**1. Introduction**

Lipid bilayers supported on solid substrates are considered as an important model to mimic the natural cell membranes in fundamental studies [1–4]. These systems were also proven to be suitable for the construction of biosensors and bioanalytical platforms for the examination of membrane proteins [5–7]. The immobilization of the membrane at the supporting substrate offers a unique opportunity to probe the properties of such an assembly with numerous surface-sensitive techniques. These include scanning probe microscopy, infrared reflective absorption spectroscopy, quartz crystal microbalance, and for conductive supports, electrochemical methods can be used as well [8–11]. Most popular approaches for supported lipid membrane formation involve vesicles spreading or Langmuir-Blodgett and Langmuir-Schafer techniques [12–15]. It was demonstrated in numerous research papers that both can produce well-defined planar bilayers with good electrical insulating properties manifested by low differential capacitance and high membrane resistance [7,16]. This issue is of crucial importance for the biosensors, which are sensitive to the structural or functional changes of lipid assemblies triggered by membrane proteins, or the constructs, which may act as affinity sensors detecting interactions of biological material with lipid membrane [17,18]. In particular, the modulation of the ion permeability of lipid membranes may be utilized in biosensors or the studies of pore-forming toxins since the dielectric damage can be transduced into the physical signal utilizing electrochemical methods [16].

An alternative approach was proposed involving the formation of supported lipid membranes from bicellar mixtures. Bicelles are composed of long-chain and short-chain

**Citation:** Dziubak, D.; Strzelak, K.; Sek, S. Electrochemical Properties of Lipid Membranes Self-Assembled from Bicelles. *Membranes* **2021**, *11*, 11. https://dx.doi.org/10.3390/membranes 11010011

Received: 1 December 2020 Accepted: 21 December 2020 Published: 23 December 2020

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phospholipids and tend to form disk-like aggregates [19,20]. Such lipidic assemblies are broadly utilized in structural biology studies due to their ability to host membrane proteins while retaining protein structure and function [21]. The planar bilayer formation on silicon chips from a bicellar mixture of 1,2-dipalmitoyl-*sn*-glycero-3-phosphocholine and 1,2-diheptanoyl-*sn*-glycero-3-phosphocholine lipids was first reported by Zeineldin and coworkers [22]. The optimal routes to fabricate supported lipid bilayers from bicelles were later demonstrated by Cho's group, who also revealed mechanistic details of the lipid membrane formation on hydrophilic substrates such as silicon dioxide, titanium oxide, and aluminum oxide [23,24]. These authors have shown that the adsorption behavior of bicelles can vary depending on the nature of the supporting surface, and the electrostatic attraction between the surface and adsorbing bicelles is necessary for the successful formation of the supported lipid bilayer (SLB). A similar conclusion can be drawn based on the results reported by Yamada and coworkers, who utilized atomic force microscopy and force spectroscopy to probe the properties of lipid films assembled from bicelles [25]. It was found that lamellae of phospholipid bilayers were aligned parallel to a surface in case of the negatively charged bare silicon substrate, while unoriented phospholipid bilayers were formed on Si substrate modified with terminal amine groups, where the excess of positive surface charge is expected.

In this work, we have described the mechanism of bicelles adsorption onto thioglucosemodified gold electrodes. The architecture of the resulting membrane can be considered as a floating lipid membrane, which is separated from the substrate by a monolayer of hydrophilic molecules of thioglucose [26,27]. In such a configuration, the polar heads of lipids located close to the electrode surface remain hydrated and the direct interaction with metal is eliminated. Immobilization of the lipid assembly on conductive support enabled electrochemical characterization of the resulting membrane and assessment of its permeability for ions. Additionally, the effect of freeze–thaw treatment on membrane electrical insulating properties and morphology was investigated.
