Size-dependent denaturing kinetics of bovine serum albumin adsorbed onto gold nanospheres

We have used localized surface plasmon resonance (LSPR) to monitor the kinetics of thermal denaturing of bovine serum albumin (BSA) adsorbed onto gold nanospheres of size 5 nm-100 nm. The effect of the protein on the LSPR was monitored by visible extinction spectroscopy. The wavelength of the peak extinction (resonance) is affected by the conformation of the adsorbed protein layer, and as such can be used as a very sensitive probe of thermal denaturing that is specific to the adsorbed (as opposed to free) protein. The time dependence of the denaturing is measured in the temperature range 60 °C–70 °C, and the lifetimes are used to calculate an activation barrier for thermal denaturing. The results show that thermally activated denaturing of proteins adsorbed onto nanoparticles has a nanoparticle-size-dependent activation barrier, and this barrier increases for decreasing particle size. This may have important implications for other protein-nanoparticle interactions.     

Study of electrical field distribution of gold-capped nanoparticle for excitation of localized surface plasmon resonance

The specific optical characteristics which can be observed from noble metal nanostructured materials such as nanoparticles and nanoislands have wide variety of applications such as biosensors, solar cells, and optical circuit. Because, these noble metal nanostructures induce the increment of light absorption efficiency by the enhancing effect of electrical field from localized surface plasmon resonance (LSPR) excitation. However, the enhancing effects of electrical field from LSPR using simple structured noble metal nanostructures for several applications are not satisfactory. To realize the more effective light absorption efficiency by the enhancing effect of electrical field, quite different noble metal nanostructures have been desired for applying to several applications using LSPR. In this study, to obtain the more effective enhancing effect of electrical field, conditions for LSPR excitation using a gold-capped nanoparticle layer substrate are computationally analyzed using finite-difference time-domain (FDTD) method. From the previous research, LSPR excitation using such gold-capped nanoparticle layer substrates has a great potential for application to high-sensitive label-free monitoring of biomolecular interactions. For understanding of detailed LSPR excitation mechanism, LSPR excitation conditions were investigated by analyzing the electrical field distribution using simulation software and comparing the results obtained with experimental results. As a result of computational analysis, LSPR excitation was found to depend on the particle alignment, interparticle distance, and excitation wavelength. Furthermore, the LSPR optical characteristics obtained from the simulation analysis were consistent with experimentally approximated LSPR optical characteristics. Using this gold-capped nanoparticle layer substrate, LSPR can be excited easily more than conventional noble metal nanoparticle-based LSPR excitation without noble metal nanoparticle synthesis. Hence, this structure is detectable a small change of refractive index such as biomolecular interactions for biosensing applications.     

Anomalous thermal denaturing of proteins adsorbed to nanoparticles

We have used localized surface plasmon resonance (LSPR) to monitor the structural changes that accompany thermal denaturing of bovine serum albumin (BSA) adsorbed onto gold nanospheres of size 5nm-60nm. The effect of the protein on the LSPR was monitored by visible extinction spectroscopy. The position of the resonance is affected by the conformation of the adsorbed protein layer, and as such can be used as a very sensitive probe of thermal denaturing that is specific to the adsorbed protein. The results are compared to detailed calculations and show that full calculations can lead to significant increases in knowledge where gold nanospheres are used as biosensors. Thermal denaturing on spheres with diameter > 20 nm show strong similarity to bulk calorimetric studies of BSA in solution. BSA adsorbed on nanospheres with d ⩽ 15nm shows a qualitative difference in behavior, suggesting a sensitivity of denaturing characteristics on local surface curvature. This may have important implications for other protein-nanoparticle interactions.     

