Silver-coated zeolite crystal films as surface-enhanced Raman scattering substrates

Silver-coated zeolite A and zeolite NaX crystal films prepared by vacuum deposition were investigated as surface-enhanced Raman scattering (SERS) substrates. The substrates were active for the enhancement of Raman scattering from uranyl ions. A detection limit of 10(-5) M for uranyl was obtained using silver-coated zeolite A films. One advantage of these zeolite-based substrates is that the negatively charged microporous framework provides the selectivity for adsorption based on static electric charges. The SERS effects of positively charged uranyl ions and neutrally charged benzoic acid were compared. For the zeolite A substrate, there was a 100-times-greater sensitivity.     

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.     

Patterned silver nanoparticle films by an ion complexation process in thermally evaporated fatty acid films

The formation of silver nanoparticle films in a patterned manner on suitable substrates is described. The protocol for realising such structures comprises of the following steps. In the first step, patterned films of a fatty acid are thermally evaporated onto solid supports using suitable masks (e.g. a TEM grid). Thereafter, the fatty acid film is immersed in silver nitrate solution and Ag⁺ ions entrapped in the lipid matrix by electrostatic complexation with the carboxylate ions of the fatty acid molecules. The final step involves the reduction of the Ag⁺ ions in situ thus leading to the formation of silver nanoparticles within the patterned lipid matrix. The process of metal ion incorporation and reduction may be repeated a number of times to increase the nanoparticle density in the lipid matrix. The silver nanoparticle density may also be increased by dissolution of the fatty acid molecules in suitable solvents. The process of Ag⁺ ion entrapment and formation of silver nanoparticles within the patterned lipid matrix has been followed by quartz crystal microgravimetry, UV-VIS spectroscopy, FTIR, SEM and EDX. The process described shows immense potential for extension to assemblies of nanoparticles in more intricate patterns as well as to the growth of semiconductor quantum dots in such patterns.     

A novel method for fabricating the surface-enhanced Raman scattering substrates and its enhanced properties

We have got large area surface-enhanced Raman scattering (SERS) substrates with uniform high enhancement factors by the so-called moulage method for the first time. A silver film (99.99%) with several millimeters thickness was thermally evaporated on the porous anodic alumina templates and the SERS substrate was got after moving off the templates. Surface-enhanced Raman scattering spectra of pyridine (0.01 Mol/L) were measured under 632.8 nm excitation. The experimental enhancement factors were more than 10(5) and S/N(p-p) around 100 was obtained. We have compared the SERS spectra of pyridine collected from different locations on the same SERS substrate and different substrates, which illustrate the well uniform enhance properties and the reproducibility of this method, respectively. The comparison of the SERS spectra, obtained from the SERS substrates and Ag film evaporated directly on glass slide, have proved that the electromagnetic coupling between two adjacent nanoparticles was important to the SERS effect. We also used rhodamine 6G as the probe molecules and found that the different molecules were very sensitive to the morphology of the SERS substrates.     

Superhyperfine interactions of Ag6+ clusters in silver loaded 4A zeolite

A paramagnetic silver cluster consisting of an octahedral Ag₆⁺ molecule surrounded by a cube of eight Ag⁺ ions has been detected in γ-irradiated zeolite 4A fully loaded with silver. The ‘superhyperfine’ interaction of the silver ions in the outer cage was used as a probe to study the effects on the cluster of dehydration at various temperatures.     

Novel method for preparing controllable and stable silver particle films for surface-enhanced Raman scattering spectroscopy

