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.     

Effective medium-based analysis of nanowire-mediated localized surface plasmon resonance

We explore a nanowire-based localized surface plasmon resonance (LSPR) sensor system using an effective medium for the nanowire layer. The effective medium is obtained based on the far-field characteristics of the nanowire-based LSPR system. Near-field properties as well as the sensitivity performance of the effective medium-based SPR structure are compared to exact results of the nanowire-based LSPR system. The results indicate that an effective medium can reproduce the far-field and near-field characteristics of nanowires fairly well, while it represents the nanowire-based LSPR on a limited basis in terms of sensitivity characteristics, particularly when the LSPR is significantly enhanced.     

Palm-sized biodetection system based on localized surface plasmon resonance

We present a complete palm-sized, battery-operated biodetection system based on highly sensitive localized surface plasmon resonance (LSPR). We have replaced the spectrum analyzer by four pulse-powered light-emitting diodes (LED), each with different emission spectra. The reflected light beams from all LEDs were detected by a single photodiode. Its composite output current was demultiplexed by a four-channel lock-in amplifier. Device performance was demonstrated using an LSPR chip covered with a mixture of ethanol/water and 2-propanol/water at different concentrations. The miniaturized system does not require any external power supply or personal computer and it is therefore suitable for point-of-care and field applications.     

Doubly localized surface plasmon resonance in bimodally distributed silver nanoparticles

Growth of bimodally distributed silver nanoparticles using sequential physical vapour deposition (PVD) is reported. Growth conditions of nanoparticles are defined in the following three steps: In the first step, nanoparticles are grown at a heated substrate and then exposed to atmosphere, in the second step, nanoparticles are vacuum annealed and finally re-deposition of silver is performed in the third step. This special way of deposition leads to the formation of bimodally distributed nanoparticles. It has been investigated that by changing the deposition time, different sets of bimodally distributed nanoparticles can be grown. Localized surface plasmon resonance (LSPR) of such bimodally distributed nanoparticles generates double plasmon resonance peaks with overlapped absorption spectra. Double plasmon resonance peaks provide a quick indication of the existence of two sets of nanoparticles. LSPR spectra of such bimodally distributed nanoparticles could be modeled with double Lorentz oscillator model. Inclusion of double Lorentz oscillator model indicates that there exist two sets of non-interacting nanoparticles resonating at different plasma frequencies. It is also reported that silver nanoparticles grown at a heated substrate, again attain the new shape while being exposed to atmosphere, followed by vacuum annealing at the same temperature. This is because of physisorption of oxygen at the silver surface and change in surface free energy. The re-shaping due to the adsorbed oxygen on the surface is responsible for bimodal size distribution of nanoparticles.     

Application of surface plasmon resonance sensing to studying elastohydrodynamic lubricant films

We present a new technique based on the spectral characteristics associated with the surface plasmon resonance (SPR) effect for studying lubricants in elastohydrodynamic (EHD) dimples. The pressure inside the EHD dimple causes a localized change of the refractive index (RI) of the entrapped lubricant. This also results in a shift in the spectral SPR absorption dip. By monitoring the color changes within the SPR image, one can obtain a direct measurement of the RI of the entrapped lubricant, from which the pressure distribution within the elastohydrodynamic lubrication (EHL) dimple can be deduced by means of a predetermined relation of pressure and RI of the tested lubricant. Dimples formed with the lubricants PB 2400 and H 1900 were studied in our experiments. Because SPR is sensitive only to the RI variation within a thin region (approximately one wavelength) close to the sensor's surface, the new technique does not require any measurement of the absolute film thickness of the lubricant. This is much simpler than the existing two-beam interferometric technique for measuring the RI of lubricants in EHD dimples, which requires simultaneous measurements of optical film thickness by use of two beams of different angles of incidence. In light of this advantage we anticipate that the new technique can be applied to pressure field mapping in highly loaded rolling and sliding EHL contacts.     

Plasmon line shaping using nanocrosses for high sensitivity localized surface plasmon resonance sensing

The detection of small changes in the wavelength position of localized surface plasmon resonances in metal nanostructures has been used successfully in applications such as label-free detection of biomarkers. Practical implementations, however, often suffer from the large spectral width of the plasmon resonances induced by large radiative damping in the metal nanocavities. By means of a tailored design and using a reproducible nanofabrication process, high quality planar gold plasmonic nanocavities are fabricated with strongly reduced radiative damping. Moreover, additional substrate etching results in a large enhancement of the sensing volume and a subsequent increase of the sensitivity. Coherent coupling of bright and dark plasmon modes in a nanocross and nanobar is used to generate high quality factor subradiant Fano resonances. Experimental sensitivities for these modes exceeding 1000 nm/RIU with a Figure of Merit reaching 5 are demonstrated in microfluidic ensemble spectroscopy.     

