Carbon Dots in Enantioselective Sensing

Chirality has a crucial effect on clinical, chemical and biological research since most bioactive compounds are chiral in the natural world. It is thus important to evaluate the enantiomeric ratio (or the enantiopurity) of the selected chiral analytes. To this purpose, fluorescence and electrochemical sensors, in which a chiral modifier is present, are reported in the literature. In this review, fluorescence and electrochemical sensors for enantiorecognition, in which chiral carbon dots (CDs) are used, are reported. Chiral CDs are a novel zero-dimensional carbon-based nanomaterial with a graphitic or amorphous carbon core and a chiral surface. They are nanoparticles with a high surface-to-volume ratio and good conductivity. Moreover, they have the advantages of good biocompatibility, multi-color emission, good conductivity and easy surface functionalization. Their exploitation in enantioselective sensing is the object of this review, in which several examples of fluorescent and electrochemical sensors, containing chiral CDs, are analyzed and discussed. A brief introduction to the most common synthetic procedures of chiral CDs is also reported, evidencing strengths and weaknesses. Finally, consideration concerning the potential challenges and future opportunities for the application of chiral CDs to the enantioselective sensing world are outlined.

Activation-Induced Senescent Cell Death based on Chiral CoHAu Nanoassemblies with Enantioselective Cascade-Catalytic Ability

Chirality is common in nature, which determines the high enantioselectivity of living systems. Selecting suitable chiral configurations is of great meaning for nanostructures to function better in biological systems. In this study, chiral Co3O4-H2TPPS-Au (CoHAu) nanoassemblies are constructed to accelerate the production ∙OH by consuming D-glucose (D-Glu, widely spread in nature) based on their outstanding enantioselective cascade-catalytic abilities. In particular, D-CoHAu nanoassemblies are more effective in the catalytic conversion of D-Glu than L-CoHAu nanoassemblies. This phenomenon is due to the stronger binding affinity of D-CoHAu nanoassemblies indicated by the lower Km value. Moreover, D-CoHAu nanoassemblies display excellent consumption-ability of D-Glu and production of ∙OH in living cells, which can eliminate senescent cells effectively based on their intracellular enantioselective cascade-catalysis. This research establishes the foundation for bio-mimicking nanostructures with unique functionalities to regulate abnormal biological activities better.

Enantioselective adsorption of ibuprofen enantiomers using chiral-active carbon nanoparticles induced S-α-methylbenzylamine

The chiral media is crucial to the chiral recognition and separation of enantiomers. In this study, we report the preparation of novel chiral carbon nanoparticles (CCNPs) via surface passivation using glucose as the carbon source and S-(-)-α-methylbenzylamine as the chiral ligand. The structures of the obtained CCNPs are characterized via FT-IR, Raman spectroscopy, DLS, XPS, XRD, TEM, and zeta potential analysis. These CCNPs could be employed as the chiral adsorbent and used for the enantioselective adsorption of the ibuprofen enantiomers. The results demonstrated that the CCNPs could selectively adsorb R-enantiomer from ibuprofen racemate solution and give an enantiomeric excess (e.e.) of about 50% under an optimal adsorption condition. Moreover, the regeneration efficiency of the CCNPs remained above e.e. of 43% after the fifth cycle. The present work confirmed that the prepared CCNPs show great potential in the enantioselective separation of ibuprofen racemate.

Preparation of Chiral Carbon Quantum Dots and its Application

Chiral carbon quantum dots (cCQDs) , as a new type of carbon nano-functional material with tunable emission wavelength, superior photostability, low toxicity, biocompatibility and chirality, are playing an increasingly important role in the fields of chemistry, biology and medicine. This paper reviews the preparation methods (one-step and two-step), optical properties (UV, fluorescence, chirality) and applications in chiral catalysis, chiral recognition, targeted imaging as well as other fields, while lists some of the issues and challenges in the research of chiral carbon quantum dots. Finally, due to its good fluorescence and other properties, it is expected that chiral carbon quantum dots will have broad commercial prospects in future applications.

One-step hydrothermal preparation of chiral carbon quantum dots and enantioselective sensing of glutamine enantiomeric isomers

A novel method for chiral identification of glutamine enantiomers based on chiral carbon quantum dots (cCQDs) fluorescent probes. cCQDs were prepared using a one-step hydrothermal method with L-tryptophan as the carbon source and chiral source, producing spherical nanoparticles exhibiting a blue colour luminescence. The fluorescence intensity (F) of cCQDs was enhanced or quenched following the addition of chiral enantiomeric glutamine (L/D-Gln), and therefore cCQDs, as a fluorescence probe, could be used for enantioselective sensing of the L/D-Gln. The fluorescence enhancement value (∆FE ) exhibited good linearity with L-Gln concentration in the range 0.23-10.00 mM, and the limit of detection was 0.14 mM. The fluorescence quenching value (∆FQ ) showed a good linear relationship with D-Gln concentration in the range 0.29-10.00 mM, and the detection limit was 0.18 mM. The mechanism of fluorescence enhancement/quenching was explored by molecular modelling and the type of quenching. The method was applied to the determination of L-Gln content in real samples, and the recovery rate was satisfactory. This study provided a novel approach for the synthesis of cCQDs and the recognition of amino acid enantiomers.

