Monoamine　neurotransmitters　such　as　L-epinephrine　(EPI),　norepinephrine　(NE),　dopamine　(DA),　and　serotonin　(ST)　are　important　biomarkers　for　neurological　diseases.　However,　specific　detection　of　a　single　monoamine　neurotransmitter　is　challenging.　In　this　work,　a　water-stable　fluorescent　biosensor,　Eu3+-doped　UiO-66　metal–organic　framework　(EuUPDC),　is　constructed　for　specific　recognition　of　ST　and　its　metabolite,　5-hydroxyindole-3-acetic　acid　(5-HI-3-AA).　The　detection　limits　of　EuUPDC　for　ST　and　5-HI-3-AA　are　0.013　μM　and　0.15　μM,　respectively.　The　LOD　value　for　ST　is　10-fold　improvement　compared　to　the　reported　fluorescent　sensors.　Importantly,　the　biosensor　specifically　recognizes　ST　at　very　low　concentrations　in　the　presence　of　co-existing　monoamine　neurotransmitters.　Furthermore,　the　EuUPDC　biosensor　is　successfully　applied　for　the　determination　of　ST　and　5-HI-3-AA　in　human　serum　and　urine　with　good　recoveries.　Finally,　a　EuUPDC-embedded　test　strip　is　fabricated　and　portable　smartphone-assisted　RGB　color　values　are　effectively　integrated　with　a　lowest　detectable　value　of　0.6　μM　for　ST　and　6.7　μM　for　5-HI-3-AA.　The　designed　biosensor　EuUPDC　has　great　potential　for　the　quantitative　determination　of　ST　and　5-HI-3-AA,　and　applicable　for　early　diagnosis　of　neurological　diseases.　©　2022　Elsevier　B.V.