Sensitive and selective determination of Cu2+ by electrochemiluminescence of CdTe quantum dots

For the first time, we report a sensitive and selective method to detect Cu²⁺ based on the electrochemiluminescence quenching of CdTe quantum dots (QDs) in aqueous solution. The mercaptosuccinic acid (MSA) protected CdTe QDs were prepared and characterized with UV, fluorescence and ECL. The anodic ECL quenching mechanism was attributed to the fact that MSA capping was removed from the surface of the CdTe QDs and preferentially bound with Cu²⁺. The displacement of MSA capping layer created imperfections on the CdTe QDs surface, and eventually led to the ECL quenching. The quenching effect of Cu²⁺ on the anodic ECL of CdTe QDs was found to be selective and concentration dependent, so we applied it to develop a method for the sensitive and selective detection of Cu²⁺. With the proposed method, the concentration of Cu²⁺ could be detected in the range of sub-nanomolar to micromolar levels.     

Formation of Surface Traps on Quantum Dots by Bidentate Chelation and Their Application in Low‐Potential Electrochemiluminescent Biosensing

Bidentate chelation, meso‐2,3‐dimercaptosuccinic acid (DMSA), was used as a stabilizer for the synthesis of CdTe quantum dots (QDs). The bidentate chelate QDs, characterized with FT‐IR, PL, and UV/Vis spectroscopy; element analysis; and high‐resolution transmission electron microscope, exhibited surface traps due to the large surface/volume ratio of QD particle and the steric hindrance of the DMSA molecule. The unpassivated surface of the QDs produced a narrower band gap than the core and electrochemiluminescent (ECL) emission at relatively low cathodic potential. In air‐saturated pH 7.0 buffer, the QDs immobilized on electrode surface showed an intense ECL emission peak at ?0.85 V (vs. Ag/AgCl). H₂O₂ produced from electrochemical reduction of dissolved oxygen was demonstrated to be the co‐reactant, which avoided the need of strong oxidant as the co‐reactant and produced a sensitive analytical method for peroxidase‐related analytes. Using hydroquinone/horseradish peroxidase/H₂O₂ as a model system, a new, reagentless, phenolic, ECL biosensor for hydroquinone was constructed, based on the quenching effect of ECL emission of QDs by consumption of co‐reactant H₂O₂. The biosensor showed a linear range of 0.2–10 μM with acceptable stability and reproducibility. This work opens new avenues in the search for new ECL emitters with excellent analytical performance and makes QDs a more attractive alternative in biosensing.     

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.     

Fluorescence enhancement of CdTe/CdS quantum dots by coupling of glyphosate and its application for sensitive detection of copper ion

A novel fluorescent probe for Cu²⁺ determination based on the fluorescence quenching of glyphosate (Glyp)-functionalized quantum dots (QDs) was firstly reported. Glyp had been used to modify the surface of QDs to form Glyp-functionalized QDs following the capping of thioglycolic acid on the core–shell CdTe/CdS QDs. Under the optimal conditions, the response was linearly proportional to the concentration of Cu²⁺ between 2.4 × 10⁻² μg mL⁻¹ and 28 μg mL⁻¹, with a detection limit of 1.3 × 10⁻³ μg mL⁻¹ (3δ). The Glyp-functionalized QDs fluorescent probe offers good sensitivity and selectivity for detecting Cu²⁺. The fluorescent probe was successfully used for the determination of Cu²⁺ in environmental samples. The mechanism of reaction was also discussed.     

Highly Sensitive and Selective Fluorescence Probe for 2,4‐Dinitrophenylhydrazine Detection in Wastewater Using Water‐Soluble CdTe QDs

A simple, cheap, sensitive and selective probe for determination of DNPH in wastewater using thioglycolic acid (TGA)‐coated CdTe QDs (TGA‐QDs) as fluorescence probe has been established, and the properties of CdTe QDs were characterized by TEM, FT‐IR, DLS, XRD and zeta potentials. CdTe QDs fluorescence is highly efficiently quenched after adding DNPH on account of electron transfer effect, and the fluorescence quenching behavior of CdTe QDs interaction with DNPH is static quenching process. A good linear relationship is observed between the relative fluorescence intensity (F₀/F) and 0.06–10 ng mL^?1 of DNPH. As compared with some of reported methods, LOD of this method for analysis of DNPH (0.23 ng mL^?1) is the lowest. Masking agents of DDTC and NH₄OH can eliminate the interference of Cu²⁺, Ag⁺ and Hg²⁺. Hence, DNPH can be selectively and accurately detected and the established method was successfully used for detecting DNPH in wastewater with acceptable recovery of 90.6–102%.     

