Localized Surface Plasmon Resonance Shift Research Articles

Enhancing absorption properties of composite nanosphere and nanowire arrays by localized surface plasmon resonance shift

Nanoparticles with nonmetallic core and metallic shell can improve the spectral solar absorption efficiency for traditional working fluids, due to the localized surface plasmon resonance (LSPR) effect exists at the surfaces of these core-shell composite nanoparticles. In this work, the effect of geometry and material, and hence the LSPR effect, on the optical absorption properties of core-shell nanostructures was numerically demonstrated by the finite difference time domain method. The nanostructures were formed by varying the inner and outer radii of the composite nanospheres and nanowires and by changing the particle spacing for their arrays. The result indicates that varying the inner radius itself can tune the absorption efficiency factors of the nanostructures monotonously, while an optimal outer radius may exist for maximizing the absorption efficiency factors. It also shows that varying the inner radius itself can widen the absorption spectrums for the arrays, but the absorptance tends to increase with decreasing inner radius or particle spacing. Meanwhile, the second absorption peaks may be observed for nanowires or nanosphere/nanowire arrays, which can be tuned by the resonance shifts induced by the change of either inner or outer radius and hence the LSPR effect. The coupled LSPR effect under studied can be efficiently utilized for tuning the optical absorption properties of nanoparticles used in many applications including photothermal conversion, and perspective also exists for many other applications including surface-enhanced Raman spectroscopy (SERS) enhancement.

PEGylated Artificial Antibodies: Plasmonic Biosensors with Improved Selectivity.

Molecular imprinting, which involves the formation of artificial recognition elements or cavities with complementary shape and chemical functionality to the target species, is a powerful method to overcome a number of limitations associated with natural antibodies. An important but often overlooked consideration in the design of artificial biorecognition elements based on molecular imprinting is the nonspecific binding of interfering species to noncavity regions of the imprinted polymer. Here, we demonstrate a universal method, namely, PEGylation of the noncavity regions of the imprinted polymer, to minimize the nonspecific binding and significantly enhance the selectivity of the molecular imprinted polymer for the target biomolecules. The nonspecific binding, as quantified by the localized surface plasmon resonance shift of imprinted plasmonic nanorattles upon exposure to common interfering proteins, was found to be more than 10 times lower compared to the non-PEGylated counterparts. The method demonstrated here can be broadly applied to a wide variety of functional monomers employed for molecular imprinting. The significantly higher selectivity of PEGylated molecular imprints takes biosensors based on these artificial biorecognition elements closer to real-world applications.

A simple and efficient design to improve the detection of biotin-streptavidin interaction with plasmonic nanobiosensors

In this manuscript we propose a simple and efficient strategy to improve the sensitivity of localized surface plasmon resonance (LSPR) shift-based biosensors using biotin-streptavidin recognition interaction as a proof-of-concept. Specifically, biotin molecules are immobilized on a low-cost plasmonic LSPR biosensor based on annealed self-assembled spherical gold nanoparticles (AuNSs) and successively incubated with increasing concentrations of streptavidin, achieving a limit of detection (LOD) of 5nM. Interestingly, when the detection is performed by the same biotin-functionalized plasmonic AuNSs substrate but against streptavidin previously conjugated to gold nanorods, the LSPR shift is 26-fold enhanced. Moreover, we confirm these results through numerical simulations and demonstrate that the proposed sensing architecture can operate as transducer not only to confirm the adsorption of bioanalyte but also to provide the chemical identity of the capture and targeted molecules from their vibrational Raman fingerprints. Therefore, we are confident that the development of such plasmonic biosensors that use metallic labels for improving the sensitivity of detection could become highly promising for future point-of-care diagnostic assays, pushing sensitivity towards single-molecule detection limit.

Electrically Controlled Plasmonic Behavior of Gold Nanocube@Polyaniline Nanostructures: Transparent Plasmonic Aggregates

We demonstrate the electrically controlled and reversible plasmonic signature of hybrid polymer–metal nanostructures composed of core/shell nanostructures: gold nanocubes (AuNCs) coated with electrochromic polyaniline (PANI) shells. A reversible tuning of the localized surface plasmon resonance (LSPR) peak of the AuNC core was obtained by applying an electrical potential that caused a reversible oxidation state change in the electroactive PANI nanoshell. A significant shift of the main LSPR peak was achieved with high reversibility and electrochemical stability due to the interplay of the local decay of the electromagnetic field and the controlled thickness of the surrounding polymer shell. Here, the PANI shell acts as an electroactive medium as well as a physical spacer to prevent uncontrollable plasmonic coupling. The most efficient LSPR shift can be induced by the refractive index change of nanoscale PANI shell thickness lower than the electromagnetic field decay length of the given gold nanoparticles....

