A ratiometric fluorometric heparin assay based on the use of CdTe and polyethyleneimine-coated carbon quantum dots

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.

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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 novel silica-coated multiwall carbon nanotube with CdTe quantum dots nanocomposite

A novel silica-coated multiwall carbon nanotube (MWNTs) with CdTe quantum dots nanocomposite was synthesized in this paper. Here, we show the in situ growth of crystalline CdTe quantum dots on the surfaces of oxidized MWNTs. The approach proposed herein differs from previous attempts to synthesize nanotube assemblies in that we mix the oxidized MWNTs into CdCl₂ solution of CdTe nanocrystals synthesized in aqueous solution. Reinforced the QD–MWNTs heterostructures with silica coating, this method is not invasive and does not introduce defects to the structure of carbon nanotubes (CNTs), and it ensures high stability in a range of organic solvents. Furthermore, a narrow SiO₂ layer on the MWNT–CdTe heterostructures can eliminate the biological toxicity of quantum dots and carbon nanotubes. This is not only a breakthrough in the synthesis of one-dimensional nanostructures, but also taking new elements into bio-nanotechnology.     

Ratiometric arginine assay based on FRET between CdTe quantum dots and Cresyl violet

The authors report on the design of a new Förster resonance energy transfer (FRET) based ratiometric nanoprobe for the determination of arginine. The method is based on the inhibition of the efficiency of FRET in assemblies formed between CdTe quantum dots capped with mercaptopropionic acid (QD-MPA) acting as energy donor, and the dye Cresyl Violet (CV) that acts as an energy acceptor at pH 8. Addition of arginine causes a displacement of the CV by arginine on the surface of the QD/MPA. Hence, the FRET between QD/MPA and CV is interrupted and fluorescence emission of the donor (QD/MPA) is restored. Arginine selectively binds to the QD/MPA via electrostatic and hydrogen bonding interactions between guanidinium and carboxylate. Under optimum conditions, the ratio of the fluorescence emissions peaking at 575 and 620 nm (under 400 nm excitation) is linear in the 1 to 30 μM arginine concentration range, and the detection limit is 0.51 μM. The nanoprobe displays good selectivity over 14 other amino acids, many metal ions, glucose, and ascorbic, tartaric and citric acids. The fluorescent nanoprobe was successfully applied to the determination of arginine in pure and spiked real samples and gave good recoveries. Its good selectivity, sensitivity, low-cost and rapidity make the QD-dye assembly a suitable nanoprobe for the quantitation of arginine.

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Graphical abstract Schematic of a FRET ratiometric nanoprobe for arginine. It is based on quantum dots acting as energy donors and Cresyl Violet acting as energy acceptor. The FRET process is interrupted by the addition of arginine which selectively interacts with carboxy groups via a guanidinium-carboxylate salt bridge.

     

Development and application of an electrochemiluminescent flow-injection cell based on CdTe quantum dots modified electrode for high sensitive determination of dopamine

A nano-liter sized flow-cell is developed for constructing a flow injection analysis (FIA) system with electrochemiluminescent (ECL) detection. A sensitive ECL electrode is applied as the working electrode in this flow-cell. It is obtained by immobilizing the composite of CdTe quantum dots (QDs), carbon nanotubes (CNTs) and chitosan (Chit) on indium tin oxide (ITO) glass. The CdTe QDs were synthesized in our lab and possessed a high quantum yield. It has been demonstrated as an efficient anodic ECL material with the triethylamine (TEA) as the co-reactant. The flow-cell gives the stable ECL background under optimized conditions for parameters such as electrolytic pulse, concentration of TEA and flow rate, etc. The sensitive ECL quenching response of dopamine (DA) is realized on this FIA system within the linear range from 10 pM to 4 nM and a detection limit as low as 3.6 pM. It is practically used to determine the neurotransmitters in cerebro-spinal fluid (CSF) with DA as the index and with an average recovery of 94%.     

