Tumor tissues have lower oxygen concentration compared to normal tissues, due to the inadequate oxygen supply caused by uncontrolled cell growth and proliferation, in addition to an abnormal vasculature. As a common feature of solid tumors, hypoxia can be an indicator of malignant tissues or cancer progression. Accurate hypoxia detection and imaging are essential for the diagnosis and clinical treatment of cancer patients. Fluorescence imaging has been used in cancer detection because of its high sensitivity, non-invasive and real-time characteristics. During recent years, azo groups have been widely used to construct fluorescent probes for hypoxia cell imaging, owing to the fluorescence quenching effect on fluorophores and their reductive cleavage resulting in fluorescence recovery. This review summarizes various azobenzene derivative probes according to different construction strategies, and explores their mechanism and application in imaging. The limitations and future development of these probes are also discussed.

The discovery of the aggregation-induced emission(AIE) phenomenon provides the best solution to solve the problem of fluorescence quenching of traditional organic fluorescent molecules at high concentrations and aggregation state. AIE molecules are widely used in many fields such as photoelectric devices, chemical sensing, biological imaging and targeting therapy. With the deepening of the research on the emissive mechanism of AIE, the AIE molecular system has been greatly expanded. Among them, a class of AIE molecules with donor-acceptor structures can significantly reduce the molecular energy gap and extend the emission wavelengths of molecules from the visible light region(400~700 nm) to the near infrared(NIR) region(700~1700 nm). Due to the unique advantages of NIR fluorescent molecules in the field of biomedicine, they have become the hot topic of AIE research. With the continuous exploration of the design and application of NIR molecules, AIE molecules with different functions and longer emission wavelengths have also been developed, and realized the application of NIR fluorescence imaging, photoacoustic imaging, photodynamic therapy and photothermal therapy to specific tissues of organisms. This article summarizes the structure of NIR fluorescent molecules with AIE performance in recent years and their related applications in the field of biomedicine.

A brand-new nitrogenated graphene-like two-dimensional material(C₂N) has attracted considerable attention due to its special two-dimension nitro-rich network, which possesses regularly distributed N-containing holes. This article summarizes recent proceeding of the C₂N material including synthesis, excellent mechanical, optical absorption, thermal, electrical and magnetic properties, as well as various applications, such as electronic devices, adsorption materials, green catalysts, drug carriers and so on. The C₂N material is predicted to cause a research upsurge in the future.

Microfluidic synthesis technique is attracting considerable interest in the synthesis of inorganic nanomaterials, especially in precise regulation of nanoparticles, due to their miniaturization of reaction apparatus, precisely controlling the substances exchange. Given the demand for detailed experiments, the micro-reactors can be redesigned and adjusted, as well as multiple experimental steps integrated into one system to perform multi-step chemical reactions and realize the preparation of composite materials. In summary, various micro-reactors are briefly introduced, different flow statuses of the fluid in the micro-reactors are discussed, and the typical microfluidic synthesis applications in nanomaterial synthesis were exemplified in this review. Finally, the development trend in the microfluidic system is summarized.

Peptide-based metal ion sensors, as a new type of sensor designed based on peptide sequences, have attracted more and more attention from researchers. As important small biological molecules, peptides have advantages of simple and well-developed synthetic methods with low costs, and can provide multidentate coordination to metal ions. Peptide-based sensors have high sensitivity and high selectivity to metal ions, and can be further optimized by adjusting the peptide sequence. Compared with other types of sensors, peptide-based metal ion sensors have good water solubility, biocompatibility, and low toxicity, and therefore have important applications in environmental detection and bioanalytical diagnosis, especially for metal ion imaging. This review focuses on the progress of different types of peptide-based metal ion sensors in recent years, including those based on UV-Vis absorption spectroscopy, fluorescence spectroscopy, and electrochemical analysis, and their applications, especially for detections and bioimaging of highly toxic metal ions(Hg²⁺, Cd²⁺, etc.), and metal ions playing key roles in biological systems(Cu²⁺, Zn²⁺, etc.). Moreover, the advantages of peptide-based metal ion sensors are summarized and their future developments and applications are prospected.

