Probing Conformation Change and Binding Mode of Metal Ion–Carboxyl Coordination Complex through Resonant Surface-Enhanced Raman Spectroscopy and Density Functional Theory

Abstract

Understanding carboxyl–metal ligand interaction has great significance in analytical chemistry. Herein, we use resonant surface-enhanced Raman scattering (SERS) to probe the physiochemical interaction and conformation change in several metal ion–carboxyl coordination complex systems adsorbed on the surface of plasmonically resonant metal nanostructures. Our SERS results and density function theory calculations jointly reveal that low-valence metal ions (such as K+ and Pb2+) tend to bind to the carboxyl active site of a Raman tag molecule, 4-mercaptobenzoic acid (4-MBA), in a unidentate binding mode of low binding energy whereas high-valence metal ions (such as Fe3+) favor a bidentate binding mode of relatively high binding energy. Particularly, Pb2+-ion concentration-dependent SERS suggests a repulsive interaction among the coordination complex leading to a tilted configuration of 4-MBA on the metal surface. This work indicates the resonant SERS approach is suitable not only for studying the carboxyl–metal ligand interaction but also for detecting various types of heavy metal ions at low concentrations.

Comparison of 4-Mercaptobenzoic Acid Surface-Enhanced Raman Spectroscopy-Based Methods for pH Determination in Cells

“…High values are likely due to glass background and low values are likely due to decarboxylation of MBA. Additionally the ν(COO – ) stretch is known to be subject to variability such as shifts in frequency and changes in linewidth with pH, 10,18,20,21 making reliable peak fitting more challenging.…”

“…13 The peak around 1700 cm –1 is attributed to a COOH stretch, and is therefore present in the protonated state of MBA. 9,10,12–14,18,20,21 The frequency of the ν 8a ring breathing mode (around 1580 cm –1 ) is also known to shift with changing pH. 18,19,22 Other peaks are affiliated with the protonated or deprotonated state of the molecule, but have much lower intensity relative to the spectral features described.…”

“…4,9–21 The orientation of the MBA molecule on the nanoparticle surface can alter which of the two peaks is observed in the SERS spectrum. 13,18–21 These features at 1380 cm –1 and 1410 cm –1 have been used for extracellular pH detection by a SERS probe anchored to the cell membrane. 13 The peak around 1700 cm –1 is attributed to a COOH stretch, and is therefore present in the protonated state of MBA.…”

Stability and Performance Enhancement of an Oligo (phenylene vinylene) Photocatalyst via Surface Grafting onto TiO2 for Visible‐Light Indigo Carmine Degradation

“…Meanwhile, the Raman spectrum of TiO 2 /OPV contains vibration bands associated with the carbonyl group (C=O) around 1632 cm −1 , a stretching mode signal for the carboxylate within the OPV structure and the characteristic vibrations bands of the TiO 2 support. The observed shift from 1413 cm −1 in OPV to 1323 cm −1 in TiO 2 /OPV, of the carboxylate stretching mode suggests the formation of the desired unidentate configuration between the carboxylate and the Ti atom in the surface [47] . Interestingly, the Raman spectrum for SiO 2 /OPV did not display any OPV peaks (Supporting information, Figure S1).…”

Magnetic nanoprobes for rapid detection of copper ion in aqueous environment by surface-enhanced Raman spectroscopy

“…Therefore, the stability of the chelating complexes determined the level of aggregation of magnetic nanoprobes and the stimulation of SERS signals. Copper ions and carboxylic acid prefer to form bidentate chelates, 44 which illustrates that copper ion requires two carboxyl acid groups to stabilize the chelating complex. Copper ions could process spontaneous aggregation of magnetic nanoprobes due to maintenance of the stability for copper-based chelation complexes.…”