Construction of Acetaldehyde-Modified g-C3N4 Ultrathin Nanosheets via Ethylene Glycol-Assisted Liquid Exfoliation for Selective Fluorescence Sensing of Ag. ACS Appl Mater Interfaces 2018;10:44624-44633. [PMID: 30511564 DOI: 10.1021/acsami.8b15501]

Graphite Phase Carbon Nitride Nanosheets-Based Fluorescent Sensors for Analysis and Detection

Fluorescent sensors reflect information such as the concentration or content of the analysis by interacting with a specific recognition group to change the signal of the fluorophore. It has attracted much attention because of its advantages of high sensitivity, fast detection speed and low cost, and it has become an effective alternative to traditional detection methods. Graphitic phase carbon nitride nanosheets (g-CNNs) are a class of carbon-based fluorescent nanomaterials derived from bulk graphite phase carbon nitride (g-C3N4), which have attracted much attention from scholars because of their advantages of low cost, simple fabrication, high quantum yield, strong stability and nontoxicity. Functional modified g-CNNs can greatly improve the photocatalytic performance. At present, although there have been some researches on fluorescent sensors based on g-CNNs. Nevertheless, there are few reviews about the g-CNNs-based fluorescent sensors. Therefore, in addition to summarizing the sensing mechanism of fluorescent sensors (such as photoinduced electron transfer, fluorescence resonance energy transfer, and intramolecular charge transfer) and the advantages and disadvantages of common signal substances, this paper focused on the application progress of g-CNNs-based fluorescent sensors in the field of analysis and detection.

Terpyridine Ni(II) Complex Grafted CdS Nanorods for Cooperative Selective Benzyl Alcohol Oxidation and Hydrogen Production

Efficient utilization of photogenerated charge carriers to realize photocatalytic solar fuel production and oxidative chemical synthesis is a challenging task. Herein, a conventional amidation reaction route is adopted to successfully construct a novel composite photocatalyst composed of a Ni(II)-terpyridine complex with carboxyl groups grafted on CdS nanorods (labeled as CdS@Ni(terpyC)2). Experimental results have unequivocally revealed that the as-fabricated composite catalyst exhibited a remarkable enhancement in photocatalytic activity for the dehydrogenation of benzyl alcohol under visible light, demonstrating superior hydrogen evolution efficiency and benzaldehyde selectivity, surpassing both pristine CdS and the blend of CdS and Ni(terpyC)2. The carrier dynamics study demonstrated that the Ni(terpyC)2 on the surface of CdS could quickly extract the photogenerated electrons of CdS, which reduced the carrier recombination efficiency, further improving the photocatalytic activity of the catalyst. This work illustrates the effect of surface active site engineering on photocatalysis and is expected to shed substantial inspiration on future surface modulation and design of semiconductor photocatalysts.

Smartphone-integrated ratiometric sensing strategy for on-line quantitation of tetracycline based on functionalized g-C3N4/Eu electrospun film

On-line quantitation of tetracycline (TC) is significant to ensure environmental health and food security. Hence, a novel smartphone-integrated ratiometric sensing platform for on-line quantitative analysis of TC was designed. A CitNa-functionalized g-C3N4/Eu3+ (g-C3N4/CitNa/Eu) composites with blue and red dual-emissive feature were fabricated as dual indicators for shielding background interference, enhancing anti-interference capability. The fluorescent response (F620/F450) ratio and TC concentration demonstrated good linear relationship ranged from 0.0 to 100.0 μM with a detection limit of 1.96 nM. Furthermore, the combination of g-C3N4/CitNa/Eu and polyacrylonitrile polymers forming electrospun film was achieved via electrospinning method. Smartphone-integrated ratiometric sensing platform was developed based on the fluorescent color of electrospun film from blue to light red with TC. This solid sensing platform achieved excellent sensitivity with a detection limit of 7.42 nM. Combining the solid ratiometric fluorescent film with smartphone reader provides a potential way for on-line quantitation of TC in food and other fields.

