Soft Stylus Probes for Scanning Electrochemical Microscopy

Abstract

A soft stylus microelectrode probe has been developed to carry out scanning electrochemical microscopy (SECM) of rough, tilted, and large substrates in contact mode. It is fabricated by first ablating a microchannel in a polyethylene terephthalate thin film and filling it with a conductive carbon ink. After curing the carbon track and lamination with a polymer film, the V-shaped stylus was cut thereby forming a probe, with the cross section of the carbon track at the tip being exposed either by UVphotoablation machining or by blade cutting followed by polishing to produce a crescent moon-shaped carbon microelectrode. The probe properties have been assessed by cyclic voltammetry, approach curves, and line scans over electrochemically active and inactive substrates of different roughness. The influence of probe bending on contact mode imaging was then characterized using simple patterns. Boundary element method simulations were employed to rationalize the distance-dependent electrochemical response of the soft stylus probes.Scanning electrochemical microscopy (SECM) is a scanning probe technique that provides spatially resolved detection of electrochemical and chemical surface reactivity.1,2 Typically, the probe is an amperometric disk-shaped ultramicroelectrode (UME) enclosed in an insulating sheath, e.g. glass. Due to the hemispherical diffusion occurring at the microdisc electrode, a steadystate current can be monitored as a function of the horizontal (x, y) and vertical (z) probe positions. SECM has found many applications ranging from surface reactivity imaging 3-13 to the study of the kinetics of homogeneous and heterogeneous reactions at solid/liquid, gas/liquid, and liquid/liquid interfaces. 14-20 Because the response of a SECM probe can be described by continuum models of mass transport coupled to electrochemical kinetics, quantitative kinetic information can be extracted by comparing experimental and simulated curves in many practically relevant situations. 21-25The response of a SECM probe depends on the surface reactivity of the sample and the distance d between the surface and the active area of the probe electrode. In order to obtain a reactivity image that is not influenced by topography, d must be kept constant. In conventional SECM operations, this is achieved by working on samples with a roughness that is considerably smaller than the radius r T of the UME and by leveling the plane of the sample surface with respect to the x,y-scanning plane of the positioning system. However, this approach becomes inappropriate on rough surfaces of large aspect ratios or when large scan areas (in the mm 2 range) have to be investigated so that leveling becomes a major obstacle. Similar limitations apply when imaging curved samples. This drawback has been recognized for a long time. In the case of a small r T , a constant d can be maintained by combining

Parallel Imaging and Template‐Free Patterning of Self‐Assembled Monolayers with Soft Linear Microelectrode Arrays

“…Afterwards, the array probe is slid laterally across the surface as indicated by the arrow in Figure 1 b diminishing topographic artifacts in reactivity imaging. [31] When the array is moved in the opposite direction (repositioning during imaging), it is lifted off the sample until it is freely suspended in solution. For imaging areas larger than the width of the electrode array, several image frames can be tiled by the control software (SI-4).…”

Seeing Big with Scanning Electrochemical Microscopy

“…[19][20][21] The new probes are made by classical microfabrication techniques and are appropriate for a wide range of samples, thereby broadening the applicability of SECM to new areas.…”

“…This provides a constant d between the active electrode area and the sample surface without the use of an electronic feedback loop for vertical positioning. 19 The main advantage of scanning in a contact regime is that a small and almost constant d can be maintained without tip or sample destruction by probe-substrate crashes. In addition, the broad range of soft plastic materials that can be used in microfabrication processes allows the tuning of the probe stiffness to avoid the destruction of even modestly fragile samples.…”

“…Additionally, soft stylus probes can be employed in a contactless regime as they are normally used during SECM experiments, showing the versatility of the soft stylus probes in different SECM applications. 19 Soft stylus probes with different active electrode materials can be easily fabricated by modern microfabrication methods such as photolithography or laser photoablation. Like normal microelectrodes sealed in glass, the soft probes can be used many times.…”

“…45 More recently, a fountain pen probe for SECM that integrates a microfluidic system with a two-electrode cell has been proposed. 20 The fountain pen is made by the same microfabrication technology as the soft stylus probes described above 19 but introduces a microfluidic channel and a counter/reference electrode prepared by ablating a parallel and aligned second microchannel on the opposite side of the carbon track used as working electrode. For surface reactivity characterization, the fountain pen is used in the contact regime similar to the soft stylus probe.…”