Figure 5.

Carbodiimide-based coupling reaction. The -COOH groups of mercaptoundecanoic acid on the QD surface are activated by EDC to form an unstable intermediate O-acylisourea which may react with the NH₂ groups on the Ab to form an amide bond or rapidly hydrolyze to yield -COOH groups. Addition of NHS yields a stable reactive NHS ester intermediate that reacts with NH₂ groups on the Ab to yield a stable amide bond.

Abbreviations: Ab, antibody; EDC, N-ethyl-N’-(3-dimethylaminopropyl)carbodiimide; NHS, N-hydroxysuccinimide; QD, quantum dot.