摘要： In the early-to-mid 1990s, fluorescence from single organic dye molecules, single fluorescent proteins (FPs) and single direct-bandgap semiconductor nanoparticles (also called quantum dots, QDs) were separately observed. There are other luminescent nanomaterial species which have also sometimes been observed (and utilized) at the single particle level, such as small metal clusters, organic polymers/nanomaterials, and lanthanide/actinide ion-doped upconverting nanocrystals, some of which have also been called quantum dots (albeit somewhat inconsistently) in the literature. However, in this book chapter, the primary focus will be on direct-bandgap semiconductor nanoparticles and so we will use the term quantum dots to refer to this specific class of fluorescent nanoparticle. The key criterion for such single molecule/nanoparticle observations (beyond the equipment perspective) is the “brightness” of the fluorophore – the product of extinction coefficient (ε) and the photoluminescence quantum yield (PL QY). It is possible to overcome a low extinction coefficient by increasing the excitation power. However, the background also increases with excitation power so, at some point, the background fluorescent signal may dominate over the signal from the single molecule or particle. Organic dye molecules and fluorescent proteins usually combine a moderate extinction coefficient at their peak absorbance wavelength (λmax) with a moderate-to-high PL QY (with some common dye molecules reaching unity PL QY). QDs exhibit a higher extinction coefficient at or above their band gap energy than organic dyes and FPs do at their λmax (approx. 3–10 fold higher). In the 1990s, PL QYs of QDs were usually below 50%, but nowadays can approach unity, meaning that QDs are often significantly brighter than organic dyes and FPs under similar experimental conditions. This fact, combined with their much higher photostability, narrower emission spectra and broad excitation bands have led to QDs becoming somewhat of a competitor to organic dyes and FPs for certain applications. Of course, the advantages of QDs come with some disadvantages that still render organic dyes and FPs superior for other applications. In particular, the size of QDs, while they can be made to a size comparable to FPs, are larger than organic dyes. Moreover, QDs are rarely monovalent/monofunctional, meaning that conjugates of 1:1 label: target are much more difficult to achieve with QDs than with organic dyes or FPs. Finally, the most common QD compositions contain heavy elements (usually Cd, although some may contain Hg and Pb), and much more care must be taken in their handling and disposal. This latter disadvantage was first mitigated by “shelling” the QD with a lower toxicity material (usually ZnS) and, more recently, by replacing the toxic elements with less toxic ones, such as In, Cu and/or Ag.