Rhodamine 6G conjugated to gold nanoparticles as labels for both SERS and fluorescence studies on live endothelial cells

Microchimica Acta - Fluorescence and surface-enhanced Raman scattering (SERS) spectroscopy were employed to investigate the cellular uptake of rhodamine 6G (R6G) alone and of R6G loaded with gold...     

Surface-enhanced Raman scattering from ordered Ag nanocluster arrays

We have examined the effect of ordered silver nanocluster substrates on the surface-enhanced Raman spectrum of rhodamine 6G (R6G). Triangular shaped silver nanocluster arrays with order on the approximately 100 mum range were prepared using nanosphere lithography. Direct comparisons of R6G surface-enhanced Raman spectroscopy (SERS) signals between ordered nanocluster regions and amorphous Ag regions prepared under identical deposition conditions provide strong evidence of an electromagnetic field enhancement attributed to the unique nanocluster morphology. We have obtained order of magnitude enhancement factors for both 200 and 90 nm Ag nanocluster SERS substrates relative to Ag films.     

Fourier transform Raman and infrared and surface-enhanced Raman spectra for rhodamine 6G

The vibrational spectra of rhodamine 6G (R6G) are discussed on the basis of Fourier transform infrared and Fourier transform Raman spectra obtained far from resonance which are compared with resonance Raman and surface-enhanced resonance Raman spectra obtained with excitation at 457.9 nm. The behaviour of several bands is described and tentative assignments are proposed. Stong resonance Raman effects are observed for bands assignable to xanthene ring stretching modes and also xanthene ring deformation modes. Some of these are sensitive to the complexing of R6G with silver colloids.     

Temperature-dependent anti-stokes/stokes ratios under surface-enhanced Raman scattering conditions

We make systematic measurements of Raman anti-Stokes/Stokes (aS/S) ratios using two different laser excitations (514 and 633 nm) of rhodamine 6G (RH6G) on dried Ag colloids over a wide range of temperatures (100 to 350 K). We show that a temperature scan allows the separation of the contributions to the aS/S ratios from resonance effects and heating/pumping, thus decoupling the two main aspects of the problem. The temperature rise is found to be larger when employing the 633 nm laser. In addition, we find evidence for mode specific vibrational pumping at higher laser power densities. We analyze our results in the framework of ongoing discussion on laser heating/pumping under surface-enhanced Raman scattering (SERS) conditions.     

Surface- and resonance-enhanced micro-Raman spectroscopy of xanthene dyes: from the ensemble to single molecules.

Surface-enhanced resonance Raman scattering (SERRS) spectra of various rhodamine dyes, of pyronine G and thiopyronine adsorbed on isolated silver clusters were recorded at the ensemble level and at the single-molecule level with a high-resolution confocal laser microscope equipped with a spectrograph and a CCD-detector. Comparing single-molecule spectra with ensemble spectra, various inhomogeneous spectral features, such as line splitting, spectral wandering, spectral diffusion and abrupt spectral jumps between different metastable spectral states, are revealed positions and the relative intensities of the vibronic bands. Resonance enhancement is investigated with respect to single-molecule surface-enhanced Raman scattering (SERS) spectroscopy and is found to be responsible for approximately three orders of magnitude in sensitivity. A significant influence of the substituents on the single-molecule SERRS sensitivity is found, showing that various chemical effects are responsible for surface enhancement in addition to the electromagnetic enhancement effect.     

Probing the Raman scattering tensors of individual molecules

Single-molecule experiments provide new views into the mechanisms behind surface-enhanced Raman scattering. It was shown previously that spectra of individual rhodamine 6G molecules adsorbed on silver nanocrystal aggregates present stronger fluctuations in two low-frequency bending modes, at 614 and 773 cm(-1). Here we use polarization spectroscopy to show that these bands are enhanced by a resonant process whose transition dipole is rotated by 15+/-10 degrees with respect to the molecular transition dipole. We also show that the polarization function remains stable over the whole time scale of a measurement, indicating that molecular reorientation with respect to the surface is unlikely. Together these findings provide further support to the involvement of a charge-transfer resonance in the enhancement of the low-frequency bands and allow us to suggest a model for the orientation of rhodamine 6G molecules at Raman hot spots.     