Effective medium based optical analysis with finite element method simulations to study photochromic transitions in Ag-TiO2 nanocomposite films

Photochromic transitions in silver nanoparticles (AgNPs) embedded titanium dioxide (TiO2) films under green light illumination are marked by reduction in strength and blue shift in the position of the localized surface plasmon resonance (LSPR) associated with AgNPs. These transitions, which happen in the sub-nanometer length scale, have been analysed using the variations observed in the effective dielectric properties of the Ag-TiO2 nanocomposite films in response to the size reduction of AgNPs and subsequent changes in the surrounding medium due to photo-oxidation. Bergman-Milton formulation based on spectral density approach is used to extract dielectric properties and information about the geometrical distribution of the effective medium. Combined with finite element method simulations, we isolate the effects due to the change in average size of the nanoparticles and those due to the change in the dielectric function of the surrounding medium. By analysing the dynamics of photochromic transitions in the effective medium, we conclude that the observed blue shift in LSPR is mainly because of the change in the dielectric function of surrounding medium, while a shape-preserving effective size reduction of the AgNPs causes decrease in the strength of LSPR.

Optical properties of local surface plasmon resonance in Ag/ITO sliced nanosphere by the discrete dipole approximation

Optical properties of localized surface plasmon resonances (LSPR) of Ag/ITO sliced nanosphere have been studied using discrete dipole approximation and plasmon hybridization theory. It is found that different morphologies of sliced nanosphere can induce distinctive features in the extinction spectra. In the meanwhile, gap distances and refractive index of the surrounding medium could modulate the plasmon hybridization and the LSPR shifting. At large separation, the shift of LSPR peaks for the nanosphere sliced in halves consisting of ITO and Ag is small and insensitive to the gap distance in the weak coupling, whereas smaller separation exhibits a distinct red shift. Additionally, multiple resonance peaks are excited for the nanosphere sliced in quarters consisting of ITO and Ag. In this situation, electric field is mainly distributed in the gap region of sliced nanosphere and the central point. These results indicate that different morphologies of sliced nanosphere could create abundant tunable LSPR modes, which provides potential for multiplex optical sensing.

Increasing the spectral shifts in LSPR biosensing using DNA-functionalized gold nanorods in a competitive assay format for the detection of interferon-γ

We demonstrate an approach that utilizes DNA-functionalized gold nanorods (AuNRs) in an indirect competitive assay format to increase the spectra shift in localized surface plasmon resonance (LSPR) biosensing. We use interferon gamma (IFN-γ) as a model analyte to demonstrate the feasibility of our detection method. The LSPR chips with periodic gold nanodot arrays are fabricated using a thermal lithography process and are functionalized with IFN-γ aptamers for detection. The DNA-functionalized AuNRs and IFN-γ compete with each other to bind to the aptamers during detection, and the spectra shifts are mainly caused by the AuNRs rather than IFN-γ. When using our approach, the target molecules do not need to be captured by two capture ligands simultaneously during detection and thus do not require multiple binding sites. Both experiments and finite-difference time-domain (FDTD) simulations show that making the AuNRs as close to the chip surface as possible is very critical for increasing LSPR shifts, and the simulated results also show that the orientation of the AuNR affects the plasmon coupling between the gold nanodots on the chip surface and the nearby AuNRs. Although only the detection of IFN-γ is demonstrated in this study, we expect that the LSPR biosensing method can be applied to label-free detection of a variety of molecules as long as suitable aptamers are available.

Quantification of proteins using enhanced etching of Ag coated Au nanorods by the Cu(2+)/bicinchoninic acid pair with improved sensitivity.

