Luminescent semiconductor nanocrystals or quantum dots (QDs) are a recently developed class of nanomaterial whose unique photophysical properties are helping to create a new generation of robust fluorescent biosensors. QD properties of interest for biosensing include high quantum yields, broad absorption spectra coupled to narrow size-tunable photoluminescent emissions and exceptional resistance to both photobleaching and chemical degradation. In this review, we examine the progress in adapting QDs for several predominantly in vitro biosensing applications including use in immunoassays, as generalized probes, in nucleic acid detection and fluorescence resonance energy transfer (FRET) - based sensing. We also describe several important considerations when working with QDs mainly centered on the choice of material(s) and appropriate strategies for attaching biomolecules to the QDs.
The most common method of detecting and quantitating biomolecules still remains the use of fluorescence [1,2]. As such fluorescent probes have found widespread use in myriad biosensing applications including immunoassays, nucleic acid detection, resonance energy transfer studies, clinical/diagnostic assays and cellular labeling, to name but a few [1-3]. Many of the organic dye and protein-based fluorophores currently in use do, however, suffer from serious chemical and photophysical liabilities. These include pH dependence, self-quenching at high concentrations, susceptibility to photo-bleaching, short-term aqueous stability, narrow absorption windows coupled to broad red-tailed emission spectra via small Stokes shifts, and short excited state fluorescent lifetimes Over time, this has resulted in the synthesis of a vast library of fluorophores, many of which are geared towards very specific applications; for example the staining of cellular mitochondria organelles with MitoTracker dyes or using tetramethylrhodamine for resonance energy