银纳米阵列直径对SERS增强效果的影响

采用电化学沉积法分别在不同孔径的阳极氧化铝(AAO)模板上沉积一系列直径不同,排列规则的银纳米阵列。以对氨基苯甲酸(PABA)和三聚氰胺两种分子分别作为探针分子, 研究了银纳米阵列的直径大小对其表面增强拉曼散射(SERS)效果的影响。结果表明, 在波长为514.5 nm的激光激发下, 探针分子的SERS信号强度随银纳米阵列直径的改变而明显变化, 并在银纳米阵列直径约为53 nm时, SERS强度达到最大。利用电磁增强机制对此实验结果进行了分析和解释。     

A comparative study of surface‐enhanced Raman scattering from silver‐coated anodic aluminum oxide and porous silicon

Three types of Ag‐coated arrays from porous anodic aluminum oxide (AAO) were prepared and studied as substrates for surface‐enhanced Raman scattering (SERS). They were compared with Ag‐coated porous silicon (PSi) samples. AAO‐based substrates were prepared by the vapor deposition of silver directly onto the surface of porous AAO with different morphologies of the pores, whereas SERS‐active island films on the PSi were prepared by immersion plating. The resulting metallic nanostructures were characterized by UV‐vis absorption spectroscopy and scanning electron microscopy (SEM). Thermal evaporation leads to the formation of granular arrays of Ag nanoparticles on the surface of AAO. SERS activity of the substrates was tested using water‐soluble cationic Zn(II)‐tetrakis (4‐N‐methylpyridyl) porphyrin (ZnTMPyP4) as a probe molecule. The results indicate that all AAO‐based substrates studied here exhibit some degree of SERS activity. Noteworthy, for excitation at 532 nm, signals from AAO‐based substrates were comparable with those from the PSi‐based ones, whereas for 441.6 nm excitation they were about twice higher. The strongest SERS‐enhancement at 441.6 nm excitationwas provided by the AAO substrates with silver deposited on the monolith (originally nonporous) side of AAO. Preferential SERS‐enhancement of the bands ascribed to the vibrations of the N‐methylpyridinium group of ZnTMPyP4 when going to blue excitation was found. Copyright © 2010 John Wiley & Sons, Ltd.     

Silver nanostructure arrays abundant in sub-5 nm gaps as highly Raman-enhancing substrates

Two types of highly Raman-enhancing arrays substrates were fabricated using anodic aluminum oxide (AAO) templates by controlling the AAO template temperature and evaporated silver thickness during e-beam evaporating: complex patterned Ag nanoparticle arrays abundant in sub-5 nm gaps (type I); hexagonal Ag nanopore arrays (type II). The surface enhanced Raman scattering (SERS) enhancement factors (EF) of both substrates are estimated experimentally to exceed 10⁵, especially that of type I reaches 10⁷ due to the existence of numerous sub-5 nm gaps. The simulation using finite-difference time-domain (FDTD) method confirmed that gap effect has significantly improved the substrates’ SERS activity.     

Large‐area Ag nanorod array substrates for SERS: AAO template‐assisted fabrication,functionalization, and application in detection PCBs

Highly ordered arrays of thiolated β‐cyclodextrin (HS‐β‐CD) functionalized Ag‐nanorods (Ag‐NRs) with plasmonic antennae enhancement of electrical field have been achieved for encapsulation and rapid detection of polychlorinated biphenyls (PCBs). The large‐area ordered arrays of rigid Ag‐NRs supported on copper base were fabricated via porous anodic aluminum oxide (AAO) template‐assisted electrochemical deposition. The inter‐nanorod gaps between the neighboring Ag‐NRs were tuned to sub‐10 nm by thinning the pore‐wall thickness of the AAO template using diluted H₃PO₄. The nearly perfect large‐area ordered arrays of Ag‐NRs supported on copper base render these systems excellent in surface‐enhanced Raman scattering (SERS) performance with uniform electric field enhancement, as testified by the SERS spectra and Raman mappings of rhodamine 6 G. Furthermore, the Ag‐NRs were functionalized with HS‐β‐CD molecules so as to capture the apolar PCB molecules in the hydrophobic cavity of the CD. Compared to the ordinary undecorated SERS substrates, the HS‐β‐CD modified Ag‐NR arrays exhibit better capture ability and higher sensitivity in rapid detection of PCBs. Copyright © 2012 John Wiley & Sons, Ltd.     