Plasmonic nanosensors show great potential in ultrasensitive detection, especially with the plasmon peak position as the detection modality. Herein, a new sensitive but simple total protein quantification method termed the SPR-BCA assay is demonstrated by combining plasmonic nanosensors with protein oxidation by Cu(2+). The easy tuning of localized surface plasmon resonance (LSPR) features of plasmonic nanostructures makes them ideal sensing platforms. We found that the Cu(2+)/bicinchoninic acid (BCA) pair exhibits accelerated etching of Au@Ag nanorods and results in the LSPR peak shift. A linear relationship between Cu(2+) and the LSPR shift is found in a double logarithmic coordinate. Such double logarithm relationship is transferred to the concentration of proteins. Theoretical simulation shows that Au nanorods with large aspect ratios and small core sizes show high detection sensitivity. Via optimized sensor design, we achieved an increased sensitivity (the limit of detection was 3.4 ng ml(-1)) and a wide working range (0.5 to 1000 μg ml(-1)) compared with the traditional BCA assay. The universal applicability of our method to various proteins further proves its potential in practical applications.

Iodine-mediated etching of gold nanorods for plasmonic sensing of dissolved oxygen and salt iodine.

Here, we have carefully investigated iodine-mediated etching of gold nanorods (AuNRs) in the presence of iodate and applied this phenomenon to on-site detection of dissolved oxygen (DO). Under given conditions, the quantitative conversion of target analytes DO to iodine leads to the etching of AuNRs along the longitudinal direction with the aid of cetyltrimethylammonium. As a result, the longitudinal localized surface plasmon resonance shifts to a short wavelength. The peak-shift can be used for quantitative determination of DO and iodate by a spectrophotometer. The satisfactory results from DO detection in different water samples and iodate detection in table salt indicate the feasibility of the proposed methods. Moreover, the as-prepared colorimetric test paper would make the detection more economical and simpler.

In Situ Localized Surface Plasmon Resonance (LSPR) Spectroscopy to Investigate Kinetics of Chemical Bath Deposition of CdS Thin Films

Techniques that can characterize the early stages of thin film deposition from liquid phase processes can aid greatly in our understanding of mechanistic aspects of chemical bath deposition (CBD). Here we have used localized surface plasmon resonance (LSPR) spectroscopy to monitor the in situ kinetics of early stage growth of cadmium sulfide (CdS) thin films on Ag nanoparticle on quartz substrates. Real-time shift during CdS deposition showed that the LSPR wavelength red-shifted rapidly due to random deposition of CdS on the substrate but saturated at longer times. LSPR modeling showed that these features could be interpreted as an initial deposition of CdS islands followed by preferential deposition onto itself. The CdS also showed significantly enhanced Raman signals up to 170 times due to surface-enhanced Raman scattering (SERS) from the CdS/Ag NP regions. The ex situ SERS effect supported the LSPR shift, suggesting that these techniques could be used to understand nucleation and growth phenomena from ...

The Effect of Local Electric Field in the Middle Dielectric Wall on the Infrared Plasmonic Shift of the Concentric Gold Double Nanotubes.

The effect of inserted gold nanotube on the localized surface plasmon resonance (LSPR) shifting and local electric field enhancement of concentric gold double nanotubes has been studied by using the quasi-static electricity. Because of the combined effect from inter-surface and inter-tube plasmon coupling, the LSPR in gold double nanotubes could be easily tuned into the infrared wavelength region. The calculation results indicate the plasmon peak shifting is quite sensitive to the thickness of the inner gold tube and the gap between the inner and outer gold tubes. The physical mechanism has been attributed to the local field effect of the separate dielectric layer. Because of the additional attraction from the inner gold tube to the free electrons at outer gold tube, the decreasing electric field in the middle dielectric spacer layer reduces the restoring force and lead to intense red shift of LSPR.

Direct electron density modulation of surface plasmons with a scanning electron microscope

The optical properties of localized surface plasmon resonance (LSPR) were modulated by direct electron injection using scanning electron microscopy (SEM). The predictions of electromagnetic theory for LSPR on charged metal nanoparticles were experimentally verified using a novel microscopic system. Extinction spectra were obtained for gold nanostructures that were under intense electron irradiation using an SEM system equipped with an optical microscope. High-frequency shifts of LSPR were observed for a single gold nanosphere, nanodimer, and nanorod, and the amount of the shifts was explained with respect to their symmetry.

