A time-resolved phosphorescence (TRP) is applied to the highly sensitive determination of Fe(II) ions. The method is based on the use of a phosphorescent probe consisting of cysteine-bridged Mn-doped ZnS quantum dots (Mn/ZnS QDs). The presence of cysteine enhances the phosphorescence of the QDs and also increases the efficiency of quenching caused by Fe(II) ions. This results in strongly improved selectivity for Fe(II). The linear response is obtained in the concentration range of 50–1000 nM with a 19 nM detection limit. Phosphorescence is recorded at excitation/emission peaks of 301/602 nm. The interference of short-lived fluorescent and scattering background from the biological fluids is eliminated by using the TRP mode with a delay time of 200 μs. The determination of Fe(II) in human serum samples spiked at a 150 nM level gave a 92.4% recovery when using the TRP mode, but only 52.4% when using steady-state phosphorescence. This demonstrates that this probe along with TRP detection enables highly sensitive and accurate determination of Fe(II) in serum.
Graphical abstract Schematic of a novel phosphorescent method for the detection of Fe2+ ions based on cysteine-bridged Mn-doped ZnS quantum dots. The sensitivity of this assay greatly increases due to the addition of cysteine. Interferences by short-lived auto-fluorescence and the scattering light from the biological fluids is eliminated by using time-resolved phosphorescence mode.
We have prepared graphene quantum dot-europium(III) complex composites by noncovalently connecting chelating ligands dibenzoylmethane (DBM) and 1,10-phenanthroline (Phen) with graphene quantum dots (GQDs) first, followed by coordination to Eu(III). The resulting composites are well water-soluble and display red fluorescence of high color purity. The composites were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. Aqueous solutions of the composites under 365 nm excitation display fluorescence with a peak at 613 nm and a quantum yield as high as 15.5 %. The good water solubility and stable photoluminescence make the composites very different from other Eu(III)-based coordination complexes. The composites are cell viable and can be used to label both the cell membrane and the cytoplasm of MCF-7 cells. They are also shown to act as bioprobes for in-vivo localization of tumorous tissue. In our perception, such composites are expected to possess wide scope because of the many functionalizations that are possible with GQDs.
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.
Carbon polymer dots (CPDs) were prepared by a one-pot aqueous synthetic route from ascorbic acid and diethylenetriamine at room-temperature. The CPDs under 350-nm excitation exhibit blue fluorescence peaking at 430 nm with a quantum yield of 47%. Other features include an average diameter of 5 nm, a fluorescence that is independent of the excitation wavelength, good water dispersibility and photostability, and excellent biocompatibility. The CPDs are shown to be viable fluorescent probes for ferric ion which acts as a strong quencher. The response to Fe(III) is linear in the 0.2 to 10 μM concentration range, and the detection limit is 0.1 μM. The probe was applied to the determination of Fe(III) in environmental waters and to intracellular imaging of ferric ions in HeLa cells.
Graphical abstract Carbon polymer dots (CPDs) are prepared from ascorbic acid and diethylenetriamine (DETA) at room-temperature (RT). The RT-CPDs exhibit excellent optical performance, biocompatibility and selectivity of quenching by ferric ions. This can be applied for determination and intracellular imaging of ferric ion.
The study reports on the synthesis of a graphene aerogel@octadecylamine-functionalized carbon quantum dots (GA@O-CQDs). The graphene oxide aqueous dispersion, O-CQDs aqueous dispersion and toluene were strongly mixed to make a toluene-in-water Pickering emulsion. The graphene oxide sheets in the aqueous phase are reduced by hydrazine hydrate, diffuse into the toluene droplets, and self-assemble into graphene oxide microgels. This is followed by freeze-drying and thermal annealing to obtain the GA@O-CQDs hybrid that has a three-dimensional structure of several microns. It was dispersed in ethanol and deposited on a glassy carbon electrode. The modified electrode was applied to differential pulse voltammetric determination of acetaminophen, best at a peak potential of 0.15 V (vs. Ag/AgCl). Figures of the merit include a wide linear response range (0.001–10 μM) and a 0.38 nM of the detection limit (S/N?=?3). The assay has been applied to the determination of acetaminophen in tablets.
