Electrochemical and X-ray Photoelectron Spectroscopy Surface Characterization of Interchain-Driven Self-Assembled Monolayer (SAM) Reorganization

Abstract

Herein, we report a combined strategy encompassing electrochemical and x-ray photoelectron spectroscopy (XPS) experiments to investigate self-assembled monolayer (SAM) conformational reorganization onto an electrode surface due to the application of an electrical field. In particular, 3-mercaptopriopionic acid SAM (3MPA SAM) modified gold electrodes are activated with a 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysulfosuccinimide (NHSS) (EDC-NHSS) mixture by shortening the activation time, from 2 h to 15/20 min, labelled as Protocol-A, -B and -C, respectively. This step, later followed by a deactivation process with ethanolamine (EA), plays a key role in the reaction yields (formation of N-(2-hydroxyethyl)-3-mercaptopropanamide, NMPA) but also in the conformational rearrangement observed during the application of the electrical field. This study aims at explaining the high performance (i.e., single-molecule detection at a large electrode interface) of bioelectronic devices, where the 3MPA-based SAM structure is pivotal in achieving extremely high sensing performance levels due to its interchain interaction. Cyclic voltammetry (CV) experiments performed in K4Fe(CN)6:K3Fe(CN)6 for 3MPA SAMs that are activated/deactivated show similar trends of anodic peak current (IA) over time, mainly related to the presence of interchain hydrogen bonds, driving the conformational rearrangements (tightening of SAMs structure) while applying an electrical field. In addition, XPS analysis allows correlation of the deactivation yield with electrochemical data (conformational rearrangements), identifying the best protocol in terms of high reaction yield, mainly related to the shorter reaction time, and not triggering any side reactions. Finally, Protocol-C’s SAM surface coverage, determined by CV in H2SO4 and differential pulse voltammetry (DPV) in NaOH, was 1.29 * 1013 molecules cm−2, being similar to the bioreceptor surface coverage in single-molecule detection at a large electrode interface.

Controlling the Binding Efficiency of Surface Confined Antibodies through the Design of Mixed Self‐Assembled Monolayers

“…This leads to an antibodies’ layer packed to a less extent than that immobilized on the 11‐MUA/NMPA, possibly reducing the presence of steric repulsive interactions among the biorecognition elements. This evidence might be ascribable to the different SAMs supramolecular structures, [ 38,63,64 ] leading to peculiar conformational rearrangement of biolayer. This can directly impact on the degrees of freedom of the capturing antibodies, hindering an efficient interaction with the cognate ligand.…”

“…This can directly impact on the degrees of freedom of the capturing antibodies, hindering an efficient interaction with the cognate ligand. A characterization of mixed SAMs based on IR spectroscopy as well as electrochemical measurements, [ 64 ] recently proved that NMPA SAMs are endowed with a tighter structure with respect to 3‐MPA SAM, probably related to high‐energetic chain interactions. This might result in a more packed biolayer, dramatically reducing the availability of the binding sites of antibodies capable to bind cognate antigens.…”

“…On the other hand, 3‐MPA SAMs are characterized by a diffuse hydrogen bonding but a relatively low chain length. [ 64 ] The latter limits hydrophobic interaction between chains, thus decreasing the stability of compact structures and promoting the formation of pinholes and defects. Consequently, the anti‐p24 deposited on the 11‐MUA/3‐MPA mixed SAM shows better accessibility of the binding sites when exposed to the antigen, due to the higher motility of the biofilm.…”