Structure and localized surface plasmon tuning of sputtered Au nano-islands through thermal annealing

Localized surface plasmon resonance of noble metallic nanoparticles has been widely used in the fabrication of various sensors. Turning the nanostructure of localized surface plasmon resonance is significant because the resonance induced localized surface plasmon resonance shift is found to be strongly dependent on the structural characteristics, and thus the performance of the sensors. A simple sputtering, annealing, re-sputtering, and re-annealing process was proposed to tune the structural and optical characteristics of Au nano-islands deposited on the glass substrate. It was found that the size and inter-particle distance of nano-islands depend on annealing time and temperature. High temperature annealing tended to increase the size and inter-islands distance of Au islands. Re-sputtering and re-annealing under different conditions made size and inter-particle distance further tuning possible. Investigations on the optical characteristics of Au nano-islands demonstrated that the surface plasmon resonance peak and the spectral bandwidth of islands were tunable from 510 nm to 620 nm and from 50 nm to approximately 400 nm, respectively. The refractive index sensitivity of Au nano-islands determined by the surface plasmon band position change in different surrounding medium was compared. 500 °C 5 h annealing increased the value of refractive index sensitivity to approximately 58 nm/RIU from 26 nm/RIU under 100 °C 5 h annealing. Besides, Au nano-islands with the same re-sputtering condition but different re-annealing conditions showed the maximum value when the re-annealing temperature is at 500 °C for 5 h. In addition, the refractive index sensitivity, surface plasmon band position, and figure of merit were dependent with each other. These results suggest that the scheme “sputtering, annealing, re-sputtering, and re-annealing” is an effective method to adjust the structure and increase the refractive index sensitivity of sputtered Au islands.

LSPR Sensor Combining Sharp Resonance and Differential Optical Measurements

A new optical sensor based on the localized surface plasmon resonance (LSPR) in 2D arrays of silver nanoparticles (AgNPs) combined with a differential optical measurement method is developed. LSPR substrates comprised of self-assembled, 2D arrays of AgNPs exhibit coherent plasmon coupling manifested as a sharp peak in the blue spectral region. A bottom-up approach was used to fabricate reproducible and cost-effective substrates with a figure of merit (FOM) of ~24. The LSPR shift was determined by measuring the difference between light extinction at two wavelengths selected on each side of the sharp peak. The sharpness of the coherent plasmon resonance together with the differential measurement method enabled a record sensing resolution in bulk for a LSPR sensor of ~4.8E-6 RIU.

Plasmonic–Photonic Mode Coupling in Indium-Tin-Oxide Nanorod Arrays

We present a systematic study of light scattering from indium-tin-oxide (ITO) nanorods in the near-infrared with a special focus on the resonant coupling of plasmonic transverse mode and photonic modes in 2-D periodic arrays. Using theoretical analysis combined with simulations, a set of experiments has been designed to study such interactions. Near-field mapping from the simulations shows a strong interaction of localized surface plasmon resonances (LSPR) with a photonic resonance; together they explain the scattering phenomenon observed in our experiments carried out in the far field. We observed the shift of LSPR as the plasma frequency was varied, resulting in a modification of the spectral shape. Utilizing the high aspect ratios of the ITO nanorods, the LSPR strength can be turned on and off by the polarization of the incident light.

Exploration of the growth process of ultrathin silica shells on the surface of gold nanorods by the localized surface plasmon resonance

Ultrathin silica coating (UTSC) has emerged as an effective way to improve the compatibility and stability of nanoparticles without attenuating their intrinsic optical properties. Exploration strategies to probe the growth process of ultrathin silica shells on the surface of nanoparticles would represent a valuable innovation that would benefit the development of ultrathin silica coated nanoparticles and their relevant applications. In this work, we report a unique, very effective and straightforward strategy for probing the growth of ultrathin silica shells on the surface of gold nanorods (Au NRs), which exploits the localized surface plasmon resonance (LSPR) as a reporting signal. The thickness of the ultrathin silica shells on the surface of Au NRs can be quantitatively measured and predicted in the range of 0.5–3.5 nm. It is demonstrated that the LSPR shift accurately reflects the real-time change in the thickness of the ultrathin silica shells on Au NRs during the growth process. By using the developed strategy, we further analyze the growth of UTSC on the surface of Au NRs via feeding of Na2SiO3 in a stepwise manner. The responsiveness analysis of LSPR also provides important insight into the shielding effect of UTSC on the surface of Au NRs that is not accessible with conventional strategies. This LSPR-based strategy permits exploration of the surface-mediated sol–gel reactions of silica from a new point of view.

