An electrochemiluminescent (ECL) aptamer based method is described for the determination of thrombin. Three-dimensional nitrogen-doped graphene oxide (3D-NGO) was placed on a glassy carbon electrode (GCE) to provide an electrode surface that displays excellent electrical conductivity and acts as a strong emitter of ECL. The modified electrode was further coated with chitosan via electrodeposition. Finally, the amino-modified aptamer was immobilized on the modified GCE. The interaction between thrombin and aptamer results in a decrease in ECL. The assay has a linear response in the 1 fM to 1 nM thrombin concentration range and a 0.25 fM lower detection limit (at an S/N ratio of 3). The method was applied to the determination of thrombin in spiked human plasma samples, and recoveries ranged between 94 and 105% (with RSDs of <3.6%). The calibration plot was recorded at potential and wavelength of fluorescence emission (wavelength:?445 nm; potential:?0 to -2 V).
Graphical abstract A bare glassy carbon electrode (GCE) does not display electrochemiluminescence (ECL). If, however, nitrogen-doped graphene quantum dots, chitosan, and three-dimensional nitrogen-doped graphene oxide (NGQD-chitosan/3D-NGO) are electrodeposited on the GCE, strong ECL can be observed. The ECL intensity decreased after aptamer and bovine serum albumin (BSA) were dropped onto the electrode (curve a). However, the ECL further decreases after addition of thrombin (TB; curve b).
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.
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.
CdSe:Eu nanocrystals were successfully synthesized and characterized by transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectric spectroscopy. The CdSe:Eu nanocrystals showed enhanced green electrochemiluminescence (ECL) intensity when compared to pure CdSe nanocrystals. Further, the nanocrystals were used to design an ECL immunosensor for the detection of carcinoembryonic antigen (CEA) that has a linear response over the 1.0 fg·mL?1 to 100 ng·mL?1 CEA concentration range with a 0.4 fg·mL?1 detection limit. The assay was applied to the determination of CEA in human serum samples.
Graphical abstract Schematic of the assay: GCE-glassy-carbon electrode, Ab- Antibody, BSA- Bovine serum albumin, Ag- Antigen. CdSe:Eu nanocrystals were used to design an ECL immunosensor for the detection of carcinoembryonic antigen.
A novel photoelectrochemical (PEC) aptasensor with graphitic-phase carbon nitride quantum dots (g-C3N4; QDs) and reduced graphene oxide (rGO) was fabricated. The g-C3N4 QDs possess enhanced emission quantum yield (with an emission peak at 450 nm), improved charge separation ability and effective optical absorption, while rGO has excellent electron transfer capability. Altogether, this results in improved PEC performance. The method is making use of an aptamer against sulfadimethoxine (SDM) that was immobilized on electrode through π stacking interaction. Changes of the photocurrent occur because SDM as a photogenerated hole acceptor can further accelerate the separation of photoexcited carriers. Under optimized conditions and at an applied potential of +0.2 V, the aptasensor has a linear response in the 0.5 nM to 80 nM SDM concentration range, with a 0.1 nM detection limit (at S/N =?3). The method was successfully applied to the analysis of SDM in tap, lake and waste water samples.
Graphical abstract Graphitic-phase carbon nitride (g-C3N4) quantum dots (QDs) and reduced graphene oxide (rGO) were used to modify fluorine-doped SnO2 (FTO) electrodes for use in a photoelectrochemical (PEC) aptasensor. SDM oxidized by the hole on valance band (VB) of g-C3N4 QDs promote the separation of electron in the conductive band (CB), which made the changes of photocurrent signal.
The need for external excitation sources limits the utility of quantum dots (QDs) in multiplexed detection schemes and in in vivo imaging, because it can lead to strong background by surface illumination and tissue autofluorescence. In this work, the authors describe the use of oxidized dextran as a support to conjugate the photoprotein aequorin to QDs in order to obtain self-illuminating QDs and an efficient QD-based bioluminescence (BL) resonance energy system. On addition of Ca2+, BL is generated by immobilized aequorin and transferred to the QDs which thereby become photoexcited. Hence, these QDs will fluoresce without being excited by an external light source and therefore have the typical merits (such as very low background) of bioluminescent systems. The half-life of the BL of aequorin peaking at 460 nm is 1.6 s, and that of the QD-conjugated aequorin (peaking at 528 nm) is 6.4 s. We perceive that by labeling antibodies with these nanocomposites, highly advanced multiplex immunoassays will become possible.
