One-step synthesis of graphitic carbon nitride nanosheets with the help of melamine and its application for fluorescence detection of mercuric ions

Discerning biomimetic nanozyme electrodes based on g-C3N4 nanosheets and molecularly imprinted polythiophene nanofibers for detecting creatinine in microliter droplets of human saliva

The growing risk of death associated with kidney dysfunction underlines the requirement for a cost-effective and precise point-of-care (POC) diagnostic tool to identify chronic kidney disease (CKD) at an early stage. This work reports the development of a non-invasive POC diagnostic based on cost-efficient, disposable electrodes and in situ-designed biomimetic nanozymes. The nanozymes are composed of graphitic carbon nitride nanosheets (gCN) and creatinine-imprinted polythiophene nanofibers (miPTh). Microscopic analyses reveal porous nanofibrous surface morphology of biomimetic miPTh/gCN nanozymes. Bulk imprinting and the inclusion of conductive gCN nanosheets drastically reduced the charge transfer resistance and improved the electron exchange kinetics at the nanozyme-electrolyte interface. The electrochemical oxidation of creatinine is studied via cyclic voltammetry (CV), and differential pulse voltammetry (DPV), which exhibit excellent creatinine recognition ability of biomimetic miPTh/gCN nanozyme sensors compared to pristine polymeric or non-imprinted nanozymes. The sensor reveals linear response toward 200-1000 nmol L-1 creatinine, high sensitivity (4.27 μA cm-2 nmol-1 L), sub-nanomolar detection limit (340 pmol L-1), and excellent selectivity over common salivary analytes. To corroborate its real-world utility, the miPTh/gCN nanozyme sensor shows an impressive 94.8% recovery of spiked creatinine concentrations in microliter droplets of human saliva samples. This disposable sensor reveals great potential in the realm of reliable and efficient non-invasive POC diagnostics for healthcare delivery.

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.

An “on-off-on” fluorescence probe for glyphosate detection based on Cu2+ modulated g-C3N4 nanosheets

The analysis of glyphosate is essential to agricultural production, environment protection and public health. Herein, we proposed a fast and convenient “on-off-on” fluorescence platform for sensitive detection of glyphosate via Cu²⁺ modulated g-C₃N₄ nanosheets. The fluorescence of the system was quenched by Cu²⁺. With the presence of glyphosate, the fluorescence could be restored due to the formation of Cu²⁺- glyphosate complex. The proposed method was cost-effective with label-free and enzyme-free. Moreover, it exhibits high sensitivity with a low detection limit of 0.01 μg/ml. Furthermore, the proposed method has been successfully monitored glyphosate in real samples.

Dual recognition strategy for selective fluorescent detection of dopamine and antioxidants based on graphite carbon nitride in human blood serum

In this work, a strong blue-emitting fluorescent biosensor based on graphite carbon nitride nanoparticles (GCNNs) (E_x = 340 nm and E_m = 435 nm) was synthesized by a facile one-step hydrothermal method. With the aid of hydrogen peroxide and horseradish peroxidase, pyrocatechol structure of dopamine (DA) was oxidized to o-quinone structure of polydopamine (PDA) by hydroxyl radical. PDA was able to rapidly and significantly quench fluorescence of GCNNs. In the meanwhile, oxidative self-polymerization from DA to PDA would be blocked by antioxidants, such as glutathione (GSH) and ascorbic acid (AA). Thus, the fluorescence of GCNNs@DA sensor would be recovered owing to the decrease of o-quinone. Based on above-mentioned dual recognition strategy of "turn-off" and "turn off-on", a fast, simple and ultrasensitive method was developed to measure DA and antioxidants. Under the optimal experimental conditions, the detection limits of DA, GSH and AA were 0.064 μmol L^-1, 0.11 μmol L^-1 and 0.16 μmol L^-1 with relative standard deviations of 1.7%, 9.3% and 8.0%, respectively. As one of metal-free quantum dots, our GCNNs-based sensors were also successfully applied to the determination of DA as well as GSH and AA in human serum. The recoveries for the spiked samples were in the range of 93.8%-109% and 95.0%-110% of DA and antioxidants, which shows great promise to clinicalapplication.

