Azetidines, as an important class of saturated four-membered azaheterocycles, are not only important raw materials, intermediates, chiral auxiliaries and catalysts in organic synthesis, but also crucial active structural units of amino acids, alkaloids, natural and synthetic biological and medicinal active compounds. Thus, it is critical to develop methods for constructing azetidine structural motifs. The review introduces the development of the synthesis of azetidines and mainly focuses on advances of the synthesis of azetidines during recent 10 years. Furthermore, it demonstrates the new achievements in the synthesis of azetidines mainly including cyclizations by the C-N bond formation and by the C-C bond formation, amine-catalyzed cycloaddition of allenoates and imines, photocycloadditions of imines and alkenes, ring contraction and expansion rearrangements, and reduction of azetidin-2-ones(β-lactams).
Contents
1 Introduction
2 Cyclizations by the C-N bond formation
2.1 Nucleophilic substitution reactions
2.2 Reductive cyclizations of β-haloalkylimines
2.3 Cyclizations of allylic and homoallylic amines
2.4 Opening of epoxides with amines
2.5 Opening of aziridines
2.6 Pd-Catalyzed cyclizations
3 Cyclizations by the C-C bond formation
3.1 Nucleophilic displacements
3.2 Cyclizations involving Michael additions
4 Cycloadditions
4.1 Amine-catalyzed cycloadditions of allenoates and imines
4.2[2+2]Photocycloadditions of imines and alkenes
5 Ring contraction and expansion rearrangements
5.1 Ring contractions of five-membered heterocycles
5.2 Ring expansions of three-membered heterocycles
6 Reduction of azetidin-2-ones
7 Conclusion

Indole diketopiperazine alkaloids(Indole DKPs) are natural products isolated from secondary metabolites of endophytic fungi, especially in the genera Aspergillus and Penicillium of the phylum Ascomycota or sac fungi, showing diverse chemical structures, which contain two moieties indole and cyclic dipeptides and are derived from several amino acids, such as L -tryptophan、L -proline and L -alanine. They not only have a class of naturally occurring privileged structures but also exhibit a broad spectrum of biological activities that make them attractive scaffolds for drug discovery. Recently, many studies have shown that the sealkaloids having significant biological activities such as anticancer, antibacterial, immunomodulatory, antioxidant, and insecticidal activities. And the discovery of these activities has spawned numerous investigations into their synthesis. Meanwhile, the discovery of increasing numbers of new indole DKPs have led to an expanding range of bioactivities. The review summarizes briefly the considerable progress on bioactivities of these alkaloids isolated from various fungal strains in recent years. In addition, we also investigate structure-activity relationship of some compounds.The aim is to give an overview to provide useful reference for the extraction, synthesis and drug discovery of these compounds.
Contents
1 Introduction
2 Structure and classification of Indole DKPs
3 Biological activities of Indole DKPs
3.1 Anticancer and antitumor activity
3.2 Antibacterial activity
3.3 Antioxidant activity
3.4 Insecticidal activity
3.5 Other activities
4 Conclusion

Zwitterionic polymer is a polymer with equivalent positive and negative charge groups on the polymer chains. It is widely used in the areas of anti-fouling and biomedical applications due to its super-hydrophilic and biocompatible properties. Based on their structural features, they can be broadly divided into mixed-charge type and betaine type zwitterionic polymers. At present, zwitterionic polymers can be prepared in a variety of methods, among which living/controlled radical polymerization is a robust and versatile method for the preparation of zwitterionic materials. This approach allows preparation of polymers with precise control over molecular weight, molecular weight distribution, polymer composition, topology and functionality. In this paper, the synthesis of zwitterionic polymers and its recent development are critically reviewed. We mainly focus on the advantages and development of living/controlled radical polymerization for preparing zwitterionic polymers. The main applications of zwitterionic polymers as anti-fouling materials, biomedical materials, detection and separation are summarized and the prospect of their future development is also outlined.
Contents
1 Introduction
2 Synthesis of zwitterionic polymers by living/controlled radical polymerization.
2.1 Preparation of mixed-charge type polymers
2.2 Preparation of betaine type polymers
3 Applications of zwitterionic polymers
3.1 Industrial materials
3.2 Biomedical materials
4 Conclusion and outlook

