DFT studies of surface‐enhanced Raman spectroscopy of 1,4‐benzenedithiol–Au2 complex under the effect of static electric fields

In this work, we demonstrate that the applied electric‐field strength and orientation can multiply modulate the Raman intensity and vibrational wavenumber of small molecule–metal complex, 1,4‐benzenedithiol–Au₂ (1,4BDT–Au₂), by density functional theory and time‐dependent density functional theory simulations. The polarizabilities are changed by the applied electric fields, leading to enhanced specific vibrational intensity and shifted vibrational wavenumber of the surface‐enhanced Raman scattering effect. The applied electric fields perturb the bonds and angles of the 1,4BDT–Au₂ complex. Owing to this reason, the peaks of Raman spectra related to these structures exhibit distinguishable responses in quasi‐static field (low‐frequency oscillating electric field). We use the visualized method of charge difference density to show that the electric fields tune the traditional excited state to pure charge‐transfer excited state. The charge‐transfer resonance transition produces enhanced Raman intensities for non‐totally symmetric modes and totally symmetric modes. These simulation results of the function of static electric field provide new guidance for the surface‐enhanced Raman scattering measurements. Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental and theoretical evidence for the chemical mechanism in SERRS of rhodamine 6G adsorbed on colloidal silver excited at 1064 nm

The evidence for the existence of a chemical mechanism in surface‐enhanced resonance Raman scattering (SERRS) of rhodamine 6G (R6G) adsorbed on colloidal silver excited at 1064 nm is reported on the basis of experimental and theoretical analyses. A weak absorption peak at around 1060 nm for R6G‐functionalized silver nanoparticles was observed, which is not present in the individual spectra of R6G or silver nanoparticles. Theoretically, the charge difference density reveals that this weak absorption is a metal‐to‐molecule charge transfer excited state. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of SERRS and RRS excitation profiles of [Fe(tpy)2]2+ (tpy = 2,2':6',2'‐terpyridine) supported by DFT calculations: effect of the electrostatic bonding to chloride‐modified Ag nanoparticles on its vibrational and electronic structure

Nonresonance (or normal) Raman scattering (NRS), resonance Raman scattering (RRS), surface‐enhanced Raman scattering (SERS), and surface‐enhanced RRS (SERRS) spectra of [Fe(tpy)₂]²⁺ complex dication (tpy = 2,2':6',2''‐terpyridine) are reported. The comparison of RRS/NRS and SERRS/SERS excitation profiles of [Fe(tpy)₂]²⁺ spectral bands in the range of 445–780 nm is supported by density functional theory (DFT) calculations, Raman depolarization measurements, comparison of the solid [Fe(tpy)₂](SO₄)₂ and solution RRS spectra, and characterization of the Ag nanoparticle (NP) hydrosol/[Fe(tpy)₂]²⁺ SERS/SERRS active system by surface plasmon extinction spectrum and transmission electron microscopy image of the fractal aggregates (D = 1.82). By DFT calculations, both the Raman active modes and the electronic states of the complex have been assigned to the symmetry species of the D_2d point group. It has been demonstrated that upon the electrostatic bonding of the complex dication to the chloride‐modified Ag NPs, the geometric and ground state electronic structure of the complex and the identity of the three different metal‐to‐ligand charge transfer (¹MLCT) electronic transitions remain preserved. On the other hand, the effect of ion pairing manifests itself by a slight change in localization of one of the electronic transitions (with max. at 552 nm) as well as by promotion of the Herzberg–Teller activation of E modes resulting from coupling of E and B₂ excited electronic states. Finally, the very low, 1 × 10⁻¹¹ M SERRS spectral detection limit of [Fe(tpy)₂]²⁺ at 532‐nm excitation is attributed to a concerted action of the electromagnetic and molecular resonance mechanism, in conjunction to the electrostatic bonding of the complex dication to the chloride‐modified Ag NP surface. Copyright © 2014 John Wiley & Sons, Ltd.     

Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer: (II) Binding‐site and quantum‐size effects

We describe quantum‐size and binding‐site effects on the chemical and local field enhancement mechanisms of surface‐enhanced resonance Raman scattering (SERRS), in which the pyridine molecule is adsorbed on one of the vertices of the Ag₂₀ tetrahedron. We first investigated the influence of the binding site on normal Raman scattering (NRS) and excited state properties of optical absorption spectroscopy. Second, we investigated the quantum‐size effect on the electromagnetic (EM) and chemical mechanism from 300 to 1000 nm with charge difference density. It is found that the strong absorption at around 350 nm is mainly the charge transfer (CT) excitation (CT between the molecule and the silver cluster) for large clusters, which is the direct evidence for the chemical enhancement mechanism for SERRS; for a small cluster the strong absorption around 350 nm is mainly intracluster excitation, which is the direct evidence for the EM enhancement mechanism. This conclusion is further confirmed with the general Mie theory. The plasmon peak in EM enhancement will be red‐shifted with the increase of cluster size. The influence of the binding site and quantum‐size effects on NRS, as well as chemical and EM enhancement mechanisms on SERRS, is significant. Copyright © 2009 John Wiley & Sons, Ltd.     

单个银纳米聚合体的偶极表面等离子体共振灵敏表面增强拉曼散射(英文)

pacc:5240F,7830Weinvestigatedpolarizationdependencesof surfaceenhancedresonanceRamanscattering(SERRS)andsurfaceplasmonresonance(SPR)toidentifytheSERRSyieldingSPRbands.We alsostudiedSERRSexcitationspectraforsingle Agnano-aggregateswiththeSPRbandstoex plore…     

Direct visual evidence for chemical mechanisms of SERRS via charge transfer in Au20–pyrazine–Au20 junction

The essence of the chemical mechanism for surface‐enhanced resonance Raman scattering (SERRS) is the charge transfer (CT) between the metal and the molecule at the resonant electronic transition, which results in the mode‐selective enhancement in the SERRS spectrum. The site‐orientated CT can directly interpret the mode‐selective chemical enhancement in SERRS. However, it is a great challenge to intutively visualize the orientation and site of the CT. In this paper, for the pyrazine–Au₂ complex, a three‐dimensional (3D) cubic representation is built to provide direct visual evidence for chemical mechanisms of SERRS via CT from the Au₂ cluster to pyrazine at the resonant electronic transition. The relationship between the mode‐selective enhancements in SERRS and the site‐orientated CT was clearly revealed. The intracluster excitation (analog of plasmon excitation in large naonoparticles) was also visualized by the 3D cubic presentation, which provided the direct evidence of local electromagnetic field enhancement of SERRS. To study the quantum size effect and the coupling effect of the nanoparticles, the photoexcitation mechanisms of the Au₂₀–pyrazine complex and the Au₂₀–pyrazine–Au₂₀ junction were also investigated. The tunneling charge transfer from one Au₂₀ cluster to another Au₂₀ cluster outside the pyrazine in Au₂₀–pyrazine–Au₂₀ junction was also revealed visually. The calculated normalized extinction spectra of Au nanoparticles using the generalized Mie theory reveal that the resonance peak is red‐shifted due to the coupling between particles. Copyright © 2009 John Wiley & Sons, Ltd.     

1,4-苯二硫醇分子的表面增强共振拉曼散射的化学增强

利用密度泛函和含时密度泛函理论方法研究了1,4-苯二硫醇分子在两个金团簇之间的表面增强拉曼散射及表面增强共振拉曼散射光谱. 采用对应四种不同形式的电荷转移激发态能量的入射光,计算了表面增强共振拉曼光谱. 结果显示,光谱增强的效果与电荷转移的形式密切相关. 不同的电荷转移形式对增强因子的贡献是有差异的.     

