Surface-enhanced Raman spectroscopy of self-assembled monolayers: sandwich architecture and nanoparticle shape dependence

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates is presented for SAMs onto which gold nanoparticles of various shapes have been electrostatically immobilized. SERS spectra of 4-MBA SAMs are enhanced in the presence of immobilized gold nanocrystals by a factor of 10(7)-10(9) relative to 4-MBA in solution. Large enhancement factors are a likely result of plasmon coupling between the nanoparticles (localized surface plasmon) and the smooth gold substrate (surface plasmon polariton), creating large localized electromagnetic fields at their interface, where 4-MBA molecules reside in this sandwich architecture. Moreover, enhancement factors depend on nanoparticle shape and vary by a factor of 10(2). This SERS geometry offers large surface enhancements for molecules adsorbed onto planar substrates and could be quite useful for determining chemical information for poor Raman scatterers from assays on 2-D substrates.     

Single nanoparticle based optical pH probe

This paper describes the development of a nanoscale optical pH probe based upon the surface-enhanced Raman scattering (SERS) properties of silica-gold core-shell nanoparticles. In this approach, a thin layer of gold is deposited onto a core of silica to form a metallic nanoshell with surface plasmon modes in the red-to-near-infrared spectral region. The surface of the nanoshell is functionalized with a pH-sensitive SERS reporter molecule, 4-mercaptopyridine (4-MPy). The SERS spectra of 4-MPy is shown to be sensitive to the pH of the surrounding media within the range of 3 to 7. In addition, it is shown that individual silica-gold core-shell nanoparticles yield more reliable SERS spectra than aggregates of core-shell nanoparticles.     

Optical properties of porous anodic alumina membrane uniformly decorated with ultra-thin porous gold nanoparticles arrays

In this paper, we directly develop a facile method to decorate a modified porous anodic alumina membrane (PAA) with an ultrathin porous film of gold nanoparticles with sub-gaps less than 25 nm and particle size less than 40 nm on the top surface and Au nanoparticles uniformly attached to the pore walls as well as the bottom of the pores, utilizing radio-frequency magnetron sputtering. The size as well as the interparticle distance of the gold nanostructures is adjusted by changing the structural properties of PAA membrane and the sputtering time. According to the measured reflection spectra, the saturation of interference color is significantly enhanced. As a result, the Au-coated PAA membrane exhibits a brilliant and tunable color. Field enhancement can be achieved in these structures through the excitation and constructive interference of surface plasmon waves. In addition, the role of localized surface plasmon and propagating surface plasmon was discussed. A four-layered model is presented to describe the reflectance data that show excellent agreement with the experimental data. The brilliant Au-coated PAA membrane is useful for decorative purposes and holds promise as an effective surface enhanced Raman scattering (SERS) substrate.     

Surface-enhanced Raman scattering of rhodamine 6G on nanowire arrays decorated with gold nanoparticles

We investigate the surface-enhanced Raman scattering (SERS) of rhodamine 6G (R6G) adsorbed on Au nanoparticles attached to InP nanowires. We find that nanowire arrays act as frameworks for effective SERS substrates with a significantly higher Raman signal sensitivity than a planar framework of Au nanoparticles adsorbed two-dimensionally on a flat surface. The SERS signal displays a clear polarization-dependent effect when the nanowires are arranged in a row. We also find that the SERS signal increases with time during continuous laser illumination. The plasmon-enhanced optical forces between Au nanoparticles may either move pairs of nanoparticles closer together or attract adsorbed molecules by moving them to the junctions of Au nanoparticle aggregates. Such effects by plasmon optical forces may cause the observed increase of the SERS signal with continuous laser illumination.     

