Synthesis of highly luminescent and biocompatible CdTe/CdS/ZnS quantum dots using microwave irradiation: a comparative study of different ligands

We compared the effects of several ligands frequently used in aqueous synthesis, including L‐cysteine, L‐cysteine hydrochloride, N‐acetyl‐L‐cysteine (NAC), glutathione and 3‐mercaptopropionic acid, for microwave synthesis of CdTe quantum dots (QDs) in a sealed vessel with varied temperatures and times, and then developed a rapid microwave‐assisted protocol for preparing highly luminescent, photostable and biocompatible CdTe/CdS/ZnS core–multishell QDs. The effects of molecular structures of these ligands on QD synthesis under high temperatures were explored. Among these ligands, NAC was found to be the optimal ligand in terms of the optical properties of resultant QDs and reaction conditions. The emission wavelength of NAC‐capped CdTe QDs could reach 700 nm in 5 min by controlling the reaction temperature, and the resultant CdTe/CdS/ZnS core–multishell QDs could achieve the highest quantum yields up to 74% with robust photostability. In addition, the effects of temperature, growth time and shell–precursor ratio on shell growth were examined. Finally, cell culturing indicated the low cytotoxicity of CdTe/CdS/ZnS core–multishell QDs as compared to CdTe and CdTe/CdS QDs, suggesting their high potential for applications in biomedical imaging and diagnostics. Copyright © 2014 John Wiley & Sons, Ltd.     

Aqueous synthesis of CdTe/CdS/ZnS quantum dots and their optical and chemical properties

In this paper, we described a strategy for synthesis of thiol‐coated CdTe/CdS/ZnS (core–shell–shell) quantum dots (QDs) via aqueous synthesis approach. The synthesis conditions were systematically optimized, which included the size of CdTe core, the refluxing time and the number of monolayers and the ligands, and then the chemical and optical properties of the as‐prepared products were investigated. We found that the mercaptopropionic acid (MPA)‐coated CdTe/CdS/ZnS QDs presented highly photoluminescent quantum yields (PL QYs), good photostability and chemical stability, good salt tolerance and pH tolerance and favorable biocompatibility. The characterization of high‐resolution transmission electron microscopy (HRTEM), X‐ray powder diffraction (XRD) and fluorescence correlation spectroscopy (FCS) showed that the CdTe/CdS/ZnS QDs had good monodispersity and crystal structure. The fluorescence life time spectra demonstrated that CdTe/CdS/ZnS QDs had a longer lifetime in contrast to fluorescent dyes and CdTe QDs. Furthermore, the MPA‐stabilized CdTe/CdS/ZnS QDs were applied for the imaging of cells. Compared with current synthesis methods, our synthesis approach was reproducible and simple, and the reaction conditions were mild. More importantly, our method was cost‐effective, and was very suitable for large‐scale synthesis of CdTe/CdS/ZnS QDs for future applications. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and photophysical studies of CdTe quantum dot-monosubstituted zinc phthalocyanine conjugates

The linkage of unsymmetrically monosubstituted 4-aminophenoxy zinc phthalocyanine (ZnAPPc, 5) to CdTe quantum dots capped with mercaptopropionic acid (MPA), l-cysteine (l-cys) or thioglycolic acid (TGA) has been achieved using the coupling agents ethyl-N(3-dimethylaminopropyl) carbodiimide and N-hydroxy succinimide, which facilitate formation of an amide bond to form the QD-ZnAPPc-linked conjugate. The formation of the amide bond was confirmed using Raman and IR spectroscopies. Atomic force microscopy (AFM) and UV-Vis spectroscopy were used further to characterise the conjugate. Förster resonance energy transfer (FRET) resulted in stimulated emission of ZnAPPc in both the linked (QD-ZnAPPc-linked) and mixed (QD:ZnAPPc-mixed) conjugates. The linked l-cys and TGA QDs conjugates (QD-ZnAPPc-linked) gave the largest FRET efficiencies hence showing the advantages of covalent linking. Fluorescence quantum yields of QDs were decreased in QD:ZnAPPc-mixed and QD:ZnAPPc-linked.     

