Highly photoluminescent nitrogen and sulfur co-doped carbon nanoparticles(CNPs) ca. 56 nm have been prepared through a green one-step hydrothermal synthesis route by using millet powder as carbon sources, in which the nitrogen and sulfur co-doping improves the photoluminescent efficiency of the CNPs. The as-prepared CNPs display excellent fluorescent properties and low biotoxicity with a relatively high quantum yield of 30.4%, which have been applied for bioimaging and highly sensitive and selective detection of iron(III) ions. Highly photoluminescent nitrogen and sulfur co-doped carbon nanoparticles (CNPs) ca. 56 nm have been prepared through a green one-step hydrothermal synthesis route by using millet powder as carbon sources, in which the nitrogen and sulfur co-doping improves the photoluminescent efficiency of the CNPs. The as-prepared CNPs display excellent fluorescent properties and low biotoxicity with a relatively high quantum yield of 30.4%, which have been applied for bioimaging and highly sensitive and selective detection of iron (III) ions.