Band-Gap Engineering of Metal Oxides for Dye-Sensitized Solar Cells

Mixed oxides Ti(1-x)Zr(x)O2 with 0 < x < or = 0.2 were synthesized by means of thermal hydrolysis for use in dye-sensitized solar cells. The lattice parameter d is observed to increase linearly with Zr content x. The band gap of the mixed oxides was measured to increase by up to 0.2 eV. The respective shift of the conduction band edge leads to an increase of the open circuit voltage (V(OC)) by up to 0.1 V. Among others, temperature-dependent measurements of V(OC) clearly identify the correlation between band edge shift and change in V(OC).

Up-conversion fluorescence: noncoherent excitation by sunlight

We demonstrate up-conversion of noncoherent sunlight realized by ultralow excitation intensity. The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has dramatically different photophysical characteristics relative to the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc.).

Metal-enhanced up-conversion fluorescence: effective triplet-triplet annihilation near silver surface

Up-conversion phenomena are traditionally related to two- or multiphoton processes occurring under relatively high excitation intensities. Here we present the first results of ultralow excitation intensity (in order of Wcm(-2)) continuous-wave (CW) excited up-conversion fluorescence in Kretschmann surface plasmon geometry. The active system is a blue-emitting polymer matrix blended with metalated porphyrine macrocycles. The up-conversion fluorescence is a consequence of a two-particle triplet-triplet annihilation process (TTA).

Low-temperature fabrication of dye-sensitized solar cells by transfer of composite porous layers

Dye-sensitized solar cells have established themselves as a potential low-cost alternative to conventional solar cells owing to their remarkably high power-conversion efficiency combined with 'low-tech' fabrication processes. As a further advantage, the active layers consisting of nanoporous TiO2 are only some tens of micrometres thick and are therefore in principle suited for flexible applications. However, typical flexible plastic substrates cannot withstand the process temperatures of up to 500 degrees C commonly used for sintering the TiO2 nanoparticles together. Even though some promising routes for low-temperature sintering have been proposed, those layers cannot compete as regards electrical properties with layers obtained with the standard high-temperature process. Here we show that by a lift-off technique, presintered porous layers can be transferred to an arbitrary second substrate, and the original electrical properties of the transferred porous layers are maintained. The transfer process is greatly assisted by the application of composite layers comprising nanoparticles and nanorods.

Diffusion Properties of Dye Molecules in Nanoporous TiO2 Networks

The adsorption of ruthenium-dye molecules out of ethanol solution onto TiO2 particles of nanoporous TiO2 films was used to study the molecules' diffusion through these layers by means of optical absorption spectrometry. Dependent on pore size, porosity, and particle size, effective diffusion constants as low as D(eff) = 4 x 10(-9) cm2/s were deduced from the uptake curves by applying a simple model for combined diffusion and adsorption. These diffusion constants for diffusion through the nanoporous network are up to 3 orders of magnitude lower than in bulk ethanol and are discussed with respect to the properties of the nanoporous material.

Odd-even chain length-dependent order in pH-switchable self-assembled layers

The reversible self-assembly of a series of bipolar amphiphiles, alpha,omega-bis(3- or 4-amidinophenoxy)alkanes (chain length n = 5-12), on mercaptoalkanoic acid-functionalized gold surfaces (chain length n = 10, 11, 14, 15) has been studied by in-situ ellipsometry, IR reflection absorption spectroscopy (IRAS), and atomic force microscopy (AFM). The layer order, amphiphile orientation, and tendency to form bilayers depends on the position of the amidine substituent, the alkyl chain length of both the amidine amphiphile and the underlying acid self-assembled monolayer (SAM), and whether the amidine alkyl chain contained an even or odd number of methylene groups. Thus, para-substituted bisbenzamidines containing more than six methylene groups (n>6) and with an odd number (n = 7, 9, 11) tended to form bilayered structures, whereas those containing an even number formed monolayers when adsorbed on SAMs of the long-chain acids (n = 14, 15). This behavior also correlated with the average tilt angle of the benzene moieties of the amphiphiles, as estimated by IRAS. The odd-numbered chains gave lower tilt angles than the even-numbered ones, and a possible model that accounts for these results is proposed. IRAS also revealed a higher order of the odd-numbered chains and an increasing hydrogen-bonding contribution with increasing chain length. Additional evidence for the proposed bilayered assemblies and their reversibility was obtained by AFM. Images obtained from the assembly of decamidine on a SAM of mercaptohexadecanoic acid in a pH 9 borate buffer revealed domains of similar size to that of the underlying acid SAM (20-30 nm), but less densely packed. By acidifying the solution, the second layer was destabilized and a very smooth layer with few defects appeared. Further acidification to pH 3 also destabilized the first layer.

