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The paper describes the first use of silanized semiconductor core-shell quantum dots as fluorescent labels for macromolecule, C-reactive protein determination in blood plasma. The controlled synthesis of CdSe cores, with successive shells of CdS, CdZnS, ZnS and coating with transparent, stable, and inert silica shell, provides quantum dots with a narrow emission band, high quantum yield, and prolonged signal stability. Finally, the quantum dots were conjugated with specific antibodies via carboxylic groups on the silica surface. The method was further used for the immunochromatographic assay of C-reactive protein, a diagnostically important inflammatory biomarker. Assays with both the fluorescent QDs and a widely used colloidal gold label were developed in parallel and compared. The silanized quantum dots provide a more sensitive assay with a detection limit of 1 ng/mL for C-reactive protein in standard solutions, whereas the common assay has a detection limit of 10 ng/mL. The possibility of quantitative evaluation of analyte content by a portable device was demonstrated; the accuracy of the measurements was in the range of 5%–10%. The tests were used to determine C-reactive proteins in human plasma samples. The selected optimized protocol for these samples is based on a 4-fold dilution. The final working range of the assay, 4–1,200 ng/mL, covers practically all important interval of C-reactive protein values for the characterization of acute, chronic, and local inflammatory processes. Due to their high physical stability and inertness as well as intense, stable, and reproducible fluorescence, silanized quantum dots may be applied for high-sensitive assays for different analytes. [ABSTRACT FROM AUTHOR]

A sensitive tool for simultaneous qualitative detection of two mycotoxins based on use of non-cadmium quantum dots (QDs) is presented for the first time. QDs have proven themselves as promising fluorescent labels for biolabeling and chemical analysis. With an increasing global tendency to regulate and limit the use of hazardous elements, indium phosphide (InP) QDs are highlighted as environmentally-friendly alternatives to the highly efficient and well-studied, but potentially toxic Cd- and Pb-based QDs. Here, we developed water-soluble InP QDs-based fluorescent nanostructures. They consisted of core/shell InP/ZnS QDs enrobed in a silica shell that allowed the water solubility (QD@SiO2). Then we applied the QD@SiO2 as novel, silica shell-encapsulated fluorescent labels in immunoassays for rapid multiplexed screening. Two mycotoxins, zearalenone and deoxynivalenol, were simultaneously detected in maize and wheat, since the two QD@SiO2 labelled conjugates emit at two different, individually detectable wavelengths. The cutoff values for the simultaneous determination were 50 and 500 μg kg-1 for zearalenone and deoxynivalenol, respectively, in both maize and wheat. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to confirm the result. [ABSTRACT FROM AUTHOR]

Electron and hole emission from states of a ten-layer system of tunneling-coupled vertically correlated InAs/GaAs quantum dots (QDs) is studied experimentally by capacitance—voltage measurements and deep-level transient spectroscopy. The thickness of GaAs interlayers separating sheets of InAs QDs was ≈3 nm, as determined from transmission electron microscope images. It is found that the periodic multimo-dal DLTS spectrum of this structure exhibits a pronounced linear shift as the reverse-bias voltage Ur applied to the structure is varied. The observed behavior is a manifestation of the Wannier—Stark effect in the InAs/GaAs superlattice, where the presence of an external electric field leads to the suppression of coupling between the wave functions of electron states forming the miniband and to the appearance of a series of discrete levels called Wannier—Stark ladder states. [ABSTRACT FROM AUTHOR]

