An ultra-sensitive spectrofluorimetry method was developed and validated for the rapid determination of ferrocyanide ions (hexacyanidoferrate(II), FeCN64-, FCNs) by using sulphur (S) and nitrogen (N) dual-doped fluorescence carbon nanoparticles (S,N-FCNPs) as nanoprobe. The fluorescence of S,N-FCNPs was strongly reduced due to the dynamic interaction between S,N-FCNPs and FeCNs. The developed method proved to be highly sensitive and selective for FeCNs detection with linearity in three concentration ranges over four orders of magnitude, covering 0.01–200.0 μg/mL....