In recent years, a newly emerging polymer-based MONOLITH material (also known as the monolithic column, continuous bed) has been developed rapidly and it has already been utilized widely in various fields. Due to its unique three-dimensional continuous interconnected porous structure in a single piece, polymer-based MONOLITH has attracted more and more attention and it is regarded as one of the major breakthroughs in separation and filtration techniques. Compared with silica-based MONOLITH material, polymer-based MONOLITH material has the advantages of simple preparation process, good biocompatibility, high chemical stability and easy regulation of surface chemical properties. Therefore, polymer-based MONOLITH has always done a better job as a stationary phase of affinity chromatography for separating and analyzing biological macromolecules. In this review, the preparation techniques and the newest development of the polymer-based MONOLITH material are summarized. In addition, its application as stationary phase of affinity chromatography for the separation of biological macromolecules in the last 5 years is described.
Contents
1 Introduction
2 Preparation techniques of the polymer-based MONOLITH materials
2.1 Free radical polymerization
2.2 Controlled/living radical polymerization
2.3 Polymerized high internal phase emulsions
2.4 Condensation polymerization
2.5 Cryogels
2.6 Other methods
3 Application of the polymer-based MONOLITH materials in separation of biomolecules by affinity chromatography
3.1 Bioaffinity chromatography
3.2 Immunoaffinity chromatography
3.3 Dye affinity chromatography
3.4 Boronate affinity chromatography
3.5 Immobilized metal-ion affinity chromatography
3.6 Pseudo affinity chromatography
4 Conclusion

Mimetic, or artificial enzymes are molecules or molecular aggregates that mimic the structural features of enzyme active center, such as shape, size, and microenvironment at molecular level. With the development of nanoscience and supramolecular technologies, the construction of supramolecular mimetic enzymes with specific catalytic activity has become a hotspot in both scientific research and application. Artificial peptide metalloenzymes have peptide molecules as the basic units, and the self-assembly of peptide supramolecular structure with enzymatic catalytic activity is driven by a series of non-covalent interactions synergistically. The structure and biochemical properties of peptide metalloenzyme are akin to those of natural enzymes. Furthermore, since peptide molecules are biocompatible and easy to be modified, artificial peptides metalloenzymes would be ideal candidates for artificial enzyme fabrication with specific functions. In this review, the progress of the mimetic metalloenzymes fabrication through peptide self-assembly has been summarized. The effects of peptide self-assembly, supramolecular structure, microenvironment of metal active center, as well as pH value on the artificial enzyme catalytic activity has been reviewed. The enhancement of the stability of self-assembled nanostructures, the improvement of catalytic activity and the broadening of the reaction types catalyzed by artificial enzymes are the main challenge in artificial peptides metalloenzyme study. Fabrication more stable peptide self-assembled nanostructure and more precise active centers to mimic those of the natural enzymes might be the right strategies.
Contents
1 Introduction
2 Effects of self-assembly nanostructures on metalloenzyme
2.1 Nanotube
2.2 Nanofibers
2.3 Coiled-coil
3 The secondary structure of peptide self-assemblies in metalloenzyme
3.1 β-hairpin
3.2 α-helix
4 Effects of spatial structure on metalloenzyme
4.1 Regulation of metal ions
4.2 Formation of protein interface
4.3 Hydrophobic interface
5 Metal-free peptide self-assembly artificial enzyme
6 Conclusion and outlook

Micro-and nanoparticles with unique properties have broad applications in drug delivery, absorption separation, optical/electrical materials, and magnetic device, etc. Due to the huge volume-to-surface ratio and continuous flow characteristic, remarkable advantages have been made in organic chemistry, polymer synthesis and material preparation by employing microfluidic technology compared with the traditional batch reactor. This review summarizes the recent progress in microfluidic synthesis of micro-and nanoparticles. Microreactor system can precisely monitor and control the particle formation process. By using single emulsion as the template, spherical and non-spherical polymer/inorganic/noble metal/semiconductor particles are fabricated. Multi-emulsions are used as templates for fabricating core shell particles, Janus particles and microcapsules.
Contents
1 Introduction
2 Particle assembly in dispersed phase of single emulsions
2.1 Spherical particles
2.2 Non-spherical particles
3 Particle assembly in dispersed phase of multi-emulsions
3.1 Core-shell particles
3.2 Janus particles
3.3 Microcapsules
4 Conclusion