An Ethyl-Thioglycolate-Functionalized Fe3O4@ZnS Magnetic Fluorescent Nanoprobe for the Detection of Ag+ and Its Applications in Real Water Solutions

Ethyl-thioglycolate-modified Fe₃O₄@ZnS nanoparticles (Fe₃O₄@ZnS-SH) were successfully prepared using a simple chemical precipitation method. The introduction of ethyl thioglycolate better regulated the surface distribution of ZnS, which can act as a recognition group and can cause a considerable quenching of the fluorescence intensity of the magnetic fluorescent nanoprobe, Fe₃O₄@ZnS-SH. Benefiting from stable fluorescence emission, the magnetic fluorescent nanoprobe showed a highly selective fluorescent response to Ag+ in the range of 0-400 μM, with a low detection limit of 0.20 μM. The magnetic fluorescent nanoprobe was used to determine the content of Ag⁺ in real samples. A simple and environmentally friendly approach was proposed to simultaneously achieve the enrichment, detection, and separation of Ag⁺ and the magnetic fluorescent nanoprobe from an aqueous solution. These results may lead to a wider range of application prospects of Fe₃O₄ nanomaterials as base materials for fluorescence detection in the environment.

Microenvironment Restruction of Emerging 2D Materials and their Roles in Therapeutic and Diagnostic Nano-Bio-Platforms

Engineering advanced therapeutic and diagnostic nano-bio-platforms (NBPFs) have emerged as rapidly-developed pathways against a wide range of challenges in antitumor, antipathogen, tissue regeneration, bioimaging, and biosensing applications. Emerged 2D materials have attracted extensive scientific interest as fundamental building blocks or nanostructures among material scientists, chemists, biologists, and doctors due to their advantageous physicochemical and biological properties. This timely review provides a comprehensive summary of creating advanced NBPFs via emerging 2D materials (2D-NBPFs) with unique insights into the corresponding molecularly restructured microenvironments and biofunctionalities. First, it is focused on an up-to-date overview of the synthetic strategies for designing 2D-NBPFs with a cross-comparison of their advantages and disadvantages. After that, the recent key achievements are summarized in tuning the biofunctionalities of 2D-NBPFs via molecularly programmed microenvironments, including physiological stability, biocompatibility, bio-adhesiveness, specific binding to pathogens, broad-spectrum pathogen inhibitors, stimuli-responsive systems, and enzyme-mimetics. Moreover, the representative therapeutic and diagnostic applications of 2D-NBPFs are also discussed with detailed disclosure of their critical design principles and parameters. Finally, current challenges and future research directions are also discussed. Overall, this review will provide cutting-edge and multidisciplinary guidance for accelerating future developments and therapeutic/diagnostic applications of 2D-NBPFs.

Ultrathin g-C3N4 nanosheet-CoOOH nanocomposite for fluorescence imaging of ascorbic acid in living cells

Ascorbic acid (AA), a critical cellular metabolite involved in many biochemical pathways, is an important antioxidant in human body. Therefore, it is of great significance to monitor AA in living cells. Nowadays, there are various technologies developed for the detection of AA, but few methods could sensitively and selectively detect the intracellular AA. Here, we reported a highly efficient biosensor (g-C₃N₄-CoOOH nanocomposite) based on ultrathin graphitic carbon nitride (g-C₃N₄) nanosheets and CoOOH nanoflakes, for sensitive detection and fluorescence imaging of AA in living cell. The g-C₃N₄ used here as fluorescence donor is a promising bioimaging nanomaterial because of their high fluorescence quantum yield, good biocompatibility and low toxicity. In addition, the CoOOH was used to be perfect fluorescence quencher. Herein, we enabled the CoOOH in situ to form a layer on the surface of g-C₃N₄, resulting in fluorescence quench of the g-C₃N₄. Upon the addition of AA, the CoOOH nanoflakes were reduced to Co²⁺, and the system gave a "turn on" fluorescence signal. It developed as an efficient sensing platform for AA, and the linear range was from 5 to 50 μM with a 1.6 μM detection limit. This novel biosensor, g-C₃N₄-CoOOH nanocomposite exhibited highly selective response toward AA relative to other biomolecules. Furthermore, this biosensor was used successfully to visualize and monitor AA in living cells. Hopefully, we believe that this biosensor would provide a low-cost and highly sensitive platform for AA detection and bioimaging. Schematic illustration of the sensing strategy based on the g-C₃N₄-CoOOH nanocomposite for AA detection.