The absorption of uncoupled and tunnel-coupled vertically correlated quantum dots (QDs), measured at room temperature, has been experimentally compared. It is revealed that matching of the laser wavelength and Stark shift for laser structures with tunnel-coupled QDs leads to resonant absorption with formation of bound and antibound exciton states with a splitting energy of ∼62 meV between them in QD molecules. For these states, an external field causes a large linear Stark shift (up to 68 meV). For uncoupled QDs, one resonant absorption peak with the formation of an exciton (for which the Stark shift does not exceed 13 meV) is observed. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy (DLTS) measurements were performed to study electron emission from quantum states in a 20-layer Ge quantum-dot superlattice (QDSL) in a Ge/Si p-n heterostructure. It was established that the changes in the DLTS spectra depend heavily on the magnitude of the applied reverse bias U r . Three regions of the reverse bias U r were identified, corresponding to the manifestation of the three modes of the Wannier-Stark effect: Wannier-Stark ladder mode, Wannier-Stark localization, and nonresonant Zener tunneling mode. Furthermore, it was found that the appearance of DLTS peaks for all three modes is associated with electron emission from deep-level defects via Wannier-Stark localized states arising as a result of the splitting of the electron miniband of the Ge/Si QDSL. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p–n heterostructures grown on (100)- and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy is used to study the emission of holes from the states of a vertically coupled system of InAs quantum dots in p-n InAs/GaAs heterostructures. This emission was considered in relation to the thickness of a GaAs interlayer between two layers of InAs quantum dots and to the reverse-bias voltage Ur. It is established that hole localization at one of the quantum dots is observed for a quantum-dot molecule composed of two vertically coupled self-organized quantum dots in an InAS/GaAs heterostructure that has a 20-A-thick or 40-A-thick GaAs interlayer between two layers of InAs quantum dots. For a thickness of the GaAs interlayer equal to 100 A, it is found that the two layers of quantum dots are incompletely coupled, which results in a redistribution of the hole localization between the upper and lower quantum dots as the voltage Ur applied to the structure is varied. The studied structures with vertically coupled quantum dots were grown by molecular-beam epitaxy using self-organization effects. [ABSTRACT FROM AUTHOR]

The effect of In and Al on the properties of the As[sub Ga] defect complex (arsenic substituted for gallium at a crystal lattice site) in GaAs-based quantum dots (QDs) was investigated using the nonempirical quantum-chemical SCF–MO–LCAO technique. It is shown that an As[sub Ga] defect can exist in stable and metastable states. Raising the indium or aluminum content in QD enhances the probability of As[sub Ga] defect formation in the stable state; in case of In introduction, this effect is manifested more strongly. The activation energy of the transition between stable and metastable states varies between 0.886 and 2.049 eV, depending on the QD stoichiometry. The formation of an As[sub Ga] defect gives rise to two deep levels in the band gap. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects. [ABSTRACT FROM AUTHOR]

The electron emission from and capture by quantum dots in InGaAs/GaAs p-n heterostructures were studied using deep-level transient spectroscopy in relation to the conditions of isochronous annealing with a switched-on or -off bias voltage (U[sub ra] < 0 or U[sub ra] = 0). The results indicate that, as a result of annealing with U[sub ra] < 0, a dipole that consists of charge carriers localized in the quantum dots and the ionized lattice defects is formed. The electrostatic potential of this dipole reduces the barrier for the electron emission from and capture by a quantum dot. If the annealing is performed at U[sub ra] = 0, no dipole is formed and the band offset is controlled by the conditions of heteroboundary formation during the structure growth. The dependence of the barrier height on the filling-pulse duration was also observed; this dependence is related to the manifestation of the Coulomb blockade for the electron capture by the ground and excited states of a quantum dot. The structures with quantum dots studied were grown by gaseous-phase epitaxy from metal-organic compounds using self-organization effects. [ABSTRACT FROM AUTHOR]

A report is presented on the investigation of the influence of in situ annealing of the InGaAs layer in p-n InGaAs/GaAs structures grown by the metalloorganic chemical vapor deposition upon the formation of coherently strained three-dimensional islands. The structures were studied by the methods of capacitance-voltage measurements, deep-level transient spectroscopy, transmission electron microscopy, and photoluminescence. It is established that three-dimensional islands with misfit dislocations are formed in the unannealed structure A, while quantum dots are formed in the annealed structure B. The deep-level defects were investigated. In structure A, defects of various types (EL2, EL3 (I3), I2, HL3, HS2, and H5) are present in the electronaccumulation layer. Concentrations of these traps are comparable to the shallow donor concentration, and the number of hole traps is higher than that of the electron traps. On the in situ annealing, the EL2 and EL3 defects, which are related to the formation of dislocations, disappear, and concentrations of the other defects decrease by an order of magnitude or more. For structure A, it is established that the population of the quantum states in the islands is controlled by the deep-level defects. In structure B, the effect of the Coulomb interaction of the charge carriers localized in the quantum dot with the ionized defects is observed. [ABSTRACT FROM AUTHOR]