Chiral Carbon Dots and Chiral Carbon Dots with Circularly Polarized Luminescence: Synthesis, Mechanistic Investigation and Applications

Chiral carbon dots (CCDs) can be widely used in various fields such as chiral recognition, chiral catalysis and biomedicine because of their unique optical properties, low toxicity and good biocompatibility. In addition, CCDs with circularly polarized luminescence (CPL) can be synthesized, thus broadening the prospects of CCDs applications. Since the research on CCDs is still in its infancy, this paper reviews the chiral origin, formation mechanism, chiral evolution, synthesis and emerging applications of CCDs, with a special focus on CCDs with CPL activity. It is hoped that it will provide some reference to solve the current problems faced by CCDs. Finally, the opportunities and challenges of the current research on CCDs are described, and their future development trends have also been prospected.

Nitrogen and sulfur co-doped carbon quantum dots as "on-off-on" fluorescence probes to detect Hg2+ and MnO4- and improving the photostability of Rhodamine B

BACKGROUND

Carbon quantum dot (CQDs) are zero-dimensional carbon nanomaterials with a size of less than 10 nm CQDs are widely used in the field of ion detection by virtue of their fluorescence characteristics such as strong fluorescence intensity, good optical stability and tunable emission wavelength. Although the traditional atomic absorption method, electrochemical method and other metal ion detection methods are highly sensitive, the operation is complex, expensive and limited by the site. Therefore, we prepared the N, S-CQDs capable of detecting Hg2+ and MnO4- in water with the advantages of simple operation, low cost, and direct visual signal.

RESULTS

N, S-CQDs with high-quantum yield (77.68%), uniform particle size (0.4 nm-2.6 nm) and green fluorescence were created utilizing a one-pot hydrothermal process with the precursors ASDA-Na4 and m-phenylenediamine. N, S-CQDs has good optical properties such as high fluorescence intensity, wavelength independence, up-conversion luminescence and fluorescence stability. We examined 27 common ions in water and found that the fluorescence of N, S-CQDs could be selectively quenched by Hg2+ and MnO4-, and the detection limits are 0.41 μM and 1.2 μM, respectively. The mechanism of quenching is further investigated. The fluorescence of N, S-CQDs-Hg2+ system can be restored by halogen ions (Cl-, Br-, I-), while the fluorescence of N, S-CQDs-MnO4- system can be partially restored by Fe2+. This forms an "on-off-on" mode of fluorescent probes. In addition, we also studied that trace amounts of N, S-CQDs can improve the photostability of RhB.

SIGNIFICANCE

The N, S-CQDs are fluorescent probes in an "on-off-on" mode. N, S-CQDs with green fluorescence (on) can be quenched by Hg2+ and MnO4- (off). The fluorescence quenched by Hg2+ can be restored by halogen ions again, while the fluorescence quenched by MnO4- can partially be restored (on). This ion detection method can be used to visually detect the two ions in the field, with the advantages of low cost, simple operation and visual intuition.

Recent Progress on Chiral Carbon Dots: Synthetic Strategies and Biomedical Applications

The discovery of chiral carbon dots (Ch-CDs) has opened up an exciting new research direction in the field of carbon dots. It not only retains the chirality of the precursor and exhibits highly symmetric chiral optical properties but also has properties such as chemical stability, antibacterial and antitumor properties, and good biocompatibility of carbon dots. Based on these advantages, the application of Ch-CDs in the biomedical field has attracted significant interest among researchers. However, a comprehensive review of the selection of precursors for Ch-CDs, preparation methods, and applications in biomedical fields is still lacking. Here, we summarize their precursor selection and preparation methods based on recent reports on Ch-CDs and provide the first comprehensive review for specific applications in biomedical engineering, such as biosensing, bioimaging, drug carriers, antibacterial and antibiofilm, and enzyme activity modulation. Finally, we discuss application prospects and challenges that need to be overcome. We hope this review will provide valuable guidance for researchers to prepare novel Ch-CDs and facilitate their application in biomedical engineering.

Fluorescent/Colorimetric Dual-Mode Discriminating Gln and Val Enantiomers Based on Carbon Dots

Discrimination and quantification of amino acid (AA) enantiomers are particularly important for diagnosing and treating diseases. Recently, dual-mode probes have gained a lot of research interest because they can catch more detecting information compared with the single-mode probes. Thus, it is of great significance to develop a dual-mode sensor realizing AA enantiomer discrimination conveniently and efficiently. In this work, carbon dot L-TCDs were prepared by N-methyl-1,2-benzenediamine dihydrochloride (OTD) and l-tryptophan. With the assistance of H2O2, L-TCDs show an excellent discrimination performance for enantiomers of glutamine (Gln) and valine (Val) in both fluorescent and colorimetric modes. The fluorescence enantioselectivity of Gln (FD/FL) and Val (FL/FD) is 5.29 and 4.13, respectively, and the colorimetric enantioselectivity of Gln (ID/IL) and Val (IL/ID) is 13.26 and 3.42, individually. The chiral recognition mechanism of L-TCDs was systematically studied. L-TCDs can be etched by H2O2, and the participation of AA enantiomers results in different amounts of the released OTD, which provides fluorescent and colorimetric signals for identifying and quantifying the enantiomers of Gln and Val. This work provides a more convenient and flexible dual-mode sensing strategy for discriminating AA enantiomers, which is expected to be of great value in facile and high-throughput chiral recognition.

