Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, simple synthesis, etc. Since their discovery, various synthesis methods using different precursors have been developed, and are mainly classified as top-down and bottom-up approaches. For the mechanistic origin of CDs photoluminescence, three mechanisms have been proposed such as quantum confinement effect, surface state and molecule state. CDs have presented many applications, and this review article mainly focuses on the development of CDs based fluorescent sensors in recent 5 years. The sensing mechanisms, senor design and sensing properties to various targets are summarized. A broad range of analytes including cations, anions, small molecules, macromolecules, cells and bacteria have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented.

Fluorescent carbon dots and their sensing applications

In the past few decades, researchers have developed lots of artificial enzymes with various materials to mimic the structures and functions of natural enzymes. Recently, nanozymes, nanomaterials with enzyme-like characteristics, are emerging as novel artificial enzymes, and attracting researchers’ enormous interest. Remarkable advances have been made in the area of nanozymes due to their unique properties compared with natural enzymes and classic artificial enzymes. Until now, lots of nanomaterials have been studied to mimic various natural enzymes for wide applications. To highlight the recent progress of nanozymes (especially in bionanotechnology), here we discuss the diverse applications of nanozymes, which range from sensing, imaging, and therapeutics, to logic gates, pollutant removal, water treatment, etc. Finally, we address the current challenges facing nanozyme research as well as possible directions to fulfill their great potential in future.

Nanozymes in bionanotechnology: from sensing to therapeutics and beyond

Dependence of solvents, pH, molar ratio and temperature in tuning metal organic framework architecture

The nanozymes are a kind of synthetic nanomaterials with enzyme-like properties. Metal–organic frameworks (MOFs) are an important class of inorganic–organic hybrid crystals. Their unique composition, structural diversity and size tailorability enable them to be promising for the construction of novel nanozymes. This review intends to summarize the recent advance for the construction of MOFs-based nanozymes and their primary applications in chemical sensing and biosensing. Based on their synthetical strategies, the MOFs-based nanozymes are categorized into four classes: pristine MOFs, MOFs with chemical modification, MOFs-based composites and MOF derivatives. In each categorization, the design and enzyme mimetic activity are discussed. Moreover, the analytical applications of these nanozymes are covered, such as the detection of H2O2, small biomolecules releasing H2O2, reductive small biomolecules, biomacromolecules, heavy metal ions, toxic metabolite of fungi, antibiotics, and so on. Finally, a summary and future perspective on the applications of MOFs-based nanozymes are briefly discussed.

Recent advances in the construction and analytical applications of metal-organic frameworks-based nanozymes

The development of two-dimensional (2D) inorganic materials-based quantum dots (QDs) is still in its infancy but is triggering immense enthusiasm due to their high chemical stability, good aqueous dispersibility, excellent optical property, good biocompatibility and easy functionalization. This review covers almost all the extant 2D-QDs based on graphene, phosphorene, silicene, carbides, nitrides, transition metal dichalcogenide, transition metal oxides and MXenes, etc. Their categories, synthetic routes, properties, functionalization and applications are critically highlighted. In the application section, special emphasis is placed on the progress in bioimaging, cancer therapy, fluorescent sensing and optoelectronics. Meanwhile, the latest advances in 2D QDs-based catalysis and energy since 2015 are addressed. Moreover, 2D nanoclusters, in particular 2D-QDs, are also included. This review provides guidance for 2D-QDs studies to meet the increasing demands in the many diverse applications.

Recent progress in two-dimensional inorganic quantum dots.

