Quick Freezing-Induced AuNP Aggregates as Near-Infrared (NIR) Surface-Enhanced Raman Scattering (SERS) Substrates

The applications of Raman spectroscopy had been limited to qualitative analysis before Surface Enhanced Raman Scattering (SERS) was introduced to enhance intrinsically weak Raman scattering signals of analytes for ultrasensitive detection. Nanoscale surfaces constructed of gold or silver have been used as SERS substrates because their plasmon wavelength matches well with the wavelength of an excitation laser source in the visible region. In particular, gold nanoparticles (AuNPs) are commonly used, versatile SERS substrates. They are chemically inert and relatively easy to synthesize in a monodisperse form with various average diameters and different surface-capping agents. It has been previously reported that the aggregation of AuNPs increases the SERS signal of analytes by increasing the number of “hot spots” such as gaps between closely-spaced particles. The hot spots are where analytes can experience the enhanced electromagnetic field from nearby SERS substrates. However, the rate and the extent of AuNP aggregation, if induced by a chemical method, is difficult to control, which leads to poor reproducibility in measuring SERS signals. My research introduces a novel method to aggregate AuNPs without using any chemicals. The process includes quick-freezing an AuNP solution in a liquid N2, followed by a thawing step. These quick freezing-induced AuNPs (QFIAAs) can be prepared reproducibly and are stable for at least three months. In this study the QFIAA system has been examined by using UV-VIS, Raman, and fluorescence spectroscopy. Importantly, the QFIAA system exhibits good SERS activity in the near-infrared (NIR) region, suggesting its potential applications for detecting analytes in biological matrices that show strong background fluorescence with visible wavelength laser excitation. My research suggests that the NIRSERS activity of the QFIAA originates from the red-shift of the plasmon wavelength of AuNPs rather than the increase of the number of hot spots after the aggregation. Finally, the effects of the size of AuNPs and the capping agents of AuNPs on the formation of a QFIAA system will be discussed