Self-limiting aggregation leads to long-lived metastable clusters in colloidal solutions.

The existence of a metastable state with limited Coulomb-blocked aggregation at the onset of instability in a colloidal solution is proposed and demonstrated both experimentally and theoretically (through Monte Carlo simulations). Such a stable state of small clusters of metallic colloids happens to be extremely important for techniques such as surface-enhanced Raman scattering (SERS), which profits explicitly from collective plasmon resonances in these clusters to boost Raman signals of specific analytes. In fact, SERS provides a unique tool to understand, monitor, and study the onset of aggregation in colloidal silver/gold and to prove the existence of the proposed state at the boundary of colloid coalescence.