Simultaneous Nanoscale Surface Charge and Topographical Mapping

Abstract

Nanopipettes are playing an increasingly prominent role in nanoscience, for sizing, sequencing, delivery, detection and mapping interfacial properties. Herein, the question of how to best resolve topography and surface charge effects when using a nanopipette as a probe for mapping in scanning ion conductance microscopy (SICM) is addressed. It is shown that using a bias modulated (BM) SICM scheme it is possible to map the topography faithfully, while also allowing surface charge to be estimated. This is achieved by applying zero net bias between the electrode in the SICM tip and the one in bulk solution for topographical mapping, with just a small harmonic perturbation of the potential to create an AC current for tip positioning.Then a net bias is applied, whereupon the ion conductance current becomes sensitive to surface charge. Practically this is optimally implemented in a hopping-cyclic voltammetry mode where the probe is approached at zero net bias at a series of pixels across the surface to reach a defined separation, and then a triangular potential waveform is applied and the current response is recorded. Underpinned with theoretical analysis, including finite element modeling of the DC and AC components of the ionic current flowing through the nanopipette tip, the powerful capabilities of this approach are demonstrated with the probing of interfacial acid-base equilibria and high resolution imaging of surface charge heterogeneities, simultaneously with topography, on modified substrates.

Characterization of Nanopipettes

“…64 Ion current rectification phenomena (ICR) are manifest when the Debye length is even a small fraction of the dimension of the nanopipette opening, 60 resulting in a diminished ionic current with positive tip bias applied and an enhanced current when the polarity is reversed, compared to expectations if the nanopipette were uncharged. 43,45,60,62 Analytical approaches, such as utilizing equation 3 or similar equations for calculating the nanopipette radius or resistive properties, often at a fixed applied bias, 55,56 may evidently become inaccurate under these conditions, as the surface charge of the nanopipette is not generally considered. While there has been much work on the study and simulation of the ICR phenomena at nanopipettes in low ionic strength, 60,62,63 quantification of the nanopipette surface charge and understanding the nanopipette current response is a difficult task owing to a lack of more complete tip characterization methods.…”

Multifunctional scanning ion conductance microscopy

“…Indeed, as we describe herein, depending on the tip polarity and substrate surface charge, the current may actually increase as the tip-substrate separation decreases [40,41]. These charge effects must be carefully considered when using SICM for topographical imaging [40][41][42]47]. We discuss this aspect of SICM and a new SICM methodology (tuneable potential-control), that opens up charge mapping with SICM, while removing charge effects in topographical imaging ( §5 The SICM probe is typically employed in one of two scanning regimes, either with a fixed probe-substrate separation in constant-distance mode (a) with a raster or spiral scan profile (b) or by hopping across the surface (c).…”

“…To place recent work in context, we briefly review the use of SICM as a topographical tool, before a comprehensive examination of how the SICM response is affected by a multitude of surface and bulk phenomena. This leads to new uses, in which careful interpretation of the SICM signal opens up avenues to map a number of surface properties, along with topography, as exemplified through studies of interfacial charge and reactivity [40][41][42][43][44]. Multichannel probes provide further means of combining SICM with other SPMs and the SICM platform is readily combined with optical techniques.…”

Simultaneous Interfacial Reactivity and Topography Mapping with Scanning Ion Conductance Microscopy

“…[19][20][21] SICM is a sensitive tool for the detection of local ion fluxes, 22 but unlike some other electrochemical methods, does not require the analyte species to be electroactive, since the probe monitors conductivity changes in the confined region between the sample and the pipette opening. Thus, SICM has been used to study individual pores and ion channels in artificial and living cell membranes.…”

“…[23][24][25][26][27] Measurements of ion flux through the pipette orifice can be also used to explore ion current rectification phenomena 28,29 at interfaces 30,31 opening up exciting opportunities to map spatial distributions of surface charge and to probe heterogeneous acid-base equilibria. 20,21 In this work we introduce new functional capabilities of ion conductance microscopy demonstrating its potential for imaging spatially distributed (electro)chemical reactions through the detection of ionic fluxes near active sites. We provide proof-of-concept applications of this technique for dynamic imaging of electrochemical reactions at electrodes, first by recording the ion conductance response to a series of voltammetric sweeps, over wide potential range, with the pipette at a set of coordinates (image pixels) to map out oxidation and reduction reactions occurring at the electrode.…”