Carbon quantum dots doped with nitrogen and phosphorus were prepared from adenosine 5′-monophosphate (AMP) in a single simple synthesis step. The nitrogen and phosphorus doped C-dots (N,P-C-dots) were characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectroscopy and X-ray powder diffraction. These carbon dots display blue fluorescence, with excitation/emission maxima at 360/430 nm, a quantum yield of 26.5% and an average decay time of 4.3 ns. Fluorescence is strongest at neutral pH values but quenched at very high and very low pH values. It is also quenched by ferric ions which suggests the use of the N,P-C-dots as fluorescent probes for Fe(III). A hemolysis test inferred favorable blood compatibility. The fluorescence of the doped C-dots is excitation wavelength dependent and also is susceptible to 2-photon excitation. The nanoparticles were applied in the fluorescent multicolor bioimaging of A549 (adenocarcinomic alveolar basal epithelial) cells under different excitation wavelengths, typically at 405, 488 and 543 nm. Emission colors ranging from blue to green and red can be adjusted in this way.
Graphical abstract Nitrogen and phosphorus doped carbon dots were synthesized and showed excitation wavelength-dependent behavior. They were applied to multi-color fluorescence imaging of adenocarcinomic alveolar basal epithelial cells.
This article reports on the synthesis of water dispersible carbon quantum dots (CDs) by a one-step hydrothermal method using polyamidoamine (PAMAM) and (3-aminopropyl)triethoxysilane (APTES) as a platform and passivant. The resulting CDs are highly uniform and finely dispersed. The synergistic effect between PAMAM and APTES on the surface of the CDs results in a fluorescence that is much brighter than that of CDs modified with either APTES or PAMAM only. The fluorescence of the co-modified CDs is quenched by Hg(II) ions at fairly low concentrations. Under the optimum conditions, the intensity of quenched fluorescence drops with Hg(II) concentration in the range from 0.2 nM to 10 μM, and the detection limit is 87 fM. The effect of potentially interfering cations on the fluorescence revealed a high selectivity for Hg2+. The fluorescent probe was applied to the determination of Hg(II) in (spiked) waters and milk and gave recoveries between 95.6 and 107 %, with relative standard deviation between 4.4 and 6.0 %.
Graphical abstract Strongly fluorescent carbon quantum dots (CDs) modified with polyamidoamine (PAMAM) and 3-aminopropyltriethoxysilane (APTES) were synthesized by one-step hydrothermal strategy. The resulting co-modified CD s were used as fluorescent probe for sensitive and selective detection of Hg2+.
An electrochemical approach is introduced for synthesis of carbon dots (CDs) by exfoliating graphite rods at a voltage of 15 V in an electrolyte consisting of a mixture of water and two ionic liquids. It is found that the size of the CDs can be tuned by varying the fraction of water in the mixed electrolyte; CDs in sizes of 4.9, 4.1 and 3.1 nm are obtained if the electrolyte contains water in fractions of 24, 38 and 56 %, respectively. The CDs have a quantum yield of almost 10 % and display the typical excitation wavelength-dependent maxima of photoluminescence, strongest at excitation/emission wavelengths of 360/440 nm. Fourier transform infrared and X-ray photoelectron spectroscopy show the CDs to have oxygen functional groups on their surface which strongly improve solubility. The CDs were applied to image cells of the electricity-producing bacteria Shewanellaoneidensis MR-1.
Graphical Abstract An electrochemical approach is introduced to synthesize carbon dots by exfoliating graphite rods in mixed electrolyte of water and ionic liquids. The increasing size of carbon dots was realized by reducing the volume of water in the mixed electrolyte. The carbon dots were used to fluorescently image the electricity-producing bacterium Shewanellaoneidensis MR-1.
We report on a sensitive and selective fluorescent assay utilizing native carbon dots (CDs) as signal transducers. The optical probes 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) or 3,3′-diaminobenzidine (DAB) were employed as substrates of horseradish peroxidase (HRP). It was found that the corresponding oxidation products (ox-ABTS or ox-DAB) quench the fluorescence of CDs, mainly via photoinduced electron transfer in case of ox-ABTS, and via aggregation and inner filter effect in case of ox-DAB. By coupling with bienzyme (glucose oxidase and HRP)-mediated biocatalytic reactions, the method was applied to the determination of hydrogen peroxide and glucose. In case of ABTS as the substrate of HRP, a wide linear range (0.05 to 100 μM) and a very low detection limit (10 nM) for glucose were attained. The method was applied to the determination of glucose in human serum and the results were found to agree well with data provided by a local hospital.