L‐Cysteine capped CdTe–CdS core–shell quantum dots: preparation,characterization and immuno‐labeling of HeLa cells

Functionalized CdTe–CdS core–shell quantum dots (QDs) were synthesized in aqueous solution via water‐bathing combined hydrothermal method using L‐cysteine (L‐Cys) as a stabilizer. This method possesses both the advantages of water‐bathing and hydrothermal methods for preparing high‐quality QDs with markedly reduced synthesis time, and better stability than a lone hydrothermal method. The QDs were characterized by transmission electronic microscopy and powder X‐ray diffraction and X‐ray photoelectron spectroscopy. The CdTe–CdS QDs with core–shell structure showed both enhanced fluorescence and better photo stability than nude CdTe QDs. After conjugating with antibody rabbit anti‐CEACAM8 (CD67), the as‐prepared l ‐Cys capped CdTe–CdS QDs were successfully used as fluorescent probes for the direct immuno‐labeling and imaging of HeLa cells. It was indicated that this kind of QD would have application potential in bio‐labeling and cell imaging. Copyright © 2009 John Wiley & Sons, Ltd.     

Facile synthesis and photophysical characterization of luminescent CdTe quantum dots for Forster resonance energy transfer based immunosensing of staphylococcal enterotoxin B

Aqueous phase synthesis of CdTe quantum dots (QDs) with surface functionalization for bioconjugation remains the best approach for biosensing and bioimaging applications. We present a facile aqueous phase method to prepare CdTe QDs by adjusting precursor and ligand concentrations. CdTe QDs had photoluminescence quantum yield up to ≈33% with a narrow spectral distribution. The powder X‐ray diffraction profile elucidated characteristic broad peaks of zinc blende cubic CdTe nanoparticles with 2.5–3 nm average crystalline size having regular spherical morphology as revealed by transmission electron microscopy. Infra‐red spectroscopy confirmed disappearance of characteristic absorptions for –SH thiols inferring thiol coordinated CdTe nanoparticles. The effective molar concentration of 1 : 2.5 : 0.5 respectively for Cd²⁺/3‐mercaptopropionic acid/HTe^– at pH 9 ± 0.2 resulted in CdTe quantum dots of 2.2–3.06 nm having band gap in the range 2.74–2.26 eV respectively. Later, QD₅₂₃ and QD₆₀₁ were used for monitoring staphylococcal enterotoxin B (SEB; a bacterial superantigen responsible for food poisoning) using Forster resonance energy transfer based two QD fluorescence. QD₅₂₃ and QD₆₀₁ were bioconjugated to anti‐SEB IgY antibody and SEB respectively according to carbodiimide protocol. The mutual affinity between SEB and anti‐SEB antibody was relied upon to obtain efficient energy transfer between respective QDs resulting in fluorescence quenching of QD₅₂₃ and fluorescence enhancement of QD₆₀₁. Presence of SEB in the range 1–0.05 µg varied the rate of fluorescence quenching of QD₅₂₃, thereby demonstrating efficient use of QDs in the Forster resonance energy transfer based immunosensing method by engineering the QD size. Copyright © 2012 John Wiley & Sons, Ltd.     

Photoluminescence properties of hybrid SiO2‐coated CdTe/CdSe quantum dots

Hybrid SiO₂‐coated CdTe/CdSe quantum dots (QDs) were prepared using CdTe/CdSe QDs prepared by hydrothermal synthesis. A CdSe interlayer made CdTe/CdSe cores with unique type II heterostructures. The hybrid SiO₂‐coated CdTe/CdSe QDs revealed excellent photoluminescence (PL) properties compared with hybrid SiO₂‐coated CdTe QDs. Because of the existence of spatial separations of carriers in the type II CdTe/CdSe core/shell QDs, the hybrid QDs had a relatively extended PL lifetime and high stability in phosphate‐buffered saline buffer solutions. This is ascribed to the unique components and stable surface state of hybrid SiO₂‐coated CdTe/CdSe QDs. During the stabilization test in phosphate‐buffered saline buffer solutions, both static and dynamic quenching occurred. The quenching mechanism of the hybrid QDs was not suited with the Stern–Volmer equation. However, the relative stable surface of CdTe/CdSe QDs resulted in lower degradation and relative high PL quantum yields compared with hybrid SiO₂‐coated CdTe QDs. As a result, hybrid SiO₂‐coated CdTe/CdSe QDs can be used in bioapplications. Copyright © 2013 John Wiley & Sons, Ltd.     

