Nanoelectrochemistry at the liquid/liquid interfaces

Shigeru Amemiya, Yixian Wang and Michael V. Mirkin

DOI: 10.1039/9781849737333-00001

1.1 Introduction

Last 15 years have witnessed the successful transition of electrochemistry at the liquid/liquid interface to the "nano era". This development has built upon and also contributed to the electrochemical understanding of charge transfer (CT) reactions at the interface between two immiscible electrolyte solutions (ITIES). In contrast to solid/liquid electrochemistry, CT reactions at the ITIES include both electron transfer (ET) and ion transfer (IT) processes. After the discovery of the polarizable ITIES, rapid progress in liquid/liquid electrochemistry was achieved by adopting methodologies and concepts from other branches of electrochemistry, including microelectrode techniques. More recently, the tools and approaches of liquid/liquid electrochemistry contributed to the development of the broader field of nanoelectrochemistry, including electrocatalysis, electrochemical imaging, and electroanalysis. These applications have been enabled by remarkable progress toward the miniaturization of liquid/liquid interfaces to the nanometre scale.

Nanoscale ITIES and their arrays can be formed by using nanopipets, nanopores, and porous membranes, some of which are created using modern nanofabrication techniques. Both nanoscopic and macroscopic ITIES can serve as a platform for studying the electrochemical behaviours of a variety of nanoscale entities, e.g., nanoparticles and biological macromolecules employed in electrocatalysis and electrochemical sensing. In this chapter, we survey recent progress in electrochemistry at the nanoscale liquid/liquid interfaces in the general context of nanoelectrochemistry.

1.2 Nanopipet-Supported ITIES

1.2.1 Charge transfer processes at nano-ITIES

All CT processes occurring at macroscopic ITIES can also be observed at a nanopipet-supported ITIES, including simple IT, facilitated IT, and ET reactions. A simple IT process is a one-step reaction in which an ion In+ is transferred directly from one phase (e.g., water) to the second phase (e.g., organic):

[MATHEMATICAL EXPRESSION OMITTED] (1.1)

This process can also involve ion pairs or ion clusters.

Facilitated IT reactions require a ligand (Lm-) in the second phase (e.g., 1,2-dichloroethane, DCE), which can react with In+ to form a complex, resulting in the transfer of In+:

[MATHEMATICAL EXPRESSION OMITTED] (1.2)

The ET reaction between redox molecules confined to two immiscible liquid phases can be described as:

[MATHEMATICAL EXPRESSION OMITTED] (1.3)

1.2.2 Fabrication and characterization of nanopipets

1.2.2.1 Pulling a nanopipet. Nanopipets can be fabricated by pulling borosilicate or quartz capillaries with a laser pipet puller (e.g., P-2000, Sutter Instrument Co.). When choosing the proper capillaries for different experiments, one needs to consider several factors, including the material (quartz or borosilicate) and properties of a specific capillary (thickness of the wall, with or without a filament, single or double barrel). Borosilicate glass has a low melting point and requires HEAT (one of the P-2000 parameters) between 300 to 400, while quartz requires HEAT between 550 and 900. Borosilicate glass is easier to work with because its properties change gradually with temperature, but it is difficult to use for producing ultra-small nanopipets with relatively short taper (which is essential for attaining a relatively small resistance). Quartz is preferred in most cases because it allows one to make very small and not exceedingly long pipets. The shortcoming of quartz is that it is very sensitive to uneven heating, which might result in asymmetrical pipets. In this case, using quartz capillaries with a thicker wall (≥0.5 mm) can help.

To support an ITIES, a nanopipet has to be filled with solution. Capillaries with filaments are preferred in most cases since they help to bring aqueous solution to the end of the nanopipet tip; otherwise it can be very difficult to remove the air and to fill the nanopipet completely. Capillaries without filaments were used to fill pipets with organic solution, which is relatively easy to inject in a glass or quartz pipet, and at the same time the solvent evaporation is slower in the absence of a filament.

The pulling process is controlled by adjusting five pulling parameters in the program, which are HEAT, FILAMENT, VELOCITY, DELAY and PULL. Generally speaking, to obtain smaller tips, one can increase the value of HEAT, VELOCITY or PULL, or decrease the value of FILAMENT or DELAY. To control the length of the taper while maintaining the nanometer-scale size, one can limit the value of VELOCITY and increase PULL at the same time.

The glass roughness after pulling might be an issue in some cases. It has been shown that the roughness of the pipet tip can be reduced by polishing or by focused ion beam (FIB) milling. A potential problem is that the pipet orifice can be contaminated by polishing agent.

1.2.2.2 Surface modification. When a water-filled pipet is immersed in an organic solution a thin aqueous film forms on its hydrophilic outer wall, making the true area of the liquid/liquid interface much larger than the geometrical area of the pipet orifice. The film formation can be avoided by silanizing the outer pipet wall to render it hydrophobic while keeping the interior wall non-silanized. In most previous publications this was done by dipping the pipet tip into a silanizing agent (chlorotrimethylsilane) while passing a flow of argon through the pipet, which is straightforward for micrometer-sized pipets, but not easy for nanopipets. Silanization of smaller pipets must be done cautiously to avoid the formation of a film on the inner wall, which can partially block the pipet orifice and induce solvent penetration into its narrow shaft. A recently developed protocol for silanizing pipets in the vapour phase...