Determination of nitrite based on its quenching effect on anodic electrochemiluminescence of CdSe quantum dots

A novel method for electrochemiluminescent (ECL) detection of nitrite was proposed based on its quenching effect on anodic ECL emission of CdSe quantum dots (QDs). The ECL emission could be greatly enhanced by sulfite and dissolved oxygen in a neutral system and occurred at a relatively low potential in comparison with traditional anodic ECL emitter, leading to high sensitivity and good selectivity. The quenching mechanism followed an “electrochemical oxidation inhibition” process, which was completely different from those of some analytes on the ECL emission of QDs. The coincidence of photoluminescence and ECL spectra of the QDs indicated that the ECL emission resulted from the redox process of QDs core and the sulfite acted as a coreactant. The nitrite quenched ECL emission could be analyzed according to the treatment of Stern-Volmer equation with a linear range from 1 μM to 0.5 mM for detection of nitrite. This work presented a new efficient ECL methodology for quencher-related detection.     

Immuno-capture and in situ detection of Salmonella typhimurium on a novel microfluidic chip

The new method presented in this article achieved the goal of capturing Salmonella typhimurium via immunoreaction and rapid in situ detection of the CdSe/ZnS quantum dots (QDs) labeled S. typhimurium by self-assembly light-emitting diode-induced fluorescence detection (LIF) microsystem on a specially designed multichannel microfluidic chip. CdSe/ZnS QDs were used as fluorescent markers improving detection sensitivity. The microfluidic chip developed in this study was composed of 12 sample channels, 3 mixing zones, and 6 immune reaction zones, which also acted as fluorescence detection zones. QDs–IgG–primary antibody complexes were generated by mixing CdSe/ZnS QDs conjugated secondary antibody (QDs–IgG) and S. typhimurium antibody (primary antibody) in mixing zones. Then, the complexes went into immune reaction zones to label previously captured S. typhimurium in the sandwich mode. The capture rate of S. typhimurium in each detection zone was up to 70%. The enriched QDs-labeled S. typhimurium was detected using a self-assembly LIF microsystem. A good linear relationship was obtained in the range from 3.7 × 10 to 3.7 × 10⁵ cfu mL⁻¹ using the equation I = 0.1739 log (C) − 0.1889 with R² = 0.9907, and the detection limit was down to 37 cfu mL⁻¹. The proposed method of online immunolabeling with QDs for in situ fluorescence detection on the designed multichannel microfluidic chip had been successfully used to detect S. typhimurium in pork sample, and it has shown potential advantages in practice.     

Quantum dots based electrochemiluminescent immunosensor by coupling enzymatic amplification for ultrasensitive detection of clenbuterol

An ultrasensitive electrochemiluminescence (ECL) immunosensor based on CdSe quantum dots (QDs) has been designed for the detection of clenbuterol. The immunosensor was fabricated by layer by layer and characterized with atomic force microscopic images (AFM) and electrochemical impedance spectra (EIS). In oxygen-saturated pH = 9.0 Tris-HCl buffer, a strong ECL emission of QDs could be observed during the cathodic process due to the H₂O₂ product from electrochemical reduction of dissolved oxygen. Upon the formation of immunocomplex, the second antibody labeled with horseradish peroxidase was simply immobilized on the electrode surface. The ECL emission decreased since steric hindrance of the immunocomplex slowed down the electron-transfer speed of dissolved oxygen, and also could be greatly amplified by an enzymatic cycle to consume the self-produced coreactant. Using clenbuterol as model analyte, the ECL intensity was determined by the concentration of competitive immunoassay of clenbuterol with a wide calibration in the range of 0.05 ng mL⁻¹ to 1000 ng mL⁻¹, and a low detection limit was 0.02 ng mL⁻¹. The immunosensor shows good stability and fabrication reproducibility. It was applied to detecting practical samples with the satisfactory results. This immunosensing strategy opens a new avenue for detection of residue and application of QDs in ECL biosensing.     