Graphical abstract In this work, a sensitive and selective fluorescent assay was developed for probing the enzymatic substrates of hydrogen peroxide and glucose which utilized native carbon dots (CDs) as signal transducer.
We describe a highly sensitive glucose probe based on carbon dots modified with MnO2. A strong reduction of the green fluorescence of the carbon dots (CDs) happened due to the surface energy transfer (SET) from CDs to the deposited MnO2. In the presence of H2O2 (formed via enzymatic oxidation of glucose), fluorescence is restored because the MnO2 nanosheets are reduced to form colorless Mn(II) ions. These findings were used to design a fluorometric glucose assay that has a detection limit as low as 44 nM (at an S/N ratio of 3).
Graphical Abstract A strong reduction of the green fluorescence of the carbon dots (CDs) occurs due to surface energy transfer (SET) from CDs to the deposited MnO2. In the presence of H2O2 (formed by enzymatic action of glucose oxidase) the MnO2 nanosheets are reduced to form colorless Mn(II) ions, and glucose can be quantified by the fluorescence restored.
Cobalt oxyhydroxide (CoOOH) nanosheets are efficient fluorescence quenchers due to their specific optical properties and high surface area. The combination of CoOOH nanosheets and carbon dots (CDs) has not been used in any aptasensor based on fluorescence quenching so far. An aptamer based fluorometric assay is introduced that is making use of fluorescent CDs conjugated to the aptamer against methamphetamine (MTA), and of CoOOH nanosheets which reduce the fluorescence of the CDs as a quencher. The results revealed that the conjugated CDs with aptamers were able to enclose the CoOOH nanosheets. Consequently, fluorescence is quenched. If the aptamer on the CD binds MTA, the CDs are detached from CoOOH nanosheets. As a result, fluorescence is restored proportionally to zhe MTA concentration. The fluorometric limit of detection is 1 nM with a dynamic range from 5 to 156 nM. The method was validated by comparing the results obtained by the new method to those obtained by ion mobility spectroscopy. Theoretical studies showed that the distance between CoOOH nanosheet and C-Ds is approximately 7.6 Å which can illustrate the possibility of FRET phenomenon. The interactions of MTA and the aptamer were investigated using molecular dynamic simulation (MDS).
Graphical abstract Carbon dots (C-Ds) were prepared from grape leaves, conjugated to aptamer, and adsorbed on CoOOH nanosheets. So, the fluorescence of C-Ds is quenched. On addition of MTA, fluorescence is restored.
Carbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Förster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging.
Graphical abstract Schematic representation of the quenching mechanisms of carbon dots (CDs) which include static quenching, dynamic quenching, Förster resonance energy transfer (FRET), photoinduced electron transfer(PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). All these effects can be used to detect and image analytes.
Carbon dots (CDs) modified with ethylene diamine (EDA) and the amino acids (AAs) Cys, His, Lys or Arg were synthesized, and their structures were confirmed by high resolution transmission electron microscopy, Raman spectrometry and X-ray photoelectron spectrometry. It is found that derivatization of the CDs with various AAs systemically modulates their electronic properties, and this results in a tunable selectivity in detection of metal cations via fluorescence quenching. The probes can be performed in aqueous solutions around pH 7. CDs can be excited under 345 nm excitation at room temperature and exhibit fluorescent peak at 450 nm. The decreasing fluorescence intensity is directly proportional to the concentration of metal cations. The limits of detection is 8.8 μg L?1 for Pb(II), 20 μg L?1 for Hg(II), 3.7 μg L?1 for Cu(II), 5.3 μg L?1 for Zn(II), 16 μg L?1 for Fe(III), and 7.2 μg L?1 for Cr(III), respectively. The different fluorescence response of the AA-modified CDs can be converted to logic gates and applied to photoelectronic nanoprobes by using microprocessors. In our perception, this assay has a large potential in terms of high-throughput screening for trace amounts of metal ions.
Graphical abstract Amino acid derivatized carbon dots with tunable selectivity were synthesized by a one pot method for fluorescent sensing of metal cations. The sensing events can be directly converted into different logic gates.
The authors describe a method for the preparation of orange-red emissive carbon dots (CDs) with excitation/emission peaks at 520/582 nm. The CDs were hydrothermally prepared by a one-pot strategy from trimesic acid and 4-aminoacetanilide. The fluorescence of the CDs is strongly quenched by hydrogen peroxide. The oxidation of glucose by glucose oxidase (GOx) produces H2O2 that quenches the fluorescence via static quenching. Based on this phenomenon, a fluorometric method was established for the determination of glucose. Under the optimum conditions, response is linear in the 0.5 to 100 μM glucose concentration range, with a 0.33 μM limit of detection. The method is selective for glucose over its analogues and was successfully applied to the determination of glucose in diluted human serum and in urine from diabetics and healthy individuals. Recoveries from spiked samples range from 98.7 to 102.5%.
