Gold-silver alloy semi-nanoshell arrays for label-free plasmonic biosensors

Impact of thermal annealing and laser treatment on the morphology and optical responses of mono- and bi-metallic plasmonic honeycomb lattice

Plasmonic nanoparticle arrays with a specific lattice arrangement can support surface lattice resonances (SLRs). SLR exhibits a sharp spectral peak and finds many applications including optical sensing and plasmonic lasers. To optimize SLR for application, a robust method that allows the mass production of plasmonic nanoparticle arrays with refined particle morphology and well-defined lattice arrangement is required. In this work, we combine nanosphere lithography (NSL) with thermal annealing or nanosecond-pulsed laser treatment to refine plasmonic nanoparticles in a honeycomb lattice. We comparatively study the effects of the two treatment methods on the particle morphology and lattice arrangement of mono (Ag and Pd) and bi-metallic (Ag-Pd) nanoparticle lattices. In general, thermal annealing preserves the lattice arrangement but fairly changes the particle roundness, while laser treatment produces particles with varying morphologies and spatial distribution. We also theoretically and experimentally investigate the optical responses of Ag nanoparticle lattices produced by different treatment methods. The observed difference in spectra can be attributed to the varying particle morphology, which shifts the localized surface plasmon resonance differently, resulting in a significant change in SLR. These findings provide valuable insights for optimizing plasmonic nanoparticle arrays for various applications.

Recent Developments in Plasmonic Alloy Nanoparticles: Synthesis, Modelling, Properties and Applications

Despite the traditional plasmonic materials are counted on one hand, there are a lot of possible combinations leading to alloys with other elements of the periodic table, in particular those renowned for magnetic or catalytic properties. It is not a surprise, therefore, that nanoalloys are considered for their ability to open new perspectives in the panorama of plasmonics, representing a leading research sector nowadays. This is demonstrated by a long list of studies describing multiple applications of nanoalloys in photonics, photocatalysis, sensing and magneto-optics, where plasmons are combined with other physical and chemical phenomena. In some remarkable cases, the amplification of the conventional properties and even new effects emerged. However, this field is still in its infancy and several challenges must be overcome, starting with the synthesis (control of composition, crystalline order, size, processability, achievement of metastable phases and disordered compounds) as well as the modelling of the structure and properties (accuracy of results, reliability of structural predictions, description of disordered phases, evolution over time) of nanoalloys. To foster the research on plasmonic nanoalloys, here we provide an overview of the most recent results and developments in the field, organized according to synthetic strategies, modelling approaches, dominant properties and reported applications. Considering the several plasmonic nanoalloys under development as well as the large number of those still awaiting synthesis, modelling, properties assessment and technological exploitation, we expect a great impact on the forthcoming solutions for sustainability, ultrasensitive and accurate detection, information processing and many other fields.

Towards multi-molecular surface-enhanced infrared absorption using metal plasmonics

Surface-enhanced infrared absorption (SEIRA) leads to a largely improved detection of polar molecules, compared to standard infrared absorption. The enhancement principle is based on localized surface plasmon resonances of the substrate, which match the frequency of molecular vibrations in the analyte of interest. Therefore, in practical terms, the SEIRA sensor needs to be tailored to each specific analyte. We review SEIRA sensors based on metal plasmonics for the detection of biomolecules such as DNA, proteins, and lipids. We further focus this review on chemical SEIRA sensors, with potential applications in quality control, as well as on the improvement in sensor geometry that led to the development of multiresonant SEIRA substrates as sensors for multiple analytes. Finally, we give an introduction into the integration of SEIRA sensors with surface-enhanced Raman scattering (SERS).

Influence of elemental composition on structural, thermal and hydration behavior of gold-silver bimetallic nanoparticles

In this study, molecular dynamics simulations (MDs) have been employed to explore the influence of elemental composition on the structural, thermal, and hydration behavior of the core-shell gold-silver bimetallic nanoparticles with three different concentrations Au₁₃₅Ag₁₁₄, Au₈₇Ag₁₆₂, and Au₅₅Ag₁₉₄. The pure gold and silver metal nanoparticles have also been studied for the sake of comparison. The calculated cohesive and formation energy values reveal the enhancement in the stability of gold-silver bimetallic nanoparticles with the increase in the concentration of gold. The specific heat capacity value of the bimetallic gold-silver nanoparticles has been found to increase linearly with the concentration of silver. This suggests that the specific heat capacity value of the gold-silver bimetallic nanoparticles may be improved by raising their silver concentration. The enhancement of specific heat capacity value with respect to the concentration of silver may be attributed to the enhanced phonon density. There is no significant difference in the computed hydration shell and water residence time of the gold-silver bimetallic nanoparticles with respect to their mixing ratio. The obtained stable structure, tunable specific heat capacity values, and the predicted hydration properties of the gold-silver bimetallic nanoparticles may be harnessed for their technological and biological applications.

