Silica-void-gold nanoparticles: temporally stable surface-enhanced Raman scattering substrates

Reproducible detection of a target molecule is demonstrated using temporally stable solution-phase silica-void-gold nanoparticles and surface-enhanced Raman scattering (SERS). These composite nanostructures are homogeneous (diameter = 45 +/- 4 nm) and entrap single 13 nm gold nanoparticle cores inside porous silica membranes which prevent electromagnetic coupling and aggregation between adjacent nanoparticles. The optical properties of the gold nanoparticle cores and structural changes of the composite nanostructures are characterized using extinction spectroscopy and transmission electron microscopy, respectively, and both techniques are used to monitor the formation of the silica membrane. The resulting nanostructures exhibit temporally stable optical properties in the presence of salt and 2-naphthalenethiol. Similar SERS spectral features are observed when 2-naphthalenethiol is incubated with both bare and membrane-encapsulated gold nanoparticles. Disappearance of the S-H Raman vibrational band centered at 2566 cm(-1) with the composite nanoparticles indicates that the target molecule is binding directly to the metal surface. Furthermore, these nanostructures exhibit reproducible SERS signals for at least a 2 h period. This first demonstration of utilizing solution-phase silica-void-gold nanoparticles as reproducible SERS substrates will allow for future fundamental studies in understanding the mechanisms of SERS using solution-phase nanostructures as well as for applications that involve the direct and reproducible detection of biological and environmental molecules.     

Nanoparticle-mirror sandwich substrates for surface-enhanced Raman scattering

Sandwich surface-enhanced Raman scattering (SERS) substrates (3S) utilizing coupling between continuous metal films and plasmonic particles were fabricated using silver mirrors, electrochemically roughened films, and various sizes of silver nanoparticles. The effect of excitation wavelength and nanoparticle size on SERS spectra of poly(vinylpyridine), selected as a model compound, was studied to determine the optimum conditions for the strongest SERS signal. The Raman enhancement resulted from the plasmon coupling of silver nanoparticles to the underlying continuous film as well as the lateral plasmon coupling between the silver nanoparticles. The formation of the charge transfer complex was also observed. The 3S configuration was used to obtain SERS spectra of dipicolinic acid (DPA), a chemical signature for Bacillus anthracis.     

Effect of substrate on surface-enhanced Raman scattering of molecules adsorbed on immobilized silver nanoparticles

Silver nanoparticles were assembled on polyvinylpyridine (PVP) derivatized glass slides. Charge transfer between the adsorbed 4-aminothiophenol (PATP) and the immobilized silver nanoparticles was studied by surface-enhanced Raman spectroscopy with 1064 nm excitation, and compared with that of the silver nanoparticles in the colloid. It was demonstrated that the positive charges of the PVP layer could alter the charge distribution in the immobilized nanoparticles and induce the formation of the dipole in the nanoparticles, leading to less charge transfer from the metal to the adsorbed molecules. The coadsorption of chloride ions on the surface of the immobilized silver nanoparticles resulted in the redistribution of the charges in the nanoparticles and, in turn, altered the charge transfer between the adsorbed PATP molecules and the silver nanoparticles.     

Sonoelectrochemical synthesis of spike-like gold-silver alloy nanoparticles from bulk substrates and the application on surface-enhanced Raman scattering

The synthesis of non-spherical spike-like gold-silver alloy nanoparticles on platinum substrates was first developed by sonoelectrochemical methods in this study. First, a silver substrate was roughened by a triangular-wave oxidation-reduction cycle (ORC) in an aqueous solution containing 0.1 M HCl. Silver-containing complexes were found in the solution after the ORC treatment. Then a gold substrate was subsequently roughened by the similar ORC treatment in the same silver complexes-containing solution. After this procedure, Au- and Ag-containing complexes were left in the solution. Subsequently, the Au working electrode was immediately replaced by a Pt electrode. A cathodic overpotential was applied under controlled sonication and slight stirring to synthesize Au-Ag alloy nanoparticles on the Pt substrate. Encouragingly, the surface-enhanced Raman scattering (SERS) of Rhodamine 6G on the Au-Ag alloy nanoparticles-deposited Pt substrate exhibits a higher intensity by eight-fold of magnitude and a better resolution, as compared to that obtained on the Au nanoparticles-deposited Pt substrate.     