Graphical abstract Schematic presentation of the synthesis of graphene aerogel@octadecylamine-functionalized carbon quantum dots. The synthesis achieves to the intimate chemical and electrical contact between graphene and carbon quantum dots. An electrode modified with the hybrid exhibits ultra high sensitivity for detection of acetaminophen.
The paper describes a fluorescent method for determination of Au(III) using molybdenum disulfide quantum dots (MoS2 QDs) that were prepared by a hydrothermal route using glutathione as a reductant. The photoluminescence of MoS2 QDs peaks at 416 nm if excited at 340 nm and is temporally stable even in presence of NaCl or when stored in the refrigerator for one year. Its quantum yield is 12.7 %. The blue-green fluorescence of MoS2 QDs is fairly specifically quenched by Au(III) ions and therefore presents a useful nanoprobe for this ion. Fluorescence intensity drops linearly with the concentration of Au(III) in the range from 0.5 to 1000 μM, and the lower detection limit is 64 nM. The quenching mechanism was investigated and it is concluded that the process is due to the reduction of Au(III) and the deposition of Au(0) on the surface of the MoS2 QDs. The nanoprobe was successfully applied to the determination of Au(III) in (spiked) environmental samples. A test stripe for Au(III) was obtained by soaking a piece of paper with a colloidal solution of the MoS2 QDs, and it was found that this stripe, after drying, can also be used to quantify Au(III) via fluorescence.
Graphical abstract Molybdenum disulfide quantum dots (MoS2 QDs) have a high quantum yield and show good stability. MoS2 QDs are shown to be a sensitive fluorescent probe for the determination of Au3+ ions in solution and with a test stripe via fluorescence quenching.
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.
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).
A voltammetric analytical assay for the selective quantification of vanillin is described. It is based on the use of a gold nanoparticle-modified screen-printed carbon electrode (SPCE) modified with graphene quantum dots (GQD) in a Nafion matrix. The GQD were synthesized by an acidic thermal method and characterized by UV-Vis, photoluminescence, and FTIR spectroscopy. The modified SPCE displays a strongly enhanced response to vanillin. Linear sweep voltammetry (LSV) and differential pulse voltammetry (DPV) were applied to optimize the methods. The analytical assay has linear responses in the 13 to 660 μM and 0.66 to 33 μM vanillin concentration ranges. The detection limits are 3.9 μM and 0.32 μM when using LSV and DPV, respectively. The analytical assay is selective and stable. It was applied to the determination of vanillin in several food samples with satisfactory results. Recoveries from spiked samples ranged between 92.1 and 113.0%.
Graphical abstract The selective and sensitive quantification of vanillin is carried out by the use of a gold nanoparticle-modified screen-printed carbon electrode modified with graphene quantum dots in a Nafion matrix.
Nitrogen- and iron-containing carbon dots (N,Fe-CDs) are synthesized by hydrothermal treatment of branched polyethylenimine (BPEI) and hemin at 180 °C. The N,Fe-CDs are mainly doped with nitrogen and trace amounts of iron(III). The N,Fe-CDs also display intrinsic fluorescence with excitation/emission maxima at 365/452 nm and a quantum yield of 27 %. The nanodots are shown to act as peroxidase mimics by catalyzing the oxidation of tetramethylbenzidine (TMB) by hydrogen peroxide to form a blue product whose quantity can be determined by photometry at 652 nm. This was exploited to design colorimetric and fluorometric assays for dopamine (DA). The colorimetric assay is based on the oxidation of DA by H2O2 in presence of the N,Fe-CDs and TMB. It has an instrumental detection limit of 40 nM (at an S/N ratio of 3), and a visual detection limit of 0.4 μM. The fluorometric assay is based on an inner filter effect that is caused by the formation of oxidized TMB which overlaps (and absorbs) the emission of the N,Fe-CDs located at 452 nm. The fluorometric detection limit is as low as 20 nM (at an S/N ratio of 3).
