Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation

Abstract

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the −0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (−0.1 V) than thiol-based monolayers (−0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure–functionality studies of NHCs on gold.

A Resilient Platform for the Discrete Functionalization of Gold Surfaces Based on N-Heterocyclic Carbene Self-Assembled Monolayers

“…The first feature at a binding energy of 401.8 eV mirrors previous characterizations of NHC-based monolayers corresponding to the nitrogen atoms in the imidazole ring. In this case the N (1s) peak shows a slightly higher binding energy corresponding to the more electron deficient imidazopyridine heterocycle compared to imidazole- and benzimidazole-derived NHCs reported elsewhere. ,, The second N 1s feature occurs at 398.8, corresponding to the pyridinic nitrogen, in good agreement with previously reported thiol-anchored alkylpyridine-based monolayers. , We found a N/Au atom ratio of 0.07:1, which corresponds to monolayer/submonolayer surface coverages reported previously . We also observed a F (1s) signal in the XPS survey sweep with a N/F ratio of 0.68:1, corresponding to an NHC/PF 6 ratio of 1.36:1.…”

Nanostructured Copper Screen-Printed Electrodes as a Platform for Plasmon-Enhanced Spectroelectrochemistry

“…Figure S11a–c shows the XPS spectrum for the positively charged precursor with a binding energy of 401.50 eV, electrochemically formed carbene with a binding energy of 400.57 eV, and immersed electrode with a binding energy of 400.78 eV, respectively. The shift of the N 1s peak from the precursor to the NHC monolayer aligns with the shifts reported in the literature. − It is interesting to note a shift of the N 1s peak with the immersed electrode (absence of potential). This observation suggests that the carbene can form under nonelectrochemical conditions, however to a significantly lesser extent.…”

“…The presence of the M–C bond is revealed by a shift of the N 1s peak. The unbound, positively charged precursor has a binding energy of ∼402 eV and the bound NHC has a binding energy of ∼400 eV. − Berg et al provided calculations to fit the N 1s XPS spectrum of a bound NHC, 398.9 and 400.7 eV are assigned to C–NC and N–C bonds . In this current work, a cathodic stepping was completed using the described NHC precursor in an electrolytic solution; at each potential, a SERS spectrum was acquired.…”

“…XPS can distinguish between chemisorption and physisorption of the NHC to the metal surface . Many reports discuss using gold surfaces in combination with benzimidazolium backbones or copper surfaces in combination with imidazolium backbones. − Regardless, shifts may vary between 0.1 and 0.5 eV depending on the backbone and metal used. The presence of the M–C bond is revealed by a shift of the N 1s peak.…”

Unmasking the Electrochemical Stability of N‐Heterocyclic Carbene Monolayers on Gold

“…The authors have cited additional references within the Supporting Information [11d,15–17,19b,c,21,24,27] …”