Labeled gold nanoparticles immobilized at smooth metallic substrates: systematic investigation of surface plasmon resonance and surface-enhanced Raman scattering

This paper experimentally and theoretically investigates the influence of an underlying metallic substrate (i.e., gold and silver) on the surface plasmon resonance (SPR) of labeled gold nanoparticles and the concomitant impact on the surface-enhanced Raman scattering (SERS) signal from the labels. These experiments employ nanoparticles of varied sizes (30-100 nm) that are coated with a bifunctional Raman scatterer composed of (1) a disulfide for chemisorption to the nanoparticle surface, (2) a succinimidyl ester for formation of a covalent linkage to an amine-terminated self-assembled monolayer on the underlying substrate, and (3) an aryl nitro group with an intrinsically strong Raman active vibrational mode. This approach allows facile systematic assessments of how variations in nanoparticle size, substrate composition, and the gap between the nanoparticle and substrate affect the SPR of the bound particles. Both UV-vis transmission and reflection absorption (incident angle of 58 degrees ) spectroscopy are used to characterize the effect of each of these parameters on SPR. These results are then correlated with SERS enhancement factors (EFs) that were determined by accounting for particle surface concentrations, which were measured by atomic force microscopy, and the absolute number of labels, which were calculated on the basis of the surface area of each of the different-sized particles. All SERS spectra were collected at an incident angle of 58 degrees with respect to the surface normal. As expected, the SPR for particles in solution red-shifts with increasing particle size. More importantly, the SPR moves to even longer wavelengths as the size of immobilized particles increases and as the gap between the immobilized particle and substrate decreases. The red shift is also greater for a gold nanoparticle tethered to a gold substrate compared to a silver substrate. A theoretical model for the extinction of a particle above a flat substrate, corrected for surface scattering, radiation damping, and dynamic depolarization, is also briefly detailed. SPR results calculated with the model are consistent with the shifts observed in the SPR position for each of the manipulated experimental variables. The largest EFs are found for samples with an SPR maximum (lambda(max)) between the wavelengths for laser excitation (633 nm) and the Raman band for the symmetric nitro stretch of the particle coating (690 nm). As an example, an order of magnitude in the SERS enhancement factor is gained for a 60-nm particle immobilized 1.2 nm above a gold substrate (SPR lambda(max) = 657 nm) compared to that for a 30-nm particle (SPR lambda(max) = 596 nm).     

A new surface-enhanced Raman scattering system for carbon nanotubes

A new surface-enhanced Raman scattering (SERS) system of carbon nanotubes (CNTs) is reported for the first time. According to the remarkable mechanical property, CNTs were grinded on a sheet of silver directly. Thus rough silver surface was obtained, at the same time, the CNTs attached to the rough silver surface. High quality SERS spectra were obtained from CNTs attached to the rough silver surface. Because there were no solvents affecting the SERS of CNTs, the dependability of the result is improved. The theory and experiment results indicate that this is an accurate and practicable method for SERS study of CNTs.     

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.     

Fabrication of flower-like silver nanoparticles for surface-enhanced Raman scattering

The flower-like silver nanoparticles have been synthesized by reducing silver nitrate (AgNO₃) with ascorbic acid (AA) as the reductant and polyvinyl pyrrolidone (PVP) as the capping agent under vigorous stirring. Such flower-like nanoparticles are aggregates of small nanoplates and nanorods. They were tested as substrates for the surface-enhanced Raman scattering (SERS), showing high sensitivity for detecting Rhodamine 6G (R6G) at a concentration as low as 10^-7 mol/L. It has been found that replacing mechanical stirring with ultrasound sonication would drastically change the particle morphology, from flower-like nanoparticles to well-dispersed smaller nanoparticles. Furthermore, when trace amounts of NaCl were added into the reagents, well-dispersed Ag nanoparticles formed even in vigorous stirring. These phenomena can be explained with the diffusion and reactant supply during nucleation and growth of Ag nanoparticles.     

Phospholipid membrane encapsulation of nanoparticles for surface-enhanced Raman scattering

Lipid-encapsulated surface-enhanced Raman scattering (SERS) nanoparticles, with promising applications in biomedical diagnostics, were produced. Gold nanoparticles, 60 nm in diameter, were coated with a ternary mixture of DOPC, sphingomyelin, and cholesterol. The lipid layer is versatile for engineering the chemical and optical properties of the particles. The stability of the lipid-encapsulated particles is demonstrated over a period of weeks. The versatility of the layer is demonstrated by the incorporation of three different Raman-active species using three different strategies. The lipid layer was directly observed by TEM, and the SERS spectrum of the three dye species was confirmed by Raman spectroscopy. UV-vis absorption and dynamic light scattering provide additional evidence of lipid encapsulation. The encapsulation is achieved in aqueous solution, avoiding phase transfer and possible contamination from organic solvents. Furthermore, when fluorescent dye-labeled lipids were employed in the encapsulant, the fluorescence and SERS activity of the particles were controlled by the use of dissolved ions in the preparation solution.     

