Improving the quality factors of plasmonic silver cavities for strong coupling with quantum emitters.

Plasmonic cavities (PCs) made of metallic nanostructures can concentrate electromagnetic radiation into an ultrasmall volume, where it might strongly interact with quantum emitters. In recent years, there has been much interest in studying such a strong coupling in the limit of single emitters. However, the lossy nature of PCs, reflected in their broad spectra, limits their quality factors and hence their performance as cavities. Here, we study the effect of the adhesion layer used in the fabrication of metal nanostructures on the spectral linewidths of bowtie-structured PCs. Using dark-field microspectroscopy, as well as electron energy loss spectroscopy, it is found that a reduction in the thickness of the chromium adhesion layer we use from 3 nm to 0.1 nm decreases the linewidths of both bright and dark plasmonic modes. We further show that it is possible to fabricate bowtie PCs without any adhesion layer, in which case the linewidth may be narrowed by as much as a factor of 2. Linewidth reduction increases the quality factor of these PCs accordingly, and it is shown to facilitate reaching the strong-coupling regime with semiconductor quantum dots.

Application of Surface Click Reactions to Localized Surface Plasmon Resonance (LSPR) Biosensing.

Localized surface plasmon resonance (LSPR) spectroscopy is an effective tool for sensitive, affordable, and label-free biosensing. LSPR transducers based on nanoparticulate Au films have been applied to biosensing of receptor-analyte interactions, employing primarily thiolated receptors for constructing biorecognition interfaces on nanostructured Au surfaces. This popular method suffers from a major drawback, that is, the need to prepare a thiolated receptor for each system used, which is typically synthetically complex and time-consuming. Herein, we present an alternative approach based on the click reaction between azide and terminal alkyne, which avoids the need to synthesize thiol-derivatized receptors and is applicable to the heterogeneous morphology of LSPR transducers. The receptors are tethered with an alkyne group, which is considerably simpler than thiolation, while producing a stable product. The transducer surface is modified with a layer of a commercial long-chain thiol-azide molecule, then clicked with an alkyne-dertivatized receptor to produce the biorecognition interface. This method is employed for immobilization of four different alkyne-bearing receptor molecules on Au nano-island film based LSPR transducers, followed by testing of their performance in biorecognition of specific analytes using LSPR and FTIR spectroscopies. The results establish the usefulness of click chemistry for the preparation of biorecognition interfaces on nanostructured LSPR transducers.

pH-Dependent Galvanic Replacement of Supported and Colloidal Cu2O Nanocrystals with Gold and Palladium.

Galvanic replacement reactions (GRRs) on nanoparticles (NPs) are typically performed between two metals, i.e., a solid metal NP and a replacing salt solution of a more noble metal. The solution pH in GRRs is commonly considered an irrelevant parameter. Yet, the solution pH plays a major role in GRRs involving metal oxide NPs. Here, Cu(2)O nanocrystals (NCs) are studied as galvanic replacement (GR) precursors, undergoing replacement by gold and palladium, with the resulting nanostructures showing a strong dependence on the pH of the replacing metal salt solution. GRRs are reported for the first time on supported (chemically deposited) oxide NCs and the results are compared with those obtained with corresponding colloidal systems. Control of the pH enables production of different nanostructures, from metal-decorated Cu(2)O NCs to uniformly coated Cu(2)O-in-metal (Cu(2)O@Me) core-shell nanoarchitectures. Improved metal nucleation efficiencies at low pHs are attributed to changes in the Cu(2)O surface charge resulting from protonation of the oxide surface. GR followed by etching of the Cu(2)O cores provides metal nanocages that collapse upon drying; the latter is prevented using a sol-gel silica overlayer stabilizing the metal nanocages. Metal-replaced Cu(2)O NCs and their corresponding stabilized nanostructures may be useful as photocatalysts, electrocatalysts, and nanosensors.

Noble Metal Nanoparticles with Nanogel Coatings: Coinage Metal Thiolate-Stabilized Glutathione Hydrogel Shells.