Fluorescence origin and chirality mechanism of graphene quantum Dots: Twist or Non-Twist?

In this paper, we theoretically investigate the fluorescence origin and chirality mechanism of graphene quantum dots with non-twist and twist geometries, respectively. It is revealed that twist is not necessary for fluorescence; but twist is must for the chirality, which can significantly enhance the intensity of chirality, demonstrated by ECD spectra. Our results provide deeper understanding on the physical mechanism of fluorescence and chirality of graphene quantum dot influenced by geometric twist.

Chiral template-induced porphyrin-based self-assembled materials for electrochemical chiral sensing

Chirality plays a key role in many fields of natural sciences as well as life sciences. Chiral materials are widely developed and used for electrochemical chiral recognition. In recent years, carbon quantum dots (CQDs) have been widely used as a novel carbon nanomaterial due to their excellent charge transfer properties, good biocompatibility, and low cost. The special structure of π-conjugated porphyrin attracts attention. Supramolecular self-assembly shows a way to construct chiral materials by self-assembling simple molecules into chiral composites. Herein, we demonstrate the self-assembly of achiral porphyrins induced by chiral carbon quantum dots assembled from L- and or D-tryptophan (L- and or D-Trp) with carbon quantum dots, resulting in 5,10,15,20-tetrakis (4-carboxyPheyl) (TCPP) self-assembled structure. The electrochemical chiral recognition of chiral self-assembled materials was studied using Phenylalanine (Phe) enantiomer as a chiral analyte. Electrochemical chiral recognition results showed that the chiral self-assembled materials induced by chiral templates have a good ability to discriminate Phe enantiomers. Therefore, this research provides a new idea for the synthesis of chiral composites and further expands applications to electrochemical chiral recognition.

Fabrication of Carbon-Based Quantum Dots via a "Bottom-Up" Approach: Topology, Chirality, and Free Radical Processes in "Building Blocks"

The discovery of carbon-based quantum dots (CQDs) has allowed opportunities for fluorescence bioimaging, tumor diagnosis and treatment, and photo-/electro-catalysis. Nevertheless, in the existing reviews related to the "bottom-up" approaches, attention is mainly paid to the applications of CQDs but not the formation mechanism of CQDs, which mainly derived from the high complexities during the synthesis of CQDs. Among the various synthetic methods, using small molecules as "building blocks", the development of a "bottom-up" approach has promoted the structural design, modulation of the photoluminescence properties, and control of the interfacial properties of CQDs. On the other hand, many works have demonstrated the "building blocks"-dependent properties of CQDs. In this review, from one of the most important variables, the relationships among intrinsic properties of "building blocks" and photoluminescence properties of CQDs are summarized. The topology, chirality, and free radical process are selected as descriptors for the intrinsic properties of "building blocks". This review focuses on the induction and summary of recent research results from the "bottom-up" process. Moreover, several empirical rules pertaining thereto are also proposed.

Chiral Carbon Dots: Synthesis and Applications in Circularly Polarized Luminescence, Biosensing and Biology

Chiral carbon dots (CDs) are a novel luminescent zero-dimensional carbon-based nanomaterial with chirality. They not only have the advantages of good biocompatibility, multi-color-emission, easy functionalization, but also exhibits highly symmetrical chiral optical characteristics, which broadens their applicability to enantioselectivity of some chiral amino acids like cysteine and lysine, asymmetric catalysis as well as biomedicine in gene expression and antibiosis. In addition, the exploration of the excited state chirality of CDs has developed its excellent circularly polarized luminescence (CPL) properties, opening up a new application scenario like recognition of chiral light sources and anti-counterfeit printing with information encryption. This review mainly focuses on the mature synthesis approaches of chiral CDs, including chiral ligand method and supramolecular self-assembly method, then we consider emerging applications of chiral CDs in CPL, biosensing and biological effect. Finally, we concluded with a perspective on the potential challenges and future opportunities of such fascinating chiral CDs.

Synthesis of CoFe2O4/Peanut Shell Powder Composites and the Associated Magnetic Solid Phase Extraction of Phenoxy Carboxylic Acid Herbicides in Water

The magnetic biochar material CoFe₂O₄/PCPS (peanut shell powder) was prepared based on the hybrid calcination method. The properties of prepared composites and the extraction effect of magnetic solid phase extraction on phenoxy carboxylic acid herbicides were assessed. The morphology, crystal structure, specific surface area, and pore size distribution of the material were analysed using a transmission electron microscope (TEM), infrared Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and N₂ absorption surface analysis (BET). The results of the magnetic solid phase extraction of a variety of phenoxy carboxylic acid herbicides in water using CoFe₂O₄/PCPS composites showed that, when the mass ratio of CoFe₂O₄ and PCPS was 1:1, 40 mg of the composite was used, and the adsorption time was 10 min at pH 8.50. Methanol was used as the eluent, and the recovery rates of the three phenoxy carboxylic acid herbicides were maintained at 81.95–99.07%. Furthermore, the actual water sample analysis results showed that the established method had good accuracy, stability, and reliability.

