Plasmon Extinction Research Articles

Polymer-Coated Gold Island Films as Localized Plasmon Transducers for Gas Sensing

Ultrathin (typically < or = 10-nm thick) gold island films evaporated on transparent substrates show a prominent localized surface plasmon (SP) extinction in the visible-to-NIR range. Changes in the dielectric properties of the contacting medium influence the SP absorption band, providing a scheme for optical sensing based on refractive index change. In the present work, the gas sensing capability of gold island based localized surface plasmon resonance (LSPR) transducers was explored using polymeric coatings as the active interface. LSPR transducers were fabricated by spin-coating of polystyrene (PS) or polystyrene sulfonic acid, sodium salt (PSS) onto 5-nm-thick (nominal thickness) gold island films evaporated on silanized glass and annealed. Detailed characterization of the transducers was carried out using high-resolution scanning electron microscopy, cross-sectional transmission electron microscopy, and in situ atomic force microscopy under controlled atmosphere. The hydrophobic PS film exhibits swelling and significant thickness increase upon exposure to chloroform vapor and little or no change in water vapor, whereas the hydrophilic PSS film shows the opposite behavior when exposed to the same vapors. Polymer swelling upon absorption of vapors of good solvents shows a net effect of lowering the effective refractive index in the vicinity of the gold islands, manifested as a characteristic decrease of the SP band intensity and a blue shift of the band maximum. The response, measured for four different vapors, is fast (approximately 15 s) and reversible. It is shown that gold island systems coated with polymeric films can be applied to vapor recognition in an array configuration.

Ion-Specific Effects on Laser Ablation of Silver in Aqueous Electrolyte Solutions

Silver nanoparticle hydrosol formation by laser ablation (LA) of a Ag target immersed in pure water and in aqueous electrolyte solutions (HCl, NaCl, NaOH, AgNO3, and/or Na2S2O3) of various concentrations has been followed by surface plasmon extinction (SPE) spectral measurements and, for selected samples, by transmission electron microscopy (TEM), scanning electron microscopy (SEM) imaging, and energy dispersive X-ray (EDX) analysis. The laser ablation process accompanied by the Ag nanoparticle fragmentation (NF) has been performed with nanosecond laser pulses, by employing either a continuous or an intermittent irradiation regime. SPE spectra have been recorded either after each irradiation step of the stepwise procedure or at the end of the continuous one and, additionally, during a subsequent aging of Ag hydrosols. The presence of HCl, NaCl, and/or NaOH during LA/NF has led to the stabilization of the resulting Ag nanoparticles, while the presence of AgNO3 and Na2S2O3 has shown a destabilizing effect. The alternations of light and dark periods in LA/NF process, along with the presence of electrolytes (having different affinities toward Ag nanoparticle surfaces and toward Ag+ ions as well) can be considered as a new physicochemical parameter affecting the outcome of LA/NF. Moreover, the light/dark alternations are useful for separating the effects of ions on Ag nanoparticle hydrosols directly under irradiation from those occurring during further aging. Ag hydrosols prepared by LA/NF in 1 × 10-3 M NaCl and aged for 1−2 days have been shown to be excellent substrates for surface-enhanced Raman scattering (SERS) measurements owing to the presence of compact aggregates with numerous interstices between nanoparticles and of reduced Ag(0) adsorption sites on chloride-modified Ag nanoparticle surfaces.

Glutathione- and Cysteine-Induced Transverse Overgrowth on Gold Nanorods

We have demonstrated glutathione- and cysteine-induced transverse overgrowth on gold nanorods. In aqueous solutions, glutathione and cysteine are preferentially bound to the ends of gold nanorods that are stabilized by a cationic surfactant bilayer. This preferential end binding blocks the growth of the nanorods in the longitudinal direction completely and allows for the growth only in the transverse direction. As a result, the diameters of the nanorods become larger and larger while their lengths remain unchanged, as more and more gold precursor is supplied. In addition, the shape of the nanorods undergoes a gradual change from rods, peanuts, and truncated octahedra to faceted spheres during overgrowth. We believe that this transverse overgrowth provides an alternative means for tailoring the longitudinal plasmon wavelengths and extinction cross sections of gold nanoparticles and will therefore facilitate their use in optics, optoelectronics, and biotechnology.

