Biocatalytically induced growth of gold nanoshells: using enzyme reaction for the controllable fabrication of nanomaterials

In the present work, the enzymatically controlled growth process of gold nanoshells (GNSs) in the presence of O2/glucose/glucose oxidase (GOx) and its chloroaurate ion electron acceptor is described. The biocatalytically stimulated growth process is one of the bio-inspired synthetic procedures directed by biological molecules which occur under ambient conditions. It is found that hydrogen peroxide (H2O2) could enlarge the gold nanoparticles (GNPs) on the surface of GNSs precursor composites, of which the preadsorbed GNPs serve as nucleation sites for further gold deposition. Here, GOx is harnessed for its unparalled level of catalytic activity and substrate specificity while H2O2 is produced as a by-product during the oxidation of D-glucose to gluconic acid by GOx. Then the bio-generated H2O2 is used as the reducing agent in the catalytic deposition process of GNSs formation. During the procedure, the localized surface plasmon resonance peaks range across hundreds of nanometers from visible to near infrared region accompanying by the resultant formation of uniform and continuous core-shell nanostructures. The corresponding optical, morphological and enzyme kinetic properties are all well investigated. The novel protocol offers a new perspective for the bio-directed synthesis method in nanotechnology.     

Electrochemical synthesis of gold nanoparticles onto indium tin oxide glass and application in biosensors

A simple one-step method for the electrochemical deposition of gold nanoparticles (GNPs) onto bare indium tin oxide film coated glass substrate without any template or surfactant was investigated. The effect of electrolysis conditions such as potential range, temperature, concentration and deposition cycles were examined. The connectivity of GNPs was analyzed by UV-Vis absorption spectroscopy and scanning electron microscopy. The nanoparticles were found to connect in pairs or to coalesce in larger numbers. The twin GNPs display a transverse and a longitudinal localized surface plasmon resonance (LSPR) band, which is similar to that of gold nanorods. The presence of longitudinal LSPR band correlates with high refractive index sensitivity. Conjugation of the twin-linked GNPs with albumin bovine serum-biotin was employed for the detection of streptavidin as a model based on the specific binding affinity in biotin/streptavidin pairs. The spectrophotometric sensor showed concentration-dependent binding for streptavidin.     

Wafer scale imprint uniformity evaluated by LSPR spectroscopy: a high volume characterization method for nanometer scale structures

We exploit the localized surface-plasmon resonance (LSPR) of terahertz gold gammadion structures for wafer scale critical dimension metrology of nanostructures. The proposed characterization method, LSPR spectroscopy, is based on optical transmission measurements and is benchmarked against numerical simulations of imprinted structures characterized by atomic force microscopy. There is a fair agreement between the two methods and the simulations enable the translation of optical spectra to critical dimensions of the physical structures, a concept known from scatterometry. The results demonstrate the potential of LSPR spectroscopy as an alternative characterization method to scanning electron microscopy, atomic force microscopy and scatterometry.     

Probe for sensitive direct determination of sulphide ions based on gold nanoparticles

A highly selective and sensitive optical sensor based on localised surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) for the determination of a sulphide ion in aqueous solution is presented. The existence of sulphide is change the intensity of the LSPR band around 520 nm. In this sensor, sulphide ions were recognised and measured by UV–vis spectrophotometry. Three factors such as pH, time and concentration of AuNPs have been studied. The optimisation of effective parameters is done by one at a time method. This method was simple, rapid and cost efficient for the detection of sulphide. The linear range is 1.00–10.00 (4.16–41.63 µM) ppm with the correlation coefficient of 0.9874 and the detection limit of 0.54 ppm. The relative standard deviation of the reported method is 1.01%.Inspec keywords: surface plasmon resonance, chemical sensors, pH, nanoparticles, spectrophotometry, gold, optical sensors, nanosensors, nanofabrication, photodetectors, nanophotonics, optical fabrication, ultraviolet spectra, visible spectraOther keywords: gold nanoparticles, localised surface plasmon resonance, LSPR band, highly sensitive optical sensor, time method, sulphide ion determination, highly selective optical sensor, aqueous solution, UV–vis spectrophotometry, optical correlation coefficient, pH, Au     

