Plasmon-Enhanced Optical Tweezers for Single Molecules on and near a Colloidal Silver Nanoaggregate

At the junction of an Ag nanoaggregate, single molecules can emit surface-enhanced Raman scattering and fluorescence (SERS and SEF) and can be optically trapped by an enhanced electromagnetic field via plasmon resonance. Blinking SERS and SEF from a single molecule on the same nanoaggregate were observed simultaneously in a bicolor movie. By super-resolution imaging, the positions of the SERS- and SEF-active molecules were detected beyond the diffraction limit. The spatial fluctuation of the molecule on the nanoaggregate was suppressed as the excitation laser intensity increased. The reason is that the single molecule was optically trapped at the junction via plasmon resonance because the mislocalization effect and the signal intensity do not influence a change in the spatial fluctuation in the super-resolution imaging. The spatial fluctuation of the SEF-active molecule near the Ag surface was larger than that of the SERS-active molecule adsorbed on the surface. The power spectral density revealed that the plasmon-enhanced optically trapped molecule by excitation at high laser intensity moved randomly rather than harmonically

Plasmon-Enhanced Optical Tweezers for Single Molecules on and near a Colloidal Silver Nanoaggregate

At the junction of an Ag nanoaggregate, single molecules can emit surface-enhanced Raman scattering and fluorescence (SERS and SEF) and can be optically trapped by an enhanced electromagnetic field via plasmon resonance. Blinking SERS and SEF from a single molecule on the same nanoaggregate were observed simultaneously in a bicolor movie. By super-resolution imaging, the positions of the SERS- and SEF-active molecules were detected beyond the diffraction limit. The spatial fluctuation of the molecule on the nanoaggregate was suppressed as the excitation laser intensity increased. The reason is that the single molecule was optically trapped at the junction via plasmon resonance because the mislocalization effect and the signal intensity do not influence a change in the spatial fluctuation in the super-resolution imaging. The spatial fluctuation of the SEF-active molecule near the Ag surface was larger than that of the SERS-active molecule adsorbed on the surface. The power spectral density revealed that the plasmon-enhanced optically trapped molecule by excitation at high laser intensity moved randomly rather than harmonically

Surface-Enhanced Phosphorescence Measurement by an Optically Trapped Colloidal Ag Nanoaggregate on Anionic Thiacarbocyanine H‑Aggregate

A citrate-reduced Ag nanoaggregate was optically trapped on a fiber-shaped H-aggregate of an anionic thiacarbocyanine dye against Coulomb repulsion by focusing a near-infrared (NIR) laser beam. As the NIR laser power increased, namely, as the Ag nanoaggregate approaches the H-aggregate, phosphorescence from the H-aggregate with the Ag nanoaggregate excited moderately at 514 and 647 nm was strengthened, although that at 568 nm was weakened. By excitation at 568 nm, which was close to a surface plasmon resonance peak of the Ag nanoaggregate, surface-plasmon-enhanced optical trapping potential well might have deepened, and then the Ag nanoaggregate might have approached the H-aggregate too closely to enhance the phosphorescence because of energy transfer to the metal. As the excitation laser intensity increased, namely, as the surface-plasmon-enhanced optical trapping potential well was deepened, the phosphorescence enhancement factor trended upward and then downward by enhancement due to plasmon at a close distance from the Ag surface and the energy transfer at the closer distance, respectively

Generation of Pronounced Resonance Profile of Charge-Transfer Contributions to Surface-Enhanced Raman Scattering

