Semiconductor Quantum Dots and FRET

Abstract

Quantum dots (QDs) are one of the most interesting new materials that have emerged over the last 20 years. These brightly luminescent nanoparticles have garnered significant attention from researchers working in the fields of biology, chemistry, physics, engineering, and at the interface between these fields. A powerful spectroscopic tool that also spans these disciplines is F€ orster resonance energy transfer (FRET). In this chapter, the compelling partnership between quantum dots and FRET is considered at both the fundamental and applied levels, emphasizing utility in biological applications. The text begins with a short review of FRET in Section 12.2, which reiterates the basic equations that relate physical parameters to experimental observables. Conventional FRET with a well-defined donor-acceptor pair serves as a reference point for comparison with quantum dots. Next, Section 12.3 describes the structure, chemistry, and optical properties of quantum dots. The purpose of this section is threefold: (i) impart the reasons for the widespread interest in quantum dots; (ii) outline the methods for using quantum dots as functional materials in biological applications; and (iii) provide a primer on quantum dot photophysics, which clearly have an important role in energy transfer. Cumulatively, Sections 12.2-12.3 provide sufficient background to fully appreciate the utility, advantages, and disadvantages of FRET with quantum dots, which are discussed in Section 12.4. Adherence and potential departures from the F€ orster formalism are also discussed. Section 12.5 provides a comprehensive review of the use of quantum dots as FRET donors in biological applications. These applications include assays, biosensing and chemosensing, distance measurements/conformational studies, and photodynamic therapy. Ensemble, single-pair, and solid-phase FRET configurations are considered. Section 12.6 reviews similar applications of quantum dot acceptors with chemiluminescent, bioluminescent, or lanthanide donors. Energy transfer between quantum dots and other nanomaterials is reviewed in Section 12.7; these mechanisms are similar to the F€ orster mechanism, but not strictly equivalent. Several examples of biological applications are given.

Inorganic Nanoparticles as Donors in Resonance Energy Transfer for Solid-Phase Bioassays and Biosensors

“…1 The relatively high QY of QDs is also useful and must be considered in the context of the linear dependency of the sixth power of the Forster distance on the donor QY. 1 From the standpoint of multiplexed QD-FRET bioassays, the narrow and symmetric PL spectra of QDs allows for an integration of a greater number of color channels within a given spectral window as compared to molecular fluorophores. 1 The large surface area afforded by QDs serves as a scaffold to allow multiple acceptors, a, to be concurrently arrayed around an exciton donor in a centrosymmetric configuration, which improves the efficiency of energy transfer.…”

“…1 From the standpoint of multiplexed QD-FRET bioassays, the narrow and symmetric PL spectra of QDs allows for an integration of a greater number of color channels within a given spectral window as compared to molecular fluorophores. 1 The large surface area afforded by QDs serves as a scaffold to allow multiple acceptors, a, to be concurrently arrayed around an exciton donor in a centrosymmetric configuration, which improves the efficiency of energy transfer. 3 This enables acceptors to be FRET paired with a given QD donor despite exhibiting a weak spectral overlap, owing to the additive channels of energy transfer offered by multiple acceptors.…”

“…3 However, this limitation to some extent is mitigated by the ability to array multiple acceptors around a single QD donor. 1 It is also important to note that a majority of studies that use QD-dye FRET pairs utilize a value of κ 2 = 2/3 in the calculation of energy transfer, which is valid provided that the transition dipoles of donor and acceptor are dynamic and random in terms of orientation. 3 For molecular fluorophores, free rotational motion around single bonds fulfills this condition despite having a fixed emission dipole orientation.…”

“…In contrast, CdSe QDs have been reported to have a degenerate transition dipole that is oriented isotropically in two dimensions. 1 This implies that the assumption of random orientation of a transition dipole is not strictly valid for QDs. Nonetheless, a value of κ 2 = 2/3 is a useful approximation for QD-dye FRET pairs given that multiple acceptors are arrayed around a central QD over a distribution of positions, where the acceptors typically have random and dynamic orientations of transition dipoles with respect to the QD transition dipole and the QD donor has a partially random orientation of the transition dipole.…”

“…31,32 The physisorption of biomolecules on the surface of QDs relies on electrostatic, polar, or hydrophobic interaction and involves a spontaneous association of biomolecules with the surface coating of a QD. 1 In the case of covalent interaction, a new bond is formed between the functional group of a biomolecule and a functional group that is associated with the surface coating of a QD. Coordination linkage is based on dative interaction and involves the spontaneous self-assembly of a functional group of a biomolecule on the surface of an inorganic shell of a QD or the surface coating of a QD.…”