Surface-enhanced infrared spectroscopic studies of the catalytic behavior of silver nanoparticles on a germanium substrate

The catalytic activity of silver nanoparticles (AgNPs) on a germanium substrate is reported. Para-nitrothiophenol (pNTP) that had been adsorbed on this substrate is converted to p-aminothiophenol (pATP) under very mild reaction conditions, such as simply soaking in water. The AgNPs may be formed either by physical vapor deposition or by electroless deposition from a solution of silver nitrate. Analogous reactions were not observed on copper nanoparticles on germanium or AgNPs on silicon or zinc selenide even though very slow conversion of pNTP to pATP was observed with Au nanoparticles (AuNPs) on Ge under controlled reaction conditions. The effects of factors that could influence the catalytic reaction were examined; these included the particle size of the AgNPs, reaction temperature, concentration and chemical nature of other ions present in the solution, the pH of the water, and the nature of the substrate. The reaction rate was approximately independent of the particle size for AgNPs between 50 and 150 nm in diameter. Increasing the temperature accelerates the reaction significantly; at temperatures above 40 °C, the adsorbed pNTP is completely converted by water within five minutes. Not surprisingly, the reaction rate was increased as the pH of the solution was decreased, as the reduction of each nitro group to an amino group requires six protons. The presence of Br(-) and I(-) ions accelerated the reaction to the point that even at 4 °C, the conversion of the nitro group was still observable, while solutions containing chloride ions had to be heated to 40 °C before their effect became apparent. Apparently, Br(-) and I(-) ions remove the oxide layer from the surface of the germanium substrate, facilitating transfer of electrons from the germanium to the nitro group of the pNTP.     

液相还原法制备纳米银颗粒

以聚丙烯酸铵为保护剂,利用液相化学还原法成功的制备了纳米银颗粒.利用透射电子显微镜(TEM),X射线衍射(XRD)对样品的形貌和结构进行了分析,并讨论了聚丙烯酸铵的加入量对纳米银粒径的影响.利用紫外分光光度计(UV光谱)对纳米银溶胶的粒径分布及其稳定性进行了分析.研究结果表明,聚丙烯酸铵的加入量对纳米银的粒径和形貌有重要的影响.同时,所制备的纳米银溶胶的稳定性较好.     

Kinetics of gas phase reduction of nickel chloride in preparation for nickel nanoparticles

We investigated the chemical kinetics of NiCl₂ reduction to apply to the synthesis of nickel nanoparticles in a tubular furnace reactor. The conversion of NiCl₂ increased monotonically with reaction temperature up to 99% at 950 °C, and in turn, the rate constant of the reaction increased from 78 to 286 with an increase in the temperature from 800 to 950 °C. The reaction rate was estimated to be the first order with respect to chloride concentration, and the rate constant obeyed the Arrhenius law, of which the activation energy and pre-exponential factor were 103.79 kJ/mol and 7.34 × 10⁶ min⁻¹, respectively. Taking advantage of the kinetics, we synthesized crystalline nickel nanoparticles with average primary particle size ranging from 31 to 106 nm by systematically controlling the reactor temperature and chloride concentration.     

A review of the preparation and application of magnetic nanoparticles for surface-enhanced Raman scattering

In recent years, the unique properties of magnetic functional nanomaterials have received considerable attentions and show promising applications in separation, detection, diagnosis, catalysis, environment remediation and so on. Specifically, introducing magnetic nanomaterials (MNPs) into traditional sensing techniques greatly simplifies detection operation and improves sensing performances, which makes magnetic nanomaterial-based sensing techniques become a hot research topic. Compared with other sensing techniques such as chromatography, fluorescence, mass spectrum and electrochemistry, surface-enhanced Raman scattering (SERS) displays unique properties of high-sensitivity, fingerprint specificity and nondestructive detection. The introduction of MNPs in SERS has proven to be an efficient way to resolve several critical challenges in practical SERS analysis leading to highly efficient target separation and enrichment, high-sensitive detection and precise outcomes analysis. This makes the MNPs involved SERS analysis a powerful technique with very appealing and promising application in various branches of analytical science. In this review, we first briefly introduced the preparation, encapsulation and surface modification of magnetic nanoparticles, assembly of magnetic nanoparticle–plasmonic substrates and then discussed their applications in SERS analysis, including biomedical application, environmental analysis, food safety and chemical reaction monitoring. Finally, we presented some outlooks on further developments of magnetic nanoparticles in SERS applications.     