Impact of micropatterned surfaces on neuronal polarity

Experimental control over cellular polarity in a neuronal network is a promising tool to study synapse formation and network behavior. We aimed to exploit a mechanism described by Stenger et al. [J. Neurosci. Methods 82 (1998) 167] to manipulate the direction of axonal versus dendritic outgrowth on a micropattern. The group had used laser ablation to create patterns of aminated silanes for cell attachment on a background of repellent fluorinated silanes. The pattern offered continuous adhesive pathways for axonal and interrupted pathways for dendritic outgrowth. By microcontact printing, we created similar patterns containing continuous and interrupted pathways consisting of extracellular matrix proteins on a background of polystyrene. Neuronal polarity was determined on the functional level through double patch clamp measurements, detecting synapses and their orientation. Although our pattern reproduced the properties that were assumed to be critical for the described effect, namely contrasting pathways of different adhesiveness, we failed to reproduce the above results. It is indicated that other qualities of alternative pathways than mere differences in adhesiveness are required to orient neuronal polarity in vitro. We suggest that the effect observed by Stenger et al. has to be attributed to less universal characteristics of the micropattern, e.g. to the specific chemical groups that were utilized.

Diffusion-limited transport of I[sub 3][sup −] through nanoporous TiO[sub 2]-polymer gel networks

Tri-iodide transport in a polymer gel electrolyte embedded in nanoporous TiO(2) networks and its diffusion limits are investigated by means of current-voltage (I-V) characteristics of simple Pt-gel-Pt sandwich devices with a thin porous TiO(2) layer sintered directly onto one of the Pt electrodes. At voltages between 0.2 and 0.7 V, the I-V curves of such devices show the typical plateau of diffusion-limited redox reactions, in this case I(-)/I(3) (-), at the platinum electrodes. From the dependence of the limiting current density on layer thickness, the diffusion constants D(bulk) and D(p,eff) of tri-iodide in the bulk polymer gel and through a polymer gel penetrated TiO(2) network, respectively, have been found to be D(bulk)=3.2(+/-0.2)x10(-6) cm(2)/s and D(p,eff)=1.5(+/-0.1)x10(-6) cm(2)/s. Temperature-dependent measurements show diffusion in the gel to be activated by about 0.16 eV. The results are discussed in comparison to diffusion in liquid electrolytes as well as with respect to the implications for dye-sensitized solar cell devices.

Analysis of electrotonic coupling in patterned neuronal networks

Microcontact printing of laminin is known as an efficient approach for guiding neuronal cell migration and neurite outgrowth on artificial surfaces. In the present study, ultrathin (approximately 250 microm) brain stem slices of Sprague-Dawley rats (E15-E18) were cultured on laminin-patterned substrates, such that neuronal cells migrating out of the slices formed grid-shaped neuronal networks along the geometry defined by the pattern. The interconnections between neighbouring pairs of neurons within these artificial networks were assessed electrophysiologically by double patch-clamp recordings and optically by microinjection of fluorescent dyes. Both functional and electrotonic synapses were detected. Based on the recorded data and simulations in PSpice, an electrical model for electrotonically coupled cells was derived. In this model the neuritic pathway is described as a cylindric cable, and gap junctions are represented by an ohmic resistor. Applying this model in the data analysis, the average inner radius of neurites could be determined to be approximately 0.1 microm. In addition, evidence was found for a correlation between the path-width of the applied pattern and the diameter of neurites growing along these paths.

A functional MRI study of three motor tasks in the evaluation of stroke recovery

Functional brain imaging studies have provided insights into the processes related to motor recovery after stroke. The comparative value of different motor activation tasks for probing these processes has received limited study. We hypothesized that different hand motor tasks would activate the brain differently in controls, and that this would affect control-patient comparisons. Functional magnetic resonance imaging (MRI) was used to evaluate nine control subjects and seven patients with good recovery after a left hemisphere hemiparetic stroke. The volume of activated brain in bilateral sensorimotor cortex and four other motor regions was compared during each of three tasks performed by the right hand: index-finger tapping, four-finger tapping, and squeezing. In control subjects, activation in left sensorimotor cortex was found to be significantly larger during squeezing as compared with index-finger tapping. When comparing control subjects with stroke patients, patients showed a larger volume of activation in right sensorimotor cortex during index-finger tapping but not with four-finger tapping or squeezing. In addition, patients also showed a trend toward larger activation volume than controls within left supplementary motor area during index-finger tapping but not during the other tasks. Motion artifact was more common with squeezing than with the tapping tasks. The choice of hand motor tasks used during brain mapping can influence findings in control subjects as well as the differences identified between controls and stroke patients. The results may be useful for future studies of motor recovery after stroke.

