Based on these experimental results, it is plausible that the interaction between FAM-ssDNA and DFNS@DA is primarily driven by the π-π stacking interaction between the exposed nucleobases in the ssDNA and the quinone-related structures in the PDA coating of DFNS@DA. The presence of Ca 2+ is likely to facilitate ssDNA adsorption through interactions with the DNA bases, which further enhances the π-π stacking interaction . The phosphate (a hard Lewis base) in the backbone of the ssDNA may also contribute to the binding to DFNS@DA by interacting with Ca 2+ (a hard Lewis acid) . As a result of the adsorption, the distance between the bound FAM-ssDNA and the PDA coating is shortened, thereby enabling an effective fluorescence quenching.