Type I collagen-templated assembly of silver nanoparticles and their application in surface-enhanced Raman scattering

Silver nanoparticles (Ag NPs) are one of the active substrates that are employed extensively in surface-enhanced Raman scattering (SERS), and aggregations of Ag NPs play an important role in enhancing the Raman signals. In this paper, we fabricated two kinds of SERS-active substrates utilizing the electrostatic adsorption and superior assembly properties of type I collagen. These were collagen-Ag NP aggregation films and nanoporous Ag films. Two probe molecules, 4-aminothiophenol (4-ATP) and methylene blue (MB), were studied on these substrates. These substrates showed reproducible SERS intensities with relative standard deviations (RSDs) of 8-10% and 11-14%, respectively, while the RSDs of the traditional thick Ag films were 12-28%. Also, the intensities for the 4-ATP spectrum on the collagen-templated nanoporous Ag film were approximately one order higher than those on the DNA-templated Ag?film.     

Preparation of silver nanoparticles by photo-reduction for surface-enhanced Raman scattering

A substrate for surface-enhanced Raman scattering (SERS) has been developed. Based on the surface-catalyzed reduction of Ag⁺ by citrate on the silver nanoparticles surface under light irradiation, small silver seeds on a quartz slide can be enlarged. The optical properties and characteristics of the silver films have been investigated by ultraviolet-visible spectroscopy, scan electron microscope and atomic force microscopy (AFM). The results indicate that the particle size and shape are different at different reduction time. At the first 3 h, some triangular and hexagonal nanoparticles formed; with the reduction proceeding, the shape of the silver particles became irregular and the size became larger. The silver films obtained are very suitable as SERS active substrate. The relationship between SERS intensity and the reduction time has been investigated for 1,4-bis[2-(4-pyridyl)ethenyl]-benzene molecule adsorbed on the silver film. The SERS intensity reached a maximum at 8 h reduction. The AFM measurements indicate that roughness features with an average size of 100 nm are present on the surface, which yielded the strongest SERS signal. Pyridine was used as a probe molecule to investigate the enhancement factor (EF) of the silver films. According to the formalism of Tian and co-workers, the EF of the silver films is estimated to be 3.4 × 10⁵. The silver film that can remain active for more than 50 days would seem to be suitable for various analytical applications.     

Reproducible surface-enhanced Raman scattering spectra of bacteria on aggregated silver nanoparticles

Surface-enhanced Raman scattering (SERS) is proven to be a powerful technique for rapid identification and discrimination of microorganisms. However, due to the heterogeneous nature of the samples, the acquisition of reproducible spectra hinders the further development of the technique. In this study, we demonstrate the influence of the experimental conditions on SERS spectra. Then, we report a simple sample preparation method coupled with a light microscope attached to a Raman spectrometer to find a proper spot on the sample to acquire reproducible SERS spectra. This method utilizes the excited surface plasmons of the aggregated silver nanoparticles to visualize the spots on the sample. The samples are prepared using the concentrated silver colloidal solutions. The collection time for one spectrum is 10 s and each spectrum is a very good representative of the other spectra acquired from the same sample. The nature of the surface charge of the silver nanoparticles influences the spectral features by determining the strength of the interactions between nanoparticles and bacteria and the aggregation properties of the nanoparticles. Although increasing the colloid concentration in the sample resulted in reproducible spectra from arbitrary points on the sample, a great variation from sample to sample prepared with the different colloidal solution concentrations is observed.     

In situ growth of silver nanoparticles in porous membranes for surface-enhanced raman scattering

We demonstrate the in situ growth of silver nanoparticles in porous alumina membranes (PAMs) for use as a surface-enhanced Raman scattering (SERS) detection substrate. This fabrication method is simple, cost-effective, and fast, while providing control over the size of silver nanoparticles through the entire length of the cylindrical nanopores with uniform particle density inside the pores unachievable by the traditional infiltration technique. The in situ growth of silver nanoparticles was conducted from electroless-deposited nanoscale seeds on the interior of the PAM and resulted in the formation of numerous hot spots, which facilitated significantly higher SERS enhancement for these substrates compared with previously reported porous substrates.     

Active control of silver nanoparticles spacing using dielectrophoresis for surface-enhanced Raman scattering

We demonstrate an active microfluidic platform that integrates dielectrophoresis for the control of silver nanoparticles spacing, as they flow in a liquid channel. By careful control of the nanoparticles spacing, we can effectively increase the surface-enhanced Raman scattering (SERS) signal intensity based on augmenting the number of SERS-active hot-spots, while avoiding irreversible aggregation of the particles. The system is benchmarked using dipicolinate (2,6-pyridinedicarboxylic acid) (DPA), which is a biomarker of Bacillus anthracis. The validity of the results is discussed using several complementing characterization scenarios.     

Plasma reduction of silver compounds for fabrication of surface-enhanced Raman scattering substrates

Several silver compounds were reduced by low-pressure air plasma to produce porous nanostructured surfaces as surface-enhanced Raman scattering (SERS) substrates. This method is advantageous because substrates are easy to prepare and the silver metal surface is inherently clean without spectroscopic background. Silver compounds were melted into 1-2 mm slugs on quartz slides and plasma treated for different lengths of time. Silver chloride was found to be the best compound to make reproducible and stable SERS substrates. SERS activity of the substrates was tested using L-tryptophan, 4-mercaptobenzoic acid, and adenine.     

