Exploring the pH Sensitivity of Ion-Pair Interactions on a Self-Assembled Monolayer by Scanning Electrochemical Microscopy

Abstract

Insights into the chemical essence of weak interactions on the surface of biomacromolecules may help to regulate biological processes. In this work, the pH sensitivity of ion-pair interactions occurring on a cysteine self-assembled monolayer (Cys SAM) that simulates the local surface of a protein was probed by scanning electrochemical microscopy (SECM). Cys SAM and the ion-pair interactions subsequently formed with the introduced aspartic acid (Asp) were both pH-sensitive, as confirmed by the tip current changes in the feedback mode. After continuous pH measurements, the most significant negative feedback was observed at pH 5.50, indicating the most robust ionpair interactions, which were simultaneously identified by voltammetry. In this case, the extra addition of the inorganic cation (i.e., Ca 2+ ) did not disrupt the existing ion-pair interactions, and the binding constant (K) and Gibbs free energy (ΔG o ) of the ion pair were finally determined to be 6.44 × 10 5 M −1 and −33.14 kJ mol −1 , respectively. Overall, the pH sensitivity of ion-pair interactions was found to be mainly attributable to pH-induced changes in the deprotonated/protonated states of the α-amino acid moieties, which may provide insights into the artificial manipulation of complex binding events at the molecular level on the biological surface.

SAM-Support-Based Electrochemical Sensor for Aβ Biomarker Detection of Alzheimer’s Disease

“…The existence of terminally functional groups on the SAM surface allows for the immobilization of antibodies, enzymes, DNA, polypeptides, and proteins [ 56 ]. The ion–pair interaction on the SAM surface is also strongly affected by the pH range [ 104 ]. Both single and mixed SAMs can be used in micro/nano electronic devices owing to tunable SAM configurations, enhancement of stability, and modulation of the rectification properties, improving device performance [ 59 ].…”

“…Composite materials are widely utilized for electrochemical sensors in general and for electrochemical biosensors in particular thanks to the synergetic effect between diverse materials to enhance the electrical signal and, therefore, improvement in the sensitivity and stability. Zhou et al prepared an electrochemical aptasensor of Au-deposited vertical graphene/carbon cloth (VG/CC), and then cellular prion protein (PrPC) residues [95][96][97][98][99][100][101][102][103][104][105][106][107][108][109][110] were immobilized on the electrode surface based on the Au-S bond to Aβ oligomer [69]. Finally, poly(themine)-template Cu NPs were used as electrochemical probes for the aptasensor.…”