Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors

A nanowire-based surface plasmon resonance (SPR) is investigated as a structure that offers improved sensor performance. The results calculated by rigorous coupled-wave analysis on a model using a hexanedithiol self-assembled monolayer (SAM) indicate that the resonant coupling between localized surface plasmons (LSPs) of nanowires affects the sensitivity enhancement substantially, while the LSP resonance in a single nanowire also contributes. SPR characteristics change significantly by applying a SAM, which can give rise to zero sensitivity for a given SAM. The results suggest that a properly designed nanowire-based SPR biosensor can enhance sensitivity by an order of magnitude with reasonable detection properties.     

Field-matter integral overlap to estimate the sensitivity of surface plasmon resonance biosensors

We have analyzed the effectiveness of field-matter integral overlap between target index distribution and local near-fields to assess detection sensitivity of surface plasmon resonance (SPR) biosensors. The correlation of the overlap with sensitivity was clear. An overlap integral defined with lateral electric field intensity produced the highest correlation due to tangential continuity across a boundary. Among the three detection scenarios considered, the correlation for localized SPR sensing was slightly lower than that of thin film-based detection and improved with an increased fill factor in the structure. The results will be useful to maximize the optical signature created by target interactions and to produce highest sensitivity of SPR detection to variations when target or field distribution is not uniform.     

Refractive-index and thickness sensitivity in surface plasmon resonance spectroscopy

The sensitivities of surface plasmon resonance spectroscopy were examined at incident angles of 66-76 deg. The sensitivities were calculated for various refractive indices of liquid samples and for various thicknesses of deposited dielectric thin layers. Furthermore, the sensitivities were confirmed experimentally. The experimentally measured refractive indices and thicknesses were 1.3311-1.3463 and 0-89 nm, respectively. From these results it was demonstrated that the sensing system showed higher sensitivities with smaller incident angles. For example, the sensitivity for a refractive index at a 66 degrees incident angle was seven times larger than that at 76 degrees . It was also demonstrated that the resonant wavelength has a linear relation to the refractive index and the thickness at refractive indices and thicknesses of 1.33-1.345 and 0-100 nm, respectively, except for a 66 degrees incident angle.     

Target dependence of the sensitivity in periodic nanowire-based localized surface plasmon resonance biosensors

We investigate the target dependence of the sensitivity in a localized surface plasmon resonance (LSPR) biosensor and compare it with that of a conventional thin-film-based plasmon resonance structure. An LSPR biosensor was modeled as subwavelength periodic nanowires on a metal/dielectric substrate and targets either as bulk refractive index changes or as a biomolecular interaction that forms a monolayer. The results found that significant target-dependent variation arises in sensitivity and sensitivity enhancement by LSPR. The variation is attributed to the nonlinearity in the plasmon dispersion relation as well as the effective permittivity due to strong LSPR signals. The target dependence suggests that an LSPR structure be designed based on estimated index changes induced by target interactions. Associated broadening of resonance width can be controlled by way of profile engineering, which is discussed in connection with experimental data.     

Improving the sensitivity limit of surface plasmon resonance biosensors by detecting mixed interference signals

We demonstrate that the sensitivity limit of intensity-based surface plasmon resonance (SPR) biosensors can be enhanced when we combine the effects of the phase and amplitude contributions instead of detecting the amplitude variation only. Experimental results indicate that an enhancement factor of as much as 20 times is achievable, yet with no compromise in measurement dynamic range. While existing SPR biosensor systems are predominantly based on the angular scheme, which relies on detecting intensity variations associated with amplitude changes only, the proposed scheme may serve as a direct system upgrade approach for these systems. The new measurement scheme may therefore lead to a strong impact in the design of SPR biosensors.     

Electrochemical thinning of thicker gold film with qualified thickness for surface plasmon resonance sensing

To meet the requirement of surface plasmon resonance (SPR) sensing, controlling the thickness of the gold film is very important. Here, we report an efficient and simple approach to prepare a SPR-active substrate when the thickness of the gold film is larger than the optimizing 50 nm and smaller than 100 nm. This method is based on anodic electrodissolution of gold in electrolyte containing chloride ions. Using this method, the thickness of gold films can be easily changed at a nanometer scale by controlling the number of potential scans and the concentrations of chloride ions in the electrolyte. At the same time, the influence of gold film thickness on the SPR signal is recorded by SPR in real time. To assess the change of the surface roughness and morphology of gold film through anodic electrodissolution, atomic force microscopy was used. The surface roughness of the same Au film before and after anodic electrodissolution is 1.179 and 2.767 nm, respectively. The change of the surface roughness of Au film brings out a slight angle shift of SPR. This indicates that surface electrodissolution of the gold does not affect the character of the original bulk film and this film can be used for SPR experiments. To confirm our expectation, a simple adsorption experiment of cytochrome c (Cyt c) on the gold film treated with anodic electrodissolution modified by 11-mercaptoundecanic acid was carried out. The angle shift of SPR confirmed the adsorption of Cyt c, and the cyclic voltammetry of Cyt c provided a complementary confirmation for the adsorption of Cyt c. These results show that this approach provides a good way to change the thicker gold film to an optimized thickness of SPR sensing. The great advantage brought by this approach is in that it can convert the waste gold films with greater thicknesses fabricated by the vacuum deposition method or other methods into useful materials as active SPR substrates.     

