Aromatic amino acids providing characteristic motifs in the Raman and SERS spectroscopy of peptides

Raman and surface-enhanced Raman spectroscopies (SERS) are potentially important tools in the characterization of biomolecules such as proteins and DNA. In this work, SERS spectra of three cysteine-containing aromatic peptides: tryptophan-cysteine, tyrosine-cysteine, and phenylalanine-cysteine, bound to Au nanoshell substrates, were obtained, and compared to their respective normal Raman spectra. While the linewidths of the SERS peaks are significantly broadened (up to 70%), no significant spectral shifts (<6 cm (-1)) of the major Stokes modes were observed between the two modalities. We show that the Raman and SERS spectra of penetratin, a cell-penetrating peptide oligomer, can be comprised quite reliably from the spectra of its constituent aromatic amino acids except in the backbone regions where the spectral intensities are critically dependent on the length and conformations of the probed molecules. From this study we conclude that, together with protein backbone groups, aromatic amino acid residues provide the overwhelmingly dominant features in the Raman and SERS spectra of peptides and proteins when present. It follows that the Raman modes of these three small constructed peptides may likely apply to the assignment of Raman and SERS features in the spectra of other peptides and proteins.     

自组装DNA吸附取向的表面增强拉曼光谱法研究

对金基体上自组装寡聚核苷酸探针杂交前后进行电化学非现场及现场表面增强拉曼光谱(SERS)研究.非现场SERS研究表明,杂交形成的dsDNA在基体表面以A型和B型两种构象同时存在,杂交过程可能伴随DNA链在基体表面吸附取向的变化.根据现场SERS研究结果可知,ssDNA及dsDNA的大多数SERS谱带强度随电极电位正移而降低,尤其是归属于碱基A的两种面外振动模式,谱带变化更为明显.利用SERS表面选择定则判断出随着电极电位由负向正变化,ssDNA及dsDNA螺旋吸附取向由垂直吸附向平躺吸附于金基体表面变化.     

Surface-enhanced Raman scattering system of sample molecules in silver-modified silver film

Surface-enhanced Raman scattering (SERS) spectrum of very good quality of "silver nano-particles/sample molecules/silver film" system was reported by nesting the sample molecules to the gap of silver nano-particles and silver film, indicating that "silver nano-particles/sample molecules/silver film" is a highly SERS-active system. Not only was the number of the vibrational modes increased, but also were the frequencies of Raman bands up and down shifted. It is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The shift by 5-30cm(-1) of the SERS bands and the change in their relative intensity compared with the ordinary Raman spectrum indicate the chemisorption of the sample molecules on the silver-modified silver surface. Furthermore, the silver nano-particles modified on the rough silver film surface play an important role in magnifying the surface local electric field near the silver surface through resonant surface plasmon excitation. From the rich information, obtained from high-quality SERS of PHBA in ternary system, we inferred that PHBA molecules in ternary system adsorb onto the metal surfaces through carboxyl at a perpendicular orientation.     

Synthesis of p-aminothiophenol-embedded gold/silver core-shell nanostructures as novel SERS tags for biosensing applications

We describe a novel surface-enhanced Raman scattering (SERS) tag that is based on Au/Ag core-shell nanostructures embedded with p-aminothiophenol. The Au/Ag core-shell sandwich nanostructures demonstrate bright and dark stripe structure and possess very strong SERS activity. Under optimum conditions, the maximum SERS signal was obtained with a 10?nm thick Ag nanoshell, and the enhancement factor is 3.4?×?10⁴ at 1077?cm^?1. After conjugation to the antibody of muramidase releasing protein (MRP), the Au/Ag core-shell nanostructures were successfully applied to an SERS-based detection scheme for MRP based on a sandwich type of immunoassay.

Exploring three-dimensional nanosystems with Raman spectroscopy: methylene blue adsorbed on thiol and sulfur monolayers on gold

