The Suitable Condition of Using LSPR Model in SERS: LSPR Effect Versus Chemical Effect on Microparticles Surface-Modified with Nanostructures

The conditions under which the localized surface plasmon resonance (LSPR) model can be applied to the calculation of surface-enhanced Raman scattering (SERS) enhancement factors have been questioned because the chemical effect presents simultaneously with LSPR effect, resulting in calculations that are not always consistent with the measured data. The SERS spectra of crystal violet (CV) molecules on single, dimer, trimer, and aggregates of silver microparticles surface-modified with nanostructures (MSMN) were obtained. It is found that the chemical effect is determined by the chemical adsorption behavior of CV molecules on single particle. As more particles are introduced as amplifiers, to assemble dimer, trimer, and aggregates, no new SERS signals related to the chemical effect can be observed, except for the further enhancement to the original signals. The further enhancement is attributed to the LSPR effect from the electromagnetic coupling with introduced particles. This is also demonstrated by dark field scattering. The LSPR theoretical values of single, dimer, trimer, and aggregates of MSMNs should fit the measured enhancement factor (G (LSPR)) after correcting the SERS enhancement factor (G (SERS)) with the chemical enhancement factor on the single particle (G (Chem-Sgl)), i.e., G (LSPR) = G (SERS)/G (Chem-Sgl). Tip-enhanced Raman spectroscopy with a gold nanoparticle further implies that this could be extended to nanoparticle systems. This work provides an effective and simple route, whereby only the chemical effect from a single particle needs to be considered when studying the fit between the LSPR model and the measured LSPR enhancement factor.