水溶液中碳纳米管的表面增强拉曼光谱研究

合成了碳纳米管和金纳米颗粒的复合物, 测量了水溶液相中复合物的表面增强拉曼光谱, 结果表明, 碳纳米管的巯基化修饰可以提高碳纳米管与金纳米颗粒复合的效率, 随着金纳米颗粒负载量的增加, 碳纳米管的拉曼信号逐渐增强. 加入己二胺分子可以减小金纳米颗粒之间的距离使表面增强效应更显著, 碳纳米管的拉曼光谱得到进一步的增强. 还可进一步在复合体系中加入对巯基苯胺和罗丹明B等小分子拉曼探针, 利用金纳米颗粒的表面增强效应, 这种多元复合体系有望作为多通道拉曼成像探针材料.     

金银纳米粒子的复合组装及表面增强拉曼光谱研究

采用自组装技术在硅基底上进行金银纳米粒子的混合组装, 通过控制组装溶液中金银溶胶的体积比而控制基底上金银纳米粒子的密度. SEM结果显示金银呈亚单层均匀分布, 以吡啶为探针分子, 在不同波长的激发光下研究了纯金、银以及混合组装时的SERS效应. 利用金银在不同激发线下增强效应的不同以及探针分子吸附在金银纳米粒子表面主要谱峰相对强度差别的特点, 通过一系列校正以及差谱方法研究了金银共存时SERS效应的变化, 并分离出混合体系中金的增强行为, 结果表明在金银同时组装时吡啶的SERS谱峰特征主要表现为银纳米粒子的行为, 分离出的金SERS光谱特征接近银的行为, 说明金银纳米粒子之间产生了一定的耦合作用.     

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).     

Selective etching of graphene edges by hydrogen plasma

We devised a controlled hydrogen plasma reaction at 300 °C to etch graphene and graphene nanoribbons (GNRs) selectively at the edges over the basal plane. Atomic force microscope imaging showed that the etching rates for single-layer and few-layer (≥2 layers) graphene are 0.27 ± 0.05 nm/min and 0.10 ± 0.03 nm/min, respectively. Meanwhile, Raman spectroscopic mapping revealed no D band in the planes of single-layer or few-layer graphene after the plasma reaction, suggesting selective etching at the graphene edges without introducing defects in the basal plane. We found that hydrogen plasma at lower temperature (room temperature) or a higher temperature (500 °C) could hydrogenate the basal plane or introduce defects in the basal plane. Using the hydrogen plasma reaction at the intermediate temperature (300 °C), we obtained narrow, presumably hydrogen terminated GNRs (sub-5 nm) by etching of wide GNRs derived from unzipping of multiwalled carbon nanotubes. Such GNRs exhibited semiconducting characteristics with high on/off ratios (～1000) in GNR field effect transistor devices at room temperature.     

SE(R)RS devices fabricated by a laser electrodispersion method

A novel laser electrodispersion (LE) technique was employed to deposit gold nanoparticles onto Si and SiO(x) surfaces. The LE technique combines laser ablation with cascade fission of liquid metal micro-drops, which results in the formation of nanoparticles upon rapid cooling. The shape and the size distribution of the Au nanoparticles prepared by LE depend on the nature of the support. Gold nanoparticles were also deposited in the channels of microreactors fabricated by wet etching of Si and used as SE(R)RS sensors. The influence of the nanoparticle surface density as well as of the nature of the substrate on the Raman response was studied. At an appropriate surface density of the deposited nanoparticles a significant enhancement of Raman signal was observed showing the possibility to create efficient SERS substrates. Application of microfluidic devices in surface enhanced Raman spectroscopy (SERS) in continuous-flow mode with sensor regeneration is described.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.     

三明治结构纳米银/氧化石墨烯基底的制备及SERS性能

采用易操作且低成本的静电自组装方法, 在质子化的玻璃基片上, 通过交替沉积氧化石墨烯(GO) 和带正电荷的银纳米粒子(AgNPs) 获得少数层GO和AgNPs复合薄膜(AgNPs/GO). 采用紫外-可见光吸收光谱、 原子力显微镜和扫描电子显微镜对复合薄膜的生长和表面形貌进行了表征. 结果表明, 通过调控AgNPs 溶胶浓度和自组装循环次数, 可以获得AgNPs/GO/AgNPs 的三明治结构, 并在基底表面形成均匀的AgNPs 聚集体. 表面增强拉曼散射(SERS)研究结果表明, AgNPs/GO-4基底具有最佳的SERS性能, 其对罗丹明6G(R6G) 和结晶紫的平均拉曼增强因子分别为3.4×10⁸和1.3×10⁹, 对R6G的最低检测浓度约为10^-12 mol/L. 多层三明治结构和较小颗粒间距使得AgNPs层之间产生强烈的耦合作用, 并在GO片层间产生大量的“热点”, 显著提高SERS性能, 而少数层GO具有强吸附性, 有利于分子在基底中富集, 从而起到化学增强作用, 提高SERS灵敏度.     

