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We describe the fabrication of optimized plasmonic substrates in the form of immobilized nanorod assemblies (INRA) for surface-enhanced Raman spectroscopy (SERS). Induded are high-resolution scanning electron micrograph (SEM) images of the surface structures, along with a mechanistic description of their growth. It is shown that, by varying the size of support microspheres, the surface plasmon resonance is tuned between 330 and 1840 nm. Notably, there are predicted optimal microsphere sizes for each of the commonly used SERS laser wavelengths of 532, 633, 785, and 1064 nm. [ABSTRACT FROM AUTHOR]

We report a method based on density functional theory molecular dynamics that allows us to calculate the plasmonic properties of liquid metals and metal alloys from first principles with no a priori knowledge of the system. We show exceptional agreement between the simulated and measured optical constants of liquid Ga and the room temperature liquid In–Ga eutectic alloy (Tm = 289 K). We then use this method to analyze the plasmonic properties of various alloy concentrations in the In–Ga system. The plasmonic performance of the In–Ga system decreases with increasing In concentration. However, the benefits of a room-temperature plasmonic liquid are likely to outweigh the minor reduction in plasmonic performance when moving from pure Ga to the eutectic composition. Our results show that density functional theory molecular dynamics can be used as a predictive tool for studying the optical properties of liquid metal systems amenable to plasmonics. [ABSTRACT FROM AUTHOR]

Here we discuss some of the challenges of using dimers of spheres for SERS at long wavelengths. We show that the field enhancement in the gap between two spheres scales with a local surface plasmon metric, effectively limiting the range of wavelengths for which the largest SERS enhancements can be obtained for this class of structures to the near-IR. [ABSTRACT FROM AUTHOR]

The properties of nanoparticle superstructuresdepend on many factors,including the structural metrics of the nanoparticle superstructure(particle diameter, interparticle distances, etc.). Here, we introducea family of gold-binding peptide conjugate molecules that can directnanoparticle assembly, and we describe how these molecules can besystematically modified to adjust the structural metrics of lineardouble-helical nanoparticle superstructures. Twelve new peptide conjugatesare prepared via linking a gold-binding peptide, AYSSGAPPMPPF(PEPAu), to a hydrophobic aliphatic tail. The peptide conjugateshave 1, 2, or 3 PEPAuheadgroups and a C12,C14, C16, or C18aliphatic tail.The soft assembly of these peptide conjugates was studied using transmissionelectron microscopy (TEM), atomic force microscopy (AFM), and infrared(IR) spectroscopy. Several peptide conjugates assemble into 1-D twistedfibers having measurable structural parameters such as fiber width,thickness, and pitch that can be systematically varied by adjustingthe aliphatic tail length and number of peptide headgroups. The linearsoft assemblies serve as structural scaffolds for arranging gold nanoparticlesinto double-helical superstructures, which are examined via TEM. Thepitch and interparticle distances of the gold nanoparticle doublehelices correspond to the underlying metrics of the peptide conjugatesoft assemblies, illustrating that designed peptide conjugate moleculescan be used to not only direct the assembly of gold nanoparticlesbut also control the metrics of the assembled structure. [ABSTRACT FROM AUTHOR]

Bottom-up assemblies of plasmonic nanoparticles exhibit unique optical effects such as tunable reflection, optical cavity modes, and tunable photonic resonances. Here, we compare detailed simulations with experiment to explore the effect of structural inhomogeneity on the optical response in DNA-gold nanoparticle superlattices. In particular, we explore the effect of background environment, nanoparticle polydispersity (>10%), and variation in nanoparticle placement (~5%). At volume fractions less than 20% Au, the optical response is insensitive to particle size, defects, and inhomogeneity in the superlattice. At elevated volume fractions (20% and 25%), structures incorporating different sized nanoparticles (10-, 20-, and 40-nm diameter) each exhibit distinct far-field extinction and near-field properties. These optical properties are most pronounced in lattices with larger particles, which at fixed volume fraction have greater plasmonic coupling than those with smaller particles. Moreover, the incorporation of experimentally informed inhomogeneity leads to variation in far-field extinction and inconsistent electric-field intensities throughout the lattice, demonstrating that volume fraction is not sufficient to describe the optical properties of such structures. These data have important implications for understanding the role of particle and lattice inhomogeneity in determining the properties of plasmonic nanoparticle lattices with deliberately designed optical properties. [ABSTRACT FROM AUTHOR]

Determining the existence of any direct spectral relationship between the far-field scattering properties and the near-field Raman-enhancing properties of surface-enhanced Raman spectroscopy (SERS) substrates has been a challenging task with only a few significant results to date. Here, we prove that hot spot dominated systems show little dependence on the far-field scattering properties because of differences between near- and far-field localized surface plasmon resonance (LSPR) effects as well as excitation of new plasmon modes via a localized emitter. We directly probe the relationship between the near- and far-field light interactions using a correlated LSPR-transmission electron microscopy (TEM) surface-enhanced Raman excitation spectroscopy (SERES) technique. Fourteen individual SERS nanoantennas, Au nanoparticle aggregates ranging from dimers to undecamers, coated in a reporter molecule and encased in a protective silica shell, were excited using eight laser wavelengths. We observed no correlation between the spectral position of the LSPR maxima and the maximum enhancement factor (EF). The single nanoantenna data reveal EFs ranging from (2.5 ± 0.6) × 104 to (4.5 ± 0.6) × 108 with maximum enhancement for excitation wavelengths of 785 nm and lower energy. The magnitude of maximum EF was not correlated to the number of cores in the nanoantenna or the spectral position of the LSPR, suggesting a separation between near-field SERS enhancement and far-field Rayleigh scattering. Computational electrodynamics confirms the decoupling of maximum SERS enhancement from the peak of the scattering spectrum. It also points to the importance of a localized emitter for radiating Raman photons to the far-field which, in nonsymmetric systems, allows for the excitation of radiative plasmon modes that are difficult to excite with plane waves. Once these effects are considered, we are able to fully explain the hot spot dominated SERS response of the nanoantennas. [ABSTRACT FROM AUTHOR]

This paper demonstrates the direct sensing of glucose at physiologically relevant concentrations with surface-enhanced Raman spectroscopy (SERS) on gold film-over-nanosphere (AuFON) substrates functionalized with bisboronic acid receptors. The combination of selectivity in the bisboronic acid receptor and spectral resolution in the SERS data allow the sensors to resolve glucose in high backgrounds of fructose and, in combination with multivariate statistical analysis, detect glucose accurately in the 1–10 mM range. Computational modeling supports assignments of the normal modes and vibrational frequencies for the monoboronic acid base of our bisboronic acids, glucose and fructose. These results are promising for the use of bisboronic acids as receptors in SERS-based in vivo glucose monitoring sensors. [ABSTRACT FROM AUTHOR]