Celeste A. Morris

Applications of nanopipettes in the analytical sciences

In this review, we describe measurements and applications of interest to the analytical community that makes use of simple nanopipettes. Fabricated by applying heat during the separation of a glass capillary, nanopipettes provide a route for nanoscale studies of ion transport and for development of chemical and biochemical sensors. When mounted on a translation stage, nanopipettes also enable unique modes of imaging and material deposition. These facets of nanopipette research, as well as some of the unique properties of nanopipettes, will be discussed.

Transport of redox probes through single pores measured by scanning electrochemical-scanning ion conductance microscopy (SECM-SICM)

We report scanning electrochemical microscopy-scanning ion conductance microscopy (SECM-SICM) experiments that describe transport of redox active molecules which emanate from single pores of a track-etch membrane. Experiments are performed with electrodes which consist of a thin gold layer deposited on one side of a nanopipet. Subsequent insulation of the electrode with parylene results in a hybrid electrode for SECM-SICM measurements. Electrode fabrication is straightforward and highly parallel. For image collection, ionic current measured at the nanopipet both controls the position of the electrode with respect to the membrane surface and reports the local conductance in the vicinity of the nanopipet, while faradaic current measured at the Au electrode reports the presence of redox-active molecules. Application of a transmembrane potential difference affords additional control over migration of charged species across the membrane.

Scanning ion conductance microscopy measurement of paracellular channel conductance in tight junctions

Elucidation of epithelial transport across transcellular or paracellular pathways promises to advance the present understanding of ion transport and enables regulation of cell junctions critical to the cell and molecular biology of the epithelium. Here, we demonstrate a new instrumental technique, potentiometric scanning ion conductance microscopy (P-SICM), that utilizes a nanoscale pipet to differentiate paracellular and transcellular transport processes at high spatial resolution. The technique is validated for well-defined polymer membranes and then employed to study wild type and claudin-deficient mutants of Madin-Darby Canine Kidney strain II (MDCKII) cells. Paracellular permeabilities conferred by claudin-2 are captured by P-SICM which demonstrates the utility to monitor apparent conductance at subcellular levels.

Carbon Electrode Fabrication from Pyrolyzed Parylene C

Carbon electrodes coupled with electrochemical detection have been used extensively for the investigation of biogenic amines. Herein we report the fabrication and characterization of carbonaceous electrodes prepared from pyrolyzed parylene C (PPC) films. High-aspect ratio carbonaceous microelectrodes have been prepared by masking PPC coated pipets with an insulating parylene C film. PPC thin film electrodes were characterized electrochemically, spectroscopically, and with electron microscopy. The procedures described here offer a route to fabrication of thin film carbon electrodes that can be patterned and produced in parallel. These electrodes are similar to carbon electrodes derived from pyrolyzed photoresist films but do not require spin-coating or lithography and can readily coat three-dimensional surfaces.

Rapid fabrication of nanoporous membrane arrays and single-pore membranes from parylene C

We report the fabrication of highly ordered arrays of uniformly sized nanopores from polymer vapor deposition and extend this straightforward approach to the production of single-pore membranes. The fabrication protocol for single-pore membranes and porous membrane arrays (PMAs) offers precise control over pore size and can be readily tailored for batch fabrication. PMAs were utilized as templates to construct both epoxy resin and silica micro/nanostructures that replicate pore geometries. As established from silica micro/nanostructures and electron microscopy, pore sizes were tuned from tens of micrometers to as low as ∼100 nm for PMAs and 40 nm for single-pore membranes. Ion transport of redox molecules and non-redox active ions through hydrophobic porous membranes was investigated with scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM), respectively. In addition, the application of PMAs toward spatial patterning of polystyrene microspheres deposited from a colloidal solution onto the membrane surface was demonstrated. This research identifies a unique strategy in nanopore fabrication. The method described to produce PMAs provides an effective and inexpensive process for production of a well-ordered three-dimensional architecture that typically requires complex instrumentation, elaborate lithographic procedures or lengthy processing techniques.