Localized Surface Plasmon Resonance Wavelength Research Articles

Plasmon Tuning of Liquid Gallium Nanoparticles through Surface Anodization.

In this work, tunable plasmonic liquid gallium nanoparticles (Ga NPs) were prepared through surface anodizing of the particles. Shape deformation of the Ga NPs accompanied with dimpled surface topographies could be induced during electrochemical anodization, and the formation of the anodic oxide shell helps maintain the resulting change in the particle shape. The nanoscale dimple-like textures led to changes in the localized surface plasmon resonance (LSPR) wavelength. A maximal LSPR red-shift of ~77 nm was preliminarily achieved using an anodization voltage of 0.7 V. The experimental results showed that an increase in the oxide shell thickness yielded a negligible difference in the observed LSPR, and finite-difference time-domain (FDTD) simulations also suggested that the LSPR tunability was primarily determined by the shape of the deformed particles. The extent of particle deformation could be adjusted in a very short period of anodization time (~7 s), which offers an efficient way to tune the LSPR response of Ga NPs.

Spatially selective nanoplasmonic response in Ag embedded GLAD TiO2 nanocomposite thin films

The nanoplasmonic behaviour of Ag decorated TiO2 film structure was spatially varied across the sample surface by utilizing the tailored optical response of the underlying dielectric TiO2 thin film prepared using a novel collimated glancing angle deposition technique (collimated-GLAD). A blue shift in maximum attenuation coefficient region and minimum film transmittance region of the Ag/collimated-GLAD TiO2 nanocomposite coating with increase in height from the evaporation source was detected. The presence of minimum transmittance region was found to have occurred due to localized surface plasmon resonance (LSPR) phenomenon with additional contribution from thin film interference. The measured transmittance spectra showed spatial selectivity in LSPR wavelength of ∼22 nm with increase in height from the evaporation source. The variation of surrounding medium's (i.e. the TiO2 layer) refractive index (with height from the evaporation source) of the Ag nanostructures was the key factor responsible for the spatially varying nanoplasmonic response of Ag/collimated-GLAD TiO2 film across the sample surface. The Surface enhanced Raman scattering (SERS) characterization of the Ag/collimated-GLAD TiO2 film revealed photocatalysis driven dimerization conversion of p-aminothiophenol (p-ATP) molecules to p,p’-dimercaptoazobisbenzene (DMAB) due to the localized surface plasmons of the film structure along with the substantial enhancement in the Raman peak intensities compared to the blank substrate.

Silver nanotriangle array based LSPR sensor for rapid coronavirus detection

A rapid, portable, and cost-effective method to detect the infection of SARS-CoV-2 is fundamental toward mitigating the current COVID-19 pandemic. Herein, a human angiotensin-converting enzyme 2 protein (ACE2) functionalized silver nanotriangle (AgNT) array localized surface plasmon resonance (LSPR) sensor is developed for rapid coronavirus detection, which is validated by SARS-CoV-2 spike RBD protein and CoV NL63 virus with high sensitivity and specificity. A linear shift of the LSPR wavelength versus the logarithm of the concentration of the spike RBD protein and CoV NL63 is observed. The limits of detection for the spike RBD protein, CoV NL63 in buffer and untreated saliva are determined to be 0.83 pM, 391 PFU/mL, and 625 PFU/mL, respectively, while the detection time is found to be less than 20 min. Thus, the AgNT array optical sensor could serve as a potential rapid point-of-care COVID-19 diagnostic platform.

