Coagulative transition of gold nanoparticle spheroids into monolithic colloids: Structure, lifetime, and transition model

This paper deals with the structural changes of spherical aggregates composed of elemental gold nanoparticles, called spheroids. The spheroid diameter is reduced to almost 70% of the original diameter by maintaining the spheroid suspension at a constant temperature ranging from 65 to 91 degreesC for 2-12 h (from 2 h at 91 degreesC to 12 h at 65 degreesC). The size-reduced spheroids become monolithic colloids of similar size at each temperature by the end of the reduction process. This is due to an irreversible transition, resulting from fusion among the nanocolloids in spheroids: coagulative transition. The 30% reduction of the diameter implies that one spheroid is initially composed of 67 nanocolloids at 0.26 volume fraction of the nanocolloids. Assuming that one spheroid gives one monolithic colloid (one-to-one coagulative transition), the lifetime of spheroids can be estimated from the dependence of the transition time on the suspension temperature, for example, 1 month at 22 degreesC and 6 months at 4 degreesC. A nonequilibrium thermodynamic model of the spheroid suspension describes the coagulative transition, suggesting that the exclusion and osmotic pressures against intercalated molecules in spheroids are additional factors keeping spheroids in water.