Pairs of nanoparticles separated by a controllable gap size that can be as small as 3 nm are fabricated via a top-down lithographic procedure. This fabrication method would be useful not only for surface-enhanced Raman scattering, where it could potentially enable single-molecule sensitivity, but also for other applications in plasmonics and nonlinear optics.
The interaction between localized and propagating surface plasmons is investigated in a structure consisting of a two-dimensional periodic gold nanoparticle array, an SiO2 spacer, and a gold film. The resonance wavelengths of the two types of surface plasmons supported by the structure are tailored by changing the gold nanoparticle size and the array period. An anticrossing of the resonance positions is observed in the reflection spectra, demonstrating the strong coupling between localized and propagating surface plasmons.
Raman signals from molecules adsorbed on a noble metal surface are enhanced by many orders of magnitude due to the plasmon resonances of the substrate. Additionally, the enhanced spectra are modified compared to the spectra of neat molecules: many vibrational frequencies are shifted and relative intensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attributed to the frequency dependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computationally using molecule-metal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate.
Abstract : We performed research on the design and realization of high performance substrates for surface enhanced Raman scattering (SERS), and elucidated the role of chemical interactions between analyte molecules and a plasmonic substrate, the so-called chemical effect. Two approaches were taken for the realization of high performance SERS substrates. In the first, metal nanostructures supporting surface plasmons were fabricated by electron beam lithography. We demonstrated that by optimizing the design of metallic nanostructures, the average enhancement factor (EF) for surface-enhanced Raman scattering (SERS) could be as large as 8.4x108. The angular dependencies of the local field enhancement and the Raman emission enhancement were also investigated. We demonstrated that a stronger SERS signal resulted when the plasmonic substrate was illuminated with a collimated, rather than focused, laser beam. In the second approach, a pulsed laser was used to texture a silicon wafer to form sharp features. Silver was evaporated onto the wafer, and the resulting structures were found to exhibit very high SERS performance. In the theory effort, a comprehensive analysis of the chemical effect, including analytical and computational modeling, was accomplished.
We demonstrate the collimation of Raman scattering by a SERS substrate consisting of optical antennas, a metallic reflector and a 1D grating of metal strips. A ~6.1° FWHM angle perpendicular to the strips is measured.
Molecular beacons represent a new family of fluorescent probes for nucleic acids, and have found broad applications in recent years due to their unique advantages over traditional probes. Detection of nucleic acids using molecular beacons has been based on hybridization between target molecules and molecular beacons in a 1:1 stoichiometric ratio. The stoichiometric hybridization, however, puts an intrinsic limitation on detection sensitivity, because one target molecule converts only one beacon molecule to its fluorescent form. To increase the detection sensitivity, a conventional strategy has been target amplification through polymerase chain reaction. Instead of target amplification, here we introduce a scheme of signal amplification, nicking enzyme signal amplification, to increase the detection sensitivity of molecular beacons. The mechanism of the signal amplification lies in target-dependent cleavage of molecular beacons by a DNA nicking enzyme, through which one target DNA can open many beacon molecules, giving rise to amplification of fluorescent signal. Our results indicate that one target DNA leads to cleavage of hundreds of beacon molecules, increasing detection sensitivity by nearly three orders of magnitude. We designed two versions of signal amplification. The basic version, though simple, requires that nicking enzyme recognition sequence be present in the target DNA. The extended version allows detection of target of any sequence by incorporating rolling circle amplification. Moreover, the extended version provides one additional level of signal amplification, bringing the detection limit down to tens of femtomolar, nearly five orders of magnitude lower than that of conventional hybridization assay.
Analysis and Applications. interfaces. Experimentally determined values for the
dielectric functions of gold and silver are taken from the work of Johnson and
Christy.47 When considering only the real part of the propagation constant, the ...
The strong coupling between localized surface plasmons and surface plasmon polaritons in a double resonance surface enhanced Raman scattering (SERS) substrate is described by a classical coupled oscillator model. The effects of the particle density, the particle size and the SiO2 spacer thickness on the coupling strength are experimentally investigated. We demonstrate that by tuning the geometrical parameters of the double resonance substrate, we can readily control the resonance frequencies and tailor the SERS enhancement spectrum.
We report a surface-enhanced Raman scattering (SERS) substrate with plasmon resonances at both excitation and Stokes frequencies. This multilayer structure combines localized surface plasmons on the nanoparticles with surface plasmon polaritons excited on a gold film. The largest SERS enhancement factor for a gold device is measured to be 7.2 × 107, which is more than 2 orders of magnitude larger than that measured on a gold nanoparticle array on a glass substrate. The largest SERS enhancement for a silver device is measured to be 8.4 × 108.
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