Surface-enhanced second-harmonic diffraction: Experimental investigation of selective enhancement

Nonlinear optics on nano/micro-hierarchical structures on metals: focus on symmetric and plasmonic effects

In this review, the authors study their creation of nano/micro-hierarchical structures on Ag and Ni substrates by femtosecond laser treatment and their investigation of their optical second-harmonic generation (SHG) signal intensities by SHG spectroscopy. The authors obtained the nanostructure-covered microgroove and microcube structures. These hierarchical surface structures were found to modify significantly the optical nonlinearity of the metal surfaces. The macroscopic symmetry of the surface’s shapes influenced SHG, and the excitation of surface plasmons enhanced SHG. On the other hand, the nanostructures on the microstructures had an additional effect on the generated SHG. For the microcube structures, the additional SHG emission was suppressed by removing a large amount of nanostructures. Left SHG was discussed by the effect of the propagating delay.

Nonlinear Surface Lattice Resonance in Plasmonic Nanoparticle Arrays

We study experimentally second-harmonic generation from arrays of split-ring resonators at oblique incidence and find conditions of more than 30-fold enhancement of the emitted second harmonic with respect to normal incidence. We show that these conditions agree well with a nonlinear Rayleigh-Wood anomaly relation and the existence of a surface lattice resonance at the second harmonic. The existence of a nonlinear surface lattice resonance is theoretically confirmed by extending the coupled dipole approximation to the nonlinear case. We further show that the localized surface plasmon modes that collectively contribute to the surface lattice resonance are inherently dark modes that become highly bright due to the collective interaction.

Enlightening surface plasmon resonance effect of metal nanoparticles for practical spectroscopic application

Pictorial depiction of applications of metal nanoparticles in different fields enlightening surface plasmon resonance effect.

Grating coupler integrated photodiodes for plasmon resonance based sensing

In this work, we demonstrate an integrated sensor combining a grating-coupled plasmon resonance surface with a planar photodiode. Plasmon enhanced transmission is employed as a sensitive refractive index (RI) sensing mechanism. Enhanced transmission of light is monitored via the integrated photodiode by tuning the angle of incidence of a collimated beam near the sharp plasmon resonance condition. Slight changes of the effective refractive index (RI) shift the resonance angle, resulting in a change in the photocurrent. Owing to the planar sensing mechanism, the design permits a high areal density of sensing spots. In the design, absence of holes that facilitate resonant transmission of light, allows an easy-to-implement fabrication procedure and relative insensitivity to fabrication errors. Theoretical and experimental results agree well. An equivalent long-term RI noise of 6.3 × 10(-6) RIU/√Hz is obtained by using an 8 mW He-Ne laser, compared to a shot-noise limited theoretical sensitivity of 5.61 × 10(-9) RIU/√Hz. The device features full benefits of grating-coupled plasmon resonance, such as enhancement of sensitivity for non-zero azimuthal angle of incidence. Further sensitivity enhancement using balanced detection and optimal plasmon coupling conditions are discussed.

Au nanoparticle based localized surface plasmon resonance substrates fabricated by dynamic shadowing growth

Au nanoparticle (NP) substrates, Au NP/TiO(2)/Au NP sandwich structures, and Ti coated Au NP substrates are fabricated by glancing angle deposition (GLAD) and oblique angle deposition (OAD) methods. Under the same deposition condition, the Au NP substrates produced by GLAD are more uniform and reproducible compared to those fabricated by OAD. The localized surface plasmon resonance (LSPR) wavelength of Au NP substrates can be easily tuned by changing the film thickness, the deposition angle, and the coating of the dielectric layer (TiO(2)) and metallic layer (Ti). In addition, the thickness and the deposition angle of the Ti coating on Au NP also affect the LSPR wavelength. Our results demonstrate that GLAD is a very versatile fabrication technique to produce reproducible and fine-tuned LSPR substrates.

Crystal growth from cluster to bulk materials via nanomaterials

In this short paper we propose the hypothesis that, in highly pure media, the cluster shape can be retained at various scales. Impurities and/or the additives can control the shape of the developing crystals by adsorption on selective sites.

