  Ag nanorod arrays/dielectrics/mirror-structured multilayer thin-film are well
known, highly sensitive surface-enhanced Raman scattering (SERS) substrates
that enhance the Raman scattering cross-section by the interference of light.
However, extracting biomarkers directly from human skin using these solid
substrates is difficult. To overcome this problem, we propose a multilayer
thin-film flake dispersion gel by centrifugal mixing of the multilayer
thin-film and hydroxyethyl cellulose (HEC) gel. The multilayer thin-film was
prepared by serial bideposition using the dynamic oblique angle deposition
technique. The mixing process was optimized to obtain flakes of ~10 {\mu}m so
that the optical properties of the multilayer film can be preserved, and there
is no risk of adverse effects on humans. The SERS features of the flakes
dispersion gel were tested using 4, 4'-bipyridine (BPY). The BPY molecules
diffused through the highly porous gel within a few seconds, generating
significant SERS signals. The multilayer film flakes dispersion gel showed a
SERS signal about 20 times better than the gel-dispersed Ag nanorod arrays
without a multilayer film structure. These SERS active flakes dispersion gel
can be used directly on the skin surface to collect body fluids from sweat, for
biomarker sensing.
