Nitrogen-doped carbon quantum dots: Facile synthesis and application as a “turn-off” fluorescent probe for detection of Hg2+ ions

Nitrogen-Doped Carbon Dots from Averrhoa carambola Fruit Extract as a Fluorescent Probe for Methyl Orange

In this study, a simple and green hydrothermal treatment was performed to prepare nitrogen-doped carbon dots (NCDs) from Averrhoa carambola (AC) fruit extract as a carbon precursor and L-arginine (Arg) as a nitrogen dopant. The AC-NCDs were characterized by UV light, fluorescence spectroscopy, transmission electron microscopy, FTIR spectroscopy, Raman spectroscopy, UV-vis spectroscopy, and zeta potential analyzer. The AC-NCDs were spherical and the average diameter was estimated to be 6.67 nm. The AC-NCDs exhibited the maximum emission intensity at 446 nm with 360 nm excitation wavelength. The fluorescence quenching behavior of AC-NCDs after interacting with methyl orange (MO) dye was studied. The interaction of AC-NCDs and MO was achieved within 3 min and the fluorescence quenching was maintained to a fixed value even after 30 min. The linearity was obtained in the range of 1 to 25 μM MO with a 0.30 μM detection limit. Furthermore, the pH values affected the quenching behavior of the AC-NCDs/MO system where the interaction mechanisms were driven by the electrostatic interaction, π-π interaction, inner filter effect, and energy transfer. The pH 5 maintained higher quenching efficiency while other pH values slightly decreased the quenching efficiency. Incoming applications, the AC-NCDs can be used in various important fields, especially for environmental protection.

Stepwise preparation of Ti-doped functionalized carbon nitride nanoparticles and hybrid TiO2/graphitic-C3N4 for detection of free residual chlorine and visible-light photocatalysis

Ti-doped functionalized carbon nitride nanoparticles and hybrid TiO2/graphitic-C3N4 were prepared stepwise and applied to the detection of free residual chlorine and visible-light photocatalysis. The photocatalytic degradation rate of methylene blue by the latter could reach 24.5 times that without a catalyst.

Chemical Sensing Platforms Based on Organic Thin-Film Transistors Functionalized with Artificial Receptors

Organic thin-film transistors (OTFTs) have attracted intense attention as promising electronic devices owing to their various applications such as rollable active-matrix displays, flexible nonvolatile memories, and radiofrequency identification (RFID) tags. To further broaden the scope of the application of OTFTs, we focus on the host-guest chemistry combined with the electronic devices. Extended-gate types of OTFTs functionalized with artificial receptors were fabricated to achieve chemical sensing of targets in complete aqueous media. Organic and inorganic ions (cations and anions), neutral molecules, and proteins, which are regarded as target analytes in the field of host-guest chemistry, were electrically detected by artificial receptors. Molecular recognition phenomena on the extended-gate electrode were evaluated by several analytical methods such as photoemission yield spectroscopy in the air, contact angle goniometry, and X-ray photoelectron spectroscopy. Interestingly, the electrical responses of the OTFTs were highly sensitive to the chemical structures of the guests. Thus, the OTFTs will facilitate the selective sensing of target analytes and the understanding of chemical conversions in biological and environmental systems. Furthermore, such cross-reactive responses observed in our studies will provide some important insights into next-generation sensing systems such as OTFT arrays. We strongly believe that our approach will enable the development of new intriguing sensor platforms in the field of host-guest chemistry, analytical chemistry, and organic electronics.

Facile Synthesis of Nitrogen-Doped Carbon Quantum Dots with Chitosan for Fluorescent Detection of Fe3

A facile, economical, and one-step hydrothermal method was used to prepare highly luminescent nitrogen-doped carbon quantum dots (N-CQDs) with chitosan as both carbon and nitrogen sources. The as-prepared N-CQDs have an average size of 2 nm and exhibit excitation wavelength-dependent fluorescence with a maximum excitation and emission at 330 and 410 nm, respectively. Furthermore, due to the effective quenching effect of Fe³⁺ ions, the prepared N-CQDs can be used as a fluorescent sensor for Fe³⁺ ion-sensitive detection with a detection limit of 0.15 μM. The selectivity experiments revealed that the fluorescent sensor is specific to Fe³⁺ even with interference by high concentrations of other metal ions. Most importantly, the N-CQD-based Fe³⁺ ion sensor can be successfully applied to the determination of Fe³⁺ in real water samples. With excellent sensitivity and selectivity, such stable and cheap carbon materials are potentially suitable for the monitoring of Fe³⁺ in environmental application.