Design and performance simulation of a highly sensitive nano-biosensor based on a realistic array of plasmonic synthesized nanostructures

In this study, simulation, analysis, and synthesis of silver plasmonic nanostructures were investigated for use in biomolecule detection. At first, silver nanoparticles (SNPs) were synthesized by a chemical reduction method using polyethylenimine as both a reducing and a stabilizing agent. The SNPs were characterized by UV–visible spectroscopy, dynamic light scattering, and field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy measurement techniques and showed spherical morphology with an average particle size of 38 nm and a localized surface plasmon resonance (LSPR) peak around 400 nm. The plasmonic effects of the silver nanostructures were also investigated for detection of glucose as a model analyte, with a concentration range of 0.0–2.603 mol/L. Wavelength shifts of 0.033 and 12.04 nm were obtained for glucose concentrations of 0.0039 and 2.603 mol/L, respectively; these shifts yielded the sensitivities of 482.75 and 189.02 nm/RIU and figures of merit (FOMs) of 13.5 and 5.24, respectively. The simulation results indicated the ability of SNPs of 30 nm to detect glucose concentrations as low as 0.0039 mol/L with an LSPR wavelength shift of about 0.033 nm. Also, the sensitivity and FOM were calculated for a 10 × 10 array of non-uniform size nanoparticles according to the experimental sizes. The simulation results revealed the significance of both the nanoparticle size and the particle size uniformity of a realistic array on the sensitivity and LSPR peak shift of SNPs. These findings show that it is possible to use an LSPR-based sensor to detect other biological samples by tuning their plasmonic features.

Polysorbate- and DNA-Mediated Synthesis and Strong, Stable, and Tunable Near-Infrared Photoluminescence of Plasmonic Long-Body Nanosnowmen.

Direct photoluminescence (PL) from metal nanoparticles (NPs) without chemical dyes is promising for sensing and imaging applications since this offers a highly tunable platform for controlling and enhancing the signals in various conditions and does not suffer from photobleaching or photoblinking. It is, however, difficult to synthesize metal NPs with a high quantum yield (QY), particularly in the near-infrared (NIR) region where deep penetration and reduced light scattering are advantageous for bioimaging. Herein, we designed and synthesized Au-Ag long-body nanosnowman structures (LNSs), facilitated by polysorbate 20 (Tween 20). The DNA-engineered conductive junction between the head and body parts results in a charge transfer plasmon (CTP) mode in the NIR region. The junction morphology can be controlled by the DNA sequence on the Au core, and polythymine and polyadenine induced thick and thin junctions, respectively. We found that the LNSs with a thicker conductive junction generates the stronger CTP peak and PL signal than the LNSs with a thinner junction. The Au-Ag LNSs showed much higher intensities in both PL and QY than widely studied Au nanorods with similar localized surface plasmon resonance wavelengths, and notably, the LNSs displayed high photostability and robust, sustainable PL signals under continuous laser exposure for >15 h. Moreover, the PL emission from Au-Ag LNSs could be imaged in a deeper scattering medium than fluorescent silica NPs. Finally, highly robust PL-based cell images can be obtained using Au-Ag LNSs without significant signal change while repetitively imaging cells. The results offer the insights in plasmonic NIR probe design, and show that chemical dye-free LNSs can be a very promising candidate with a high QY and a robust, reliable NIR PL signal for NIR sensing and imaging applications.

Electrostatic and Covalent Assemblies of Anionic Hydrogel-Coated Gold Nanoshells for Detection of Dry Eye Biomarkers in Human Tears.

Although dry eye is highly prevalent, many challenges exist in diagnosing the symptom and related diseases. For this reason, anionic hydrogel-coated gold nanoshells (AuNSs) were used in the development of a label-free biosensor for detection of high isoelectric point tear biomarkers associated with dry eye. A custom, aldehyde-functionalized oligo(ethylene glycol)acrylate (Al-OEGA) was included in the hydrogel coating to enhance protein recognition through the formation of dynamic covalent (DC) imine bonds with solvent-accessible lysine residues present on the surface of select tear proteins. Our results demonstrated that hydrogel-coated AuNSs, composed of monomers that form ionic and DC bonds with select tear proteins, greatly enhance protein recognition due to changes in the maximum localized surface plasmon resonance wavelength exhibited by AuNSs in noncompetitive and competitive environments. Validation of the developed biosensor in commercially available pooled human tears revealed the potential for clinical translation to establish a method for dry eye diagnosis.