Anti-biofouling properties of a telomer brush with pendent glucosylurea groups

A thiol-group-carrying telomer with pendent D-glucosylurea groups [poly(glucosylureaethyl methacrylate)-SH (PolyGUMA-SH)] was obtained by reversible addition-fragmentation chain-transfer (RAFT) polymerization of GUMA in the presence of 4,4'-azobis(4-cyanopentanoic acid) (initiator) and 4-cyanopentanoic acid dithiobenzoate (chain-transfer agent) and subsequent reduction with NaBH4. The thiol-carrying telomer was accumulated on both a gold electrode and a colloidal gold-immobilized glass substrate as proven by cyclic voltammetry using hydroquinone as a probe, and the increase in absorbance at 550 nm was ascribable to localized surface plasmon resonance (LSPR), respectively. The adsorption of various proteins to the surface of the telomer brush was examined by the LSPR method, too. The PolyGUMA brush showed a significant resistance against nonspecific adsorption of proteins, such as lysozyme, bovine serum albumin, immunoglobulin G, and fibrinogen. Furthermore, sugar-binding proteins, concanavalin A (Con A, with an affinity for mannose and glucose) and wheat germ agglutinin (WGA, with an affinity for N-acetylglucosamine), were not adsorbed to the GUMA-carrying brush, which is in contrast with the prompt and distinct binding of these proteins to the telomer brushes composed of 2-methacryloyloxyethyl D-glucopyranoside (Con A) and 1-O-(6'-methacrylamido)hexyl-2-N-acetoamido-2-deoxy-D-glucopyranoside residues (WGA). The glucosylurea-group-carrying telomer brush prepared here might be quite useful to provide a "bio-inert (anti-biofouling)" surface in biomedical fields.

Plasmonic band gap structures for surface-enhanced Raman scattering

Surface-enhanced Raman Scattering (SERS) of rhodamine 6G (R6G) adsorbed on biharmonic metallic grating structures was studied. Biharmonic metallic gratings include two different grating components, one acting as a coupler to excite surface plasmon polaritons (SPP), and the other forming a plasmonic band gap for the propagating SPPs. In the vicinity of the band edges, localized surface plasmons are formed. These localized plasmons strongly enhance the scattering efficiency of the Raman signal emitted on the metallic grating surfaces. It was shown that reproducible Raman scattering enhancement factors of over 10(5) can be achieved by fabricating biharmonic SERS templates using soft nano-imprint technique. We have shown that the SERS activities from these templates are tunable as a function of plasmonic resonance conditions. Similar enhancement factors were also measured for directional emission of photoluminescence. At the wavelengths of the plasmonic absorption peak, directional enhancement by a factor of 30 was deduced for photoluminescence measurements.

Interaction of wheat germ agglutinin with an N-acetylglucosamine-carrying telomer brush accumulated on a colloidal gold monolayer

A disulfide-carrying telomer with many pendent N-acetylglucosamine (GlcNAc) residues (Cys-PMHGlcNAc) was obtained by photo-polymerization of 1-(6'-methacryloylaminohexyl)-2-N-acetoamido-2-deoxy d-glucopyranoside) (MHGlcNAc) using a benzyl N,N-diethyldithiocarbamoyl (BDC) derivative that shows abilities of initiation, transfer, and termination (iniferter). The disulfide-carrying telomer was accumulated on a monolayer of colloidal Au on a glass substrate, and the interaction of wheat germ agglutinin (WGA) with GlcNAc residue at the polymer brush-solution interface was examined by using the localized surface plasmon resonance (LSPR) technique. For comparison, an amphiphile carrying many pendent GlcNAc residues was also prepared with MHGlcNAc and a lipophilic radical initiator and was incorporated in a phospholipid liposome to examine interaction of the GlcNAc residue with WGA on the liposome surface using turbidity measurements. Both the colloidal gold optical device and the liposome showed a concentration-dependent specific binding of WGA, and the GlcNAc-carrying liposome had a detection limit of 100 nM for WGA, whereas that of the colloidal gold device was 10nM. The sugar-carrying telomer-coated device examined here is not only useful as a simple biosensor chip but is also expected to expand our knowledge of bio-related phenomena at the liquid-telomer brush interfaces on a colloidal Au.

Size-selective synthesis and stabilization of gold organosol in C(n)TAC: enhanced molecular fluorescence from gold-bound fluorophores

Gold nanoparticles of variable sizes have been synthesized in toluene employing two-phase (water-toluene) extraction of AuCl4- followed by its reduction with sodium borohydride in the presence of a series of cationic surfactants of a homologous series having the general formula C(n)TAC. The solubility features of the gold particles in the organic solvent have been accounted qualitatively by calculating the van der Waals interaction potential between the particles. The effect of thermal energy and medium dielectric constant on the stability of metal particles has been studied by measuring the surface plasmon resonance. The stabilization of surfactant-mediated gold particles as hydrosol or organosol has been elucidated by considering the double-layer interaction as a function of the dielectric constant of the solvent medium. The influence of the counterion of the phase transfer reagent and stabilizing ligand on the photochemical stability of the gold colloids has been investigated. The fluorescence probe 1-methylaminopyrene (MAP) was considered for the surface functionalization of the gold particles, and it has been found that there is an enhancement of molecular fluorescence from the gold-probe assembly.