Boosting the Quantum Yield of Oxygen-Doped g-C3N4 via a Metal-Azolate Framework-Enhanced Electron-Donating Strategy for Highly Sensitive Sulfadimethoxine Tracing

Two of the most persistent challenges for the sensing applications of luminescent carbon nitride-based materials are poor quantum yields and aggregation-induced luminescence quenching in aqueous environments. Herein, a highly emissive oxygen-doped carbon nitride composite (OCNP@M7) was synthesized, with a metal-azolate framework (MAF-7) serving as a luminous booster. Both experimental studies and theoretical calculations suggest that the MAF-enhanced electron-donating effect dramatically promoted the electron density on the π-structure of oxygen-doped carbon nitride. In addition, the structural rigidity of MAF-7 effectively inhibits both aggregation and nonradiative energy dissipation. Consequently, OCNP@M7 exhibits strong and stable blue emission under UV light irradiation and an absolute quantum yield up to 95.2%, which is, as far as we know, the highest value among fluorescent carbon nitride materials in solution ever reported. OCNP@M7 could further function as a high-efficiency fluorescent probe for the sensitive detection of sulfadimethoxine residues in complex environments. It is anticipated that this strategy can be extended to fabricate various carbon nitride-based antibiotic monitoring systems with tailor-made functions.

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

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

Two-dimensional biomaterials: material science, biological effect and biomedical engineering applications

To date, nanotechnology has increasingly been identified as a promising and efficient means to address a number of challenges associated with public health. In the past decade, two-dimensional (2D) biomaterials, as a unique nanoplatform with planar topology, have attracted explosive interest in various fields such as biomedicine due to their unique morphology, physicochemical properties and biological effect. Motivated by the progress of graphene in biomedicine, dozens of types of ultrathin 2D biomaterials have found versatile bio-applications, including biosensing, biomedical imaging, delivery of therapeutic agents, cancer theranostics, tissue engineering, as well as others. The effective utilization of 2D biomaterials stems from the in-depth knowledge of structure-property-bioactivity-biosafety-application-performance relationships. A comprehensive summary of 2D biomaterials for biomedicine is still lacking. In this comprehensive review, we aim to concentrate on the state-of-the-art 2D biomaterials with a particular focus on their versatile biomedical applications. In particular, we discuss the design, fabrication and functionalization of 2D biomaterials used for diverse biomedical applications based on the up-to-date progress. Furthermore, the interactions between 2D biomaterials and biological systems on the spatial-temporal scale are highlighted, which will deepen the understanding of the underlying action mechanism of 2D biomaterials aiding their design with improved functionalities. Finally, taking the bench-to-bedside as a focus, we conclude this review by proposing the current crucial issues/challenges and presenting the future development directions to advance the clinical translation of these emerging 2D biomaterials.

A simple enzyme-catalyzed reaction induced "switch" type fluorescence biosensor based on carbon nitride nanosheets for the assay of alkaline phosphatase activity

An enzyme-catalyzed fluorescence "switch" type sensor was constructed for the determination of alkaline phosphatase (ALP) activity by combining the fluorescence quenching effect of Ag+ on ultrathin g-C3N4 nanosheets (CNNSs) with the simple redox reaction of AA and Ag+. Briefly, Ag+ exhibits a significant quenching effect on the fluorescence of CNNSs. Thus the fluorescence signal of the CNNS-Ag+ system is extremely weak even in the presence of l-ascorbic acid-2-phosphate (AAP) ("off" state). When ALP coexists in the system, the enzyme can specifically catalyze the hydrolysis of AAP to form ascorbic acid (AA), which reduces Ag+ to Ag0. In this case, the fluorescence signal of the system is recovered ("on" state). Based on this principle, a signal-enhanced CNNS fluorescence sensor was developed to determine the activity of alkaline phosphatase. The experimental results show that the detection range of alkaline phosphatase is 0.5-20 U L-1, and the detection limit is 0.05 U L-1 (S/N = 3). Meanwhile, this method was used to assay ALP in serum samples.