Chitosan-coated anisotropic silver nanoparticles as a SERS substrate for single-molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a technique that has become widely used for identifying and providing structural information about molecular species in low concentration. There is an ongoing interest in finding optimum particle size, shape and spatial distribution for optimizing the SERS substrates and pushing the sensitivity toward the single-molecule detection limit. This work reports the design of a novel, biocompatible SERS substrate based on small clusters of anisotropic silver nanoparticles embedded in a film of chitosan biopolymer. The SERS efficiency of the biocompatible film is assessed by employing Raman imaging and spectroscopy of adenine, a significant biological molecule. By combining atomic force microscopy with SERS imaging we find that the chitosan matrix enables the formation of small clusters of silver nanoparticles, with junctions and gaps that greatly enhance the Raman intensities of the adsorbed molecules. The study demonstrates that chitosan-coated anisotropic silver nanoparticle clusters are sensitive enough to be implemented as effective plasmonic substrates for SERS detection of nonresonant analytes at the single-molecule level.     

Effect of oxidation on surface-enhanced Raman scattering activity of silver nanoparticles: a quantitative correlation

We quantitatively studied, using X-ray photoelectron spectroscopy (XPS), oxidation of substrate-immobilized silver nanoparticles (Ag NPs) in a wide range of conditions, including exposure to ambient air and controlled ozone environment under UV irradiation, and we correlated the degree of silver oxidation with surface-enhanced Raman scattering (SERS) enhancement factors (EFs). The SERS activity of pristine and oxidized Ag NPs was assessed by use of trans-1,2-bis(4-pyridyl)ethylene (BPE) and sodium thiocynate as model analytes at the excitation wavelength of 532 nm. Our study showed that the exposure of Ag NPs to parts per million (ppm) level concentrations of ozone led to the formation of Ag(2)O and orders of magnitude reduction in SERS EFs. Such an adverse effect was also notable upon exposure of Ag NPs under ambient conditions where ozone existed at parts per billion (ppb) level. The correlated XPS and SERS studies suggested that formation of just a submonolayer of Ag(2)O was sufficient to decrease markedly the SERS EF of Ag NPs. In addition, studies of changes in plasmon absorption bands pointed to the chemical enhancement as a major reason for deterioration of SERS signals when substrates were pre-exposed to ambient air, and to a combination of changes in chemical and electromagnetic enhancements in the case of substrate pre-exposure to elevated ozone concentrations. Finally, we also found UV irradiation and ozone had a synergistic effect on silver oxidation and thus a detrimental effect on SERS enhancement of Ag NPs and that such oxidation effects were analyte-dependent, as a result of inherent differences in chemical enhancements and molecular binding affinities for various analytes.     

Sonochemical synthesis of silver nanoparticles in Y-zeolite substrate

Silver ions can be reduced by 24 kHz ultrasonic waves in ion-exchanged Ag⁺–Y zeolite. In this research, silver ions were introduced into the nano-porous (1.2 nm) zeolite lattice by ion-exchange route. After the reduction process, silver nanoparticles were placed in the cavities, with a size of about 1 nm and also on the external surfaces of the zeolite, with sizes about less than 10 nm. Fast and simple lab-scale reduction of silver ions in the zeolite is important for researchers who work on catalytic properties of metallic silver–zeolite. Several reduction methods have been reported but reduction by ultrasonic waves is a new, simple, and size-controllable method with a high practical value which does not need any complicated facilities. In a sonochemical process, a huge density of energy is provided by the collapse of bubbles which formed by ultrasonic waves. The released energy causes the formation of reducing radicals that consequently reduce the silver ions. It is concluded that the higher silver content may result in the formation of larger silver crystals on the external surface of zeolite crystals. Also, the addition of 1-propanol and 2-propanol to the aqueous reaction medium does not cause better reduction. In addition, increasing the irradiation time and ultrasonic power does not affect the silver crystal growth significantly but the extent of silver ion reduction increases when the power of ultrasonic waves increases. All samples were irradiated under the same ultrasonic conditions. The samples were analyzed by XRD, EDS, SEM, and TEM.     

