Fig. 1.

(a) Schematic representation of a multimodal QD, where the QD serves as both a diagnostic agent (imaging) and a nanoscaffold to incorporate multiple functional modalities, such as a targeting ligand (peptide, antibody, or protein) and a therapeutic agent. Upon interacting with the target cell, the cell-penetrating ligand can then be exposed, allowing the multifunctional QD to enter the cell. Stimuli-sensitive antennae may be triggered by local stimuli (pH, temperature, or enzyme), allowing subsequent intracellular release of the drug from the drug-loaded vesicle. (b) Requirements of an ideal theranostic process in the human body may include: (1) escape from the clearance of reticuloendothelial system (RES, mainly liver, spleen, and bone marrow), allowing longer blood circulation time, (2) accumulate in the pathological zone, targeting specific cell types, (3) penetrate the cell efficiently, leaving minimum damage to the cell, (4) overcome the delivery barriers, leading to efficient intracellular release, and (5) bear a diagnostic agent (imaging, optical or magnetic), allowing for real-time monitoring of the treatment, while maintaining minimum toxicity to the healthy cells.