One Spot Microwave Synthesis and Characterization of Nitrogen-Doped Carbon Dots with High Oxygen Content for Fluorometric Determination of Banned Sudan II Dye in Spice Samples

A simple microwave-assisted synthesis of nitrogen-doped carbon dots with high oxygen content (O-N-CDs) was carried out with citric acid as a carbon source and 2,4-diamino-6-methyl-1,3,5-triazine as a nitrogen source in triethylene glycol (TEG) media. It was determined by SEM analysis that O-N-CDs consisted of particles of different sizes and shapes. Transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction (XRD) analysis confirmed that O-N-CDs have a graphitic structure. Moreover, they showed a high fluorescence property based on the excitation wavelength. Therefore, a new fluorometric method was developed for the determination of banned food dye Sudan II by using the O-N-CDs. The proposed method was used in the determination of Sudan II in spiked spice samples. The detection limit was 0.6 mg L^-1 and the linear range was 0-8 mg L^-1.

Application of Nanotechnology in Analysis and Removal of Heavy Metals in Food and Water Resources

Toxic heavy metal contamination in food and water from environmental pollution is a significant public health issue. Heavy metals do not biodegrade easily yet can be enriched hundreds of times by biological magnification, where toxic substances move up the food chain and eventually enter the human body. Nanotechnology as an emerging field has provided significant improvement in heavy metal analysis and removal from complex matrices. Various techniques have been adapted based on nanomaterials for heavy metal analysis, such as electrochemical, colorimetric, fluorescent, and biosensing technology. Multiple categories of nanomaterials have been utilized for heavy metal removal, such as metal oxide nanoparticles, magnetic nanoparticles, graphene and derivatives, and carbon nanotubes. Nanotechnology-based heavy metal analysis and removal from food and water resources has the advantages of wide linear range, low detection and quantification limits, high sensitivity, and good selectivity. There is a need for easy and safe field application of nanomaterial-based approaches.

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: A Highly Electroactive Nanomaterial for Environmental and Clinical Sensing

Graphitic carbon nitride (g-C3N4) is a two-dimensional conjugated polymer that has attracted the interest of researchers and industrial communities owing to its outstanding analytical merits such as low-cost synthesis, high stability, unique electronic properties, catalytic ability, high quantum yield, nontoxicity, metal-free, low bandgap energy, and electron-rich properties. Notably, graphitic carbon nitride (g-C3N4) is the most stable allotrope of carbon nitrides. It has been explored in various analytical fields due to its excellent biocompatibility properties, including ease of surface functionalization and hydrogen-bonding. Graphitic carbon nitride (g-C3N4) acts as a nanomediator and serves as an immobilization layer to detect various biomolecules. Numerous reports have been presented in the literature on applying graphitic carbon nitride (g-C3N4) for the construction of electrochemical sensors and biosensors. Different electrochemical techniques such as cyclic voltammetry, electrochemiluminescence, electrochemical impedance spectroscopy, square wave anodic stripping voltammetry, and amperometry techniques have been extensively used for the detection of biologic molecules and heavy metals, with high sensitivity and good selectivity. For this reason, the leading drive of this review is to stress the importance of employing graphitic carbon nitride (g-C3N4) for the fabrication of electrochemical sensors and biosensors.

Ratiometric Fluorescent Paper-Based Sensor Based on CdTe Quantum Dots and Graphite Carbon Nitride Hybrid for Visual and Rapid Determination of Cu2+ in Drinks

A simple and effective ratiometric fluorescence sensor of CdTe QDs/GCNNs for on-site and rapid analysis of Cu²⁺ has been established by mixing physically CdTe QDs and graphite carbon nitride (GCNNs). Two emissions peaks of CdTe QDs at 572 nm and GCNNs at 436 nm are both excitated at 340 nm. Under a UV lamp, fluorescent of traffic yellow CdTe QDs is linearly quenched by Cu²⁺ (as the detection signal), while blue GCNNs remains unchanged (as the reference), resulting in a distinguishable color change gradually from pink yellow to blue. The limit of detection (LOD) of this new sensor for Cu²⁺ is as low as 0.47 ng mL^-1 with 1.4 % RSD. The established method has been successfully applied to detection of Cu²⁺ in various drinks with satisfactory results. Moreover, a paper-based sensor, which has been prepared by soaking cellulose acetate membrane in CdTe QDs/GCNNs sensor solution, has a wide semiquantitative detection range for Cu²⁺ (0.01 ~ 5.0 μg mL^-1 ). It has realized successfully on-site and rapid determination of Cu²⁺ in red wine without any pretreatment procedure and is of great promotion and application value in determination of Cu²⁺ in liquid samples.

