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Sci Rep
2015 Jun 10;5:11242. doi: 10.1038/srep11242.
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Electronic polymers in lipid membranes.
Johansson PK
,
Jullesson D
,
Elfwing A
,
Liin SI
,
Musumeci C
,
Zeglio E
,
Elinder F
,
Solin N
,
Inganäs O
.
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Electrical interfaces between biological cells and man-made electrical devices exist in many forms, but it remains a challenge to bridge the different mechanical and chemical environments of electronic conductors (metals, semiconductors) and biosystems. Here we demonstrate soft electrical interfaces, by integrating the metallic polymer PEDOT-S into lipid membranes. By preparing complexes between alkyl-ammonium salts and PEDOT-S we were able to integrate PEDOT-S into both liposomes and in lipid bilayers on solid surfaces. This is a step towards efficient electronic conduction within lipid membranes. We also demonstrate that the PEDOT-S@alkyl-ammonium:lipid hybrid structures created in this work affect ion channels in the membrane of Xenopus oocytes, which shows the possibility to access and control cell membrane structures with conductive polyelectrolytes.
Figure 1. Characterization of the PEDOT-S@alkyl-ammonium complexes.(a) i) The monomer of PEDOT-S, ii) dioctyl-ammonium chloride, the alkyl-ammonium molecule used in subsequent experiments, iii) DOPC, the molecule used to create P-S@dioct:DOPC structures, iv) the presumed structure of the PEDOT-S@dioctyl-ammonium complex. (b) PEDOT-S is soluble in water (top), but precipitates in 2.5 mM dioctyl-ammonium (middle) and the precipitate is soluble in chloroform (bottom). (c) UV-vis spectra of PEDOT-S@alkyl-ammonium complexes dissolved (100 μg/mL based on PEDOT-S), in chloroform:methanol (2:1) after being precipitated from water solutions at pH 4 (solid) or pH 9 (dashed), corresponding to doped and dedoped PEDOT-S respectively. The alkyl-ammonium salts used were tetrabutyl-ammonium (green), hexadecyl-trimethyl-ammonium (red), dioctyl-ammonium (blue) and nonyl-ammonium (black). PEDOT-S@dioctyl-ammonium (d) and PEDOT-S@hexadecyl-trimethyl-ammonium (e) complexes dissolved (100 μg/mL based on PEDOT-S) in chloroform:methanol (2:1) were characterized after being precipitated from water solutions with pH in the range 9–4. The arrows indicate increased doping as the pH is decreased.
Figure 2. Characterization of P-S@dioct:DOPC interactions with supported lipid bilayers.(a) 100 nm DOPC liposomes (500 μg/mL) reach the SiO2 QCM-D sensor at 6 min and a lipid bilayer is formed at about 9 min (A). P-S@dioct:DOPC structures (500 μg/mL DOPC, 50 μg/mL PEDOT-S) were introduced and reached the sensor at 19 min (B) which caused shifts in frequency and dissipation indicative of structure adsorption. After additional 18 min, rinsing with pure PBS started (C) and the curves returned to the values for a clean lipid bilayer. b) P-S@dioct:DOPC structures (500 μg/mL DOPC, 50 μg/mL PEDOT-S) were introduced directly and reached the SiO2 QCM-D sensor at 5 min. A lipid bilayer was formed at about 11 min (D) and the shifts in dissipation and frequency remained high, which indicated presence of P-S@dioct:DOPC structures on the surface. Rinsing with PBS started after additional 16 min (E) and continued for 50 min, but the shifts of the curves remained high. c) AFM image of the P-S@dioct:DOPC structures adsorbed onto a supported lipid bilayer prepared on a SiO2 substrate.
Figure 3. Electrical characterization of P-S@dioct:DOPC structures.a) Supported lipid bilayers were prepared on nano-electrodes with 100 nm gaps, and 100 nm P-S@dioct:DOPC structures (500 μg/mL DOPC, 50 μg/mL PEDOT-S) prepared at pH 7.4 were then adsorbed. A time-series during 300 s with 0.5 V applied voltage show stable electronic currents (A). References with either only lipid bilayers (B) or lipid bilayers treated with PEDOT-S dissolved in MilliQ (100 μg/mL) during 15 min (C) were not conductive and the measurements were terminated after 7 s. (D) shows a voltage sweep for the P-S@dioct:DOPC structures between 0–1 V. b) 400 nm P-S@dioct:DOPC structures (500 μg/mL DOPC, 50 μg/mL PEDOT-S) were applied on Pt substrates and measured on by C-AFM. i) shows two representative 400 nm structures that had collapsed on the surface and had heights of 17 nm and 21 nm respectively. ii) shows the various spots that were measured and iii) shows the average currents in voltage sweeps between −100 to 100 mV for the two structures (17 nm black, 21 nm red), the surrounding lipid bilayer (green) and the clean Pt substrate (blue).
Figure 4. PEDOT-S quenches the fluorescent dye Nile Red in the P-S@dioct:DOPC structures.a) When the P-S@dioct:DOPC structures are prepared together with Nile Red, the fluorescence is significantly decreased compared with reference DOPC liposomes stained with Nile Red but without PEDOT-S@dioctyl-ammonium. b) The fluorescence decay of Nile Red is quicker (multiexponential) for P-S@dioct:DOPC structures than the monoexponential decay for Nile Red in DOPC liposomes (τ = 3.5 ns), which indicates quenching. Excitation wavelength was 550 nm in a) and 500 nm in b).
Figure 5. P-S@dioct:DOPC structures affect the voltage dependence of the Shaker K channel.(a) 0.33 μM P-S@dioct:DOPC applied to the extracellular solution increases the steady-state K+ current at −20 mV. (b) Current traces at −20 mV from a holding voltage of −80 mV. Same recording as in (a). (c) Steady-state K+ conductance vs. membrane voltage. (d) Dose-response curve of the induced shift. The best-fitted curve is ΔV = ΔVmax / (1 + (KD/c)n), where ΔVmax = −5.0 mV, KD = 0.30 μM, and n = 3.2. Neither control samples with DOPC liposomes without PEDOT-S@dioctyl-ammonium nor PEDOT-S added to the water phase, resulted in significant shifts. Data shown as mean ± SEM, n = 4 − 10. All concentrations given are based on the PEDOT-S monomer and the pH was 7.4 for all experiments.
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