Multi-spectroscopic Study on the Interaction between CdTe Quantum Dots and Bovine Serum Albumin

The interaction between CdTe quantum dots (QDs) and bovine serum albumin (BSA) was systematically investigated by fluorescence, UV‐vis absorption and circular dichroism (CD) spectroscopy under physiological conditions. The experimental results showed that the fluorescence of BSA could be quenched by CdTe QDs with a static quenching mechanism, indicating that CdTe QDs could react with BSA. The quenching constants according to the modified Stern‐Volmer equation were obtained as 1.710×10⁶, 1.291×10⁶ and 1.010×10⁶ L·mol^?1 at 298, 304, and 310 K, respectively. ΔH, ΔS and ΔG for CdTe QDs‐BSA system were calculated to be ?33.68 kJ·mol^?1, 6.254 J·mol^?1·K^?1 and ?35.54 kJ·mol^?1 (298 K), respectively, showing that electrostatic interaction in the system played a major role. According to F?rster theory, the distance between Trp‐214 in BSA and CdTe QDs was given as 2.18 nm. The UV‐vis, synchronous fluorescence and CD spectra confirmed further that the conformations of BSA after addition of CdTe QDs have been changed.     

CdTe量子点与蛋白质相互作用的荧光猝灭效应

本文在水热法合成水溶性CdTe及核壳结构CdTe/CdS量子点的基础上,分别研究了细胞色素c对CdTe量子点及CdTe/CdS核壳量子点荧光的猝灭效应和CdTe量子点对牛血清白蛋白荧光的猝灭效应,并阐述了猝灭机理。结果显示,细胞色素c对CdTe量子点的荧光猝灭效应具有一定的粒径依赖性,粒径越小,猝灭效应越强;细胞色素c对CdTe/CdS核壳量子点的猝灭效应比对CdTe量子点的更强,揭示了受激电子的表面传递机理。CdTe量子点通过松散牛血清白蛋白的螺旋结构而猝灭其荧光。     

Selective Detection of Mercury（Ⅱ） and Copper（Ⅱ） Based on the Opposite Size-dependent Fluorescence Quenching of CdTe Quantum Dots

Three different size CdTe quantum dots （QDs） capped by 3-mercaptopropionic acid （MPA） have been prepared in aqueous solutions, and their interactions with Cu^2＋ and Hg^2＋ have been investigated. The opposite size-dependent fluorescence quenching of CdTe QDs by Hg^2＋ and Cu^2＋ was observed： Hg^2＋ quenched smaller particles more efficiently than larger ones while larger particles were more markedly quenched by Cu^2＋. Based on the different size responses, Hg^2＋ and Cu^2＋ were respectively detected with high sensitivity and selectivity, for the first time, using the QDs with different sizes but the same components and capping ligands.     

High Sensibility of Quantum Dots to Metal Ions Inspired by Hydroxyapatite Microbeads

An approach for the sensitive and selective determination of Ag⁺, Cu²⁺ and Hg²⁺ ions was developed based on the fluorescence quenching of mercaptopropionic acid (MPA) capped CdTe quantum dots in the existence of hydroxyapatite (HAP) nanoribbon spherulites. Among various metal ions investigated, it was found that the fluorescence of CdTe QDs was only sensitive to Ag⁺, Cu²⁺ and Hg²⁺ ions. The addition of HAP into the CdTe system could bring forward a sensitivity improvement of about 1 to 2 orders of magnitude in the detection of Ag⁺ and Cu²⁺ compared with the plain CdTe system without the existence of HAP; while there was no sensitization effect for Hg²⁺. Under optimal conditions, the detection limits for Ag⁺, Cu²⁺ and Hg²⁺ were 20, 56 and 3.0 nmol·L^?1, respectively, and the linear ranges were 0.02–50, 0.056–54 and 0.003–2.4 µmol·L^?1, respectively. Mechanisms of both QDs fluorescence quenching by metal ions and the sensitization effect by HAP were also discussed.     