Gamma-polyglutamic acid (γ-PGA) hydrogel is a kind of material with three-dimensional network structure formed by the crosslinking of γ-polyglutamic acid. Because the molecule contains a large number of carboxyl groups, it has good absorptive capacity. Since γ-PGA is derived from microorganisms, its hydrogel is a biocompatible, environmentally friendly, and multi-functional material that can be applied to the biomedicine, daily chemical, environmental and textile fields. In this paper, three crosslinking methods for preparing γ-PGA hydrogels in recent years:physical crosslinking, chemical crosslinking and enzymatic crosslinking are reviewed. Then the application of γ-PGA hydrogels in tissue engineering, supercapacitor and textile is summarized, and the future prospective tendency is presented.
Contents
1 Introduction
2 Methods to prepare γ-polyglutamic acid hydrogel
2.1 Physical crosslinking
2.2 Chemical crosslinking
2.3 Enzymatic crosslinking
3 Application of γ-polyglutamic acid hydrogel
3.1 Tissue Engineering
3.2 Super capacitor
3.3 Textile
3.4 Others
4 Conclusion

Two-photon absorption (TPA) materials have wide range of applications in the upconversion luminescent material, bioimaging, photodynamic therapy, three-dimensional lithographic microfabrication, etc. By using metal centers as template, metal complexes can combine several TPA active organic ligands into a complicate multipolar system to achieve enhanced TPA effect. The resulting two-photon absorptive material also displays better stability, extended emission lifetime and optimized spectral range when compared with the pure ligands. The "cooperative enhancement" effect in the TPA cross-section of multinuclear metal complex has drawn broad attention. In this paper, representative multinuclear complexes(homonuclear and heteronuclear) are selected to study the influencing factors on their TPA properties, such as the type and amount of metal ions, the structure of ligand molecules and complexes, etc. Special attention is paid to the mechanism of their photophysical properties that explain the structure and energy level of the excited states, the mode and direction of energy transfer in the multinuclear complexes, hoping to summarize the molecular design rules for TPA active multinuclear complexes. Finally, the existing problems in the research of multinuclear complexes with two-photon activity are described and studies on the "bifunctional multinuclear" complex are prospected, which will provide an important base for future development of novel two-photon absorptive materials.
Contents
1 Introduction
2 Homometallic multinuclear complexes
2.1 Multinuclear platinum complex
2.2 Multinuclear ruthenium complexes
2.3 Multinuclear zinc complexes
2.4 Multinuclear iridium complexes
2.5 Multinuclear iron complexes
2.6 Copper clusters
2.7 Gold clusters
2.8 Other metal complexes
3 Heterometallic multinuclear complexes
3.1 d-f type heteronuclear complex
3.2 d-d type heteronuclear complex
3.3 Clusters and MOF Structures
3.4 Biological applications of heterometallic complexes
4 Conclusion and outlook

