size (resulting CQDs: 1-4 nm), whereas top-down synthesis approach (resulting CQDs: 2-10 nm) have more potential for large scale applications and tunability. Transmission electron microscopy (TEM) remains the most expensive characterization technique, which provides the best resolution of the CQD's surface. The study emphasizes on the environmental impact and safety considerations pertaining to CQDs by emphasizing the need for thorough toxicity evaluation, and necessary environmental precautions. The study also identifies the lacunae pertaining to critical challenges in practical implementation of CQDs, such as scalability, competition of co-existing contaminants, and stability. Finally, future research directions are proposed, advocating green synthesis approaches, tailored surface functionalization, and, lowering the overall cost for analysis, synthesis and application of CQDs.

Microenvironment Control over Electrocatalytic Activity of g-C3N4/2H-MoS2 Superlattice-like Heterostructures for Hydrogen Evolution

Microenvironments in heterogeneous catalysis have been recognized as equally important as the types and amounts of active sites for regulating catalytic activity. Two-dimensional (2D) nanospaces between van der Waals (vdW) gaps of layered materials provide an ideal microenvironment to create novel functionalities. Here, we explore a facile method for fabricating g-C3N4/2H-MoS2 superlattice-like heterostructures based on thermochemical intercalation and polymerization reactions of formamide within enlarged vdW gaps of 2H-MoS2 nanosheets without any transfer process. DFT calculations demonstrate that the interlayer electron-electron correlations due to the intercalation effect of g-C3N4, rather than high-κ dielectric environments, lead to the improvement of intrinsic conductivity of 2H-MoS2 nanosheets. As the proof of concept in applications for the electrocatalysis field, the heterostructure for hydrogen electrochemical reaction (HER) exhibits high stability and catalytic activity in both acid and alkaline media, such as a quite low onset overpotential of 98 mV, a high exchange current density of 77.6 μA cm-2, and a small Tafel slope (52.9 mV dec-1) in an acid medium. The enhanced HER activity is attributed to the improved conductivity and nanoconfinement effect of 2D nanospaces that decrease the reaction activation energy and activate the inert basal planes.

Graphitic carbon nitride-based electrochemical sensors: A comprehensive review of their synthesis, characterization, and applications

Graphitic carbon nitride (g-C3N4) has garnered much attention as a promising 2D material in the realm of electrochemical sensors. It contains a polymeric matrix that can serve as an economical and non-toxic electrode material for the detection of a diverse range of analytes. However, its performance is impeded by a relatively limited active surface area and inherent instability. Although electrochemistry involving metal-doped g-C3N4 nanomaterials is rapidly progressing, it remains relatively unexplored. The metal doping of g-C3N4 augments the electrochemically active surface area of the resulting electrode, which has the potential to significantly enhance electrode kinetics and bolster catalytic activity. Consequentially, the main objective of this review is to provide insight into the intricacies of synthesizing and characterizing metal-doped g-C3N4. Furthermore, we comprehensively delve into the fundamental attributes of electrochemical sensors based on metal-doped g-C3N4, with a specific focus on healthcare and environmental applications. These applications encompass a meticulous exploration of detecting biomolecules, drug molecules, and organic pollutants.

Hydrogen spillover inspired bifunctional Platinum/Rhodium Oxide-Nitrogen-Doped carbon composite for enhanced hydrogen evolution and oxidation reactions in base

The poor activity of Pt-based-catalysts for alkaline hydrogen oxidation/evolution reaction (HOR/HER) encourages scientific society to design an effective electrocatalyst to develop alkaline fuel cells/electrolyzers. Herein, platinum/rhodium oxide-nitrogen-doped carbon (Pt/Rh2O3-CNx) composite is prepared for alkaline HER and HOR inspired by hydrogen spillover. The HER performance of Pt/Rh2O3-CNx is ∼ 6 times higher than Pt/C. In HOR, Pt/Rh2O3-CNx possesses an exchange current density of 657.60 mA/mgmetal, which is ∼ 3.4 times higher than Pt/C. Hydrogen and hydroxyl binding energy (HBE and OHBE) contribute equally to alkaline HOR/HER. The experimental and theoretical evidence suggests that the enhanced HER and HOR activity of Pt/Rh2O3-CNx may be due to hydrogen spillover from Pt to Rh2O3. Small work function difference [0.08 eV] of the system suggested hydrogen-spillover is feasible, which has been justified by reaction-free energy calculations. We proposed that the dissociation of hydrogen (H2) and water (H2O) occurs at Pt to form Pt-adsorbed hydrogen species (Pt-Had). Then, some Had moves to Rh2O3 through hydrogen spillover and reacts with neighboring Had or adsorbed hydroxyl species (OHad) to form H2 or H2O, which enhances the HER and HOR activity, respectively. The role of water-metal-hydroxyl species in the electrical double layer was also demonstrated on alkaline HOR/HER. This work may help to design the hydrogen-spillover-based catalysts for several renewable energy technologies.

