Silver-Surface-Enhanced Raman Spectra of Berberine: Analyte-Induced Surface Changes, Variable Concentration Correlation, and Excitation Wavelength Dependence

The utilization of surface-enhanced Raman scattering (SERS) for the analysis of biologically important compounds is strictly dependent on the properties of the substance being analyzed. One of them is berberine, a highly valued bioactive alkaloid sourced from various botanical species, which is renowned for its multifaceted health-enhancing attributes, although its potential negative effects have been widely discussed. Here, we aimed to investigate the properties of berberine influencing the SERS intensity. By modifying silver colloids by the wide range of berberine concentrations, we have revealed its ability to significantly affect the nanoparticle surface's properties, which results in complex concentration-dependent behavior. Characterization using extinction spectroscopy and transmission electron microscopy was performed to describe the ongoing effects. These results show that the tendency of silver nanoparticles to preferentially form assemblies with different geometries is the main reason for the nonlinear concentration dependence of the SERS signal. Additionally, we have investigated the effect of the excitation wavelength (532, 785, and 1064 nm) used. Such experiments not only provided the first comparison of the berberine SERS spectra measured with three different excitation wavelengths but also demonstrated that the observed intensity dependence is valid over a wide interval of excitation wavelengths. Apart from the physicochemical point of view, we also paid attention to effects important for possible analytical applications, such as reproducibility and long-term validity of the observed trends.