Factor Analysis of SERS Spectra Sheds Light on Changes in Ag Nanoparticle Systems

Researchers at Charles University in Hlavova, Czech Republic, have utilized factor analysis (FA) to gain a better understanding of changes in surface plasmon extinction (SPE) and water arrangement in silver (Ag) nanoparticle systems (NPs). The study, published in the Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal, highlights the importance of the selected excitation wavelength in surface-enhanced Raman scattering (SERS) measurements, which is determined by the SPE band position (1).

3d illustration of nanoparticles | Image Credit: © Artur - stock.adobe.com

SPE is a phenomenon in which plasmonic nanoparticles absorb and scatter incident light at their resonant frequency. When a molecule is close to a plasmonic nanoparticle, the enhancement of its Raman signal is highly influenced by the position and intensity of the SPE band. The SPE band is usually determined from the absorption UV-visible (UV-vis) spectra or from the extinction spectra. The optimal excitation wavelength for SERS measurements is given by the position of the SPE band of the studied system. A small change in the SPE band intensity, position, and shape during the measurement may significantly influence the SERS signal.

The team prepared several systems of Ag nanoparticles, which were used to demonstrate how information about SPE changes can be obtained by FA from SERS spectral sets, resulting in more precise and comprehensive interpretation of the results. In non-aggregated Ag colloidal systems measured at the excitation wavelength of 445 nm, SPE band changes were monitored by analyzing the water stretching vibration together with vibrations in the fingerprint region. The FA of the water stretching band region provided unique information on both the arrangement and disarrangement of water molecules in the vicinity of Ag NPs during the time evolution of these SERS active systems.

In aggregated Ag colloidal systems measured at the excitation wavelength of 785 nm, the FA of SERS spectral sets enabled the team to reveal the contribution of the 2nd electromagnetic enhancement to the overall SERS signal. The reliability of the conclusions was verified by comparing the results obtained from FA of SERS spectral sets with the data obtained from the parallel SPE measurements of the studied systems.

The localized surface plasmon resonance (LSPR) responsible for the unique optical properties of plasmonic metal NPs is attributed to the collective oscillation of electrons on the surface of metallic NPs excited by the incident radiation at their resonant frequency. The LSPR can be determined from the absorption UV-vis spectra or, more precisely, from the SPE spectra. The excitation of the LSPR bands is the main cause of the amplification of the signal in SERS. The highest SERS enhancement occurs if both the incident and Raman scattered photons fall near the maximum of the LSPR extinction.

The study highlights the importance of FA in gaining a better understanding of changes in Ag nanoparticle systems and how this can lead to more precise and comprehensive interpretation of results. It also sheds light on the role of the selected excitation wavelength in SERS measurements and how it can influence the SERS signal significantly. This research is expected to have a significant impact on the development of SERS-based analytical techniques for the detection of trace amounts of various analytes in environmental and medical applications.

Reference

(1) Kozisek, J.; Slouf, M.; Sloufova, I. Factor analysis of the time series of SERS spectra reveals water arrangement and surface plasmon changes in Ag nanoparticle systems. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2023, 293, 122454. DOI: 10.1016/j.saa.2023.122454