QD Surface Research Articles

Variation of Raman spectra of CdSe/ZnS quantum dots at the bioconjugation

AbstractThis paper presents the results of comparative analysis of Raman scattering spectra of CdSe/ZnS QDs covered by polymer with and without bio‐conjugation to the mouse anti PSA (Prostate‐Specific Antigen) antibodies (mab). Commercial CdSe/ZnS QDs used in the study are characterized by the color emission with the maximum at 565 nm (2.19 eV) at 300 K. Raman scattering spectra measured at room temperature demonstrate two groups of peaks: i) related to the Si substrate at 230–460, 522, 610, 670, 940‐1040 cm–1 and 2) to polymer on the QD surface in the spectral range 1268–3310 cm–1. It is revealed that the QD bio‐conjugation to the anti PSA mab is accompanied by the variation dramatically in the intensity of Raman lines of both types. The explanation of Raman peak stimulation in bio‐conjugated QDs has been proposed on the base of surface enhanced Raman scattering (SERS) effect (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

L-Cysteine-induced photoluminescence enhancement of CdSe/ZnSe quantum dots in aqueous solution

l-Cysteine molecules dramatically enhance the photoluminescence of colloidal CdSe/ZnSe quantum dots (i.e., CTAB/TOPOQD). Based on our spectroscopic studies of temporal variations in QD quantum yields as well as the in situ infrared spectral features of QDs, we propose that adsorption and rearrangement of l-cysteine molecules at the QD–water interface induces the observed unusual enhancement of the photoluminescence quantum yield. Upon addition of l-cysteine to the CTAB/TOPOQD solution, the adsorption of l-cysteine to the CTAB/TOPOQD colloidal particles is driven by the formation of a kinetically favorable intermediate species, which is formed by the coordination of thiol groups to the QD surface Cd atoms. The above species then reacts further to form a thermodynamically stable QD species, which probably involves coordination of both the amine and thiol groups of l-cysteine on the QD surface. Additional comparison studies using MPAQD and other small ligands (i.e., l-alanine, l-serine, and MPA) confirmed our proposed mechanism of l-cysteine adsorption at the CTAB/TOPOQD–water interfaces. In addition to these adsorption structures, we also propose that the dramatic enhancement of QY observed in this study is probably induced by the rearrangement and structural organization of l-cysteine and CTAB molecules at the QD–water interface, which improves the homogeneity and self-organization of the interfacial molecules.

Functionalized silicon quantum dots tailored for targeted siRNA delivery

For RNA interference (RNAi) mediated silencing of the ABCB1 gene in Caco-2 cells biocompatible luminescent silicon quantum dots (SiQDs) were developed to serve as self-tracking transfection tool for ABCB1 siRNA. While the 2–3 nm sized SiQD core exhibits green luminescence, the QD surfaces are completely saturated with covalently linked 2-vinylpyridine that may electrostatically bind siRNA. For down-regulating P-glycoprotein (Pgp) expression of the ABCB1 gene the SiQDs were complexed with siRNA. The cellular uptake and allocation of SiQD–siRNA complexes in Caco-2 cells were monitored using confocal laser scanning microscopy and transmission electron microscopy. The release of siRNA to the cytoplasm was verified through real-time PCR quantification of the reduced ABCB1 mRNA level. Additional evidence was obtained from time-resolved in situ fluorescence spectroscopic monitoring of the Pgp efflux dynamics in transfected Caco-2 cells which yielded significantly reduced transporter efficiencies for the Pgp substrate Rhodamine 123.

Core/CdS Quantum Dot/Shell Mesoporous Solar Cells with Improved Stability and Efficiency Using an Amorphous TiO2 Coating

Here we present a CdS quantum dot sensitized solar cell based on a mesoporous TiO2 film with remarkable stability using I−/I3− electrolyte. Chemical Bath Deposition (CBD) was used to deposit the CdS quantum dots within the porous network. We show that a thin coating of the QD sensitized film with an amorphous TiO2 layer strongly improves the performance and photostability of the solar cell. We propose that the coating passivates QD surface states which act as hole traps and are responsible for photodegradation of the device. In addition, this coating decreases the recombination of electrons from the CdS quantum dots and the mesoporous TiO2 into the electrolyte solution. We obtain a significant improvement of all cell parameters resulting in a total light to electric power conversion efficiency of 1.24%.

