Ultrahigh-Vacuum Tip-Enhanced Raman Spectroscopy.

Molecule-surface interactions and processes are at the heart of many technologies, including heterogeneous catalysis, organic photovoltaics, and nanoelectronics, yet they are rarely well understood at the molecular level. Given the inhomogeneous nature of surfaces, molecular properties often vary among individual surface sites, information that is lost in ensemble-averaged techniques. In order to access such site-resolved behavior, a technique must possess lateral resolution comparable to the size of surface sites under study, analytical power capable of examining chemical properties, and single-molecule sensitivity. Tip-enhanced Raman spectroscopy (TERS), wherein light is confined and amplified at the apex of a nanoscale plasmonic probe, meets these criteria. In ultrahigh vacuum (UHV), TERS can be performed in pristine environments, allowing for molecular-resolution imaging, low-temperature operation, minimized tip and molecular degradation, and improved stability in the presence of ultrafast irradiation. The aim of this review is to give an overview of TERS experiments performed in UHV environments and to discuss how recent reports will guide future endeavors. The advances made in the field thus far demonstrate the utility of TERS as an approach to interrogate single-molecule properties, reactions, and dynamics with spatial resolution below 1 nm.

Plasmon-enhanced colorimetric ELISA with single molecule sensitivity.

Robust but ultrasensitive biosensors with a capability of detecting low abundance biomarkers could revolutionize clinical diagnostics and enable early detection of cancer, neurological diseases, and infections. We utilized a combination of localized surface plasmon resonance (LSPR) refractive index sensing and the well-known enzyme-linked immunosorbent assay to develop a simple colorimetric biosensing methodology with single molecule sensitivity. The technique is based on spectral imaging of a large number of isolated gold nanoparticles. Each particle binds a variable number of horseradish peroxidase (HRP) enzyme molecules that catalyze a localized precipitation reaction at the particle surface. The enzymatic reaction dramatically amplifies the shift of the LSPR scattering maximum, λ(max), and makes it possible to detect the presence of only one or a few HRP molecules per particle.

Syntheses, Crystal and Band Structures, and Magnetic and Optical Properties of New CsLnCdTe(3) (Ln = La, Pr, Nd, Sm, Gd-Tm, and Lu).

A series of new quaternary semiconductor materials CsLnCdTe(3) (Ln = La, Pr, Nd, Sm, Gd-Tm, and Lu) was obtained from high-temperature solid-state reactions by the reactive halide flux method. These compounds belong to the layered KZrCuS(3) structure type and crystallize in the orthorhombic space group Cmcm (No. 63). Their structure features two-dimensional infinity(2)[LnCdTe(3)-] layers of edge- and vertex-sharing LnTe(6) octahedra with Cd atoms filling the tetrahedral interstices, which stack along b-axis. The Cs atoms are located between the infinity(2)[LnCdTe(3)-] layers and are surrounded by eight Te atoms to form a CsTe(8) bicapped trigonal prism. Such Te layers are more flexible than the Se analogues in the isostructural CsLnMSe(3) to accommodate nearly the entire Ln series. Theoretical studies performed on CsTmCdTe(3) show that the material is a direct band gap semiconductor and agrees with the result from a single-crystal optical absorption measurement. Magnetic susceptibility measurements show that the CsLnCdTe(3) (Ln = Pr, Nd, Gd, Dy, Tm) materials exhibit temperature-dependent paramagnetism and obey the Curie-Weiss law, whereas CsSmCdTe(3) does not.

Interaction of plasmon and molecular resonances for rhodamine 6G adsorbed on silver nanoparticles.

Localized surface plasmon resonance (LSPR) is a key optical property of metallic nanoparticles. The peak position of the LSPR for noble-metal nanoparticles is highly dependent upon the refractive index of the surrounding media and has therefore been used for chemical and biological sensing. In this work, we explore the influence of resonant adsorbates on the LSPR of bare Ag nanoparticles (lambda(max,bare)). Specifically, we study the effect of rhodamine 6G (R6G) adsorption on the nanoparticle plasmon resonance because of its importance in single-molecule surface-enhanced Raman spectroscopy (SMSERS). Understanding the coupling between the R6G molecular resonances and the nanoparticle plasmon resonances will provide further insights into the role of LSPR and molecular resonance in SMSERS. By tuning lambda(max,bare) through the visible wavelength region, the wavelength-dependent LSPR response of the Ag nanoparticles to R6G binding was monitored. Furthermore, the electronic transitions of R6G on Ag surface were studied by measuring the surface absorption spectrum of R6G on an Ag film. Surprisingly, three LSPR shift maxima are found, whereas the R6G absorption spectrum shows only two absorption features. Deconvolution of the R6G surface absorption spectra at different R6G concentrations indicates that R6G forms dimers on the metal surface. An electromagnetic model based on quasi-static (Gans) theory reveals that the LSPR shift features are associated with the absorption of R6G monomer and dimers. Electronic structure calculations of R6G under various conditions were performed to study the origin of the LSPR shift features. These calculations support the view that the R6G dimer formation is the most plausible cause for the complicated LSPR response. These findings show the extreme sensitivity of LSPR in elucidating the detailed electronic structure of a resonant adsorbate.

Preliminary studies and potential applications of localized surface plasmon resonance spectroscopy in medical diagnostics.

Miniature optical sensors that specifically identify low concentrations of environmental and biological substances are in high demand. Currently, there is no optical sensor that provides identification of the aforementioned species without amplification techniques at naturally occurring concentrations. Recently, it has been demonstrated that triangular silver nanoparticles have remarkable optical properties and that their enhanced sensitivity to their nanoenvironment has been used to develop a new class of optical sensors using localized surface plasmon resonance spectroscopy. The examination of both model and nonmodel biological assays using localized surface plasmon resonance spectroscopy will be presented in this review. It will be demonstrated that the use of a localized surface plasmon resonance nanosensor rivals the sensitivity and selectivity of, and provides a low-cost alternative to, commercially available sensors.