Physical Geography and Sciences



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Carleigh Cimmerer, Forensic Chemistry
Faculty Mentor: Professor Jinseok Heo, Chemistry

Here we report the effect of capping agents on the freezing-induced aggregation of gold nanoparticles (AuNPs). Previously, we demonstrated that quick freezing-induced AuNP aggregates (QFIAA), prepared from citrate-capped AuNPs, could function as surface-enhanced Raman scattering (SERS) substrates in the near-IR region. Near-IR SERS substrates are important for the Raman detection of low concentration analytes in a biological matrix that emits strong fluorescence. Further research showed that the SERS signals of analytes on the QFIAA strongly depended on the sizes of individual AuNPs in the AuNP clusters. In this study we wanted to examine how the capping agents, which were used to prevent the aggregation of AuNPs during the synthesis, would affect the freezing-induced aggregation of AuNPs. To test the hypothesis, three different types of AuNPs capped with citrates, tartarate, or PVP were used to prepare for the QFIAA. All these capping agents were negatively charged but had different molecular sizes. The extent of the AuNP aggregation was examined using UV-VIS absorption and Raman spectroscopy. Furthermore, four different dye analytes were tested for Raman detection in the presence of quick freezing-treated AuNPs. We found that the citrate-capped QFIAA performed the best as near-IR SERS subtrates, followed by tartarate-capped QFIAA. However, PVP-capped AuNPs did not form clusters thanks to the steric hindrance of PVP polymer. While the positively charged analytes could be detected with citrate-capped or less sensitively with tartate-capped QFIAA, the negatively charged molecules could not be detected with any types of AuNPs tested here. Further research will focus on expanding the applications of QFIAA for the detection of a broad range of dye analytes.

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The Effect of Capping Ligands on the Freezing-Induced Aggregation of Gold Nanoparticles (AuNPs) for Near-IR SERS Substrates
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