Developing biocompatible nanocoatings is crucial for biomedical applications. Noble metal colloidal nanoparticles with biomolecular shells are thought to combine diverse chemical and optothermal functionalities with biocompatibility. Herein, we present nanoparticles with peptide hydrogel shells that feature an unusual combination of properties: the metal core possesses localized plasmon resonance, whereas a few-nanometer-thick shells open opportunities to employ their soft framework for loading and scaffolding. We demonstrate this concept with gold and silver nanoparticles capped by glutathione peptides stacked into parallel ß-sheets as they aggregate on the surface. A key role in the formation of the ordered structure is played by coinage metal(I) thiolates, i.e., Ag(I), Cu(I), and Au(I). The shell thickness can be controlled via the concentration of either metal ions or peptides. Theoretical modeling of the shell's molecular structure suggests that the thiolates have a similar conformation for all the metals and that the parallel ß-sheet-like structure is a kinetic product of the peptide aggregation. Using third-order nonlinear two-dimensional infrared spectroscopy, we revealed that the ordered secondary structure is similar to the bulk hydrogels of the coinage metal thiolates of glutathione, which also consist of aggregated stacked parallel ß-sheets. We expect that nanoparticles with hydrogel shells will be useful additions to the nanomaterial toolbox. The present method of nanogel coating can be applied to arbitrary surfaces where the initial deposition of the seed glutathione monolayer is possible.

Stabilization of metal nanoparticle films on glass surfaces using ultrathin silica coating.

Metal nanoparticle (NP) films, prepared by adsorption of NPs from a colloidal solution onto a preconditioned solid substrate, usually form well-dispersed random NP monolayers on the surface. For certain metals (e.g., Au, Ag, Cu), the NP films display a characteristic localized surface plasmon resonance (LSPR) extinction band, conveniently measured using transmission or reflection ultraviolet-visible light (UV-vis) spectroscopy. The surface plasmon band wavelength, intensity, and shape are affected by (among other parameters) the NP spatial distribution on the surface and the effective refractive index (RI) of the surrounding medium. A major concern in the formation of such NP assemblies on surfaces is a commonly observed instability, i.e., a strong tendency of the NPs to undergo aggregation upon removal from the solution and drying, expressed as a drastic change in the LSPR band. Since various imaging modes and applications require dried NP films, preservation of the film initial (wet) morphology and optical properties upon drying are highly desirable. The latter is achieved in the present work by introducing a convenient and generally applicable method for preventing NP aggregation upon drying while preserving the original film morphology and optical response. Stabilization of Au and Ag NP monolayers toward drying is accomplished by coating the immobilized NPs with an ultrathin (3.0-3.5 nm) silica layer, deposited using a sol-gel reaction performed on an intermediate self-assembled aminosilane layer. The thin silica coating prevents NP aggregation and maintains the initial NP film morphology and LSPR response during several cycles of drying and immersion in water. It is shown that the silica-coated NP films retain their properties as effective LSPR transducers.

Direct observation of aminoglycoside-RNA binding by localized surface plasmon resonance spectroscopy.

RNA is involved in fundamental biological functions when bacterial pathogens replicate. Identifying and studying small molecules that can interact with bacterial RNA and interrupt cellular activities is a promising path for drug design. Aminoglycoside (AMG) antibiotics, prominent natural products that recognize RNA specifically, exert their biological functions by binding to prokaryotic ribosomal RNA and interfering with protein translation, ultimately resulting in bacterial cell death. The decoding site, a small internal loop within the 16S rRNA, is the molecular target for the AMG antibiotics. The specificity of neomycin B, a highly potent AMG antibiotic, to the ribosomal decoding RNA site, was previously studied by observing AMG-RNA complexes in solution. Here, we study this interaction using localized surface plasmon resonance (LSPR) transducers comprising gold island films prepared by evaporation on glass and annealing. Small molecule AMG receptors were immobilized on the Au islands via polyethylene glycol (PEG)-thiol linkers, and the interaction with target RNA in solution was studied by monitoring the change in the LSPR optical response upon binding. The results show high-affinity binding of neomycin to 27-nucleotide model A-site RNA sequence in the nanomolar range, while no specific binding is observed for synthetic RNA oligomers (e.g., poly-U). The impact of specific base substitutions in the A-site RNA constructs on binding affinity and selectivity is determined quantitatively. It is concluded that LSPR is a powerful tool for providing molecular insight into small molecule-RNA interactions and for the design and screening of selective antimicrobial drugs.

