In the past few decades,protein-templated fluorescent gold nanoclusters have attracted much attention in the fields of bio-sensing as their excellent optical properties and good biocompatibility.In this work,a fluorescence turn-on biosensor was fabricated for the sensitive detection of 5-hydroxytryptamine(5-HT)with transferrin encapsulated gold nanoclusters(Tf-Au NCs).The Tf-Au NCs were facilely prepared with the assistance of microwave radiation in several minutes and exhibited good monodispersity as well as strong red-emission.It was demonstrated that an aggregation-enhanced emission of Tf-Au NCs was triggered by 5-HT due to the high affinity between 5-HT and the sialic acid(SA)residues of transferrin.Based on these findings,a highly selective turn-on fluorescent sensor was fabricated for the quantitative determination of 5-HT in the range of 0.2 to 50 μM(R2 = 0.994)with a detection limit of 0.049 μM(S/N = 3).The developed sensor was also successfully applied for the 5-HT detection in human serum with satisfactory recoveries of 96.20~108.6%.