Carbon dots(CDs), as a class of rising nano-materials with strong fluorescence and tunable emission wavelength, exhibit potential application in many fields including optoelectronic device, biosensing and drug carrier because of their superior resistance to photobleaching, low toxicity and excellent biocompatibility. The unique excitation-dependent fluorescence property of CDs denotes the variation of the emission wavelength of CDs with the excitation wavelength, thus the realization of full-color emission through tuning excitation wavelength, which will provide a possibility for the application of CDs in multicolor imaging. Meanwhile, the emission of CDs can extend to near-infrared region when excited by long-wavelength, exhibiting the potential application in living imaging. Therefore, it is important to explore the mechanism of excitation-dependent fluorescence property of CDs, so as to provide a theoretical guidance for realizing their excitation-dependent fluorescence property. In this review, the recent progress in excitation-dependent fluorescence property of CDs was summarized, with especial emphasis on their mechanism, realization approach and application. At present, the excitation-dependent fluorescence in CDs has induced different hypotheses, such as the presence of multiple surface states, wide size distribution, formation of carbon nuclei in CDs, and slow solvent relaxation around CDs. Therefore, the excitation-dependent fluorescence can be realized by tuning surface states, size distribution and carbon nuclei. In addition, the unique property exhibits potential application in imaging and optoelectronic field. In the end, the existing problems are pointed out and the prospects are predicted.
Contents
1 Introduction
2 Mechanism of excitation-dependent fluorescence property in CDs
2.1 Distribution of different surface states
2.2 Distribution of wide particle size
2.3 Structure of carbon nuclei
2.4 Slow solvent relaxation around CDs
3 Approaches to excitation-dependent fluorescence property in CDs
3.1 Tuning of surface states
3.2 Tuning of size distribution
3.3 Tuning of carbon nuclei
4 The application of excitation-dependent fluorescence property in CDs
4.1 Multicolor imaging in vitro
4.2 Imaging in vivo
5 Conclusion and outlook

Comparing with vacuum evaporation, fabricating organic light-emitting diodes (OLEDs) by solution-processing methods has plenty of merits, such as high material utilization, simple equipment and feasibility of low cost large size panels, which has drawn much attention. Small molecule hole-transporting materials (HTMs) are an important part of OLED devices for hole transporting and electron blocking. Cross-linking groups such as styryl, oxetane, etc. are added to the structure to form cross-linkable small molecular hole transporting materials, which are suitable for solution-processed technology. Initiating with the thermal or the ultra-violet irradiation cross-linking curing reaction, the polymer network structure is formed. As applied in the multilayer OLEDs, the inducing of cross-linkable hole-transporting materials can solve the miscibility problem of the interlayers between the emitting material layer(EML) and the hole transporting layer(HTL), then improve the efficiency and stability of the device consequently. In this paper, according to the requirements of the solution process for the materials, many techniques about spin-coating and ink-jet printing are presented, such as the methods of fabrication, the quality of films affected by manual factors and the comparison of different manufactory measures. Afterwards, to summarize their performance in devices, various cross-linkable HTMs are listed and investigated, such as trifluoroethylene group, styrene group, oxetane group, siloxane group and unsaturated ester group. Finally, the developing trend of cross-linkable HTMs and solution process is introduced.
Contents
1 Introduction
2 Solution-processed multi-layer organic light-emitting diodes
2.1 Spin-coating process
2.2 Ink-jet printing process
3 Cross-linkable hole-transporting materials
3.1 Trifluoroethylene group
3.2 Styrene group
3.3 Oxetane group
3.4 Siloxane group
3.5 Unsaturated ester group
3.6 Other cross-linking methods
4 Conclusion and outlook

Microbial extracellular electron transfer (EET) is the process that electrons generated from redox reactions transfer between inside of cells and extracellular electron donors/acceptors. This process accompanies with the energy transformation and substance conversion. Studies of microbial EET have attracted increasing interests in recent years because it is found to be significant to understand element biogeochemical cycle, anti-corrosion of metals, bioelectrochemical systems, etc. Electrochemical techniques have played important roles in EET mechanisms analysis, because these techniques are simple and available in studying electron transfer reactions at the interfacial region between electrode and solution. In this review, electrochemical integrated techniques used to study EET pathways including microelectrode, scanning electrochemical microscopy, three-electrode cell combined with optical microscopy and electrochemical spectroscopy are summarized. The functions and advantages of these integrated techniques are illustrated in detail. The EET pathways from macroscopic to microcosmic aspects are reviewed, which include four hierarchies:whole biofilm, redox reaction in microenvironment of biofilm, single cell, redox proteins or molecular. This review proposes that several advanced electrochemistry combined techniques can be useful for the investigation of microbial EET mechanisms in the future.
Contents
1 Introduction
2 Study of the electron transfer of microenvironment in biofilm by microelectrode and scanning electrochemical microscopy
3 Study of the electron transfer of single cell by three-electrode cell combined optical microscope
4 Study of the redox of protein or molecule by electrochemical spectroscopy
4.1 Electrochemical UV-visible spectroscopy
4.2 Electrochemical infrared spectroscopy
4.3 Electrochemical Raman spectroscopy
5 Conclusion and outlook

