Quantum Dot-Dye Conjugates for Biosensing, Imaging, and Therapy

Adding value to the intrinsic properties of quantum dots (QDs), a strategy to conjugate dyes on the surface of QDs offers new opportunities, since the coupling between QD and dyes can be designed to allow Förster resonance energy transfer (FRET) and/or electron transfer (eT). These processes are accompanied by the change of QD and/or dye fluorescence and subsequent photochemical reactions (e.g., generation of ¹ O₂ ). Based on the change of fluorescence signals by the interaction with biomolecules, QD-dye conjugates are exploited as biosensors for the detection of pH, O₂ , nicotinamide adenine dinucleotide (phosphate), ions, proteases, glutathione, and microRNA. QD-dye conjugates also can be modulated by the irradiation of external light; this concept is demonstrated for fluorescence super-resolution imaging as photoactivatable or photoswitchable probes. When QDs are conjugated with photosensitizing dyes, the QD-dye conjugates can generate ¹ O₂ in a repetitive manner for better cancer treatment, and can also be available for approaches using two-photon excitation or bioluminescence resonance energy transfer mechanisms for deep tissue imaging. Here, the recent advances in QD-dye conjugates, where FRET or eT produces fluorescence readouts or photochemical reactions, are reviewed. Various QD-dye conjugate systems and their biosensing/imaging and photodynamic therapeutics are summarized.