Quantum-dots (QDs) have fuelled up intensive research efforts over the past two decades. Nevertheless, currently developed two classes of fluorescent QDs, colloidal semiconductor QDs and carbonaceous QDs suffer from either toxicity or short luminescence lifetime. Here, we report a new class of fluorescent bio-dots that are derived from DNA via self-assembly at low temperatures down to 80°C, which has an optical bandgap of 3.4 eV, and in particular possesses strong photoluminescence with a much longer luminescence lifetime (τ₁ = 10.44 ns) than the carbonaceous QDs (τ₁ < 0.5 ns). It is discovered that it is the interactions of base pair cytosines with each other to form sp² carbon-like centers as luminescence centers or chromophores for the photoluminescence. The use of bio-dots in cell imaging with strong photoluminescence signal and good biocompatibility demonstrates great potentials of broad biological and optoelectronic applications.