Graphical abstract Carbon dots containing nitrogen and iron (N,Fe-CDs) were synthesized by hydrothermal treatment of branched polyethylenimine and hemin. The N,Fe-CDs display excellent fluorescent properties, peroxidase-like activity and potential application in colorimetric and fluorometric detection of dopamine.
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.
A strategy was developed for the voltammetric determination of the antibiotic drug levofloxacin (LV) based on a glassy carbon electrode modified with a composite consisting of poly(o-aminophenol) and graphene quantum dots (PoAP/GQD) that was fabricated by electropolymerization. The PoAP/GQD composite provides a large surface area and sensing interface and strongly promotes the oxidation current of LV. Under optimal conditions, the modified GCE displays an oxidation peak current (best measured at a working voltage of 1.05 V vs. SCE) that is linearly related to the levofloxacin concentration in the range from 0.05 to 100 μM, and the detection limit is 10 nM (at an S/N of 3). The method was applied to the determination of levofloxacin in spiked milk samples where is gave recoveries between 96.0 and 101.0 %.
Graphical Abstract We describe a one-step electrochemical polymerization method to synthesize a layer of conductive film of poly(o-aminophenol) and graphene quantum dots (PoAP/GQD) onto a glassy carbon electrode (GCE) surface. The composite film exhibited high electro catalytic activity for the quantitative determination of levofloxacin by stripping voltammetry.
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 method for the differentiation of penicillamine (PA) enantiomers by using CdSe/ZnS quantum dots (QDs) modified with β-cylodextrin (β-CD-CdSe/ZnS QDs). Selective enantiorecognition of L-PA and D-PA was accomplished by virtue of selective host-guest interaction between the PAs and the β-CD pockets on the QDs. The fluorescence intensity of the modified QDs decreases in the presence of L-PA. On the contrary, it increases in the presence of D-PA. These findings form the basis for a new method for recognition of PA enantiomers. Under optimized conditions, a linear relationship exists between fluorescence intensity and D-PA concentration in the 0.1 to 5.0 mg L?1 range, and between 0.8 and 5.0 mg L?1 for L-PA. Detection limits are 0.06 mg L?1 for D-PA, and 0.2 mg L?1 for L-PA. The potential of this method has been demonstrated by the determination of D-PA in pharmaceutical formulations and L-PA in (spiked) environmental samples.
Graphical abstract Selective and specific enantiorecognition of penicillamine (PA) enantiomers using β-cylodextrin modified CdSe/ZnS quantum dots is described. Fluorescence intensity increases in the presence of D-PA, but it decreases in the presence of L-PA. Results were the basis for analytical applications.
The authors describe the synthesis of a multifunctional nanocomposite with an architecture of type Fe3O4@SiO2@graphene quantum dots with an average diameter of about 22 nm. The graphene quantum dots (GQDs) were covalently immobilized on the surface of silica-coated magnetite nanospheres via covalent linkage to surface amino groups. The nanocomposite displays a strong fluorescence (with excitation/emission peaks at 330/420 nm) that is fairly selectively quenched by Hg2+ ions, presumably due to nonradiative electron/hole recombination annihilation. Under the optimized experimental conditions, the linear response to Hg2+ covers the 0.1 to 70 μM concentration range, with a 30 nM lower detection limit. The high specific surface area and abundant binding sites of the GQDs result in a good adsorption capacity for Hg2+ (68 mg?g?1). The material, due to its superparamagnetism, can be separated by using a magnet and also is recyclable with EDTA so that it can be repeatedly used for simultaneous detection and removal of Hg2+ from contaminated water.
Graphical abstract A schematic view of preparation process for the Fe3O4@SiO2@graphene quantum dots nanocomposite (denoted as Fe3O4@SiO2@GQDs). The graphene quantum dots were covalently immobilized on the surface of silica-coated magnetite nanospheres (Fe3O4@SiO2) via covalent linkage to surface amino groups.