AAO模板吸收光谱变化及红移现象研究

阳极氧化铝(AAO)模板作为一种制备表面增强拉曼散射光谱(SERS)基底的工具被广泛使用。为了制备SERS效果更好的基底, 研究AAO模板自身的属性显得极为重要。本文以0.4 mol/L的草酸为电解液制备出高度有序的AAO模板。发现其吸收光谱在203、250、275 nm处有三个吸收峰, 其中203和275 nm处的吸收峰, 分别由F心(带两个电子的氧空位)和F+心(带一个电子的氧空位)产生。而250 nm处的吸收峰, 由制备过程中引入的草酸根离子产生。并给出了随着退火温度升高250、275 nm的吸收峰消失的合理解释。通过对不同氧化时间、氧化电压下制备的AAO模板吸收光谱的研究, 发现延长氧化时间对AAO模板吸收带边的红移影响不大, 而升高氧化电压则使AAO模板的吸收带边出现了明显红移。表明氧化电压的升高对AAO模板的内部结构及缺陷的变化影响极大。     

SERS imaging of silver coated nanostructured Al and Al2O3 substrates. The effect of nanostructure

The last few years have witnessed rapid development of highly ordered and reproducible surface‐enhanced Raman scattering (SERS) nanostructured substrates for their potential medical and analytical application such as biosensing and bioimaging. In this work, 5‐nm silver films deposited on nanostructured Al and Al₂O₃ templates are investigated as substrates for SERS. The chosen templates show different honeycomb nanostructures with two sets of dimension, i.e. pore diameter of ca. 25 and 50 nm and interpore distance of ca. 56 and 100 nm. The SERS imaging results reveal that the signal of the probe molecule (4‐thiazolidinone‐2‐thione) is distributed inhomogeneously on the substrate surface, and this fact is correlated with the morphology of nanostructures determined by atomic force microscopy. The variation of SERS intensity among the substrates is strongly correlated with the shape and size of potential SERS‐active sites, e.g. nanocups and nanopores. The strongest SERS response is found for the Ag/Al₂O₃ template anodized in sulfuric acid, which represents the nanopore array with the smallest dimensions (e.g. pore diameter, interpore distance etc.). Furthermore, depending on size and nanostructure shape, changes in the adsorption mechanism of the probe molecule are observed. Copyright © 2014 John Wiley & Sons, Ltd.     

AAO模板法制备CdS纳米微粒的SERS光谱研究

在自制的孔径约15nm多孔氧化铝模板上沉积银纳米粒子,然后用电化学方法在此衬底上沉积CdS纳米微粒。研究了CdS纳米阵列在457.5nm波长激光激发下的表面增强拉曼散射(SERS)性质。实验结果显示CdS的SERS信号有三个振动模式,分别对应1LO、2LO和3LO纵光学声子模,它们的强度随着作为SERS衬底的银纳米粒子高度的增加而增强,当银纳米粒子的长/径比(长度与直径的比值)达到4时,这种增强趋近饱和。最后对CdS纳米微粒光学声子模的增强机理进行了分析和讨论。     

Nanogaps in 2D Ag‐nanocap arrays for surface‐enhanced Raman scattering

The surface‐enhanced Raman scattering substrate of Ag–Ag nanocap arrays are prepared by depositing Ag film onto two‐dimensional (2D) polystyrene colloidal nanosphere templates. When the original colloidal arrays are used as the substrate for Ag deposition, surface‐enhanced Raman scattering (SERS) enhancements show the strong size‐dependence behaviours. When O₂‐plasma etched 2D polystyrene templates are used as the substrate for Ag deposition to form nanogaps, the gap sizes between adjacent Ag nanocaps from 5 to 20 nm generate even greater SERS enhancements. When SiO₂ coverage is deposited to isolate the Ag nanocaps from the neighbours, the SERS signals are enhanced more. The significant SERS effects are due to the coupling between Ag nanocaps controlled by the distance, which enhances the local electric‐field intensity. Copyright © 2013 John Wiley & Sons, Ltd.     