Nanosensor for dopamine and glutathione based on the quenching and recovery of the fluorescence of silica-coated quantum dots

We have constructed a fluorescent nanosensor for dopamine (DA) and glutathione (GSH) in physiologically relevant concentrations. CdTe quantum dots (QDs) were coated with silica, and dopamine-quinone (formed by oxidation of DA) is captured on the surface of silica via dual interactions (hydrogen bonding and electrostatic interaction) and quenches the photoluminescence of the modified QDs by an electron transfer process. GSH, in being a strong reducing agent, can chemically reduce the dopamine-quinone on the QDs, and this results in recovered photoluminescence. There are linear relationships between the concentrations of dopamine and GSH respectively and the intensity of the photoluminescence intensity of the QDs both in the quenched and regenerated form, the ranges being 0.0005 to 0.1 mmol?L^?1 for dopamine, and 0.1 to 10 mmol?L^?1 for GSH. The method was applied to the determination of dopamine and GSH in human serum samples with satisfactory results.

Glutathione-capped CdTe quantum dots for the sensitive detection of glucose

A simple and sensitive assay system for glucose based on the glutathione (GSH)-capped CdTe quantum dots (QDs) was developed. GSH-capped CdTe QDs exhibit higher sensitivity to H₂O₂ produced from the glucose oxidase catalyzed oxidation of glucose, and are also more biocompatible than other thiols-capped QDs. Based on the quenching of H₂O₂ on GSH-capped QDs, glucose can be detected. The detection conditions containing reaction time, the concentration of glucose oxidase and the sizes of QDs were optimized and the detection limits for glucose was determined to be 0.1 μM; two detection ranges of glucose from 1.0 μM to 0.5 mM and from 1.0 mM to 20 mM, respectively were obtained. The detection limit was almost a 1000 times lower than other QDs-based optical glucose sensing systems. The developed glucose detection system was simple and facile with no need of complicated enzyme immobilization and modification of QDs.     

水溶性CdTe量子点的合成及其用于荧光猝灭法测定肌红蛋白

以3-巯基丙酸为稳定剂,合成了具有特殊光学性质的水溶性CdTe量子点,其最大发射波长位于544 nm.利用荧光光谱、紫外可见光谱及圆二色光谱法系统的研究了CdTe量子点与肌红蛋白(Mb)二者结合前后体系光谱的变化,从而证实了CdTe量子点与Mb之间静电结合反应的特征.在pH 7.0的PBS缓冲液中,用CdTe量子点作为荧光探针研究了肌红蛋白与量子点的相互作用,并基于肌红蛋白对CdTe量子点有显著的荧光猝灭作用,建立了肌红蛋白的快速检测方法.在最佳实验条件下,该体系荧光强度的猝灭程度(△F)与肌红蛋白质量浓度呈良好的线性关系,线性范围为0.3～24 μg/mL,检出限为0.13 μg/mL.该方法已对合成样品中肌红蛋白进行检测,并用于人体尿样中肌红蛋白的测定.     

Antioxidant activity assay based on the inhibition of oxidation and photobleaching of l-cysteine-capped CdTe quantum dots

Quantum dots (QDs) have recently been the focus of attention of many investigators for development of diagnostic tools in many research areas. In this work, we established a new QD-based assay to evaluate the antioxidant/polyphenolic activity. This assay is based on measurement of the inhibitory effect of the antioxidant/polyphenolic compounds on the UV-induced bleaching of CdTe QDs with l-cysteine capping. QDs exhibited excellent photostability without any UV exposure, while they bleached rapidly under UV irradiation. Generation of reactive oxygen species (ROS) under UV irradiation is probably the main cause of the photobleaching of QDs. By comparing the photostability of QDs in buffer solution in the absence and presence of sodium azide, as a known (1)O(2) quencher, the involvement of (1)O(2) in photobleaching of QDs was confirmed. The photobleaching effect induced by ROS could be reduced in the presence of antioxidant/polyphenolic compounds. We tested several antioxidant/polyphenolic compounds as well as known antioxidants such as trolox and 4 different types of tea. The results obtained by the QD-based assay revealed a very good correlation with the data acquired by Folin-Ciocalteu assay. Furthermore, a deeper understanding of the mechanism and the solution for photobleaching of QDs under UV irradiation might be very meaningful in promoting their clinical applications.     