Laser ablation of silver in aqueous ambient: effect of laser pulse wavelength and energy on efficiency of the process

Laser ablation (LA) of a Ag target in ultrapure water has been performed with nanosecond laser pulses of 355, 532 and 1064 nm in the range of fluences achievable for the particular wavelength. Efficiency of LA process was quantified in terms of the amount of ablated Ag as determined by atomic absorption spectroscopy (AAS) as well as of the area of the surface plasmon extinction (SPE) band of the resulting Ag nanoparticle hydrosol, and a fairly good agreement between the results produced by the two methods was obtained. Sigmoidally shaped plots of the LA efficiency as a function of laser fluence were obtained for LA with all wavelengths of laser pulses examined. Nevertheless, the maximum amount of Ag transferred from the target into the aqueous medium (yielding Ag nanoparticle hydrosol) is substantially (at least 6.5 x) larger for LA performed with 1064 nm pulses than that with 532 nm and 355 nm pulses. On the other hand, polydispersity of the hydrosol ablated with 1064 nm pulses is higher than that of the sols obtained with 532 and 355 nm pulses, most probably due to a limited extent of Ag nanoparticle fragmentation.

Effect of citrate ions on laser ablation of Ag foil in aqueous medium

Promoting effect of citrate in 1 × 10−5−1×10−2 M concentrations on laser ablation (LA) of a Ag foil in aqueous solution performed by ns laser pulses at 1064 nm is reported. Furthermore, adsorption of citrate ions was found to increase markedly the stability of the resulting LA-Ag hydrosol. The results are discussed on the basis of comparison of surface plasmon extinction spectral characteristics, transmission electron microscopy images, nanoparticle size distributions and surface-enhanced Raman scattering (SERS) spectral tests of hydrosols resulting from LA in neutral and acidic aqueous citrate solutions and in pure water.

Chemically reduced and laser-ablated gold nanoparticles immobilized to silanized glass plates: Preparation, characterization and SERS spectral testing

SERS-active surfaces based on gold colloidal nanoparticles attached to silanized glass plates were prepared and characterized by surface plasmon extinction (SPE) and atomic force microscopy (AFM). Aminopropyltrimethoxysilane (APTMS) was found to be more efficient in the silanization process than mercaptopropyltrimethoxysilane (MPTMS). Gold colloidal nanoparticles prepared by three different procedures, i.e. laser-ablation and chemical reduction of HAuCl 4 by sodium borohydride or by sodium citrate, were immobilized to glass plates using APTMS. The laser-ablated colloids exhibited poor stability during deposition and could not be used for routine preparation of surfaces. Drying at 100 °C after immobilization of chemically reduced nanoparticles stabilizes morphological properties of the surface and thus significantly improves the preparation procedure. Reproducibility and stability of substrates were tested by measurement of SERRS spectra of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (TMPyP). Spectra measured from different spots of a sample showed excellent reproducibility and proved high homogeneity of the Au surface. Measurement of several days aged samples revealed very good stability of the SERRS signal in time.

Plasmonic Properties of Supported Pt and Pd Nanostructures

The plasmonic properties of nanodisk arrays of Pt, Pd, and, for comparison, Ag are studied over a large size and spectral range and analyzed theoretically by an electrostatic model. Pt and Pd nanodisks exhibit broad localized surface plasmons with a higher sensitivity of the plasmon to the disk aspect ratio compared to Ag. Extinction cross-sections are generally about 50% smaller for Pt and Pd. The spectral plasmon positions, line-widths, and extinction cross-sections are well reproduced by the model.

Depolarized light scattering from silver nanoparticles

We present an observation of depolarized scattering by silver colloids. The profile of the wavelength-dependent anisotropy of the colloidal solution of silver nanoparticles traces the plasmonic extinction spectrum. The scattering component of the colloid extinction spectrum is responsible for the depolarization effect. We believe that the presence of the orthogonal component in the silver colloid scattering can be used in dye-less sensing devices.

Sensitivity of Transmission Surface Plasmon Resonance (T‐SPR) Spectroscopy: Self‐Assembled Multilayers on Evaporated Gold Island Films

The distance dependence of the localized surface plasmon (SP) extinction of discontinuous gold films is a crucial issue in the application of transmission surface plasmon resonance (T-SPR) spectroscopy to chemical and biological sensing. This derives from the usual sensing configuration, whereby an analyte binds to a selective receptor layer on the gold film at a certain distance from the metal surface. In the present work the distance sensitivity of T-SPR spectroscopy of 1.0-5.0 nm (nominal thickness) gold island films evaporated on silanized glass substrates is studied by using coordination-based self-assembled multilayers, offering thickness tuning in the range from approximately 1 to approximately 15 nm. The morphology, composition and optical properties of the Au/multilayer systems were studied at each step of multilayer construction. High-resolution scanning electron microscopy (HRSEM) showed no apparent change in the underlying Au islands, while atomic force microscopy (AFM) indicated flattening of the surface topography during multilayer construction. A regular growth mode of the organic layers was substantiated by X-ray photoelectron spectroscopy (XPS). Transmission UV-visible spectra showed an increase of the extinction and a red shift of the maximum of the SP band upon addition of organic layers, establishing the distance dependence of the Au SP absorbance. The distance sensitivity of T-SPR spectroscopy can be varied by using characteristic substrate parameters, that is, Au nominal thickness and annealing. In particular, effective sensitivity up to a distance of at least 15 nm is demonstrated with 5 nm annealed Au films. It is shown that intensity measurements, particularly in the plasmon intensity change (PIC) presentation, provide an alternative to the usually measured plasmon band position, offering good accuracy and the possibility of measuring at a single wavelength. The present distance sensitivity results provide the basis for further development of T-SPR transducers based on receptor-coated Au island films.