Theoretical study on SERRS of rhodamine 6G adsorbed on Ag2 cluster: chemical mechanism via intermolecular or intramolecular charge transfer

The problem of the chemical enhancement of rhodamine 6G (R6G) adsorbed on silver cluster has been theoretically investigated by charge difference densities (CDDs) to show the direct charge transfer (CT) evidence. For surface‐enhanced resonance Raman scattering (SERRS) of R6G excited at 514.5 nm, the enhancements of v(151) and v(154) result from weak intermolecular (from Ag to R6G) CT and the strong intramolecular CT [similar to that of resonance Raman scattering (RRS) of R6G], respectively. The possibility of the SERRS of R6G contributed from pure intermolecular CT is also discussed, when the incident light is close to the new metal–R6G CT excited state at 1571.4 nm. Meanwhile compared with the absorption process the fluorescence yield of R6G is investigated by transition densities and CCDs. Copyright © 2008 John Wiley & Sons, Ltd.     

Incorporation of Ag nanoparticles into membrane mimetic systems composed by phospholipid layer‐by‐layer (LbL) films to achieve surface‐enhanced Raman scattering as a tool in drug interaction studies

The synergistic effect produced by nanoparticles when incorporated into different systems used as analytical tools represents a growing research field nowadays. On the other hand, the study of interactions involving pharmacological drugs and biological membranes using phospholipids as mimetic systems is a research field already well established. Here, we combine both the anionic phospholipid dipalmitoyl phosphatidyl glycerol (DPPG) and negative Ag nanoparticles (AgNP) to form layer‐by‐layer (LbL) multilayered films using the cationic polymer poly(allylamine hydrochloride) (PAH) as the supporting polyelectrolyte, which were further investigated in the presence of a phenothiazine compound (methylene blue—MB). The molecular architecture of the LbL films in terms of controlled growth, morphology with micro and nanometer spatial resolutions, and dispersion of both AgNP and MB within the DPPG matrix was determined combining spectroscopy [ultraviolet–visible (UV–Vis) absorption and micro‐Raman spectroscopy] and microscopy [scanning electron microscopy (SEM) and atomic force microscopy (AFM)]. The results showed that the LbL films can be grown in a controlled way at nanometer thickness scale with the surface morphology susceptible to the presence of both AgNP and MB. The surface‐enhanced phenomenon was applied to investigate the LbL films taking the advantage of the strong surface‐enhanced resonance Raman scattering (SERRS) signal presented by the MB molecules. Besides, as MB is a pharmacological drug of interest, its molecular arrangements when dispersed in LbL films containing DPPG, which is the biological membrane mimetic system here, were investigated. In this case, the AgNP played a key role in achieving the MB SERRS signal. Copyright © 2009 John Wiley & Sons, Ltd.     

Portable smart films for ultrasensitive detection and chemical analysis using SERS and SERRS

Metallic nanostructures, much smaller than the wavelength of visible light, which support localized surface plasmon resonances, are central to the giant signal enhancement achieved in surface‐enhanced Raman scattering (SERS) and surface‐enhanced resonance Raman scattering (SERRS). Plasmonic driven SERS and SERRS is a powerful analytical tool for ultrasensitive detection down to single molecule detection. For all practical SERS applications a key issue is the development of reproducible and portable SERS‐active substrates, where the most widely used metals for nanostructure fabrication are silver and gold. Here, we report the fabrication of a ‘smart film’, containing gold nanoparticles (AuNPs), produced by in situ reduction of gold chloride III (Au⁺³) in natural rubber (NR) membranes for SERS and SERRS applications. The composite films (NR/AuNP membranes) show characteristic plasmon absorption of Au nanostructures, which notably do not influence the mechanical properties of the NR membranes. The term ‘smart film’ has to do with the fact that the SERS substrate (smart film) is flexible and standalone, which allows one to take it anywhere and to dip it into solutions containing the analyte to be characterized by SERS or SERRS technique. Besides, the synthesis of the AuNPs at the surface of NR films is much simpler than making an Au colloid and cast it onto a substrate surface or preparing an Au evaporated film. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of Raman dispersion spectroscopy in 3‐way multivariate data analysis