A fluorometric ATP assay is described that makes use of carbon dots and graphene oxide along with toehold-mediated strand displacement reaction. In the absence of target, the fluorescence of carbon dots (with excitation/emission maxima at 360/447 nm) is strong and in the “on” state, because the signal probe hybridizes with the aptamer strand and cannot combine with graphene oxide. In the presence of ATP, it will bind to the aptamer and induce a strand displacement reaction. Consequently, the signal probe is released, the sensing strategy will change into the “off” state with the addition of graphene oxide. This aptasensor exhibits selective and sensitive response to ATP and has a 3.3 nM detection limit.
Graphical abstract Schematic of signal amplification by strand displacement in a carbon dot based fluorometric assay for ATP. This strategy exhibits high sensitivity and selectivity with a detection limit as low as 3.3 nM.
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.
Carbon nanodots modified with triethylenetetramine (referred to as TCDs) are shown to be viable fluorescent probes for relay recognition of Cu(II) ion and glutathione (GSH). The assay is based on an “on-off-on” mechanism where the “on-off” effect, i.e. quenching by Cu(II) by up to 67%, is exploited to quantify it in concentrations as low as 3.4 nM. The unique quenching of fluorescence (measured at excitation/emission wavelengths of 380/470 nm; quantum yield 16%) is attributed to the fairly selective capture of Cu(II) by the amino and amide groups on the surface of the TCDs. On addition of GSH to the quenched TCD/Cu(II) complex, fluorescence is restored. This effect enables GSH to be quantified in the 0.2 to 175 μM concentration range, with a 0.11 μM detection limit. The turn-on response to GSH is highly selective over other natural amino acids and common anions. Furthermore, the TCDs were successfully applied to image Cu(II) and GSH in living yeast cells.
Graphical Abstract Carbon nanodots modified with triethylenetetramine show strong blue fluorescence which is quenched by Cu(II) but restored on addition of glutathione. Both Cu(II) (down to 3.4 nM) and glutathione (down to 110 nM) can be detected via these effects.
The authors describe a novel assay for the detection of methylated DNA site. Rolling circle amplification and CdSe/ZnS quantum dots with high fluorescence efficiency are applied in this method. The CdSe/ZnS quantum dots act as electron donors, and hemin and oxygen (derived from hydrogen peroxide act as acceptors in photoinduced electron transfer. The assay, best performed at excitation/emission peaks of 450/620 nm, is sensitive and specific. Fluorometric response is linear in the 1 pM to 100 nM DNA concentration range, and the lowest detectable concentration of methylated DNA is 142 fM (S/N =?3). The method is capable of recognizing 0.01% methylated DNA in a mixture of methylated/unmethylated DNA.
Graphical abstract A novel method for methylated sites detection in DNA is established. Rolling circle amplification and photoinduced electron transfer. CdSe/ZnS quantum dots with high fluorescence efficiency act as the electron donor, while G-quadruplex/hemin and hydrogen peroxide derived oxygen act as electron acceptor. It presents a linear response towards 1 pM to 100 nM methylated DNA with a correlation coefficient of 0.9968, and the lowest detectable concentration of methylated DNA was 142 fM, with selectivity significantly superior to other methods.
A method is described for the determination of the activity of alkaline phosphatase (ALP). It is based on the reversible modulation of the fluorescence of WS2 quantum dots (QDs). The fluorescence of the QDs is quenched by Cr(VI) but restored by free ascorbic acid (AA). The detection scheme relies on the fact that ALP hydrolyzes the substrate ascorbic acid 2-phosphate to produce AA, and that enzymatically generated AA can restore the fluorescence of the QDs. The signal (best measured at excitation/emission peak wavelengths of 365/440 nm) increases linearly in the 0.5 to 10 U·L?1 ALP activity range, with a detection limit of 0.2 U·L?1. The method was applied to the determination of ALP activity in human serum samples and demonstrated satisfactory results.
Graphical abstract The fluorescence of chromate-loaded tungsten disulfide quantum dots (QDs) is quenched but restored after reaction with ascorbic acid that is formed by the catalytic action of alkaline phosphatase (ALP) on ascorbic acid 2-phosphate (AAP). The increase in fluorescence can be related to the activity of ALP.
