In this report we discuss the influence of plasmon excitations in a silver island film on the fluorescence of photosynthetic complex, peridinin-chlorophyll-protein (PCP). Control of the separation between these two components is obtained by fabricating a wedge layer of silica across the substrate, with a thickness from 0 to 46 nm. Continuous variation of the silica thickness allows for gradual change of interaction strength between plasmon excitations in the metallic film and the excited states of pigments comprising photosynthetic complexes. While the largest separation between the silver film and photosynthetic complexes results in fluorescence featuring a mono-exponential decay and relatively narrow distribution of intensities, the PCP complexes placed on thinner silica spacers show biexponential fluorescence decay and significantly broader distribution of total fluorescence intensities. This broad distribution is a signature of stronger sensitivity of fluorescence enhancement upon actual parameters of a hybrid nanostructure. By gradual change of the silica spacer thickness we are able to reproduce classical distance dependence of fluorescence intensity in plasmonic hybrid nanostructures on ensemble level. Experiments carried out for different excitation wavelengths indicate that the interaction is stronger for excitations resonant with plasmon absorption in the metallic layer.
We have studied the ability of a lamellar near-field superlens to transfer an enhanced electromagnetic field to the far side
of the lens. In this work, we have experimentally and numerically investigated superlensing in the visible range. By
using the resonant hot-spot field enhancements from optical nanoantennas as sources, we investigated the translation of
these sources to the far side of a layered silver-silica superlens operating in the canalization regime. Using near-field
scanning optical microscopy (NSOM), we have observed evidence of superlens-enabled enhanced-field translation at a
wavelength of about 680 nm. Specifically, we discuss our recent experimental and simulation results on the translation of
hot spots using a silver-silica layered superlens design. We compare the experimental results with our numerical
simulations and discuss the perspectives and limitations of our approach.
Historically, the methods used to describe the electromagnetic response of random, three-dimensional (3D), metal-dielectric composites (MDCs) have been limited to approximations such as effective-medium theories that employ easily-obtained, macroscopic parameters. Full-wave numerical simulations such as finite-difference time domain (FDTD) calculations are difficult for random MDCs due to the fact that the nanoscale geometry of a random composite is generally difficult to ascertain after fabrication. We have developed a fabrication method for creating semicontinuous metal films with arbitrary thicknesses and a modeling technique for such films using realistic geometries. We extended our two-dimensional simulation method to obtain realistic geometries of 3D MDC samples, and we obtained the detailed near- and far-field electromagnetic responses of such composites using FDTD calculations. Our simulation results agree quantitatively well with the experimentally measured far-field spectra of the real samples.
Adaptive silver films (ASFs) have been studied as a substrate for protein microarrays. Vacuum evaporated silver films fabricated at certain range of evaporation parameters allow fine rearrangement of the silver nanostructure under protein depositions in buffer solution. Proteins restructure and stabilize the ASF to increase the surface-enhanced Raman scattering (SERS) signal from a monolayer of molecules. Preliminary evidence indicates that the adaptive property of the substrates make them appropriate for protein microarray assays. Head-to-head comparisons with two commercial substrates have been performed. Protein binding was quantified on the microarray using the streptavidinCy3/biotinylated goat IgG protein pair. With fluorescence detection, the performance of ASF substrates was comparable with SuperAldehyde and SuperEpoxy substrates. Additionally, the ASF is also a SERS substrate and this provides an additional tool for analysis. It is found that the SERS spectra of the streptavidinCy5 fluorescence reporter bound to true and bound to false sites show distinct difference.
Adaptive surface-enhanced Raman scattering (SERS) substrates exhibit unique properties which make them well suited for SERS studies of proteins on surfaces. Specifically, adaptive silver films (ASFs) allow nanoscale restructuring of metal particles during protein deposition which yields a three-fold benefit of soft protein adsorption, protein-metal complex stabilization, and increased SERS signal. In this work ASF fabrication and characterization methods are introduced, with special attention paid to characterization methods that provide insight into the adaptive nature of the substrates, such as UV-vis spectrophotometry, field-emission scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. These ASF substrates show SERS enhancement factors in the range of 106 for an area-averaged signal, and have been successfully used for sub-monolayer protein detection. The addition of a thick metal layer in the ASF fabrication structure typically increases the SERS signal by a factor of four or five. Finally, several examples of current SERS protein studies using ASF substrates are provided.
High SERS sensitivity for protein detection has been accomplished with semicontinuous silver films. Specifically, an insulin surface density as low as 80 fmol/mm2 and 25 amol in a probed area has been readily detected.