Profile effect on the feasibility of extinction-based localized surface plasmon resonance biosensors with metallic nanowires

We investigate the effect of the cross-sectional profile of an array of metallic nanowires on the feasibility of a localized surface plasmon resonance (LSPR) biosensor. Calculations were performed using rigorous coupled-wave analysis with an emphasis on the extinction properties of the LSPR structure. The results indicate that the nanowire structure, particularly that of a T-profile, delivers an extremely linear sensing performance over a wide range of the target refractive index with much enhanced sensitivity. The extinction-based LSPR structure also involves a relatively large dimension and thus is expected to provide a feasible biosensor using current semiconductor technology.     

Comparative study of Ag and Au nanoparticles biosensors based on surface plasmon resonance phenomenon

The specific sensitivity of surface plasmon resonance to changes in the local environment of nanoparticles allows their use as platforms to probe chemical and biochemical binding events on their surfaces without any labeling [1], [2], [3], [4]. In this paper, we perform a comparative study of gold and silver nanoparticle based biosensors, prepared within the same conditions, in order to determine which metal seems the best for biological sensing. The prototypical biocytin–avidin interaction is used to study gradual changes over time and with avidin concentration in the absorption spectra bands of biocytinylated 10 nm silver and gold nanospheres. First, the Ag nanoparticles plasmon resonance absorbance signal is about 10 times larger than the Au one. Secondly, for an equivalent concentration of avidin, the optical property modifications are more pronounced for silver nanoparticles than for gold ones of the same geometry. These observations attest the superiority of Ag on Au nanoparticles when optical considerations are only taken into account. Finally, with both biosensors, the specificity of the interaction, checked by replacing avidin with bovine serum albumin, is relatively poor and needs to be improved.     

Designed biointerface using near-infrared quantum dots for ultrasensitive surface plasmon resonance imaging biosensors

The surface plasmon resonance imaging chip biointerface is fully designed using near-infrared (NIR) quantum dots (QDs) for the enhancement of surface plasmon resonance imaging (SPRi) signals in order to extend their application for medical diagnostics. The measured SPRi detection signal following the QD binding to the surface was amplified 25-fold for a 1 nM concentration of single-stranded DNA (ssDNA) and 50-fold for a 1 μg/mL concentration of prostate-specific antigen (PSA), a cancer biomarker, thus substantiating their wide potential to study interactions of a diverse set of small biomolecules. This significant enhancement is attributed to the QD's mass-loading effect and spontaneous emission coupling with propagating surface plasmons, which allowed the SPRi limit of detection to be reduced to 100 fM and 100 pg/mL for ssDNA and PSA, respectively. Furthermore, this study illustrates the potential of SPRi to be easily integrated with fluorescent imaging for advanced correlative surface-interaction analysis.     

影响贵金属纳米颗粒表面等离子体共振因素评述

贵金属纳米颗粒具有可调的共振吸收谱,被广泛用于光能传送器、近场扫描光学显微学、表面增强谱学、化学和生物传感器等。系统地评述了颗粒尺寸、颗粒分布、颗粒形状、颗粒体积分数、颗粒组成和颗粒结构等因素对金属纳米颗粒等离子体共振吸收性能的影响,有利于深入理解等离子体共振吸收的物理实质和实现对等离子体共振频率的调控。     

A kinetic study of analyte-receptor binding and dissociation for surface plasmon resonance biosensors applications

A fractal analysis, which takes into account the effect of surface heterogeneity brought about by ligand immobilization on the reaction kinetics in surface plasmon resonance (SPR) biosensors, is presented. The binding and dissociation of estrogen receptors (ERs), ERa and ER/spl alpha/ and ER/spl beta/, in solution to different ligands immobilized on the SPR biosensor is analyzed within the fractal framework. The heterogeneity on the biosensor surface is made quantitative by using a single number, the fractal dimension D/sub f/. The analysis provides physical insights into the binding of these receptors to different ligands and compounds, particularly the endocrine disrupting compounds (EDCs). These EDCs have deleterious effects on humans and on wildlife. Single- and dual-fractal models were employed to fit the ER-binding data obtained from the literature. Values of the binding and dissociation rate coefficient and fractal dimensions were obtained from a regression analysis provided by Corel Quattro Pro, 8.0. Values for the affinity K/sub D/(=k/sub d//k/sub a/) were also calculated. This provides us with some extra flexibility in designing biomolecular assays. The analysis should provide further information on the mode of action and interaction of EDCs with the ERs. This would help in the design of agents and modulators against these EDCs.     