Dendrimer owns a precise chemical structure, including a small molecular core, an internal space induced by multiple branches, and large numbers of functional groups on the surface, which can be used to load a variety of nanoparticles and perform functional modification. In addition, based on the excellent properties of dendrimer, it demonstrates good biocompatibility and stability. After functional modification, it could achieve long blood circulation and high tissue specificity in vivo. Therefore, dendrimer has often been used as an ideal carrier for constructing SPECT imaging agents in recent years. Through multifunctional modification with various functional groups, the cytotoxicity of the generated nanocarrier is decreased and its accumulation at tumor site is improved, which faciliates accurate and efficient SPECT imaging. SPECT imaging monitors the physiological and pathological changes of lesions at the molecular level via monitoring the distribution, flow and metabolism of radionuclides in vivo. Compared with small molecular radionuclides, dendrimer-based SPECT imaging agents demonstrate longer circulation time and could achieve specific distribution after functional modification in vivo. In this review, various radionuclides labeled functional dendrimers are described in detail, their preparation methods and biomedical applications are summarized. Finally, the applications of these dendrimer-based SPECT imaging agents in the early diagnosis of tumor are prospected.

Being the intermediates and final products of life metabolic activities, endogenous metabolites precisely mirror metabolic changes in vivo, providing clues for the study of diseases and other life processes. Mass spectrometry has become a universal technique for qualitative and quantitative analysis of endogenous metabolites, with high sensitivity, high specificity and wide dynamic range. However, it remains a limitation on the detection and differentiation of subtle structural differences of metabolite isomers in heterogeneous biological tissue using mass spectrometry. Therefore, how to improve sensitivity and specificity of mass spectrometry needs to be solved in the study of metabolite isomers’ analysis. Chemical derivatization takes advantage of bringing high proton affinity groups, hydrophobicity or chiral selectors to desired analytes, which is expected to broaden the applications of mass spectrometry for analysis of metabolite isomers. In this review, advances in chemical derivatization-based mass spectrometric analysis of metabolic isomers are summarized, including(1) reaction approaches and(2) methods and applications of lipids, carbohydrates and chiral amino acids. Special emphasis is placed on on-line derivatization microdroplet accelerate reaction by mass spectrometry, in which chemical derivatization of analytes occurring simultaneously with spray ionization, providing a promising way for instant and in-situ metabolite derivatization. The emerging study of on-tissue derivatization mass spectrometry imaging is of great value for providing spatial distribution of low abundance, non-polar and isomer metabolites. In the second part, different approach of detailed structural characterization of various compound classes are discussed, with a focus on lipids, carbohydrates and chiral amino acids.

Carbon-based materials have excellent conductivity, good stability, and low price. They are widely used in the preparation of stretchable conductive nanocomposites, and have tremendous application potential in the field of stretchable and wearable electronic devices, which has attracted lots of attention. This paper introduces the carbon-based materials, such as carbon black, carbon nanotubes and graphene. The preparation methods of these nanocomposites are also summarized, such as in-situ polymerization, melt blending and solution mixing, followed by the introduction of traditional and new printing technology. Then this paper analyzes the conductive mechanisms of composite materials including the percolation threshold, and focuses on their application in the field of stretchable sensors and stretchable energy storage devices. The shortcomings of the current research on stretchable conductive composites based on nano-carbon fillers are pointed out: poor dispersion of conductive fillers, unstable conductive network and inability to mass-produce, and the corresponding solutions are put forward. Finally, the applications of stretchable conductive composites based on nano-carbon fillers are prospected, including miniaturized, stretchable and wearable electronic devices.