Zinc oxide nanotubes decorated with silver nanoparticles as an ultrasensitive substrate for surface-enhanced Raman scattering

We introduce a novel voltammetric method, so-called sinusoidal envelope voltammetry, for use in electronic tongues. Fourier transformation was used to transform the data of the signal from the time domain to the frequency domain. The four taste substances, acesulfame potassium, monosodium glutamate, potassium chloride and tartaric acid, are shown to exhibit abundant frequency characteristics in the power spectrum of a Fourier transformation. This indicates that the power spectrum from sinusoidal envelope voltammetry can be used as fingerprints of samples for classification. Principal component analysis along with discrimination index and multi-frequency large amplitude pulse voltammetry as a reference technique is used to evaluate the separation ability of sinusoidal envelope voltammetry. The score plots of the method for the four taste substances and for the five brands of Jiafan rice wine show better discrimination ability than multi-frequency large amplitude pulse voltammetry. Sinusoidal envelope voltammetry is considered to be a promising technique for use in voltammetric electronic tongues.

A facile route to the hierarchical assembly of Au nanoparticles on carbon nanotubes through Cu~(2+) coordination for surface-enhanced Raman scattering

We report the hierarchical assembly of Au nanoparticles on carboxylized carbon nanotubes(c-CNTs)through Cu~(2+) coordination. This route is facile and green, and can easily control the loading density of Au nanoparticles. The c-CNT matrix ensures uniform distribution of Au nanoparticles, which is particularly important for the enrichment of hot spots while preventing their serious agglomeration. Moreover, the cCNT matrix also contributes to the electromagnetic enhancement due to its surface plasmon resonance,and the chemical enhancement due to the adsorption of the target molecules. The resulting Au@c-CNT nanohybrids exhibit a remarkable synergy in SERS compared to neat Au nanoparticles.     

Nanoparticle-based substrates for surface-enhanced Raman scattering detection of bacterial spores

The development of ultrasensitive and rapid methods for the detection of bacterial spores is important for medical diagnostics of infectious diseases. While Surface-Enhanced Raman Spectroscopic (SERS) techniques have been increasingly demonstrated for achieving this goal, a key challenge is the development of sensitive and stable SERS substrates or probes. This Minireview highlights recent progress in exploring metal nanoparticle-based substrates, especially gold nanoparticle-based substrates, for the detection of biomarkers released from bacterial spores. One recent example involves assemblies of gold nanoparticles on a gold substrate for the highly sensitive detection of dipicolinic acid (DPA), a biomarker for bacterial spores such as Bacillus anthracis. This type of substrate exploits a strong SERS effect produced by the particle-particle and particle-substrate plasmonic coupling. It is capable of accurate speciation of the biomarker but also selective detection under various reactive or non-reactive conditions. In the case of detecting Bacillus subtilis spores, the limit of detection is quite comparable (0.1 ppb for DPA, and 1.5 × 10(9) spores per L (or 2.5 × 10(-14) M)) with those obtained using silver nanoparticle-based substrates. Implications of the recent findings for improving the gold nanoparticle-based SERS substrates with ultrahigh sensitivity for the detection of bacterial spores are also discussed.     

High-yield synthesis of multi-branched gold nanoparticles and their surface-enhanced Raman scattering properties

Multi-branched gold nanoparticles were synthesized in high-yield through the reduction of HAuCl(4) by using hydrazine as a reducing agent. Practically 100% of the particles have numerous branches. The high reduction capability of hydrazine is found to be crucial for the formation of these branched gold nanoparticles. Their size can be controlled from 20 to 130 nm by varying the amounts of hydrazine. The prepared nanoparticles exhibit efficient surface-enhanced Raman scattering (SERS) properties and the SERS activity of the particles depends on the aspect ratio of their branches, which are most likely related to a great increase in the localized electromagnetic field enhancement from their unique sharp surface features arising from the branches.     