Sensitive localized surface plasmon resonance multiplexing protocols

Herein are reported two new protocols to obtain different zones of localized surface plasmon resonance (LSPR) gold nanostructures on single glass substrate by using a vacuum evaporation technique followed by a high-temperature annealing (550 °C). The thickness of the gold film, considered as the essential parameter to determine specific LSPR properties, is successfully modulated. In the first protocol, a metal mask is integrated onto the glass substrate during vacuum evaporation to vary the gold film thickness by a "shadowing effect", while in the second protocol several evaporation cycles (up to four cycles) at predefined areas onto the single substrate are performed. The resulting gold-modified samples are characterized using a transmission UV-vis extinction optical setup and scanning electron microscopy (SEM). The size distribution histograms of nanoparticles are also acquired. By employing the first protocol, thanks to the presence of different zones of gold nanoparticles on a single substrate, optimized LSPR responses to different (bio)functionalization zones are rapidly screened. Independently, the second protocol exhibited an excellent correlation between the nominative evaporated gold film thickness, gold nanoparticle sizes, and plasmonic properties (resonant wavelength and peak amplitude). Such substrates are further used in the construction of LSPR immunosensors for the detection of atrazine herbicide.     

Organic-dye-coupled magnetic nanoparticles encaged inside thermoresponsive PNIPAM Microcapsules

We present a new approach for the fabrication of thermoresponsive polymer microcapsules with mobile magnetic cores that undergo a volume phase-transition upon changing the temperature and are collected under an external magnetic field. We have prepared organic/inorganic composite microspheres with a well-defined core-shell structure that are composed of a crosslinked poly(N-isopropylacrylamide) (PNIPAM) shell and silica cores dotted centrally by magnetite nanoparticles. Since the infiltration of template-decomposed products is dependent on the permeability of PNIPAM shells triggered by changes of exterior temperature, the silica layer sandwiched between the magnetic core and the PNIPAM shell was quantitatively removed to generate PNIPAM microcapsules with mobile magnetic cores by treatment with aqueous NaOH solution. For development of the desired multifunctional microcapsules, modification of the unetched silica surface interiors can be realized by treatment with a silane coupling agent containing functional groups that can easily bind to catalysts, enzymes, or labeling molecules. Herein, fluorescein isothiocyanate (FITC), which is a common organic dye, is attached to the insides of the mobile magnetic cores to give PNIPAM microcapsules with FITC-labeled magnetic cores. In this system, it can be expected that an extension of the functionalization of the cavity properties of smart polymer microcapsules is to immobilize other target molecules onto the mobile cores in order to introduce other desired functions in the hollow cage.     

Detection of phosphopeptides by localized surface plasma resonance of titania-coated gold nanoparticles immobilized on glass substrates

We herein demonstrate a new sensing method for phosphopeptides by localized surface plasmon resonance (LSPR) using titania-coated gold nanoparticles immobilized on the surface of a glass slide as the sensing substrate and using UV-visible spectrophotometry as the detection tool. Titania has been known to be an effective substrate for binding with phosphorylated species. The detection principle is the shift of wavelength of optical absorption due to SPR of the gold nanoparticles induced by binding of phosphorylated species with titania on the surface of the gold nanoparticles. The feasibility of the approach is demonstrated by detection of tryptic digest products of beta-casein and milk. Gold nanoparticles coated with thin films of titania, immobilized on a glass slide, can selectively bind traces of phosphopeptides from complex samples, resulting in a wavelength shift of the absorption band in the SPR spectrum with good reproducibility. The LSPR results are confirmed by matrix-assisted laser desorption/ionization mass spectrometry. The detection limit for the tryptic digest product of beta-casein is 50 nM.     

