Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

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Visible-light driven heterojunction photocatalysts for water splitting – a critical review

Pickering emulsions possess many advantages over traditional surfactant stabilized emulsions. For example, Pickering emulsions impart better stability against coalescence and, in many cases, are biologically compatible and environmentally friendly. These characteristics open the door for their use in a variety of industries spanning petroleum, food, biomedicine, pharmaceuticals, and cosmetics. Depending on the application, rapid, but controlled stabilization and destabilization of an emulsion may be necessary. As a result, Pickering emulsions with stimuli-responsive properties have, in recent years, received a considerable amounts of attention. This paper provides a concise and comprehensive review of Pickering emulsion systems that possess the ability to respond to an array of external triggers, including pH, temperature, CO2 concentration, light intensity, ionic strength, and magnetic field. Potential applications for which stimuli-responsive Pickering emulsion systems would be of particular value, such as emulsion polymerization, enhanced oil recovery, catalyst recovery, and cosmetics, are discussed.

Stimuli-responsive Pickering emulsions: recent advances and potential applications

The technological evolution has been progressing for centuries and will possibly increase at a higher rate in the 21st century. Currently, in this age of nanotechnology, the discovery of more economical and sustainable novel materials has considerably increased. The abundance of two-dimensional (2D) materials has endowed them with a broad material platform in technical studies and in the expansion of nano- and atomic-level applications. The innovation of graphene has motivated considerable attention to the study of other novel 2D materials, known as modern day “alchemy”, by which scientists are trying to convert most possible periodic table elements into 2D material structures and forms. 2D material devices with high quality and good optical encoder performance have a multitude of industrial applications. However, their stability and large size restrict their applications, but these problems can be overcome by functionalization and substrate-based formation of 2D materials. Therefore, via this review, first, basic attributes of 2D materials are described, and the mechanisms to further enhance their properties are also summarized. Second, the applications of 2D materials are discussed, along with their advantages and disadvantages. Finally, some effective device-fabrication approaches, such as heterostructure approaches, are applied to further enhance the properties of 2D materials; their novel device applications and opportunities are also presented. This updated review may provide new avenues for 2D material synthesis and development of more efficient devices compared to conventional devices in different fields.

Recent developments in emerging two-dimensional materials and their applications

Inner filter effect-based fluorescent sensing systems: A review.

In this study, we systematically investigate the mechanism of single-layer MnO2 nanosheets suppressing fluorescence of 7-hydroxycoumarin and, based on this, demonstrate a new fluorescent method for in vivo sensing of ascorbic acid (AA) in rat brain. The mechanism for the fluorescence suppression is attributed to a combination of inner filter effect (IFE) and static quenching effect (SQE), which is different from those reported for the traditional two-dimensional nanosheets, and Forster resonant energy transfer (FRET) mechanism reported for MnO2 nanosheets. The combination of IFE and SQE leads to an exponential decay in fluorescence intensity of 7-hydroxycoumarin with increasing concentration of MnO2 nanosheets in solution. Such a property allows optimization of the concentration of MnO2 nanosheets in such a way that the addition of reductive analyte (e.g., AA) will to the greatest extent restore the MnO2 nanosheets-suppressed fluorescence of 7-hydroxycoumarin through the redox reaction between AA and MnO2 nanosheets. On the basis of this feature, we demonstrate a fluorescent method for in vivo sensing of AA in the cerebral systems with an improved sensitivity. Compared with the turn-on fluorescent method through first decreasing the fluorescence to the lowest level by adding concentrated MnO2 nanosheets, the method demonstrated here possesses a higher sensitivity, lower limit of detection, and wider linear range. Upon the use of ascorbate oxidase to achieve the selectivity for AA, the turn-on fluorescence method demonstrated here can be used for in vivo sensing of AA in a simple but reliable way.

Single-Layer MnO2 Nanosheets Suppressed Fluorescence of 7-Hydroxycoumarin: Mechanistic Study and Application for Sensitive Sensing of Ascorbic Acid in Vivo

This study demonstrates a first exploitation of the unique properties inherent in Pickering emulsions to develop a new kind of photocatalytic system. We engineered the system by using silver phosphate (Ag3PO4) as a photocatalytic active metal oxide semiconductor and multiwalled carbon nanotubes (MWNTs) as a hydrophobic conducting nanostructure to form the Pickering emulsions. The photocatalytic activity of the as-formed Ag3PO4/MWNT-stabilized Pickering emulsion-based system is studied toward dye decomposition and oxygen evolution. Results imply that the Pickering emulsion-based photocatalytic system exhibits a much higher efficiency, as compared with traditional solution-dispersed photocatalytic system. This high efficiency is elucidated in terms of the unique properties inherent in Pickering emulsions including (i) the self-assembled Ag3PO4/MWNT nanohybrid at water/oil interface, well ensuring a large surface area of the photocatalyst, (ii) the use of MWNTs to facilitate the formation of amphiphilic nano...

Silver Phosphate/Carbon Nanotube-Stabilized Pickering Emulsion for Highly Efficient Photocatalysis

Manganese dioxide (MnO2) nanosheets have recently been demonstrated to be particularly attractive for fluorescent sensing and imaging; however, almost all MnO2 nanosheets-based fluorescent assays have been developed with emissive nanoparticles as the probes. In this study, we developed a novel strategy to use organic dyes, instead of emissive nanoparticles, as the probe to construct a platform for biosensing with excellent analytical properties. With 5-carboxyfluorescein (FAM) as a model organic dye, we firstly investigate the effect of MnO2 nanosheets on the fluorescence of FAM and find that the fluorescence intensity of FAM is considerably suppressed by MnO2 nanosheets based on the inner filter effect (IFE). To demonstrate that the MnO2 nanosheets-based fluorescence sensing platform can easily achieve a high selectivity with organic dyes as the probe, we use single-stranded DNA (ssDNA) oligonucleotide as a typical biorecognition unit, which is labeled with the FAM probe to form FAM-ssDNA. The fluorescent intensity of FAM-ssDNA is first suppressed by MnO2 nanosheets through the combination of IFE and Forster resonant energy transfer (FRET), and then recovered with subsequent hybridization with the complementary DNA oligonucleotide. To demonstrate the potential applications of the MnO2 nanosheets-based fluorescence sensing platform with organic dyes as the probes, we developed methods for simple but effective microRNA and thrombin assays. With the platform demonstrated here, the limits of detection for miR124a and thrombin are 0.8 nM and 11 nM, respectively. Moreover, the fluorescent sensing assay for thrombin exhibits high selectivity. This study essentially demonstrates a new 2D nanostructure-based fluorescent sensing platform that is robust, technically simple, and easily manipulated to achieve high selectivity and sensitivity for practical applications.

Wanying Zhai

Papers

Single-Layer MnO2 Nanosheets Suppressed Fluorescence of 7-Hydroxycoumarin: Mechanistic Study and Application for Sensitive Sensing of Ascorbic Acid in Vivo

Silver Phosphate/Carbon Nanotube-Stabilized Pickering Emulsion for Highly Efficient Photocatalysis

MnO2 nanosheets based fluorescent sensing platform with organic dyes as a probe with excellent analytical properties.