Mo5N6 nanosheets for fluorescent quenching and target recognition: Highly selectively sensing of sodium hexametaphosphate.

Typical fluorescent biosensors use fluorescently labeled ssDNA for target recognition and nanomaterials for signal transduction. Herein, we propose a reverse sensing strategy that Mo5N6 nanosheets are used for target recognition while fluorescein (FAM)-labeled ssDNA only serves for signal generation. We discover that Mo5N6 nanosheets show high fluorescence quenching ability (>95%) and selective recognition for sodium hexametaphosphate (SHMP). After FAM-labeled ssDNA is adsorbed on Mo5N6 nanosheets, the fluorescence is quenched due to the photoinduced electron transfer (PET) effect between FAM and Mo5N6 nanosheets. SHMP can specifically displace the adsorbed FAM-labeled ssDNA from Mo5N6 nanosheets, resulting in more than 80% fluorescence recovery on addition of 5 µmol L-1 SHMP. This biosensor can sensitively detect SHMP down to 150 nmol L-1 and selectively recognize SHMP over glucose, lactose, common amino acids, Zn2+, Mg2+, Ca2+ and other phosphates (such as Na2HPO4, sodium pyrophosphate, sodium tripolyphosphate). This biosensor also shows great potential for the detection of SHMP in bacon sample. This work not only provides a facile sensitive and selective biosensor for SHMP but also exploits the application of transition metal nitrides in the field of sensing and biosensing.

Quantitative analysis of microplastics and nanoplastics released from disposable PVC infusion tubes.

The exposure of micro- and nanoplastics (MNPs) via medical device is still unknown to us. Herein, a visual quantitative detection of polyvinyl chloride (PVC) MPs and a fluorescent quantitative detection of PVC NPs were developed. To overcome the aggregation of PVC NPs, sodium dodecylbenzene sulfonate was used as the stabilizer of PVC NPs. The brand-new disposable PVC infusion tubes were found to carry PVC MPs with the average total number (ATN) of 931.4 particles and PVC NPs with the average mass of 0.040 µg, respectively. For four typical infusion fluids such as 0.9% sodium chloride, 5% glucose, 5% sodium bicarbonate, hydroxyethyl starch 40 sodium chloride, the released PVC MPs and NPs were ranged from 1003.6 ∼ 3494.6 particles and 0.042 ∼ 0.087 µg, respectively in stimulating normal infusion scenario (room temperature 4 h). The released PVC MPs and NPs were also increased with the infusion duration and temperature. The released PVC MPs are mainly in granular form, accounting for 38 ∼ 49% of the total PVC MPs. Our findings indicate PVC MNPs can enter the blood vessel directly with the infusion fluids during intravenous infusion and the PVC MNPs exposure risk towards patients deserves more attention.

Quantification analysis of microplastics released from disposable polystyrene tableware with fluorescent polymer staining.

Ingesting microplastics (MPs) from plastic tableware is an important source of health risk to human bodies. However, the comprehensive information of MPs released from disposable tableware has not been explored. Herein, a new visual quantification method for polystyrene MPs is proposed with carbon nitride fluorescent polymers staining, which can overcome the disadvantages of high signal background and photobleaching derived from organic dyes staining. Combining with fluorescence microscope and ImageJ software, the quantity, shape, and size distribution of MPs carried by the brand-new disposable polystyrene tableware (DPT) samples before usage and released from the clean DPT samples in different simulated usage scenes were studied. The brand-new DPT samples were found to carry a large number of MPs particles and the clean DPT samples could release MPs during usage. Fiber and fragment are the main morphology of the detected MPs and fiber accounts for 45-52 %. The particles with size <50 µm are the majority of the detected MPs and the distribution fraction of MPs particles is gradually decreased with the raising of particle size within 50 µm. The released MPs particles are increased with the raising of contact time and temperature, and greatly boosted for the DPT samples with cracks. The DPT samples are more like to release MPs in weak acidic condition (pH 4.0) than in weak alkaline (pH 8.3) and neutral (pH 7.0) conditions. The obtained results help to assess the food safety of tack-out food and the health risk of MPs exposure to human.