The authors describe a sensitive method for determination of glutathione (GSH) that is based on a thiol-triggered inner filter effect on the fluorescence of N-doped carbon dots (N-doped CDs). N-doped CDs with a quantum yield as high as 31% were prepared by a one-pot procedure, and 5,5′-dithiobis-(2-nitrobenzoic acid) was employed as a reagent for GSH recognition. The reaction product (5-thio-2-nitrobenzoic acid; TNB) acts as an absorber of the 410-nm light used to photo-excite the N-doped CDs. Hence, the fluorescence of N-doped CDs (peaking at 510 nm) is reduced with increasing concentrations of GSH. As little as 30 nM of GSH can be detected by this method. The approach was successfully applied to (a) food analysis, (b) an investigation of an oxidative stress model, and (c) to live cells imaging. The method does not require the surface of N-doped CDs to be chemically modified, and a linkage between receptor and fluorophore is not needed. In our perception, the method may become a viable tool for the detection and imaging of thiols.
Graphical abstract Fluorescence sensing strategy for glutathione detection based on a thiol-triggered inner filter effect via new N-doped carbon dots and application to food analysis, oxidative stress study and cell imaging.
A colorimetric and fluorescent pH probe was designed by doping carbon dots (C-dots) with Eu(III), Tb(III) and 2,6-pyridinedicarboxylic acid (DPA). The resulting nanoparticles were applied as fluorescent indicators for pH values (best detected at excitation/emission wavelengths of 272/545, 614 nm). The pH induced optical effects are due to pH induced variations in energy transfer. The fluorescence of the probe shows a continuous color variation, and a linear change with pH values in the range from 3.0 to 10.0 can be established by using a Commission Internationale de L’Eclairage (CIE) chromaticity diagram. This new kind of pH nanoprobe is more accurate than previously reported pH indicator probes because the pH value can be calculated by using chromaticity coordinates that only depend on the chromaticity. The pH nanoprobe was applied to visualize pH values in human breast adenocarcinoma cells (MCF-7).
Graphical abstract Carbon dots modified with Eu(III) and Tb(III) complexes of 2,6-pyridinedicarboxylic acid (DPA) were prepared. The doped carbon dots were used as a pH-sensitive nanosensor. The fluorescence chromaticity of the nanoparticles changes with the variation of pH value.
This work describes the preparation of carbon dots doped with terbium(III) (Tb-CDs) via a hydrothermal method, starting from terbium ion and ethylenediamine. The size, composition and spectral properties of the Tb-CDs were characterized by transmission electron microscopy, infrared spectra, and fluorescence spectra. The results show that doping of the CDs with Tb(III) reduces the particle size and results in more uniform particles, while fluorescence (at excitation/emission peaks of 380/475 nm) is strongly enhanced. The interaction between Tb-CDs and ct-DNA results in fluorescence quenching of Tb-CDs. The findings were exploited to design a quenchometric method for the determination of ct-DNA. The signal drops linearly in the 80 ng·mL?1 to 50 μg·mL?1 ct-DNA concentration range, and the detection limit is 53 ng·mL?1. The method was applied to the determination of ct-DNA in spiked samples and gave satisfactory results. The possible fluorescence quenching mechanism (which is mainly static) was investigated using the Stern–Volmer equation and thermodynamic equations.
Graphical abstract A kind of carbon dots doped with terbium(III) (Tb-CDs) were prepared via a hydrothermal method, using terbium ion and ethylenediamine as precursor. Doping with Tb(III) reduced the particle size of CDs and results in uniform particle size and stronger fluorescence. The interaction between the Tb-CDs and dsDNA results in quenching of the fluorescence of Tb-CDs and can be applied to determination of dsDNA.