Chiral carbon dots: synthesis, optical properties, and emerging applications

Carbon dots are luminescent carbonaceous nanoparticles that can be endowed with chiral properties, making them particularly interesting for biomedical applications due to their low cytotoxicity and facile synthesis. In recent years, synthetic efforts leading to chiral carbon dots with other attractive optical properties such as two-photon absorption and circularly polarized light emission have flourished. We start this review by introducing examples of molecular chirality and its origins and providing a summary of chiroptical spectroscopy used for its characterization. Then approaches used to induce chirality in nanomaterials are reviewed. In the main part of this review we focus on chiral carbon dots, introducing their fabrication techniques such as bottom-up and top-down chemical syntheses, their morphology, and optical/chiroptical properties. We then consider emerging applications of chiral carbon dots in sensing, bioimaging, and catalysis, and conclude this review with a summary and future challenges.

Green Synthesis of Nitrogen–Doped Carbon Dots from Fresh Tea Leaves for Selective Fe3+ Ions Detection and Cellular Imaging

In this research, we successfully developed a green, economical and effective one–step hydrothermal method for the synthesis of fluorescent nitrogen–doped carbon dots (N–CDs) by utilizing fresh tea leaves and urea as the carbon and nitrogen sources, respectively. The obtained N–CDs were characterized by TEM, XPS and FT–IR. We found that the N–CDs were near–spherical with an average size of about 2.32 nm, and contained abundant oxygen and nitrogen functional groups. The N–CDs exhibited bright blue fluorescence under ultraviolet illumination, with the maximum emission at 455 nm. Meanwhile, the as–prepared N–CDs could be selectively quenched by Fe3+ ions. The quenching of N–CDs is linearly correlated with the concentration of Fe3+ in the range of 0.1–400 μM with a low detection limit of 0.079 μM. Significantly, the N–CDs present excellent biocompatibility and high photostability. The results also depict that multicolor fluorescence is displayed under a fluorescence microscope and successfully applied for the detection of intracellular Fe3+. To sum up, the fluorescent N–CDs are expected to be a sensitive detection probe for Fe3+ in biological systems.

Orange-emissive carbon quantum dots for ligand-directed Golgi apparatus-targeting and in vivo imaging

Golgi apparatus is one of the most important organelles in cells. Targeting and monitoring the morphology and structure of Golgi apparatus are crucial and challenging. Aimed at the cysteine (Cys)...

Chiral carbon quantum dots as fluorescent probe for rapid chiral recognition of isoleucine enantiomers

Chiral recognition is always a significant and challenging work in analytical chemistry. A fluorescent chiral recognition method based on chiral carbon quantum dots (CCQDs) towards isoleucine (Ile) enantiomers was developed in this work. CCQDs were synthesized by one-step hydrothermal method using l-cysteine as chiral source. The fluorescence intensity of CCQDs enhanced obviously in the presence of L-Ile, but had no observable change in the presence of D-Ile. The response speed of this chiral sensing system is fast, Ile enantiomers can be discriminated by CCQDs within 5 min, the enantioselectivity (I_L/I_D) can reach up to 2.2. Good linearity for detecting L-Ile was obtained over the concentration range from 0 to 30 mM with a LOD of 0.29 mM. The fluorescence intensity also increased linearly with the enantiomeric percentages of L-Ile in the mixture of Ile enantiomers. Thus, the developed method not only can achieve quantitative detection of L-Ile but also can determine the enantiomeric percentage in racemates. The chiral recognition mechanism can be explained by the difference in binding energy and interaction types between D-Ile and L-Ile with CCQDs by molecular modeling. The current method was applied in detecting L-Ile in real samples of functional drinks, the detection results were in consistent with the results obtained from high performance liquid chromatography, and the recoveries of standard addition were also satisfactory, which verified the reliability of the developed method.

Carbon dots: synthesis, properties and biomedical applications

Carbon dots (CDs) are a new type of carbon nanomaterial that have unique physical and chemical properties, good biocompatibility, low toxicity, and easy surface functionalization, making them widely used in biological imaging, environmental monitoring, chemical analysis, targeted drug delivery, disease diagnosis, therapy, etc. In this review, our content is mainly divided into four parts. In the first part, we focused on the preparation methods of CDs, including arc discharge, laser ablation, electrochemical oxidation, chemical oxidation, combustion, hydrothermal/solvent thermal, microwave, template, method etc. Next, we summarized methods of CD modification, including heteroatom doping and surface functionalization. Then, we discussed the optical properties of CDs (ultraviolet absorption, photoluminescence, up-conversion fluorescence, etc.). Lastly, we reviewed the common applications of CDs in biomedicine from the aspects of in vivo and in vitro imaging, sensors, drug delivery, cancer theranostics, etc. Furthermore, we also discussed the existing problems and the future development direction of CDs.

The formation mechanism and chirality evolution of chiral carbon dots prepared via radical assisted synthesis at room temperature

We report on a Cu(ii) catalyzed process for the production of cysteine based chiral carbon dots; the process does not require any thermal treatment and the carbon dot formation is driven by the production of reactive radical species that are generated in the reaction media by the catalytic role played by the multivalent transition metal. The nanomaterial presents a well-defined chirality and the enantioselectivity of the synthesis is proved by the isolation of both the carbon dot enantiomers. We focused our attention on the processes that take place during the carbon dot formation and the relationship with the structure of the organic starting material. Thanks to the comparison of reactions conducted with different organic substrates whose thiyl radical chemistry is known, we recognized a non-trivial role of the radical hydrogen abstraction reactions in the carbon dot formation process. The reported process allows access to a large variety of analyses to monitor the reaction mixtures during the reaction course. Finally, we report a detailed analysis on the evolution of optical chirality during the synthesis and related this feature with the formation mechanism of the nanomaterial revealing significant evidence on the chirality origin and structure of chiral carbon dots.

