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.

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.

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.

Barbituric acid-modified graphitic carbon nitride nanosheets for ratiometric fluorescent detection of Cu2.

In this work, barbituric acid-modified graphitic carbon nitride (BCN) nanosheets with blue-green fluorescence were prepared by a copolymerization of dicyandiamide with barbituric acid and then by a chemical oxidation process. A ratiometric fluorescent sensor for Cu2+ was designed by using BCN nanosheets as the report probe and Ru(bpy)3Cl2 as the reference probe. In the presence of different concentrations of Cu2+, the blue-green fluorescence of BCN nanosheets was drastically quenched, while the red fluorescence of Ru(bpy)3Cl2 remained constant. The color of the sensor changed from blue-green to red, resulting in ratiometric and visual detection of Cu2+ ions. The detection limit for Cu2+ was estimated to be 70 nM, and as low as 5 µM Cu2+ was distinguished with the naked eye. Additionally, this sensor was also applied for the detection of Cu2+ in tap water samples.

Visual Monitoring of Food Spoilage Based on Hydrolysis-Induced Silver Metallization of Au Nanorods.

Colorimetric detection of biogenic amines, well-known indicators of food spoilage, plays an important role for monitoring of food safety. However, common colorimetric sensors for biogenic amines suffer from low color resolution or complicated design and intricate output for the end-users. Herein, we explored a simple but effective strategy for visual monitoring of biogenic amines with multiple color change based on hydrolysis-induced silver metallization reaction to tune the localized surface plasmon resonance (LSPR) adsorption of Au nanorods (NRs). The color change and blue shift of longitudinal LSPR peak of Au NRs were closely related to the concentration of biogenic amines. This strategy provided a simple, sensitive, robust, nondestructive, cost-effective, and user-friendly platform for in situ evaluating the freshness of foodstuffs.

Invisible Security Ink Based on Water-Soluble Graphitic Carbon Nitride Quantum Dots.

Stimuli-responsive photoluminescent (PL) materials have been widely used as fluorescent ink for data security applications. However, traditional fluorescent inks are limited in maintaining the secrecy of information because the inks are usually visible by naked eyes either under ambient light or UV-light illumination. Here, we introduced metal-free water-soluble graphitic carbon nitride quantum dots (g-CNQDs) as invisible security ink for information coding, encryption, and decryption. The information written by the g-CNQDs is invisible in ambient light and UV light, but it can be readable by a fluorescence microplate reader. Moreover, the information can be encrypted and decrypted by using oxalic acid and sodium bicarbonate as encryption reagent and decryption reagent, respectively. Our findings provide new opportunities for high-level information coding and protection by using water-soluble g-CNQDs as invisible security ink.

High-Throughput and Rapid Screening of Low-Mass Hazardous Compounds in Complex Samples.

Rapid screening and identification of hazardous chemicals in complex samples is of extreme importance for public safety and environmental health studies. In this work, we report a new method for high-throughput, sensitive, and rapid screening of low-mass hazardous compounds in complex media without complicated sample preparation procedures. This method is achieved based on size-selective enrichment on ordered mesoporous carbon followed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry analysis with graphene as a matrix. The ordered mesoporous carbon CMK-8 can exclude interferences from large molecules in complex samples (e.g., human serum, urine, and environmental water samples) and efficiently enrich a wide variety of low-mass hazardous compounds. The method can work at very low concentrations down to part per trillion (ppt) levels, and it is much faster and more facile than conventional methods. It was successfully applied to rapidly screen and identify unknown toxic substances such as perfluorochemicals in human serum samples from athletes and workers. Therefore, this method not only can sensitively detect target compounds but also can identify unknown hazardous compounds in complex media.

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.

Oxidized 3,3',5,5'-tetramethylbenzidine nanobelts enhance colorimetric sensing of H2O2.

