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.

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.

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.

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.

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.

Template-Free Electroless Plating of Gold Nanowires: Direct Surface Functionalization with Shape-Selective Nanostructures for Electrochemical Applications.

Metal nanowires (NWs) represent a prominent nanomaterial class, the interest in which is fueled by their tunable properties as well as their excellent performance in, for example, sensing, catalysis, and plasmonics. Synthetic approaches to obtain metal NWs mostly produce colloids or rely on templates. Integrating such nanowires into devices necessitates additional fabrication steps, such as template removal, nanostructure purification, or attachment. Here, we describe the development of a facile electroless plating protocol for the direct deposition of gold nanowire films, requiring neither templates nor complex instrumentation. The method is general, producing three-dimensional nanowire structures on substrates of varying shape and composition, with different seed types. The aqueous plating bath is prepared by ligand exchange and partial reduction of tetrachloroauric acid in the presence of 4-dimethylaminopyridine and formaldehyde. Gold deposition proceeds by nucleation of new grains on existing nanostructure tips and thus selectively produces curvy, polycrystalline nanowires of high aspect ratio. The nanofabrication potential of this method is demonstrated by producing a sensor electrode, whose performance is comparable to that of known nanostructures and discussed in terms of the catalyst architecture. Due to its flexibility and simplicity, shape-selective electroless plating is a promising new tool for functionalizing surfaces with anisotropic metal nanostructures.

Real-time plasmon spectroscopy study of the solid-state oxidation and Kirkendall void formation in copper nanoparticles.

Oxidation and corrosion reactions have a major effect on the application of non-noble metals. Kinetic information and simple theoretical models are often insufficient for describing such processes in metals at the nanoscale, particularly in cases involving formation of internal voids (nano Kirkendall effect, NKE) during oxidation. Here we study the kinetics of solid-state oxidation of chemically-grown copper nanoparticles (NPs) by in situ localized surface plasmon resonance (LSPR) spectroscopy during isothermal annealing in the range 110-170 °C. We show that LSPR spectroscopy is highly effective in kinetic studies of such systems, enabling convenient in situ real-time measurements during oxidation. Change of the LSPR spectra throughout the oxidation follows a common pattern, observed for different temperatures, NP sizes and substrates. The well-defined initial Cu NP surface plasmon (SP) band red-shifts continuously with oxidation, while the extinction intensity initially increases to reach a maximum value at a characteristic oxidation time τ, after which the SP intensity continuously drops. The characteristic time τ is used as a scaling parameter for the kinetic analysis. Evolution of the SP wavelength and extinction intensity during oxidation at different temperatures follows the same kinetics when the oxidation time is normalized to τ, thus pointing to a general oxidation mechanism. The characteristic time τ is used to estimate the activation energy of the process, determined to be 144 ± 6 kJ mol-1, similar to previously reported values for high-temperature Cu thermal oxidation. The central role of the NKE in the solid-state oxidation process is revealed by electron microscopy, while formation of Cu2O as the major oxidation product is established by X-ray diffraction, XPS, and electrochemical measurements. The results indicate a transition of the oxidation mechanism from a Valensi-Carter (VC) to NKE mechanism with the degree of oxidation. To interpret the optical evolution during oxidation, Mie scattering solutions for metal core-oxide shell spherical particles are computed, considering formation of Kirkendall voids. The model calculations are in agreement with the experimental results, showing that the large red-shift of the LSPR band during oxidation is the result of Kirkendall voiding, thus establishing the major role of the NKE in determining the optical behavior of such systems.

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.

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.

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.

Chemical deposition of Cu(2)O nanocrystals with precise morphology control.

Copper(I) oxide nanoparticles (NPs) are emerging as a technologically important material, with applications ranging from antibacterial and fungicidal agents to photocatalysis. It is well established that the activity of Cu2O NPs is dependent on their crystalline morphology. Here we describe direct preparation of Cu2O nanocrystals (NCs) on various substrates by chemical deposition (CD), without the need of additives, achieving precise control over the NC morphology. The substrates are preactivated by gold seeding and treated with deposition solutions comprising copper sulfate, formaldehyde, NaOH, and citrate as a complexant. Production of NC deposits ranging from complete cubes to complete octahedra is demonstrated, as well as a full set of intermediate morphologies, i.e., truncated octahedra, cuboctahedra, and truncated cubes. The NC morphology is defined by the NaOH and complexant concentrations in the deposition solution, attributed to competitive adsorption of citrate and hydroxide anions on the Cu2O {100} and {111} crystal faces and selective stabilization of these faces. A sequential deposition scheme, i.e., Cu2O deposition on pregrown Cu2O NCs of a different morphology, is also presented. The full range of morphologies can be produced by controlling the deposition times in the two solutions, promoting the cubic and octahedral crystal habits. Growth rates in the ⟨100⟩ and ⟨111⟩ directions for the two solutions are estimated. The Cu2O NCs are characterized by SEM, TEM, GI-XRD, and UV-vis spectroscopy. It is concluded that CD furnishes a simple, effective, generally applicable, and scalable route to the synthesis of morphologically controlled Cu2O NCs on a variety of conductive and nonconductive surfaces.