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.     

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.     

Origin of strong and narrow localized surface plasmon resonance of copper nanocubes

Nano Research - Inexpensive copper nanoparticles are generally thought to possess weak and broad localized surface plasmon resonance (LSPR). The present experimental and theoretical studies show...     

A localized surface plasmon resonance based immunosensor for the detection of casein in milk

In this research, a localized surface plasmon resonance (LSPR) immunosensor based on gold-capped nanoparticle substrate for detecting casein, one of the most potent allergens in milk, was developed. The fabrication of the gold-capped nanoparticle substrate involved a surface-modified silica nanoparticle layer (core) on the slide glass substrate between bottom and top gold layers (shell). The absorbance peak of the gold-capped nanoparticle substrate was observed at ～520 nm. In addition, the atomic force microscopy (AFM) images demonstrated that the nanoparticles formed a monolayer on the slide glass. After immobilizing anti-casein antibody on the surface, our device, casein immunosensor, could be applied easily for the detection of casein in the raw milk sample without a difficult pretreatment. Under the optimum conditions, the detection limit of the casein immunosensor was determined as 10 ng/mL. Our device brings several advantages to the existing LSPR-based biosensors with its easy fabrication, simple handling, low-cost, and high sensitivity.     

A localized surface plasmon resonance based immunosensor for the detection of casein in milk

In this research, a localized surface plasmon resonance (LSPR) immunosensor based on gold-capped nanoparticle substrate for detecting casein, one of the most potent allergens in milk, was developed. The fabrication of the gold-capped nanoparticle substrate involved a surface-modified silica nanoparticle layer (core) on the slide glass substrate between bottom and top gold layers (shell). The absorbance peak of the gold-capped nanoparticle substrate was observed at ∼520 nm. In addition, the atomic force microscopy (AFM) images demonstrated that the nanoparticles formed a monolayer on the slide glass. After immobilizing anti-casein antibody on the surface, our device, casein immunosensor, could be applied easily for the detection of casein in the raw milk sample without a difficult pretreatment. Under the optimum conditions, the detection limit of the casein immunosensor was determined as 10 ng/mL. Our device brings several advantages to the existing LSPR-based biosensors with its easy fabrication, simple handling, low-cost, and high sensitivity.     

Improving the instrumental resolution of sensors based on localized surface plasmon resonance

The colorimetric variations induced upon changes in interfacial refractive index of nanoscale noble metal structures exhibiting localized surface plasmon resonance (LSPR) provides a convenient means of label-free, affinity-based detection of biomolecular recognition reactions. However, despite being similar in nature to conventional SPR, LSPR has so far suffered from significantly lower data quality in terms of its signal-to-noise ratio (S/N) in typical biomolecular recognition analysis. In this work, generic data analysis algorithms and a simple experimental setup that provide a S/N upon protein binding that is comparable to that of state-of-the art SPR systems are presented. Specifically, it is demonstrated how temporal variations (rate approximately 0.5 Hz) in parameters proportional to the resonance peak position can be recorded simultaneously, yielding a peak position precision of <5 x 10(-4) nm and an extinction noise level of <5 x 10(-6) absorbance units (Abs). This, in turn, is shown to provide a S/N of approximately 2000 (equivalent to a detection limit of <0.1 ng/cm(2)) for typical protein binding reactions. Furthermore, the importance of utilizing changes in both peak position and magnitude is highlighted by comparing different LSPR active noble metal architectures that respond differently to bulk and interfacial refractive index changes.     

In situ sensing of single binding events by localized surface plasmon resonance

Single binding events of nanoparticle-labeled DNA strands were detected as stepwise peak shifts in localized surface plasmon resonance by single particle measurement. We confirmed the number of binding events by observing label particles by scanning electron microscopy. Our simulation based on a multiple multipole program showed that the peak shift is dependent on interparticle gap size and binding position. The experimental peak shift distribution was also reproduced by simulation.     