Resonant Raman and surface-enhanced Raman scattering (SERS) spectroscopies, complemented with scanning tunnel microscopy and electrochemical techniques, have been used to obtain information about the amount and spatial distribution of methylene blue (MB) molecules immobilized on sulfur and four ultrathin molecular alkanethiolate films self-assembled on Au(111) and rough Au electrodes. The intensity of the Raman signals allow one to estimate the amount of immobilized MB at different organic films, whereas the decrease in the SERS intensity as a function of distance for the rough Au electrodes is used to locate the average position of the MB species with respect to the Au substrate. We found that significant amounts of cationic MB species are able to diffuse into methyl-terminated thiols, but they are stopped at the outer plane of the self-assembled monolayer (SAM) by negatively charged carboxylate groups. The relative shift of C-N stretching Raman modes indicates that the binding of MB to S is different from that found for MB on thiols. Most of the molecules immobilized on methyl- and carboxylate-terminated thiols are electrochemically inactive, suggesting that strong coupling between the Au electrode and the MB molecules is needed for charge transfer. Our results are consistent with a small population of electrochemically active MB species very close to the Au surface that reach this position driven by their lipophilic (hydrophobic) character through defects at SAMs.     

Surface-enhanced Raman scattering and the adsorption behavior of (RS)-phenylsuccinic acid

The surface geometry of (RS)-phenylsuccinic acid molecule was studied by analysis of the SERS spectra of aromatic dicarboxylic acid adsorbed on silver colloid surfaces. For a reliable analysis of the SERS spectrum, we also performed density functional theoretical calculations. The SERS spectral features indicated that the RSPSA molecules should bound to the silver as dicarboxylate, with a strongly tilted orientation with respect to the normal to the surface. Such a tilted orientation was presumed to occur by the simultaneous sigma and pi-type coordination of carboxylate groups to silver surface caused by the steric hindrance and electrostatic repulsion between the two carboxylate groups, and thereby RSPSA on silver was easily displaced with aromatic carboxylic acids. A sigma-type coordination therefore seemed to be more important than a pi-type coordination for aromatic carboxylic acid derivatives to assemble on a silver surface. The large enhancement of in-plane bending, out of plane bending and ring breathing modes in the surface-enhanced Raman scattering spectrum indicates that the molecule is adsorbed on the silver surface in a 'at least vertical' configuration, with the ring perpendicular to the silver surface.     

Surface-enhanced Raman scattering and density functional theory calculation of uracil on gold and silver nanoparticle surfaces

The adsorption structure of uracil on gold and silver nanoparticle surfaces has been comparatively studied by means of surface-enhanced Raman scattering (SERS). Uracil appeared to assume a perpendicular orientation with respect to the surfaces. The presence of the nu(CH) band in the SERS spectra indicated a vertical orientation of the aromatic ring of uracil on Au and Ag. The density functional theory (DFT) calculation was performed at the levels of B3LYP and MP2 to estimate the energetic stability of the N3- and N1-deprotonated tautomers and their vibrational frequencies on the surfaces. Almost all the vibrational bands in the SERS spectra at high concentrations could be ascribed to the N3-deprotonated uracil. The N3-deprotonated tautomer was predicted to be more favorable on Au than on Ag from the DFT calculation. The metal-N bond distance was assumed to be shorter for Au than for Ag upon adsorption of uracil.     

有序Au/Ag纳米碗阵列的简易制备及其表面增强拉曼散射性能

通过湿法化学合成基于SiO₂胶体晶体的大面积有序Au/Ag纳米碗(Au/AgNB)阵列。首先,在玻璃基板上以3D SiO₂胶体晶体作为模板。然后,在Au纳米颗粒(AuNP)种子的帮助下,通过原位生长方法在模板上沉积一层Au纳米壳(AuNS)。再通过HCHO还原Ag⁺使AuNS表面进一步沉积Ag纳米壳,形成Ag/Au双纳米壳(Ag/AuNS)阵列。通过丙烯酸酯改性双向取向聚丙烯(BOPP)方便地获得了单层有序反转Ag/AuNB阵列。这种有序Au/AgNB阵列具有更佳的表面增强拉曼散射(SERS)活性,其SERS分析增强因子(AEF)可达2.23×10⁷。     

Nanosphere arrays with controlled sub-10-nm gaps as surface-enhanced raman spectroscopy substrates

We demonstrate a convenient and cost-effective chemical approach for fabricating highly ordered Au nanoparticle arrays with sub-10-nm interparticle gaps. Near-field enhancements inside the interparticle gaps create uniform periodic arrays of well-defined "hot spots" exploitable for large surface-enhanced Raman spectroscopy (SERS) enhancements. A cetyltrimethylammonium bromide (CTAB) bilayer surrounding each individual nanoparticle upon array crystallization is responsible for this periodic gap structure; displacement of the CTAB by smaller thiolated molecules does not affect the structural integrity of the arrays. As SERS substrates, the as-fabricated Au nanoparticle arrays exhibit high SERS sensitivity, long-term stability, and consistent reproducibility.     

