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Abstract
Point-of-care diagnostics is a field that has attracted much attention in the past few decades -- however realising the dream of a small, portable, user-intervention free, and rapid diagnostic device for the detection of small, organic analytes, remains elusive. To date, optical and electrochemical methods have not been able to satisfy all of the criteria necessary to build such a device, with the main challenge being able to overcome background noise due to non-specific events when attempting to transduce the signal for a particular analyte. In attempting to solve this problem, Blocked Electrochemical Sensing Technology (BEST) has a redox-active reporter molecule that is dispersed on the electrode surface amongst an anti-fouling diluent. The main advantage of the design is that signal is proportional only to the degree to which binding of the analyte occurs around the reporting molecule (facilitated by an epitope in proximity to it), and not on the electrode surface in general. Of course, non-specific adsorption can still contribute to background signal, but the chance of this occurring is minimised for this design.
Although BEST has been shown to work with various analytes, its current configuration is not suitable for commercialisation. The interface makes use of a difficult to synthesise, and unstable, molecular wire, which is necessary to facilitate electron transfer between the electrode surface and the redox reporting molecule, through the passivating oligo(ethylene glycol)-based anti-fouling layer.
Zwitterionic molecules have shown great promise as an alternative to oxidation-prone oligo(ethylene glycol)-based antifouling molecules, however to date they have been attached to surfaces via long alkane tails, or have been polymer-based; either way, they passivate the surfaces to which they are attached.
In this work, a range of small, non-passivating, zwitterionic molecules are examined for their antifouling ability, with the aim of simplifying the BEST design by replacing the passivating antifouling layer with one that is non-passivating, thus obviating the need for a molecular wire. Phosphorylcholineethanethiol is identified as a suitable candidate, shown to possess antifouling qualities similar to oligo(ethylene glycol)-based coatings, and to be non-passivating when attached to a gold electrode.