Reproducible detection of a target molecule is demonstrated using temporally stable solution-phase silica-void-gold nanoparticles and surface-enhanced Raman scattering (SERS). These composite nanostructures are homogeneous (diameter = 45 +/- 4 nm) and entrap single 13 nm gold nanoparticle cores inside porous silica membranes which prevent electromagnetic coupling and aggregation between adjacent nanoparticles. The optical properties of the gold nanoparticle cores and structural changes of the composite nanostructures are characterized using extinction spectroscopy and transmission electron microscopy, respectively, and both techniques are used to monitor the formation of the silica membrane. The resulting nanostructures exhibit temporally stable optical properties in the presence of salt and 2-naphthalenethiol. Similar SERS spectral features are observed when 2-naphthalenethiol is incubated with both bare and membrane-encapsulated gold nanoparticles. Disappearance of the S-H Raman vibrational band centered at 2566 cm(-1) with the composite nanoparticles indicates that the target molecule is binding directly to the metal surface. Furthermore, these nanostructures exhibit reproducible SERS signals for at least a 2 h period. This first demonstration of utilizing solution-phase silica-void-gold nanoparticles as reproducible SERS substrates will allow for future fundamental studies in understanding the mechanisms of SERS using solution-phase nanostructures as well as for applications that involve the direct and reproducible detection of biological and environmental molecules.

The heterogeneous processing of atmospheric aerosols by reaction with nitrogen oxides results in the formation of particulate and adsorbed nitrates. The water content of these hygroscopic nitrate aerosols and consequently the nitrate ion concentration depend on relative humidity, which can impact the physicochemical properties of these aerosols. This report focuses on the 310 nm photolysis of aqueous sodium and calcium nitrate solutions at pH 4 over a wide concentration range of nitrate ion concentrations representative of atmospheric aerosols. In particular, the quantum yield (phi) of nitrite formation was measured and found to significantly decrease at high concentrations of nitrate for Ca(NO(3))(2). In particular, phi for Ca(NO(3))(2) was found to have a maximum value of (7.8 +/- 0.1) x 10(-3) for nitrate ion solution concentrations near one molal, with the smallest quantum yield for the highest concentration solution above 14 m nitrate ion, phi = (2.3 +/- 2.0) x 10(-4). The effect of the addition of the radical scavenger, formate, on the 310 nm photolysis of these solutions was also investigated and found to increase phi by a factor of 2 or more for both sodium and calcium nitrate solutions. In the presence of formate, Ca(NO(3))(2) solutions again showed a significant decrease in phi with increasing NO(3)(-) concentration: phi = (1.4 +/- 0.1) x 10(-2) at (1.0 +/- 0.1) x 10(-2) m NO(3)(-) compared to phi = (4.2 +/- 0.3) x 10(-3) at 14.9 +/- 0.1 m NO(3)(-). This decrease in phi was not observed in NaNO(3) solutions. The change in electronic structure, as evident by the more pronounced shift of the n-pi* absorption band away from actinic wavelengths with increasing concentration for Ca(NO(3))(2) compared to NaNO(3), is most likely the origin of the greater decrease in phi for Ca(NO(3))(2) compared to NaNO(3) at elevated NO(3)(-) concentrations. The role of nitrate photochemistry in atmospheric aerosols and the atmospheric implications of these concentration dependent quantum yields are discussed.

Control over the composition, shape, size, stability, and local dielectric environment of solution-phase metallic substrates is vital to consistent surface-enhanced Raman scattering (SERS) signals. Because of their inherent instability, solution-phase nanoparticles can undergo uncontrolled aggregation when target molecules are added. Here, we demonstrate that both molecular surface coverage of the Raman active molecule, 2-naphthalenethiol (2-NT), and nanoparticle concentration are critical parameters for obtaining reproducible SERS signals using solution-phase gold nanoparticles. Both gold nanoparticle and 2-naphthalenethiol concentrations are varied, and the extinction of the nanoparticle substrate and the SERS intensity of the target molecule are monitored as a function of time. These results indicate that extinction and SERS spectral intensities increase predictably below full monolayer surface coverage. When excess molecules are added, uncontrolled and irreproducible nanoparticle aggregation leads to optimal overlap between the plasmonic properties of the nanoparticles and the SERS excitation wavelength. Importantly, this is the first report which correlates solution-phase nanoparticle concentration and stability to molecular surface coverage for simultaneous localized surface plasmon resonance (LSPR) and SERS spectroscopic measurements. As a result, these data should facilitate the experimental design and use of solution-phase SERS substrates for more predictable molecular detection.

This review focuses on the integration of noble metal nanoparticle aggregates as tags and transport vessels in cellular applications. The natural tendency of nanoparticles to aggregate can be reduced through surface modification; however, this stabilization is often compromised in the cellular environment. The degree of nanoparticle aggregation has both positive and negative consequences. Nanoparticle aggregates are more efficiently removed by the organism compared with single nanoparticles, preventing delivery to their cellular target. In addition, these aggregates are recognized by cells in different ways versus isolated nanoparticles. Despite these negatives, aggregates exhibit enhancement for many detection and treatment techniques in comparison with single nanoparticles. In coming years, the role of aggregates and better control over the degree of aggregation in cellular studies will be required for the realization of medical applications.