Localized Surface Plasmon Resonance Spectroscopy and Sensing

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance.

Recognition of sugars on surface-bound cap-shaped gold particles modified with a polymer brush

A dithiolated random copolymer with pendent phenylboronic acid residues (Cys-Poly(3-acrylamidophenylboronic acid-co-2-dimethylaminopropyl methacrylamide), Cys-Poly(APBA-co-DMAPMA)) obtained by photo-iniferter method was accumulated as a polymer brush on a cap-shaped gold particles deposited on a vacuum-evaporated gold film, and the usefulness of the polymer brush as a sensing element for glycoprotein, ovalbumin (OVA), was examined by using UV-vis spectroscopy with a help of surface plasmon resonance. A similar system was constructed with a dithiolated mannose-carrying polymer, dithiolated-poly(2-methacryloyloxyethyl-D-mannopyranoside) (DT-PMEMan), prepared by the atom transfer radical polymerization (ATRP). The brush composed of this polymer was examined as a sensing element for lectin, concanavalin A (Con A). The sensor cells modified with Cys-Poly(APBA-co-DMAPMA) and DT-PMEMan showed a concentration-dependent binding of OVA and Con A, respectively, with a comparable detection limit to those with a monolayer of polymer brush-coated gold particle deposited on a glass substrate. Using this system, it can be expected to open a new perspective to various functional polymer brushes fixed to the cap-shaped gold particle on a solid substrate.

Resistance of surface-confined telomers with pendent glucosylurea groups against non-specific adsorption of proteins

A disulfide-carrying random telomer with pendent D-glucosylurea groups (Cys-poly(glucosylureaethyl methacrylate-r-ureaethyl methacrylate), Cys-Poly(GUMA-r-UMA)) was obtained by UV-irradiation of GUMA and UMA in the presence of benzyl N,N-diethyldithiocarbamoyl (BDC) derivative which shows the abilities of initiation, transfer, and termination (iniferter). The disulfide-carrying telomer was accumulated on a gold electrode and colloidal gold-immobilized glass substrate, and the binding of various proteins to the surface of the polymer brush was examined by both cyclic voltammetry (CV) with hydroquinone as a probe, and localized surface plasmon resonance (LSPR) absorption method. The Cys-Poly(GUMA-r-UMA) brush did not show a significant non-specific adsorption of proteins such as bovine serum albumin and egg white lysozyme. Furthermore, sugar binding proteins, concanavalin A (with an affinity for mannose and glucose) and wheat germ agglutinin (with an affinity for N-acetylglucosamine), were only slightly adsorbed to the GUMA-carrying brush, which is in contrast with the prompt and distinct binding of these proteins to the brushes composed of 2-methacryloyloxyethyl D-glucopyranoside and 1-(6'-methacrylamido)hexyl-2-N-acetoamido-2-deoxy-D-glucopyranoside, respectively. The glucosylurea group-carrying telomer brush prepared here might be useful to provide a "bio-inert" surface in bio-medical fields.

Sensing Capabilities of Colloidal Gold Monolayer Modified with a Phenylboronic Acid-Carrying Polymer Brush

A dithiolated random copolymer with pendent phenylboronic acid residues [Cys-poly(3-acrylamidophenylboronic acid-co-N,N-dimethylaminopropyl methacrylamide), Cys-poly(APBA-co-DMAPMA)] that shows the abilities of initiation, transfer, and termination (iniferter) was obtained by using a benzyl N,N-diethyldithiocarbamoyl (BDC) derivative. The obtained disulfide-carrying copolymer was accumulated on a colloidal gold-immobilized glass substrate, and the usefulness of the polymer brush as a sensing element for glycoproteins such as ovalbumin (OVA) was examined by UV-visible spectrophotometry with the help of localized surface plasmon resonance (LSPR). The sensor showed a concentration-dependent binding of OVA with a detection limit of 100 nM, and it had a very high stability at high ionic strength. The sensor chip could be used for a detection of another glycoprotein, avidin, as well. Furthermore, the binding of biotin-modified human serum albumin (biotinylated HSA) to the avidin-phenylboronic acid- (PBA-) carrying polymer brush complex and further specific binding of anti-HSA immunoglobulin G to the biotinylated HSA-avidin-PBA-carrying polymer brush ternary complex could clearly be observed. The polymer-brush-coated device examined here not only was useful as a simple sensor chip, but also is expected to open a new perspective on interfacial phenomena performed by various functional polymer brushes fixed to colloidal gold on glass substrates.