Mechanism of morphology transformation during annealing of nanostructured gold films on glass.

Nanostructured, just-percolated gold films were prepared by evaporation on bare glass. Annealing of the films at temperatures close to or higher than the softening temperature of the glass substrate induces morphological transformation to discrete Au islands and gradual embedding of the formed islands in the glass. The mechanism and kinetics of these processes are studied here using a combination of in situ high-temperature optical spectroscopy; ex situ characterization of the island shape by high-resolution scanning electron microscopy (HRSEM), atomic force microcopy (AFM) and cross-sectional transmission electron microscopy (TEM); and numerical simulations of transmission spectra using the Multiple Multipole Program (MMP) approach. It is shown that the morphological transformation of just-percolated, 10 nm (nominal thickness) Au films evaporated on glass and annealed at 600 °C, i.e., in the vicinity of the substrate glass transition temperature (Tg = 557 °C), proceeds via three processes exhibiting different time scales: (i) fast recrystallization and dewetting, leading to formation of single-crystalline islands (minutes); the initial spectrum characteristic of a continuous Au film is transformed to that of an island film, displaying a surface plasmon (SP) absorption band. (ii) Reshaping and faceting of the single-crystalline islands accompanied by formation of circumferential glass rims around them (first few hours); the overall optical response shows a blue shift of the SP band. (iii) Gradual island embedding in the glass substrate (tens of hours), seen as a characteristic red shift of the SP band. The influence of the annealing atmosphere (air, vacuum) on the embedding process is found to be minor. Numerical modeling of the extinction cross-section corresponding to the morphological transformations during island recrystallization and embedding is in qualitative agreement with the experimental data.

Optimization of localized surface plasmon resonance transducers for studying carbohydrate-protein interactions.

Noble metal nanostructures supporting localized surface plasmons (SPs) have been widely applied to chemical and biological sensing. Changes in the refractive index near the nanostructures affect the SP extinction band, making localized surface plasmon resonance (LSPR) spectroscopy a convenient tool for studying biological interactions. Carbohydrate-protein interactions are of major importance in living organisms; their study is crucial for understanding of basic biological processes and for the construction of biosensors for diagnostics and drug development. Here LSPR transducers based on gold island films prepared by evaporation on glass and annealing were optimized for monitoring the specific interaction between Concanavalin A (Con A) and D-(+)-mannose. The sugar was modified with a PEG-thiol linker and immobilized on the Au islands. Sensing assays were performed under stationary and flow conditions, the latter providing kinetic parameters for protein binding and dissociation. Ellipsometry and Fourier transform-infrared (FT-IR) data, as well as scanning electron microscopy (SEM) imaging of fixated and stained samples, furnished independent evidence for the protein-sugar recognition. Enhanced response and visual detection of protein binding was demonstrated using Au nanoparticles stabilized with the linker-modified mannose molecules. Mannose-coated transducers display an excellent selectivity toward Con A in the presence of a large excess of bovine serum albumin (BSA).

A quantitative, real-time assessment of binding of peptides and proteins to gold surfaces.

