Stability of silver colloids as substrate for surface enhanced Raman spectroscopy detection of dipicolinic acid

Silver colloids have been commonly used as substrates for surface enhanced Raman spectroscopy (SERS). It has been shown that SERS requires partial aggregation of the silver colloids. This study evaluates factors affecting the aggregative state of the silver colloids such as the age of the silver colloids and the aggregation as a result of addition of the analyte. The silver colloids are obtained from the chemical reduction of silver nitrate by sodium borohydride. Further oxidation of the sodium borohydride solution at room temperature results in concentration changes of the resulting silver colloids. Methods of controlling the sodium borohydride depletion are presented in this paper. The analyte used is dipicolinic acid, a molecular signature of Bacillus spores.     

In situ laser-induced photochemical silver substrate synthesis and sequential SERS detection in a flow cell

A new, simple, and effective approach for multianalyte sequential surface-enhanced Raman scattering (SERS) detection in a flow cell is reported. The silver substrate was prepared in situ by laser-induced photochemical synthesis. By focusing the laser on the 320 μm inner diameter glass capillary at 0.5 ml/min continuous flow of 1 mM silver nitrate and 10 mM sodium citrate mixture, a SERS active silver spot on the inner wall of the glass capillary was prepared in a few seconds. The test analytes, dacarbazine, 4-(2-pyridylazo)resorcinol (PAR) complex with Cu(II), and amoxicillin, were sequentially injected into the flow cell. Each analyte was adsorbed to the silver surface, enabling the recording of high intensity SERS spectra even at 2 s integration times, followed by desorption from the silver surface and being washed away from the capillary. Before and after each analyte passed the detection window, citrate background spectra were recorded, and thus, no “memory effects” perturbed the SERS detection. A good reproducibility of the SERS spectra obtained under flow conditions was observed. The laser-induced photochemically synthesized silver substrate enables high Raman enhancement, is characterized by fast preparation with a high success rate, and represents a valuable alternative for silver colloids as SERS substrate in flow approaches.     

Stable and efficient silver substrates for SERS spectroscopy

Silver substrates have been obtained, by depositing silver colloidal nanoparticles on a roughened silver plate treated with 1,10-phenanthroline, and checked by means of AFM microscopy and Raman spectroscopy. The ligand molecules are located between two silver substrates and undergo the SERS (Surface Enhanced Raman Scattering) enhancement of both the roughened silver plate and the silver colloidal layer deposited on it. These SERS-active substrates, which show the advantages of being stable with respect to the metal colloidal suspensions, along with an easy and reproducible preparation, can be very useful for catalytic and analytical applications of the SERS spectroscopy.     

Novel Bottom‐Up SERS Substrates for Quantitative and Parallelized Analytics

Surface‐enhanced Raman spectroscopy (SERS) is an emerging technology in the field of analytics. Due to the high sensitivity in connection with specific Raman molecular fingerprint information SERS can be used in a variety of analytical, bioanalytical, and biosensing applications. However, for the SERS effect substrates with metal nanostructures are needed. The broad application of this technology is greatly hampered by the lack of reliable and reproducible substrates. Usually the activity of a given substrate has to be determined by time‐consuming experiments such as calibration or ultramicroscopic studies. To use SERS as a standard analytical tool, cheap and reproducible substrates are required, preferably with a characterization technique that does not interfere with the subsequent measurements. Herein we introduce an innovative approach to produce low‐cost and large‐scale reproducible substrates for SERS applications, which allows easy and economical production of micropatterned SERS active surfaces on a large scale. This approach is based on an enzyme‐induced growth of silver nanostructures. The special structural feature of the enzymatically deposited silver nanoparticles prevents the breakdown of SERS activity even at high particle densities (particle density >60 %) that lead to a conductive layer. In contrast to other approaches, this substrate exhibits a relationship between electrical conductivity and the resulting SERS activity of a given spot. This enables the prediction of the SERS activity of the nanostructure ensemble and therewith the controllable and reproducible production of SERS substrates of enzymatic silver nanoparticles on a large scale, utilizing a simple measurement of the electrical conductivity. Furthermore, through a correlation between the conductivity and the SERS activity of the substrates it is possible to quantify SERS measurements with these substrates.     