Ultrafast Charge Separation in Multiexcited CdSe Quantum Dots Mediated by Adsorbed Electron Acceptors

Ultrafast ET with a characteristic time constant of approximately 70 fs between CdSe QDs (mean radii of 1.4 nm) photoexcited in the lowest 1S electron state (lambda(exc) = 539 nm), and the molecular electron acceptor MV(2+) adsorbed on the QD surface was observed. The photophysics of such a system was investigated by time-resolved transient absorbance spectroscopy in the UV-visible spectral region. Our studies for the coupled system as a function of excitation intensity at lambda(exc) = 387 nm show that the ET processes compete efficiently with Auger recombination in CdSe QDs and at least 4 e-h pairs can be separated by ET to the electron acceptor MV(2+).

Oxidative Quenching and Degradation of Polymer-Encapsulated Quantum Dots: New Insights into the Long-Term Fate and Toxicity of Nanocrystals in Vivo

We report quenching and chemical degradation of polymer-coated quantum dots by reactive oxygen species (ROS), a group of oxygen-containing molecules that are produced by cellular metabolism and are involved in both normal physiological and disease processes such as oxidative signaling, cancer, and atherosclerosis. A major new finding is that hypochlorous acid (HOCl) in its neutral form is especially potent in degrading encapsulated QDs, due to its small size, neutral charge, long half-life, and fast reaction kinetics under physiologic conditions. Thus, small and neutral molecules such as HOCl and hydrogen peroxide (H2O2) are believed to diffuse across the polymer coating layer, leading to chemical oxidation of sulfur or selenium atoms on the QD surface. This "etching" process first generates lattice structural defects (which cause fluorescence quenching) and then produces soluble metal (e.g., cadmium and zinc) and chalcogenide (e.g., sulfur and selenium) species. We also find that significant fluorescence quenching occurs before QD dissolution and that localized surface defects can be repaired or "annealed" by UV light illumination. These results have important implications regarding the long-term fate and potential toxicity of semiconductor nanocrystals in vivo.

DNA-tile-directed self-assembly of quantum dots into two-dimensional nanopatterns.

DNA-tile-directed self-assembly of quantum dots into two-dimensional nanopatterns.

Biocompatible Quantum Dot Nanocolloids Stabilized by Gum Arabic

However, these surface modi-fication processes often involve covalent bond formationbetween ligand and QD surface and cause significant changesin the physicochemical states of the QD surface atoms. As aresult, dramatic degradation in the desired properties of QDswere frequently observed, such as significant decreases inphotochemical and/or colloidal stability, quantum yields(QY) and acceleration of coagulation/precipitation process.

Biocompatible Luminescent Silicon Quantum Dots for Imaging of Cancer Cells

Luminescent silicon quantum dots (Si QDs) have great potential for use in biological imaging and diagnostic applications. To exploit this potential, they must remain luminescent and stably dispersed in water and biological fluids over a wide range of pH and salt concentration. There have been many challenges in creating such stable water-dispersible Si QDs, including instability of photoluminescence due their fast oxidation in aqueous environments and the difficulty of attaching hydrophilic molecules to Si QD surfaces. In this paper, we report the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained. These luminescent micelle-encapsulated Si QDs were used as luminescent labels for pancreatic cancer cells. This paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.

On the Incorporation Mechanism of Hydrophobic Quantum Dots in Silica Spheres by a Reverse Microemulsion Method

In this work, we show strong experimental evidence in favor of a proposed incorporation mechanism of hydrophobic semiconductor nanocrystals (or quantum dots, QDs) in monodisperse silica spheres (diameter ∼35 nm) by a water-in-oil (W/O) reverse microemulsion synthesis. Fluorescence spectroscopy is used to investigate the rapid ligand exchange that takes place at the QD surface upon addition of the various synthesis reactants. It is found that hydrolyzed TEOS has a high affinity for the QD surface and replaces the hydrophobic amine ligands, which enables the transfer of the QDs to the hydrophilic interior of the micelles where silica growth takes place. By hindering the ligand exchange using stronger binding thiol ligands, the position of the incorporated QDs can be controlled from centered to off-center and eventually to the surface of the silica spheres. The proposed incorporation mechanism explains how we can have high control over the incorporation of single QDs exactly in the middle of silica spheres. ...

Monodisperse Oligo(phenylene vinylene) Ligands on CdSe Quantum Dots: Synthesis and Polarization Anisotropy Measurements

Well-defined oligo(phenylene vinylene) (OPV) molecules with phosphine oxide chain-ends were prepared and used for surface functionalization of CdSe quantum dots. Interestingly, OPV-covered CdSe nanostructures exhibit photoluminescence emission properties characteristic of linear dipoles, similar to those found in organic dyes and collapsed polymer chains. This is seen in the spatial photon distribution from a single OPV-covered CdSe nanostructure under slight defocusing in a high numerical aperture (NA) optical system (Inverted Nikon TE300 microscope, 1.4NA), excited in epi-illumination geometry using a 405 nm GaN diode laser. We speculate that the appearance of linear polarization in the QD emission is associated with photoinduced charge transfer from the ligand to the QD surface.