Analysis of metallic nanoparticles embedded in thin film semiconductors for optoelectronic applications

This paper reports about a study of the local plasmonic resonance (LSPR) produced by metal nanoparticles embedded in a dielectric or semiconductor matrix. It is presented an analysis of the LSPR for different nanoparticle metals, shapes, and embedding media composition. Metals of interest for nanoparticle composition are Aluminum and Gold. Shapes of interest are nanospheres and nanotriangles. We study in this work the optical properties of metal nanoparticles diluted in water or embedded in amorphous silicon, ITO and ZnO as a function of size, aspect-ratio and metal type. Following the analysis based on the exact solution of the Mie theory and DDSCAT numerical simulations, it is presented a comparison with experimental measurements realized with arrays of metal nanospheres. Simulations are also compared with the LSPR produced by gold nanotriangles (Au NTs) that were chemically produced and characterized by microscope and optical measurements.     

Using Solution-Phase Nanoparticles, Surface-Confined Nanoparticle Arrays and Single Nanoparticles as Biological Sensing Platforms

The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe three sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of gold and silver nanoparticles. Specifically, the color associated with solution-phase nanoparticles, surface-confined nanoparticle arrays, and single nanoparticles will be shown to be tunable and useful as platforms for biological sensing.     

Nanoplasmonic sensing and QCM-D as ultrasensitive complementary techniques for kinetic corrosion studies of aluminum nanoparticles

Corrosion (oxidation) kinetics of Al nanodisks, 262 nm in diameter and 20 nm in height, was measured in degassed Milli-Q water at 23 °C and neutral pH by quartz crystal microbalance with dissipation monitoring (QCM-D) and nanoplasmonic sensing. The former detects the changes of the resonance frequency and the damping of the oscillation of a piezoelectric quartz crystal resonator. The latter detects the changes of the localized surface plasmon resonance (LSPR) in the metallic part of the Al nanoparticle, caused both by the shrinking metallic core and the changes in the dielectric environment as the oxide grows. Highly resolved kinetic data were obtained which show different corrosion stages. The two techniques yield complementary information not obtainable with one technique alone. Two main corrosion mechanisms, namely homogeneous oxide growth and nanoparticle fragmentation and roughening, are distinguished. The time dependence of the corrosion kinetics, determined using QCM-D, is in agreement with weight gain studies of bulk Al found in literature. The nanoplasmonic sensing measurements are compared to analytical model calculations of LSPR shifts which yield an estimate for the increase of oxide thickness during homogeneous oxide growth.     

Aggregation and adhesion of gold nanoparticles in phosphate buffered saline

In applications in medicine and more specifically drug delivery, the dispersion stability of nanoparticles plays a significant role on their final performances. In this study, with the use of two laser technologies, dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA), we report a simple method to estimate the stability of nanoparticles dispersed in phosphate buffered saline (PBS). Stability has two features: (1) self-aggregation as the particles tend to stick to each other; (2) disappearance of particles as they adhere to surrounding substrate surfaces such as glass, metal, or polymer. By investigating the effects of sonication treatment and surface modification by five types of surfactants, including nonylphenol ethoxylate (NP9), polyvinyl pyrrolidone (PVP), human serum albumin (HSA), sodium dodecyl sulfate (SDS) and citrate ions on the dispersion stability, the varying self-aggregation and adhesion of gold nanoparticles dispersed in PBS are demonstrated. The results showed that PVP effectively prevented aggregation, while HSA exhibited the best performance in avoiding the adhesion of gold nanoparticle in PBS onto glass and metal. The simple principle of this method makes it a high potential to be applied to other nanoparticles, including virus particles, used in dispersing and processing.     