Surface-enhanced Raman spectroscopy study on the structure changes of 4-mercaptopyridine adsorbed on silver substrates and silver colloids

Surface-enhanced Raman scattering (SERS) of 4-mercaptopyridine (4-mpy) adsorbed on HNO3 etched silver foil, chemically deposited silver films (silver mirror) and silver colloids were measured. The SERS study has revealed that 4-mpy was adsorbed onto the three kinds of silver surfaces by a sulfur-silver bond with the plane of pyridine ring being normal to the silver substrates. The structure of 4-mpy adsorbed on the silver surfaces depends largely on the pH values of environment. When the pH values of the environment are changed, the structure of 4-mpy adsorbed on silver surfaces can easily be altered through a protonation or deprotonation reaction occurring on the N atom of the pyridine ring, and the modified structure shows unique characters on the SERS spectrum. Owing to the remarkable enhancement ability of SERS technique and characteristic spectrum of different species, a monolayer of 4-mpy assembled on a silver mirror holds potential as a H+ sensor for highly sensitive detection of the proton concentration in an aqueous solution.     

Silver nanoparticles active as surface-enhanced Raman scattering substrates prepared by high energy irradiation

In surface-enhanced Raman scattering (SERS) technique the preparation of metal substrates containing minimum hindrance from impurities is an important issue. The synthesis of silver nanoparticles (Ag NPs) active as SERS substrates and having the above-mentioned advantage, were obtained by electron beam irradiation of Ag⁺ aqueous solutions. Ag⁺ ions were reduced by free radicals radiolytically generated in solution without the addition of chemical reductants or stabilizing agents.The metal colloids were characterised by UV-Vis spectroscopy and scanning electron microscopy, monitoring the nanoparticles’ growth process that depends on the irradiation dose and the initial AgNO₃ concentration. Nanoparticles of long-time stability and with different size and shape, included silver nanocubes, were synthesised by varying the irradiation dose. Different tests on the SERS activity of Ag NPs obtained by electron beam irradiation were performed by using benzenethiol as a probing molecule, achieving a good magnification of the adsorbate Raman bands.     

Surface enhanced Raman spectroscopy on copper hydrosols

Surface enhanced Raman spectroscopy (SERS) allows the detection of trace quantities of molecular species adsorbed onto a surface. The potential use of silver colloids as substrates for analytical SERS measurements is demonstrated. Detection limits and other analytical figures of merit for pyridine, p-aminobenzoic acid and p-nitrobenzoic acid are presented.     

Sub-wavelength Raman spectroscopy on isolated silver islands

Surface Enhanced Raman Spectroscopy (SERS) is a valuable analytical tool for the investigation of molecules adsorbed on roughened noble metal surfaces. The shape, size, and surrounding of the metal protrusions play an important role in the Raman scattering enhancement. By combining scanning near-field optical microscopy (SNOM) with Raman spectroscopy the spatial resolution suffices for investigating isolated silver islands on SERS active substrates. We demonstrate an optical resolution below 70 nm for recording spectra on specifically prepared and fully characterized SERS substrates. For a quantitative evaluation of the SERS signal the spatial distribution of Rhodamine 6G (R6G) deposited on the SERS substrate was determined by friction force measurements. By comparing the Raman intensities of the SERS substrates with those of unmetallized support plates absolute SERS enhancement factors at specific locations on top and in the vicinity of the silver islands were determined directly.     

Magnetron sputtering of silver nanowires using anodic aluminum oxide template: a new active substrate of surface enhanced Raman scattering and an investigation of its enhanced mechanism

A high quality anodic aluminum oxide (AAO) template with ordered apertures about 50-80 nm was fabricated by anodizing aluminum in electrolytes through a two-step method, and silver nanowires with diameters from 40nm to 70nm were prepared on this AAO template by magnetron sputtering. On the glass covered with silver nanowires, high quality surface enhanced Raman scattering (SERS) spectra of sudan II (C₁₈H₁₆N₂O) with enhancement factors of 10⁵ were obtained. And comparison of SERS spectra on silver nanowires with the SERS spectra of silver colloids indicates that main enhanced mode is lightning rod effect of nanorods on the Sudan II/silver nanowires system.     