Chiral carbon dots based on L/D-cysteine produced via room temperature surface modification and one-pot carbonization

Since chirality is one of the phenomena often occurring in nature, optically active chiral compounds are important for applications in the fields of biology, pharmacology, and medicine. With this in mind, chiral carbon dots (CDs), which are eco-friendly and easy-to-obtain light-emissive nanoparticles, offer great potential for sensing, bioimaging, enantioselective synthesis, and development of emitters of circularly polarized light. Herein, chiral CDs have been produced via two synthetic approaches using a chiral amino acid precursor l/d-cysteine: (i) surface modification treatment of achiral CDs at room temperature and (ii) one-pot carbonization in the presence of chiral precursor. The chiral signal in the absorption spectra of synthesized CDs originates not only from the chiral precursor but from the optical transitions attributed to the core and surface states of CDs. The use of chiral amino acid molecules in the CD synthesis through carbonization results in a substantial (up to 8 times) increase in their emission quantum yield. Moreover, the synthesized CDs show two-photon absorption which is an attractive feature for their potential bioimaging and sensing applications.

One-step hydrothermal synthesis of chiral carbon dots with high asymmetric catalytic activity for an enantioselective direct aldol reaction

Chiral carbon dots are prepared by a simple and one-step hydrothermal reaction at 180 °C for 2 h using citric acid and d-proline as precursors, which show high asymmetric catalytic activity for enantioselective direct aldol condensation. This work provides a hint for the simple preparation of heterogeneous chiral catalysts.

Tuning residual chirality in carbon dots with anti-microbial properties

Chirality remains a critical consideration in drug development and design, as well as in applications of enantioselective recognition and sensing. However, the preparation of chiral nanomaterials requires extensive post synthetic modifications with a chiral agent, coupled with extensive purification. This limits the use and application of chiral nanomaterials. Herein, we report a facile, one-step microwave-assisted synthesis of chiral carbon dots through the reaction of l- and d-cysteine amino acid precursors and citric acid. We modulated the synthetic parameters to preserve and tune the residual chiral properties of the dots and demonstrate that the reaction conditions play a critical role in dictating the chiral behaviour of the dots. Finally, in a proof of concept application we demonstrated that the synthesized carbon dots, particularly d-carbon dots inhibit bacterial growth at a lower concentration than l-carbon dots. By varying bacterial strains and chirality of the carbon dots, concentrations ranging from 0.25-4 mg mL-1 of the nanoparticles were required to inhibit microbial growth. The ability to preserve and tune chirality during synthesis can open up novel avenues and research directions for the development of enantioselective materials, as well as antibacterial films and surfaces.

Preparation and Properties of Cyanobacteria-Based Carbon Quantum Dots/Polyvinyl Alcohol/ Nanocellulose Composite

Blue luminescent carbon quantum dots (CQDs) were prepared from cyanobacteria by a hydrothermal method. The PL quantum yields of the obtained CQDs was 5.30%. Cyanobacteria-based carbon quantum dots/polyvinyl alcohol/nanocellulose composite films were prepared, which could emit bright blue under UV light. FTIR characterization showed that the composite films had hydroxyl groups on the surface and no new groups were formed after combining the three materials. The photoluminescence (PL) spectra revealed that the emission of the prepared CQDs was excitation dependent. Studies on the water resistance performance and light barrier properties of the composite films showed that they possessed higher water resistance properties and better UV/infrared light barrier properties. Therefore, we report the cyanobacteria-based carbon quantum dots/polyvinyl alcohol/nanocellulose composite films have the potential to be applied in flexible packaging materials, anti-fake materials, UV/infrared light barrier materials and so on.

Application of temperature-controlled chiral hybrid structures constructed from copper(ii)-monosubstituted Keggin polyoxoanions and copper(ii)-organoamine complexes in enantioselective sensing of tartaric acid

Temperature usually occupies a crucial position in the construction of chiral compounds. By controlling the temperature of the reaction system, chiral and non-chiral compounds can be designed and synthesized. Given the above, three new chiral and non-chiral compounds based on copper(ii) monosubstituted polyoxoanions and Cu(en) complexes (en = ethylenediamine), d/l-[Cu(H₂O)(en)₂]₂{[Cu(H₂O)₂(en)][SiCuW₁₁O₃₉]}·5H₂O (1, d-1 and l-1) and [Cu(H₂O)(en)₂]{[Cu(en)₂]₂[SiCuW₁₁O₃₉]}·2.5H₂O (2), were successfully synthesized under hydrothermal conditions. The main synthesis conditions of compound 1 (d-1 and l-1) and compound 2 are the same, however, the only difference is that the reaction temperatures are 80 °C and 140 °C, respectively. What's more, compounds 1 and 2 can form a 1D chiral chain by Cu–O and W/Cu–O–W/Cu bonds, respectively, and further obtain a 3D-supramolecular framework through hydrogen bonding interaction. Meanwhile, due to the asymmetry of chiral compound 1, optical second-harmonic generation (SHG) was used to investigate the second-order nonlinear optical effect and it was found that the observed SHG efficiency of compound 1 is 0.3 times that of urea. To further investigate the chiral properties, d-1 and l-1 were used in the electrochemical enantioselective sensing of d-/l-tartaric acid (d-/l-tart) molecules, respectively, which demonstrates that d-1 and l-1 have a good application prospect in sensing chiral substances.