Correlation of surface-enhanced Raman spectroscopy and laser desorption-ionization mass spectrometry acquired from silver nanoparticle substrates

Applying complementary experiments, like laser desorption-ionization mass spectrometry (LDI-MS) and confocal surface-enhanced Raman microscopy, to the same physical sample location has the potential to elucidate the behavior of complex chemical and biochemical systems in ways that are not available to either method applied in isolation. In these experiments surface-enhanced Raman scattering (SERS) and LDI-MS are applied to the same sample spot using a common structure, deposited Ag colloids, both as ionization matrix and simultaneously as enhancing media for surface-enhanced Raman scattering of small organic molecules, dyes and lipids, and the behavior is compared. Three compounds-p-aminothiophenol (ATP), rhodamine 6G and cholesterol-which exhibit different strengths of interaction with Ag are examined in detail by correlated SERS and LDI-MS. The related mechanisms of nanoparticle-assisted desorption-ionization and Raman enhancement are explored by correlating mass and Raman spectra. The correlated spectra highlight the manner in which the different test compounds interact with plasmonic metal nanostructures. These coupled studies yield new insight into the transition of analyte from the metal-solution interface to gaseous ions, including, in the case of organothiols, a rich set of mixed clusters that provide chemical insight into the ion formation process.     

Spectroscopic studies of rhodamine 6G dispersed in polymethylcyanoacrylate

We report here electronic absorption, fluorescence and resonance Raman studies of rhodamine 6G laser dye dispersed in the polymethylcyanoacrylate matrix. In the electronic absorption and fluorescence spectra of dispersed rhodamine 6G, band maxima are red shifted compared to solution. Raman spectra show some new bands. These spectral changes arise due to matrix effect and interaction between rhodamine 6G and the host material involving amine group of rhodamine.     

Resonance Raman scattering of rhodamine 6G as calculated using time-dependent density functional theory

In this work, we present the first calculation of the resonance Raman scattering (RRS) spectrum of rhodamine 6G (R6G) which is a prototype molecule in surface-enhanced Raman scattering (SERS). The calculation is done using a recently developed time-dependent density functional theory (TDDFT) method, which uses a short-time approximation to evaluate the Raman scattering cross section. The normal Raman spectrum calculated with this method is in good agreement with experimental results. The calculated RRS spectrum shows qualitative agreement with SERS results at a wavelength that corresponds to excitation of the S(1) state, but there are significant differences with the measured RRS spectrum at wavelengths that correspond to excitation of the vibronic sideband of S(1). Although the agreement with the experiments is not perfect, the results provide insight into the RRS spectrum of R6G at wavelengths close to the absorption maximum where experiments are hindered due to strong fluorescence. The calculated resonance enhancements are found to be on the order of 10(5). This indicates that a surface enhancement factor of about 10(10) would be required in SERS in order to achieve single-molecule detection of R6G.     

Silver Nanostructures on Graphene Oxide as the Substrate for Surface-Enhanced Raman Scattering (SERS)

Nanosized surface-enhanced Raman scattering (SERS) substrates fabricated by the controlled growth of metal nanostructures on water-dispersed two-dimensional nanomaterials can open a new avenue for SERS analysis of liquid samples in biological fields. In this work, regular and uniform Ag nanostructures were grown on the surface of graphene oxide (GO) through a microwave-assisted hydrothermal method. Polyamidoamine (PAMAM) dendrimers were assembled on the surface of GO to form GO/PAMAM templates for growing Ag nanostructures, which are primarily comprised of Ag dimers and trimers. The prepared Ag/GO nanocomposites are highly dispersed and stable in aqueous solution and may be used as substrates for enhanced Raman detection of rhodamine 6?G (R6G) in aqueous solution. This special substrate provides high-performance SERS and suppresses R6G fluorescence in aqueous solution and is promising as a nanosized material for the enhanced Raman detection of liquid samples in biological diagnostics.     

