Nanoporous　plasmonic　nanostructures　based　on　noble　metals　have　received　extensive　attention　on　the　high-performance　detection　of　chemical　analytes.　However,　the　size　of　such　pores　is　rarely　at　the　angstrom　scale　given　the　limitation　of　current　fabrication　methods.　This　leads　to　a　relatively　poor　performance　in　the　detection　of　ions.　Here,　we　demonstrate　the　formation　of　angstrom-scale　pores　in　ultra-thin　plasmonic　ammonium　doped　molybdenum　oxide　through　crystal　nucleation.　The　molybdenum　oxide　octahedra　assemble　into　hexagonal　rings,　in　which　dopants　fill　in　parts　of　the　rings　to　stabilize　the　crystal　structure　and　simultaneously　generate　a　broad　plasmon　resonance　across　the　visible　to　near-infrared　regions.　As　a　result,　the　unfilled　centers　of　the　rings　effectively　become　angstrom-scale　pores.　Na+　ion　sensing　capability　is　investigated　by　integrating　plasmonic　ammonium　doped　molybdenum　oxide　onto　D-shaped　optical　fibers.　The　ions　are　facilely　accommodated　within　the　pores　and　induce　a　charge　re-distribution　in　the　host.　This　alters　the　plasmon　resonance　behavior　and　modulates　the　optical　output　of　the　fiber　transducing　platform,　through　a　strong　light-matter　interaction.　The　structure　is　sensitive　to　a　wide　concentration　range　of　Na+　ions　from　subnanomolar　(sub-nM)　to　submolar　(sub-M)　with　the　limit　of　detection　(LOD)　of　similar　to　5　fM,　and　high　selectivity　in　both　the　aqueous　solution　and　simulated　serum　conditions,　which　is　a　superior　sensitivity　over　other　reported　optical　ion　sensors.　This　work　demonstrates　the　strong　potential　of　angstrom-scale　porous　plasmonic　materials　for　chemical　detection,　and　the　possibility　of　being　integrated　with　popular　optical　transducing　platforms　for　practical　high-performance　sensing.