光纤端面可调LSPR金纳米半球壳阵列的制备

提出了一种具有可调局域表面等离子体共振(LSPR)特性的光纤端面纳米结构的制备方法.利用二维胶体聚苯乙烯球(PS)单层膜的可转移性,结合溅射沉积,在光纤端面上获得了金纳米阵列结构.扫描电子显微镜和透射电子显微镜图像表明,单个纳米颗粒具有半球壳结构,阵列结构呈现六角密堆积排列;光谱测量结果显示,制备的样品具有明显的LSPR效应,改变溅射时间可实现对LSPR峰位从可见至近红外的调控,且金纳米半球壳直径改变时其LSPR峰位调控规律相似,重复测量结果表明制备的纳米结构具有良好的稳定性.     

Biological sensing and interface design in gold island film based localized plasmon transducers

Discontinuous, island-type gold films (typically < or = 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.     

Label-free detection of leptin antibody-antigen interaction by using LSPR-based optical biosensor

In the recent research, the development of optical biosensing devices has been focused on finding new method and technologies to exploit the optical properties of noble metal nanostructure, especially localized surface plasmon resonance (LSPR). In this study, we fabricated a LSPR-based label-free optical biosensor with the multi-spot gold-capped nanoparticle array (MG-NPA) biochip based on the deposition of a thin gold (Au) film on the silica nanoparticles layer with the simple process. The MG-NPA biochip used the silica nanoparticles as the core and a thin Au film as a shell on the surface. This structure can excite the LSPR signal easily with the high reproducibility. The anti-leptin antibody was immobilized on the surface of MG-NPA biochip, which could recognize only leptin antigen. The leptin antibody-antigen interaction was performed by the introduction of different concentration (1 pg/mL-100 microg/mL) of leptin antigen solutions for 1 h. The detection limit was found to be 100 pg/mL by using the anti-leptin antibody immobilized MG-NPA biochip. This LSPR-based label-free optical biosensor employing the MG-NPA biochip brings several advantages such as low cost, easy to fabricate, using a simple optical system and can be applied in a wide immunoassay with the similar antibody-antigen model.     

A gold-nanoparticle-enhanced immune sensor based on fiber optic interferometry

A method using gold nanoparticles (GNPs) to enhance fiber optic interferometry (GNPFOI) for immune-sensing is reported in this paper. It is suggested that an enlarged index mismatch and an elongated optical path by GNPs conjugated on recognition proteins will contribute most to signal enhancement in the interference fringe shift. Theoretical and experimental results show that the interference fringe shift is linearly related to both the amount and size of the GNPs binding on the sensor surface. The detected signal for 30?nm GNPs can reach a lowest detection limit of 18?pM (10(10)?particles?ml(-1)). Immune-sensing for rabbit IgG as the antigen to anti-rabbit IgG has been demonstrated and a detection cycle has been completed by elution buffer for surface regeneration. The repeatability of the immune-sensing on one GNPFOI sensor has also been verified by three identical cycles, and the detection limit for 13?nm GNPs conjugated anti-rabbit IgG reaches 0.17?nM (～25.5?ng?ml(-1)). The sensory mechanism has the potential to be engineered on the tip of a needle-type micro-device, which would allow it to monitor immune recognition signals in the future.     

Coinage Metals in the Nanometer Length Scale

Cetylpyridinium chloride (CPC)-stabilized gold organosol in toluene has been prepared by using a two-phase (water-toluene) extraction of AuCl₄^- followed by its reduction with sodium borohydride in the presence of the surfactant, CPC. The surfactant-stabilized gold nanoparticles were exploited to examine their optical properties when exposed to various solvent systems by measuring the changes in the localized surface plasmon resonance (LSPR) spectrum.     

Ordered Arrangement and Optical Properties of Silica‐Stabilized Gold Nanoparticle–PNIPAM Core–Satellite Clusters for Sensitive Raman Detection