Fabrication of rice-like gold nanoparticles and application in surface-enhanced Raman scattering

A simple method for the synthesis of rice-like gold nanoparticles using gold nanorods (GNRs) as precursors in the aqueous phase was exploited. The method used in this work involves eroding GNRs with potassium ferricyanide in the aqueous phase. Surface plasmon resonance (SPR) bands of the resulting nanoparticles present a notable blue-shift from 670 to 570 nm with increasing amounts of potassium ferricyanide, and subsequently the shape of the resulting nanoparticles can be readily controlled. Most importantly, the SPR response is an almost linear function of the quantity of potassium ferricyanide added. The synthesis of the resulting nanoparticles with various aspect ratios has been extensively studied and is well established. The surface-enhanced Raman scattering (SERS) intensity enhancement of the adsorbate on the surface of these gold nanoparticles was also studied.     

Distance-dependent plasmon resonant coupling between a gold nanoparticle and gold film

We present an experimental analysis of the plasmonic scattering properties of gold nanoparticles controllably placed nanometers away from a gold metal film. We show that the spectral response of this system results from the interplay between the localized plasmon resonance of the nanoparticle and the surface plasmon polaritons of the gold film, as previously predicted by theoretical studies. In addition, we report that the metal film induces a polarization to the single nanoparticle light scattering, resulting in a doughnut-shaped point spread function when imaged in the far-field. Both the spectral response and the polarization effects are highly sensitive to the nanoparticle-film separation distance. Such a system shows promise in potential biometrology and diagnostic devices.     

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.     

Controlling the aggregation behavior of gold nanoparticles

The aggregation of Au nanoparticles (NPs) in solution is influenced by cationic and oligocationic species. The polarization of the conduction electron oscillations in adjacent gold nanoparticles causes a new red-shifted plasmon absorbance attributed to the coupling of the plasmon absorbance of the particles. This appearance of an additional plasmon band is of particular interest to the field of SERS and has led to research works directed at the stabilization of small colloid aggregates in solution. The surface plasmon coupling can be tuned by controlling the aggregation of gold nanoparticles by the addition of some “cross-linking” agent. Here we develop a simple method to fabricate linear-chainlike aggregates of gold nanoparticles (so-called nanochains), tuning the linear optical properties in a wide wavelength range from visible to the near-infrared. The aggregation behavior and linear self-assembly mechanism of citrate-stabilized gold colloids as provoked by the addition of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA) are also analyzed. The line-assembly mechanism of gold nanochain is attributed to the preferential binding of CTAB molecules on a certain facet of gold NPs and the Au NP electrostatic interactions. We also found that the 11-mercaptoundecanoic acid was effective to prevent the further aggregation of CTAB-modified gold colloids.     

Gold nanoparticle-based pH sensor in highly alkaline region at pH > 11: surface-enhanced Raman scattering study

A surface plasmon resonance spectroscopy study showed that citrate-reduced gold nanoparticles ( approximately 15 nm diameter, approximately 9 x 10(-9) M concentration, approximately 2 x 10(-2) M ionic strength) were found to be utilized as a colorimetric sensor by exhibiting a distinct color change at a highly alkaline pH > 11.5. Surface-enhanced Raman scattering (SERS) of 4-ethynylpyridine (4-EP) on gold nanoparticle surfaces indicated that the multiple peaks in the v(C identical withC) stretching bands should vary significantly in the highly alkaline region from pH 12 to 14. As the pH value increased, the v(C identical withC) stretching band intensity at approximately 2080 cm(-1) became stronger than that at approximately 2010 cm(-1). The pK(1/2) value was determined to be around 13 by the SERS titration of taking intensity ratios of I(2080) with respect to I(2010). Using SERS enhancements and conspicuous spectral changes, self-assembled monolayers (SAMs) of 4-EP on Au nanoparticles holds potential as a pH sensor for sensitive detection of the hydroxide OH(-) concentration at around pH 13 in an aqueous solution. The pH calibration from SERS titration of 4-EP is expected to have advantages in terms of higher alkaline detection limit and more precise measurements, if compared with the indigo carmine, the pK(1/2) value of which is 12.2.     