Fluorescent carbon dots driven from ayurvedic medicinal plants for cancer cell imaging and phototherapy

Ayurveda based nanomaterials are recently conceptualized phenomena for biomedical applications especially for imaging and treatment of in vitro cancer cell. Wide range florescent (blue to red emission) quantum dots are versatile materials for imaging and sensing applications. Various procedures and precursors of fluorescent carbon quantum dots (CQDs) are well established and documented in the literature. However, expensive precursors and production, and time consuming process limit their economical design that need to be addressed. Herein, we report a cost effective simple route for fluorescent CQDs by using affordable ayurvedic plant's precursors such as Azadirachta Indica, OcimumTenuiflorum and Tridax Procumbens. Obtained quantum dots from ayurvedic plant leaves namely CQDs-1 (AzadirachtaIndica), CQDs-2 (OcimumTenuiflorum) and CQDs-3 (TridaxProcumbens) showed homogeneous size distribution (∼6–12 nm) and green fluorescent nature, average photo-stability, biocompatibility (more than 85 %), cancer cell imaging and promising phototherapy for cancer and bacterial cell lines.

Nitrogen Doped Carbon Quantum Dots Modified by Lens culinaris β-Galactosidase as a Fluorescent Probe for Detection of Lactose

Nitrogen doped carbon quantum dots (NCQDs) were synthesized via hydrothermal route. The NCQDs are thermally and optically stable with high flouresence yield. For the synthesis of NCQDs, citric acid and urea was taken as carbon and nitrogen sources, respectively. The Transmission Electron Microscopy (TEM) of these quantum dots revealed nearly spherical shape and average size of 1.5 nm, which was calculated using Image J software. The quantum dots were also well-characterized using spectroscopic techniques such as FTIR, UV-Visible absorption and fluorescence. These synthesized and characterized dots were utilized for selective detection of lactose in Milli Q water. The bioprobe provide a wide linear range varying from (10.00-77.41) μM with limit of detection 11.36 μM and sensitivity equal to (0.0065 ± 0.0002) μM^-1. Graphical Abstract.

A novel enhanced fluorescence method based on multifunctional carbon dots for specific detection of Hg2+ in complex samples

Carbon quantum dots (CQDs), especially originated from biomass, have emerged as a rising star for the construction of metal ion sensor because they can serve as sensitive, selective and biocompatible probes. The present work describes a novel kind of ascorbic acid (AA)-enhanced CQDs which are synthesized with a kind of famous green teas, Maojian, serving as carbon source. Compared with the CQDs only based on Maojian teas, citric acid (CA)-enhanced and ascorbic acid (AA)-enhanced CQDs had the enhanced fluorescence intensity, and different response characteristics. In addition, the (AA)-enhanced CQDs showed more sensitive and specific fluorescence response to Hg²⁺ than simple ones, with a detection limit of 6.32 × 10^-9 nmol·L^-1. A linear response range from 2.00 × 10^-7 mol·L^-1 to 6.00 × 10^-5 mol·L^-1 was also achieved. The (AA)-enhanced CQDs also demonstrate good stability. They could effectively sense the Hg²⁺ in complex samples including waste water, tea and rice. Therefore, these versatile (AA)-enhanced CQDs fluorescence method hold a promising potential in other promising applications such as pharmaceutical quality, environmental quality, and food safety monitoring.