In this paper a new application of Raman dispersion spectroscopy (RADIS) is proposed, namely as a source of 3‐way data in multivariate analysis of classification problems, i.e. problems where different molecular species are identified in a number of samples. Owing to the coherent nature of the absorption–emission sequence in the Raman process, the unpolarized data generated by RADIS are born as 3‐way multivariate data. To demonstrate the potential of RADIS, a series of classification problems have been constructed and analyzed. By using computer‐simulated Raman and RADIS data it is shown that the 3‐way RADIS data obtained from very few samples, using only a few laser frequencies and a few Raman bands, can in general provide highly reliable results in classification problems without any a priori knowledge about the number of different molecular species in the samples. Quantitative comparisons have been made between a principal component analysis (PCA) of the 2‐way Raman data and a Tucker 3 analysis of the corresponding 3‐way RADIS data. It is found that the 3‐way RADIS data will give rise to a recognition ratio equal to 100% even in the worst case where no conclusions about the number of different molecular species in the samples can be drawn from the PCA of the 2‐way Raman data. It is shown that only the raw RADIS data are needed for the multivariate analysis, so that the use of internal standards, corrections for self‐absorption and other corrections necessary in molecular dynamics problems can be avoided. The results do not depend very much on the particular excitation wavenumbers used. Furthermore in most cases the fluorescence background does not influence the results. These facts together with the increased availability of solid state lasers with a variety of wavenumbers makes it easy to implement the proposed application of RADIS in practice. Copyright © 2007 John Wiley & Sons, Ltd.     

Analysis of alizarin by surface‐enhanced and FT‐Raman spectroscopy

This paper presents the application of Raman spectroscopy (RS) for the structural study of alizarin adsorbed on a metallic surface. As a biologically active molecule, alizarin has remarkable antigenotoxic activity like other anthraquinone dyes. Alizarin is highly fluorescent and that limits the application of RS as an investigation method; however, the Fourier transform‐RS (FTRS) can be applied since the near‐infrared excitation line lies far away from the absorption region of alizarin. The surface enhanced‐RS (SERS) technique also makes the fluorescence quenching possible. In this work, monolayers of alizarin were deposited on the surface of an electrode by the immersion of silver substrates in methanolic solution of the analyte. From such prepared samples, by using the excitation of 488, 514.5 and 647.1 nm the Raman spectra were registered. Depending on the excitation line, SERS or surface‐enhanced resonance Raman scattering (SERRS) spectra of alizarin were observed. The interpretation of experimental data was supported by theoretical calculations. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface-enhanced resonance Raman scattering spectroscopy of single R6G molecules

Surface-enhanced resonance Raman scattering (SERRS) of Rhodamine 6G (R6G) adsorbed on colloidal silver clusters has been studied. Based on the great enhancement of the Raman signal and the quench of the fluorescence, the SERRS spectra of R6G were recorded for the samples of dye colloidal solution with different concentrations. Spectral inhomogeneity behaviours from single molecules in the dried sample films were observed with complementary evidences, such as spectral polarization, spectral diffusion, intensity fluctuation of vibrational lines and even ``breathing' of the molecules. Sequential spectra observed from a liquid sample with an average of 0.3 dye molecules in the probed volume exhibited the expected Poisson distribution for actually measuring 0, 1 or 2 molecules. Difference between the SERRS spectra of R6G excited by linearly and circularly polarized light were experimentally measured.     

Quantitative surface enhanced Raman scattering measurements at the early stage of active agent release processes

The surface enhanced Raman scattering spectroscopy (SERS) is introduced as a new method to probe the initial release of active agents from controlled delivery systems. As a model system, mitoxantrone‐loaded polypropylene specimens immersed in water have been utilized. Surface enhanced resonance Raman scattering (SERRS) measurements allowed the quantitative delineation of the initial drug release profile. SERRS was also compared in early stage release processes with UV–vis absorption often used in traditional quantitative analysis via HPLC, a common technique for controlled release evaluation. More and above the high selectivity of the Raman Effect, SERS has been proved as a highly sensitive method to quantitatively monitor the initial release of the medicine even at the very early stage of the delivery process; UV–vis absorbance was unable to respond accordingly. Copyright © 2012 John Wiley & Sons, Ltd.     

Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer

We describe the chemical and electromagnetic enhancements of surface‐enhanced resonance Raman scattering (SERRS) for the pyridine molecule absorbed on silver clusters, in which different incident wavelength regions are dominated by different enhancement mechanisms. Through visualization we theoretically investigate the charge transfer (CT) between the molecule and the metal cluster, and the charge redistribution (CR) within the metal on the electronic intracluster collective oscillation excitation (EICOE). The CT between the metal and the molecule in the molecule–metal complex is considered as an evidence for chemical enhancement to SERRS. CR within the metal on EICOE is considered as an evidence for the electromagnetic enhancement by collective plasmons. For the incident wavelength from 300 to 1000 nm, the visualized method of charge difference density can classify the different wavelength regions for chemical and electromagnetic enhancement, which are consistent with the formal fragmented experimental studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Theory of the resonance electronic Raman effect

The behaviour of electronic Raman scattering, excited under resonance conditions, is discussed in relation to molecular properties of the ground electronic state, the resonant excited state and the final electronic state. It is shown how the intensity distribution within vibronically structured electronic Raman bands depends on differences of molecular geometry and force field between these states.     

The influence of pH and anions on the adsorption mechanism of rifampicin on silver colloids

The influence of pH and anions on the adsorption mechanism of rifampicin on colloidal silver nanoparticles has been analysed by electronic absorption, resonance Raman (RR) and surface‐enhanced resonance Raman spectroscopy (SERRS). Rifampicin is a widely used antibiotic with a zwitterionic nature. SERRS spectra of rifampicin adsorbed on silver sols, prepared using hydroxylamine hydrochloride as reducing agent, undergo dramatic changes upon lowering the pH. The spectral form changes progressively from that characteristic of chemisorbed rifampicin (at pH > 7) to one very similar to the rifampicin RR spectrum (at lower pH), indicative of a modification of the adsorption mechanism on the surface of the Ag nanoparticles. The RR‐type SERRS spectrum is proposed to result from formation of an ion pair between rifampicin and Cl⁻ anions, which, deriving from the colloid preparation, are adsorbed on the Ag surface. The addition of anions to the hydroxylamine hydrochloride sol facilitates conversion from the chemisorbed to ion pair form and leads to an order of magnitude increase in the SERRS signal. Copyright © 2007 John Wiley & Sons, Ltd.     

DFT study on the influence of electric field on surface‐enhanced Raman scattering from pyridine–metal complex

The influence of a static external electric field on surface‐enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine–Au₂₀ complex with the density functional theory and time‐dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm⁻¹) but suppresses the trigonal ring breathing mode (~1051 cm⁻¹). On the contrary, the mode at 1051 cm⁻¹ is more enhanced than the mode at 1009 cm⁻¹ when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field. Copyright © 2013 John Wiley & Sons, Ltd.     

Totally symmetric vibrational modes of fullerene C60

Semiempirical molecular orbital calculations are carried out for the totally symmetric vibrations of the C₆₀ molecule. The calculated equilibrium geometry coincides with the precision experimental data to within measurement error. The ratio of force constants calculated for the two different types of C-C bonds in fullerene is equal to 1.389. A comparison of the computational results with the Raman scattering data indicates that there may be Fermi resonance between the totally symmetric vibrations.     

Influence of local environment on the intensity of the localized surface plasmon polariton of Ag nanoparticles

A combined Ag nanoparticle with an insulating or conductive layer structure has been designed for molecular detection using surface enhanced Raman scattering microscopy. Optical absorption studies revealed localized surface plasmon resonance, which shows regular red shift with increasing environmental dielectric constant. With the combined structure of surface enhanced Raman scattering substrates and rhodamine 6G as a test molecule, the results in this paper show that the absorption has a linear relationship with the local electromagnetic field for insulating substrates, and the electrical property of the substrate has a non-negligible effect on the intensity of the local electromagnetic field and hence the Raman enhancement.