Trace molecules detectable by surface-enhanced Raman scattering based on newly developed Ag and Au nanoparticles-containing substrates

In this work, Ag and Au nanoparticles-containing substrates were first developed for obtaining a stronger surface-enhanced Raman scattering (SERS) intensity of Rhodamine 6G (R6G) and reducing the limit of detection (LOD) of trace molecules. First, the optimum electrochemically roughening conditions employed on Ag substrates for obtaining strongest SERS of R6G were investigated. Then the optimally roughened Ag substrates were incubated in the prepared Cl- and Au-containing solutions for different couples of minutes to undergo the galvanic replacement reactions. Encouragingly, the SERS of R6G adsorbed on this roughened Ag substrate modified by the replacement of Ag with Au for 5 min exhibits a higher intensity by 8-fold of magnitude, as compared with the SERS of R6G adsorbed on an unmodified roughened Ag substrate. Moreover, the practical LOD of R6G can be reduced by one order of magnitude from 1 ppq to 0.1 ppq. Further investigations indicate that the compositions of complexes formed on the substrates demonstrate decided effects on the improved SERS.     

Improved surface-enhanced Raman scattering on optimum electrochemically roughened silver substrates

In this work, the effects of preparation conditions used in roughening silver substrates by electrochemical triangular-wave oxidation-reduction cycles (ORC) on surface-enhanced Raman scattering (SERS) were first investigated. The optimum roughening conditions for obtaining strongest SERS of Rhodamine 6G (R6G) are as follows. Ag electrodes were cycled in deoxygenated aqueous solutions containing 0.1 M NaCl from −0.3 to +0.2 V versus Ag/AgCl at 25 mV s⁻¹ for five scans. The SERS of R6G adsorbed on this optimum procedure-prepared roughened Ag substrate exhibits a higher intensity by one order of magnitude, as compared with that of R6G adsorbed on a normally roughened Ag substrate.     

Substrates with discretely immobilized silver nanoparticles for ultrasensitive detection of anions in water using surface-enhanced Raman scattering

Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.     

Highly reproducible surface-enhanced Raman scattering-active Au nanostructures prepared by simple electrodeposition: Origin of surface-enhanced Raman scattering activity and applications as electrochemical substrates

The fabrication of effective surface-enhanced Raman scattering (SERS) substrates has been the subject of intensive research because of their useful applications. In this paper, dendritic gold (Au) rod (DAR) structures prepared by simple one-step electrodeposition in a short time were examined as an effective SERS-active substrate. The SERS activity of the DAR surfaces was compared to that of other nanostructured Au surfaces with different morphologies, and its dependence on the structural variation of DAR structures was examined. These comparisonal investigations revealed that highly faceted sharp edge sites present on the DAR surfaces play a critical role in inducing a high SERS activity. The SERS enhancement factor was estimated to be greater than 10⁵, and the detection limit of rhodamine 6G at DAR surfaces was 10⁻⁸ M. The DAR surfaces exhibit excellent spot-to-spot and substrate-to-substrate SERS enhancement reproducibility, and their long-term stability is very good. It was also demonstrated that the DAR surfaces can be effectively utilized in electrochemical SERS systems, wherein a reversible SERS behavior was obtained during the cycling to cathodic potential regions. Considering the straightforward preparation of DAR substrates and the clean nature of SERS-active Au surfaces prepared in the absence of additives, we expect that DAR surfaces can be used as cost-effective SERS substrates in analytical and electrochemical applications.     

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.     

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.     

Ag nanoparticles prepared by laser photoreduction as substrates for in situ surface-enhanced Raman scattering analysis of dyes

In this work Ag nanoparticles (AgNP) with surface-enhanced Raman scattering (SERS) activity were prepared and immobilized by laser irradiation on a water/ solid interface where the aqueous phase contains the Ag+ cation and the solid surface is of hydrophilic nature (glass and cellulose). The so-prepared AgNP demonstrated a high SERS effectiveness in the detection of dispersed adsorbates such as the case of the anthraquinonic dye alizarin. The size and SERS effectiveness of AgNP increases with the irradiation time, the laser power, and the cation concentration. Laser-induced AgNP can be classified into two classes attending to the morphology: spherical and planar. The latter are formed after longer irradiation times, being more active regarding the SERS efficiency. Ag photoreduction can be employed for in situ detection of the dye alizarin, but when the dye is placed on a hydrophilic substrate. Even so, this in situ SERS technique could be attractive for analytical applications involving the in situ detection of the analyzed species, such as the case of dyes in artistical objects, textiles, foods, and surface analysis in general.