Electrogenerated Chemiluminescence Sensor Based on Tris(2,2′‐bipyridine)ruthenium(II)‐Immobilized Natural Clay and Ionic Liquid

A novel electrogenerated chemiluminescence (ECL) sensor based on natural clay and ionic liquid was fabricated. Tris(2,2′‐bipyridine)ruthenium(II) (Ru(bpy)₃²⁺) was immobilized on natural clay surface through simple adsorption. An ECL sensor was prepared by mixing Ru(bpy)₃²⁺‐incorporated clay, graphite powder and an ionic liquid (1‐butyl‐3‐methylimidazolium hexafluorophosphate) as the binder. The electrochemical behavior and ECL of the immobilized Ru(bpy)₃²⁺ was investigated. It was observed that the ECL of immobilized Ru(bpy)₃²⁺ was activated by the ionic liquid. The proposed ECL sensor showed high sensitivity to tri‐n‐propylamine (TPrA) and the detection limit was found to be 20 pM. In addition, the ECL sensor displayed good stability for TPrA detection and long‐term storage stability.     

Probing the mechanism of the interaction between l‐cysteine‐capped‐CdTe quantum dots and Hg2+ using capillary electrophoresis with ensemble techniques

A good understanding of the mechanism of interaction between quantum dots (QDs) and heavy metal ions is essential for the design of more effective sensor systems. In this work, CE was introduced to explore how l ‐cysteine‐capped‐CdTe QDs (l ‐cys‐CdTe QDs) interacts with Hg²⁺. The change in electrophoretic mobility can synchronously reflect the change in the composition and property of QDs. The effects of the free and capping ligands on the system are discussed in detail. ESI‐MS, dynamic light scattering (DLS), zeta potential, and fluorescence (FL) were also applied as cooperative tools to study the interaction mechanism. Furthermore, the interaction mechanism, which principally depended on the concentration of Hg²⁺, was proposed reasonably. At the low concentration of Hg²⁺, the formation of a static complex between Hg²⁺ and the carboxyl and amino groups of l ‐cys‐CdTe QDs surface was responsible for the FL quenching. With the increase of Hg²⁺ concentration, the capping l ‐cys was stripped from the surface of l ‐cys‐CdTe QDs due to the high affinity of Hg²⁺ to the thiol group of l ‐cys. Our study demonstrates that CE can reveal the mechanism of the interaction between QDs and heavy metal ions, such as FL quenching.     

Size-dependent electrochemiluminescence behavior of water-soluble CdTe quantum dots and selective sensing of l-cysteine

Water-soluble CdTe quantum dots (QDs) with five sizes (2.25, 2.50, 2.77, 3.12, and 3.26 nm) were synthesized with the hydrothermal method. The electrochemiluminescence (ECL) of CdTe QDs was investigated in detail in air-saturated solution without adding foreign oxidant. It was found that the ECL of CdTe QDs displayed a size-dependent property. With the increasing in the particle size of the CdTe QDs, the ECL intensity was gradually increased, in addition, both ECL peak potentials and ECL onset potentials of CdTe QDs were shifted positively. Influences of some factors on the ECL intensity were investigated. Under the optimal conditions, the ECL intensity had a linear relationship with the concentration of l-cysteine (l-Cys) in the range from 1.3 × 10⁻⁶ to 3.5 × 10⁻⁵ mol L⁻¹ (R² 0.996) with a detection limit of 8.7 × 10⁻⁷ mol L⁻¹ (S/N = 3). The proposed method was applied to the determination of l-Cys in real samples with satisfactory results. Compared with previous reports, it has better selectivity for the determination of l-Cys.     

