Facile fabrication of large area of aggregated gold nanorods film for efficient surface-enhanced Raman scattering

An effective and facile method for fabrication of large area of aggregated gold nanorods (AuNRs) film was proposed by self-assembly of AuNRs at a toluene/water interface for the first time. It was found that large area of aggregated AuNRs film could be formed at the interface of toluene and water due to the interfacial tension between the two phases. The obtained large area of aggregated AuNRs film exhibits strong surface-enhanced Raman scattering (SERS) activity with 4-aminothiophenol (4-ATP) and 2-aminothiophenol (2-ATP) as the probe molecules based on the strong electromagnetic coupling effect between the very adjacent AuNRs. Enhancement factors (EF) were used to estimate the SERS activity of the aggregated AuNRs film, which is obtained to be 1.7x10(5) for 7a vibration of 4-ATP. SERS intensity is compared with AuNRs deposited directly on glass, indicating high SERS activity and reproducibility of the aggregated AuNRs film. In addition, SERS activity has also been successfully demonstrated for dye molecule (Rhodamin 6G (R6G)) and biological small molecule (adenine) on the aggregated AuNRs film, showing great potential of the aggregated AuNRs film as a convenient and powerful SERS substrate for biological tags and biological molecular detection.     

Probing nanoscale interfaces with electrochemical surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is a powerful tool for studying nanoscale molecule-metal interfaces across a range of electrochemical applications. SERS combines molecular-level information with a high degree of surface specificity, making it an ideal tool for understanding interfacial processes, from understanding how analytes and electrolytes organize near metal surfaces to following surface-mediated reactions in real time. However, because SERS relies on the excitation of localized surface plasmons, additional effects such as the production of hot charge carriers and photothermal heating can impact electrochemical SERS signals. These effects must also be considered when using SERS for quantitative electrochemical studies.     

Approaching the electromagnetic mechanism of surface-enhanced Raman scattering: from self-assembled arrays to individual gold nanoparticles

Surface-enhanced Raman scattering (SERS) has been intensively explored both in theory and applications and has been widely used in chemistry, physics and biology for decades. A variety of SERS substrates have been developed in order to investigate the mechanisms behind, which give rise to the enormous enhancement even enabling single molecule detection. The Raman enhancement, which involves an electromagnetic enhancement (EM) and a chemical enhancement (CM), reflects both the physical principle of light/metal interactions and the molecule/metal interactions. In this tutorial review, we focus on the EM enhancement of SERS active substrates made of colloidal gold nanoparticles (GNPs), varying from self-assembled arrays down to single particles, for the purpose of investigating the EM coupling effect and probing the distribution of the induced electric field of single GNPs.     

Probing the structure of single-molecule surface-enhanced Raman scattering hot spots

We present here a detailed study of the specific nanoparticle structures that give rise to single-molecule surface-enhanced Raman scattering (SMSERS). A variety of structures are observed, but the simplest are dimers of Ag nanocrystals. We chose one of these structures for detailed study using electrodynamics calculations and found that the electromagnetic SERS enhancement factors of 10(9) are easily obtained and are consistent with single-molecule SERS activity.     

Rapid detection of melamine with 4-mercaptopyridine-modified gold nanoparticles by surface-enhanced Raman scattering

A surface-enhanced Raman scattering (SERS) strategy based on 4-mercaptopyridine (MPY)-modified gold nanoparticles (AuNPs) was developed for the rapid and sensitive detection of melamine in milk powder. The SERS measurement of melamine strongly relied on the “hotspot” effect, in which AuNPs immediately aggregated upon the addition of melamine, leading to significantly enhanced Raman intensity of the reporter molecule MPY and a color change for the solution from red to blue-gray. The limit of detection based on a signal to noise of 3 (S/N = 3) was found to be as low as 0.1 ppb of melamine, with an excellent linearity of 0.5–100 ppb, demonstrating a higher sensitivity and a wider quantitation range than direct SERS sensing methods based on enhanced substrate. An impressive specificity for melamine detection over various common metal ions and excipients in dairy products, even at concentrations of 100-fold higher than melamine, was achieved. Good recoveries of 88.5% and 111.7% were obtained from milk samples spiked to 20 and 100 ppb levels, respectively. The proposed method is potentially applicable for the rapid in situ determination of melamine in complex matrices.     

