Preparation of graphite-like carbon nitride nanoflake film with strong fluorescent and electrochemiluminescent activity

Synergic Effect between Adsorption and Photocatalysis of Metal-Free g-C3N4 Derived from Different Precursors

Graphitic carbon nitride (g-C₃N₄) used in this work was obtained by heating dicyandiamide and melamine, respectively, at different temperatures. The differences of g-C₃N₄ derived from different precursors in phase composition, functional group, surface morphology, microstructure, surface property, band gap and specific surface area were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-visible diffuse reflection spectroscopy and BET surface area analyzer, respectively. The photocatalytic discoloration of an active cationic dye, Methylene Blue (MB) under visible-light irradiation indicated that g-C₃N₄ derived from melamine at 500°C (CN-M500) had higher adsorption capacity and better photocatalytic activity than that from dicyandiamide at 500°C (CN-D500), which was attributed to the larger surface area of CN-M500. MB discoloration ratio over CN-M500 was affected by initial MB concentration and photocatalyst dosage. After 120 min reaction time, the blue color of MB solution disappeared completely. Subsequently, based on the measurement of the surface Zeta potentials of CN-M500 at different pHs, an active anionic dye, Methyl Orange (MO) was selected as the contrastive target pollutant with MB to reveal the synergic effect between adsorption and photocatalysis. Finally, the photocatalytic mechanism was discussed.

Investigating the Dispersion Behavior in Solvents, Biocompatibility, and Use as Support for Highly Efficient Metal Catalysts of Exfoliated Graphitic Carbon Nitride

The liquid-phase exfoliation of graphitic carbon nitride (g-C3N4) to afford colloidal dispersions of two-dimensional flakes constitutes an attractive route to facilitate the processing and implementation of this novel material toward different technological applications, but quantitative knowledge about its dispersibility in solvents is lacking. Here, we investigate the dispersion behavior of exfoliated g-C3N4 in a wide range of solvents and evaluate the obtained results on the basis of solvent surface energy and Hildebrand/Hansen solubility parameters. Estimates of the three Hansen parameters for exfoliated g-C3N4 from the experimentally derived data yielded δD ≈ 17.8 MPa(1/2), δP ≈ 10.8 MPa(1/2), and δH ≈ 15.4 MPa(1/2). The relatively high δH value suggested that, contrary to the case of other two-dimensional materials (e.g., graphene or transition metal dichalcogenides), hydrogen-bonding plays a substantial role in the efficient interaction, and thus dispersibility, of exfoliated g-C3N4 with solvents. Such an outcome was attributed to a high density of primary and/or secondary amines in the material, the presence of which was associated with incomplete condensation of the structure. Furthermore, cell proliferation tests carried out on thin films of exfoliated g-C3N4 using murine fibroblasts suggested that this material is highly biocompatible and noncytotoxic. Finally, the exfoliated g-C3N4 flakes were used as supports in the synthesis of Pd nanoparticles, and the resulting hybrids exhibited an exceptional catalytic activity in the reduction of nitroarenes.

Potential-Modulated Electrochemiluminescence of Carbon Nitride Nanosheets for Dual-Signal Sensing of Metal Ions

As an emerging semiconductor, graphite-phase polymeric carbon nitride (GPPCN) has drawn much attention not only in photocatalysis but also in optical sensors such as electrochemiluminescence (ECL) sensing of metal ions. However, when the concentrations of interfering metal ions are several times higher than that of the target metal ion, it is almost impossible to distinguish which metal ion changes the ECL signals in real sample detection. Herein, we report that the dual-ECL signals could be actuated by different ECL reactions merely from GPPCN nanosheets at anodic and cathodic potentials, respectively. Interestingly, the different metal ions exhibited distinct quenching/enhancement of the ECL signal at different driven potentials, presumably ascribed to the diversity of energy-level matches between the metal ions and GPPCN nanosheets and catalytic interactions of the intermediate species in ECL reactions. On this basis, without any labeling and masking reagents, the accuracy and reliability of sensors based on the ECL of GPPCN nanosheets toward metal ions were largely improved; thus, the false-positive result caused by interferential metal ions could be effectively avoided. As an example, the proposed GPPCN ECL sensor with a detection limit of 1.13 nM was successfully applied for the detection of trace Ni(2+) ion in tap and lake water.

