Signal Amplification via Biological Self-Assembly of Surface-Engineered Quantum Dots for Multiplexed Subattomolar Immunoassays and Apoptosis Imaging

The parallel and highly sensitive detection of biomolecules is of paramount importance to understand biological functions at the single cell level and for various medical diagnoses. Surface-engineered semiconductor quantum dots (QDs) have been demonstrated to act as a signal amplifiable reporter in immunoassays. This takes advantage of the QDs' robustness against self-quenching in proximity and the tunability of their surface properties. A streptavidin (SA) and biotin QD conjugate pair containing a zwitterionic surface modification was designed for QD self-assembly with minimal nonspecific adsorption. Typical sandwich-type immunoassay procedures were adopted, and the targeted protein binding events were effectively transduced and amplified by the fluorescence of the SA biotin QD conjugates. The detection limit of myoglobin in 100% serum was determined to be at the subattomolar (tens of copies per milliliter) level, which was achieved by using 100 cycles of the layer-by-layer QD assembly. Adsorption kinetics studies and Monte Carlo simulations revealed that this highly sensitive signal amplification was accomplished by the zwitterionic surface, which gave equilibrium constants 5 orders of magnitude larger for specific binding than for nonspecific binding. The QD conjugates showed an effective multivalency of two, which resulted in a broad linear dynamic range spanning 9 orders of magnitude of target protein concentrations. The assay can be highly miniaturized and multiplexed, and as a proof-of-concept, parallel and rapid detection of four different cancer markers has been successfully demonstrated. To demonstrate that this QD signal amplification can be a universal platform, sensitive imaging and early detection of apoptotic cells were also showcased.