(A) Schematic of the aniline-catalyzed hydrazone ligation between aldehyde and hydrazine functionalities color-coded in blue and red, respectively. (B) Schematic of the two-step, modular design for the attachment of biomolecules to the surface of CdSe/ZnS core/shell QDs surface functionalized with charged DHLA or neutral DHLA-PEG ligands. Initially, a hexahistidine-appended, synthetic, starter peptide is ligated to the appropriately functionalized target biomolecule of interest using chemoselective ligation. Isolated conjugates are then ratiometrically self-assembled to QDs for use in targeted bioassays. (C) Structures of DHLA (deprotonated/negatively charged) and DHLA-PEG600 QD surface modification ligands.