Salt-resistant nanosensor for fast sulfadimethoxine tracing based on oxygen-doped g-C3N4 nanoplates

A novel oxygen-doped g-C₃N₄ nanoplate (OCNP) structure that can serve as an efficient sulfadimethoxine (SDM) sensing platform has been developed. Taking advantage of its inherent oxygen-containing functional groups and 2D layered structure with π-conjugated system, OCNP exhibits effective radiative recombination of surface-confined electron-hole pairs and efficient π-π interaction with SDM. This causes rapid fluorescence response and thus ensures the fast and continuous monitoring of SDM. Based on the fluorescence experiments and band structure calculation, the mechanism of the SDM-induced quenching phenomenon was mainly elucidated as the photoinduced electron transfer process under a dynamic quenching mode. Under optimized conditions, the as-proposed nanosensor, which emitted strong fluorescence at 375 nm with an excitation wavelength at 255 nm, presents an excellent analytical performance toward SDM with a wide linear range from 3 to 60 μmol L^-1 and a detection limit of 0.85 μmol L^-1 (S/N = 3). In addition, this strategy exhibits satisfactory recovery varied from 94 to 103% with relative standard derivations (RSD) in the range 0.9 to 6.8% in real water samples. It also shows marked tolerability to a series of high concentrations of metals and inorganic salts. This strategy not only broadens the application of oxygen-doped g-C₃N₄ nanomaterial in antibiotic sensing field but also presents a promising potential for on-line contaminant tracing in complex environments.

Fluorescent and visual assay of H2O2 and glucose based on a highly sensitive copper nanoclusters-Ce(III) fluoroprobe

Herein, we synthesized and characterized glutathione-capped copper nanoclusters (CuNCs) using a convenient one-pot chemical reduction approach based on glutathione as capping and reducing agents. The Ce(III) induced aggregation-induced emission of CuNCs to form a CuNCs-Ce³⁺ fluoroprobe due to electrostatic and coordination interactions between Ce³⁺ and CuNCs. In contrast to CuNCs, the fluorescent intensities (FLs) of CuNCs-Ce³⁺ were enhanced by ~ 40-fold concomitant with 20-nm blue-shift of the maximum emission, and a 3.45-fold lengthening of the average fluorescent lifetime. The FLs of CuNCs-Ce³⁺ were selectively quenched at 650 nm by hydrogen peroxide (H₂O₂) via the redox reaction. Based on this phenomenon, the sensitive assay of H₂O₂ was realized, and the linear range spanned over the range of 14-140 μM. Notably, the visualization of the fluorescence quenched effect of H₂O₂ could be easily attained. Additionally, glucose could be specifically oxidized by glucose oxidase to produce H₂O₂, and thus the detection of glucose was achieved according to changes in the concentrations of H₂O₂. Under optimized conditions, the fluorescent assay of glucose based on the CuNCs-Ce³⁺ system offered the linear range of 8-48 μM with detection limit of 2.4 μM. Meanwhile, high selectivity of the as-constructed fluorescent assay allows the sensitive detection of H₂O₂ and glucose in real-world care products and human serum samples, showing a great application potential in their conventional monitoring.

A ratiometric electrochemiluminescence strategy based on two-dimensional nanomaterial-nucleic acid interactions for biosensing and logic gates operation