Facile synthesis of highly fluorescent free-standing films comprising graphitic carbon nitride (g-C3N4) nanolayers

The free-standing g-C₃N₄ films were fabricated by thermal condensation of C₂H₄N₄ at 600 °C in a low pressure of Ar atmosphere. The as-synthesized g-C₃N₄ films exhibited stable and strong photoluminescence emission centered around 455–460 nm.

Enhancement of photocatalytic activity of g-C3N4 by hydrochloric acid treatment of melamine

Modified g-C₃N₄ samples (g-X, where X corresponds to the number of hours of acid treatment of the melamine) with outstanding photocatalytic performance were prepared by using hydrochloric acid-treated melamine as a precursor and calcining at 550 °C for 2 h. An x-ray diffractometer, field-emission scanning electron microscope, infrared spectrometer, N₂ adsorption-desorption test, x-ray photoelectron spectroscopy, and ultraviolet-visible diffuse-reflectance spectroscopy analysis were carried out to characterize the phase composition, microstructure, chemical structure, specific surface area (SSA), chemical states, elemental composition and optical properties of the samples, respectively. The photocatalytic performance of the samples was evaluated by degrading the Rhodamine B (RhB) aqueous solution. The results showed that the crystal structure and vibration bands of melamine changed due to the reaction with hydrochloric acid. The crystallinity and grain size of g-C₃N₄ in g-X (X = 1, 2, 4, 6, 8, 10) reduced, and the SSA values of g-X increased compared to that of the g-0 sample, which was synthesized from pristine melamine. The g-X samples exhibited excellent photocatalytic activity towards degradation of RhB compared to g-0. The photocatalytic activity of the g-X samples increased gradually as the acid treatment time of the melamine increased from 1 h to 2 h, and then decreased gradually with the extension of the acid treatment time. The rate constant (k) values of g-X are higher than that of g-0. g-2 presented the highest rate constant (k = 0.052 min^-1), which was 5.5 times higher than that of g-0. The improved photocatalytic activity of the g-X samples was attributed to the higher SSA value, the appearance of surface defects, the outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X, with the last two factors being more significant. Acid treatment of melamine is helpful in the preparation of high performance g-C₃N₄ photocatalyst, and the microstructure and photocatalytic performance of g-C₃N₄ were affected significantly by the acid treatment time.

Facile preparation of graphitic-C3N4 quantum dots for application in two-photon imaging

A novel one-step method for the preparation of g-C₃N₄ QDs for effective two-photon imaging.

Microwave-assisted synthesis of thymine-functionalized graphitic carbon nitride quantum dots as a fluorescent nanoprobe for mercury(II)

A microwave-assisted hydrothermal method was employed to prepare thymine-modified graphitic carbon nitride quantum dots (T-gCNQDs) which are shown to be a novel fluorescent nanoprobe for Hg(II). They exhibit excellent optical properties (blue emission with a fluorescence quantum yield of 46%) and water solubility. The incorporation of thymine into the gCNQDs results in an enhancement in photoluminescence properties. It is found that fluorescence, best measured at excitation/emission wavelengths of 350/445 nm, is much more strongly quenched by Hg(II) compared to the thymine-free nanoprobe. The quenching is highly selective even in the presence other metal ions. This is ascribed to the formation of T-Hg(II)-T base complexes. Fluorescence drops linearly in the 1.0 to 500 nM Hg(II) concentration range, and the limit of detection is 0.15 nM. The method was applied to the determination of Hg(II) in spiked samples of tap and pond water. Recoveries were found to be >95%, thus demonstrating the practical applicability of the assay. Graphical abstract A microwave-assisted hydrothermal route was employed to prepare thymine-functionalized graphitic carbon nitride QDs (T-gCNQDs). A selective fluorescence quenching mechanism occurred between T-gCNQDs and Hg(II) due to thymine functionalization. T-gCNQDs was utilized to detect Hg(II) in real samples.