CdSe quantum dots as labels for sensitive immunoassay of cancer biomarker proteins by electrogenerated chemiluminescence

A sensitive and specific immunoassay method for detecting α-fetoprotein (AFP) based on electrogenerated chemiluminescence (ECL) was described. ECL could perform detection for a series of different concentrations of AFP. CdSe quantum dots (QDs) were used as labels and were linked to AFP antibody (anti-AFP, the secondary antibody, Ab2*). Immunoassay was carried out on a modified electrode using a sandwich assay approach, where anti-AFP (Ab1) was covalently bound to the surface of an Au electrode to be allowed to capture AFP specifically. Afterwards, Ab2* was allowed to bind selectively to the captured AFP. The non-specific adsorption was negligible. In the presence of H(2)O(2), the ECL intensity increased with the increase of AFP, which indicated that an immunosensor for AFP was constructed. The detection of AFP based on measuring the ECL intensity of CdSe without the enzyme and mediator can promote the stability of the immunosensor. The linear range of the AFP assay was from 0.002 to 32 ng mL(-1). Furthermore, the immunosensor showed high sensitivity, good precision, stability, and reproducibility and could be used for the detection of real samples with consistent results in comparison with those obtained by the enzyme-linked immunosorbent assay (ELISA) method. The strategy was successfully demonstrated as a simple, cost-effective, specific, and potential method to detect AFP in practical samples.     

Graphene enhanced electrochemiluminescence of CdS nanocrystal for H2O2 sensing

Graphene-CdS (G-CdS) nanocomposites were successfully prepared by CdS nanocrystals (CdS NCs) formed in situ on the surface of graphene sheets, using graphene oxide (GO) sheets with rich negatively charged carboxylic acid groups as starting materials. Compared with pure CdS NCs, the presence of the graphene doped in G-CdS nanocomposites could facilitate the electrochemical redox process of CdS NCs; further, the as-prepared G-CdS nanocomposite can react with H₂O₂ to generate strong and stable electrochemiluminescent (ECL) emission, which not only enhances its ECL intensity by about 4.3-fold but also decreases its onset potential for about 320 mV. The as-prepared solid-state ECL H₂O₂ sensor shows acceptable linear response from 5 μM up to 1 mM with a detection limit of 1.7 μM (S/N = 3). The ECL H₂O₂ sensor exhibits excellent reproducibility and long-term stability. Such a property would promote the potential application of the graphene as enhanced materials in fabricating sensors for chemical and biochemical analysis.     

Use of Cdse/ZnS quantum dots for sensitive detection and quantification of paraquat in water samples

Based on the highly sensitive fluorescence change of water-soluble CdSe/ZnS core-shell quantum dots (QD) by paraquat herbicide, a simple, rapid and reproducible methodology was developed to selectively determine paraquat (PQ) in water samples. The methodology enabled the use of simple pretreatment procedure based on the simple water solubilization of CdSe/ZnS QDs with hydrophilic heterobifunctional thiol ligands, such as 3-mercaptopropionic acid (3-MPA), using microwave irradiation. The resulting water-soluble QDs exhibit a strong fluorescence emission at 596 nm with a high and reproducible photostability. The proposed analytical method thus satisfies the need for a simple, sensible and rapid methodology to determine residues of paraquat in water samples, as required by the increasingly strict regulations for health protection introduced in recent years. The sensitivity of the method, expressed as detection limits, was as low as 3.0 ng L⁻¹. The lineal range was between 10–5 × 10³ ng L⁻¹. RSD values in the range of 71–102% were obtained. The analytical applicability of proposed method was demonstrated by analyzing water samples from different procedence.     

Carbon nanospheres enhanced electrochemiluminescence of CdS quantum dots for biosensing of hypoxanthine

This work developed a novel method to greatly enhance the electrochemiluminescence (ECL) of CdS quantum dots (QDs). The ECL amplification was achieved by the assembly of QDs on poly (diallyldimethylammonium chloride)-functionalized carbon nanospheres (PFCNSs), and successfully employed for sensitive ECL biosensing of oxidase substrates. The carbon nanospheres were prepared by a “green” method, and the high loading of QDs on carbon nanospheres led to a 4-times increased ECL intensity with dissolved O₂ as the coreactant. Using xanthine oxidase (XOD) as a model, an ECL biosensor was fabricated by immobilizing the enzyme on the mixing membrane of PFCNSs and QDs. The ECL biosensor showed a fast response to hypoxanthine with a linear concentration range from 2.5 × 10⁻⁸ to 1.4 × 10⁻⁵ M. The limit of detection was 5 nM at a signal-to-noise ratio of 3. The assay results of hypoxanthine in fish samples were in a good agreement with the reference values by amperometric technique. This facile approach to prepare the PFCNSs/QDs system for ECL biosensing could be of promising application in bioanalysis and electronic device.     

