Electrochemical Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy of Imidazole Ring Functionalized Monolayer on Smooth Gold Electrode

Abstract

The imidazole ring (Im) of histidine side chains plays a unique role in the function of proteins through covalent bonding with metal ions and hydrogen bonding interactions with adjusted biomolecules and water. At biological interfaces, these interactions are modified because of the presence of an electric field. Self-assembled monolayers (SAMs) with the functional Im group mimic the histidine side chain at electrified interfaces. In this study, we applied in-situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to probe the structure and hydrogen bonding of Im-functionalized SAM on smooth Au at the electrochemical interface. The self-assembly of molecules on the Au induced the proton shift from N1 atom (Tautomer-I), which is the dominant form of Im in the bulk sample, to N3 atom (Tautomer-II). The impact of electrode potential on the hydrogen bonding interaction strength of the Im ring was identified by SHINERS. Temperature-Raman measurements and density functional theory (DFT) analysis revealed the spectral marker for Im ring packing (mode near 1496–1480 cm−1) that allowed us to associate the confined and strongly hydrogen bonded interfacial Im groups with electrode polarization at −0.8 V. Reflection adsorption IR (RAIR) spectra of SAMs with and without Im revealed that the bulky ring prevented the formation of a strongly hydrogen bonded amide group network.

Electrochemical SEIRAS Analysis of Imidazole-Ring-Functionalized Self-Assembled Monolayers

“…In our recent study based on the temperature-Raman and theoretical modeling, the Im ring ν(C2–N3) + β(C2H) mode was found to depend on the H-bond strength at the N1/N3 atoms [ 21 ]. DFT predicted that interaction of the Im ring with explicit water molecules resulted in a vibrational frequency shift by 2–6 cm −1 to higher frequencies, while an addition of two water molecules shifted the vibrational mode by 9 cm −1 .…”

“…For the molecule in the T-II state, these vibrations become coupled and engage in out-of-phase and in-phase vibrational motions, respectively. Surface adsorption induces the transition from the T-I to the T-II form; therefore, we consider the T-II (N1, N3–D) as the main imidazole tautomeric form in our system [ 4 , 21 ]. The frequency tuning rate of −10.4 and −6.3 cm −1 V −1 for the low and high-frequency modes was identified when electrode polarization was changed from 0.3 to −0.5 V ( Figure 8 B).…”

“…N-(2-(1H-imidazol-4-yl)ethyl)-6-mercaptohexanamide (IMHA) and 6-mercapto-N-methylhexanamide (molecular fragment, Frag) were synthesized in-house. The synthesis details are published elsewhere [ 21 ]. IMHA and Frag hydrogen atoms at N and S were substituted with deuterium by dissolving the compounds in ethanol-d 6 and then evaporating the solvent in a heated ultrasonic bath.…”

Adsorption of 2-Mercapto-1-methylimidazole on the (√3 × 22) Reconstructed and (1 × 1) Phases of an Ordered Au(111) Electrode under Potential Control

“…19,20 In situ Raman spectroscopy has been used to reveal the tautomerization of Im adsorbed on a Au electrode, as the proton shifts from the N1 to N3 site of the Im group at a positive potential. 21 The use of Im as a linker to fabricate more sophisticated bilayer structures on a substrate has been illustrated. 22 Although in situ STM has been used to reveal molecular adsorptions at the electrified interface, 23−27 only a few reports have addressed the structure of thiol with Im groups adsorbed on Au(111) electrodes.…”

“…A number of studies on SAMs of azole thiols, such as 2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and 2-mercaptobenzimidazole, and 6-mercaptopurine on Au(111) have been examined by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy (STM). These molecules can tether to a Au substrate with not only the S-head group but also the S–, O–, and N– ends of the heterocycle. , In situ Raman spectroscopy has been used to reveal the tautomerization of Im adsorbed on a Au electrode, as the proton shifts from the N1 to N3 site of the Im group at a positive potential . The use of Im as a linker to fabricate more sophisticated bilayer structures on a substrate has been illustrated …”