A Smartphone-Based Sensing for Portable and Sensitive Visual Detection of Hg (II) via Nitrogen Doped Carbon Quantum Dots Modified Paper Strip

The development of portable and cost-effective sensing system for Hg2+ quantitation is highly demanded for environmental monitoring. Herein, an on-site, rapid and portable smartphone readout device based Hg2+ sensing system integrating nitrogen-doped carbon quantum dots (NCDs) modified paper strip was proposed, and the physicochemical properties of NCDs were characterized by high resolution TEM, FTIR, UV-vis absorption spectrum and fluorescence spectral analysis. The modified paper strip was prepared via "ink-jet" printing technology and exhibits sensitive fluorescence response to Hg2+ with fluorescence color of bright blue (at the excitation/emission wavelength of 365/440 nm). This portable smartphone-based sensing platform is highly selective and sensitive to Hg2+ with the limit of detection (LOD) of 10.6 nM and the concentration range of 0-130 nM. In addition, the recoveries of tap water and local lake water were in the range of 89.4% to 109%. The cost-effective sensing system based on smartphone shows a great potential for trace amounts of Hg2+ monitoring in environmental water samples.

Oxygen-doped Carbon Nitrides with Visible Room-temperature Phosphorescence and Invisible Thermal-Stimuli-Responsive Ultraviolet Delayed Fluorescence for Security Applications

Multi-mode emissive materials with stimuli-responsive producing invisible signals are very attractive for advanced security applications, but development of such materials remains highly challenging. In this work, oxygen-doped carbon nitrides (O-CNs) are prepared via microwave-assisted heating of urea, which exhibit ultraviolet (UV) solid-state fluorescence (SSFL), visible room temperature phosphorescence (RTP) and thermal-stimuli production of invisible UV delayed fluorescence (DF) properties. Further studies confirmed that the SSFL and RTP could be attributed to the introduction of oxygen functional group (e. g., C=O) in the skeleton of O-CNs, thus minimizing the aggregation caused quenching effect, facilitating intersystem crossing, and stabilizing the excited triplet states. The specific thermal-stimuli production of UV DF is deemed to be the relatively large energy gap between ground and excited singlet states as well as an effective triplet-triplet annihilation. Notably, the emission maximum of UV DF locates at ~310 nm with an ultra-narrow full width at half maximum (FWHM) down to 19 nm, so it is completely invisible to the naked eyes, but detectable by a UV camera. To employ the unique characteristics of O-CNs, security protection strategies with superior concealment by virtue of the thermal-stimuli quenching visible RTP and meanwhile producing invisible UV DF are demonstrated.

Graphitic Carbon Nitride Quantum Dots (g-C3N4 QDs): From Chemistry to Applications

Since their emergence in 2014, graphitic carbon nitride quantum dots (g-C3N4 QDs) have attracted much interest from the scientific community due to their distinctive physicochemical features, including structural, morphological, electrochemical, and optoelectronic properties. Owing to their desirable characteristics, such as non-zero band gap, ability to be chemically functionalized or doped, possessing tunable properties, outstanding dispersibility in different media, and biocompatibility, g-C3N4 QDs have shown promise for photocatalysis, energy devices, sensing, bioimaging, solar cells, optoelectronics, among other applications. As these fields are rapidly evolving, it is very strenuous to pinpoint the emerging challenges of the g-C3N4 QDs development and application during the last decade, mainly due to the lack of critical reviews of the innovations in the g-C3N4 QDs synthesis pathways and domains of application. Herein, an extensive survey is conducted on the g-C3N4 QDs synthesis, characterization, and applications. Scenarios for the future development of g-C3N4 QDs and their potential applications are highlighted and discussed in detail. The provided critical section suggests a myriad of opportunities for g-C3N4 QDs, especially for their synthesis and functionalization, where a combination of eco-friendly/single step synthesis and chemical modification may be used to prepare g-C3N4 QDs with, for example, enhanced photoluminescence and production yields.

Fluorescent Carbon Nitride Nanoparticles for Picric Acid Sensing

Detection of nitroaromatic explosives is essential in the area of environmental safety. Fluorescent carbon nitride nanoparticles is a promising material for this purpose. Herein, we have prepared fluorescent carbon nitride nanoparticles (CNNPs) by one step thermal treatment of formamide. These fluorescent CNNPs is sensitive towards picric acid (PA) than other analytes both in aqueous medium and on test paper which is witnessed by fluorescence quenching based on inner filter effect (IFE). The PA detection with the fluorescent CNNPs is observed in the concentration ranges, 0 µM to 60 µM with linear range of 10 nM to 25 µM. The minimum detection limit in aqueous medium and solid phase are determined to be 26.20 nM and 10 µM respectively. Finally, the fluorescent CNNPs is applied for detection of PA in real water samples. The recoveries are in the ranges from 99.54 to 116.35% with relative standard deviation less than 3.85%. This proposed fluorescent method can act as suitable analytical technique to monitored PA concentration in water samples.

