Homogeneous functional self-assembled monolayers: Faradaic impedance baseline signal drift suppression for high-sensitivity immunosensing of C-reactive protein

Abstract

The limit of detection (LOD) of affinity sensors based on alkanethiol self-assembled monolayers (SAMs) system can be improved by either signal amplification and/or noise reduction. The latter includes baseline signal drift arising, in part, from, monolayer imperfections (and variations in this through repeats) as well as electrical noise of both the sensor and transducer. In reagentless “label free” assays signal drift carries with it the possibility of assay false-positive results (if signal drift is positive) or false-negatives (if specific signal is swamped by downward drift). Faradaic electrochemical impedance spectroscopy (FEIS) sensors based on SAM interfaces have been shown to exhibit considerable baseline signal drift, which consequently affects their assaying capabilities. This study reports on the development of a simple two-step pre-treatment method for generating functional SAMs of 11-mercaptoundecanoic acid (MUA) on Au with a highly suppressed baseline signal drift. For electrochemical characterisation of the SAMs, the charge-transfer resistance (Rct), defect presence (pinhole radius and separation), capacitance, and dielectric constant were evaluated. The interface pre-treatment generates films of substantially improved homogeneity that are subsequently functionalised with anti-human C-reactive protein. CRP, an acute-phase protein, is detectable down to femtomolar levels without any amplification; this is a 2–3 order of magnitude lower detection limit than that typically accessible for analyses of this type. The developed protocols thus present a convenient and general route to suppress baseline signal drifts and improve the limits of detection of affinity sensors based on Faradaic impedance.