Signal-on electrochemiluminescent aptasensors based on target controlled permeable films

A novel permeability gate-based electrochemiluminescent (ECL) aptasensor has been constructed by utilizing target-responsive polyelectrolyte-aptamer film deposited on the solid-state ECL electrode to control the rate of diffusion of a coreactant that triggers the ECL.

Installing logic gates in permeability controllable polyelectrolyte-carbon nitride films for detecting proteases and nucleases

Proteases and nucleases are enzymes heavily involved in many important biological processes, such as cancer initiation, progression, and metastasis; hence, they are indicative of potential diagnostic biomarkers. Here, we demonstrate a new label free and sensitive electrochemiluminescent (ECL) sensing strategy for protease and nuclease assays that utilize target-triggered desorption of programmable polyelectrolyte films assembled on graphite-like carbon nitride (g-C3N4) film to regulate the diffusion flux of a coreactant. Furthermore, we have built Boolean logic gates OR and AND into the polyelectrolyte films, capable of simultaneously sensing proteases and nucleases in a complicated system by breaking it into simple functions. The developed intelligent permeability controlled enzyme sensor may prove valuable in future medical diagnostics.

A practical and highly sensitive C3N4-TYR fluorescent probe for convenient detection of dopamine

The C3N4-tyrosinase (TYR) hybrid is a highly accurate, sensitive and simple fluorescent probe for the detection of dopamine (DOPA). Under optimized conditions, the relative fluorescence intensity of C3N4-TYR is proportional to the DOPA concentration in the range from 1 × 10(-3) to 3 × 10(-8) mol L(-1) with a correlation coefficient of 0.995. In the present system, the detection limit achieved is as low as 3 × 10(-8) mol L(-1). Notably, these quantitative detection results for clinical samples are comparable to those of high performance liquid chromatography. Moreover, the enzyme-encapsulated C3N4 sensing arrays on both glass slide and test paper were evaluated, which revealed sensitive detection and excellent stability. The results reported here provide a new approach for the design of a multifunctional nanosensor for the detection of bio-molecules.

Electrochemiluminescence Resonance Energy Transfer System: Mechanism and Application in Ratiometric Aptasensor for Lead Ion

In this paper, a novel electrochemiluminescence resonance energy transfer (ECL-RET) system from O2/S2O8(2-) to a kind of amino-terminated perylene derivative (PTC-NH2) was demonstrated for the first time, which was then applied to construct a ratiometric aptasensor for lead ion (Pb(2+)) detection. First, gold-nanoparticles-functionalized fullerene nanocomposites (AuNPs@nano-C60) were coated on a glassy carbon electrode (GCE), and then thiol-modified assistant probes (APs) were attached on AuNPs@nano-C60/GCE. Then the resultant electrode was hybridized with capture probes (the aptamer of the Pb(2+), abbreviated as CPs) to generate DNA duplexes, which could induce PTC-NH2 to be intercalated into the dsDNA grooves by the electrostatic adsorption. Herein, ECL dual peaks at -0.7 V (vs Ag/AgCl) and -2.0 V (vs Ag/AgCl) were obtained when the prepared aptasensor was detected in air-saturated S2O8(2-) solution, which could be attributed to the emission of excited dimmers (π-excimers) ((1)(NH2-PTC)2*) and (1)(O2)2*, respectively. In the presence of Pb(2+), the dsDNA was unwound, and Pb(2+) G-quadruplex structure was generated because of the highly specific affinity between Pb(2+) and CPs, which made the PTC-NH2 release from the electrode surface. As a result, the ECL signal at -0.7 V was decreased, and the ECL signal around -2.0 V was increased. By measuring the ratio of ECL intensities at two excitation potentials, the developed aptasensor exhibited the linear response range from 1.0 × 10(-12) M to 1.0 × 10(-7) M with a detection limit of 3.5 × 10(-13) M (S/N = 3) for Pb(2+), which could offer an alternative analytical method with excellent properties of high selectivity, accuracy, and sensitivity.

