pH and concentration dependence of the optical properties of thiol-capped CdTe nanocrystals in water and D2O.

The optical properties of semiconductor nanocrystals (SC NCs) are largely controlled by their size and surface chemistry, i.e., the chemical composition and thickness of inorganic passivation shells and the chemical nature and number of surface ligands as well as the strength of their bonds to surface atoms. The latter is particularly important for CdTe NCs, which - together with alloyed CdxHg1-xTe - are the only SC NCs that can be prepared in water in high quality without the need for an additional inorganic passivation shell. Aiming at a better understanding of the role of stabilizing ligands for the control of the application-relevant fluorescence features of SC NCs, we assessed the influence of two of the most commonly used monodentate thiol ligands, thioglycolic acid (TGA) and mercaptopropionic acid (MPA), on the colloidal stability, photoluminescence (PL) quantum yield (QY), and PL decay behavior of a set of CdTe NC colloids. As an indirect measure for the strength of the coordinative bond of the ligands to SC NC surface atoms, the influence of the pH (pD) and the concentration on the PL properties of these colloids was examined in water and D2O and compared to the results from previous dilution studies with a set of thiol-capped Cd1-xHgxTe SC NCs in D2O. As a prerequisite for these studies, the number of surface ligands was determined photometrically at different steps of purification after SC NC synthesis with Ellman's test. Our results demonstrate ligand control of the pH-dependent PL of these SC NCs, with MPA-stabilized CdTe NCs being less prone to luminescence quenching than TGA-capped ones. For both types of CdTe colloids, ligand desorption is more pronounced in H2O compared to D2O, underlining also the role of hydrogen bonding and solvent molecules.

Flexible and fragmentable tandem photosensitive nanocrystal skins.

We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 µW cm(-2) at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.

Resonance energy transfer in self-organized organic/inorganic dendrite structures.

Hybrid materials formed by semiconductor quantum dots and J-aggregates of cyanine dyes provide a unique combination of enhanced absorption in inorganic constituents with large oscillator strength and extremely narrow exciton bands of the organic component. The optical properties of dendrite structures with fractal dimension 1.7-1.8, formed from J-aggregates integrated with CdTe quantum dots (QDs), have been investigated by photoluminescence spectroscopy and fluorescence lifetime imaging microscopy. Our results demonstrate that (i) J-aggregates are coupled to QDs by Förster-type resonant energy transfer and (ii) there are energy fluxes from the periphery to the centre of the structure, where the QD density is higher than in the periphery of the dendrite. Such an anisotropic energy transport can be only observed when dendrites are formed from QDs integrated with J-aggregates. These QD/J-aggregate hybrid systems can have applications in light harvesting systems and optical sensors with extended absorption spectra.

Saturated near-resonant refractive optical nonlinearity in CdTe quantum dots.

In this work, we report on an experimental investigation of the nonlinear optical properties near the first electronic resonance of thiol-capped CdTe quantum dots (QDs) being in the strong confinement regime. Using a cw laser excitation in a Z-scan experimental setup, we show the presence of saturated Kerr-type nonlinear optical properties of the QDs, at low intensity levels. The large optical nonlinearity and the control of the linear and nonlinear optical properties by the size of the QDs are of special interest for applications in integrated nanophotonic devices.

Protective effect of tripeptide in the presence of cyclophosphamide on the growth of cultured lymphoid tissue from rats of different age.

We studied the effect of tripeptide T-38 (Lys-Glu-Asp) in the presence of cyclophosphamide on cell proliferation and apoptosis in explants of splenic lymphoid tissue from young and old rats. Peptide T-38 in a concentration of 0.05 ng/ml produced a stimulatory effect on the growth zone of the explants. Addition of 1 mg/ml cyclophosphamide to the culture medium suppressed cell proliferation, which was associated with enhanced expression of proapoptotic p53 protein. Under conditions of combined treatment with cyclophosphamide and T-38 no inhibiting effect of the cytostatics was observed. Thus, tripeptide T-38 in the presence of cytostatics produces a protective effect on cell proliferation in lymphoid tissue explants.

Switchable photoluminescence of CdTe nanocrystals by temperature-responsive microgels.

In the present study, we report a method for preparing a fluorescent thermosensitive hybrid material based on monodisperse, thermosensitive poly( N-isopropyl acrylamide) (PNIPAM) microgels covered with CdTe nanocrystals of 3.2 nm diameter. The CdTe nanocrystals were covalently immobilized on the surface of PNIPAM microgels. The chemical environment around the CdTe nanocrystals was modified by changing the temperature and inducing the microgel volume-phase transition. This change provoked a steep variation in the nanocrystal photoluminescence (PL) intensity in such a way that when the temperature was under the low critical solution temperature (LCST) of the polymer (36 degrees C) the PL of the nanocrystals was strongly quenched, whereas above the LCST the PL intensity was restored.

Structure of nocturnal sleep in patients with cerebral insult.

The aim of the present work was to study the structure of nocturnal sleep in patients who had sustained brain insults, and to relates sleep structure to the stage of disease and the location of the focal lesion. Studies were performed on 18 patients with ischemic insult, 8 affected in the right hemisphere, 6 affected in the left hemisphere, and 4 with brainstem lesions; controls consisted of 5 healthy subjects. Diagnoses were in all cases confirmed by computer tomography. Clinical and neurological studies were performed, along with recording of polygraph traces of nocturnal sleep (electroencephalograms, electrooculograms, electromyograms). Sleep parameters were analyzed using a program developed at the Sleep Studies Center, I. M. Sechenov Moscow Medical Academy; along with standard parameters, this program included analysis of the segmental structure of sleep. Sleep quality-the level of normality or disturbance to its structure-was assessed using a sleep index (SI). Patients showed profound disorganization of nocturnal sleep structure, with disruption of the mechanisms organizing sleep as a whole and generating and maintaining the individual stages. The greatest sleep disturbances were seen in ischemic insult in the right hemisphere and in patients with lesions in medial deep structures.