Ultrasensitive detection of Ag+and Ce3+ions using highly fluorescent carboxyl-functionalized carbon nitride nanoparticles

The fluorescence quenching of carboxyl-rich g-C3N4nanoparticles was found to be selective to Ag+and Ce3+with a limit of detection as low as 30 pM for Ag+ions. A solid-state thermal polycondensation reaction was used to produce g-C3N4nanoparticles with distinct green fluorescence and high water solubility. Dynamic light scattering indicated an average nanoparticle size of 95 nm. The photoluminescence absorption and emission maxima were centered at 405 nm and 540 nm respectively which resulted in a large Stokes shift. Among different metal ion species, the carboxyl-rich g-C3N4nanoparticles were selective to Ag+and Ce3+ions, as indicated by strong fluorescence quenching and a change in the fluorescence lifetime. The PL sensing of heavy metal ions followed modified Stern-Volmer kinetics, and CNNPs in the presence of Ag+/Ce3+resulted in a higher value ofKapp(8.9 × 104M-1) indicating a more efficient quenching process and stronger interaction between CNNP and mixed ions. Sensing was also demonstrated using commercial filter paper functionalized with g-C3N4nanoparticles, enabling practical on-site applications.

Vacuum-Processed Propylene Urea Additive: A Novel Approach for Controlling the Growth of CH3NH3PbI3 Crystals in All Vacuum-Processed Perovskite Solar Cells

In this study, we present a novel method for controlling the growth of perovskite crystals in the vacuum thermal evaporation process by utilizing a vacuum-processable additive, propylene urea (PU). By coevaporation of perovskite precursors with PU to form the perovskite layer, PU, acting as a Lewis base additive, retards the direct reaction between the perovskite precursors. This facilitates a larger domain size and reduced defect density. Following the removal of the residual additive, the perovskite layer, exhibiting improved crystallinity, demonstrates reduced charge recombination, as confirmed by a time-resolved microwave conductivity analysis. Consequently, there is a notable enhancement in open-circuit voltage and power conversion efficiency, increasing from 1.05 to 1.15 V and from 17.17 to 18.31%, respectively. The incorporation of a vacuum-processable and removable Lewis base additive into the fabrication of vacuum-processed perovskite solar cells offers new avenues for optimizing these devices.

Carbon Quantum Dots: Properties, Preparation, and Applications

Carbon quantum dots are a novel form of carbon material. They offer numerous benefits including particle size adjustability, light resistance, ease of functionalization, low toxicity, excellent biocompatibility, and high-water solubility, as well as their easy accessibility of raw materials. Carbon quantum dots have been widely used in various fields. The preparation methods employed are predominantly top-down methods such as arc discharge, laser ablation, electrochemical and chemical oxidation, as well as bottom-up methods such as templates, microwave, and hydrothermal techniques. This article provides an overview of the properties, preparation methods, raw materials for preparation, and the heteroatom doping of carbon quantum dots, and it summarizes the applications in related fields, such as optoelectronics, bioimaging, drug delivery, cancer therapy, sensors, and environmental remediation. Finally, currently encountered issues of carbon quantum dots are presented. The latest research progress in synthesis and application, as well as the challenges outlined in this review, can help and encourage future research on carbon quantum dots.

Recent progress and prospect of graphitic carbon nitride-based photocatalytic materials for inactivation of Microcystis aeruginosa

The proliferation of harmful algal blooms is a global concern due to the risk they pose to the environment and human health. Algal toxins which are hazardous compounds produced by dangerous algae, can potentially kill humans. Researchers have been drawn to photocatalysis because of its clean and energy-saving properties. Graphite carbon nitride (g-C3N4) photocatalysts have been extensively studied for their ability to eliminate algae. These photocatalysts have attracted notice because of their cost-effectiveness, appropriate electronic structure, and exceptional chemical stability. This paper reviews the progress of photocatalytic inactivation of harmful algae by g-C3N4-based materials in recent years. A brief overview is given of a number of the modification techniques on g-C3N4-based photocatalytic materials, as well as the process of inactivating algal cells and destroying their toxins. Additionally, it provides a theoretical framework for future research on the eradication of algae using g-C3N4-based photocatalytic materials.

Carbon Dots and Their Polymeric Nanocomposites: Insight into Their Synthesis, Photoluminescence Mechanisms, and Recent Trends in Sensing Applications

Carbon dots (CDs), a novel class of carbon-based nanoparticles, have received a lot of interest recently due to their exceptional mechanical, chemical, and fluorescent properties, as well as their excellent photostability and biocompatibility. CDs' emission properties have already found a variety of potential applications, in which bioimaging and sensing are major highlights. It is widely acknowledged that CDs' fluorescence and surface conditions are closely linked. However, due to the structural complexity of CDs, the specific underlying process of their fluorescence is uncertain and yet to be explained. Because of their low toxicity, robust and wide optical absorption, high chemical stability, rapid transfer characteristics, and ease of modification, CDs have been recognized as promising carbon nanomaterials for a variety of sensing applications. Thus, following such outstanding properties of CDs, they have been mixed and imprinted onto different polymeric components to achieve a highly efficient nanocomposite with improved functional groups and properties. Here, in this review, various approaches and techniques for the preparation of polymer/CDs nanocomposites have been elaborated along with the individual characteristics of CDs. CDs/polymer nanocomposites recently have been highly demanded for sensor applications. The insights from this review are detailed sensor applications of polymer/CDs nanocomposites especially for detection of different chemical and biological analytes such as metal ions, small organic molecules, and several contaminants.

