Self-Assembled N-Heterocyclic Carbene Monolayers on Gold as a Tunable Platform for Designing Biosensor Surfaces
Surface plasmon resonance (SPR)-based biosensing is an excellent tool to probe the recognition process between biomolecules in real-time, without need for labels. Thiol-based self-assembled monolayers (SAMs) on gold have been widely used as linker layers for biosensor chips used in SPR. However, their use is hampered by the gold-thiol linkage’s high propensity for oxidation in ambient atmosphere. To this end, a new family of materials based on N-heterocyclic carbenes (NHCs) has emerged as an alternative anchor for surface modiﬁcation. Most importantly, the NHC SAMs outperform thiol analogues under a wide range of harsh conditions. Here, we compare the performance of an alkylated NHC SAM on gold with a commercial thiol-based analogue, a hydrophobic association (HPA) chip. Compared to the thiol-based sensor surface, the NHC sensor surface features a desired list of performance merits, including lower non-specific binding capacity, better chemical stability, higher reproducibility, shorter equilibration time, and longer life span. We also demonstrate that the NHC sensor surface can be used for rapid and efficient formation of a hybrid lipid bilayer for use in membrane interaction studies. A sensor surface consisting of an NHC SAM coupled to carboxymethylated dextran was also developed. Standard SPR instrument tests demonstrated the NHC-dextran surfaces to be homogeneous and sufficiently responsive to meet performance standards. In terms of the real life biosensing validation, we found that the NHC-supported dextran surfaces yielded comparable performance to commercial thiol supported biosensor surfaces in kinetic analysis of drug/plasma protein and antibody/antigen interactions. Three types of NHC-based dextran surfaces derivatized with affinity capture functional molecules to enable specifically oriented binding of ligands for biosensing applications were developed: NHC-Streptavidin (SA), NHC-Nitrilotriacetic acid (NTA), and NHC-Protein A. Our results show that, when properly designed and applied, the NHC-based platform has the potential that allows facile tuning of surface properties in a highly divergent fashion, enabling various ligand to be efficiently immobilized for reliable biomolecular interactions. These results highlight the potential viability of chemical modifications to gold surfaces using NHC ligands as a robust and versatile platform to enable efficient evaluation of a wide range of biomolecular interactions.