Trace detection of nitro aromatic explosives by highly fluorescent g-C3N4 nanosheets

Highly fluorescent g-C3N4 nanosheets were facilely fabricated by exfoliating bulk g-C3N4 under ultrasonic irradiation for 1 h. The atomic force microscopy (AFM) image shows that the resultant g-C3N4 nanosheets are ∼6-14 nm thick, and the suspension is stable in air for several weeks. Remarkably, the obtained nanosheets exhibited strong fluorescence with an extremely high quantum yield (QY) up to 32%, and high sensitivity, selectivity, as well as a fast response to nitro aromatic explosives were observed. Typically, the quenching efficiency coefficient Ksv for PNP was 30,460 M(-1), which proved that the resultant nanosheets possessed an extremely high sensitivity for nitro-phenol PNP detection.

Recent advances in electrochemiluminescence

The great success of electrochemiluminescence (ECL) for in vitro diagnosis (IVD) and its promising potential in light-emitting devices greatly promote recent ECL studies. More than 45% of ECL articles were published after 2010, and the first international meeting on ECL was held in Italy in 2014. This critical review discusses recent vibrant developments in ECL, and highlights novel ECL phenomena, such as wireless ECL devices, bipolar electrode-based ECL, light-emitting electrochemical swimmers, upconversion ECL, ECL resonance energy transfer, thermoresponsive ECL, ECL using shape-controlled nanocrystals, and ECL as an ion-selective electrode photonic reporter, a paper-based microchip, and a self-powered microfluidic ECL platform. We also comment on the latest progress in bioassays, light-emitting devices and, the computational approach for the ECL mechanism study. Finally, perspectives and key challenges in the near future are addressed (198 references).

A signal-on electrochemiluminescence biosensor for detecting Con A using phenoxy dextran-graphite-like carbon nitride as signal probe

A novel signal-on electrochemiluminescence (ECL) biosensor for detecting concanavalin A (Con A) was fabricated with phenoxy dextran-graphite-like carbon nitride (DexP-g-C3N4) as signal probe. In this construction strategy, the nanocomposites of three-dimensional graphene and gold nanoparticles (3D-GR-AuNPs) were used as matrix for high loading of glucose oxidase (GOx), which served as recognition element for bounding Con A. Con A further interacted with DexP-g-C3N4 through a specific carbohydrate-Con A interaction to achieve a sandwiched scheme. With the increase of Con A incubated onto the electrode, the ECL signal resulted from DexP-g-C3N4 would enhance, thus achieving a signal-on ECL biosensor for Con A detection. Due to the integration of the virtues of 3D-GR-AuNPs and the excellent ECL performance of DexP-g-C3N4, the prepared biosensor exhibits a wide linear response range from 0.05 ng/mL to 100 ng/mL and a low detection limit of 17 pg/mL (S/N=3).

Post-annealing reinforced hollow carbon nitride nanospheres for hydrogen photosynthesis

Hollow-structured g-C3N4 polymers with a high thermal stability up to 550 °C and an enhanced photocatalytic activity have been developed by post-annealing treatment, which effectively modifies the textural, crystal, and electronic properties of the g-C3N4 semiconductors without extra chemical assistance. This is a unique example of thermally and chemically stable conjugated polymers with hollow nanostructures and optoelectronic properties, promising the development of functional hollow g-C3N4 nanocomposites by chemical modifications like doping, surface grafting, and coupling with other inorganic/polymeric semiconductors with the aid of thermal treatment at high temperatures.

Synthesis and characterization of single-crystalline graphitic C3N4 nanocones

Single-crystalline g-C₃N₄ nanocones were successfully synthesized using a novel plasma sputtering reaction deposition method.

High yield synthesis of nano-size g-C3N4 derivatives by a dissolve-regrowth method with enhanced photocatalytic ability

Bulk g-C₃N₄ was “cut” into nano-size g-C₃N₄ by HNO₃ scissors.

Electrochemiluminescence sensor for dopamine with a dual molecular recognition strategy based on graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids

Preparation of an ECL sensor and graphite-like carbon nitride nanosheets/3,4,9,10-perylenetetracarboxylic acid hybrids.