A sensitive sensor for anthraquinone anticancer drugs and hsDNA based on CdTe/CdS quantum dots fluorescence reversible control

Based on CdTe/CdS quantum dots (CdTe/CdS QDs) fluorescence (FL) reversible control, a new and sensitive FL sensor for determination of anthraquinone (AQ) anticancer drugs (adriamycin and daunorubicin) and herring sperm DNA (hsDNA) was developed. Under the experimental conditions, FL of CdTe/CdS QDs can be effectively quenched by AQ anticancer drugs due to the binding of AQ anticancer drugs on the surface of CdTe/CdS QDs and photoinduced electron transfer (PET) process from CdTe/CdS QDs to AQ anticancer drugs. Addition of hsDNA afterwards brought the restoration of CdTe/CdS QDs FL intensity, as AQ anticancer drugs peeled off from the surface of CdTe/CdS QDs and embedded into hsDNA double helix structure. The liner ranges and the detection limits of FL quenching methods for two AQ anticancer drugs were 0.33-9 μg mL⁻¹ and 0.09 μg mL⁻¹ for ADM and 0.15-9 μg mL⁻¹ and 0.04 μg mL⁻¹ for DNR, respectively. The restored FL intensity was proportional to concentration of hsDNA in the range of 1.38-28 μg mL⁻¹and the detection limit for hsDNA was 0.41 μg mL⁻¹. It was applied to the determination of AQ anticancer drugs in human serum and urine samples with satisfactory results. The reaction mechanism of CdTe/CdS QDs FL reversible control was studied.     

Turn-on fluorometric NADPH assay using orange emitting graphene oxide quantum dots

The authors describe a method for synthesizing graphene oxide quantum dots (GOQDs) possessing orange fluorescence with emission wavelength that can be tuned over the range from 537 to 593 nm by variation of the excitation wavelength. The GOQD display peroxidase-mimicking catalytic activity. Specifically, they catalyze the oxidation of dopamine to produce 4-(2-aminoethyl)benzene-1,2-quinone (AQ) which is colored and can quench the fluorescence of GOQDs. However, quenching is reversed by addition of NADP⁺, but not by its reduced form (NADPH). Based on these findings, an assay was worked out to monitor enzymatic reactions involving NADP⁺. The method allows NADPH to be detected in the 2–175 μM concentration range, with a 0.6 μM detection limit.

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Graphical abstract Schematic of a top-down method for synthesizing fluorescent graphene oxide quantum dots (GOQD) by chemical degrading, exfoliation and self-assembly of graphene oxide (GO). The GOQDs display peroxidase-like activity and can oxidize dopamine to form a colored quinone that quenches the fluorescence of the GOQDs. The quenching efficiency is reduced, however, in the presence of NADP⁺.

     

A flow cytometry-based dopamine transporter binding assay using antagonist-conjugated quantum dots

Here we present the development and validation of a flow cytometry-based dopamine transporter (DAT) binding assay that uses antagonist-conjugated quantum dots (QDs). We anticipate that our QD-based assay is of immediate value to the high throughput screening of novel DAT modulators.     

