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

Conversion of the Aggregation State of Merocyanine Dye, Modification of the Subcell Packing of Arachidic Acid, and Removal of the Majority of <i>n</i>-Octadecane by Hydrothermal Treatment in the Liquid Phase in a Mixed Langmuir−Blodgett Film of the Ternary System

We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir−Blodgett (LB) film of merocyanine dye with an octadecyl group (MS18)−arachidic acid (C20)−n-octadecane (AL18) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS18 hydrocarbon chain, the slight orientation change in the C20 hydrocarbon chain, and the complete evaporation of AL18. The dissociation of MS18 is probably ascribed to the complete evaporation of AL18 from the mixed LB film and the increase in thermal mobility of the long axis of the MS18 hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS18 aggregation state from H- to J-aggregates, the modification of the C20 subcell packing from hexagonal to orthorhombic, and the removal of most of the AL18 molecules. The conversion of the MS18 aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS18 aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL18 molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS18 permanent dipole interaction. Moreover, the modification of the C20 subcell packing is possibly due to the hydrophobic effect, where the C20 hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS18, to modify the subcell packing of C20 and to erase the majority of AL18 molecules in the mixed LB film of the MS18−C20−AL18 ternary system in a short time

Surface Plasmon Resonance Near-Infrared Spectroscopy

Near-infrared (NIR) spectroscopy is ill-suited to microanalysis because of its low absorptivity. We have developed a highly sensitive detection method for NIR spectroscopy based on absorption-sensitive surface plasmon resonance (SPR). The newly named SPR−NIR spectroscopy, which may open the way for NIR spectroscopy in microanalysis and surface science, is realized by an attachment of the Kretschmann configuration equipped with a mechanism for fine angular adjustment of incident light. The angular sweep of incident light enables us to make a tuning of a SPR peak for an absorption band of sample medium. From the dependences of wavelength, incident angle, and thickness of a gold film on the intensity of the SPR peak, it has been found that the absorbance can be enhanced by ∼100 times compared with the absorbance obtained without the gold film under optimum conditions. This article reports the details of the experimental setup and the characteristics of absorption-sensitive SPR in the NIR region, together with some experimental results obtained by using it

Infrared, Raman, and Near-Infrared Spectroscopic Evidence for the Coexistence of Various Hydrogen-Bond Forms in Poly(acrylic acid)

Fourier-transform infrared (FT-IR), near-infrared (NIR)-excited FT-Raman, and FT-NIR spectra have been measured for poly(acrylic acid) (PAA) in a cast film over a temperature range of 40−140 °C, to investigate structures of hydrogen bonds and their dissociation. The CO stretching bands in the FT-IR spectra are unraveled by a prevalent multiple species model for small aliphatic acids with various kinds of associated forms of carboxylic acid groups, namely cyclic dimer, linearly associated oligomers of COOH, and free COOH groups. These different structures of hydrogen bond persist even when the temperature rises well above the glass transition temperature. The FT-Raman spectra confirm the existence of such COOH groups. Temperature-dependent intensity changes in the first overtone of an OH stretching mode of PAA reveal that the COOH groups dissociate significantly at high temperatures. We propose that the coexistence of various possible hydrogen-bond forms analogous to those in small aliphatic acids best interprets the vibrational spectral features of PAA. The oligomeric chains of COOH groups in PAA may explain the previously proposed cooperative hydrogen bond in PAA or polymer blends containing PAA

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

Sequential Identification of Model Parameters by Derivative Double Two-Dimensional Correlation Spectroscopy and Calibration-Free Approach for Chemical Reaction Systems

A sequential identification approach by two-dimensional (2D) correlation analysis for the identification of a chemical reaction model, activation, and thermodynamic parameters is presented in this paper. The identification task is decomposed into a sequence of subproblems. The first step is the construction of a reaction model with the suggested information by model-free 2D correlation analysis using a novel technique called derivative double 2D correlation spectroscopy (DD2DCOS), which enables one to analyze intensities with nonlinear behavior and overlapped bands. The second step is a model-based 2D correlation analysis where the activation and thermodynamic parameters are estimated by an indirect implicit calibration or a calibration-free approach. In this way, a minimization process for the spectral information by sample–sample 2D correlation spectroscopy and kinetic hard modeling (using ordinary differential equations) of the chemical reaction model is carried out. The sequential identification by 2D correlation analysis is illustrated with reference to the isomeric structure of diphenylurethane synthesized from phenylisocyanate and phenol. The reaction was investigated by FT-IR spectroscopy. The activation and thermodynamic parameters of the isomeric structures of diphenylurethane linked through a hydrogen bonding equilibrium were studied by means of an integration of model-free and model-based 2D correlation analysis called a sequential identification approach. The study determined the enthalpy (Δ<i>H</i> = 15.25 kJ/mol) and entropy (<i>T</i>Δ<i>S</i> = 13.20 kJ/mol) of CO···H hydrogen bonding of diphenylurethane through direct calculation from the differences in the kinetic parameters (δΔ^⧧<i>H</i>, −<i>T</i>δΔ^⧧<i>S</i>) at equilibrium in the chemical reaction system

