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We seek to understand the factors that most strongly govern the processes of ion transport and charge separation across synthetic membranes using scanning electrochemical microscopy (SECM). By systematically varying both membrane composition and environmental conditions, the permeability, conductivity, and operating range of candidate materials can be evaluated. We believe the information acquired through such experiments will ultimately allow for the development of more efficient fuel cells as a result of improved membrane design.

Our efforts to date have largely focused on outfitting our new research laboratory (which became available in November 2009 following major renovation) and training undergraduate research students. In addition to standard lab equipment, our research space now houses two conventional potentiostats and, perhaps most importantly, one SECM (with its own bipotentiostat). All of these instruments have been used towards preliminary membrane studies. Three undergraduate students (Bianca Lahiji, BC '10, Nanette Jarenwattananon, BC '11, and Michelle Sykes, BC '12) have been trained on the equipment, and a fourth student (Camille Gandara, BC '12) has most recently joined the SECM/membrane project. A significant portion of the past year has been dedicated to introducing these students to the lab environment, the fundamentals of electrochemistry, and the operating principles of SECM. As a result, our most early studies focused on calibrating and characterizing well-behaved redox systems, such as ferrocene and potassium ferricyanide. Parallel studies on macro- and microelectrodes allowed the students to gain an appreciation of the influence of surface geometry on the observed voltammetry. Within the past few months, our SECM-based investigative efforts have begun and, encouragingly, have already yielded positive results.

SECM is well-suited for studying transport across membranes, as has been demonstrated previously. The intrinsic two-electrode design allows for electrochemical interrogation on both sides of a suspended surface, as well as the establishment of a potential gradient across the membrane. Both of these features have significant implications for studies of ion transport. We aim to devise a cell arrangement analogous to that shown in Figure 1, whereby the synthetic membrane under study would be mounted between the substrate and tip electrodes in the presence of a given solvent. Initial studies, however, have utilized naturally self-assembling phospholipid bilayers as a model system. Importantly, solutions of these lipids can be directly deposited onto the SECM substrate electrode and do not require a modified cell set-up. The ease of deposition and surface modification translates to increased repeatability and data collection, which is of particular importance in the early stages of these experiments as we refine the practical details. 

In order to establish a membrane surface that can in fact be electrochemically analyzed from either side, it is useful to employ a passivating agent that serves both to suspend the membrane above the substrate electrode as well as block diffusion of redox-active species to the electrode surface. Effective passivating agents are also known to provide an added level of structural support to self-assembled monolayers, and thus, in this case, to the overall construct of the membrane. A number of different alkanethiol chains were tested at various concentrations and incubation times. It was found that octadecanethiol (ODT) or mixed monolayers of ODT and mercaptohexanol demonstrated the most reliable and reproducible passivation against solution-borne potassium ferricyanide (FeCN). Figure 2 shows a typical voltammetric response at a gold electrode that has been incubated with 200 mM ODT for one hour and then exposed to 100 mM ferricyanide. It is evident from the absence of faradaic features in the potential window shown that the FeCN molecules are no longer free to diffuse to the surface. (The inset shows the voltammetric response of FeCN at bare gold for comparison).  Commercially-available phospholipids, specifically L-a-phospatidylcholine and 1,2-dioleoyl-sn-glycero-3-phospho-L-serine, were then employed as self-assembling membrane materials. Following passivation of the surface, extruded solutions of lipid vesicles were deposited onto the modified electrode and analogous voltammetric measurements were taken. It was consistently observed that the degree of passivation against FeCN was either retained or even improved following addition of lipids to the surface.

Most recently, we have begun to conduct such studies using SECM. Similar voltammetric responses were observed when the substrate gold electrode was modified with octadecanethiol. In order to provide further evidence for the presence of this passivating layer, we recorded approach curves, whereby the current response at the probe electrode is measured as it is lowered closer and closer to the substrate surface, in the presence of FeCN. When such experiments are recorded over conductive substrates (e.g. bare gold), the current readings will display positive feedback that arises from the ability of reduced FeCN to be re-oxidized at the substrate electrode. Over an insulating surface (e.g. PTFE), the current will exhibit negative feedback, as the FeCN molecules are blocked from the surface. Approach curves were measured over the bare gold substrate electrode, the Teflon casing that surrounds the substrate, and the gold surface modified with ODT. The current response for the latter two was nearly identical, indicating the surface had been effectively passivated against FeCN (see Figure 3). Current studies are focused on using SECM to evaluate more closely the formation of these bilayers as well as monitor ion transport across them.  We are also designing an electrochemical cell that would accommodate synthetic membranes so that we can begin surveying commercially-available materials. Future work will employ electrochemical impedance spectroscopy, quartz crystal microbalance, and atomic force microscopy for detailed characterization studies of these surfaces.

This grant has been instrumental to the early development of our research program and we are grateful to the Petroleum Research Fund for this support.  In addition to funding various aspects of the experiments described above, the award financed travel to and attendance at the National ACS Meeting in Boston this past August, where our results to date were presented. We believe our preliminary findings show great promise for the application of scanning electrochemical microscopy to the proposed membrane studies and look forward to pursuing these experiments further.