Protein–ligand structure guided by backbone and side-chain proton chemical shift perturbations

The fragment-based drug design approach consists of screening libraries of fragment-like ligands, to identify hits that typically bind the protein target with weak affinity ( \(100\,\upmu \hbox {M}\) –5 mM). The determination of the protein–fragment complex 3D structure constitutes a crucial step for uncovering the key interactions responsible for the protein–ligand recognition, and for growing the initial fragment into potent active compounds. The vast majority of fragments are aromatic compounds that induce chemical shift perturbations (CSP) on protein NMR spectra. These experimental CSPs can be quantitatively used to guide the ligand docking, through the comparison between experimental CSPs and CSP back-calculation based on the ring current effect. Here we implemented the CSP back-calculation into the scoring function of the program PLANTS. We compare the results obtained with CSPs measured either on amide or aliphatic protons of the human peroxiredoxin 5. We show that the different kinds of protons lead to different results for resolving the 3D structures of protein–fragment complexes, with the best results obtained with the \(\hbox {H}_{\alpha }\) protons.     

CdSe and CdSe/CdS core–shell QDs: New approach for synthesis,investigating optical properties and application in pollutant degradation

In this work, CdSe quantum dots (QDs) were synthesized by a simple and rapid microwave activated approach using CdSO₄, Na₂SeO₃ as precursors and thioglycolic acid (TGA) as capping agent molecule. A novel photochemical approach was introduced for the growth of CdS QDs and this approach was used to grow a CdS shell around CdSe cores for the formation of a CdSe/CdS core–shell structure. The core–shells were structurally verified using X‐ray diffraction, transmission electron microscopy and FTIR (Fourier‐transform infrared (FTIR)) spectroscopy. The optical properties of the samples were examined by means of UV–Vis and photoluminescence (PL) spectroscopy. It was found that CdS QDs emit a broad band white luminescence between 400 to 700 nm with a peak located at about 510 nm. CdSe QDs emission contained a broad band resulting from trap states between 450 to 800 nm with a peak located at 600 nm. After CdS shell growth, trap states emission was considerably quenched and a near band edge emission was appeared about 480 nm. Optical studies revealed that the core–shell QDs possess strong ultraviolet (UV) ? visible light photocatalytic activity. CdSe/CdS core–shell QDs, showed an enhancement in photodegradation of Methyl orange (MO) compared with CdSe QDs.     

SERS and integrative imaging upon internalization of quantum dots into human oral epithelial cells

CdTe quantum dots (QDs) are widely used in bio‐applications due to their size and highly efficient optical properties. However internalization mechanisms thereof for the variety of freshly extracted, not cultivated human cells and their specific molecular interactions remains an open topic for discussion. In this study, we assess the internalization mechanism of CdTe quantum dots (3.3 nm) capped with thioglycolic acid using non cultivated oral epithelial cells obtained from healthy donors. Naked gold nanoparticles (40 nm) were successfully used as nanosensors for surface‐enhanced Raman spectroscopy to efficiently identify characteristic Raman peaks, providing new evidence indicating that the first interactions of these QDs with epithelial cells occurred preferentially with aromatic rings and amine groups of amino acid residues and glycans from trans‐membrane proteins and cytoskeleton. Using an integrative combination of advanced imaging techniques, including ultra‐high resolution SEM, high resolution STEM coupled with EDX spectroscopy together with the results obtained by Raman spectroscopy, it was determined that thioglycolic acid capped CdTe QDs are efficiently internalized into freshly extracted oral epithelial cells only by facilitated diffusion, distributed into cytoplasm and even within the cell nucleus in three minutes.

     

Quantifying engineered nanomaterial toxicity: comparison of common cytotoxicity and gene expression measurements

Background

When evaluating the toxicity of engineered nanomaterials (ENMS) it is important to use multiple bioassays based on different mechanisms of action. In this regard we evaluated the use of gene expression and common cytotoxicity measurements using as test materials, two selected nanoparticles with known differences in toxicity, 5 nm mercaptoundecanoic acid (MUA)-capped InP and CdSe quantum dots (QDs). We tested the effects of these QDs at concentrations ranging from 0.5 to 160 µg/mL on cultured normal human bronchial epithelial (NHBE) cells using four common cytotoxicity assays: the dichlorofluorescein assay for reactive oxygen species (ROS), the lactate dehydrogenase assay for membrane viability (LDH), the mitochondrial dehydrogenase assay for mitochondrial function, and the Comet assay for DNA strand breaks.

