Janus　nanogap　gold　nanoparticles　(JAuNNPs)　with　varying　proportions　of　Au　shell　coverage　of　(ca.　100/75/50/25　%)　are　presented.　The　internal　nanogap　between　the　partial　Au　shell　and　core　caused　asymmetric　optical　behavior;　tunability　depends　on　the　degree　of　Au　shell　coverage　and　structural　asymmetry.　The　shell-to-shell　or　core-to-core　JAuNNDs(50　%)　were　self-assembled　from　amphiphilic　JAuNNPs(50　%)　by　tuning　the　hydrophilic　and　hydrophobic　polymer　brushes　on　the　Au　core　or　shell.　The　positions　of　electromagnetic　field　enhancement　of　JAuNNDs　varied　with　geometrical　configurations　because　of　hybridized　plasmonic　coupling　effects.　Furthermore,　DNA　linkers　were　utilized　to　form　JAuNND12(50　%).　By　combining　with　Raman　molecules,　ratiometric　SERS　signals　could　be　generated,　enabling　JAuNND12(50　%)　to　image　the　distribution　of　miR-21　in　living　cells　and　tumors.　Asymmetric　JAuNNPs　allowed　facile　conjugation　of　various　linkage　molecules　to　fabricate　dimeric　nanostructures.　©　2021　Wiley-VCH　GmbH