Quantitative Biosensing Based on a Liquid Crystal Marginally Aligned by the PVA/DMOAP Composite for Optical Signal Amplification

Abstract

The working principle for a liquid crystal (LC)-based biosensor relies on the disturbance in the orderly aligned LC molecules induced by analytes at the LC-aqueous or LC-solid interface to produce optical signals that can be typically observed under a polarizing optical microscope (POM). Our previous studies demonstrate that such optical response can be enhanced by imposing a weak electric field on LCs so that they are readily tilted from the homeotropic alignment in response to lower concentrations of analytes at the LC-glass interface. In this study, an alternative approach toward signal amplification is proposed by taking advantage of the marginally tilted alignment configuration without applying an electric field. The surface of glass substrates was modified with a binary aligning agent of poly(vinyl alcohol) (PVA) and dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP), in which the amount of PVA was fine-tuned so that the interfacing LC molecules were slightly tilted but remained virtually homeotropically aligned to yield no light leakage under the POM in the absence of an analyte. Two nematic LCs, E7 and 5CB, were each sandwiched between two parallel glass substrates coated with the PVA/DMOAP composite for the detection of bovine serum albumin (BSA), a model protein, and cortisol, a small-molecule steroid hormone. Through image analysis of the optical appearance of E7 observed under the POM, a limit of detection (LOD) of 2.5 × 10−8 μg/mL for BSA and that of 3 × 10−6 μg/mL for cortisol were deduced. Both values are significantly lower than that obtained with only DMOAP as the alignment layers, which correspond to signal amplification of more than six orders of magnitude. The new approach for signal amplification reported in this work enables analytes of a wide range of molecular weights to be detected with high sensitivity.

Liquid Crystal Biosensors: Principles, Structure and Applications

“…Azobenzene LC polymer-grafted gold nanoparticles are homeotropically aligned and the transition of the alignment (from homeotropic to random) can be efficiently achieved by the illumination [ 80 ]. Chang et al developed a signal amplification approach in an LC biosensor [ 78 ] ( Figure 2 e) by employing PVA to preserve a small oblique alignment of LCs. Polyvinyl alcohol (PVA), a negatively charged planar alignment agent, which is able to introduce tiny pretilt angles, can be used with positively charged vertical surfactant DMOAP to produce a hybrid alignment layer on glass substrates, to adjust the tilted alignment state of LC molecules via the electrostatic interaction.…”

“…( b ) POM images of LC aptasensor cells with tetracycline at various concentrations of: (b1) 0.1 pM; (b2) 0.5 pM; (b3) 5 pM; (b4) 10 pM; (b5) 100 pM; (b6) 250 pM; (b7) 500 pM [ 74 ]; reproduced with permission from Elsevier. ( c ) POM images of the LC aptamer sensor [ 75 ]; copyright with permission from Abbasi et al ( d ) Schematic illustration of the LC sensing strategy for detecting AMX (d1) Cleaned the bottom glass slide; (d2) upper glass slide; (d3) build self-assembled monolayer; (d4) grafting of GA onto APTES/DMOAP-modified glass substrate; (d5) immobilization of the AMX aptamer; (d6) absence of AMX results in LCs orienting homeotropically; (d7) POM image of the LC cell in the absence of AMX; (d8) binding of AMX to AMX aptamers disrupts the orientation of LCs; (d9) POM image of the LC cell in the presence of AMX [ 77 ]; copyright with permission from Nguyen et al ( e ) Illustration of the DMOAP-coated and the PVA/DMOAP-coated substrates used to achieve the tilted state of the LCs [ 78 ]; copyright with permission from Chang et al…”