Surface-enhanced Raman scattering of carbon nanotubes by decoration of ZnS nanoparticles

ZnS nanoparticles anchored on the single-walled carbon nanotubes (SWNTs) were fabricated by a chemical vapor deposition (CVD) method. The CVD method shows no selectivity for growth of ZnS nanoparticles on types and defects of the SWNTs, and thus ensures the uniform decoration of all SWNTs on the substrate. ZnS nanoparticles with a diameter of 10 nm were decorated on the SWNTs surface with an interparticle distance of about 20 nm. This method provides the possibility to realize the optimal configurations of ZnS nanoparticles on SWNTs for obtaining surface-enhanced Raman spectroscopy (SERS) of SWNTs. Investigations of mechanism reveal that charge transfer (a small amount of excitation electrons) from ZnS nanoparticles to SWNTs weakly affects Raman intensity, and the coupled surface plasmon resonance (SPR) formed from plenty of excitation electrons on the surface of ZnS nanoparticles contributes to the strong surface enhancement. It would be an alternative approach for SERS after metal (normally gold or silver) nanoparticles' decoration on the SWNTs surface.     

Optical probes for biological applications based on surface-enhanced Raman scattering from indocyanine green on gold nanoparticles

We report surface-enhanced Raman scattering (SERS) studies on indocyanine green (ICG) on colloidal silver and gold and demonstrate a novel optical probe for applications in living cells. In addition to its own detection by the characteristic ICG SERS signatures, the ICG gold nanoprobe delivers spatially localized chemical information from its biological environment by employing SERS in the local optical fields of the gold nanoparticles. The probe offers the potential to increase the spectral specificity and selectivity of current chemical characterization approaches of living cells and biomaterials based on vibrational information.     

Inverted size-dependence of surface-enhanced Raman scattering on gold nanohole and nanodisk arrays

Surface-enhanced Raman scattering (SERS) on gold nanohole and nanodisk arrays with precisely controlled size and spacing fabricated via electron beam lithography was investigated. These nanostructures exhibit strong SERS signals at 785 nm excitation but not at 514 nm. When the edge-to-edge distance is maintained, enhancement increases for nanoholes but decreases for nanodisks as diameter is increased. It is shown that the observed enhancement results from the local surface plasmon resonance wavelength shifts to the near-infrared regime as nanohole diameter increases. The large tolerance on dimensions and the empty space confined by nanoholes suggest promise for their use as a functional component in sensing, spectroscopy, and photonic devices.     

Surface-enhanced Raman scattering dendritic substrates fabricated by deposition of gold and silver on silicon

This paper reports a study on the preparation of gold nanoparticles and silver dendrites on silicon substrates by immersion plating. Firstly, gold was deposited onto silicon wafer from HF aqueous solution containing HAuCl4. Then, the silicon wafer deposited gold was dipped into HF aqueous solution of AgNO3 to form silver coating gold film. Scanning electron microscopy reveals a uniform gold film consisted of gold nanoparticles and rough silver coating gold film containing uniform dendritic structures on silicon surface. By SERS (surface-enhanced Raman scattering) measurements, the fabricated gold and silver coating gold substrates activity toward SERS is assessed. The SERS spectra of crystal violet on the fabricated substrates reflect the different SERS activities on gold nanoparticles film and silver coating gold dendrites film. Compared with pure gold film on silicon, the film of silver coating gold dendrites film significantly increased the SERS intensity. As the fabrication process is very simple, cost-effective and reproducible, and the fabricated silver coating gold substrate is of excellent enhancement ability, spatial uniformity and good stability.     

