A　layer-pillared　three-dimensional　(3D)　metal-organic　framework　(MOF)BUT-26was　synthesized　from　a　two-dimensional　(2D)　MOFBUT-25through　single-crystal　to　single-crystal　(SC-SC)　transformation.　Two　honeycomb-shaped　layers　were　linked　by　formate　ions　forming　a　bilayer　structure　ofBUT-25.　The　formate　pillar　was　then　exchanged　by　the　subsequently　added　ligand　thiophene-2,5-dicarboxylic　acid　(H2TDC),　which　successfully　stitched　the　2D　layers　ofBUT-25into　3DBUT-26without　disturbing　the　original　honeycomb-shaped　layers.　Interestingly,BUT-26with　free　nitrogen　and　oxygen　sites　proved　to　be　a　highly　sensitive　MOF-based　sensor　with　a　limit　of　detection　(LOD)　of　0.12　mu　M　and　0.1　mu　M　for　Cu(2+)and　Fe(3+)ions,　respectively.　These　LOD　results　are　much　lower　than　the　minimum　values　of　these　ions　in　drinking　water　specified　by　the　US-EPA.　Sensitive　detection　accompanied　by　a　visible　color　change　in　the　solution　and　solid　phase　confirms　thatBUT-26can　be　used　for　colorimetric　sensing　of　both　ions.