Detecting small amounts of chemicals is critical for many technologies, from environmental and food monitoring to medicine and defence. The ideal sensor makes a lot from a little -generating a big signal when it detects only a little of the substance under surveillance. Usually this amplification is performed electronically. Now a team of European chemists has made a molecule that acts as both sensor
The new sensor has been devised by the groups of Vincenzo Balzani, a specialist in 'molecular switching' at the University of Bologna in Italy, and Fritz Vögtle, a chemist at the University of Bonn in Germany.
Balzani's team creates molecules that send out a signal when they stick to target molecules. For example, the signalling molecule might emit light when alone, but not when it binds to the right substance.
Generally, these molecules work in one-to-one relationships: to detect all the targets, you need an equal number of sensors. But Balzani's collaboration with Vögtle has produced a molecule with 32 light-emitting units, all of which are turned off when the molecule binds a single target, as they report in the journal
In the late 1970s, Vögtle pioneered the synthesis of molecules with highly branched structures, like a bush. He called them 'cascade molecules', but they have since become known as 'dendrimers', derived from the Greek word for 'tree'.
In a dendrimer, a few molecular chains are linked together to form a multi-armed core. Two or more new chains sprout from the ends of each arm, and each of these similarly branches at the tip. Thus the dendrimer contains a cascade of branching chains. These chains bunch together to give the molecule a roughly spherical shape with the tips of the outermost branches exposed at the surface.
Dendrimers have attracted increasing interest in recent years, because their relatively large size and globular shape could have many applications. They are being investigated, for example, as capsules for transporting drug molecules, and as catalysts small enough to dissolve in solvents but big enough to be filtered out.
In the dendrimer devised by Balzani's and Vögtle's groups, all the branch tips -- 32 of them -- end in a fluorescent 'dansyl unit', which glows green under ultraviolet light.
The chains inside the dendrimer, meanwhile, contain nitrogen atoms, which hook up with cobalt ions. The researchers found that their dendrimers capture one cobalt ion each if the concentration of cobalt is low. On doing so, the molecules stop fluorescing altogether: all 32 dansyl groups are turned off.
The attachment of the cobalt ions inside the dendrimer must somehow pull the branches together, disrupting the light emission from the dansyl units at the surface. Perhaps by being pulled closer together, the units squander any energy they absorb by partitioning it between them instead of re-emitting it in one lump.
About 25 micrograms of cobalt in a litre of water -- 3 to 5 times the safe daily intake for humans -- is enough to produce a clearly detectable change in fluorescence intensity from their dendrimer sensor.
But the value of this work lies in the principles rather than in immediate practical applications. For example, it might now be possible to design dendrimer branches that can recognize and attach to metals or other targets selectively (cobalt is unlikely to be alone in triggering such a response). Plus, more branched dendrimers, with a greater number of light-emitting tips, might amplify the 'sensing' signal even more.