Introduction

Part of the SPM family, the term scanning electrochemical microscopy (SECM) was coined by Bard and co-workers in 1989. SECM works by measuring a current through an ultramicroelectrode (UME) held or scanned in a solution (containing electroactive species) in close proximity to a substrate surface. Substrates can be a variety of solid surfaces such as metals, glass, polymers or biological materials, or liquids such as mercury or oils that form an immiscible phase. The substrate perturbs the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate.

At Warwick SECM is used to study crystal dissolution, conducting polymers, hydrodynamic imaging, metal nanoparticle deposition, processes occurring at air/liquid and liquid/liquid interfaces, and many biological systems such as cartilage etc, a few examples are given below.

Figure 1: Typical SECM set-up.

Transport and Permeability in Cartilage

Imaging Localised Transport and Permeability in Cartilage

Cartilage is a tissue which cushions the ends of bones in articular joints. It is composed of a matrix of cells (chondrocytes), proteoglycan macromolecules and interstitial fluid embedded in a network of collagen fibres. Cartilage owes its shock-absorbing and low-friction properties to this structure. Since there are no blood vessels in cartilage, the normal physiological function and structural integrity of the tissue requires that nutrients, metabolites and signal molecules are transported to the chondrocytes by diffusion and fluid flow through the matrix. Knowledge of these processes is thus vital in understanding the biochemistry and viscoelastic properties of cartilage, particularly since diseases such as arthritis may result from disturbances in fluid movement in the tissue.

In a preliminary study, we have been able to measure osmotically-driven water transport through laryngeal cartilage. By developing a model for mass transport in this complex geometry, it has been possible to quantitatively interpret SECM current measurements to produce a spatially-resolved map of fluid flow through this tissue.¹

More recently, in a project funded by the Wellcome Trust, we have made localised oxygen permeability measurements in bovine articular cartilage, using the SECM-induced transfer technique.²

The topography of the sample was first determined by employing a mediator that was impermeable in the tissue. This revealed a number of pit-like features in the surface corresponding to the location of cells. When the same area was then scanned for oxygen, the measured currents could be corrected for topographical effects, to provide a permeability map of the area. We have shown, for the first time, that oxygen permeability is enhanced in the regions corresponding to the cells. Histological-staining of the cartilage has shown that such areas have a low collagen content.³

Figure 2 : SECM images showing normalised current maps for an impermeable (top left) and permeable (bottom left) mediator scanned over bovine articular cartilage. From these, the topography (top right) and permeability towards oxygen (bottom right) of the cartilage can be calculated.

Figure 3: Cartilage section stained with van Gieson's stain to highlight the collagen content (pink). The dark red spots indicate the location of cells or cell pairs.

Fluid Flow Through Dentine

Measuring Fluid Flow Through Porous Materials: Dentine

Dentinal hypersensitivity is a common problem in the adult population. Dentine contains tubules, usually 1-2 microns in diameter, which run between the pulp and enamel. When the enamel is eroded or gums recess, the tubules become exposed, and dentinal fluid flows outward. This fluid flow increases in response to tactile, thermal and osmotic stimuli. This is thought to result in a mechano-receptor response in the nerve fibres of the pulp. This is detected as pain.

Figure 4: FE-SEM image of human dentine tubules.

Quantitative measurements of fluid flow rates across dentine are important in the development of treatments for dentinal hypersensitivity. The established methods for determining fluid flow rates through dentine only measure the net permeability of the sample. However, recent studies have demonstrated that the tubules vary in structure, possibly leading to different fluid flow rates and, consequently, distinct responses to treatment.

By using the SECM as a microscale detector of fluid flow through dentine tubules⁴ it is possible to quantify and spatially resolve heterogenities in fluid flow across a typical sample, in-vitro. Figure 5 shows tip current images in the absence (a) and presence (b) of a hydrostatic pressure across the dentine slice. Figure 5(a) corresponds to a topography map of the surface, resolution governed by the size of the electrode (in this case 2 microns) and (b) a flow map indictaing only a few tubules are active to flow in a region which should contains hundreds of channels. Using similar principles the action of fluid flow blocking agents can also be assessed using SECM.^5,6

Figure 5: (a) Normalised tip current representing topography of dentine surface in absence of fluid flow (b) in presence of 2.5 kPa. Taken from reference 4.

Intermittent Contact-SECM (IC-SECM)

The general problem: SECM tip to substrate distance

Within SECM imaging (and approach curves) the tip response, recorded as a function of position, provides a current image which depends on both the sample topography (distance between the tip and the interface) and sample activity; and one cannot unambiguously determine these two parameters. This technique is a new and simple approach for both tip positioning and SECM imaging which addresses this challenge.

Intermittent Contact SECM

A small AC positional perturbation is applied to the piezoelectric positioner, controlling the tip normal to the surface of interest (z-axis). Under this AC positional perturbation, the amplitude of the oscillation becomes damped just as the tip encounters the surface (in some respects akin to Tapping Mode AFM but different in that the tip is oscillated at much lower frequencies). The damping is detected, through the positional feedback from the piezoelectric positioner, and provides information on the tip-surface separation. This feedback signal can be used to detect the surface in approach curve measurements and can also be used as a set point to maintain a fixed distance between the tip and surface during electrochemical imaging.

This technique is attractive because it requires minimal additional hardware and is easily implemented.

IC-SECM in Action

Positive and Negative Feedback from Gold Bands

Gold bands on glass. Obtained using a 1 µm radius Pt disk electrode (70 Hz, d =39 nm, 2 mM ferrocenylmethyltrimethylammonium)

Flux of Ruthenium Hexamine through Dentine

Obtained using a 1 µm radius Pt disk electrode (70 Hz, d = 39 nm). A 15 mM ruthenium hexamine solution was used.

For more information

• Intermittent Contact-Scanning Electrochemical Microscopy (IC-SECM): A New Approach for Tip Positionong and Simultaneous Imaging of Interfacila Topography and Activity
  K. McKelvey. M. A. Edwards and P. R. Unwin, Anal. Chem., 2010, ASAP.