Resistance of zwitterionic telomers accumulated on metal surfaces against nonspecific adsorption of proteins

Telomers of N,N-dimethyl-N-(3-sulfopropyl)-3'-methacryloylaminopropanaminium inner salt (SPB), 2-methacryloyloxyethyl phosphorylcholine (MPC), and N,N-dimethyl-N-(1-carboxymethyl)-2'-methacryloyloxylethanaminium inner salt (CMB) were prepared by UV irradiation in the presence of N,N,N',N'-tetraethylthiuram disulfide and a disulfide-group-carrying iniferter (a compound which pursues initiation, chain transfer, and termination), Cys-BDC, which had been prepared by coupling N,N-diethyldithiocarbamoylmethylbenzoic acid succinimidyl ester with cystamine dihydrochloride. The telomers formed a self-assembled monolayer (SAM) on a gold electrode and a monolayer of colloidal gold deposited on a glass plate, as confirmed by the increase in potential difference (DeltaE(p)) of the voltammogram for hydroquinone as a probe using cyclic voltammetry (CV) and the increase in absorbance using localized surface plasmon resonance (LSPR) absorption spectroscopy, respectively. Nonspecific adsorption of various proteins onto the surfaces of various telomer SAMs was examined from the decrease in peak current (DeltaI) using CV and the absorption increase at 550 nm using LSPR absorption spectroscopy. The zwitterionic telomer SAM in general did not adsorb proteins significantly, suggesting the usability of zwitterionic polymer SAMs and brushes to coat various materials used in biomedical fields. The correlation between the structure of water in the vicinity of zwitterionic telomers and the resistance of the zwitterionic telomer SAMs against the nonspecific adsorption of proteins was discussed.

Fishing of β-amylase with a SAM of α-cyclodextrin-poly- (ethylene glycol) conjugate

Alpha-cyclodextrin (alpha-CD) with an amino group was conjugated to an alpha, omega-dicarboxylated poly(ethylene glycol) (PEG). The inhibition constant (Ki) of the alpha-CD-PEG conjugate for the catalysis by beta-amylase was larger than that of alpha-CD, due to a steric obstruction of the PEG moiety to the binding of alpha-CD moiety to beta-amylase. alpha-CD-PEG was further modified with cystamine (CD-PEG-Cys) or cysteamine methyl disulfide (CD-PEG-MDS), and the disulfide-carrying alpha-CD-PEG was accumulated on a gold surface as a self-assembled monolayer (SAM). The binding of beta-amylase to the alpha-CD-PEG SAM was followed by a decrease in cathodic peak current in the voltammogram of hydroquinone as a probe using a cyclic voltammetry (CV). The beta-amylase bound to the alpha-CD-PEG SAM was desorbed by the addition of free alpha-CD, and the ratio of desorbed beta-amylase from the SAM of alpha-CD-PEG-Cys to the total amount of the enzyme bound to the SAM was 40% whereas that from the alpha-CD-PEG-MDS SAM was 83-85%. The percentage of desorption was increased to 100% by the treatment of the alpha-CD-PEG-MDS SAM-carrying electrode with 2-hydroxyethyldisulfide prior to the immersion in the enzyme solution. Adsorption and desorption processes of beta-amylase to the surface of alpha-CD-PEG-MDS SAM were clearly observed using localized surface plasmon resonance absorption spectroscopy. The binding constant of the enzyme to the surface-confined alpha-CD-PEG was much larger than that to free alpha-CD, probably due to a large local concentration of the alpha-CD moiety on the gold surface.

Sensing capabilities of colloidal gold modified with a self-assembled monolayer of a glucose-carrying polymer chain on a glass substrate

A disulfide-carrying polymer with pendent glucose residues (poly(2-methacryloyloxyethyl D-glucopyranoside)) was obtained by using a benzyl N,N-diethyldithiocarbamoyl derivative which shows the abilities of initiation, chain transfer, and termination (iniferter). The disulfide-carrying polymer was accumulated on a colloidal Au-immobilized glass substrate, and the usefulness of the polymer as a sensing element of concanavalin A (Con A) was examined by using a UV-visible spectrophotometer with the help of surface plasmon resonance. The sensor showed a concentration-dependent specific binding of Con A with a detection limit of 1.9 nM, and furthermore, it had a very high stability at high ionic strength. The polymer-coated device examined here was not only useful as a simple biosensor chip but is also expected to expand our knowledge of interfacial phenomena by introducing various functional polymers on colloidal Au.

Nanocrystal superlattices

Ordered arrays, or superlattices, of metallic, insulating, or semiconducting quantum dots, represent an exciting new class of materials. These superlattices are often referred to as artificial solids, in which the nanocrystals take the place of atoms in traditional solids, and the packing arrangement of the nanocrystals determines the unit cell parameters of the superstructure. In this review, we discuss various approaches toward assembling nanocrystal superlattices and we discuss their physical properties.