Gold–polymer hybrid nanoparticles attract wide interest as building blocks for the engineering of photonic materials and plasmonic (active) metamaterials with unique optical properties. In particular, the coupling of the localized surface plasmon resonances of individual metal nanostructures in the presence of nanometric gaps can generate highly enhanced and confined electromagnetic fields, which are frequently exploited for metal‐enhanced light–matter interactions. The optical properties of plasmonic structures can be tuned over a wide range of properties by means of their geometry and the size of the inserted nanoparticles as well as by the degree of order upon assembly into 1D, 2D, or 3D structures. Here, the synthesis of silica‐stabilized gold–poly(N‐isopropylacrylamide) (SiO₂‐Au‐PNIPAM) core–satellite superclusters with a narrow size distribution and their incorporation into ordered self‐organized 3D assemblies are reported. Significant alterations of the plasmon resonance are found for different assembled structures as well as strongly enhanced Raman signatures are observed. In a series of experiments, the origin of the highly enhanced signals can be assigned to the interlock areas of adjacent SiO₂‐Au‐PNIPAM core–satellite clusters and their application for highly sensitive nanoparticle‐enhanced Raman spectroscopy is demonstrated.     

Probing dynamically tunable localized surface plasmon resonances of film-coupled nanoparticles by evanescent wave excitation

The localized surface plasmon resonance (LSPR) spectrum associated with a gold nanoparticle (NP) coupled to a gold film exhibits extreme sensitivity to the nanogap region where the fields are tightly localized. The LSPR of an ensemble of film-coupled NPs can be observed using an illumination scheme similar to that used to excite the surface plasmon resonance (SPR) of a thin metallic film; however, in the present system, the light is used to probe the highly sensitive distance-dependent LSPR of the gaps between NPs and film rather than the delocalized SPR of the film. We show that the SPR and LSPR spectral contributions can be readily distinguished, and we compare the sensitivities of both modes to displacements in the average gap between a collection of NPs and the gold film. The distance by which the NPs are suspended in solution above the gold film is fixed via a thin molecular spacer layer and can be further modulated by subjecting the NPs to a quasistatic electric field. The observed LSPR spectral shifts triggered by the applied voltage can be correlated with angstrom scale displacements of the NPs, suggesting the potential for chip-scale or flow-cell plasmonic nanoruler devices with extreme sensitivity.     

Plasmonic Heterodimers with Binding Site‐Dependent Hot Spot for Surface‐Enhanced Raman Scattering

A novel plasmonic heterodimer nanostructure with a controllable self‐assembled hot spot is fabricated by the conjugation of individual Au@Ag core–shell nanocubes (Au@Ag NCs) and varisized gold nanospheres (GNSs) via the biotin–streptavidin interaction from the ensemble to the single‐assembly level. Due to their featured configurations, three types of heterogeneous nanostructures referred to as Vertice, Vicinity, and Middle are proposed and a single hot spot forms between the nanocube and nanosphere, which exhibits distinct diversity in surface plasmon resonance effect. Herein, the calculated surface‐enhanced Raman scattering enhancement factors of the three types of heterodimers show a narrow distribution and can be tuned in orders of magnitude by controlling the size of GNSs onto individual Au@Ag NCs. Particularly, the Vertice heterodimer with unique configuration can provide extraordinary enhancement of the electric field for the single hot spot region due to the collaborative interaction of lightning rod effect and interparticle plasmon coupling effect. This established relationship between the architecture and the corresponding optical properties of the heterodimers provides the basis for creating controllable platforms which can be exploited in the applications of plasmonic devices, electronics, and biodetection.     

Localized surface plasmon resonance optical characteristics for hydrogen peroxide using polyvinylpyrrolidone coated silver nanoparticles

In this study, monitoring of localized surface plasmon resonance (LSPR) optical characteristics and dispersion condition change for hydrogen peroxide using polyvinylpyrrolidone (PVP)-coated silver nanoparticles (NPs) was described. PVP-coated silver NPs exhibit the specific light absorption in visible region. Hence, using LSPR optical characteristics, several applications such as optoelectronics, food control and life science can be realized. In addition, by introducing hydrogen peroxide solution into the PVP-coated silver NP dispersion, LSPR optical characteristics were drastically changed. From these LSPR optical characteristic changes of PVP-coated silver NPs for hydrogen peroxide, in this study, observation of dispersion kinetics of PVP-coated silver NPs was carried out. As a result, aggregation which is attributed by the radical polymerization of PVP layer could be observed by introduction of hydrogen peroxide. In addition, silver cluster structure which is included in the PVP layer was stably contained in the aggregated PVP layer. From these optical characteristic change and dispersion kinetics, this PVP-coated silver NPs have great potential for application to biosensing applications as a color indicator.     

