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\u27s 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 ?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

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

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-<i>n</i>) 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-<i>n</i> dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-<i>n</i> 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 (<i>n</i>_m). The significant modulation of the MDR intensity when <i>n</i>_m 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 <i>n</i>_m, 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-<i>n</i> dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)­sensing applications

A General Kinetic-Optical Model for Solid-State Reactions Involving the Nano Kirkendall Effect. The Case of Copper Nanoparticle Oxidation

Oxidation of copper nanoparticles (NPs) and other solid-state reactions have been shown recently to involve the nanoscale Kirkendall effect (NKE). The process, resulting in the formation of internal voids, has been rarely considered in most of the simple reaction kinetic models currently available. Here we present a general solid-state reaction kinetic model based on the assumption of steady-state diffusion profiles for rationalizing the evolution of the optical behavior observed in plasmonic Cu NPs undergoing solid-state oxidation to Cu₂O. While the model is applied here to an oxidation process, it is applicable in principle to any system showing Kirkendall voiding. An analytical expression of the rate law for spherical NPs is derived, and implications of the model are discussed. By combining the general kinetic model with Mie scattering solutions under the quasi-static approximation, the extinction cross section of Cu NPs as a function of the Cu-to-Cu₂O conversion fraction is modeled, for NPs of different initial sizes and for different degrees of shrinkage upon formation of a Cu₂O shell layer. The experimental observation of an initial increase in the surface plasmon (SP) band extinction intensity, followed by a decrease, is qualitatively reproduced using the combined model. This characteristic behavior of Cu nanoplasmonic systems is not expected theoretically in sufficiently small NPs; however, NPs of > ∼6 nm in diameter are expected to exhibit this type of behavior. Our results suggest that the NKE may be important in describing the optical behavior of plasmonic NPs subject to solid-state oxidation