Near-infrared photoluminescence is intrinsic only to a minority of carbonaceous nanomaterials. Longwave fluorescence is, however, well suited for bio-sensing and bio-imaging owing to the low autofluorescence and low absorbance by biomatter. The authors describe here sulfur doped carbon quantum dots (S-CQDs) and their derivatives (referred to as 3D carbon nanoflowers; S-CNFs). Their average diameters are 2 and 28.5 nm, respectively. They display two emission peaks, one being purple and peaking at 407 nm, the other in the near-infrared and peaking at 780 nm. Quantum yields are 4% for S-CQDs and 6.4% for S-CNFs. The nanoparticles are shown to be viable fluorescent probes for hydrogen peroxide which acts as a quencher. The 3D structure of the S-CNFs and near-infrared detection result in a better linear relationship and lower detection limits. The detection limits for H2O2 are 1.1 μM for S-CQDs, and 0.6 μM for S-CNFs. The results presented here contribute to an improved understanding on how the nanostructure and selection of wavelengths affect the sensitivity and detection limits of such probes.
Graphical abstract “Button-up” - synthesized sulfur-doped carbon quantum dots and carbon nanoflowers display two emission peaks, one being purple, the other in the near-infrared. The nanoparticles are shown to be viable fluorescent probes for hydrogen peroxide which acts as a quencher.
The authors describe a fluorometric aptamer based assay for adenosine triphosphate (ATP). It is based on the use of carbon dots (CDs) and graphene oxide (GO). The resultant CD-aptamer is adsorbed on the surface of GO via π-stacking and hydrophobic interaction, and the fluorescence of CD-aptamer is quenched via fluorescence resonance energy transfer (FRET) between CDs and GO. If ATP is present, it will bind to the aptamer and the CD-aptamer will be desorbed from GO. This will suppress FRET and the fluorescence of the CDs is restored. Under the optimal conditions and at typical excitation/emission wavelengths of 358/455 nm, the assay has a 80 pM detection limit and a linear range that extends from 0.10 to 5.0 nM concentrations of ATP. The method was successfully applied to the determination of ATP in yogurt samples. This method can also be conceivably applied to the detection of other analytes for which appropriate aptamers are available.
Graphical abstract Schematic of a novel fluorometric ATP assay based on the fluorescence resonance energy transfer (FRET) between aptamer modified carbon dots (CD-aptamer) and graphene oxide (GO). CD-aptamer was used as the energy donor and molecular recognition probe, and GO acted as energy acceptor. This assay exhibits high sensitivity and selectivity with a detection limit as low as 80 pM.
The authors describe new bifunctional mesoporous silica nanoparticles (NPs) for specific targeting of tumor cells and for intracellular delivery of the cancer drug doxorubicin (DOX). Mesoporous silica nanoparticles (MSNPs) were coated with blue fluorescent N-graphene quantum dots, loaded with the drug DOX, and finally coated with hyaluronic acid (HA). Cellular uptake of the NPs with an architecture of the type HA-DOX-GQD@MSNPs enabled imaging of human cervical carcinoma (HeLa) cells via fluorescence microscopy. The cytotoxicity of the nanoparticles on HeLa cells was also assessed. The results suggest that the NPs are higher cytotoxicity effect and exert in living cell imaging ability. Compared to the majority of other drug nanocarrier systems, the one described here enables simultaneous DOX release and fluorescent monitoring.
Graphical abstract Schematic of the bifunctional mesoporous silica nanoparticles were obtained via the Stöber method, along with the doxorubicin loaded and the hyaluronic acid capped. The sensor shows good specificity and significant cytotoxicity effect on Hela cells. (TEOS: tetraethyl orthosilicate; GQDs: graphene quantum dots; DOX: doxorubicin; HA: Hyaluronic acid).
We describe the preparation of carbon quantum dots (C-dots) by a one-step hydrothermal method starting from o-aminophenol as the precursor. The C-dots exhibit bright both blue fluorescence (with excitation/emission peaks at 300/410 nm and with quantum yield of 0.40) and green fluorescence (420/500 nm; QY 0.28) without any other element doping. The unique emission properties are attributed to a synergistic effect of amino and hydroxy groups on the surface of the C-dots. The C-dots are shown to be viable fluorescent probes for heparin. The positively charged surface amino groups are assumed to interact with sulfate and carboxy groups in heparin via electrostatic interactions and hydrogen bonding. This causes the blue fluorescence of C-dots to be turned off (quenched). Fluorescence is strongest at a pH value of 6. The fluorometric calibration plot is linear in the 10 to 100 nM concentration range, with an 8.2 nM detection limit (at a signal-to-noise ratio of 3).
Graphical abstract Carbon quantum dots with dual fluorescence emission bands were synthesized and are shown to be a viable fluorescent probe for heparin.