Sequential injection immunoassay for human bone morphogenic protein-7 using an immunoreactor immobilized with anti-human bone morphogenic protein-7 antibody–CdSe/ZnS quantum dot conjugates

The detection of human bone morphogenic protein-7 (BMP-7) was achieved using a sequential injection immunoassay (SIIA) system. The SIIA system is based on the binding between BMP-7 and anti-human BMP-7 (AbBMP7)–CdSe/ZnS quantum dot (QD) conjugates immobilized onto a glass disk or an optical fiber, using fluorescence detection at excitation and emission wavelengths of 470 nm and 580 nm, respectively. The AbBMP7–QD conjugates were prepared by conjugating anti-human BMP-7 antibody (AbBMP7) to hydrophilic CdSe/ZnS core/shell quantum dots (QDs). The SIIA system was fully automated using software written in the LabVIEW™ development environment. The analytical performance of the SIIA system was characterized with a number of variables such as carrier flow rate and elution buffer. Under partially optimized operating conditions, the SIIA system had a linear calibration graph at up to 10.0 ng mL⁻¹ BMP-7 (R² ≥ 0.975) and a sample frequency of two samples per hour. The SIIA system with an optical fiber immunosensor was used to detect and quantify BMP-7 in spiked real samples obtained from a biological process with recoveries in the range of 95–102%.     

Quantum dots/particle-based immunofluorescence assay: synthesis, characterization and application

Recently, it has been proved that quantum dots (QDs) hold the potential to be used in the bioanalysis as fluorescent probes for their many unique optical properties. In this paper, immunofluorescence assay, an integration of particle-based immunoassays and fluorescent QD-probes, was constructed. Firstly, high quality CdSe/ZnS QDs were prepared. Then after being water-solubilized by amphiphilic polymer based on self-assembling, the QDs were labeled by immunoglobulin G (IgG) antibody. At the same time, both carboxyl-polystyrene (PS) and magnetic carboxyl-PS microspheres were prepared and coated by antigens. The antigen sensitized PS microspheres were specifically captured by the QD-IgG bioconjugates based on the antibody-antigen reaction, which was confirmed by the immunofluorescence test in vitro. The sensitivity of current assay was tested by sandwich immunofluorescence assay using human alpha fetoprotein (AFP) as antigen model. The detection limit of AFP antigen is 4.9 ng/mL.     

Selective capturing and detection of Salmonella typhi on polycarbonate membrane using bioconjugated quantum dots

Present work demonstrates the utilization of surface modified polycarbonate (PC) membrane as solid phase and antibody conjugated CdSe/ZnS quantum dots (QDs) as fluorescent label for the sensitive and selective detection of Salmonella typhi (S. typhi) in water in a period of 2.5 h. PC membrane was surface modified with glycine and activated by EDC/NHS for immobilization of S. typhi specific IgG. Antibody immobilized porous PC membrane was incubated with bacteria contaminated water for immunocapturing of S. typhi. Antibody conjugated QDs were also prepared by using carbodiimide chemistry. Both modified PC membrane and quantum dots were characterized by using various modern analytical tools. It was estimated that 1.95 molecules of QDs were successfully bio-conjugated per unit of IgG. PC membrane with captured bacteria was incubated with prepared IgG conjugated QDs for the formation of sandwich complex. Analysis of the regions of interest (ROI) in fluorescent micrographs showed that newly developed method based on PC and fluorescent QDs has 100 times higher detection sensitivity (100 cells/mL) as compared with detection using conventional dye (FITC) based methods.     