A new surface-enhanced Raman scattering (SERS) active substrate has been developed based on our previous study. Small silver nanoparticles on a quartz slide can be enlarged by using a mixture of commercially available reagents called Silver Enhancer and Initiator. The optical properties and characteristics of the new substrate have been investigated by ultraviolet-visible (UV-Vis) spectroscopy and atomic force microscopy (AFM). The results indicate that the small silver nanoparticles grow and some silver aggregates emerge on the quartz slide after the slide is immersed into the Silver Enhancer and Initiator Mixture (SEIM). The average diameter of the silver nanoparticles on the substrate becomes approximately double after the immersion into SEIM for 20 s. 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB) was used as a Raman probe to evaluate the enhancement ability of the new silver substrate. It has been found that the SERS intensity can be increased about 10 times by using the substrate treated by SEIM compared with that without being treated by SEIM. Interestingly enough, the SERS enhancement increases with time. This may be due to the reorganization of silver nanoparticles on the quartz surface. The new substrate can remain active for more than 90 days. The adsorption mode of BPENB on the new substrate and the dependence of the BPENB configurations on the concentration of BPENB in methanol solution have also been investigated by SERS or UV-Vis spectroscopy. The SERS spectra of a self-assembled monolayer (SAM) BPENB film adsorbed on a silver substrate treated by SEIM show that BPENB molecules are chemically absorbed through the Ag-N bond. Consequently, a nearly perpendicular orientation of BPENB on the silver surface is proposed. The SERS spectra of BPENB SAMs on the new substrates fabricated from methanol solutions with different concentrations are compared. The concentration dependence of the SERS spectra reveals that the BPENB molecules are adsorbed on the silver film as monomers when the film is prepared from the solution with a lower concentration (<4 x 10(-6) M) and as aggregates when it is prepared from the solution with a higher concentration (>1 x 10(-5) M).     

Silver nanoparticles deposited on anodic aluminum oxide template using magnetron sputtering for surface-enhanced Raman scattering substrate

Low-cost and highly sensitive surface-enhanced Raman scattering (SERS) substrates have been fabricated by a simple anodizing process and a magnetron sputtering deposition. The substrates, which consist of silver nanoparticles embedded on anodic aluminum oxide (AAO) templates, are investigated by a scanning electron microscope and a confocal Raman spectroscopy. The SERS activities are demonstrated by Raman scattering from adsorbed solutions of methylene blue and pyridine on the SERS substrate surface. The most optimized SERS substrate contains the silver nanoparticles, with a size distribution of 10-30 nm, deposited on the AAO template. From a calculation, the SERS enhancement factor is as high as 8.5 × 10⁷, which suggests strong potentials for direct applications in the chemical detection and analyses.     

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.     

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.     

A multifunctional Ag/PAOCG reusable substrate for <Emphasis Type="Italic">p</Emphasis>-nitrophenol reduction and SERS applications

We demonstrate a facile galvanic replacement reaction route to direct growth of silver nanoparticles (AgNPs) into porous alumina on conductive glass (PAOCG). Porous Al₂O₃ layer was prepared by using boehmite as precursor, deposited on conductive glass via spin coating, and followed by a heat treatment. PAOCG was attached firmly with a tiny sheet of steel and was then soaked in AgNO₃ solution. Ag⁺ ions in the nanopores of PAOCG adsorbed by capillarity were automatically reduced to metallic AgNPs, thus forming Ag/PAOCG. The catalytic property of Ag/PAOCG was investigated for p-nitrophenol (PNP) reduction using UV–Vis absorption spectroscopy, and the rate constant was evaluated using the pseudo-first-order kinetic model. This film catalyst could be readily regenerated and reused for up to ten times without significantly depreciate its efficiency. The SERS performance of Ag/PAOCG was investigated using aqueous crystal violet (CV) as a probe molecule. The optimum Ag/PAOCG substrate was capable of detecting as low as 10^?10 M aqueous CV. The reusability of Ag/PAOCG was achieved by heating the substrate at 400 °C for 5 min in air. This substrate could be reused for at least five cycles without significantly reduced SERS performance. Therefore, this powerful multifunctional surface can serve as a portable, durable and reusable substrate for PNP reduction and SERS applications.     

Surface-enhanced Raman scattering substrate of silver nanoparticles depositing on AAO template fabricated by magnetron sputtering

In this report, we describe a fabrication process of low-cost and highly sensitive SERS substrates by using a simple anodizing setup and a low-energy magnetron sputtering method. The structure of the SERS substrates consists of silver nanoparticles deposited on a layer of anodic aluminum oxide (AAO) template. The fabricated SERS substrates are investigated by a scanning electron microscope (SEM), a transmission electron microscope (TEM), and a confocal Raman spectroscope. We have verified from the surface morphology that the fabricated SERS substrates consist of high-density round-shape silver nanoparticles where their size distribution ranges from 10 to 30 nm on the top and the bottom of nanopores. The surface-enhanced Raman scattering activities of these nanostructures are demonstrated using methylene blue (MB) as probing molecules. The detection limit of 10⁻⁸ M can be achieved from this SERS substrate.     