利用表面等离子激元共振生物传感器研究聚合酶与敏感表面上自组装的DNA的相互作用

利用SPR生物传感器研究了人的型DNA聚合酶与DNA模板一引物二聚体以及单链DNA的相互作用情况.同时,观察了两种抑制剂(神经酸和亚油酸)对这些相互作用的影响.结果表明,神经酸和亚油酸可以使β聚合酶与DNA二聚体的亲和力分别下降20倍和5倍,神经酸的抑制作用更加明显.利用这些方法,有助于清楚了解抑制剂对β聚合酶与DNA相互作用的影响和抗癌及抗病毒药物的研制.     

Bimetallic multifunctional core@shell plasmonic nanoparticles for localized surface plasmon resonance based sensing and electrocatalysis

Smart bimetallic core@shell nanoparticles were fabricated based on gold nanoparticles (AuNPs) decorated with pH-sensitive polymer shell. Concretely, AuNPs having poly(4-vinylpyridine) (P4VP) on the surface were first fabricated through surface-initiated atom transfer radical polymerization (SI-ATRP). Then, they were mixed with selected metal precursor solutions followed by reduction using reducing agent. The metal NPs thus introduced were uniformly distributed in P4VP polymer shells. In order to explore the diversity and viable function of the resultant nanostructures, we controlled the size of AuNP, pH, selectivity of metal precursors, etc. We investigated the structural alteration during the sequential synthetic process. The bimetallic nanostructures of AuNP@P4VP nanocomposites containing another type of metal NP at the P4VP periphery exhibit a controlled sensing property in terms of the change in the refractive index of surrounding media and a typical electrocatalytic activity for methanol oxidation reaction.     

Label-free DNA biosensor based on localized surface plasmon resonance coupled with interferometry

In this report, we developed a new optical biosensor in connection with a gold-deposited porous anodic alumina (PAA) layer chip. In our sensor, we observed that the gold deposition onto the chip surface formed a "caplike" layer on the top of the oxide nanostructures in an orderly fashion, so we called this new surface formation a "gold-capped oxide nanostructure". As a result of its interferometric and localized surface plasmon resonance properties, the relative reflected intensity (RRI) at surface of the chip resulted in an optical pattern that was highly sensitive to the changes in the effective thickness of the biomolecular layer. We demonstrated the method on the detection of picomolar quantities of untagged oligonucleotides and the hybridization with synthetic and PCR-amplified DNA samples. The detection limit of our PAA layer chip was determined as 10 pM synthetic target DNA. The capability of observing both RRI increment and wavelength shift upon biomolecular interactions promises to make our chip widely applicable in various analytical tests.     

Analysis of biomolecular interactions using a miniaturized surface plasmon resonance sensor

A commercially available miniaturized surface plasmon resonance sensor has been investigated for its applicability to biological interaction analysis. The sensor was found to exhibit excellent repeatability and linearity for high-refractive index solutions and good reproducibility for the binding of proteins. Its detection limit for the monoclonal antibody M1 was found to be 2.1 fmol, which corresponds to a surface concentration of 21 pg/mm2. Simple surface immobilization procedures relying on biotin/avidin or glycoprotein/lectin chemistry have been explored. Equilibrium dissociation constants for the binding of the FLAG peptide to its monoclonal antibody (M1) and for the binding of concanavalin A to a glycoprotein have been determined. The close agreement of these measurements with values obtained by surface fluorescence microscopy and fluorescence correlation spectroscopy helps to validate the use of this device. Thus, this sensor shows promise as an inexpensive, portable, and accurate tool for bioanalytical applications in laboratory and clinical settings.     

Simultaneous excitation of propagating and localized surface plasmon resonance in nanoporous gold membranes

Materials multifunctionality for optical sensing of adsorbates has obvious advantages-in addition to the potential for greater sensitivity, the different length scales associated with a variety of optical phenomena allow a greater variety of adsorption characteristics to be examined. Here, we show that ultrathin (approximately 100 nm) nanoporous gold membranes possess features of both planar metal films that exhibit propagating SPR excitations and nanofeatured metals that exhibit localized SPR excitations. This is the first report of such multifunctionality in an optically active metal. We give illustrative examples of using this material to probe biorecognition reactions and to probe the structure evolution of a layer-by-layer deposition of charged dendrimers. Our results are consistent with the very different lengths of the tail of the evanescent field decays associated with each of these plasmon excitation modes.     

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.