Defined nanoparticle cluster arrays (NCAs) with total lateral dimensions of up to 25.4 microm x 25.4 microm have been fabricated on top of a 10 nm thin gold film using template-guided self-assembly. This approach provides precise control of the structural parameters in the arrays, allowing a systematic variation of the average number of nanoparticles in the clusters (n) and the edge-to-edge separation (Lambda) between 1 < n < 20 and 50 nm < or = Lambda < or = 1000 nm, respectively. Investigations of the Rayleigh scattering spectra and surface-enhanced Raman scattering (SERS) signal intensities as a function of n and Lambda reveal direct near-field coupling between the particles within individual clusters, whose strength increases with the cluster size (n) until it saturates at around n = 4. Our analysis shows that strong near-field interactions between individual clusters significantly affect the SERS signal enhancement for edge-to-edge separations Lambda < 200 nm. The observed dependencies of the Raman signals on n and Lambda indicate that NCAs support a multiscale signal enhancement which originates from simultaneous inter- and intracluster coupling and |E|-field enhancement. The NCAs provide strong and reproducible SERS signals not only from small molecules but also from whole bacterial cells, which enabled a rapid spectral discrimination between three tested bacteria species: Escherichia coli, Bacillus cereus, and Staphylococcus aureus.

The ability to assess the risks of human exposure to engineered nanomaterials requires fundamental understanding of the fate and potential cytotoxicity of nonbiodegradable nanoparticles, for instance, after oral uptake. In this study, we quantify the impact of nanoparticles with low chemical toxicity on the intestinal membrane in a human intestinal in vitro model. Differentiated human colorectal adenocarcinoma cells, Caco-2, were cultured on a permeable support where they form an epithelial monolayer separating an apical and basal compartment. This model system allows a systematic characterization of the effect of nanoparticles on the cell viability as a function of size, surface chemistry, concentration, and incubation time. We used polystyrene (PS) nanoparticles (20 and 40 nm diameter) with two different surface chemistries (carboxylic acid and amines). The experiments performed show a strong decrease in cell viability as a response to nanoparticle exposure. Incubation times of

of the activation energy barrier, and Dx U, the distance to the transition state. For all protecting osmolytes we measure DG U increases, demonstrating that the I27 protein is stabilized. More striking is the measurement of Dx U . Unfolding the I27 protein in water gives a Dx U ¼ 2.5 Å , a distance similar to the size of a water molecule. Water molecules have been identified as integral components of the unfolding transition state of the I27 protein, forming a solvent bridge between two b-strands. By varying osmolyte molecule size we rigorous test this solvent bridging hypothesis. We find that Dx U correlates with osmolyte size for molecules ranging in size from 2.5 Å to 5.6 Å . However, for larger molecules (> 5.6 Å ) Dx U remains unchanged relative to the value measured in water, suggesting these osmolytes do not participate in solvent bridging in the transition state. These studies uniquely probe the length scales over which solvent molecules can modify the molecular architecture of the unfolding transition of a protein, an area which remains beyond the reach of other experimental techniques.