Deep eutectic solvents-derived carbon dots for detection of mercury (II), photocatalytic antifungal activity and fluorescent labeling for C. albicans

A novel nitrogen and chloride co-doped carbon dots (N/Cl-CDs) based choline chloride-urea deep eutectic solvents (DESs) were synthesized by one-step hydrothermal method with high quantum yield of 37% and excellent photoluminescent properties. This N/Cl-CDs fluorescent probe had been successfully applied to sensitively and selectively detect the concentration of Hg²⁺ with a linear range of 0.2-40 μM and a detection limit of 0.05 μM. Moreover, the N/Cl-CDs displayed a strong photocatalytic antifungal activity against C. albicans and their photoinduced antifungal functions were evaluated under conditions of varying other experimental parameters. The antifungal efficiency of N/Cl-CDs (7 mg/mL) against C. albicans is upon 100% when extending the visible light irradiation time to 80 min. In addition, the excellent luminescence properties of N/Cl-CDs can also label C. albicans and displayed multicolour fluorescence imaging at different excitation wavelengths. Based on their functions of fluorescence probe, antifungal and fluorescence imaging, N/Cl-CDs would provide potentials for a wide range of applications in the detection and microbe in the near future.

Visual detection of Hg2+ by manipulation of pyocyanin biosynthesis through the Hg2+-dependent transcriptional activator MerR in microbial cells

Mercury pollution has always been a huge threat to human health due to its significant toxicity. Thus, it's the continuing goal to obtain new mercury detection techniques that are cost-effective, operational stable, performance efficient, and applicable to the environmental and biological milieus. In this research, the soluble pigment pyocyanin with anti-bacterial and anti-fungal activities, the biosynthesis pathway of which was engineered under the regulation of Hg²⁺-dependent transcriptional activator MerR, was firstly used as the visual detection signal in the whole-cell biosensor. The engineered biosensor displayed optical sensing window and a good linearity for Hg²⁺ in the range of 25-1000 nM, and the detection limit could reach as low as 10 nM. It permitted on-site detection of bioavailable Hg²⁺ with extraordinary selectivity and could resist the interferences of extra metal ions. What's more, the developed biosensor performed function well in a wide pH range (pH 4-10) as well as the environmental water. By fully imitating and utilizing the biosystems from nature, the engineered colorimetric biosensor has great economic and performance advantages over most chemosensors as well as whole-cell biosensors in the practical application of detecting Hg²⁺ in the contaminated aquatic systems.

Highly Fluorescent Green Carbon Dots as a Fluorescent Probe for Detecting Mineral Water pH

In this report, high-brightness green carbon dots were successfully prepared using 3,5-diaminobenzoic acid as the sole precursor and synthesized in one step using a solvothermal strategy. Under the excitation of 365 nm ultraviolet light, the quantum yield of carbon dots is as high as 53.8%. Experiments revealed that the carbon dots are highly carbonized and the surface is rich in amino and carboxyl groups. The synthesized carbon dots have good water solubility, and are resistant to ions and temperature. The fluorescence intensity of CDs is sensitive to pH changes and is linearly correlated with the pH in the near-neutral range (pH = 6.0 to 9.0). Our experiments showed that carbon dots were sensitive and accurate fluorescent probes for measuring the pH value of drinking water, which could provide an effective method for measuring the pH value of water in the future.

Self-Assembly of Emissive Nanocellulose/Quantum Dot Nanostructures for Chiral Fluorescent Materials

Chiral fluorescent materials with fluorescent nanoparticles assembled into a chiral structure represent a grand challenge. Here, we report self-assembled emissive needle-like nanostructures through decorating cellulose nanocrystals (CNCs) with carbon quantum dots (CQDs). This assembly is facilitated by the heterogeneous amphiphilic interactions between natural and synthetic components. These emissive nanostructures can self-organize into chiral nematic solid-state materials with enhanced mechanical performance. The chiral CQD/CNC films demonstrate an intense iridescent appearance superimposed with enhanced luminescence that is significantly higher than that for CQD films and other reported CQD/CNC films. A characteristic fluorescent fingerprint signature is observed in the CQD/CNC film, proving the well-defined chiral organization of fluorescent nanostructures. The chiral organization of CQDs enables the solid CQD/CNC film to form a right-hand chiral fluorescence with an asymmetric factor of -0.2. Additionally, we developed chemical 2D printing and soft lithography patterning techniques to fabricate the freestanding chiral fluorescent patterns that combines mechanical intergrity and chiral nematic structure with light diffraction and emission.

