Label-free qualitative and quantitative protein assays were established with a single-substrate biosensing platform based on cholesteric liquid crystal (CLC) films with different ratios of the thickness d to the helical pitch P. By adjusting the amount of chiral dopant incorporated in CLC, the d/P ratio or the center wavelength of Bragg’s reflection band can be fine-tuned to achieve signal amplification or to improve detection sensitivity. The single-substrate platform eliminates the need for LC cell assembly, which requires a pair of glass substrates, and can be conveniently integrated with the format of most clinical assays performed on a single solid surface.
Liquid crystals (LCs) have been suggested to have a place in biological sensing for detection and quantitation of biomolecules. Through many years, texture observation has long been the core technique in LC-based bioassays. Its principle stems from the texture change of LCs due to the interruption of the initially homeotropic alignment in nematic bulks or the radial-to-bipolar configuration transition in LC droplets in the presence of biomolecules. Biodetection through optical texture observation is convenient but remains a qualitative method, and alternative approaches of detection and quantitative analysis are necessary for the development of a practical LC-based biosensor. In this study, we explore the potential of two dye-enhanced LCs, a dye LC (DLC) and a dye-doped LC (DDLC), in biosensing and protein quantitation. Depending on the chromophore or dye incorporated in DLC and DDLC, the dichroic features of these LCs enable us to analyze the change in their orientation in the presence of biomolecules by transmission spectrometry, from which spectral parameters are derived to establish novel LC-based protein quantitative methods.
The texture observation has long been the core technique in liquid crystal (LC)-based bioassays. Its working principle stems from the dark-to-bright texture change induced by the interruption of the initially homeotropic alignment in nematic bulks or from the radial-to-bipolar configuration transition in LC droplets in the presence of biomolecules. One of the drawbacks of this observational scheme, which requires a polarizing optical microscope, is the difficulty in quantitative analysis. In this invited paper, we report on our recent development of alternative optical biosensing techniques based on cholesteric LCs (CLCs) without the use of a polarizer. The increase in structural order in a vertically anchored CLC cell in the quasi-planar state provides a means to allow detection and quantification of the concentration of biomolecules immobilized on the interface between the mesophase and the surfactant DMOAP for LC vertical alignment.
The texture observation has long been the core technique in liquid crystal (LC)-based biosensing. One of the drawbacks of this observational means is the difficulty in quantitative analysis. In this invited paper, we report on our recent attempt to improve the LC-based biosensing technique through the analysis of bovine serum albumin (BSA), a protein standard commonly used in the assay of protein concentration. We propose to overcome the technical limitations in the analysis of LC-based biosensors by considering alternative measuring schemes. By means of the induced changes in electro-optical properties of LCs in the presence of different concentrations of BSA, novel quantitative techniques specific for LC-based biosensors are developed.
Liquid crystal (LC)-based biosensing has attracted much attention in recent years. We focus on improving the detection limit of LC-based immunoassay techniques by surface modification of the surfactant alignment layer consisting of dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP). The cancer biomarker CA125 was detected with an array of anti-CA125 antibodies immobilized on the ultraviolet (UV)-modified DMOAP monolayer. Compared with a pristine counterpart, UV irradiation enhanced the binding affinity of the CA125 antibody and reproducibility of immunodetection in which a detection limit of 0.01 ng/ml for the cancer biomarker CA125 was achieved. Additionally, the optical texture observed under a crossed polarized microscope was correlated with the analyte concentration. In a proof-of-concept experiment using CA125-spiked human serum as the analyte, specific binding between the CA125 antigen and the anti-CA125 antibody resulted in a distinct and concentration-dependent optical response despite the high background caused by nonspecific binding of other biomolecules in the human serum. Results from this study indicate that UV modification of the alignment layer, as well as detection with LCs of large birefringence, contributes to the enhanced performance of the label-free LC-based immunodetection, which may be considered a promising alternative to conventional label-based methods.
Liquid crystals (LCs) can be employed in biological sensing and applied to label-free immunodetection owing to their unique birefringent, anchoring, alignment and collective properties. Like all different kinds of immunoassays, both sensitivity and specificity are universally the most important key points of concern. In this study, we developed various approaches toward ultrahigh sensitivity in LC-based immunoassays for potential clinical applications. The LC-based immunodetection technique was demonstrated with the cancer biomarker CA125, which is a mucin-like glycoprotein commonly present in the serum of patients with ovarian and other types of cancer. By using LCs with larger birefringence, such as HDN, the sensitivity of immunodetection was drastically enhanced compared to 5CB, which has a relatively lower birefringence and is commonly used in LC biosensing studies. In addition, UV modification of the monolayer of dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP), which functions as the alignment layer for LCs, is suggested to increase functional groups suitable for covalent binding of biomolecules, stabilizing the immobilized anti-CA125 antibody and the immunocomplex thus formed, and contributing to the lowered detection limit. Finally, we show that it is possible to directly identify the formation of CA125 immunocomplex with HDN in a mixture of antigen and antibody without the need to eliminate unbound or unreacted biomolecules through washing, thereby creating a simplified procedure for faster LC-based immunoassay. It is evident from our results that label-free immunodetection based on birefringent LCs represents a novel biosensing technique with potentials to detect a wide range of biomolecules, providing an alternative to conventional label-based immunoassays.
A label-free and array-based optical liquid-crystal (LC) immunodetection technique for the detection of CA125 antigen, a protein biomarker most frequently used for ovarian cancer detection, was demonstrated with a nematic LC with larger birefringence (Δn) to promote sensitivity in detecting biomolecules. The LC-based immunodetection offers an alternative and sensitive approach for the detection of biomarker proteins, with the potential of replacing conventional immunoassays used in biochemical studies and clinical laboratories.
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