Interactions of peptides and proteins with inorganic surfaces are important to both natural and artificial systems; however, a detailed understanding of such interactions is lacking. In this study, we applied new approaches to quantitatively measure the binding of amino acids and proteins to gold surfaces. Real-time surface plasmon resonance (SPR) measurements showed that TEM1-ß-lactamase inhibitor protein (BLIP) interacts only weakly with Au nanoparticles (NPs). However, fusion of three histidine residues to BLIP (3H-BLIP) resulted in a significant increase in the binding to the Au NPs, which further increased when the histidine tail was extended to six histidines (6H-BLIP). Further increasing the number of His residues had no effect on the binding. A parallel study using continuous (111)-textured Au surfaces and single-crystalline, (111)-oriented, Au islands by ellipsometry, FTIR, and localized surface plasmon resonance (LSPR) spectroscopy further confirmed the results, validating the broad applicability of Au NPs as model surfaces. Evaluating the binding of all other natural amino acid homotripeptides fused to BLIP (except Cys and Pro) showed that aromatic and positively-charged residues bind preferentially to Au with respect to small aliphatic and negatively charged residues, and that the rate of association is related to the potency of binding. The binding of all fusions was irreversible. These findings were substantiated by SPR measurements of synthesized, free, soluble tripeptides using Au-NP-modified SPR chips. Here, however, the binding was reversible allowing for determination of binding affinities that correlate with the binding potencies of the related BLIP fusions. Competition assays performed between 3H-BLIP and the histidine tripeptide (3 His) suggest that Au binding residues promote the adsorption of proteins on the surface, and by this facilitate the irreversible interaction of the polypeptide chain with Au. The binding of amino acids to Au was simulated by using a continuum solvent model, showing agreement with the experimental values. These results, together with the observed binding potencies and kinetics of the BLIP fusions and free peptides, suggest a binding mechanism that is markedly different from biological protein-protein interactions.

Versatile scheme for the step-by-step assembly of nanoparticle multilayers.

A versatile scheme for the preparation of nanoparticle (NP) multilayers is presented. The method is based on the step-by-step assembly of NPs and bishydroxamate disulfide ligand molecules by means of metal-organic coordination using easily synthesized tetraoctylammonium bromide (TOAB)-stabilized gold NPs. The assembly of NP multilayers was carried out via a Zr(IV)-coordinated sandwich arrangement of the hydroxamate ligands on Au and glass surfaces. The latter were precoated with electrolessly deposited Au clusters to enable binding of the first NP layer. The new method avoids the need to perform elaborate colloid reactions to prepare the NP building blocks. Au NP monolayer and multilayer films prepared in this manner were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM), showing a regular growth of NP layers. The use of coordination chemistry as the binding motif between repeat layers allows for the convenient assembly of hybrid nanostructures comprising molecular and NP components. This was demonstrated by the construction of Au NP multilayers with controlled spacing from the surface or between two NP layers. Drying the samples during or after the construction process induces NP aggregation and changes in the film morphology and optical properties.

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on "Naked" Silver Nanoparticles.

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the ß-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using 13C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native ß-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Protein-surface interactions: challenging experiments and computations.

Protein-surface interactions are fundamental in natural processes, and have great potential for applications ranging from nanotechnology to medicine. A recent workshop highlighted the current achievements and the main challenges in the field.

Rapid formation of coordination multilayers using accelerated self-assembly procedure (ASAP).

Layer-by-layer (LbL) assembly of multilayers on surfaces using metal-organic coordination between consecutive layers is a well-established method for multilayer construction. The basic scheme includes self-assembly of a ligand (anchor) monolayer on the surface, followed by alternate binding of metal ions and multifunctional ligand layers to form a coordination multilayer. Binding of the ligand repeat unit to form a new layer is commonly a slow process, taking typically overnight to complete. This renders the process of multilayer preparation exceedingly slow and, in many cases, impractical. Here we describe a method for LbL synthesis of self-assembled coordination multilayers denoted accelerated self-assembly procedure (ASAP), where binding of a full organic ligand layer occurs in ca. 1 min. In the new protocol a small volume of a dilute ligand solution is spread on the substrate surface and evaporated under natural convection conditions, leaving the surface covered with excess ligand. Extensive rinsing in pure solvent results in complete removal of unbound molecules from the surface, leaving only the new coordinated layer. ASAP is demonstrated here by the construction of two kinds of coordination multilayers, comprising mercaptoundecanoic acid-Cu(II) and bishydroxamate-Zr(IV). Multilayers prepared by ASAP and by the standard (overnight adsorption) procedure are compared using ellipsometry, contact-angle, and FTIR data, showing regular multilayer growth in both cases. However, the rapid binding associated with ASAP may lead to a different structure than the one reached after prolonged assembly. Study of the ASAP mechanism suggests that the fast ligand binding kinetics are attributed to a large increase of the local ligand concentration at the moving liquid front when the solvent evaporates on the surface.