Highly fluorescent carbon nitride oligomer with aggregation-induced emission characteristic for plastic staining.

Polymeric carbon nitride often displays weak photoluminescence in solid state due to the aggregation-caused quenching effect. Herein, highly fluorescent carbon nitride oligomer (CNO) with aggregation-induced emission (AIE) characteristic was prepared via one-step solid-phase thermal condensation of 2,4-diamino-6-phenyl-1,3,5-triazine (DPT) at 350 °C. CNO is mainly composed of DPT dimer connected by rotatable imine groups, and exhibits weak fluorescence in the dispersed state and strong blue-green emission in the aggregated state and solid state. Density functional theory calculations indicate that the restriction of phenyl and triazine ring twisting motions is the main origin of the AIE phenomenon of CNO. Finally, CNO was preliminarily applied for fluorescent staining of plastic pellets. This work not only provides a solid-state strategy to synthesize fluorescent material with AIE characteristic but also extends the application of polymeric carbon nitride.

Fluorescent graphitic carbon nitride with photocatalytic oxidase-like activity for anti-counterfeiting application.

Herein bulk phenyl- and carbon-modified graphitic carbon nitride (PCCN) powders with tunable fluorescent emission from green-color to yellow-color were prepared by copolymerization of 2,4-diamino-6-phenyl-1,3,5-triazine and 2,2,6-triaminopyrimidine. The corresponding nanosheets with blue-color to green-color fluorescence were obtained by the oxidation of their bulk powders in sulfuric or nitric acid and then ultrasonic exfoliation. The typical PCCN0.6 nanosheets not only displayed strong green-color fluorescence but also exhibited photocatalytic oxidase-like activity, which can catalyze the oxidation of substrates 3,3',5,5'-tetramethylbenzidine and Amplex UltraRed by O2 to produce blue-color colorimetric product and pink-color fluorescent product, respectively. By taking advantage of green-color fluorescence and photocatalytic activity of PCCN0.6 nanosheets, a prototype for high-level anti-counterfeiting application was demonstrated by using the mixture of PCCN0.6 nanosheets and Amplex UltraRed as the fluorescent ink.

Salt-template preparation of Mo5N6 nanosheets with peroxidase- and catalase-like activities and application for colorimetric determination of 4-aminophenol.

Mo5N6 nanosheets were synthesized by a nickel-induced growth method and were found to possess peroxidase-like activity in acidic condition and catalase-like activity in weak basic condition. In acidic condition, Mo5N6 nanosheets can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2 to form a blue color product (TMBOX). At the co-existence of 4-aminophenol (4-AP), 4-AP can react with H2O2 and TMBOX, resulting in the decrease of TMBOX and the fading of blue color. Therefore, a facile, sensitive colorimetric method for the quantitative detection of 4-AP was developed. The linear range for 4-AP was 1.0 to 80.0 µmol⋅L‒1 (R2 = 0.999), and the detection limit was 0.56 µmol⋅L‒1 based on 3σ/k. Resorcinol, aniline, humic acid, and common ions and anions in surface water did not interfere the determination of 4-AP. This colorimetric method was applied to measure the 4-AP in real water sample from Wulong River in Fujian Province of China. The relative standard deviation for the determination of 4-AP was ranged from 0.03 to 1.88%, and the recoveries from spiked samples were ranged between 99.2 and 107.6%. The determination results were consistent with those obtained by HPLC.

Highly fluorescent g-C3N4 nanobelts derived from bulk g-C3N4 for NO2 gas sensing.