银表面罗丹明6G的电化学表面增强拉曼光谱研究

罗丹明6G（Rhodamine 6G,R6G）是单分子表面增强拉曼光谱（SM-SERS）研究中最常用的探针分子之一,对R6G分子在表面吸附行为的研究有助于了解R6G分子和表面的相互作用. 本文应用电化学和电化学表面增强拉曼光谱技术,研究不同电位下R6G的银电极表面的吸附行为. 结果表明,随着电位负移罗丹明6G在银表面上从垂直吸附转为倾斜吸附,该变化和碱性条件下吸附于金纳米粒子上R6G的吸附构象一致. 这说明,在部分单分子实验中所发现的R6G反常光谱其来源是单个R6G分子在表面吸附取向变化. 本研究对后续详细分析SM-SERS研究中单分子SERS谱峰变化的机制有一定的参考价值.     

Colloidal AgCl induced-growth of thorny gold nanoparticles and their SERS activity

In this paper, we exploited a unique procedure for obtaining thorny gold nanoparticles (Au NPs) with controllable length of thorns without using seeds and surfactants. The obtained Au NPs exhibited shape-determined surface-enhanced Raman spectroscopy activity toward rhodamine 6G.     

Surface-enhanced raman spectra of dyes and organic acids in silver solutions: chloride ion effect

We have recorded surface-enhanced Raman (SER) spectra of two different classes of compounds, cationic dyes and organic acids, and studied their chloride ion effects on the surface-enhanced Raman scattering (SERS) activities of the silver solution. For the positive charge dyes, rhodamine 6G (R6G) and 1,1'-dimethyl-2,2'-cyanine iodide (DECI), no SERS could be observed without the addition of chloride ions because of lack of the electrostatic interaction between the dye species and the silver particles in the silver solution. The chloride ions served to enlarge silver particles and to contribute the existence of the surface active sites, making the silver solution SERS active to the dye samples. Surface-enhanced resonance Raman scattering (SERRS) intensity of the dye molecules increased with the chloride ion concentration. After reaching a maximum intensity, a Cl- quenching effect on the intensity took place. For the organic acids, benzoic acid and p-aminobenzoic acid (PABA), SERS could be observed without the coexistence of chloride ions. The intensity of the Raman scattering did not vary significantly in the presence of small amount of chloride ion. At high Cl- concentration, quenching SERS intensity began to take effect.     

DFT vibrational calculations of rhodamine 6G adsorbed on silver: analysis of tip-enhanced Raman spectroscopy

The tip-enhanced near-field Raman (TERS) bands of Rhodamine 6G (R6G), that we reported earlier [Chem. Phys. Lett. 2001, 335, 369.], are assigned on the basis of density-functional theory (DFT) calculations at the 6-311++G(d,p) level. The Raman and infrared intensities as well as frequencies of the vibrational modes are used for band assignments. These vibrational modes, in combination with characterization of resonant electronic transitions using time-dependent DFT calculations, predict spectral changes in resonant Raman and surface-enhanced resonant Raman scatterings of R6G. Moreover, the TERS spectra of R6G are analyzed in detail, where interactions between the tip and R6G molecules and their enhancement mechanisms are discussed. Finally, we propose a novel Raman spectroscopy technique capable of detecting molecular vibrations at sub-nanometer scale.     

树枝状银微纳结构的表面增强拉曼散射效应研究

利用两电极电化学沉积法制备出一种树枝状银微纳结构基体.扫描电子显微镜(SEM)的表征结果证实所制备的银基体呈现出完整的树枝状结构,具有对称性的树枝和树干,且树叶清晰可见.实验结果表明,树枝状银微纳结构的表面增强拉曼散射(Surface-enhanced Raman scattering,SERS)可以检测到超低浓度的罗丹明6G(Rhodamine 6G,R6G,10-10 mol/L)光谱信号,即树枝状银微纳结构作为SERS基体表现出较好的灵敏性;当R6G的浓度在10-5~10-10 mol/L范围依次降低一个数量级时,谱带610 cm-1处的拉曼散射强度的相对标准偏差分别为12.1%,12.0%,11.7%,10.9%,13.2%和14.3%,表明所制备银基体的SERS"热点"(Hot spots)分布较均一,树枝状银微纳结构作为SERS基体具有较好的重现性;当低SERS活性的3-巯基丙酸(3-Mercaptopropionic acid,3MPA)的检测浓度为10-5 mol/L时,利用树枝状银基体能检测到3MPA的SERS光谱,说明所制备的银基体对低活性物质也具有较好的SERS灵敏性.     

