ZnS Core Shell Quantum Dots Research Articles

Luminescence Modulation with Semiconductor Quantum Dots and Photochromic Ligands

We have attached a photochromic spiropyran to the surface of CdSe–ZnS core–shell quantum dots, relying on a dithiolane anchoring group. In the resulting assembly, the reversible interconversion of the photochromic component activates and suppresses an energy transfer pathway from the excited quantum dots to the coloured state of the ligand. As a result, the luminescence intensity decreases by 45% with the photoinduced colouration of the spiropyran and returns to the original value after the thermal reisomerization of the ligand.

Photochromic nanocomposites of bipyridinium dications and semiconductor quantum dots

We have developed photochromic materials based on the photoinduced transfer of electrons from CdSe–ZnS core–shell quantum dots to bipyridinium dications.

Photoluminescence characteristics of coupled CdSe∕ZnS quantum dots on self-assembled silica nanospheres

The photoluminescence (PL) characteristics of the self-assembled silica nanospheres containing coupled CdSe∕ZnS core-shell quantum dots (QDs) on the surface of the sphere were studied. The degree of QD-coupling and the distribution of the coupled QDs were controlled by generating spatially separated QD-binding sites on the surface of the spheres and varying the QD concentration during the attachment process. When the sizes of QD spots on the surface probed by atomic force microscopy gradually increased, the band gap energy shifted to lower energy via strong Förster energy transfer between nearby QDs, and when UV-exposure time increased, the band gap energy shifted to slightly higher energy, accompanied by a large enhancement in the emission intensity. Moreover, the Bragg diffraction of the self-assembled crystal structure of the spheres affected the PL characteristics of QDs by shifting the entire emission spectra toward longer wavelength as the incident angles were increased.

(CdSe)ZnS Quantum Dots and Organophosphorus Hydrolase Bioconjugate as Biosensors for Detection of Paraoxon

In this paper, we first report a novel biosensor for the detection of paraoxon based on (CdSe)ZnS core-shell quantum dots (QDs) and an organophosphorus hydrolase (OPH) bioconjugate. The OPH was coupled to (CdSe)ZnS core-shell QDs through electrostatic interaction between negatively charged QDs surfaces and the positively charged protein side chain and ending groups (-NH2). Circular dichroism (CD) spectroscopy showed no significant change in the secondary structure of OPH after the bioconjugation, which indicates that the activity of OPH was preserved. Detectable secondary structure changes were observed by CD spectroscopy when the OPH/QDs bioconjugate was exposed to organophosphorus compounds such as paraoxon. Photoluminescence (PL) spectroscopic study showed that the PL intensity of the OPH/QDs bioconjugate was quenched in the presence of paraoxon. The overall quenching percentage as a function of paraoxon concentration matched very well with the Michaelis-Menten equation. This result indicated that the quenching of PL intensity was caused by the conformational change in the enzyme, which is confirmed by CD measurements. The detection limit of paraoxon concentration using OPH/QDs bioconjugate was about 10(-8) M. Although increasing the OPH molar ratio in the bioconjugates will slightly increase the sensitivity of biosensor, no further increase of sensitivity was achieved when the molar ratio of OPH to QDs was greater than 20 because the surface of QDs was saturated by OPH. These properties make the OPH/QDs bioconjugate a promising biosensor for the detection of organophosphorus compounds.

Cyclodextrin driven hydrophobic∕hydrophilic transformation of semiconductor nanoparticles

Quantum dots (QDs) have been the subject of considerable study in theoretical physics, and water soluble QDs now appear to have numerous applications in biological tagging, molecular electronic devices, and nanoscale engineering. The work reported here supports the notion that the aliphatic chains of the trioctylphosphine oxide molecules decorating these (CdSe)ZnS core-shell QDs are stabilized by the hydrophobic cyclodextrin (CD) lumen. Photoluminescence studies show a redshift of over 15nm in the emission wavelength of the QDs upon complexation with the CD, and first-principles calculations reveal an exothermic exchange of the S in the ZnS shell with the CD hydroxyl oxygen. Unlike simple water-driven surface transformations, the directed bonding of hydroxyl groups to the ZnS shell results in stable structures, verified by photoluminescence and Fourier transform infrared spectroscopy.

Förster energy transfer from blue-emitting polymers to colloidal CdSe∕ZnS core shell quantum dots

We have studied the interactions in the solid phase between a blue-light-emitting organic conjugated polymer {poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(9,ethyl-3,6-carbazole)]} and colloidal CdSe∕ZnS core shell quantum dots. We demonstrate by time resolved photoluminescence measurements that efficient Förster energy transfer takes place from the polymer acting as the donor and the CdSe∕ZnS dots, acting as the acceptors. A Förster radius of about 80±15Å is extracted from the analysis of the relaxation dynamics based on the solution of the donor-acceptor coupled rate equations. This value is in good agreement with the value extracted from the steady state spectra and indicates that efficient energy tranfer from a polymeric host to a nanocrystal guest can occur. Our results are relevant to the application of hybrid organic/inorganic systems to light-emitting devices.