The fluorescent emission wavelengths of nanostructures derived from bulk graphitic carbon nitride were commonly lower than those of their bulk due to the quantum confinement effect, which are disadvantageous for bioimaging and sensing applications. Herein, a new strategy to engineer graphitic carbon nitride nanomaterials with tunable fluorescent wavelength and intensity was proposed via thermal treatment of bulk graphitic carbon nitride at high temperature and then hydrolysis in alkali solution. Highly fluorescent g-C3N4 nanobelts with emission peak at 494 nm, 19 nm higher than that of bulk graphitic carbon nitride and 23.6% quantum yield were successfully obtained by controlling the heating temperature at 750 °C for 2 h and the hydrolysis in 4 mol L-1 NaOH solution for 8 h. Finally, a home-made portable gas sensor for reversibly sensing of toxic NO2 gas at room temperature was designed by utilizing graphitic carbon nitride nanobelts as the fluorescent nanoprobe, which can overcome the disadvantages of high operation temperature or the interference of humidity resulting from the common chemiresistive sensors.

A highly sensitive method for simultaneous detection of hAAG and UDG activity based on multifunctional dsDNA probes mediated exponential rolling circle amplification.

DNA glycosylase is an indispensable DNA damage repair enzyme which can recognize and excise the damaged bases in the DNA base excision-repair pathway. The dysregulation of DNA glycosylase activity will give rise to the dysfunction of base excision-repair and lead to abnormalities and diseases. The simultaneous detection of multiple DNA glycosylases can help to fully understand the normal physiological functions of cells, and determine whether the cells are abnormal in pre-disease. Regrettably, the synchronous detection of functionally similar DNA glycosylases is a great challenge. Herein, we developed a multifunctional dsDNA probe mediated exponential rolling circle amplification (E-RCA) method for the simultaneously sensitive detection of human alkyladenine DNA glycosylase (hAAG) and uracil-DNA glycosylase (UDG). The multifunctional dsDNA probe contains the hypoxanthine sites and the uracil sites which can be recognized by hAAG and UDG respectively to generate apyrimidinic (AP) sites in the dsDNA probe. Then the AP sites will be recognized and cut by endonuclease Ⅳ (Endo IV) to release corresponding single-stranded primer probes. Subsequently, two padlock DNA templates are added to initiate E-RCA to generate multitudinous G-quadruplexes and/or double-stranded dumbbell lock structures, which can combine N-methyl mesoporphyrin IX (NMM) and SYBR Green Ⅰ (SGI) for the generation of respective fluorescent signals. The detection limits are obtained as low as 0.0002 U mL-1 and 0.00001 U mL-1 for hAAG and UDG, respectively. Notably, this method can realize the simultaneous detection of two DNA glycosylases without the use of specially labeled probes. Finally, this method is successfully applied to detect hAAG and UDG activities in the lysates of HeLa cells and Endo1617 cells at single-cell level, and to detect the inhibitors of DNA glycosylases.

3-Amino-1,2,4-triazole-derived graphitic carbon nitride for photodynamic therapy.

Graphitic carbon nitride (g-C3N4) has been shown as a promising visible-light photosensitizer for photodynamic therapy (PDT) application. Nevertheless, its therapeutic efficiency is limited by the low efficiency of visible-light utilization. To overcome this issue, 3-amino-1,2,4-triazole-derived graphitic carbon nitride nanosheets (g-C3N5 NSs) are prepared for PDT application. The addition of nitrogen-rich triazole group into the g-C3N4 motif significantly makes the light absorption of g-C3N5 NSs red-shift with the band gap down to 1.95 eV, corresponding to a absorption edge at a wavelength of 636 nm. g-C3N5 NSs generate superoxide anion radicals (O2•-) and singlet oxygen (1O2) under the irradiation of a low-intensity white light emitting diode. Owing to the high efficiency of visible-light utilization, g-C3N5 NSs show about 9.5 fold photocatalytic activity of g-C3N4 NSs. In vitro anticancer studies based on the results of CCK-8 assay, Calcein-AM/PI cell-survival assay and photo-induced intracellular ROS level analysis in living HeLa cells demonstrate the potential of g-C3N5 NSs as a low-toxic and biocompatible high-efficient photosensitizer for PDT.

Phenyl doped graphitic carbon nitride nanosheets for sensing of copper ions in living cells.