A pair of temperature-controlled chiral compounds, d- and l-[Cu(en)₂(H₂O)]₂{[Cu(en)(H₂O)₂][SiCuW₁₁O₃₉]}·5H₂O (en = ethanediamine) are isolated by hydrothermal method, having a good application prospect in sensing d-/l-tartaric acid.

One-step electrodeposition of the MOF@CCQDs/NiF electrode for chiral recognition of tyrosine isomers

Electrochemical enantiorecognition of tyrosine (Tyr) isomers using a MOF@CCQDs/NiF electrode prepared by electrodepositing a metal-organic framework (MOF) and chiral carbon quantum dots (CCQDs) on Ni foil is reported. MOF@CCQDs/NiF not only shows highly selective, sensitive and quantitative analysis towards Tyr enantiomers but also presents the ability to determine l-Tyr% in racemic mixtures. This proposed that chiral sensors could be considered for practical applications in the field of Tyr related medical recognition.

Carbon dots: advances in nanocarbon applications

Carbon dots (C-Dots), defined by characteristic sizes of <10 nm, have become a rising star in carbon nanomaterials. C-Dots possess many unique physiochemical and photochemical properties which make them a promising platform for imaging, environmental, catalytic, biological and energy-related applications. To date, C-Dots have been investigated extensively, and their related applications have developed rapidly. However, quantitative understanding of the physiochemical properties of C-Dots still remains a difficult challenge because of their complex structures. Here, we will highlight the recent progress in the practical applications of C-Dots, with particular attention to the research in light-emitting devices, bioimaging and biodetection, catalysis, functional materials, and agriculture.

Single- and multi-component chiral supraparticles as modular enantioselective catalysts

Nanoscale biological assemblies exemplified by exosomes, endosomes and capsids, play crucial roles in all living systems. Supraparticles (SP) from inorganic nanoparticles (NPs) replicate structural characteristics of these bioassemblies, but it is unknown whether they can mimic their biochemical functions. Here, we show that chiral ZnS NPs self-assemble into 70–100 nm SPs that display sub-nanoscale porosity associated with interstitial spaces between constituent NPs. Similarly to photosynthetic bacterial organelles, these SPs can serve as photocatalysts, enantioselectively converting L- or D-tyrosine (Tyr) into dityrosine (diTyr). Experimental data and molecular dynamic simulations indicate that the chiral bias of the photocatalytic reaction is associated with the chiral environment of interstitial spaces and preferential partitioning of enantiomers into SPs, which can be further enhanced by co-assembling ZnS with Au NPs. Besides replicating a specific function of biological nanoassemblies, these findings establish a path to enantioselective oxidative coupling of phenols for biomedical and other needs.

Nanoscale biological assemblies play crucial roles in all living systems and display a variety of chemical functionalities. Here the authors show that it is possible to replicate some of the biochemical functions in similarly-sized assemblies made from inorganic nanoparticles.

High Amplification of the Antiviral Activity of Curcumin through Transformation into Carbon Quantum Dots

It is demonstrated that carbon quantum dots derived from curcumin (Cur-CQDs) through one-step dry heating are effective antiviral agents against enterovirus 71 (EV71). The surface properties of Cur-CQDs, as well as their antiviral activity, are highly dependent on the heating temperature during synthesis. The one-step heating of curcumin at 180 °C preserves many of the moieties of polymeric curcumin on the surfaces of the as-synthesized Cur-CQDs, resulting in superior antiviral characteristics. It is proposed that curcumin undergoes a series of structural changes through dehydration, polymerization, and carbonization to form core-shell CQDs whose surfaces remain a pyrolytic curcumin-like polymer, boosting the antiviral activity. The results reveal that curcumin possesses insignificant inhibitory activity against EV71 infection in RD cells [half-maximal effective concentration (EC₅₀ ) >200 µg mL^-1 ] but exhibits high cytotoxicity toward RD cells (half-maximal cytotoxic concentration (CC₅₀ ) <13 µg mL^-1 ). The EC₅₀ (0.2 µg mL^-1 ) and CC₅₀ (452.2 µg mL^-1 ) of Cur-CQDs are >1000-fold lower and >34-fold higher, respectively, than those of curcumin, demonstrating their far superior antiviral capabilities and high biocompatibility. In vivo, intraperitoneal administration of Cur-CQDs significantly decreases mortality and provides protection against virus-induced hind-limb paralysis in new-born mice challenged with a lethal dose of EV71.