Ag/Si核壳结构纳米颗粒粒子系的吸收特性研究

当电子振动频率与入射光的频率相同时,部分金属纳米颗粒可以在其表面激发局部表面等离子共振效应(LSPR),该波长下颗粒的吸收增强。这种效应也被应用于增强拉曼光谱信号的强度。本文研究了以Ag为外壳材料、Si为内核的核壳结构纳米颗粒粒子系的吸收特性。采用时域有限差分方法求解了颗粒随机分布粒子系的吸收率,分析了颗粒体积分数、内核外壳尺寸、椭球化等因素对粒子系吸收特性的影响以及对吸收峰的调控作用。     

Interaction of gold nanoparticles with bovine serum albumin

The interaction between gold nanoparticles and bovine serum albumin (BSA) in aqueous solutions was studied. The formation of nanoparticle—BSA associates was demonstrated, which is expressed in a bathochromic shift of the surface plasmon resonance band by 5–6 nm in the absorption spectrum. The results were approximated using the Drude model for metal spheres. The thickness of the dielectric (protein) shell of the nanoparticle and its permittivity (refractive index) were calculated.     

Matrix effects on the surface plasmon resonance of dry supported gold nanocrystals

We present a method to characterize surface-chemical properties of gold nanocrystals. Spherical, 60 nm gold nanocrystals were immobilized on quartz substrates by a coupling agent and cleaned in a hydrogen plasma. The nanocrystals were then functionalized with alkanethiol self-assembled monolayers (SAM) of varying chain lengths by adsorption from the gas phase, and localized surface plasmon resonance (LSPR) spectroscopy was performed on the samples. Depending on the alkanethiol chain length, the adsorption of the SAM redshifted the LSPR to different extents, in accordance with Mie theory. SAM thickness differences below 1 nm could be easily resolved. Our results demonstrate that LSPR spectroscopy can be applied to characterize thin organic layers on dry supported gold particles with high sensitivity.     

A quantitative modeling of the contributions of localized surface plasmon resonance and interband transitions to absorbance of gold nanoparticles

A quantitative modeling of the contributions of localized surface plasmon resonance (LSPR) and interband transitions to absorbance of gold nanoparticles has been achieved based on Lorentz–Drude dispersion function and Maxwell-Garnett effective medium approximation. The contributions are well modeled with three Lorentz oscillators. Influence of the structural properties of the gold nanoparticles on the LSPR and interband transitions has been examined. In addition, the dielectric function of the gold nanoparticles has been extracted from the modeling to absorbance, and it is found to be consistent with the result yielded from the spectroscopic ellipsometric analysis.     

Peptide-induced patterning of gold nanoparticle thin films

In this work, the use of patterned proteins and peptides for the deposition of gold nanoparticles on several substrates with different surface chemistries is presented. The patterned biomolecule on the surface acts as a catalyst to precipitate gold nanoparticles from a precursor solution of HAuCl₄ onto the substrate. The peptide patterning on the surfaces was accomplished by physical adsorption or covalent attachment. It was shown that by using covalent attachment with a linker molecule, the influence of the surface properties from the different substrates on the biomolecule adsorption and subsequent nanoparticle deposition could be avoided. By adjusting the reaction conditions such as pH or HAuCl₄ concentration, the sizes and morphologies of deposited gold nanoparticle agglomerates could be controlled. Two biomolecules were used for this experiment, 3XFLAG peptide and bovine serum albumin (BSA). A micro-transfer molding technique was used to pattern the peptides on the substrates, in which a pre-patterned poly(dimethylsiloxane) (PDMS) mold was used to deposit a lift-off pattern of polypropylmethacrylate (PPMA) on the various substrates. The proteins were either physically adsorbed or covalently attached to the substrates, and an aqueous HAuCl₄ solution was applied on the substrates with the protein micropatterns, causing the precipitation of gold nanoparticles onto the patterns. SEM, AFM, and Electron Beam Induced Current (EBIC) were used for characterization.     

Concurrent Detection of Protein Adsorption on Mixed Nanoparticles by Differential Centrifugal Sedimentation

In mixtures of nanoparticles of various sizes or compositions, monitoring protein partitioning on their surfaces provides important information about particle–protein interactions during competitive adsorption. Utilizing the size‐resolving capability of differential centrifugal sedimentation, the adsorption of bovine serum albumin on multisize gold nanoparticles with diameters ranging from 20 to 100 nm or gold, silver, and silica nanoparticles with similar diameter can be concurrently observed. This method can be used to gain insight into nanoparticle–protein interactions based on analyses of curvature and relative abundance.     