Indium arsenide/gallium arsenide structures with vertically coupled quantum dots imbedded in the active zone of a laser diode are investigated by deep-level transient spectroscopy (DLTS), and the capacitance-voltage characteristics are analyzed. The DLTS spectrum was found to undergo significant changes, depending on the temperature of preliminary isochronous annealing of the sample, T[sub a] < T[sub ac] = 245 K or T[sub a] > T[sub ac], and on the cooling conditions, with a bias voltage V[sub b] = 0 or with an applied carrier pulse V[sub f] > 0. The changes are attributed to the onset of Coulomb interaction of carriers trapped in a quantum dot with point defects localized in the nearest neighborhoods of the quantum dots and also to the formation of a dipole when T[sub a] < T[sub ac] and cooling takes place with V[sub f] > 0, or to the absence of a dipole when T[sub a] > T[sub ac] and V[sub b] = 0. It is discovered that the tunneling of carriers from the deeper states of defects to the shallower states of quantum dots takes place in the dipole, and the carriers are subsequently emitted from the dots into bands. [ABSTRACT FROM AUTHOR]

The Stark effect has been studied in multilayer InGaAs/GaAs laser structures with self-assembled quantum dots (QDs). A shift in the absorption edge depending on the reverse bias voltage has been measured in a two-section laser diode. The QD absorption edge shifts toward longer wavelengths with increasing electric field strength. It is established that the QD absorption depends on the polarization of light. The intensity at which TE-polarized luminescence in laser structures is studied is more than ten times higher than that of the TE-polarized emission component, which is explained by higher amplification of the TE mode. [ABSTRACT FROM AUTHOR]

Electron emission in a system of vertically coupled quantum dots (VCQDs) in InAs/GaAs p- n-heterostructures obtained by molecular beam epitaxy has been studied by means of deep-level transient spectroscopy (DLTS) as a function of the number of quantum dot (QD) rows and the reverse bias voltage. For a GaAs spacer thickness of d GaAs = 40 Å, the system occurs in a molecular state, irrespective of the number of QD rows. An increase in this number leads to a decrease in the Stark shift, which is probably related to a decrease in the lattice strain potential in the vicinity of VCQDs. [ABSTRACT FROM AUTHOR]

Argonaute protein (AGO2) bound circulating cell-free miRNAs (ccf- miRs), in the recent years, has attracted great attention due to their differential abundance in biological fluids. In the present work, a selective and technically uncomplicated quantum dot (QD) nanoconjugate has been fabricated combining the specific affinity of the antibody and fluorescent property of QDs for the precise immuno-detection of AGO2-bound ccf- miRs in plasma samples. The electrophoretic mobility assay confirmed the conjugation of antibody with QDs. The detection methodology involves a highly specific antigen-antibody reaction between the AGO2 proteins of miRNA-induced silencing complex and the anti-AGO2 antibody conjugated with QDs. The recognition efficiency of QD-Ab nanoconjugates was analysed using flow cytometry and fluorometry. The flow cytometry results demonstrated a significant change in the fluorescence intensity of the prepared nanoconjugates upon capture of ccf- miRs in the plasma samples with respect to the samples devoid of any miRNAs. Fluorometry measurements exhibited corroboration with the flow cytometry results indicating the selectivity and reproducibility of the developed method. Current research highlights the translational significance of the methodology as a novel flow cytometry based immunoassay for detection of differentially expressed AGO2-bound miRNAs in clinical and field settings. Image 1 • Circulating cell-free miRNAs (ccf- miRs) are associated with different pathologies. • AGO2-bound miRs represent the functional fraction of ccf- miRs. • Quantum dot - anti-AGO2 antibody nanoconjugates were fabricated. • A flow cytometric based method for detection AGO-bound miRNAs was developed. [ABSTRACT FROM AUTHOR]