Mapping surface charge density of lipid bilayers by quantitative surface conductivity microscopy

Abstract

Local surface charge density of lipid membranes influences membrane–protein interactions leading to distinct functions in all living cells, and it is a vital parameter in understanding membrane-binding mechanisms, liposome design and drug delivery. Despite the significance, no method has so far been capable of mapping surface charge densities under physiologically relevant conditions. Here, we use a scanning nanopipette setup (scanning ion-conductance microscope) combined with a novel algorithm to investigate the surface conductivity near supported lipid bilayers, and we present a new approach, quantitative surface conductivity microscopy (QSCM), capable of mapping surface charge density with high-quantitative precision and nanoscale resolution. The method is validated through an extensive theoretical analysis of the ionic current at the nanopipette tip, and we demonstrate the capacity of QSCM by mapping the surface charge density of model cationic, anionic and zwitterionic lipids with results accurately matching theoretical values.

Extracellular Surface Potential Mapping by Scanning Ion Conductance Microscopy Revealed Transient Transmembrane Pore Formation Induced by Conjugated Polymer Nanoparticles

“…Therefore, SICM techniques have been extended to map the electrical and electrochemical properties of a surface, along with topography imaging. For example, the surface charge distributions of artificial lipid bilayer membranes and live PC‐12 cell surface were successfully mapped. However, due to the complexity of live cell, these emerging multifunctional SICM techniques have not been applied in in‐depth live cell electrical studies.…”

Mapping Surface Charge of Individual Microdomains with Scanning Ion Conductance Microscopy

“…Interactions between the ion current flowing through the tip of the nanopipette and the charge of a surface have been reported previously by our group [10][11][12] and others, [13][14][15][16][17][18][19][20] with charge mapping initially reported by Unwin and coworkers. [13] Charge mapping has been applied to chemically modified surfaces, [15] cell interfaces, [14,[16][17] chromosomes, [18] and supported lipid bilayers.…”

“…The protocol measures current-voltage curves at positions close to and far from the surface of interest and reports the differential response. If the tip of the pipette is small (e. g. a nanopipette), then the feedback regime occurs at distances where the EDL of the tip and the surface interact (typically on the order of the radius of the pipette opening), [9] and this interaction forms the basis for measuring or detecting the charge presented at the surface, as reflected in the EDL.Interactions between the ion current flowing through the tip of the nanopipette and the charge of a surface have been reported previously by our group [10][11][12] and others, [13][14][15][16][17][18][19][20] with charge mapping initially reported by Unwin and coworkers. With this protocol, we further investigate the effect of electrolyte concentration and study the influence of scan potential on surface charge measurement on chemically modified surfaces.…”

“…[13] Charge mapping has been applied to chemically modified surfaces, [15] cell interfaces, [14,[16][17] chromosomes, [18] and supported lipid bilayers. [19,21] In previous studies, phase [13,15,17] or changes in apparent imaging height [19,21] have been used to infer surface charge.…”

Development of high-speed ion conductance microscopy

“…To improve fundamental performances of SICM, several devices have recently been introduced, including a technique to control the pore size of pipettes [9][10][11] and a feedback control technique based on tip-sample distance modulation [12][13][14]. Moreover, the SICM nanopipette has recently been used to measure surface charge density [15][16][17][18][19][20][21] and electrochemical activity [22,23] as well as to deliver species [24][25][26][27]. Thus, SICM is now becoming a useful tool in biological studies, especially for characterizing single cells with very soft and fragile surfaces [28].…”