Copper (Cu) is a vital metal element for humans and animals. Monitoring and evaluating the concentration level of Cu2+ in a biological body is an effective way to prevent a variety of diseases. In this work, phenyl doped graphitic carbon nitride (PDCN) nanosheets with strong green fluorescence exhibited a sensitive and selective detection for Cu2+ with a linear range from 0.1-2.0 µmol L-1. Furthermore, fluorescent imaging was applied to semiquantitatively detect Cu2+ in HeLa cells using PDCN nanosheets as the probe, which can avoid the interference of background autofluorescence. This work provided a low-cost and biologically friendly fluorescent probe to monitor the concentration level of Cu2+ in living cells.

A tri-functional probe mediated exponential amplification strategy for highly sensitive detection of Dnmt1 and UDG activities at single-cell level.

Multiplex DNA methylation and glycosylation are ubiquitous in the human body to ensure the normal function and stability of the genome. The methyltransferases and glycosylases rely on varied enzymes with different action mechanism, which still remain challenges for multiple detection. Herein, we developed a tri-functional dsDNA probe mediated exponential amplification strategy for sensitive detection of human DNA (cytosine-5) methyltransferase 1 (Dnmt1) and uracil-DNA glycosylase (UDG) activities. The tri-functional dsDNA probe was rationally designed with M-DNA and U-DNA. M-DNA contains the 5'-GCmGCGC-3' site for Dnmt1 recognition. U-DNA possesses one uracil as the substrate of UDG and a primer sequence for initiating the amplification reaction. M-DNA was complementary to partial sequence of U-DNA. In the presence of Dnmt1 and UDG, BssHⅡ and Endo Ⅳ were used to nick the 5'-GCGCGC-3' and AP sites respectively, resulting in the release of single-stranded DNA sequence (primer sequence), respectively. After magnetic separation, the released primer sequence hybridizes with padlock DNA (P-DNA), initiating exponential rolling circle amplification to produce numerous G-quadruplexes for recordable signals. The strategy exhibited the limit of detection as low as 0.009 U mL-1 and 0.003 U mL-1 for Dnmt1 and UDG, respectively. Meanwhile, this strategy was successfully applied to detect Dnmt1 and UDG activities in living cell samples at single-cell level and assay the inhibitors of Dnmt1 and UDG. Therefore, the strategy provided a potential method to detect Dnmt1 and UDG activities in biological samples for early clinic diagnosis and therapeutics.

Colorimetric assay of acetylcholinesterase inhibitor tacrine based on MoO2 nanoparticles as peroxidase mimetics.

Molybdenum dichalcogenides MoX2 (X=S, Se) have been found to possess intrinsic peroxidase-like activity. However, molybdenum oxides (MoO2) as peroxidase mimetics have not been exploited yet. Herein, MoO2 nanoparticles were synthesized by a simple hydrothermal method and found to possess the peroxidase-like activity for the first time. MoO2 nanoparticles could catalyze the oxidation of 3,3',5,5'-tetrametylbenzidine (TMB) by H2O2 to produce a blue-color product (oxTMB). The catalytic property and mechanism were investigated by stead-state kinetics experiment and free radicals scavenging experiment, respectively. Acetylcholinesterase (AChE) could catalyze the hydrolysis of acetylthiocholine chloride (ATCh) into thiocholine (TCh), which could reduce oxTMB to decrease the absorbance in solution. In the presence of AChE inhibitor tacrine, the generation of TCh was inhibited and the absorbance was preserved. Based on these properties, a colorimetric assay method was developed for AChE inhibitor tacrine. This work not only broadens the application of the peroxidase mimetics, but also overcome the disadvantages of traditional methods such as expensive, complex and vulnerable to background interference for colorimetric assay of AChE inhibitor.

Fluorescence imaging of Cys in keratinocytes upon UVB exposure using phenyl doped graphitic carbon nitride Nanosheets-Au nanoparticles nanocomposite.

