Page 192 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Photonic Crystal Fibers: Pr operties and Applications 167

optical activity [58], or refractive index measurements using the shift
of bandgaps [99] and/or of LPG resonances [61,162,163]. Such dip-
sensors could in principle be manufactured by the millions in one
single drawing process, and thus would be cheap, disposable
(avoiding the issues of sample cross contamination in the microflu-
idic channels), safe, and biocompatible. Polymer MOFs, which are
more readily coated with biosensitive organic molecules and avoid
the risk of leaving glass shards in the sample (possibly a living organ-
ism) [58], are especially well suited for this application.
The shift of LPG resonances in fluidic photonic bandgap fibers is
one of the most sensitive in-fiber refractive-index-sensing schemes
[61]. However, this mechanism cannot be used for fluids with refrac-
tive indices below that of the fiber material (as the fibers then become
index guiding), de facto excluding most biomedically relevant water-
based analytes. One solution to circumvent this problem is to add a
high-refractive-index coating to the holes of the PCFs, which can
restore bandgap guidance [164].
Some of the more noticeable recent efforts to increase the sensitiv-
ity of PCF sensors include the combination of PCFs with existing
highly sensitive techniques, such as SPR and SERS. Indeed, compel-
ling arguments can be made in favor of including SPR techniques
into PCFs—for the small sample volumes, propagation constant engi-
neering, and ready access to waveguide fields [164–167]—however,
achieving metallic coatings of sufficient quality (~50-nm thickness
and negligible surface roughness) to allow unhindered propagation
of surface plasmons at the metal/analyte interface remains a chal-
lenge. So far the only surface plasmon resonances that have been
clearly demonstrated in PCFs have used bulk metallic inclusions
(fully filled holes), which can be very smooth at the metal/silica inter-
face, but can hardly be used for sensing [168, 169]. A number of metallic
coating techniques have been applied to PCFs, mostly in a non-SPR
context: liquid phase deposition is reasonably easy to implement and
can coat several meters of PCF holes at a time, but leads to surfaces
too rough for SPR [170, 171]. Sazio et al. demonstrated high-pressure
chemical vapor deposition of thin gold coatings in PCF holes with
what appear to be extremely smooth interfaces [170]. This seems to be
the most promising technique to achieve SPR capable PCFs, but is not
easily implemented, and to the best of our knowledge no one has yet
tried to use such fibers in the context of SPR sensing.
While the silver surfaces obtained with liquid phase depositions
are not suitable for SPR, their roughness makes them a very good
candidate for SERS. “Conventional” SERS techniques exploit hot
spots (spots of extreme plasmonic field enhancement) to locally
increase Raman scattering cross sections by up to 14 orders of magni-
tude. These hot spots are obtained using either a suspension of metal
nanoparticles or a rough gold or silver surface. The difficulty is that