Opposite regulation effects of Al3+ on different types of carbon quantum dots and potential applications in information encryption

Regulating the photoluminescence (PL) of carbon quantum dots (CQDs) through ion modification is a well-established and effective approach. Herein, we report the opposite regulation effects of Al3+ ions on the PL properties of two distinct types of CQDs (graphene quantum dots, GQDs, and nitrogen-doped carbon quantum dots of 2,3-diaminophenazine, DAP), and elucidate the underlying mechanism of the binding of Al3+ ions to different PL sites on CQDs by employing ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Specifically, Al3+ ions are primarily situated around the oxygen-containing groups, which do not impact the π-π regions of GQDs. However, Al3+ ions are preferentially adsorbed on the top of pyridine nitrogen in the phenazine rings of DAP, thus reducing the PL regions of DAP. Based on the opposite PL effects of Al3+ on GQDs and DAP, we explore potential applications of information encryption and successfully realize multi-level information encryption and decryption, which may provide new strategies for CQDs in information security.

Graphitic Carbon Nitride/Zinc Oxide-Based Z-Scheme and S-Scheme Heterojunction Photocatalysts for the Photodegradation of Organic Pollutants

Graphitic carbon nitride (g-C3N4), a metal-free polymer semiconductor, has been recognized as an attractive photocatalytic material for environmental remediation because of its low band gap, high thermal and photostability, chemical inertness, non-toxicity, low cost, biocompatibility, and optical and electrical efficiency. However, g-C3N4 has been reported to suffer from many difficulties in photocatalytic applications, such as a low specific surface area, inadequate visible-light utilization, and a high charge recombination rate. To overcome these difficulties, the formation of g-C3N4 heterojunctions by coupling with metal oxides has triggered tremendous interest in recent years. In this regard, zinc oxide (ZnO) is being largely explored as a self-driven semiconductor photocatalyst to form heterojunctions with g-C3N4, as ZnO possesses unique and fascinating properties, including high quantum efficiency, high electron mobility, cost-effectiveness, environmental friendliness, and a simple synthetic procedure. The synergistic effect of its properties, such as adsorption and photogenerated charge separation, was found to enhance the photocatalytic activity of heterojunctions. Hence, this review aims to compile the strategies for fabricating g-C3N4/ZnO-based Z-scheme and S-scheme heterojunction photocatalytic systems with enhanced performance and overall stability for the photodegradation of organic pollutants. Furthermore, with reference to the reported system, the photocatalytic mechanism of g-C3N4/ZnO-based heterojunction photocatalysts and their charge-transfer pathways on the interface surface are highlighted.

Graphitic carbon nitride derived probes for the recognition of heavy metal pollutants of environmental concern in water bodies

Graphitic carbon nitride (g-CN) has a number of valuable features that have been recognized during the studies related to its photocatalytic activity enhancement derived by visible light. Because of these characteristics, g-CN can be used as a detecting signal transducer with different transmission modalities. The latest up-to-date detection capabilities of modified g-CN nanoarchitectures are covered in this study. The structural features and synthetic methodologies have been discussed in a number of reports. Herein, employment of the g-CN as a promising probing modality for the recognition of different toxic heavy metals is the promising feature of the present study.

Graphitic carbon nitride (g-C3N4)-assisted materials for the detection and remediation of hazardous gases and VOCs

Graphitic carbon nitride (g-C3N4)-based materials are attracting attention for their unique properties, such as low-cost, chemical stability, facile synthesis, adjustable electronic structure, and optical properties. These facilitate the use of g-C3N4 to design better photocatalytic and sensing materials. Environmental pollution by hazardous gases and volatile organic compounds (VOCs) can be monitored and controlled using eco-friendly g-C3N4- photocatalysts. Firstly, this review introduces the structure, optical and electronic properties of C3N4 and C3N4 assisted materials, followed by various synthesis strategies. In continuation, binary and ternary nanocomposites of C3N4 with metal oxides, sulfides, noble metals, and graphene are elaborated. g-C3N4/metal oxide composites exhibited better charge separation that leads to enhancement in photocatalytic properties. g-C3N4/noble metal composites possess higher photocatalytic activities due to the surface plasmon effects of metals. Ternary composites by the presence of dual heterojunctions improve properties of g-C3N4 for enhanced photocatalytic application. In the later part, we have summarised the application of g-C3N4 and its assisted materials for sensing toxic gases and VOCs and decontaminating NOx and VOCs by photocatalysis. Composites of g-C3N4 with metal and metal oxide give comparatively better results. This review is expected to bring a new sketch for developing g-C3N4-based photocatalysts and sensors with practical applications.

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.