Ultraviolet B (UVB) irradiation induces the generation of reactive oxygen species (ROS) and causes damages to human skin. The depletion of glutathione (GSH) to scavenge ROS can protect skin cells from oxidative damage. However, little is known about the concentration level changes of cysteine (Cys), a precursor of GSH in skin cells after exposure to UVB irradiation. Herein, phenyl doped graphitic carbon nitride nanosheets-Au nanoparticles nanocomposite was prepared by in situ deposition of Au nanoparticles on the surface of phenyl doped graphitic carbon nitride nanosheets and showed a turn-on fluorescence response toward Cys over homocysteine, glutathione under physiological conditions. In the presence of Cys, remarkable enhancement of green fluorescence was observed. This nanocomposite was successfully applied for fluorescence imaging of Cys in human skin epidermal cells and monitoring the changes of Cys concentration level under the oxidative stress upon exposure to UVB irradiation in keratinocytes. It was found that the concentration of Cys was increased in the initial period after exposure to UVB irradiation and then gradually decreased to the normal level for the synthesis of GSH to defense the oxidative stress. Our result helps to understand the physiological function of Cys in human skin cells under UVB exposure.

A ratiometric multicolor fluorescence biosensor for visual detection of alkaline phosphatase activity via a smartphone.

Herein we designed a selective and smartphone-based strategy for visual detection of alkaline phosphatase (ALP) by utilizing the property of amino-functionalized copper (II)-based metal-organic frameworks (NH2-Cu-MOFs) with oxidase mimic property and fluorescence property. Surprisingly, the oxidase mimic property of NH2-Cu-MOFs can work well at a high pH value 8.0. Thus, a cascade reaction between ALP and NH2-Cu-MOFs was realized for the construction of a ratiometric multicolor sensing platform through the controllable catalytic activity of NH2-Cu-MOFs by pyrophosphate (PPi) and ALP. The catalytic activity of NH2-Cu-MOFs was greatly inhibited because of the binding ability of Cu2+ with PPi. When the ALP was added, the catalytic activity of NH2-Cu-MOFs was restored and then further catalyzed the o-phenylenediamine to form the 2, 3-diaminophenazine due to the hydrolysis function of ALP towards PPi into orthophosphates. RGB analysis of the fluorescent sample images was adopted for ALP quantitative analysis. Besides, a hydrogel test kit and mobile app for ALP detection were designed as conceptual products for point-of-care. The LODs of the fluorescence sensing platform was 0.078 mU mL-1 and 0.35 mU mL-1 by solution analysis and hydrogel test kit analysis, respectively. This fluorescent visual method was applied to ALP detection in serum samples with satisfying results, which opened a promising horizon for the diagnosis of other biomarkers in clinical serum samples based on ALP-mediated enzyme-linked immunosorbent assay for the development of biomedicine and clinical diagnosis.

Colorimetric determination of xanthine in urine based on peroxidase-like activity of WO3 nanosheets.

Two-dimensional WO3 nanosheets were prepared by ultrasonic exfoliation of bulk WO3·2H2O in water and characterized by transmission electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman, dynamic light scattering. The nanosheets were discovered to possess the peroxidase-like catalytic activity, which can catalyze the oxidation of 3, 3', 5, 5'-tetramethylbenzidine by H2O2. The catalytic mechanism was also investigated by the scavenger experiments. Taking advantage of the peroxidase-like activity of WO3 nanosheets, a facile colorimetric method for xanthine was developed by combining the oxidation reaction of xanthine catalyzed by xanthine oxidase. The linear range for xanthine was ranged from 25 to 200 µmol L-1. The limit of detection for xanthine was 1.24 µmol L-1. The colorimetric method was applied to determine xanthine in urine samples.

Thermo- and Photoresponsive Actuators with Freestanding Carbon Nitride Films.

The development of versatile actuators that convert environmental energy (heat or light, especially sunlight) into mechanical energy remains a great challenge. Herein, freestanding carbon nitride films were prepared by the physical vapor deposition method using bulk g-C3N4 powder as a starting material. The carbon nitride films exhibited controllable deformation under the stimuli of heat and light. An alarm device for high temperature and an artificial hand for grasping and releasing objects were designed. By utilizing the photothermal effect, a smart curtain that could block the UV rays and a manipulator that could perform sophisticated mechanical work were demonstrated under the direct irradiation of natural and simulated sunlight. This work exploited the potential applications of carbon nitride polymers for the conversion of solar energy to mechanical energy.