Investigation on the chirality mechanism of chiral carbon quantum dots derived from tryptophan

Chiral carbon quantum dots (CQDs) with chirality, fluorescence and biocompatibility were synthesized by a one-step method with l-/d-tryptophan (l-/d-Trp), as both carbon source and chiral source. Levogyration-/dextrorotation-CQDs (l-/d-CQDs) were characterized by transmission electron microscopy, Fourier transform infrared spectrometry, ultraviolet-visible absorption, excitation and emission spectrometry and circular dichroism (CD) spectrometry. Results show that l-CQDs and d-CQDs present similar spherical morphology, functional groups and optical properties. The CD signal, around 220, 240 and 290 nm are opposite and symmetric, which conclusively demonstrates that l-CQDs and d-CQDs are enantiomers. Besides the CD signal around 220 nm from the inheritance of l-/d-Trp, two new chiral signals around 240 and 290 nm were induced by chiral environment.

To clarify the chirality mechanism of chiral CQDs prepared by l-/d-tryptophan, the chirality origin in CQD structure was revealed.

pH-Dependent photoluminescence “switch-on” nanosensors composed of silver nanoparticles and nitrogen and sulphur co-doped carbon dots for discriminative detection of biothiols

A nanomaterial surface energy transfer (NSET) system composed of silver nanoparticles (AgNPs) and nitrogen and sulphur co-doped carbon dots (N,S-CDs) was established to discriminate biothiols, featuring the pH-promoted distinct PL “switch-on” response.

A universal strategy to obtain chiroptical carbon quantum dots through the optically active surface passivation procedure

A two-step pyrolytic route has been demonstrated for producing optically active photoluminescence CQDs by surface passivation with an enantiomeric ligand.

Chirality Inversion on the Carbon Dot Surface via Covalent Surface Conjugation of Cyclic α-Amino Acid Capping Agents

Manipulating the chiroptical properties at the nanoscale is of great importance in stereoselective reactions, enantioseparation, self-assembly, and biological phenomena. In recent years, carbon dots have garnered great attention because of their favorable properties such as tunable fluorescence, high biocompatibility, and facile, scalable synthetic procedures. Herein, we report for the first time the unusual behavior of cyclic amino acids on the surface of carbon dots prepared via microwave-based carbonization. Various amino acids were introduced on the surface of carbon dots via EDC/NHS conjugation at room temperature. Circular dichroism results revealed that although most of the surface conjugated amino acids can preserve their chirality on negatively charged, "bare" carbon dots, the "handedness" of cyclic α-amino acids can be flipped when covalently attached on carbon dots. Moreover, these chiroptical carbon dots were found to interact with the cellular membrane or its mimic in a highly selective manner due to their acquired asymmetric selectivity. A comprehensive inhibitor study was conducted to investigate the pathway of cellular trafficking of these carbon dots. Overall, it was concluded that the chirality of the amino acid on the surface of carbon dots could regulate many of the cellular processes.

Novel properties and applications of carbon nanodots

In the most recent decade, carbon dots have drawn intensive attention and triggered substantial investigation. Carbon dots manifest superior merits, including excellent biocompatibility both in vitro and in vivo, resistance to photobleaching, easy surface functionalization and bio-conjugation, outstanding colloidal stability, eco-friendly synthesis, and low cost. All of these endow them with the great potential to replace conventional unsatisfactory fluorescent heavy metal-containing semiconductor quantum dots or organic dyes. Even though the understanding of their photoluminescence mechanism is still controversial, carbon dots have already exhibited many versatile applications. In this article, we summarize and review the recent progress achieved in the field of carbon dots, and provide a comprehensive summary and discussion on their synthesis methods and emission mechanisms. We also present the applications of carbon dots in bioimaging, drug delivery, microfluidics, light emitting diode (LED), sensing, logic gates, and chiral photonics, etc. Some unaddressed issues, challenges, and future prospects of carbon dots are also discussed. We envision that carbon dots will eventually have great commercial utilization and will become a strong competitor to some currently used fluorescent materials. It is our hope that this review will provide insights into both the fundamental research and practical applications of carbon dots.

A chiral responsive carbon dots-gold nanoparticle complex mediated by hydrogen peroxide independent of surface modification with chiral ligands

Chiral recognition of enantiomers is fundamentally important. In this study, a novel strategy for the chiral discrimination of glucose enantiomers was constructed based on the hydrogen peroxide (H₂O₂)-mediated generation of a carbon dots-gold nanoparticle (C-dots@Au NP) complex independent of surface modification with chiral ligands. H₂O₂ is essential as a reductant to promote the growth of Au NPs from gold salts. Besides, the modification of C-dots with sulfhydryl groups is necessary for its anchoring on the surface of Au NPs. Therefore, in the presence of H₂O₂, the C-dots@Au NP complex can be self-generated from a simple mixture containing C-dots and Au salts. It is worth noting that glucose oxidase can selectively catalyze d-glucose but not l-glucose to generate H₂O₂. In this regard, the chiral recognition process can trigger the formation of the C-dots@Au NP complex. Furthermore, based on the production of reddish Au NPs and the reduction of C-dot fluorescence quenched by Au NPs, the resultant C-dots@Au NP complex enables achieving the chiral discrimination of glucose enantiomers by combining colorimetric and fluorometric assays. Compared with the conventional approaches that use chiral ligands to decorate NPs, the generation of the chiral-responsive C-dots@Au NP complex is much simpler and faster. Upon combination with specific enzymatic reactions that produce a reductive product, the current strategy provides a general approach for the identification of chiral enantiomers.