有序Au/Ag纳米碗阵列的简易制备及其表面增强拉曼散射性能

通过湿法化学合成基于SiO₂胶体晶体的大面积有序Au/Ag纳米碗(Au/AgNB)阵列。首先,在玻璃基板上组装3D SiO₂胶体晶体作为模板。然后,以Au纳米颗粒(AuNP)为种子,通过原位生长法在SiO₂模板上沉积一层Au纳米壳(AuNS)。再通过HCHO还原Ag⁺成Ag⁰,进一步在AuNS表面沉积Ag纳米壳,形成Ag/Au双纳米壳(Ag/AuNS)阵列。最后通过丙烯酸酯改性双向取向聚丙烯(BOPP)膜方便地获得了单层有序反转Ag/AuNB阵列。这种有序Au/AgNB阵列具有更佳的表面增强拉曼散射(SERS)活性,其SERS分析增强因子(AEF)可达2.23×10⁷。     

A study of DFT and surface enhanced Raman scattering in silver colloids for thymine

The vibrational spectrum of crystal thymine is calculated by density functional theory (DFT) at the B3LYP complex function. Considering the effect of intermolecular H-bonds, we add two water molecules that can form H-bonds with the CO and NH groups of thymine. The experimental spectra of normal Raman of thymine in solid state and surface enhanced Raman (SERS) of thymine adsorbed in silver colloids are presented in this study. The calculated Raman spectrum of thymine by DFT is in agreement with the experimental result of normal Raman spectrum. The appearance of new bands of thymine in SERS shows that molecules of thymine are adsorbed in the surface of silver nanoparticles with a perpendicular orientation through an oxygen atom (O7).     

TiO2电极表面氰基吡啶吸附的电化学SERS光谱

采用基于核壳纳米粒子的壳层隔绝纳米粒子增强拉曼(SHINERS)以及Au纳米粒子增强技术, 对比研究了4-氰基吡啶(4-CNPy)在TiO2表面的吸附行为. 结果表明, 采用2种技术所获得的光谱存在明显的差别. 利用前者得到了4-CNPy在TiO2电极上随电极电位变化的吸附方式. 在电位为0时, 分子以吡啶环上的N垂直吸附; 随电位负移, 部分分子变为倾斜吸附, 且在电位为-1.0 V时倾斜角度变大. 在正电位区间, 分子始终以吡啶环上的N垂直吸附. 而采用Au纳米粒子滴加在TiO2电极上的方式, 则得到吸附在TiO2, Au及TiO2/Au复合结构上的总光谱信息.     

In-situ monitoring of redox processes of viologen at Au(hkl) single-crystal electrodes using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy

In-situ Raman/SERS studies of molecular adsorption/reaction behaviors at well-defined electrochemical interfaces are important for understanding the fundamentals of electrochemical processes. However, it is still a great challenge to perform such studies on model single-crystal surfaces as the smooth surface cannot support surface plasmon resonance (SPR). In this work, shell-isolated nanoparticle-enhanced Raman spectroscopy was combined with an electrochemical method (EC-SHINERS) to study the adsorption and redox transformation of a resonant molecule viologen HS-8V8H at Au(hkl) single-crystal electrodes. Changes in the molecular structure with potential were identified on different single-crystal surfaces, which explained the transformation process of viologen from V^2 + state to V⁺ and then V⁰. Facet-dependent SERS enhancement was also observed, which results from the different imaginary part of the dielectric function on Au(111), Au(100) and Au(110), and is supported by the FEM simulations. Furthermore, a nonlinear resonant Raman process has been directly observed in our experiments, which is consistent with the simulation results. These findings increase our understanding of the electrochemical behavior of molecules in model systems.     

Surface-enhanced Raman scattering of 4,4′-bipyridine on gold nanoparticle surfaces

The adsorption structure and mechanism of 4,4′-bipyridine (BiPy) on gold nanoparticle surfaces has been investigated by means of surface-enhanced Raman scattering (SERS). The aromatic ring of BiPy appeared to assume a perpendicular orientation with respect to the gold surface from the presence of the ν(CH) band at ∼3060 cm⁻¹. The SERS intensities of several vibrational modes of BiPy on Au were found to vary as the bulk concentration. The SERS intensities for BiPy on Au could be ascribed to both the electromagnetic (EM) and charge transfer (CT) enhancement mechanism.     