Aptasensor for visual and fluorometric determination of lysozyme based on the inner filter effect of gold nanoparticles on CdTe quantum dots

The authors describe an aptasensor for visual and fluorescent detection of lysozyme via an inner filter effect (IFE). The assay is based on the fact that red gold nanoparticles (AuNPs) act as powerful absorbers of the green fluorescence of CdTe because of spectral overlap. If the lysozyme-binding aptamer is adsorbed onto the surface of the AuNPs, the salt-induced aggregation of AuNPs (that leads to a color change from red to blue) does not occur and the IFE remains efficient. If lysozyme is present, it will bind the aptamer and thereby prevent its adsorption on the AuNPs. As a result, the salt-triggered aggregation of the AuNPs will occur. Consequently, color will change from red to blue, and green fluorescence will pop up because the IFE is suppressed. Under optimum conditions, fluorescence is linearly related to lysozyme concentration in the 1.0 nM to 20 nM concentration range, with a 0.55 nM limit of detection. The method is perceived to be of wider applicability in that it may be used to design other visual and fluorescent assays if appropriate aptamers are available.

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Graphical abstract The fluorescence intensity of QDs is quenched by gold nanoparticles (AuNPs) due to an inner filter effect. Aptamers can adsorb on AuNPs to prevent the salt-induced aggregation. AuNPs serve a dual function as fluorescence quencher and colorimetric reporter.

     

A simple fluorometric assay for DNA exonuclease activity based on graphene oxide

A new assay platform for DNA exonuclease activity is developed based on the preferential binding of single-stranded DNA (ssDNA) over double-stranded DNA (dsDNA) to graphene oxide. This approach allows a simple and quantitative activity measurement in a short time at low cost.     

Wavelength encoded analytical imaging and fiber optic sensing with pH sensitive CdTe quantum dots

CdTe quantum dots (QDs), capped with mercaptopropionic acid (MPA), were synthesized and the variation of their fluorescence properties (steady state and lifetime) with pH was assessed in solution and when immobilized in a sol-gel host. Three different sizes of CdTe QDs with excited state lifetimes ranging from 42 to 48 ns and with emission maximum at 540 nm (QD₅₄₀), 580 nm (QD₅₈₀) and 625 nm (QD₆₂₅) were selected. The solution pH affects the maximum emission wavelength (shifts to higher wavelengths of 23, 24 and 27 nm for QD₅₄₀, QD₅₈₀ and QD₆₂₅, respectively), the excited state lifetime and the fluorescence intensity in a reversible way. Linearization of the maximum emission wavelength variation with the pH allows the estimation of an apparent ionization constant (pK_a) for each QD: 6.5 ± 0.1 (QD₅₄₀), 6.1 ± 0.5 (QD₅₈₀) and 5.4 ± 0.3 (QD₆₂₅). The variation of the QDs fluorescence properties was further explored using confocal laser scanning microscopy allowing the implementation of a new calibration method for pH imaging in solution. QDs were successfully immobilized on the tip of an optical fiber by dip-coating using sol-gel procedure. The immobilized QDs showed a similar pH behaviour to the one observed in solution and an apparent lifetime of 80, 68 and 99 ns, respectively. The proposed QDs based methodology can be successfully used to monitor pH using wavelength encoded data in imaging and fiber optic sensing applications.     

A simple fluorescence quenching method for berberine determination using water-soluble CdTe quantum dots as probes

A novel method for the determination of berberine has been developed based on quenching of the fluorescence of thioglycolic acid-capped CdTe quantum dots (TGA-CdTe QDs) by berberine in aqueous solutions. Under optimum conditions, the relative fluorescence intensity was linearly proportional to the concentration of berberine between 2.5 × 10^?8 and 8.0 × 10^?6 mol L^?1 with a detection limit of 6.0 × 10^?9 mol L^?1. The method has been applied to the determination of berberine in real samples, and satisfactory results were obtained. The mechanism of the proposed reaction was also discussed.     

Highly sensitive detection of lead(II) ion using multicolor CdTe quantum dots

Multicolor and water-soluble CdTe quantum dots (QDs) were synthesized with thioglycolic acid (TGA) as stabilizer. These QDs have a good size distribution, display high fluorescence quantum yield, and can be applied to the ultrasensitive detection of Pb(II) ion by virtue of their quenching effect. The size of the QDs exerts a strong effect on sensitivity, and quenching of luminescence is most effective for the smallest particles. The quenching mechanism is discussed. Fairly selective detection was accomplished by utilizing QDs with a diameter of 1.6?nm which resulted in a detection limit of 4.7?nmol?L^?1 concentration of Pb(II). The method was successfully applied to the determination of Pb(II) in spinach and citrus leaves, and the results are in good agreement with those obtained with atomic absorption spectrometry.