One-step hydrothermal synthesis of chiral carbon dots and their effects on mung bean plant growth

Chiral compounds/materials have important effects on the growth of plants. Chiral carbon dots (CDs), as an emerging chiral carbon nanomaterial, have great potential in bio-application and bio-nanotechnology. Herein, we report a hydrothermal method to synthesize chiral CDs from cysteine (cys) and citric acid. These chiral CDs were further demonstrated to have systemic effects on the growth of mung bean plants, in which case both l- and d-CDs can promote the growth of the root in mung bean plants, stem length of mung bean sprouts and water absorption of bean seeds. The elongation of mung bean sprouts presented an increasing trend with the treatment of chiral CDs of increasing concentration (below 500 μg mL-1). Furthermore, in the optimal concentration (100 μg mL-1), the l-CDs can improve root vigor and the activity of the Rubisco enzyme of bean sprouts by 8.4% and 20.5%, while the d-CDs increased by 28.9% and 67.5%. Due to more superior properties in improving root vigor and the activity of the Rubisco enzyme of mung bean sprouts, d-CDs are able to enhance photosynthesis better and accumulate more carbohydrate in mung bean plants.

Tunable excitation properties of ZnCdS:Mn/ZnS quantum dots for cancer imaging

Water-soluble ZnS:Mn quantum dots (QDs) were synthesized using a hydrothermal method with 3-mercaptopropionic acid as stabilizer. The optical properties of ZnS:Mn QDs were thoroughly investigated by tuning the doping concentration of Mn²⁺ and the Zn/S precursor ratio, to obtain an optimal parameter for QDs with excellent fluorescence characteristics. ZnS:Mn QDs excited at only one wavelength, however, which seriously limited their further application. Here, a trace Cd ion was doped into a ZnS host, resulting in QD excitation covering a wide adjustable waveband. Furthermore, when a ZnS shell was coated onto the surface of the ZnCdS:Mn QDs, photoluminescence intensity and stability were further enhanced. After coupling with an anti-CK 19 antibody, the ZnCdS:Mn/ZnS core/shell QDs were able to function by labeling cancer cells, indicating that they could be considered as a suitable bio-probe for cells and tissue imaging.

Chiral evolution of carbon dots and the tuning of laccase activity

Chirality has attracted extensive attention in many fields ranging from chemistry to life sciences. Carbon dots (CDs) with good biocompatibility and unique photochemical properties have become a new star in the nanocarbon family. Endowed with chirality, CDs will exhibit more marvellous properties and bridge the fields of material chemistry and life sciences tightly. Herein, we report a facile one-step alkali-assisted electrochemical method to fabricate chiral CDs from cysteine (cys). We showed the chiral evolution of CDs with highly symmetrical circular dichroism (CD) signals in the range from 205 to 350 nm. These chiral CDs have been further demonstrated to be capable of tuning the activity of laccase: the l-CDs can improve the activity of the enzyme up to 20.2%, whereas the d-CDs decrease the activity to 10.4%. A series of experiments confirm that it is the synergistic effect of nanosize and chirality of CDs that induces the change in the structure of laccase and thus leads to the tuning of the laccase activity.

A new nanosensor for the chiral recognition of cysteine enantiomers based on gold nanorods

A schematic illustration of the chiral recognition of d-Cys and l-Cys using GNRs in the presence of Cu²⁺ (0.125 mM).

A photoluminescence “switch-on” nanosensor composed of nitrogen and sulphur co-doped carbon dots and gold nanoparticles for discriminative detection of glutathione

A nanosensor was established to discriminate glutathione (GSH) from other competitive biothiols based on a photoluminescence (PL) “switch-on” signal readout.

Prolonged fluorescence lifetime of carbon quantum dots by combining with hydroxyapatite nanorods for bio-applications

Carbon quantum dots (CQDs) are a new type of fluorescent nanoparticle for cell imaging and tracking. However, they would easily diffuse and quench, followed by the loss of their fluorescence ability. By connecting their functional groups with other nanoparticles, the CQDs will be protected from destruction and exhibit long-time fluorescence. Here, carbon quantum dot-hydroxyapatite (CQD-HAp) hybrid nanorods were prepared by the self-assembly of CQDs on the surface of HAp nanorods through a facile one-pot process. The morphology and size of the CQD-HAp hybrid nanorods can be well controlled by using oleic acid, which meanwhile is the source of CQDs. The hydrophilic CQD-HAp hybrid nanorods have prolonged fluorescence life due to the connection between CQDs and HAp nanorods, and exhibit a higher fluorescence quantum yield than pure CQDs. In addition, when hybrid nanorods load doxorubicin (Dox) to form Dox-CQD-HAp hybrid nanorods, they can more efficiently kill human cervical cancer (HeLa) cells, rather than human prostatic cancer (PC-3) cells. Long time fluorescence for cell imaging and high efficiency in killing cancer cells as a drug-delivery medium make CQD-HAp hybrid nanorods have great potential applications in the bio-field.

Chiral carbon dots and their effect on the optical properties of photosensitizers

In this work, we have successfully prepared intrinsically chiral carbon dots from chiral precursors. We have also demonstrated that the chirality of these carbon dots can affect the optical properties of photosensitizer molecules like azobenzene.

Nitrogen and sulfur co-doped chiral carbon quantum dots with independent photoluminescence and chirality

Chiral CQDs exhibit strong PL and outstanding enantioselective electrochemical recognition ability. Moreover, chiral CQDs possess independent PL and chiral properties.