Versatile Gap Mode Plasmon under ATR Geometry towards Single Molecule Raman, Laser Trapping and Photocatalytic Reactions.

We have investigated various aspects of a gap mode plasmon to establish it as an analytical tool. First, markedly large (107 - 109) enhancement factors for the Raman scattering intensity from a thiophenol (TP) monolayer sandwiched by Ag films on a prism and silver nanoparticles (AgNPs) were obtained under attenuated total reflection (ATR) geometry. Second, AgNPs with a radius of ∼20 nm were optically trapped and immobilized on TP-covered Ag films under a gap mode resonance with extremely weak laser power density of ∼1 µW/µm2 at 532 nm. The observed optical trapping and immobilization were theoretically rationalized using a dipole-dipole coupling and van der Waals interaction between AgNPs and Ag films. Third, p-alkyl TP molecules such as p-methyl TP, p-ethyl TP, p-isopropyl TP, and p-tertiary butyl TP were photocatalytically oxidized into p-carboxyl TP, whereas o- and m-methyl TP did not show such reactions.

The critical importance of gap modes in surface enhanced Raman scattering.

For efficient utilization of surface plasmons in surface enhanced Raman scattering (SERS), we investigated the gap modes in flocculates of metal nanoparticles (MNPs), and between MNPs and metal substrates under an external and an attenuated total reflection (ATR) geometry. First, the adsorbed state of thiol molecules and counter ions trapped in solutions were elucidated using flocculation-SERS, in which closely adjacent nanoparticles are formed by using interactions between MNPs and target species. Second, we obtained a pronounced enhancement of 10(5)-10(8) at a nanogap between gold nanoparticles (AuNPs) and various metal substrates even with a large damping under an external geometry. Markedly larger enhancement was obtained for larger AuNPs, by a factor of 10(3) for particles with a radius (r) of 50 nm compared with those of r=15 nm in this geometry. Finally, we attained an additional enhancement factor under an ATR geometry by a coupling of propagating surface plasmons with gap modes.

Closely adjacent gold nanoparticles linked by chemisorption of neutral rhodamine 123 molecules providing enormous SERS intensity.

Addition of neutral R123 molecules (10(-7) M) to an as-prepared gold nanoparticles (AuNPs) suspension generated flocculates that are a small number of closely adjacent particles. Formation of AuNP flocculates was evidenced by the coupled localized plasmon peak at 720-750 nm. The AuNP flocculates provided pronounced SERS spectra of adsorbed neutral R123 molecules (SERS-A) as anticipated by FDTD (Finite Difference Time Domain) simulations. The observed SERS spectra are significantly different from those of cationic R123(+) molecules (SERS-B), which electrostatically adsorbed on Cl(-)-treated AuNPs. The difference is not simply due to deprotonation but reflects a distinct difference in adsorption nature between neutral R123 and cationic R123(+) molecules. Indeed neutral R123 molecules exclusively gave an Au-N stretching band at 202 cm(-1), showing the chemisorption on Au surfaces through lone pair electrons at the amino groups. The different adsorption nature is further evidenced by the observation that cationic R123(+) molecules adsorbed on as-prepared (without NaCl addition) AuNP flocculates gave both SERS-A and SERS-B spectra. Thus, the cationic R123(+) molecules form the flocculates both by chemisorption and electrostatic adsorption owing to modest surface charge on as-prepared AuNPs.

Single molecule sensitivity in SERS: importance of junction of adjacent Ag nanoparticles.

An enormous SERS (Surface Enhanced Raman Scattering) signal from dye and adenine on hot or blinking silver nanoparticles is extinguished by duration of measurement possibly due to thermal diffusion or desorption of adsorbed molecules. Simultaneously, the elastic scattering peak at ca. 630 nm disappears. The Three-dimensional Finite Difference Time Domain (FDTD-3D) method manifests this scattering peak as originating from enhanced coupling of localized surface plasmon (LSP) on adjacent Ag particles through absorption of adspecies located at their junction. Distinct emission peaks were observed at 550-600 nm and 600-750 nm. Based on the spectral variations for different surface coverage and for different dye species, the shorter and longer wavelength peaks were attributed to excited electrons on metal, and from fluorescence of molecules, respectively. Furthermore, we found the shorter wavelength peak shows invariant Stokes shift irrespective of excitation wavelengths, indicating it arises from inelastic scattering of excited electron by surface roughness or by adsorbed molecules.

Apertureless tip-enhanced Raman microscopy with confocal epi-illumination/collection optics.

It is demonstrated that confocal epi-illumination/collection optics can be effectively used to generate surface-enhanced Raman scattering at the near-field region of a gold-coated tip for an atomic force microscope operated in semi-contact tapping mode. When the tip, with a 50-nm apex radius, was illuminated by a highly focused laser beam at 532 nm and approached the isolated diamond particle, with a size of approximately 1 microm, the Raman signal was enhanced by approximately 10(3). This result is in good agreement with numerical simulations performed by the finite difference time-domain method. Since our apertureless microscope is based on readily available conventional components, there is wide room for improvements and modifications by common users in various applications of micro-Raman analysis.

Attenuated total reflection surface-enhanced infrared absorption spectroscopy of carboxyl terminated self-assembled monolayers on gold.

A new recipe for surface-enhanced infrared absorption (SEIRA) active island Au films with improved adhesion in aqueous solution, low resistivity, and enhancement of the infrared (IR) absorption of about 300 was developed. The Au films prepared were utilized in studies of the ionization of self-assembled monolayers of 11-mercaptoundecanoic acid in Na2SO4 aqueous solutions by attenuated total reflection surface-enhanced infrared absorption (ATR-SEIRA) spectroscopy. It was found that the carboxyl end groups of the self-assembled monolayer turn into carboxylate anions on going from anodic to cathodic potentials or from acidic to alkaline pH. The water molecules close to the self-assembled monolayer in acidic solutions or at anodic potentials are preferentially aligned with their dipole moments parallel to the interface. This type of alignment can be ascribed to the dipole-dipole interaction between the carboxyl groups and the water molecules. On the other hand, in alkaline solutions or at cathodic potentials the structure of water close to the self-assembled monolayer is essentially bulk-like, with randomly oriented water molecules. This observation suggests that in alkaline solutions or at cathodic potentials the charge of the carboxylate anions is almost completely compensated for by strongly adsorbed counter cations. As a result, the electric field close to the surface of the ionized self-assembled monolayer is weak and has little influence on the orientation and hydrogen bonding of the water molecules.

Highly efficient and aberration-corrected spectrometer for advanced Raman spectroscopy.

We designed an asymmetric Czerny-Turner-type spectrometer with a spectral resolution of approximately 1 cm(-1) and a focal length of 500 mm (F/4.1) to improve the aberration properties: (1) coma aberration was corrected by use of a particular incident angle for a condensing mirror based onShafer's equation, (2) astigmatism was corrected by use of a toroidal condensing mirror, (3) the optimum distance was found between a grating and condensing mirror so that the centered light and marginal light at the detector possess the same incident angles to the condensing mirror (the aberration is therefore excellently corrected over the whole detector surfaces), and (4) these optimal configurations are ensured in a wide wavelength between 400 and 800 nm by use of gratings with different grooves. Then the spectrometer was constructed, and the excellent optical properties were confirmed with aligned fiber images and Raman spectra from copper phthalocyanine.