Electrochemical properties of CdSe and CdTe quantum dots

Semiconductor nanocrystal quantum dots (QDs), owing to their unique opto-electronic properties determined by quantum confinement effects, have been the subject of extensive investigations in different areas of science and technology in the past two decades. The electrochemical behaviour of QDs, particularly for CdSe and CdTe nanocrystals, has also been explored, although to a lesser extent compared to the optical properties. Voltammetric measurements can be used to probe the redox levels available for the nanocrystals, which is an invaluable piece of information if these systems are involved in electron transfer processes. Electrochemical data can also foster the interpretation of the spectroscopic properties of QDs, and give insightful information on their chemical composition, dimension, and surface properties. Hence, electrochemical methods constitute in principle an effective tool to probe the quality of QD samples in terms of purity, size dispersion, and surface defects. The scope of this critical review is to discuss the results of electrochemical studies carried out on CdSe and CdTe core and core-shell semiconductor nanocrystals of spherical shape. Examples of emerging or potential applications that exploit electroactive quantum dot-based systems will also be illustrated.     

Conjugating luminescent CdTe quantum dots with biomolecules

Newly prepared CdTe quantum dots ( QD) bearing shells of water solubility providing capping agents (i.e., thioglycolic acid ( TGA) and 2-(dimethylamino)ethanethiol hydrochloride (DMAET) were subjected to electrostatic assays with several proteins (i.e., cytochrome c (cyt c) and human serum albumin (HSA). In particular, we employed absorption, emission, transient absorption and time-resolved emission spectroscopic means to test their response to light. Only for negatively capped QDs spectroscopic and kinetic evidence were gathered that corroborate the successful bioconjugation of QDs with cyt c to yield QD- cyt c bioconjugates. In fact, photoexcitation of QD-cyt c leads to a fast deactivation of the QD band gap emission and of the QD excited state. Notably, these interactions depend on the size of the QDs. Repulsive forces, on the other hand, are operative between the positively capped QDs and cyt c, hampering any bioconjugation.     

A novel dual-function molecularly imprinted polymer on CdTe/ZnS quantum dots for highly selective and sensitive determination of ractopamine

A novel dual-function material was synthesized by anchoring a molecularly imprinted polymer (MIP) layer on CdTe/ZnS quantum dots (QDs) using a sol–gel with surface imprinting. The material exhibited highly selective and sensitive determination of ractopamine (RAC) through spectrofluorometry and solid-phase extraction (SPE) coupled with high performance liquid chromatography (HPLC). A series of adsorption experiments revealed that the material showed high selectivity, good adsorption capacity and a fast mass transfer rate. Fluorescence from the MIP-coated QDs was more strongly quenched by RAC than that of the non-imprinted polymer, which indicated that the MIP-coated QDs acted as a fluorescence sensing material could recognize RAC. In addition, the MIP-coated QDs as a sorbent was also shown to be promising for SPE coupled with HPLC for the determination of trace RAC in feeding stuffs and pork samples. Under optimal conditions, the spectrofluorometry and SPE-HPLC methods using the MIP-coated QDs had linear ranges of 5.00 × 10⁻¹⁰–3.55 × 10⁻⁷ and 1.50 × 10⁻¹⁰–8.90 × 10⁻⁸ mol L⁻¹, respectively, with limits of detection of 1.47 × 10⁻¹⁰ and 8.30 × 10⁻¹¹ mol L⁻¹, the relative standard deviations for six repeat experiments of RAC (2.90 × 10⁻⁹ mol L⁻¹) were below 2.83% and 7.11%.