ABSTRACT
The dependence of the interfacial electron transfer in alizarin-sensitized TiO2 nanoparticles on the sample pH has been examined via transient absorbance spectroscopy in the visible spectral region (443-763 nm). Excitation of the alizarin/TiO2 system with visible pump pulses (lambdaexc = 500 nm) leads to a very fast electron injection (tauinj < 100 fs) over a wide pH range. Back electron transfer shows complicated multiphasic kinetics and strongly depends on the acidity of the solution. The strong dependence of back-electron-transfer dynamics on the ambient pH value is explained by a Nernstian-type change in the semiconductor band energy. Indeed, a variation of pH values over 7 units leads to a approximately 0.42 eV change of the conduction band edge position (i.e., the nominal free energy of the electron in the electrode). Assuming a pH-independent redox potential of the dye, this change was sufficient to push the system to a condition where direct photoinitiated electron injection to intraband gap surface states could be investigated. The existence of an electron-transfer pathway via surface trap states is supported by the similarity of the observed back-electron-transfer kinetics of alizarin/TiO2 at pH 9 and alizarin/ZrO2 reported in earlier work (J. Phys. Chem. B 2000, 104, 8995), where the conduction band edge is approximately 1 eV above the excited state of the dye. The influence of surface trap states on interfacial electron transfer has been studied, and a detailed analysis of their population, depopulation, and relaxation kinetics is performed. Therefore, alizarin adsorbed on the surface of TiO2 nanoparticles is an ideally suited system, where pH-dependent investigations allow a detailed study of the electron dynamics in trap states of TiO2 nanoparticles.
ABSTRACT
UNLABELLED: Memory and attention are cognitive functions that depend heavily on the cholinergic system. Local activity of acetylcholine esterase (AChE) is an indicator of its integrity. Using a recently developed tracer for positron emission tomography (PET), C-11-labeled N-methyl-4-piperidyl-acetate (C11-MP4A), we measured regional AChE activity in 4 non-demented subjects, 4 patients with dementia of Alzheimer type (DAT) and 1 patient with senile dementia of Lewy body type (SDLT), and compared the findings with measurements of blood flow (CBF) and glucose metabolism (CMRGlc). Initial tracer extraction was closely related to CBF. AChE activity was reduced significantly in all brain regions in demented subjects, whereas reduction of CMRGlc and CBF was more limited to temporo-parietal association areas. AChE activity in SDLT was in the lower range of values in DAT. Our results indicate that, compared to non-demented controls, there is a global reduction of cortical AChE activity in dementia. KEYWORDS: Dementia, cholinergic system, acetylcholine esterase, positron emission tomography, cerebral blood flow, cerebral glucose metabolism.
