Band 222, Heft 5-6

This special issue of Zeitschrift für Physikalische Chemie is a compilation of works presented at the International Bunsen Discussion Meeting. Held on June 29-30, 2007 at the University of Heidelberg, the meeting was organized to honor the scientific achievements of Professor Michael Grunze on his 60th anniversary.The title of the meeting, “Modern surface science: Out of the vacuum – through the liquid – into the cell”, reflects the evolution of surface science over the past three decades; Michael Grunze has been an integral part of this evolution, making seminal contributions at all stages.

The local electronic structure at Zn 2+ ions during complex formation with histidine and cysteine has been studied by near edge x-ray absorption fine structure spectroscopy at the Zn L-edge. At pH ≅ 5 both histidine and cysteine have carboxylate and amino groups able to form complexes with cations in solution such as Zn 2+ . Compared to histidine, cysteine has an extra thiolate group which can chelate Zn 2+ . This investigation shows that histidine is chelating the Zn 2+ ion mainly via the amino group, while cysteine chelates primarily via the thiolate group. The nature of the empty molecular orbitals involved in the transitions has been analyzed using density functional theory including the solvent effects. By comparing the calculated results with the experimental observations, we conclude that histidine affects primarily the Zn 2+ electronic states of d-symmetry, while cysteine affects both s- and d-states. This mechanism is important for understanding the zinc sulfhydryl bond in zinc finger proteins, where cysteine and histidine in the protein are chelating Zn 2+ .

A homologue series of n -alkane thiols (CH 3 (CH 2 ) m SH, MC m , m between 8 and 22) adsorbed on Au/mica substrates was studied in an alkaline environment using cyclic voltammetry (CV) and second harmonic generation (SHG). The known split of the reductive desorption signal in the cyclic voltammogram for thiols with chain length of m > 12 disappears again for m = 22. Simultaneous recording of SHG and CV data allowed correlation of thiol coverage with charge which was found to be linear, irrespective of the shape of the desorption signal. This evidences that the double peak structure of the desorption signal does not arise from the separation of faradaic and capacitive processes. The same is found for the double wave of the readsorption signal. The transient appearance of a split desorption signal for the one electron process of thiol desorption in the range of 12 < m < 22 is suggested to originate from intermolecular interactions.

We compare the interactions of water with the ferroelectric copolymer poly(vinylidene fluoride (PVDF) – trifluoroethylene (TrFE)) and poly(methylvinylidenecyanide) (PMCV), a strongly dipole ordered polymer. At the microscopic scale, dipole interactions matter and affect the surface chemistry at these polymer surfaces, as does lattice strain caused by water absorption. Light polarization dependent photo-assisted thermal desorption helps demonstrate that water desorption from surface and bulk can be influenced by the formation of electronic metastable states. Changes in local dipole orientation and the formation of long lived metastable states affect the strength of the coupling between the dipoles of water molecules and the dipoles of the copolymer poly(vinylidene fluoride – trifluoroethylene) but these effects were not observed for poly(methylvinylidenecyanide).

Holography with the low energy electron point source (LEEPS) microscope has been explored for imaging of unstained biomolecules. DNA ropes are prepared on microfabricated sample supports and imaged with coherent 40 eV electrons emitted from a W(111) point source. We reconstruct the electron wave front and show that the correct source-to-object distance can be determined by analyzing the focus of the reconstruction. Upon variation of the source position, the reconstruction shows reproducible object features with an object focus located at the correct tip-to-object distance with a depth of sharpness of ∼40 nm. It is argued that reconstruction artifacts can be identified by their focus behavior and that only features in the first focus – coming from the screen – truly relate to the object's structure.

This review presents the molecular basis of the high affinity between (strept)avidin and biotin as it was discovered from different protein crystal structures using wild type and mutant streptavidin. Optimization strategies for further improving the applicability of the (strept)avidin-biotin system and prospects for modulating the affinity are discussed. The characterization and the application of the streptavidin-biotin system in surface-based biosensing assays are demonstrated with selected examples focussing on surface plasmon resonance (SPR) and atomic force microscopy (AFM). Recent trends to further enhance the utility of convential SPR e.g. parallel detection of biological molecules and sensitivity enhancement towards small molecules are covered as well.

Muscovite mica surfaces were coated with methyl- and hydroxyl-terminated dialkyldimethylammonium monolayers via ion exchange by immersion of the substrates into solutions of the corresponding salts. The samples were examined by water contact-angle measurements and near edge X-Ray absorption fine structure spectroscopy (NEXAFS). The purely methyl-terminated species was found to form self-assembled monolayers (SAMs) of comparable quality to those of a similar thiol/gold system as has been reported earlier. The hydroxyl-terminated surfactant established films with a lower degree of orientation under the same preparation conditions and showed a high water contact angle value for a hydrophilic surface (47°). The molecularly mixed film where one alkyl chain of the surfactant is methyl terminated and one hydroxyl terminated had a significantly lower wettability but established a similar degree of orientation as the film prepared from the purely OH-terminated species.

