3.1.2. Suspended Planar Lipid Bilayers—FFT-EIS of BLMs

Fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS) is applied to probe the effect of simple carbohydrates on the impedance characteristics of POPC bilayer lipid membranes. Montal–Mueller technique for planar lipid bilayers yields stable bilayer membranes for assessment of their impedance properties to be probed here by an FFT electrochemical impedance spectrometer. Control samples are obtained in 1 mmol/L NaCl. Impedance measurements are performed also in the presence of 1 mmol/L NaCl and 200 mmol/L glucose, fructose or sucrose. Solvent-free BLMs are produced by Montal– Mueller technique [27,28] across a 100 µm aperture in a 0.025 mm thin Teflon film. The application of Montal–Mueller cell for the measurement of BLM electrical properties requires considering the capacitance *C*TM of the Teflon membrane. The value measured for *C*TM = 56.33 ± 0.23 pF is comparable to the expected capacitance of the bilayer lipid membrane suspended on the hole. Hence, it has been considered in the equivalent circuit modeling as shown in Figure 1. In order to exclude any preparation-related artifacts, the impedance of at least four POPC BLMs is measured for each carbohydrate tested. Impedance spectra plots in the complex plane (Nyquist diagrams) measured for POPC BLMs in 1 mmol/L NaCl (solid circles) and 200 mmol/L of sucrose (hollow circles) with equivalent model circuit fits (lines) are displayed in Figure 3. The average values of the specific capacitance and resistance of POPC BLMs with their standard deviations obtained in 1 mmol/L NaCl aqueous solution and in 200 mmol/L sugar-containing media are summarized in Table 2 and in Figure 4.

**Figure 3.** Impedance spectra plots in the complex plane (Nyquist diagrams) measured for POPC BLMs in 1 mmol/L NaCl (solid circles) and 200 mmol/L of sucrose (hollow circles). Lines represent equivalent model circuit fits.

а

<sup>௦</sup> Ω

≈ Ω

= 56.33 ± 0.23

**Table 2.** Specific capacitance and resistance of POPC membranes in aqueous solutions containing 1 mmol/L NaCl and 200 mmol/L glucose, fructose or sucrose; data obtained from FFT-EIS of BLMs by equivalent model circuit fits; control samples contain only 1 mmol/L NaCl.


All measurements are performed in aqueous solutions containing 1 mmol/L NaCl. The equivalent model circuit fits give for the studied samples values of the aqueous surrounding series resistance *<sup>R</sup><sup>s</sup>* (Figure 1) ranging from 36 ± 1 to 136 ± 8 kΩ. In all aqueous solutions, we measure POPC membrane resistance *<sup>R</sup><sup>m</sup>* <sup>≈</sup> <sup>10</sup><sup>6</sup> <sup>Ω</sup> cm<sup>2</sup> (cf. Table 2), which is similar to the values for lipid bilayers composed of other synthetic phosphatidylcholines as reported so far [48–50]. Upon addition of sugars in the aqueous phase we observe a slight increase of ~10–14% in BLM resistance as shown in Table 2.

FFT-EIS measurements of POPC BLMs impedance yield higher capacitance values compared to *C<sup>m</sup>* deduced from experiments with GUVs for the same lipid bilayer and aqueous solution compositions (cf. Tables 1 and 2). Membrane specific capacitance remains unchanged in 200 mmol/L glucose or fructose solutions, while in the presence of the same concentration of sucrose, we obtain capacitance value almost 40% higher compared to the sugar-free control sample (cf. Table 2 and Figure 4). The latter is qualitatively consistent with the respective results for free-standing POPC membranes acquired from electrodeformation of GUVs, which have shown slightly lower increase of 25% of membrane specific capacitance.

**Ω**

 **Figure 4.** Specific capacitance of POPC BLMs in 1 mmol/L NaCl (control); 1 mmol/L NaCl and 200 mmol/L of sugar (sucrose, glucose and fructose); data acquired by FFT-EIS; *C<sup>m</sup>* values calculated as the weighted average of 4–7 independent measurements, each of them averaged over 10 repetitions.
