C-dots can be doped with P , N , S , F , B , nitrogen and sulfur codoped carbon dots (N, S-CDs) , and N/B . C dots exhibit good optical properties including photoluminescence in the visible range , and high quantum yields (QY) . The photoluminescence properties of CDs can be tunable by changing their size, surface modification with functional groups at the graphitic edges of the materials, doping with heteroatoms, or selecting a suitable synthesis method . They can be tuned offering fluorescence emission from blue to green . It has been used for tackling COVID-19 , the virus . It offered naked eye sensors . N-doped CDs especially exhibit remarkable acid-evoked fluorescence enhancement under acidic conditions .