A ratiometric fluorescence and light scattering sensing platform based on Cu-doped carbon dots for tryptophan and Fe(III)

A new Cu-doped carbon dots (Cu-CDs) were synthesized rapidly and simply via one-step thermolysis of Na₂[Cu(EDTA)] and ascorbic acid (AA) at 250°C for 2h with a high quantum yield of 9.8%. The Cu-CDs exhibits two signals of fluorescence at 396nm and second-order scattering (SOS) at 617nm under a single excitation wavelength of 308nm, and can be obviously enhanced by tryptophan (Trp) or Fe(III) leading to the ratiometric fluorescence and SOS response with a good linear wider range of 0.5-250μM and 0.1-50μM, respectively. This sensing system exhibits good selectivity and sensitivity toward Trp and Fe(III) over other analytes with a low detection limit of 275nM and 46nM, respectively. Furthermore, the proposed sensing system displays a prospective application for quantitative assay of Trp and Fe(III) in practical sample.

Review of Carbon and Graphene Quantum Dots for Sensing

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electrochemiluminescent properties are what gives them potential in sensing. In this Review, we summarize the basic knowledge on CQDs and GQDs before focusing on their application to sensing thus far followed by a discussion of future directions for research into CQDs- and GQD-based nanomaterials in sensing. With regard to the latter, the authors suggest that with the potential of these nanomaterials in sensing more research is needed on understanding their optical properties and why the synthetic methods influence their properties so much, into methods of surface functionalization that provide greater selectivity in sensing and into new sensing concepts that utilize the virtues of these nanomaterials to give us new or better sensors that could not be achieved in other ways.

Carbon dots with molecular fluorescence and their application as a "turn-off" fluorescent probe for ferricyanide detection

Highly fluorescent carbon dots (CDs) exhibiting molecular fluorescence were synthesized and successfully used for sensing ferricyanide based on fluorescence quenching. We conducted dialysis to purify the CDs and found that the dialysate is also fluorescent. From the mass spectra and quantum yield analyses of the dialysate, it is demonstrated that molecular fluorophores were also synthesized during the synthesis of CDs. By the comparison of fluorescence spectra between CDs and dialysate, it is established that the fluorescence emission of CDs partly originates from fluorophores that are attached to CDs' surface. The fluorescence quenching caused by ferricyanide is proved to be the overlap of absorption spectra between ferricyanide and CDs. The changes of the absorbance and fluorescence spectra are combined to enhance the detection sensitivity, and the limit of detection is calculated to be 1.7 μM. A good linear response of fluorescence-absorbance combined sensing toward ferricyanide is achieved in the range of 5-100 µM. This method is highly selective to ferricyanide among other common cations and anions, and it is also successfully applied in detecting ferricyanide in real water samples.

Green Hydrothermal Synthesis of N-doped Carbon Dots from Biomass Highland Barley for the Detection of Hg2

Totally water-soluble N-doped Carbon dots (N-CDs) were synthesized by a green hydrothermal method from biomass using Highland barley as a carbon source and ethanediamine as nitrogen source. TEM and XRD showed the graphitic amorphous structure and narrow diameter distribution of these N-CDs. N-doping to the crystal lattice and carrying many hydrophilic groups on the surface of N-CDs were verified by XPS and FT-IR. The as-synthesized N-CDs emitted strong blue fluorescence at 480 nm and owned a relatively high quantum yield of 14.4%. The product also could sensitively and selectively detect Hg2+ ions in the range of 10-160 μM and the limit of detection was equal to 0.48 μM.