A rapid and simple chemical method for the preparation of Ag colloids for surface-enhanced Raman spectroscopy using the Ag mirror reaction

Colloidal silver (Ag) nanoparticles (AgNP) have been widely used for surface-enhanced Raman spectroscopy (SERS) applications. We report a simple, rapid and effective method to prepare AgNP colloids for SERS using the classic organic chemistry Ag mirror reaction with Tollens’ reagent. The AgNP colloid prepared with this process was characterized using SEM, and the reaction conditions further optimized using SERS measurements. It was found that Ag mirror reaction conditions that included 20 mM AgNO₃, 5 min reaction time, and 0.5 M glucose produced AgNP colloids with an average size of 319.1 nm (s.d ± 128.1). These AgNP colloids exhibited a significant SERS response when adenine was used as the reporter molecule. The usefulness of these new AgNP colloids was demonstrated by detecting the nucleotides adenosine 5′-mono-phosphate (AMP), guanosine 5′-monophosphate (GMP), cytidine 5′-monophosphate (CMP), and uridine 5′-monophosphate (UMP). A detection limit of 500 nM for AMP was achieved with the as-prepared AgNP colloid. The bacterium Mycoplasma pneumoniae was also easily detected in laboratory culture with these SERS substrates. These findings attest to the applicability of this AgNP colloid for the sensitive and specific detection of both small biomolecules and microorganisms.     

Comparative study of the morphology, aggregation, adherence to glass, and surface-enhanced Raman scattering activity of silver nanoparticles prepared by chemical reduction of Ag+ using citrate and hydroxylamine

Two different silver colloids were prepared by chemical reduction of silver nitrate with trisodium citrate and hydroxylamine hydrochloride to compare their characteristics in relation to their possible use in surface-enhanced Raman scattering (SERS) spectroscopy. The morphology and plasmon resonance of the single nanoparticles and aggregates integrating these colloids were characterized by means of UV-vis absortion spectroscopy and scanning electron microscopy, revealing important differences between each type of nanoparticle as concerns their physical properties. These metallic systems also manifested differences in the aggregation and the adherence to glass surfaces, revealing significant differences in the chemical surface properties of these nanoparticles. SERS and surface-enhanced IR also indicated the presence of interference bands which can overlap the spectra of the analyte, mainly in the case of the citrate colloid. All these differences have an important influence on the applicability of these nanostructured systems in SERS. In fact, the enhancement factor and spectral pattern of the SERS obtained by using alizarin as a molecule probe are different.     

Angular-dependent metal-enhanced fluorescence from silver colloid-deposited films: opportunity for angular-ratiometric surface assays

We describe an exciting opportunity for Metal-Enhanced Fluorescence (MEF)-based surface assays using an angular-ratiometric approach to the observed enhanced emission from fluorophores in close proximity to silver colloids deposited on glass substrates. This approach utilizes the radiationless energy transfer (coupling) between the excited states of the fluorophore and the induced surface plasmons of the silver colloids, and the subsequent angular-dependent fluorescence emission from the fluorophore-silver colloid system. Since MEF is related to surface plasmons' ability to scatter light, angular-dependent light scattering from three different silvered surfaces and glass substrates were investigated using two common excitation angles, 45 and 90 degrees . The scattered light from silvered surfaces with a high loading was observed at wider angles on both sides of the glass substrates, while forward scattering (from the back of the glass) was dominant for the silvered surfaces with low loading, as explained by both Mie and Rayleigh theories. When silver colloids were placed between the fluorophore and glass interface, the coupled fluorescence emission through the higher refractive index glass (and in air), increased in an angular-dependent fashion, following closely the angular-dependent light scattering pattern of the silver colloids themselves. Similar observations for fluorescence emission from fluorophores deposited onto glass surfaces alone were made, but at much narrower angles on both sides of the fluorophore-glass interface and were simply explained by Lambert's cosine law. As the loading of silver on glass was increased, the enhanced fluorescence emission was observed at wider angles (towards 0 and 180 degrees ) at both sides of the silvered surfaces. Glass surfaces without silver colloids were used as control samples to demonstrate the benefits of MEF for enhancing fluorescence signatures in an elegant, angular-dependent fashion. Finally, the utility of the angular-dependent MEF phenomenon for intensity-based angular-ratiometric surface assays is demonstrated.     

Towards SERS based applications in food analytics: Lipophilic sensor layers for the detection of Sudan III in food matrices

Food safety is a topic of great importance for our society which places high demands on analytical methods. Surface enhanced Raman spectroscopy (SERS) meets the requirements for a rapid, sensitive and specific detection technique. The fact that metallic colloids, one of the most often used SERS substrates, are usually prepared in aqueous solution makes the detection of water-insoluble substances challenging. In this paper we present a SERS based approach for the detection of water-insoluble molecules by applying a hydrophobic surface modification onto the surface of enzymatic generated silver nanoparticles. By this approach the detection of the illegal water-insoluble food dyes, such as Sudan III in presence of riboflavin, as water-soluble competitor, is possible. Moreover, we demonstrate the usability of this kind of SERS substrates for determination of Sudan III out of spiked paprika extracts.     

