222 CHAPTER 11 Bionanotechnology specific clusters of particles, although unless the fastening is geometrically precise, rather than merely statistical, the clusters are also statistical in their composition, except for the simplest ones. 11.5 BIOSENSORS The dictionary definition of a biosensor is "a device which uses a living organism or biological molecules, especially enzymes or antibodies, to detect the presence of chemicals" [35] (cf. Section 7.9.2). The classic biosensor is the amperometric glu- cose sensor, comprising glucose oxidase coating an electrode; the enzyme oxidizes glucose. The main reason for wishing to use an enzyme is to exploit the exquisite specificity of biomolecular binding interactions ("molecular recognition"). The Holy Grail of research in the field is to couple the enzyme directly to the electrode such that it can be regenerated by passing electrons to it; in current prac- tice the enzyme concomitantly reduces water to hydrogen peroxide, which is in turn reduced at the electrode, engendering the measured amperometric signal. This is not nanoscale technology, but if the enzyme could indeed be coupled directly to the elec- trode, this would typically require the active site of the enzyme to be within 1 nm of the electrode, hence it enters the realm of nanoengineering, in which a carbon nano- tube might be used as the electrode, and which opens the way to reducing the size of the device, such that ultimately it might incorporate a single enzyme, able to detect single glucose molecules. Another kind of biosensor exploits the combinatorial uniqueness of base strings of even fairly modest length to fabricate "gene chips" [33] used to identify genes and genomes. In these devices, the sample to be identified (e.g., the nucleic acids extracted from bacteria found in the bloodstream of a patient) is dispersed over the surface of the chip, which comprises an array of contiguous microzones containing known oligomers of nucleic acids complementary to the sought-for sequences (e.g., a fragment GATTACA is complementary to CTAATGA). Binding can be detected by double helix-specific dyes. 11.6 BIOPHOTONIC DEVICES Apart from the marvellous intricacy of the biological machinery that converts light into chemical energy, which at present only serves to inspire nanotechnological mim- ics, there are other, simpler, photoactive proteins, robust enough to be incorporated into artificial devices. Molecules based on the chromophore rhodopsin (such as the primary optical receptor in the eye) seem to have a special place here. One of the most remarkable of these photoactive proteins is bacteriorhod- opsin, which constitutes about a third of the outer membranes of the archaeon (extremophilic prokaryote) Halobium salinarum, living in salt lakes. The optically