Structural Properties of Alkanethiolates and Their Influence on The Stability and Reproducibility of Impedimetric Sensing

Abstract

The utilization and commercialization of electronic based point of care (PoC) diagnostic devices has been hindered by the lack of repeatability and stability associated with such devices. Consequently, the enzyme-linked immunosorbent assay (ELISA) remains the dominant immunoassay used for the detection of serological diseases despite the numerous drawbacks associated with this platform.1, 2 Most notably, the ELISA relies on trained laboratory personnel to perform the assay in a centralized laboratory, which increases both the cost and time to actionable treatment. An electrochemical impedance biosensor is an example of a label-free biosensing platform that has high sensitivity and can be easily miniaturized and mass produced at a low cost, thus improving the time to actionable treatment.3-7 It has been shown that the repeatability and stability of electronic based biosensors is not associated with the transducing element but rather is dependent on the sensor-molecule interface.8 This interface typically consists of an alkanethiolate-based self-assembled monolayer (SAM) chemisorbed onto a gold (Au) substrate. In the case of Electrochemical Impedance Spectroscopy (EIS) sensing, the SAM forms a barrier to the ionic transport of the selected redox coupling agent. The molecular structure of the alkanethiolate determine the uniformity and density of the SAM, which directly affects its ability to impede the ionic transport of the redox coupling agent. Molecular dynamics studies have shown that shorter, less-ordered alkanethiolates rotate more easily than longer alkylthiolates.9 Their ability to rotate facilitates the gradual coverage of pinholes and defects present on the sensor-molecule interface.9 A separate study has shown that short chain (6C) alkanethiolates leads to significant drift in EIS measurements.10 The drift in charge transfer resistance of the 6C SAM was hypothesized to be associated with the surface reorganization of these thiols. The observed drift in the SAM measurements has been reported to be greater in magnitude than the measured change in charge transfer resistance upon binding of a receptor, thus compromising the reliability of biosensing measurements.10 This work uses a 16C alkanethiolate SAM to show that the stability and reproducibility of EIS measurements is directly dependent on the coverage of pinholes and defects present on the Au substrate as well as the crystallinity of the SAM. Cyclic Voltammetry (CV) and X-ray Photoelectron Spectroscopy (XPS) were used to measure the density and uniformity of the SAM, respectively. We show that stability in EIS measurements of the sensor-molecule interface directly corresponds to stable and reproducible measurements associated with the attachment of the selected receptor and lastly of the target analyte. Thus, through the rational design of a stable sensor-molecule interface we can minimize the drift associated with the SAM, and consequently the receptor interface. Thus, improving both the sensitivity and limit of detection of EIS based sensors.References Yalow, R. S.; Berson, S. A., Immunoassay of endogenous plasma insulin in man. The Journal of clinical investigation, 1960, 39 (7), 1157-1175.Prevention, C. f. D. C. a. Understanding the EIA Test. https://www.cdc.gov/lyme/diagnosistesting/labtest/twostep/eia/index.html Cui, Y., Wei, Q. Q., Park, H. K., and Lieber, C. 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