Abstract
Electrochemical, aptamer-based (E-AB) sensors uniquely enable reagentless, reversible, and continuous molecular monitoring in biological fluids. Because of this ability, E-AB sensors have been proposed for therapeutic drug monitoring. However, to achieve translation from the bench to the clinic, E-AB sensors should ideally operate reliably and continuously for periods of days. Instead, because these sensors are typically fabricated on gold surfaces via self-assembly of alkanethiols that are prone to desorption from electrode surfaces, they undergo significant signal losses in just hours. To overcome this problem, our group is attempting to migrate E-AB sensor interfaces away from thiol-on-gold assembly towards stronger covalent bonds. Here, we explore the modification of carbon electrodes as an alternative substrate for E-AB sensors. We investigated three strategies to functionalize carbon surfaces: (I) anodization to generate surface carboxylic groups, (II) electrografting of arenediazonium ions, and (III) electrografting of primary aliphatic amines. Our results indicate that electrografting of primary aliphatic amines is the only strategy achieving monolayer organization and packing densities closely comparable to those obtained by alkanethiols on gold. In addition, the resulting monolayers enable covalent tethering of DNA aptamers and support electrochemical sensing of small molecule targets or complimentary DNA strands. These monolayers also achieve superior stability under continuous voltammetric interrogation in biological fluids relative to benchmark thiol-on-gold monolayers when a positive voltage scan window is used. Based on these results, we postulate the electrografting of primary aliphatic amines as a path forward to develop carbon-supported E-AB sensors with increased operational stability.