DNA is a negatively charged polyanion due to its phosphate backbone. The negative surface charge of PDA is expected to influence the interaction between DNA and PDA. The presence of Na + is known to reduce the charge repulsion and promote DNA adsorption on negatively charged surfaces, e.g. on negatively charged gold nanoparticles and graphene oxide . However, as shown in Fig. , increasing the concentration of Na + from 0 to 2 mM did not result in fluorescence quenching. Therefore, the attractive force between FAM-ssDNA and the PDA-coated DFNS-20@DA needs to be increased. Considering that the catechol groups of dopamine can interact strongly with divalent metal ions, we tested the effect of different concentrations of Ca 2+ and Mg 2+ on the efficiency of quenching. The quenching efficiency increased as the concentration of Ca 2+ and Mg 2+ increased. At the same concentration, Ca 2+ was found to be even more effective than Mg 2+ for ssDNA adsorption and quenching. With 2 mM Ca 2+ , the QE reached 98%. Therefore, the buffer containing 2 mM Ca 2+ was selected as the solvent for all subsequent quenching experiments. Fig. shows the Zeta potential of the PDA-coated nanosilica (NS@DA) measured in buffer with and without Ca 2+ .