Comparison of Surface-Enhanced Raman Scattering Spectra of Azo Dye Sudan III on Several Substrates

On several substrates, including dried silver-coated filter paper, electro-polishing aluminum foils, and anodic aluminum oxide (AAO) nanometer array templates, surface-enhanced Raman scattering (SERS) spectra of Azo Dye Sudan III molecules were studied, and high-quality SERS spectra were obtained, which indicated that electro-polishing aluminum foils and AAO nanometer array templates were also active SERS substrates. A number of additional modes were presented, as well as some split peaks, which were especially obvious on electro-polishing aluminum foil and AAO nanometer array template. The enhancement mechanisms were discussed and the enhancement factors G were calculated as 10⁴, 10⁶, and 10⁵, respectively.     

Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency

In this study, monodisperse bimetallic nanorods with gold (Au) nanorod core and silver (Ag) shell (Au@AgNRs) were synthesized through seed‐mediated growth process by reduction of AgNO₃ using Au nanorods with narrow size and shape distribution as seeds. With increasing the used amount of AgNO₃, the Ag shell thickness of their lateral facets is raised faster than that of their two tips, leading to a decrease of their aspect ratios. Four plasmon bands are observable on the extinction spectra of Au@AgNRs, which are attributed to the longitudinal dipolar plasmon mode, transverse dipolar plasmon mode, and octupolar plasmon mode of the core‐shell structured bimetallic nanorods, respectively. As their Ag shell thickness increases, their longitudinal plasmon band blue‐shifts notably with the transverse plasmon band blue‐shifting and the two octupolar plasmon bands red‐shifting slightly, due to the decrease of their aspect ratios and enhancement of Ag plasmon resonance contribution. When used as surface‐enhanced Raman scattering (SERS) substrate for probing minute amounts of 4‐mercaptobenzoic acid in aqueous solution, Au@AgNRs have much stronger SERS activity than Au nanorods, and the obtained Raman signals are highly reproducible arising from their excellent monodispersity. Their SERS activity is remarkably increased with their Ag shell thickness thanks to the enhancing surface electric field and the chemical enhancement associated with electronic ligand effect. Copyright © 2014 John Wiley & Sons, Ltd.     

Strategy for improved analysis of peptides by surface‐enhanced Raman spectroscopy (SERS) involving positively charged nanoparticles

A strategy for improved surface‐enhanced Raman spectroscopy (SERS) measurements that extends the variety of analytes accessible to SERS analysis is developed. The strategy involves inducing aggregation by mixing positively charged nanoparticles which form SERS‐active clusters when mixed with negatively charged silver nanoparticles fabricated using the Lee–Meisel process. To make positively charged nanoparticles, silver nanoparticles using the traditional Lee–Meisel process are fabricated and coated with a thin layer of silica and the silica modified with silane chemistry. Analytes with a significant amount of negative charge exhibit strong Raman bands when the strategy using these fabricated, positively charged nanoparticles for inducing cluster formation is used, enabling their detection and analysis. We envision the use of positively charged nanoparticles in cluster formation for expanding the range of analytes that can be detected using SERS and hence the range of applications SERS can play a role in. Copyright © 2008 John Wiley & Sons, Ltd.     

Controllable synthesis of Ag nanorods using a porous anodic aluminum oxide template

This paper describes a new approach to the synthesis of Ag nanorods. A solvothermal method was used to make Ag nanoparticles inside anodic aluminum oxide (AAO) templates. The nanoparticles were then annealed at 300 °C to produce Ag nanorods. The size of AAO templates, which is focused on in this study, would determine the diameter of Ag nanorods. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In this study, a nanorod growth mechanism is deduced, and understanding of the growth of nanorods inside AAO templates is furthered. This work demonstrates that it is possible to make crystalline nanorods that the size can be varied.     

Characterization and optimization of AuNPs labeled by Raman reporters on glass based on silver enhancement

In this report, gold nanoparticles (AuNPs) labeled by Raman reporters (AuNPs‐R6G) were assembled on glass and used as the seeds to in situ grow silver‐coated nanostructures based on silver enhancer solution, forming the nanostructures of AuNPs‐R6G@Ag, which were characterized by scanning electron microscopy (SEM) and UV‐visible spectroscopy. More importantly, the obtained silver‐coated nanostructures can be used as a surface enhancement Raman scattering (SERS) substrate. The different SERS activities can be controlled by the silver deposition time and assembly time of AuNPs‐R6G on glass. The results indicate that the maximum SERS activity could be obtained on AuNPs‐R6G when these nanostructures were assembled on glass for 2 h with silver deposition for 2 min. In addition, the reproducibility of SERS signal on the fabricated nanostructures is very high with the intensity error lower than 15%, which has great promise as a probe for application in bioanalysis. Copyright © 2008 John Wiley & Sons, Ltd.     