Herein, oxidized 3,3',5,5'-tetramethylbenzidine (oxTMB) nanobelts were developed to enhance the colorimetric and paper-based sensing of H2O2. It was found that the minor component of Fe2+ in Na2SO4 reagent could catalyze the oxidization of TMB by H2O2 into positively charged oxTMB, which was further assembled into dark blue oxTMB nanobelts via electrostatic interaction with SO42-. The extinction originating from the absorption and scattering of oxTMB nanobelts was utilized to quantitatively detect H2O2 with a wide linear detection range (1.0-300 µM) and a low limit of detection (0.48 µM). In addition, no coffee-ring effect was observed in the test zone of the paper-based colorimetric array, which was beneficial to judge the color by naked eye. Finally, the colorimetric method was applied to detect H2O2 in contact lens care solution. This work not only proposed a new colorimetric sensing platform for H2O2, but also highlighted the minor component in the reagent might influence the experimental result.

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.

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.

[Study of pH measuring based on i-motif DNA conformation switch and UV-Vis absorption spectroscopy of gold nanoparticles].

A fast, sensitive, colorimetric method for the detection of pH based on the differentiate effect of gold nanoparticles to the configuration of DNA was developed in this study. The UV-Vis absorption spectroscopy of the i-motif DNA-Au NPs system has been investigated, and the effect of the concentration of salt and i-motif DNA, reaction time and DNA sequence on the pH response of the system have been also optimized. Under the optimum conditions, the UV-Vis absorption spectroscopy of the Au NPs is changed regularly with pH in the range of 5.3 - 7.0, the absorbance at 520 nm is increased gradually while at 700 nm decreased. Correspondingly, the color of the Au NPs is varied from violet to red. The pH sensor is no need to modification, low cost, fast and can be carried out by naked eyes. It is promising to use in monitoring some life process which associated with pH variation.

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.

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.

Sensitive turn-on fluorescent detection of melamine based on fluorescence resonance energy transfer.

We here report a novel fluorescent method for the detection of melamine based on the high fluorescence quenching ability of gold nanoparticles. The fluorescence was significantly quenched via fluorescence resonance energy transfer when fluorescein molecules were attached to the surface of gold nanoparticles by electrostatic interaction. Upon addition of melamine, the fluorescence was enhanced due to the competitive adsorption of gold nanoparticles between melamine and fluorescein. Under the optimum conditions, the fluorescence enhancement efficiency [(I-I(0))/I(0)] showed a linear relationship with the concentration of melamine in the range of 1.0 × 10(-7) mol L(-1)~4.0 × 10(-6) mol L(-1), and the detection limit was calculated to be 1.0 × 10(-9) mol L(-1). The proposed method showed several advantages such as high sensitivity, short analysis time, low cost and ease of operation.

Label-free fluorescent sensor for mercury(II) ion by using carbon nanotubes to reduce background signal.

A simple, selective and sensitive turn-on fluorescent sensor for the detection of mercury(II) ion was developed using Sybr Green I as the signal reporter and SWCNTs as the quencher. Due to the affinity of SWCNTs towards ssDNA and organic dye, Sybr Green I, thymine-rich ssDNA and SWCNTs could form a self-assembly of three components, resulting in fluorescence quenching. Upon addition of another thymine-rich ssDNA and mercury(II) ion, formation of dsDNA via T-Hg(2+)-T base pairs enabled Sybr Green I to intercalate into the dsDNA, resulting in the restoration of fluorescence. SWCNTs were found to reduce the background signal and improve the analytical sensitivity. A linear relationship between the fluorescence intensity and the concentration of mercury(II) ion was observed in the range of 20-1250 nM (R = 0.9985) with a detection limit of 7.9 nM. The proposed method was applied to detect mercury(II) ion in tap water samples with good results.

Dual-mode unsymmetrical squaraine-based sensor for selective detection of Hg2+ in aqueous media.

A novel chemosensor based on unsymmetrical squaraine dye (USQ-1) for the selective detection of Hg(2+) in aqueous media is described. USQ-1 in combination with metal ions shows dual chromogenic and "turn-on" fluorogenic response selectivity toward Hg(2+) as compared to Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ba(2+), Al(3+), Cu(2+), Cd(2+), Mn(2+), Fe(3+), Ag(+), Pb(2+), Zn(2+), Ni(2+) and Co(2+) due to the Hg(2+)-induced deaggregation of the dye molecule. A recognition mechanism based on the binding mode is proposed based on the absorption and fluorescence changes, (1)H NMR titration experiments, ESI-MS study, and theoretical calculations.

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.

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.

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.