Surface Functionalization and Ion Selectivity in Sting Sensors

Abstract

Solid-state nanopores are a promising platform for sensitive detection of many analytes, but modifying the surface of pores to impart selectivity is a challenge. We are applying the principle of signal transduction by ion nano-gating (STING) to use modified quartz nanopipettes as selective electrical nanosensors. This mechanism uses modulations in ion current through a rectifying nanopore (roughly 50 nm in diameter) to detect analytes as they interact with surface-bound receptors. Because the current rectification is highly sensitive to changes in surface charge, receptors that reversibly bind metal cations can be used for label-free and continuous ion sensing. Using a combination of polyelectrolyte self-assembly on the pore surface and bioconjugation, we have modified the surface of quartz nanopipettes with calmodulin, a calcium-binding protein. At neutral pH, the sensor is selective for calcium over magnesium, with a limit of detection in the low micromolar range. Additional methods for detection of salts such as copper and zinc are also being pursued. These involve alternative recognition mechanisms include surface functionalization with chelating biopolymers such as chitosan, and voltage-directed nanoprecipitation of metal salts. The surface functionalization, sensor performance, and potential applications will be discussed. This electrical nanosensor may be applicable in many types of ion detection schemes, such as remote sensing applications or intracellular measurements for biological research.