Laser-induced synthesis of silver nanoparticles and their conjugation with protein

Partially oxidized spherical silver nanoparticles (AgNPs) of different size are prepared by pulsed laser ablation in water and directly conjugated to protein S-ovalbumin for the first time and characterized by various optical techniques. UV–Visible spectrum of AgNPs showed localized surface plasmon resonance (LSPR) peak at 396 nm which red shift after protein addition. Further the increased concentration of AgNPs resulted a decrease in intensity and broadening of S-ovalbumin peak (278 nm), which can be related to the formation of protein NPs complex caused by the partial adsorption of S-ovalbumin on the surface of AgNPs. The red shift in LSPR peak of AgNPs after mixing with S-ovalbumin and decrease in protein-characteristic peak with increased silver loading confirmed the formation of protein–AgNPs bioconjugates. The effect of laser fluence on the size of AgNPs and nanoparticle–protein conjugation in the size range 5–38 nm is systematically studied. Raman spectra reveal broken disulphide bonds in the conjugated protein and formation of Ag–S bonds on the nanoparticle surface. Fluorescence spectroscopy showed quenching in fluorescence emission intensity of tryptophan residue of S-ovalbumin due to energy transfer from tryptophan moieties of albumin to AgNPs. Besides this, small blue shift in emission peak is also noticed in presence of AgNPs, which might be due to complex formation between protein and nanoparticles. The binding constant (K) and the number of binding sites (n) between AgNPs and S-ovalbumin have been found to be 0.006 M^?1 and 7.11, respectively.     

Effects of thickness & shape on localized surface plasmon resonance of sexfoil nanoparticles

Localized surface plasmon(LSPR) resonance and sensing properties of a novel nanostructure(sexfoil nanoparticle)are studied using the finite-difference time-domain method. For the sandwich sexfoil nanoparticle, the calculated extinction spectrum shows that with the thickness of the dielectric layer increasing, long-wavelength peaks blueshift, while shortwavelength peaks redshift. Strong near-field coupling of the upper and lower metal layers leads to electric and magnetic field resonances; as the thickness increases, the electric field resonance gradually increases, while the magnetic field resonance decreases. The obtained refractive index sensitivity and figure of merit are 332 nm/RIU and 3.91 RIU~(-1), respectively. In order to obtain better sensing ability, we further research the LSPR character of monolayer Ag sexfoil nanoparticle. After a series of trials to optimize the thickness and shape, the refractive index sensitivity approximates 668 nm/RIU, and the greatest figure of merit value comes to 14.8 RIU~(-1).     

金银三层纳米管局域表面等离激元共振特性研究

相对于单一金属纳米材料,二金属复合纳米材料具有更大的潜在应用价值.基于时域有限差分方法,研究了SiO₂-Ag-Au和SiO₂-Au-Ag二金属三层纳米管的消光光谱,并对其局域表面等离激元共振(Localized Surface Plasmon Resonance,LSPR)特性进行了分析.研究发现,内核尺寸变大将导致上述两种金属纳米管LSPR峰红移;内层金属及外层金属壳层厚度增大均会导致其LSPR峰蓝移.银壳厚度变化对纳米管LSPR的调制作用大于金壳厚度变化造成的影响.上述现象可以利用等离激元杂化理论及自由电子和振荡电子变化的竞争机制进行分析.     