Growth-sensitive 3D ordered gold nanoshells precursor composite arrays as SERS nanoprobes for assessing hydrogen peroxide scavenging activity

A novel method based on surface-enhanced Raman scattering (SERS) was developed to estimate the antioxidant activity of antioxidants by using self-assembled three-dimensionally (3D) ordered gold nanoparticles (GNPs) precursor composite (SiO(2)/GNPs) arrays as nanoprobes. H(2)O(2) could reduce AuCl(4)(-) to Au(0) which deposited onto the surface of the SiO(2)/GNPs arrays and enlarged the GNPs. As the concentration of H(2)O(2) increased, the surface coverage of the resultant gold on the silica cores increased accordingly until continuous gold nanoshells (GNSs) were formed. The change of the intensities of the SERS spectra correlated well with H(2)O(2) concentration which indicated that this SiO(2)/GNPs array was a potential SERS nanoprobe for H(2)O(2). The presence of antioxidant will prevent the growth of GNPs on the surface of the silica arrays from forming the structure which has strongest SERS-activity and the corresponding change in SERS intensity correlated well with the H(2)O(2) scavenging activity of the antioxidants. The H(2)O(2) scavenging activities of four plant-based antioxidants, tannic acid, citric acid, ferulic acid, and tartaric acid were studied. Our results showed the H(2)O(2) scavenging activities (SA(HP) values) of these four compounds were: tannic acid > ferulic acid > citric acid > tartaric acid.     

Electronic excited states of [Au(2)(dmpm)(3)](ClO(4))(2) (dmpm= bis(dimethylphosphine)methane)

We present studies of the resonance Raman and electronic luminescence spectra of the [Au(2)(dmpm)(3)](ClO(4))(2) (dmpm = bis(dimethylphosphine)methane) complex, including excitation into an intense band at 256 nm and into a weaker absorption system centered about approximately 300 nm. The resonance Raman spectra confirm the assignment of the 256 nm absorption band to a (1)(dsigma --> psigma) transition, a metal-metal-localized transition, in that nu(Au-Au) and overtones of it are strongly enhanced. A resonance Raman intensity analysis of the spectra associated with the 256 nm absorption band gives the ground-state and excited-state nu(Au-Au) stretching frequencies to be 79 and 165 cm(-1), respectively, and the excited-state Au-Au distance is calculated to decrease by about 0.1 A from the ground-state value of 3.05 A. The approximately 300 nm absorption displays a different enhancement pattern, in that resonance-enhanced Raman bands are observed at 103 and 183 cm(-1) in addition to nu(Au-Au) at 79 cm(-1) The compound exhibits intense, long-lived luminescence (in room-temperature CH(3)CN, for example, tau = 0.70 micros, phi(emission) = 0.037) with a maximum at 550-600 nm that is not very medium-sensitive. We conclude, in agreement with an earlier proposal of Mason (Inorg. Chem. 1989, 28, 4366-4369), that the lowest-energy, luminescent excited state is not (3)(dsigma --> psigma) but instead derives from (3)(d(x2-y2,xy --> psigma) excitations. We compare the Au(I)-Au(I) interaction shown in the various transitions of the [Au(2)(dmpm)(3)](ClO(4))(2) tribridged compound with previous results for solvent or counterion exciplexes of [Au(2)(dcpm)(2)](2+) salts (J. Am. Chem. Soc. 1999, 121, 4799-4803; Angew. Chem. 1999, 38, 2783-2785; Chem. Eur. J. 2001, 7, 4656-4664) and for planar, mononuclear Au(I) triphosphine complexes. It is proposed that the luminescent state in all of these cases is very similar in electronic nature.     