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).

Stabilization of gold nanoparticle films on glass by thermal embedding.

The poor adhesion of gold nanoparticles (NPs) to glass has been a known obstacle to studies and applications of NP-based systems, such as glass/Au-NP optical devices. Here we present a simple scheme for obtaining stable localized surface plasmon resonance (LSPR) transducers based on Au NP films immobilized on silanized glass and annealed. The procedure includes high-temperature annealing of the Au NP film, leading to partial embedding in the glass substrate and stabilization of the morphology and optical properties. The method is demonstrated using citrate-stabilized Au NPs, 20 and 63 nm mean diameter, immobilized electrostatically on glass microscope cover slides precoated with an aminosilane monolayer. Partial thermal embedding of the Au NPs in the glass occurs at temperatures in the vicinity of the glass transition temperature of the substrate. Upon annealing in air the Au NPs gradually settle into the glass and become encircled by a glass rim. In situ transmission UV-vis spectroscopy carried out during the annealing in a specially designed optical oven shows three regions: The most pronounced change of the surface plasmon (SP) band shape occurs in the first ca. 15 min of annealing; this is followed by a blue-shift of the SP band maximum (up to ca. 5 h), after which a steady red-shift of the SP band is observed (up to ca. 70 h, when the experiment was terminated). The development of the SP extinction spectrum was correlated to changes in the system structure, including thermal modification of the NP film morphology and embedding in the glass. The partially embedded Au NP films pass successfully the adhesive-tape test, while their morphology and optical response are stable toward immersion in solvents, drying, and thiol self-assembly. The enhanced adhesion is attributed to the metal NP embedding and rim formation. The stabilized NP films display a refractive index sensitivity (RIS) of 34-48 nm/RIU and 0.1-0.4 abs.u./RIU in SP band shift and extinction change, respectively. The RIS can be improved significantly by electroless deposition of Au on the embedded NPs, while the system stability is maintained. The method presented provides a simple route to obtaining stable Au NP film transducers.

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.

Sensitivity and optimization of localized surface plasmon resonance transducers.

Gold nanoisland films displaying localized surface plasmon resonance optical response were constructed by evaporation on glass and annealing. The surface plasmon distance sensitivity and refractive index sensitivity (RIS) for island films of different nominal thicknesses and morphologies were investigated using layer-by-layer polyelectrolyte multilayer assembly. Since the polymer forms a conformal coating on the Au islands and the glass substrate between islands, the relative sensitivity of the optical response to adsorption on and between islands was evaluated. The RIS was also determined independently using a series of solvents. An apparent discrepancy between the behavior of the RIS for wavelength shift and intensity change is resolved by considering the different physical nature of the two quantities, leading to the use of a new variable, that is, RIS (for intensity change) normalized to the surface density of islands. In the present system the surface plasmon decay length and RIS are shown to be directly correlated; both parameters increase with increasing average island size. This result implies that a higher RIS is not always beneficial for sensing; maximizing the transducer optical response requires the interrelated RIS and decay length to be optimized with respect to the dimensions of the studied analyte-receptor system. It is shown that, as a rule, transducers comprising larger islands furnish better overall sensitivity for thicker adlayers, whereas thinner adlayers produce a larger response when sensed using transducers comprising smaller islands, despite the lower RIS of the latter.

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.

Improved Sensitivity of Localized Surface Plasmon Resonance Transducers Using Reflection Measurements.

The refractive index sensitivity (RIS) of a localized surface plasmon resonance (LSPR) transducer is one of the key parameters determining its effectiveness in sensing applications. LSPR spectra of nanoparticulate gold films, including Au island films prepared by evaporation on glass and annealing as well as immobilized Au nanoparticle (NP) films, were measured in the transmission and reflection modes. It is shown that the RIS, measured as the wavelength shift in solvents with varying refractive index (RI), is significantly higher in reflection measurements.