Results

The cytotoxicity assays showed similar trends when exposed to nanoparticles for 24 h at 80 µg/mL with a threefold increase in ROS with exposure to CdSe QDs compared to an insignificant change in ROS levels after exposure to InP QDs, a twofold increase in the LDH necrosis assay in NHBE cells with exposure to CdSe QDs compared to a 50% decrease for InP QDs, a 60% decrease in the mitochondrial function assay upon exposure to CdSe QDs compared to a minimal increase in the case of InP and significant DNA strand breaks after exposure to CdSe QDs compared to no significant DNA strand breaks with InP. High-throughput quantitative real-time polymerase chain reaction (qRT-PCR) data for cells exposed for 6 h at a concentration of 80 µg/mL were consistent with the cytotoxicity assays showing major differences in DNA damage, DNA repair and mitochondrial function gene regulatory responses to the CdSe and InP QDs. The BRCA2, CYP1A1, CYP1B1, CDK1, SFN and VEGFA genes were observed to be upregulated specifically from increased CdSe exposure and suggests their possible utility as biomarkers for toxicity.

Conclusions

This study can serve as a model for comparing traditional cytotoxicity assays and gene expression measurements and to determine candidate biomarkers for assessing the biocompatibility of ENMs.

     

Penta- and heteropentadienyl ligands coordinated to beryllium

In this work we have performed a systematic study of new organometallic complexes containing penta- and heteropentadienyl (CH₂CHCHCHX, X?=?CH₂, O, NH, S) ligands coordinated to beryllium. Calculated complexes were studied using the density functional theory (PBE) in combination with the 6-311++G(3d,2p) basis set. The coordination number on the beryllium atom varies according to the type of ligand. Pentadienyl ligand shows hapticities η¹ and η⁵, while heteropentadienyl ligands display η¹ and η² hapticities. A Wiberg bond indices study was performed in order to get information about their bond orders.

Entropy versus aromaticity in the conformational dynamics of aromatic rings

Comparison of the results of Car-Parrinello molecular dynamics simulations of isolated benzene, pyrimidine and 1,2,4-triazine molecules reveals that the unusually low population of planar geometry of the benzene ring is caused by entropy effects despite its high aromaticity. The decrease in symmetry of the molecule results in smaller changes in entropy and Gibbs free energy due to out-of-plane deformations of the ring, leading to an increase in the population of planar geometry of the ring. This leads to differences in the topology of potential energy and Gibbs free energy surfaces.

Core–shell structured CdTe/CdS@SiO2@CdTe@SiO2 composite fluorescent spheres: Synthesis and application for Cd2+ detection

Core–shell structured quantum dot (QD)–silica fluorescent nanoparticles have attracted a great deal of attention due to the excellent optical properties of QDs and the stability of silica. In this study, core–shell structured CdTe/CdS@SiO₂@CdTe@SiO₂ fluorescent nanospheres were synthesized based on the Stöber method using multistep silica encapsulation. The second silica layer on the CdTe QDs maintained the optical stability of nanospheres and decreased adverse influences on the probe during subsequent processing. Red‐emissive CdTe/CdS QDs (630 nm) were used as a built‐in reference signal and green‐emissive CdTe QDs (550 nm) were used as a responding probe. The fluorescence of CdTe QDs was greatly quenched by added S^2?, owing to a S^2?‐induced change in the CdTe QDs surface state in the shell. Upon addition of Cd²⁺ to the S^2?‐quenched CdTe/CdS@SiO₂@CdTe@SiO₂ system, the responding signal at 550 nm was dramatically restored, whereas the emission at 630 nm remained almost unchanged; this response could be used as a ratiometric ‘off–on’ fluorescent probe for the detection of Cd²⁺. The sensing mechanism was suggested to be: the newly formed CdS‐like cluster with a higher band gap facilitated exciton/hole recombination and effectively enhanced the fluorescence of the CdTe QDs. The proposed probe shows a highly sensitive and selective response to Cd²⁺ and has potential application in the detection of Cd²⁺ in environmental or biological samples.     

Interaction properties of biosynthesized cadmium sulphide quantum dots with human serum albumin: further investigation of antibacterial activities and sensing applications

