Quantum dots as fluorescent probes: Synthesis, surface chemistry, energy transfer mechanisms, and applications

Luminescent semiconductor nanocrystals or quantum dots (QDs) provide exquisite electro-optical properties that are ideal for biological sensing applications. Unlike traditional fluorescent dyes that lack in long-term stability and the ability to detect multiple signals simultaneously, QDs have overcome these obstacles, and thus, their potential use as in vivo and in vitro fluorophores has greatly advanced since their discovery in the 1980’s. In this review article, we discuss the classic and current protocols used to synthesize QDs, as well as the adaptability of QD surfaces for versatile bioconjugation. Energy transfer mechanisms represent the basis for the strong attraction of QDs to the biosensing community and thus, we examine the parameters that are required for efficient fluorescence resonance energy transfer, bioluminescence resonance energy transfer, and chemiluminescence resonance energy transfer. In addition, the recent advances in the detection of heavy metal ions, pathogens, and cancer biomarkers are also highlighted in this review. While QDs have shown much progress, the materials selection and commercialization of QDs for biological applications remain an ambitious challenge.