Label-free biosensors based on optically responsive nanocomposite layers: sensitivity and dynamic range.

Core-shell nanoparticle layers have proven to be a promising tool for the label-free detection of binding events. Upon reflection of white light, they exhibit pronounced extinction peaks in the UV/vis and NIR regime of the electromagnetic spectrum, which shift to higher wavelengths when molecules are adsorbed. Beside drastic simplification of the instrumentation and related reduction in cost, a significantly stronger response toward alkanethiol adsorption has been observed in previous experiments than in conventional surface plasmon resonance (SPR). However, as the amount of molecules deposited onto the nanoparticle films was unknown, no quantitative relationship could be established between the measured wavelength shifts and the surface mass density of the adsorbate. In order to facilitate quantitative molecule detection, self-assembled monolayers (SAMs) of simple and ethylene glycol (EG) terminated alkanethiols with various chain lengths were prepared on the nanoparticle-coated substrates. The measured red-shift of the extinction spectrum upon molecule adsorption was related to the amount of adsorbate as determined by X-ray photoelectron spectroscopy (XPS). For the whole range of film thicknesses studied, a linear relationship is found yielding a sensitivity factor of 0.027 nm/(ng/cm (2)). As proven by enzyme-linked immunosorbent assay (ELISA), such determined sensitivity factor can also be used to correctly predict the amount of surface-bound protein in immunoreactions from the measured wavelength shifts. It is concluded that the decay length of the evanescent electric field associated with the nanoparticle sensors is more than 100 nm and, thus, significantly larger than that observed for localized surface plasmons excited in small isolated metal clusters.

Fabrication of versatile 3-D ternary nanostructures via gas-mediated metal evaporation.

By combining nanosphere lithography (NSL) with metal evaporation at two different chamber pressures, ternary metallic nanostructures with 3D topography were fabricated and characterized by field emission gun scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The nanostructures consist of 340 nm silicon patches surrounded by a chromium mesh of varying height and gold triangular pillars with a height of 40 nm and a diameter of about 200 nm. While NSL was applied due to its ease of use, evaporation at different chamber pressures can be combined with any kind of template mask comprising convex shape to yield novel kinds of ternary nanostructures analogous to the ones presented here. The method shows to be more versatile than plasma-based deposition techniques due to avoidance of substrate charging effects and a freely adjustable chamber pressure.

Density depletion at solid-liquid interfaces: a neutron reflectivity study.

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Spectroscopic characterization of omega-substituted biphenylthiolates on gold and their use as substrates for "on-top" siloxane SAM formation.

Self-assembled monolayers (SAMs) of omega-substituted biphenylthiolates (omega-MBP) on gold were characterized by spectral ellipsometry, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and vibrational sum frequency generation spectroscopy (VSFG). The vibrational studies of the SAMs were supported by an ab initio frequency analysis at HF/6-31G and BP86/6-31G levels, yielding an assignment of all relevant spectral features in the range from 3500 to 1200 cm(-1). We were able to demonstrate that hydroxy-terminated MBP (HMBP) SAMs are basically featureless in the range of the CH stretching vibrations. Accordingly, the adsorption of a SAM of octadecyltrichlorosilane (OTS) on top of this model surface could be studied. A red shift of the C-O stretching vibration from 1281 to 1264 cm(-1) was observed during the chemisorption of OTS, thus allowing for a quantification of the number of OTS molecules involved in surface binding of OTS, which was found to be about 26% on average.

Interaction of self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiols with water studied by vibrational sum-frequency generation.

Vibrational sum-frequency generation (VSFG) was used to investigate the conformational changes in self-assembled monolayers (SAMs) of (1-mercaptoundec-11-yl) hexa(ethylene glycol) monomethylether (EG6-OMe) on gold when exposed to liquid water. VSFG spectra of the EG6-OMe SAMs were recorded before, during, and after exposure of the films to water and after a subsequent evacuation step. While in contact with water the entire ethylene glycol chains are found in a random, solvated state, after removal from the fluid water molecules remain absorbed only at the terminal groups of the film giving rise to distinct conformational changes. After evacuation, the structure of the EG6-OMe SAM reverts to its original state, indicating that water has been removed from the monolayer. Our findings support recent ab initio calculations and Monte Carlo simulations on the interaction of ethylene glycol-terminated monolayers with water.

Swelling behavior of self-assembled monolayers of alkanethiol-terminated poly(ethylene glycol): a neutron reflectometry study.

The swelling behavior of alkanethiol-terminated poly(ethylene glycol) with an average molecular weight of 2180 Da (i.e., approximately 45 ethylene glycol, EG, units) in contact with water was investigated by neutron reflectometry as a function of the morphology of the PEG-SH layer. Amorphous films at a low grafting density show significant swelling with an increase of the film thickness from approximately 25 A in the dry state to approximately 70 A in contact with D2O, which corresponds to a total water uptake of approximately 38 mass %. In contrast, quasi-crystalline monolayers exhibit only a small amount of water penetrating into the film (approximately 8 mass %) with a corresponding change of the layer thickness from approximately 110 to approximately 125 A. The water uptake per EG unit corresponds to the literature value of 1.5 for the amorphous layer and to only 0.25 in the case of the quasi-crystalline film.