Graphical abstract The photoprotein aequorin was conjugated to CdTe quantum dots coated wit denatured and reduced bovine serum albumin (dBSA) by using oxidized dextran as a cross linker, which leads to the formation of self-illuminating QDs.
A novel electrochemiluminescent (ECL) method for highly sensitive detection of gene mutations was designed based on the amplification strategy of dual-functional aluminum(III). A film composed of nafion and polyaniline (Nafion-PANI) was placed onto glassy carbon electrode (GCE) in order to improve conductivity and stability, and then cadmium sulfide quantum dots (CdS QDs) were attached as an ECL label. Al(III) was introduced in order to enhance the ECL signal intensity of the CdS QDs by filling the surface electronic defects of CdS QDs. The Al(III) ions also assist by improving sensitivity by promoting the electron transfer at the GCE and by retaining plenty of single-stranded DNA (ssDNA). The ECL is generated at typically ?1.5 V in the presence of containing K2S2O8. Compared to conventional ECL based DNA biosensors, the one described here – based on the use of dually functional Al(III) ions – enables ssDNA to be detected in the 1 f. to 10 nM concentration range, with a 6 f. detection limit. This method was applied to the quantitation of target ssDNA with different mismatching status in human serum. In our perception, it represents a highly attractive tool for the detection of ssDNA and has a particular potential in the diagnosis of hereditary diseases.
Graphical abstract Preparation and schematic illustration of dual-functional aluminum(III)-based electrochemiluminescent for detection of target ssDNA.
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.
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.
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.
A fluorescent probe for the sensitive and selective determination of sulfide ions is presented. It is based on the use of graphene quantum dots (GQDs) which emit strong and stable blue fluorescence even at high ionic strength. Copper(II) ions cause aggregation of the GQDs and thereby quench fluorescence. The GQDs-Cu(II) aggregates can be dissociated by adding sulfide ions, and this results in fluorescence turn on. The change of fluorescence intensity is proportional to the concentration of sulfide ions. Under optimal conditions, the increase in fluorescence intensity on addition of sulfide ions is linearly related (r2 = 0.9943) to the concentration of sulfide ions in the range from 0.20 to 20 μM, and the limit of detection is 0.10 μM (at 3 σ/s). The fluorescent probe is highly selective for sulfide ions over some potentially interfering ions. The method was successfully applied to the determination of sulfide ions in real water samples and gave recoveries between 103.0 and 113.0 %.
Graphene quantum dots (GQDs) emit strong blue fluorescence which, however, is quenched by copper(II) ions due to the formation of GQDs-Cu(II) aggregates. Fluorescence is recovered by sulfide ions due to the dissociation of GQDs-Cu(II) aggregates.
CdTe quantum dots (QDs) were integrated with polyethyleneimine-coated carbon dots (PEI-CDs) to form a dually emitting probe for heparin. The red fluorescence of the CdTe QDs is quenched by the PEI-CDs due to electrostatic interactions. In the presence of heparin, the blue fluorescence of PEI-CDs remains unaffected, while its quenching effect on the fluorescence of CdTe QDs is strongly reduced. A ratiometric fluorometric assay was worked out. The ratio of the fluorescences at 595 and 436 nm serves as the analytical signal. Response is linear in the concentration range of 50–600 ng·mL?1 (0.1–1.2 U·mL?1) of heparin. The limit of detection is 20 ng·mL?1 (0.04 U·mL?1). This makes the method a valuable tool for heparin monitoring during postoperative and long-term care. This assay is relatively free from the interference by other analogues which commonly co-exist with heparin in samples, and it is more robust than single-wavelength based assays.
Graphical abstract In the presence of heparin, the fluorescence of polyethyleneimine-coated carbon dots (PEI-CDs) at 436 nm remains unaffected, while its quenching effect on the fluorescence of CdTe at 595 nm is strongly reduced.
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.
