Rebecca Somers, Moungi Bawendi and Daniel Nocera of MIT, US, explain how to solve a paradox: making quantum dots both bright and sensitive

Inorganic semiconductor nanocrystals (NCs), popularly known as quantum dots, have found application in biology mostly as optical imaging agents. Compared to conventional organic dyes, NCs exhibit broad absorption profiles, narrow tunable emission, photostability, and high quantum yields. Imaging applications exploit these optical qualities, and the NCs act as bright beacons of light that may be followed within the biological milieu. Although the synthesis of quantum dots dates back to the early 1990s, their application to biology rocketed with two significant advances in recent years. One is the development of core-shell structures formed by coating the original quantum dot with a thin layer of a higher band gap inorganic material. This 'overcoating' makes the NCs extremely bright and more robust by chemically and electronically shielding the cadmium selenide (CdSe) core from its surroundings. The other advance came in 1998, when two different methods to water-solubilize dots were published. The water solubility of core-shell dots enabled their ready application to the aqueous world of biology.

The foregoing advances present a paradox for the application of NCs beyond imaging and labeling. Now, current research is focused on making these small fluorescent dots sensitive, to be 'smart' and optically report on the chemical and biological environment that surround them. But here is the paradox: the properties of the NCs for imaging and labeling applications are achieved by making the NC impervious to its environment. How can NCs be sensitive to their environment if they are encased within the cocoon of a passivating overcoat?

The way out of this quandary is to design NCs that can participate in fluorescence resonant energy transfer (FRET). The binding of a second chromophore, which can resonantly accept energy from the NC excited state, introduces a new pathway for the flow of energy resulting from light absorbed by the NC. The efficiency of FRET between the NC donor and energy acceptors, which is highly dependent upon donor-acceptor distance and the spectral overlap between the donor emission and acceptor absorption, can be used to give specific information about the NC surroundings. In this way, chemically passivated quantum dots can report on their environment, thus turning NCs into sensors.

Information on nucleic acid processes such as telomerization, replication, hybridization and cleavage is usually obtained by modifying one strand with a NC and by conjugating the complementary strand with an energy acceptor dye. As the two strands begin to interact and intertwine, the distance between the NC and the dye changes to modulate the efficiency of FRET.

Several different strategies are used to engender NC sensitivity to small molecules and ions. First, a receptor (such as an antibody fragment) with affinity for the target analyte can be tethered to the CdSe NC surface. The receptor is pre-loaded with a quencher dye, effectively turning off the emission of the NC. When the target is added, the quencher is displaced and the luminescence restored. A different strategy involves controlling the aggregation of differently sized (and hence differently colored) NCs in the presence of an analyte. If the analyte induces aggregation of NCs, an increase in FRET from the smaller to larger CdSe NCs will cause a redshift in the overall emission. Another emerging strategy relies on the energy transfer between NCs and permanently tethered, analyte-sensitive chromophores or fluorophores, which have been exemplified as pH sensors. In these sensors, a pH-sensitive spectral overlap between the NC and acceptor dyes affects the efficiency of energy transfer. This final strategy has the advantage of reversibility in the sensing mechanism, and in the case that the tethered dye is a fluorophore, a ratiometric signal from the NC and dye emission can be obtained, allowing for self-calibration.

While current research is expanding the repertoire of NC sensing to other analytes and other types of quantum dot NCs, the field of NC sensors has been established and is now a rapidly expanding one.

Read the full tutorial review 'CdSe nanocrystal based chem-/bio- sensors' in issue 4 of Chemical Society Reviews