Probing strong optical fields in compact aggregates of silver nanoparticles by SERRS of protoporphyrin IX

Abstract

TEM images and measurements of SERRS (surface-enhanced resonance Raman scattering) spectra as a function of the porphyrin concentrations in systems with unmodified and chloride-modified Ag nanoparticles and protoporphyrin IX (PPIX) are reported. TEM images have shown formation of compact aggregates in systems with chloride modified Ag nanoparticles, as opposed to systems with the unmodified particles constituted by isolated particles. SERRS spectra of PPIX as a function of PPIX concentration were measured and subjected to factor analysis. Two spectral components were identified and tentatively attributed to unperturbed PPIX and to Ag⁺–PPIX surface species. Concentration value of the SERRS spectral detection limit of the latter species was determined to be nearly three orders of magnitude lower in the system with the compact aggregates than in the system with separated nanoparticles and achieves the value of 1 × 10⁻¹⁰ M in a macrosampling Raman experiment. TEM images and SERRS-micro-Raman spectra of single compact aggregates of chloride-modified Ag nanoparticles incorporating PPIX molecules were acquired from a sample prepared by attachment of the aggregates to amine groups of derivatized, SiO_x/formvar coated copper grids for TEM. The SERRS signal has shown large temporal fluctuations as well as variations from one aggregate to another. Within the signal fluctuations, a SERRS spectrum showing the characteristic bands of both SERRS spectral forms of PPIX and originating most probably from a few PPIX molecules located in hot spots in the interstices between the Ag nanoparticles, was obtained.