Far-Ultraviolet Spectroscopy and Quantum Chemical Calculation Studies of the Conformational Dependence on the Electronic Structure and Transitions of Cyclohexane, Methyl and Dimethyl Cyclohexane, and Decalin; Effects of Axial Substitutions on the Electronic Transitions

Far-ultraviolet (FUV) spectra were measured for cyclohexane, methyl cyclohexane, six isomers of dimethyl cyclohexane, and cis- and trans-decalin. Attenuated total reflection-FUV (ATR-FUV) spectroscopy, which we originally proposed, provides systematic information about the excitation states of saturated organic molecules and the hyperconjugation of σ bonds. The FUV spectra of cyclohexane and methyl cyclohexane in neat liquids showed a band with central wavelengths near 155 and 162 nm. The simulation spectrum of cyclohexane calculated by time-dependent density-functional theory (TD-DFT) (CAM-B3LYP/aug-cc-pVTZ) gives two bands at 146 and 152 nm owing to the transition from HOMO-2 to Rydberg 3pz (Tb) and those from HOMO and HOMO-1 to Rydberg 3px/3py (Ta), respectively. The simulation spectrum of methyl cyclohexane with the equatorial substituent has peaks at approximately the same positions as cyclohexane. The calculated molar absorption coefficient is larger than that of cyclohexane, estimating the observed FUV spectra very well. The FUV spectra of dimethyl cyclohexane with two methyl substituents at the equatorial positions (trans-1,2-, cis-1,3-, and trans-1,4-) and trans-decalin had similar features to those of cyclohexane and methylcyclohexane. The TD-DFT calculations revealed that the shoulders at the shorter- and longer-wavelength sides of the band center of dimethyl cyclohexane (with methyl substituents at equatorial positions) and trans-decalin are assigned to Tb and Ta, respectively. In the case of dimethyl cyclohexane with one methyl substituent in the axial position (cis-1,2-, trans-1,3-, and cis-1,4-) and cis-decalin, the band caused by Tb decreased compared to those of the other compounds. The decrease in intensity and the longer-wavelength shift of the Tb band for dimethyl cyclohexane (with one methyl group at the axial position) and cis-decalin revealed that the band on the longer-wavelength side was assigned to the overlap band of Ta and Tb. The reason for such a large spectral alternation for the axial substitution may be the increase in the orbital energy of HOMO-2, which has its electron density concentrated at the axial C–H bond. Regarding the effect of the hyperconjugation of C–C and C–H σ orbitals, the second perturbation energies of the interaction between Cα–Hax and Cβ–Hax were estimated for molecules by natural bond orbital (NBO) analysis. There is a correlation between the orbital energies of HOMO-2 and the changes in vicinal interaction by axial substitution

Silver Nanoplates with Special Shapes: Controlled Synthesis and Their Surface Plasmon Resonance and Surface-Enhanced Raman Scattering Properties

Shape-controlled synthesis of metal nanostructures has opened many new possibilities to design ideal building blocks for future nanodevices. In this work, new types of monodisperse silver nanoplates with complex shapes, namely, a disklike shape and flowerlike shapes, were controllably synthesized in high yield by reducing [Ag(NH3)2]+ with ascorbic acid in the presence of silver seed at room temperature. Unlike previous methods for synthesizing the silver nanoplates in the presence of cetyltrimethylammonium bromide (CTAB) micelles, the use of the precursor [Ag(NH3)2]+, other than Ag+, provides a flexible strategy to control the procession of the reduction reaction in a mild way. These silver nanoplates with shapes of disk and flower were shown to possess surface plasmon resonance (SPR) that directly relates to their geometric shapes. As a result of their high anisotropy in shape, the flowerlike silver nanoplates exhibit excellent surface-enhanced Raman scattering (SERS) enhancement ability relative to spherical silver nanoparticles and the disklike silver nanoplates. We believe that with the efficient synthesis and excellent SERS enhancement ability, these novel flowerlike silver nanoplates may find potential applications for biological sensing and labeling systems

Highly Sensitive Protein Concentration Assay over a Wide Range via Surface-Enhanced Raman Scattering of Coomassie Brilliant Blue

In the Bradford protein assay, protein concentrations are determined by the absorbance at 595 nm due to the binding of Coomassie brilliant blue G-250 (CBBG) to proteins. In a protein−CBBG liquid mixture, surface-enhanced Raman scattering (SERS) is sensitive to the amount of unbound CBBG molecules adsorbed on silver surfaces, and the bound CBBG amount is directly related to the target protein concentration. Accordingly, a novel method for detecting total protein concentration in a solution has been developed based on SERS of unbound CBBG with an internal standard of silicon. Two obvious advantages of the proposed protein assay over conventional Bradford protein assay are its much wider linear concentration range (10−5−10−9 g/mL) and 200 times lower limit of detection (1 ng/mL), which demonstrates its great potential in rapid, highly sensitive concentration determination of high and low-abundance proteins