Reed-derived fluorescent carbon dots as highly selective probes for detecting Fe3+ and excellent cell-imaging agents

A kind of highly selective and sensitive fluorescent probe for detecting Fe3+, carbon dots (CDs), was prepared with renewable reed naturally containing C, N, O, and S elements as a green and eco-friendly carbon source by a simple hydrothermal process. The fluorescence of CDs without purification and surface modification can be quenched by Fe3+ in a wide concentration range of 0 to 362 μmol L-1 (concentration of Fe3+), with detection limits as low as 0.014 μmol L-1 in 0-50 μmol L-1. Characterizations, such as TEM, XPS, Raman and FTIR, confirmed that the static quenching mechanism involved the generation of non-luminescent complexes between Fe3+ and functional groups (carboxyl group, sulfur-oxyl group and hydroxyl group) on the surface of CDs and with the aggregation of CDs. More importantly, CDs had good biocompatibility and nontoxicity according to an MTT cell-viability assay, and cells labeled with CDs emitted blue, green and red color fluorescence. Thus, the static quenching mechanism was confirmed. So, this reed-derived natural CD solution can be utilized in detecting Fe3+, culture cells, and cell imaging.

Facile ultrasonic synthesized NH2-carbon quantum dots for ultrasensitive Co2+ ion detection and cell imaging

The amine decorated carbon quantum dots (NH₂-CQDs) were synthesized through ultrasonic method from graphite rods derived CQDs and ammonia hydroxide and utilized as the sensing probes for cobalt (II) ions and nucleic acids. The sensing technique was investigated to be the fluorescence quenching effect, which demonstrated linear relationship between cobalt (II) ions concentration and the emission intensity deviation ratio in the concentration range of 50 nM to 40 μM with the detection limit of 12 nM. In brief, this sensitive and selective detection method was confirmed to demonstrate high potential in cobalt (II) ions detection in real samples and nucleic acid sensing in biological cells.

Recyclable fluorescent chemodosimeters based on 8-hydroxyquinoline derivatives for highly sensitive and selective detection of mercury(II) in aqueous media and test strips

Four novel highly selective 8-hydroxyquinoline-based fluorescent chemodosimeters (1-4) were synthesized for the rapid analysis of Hg²⁺ in aqueous solution and on paper strips, which probably attributed to the excited state intramolecular proton transfer (ESIPT) process. Chemodosimeter 1 was evaluated as a Hg²⁺-ratiometric fluorescent sensor while others (2, 3 and 4) displayed fluorescence turn-on response for Hg²⁺ among the various survey metal ions. We demonstrated that chemodosimeters (1-4) could recognized Hg²⁺ ions based on a 1:1 stoichiometric binding event with fast detection time. More importantly, the detection limits for Hg²⁺ could reach at 10^-9 M level except chemodosimeter 1 (4.05 × 10^-8 M). In addition, it was found that chemodosimeters (1-4) were recycled efficiently because the Hg²⁺ induced emission spectra were reversed after adding NaBH₄. Finally, these four sensors were successfully applied for fabrication of simple device test strips for rapid and on-site detection of Hg²⁺ ions.

Nitrogen-Doped Graphene Oxide Dots-Based "Turn-OFF" H2O2, Au(III), and "Turn-OFF-ON" Hg(II) Sensors as Logic Gates and Molecular Keypad Locks

Fluorescent nitrogen-doped graphene oxide dots (NGODs) have been demonstrated as an on-off nanosensor for the detection of Hg²⁺, Au³⁺, and H₂O₂. As compared to l-cystine, where the luminescence signal recovery results from the detachment of Hg²⁺ from the NGODs, signal recovery through l-ascorbic acid (turn-off-on model) has been attributed to the reduction of Hg²⁺ to Hg⁰. The sustainable recovery of the photoluminescence signal is demonstrated using common citrus fruits containing vitamin C (l-AA), suggesting a promising practical usage of this sensing system. Additionally, the sensitivity of NGOD- and AA-originated signal recovery from the Hg(II)-NGODs mixture has been successfully tested in Hg²⁺ ion-spiked tap water from three different places. Mimic devices were executed and verified on the basis of characteristic spectral changes, and the possible utility of this system in electronic security and memory element devices has also been demonstrated. Considering an easy synthesis process and excellent performance of NGODs, this investigation opens up new opportunities for preparing high-quality fluorescent NGODs to meet the requirements of many applications.

