Description
Electrokinetic and electrochemical sensors have several advantages over lab-based assays. In many cases, sensors are more rapid, with fewer reagent additions and sample pretreatment steps. Sensors that leverage the influence of a target recognition event on charge transport are among the most sensitive because they translate localized target binding into a change in a system-scale property.
Ion concentration polarization (ICP) is an electrokinetic phenomenon forming an ion depletion zone (IDZ) and an ion-enriched zone (IEZ) at opposing ends of a perm-selective membrane (ICP) or an electrode (fICP) when an electric field is applied across it. The low ionic conductivity of the IDZ leads to a strong (>10-fold) local enhancement of the electric field and the formation of concentration and electric field gradients at the IDZ boundary. The non-linear migration of ions and non-linear effects in these gradients have been leveraged for focusing and continuous separation of charged species, and more recently, for sensing applications.
An ICP based microscale ion conduction sensor consists of a microfluidic channel containing packed beds of microbeads. Hybridization or binding of a target to a packed bed of microscale beads modulates the surface charge, and in turn, the conduction of electrolyte counterions along the bead surfaces. Binding or hybridization of a target, therefore, leads to changes in a current-voltage curve (CVC) measured across the bed, including shifts in the slope and in the onset voltages where non-linear processes begin to influence ion transport. Our results indicate that this strategy is broadly relevant to label-free sensing of nucleic acids, virions, proteins, and small molecules, in both the presence and absence of pre-enrichment. Findings from these studies introduced important fundamental questions regarding the impact of analyte charge and size, ionic strength, bead surface charge, and device dimensions on the sensitivity and specificity of the surface charge-based sensors. Here, we present a systematic study and a range of operational regimes for microscale surface ion conduction sensing for bioanalytical applications.