Morphological effects on surface-enhanced Raman scattering from silver butterfly wing scales synthesized via photoreduction

Through a simple room-temperature photoreduction process, this letter conformally replicates 3D submicrometer structures of wing scales from two butterfly species into Ag to generate practical surface-enhanced Raman scattering (SERS) substrates. The Ag replicas of butterfly scales with higher structural periodicity are able to detect rhodamine 6G at a low concentration down to 10(-9) M, which is three orders of magnitude lower than the detectable concentration limit of using quasi-periodic Ag butterfly structures. This result presents a way to select suitable scale morphologies from 174,500 species of Lepidopterans to replicate, as consumable SERS substrates with low cost and high reproducibility.     

Combined linear response quantum mechanics and classical electrodynamics (QM/ED) method for the calculation of surface-enhanced Raman spectra

A multiscale method is presented that allows for evaluation of plasmon-enhanced optical properties of nanoparticle/molecule complexes with no additional cost compared to standard electrodynamics (ED) and linear response quantum mechanics (QM) calculations for the particle and molecule, respectively, but with polarization and orientation effects automatically described. The approach first calculates the total field of the nanoparticle by ED using the finite difference time domain (FDTD) method. The field intensity in the frequency domain as a function of distance from the nanoparticle is calculated via a Fourier transform. The molecular optical properties are then calculated with QM in the frequency domain in the presence of the total field of the nanoparticle. Back-coupling due to dipolar reradiation effects is included in the single-molecule plane wave approximation. The effects of polarization and partial orientation averaging are considered. The QM/ED method is evaluated for the well-characterized test case of surface-enhanced Raman scattering (SERS) of pyridine bound to silver, as well as for the resonant Raman chromophore rhodamine 6G. The electromagnetic contribution to the enhancement factor is 10(4) for pyridine and 10(2) for rhodamine 6G.     

Quantitative evaluation of blinking in surface enhanced resonance Raman scattering and fluorescence by electromagnetic mechanism

We analyze blinking in surface enhanced resonance Raman scattering (SERRS) and surface enhanced fluorescence (SEF) of rhodamine 6G molecules as intensity and spectral instability by electromagnetic (EM) mechanism. We find that irradiation of intense NIR laser pulses induces blinking in SERRS and SEF. Thanks to the finding, we systematically analyze SERRS and SEF from stable to unstable using single Ag nanoparticle (NP) dimers. The analysis reveals two physical insights into blinking as follows. (1) The intensity instability is inversely proportional to the enhancement factors of decay rate of molecules. The estimation using the proportionality suggests that separation of the molecules from Ag NP surfaces is several angstroms. (2) The spectral instability is induced by blueshifts in EM enhancement factors, which have spectral shapes similar to the plasmon resonance. This analysis provides us with a quantitative picture for intensity and spectral instability in SERRS and SEF within the framework of EM mechanism.     

二步电沉积法制备Au/氧化石墨烯复合薄膜作为SERS基底

采用二步电沉积方法在Ti片表面制备了Au-氧化石墨烯（Au-GO）复合薄膜,通过XRD、SEM、XPS等对薄膜的组成、结构和形貌进行了表征,并以罗丹明6G（R6G）为探针分子,对Au-GO/Ti基底的SERS活性进行了表征。结果显示,Au纳米颗粒尺寸约为60 nm,均匀、致密分布于GO表面,该基底显示出较高的SERS活性,对R6G分子的检测极限可达~10^-10 mol·L^-1,增强因子高达约10⁶,且基底显示出良好的稳定性,在冰箱中存放90 d后,SERS活性仅降低30%左右。     

二步电沉积法制备Au/氧化石墨烯复合薄膜作为SERS基底

采用二步电沉积方法在Ti片表面制备了Au-氧化石墨烯（Au-GO）复合薄膜,通过XRD、SEM、XPS等对薄膜的组成、结构和形貌进行了表征,并以罗丹明6G（R6G）为探针分子,对Au-GO/Ti基底的SERS活性进行了表征。结果显示,Au纳米颗粒尺寸约为60 nm,均匀、致密分布于GO表面,该基底显示出较高的SERS活性,对R6G分子的检测极限可达~10^-10 mol·L^-1,增强因子高达约10⁶,且基底显示出良好的稳定性,在冰箱中存放90 d后,SERS活性仅降低30%左右。