A simple biphasic route to water soluble dithiocarbamate functionalized quantum dots

Hydrophobic trioctylphosphine oxide-functionalized CdSe quantum dots (CdSe-TOPO QDs) were transferred from organic solvent to aqueous solution via a simple yet novel biphasic ligand exchange process in one step, which involved the in-situ formation of hydrophilic dithiocarbamate moieties and subsequent ligand exchange with TOPO at the chloroform/water interface. The resulting water dispersible, dithiocarbamate functionalized CdSe QDs (i.e., D-CdSe) exhibited an increased photoluminescence (PL) quantum yield as compared to the original CdSe-TOPO QDs, suggesting an effective passivation of dithiocarbamate ligands on the QD surface. The D-CdSe QDs were then mixed with hydroxyl terminated TiO2nanoparticles. A decrease in the PL of the mixture was observed, indicating a possible charge transfer from the D-CdSe QDs to the TiO2nanoparticles. The reaction of the carboxyl group on the D-CdSe surface with the hydroxyl group on the TiO2 rendered QDs in direct contact with TiO2, thereby facilitating the electronic interaction between them.

Electrochemical determination of mesoscopic phenomena, defect states in CdSe nanocrystals and charge carrier manipulability

The review gives an overview of cyclic voltammetry measurements performed on CdSe nanocrystals (so called Quantum Dots). Data of relevant publications is gathered and newly interpreted to give complete insights in charge transfer processes at QD surfaces. In specific, it is focused on the size-dependent electronic properties (Quantum-Size-Effect), the characterisation of surface defects, and the characterisation of hybrid nanocrystal/conducting polymer composites.

Surface Engineering of Quantum Dots for In Vivo Vascular Imaging

Quantum dot-antibody bioconjugates (QD-mAb) were synthesized incorporating PEG cross-linkers and Fc-shielding mAb fragments to increase in vivo circulation times and targeting efficiency. Microscopy of endothelial cell cultures incubated with QD-mAb directed against cell adhesion molecules (CAMs), when shielded to reduce Fc-mediated interactions, were more specific for their molecular targets. In vitro flow cytometry indicated that surface engineered QD-mAb labeled leukocyte subsets with minimal Fc-mediated binding. Nontargeted QD-mAb nanoparticles with Fc-blockade featured 64% (endothelial cells) and 53% (leukocytes) lower nonspecific binding than non-Fc-blocked nanoparticles. Spectrally distinct QD-mAb targeted to the cell adhesion molecules (CAMs) PECAM-1, ICAM-1, and VCAM-1 on the retinal endothelium in a rat model of diabetes were imaged in vivo using fluorescence angiography. Endogenously labeled circulating and adherent leukocyte subsets were imaged in rat models of diabetes and uveitis using QD-mAb targeted to RP-1 and CD45. Diabetic rats exhibited increased fluorescence in the retinal vasculature from QD bioconjugates to ICAM-1 and VCAM-1 but not PECAM-1. Both animal models exhibited leukocyte rolling and leukostasis in capillaries. Examination of retinal whole mounts prepared after in vivo imaging confirmed the fluorescence patterns seen in vivo. Comparison of the timecourse of retinal fluorescence from Fc-shielded and non-Fc-shielded bioconjugates indicated nonspecific uptake and increased clearance of the non-Fc-shielded QD-mAb. This combination of QD surface design elements offers a promising new in vivo approach to specifically label vascular cells and biomolecules of interest.

Photosensitized NO Release from Water-Soluble Nanoparticle Assemblies

The photoluminescence from CdSe/ZnS core/shell quantum dots, modified to make them water-soluble by exchanging the surface ligands with dihydrolipoic acid, is partially quenched by adding the chromium(III) complex trans-Cr(cylcam)(ONO)2+ (1, BF4- salt). This quenching, attributed to the formation of electrostatic assemblies due to ion pairing of 1 with the negatively charged QD surface, is accompanied by the release of NO owing to photosensitized reaction of the Cr(III) nitrito complex.

Design of Biotin-Functionalized Luminescent Quantum Dots

We report the design and synthesis of a tetraethylene glycol- (TEG-) based bidentate ligand functionalized with dihydrolipoic acid (DHLA) and biotin (DHLA—TEG—biotin) to promote biocompatibility of luminescent quantum dots (QD's). This new ligand readily binds to CdSe—ZnS core-shell QDs via surface ligand exchange. QDs capped with a mixture of DHLA and DHLA—TEG—biotin or polyethylene glycol- (PEG-) (molecular weight average ∼600) modified DHLA (DHLA—PEG600) and DHLA—TEG—biotin are easily dispersed in aqueous buffer solutions. In particular, homogeneous buffer solutions of QDs capped with a mixture of DHLA—PEG600 and DHLA—TEG—biotin that are stable over broad pH range have been prepared. QDs coated with mixtures of DHLA/DHLA—TEG—biotin and with DHLA—PEG600/DHLA—TEG—biotin were tested in surface binding assays and the results indicate that biotin groups on the QD surface interact specifically with NeutrAvidin-functionalized microtiter well plates.

