Surface Charge Mapping with a Nanopipette

Abstract

Nanopipettes are emerging as simple but powerful tools for probing chemistry at the nanoscale.In this contribution the use of nanopipettes for simultaneous surface charge mapping and topographical imaging is demonstrated, using a scanning ion conductance microscopy (SICM) format. When a nanopipette is positioned close to a surface in electrolyte solution the direct ion current (DC), driven by an applied bias between a quasi-reference counter electrode (QRCE) in the nanopipette and a second QRCE in the bulk solution is sensitive to surface charge. The charge sensitivity arises because the diffuse double layers at the nanopipette and the surface interact, creating a perm-selective region which becomes increasingly significant at low ionic strengths (10 mM 1:1 aqueous electrolyte herein). This leads to a polarity-dependent ion current and surface-induced rectification as the bias is varied. Using distance-modulated SICM, which induces an alternating ion current component (AC) by periodically modulating the distance between the nanopipette and the surface, the effect of surface charge on the DC and AC is explored and rationalized. The impact of surface charge on the AC phase (with respect to the driving sinusoidal signal) is highlighted in particular; this quantity shows a shift that is highly sensitive to interfacial charge and provides the basis for visualizing charge simultaneously with topography. The studies herein highlight the use of nanopipettes for functional imaging with applications from cell biology to materials characterization where understanding surface charge is of key importance.3

Simultaneous Nanoscale Surface Charge and Topographical Mapping

“…Note that by maintaining V = 0 on approach for topographical imaging in this work, the scenario of traditional SICM experiments is avoided where, based on recent work, 42,44 an applied bias upon approach to heterogeneously charged substrates, may result in a non-constant working distance and hence distorted topography (Figure 1c).…”

“…The application of bias, however, is known to lead to ICR inside the nanopipette itself [39][40][41] (when it is freely suspended in bulk) and a surface-induced rectification. 44 This causes a drastic change in the nanopipette conductance state depending on bias polarity and surface charge due to a significant change of ionic conductivities (and therefore, the overall resistance) within and near the tip opening (see Figure 3e and f for V values of +0.3 V and -0.3 V, respectively). In turn, the AC ion current components, particularly the phase shift, which are highly sensitive to the overall resistance, as…”

“…43 Finally, for smaller probes, it becomes especially difficult to separate topography and surface charge. 44 We have recently proposed an alternative approach for positionable feedback control of nanopipettes in SICM, whereby the tip-to-substrate separation is controlled through the application of an oscillating bias between the two QRCEs to generate an AC signal. 45 It has been demonstrated that at high electrolyte concentrations, bias modulated (BM)-SICM provides a stable feedback for tracking surface topography with oscillation around 0 V between the two QRCEs, at a range of frequencies using either the AC amplitude or AC phase signals.…”

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.…”