Quantum dots (QDs) are becoming an increasingly popular fluorescent probe in biological imaging and single molecule applications. With advantages and disadvantages over traditional organic fluorophores, quantitative characterization of the photophysical properties of QDs is a required task for optimizing their performance. For example, maximizing the number of collected photons is essential for high-quality fluorescence imaging and yet is often a limiting factor in biological applications. Using fluorescence correlation spectroscopy (FCS), we compare important photophysical properties (count rates, photobleaching quantum yields, dark state occupancy and dark-state-to-bright-state interconversion rates, among others) of typical commercial CdSe/ZnS QDs against commonly used organic fluorophores relevant to biological applications. Two-photon action cross sections are measured using a novel version of the reference method in a laser-scanning confocal microscope geometry. FCS results for QDs show a correlation between reduced brightness, high fraction of molecules in dark states, and slow interconversion rates between the bright state and dark state(s) consistent with previous work. We confirm large two-photon action cross sections (10 3 −10 4 GM) and broad two-photon excitation spectra that suggest QDs as advantageous probes for multicolor multiphoton imaging. FCS results show Alexa546 is a particularly bright probe suited for use when probe size is a limitation. While superior in count rate to Alexa555, Alexa546 bleaches faster when used in one-photon excitation.

CdSe(ZnS) quantum dots conjugated to the electron donor dopamine show enhancement of photoluminescence over a time course of seconds to minutes when exposed to ultraviolet or blue irradiation. This phenomenon is observed when the dots are in aqueous solution as well as after endocytosis by living cells. The rate of enhancement is directly related to the number of dopamine conjugates per quantum dot, and can be semi-quantitatively adjusted over a wide range. Uptake of the particles by cells is also dependent upon surface coverage, with moderate coverages proving ideal for both uptake and relative signal enhancement. The enhancement is strongly power-dependent and may also show dependence upon wavelength; however, direct excitation of the dopamine molecule is not necessary for the enhancement effect.

The accurate theoretical and computational treatment of chemical dynamics and reactivity generally makes use of quantum mechanical analyses of the anharmonic motions of atoms within molecular systems. As reactive dynamics are inherently anharmonic, special potential energy functions that can address the issue are required. The purpose of this paper, therefore, is to discuss a model potential energy function that may accurately represent both local molecular anharmonic and discrete harmonic motions while incorporating the collective harmonic dynamics of the rest of a macromolecule in a parametric form. The model is approximately analogous to a small molecule chemisorbed on or embedded in an elastic surface. As the embedding region in the macromolecule in which the local group resides is generally highly anisotropic, the usual development of continuum elasticity theory is not straightforward. Instead, this approach adopts a local surface model with anisotropic points of attachment between the surface and atoms of the local group. The dynamics of the points of attachment, i.e., anchor pseudo-atoms in a cavity, are proposed to be treated phenomenologically to yield local force constants as fitting parameters to represent the effect of the collective motion ofthe remainder of the macromolecule. A potential energy function of this type should be useful for the analysis of local reactive dynamics in macromolecular systems, such as a reactive polymer or an enzyme. Extension of the model to a closed cavity may also be useful for the treatment of local discrete effects of solvent coordination on solute spectra and dynamics.

He-atom scattering (HAS), X-ray photoelectron spectroscopy (XPS and NEXAFS), periodic density functional theory and quantum chemical ab initio cluster calculations have been used for characterizing the adsorption of CO 2 on the mixed terminated ZnO(10-10) surface. The HAS data allow to identify three different structural phases of CO 2 in the monolayer regime: A high-density (1×1) phase at low temperature, a lower density (2×1) phase at higher temperatures, and in addition an incommensurate phase (which has not been reported before) with a slightly lower coverage and a 2.3-fold periodicity at surface temperatures above 270 K. XPS and NEXAFS measurements performed for the two low-coverage phases show that CO 2 is adsorbed in form of a carbonate species. This is supported by the ab initio calculations for CO 2 adsorbed on ZnO(10-10). The calculated XP and NEXAFS spectra agree with the experimental data and are only compatible with an adsorbed carbonate species, the presence of physisorbed CO 2 can be excluded. In addition, the calculations demonstrate that X-ray emission spectroscopy (XES) is a sensitive method to distinguish between linear CO 2 and carbonate.

We demonstrate the metallization of amino-terminated, cross-linked biphenylthiol self-assembled monolayers (SAMs) via Pd catalysed electroless deposition (ELD) of Cu. Angle resolved X-ray photoelectron spectroscopy and atomic force microscopy were used for characterization. Lateral cross-linking and generation of terminal amino groups are induced by irradiation with low energy electrons. The cross-linked SAM suppresses the ELD of Cu onto the Au substrate. By the immobilization of Pd atoms to the terminal amino groups, catalytic sites are created in cross-linked areas. These sites then facilitate the electroless deposition of Cu on top of the SAM.

High resolution electron energy loss spectroscopy (HREELS) is a powerful method for the study of vibrational and electronic excitations at solid surfaces and has been extensively applied to metal single crystal surfaces. As a result of experimental difficulties, unfortunately, much less information is available on adsorbate vibrations at oxide surfaces. This review focuses on recent results showing the successful application of HREELS to study adsorption and reaction of molecules on metal oxide single crystal surfaces. The chemical reactivity of perfect surfaces is first investigated systematically using HREELS combined with thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). Furthermore, it is demonstrated that the interaction of adsorbates with surface defects (in particular oxygen vacancies) can also be monitored by vibrational spectroscopy.

Self-assembeld monolayers (SAMs) formed by the adsorption of 4 ′ -nitro-1,1 ′ -biphenyl-4-thiol (NBT) on gold substrates were studied using infrared reflection-absorption spectroscopy (IRRAS). The IRRAS results revealed that the straightforward approach for the fabrication of SAMs, namely, immersions of the gold film in organothiol solutions, leads to the formation of highly oriented SAMs. To prepare highly oriented amino-terminated SAMs, the nitro-terminated SAMs were immersed into ZnCl 2 /HCl solution to chemically convert the nitro group to an amino group. IR spectra recorded for this conversion revealed that the conversion is completed after a fairly short period of time (45 min.), immersion for longer time results in the etching of the SAMs by the acidic solution.