Trapping and release of citrate-capped gold nanoparticles

An electrical method to trap and release charged gold nanoparticles onto and from the surface of gold electrodes modified by an alkanethiol self-assembled monolayer (SAM) is presented. To form electrodes coated with gold nanoparticles (GNPs), amine-terminated SAMs on gold electrodes were immersed in a solution of negatively charged citrate-capped GNPs. Accumulation of GNPs on the electrode surface was monitored by a decrease in the impedance of the SAM-modified electrode and by an increase in the electrochemical activity at the electrode as shown through cyclic voltammetry (CV). Electrostatic interactions between the GNPs and the amine-terminated SAM trap the GNPs on the electrode surface. Application of a subsequent negative bias to the electrode initiated a partial release of the GNPs from the electrode surface. Impedance spectroscopy, cyclic voltammetry, ultraviolet-visible (UV-Vis) spectroscopy and atomic force microscopy (AFM) were used to monitor and confirm the attraction of GNPs to and release from the aminealkanethiolated gold electrodes. This work describes a method of trapping and release for citrate-capped GNPs that could be used for on-demand nanoparticle delivery applications such as in assessing and modeling nanoparticle toxicology, as well as for monitoring the functionalization of gold nanoparticles.     

Confined clustering of AuCu nanoparticles under ambient conditions

The study of bimetallic clusters has been increasing in recent years due to the wide range of novel applications, when compared with monometallic nanoparticles This paper presents a simple method of low toxicity to obtain bimetallic nanoparticle clusters of AuCu. The localized surface plasmon resonance (LSPR) of the nanoalloy was located in a value in between those reported for AuNP and CuNP. Raman bands of nanoparticle clusters of AuCu were observed in the range 130-300 cm^?1. The structural and optical analysis of the synthesized nanoparticles confirmed the presence of the bond Au-Cu, with a particle size ranging from 1 to 2 nm. The geometries and vibrational modes were predicted for small metallic clusters of Au, Cu and bimetallic AuCu. The AgCu nanoparticles have been recently implemented in glasses to study optical properties, as well as antibacterial applications.     

Correlating SAXS analysis with LSPR behavior: poly(vinyl alcohol)-stabilized Ag nanoparticles

We explore the localized surface plasmon resonance (LSPR) behavior of aqueous sols of PVA-stabilized AgNPs synthesized in presence of glucose as a reducing agent. The in situ synthesis parameters are tuned by adding increasing amounts of NaOH, which causes interesting variations in pH, LSPR curve broadening, and the evolution of AgNPs embedded PVA cross-linked nanostructures. Small angle X-ray scattering (SAXS) analysis is used to investigate sol aggregate characteristics. In all samples, mass fractal aggregates of two radius of gyration are formed. Assuming spherical form, the SAXS nanoparticle size distribution predicts a range from 6 to 60 nm. However, only the fractal aggregate features closely correspond to the observed LSPR characteristics.     

Enhanced chiral response from the Fabry–Perot cavity coupled meta-surfaces

The circular dichroism(CD) signal of a two-dimensional(2D) chiral meta-surface is usually weak, where the difference between the transmitted(or reflected) right and left circular polarization is barely small. We present a general method to enhance the reflective CD spectrum, by adding a layer of reflective film behind the meta-surface. The light passes through the chiral meta-surface and propagates towards the reflector, where it is reflected back and further interacts with the chiral meta-surface. The light is reflected back and forth between these two layers, forming a Fabry–Perot type resonance,which interacts with the localized surface plasmonic resonance(LSPR) mode and greatly enhances the CD signal of the light wave leaving the meta-surface. We numerically calculate the CD enhancing effect of an L-shaped chiral meta-surface on a gold film in the visible range. Compared with the single layer meta-surface, the L-shaped chiral meta-surface has a CD maximum that is dramatically increased to 1. The analysis of reflection efficiency reveals that our design can be used to realize a reflective circular polarizer. Corresponding mode analysis shows that the huge CD originates from the hybrid mode comprised of FP mode and LSPR. Our results provide a general approach to enhancing the CD signal of a chiral meta-surface and can be used in areas like biosensing, circular polarizer, integrated photonics, etc.