Effects of chromophore orientation and molecule conformation on surface-enhanced Raman scattering studied with alkanoic acids and colloidal silver nanoparticles

Abstract

Experimental studies have been carried out to gain a better understanding of the effects of chromophore orientation and molecular conformation on surface-enhanced Raman scattering (SERS) based on metal nanostructures. A series of alkanoic acids that contain a phenyl ring separated by methylene groups from the carboxylic acid, including phenylacetic acid, 3-phenylpropionic acid, 4-phenylbutyric acid, 5-phenylvaleric acid, and 6-phenylhexanoic acid, was investigated as model molecules with colloidal silver nanoparticles as SERS substrates. As the number of methylene groups increases, the molecules display an interesting zigzag intensity pattern of the phenyl ring bending mode around 1000 cm(-1) as well as a trend of appearance and disappearance of either the degenerate ring breathing mode or C[Double Bond]O vibrational mode near 1585 and 1630 cm(-1), respectively. Molecules containing an odd number of methylene units display a higher ring bending intensity and degenerate ring breathing mode and are suggested to have a trans conformation on the particle surface. Molecules with an even number of methylene units show a C[Double Bond]O vibrational mode and weaker ring bending in their SERS spectra and are suggested to have a gauche conformation on the silver nanoparticle surface. The different conformation is attributed to the varying interactions of the carboxylic group or the phenyl ring pi electrons with the silver surface. The SERS intensity was found to change little as the length between the phenyl ring and the carboxylic group was increased by adding CH(2) spacers. This is possibly because the effective distance between the phenyl ring and the silver surface does not change much with increasing number of CH(2) spacers as a result of changes in molecular conformation and variations in the phenyl ring orientation with CH(2) addition. The insight gained from this study is important for understanding SERS of complex molecules for which chromophore orientation and molecular conformation must be taken into careful consideration.

Sensing Remote Chirality: Stereochemical Determination of β‐, γ‐, and δ‐Chiral Carboxylic Acids

“…The inversion in the ECCD signals observed for amides bearing an aryl group has been documented previously 8. 9 Aryl‐group‐directed change in conformational preference was also detected in nanoassemblies, where complexation of β‐ δ chiral aryl versus alkyl carboxylic acids with gold nanoparticles led to complexes of different geometries, probably as a result of the secondary interaction between metal nanoparticles and closely situated aryl substituents 12. With respect to the β‐, γ‐, and δ‐chiral amides analyzed herein, the conformational change appears to be induced by the secondary interaction with the N‐Ar carrier, since no change in the ECCD sign between pseudoidentical alkyl‐ and aryl‐substituted substrates was observed when 1,3‐diaminopropane was used as the carrier in studies with α‐chiral acids 13…”

Plasmonic Optical Properties and Applications of Metal Nanostructures

“…Therefore, care needs to be taken to interpret SERS spectra. [238,239] There are several different theories developed over the years to explain the origin of SERS. It is generally agreed today that the main source of enhancement is due to amplified EM field at the surface of the metal substrate, usually in the form of nanostructures.…”

“…This is in part because it is actually challenging to unambiguously determine the distance dependence of SERS due to complications caused by changes in other factors when distance is varied on the atomic scale. [238,239] A second important factor is the location of the analyte molecule on the substrate surface. As an example, for spherical particles with incident laser light polarized in a particular direction, the EM field enhancement is anisotropic.…”

Sensing Remote Chirality: Stereochemical Determination of β‐, γ‐, and δ‐Chiral Carboxylic Acids

“…[8,9] Aryl-groupdirected change in conformational preference was also detected in nanoassemblies,w here complexation of b-d chiral aryl versus alkyl carboxylic acids with gold nanoparticles led to complexes of different geometries,probably as ar esult of the secondary interaction between metal nanoparticles and closely situated aryl substituents. [12] With respect to the b-, g-, and d-chiral amides analyzed herein, the conformational change appears to be induced by the secondary interaction with the N-Ar carrier,since no change in the ECCD sign between pseudoidentical alkyl-and aryl-substituted substrates was observed when 1,3-diaminopropane was used as the carrier in studies with a-chiral acids. [13] With the empirical observations in hand, we focused on understanding the mode of stereodifferentiation with the goal of generating aw orking mnenonic.…”