Self-Assembled Monolayers for Anti-Fouling and Highly Selective Electrode Interfaces
Authors: Harris Jonathan Kyle , Brothers Michael , Ratcliff Erin L , St. Lawrence Michael , Kim Steve 
Published: May 2020

The detection, collection and/or conversion of biological-based molecules has applications to sensing, wearables, and biofuels. Flexible, wearable electronics for real-time detection of, for example, Neuropeptide Y in human sweat [1], or flagella protein in biofuels, enables deviation from bulky laboratory benchtop analyses such as ELISA. Challenges include required limits of detection, as biological materials can be present in small amounts (nM to pM concentrations) in complex high ionic strength environments[2], the need for a highly selective sensor interface that avoids non-specific binding, and a quick response to account for degradation of analytes.
This work will discuss the implementation of a mixed self-assembled monolayer (SAM) interface with highly selective bio-recognition elements (BREs) on gold disk electrodes, focusing on target application for aqueous sensing in biofuels, with room for expansion to sensing in other biological fluids. The SAM composition was systematically varied to assess functionality towards preventing fouling/non-specific binding and increase the selectivity of the interface for the analyte of interest. Briefly, mixed monolayer combinations included pseudo-zwitter surfaces to prevent fouling, blocking groups similarly employed in the literature[3] to help rectify BREs, and dithiobis(succinimidyl propionate) (DSP) cross-linking chemistry[4] to attach various BREs to the gold electrode. BREs include concanavilin A, streptavidin, nanobodies, and synthetic lectins. Streptavidin and it’s interaction with biotin was chosen as a control, due to their known strong interaction. Electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) were used to estimate changes in the resistance to charge transfer to redox mediators either immobilized or in solution. EIS is one of the most common modern techniques used in biosensing, but as a time-intensive measurement it can be undesirable for accurate sensing in analyte-degrading environments. Therefore, SWV was employed as a substitute which reduces times of measurements by an order of magnitude, and is shown to provide the same information as EIS. To corroborate these electrochemical effects with a definite surface coverage/binding event, ellipsometric experiments of functionalized electrodes are underway.
Our SAM interface produces nM level detection for yeast through the employment of gold immobilized charge transfer mediators and concanavalin A with selective binding to the yeast cell wall. Specifically, a decrease in charge transfer resistance (or increase in peak current as measured in SWV) occurs upon binding of yeast cells to concanavalin A. The streptavidin-biotin interaction also manipulated the charge transfer resistance similarly, confirming our method effectiveness. Nanobodies (binding fragment of antibodies) were also shown to have some success in detecting flagella protein with redox mediators in solution.
These proof-of-concept results demonstrate feasibility and indicate forward paths towards highly selective sensor electrodes with simultaneous anti-fouling properties that prevents non-specific binding. Ongoing efforts include modifying electronic device interfaces with SAMs for combined signaling and amplification, such as modification of gate electrodes in printable electrochemical transistors.
References:
[1] G. Cizza et al., “Elevated Neuroimmune Biomarkers in Sweat Patches and Plasma of Premenopausal Women with Major Depressive Disorder in Remission: The POWER Study,” Biol. Psychiatry 64, 907–911 (2008); doi: 10.1016/j.biopsych.2008.05.035
[2] Z. Sonner et al., “The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications,” Biomicrofluidics 9, 031301(2015); doi: 10.1063/1.4921039
[3] R. Levicky, T. M. Herne, M. J. Tarlov, and S. K. Satija, “Using Self-Assembly To Control the Structure of DNA Monolayers on Gold: A Neutron Reflectivity Study,” J. Am. Chem. Soc. 120, 9787–9792 (1998); doi: 10.1021/ja981897r
[4] A. J. Lomant and G. Fairbanks, “Chemical probes of extended biological structures: Synthesis and properties of the cleavable protein cross-linking reagent [35S] dithiobis(succinimidyl propionate),” J. Mol. Biol. 104, 243–261, (1976); doi: 10.1016/0022-2836(76)90011-5

Figure 1