Localized surface plasmon resonance (LSPR) biosensing using gold nanotriangles: detection of DNA hybridization events at room temperature.

We present a proof-of-concept of the application of gold nanotriangles in sequence specific DNA detection, using localized surface plasmon resonance (LSPR) spectroscopy and dark-field optical microscopy. The sensing platform comprises gold nanotriangles immobilized on a glass chip and oligonucleotides as probes. Probe formation and testing complementary and non-complementary targets followed common chip technology protocols. Gold nanotriangles showed a remarkable sensitivity of 468 nm per RIU and allowed detection of 20-mer targets. When the target sequence was part of a 50-mer synthetic DNA oligonucleotide, LSPR shifts as high as 35 nm were observed. Conversely, when the target was present in PCR products of ca. 350 bp, obtained from clinical samples, LSPR shifts larger than 20 nm were observed. Moreover, LSPR shifts were less than ±1 nm for the respective non-complementary targets. These results with gold nanotriangles as sensors are a notable improvement to the LSPR shifts of less than 5 nm usually obtained for spherical gold nanoparticles of comparable sizes. Optimal conditions for the detection of synthetic and PCR product targets using gold nanotriangles and oligonucleotide probes were achieved with low percentages of intercalating thioalkanes; target hybridization at room temperature, 3 hours of incubation, and 2× SSC buffer stringency conditions.

Surface-Dependent Localized Surface Plasmon Resonances in CuS Nanodisks

We demonstrated revertible shifts of surface-dependent localized surface plasmon resonances (LSPRs) in CuS nanodisks. Oleylamine (OYA) served as a solvent and surface ligand covering on CuS nanodisks during the thermolysis of single-source precursor copper ethylxanthate (Cu(ex)2). When OYA ligand was unloaded and reloaded on the surface of CuS nanodisks, the wavelength of LSPRs blue-shifted due to more oxygen exposure and then reverted through surface repassivation. The surface-dependent shift of LSPRs was dominated by the concentration of free holes in CuS nanodisks, which was modulated by the coverage and exchange of surface ligands, and the oxygen exposure dose and time. The semiconductor nanocrystals with tunable LSPRs have great potential in advanced plasmonics.

Tuning the plasmon shift and local electric field distribution of gold nanodumbbell: The effect of surface curvature transition from positive to negative

The localized surface plasmon resonance (LSPR) properties and local electric field distributions of the gold nanodumbbell are theoretically studied by using the finite-difference time-domain (FDTD) calculation. The decrease of both positive curvature radius in the spherical end and the negative curvature radius in the midsection results in the absorption peak corresponding to longitudinal plasmon mode red shifts distinctly. However, the transverse plasmonic absorption peak is not sensitive to the change of the curvature radii. The physical mechanism has been illuminated by studying the shape transformation dependent local field distribution between nanorod and nanodimer. It has been found that the decrease of the radii of curvature leads to the dumbbell-shaped particle transforming to a dimer-like structure, while the change in inter-particle plasmon coupling results in the “hot spots” of the local field migrating from the spherical ends to the midsection of the particle. There is an associated red shift of longitudinal LSPR. These results indicate that the transition of surface curvature between positive and negative greatly affects the surface charge distribution, local field enhancement and plasmon coupling.

Benchmarking Common Approximations for Determining the Particle-Size Dependence of Adsorbate-Induced Localized Surface Plasmon Resonance Shifts

Anomalies are investigated that exist between many long-standing theoretical models of the optical behavior of sensors based on changes in the localized surface plasmon resonance upon analyte adsorption. In particular, we focus on single metal nanoparticles which represent the core building-block of many recent sensing devices. Theoretical approaches include the Retarded Mie theory, the Non-Retarded quasi-static-dipole approximation, and two radiative corrections to the Non-Retarded case (radiative damping and radiative damping + depolarization). We find that the most accurate Non-Retarded approximation to the Retarded Mie theory varies strongly on a case by case basis; anyway, for particle radii beyond a few tens of nanometers, none of the considered approximations represents properly the adsorbate induced plasmon shift. We also find that the size-dependent peak shift has a complex dependence on the metal dielectric function. Accordingly, the trend of the adsorbate-induced plasmon peak shift as a functio...