High temperature proton exchange membrane fuel cells(HT-PEMFCs) have many advantages over traditional proton exchange membrane fuel cells, which can not only enhance the catalysts tolerance to carbon monoxide poisoning, but also simplify the water and heat management as well as improve the energy conversion efficiency. Proton exchange membrane(PEM) is one of the key components of PEMFCs. Phosphoric acid(PA) doped PEMs have recently shown remarkable advantages due to the high proton conductivity and longevity at high operating temperatures(100~200 ℃) and low relative humidity. Generally, high PA doping level can improve the proton conductivity of PEMs, whereas the mechanical strength of the membranes dramatically deteriorates as an expense, therefore, enormous research on the synthesis of modified polymer electrolyte membranes with improved comprehensive performance has been carried out. This review focuses on the research progress of PA doped high temperature proton exchange membranes(HT-PEMs) such as polybenzimidazole and alkaline poly(aryl ether). Particularly, the application of porous materials including metal organic frameworks(MOFs) and covalent organic frameworks(COFs) in PEMs is also summarized. Finally, the remaining challenges in this filed are indicated.

Lithium-sulfur batteries have a theoretical energy density of up to 2600 Wh·kg^-1, and the theoretical capacity of sulfur can reach 1675 mAh·g^-1, which is much higher than that of commercial cathode materials of lithium-ion batteries. However, problems such as the "shuttle-effect" of polysulfides have a serious impact on the performance. Current researches mainly use physical limitation and chemisorption to limit polysulfides to the cathode region based on the "blocking" strategies. Inspired by the concept of “dredging”, the catalytic-conversion strategy can realize functions such as reducing overpotential and inducing uniform deposition of Li ₂S while suppressing the "shuttle-effect" by speeding up the oxidation-reduction reaction kinetics. Herein, we review the progress of catalysis in lithium-sulfur batteries and divide them into adsorption-conversion mechanisms and redox-mediated mechanisms based on whether redox intermediates are produced. Related materials and characterization techniques and research methods commonly used are also introduced.

With the continuous development of renewable energy technology, vanadium redox flow battery, as a large-scale energy storage device with great development prospects, has widely drawn attention of domestic and foreign scholars. Ion-conducting membrane is one of the important components of vanadium redox flow battery, which has a key influence on the performance, life time and cost of the battery. Based on the research work reported domestically and abroad, this review reports in detail the research and application progress as well as the technical problems of ion-conducting membrane for vanadium redox flow battery. In addition, some new developments of ion-conducting membrane for vanadium redox flow battery are introduced.

The emergence of emerging organic contaminants(EOCs) has attracted increasing attention due to their wide distribution and persistence in aquatic ecosystems, as well as the potential threat to the health and safety of aquatic organisms. Traditional water treatment processes represented by activated sludge processes are generally insufficient to eliminate these persistent pollutants. For efficient removal of EOCs, advanced oxidation technology based on new materials is one of the most important advanced treatment technologies. Fe-MOFs and their composites have been widely used in many fields, especially in the catalytic oxidation of pollutants in wastewater. With the aid of the improvement of synthesis methods, post-synthetic modification and being composited with specific functional materials, Fe-MOFs can be used to effectively improve the adsorption performance, enhance the light absorption characteristics, and promote the effective separation of charge carriers. The review focuses on the progress of advanced oxidation processes(photocatalysis, Fenton-like reaction and sulfate radical($SO_{4}^{-·}$) mediated oxidation) of Fe-MOFs and their composites to remove emerging organic contaminants in wastewater. As well, the opportunities and challenges of Fe-MOFs in the field of EOCs removal are proposed.

Printed organic electronic technology is the electronic manufacturing technology based on the printing method, and particularly refers to the printing of electronic devices by using functional inks consisting of organic semiconductor materials. The development of this technology involves multidisciplinary approaches including material chemistry and microelectronics. The unique fabrication method and device form brings the flexibility, low-cost and large-area processability to the devices, complementary to Si based electronics in the applications of biosensing, electronic skin and flexible display. To capture the rapid progress of this research field, a timely review of recent work is necessary. In addition, a better understanding of both material processing and digital electronics is essential for the development of this field. In this work, we provide a comprehensive overview from the perspective of printing technology and circuit systems. Printing technologies such as inkjet printing, screen printing, and transfer printing, including both the printing mechanisms and their applications related to organic devices are introduced. The functionality and the development challenge of organic digital circuits such as inverter, NAND gate, ring oscillator and D-type flip-flop, are summarized. We then review the applications of printed organic electronics, such as RFID, electronic skin, and OLED displays. Finally, the challenges and the outlook of printed organic electronics are briefly discussed.