Near infrared (NIR) emitting semiconductor quantum dots can be excellent fluorescent nanoprobes, but the poor biodegradability and potential toxicity limits their application. The authors describe a fluorescent system composed of graphene quantum dots (GQDs) as NIR emitters, and novel MnO2 nanoflowers as the fluorescence quenchers. The system is shown to be an activatable and biodegradable fluorescent nanoprobe for the “turn-on” detection of intracellular glutathione (GSH). The MnO2-GQDs nanoprobe is obtained by adsorbing GQDs onto the surface of MnO2 nanoflowers through electrostatic interaction. This results in the quenching of the NIR fluorescence of the GQDs. In the presence of GSH, the MnO2-GQDs nanoprobe is degraded and releases Mn2+ and free GQDs, respectively. This gives rise to increased fluorescence. The nanoprobe displays high sensitivity to GSH and with a 2.8 μM detection limit. It integrates the advantages of NIR fluorescence and biodegradability, selectivity, biocompatibility and membrane permeability. All this makes it a promising fluorescent nanoprobe for GSH and for cellular imaging of GSH as shown here for the case of MCF-7 cancer cells.
Graphical abstract A biodegradable NIR fluorescence nanoprobe (MnO2-GQDs) for the “turn-on” detection of GSH in living cell was established, with the NIR GQD as the fluorescence reporter and the MnO2 nanoflower as the fluorescence quencher.
An electrochemical nanoaptasensor is described that is based on the use of a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs). An aptamer (Apt) against trinitrotoluene (TNT) was then immobilized on the AgNPs. The addition of TNT to the modified GCE leads to decrease in peak current (typically measured at a potential of ?0.45 V vs. Ag/AgCl) of riboflavin which acts as an electrochemical probe. Even small changes in the surface (as induced by binding of Apt to TNT) alter the interfacial properties. As a result, the LOD is lowered to 33 aM, and the dynamic range extends from 0.1 fM to 10 μM without sacrificing specificity.
Graphical abstract Schematic presentation of a nanoaptasensor which is based on a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs) and aptamer (Apt). It was applied to the detection of 2,4,6-trinitrotoluene (TNT) with the help of riboflavin (RF) as a redox probe.
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.
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 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.
A selective phosphorescent on-off-on probe with long decay lifetime has been designed for the detection of pyrophosphate ions (PPi). The detection scheme is based on the use of europium(III)-modulated Mn(II)-doped ZnS quantum dots capped with N-acetyl-L-cysteine. Both the aggregation of quantum dots and electron transfer induced by Eu(III) ions cause phosphorescence to be quenched (“off” state). Phosphorescence is, however, restored on addition of PPi to the system (“on” state). The effect is attributed to the removal of Eu(III) from the carboxy groups on the surface of the quantum dots owing to the stronger interaction between PPi and Eu(III). A linear relationship exists between phosphorescence intensity (best measured at excitation/emission wavelengths of 316/594 nm) and PPi concentration in the 400 nM to 6000 nM with a detection limit of 145 nM. An additional attractive feature is provided by the long-lived phosphorescence (1920 μs) of the quantum dots. It can be used to eliminate interference by short-lived fluorescence in biological samples by performing time resolved measurements. The probe was applied to the determination of PPi in spiked in urine samples and gave recoveries in the range from 98 to 105% with RSDs of <2.0%.
Graphical abstract Schematic of a long-lived phosphorescent on-off-on probe for the sensitive and selective detection of pyrophosphate ions (PPi). It is based on the use of Eu(III)-modulated Mn(II)-doped ZnS quantum dots (QDs). Phosphorescence is quenched of QDs after the addition of Eu3+but restored after the addition of PPi.
The authors describe a fluorescence based aptasensor for adenosine (AD), a conceivable biomarker for cancer. The assay is based on the immobilization of capture DNA on newly synthesized quaternary CuInZnS quantum dots (QDs) and the conjugation of probe DNA on gold nanoparticles (AuNPs). The capture DNA is an adenosine-specific aptamer that is partly complementary to the probe DNA. Once the capture aptamer hybridizes probe DNA, the fluorescence of the QDs (measured at excitation/emission wavelengths of 522/650 nm) is quenched by the AuNPs. However, when AD is added, it will bind to the aptamer and restrain the hybridization between capture DNA and probe DNA. Therefore, the fluorescence of the QDs will increase with increasing AD concentration. Under optimal conditions, fluorescence is linearly related to the AD concentration in the range from 50 to 400 μM, the detection limit being 1.1 μM. This assay is sensitive, selective, reproducible and acceptably stable. It was applied to the determination of AD in spiked human serum samples where it gave satisfactory results.