Insights into engineered graphitic carbon nitride quantum dots for hazardous contaminants degradation in wastewater

Increased environmental pollution is a critical issue that must be addressed. Photocatalytic, adsorption, and membrane filtration methods are suitable in environmental governance because of their high selectivity, low cost, environment-friendly nature, and excellent treatment efficiency. Graphitic carbon nitride (g-C₃N₄) quantum dots (QDs) have been considered as photocatalysts, adsorbents, and membrane materials for wastewater treatments, owing to their stability, adsorption capacity, photochemical properties, and low toxicity and cost. This review summarizes g-C₃N₄ QD synthesis techniques, operating parameters affecting the removal performance in the treatment process, modification effects with other semiconductors, and benefits and drawbacks of g-C₃N₄ QD-based materials. Furthermore, this review discusses the practical applications of g-C₃N₄ QDs as adsorbents, photocatalysts, and membrane materials for organic and inorganic contaminant treatments and their value-added product formation potential. Modified g-C₃N₄ QD-based material adsorbents, photocatalysts, and membranes present potentially applicable effects, such as removal of most waterborne contaminants. Excellent results were obtained for the reduction of methyl orange, bisphenol A, tetracycline, ciprofloxacin, phenol, rhodamine B, E. coli, and Hg. Overall, this paper provides comprehensive background on g-C₃N₄ QD-based materials and their diverse applications in wastewater treatment, and it presents a foundation for the enhancement of similar unique materials in the future.

Carbon-Based Stimuli-Responsive Nanomaterials: Classification and Application

Carbon-based nanomaterials, including carbon nanotubes, carbon nanospheres, and carbon nanofibers, are becoming a research hotspot due to their unique structure and good mechanical, thermal, electrical, optical, and chemical properties. With the development of material synthesis technology, they can be functionalized and used in various fields such as energy, environment, and biomedicine. In particular, stimuli-responsive carbon-based nanomaterials have stood out in recent years because of their smart behavior. Researchers have applied carbon-based nanomaterials to different disease treatments based on their stimulus-response properties. In this paper, based on stimuli-responsive carbon-based nanomaterials' morphology, we categorize them into carbon nanotubes, carbon nanospheres, and carbon nanofibers according to their morphology. Then, their applications in probes, bioimaging, tumor therapy, and other fields are discussed. Finally, we address the advantages and disadvantages of carbon-based stimuli-responsive nanomaterials and discuss their future perspective.

Emerging Trends of Carbon-Based Quantum Dots: Nanoarchitectonics and Applications

Carbon-based quantum dots (QDs) have emerged as a fascinating class of advanced materials with a unique combination of optoelectronic, biocompatible, and catalytic characteristics, apt for a plethora of applications ranging from electronic to photoelectrochemical devices. Recent research works have established carbon-based QDs for those frontline applications through improvements in materials design, processing, and device stability. This review broadly presents the recent progress in the synthesis of carbon-based QDs, including carbon QDs, graphene QDs, graphitic carbon nitride QDs and their heterostructures, as well as their salient applications. The synthesis methods of carbon-based QDs are first introduced, followed by an extensive discussion of the dependence of the device performance on the intrinsic properties and nanostructures of carbon-based QDs, aiming to present the general strategies for device designing with optimal performance. Furthermore, diverse applications of carbon-based QDs are presented, with an emphasis on the relationship between band alignment, charge transfer, and performance improvement. Among the applications discussed in this review, much focus is given to photo and electrocatalytic, energy storage and conversion, and bioapplications, which pose a grand challenge for rational materials and device designs. Finally, a summary is presented, and existing challenges and future directions are elaborated.

Employing functionalized graphene quantum dots to combat coronavirus and enterovirus

The ongoing COVID-19 (i.e., coronavirus) pandemic continues to adversely affect the human life, economy, and the world's ecosystem. Although significant progress has been made in developing antiviral materials for the coronavirus, much more work is still needed. In this work, N-functionalized graphene quantum dots (GQDs) were designed and synthesized as the antiviral nanomaterial for Feline Coronavirus NTU156 (FCoV NTU156) and Enterovirus 71 (EV71)) with ultra-high inhibition (>99.9%). To prepare the GQD samples, a unique solid-phase microwave-assisted technique was developed and the cell toxicity was established on the H171 and H184 cell lines after 72 h incubation, indicating superior biocompatibility. The surface functionality of GQDs (i.e., the phenolic and amino groups) plays a vital role in interacting with the receptor-binding-domain of the spike protein. It was also found that the addition of polyethylene glycol is advantageous for the dispersion and the adsorption of functionalized GQDs onto the virus surface, leading to an enhanced virus inhibition. The functionality of as-prepared GQD nanomaterials was further confirmed where a functionalized GQD-coated glass was shown to be extremely effective in hindering the virus spread for a relatively long period (>20 h).

Synthesis of N-doped carbon dots for highly selective and sensitive detection of metronidazole in real samples and its cytotoxicity studies

The current investigation reports the synthesis of N-CDs using glucosamine, ascorbic acid, and ethylenediamine precursors by a simple hydrothermal technique. The formation of N-CDs was proved by various characterisation techniques such as X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Fourier-Transform Infrared Spectrophotometer (FT-IR). The optical properties were investigated by fluorescence and UV-vis spectrophotometer. Also, N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. However, the metronidazole serves as an antibiotic against several microbial diseases but also a genotoxic, carcinogenic to the human when used in excessive dosage. The synthesised N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. Besides, the cytotoxicity of the N-CDs was studied to evaluate its toxicity against the HeLa cancer cells. It showed 65.6% cell viability and 34.3% toxicity against the cancerous cells, and similarly 71% of cells viability against H9C2 cells. Thus, the current investigation explores the promising selective sensing of N-CDs against MTZ, along with that, it proved its cytotoxicity against HeLa cancerous cells and non-toxicity against H9C2 cells. The synthesised CDs can be better MTZ sensors and anti-cancer agents on further development at the industrial scale.

