Chapter 4 - Quantum dots in single molecule spectroscopy

Abstract

Quantum dots (QDs) are luminescent nanoparticles that are bright, photostable, color tunable by size and have long fluorescence lifetimes. These properties allow them to be observed at the single particle level for long periods of time, both when observed alone or when conjugated to biomolecules to be used for in vitro and in vivo bioimaging. As with all single molecule/particle species, heterogeneity in their behavior is immediately apparent that can provide information both on the structure-property relationships of single QDs, as well as the biomolecules that they are conjugated to. This book chapter will start by briefly reviewing the synthetic methods used to control the structure-property relationships of QDs and the surface functionalization/bioconjugation approaches used to study biological processes of interest. Then, the electronic structure of QDs will be described and how the electronic structure affects single QD photophysics. Several initially unexpected observations, such as blinking, dark fraction formation and spectral diffusion will be described, together with the significant efforts to study and control the mechanisms underlying them. Then, a range of applications will be described where QDs (and, to a lesser extent quantum dots, QRs) have been used as biological fluorescent labels. Examples of such applications include 3D and 4D single particle tracking (SPT), fluorescence/image correlation spectroscopy (FCS, ICS), single molecule fluorescence resonance energy transfer (SMFRET), super-resolution microscopy, fluorescence lifetime imaging (FLIM), time gated imaging (TGI) and as probes for local environmental and chemical changes. The chapter will conclude with a brief discussion on the potential for using single QDs as single photon sources and to create quantum-entangled photons deterministically for potential applications in quantum computing, cryptography and communications.