We demonstrate visualization of localized intense electromagnetic fields in real space in well-tailored dimeric and trimeric gold nanospheres by using near-field optical techniques. With two-photon induced luminescence and Raman measurements, we show that the electric field is confined at an interstitial site in the aggregate. We also demonstrate optical switching operations for the electric-field localized sites in the trimer structure.

In this paper, a direct and simple detection method based on surface-enhanced Raman scattering (SERS) named "heat-induced SERS sensing method" is proposed for rapid determination of glutathione in aqueous solutions. It was found that highly enhanced SERS spectra of glutathione can be obtained if the silver colloids adsorbed with the analyte were heated up before the SERS measurement. Besides, it was revealed that silver particles with a size of approximately 60 nm are suitable for this study and that the SERS intensity is also influenced by the dropped sample volume, drying temperature, buffer concentration, and pH of the solution. It is noted that the thiol group of glutathione has a particularly strong interaction with a silver surface compared with other small biological molecules without a thiol group, validating this method to detect glutathione selectively. Under the optimal conditions, the detection of glutathione can be finished within 5 min, and the detection limit of ca. 50 nM can be reached, which is much better than the reported detection limit of glutathione (approximately 1 microM) by SERS. The enhancement factor of the proposed heat-induced SERS sensing method for the detection of glutathione is about 7.5 x 10(6). The proposed method holds a specific selectivity toward glutathione, facilitating its rapid detection in practical applications.

Whereas serious health consequences of widespread consumption of groundwater elevated in As have been documented in several South Asian countries, the mechanisms responsible for As mobilization in reducing aquifers remain poorly understood. We document here a previously unrecognized and consistent relationship between dissolved As concentrations in reducing groundwater and the phosphate-mobilizable As content of aquifer sediment for a set of precisely depth-matched samples from across Bangladesh. The relationship holds across nearly 3 orders of magnitude in As concentrations and suggests that regional as well as local patterns of dissolved As in shallow groundwater are set by the solid phase according to a remarkably constant ratio of ∼250 μg/L dissolved As per 1 mg/kg P-mobilizable As. We use this relationship in a simple model of groundwater recharge to propose that the distribution of groundwater As in shallow aquifers of the Bengal Basin could primarily reflect the different flushing histories of sand formations deposited in the region over the past several thousand years.

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Surface-enhanced Raman scattering (SERS) enhancement and the reproducibility of the SERS signal strongly reflect the quality and nature of the SERS substrates because of diverse localized surface plasmon resonance (LSPR) excitations excited at interstitials or sharp edges. LSPR excitations are the most important ingredients for achieving huge enhancements in the SERS process. In this report, we introduce several gold and silver nanoparticle-based SERS-active substrates developed solely by us and use these substrates to investigate the influence of LSPR excitations on SERS. SERS-active gold substrates were fabricated by immobilizing colloidal gold nanoparticles on glass slides without using any surfactants or electrolytes, whereas most of the SERS-active substrates that use colloidal gold/silver nanoparticles are not free of surfactant. Isolated aggregates, chain-like elongated aggregates and two-dimensional (2D) nanostructures were found to consist mostly of monolayers rather than agglomerations. With reference to correlated LSPR and SERS, combined experiments were carried out on a single platform at the same spatial position. The isolated aggregates mostly show a broadened and shifted SPR peak, whereas a weak blue-shifted peak is observed near 430 nm in addition to broadened peaks centered at 635 and 720 nm in the red spectral region in the chain-like elongated aggregates. In the case of 2D nanostructures, several SPR peaks are observed in diverse frequency regions. The characteristics of LSPR and SERS for the same gold nanoaggregates lead to a good correlation between SPR and SERS images. The elongated gold nanostructures show a higher enhancement of the Raman signal than the the isolated and 2D samples. In the case of SERS-active silver substrates for protein detection, a new approach has been adopted, in contrast to the conventional fabrication method. Colloidal silver nanoparticles are immobilized on the protein functionalized glass slides, and further SERS measurements are carried out based on LSPR excitations. A new strategy for the detection of biomolecules, particularly glutathione, under aqueous conditions is proposed. Finally, supramolecular J-aggregates of ionic dyes incorporated with silver colloidal aggregates are characterized by SERS measurements and correlated to finite-difference time-domain analysis with reference to LSPR excitations.

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We observed near-field images of aggregates of gold nanospheres, by detecting TPI-PL from gold and Raman scattering from Rhodamine 6G adsorbed on the nanoparticles. The results were analyzed in relation to the localized Raman active site (hot spot) of surface-enhanced Raman scattering.Although chemical specificity of Raman spectra is expected to be useful in various fields of science, small cross section of Raman scattering sometimes causes difficulties in practical applications. To extend the applicability of Raman spectroscopy to wider purposes, it is essential to enhance the scattering cross section. In surface-enhanced Raman scattering (SERS), 1 the Raman signal is so drastically enhanced that it would give an effective solution to the difficulties. Even single-molecule level Raman spectroscopy has been reported recently by using the SERS scheme.2 In the mechanism of SERS, it is considered that enhancement of the electric field by plasmon in noble metal nanoparticles plays a crucial role. This mechanism of SERS is called electromagnetic (EM) enhancement.3 In addition to the EM mechanism, chemical effects are also considered to have some contribution to the SERS enhancement. To understand and control the SERS process, knowledge about spatial structures and other characteristics of plasmons is indispensable.In single-molecule level SERS, spatially localized plasmon resonance, called ''hot spot,'' is believed to be essential for the Raman enhancement.3 Understanding of the physical properties of hot spots and imaging of them are hence important. From electromagnetic calculations on dimers of noble metal spherical nanoparticles, it has recently been revealed that the radiation field is strongly enhanced in the interstitial space between the particles (sometimes called as ''junction'').3 Such a structure may be regarded as a model system for the hot spot, but up to now direct microscopic observation of the dimeric structures and radiation field distribution has not been reported.The present authors recently reported that the plasmonic modes are visualized using near-field two-photon induced photoluminescence (TPI-PL). 4 Applying this method to the aggregates of metal nanoparticles, distribution of the electric field can be imaged. By monitoring Raman signals, direct visualization of hot spot distribution is also possible. In this letter, we report imaging of hot spots on gold nanosphere aggregates by near-field TPI-PL and Raman imaging. We consider that such direct optical imaging with high spatial resolution is essential to understand structures of hot spots and the electromagnetic effects of junctions in SERS process.Gold nanosphere (diameter 100 nm) solution was purchased from BB International. Gold dimers were prepared by aggregating the gold nanospheres on a cover-slip treated with trimethoxy[3-(methylamino)propyl]silane. The cover-slip was then spin-coated by a methanol/water solution of Rhodamine 6G (R6G) molecules. The area density of R6G molecules was estimated to be less than 15 molecules per 100 Â 100...