Fluorescence detection of total count of Escherichia coli and Staphylococcus aureus on water-soluble CdSe quantum dots coupled with bacteria

The aim of this paper was to demonstrate a fluorescence measurement method for rapid detection of two bacterial count by using water-soluble quantum dots (QDs) as a fluorescence marker, and spectrofluorometer acted as detection apparatus, while Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were as detection target bacteria. Highly luminescent water-soluble CdSe QDs were first prepared by using thioglycolic acid (TGA) as a ligand, and were then covalently coupled with target bacteria. The bacterial cell images were obtained using fluorescence microscopy. Our results showed that CdSe QDs prepared in water phase were highly luminescent, stable, and successfully conjugated with E. coli and S. aureus. The fluorescence method could detect 10²-10⁷ CFU/mL total count of E. coli and S. aureus in 1-2 h and the low detection limit is 10² CFU/mL. A linear relationship of the fluorescence peak intensity and log total count of E. coli and S. aureus have been established using the equation Y = 118.68X − 141.75 (r = 0.9907).     

Tunable electrochemiluminescence of CdSe@ZnSe quantum dots by adjusting ZnSe shell thickness

CdSe quantum dots as cores capped with ZnSe shell (CdSe@ZnSe QDs) via a facile and eco-friendly strategy have been synthesized in aqueous solution for the first time. The electrochemiluminescence (ECL) of CdSe@ZnSe QDs was greatly enhanced compared to that of CdSe QDs. In particular, the ECL properties of the resulting CdSe@ZnSe QDs were found to be controllable by adjusting the thickness of ZnSe shells. Benefiting from the enhanced ECL intensity, the sensor based on CdSe@ZnSe QDs could accurately quantify dopamine from 10.0 nM to 3.0 μM with a detection limit of 3.6 nM.     

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.     

CdSe quantum dots as luminescent probes for spironolactone determination

Based on the quenching of the fluorescence of CdSe quantum dots (QDs) by spironolactone, a simple, rapid and specific method for spironolactone determination was proposed. In the optimum conditions, spironolactone concentration versus quantum dot fluorescence gave a linear response with an excellent 0.997 correlation coefficient, between 2.5 and 700 mg/mL (6.0-1680 μmol/L) and the limit of detection (S/N = 3) was 0.2 μg/mL (0.48 μmol/L). The contents of spironolactone in pharmaceutical tablets were determined by the proposed method and the results agreed with the claimed values. The possible mechanism for the reaction was also discussed.     

Imaging pancreatic cancer using surface-functionalized quantum dots

In this study, CdSe/CdS/ZnS quantum dots (QDs) were used as optical contrast agent for imaging pancreatic cancer cells in vitro using transferrin and anti-Claudin-4 as targeting ligands. CdSe/CdS/ZnS was chosen because the CdSe/CdS/ZnS QDs have better photoluminescence (PL) efficiency and stability than those of CdSe/ZnS. The transferrin-mediated targeting is demonstrated in both a cell-free coprecipitation assay as well as using in vitro confocal microscopy. Pancreatic cancer specific uptake is also demonstrated using the monoclonal antibody anti-Claudin-4. This targeted QD platform will be further modified for the purpose of developing as an early detection imaging tool for pancreatic cancer.     

ZnS quantum dots derived a reagentless uric acid biosensor

A reagentless amperometric uric acid biosensor based on zinc sulfide (ZnS) quantum dots (QDs) was firstly developed. It could detect uric acid without the presence of an electron mediator. The carboxyl group functionalized ZnS QDs were synthesized, and they were soluble biocompatible and conductive. ZnS QDs conjugates could provide increased enzyme binding sites, which may result in higher enzyme loading. Thus, the proposed uricase/ZnS QDs/l-cys biosensor exhibited higher amperometric response compared to the one without QDs (uricase/l-cys biosensor). In addition, there was little AA interference. It showed a linear dependence on the uric acid concentration ranging from 5.0 × 10⁻⁶ to 2.0 × 10⁻³ mol L⁻¹ with a detection limit of 2.0 × 10⁻⁶ mol L⁻¹ at 3σ.     

A novel fluorescent assay for edaravone with aqueous functional CdSe quantum dots

Aqueous thiol-capped CdSe QDs with a narrow, symmetric emission were prepared under a low temperature. Based on the fluorescence enhancement of thiol-stabilized CdSe quantum dots (QDs) caused by edaravone, a simple, rapid and specific quantitative method was proposed to the edaravone determination. The concentration dependence of fluorescence intensity followed the binding of edaravone to surface of the thiol-capped CdSe QDs was effectively described by a modified Langmuir-type binding isotherm. Factors affecting the fluorescence detection for edaravone with thiol-stabilized CdSe QDs were studied, such as the effect of pH, reaction time, the concentration of CdSe QDs and so on. Under the optimal conditions, the calibration plot of C/(I − I₀) with concentration of edaravone was linear in the range of (1.45–17.42) μg/mL (0.008–0.1 μmol/L) with correlation coefficient of 0.998. The limit of detection (LOD) (3σ/κ) was 0.15 μg/mL (0.0009 μmol/mL). Possible interaction mechanism was discussed.     

