Probing N-Heterocyclic Carbene Surfaces with Laser Desorption Ionization Mass Spectrometry

Abstract

The proliferation of N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold surfaces stems from their exceptional stability compared to conventional thiol-SAMs. The prospect of biological applications for NHC-SAMs on gold shows the need for biocompatible techniques (e.g., large biomolecule detection and high throughput) that assesses SAM molecular composition. Herein, we demonstrate that laser desorption ionization mass spectrometry (LDI-MS) is a powerful and facile probe of NHC surface chemistry. LDI-MS of prototypical imidazole-NHC-and benzimidazole-NHC-functionalized AuNPs yields exclusively [NHC 2 Au] + ions and not larger gold clusters. Employing benzimidazole-NHC isotopologues, we explore how monolayers pack on a single AuNP and the lability of the NHCs once ligated. Quantitative analysis of the homoleptic and heteroleptic [NHC 2 Au] + ions is performed by comparing to a binomial model representative of a randomized monolayer. Lastly, the reduction of nitro-NHC-AuNPs to amine-NHC-AuNPs is tracked via LDI-MS signals, illustrating the ability of LDI-MS to probe postsynthetic modifications of the anchored NHCs, which is critical for current and future applications of NHC surfaces.

Ad aurum: tunable transfer of N-heterocyclic carbene complexes to gold surfaces

“…3, middle). 52 Most notably, treatment of AuNPs with ( 1 )AgBr gives predominantly [( 1 ) 2 Au] + at 601 m / z (Fig. 3, bottom).…”

“…LDI-MS characterization is a powerful tool to probe the chemical composition of NHC monolayers and reveal reactions at the surface. 52 Here, we use LDI-MS to probe the ligand transfer process. SERS is a highly surface sensitive technique capable of measuring the vibrational spectroscopy of ligands on AuNP surfaces.…”

Fundamentals and applications of N-heterocyclic carbene functionalized gold surfaces and nanoparticles

“…These post-synthetic modifications of NHC–AuNPs have been explored by Camden and Jenkins using SERS 67 and laser desorption ionization mass spectrometry (LDI-MS). 108 A nitro-functionalized NHC–Au( i ) complex was appended to AuNPs and reduced in situ to yield amine-functionalized NHC–AuNPs followed by an in situ amide coupling with benzoic acid.…”

Giving Gold Wings: Ultrabright and Fragmentation Free Mass Spectrometry Reporters for Barcoding, Bioconjugation Monitoring, and Data Storage

“…This high ion yield likely arises from the formation of chemically stable, positively charged, bis‐NHC ions [16a, 19] during the desorption/ionization process [20] . Critically, previous research from our groups with deuterium isotopologues of NHC ligands demonstrated that bis‐NHC gold ions form via desorption of two neighboring ligands and not from a plume chemistry process of the LDI [20] . The high yield of a variety of bis‐NHC gold ions from our NHC‐AuNP library highlights the modularity of a NHC mass barcode library with predictable, easily analyzed mass spectra.…”