Surface-enhanced
Raman spectroscopy (SERS) is a vibrational spectroscopy
technique that enables specific identification of target analytes
with sensitivity down to the single-molecule level by harnessing metal
nanoparticles and nanostructures. Excitation of localized surface
plasmon resonance of a nanostructured surface and the associated huge
local electric field enhancement lie at the heart of SERS, and things
will become better if strong chemical enhancement is also available
simultaneously. Thus, the precise control of surface characteristics
of enhancing substrates plays a key role in broadening the scope of
SERS for scientific purposes and developing SERS into a routine analytical
tool. In this review, the development of SERS substrates is outlined
with some milestones in the nearly half-century history of SERS. In
particular, these substrates are classified into zero-dimensional,
one-dimensional, two-dimensional, and three-dimensional substrates
according to their geometric dimension. We show that, in each category
of SERS substrates, design upon the geometric and composite configuration
can be made to achieve an optimized enhancement factor for the Raman
signal. We also show that the temporal dimension can be incorporated
into SERS by applying femtosecond pulse laser technology, so that
the SERS technique can be used not only to identify the chemical structure
of molecules but also to uncover the ultrafast dynamics of molecular
structural changes. By adopting SERS substrates with the power of
four-dimensional spatiotemporal control and design, the ultimate goal
of probing the single-molecule chemical structural changes in the
femtosecond time scale, watching the chemical reactions in four dimensions,
and visualizing the elementary reaction steps in chemistry might be
realized in the near future.