Surface-enhanced Raman scattering substrate of silver nanoparticles depositing on AAO template fabricated by magnetron sputtering

In this report, we describe a fabrication process of low-cost and highly sensitive SERS substrates by using a simple anodizing setup and a low-energy magnetron sputtering method. The structure of the SERS substrates consists of silver nanoparticles deposited on a layer of anodic aluminum oxide (AAO) template. The fabricated SERS substrates are investigated by a scanning electron microscope (SEM), a transmission electron microscope (TEM), and a confocal Raman spectroscope. We have verified from the surface morphology that the fabricated SERS substrates consist of high-density round-shape silver nanoparticles where their size distribution ranges from 10 to 30 nm on the top and the bottom of nanopores. The surface-enhanced Raman scattering activities of these nanostructures are demonstrated using methylene blue (MB) as probing molecules. The detection limit of 10⁻⁸ M can be achieved from this SERS substrate.     

High-density silver nanoparticle film with temperature-controllable interparticle spacing for a tunable surface enhanced Raman scattering substrate

The formation of high-density silver nanoparticles and a novel method to precisely control the spacing between nanoparticles by temperature are demonstrated for a tunable surface enhanced Raman scattering substrates. The high-density nanoparticle thin film is accomplished by self-assembling through the Langmuir-Blodgett (LB) technique on a water surface and transferring the particle monolayer to a temperature-responsive polymer membrane. The temperature-responsive polymer membrane allows producing a dynamic surface enhanced Raman scattering substrate. The plasmon peak of the silver nanoparticle film red shifts up to 110 nm with increasing temperature. The high-density particle film serves as an excellent substrate for surface-enhanced Raman spectroscopy (SERS), and the scattering signal enhancement factor can be dynamically tuned by the thermally activated SERS substrate. The SERS spectra of Rhodamine 6G on a high-density silver particle film at various temperatures is characterized to demonstrate the tunable plasmon coupling between high-density nanoparticles.     

Surface-enhanced Raman spectroscopy of soft-landed polyatomic ions and molecules

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.     

Preparation of Ag2S–Graphene nanocomposite from a single source precursor and its surface-enhanced Raman scattering and photoluminescent activity

Ag₂S–Graphene nanocomposite was prepared via a relatively facile hydrothermal method, using a single-source molecular (silver diethyldithiocarbamate [Ag(DDTC)]) as precursor and graphene sheets as a support material. The composite was characterized by X-ray power diffraction, X-ray photoelectron spectroscopy, Field-emission scanning electron microscope, transmission electron microscopy, Fourier transform infrared, Raman spectra and fluorescence spectroscopy. The experimental results show that the Ag₂S–Graphene nanocomposite displays surface-enhanced Raman scattering (SERS) activity for graphene oxide and reveals relatively better fluorescence property compared with pure Ag₂S.     

Spherical Nanoparticle Arrays with Tunable Nanogaps and Their Hydrophobicity Enhanced Rapid SERS Detection by Localized Concentration of Droplet Evaporation

Periodic hexagonal spherical nanoparticle arrays are fabricated by a sacrificial colloidal monolayer template route by chemical deposition and further physical deposition. The regular network‐structured arrays are first templated by colloidal monolayers and then they are changed to novel periodic spherical nanoparticle arrays by further sputtering deposition due to multiple direction deposition and shadow effect between adjacent nanoparticles. The nanogaps between two adjacent spherical nanoparticles can be well tuned by controlling deposition time. Such periodic nanoparticle arrays with gold coatings demonstrate a very stable and high sensitive surface‐enhanced Raman scattering spectroscopy (SERS) performance. The periodic nanoparticle arrays with 10 nm gaps display much stronger SERS enhancement due to electromagnetic coupling. The chemically modified nanoparticle arrays show good hydrophobicity, which shorten process of detecting probe molecules using them as SERS‐active substrates by localized concentration of droplet evaporation and a low detection limit of 10⁻¹² m R6G can be achieved without solution wasting in a short time. The hydrophobic substrate offers a simple, convenient, and economical method to examine SERS performance by rapid concentration of solution on it and it is highly helpful to improve its practical applications in portable Raman detecting devices to detect organic molecules.     