Use of CdSe/ZnS luminescent quantum dots incorporated within sol-gel matrix for urea detection

In this work, urea detection techniques based on the pH sensitivity of CdSe/ZnS QDs were developed using three types of sol-gel membranes: a QD-entrapped membrane, urease-immobilized membrane and double layer consisting of a QD-entrapped membrane and urease-immobilized membrane. The surface morphology of the sol-gel membranes deposited on the wells in a 24-well microtiter plate was investigated. The linear detection range of urea was in the range of 0-10 mM with the three types of sol-gel membranes. The urea detection technique based on the double layer consisting of the QD-entrapped membrane and urease-immobilized membrane resulted in the highest sensitivity to urea due to the Michaelis-Menten kinetic parameters. That is, the Michaelis-Menten constant (K_m =2.0745 mM) of the free urease in the QD-entrapped membrane was about 4-fold higher than that (K_m =0.549 mM) of the immobilized urease in the urease-immobilized membrane and about 12-fold higher than that (K_m =0.1698 mM) of the immobilized urease in the double layer. The good stability of the three sol-gel membranes for urea sensing over 2 months showed that the use of sol-gel membranes immobilized with QDs or an enzyme is suitable for biomedical and environmental applications.     

3-巯基丙酸修饰的CdSe/ZnS量子点的合成及测定牛血清白蛋白的研究

以3-巯基丙酸作为修饰剂,在水溶液中合成了稳定的CdSe/ZnS量子点(QDs),透射电镜观察所合成量子点的形貌近似球形,粒径约为25 nm.吸收光谱与荧光光谱的研究表明,CdSe QDs在410 nm处有最大吸收峰,而CdSe/ZnS QDs的最大吸收峰在470 nm处,CdSe/ZnS QDs的荧光强度是CdSe QDs的11倍.考察了缓冲溶液的体积、pH值、反应温度、反应时间对体系荧光的影响.在最佳实验条件下,体系的荧光强度与BSA的浓度呈线性关系,线性响应范围为0.746×10-7～4.48×10-7 mol/L,检出限为3.846×10-10 mol/L.并且CdSe/ZnS QDs荧光强度基本保持稳定,可达两个多月.该方法应用于合成样品的测定,结果满意.     

Sandwich-type electrochemiluminescence immunosensor based on Ru-silica@Au composite nanoparticles labeled anti-AFP

A simple and sensitive sandwich-type electrochemiluminescence immunosensor for α-1-fetoprotein (AFP) on a gold nanoparticles (nano-Au) modified glassy carbon electrode (GCE) was developed by using Ru-silica (Ru(bpy)₃²⁺-doped silica) doped Au (Ru-silica@Au) composite as labels. The primary antibody, anti-AFP was first immobilized on the gold nanoparticles modified electrode due to the covalent conjugation, then the antigen and the Ru-silica@Au composite nanoparticles labeled secondary antibody was conjugated successively to form a sandwich-type immunocomplex through the specific interaction. The surfaces of Ru-silica nanoparticles were modified via the assemble of Au nanoparticles. The prepared Ru-silica@Au composite nanoparticles own the large surface area, good biocompatibility and highly effective electrochemiluminescence properties. The morphologies of the Ru-silica@Au composite nanoparticles were investigated by using transmission electronic microscope (TEM). The Ru-silica@Au composite nanoparticles labeled anti-AFP/AFP/bovine serum albumin (BSA)/anti-AFP/nano-Au modified GCE electrode was evaluated by means of cyclic voltammetry (CV) and electrogenerated chemiluminescence (ECL). The immunosensor performed high sensitivity and wide liner for detection AFP in the range of 0.05-50 ng/mL and the limit detection was 0.03 ng/mL (defined as S/N = 3).