Biomedical subjects

Claudia Querner

Publications and source records attributed to Claudia Querner.

5 recordsLinked to original sources

PubMed2006

Fluorescence blinking statistics from CdSe core and core/shell nanorods.

We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratios ranging from 3 to 11. This study also included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that, for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions are best described by a truncated power law, P(tau(on)) approximately tau(on)(-alpha)e((-tau)(on)/(tau)(c)). The measured crossover time, tau(c), is indistinguishable within experimental uncertainty for core and core/shell NRs, as well as for core NRs with different ligands, for the same core size, indicating that surface passivation does not affect the blinking statistics significantly. We find that, at fixed excitation intensity, 1/tau(c) increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/tau(c) increases very gradually with increasing excitation intensity. Examining 1/tau(c)versus the single-particle photon absorption rate for all samples indicates that the change in NR absorption cross section with sample size can account for some but not all of the differences in crossover time. This suggests that the degree of quantum confinement may be partially responsible for the aspect ratio dependence of the crossover time.

Absorption↗

PubMed2006

Electric-field-driven accumulation and alignment of CdSe and CdTe nanorods in nanoscale devices.

Local electric fields generated by nanopatterned electrodes were used to control the position and orientation of well-isolated as well as closely packed colloidal semiconducting CdTe and CdSe nanorods (NRs) drop-cast from solution. Postdeposition imaging using transmission-electron microscopy and atomic-force microscopy revealed strong NR alignment to the direction of the applied field and dense accumulation around and onto voltage-biased electrodes when deposited from dilute and concentrated solutions, respectively. The degree of alignment under the applied electric field is characterized by a nematic order parameter S approximately 0.8 in contrast to the zero-field case when S approximately 0.1.

Cadmium Compounds↗

PubMed2005

Size and ligand effects on the electrochemical and spectroelectrochemical responses of CdSe nanocrystals.

The electrochemical properties of CdSe quantum dots with electrochemically inactive surface ligands (TOPO) have been investigated in comparison with the analogous nanocrystals containing electrochemically active oligoaniline ligands. The TOPO-capped nanocrystals have been studied in a wide size range (from 3 to 6.5 nm) with the goal to amplify the influence of the quantum confinement effect on the electrochemical response. The determined HOMO and LUMO levels have been found in good agreement with the ones obtained from photoluminescence studies and those predicted theoretically. Ligand exchange with aniline tetramer significantly influences the voltammetric peaks associated with the HOMO oxidation and the LUMO reduction of the quantum dots, which are shifted to higher and lower potentials, respectively. These shifts are interpreted in terms of the positive ligand charging which precedes the oxidation of the nanocrystals and the insulating nature of the ligand in the case of the nanocrystal reduction. The ligand-nanocrystal interactions have also been studied by UV-Vis-NIR and Raman spectroelectrochemistry in comparison with a specially prepared model compound which, apart from the anchoring function is identical to the grafted oligoaniline ligand. Both spectroelectrochemical techniques clearly indicate the same nature of the oxidation/reduction pathway for both the model compound and the grafted ligand. The influence of the grafting is manifested by a shift in the onset of the ligand oxidation as compared to the case of the "free" model compound. Since both components (ligands and nanocrystals) mutually influence their electrochemical and spectroelectrochemical properties, the newly developed system can be considered as a true molecular hybrid. Such hybrids are of interest because the potential zone of the ligand electroactivity is well separated from that of the nanocrystals and, as a result, the organic part can be electrochemically switched between the semiconducting and the conducting states with no change in the oxidation state of the nanocrystal. The newly developed system offers therefore the possibility of an electrical addressing of individual nanocrystals via the conducting ligands.

Aniline Compounds↗

PubMed2004

Chelating ligands for nanocrystals' surface functionalization.

A new family of ligands for the surface functionalization of CdSe nanocrystals is proposed, namely alkyl or aryl derivatives of carbodithioic acids (R-C(S)SH). The main advantages of these new ligands are as follows: they nearly quantitatively exchange the initial surface ligands (TOPO) in very mild conditions; they significantly improve the resistance of nanocrystals against photooxidation because of their ability of strong chelate-type binding to metal atoms; their relatively simple preparation via Grignard intermediates facilitates the development of new bifunctional ligands containing, in addition to the anchoring carbodithioate group, a second function, which enables the grafting of molecules or macromolecules of interest on the nanocrystal surface. To give an example of this approach, we report, for the first time, the grafting of an electroactive oligomer from the polyaniline family-aniline tetramer-on CdSe nanocrystals after their functionalization with 4-formyldithiobenzoic acid. The grafting proceeds via a condensation reaction between the aldehyde group of the ligand and the terminal primary amine group of the tetramer. The resulting organic/inorganic hybrid exhibits complete extinction of the fluorescence of its constituents, indicating efficient charge or energy transfer between the organic and the inorganic semiconductors.

Journal Article↗

PubMed2004

Characterization of structural changes of poly(vinyl methyl ether) gamma-irradiated in diluted aqueous solutions.

An aqueous deoxygenated dilute PVME solution (cP = 0.5 mass %) was irradiated with gamma-rays at different radiation doses (D = 0.2-10 kGy). At these concentrations and doses contracted molecules are formed but no macroscopic networks. For the analysis of the structural changes, the irradiated samples were analyzed by size exclusion chromatography (SEC) with a triple detector system, static light scattering, and viscometry. SEC measurements necessitate for the application of the universal calibration principle the knowledge of the Kuhn-Mark-Houwink (KMH) parameters. To obtain these parameters commercially available poly(vinyl methyl ether) was fractionated and analyzed by the same means. We found at 30 degrees C a KMH relationship to [eta] = 0.0226 (mL/g) x Mw(0.67) in THF. We observed an increase in molar masses without any significant structural changing for D < 0.5 kGy. With increasing radiation dose, the intramolecular cross-linking reaction becomes more and more important. The results of viscosity measurements show a slight increase in contraction for 1.0 kGy. For irradiation dose higher than D > 1.5 kGy, a strong contraction occurs. For D > 5.0 kGy, the favored intramolecular reactions lead to the formation of microgels.

Gamma Rays↗