Surface-enhanced Raman spectroscopy (SERS) is now a well-established technique for the detection, under appropriate conditions, of single molecules (SM) adsorbed on metallic nanostructures. However, because of the large variations of the SERS enhancement factor on the surface, only molecules located at the positions of highest enhancement, so-called hot-spots, can be detected at the single-molecule level. As a result, in all SM-SERS studies so far only a small fraction, typically less than 1%, of molecules are actually observed. This complicates the analysis of such experiments and means that trace detection via SERS can in principle still be vastly improved. Here we propose a simple scheme, based on selective adsorption of the target analyte at the SERS hot-spots only, that allows in principle detection of every single target molecule in solution. We moreover provide a general experimental methodology, based on the comparison between average and maximum (single molecule) SERS enhancement factors, to verify the efficiency of our approach. The concepts and tools introduced in this work can readily be applied to other SERS systems aiming for detection of every single target molecule.
The enhanced local fields around plasmonic structures
can lead
to enhancement of the excitation and modification of the emission
quantum yield of fluorophores. So far, high enhancement of fluorescence
intensity from dye molecules
was demonstrated using bow-tie gap antenna made by e-beam lithography.
However, the high manufacturing cost and the fact that currently there
are no effective ways to place fluorophores only at the gap prevent
the use of these structures for enhancing fluorescence-based biochemical
assays. We report on the simultaneous modification of fluorescence
intensity and lifetime of dye-labeled DNA in the presence of aggregated
silver nanoparticles. The nanoparticle aggregates act as efficient
plasmonic antennas, leading to more than 2 orders of magnitude enhancement
of the average fluorescence. This is comparable to
the best-reported fluorescence enhancement for a single molecule but
here applies to the average signal detected from all fluorophores
in the system. This highlights the remarkable efficiency of this system
for surface-enhanced fluorescence. Moreover, we show that the fluorescence
intensity enhancement varies with the plasmon resonance position and
measure a significant reduction (300×) of the fluorescence lifetime.
Both observations are shown to be in agreement with the electromagnetic
model of surface-enhanced fluorescence.
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