Two-dimensional (2D) nanomaterial-nucleic acid interactions have been widely used in the construction of fluorescent sensors, but they are rarely used in the construction of electrochemiluminescent (ECL) sensors and have never been used in the design of ratiometric ECL sensors. Therefore, a ratiometric ECL sensing platform was developed in this study based on the ECL resonance energy transfer (ECL-RET) of graphitic carbon nitride nanosheets (GCNNs)/Ru(bpy)₃²⁺ donor/acceptor pair. The 2D GCNNs showed much weaker affinity to the long dsDNA duplexes formed by hybridization chain reaction (HCR) than Ru(bpy)₃²⁺-lableled fuel hairpin DNAs (H1 and H2) for HCR. Therefore, the target-initiated HCR resulted in the luminescence enhancement of the GCNNs at 460 nm and the luminescence attenuation of the Ru(bpy)₃²⁺ at 610 nm. By measuring the I_{460 nm}/I_{610 nm} ratios, quantitative analysis of microRNA-133a was realized with a limit of detection of 0.41 pM. In addition, this ECL-RET sensing platform can be easily extended to detect metal ions or aptamer substrates by simply redesigning helper DNAs without changing the sequences of fuel hairpin DNAs. Moreover, due to the programmability of HCR, a series of sensitive logic gates ("OR", "INHIBIT", "AND", "NAND" and "INHIBIT-OR") based on the ECL-RET ratiometry can be constructed and responded to as low as 100 pM of Hg²⁺ or Ag⁺.

Smartphones and Test Paper-Assisted Ratiometric Fluorescent Sensors for Semi-Quantitative and Visual Assay of Tetracycline Based on the Target-Induced Synergistic Effect of Antenna Effect and Inner Filter Effect

Development of selective and sensitive methods for on-site assay of tetracycline (TC) is of great significance for public health and food safety. Herein, a valid ratiometric fluorescence strategy using g-C₃N₄ nanosheets coupled with Eu³⁺ is designed for the assay of TC. In this strategy, both Eu³⁺ and g-C₃N₄ nanosheets serve as the recognition units of TC. The blue fluorescence of g-C₃N₄ nanosheets can be quenched by TC via the inner filter effect (IFE); meanwhile, the red fluorescence of Eu³⁺ can be enhanced by TC through the antenna effect (AE). The synergistic effect of AE and IFE caused by TC makes the developed ratiometric fluorescent sensor display a wide linear range for TC from 0.25 to 80 μM with a detection limit of 6.5 nM and a significant fluorescence color evolution from blue to red. Given its simplicity, free-label, excellent selectivity, high sensitivity, and recognizable color change, point-of-care testing systems, including smartphones and test paper-based assays, are developed for the visual sensing of TC. The integration of smartphones and test paper on a ratiometric fluorescent sensor greatly reduces the detection cost and time, providing a promising method for the qualitative discernment and semi-quantitative assay of TC on-site. Moreover, the potential application of the approach is also verified by detecting TC in milk.

Fluorescent assay based on phenyl-modified g-C3N4 nanosheets for determination of thiram

Phenyl-modified graphitic carbon nitride nanosheets (Ph-g-C₃N₄ NSs) were synthesized by a thermal copolymerization and ultrasonic exfoliation method. The Ph-g-C₃N₄ NSs are used as a fluorescent assay for determination of thiram. The results of X-ray photoelectron spectroscopy, ¹³C solid-state nuclear magnetic resonance and Fourier transform infrared spectra confirm that phenyl group is integrated into the heptazine network of g-C₃N₄. Compared to the g-C₃N₄ NSs, the Ph-g-C₃N₄ NSs show bigger stokes shift about 185 nm and higher fluorescence intensity. The fluorescence of Ph-g-C₃N₄ NSs is quenched by Cu²⁺ via the photo-induced electron transfer mechanism, which then recovers in the presence of thiram. The fluorescence restoring of Ph-g-C₃N₄ NSs is correlated with the concentration of thiram. Under the optimized conditions, the fluorescent intensity of g-C₃N₄ NSs at excitation/emission wavelengths of 310/455 nm give a linear range of 33.0-670 nM with detection limit of 9.90 nM. While fluorescent assay based on the Ph-g-C₃N₄ NSs show the linear range of 6.70-1300 nM at excitation/emission wavelengths of 310/495 nm with detection limit of 2.01 nM. Graphical abstract Schematic representation of fluorescent "on-off-on" assay based on phenyl-modified graphitic carbon nitride nanosheets (Ph-g-C₃N₄ NSs) for determination of thiram.