The photothermal-based AFM-IR technique provides chemical characterization with 10 nm spatial resolution and monolayer sensitivity, and for the first time has been applied to study the self-assembled monolayers (SAMs) of N-Heterocyclic carbenes (NHCs) on metal surfaces. The NHCs are a group of molecular ligands with strong affinity to metals, and NHC-SAMs are often utilized as biosensors and molecular probes for surface reactivity. The authors developed a novel approach through electrochemical deposition to prepare NHC-SAMs on various metal surfaces (Au, Pt, Pd and Ag). In the electrochemical deposition, high concentration of hydroxide ions (OH−) was generated in proximity to a metal electrode by applying a negative potential (-1 V); the OH- ions would then deprotonate the precursor cations of NHCs, leading to the formation of NHC-SAMs of high surface density and improved chemical stability.
As one example, the Nitro-functionalized NHCs (NO2-NHCs) were electrochemically deposited on Si-supported Au film. The AFM-IR spectra measured on the gold surface showed two clear IR bands at 1533 and 1603 cm−1, corresponding to asymmetric N–O and aromatic C=C vibrations. The absence of a symmetric N–O vibration indicates that the –NO2 groups in NO2-NHCs were not oriented in a standing position, which is consistent with the result of theoretical DFT calculations. AFM-IR mapping at 1533 and 1603 cm−1 revealed homogeneous distribution of NO2-NHCs on the Au film and no signal on the Si surface. Some randomly distributed structures with a size of 10–70 nm were shown to be scattered on both the Au film and Si substrate by the AFM topography image. Those structures were attributed to bromide residues from the electrochemical deposition process and can locally block the NHCs’ adsorption on the Au film, resulting in weak AFM-IR signal at the locations of those random structures.
As a comparison, SAM of NO2-NHCs prepared on a gold film by base-induced deprotonation was also studied by AFM-IR measurements. Besides IR bands at 1533 and 1603 cm−1, two extra bands were observed at 1346 and 1466 cm−1, corresponding to symmetric N–O and C-NH vibrations. The 1466 cm−1 band was due to the reduction of nitro groups to amines, which has also been identified by X-ray photoelectron spectroscopy (XPS) and Linear sweep voltammetry (LSV) measurements. The presence of the symmetric N-O vibration at 1346 cm−1 (in addition to asymmetric N-O vibration at 1533 cm−1) is correlated to the random orientation of the –NO2 groups. This variation reflects the higher surface density of NHCs prepared by electrochemical deposition than by conventional base-induced deprotonation.