pH effect on surface potential of polyelectrolytes-capped gold nanoparticles probed by surface-enhanced Raman scattering

Nanoparticles are commonly stabilized through the adsorption of acidic/basic polyelectrolytes around the surface of the particle. One example of these nanoparticles is poly(ethylenimine) (PEI)-capped Au nanoparticles. In this work, we have examined by means of surface-enhanced Raman scattering (SERS) of 2,6-dimethylphenylisocyanide (2,6-DMPI) how much the surface potential of Au nanoparticles is affected by the solution pH through the mediation of the protonation and deprotonation of PEI in contact with Au nanoparticles. In fact, the surface-potential-dependent isocyanide (NC) stretching peak of 2,6-DMPI has shifted sharply around pH 8.5, close to the pK(a) value of the primary amine of PEI. When a negatively charged poly(acrylic acid) (PAA) was deposited onto the PEI, the peak shift of the NC stretching band took place around pH 6.5, close to the average pK(a) value of PEI and PAA. When additional PEI was deposited on PAA, the peak shift of the NC stretching band occurred once again around pH 8.5, indicative of the stronger interaction of upper two polyelectrolyte layers. These data clearly illustrate the usefulness of SERS in the elucidation of a delicate interaction of cationic and anionic polyelectrolytes, especially in layer-by-layer deposition.     

Structural transition in the surfactant layer that surrounds gold nanorods as observed by analytical surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) of gold nanorods in cetyltrimethylammonium bromide solution has been used to analyze the interfacial surfactant structure based on the distance-dependent electromagnetic enhancement. The spectra were consistent with a surfactant bilayer oriented normal to the surface. As the surfactant concentration was reduced, a structural transition in the surfactant layer was observed through a sudden increase in the signal from the alkane chains. The structural transition was shown to influence the displacement of the surfactant layer by thiolated poly(ethylene glycol). The monodisperse and thoroughly characterized gold nanorod samples yield consistent enhancement factors that were compared to electromagnetic simulations.     

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.     

The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.     

Spectroscopic analysis of L-histidine adsorbed on gold and silver nanoparticle surfaces investigated by surface-enhanced Raman scattering

The adsorption of l-histidine on gold (Au) and silver (Ag) nanoparticle surfaces has been comparatively analyzed by means of surface-enhanced Raman scattering (SERS). The SERS spectra of l-histidine on Ag were found to be quite different from those on Au, indicating dissimilar adsorption structures depending on metal substrates. Most peaks of l-histidine on Ag appeared to be due to coordination via the carboxylate (COO(-)) group with an imidazole ring of fairly upright geometry, whereas on Au it was assumed to adsorb with a rather flat geometry. A density functional theory (DFT) calculation was performed at the level of B3LYP/LANL2DZ to estimate the energetic stability of the binding of the imidazole ring and the carboxylate group of l-histidine with the Ag and Au atoms, respectively. Based on the DFT calculation, the carboxylate group of l-histidine was predicted to bind more favorably to Ag than to Au, and this was in line with our SERS spectral analysis.     

Relating surface-enhanced Raman scattering signals of cells to gold nanoparticle aggregation as determined by LA-ICP-MS micromapping

The cellular response to nanoparticle exposure is essential in various contexts, especially in nanotoxicity and nanomedicine. Here, 14-nm gold nanoparticles in 3T3 fibroblast cells are investigated in a series of pulse-chase experiments with a 30-min incubation pulse and chase times ranging from 15 min to 48 h. The gold nanoparticles and their aggregates are quantified inside the cellular ultrastructure by laser ablation inductively coupled plasma mass spectrometry micromapping and evaluated regarding the surface-enhanced Raman scattering (SERS) signals. In this way, both information about their localization at the micrometre scale and their molecular nanoenvironment, respectively, is obtained and can be related. Thus, the nanoparticle pathway from endocytotic uptake, intracellular processing, to cell division can be followed. It is shown that the ability of the intracellular nanoparticles and their accumulations and aggregates to support high SERS signals is neither directly related to nanoparticle amount nor to high local nanoparticle densities. The SERS data indicate that aggregate geometry and interparticle distances in the cell must change in the course of endosomal maturation and play a critical role for a specific gold nanoparticle type in order to act as efficient SERS nanoprobe. This finding is supported by TEM images, showing only a minor portion of aggregates that present small interparticle spacing. The SERS spectra obtained after different chase times show a changing composition and/or structure of the biomolecule corona of the gold nanoparticles as a consequence of endosomal processing.     

