Masayuki Futamata

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.

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.

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 ∼1 μm, the Raman signal was enhanced by ∼103. 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.

Single-molecule imaging and spectroscopy of adenine and an analog of adenine using surface-enhanced Raman scattering and fluorescence

Abstract We implemented Raman imaging and spectroscopy of adenine molecules adsorbed on Ag colloidal nanoparticles to find feasibility of detecting and identifying nucleic-acid bases at the single-molecule level. Surface enhanced Raman scattering of adenine molecules showed an intermittent on-and-off behavior called “blinking” and fluctuation of the center of spectra called “spectral diffusion”. The observation of blinking and spectral diffusion provides substantial evidence for detecting single adenine molecules. Also, we extended single molecule fluorescence imaging and time-resolved fluorometry from green to violet-excitation regime to detect a fluorescent analog of nucleic-acid bases at the single-molecule level. Using violet excitation we observed fluorescence spectra and lifetimes from single complexes of an analog of adenine and the Klenow fragment of DNA polymerase I.

Application of attenuated total reflection surface-plasmon-polariton Raman spectroscopy to gold and copper.

An attenuated total reflection (ATR) method to excite the surface plasmon polariton (SPP) was applied to gold and copper to improve the sensitivity level of classical Raman spectroscopy. Reflectivity and Raman band intensity from copper phthalocyanine (CuPc) adsorbates on these metals were measured in an Otto configuration as a function of the angle of incidence and gap size at various excitation wavelengths. The SPPs on gold and copper were most efficiently excited at a wavelength of 632.8 nm according to the sufficiently large real part and the small imaginary part in the dielectric constants. It yields an enhancement factor of approximately 50 and 23 for Raman band intensity from CuPc 6 nm thick on gold and copper, respectively. Furthermore the significantly larger enhancement factor of ~100 was obtained for a self-assembled monolayer of p-nitrothiophenol on gold at 632.8-nm excitation, probably owing to its smaller imaginary part value. It enabled us to observe the electrochemical reduction to p -aminothiophenol in situ and gave clear evidence of charge transfer resonance as observed for the silver system. Conclusively, the ATR-SPP method is essential to enhancing a quite weak Raman signal from adsorbates on gold and copper surfaces as well as silver.

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.

Electronic Mechanisms of SERS

Within the last decade, the field of Mie resonances in small metal particles [1] and of surface-plasmon polaritons at plane metal surfaces [2] has matured into the topic of plasmonics, see for example [3, 4]. This field is thriving, because patterned structures can be produced by nanotechnology. Large enhancements of the local field at the laser frequency can be achieved, which support surface-enhanced Raman scattering by adsorbed molecules [5, 6]. The special SERS intensities for the coinage and alkali metals [7, 8], and less for transition metals (for instance [9]), the strong dependence on the mesoscopic structure of the systems involved, the strong dependence of the SERS intensity on the state of aggregation of colloidal nanoparticles (for instance [10]) are essentially incomprehensible without invoking the electromagnetic (em) theory. Though a “chemical effect” has sometimes been postulated for smooth surfaces [11], detailed experiments and theory [12] have not confirmed this opinion (with the exception of molecules without π∗ states). At a plane interface, enhanced Raman scattering has only been achieved by tip-enhanced Raman scattering [13] or by exciting a surface-plasmon polariton by using a coupling prism above the surface [14]. Large em enhancement without interfering electronic (“chemical”) effects is the aim of analytical chemistry, where molecules must be recognized and their concentration be measured irrespective of the chemical nature of the adsorbed molecule [5]. The research in the electronic contribution to SERS is driven by the surface-science interest in the metal–adsorbate electronic and vibrational interaction, also in dependence of the atomistic structure of the surface, reactions between different adsorbates and the hope to contribute to the understanding of important catalytic reactions, as Raman spectroscopy can bridge the “pressure gap” or allows investigations at the solid/liquid interface. The unavoidable electronic effect can spoil in some cases the great expectations based on the theory of the em enhancement. In the case of horseradish

Single-Molecule Imaging and Spectroscopy Using Fluorescence and Surface-Enhanced Raman Scattering

We extended single molecule fluorescence imaging and time-resolved fluorometry from the green to the violet-excitation regime to find feasibility of detecting and identifying fluorescent analogs of nucleic-acid bases at the single-molecule level. Using violet excitation, we observed fluorescent spotsfrom single complexes composed of a nucleotide analogue and the Klenow fragmentof DNA polymerase I. Also, we implemented Raman imaging and spectroscopy of adenine molecules adsorbed on Ag colloidal nanoparticles to find feasibility of identifying nucleic-acid bases at the single-molecule level. Surface enhanced Raman scattering (SERS) of adenine molecules showed an intermittent on-and-off behavior called blinking. The observation of blinking provides substantial evidence for detecting single adenine molecules.

ATR-SNOM-Raman spectroscopy

In this study, (1) an ATR-SNOM-Raman equipment was built to obtain topography and SNOM images simultaneously, (2) Raman spectra from copper phthalocyanine (CuPc) were obtained with 600 s without using the resonance effect, (3) the optimum conditions to excite the LSP (Localized Surface Plasmon) on the Ag island films were found, (4) the Raman signal from CuPc (1.5 nm) was enhanced by a factor of ca. 300 using the Ag island film (5 nm), which gives 36 counts/s for ca. 7000 molecules assuming a sampled volume of φ 50 nm×1.5 nm thickness, and enables us to obtain the Raman spectra in 2 s. At last, (5) SNOM-Raman signal arises from the surface < ca. 2 nm similar to a localized SPP field.

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 onShafers 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.