Enhanced sensing performance of supported graphitic carbon nitride nanosheets and the fabrication of electrochemiluminescent biosensors for IgG

A greatly enhanced ECL signal was obtained after the incorporation of C₃N₄ nanosheets into the NPG matrix and a label-free ECL immunosensor was proposed.

One-step electrochemical synthesis of ultrathin graphitic carbon nitride nanosheets and their application to the detection of uric acid

A one-step electrochemical method for synthesis of ultrathin g-C₃N₄ nanosheets is reported. This method does not need dangerous reagents and largely reduces the reaction time.

A label-free fluorescence sensing approach for selective and sensitive detection of 2,4,6-trinitrophenol (TNP) in aqueous solution using graphitic carbon nitride nanosheets

An effective and facile fluorescence sensing approach for the determination of 2,4,6-trinitrophenol (TNP) using the chemically oxidized and liquid exfoliated graphitic carbon nitride (g-C3N4) nanosheets was developed. The strong inner filter effect and molecular interactions (electrostatic, π-π, and hydrogen bonding interactions) between TNP and the g-C3N4 nanosheets led to the fluorescence quenching of the g-C3N4 nanosheets with efficient selectivity and sensitivity. Under optimal conditions, the limit of detection for TNP was found to be 8.2 nM. The proposed approach has potential application for visual detection of TNP in natural water samples for public safety and security.

Fluorescence sensing of chromium (VI) and ascorbic acid using graphitic carbon nitride nanosheets as a fluorescent "switch"

Using graphitic carbon nitride (g-C3N4) nanosheets, an effective and facile fluorescence sensing approach for the label-free and selective determination of chromium (VI) (Cr(VI)) was developed. The fluorescence of the solution of g-C3N4 nanosheets was quenched effectively by Cr(VI) via the inner filter effect. Under optimal conditions, a wide detection linear range for Cr(VI) was found to be from 0.6 μM to 300 μM with a limit of detection (LOD) of 0.15 μM. In addition, the fluorescence of the solution of g-C3N4 nanosheets-Cr(VI) could be sensitively turned on in the presence of a reductant such as ascorbic acid (AA) via an "on-off-on" fluorescence response through the oxidation-reduction between Cr(VI) and AA. And a wide detection linear range for AA was found to be from 0.5 μM to 200 μM with an LOD of 0.13 μM. Furthermore, the proposed method has the potential application for detection of Cr(VI) in lake waters and AA in biological fluids.

Photoelectrochemical aptasensing of kanamycin using visible light-activated carbon nitride and graphene oxide nanocomposites

Photoactive material and recognition element are two crucial factors which determine the sensitivity and selectivity of the photoelectrochemical (PEC) sensor. Herein we developed a novel PEC aptamer sensor for the specific detection of kanamycin using water-dispersible graphite-like carbon nitride (w-g-C3N4) as visible light-active material and aptamer as the biorecognition element. While a suitable amount of graphene oxide (GO) was doped in w-g-C3N4, the visible light photocurrent response was enhanced, which was beneficial to the construction of PEC sensor. On the other hand, the large specific surface area and π-conjugated structure of GO/w-g-C3N4 provided an excellent platform for immobilizing the kanamycin-binding DNA aptamer on the surface of the sensor via π-π stacking interaction. On such a sensor, the capture of kanamycin molecules by aptamer resulted in increased photocurrent. The PEC response of the sensor was found to be linearly proportional to the concentration of kanamycin in the range from 1 nM to 230 nM with a detection limit (3S/N) of 0.2 nM. Moreover, the proposed sensor displayed high selectivity, good reproducibility, and high stability, demonstrating the successful combination of GO/w-g-C3N4 with aptamer in fabricating high performance PEC sensors.

