Here we report for the first time that ion current rectification (ICR) can be observed at the micrometer scale in symmetric electrolyte solution with polyimidazolium brush (PimB)-modified micropipets, which we call micrometer-scale ion current rectification (MICR). To qualitatively understand MICR, a three-layer model including a charged layer, an electrical double layer, and a bulk layer is proposed, which could also be extended to understanding ICR at the nanoscale. Based on this model, we propose that when charges in the charged layer are comparable with those in the bulk layer, ICR would occur regardless of whether the electrical double layers are overlapped. Finite element simulations based on the solution of Poisson and Nernst-Planck equations and in situ confocal laser scanning microscopy results qualitatively validate the experimental observations and the proposed three-layer model. Moreover, possible factors influencing MICR, including the length of PimB, electrolyte concentration, and the radius of the pipet, are investigated and discussed. This study successfully extends ICR to the micrometer scale and thus opens a new door to the development of ICR-based devices by taking advantage of ease-in-manipulation and designable surface chemistry of micropipets.

Single nanoparticle (NP) events are successfully observed at the orifice of a nanopipet by blocking the ionic current with a single NP.

Development of new principles and methods for cerebral ATP assay is highly imperative not only for determining ATP dynamics in brain but also for understanding physiological and pathological processes related to ATP. Herein, we for the first time demonstrate that micrometer scale ion current rectification (MICR) at a polyimidazolium brush-modified micropipette can be used as the signal transduction output for the cerebral ATP assay with a high selectivity. The rationale for ATP assay is essentially based on the competitive binding ability between positively charged polyimidazolium and ATP toward negatively charged ATP aptamer. The method is well responsive to ATP with a good linearity within a concentration range from 5 nM to 100 nM, and high selectivity toward ATP. These properties essentially enable the method to determine the cerebral ATP by combining in vivo microdialysis. The basal dialysate level of ATP in rat brain cortex is determined to be 11.32 ± 2.36 nM (n = 3). This study demonstrates that the MICR-based sensors could be potentially used for monitoring neurochemicals in cerebral systems.