Au Polyhedron Array with Tunable Crystal Facets by PVP-Assisted Thermodynamic Control and Its Sharp Shape As Well As High-Energy Exposed Planes Co-Boosted SERS Activity

A route is developed for directly growing 2D Au polyhedron arrays with controllable exposed facets of polyhedron by utilizing the substrate-supported 2D Au quasi-spherical nanoparticle arrays as the Au seed arrays, which cannot be realized by traditional lithography. In the reaction system, polyvinyl pyrrolidone (PVP) plays a vital role in guiding the reduced Au atoms and stabilizing the substrate-supported Au seeds. More importantly, by thermodynamic control, PVP as a capping agent can further direct the formation of {111} facets. The key to guarantee the integrity and periodicity of array is a proper reduction of Au ions and low growth rate of crystal. Benefiting from the higher electric field intensity near the sharp vertexes and edges of Au polyhedra and the exposed {110} facets with high energy, the Au polyhedron array with {110} facets encasing polyhedron exhibits good, stable surface enhanced Raman scattering activity toward 4-aminothiophenol among the involved arrays. The proposed fabrication approach tremendously enriches the structural diversity of Au nanoarrays on substrates and greatly overcomes the shortcoming of traditional lithography.

Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays

Combining finite elements method electrodynamic simulations and cost-effective and scalable nanofabrication techniques, we carried out a systematic investigation and optimization of the sensing properties of non-interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a very effective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their main geometrical parameters (diameter and height) to monitor the plasmonic resonance position and to find the configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to the variation of the refractive index of an embedding medium (bulk sensitivity). The spectral position of the plasmonic resonance can be tuned over a wide range from the visible to the near-IR region (500-1300 nm) and state-of-the-art performances can be obtained using the optimized nanodisks; we obtained local and bulk sensitivities of S 0 = 11.9 RIU-1 and S bulk = 662 nm RIU-1, respectively. Moreover, the results of the simulations are compared with the performances of experimentally synthesized non-interacting Au nanodisk arrays fabricated by combining sparse colloidal lithography and hollow mask lithography, with the parameters obtained by the sensitivity numerical optimization. An excellent agreement between the experimental and the simulated results is demonstrated, confirming that the optimization performed with the simulations is directly applicable to nanosensors realized with cost-effective methods, due to the quite large stability basin around the maximum sensitivities.

Diffractive dipolar coupling in non-Bravais plasmonic lattices

Honeycomb plasmonic lattices are paradigmatic examples of non-Bravais lattices. We experimentally measure surface lattice resonances in effectively free-standing honeycomb lattices composed of silver nanospheres. By combining numerical simulations with analytical methods, we analyze the dispersion relation and the near-field properties of these modes along high symmetry trajectories. We find that our results can be interpreted in terms of dipole-only interactions between the two non-equivalent triangular sublattices, which naturally lead to an asymmetric near-field distribution around the nanospheres. We generalize the interaction between the two sublattices to the case of variable adjacent interparticle distance within the unit cell, highlighting symmetry changes and diffraction degeneracy lifting associated to the transition between Bravais and non-Bravais lattices.

Circular Dichroism in Low-Cost Plasmonics: 2D Arrays of Nanoholes in Silver

Arrays of nanoholes in metal are important plasmonic devices, proposed for applications spanning from biosensing to communications. In this work, we show that in such arrays the symmetry can be broken by means of the elliptical shape of the nanoholes, combined with the in-plane tilt of the ellipse axes away from the array symmetry lines. The array then differently interacts with circular polarizations of opposite handedness at normal incidence, i.e., it becomes intrinsically chiral. The measure of this difference is called circular dichroism (CD). The nanosphere lithography combined with tilted silver evaporation was employed as a low-cost fabrication technique. In this paper, we demonstrate intrinsic chirality and CD by measuring the extinction in the near-infrared range. We further employ numerical analysis to visualize the circular polarization coupling with the nanostructure. We find a good agreement between simulations and the experiment, meaning that the optimization can be used to further increase CD.

