Photovoltage transient technique: an alternative approach to real-time analysis of membrane interface dynamic processes
Authors: Dou Yu-Jiang , Li Wen-Wen , Wei Yu-Shuang , Yang Kai , Yuan Bing ,   School of Electronic Information, Dongguan Polytechnic, Dongguan 523808, China,   Songshan Lake Materials Laboratory, Dongguan 523808, China,   Center for Soft Condensed Matter Physics and Interdisciplinary Research & School of Physical Science and Technology, Soochow University, Suzhou 215006, China
Published: October 2024

<sec>The dynamic interactions between active molecules and the cell membrane play a crucial role in various fundamental biological processes. The photovoltage transient technique developed in recent years has provided an <i>in situ</i>, real-time, and non-invasive approach to studying dynamic processes at the membrane interface. In this technique, the photoelectric response of silicon wafers is used to generate charges and record voltage transient pulses in the charging and discharging process of phospholipid membranes. These pulses directly reflect the instantaneous structure and properties of the membrane. By analyzing the temporal evolution of voltage pulses, the dynamic changes in membrane structure induced by molecular actions can be elucidated. In particular, this technique provides valuable insights into the timing of transitions between different functional states. In this paper, the working principle, equipment setup, and data processing methods used in photovoltage transient analysis are comprehensively reviewed. Furthermore, the use of supported phospholipid bilayers as model cell membranes highlights recent progress of this technique in investigating the mechanisms underlying membrane interactions of active molecules such as surfactants, polymers, peptides, and nanoparticles. Finally, its advantages and limitations are evaluated, and its development prospects are discussed.</sec><sec>The photovoltage transient technique is initially used to analyze the charging and discharging curves, as well as the hydration process, of single- and multi-layered membranes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) phospholipids. Previously, X-ray diffraction (XRD) and quartz crystal microbalance with dissipation (QCM-D) technology were commonly utilized for the real-time monitoring of the swelling process in phospholipid membranes, providing the information about changes in mass and thickness of <i>Z</i>-direction layers. In contrast, the photovoltage transient technique presents additional insights into the kinetics of the swelling process and timing of transitions between different stages. The present study demonstrates the effectiveness of photovoltage transient technique in real-time monitoring of membrane interface processes, specifically, it quantitatively measures the characteristic <i>τ</i> value of DOPC phospholipid membrane, thereby enabling further development of quantitative analysis method for this technique. Then, the photovoltage transient technique, in conjunction with giant unilamellar vesicle (GUV) leakage assays, atomic force microscopy (AFM) and QCM-D, is employed to monitor the structural perturbation of surfactants (TTAB) and polymers (Brij35 and PVPk30) on the membranes. Specifically, Brij35 primarily undergoes an adsorption-accumulation-penetration process; whereas PVPk30 exhibits a dynamic equilibrium between molecular adsorption-desorption and/or membrane permeation-healing competing mechanisms. This disparity in membrane action processes elucidates the discrepancy observed in their cytotoxicity during live cell experiments. The ability of photovoltage transient technology to investigate the entire membrane as a research subject along with its high sensitivity enables it to capture fluctuations in data points that reflect the coexistence of competitive mechanisms. Furthermore, photovoltage monitoring reveals the occurrence of peptide-induced membrane permeabilization. The distinct mechanism of action on the membrane between melittin (as a representative antimicrobial peptide) and TAT (a typical cell penetrating peptide) is elucidated. Finally, the conductive carbon dots (CDs) induced phenomena of membrane overcharging and overdischarging, are likely to be attributed to charge transfer between the silicon substrate and the embedded conductive CDs.</sec>