The SERS response of semiordered Ag nanorod arrays fabricated by template oblique angle deposition

Semiordered Ag nanorod arrays are fabricated by template oblique angle deposition (OAD) using regular Au nano‐post arrays with different diameters as seed patterns. The Au nano‐post arrays do not give an observable surface‐enhanced Raman scattering (SERS) activity under our detection configuration, whereas the patterned Ag nanorod arrays can produce a very strong SERS signal. These SERS intensities increase monotonically with the decrease in the diameter and separation of the Ag nanorods, which demonstrates that one can improve the SERS detection by tuning the diameter and separation of the Ag nanorods, and the template OAD method can help produce more uniform, reproducible, and sensitive Ag nanorod SERS substrates. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of periodical Ag–Au compound nanostructure films with controllable Ag nanoparticle aggregate patterns: a study on surface‐enhanced Raman scattering

The Ag–Au compound nanostructure films with controllable patterns of Ag nanoparticle (NP) aggregates were fabricated. A strategy of two‐step synthesis was employed toward the target products. Firstly, the precursor Au NP (17 nm) films were synthesized as templates. Secondly, the Ag NPs (45 nm) were deposited on the precursor films. Three types of Ag NP aggregates were obtained including discrete Ag NPs (discrete type), necklace‐like Ag NP aggregates (necklace type), and huddle‐like Ag NP aggregates (huddle type). The surface‐enhanced Raman scattering (SERS) property was studied on these nanostructures by using the probing molecule of rhodamine 6G under the excitation laser of 514.5 nm. Interestingly, the different types of samples showed different enhancement abilities. A statistical method was employed to assess the enhancement. The relative enhancement factor for each Ag NP was estimated quantitatively under the ratio of 1 : 25 : 18 for the discrete‐type, necklace‐type, and huddle‐type samples at the given concentration of 10⁻⁸ mol/l. This research shows that the enhancement ability of each Ag NP is dependent on the aggregate morphology. Moreover, the different enhancement abilities displayed different limit detection concentrations up to 10⁻⁸, 10⁻¹¹, and 10⁻⁹ mol/l, separately. The understanding of the relationship between the defined nanostructures and the SERS enhancement is very meaningful for the design of new SERS substrates with better performance. Copyright © 2015 John Wiley & Sons, Ltd.     

二维银球腔阵列的制备及其在SERS检测中的应用

以密堆积的700 nm单分散聚苯乙烯微球为模板,采用多电流阶跃方法制备了不同深度的二维有序微/纳尺度银球腔阵列。通过扫描电子显微镜,反射紫外对球腔形貌及表面等离子体共振进行了表征,以对氨基苯硫酚及罗丹明6G为探针分子进行了表面增强拉曼光谱(SERS)的研究。结果表明,通过控制电化学沉积的条件可以实现对球腔形貌的控制。以该种球腔阵列作为SERS基底,其增强因子可达107,并具有良好的信号重现性,信号间相对标准偏差小于8%。该基底用于对罗丹明6G的定量检测,检测限可达0.1 ng·mL-1。     

Adsorption of 1,2,4‐triazole on a silver electrode: surface‐enhanced Raman spectroscopy and density functional theory studies

The pH‐dependent surface‐enhanced Raman scattering (SERS) of 1,2,4‐triazole adsorbed on silver electrode and normal Raman (NR) spectra of this compound in the aqueous solutions were investigated. The observed bands in the NR and SERS spectra were assigned with the help of density functional theory calculations for model molecules in the neutral, anionic, and cationic forms and their complexes with silver. The Raman wavenumbers and intensities were computed at the optimized molecular geometry. Vibrational assignments of the SERS and NR spectra are provided by calculated potential energy distributions. The combination of experimental SERS results and density functional theory calculations provide an insight into the molecular structure of adlayers formed by 1,2,4‐triazole on a silver surface at varying pH values and enable the determination of molecular orientation with respect to the surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Fabrication and characterization of SERS‐active silver clusters on glassy carbon