The octahedral structure of MIL-53(Fe) was facilely prepared by a microwave (MW)-assisted approach, and confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The MIL-53(Fe) MOFs were further characterized by thermo gravimetric (TG) analysis and fourier transform infrared (FTIR) spectroscopy. It is found that the as-prepared MIL-53(Fe) exhibits intrinsic peroxidase-like activity, and could catalytically oxidize 3,3′,5,5′-tetramethylbenzidine (TMB), ABTS and OPD by H2O2 to produce a typical coloured reaction. The Michaelis–Menten behavior of the as-prepared MIL-53(Fe) was studied. The Km value of the as-prepared MIL-53(Fe) with H2O2 as the substrate was 0.03 mM, which was at least seven times lower than that of Fe-MIL–88NH2 and hemin@MIL-53(Al)–NH2. Interesting, the Km values of the as-prepared MIL-53(Fe) with H2O2 and TMB as the substrates were both lower than those of the MIL-53(Fe) obtained by conventional electric (CE) heating-based solvothermal method. This is probably attributed to the purely octahedral structure and small sized crystals of the MIL-53(Fe) obtained by MW-based synthesis method, confirming that the MW-based synthesis method promised advantages of simplicity, fast crystallization and good phase selectivity. Results of electron spin resonance (ESR) experiments indicated that the as-prepared MIL-53(Fe) exhibited catalytic ability to decompose H2O2 into ˙OH radicals. On this basis, a simple, sensitive and selective method for glucose detection was developed by coupling the oxidation of glucose catalyzed by glucose oxidase (GOx). As low as 0.25 μM glucose could be detected with a linear range from 0.25–20 μM. The proposed method was successfully used to determine glucose in real human serum samples.

Metal–organic framework MIL-53(Fe): facile microwave-assisted synthesis and use as a highly active peroxidase mimetic for glucose biosensing

In this paper, the stable and good water-soluble polyacrylic acid functioned cerium oxide nanoparticles (CeO2 NPs) were successfully synthesized and characterized by dynamic light scattering (DLS), FT-IR and XPS. The experimental results indicated that the prepared CeO2 NPs exhibit intrinsic peroxidase-like activity, which can catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) in the presence of H2O2 to produce blue reaction. Electron paramagnetic resonance (EPR) and Raman techniques were used to investigate the catalytic mechanism. On the basis of H2O2 concentration-dependent peroxidase-like activity of CeO2 NPs, a simple, low cost and selectively colorimetric method was constructed for sensing of glucose. Under the optimal conditions, the linear range for the determination of glucose was 0.5-10 mmol/L with a limit of detection (LOD, 3 σ ) of 0.1 mmol/L. The RSD was 2.4% for 1.0 mmol/L glucose ( n = 11). The proposed method has been successfully used for determination of glucose in serum samples.

Intrinsic peroxidase-like activity exhibited by cerium oxide nanoparticles and their application to glucose detection

A new method for selective determination of Cr(VI) in environmental water samples was developed based on its quenching effect on the fluorescent N-doping graphene quantum dots (N-GQDs). The N-GQDs were synthesized by a simple one-step method using citric acid as the carbon source and ammonia as the nitrogen source with a 65% yield, showing that mass production of the N-GQDs is possible. The obtained N-GQDs with oxygen-rich functional groups exhibited a strong blue emission with a quantum yield of 18.6%, which was 7 times greater than that of graphene quantum dots (GQDs). Due to the selective coordination to Cr(VI), the N-GQDs can be used as a green and facile sensing platform for label-free sensitive and selective detection of Cr(VI) ions in aqueous solution and real water samples. Compared to GQDs, the N-GQDs as a fluorescent probe promises much improved selectivity for sensing of Cr(VI). The N-GQDs fluorescence probe shows a sensitive response to Cr(VI) in a wide concentration range of 0–140 μM with a detection limit of 40 nM. The N-GQDs-based fluorescence method was successfully used to selectively detect Cr(VI), and discriminate it and Cr(III) as well in aqueous samples.

/pdf/a-simple-one-pot-synthesis-of-highly-fluorescent-nitrogen-1e1bge33dz.pdf

A simple one-pot synthesis of highly fluorescent nitrogen-doped graphene quantum dots for the detection of Cr(VI) in aqueous media

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Metal–organic framework MIL-53(Fe): facile microwave-assisted synthesis and use as a highly active peroxidase mimetic for glucose biosensing

Intrinsic peroxidase-like activity exhibited by cerium oxide nanoparticles and their application to glucose detection

A simple one-pot synthesis of highly fluorescent nitrogen-doped graphene quantum dots for the detection of Cr(VI) in aqueous media