Emerging Graphitic Carbon Nitride-based Nanobiomaterials for Biological Applications

Graphitic carbon nitride (g-CN) based nanostructures are distinctive materials with unique compositional, structural, optical, and electronic properties with exceptional band structure, moderate surface area, and exceptional thermal and chemical stability. Because of these properties, g-CN based nanomaterials have shown promising applications and higher performance in the biological avenue. This review covers the state-of-the-art synthetic strategies used for the preparation of the materials, the basic structure, and a panorama of different optimization strategies leading to improved physicochemical properties responsible for the biological application. The following sections include the recent progress in the use of g-CN based nanobiomaterials for biosensors, bioimaging, photodynamic therapy, drug delivery, chemotherapy, and the antimicrobial segment. Furthermore, we have summarized the role and evaluation of biosafety and biocompatibility of the material. Finally, the unresolved issues, plausible challenges, current status, and future perspectives for the development and design of g-CN have been summarized and are expected to promote a clinical path for the medical sector and human well-being.

Photomemristive sensing via charge storage in 2D carbon nitrides

Photomemristive sensors have the potential to innovate current photo-electrochemical sensors by incorporating new sensing capabilities including non-invasive, wireless and time-delayed (memory) readout. Here we report the charge storing 2D carbon nitride potassium poly(heptazine imide), K-PHI, as a direct photomemristive sensing platform by capitalizing on K-PHI's visible light bandgap, large oxidation potential, and intrinsic optoionic charge storage properties. Utilizing the light-induced charge storage function of K-PHI nanosheets, we demonstrate memory sensing via charge accumulation and present potentiometric, impedimetric and coulometric readouts to write/erase this information from the material, with no additional reagents required. Additionally, wireless colorimetric and fluorometric detection of the charging state of K-PHI nanoparticles is demonstrated, enabling the material's use as particle-based autonomous sensing probe in situ. The various readout options of K-PHI's response enable us to adapt the sensitivities and dynamic ranges without modifying the sensing platform, which is demonstrated using glucose as a model analyte over a wide range of concentrations (50 μM to 50 mM). Since K-PHI is earth abundant, biocompatible, chemically robust and responsive to visible light, we anticipate that the photomemristive sensing platform presented herein opens up memristive and neuromorphic functions.

Fluorescent Biosensors for the Detection of Viruses Using Graphene and Two-Dimensional Carbon Nanomaterials

Two-dimensional carbon nanomaterials have been commonly employed in the field of biosensors to improve their sensitivity/limits of detection and shorten the analysis time. These nanomaterials act as efficient transducers because of their unique characteristics, such as high surface area and optical, electrical, and magnetic properties, which in turn have been exploited to create simple, quick, and low-cost biosensing platforms. In this review, graphene and two-dimensional carbon material-based fluorescent biosensors are covered between 2010 and 2021, for the detection of different human viruses. This review specifically focuses on the new developments in graphene and two-dimensional carbon nanomaterials for fluorescent biosensing based on the Förster resonance energy transfer (FRET) mechanism. The high-efficiency quenching capability of graphene via the FRET mechanism enhances the fluorescent-based biosensors. The review provides a comprehensive reference for the different types of carbon nanomaterials employed for the detection of viruses such as Rotavirus, Ebola virus, Influenza virus H3N2, HIV, Hepatitis C virus (HCV), and Hepatitis B virus (HBV). This review covers the various multiplexing detection technologies as a new direction in the development of biosensing platforms for virus detection. At the end of the review, the different challenges in the use of fluorescent biosensors, as well as some insights into how to overcome them, are highlighted.

Current advances on g-C3N4-based fluorescence detection for environmental contaminants

The development of highly-sensitive fluorescence detection systems for environmental contaminants has become high priority research in the past years. Special attention has been paid to graphitic carbon nitride (g-C₃N₄)-based nanomaterials, whose unique and superior optical property makes them promising and attractive candidates for this purpose. It is necessary to enhance the current understanding of the various classes of g-C₃N₄-based fluorescence detection systems and their mechanisms, as well as find suitable approaches to improve detection performance for environmental monitoring, protection, and management. In this review, the recent progresses on g-C₃N₄-based fluorescence detections for environmental contaminants, mainly including their basic principles, mechanisms, applications, modification strategies, and conclusions, are summarized. A particular emphasis is placed on the design and development of modification strategies for g-C₃N₄ with the objective of improving detection performance. High photoluminescence quantum yield, tunable fluorescence emission characteristics, and strong adsorption capacity of g-C₃N₄ could ensure the ultrasensitivity and selectivity of fluorescence detection of environmental contaminants. Concluding perspectives on the challenges and opportunities to design highly efficient g-C₃N₄-based fluorescence detection system are intensively put forward as well.

