Application Cases

Experiment Name: High-Throughput Protein Photoelectric Biosensor Based on Liquid Crystals

Research Direction: Biorecognition and Detection

Test Objective:
Protein analysis is an important method for disease diagnosis and medical research. This paper proposes a single-substrate liquid crystal biophotoelectric sensor capable of rapidly detecting protein concentrations. Experiments have shown that the single-substrate liquid crystal biophotoelectric sensing method offers advantages such as electrical modulation and high efficiency, providing a new technical approach for the application of liquid crystal devices.

Testing Equipment: Signal generator, ATA-7020 high-voltage amplifier, glass substrate, heating stage, ultraviolet (UV) lamp, polarizing microscope

Experimental Procedure:

Figure 1: Schematic Diagram of the Interdigitated Electrode on the Sample

Interdigitated electrodes were fabricated on ITO glass (2 cm × 2 cm). The entire electrode area has a length of 2 cm and a width of 1.2 cm. To ensure that the liquid crystal molecules receive a sufficiently high voltage between the electrodes, the spacing between electrodes was set to 50 μm, as shown in Figure 1. A DMOAP layer was first prepared on the substrate to form a vertical alignment layer. BSA solution was diluted to specific concentrations using deionized water, and the BSA solution was dropped onto the DMOAP-modified glass substrate (one BSA droplet covered one interdigitated electrode). The glass substrate was then placed on a heating stage and heated at 30°C for 30 minutes to cure the BSA. Next, the LC/NOA65 mixture was dropped onto the glass substrate and spin-coated evenly. It was exposed to UV light with an intensity of 15 mW/cm² for 30 seconds to form a polymer network. A polarizing microscope (POM) was used to observe the transmittance of the sample under crossed polarizers to calibrate the BSA concentration.

Figure 2: Experimental Setup of the Biosensing System

Figure 2 shows the experimental setup for protein concentration analysis. The signal generator and ATA-7020 high-voltage power amplifier were connected via a coaxial cable to apply an electric field to the liquid crystal device.

Experimental Results:

1. Single-Substrate Concentration Detection:
First, a DMOAP vertical alignment layer was prepared on the ITO glass substrate. Different concentrations of BSA were dropped and fixed. The LC/NOA65 mixture (with NOA65 content of 3 wt%) was then prepared, spin-coated onto the glass substrate, and cured by UV irradiation. A voltage was applied between the positive and negative electrodes of the interdigitated electrode, with the voltage range controlled between 0 and 50 V. The brightness change of the liquid crystal under voltage was observed using a polarizing microscope.

2. High-Throughput Analysis Approach:
When dropping BSA, the area of droplet spreading varies with concentration, making it difficult to detect multiple concentration samples simultaneously. To overcome this difficulty, a photolithography method was used to fabricate an array of square cells (liquid crystal cells) between the ITO surface electrodes, with cell dimensions of 800 μm × 800 μm. Different concentrations of BSA could be dropped into each liquid crystal cell. To ensure that the dropped BSA remained completely within a single cell without mixing with other concentrations, a capillary with a diameter of 300 μm was used for spotting. Subsequently, a liquid crystal polymer layer was prepared on top, and observations were made under a POM.

The fabricated single-substrate liquid crystal biosensor with in-plane electrodes exhibited improved linearity of sensing characteristics when an external electric field was applied. The detection range for BSA reached 10⁻³ – 10⁻⁷ g/mL, with a detection limit of 10⁻⁷ g/mL. The use of photolithography to prepare an array of liquid crystal cells on the substrate improved the efficiency of detecting protein concentrations. Due to the limitations of the polarizing microscope's field of view, the number of samples that could be detected simultaneously was relatively small. In the future, increasing the number of simultaneously detectable channels could be achieved by improving photolithography precision to reduce the size of the liquid crystal cells.

Aigtek ATA-7020 High-Voltage Amplifier:

Figure: Specifications of the ATA-7020 High-Voltage Amplifier