We　demonstrated　the　sensitive　CO　detection　in　a　novel　hollow-core　negative　curvature　fiber　(HC-NCF)　with　infrared　laser　absorption　at　2.3　mu　m.　The　HC-NCF　consists　of　a　single　ring　of　eight　nontouching　silica　capillaries　around　the　air　core,　providing　a　single-mode　light　delivery　of　the　2.3-mu　m　distributed　feedback　laser.　A　high　coupling　efficiency　of　90%　was　achieved　with　the　aid　of　optimal　free-space　coupling　optics.　The　hollow-core　fiber　was　used　as　a　gas　cell　for　gas　absorption　measurement　of　a　total　path　length　of　85　cm.　Both　direct　absorption　spectroscopy　(DAS)　and　wavelength　modulation　spectroscopy　(WMS)　were　adopted　to　demonstrate　the　sensor　performance　by　detecting　the　CO　line　R(10)　at　4297.7　cm(-1).　In　scanned-wavelength　DAS,　we　obtained　a　minimum　detection　limit　(MDL)　of　13　ppm　CO,　which　was　limited　mainly　by　the　existing　mode　noise　in　the　HC-NCF.　By　applying　a　pressure　difference　of　0.8　bar　between　the　two　ends　of　the　fiber,　we　demonstrated　a　very　short　gas　loading　time　of　only　5　s.　Finally,　we　achieved　a　MDL　of　0.4　ppm　CO　using　the　WMS　technique,　corresponding　to　a　noise　equivalent　absorption　of　1.6　x　10(-7)　cm(-1).