Quantitative Concentration Mapping with AFM-SECM

Abstract

Efficient transport of reaction intermediates between active sites can improve the efficiency of multi-site reactions1–3. Experimental detection of these nanometer length scale transport mechanisms is enabled by Scanning Electrochemical Microscopy (SECM). Electrochemical measurements can allow for concentration profile mapping to elucidate the transport processes in experimental systems. SECM can correlate position dependent responses to properties of the system. There is a significant impact on the response profile due to the SECM tip’s presence at the nanoscale. Finite Element Analysis (FEA) has been used to understand and remove these in situ tip effects.The calculated concentration, using current-flux relations, can be complex near sites generating flux through reactions and/or transport. Additionally, uncertainties such as tip positioning in relation to the interface site can further complicate the response. Quantitative concentration fields can more readily be interpreted through FEA, by matching the experimental tip response.Electrodeposited platinum nanoparticles were imaged topographically and electrochemically with a Bruker AFM-SECM (Figure 1). The positive response over the nanoparticles vs. the planar HOPG sites allows for kinetic response imaging. The model is parameterized through approach curves and verified though SECM mapping correlation.We have previously applied FEA to correlate experimental in situ results to in operando conditions. Here, we use this approach to generate corrected concentration profiles. We use Pt nanoparticle catalysts deposited on HOPG, to develop surface property correlations. In this way, a comparison can be drawn between in situ and in operando approaches. References (1) Schuhmann, W. Amperometric Enzyme Biosensors Based on Optimised Electron-Transfer Pathways and Non-Manual Immobilisation Procedures. Rev. Mol. Biotechnol. 2002, 82 (4), 425–441.(2) Wheeldon, I.; Minteer, S. D.; Banta, S.; Barton, S. C.; Atanassov, P.; Sigman, M. Substrate Channelling as an Approach to Cascade Reactions. Nat. Chem. 2016, 8 (4), 299–309.(3) Liu, Y.; Hickey, D. P.; Guo, J. Y.; Earl, E.; Abdellaoui, S.; Milton, R. D.; Sigman, M. S.; Minteer, S. D.; Calabrese Barton, S. Substrate Channeling in an Artificial Metabolon: A Molecular Dynamics Blueprint for an Experimental Peptide Bridge. ACS Catal. 2017, 7 (4), 2486–2493. Figure 1