The fluorescence signal of the FAM-ssDNA/DFNS-20@DA mixture remained at the background level after addition of inorganic phosphate up to 1 mM, then began to increase when the concentration of inorganic phosphate reached 10 mM. The addition of dNTPs appeared to have a great impact on ssDNA binding. When the concentration of dNTPs changed from 0.1 mM to 0.5 mM, the fluorescence intensity increased by more than 20 folds. Therefore, the nucleobases in the ssDNA play a more critical role than the phosphate backbone for ssDNA adsorption on the PDA coating. The UV-vis spectrum of DFNS@DA showed strong absorption from 500 to 600 nm that overlapped with the fluorescence emission band of FAM-ssDNA. In contrast, the uncoated DFNS had almost no UV-vis adsorption in the same range (Fig. ). From time-resolved fluorescence measurements, the fluorescence lifetime of FAM-ssDNA was found to decrease from 4.5 ns to 0.15 ns after adsorption to DPNS@DA (Fig. ). Thus, the fluorescence quenching is most likely caused by the Förster resonance energy transfer (FRET) between the "donor" (FAM) and the "acceptor" (PDA) .