Experimental and theoretical study of the characteristics of LSPR peaks for metal NPs produced by controlling Ar ambient gas pressure to enhance the efficiency of solar cells

In this study, silver (Ag) nanoparticle thin films were deposited on microscope slide glass and Si wafer substrates using the pulsed-laser deposition (PLD) technique in Ar ambient gas pressures of 1 × 10−3 and 7.5 × 10−1 mbar. AFM analysis has shown that the number of Ag nanoparticles reaching the substrate decreased with increasing Ar gas pressure. As a result of Ar ambient gas being allowed into the vacuum chamber, it was observed that the size and height of Ag nanoparticles decreased and the interparticle distances decreased. According to the absorption spectra taken by a UV–vis spectrometer, the wavelength where the localised surface plasmon resonance (LSPR) peak appeared was shifted towards the longer wavelength region in the solar spectrum as Ar background gas pressure was decreased. This experiment shows that LSPR wavelength can be tuned by adjusting the size of metal nanoparticles, which can be controlled by changing Ar gas pressure. The obtained extinction cross section spectra for Ag nanoparticle thin film was theoretically analysed and determined by using the metal nanoparticle–boundary element method (MNPBEM) toolbox simulation program. In this study, experimental spectrum and simulation data for metal nanoparticles were acquired, compared, and determined to be in agreement.

Scanning probe microscopy by localized surface plasmon resonance at fiber taper tips.

Plasmonic antenna probes have been widely investigated for detecting electrical permittivity changes on the nanometer scale by employing high-sensitivity localized surface plasmon resonance (LSPR). Although it is intuitive to integrate such a probe onto an atomic force microscope (AFM) to add one more measurable quantity to the family of scanning probe microscopy techniques, the strong scattering background of the AFM tip overwhelms the LSPR scattering signal. To solve this problem, we combined evanescent coupling, polarization and spatial filtering, confocal spectroscopy, and numerical methods to extract clean LSPR spectra from a gold nanosphere-antenna probe attached to the tip of a fiber taper. By mounting the fiber taper on a custom quartz-tuning-fork SPM, we achieved high-quality nanometer-scale imaging of gold nanospheres on glass slides by mapping the LSPR wavelength shift. In addition, we reported an LSPR wavelength shift enhancement by more complicated probe designs and the consequent promise for higher-sensitivity microscopy. Our optical system and spectral processing method provide an effective solution to the long-standing quest for LSPR scanning microscopy.

The approaches for localized surface plasmon resonance wavelength position tuning. Short review

A unique feature of nanoparticles made from highly conductive materials (plasmonic nanoparticles) is that their localized surface plasmon resonance (LSPR) wavelength position can be tuned by changing the shape, size, composition and environment in accordance with the purpose of the application. In this paper, the main mechanisms of LSPR tuning that are available at the present time are reviewed. In particular, a widely used method for tuning the LSPR wavelength position is based on selecting the type of a plasmonic nanoparticle material such as gold, silver, copper, aluminum and gold-silver alloy. The examples of changing the resonance absorption position by using nanoparticles with different shapes and dimensions have been аlso demonstrated. Furthermore, works with less used LSPR tuning methods, such as controlled regulation of the distance between nanoparticles in one and two dimensions have been considered. The number of works is given, where the LSPR wavelength position can be also controlled by changing the environment in the vicinity of plasmonic nanoparticle: the substrate thickness, the thickness and dielectric parameters of the layer on the surface of the nanoparticle. Examples of active influence on the change in the wave position of LSPR by applying an electric potential and regulating plasma modes have been also discussed.

