| Droplet interface bilayer configuration; two lipid-monolayer-coated aqueous droplets suspended in hexadecane from Ag/AgCl-tipped microtip electrodes |
Droplet interface bilayers were formed by contacting the two lipid-monolayer-coated droplets; the droplet connected to the current-amplifier headstage was defined as cis and the grounded droplet as trans, with positive current corresponding to cation flow from cis to trans |
| Baseline DPhPC-only or DoPhPC-only films |
Final lipid concentration 2 mg/mL, corresponding to 2.34 mM DPhPC or 2.45 mM DoPhPC |
| Electrical monitoring of bilayer formation and stability; 10 mV, 10 Hz triangular voltage waveform applied across the membrane; bilayer modeled as a parallel resistor and capacitor |
Resistance and capacitance were extracted by fitting the square-wave current response to the parallel-RC circuit model; the capacitance–voltage curve across applied DC voltages was fit to a quadratic function to determine Vₘᵢₙ, the voltage at minimum capacitance, and Cₘᵢₙ, the corresponding minimum capacitance |
| Empirical relationship between nominal bilayer capacitance C and applied voltage V |
C = C₀(1 + α(V + Vᵣ)²), where C₀ is capacitance at V = 0, α is the voltage-dependent fractional capacitance increase from electrowetting and electrocompression, and Vᵣ is the intrinsic membrane potential defined as the total difference between the dipole and surface potentials of the opposing lipid leaflets; capacitance is minimized at V = −Vᵣ |
| Capacitance–voltage acquisition and analysis; 10 mV, 10 Hz triangular AC waveform superimposed on DC steps in 25 mV increments, each held for 20 s to allow membrane capacitance to equilibrate; steady-state current waveform from each DC step fit to a parallel-RC equivalent circuit to extract the corresponding capacitance value; before data collection, a priming sweep from 0 to +250 mV pre-conditioned the membrane, followed by a continuous descending sweep from +250 to −100 mV during which all C–V data were collected; capacitance at each step normalized to zero-volt membrane capacitance (C/C₀) to account for differences in nominal bilayer area among separate DIBs; resulting C–V curves fit with a quadratic polynomial to extract Vₘᵢₙ |
Vₘᵢₙ = V(min(C)), the applied voltage at minimum capacitance |
| Reference-bilayer Vₘᵢₙ: symmetric DPhPC|DPhPC; symmetric DoPhPC|DoPhPC; fully asymmetric DPhPC|DoPhPC (trans|cis), with Vₘᵢₙ = |Vᵣ|; fully asymmetric DoPhPC|DPhPC (trans|cis), with Vₘᵢₙ = |Vᵣ| |
−1.5 ± 2.3 mV; −2.9 ± 3.1 mV; +135.3 ± 5.4 mV; −133.4 ± 3.5 mV, respectively; the two symmetric bilayers confirmed near-zero net membrane potentials (Vᵣ ∼ 0); the sign depended on which lipid leaflet was connected to the headstage electrode versus the ground electrode |