Material-selective surface chemistry for nanoplasmonic sensors: optimizing sensitivity and controlling binding to local hot spots

Optical sensors utilizing the principle of localized surface plasmon resonance (LSPR) offer the advantage of a simple label-free mode of operation, but the sensitivity is typically limited to a very thin region close to the surface. In bioanalytical sensing applications, this can be a significant drawback, in particular since the surface needs to be coated with a recognition layer in order to ensure specific detection of target molecules. We show that the signal upon protein binding decreases dramatically with increasing thickness of the recognition layer, highlighting the need for thin high quality recognition layers compatible with LSPR sensors. The effect is particularly strong for structures that provide local hot spots with highly confined fields, such as in the gap between pairs of gold disks. While our results show a significant improvement in sensor response for pairs over single gold disks upon binding directly to the gold surface, disk pairs did not provide larger signal upon binding of proteins to a recognition layer (already for around 3 nm thin layers) located on the gold. Local plasmonic hot spots are however shown advantageous in combination with directed binding to the hot spots. This was demonstrated using a structure consisting of three surface materials (gold, titanium dioxide, and silicon dioxide) and a new protocol for material-selective surface chemistry of these three materials, which allows for controlled binding only in the gap between pairs of disks. Such a design increased the signal obtained per bound molecule by a factor of around four compared to binding to single disks.     

Synthesis of size-tunable gold nanoparticles by poly(vinylphenol) and electrostatic multilayer deposition of the gold-poly(vinylphenol) nanocomposites

Water-dispersible gold nanoparticles (GNPs) were synthesized by the polymer in situ redox technique at room temperature using poly(4-vinylphenol) (PVPh) as a simultaneous template/stabilizer and reducing agent. GNPs were characterized by its surface plasmon absorption peak at 536 nm. The average particle size of the gold-poly(4-vinylphenol) (Au-PVPh) nanocomposites decreases with increase in the ratio of PVPh to gold salt. A uniform multilayer of Au-PVPh nanocomposites was fabricated on glass surface by electrostatic layer-by-layer assembly using poly(diallyldimethylammonium chloride) (PDDAC) as the oppositely charged polycation. UV-vis spectra of the consecutive multilayer showed that the absorbance at 549 nm corresponding to GNPs increases linearly with the number of Au-PVPh nanocomposite layers, indicating regular bilayer growth.     

Regenerating optical properties of individual gold nanoparticles in alcoholic solvents without any surfactant

We compared the optical properties of gold nanoparticles (GNPs) in various solvents with those of strawberry-like composite particles (Au/SiO2) consisting of a silica core and single attached GNPs. The results show that Au/SiO2 without any surfactant could regenerate well optical properties of individual GNPs in alcoholic solvents. By the electrophoretic light-scattering measurements, the high dispersibility of Au/SiO2 composite particles dispersed in alcoholic solvents has been demonstrated. In addition, using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, we proposed a possible mechanism to qualitatively account for the dispersibility of Au/SiO2 in organic solvents such as alcoholic solvents and cyclohexane, which may provide an opportunity to manipulate optical signals of single nanoparticle in organic solvents.     

Enhancement of localized surface plasmon resonance detection by incorporating metal-dielectric double-layered subwavelength gratings

In this study, we investigated the enhanced sensing performance of a localized surface plasmon resonance (LSPR) biosensor by employing metal-dielectric double-layered subwavelength grating structures. The numerical results showed that the LSPR substrate with a dielectric spacer can provide not only a better sensitivity but also a significantly improved reflectance characteristic. While the presence of metallic gratings leads to a broad and shallow reflectance curve inevitably, the dielectric spacer can prevent the propagating surface plasmons from being interfered by the locally enhanced fields excited at the gold gratings, finally resulting in a strong and deep absorption band at resonance. Therefore, the proposed structure could potentially open a new possibility of the enhanced LSPR detection for monitoring biomolecular interactions of low molecular weights.