3D-phosphorus doped mesoporous graphitic carbon nitride based immunosensor for swine flu detection

Illustrative representation of the immunosensing platform and its electrochemical response for the detection of swine flu.

Single-layer carbon nitride: synthesis, structure, photophysical/photochemical properties, and applications

This Perspective provides a critical summary of the current state of the art in the synthesis and properties of polyheptazine single-layer carbon nitride (SLCN). The summary combines the authors' research and literature reports on SLCN concerning the synthesis of single-layer polyheptazine sheets, light absorption and emission by SLCN, photochemical and photocatalytic properties of SLCN as well as examples of applications of SLCN sheets as "building blocks" in heterostructures with nanocrystalline semiconductors and metals. The Perspective is concluded with an outlook discussing the most promising directions for further studies and applications of SLCN and related composites.

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.

Microporous Carbon Nitride (C3 N5.4 ) with Tetrazine based Molecular Structure for Efficient Adsorption of CO2 and Water

Mesoporous carbon nitrides with C₃ N₅ and C₃ N₆ stoichiometries created a new momentum in the field of organic metal-free semiconductors owing to their unique band structures and high basicity. Here, we report on the preparation of a novel graphitic microporous carbon nitride with a tetrazine based chemical structure and the composition of C₃ N_5.4 using ultra-stable Y zeolite as the template and aminoguanidine hydrochloride, a high nitrogen-containing molecule, as the CN precursor. Spectroscopic characterization and density functional theory calculations reveal that the prepared material exhibits a new molecular structure, which comprises two tetrazines and one triazine rings in the unit cell and is thermodynamically stable. The resultant carbon nitride shows an outstanding surface area of 130.4 m²  g^-1 and demonstrates excellent CO₂ adsorption per unit surface area of 47.54 μmol m^-2 , which is due to the existence of abundant free NH₂ groups, basic sites and microporosity. The material also exhibits highly selective sensing over water molecules (151.1 mmol g^-1 ) and aliphatic hydrocarbons due to its unique microporous structure with a high amount of hydrophilic nitrogen moieties and recognizing ability towards small molecules.

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Tailoring the stoichiometry of C3N4 nanosheets under electron beam irradiation

Two-dimensional polymeric graphitic carbon nitride (g-C3N4) is a low-cost material with versatile properties that can be enhanced by the introduction of dopant atoms and by changing the degree of polymerization/stoichiometry, which offers significant benefits for numerous applications. Herein, we investigate the stability of g-C3N4 under electron beam irradiation inside a transmission electron microscope operating at different electron acceleration voltages. Our findings indicate that the degradation of g-C3N4 occurs with N species preferentially removed over C species. However, the precise nitrogen group from which N is removed from g-C3N4 (C-N-C, [double bond, length as m-dash]NH or -NH2) is unclear. Moreover, the rate of degradation increases with decreasing electron acceleration voltage, suggesting that inelastic scattering events (radiolysis) dominate over elastic events (knock-on damage). The rate of degradation by removing N atoms is also sensitive to the current density. Hence, we demonstrate that both the electron acceleration voltage and the current density are parameters with which one can use to control the stoichiometry. Moreover, as N species were preferentially removed, the d-spacing of the carbon nitride structure increased. These findings provide a deeper understanding of g-C3N4.