Photoactivated oxidase mimetics derived from dicyandiamide and barbituric acid for colorimetric detection of glutathione.

In this work, photoactivated oxidase mimetics was prepared by copolymerizing dicyandiamide with barbituric acid (BA) and characterized by X-ray diffraction pattern, Fourier transformed infrared spectrum, X-ray photoelectron spectroscopy, transmission electron microscopy, photoluminescence spectrum, diffuse reflectance spectrum. Experimental results and density functional theory calculation indicated that the substitution of nitrogen atoms by carbon atoms in tri-s-triazine structure due to the copolymerization of BA enhanced visible light absorption and weakened the barrier of photocarrier transfer. In the presence of visible light and oxygen, 3, 3', 5, 5'-tetramethylbenzidine was oxidized under the catalysis of photoactivated oxidase mimetics to produce a green colored product, which could be reduced by glutathione (GSH). Therefore, a facile method based on the photoactivated oxidase mimetic has been developed for colorimetric detection of GSH. The linear range for GSH was ranged from 2.0 to 50.0 µmol L-1 (R2 = 0.998) with the detection limit of 1.4 µmol L-1. The proposed method was applied to detect the cellular GSH with satisfactory results.

Exfoliation of transition-metal dichalcogenides using ATP in aqueous solution.

A facile, environmentally friendly liquid ultrasonic exfoliation of transition metal dichalcogenides (TMDCs) was proposed using ATP as the exfoliating reagent. A biosensing platform for the detection of adenosine based on the fluorescence quenching ability of as-exfoliated MoS2 nanosheets was also demonstrated.

Boron- and Phenyl-Doped Graphitic Carbon Nitride (g-C3N4) Nanosheets for Colorimetric Detection of Hydrogen Peroxide in Soaked Foods.

Boron- and phenyl-doped graphitic carbon nitride nanosheets (BPCN NSs) were prepared by thermal polymerization of cyanamide with 3-aminobenzeneboronic acid followed by ultrasonic exfoliation. BPCN NSs exhibited enhanced peroxidase-like activity and catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) and o-phenylenediamine by H2O2. A simple sensitive colorimetric senor was developed for H2O2 by utilizing TMB as the substrate and BPCN NSs as enzyme mimetic. The linear relations between the absorbance and H2O2 concentration over the range from 0 to 280 µmol L-1 and from 280 to 1000 µmol L-1 were obtained with the limit of detection of 1.0 µmol L-1 according to the 3σ rule. The colorimetric sensor was applied for the detection of H2O2 residue in simulated soaked foods with satisfied results. Finally, the portable test kits for H2O2 were prepared and applied for the semi-quantitative assay of H2O2 residues in soaked chicken feet.

A ratiometric nanoprobe for biosensing based on green fluorescent graphitic carbon nitride nanosheets as an internal reference and quenching platform.

The integrating of high fluorescent quenching capability of two-dimensional nanomaterials with dye-labelled ssDNA as nanoprobes has attracted increasing interest for biosensing applications. However, this absolute intensity-dependent single fluorescent signal may be influenced by target concentration-independent factors. To overcome this challenge, a ratiometric nanoprobe was developed by utilizing green fluorescent phenyl-doped carbon nitride (PDCN) nanosheets as an internal reference and quenching platform. 5-carboxy-X-rhodamine-labelled anti-adenosine aptamer was used as a signal probe. PDCN nanosheets quenched the fluorescence of the absorbed signal probe while kept their own fluorescence constantly. Upon addition of adenosine, the formation of adenosine-aptamer complexes led to desorption of the signal probe from the surface of PDCN nanosheets, resulted in the fluorescent recovery of the signal probe. The ratio of the fluorescent enhancement of the signal probe to the inherent fluorescence of PDCN nanosheets was used to quantitatively measure adenosine. The limit of detection for adenosine was 6.86 µM. Finally, the ratiometric nanoprobe was applied to determine adenosine in serum samples.