Nonregeneration protocol for surface plasmon resonance: study of high-affinity interaction with high-density biosensors

Surface plasmon resonance (SPR) has been used in determining kinetics and thermodynamics of biological interaction in the past decades. One difficulty encountered in this technology is the need for a proper regeneration, which means the removal of analytes from the bound complexes to regenerate the activity of the ligands. Regeneration is not always practical since the harsh regeneration reagents may destroy the bioactivity of the ligands. It is even more difficult for complexes with high affinity constants. In this paper, we report a nonregeneration protocol for SPR techniques in which subsequent ligand/analyte interactions can be measured without regeneration; thus ligand biological activity could be retained. Kinetics, binding models, and mathematics of this protocol are discussed in detail using rabbit IgG as the analyte and engineered recombinant antibody A10B single-chain fragment variables (scFv) as the ligand. The affinity constant of rabbit IgG binding with A10B scFv measured by using a nonregeneration protocol was (2.5 +/- 0.2) x 10(7) M(-1), which was comparable with the value determined with a conventional regeneration SPR method ((2.2 +/- 1.5) x 10(7) M(-1)) and quartz crystal microbalance (1.9 x 10(7) M(-1)). A paradigm of streptavidin-biotin binding was analyzed to validate this protocol. The affinity constant for each binding subunit of streptavidin to the immobilized biotin was determined to be (7.3 +/- 0.2) x 10(6) M(-1), which was comparable with the solution-based value of 2 x 10(7) M(-1). The nonregeneration protocol requires a relatively high ligand density on the biosensor surface so that more data points can be obtained before surface saturation. The small size of scFv enables them to be constructed in the biosensors for such purpose.     

Scanning electrochemical microscopy combined with surface plasmon resonance: studies of localized film thickness variations and molecular conformation changes

The combination of scanning electrochemical microscopy (SECM) with surface plasmon resonance (SPR) is described. By oxidizing ferrocenylalkanethiol self-assembled monolayer (SAM) with SECM-generated Ce4+, the coupled technique, SECM-SPR, is shown to be viable for determining local variations in thin film thickness. Factors (tip/substrate distance, tip potential scan rate, and solution composition change) affecting the SECM-SPR response and operation are also discussed. The approach was further extended to the determination of conformational changes of cytochrome c molecules attached electrostatically onto a negatively charged SAM during its reduction by the tip-generated methyl viologen monocation. The high sensitivity of the SPR equipped with a bicell detector facilitates the measurement of infinitesimal film thickness changes accompanying redox reactions, while the SECM provides a means to obviate the necessity of applying a potential to the SPR substrate, which tends to cause unwanted interferences and complications. The approach also affords an avenue for determining film thickness variations that are not subject to certain effects, such as the surface charge, the heterogeneity of the substrate, and the distance between the redox center of the immobilized molecule and the underlying substrate electrode.     

Improved sensitivity of wavelength-modulated surface plasmon resonance biosensor using gold nanorods

In this report, gold nanorods (GNRs) were used to enhance the sensitivity of the wavelength-modulated surface plasmon resonance (SPR) biosensor. The GNRs were designed and fabricated through seed-medicated growth and surface activation by a layer of a weak polyelectrolyte, poly(acrylic acid) for the attaching antibody. Rabbit anti-goat IgG was immobilized on GNRs, and sandwich assays were carried out to detect goat IgG using a wavelength-modulated SPR biosensor. The detection sensitivity of the nanorod-conjugated antibody is 25-100 times more sensitive than the SPR biosensor without GNRs. Drastic sensitivity enhancement, owing to the electromagnetic interaction between the nanotag and the sensing film, was maximized using the longitudinal plasmonic resonance of the GNRs. GNRs could significantly enhance the sensitivity of the SPR biosensor, and the maximum enhancement effect can be achieved when the longitudinal SPR peak wavelength of GNRs functionally matches the surface plasmon wavelength.     

