  We demonstrated remote plasmonic enhancement (RPE) by a dense random array of
Ag nanoislands (AgNIs) that were partially gold-alloyed and attached with
column-structured silica (CSS) overlayer of more than 100 nm in thickness. The
physical and chemical protection of the CSS layer could lead to reducing the
mutual impact between analyte molecules and metal nanostructures. RPE plate was
realized just by sputtering and chemical immersion processes, resulting in high
productivity. We found a significant enhancement on the order of 10$^7$-fold
for Raman scattering and 10$^2$-fold for fluorescence by RPE even without the
proximity of metal nanostructures and analyte molecules. We confirmed the
feasibility of RPE for biophotonic analysis. RPE worked for dye molecules in
cells cultured on the CSS layer, enabling the enhanced fluorescence biosensing
of intracellular signaling dynamics in HeLa cells. RPE also worked for
biological tissues, enhancing Raman histological imaging of esophagus tissues
with esophageal adventitia of a Wistar rat attached atop the CSS layer. We also
investigated the wavelength dependency of RPE on the on- or off-resonant with
the dye molecular transition dipoles with various molecular concentrations. The
results suggested that the RPE occurred by remote resonant coupling between the
localized surface plasmon of AgNIs and the molecular transition dipole of the
analyte via the CSS structure. The RPE plate affords practical advantages for
potential biophotonic analyses such as high productivity and biocompatibility.
We thus anticipate that RPE will advance to versatile analytical tools in
chemistry, biology, and medicine.