Highly confined electromagnetic fields in arrays of strongly coupled Ag nanoparticles

Linear arrays of very small Ag nanoparticles (diameter ~10 nm, spacing 0–4 nm) were fabricated in sodalime glass using an ion irradiation technique. Optical extinction spectroscopy of the arrays reveals a large polarization-dependent splitting of the collective plasmon extinction band. Depending on the preparation condition, a redshift of the longitudinal resonance as large as 1.5 eV is observed. Simulations of the three-dimensional electromagnetic field evolution are used to determine the resonance energy of idealized nanoparticle arrays with different interparticle spacings and array lengths. Using these data, the experimentally observed redshift is attributed to collective plasmon coupling in touching particles and/or in long arrays of strongly coupled particles. The simulations also indicate that for closely coupled nanoparticles (1–2 nm spacing) the electromagnetic field is concentrated in nanoscale regions (10 dB radius: 3 nm) between the particles, with a 5000-fold local field intensity enhancement. In arrays of 1-nm-spaced particles the dipolar particle interaction extends to over 10 particles, while for larger spacing the interaction length decreases. Spatial images of the local field distribution in 12-particle arrays of touching particles reveal a particlelike coupled mode with a resonance at 1.8 eV and a wirelike mode at 0.4 eV.

Probing of Porphyrin Surface Chemistry in Systems with Laser-Ablated Ag Nanoparticle Hydrosol: Role of Thiosulfate Anions

The influence of sodium thiosulfate (THS) concentration in Ag colloid/THS/H(2)TMPyP and Ag colloid/H2TMPyP/THS systems (H2TMPyP = 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin) was investigated by a combination of surface-enhanced resonance Raman scattering (SERRS) spectroscopy, surface plasmon extinction (SPE) measurements, and transmission electron microscopy (TEM). THS was found to have a strong impact on Ag nanoparticle surface structure and aggregation state and on interaction with H2TMPyP probe molecules, as evidenced by variations of the SERRS spectrum. In the Ag colloid/THS/H2TMPyP system, when laser-ablated Ag colloid was THS pretreated prior to the porphyrin addition, a critical threshold THS concentration (4 x 10(-5) M) was discovered. At concentrations below the threshold, THS mainly reduces the number of Ag+ adsorption sites. This leads to increased Ag nanoparticle aggregation prior to the porphyrin addition and significant weakening of the overall SERRS signal. Dominant contributions in the SERRS spectrum correspond to free base H2TMPyP and Ag+ containing the AgTMPyP form. At concentrations above the threshold, THS mediates also the formation and stabilization of new adsorption sites, probably Ag(0) sites. This induces a turn in the aggregation state of the pretreated Ag-c/THS system, an increase of the overall SERRS signal, and the appearance of a new spectral form of Ag metalated porphyrin.

In-situ study of Ag nanoparticle hydrosol optical spectra evolution during laser ablation/fragmentation

The results of in-situ monitoring of a laser fragmentation process of a largely polydisperse and morphologically heterogeneous citrate-reduced Ag hydrosol containing a fraction of Ag nanowires are presented. The laser fragmentation was performed using several wavelengths of the incident laser pulses (1064, 532 and 355 nm). Surface plasmon extinction spectra monitoring the nanoparticle fragmentation process were acquired pulse by pulse and related to transmission electron microscopy (TEM) images and statistical TEM image analysis of Ag nanoparticles collected in selected stages of the fragmentation. It was found that, due to different interactions of the laser pulses of various wavelengths with a specific fraction of the Ag nanoparticles in the hydrosol, the course of the fragmentation process depends on the wavelength, leading to different size distributions of the nanoparticles in the resulting hydrosol. The laser pulses of 532 nm wavelength are the most effective for the fragmentation process of the citrate-reduced Ag hydrosol, yielding the narrowest size distribution and the smallest mean radius of the Ag nanoparticles.

Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes.