Polymer nanodots of graphitic carbon nitride as effective fluorescent probes for the detection of Fe³⁺ and Cu²⁺ ions

A simple and green route was developed for the first time to produce fluorescent graphitic carbon nitride (F-g-C₃N₄) by hydrothermal treatment of bulk g-C₃N₄. The produced F-g-C₃N₄ dots have blue emission and a high quantum yield, and were applied as a very effective fluorescent probe for label-free selective and sensitive detection of Cu(2+) and Fe(3+) ions; the limits of detection were as low as 0.5 nM and 1.0 nM, respectively. By using sodium hexametaphosphate (SHPP) as a masking agent of Fe(3+), Cu(2+) was exclusively detected in the presence of Fe(3+) ions. Cu(2+) and Fe(3+) ions in real water samples were also detected successfully. This exceptional fluorescent performance makes the probes based on F-g-C₃N₄ dots attractive for highly sensitive detection of Cu(2+) and Fe(3+) ions in real water.

Turn-on fluorescence sensor for intracellular imaging of glutathione using g-C₃N₄ nanosheet-MnO₂ sandwich nanocomposite

Herein, a novel fluorescence sensor based on g-C3N4 nanosheet-MnO2 sandwich nanocomposite has been developed for rapid and selective sensing of glutathione (GSH) in aqueous solutions, as well as living cells. The graphitic-phase C3N4 (g-C3N4) nanosheet used here is a new type of carbon-based nanomaterial with high fluorescence quantum yield and high specific surface area. We demonstrate a facile one-step approach for the synthesis of a g-C3N4 nanosheet-MnO2 sandwich nanocomposite for the first time. The fluorescence of g-C3N4 nanosheet in this nanocomposite is quenched, which attributing to fluorescence resonance energy transfer (FRET) from a g-C3N4 nanosheet to the deposited MnO2. Upon the addition of GSH, MnO2 is reduced to Mn(2+), which leads to the elimination of FRET. As a result, the fluorescence of g-C3N4 nanosheet is restored. Importantly, the chemical response of the g-C3N4-MnO2 nanocomposite exhibits great selectivity toward GSH relative to other electrolytes and biomolecules. Under the optimal conditions, the detection limit of 0.2 μM for GSH in aqueous solutions can be reached. Furthermore, the g-C3N4-MnO2 nanocomposite is confirmed to be membrane-permeable and have low cytotoxicity. Moreover, we successfully apply this sensor for visualizing and monitoring change of the intracellular GSH in living cells. Moreover, the proposed sensor shows satisfying performance, such as low cost, easy preparation, rapid detection, good biocompatibility, and turn-on fluorescence response.

Anodic electrochemiluminescence of graphitic-phase C₃N₄ nanosheets for sensitive biosensing

This work observed the anodic electrochemiluminescence (ECL) of C₃N₄ nanosheets (CNNS) for the first time. The ECL emission was 40 times stronger than that from bulk g-C₃N₄ in the presence of triethylamine (Et₃N) as a coreactant due to large surface-to-volume ratio, which enhanced the sensitivity for biosensing. At pH 7.0, the CNNS modified electrode prepared with 0.75 mg mL(-1) CNNS in 0.025% chitosan solution possesses good stability and acceptable reproducibility in the presence of 30 mM Et₃N. The ECL mechanism of CNNS/Et₃N system was proposed to be emitted from the excited CNNS, which was produced during the reaction between the electro-oxidation products of CNNS and coreactant Et₃N. Based on the annihilation between the oxidation product of dopamine (DA(+)) and Et₃N radical, a quenching-based method was established for sensitive and specific detection of dopamine ranging from 1.0 nM to 100 nM with a detection limit of 96 pM by using the CNNS nanosheets as an ECL emitter. The proposed method showed excellent specificity, high sensitivity and low detection limit, and could be applied in analysis of real samples.

Cathodic electrochemiluminescence immunosensor based on nanocomposites of semiconductor carboxylated g-C3N4 and graphene for the ultrasensitive detection of squamous cell carcinoma antigen

A novel label-free electrochemiluminescence (ECL) immunosensor was developed for the detection of squamous cell carcinoma antigen (SCCA) based on nanocomposites of semiconductor carboxylated graphitic carbon nitride (g-C3N4) and graphene (g-C3N4-graphene). The ECL intensity of carboxylated g-C3N4 was much enhanced after being combined with graphene which had excellent electron-transfer ability. The sensing platform was constructed by depositing g-C3N4-graphene on electrodes and immobilizing antibodies on the surface of carboxylated g-C3N4 through amidation. The specific immunoreaction between SCCA and antibody resulted in the decrease of ECL intensity and the intensity decreased linearly with the logarithm of SCCA concentration in the range of 0.025-10 ng mL(-1) with a detection limit of 8.53 pg mL(-1). The developed ECL immunosensor exhibited high sensitivity, good reproducibility and long-term stability, which possessed great potential for cancer detection in clinical laboratory diagnosis.