Electrochemiluminescence of Supramolecular Nanorods and Their Application in the “On–Off–On” Detection of Copper Ions

In this work, an “on–off–on” switch system has been successfully applied through the construction of an electrochemiluminscent biosensor for copper ion (Cu²⁺) detection based on a new electrochemiluminescence (ECL) emitter of supramolecular nanorods, which was achieved through supramolecular interactions between 3,4,9,10‐perylenetetracarboxylic acid (PTCA) and aniline. The initial “signal‐on” state with strong and stable ECL emission was obtained by use of the supramolecular nanorods with a new signal amplification strategy involving a co‐reaction accelerator. In addition, ECL quencher probes (Fc‐NH₂/Cu‐Sub/nano‐Au) were fabricated by immobilizing aminoferrocene (Fc‐NH₂) on Cu‐substrate strand modified Au nanoparticles. The quencher probes were hybridized with the immobilized Cu‐enzyme strand to form Cu²⁺‐specific DNAzyme. Similarly, the “signal‐off” state was obtained by the high quenching effect of Fc‐NH₂ on the ECL of the excited‐state PTCA (¹PTCA*). As expected, the second “switch‐on” state could achieved by incubating with the target Cu²⁺, owing to the Cu²⁺‐specific DNAzyme, which was irreversibly cleaved, resulting in the release of the quencher probes from the sensor interface. Herein, on the basis of the ECL intensity changes (ΔI_ECL) before and after incubating with the target Cu²⁺, the prepared Cu²⁺‐specific DNAzyme‐based biosensor was used for the determination of Cu²⁺ concentrations with high sensitivity, excellent selectivity, and good regeneration.     

Electrochemiluminescence of Dendritic Magnetic Quantum Dots Nanostructure and Its Quenching by Gold Nanoparticles for Cancer Cells Assay

A novel dendritic CdS‐ZnS‐Quantum Dots (QDs) nanocomposite with intense electrochemiluminescence (ECL) and excellent magnetism was prepared, which was applied to the cancer cells assay based on ECL quenching of QDs by gold nanoparticles (NPs). DNA conjugation, gold NPs linking and sensing target cells can be directly performed on the magnetic nanocomposites, which is more rapid, convenient, and has better reproducibility than the conventional methods. So far, this is the first report on magnetic electrochemiluminescent QDs nanocomposites for cell detection based on ECL quenching, which opens a new approach for developing multifunctional QDs nanocomposite for ECL assays of cancer cells.     

CdTe/CdS荧光探针测定痕量铜(Ⅱ)

以巯基乙酸为稳定剂,在水溶液中合成CdTe/CdS量子点,基于量子点与Cu2+混合后发生荧光猝灭作用,建立CdTe/CdS量子点作为荧光探针检测微量铜的新方法。在pH 4.60的HAc-NaAc缓冲溶液中,反应时间为10 min时,Cu2+质量浓度在0.01～1.00μg/mL范围与CdTe/CdS量子点的荧光猝灭程度呈良好的线性关系,相关系数为0.9978,检出限为9.90×10-3μg/mL。方法可以用于雨水、自来水和延河水中Cu2+的分析。     

Ultrafast synthesis of near-infrared-emitting aqueous CdTe/CdS quantum dots with high fluorescence

The traditional aqueous route to synthesis CdTe/CdS Core/shell (c/s) quantum dots (QDs) via decomposition of Cd-thiol complexes is usually time consuming. Herein, an ultrafast and facile aqueous synthetic approach under atmospheric pressure for CdTe/CdS c/s QDs with emission from the green to the near-infrared window (535–820 nm) is reported. With purified CdTe core QDs diluted in solution of Cd-3-mercaptopropionic acid (MPA) complexes, CdTe/CdS c/s QDs with emission wavelengths at 700 and 800 nm can be obtained within 20- and 45-min refluxing under the optimized experimental conditions, respectively. This is the most rapid way to prepare CdTe/CdS c/s QDs in aqueous phase, and the obtained QDs were highly luminescent without postsynthesis treatment. The influences of various experimental factors, including Cd²⁺ concentration, MPA-to-Cd ratio, pH value, and dilution ratio on the growth rate and luminescent properties of the obtained CdTe/CdS c/s QDs, have been taken into consideration. The three processes “purification-dilution-addition” ensure the synthesis environment with high pH value and low core concentration and have a marked impact on the rapid synthesis rate and the resulting high fluorescence of CdTe/CdS c/s QDs.     

Cadmium sulfide quantum dots modified by chitosan as fluorescence probe for copper (II) ion determination

CdS quantum dots (QDs) have been prepared and modified with chitosan. Based on the quenching of fluorescence signals of the functionalized CdS QDs at 531 nm wavelength and enhancement of signals the 400–700 nm wavelength range by Cu²⁺ at pH 4.2, a simple, rapid and specific method for Cu²⁺ determination is presented. Under optimum conditions, the relative fluorescence intensity of CdS QDs is linearly proportional to copper concentration from 8.0 nmol L^?1 to 3.0 μmol L^?1 with a detection limit of 1.2 nmol L^?1. The mechanism can be explained in terms of strong binding of Cu²⁺ onto the surface of CdS, resulting in a chemical displacement of Cd²⁺ ions and the formation of CuS on the surface of the QDs.     