Sensing of 2,4-dichlorophenoxyacetic acid by surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) spectra of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was obtained by employing a bi-layer gold substrate, assembled by the reduction of Au(III) over gold-seeded nanoparticles immobilized on functionalized glass substrates. The SERS signal was linear with the logarithm of the solution concentrations between 1.0 × 10⁻⁷ mol L⁻¹ and 1.0 × 10⁻³ mol L⁻¹, indicating that the bi-layer gold substrate affords a significant dynamic range for SERS, providing an excellent analytical response within this concentration range, and revealing the high sensitivity of the gold surface towards such analyte. In addition, using the same gold substrate, a similar calibration curve was obtained for crystal-violet (CV), and it was possible to identify the concentration limit corresponding to the transition from the average SERS to the nonlinear SERS response.     

Essential nanogap effects on surface-enhanced Raman scattering signals from closely spaced gold nanoparticles

We have demonstrated the essential nanogap effects on surface-enhanced Raman scattering (SERS) signals obtained from two diagonally aligned gold nanoparticles with several nanometre separations, which were precisely fabricated on a glass substrate. This is the first proof of principle for extracting the light localization effects on SERS due to the formation of nanogaps from experimentally observed SERS signals.     

Temporal evolution of Raman intensities on surface-enhanced Raman scattering active copper and gold electrodes at negative potentials

The effect of a roughening procedure on surface-enhanced Raman scattering (SERS) intensity of pyridine at copper and gold electrodes subjected to negative potential has been investigated. Among four procedures tested for a copper electrode the one consisting of electrochemical activation in a solution of LiCl and CuCl₂ resulted in the most stable and effective surface. It was proved that the presence of pyridine during the pretreatment procedure caused a very fast, irreversible decay of SERS intensity for both copper and gold electrodes. Quite stable, at least at room temperatures, gold surfaces were obtained by oxidation-reduction cycles activation in KCl solution.     

Templated assembly of gold nanoparticles into microscale tubules and their application in surface-enhanced Raman scattering

We report a simple procedure to assemble gold nanoparticles into hollow tubular morphology with micrometer scale, wherein the citrate molecule is used not only as a reducing and capping agent, but also as an assembling template. The nanostructure and growth mechanism of microtubes are explored via SEM, TEM, FTIR spectra, and UV-vis spectra studies. The incorporation of larger gold nanoparticles by electroless plating results in an increase in the diameter of microtubes from 900 nm to about 1.2 microm. The application of the microtubes before and after electroless plating in surface-enhanced Raman scattering (SERS) is investigated by using 4-aminothiophenol (4-ATP) as probe molecules. The results indicate that the microtubes both before and after electroless plating can be used as SERS substrates. The microtubes after electroless plating exhibit excellent enhancement ability.     

Controllable synthesis of water-soluble gold nanoparticles and their applications in electrocatalysis and surface-enhanced Raman scattering

We report a facile method to synthesize water-soluble gold nanoparticles (AuNPs) using a biosurfactant sodium cholate as reducing reagents and protective groups in aqueous solution at ambient temperature. The diameters (13-70 nm) of uniform AuNPs can be readily adjusted by changing the initial molar ratio of sodium cholate to chloroauric acid (HAuCl(4)). Also, the alkaline condition of preparative solution is found to affect the size of as-synthesized AuNPs. This synthetic approach is one-step and "green". The obtained AuNPs exhibit a good electrocatalytic activity toward methanol oxidation. Meanwhile, the AuNPs thin films can serve as an efficient substrate for surface-enhanced Raman scattering (SERS). Furthermore, platinum nanoparticles (PtNPs) are also prepared by reducing sodium tetrachloro platinate hydrate with sodium cholate.     

On the theory of surface-enhanced Raman scattering

A model for Raman scattering from molecules chemisorbed on surfaces is proposed. It is shown that part of the en hancement may be due to charge-transfer excitations between the metal and the adsorbed molecules.