Optimized Immobilization of Biomolecules on Nonspherical Gold Nanostructures for Efficient Localized Surface Plasmon Resonance Biosensing

Plasmonic biosensing techniques employ metal nanostructures, commonly gold (Au), often with biomolecules attached to their surfaces either directly or via other linkers. Various surface chemistry methods based on dispersion and covalent interactions are used to attach biomolecules to Au. As a result, when immobilizing a molecule on a metal surface, quantitative estimates of binding efficiency and stability of these surface chemistry methods are needed. Most prior work to compare such methods deals with bulk/thin film configurations or spherical nanoparticles, and very little is known about immobilization of biomolecules on plasmonic nanostructures of different shapes. Besides, due to rapid advancement of modern nanofabrication techniques, there is a growing need to determine an efficient surface chemistry method for immobilization of biomolecules on nonspherical plasmonic nanostructures. Previous comparison of immobilization methods on spherical Au nanoparticles has shown that physical adsorption resulted in the highest concentration of immobilized antibodies. In our work, we conducted a similar study and compared four representative Au surface functionalization methods as well as estimated how efficient these methods are at attaching biomolecules to nonspherical plasmonic Au nanostructures. We estimated the concentration of immobilized antibody that is specific to human C-reactive protein (anti-hCRP) by measuring the localized surface plasmon resonance (LSPR) shifts after exposing the surface of Au nanostructures to the antibody. Our results differ from the previously reported ones since the highest concentration of anti-hCRP was immobilized using 11-mercaptoundecanoic acid (MUA) chemistry. We demonstrated that immobilized antibodies retained their stability and specificity toward hCRP throughout the immunoassay when diluted hCRP or hCRP-spiked human serum samples were used. These findings have important implications for the fields of biosensing and diagnostics that employ nonspherical plasmonic nanostructures since an overall performance of these devices depends on efficient biomolecule immobilization.

Polarization dependence of second harmonic generation from plasmonic nanoprism arrays

The second order nonlinear optical response of gold nanoprisms arrays is investigated by means of second harmonic generation (SHG) experiments and simulations. The polarization dependence of the nonlinear response exhibits a 6-fold symmetry, attributed to the local field enhancement through the excitation of the surface plasmon resonances in bow-tie nanoantennas forming the arrays. Experiments show that for polarization of the input light producing excitation of the plasmonic resonances in the bow-tie nanoantennas, the SHG signal is enhanced; this despite the fact that the linear absorption spectrum is not dependent on polarization. The results are confirmed by electrodynamic simulations which demonstrate that SHG is also determined by the local field distribution in the nanoarrays. Moreover, the maximum of SHG intensity is observed at slightly off-resonance excitation, as implemented in the experiments, showing a close relation between the polarization dependence and the structure of the material, additionally revealing the importance of the presence of non-normal electric field components as under focused beam and oblique illumination.

Arrays of highly complex noble metal nanostructures using nanoimprint lithography in combination with liquid-phase epitaxy

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core-shell, core-void-shell, and core-void-nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.

Rare-earth fluorescence thermometry of laser-induced plasmon heating in silver nanoparticles arrays

The laser-induced plasmon heating of an ordered array of silver nanoparticles, under continuous illumination with an Ar laser, was probed by rare-earth fluorescence thermometry. The rise in temperature in the samples was monitored by measuring the temperature-sensitive photoluminescent emission of a europium complex (EuTTA) embedded in PMMA thin-films, deposited onto the nanoparticles array. A maximum temperature increase of 19 °C was determined upon resonant illumination with the surface plasmon resonance of the nanoarray at the highest pump Ar laser power (173 mW). The experimental results were supported by finite elements method electrodynamic simulations, which provided also information on the temporal dynamics of the heating process. This method proved to be a facile and accurate approach to probe the actual temperature increase due to photo-induced plasmon heating in plasmonic nanosystems.

Interplay between magnetic anisotropies in CoAu and Co films and antidot arrays: effects on the spin configuration and hysteretic behavior

We studied (i) a set of three Co : Au continuous films, grown by sputtering co-deposition (∼80 nm thick) with concentration ratios of 2 : 1, 1 : 1 and 1 : 0 (i.e., a pure Co film was also included), and (ii) a corresponding set of antidot arrays, produced by nanosphere lithography with the same hexagonal pattern (nominal lattice periodicity ∼520 nm). The samples were investigated by atomic and magnetic force microscopy and SQUID magnetometry. A twofold aim was fulfilled: to gain information on the magnetism of the CoAu compound (saturation magnetization, effective in-plane and out-of-plane anisotropy, exchange stiffness constant and magnetostrictive behavior) and to compare the magnetic behavior of the continuous and patterned samples. The continuous films exhibited a variety of hysteretic behaviours and magnetic configurations, ruled by the interplay between different magnetic anisotropy terms (magnetocrystalline, magnetoelastic and shape). The Co1Au1 film was anisotropic in the plane, whereas Co2Au1 and Co were isotropic and had an out-of-plane magnetization component; stripe domains were observed in Co2Au1, resulting in a transcritical hysteresis loop. A key role in determining these properties was ascribed to the magnetoelastic anisotropy term. Unlike the continuous films, the antidot arrays showed a similar hysteretic behavior and important similarities in the spin configuration were pointed out, despite the different compositions. We argue, also based on micromagnetic simulations, that this occurred because the nanopatterning enabled a local modification of the shape anisotropy, thus smoothing out the differences observed in the continuous films.