Surface-enhanced Raman scattering of p-aminothiophenol on a Au(core)/Cu(shell) nanoparticle assembly.

Surface-enhanced Raman scattering (SERS) of p-aminothiophenol (PATP) molecules adsorbed onto assemblies of Au(core)/Cu(shell) nanoparticles is reported. We compare it with the SERS spectrum of PATP adsorbed onto gold nanoparticles: both the absolute and relative scattered intensities of various bands in the two spectra are very different. The difference in relative intensity can be ascribed to chemical effects; the chemical enhancement ratio of the two substrates is approximately 3-5. A theoretical analysis based on a charge-transfer model is carried out, which yields a consistent result and shows that the difference in chemical enhancement is mainly due to the state densities and Fermi levels of the substrates. The difference in absolute intensity originates from electromagnetic (EM) enhancement. EM enhancement of Au(core)/Cu(shell) nanoparticles is unlike that of single-component gold or copper SERS-active substrates. The core/shell particle size for optimal enhancement is about 20 nm in the case of a 632.8 nm incident laser (the size ratio of the core and shell layers is about 0.6).     

胞嘧啶吸附在粗糙金电极上的表面增强拉曼光谱

应用电化学伏安法和表面增强拉曼光谱(SERS)研究在-1.0 V~0 V电位区间内胞嘧啶于粗糙金电极表面的吸附行为.结果表明,在本实验的电位区间,胞嘧啶是以其N3位垂直吸附在粗糙金电极表面的.在负电位区间环呼吸振动模的强度出现极大值,与其它振动模强度相比,作者认为电磁场的增强和电荷转移均使该谱峰的拉曼信号增强.胞嘧啶的环呼吸振动频率随着电位负移而红移,这意味着它与金电极的成键作用减弱.同时也表明SERS谱可用于研究生物分子在金属电极表面的吸附行为.     

苯胺在银和金电极上的表面增强拉曼光谱

聚苯胺作为性能较好的导电高聚物有很大的应用潜力,有关其聚合和导电机理的研究受到广泛重视。表面增强拉曼光谱(SERS)能现场(in situ)检测表面吸附分子、提     

Au(111)表面Verdazyl自由基的构象转换

Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.     

Adsorption of fumaramide [2]rotaxane and its components on a solid substrate: a coverage-dependent study

The coverage-dependent adsorption on Au(111) of a fumaramide [2]rotaxane and its components, a benzylic amide macrocycle and a fumaramide thread, is studied using high-resolution electron energy loss spectroscopy (HREELS). Up to monolayer coverage, the relative intensity of out-of-plane to in-plane phenyl ring vibrational modes indicates that the macrocycle adopts an orientation with the phenyl rings largely parallel to the surface. The formation of a chemisorption bond is evidenced by the presence of a Au-O stretching vibration. In contrast, the thread shows no evidence of chemisorption or a preferential orientation. The introduction of the thread into the macrocycle partly disrupts the film order so that the resulting chemisorbed rotaxane shows intermediate behavior with a preferential orientation up to 0.5 ML coverage. A decrease in film order and the absence of a preferred molecular orientation is observed for all three molecules at multilayer coverages. The spectral differences are addressed by molecular dynamics simulations in terms of the mobility of the phenyls of the three molecules on Au(111).     

Experimental and theoretical surface enhanced Raman scattering study of 2-amino-4-methylbenzothiazole adsorbed on colloidal silver particles

The adsorption of 2-amino-4-methylbenzothiazole (2-AMBT) on colloidal silver particles has been investigated by a surface enhanced Raman scattering (SERS) study. The SERS spectra of the 2-AMBT molecule at varied adsorbate concentrations recorded in different time domains are compared with its Fourier transform infrared (FTIR) spectrum and normal Raman spectrum (NRS) in the bulk and in solution. The experimentally observed SERS spectra are compared with the theoretically modeled surface complexes using ab initio restricted Hatree-Fock (RHF) and density functional theory (DFT) calculations. The most favorable adsorptive sites of the 2-AMBT molecule have been estimated by natural population analysis (NPA) using the above-mentioned high level of theories. The enhancement of the in-plane modes together with the appearance of Ag-N stretching frequency at 215 cm(-1) indicates that the 2-AMBT molecule is adsorbed on the silver surface through the lone pair electrons of both nitrogen atoms with the molecular plane nearly vertical to the surface.