The effects of Au aggregate morphology on surface-enhanced Raman scattering enhancement

We have identified empirically a relationship between the surface morphology of small individual aggregates (<100 Au nanoparticles) and surface-enhanced Raman scattering (SERS) enhancement. We have found that multilayer aggregates generated greater SERS enhancement than aggregates limited to two-dimensional (2D) or one-dimensional structures, independent of the number of particles. SERS intensity was measured using the 730 cm(-1) vibrational mode of the adsorbed adenine molecule on 75 nm Au particles, at an excitation wavelength of 632.8 nm. To gain insight into these relationships and its mechanism, we developed a qualitative model that considers the collections of interacting Au nanoparticles of an individual aggregate as a continuous single entity that retains its salient features. We found the dimensions of the modeled surface features to be comparable with those found in rough metal surfaces, known to sustain surface plasmon resonance and generate strong SERS enhancement. Among the aggregates that we have characterized, a three 75 nm nanoparticle system was the smallest to generate strong SERS enhancement. However, we also identified single individual Au nanoparticles as SERS active at the same wavelength, but with a diameter twice in size. For example, we observed a symmetric SERS-active particle of 180 nm in diameter. Such individual nanoparticles generated SERS enhancement on the same order of magnitude as the small monolayer Au aggregates, an intensity value significantly stronger than predicted in recent theoretical studies. We also found that an aspect of our model that relates the dimensions of its features to SERS enhancement is also applicable to single individual Au particles. We conclude that the size of the nanoparticle itself, or the size of a protrusion of an irregularly shaped single Au particle, will contribute to SERS enhancement provided that its dimensions satisfy the conditions for plasmon resonance. In addition, by considering the ratio of the generated intensities of typical 2D Au aggregates to the enhancement of individual SERS-active particles, a value of approximately 2 is determined. Its moderate value suggests that it is not the aggregation effect that is responsible for much of the observed SERS enhancement but the surface region associated with the SERS-active site.     

Raman scattering of 4-aminobenzenethiol sandwiched between Ag/Au nanoparticle and macroscopically smooth Au substrate

Raman scattering measurements were conducted for a 4-aminobenzenethiol (4-ABT) monolayer assembled on a macroscopically smooth Au substrate. Although no peak was detected at the beginning, Raman peaks were distinctly observed by attaching Ag or Au nanoparticles onto the 4-ABT monolayer (Ag(Au)@4-ABT/Au(flat)). Considering the fact that no Raman signal is observed when Ag (Au) nanoparticles are adsorbed on a (4-aminophenyl)silane monolayer assembled on a silicon wafer, the Raman spectrum observed for Ag(Au)@4-ABT/Au(flat) must be a surface-enhanced Raman scattering (SERS) spectrum, derived from the electromagnetic coupling of the localized surface plasmon of Ag (Au) nanoparticles with the surface plasmon polariton of the underneath Au metal. The electromagnetic coupling responsible for SERS appeared to be governed more by the bulk Au substrate than the sparsely distributed Ag or Au nanoparticles. The chemical enhancement appeared on the other hand to be derived more from the formation of Au-S bonds than any charge-transfer interaction between the protonated amine group and the Au or Ag nanoparticles. The enhancement factors derived from the attachment of a single Ag or Au nanoparticle onto 4-ABT on Au were estimated to be as large as 8.3 x 10(5) and 5.0 x 10(5), respectively, (for the ring 3 band (b(2)) near 1390 cm(-1)) in which a factor of approximately 10(2) was presumed to be due to the chemical effect, with the remaining contributed by the electromagnetic effect.     

小尺寸金石墨纳米颗粒的合成与表征

在表面增强拉曼光谱(SERS)的研究领域中,基于局域表面等离子体共振效应的等离子体SERS基底的制备成为过去几十年的研究热点。然而,通常开发的等离子体金属基底具有较差的稳定性和重现性。对于SERS而言,石墨烯类材料具有拉曼化学增强效应,除此之外,还具有分子富集、强的稳定性与荧光猝灭能力等优点,因此基于石墨金属复合纳米材料的SERS基底受到了研究人员的重视。我们利用化学气相沉积(CVD)法制备了小尺寸的金石墨核壳纳米颗粒(Au@G),其粒径约为17 nm。我们通过在Au NP上包覆介孔二氧化硅来控制Au@G的尺寸,同时还研究了包覆二氧化硅过程中,正硅酸乙酯(TEOS)的浓度对于石墨壳层形成的影响。结果表明当TEOS在一定浓度范围内,其浓度的降低有利于得到石墨化程度高的Au@G。进一步利用Au@G对结晶紫分子进行拉曼检测,也表明了Au@G具有较好的拉曼增强效果。这种小尺寸的Au@G在分子检测与细胞成像分析领域中具有广泛的应用潜力。     