In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst and its synergistic catalytic performance

Water-soluble magnetic-functionalized graphitic carbon nitride (g-C3N4) composites were synthesized successfully by in situ decorating spinel ZnFe2O4 nanoparticles on g-C3N4 sheets (CN-ZnFe) through a one-step solvothermal method. The magnetic properties of CN-ZnFe can be effectively controlled via tuning the coverage density and the size of ZnFe2O4 nanoparticles. The results indicate that the CN-ZnFe exhibits excellent photocatalytic efficiency for methyl orange (MO) and fast separation from aqueous solution by magnet. Interestingly, the catalytic performance of the CN-ZnFe is strongly dependent on the loading of ZnFe2O4. The optimum activity of 160CN-ZnFe photocatalyst is almost 6.4 and 5.6 times higher than those of individual g-C3N4 and ZnFe2O4 toward MO degradation, respectively. By carefully investigating the influence factors, a possible mechanism is proposed and it is believed that the synergistic effect of g-C3N4 and ZnFe2O4, the smaller particle size, and the high solubility in water contribute to the effective electron-hole pairs separation and excellent photocatalytic efficiency. This work could provide new insights that g-C3N4 sheets function as good support to develop highly efficient g-C3N4-based magnetic photocatalysts in environmental pollution cleanup.

Ultrathin graphitic carbon nitride nanosheets: a novel peroxidase mimetic, Fe doping-mediated catalytic performance enhancement and application to rapid, highly sensitive optical detection of glucose

In this article, we demonstrate for the first time that ultrathin graphitic carbon nitride nanosheets (g-C3N4) possess peroxidase activity. Fe doping of the nanosheets leads to peroxidase mimetics with greatly enhanced catalytic performance and the mechanism involved is proposed. We further demonstrate the novel use of such Fe-g-C3N4 as a cheap nanosensor for simple, rapid, highly selective and sensitive optical detection of glucose with a pretty low detection limit of 0.5 μM.

Synthesis and luminescence mechanism of multicolor-emitting g-C3N4 nanopowders by low temperature thermal condensation of melamine

Graphite like C3N4 (g-C3N4) was synthesized facilely via the low temperature thermal condensation of melamine between 300-650°C. The results showed that the products maintained as melamine when the temperature is below 300°C. With the increase of temperature, the products were transformed into carbon nitride and amorphous g-C3N4 successively. The morphology of products was changed from spherical nanoparticles of melamine into layer carbon nitride and g-C3N4 with the increase of temperature. The photoluminescence spectra showed that the carbon nitride products have continuous tunable photoluminescence properties in the visible region with increasing temperature. With the help of steady state, transient state time-resolved photoluminescence spectra and Raman microstructural characterization, a novel tunable photoluminescence mechanism was founded systematically, which is mainly related to the two dimensional π-conjugated polymeric network and the lone pair of the carbon nitride.

Ultrathin graphitic carbon nitride nanosheet: a highly efficient fluorosensor for rapid, ultrasensitive detection of Cu(2+)

A highly efficient fluorosensor based on ultrathin graphitic carbon nitride (g-C₃N₄) nanosheets for Cu(2+) was developed. In the absence of metal ions, the nanosheets exhibit high fluorescence; the strong coordination of the Lewis basic sites on them to metal ions, however, causes fluorescence quenching via photoinduced electron transfer leading to the qualitative and semiquantitative detection of metal ions. This fluorosensor exhibits high selectivity toward Cu(2+). The whole detection process can be completed within 10 min with a detection limit as low as 0.5 nM. The use of test paper enables the naked-eye detection of Cu(2+) with a detection limit of 0.1 nmol. The practical use of this sensor for Cu(2+) determination in real water samples was also demonstrated.