Silver CD-R based substrate as a SERS active material

A silver Compact Disc Recortable (CD-R) based substrate has been proposed as an alternative to silver colloids as active material successfully used in surface-enhanced Raman spectroscopy (SERS). Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDXS) measurements revealed that silver nanoparticles are present over the entire surface of the uncovered reflective layer of commonly used CD-R. The process of preparation of the CD-R based surface is simple, fast and repeatable. Recorded Raman spectra of 10 µM Rhodamine 6G applied to the substrate corroborate strong enhancement of Raman signal. The maximum value of EF was calculated to be about 5.76 × 10⁶. Raman maps are consistent with SEM micrographs and confirmed the presence of a numerous SERS hot spots occurring on the trucks of CD-R based substrate.     

Enhancement mechanism of SERS from cyanine dyes adsorbed on Ag2O colloids

Surface enhancement mechanism of Raman scattering from molecules adsorbed on silver oxide colloids is reported. Absorption spectra and Raman spectra of the cyanine dye D266 and pyridine molecules adsorbed on Ag₂O colloids, and the influences of S₂O₃²⁻ and OH⁻ on the SERS are studied respectively. The results indicate that ‘chemical' enhancement is dominant in Ag₂O colloidal solution. Surface complexes of adsorbed molecules and small silver ion clusters Ag_n⁺ as the SERS active sites make an important contribution to surface enhanced Raman scattering (SERS). At these active sites, charge transfer between the adsorbed molecules and the small silver ion clusters is the main enhancement origin. The enhancement factor of D266 adsorbed on Ag₂O colloids is theoretically estimated with the excited-state charge transfer model, which is roughly in accordance with the experiments.     

Microspectrometric investigation of active substrates for surface enhanced Raman scattering

The results of investigations of several new active silver substrates and some previously reported active silver substrates for surface enhanced Raman spectrometry (SERS) using a Raman microprobe are given. Filter-papers of different composition and porosity, silver membranes and glass slides are evaluated as supports for SERS active substrates. Methods of silver preparation include evaporation and chemical reduction. The Raman microprobe facilitates the acquisition of SER spectra of the adsorbate over the metal microstructure being observed on a TV monitor. This capability allows the establishment of practical relationships between the surface morphology and SERS activity which can be used as guidelines for SERS experiments with the microprobe. For the most monodisperse substrates, it is possible to establish a linear relationship between SERS intensity and adsorbate concentration. In the lower extreme of the calibration graph, the amount of adsorbate being observed under the microscope objective is only 0.3 amol or 1.9 × 10⁵ molecules.     

不同银膜粒径对单壁碳纳米管SERS谱的影响

利用化学沉积法和溶胶法制备了粒径在20～100nm范围内不同的表面增强纳米结构活性银膜,系统地研究了单壁碳纳米管（SWCNT）的表面增强拉曼光谱（SERS）的G—band和D—band、比较玻璃和石英两种不同基片上的结果发现,单壁碳管的SERS谱随银膜粒径的变化有相同的变化趋势,G-band峰移对20～100nm范围内活性银膜粒径的差异不敏感,表明该波段所对应的碳管六元环本征振动比较稳定,与界面的化学相互作用较弱．D—band的峰形随基片和活性银膜粒径不同均有改变,且随着粒径变小,高频振动贡献有增大的趋势,表明无序碳与活性银膜间存在很强的相互作用。     

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。     

水汽界面二维银颗粒表面上的单分子拉曼光谱检测

随着各种超灵敏分析仪器的发展 ,已经可以在低温固体中、室温液体中和电介质表面检测、鉴定单分子及其动力学行为 .这种新进展为科学家在分析化学、分子生物学和纳米结构材料等各种学科的应用开辟了许多新的视窗 .单分子谱学的研究在基础科学和应用科学方面引起了人们广泛的兴趣 .人们不仅希望能够“看到”单分子 ,而且希望了解单分子的物理化学行为 .在各种超灵敏检测技术中 ,拉曼光谱成为一种重要的技术 .由于原子力显微等微区技术的发展 ,并结合高灵敏度检测技术的进步 ,拉曼光谱已经发展成为一种检测灵敏度可以达到分子级的检测技术 [1,…     

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。