Characterization of the surface enhanced raman scattering (SERS) of bacteria

The surface enhanced Raman scattering (SERS) of a number of species and strains of bacteria obtained on novel gold nanoparticle (approximately 80 nm) covered SiO(2) substrates excited at 785 nm is reported. Raman cross-section enhancements of >10(4) per bacterium are found for both Gram-positive and Gram-negative bacteria on these SERS active substrates. The SERS spectra of bacteria are spectrally less congested and exhibit greater species differentiation than their corresponding non-SERS (bulk) Raman spectra at this excitation wavelength. Fluorescence observed in the bulk Raman emission of Bacillus species is not apparent in the corresponding SERS spectra. Despite the field enhancement effects arising from the nanostructured metal surface, this fluorescence component appears "quenched" due to an energy transfer process which does not diminish the Raman emission. The surface enhancement effect allows the observation of Raman spectra of single bacterial cells excited at low incident powers and short data acquisition times. SERS spectra of B. anthracis Sterne illustrate this single cell level capability. Comparison with previous SERS studies reveals how the SERS vibrational signatures are strongly dependent on the morphology and nature of the SERS active substrates. The potential of SERS for detection and identification of bacterial pathogens with species and strain specificity on these gold particle covered glassy substrates is demonstrated by these results.     

Synthesis, characterization, and 3D-FDTD simulation of Ag@SiO2 nanoparticles for shell-isolated nanoparticle-enhanced Raman spectroscopy

Au-seed Ag-growth nanoparticles of controllable diameter (50-100 nm), and having an ultrathin SiO(2) shell of controllable thickness (2-3 nm), were prepared for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Their morphological, optical, and material properties were characterized; and their potential for use as a versatile Raman signal amplifier was investigated experimentally using pyridine as a probe molecule and theoretically by the three-dimensional finite-difference time-domain (3D-FDTD) method. We show that a SiO(2) shell as thin as 2 nm can be synthesized pinhole-free on the Ag surface of a nanoparticle, which then becomes the core. The dielectric SiO(2) shell serves to isolate the Raman-signal enhancing core and prevent it from interfering with the system under study. The SiO(2) shell also hinders oxidation of the Ag surface and nanoparticle aggregation. It significantly improves the stability and reproducibility of surface-enhanced Raman scattering (SERS) signal intensity, which is essential for SERS applications. Our 3D-FDTD simulations show that Ag-core SHINERS nanoparticles yield at least 2 orders of magnitude greater enhancement than Au-core ones when excited with green light on a smooth Ag surface, and thus add to the versatility of our SHINERS method.     

Plasma-induced formation of Ag nanodots for ultra-high-enhancement surface-enhanced Raman scattering substrates

We report here plasma-induced formation of Ag nanostructures for surface-enhanced Raman scattering (SERS) applications. An array of uniform Ag patterned structures of 150 nm diameter was first fabricated on a silicon substrate with imprint lithography; then the substrate was further treated with an oxygen plasma to fracture the patterned structures into clusters of smaller, interconnected, closely packed Ag nanoparticles (20-60 nm) and redeposited Ag nanodots ( approximately 10 nm) between the clusters. The substrate thus formed had a uniform ultrahigh SERS enhancement factor (1010) over the entire substrate for 4-mercaptophenol molecules. By comparison, Au patterned structures fabricated with the same method did not undergo such a morphological change after the plasma treatment and showed no enhancement of Raman scattering.     

Gold particle interaction in regular arrays probed by surface enhanced Raman scattering

Lithographically designed two-dimensional arrays consisting of gold nanoparticles deposited on a smooth gold film are used as substrate to examine the SERS effect of the trans-1,2-bis (4-pyridyl) ethylene molecule. These arrays display two plasmon bands instead of the single one observed for the same arrays of particles but deposited on indium tin oxide coated glass. Laser excitation within the short wavelength band does not bring about any SERS spectrum, while excitation within the long wavelength band yields SERS spectra with a gain per molecule rising up to 10(8). The simultaneous investigation of extinction and Raman spectra of arrays exhibiting various topography parameters enables us to suggest an interpretation for both the occurrence of the two plasmon resonances and for the high Raman enhancement. We suggest to assign the short wavelength band to a plasmon wave propagating at the gold glass interface and the long wavelength one to an air/gold surface plasmon mode modified by particle-particle interaction.     