Synthesis and characterization of novel bithiazolidine derivatives-capped CdTe/CdS quantum dots used as a novel Hg2+ fluorescence sensor

In the present study, (E)-2,2'-(4,4'-dioxo-2,2'-dithioxo-2H,2'H-[5,5'-bithiazolylidene]-3,3'(4H,4'H)-diyl) bis(3-mercaptopropanoic acid) (DTM) as a new derivative of thiazolidine was synthesis and characterized for the detrtmination of Hg²⁺ ions. Then, the CdTe@CdS QDs and DTM capped CdTe@CdS QDs were produced. The DTM-CdTe@CdS/QDs used as an effective fluorescence sensing material due to the selective interaction of DTM with Hg (II). The results indicated that the DTM-CdTe@CdS/QDs shows strong fluorescence emissions in the absence of mercury ions and efficiently quenched in presence of Hg², with the formation of a strong and stable complex between Hg²⁺ and DTM. Experimental results showed that under optimal conditions, Hg²⁺ could be detected with a detection limit of 0.08 nM in a linear range from 0.3 nM to 21 nM. The constructed aptasensor illustrated the high selectivity for mercury ions even in the presence of the other interfering metal ions when their concentration ratio was more than 300 times. The satisfactory results illustrated that the designed fluorescence sensor as a sensitive, reliable and easy to use approach could be applied for the facile and rapid determination of Hg²⁺ in tap water.

White light emitting lanthanide based carbon quantum dots as toxic Cr (VI) and pH sensor

Although, great attention is paid to synthesize fluorescent carbon quantum dots (CQDs) for versatile applications, the field remains still attractive to achieve white light using these nano materials. In the present work, CQDs are synthesized from citric acid and lanthanide ions viz. Europium (Eu) and Terbium (Tb) are doped in CQD moiety to explore superior optical response for multifunctional applications. By proper tuning of excitation wavelength, perfect white light with Commission Internationale de l'Elcairage (CIE) index (0.345, 0.344) is obtained using these Europium Terbium co-doped CQDs (Eu-Tb-CQD). The observed photoluminescence of white light emitting lanthanide based CQD is pH dependent and will be used as a visual pH sensor. These luminescent Eu and Tb co-doped CQDs are also very useful to detect toxic Cr (VI) with excellent selectivity and sensitivity as compared to pure CQDs. It shows high quenching efficiency (∼95%) in presence of only 160 µM Cr(VI). The selectivity and lower detection limit are also obtained as ∼80% and 0.175 µM respectively.

Recent Advancements in Doped/Co-Doped Carbon Quantum Dots for Multi-Potential Applications

Carbon quantum dots (CQDs)/carbon nanodots are a new class of fluorescent carbon nanomaterials having an approximate size in the range of 2–10 nm. The majority of the reported review articles have discussed about the development of the CQDs (via simple and cost-effective synthesis methods) for use in bio-imaging and chemical-/biological-sensing applications. However, there is a severe lack of consolidated studies on the recently developed CQDs (especially doped/co-doped) that are utilized in different areas of application. Hence, in this review, we have extensively discussed about the recent development in doped and co-doped CQDs (using elements/heteroatoms—e.g., boron (B), fluorine (F), nitrogen (N), sulphur (S), and phosphorous (P)), along with their synthesis method, reaction conditions, and/or quantum yield (QY), and their emerging multi-potential applications including electrical/electronics (such as light emitting diode (LED) and solar cells), fluorescent ink for anti-counterfeiting, optical sensors (for detection of metal ions, drugs, and pesticides/fungicides), gene delivery, and temperature probing.

Ratiometric fluorometric determination of mercury(II) by exploiting its quenching effect on glutathione-stabilized and tetraphenylporphyrin modified gold nanoclusters

A ratiometric fluorometric assay for mercury(II) ion is described. It is making use of glutathione-stabilized gold nanoclusters (GSH-AuNCs) modified with tetraphenylporphyrin tetrasulfonic acid (TPPS). The resultant GSH-AuNC/TPPS nanocomposite displays dual emission (at 572 and 664 nm) under a single excitation wavelength of 365 nm. Mercury(II) ion intensively quenches the yellow fluorescence of GSH-AuNCs (peaking at 572 nm) but has a negligible effect on the red fluorescence of TPPS (at 664 nm). The ratio of fluorescence intensities at 572 and 664 nm drops linearly with Hg(II) ion concentration in the 0.02-2.0 μmol·L^-1 range, and the detection limit is 7 nmol·L^-1 (3s_b/S). The relative standard deviation (RSD) of the assay is 2.0% at a 0.5 μmol·L^-1 concentration level (n = 11). The method was successfully applied to the determination of Hg(II) ion in spiked water samples, with recoveries within the range of 87.5-107.5%. Graphical abstract Ratiometric fluorescence detection of mercury(II).