A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a series of metal-charge-transfer (CT) complex systems fabricated by a self-assembly method. The developed Ag/4-mercaptophenols (MPH)/<i>n</i>-TiO₂ system presented layer number-dependent SERS spectra. By using the electron density values of the Ag₁₃/MPH and Ag₁₃/MPH/TiO₂ system calculated using the density functional theory (DFT) and by using these values in combination with the results of our previous investigations on the mechanism of the Ag/MPH/TiO₂ system, the absorption threshold of the CT complexes was clearly defined. The degree of CT was selected to study the layer number-dependent SERS spectra. Based on the layer number-dependent SERS data, it has been inferred that the degree of CT represents a resonance phenomenon. In addition, the CT resonance occurs at higher energy in the Ag/MPH/<i>n</i>-TiO₂ system than in the monolayer TiO₂ system owing to the blue-shift of CT states with the continuous introduction of TiO₂. Thus, we provide a good example of the use of a CT complex system to investigate the chemical mechanism of SERS

Nanoscale pH Profile at a Solution/Solid Interface by Chemically Modified Tip-Enhanced Raman Scattering

A nanoscale pH profile on a 4 × 4 μm² area of NH₂-anchored glass slide in an aqueous solution is constructed using chemically modified tip-enhanced Raman scattering (TERS). <i>p</i>-Mercapto­benzoic acid (<i>p</i>MBA) and <i>p</i>-amino­thiophenol (<i>p</i>ATP) are bonded to the tip surface. A pH change can be detected from a peak at 1422 cm^–1 due to the −COO^– stretching vibration from <i>p</i>MBA and that at 1442 cm^–1 due to the NN stretching vibration arising from the formation of 4,4′-dimercapto­azobenzene (DMAB) on the <i>p</i>ATP-modified tip. The <i>p</i>MBA- and <i>p</i>ATP-modified tip can be used to determine pH in the range of 7–9 and 1–2, respectively. The spatial resolution to differentiate pH of two areas can be considered as ∼400 nm. The measured pH becomes the pH of the bulk solution when the tip is far by ∼200 nm from the surface. This technique suggests a possibility for the pH sensing in wet biological samples. TERS tips could also be chemically modified with other molecules to determine other properties in a solution

Structural Characterization of a Mixed Langmuir−Blodgett Film of a Merocyanine Dye Derivative−Deuterated Arachidic Acid Binary System and the Influence of Successive Hydrothermal Treatment in the Liquid Phase on the Film as Investigated by Polarized UV−Visible and IR Absorption Spectroscopy

We have investigated the structure of the mixed Langmuir−Blodgett (LB) film of a merocyanine dye derivative (MO18)−deuterated arachidic acid (C20-d) binary system and the influence of successive hydrothermal treatment in the liquid phase (HTTL) on the mixed LB film by means of polarized UV−visible and IR absorption spectroscopy. The visible absorption band with in-plane anisotropy at 503 nm before HTTL transforms into an absorption band with in-plane isotropy at 557 nm after HTTL for 16−18 min through a peak maximum near 520 nm after HTTL for 2−12 min. The degree of total MO18 intramolecular charge transfer for the 503 nm band is the largest among those for all of the bands. Therefore, the 503 nm band is ascribed to the MO18 H-like aggregation, based on its shape, peak height, and in-plane anisotropy, the subsequent change to two kinds of visible peaks by successive HTTL, and the most degree of MO18 intramolecular charge transfer among all of the aggregation states. While the MO18 hydrocarbon chain takes the all-trans conformation before HTTL, its conformation and orientation are most disarranged after HTTL for 2 min. Subsequently, the original conformation and orientation are recovered by degrees with successive HTTL, except after final HTTL for 18 min, when the orientation is again changed. On the other hand, the C20-d hydrocarbon chain maintains the all-trans conformation before and after HTTL. The orientation of the C20-d hydrocarbon chain after HTTL for 2 min is more ordered than that before HTTL, with the nature of the C20-d subcell packing changing from hexagonal to orthorhombic. During successive HTTL from 2 to 18 min, the C20-d orientation is gradually disorganized but with the orthorhombic nature remaining constant. Thus, the variations in the conformation and orientation of the MS18 hydrocarbon chain and in the orientation of the C20-d hydrocarbon chain tend to change from ordered and disordered structures and turn to more disordered and ordered ones, respectively, where the former is mainly caused by the priority action of thermal energy and the latter by hydrophobic effect due to the presence of warm water. Consequently, it is suggested that there is a correlation between the degree of structural order for both hydrocarbon chains and the preferential action that takes place during HTTL

