Chemical enhancement effects on protoporphyrin IX surface‐enhanced Raman spectra: Metal substrate dependence and a vibronic theory analysis

Abstract

Surface‐enhanced Raman spectra (SERS) of protoporphyrin IX (PPIX) and analogous metal‐containing PPIX compounds excited at 785 nm exhibit robust, strongly enhanced (~5 × 109) spectra that are highly dependent on the nanostructured metal (Au or Ag). Although the relative intensities of the 785‐nm SERS vibrational spectral features are very similar for all observed PPIX‐based compounds, the relative intensities are dramatically different on Au and Ag SERS substrates. Time‐dependent density functional theory (TDDFT) calculations are carried out for Au2/Ag2–PPIX complexes to understand the origins of this chemical enhancement contribution to the observed SERS spectra. The observed metal dependence of the SERS spectra in the heme ring stretching region (1,500–1,620 cm−1) are reproduced by calculated resonance Raman intensities due to low‐lying metal (σ) to molecule (π*) charge transfer excitations of the metal dimer–PPIX complexes when the metal dimer is near a porphyrin ring Cβ position. The nuclear coordinate dependence of electronic transition moments is essential for capturing the effects of these CT excitations. A simple method for determining the importance of the transition moment normal mode dependence relative to Franck–Condon factors (A term) is described and implemented. The consequences of this vibronic analysis for the observation of overtone/combination bands in SERS spectra are discussed.

Retracted: Zeptomolar detection of 4-aminothiophenol by SERS using silver nanodendrites decorated with gold nanoparticles

“…The amplification of the Raman signal is attributed to two effects: the chemical factor, which consists in the chemisorption of the analyte in the noble metal nanostructure where an adsorbate-metal complex is formed, amplifying the molecular orbitals of the analyte caused by an interaction with the conduction bands of the metal [17][18][19], and the electromagnetic effect, which consists in an increase of the local electric field caused by the polarization in the metal nanoparticles [20] and the interaction with the Raman signal scattered by the re-radiation process [21].…”

NIR surface enhanced Raman spectra of biological hemes: Solvation and plasmonic metal effects

“…Calculations reveal that the vibronic coupling resulting from the new low‐lying metal to heme π* charge transfer bands of the physi‐adsorbed analyte‐surface complex is metal dependent and accounts for the different relative 785 nm SERS intensities on Au and Ag apart from any additional orientational effects. [ 32 ] The observed pattern of relative intensities in these 785 nm SERS spectra are different than those seen for SERRS excited in the Soret or α , β band regions. [ 37–44 ]…”

“…It was previously established for hemoglobin at least, that the 785‐nm SERS spectrum in 50% acetic acid on Au and on Ag substrates have distinct patterns of relative intensities due, in part, to the metal specificity of 785‐nm chemical enhancement effects. [ 28,32 ] The studies describe here will address whether such metal surface dependent differences extend to all the heme proteins and solvent environments observed here.…”

“…In addition to the high level of sensitivity previously demonstrated for the SERS spectra of dried blood via HAc extraction and transfer to the SERS substrates, the corresponding robust 785-nm SERS signature of dried blood on the Au and Ag SERS substrates, that is, the pattern of relative vibrational intensities, was shown to be nearly completely different on the two metals. [28,32] Figure 2 shows the normalized 785 nm excited SERS spectrum of myoglobin in 50% acetic acid (HAc) on Au and Ag substrates. These are virtually identical (vide infra) to the 785 nm excited SERS spectra of dried blood, red blood cells, and hemoglobin shown previously.…”

Attomolar detection of 4-aminothiophenol by SERS using silver nanodendrites decorated with gold nanoparticles

“…For details of the changes made between the two works, please see the supplementary data. an adsorbate-metal complex is formed, amplifying the molecular orbitals of the analyte caused by an interaction with the conduction bands of the metal [17][18][19], and the electromagnetic effect, which consists in an increase of the local electric field caused by the polarization in the metal nanoparticles [20] and the interaction with the Raman signal scattered by the re-radiation process [21].…”