A Zwitterionic Anti-Fouling Coating Promotes Electrochemical Biosensing in Complex Media

Abstract

Biofoulants present in complex media, such as blood, plasma, saliva, etc., can cause an increase in the background of the signal or a reduction in signal-to-noise ratio, due to which most biosensors require a sample processing step. One of the major interactions for biofouling in biosensing platforms is non-specific hydrophobic interactions, which can be prevented using hydrophilic polymeric coatings [1], [2]. However, most hydrophilic polymers are not conductive enough to provide a strong electrochemical current signal, whereas the existing conductive polymers are often hydrophobic. To avoid this, zwitterionic polymers that are both ionically conductive [3] and hydrophilic [3] can be used as anti-fouling polymers for electrochemical biosensing. Additionally, these polymers are highly effective at preventing fouling by electrostatically-induced water structuring [4]. Herein, we report a zwitterionic polymer (bearing thiols, aldehydes and carboxylic acid groups) that forms a thin conductive and hydrophilic layer on the electrodes, while also housing biorecognition layers for capturing target analytes of interest.The polymer was synthesized by free radical copolymerization and coated on gold electrodes to form a thin coating (16 nm when dry), which upon hydration results in a hydrophilic interface (contact angle <15 degrees). Anti-fouling properties were validated by determining the adsorption of radiolabeled human serum albumin (HSA) protein (spiked in plasma) on polymer-coated electrodes. The polymer-coated electrodes showed a ~67% reduction in protein adsorption relative to bare gold electrodes. Additionally, no significant change in anodic current was observed when 1% HSA was incubated on polymer-coated electrodes for 1 hour; whereas an 83% decrease in anodic current was observed on bare-gold electrodes under the same conditions. The polymer-modified electrode was used in detecting redox-labeled DNA sequences with a LOD of 11 nM in buffer and unprocessed plasma. The coating was also utilized to detect COVID-19, where the prepared setup detected 104 cp/mL of SARS-CoV-2 pseudovirus spiked in 50% unprocessed saliva. Similar coatings created from poly(ethylene glycol) (PEG) were unresponsive toward SARS-CoV-2 spiked in 50% saliva [5]. The anti-fouling polymer coatings developed herein show enhanced performance compared to previously reported strategies, based on self-assembled monolayers, in detecting target analytes in complex media, demonstrating the potential for use in clinical diagnostics.