State-of-the-art developments in carbon quantum dots (CQDs): Photo-catalysis, bio-imaging, and bio-sensing applications

Carbon quantum dots (CQDs), the intensifying nanostructured form of carbon material, have exhibited incredible impetus in several research fields such as bio-imaging, bio-sensing, drug delivery systems, optoelectronics, photovoltaics, and photocatalysis, thanks to their exceptional properties. The CQDs show extensive photonic and electronic properties, as well as their light-collecting, tunable photoluminescence, remarkable up-converted photoluminescence, and photo-induced transfer of electrons were widely studied. These properties have great advantages in a variety of visible-light-induced catalytic applications for the purpose of fully utilizing the energy from the solar spectrum. The major purpose of this review is to validate current improvements in the fabrication of CQDs, characteristics, and visible-light-induced catalytic applications, with a focus on CQDs multiple functions in photo-redox processes. We also examine the problems and future directions of CQD-based nanostructured materials in this growing research field, with an eye toward establishing a decisive role for CQDs in photocatalysis, bio-imaging, and bio-sensing applications that are enormously effective and stable over time. In the end, a look forward to future developments is presented, with a view to overcoming challenges and encouraging further research into this promising field.

Gram-Scale Synthesis of Graphitic Carbon Nitride Quantum Dots with Ultraviolet Photoluminescence for Fe3+ Ion Detection

A method for gram-scale synthesis of graphitic carbon nitride quantum dots (g-C₃N₄QDs) was developed. The weight of the g-C₃N₄QDs was up to 1.32 g in each run with a yield of 66 wt%, and the purity was 99.96 wt%. The results showed that g-C₃N₄QDs exhibit a stable and strong ultraviolet photoluminescence at a wavelength of 365 nm. More interestingly, the g-C₃N₄QDs can be used as a high-efficiency, sensitive, and selective fluorescent probe to detect Fe³⁺ with a detection limit of 0.259 μM.

Porphyrin-Based Nanoparticles: A Promising Phototherapy Platform

Phototherapy, including photodynamic therapy and photothermal therapy, is an emerging form of non-invasive treatment. The combination of imaging technology and phototherapy is becoming an attractive development in the treatment of cancer, as it allows for highly effective therapeutic results through image-guided phototherapy. Porphyrins have attracted significant interest in the treatment and diagnosis of cancer due to their excellent phototherapeutic effects in phototherapy and their remarkable imaging capabilities in fluorescence imaging, magnetic resonance imaging and photoacoustic imaging. However, porphyrins suffer from poor water solubility, low near-infrared absorption and insufficient tumor accumulation. The development of nanotechnology provides an effective way to improve the bioavailability, phototherapeutic effect and imaging capability of porphyrins. This review highlights the research results of porphyrin-based small molecule nanoparticles in phototherapy and image-guided phototherapy in the last decade and discusses the challenges and directions for the development of porphyrin-based small molecule nanoparticles in phototherapy.

Metal ion sensing with graphene quantum dots: detection of harmful contaminants and biorelevant species

Graphene quantum dots (GQDs) are attractive materials for use as highly selective and sensitive chemical sensors, owing to their simple preparation and affordability. GQDs have been successfully deployed as sensors for toxic metal ions, which is a significant issue due to the ever-increasing environmental contamination from agricultural and industrial activities. Despite the success of GQDs in this area, the mechanisms which underpin GQD-metal ion specificity are rarely explored. This lack of information can result in difficulties when attempting to replicate published procedures and can limit the judicious design of new highly selective GQD sensors. Furthermore, there is a dearth of GQD examples which selectively detect biologically relevant alkali and alkaline earth metals. This review will present the current state of GQDs as metal ion sensors for harmful contaminants, highlighting and discussing the discrepancies that exist in the proposed mechanisms regarding metal ion selectivity. The emerging field of GQD sensors for biorelevant metal ion species will also be reviewed, with a perspective to the future of this highly versatile material.

Carbon dots-modified paper-based chemiluminescence device for rapid determination of mercury (II) in cosmetics

Herein, a simple and portable paper-based analytical device (PAD) based on the inherent capability of carbon quantum dots (CQDs) to serve as a great emitter for the bis(2,4,6-trichlorophenyl) oxalate (TCPO)-hydrogen peroxide (H₂O₂) chemiluminescence (CL) reaction is introduced for the detection of harmful mercury ions (Hg²⁺ ). The energy is transferred from the unstable reaction intermediate (1,2-dioxetanedione) to CQDs, as acceptors, and an intensive orange-red CL emission is generated at about 600 nm, which is equal to the fluorescence emission wavelength of CQDs. The analytical applicability of this system was examined for the determination of Hg²⁺ . It was observed that Hg²⁺ could significantly quench the produced emission, which can be attributed to the formation of a stable and non-luminescent Hg²⁺ -CQDs complex. Accordingly, a simple and rapid PAD was established for monitoring Hg²⁺ , with a limit of detection (LOD) of 0.04 μg mL^-1 . No interfering effect on the signal was found from other examined cations, indicating the acceptable specificity of the method. The designed assay was appropriately utilized to detect the Hg²⁺ in cosmetic samples with high efficiency. It is characterized by its low-cost, easy-to-use, facile but, accurate, and high selective for the detection of Hg²⁺ ions. Besides, the portability of this probe makes it suitable for on-site screening purposes.

