ARTICLE pubs.acs.org/Langmuir
Parylene Insulated Probes for Scanning Electrochemical-Atomic Force Microscopy Maksymilian A. Derylo, Kirstin C. Morton, and Lane A. Baker* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
bS Supporting Information ABSTRACT: Scanning electrochemical-atomic force microscopy (SECM-AFM) is a powerful technique that can be used to obtain in situ information related to electrochemical phenomena at interfaces. Fabrication of probes to perform SECM-AFM experiments remains a challenge. Herein, we describe a method for formation of microelectrodes at the tip of commercial conductive AFM probes and demonstrate application of these probes to SECM-AFM. Probes were first insulated with a thin parylene layer, followed by subsequent exposure of active electrodes at the probe tips by mechanical abrasion of the insulating layer. Characterization of probes was performed by electron microscopy and cyclic voltammetry. In situ measurement of localized electrochemical activity with parylene-coated probes was demonstrated through measurement of the diffusion of Ru(NH)63+ across a porous membrane.
’ INTRODUCTION Measurement of micro-/nanoscopic processes with electrochemical scanning probe microscopies has resulted in a deeper knowledge of numerous physical and biological processes.19 In these microscopy techniques, an electrode is moved close to a sample of interest, and an electrochemical measurement is recorded. To control the position of the electrode with respect to the sample, a feedback loop is utilized. Faradaic current in scanning electrochemical microscopy (SECM),10 tunneling current in scanning tunneling microscopy (STM),11 or ion current in scanning ion conductance microscopy (SICM)3 are all electrochemical signals that can be used to generate feedback control. Significant efforts have been made to incorporate electrochemical measurements with atomic force microscopy (AFM).1227 In AFM, a sharp tip on the cantilever serves as the probe, and position is commonly controlled by measuring deflection of the cantilever with a laser-photodetector system. Thus, control of probe position is independent of the electrical or electrochemical properties of interest, which can prove beneficial. For instance, decoupled feedback offers advantages in studies of heterogeneous electrochemical processes, as variations in surface reactivity and topography can be measured simultaneously and correlated. Further, in comparison to SECM,28,29 control of the probesubstrate distance at small probesample separations can be achieved without risk of tip collision into the surface. Ex situ conductive probe AFM, which makes use of probes with a conductive metal layer, has proven useful in diverse applications such as fabrication of nanometer scale devices,30 tip-enhanced Raman spectroscopy,31,32 analysis of protein/DNA nanoarrays,33 and measurement of localized Li+ transport.34,35 In situ electrochemical measurement with conductive probes typically proves a more significant challenge, as most conductive probes consist of a metallic layer that covers at least one side of the cantilever. Such probes can be used to electrochemically perturb a system, but as the entire cantilever is electroactive, the ability to measure processes r 2011 American Chemical Society
specifically at the probesample interface is lost.12 Even if the electrochemical process of interest is a surface phenomenon (e.g., corrosion or a surface bound redox species), significant charging currents that originate from the entire active electrode are observed, which can overwhelm measurement of the faradaic response at the tipsample interface. An in situ scanning electrochemical AFM (SECMAFM)14,16,23 probe would consist of a cantilever that is insulated except for a small exposed electrode at or near the probe tip. Further, this tip-integrated ultramicroelectrode (UME) must maintain connection to an external potentiostat. Construction of such probes has proven nontrivial. Methods for fabrication of high-resolution SECM-AFM probes that contain well-defined electrode geometries are labor intensive and often require advanced instrumentation, such as a focused ion beam (FIB). Cantilevers insulated with electrophoretic paint,14,36 silicon nitride,16,23,3740 silicon oxide,41,42 polyfluoroethane,43 photoresist,44 and parylene21,4548 have been explored with some success. Here, we describe a method for fabrication of SECM-AFM probes through parylene insulation of commercially available conductive probes and subsequent mechanical exposure of the tip. Insulation of metallic AFM probes with parylene was previously described by Kranz et al.,21 where parylene layers of 700 nm thickness were deposited onto metal-coated cantilevers. A high radius of curvature at the probe tip was retained by reshaping the parylene insulated tip with a focused ion beam. A recent report by Salomo et al.47 described unsuccessful attempts to electrically insulate SECM-AFM probes with 1 μm thick parylene films. Wain and co-workers48 used a modified parylene insulation method where parylene was deposited on metalReceived: August 3, 2011 Revised: September 27, 2011 Published: September 30, 2011 13925
dx.doi.org/10.1021/la203032u | Langmuir 2011, 27, 13925–13930
Langmuir coated cantilevers, followed by electrophoretic deposition of paint to coat any pinhole openings in the film, with removal of insulation material at the probe tip with FIB. We have recently reported microelectrodes on pipettes that make use of insulating layers of parylene.49 Here, we use a combination of parylene and electrodeposition of paint to achieve thin (
