  Surface enhanced Raman spectroscopy (SERS) has established itself as a
promising tool in optical sensing technology. Efforts have been made to improve
practicalities of the technology with regards to costs of production,
stability, reproducibility, flexibility and robustness. Here, we demonstrate a
method to fabricate Ag/Au bimetallic inverted nanopyramid (i-NPyr) and upright
nanopyramid (u-NPyr) using a multi-step molding process on flexible plastics as
SERS sensors for the ultrasensitive detection of haemoglobin at a very low
concentration (down to nM). First, a Si i-NPyr master was created using
electron beam lithography, then i-NPyr and u-NPyr structures were imprinted on
flexible polymer substrates using nanoimprinting lithography, and last, Ag/Au
coatings were coated on top of them. The SERS activity of the imprinted i-NPyr
and u-NPyr substrates were evaluated using rhodamine 6G (Rh6G) as probe
molecules. Enhancement factor (EF) values of 3.88 x 10^6 and 7.86 x 10^5
respectively for the iNPyr and u-NPyr substrates, and the i-NPyr SERS substrate
was able to detect Rh6G at concentrations as low as 1pM. In addition, we
investigated the stability and resilience of i-NPyr by thoroughly analysing
signal performance under angled bending and delicate crumpling. Finally, a
proof-of-concept application as a wearable i-NPyr SERS sensor was demonstrated
by detecting the analyte in sweat excreted during running and walking. We
believe that this extremely sensitive i-NPyr SERS substrate with dense electric
field confinement around hotspots (confirmed using computational simulations),
as well as its good stability, durability, and low production cost, may enable
in-situ measurement of analytes in wearable technologies.