Quantum dot sensitized semiconductors for solar energy conversion

Metal sulfide (CdS or PbS) quantum dots were synthesized in nanoporous TiO 2 films for applications in solar energy conversion devices. Sandwich type regenerative solar cells, based on the quantum dots sensitized TiO 2 film, exhibit a high IPCE over visible wavelengths by optimizing the polysulfide electrolyte composition. The CdS QD shows a higher IPCE, compared to PbS, related to an increased light harvesting efficiency when the number and size of the QDs intensified. In contrast, QD size dependence on the IPCE was observed for the PbS, likely resulting from the QD size dependence on a conduction band edge potential (associated with quantum size effect) relative to the TiO 2 conduction band edge, or the kinetic competition between the hot electron injection and the electron relaxation in the PbS conduction band. We also propose that an I 3 - /I - redox electrolyte, with NaSCN addition, can be employed to enhance the solar cell performance. SCN - ions may attach to the QD surface forming a shell type structure to prevent the photocorrosion reaction, and act as an intermediate electronic state to induce the sequential step electron transfer reactions for the QD re-reduction.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photoluminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system.

Design of Stable Bionanoparticles by PEG based Surface Modifications

Acetal-poly(ethylene glycol)/poy[(2-N,N-dimethylaminoethyl) methacrylate] (acetal-PEG/ PAMA) block copolymer was prepared via an oxianionic polymerization technique, which was established by our laboratory. The acetal-end group was converted to an aldehyde group by an acidic hydrolysis reaction, followed by the installation of biotin moiety at the end of the PEG chain via a reductive amination reaction of biocytin hydrazide. The biotin-PEG/PAMA block copolymer thus prepared was mixed with CdSe/ZnS nano-particles (QD) in aqueous media. The obtained polymer-QD complex showed extremely high dispersion stability even in the physiological saline condition. The adsorbed biotin-PEG/PAMA block copolymer on the QD formed the tethered chain fashion on the surface to improve the dispersion stability by the steric factor of the tethered chains. Since the PEG chain end on the QD surface possessed biotin moiety, it could be utilized for not only immunohistochemistry but also various other biological applications.

Surface-Tunable Photoluminescence from Block Copolymer-Stabilized Cadmium Sulfide Quantum Dots

The static and time-resolved photoluminescence properties of polystyrene-b-poly(acrylic acid) (PS-b-PAA)-stabilized cadmium sulfide quantum dots (CdS QDs) have been characterized for the first time, demonstrating tunable emission spectra and quantum yields via different chemical treatments of the PAA layer. Samples with the PAA layer in its cadmium carboxylate form showed more-intense band-edge emission and relatively high quantum yields compared with samples in which the PAA layer was in its acid form. This activation effect is explained in terms of passivation of trap sites on the QD surface by specific interactions between the QD and the cadmium-neutralized PAA layer. Lifetimes of band-edge and trap state emission for the various samples ranged from 40 to 61 ns and 244 to 360 ns, respectively. Impressive long-term stability was also shown for a sample of cadmium-neutralized PS-b-PAA-stabilized QDs dispersed in toluene, which maintained 90% of its photoluminescence over 57 days aging under ambient conditions. It is also shown that Cd2+ activation of photoluminescence does not occur when Mg2+ ions are added to similar QD solutions, indicating potential of these block copolymer-stabilized QDs as Cd2+-selective sensors. Irrespective of chemical treatment of the PAA layer, the external PS brush layer effectively stabilized all samples in various organic solvents, resulting in clear CdS colloids with no observed precipitation over several months. Dynamic light scattering and gel permeation chromatography revealed differences in the aggregation numbers and hydrodynamic radii of colloidal QDs for different treatments of the PAA layer, attributed to the lower solubility of the poly(cadmium acrylate) blocks compared to the PAA blocks in the acid form. Finally, it was demonstrated that the PS-b-PAA-stabilized QDs could be well dispersed in PS homopolymer, producing optically transparent photoluminescent films which retained the emission features of the colloidal QDs. Stable and surface-tunable optical properties via the PAA layer and polymer solubility and processability via the PS layer make these PS-b-PAA-stabilized CdS QDs exciting "building blocks" for the bottom-up assembly of functional hierarchical materials for photonics, sensors, and bio-labeling applications.