A collaboration of Finnish-German scientists have demonstrated that clusters of silver particles can be used as surface sensors for molecular analysis. Because clusters of only a few atoms are being used, the technology could potentially lead to tiny injectable sensors tuned to look for specific biochemical markers.
Ras [Robin Ras, senior researcher in the Molecular Materials group at Helsinki University of Technology] and his collaborators have systematically investigated the optical properties of silver (Ag) nanoclusters in solution – prepared in different water/methanol mixtures –and their response to the environment. What they found is that the spectral properties or color of the cluster solutions can be tuned to a great extent by selecting appropriate solvents. This property is called solvatochromism. The spectral shifts are not related to a change in nanocluster size.
"The clusters we examined were only few atoms in size such as Ag2 and Ag3" says Ras. "We found that the solvatochromism of the nanoclusters is analogous with that of metal nanoparticles but has a completely different origin. The solvatochromic effect for metal nanoparticles is well-known, and their photophysical properties are determined by surface plasmons. On the other hand, the photophysical properties of metal nanoclusters differ in character and are controlled by quantum confinement that results in discrete energy levels, therefore it was not evident that metal nanoclusters would also have solvatochromic properties."