The authors describe a method for the determination of norfloxacin (NOR) or ciprofloxacin (CIP). It is making use of a combination of fluorescence enhancement and magnetic solid-phase extraction (MSPE). Sulfur-doped carbon dots (S-CDs) are used as a fluorescent probe. They were prepared by a one-pot method using poly(4-styrenesulfonic acid-co-maleic acid) (PSMA) as a source for carbon and sulfur. NOR or CIP act as sensitizers of fluorescence (with excitation/emission maxima at 324/412 nm), probably due to strong hydrogen bond interaction and charge transfer with the S-CDs. The S-CDs were characterized by using TEM, XRD, XPS, FT-IR, UV-Vis and fluorescence spectroscopies. Response is linear in the 0.02–1.25 μM NOR concentration range, and the detection limit is 4.6 nM. The respective data for CIP are 0.02–1.0 μM and 6.7 nM. The average recoveries of NOR and CIP residues from spiked bovine raw milk are 96.2%~105.2% and 92.3%~102.5%.
Graphical abstract Sulfur doped carbon dots (S-CDs) were synthesized by a hydrothermal method using poly(4-styrenesulfonic acid-co-maleic acid) (PSMA). Norfloxacin (NOR) or ciprofloxacin (CIP) was extracted by magnetic nanoparticles (MNPs), they were detected by carbon dots fluorescence enhancement.
A method is described for the fluorometric determination of hypochlorite. It is making use of molybdenum disulfide quantum dots (MoS2 QDs) as a fluorescent probe. The QDs are prepared by hydrothermal reaction of sodium molybdate with glutathione. They possess diameters typically ranging from 1.4 to 3.8 nm, excellent stability in water, and blue photoluminescence (with excitation/emission peaks located at 315/412 nm and a quantum yield of 3.7%). The fluorescence of the QDs is statically quenched by hypochlorite, and the Stern-Volmer plot is linear. Hypochlorite can be detected in the 5–500 μM concentration range with a 0.5 μM detection limit. The method has been successfully applied to the determination of hypochlorite in spiked samples of tap water, lake water, and commercial disinfectants.
Graphical abstract Schematic of a method for the fluorometric determination of hypochlorite using MoS2 quantum dots as a fluorescent probe. It has been applied to hypochlorite assay in spiked samples of tap water, lake water, and commercial disinfectants.
Europium(III)-doped carbon dots (Eu-CDs) were prepared from citric acid and europium nitrate via a one-pot pyrolytic method. The Eu-CDs emit intense blue fluorescence (with excitation/emission peaks at 365/465 nm), are water soluble and biocompatible. On addition of 2,6-dipicolinic acid (DPA; an anthrax biomarker), ligand-to-ion energy transfer occurs from DPA to Eu(III) which has a red emission peaking at 615 nm. This results in an increase of the intensity of the red fluorescence. DPA can be detected by the ratio of fluorescence intensities at 616 and 475 nm. The method has an analytical range that extends from 5 to 700 nmol·L?1, with a 5 nmol·L?1 detection limit. The Eu-CDs also were incorporated into a test paper for visual detection of DPA with a portable UV lamp and a smartphone. In this case, the detection limit is 1 μmol·L?1. The Eu-CDs internalize well into HeLa cells, and this paves the way to bioimaging.
Graphical abstract Schematic of a method for visual detection of 2,6-dipicolinic acid (DPA, an anthrax biomarker) by using a test stripe impregnated with europium(III)-doped carbon dots (Eu-CDs).
It is found that the fluorescence of carbon dots (CD) with an emission peak at 459 nm is strongly quenched by silver nanoparticles (AgNPs) with their absorption peak at 430 nm. The finding was applied in a fluorescence quenchometric lateral flow immunochromatographic assay for detection of zearalenone (ZEN) with CDs conjugated to ovalbumin (OVA) as donor signal probe and AgNP-Ab as acceptor signal probe. The assay has an LOD of 0.1 μg·L?1 for ZEN. This is 10 times better than the respective “turn-off” AgNP-based LFIA. In case of cereal samples and their products, the LODs range from 1 to 2.5 μg·kg?1. Only minor cross reactivity is found for fusarium toxins, and no cross-sensitivity for aflatoxin B1, T-2 mycotoxin, ochratoxin A, deoxynivalenol, and fumonisin B1. The assay represents a simple, sensitive, and rapid tool for determination of ZEN in cereal samples and their products.
Graphical abstract Schematic presentation of fluorescence quenching lateral flow immunochromatographic assay (FLFIA) based on carbon dots (CD) and silver nanoparticle (AgNP) fluorescence resonance energy transfer (FRET) system for the rapid high sensitive detection of zearalenone (ZEN) in cereal samples.
