Surface charge induced Stark effect on luminescence of quantum dots conjugated on functionalized carbon nanotubes

Ternary Chalcogenide-Based Quantum Dots and Carbon Nanotubes: Establishing a Toolbox for Controlled Formation of Nanocomposites

A general procedure based on electrostatic self-assembly for preparing nanocomposites based on carbon nanotubes (CNTs) and ternary chalcogenide semiconductor nanoparticles is shown. This was achieved by surface functionalization of the single components through well-established protocols, for CNTs, and a transferable general strategy for the nanoparticles. Heterostructures were then synthesized through electrostatic interaction between oppositely charged components. Structural, colloidal, and optical properties were characterized by transmission electron microscopy, X-ray diffraction, infrared spectroscopy, dynamic light scattering, ζ-potential, and absorption- and (time-resolved) photoluminescence measurements. Interestingly, the nanocomposites showed a blue shift in their excitation and emission spectra when compared to the pure nanoparticles but only when analyzed in powder form. Further investigations in the form of density functional theory (DFT) calculations were performed to evaluate the origin of the change in the optical properties.

Tuning the Optical Band Gap of Semiconductor Nanocomposites-A Case Study with ZnS/Carbon

The linear photochemical response of materials depends on two critical parameters: the size of the optical band gap determines the onset of optical excitation, whereas the absolute energetic positions of the band edges define the reductive or oxidative character of photo-generated electrons and holes. Tuning these characteristics is necessary for many potential applications and can be achieved through changes in the bulk composition or particle size, adjustment of the surface chemistry or the application of electrostatic fields. In this contribution the influence of surface chemistry and fields is investigated systematically with the help of standard DFT calculations for a typical case, namely composites prepared from ZnS quantum dots and functionalized carbon nanotubes. After comparing results with existing qualitative and quantitative experimental data, it is shown conclusively, that the details of the surface chemistry (especially defects) in combination with electrostatic fields have the largest influence. In conclusion, the development of novel or improved photoresponsive materials therefore will have to integrate a careful analysis of the interplay between surface chemistry, surface charges and interaction with the material environment or substrate.

Effective reduction of nonspecific binding by surface engineering of quantum dots with bovine serum albumin for cell-targeted imaging

Quantum dots (QDs) have been widely used as fluorescent probes in cell-targeted imaging. However, nonspecific binding to cellular membranes has been a major challenge. In this study, a new approach is developed for effective reduction of nonspecific binding by bovine serum albumin (BSA)-coated QDs in cell targeting. The experimental results show efficient transfer of hydrophobic QDs from organic to aqueous phase in the presence of BSA aqueous solution under ultrasonication. This ultrasonication-based approach is facile, rapid, and efficient. Stabilization of QDs is mainly achieved by multiple mercapto groups in BSA macromolecules as multidentate ligands and partially by hydrophobic interaction between BSA and pending fatty ligands on QDs. The water solubility of QDs is enhanced by the surface amino and carboxyl groups, which also provide reaction sites for conjugation of targeting ligands. The BSA-coated QDs, with an overwhelming majority of hydrodynamic diameter size of ca. 18 nm, are colloidally stable under both acidic and basic conditions and found to exhibit strong fluorescent intensities. The nonspecific cellular binding is effectively reduced by BSA-coated QDs, compared with the mercaptopropionic acid (MPA)-coated CdTe QDs. BSA-coated QDs are further functionalized with cyclic Arg-Gly-Asp (cRGD) peptide. The cell assays indicate their high target-selectivity in integrin α(v)β(3)-expressed cell imaging.

Spectral properties of multiply charged semiconductor quantum dots

Spectrally resolved fluorescence imaging of single CdSe/ZnS quantum dots (QDs), charged by electrospray deposition under negative bias has revealed a surprising net blue shift (∼60 meV peak-to-peak) in the distribution of center frequencies in QD band-edge luminescence. Electrostatic force microscopy (EFM) on the electrospray QD samples showed a subpopulation of charged QDs with 4.7 ± 0.7 excess electrons, as well as a significant fraction of uncharged QDs as evidenced by the distinct cantilever response under bias. We show that the blue-shifted peak recombination energy can be understood as a first-order electronic perturbation that affects the band-edge electron- and hole-states differently. These studies provide new insight into the role of electronic perturbations of QD luminescence by excess charges.

Engineered multifunctional nanocarriers for cancer diagnosis and therapeutics

This article reviews advances in the design and development of multifunctional carbon-based and/or magnetic nanoparticle systems (or simply 'nanocarriers') for early cancer diagnosis and spatially and temporally controlled therapy. The critical issues in cancer diagnosis and treatment are addressed based on novel nanotechnologies such as real-time in-vivo imaging, drug storage and release, and specific cancer-cell targeting. The implementation of nanocarriers into animal models and the subsequent effectiveness in treating tumors is also reviewed. Recommendations for future research are given.