Synergistic Doping and Surface Decoration of Carbon Nitride Macrostructures by Single Crystal Design

Tailored design of hybrid carbon nitride (CN) materials is quite challenging because of the drawbacks of the solid-state reaction, and the utilization of single crystals containing C-N monomers as reactants for the high-temperature reaction has been proven to imprint a given chemical composition, morphology, or electronic structure. We report the one-pot synthesis of alkali-containing CN macrostructures with ionic crystals on its surface by utilizing a tailored melamine-hydrochloride-based molecular single crystal containing NaCl and KCl as reactants. Structural and optical investigations reveal that upon calcination, molecular doping with Na⁺ and K⁺ is achieved, and additionally, the ionic species remain on the surface of the materials, resulting in an enhanced H₂ evolution performance through water splitting owing to a high ionic strength of the reaction media. Additionally, the most stable configuration of the alkaline metals in the CN lattice is evaluated by DFT calculations. This work provides an approach for the rational design of CN and other related metal-free materials with controllable properties for energy-related applications and devices.

Molecular Mechanism for the Self-Supported Synthesis of Graphitic Carbon Nitride from Urea Pyrolysis

Quantum chemical calculations combined with kinetic Monte Carlo simulations are performed to decipher the kinetics for the one-pot synthesis of two-dimensional graphitic carbon nitride (g-C₃N₄) from urea pyrolysis. Two mechanisms are considered, one involving ammelide as the intermediate compound and the other considering cyanuric acid. Different grid growing patterns are investigated, and the size, shape, and density of the grids as well as the number and position of the defects are evaluated. We find that the mechanistic pathway involving ammelide is preferred. Larger g-C₃N₄ grids with lower density are achieved when the rate constant for melon growing is inversely proportional to the number of local reaction sites, while nearly filled smaller grids are obtained in the opposite scenario. Larger defects appear at the grid periphery while smaller holes appear throughout the grid. The synthesis of extended g-C₃N₄ structures is favored if the g-C₃N₄ growing propensity is directly proportional to the number of reaction sites.

Nanostructured graphitic carbon nitride based ultrasensing electrochemical biosensor for food toxin detection

We report results of the studies related to the fabrication of thionine functionalized graphitic carbon nitride nanosheets based ultrasensing platform for food toxin (Aflatoxin B1, AfB₁) detection. The synthesis of graphitic carbon nitride nanosheets (g-C₃N₄) was carried out by polycondensation of melamine followed by chemical exfoliation. Further, thionine was used for the functionalization of g-C₃N₄ (Thn/g-C₃N₄) and deposited electrophoretically onto the indium tin oxide (ITO) coated glass electrode. The fabricated Thn/g-C₃N₄/ITO electrode was covalently immobilized by EDC-NHS chemistry with anti-aflatoxin B1 (anti-AfB₁) followed by blocking of non-specific sites using BSA molecules. For structural, morphological, functional and electrochemical properties analysis of synthesized nanomaterials and fabricated electrodes X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, atomic force microscopy and cyclic voltammetry techniques were used. The electrochemical response studies of the fabricated biosensing platform (BSA/anti-AfB₁/Thn/g-C₃N₄/ITO) were carried out towards detection of AfB₁ antigen using cyclic voltammetry technique. The obtained electrochemical results indicate that the fabricated biosensing electrode having ability to detect AfB₁ with lower limit of detection of 0.328 fg mL^-1, linear detection range in between 1 fg mL^-1 to 1 ng mL^-1, sensitivity of 4.85 μA log [ng^-1 mL] cm^-2 with stability upto 7 weeks.

Graphitic carbon nitride nanotubes: a new material for emerging applications

We provide a critical review of the current state of the synthesis and applications of nano- and micro-tubes of layered graphitic carbon nitride. This emerging material has a huge potential for light-harvesting applications, including light sensing, artificial photosynthesis, selective photocatalysis, hydrogen storage, light-induced motion, membrane technologies, and can become a major competitor for such established materials as carbon and titania dioxide nanotubes. Graphitic carbon nitride tubes (GCNTs) combine visible-light sensitivity, high charge carrier mobility, and exceptional chemical/photochemical stability, imparting this material with unrivaled photocatalytic activities in photosynthetic processes, such as water splitting and carbon dioxide reduction. The unique geometric GCNT structure and versatility of possible chemical modifications allow new photocatalytic applications of GCNTs to be envisaged including selective photocatalysts of multi-electron processes as well as light-induced and light-directed motion of GCNT-based microswimmers. Closely-packed arrays of aligned GCNTs show great promise as multifunctional membrane materials for the light energy conversion and storage, light-driven pumping of liquids, selective adsorption, and electrochemical applications. These emerging applications require synthetic routes to GCNTs with highly controlled morphological parameters and composition to be available. We recognize three major strategies for the GCNT synthesis including templating, supramolecular assembling of precursors, and scrolling of nano-/microsheets, and outline promising routes for further progress of these approaches in the light of the most important emerging applications of GCNTs.