Raman spectroelectrochemistry of molecules within individual electromagnetic hot spots.

The role of chemical enhancement in surface-enhanced Raman scattering (SERS) remains a contested subject. We study SERS spectra of 4-mercaptopyridine molecules excited far from the molecular resonance, which are collected from individual electromagnetic hot spots at concentrations close to the single-molecule limit. The hot spots are created by depositing Tollen's silver island films on a transparent electrode incorporated within an electrochemical cell. Analysis of the intensity of the spectra relative to those obtained from individual rhodamine 6G molecules on the same surface provides a lower limit of approximately 3 orders of magnitude for the chemical enhancement. This large enhancement is likely to be due to a charge transfer resonance involving the transfer of an electron from the metal to an adsorbed molecule. Excitation at three different wavelengths, as well as variation of electrode potential from 0 to -1.2 V, lead to significant changes in the relative intensities of bands in the spectrum. It is suggested that while the bulk of the enhancement is due to an Albrecht A-term resonance Raman effect (involving the charge transfer transition), vibronic coupling provides additional enhancement which is sensitive to electrode potential. The measurement of potential-dependent SERS spectra from individual hot spots opens the way to a thorough characterization of chemical enhancement, as well to studies of redox phenomena at the single-molecule level.

Mass thickness analysis of gold thin films using room temperature gas-phase chlorination.

Chemical interaction of ultrathin gold films with gaseous chlorine at room temperature results in quantitative metal chlorination and formation of Au(III) chloride. Combination of gas-phase chlorination of Au nanostructures with dissolution of the chloride product and spectrophotometric determination of the chloroaurate concentration presents a simple procedure for quantitative determination of the mass thickness of Au nanostructures. The method is demonstrated by mass thickness analysis of the condensation coefficient of Au adatoms on solid substrates, the extinction coefficient of Au nanoparticles, as well as study of the integrity of Au island films upon interaction with solvents.

Empowering Electroless Plating to Produce Silver Nanoparticle Films for DNA Biosensing Using Localized Surface Plasmon Resonance Spectroscopy.

To facilitate the implementation of biosensors based on the localized surface plasmon resonance (LSPR) of metal nanostructures, there is a great need for cost-efficient, flexible, and tunable methods for producing plasmonic coatings. Due to its simplicity and excellent conformity, electroless plating (EP) is well suited for this task. However, it is traditionally optimized to produce continuous metal films, which cannot be employed in LSPR sensors. Here, we outline the development of an EP strategy for depositing island-like silver nanoparticle (NP) films on glass with distinct LSPR bands. The fully wet-chemical process only employs standard chemicals and proceeds within minutes at room temperature. The key step for producing spread-out NP films is an accelerated ripening of the silver seed layer in diluted hydrochloric acid, which reduces the nucleation density during plating. The reaction kinetics and mechanisms are investigated with scanning (transmission) electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy, with the latter enabling a convenient live monitoring of the deposition, allowing its termination at a stage of desired optical properties. The sensing capabilities of chemically deposited NP films as LSPR transducers are exemplified in DNA biosensing. To this end, a sensing interface is prepared using layer-by-layer (LbL) buildup of polyelectrolytes (PE), followed by adsorption and covalent immobilization of ssDNA. The obtained LSPR transducers demonstrate robustness and selectivity in sensing experiments with binding complementary and unrelated DNA strands.

