Photocontrolled capacitive biosensor based on photoresponsive azobenzene-doped liquid crystals for label-free protein assay

Azobenzene-doped liquid crystals (ADLCs) have found widespread applications in fields such as holographic technology, photoactuators, and smart windows, but their potential in biosensing has not been explored. In this study, ADLCs were prepared by doping the nematic liquid crystal (LC) host E7 with a photochromic azobenzene dye ChAD-3C-S, which, upon exposure to ultraviolet (UV) light, underwent trans - cis photoisomerization and induced the phase transition of ADLCs from the cholesteric to the nematic state. The isomerization and phase transition are reversed when the nematic ADLCs are irradiated with green light. A biosensing platform based on the photoresponsive ADLCs for the detection and quantitation of bovine serum albumin (BSA) at the LC-glass interface was constructed by assembling a sandwiched LC cell with a pair of glass substrates modified with the homeotropic alignment reagent dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP). By immobilizing BSA on one of the DMOAP-coated glass substrates, the concentration-dependent effect of the biomolecule on the physical properties of ADLCs was analyzed quantitatively through multiple modes of detection including optical texture observation under a polarizing optical microscope as well as transmission spectrometry and capacitance measurements. In the photocontrolled ADLC-based capacitive biosensor, BSA was quantitated by measuring the time-evolved capacitance of an ADLC during the trans–cis-trans isomerization of ChAD-3C-S by alternating the wavelength of incident light, which improved the limit of detection (LOD) to 8.8 × 10⁻³ μg/ml, an order of magnitude lower than that obtained by spectrometric analysis. Results from this study suggest that the photocontrollable and fast-response features unique to ADLCs may create new possibilities for the development of innovative LC-based biosensing technologies.