Optimization of Gold Nanoparticle-Based DNA Detection for Microarrays

DNA microarrays are promising tools for fast and highly parallel DNA detection by means of fluorescence or gold nanoparticle labeling. However, substrate modification with silanes (as a prerequisite for capture DNA binding) often leads to inhomogeneous surfaces and/or nonspecific binding of the labeled DNA. We examined both different substrate cleaning and activating protocols and also different blocking strategies for optimizing the procedures, especially those for nanoparticle labeling. Contact angle measurements as well as fluorescence microscopy, atomic force microscopy (AFM), and a flatbed scanner were used to analyze the multiple-step process. Although the examined different cleaning and activating protocols resulted in considerably different contact angles, meaning different substrate wettability, silanization led to similar hydrophobic surfaces which could be revealed as smooth surfaces of about 2–4 nm roughness. The two examined silanes (3-glycidoxypropyltrimethoxysilane (GOPS) and 3-aminopropyltriethoxysilane (APTES)) differed in their DNA binding homogeneity, maximum signal intensities, and sensitivity. Nonspecific gold binding on APTES/PDC surfaces could be blocked by treatment in 3% bovine serum albumin (BSA).     

Condensation of gases on metals: The one-dimensional square well model

Trapping probabilities of gas atoms at surfaces are calculated assuming a classical onedimensional square well potential as a function of gas and surface temperatures. It is shown that initial sticking coefficients of chemisorbed gases on transition metal surfaces can in most cases be fit fairly well by this model using reasonable values of the interaction energy, although the model does not predict observed behavior for surface temperatures>600°K. For some systems the initial sticking coefficients are higher than predicted by this model, indicating that other mechanisms of energy transfer are probably operative. The angular dependent sticking coefficient which would be measured in a molecular beam experiment is also computed.     

A simple formula for low-energy sputtering yields

A simple formula for low-energy sputtering yields of elemental targets is presented. The formula follows directly from Sigmund's theory but includes a modified form of the function(M _t /M _p ) and an accurate expression for the nuclear stopping cross section. Good agreement with experimental results is obtained for the projectiles Ne, Ar, Kr sputtering not too reactive surfaces at energies 1 keV. Further experiments are suggested as well as theoretical work concerning the meaning of the surface binding energy and the effect of the surface on the sputtering process in general.     

The adsorption of CO on the (110) plane of tungsten; LEED,XPS, and Auger measurements

Adsorption of CO on W(110) at 100 K produces a number of ordered LEED patterns as coverage increases, culminating in a p(5 × 1) pattern for a full virgin CO layer. The beta-1 layer obtained by heating a virgin layer to 400 K has a p(2 × 1) structure. Absolute coverages, obtained by comparison of XPS intensities (and Auger intensities where feasible) with those of oxygen on tungsten at O/W = 0.5 indicate that CO/W ? 0.8 for the full virgin layer and ? 0.3 for beta-1. These results, together with the LEED data, indicate that low temperature adsorption of virgin CO is not very site specific, and that beta-1 must be dissociated with C and O lying along alternate closepacked rows of W. XPS results for the oxygen 1s peak show that the latter shifts in beta and beta-1 from its position in virgin CO to an energy equal to that seen for pure oxygen on tungsten. A number of electron impact desorption results are also presented, and the nature of the various binding states of CO on this plane is discussed.     

Desorption rates and mechanisms for dissociatively chemisorbed molecules

The desorption kinetics of dissociatively chemisorbed diatomic molecules are examined from a kinetic-modeling point of view. A comparison is made between a one-step process, resulting in the usual second-order kinetics, and a two-step process which takes into account explicity recombination of atoms and subsequent desorption of molecules. The kinetics from the two-step mechanism are found to be equivalent to second-order desorption with a coverage-dependent activation energy which, in many cases, is a linear function of coverage. The two-step process leads to second-order kinetics with a constant activation energy only for special values of the model rate parameters, or if chemisorption is activated. The steady onate approximation for the intermediate in the two-step process is often adequate, but the transient period during which a steady state is reached also contains important kinetic information. The implications of these results for desorption kinetics and molecular beam reaction experiments are discussed. Work performed under the auspices of the Office of Basic Energy Sciences of the Department of Energy     

Oxygen adsorption on the tungsten (110) plane. Electron impact and thermal desorption at high temperatures

When a layer of oxygen on the (110) plane of tungsten at coverages O/W≦0.5 is heated from 100 K, O⁺ evolution under electron impact becomes almost negligible at 600 K. On further heating, however, a slow, temperature-dependent evolution of O⁺ current is observed atT≳1500 K. For O/W>0.3 there is also desorption under massive bombardment. Once an equilibrium value of O⁺ current has been established, there is rapid adjustment to the appropriate equilibrium value when the temperature changes in the range 1500–1700 K. On cooling toT<1000 K, O⁺ decreases rapidly; on reheating toT>1500 K, O⁺ increases slowly again. Above 1700 K there is thermal desorption which is also reflected in the O⁺ signal. These facts indicate that there is a slow activated evolution of an electron sensitive state above 1500 K, from a reconstructed state formed by heating the low temperature layer toT≧1000 K. The latter state seems to be reformed on cooling below 1500 K.     

Enzyme-induced growth of silver nanoparticles studied on single particle level

Based on their interesting properties, metal nanoparticles show the potential as an analytical tool in electronic (Burmeister et al. 2004), optical (Yguerabide and Yguerabide 1998), and catalytic applications (Liu 2006). Their characteristics depend on the composition, shape, and size of the single particles. These various properties are utilized in many different approaches such as optics, magnetics (Lang et al. 2007), and laser technology (Csaki et al. 2007). We investigated an alternative method for the synthesis of nanoparticles. In this case, an enzyme, horseradish peroxidase, induces a silver deposition and replaces a metal nanoparticle as the reaction seed. Depending on the reaction time, we could obtain particles in a range of few nanometers up to more than 250 nm. For a better understanding of the enzymatic silver deposition process, the silver particles produced by this process were analyzed by SEM, TEM, and atomic force microscopy (AFM) on a single particle level after different enhancement times. The AFM images were utilized for the characterization of particle height and volume to study the enzyme kinetics, i.e., the particle growth process. Thereby, two different phases are described: a first growth phase probably induced by the enzyme-related growth, and a second, more unspecific growth based on the metal deposition onto the silver deposits. These findings may help to use the enzyme-induced silver deposition in a quantitative manner for bioanalytical applications.     

The scattering of phonons of arbitrary wavelength at a solid-solid interface: Model calculation and applications

A one-dimensional lattice model of a solid-solid interface is presented within which it is possible to characterize the scattering of phonons at the interface as a function of wavelength. The probability for a phonon to be transmitted across the interface is found generally to decrease with decreasing wavelength, although phenomena such as total reflexion and resonant transmission may occur. Conditions for the existence of a localized interface mode are given. The thermal boundary resistance for heat flow across the interface is expressed in terms of an average temperature-dependent phonon transmission coefficient which generally increases with decreasing temperature and approaches the continuum value at very low temperature. Applications of these results to three-dimensional interfaces in general, and particularly to heat dissipation in catalysts, high-frequency phonon radiators, and Kapitza resistance, are discussed.