Gold and magnetic oxide/gold core/shell nanoparticles as bio-functional nanoprobes

The ability to create bio-functional nanoprobes for the detection of biological reactivity is important for developing bioassay and diagnostic methods. This paper describes the findings of an investigation of the surface functionalization of gold (Au) and magnetic nanoparticles coated with gold shells (M/Au) by proteins and spectroscopic labels for the creation of nanoprobes for use in surface enhanced Raman scattering (SERS) assays. Highly monodispersed Au nanoparticles and M/Au nanoparticles with two types of magnetic nanoparticle cores (Fe(2)O(3) and MnZn ferrite) were studied as model systems for the bio-functionalization and Raman labeling. Comparison of the SERS intensities obtained with different particle sizes (30-100?nm) and samples in solution versus on solid substrates have revealed important information about the manipulation of the SERS signals. In contrast to the salt-induced uncontrollable and irreversible aggregation of nanoparticles, the ability to use a centrifugation method to control the formation of stable small clustering sizes of nanoparticles was shown to enhance SERS intensities for samples in solution as compared with samples on solid substrates. A simple method for labeling protein-capped Au nanoparticles with Raman-active molecules was also described. The functionalized Au and M/Au nanoparticles are shown to exhibit the desired functional properties for the detection of SERS signals in the magnetically separated reaction products. These results are discussed in terms of the interparticle distance dependence of 'hot-spot' SERS sites and the delineation of the parameters for controlling the core-shell reactivity of the magnetic functional nanocomposite materials in bio-separation and spectroscopic probing.     

Precision Synthesis: Designing Hot Spots over Hot Spots via Selective Gold Deposition on Silver Octahedra Edges

A major challenge in plasmonic hot spot fabrication is to efficiently increase the hot spot volumes on single metal nanoparticles to generate stronger signals in plasmon‐enhanced applications. Here, the synthesis of designer nanoparticles, where plasmonic‐active Au nanodots are selectively deposited onto the edge/tip hot spot regions of Ag nanoparticles, is demonstrated using a two‐step seed‐mediated precision synthesis approach. Such a “hot spots over hot spots” strategy leads to an efficient enhancement of the plasmonic hot spot volumes on single Ag nanoparticles. Through cathodoluminescence hyperspectral imaging of these selective edge gold‐deposited Ag octahedron (SEGSO), the increase in the areas and emission intensities of hot spots on Ag octahedra are directly visualized after Au deposition. Single‐particle surface‐enhanced Raman scattering (SERS) measurements demonstrate 10‐fold and 3‐fold larger SERS enhancement factors of the SEGSO as compared to pure Ag octahedra and non‐selective gold‐deposited Ag octahedra (NSEGSO), respectively. The experimental results corroborate well with theoretical simulations, where the local electromagnetic field enhancement of our SEGSO particles is 15‐fold and 1.3‐fold stronger than pure Ag octahedra and facet‐deposited particles, respectively. The growth mechanisms of such designer nanoparticles are also discussed together with a demonstration of the versatility of this synthetic protocol.     

Gold nanorods with finely tunable longitudinal surface plasmon resonance as SERS substrates

Advances in nanophotonics have shown the potential of colloidal metal nanoparticles with sharp tips, such as rods, to focalize plasmonic electromagnetic fields. We report on the synthesis of Au nanorods via a seed mediated approach and the influence of silver ions on the aspect ratio of the Au nanorods. The longitudinal surface plasmon resonance (LSPR) of the Au nanorods was successfully tuned with the concentration of silver ions. The surface enhanced Raman scattering (SERS) effect of 2-aminothiophenol (2-ATP) as a probe molecule on Au nanorods was systematically studied by varying the longitudinal surface plasmon resonance of the nanorods. The highest electromagnetic enhancement was observed when the longitudinal surface plasmon resonance of the Au nanorods overlapped with the laser excitation wavelength. The variation of the SERS enhancement factor with the longitudinal surface plasmon resonance and laser excitation lines is also discussed in detail.     