A Label-Free Fluorescent Sensor Based on Si,N-Codoped Carbon Quantum Dots with Enhanced Sensitivity for the Determination of Cr(VI)

A signal shut-off probe of Si, N-codoped carbon quantum dots (Si, N-CQDs) was exploited to detect Cr(VI) by fluorescence quenching without the aid of any biomolecules or labeling materials. The sensing system prepared the precursor of diacetone acrylamide and the silane coupling agent 3-aminopropyltriethoxysilane (KH-550) by a simple hydrothermal method, and the quantum yield is as high as 75% Si, N-CQDs. The fluorescence stability and microstructure of the Si, N-CQDs were studied. The Si, N-CQDs has a high sensitivity for detecting Cr(VI) with the linear range of 0–200 μM and the detection limit of 0.995 μM. The quenching mechanism of Si, N-CQDs is attributed to FRET.

A paper-based visualization chip based on nitrogen-doped carbon quantum dots nanoprobe for Hg(Ⅱ) detection

Hg(II) is one of the most toxic heavy metal ions. The bioconcentration and degradation-resistant of Hg(II) bring about serious harm to the ecosystem and humans. Therefore, the establishment of an accurate and effective method for detecting mercury ions is of great significance to environmental protection, food safety and human health. In this work, a new fluorescent nanoprobe was presented using nitrogen-doped carbon quantum dots (N-CQDs) for Hg(II) sensing with high stability and selectivity. On this basis, a paper-based chip was innovatively developed for visualization detection of Hg(II). The N-CQDs were prepared through a one-step hydrothermal reaction using catechol and ethylenediamine as carbon and nitrogen sources, respectively. As-prepared N-CQDs exhibit the strong green fluorescence at the excitation/emission wavelength of 370/511 nm. In aqueous solution, a rapid and highly sensitive detection method of Hg(II) was established by the joint of dynamic and static quenching effect of Hg(II) on N-CQDs fluorescence. Under the optimized conditions, there was a stable correlation between the fluorescence intensity change of N-CQDs and the concentrations of Hg(II) in the range of 15 ∼ 10⁴ nM, and the detection limit was down to 8 nM (S/N = 3). The recoveries of water, sorghum and rice were 91.60 to 102.46%, which was consistent with ICP-MS. More importantly, the N-CQDs nanoprobe was further integrated in nitrocellulose membrane to develop paper-based chip for Hg(II) visualization detection, and the detection performance was also excellent. This strategy had significant implications for achieving low-cost, on-site real-time monitoring of mercury (II) in the environment and food.

Nanostructure Engineering of Graphitic Carbon Nitride for Electrochemical Applications

Graphitic carbon nitride with ordered two-dimensional structure displays multiple properties, including tunable structure, suitable bandgap, high stability, and facile synthesis. Many achievements on this material have been made in photocatalysis, but the advantages have not yet been fully explored in electrochemical fields. The bulk structure with low conductivity impedes charge-transfer kinetics during electrochemical processes. Excessive nitrogen content leads to insufficient charge transfer, while bulk structures produce tortuous channels for mass transport. Some attempts have been made to address these issues by nanostructure engineering, such as ultrathin structure design, heterogeneous composition, defect engineering, and morphology control. These structure-engineered nanomaterials have been successfully applied in electrochemical fields, including ionic actuators, flexible supercapacitors, lithium-ion batteries, and electrochemical sensors. Herein, a timely review on the latest advances in graphitic carbon nitride through various engineering strategies for electrochemical applications has been summarized. A perspective on critical challenges and future research directions is highlighted for graphitic carbon nitride in electrochemistry on the basis of existing research works and our experimental experience.

Pristine Graphic Carbon Nitride Quantum Dots for the Visualized Detection of Latent Fingerprints

User-friendly fingerprint powders, namely efficient, low-cost and nontoxic ones, are always desirable for the development of latent fingerprints (LFPs). Here, we described the use of pristine graphic carbon nitride quantum dots (g-C₃N₄ QDs) as a new kind of user-friendly fingerprint powder. The g-C₃N₄ QDs can be easily prepared from urea and sodium citrate precursors through low temperature solid-phase reaction. Due to their good optical properties and selective interactions with secretion residuals, the g-C₃N₄ QDs powders were exploited to develop LFPs on different substrates by the powder dusting technique. The LFP images on a plastic bag exhibited a high ridge and furrow contrast ratio, allowing for easy identification of level 1 - 3 details of LFPs. This work indicates that the g-C₃N₄ QD powders provide good performance for LFP visualization and is likely to be adopted for some applications in forensic investigations.