Using electrodynamics calculations, we have discovered one dimensional array structures built from spherical silver nanoparticles that produce remarkably narrow ( approximately meV or less) plasmon resonance spectra upon irradiation with light that is polarized perpendicular to the array axis. The narrow lines require a minimum particle radius of about 30 nm to achieve. Variations of the plasmon resonance wavelength, extinction efficiency and width with particle size, array structure, interparticle distance and polarization direction are examined, and conditions which lead to the smallest widths are demonstrated. A simple analytical expression valid for infinite lattices shows that the sharp resonance arises from cancellation between the single particle width and the imaginary part of the radiative dipolar interaction.

Bimetallic (Ag)Au Nanoparticles Prepared by the Seed Growth Method: Two-Dimensional Assembling, Characterization by Energy Dispersive X-ray Analysis, X-ray Photoelectron Spectroscopy, and Surface Enhanced Raman Spectroscopy, and Proposed Mechanism of Growth

Layered core-shell bimetallic silver-gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2'-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.

Metal nanostructures with single molecule sensitivity in surface enhanced Raman scattering

Modified Raman microscope was used to correlate surface morphology, surface plasmon extinction spectra of Ag particles and Raman spectra from adsorbed molecules. All the hot particles providing large enhancement consist of Ag aggregates with a connecting axis parallel to the polarization of the excitation light. They give broad extinction peaks at ca. 500 nm from localized surface plasmon (LSP) to be in resonance at λ ex=488 nm. Moreover, huge enhancement is obtained at the junctions due to coupling of the LSP on neighboring Ag particles. Considerable number of blinking particles at room temperature showed the invariant signal intensity, i.e. bright and dark images, at 77 K. It indicates the blinking is thermally activated, most probably due to thermal diffusion of adsorbed molecules on the Ag particle with distinct enhancement as suggested by numerical simulations of the local electric field. In addition, DNA base molecules such as adenine, thymine, guanine or cytosine showed similar blinking features in SERS signal, which have no electronic transition in visible wavelength at a bulk state. Thus, SMS-SERS using a junction of touching Ag nanoparticles is quite promising to characterize various adsorbed species.

Characterization and surface-enhanced Raman spectral probing of silver hydrosols prepared by two-wavelength laser ablation and fragmentation

A four step Ag foil laser ablation-Ag nanoparticle fragmentation procedure in ultrapure water was carried out both under argon and in air. Pulses of a high power Nd/YAG laser were used for laser ablation (1064 nm) and for the three step Ag hydrosol treatment in the absence of Ag foil in the sequence 1064–532–1064 nm. Transmission electron microscopy (TEM) and surface plasmon (SP) extinction spectra provide evidence of Ag nanoparticle fragmentation in the second and third step of the procedure carried out under argon. While polydispersity of Ag hydrosol increases in the second step, both the polydispersity and the mean size of the nanoparticles are reduced in the third step. Qualitative and quantitative surface-enhanced Raman scattering (SERS)/surface-enhanced resonance Raman scattering (SERRS) spectral probing of systems with Ag hydrosols and the selected adsorbates at 514.5 nm excitation shows that Ag hydrosols obtained in the second step of the preparation procedure carried out in air are the most suitable substrates for SERS/SERRS experiments performed at this excitation wavelength.

Resonance absorption spectra of composites containing metal-coated nanoparticles

The possibility of controlling the selective absorbance of composites containing nanoparticles with a metallic shell is investigated. The dependence of spectral position and absolute value of surface plasmon extinction maximum on the particle radius and material of core and shell are analysed in the visible region. The effect of absorbed energy redistribution between external and internal surface plasmons is established to be a change in the ratio of dielectric core/matrix refractive indices.

Dichroic Thin Layer Films Prepared from Alkanethiol-Coated Gold Nanoparticles

A simple method for preparing thin dichroic metal composite films is described. Alkanethiol-derivatized gold particles are incorporated into poly(tetrafluoroethylene) (PTFE) matrixes that have been friction-transferred onto glass slides. Exposure of these gold particle/PTFE composites to flame (ca. 1700 °C) leads to strong dichroic properties in the visible spectrum. Extinction spectra obtained with the incident electric field polarized perpendicular to the direction of friction orientation show a plasmon extinction band with a maximum at ca. 545 nm. Spectra collected with the incident field polarized parallel to the friction orientation direction show a strong extinction band with a plateau from λ = 550 to 850 nm. The experimental polarization spectra are in general accord with the predictions of the Bruggeman effective medium approximation (EMA). EMA theory modeling suggests that the gold structures are not surrounded primarily by air, but rather are coated with, or imbedded within, a material that itse...

Plasmon extinction spectra of three-dimensional fractal clusters

Plasmon extinction spectra of three-dimensional fractal clusters