In this article, a novel technique for the fabrication of surface enhanced Raman scattering (SERS) active silver clusters on glassy carbon (GC) has been proposed. It was found that silver clusters could be formed on a layer of positively charged poly(diallyldimethylammonium) (PDDA) anchored to a carbon surface by 4‐aminobenzoic acid when a drop containing silver nanoparticles was deposited on it. The characteristics of the obtained silver clusters have been investigated by atomic force microscopy (AFM), SERS and an SERS‐based Raman mapping technique in the form of line scanning. The AFM image shows that the silver clusters consist of several silver nanoparticles and the size of the clusters is in the range 80–100 nm. The SERS spectra of different concentrations of rhodamine 6G (R6G) on the silver clusters were obtained and compared with those from a silver colloid. The apparent enhancement factor (AEF) was estimated to be as large as 3.1 × 10⁴ relative to silver colloid, which might have resulted from the presence of ‘hot‐spots’ at the silver clusters, providing a highly localized electromagnetic field for the large enhancement of the SERS spectra of R6G. The minimum electromagnetic enhancement factor (EEF) is estimated to be 5.4 × 10⁷ by comparison with the SERS spectra of R6G on the silver clusters and on the bare GC surface. SERS‐based Raman mapping technique in the form of line scanning further illustrates the good SERS activity and reproducibility on the silver clusters. Finally, 4‐mercaptopyridine (4‐Mpy) was chosen as an analyte and the lowest detected concentration was investigated by the SERS‐active silver clusters. A concentration of 1.6 × 10⁻¹⁰ M 4‐Mpy could be detected with the SERS‐active silver clusters, showing the great potential of the technique in practical applications of microanalysis with high sensitivity. Copyright © 2006 John Wiley & Sons, Ltd.     

Adjustment and control of SERS activity of metal substrates by pressure

Metal pellets of silver and copper for surface‐enhanced Raman scattering (SERS) spectroscopy were prepared by compression with different pressures. It was found that the SERS activity of the pellet could be controlled by pressure. Enhanced Raman scattering properties of the metal pellets in the presence of adsorbed 4‐mercaptobenzoic acid (4‐MBA) with excitation at 632.8 or 514 nm could be obtained by choosing proper pressure of pellatization. The SERS peak intensity of the band at ∼1584 cm⁻¹ of 4‐MBA adsorbed on the metal pellets varies as a function of applied pressure, and which is about 1.2–32 times greater than when it is adsorbed on silver and copper particles. The calculated results of three‐dimensional finite‐difference time‐domain method (3D‐FDTD) are in good agreement with the experimental data. Moreover, no spurious peaks appear in the SERS spectra of the samples because no other chemicals are involved in the simple preparation process of the metal pellets, which will facilitate its use as an SERS‐active substrate for analytical purposes. In summary, SERS‐active metal pellets can be produced simply and cost effectively by the method reported here, and this method is expected to be utilized in the development of SERS‐based analytical devices. Copyright © 2009 John Wiley & Sons, Ltd.     

Controllable fabrication of highly ordered thin AAO template on Si substrate for electrodeposition of nanostructures

In this work, simple fabrication of hexagonally highly ordered porous anodic aluminum oxide (AAO) of Al thin film (1 μm) on Si substrate is described using two-step anodization method for electrochemical synthesis of nanostructures. In this method, the templates were prepared under the controllable conditions of the parameters, which give rise to the possibility of highly ordered nanopore arrays with a well aspect ratio. Pore widening was then fulfilled in 5 wt% phosphoric acid solution at 25 °C. The pore diameter and spacing are proportional to the applied voltage, which is due to the mechanical stress associated with the volume expansion of the aluminum during the anodization according to the mechanical stress model. Pore-widening solution adjusted the pore diameter and thinned the AAO barrier layer at room temperature under the control of etching time. As an application, Cu nanorods arrays embedded in anodic alumina (AAO) template were fabricated by dc electrodeposition. The characterization of the AAO templates and the Cu nanorods produced was made by X-ray diffraction, field emission scanning microscope, energy dispersive X-ray spectroscopy and atomic force microscope (AFM). The images of AFM show that porous AAO template under constant voltage is 40 V which presents the optimum ordering.