Synchronous determination of mercury (II) and copper (II) based on quantum dots-multilayer film

A sensitive sensor for mercury (II) and copper (II) synchronous detection was established via the changed photoluminescence of CdTe quantum dots (QDs) multilayer films in this work. QDs were deposited on the quartz slides to form QDs-multilayer films by electrostatic interactions with poly(dimethyldiallyl ammonium chloride) (PDDA). Hg²⁺ or Cu²⁺ could quench the photoluminescence of the QDs-multilayer films, and glutathione (GSH) was used to remove Hg²⁺ or Cu²⁺ from QDs-multilayer films due to strong affinity of GSH-metal ions, which resulted in the recovered photoluminescence of QDs-multilayer films. There are good linear relationships between the metal ions concentration and the photoluminescence intensity of QDs in the quenched and recovered process. It was found that the Stern–Volmer constants for Hg²⁺ are higher than that for Cu²⁺. Based on different quenching and recovery constant between Hg²⁺ and Cu²⁺, the synchronous detection of Hg²⁺ and Cu²⁺ can be achieved. The linear ranges of this assay were obtained from 0.005 to 0.5 μM for Hg²⁺ and from 0.01 to 1 μM for Cu²⁺, respectively. And the artificial water samples were determined by this method with satisfactory results, the recoveries for Hg²⁺ and Cu²⁺ ions were found in the range of 90.4–106.4%. To the best of our knowledge, it is the first report about the synchronous detection of Hg²⁺ and Cu²⁺ by using quenched and recovered photoluminescence of quantum dots multilayer films.     

Amine‐Rich Nitrogen‐Doped Carbon Nanodots as a Platform for Self‐Enhancing Electrochemiluminescence

Amine‐rich nitrogen‐doped carbon nanodots (NCNDs) have been successfully used as co‐reactant in electrochemiluminescence (ECL) processes. Primary or tertiary amino groups on NCNDs have been studied as co‐reactant sites for Ru(bpy)₃²⁺ ECL, showing their eligibility as powerful alternatives to tripropylamine (TPrA). We also report the synthesis and ECL behavior of a new covalently linked hybrid of NCNDs and Ru(bpy)₃²⁺. Notably, the NCNDs in the hybrid act both as carrier for ECL labels and as co‐reactant for ECL generation. As a result, the hybrid shows a higher ECL emission as compared to the combination of the individual components, suggesting the self‐enhancing ECL of the ruthenium complex due to an intramolecular electron transfer process.     

L-Cysteine-coated CdSe/CdS core-shell quantum dots as selective fluorescence probe for copper(II) determination

Quantum dots (QDs) or semiconductor nanocrystals have been receiving great interest in the last few years. In this paper, L-cysteine-coated CdSe/CdS core-shell QDs (λ_em = 585 nm) have been prepared, which have excellent water-solubility. The full width at half maximum (FWHM) of the photoluminescence of these nanocrystals is very narrow (about 30 nm), and the quantum yield (QY) is 15% relative to Rhodamine 6G in ethanol (QY = 95%). With excess free L-cysteine in the solution, the fluorescence intensity of L-cysteine-coated CdSe/CdS QDs showed improved stability. It was found that the fluorescence of L-cysteine-capped CdSe/CdS QDs could be quenched only by copper (II) ions and was insensitive to other physiologically important cations, such as Ca²⁺, Mg²⁺, Zn²⁺, Al³⁺, Fe³⁺, Mn²⁺ and Ni²⁺ etc. Based on this finding, the quantitative analysis of Cu²⁺ with L-cysteine-capped CdSe/CdS QDs has been established. The linear range was from 1.0 × 10^− 8 to 2.0 × 10^− 7 mol L^− 1 and the limit of detection (LOD) was 3.0 × 10^− 9 mol L^− 1 (S/N = 3). The proposed method has first been applied to the determination of Cu²⁺ in vegetable samples with recoveries of 99.6–105.8%.