Nanopipette exploring nanoworld

Abstract

Nanopipettes, with tip orifices on the order of tens to hundreds of nanometers, have been utilized in the fields of analytical chemistry and nanophysiology. Nanopipettes make nanofabrication possible at liquid/solid interfaces. Moreover, they are utilized in time-resolved measurements and for imaging biological materials, e.g., living cells, by using techniques such as scanning ion-conductance microscopy and scanning electrochemical microscopy. We have successfully fabricated ion-selective nanopipettes that can be used to identify targeted ions such as sodium and potassium in- and outside of living cells. In this review, we discuss the extent of utilization of nanopipettes in investigating the nanoworld. In addition, we discuss the potential applications of future nanopipettes.

Keywords: Ion-selective electrode; Nanopipette; Potassium; Scanning ion conductance microscopy; Sodium.

Figure 1

Concept of a nanopipette: a bridge between macro- and nano-world.

Figure 2

Ion current oscillations in a nanopipette induced by nano-precipitation. (a) Electrochemical setup to measure ion current through a quartz nanopipette. All solutions contain 0.1 M KCl and are buffered at pH 7, with 10 mM potassium phosphate in the barrel and 10 mM Tris–HCl in the bath. Zinc chloride is included in the bath at concentrations of 2--20 μM. (b) Configuration causing ion current oscillations. A negative potential in the nanopipette barrel draws zinc cations from the bath into the pore, while phosphate ions are pushed out into the bath. When a precipitate of sufficient size is formed, the pore is blocked and ionic current decreases. (c) Current oscillations in a nanopipette setup with 2 μM zinc chloride in the bath and a potential of 350 mV. Inset: expanded view of one of the open states. From Ref. [37], copyright@2011 American Chemical Society.

Figure 3

Schematic of reversible calcium ion binding at the tip of a nanopipette. As calcium ions (yellow spheres) are bound by calmodulin protein (blue), changes to the surface charge at the tip will affect the ion current. From Ref. [47], copyright@2011 American Chemical Society.

Figure 4

Fabrication of a gold island consisting of nanoparticles by the electrophoretic deposition method with a nanopipette. (a) Topographic image of the gold island deposited at a voltage of 30 V for 4 s. (b) Cross-sectional profile of the island structure shown in image (a). From Ref. [50], copyright@2007 IOP Publishing.

Figure 5

Comparison of SICM and AFM imaging capabilities. Collagen fibrils on a glass slide were first imaged by AFM (upper left image) and then by SICM (upper right image). Section profiles 1 and 2 are based on lines labeled “profile 1” and “profile 2” in the two upper images. From Ref. [62], copyright@2012 Elsevier.

Figure 6

Observations of local ion concentrations in- and outside living cells. (a) Potassium ion concentration in- and outside HeLa cell. (b) Sodium ion concentration in- and outside rat vascular myocyte. (c) Potassium ion concentration in- and outside motor neuron cell from ES cell. The optical microscopy image of the corresponding cell is shown in the upper part in each figure. Each lower part shows the time chart of the relative concentration, and the times to insert or extract the nanopipette are pointed with arrows. From Ref. [73], copyright@2013 World Scientific.

Figure 7

Left figure: schematic showing the concept of a “ nano-mosquito ”; three-legged beetle-type robot with a nanopipette. The design of the nano-mosquito is inspired from the beetle STM developed by Besocke. The quote “Learn from nature and create what is not in nature” is by Dr. Heinrich Rohrer who invented STM. See Ref. [79] for more details on the nano-mosquito. Lower figure: conceptual image of the nano-mosquito reduced to the millimeter scale. Both illustrations were partly prepared by Miyuki Miyata.

Figure 8

Schematic showing ion-selective SICM withthe ion-selective nanopipette developed by us (left) and expected ion-selective SICM images (right).