Quantum dots are nanometre-scale semiconductor crystals with unique optical properties that are advantageous for the development of novel chemical sensors and biosensors. The surface chemistry of luminescent quantum dots has encouraged the development of multiple probes based on linked recognition molecules such as peptides, nucleic acids or small-molecule ligands. This review overviews the design of sensitive and selective nanoprobes, ranging from the type of target molecules to the optical transduction scheme. Representative examples of quantum dot-based optical sensors from this fast-moving field have been selected and are discussed towards the most promising directions for future research.

The use of highly activated carbon nanofibers for the design of catalytic electrochemical biosensors is demonstrated. The direct immobilization of enzymes onto the surface of carbon nanofibers is shown to be a highly efficient method for the development of a new class of very sensitive, stable, and reproducible electrochemical biosensors. These results establish the fact that the carbon nanofiber is the best matrix so far described for the development of biosensors, far superior to carbon nanotubes or graphite powder.

Aligned multi-wall carbon nanotubes (MWNT) grown on platinum substrate are used for the development of an amperometric biosensor. The opening and functionalization by oxidation of the nanotube array allows for the efficient immobilization of the model enzyme, glucose oxidase. The carboxylated open-ends of nanotubes are used for the immobilization of the enzymes, while the platinum substrate provides the direct transduction platform for signal monitoring. It is also shown that carbon nanotubes can play a dual role, both as immobilization matrices and as mediators, allowing for the development of a third generation of biosensor systems, with good overall analytical characteristics.

The response properties of a new solvent/polymeric membrane electrode with unique selectivity toward anionic salicylate are reported. The electrode is prepared by incorporating 5, 10, 15, 20-tetraphenyl(porphyrinato)tin(IV) dichloride (Sn[TPP]Cl2) into a plasticized poly(vinyl chloride) membrane. The resulting sensor exhibits an anti-Hofmeister selectivity pattern, with high specificity for salicylate over lipophilic inorganic anions (perchlorate, periodate, thiocyanate, iodide, etc.) and biological organic anions (citrate, lactate, acetate). Moderate selectivity over structural analogues of salicylate (3- and 4-hydroxybenzoate, benzoate) is also observed. Radiotracer uptake experiments using [14C]salicylate clearly show that the metal center of the metalloporphyrin is critical for selective salicylate transport in the membrane phase. Minimal response to chloride ions makes the new electrode potentially useful for estimating salicylate levels in biological samples.