Preparation of gold nano-cones as surface-enhanced Raman scattering sensors for molecule detection

Surface-enhanced Raman spectroscopy (SERS) is a powerful novel analytical tool which integrates high levels of sensitivity for trace analysis of chemical and biomolecular species due to the massive enhancement of Raman signals by using nanometre-sized metal particles. However, SERS can be envisaged as an analytical tool only if substrates with strong, predictable and reproducible SERS enhancement can be produced. Here we have developed one simple Ar+ ions sputtering technology to prepare gold nano-cones array on silicon substrates as surface-enhanced Raman scattering (SERS)-active substrates. The tip of the gold cone-structures exhibited an extremely sharp curvature with an apex diameter of 20 nm and the interior apex angle of the nanocones was around 20 degrees. These samples were evaluated as potential SERS substrates using Rhodamine 6G molecules as molecule probe and exhibited SERS enhancement factor of greater than 10, originated from the localized electron field enhancement around the apex of cones and the surface plasmon coupling of periodic structures.     

Combined Visible Plasmons of Ag Nanoparticles and Infrared Plasmons of Graphene Nanoribbons for High-Performance Surface-Enhanced Raman and Infrared Spectroscopies

Ag nanoparticles (NPs) modified graphene nanoribbons (GNRs) are proposed to function as the high-performance shared substrates for surface-enhanced Raman and infrared absorption spectroscopy (SERS and SEIRAS). This is realized by modulating the localized plasmonic resonances of Ag NPs in visible region and GNRs in mid-infrared region simultaneously, so as to selectively employ each resonance to acquire SERS and SEIRAS on a single substrate. As a proof of concept, shared substrates are prepared by fabricating GNRs on a Fabry–Pérot like cavity, followed by depositing a thin Ag film with annealing treatment to achieve Ag NPs. Complementary Raman and infrared active vibrational modes of rhodamine 6G molecules can be extracted from the SERS and SEIRAS spectra. By optimizing the dimension of Ag NPs, SERS enhancement factors at the order of 10⁵ can be achieved, which are comparable with or even larger than that of the reported shared substrates. Meanwhile, various polyethylene oxide vibrational modes can be recognized with maximum SEIRAS amplification up to 170 times, which is one order larger than that of the reported graphene plasmonic infrared sensors. Such plasmonic nanosensor with excellent SERS and SEIRAS performance exhibits promising potential for biosensing applications on an integrated lab-on-a-chip strategy.     

Bactericidal effects of different silver-containing materials

The evaluation of the bactericidal effect of different silver-containing materials where silver is available as Ag⁺ (silver nitrate and different silver-exchanged zeolites), as metallic Ag⁰ (commercial silver nanoparticles) or as oxide (silver (I) oxide) was carried out in order to elucidate the importance of the bioavailability of silver (i.e., as free ions, metallic particles, combination of them, clusters, complexes, partially soluble or insoluble salts, etc.) on its bactericidal action.For the different materials tested, their bactericidal effect is ordered in the following sequence: AgNO₃ > Ag-ZSM-5 > Ag₂O > commercial silver-exchanged zeolite (granular) > commercial silver-exchanged zeolite (pellets) > Ag nanoparticles. In general, as the content of bioavailable ionic silver increases, the biocidal effectiveness of the corresponding silver-releasing material increases too.     

Silver nanocrystal-modified silicon nanowires as substrates for surface-enhanced Raman and hyper-Raman scattering

Metal catalyzed, CVD-grown silicon nanowires decorated by chemical assembly of closely spaced Ag nanocrystals were modified with the well-known "silver mirror" reaction and investigated as substrates for surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) spectroscopy. Four chromophores were examined: Rhodamine 6G, crystal violet, a cyanine dye, and a cationic donor-acceptor substituted stilbene. After soaking the substrates overnight in 10(-4) M aqueous chromophore solutions, all four chromophores gave good-quality SERS spectra in < or =60 s using <1 microW of 458-nm cw laser power, and SEHRS spectra are obtained in < or =120 s using <1 mW of mode-locked 916-nm laser power. Results from this substrate are compared with those on colloidal silver nanoparticles deposited as a film, as well as surfaces grown by the silver mirror reaction.