Microwave-assisted synthesis of polyamine-functionalized carbon dots from xylan and their use for the detection of tannic acid

A facile and straightforward microwave-assisted method was used to prepare polyamine-functionalized carbon dots (CDs) from a precursor comprising renewable xylan and branched polyethyleneimine (BPEI). The as-prepared BPEI-CDs were monodispersed sphere particles with an average diameter of about 8.62 nm, and exhibited excellent fluorescent property and high stability, as well as excitation-independent emission behavior. Furthermore, it is attractive that the BPEI-CDs can be used as novel fluorescent probes for detecting tannic acid (TA) sensitively and selectively. At the optimum condition, the TA detection system was established in water solution and ethanol solution with a dynamic range from 0.1 to 5 μM, and their detection limit of 36.8 nM and 44.9 nM were also determined, respectively. Most importantly, the BPEI-CDs-based sensors can be successfully applied to detect TA in real lake water and white wine samples, suggesting the low-cost and excellent BPEI-CDs are potential suitable for TA detection in practical application.

Assembly of shell/core CDs@CaF2 nanocomposites to endow polymers with multifunctional properties

The shell/core structure of CDs@CaF₂ nanocomposites (CCNCs) were prepared by assembling fluorescent carbon dots (CDs) inside the inorganic CaF₂ substrates using co-precipitation interaction. CDs endow CaF₂ with properties of good UV-absorbing behavior and efficient blue light emission instead of rare-earth such as Eu that is expensive and susceptible to polluting the environment during the mining process. Due to the nanometer size and surface effect of nano CaF_2, and the approximate refractive index between CaF₂ and polyethylene (PE), CCNC/PE film exhibits better elongation at the break than pure PE film while maintaining high transparency and visible light transmittance. Simultaneously, the CCNC/PE film was experimentally demonstrated to have outstanding performance of anti-UV and blue light conversion, which shows that CCNCs can be a novel and promising multifunctional additive applied in polymers especially for greenhouse film.

Nitrogen- and Sulfur-Codoped Carbon Dots for Highly Selective and Sensitive Fluorescent Detection of Hg2+ Ions and Sulfide in Environmental Water Samples

Nitrogen- and sulfur-codoped carbon dots (N,S-CDs) with a fluorescence quantum yield of 16.1% and good photoluminescent properties were synthesized by a simple hydrothermal method. Cytotoxicity of the N,S-CDs was evaluated by the MTT assay, and human hepatoma HepG2 cells were chosen as the target. The cell viability was more than 85% after 24 h of incubation when its concentration was up to 300 μg/mL, suggesting low cytotoxicity and good biocompatibility of the N,S-CDs. The fluorescence spectra of the N,S-CDs are excitation-dependent in the excitation-wavelength range of 295-400 nm, and it emits bright blue fluorescence centered at 435 nm. Its selective fluorescence recognition for Hg²⁺ ions was found. When Hg²⁺ ions were added to the N,S-CDs solution, its bright blue fluorescence was obviously quenched and could be recovered by the addition of sulfide. Accordingly, a new strategy based on N,S-CDs-Hg²⁺ system as a highly selective and ultrasensitive "turn off-on" fluorescence sensing for the detection of sulfide was fabricated. The limits of detection (S/N = 3) are 83 nM for Hg²⁺ ions and 11 nM for sulfide. Almost no statistically significant interference for Hg²⁺-ion and sulfide detection was observed among possible coexisting substances in the water samples, including 17 common metal ions and 11 anions. This probe was successfully applied for the cellular imaging of Hg²⁺ ions in HepG2 cells by fluorescence microscopy.