The synthesis of small-sized quantum dots (QDs) (1–10 nm) via the green route has garnered great interest regarding their prospective use in many biological applications (diagnosis, drug delivery and in vivo sensing); this is difficult to achieve using chemical synthesis methods, which produce larger size QD particles and also require hazardous reagents. Here, we synthesized biogenic cadmium sulphide (CdS) QDs using green tea extract as the reducing agent to produce particles that were homogeneous and a smaller size of 2–4 nm. We also elucidated the (a) protein binding, (b) antibacterial use and (c) sensing applications of biogenic CdS QDs in this present work. The biosynthesized CdS QDs were found to have extensive antibacterial activity against both Gram-negative Escherichia coli and Gram-positive Enterococcus faecalis bacterial strains. The introduction of QDs in biological medium can lead to the formation of protein–QD complexes; therefore we investigated the binding interaction of CdS QDs with the carrier protein human serum albumin (HSA) in vitro. The synthesized CdS QDs quenched the intrinsic fluorescence of HSA through a static quenching mechanism and the binding constant (K_b) was in the order of 10⁴ M⁻¹. It was also observed that the presence of biogenic CdS QDs affected the HSA–ligand interactions in vitro. The synthesized CdS made highly effective sensors for tetracycline, rifampicin, and bilirubin with limit of detection (LOD) values of 99, 141 and 29 ng/ml, respectively.     

Multi-Color Single Particle Tracking with Quantum Dots

Quantum dots (QDs) have long promised to revolutionize fluorescence detection to include even applications requiring simultaneous multi-species detection at single molecule sensitivity. Despite the early promise, the unique optical properties of QDs have not yet been fully exploited in e. g. multiplex single molecule sensitivity applications such as single particle tracking (SPT). In order to fully optimize single molecule multiplex application with QDs, we have in this work performed a comprehensive quantitative investigation of the fluorescence intensities, fluorescence intensity fluctuations, and hydrodynamic radii of eight types of commercially available water soluble QDs. In this study, we show that the fluorescence intensity of CdSe core QDs increases as the emission of the QDs shifts towards the red but that hybrid CdSe/CdTe core QDs are less bright than the furthest red-shifted CdSe QDs. We further show that there is only a small size advantage in using blue-shifted QDs in biological applications because of the additional size of the water-stabilizing surface coat. Extending previous work, we finally also show that parallel four color multicolor (MC)-SPT with QDs is possible at an image acquisition rate of at least 25 Hz. We demonstrate the technique by measuring the lateral dynamics of a lipid, biotin-cap-DPPE, in the cellular plasma membrane of live cells using four different colors of QDs; QD565, QD605, QD655, and QD705 as labels.     

Comparison of quantum dot-binding protein tags: Affinity determination by ultracentrifugation and FRET

Background

Hybrid complexes of proteins and colloidal semiconductor nanocrystals (quantum dots, QDs) are of increasing interest in various fields of biochemistry and biomedicine, for instance for biolabeling or drug transport. The usefulness of protein–QD complexes for such applications is dependent on the binding specificity and strength of the components. Often the binding properties of these components are difficult and time consuming to assess.

Methods

In this work we characterized the interaction between recombinant light harvesting chlorophyll a/b complex (LHCII) and CdTe/CdSe/ZnS QDs by using ultracentrifugation and fluorescence resonance energy transfer (FRET) assay experiments. Ultracentrifugation was employed as a fast method to compare the binding strength between different protein tags and the QDs. Furthermore the LHCII:QD stoichiometry was determined by separating the protein–QD hybrid complexes from unbound LHCII via ultracentrifugation through a sucrose cushion.

Results

One trimeric LHCII was found to be bound per QD. Binding constants were evaluated by FRET assays of protein derivatives carrying different affinity tags. A new tetra-cysteine motif interacted more strongly (K_a = 4.9 ± 1.9 nM^− 1) with the nanoparticles as compared to a hexahistidine tag (His₆ tag) (K_a ~ 1 nM^− 1).

Conclusion

Relative binding affinities and binding stoichiometries of hybrid complexes from LHCII and quantum dots were identified via fast ultracentrifugation, and binding constants were determined via FRET assays.

General significance

The combination of rapid centrifugation and fluorescence-based titration will be useful to assess the binding strength between different types of nanoparticles and a broad range of proteins.     

Modeling the effect of H-bonding interactions and molecular packing on the molecular structure of [Ag(ethylnicotinate)2]NO3 complex

