Linking Nanoscale Film Properties to Electrochemical Response in Cytochrome P450 Membrane Enzyme Films

Membrane-bound human liver cytochrome P450 (CYP) enzymes catalyze key steps in xenobiotic metabolism, yet their integration into analytical devices depends critically on surface chemistry and film mechanics. Bacterial membrane preparations ("bactosomes") provide abundant CYP sources for in vitro measurements and biosensors. Here, cystamine self-assembled monolayers (SAMs) on gold-coated quartz were compared with polyethylenimine (PEI)-coated gold for immobilizing CYP bactosomes. Quartz crystal microbalance with dissipation monitoring (QCM-D) tracked adsorption and film evolution, enabling quantitative estimates of the mass loading, thickness, and viscoelastic properties. After washing, bactosomes/SAM reached ∼113 nm thickness and 11.9 μg cm^-2 mass, whereas bactosomes/PEI yielded ∼40 nm and 4.2 μg cm^-2. Viscosity values were similar (2.5 cP), but elastic moduli differed: 2.0 × 10⁴ Pa for bactosomes/SAM vs 2.5 × 10⁵ Pa for bactosomes/PEI, indicating rigid films on PEI. Electrochemical characterization under argon and O₂-saturated conditions showed higher faradaic responses for SAM-modified electrodes (I _p = 5.6 ± 0.6 μA in Ar; I _limiting = 95.9 ± 3.9 μA in O₂) than PEI-coated electrodes (I _p = 3.2 ± 1.0 μA in Ar and I _limiting = 72.6 ± 3.3 μA in O₂). These differences are attributed to the greater immobilized mass of bactosomes on the SAM-modified surface and the proportional electronic connectivity of their redox enzymes with the underlying gold electrode despite their higher film thickness. This work provides quantitative benchmarks for tuning film mechanics and coverage to optimize CYP-bactosome biocatalytic electrodes, informing the design of robust, high-signal platforms for biosensing and high-throughput drug metabolism assays. Correlation of QCM-D metrics with voltammetric outputs establishes a direct link among film mechanics, coverage, and electron-transfer efficiency, offering a generalizable framework for membrane enzyme immobilization. These measurements provide practical guidance for selecting surface chemistries when prioritizing either robustness or signal amplitude in CYP-based sensing architectures.