具有等离子共振效应的纳米金属在光解水中的应用

简述光催化分解水反应的基本原理、肖特基结和等离子体共振的概念、等离子体共振在光解水过程的工作机制。介绍Au和Ag等贵金属复合半导体、单一银系化合物、非贵金属等离子体光催化剂。提出具有等离子共振效应的金属纳米颗粒能提高光解水性能的原因是形成肖特基结和具有独特的局域表面等离子共振效应;相比半导体,金属的功函数较大,形成的肖特基结可实现电荷定向迁移;局域表面等离子共振效应通过共振能量转移、增强局部场强和热电子注入等方式拓宽光谱吸收,提高电荷分离效率和光催化效率。认为等离子光催化的研究目前主要集中在Au、Ag等贵金属,其原理有待深入探究,且应理论与实验相结合;开发非贵金属替代物将更有利于降低成本。     

Theoretical limit of localized surface plasmon resonance sensitivity to local refractive index change and its comparison to conventional surface plasmon resonance sensor

In this paper, the theoretical sensitivity limit of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment is discussed. The presented theoretical analysis of the LSPR phenomenon is based on perturbation theory. Derived results can be further simplified assuming quasistatic limit. The developed theory shows that LSPR has a detection capability limit independent of the particle shape or arrangement. For a given structure, sensitivity is directly proportional to the resonance wavelength and depends on the fraction of the electromagnetic energy confined within the sensing volume. This fraction is always less than unity; therefore, one should not expect to find an optimized nanofeature geometry with a dramatic increase in sensitivity at a given wavelength. All theoretical results are supported by finite-difference time-domain calculations for gold nanoparticles of different geometries (rings, split rings, paired rings, and ring sandwiches). Numerical sensitivity calculations based on the shift of the extinction peak are in good agreement with values estimated by perturbation theory. Numerical analysis shows that, for thin (≤10 nm) analyte layers, sensitivity of the LSPR is comparable with a traditional surface plasmon resonance sensor and LSPR has the potential to be significantly less sensitive to temperature fluctuations.     

Sequential monitoring of film thickness variations with surface plasmon resonance imaging and imaging ellipsometry constructed with a single optical system

Sequential surface plasmon resonance imaging (SPRI) and imaging ellipsometry (IE) measurements are realized with a single set of optical components mounted on a goniometer. The sample situated on top of a triangular prism is positioned in the center of the goniometer. The resultant setup (SPRI/IE) can be used to examine the same surface region above (IE) and below (SPRI) the sample. Thickness values of silver stripes sputtered onto a gold substrate were determined by SPRI and IE, and the results were compared to establish the validity of the method. The SPRI/IE setup was also used to monitor the thickness of phospholipid films of different layer numbers. SPRI measurements were found to be more accurate for ultrathin films, whereas the IE results are more reliable for films whose thicknesses approach or exceed the distance encompassed by the evanescent wave of the surface plasmon. Thus, utilizing these two techniques sequentially facilitates the continuous monitoring of film thickness variation over a wide thickness range with high fidelity and provides a viable approach to image the surface regions with large topographic fluctuations.     

Microsurface plasmon resonance biosensing based on gold-nanoparticle film

We use a gold-nanoparticle coated film to achieve highly spatially resolved biosensing that is based on localized surface-plasmon resonance. Unlike the planar gold film employed for conventional surface-plasmon resonance sensing, the gold-nanoparticle film relies exclusively on shifting of the peak extinction wavelength for detection of biointeraction and does not depend critically on the angle of incidence. These characteristics permit integration of surface-plasmon resonance with large-numerical-aperture optics to achieve biosensing with high sensitivity and spatial resolution as high as 25 microm.     

Differential-phase surface plasmon resonance biosensor

In this paper, a novel differential-phase-sensitive surface plasmon resonance biosensor (DP-SPRB) is proposed and developed, in which a two-frequency laser is integrated with a differential amplifier in order to analytically convert the phase modulation into amplitude modulation. With the use of the conventional envelope detection technique, the differential phase is precisely decoded in real time in terms of the demodulated amplitude. In order to verify high detection sensitivity of the DP-SPRB, a sucrose-water solution and glycerin-water solution at low concentrations were both tested, and the experimental results confirm that the detection sensitivity on wt % concentration of the sucrose solution is 0.00001%. Moreover, the real-time monitoring mouse IgG/antimouse IgG interaction shows the minimum concentration of mouse IgG to be at 10 fg/mL. To our knowledge, this is the highest sensitivity ever measured by a surface plasmon resonance biosensor. However, because of the limited dynamic range of DP-SPRB, it can only apply to biomolecule interactions at extremely low concentration.