Mechanically Tunable Nanogap Antennas: Single‐Structure Effects and Multi‐Structure Applications

AbstractBy mechanically changing the width of a nanometer gap between two optical antennas, the coupling condition for the antennas can be seamlessly modified. This leads to a shift in the antenna's localized surface plasmon resonance wavelength, and a modification of the intensity in the antenna hotspot, which can reach extraordinarily high values for nanometer gaps. Such systems have in recent years been realized, for example, by using flexible polymers as substrates that are mechanically stretched or bent, or by combining break junctions with plasmonic antennas. Applications can be found, amongst others, in plasmonic strain sensing via color shift sensors, or tunable surface‐enhanced Raman spectroscopy platforms. In this article an overview is given over recent work on mechanically tunable gap antennas, with a specific focus on studies at the single‐nanostructure level. Different techniques used for mechanical tuning, optical read‐out signals that are influenced by the gap width, and potential applications of the resulting structures are presented. Plasmonic break junctions offer a new perspective that allows for added functionalities. The mechanical activation is discussed within the context of single nanoantenna gap tuning by using alternative stimuli.

Rapid controlled synthesis of gold-platinum nanorods with excellent photothermal properties under 808 nm excitation.

Noble metal nanomaterials are particularly suitable as photothermal transduction agents (PTAs) with high photothermal conversion efficiency (PCE) due to local surface plasmon resonance (LSPR). Studies on different gold–platinum (Au–Pt) bimetal nanoparticles exhibiting the LSPR effect have provided a new idea for the synthesis of excellent PTAs. But there is no simple and scalable method for the controllable synthesis of Au–Pt nanoparticles with adjustable LSPR wavelength range. In this work, the effects of Ag+ and K2PtCl4 on the deposition of Pt on the surface of gold nanorods (AuNRs) were investigated. A fast, precise, and controlled synthesis of dumbbell-like Pt-coated AuNRs (Au@Pt NRs) under mild conditions is proposed. The synthesized Au@Pt NRs have a longitudinal LSPR wavelength of 812 nm, which is very close to a common laser wavelength of 808 nm. The Au@Pt NRs exhibit excellent photothermal properties when irradiated with a laser. The temperature increased by more than 36 °C after irradiation for 10 min, with a PCE of about 78.77%, which is much higher than that of AuNRs (57.33%). In addition, even after four on/off cycles, the Au@Pt NRs are able to maintain the photothermal properties and retain their optical properties, indicating that they have excellent photothermal stability and reusability.

Impressively Printing Patterns of Gold and Silver Nanoparticles.

The optical and chemical properties of gold and silver nanoparticles make them useful for many applications, including surface enhanced spectroscopy-based biosensors, photostable colorants, enhanced photovoltaics, and nanoscale optical elements. We report a simple technique to generate patterns of gold and silver nanoparticles with controlled shape and shape-dependent optical properties using metal stamps to impress them onto a glass substrate or flexible polymers. The pressure flattens the nanoparticles, converting initially spherical nanoparticles into discs with reduced height and increased diameter. This deformation causes their localized surface plasmon resonance wavelength to red-shift. Nanoparticles were characterized by electron microscopy, atomic force microscopy, and dark field optical scattering spectroscopy. The deformed nanoparticle patterns had a lateral resolution limited by the nanoparticle diameter (single particles are partly flattened only where they contact the stamp). The method also (i) transfers the stamp's topography, with smooth stamps generating flattened nanoparticles with uniform height, and small changes in stamp height are evident in the nanoparticle height and scattering wavelength, and (ii) allows facile removal of undeformed nanoparticles using scotch tape, and patterns of deformed nanoparticles can be transferred to a thin polymer-film. The patterning process is simple and inexpensive. It can be performed by hand for demonstrations or artistic applications, with controlled force for plasmonics research, and potentially automated on reel-to-reel presses for large scale production.

In situ evaluation of plasmonic enhancement of gold tips for plasmon-enhanced imaging techniques.