Carbon quantum dots and their biomedical and therapeutic applications: a review

In recent years, nano carbon quantum dots (CQDs) have received increasing attention due to their properties such as small size, fluorescence emission, chemical stability, water solubility, easy synthesis, and the possibility of functionalization. CQDs are fluorescent 0D carbon nanostructures with sizes below 10 nm. The fluorescence in CQDs originates from two sources, the fluorescence emission from bandgap transitions of conjugated π-domains and fluorescence from surface defects. The CQDs can emit fluorescence in the near-infrared (NIR) spectral region which makes them appropriate for biomedical applications. The fluorescence in these structures can be tuned with respect to the excitation wavelength. The CQDs have found applications in different areas such as biomedicine, photocatalysis, photosensors, solar energy conversion, light emitting diodes (LEDs), etc. The biomedical applications of CQDs include bioimaging, drug delivery, gene delivery, and cancer therapy. The fluorescent CQDs have low toxicity and other exceptional physicochemical properties in comparison to heavy metals semiconductor quantum dots (QDs) which make them superior candidates for biomedical applications. In this review, the synthesis routes and optical properties of the CQDs are clarified and recent advances in CQDs biomedical applications in bioimaging (in vivo and in vitro), drug delivery, cancer therapy, their potential to pass blood-brain barrier (BBB), and gene delivery are discussed.

Fluorescent Sensors for the Detection of Heavy Metal Ions in Aqueous Media

Due to the risks that water contamination implies for human health and environmental protection, monitoring the quality of water is a major concern of the present era. Therefore, in recent years several efforts have been dedicated to the development of fast, sensitive, and selective sensors for the detection of heavy metal ions. In particular, fluorescent sensors have gained in popularity due to their interesting features, such as high specificity, sensitivity, and reversibility. Thus, this review is devoted to the recent advances in fluorescent sensors for the monitoring of these contaminants, and special focus is placed on those devices based on fluorescent aptasensors, quantum dots, and organic dyes.

Efficient and visual monitoring of cerium (III) ions by green-fluorescent carbon dots and paper-based sensing

An efficient strategy for the synthesis of excitation-independent carbon nanodots has been provided. This functional nano-scale particle has been assembled by using citric acid and urea as raw materials through a microwave irradiation process in the absence of further surface modifications. The achieved nanomaterial demonstrates intensive green emissions and can be dispersible in aqueous environment. In particular, cerium (III) ion is able to quench its fluorescence emission intensity selectively and an "on-off" change has been observed. A good linearity between the concentration range of 10^-6-10^-4 M has been realized and the detection limit is determined to be 0.7 μM. More importantly, we report a color-evolution based paper sensor for effective monitoring of Ce³⁺ by incorporating fluorescent nanoprobe onto the cellulose paper substrate.

Facile one-pot synthesis of highly fluorescent nitrogen-doped carbon dots by mild hydrothermal method and their applications in detection of Cr(VI) ions

Nitrogen-doped carbon dots (N-CDs) with well-distribution size and strong blue emission were successfully synthesized via a simple mild hydrothermal strategy using citric acid and ethylenediamine as co-precursors. The highly fluorescent N-CDs exhibit high fluorescence quantum yield (QY, 58.6%), excitation-independent emission behavior, and good photostability. The experimental results showed that the N-CDs can be served as a fluorescent sensing platform for detection of Cr(VI) ions due to the effective fluorescence quenching effect of Cr(VI) ions. The quenching mechanism probably arises from the inner filter effect (IFE) and the electron transfer due to the strong interactions between functional groups (COOH, OH and NH₂ groups) of the N-CDs and Cr(VI) ions. It is also found that the N-CDs showed high sensitivity toward Cr(VI) ions with a detection limit of 0.26 μM. Moreover, the obtained N-CD can be employed as chemsensor to detect Cr (VI) in real river water samples, which have potential applications in the environmental water.

Red-emissive nitrogen doped carbon quantum dots for highly selective and sensitive fluorescence detection of the alachlor herbicide in soil samples

We report the synthesis of red-emissive nitrogen-doped carbon quantum dots and their applications in the highly selective and sensitive detection of the alachlor herbicide in soil samples.

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.