Although most of these studies clearly demonstrate that
SERS substrates hosting closely separated metal nanostructures
and/or sharp tips result in large enhancements, an
important practical consideration apart from the cost, which
is often overlooked, is the ease and efficiency of the sample
collection. In real-world applications such as explosive detection,
the efficiency of sample collection becomes a decisive
factor. For example, in the case of explosives such as trinitrotoluene (TNT), which inherently have low vapor pressure
(∼10 ppbv at room temperature), intentional packaging
further lowers the actual vapor concentration by more than
an order of magnitude (21). For detection of such explosives,
it is extremely important to collect particulates (few μg), that
are invariably present on the surface of objects exposed to
the explosive. Physical swabbing, puffer systems (aerodynamic),
and direct vapor sniffing are recognized as efficient
methods to collect trace amounts of analytes. In particular,
swabbing the surface under investigation with a soft and
flexible substrate (swab) is a highly practical and efficient
method to maximize the sample collection from a real-world
surface. In fact, this strategy is being extensively employed
for passenger screening at airports using ion mobility spectroscopy
(22). On the contrary, conventional SERS substrates
based on silicon, glass, and porous alumina, which are
conceived for homeland security applications, are not compatible
with such an efficient sample collection process
because of their nonconformal, rigid, and brittle nature.