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Single molecule fluorescence experiments yield a stream of photocounts, whose statistical properties contain valuable information about processes within the molecule and in its microscopic environment. The photon statistics display features specific of single quantum systems and are therefore best discussed in the frame of a quantum mechanical theory of radiation. The present summary of a lecture given in April 2002 at the Hofgeismar Spring School on single molecules presents the main concepts used in quantum theory of light, together with a few useful references, and discusses some illustrations and applications to single molecule measurements drawn from the recent literature.

The use of molecular building blocks opens a new dimension for nanotechnology. Biomolecules offer a variety of possibilities for manipulation, provide a new size dimension and are especially suitable for "bottom up" approaches. Nucleic acids are of special interest due to their ability of self-organization, the achieved combinatorial information capacity and its molecular-biological processability. Here we present an approach for a molecular component systems with DNA-based elements and products that is suitable for molecular nanotechnology. Oligonucleotides thereby serve as biological modifiers of nanoparticles and surfaces to form self-assembling monolayers, and genomic DNA acts as framework for the building blocks. A first application of DNA-nanoparticle complexes could be the use as a novel, highly-stable label for chip technologies, with the potential for single-molecule detection. Another field is the fabrication of novel electronic devices, based on extreme miniaturization. This paper describes the different fields of use for DNA-based molecular modules, and presents first results of the realization of this concept.

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Scanning near-field optical microscopes (SNOM) illuminate a sample in the very near-field using a nanometer sized tip. Ideally, the light source should be point-like and many efforts have been made to optimize tip efficiency (see, for example, the article of Heimel et al in this issue). Very recently, Sandoghdar et al have realized a molecular probe tip in which a terrylene molecule inserted in a paraterphenyl microcrystal is attached at the extremity of the probe tip [1]. The excited molecule behaves as a point-like light source which is raster scanned over an aluminium patterned structure. We propose here an analysis of this experiment based on the field-susceptibility formalism (also called Green's Dyadic Method) [2,3].

First results of a new dynamic approach to calculate the field enhancement at a metal ellipsoid close to a metal surface are presented. The theoretical approach is based on the solution of Maxwells equations. The numerical calculation is influenced by the dielectric constants of both metals, the angle of incidence of the monochromatic electromagnetic wave, the distance of the ellipsoid to the metal surface and the semi axis ratio of the ellipsoid.

Aldosterone is an important mediator of osmoregulation in vertebrate cells and is thought to be involved in cell differentation. Its signaling pathway starts with the formation of a complex with the mineralocorticoid receptor (MCR) which is then translocated into the cell nucleus where transcription of target genes is initiated. Finally, mRNAs of specifically activated genes appear in the cytoplasm. Using atomic force microscopy (AFM), we were able to visualize the signaling cascade described above. We injected stage VI oocytes of Xenopus laevis with aldosterone and isolated their nuclei after different incubation times. Nuclei obtained 2 min after application of aldosterone clearly exhibited macromolecules ("flags") attached to the nuclear pore complexes (NPCs) when examined by AFM. The estimated size of the observed particles (80-160 kDa) corresponds with the actual size of the MCR ([sim]120kDa). NPCs of oocytes injected about 20 min prior to preparation showed large macromolecules ("plugs") within their central channels. We concluded that these plugs contain transcripts induced by aldosterone.

In this work we present a framework for the calculation of the conduction properties of a metal-molecule-metal junction which is in contact with its thermal environment. The effects of thermal relaxation and dephasing on the transmission properties of the junction were studied using a simple tight binding model for the molecular conductor. The interaction between the molecular system and the thermal environment is described on the level of the Redfield theory, which is a weak electron-phonon coupling scheme, modified for the description of steady-state situations. We show that the transmitted flux consists of two (generally non-separable) components: a flux associated with the elastic tunneling and a thermally activated flux component. The coherent (tunneling) component dominates the transport at low temperatures, large energy gaps and short molecular chains. The incoherent (activated) component is important in the opposite limits. The integrated transmission provides a generalization of the Landauer conduction formula in the presence of thermal relaxation.

Porphyrin dendrimers were synthesized to mimic naturally occurring proteins, which catalyze a number of biochemically important reactions. In addition, chiral dendrimers were prepared as model compounds for the study of nanoscopic chirality. The structures of these dendrimers cannot be characterized by x-ray, as most dendrimers do not crystallize. We succeeded to image single, physisorbed dendrimers on noble metal surfaces with STM. All examined dendrimers can easily be (re)moved with the STM tip, even at low scanning currents (low pA range). One possibility to avoid this, is to change the peripheral groups of the dendrimer, so that they chemisorb on the surface. We decided to study physisorbed macromolecules with unchanged molecular properties, and investigated how the approach by Tokuhisa et al. for chemisorbed polyamidoamin (PAMAM) dendrimers, could be adapted to our molecules. Indeed, even physisorbed molecules are stabilized by embedding them in a self-assembled monolayer of covalently bonded 1-hexadecanethiol. The so embedded dendrimers show a reduced mobility on the substrate with a conformation closer to the one in solution, permitting STM imaging at various temperatures.