This contribution has been forwarded by Ivo Rivetta.
The primary forces on the nanometer scale are scaled versions of what we experience on a day to day basis. Instead of gravity, surface forces such as water tension and electric charge dominate. As an example, compare wet basketballs and wet sand. The weight of the basketballs overpowers the surface forces that water introduces but wet sand particles clump together despite the fact that gravity is still trying to pull them apart.
Researchers at the National Institute of Standards and Technology (NIST) take advantage of this phenomena in the design of a Gold nanoparticle biosensor for HCG, a pregnancy hormone. The detection mechanism is based on the fact that nanoparticles absorb characteristic frequencies of light based on the size of their clusters.
The larger the cluster, the more light is absorbed by lower frequencies.
Light absorption is a common way to gauge the size of individual nanoparticles but NIST researchers are the first to correlate the absorbed frequencies of light specifically to cluster size. The next step in designing a sensor is establishing the minimum detection threshold for nanoparticles bound to HCG versus unbound nanoclusters.
Quote from: NIST Press Release: http://www.nist.gov/mml/biochemical/cluster-102511.cfm