Electroluminescence from isolated CdSe∕ZnS quantum dots in multilayered light-emitting diodes

Electro- and photoluminescence spectra of the CdSe∕ZnS core-shell quantum dots (QDs) covered by various organic ligands and incorporated into multilayered light-emitting diodes (LEDs) were recorded by a confocal optical microscope. The QDs were dispersed in a hole transporting material, N,N’-Diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD), to investigate the LED performance at different QD concentrations and the effect of different surface modifications on the isolated QDs. No wavelength shift was observed in the electroluminescence spectra from the QD LEDs with or without the TPD. The peak energies of the electro- and photoluminescence showed only small spectral shifts (several nanometer) for the diluted QDs and no dependence on the QD-concentration, surface ligands, or conductive polymers that were used. This suggests that the relative peak shifts are related to the different filling processes in the CdSe QDs under photo excitation and electric injection, rather than to the “chemical” effects on the surface of the CdSe∕ZnS QDs.

Self-assembled luminescent CdSe–ZnS quantum dot bioconjugates prepared using engineered poly-histidine terminated proteins

We report a simple and versatile approach for the conjugation of luminescent CdSe–ZnS core-shell quantum dots (QDs) to proteins through coordination of engineered C-terminal oligohistidine sequences. Several histidine tail containing proteins were self-assembled onto the QD surface using this method. A recombinant antibody specific for the high explosive 2,4,6-trinitrotoluene (TNT) was conjugated to QDs through a carboxy terminal histidine tail and the bioconjugate used to detect TNT by competitive immunoassay. TNT was detected over the range of 10 μg/ml down to 41 ng/ml using the scFv conjugated to QDs. These results open up the possibility to conjugate luminescent QDs to a whole range of proteins to form QD bioconjugates that can be effectively used in bio-oriented applications, such as sensing, imaging, immunoassay and other diagnostics.

Self-assembled luminescent CdSe?ZnS quantum dot bioconjugates prepared using engineered poly-histidine terminated proteins

We report a simple and versatile approach for the conjugation of luminescent CdSe–ZnS core-shell quantum dots (QDs) to proteins through coordination of engineered C-terminal oligohistidine sequences. Several histidine tail containing proteins were self-assembled onto the QD surface using this method. A recombinant antibody specific for the high explosive 2,4,6-trinitrotoluene (TNT) was conjugated to QDs through a carboxy terminal histidine tail and the bioconjugate used to detect TNT by competitive immunoassay. TNT was detected over the range of 10 μg/ml down to 41 ng/ml using the scFv conjugated to QDs. These results open up the possibility to conjugate luminescent QDs to a whole range of proteins to form QD bioconjugates that can be effectively used in bio-oriented applications, such as sensing, imaging, immunoassay and other diagnostics.

Luminescent Quantum Dot-Adaptor Protein-Antibody Conjugates for Use in Fluoroimmunoassays

A method for the preparation and characterization of bioinorganic conjugates made with highly luminescent semiconductor CdSe–ZnS core-shell quantum dots (QDs) and antibodies for use in fluoroimmunoassays is presented. The conjugation strategy employs two routes: 1. Use of an engineered molecular adaptor protein, attached to the QDs via electrostatic/hydrophobic self-assembly, to link the inorganic fluorophore with antibodies, and 2. use of avidin, also electrostatically self-assembled onto the nanocrystal surface, which allows QD conjugation to biotinylated antibodies via avidin–biotin binding scheme. With this approach, the average number of antibodies conjugated to a single QD can be varied. In addition, we have developed a simple purification strategy based on mixed composition conjugates of the molecular adaptor and a second “inert” two-domain fusion protein that allows the use of affinity chromatography. QD/adaptor-antibody conjugates were successfully employed in fluoroimmunoassays for the detection of small molecule analytes, 2,4,6-trinitrobenzene (TNB) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We also demonstrate the use of QD/avidin-antibody conjugates for fluoroimmunoassays using a model protein system.

Highly luminescent (ZnSe)ZnS core-shell quantum dots for blue to UV emission: synthesis and characterization

Abstract We synthesized nearly monodisperse bare ZnSe nanocrystallites having luminescence which ranges in wavelength from 340 to 430 nm via nucleation due to supersaturation and growth followed by size selective precipitation. Bare ZnSe dots' outermost surface is passivated with organic HDA/TOP. In order to enhance the radiative emission from the semiconductor nanocrystals, we capped the bare ZnSe quantum dots with ZnS semiconductor materials of a wider band gap and 5% of lattice mismatch and produced highly luminescent core-shell (ZnSe)ZnS quantum dots. The core-shell (ZnSe)ZnS nanocrystals show 20 times or more as greatly enhanced luminescence quantum yields as those of bare ZnSe nanocrystals. The ZnSe bare dots and the (ZnSe)ZnS core-shell dots have cubic zinc blende structures as expected from the bulk structure. The observed shapes of bare ZnSe and core-shell (ZnSe)ZnS dots are nearly spherical or ellipsoidal with the aspect ratios of 1.2 and 1.4, respectively. They are not faceted.

ChemInform Abstract: (CdSe)ZnS Core—Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites.

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