Current Rectification with Poly-l-Lysine-Coated Quartz Nanopipettes

Abstract

Ion current rectification with quartz nanopipette electrodes was investigated through the control of the surface charge. The presence and absence of a positively charged poly-L-lysine (PLL) coating resulted in the rectified current with opposite polarity. The results agreed with the theories developed for current-rectifying conical nanopores, suggesting the similar underlying mechanism among asymmetric nanostructure in general. This surface condition dependence can be used as the fundamental principle of multi-purpose real-time in vivo biosensors. Nanomaterials are being widely exploited by recent technologies because of their extraordinary properties. Although the development of fabrication processes for particular nanostructures poses great challenges by itself, the practical use of these nanomaterials is also of great interest for biology and medicine. 1 Because of their structural diversity, these materials are often categorized and referred to as nanoparticles, 2 nanowires, 3 nanotubes, 4 nanopores, 5 or nanopatterned surfaces. 6 Nanopipettes are among these; a nanopipette is defined as a pipette with a very fine tip that has a nanoscale opening. Nanolithography is one of the typical applications of nanopipettes as a delivery tool of a tiny amount of chemicals. 7,8 Nanopipettes are versatile enough to be used as a tool for sensitive detection in biomedical applications. Optical detection of fluorescently labeled macromolecules such as DNA or proteins with nanopipettes has been reported. 9,10 Fully electrical detection has also been shown with similarly sized nanoparticles, whose flow through the nanopipette opening creates temporal current blockades. 11 The ultimate goal of these efforts, the label-free real-time electrical detection of single molecules, could be achieved eventually by a deeper understanding of the fundamental characteristics of nanopipette electrodes under an external electric field. It not only helps to unveil the dynamics of biological systems but can also have a remarkable impact on drug screening and pathogen detection. Interestingly, although a general understanding of nanopipette electrodes can be based on the understanding of microelectrodes, the unique nanoscale geometry often causes characteristic behavior that requires further focused studies. For example, related studies have examined the physicochemical properties of nanopipettes under varying conditions such as electrolyte concentrations and pH, 12 or polyethylene glycol polymer coatings. 13 Similarly shaped goldplated conical nanopores have been studied in a more detailed manner, involving observations of the role of surface charge in the ionic current. 14 Our target here is the effect of cationic polymer coating on a glass nanopipette surface, providing the basis for functionalized nanopipettes that will be used as sensitive biosensors. By understanding how ions flow through the nanometer-sized opening, how these ions interact with the surface inside and outside the tip, and what happens if the surface is modified by...

Surface Charge Mapping with a Nanopipette

“…As expected, these nanopipettes show slight ICR due to the negative surface charge at the walls of the nanopipette, 21,22 with the current magnitude at positive values of the applied potential (V QRCE, nanopipette -V QRCE, bulk ) being less than the current magnitude at negative potential values, as discussed in some detail in the literature, 19,20,22,38,39 and briefly below. The additional effect of a charged surface on the DC and AC ion currents in SICM is investigated herein.…”

Label-free biosensing with functionalized nanopipette probes

“…Quartz nanopipettes with an opening of Ϸ50 nm in diameter at the tip were fabricated as previously reported (8,19), except that capillaries with filaments were used for smooth backfilling by capillary force. The nanopipettes were backfilled with 100 mM potassium chloride (KCl) aqueous solution supplemented with 2 mM phosphates (pH 7; called working buffer hereafter), and an Ag/AgCl wire electrode was inserted to comprise a nanopipette electrode module.…”

Noise Properties of Rectifying Nanopores

“…24,25 Polymer nanopores and nanopipettes rectify due to the asymmetric electric potential resulting from the shape and finite surface charges of the pore walls. 4,5,24 Rectification in SiN nanopores results from the gold layer at the membrane surface deposited before FIB drilling, which is characterized by a higher surface charge density in KCl solutions 26 than the SiN pore walls.…”