Biological sensing and interface design in gold island film based localized plasmon transducers.

Discontinuous, island-type gold films (typically < or = 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody-antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.

Polymer-coated gold island films as localized plasmon transducers for gas sensing.

Ultrathin (typically < or = 10-nm thick) gold island films evaporated on transparent substrates show a prominent localized surface plasmon (SP) extinction in the visible-to-NIR range. Changes in the dielectric properties of the contacting medium influence the SP absorption band, providing a scheme for optical sensing based on refractive index change. In the present work, the gas sensing capability of gold island based localized surface plasmon resonance (LSPR) transducers was explored using polymeric coatings as the active interface. LSPR transducers were fabricated by spin-coating of polystyrene (PS) or polystyrene sulfonic acid, sodium salt (PSS) onto 5-nm-thick (nominal thickness) gold island films evaporated on silanized glass and annealed. Detailed characterization of the transducers was carried out using high-resolution scanning electron microscopy, cross-sectional transmission electron microscopy, and in situ atomic force microscopy under controlled atmosphere. The hydrophobic PS film exhibits swelling and significant thickness increase upon exposure to chloroform vapor and little or no change in water vapor, whereas the hydrophilic PSS film shows the opposite behavior when exposed to the same vapors. Polymer swelling upon absorption of vapors of good solvents shows a net effect of lowering the effective refractive index in the vicinity of the gold islands, manifested as a characteristic decrease of the SP band intensity and a blue shift of the band maximum. The response, measured for four different vapors, is fast (approximately 15 s) and reversible. It is shown that gold island systems coated with polymeric films can be applied to vapor recognition in an array configuration.

Nucleation-Controlled Solution Deposition of Silver Nanoplate Architectures for Facile Derivatization and Catalytic Applications.

Due to their distinctive electronic, optical, and chemical properties, metal nanoplates represent important building blocks for creating functional superstructures. Here, a general deposition method for synthesizing Ag nanoplate architectures, which is compatible with a wide substrate range (flexible, curved, or recessed; consisting of carbon, silicon, metals, oxides, or polymers) is reported. By adjusting the reaction conditions, nucleation can be triggered in the bulk solution, on seeds and by electrodeposition, allowing the production of nanoplate suspensions as well as direct surface modification with open-porous nanoplate films. The latter are fully percolated, possess a large, easily accessible surface, a defined nanostructure with {111} basal planes, and expose defect-rich, particularly reactive edges in high density, making them compelling platforms for heterogeneous catalysis, and electro- and flow chemistry. This potential is showcased by exploring the catalytic performance of the nanoplates in the reduction of carbon dioxide, 4-nitrophenol, and hydrogen peroxide, devising two types of microreactors, and by tuning the nanoplate functionality with derivatization reactions.

Refractive Index Sensing Using Visible Electromagnetic Resonances of Supported Cu2O Particles.

Plasmonic metal nanostructures, in colloidal or surface-supported forms, have been extensively studied in the context of metamaterials design and applications, in particular as refractometric sensing platforms. Recently, high refractive index (high-n) dielectric subwavelength structures have been experimentally shown to support strong Mie scattering resonances, predicted to exhibit analogous refractive index sensing capabilities. Here we present the first experimental demonstration of the use of supported high-n dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-n material. Single-crystalline Cu2O particles were deposited on transparent substrates using a chemical deposition scheme, showing well-defined electric and magnetic dipolar resonances (EDR and MDR, respectively) in the visible range, which change in intensity and wavelength upon changing the medium refractive index (nm). The significant modulation of the MDR intensity when nm is modified appears to be the most valuable empirical sensing parameter. The Mie scattering properties of Cu2O particles, particularly the spectral dependence of the MDR on nm, are theoretically modeled to support the experimental observations. MDR extinction changes (i.e., refractive index sensitivity) per particle are >100 times higher compared to localized surface plasmon resonance (LSPR) changes in supported Au nanoislands, encouraging the evaluation of Cu2O and other high-n dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)sensing applications.