Compensatory visual field training for patients with hemianopia after stroke

Twenty-one patients with hemianopia received 4 weeks of compensatory visual field training. Detection of and reaction time to visual stimuli were measured with eyes fixating (condition A) and with use of exploratory eye movements (condition B) before and after training. Twenty-three healthy individuals served as control subjects for measurements of parameters during both conditions. Patients with hemianopia to either side showed a marked improvement of detection and reaction time during condition B, but minimum or no change during condition A. Improvements were maintained 8 months after training. Activity of daily living skills also improved in all patients. The size of scotoma on computerized perimetry, in contrast, remained unchanged. Training improved detection of and reaction to visual stimuli without restitution of the visual field defect.

Arm Training Induced Brain Plasticity in Stroke Studied with Serial Positron Emission Tomography

We used serial positron emission tomography (PET) to study training-induced brain plasticity after severe hemiparetic stroke. Ten patients were randomized to either task-oriented arm training or to a control group and scanned before and after 22.6 +/- 1.6 days of treatment using passive movements as an activation paradigm. Increases of regional cerebral blood flow (rCBF) were assessed using statistical parametric mapping (SPM99). Before treatment, all stroke patients revealed bilateral activation of the inferior parietal cortex (IPC). After task-oriented arm training, activation was found bilaterally in IPC and premotor cortex, but also in the contralateral sensorimotor cortex (SMC). The control group only showed weak activation of the ipsilateral IPC. After treatment, the training group revealed relatively more activation bilaterally in IPC, premotor areas, and in the contralateral SMC. Five normal subjects showed no statistical significant differences between two separate PET studies. In this group of patients, task-oriented arm training induced functional brain reorganization in bilateral sensory and motor systems.

Evolution of functional reorganization in hemiplegic stroke: a serial positron emission tomographic activation study

We used serial positron emission tomography (PET) to study the evolution of functional brain activity within 12 weeks after a first subcortical stroke. Six hemiplegic stroke patients and three normal subjects were scanned twice (PET 1 and PET 2) by using passive elbow movements as an activation paradigm. Increases of regional cerebral blood flow comparing passive movements and rest and differences of regional cerebral blood flow between PET 1 and PET 2 in patients and normal subjects were assessed by using statistical parametric mapping. In controls, activation was found in the contralateral sensorimotor cortex, supplementary motor area, and bilaterally in the inferior parietal cortex with no differences between PET 1 and PET 2. In stroke patients, at PET 1, activation was observed in the bilateral inferior parietal cortex, contralateral sensorimotor cortex, and ipsilateral dorsolateral prefrontal cortex, supplementary motor area, and cingulate cortex. At PET 2, significant increases of regional cerebral blood flow were found in the contralateral sensorimotor cortex and bilateral inferior parietal cortex. A region that was activated at PET 2 only was found in the ipsilateral premotor area. Recovery from hemiplegia is accompanied by changes of brain activation in sensory and motor systems. These alterations of cerebral activity may be critical for the restoration of motor function.

Reorganization of sensory and motor systems in hemiplegic stroke patients. A positron emission tomography study

BACKGROUND AND PURPOSE

Cortical reorganization of motor systems has been found in recovered stroke patients. Reorganization in nonrecovered hemiplegic stroke patients early after stroke, however, is less well described. We used positron emission tomography to study the functional reorganization of motor and sensory systems in hemiplegic stroke patients before motor recovery.

METHODS

Regional cerebral blood flow (rCBF) was measured in 6 hemiplegic stroke patients with a single, subcortical infarct and 3 normal subjects with the [(15)O]H(2)O injection technique. Brain activation was achieved by passive elbow movements driven by a torque motor. Increases of rCBF comparing passive movements and rest were assessed with statistical parametric mapping. Significant differences were defined at P<0.01.

RESULTS

In normal subjects, significant increases of rCBF were found in the contralateral sensorimotor cortex, supplementary motor area, cingulate cortex, and bilaterally in the inferior parietal cortex. In stroke patients, significant activation was observed bilaterally in the inferior parietal cortex and in the contralateral sensorimotor cortex, ipsilateral prefrontal cortex, supplementary motor area, and cingulate cortex. Significantly larger increases of rCBF in patients compared with normal subjects were found bilaterally in the sensorimotor cortex, stronger in the ipsilateral, unaffected hemisphere, and in both parietal lobes, including the ipsilateral precuneus.

CONCLUSIONS

Passive movements in hemiplegic stroke patients before clinical recovery elicit some of the brain activation patterns that have been described during active movements after substantial motor recovery. Changes of cerebral activation in sensory and motor systems occur early after stroke and may be a first step toward restoration of motor function after stroke.