Gold-coated array patterned with tightly-packed nanospheres was developed as a substrate base for constructing SERSenriched
nanogaps with Au-nanoparticles (GNPs). Using 1,2-ethanedithiol as a linker, Au-NPs (=17-40nm) were
anchored covalently on the sphere-array. Thin Au layer was sputtered on the substrate to mask the citrate coating of
GNPs that could demote the sensing mechanism. The negatively-charged GNP surface warrants the colloidal stability,
but the resulting repulsive force keeps the immobilized NPs apart by about 40nm. The attained gap size is inadequately
narrow to sustain any intense enhancement owing to the near-field nature of SERS. Minimal amount of NaCl was then
added to slightly perturb the colloidal stability by reducing their surface charge. Notably, the interparticle-gap reduces at
increasing amount of salt, giving rise to increased packing density of GNPs. The SERS enhancement is also found to
exponentially increase at decreasing gap size. Nevertheless, the minimum gap achieved is limited to merely 7nm.
Excessive addition of salt would eventually induce complete aggregation of particles, forming clustered NPs on the
array. A simple sputtering-growth approach is therefore proposed to further minimize the interparticle gap by enlarging
the seeded NPs based on mild sputtering. The SEM images confirm that the gap below 7nm is achievable. With advent
of the colloidal chemistry, the combined salt-induced aggregation and sputtering-growth techniques can be applied to
engineer interparticle gap that is crucial to realize an ultrasensitive SERS biosensor. The proposed two-step preparation
can be potentially adopted to fabricate the SERS-enriched nanogaps on the microfluidics platform.