Investigating the 8CB liquid crystal-insulin interaction: The role of the smectic a phase in enhancing detection...

Understanding the interactions between liquid crystals (LCs) and proteins is crucial for developing novel materials for biomedical and sensing applications. This study presents the first comprehensive multi-technique investigation of human insulin (HI) detection using octyl-4-biphenylcarbonitrile (8CB) LC, highlighting the enhanced sensitivity achieved when 8CB operates in its smectic A phase compared to its nematic homologs. The importance of this work lies in providing fundamental insights into how LC phase molecular ordering governs protein-LC interactions, rather than in establishing a new standard for biosensing sensitivity. A comprehensive investigation into the 8CB LC and HI interaction was conducted using polarizing optical microscopy (POM), molecular docking (MD), dielectric spectroscopy, Raman spectroscopy, and statistical analysis. POM images of 8CB interacting with HI revealed key textural transitions, including radial and focal conic domains across HI concentrations (20-300 μM), indicating a concentration-dependent disruption of LC alignment. Selectivity analysis using Grey Index (GI) and RGB values showed a strong correlation with logarithmic HI concentration (R² = 0.9446), confirming high specificity, especially at pH 7.4 and 70 mM NaCl. A significant time-dependent effect was established via one-way ANOVA (p = 0.047) and confirmed with Tukey's HSD (HSD = 1.283; 95 % CI [0.117, 2.683]), showing a large effect size (η² = 0.368). At the molecular level, docking identified a stable binding of 8CB to HI with an energy of -6.86 kcal/mol (RMSD = 1.12 Å), mediated by π-alkyl and hydrogen bonds involving Arginine, Cysteine, Phenylalanine, and Leucine residues. Dielectric analysis showed increasing relaxation times and tangent loss (tan δ) with rising HI concentration, while Nyquist plots confirmed the system's capacitive behavior. Raman spectra revealed concentration-dependent shifts in the 2500-3200 cm^-1 region, linked to CH stretching. Notably, the enhanced molecular ordering of 8CB in the smectic A phase facilitated HI detection at concentrations as low as 20 μM, representing a lower limit of detection (LOD) than that achieved with 5CB and 7CB. Overall, these observations confirm 8CB's high specificity and sensitivity, underscoring its potential as a robust platform for label-free insulin biosensing.