Plasmonic nanoantennas are at the core of various optical near-field scanning techniques such as tip-enhanced Raman spectroscopy as they provide the amplification and confinement of the electromagnetic field, which ultimately provides sensitivity and spatial resolution. With a cornucopia of different fabrication methods available, the actual performance of a nanoantenna is often only assessed by whether or not near-field imaging is possible, implying the complete alignment and landing procedure of the scanning probe. We present a semi-quantitative approach to assess the plasmonic enhancement of gold tips via localized surface plasmon resonance (LSPR) enhancement of intrinsic gold photoluminescence without the need for interaction with the sample. As the intensity of the plasmon at the apex decreases, a significant change in the shape of the tip signal spectrum is observed, reflecting itself as a decrease in the R2 value (fit quality) for numerical fitting with a Lorentzian, which also provides an approximation for the LSPR wavelength. Our findings suggest that the potential of a tip to perform well as an optical near field antenna may already be assessed in an early stage of the experiment.

Detection of Chemotherapeutic Drug 6-Mercaptopurine Through Manipulating Localized Surface Plasmon Resonance Absorption of Silver Nanoplates

A new colorimetric method for the detection of chemotherapeutic drug 6-mercaptopurine (6-MP) is developed based on controlling localized surface plasmon resonance (LSPR) of triangular silver nanoplates (AgNPs). The triangular AgNPs can be etched by trace amount of Cl− and transformed to a disc-like shape, accompanied by a blueshift of the corresponding LSPR absorption band. With the protective effect of 6-MP, the AgNPs are hardly or partly to be etched by Cl−, resulting in a reversed shift of LSPR. Based on a linear relationship between LSPR wavelength change and the concentration of 6-MP, a fast, visual and selective method for 6-MP assay is established. This method has been successfully applied to the detection of 6-MP in tablets.

Color Changes in Ag Nanoparticle Aggregates Placed in Various Environments: Their Application to Air Monitoring.

Fresh Ag nanoparticles (NPs) dispersed on a transparent SiO2 exhibit an intense optical extinction band originating in localized surface plasmon resonance (LSPR) in the visible range. The intensity of the LSPR band weakened when the Ag NPs was stored in ambient air for two weeks. The rate of the weakening and the LSPR wavelength shift, corresponding to visual chromatic changes, strongly depended on the environment in which Ag NPs were set. The origin of a chromatic change was discussed along with both compositional and morphological changes. In one case, bluish coloring followed by a prompt discoloring was observed for Ag NPs placed near the ventilation fan in our laboratory, resulted from adsorption of large amounts of S and Cl on Ag NP surfaces as well as particle coarsening. Such color changes deduce the presence of significant amounts of S and Cl in the environment. In another case, a remarkable blue-shift of the LSPR band was observed for the Ag NPs stored in the desiccator made of stainless steel, originated in the formation of CN and/or HCN compounds and surface roughening. Their color changed from maroon to reddish, suggesting that such molecules were present inside the desiccator.

Large range LSPR of KTN@Ag core-shell nanoarray dependent of structure size

The localized surface plasmon resonance (LSPR) of Ag nanosphere array and KTN@Ag (nano KTa0.65Nb0.35O3 with Ag at it’s the surface) core-shell nanoarray are elaborately analyzed based on finite-difference time-domain (FDTD). The effects of particle spacing, core size and shell size on LSPR for the KTN@Ag core-shell nanoarray structure are systematically investigated. The results indicate that the core size and shell size for the KTN@Ag core-shell nanoarray structure have a great influence on LSPR. With the increase of the Ag shell radius, the resonance absorption appears blue-shift from 878 nm to 660 nm. With the increase of KTN core radius, the resonance absorption peak exhibits significantly red-shift from 538 nm to 867 nm. Thus, the designed KTN@Ag core-shell nanoarray structure can realize the effective regulation of LSPR in a wider range, which is obviously better than the Ag nanosphere array. The controllable LSPR wavelength enables the KTN@Ag core-shell nanoarray to serve as plasmonic nanosensors. This study provides a theoretical basis for the future application of KTN@Ag core-shell nanoarray structure in optical and optoelectronic devices.

