Here we present that the extent of the aggregation of citrate-capped gold nanoparticles (AuNPs) could be controlled by the freezing rate for the AuNP solution. A citrate buffer solution containing AuNPs with an average diameter of 70 nm was frozen and thawed. The freezing-induced AuNP aggregation was examined using UV-VIS absorption spectroscopy and Raman spectroscopy.  We paid attention to the change of the plasmon band of AuNPs and the characteristic Raman peaks of citrate on AuNP surfaces after the cycle of freezing and thawing process was completed. Overall, the extinction intensity of the representative plasmon band of 70 nm AuNPs was significantly reduced and a broad plasmon band in the near-IR region newly appeared, which suggested the aggregation of the AuNPs. The aggregation of the AuNPs was also confirmed by the appearance of citrate peaks in the Raman spectrum.  In addition, Rhodamine 6G (R6G), a well-known Raman dye, was tested to see the effect of surface-enhanced Raman scattering (SERS) in the near-IR region as a result of the aggregation of the AuNPs. The near-IR Raman signals of R6G were more enhanced in the solution of AuNP aggregates than in the solution of free AuNPs. This was due to the resonance of plasmon band of AuNP aggregates with the near IR laser. Our results also showed that a quick freezing rate produced less extensive AuNP aggregates than a slow freezing rate. This was ascribed to the formation of small crystal grain boundaries for the quick-frozen AuNP solution, where the AuNPs were forced to aggregate.  <br/>

Underlying mechanism of freezing-induced aggregation of gold nanoparticles

255th National Meeting (2018)

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