CeO2-Based Two-Dimensional Layered Nanocomposites Derived from a Metal-Organic Framework for Selective Electrochemical Dopamine Sensors

In this work, we demonstrate the incorporation of two-dimensional (2D) layered materials into a metal-organic framework (MOF) derived from one-dimensional (1D) cerium oxide (CeO2) for the electrochemical detection of dopamine. Ce-MOF was employed as a sacrificial template for preparing CeO2 with 2D materials by the pyrolysis process. The influence of the pyrolysis temperature was studied to achieve a better crystal structure of CeO2. Siloxene improved the dopamine sensing performance of CeO2 compared with graphitic carbon nitride (g-C3N4) due to the basal plane surface oxygen and hydroxyl groups of 2D siloxene. Under optimal conditions, the fabricated CeO2/siloxene electrode exhibited a detection limit of 0.292 μM, with a linear range from 0.292 μM to 7.8 μM. This work provides a novel scheme for designing the CeO2 material with siloxene for excellent dopamine sensors, which could be extended towards other biosensing applications.

Long-Lasting and Intense Chemiluminescence of Luminol Triggered by Oxidized g-C3N4 Nanosheets

Most of the known chemiluminescence (CL) systems are flash-type, whereas a CL system with long-lasting and strong emission is very favorable for accurate CL quantitative analysis and imaging assays. In this work, we found that the oxidized g-C₃N₄ (g-CN_OX) could trigger luminol-H₂O₂ to produce a long-lasting and intense CL emission. The CL emission lasted for over 10 min and could be observed by the naked eye in a dark room. By means of a CL spectrum, X-ray photoelectron spectra, and electron spin resonance spectra, the possible mechanism of this CL reaction was proposed. This strong and long-duration CL emission was attributed to the high catalytic activity of g-CN_OX nanosheets and continuous generation of reactive oxygen species from H₂O₂ on g-CN_OX surface. Taking full advantage of the long-lasting CL property of this system, we proposed one "non-in-situ mixing" mode of CL measurement. Compared with the traditional "in-situ mixing" CL measurement mode, this measurement mode was convenient to operate and had good reproducibility. This work not only provides a long-lasting CL reaction but also deepens the understanding of the structure and properties of g-C₃N₄ material.

Delicate Balance of Non-Covalent Forces Govern the Biocompatibility of Graphitic Carbon Nitride towards Genetic Materials

Despite a plethora of suggested technological and biomedical applications, the nanotoxicity of two-dimensional (2D) graphitic carbon nitride (g-C₃ N₄ ) towards biomolecules remains elusive. To address this issue, we employ all-atom classical molecular dynamics simulations and investigate the interactions between nucleic acids and g-C₃ N₄ . It is revealed that, toxicity is modulated through a subtle balance between electrostatic and van der Waals interactions. When the exposed nucleobases interact through predominantly short-ranged van der Waals and π-π stacking interactions, they get deviated from their native disposition and adsorb on the surface, leading to loss of self-stacking and intra-quartet H-bonding along with partial disruption of the native structure. In contrast, for the interaction with double-stranded structures of both DNA and RNA, long-range electrostatics govern the adsorption phenomena since the constituent nucleobases are relatively concealed and wrapped, thereby resulting in almost complete preservation of the nucleic acid structures. Construction of free energy landscapes for lateral translation of adsorbed nucleic acids suggests decent targeting specificity owing to their restricted movement on g-C₃ N₄ . The release times of nucleic acids adsorbed through predominant electrostatics are significantly less than those adsorbed through stacking with the surface. It is therefore proposed that g-C₃ N₄ would induce toxicity towards any biomolecule having bare residues available for strong van der Waals and π-π stacking interactions relative to those predominantly interacting through electrostatics.