Seed-mediated Growth of Silver Nanoplates and Investigation on Their Nonlinear Optical Behaviors

Silver nanoplates have obvious advantages compared with other silver structural nanoparticles due to their wide tunability of the local surface plasmon resonance (LSPR) and significant local field enhancement around the sharp corners. Nonlinear optical absorption (NLA) and nonlinear optical refraction (NLR) are widely employed to characterize the nonlinear optical properties due to their simply testing requirement and comparable valuable results. Z-scan method is widely used in optical characterization of nonlinear properties of different materials. Here, a seed-mediated growth method of the silver nanoplates with well controlled size and local surface plasmon wavelength is presented. The nanoparticle size increases gradually and becomes more uniform distribution as the red-shift of the plasmon resonance wavelength from 650 nm to 950 nm. In particular, we show the dependence of NLA and NLR of the silver nanoplates on the LSPR. The absolute value of effective NLA coefficient increases from 1.38 to 4 cm/GW and that of the NLR index increases as large as 3 times when the LSPR wavelength changes from 728 nm to 898 nm, which is attributed to the strong plasmon absorption and local filed enhancement of larger size silver nanoplates. These findings have great potentials in the explorations of functional non-linear devices.

Single gold nanostars with multiple branches as multispectral orientation probes in single-particle rotational tracking.

Herein, we performed a single-particle correlation study to characterize the optical properties of gold nanostars (AuNSs) with multiple sharp branches under dark-field (DF) and differential interference contrast (DIC) microscopy, and to examine their use as multispectral orientation probes. We presented the polarization-dependent, periodic DIC images and intensities of single AuNSs at their localized surface plasmon resonance (LSPR) wavelengths with high sensitivity. Furthermore, we demonstrated that single AuNSs protrude multiple branches that can be used as individual sensors with DIC polarization anisotropy. Thus, unlike conventional Au nanorod (AuNR) probes, single AuNSs were presented as multispectral optical sensors that can provide detailed information such as rotational motions and rotational speeds at different branches of their star-like structure in dynamic environments.

Sulfur Precursor Reactivity Affecting the Crystal Phase and Morphology of Cu2-x S Nanoparticles.

For plasmonic copper-deficient Cu2-x S nanoparticles (NPs), accurate control of the crystal phase and morphology is highly desirable as both of which are known to determine the localized surface plasmon resonance (LSPR) wavelength and amplitude. Here, how the sulfur precursor reactivity in the synthesis of Cu2-x S NPs affects the resulting crystal phase and morphology is examined. Djurleite Cu1.94 S, roxbyite Cu1.8 S, digenite Cu1.8 S as well as covellite CuS nanodisks were synthesized by using 1-dodecanethiol, N,N-dibutylthiourea, and crystal sulfur 1-octadecene/oleylamine solutions and their crystal phase dependent LSPR properties were exhaustively discussed. In addition, crystal phase interconversion between covellite CuS and djurleite/roxbyite Cu2-x S was realized in the presence of the above sulfur precursors. On the other hand, djurleite Cu1.94 S nanorods rather than nanodisks were prepared by replacing 1-dodecanethiol with more reactive tert-dodecanethiol. The structural and morphological Cu2-x S NPs here holds great promise in the application of photothermal therapy, photocatalysis, surface-enhanced Raman scattering (SERS), and many others.

Tunable surface plasmon resonance of Al-Cu bimetallic nanoparticles thin films induced by pulsed-laser

Al-Cu bimetallic nanoparticles (NPs) thin film was fabricated by Nd:YAG fiber pulsed-laser irradiation of Al/Cu bilayers at ambient conditions. The effects of laser irradiation on the morphology, structure, composition and optical properties of the samples were investigated. The tunability of both localized surface plasmon resonance (LSPR) wavelength and intensity were realized by varying the pulse shot or relative thickness of Al/Cu bilayers. The SEM analysis with EDS results further revealed that the obtained particles consisted of a mean diameter of 36–80 nm bimetallic NPs system, being dependent on the Cu proportion. In addition, Surface-enhanced Raman spectroscopy (SERS) measurement and corresponding finite-difference time-domain (FDTD) simulation of before and after laser irradiation samples were discussed to verify the LSPR properties.