The gas phase molecular structure of a single isolated molecule of [Ag(Etnic)₂NO₃];1 where Etnic = Ethylnicotinate was calculated using B3LYP method. The H-bonding interaction between 1 with one (complex 2) and two (complex 3) water molecules together with the dimeric formula [Ag(Etnic)₂NO₃]₂;4 and the tetrameric formula [Ag(Etnic)₂NO₃]₄;5 were calculated using the same level of theory to model the effect of intermolecular interactions and molecular packing on the molecular structure of the titled complex. The H-bond dissociation energies of complexes 2 and 3 were calculated to be in the range of 12.220–14.253 and 30.106–31.055 kcal?mol^?1, respectively, indicating the formation of relatively strong H-bonds between 1 and water molecules. The calculations predict bidentate nitrate ligand in the case of 1 and 2, leading to distorted tetrahedral geometry around the silver ion with longer Ag–O distances in case of 2 compared to 1, while 3 has a unidentate nitrate ligand leading to a distorted trigonal planar geometry. The packing of two [Ag(Etnic)₂NO₃] complex units; 4 does not affect the molecular geometry around Ag(I) ion compared to 1. In the case of 5, the two asymmetric units of the formula [Ag(Etnic)₂NO₃] differ in the bonding mode of the nitrate group, where the geometry around the silver ion is distorted tetrahedral in one unit and trigonal planar in the other. The calculations predicted almost no change in the charge densities at the different atomic sites except at the sites involved in the C–H?O interactions as well as at the coordinated nitrogen of the pyridine ring.

Relation between topology and stability of bent titanocenes

Bent metallocenes are a class of organometallic compounds that are widely used as catalysts in olefin polymerization procedures. We found a linear relation between the relative stability of bent titanocenes and the average delocalization index (DI) for Ti–C (from the cyclopentadienyl ring) atomic pairs within the evaluated compounds. As a consequence, the stability of the bent titanocenes can be estimated from their topologies. However, secondary interactions between the ligands of some of the bent titanocenes reduce the coefficient of determination for the average DI–stability relation.

Highly luminescent hybrid SiO2‐coated CdTe quantum dots: synthesis and properties

Novel hybrid SiO₂‐coated CdTe quantum dots (QDs) were created using CdTe QDs coated with a hybrid SiO₂ shell containing Cd²⁺ ions and a sulfur source via a sol–gel process in aqueous solution. Aqueous CdTe QDs with tunable emitting color created through a reaction between cadmium chloride and sodium hydrogen telluride was used as cores for the preparation of hybrid SiO₂‐coated CdTe QDs. In our experiments we found that the surface state of the cores and preparation conditions that affect the formation of the hybrid SiO₂ shell also greatly affect photoluminescence of the hybrid SiO₂‐coated CdTe QDs. The generation of CdS‐like clusters in the vicinity of the CdTe QDs, caused the quantum size effect of the QDs to be greatly reduced, which changes photoluminescence properties of the hybrid QDs fundamentally. Namely, the novel hybrid SiO₂ shell played an important role in generating a series of specific optical properties. In addition, the novel hybrid SiO₂ shell can be created if no CdTe QD is added. In order to gain an insight into the inter structure of the hybrid shell, we characterized the hybrid SiO₂‐coated CdTe QDs using X‐ray diffraction analysis and discuss the formation mechanism of such a hybrid structure. This work is significant because the novel hybrid SiO₂‐coated CdTe QDs with its excellent properties can be used in many applications, such as biolabeling and optoelectronic devices. Copyright © 2013 John Wiley & Sons, Ltd.     

Molecular spectroscopic studies on the interactions of rhein and emodin with thioglycolic acid‐capped core/shell CdTe/CdS quantum dots and their analytical applications

Water‐soluble thioglycolic acid (TGA)‐capped core/shell CdTe/CdS quantum dots (QDs) were synthesized. The interactions of rhein and emodin with TGA‐CdTe/CdS QDs were evaluated by fluorescence and ultraviolet‐visible absorption spectroscopy. Experimental results showed that the high fluorescence intensity of TGA‐CdTe/CdS QDs could be effectively quenched in the presence of rhein (or emodin) at 570 nm, which may have resulted from an electron transfer process from excited TGA‐CdTe/CdS QDs to rhein (or emodin). The quenching intensity was in proportion to the concentration of both rhein and emodin in a certain range. Under optimized conditions, the linear ranges of TGA‐CdTe/CdS QDs fluorescence intensity versus the concentration of rhein and emodin were 0.09650–60 µg/mL and 0.1175–70 µg/mL with a correlation coefficient of 0.9984 and 0.9965, respectively. The corresponding detection limits (3σ/S) of rhein and emodin were 28.9 and 35.2 ng/mL, respectively. This proposed method was applied to determine rhein and emodin in human urine samples successfully with remarkable advantages such as high sensitivity, short analysis time, low cost and easy operation. Based on this, a simple, rapid and highly sensitive method to determine rhein (or emodin) was proposed. Copyright © 2014 John Wiley & Sons, Ltd.