Wavelength-Dependent Correlations between Ultraviolet-Visible Intensities and Surface Enhanced Raman Spectroscopic Enhancement Factors of Aggregated Gold and Silver Nanoparticles

The wavelength-dependent correlations between UV-vis intensities and surface enhanced Raman spectroscopic (SERS) enhancement factors (EFs) of aggregated gold and silver nanoparticles (AgNPs and AuNPs) were investigated using two experimental approaches. The first is to study the time-resolved SERS EFs under three fixed excitation wavelengths (532, 632, and 785 am), each as a function of nanoparticle (NP) aggregation states. The second is to compare SERS EFs at these three excitation wavelengths for a series of protein-stabilized AuNP or AgNP aggregates. The SERS EFs were determined using a solvent internal reference method. The NP UV-vis intensity is an excellent indicator for identifying the optimal aggregation state for the AgNP-based SERS acquisitions under each of the three excitation wavelengths and for the AuNP-based SERS under 632 nm excitation. However, the NP UV-vis intensity is an unreliable predictor of the optimal excitation wavelength for either AuNPs or AgNPs. Computational simulations reveal that the NP SERS enhancement is much more sensitive than NP UV-vis intensity to small changes in the NP aggregation states. In addition to enhancing the understanding of the correlation among NP aggregation, UV-vis intensity, and SERS activity, the techniques and insights derived from this work should be important for developing sensitive and reproducible colloidal-NP-based SERS applications.