Polarization dependence of surface-enhanced Raman scattering in gold nanoparticle-nanowire systems

We study the polarization dependence of surface-enhanced Raman scattering (SERS) in coupled gold nanoparticle-nanowire systems. The coupling between the continuous nanowire plasmons and the localized nanoparticle plasmons results in significant field enhancements and SERS enhancements comparable to those found in nanoparticle dimer junctions. The SERS intensity is maximal when the incident light is polarized across the particle and the wire, and the enhancement is remarkably insensitive to the detailed geometrical structures of the nanoparticles.     

Shape-dependent surface-enhanced Raman scattering in gold-Raman probe-silica sandwiched nanoparticles for biocompatible applications

To meet the requirement of Raman probes (labels) for biocompatible applications, a synthetic approach has been developed to sandwich the Raman-probe (malachite green isothiocyanate, MGITC) molecules between the gold core and the silica shell in gold-SiO? composite nanoparticles. The gold-MGITC-SiO? sandwiched structure not only prevents the Raman probe from leaking out but also improves the solubility of the nanoparticles in organic solvents and in aqueous solutions even with high ionic strength. To amplify the Raman signal, three types of core, gold nanospheres, nanorods and nanostars, have been chosen as the substrates of the Raman probe. The effect of the core shape on the surface-enhanced Raman scattering (SERS) has been investigated. The colloidal nanostars showed the highest SERS enhancement factor while the nanospheres possessed the lowest SERS activity under excitation with 532 and 785 nm lasers. Three-dimensional finite-difference time domain (FDTD) simulation showed significant differences in the local electromagnetic field distributions surrounding the nanospheres, nanorods, and nanostars, which were induced by the localized surface plasmon resonance (LSPR). The electromagnetic field was enhanced remarkably around the two ends of the nanorods and around the sharp tips of the nanostars. This local electromagnetic enhancement made the dominant contribution to the SERS enhancement. Both the experiments and the simulation revealed the order nanostars > nanorods > nanospheres in terms of the enhancement factor. Finally, the biological application of the nanostar-MGITC-SiO? nanoparticles has been demonstrated in the monitoring of DNA hybridization. In short, the gold–MGITC-SiO? sandwiched nanoparticles can be used as a Raman probe that features high sensitivity, good water solubility and stability, low-background fluorescence, and the absence of photobleaching for future biological applications.     

Metallic nanocrystals near ultrasmooth metallic films for surface-enhanced Raman scattering application

We studied the effect of the substrate on the surface-enhanced Raman scattering (SERS) signals of metallic nanocrystal films by making a direct comparison between cases with metallic and semiconducting substrate surfaces. Ag nanoparticles smaller than 10?nm were synthesized and uniform arrays were formed on both ultrasmooth metallic and Si surfaces. These substrates provide reproducible SERS signals with high enhancement factors over large areas. Moreover, a SERS signal about one order of magnitude higher was obtained in the metallic surface case as compared with the Si substrate case, which is attributed to stronger plasmon coupling between the nanoparticles and their charge-conjugate images in the underlying metallic surface. The interpretation of our experimental results was confirmed by our finite difference time domain calculations. The dependence of the interaction between the nanoparticles and the substrate surface on the direction of the incident electromagnetic field is also discussed.     

Investigation into a surface plasmon related heating effect in surface enhanced Raman spectroscopy

The irreversible loss of surface enhanced Raman spectroscopy (SERS) intensity derived from electrostatically immobilized Au colloidal substrate is studied. The total intensity of the SERS scattering from crystal violet molecules adsorbed onto the substrate was monitored for up to 240 s under continuous laser irradiation. The rate of signal decay was found to depend upon the thickness of the liquid layer over the coated substrate. On the basis of this observation, we propose a plausible mechanism in which surface plasmon related heating induced a small but significant lateral particle diffusion in closely spaced Au particles. The result of the shifting is an increase in the average interparticle distance, which subsequently reduces the electromagnetic coupling between the Au nanoparticles, and in turn causes a reduction in the SERS activity. Finally, a three-state model is proposed and shown to satisfactorily describe the observed decays.