Truncated Power Law Analysis of Blinking SERS of Thiacyanine Molecules Adsorbed on Single Silver Nanoaggregates by Excitation at Various Wavelengths

From blinking surface-enhanced Raman scattering (SERS) of anionic thiacyanine adsorbed on single Ag nanoaggregates, the electromagnetic field and the molecular behavior in a nonemissive state were investigated by a truncated power law analysis. The power law that reproduces probability distribution of dark SERS events versus duration time was not truncated often by excitation at long wavelengths; otherwise it was truncated at the long tail. The truncation suggests a high energy barrier from nonemissive to emissive state and a short passage time of molecular random walk to overcome the energy barrier. The energy barrier in blinking SERS likely originates from a nanometer-ordered periodic optical trapping potential well, namely, electromagnetic field around a junction of the Ag nanoaggregate due to coupling of multipolar surface plasmon resonance, which is hardly induced by excitation at long wavelengths. This is consistent with the experimental excitation wavelength dependence of the truncation. At a low concentration of anionic thiacyanine, the power law was truncated at the short tail. The reason may be the short passage time of the molecule on the Ag surface adsorbing a small number of obstacles to reach the junction

Surface Plasmon Excitation and Surface-Enhanced Raman Scattering Using Two-Dimensionally Close-Packed Gold Nanoparticles

We investigated plasmon excitation and surface-enhanced Raman scattering (SERS) of crystal violet dye adsorbed on two-dimensionally (2D) close-packed gold nanostructures. With reference to the absorption maximum of colloidal nanoparticles, the 2D nanostructures showed red-shifted maxima at ∼560 and 740 nm. Interestingly, we found that the surface plasmon resonance (SPR) and SERS signals were preferentially enhanced near the edge of the 2D nanostructures. Both the SPR and SERS images suggest good correlation between the SPR-mediated localized electromagnetic field and the localized optical field, particularly near the edge of the 2D nanostructures. We observed broad SPR bands near the edge and red-shifted and intense bands near the center of the 2D nanostructures. Also, the overall SERS enhancement on the 2D nanostructures was comparable to that reported on engineered gold nanostructures. The integral SERS intensity near the edge was higher than that measured near the center. We carried out finite-different time-domain simulations and demonstrated that the experimental data are consistent with the simulated ones. Here, several proximal localization sites appeared for nanoparticles with their out-of-plane polarization parallel to the interparticle axis. Thus, the experimental results and the simulation predicted the existence of multipolar plasmon excitation and a narrow passage for energy percolation, which are often obscured in ensemble SERS measurements

pH-Response Mechanism of <i>p</i>‑Aminobenzenethiol on Ag Nanoparticles Revealed By Two-Dimensional Correlation Surface-Enhanced Raman Scattering Spectroscopy

The existence of pH-dependent surface-enhanced Raman scattering (SERS) of <i>p</i>-aminobenzenethiol (PATP) on Ag nanoparticles has been confirmed by numerous studies, but its mechanism still remains to be clarified. Discussion of the mechanism is at a standstill because of the lack of a systematic investigation of the process behind the pH-induced variation of the PATP behavior. Two-dimensional correlation spectroscopy is one of the most powerful and versatile spectral analysis methods for investigating perturbation-induced variations in dynamic data. Herein, we have analyzed the pH-dependent behavior of PATP using a static buffer solution with pH ranging from 3.0 to 2.0. The order of the variations in the different vibrational intensities was carefully investigated based on 2D correlation SERS spectroscopy. These results have demonstrated that the very first step of the pH-response process involves protonation of the amine group. The pH-response mechanism revealed is an important new component to our understanding of the origin of the b₂-type bands of PATP