A "green nano" strategy was extended to prepare monodisperse polypyrrole (PPy) nanospheres with tunable sizes via simply adjusting pyrrole monomer's concentration. And a linear model was obtained and used to predict the diameter of PPy nanosphere. Nitrogen-doped carbon nanospheres (NCSs) with well-controlled diameters were further synthesized through direct carbonzilation of harvested PPy nanospheres, which demonstrated a facile route for the preparation of highly dispersible NCSs with controlled sizes. Then the potential electrochemical response of as-fabricated NCSs toward food additive-Sunset yellow (SY) was explored. NCSs with medium size (M-NCSs) showed the best electrochemical performance and an ultra-high sensitivity of 679.16 A M-1 cm(-2) and lower detection limit of 0.95 nM were gained. This study therefore provides a facile route to high quality, uniform, and size-controlled PPy nanospheres and NCSs, which are promising to be applied to electrochemical and other fields. (C) 2019 The Electrochemical Society.