Random lasing carbon dot fibers for multilevel anti-counterfeiting

Random lasers, which achieve light amplification through random light scattering in disordered laser gain medium, have found widespread applications in varieties of areas due to the convenient design of laser cavities. In this work, carbon dot fibers with superior fluorescence stability under a high temperature or high humidity environment are fabricated by using a single-step partial thermal decomposition method. Random lasers, which are used for multilevel anti-counterfeiting, are achieved from different types of carbon dot fibers by 370 nm excitation at room temperature. Multilevel anti-counterfeiting is realized by tuning the lasing spectra (between ∼428 and 560 nm) and threshold (between ∼2 and 25 mJ cm^-2) at designated decomposition times and fiber diameters. It is extremely difficult to counterfeit lasing spectra and the specific laser threshold simultaneously. As a result, this will comprehensively improve system safety.

One-Step Synthesis of Green Fluorescent Carbon Dots for Chloride Detecting and for Bioimaging

The chloride ion is an essential ion in organisms, which plays an important role in maintaining normal cell functions. It is involved in many cell activities, such as cell proliferation, cell excitability regulation, immune response, and volume regulation. Accurate detection of the chloride ion can balance its concentration in vivo, which is of great significance. In this study, we developed a green fluorescent carbon quantum dot to detect chloride concentration through the “off–on” mechanism. First, the fluorescence of carbon dots is quenched by the complex of sulfhydryl and silver ions on the surface of carbon dots. Then, the addition of chloride ions pulls away the silver ions and restores the fluorescence. The fluorescence recovery is linearly related to the concentration of chloride ions, and the limit of detection is 2.817 μM, which is much lower than those of other reported chloride probes. Besides, cell and zebrafish experiments confirmed the biosafety and biocompatibility of the carbon dots, which provided a possibility for further applications in bioimaging in vivo.

Carbon Nitride Quantum Dot-Embedded Poly(vinyl alcohol) Transparent Thin Films for Greenish-Yellow Light-Emitting Diodes

Recently, freestanding polymer thin films encapsulated with nanostructures have attracted the significant attention of the scientific community due to their promising application in portable optoelectronic devices. In this research contribution, we have fabricated a freestanding polymer thin film of poly(vinyl alcohol) (PVA) encapsulated with carbon nitride quantum dots (CN-QDs) using the casting method, for the first time. The PVA polymer matrix provides mechanical support as well as dispersion of the CN-QDs preventing its solid-state quenching. From UV-visible spectra, it is revealed that optical transparency decreases with an increase in the concentration of CN-QDs within the PVA polymeric thin film. Such kind of decrease in optical transparency is one of the crucial factors for the optical concert of a nanomaterial. Interestingly, we have optimized the synthesis protocol to retain 40% transparency of the thin film by incorporating 10 wt % CN-QDs along with PVA without deteriorating its optical behavior. It is observed that when CN-QDs are embedded in the PVA matrix, emission becomes independent of excitation wavelength and is localized in the 510-530 nm region of the spectrum. Thus, the films exhibit excellent greenish-yellow emission when excited at 420 nm with the Commission Internationale de l'èclairage (CIE) coordinates (0.39, 0.46) and a correlated color temperature (CCT) of 4105 K. These excellent optoelectronic properties make them a promising candidate for practical phosphor applications. In a nutshell, this study demonstrates a promising way to exhibit the luminescence potential of freestanding polymer/CN-QD films in CN-QD-based solid-state lighting systems.

Recyclable Supramolecular Assembly-Induced Emission System for Selective Detection and Efficient Removal of Mercury(II)

An efficient strategy for simultaneously detecting and removing Hg²⁺ from water is vital to address mercury pollution. Herein a supramolecular assembly G⊂H with photoluminescent properties is facilely constructed through the self-assembly of a functional pillar[5]arene bearing two N,N-dimethyldithiocarbamoyl binding sites (H) and an AIE-active tetraphenylethene derivative (G). Remarkably, the fluorescence of G⊂H can be exclusively quenched by Hg²⁺ among the 30 cations due to the formation of non-luminous ground state complex and only L-cysteine can restore fluorescence in the common 20 amino acids. Meanwhile, the probe G⊂H has a considerable thermal and pH stability, a good anti-interference property from various cations, and a satisfactory sensitivity. More importantly, G⊂H exhibits a prominent capability of Hg²⁺ removal with rapid capture rate (within 1 h) and excellent adsorption efficiency (98 %), as well as a highly efficient recyclability without losing any adsorption activity.

Advanced Inorganic Nitride Nanomaterials for Renewable Energy: A Mini Review of Synthesis Methods

Inorganic nitride nanomaterials have attracted widespread attention for applications in renewable energy due to novel electrochemical activities and high chemical stabilities. For different renewable energy applications, there are many possibilities and uncertainties about the optimal nitride phases and nanostructures, which further promotes the exploration of controllable preparation of nitride nanomaterials. Moreover, unlike conventional nitrides with bulk or ceramic structures, the synthesis of nitride nanomaterials needs more accurate control to guarantee the target nanostructure along with the phase purity, which make the whole synthesis still a challenge to achieve. In this mini review, we mainly summarize the synthesis methods for inorganic nitride nanomaterials, including chemistry vapor deposition, self-propagation high-temperature synthesis, solid state metathesis reactions, solvothermal synthesis, etc. From the perspective of nanostructure, several novel nitrides, with nanostructures like nanoporous, two-dimensional, defects, ternary structures, and quantum dots, are showing unique properties and getting extensive attentions, recently. Prospects of future research in design and synthesis of functional inorganic nitrides are also discussed.