Dopamine is a catecholamine that acts as an important neurotransmitter in the nervous system. Since the introduction of the simple preparation method of polydopamine with the oxidative self-polymerization of dopamine, dopamine based nanomaterials have emerged as the novel biomaterials. They have attracted considerable interests in the fields of biosensing, drug delivery, photothermal therapy, antimicrobials, and tissue engineering due to their unique physicochemical properties, such as versatile adhesion property, high chemical reactivity, excellent biocompatibility and biodegradability, and strong photothermal conversion capacity. This review summarizes the recent advances on the fabrication, functionalization, and biomedical applications of dopamine based nanomaterials. Firstly, several typical dopamine based nanomaterials are introduced with a discussion of the factors that influence the assembly process. Then detailed elaboration is followed on their applications in biomedical fields, especially in cancer diagnosis and therapy. Finally, the review proposes some research topics for clinical applications of dopamine based nanomaterials.
Contents
1 Introduction
2 Dopamine based nanomaterials
2.1 Nanoparticles
2.2 Microcapsules
2.3 Polydopamine films
2.4 Other materials
3 Biomedical applications
3.1 Biosensing
3.2 Bioimaging
3.3 Drug delivery
3.4 Photothermal therapy
3.5 Theranostics
3.6 Antimicrobials
3.7 Tissue engineering
4 Conclusion and perspective

The structure of liquid water particularly the number of bonds per water molecule has been a debating issue during 1933~1935 when Bernal, Fowler, and Pauling firstly proposed the scenario of proton "transitional quantum tunneling" in THz frequency at asymmetrical sites between two oxygen ions. Although conventions of the rigid or flexible dipole-dipole interaction, nanophase mixed amorphous structure or homogeneous fluctuating phase models, solute diffusion dynamics or hydration length scale premises have been becoming dominant, mysteries such as floating of ice, regelation of ice (compression melting), slipperiness of ice, fast cooling of warm water, etc. have yet to be resolved. The definition of hydrogen bond needs yet to be certain. In this perspective, we emphasize that it would be more efficient to transit the conventional "dipole-dipole" interaction to "hydrogen bond (O:H-O) asymmetrical, short-range, correlative" interaction, from the "proton translational tunneling" to "hydrogen bond cooperative relaxation". Progress also revealed that the O:H-O bond configuration and the numbers of protons and nonbonding electron lone pairs conserve and that water forms the tetrahedrally-coordinated, strongly correlated, fluctuating single liquid crystal. The O:H nonbond and the H-O bond segmental specific heat disparity derives a quasisolid phase between the liquid and the solid. With tunable boundaries, the quasisolid phase possesses the negative thermal expansion coefficient. Remarkably, molecular undercoordination results in a supersolid phase that is highly polarized, thermally stable, viscoelastic, and lesser dense. Extending hydrogen-bond knowledge to the energy storage-explosion reaction mechanics of energetic materials may further verify the comprehensiveness and universality of the current notion of hydrogen bond cooperativity-nonbonding interaction is ubiquitously important.
Contents
1 Introduction:challenges and opportunities
2 Strategies:manner of thinking
3 Principles:rules of conservation and prohibition
3.1 N number and O:H-O bond configuration conservation
3.2 Molecular orientation and proton tunneling prohibition
3.3 Water single crystal and O:H-O bond potential Path
3.4 O:H-O bond coorpertivity and ice water anomalies
4 Method:spectrometrics and analysis
5 Progress:mysteries resolution
5.1 Quasisolid cooling expansion:ice floating and density oscillation
5.2 Undercoordination supersolidity:slipperiness and skin hydorphobicity
5.3 O:H-O symmetrization:quasisolid phase boundary dispersion and regelation
5.4 Hot water cools faster:O:H-O bond memory and skin supersolidity
6 Conclusion:insight and perspective