Time-gated pre-resonant femtosecond stimulated Raman spectroscopy of diethylthiatricarbocyanine iodide.

We present time-gated femtosecond stimulated Raman spectroscopy (fSRS) under the pre-resonance Raman conditions of diethylthiatricarbocyanine (DTTC) iodide. A 'pseudo emission-free' condition is achieved by delivering the probe beam ahead of the pump beam. Regeneratively amplified pulse trains are employed to create an angle-geometry (non-collimated) mixing between the pump and probe beams, leading to highly sensitive measurement of the stimulated Raman gain. Time-integrated spectroscopy allows for a more quantitative distinction between the contributions of stimulated Raman scattering and stimulated emission. We successfully obtain a highly sensitive (signal-to-noise ratio >100) stimulated Raman spectrum under the optimized conditions, which compares favourably to results obtained using two-dimensional correlation spectroscopy (2DCOS). Given the optical pre-resonance of ∼0.1 eV, the background signals mostly originate from the stimulated emission of excited electrons and are significantly reduced by partial overlapping of the pump and probe beams; a genuine fSRS spectral profile is obtained for a temporal delay of ∼0.2 ps between the two beams.

Single-molecule and single-particle-based correlation studies between localized surface plasmons of dimeric nanostructures with ~1 nm gap and surface-enhanced Raman scattering.

Understanding the detailed electromagnetic field distribution inside a plasmonically coupled nanostructure, especially for structures with ~ 1 nm plasmonic gap, is the fundamental basis for the control and use of the strong optical properties of plasmonic nanostructures. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between localized surface plasmon (LSP) and surface-enhanced Raman scattering (SERS), the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au-Ag core-shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ~ 1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ~ 1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures.

Single-molecule surface-enhanced Raman spectroscopy: a perspective on the current status.

This perspective presents an overview of single-molecule surface-enhanced Raman scattering (sm-SERS). Our overview is organized as a brief theoretical background, discussion of the factors that enhance SERS, various experimental preparations for inserting a single molecule in a hot spot, recent sm-SERS experiments, and a perspective. Although, there have been numerous review papers on sm-SERS, we mainly concentrated on the logical development of sm-SERS on the basis of the fundamental concepts and their physical significance, so that readers outside this field can understand the motivation and the underlying physics when describing current sm-SERS measurements. Indeed, understanding such current sm-SERS experiments conducted by representative groups would be very helpful for readers to answer for themselves the fundamental and practical questions surrounding sm-SERS: (1) what information can sm-SERS provide? (2) Which factors based on the SERS mechanism should be considered to significantly amplify the SERS signal? (3) What kinds of related microscopy techniques could be combined with sm-SERS to attain more meaningful results? (4) Which statistical approaches can be used and how they can be applied to properly analyze sm-SERS data? We hope that this review article can help readers answer these questions.

High-precision measurement-based correlation studies among atomic force microscopy, Rayleigh scattering, and surface-enhanced Raman scattering at the single-molecule level.

We investigated the correlations among the structure, Rayleigh scattering, and single-molecule surface-enhanced Raman scattering (SERS) of DNA-tethered Au-Ag core-shell nanoparticles, especially in dimer and trimer forms. For the optimal correlation measurements, accurate information on the position of the nanoparticle is crucial for locating the nanoparticle at the center of the excitation source for the optical measurements. To achieve this, we developed a multistep correlation strategy that enables us to unambiguously correlate the AFM images with optical images within a few nanometers. We also newly defined the correlation accuracy in this paper as a useful concept for the correlation measurements. With this reliable correlation accuracy, we performed various statistical analyses to thoroughly elucidate the relationships between particle structure, Rayleigh scattering and SERS in terms of the incident polarization and scattering intensity ratio.

Detection of Biomolecular Binding Through Enhancement of Localized Surface Plasmon Resonance (LSPR) by Gold Nanoparticles.

To amplify the difference in localized surface plasmon resonance (LSPR) spectra of gold nano-islands due to intermolecular binding events, gold nanoparticles were used. LSPR-based optical biosensors consisting of gold nano-islands were readily made on glass substrates using evaporation and heat treatment. Streptavidin (STA) and biotinylated bovine serum albumin (Bio-BSA) were chosen as the model receptor and the model analyte, respectively, to demonstrate the effectiveness of this detection method. Using this model system, we were able to enhance the sensitivity in monitoring the binding of Bio-BSA to gold nano-island surfaces functionalized with STA through the addition of gold nanoparticle-STA conjugates. In addition, SU-8 well chips with gold nano-island surfaces were fabricated through a conventional UV patterning method and were then utilized for image detection using the attenuated total reflection mode. These results suggest that the gold nano-island well chip may have the potential to be used for multiple and simultaneous detection of various bio-substances.

Electron beam lithography-assisted fabrication of Au nano-dot array as a substrate of a correlated AFM and confocal Raman spectroscopy.

This paper documents a study of an Au nano-dot array that was fabricated by electron beam lithography on a glass wafer. The patterns that had features of 100 nm dots in diameter with a 2-mum pitch comprised a total area of 200 x 200 microm(2). The dot-shaped Cr underlayer was open to the air after developing Poly(methyl methacrylate) (PMMA). When dipped into the Cr etchant, the exposed Cr layer was eliminated from the glass wafer in a short period of time. In order to ultimately fabricate the Ti/Au dot arrays, Ti and Au were deposited onto the arrays with a thickness of 2 and 40 nm, respectively. The lift-off procedure was carried out in the Cr etchant using sonication in order to completely remove the residual Cr/PMMA layer. The fabricated Au nano-dot array was then immersed in an Ag enhancing solution and then into an ethanol solution containing (N-(6-(Biotinamido)hexyl)-3'-(2'-pyridyldithio)-propionamide (Biotin-HPDP). The substrate was analyzed using a correlated atomic force microscopy (AFM) and confocal Raman spectroscopy. Through this procedure, position-dependent surface-enhanced Raman spectroscopy (SERS) signals could be obtained.

Combined micro-Raman/UV-visible/fluorescence spectrometer for high-throughput analysis of microsamples.

Combined micro-Raman/UV-visible (vis)/fluorescence spectroscopy system, which can evaluate an integrated array of more than 10,000 microsamples with a minimuma size of 5 microm within a few hours, has been developed for the first time. The array of microsamples is positioned on a computer-controlled XY translation microstage with a spatial resolution of 1 mum so that the spectra can be mapped with micron precision. Micro-Raman spectrometers have a high spectral resolution of about 2 cm(-1) over the wave number range of 150-3900 cm(-1), while UV-vis and fluorescence spectrometers have high spectral resolutions of 0.4 and 0.1 nm over the wavelength range of 190-900 nm, respectively. In particular, the signal-to-noise ratio of the micro-Raman spectroscopy has been improved by using a holographic Raman grating and a liquid-nitrogen-cooled charge-coupled device detector. The performance of the combined spectroscopy system has been demonstrated by the high-throughput screening of a combinatorial ferroelectric (i.e., BaTi(x)Zr(1-x)O(3)) library. This system makes possible the structure analysis of various materials including ferroelectrics, catalysts, phosphors, polymers, alloys, and so on for the development of novel materials and the ultrasensitive detection of trace amounts of pharmaceuticals and diagnostic agents.