New surface-enhanced Raman spectroscopy substrates via self-assembly of silver nanoparticles for perchlorate detection in water

Perchlorate (ClO4-) has recently emerged as a widespread contaminant in drinking water and groundwater supplies in the United States, and a need exists for rapid detection and monitoring of this contaminant. In this study, surface-enhanced Raman spectroscopy (SERS) was studied as a means of ClO4- detection, and new sol-gel-based SERS substrates were developed by self-assembly of silver colloidal nanoparticles with various functionalized silane reagents. These substrate materials were tailored to allow detection of ClO4- in water with improved sorptivity, stability, and sensitivity and with the ability to detect ClO4- at concentrations as low as 10(-6) M (or 100 microg/L) with good reproducibility. Similar techniques were used to fabricate capillary SERS flow cells by assembling functionalized silver nanoparticles capable of attracting ClO4- to the SERS surface or the internal wall of glass capillaries. These capillary flow cells could be readily configured to allow for in situ, nondestructive detection of ClO4- via fiber optics.     

工艺参数对液相还原法制备超细银粉形貌特征的影响

研究硝酸银浓度、抗坏血酸浓度、反应体系温度及pH值、添加剂种类及用量对液相还原法制备超细银粉形貌特征的影响,借助SEM、粒度仪、比表面仪和颗粒图像分析仪对银粉进行表征和分析。结果表明,pH值是同时影响银粉形状、粒度和分散性的显著因素,添加剂用量和抗坏血酸浓度分别是影响银粉形状及分散性的显著因素;随着pH值由2逐渐增大至10,银粉呈现由无规则多面体至棒状、类球形的形状变化,平均粒径逐渐减小;随着添加剂用量增加,银粉呈现由异形至球形、刺球型的形状变化,其用量质量分数为2.5%时颗粒球形度值最大;随着抗坏血酸浓度增加,银粉的分散性逐渐变差,其浓度为0.5 mol/L时分散性最好。     

The preparation of silver sulfide nanoparticles in lamellar liquid crystal and application to lubrication

The silver sulfide (Ag₂S) nanoparticles were prepared by the reaction of AgNO₃ and Na₂S in the lamellar liquid crystal (LLC) formed by Triton X-100, n-C₁₀H₂₁OH and H₂O. The size of the particles is about 2-3 nm. The existence of Ag₂S nanoparticles can improve the lubrication of the lamellar liquid crystal.     

Morphology effects on the optical properties of silver nanoparticles

Employing methods developed for the control of shape and size in silver nanoparticles, we have compared the optical properties of nanorods, nanoprisms, nanodisks, and nanospheres. Solutions of these particles show distinct surface plasmon resonant absorption signatures that are directly correlated with the symmetries of their morphology. Nonlinear optical behavior for suspensions of these nanostructures, for nanosecond pulses at 532 and 1064 nm, have been correlated to plasmon resonances determined by shape and size.     

Silver nanoparticles deposited on porous silicon as a surface-enhanced Raman scattering (SERS) active substrate

Silver nanoparticles were deposited spontaneously from their aqueous solution on a porous silicon (PS) layer. The PS acts both as a reducing agent and as the substrate on which the nanoparticles nucleate. At higher silver ion concentrations, layers of nanoparticle aggregates were formed on the PS surface. The morphology of the metallic layers and their SERS activity were influenced by the concentrations of the silver ion solutions used for deposition. Raman measurements of rhodamine 6G (R6G) and crystal violet (CV) adsorbed on these surfaces showed remarkable enhancement of up to about 10 orders of magnitude.     

Preparation of silver nanoparticles via a non-template method

Silver nanoparticles have been prepared by a non-template method. Silver nitrate can be easily decomposed into silver nanosized materials. Small nanoparticles (less than 2 nm) can be formed in aqueous solution. Larger silver particles of about 100 nm can be formed in ethanol solution. By rationally adjusting the experimental conditions we finally obtain silver particles of about 20 nm with a relatively narrow distribution in ethanol solution. Differing from the previous reports, we find that silver nanoparticles can be formed by direct decomposition of AgNO₃ under UV light irradiation. No catalyst like TiO₂ is needed at all. We believe that it is a further step to precede the preparation of silver nanometer sized materials.     

Base effects on fabrication of silver nanoparticles embedded silica nanocomposite for surface-enhanced Raman scattering (SERS)

In this paper, we studied on the effect of organic bases in the case of ethylene glycol based fabrication of silver nanoparticles embedded silica nanocomposite (Ag SNC) without heating. Considering their chemical structures, butylamine (BA), ethanolamine (EA), triethanolamine (TEA), tributylamine (TBA), octylamine (OA) and Jeffamine 500 (JA) were used as an organic base. In addition, the effect of the concentrations of AgNO3 and organic bases on the formation of Ag SNC was also examined. In conformity with the characteristics of Ag SNC, SERS signal intensity of benzenethiol on Ag SNC was measured. As a result, the SERS signal intensity of Ag SNCs was strongly dependent on the reaction conditions. Furthermore, when reacted under the best reaction condition with concentrations of AgNO3 and OA, 3 mM and 5 mM, respectively, a large-scale production of Ag SNC was possible under the mild conditions.