介孔SiO_2薄膜增敏SERS基底在消逝波激励下的特性表征

采用溶胶-凝胶分子模板法在50 nm厚金膜表面制备约40 nm厚介孔二氧化硅(MPS)薄膜,然后在MPS薄膜表面静电自组装金纳米粒子(GNP)单层膜,形成的多层膜结构用作表面增强拉曼散射(SERS)基底.利用扫描电镜观测到MPS薄膜具有表面开口多孔结构,有助于小分子向薄膜内快速扩散.基于时域有限差分(FDTD)方法对电场分布的仿真结果指出,在表面等离子体共振(SPR)条件下分布于金膜与GNP之间的消逝场显著增强.由于空间重叠,该增强场能够高效激发MPS内富集的小分子拉曼信号,产生的拉曼信号还可免受金属作用的干扰.利用Kretschmann结构和尼罗蓝(NB)拉曼活性分子测试了Au/MPS/GNP基底在785 nm激发波长下的SERS效果,并与Au/GNP基底进行了比较.结果表明,在SPR条件下,Au/MPS/GNP基底能够导致较强的定向和背向拉曼信号,而且在586 cm-1处的背向拉曼信号强度是Au/GNP基底的40倍,这归功于MPS薄膜.进一步测试表明背向拉曼信号强度与NB浓度成正相关.这意味着Au/MPS/GNP基底具有良好的半定量检测本领.     

基于自组装纳米金单层膜的全内反射SERS研究

采用静电自组装技术分别在玻璃基片和30 nm厚的金膜表面固定一层金纳米粒子(GNP)制得两种表面增强拉曼散射(SERS)基底,然后通过棱镜全内反射(TIR)激励和背向收集模式分别测试了两种基底上吸附的染料单分子层SERS光谱.实验结果表明两种SERS基底的拉曼增强效果均高度依赖于入射激光的偏振状态,对于玻璃/纳米金SERS基底,s光全内反射导致的拉曼增强因子是线偏振光(p)光的2-5倍,说明该基底上的"热点"位于纳米金单层膜内相邻粒子之间;对于玻璃/金膜/纳米金SERS基底,只有采用p光在特定的全内反射角下才能激发SERS信号,而且测得的SERS信号比玻璃/纳米金基底增强了近30倍.究其原因是p光在金膜表面共振激发的传播表面等离子体与纳米金局域表面等离子体耦合,进而导致显著场增强.实验结果指出在背向收集模式下,由p光激发的SERS信号是非偏振光,包含强度几乎相等的s和p成分.利用玻璃/金膜/纳米金基底还实现了拉曼光定向发射和收集,测得的SERS信号是p光.     

Synthesis, characterization and SERS activity of Au-Ag nanorods

The formation mechanism and morphology of Au-Ag bimetallic colloidal nanoparticles depend on the composition. Ag coated Au colloidal nanoparticles have been prepared by deposition of Ag through chemical reduction on performed Au colloid. The composition of the Au(100-x)-Ag(x) particles was varied from x=0 to 50. The obtained colloids were characterized by UV-vis spectroscopy and transmission electron microscopy (TEM). The Au(80)-Ag(20) colloid consists of alloy nanorods with dimension of 25nmx100nm. The activity of these nanorods in surface enhanced Raman spectroscopy (SERS) was checked by using sodium salicylate as an adsorbate probe. Intense SERS bands are observed indicating its usefulness as a SERS substrate in near infrared (NIR) laser excitation.     

基于自组装纳米金单层膜的全内反射SERS研究

采用静电自组装技术分别在玻璃基片和30 nm厚的金膜表面固定一层金纳米粒子（GNP）制得两种表面增强拉曼散射（SERS）基底,然后通过棱镜全内反射（TIR）激励和背向收集模式分别测试了两种基底上吸附的染料单分子层SERS光谱. 实验结果表明两种SERS基底的拉曼增强效果均高度依赖于入射激光的偏振状态,对于玻璃/纳米金SERS基底,s 光全内反射导致的拉曼增强因子是线偏振光（p）光的2-5 倍,说明该基底上的“热点”位于纳米金单层膜内相邻粒子之间;对于玻璃/金膜/纳米金SERS基底,只有采用p光在特定的全内反射角下才能激发SERS信号,而且测得的SERS信号比玻璃/纳米金基底增强了近30 倍. 究其原因是p 光在金膜表面共振激发的传播表面等离子体与纳米金局域表面等离子体耦合,进而导致显著场增强. 实验结果指出在背向收集模式下,由p 光激发的SERS信号是非偏振光,包含强度几乎相等的s 和p 成分. 利用玻璃/金膜/纳米金基底还实现了拉曼光定向发射和收集,测得的SERS信号是p光.